Post-combustion Carbon Dioxide Capture Materials

By Royal Society of Chemistry

Inorganic solid adsorbents/sorbents are attractive materials for capturing carbon dioxide (CO2) from flue gases after fossil fuel combustion.

Post-combustion Carbon Dioxide Capture Materials introduces the key inorganic materials used as adsorbents/sorbents with specific emphasis on their design, synthesis, characterization, performance, and mechanism. Dedicated chapters cover carbon-based adsorbents, zeolite- and silica-based adsorbents, metal–organic framework (MOF)-based adsorbents, and alkali-metal-carbonate-based adsorbents. The final chapter discusses the practical application aspects of these adsorbents used in carbon dioxide capture from flue gases.

Edited and written by world-renowned scientists in each class of the specific material, this book will provide a comprehensive introduction for advanced undergraduates, postgraduates and researchers from both academic and industrial fields wishing to learn about the topic.

• Language: English
• Publisher: Royal Society of Chemistry
• Release date: Oct 22, 2018
• ISBN: 9781788015455

Book preview

Preface

Carbon dioxide (CO2) is one of the major greenhouse gases that contribute to global warming and anthropogenic climate change. Unfortunately, it is predicted that the atmospheric CO2 concentration will continue to increase in the next few decades because fossil fuels will still be the dominant energy source. In recent years, worldwide efforts have been made to reduce the CO2 emissions, among which capturing using solid adsorbents/sorbents has attracted intense attention from both academia and industry. I have been working on CO2 capture materials since 2009, and have witnessed nearly one-thousand papers per year being published in this field. With this rapid development, I believe that it is was necessary to edit a book to summarize all the important progresses made with each type of CO2 capture material. Professor Dermot O'Hare, University of Oxford suggested that this could be part of the Royal Society of Chemistry's Inorganic Materials Series. He suggested two books entitled Pre-combustion Carbon Dioxide Capture Materials and Post-combustion Carbon Dioxide Capture Materials. I am confident that these works will benefit advanced undergraduates, postgraduates and researchers working in both academia and industry on this topic.

This book is organized into five chapters and focuses mainly on the low-temperature CO2 adsorbents used for post-combustion CO2 capture from flue gases. This book aims to present the full picture of various post-combustion CO2 capture materials including carbon, zeolite and silica, metal organic frameworks, and alkali metal carbonate, etc. The discussion of each type of adsorbent starts with the fundamental mechanism for CO2 capture, followed by the preparation and modification of the materials, and their capture capacity, kinetics, and recycling stability, etc. The application status of the above mentioned materials for post-combustion CO2 capture is presented in the fifth chapter.

The editor thanks all the contributors to this book, particularly Professor Jin Zhou (Shandong University of Technology, China), Professor Wei Xing (China University of Petroleum, China), Professor Guillaume Laugel (Université Pierre et Marie Curie, France), Professor Benoît Louis (Université de Strasbourg, France), Professor Wha-Seung Ahn (Inha University, South Korea), Professor Hirofumi Kanoh (Chiba University, Japan), Professor Rajender Gupta (University of Alberta, Canada), and all the students and researchers involved in each chapter.

Also, I would like to express special acknowledgements to Professor Duncan Bruce (University of York, UK), Professor Dermot O'Hare (University of Oxford, UK), and Professor Richard Walton (University of Warwick, UK), who accepted and supported this project, and to Connor Sheppard, Leanne Marle, Sylvia Pegg, and Robin Driscoll for all their support during the editing of this book. Finally, I thank the Royal Society of Chemistry for supporting this edition.

