Carbon-Based Adsorbents for Postcombustion CO2 Capture: A Critical

Critical Review pubs.acs.org/est

Carbon-Based Adsorbents for Postcombustion CO2 Capture: A Critical Review Anne Elise Creamer and Bin Gao* Department of Agricultural and Biological Engineering, University of Florida, Gainesville, Florida 32611, United States ABSTRACT: The persistent increase in atmospheric CO2 from anthropogenic sources makes research directed toward carbon capture and storage imperative. Current liquid amine absorption technology has several drawbacks including hazardous byproducts and a high-energy requirement for regeneration; therefore, research is ongoing to develop more practical methods for capturing CO2 in postcombustion scenarios. The unique properties of carbon-based materials make them specifically promising for CO2 adsorption at low temperature and moderate to high partial pressure. This critical review aims to highlight the development of carbon-based solid sorbents for postcombustion CO2 capture. Specifically, it provides an overview of postcombustion CO2 capture processes with solid adsorbents and discusses a variety of carbon-based materials that could be used. This review focuses on low-cost pyrogenic carbon, activated carbon (AC), and metal−carbon composites for CO2 capture. Further, it touches upon the recent progress made to develop metal organic frameworks (MOFs) and carbon nanomaterials and their general CO2 sorption potential.

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INTRODUCTION There is a continued rise in greenhouse gas concentrations in the atmosphere, which causes severe problems, such as increases in the frequency of extreme weather events.1 These phenomena are greatly due to the increased release of CO2 through human activities. Research that aims to reduce CO2 release is imperative, as it accounted for 82% of all anthropogenic greenhouse gas emissions in the US in 2013.2 There are numerous ways to reduce anthropogenic carbon footprint, from increasing household energy efficiency to sequestering carbon in soils;3 some studies even focus on capturing CO2 from ambient air.4 Because the combustion of fossil fuels to produce electricity contributes to 37% of all anthropogenic CO2 released, power plants have been proposed as important locations for CO2 capture.2 Currently, chemical absorption is the most applicable technology for CO2 capture in power plants, but has several drawbacks (i.e., corrosion rate and high energy regeneration requirements).5−7 Alternative technologies using other absorbents, adsorbents, and membranes have been studied for their ability to capture postcombustion CO2.8−10 Because flue gas has different properties, such as CO2 concentration, temperature, and pressure, it is important to be able to design sorbents with a variety of applications.11 Four main characteristics are considered when evaluating postcombustion CO2 capture materials: capture efficiency, sorption rate, energy required for regeneration, and volume of sorbent.5,11 Bhown et al.12 evaluated a diversity of postcombustion CO2 capture technologies and assigned them a “Technology Readiness Level”, similar to that developed originally by NASA. Table 1 gives an overview of the progress that has been made in developing three categories of postcombustion © 2016 American Chemical Society

Table 1. State of Postcombustion CO2 Capture Developmenta commercialization operational confidence primary source of energy penalty a

absorbent

adsorbent

membrane

high high regeneration

moderate high regeneration

low low/moderate compression

Adapted from ref 12.

CO2 capture materials. Out of the 120 postcombustion CO2 capture technologies they assessed, the majority are absorption processes (60%).12 It is necessary to utilize regenerative materials, rather than chemicals that must be disposed of, as the global supply of sorbent chemicals would be depleted; hence, the biggest challenge in applying absorption processes for postcombustion CO2 capture is to reduce the heat of regeneration. 12−14 In addition to the high energy of regeneration associated with chemical absorbents, another problem is the release of hazardous byproducts.15,16 For these reasons, solid adsorbents are considered promising for the capture of postcombustion CO2.5,17−20 CO2 capture by solid adsorbents typically relies on the mechanism of physical adsorption, or weak bonding between CO2 and the adsorbent surface, which in turn results in lower regeneration energy. Numerous promising solid adsorbents have been developed that possess desirable characteristics, such Received: Revised: Accepted: Published: 7276

February 9, 2016 May 27, 2016 June 3, 2016 June 3, 2016 DOI: 10.1021/acs.est.6b00627 Environ. Sci. Technol. 2016, 50, 7276−7289

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Figure 1. CO2 sorption process using a fluidized bed reactor.29

as high adsorption capacity, insensitivity to moisture,21 property variability, low cost, 13 good selectivity for CO2 , easy regeneration, and good reusability.22 Some examples of solid physical adsorbents include metal−organic frameworks, zeolites, and carbonaceous materials such as activated carbon23 and engineered carbon nanomaterials.11,24,25 When evaluating solid physical adsorbents, it is important to consider their surface area, density, pore size and volume, feasibility of regeneration, production method, stability, and abundance and sustainability.26 Studies based on carbonaceous adsorbents focus on high partial pressure CO2 capture scenarios, as the extent of physical adsorption usually increases with increase in gas pressure.27 The general focus of this review is to summarize the progresses made in developing carbon-based solid sorbents for postcombustion CO2 capture. Specifically, this work will provide overviews of (1) postcombustion CO2 capture processes with solid adsorbents, (2) low-cost pyrogenic carbon in CO2 capture, (3) activated carbon (AC) in capturing CO2, (4) the development and applications of metal−carbon composites for CO2 capture, (5) recent progress made to develop metal organic frameworks (MOFs) for CO2 capture, and (6) carbon nanomaterials and their general CO2 sorption potential.

