An overview of a novel concept in biomass pyrolysis: microwave irradiation

Abstract

The increasing demand for renewable fuels and chemicals necessitates the exploration of alternative sources to replace petroleum sources. Biomass has been viewed as the most promising source to produce sustainable fuels and chemicals. Underpinning the key advantages of microwave heating (e.g., rapid and controlled heating, energy saving, and no requirement for agitation or fluidization), microwave-assisted pyrolysis (MAP) is one of the most attractive techniques for the valorization of biomass, which are more amenable to produce three high quality products: bio-oil, gas, and bio-char. In this respect, this article reviews the biomass pyrolysis using microwave irradiation from several points of view, starting from fundamentals of microwave irradiation, types of microwave absorbers, and chemistry of non-catalytic MAP and focusing on chemistry of catalytic MAP plus various categories of catalysts. Recent progress in the experimental studies on both non-catalytic MAP and catalytic MAP of biomass is also demonstrated with emphasis on the bio-oil yield and quality. Additionally, reaction kinetics and future prospects in the light of current studies are also given in this review. Consequently, this review illustrates both the highlights of significant achievements from biomass pyrolysis using microwave irradiation and the milestones that are necessary to be obtained in the future.

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Xuesong Zhang

Xuesong Zhang is a Postdoctoral Research Associate at the Illinois Sustainable Technology Center, University of Illinois Urbana-Champaign. He received his B.E. degree in 2012 from Northwest A&F University (China), and his PhD degree from Washington State University in 2016. His research focuses on the thermochemical conversions including microwave pyrolysis and hydrothermal liquefaction, catalysis, bio-oil upgrading for advanced jet fuels plus valuable chemicals, as well as treatment of aqueous phase from biocrude/bio-oil, and reverse osmosis of treated aqueous phase to recover nontoxic water and value-added products. He has co-authored over 20 relevant peer-reviewed publications.

Kishore Rajagopalan

Dr Rajagopalan is an Associate Director at the Illinois Sustainable Technology Center and was appointed as the State Pollution Prevention Scientist in 2013. He has over 30 years of experience in plant operations, green process development, separations, and pollution prevention research. His personal research interests are in the integration of separation research to advance green process development with a special focus on membrane processes. He has a BS in Chemical Engineering and a Ph.D. in Food Science along with a postgraduate diploma in management. He has over 50 peer-reviewed publications and holds 5 U.S. patents.

Hanwu Lei

Hanwu Lei received his PhD degree in Biosystems and Agricultural Engineering from University of Minnesota, Twin Cities (USA) in 2006. He is an associate professor of Biological Systems Engineering at Washington State University. His research is on discovering and applying novel approaches for renewable energy and value-added process development from agricultural feedstocks and products. Dr Lei's research activities include biomass thermochemical and biochemical conversions to biofuels and bioproducts. His research is focused on microwave pyrolysis, catalysis processes, bio-oil upgrading, and biomass carbon utilization for jet fuels, phenols, aromatics, cycloalkanes, hydrocarbons, activated carbon, and bio-based chemical production.

Roger Ruan

Roger Ruan is a Distinguished Guest Professor of Nanchang University and a Professor and Director of Center for Biorefining at the University of Minnesota. He received his Ph.D. degree from the University of Illinois at Urbana-Champaign in 1991. His current interests are in nonthermal plasma technology development and application, algae and aquaponics system for waste utilization and treatment, solid waste conversion for energy fuels, chemicals, and materials production, and food shelf stability and quality enhancement and safety assurance. He has published over 400 peer-reviewed papers, books and patents.

Brajendra K. Sharma

B. K. Sharma joined the Prairie Research Institute – Illinois Sustainable Technology Center at the University of Illinois, Urbana-Champaign in 2009 as a Senior Research Engineer. In February 2016, he was elected a fellow of the Society of Tribology & Lubrication Engineers. His research interests include thermochemical conversion of biomass and waste materials, biolubricants, bio-oil derived chemicals and biobased products. His current research is focused on conversion of waste plastics, tires, and waste lipids into liquid fuels using pyrolysis and hydrothermal liquefaction; recovery of value-added products, antioxidants from bio-oils; upgradation of bio-oils through a low-cost catalytic and extraction process; and bio-oil derived bioasphalts.

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1. Introduction

The depletion of fossil fuel resources and resulting climate changes are of preliminary economic, academic and politic concern worldwide. Alterations to the climate because of the greenhouse effect are threatening to humans and other species, originating from anthropogenic activities including the burning of fossil fuels for power generation.¹ In this context, energy consumption is projected to continuously increase with an average annual growth of approximately 1.6% in the next two decades; whilst the supplies of fossil fuel resources for energy generation are limited.² Alternative solutions are sought to switch from conventional to renewable resources, e.g., biomass. The valorization of biomass is currently an area of utmost interest since the utilization of biomass has the potential to substitute significant amounts of petroleum-derived fuels. Moreover, biomass is abundantly available currently, offering over 10% of the global energy supply, and being recognized as the top four energy sources in the light of world final energy consumption in 2011.^1,3

Biomass generally refers to wood waste, agricultural crops and their residues, municipal solid waste, food waste, animal waste, algae, aquatic plants etc. For instance, lignocellulosic biomass (e.g., wood, grass, and agricultural residues) is an abundant and carbon-neutral resource found on the earth,² which makes the production of second generation biofuels highly attractive in a broad sense.⁴ Lignocellulosic biomass is primarily comprised of cellulose, hemicellulose and lignin. Cellulose and hemicellulose consist of complex polysaccharides, while lignin as highly substituted and mononuclear phenolic amorphous polymers is made up of phenylpropane units.⁵ Microalgae are another very important biomass source containing proteins, carbohydrates, enzymes, vitamins, and minerals.⁶ In particular, biofuels evolved from microalgal biomass are identified as third generation biofuels.^7,8 It is discerned that microalgae have a much higher capacity to manufacture 30–100 times more energy per hectare in comparison with terrestrial corps.⁹ Despite some limited use of biomass typically for power generation and low quality bio-crude production, biomass still appears to be the most attractive and feasible form to produce advanced biofuels, fine chemicals and byproducts with net zero carbon emission.¹⁰

There are several technologies that have been extensively evaluated and implemented in past decades to valorize biomass, which can be mainly divided into two platforms: thermochemical and biological technologies. A biological pathway generally involves the application of various microorganisms and bio-catalysts for the transformation of biomass into high value-added fuels, chemicals, gases and other valuable outputs.¹¹ Nevertheless, biological conversions of biomass are not cost-effective mainly because of the fact that the biochemical techniques can be merely devoted to the utilization of cellulose and hemicellulose fractions.¹² In contrast, thermochemical approaches are obviously more energy efficient and flexible pertaining to feed and products, including pyrolysis, gasification, combustion, reforming and hydrothermal conversions.¹³ Among the primary thermochemical techniques, pyrolysis is a prominent and well-recognized platform that has been extensively investigated and employed to valorize biomass into valuable products.^12,14 Typical pyrolysis is to heat biomass in the absence of oxygen at 400–600 °C, yielding bio-oil, biochar, and biogas. Fast pyrolysis of biomass can garner a very high yield (up to 80 wt% of dry feed) of liquid product retaining main energy (up to 70%) in the so-called bio-oils.^15,16 Bio-oil has been considered as the cheapest biofuel and has attracted crucial research concerns in the past two decades.¹⁷ Moreover, the technique of biomass pyrolysis is on the verge of commercialization.¹⁸

Bio-oil derived from the immediate quenching of pyrolytic vapors is a viscous black liquid and it is composed of over 300 oxygenated compounds.¹⁹ Typical bio-oil mainly contains three major classifications of compounds, i.e., sugar-evolved compounds, small carbonyl compounds, and lignin-derived compounds.²⁰ The distribution of these compounds in the bio-oil is dominantly dependent on the category of biomass and pyrolysis severity.²¹ It is evident that the high water content (15–30 wt%) triggers a low heating value of the bio-oil.^20,22 A large variety of carboxylic acids renders low pH values of 2–2.5.²³ In general, the instability of bio-oil is mostly attributed to the oxygen-rich nature, thereby rendering the main difference between bio-oils and hydrocarbon fuels.^20,24 As a result, these detrimental properties (e.g., high acidity, low heating value, high reactivity, and high viscosity) of bio-oils hamper their straightforward application as transportation fuels.^15,20

Enhancing bio-oil quality toward its properties identical to those of petroleum-derived fuels is necessitated.²⁵ Efforts of rejecting its high oxygen content should be devoted to via certain upgrading technologies.^15,19 These technologies should be developed as efficient processes that transform biomass into biofuels, showing high compatibility and energy density with the current energy infrastructure.²⁶ Advanced biomass conversion technologies have been undertaken to achieve these goals in the last two decades.^15,16 Of these, catalytic deoxygenation has been widely investigated to improve the bio-oil quality and primarily includes two platforms: catalytic cracking and hydrodeoxygenation (HDO).^27,28 Catalytic cracking is a regular technique that implements solid acid catalysts in the pyrolysis system and it is performed at atmospheric pressure without the introduction of hydrogen.²⁹ Yet, it is noted that catalytic cracking of bio-oils is not promising owing to the low quality products observed with a low carbon yield and significant coke formation, leading to a short catalyst lifetime.³⁰ HDO of bio-oil adopts petroleum oil hydrotreating protocol by using supported metal catalysts in a pressurized hydrogen atmosphere to achieve desired hydrocarbons.^17,31 This technology is attractive and has been developed in terms of the substantial increase in high quality products.³² Yet, the HDO process has to be carried out under high-severity conditions (e.g., elevated reaction temperature and high hydrogen pressure). High operating costs due to costly noble catalysts utilization, substantial hydrogen consumption, and significant catalyst deactivation are endured in the HDO process as well.³³

Instead, catalytic fast pyrolysis (CFP) is an alternative approach to directly upgrade hot pyrolytic vapors assisted by a suitable catalyst prior to quenching pyrolytic vapors to gain bio-oil.¹⁴ The in situ catalytic fast pyrolysis (referred to as in situ CFP) is conducted by directly combining biomass with the catalyst; when the catalyst placed in a downstream reactor only upgrades the pyrolytic vapors, the process is referred to as ex situ catalytic fast pyrolysis (ex situ CFP).³⁴ CFP has been suggested to be more amenable for directly converting biomass into high grade bio-oils with improved stability since it can get rid of polymerization and re-evaporation of bio-oils.^{21,23,35–40} It is noteworthy that zeolite catalysts (e.g., HZSM-5) have been considered as the most efficient catalysts to generate considerable petrochemicals (aromatics and olefins).^41–45

It is found that conventional pyrolysis suffers from some intrinsic drawbacks such as heat transfer resistance, heat losses to the surroundings, non-selective heating, damage to reactor walls due to continuous electrical heating, etc.^46,47 Due to the above-mentioned problems, the integration of microwave irradiation and pyrolysis process is a novel conceptual development to effectively valorize biomass/organic waste as sketched in Fig. 1.^11,48,49 MAP technology is emerging as one of the most promising approaches of enhancing and accelerating chemical reactions due to effective heat transfer profiles through microwave irradiation.^50,51 Furthermore, MAP encloses the potentials of easy control of reaction conditions, fast and selective heating, low reaction temperatures, and low energy requirements.^11,52

Fig. 1 The representative concept of microwave-assisted pyrolysis. Adapted from ref. 11.

Identical to CFP, a novel concept of catalytic microwave-assisted pyrolysis (CMAP) has been presented to generate outstanding products by using catalysts such as HZMS-5. As expected, CMAP not only obtains high quality biofuels and fine byproducts, but also possesses perceived advantages of MAP. Very recently, catalytic microwave-assisted co-pyrolysis (CMACP) is attracting substantial attention worldwide, and it is recognized as one of the most advanced pyrolytic technologies, outperforming conventional CFP in terms of highly desirable products.^53–55 Given high grades of bio-oil, bio-char, and biogas derived from the CMAP of biomass, this technique paves a novel and economically feasible pathway for the valorization of biomass on a large scale.⁵³ In the light of these premises, the CMAP of biomass is clearly beneficial for environment and energy recapture.

Numerous comprehensive reviews have reported pyrolysis and catalytic pyrolysis of biomass. Some recent reviews have also highlighted MAP of lignocellulosic biomass for biofuel production.^4,51,56–61 To the best of our knowledge, the CMAP of biomass (with the concentration on lignocellulosic biomass and microalgae) has never been reviewed. This is an unprecedented review that aims to shed light on the technology with respect to the CMAP of biomass. Herein, it starts with the principles of microwave irradiation, giving special attention to the comparison between conventional heating and microwave heating. The role and categories of microwave absorbers are also discussed. Furthermore, fundamental chemistry and kinetics of the reactions in CMAP are elucidated as well. In order to understand the state-of-the-art situation of CMAP, up-to-date research efforts regarding CMAP of biomass are summarized. With the consideration of microwave irradiation contribution in catalytic pyrolysis, this review will focus principally upon recent advances in the improvements of product quality and yields for the sake of future commercialization.

2. Microwave irradiation

2.1 Fundamentals of microwave irradiation

Microwaves are generally defined as non-ionizing radiations, namely electromagnetic waves that are comprised of two perpendicular components (i.e., electric and magnetic fields).⁵¹ Microwave is recognized as an unconventional source of energy since it can be transmitted, reflected or absorbed toward the reagents.⁶² In the electromagnetic spectrum as shown in Fig. 2, microwaves fall in between the infrared and radio wave regions of the electromagnetic spectrum with the wavelengths ranging from 0.01 to 1 m, and the corresponding frequency varying from 0.3 to 300 GHz.^51,63

Fig. 2 The frequency and wavelength of microwave radiation region. Adapted from ref. 64 and 65.

With this region, the microwave frequencies interfere with the frequencies allocated to cellular phones and telecommunications.^57,59,65 To avoid any interference, domestic microwave ovens and most microwave reactors for chemical synthesis are operated at the frequency of 2.45 GHz, corresponding to a wavelength of 12.25 cm.^65,66 At 2.45 GHz, designed microwave ovens are prone to heating foods; meanwhile the water inside acts as a good absorber of microwave irradiation.⁶⁷ The four frequencies of 0.915 GHz, 2.45 GHz, 5.8 GHz, and 24.124 GHz are reserved by the Federal Communications Commission (FCC) for industrial, scientific, and medical (ISM) purposes.⁵¹ Most industrial microwave processing for chemical synthesis is conducted at the frequencies of 0.915 GHz and 2.45 GHz, which are assigned for heating applications.⁵⁹

It is observed that the conversion efficiencies of electrical energy to microwave energy are approximately 85% and 50% for 0.915 GHz and 2.45 GHz, respectively.⁵⁷ In comparison with 2.45 GHz, the utilization of 0.915 GHz can offer a substantially large penetration depth, which is an essential indicator in the evaluation of the microwave assimilation capacity of materials, design of the microwave cavity size, and the process of scale-up.⁵⁹ Microwave penetration depth is also affected by material types, microstructure properties, and temperature. For instance, microwave penetration depth in water is just 1.4 cm by using the frequency of 2.45 GHz at 25 °C, whereas its penetration depth is enhanced to 5.7 cm using the same frequency at 90 °C.⁵⁹ In the case of quartz glass, the microwave penetration depth can reach up to 160 m. As for the microwave penetration depth in biomass, it is mainly dependent on biomass density, composition, water content, etc.⁵⁹ The microwave penetration depth in representative materials are given in Table 1.

Table 1 Dielectric loss tangent and the penetration depth of various representative materials at 2.45 GHz and room temperature^74,88–93

Microwave transparent			Reflector		Microwave absorber
Material	tan δ	d p (m)	Bulk metal	d p (μm)	Material	tan δ	d p (cm)
Alumina	1.0 × 10^−3	12.65	Al	1.7	Water	0.15	3
Fused quartz	3.0 × 10^−4	75.73	Cu	1.3	SiC	0.37	1.93
Borosilicate glass	1.2 × 10^−3	15.7	Au	1.5	Carbon black (20 μm)	0.23	5.75
Teflon	4.8 × 10^−4	56.4	Ag	1.3	Graphite powder (20–80 μm)	0.36–0.67	1.34–2.09
PVC	5.6 × 10^−3	4.03	Zr	6.7	Graphite carbon powder (60–80 mesh)	0.4–1	0.5–0.9
Polystyrene	3.0 × 10^−4	76.2			Activated carbon	0.31–0.9	0.7–3.43
Silicon	<1.2 × 10^−2	>3.96			Charcoal	0.14–0.38	6–11

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2.2 Principles of microwave heating

Microwave heating is a non-contact energy transfer process that transforms electromagnetic energy into thermal energy at certain frequencies, implying fast heating rates, where electromagnetic energy is effectively absorbed by the materials under microwave irradiation.⁶⁹ Microwave heating consists of two general mechanisms: dipolar polarization and ionic migration as sketched in Fig. 3.^59,69–71 It is mainly attributed to the alignment of dipoles (Fig. 3A) or ions (Fig. 3B) in the electric field.⁶⁸ With regard to polar molecules, the electric field component of microwave results in water and other polar fluid (e.g., ethanol) molecules rotating and aligning in both induced and permanent dipoles with an alternating field, which is named as dipolar polarization.⁶⁹ The molecule friction and collisions caused by increased molecular rotation and movements thereby lead to microwave heating and heat loss.⁶⁶ As for ionic migration, dissolved charged particles oscillate back and forth under microwave irradiation, and they also collide with neighboring molecules, thereby generating heat.⁴ It is noteworthy that the ionic migration mechanism has a more significant influence than the dipolar polarization pertaining to the heat-generating capacity.⁶⁸

Fig. 3 The fundamental mechanism of microwave heating: (A) dipole rotation; (B) ionic migration. Adapted from ref. 68.