Qiang Wang

CHAPTER 1

Carbon-based CO2 Adsorbents

Jin Zhou*a, Xuan Wanga and Wei Xing*b

a Shandong University of Technology, School of Chemistry and Chemical Engineering, Zibo, 255000, PR China

b China University of Petroleum, School of Science, State Key Laboratory of Heavy Oil Processing, Qingdao, 266580, PR China

*E-mail: [e-mail]; [e-mail]

1.1 Introduction

Carbon dioxide (CO2) has been recognized to be the biggest driver of global warming, which is one of the most serious problems that our world is facing.¹,³ Furthermore, CO2 is an important source of C1 chemical engineering, and could be converted into high-value chemical products via chemical,²,³ photochemical,⁴,⁵ or electrochemical processes.⁶,⁷ However, the efficient capture of CO2 is essential for these processes. So, there is an urgent need to develop CO2 capture and storage (CCS) technologies. The basic concept of CCS is to capture CO2 from emissions without releasing it into the atmosphere. CCS can be classified as post-combustion, pre-combustion, and oxy-fuel combustion technologies. Among the current CCS technologies, post-combustion capture, a technology for capturing CO2 from post-combustion emission gases (e.g., flue gas from power plants) is the most easily applied technology for existing emission sources.

In general, post-combustion capture technologies include chemical absorption, dry adsorption, membrane-based technologies, and cryogenic technologies. Currently, chemical absorption is the most applicable technology for CO2 capture in power plants, but this technology suffers from several drawbacks. The biggest challenge in applying a chemical absorption process for post-combustion is how to reduce the heat of regeneration. Another problem is the release of hazardous byproducts. For these reasons, dry adsorption using solid adsorbents is considered to be promising for the capture of post-combustion CO2.

The dry adsorption technique is a process of selective adsorption of CO2 from post-combustion gases using solid adsorbents, which has advantages such as a simple device, easy operation, it is environmentally friendly and has a high energy efficiency. When evaluating solid adsorbents, it is important to consider their surface area, apparent density, pore size and volume, feasibility of regeneration, stability, abundance and sustainability. CO2 capture by solid adsorbents mainly relies on the mechanism of physical adsorption that is also interfered with by some weak interactions between CO2 and the adsorbent's surface (i.e. hydrogen bonding or electric quadrupole interactions). Due to the main contribution of van der Waals forces to the physical adsorption, materials that possess a developed microporous texture are preferred for CO2 capture. Nowadays, many kinds of solid adsorbent materials with porous textures, such as porous carbonaceous materials,⁸,⁹ zeolites,¹⁰ zeolitic imidazolate frameworks (ZIFs),¹¹ metal–organic frameworks (MOFs),¹² covalent organic frameworks (COFs),¹³ and porous coordination polymers (PCPs),¹⁴ have been investigated, and show excellent CO2 capture performances.

Among these solid adsorbents, porous carbonaceous materials have been studied intensively because of their desirable physical and chemical properties, such as low cost, variety of form (powder, fibers, aerogels, composites, sheets, monoliths, tubes, etc.), ease of processability, controllable porosity (adjustable pore size and its distribution, high specific surface area and pore volume), and tailored surface chemistry (O, N, S, P, F or other heteroatom doping). They also possess some other advantages, particularly for adsorption applications: (1) carbon materials have excellent stability especially in hot and humid environments; (2) gas sorption on carbon materials is not moisture-sensitive because the surface is usually hydrophobic; (3) the energy consumption of regeneration is low due to the nature of physical adsorption; (4) the adsorption/desorption temperatures are always below 373 K; (5) these materials can be used at atmospheric pressure.

In this chapter, we summarize the recent research progress made in developing carbon-based sorbents for post-combustion CO2 capture. Specifically, this chapter will provide overviews of (1) porous carbons, (2) graphene-based porous materials, (3) carbon nanotubes, (4) carbon-based hybrid sorbents, and (5) important factors influencing CO2 uptake over carbon adsorbents.

1.2 Porous Carbons

Porous carbons have been extensively studied in the field of CO2 capture. In order to control the pore structure in carbon materials, a variety of preparation methods have been developed and certain successes have been achieved. Herein, we summarize typical preparation methods for porous carbon adsorbents, including chemical activation, physical activation, metal ion activation, templating methods, and the combined method of templating and activation. In each section, synthesis principles, carbon precursors, pore structures, as well as their CO2 adsorption performance, are discussed.

