CO₂ capture by solid adsorbents and their applications: current status and new trends

Abstract

In the last few years there has been a rapid growth in governmental funding and research activities worldwide for CO₂ capture, storage and utilization (CSU), due to increasing awareness of the link between CO₂ accumulation in the atmosphere and global warming. Among the various technologies and processes that have been developed and are emerging for CSU of CO₂, solid CO₂-adsorbents are widely applied. In this review, these solid CO₂-adsorbents are classified into three types according to their sorption/desorption temperatures: low-, intermediate- and high-temperature adsorbents with temperatures ranging from below 200 °C, between 200–400 °C and above 400 °C, respectively. For each type of solid CO₂-adsorbent, the synthesis, interaction mechanism with CO₂ and sorption performance, potential applications and problems are reviewed. In the last section, several representative CO₂-sorption-enhanced catalytic reactions are discussed. It is expected that this review will not only summarize the main research activities in this area, but also find possible links between fundamental studies and industrial applications.

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Qiang Wang

Qiang Wang is a Research Fellow in the Institute of Chemical and Engineering Sciences (ICES) under A*STAR, Singapore. He received his BSc (2003) and MSc (2005) from Harbin Institute of Technology (HIT) in China, and his PhD (2009) from Pohang University of Science and Technology (POSTECH) in South Korea. His research interests are heterogeneous catalysis and materials chemistry, with a particular focus on CO2 capture and utilization, hydrogen production, vehicle emission abatement, biomass conversion, and synthesis and applications of porous materials and nanocomposites. He was awarded the Chinese Government Award for Outstanding Self-Financed Students Abroad in 2008.

Jizhong Luo

Jizhong Luo obtained his PhD from Xiamen University in 1997. After two years of postdoctoral research at Hong Kong Baptist University, he joined the Physics Department of the National University of Singapore as a research fellow in 2000. In 2003, he joined the Institute of Chemical & Engineering Science (ICES), and he is now a senior research follow in the Applied Catalysis group. His research interests include heterogeneous catalysis, alternative energy, microwave chemistry, and the carbon credit market.

Ziyi Zhong

Ziyi Zhong received his BSc and MSc degrees from Wuhan University and obtained his PhD from Nanjing University in China in 1995. He spent five years carrying out postdoctoral research at Bar-Ilan University, the University of Washington and finally at the National University of Singapore. In 2002, he was awarded a Visiting Fellowship by NSERC Canada and worked in an institute (BRI) in Montreal. Since 2003, he has been working in the Institute of Chemical and Engineering Sciences in Singapore, researching environmentally related heterogeneous catalysis. So far he has published 90 journal papers, a book chapter and filed two patents.

Armando Borgna

Armando Borgna (PhD in Chem. Eng., Universidad Nacional del Litoral, Argentina, 1987) is a Senior Scientist and Programme Manager at ICES (A*STAR, Singapore). He is also an Adjunct Associate Professor at the Department of Chemical and Biomolecular Engineering, National University of Singapore. He was previously a postdoctoral researcher at the Institut de Recherches sur la Catalyse, CNRS (1987–1990), Researcher at the Institute of Research on Catalysis and Petrochemistry, CONICET-Argentina (1991–2001), and Senior Research Associate at TU-Eindhoven (2001–2004). He was a Fulbright Fellow at the University of Oklahoma (1999). His research interest is in heterogeneous catalysis, particularly characterization of working catalysts, kinetic modeling and mechanistic studies.

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Broader context

There is a growing awareness that anthropogenic CO₂ emissions should be reduced as CO₂ is a major greenhouse gas in the atmosphere. In order to do that, there are various measures which we can adopt: (1) change part of our current life styles, e.g., by using less fossil fuels and adapting to greener technologies/processes; (2) seek alternative energy and chemical resources; (3) capture, store and utilize (CSU) part of the emitted CO₂. For the 3rd option, solid CO₂-adsorbents will play an important role. For any practical application of these solid CO₂-adsorbents, not only should we take into consideration their sorption capacities, selectivities and recyclibilities, we should also look into other factors, e.g., their working temperatures, which should be compatible to that of the particular process. In addition, in some reactions that produce CO₂ as one of its products, CO₂-removal is likely to influence the outcome of the reactions. Thus, this will probably widen the application of these materials in catalysis. In light of the increasingly obvious global warming trends worldwide, capture, storage and utilization of CO₂ by means of solid CO₂-adsorbents via a number of environmentally friendly processes is of increasing importance. Meanwhile, it will create new business opportunities.

1. Introduction

CO₂ is the major anthropogenic greenhouse gas (GHG) in the atmosphere. Its atmospheric concentration has increased to 384 ppm in 2007 from its pre-industrial level of ca. 280 ppm, and is expected to reach 550 ppm by 2050 even if CO₂ emission is stable for the next four decades.¹ One of the environmental impacts of atmospheric CO₂ accumulation is global warming. Indeed, it is evident that a linear warming trend over the last 50 years (from 1956 to 2005) has almost doubled compared to that for the period from 1906 to 2005.²

The CO₂ emissions associated with human activities are mainly due to the burning of fossil fuels and various chemical processes, e.g., ca. 44% of anthropogenic CO₂ emissions come from coal-, oil- or natural gas-fired power plants.² Unfortunately there will be no big change in the next few decades for the energy consuming spectrum; fossil fuels will still be the dominant source as other energy sources such as biomass-based fuels, solar energy and nuclear energy, which are CO₂-neutral or do not emit CO₂, still cannot replace fossil fuels on a large scale. Moreover, the energy demand will increase further by 53% by 2030.² Since an immediate CO₂-emission halt is impossible, in recent years worldwide efforts have been devoted to developing new technologies/processes for CO₂ capture, storage (sequestration) and utilization (CSU), and to improving the energy utilization efficiency.³ Among them, sorption-based technologies and processes account for the majority of these research activities, and they usually involve solid CO₂-adsorbents. For example, in order to remove CO₂ from power plant flue gas, which is crucial for large scale CO₂-emission reduction, three kinds of technologies have been developed and tested, including post combustion, pre-combustion and oxyfuel.⁴ Some solid CO₂-adsorbents have been tested in the post combustion and pre-combustion capture processes,^5,6 though presently almost all commercial processes for capturing CO₂ still use liquid alkaline solutions (e.g. amine). Unlike liquid adsorbents, solid adsorbents can be used over a wider temperature range from ambient temperature to 700 °C, yield less waste during cycling, and the spent solid adsorbents can be disposed of without undue environmental precautions.⁷

In the last few years there has been a rapid growth in publications related to CSU of CO₂, and several recent review papers^2,4,8 provide very good insights into the progress in this area. However, we feel that a new review focusing on solid CO₂-adsorbents, addressing their pros and cons, and potential applications in industry is still needed. Furthermore, in this review, we will not only summarize the latest progress in the synthesis and applications of various solid CO₂-adsorbents, but also organize these materials into different types according to their properties and performances, e.g., their working and regeneration temperatures, and associate them with various application processes. In addition, we will also address representative catalytic applications for the solid adsorbents, which is an active research topic with plenty of room for further development. This is based on the fact that since some catalytic reactions generate CO₂ as one of the products, integration of a solid CO₂-adsorbent with the catalyst is expected to shift the reaction balance. This will probably widen the application of solid CO₂-adsorbents. However, some other topics such as the storage of enriched CO₂ in proper geologic formations, catalytic conversion of CO₂ to some value-added chemicals and utilization of CO₂ as a solvent in chemical reactions will not be included.

