Solid Adsorbents for CO2 Capture

Solid Adsorbents for CO2 Capture: Comparison

Please note this is a comparison between Version 2 by Lindsay Dong and Version 4 by Lindsay Dong.

Carbon capture and storage (CCS) is considered to be a promising technology in reducing atmospheric CO2 concentration. Among the CO2 capture technologies, adsorption has grabbed significant attention owing to its advantageous characteristics discovered in recent years. Solid adsorbents have emerged as one of the most versatile CO2 adsorbents.

porous carbon
amine functionalization
physisorption
chemisorption
CO2 capture
activated carbon
Greenhouse effect

1. Introduction

1.1. Physical and Chemical Properties of CO2

Carbon dioxide (CO2) is a triatomic gas under ambient conditions ^[1], which is abundant, non-toxic, recyclable, and economical ^[2]. Moreover, CO2 sublimates from solid-state to gas at −78 °C under atmospheric pressure and is comparatively inert. As a commonly known fact, CO2 gas that naturally occurs in the Earth’s atmosphere is of paramount importance to photosynthesis ^[1]. From an economic point of view, CO2 can be converted into high-value chemical products such as urea, carbonates, and acrylates ^[3] through catalytic conversion, mineralization, photochemical, or electrochemical reactions, and supercritical CO2 can be also utilized in various industrial fields, including food beverages, refrigerants, transportation fuels, fire extinguishers, polymer synthesis, medical, and exploitation of heavy oil. Solid-state CO2 can be used in artificial rainfall and concrete production ^[4][5].

1.2. Trend of Atmospheric CO2 Concentration and Potential CO2 Emissions Sources

Although the natural carbon cycle controls the CO2 concentration level in the Earth’s atmosphere ^[1], due to both anthropogenic activities and natural emissions, the current atmospheric CO2 concentration reached around 416.5 ppm in mid-2020 ^[6], which is ~40% greater than the beginning of the industrial revolution (280 ppm) in 1750 ^[7][8][9], with an average growth rate of 2 ppm per year ^[9][10]. In other words, the global emission of CO2 was estimated to be more than 36 MT in 2017, which is 18-fold greater than compared to the 1800s ^[11]. Although it is a consensus that the amount of atmospheric CO2 should not exceed 350 ppm ^[12], according to the predictions by the International Panel on Climate Change (IPCC), it is expected to reach up to 570 ppm by 2100 ^[12][13][14]. It is identified that the main causes for the tremendous increase in such atmospheric CO2 concentration are mainly associated with various anthropogenic activities, including vehicular emissions, fossil-fuel power plants, deforestation, chemical processes ^[15], and waste treatment ^[16], which have been growing steadily due to rapid industrialization and urban development ^[15][17]. The natural emission sources, including soil degradation processes and volcanic activities, are also responsible for supplying atmospheric CO2 to some extent ^[18].

1.3. Significant Outcomes Owing to the Trend of Increasing CO2 Emissions

Unfortunately, the non-controllable anthropogenic activities have negatively affected human beings ^[19] and the entire ecosystem ^[3][6] by releasing greenhouse gases, including CO2, into the atmosphere. Among the greenhouse gases, CO2 is considered as one of the primary sources, contributing to roughly 64% of the total greenhouse effect ^[14][20]. The progressive increase in atmospheric CO2 concentration is responsible for climate change, which might adversely impact the global environmental processes, such as the long-term rise in global temperatures, changes in rainfall patterns, rising sea levels ^[21][22], ocean acidification ^[23], species extinction, melting of polar ice ^[9], shrinkage of snow covers ^[24], and severe weather events, ranging from flash floods ^[25], hurricanes, freezing winters, severe droughts ^[22], heat waves ^[26], urban smog ^[17], and cold streaks ^[27]. According to the predictions made by IPCC, the rise in sea level of 3.8 m ^[14][28] and rise in mean global temperature by 3.7 °C ^[29][30] are expected by 2100 ^[24]. Besides, the increasing trend of CO2 in the air might cause various air-borne diseases, which will increase the risk of health complications ^[31]. The economic loss due to climate change is expected to be 5–20% of the global domestic production ^[12][28]. Therefore, extensive research projects are currently underway to reduce and control CO2 emissions from power plants, industries, and transportation ^[32].