1.2.1 Chemical Activation

Activated carbon is the oldest and most widely used carbon material. Generally, the production routes of activated carbons are divided into physical activation and chemical activation. In chemical activation, the carbon precursor is mixed/impregnated with an activating agent (such as KOH, H3PO4, ZnCl2, K2CO3, etc.), then the precursor is simultaneously carbonized and activated at an elevated temperature (from 400 to 900 °C) and under an inert atmosphere (usually N2 or Ar). In physical activation, the carbon precursor is usually pre-carbonized at temperatures over 500 °C in an inert atmosphere to remove non-carbon species, followed by etching by an oxidizing gas (such as CO2, steam, and air) at a higher temperature (from 700 to 1200 °C). Comparatively, chemical activation needs a lower temperature and shorter activation time, and generally results in a higher specific surface area and more uniform pore size distribution (PSD), while physical activation is simple and does not require chemical agents and repeated washing procedures to remove the inorganic residues after activation. The structure of activated carbons, containing the surface area, pore size and its distribution, and surface chemistry, etc., strongly depends on the activation conditions, activating agents as well as the carbon precursors used.

1.2.1.1 KOH as an Activating Agent

KOH is the most common activating agent in chemical activation. Jaroniec et al. treated a commercial carbon sorbent (Ambersorb 563) with five of the most commonly-used activating agents, including CO2, H2O, NH3, KOH, and ZnCl2, and compared their activating power for the evolution of microporosity responsible for CO2 adsorption.¹⁵ N2 adsorption analysis showed that the investigated activating agents enlarged microporosity and consequently surface area and pore volume of the carbons in the following order: KOH > CO2 > NH3 > H2O > ZnCl2. It was shown that KOH activation yielded the highest volume of micropores and small micropores. Besides, the CO2 uptake for the KOH-activated sample was the highest, indicating that KOH activation appears to be the most effective to obtain carbon adsorbents for CO2 capture.

However, the mechanism of KOH activation has not been totally understood due to the complexity of this process. In a previous review about KOH activated carbon materials for energy storage, Wang et al. suggested that KOH activation is a synergistic process of chemical activation, physical activation, and carbon lattice expansion by metallic K intercalation.¹⁶ Firstly, the potassium species serving as chemical activating reagents vigorously etch the carbon framework by the redox reactions shown in eqn (1.1)–(1.3). Secondly, H2O (from dehydration of carbon precursors or eqn (1.4)) and CO2 (eqn (1.5) and (1.6)) produced in situ in the activation system further develop the porosity through the gasification of carbon, namely physical activation (eqn (1.7) and (1.8)). Meanwhile, the produced metallic K intercalates into the lattices of the carbon matrix, responsible for both stabilization and widening of the interlayer spacing (Figure 1.1). After removal of the intercalated metallic K and other K compounds by washing, the expanded carbon lattices cannot return to their previous non-porous structure and thus create a narrow microporosity and large specific surface areas.

1.1 6KOH + 2C + → 2K + 2H2 + 2K2CO3

1.2 K2CO3 + 2C → 2K + 3CO

1.3 C + K2O → 2K + CO

1.4 2KOH → K2O + 3H2O

1.5 CO + H2O → CO2 + H2

1.6 K2CO3 → K2O + CO2

1.7 C + H2O → CO + H2

1.8 C + CO2 → 2CO

Figure 1.1 Activation mechanism by the penetration of metallic K into the carbon lattices. (a) Carbon lattices, (b) metallic K intercalated in the carbon lattices, (c) activated carbon.

