Nanoporous catalysts for biomass conversion

Abstract

Biomass conversion is potentially important for producing renewable fuels and chemicals in the future, and the nanoporous materials as acidic, basic, or metallic catalysts play important roles in the biomass conversion. This paper briefly summarizes recent developments on the nanoporous material-based catalysts used for biomass conversion, including mesoporous/macroporous resins, mesoporous metal oxides, microporous/mesoporous carbons, mesoporous silicas, hierarchically mesoporous polydivinylbenzene, and microporous zeolites. Due to their superior thermal and hydrothermal stability, controllable acidity, and relatively easy introduction of mesoporosity into the crystals, compared with other nanoporous catalysts, zeolites have good opportunities for application in biomass conversion.

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Liang Wang

Dr. Liang Wang received his B.S. (2008) and Ph.D. (2013) degrees from the College of Chemistry, Jilin University, China. In July 2013, he joined the Institute of Catalysis in Zhejiang University for postdoctoral work under the guidance of Prof. Feng-Shou Xiao. His current research is mainly focused on heterogeneous catalysis, biomass conversion, and green oxidations.

Feng-Shou Xiao

Prof. Feng-Shou Xiao received his B.S. and M.S. degrees from the Department of Chemistry, Jilin University, China. From there he moved to the Catalysis Research Center, Hokkaido University, Japan, where he was involved in collaborative research between China and Japan. He was a Ph.D. student there for two years, and was awarded with a Ph.D. degree at Jilin University in 1990. After postdoctoral work at the University of California at Davis, USA, he joined the faculty at Jilin University in 1994, where he was a full and distinguished professor of Chemistry. He moved to Zhejiang University in 2009, and now he is a full and distinguished professor of Chemistry. For his research in nanoporous materials, Prof. Xiao was recognized with the National Outstanding Award of Young Scientists in the National Science Foundation of China in 1998 and the Thomson Scientific Research Fronts Award in 2008. Currently he is the secretary of the Asia-Pacific Association of Catalysis Societies.

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1. Introduction

Nanoporous materials mainly including micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) have been widely applied in heterogeneous catalysis.^1–9 For example, microporous zeolites are excellent catalysts for oil refining, where the acid sites in the framework of zeolite could catalyze the selective formation of desired alkanes because of the micropore size limitation.^10–13 However, due to the shortage of fossil resources and environmental concerns related to CO₂ emissions,¹⁴ much attention has been paid to the catalytic conversion of biomass because there is a huge amount of carbohydrates in biomass (nearly 130 billion metric tons) synthesized with CO₂ and H₂O using sunlight as the energy source per year, but only about 4% of these compounds are used currently.¹⁵ These data confirm the importance of catalytic conversion of biomass as renewable energy.^16–23

The biomass mainly includes cellulose formed with glucose, hemicellulose formed with polymerization of glucose and xylose, and lignin formed with phenols and derivative phenolic molecules. Besides these sugars and phenol components, the plant can also provide energy storage biomass of triglycerides, lipids, and terpenes.^24,25 These compounds can be directly converted into desired fuels and chemicals to replace the existing chemicals related to crude oils. For example, the cellulose can be depolymerized into glucose and cellobiose;^18,26–31 the sugars (e.g. glucose, cellulose, and xylose) can be transformed into platform chemicals of 5-hydroxymethylfurfural (HMF), furfural, glycol, and lactic acid;^23,32–42 the triglycerides can be transesterificated into biodiesels;^43–51 the pyrolysis oil can be hydrodeoxygenated into alkane fuels.^52–55

Notably, although these processes start from renewable biomass, much attention should still be focused on waste minimisation and high atom efficiency.^56–61 In this field, an important topic is to develop highly efficient catalysts in order to achieve a high yield of the target product under sustainable reaction conditions. Since the end of the 20th century, a large number of catalysts have been confirmed efficiently for transforming the biomass-derived carbohydrate into the desired fuels and chemicals. Especially the past few years have seen an enormous increase in the number of nanoporous catalysts, which usually exhibit exciting catalytic activity, excellent product selectivity, and superior recyclability.^{15,24,25,62,63} In these cases, the catalyst nanoporosity has obvious advantages for designing efficient heterogeneous catalysts in the following: (1) the nanoporous catalysts generally have high surface areas and large pore volumes, which is very helpful for access of reactants, mass transfer and functionalization of active sites; (2) the catalyst micropore sizes effectively control the product selectivities; and (3) the wettability of the nanoporous catalysts is easily adjusted, which is favourable for adsorption, desorption, and surface diffusion of reactants and products in catalysis.

Although a number of encouraging reviews have already summarized the production of fuels and chemicals from biomass, they are mostly focused on the conversion routes.^{15,24,64–68} Very differently, the aim of this review is to discuss and evaluate the nanoporous catalysts used in the biomass transformation. Here, the nanoporous materials with acidic, basic, or metallic active sites mainly include functionalized resins, metal oxides, carbons, mesoporous silicas, polydivinylbenzene, and zeolites.

2. Sulfonated resins

The sulfonated resins, such as Amberlyst,^69,70 Nafion,^71–75 and EBD resins,⁷⁶ have been widely used in the conversion of biomass because of their commercial synthesis. More importantly, the abundant porosity in the resins is favourable for the functionalization of Brønsted sulfonated groups on the organic framework with high concentration, as well as for the full interaction between substrates and acid sites, leading to high catalytic activity.