As the response of different materials to microwave irradiation is not comparable, only partial materials are amenable to microwave heating. According to the response to microwave irradiation, there are three types into which a material can be broadly categorized: (i) insulators or microwave-transparent materials in which the microwaves pass through without any energy losses (e.g., quartz, Teflon, etc.); (ii) conductors where the microwaves cannot penetrate through and reflect back (e.g., metals); and (iii) absorbers where the microwaves can pass through and be absorbed (e.g., water, oils, etc.).^51,72 Since materials that absorb microwave irradiation are usually called as dielectrics or absorbers, microwave heating is also defined as dielectric heating.⁶⁴

2.3 Comparison between microwave heating and conventional heating for biomass pyrolysis

Conventional thermal heating is widely known as a traditional heating pattern with heat transfer from the surface to the center of a material by conduction, convection, and radiation.^4,51 Nevertheless, conventional heating is extremely slow and inefficient, which is relatively dependent on the convection currents and thermal conductivity of the material.⁵¹ In contrast, microwave heating, transforming electromagnetic energy into thermal energy, is viewed as one energy conversion rather than heating transfer.⁵¹ Since microwaves can penetrate materials and deliver energy, heat can be generated through the volume of materials (referred to as in-core volumetric heating). The formation of hot spots is a vital property of microwave heating, which is derived from the inhomogeneity of microwave field or dielectric characteristics within the feedstock.^4,73 Thus, the accurate temperature inside the feedstock is much higher than the measured temperature in the bulk. It is proven that hot spots play a significant role in the yield and quality of microwave processing products. To control or prefer the hot spot phenomenon, several important factors (such as cavity design and operating conditions) can be implemented to modify the phenomenon.⁷³

The different mechanisms for microwave and conventional heating patterns are demonstrated in Fig. 4. Unlike conventional heating, microwave heating shows a much higher temperature at the central of the material comparing to the surrounding material. Table 2 lists the specific details regarding the advantages and disadvantages for both microwave heating and conventional heating. There are many perceived merits of microwave heating, including uniform internal heating of large biomass particles, rapid and controlled heating, and no requirement for agitation or fluidization.⁷⁵ Microwave heating is more efficient than conventional heating because microwave heating can overcome the major issues of conventional heating, including limitations of specific heat and density, heterogenic heating of the surface, and thermal conductivity of materials.⁷⁶ Moreover, it requires less energy input for heating feedstock in comparison with conventional heating approach.⁶² It has been estimated that 15% of energy in biomass feed is required to drive a conventional pyrolysis process.²⁵ Approximately 2.7–3.1 kJ g⁻¹ energy input is required for woody biomass pyrolysis (assuming a gross calorific value of 18–20.5 kJ g⁻¹).⁷⁷ However, the energy consumption for MAP of woody biomass to achieve an acceptable degree of pyrolysis is in the range of 2.2–2.5 kJ g⁻¹;⁷⁸ increasing the power density to 100 MW m⁻³ can even allow a decrease in specific energy to as little as 1.07 kJ g⁻¹.⁷⁹ Underpinning these key advantages, studies of MAP of biomass have been recently reported, and indicate that MAP is a highly scalable technology suitable for distributed conversion of biomass.^4,11,51,59

Fig. 4 Microwave-induced and conventional heating patterns. Adapted from ref. 74.

Table 2 Advantages and limitations shown by microwave heating versus conventional heating techniques for biomass pyrolysis

Technique	Advantages	Disadvantages
Microwave heating	• Broad range of feedstock for valorization and wide range of higher quality products is achieved^11,67	• Inherent issue of temperature measurement and uniformity^4,11
	• Non-contact and volumetric heating^4,51,57,59,66,67	• Penetration depth inhibits the large reaction vessel for scaling up^65,67,80
	• Energy transfer rather than heat transfer^4,51,57,67	• Uncontrolled heating leads to safety concerns^65
	• Energy savings^11,67,80	• Low energy conversion efficiency^81,82
	• Rapid and efficient heating^4,51,57,59,66,67	• Difficulties of the scaling up of microwave-assisted heating for biomass pyrolysis^11
	• Material selective heating^57,67
	• Quick start up and stopping^11,57,66,67
	• Higher level of safety and automation^57,67
Conventional heating	• High flexibility of feedstock and products^11	• Relatively slow and inefficient convection currents and thermal conductivity^4,59,65
	• Well developed and easy to scale up^11	• Lower quality products^4,11
	• High possibility of continuous processing on a large scale^11	• High energy consumption^4,11

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On the other hand, microwave heating also suffers from some disadvantages as given in Table 2. For example, the current methods used to measure the accurate temperature in a microwave reactor are not effective because of the interference between electromagnetic irradiation and thermocouple sensors.⁴ The declared efficiency of electric energy as compared to that of microwave energy is approximately over 70% when the modern magnetrons are operated at 2.45 GHz; consequently, the energy conversion efficiency is low, which can amount to 20–60% from the input electric energy to heat by microwave heating.^81–84 It has been also assessed that the energy losses in transformers and magnetron during MAP of biomass is ca. 26%, and the power loss caused by the heat loss accounted for approximately 29%.⁸⁵ In addition, the scaling up of biomass pyrolysis using microwave heating is highly difficult and still in the infancy stage since the limited penetration depths in materials trigger a very restrictive size of the reaction vessel.¹¹ As a consequence, a simple and direct comparison between microwave heating and conventional heating cannot give rise to correct interpretations. To date, the theory with regard to the observed rate enhancement using a microwave heating technique purely due to thermal effects is widely accepted by the literature.⁸⁶ Even though non-thermal effects may not be thoroughly ruled out, they are much less essential for chemical reaction enhancement.^86,87

3. Microwave absorbers

The ability of a specific substance to transform electromagnetic energy into heat plays a vital role in microwave heating; and it is determined by the dielectric properties, i.e., dielectric loss (ε′′) and dielectric constant (ε′).^4,59,63 The former measures the efficiency with which electromagnetic energy can be converted into heat, whilst the latter illustrates the capacity of molecule polarization by an electric field.⁴ The ratio of the dielectric loss (ε′′) to the dielectric constant (ε′) is denoted as the loss tangent factor (tan δ), which is a parameter used to express the overall efficiency of a material to absorb energy from microwave radiation.⁵⁹ In general, the materials used for microwave heating can be classified as high (tan δ > 0.5), medium (0.1 < tan δ < 0.5), and low microwave absorbing (tan δ < 0.1).⁶³ The dielectric loss tangent of diverse materials is also summarized in Table 1. Materials with low tan δ values cannot be heated to desired temperatures by a rapid heating rate, using sufficient amount of microwave energy. Most biomass show relatively low microwave absorption when they are subjected to microwave-assisted heating.⁵⁹ Yet raw biomass can be blended with materials with high tan δ values before being fed into a MAP reactor to achieve a rapid heating rate. These materials with high tan δ are usually named as microwave absorbers.

3.1 Solid carbonaceous microwave absorbers

Solid Carbon Based Materials (SCBMs) are regarded as good microwave absorbers. SCBMs can indirectly heat feedstock that are relatively transparent to microwave. A variety of SCBMs have been employed as ideal candidates for microwave absorbers to promote biomass pyrolysis, e.g., charcoal, activated carbon, coke, and graphite.^46,94–96 As a matter of fact, generated biochar from MAP of biomass can be reused as cost-effective microwave absorbers. Although biochar formed in the pyrolysis is less active than graphite, biochar is a modest absorber potentially having some impact at some instances. The tan δ of most SCBMs as listed in Table 1 is much higher than that of biomass, which suggests that the addition of SCBMs can garner some perceived benefits: (i) the increase of microwave assimilation capacity of bulk materials; (ii) heat transmitted to the surrounding materials; and (iii) the supply of a rapid heating rate and sufficient temperature at the low microwave powers.^59,66

Regarding SCBMs, there are no other contaminations with extra elements; on the contrary, they can even contribute to the enhancement of the calorific value of biochar. However, if additives such as SiC are used as the microwave absorbers, the element of silicon is added into the solid fraction.⁵⁶ Accordingly, the consequence of this situation has to be carefully considered when using additives as microwave absorbers. Under microwave irradiation, SCBMs heated by microwave energy can generate hot spots termed as micro-plasmas, appearing in the form of small sparks or electric arcs.⁹⁴ The micro-plasmas is limited to tiny space and usually lasts for fractional second; and, they are of two classification depending on nature and shape: ball lighting and arc discharge effects.⁹⁷ It is possibly like that the MAP of biomass in the presence of SCBMs may lead to thermal instability and localized heating of biomass. Yet, the choice plus distribution of SCBMs with biomass can be conductive to reducing hot spots.⁵⁹ Additionally, it is manifested that some SCBMs can also take part in biomass gasification and pyrolysis under microwave irradiation, influencing the yields and quality of products.

3.2 Metal oxide microwave absorbers

Metallic oxides can also act as good microwave absorbers, providing several merits: the effective absorption of microwave energy, the increase of materials' heating rate, and the improvement of devolatilization from biomass.⁵⁹ Compared with SCBMs, metal oxide microwave absorbers (MOMAs) can substantially affect product yields and quality during the MAP of biomass.⁵⁹ It is mainly due to the fact that the evolved vapor and gas can simultaneously undergo secondary and catalyzed reactions when contacting MOMAs.

The most common metal oxides utilized as microwave absorbers are copper oxide (CuO), magnesium oxide (MgO), calcium oxide (CaO), iron oxide (Fe₃O₄), nickel oxide (NiO), etc. Moreover, some metals and metal hydroxides can serve as microwave absorbers including elemental iron (Fe), elemental aluminum (Al), sodium hydroxide (NaOH), potassium hydroxide (KOH), iron chloride (FeCl₃), zinc chloride (ZnCl₂), etc.⁹⁸ As mentioned above, the concerns of MOMA utilization are the recycling of these absorbers, along with the utilization or disposal of biochar containing the additives.⁶⁶

4. Microwave-assisted pyrolysis chemistry

4.1 Chemistry of non-catalytic microwave-assisted pyrolysis

MAP is a promising alternative technique to conventional pyrolysis mainly because of dielectric heating. Basically, a heating rate of 10–200 °C s⁻¹ can be achieved by using microwave absorbers, favoring the liquid and gaseous products.^25,51 Normally, quartz reactors are employed inside a microwave chamber where biomass is placed; and a product cooling and collection system is introduced, where the condensable bio-oil is collected.^99–102 The MAP system is usually purged with N₂, He, or CO₂ to maintain an oxygen-free environment.¹⁰³ The inert gases can also assist pyrolysis volatile vapors flowing from the reactor to condensable system to obtain bio-oil. The non-condensable vapors escape as gas at the end of the condensers, while biochar is left in the quartz reactor. As biomass is often a poor absorbing material, microwave absorbers should be introduced and homogeneously blended with biomass prior to the pyrolysis, to gain a rapid heating rate. It is discerned that selecting a proper microwave absorber is key in the improvement of bio-oil quality and fine biochar for soil amendment and remediation applications.¹⁰⁴

The MAP of biomass reveals a potential for bio-oil production with higher carbon content, higher heating value, and lower oxygen content than conventional pyrolysis of biomass.^20,105–107 The MAP of biomass can produce biochar with higher surface area and pore volume than conventional pyrolysis of biomass.¹¹ Furthermore, it is indicated that biochar from MAP is uniform and very clean.^108,109 At the same time, part of biochar can be recycled into the microwave reactor, serving as the microwave absorber to recover some sensible heat. Generally, MAP of biomass is a novel technique for producing more porous biochar that is used in sorption applications or as a precursor for activated carbon production. The gaseous product from the MAP of biomass dominantly contains H₂, CO, CH₄, and CO₂, which is thought of as a promising technique for producing H₂-enriched fuel gas.^107,110 The gaseous product from MAP of biomass can liberate more energy due to the higher heating value than gas from conventional pyrolysis of biomass. It is also proved that increasing reaction temperature can substantially increase H₂ and CO concentrations, while CO₂ concentration significantly decreases; that is possibly due to the self-gasification of biochar.⁹⁴

The MAP of biomass can allow a careful control of pyrolysis parameters to maximize bio-oil or gas production; operating pyrolysis parameters can induce and alter partial reaction pathways, garnering different chemical profiles of pyrolytic vapors/bio-oil.¹¹ It has been reported that the discrepancy in product yields is attributable to the difference in biomass characteristics, particle size, reaction temperature, reaction time, reactor design, product vapor residence time, microwave heating manner, microwave power level, etc.^{57,107,111–115} Among these factors, reaction temperature plays the most critical role in product yields and distribution. In general, the increase of reaction temperature can substantially increase the gas yield; while the bio-oil yield drastically decreases and the alteration of the biochar yield is not obvious. It is suggested that when pyrolysis vapors are released by the devolatilization of biomass in MAP, the heavy intermediates in the vapors can be thermally cracked into permanent gaseous compounds at elevated reaction temperature.⁵⁷ Prolonged residence time of pyrolysis vapors under microwave irradiation can extend their secondary reactions, resulting in the decrease of bio-oil production but increase of biochar formation.¹¹² Hence, the pyrolysis vapors should be swept out of the microwave chamber instantly to reduce secondary cracking of the hot organic vapors and to deliver water vapor together with water-soluble small polar molecules out of the microwave reactor.⁴

More importantly, the microwave power level can obviously affect product yields and qualities. The improvement of the microwave power level increases the microwave heating rate, thus promoting the yields of pyrolysis vapors and bio-oil production. Besides, the variation of microwave power level not only affects the biochar yield, but also alters the heating value of biochar. Since the heating values of cellulose and hemicellulose are much lower than that of lignin (the more thermo-resistant biopolymer), the devolatilization of cellulose and hemicellulose in biomass is beneficial for the increase of biochar heating value.⁴ At relatively low microwave power levels, the degradation of cellulose and hemicellulose is dominant, whereas the thermal degradation of lignin is not remarkable. However, the devolatilization of lignin is considerable at high microwave power levels, resulting in the drop in the heating value of biochar. Some chemical pretreatments, such as using ionic liquids or acids, are also good for the MAP of biomass.⁴

On the other hand, the regular MAP of biomass is not beneficial for high bio-oil yield and quality, and the thorny challenge is to promote the heating rate during MAP.¹¹⁶ Recently, a novel concept of a fast microwave-assisted pyrolysis (fMAP) utilizing microwave absorbers has been proposed by Ruan's group, as depicted in Fig. 5.^116–118 In this new concept with minimal energy input under microwave irradiation, the microwave absorbers particles (e.g., SiC) are kept heated on a fixed bed, and the chamber temperature inside the microwave oven is steady when the biomass is fed. Owing to a simultaneous indirect and direct heating methods: microwave irradiation and thermal conduction from heated absorbers particles, biomass can be loaded in contact with the hot absorbers and it can be almost instantly heated to the desirable temperature. Biomass subjected to the fMAP can be instantaneously converted into pyrolytic volatiles, giving rise to a very high bio-oil yield, and this technique is of great interest for the scale-up.

Fig. 5 Schematic diagram of fMAP setup. (1) Semi-continuous feeder; (2) biomass; (3) quartz inlet connector; (4) microwave oven; (5) control panel; (6) quartz reactor; (7) microwave absorber bed; (8) quartz outlet connector; (9) thermocouple; (10) quartz connector; (11) connecting tube; (12) bio-oil collector; (13) condenser; (14) gas exporting tube. Reprinted with the permissions from ref. 53. Copyright (2016) Elsevier.

The mechanism corresponding to non-catalytic MAP of biomass has been widely investigated by a number of researchers. In terms of MAP, the rapid and in-core volumetric heating contribute to a very prompt moisture release from the biomass, improving its surface area and the surface or pore structure; meanwhile, a very quick release of organic vapors are favored during MAP.^4,11 Moreover, the inverted heat transfer properties of microwave heating, namely higher temperature at the material interior and lower temperatures at the material surface, avoids the direct contact between the hot biochar and released pyrolysis vapors so that char-catalyzed secondary cracking is hampered. In addition, microwave heating is easily controlled and the reaction temperature can be readily maintained at a desired moderate level, which is also beneficial for the fast pyrolysis process. It is noticeable that the non-catalytic MAP of biomass undergoes a sequence of exothermic and endothermic reactions.³⁰ It is found that endothermic reactions are energy-consuming and require prolonged microwave irradiation to achieve complete reactions;^85,119 whilst exothermic reactions rapidly elevates the temperature of materials and reduce the microwave energy consumption. It is reported that the maximum temperature of MAP is highly correlated to the heating value of feedstocks and microwave power level.¹²⁰

More specifically, the overall reaction pathway with regard to non-catalytic MAP of lignocellulosic biomass is presented in Fig. 6. It can be seen that the decomposition of cellulose and hemicellulose is mainly composed of two steps.¹²¹ In the first step with the low temperature ranged from 329 to 350 °C, cellulose and hemicellulose undergo a series of depolymerization and dehydration reactions to form furfuran and 2-furancarboxaldehyde. During the second step at the high reaction temperature varied from 400 to 471 °C, C=O linkage is broken and recombined to generate β-methoxy-(S)-2-furanethanol and tetrahydro-2,5-dimethoxy-furan. Unlike the decomposed pathways of cellulose and hemicellulose, the decomposition of lignin involving depolymerization, dehydration, cracking, and hydrogenation can be relatively complex. Lignin is primarily depolymerized and dehydrated into propenyl-guaiacols at the low temperature from 329 to 350 °C. The propenyl-guaiacols can be further hydrogenated into propyl-guaiacols at 350–400 °C. The cracking of lignin and guaiacols takes place at 350–471 °C, and the positions of C–C bond broken are highly associated with the temperature. In addition, the scission of C_β–C_γ bond occurs at 350 °C, followed by the scission of C_α–C_β bond at 400 °C versus C₄–C_α bond at 450 °C. Ultimately, the cleavage of C–OCH₃ emerges at the temperature of 471 °C.