Generally, the raw materials of KOH activation could be classified into non-renewable fossil-based materials and renewable biomass resources. As shown in Table 1.1, various fossil-based precursors, such as petrol coke,¹⁷,¹⁸ pitch,¹⁹ and synthetic polymers,²⁰,²² have been used as precursors for the preparation of porous carbon adsorbents. Wahby et al. prepared a series of carbon molecular sieves (CMS) from petroleum pitch using KOH as the activating agent. Depending on the type of petroleum residue and the conforming step applied, the prepared CMS possessed a well-defined pore size (0.35–0.7 nm), together with a very large surface area up to 3100 m² g−1, thus exhibited a high CO2 adsorption capacity up to 4.09 mmol g−1 at 1 bar and 25 °C.²³ After further optimizing the process parameters, including the nature of the petroleum residue, the KOH–pitch ratio, the mesophase content, the temperature and time of activation, the CO2 uptake could increase to 5.23 mmol g−1 at 1 bar and 25 °C.²⁴ The activated carbons by the KOH activation of petroleum coke possessed high surface areas, over 3000 m² g−1, and a high CO2 capacity, over 15 wt% at 1 bar.²⁵ High-resolution analysis of N2 sorption isotherms concluded that the micropores smaller than 1 nm played a critical role in CO2 capture under ambient conditions due to the high-density filling of CO2 in these small pores.²⁵ Using petroleum coke as the precursor, Yang et al. prepared N-doped porous carbons by combining ammoxidation with KOH activation.¹⁸ The sample prepared under mild conditions (a low temperature of 650 °C and a low KOH–precursor ratio of 2) showed the highest CO2 uptake of 4.57 mmol g−1 at 25 °C and 1 bar, while the CO2/N2 selectivity and CO2 heats of adsorption of the sorbent were 22 and 37 kJ mol−1, respectively. The high CO2 capture capacity was attributed to the synergetic effect of N-doping and high narrow microporosity, while the latter was suggested to contribute more.¹⁸

Table 1 Porous carbons prepared by KOH activation of fossil-based resources for CO 2 capture

aThe selectivity calculated by the ratio of CO2–N2 sorption capacity.

bThe selectivity based on ideal adsorbed solution theory (IAST).

cThe selectivity of Henry's law.

Besides petroleum coke and pitch, synthetic polymers, such as phenolic resins,²⁷ poly(vinylidene chloride),²⁹ styrene-divinylbenzene resin,³⁰ polypyrrole,²⁰ polyaniline,²¹ polyurethane,³² urea furfural resins,³³ polyacrylonitrile,²² and polyimine,³⁷,³⁸ have also been widely used as precursors for the preparation of porous carbon adsorbents. For instance, Jaroniec et al. prepared activated carbon spheres by direct KOH activation of phenolic resin spheres obtained by a modified Stöber method.²⁷ Due to the small micropore (<0.8 nm) and large specific surface areas, the prepared porous carbon spheres exhibited superior CO2 uptakes reaching 4.6 and 8.9 mmol g−1 at 23 and 0 °C under 1 bar, respectively.

Many studies have reported that N-doping could significantly improve the CO2 capture performance of porous carbons, especial CO2 uptake at low partial pressure and the selectivity of CO2-to-N2 (Table 1.1). Activation/carbonization of nitrogen-containing synthetic polymers is an important approach to preparing N-doped porous carbons. In 2010, Lu et al. reported a facile and rapid preparation of N-doped porous carbon monoliths using a basic amino acid as both the catalyst and nitrogen source.³⁹ This monolithic carbon directly pyrolyzed at 500 °C exhibited a CO2 adsorption capacity of 3.13 mmol g−1 at room temperature. KOH activation of nitrogen-containing polymers could result in highly developed microporosity at the same time maintaining the N-doping. Sevilla et al. prepared highly porous N-doped carbon by using polypyrrole (PPy) as the carbon precursor and KOH as the activating agent.²⁰ The mildly activated carbon (KOH–PPy = 2) showed a larger CO2 uptake than the severely activated ones (KOH–PPy = 4) as a result of two important characteristics for the mildly activated samples: (a) a high-level N-doping (up to 10.1 wt% N) identified as main pyridonic-N and a small proportion of pyridinic-N groups, and (b) a narrower micropore size. Moreover, these polypyrrole-based carbons showed a high adsorption rate for the capture of CO2, more than 95% of the CO2 being adsorbed in 2 min with a high kinetic CO2–N2 selectivity of 12. Other nitrogen-containing polymers, such as polyacrylonitrile,²² polyaniline,²¹ polyurethane,³² urea furfural resin,³³ and polyimine,³⁵,³⁶ have also been used as precursors and the corresponding N-doped porous carbons showed CO2 uptakes of 4.50, 4.30, 4.33, 4.70, and 3.10 mmol g−1 at 25 °C and 1 bar, respectively.