2.1 Amberlyst resins

Amberlyst-15, synthesized from styrene polymerization and sulfonation, is one of the most famous resins with rich macropores and mesopores (surface area of about 46 m² g⁻¹), forming a high density of acid sites (4.0 mmol g⁻¹) exposed to the reactants.⁷⁷ Notably, Amberlyst-15 has been used in almost all acid-catalyzed liquid-phase reactions of biomass, in particular to produce sugars, HMF, and biodiesels. For example, more than 20% yield of sugars could be obtained from the conversion of wood and cellulose over the Amberlyst-15 catalyst in ionic liquid solvents at 100 °C.^29,78,79 In the conversion of fructose to HMF, Amberlyst-15 gave HMF yield at nearly 100% in a water-separation reactor.⁸⁰ For the biodiesel production from esterification of dodecanoic acid with 2-ethylhexanol at 150 °C, Amberlyst-15 is highly active, giving a yield higher than 80%.⁴³

However, the nanoporous structure of the Amberlyst-15 catalyst has a relatively low stability. For example, in the transesterification of tripalmitin with methanol to produce biodiesel at 65 °C, the activities of Amberlyst-15 decreased with recycles significantly. N₂ sorption analysis indicates that the porous structure of Amberlyst-15 is partially destroyed after recycling in the transesterification.^43,81

2.2 Nafion resins

Compared with Amberlyst-15, Nafion resins have relatively high thermal stability in the nanoporous structure.^82,83 For example, NR-50, a typical Nafion resin of sulfonated and fluorinated polymers, is stable up to the temperature of 280 °C. When NR-50 was used to catalyze an esterification of acrylic acid with methanol to methyl acrylate, it gave the methyl acrylate yield over 90%, which is even comparable with the homogeneous sulfuric acid catalyst.⁸⁴ Nafion perfluorinated membrane NR-117 gave furfural yield of 60% in the dehydration of xylose in DMSO solvent.

Interestingly, the stability of Nafion resins is significantly improved compared with Amberlyst-15. For example, NR-117 exhibits very good recyclability in the conversion of xylose to furfural at 150 °C;⁸⁵ SAZ-13 are highly active for long reaction time in the esterification of short-chain carboxylic acids (e.g. acetic, propionic, and butyric acids) with methanol. However, in the esterification of long-chain carboxylic acids (e.g. hexanoic and caprylic acids) with methanol, SAZ-13 shows poor recyclability, because of the accumulation of the long-chain acids on the surface of SAZ-13, which could not be removed even by washing with a large amount of solvents.^71,86 This phenomenon is strongly related to its relatively low surface area and pore volume. Therefore, the resins with improved surface area and pore volume are effective for enhancing catalyst activities and recyclability in the biomass conversion.⁸⁷

2.3 EBD resins

EBD resins are copolymers of styrene–divinylbenzene with functionalized sulfonic acid groups. The copolymerization of styrene with divinylbenzene could effectively increase cross-linking degree, leading to high surface area and pore volume, which is helpful for mass transfer during the reaction process. The typical EBD-100 exhibited high catalytic activity in the esterification of vegetable oils. For example, in the esterification of stearic acid as a model molecule, EBD-100 gave higher catalytic activity than Amberlyst-15, giving a stearic acid conversion higher than 90% in a reaction time of 10 h.

2.4 Double-shell resin

Because of the abundant porosity and high surface area in the sulfonated resins (e.g. Amberlyst, Nafion, and EBD resins), they have high concentration of the exposed acidic sites. However, it is still challenging to improve their acidic strength that is very sensitive to the catalytic performances.^88–95 Recently, Zhang et al.⁹⁵ synthesized a polystyrene sulphonic acid with a double-shell nanostructure (PS-SO₃H@mesosilica DSNs, Fig. 1), which exhibited higher acid strength than the Amberlyst-15 and Nafion NR50 resins. This phenomenon is assigned to the fact that the sulfonic acid groups in the unique nanoporous structure have strong H-bond interactions, which significantly enhance the acid strength. The DSN catalyst exhibited a conversion of 89.7% in the esterification of lauric acid with ethanol at 80 °C for 6 h, which is even higher than that (71.7%) of the homogeneous H₂SO₄ catalyst. After treating in DMF to break the H-bond interaction, the obtained catalyst lost most of its activity. This result indicates the importance of the H-bond interaction for the catalytic activity. In the recycling test, 5.4% of the laurate yield dropped in the second run. For further recycling test until the eighth run, no further reduction of yield was observed. After the eighth run, the activity dropped severely, because the resin swelled and caused mesopore blockage on the silicas. This observation suggests that the acid strength of sulfonated resin could also be improved by adjusting the nanoporous structures.

Fig. 1 (a) Scheme for the synthesis of PS-SO₃H@mesosilica DSNs. TEM images of (b) polystyrene (PS) template, (c) PS@mesosilicas with yolk–shell nanostructures (YSNs), and (d) PS-SO₃H@mesosilica DSNs. The scale bars in images (b), (c), and (d) are 100, 200, and 200 nm, respectively (reproduced with the permission of ref. 95. Copyright 2014 The Nature Publishing Group).

As shown in this section, it is clear that the sulfonated resins are good acidic catalysts for biomass conversion, where the nanoporous structure is very important for the acid concentration and/or strength on the resins. However, we have to note the fact that the sulfonated resin catalysts still have relatively low stability. For example, even for the most stable Nafion resins, it is still difficult to catalyze the reaction at relatively high temperature, which strongly hinders their wide application in biomass conversion.

3. Nanoporous metal oxides

Metal oxide catalysts play important roles in heterogeneous catalysis.^96–98 Apart from the catalytic reactions, the techniques used in catalyst characterization are also developed with the growth of metal oxide-based catalysts.^99,100 Compared with the resin catalysts, the nanoporous structures of metal oxides have obvious advantages in thermal stability, because the synthesis of nanoporous metal oxides generally requires calcination at high temperature. In the field of biomass conversion, the nanoporous metal oxides with acidic and basic sites have already been successfully used.

3.1 Sulfated or protonated metal oxides

The sulfated metal oxides such as ZrO₂, SnO₂, TiO₂, and Nb₂O₅ are typical solid acids which have already been used in various reactions.^101–114 Notably, compared with the sulfonated resin catalysts with only Brønsted acid sites, a unique feature of sulfated metal oxides is that they have both Brønsted and Lewis acid sites. In these cases, the Lewis acidic sites are metal atoms, while the Brønsted acidic sites are protons on the surface hydroxyl groups of sulfated metal oxides, as shown in Fig. 2.^101–114 Usually, the hydroxyl groups on the surface of metal oxides have extremely weak Brønsted acidity. After sulfonation, the S–O bonds on the sulphuric groups are strongly bonded to metal atoms, leading to a coordination of the S=O bond with the hydroxyl groups on metal oxides, which causes the enhancement of Brønsted acidic strength. Obviously, the Brønsted acidic strength is sensitive to the metal type. For example, SO₄²⁻/SnO₂ and SO₄²⁻/ZrO₂ exhibit stronger acidic strength than SO₄²⁻/TiO₂, because the lower electronegativity of Sn and Zr metals than Ti metal causes the proton to be realized more easily.¹⁰¹ It is worth pointing out that the nanoporous structure of metal oxides with high surface areas is very positive for improving the Brønsted acid density.