2. Low-temperature solid adsorbents (< 200 °C)

2.1 Carbon based adsorbents

Owing to their low cost, high surface area, high amenability to pore structure modification and surface functionalization, and relative ease of regeneration, carbon-based materials are considered to be one of the most promising adsorbents for capturing CO₂ in integrated gasification combined cycle (IGCC) processes for energy generation and hydrogen production.⁹ However, the CO₂ adsorption on carbon materials is “physical” and weak, which makes these adsorbents sensitive to temperature and relatively poor in selectivity. The CO₂ sorption capacity drops dramatically at temperatures associated with power plant flue gas (50–120 °C).¹⁰ Thus current research efforts are focused on how to enhance the adsorbate–adsorbent interaction and the selectivity for CO₂. One approach is to increase the surface area and tune the pore structure of the carbon adsorbents either by using different precursors,¹¹ or forming different structures such as single-walled carbon nanotubes (CNTs),¹² multi-walled CNTs,^13–15 ordered mesoporous carbon,¹⁶ microporous carbon,¹⁷etc. Another efficient approach is to increase the alkalinity by surface modification. This can be achieved either by impregnation of additives such as 3-chloropropylamine-hydrochloride (3-CPAHCl)¹⁸ and polyethylene glycol (PEG),¹⁰ or by incorporation of basic nitrogen groups into the carbon framework.^19–21 Arenillas et al.¹⁰ impregnated several organic bases on fly ash-derived carbon materials, and observed a CO₂ capture capacity of 1.13 mmol g⁻¹ at 75 °C. It should be noted that, although the impregnation method is often used, the adsorbate–adsorbent interaction is weak thus the recycling ability is not strong. Moreover, the introduction of additives may block the porous carbon structure, thus lowering the adsorption capacity.^22,23 Nitrogen can be incorporated into carbon structures by preparing carbon adsorbents from N-containing polymers, or by heat treatment of carbon adsorbents with NH₃,^23,24 or amines.^25–27 Recently Hao et al.²⁸ synthesized a new nitrogen-doped porous carbon monolith, for which a very high CO₂ adsorption capacity of 3.13 mmol g⁻¹ was achieved at room temperature and 1 atm. To decrease the cost of the adsorbents, cheaper carbon resources such as fly ash,²⁹ almond shell-derived carbon,³⁰ or anthracite²⁵ can be selected. This is especially the case for the waste recycling of fly ash which cannot be marketed as a cement extender and has to be disposed of.^29,31 All of the above mentioned research activities on carbon-based CO₂-adsorbents are summarized in Fig. 1.

Fig. 1 A summary of current research activities on carbon-based CO₂-adsorbents. The middle part shows the types of carbons adsorbents that have been studied, the left part shows the impregnation or grafting of basic additives, and the right part shows the incorporation of N-containing groups.^{13,22,24,26,32}

2.2 Zeolite based adsorbents

Zeolites are porous crystalline aluminosilicates, whose framework consists of interlocking tetrahedrons of SiO₄ and AlO₄ joined together in various regular arrangements through shared oxygen atoms. They have open crystal lattices containing pores with molecular dimensions, into which molecules can penetrate. The negative charge created by the substitution of an AlO₄ tetrahedron for a SiO₄ tetrahedron is balanced by exchangeable cations (e.g., Na⁺, K⁺, Ca²⁺, Mg²⁺), which are located in the channels and cavities throughout the structure. The adsorption and gas separation properties of zeolites are heavily dependent on the size, charge density, and distribution of these cations in the porous structure.³³ The CO₂ adsorption mechanism on zeolites has been investigated by various groups and it has been revealed that the physisorption of CO₂ occurs with CO₂ in a linear orientation by an ion–dipole interaction (reaction 1).^34,35

(metal ion)^x+ ⋯⋯ ^δ−O=C=O^δ+ (1)

In addition to physical adsorption, more strongly bound carbonate species are also observed. These adsorbed CO₂ sites are bent and associated with bi-coordination, as shown in Scheme 1.^34,35

Scheme 1 Two types of carbonate species associated with bi-coordination.

Although zeolites have shown promising results for separating CO₂ from gas mixtures and can potentially be used in the pressure swing adsorption (PSA) process, their selectivity to CO₂ over other gases (N₂, CH₄, H₂O, etc.) is still low, and their adsorption capacities rapidly decline with increasing temperature above 30 °C and become negligible above 200 °C.^36–38 Generally speaking, separation of gases by zeolites depends on three factors: structure and composition of the framework, cationic form, and zeolite purity.³⁹ A number of zeolites, especially those which are highly crystalline, with high surface area, and three-dimensional pore structures, have been investigated, including zeolite X,^37,40,41 Y,^42,43 A,⁴⁴ β,⁴⁵ ZSM,^46,47 CHA,⁴⁸ and natural zeolites (e.g. ZAPS, ZNT, ZN-19).³⁹ Harlick et al.⁴⁹ studied 13 types of zeolite-based adsorbents, and concluded that the most important adsorbent characteristics are a near linear CO₂ isotherm and a low SiO₂/Al₂O₃ ratio with cations that have a strong electrostatic interaction with CO₂. Siriwardane et al.⁵⁰ reported that the CO₂ adsorption capacity for zeolites 13X and 4A were about 3.64 and 3.07 mmol g⁻¹ respectively at 25 °C and 1 atm of CO₂ partial pressure. Inui et al.³⁶ studied the adsorption behavior of different zeolites in the PSA process and claimed that CHA and 13X were the most appropriate for CO₂ separation among the zeolites studied. Merel et al.⁴⁴ investigated the CO₂ capture of zeolites 13X and 5A by indirect thermal swing adsorption, and found that zeolite 5A performed better than 13X. Calleja et al.⁵¹ reported that the preference of the adsorbent for polar molecules (e.g. CO₂) increases as the Si/Al ratio decreases due to the higher surface heterogeneity and the stronger electrostatic field inside the pores of the zeolite.

Another research area is the zeolites exchanged with alkali and alkaline-earth cations. The cations influence the electric field inside the pores as well as the available pore volume, and provide a convenient means for tuning adsorptive properties of these porous materials. Walton et al.⁵² studied the CO₂ adsorption behavior of the Y and X zeolites exchanged with Li, K, Na, Rb, and Cs, and found that the largest CO₂ capacity was obtained for the zeolite containing Li. It was suggested that the CO₂ molecule possesses a quadrupole moment that can interact more strongly with smaller cations like Li. Díaz et al.⁴² evaluated the Na- and Cs-treated Y zeolites, and suggested that Cs–Y is a promising material for CO₂ adsorption processes at relatively high temperatures. Yamazaki et al.⁵³ and Wirawan et al.^46,54 investigated the CO₂ adsorption on cation-exchanged MZSM-5 (M = Li, Na, K, Rb, Cs, H, Ba) and observed that, compared to other cation-exchanged zeolites, BaZSM-5 has a higher CO₂ adsorption capacity and the adsorbed species also has a higher thermal stability. Zhang et al.⁴⁸ found that NaCHA and CaCHA have comparative advantages for high temperature CO₂ separation whilst NaX shows superior performance at relatively low temperatures.