Fig. 6 Schematic presentation with respect to non-catalytic microwave-assisted pyrolysis of lignocellulosic biomass. Reproduced by the permissions from ref. 121 Copyright (2012) Elsevier.

To date, it is widely known that simplicity and effectiveness are two key criteria to evaluate a technique for the production of liquid fuel.³⁰ Co-pyrolysis is considered as another ideal process by using diverse materials as feedstock, satisfying the aforementioned criteria.¹⁴ There are a number of studies showing that microwave-assisted co-pyrolysis (MACP) of biomass with polymers have successfully improved product yield and liquid quality.^122–124 Unlike catalytic cracking and HDO, the MACP technique has a promising application for biomass utilization due to its attractive outcomes.⁵³ Unlike MAP process, MACP is performed by using the mixture of biomass and other materials (e.g., plastics and scum) as certain mass ratios. However, the other operating steps and conditions of MACP process are comparable with those of MAP process.

The synergistic effect between biomass and polymers in MACP process is the main factor for the improvement in target product quantities and qualities. The mechanism corresponding to non-catalytic MACP of biomass and polymers (e.g., plastics) has been investigated.^53–55 The mechanism of radical interactions during MACP contributing to synergistic effect is mostly proposed. It is shown that the decomposition mechanisms of biomass and plastics are readily different. The decomposition of biomass under microwave irradiation undergoes a series of exothermic and endothermic reactions; while the thermal degradation of plastics occurs via a sequence of radical mechanisms (initiation, propagation, and termination).^53,55 Since the thermal stability of biomass is much lower than that of plastics during MACP process, the free radicals from the decomposition of biomass can enhance the degradation of plastics-derived macromolecules. Meanwhile, the active hydrogen proton derived from the thermal degradation of plastics can react with unstable biomass-derived oxygenates to inhibit the polymerization reactions to form less valuable products.

4.2 Chemistry of catalytic microwave-assisted pyrolysis

CMAP is a relatively emerging technique.¹²⁵ In terms of chemistry or steps for CMAP, they are comparable with those of CFP. Basically, CMAP is performed by means of introducing a catalyst or additive during the MAP process. The use of catalysts or additives in the CMAP process could enhance the desirable product yield and selectivity.¹¹ Among the widely used catalysts and additives, zeolite-based catalysts (e.g., HZMS-5, γ-Al₂O₃, and TiO₂) and additives (such as activated carbon, Ni₂O₃, CaO, and Na₂CO₃) are especially attractive owing to their excellent performances at the improvement of product selectivity and quality in the CMAP process.^{54,95,126–128}

Similar to CFP, the CMAP technique is also divided into two categories: in situ CMAP and ex situ CMAP. In situ CMAP of biomass is carried out by means of premixing biomass and catalyst samples into the quartz reactors. Alternatively, a fixed bed reactor filled with microwave absorbers and a catalyst mixture is placed into microwave cavity, which is also named as in situ CMAP. This platform is developed and mainly applied by Ruan and his co-workers.^{58,116,125,130,131} In this in situ CMAP system, the microwave absorbers are preheated to a set temperature and simultaneously catalysts are maintained at the same temperature by heat conduction; biomass is subsequently fed into the microwave oven and subjected to fast pyrolysis and catalytic reforming. Nonetheless, one main shortcoming of this approach is the uncontrollable catalytic temperature, which cannot optimize the product yields and selectivity. Instead, ex situ CMAP of biomass incorporates an upfront MAP process with a catalytic reforming process.^{45,116,132–134} Herein, hot pyrolysis vapors derived from the MAP of biomass can be subjected to the packed-bed catalysis reactor which is filled with catalysts for a preferred product selectivity. As expected, the ex situ CMAP of biomass indicates the controllable catalytic temperature, which could optimize the bio-oil yield and selectivity.

Various mechanisms and reaction pathways during the CMAP of different biomass have been investigated. The overall reaction pathways and reaction mechanism regarding the CMAP of cellulose by using HZSM-5 as the catalyst have been identified in the light of these observations from cellulose to high-selectivity aromatic hydrocarbons.¹²⁹ It is affirmed that cellulose is thermally decomposed and dehydrated to form anhydrosugars, which subsequently undergo dehydration and re-arrangement reactions to generate furanic compounds such as furan and its derivatives with appended side groups as depicted in Fig. 7. Anhydrosugars are not prone to entering the zeolite pores due to the large molecule size,¹³⁶ thus the dehydration and re-arrangement reactions should occur at the external surface of the modified HZSM-5. The furanic compounds can thereafter diffuse into the modified catalyst pores and go through a series of decarbonylation, decarboxylation, dehydration, and oligomerization reactions to yield both monocyclic aromatics and olefins. Monocyclic aromatics either leave the catalyst pores as final products or further react with oxygenates or other reaction intermediates to form the polycyclic aromatics like naphthalene and its derivatives. In the whole deoxygenation processes, the competing reaction in this phase is the catalytic coke polymerization from thermal decomposition of homogeneous vapour phase and heterogeneous catalytic reactions deactivating strong acid sides and suppressing the diffusion of aromatics. Afterwards, Zhang and his colleagues have investigated the overall reaction mechanism regarding the CMAP of intact biomass as sketched in Fig. 8. Unlike the decomposed pathways of cellulose and hemicellulose in the CMAP system, lignin could be initially decomposed into phenolic compounds (such as phenols and guaiacols); and these phenolic compounds can be catalyzed by HZMS-5 catalyst via dehydration, cracking, and oligomerization reactions to form aromatic hydrocarbons.

Fig. 7 Overall reaction pathway for catalytic microwave pyrolysis of cellulose over modified HZSM-5. Reproduced with the permissions from ref. 129 Copyright (2014) American Society of Agricultural and Biological Engineers.

Fig. 8 Possible reaction scheme regarding catalytic microwave-assisted pyrolysis of lignocellulosic biomass by using HZSM-5 as the catalyst. Reprinted by the permissions from ref. 135 Copyright (2015) Royal Society of Chemistry.

The reaction mechanism for furfural conversion to xylenes and toluene during CMAP has been investigated as shown in Fig. 9.⁹⁹ It can be seen that furfural can initially undergo decarbonylation reaction to generate allene (C₃H₄) and CO. The allene can go through either aromatization to form benzene or oligomerization and crackings giving rise to propylene, hydrogen and cyclopentadiene. The alkylation of benzene can readily take place at high temperatures because of the active feature of benzene. Therefore, propylene serving as the intermediate compound can react with benzene via alkylation to form xylenes, toluene and ethylene. Meanwhile, generated toluene could further react with propylene to produce xylenes through alkylation. From the conversion of furfural into xylenes, the proportion of ethylene as the by-product is regarded as an index of aromatic formation during the CMAP process. More specifically, the furanic compounds go through HZSM-5 framework during the CMAP system that is demonstrated in Fig. 10.

Fig. 9 The proposed reaction pathway from furfural to xylenes and toluene in the catalytic microwave-assisted pyrolysis employing modified HZSM-5 as the catalyst. Reprinted by the permissions from ref. 133 Copyright (2015) Royal Society of Chemistry.

Fig. 10 The generation of aromatic hydrocarbons in the HZSM-5 framework during the catalytic microwave-assisted pyrolysis system: the catalyst consists of a local area of zeolite framework with at least one Brønsted acid site and an organic co-catalyst. Reprinted by the permissions from ref. 133 Copyright (2015) Royal Society of Chemistry.

However, CMAP of biomass cannot manufacture a very high yield of liquid organic fraction even in the presence of highly efficient catalysts; nevertheless, high amounts of solid residues, including both biochar and coke, are achieved in the process.^{45,100,118,125,132,133,137,138} Indeed, a low yield of liquid organic fraction from CMAP is not cost-effective to scale up the process in a biorefinery. It is discerned that the petrochemicals with a low yield of aromatics and high solid formation are mainly related to the oxygen-enriched intrinsic nature and hydrogen deficiency of biomass.^2,139,140 In addition, the hydrogen to carbon effective (H/C_eff) ratio plays a vital role in coke formation and converting efficiency of biomass into valuable products.^141–143 In order to improve the carbon efficiency of liquid organic fraction and minimize the solid formation, it is reasonable that the addition of high H/C_eff ratio co-reactants with biomass in the CMAP can help mitigate these issues.^54,122 Waste plastics represent an abundant and cheap hydrogen sources, which can be implemented to improve the yield of liquid organic fraction and lower the solid formation in the CMAP process.^53,55 Accordingly, this technique using two or more feedstock (usually biomass and polymers) in CMAP is referred to as catalytic microwave-assisted co-pyrolysis (CMACP). Unlike non-catalytic MACP, CMACP introduces the catalysts in the process by using either in situ or ex situ manner. The basic experimental steps and reaction conditions of the CMACP process are similar to those of the CMAP process.

The reaction mechanism and overall reaction pathways pertaining to the CMACP can be relatively complicated due to various types of materials employed in the process.¹⁴² The CMACP mechanism can be classified into two ways: the mechanism regarding biomass and polymers during thermal decomposition, and the mechanism regarding the pyrolysis vapors at the catalytic sites.¹⁴⁰ As for CMACP of biomass and plastics model compounds, it is observed that there is a positive synergy for aromatic production in the co-feeding of cellulose and low-density polyethylene (LDPE) during the process.⁵⁵ The phenomenon is mainly attributed to the interactions between cellulose-derived furanic compounds and LDPE-derived olefins. It is suggested that furanic compounds (e.g., furan and furfural) derived from cellulose could react with light olefins (e.g., ethylene and propylene) originating from LDPE to generate aromatics through Diels–Alder reactions followed by a dehydration reaction. It is proven that furanic compounds and light olefins act as the diene and dienophile compounds, respectively.^141,144,145 The Diels–Alder reactions followed by dehydration reaction can improve the yield of aromatic production and reduce the coke formation through polymerization of furans.^41,146 It is also evidenced that LDPE-derived hydrocarbons including as olefins and alkanes can also serve as the hydrogen donor for cellulose-derived oxygenates, leading to the reduction of coke yield in the zeolite-catalyzed conversions.^147,148

The CMACP process using real woody biomass with plastics as feedstock has been performed as well.^53,54,149 Zhang et al.⁵³ has demonstrated the overall reaction network and mechanism (CMACP of lignocellulosic biomass with hydrocarbon-based plastics in the presence of zeolite-based catalysts) as outlined in Fig. 11. Regarding the dominant route in lignocellulosic biomass, cellulose should first undergo a sequence of dehydration, decarbonylation, and decarboxylation to yield furanic compounds in the thermal degradation.^44,133 As such, it is evidenced that hemicellulose is likely to depolymerize into furanic compounds,¹²¹ which is identical to the result of cellulose degradation.¹³³ Unlike cellulose and hemicellulose, lignin in lignocellulosic biomass is mainly depolymerized into phenolic compounds. In terms of the degradation of plastics in another route, thermal degradation of plastics usually takes place through two mechanisms (random scission and chain-end scission).^150,151 The two abovementioned mechanisms can occur simultaneously, giving rise to free radicals along with the long carbon chains.¹⁵² At the same time, the radical fragments can be converted into straight chain hydrocarbons through hydrogen transfer reactions.¹⁵² In addition, the hydrogen from the thermal decomposition of plastics can be provided for biomass-derived oxygenates which serve as the strong acceptor, suppressing the char formation.

Fig. 11 Proposed reaction pathways for catalytic microwave-assisted co-pyrolysis of lignocellulosic biomass and plastics.⁵³ Reprinted with permission from ref. 53 Copyright (2016) Elsevier.

It is worth noting that the waxes with large molecule weight from the thermal decomposition of plastics can predominantly go through catalytic cracking over zeolite catalysts via two carbocationic mechanisms, generating light olefins.^150,153 On the other hand, it is found that a small partial amount of waxes could directly undergo the catalytic cracking and oligomerization reactions to produce liquid alkanes and olefins. The light olefins can thereafter react with furanic compounds by the Diels–Alder reaction followed by the dehydration reaction to generate aromatic hydrocarbons. Meanwhile, these plastics-derived light olefins can individually undergo cyclization, aromatization, and oligomerization reactions to form aromatic hydrocarbons.¹⁴⁰ As such, these furanic compounds could partially go through decarbonylation, aromatization, and oligomerization reactions on the zeolite catalysts to obtain aromatic hydrocarbons as well. As previously reported, it is confirmed that the interaction via the hydrocarbon pool mechanism could be found in addition to the Diels–Alder reaction.⁴¹ Since the carbon yield of liquid organic fraction significantly increases but the coke yield dramatically decreases during the CMACP process, the Diels–Alder reaction is the dominant reaction pathway during the CMACP process compared to the hydrocarbon pool mechanism. Additionally, phenolic compounds can be also transformed into aromatic hydrocarbons over zeolite-based catalysts by the dehydration, cracking, and oligomerization reactions.^44,154

During the CMAP of lignocellulosic biomass individually, the hydrogen-deficient oxygenates from the thermal degradation of cellulose and hemicellulose can readily polymerize and react with phenolic compounds to form coke.^128,132 Most phenolic compounds from the thermal cracking of lignin are too large to diffuse into zeolite pores.¹⁵⁵ Yet, these phenolic compounds are relatively unstable and can easily attach on the catalyst surface, and further polymerize or react with small molecular oxygenates to yield coke.¹⁴⁰ On the other hand, the resultant furanic compounds can readily react with plastics-derived light olefins via Diels–Alder reactions rather than polymerization reactions, resulting in the decrease of coke formation during the CMACP process.⁵³ As such, lignin-derived phenolic compounds can abstract hydrogen atoms derived from the degradation of plastics; thus these phenolic compounds are stabilized by the hydrogen donors and the polymerization reaction can be inhibited to generate more coke.^53,140 Nonetheless, it should also be kept in mind that the issue of coke formation in the CMACP can only be mitigated, rather than being thoroughly eliminated.^53,55 It is reinforced by the overall reaction pathway plus the reaction mechanism during the CMACP process that the plastics-derived olefins and biomass-derived oxygenates can partially go through polymerization reactions to generate coke as well.

4.3 Catalysts for catalytic microwave-assisted pyrolysis

4.3.1 Soluble inorganics. To obtain more desirable products and removing tars, soluble inorganics are often mixed with biomass samples in the CMAP process. In this sense, a variety of soluble inorganics (NaCl, NaOH, Na₂CO₃, KOH, K₂CO₃, K₃PO₄, ZnCl₂, MgCl₂, etc.) have been used as the catalysts in the CMAP of biomass. The soluble inorganics can function as the microwave absorbers to speed up the heating rate and catalysts to participate in the so-called in situ catalytic reforming of hot pyrolysis vapors during the MAP of biomass. In general, the presence of soluble inorganics could dramatically affect reaction temperature, product yields and distribution. The presence of inorganics can increase the yield of solid products but decrease the yield of gaseous products in the CMAP process. It is found that these inorganics, especially Ca and K, can facilitate the decomposition of biomass and the generation of biochar.¹⁵⁶

By using K₂CO₃ and NaOH as the catalyst in the CMAP of biomass, gas is the dominate fraction; that is due to the fact that the catalysts can strongly absorb microwaves, resulting in the extremely high temperature inside biomass. Taking K₂CO₃ for example, it is a strong polar material, which can absorb more microwave energy. Through employing K₂CO₃, the activation energy during the CMAP of biomass is much lower than that in the direct MAP of biomass, indicating that the catalytic effect of K₂CO₃ is the preliminary role in the CMAP of biomass.¹⁵⁷ At a high reaction temperature, the increase of gaseous products during CMAP is mainly attributed to the catalysis of K₂CO₃, which is also used as the catalyst for tar conversion into CO, H₂, and CH₄. Moreover, the introduction of K₂CO₃ can promote the gasification reaction between CO₂ and C, the dry reforming and cracking reaction of CH₄.¹⁵⁸ On the other hand, it is suggested that the catalytic performance of K₂CO₃ is weak at the low temperatures, and the heat and mass transfer resistance is enhanced with the addition of K₂CO₃ at low temperatures, thereby postponing the pyrolysis reactions. It is also noted that alkaline earth metal chlorides including MgCl₂ and ZnCl₂ could accelerate the biochar formation at low temperatures but reduce its generation at high temperatures. Sometimes, chlorides salts can favor the liquid production during MAP of biomass.¹⁵⁹

K₃PO₄ mixed with biomass can remain in the biochar after MAP and provide two essential macro-nutrients of K and P for plants. K₃PO₄ has been also identified as a great microwave absorber, which can remarkably reduce bio-oil acidity. It is found that high load of K₃PO₄ during MAP enhances the endothermic reactions, resulting in a slow heating rate. Large amounts of gases products are also attributed to the endothermic reaction of CH₄ dry reforming caused by high temperatures of K₃PO₄ particles in comparison with biomass particles, and the accumulation of coke deposited on K₃PO₄ surface that further enhances the gaseous production. Moreover, it is noted that the element of K is prone to catalyzing gasification reaction and improving the cracking reaction of large molecular weight compounds at high heating rates under microwave irradiation, creating small molecules of gas (e.g., CO and H₂). The higher biochar yield from MAP of biomass in the presence of K₃PO₄ is probably because of the low heating rates and the high presence of K that is responsible for the enhancement of the secondary carbon formation and the inhibition of devolatilization of cellulose and hemicellulose, thereby triggering more biochar production.¹⁰⁴

4.3.2 Metal oxides. Metal oxides, especially transition metal oxides, have been essentially utilized as heterogeneous catalysts in various reactions.² In general, metal oxides possess redox characteristics owing to their multivalent nature and/or the acidic properties that can potentially catalyze the thermal degradation of biomass and pyrolytic intermediates to generate more stable products. Herein, a large number of metal oxides, such as CaO, MgO, NiO, CuO, Al₂O₃, TiO₂, and Fe₃O₄, have been investigated as the catalysts for CMAP of biomass.