From Table 1.1, we find that the CO2 uptakes for most of the activated carbons are lower than 5.0 mmol g−1. Recently, Sethia and Syari prepared a series of strictly microporous N-doped activated porous carbons by using a nitrogen-containing ionic liquid of 1,3-bis(cyanomethyl imidazolium) chloride as a precursor and KOH as the activating agent.³⁴ The optimized material exhibited an extraordinary CO2 uptake of 5.39 mmol g−1 at 25 °C and 1 bar, which is the highest uptake reported so far for activated carbons. They concluded that both nitrogen content and ultramicropores played important roles in the CO2 capture, with the latter being predominant.³⁴

Besides the N-doped porous carbons, S-doped porous carbons have also been prepared by the KOH activation of sulfur-containing synthetic polymers and are applied in CO2 capture. For instance, S-doped microporous carbon materials can be prepared by the KOH activation of polythiophene–graphene composite.³⁷ This material displayed a high CO2 uptake of 4.5 mmol g−1 at 25 °C and 1 bar, as well as an impressive CO2 adsorption selectivity over N2 (51), CH4 (12), and H2 (214) based on the initial slopes method of Henry's law.⁴⁰,⁴²

Fossil-derived materials, such as petroleum coke, pitch, coal, and synthesized polymers are non-renewable and expensive. Considering the demand for a huge amount of solid CO2 sorbents, the development of low-cost carbon materials from renewable and sustainable sources is urgent.⁴³ Biomass materials are renewable, easily available and very cheap, which are promising raw materials for the preparation of carbon adsorbents. As shown in Table 1.2, there have been over 40 kinds of biomass employed as precursors for porous carbon adsorbents, and most of them are obtained via chemical or physical activation, especially KOH activation. The specific surface areas and pore volumes of the biomass-based carbons were tuned with controlled carbonization and/or activation processes. The biomass used previously could be roughly divided into three categories: (1) polysaccharides, such as sucrose,⁴⁴ cellulose,⁴⁵ chitosan,⁴⁶ starch,⁴⁷,⁴⁸ etc., (2) raw biomass or waste biomass, such as nutshells,⁴⁹,⁵⁰ wood residues,⁵¹ Jujun grass,⁵² Enteromorpha prolifera,⁵³ sugar cane bagasse,⁵⁴ granular bamboo,⁵⁵ leaves,⁵⁶ and others, and (3) microorganisms, including fungi⁵⁷ and yeast.⁵⁸

Table 2 Biomass-based porous carbons prepared by KOH activation for CO 2 capture

aAt 25 °C and 0.15 bar.

bAt 2 °C, N2–CO2 = 85 : 15.

cAt 0 °C and 1 bar.

dAt 0 °C and 0.1 bar.

Polysaccharides could be converted into carbonaceous materials via a hydrothermal treatment approach using water as the carbonization medium under a self-generated pressure. In 2011, Sevilla and Fuertes reported a series of porous carbons prepared by the KOH activation of hydrothermally carbonized polysaccharides (starch and cellulose) and biomass (sawdust).⁴⁸ The prepared porous carbons were used as CO2 adsorbents. Although the porous carbons prepared at a ratio of KOH–precursor = 2 have a considerably low-level pore development than those obtained at a ratio of KOH–precursor = 4, they exhibited significantly better CO2 capture capacities. This is mainly due to the presence of a larger number of narrow micropores (<1 nm) in the mildly activated carbons. The sawdust-based carbon activated at 600 °C using KOH–precursor = 2 gave the highest CO2 uptake of 4.8 mmol g−1 (21.2 wt%) at 25 °C and 1 atm, which is still among the highest ever reported for activated carbons now.