In terms of practical applications, moisture is another challenge to zeolite based adsorbents. H₂O is an important component in boiler flue gas and some other industrial gases, and it may compete with CO₂ for the active adsorption sites. Gallei et al.⁵⁵ have reported that the physical adsorption of carbon dioxide on CaY- and NiY-zeolite is scanty in the presence of water, because water is preferentially adsorbed onto these surfaces. Later Rege et al.⁵⁶ and Brandani et al.⁵⁷ observed that H₂O has a strong effect on CO₂ adsorption on type X zeolites. From a thermodynamic viewpoint, the adsorption of CO₂ in the presence of H₂O is not favored.⁵⁸ However, Díaz et al.⁴² found that the CO₂ adsorption on Cs- and Na-treated Y zeolites increased after water treatment, and believed that it was due to the enhancement in the Brønsted acidity in zeolites after treatment with alkali. The number of acid sites presumably increased after alkaline treatment (both with CsOH and NaOH), and the water molecules could coordinate to the Lewis sites and create new Brønsted acid sites. In addition, H₂O may have a detrimental effect on the stability of zeolite frameworks. In the presence of CO₂, the acidic conditions may cause dealumination of the zeolite structures, leading to a partial or total destruction of the framework. Ertan et al.⁵⁹ studied CO₂ adsorption on acid (HCl, HNO₃, H₂SO₄, H₃PO₄) treated zeolites, and found that their CO₂ adsorption capacities decreased after the acid treatment.

2.3 Metal organic framework based adsorbents

Metal organic frameworks (MOFs), which are constructed from transition metal ions and bridging organic ligands, are a new family of porous materials.^60,61 One type of MOF structure (indium soc-MOF) is shown in Fig. 2, in which (a) is a cluster of metal ions (indium-carboxylate trimer, TMBB, In₃O(CO₂)₆(H₃O)₃), (b) is an organic linker (3,3′5,5′-azobenzenetetracarboxylic acid), (c) and (d) are representations of a cuboidal cage of indium soc-MOF and (e) is a space filling representation of the framework viewed along the y-direction. The CO₂ adsorption and desorption in MOFs, which are ascribed to changes in the framework structures, are explained by a “breathing-type mechanism” or a “gate effect mechanism”.^62–64 Normally the interaction between adsorbed CO₂ and adsorbents is weak, and the species starts to desorb when the temperature is higher than 30 °C. It should be noted that the MOFs used for CO₂ capture are not limited to this type of structure.

Fig. 2 X-ray crystal structure of indium soc-MOF. Color scheme: carbon = gray, oxygen = red, nitrogen = blue, indium = dark green.⁶⁵

Owing to their very high porosity and surface area, ordered and well characterized porous structure, and adjustable chemical functionality, MOFs are attracting increasing attention in CO₂ capture.⁶⁶ Millward et al.⁶⁷ reported that, at 35 bar, a container filled with MOF-177 could capture nine times the amount of CO₂ in the container (33.5 mmol g⁻¹) compared to that without the adsorbent. Llewellyn et al.⁶⁸ obtained a record capacity of 40 mmol g⁻¹ for CO₂ capture with MIL-101 at 50 bar and 30 °C.

In order to be applicable to realistic flue streams, MOF-based CO₂-adsorbents should fulfil the following requirements: high CO₂ capture capacity and selectivity for CO₂ over other components in the flue gas, corrosion resistance, and high thermal/hydrothermal stability, etc. To date, although MOFs have shown exceptionally high CO₂-storage capacity under equilibrium conditions with pure CO₂, most of them show little uptake in the low pressure regime of 0.1–0.2 bar, which is the pressure range used for CO₂ capture from flue gas streams.⁶⁶ Even worse, their capacities reduce dramatically when exposed to mixtures of gases under dynamic conditions, which would be the case in power plant flue gas and methane mining applications.⁶⁹ Therefore, tremendous efforts have been made to further increase their CO₂ capture selectivity while maintaining or improving their capture capacity under equilibrium conditions. The modifications are usually conducted in three aspects: (a) metal ions, (b) organic linkers, or (c) novel combinations of both.

For the metal ions, creating open metal sites has been proven to be an efficient approach, as the vacant coordination sites at metal ions are the primary coordination sites for guest molecules.^69–72 Britt et al.⁶⁹ reported that a MOF replete with open magnesium sites, Mg-MOF-74 (Mg₂(DOT); DOT: 2,5-dioxidoterephthalate), has excellent selectivity, facile regeneration, and is ranked among the highest dynamic capacities reported for CO₂ in porous materials (Fig. 3(a)). In particular, when Mg-MOF-74 was subjected to a gas stream containing 20% CO₂ in CH₄, a percentage in the range of industrial separation conditions, it captured only CO₂ and not CH₄. The porous material retained 89 g of CO₂ per kilogram of material before breakthrough (2.23 mmol g⁻¹). Furthermore, these unsaturated metal sites can be functionalized with groups such as amine to further improve their performance.^{66,69,73–75} Demessence et al.⁶⁶ functionalized the triazolate-bridged MOF 1 (Cu-BTTri) with ethylenediamine, and obtained a very high uptake of CO₂ at low pressures (see Fig. 3(b)). This material also displayed a record isosteric heat of 90 kJ mol⁻¹. Bae et al.⁷⁵ reported that the CO₂/N₂ selectivity could also be enhanced by a cavity modification. By replacing the coordinated solvent molecules with highly polar ligands, a high CO₂/N₂ selectivity of ∼42 was obtained at low pressure. Couck et al.⁷³ demonstrated that the MOF MIL-53(Al) functionalized with amino groups increased its selectivity of CO₂/CH₄ by orders of magnitude while maintaining a very high capacity for CO₂ capture. In addition to amines, the occupation of open metal sites by coordinated water molecules has also been studied (Fig. 3(c)).^76,77 Yazaydin et al.⁷⁶ first predicted by molecular simulations and then validated by experiments that water molecules coordinated to open metal sites could significantly increase CO₂ adsorption in Cu-BTC.

Fig. 3 (a) Single crystal structure of Mg-MOF-74, formed by reaction of the DOT linker with Mg(NO₃)₂·6H₂O. C atoms are shown in gray, O atoms in red, 6-coordinate Mg atoms and terminal ligands in pink, and 5-coordinated Mg atoms in blue. H atoms and terminal ligands on the fragment in the top right are omitted for clarity.⁶⁹ (b) A portion of the structure of the sodalite-type framework of Cu-BTTri (1) showing surface functionalization of a coordinatively unsaturated Cu²⁺ site with ethylenediamine, followed by attack of an amino group on CO₂. Purple, green, gray, and blue spheres represent Cu, Cl, C, and N atoms, respectively; framework H atoms are omitted for clarity.⁶⁶ (c) Hydrated Cu-BTC (4 wt %) with a coordinated water molecule from DFT. Cu atoms are orange, O red, C gray, and H white. The oxygen atom of the coordinated water molecule is shown in blue.⁷⁶

For the modification of organic linkers, Bae et al.^60,78 examined the possibility of using MOFs that are carborane-based or coordinated with a mixture of ligands, and obtained a high selectivity for CO₂ over CH₄ (∼17).⁷⁸ Carboranes possess several favorable material properties including rigidity, thermal stability, and chemical stability. In addition, carboranes can produce MOFs with smaller pores than phenyl-based MOFs. The use of two different linkers opens up more possibilities to tune pore size and chemical functionality independently. The mixed ligand MOF Zn₂(NDC)₂(DPNI), 2, (NDC = 2,6-naphthalenedicarboxylate, DPNI = N,N′-di-(4-pyridyl)-1,4,5,8-naphthalene tetracarboxydiimide) exhibited a selectivity of ∼30 for CO₂ over CH₄.⁷⁸