Acidic metal oxides including Al₂O₃ have been used as the catalyst in CMAP of biomass. The presence of acidic metal oxides can result in the decrease of the liquid product, but the increase of gaseous and solid products. Several studies have suggested that Al₂O₃ catalyst has high catalytic performance towards the thermal decomposition of biomass or tarry constituents.^15,22 Besides AlO₄ tetrahedron is included in commercial zeolite catalysts (HZMS-5) that have long been known for biomass conversions and bio-oil cracking.¹⁶⁰ By using the γ-Al₂O₃ as the catalyst in the CMAP of biomass,¹⁶¹ the introduction of γ-Al₂O₃ promotes the thermal decomposition of biomass to produce organic volatile vapors but has little impact at the variation of gaseous composition at the low temperature comparing to that of no catalyst. On the contrary, the application of γ-Al₂O₃ results in the increase in the shares of H₂ and CH₄ relative to the absence of catalyst at the high temperature. These observations imply that γ-Al₂O₃ exhibits a very high catalytic performance at the cracking of tarry constituents in a high temperature range of >300 °C during CMAP of biomass. Al₂O₃ is also found to enhance the bio-oil production through either suppressing biochar yield or gas yield or both.¹⁵⁹ Even though Al₂O₃ does not improve the converting efficiency from biomass to pyrolysis vapors (slight variation in the biochar yield), it promotes the formation of liquid products.

Basic metal oxides are well known to be active catalysts for ketonization and aldol condensation of carboxylic acid and carbonyl compounds.² Alkaline earth metal oxide, e.g., CaO and MgO, are classic base catalysts where oxide ions serve as bases and metal cations behave as Lewis acids.² It is manifested that the employment of either MgO or CaO as the catalyst in the CMAP of biomass can enhance the gaseous production.^{54,149,158,161,162} It should be mainly due to several secondary reactions including self-gasification and interactions among hot pyrolytic intermediates occurring during the CMAP of biomass to alter gaseous compositions and the three-phase production distribution. As for the comparisons between CaO and MgO as the catalysts during the comparable CMAP system,¹⁶² the addition of CaO in the CMAP of biomass slightly enhance the share of H₂, while adding MgO as the catalyst slightly causes the decrease of H₂ share. The quantity of CaO has a moderate negative effect on the reaction temperature, suggesting that CaO may affect the dielectric heating of the reaction system; while MgO has a weak negative effect. The high production of H₂ and CO implies that the use of CaO as the catalyst makes the equilibrium of CH₄ steam reforming move to the right-hand side of the reaction. Therefore, using CaO as the catalyst during the CMAP of biomass is beneficial for the formation of syngas, while MgO exhibits a reverse function in syngas production. It is worth noting that the CaO quantity plays a strongly negative role in solid yield whereas it shows a significantly positive effect on liquid or gaseous yield. The MgO quantity is likely to present considerable influence in gaseous or liquid yield, whilst the impact on solid yield can be negligible.¹⁶²

Regarding the specific role of CaO in the CMAP process,¹⁶¹ the increasing share of H₂ in the gas product is partially due to the thermal cracking of hydrocarbons. It is discerned that CaO can effectively catalyze the reactions of hydroxyl group and therefore facilitate the water–gas shift reaction that results in the decrease of CO during the low-temperature CMAP stage. As an alkaline oxide, CaO can absorb a part of CO₂ evolved from the CMAP and water–gas shift reaction to form CaCO₃; the decreased share of CO₂ and increased share of H₂ can be observed.^158,161 Accordingly, CaO is a promising catalyst used to produce pure H₂ in MAP or gasification. The low yield of CH₄ in the gas product is principally due to the gas-phase conversion of CH₄ into CH₃OH and methyl radical intermediated by CaO. Moreover, with the addition of CaO as the catalyst in the CMAP of biomass, the heat and mass transfer resistance between biomass particles is hampered, which limits the further decomposition of more biomass.¹⁵⁸ In addition, CaO as the catalyst also contributes to the deoxygenation of bio-oil; moreover, the production of light phenols in bio-oil is enhanced at the expense of large oxygenated compounds.⁵⁴ The reaction mechanism is surmised that when pyrolysis vapors pass through the mesoporous CaO catalyst, heavy compounds (e.g., large phenols and anhydrosugars) are cracked into light molecules.

When MgO is used as the catalyst in the CMAP of biomass at the high temperature, the alkylated phenols share in the phenolic compounds decreases whilst phenol share increases with the increases of temperature, implying that the MgO catalyst is less effective at the high temperature.¹⁴⁹ It is also noticeable that the phenol is dominant in the phenolic compounds due to high bond dissociation energy of the HO-Ph. However, with the high loading of the MgO catalyst, bio-oil yield gradually decreases. The phenomenon is mainly due to the reason that more phenols derived from MAP undergo alkylation reaction with the increasing addition of MgO, eventually resulting in the drop of bio-oil yield.¹⁴⁹ Additionally, the production of chain hydrocarbons is enhanced because the pore size of MgO favors olefins pass.

Apart from acidic and basic metal oxides, transition metal oxides including NiO, Ni₂O₃, CuO, TiO₂, and Fe₃O₄ have also been researched in the CMAP of biomass.^{85,95,161–163} It has been widely reported that most transition metal oxides can accelerate the heating rate under microwave irradiation. Ni-based catalysts such as NiO and Ni₂O₃ show high catalytic activities towards the thermal decomposition of biomass during the CMAP process and improve the yields of both bio-oil and gas, particularly with the case of the Ni₂O₃ catalyst.¹⁶¹ The introduction of both the Ni-based catalyst results in the higher gas production rate and larger content of combustible gas (H₂, CO, and CH₄).¹⁶¹ In comparison with the absence of catalysts, the use of NiO and Ni₂O₃ can contribute to a considerable increase in CO and H₂ yields, while causing a significant drop in CH₄ and CO₂ yields. It is obvious that NiO and Ni₂O₃ not only facilitate the cracking of organic matter but also enhance the CO₂ reforming and stem reforming reactions. It is interesting to note that Ni₂O₃ presents more excellent catalytic performance toward the CO₂ and steam reforming reaction than NiO during the CMAP of biomass.

When NiO and CuO used as the catalysts are compared in the CMAP of biomass,¹⁶² the addition of NiO as the catalyst enhances the production of H₂ or syngas, while the utilization of CuO slightly reduces the H₂ or syngas production. The quantity of NiO has a positive effect on liquid yield while it has a strongly negative effect on gas or solid yield. CuO quantity is also positive correlated to liquid yield while it is medium negative associated with either solid or gas yield. In general, the solid yield is slightly decreased with the increase of NiO or CuO quantity, whereas the liquid yield is substantially promoted with the increasing loading of either NiO or CuO. By using NiO and CuO as the catalyst in the CMAP of biomass, polycyclic aromatic hydrocarbons (PAHs) can be detected by using CuO, while PAHs cannot be observed in the presence of NiO.¹⁶³ As for TiO₂ as the catalyst, it is also good for the enhancement of H₂ production during the CMAP of biomass;⁹⁵ it can promote the degradation of biomass to produce more organic volatiles as well.¹⁶¹

4.3.3 Microporous materials. Massive microporous materials have been reported for their application in the CMAP of biomass.^{45,100,116,118,132,135} In this respect, zeolite-based catalysts have been most extensively investigated especially HZSM-5 because of its strong acidity and shape selectivity. HZMS-5 is the most efficient catalyst for aromatic production during the CMAP of biomass.^128,133 That's because it reveals intermediate pore size (0.54 × 0.56 nm) plus good thermal and hydrothermal stabilities, implying that only small molecules are allowed to diffuse into the micropores and restructure to large molecules with folded effective sizes.¹³⁶ Besides, the acidity of HZMS-5, particularly Brønsted acidity, is important for deoxygenation during pyrolysis.¹⁶⁴ It is affirmed that the acidic sites of HZMS-5 can enhance deoxygenation, decarbonylation, and decarboxylation of oxygenates, alongside cracking, alkylation, isomerization, oligomerization, cyclization, and aromatization through a carbonium ion mechanism.¹⁶⁵

In order to determine the influence of catalyst properties in the reaction chemistry and product distribution during the CMAP of biomass, and to further engineer highly efficient and stable catalysts, the properties of representative zeolite-based catalysts are interpreted in Table 3. In the CMAP of biomass using HZMS-5 as the catalyst, primary pyrolysis vapors pass through the catalyst layer, they can be adsorbed by the catalyst first, and thereafter a series of reactions take place inside the micropores, internal structure, and internal volume, including dehydration, decarbonylation, decarboxylation, isomerization, rejecting the oxygen content in the form of water and light gas (e.g., CO and CO₂).¹⁶⁶

Table 3 Characterizations of various zeolite-based catalysts investigated in CMAP^a

Catalyst	S BET (m^2 g^−1)	V pore (cm^3 g^−1)	d pore (nm)	A Brønsted (mmol NH3 per g)	A Lewis (mmol NH3 per g)	T acidity (mmol NH3 per g)	Ref.
a S BET: BET surface area; Vpore: pore volume; dpore: average pore size; ABrønsted: Brønsted site acidity; ALewis: Lewis site acidity; Tacidity: total acidity.
HZSM-5	396.2	0.097	5.2	—	—	0.18	53 and 133
HZSM-5	308	—	2.73	0.80	0.60	1.40	125
1.3-SiO2/HZSM-5	302	—	2.50	0.75	0.55	1.30
2.5-SiO2/HZSM-5	290	—	2.15	0.70	0.45	1.15
3.7-SiO2/HZSM-5	283	—	1.88	0.60	0.40	1.00
5.1-SiO2/HZSM-5	278	—	1.69	0.55	0.35	0.90
5.9-SiO2/HZSM-5	268	—	1.56	0.50	0.25	0.75
HZSM-5	342	0.17	—	0.97	0.65	1.62	167
1EDTA-HZSM-5	330	0.18	—	0.85	0.62	1.47
2EDTA-HZSM-5	321	0.20	—	0.72	0.60	1.32
3EDTA-HZSM-5	310	0.205	—	0.62	0.59	1.21
4EDTA-HZSM-5	308	0.21	—	0.58	0.58	1.16
HZSM-5	308	—	2.75	—	—	—	131
PC-HZMS-5	288.5	—	2.02	—	—	—
SiO2-HZSM-5	289.5	—	2.16	—	—	—
EDTA-HZSM-5	289.6	—	2.14	—	—	—

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To give more insight into the chemical reactions over HZMS-5 catalyst during the CMAP of biomass, the actual mechanism of the CMAP of biomass in the presence of the HZMS-5 catalyst has been proposed according to the hydrocarbon pool model.¹³³ The hydrocarbon pool model has been successfully accepted to demonstrate the reaction mechanisms based on zeolite catalysts.^168–170 The co-catalytic hydrocarbon pool generally consisting of entrained organic species within the inorganic zeolite framework provides an alternative pathway towards hydrocarbons with lower energy barriers, forgoing the problematic direct reaction routes.^168–170 In this regard, the cooperation of the entrained aromatic component inside the inorganic zeolite framework possibly creates a rather unique reaction route as illustrated in Fig. 10. Herein the inorganic zeolite framework can act as the dehydration, oligomerization, aromatization and methylation center for the generation of aromatic hydrocarbons. Polymethylbenzenes mainly work as the entrained aromatic compounds because naphthenic species usually have a high energy barrier.¹⁷⁰ The crucial reaction mechanism of the hydrocarbon pool may be that appropriate intermediates (e.g., furans) can react with entrained hydrocarbon species in the catalyst including a series of complicated steps to form the aromatic hydrocarbons and olefinic products during the catalytic cycle.¹³³

The product distribution and selectivity can be tuned by fabricating the structure or components of zeolite-based catalysts. Varying the Si/Al ratio and doping other metal cations or oxides in zeolite-based catalysts is an effective approach to fabricate the strength and density of active acid sites. A lot of metal modified HZMS-5 catalysts have been tested during the CMAP of biomass. Metal modified HZMS-5 (e.g., Zn/HZMS-5) catalysts are normally prepared by using a wet incipient impregnation method , resulting in the replacement of a portion of protons in HZMS-5 with metal ions.¹⁷¹ Zinc ions and Lewis acid formed by zinc implementation can stimulate H-atom migration through C–H activation, which can catalyze the oligomerization of pyrolysis vapors to form aromatic hydrocarbons.¹³² It is also deemed that the doping of zinc can accelerate the decarbonylation reaction of intermediates (e.g., furans) to produce aromatic hydrocarbons than the parent HZMS-5 catalyst.¹⁷¹ Hence, Zn/HZSM-5 is viewed as the one of most promising catalysts for aromatic hydrocarbon production.¹³² Moreover the zinc ions on HZMS-5 can also maintain the function of preventing coke formation over protons.¹³²

It is noted that the formation of aromatic hydrocarbons is mainly attributed to the role of internal acid sites of HZMS-5, and external acid sites are primarily responsible for coke generation because of the shape selectivity of HZMS-5 that triggers it to be difficult to form coke in the internal pores.¹⁷² The accumulated coke on the external surface of the catalyst can lead to the blockage of pore opening, causing rapid deactivation. To regard coke formation and improve aromatic hydrocarbon production, the external acid sites of HZMS-5 should be eliminated but meanwhile the internal acid sites should be maintained. One of the promising approaches to lower strong external acid sties but retain the internal weak sites is to selectively remove strong acid sites by dealumination.¹⁷³ Ethylene diamine tetraacetic acid (EDTA) presents prevalent capacity to eliminate the framework.¹⁷⁴

With the EDTA modification, the BET surface area decreases, while the total pore volume rises. The underlying reaction of this phenomenon is given as follows:¹⁶⁷ first of all, the aluminium–oxygen tetrahedron goes through hydrolysis reaction to form aluminium hydroxide, and subsequently the aluminium hydroxide can react with the proton acid centers to generate Al(OH)₂⁺ and H₂O. Ultimately, the Al(OH)₂⁺ cation is exchanged by Na⁺ and then undergoes chelation reaction with EDTA to eliminate the external framework aluminums, contributing to the emergence of hydroxyl holes and the increase of pore size, thus resulting in the improvement of total pore volume. Simultaneously, the generation of hydroxyl holes and the increase of pore size cause the loss of original micropores, thereby incurring a drop in the total specific surface area.¹⁶⁷ It is also found that the EDTA modified HZMS-5 (referred as EDTA-HZSM-5) can reduce the external strong acid sites, contributing to the enhanced stability of the catalyst and retarding the catalyst deactivation, as well as promoting the catalyst life to accelerate the catalytic reactions during the CMAP process.¹⁶⁷

Soon afterwards, the external acid sites of HZMS-5 catalyst was passivated by means of pre-coking treatment, chemical vapor deposition of inert silica (SiO₂-CVD), and EDTA chemical modification.¹³¹ Pre-coking modified HZSM-5 (referred as PC-HZMS-5) has some coke deposited on its external surface, thereby reducing its external acid sties. It is found that the appropriate pre-coking treatment of HZMS-5 can reduce the coke yield and enhance the target products. HZSM-5 modified by chemical vapor deposition of tetra-ethyl-orthosilicate (TEOS) is designated as SiO₂-HZMS-5. As a large molecule compound, TEOS cannot enter the narrow intracrystalline void space of HZMS-5. As a result, the external acid sites of HZSM-5 are replaced by SiO₂via thermal decomposition of TEOS; nonetheless, the acid sites on the internal surface are not affected by the treatment.¹³¹ EDTA chemical modification has also been implemented to eliminate the outer framework aluminums of HZMS-5. Since EDTA is a bulky compound and it is not capable of entering the internal channels of HZMS-5, the external acid sites of HZMS-5 can be selectively removed by EDTA while the internal acid sites can be held intact.¹³¹

It is discerned that all the catalyst treatments of pre-coking, SiO₂-CVD and EDTA chemical modifications can lead to a decrease of coke and bio-oil yields, but an increase of water and gas yields compared with parent HZMS-5 in the CMAP of biomass.¹³¹ It is illuminated that all these surface modification methods could selectively remove the outer acid sites of HZMS-5. As the removal of external acid sites and the drop in coke yield could prolong the catalyst life and improve catalytic reactions, more pyrolysis vapors can diffuse into the internal pores of HZMS-5 and undergo a sequence of reactions to generate more water and light gas. To conclude, these modified HZMS-5 catalysts, especially SiO₂-HZMS-5, show better catalytic activities than parent HZMS-5 in the CMAP of biomass.¹³¹

4.3.4 Carbonaceous materials. It is interesting to observe that carbonaceous materials (such as activated carbon and biochar) as admirable absorber have high dielectric loss tangents, implying the rapid heating rates in the microwave field; they can also act as decent catalysts inducing a series of reactions to obtain target products.¹⁵⁷ Typically, a decrease of bio-oil yield is achieved from the CMAP of biomass in the presence of activated carbon, this phenomenon is possibly due to the competing reaction or heterogeneous gas–solid reaction between the generation of bio-oil and gas.¹⁷⁵ As activated carbon is a promising microwave absorber, the temperature of activated carbon is much higher than the surrounding solid and gaseous fractions; moreover, the generation of hot spots owing to microwave heating can incur higher temperature of solid fraction than the average measured temperature.¹¹² Thus, the elevated temperature can enhance the secondary cracking of bio-oil and catalytic cracking of biomass, resulting in the decrease of bio-oil yield.^112,176 In addition, it is found that the activation energy decreases during the CMAP of biomass in the presence of activated carbon, suggesting that the catalytic effects of activated carbon are a preliminary mechanism for the CMAP process.¹⁵⁷

In the CMAP of biomass employing activated carbon as the catalyst, the high concentration of phenols is probably produced from the radical reaction of O–CH₃ homolysis where cellulose-derived oxygenated and phenolic compounds derived from lignin function as H-donors and H-acceptors, respectively.^52,127,175 As for the reaction mechanism, when superheated water heated under microwave irradiation is subjected to the activated carbon catalyst, carboxylic anhydride can react with water to generate carboxylic acids that become proton (H⁺) donors and acidic catalyst. It is also shown that the amount of carbonyl group on the activated carbon catalyst increases, which is relatively correlated with guaiacol conversion to phenols.⁵²

The protonation reaction of carbonyl oxygen can be enhanced by acids in bio-oil. Some of the electron density on oxygen which is available for donation to carbonyl carbon is taken up by new O–H bonds.⁵² Thus, the net effect of protonation can weaken the carbon oxygen π bond, and protonation of the carbonyl renders the carbonyl carbon to be a stronger electrophile. The carbonyl carbon will react much more quickly with whatever nucleophiles are present in bio-oil; the methoxide (O–CH₃) that is an electron rich species donates electrons to electron-poor carbon. As a result, O–CH₃ homolysis for guaiacol conversions to phenols is catalyzed by activated carbon catalyst. In addition, the protonation mechanism is also reversible; and, the electrons on the newly formed O⁻ atom can collapse back down to reform the carbonyl and kick off a good leaving group (CH₃O⁻).⁵² The phenols are then produced by the reaction of O–CH₃ homolysis in which the guaiacols are stabilized with saturated alkyl- or H-donors.