1.4. Approaches to Reduce Atmospheric CO2 Concentration

Three feasible strategies to reduce CO2 emissions are exhibited by the modified Kaya identity as expressed in equation (1) ^[28]. They are namely, (i) improving the energy efficiency of coal-fired plants ^[33][34], (ii) change of the fossil fuels to renewable and carbon-free energy resources ^[35], and (iii) utilization of carbon capture and storage (CCS) technologies ^[28][36][37].

where CD: CO2 emissions, P: Population, GDP: economic development in gross domestic production, E: energy production, C: carbon-based fuels used for energy production, and SCO2: CO2 sinks ^[28]. Apart from the above-mentioned three strategies, enhancing partial pressure in exhaust gas ^[36], geoengineering approaches including afforestation and reforestation ^[38], flue gas separation, and carbon mineralization ^[39] can also be considered. Among the different CO2 mitigation options, IPCC has suggested CCS as a promising technology for achieving a 19% reduction of global CO2 emissions by 2050 ^[34]. CCS can reduce CO2 emissions (typically 85–90%) from significant stationary point sources such as power plants, cement kilns, and NG wells ^[40][41]. Nevertheless, CCS is considered a mid-term solution in reducing global warming, climate change, and simultaneously allowing humans to continue using fossil fuels until a renewable and clean energy source is discovered to replace them ^[34]. CCS is comprised of three significant steps, namely, (i) capture of emitted CO2 from power plants and industrial processing without releasing them into the atmosphere, (ii) transportation of the captured and compressed CO2, and (iii) underground storage of the captured CO2 ^[26][42][43]. However, the process of CO2 capture, which accounts for 70–80% of the total cost, has proven to be the major barrier for the deployment of CCS ^[40][44]. Interestingly, in recent years, carbon capture storage and utilization (CCSU) has grabbed significant attention compared to CCS owing to the convertibility of the captured CO2 into commercial products ^[45][46]. The success of CCS and CCSU technologies are associated with the CO2 adsorption efficiency, ease of handling, manufacturing cost, and renderability of the associated materials ^[22].

1.5. CO2 Emission Sources

The CO2 emission sources are the primary candidates for potential applications of CCS or CCSU technologies. Therefore, from a community and industrial point of view, CO2 capture from typical gas streams, including flue gas, biogas, flare gas, syngas, and ambient air, has grabbed significant interest ^[47]. Table 1 depicts the summary of the compositions of different gas streams.

Table 1.

Compositions of different gas streams which act as potential CO2 capture opportunities (Reprinted with permission from ref.

^[47][48]

Component	Cement Rotary Kiln	Dry Atmospheric Air	Biogas Generated from Waste Water Treatment Plant Sludge	Natural Gas Fired Flue Gas	Coal-Fired Flue Gas
N2	59 vol %	70 vol %	0–1 vol %	73–80 vol %	70–80 vol %

^]. The working conditions such as pressure and temperature differ for each technique ^[51]. The main factors impacting CO2 capture efficiency are the gas composition, gas stream temperature, and energy penalty associated with regeneration ^[28].

Table 2.

Comparison of the three main carbon capture technologies.

CO2 Capture Technology	Advantages	Disadvantages
Pre-combustion capture
CO2	19 vol %	410 ppm

2 onto a material occurs through different types of interactions between the gas molecules and the adsorbent. Adsorption can be classified as (i) physisorption or (ii) chemisorption ^[59]. CO2 adsorption is an exothermic process as reported elsewhere ^[60][61]. Figure 1 presents the schematic of the two adsorption processes, while Table 3 tabulates the differences between physisorption and chemisorption.

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Figure 1. Schematic of the interactions between gas molecules and the adsorbent surface during physisorption and chemisorption (Reprinted with permission from ref. ^[62]).

Table 3.

Comparison of the CO2 physisorption and chemisorption processes.

Process	Advantages	Disadvantages
	The concentration of CO2 produced within these processes range from ~15–60% which makes it easy to capture ^[51]
Physisorption	More appropriate for high pressure applications ^[63] Adsorbent is easily regenerated, and low energy is required for desorption ^[10] Relatively stable even past 200 °C ^[10] Low cost for adsorbent preparation ^[64]	When applying to new power plants, the technology is not yet commercialized and requires a high capital investment due to major alternatives to be done into boiler and flue gas systems ^[28] Process of gasification and water gas shift reactions are expensive and quite challenging ^[51]	CO₂ capture capacity decreases with increasing temperature 2 capture capacity decreases with increasing temperature ^{[15][1565][65]} Low CO2 uptake at low pressures ^[47] High energy penalty associated with regeneration of chemical solvents ^[52]
		Low CO 2 selectivity for combustion flue gas streams ^[42] Adsorption capacity decreases in the presence of water ^[62]	Oxy-fuel combustion	Avoids the requirement of chemicals or other means of CO2 separation from flue gas ^[52
Chemisorption		^] A corrosive environment could be produced during the regeneration of spent adsorbent due to the presence of amine groups ^[ ⁵⁹ ^]

2.2. Different Regeneration Strategies

The attached CO2 molecules onto the adsorbent surface could be regenerated through the (i) pressure swing adsorption (PSA), (ii) temperature swing adsorption (TSA), (iii) vacuum swing adsorption (VSA), (iv) pressure and vacuum swing adsorption (PVSA), and (v) electric swing adsorption (ESA) processes ^[26][28][73]. Table 5 shows the advantages and disadvantages of different regeneration strategies. The regeneration method depends on the chemical and structural properties of a given adsorbent ^[69].