Much attention has also been paid to the synthesis of novel MOFs with both high CO₂ capture capacity and stability. The ability to synthesize MOFs with various organic linkers and metal joints provides tremendous flexibility in equipping the porous material with specific physical characteristics and chemical functionalities. Recently, a big breakthrough has been made by Yaghi's research group in UCLA.^79,80 A new class of MOFs known as zeolitic imidazole frameworks (ZIFs), in which metal atoms such as Zn are linked through N atoms by ditopic imidazolate (C₃N₂H₃⁻ = Im) or functionalized Im links to form neutral frameworks. These compounds have high chemical and thermal stabilities (up to 500 °C).⁸¹ Phan et al.⁸² reported that 1 L of ZIF-69, the best-performing ZIF, could store 82.6 L of CO₂ at 0 °C. In addition, ZIFs show greater selectivity than other types of MOFs for CO₂ from relevant flue gases. It is believed that only CO₂ can slip into the cages within the ZIF, while other gas molecules just pass through them without hindrance.⁸³ Therefore, ZIFs are a very promising type of material for selective adsorption of CO₂, and they are twice as efficient as BPL carbon (a commercially available carbon product of Calgon).⁸⁴ Yaghi's group have also developed another class of new porous crystals, covalent organic frameworks (COFs), by condensation reactions of phenyl diboronic acid (C₆H₄[B(OH)₂]₂) and hexahydroxytriphenylene (C₁₈H₆(OH)₆).^85,86 Recent studies indicated that COFs also have a comparable capacity for CO₂ adsorption to the most common carbon materials, zeolites, mesoporous solids, MOFs, etc.⁸⁷

2.4 Alkali metal carbonate based adsorbents

Thermodynamic analysis of dry and re-generable adsorbents has shown that alkali metal carbonates are suitable for the treatment of flue gases at temperatures below 200 °C.^88,89 Typically CO₂ capture is effective within the temperature range of 50–100 °C, while regeneration occurs in the range of 120–200 °C. This concept is potentially applicable to the capture of CO₂ from existing fossil fuel-fired power plants.

Liang et al.⁹⁰ studied the mechanism and revealed that the important reactions involved in the capture of CO₂ using Na₂CO₃ are shown in reactions (2) and (3).

Na₂CO₃(s) + CO₂(g) + H₂O(g) ⇔ 2NaHCO₃(s), ΔH_r° = −135 kJ mol⁻¹ Na₂CO₃ (2)

Na₂CO₃(s) + 0.6CO₂(g) + 0.6H₂O(g) ⇔ 0.4[Na₂CO₃·3NaHCO₃](s), ΔH_r° = −82 kJ mol⁻¹ Na₂CO₃ (3)

The theoretical CO₂ capture capacity of Na₂CO₃, calculated from reactions (2) and (3), is 9.43 mmol g⁻¹ and 5.66 mmol g⁻¹ respectively. Both reactions are reversible and highly exothermic so energy management is an issue that should be considered in any particular application. K₂CO₃-based adsorbents have also been investigated and showed similar results.^91,92 The conversion decreases with an increase in the reaction temperature and pressure, and varying the CO₂ and H₂O concentrations has little effect, while the maximum reaction rate increases with an increase in temperature and H₂O concentration, and with a decrease in pressure. The carbonation can start from 60 °C, and the maximum carbonation temperature is limited by the reaction thermodynamics.⁹⁰ As a result, a common problem is that the overall carbonation reaction rate for Na₂CO₃/K₂CO₃ is rather slow. Many attempts have been made to resolve this, e.g., by dispersing active Na₂CO₃/K₂CO₃ on a support such as Al₂O₃, active carbon (AC), TiO₂, SiO₂, MgO, ZrO₂, etc., so as to enhance the adsorption rate and provide the required attrition resistance in the fluid-bed or transport reactors.^7,90,93–98 Nevertheless, these adsorbents have a disadvantage in that the reactivity always decreases with increases in sorption/regeneration operations.^99,100 Okunev et al.¹⁰¹ studied various K₂CO₃/support composite materials and concluded that the host matrix should not be limited to an inert support and can undergo chemical transformations caused by an interaction with the impregnated salt.

So far the issues that should be considered for practical applications of such CO₂-adsorbents include durability, carbonation reaction rate and temperature control, and energy management. In addition, contaminants in flue gas such as SO₂ and HCl will react irreversibly with Na₂CO₃ and must therefore be reduced to low levels prior to CO₂ capture. Their practical application is probably still a long way off.

2.5 Amine-based solid adsorbents

Some NH_x-containing organics or polymers can chemically bind to acidic CO₂ molecules at low reaction temperatures, and thus they are potentially good candidates for CO₂ capture. CO₂ removal from flue gas by sorption and stripping with aqueous amines (commonly monoethanolamine (MEA), diethanolamine (DEA) and methyldiethanolamine (MDEA)) has been established since 1930 and is still believed to be a feasible technology.^102,103 However, this process suffers from a series of inherent problems, including the corrosive nature of the amines, high regeneration energy, fouling of the process equipment, etc.^104,105 In order to avoid the above problems, intensive efforts have been made to prepare amine-based solid adsorbents by immobilizing organic amines on certain support materials.^{26,50,106–108} Compared to the aqueous amine solutions, the solid amine adsorbents usually require lower capital cost, lower pressure for gas recovery, and lower energy consumption for regeneration.¹⁰⁹ Research activities have been focused on three aspects to improve the CO₂-capture properties of amine-based solid adsorbents: (1) supports that are able to bear high amine-loading, (2) amines that can generate high amine density, and (3) effective methods for amine immobilization.

Generally speaking, the supports should have a good affinity for the amine molecules, high surface area, proper porosity, good mechanical strength and hydrothermal stability, etc. Silica, carbon, polymers such as resin, glass and metal oxides can be considered, but usually porous materials are preferred due to their high surface areas and large pore size and volume. In other words, nonporous and even microporous materials are rarely selected for the purpose. Among various porous candidates, mesoporous silicas (SBA-15, MCM-41, etc.) are most widely investigated, mainly because of the “silane chemistry”; a number of aminosilanes can react with the silica surface via alkyl–silyl linkages, thus providing vast opportunities for loading amine molecules onto the supports. Harlick et al.¹¹⁰ did systemic work on pore-expanded MCM-41 (PE-MCM-41) grafted with 3-[2-(2-aminoethylamino)ethylamino]propyl trimethoxysilane (TRI), and found that the TRI-PE-MCM-41 adsorbent showed a higher CO₂-sorption capacity and kinetics than TRI-MCM-41. One of the drawbacks of these mesoporous silicas is their relatively weak hydrothermal stability, probably limiting their application in aqueous media. Amination of carbon materials is also reported by reacting carbon surface carboxylic groups with halogenated amine molecules.²⁶ This kind of material has been addressed in section 2.1 and is probably better suited to aqueous media.