pressure and vacuum swing adsorption (PSA and VSA, respectively) are possible designs for CO2 capture from flue gas. In temperature swing adsorption, CO2 is adsorbed at low temperature and desorbed by raising the temperature.28 Figure 1 illustrates a postcombustion CO2 capture system using temperature swing adsorption. The flow gas is propelled by a blower, cooled, and bubbled through one or more fluidized beds. The adsorbent is circulated through the cooling reactor; when it becomes rich with CO2, the adsorbent is pumped into a separate tank, where the CO2 can be desorbed at higher temperature and rise into an overhead baghouse. The adsorbent then recirculates through the initial fluidized reactor.29 Krutka et al.29 considered process designs that would capture 90% CO2 from flue gas in a coal-fired power plant at low CO2 partial pressure, using temperature swing adsorption. They chose a bubbling fluidized bed reactor to be the optimal design, as this design had the most efficient heat and mass transfer, and therefore provided the optimal configuration for effective CO2 capture. The optimal CO2 adsorbent was chosen to be an amine-based solid adsorbent, as it provided high CO2 working capacity under the influence H2O vapor.29 Nevertheless, other solid adsorbents are promising for thermal swing adsorption processes and will be discussed further in later sections of this review. Because CO2 sorbents need to be recycled, when assessing potential CO2 sorbents for large-scale applications, it is important to consider the regeneration temperature.29,22 Hasan et al.,30 who assessed the suitability for zeolite 13X in VSA, found that when the CO2 concentration makes up more than 15−20% of gas in the stream, it is more cost-effective to use VSA than traditional MEA-based absorption. Pressure

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CO2 CAPTURE PROCESS WITH SOLID SORBENTS Temperature swing adsorption, pressure swing adsorption, and vacuum swing adsorption are three processes considered for CO2 capture. Different infrastructure and different adsorbent materials would be required depending on the type of adsorption cycle. Temperature swing adsorption (TSA) and 7277

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technology and process, and power plant engineers, who can integrate these materials into postcombustion infrastructure, work alongside each other in order to reach a technology breakthrough in postcombustion CO2 capture.12

swing adsorption (PSA) and vacuum swing adsorption (VSA) use pressure change, within an adiabatic system, to remove CO2 from the flue gas stream. Typically, PSA and VSA are carried out in parallel vessels. While one adsorbs, the other desorbs. The cost of pressure swing adsorption using solid adsorbents is comparable to that of conventional MEA absorption technologies; however, at this time, the purity is lower.31 A PSA cycle involves the use of several vessels to compress and cool gas. Although it is beneficial to remove impurities such as SOx, water vapor, and NOx prior to the PSA/VSA process (via dehydration, desulfurization steps, and molecular sieves),31 Chaffee et al.32 found that the most important parameter for VSA, in determining the amount of CO2 captured, is the ultimate vacuum pressure, rather than the purity of the gas stream. A few example PSA and VSA methods include the Skarstrom cycle, Gemini Process, and Seven-step PSA cycle,31 but these are difficult to evaluate due to their complexity. For this reason, Chung et al.33 developed the “short-cut” method to model more easily the PSA process (Figure 2). In this model, PSA

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LOW-COST PYROGENIC CARBON Low-cost pyrogenic carbon materials, such as charcoal, biochar, agrichar, carbonized biomass, and other labels of pyrogenic carbon materials have been used frequently in environmental remediation,36−38 for carbon sequestration,39,40 and as soil amendment.41,42 The diversity of names assigned to pyrogenic carbon reflects not only the diversity of characteristics upon the spectrum of pyrolysis but also their diversity of applications, from energy to agriculture.43 Bird and Ascough43 have shown the progression of the characteristics of pyrogenic carbon as it progresses from partly charred biomass to black carbon (Figure 3).

Figure 3. Pyrogenic carbon continuum; diagram based on Bird and Ascough.43

As the carbonization/pyrolysis temperature increases, so does the porosity and carbon content of the pyrogenic carbon and the product becomes less subject to demineralization from environmental influences.43,44 Figure 4 depicts the process of biochar production through slow pyrolysis, in which wet biomass is dried and the pyrolysis temperature is raised slowly (less than 100 K/min) under oxygen-starved conditions.45 The peak temperature at which pyrolysis is performed also affects the properties of the pyrogenic carbon, and therefore the subsequent application.46−48 The products of pyrolysis include syngas, bio-oil, and solid char.49,50 Fast pyrolysis to 808 °C of lignite, or “brown carbon” produces pyrogenic carbon with higher open-pore volume than that of slow pyrolysis.51,52 Fast pyrolysis produces chars of larger N2 and CO2 surface area; however, very little carbon is produced and the yield is made up almost entirely of volatiles.51,53 On the other hand, with slow pyrolysis, there is a much higher yield of carbon and the properties of this product are dependent on the starting carbonbased material.51,53,54 Low heating rates tend to produce biochar that largely consists of micropores; whereas, rapid heating produces macropores in the material.55 Another major limitation of producing char through rapid heating is its tendency to rapidly chemi-sorb to large amounts of O2 when the char comes in contact with air, which would reduce the surface area and active sites for sorption.51 When the material is undergoes carbonization/pyrolysis, the carbon content increases and the oxygen content decreases