Microporous carbons were prepared from wheat flour (mainly starch) via a combined process of pre-carbonization and post-KOH activation.⁵⁹ The pore texture of the wheat-flour-based carbons was significantly improved by the post-chemical activation of KOH and varied with the KOH–carbon ratio. By increasing the KOH–carbon ratio up to 3, narrow micropores smaller than 0.8 nm were developed primarily, and the corresponding activated carbon possessed a moderate surface area but the highest volume of pores smaller than 0.8 nm, thus delivering the highest CO2 adsorption capacities of 5.70 and 3.48 mmol g−1 at 0 and 25 °C, respectively. The experimental results confirmed that CO2 adsorption uptake at ambient conditions was significantly dependent on the volume of narrow micropores with a pore size of less than 0.8 nm rather than the total volume or specific surface area.

Biomass waste is not only abundant, and sustainably renewable, but is also more cost-effective as carbon precursors. A wide range of biomass waste has been used to prepare porous carbon materials and showed excellent CO2 sorption capacities. Shell is one of them. As early as 2004, Tan and Ani reported carbon molecular sieves by direct carbonization of palm shell waste at 600–1000 °C.⁶⁰ When the material was pyrolyzed at 600 °C, the micropore surface area was 753 m² g−1; it showed CO2 adsorption near 2.3 mmol g−1 at 25 °C. When the pyrolysis temperature increased to 1000 °C, the micropore surface area and the pore volume decreased; however, the selectivity toward CO2 increased.⁶⁰ After further KOH activation at 600 °C, the CO2 uptake of palm-shell-based porous carbon significantly increased from 2.7 mmol g−1 to 4.4 mmol g−1 at 25 °C and 1 bar, due to the much more developed microporosity of the activated carbons compared to the non-activated ones.⁶¹ Hu and his co-workers prepared two series of nitrogen-doped porous carbons from coconut shells.⁵⁰,⁶² One type was prepared by urea modification and KOH activation, and the other was prepared by a successive process of pre-oxidization by H2O2, ammoxidation, and KOH activation. These carbons were found to exhibit very high CO2 uptakes at 1 bar, almost 5.0 mmol g−1 and 4.47 mmol g−1 at 25 °C, 1 bar, respectively. The pre-oxidization by H2O2 increased oxygen-containing surface groups, which were expected to increase the amount of nitrogen incorporated into the carbon product in the subsequent ammoxidation process. The resulting carbons possessed a much higher nitrogen content and a narrower microporosity than the control sample without H2O2 pre-treatment. These results made coconut shell an attractive precursor for carbon adsorbents.

Shahkarami et al. performed a comprehensive study on the effects of the nature of precursors and carbonization conditions on the physical and chemical properties of activated carbon and the influence of these factors on CO2 adsorption capacity, selectivity, and stability of the produced activated carbon in a fixed-bed reactor and mixed feed stream of N2/CO2/O2.⁶³ In that work, three types of abundant feedstocks were used: agricultural waste (wheat straw and flax straw), forest residue (sawdust and willow ring), and animal manure (poultry litter). The selected precursors were carbonized via both fast and slow pyrolysis processes and were converted to porous carbons after KOH activation. Slow-pyrolysis-based activated carbon had a lower surface area and total pore volume but higher CO2 adsorption capacity in the presence of N2. Sawdust-based activated carbon synthesized from the slow pyrolysis possessed the largest ultramicropore volume of 0.36 cm³ g−1, and the highest CO2 adsorption capacity (78.1 mg g−1) in N2 but low selectivity (2.8) over O2 because of the oxygen functional groups on the surface. The ultramicropores and surface chemistry of adsorbents were found to be much more important than particle size, total pore volume, and internal surface area of the adsorbents.