Various amines tested for CO₂ capture include alkanolamines, mono-, di-, and triaminosilanes, aminopolymers such as polyethyleneimine (PEI),^110,111 dendrimers,¹¹² hyperbranched aminosilica,¹¹³etc. In general, multi-amine-containing molecules are superior due to their higher amino group densities. Chang et al.¹¹⁴ observed a CO₂ capture capacity of 2.4–2.7 mmol g⁻¹ at 60 °C on triaminosilane/SBA-15. Sayari et al.¹¹⁵ obtained a value of 2.65 mmol g⁻¹ at 25 °C on TRI-PE-MCM-41. On a tetraethylenepentamine (TEPA)-grafted mesocellullar silica foam (MSF), a capacity as high as 4.5 mmol g⁻¹ adsorbent was achieved at 75 °C by Liu et al.¹¹⁶ For polyethylenimine coated glass fibers, Li et al.¹¹⁷ observed a capacity of 4.1 mmol g⁻¹ at 30 °C. Liang et al.¹¹⁸ synthesized various generations (G0, G1, G2, G3 and G4) of melamine-based dendrimers on SBA-15. However, at 20 °C, a CO₂ capacity of only 1.0 mmol g⁻¹ (ca. 4.4 wt% g⁻¹) was observed on G3/SBA-15. On G4/SBA-15, a capacity of only 0.4 mmol g⁻¹ (1.7 wt% g⁻¹) was obtained instead of a theoretical value of 1.36 mmol g⁻¹ (ca. 6 wt% g⁻¹), indicating many amino groups were not accessible by the CO₂ molecules. The highest CO₂ capture capacity observed was 5.55 mmol g⁻¹ on hyper-branched aminosilica (HAS) on SBA-15 at 25 °C by Hicks et al.,¹¹³ which was formed by polymerization of aziridine on SBA-15.¹¹⁹

Methods for loading amines onto supports can be roughly classified as (a) impregnation, (b) post-synthesis grafting and (c) direct condensation and hydrolysis methods.¹¹¹ The samples prepared by the impregnation method can have high amine-loading but are not usually good in recycling. For the post-synthesis grafting method, the above mentioned grafting of aminosilane molecules onto silica-based supports belongs to this category, and usually takes place in anhydrous solution so as to reduce the self polymerization behavior of the aminosilane molecules.¹²⁰ In the direct condensation and hydrolysis method, an aminosilane, a surfactant (template) and the silica precursor are mixed and reacted directly, and the amino groups can thus be incorporated into the matrix of the synthesized material. In a recent work by Tang et al.,¹²¹ bis((triethoxysilyl)propyl) ethylenediamine (BTEPED) was incorporated into a porous silica and a CO₂ capture capacity of 2.3 mmol g⁻¹ was obtained. The materials synthesized by this method should maintain their amines well; however, not all of the amino groups are accessible to CO₂ molecules as they are trapped in the silica matrix. In addition, in many cases it is still a challenge to incorporate amine molecules while forming the porous structure, and the uncalcined silica wall should have a poorer hydrothermal stability than the calcined porous silica used in the post-synthesis grafting.

The chemistry of grafted amines with CO₂ is similar to that between amines in aqueous solution and CO₂.¹²² Three kinds of carbonates can be formed from primary amines as shown in Scheme 2. From the reaction pathway in Scheme 2, it is understandable that the existence of H₂O can lead to the formation of bicarbonate and carbonate and consume more CO₂ molecules, thus increasing the CO₂ capture capacity. This is the reason why these CO₂-adsorbents can tolerate moisture; indeed, the presence of moisture can promote the CO₂ capture.¹¹⁰ Since these reactions can run at a fast rate, the CO₂-capture process is also fast, unless the CO₂ concentration is very low.

Scheme 2 CO₂ reaction pathway with a primary amine.¹¹⁰

The thermal stability of amines is well studied. Degradation of aqueous alkanolamines occurs below 140 °C under atmospheric pressure,¹²³ though a slightly higher temperature has also been reported in some grafted amines.¹¹⁰ The working temperature for CO₂-capture should usually be below 60 °C, with better results achieved as the temperature approaches ambient temperature. For regeneration, it is usually between 120–170 °C according to our unpublished data.

3. Intermediate-temperature solid adsorbents (200–400 °C)

Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds (HTs) or anionic clays are layered basic solids. They have been widely used as adsorbents, ion exchangers, base catalysts, and precursors of well-mixed oxides for various catalytic applications.¹²⁴ Their structure consists of positively charged brucite-like layers with charge compensating anions and water molecules within the interlayer space.^125–127 The metal cations occupy the centers of the octahedral structures, whose vertices contain hydroxide ions, and the octahedrons are connected by edge sharing to form an infinite sheet (Fig. 4A). The general formula of the compounds is [M²⁺_1-xM³⁺_x(OH)₂][Aⁿ⁻]_x/n·zH₂O, where M²⁺ and M³⁺ are divalent (Mg²⁺, Zn²⁺, Ni²⁺, etc.) and trivalent cations (Al³⁺, Ga³⁺, Fe³⁺, Mn³⁺, etc.) respectively, Aⁿ⁻ is a non-framework charge compensating anion (CO₃²⁻, Cl⁻, SO₄²⁻, etc.) and x is normally between 0.2–0.4. In general, LDH materials possess both high surface area and abundant basic sites at the surface, which are favorable for adsorbing acidic CO₂.^128–130

Fig. 4 (A) Schematic structural representation of LDHs; M²⁺ and M³⁺ represent divalent and trivalent cations. (B) The structural evolution of Mg-Al-CO₃ LDH as a function of temperature.¹³¹

Fresh LDH itself does not possess any basic sites. Upon thermal treatment, LDH gradually loses interlayer water, then dehydroxylates and decarbonates to a large extent, leading to the formation of a mixed oxide with a 3D network. The detailed structural evolution of Mg-Al-CO₃ LDH as a function of temperature was investigated by Yang et al.,¹³¹ as shown in Fig. 4B. Ram Reddy et al.¹³² reported the effect of calcination temperature on the CO₂ adsorption capacity of Mg-Al-CO₃ LDH. The sample calcined at 400 °C showed the highest adsorption capacity, which is presumably due to the trade-off between the surface area and availability of active basic sites. This amorphous phase has a relatively high surface area and exposes sufficient basic sites on the surface. These basic sites favor reversible CO₂ adsorption in a form as shown in reaction (4).¹³³ The interaction between the adsorbed CO₂ and the basic sites is stronger than that in the case of zeolite (see section 2.2) but weaker than that in the case of alkali metal oxide (see section 4), which explains why the working temperature of LDH-based CO₂-adsorbents used in flue-gas systems falls in the middle temperate range of 200 °C and above. Normally the regeneration temperature is around 400 °C.^132–135

Mg–O + CO₂ → Mg–O⋯CO₂(ad) (4)

The relatively low CO₂ adsorption capacity is a problem for LDH-based adsorbents. Intense efforts have been put into improving the capacity, especially on the mineral Mg-Al-CO₃ LDH and its derivatives. For instance, the synthetic conditions,¹³⁶ presence of SO_x and H₂O,^133,137 operation pressures,^127,138,139 alkali (K, Cs) doping,^140–143 particle size,¹⁴⁴ and the use of supported LDHs^145,146 were widely investigated. Sharma et al.¹³⁶ obtained the optimized synthetic parameters: 37% Al content, room temperature addition of magnesium and aluminium precursors, aging at 65 °C for 18 h, and drying at room temperature. A maximum CO₂ adsorption of 0.98 mmol g⁻¹ was obtained. However, it should be noted that, in their work, the CO₂ adsorption isotherms were measured at 30 and 60 °C, and not in the high temperature range (200–400 °C). Ram Reddy et al.^133,137 reported that the presence of water in the feed gas had a positive effect on the CO₂ adsorption capacity, which increased from 0.61 mmol g⁻¹ to 0.71 mmol g⁻¹, whereas the presence of SO₂ had a negative effect, as it could competitively interact with the CO₂ adsorption sites. The operation pressure has been proven to have a significant effect: CO₂ adsorption capacity increases with increasing operation pressure.^127,138,139 Doping alkali metal carbonates in LDHs is an effective approach to improving the CO₂ adsorption. Oliveira et al.¹⁴⁰ reported that the adsorbed CO₂ on MG 30 (commercial hydrotalcite from Sasol, Germany) increased from < 0.1 mmol g⁻¹ to 0.76 mmol g⁻¹ after doping with K₂CO₃. Walspurger et al.¹⁴³ investigated the K⁺ promotion mechanism, and suggested that potassium ions could strongly interact with aluminium oxide centers in hydrotalcite, resulting in the generation of basic sites that were able to reversibly adsorb CO₂ at high temperatures. Meis et al.¹⁴⁶ studied the support and size effects of activated hydrotalcites for pre-combustion CO₂ capture, and found that the CO₂ adsorption capacity significantly increased with a decrease in the particle size. The adsorption capacity of carbon nanofiber supported hydrotalcites increased by an order of magnitude (1.3–2.5 mmol g⁻¹) compared to that of unsupported hydrotalcites. It was proposed that the CO₂ adsorption capacities of LDHs were determined by the amount of low coordination number oxygen sites in the Mg(Al)O_x nanoparticles, which was highest on supported LDHs due to the high distribution.