When using metal modified activated carbon as the catalyst (e.g., ferrum-modified activated carbon),¹⁷⁷ the substantial increase of ketones/ethers is obtained, having a close relationship with the drop in guaiacols. It can be explained that furans are subjected to acid-catalyzed ring opening to form aldehydes; and a sequence of condensation reactions such as aldol condensation are facilitated to gain structural stable products. It is also noticed that the amount of phenols in bio-oil increases compared to that from the non-catalytic MAP process in the absence of ferrum-modified activated carbon. This phenomenon is illuminated by catalytic conversions of guaiacols under the impact of hydrogen donors from many reactions, including demethylation, alkylation, demethoxylation, and dehydration.¹⁷⁷

5. Practical non-catalytic microwave-assisted pyrolysis of biomass

The non-catalytic MAP of biomass has been extensively investigated in the past decade using various types of microwave absorbers, and most of the studies before 2013 have been reviewed by several studies.^{4,11,51,56,57,59,66} In this section, the up-to-date non-catalytic MAP of biomass will be primarily interpreted. The product profiles and compositions from the non-catalytic MAP of selected biomass are summarized in Table 4. To determine the quality of bio-oil from MAP, some of the resultant bio-oils are shown in Table 5. These results indicate that the quality of bio-oils from MAP is superior to that of conventional pyrolysis bio-oils. Even though biocrude derived from hydrothermal liquefaction (HTL) of algae could present comparable properties with bio-oil from the fMAP of microalgae, the high energy consumption and high-severity reaction conditions (e.g., high pressure and long reaction time) of HTL are inferior to those of the MAP technique in the industrial application. It is discerned that diverse biomass has been valorized in the MAP process to produce bio-oil, gas, and biochar. With regard to oil palm biomass, two kinds of oil palm biomass (shell and fibers) were selected for the MAP process with the assistance of biochar as the microwave absorber.⁴⁶ It is found that the microwave heating rate and the yields of bio-oil, gas, and biochar are significantly dependent on the ratio of biomass to microwave absorber. A positive synergistic effect was observed in using microwave absorber and oil palm biomass according to the quality of bio-oil, containing important chemical compounds such as phenols and ketones.

Table 4 Selected biomass demonstrating the product yield and distribution from a non-catalytic MAP process

Feedstock	Operating conditions		Results		Ref.
	Absorber	Reaction parameters	Bio-oil/gas/solid yield (wt%)	Bio-oil/gas composition
Rice straw	—	50–500 W, 105–563 °C, 30 min	22.56/max. 49.37/28.07	Mainly alkanes and polars/mainly H2 and CO2	178
Rice straw	—	50–500 W, 105–563 °C	∼50/10–34/20–50	Mainly mono-aromatics, aliphatics and PAHs/mainly H2 and CO	179
Corn stover	—	700 W, 515–685 °C, 4–22 min	26.94–36.98/17.71–42.36/—	Dominate phenols, aliphatic hydrocarbons/—	180
Corn stover	SiC	750 W, 450–550 °C	Max. 64/max. ∼38/max. 28	Dominant phenolic compounds/dominant CO and CO2	117
Wood sawdust	SiC	750 W, 450–550 °C	Max. 65/max. ∼25/max. ∼50	Dominant phenolic compounds/dominant CO and CO2
Douglas fir pellets	—	700 W, 350–450 °C, 10–20 min	31.4–53.9/7.9–15.0/31.2–60.7	Dominant phenolic compounds and furans/dominant CO and CO2	121
Wood	H2O	600–1200 W, 200 °C	42.23–47.10/8.39–9.68/43.23–48.39	Mainly phenolic compounds and furans/mainly CO and CO2	181
Oil palm shell	Activated carbon	180–720 W, 45 min	16.43–36.75/—/—	Mainly phenolic compounds/—	182
Oil palm shell	Activated carbon	800 W, 400–500 °C, 33 min	Max. 28/38–47/28–41	Dominant phenolic compounds/—	103
Oil palm shell	Biochar	450 W, 25 min	Max. 22/max. 29/49	Mainly phenolic compounds, ketones, and aldehydes/—	46
Oil palm fibers	Biochar	450 W, 25 min	Max. 25/max. 30/45	Mainly ketones, phenols, and carboxylic acids/—
Oil palm EFB pellets	Activated carbon	300–450 W, 200–560 °C, 25 min	Max. 21/∼50/∼30	Mainly phenolic compounds/—	183
Prairie cordgrass	—	700 W, 530–670 °C, 6–20 min	20.3–33.1/32.0–53.7/13.8–47.7	Mainly phenolic compounds and hydrocarbons/—	184
P. juliflora	Flyash	280–700 W, 500 °C	9.60–39.87/35.06–49.58/22.65–28.28	Mainly phenolic compounds/mainly H2 and C2 hydrocarbon	185
	Aluminum	560 W, 500 °C	36.83/37.33/25.84	Mainly phenolic compounds/mainly H2 and CH4
	Graphite	560 W, 500 °C	31.25/39.79/28.96	Mainly phenolic compounds/mainly H2 and CH4
	Char	560 W, 500 °C	32.95/42.45/24.60	Mainly phenolic compounds/mainly H2 and CH4
	SiC	560 W, 500 °C	26.37/47.96/25.67	Mainly phenolic compounds/mainly H2 and CH4
Microalgae (Chlorella sp.)	Biochar	500–1250 W, 462–627 °C, 20 min	Max. 28.6/∼27/∼25	Dominant aromatic hydrocarbons and phenols/dominant H2, CH4, and CO	75
Microalgae (C. vulgaris)	Lignite char	700 W, 10 min	∼30/∼50/∼20	Dominant aromatic hydrocarbons, aliphatic hydrocarbons, and heterocyclic compounds/—	47
Peanut shells	Lignite char	700 W, 10 min	∼10/∼60/∼30	Dominant phenolic compounds and aromatic hydrocarbons/—
	Fe3O4	700 W, 10 min	∼10/∼60/∼30	Dominant phenolic compounds and aromatic hydrocarbons/—
Arundo donax	Carbon	3000 W, 365–493 °C, 18–40 min	22.1–40.9/4.6–17.2/43.8–62.9	Mainly aromatics and acetic acid/mainly CO2 and CO	186
Waste paper	—	1200 W, <200 °C	42/15/43	Mainly carbohydrates and aromatic compounds/—	105
Soapstock	SiC	800 W, 400–600 °C, 30 min	44.18–69.37/9.95–27.28/15.62–39.87	Mainly aliphatic hydrocarbons/—	58

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Table 5 Main properties of bio-oils from the non-catalytic MAP of biomass

Properties	Non-catalytic MAP bio-oil							Conventional pyrolysis bio-oil^20	Biocrude from HTL of algae^187,188
	Coffee hulls^107	Corn stover^106	Pine wood sawdust^189	Wheat straw^190	A. donax rhizomes^186	Waste paper^105	Chlorella sp.^75
a Acid number. b LHV (lower heating value in MJ kg^−1). c EHCcalc (MJ mol^−1). d TAN (Total Acid Number), mg KOH per g.
Elemental composition (wt%)
C	74	60.7	48.8	58.9	26.1	49.9	65.4	54–58	77.6–79.2
H	8.4	7.7	6.8	6.85	5.1	5.8	7.84	5.5–7.0	10.0–10.6
N	8.1	2.0	0.9	1.15	0.4	—	10.28	0–0.2	4.0–4.7
S	0.8	0.2	0	0.02	—	0.04	0		0.3–0.5
O	8.7	29.4	43.5	33.2	68.4	44.2	16.48	35–40	5.3–8.0
Water (wt%)	—	15.2	26.2	<1	50.7	2.6	—	15–30	2.8–7.8
Density (kg m^−3)	—	1250	1145	1200	1030	—	980	1200	0.94–0.96
pH value	—	2.87	2.5	1.4^a	—	—	9.7	2.5	59–74^d
Viscosity (mPa s, 40 °C)	—	185	15.2	—	1.90	—	60	40–100	114–355
HHV (MJ kg^−1)	34.4	17.5	15^b	16–22^b	9.2^c	21.8	30.7	16–19	25.8–33.3

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A multimode microwave system has been implemented to carry out the MAP of oil palm empty fruit bunch pellets using activated carbon as the microwave absorber. It is found that the ratio of biomass to microwave absorber not only affected the temperature profiles, but also influenced product yields and distributions. The highest bio-oil yield of 21 wt% was gained when using 25% microwave absorber. Phenolic compounds (60–70%) was mainly presented in the bio-oil detected by GC/MS and confirmed by FI-IR analysis. Biochar could be regarded as a potentially alternative solid fuel due to the high HHV of 25 MJ kg⁻¹.¹⁸³ Ani and his co-workers have also developed a new technique to pyrolyze oil palm shells in a microwave system using an activated carbon as the microwave absorber to solve the problem of bio-oil deposition.¹⁰³ It is noticed that the temperature profile, product yields and the characteristics of products were relatively dependent upon the stirrer spend and microwave absorber percentage. The maximum bio-oil yield was 28 wt% under the conditions operated at 25% microwave absorber and 50 rpm stirrer speed. Up to 85% selectivity toward phenols in the bio-oil is identified.

Furthermore, the uniformly distributed coconut activated carbon has been applied as a microwave absorber during the MAP of oil palm shell waste biomass to investigate the effects of microwave absorber loading, microwave power, and N₂ flow rate on its heating profile, bio-oil yield and its compositions.¹⁸² A central composite design (CCD) was implemented to estimate the importance of process parameters on bio-oil yield, suggesting that the N₂ flow rate was the most significant effect. It is discerned that the phenol content was dominant in bio-oil, accounting for 32.24–58.09%. It can be summarized that the MAP of oil palm shell with carbon absorber shows the potential to manufacture valuable fuel products. The MAP of oil palm fiber has also been conducted to optimize experimental conditions for maximum H₂ and biochar yields according to CCD.¹⁹¹ Both H₂ and bio-char yields together were optimized at 450 °C and a N₂ flow rate of 200 cm³ min⁻¹ under the microwave power of 400 W. The biochar possessed a large proportion of carbon content (>60 wt%) and higher value of HHV (>20 MJ kg⁻¹); and some pores were found through SEM and BET analysis, which is effective for soil improvement.

The behavior pertaining to MAP of another woody biomass (Larch) has been reported to establish the bio-oil yield and its quality from MAP of biomass.⁷⁸ It is the first study in biomass pyrolysis to utilize a MAP technique and methodology which was fundamentally scalable, from which the fundamentals of design for a continuous processing system could be predicted to maximize bio-oil yield and its quality. The bio-oil yield from MAP was comparable to that from conventional pyrolysis; however, the corresponding quality was much higher compared with that from conventional pyrolysis due to the rapid heating and quenching during the MAP process. It is also achieved that the amounts of levoglucosan and phenolic compounds were an order of magnitude higher in MAP when compared to those achieved by conventional pyrolysis. Prosopis juliflora (one type of woody biomass) has been also valorized in the MAP process to investigate the effects of microwave power, feedstock particle size, types of microwave absorber, biomass to microwave absorber mass ratio, and biomass loading on bio-oil, gas, and biochar yields, bio-oil compositions, and energy recovery in bio-oil and biochar.¹⁸⁵ Five types of microwave absorber, namely, biochar, graphite, aluminum, silicon carbide, and fly ash (an industrial waste), were used in the MAP process. A maximum bio-oil yield of 40 wt% with a HHV of 26 MJ kg⁻¹ was gained using fly ash as the microwave absorber. A mixture of phenolic compounds, aromatic hydrocarbons, carboxylic acids, etc. were included in the bio-oil. Nearly 51% deoxygenation of feedstock were obtained with an atomic O/C ratio of 0.24 in the bio-oil. Besides several organs (rhizomes, stems and leaves) of Arundo donax (one of the several species of the so-called reed) have been subjected to MAP with carbon as microwave absorber based on different reaction conditions.¹⁸⁶ A minor yield (4.6%) of gas together with a large yield (up to 62.9%) were observed whilst up to 40.9% of bio-oil yield was gained. The bio-oil contains a large amount of aromatics, acetic acid, furans, etc.

Apart from the above-mentioned lignocellulosic biomass, microalgae (Chlorella sp.) has been subjected to MAP using biochar as the microwave absorber to gain high-quality bio-oil.⁷⁵ A maximum yield (28.6%) of bio-oil and a higher heating value (HHV) of 30.7 MJ kg⁻¹ were achieved at the microwave power of 750 W. The bio-oil was mainly comprised of aliphatic hydrocarbons, aromatic hydrocarbons, and long chain fatty acids among which aliphatic and aromatic hydrocarbons are admirable compounds for use in petroleum hydrocarbon fuels. Accordingly, it is affirmed that the MAP paves a very promising approach for microalgae conversion into advanced renewable biofuel. Moreover, MAP of algal and lignocellulosic biomass was compared to determine the effect on the product yield and distribution.⁴⁷ The bio-oil yields from the MAP of peanut shell and C. vulgaris are 11.0 wt% and 27.7 wt%, respectively. It is noted that phenolic compounds are dominant in the bio-oil derived from peanut shells, while C. vulgaris-derived bio-oil primarily contains more nitrogen-containing species. In addition, the concentration of OH, C–H, C=O, O–CH₃, and C–O functional groups in all biochar samples significantly decreased after the pyrolysis.

Regarding the novel experimental design of CCD for MAP of biomass, other groups (particularly Lei's group) have widely used CCD to predict the optimization of product yields and distributions. In this regard, prairie cordgrass has been applied in the MAP process to determine the influences of pyrolysis temperature and reaction time in the yields of bio-oil, gas, and biochar.¹⁸⁴ CCD was performed to estimate the effects of reaction variables on bio-oil and gas yields and establish prediction models. A maximal yield (33.1 wt%) of bio-oil was obtained, and the bio-oil contained a large amount of aliphatic and aromatic hydrocarbons. The MAP of corn stover has been performed by Lei et al. to determine the effects of reaction temperature, reaction time, and feedstock particle size on the yields of bio-oil, gas, and biochar.¹⁰¹ CCD was satisfactorily developed to explain the biofuel conversion yield as a function of these independent variables. It is manifested that most minerals could remain in the biochar according to the mineral analysis. A series of important chemical compounds (e.g., phenols, aromatic and aliphatic hydrocarbons, furans) could be found in the bio-oil. Lei and his co-workers also conducted non-catalytic MAP of Douglas fir sawdust pellets using CCD and response surface analysis to understand the influences of reaction temperature and time on the yields of bio-oil, gas, and biochar. It is obtained that bio-oil and gaseous yields increased with the elevated reaction temperature and prolonged reaction time. The maximum bio-oil yield of 57.8% was gained; moreover, phenolic compounds and furans were main compositions existing in the bio-oil. The gaseous product mostly consisted of CO, CO₂, CH₄, and short chain hydrocarbons.¹²¹

As for the production of H₂ from the MAP process, the MAP of rice straw has been conducted by Huang et al.¹⁷⁸ It reveals that the microwave power and feedstock particle size are two key parameters affecting product yields and distributions. The bio-oil was primarily composed of alkanes, aromatic hydrocarbon, and phenolic compounds. The gaseous fraction mainly contained H₂, CO₂, CO, and CH₄; and H₂ is dominant in gaseous fraction. Thereafter, Huang and co-workers aimed to investigate the productivity of H₂-rich fuel gas from non-catalytic MAP of rice straw.¹¹⁰ The primary compositions of gaseous product were also H₂, CO₂, CO, and CH₄; in particular, the proportion of H₂ is more than 50 vol%. TA-MS analysis has proven that dielectric heating made the MAP different from conventional pyrolysis. A chemical equation has been nearly balanced to demonstrate gaseous composition produced from MAP of rice straw. From the viewpoint of energy consumption, approximately 60% of the input energy could be transferred and utilized as bioenergy. Besides the MAP of rice straw has been carried out to evaluate the products, mechanisms, and reaction kinetics.¹⁷⁹ It is discerned that more gaseous and less biochar products were observed at higher microwave power levels, whilst bio-oil product remains stable with a maximum yield of ∼50 wt%. The atomic H/C and O/C ratios of biochar are much lower than those of feedstock. Although the primary components of gaseous product are also H₂, CO, CO₂, and CH₄, CO became dominant in the gaseous product with the percentage of 57%. The kinetic parameters regarding the MAP of rice straw increased with the augment of microwave power level.