Figure 2. Short-cut method of VSA/PSA process.31

systems rely on batch sorption vessels with a fixed volume. The processes of pressurization and adsorption and the processes of depressurization and desorption are combined. CO2 is adsorbed at higher pressure and desorbed at lower pressure, under adiabatic conditions. The adsorption and desorption can be modeled by the Langmuir isotherm. When designing the system, the feed temperature and concentration of gas in the stream must be considered. In regards to the apparatus, working parameters include the bed length, effective working length, internal diameter, wall thickness, and adsorbent load in each bed. When assessing adsorbents for pressure or vacuum swing adsorption, the adsorbent properties that are often considered include: bulk density, micropore volume, particle density, and skeletal density.34 For this reason, carbonaceous materials, like AC are promising adsorbents for PSA and VSA systems.35 It is suggested that chemists, who can design and produce the materials, process engineers, who can model the separation 7278

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Figure 4. Pyrolysis of biomass to biochar and bioenergy; diagram based on Brown, Wright, and Brown.45

Table 2. Chemical Analysis of a Selection of Low Cost Pyrogenic Carbon Materials raw olive stones pyrolyzed olive stones BC300 BC500 BC600 raw sludge sludge300 sludge500 sludge700 digested bagasse char bagasse char digested SB tailing raw SB tailing digested SBT char SBT char raw hickory wood raw bagasse raw bamboo hydrochar HW HW300 HW600 hydrochar bagasse BG300 BG600 hydrochar bamboo BB300 BB600

pH

BET SA

C

H

O

N

source

6.2 8.7 8.28 9.49 10.05 4.42 5.32 7.72 12 10.9 7.7 na na 9.95 9.45 5.8 6 6.6 6 7.1 8.4 5.7 7.3 7.5 6.3 7.9 9.2

na na 0.56 181 381.5 na na na na 17.66 14.07 na na 336 2.6 0.1 0.4 na 2.5 na 401 3.8 5.2 388.3 1.3 1.3 375.5

51.6 93 68.48 86.66 90.71 32.3 25.6 20.3 20.4 73.55 76.45 33.94 36.06 30.81 50.78 45.51 45.82 46.52 53.52 69.13 81.81 53.32 69.5 76.45 55.27 66.2 80.89

6 2.2 5.5 3.2 2.26 4.47 2.55 0.88 0.51 2.41 2.93 4.53 3.43 1.38 2.08 6.17 6.25 6.11 6.13 4.85 2.17 6.23 4.2 2.93 6.18 4.7 2.43

41.7 4.5 25.71 9.74 6.7 18.36 8.33 0.65 0 24.04 19.83 46.89 55.82 39.87 36.7 47.83 47.18 46.89 39.84 24.36 14.03 39.93 24.36 18.33 38.27 27.72 14.87

0.2 0.3 0.31 0.4 0.33 3.27 3.32 2.13 1.2 na 0.79 2.35 1.23 2.74 1.83 0.15 0.36 0.2 0.25 0.39 0.73 0.44 0.9 0.79 0.21 0.4 0.15

56 56 153 153 153 154 154 154 154 155 155 156 156 156 156 46 46 46 46 46 46 46 46 46 46 46 46

significantly.46,49 This transformation produces a material with higher surface area and higher pH.56,57 Just like with the pyrolysis of other biomass materials, as the temperature of pyrolysis increases, some functional groups are removed as volatile matter.49,51,57 Table 2 shows the chemical breakdown of the char material from the raw biomass. The abundant minerals, surface functional groups, and high surface area of pyrogenic carbon enable its sorption of contaminants, such as heavy metals.37,58 Similar properties and sorption mechanisms are desirable in a CO2 capture material. Variations in feedstock materials correspond to variations in the properties of the pyrogenic carbon. It is important to develop materials that provide minimal diffusional resistance for the adsorbate, until adsorption in the meso or micropores. For example, coal chars have a tree system where macropores branch into many super micropores.59,60 Before carbon-based materials are subjected to gasification or pyrolysis, materials tend to have very narrow pores. Under this condition, adsorption is based on activated diffusion, where activation energy is required so that the gas can diffuse. After pyrolysis, the pore diameter increases and sorption follows Knudsen (or bulk) diffusion principles. Here, the rate of gas transport is limited by diffusion into and out of a feeder pore system of larger pores (i.e., mesopores).61−63

There are a few other ways of producing pyrolyzed-carbon at a low cost and zero net emissions, including microwave-assisted pyrolysis and hydrothermal carbonization. It was found that biochar produced by microwave-assisted pyrolysis has similar properties to conventionally produced biochar.64 Another way that biomass can be converted into pyrogenic carbon is by hydrothermal carbonization (HTC), or the production of raw HTC biochar or “hydrochar”.65 In this process, the biomass feedstock (e.g., bamboo, peat, cellulose, coconut, wood, etc.) is hydrolyzed and then is carbonized at temperatures less than that of dry pyrolysis.66−68 The benefit of this process is that there are less energy inputs because the drying process is eliminated.69 Plasma-assisted pyrolysis of biomass is another method of pyrogenic-carbon production, but not well studied in the literature. The resulting carbon has high surface area and micropore volume, making it promising for CO2 capture.70 Even without activation, pyrogenic biomass has been found to be highly effective in carbon dioxide capture.71−74 Studies indicate that the main mechanism for the capture of CO2 in biomass-derived pyrogenic carbon (i.e., biochar) is physical adsorption; they express the importance of high surface area on sorption capacity.75 The production of the biochar material, which uses agricultural, forestry, and waste biomass, produces 7279