More work on modifying the composition of Mg-Al-CO₃ has also been carried out by either substituting the CO₃²⁻ anion or Mg²⁺ divalent cations. Hutson et al.¹⁴⁷ investigated CO₂ adsorption on various LDHs with different anions including CO₃²⁻, Fe(CN₆)⁴⁻, Cl⁻ and ClO₄⁻, and concluded that Mg-Al-CO₃ has the highest basic site density (692 μmol g⁻¹). Lwin et al.¹⁴⁸ and Wang et al.¹⁴⁹ studied various LDHs with different divalent cations including Mg²⁺, Co²⁺, Ca²⁺, Cu²⁺, or their combinations, and came to the conclusion that the Ca–Co–Al system had the highest CO₂ adsorption capability. Yavuz et al.¹⁵⁰ demonstrated that the CO₂ capture efficiency could be markedly improved by partially substituting Al with Ga. With 10 mol% substitution for Al, the CO₂ capture capacity increased from 0.47 to 1.40 mmol g⁻¹ for K₂CO₃-promoted Mg-Al-CO₃. Recently, our group has carried out a systematic investigation on the Mg-M-CO₃ (M = Al, Fe, Ga, Mn) LDHs for the effect of trivalent cations on their CO₂ capture performance and found that the M³⁺ cations actually determine the structure evolution of the LDH derivatives under the thermal treatment, and ultimately influence the CO₂ capture capacity. A very different calcination temperature is required for each LDH to obtain the maximum CO₂ capture capacity. The optimal calcination temperatures are as follows: Mg₃Al₁ (400 °C) > Mg₃Ga₁ (350 °C) > Mg₃Fe₁ (300 °C) > Mg₃Mn₁ (250 °C).¹⁵¹Scheme 3 lists the possible methods for modifying the composition of Mg-Al-CO₃ LDHs.

Scheme 3 The possible approaches to modifying the composition of Mg-Al-CO₃ LDH.

Besides the efforts made to increase the CO₂ capture capacity, a more important issue is to enhance the long-term stability of LDHs during CO₂ adsorption/desorption cycling operation, which is crucial for the development of practical applications. Though there is still no clear answer from the literature, the solution probably lies in the progress in materials science, either by modifying the current LDHs, or by identifying new systems.

Magnesium oxide is another CO₂-adsorbent working in the intermediate temperature range from room temperature up to 200 °C.¹⁵² However, due to its moderate CO₂ adsorption capacity and poor thermal stability during regeneration, the practical application of MgO as a CO₂-adsorbent may be limited. Gregg et al.¹⁵³ tested the CO₂ capture on MgO at 200 °C, and obtained a capacity of only 0.43 mmol g⁻¹. Bhagiyalakshmi et al.¹⁵⁴ noted that the complete desorption of adsorbed CO₂ could only be accomplished by prolonged heating at 450 °C.

4. High-temperature solid adsorbents (> 400 °C)

4.1 Calcium based adsorbents

It is well accepted that calcium based materials are good adsorbent candidates for capturing CO₂ due to their high reactivity with CO₂, high capacity and low material cost.¹⁵⁵ The reversible reaction between CaO and CO₂, as shown in reaction (5), offers a great potential for reducing CO₂ from various clean energy systems, e.g. pre-combustion carbon capture from gasification processes and post-combustion carbon capture from zero emission coal (ZEC) processes. The carbonation temperature for CaO-based adsorbents is between 600–700 °C and the regeneration temperature is normally above 950 °C.^6,156–160

CaO(s) + CO₂(g) ⇔ CaCO₃(s), ΔH°_{873.15 K} = −171.2 kJ mol⁻¹ (for carbonation) (5)

This carbonation reaction is highly exothermic and it is possible to efficiently recover the large amount of energy released during the CO₂ capture. Preliminary economic analyses suggest that this process is economically attractive because limestone (CaCO₃) is abundant and low-cost when used at the industrial scale.^161–163 The CO₂ capture reaction is characterized by an initial rapid and kinetically-controlled phase, followed by an abrupt transition into a slower diffusion-controlled phase, whereas the CO₂ release reaction is much faster and always goes to completion in minutes. In practical applications, this reaction is always carried out at temperatures ≥ 850 °C in order to produce a nearly pure stream of CO₂.^164–166

Despite the simple chemistry involved, the main problem for the calcium-based materials is the loss of reversibility for the carbonation reaction due to the sintering of the adsorbent particles.^167,168 This is caused by three major factors: the carbonation process is highly exothermic, the volume increases substantially from CaO to CaCO₃ (from 16.9 to 34.1 cm³ mol⁻¹), and the Tammann temperature of CaCO₃ (533 °C) is much lower than normal carbonation temperatures.^167,169,170 Lysikov et al.¹⁷¹ proposed a simple schematic diagram for the textural transformation of the CaO adsorbent in recarbonation–decomposition cycles. The first decomposition of the calcium carbonate produces highly dispersed, nanosized CaO particles that are highly reactive. The following carbonation is incomplete due to the slow diffusion-controlled reaction with CO₂ and pore blocking upon carbonation.⁶ The amount of unreacted CaO and its particle size increase from cycle to cycle until a rigid interconnected CaO skeleton is formed. This rigid backbone is considered a much less reactive adsorbent compared to the highly dispersed CaO. There are some other restrictions to such processes related to the kinetics and thermodynamics of the reactions, along with undesirable side reactions such as sulfation and processes such as attrition.^172–174

Numerous efforts have been undertaken to enhance the durability of CaO-based CO₂-adsorbents. Hydration is currently considered to be a promising approach to reactivating the spent adsorbents for CO₂ capture.⁶ Manovic et al.¹⁷⁵ and Fennell et al.^176,177 observed that the reactivity of the spent adsorbents could be doubled following hydration. The mechanism of reactivation by hydration appeared to be through breakup of particles to increase porosity.^178,179 Thermal pre-activation in an inert atmosphere at high temperature (e.g. 1000 or 1100 °C for 6 or 24 h) is another option and was first studied by Lysikov et al.¹⁷¹ in 2007. Later Chen et al.¹⁸⁰ found that the activity and durability of thermally pretreated limestone and dolomite were greatly enhanced. Manovic et al.¹⁷² proposed a pore-skeleton mechanism to explain the thermal pre-activation. The cycling stability could also be improved by incorporating inert materials such as MgO,^170,181,182 CaTiO₃,¹⁸³ Ca₁₂Al₁₄O₃₃,^184–187 ZrO₂,¹⁸⁸etc. One practical problem of this method is the presence of a large amount of undesirable inert material in the adsorbent, since this will increase the capital and operating costs. Some researchers have attempted to synthesize CaO from nano-sized CaCO₃ particles¹⁵⁵ or organometallic precursors.¹⁸⁹ It would make economic sense to use cheap natural adsorbents (e.g. limestone) and to sell the spent adsorbent to the cement industry.^{177,190–193}

4.2 Alkali ceramic based adsorbents

Alkali metal (Li, Na, K, etc.) containing ceramics are another type of high temperature CO₂-adsorbent. In 1998, Nakagawa and Ohashi first reported the capture of CO₂ using Li₂ZrO₃ at high temperatures (400–600 °C).¹⁹⁴ This material has great potential because it has an excellent CO₂ sorption capacity (28 wt%) as well as a small volume change during the CO₂ sorption/desorption cycles.¹⁹⁵ The reaction and process are described by reaction (6), which can occur mainly due to the lithium mobility in the ceramics (Fig. 5).^196,197 During the CO₂ sorption, the lithium atoms diffuse from the core of the particles to the surface and react with CO₂ to form Li₂CO₃, and the diffusion of CO₂ in the solid Li₂CO₃ that is produced is the rate-limiting step for Li₂ZrO₃. The reverse reaction normally happens at temperatures around 750–800 °C.