Another study for the production of H₂ by feeding rice straw into a MAP system has been reported. The optimal H₂ production was observed when rice straw was fed into the system at a microwave power of 1000 W; approximately 40.47 mg of H₂ could be produced from each gram of rice straw with the conversion rate of 67.45%.¹⁹² Moreover, oil palm fiber was recently used for the production of H₂ rich gas through the MAP process on the basis of different reaction parameters.¹⁹¹ It has been suggested that the utilization of a smaller biomass particle size, a higher reaction temperature, a higher microwave power, and a higher N₂ flow rate could dramatically produce more H₂ rich gas. A maximum H₂ yield (10.91 g kg⁻¹) was attained when using the biomass with the smallest particle size (less than 1 mm). TGA and SEM analysis were used to evaluate the properties of biochar, which shows a potential application for soil fertilization due to the existence of a porous hole in its microstructure.

To determine the effect of pyrolysis platforms on the product yields and distribution, woody biomass has been subjected to both microwave pyrolysis and conventional pyrolysis to determine the effects of each process on the product yields and composition with regard to bio-oil, gas, and biochar products.¹⁸¹ Comparable yields (∼46 wt%) of bio-oil production were gained for both cases of microwave pyrolysis and conventional pyrolysis; while a much lower gas yield was observed from microwave pyrolysis due to its fast heating rate. As the heating rate was enhanced, both the peak release of CO and CO₂ moved to a higher reaction temperature for both microwave and convention pyrolysis. It is also perceived that the clear release of CH₄ was achieved at a higher heating rate of microwave pyrolysis.

A novel concept with regard to the fMAP of corn stover and wood sawdust has been proposed by Ruan's group.¹¹⁷ This process using SiC as the microwave absorber was conducted as a function of reaction temperature, feedstock lading, feedstock particle size, and vacuum degree. A maximum bio-oil yield of 65 wt% from fMAP of wood sawdust was obtained, while the optimal bio-oil yield from the fMAP of corn stover was 64 wt%. It is thus indicated that the fMAP of biomass using SiC as the microwave absorber was a feasible and promising technique to promote the practical values and commercial application outlook of the MAP process. Furthermore, Wang et al. has conducted the fMAP of soapstock with SiC as the microwave absorber to produce hydrocarbon fuels.⁵⁸ It is worth noting that the utilization of SiC significantly enhanced the hydrocarbon production and the selectivity towards alkanes and aromatics. Both the effects of pyrolysis temperature and feeding rate on hydrocarbon yield and its compositions were investigated; and it is found that the optimal conditions to maximize the hydrocarbon yield of 64.74 wt% were at 550 °C and a feeding rate of 6 g m⁻¹, indicating high selectivity toward alkanes but low selectivity toward alkenes. The properties of the hydrocarbons are comparable with those of 0# diesel in terms of the density and dynamitic viscosity. This research presents that the fMAP of soapstock is a promising technique for commercial production of hydrocarbon fuels.

In addition, a low temperature MAP has been developed for syngas production using macroalgae as feedstock.¹⁹³ This protocol garnered unprecedented H₂ production, with switchable H₂/CO ratios from 3 to 1 depending upon pyrolysis conditions. The biochar was exerted as microwave absorber, giving rise to an optimum H₂ production with regards to a pure graphite material utilized as comparison. It is also found that arcing effects under microwave irradiation resulted in an interesting pseudo-catalytic effect induced by the metal oxides contained in macroalgae, possibly accounting for the enhanced results. Another low temperature (<200 °C) MAP of waste office paper has been reported to produce bio-oil for the utilization as an adhesive for aluminum–aluminum bonding.¹⁰⁵ The yields of organic phase and aqueous phase were 19% and 23%, respectively. Broad categories of compounds indicative of carbohydrates, aromatics, and carbonyl-containing moieties were included in the bio-oil through a sequence of characterization by ICP-MS, ATR-IR, GC-MS and NMR. A liquid–liquid fractionation of the organic phase was carried out to attain an in-depth understanding of the adhesive characteristics of bio-oil. The ‘acidic’ fraction indicates far better adhesion properties than the ‘neutral’ fraction with no bonding achieved for the aqueous fraction. The mixture of the ‘acidic’ and ‘neutral’ fractions reveals better adhesion, thereby implying a possible synergistic effect.

More interestingly, a non-catalytic MACP of sewage sludge with rice straw was investigated elsewhere.¹²⁴ It is noticed that the introduction of rice straw essentially promoted the performance of microwave heating, achieving a much higher maximum temperature. A maximum temperature (up to 500 °C) was detected from the mixture containing 20 wt% rice straw. It is inferred from the outcome that there was a positive synergistic effect between sewage sludge and rice straw for microwave heating. The high heating temperature can provide another approach for thermal treatment of waste sewage sludge. It is found that the calorific value of the biochar from the MACP of the mixture containing 30–40 wt% rice straw was significantly improved, compared with the biochar from the MAP of sewage sludge individually; and the fixed carbon content (up to 33 wt%) of the biochar was gained from MACP of the blends. It is also discerned that the atomic H/C and O/C of the biochar are very close to those of anthracite coal. Therefore, the biochar from the MACP of sewage sludge and rice straw indicates a high potential to be co-fired with coal or utilized as a replacement.

6. Practical catalytic microwave-assisted pyrolysis of biomass

6.1 Catalytic microwave-assisted pyrolysis of biomass

At the early stage, Chen et al.⁹⁵ carried out the CMAP of pine wood sawdust using eight inorganic additives (i.e., NaCl, NaOH, Na₂CO₃, H₃PO₄, Fe₂(SO₄)_3, Na₂SiO₃, TiO₂, and HZSM-5) in the light of their catalytic effects, as demonstrated in Table 6. It is observed that all catalysts improved the solid production while there was no dramatic change of bio-oil yield. The incondensable gas mainly consisted of H₂, CH₄, CO and CO₂. In particular, alkaline sodium compounds (NaOH, Na₂CO₃ and Na₂SiO₃) could significantly favored the formation of H₂. The most abundant organic component was acetol in the bio-oils from CMAP in the presence of these additives except H₃PO₄ and Fe₂(SO₄)₃. On the contrary, H₃PO₄ and Fe₂(SO₄)₃ mostly favored the formation of furfural and 4-methyl-2-methoxy-phenol in the bio-oils. Thereafter, Ruan and his co-workers have reported the effects of various metal oxides, salts, and acids including K₂Cr₂O₇, KAc, MAl₂O₃, MgCl₂, AlCl₃, CoCl₂, Na₂HPO₄, ZnCl₂, and H₃BO₃ on product yields and distribution during in situ CMAP of corn stover and aspen wood.¹⁵⁹ The five catalysts of KAc, Al₂O₃, Na₂HPO₄, MgCl₂, and H₃BO₃ were found to increase the bio-oil yield by either suppressing biochar or gas yield or both. It is also noticed that the employment of the catalysts considerably simplified the chemical compounds in the bio-oils, improving the liquid product selectivity. Moreover, furfural accounts for approximately 80% based on the GC/MS peak area at 8 g MgCl₂ per 100 g biomass level. Thus, it is suggested that these catalysts play a vital role in product yields and selectivity during the in situ CMAP process.

Table 6 Summary of studies on the CMAP of biomass

Biomass	Operating conditions			Results		Ref.
	Absorber	Catalyst/catalyst to biomass ratio	Reaction conditions	Bio-oil yield (wt%)	Bio-oil composition, calorific value (MJ kg^−1) and oxygen content (wt%)
a WHSV (h^−1). b (WHSV)^−1 (h). c Carbon yield of aromatics. d Yield of liquid organics. e The ratio of bentonite : HZSM-5 : soapstock. f Reaction conditions: 500 °C, catalyst to biomass ratio of 1 : 2.
Douglas fir pellets	Activated carbon	Activated carbon/1.3 : 1–4.68–1	700 W, 589–757 K, 1.3–14.7 min	6.8–50.2	Mainly phenolic compounds	127 and 175
Douglas fir pellets	Activated carbon	Fe modified activated carbon/1 : 2	700 W, 309–591 °C, 2.3–13.7 min	23.3–45.2	Dominant furans, phenols, guaiacols and ketones	177
Douglas fir pellets	Corn stover biochar	Biochar/1 : 4–1 : 1	700 W, 480 °C, 10 min	25.8–37.1	Dominant phenols and guaiacols	194
Douglas fir pellets	Biochar	HZSM-5/1.3 : 1–4.7 : 1	700 W, 332–668 °C, 1.3–14.7 min	28.43–40.83	Mainly phenols, guaiacols, and aromatic hydrocarbons	137
Douglas fir pellets	Biochar	HZSM-5/0.021–0.075^b	700 W, 269–481 °C, 9 min	32.20–37.75	Dominant aromatic hydrocarbons	45
Douglas fir pellets	Biochar	Zn/HZSM-5/0.021–0.075^b	700 W, 269–481 °C, 9 min	24.75–44.80	Dominant aromatic hydrocarbons	132
Cellulose	Activated carbon	HZSM-5/0.045–0.126^b	700 W, 269–481 °C, 10 min	30.35–36.18	Dominant mono-cyclic aromatic hydrocarbons	133
Alkali lignin	Activated carbon	Activated carbon/0.82–2.62^a	700 W, 309–591 °C, 8 min	15.46–41.48	Mainly phenols and guaiacols	52
Hybrid poplar	Activated carbon	HZMS-5/1 : 4	700 W, 500 °C, 10 min	24.76^c	Dominate xylenes and naphthalenes	128
Loblolly pine	Activated carbon	HZMS-5/1 : 4	700 W, 500 °C, 10 min	22.91^c	Dominate xylenes and naphthalenes
Douglas fir	Activated carbon	HZMS-5/1 : 4	700 W, 500 °C, 10 min	21.52^c	Dominate xylenes and naphthalenes
Mushroom waste	SiC	HZSM-5/1 : 1	750 W, 550 °C	∼47^d	Mainly acids and hydrocarbons	167
	SiC	EDTA-HZSM-5/1 : 1	750 W, 550 °C	Max. ∼45^d	Mainly acids and hydrocarbons
Mushroom substrate	SiC	HZMS-5/1 : 20	750 W, 500 °C, 45 min	∼18^d	Mainly oxygen-containing aliphatic and aromatic compounds	131
	SiC	PC-HZSM-5/1 : 20	750 W, 500 °C, 45 min	14.8^d	Mainly oxygen-containing aliphatic and aromatic compounds
	SiC	SiO2-HZMS-5	750 W, 500 °C, 45 min	16.0^d	Mainly oxygen-containing aliphatic and aromatic compounds
	SiC	EDTA-HZSM-5	750 W, 500 °C, 45 min	∼15.5^d	Mainly oxygen-containing aliphatic and aromatic compounds
Corn stover	SiC	HZMS-5/1 : 100–1 : 20	750 W, 400–700 °C, 45 min	Max. 31.2	Mainly oxygen-containing aliphatic and aromatic compounds	116
Corn stover	SiC	HZMS-5/1 : 20	750 W, 500 °C, 45 min	∼20^d	Mainly oxygen-containing aliphatic and aromatic compounds	125
	SiC	SiO2-HZSM-5/1 : 20	750 W, 500 °C, 45 min	15.6–22.6^d	Mainly oxygen-containing aliphatic and aromatic compounds
Peanut shell	Activated carbon	Activated carbon/1 : 8–1 : 4	2000 W, 300–600 °C, 50 min	11.13–16.62	Dominate phenols	195
	Lignite char	Lignite char/1 : 8–1 : 4	2000 W, 300–600 °C, 50 min	13.50–24.26	Dominate phenols
Pine sawdust	Activated carbon	Activated carbon/1.5 : 8–2.5 : 8	2000 W, 300–600 °C, 50 min	16.00–33.18	Dominant phenol and aromatic hydrocarbons
	Lignite char	Lignite char/1.5 : 8–2.5 : 8	2000 W, 300–600 °C, 50 min	20.00–35.38	Dominant phenol and aromatic hydrocarbons
Soapstock	SiC	Bentonite/1 : 1	1000 W, 550 °C	∼48^d	Mainly olefins and oxygenates	196
	SiC	HZMS-5/1 : 1	1000 W, 550 °C	∼43^d	Mainly olefins and oxygenates
	SiC	Bentonite and HZSM-5/1 : 1 : 2^e	1000 W, 500–600 °C	∼32–∼43^d	Mainly olefins, aromatics, and alkanes
Chlorella sp.	SiC	HZMS-5/1 : 2–1 : 1	750 W, 450–500 °C, 30 min	Max. 57	HHV = 26.80, O = 23.52^f	118
Nannochloropsis	SiC	HZMS-5/1 : 2–1 : 1	750 W, 450–500 °C, 30 min	Max. 59	HHV = 27.15, O = 17.16^f
Algae	Activated carbon	Activated carbon/1 : 20–3 : 20	400–800 W, 450 °C	Max. 54.3	Dominant carboxylic acids, aromatics, and nitrogen-containing compounds	197

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Other metal oxides (NiO, CuO, CaO and MgO) have been implemented as catalysts in CMAP of corn stover, lowering the formation of PAHs and thereby rendering the bio-oil to be less toxic.¹⁶³ The catalytic effects of NiO, CaO, CuO, and MgO on CMAP of sugarcane bagasse were also tested.¹⁶² Adding either NiO or CaO slightly promoted the production of H₂, whilst either CuO or MgO presented inverse effect. Furthermore, the introduction of either CaO or MgO improved the gaseous production, whereas the addition of either NiO or CuO assisted in the liquid production. Moreover, the additives (SiC, activated carbon, coke, K₂CO₃, and NaOH) were used as catalysts during the CMAP of sawdust.¹⁵⁷ It is observed that the additives indicated obvious effects on product yields and their characteristics. Both K₂CO₃ and NaOH favored the formation of gaseous products because they could strongly absorb microwaves, resulting in extremely high temperature inside the sawdust. It is also noticed that K₂CO₃ could simplify the bio-oil compositions, improving the bio-oil quality.

The CMAP of wheat straw using CaO and K₂CO₃ as catalysts have been operated in a fixed-bed microwave reactor at a constant temperature of 500 °C as well.¹⁵⁸ The introduction of CaO gave rise to the lowest concentration of CO₂ in the gaseous product, while the employment of K₂CO₃ facilitated the production of H₂ and CO at high temperature. In addition, the synergistic effects were manifested using a mixture of K₃PO₄ and clinoptilolite or bentonite compared to individual catalyst during the CMAP of switchgrass, implying the increase of the microwave absorption rate, bio-oil and biochar qualities.¹⁰⁴ It is worth noting that combining 10 wt% K₃PO₄ with 10 wt% clinoptilolite remarkably reduced the water content in the bio-oil by 39.5%, whilst the pH of the bio-oil increased by 43% compared with the pH of the bio-oil employing 10 wt% clinoptilolite individually, implying a potential synergistic effect between the catalyst mixtures.

With regard to carbon-based catalysts, Bu et al. has investigated the CMAP of biomass using activated carbon as the catalyst to determine the influence of pyrolytic conditions in the yields of both phenols and phenolics. Very high concentrations of phenols (38.9%) and phenolics (66.9%) were observed in the bio-oil with a catalyst to biomass ratio of 3 : 1, which were superior to those obtained from the MAP of biomass without activated carbon addition.^127,175 Bu and his co-workers thereafter used diverse activated carbon as catalysts during the CMAP of woody biomass to determine the effect on product yields and chemical compositions of bio-oils.¹⁹⁸ It is found that wood-based and lignite coal-based activated carbon contributed to high amounts of phenols, containing 74.61% and 74.77% in the bio-oils, respectively. It is also manifested that activated carbon as the catalysts could be reused 3–4 times without any regeneration, producing high concentrations of phenol and phenolic compounds. Besides lignin has also been explored to produce renewable phenols and fuels from CMAP using activated carbon as the catalyst.⁵² As expected, the main chemical compositions of bio-oils were phenols and guaiacols; up to 45% selectivity toward phenols in the bio-oil was achieved. Unlike the role of activated carbon during the CMAP of woody biomass, ferrum modified activated carbon showed different catalytic performance, primarily producing furans, phenols, guaiacols, and ketones occupying 80–87% of the bio-oils.¹⁷⁷

Yu's group has also produced phenolic-rich bio-oil from the CMAP of peanut shells and pine sawdust by using activated carbon and lignite char as catalysts.¹⁹⁵ Several parameters (e.g., pyrolysis temperature and biomass to catalyst ratio) have been investigated to garner phenolic-rich bio-oil. It is worth noticing that activated carbon could considerably enhance the production of phenolic compounds in the bio-oil. The maximum phenolic content in the bio-oil was 61.2%, and the formation of nanotubes in peanut shell biomass particles was also found. Another carbon-based material (biochar) has been utilized as the catalyst in the CMAP of biomass to estimate the effect of biochar as a catalyst on CMAP of biomass and bio-oil upgrading.¹⁹⁴ It is noted that high amounts of phenols (46%) and hydrocarbons (16%) were gained in the bio-oil and high-quality gas enriched in H₂, CO, and CH₄ was also achieved. More interestingly, fast pyrolysis oil upgraded by biochar as the catalyst was dominated by phenols (∼37%) and hydrocarbons (43%) at high biochar loadings. Additionally, rice husk biochar and the biochar-supported metallic (Ni, Fe, and Cu) catalysts have been applied in the CMAP of rice husks to maximize the high quality syngas.¹⁹⁹ It is shown that the biochar-supported metallic catalysts could promote the microwave absorption and enhance the heating rate. Biochar-supported Ni catalysts revealed the most effective impacts on gas production, with a gas yield of 53.9% and the concentration of target syngas production of ∼70%; while biochar-supported Ni and Fe catalysts played pivotal roles in tar removal.