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Shihong77 developed biochar by pyrolyzing cotton stalk biomass at 600 °C and reported that it has a CO2 adsorption capacity of about 58 mg g−1 at 20 °C; however, when the temperature was raised to 120 °C, the capacity is less than 20 mg g−1. The physical adsorption mechanism is highly desirable for postcombustion CO2 capture because of the requirement of relatively low temperature desorption of the CO2, which regenerates the material.13,27,78 When assessing the usefulness of char for CO2 adsorption applications, it is also important to consider porosity. As the carbon precursor is carbonized and further activated, the surface area increases; however, if the pore size is also enlarged, the result is a reduction in the interactions between the walls of the pores and the CO2 molecules. In this case, increases in surface area may correspond less with increases in CO2 uptake,79 as a pore diameter of 0.5 to 0.8 nm is optimal for CO2 sorption at atmospheric pressure and room temperature.80,81 Because the materials rely on physical adsorption, it is pivotal that the pyrogenic biomass has sufficiently high surface area, appropriate pore size and high pore volume.13,27,78 For example, Tan and Ani82 carbonized palm shell waste from palm oil production to produce a CO2 adsorbing material. The carbon precursor was crushed and dried at 120 °C and then pyrolyzed at 600−1000 °C under nitrogen flow; no further activation was performed. The material was then evaluated on its effectiveness as a carbon molecular sieve, rather than for adsorption capacity; nevertheless, the samples showed high selectivity and capacity for CO2 capture. When the material was pyrolyzed at 600 °C, the micropore surface area was 753 m2/g; it shows CO2 capture at 0.02 P/P0 to be near 100 mg/g at 298 K. When the pyrolysis temperature increased to 1000 °C, the micropore surface area and the pore volume decreased; however, the selectivity toward CO2 increased.82 To increase the CO2 sorption capacity, pyrogenic biomass can be modified with nitrogenous functional groups. For example, Xiong and Shihong77 modified the biochar with NH3. When the NH3 modified biochar was pyrolyzed at 600 °C, the material captured more CO2 than the unmodified one at 120 °C. This could be due to the introduction of nitrogencontaining functional groups that improve the capture of CO2 at higher temperature. On the other hand, at a CO2 flow of only 20 °C, the unmodified biochar performed better than the NH3 modified one.77 Hydrochar has also had success in CO2 capture. Recently, Sevilla and Fuertes83 produced porous carbon CO2 capture materials through hydrothermally treating polysaccharides and biomass. Although the pristine hydrochar showed low sorption ability to CO2, they found that chemically activating the material with KOH significantly increased the amount of micropores and therefore its CO2 capture capacity.83 In summary, low-cost pyrogenic carbon derived from biomass, itself, has substantial potential as a CO2 capture material. It has been shown that biochar and other pyrogenic carbon materials can be produced to have high enough surface area for the employment of CO2 physisorption, as well as natural basic surface properties and adequate pore size that allow for selectivity of CO2.13,71,72,84

more hydrogen than it consumes, making it a net energy producer for the power plant.76 Plaza and Pevida56 developed low-cost solid CO2 sorbents out of olive stones. The biomass was pyrolyzed at 600 °C to increase surface area and carbon fraction. The pyrolyzed product (GKOS) has CO2 capture capacities that are comparable to the activated samples at 25 °C and is reported to be higher that other commercial ACs. The significant sorption capacities are attributed to the effect that the formation of basic sites and favorable pore size distribution that are formed during the carbonization process.56 Creamer et al.71 produced biochar from sugarcane bagasse and hickory wood that was pyrolyzed under N2 flow at temperatures between 300 and 600 °C to capture CO2. The biochar produced from sugarcane bagasse feedstock at 600 °C had the highest CO2 capture capacity (73.55 mg g−1 at 25 °C and 101 kPa). Even when the feedstock was pyrolyzed at 300 °C, it still was able to capture 38.72 and 34 mg g−1 (for the bagasse and hickory-based biochar, respectively). A major benefit of using adsorbents produced simply by slow pyrolysis of biomass is the ease of regeneration. Figure 5 indicates the relationship

Figure 5. Temperature dependence of CO2 adsorption for sugarcane bagasse biochar.71

between temperature and CO2 adsorption for biochar produced from sugarcane bagasse.71 Because of the physical adsorption mechanism, CO2 is captured at low temperatures and desorbed at higher temperatures. Figure 5 also shows that the biochar produced by sugarcane bagasse releases CO2 by 92% when the temperature is raised to 120 °C (Figure 5).71 Day et al.73 demonstrated that a novel, nitrogen-rich, biochar material produced through pyrolysis that is capable of capturing CO2 within its pore structures. Similar to the biochar produced by Creamer et al., this sustainable material can even play the role of a carbon-sequestering, slow-release fertilizer. The mechanism of physical adsorption, or the weak bonding of CO2 to the surface of the material, is based on Henry’s law, which indicates that molecules are adsorbed under low temperature and high pressure and are desorbed at high temperature and low pressure. For example, Xiong and

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ACTIVATED CARBON Activated carbon (AC) can also be derived from biomass through pyrolysis, but requires either physical or chemical activation. The feedstock of AC production can range from waste biomass, such as spent coffee grounds,85 to nitrogen-rich 7280