Li₂ZrO₃ + CO₂ ⇔ Li₂CO₃ + ZrO₂, ΔH_{298 K} = −160 kJ mol⁻¹ (6)

Fig. 5 Proposed mechanism for CO₂ sorption (a) and desorption (b) on Li₂ZrO₃.¹⁹⁷

The main obstacle for the practical application of Li₂ZrO₃ is its kinetic limitation. Various efforts have been made to improve its CO₂ capture performance. Nair et al.¹⁹⁸ carried out a systematic study of the properties of lithium zirconates with different crystal structures. They compared the properties of three Li₂ZrO₃ powder samples: one commercial and two others prepared by the solid-state reaction method and the sol–gel method, and found that small particle size and tetragonal phase are crucial for enhancing the performance in CO₂ capture. Later, Ochoa-Fernández et al.^199,200 successfully synthesized nanocrystalline tetragonal Li₂ZrO₃ by a novel soft-chemistry route and achieved improved kinetics.

Another method is to substitute or partially substitute Li⁺ by Na⁺ or K⁺. López-Ortiz et al.²⁰¹ found that Na₂ZrO₃ achieved a superior CO₂ capture ability than Li₂ZrO₃. Pfeiffer et al.¹⁹⁶ explained that Na₂ZrO₃ has a lamellar structure and the sodium atoms are located among the ZrO₃⁻ layers with higher mobility. On the other hand, Li₂ZrO₃ has a much more packed structure, which limits lithium diffusion. Later lithium–sodium and lithium–potassium metazirconate solid solutions (Li_2-xNa_xZrO₃ and Li_2-xK_xZrO₃) were also investigated.^196,202,203 Pfeiffer et al.¹⁹⁶ found that LiNaZrO₃ presented the best ability for CO₂ chemical adsorption, exhibiting a CO₂-adsorption capacity of 4.45 mmol g⁻¹ at 600 °C, which means an efficiency of 75.3% (the efficiency is defined as the ratio between the real adsorption capacity to the theoretical value). Furthermore, LiNaZrO₃ adsorbed CO₂ faster than any other similar ceramics in short time spans. Veliz-Enriqueza et al.²⁰³ found that Li_2-xK_xZrO₃ solid solutions adsorbed CO₂ between 450 and 730 °C, but the ceramics containing potassium adsorbed CO₂ at higher temperatures. The kinetic analyses indicated that Li_2-xK_xZrO₃ solid solutions can adsorb CO₂ up to five times faster than Li₂ZrO₃ in short time spans.

Besides zirconates, some other alkali-metal-containing materials including lithium orthosilicate (Li₄SiO₄),^204–207 Na₂SiO₃,²⁰⁸ CaSiO₃,²⁰⁹ and lithium cuprates (Li₂₊xCuO_2+x/2)²¹⁰ are also potential high temperature CO₂-adsorbents. However, it seems there are still many undiscovered materials in this area, thus a systemic investigation is necessary.

Table 1 summarizes the general CO₂ adsorption capacities of the main types of adsorbents mentioned in this paper. However, it is worth mentioning that, once a higher adsorption pressure is applied, the CO₂ adsorption capacity might greatly exceed the capacity range listed in this table. This phenomenon is especially significant for low temperature CO₂-adsorbents. For instance, Millward et al.⁶⁷ reported that the amount of adsorbed CO₂ could be as high as 33.5 mmol g⁻¹ at 35 atm.

Table 1 The general CO₂ adsorption capacities of the main types of adsorbents

Adsorbent type		Adsorption temperature/°C	Adsorption pressure/atm	CO2 adsorption capacity/mmol g^−1	References
Low temperature adsorbents	Carbon based	≤ 80	1	≤ 3.5	10–29
	Zeolite based	≤ 100	1	≤ 4.9	34–59
	MOF based	≤ 100	1	≤ 4.5	62–87
	Alkali metal carbonate based	≤ 120	1	≤ 9.4	88–101
	Amine based	≤ 60	1	≤ 5.5	101–122
Intermediate temperature adsorbents	LDH based	200–400	1	≤ 1.4	131–150
High temperature adsorbents	Calcium based	600–700	1	≤ 11.6	155–193
	Alkali ceramic based	500–600	1	≤ 6.5	194–210

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5. Catalytic applications of solid CO₂-adsorbents

The captured and concentrated CO₂ can be catalytically converted to various chemical products, e.g., to methanol via hydrogenation of CO₂/CO,²¹¹ to syngas (H₂ + CO) via drying reforming of CH₄ with CO₂,²¹² and to many fine/pharmaceutical chemicals, etc.²¹³ However, here we only focus on some sorption-enhanced reactions that utilize the CO₂-capture function of the solid CO₂-adsorbents. Besides the CO₂-capture capacity, the kinetics and recycling ability of the materials and the temperature gap between the reaction temperature and the sorption/desorption temperature should also be considered. It is better if the temperature gap is small; if not, in some cases it could be overcome by a proper combined configuration of the catalysts and the CO₂-adsorbents. Otherwise, the extra energy consumption and cost have to be considered.

Several important CO₂-sorption enhanced reactions are steam reforming (SR) of hydrocarbons or biomass-derived chemicals, such as glycerol,²¹⁴ the water gas shift (WGS) reaction, and preferential oxidation (PROX) of CO in H₂. In general, these reactions are related to the production of H₂-rich gas. The enhanced-WGS reaction has been tested in the pre-combustion process to clear up flue gas of the coal-fueled gasification power plants,^156,215,216 and the PROX reaction is used to produce very pure H₂ (the CO concentration is required to be less than 100 ppm or even 10 ppm) that can be fed to polymer electrolyte membrane fuel cells (PEMFCs). It is very common for several reactions to occur simultaneously in a single reaction process, thus increasing the complexity for very good control of it. For thermodynamic analyses and process considerations of some reactions and processes, readers can refer to Harrison's review paper and references therein.¹⁵⁶ However, we should keep in mind that the effect of CO₂ removal is not just limited to these reactions. Theoretically, a reaction having CO₂ as a product could be impacted by removal of CO₂.