Fig. 2 Scheme of the Brønsted and Lewis acid sites on metal oxides and sulfated metal oxides (M = Zr, Sn, Ti, Nb).

SO₄²⁻/ZrO₂ is a typical sulfated metal oxide used in the conversion of biomass.^115–120 Suwannakarn et al.¹²⁰ reported that SO₄²⁻/ZrO₂ is catalytically active for transesterification of tricaprylin with methanol, ethanol, and n-butanol at 120 °C. However, they found that the activity loss occurred during the recycling tests because of the leaching of sulfur species from the catalysts. Notably, the conventional synthesis of SO₄²⁻/ZrO₂ catalysts from ZrO₂ is mostly performed by post-sulfonation and calcinations, which might result in a relatively weak interaction between SO₄²⁻ and ZrO₂ species. To solve this problem, Sun et al.¹²¹ reported a solvent-free route to synthesize sulfated ZrO₂ (S-ZrO₂) from a simple mixture of ZrOCl₂ and (NH₄)₂SO₄, followed by grinding and calcining, where the interaction between SO₄²⁻ and ZrO₂ could be highly improved. As a result, S-ZrO₂ is very active for the depolymerization of cellulose.²⁹

In addition, we also note that the sulfated nanoporous oxides have relatively low surface areas, which strongly limits the access of the acid sites to reactants. In order to solve this problem, Jiménez-López et al.¹²² reported a supported SO₄²⁻/ZrO₂ in mesoporous silicas such as MCM-41 (Zr-MCM), which showed the high yield of ethyl esters and good recyclability from ethanolysis of sunflower oil.

Besides the supported metal oxides, introduction of unique nanoporosity into the metal oxides is also an effective route for increasing sample surface areas. A typical example is a TiO₂ nanotube with one-dimensional nanopores, which has been widely studied in photocatalysis, sensors, and batteries.^123–125 Kitano et al.¹²⁶ synthesized a TiO₂ nanotube with Brønsted acidic sites by hydrothermal treatment of TiO₂ in basic aqueous solution, followed by proton exchange. This TiO₂ nanotube shows a surface area as high as 400 m² g⁻¹ and a pore size of about 5 nm. The protonated TiO₂ nanotube is highly efficient for catalyzing the conversion of glucose to HMF in water solvent, compared with Nafion NR50, Amberlyst-15 resins, and sulfated ZrO₂ catalyst, because of its good mass transfer on the one-dimensional channels of nanotubes (Fig. 3).

Fig. 3 TEM images of protonated TiO₂ nanotubes with one-dimensional channels (reproduced with the permission of ref. 126. Copyright 2010 American Chemical Society).

3.2 Hydrotalcite

Hydrotalcites (HT) are a class of layered materials with good surface areas, which have the trivalent and divalent cations contained within the octahedral sites of positively charged hydroxide sheets.^127–135 In the conversion of biomass, the HT is not only used as a solid basic catalyst, but also combined with other active centers to catalyze the tandem reactions.^136–142

The Mg–Al based HTs were highly efficient for the production of biofuel derived chemicals such as aldol condensation of furfural with acetone¹³⁶ and transesterification of tripalmitin with alcohols.^137–139 For example, Hora et al.¹³⁶ reported that HT gave furfural conversion at nearly 100% in the condensation of furfural with acetone.

Apart from these results, HT was also used to catalyze the tandem reactions. For example, Takagaki et al.¹⁴⁰ found that the combination of HT and Amberlyst-15 resin could catalyze the conversion of glucose to HMF, where HT catalyzed the isomerization of glucose to fructose, while the Amberlyst-15 resin catalyzed the dehydration of fructose to HMF. Later, they supported Ru nanoparticles on HT for HMF oxidation.¹⁴¹ In this case, the combined catalysts of Ru/HT and Amberlyst-15, which are ternary catalysts with a base, an acid, and Ru sites, are efficient for one-pot conversion of glucose to 2,5-diformylfuran (Fig. 4).

Fig. 4 One-pot conversion of glucose to 2,5-diformylfuran over combined catalysts of Ru/HT and Amberlyst-15.

In order to increase the sample stability, Matson et al.¹⁴² synthesized a porous metal oxide supported Cu catalyst (Cu20-PMO) by calcining a Cu-doped HT. It was found that the lignocellulose solids, including maple sawdust, pine flour, torrefied wood, and cellulose fibers, could be directly converted into alcohols, ethers, hydrogen, and C₁ gases at 300–320 °C in supercritical methanol. It is worth emphasizing that nearly no coke is formed in the catalytic process, and the stable Cu20-PMO catalyst could be continuously used.¹⁴³

4. Porous carbons

Considering the low thermal stability of sulfonated resins and poor porosity of sulfated metal oxides and HTs, nanoporous carbon-based catalysts have obvious advantages in the thermal stability and porosity. For example, the activated carbon, which is produced for nearly 1.5 million tons every year as an adsorbent, a capacitor, and a catalyst support, has very high surface area (∼1000 m² g⁻¹) and superior stability in acid/base media and/or under high temperature calcination at even 800 °C in an inert gas.¹⁴⁴ Based on these excellent features, the nanoporous carbon supported acidic sites or metal nanoparticles are employed as efficient catalysts active for biomass conversion.