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Environmental Science & Technology chitosan,86 to synthetic organic polymers,87 to chicken feathers.23 To prepare chemically activated carbon, it is necessary to introduce chemicals, such as acid, base, or salt to treat the feedstock biomass. Preparing physically active carbon usually involves two steps: (1) convert biomass into pyrogenic carbon at relatively low temperature conditions, and (2) activate the carbon with steam, CO2, O2, or other gases at high temperature conditions.88−90 AC can also be made from natural charcoal materials such as coal and lignite.91,92 Numerous studies focus on AC production using a diversity of activation techniques to optimize the capacity for CO2 sorption.93−95 The sorption affinity of AC is comparable to that of other proposed CO2 capture materials, such as zeolites. The benefit of AC for postcombustion CO2 capture tends to be its low heat of adsorption and lower relative cost.96−98 Physical activation can be performed using steam/water vapor, air, or CO2. For example, A.S. Gonzalez used olive stones (OS) and almond shells (AS) that were subjected to single-step activation at 800 °C with CO2 to capture CO2. Under 101 kPa, the OS-based carbon was able to adsorb 4.8, 3.0, and 0.7 mmol CO2 g−1 at the temperatures of 0, 25, and 100 °C, respectively. At 25 °C and 15 kPa, the capture is reduced to 1.1 mmol g−1, demonstrating the importance of temperature and pressure on physical CO2 adsorption. Plaza et al. also used almond shells and olive stones as a basis for the production of a stable CO2 trapper; however, rather than activating the biomass with CO2, the biomass was heated under a mixture of gaseous N2 and O2 (3%) at 650 °C. The ACs derived from almond shells and olive stones were able to adsorb 2.11 and 2.02 mmol g−1, respectively.99 AC produced from olive stones has been shown to not only preferentially capture CO2 but also water vapor, over N2 and O2. Fortunately, when water vapor is incorporated into the gas stream, H2O coadsorbs with CO2 and there is no significant reduction in the AC’s capacity for CO2. Regeneration of both CO2 and H2O can be accomplished by raising the temperature to 150 °C.100 Carbon can also be chemically activated by various chemicals to increase the surface area as well as add (or remove) specific surface functional groups. When carbon is activated with ammonia at high temperatures, nitrogenous groups are added and acidic oxygen groups are removed, which significantly improves basicity.101 Functional groups, such as amide, imide, pyrrolic, pyridinic, and lactame groups are formed through reaction with nitrogen-containing reagents, such as NH3, nitric acids, and amines.100,102 Acidic functional groups can be removed by heat treatment, as oxygen-containing functional groups will decompose at 800−1000 °C.101 Although most of the chemically activated carbons are produced from feedstock pretreated with various chemical agents, several studies have also explored the activation of pyrogenic carbons with chemicals. Wang et al.103 prepared AC from celtuce leaves with KOH as the activation agent at 600 °C. The activation process is shown in Figure 6. The dry feedstock is carbonized and then activated with KOH. This produces a product capable of efficient ion exchange. Sevilla et al. produced a chemically activated carbon by hydrothermally carbonizing polysaccharides and biomass at 600 °C and then activating this precursor with KOH. This led to an extensive production of the narrow micropores in the carbon material.83 It is also common to activate the surface of carbon with acids, such as nitric or hydrochloric acid, especially when the goal is to adsorb a basic contaminant (i.e., NH3).104 Oxygen groups, such as carboxyl, lactone, and phenol, can be incorporated on the

Figure 6. Production of activated carbon from biomass with KOH activation.103

surface of the AC. The material can be oxidized with gas or aqueous solution; high temperature leads to the incorporation of weak acid groups, such as phenol. Acid digestion, with HNO3, HCl, and HF, is also used in order to remove ash from samples and to concentrate any unburned carbon.105 Low temperature activation produces strong acid groups, such as carboxylic acid. By this treatment, hydrophilic groups on the material are removed.100,102 CO2 is an acidic molecule, so acid activated carbon is less effective for CO2 capture. Interactions between AC and CO2, including H−H bonding, dipole−dipole interactions, and covalent bonding, are enhanced with the incorporation of basic surface groups. The carbon structure itself acts a lewis base and can absorb protons from solution. Further, the introduction of nitrogenous groups onto AC surfaces is known to increase the basicity of the material, and therefore increase its ability to sorb CO2.100,102 Although nonactivated celtuce leaf porous carbon is able to capture efficiently CO2, when the carbon was activated with KOH at 600 °C, its CO2 adsorption capacity became significantly higher. The CO2 capture at 0 and 25 °C are shown in Figure 7. The material is also highly selective for CO2 over N2.103 The AC produced by Sevilla et al.83 showed high CO2 capture ability (4.8 mmol g−1) due to the prevalence of weak interactions between the CO2 and the carbon surface. These weak bonds make it possible for efficient regeneration.83 As the bond strength (measured by heat of sorption) between the AC surface and CO2 increases, so does the energy needed to regenerate the adsorbent. Although chemically activated carbons tend to have higher heat of sorption than nonactivated porous carbons (Figure 8), it still tends to be lower than that of other adsorbents, such as zeolites.106 An assortment of AC materials for CO2 capture are represented in Table 3. Activation with KOH appears to enhance greatly capture, whereas activation with HCl is less effective. Fan et al.106 optimized the mixture ratio of chitosan:K2O3 and the activation temperature with amines. Zhao et al.107 showed that the addition of p-diaminobenzene to furfural alcohol enhances the capture of CO2. Most of these labproduced ACs are significantly more effective than the commercial ones (such as that produced by Alfa Aesar Co.),108 which capture about 2.1 mol CO2 per gram adsorbent. The table also shows that higher activation temperature does not always correspond with higher CO2 sorption capacity, and the choice of activation agent is important. In summary, when producing an AC for CO2 capture, many conditions must be considered. The feedstock, activation agent and conditions, as well as the addition of extra solvents and chemicals can be optimized. The extensive diversity of AC 7281