Table 4.

Comparison of different regeneration strategies.

Regeneration Strategy	Advantages	Disadvantages
Temperature swing adsorption (TSA)	Simple in operation ^[69] Can use low-grade heat from power plants ^[74]	Long heating and cooling time periods ^[69] Longer desorption time than PSA ^[28] Higher energy requirement than PSA ^[28] Rapid adsorbent deactivation due to coking at higher temperatures ^[28]
19–33 vol %	3–8 vol %	11–15 vol %
^]		High selectivity towards CO2 due to strong interactions between basic species on the adsorbent surface and the acidic CO2 molecule ^[42][66] High adsorption capacity at low CO₂ partial pressures such as in the ambient air 2 partial pressures such as in the ambient air ^[42
Pressure swing adsorption (PSA)	Lower energy requirement than TSA ^[75] ^]^[4267]^[6768]^[68] Easy operation ^[75] Enhanced adsorption capacity in the presence of water ^[64]^[69] Low capital investment than TSA and VSA ^[75] Comparatively higher mechanical stability ^[45]	Applicability over a wide range of temperatures and pressures ^[ ^76] Large energy penalty requirement for providing pure oxygen ^[53] Slower than the physisorption process ^[70] Absence of complete preparation methods ^[	Compression of the flue gas streams ^[69] ^54] Functionalization of porous materials with amine groups decreases the CO₂ capture capacity due to pore blockage 2 capture capacity due to pore blockage ^[66][6671 Dilute gas streams may result in intense energy consumptions during PSA ^[72]^]^[71] Pure oxygen is expensive ^[52] High energy requirement for regeneration of the adsorbent ^[72] Limited knowledge regarding the technology ^[53] Low cyclic stability due to amine degradation ^[66]. Higher cost associated with adsorbent synthesis ^[64] Environmental impacts associated are higher due to energy intensive air separation process ^[52]	H
		Chemisorbents can permanently bind to gases such as SO 2 to decrease the capacity of active sites for CO2 capture ^[ ⁷² ^] Grafted amines volatilize and degrade above 120 °C due to instability at higher temperatures ^[72	2O
Electric swing adsorption (ESA)	13 vol %	-	-	Post-combustion capture	Readily applicable for large-scale in newly built and existing power plants without upgrading and reconstruction ^[55]	More economical than TSA and PSA ^[28] Repairing does not discontinue the procedure of the entire power plant and it can be regulated or managed easily ^[56] Independent purge gas flow ^[69] Shorter time required for creation ^[57]	Fast heating and cooling rates ^[69] Low energy consumption ^[69] Requirement of huge energy supplies for sorbent regeneration ^[53]	7–14.6 vol %	Requires the separation of impurities from captured CO2 ^[58]	5–12 vol %
		Further improvements are required before commercialization ^[28] CO₂ in the flue gas is diluted with a concentration ranging from 10–15% which requires high recovery and capital costs and 25–35% additional energy for plant operation ^[28] The adsorbents should have good electrical conductivity ^[692 in the flue gas is diluted with a concentration ranging from 10–15% which requires high recovery and capital costs and 25–35% additional energy for plant operation ^[28]	^]	O2	7 vol %	21 vol %	<0.5 vol %	4.5–15 vol %	3–6 vol %
Vacuum swing adsorption (VSA)	Applicability in large point sources ^[69]	SO2	5–1200 ppm	-	-	<10 ppm	200–4000 ppm
SO3	-	-	-	-	0–20 ppm
		NOX	100–1500 ppm	-	-	50–70 ppm	200–800 ppm
CO	-	-	-	-	50–100 ppm
H2	-	0.5 vol %	-	5–300 ppm	5–20 g/m³
Particulate matter	-	-	-	-	-
H2S	-	-	100–4000 ppm	-	-
Ar	-	0.9 vol %	-	-	-
Xe	-	0.1 vol %	-	-	-
	Ne	-	18 ppm	-	-	-
He	-	5.2 ppm	-	-	-
CH4	-	1.6 vol %	60–75 vol %	-	-
Kr	-	1.1 vol %	-	-	-
N2O	-	0.3 vol %	-	-	-

1.6. CO2 Capture Technologies

Table 2 depicts the comparison of the leading carbon capture technologies. According to Table 2, carbon capture from power plants in industries can be classified as (i) pre-combustion capture, (ii) oxy-fuel combustion, and (iii) post-combustion capture ^[49] depending on the combustion method and composition of the gas stream ^[50

2. Solid Adsorbents for CO2 Capture

2.1. Adsorption Process of CO2

Adsorption is a surface phenomenon that highly depends on surface properties and functionalities ^[50]. Adsorption of CO