4.1 Activated carbons and ordered mesoporous carbons (OMC)

Since the pioneering work of Hara et al. on sulfonated incompletely carbonized naphthalene as an efficient solid acid catalyst in 2004,¹⁴⁵ sulfonated carbons have become a new type of solid acid. Recently, much attention has been paid to developing new sulfonated porous carbons for the conversion of biomass.^146–149

Followed by Hara's observation, the sulfonated activated carbons were synthesized from sulfonation of activated carbons with sulfuric acid or chlorosulfonic acid. However, the sulfonated activated carbons usually exhibited poor catalytic activities, which is due to the relatively weak interaction between sulphuric acid and the carbon surface.¹⁴⁵ A similar phenomenon was also observed in the direct sulfonation of ordered mesoporous carbon (OMC) materials.^150,151 Recently, to increase this interaction, Chang et al.¹⁵⁰ have used H₂O₂ to pretreat the surface of OMC, obtaining the OMC with rich hydroxyl and carboxyl groups.^152,153 After sulfonation with concentrated sulfuric acid, OMC-H₂O₂–SO₃H gives an acid concentration of 2.09 mmol g⁻¹, which is much higher than that (1.06 mmol g⁻¹) of the directly sulfonated sample (OMC-SO₃H). As a consequence, OMC-H₂O₂–SO₃H exhibited a much higher catalytic activity than OMC-SO₃H in the esterification of oleic acid with methanol, as shown in Fig. 5.^152–154 Very importantly, even if recycled 5 times, there is no obvious activity loss compared with the fresh catalyst, which is reasonably attributed to the strong interaction between sulfonic groups and the carbon surface.

Fig. 5 Photographs of (A) OMC-SO₃H and (B) OMC-H₂O₂-SO₃H in methanol solvent. OMC-SO₃H precipitates in a few minutes, while OMC-H₂O₂-SO₃H forms a stable dispersion, because of its hydrophilic surface (reproduced with the permission of ref. 150. Copyright 2013 American Chemical Society).

Except for using solid acids, porous carbons help to support metal nanoparticles for the oxidation and hydrogenation. Considering that these reactions are usually performed at relatively high temperature, a high stability and inert surface of porous carbons are suitable.^155–157

One typical example is the activated carbon supported Pd nanoparticle catalyst (Pd/C). Since 1995, Pd/C has been widely applied in the conversion of biomass-based feedstocks.¹⁵⁸ An important use is to catalyze the oxidation of glycerol, which is a major by-product for the production of biodiesel. Currently, about 1.2 million tons of glycerol were formed every year, mostly coming from the production of biodiesel. Therefore, the conversion of glycerol into valuable chemicals is of great importance. Garcia et al.¹⁵⁸ reported a Pd/C catalyst used in the oxidation of glycerol by molecular oxygen, where the selectivity to glyceric acid could be as high as 70%. Interestingly, they found that adjusting the composition of metal species on activated carbons could effectively change the reaction route. For example, the deposition of Bi on Pt nanoparticles on activated carbon (Bi–Pt/C) orientated the selectivity towards the oxidation of the secondary hydroxyl group to yield dihydroxyacetone with a selectivity of 50%. Based on these observations, Bianchi et al. performed a systematic study on the glycerol oxidation over carbon-supported mono- and bimetallic catalysts based on Au, Pd, and Pt. It was found that the bimetallic Au-Pd/C and Au-Pt/C catalysts are more active than the monometallic catalysts.¹⁵⁹

Besides the conversion of glycerol, the activated carbon supported metal nanoparticles are applied also for the conversion of sugars and cellulose.¹⁶⁰ For example, Fuchigami et al. found that the Pd/C catalyst is highly active for the conversion of maleic anhydride (MAN) and succinic anhydride (SAN) in the presence of Cs₂SO₄, giving a γ-butyrolactone (GBL) yield of 97%. Ji et al.³² reported that activated carbon supported tungsten carbide with a small amount of Ni (Ni–W₂C/C) can effectively catalyze cellulose conversion into ethylene glycol (yield of 61%). Very interestingly, this catalyst has a good recyclability, which is assigned to its high stability of the activated carbon support. Taking into consideration the huge amount (20 million tons in global 2011) of ethylene glycol produced from crude oil-based feedstocks, this route would be important for “green” synthesis of ethylene glycol from biomass.

The activated carbon supported metal catalysts are also good catalysts for upgrading of biofuels.^161,162 Different from the conventional reaction systems, which generally produce alcohols and ketones from hydrogenation of phenols, Zhao et al.¹⁶² found that the Pd/C catalyst is efficient for the production of cyclohexane from hydrogenation of phenol, a model molecule of aromatic pyrolysis oil. In this case, a key factor is that the presence of homogeneous H₃PO₄ in the reaction system could catalyze the dehydration of cyclohexanol to cyclohexene at high reaction temperature (>170 °C). Furthermore, they also performed the hydrogenation of phenols (e.g. guaiacols and syringols) originating from lignin-based pyrolysis oil. In these cases, Pd/C catalysts exhibited high yield of C₉ alkanes at 250 °C in the presence of H₃PO₄. As expected, the Pd/C catalysts with high stability exhibited excellent catalytic recyclability. However, it is still a challenge to recycle liquid H₃PO₄ in this system. The solution for this shortage will be discussed in the zeolite section.

The activated carbon supported Pt and Mo₂C catalyst (Pt–Mo₂C/C) is also effective for reforming CO, H₂O, and cellulose to produce polyols.¹⁶³ Here, the Pt–Mo₂C/C was synthesized from impregnation of nitric acid-treated carbon with ammonium paramolybdate and chloroplatinic acid solution, followed by a temperature programmed reduction. As shown in Fig. 6, the small particle sizes of Pt and Mo₂C at high weight loading should be attributed to the abundant nanoporosity of the activated carbon support, which has an extremely high surface area for the dispersion of Pt and Mo₂C species. During this catalyst, the Pt–Mo₂C catalyzed the water–gas shift reaction from CO and H₂O to H₂, while the Pt–C species catalyzed the cellulose conversion to polyols. This catalytic system exhibits much higher selectivity to polyols than the conventional system using pure molecular hydrogen, because the hydrogen species in situ produced on Pt–Mo₂C in the nanopores of the activated carbon support immediately shifted to nearby Pt–C, thus taking the hydrogenation.

Fig. 6 (a) A high-angle annular dark-field-STEM image of the Pt–Mo₂C/C composite catalyst, showing two types of particles of different sizes; the large particles with a broad size distribution (20–50 nm) and the small particles with relatively uniform sizes (∼5 nm) correspond to Mo₂C and Pt, respectively. (b) A magnified HAADF-STEM image (upper-left) of the red square region in (a) and the corresponding EDX elemental mappings of Mo (upper-right) and Pt (lower-left). The combined mapping image is shown in the lower right corner. These data confirmed that Pt-Mo₂C/C have two types of Pt species on Mo₂C and C, respectively (reproduced with the permission of ref. 163. Copyright 2013 Royal Society of Chemistry).