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metal organic frameworks (MOFs), which are typically developed from pure, high-cost materials. Most synthesis methods for metal oxyhydroxide carbon are designed to produce a composite with high surface area and good dispersion of nanosized or colloidal oxide or hydroxide on the surface within the porous carbon network. When used for CO2 capture, the mechanisms of both chemisorption (from the addition of some metal oxyhydroxides) and physisorption (from increased surface area of the structure) may be involved in the process. Typically, samples with added metals, such as Ca, Mg, Al, Fe oxides and hydroxides, will have capacities for CO2 that are higher than those of the unmodified samples. Nevertheless, it was found that there is typically an optimal metal content to incorporate, as too much will decrease the surface area,112 and therefore active sites for bonding. Metal oxyhydroxides, themselves, have been shown to be effective CO2 trappers. For example, Pierre-Louis et al. studied the ability of Al/Fe oxyhydroxide to capture CO2.111 There is an optimal ratio of the metal oxyhydroxides in the composite; the addition of AlOOH to the composite enhanced the CO2 sorption until the ratio of Al to Fe reached 30%. In this study, both physical adsorption (from the high surface area and basic nature of mesoporous carbon and metal oxyhydroxides), and chemical adsorption (from the presence of metal oxyhydroxides that chemically bind with CO2) are present within the composite and contribute to CO2 sorption.111 Creamer et al.78 produced carbon−aluminum, magnesium, and iron oxyhydroxide composites from hickory wood biomass. The ratio of metal to biomass was varied to determine the concentration to lead to the highest sorption of CO2. The materials were produced in solution, dried, and pyrolyzed at 600 °C. It was found that the aluminum hydroxide−hickory composite with a AlCl3:cottonwood (CW) biomass ratio of 4:1 captured 71 mg g−1 CO2, which was higher than the other metal composites and the unmodified hickory biochar (58 mg g−1). The results of the kinetics experiment, performed at 25 °C are displayed in Figure 9.78 The capture of CO2 by the composites follows second-order kinetics. It can be seen that the represented metal-composites (FeCW, MgCW, AlCW) are all more effective at capturing CO2 than the unmodified cottonwood sample (CW(0)); however, even the unmodified sample had comparable adsorbent capacity.78 SEM-EDS imaging showed the presence of the metal particles on the surface of the carbon. When the surface area and metal concentration of the metal hydroxide composite was compared to the sorption capacity, it was elucidated that both the metal concentration in the composite and the surface area impact the CO2 sorption capacity.78 Several studies have focused on producing MgO/C composites to capture CO2 at both high and low temperatures.109,117−119 Bhagiyalakshmi110 carbonized a mixture of magnesium nitrate along with other solvents (HCl, sucrose, H2SO4, H2O) after evaporation. This led to the production of a thermally stable mesoporous supported MgO that is capable of sorbing up to 92 mg g−1 CO2 at 25 °C. Only a low concentration of metal was required; the material could be fully regenerated at low temperature (200 °C).110 Liu et al.120 developed MgO nanoparticles that were stabilized by mesoporous carbon. MgCl2 was obtained in low concentrations from seawater, mixed in solution with biomass, and pyrolyzed to produce an efficient CO2 capture material. The excellent CO2 capture is attributed to the porous structure along with the presence of basic −OH groups. The high surface

Figure 7. CO2 capture by celtuce leaf porous carbon.103

Figure 8. Heat of adsorption for AC-2-635 as CO2 is adsorbed.106

materials available for evaluation is valuable, as the material properties can be decisively altered to fit a specific application scenario.

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METAL OXYHYDROXIDE−CARBON COMPOSITES The addition of metal oxides and/or hydroxides (i.e., oxyhydroxides) to the surface of porous carbon adsorbents has been shown to enhance CO2 capture ability by increasing surface area, increasing the basicity of the material, and even promoting the production of carbonates with the reaction of CO2.109−111 These composite materials can be derived from the pyrolysis of either biomass feedstock along with a metal oxide or hydroxide,112,113 or biomass pretreated with precursors of metal oxyhydroxides,38,114−116 making them lower cost than 7282

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Environmental Science & Technology Table 3. AC Adsorbents for CO2 Capture name

moles gas/kg sorbent

celtuce leaf AC 3C-1000N H-600 AC-2-635 AC-2-800 IBN9-NC IBN9-NC-A IBN9-NC1-A AC-1-600 AC-1-800 nitrogen rich porous carbon