Typically, zeolite-based catalysts are the most popular catalysts used in the CMAP process. Wang et al. has exerted HZMS-5 as the catalyst for in situ CMAP of Douglas fir pellets to produce high-quality bio-oils.¹³⁷ It is shown that a series of important and useful chemical compositions (i.e., phenols, guaiacols, and aromatic hydrocarbons) were the most abundant compounds, accounting for 82% in the bio-oil. To produce a high amount of valuable aromatic hydrocarbons, ex situ CMAP of biomass was operated using an upfront microwave pyrolysis process coupled with a fixed-bed catalytic process in the presence of HZMS-5 catalyst.⁴⁵ Aromatic hydrocarbons were enriched and became the most abundant compounds, reaching 92.6% in the bio-oil. These outcomes affirm that the HZSM-5 catalyst favors the production of aromatic hydrocarbons during the CMAP process. Moreover, Wang and her co-workers leveraged the ex situ CMAP system by adding a more efficient catalyst (Zn/HZMS-5) to further improve the concentration of aromatic hydrocarbons in the bio-oil.¹³² Both GC/MS and FTIR analysis confirmed that aromatic hydrocarbons were dominant in the bio-oil.

Based on previous work, Zhang et al. has investigated a novel pathway of the CMAP of cellulose integrated with packed-bed catalysis in the presence of a solid phase catalyst (well-modified HZSM-5) to produce gasoline-range aromatics and hydrogen-enriched fuel gas.¹³³ It is found that the chemical compounds of the bio-oils were aromatic hydrocarbons, phenols, and aromatic oxygenates. The maximum selectivity toward aromatic hydrocarbons were 96.6%, in which up to 48.6% of chemical compounds belongs to gasoline-range aromatics. H₂ was the primary composition in the gaseous product, occupying approximately 40 vol%. It has been suggested that these findings pave a new route for biorefinery industries to manufacture advanced products (aromatics and hydrogen-rich gases) via microwave-induced technologies. It is noticed that the spent HZSM-5 derived from the catalytic process at 500 °C still presented a high BET surface area, pore volume, and pore surface area (Table 7, entry 2). The results indicate that the elevated temperature promoted the cracking reaction of pyrolytic volatiles on the catalyst towards small molecules. Subsequently, diverse lignocellulosic biomass (hybrid poplar, loblolly pine and Douglas fir) was subjected to CMAP using well-modified HZMS-5 as the catalyst to determine the optimal conditions for the production of C₈–C₁₆ aromatics.¹²⁸ The maximum carbon yield of target aromatics was 24.76%, which is observed from the CMAP of hybrid poplar at 500 °C with a catalyst-to-biomass ratio of 0.25. Similarly, a two-step fMAP of corn stover for bio-oil production in the presence of microwave absorber (SiC) and HZMS-5 catalyst has been recently developed by Zhong's group.¹¹⁶ Two variables (reaction temperature and catalyst to biomass mass ratio) were examined to determine their effects on bio-oil yield and chemical components. It is observed from this study that the pyrolysis temperature of 500 °C was suggested as the optimal conditions for maximum bio-oil yield and highest quality of bio-oil. As a result, the two-step fMAP resulted in higher quality bio-oil with a smaller catalyst to biomass ratio than that of the one-step fMAP process. Likewise, Liu et al. has conducted sequential two-step catalytic fMAP of corn stover to yield high-quality bio-oil using HZMS-5 as the catalyst.¹³⁴Table 8 reveals the physicochemical properties of bio-oils from the fMAP of corn stover, which suggests that high-quality bio-oil was observed through sequential two-step catalytic fMAP. The fresh, spent and regenerated HZMS-5 catalysts have been characterized and evaluated using the XRD technique to determine the effect of coke on the catalyst structure (Fig. 12). Interestingly, the diffractogram comparison of the catalysts showed obvious similarities, the crystal structure and crystallinity of HZMS-5 catalyst were intact before and after the CMAP and regeneration processes. It is manifested that HZMS-5 show good stability in the CMAP process.

Table 7 Post-reaction properties of zeolite-based catalysts from the CMAP of biomass^a

Entry	Catalyst	S BET (m^2 g^−1)	V pore (cm^3 g^−1)	d pore (nm)	A Brønsted (mmol NH3 per g)	A Lewis (mmol NH3 per g)	T acidity (mmol NH3 per g)	Ref.
a S BET: BET surface area; Vpore: pore volume; dpore: average pore size; ABrønsted: Brønsted site acidity; ALewis: Lewis site acidity; Tacidity: total acidity. b 2H-Z5: 2EDTA-HZMS-5.
1	Parent HZSM-5	396.2	0.097	5.2	—	—	0.18	53 and 133
2	Spent HZMS-5	306.3	0.063	4.6	—	—	—
3	Fresh 2H-Z5^b	321	0.20	—	0.72	0.60	1.32	167
4	2H-Z5-regeneration 1^st	315	0.201	—	0.41	0.58	0.99
5	2H-Z5-regeneration 2^nd	301	0.186	—	0.40	0.54	0.94
6	2H-Z5-regeneration 3^rd	283	0.175	—	0.39	0.49	0.88
7	2H-Z5-regeneration 4^th	272	0.168	—	0.33	0.46	0.79
8	2H-Z5-regeneration 5^th	259	0.159	—	0.31	0.45	0.76

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Table 8 Physicochemical characteristics of bio-oils from the catalytic fMAP of biomass^118,134

Properties	Chlorella sp.		Nannochloropsis		Corn stover
	Non-catalysis	HZMS-5 catalysis	Non-catalysis	HZMS-5 catalysis	Non-catalysis	HZSM-5 catalysis^b
a HHV (MJ kg^−1) = (3.55 × C^2 − 232 × C − 2230 × H + 51.2 × C × H + 131 × N + 20 600) × 10^−3. b Pyrolysis temperature: 550 °C, catalyst bed temperature: 425 °C, catalyst to biomass ratio: 1 : 5. c Calculated by difference.
Elemental composition (wt%)
C	65.70	59.27	81.64	59.75	48.13	68.43
H	9.34	7.75	8.20	6.75	8.16	6.51
N	8.45	9.46	5.24	16.34	1.66	1.62
O	15.78	23.52	4.90	17.16	42.05^c	23.44^c
Water (wt%)	—	—	—	—	29.82	9.11
Density (kg m^−3)	1000	1010	1180	1180	—	—
pH value	9.33	9.54	9.93	9.62	—	—
Viscosity (cP, at 40 °C)	11	11	1.18	1.18	—	—
HHV (MJ kg^−1)^a	32.37	26.80	42.00	27.15	19.79	29.85

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Fig. 12 The XRD patterns of fresh HZSM-5, spent HZMS-5, and regenerated HZMS-5. Reproduced with permission from ref. 134 Copyright (2017) Elsevier.

Modified zeolite-based catalysts are also preferred to be used during the CMAP process. In this respect, Ruan's group have tested a variety of modified HZMS-5 catalysts during the CMAP of biomass to improve bio-oil yield and quality. For instance, when SiO₂ was deposited on HZMS-5, the external acid sites of HZMS-5 dramatically decreased.¹²⁵ With the increase of the SiO₂ deposited amount, the yield of the liquid product from the CMAP of corn stover first increased and then decreased, and the amounts of aliphatic hydrocarbons, aromatic hydrocarbons, and oxygen-containing aromatic compounds first increased until the maximum values and then decreased. In order to minimize coke yield and enhance hydrocarbon production, PC-HZSM-5, SiO₂-HZMS-5, and EDTA-HZSM-5 were utilized as catalysts in the CMAP of spent edible mushroom substrates.¹³¹ All modifications for HZMS-5 led to a loss of external acid sites of HZMS-5. Of the modified catalysts, SiO₂-HZMS-5 was in favor of maximizing bio-oil yield and minimizing coke yield. Furthermore, SiO₂-HZMS-5 contributed to a higher relative content of hydrocarbons (especially toluene and xylene) in the oil phase than the other catalysts.

Another study has reported the CMAP of mushroom waste using various EDTA modified HZMS-5 catalysts.¹⁶⁷ Among these EDTA modified HZMS-5 catalysts, an EDTA treatment for 2 h performed prominent promise for removing oxygenated chemicals and improving aromatic species as well as suppressing coke formation. The regeneration of the modified HZSM-5 catalyst by EDTA treatment for 2 h was also studied as shown in Table 7 (entry 4–8). Compared to the characteristics of the fresh 2H-Z5, the BET surface area, pore volume, and the total number of acid sites gradually decreased as the regeneration times increased. It is discerned that the highest relative content of hydrocarbons and lowest coke yield were both observed under the third regeneration cycle conditions. Hence, it is concluded that a moderate regeneration time would encourage the yield of target products, and high-quality bio-oil production could be garnered. In addition, two catalysts (bentonite and HZMS-5) combined were used as the catalyst during the CMAP of soapstock.¹⁹⁶ The two parameters of catalyst and pyrolysis temperature were tested to understand the effects on product yields and bio-oil chemical compositions. It is found that the combined catalyst can reduce the water content, and bentonite could assist in the increase of bio-oil yield. Up to 84.2% selectivity toward hydrocarbons in the bio-oil was gained, the proportion of oxygenates could decrease to 15.8%, and the N-containing compounds were completely eliminated. It is also noted that the introduction of bentonite could significantly promote the BET surface area of biochar, effectively improving the removal efficiency of Cd²⁺ by 27.4 wt%.

More importantly, microalgae are also attractive to be valorized through the CMAP system. Borges et al. has studied the fMAP of Chlorella sp. and Nannochloropsis in the presence of a microwave absorber (SiC) and catalyst (HZMS-5).¹¹⁸ The maximum bio-oil yields from the CMAP of Chlorella sp. and Nannochloropsis were 57 wt% and 59 wt%, respectively. The characteristics of bio-oils from fMAP of the two types of microalgae are demonstrated in Table 8. It is discerned that fMAP is a promising technique to valorize microalgae for commercial application and economic outlook. The CMAP of natural algae was also conducted for bio-oil production by using activated carbon as the catalyst.¹⁹⁷ The maximum bio-oil yield gained was 54.3%, and the energy yield of bio-products was 236.9%. It is obtained that carboxylic acids were the main composition in the bio-oil, optimizing at 66.6%. This study has indicated that the CMAP of algae provides a route to not only ameliorate the environment but also achieve renewable fuels or chemicals.

Apart from advanced bio-oil production, magnetic biochar has been developed from the CMAP process as well. Magnetic biochar has been synthesized by Mubarak et al. via the CMAP of empty fruit bunch using FeCl₃ as the catalyst.^200,201 The effects of microwave power, retention time, and impregnation ration of FeCl₃ to biomass were investigated. The newly produced magnetic biochar has a very high surface area of 890 m² g⁻¹, efficiently removing methylene blue with an efficiency of 99.9% from aqueous solution with a maximum adsorption of 265 mg g⁻¹. Likewise, sugarcane bagasse was used to prepare magnetic biochar through the CMAP process using Fe₂O₃ as the catalyst.²⁰² The optimum conditions to maximize magnetic biochar yield (69%) were at an impregnated Fe₂O₃ to biomass ratio of 0.45 with a radiation time of 30 min. It is noticed that the magnetic biochar could remove 96.2% of hazardous Cd²⁺ from aqueous solutions. Moreover, K₃PO₄, clinoptilolite and/or bentonite have served as catalysts during the CMAP of switchgrass to engineer biochar with decent water holding capacity, cation exchange capacity and fertility of loamy sand soil.²⁰³ It is observed that the addition of these catalysts during the CMAP process improved the biochar surface area and plant nutrient contents. The catalysts remaining in the biochar could provide important nutrients for the growth of bioenergy and food crops as well.

6.2 Catalytic microwave-assisted co-pyrolysis of biomass and other polymers

As claimed above, CMACP is one of the state-of-the-art techniques which has recently aroused numerous concerns. Both Ruan's group and Lei's group are paying more attention to this novel technology. Table 9 summarizes the current studies with regard to the CMACP of biomass with other polymers. To be more specific, Zhang and his co-workers conducted the CMACP of cellulose with LDPE to manufacture C₈–C₁₆ range aromatic hydrocarbons in the presence of well-promoted HZMS-5 catalysts.⁵⁵ It is noticeable that the raw organics with enhanced carbon yield (∼44%) were more principally lumped in the jet fuel range at the catalytic temperature of 375 °C with the LDPE to cellulose mass ratio of 3 : 4. And then Zhang et al. leveraged intact biomass (Douglas fir pellets) co-fed with LDPE through the CMACP system to produce highly valuable aromatic hydrocarbons.⁵³ It is discerned that improved carbon yield (40.54%) of liquid organics primarily lumped in the jet fuel range was also obtained at the catalytic temperature of 375 °C with the LDPE to biomass ratio of 3 : 4. Accordingly, it is reaffirmed that the CMACP of biomass with plastics indicates a promising way to produce jet fuel range compounds.

Table 9 Summary of recent studies on CMACP of biomass with other polymers

Type of materials		Reaction conditions			Results		Ref.
Biomass	Polymers	Catalyst	Polymers to biomass ratio	Temperature (°C)	Bio-oil yield (C%)	Bio-oil composition
a Aromatic yield (C%). b The HZMS-5 to MgO ratio of 1 : 1. c The HZSM-5 to Cao ratio of 1 : 1. d Gas composition.
Cellulose	LDPE	HZSM-5	0.4 : 1–1.45 : 1	250–500	36.05–46.35^a	Dominant xylenes and trimethylbenzenes	55
Lignin	LDPE	HZMS-5 + MgO^b	1 : 3–2 : 1	450–600	22.8–35.9	Dominant aromatics and phenolic compounds	149
Corn stover	Scum	HZMS-5 + CaO^c	1 : 4–4 : 1	450–650	17.4–31.4	Mainly aromatic and polycyclic aromatic hydrocarbons	54
Douglas fir pellets	LDPE	HZMS-5	0.4 : 1–1.45 : 1	200–500	34.18–42.66^a	Mainly xylenes and trimethylbenzenes	53
Bamboo sawdust	Waste tire	HZSM-5	1 : 4–4 : 1	450–650	19.56–37.86	Mainly aliphatic and aromatic hydrocarbons	204
Bamboo sticks	HDPE	Ni/Al2O3	1 : 20–1 : 4	600	10.3–22.0	Dominant H2 and CO^d	123
Nannochloropsis sp.	Scum	HZMS-5	1 : 4–4 : 1	450–650	11.6–27.7	Mainly aromatic and polycyclic aromatic hydrocarbons	122

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Moreover, bifunctional catalysts (HZMS-5 and CaO) have been investigated in the CMACP of corn stover and scum for bio-oil production.⁵⁴ The effects of reaction temperature, HZSM-5 to CaO ratio, and corn stover to scum ratio on product yields and chemical selectivity were estimated. Maximum bio-oil and aromatic yields were both observed at 550 °C. It is also discerned that scum as a hydrogen donor had a positive synergistic effect with corn stover to improve the bio-oil yield and aromatic hydrocarbons production when the H/C_eff exceeded 1. Besides, the maximum yield (29.3 wt%) of aromatic hydrocarbons was achieved when the corn stover to scum ratio was set at 1 : 2. Likewise, bifunctional catalysts (HZMS-5 and MgO) were introduced in the CMACP of lignin and LDPE to evaluate the effects of pyrolysis temperature, lignin to LDPE ratio, HZSM-5 to MgO ratio, and feed to catalyst ratio on product yields and distributions.¹⁴⁹ 500 °C was found to be the optimal conditions in terms of the maximum bio-oil yield. Increasing LDPE proportion could improve the production of aromatics; at the same time methoxyl group substituted phenols were eliminated. Improving the HZMS-5 to MgO ratio could promote the yield of aromatics, but reduce the yield of alkylated phenols. With respect to other catalysts, the CMACP of bamboo with high-density polyethylene (HDPE) was studied using a Ni/Al₂O₃ catalyst to produce a high yield of H₂-enriched syngas.¹²³ It is observed that the bio-oil quality was improved and a large amount of H₂ was also produced during the CMACP process. The maximum concentration of H₂ could reach 54 vol% in the gaseous fraction when the bamboo wood to HDPE ratio was fixed at 4 : 1.

As for microalgae biomass, microalgae co-fed with scum in the CMACP using the HZSM-5 catalyst has been reported as well.¹²² Identical to the above-mentioned studies, effects of pyrolysis temperature, catalyst to feed ratio, and microalgae to scum ratio on bio-oil yield and composition were investigated. It is found that the maximum bio-oil yield and highest share of aromatic hydrocarbons in the bio-oil were gained at 550 °C. The addition of scum in the process not only improved bio-oil yield but also enhanced aromatic production. It has also been suggested that the synergistic effect between microalgae and scum during the CMACP process was remarkable only when the effective hydrogen index of feedstock was larger than ∼0.7. Additionally, waste tires were also co-fed with lignocellulosic biomass (bamboo sawdust) during CMACP using HZMS-5 as the catalyst for bio-oil production.²⁰⁴ Several variables (pyrolysis temperature, catalyst to feed ratio, and biomass to waste tires ratio) were examined to determine their influence in bio-oil production. It is found that the optimum pyrolysis temperature was maintained at 550 °C, indicating the maximum bio-oil yield and lowest share of polycyclic aromatic hydrocarbons. Moreover, waste tires as the hydrogen donor could be beneficial for the promotion of bio-oil production and aromatic hydrocarbons. It should be also noted that the maximum production of aromatic hydrocarbons was garnered at the biomass to waste tires ratio of 1 : 1.