4.2 Carbon nanotubes

Carbon nanotubes (CNTs) is a great discovery in carbon materials after fullerene.¹⁴⁴ With a hollow structure, the CNTs show a large surface area of 200–300 m² g⁻¹ for multiwalled carbon nanotubes (MWCNTs) and 700–800 m² g⁻¹ for single-walled carbon nanotubes (SWCNTs). Although the surface areas of CNTs are lower than those of activated carbons (∼1000 m² g⁻¹), they have better mass transfer for the reactants/products than activated carbons.¹⁴⁴

In addition, the basic sites were easily functionalized on CNTs for catalyzing the biomass conversion. By carbometalation and C–C bond formation steps, Tessonnier et al.¹⁶⁴ successfully synthesized a triethylamine-modified MWCNT catalyst (NEt₃-MWCNT, Fig. 8), which is highly active and stable for transesterification of glyceryl tributyrate with methanol to produce biodiesel. In contrast, the NH₂-MWCNT catalyst synthesized by the oxidation and amidation (Fig. 7) method gives lower activity and stability than NEt₃-MWCNT, which might be due to the destruction of the CNT walls during the synthesis of NH₂-MWCNT.¹⁶⁵ These observations suggested the importance of a stable nanotube structure for the catalytic activity and stability of the CNTs-based catalysts.

Fig. 7 (Top) Synthesis of NEt₃-MWCNT by the carbometalation and C–C bond formation method. (Bottom) Synthesis of NH₂-MWCNT synthesized by the oxidation and amidation method (reproduced with the permission of ref. 164. Copyright 2009 Wiley-VCH).

Fig. 8 TEM images of (a) SiO₂@resin, (b) SiO₂@HSC, and (c) HSC-SO₃H; (d) hollow size distribution of HSC-SO₃H. Clearly, the HSC-SO₃H exhibits a hollow structure with a carbon shell thickness of 10–13 nm (reproduced with the permission of ref. 176. Copyright 2013 Royal Society of Chemistry).

Besides modifications of basic sites, CNTs are also good supports to load metal nanoparticles for the conversion of biomass-based feedstocks. For example, Tan et al.¹⁶⁶ reported the CNT supported Au nanoparticles as efficient catalysts for oxidation of cellobiose, giving a gluconic acid yield of 80% at 145 °C. Wang et al.¹⁶⁷ found that CNT supported Ru nanoparticles are highly efficient for selective hydrogenolysis of glycerol to glycols. Later, Li et al.¹⁶⁸ synthesized highly dispersed bimetallic Ru–Fe nanoparticles with a small size of 3 nm on CNTs, giving higher glycol yield than the mono-metallic catalysts in hydrogenolysis of glycerol. Recently, de Vlieger et al.¹⁶⁹ extended the use of CNT supported Ru nanoparticles for reforming biomass-derived acetic acid to produce hydrogen.

4.3 Nitrogen-doped carbons

The activated carbon and carbon nanotubes have been widely used as catalyst supports. However, the shortage of strong interaction between the carbon surface and metals results in leaching of active metal sites.^2,170–172 One solution to this problem is to dope nitrogen species in the carbons.¹⁷³ As expected, N-doped porous carbon supported metal catalysts exhibited excellent catalytic performances in the biomass conversion. For example, Xu et al.¹⁷⁴ synthesized N-doped porous carbons from the carbonization of dicyanamide-containing ion liquids of 3-methyl-1-butylpyridine dicyanamide using silica spheres as a hard template. After loading Pd nanoparticles, the obtained catalyst (Pd/CN_0.132) is highly efficient for the hydrogenation of vanillin, a model reaction for biofuel upgrade, giving a product (2-methoxy-4-methylphenol) yield of 100%. In contrast, the conventional Pd/C showed the product yield of 72.5%.

More recently, Wang et al.¹⁷⁵ designed and synthesized a N-modified porous carbon and TiO₂ composite supported Pd catalyst (Pd/TiO₂@N-C). In this work, chitosan, a polysaccharide biomass with high nitrogen content, was used as a precursor for synthesizing TiO₂@N-C by hydrothermal treatment with tetrabutyl titanate in acetic acid aqueous solution, followed by calcination. After loading Pd nanoparticles, the obtained Pd/TiO₂@N-C is very active for catalytic hydrogenation of vanillin with formic acid as a hydrogen source, giving the product (2-methoxy-4-methylphenol) yield of 100%. The excellent catalytic performances over Pd/TiO₂@N-C are reasonably attributed to the high activity of Pd nanoparticles on TiO₂ and N-modified carbon for the tandem formic acid dehydrogenation and vanillin hydrogenation processes.

4.4 Carbon spheres

It is well known that an increase of mass transfer for heterogeneous catalysts is very helpful for enhancing catalytic performance, and an effective strategy is to introduce hierarchical porosity in the samples. Recently, Wang et al.¹⁷⁶ synthesized a sulfonated hollow sphere carbon (HSC-SO₃H) from carbonization of resin/SiO₂ nanocomposites (SiO₂@resin), removal of SiO₂ (SiO₂@HSC) by HF washing, and sulfonation with chlorosulfonic acid (Fig. 8). This unique sulfonated carbon catalyst with a hierarchical nanoporous structure shows comparable catalytic activities with a homogeneous sulphuric acid catalyst in acetalisation of glycerol with acetone to solketal. This phenomenon is assigned to its fast mass transfer due to the unique hollow sphere structure with rich micropores on the thin carbon shell (thickness of 10–13 nm), as shown in Fig. 8.