4.36 3.25 1.73 3.86 3.08 2.21 3.68 4.5 3.57 2.78 3.2

feedstock and carbonization method celtuce leaf at 600 °C nitrogen doped OMCs at 1000 °C nitrogen doped OMCs at 600 °C chitosan/K2CO3 = 2 chitosan/K2CO3 = 2 p-diaminobenzene p-diaminobenzene p-diaminobenzene, with furfural alcohol chitosan/K2CO3 = 1 chitosan/K2CO3 = 1 terephthalaldeyde and amine polymer pyrolyzed at 600 °C

Figure 9. CO2 sorption kinetics of metal oxyhydroxide biochar.78

activation temperature

activation agent

source

600 °C 1000 °C 600 °C 635 °C 800 °C none 600 °C 600 °C 600 °C 800 °C 180 °C prior to pyrolysis

KOH NH3 HCl amines amines none KOH KOH amines amines amines

103 157 157 106 106 107 107 107 106 106 158

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METAL ORGANIC FRAMEWORKS MOFs are promising adsorbents in the field of CO2 capture for postcombustion scenarios. Because of their huge potentials in various applications, several review papers discuss their synthesis14 and CO2 capture applications.14,121,122 This review, therefore, only provides a brief overview of the potential application of MOFs in CO2 capture. With the use of pristine components, it is possible to select for specific structural and chemical characteristics of MOFs. In other words, it is possible to “tune” or alter MOF properties to fit a specific CO2 capture scenario.123 Altering the pore dimension and surface chemistry can help optimize the capture CO2 or other gases.124−128 For example, isoreticular metal organic frameworks (IRMOF) have a tunable pore size and are made up of bridging metal-based nodes (i.e., Al3+, Cr3+, Cu2+, Zn2+) with organic linking groups (i.e., carboxylate and pyridyl) through strong coordination bonds to produce a 3D organic− inorganic network. The structure of an example metal organic framework is depicted in Figure 10.129 The fully assembled MOF-14 (Figure 10E) is broken into 5 different components of the framework (Figure 10A−D) for clarity. For all MOFs, it is necessary to optimize the production conditions to fit the necessary situation. Altering the concentration of the reactants, pH of the solution, the ratio

area is said to result from the dehydration and decomposition of MgCl2, which enhances the release of volatile matter in the pyrolysis process. Lastly, the presence of the MgO crystal facilitates the formation of carbonate (MgCO3), indicating both physical and chemical mechanisms of adsorption.120 Li et al.118 used carbon as a support for the dispersion of fine MgO particles. The carbon-based composite was capable of high temperature (150 °C) CO2 capture that was higher than MgO−SiO2 composites. The high sorption is attributed to the basicity of the samples and hierarchal structure of the carbon, which led to large surface area. Desorption was possible at temperatures around 230 °C.118 In summary, the biggest benefit of using carbon-based oxyhydroxide composites is that the material can call upon the use of not only enhanced physical adsorption but also, in some cases, the mechanism of chemical adsorption. When the biomass feedstock is pyrolyzed with the metal oxyhydroxides or the precursors, not only is the surface area increased but also additional functional groups may be added to the carbon surface, which contributes to enhanced physical adsorption. Further, chemical sorption, from the interaction of the metal oxyhydroxides and CO2, is an added benefit. 7283

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Figure 10. Metal organic framework structure.129 Single-crystal structure of Cu3(BTB)2(H2O)3·(DMF)9(H2O)2(MOF-14) composed of (A) square paddle-wheel and triangular BTB SBUs, which assemble into (B and C) a pair of augmented Pt3O4 nets that are held together by (D) numerous π−π and C−H···π interactions to yield (E) a pair of interwoven three-dimensional porous frameworks. Cu, red; C, gray or blue atoms not linked directly to Cu; O, gray atoms linked directly to Cu.129

mechanical, and chemical properties. They are typically produced through self-assembly or polymerization followed by direct carbonization. Self-assembly relies on hydrogenbonding interactions between the carbon precursors and block copolymer surfactants.135 Dai et al. showed that using phenolresorcinol-/phloroglucinol-based phenolic resins allows the synthesis of monoliths, fibers or particles. Nanospheres can be functionalized easily in bulk or on the surface through template or self-assembly.137 The synthesis of porous carbon nanofilms requires a monomer-block copolymer film casting, structure refining through solvent annealing, polymerization of the carbon precursor, and carbonization.135 Carbon nanocomposites are carbon-containing nanoparticles that are functionalized to enhance their properties, such as conductivity.138 They are typically produced through heat treatment after heat impregnation onto a silica or metal oxide matrix. Another typical way to produce these carbon nanomaterials is through “nanocasting”. This is when a nanosized mold is filled with a carbon precursor that, when the template is removed, is left with pores where the template once was and results in a porous carbon framework.139 Silica is frequently studied as a hard template for the preparation of mesoporous carbon. First, the porous template is prepared with a controlled porosity, then the carbon precursor undergoes wet impregnation, polymerization, and pyrolysis of the precursor to generate the composite. Lastly, the inorganic template is removed. Porous carbons can also be replicated from MgO nanoparticles.135 Carbon composites are shown to be efficient in the capture of CO2 from flue gas.140 A study by Meng and Park used spherical nanosilica templates to produce carbon materials with enhanced capacity for CO2 capture. Increasing the ratio of silica to polystyrene-based resin increased the pore volume and surface area, which yielded higher CO2 capture.141

of the metal-to-ligand, reaction temperature, and reaction times can provide the necessary changes in the properties of the material. The production of these MOFs, which includes sonication-assisted synthesis,130 microwave heating,131 mechanochemical procedures,132 photochemical133 or electrochemical synthesis,134 is difficult and costly at large scale. Because of the low stability of MOFs, slight perturbations to the chemical or structural properties, which might occur in a densely packed adsorbent bed or under mechanical pressure, can considerably impact its performance in CO2 capture scenarios.14 Regardless, MOFs have potential to be used in numerous applications, including postcombustion CO2 capture, so current research is working to develop and synthesize a huge diversity of these adsorbents.13