7. Kinetics of both non-catalytic and catalytic MAP of biomass

The systematic study on behaviors and kinetics of biomass in MAP system is highly important for the effective design of the thermochemical conversion units. Besides, determination of kinetic behaviors of biomass during the MAP process in high temperature regions can still be regarded as one of the most attractive research areas.⁵⁷ Recently, a few studies have investigated the characteristics and kinetics of biomass in the non-catalytic MAP process.^{47,121,162,179,184,205} The kinetic scheme to model thermal decomposition of biomass under microwave irradiation can be considered as the following reaction scheme:

Biomass → biochar + volatiles

where the reaction rate is a function of the remaining material in the reactor and follows an Arrhenius law dependence on temperature.

In general, the thermal decomposition of biomass at a constant heating rate can be explained as the following formula:

where A is frequency or pre-exponential factor of the MAP process; E is the apparent activation energy; T is the temperature; t is the time; R is the universal gas constant, and α is defined as the weight loss fraction or MAP conversion which can be calculated by the equation.
where X₀ is the initial mass of biomass; X_t is the mass at time t and X_f is the final mass at the end of MAP. The Coast–Redfern method is typically used to determine the values of kinetic parameters during MAP.^{47,121,162,179,184,205} For a constant heating rate β during MAP, β = dT/dt, rearranging the above equations and integrating gives:

Assuming 2RT/E ≪ 1, simplifying the two above-mentioned equations can be expressed as the following formulas:

As a result, a plot of ln[(−ln(1 − α)/T²)] against 1/T for n = 1 or ln[(1 − (1 − α)⁽¹⁻ⁿ⁾)/(T²(1 − n))] versus 1/T when n ≠ 1 can give rise to a straight line with a slop of −E/R and an intercept of ln(AR/βE). Therefore, the pre-exponential factor A and the activation energy E can be determined.

Table 10 gives representative studies on kinetic parameters for both non-catalytic MAP and CMAP of biomass. It can be seen that high coefficients of determination (R²) were observed for all studies, implying the significant fit for these reactions. It is also noted that the activation energy (E) and pre-exponential factor of non-catalytic MAP is much lower than those of conventional pyrolysis, indicating the difference in kinetic processes for both heating manners.²⁰⁵ The kinetic parameters of CMAP in the presence of catalysts are also listed in Table 10. As such, the coefficients of determination (R²) of these linear regressions were all larger than 0.99.¹⁶² Among the catalysts, CaO reveals the best catalytic performance whilst MgO indicates the worst effect. However, this kinetic analysis is not as satisfactory as it should be because of the difficulty in acquiring a precise relationship between biomass weight and reaction temperature.

Table 10 Representative results regarding kinetic parameters for the non-catalytic MAP and CMAP of biomass

Types of materials	Reaction order n	E (kJ mol^−1)	A (s^−1)	R ^2	Ref.
a Assume n = 1.
Non-catalytic MAP
Douglas fir pellets	1	16.5	15.27	0.81	121
	2	21.0	1.42	0.79
	3	33.5	3.03	0.85
Rice straw	1	27.1	1.06	0.97	179
Prairie cordgrass	1	3.1	2.26	0.42	184
	2	13.3	1.82	0.91
	3	12.1	1.53	0.89
Sugarcane bagasse	1	18.9	0.18	0.99	162
Bamboo sawdust	1.5	24.5	198	0.97	205

---

Types of materials	Catalyst	E (kJ mol^−1)	A (s^−1)	R ^2	Ref.
Catalytic MAP
Sugarcane bagasse^a	NiO 10%	20.24	0.25	0.99	162
	CuO 10%	20.85	0.29	0.99
	CaO 10%	21.39	0.34	0.99
	MgO 10%	19.95	0.23	0.99

---

8. Future prospects and outlook

The MAP technology that has obtained plenty of attention has been manifested to be the promising alternative of conventional pyrolysis for the valorization of biomass, and its prospects are really optimistic.⁶⁰ That is because the input energy and processing time of MAP can be much lower, and the efficiency is much higher than conventional pyrolysis.^206,207 Regardless of low energy conversion efficiency from electric energy to thermal energy via microwave irradiation, the energy recovery is very high when the output energy of biofuel is divided by input energy of biomass plus energy for pyrolysis during the MAP process. It is found that the energy recovery during MAP of biomass could achieve 91%, which is much higher than the energy recovery of 35–39% from similar biomass by means of conventional flash pyrolysis techniques.²⁰⁸ In the same regard, the energy recovery by means of conventional pyrolysis of coffee hulls was 84%, while the energy recovery increased up to 99% using a microwave technique.¹⁰⁷ According to the report from Lei's group, if biochar generated is supplied for a coal-firing plant with a 35% efficiency to electricity and 90% transmission efficiency to a MAP plant, more than 150% electricity for the MAP plant can be supplied from biochar firing.²⁰⁹ Indeed, the MAP technique is likely to fairly promising for the valorization of biomass on a large scale.

To enhance the energy conversion efficiency from electric energy to thermal energy during microwave irradiation, it has been recently found that a higher energy conversion efficiency can be obtained by using single-mode microwave irradiation.²¹⁰ It is also well known that the energy loss of microwave energy to thermal energy plays a key role in the overall energy conversion efficiency. Hence, the improvement of the energy conversion efficiency in the step will remarkably contribute to enhancing the energy conversion efficiency. In this sense, microwave absorbers used in the MAP system can assist in the reduction of the overall energy consumption. Nevertheless, commonly used microwave absorbers microwave absorbers (e.g., biochar and metal oxides) leading to a sharp increase in dielectric properties at high temperatures allow the thermal runaway effect to take place.²¹¹ This effect is hard to control and can induce the occurrence of secondary pyrolysis reactions, such as gasification of biochar matrix and reforming of valuable compounds in the bio-oil.²¹² As a consequence, commonly used microwave absorbers are presented to lower the temperature threshold of the thermal runaway effect, and these microwave absorbers are thus not viable on an industrial scale for bio-oil production. Instead, the development of the novel microwave absorbers is obviously the other vital direction to alleviate the issues.⁷⁴ Since the increase of microwave absorption ability of the absorber can facilitate the transformation of microwave energy to thermal energy, thereby enhancing the energy conversion efficiency. It is suggested that the doping of another material or nanoparticles on microwave absorbers, and the fabrication of the layered composites can achieve the promoted dielectric and thermal response. The newly developed microwave absorbers may contribute to the reduction of the thermal runaway effect during the MAP process.

Furthermore, it is widely recognized that there are some other downsides of MAP technology that are required to be overcome and promoted. The most essential attempts are to increase the bio-oil yield and improve the selectivity towards target products.^213,214 Current studies have determined the optimized conditions (e.g., temperature range, microwave powers, the type and loadings of microwave absorbers, and catalytic patterns) for maximizing desirable products so far.^58,120,215 As every feedstock source is relatively unique, the pyrolysis performance is thus affected.²¹⁴ To remedy the dilemma of feedstock's influence, there have been some feasible investigations on pretreatment technologies. One of the most promising pretreatment approaches is torrefaction.⁶⁰ Homogeneity of torrefied biomass allow the MAP process to yield similar and simplified chemical compositions. Nonetheless, future investigations should be carried out to improve the energy efficiency of pretreatment technologies and lower the production cost of the large-scale process.

Given the state-of-the-art outcomes in this field, the motivation is transferred to biomass valorization in large-scale biorefineries. Moreover, when the MAP system is enlarged to the industrial application scale, the ratio of energy for biomass pyrolysis to total energy will be considerably enhanced, i.e. the ratio of heat loss during the process can be dramatically reduced.⁸⁵ However, the scale-up of the current MAP of biomass is difficult. To uplift the MAP technique to the industrial level, these are some steps that should be considered.²¹⁶ First of all, the key parameters that affect the process, such as microwave power, reaction temperature, and residence time should be clearly understood. Based on the fundamental understanding and process requirements, a model should be established to determine how the microwave irradiation interacts with biomass in a range of pilot-scale concepts and designs. Ultimately, an ideal amount of microwave absorber should be investigated to rapidly induce the pyrolysis by consuming the least energy.

With careful consideration of current lab-scale findings, a continuous MAP process can be developed and scaled up for future industrial applications. It is discerned that coupling MAP process and continuous flow technology can overcome the main drawbacks of microwaves and give a very promising way to manufacture high value-added chemicals or advanced fuels since the continuous flow has been proved to accelerate process intensification and obtain safe, efficient and sustainable products.⁸⁰ For example, continuous MAP with an apparatus fitted with magnetrons capable of outputting a maximum microwave power of 5000 W has been developed by Lam et al.²¹⁷ Waste automotive oil was fed into the continuous MAP system at a feed ratio of 5 kg per hour; and the MAP system presented a positive energy ratio of 8 and a net energy output of over 179 MJ h⁻¹. In terms of microwave reactors for the large scale, it is worth pointing out that large scale microwave reactors are commonly designed and specifically constructed according to applications to maximize energy transfer and process efficiency.^86,218 It has been concluded by Moseley et al. that medium to large scale multimode microwave reactors are more energy efficient than conventional heating reactors.^218,219 To develop a reliable MAP concept of biomass on a large scale, high power density has been proven to be a key criterion to enable the rapid conversion of biomass without the large amount of heat loss to the surroundings according to batch tests.^220,221 It is also suggested that very rapid quenching systems is the most effective alternative to recover bio-oils at the large scale.²²² Since the quality of bio-oil production is highly dependent upon the control of residence time of volatiles. The release of pyrolytic volatiles should be conducted rather rapidly (<1 s) to present secondary fragmentation reactions within bio-oils and to maintain an acceptable quality.^223,224 As fouling the handling system can result in a decrease in bio-oil yield and may cause damage to the microwave hardware (magnetrons and waveguide), a pressure window will be commonly used to separate the waveguide from microwave cavity where pyrolysis occurs on a large scale.²¹¹ Generally, nearby metallic surfaces must be avoided during the scaling up of MAP to prevent the dielectric breakdown induced by an excess of a charge buildup over the dielectric strength of the material.²¹¹ In this regard, five various prospective concepts have been proposed by Beneroso et al. pertaining to the scaling up of the MAP of biomass depending on the means by which biomass is transported through the continuous process: (1) rotary kiln concept; (2) conveyor belt concept; (3) rotating ceramic-based disc concept; (4) microwave fluidized bed concept; (5) auger reactor concept.²¹¹ These concepts have been widely used in the chemical industries but not within the MAP process. In fact, they have been evaluated for their electromagnetic compatibility and ability to deliver sufficient power density to induce pyrolysis without the utilization of conventional microwave absorbers.

Indeed, the commercialization of the MAP technology is associated with techno-economic feasibility.¹¹ Several studies have evaluated the feasibility of both non-catalytic MAP and CMAP techniques from an economic point of view.^225–228 In this regard, Ruan et al. discussed the utilization of small biorefinery systems which are distributed in different local communities.²²⁵ The results suggested that a small distributed biomass energy production system was technologically more feasible, economically viable, and suitable than current conventional large-scale biomass energy systems. Extra income can be provided for farmers and truly involved biomass producers. Accordingly, the continuous MAP of biomass and waste feedstock shows crucial potential to integrate biorefineries for the local conversion of biomass residues or waste into fuels and chemicals. Furthermore, the techno-economic analysis regarding ex situ CMAP of biomass for bio-oil production was also evaluated.²²⁷ For this case, a distributed mobile microwave pyrolysis system coupled with an ex situ catalysis process is assumed to produce aromatic hydrocarbon enriched bio-oil, gas and biochar. This study revealed that this scenario can gain a high profit annually, and the return of investment (ROI) can reach 45.34% per year. It is also interesting that a slight improvement of bio-oil yield can significantly increase the ROI.

To improve the yield of bio-oil and obtain more advanced fuel or chemicals for the improvement of ROI, the CMACP process integrated with the hydrogenation process paves a more feasible and promising way to valorize both biomass and waste hydrogen-rich polymers (such as waste plastics and waste tires). More specifically, jet fuels are mainly composed of linear- and branched-chain alkanes (C₈ – C₁₆).^15,229 It is widely proven that cyclic alkanes can be burned very cleanly releasing more energy straight-chain alkanes because cyclic alkanes are defined as compact molecules within a robust ring strain.^230,231 To enhance the qualities of jet fuels, jet fuel range cyclic alkanes can sever as additives in civilian jet fuels.^15,232 It has been suggested that the promising and simple method for the production of cyclic alkanes is the hydro-cycloaddition of aromatic hydrocarbons.^{100,128,135,233,234} Based on the studies from the CMACP of biomass with other polymers, aromatic hydrocarbons are mostly lumped in the range of C₈–C₁₆.^53,55 Moreover, Lei and his colleagues have developed a green catalytic route for the production of renewable cyclic alkanes from woody biomass via the CMAP process and the hydro-cycloaddition process.^100,128,135 Taking these outcomes into consideration, hydrogen-rich plastics have been introduced into the CMAP of biomass to enhance the carbon yield of aromatic hydrocarbons.^53,55 Therefore, the enhanced carbon yield of aromatic hydrocarbons was hydrogenated into highly valuable cyclic alkanes for potential use as replacements or additives in jet fuels as sketched in Fig. 13.

Fig. 13 Promising routes for the conversion of lignocellulosic biomass into advanced biofuels using the system of catalytic microwave-assisted pyrolysis. Reprinted with permission from ref. 53 Copyright (2016) Elsevier.

It is also observed that the co-feeding of biomass with waste polymers during the CMACP process could lower the coke formation. Yet, the heteroatoms can considerably affect the lifetime of zeolite-based catalysts (e.g., HZMS-5) during the CMACP process. Hence, developing a catalyst that can mitigate the influence is necessitated. To enhance the value of the resultant aromatic hydrocarbons, newly developed catalysts should accelerate tailored reaction routes toward either optimizing the carbon yield of valuable monocyclic aromatics (such as BTX) or jet fuel range precursors. Besides, the ideal catalysts should enable catalysis of a sequence of desired reactions, including dehydration, cracking, decarboxylation, decarbonylation, and C–C coupling.¹⁴ These zeolite-based catalysts developed should also own appropriate acid sites and acidity for target aromatic hydrocarbons through certain reaction pathways and to extend their lifetime.

9. Conclusions

To summarize, this study reviews a state-of-the-art technique regarding the MAP of biomass. The sections from Fundamentals of microwave irradiation to Catalysts for catalytic microwave-assisted pyrolysis are specifically demonstrated. Owing to the key features of MAP (e.g. rapid volumetric heating, easy control, and energy saving), MAP has been proven as the more effective pathway to valorize biomass than conventional pyrolysis. It is also noticed that the MAP technology is a well-established one that can be used to enhance pyrolysis for target products. Several types of catalysts including metal oxides and zeolite-based catalysts have been most commonly employed during the CMAP of biomass, significantly affecting the product yields, distributions, and even qualities. Techno-economic analysis of both non-catalytic MAP and CMAP processes suggested that the distributed mobile microwave pyrolysis system whether coupled with the catalysis process or not is profitable.

According to the current technique converting aromatic hydrocarbons to cyclic alkanes under very mild conditions, the CMACP integrated with a hydro-cycloaddition process is extremely essential for the production of advanced biofuels or highly valuable chemicals from biomass and waste polymers. Nevertheless, current studies are primarily dependent on the lab scale equipment using biomass with pure plastics to produce aromatics. Apparently, the most achievable and economic method is to directly transform biomass with waste polymers into target aromatic hydrocarbons, rather than pure plastics. In this premise, the CMACP technique remains in its infancy and requires continual research work on the large scale-up of the technique. As a result, the MAP technology paves a promising route for biomass utilization in a biorefinery; however, there are still some challenges for further advances in the MAP technique, including improvement of heating uniformity, scale-up of current technology for commercialization, etc.

Abbreviations

BTX	Benzene, toluene, and xylenes
CCD	Central composite design
CFP	Catalytic fast pyrolysis
CMACP	Catalytic microwave-assisted co-pyrolysis
CMAP	Catalytic microwave-assisted pyrolysis
EDTA	Ethylene diamine tetraacetic acid
FCC	Federal Communication Commission
fMAP	Fast microwave-assisted pyrolysis
HDO	Hydrodeoxygenation
HDPE	High-density polyethylene
HHV	High heating value
HTL	Hydrothermal liquefaction
ISM	Industrial, scientific, and medical
LDPE	Low-density polyethylene
MACP	Microwave-assisted co-pyrolysis
MAP	Microwave-assisted pyrolysis
MOMAs	Metal oxide microwave absorbers
PAHs	Polycyclic aromatic hydrocarbons
PC	Pre-coking
ROI	Return on investment
SCBMs	Solid carbon based materials
SiO2-CVD	Chemical vapor deposition of inert silica
TEOS	Tetra-ethyl-orthosilicate

Acknowledgements

This study was supported partially by the postdoctoral fellowship of the Illinois Sustainable Technology Center, University of Illinois at Urbana-Champaign, and The Agriculture and Food Research Initiative of National Institute of Food and Agriculture, United States Department of Agriculture (Award Number: 2016-67021-24533; Award Number: 2016-33610-25904).

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