4.5 Porous carbons synthesized from biomass feedstocks

The cellulose, starch, glucose, and sucrose are natural carbohydrates with rich hydrophilic groups. Hara and co-workers developed a series of sulfonated amorphous carbons from these natural materials.^177–180 These results show a new route to synthesize carbon-based solid acids as highly efficient catalysts for transesterification of triolein, esterification of oleic acid, hydrolysis of cellobiose, and dehydration of fructose. It should be noted that the synthesized catalysts are non-porous with low surface area (<5 m² g⁻¹).^177–180 Therefore, it is desirable to synthesize carbon materials with rich porosity from the natural feedstocks.^181,182

Starbon, a typical porous carbon derived from porous starch, leads to a hot topic focusing on synthesizing mesoporous porous carbons from the natural feedstocks in the absence of any template.^183–192 Until now, the Starbon technique has been extended to synthesize mesoporous carbons from a series of biomass such as chitin, collagen, keratin, cellulose, lignin, furfural, and alginic acid, where the porosity, surface area, hydrophobicity/hydrophilicity, and functionalized groups could be easily controlled by adjusting the synthesis conditions.^190–192 Interestingly, the sulfonated Starbon (Starbon-SO₃H) is highly active for the esterification of diacids of succinic, fumaric, and itaconic acids, giving 5–10 times higher reaction rates than commercial solid acids of zeolites, Zr/SO₄²⁻, acidic clays, and resins.^183,184,193

Notably, the mesopores in the carbons synthesized from the Starbon technique are disordered, because of the absence of any template during the synthesis process. More recently, Kubo et al.¹⁹⁴ successfully synthesized ordered microporous and mesoporous carbons using fructose as feedstocks in the presence of the F127 triblock copolymer as a template (Fig. 9). Particularly, it is worth noting that although great success has been achieved in the synthesis of porous carbons from natural feedstocks, the study on their catalytic properties is just in the beginning stage. Considering the easy and low-cost synthesis process, rich porosity, and functionalized hydroxyl groups on these porous carbons, it is suggested that catalysts based on porous carbons from natural feedstocks would be potentially important for biomass conversion in the future.

Fig. 9 (A and B) TEM and (C and D) HR-TEM images of microporous and mesoporous carbons synthesized from fructose (reproduced with the permission of ref. 194. Copyright 2011 American Chemical Society).

5. Ordered mesoporous silicas

Compared with nanoporous resins, metal oxides, and carbons, ordered mesoporous silicas (OMS) such as MCMs and SBAs materials have the obvious advantages of ordered open channels, controllable morphology and pore sizes, large surface areas, and rich functionalized groups. These features are attractive for functionalization of the OMS by active sites as efficient catalysts for biomass conversion.

5.1 OMS with sulfonated groups

In 2003, Dufaud and Davis¹⁹⁵ synthesized mesoporous silicas with various active sites such as sulfonated acid using organic–inorganic hybrid silane as a precursor. Recently, this method was applied to synthesize ordered mesoporous silicas with sulfonated groups for the biomass conversion.^196–198 For example, Karaki et al.¹⁹⁶ synthesized SBA-15 with a controlled local environment of the acid sites using different sulfonated silanes. They show high conversion for hydrolysis of cellobiose to glucose. Dhainaut et al.¹⁹⁷ prepared hierarchical macroporous–mesoporous SBA-15 sulfonic acid catalysts in the presence of polystyrene in the starting gel. They are more active in the transesterification of tricaprylin and esterification of palmitic acid with methanol than the conventional sulfonated mesoporous SBA-15 because of the highly efficient mass transfer from the macroporous and mesoporous structure. Pirez et al.¹⁹⁸ synthesized various sulfonated mesoporous KIT-6 catalysts, an ordered mesoporous silica with Ia3d architecture, by the post-grafting route (PrSO₃H-KIT-6). They found that PrSO₃H-KIT-6 is highly efficient for the esterification of free fatty acids (propanoic acid, hexanoic acid, lactic acid, and palmitic acid) with alcohols (methanol and ethanol), because of their interconnected pore network.

5.2 OMS with metals

The metal species in OMS have Lewis acidic feature. Li et al.¹⁹⁹ found that Zr, Hf, and Al in TUD-1 and MCM-41 catalysts are active for the acetalization of glycerol with acetone to produce solketal. However, due to a lack of Brønsted acid sites, their catalytic applications are severely limited. To create Brønsted acidic sites, Wang et al.²⁰⁰ synthesized extremely highly dispersed Sn species in the mesopores of SBA-15 (S–Sn–OH) by grafting dimethyldichlorostannane onto the silica surface, followed by calcination to transform the methyl groups into hydroxyl groups, where both Lewis and Brønsted acidic sites are present (the formation of Brønsted acidic sites is due to the presence of hydroxyl groups attached on the Sn species). This sample is designated as S–Sn–OH, giving comparable catalytic activity with that of sulphuric acid. Very importantly, no decrease of glycerol conversion and solketal selectivity was observed over a reaction period of 50 h in a continuous reaction system with a fixed-bed reactor, indicating its good stability.

Interestingly, the S–Sn–OH catalyst exhibits unique product selectivities. For example, in the catalytic conversion of fructose into ethanol, ethyl lactate is the major product over SBA-15 with only Lewis sites, while ethyl levulinate is the major product over the S–Sn–OH catalyst. This phenomenon is also related to the presence of Brønsted acidity in the S–Sn–OH, which could catalyze the dehydration of fructose to HMF, followed by hydrolysis and esterification of HMF to ethyl levulinate. Possibly, this methodology for introducing Brønsted sites into the OMS samples with both Lewis and Brønsted acidity might be potentially suitable to extend other metals such as Zr and Nb.

Accordingly, the synthesis of OMS requires expensive surfactants as mesoporous templates, and removal of these templates by calcination always produces undesirable wastes. In addition, their relatively low hydrothermal stability and hydrophilic surface are not favourable for the biomass conversion. Therefore, synthesis of novel catalysts with abundant porosity, good stability, and controllable wettability is desirable.

6. Porous polydivinylbenzene

Nanoporous polydivinylbenzene (PDVB) synthesized from the solvothermal route²⁰¹ has obvious advantages of abundant porosity, large pore volume, and superoleophilicity. Particularly, compared with conventional resins, the PDVB exhibits high stability and adsorption selectivity. For example, the PDVB almost does not adsorb either liquid or gaseous water and thus has a preferential selectivity for organic compounds. These features are helpful for the biomass conversion.^202–208

Motivated by the unique features of PDVB, a series of PDVB-based basic, acidic, and metal catalysts has been developed. For example, Liu et al.²⁰² synthesized basic imidazole group modified nanoporous polydivinylbenzene by co-polymerization of divinylbenzene and 1-vinylimidazole (PDVB-VI), which showed high surface area (510–680 m² g⁻¹) and high alkaline density (0.953–2.822 mmol g⁻¹). Interestingly, the PDVB-VI has a superhydrophobic surface, which is much more active in transesterification of tripalmitin with methanol than the conventional base catalysts such as Amberlyst-400 resin, hydrotalcite, CaO, NaOH, and 1-vinylimidazolate. Later, they functionalized the ionic liquid groups to the polydivinylbenzene to obtain nanoporous ionic liquid catalysts. These novel heterogeneous catalysts are even more active than the corresponding homogeneous ionic liquid catalysts.²⁰³ This phenomenon is due to the enrichment of reactants including tripalmitin and methanol in the nanopores of PDVB-based catalysts. Very interestingly, these catalysts have excellent catalytic recyclability. There is almost no activity loss over PDVB-[C₁vim][SO₃CF₃] in the transesterification reaction after recycling 4 times. In contrast, the activity over Amberlyst-15 is remarkably reduced.