Besides terrestrial raw biomass, marine raw biomass, such as microalgae⁶⁴ and Enteromorpha prolifera,⁵³ were also converted into carbons. For instance, a sugar-rich microalgae (Chlorococcum sp.) was used to prepare N-doped activated carbons through hydrothermal carbonization and a subsequent activation of KOH or NH3.⁶⁴ Although the NH3-activated carbon possessed a much higher nitrogen content than the KOH-activated ones, the latter exhibited higher CO2 sorption capacities than the former, indicating the main contribution of narrow microporosity on the CO2 capacities. Ma et al. have successfully prepared highly porous N-doped carbon monoliths by using binary H3PO4–HNO3 mixed acid as a co-activating agent and sodium alginate, a marine biopolymer, as a carbon precursor.⁶⁵ The SA-2N–P carbon prepared at a volume ratio of HNO3–H3PO4 = 2 showed high CO2 adsorption capacities of 8.99 mmol g−1 at 0 °C and 4.57 mmol g−1 at 25 °C, along with a high CO2 capacity of 1.51 mmol g−1 at 25 °C and 0.15 bar.

Microorganisms are another promising kind of carbon precursor. Wang et al. reported a series of porous carbons with narrow microporosities by KOH activation of pre-carbonized fungi (Agaricus).⁵⁷ A moderate CO2 uptake of 5.5 mmol g−1 and a high CO2–N2 selectivity of 27.3 at 0 °C, 1 bar were obtained. Similarly, Shen et al. found that yeast-based carbon was a promising adsorbent.⁵⁸ Hierarchical microporous carbon with a specific surface area of 1348 m² g−1 and a pore volume of 0.67 cm³ g−1 was prepared by KOH activation of yeast. This type of carbon material showed a high CO2 uptake of 4.77 mmol g−1 and a fast adsorption rate with an equilibrium time less than 10 min at 25 °C.

1.2.1.2 Other Chemicals as Activating Agents

Other activating agents, such as ZnCl2,⁹⁰,⁹¹ NaOH,⁸⁵ K2CO3,³⁸,⁸⁶ H3PO4,⁹² NaNH2,⁹⁴ etc. have also been used to prepare porous carbon for CO2 capture. ZnCl2 is the second most commonly used activating agent in chemical activation.⁹⁶,⁹⁸ However, there are only a few reports on ZnCl2 activated carbons for CO2 capture, as shown in Table 1.3, which may be due to its relatively undeveloped porosity as a result of the low activating power of ZnCl2.⁹⁰,⁹¹ For example, ZnCl2-activated polypyrrole gave a specific surface area of 1283 m² g−1 and pore volume of 0.46 cm³ g−1, much lower than those of KOH-activated polypyrrole (1700 m² g−1 and 0.88 cm³ g−1).²⁰,⁹¹ The as-prepared N-doped carbon exhibited a moderate CO2 uptake of 3.80 mmol g−1 at 25 °C and 1 bar when the activation temperature was 600 °C.⁹¹ Although the ZnCl2 activated carbons showed lower CO2 uptakes compared to the KOH activated carbons, the ZnCl2 activation usually gave a higher carbon yield and higher carbon density thus could exhibit some advantage in volumetric CO2 capacities.

Table 3 Activated carbons prepared by other chemical activations

aCO2 capacities at 25 °C and 1 bar.

bSelectivity based on the ideal adsorbed solution theory (IAST).

cThe selectivity of Henry's law.

dCO2 capacity at 0 °C and 1 bar.

Using K2CO3 as an activating agent, Fan et al. activated chitosan into N-doped microporous carbon.⁴⁶ By changing the weight ratio of K2CO3–chitosan and the activation temperature, the porosity and nitrogen content of the prepared carbons could be tuned in the range of 1180–2567 m² g−1 and 1.29–6.02 wt%, respectively. The sample prepared at 635 °C with a K2CO3-chitosan ratio of 2 showed a CO2 uptake of 3.86 mmol g−1 at 25 °C, 1 atm, five consecutive recyclabilities, and a good CO2–N2 selectivity of ca. 21. Silvestre-Albero prepared a series of activated carbons by pre-carbonization of PANI at different temperatures and post-activation of KOH or K2CO3.⁸⁶ They studied the activating effects of KOH and K2CO3. It was found that carbonization temperature significantly influenced the porosity of the prepared carbons when using KOH as an activating agent, while K2CO3 mainly produced microporosity, independent of the carbonization temperature. The highest CO2 uptake of these carbons was 7.60 mmol g−1 at 1 bar and 0 °C.