Energy intensive operation

2.3. Criteria for Selecting CO2 Adsorbents

When synthesizing and selecting an effective CO2 adsorbent, the material should be economical and operational simultaneously ^[74]. Therefore, a prospective CO2 adsorbent should satisfy the following criteria (Table 5): (i) CO2 adsorption capacity: The adsorption capacity plays a vital role since it determines the amount of adsorbent to be inserted into the adsorption column to attain the desired performance ^[77][78], (ii) Regenerability: The adsorbent should be fully regenerable and require relatively mild conditions for complete regeneration ^[78], (iii) CO2 selectivity: The adsorbent should display substantially high selectivity for CO2 in the co-presence of other species (e.g., N2, methane (CH4), sulfur dioxide (SO2), hydrogen sulfide (H2S), and moisture) ^[74][79][80], (iv) Adsorption/desorption kinetics: A rapid adsorption/desorption is required for swing adsorption to decrease the cycle time ^[73][74], (v) Thermal, chemical, and mechanical stability: During the cyclic regeneration process, the microstructure and morphology of the adsorbent should be retained. Moreover, the adsorbent should withstand harsh operating conditions, including vibration, high temperatures, pressures, and flow rates. Additionally, the amine-functionalized adsorbents should be resistant against oxidizing agents and contaminants such as sulfur oxides (SOX), nitrogen oxides (NOX), water vapor, and heavy metals ^[11][81], and (vi) Adsorbent cost: The adsorbent should be synthesized using cheap raw materials while adopting a cost-effective and energy-saving synthesis routes ^[62].

Table 5.

Threshold values of criteria for selecting an effective CO2 adsorbent (Reprinted with permission from refs.

^[74][77]

Parameter	Requirement
CO2 adsorption capacity	3–4 mmol/g
Regenerability	>1000 cycles
CO2 gas selectivity over other gases	>100
Adsorption/desorption kinetics	>1 mmol/g.min
Adsorbent cost	$5–15/kg sorbent

2.4. Different Adsorbents for CO2 Capture

Numerous studies on CO2 capture conducted in academic and industrial settings have developed promising adsorbents possessing the requirements demonstrated in Table 5 ^[55]. A variety of adsorbents have been discovered and synthesized, including MOFs, zeolites, activated carbons, zeolite imidazolate frameworks (ZIFs), grafted and impregnated polyamines ^[44], activated alumina, carbonized porous aromatic frameworks (PAFs), covalent organic frameworks (COFs) ^[82][83], porous organic polymers (POPs) ^[33], mesoporous silica, carbon nanotubes ^[84], metal oxides, ionic liquids ^[85], phosphates ^[28], and molecular sieves ^[5].

2.5. Importance of Carbon-Based Adsorbents for Effective CO2 Capture

Of the previously mentioned CO2 adsorbents, though zeolites and well-ordered frameworks exhibit high CO2 adsorption capacities at relatively lower pressures ^[39], the CO2 adsorption performance gradually decreases in the co-presence of moisture ^[34][86]. Similarly, molecular sieves and silica gel also demonstrate decreased CO2 adsorption performance in the co-presence of moisture ^[5]. Additionally, the usage of MOFs has been severely limited due to structural collapse upon vacuum treatments ^[34], contact with acid gases, thermal regeneration ^[84], and their complex and expensive synthesis procedures ^[86]. The ionic liquids are also unfavorable for practical applications due to their relatively high operational costs and high viscosity, leading to corrosion-related problems ^[87]. On the other hand, the application of carbon materials in the day-to-day lives of human beings can be traced back to more than 5000 years when the early humans discovered charcoal formed through the incomplete combustion of wood. Interestingly, many carbon materials have been discovered, such as graphene, fullerene, activated carbons, graphite, carbon foams, biochar carbon nanotubes, and carbon aerogels ^[88]. The carbon-based materials can be used as appropriate candidates in catalysis, electronics, fuel cells, biology, metal recovery, and gas storage and separation ^[27][88][89]. Among the aforementioned wide range of applications, carbon-based porous materials can serve as appropriate candidates for CO2 capture due to their advantageous, including low production cost ^[27], competitive CO2 adsorption performance at a given pressure ^[39][90], easy synthesis, ease of scaling up ^[88], wide availability, controllable pore structure, high thermal stability ^[15], good chemical resistance against alkaline and acidic media ^[91], fast adsorption kinetics ^[44], lower regeneration energy requirements ^[86], high apparent density (0.3 g/cm³) ^[92][93], high surface area ^[94][95], environmental benignity ^[85], favorable surface chemistry ^[96], selectivity ^[66], and flexibility for heteroatom doping or surface functionalization ^[97]. Additionally, the high thermal and chemical conductivity of carbon-based materials can be exploited for thermal, electric, and pressure swing adsorption strategies ^[92].