Both catalytic activity and product selectivity in the biomass conversion could be adjusted by the surface wettability of PDVB-based catalysts. It is well known that the key step in producing HMF from the biomass feedstocks is the dehydration of fructose, which is generally catalyzed by the acidic sites. Once HMF is formed, the hydration of HMF to levulinic acid (LA) and formic acid (FA) is also catalyzed in the same system, which results in a significant decrease of HMF yield.^{34–36,204–208} In order to investigate the reaction between water and HMF over acid catalysts, Wang et al.²⁰⁸ performed theoretical simulations on the hydration of HMF, and it was found that keeping the distance of protons on acidic sites to water molecules beyond the value of 2.05 Å could completely prevent the hydration of HMF to FA and LA. Prompted by this result, they designed the dehydration of fructose to HMF over a superhydrophobic PDVB-based solid acid catalyst (P-SO₃H-154), which is synthesized from the co-polymerization of divinylbenzene and sodium p-styrene sulfonate, followed by ion-exchange in dilute sulphuric acid. As a result, the P-SO₃H-154 catalyst shows a HMF yield of nearly 100% from the dehydration of fructose in a mixed solvent of THF–DMSO, because the hydrophobic surface of P-SO₃H-154 could isolate the water molecules from the acid centers (Fig. 10), preventing the hydration of HMF. In contrast, the hydrophilic Amberlyst-15 shows a lower HMF yield of 45.7%, with LA and FA as by-products from the hydration of HMF.

Fig. 10 The photographs and water droplet contact angle tests of (A) P-SO₃H-154 and (B) Amberlyst-15. The P-SO₃H-154 sample is superhydrophobic with a water droplet contact angle (CA) of 154°, while Amberlyst-15 is superhydrophilic with CA ∼10° (reproduced with the permission of ref. 208. Copyright 2014 Wiley-VCH).

Furthermore, the P-SO₃H-154 catalyst was also extended to other biomass transforms such as dehydration of sorbitol to isosorbide, giving an isosorbide yield of 87.9%.²⁰⁹ In contrast, the sulfonated PDVB without any nanoporosity (nonporous-P-SO₃H) which is also superhydrophobic gives a very low isosorbide yield of 3.4% due to the very low surface area of nonporous-P-SO₃H (<5 m² g⁻¹). This result indicates the importance of the catalyst nanoporosity.

7. Zeolites

It is worth noting that there is always coke formed over the solid acid catalysts in the conversion of biomass, and calcination in air is regarded as an effective method to remove the coke. However, for the acid-functionalized resins, metal oxides, and PDVB catalysts, they could not bear the high temperature calcination.

Zeolite crystals with intricate micropores, strong Brønsted and Lewis acidity have already been widely used as heterogeneous catalysts in the petrochemical and fine chemical industries.^210–224 They have crystalline inorganic frameworks, offering a very high thermal stability.

7.1 Aluminosilicate zeolites

One of the best advantages of aluminosilicate zeolites is the product selectivity controlled by the micropore sizes. For example, Jae et al.²²⁵ reported that the ZSM-5, ZSM-11, TNU-9, ZSM-23, MCM-22, and IM-5 zeolites with small micropore sizes of 4.0–5.9 Å produced higher yield of aromatics in fast pyrolysis of glucose, while the SSZ-55, Beta, and Y zeolites with micropore sizes of 6.5–7.4 Å produced higher yield of coke, because of the large micropore sizes. In contrast, the ZK-5 zeolite with the smallest micropore size of 3.9 Å produced only oxygenated products without any aromatics, because the pore size (3.9 Å) is too small for the formation of aromatics. These results indicate the good feature of zeolites in influencing the product selectivity by micropore size limitation, which would be a renewable route to produce the desired chemicals from biomass by employing zeolites with suitable micropore sizes.

Recently, Cheng et al.²²⁶ modified the conventional ZSM-5 zeolite by the chemical liquid deposition (CLD) method to obtain silylated ZSM-5, which has smaller micropore opening than the conventional ZSM-5. Interestingly, the silylated ZSM-5 exhibited a much higher p-xylene selectivity than conventional ZSM-5, because the decreased micropore size of silylated ZSM-5 is in good agreement with the kinetic diameter of p-xylene (Fig. 11). Later, Williams et al.²²⁷ reported the synthesis of p-xylene from dimethylfuran and ethylene in an aliphatic solvent at 300 °C over Y zeolite with a larger micropore size. After the cycloaddition of ethylene and 2,5-dimethylfuran and subsequent dehydration steps, the selectivity to p-xylene can reach as high as ∼75% with a dimethylfuran conversion of ∼95%. Furthermore, Do et al.²²⁸ proposed the reaction network of the side reactions in this reaction over Y zeolite by systematically identifying the structures of various by-products. Considering the importance of p-xylene in polyester fibre production in the industrial process (more than 30 million tons of p-xylene each year from crude-oil feedstock in the world), the green route for synthesizing p-xylene from the biomass should have a good future.

Fig. 11 A scheme diagram showing the limitation of xylene isomers inside the decreased micropore of silylated ZSM-5 (reproduced with the permission of ref. 226. Copyright 2012 Wiley-VCH).