In the last decade, ammonia (NH3) treatment has been used to introduce nitrogen surface groups into the carbon framework. Several authors have reported ammonia-treated porous carbons for CO2 capture.⁷⁹,⁹⁹ Pevida observed that nitrogen groups were successfully introduced into carbon frameworks after a NH3 treatment, and the nitrogen content was proportional to the oxygen content of the pristine porous carbons.⁹⁹ Due to the incorporated basic nitrogen groups, the ammoxidized carbons showed enhanced CO2 uptakes at high adsorption temperatures. However, the low reaction efficiency between the NH3 and the carbon resulted in a relatively low N-doping level. Recently, Geng et al. reported a NH3-assisted activation process in which NH3 played the roles of activating agent and nitrogen source at the same time.⁷⁸ When carbonizing a corncob in a NH3 atmosphere, the nitrogen could be easily incorporated into a carbon framework. The N content increased as the activation temperature increased, reaching a high level of ∼12 wt% at 800 °C, along with an increase in the specific surface area and pore volume. Especially, the nitrogen was mainly incorporated in the form of phenyl amine and pyridinic N groups, which were very efficient for CO2 capture. These carbons showed a moderate CO2 uptake of 2.81 mmol g−1, but superior IAST CO2–N2 selectivity up to 82. Similarly, Hu et al. prepared a N-doped hierarchical porous carbon by carbonization of a cellulose aerogel under a NH3 atmosphere.⁴⁵ This N-doped carbon aerogel exhibited a N-doping of 4.62 wt%, and a high CO2 adsorption capacity of 4.99 mmol g−1 at 25 °C and 1 atm.

Most of the other activating agents usually show a lower activating ability than KOH. For example, KOH-activated polypyrrole carbons show a much higher specific surface area than the NaOH-activated ones under the same activation conditions (e.g., 2940 m² g−1 vs. 1453 m² g−1, activation conditions: the weight ratio activating reagent–polypyrrole = 2, and activation temperature = 700 °C).²⁰,⁸⁵ Interestingly, NaNH2, a strong base commonly used in organic synthesis, was reported to exhibit a more powerful activation ability compared to NaOH and KOH. At an activating reagent–carbon weight ratio of 2 and an activation temperature of 550 °C, NaNH2 could activate a much higher microporosity, especially more small micropores, than KOH and NaOH, resulting in a higher CO2 uptake of 3.66 mmol g−1 at 25 °C and 1 bar, verifying the superiority of NaNH2 in the activation under relatively moderate conditions.⁹⁴

1.2.2 Physical Activation

The so-called physical activation is the partial gasification of the carbon framework with CO2, steam, and air, or a combination of these, at high temperatures (from 700 to 1200 °C) as shown in eqn (1.9) to (1.12). In view of porosity development, the most important variables in the gasification process are the activating agents, the final burn-off ratio, and the inorganic impurities. Rodríguez-Reinoso and Molina-Sabio comprehensively studied the evolution in the porosity of several series of activated carbons prepared by physical activation of lignocellulosic materials (uncatalyzed and iron-catalyzed) in CO2 or in a water–nitrogen mixture.¹⁰⁰ They found that at the initial stage of CO2 activation, the micropore and macropore volume increase coincided with the proceeding burn-off. Further burn-off with CO2 opened and enlarged the micropores of the char with even a shift to meso- and macropores, the ablation of the exterior of the particle being very important at high burn-offs over 50%.

The final activated carbon had a well-developed micro- and macroporosity, with a relatively small portion of mesoporosity. The Fe-catalyzed CO2 gasification was initially very fast even at a lower gasification temperature and declined at an increasing burn-off due to the deactivation of the Fe catalyst. Steam activation produced a more selective attack of the carbon framework and a more uniform widening of porosity. It developed a large number of