The SR reactions usually take place above 500 °C, so high-temperature CO₂-adsorbents are often used in these reactions. For the SR reaction of methane, CaO and Na₂ZrO₃ exhibit the highest reaction kinetics among various CO₂-adsorbents (CaO, Li₂ZrO₃, K-doped Li₂ZrO₃, Na₂ZrO₃, and Li₄SiO₄). However, the former is easily sintered thus losing its capacity gradually.²¹⁷ Because of the relatively low reaction kinetics on Li₂ZrO₃, a long reactor and low space velocity have to be used so as to reach the equilibrium composition.²¹⁸ The sintering of the adsorbents is not only due to the high reaction temperature, but also to the even higher regeneration temperature, which is over 850 °C for CaO.²¹⁹ In order to improve the sorption-enhanced SR reaction further, adsorbents with lower sorption/desorption temperature are preferred, or SR catalysts that can catalyze the reaction at lower temperatures should be developed and applied. In the latter case, CO₂-adsorbents that have lower sorption/desorption temperature, e.g. LDHs, become applicable.¹²⁹

The WGS reaction usually takes place between 200–500 °C, so solid CO₂-adsorbents with lower sorption/desorption temperatures such as LDHs can be used. In the presence of K₂CO₃-promoted LDHs, the WGS reaction on a commercial catalyst could produce H₂ with a CO concentration as low as 10 ppm, because the CO conversion was increased from 99.87 to 99.999% after addition of the adsorbent.²²⁰ Van Selow et al.²²¹ recently reported a good stability for more than 300 cycles of adsorption/reaction and desorption on an WGS reaction system enhanced by a LDH-based CO₂-adsorbent, and the CO concentration was kept at ca. 300 ppm, a value much lower than that of the conventional WGS system (usually above 1000 ppm). It should be pointed out that in the SR reactions, the WGS reaction also often occurs, because the reformate contains CO, CO₂, H₂ and H₂O, so the existence of CO₂-adsorbents enhances the two reactions.²²²

It would be ideal to combine the CO₂-adsorbents with the PROX reaction catalysts for fuel cell applications. After the WGS reaction, the feed gas contains quite a low amount of CO, which allows a much longer working span for the CO₂-adsorbents than that in the sorption-enhanced SR and WGS reactions. The PROX reaction following should be able to further lower the CO concentration significantly. Though some PROX catalysts are efficient, there is limited room for further improvement of the selectivity for CO oxidation over H₂ oxidation in a H₂-enriched environment. One of the reasons is that, in the PROX process, due to the generation of CO₂ and H₂O, the WGS and reverse WGS reactions and even the methanation reaction may also occur. The reverse WGS (r-WGS) reaction regenerates CO, and the methanation reaction consumes H₂, so both of them should be avoided. Addition of a CO₂-adsorbent is likely to shift this WGS reaction towards H₂ production and eliminate the methanation reaction. From a thermodynamic viewpoint, the above mentioned reactions favor different reaction temperature ranges, e.g., the r-WGS and methanation reactions occur above 200 °C and 400 °C respectively, while the WGS reaction favors lower reaction temperatures. Theoretically, the enhancement will become more obvious for catalysts with working temperatures above 200 °C as the r-WGS and methanation reactions can be suppressed more significantly. Therefore, some catalysts with high working temperatures may work better and become feasible by combination with a CO₂-adsorbent. Of course, the selection of a specific CO₂-adsorbent working in low, moderate or high temperature ranges is dependent on the working temperature of the selected catalyst. It is hoped that an efficient PROX process can be developed if their working temperatures are compatible. Unfortunately, so far there are still no promising results reported for this application.

6. Concluding remarks and outlook

There is a growing awareness that anthropogenic CO₂ emissions should be reduced as CO₂ is a major greenhouse gas in the atmosphere. For the capture, storage and utilization of CO₂, solid CO₂-adsorbents are widely used. These adsorbents can be roughly classified into three types according to their sorption/desorption temperatures: low-, intermediate- and high-temperature adsorbents with temperature ranges below 200 °C, between 200–400 °C and above 400 °C respectively.

(1) The low-temperature CO₂-adsorbents include carbon, zeolites, MOFs/ZIFs, alkali metal carbonates and amine-based materials. The first three materials adsorb CO₂ mainly by physical interaction while the final two can chemically bind to CO₂, and most of them have good CO₂ adsorption capacities. However, for practical applications, each of them possesses pros and cons. The carbon-based materials are abundant, cheap, easy to make, and chemically and hydrothermally stable, but their selectivity for CO₂ is low in the presence of other gases (e.g. N₂, H₂, CH₄, etc.). Grafting functional amine groups to the carbon surface is a good option to solve this problem. Both zeolites and MOFs/ZIFs have a high capacity for CO₂ capture, and some of them are quite stable, but usually not cheap. The main challenge for alkali metal carbonates and amine-based adsorbents lies in their long-term stability. The thermal and hydrothermal stability, especially in the presence of H₂O, is a problem to support materials such as mesoporous silica. It is also worthwhile to mention that the CO₂ adsorption temperature (normally < 60 °C) of these materials is slightly lower than that of real flue gases (normally ∼90 °C), and the current choice is to first cool down the flue gas, which is also energy consuming. Certain modifications should be made to these materials to increase their adsorption temperatures slightly. The amine-based solid adsorbents have high selectivity towards CO₂ and other acidic gases, but their capacities are still lower than that of the corresponding liquid amines. Developing amine molecules with low volatility and high density of amino groups should be a direction to pursue.

(2) Among various intermediate-temperature CO₂-adsorbents, LDHs have been well investigated and are believed to be the most promising adsorbent candidate. The main problems of this kind of material are still their insufficient adsorption capacity, kinetics and stability. Doping LDHs with K₂CO₃ can improve their capacity and kinetics. For LDHs, one critical issue that cannot be bypassed is their stability under real operating conditions, especially during the pressure swing adsorption/temperature swing adsorption (PSA/TSA) cycles and in the presence of H₂O and SO₂. For the sorption-enhanced hydrogen production reactions, the compatibility between LDHs and WGS/SR catalysts should also be considered, e.g., the temperature gap. In addition, from the viewpoint of fundamental research, the structure of the active sites for CO₂ adsorption and the structural evolution of these materials during the CO₂ sorption and desorption are still not very clear and should be further investigated.

(3) For the high-temperature CO₂-adsorbents, CaO and alkali ceramics are representative materials. These CO₂-adsorbents suffer severely from textural degradation during the sorption/desorption operations. Currently, these materials can only run several tens of cycles before any obvious degradation, and are still far from practical applications. Therefore, it is highly desirable to increase their thermal stabilities. Encapsulated or supported CaO-like or LDH adsorbents probably have higher stabilities, but the support or matrix materials must be inert to these active components. Carbon is probably a good choice.

(4) The great success in CSU of CO₂ is probably determined not only by exploration of the well-known materials, but also by the discovery and synthesis of new materials that have high capacity for CO₂ capture and good thermal and recycling stability. These materials may be new polymers, organic, inorganic materials or their hybrids, and they should bear a high density of basic functional groups or active sites.

(5) For catalytic applications including the WGS, SR and PROX reactions, the first two still suffer from insufficient thermal stability of the CO₂-adsorbents (mainly intermediate and high temperature CO₂-adsorbents) in the reaction temperature ranges. The combination of the PROX catalysts with low-temperature CO₂-adsorbents is promising for practical applications, as both the process temperature and the CO₂ concentration are quite low. It is expected that more CO₂-adsorbent enhanced catalytic reactions will be identified and studied in the future. However, one should keep in mind that a successful process is not only a scientific issue but also an engineering issue. The proper configuration of the catalyst and the CO₂-adsorbent and the whole process will play an important role, and in some cases, become a key performance indicator.

Acknowledgements

This research was supported by the Agency for Science, Technology and Research in Singapore (A*STAR, project code 0921380024). The authors would like to thank Mr Desmond Jia Wei Ng, Ms Hui Huang Tay and Jaclyn Teo for their assistance and Dr P. K. Wong and Dr Keith Carpenter for their support of this project.

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