As is known, upgrading of pyrolysis oil into alkanes goes through many steps including hydrogenation and isomerization, where the isomerization was conventionally performed over homogeneous acids.¹⁶¹ Based on the observations on pyrolysis oil upgrading over Pd/C and H₃PO₄ catalysts,¹⁶¹ Lercher and co-workers^{161,229–233} developed the upgrading of pyrolysis oil over aluminosilicate ZSM-5 and Beta zeolite supported Ni or Pd catalysts with high activity and good recyclability. In this case, liquid H₃PO₄ is not necessary. The avoidance of the liquid H₃PO₄ is greatly important for simplifying the process and saving energy.

Notably, upgrading of biofuels in a hot water for a long time is damaging for the zeolite framework owing to the destruction of Si–O–Si or Si–O–Al bonds.^234,235 In order to improve the stability of Y zeolite, Zapata et al.^236,237 functionalized the external surface of Y zeolite with organosilanes. The nanoporous structure of the hydrophobic Y zeolite obtained is much more stable in the water/oil emulsions (Fig. 12) than that of the untreated sample, giving excellent catalytic activity and recyclability in the biofuel upgrading performed at 200 °C.

Fig. 12 HRTEM of the Y zeolites: untreated zeolite (a) before and (c) after reaction; organosilane-functionalized zeolite (b) before and (d) after reaction. The crystalline structure of the functionalized zeolite remains unchanged after reaction, while that of the untreated zeolite is greatly affected, which indicates the high stability of organosilane-functionalized zeolite (reproduced with the permission of ref. 236. Copyright 2012 American Chemical Society).

7.2 Sn-Beta, Ti-Beta, and Zr-Beta zeolites

Sn-, Ti-, and Zr-substituted Beta zeolites with Lewis acidic sites^{228,238–245} exhibit unique catalytic performance in biomass conversion. In 2010, Holm et al.²⁴⁶ found that Sn-, Ti-, and Zr-substituted microporous Beta zeolites could catalyze the one-pot conversion of glucose, fructose, and sucrose to lactic acid-derived products of methyl lactate and ethyl lactate at 160 °C. In contrast, the Al-Beta with Brønsted acid sites would catalyze the dehydration reaction to form HMF-derived products.

In the same year, Davis et al.^247–249 reported Sn-Beta zeolite as an efficient catalyst for isomerization of glucose into fructose in aqueous media with glucose feeds as high as 45 wt%. By ¹H and ¹³C NMR spectroscopy on labelled glucose, they proposed the intramolecular hydride shift mechanism for glucose conversion to fructose, which is different from the proton transfer mechanism over base catalysts, as shown in Fig. 13. Later, they found that the glucose isomerization in methanol solvent in the Sn-Beta zeolite micropores could form mannose, while the reaction on the external surface of zeolite crystals over extraframework SnO₂ nanoparticles could produce fructose.²⁵⁰ Based on these results, it could be easy to obtain desirable products from glucose isomerization inside and outside of the micropores of Beta zeolite by controlling the position of Sn species.

Fig. 13 Glucose isomerization mechanisms by way of (A) proton transfer and (B) intramolecular hydride shift over base and Sn-Beta catalysts, respectively (reproduced with the permission of ref. 247. Copyright 2010 Wiley-VCH).

To catalyze the one-pot production of HMF from glucose, cellobiose, and starch, a mixed catalyst of Sn-Beta and hydrochloric acid was used, where biphasic solvents were employed in order to immediately extract the HMF formed in the water phase to the organic phase, avoiding the side reaction of HMF hydration in water solvent.²⁵¹ However, the use of homogeneous hydrochloric acid as a Brønsted acid makes it difficult to be regenerated, compared with the heterogeneous solid acids. Roman-Leshkov et al.²⁵² performed the combination of Zr-Beta and aluminosilicate ZSM-5 nanosheet catalysts to one-pot production of γ-valerolactone from fructose. The whole process, which is designated as domino reaction, included dehydration and esterification catalyzed by the Brønsted acid sites on ZSM-5 nanosheets, and hydrogen transfer with 2-butanol as a hydrogen source catalyzed by Lewis acid of Zr-Beta. It is worth mentioning that they employed ZSM-5 nanosheets as Brønsted acid catalysts instead of the conventional ZSM-5 crystals, because the micropore size of ZSM-5 zeolite is too small for the relatively bulky molecule of fructose, while the ZSM-5 nanosheets have more exposed external acid sites.

The success of Davis and Roman-Leshkov's studies confirmed that adjusting metals and combining different types of zeolite catalysts could be used as a novel, simple, and efficient method to transform the biomass feedstocks by alternative routes.

8. Conclusions and perspectives

A brief overview was presented of recent studies that focused on the design, synthesis, and catalytic applications of nanoporous catalysts for the conversion of biomass-based feedstocks, including nanoporous resins, metal oxides, carbons, mesoporous silicas, polydivinylbenzene, and zeolites. For these catalysts, the nanoporous structures play an important role in dispersion of the active sites, access of reactants, mass transfer, and control of product selectivity. Notably, each type of nanoporous catalyst has its unique advantages. The sulfonated resins have high acidic density, but relatively low stability; the sulfated metal oxides have high stability, but relatively low surface areas; carbons and ordered mesoporous silicas have large surface areas, but the surface wettability of the nanopores is not easily controllable; the nanoporous polydivinylbenzenes have easily controllable wettability, but relatively low thermal stability; the zeolites have high thermal stability, but relatively low mass transfer for the sole micropores.

Obviously, the advantages of each catalyst mentioned above are separate. Therefore, it is strongly expected that the combination of the advantages of various catalysts to synthesize novel nanoporous catalysts will result in more efficient catalytic performances. For example, introduction of mesoporosity into the microporous zeolite catalysts could form mesoporous zeolites with both micropores and mesopores, exhibiting excellent catalytic performance for the conversion of both small and bulky biomass molecules. Notably, the synthesis of a series of mesoporous zeolites such as MFI and BEA structures has been successful,^253–257 but the catalytic conversion of biomass molecules is still under investigation currently. The synthesis of mesoporous zeolites offers a good opportunity for catalyzing the biomass to valuable fuels and chemicals.

Finally, it is emphasized that the discovery of novel catalysts is fundamental for developing efficient catalysts for the biomass conversion. This review briefly summarized recent studies on the nanoporous catalysts with various features, which might be valuable for the rational design of highly active catalysts in the future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21273197 and U1162201) and the China Postdoctoral Science Foundation (2014M550318).

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