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CARBON NANOMATERIALS Carbon nanomaterials, or carbon materials with one or more dimensions less than 100 nm, can take on many different forms. They possess a large diversity of properties, leading to their use in many applications, including catalyst supports, energy conversion and storage, filtration, and sorption.135 This review will only touch upon the specifics of carbon nanomaterials as a CO2 capture material. Carbon nanomaterials can be synthesized in bulk and possess a hierarchical porous structure and contain both macropores and micropores, which is important for CO2 capture as the macropores allow for low resistance pathways and high surface area to access micropores.135 The challenge of synthesizing carbon nanomaterials through polymerization is preventing aggregation.136 Individual carbon nanomaterials, including fullerenes, carbon nanotubes, graphene, and carbon nanofilms or nanofibers have been highly studied because of their unique thermal, electrical, 7284

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zeolites, as well as other pristine solid and chemical sorbents. Metal−carbon composites have enhanced CO2 capture ability, while being less expensive to produce than MOFs. The addition of metal oxides and/or hydroxides gives them increased functionality and surface area. Extensive research has also been done toward developing synthesized metal organic frameworks (MOFs) and carbon nanomaterials for CO2 capture. These materials, although more intensive to produce, have relatively high capacities for CO2. Overall, research on carbon-based adsorbents for postcombustion CO2 capture is still in its infancy. To promote this technology, further investigations are needed in many areas, such as (1) Improving the adsorption ability of pyrogenic carbon: pyrogenic carbon materials are typically renewable, inexpensive to produce, have low heat of regeneration,71 and hydrophobic properties.143,144 Research should be conducted to optimize their production processes and to understand further the interaction mechanisms for optimal CO2 capture, particularly at the field scale. (2) Develop novel composites: adsorption ability of pyrogenic carbon can be improved by impregnating metal oxyhydroxides or other CO2 capture agents. Further research is needed to prepare novel carbon-based composites, particularly carbon nanocomposites for CO2 capture. (3) Development of nanotechnology: the properties associated with carbon nanomaterials enhance reactions associated with CO2 adsorption, conversion, and binding;145 however, relatively few studies have been performed to assess the ability of carbon-based nanomaterials.22,146−150 It is necessary that future studies consider the unique properties of carbon-based nanomaterials and expand their applications into the field of CO2 capture and storage. (4) Regeneration and reuse of CO2: with the increase in CO2 capture, it is critical to develop new technologies to store CO2151 or convert it to useable materials, such as methane.152

3D carbon nanomaterials, including carbon-gels and other porous structures, can be used for numerous applications in adsorption. To produce carbon aerogels, the sol−gel process is typically used. Here, molecular precursors are transformed into high cross-linked organic gels. Another way to produce these structures is through direct transformation, which can be performed through carbonization of crystalline polymers or MOFs.135 Alhwaige et al. produced biobased chitosan hybrid aerogels decorated with graphene oxide nanosheets that were able to capture between 1.92 and 4.14 mmol g−1 CO2 with high stability.142 Carbon nanomaterials, although they might be more intensive and expensive to produce than other carbon materials, can be produced precisely for optimal CO2 capture with high selectivity and capacity. As shown in Table 4, carbon Table 4. Carbon Nanomaterials for CO2 Capture material TiO2/GO nanocomposite CTS-GO-15 (chitosan GO composite) PEI/graphene/silica sheets GODC4-600 (graphene oxide derived carbon) N-HCS (hollow carbon nanosphere) carbon nanotube CNT (APTS) (modified carbon nanotube) polypyrrole functionalized graphene sheets CS3-6A (nitrogen enriched porous carbon spheres) CS3-6A (nitrogen enriched porous carbon spheres) N-doped carbon aerogel N-doped carbon aerogel 14ACA-900 carbon aerogel

CO2 capture (mmol g−1)

conditions

source

1.88 3.96

25 °C, 1 atm 25 °C, 1 atm

25 142

3.9 8.9

75 °C, 1 atm 27 °C, 20 atm

159 160

2.67

25 °C, 1 atm

148

2.2 2.6

25 °C, 0.5 atm 20 °C, 0.5 atm

90 161

4.3

25 °C, 1 atm

162

4.1

25 °C, 1 atm

163

6.2

0 °C, 1 atm

163

3.6 4.5 3

25 °C, 1 atm 273 K, 1 atm 25 °C, 1 atm

164 164 165

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AUTHOR INFORMATION

Corresponding Author

*B. Gao. Phone: (352) 392-1864 ext. 285. Email: bg55@ufl. edu. Notes

nanostructures have very high CO2 capture ability (e.g., functionalized graphene sheets capture 4.3 mmol g−1 at 25 °C, 1 atm), high thermal stability, and good regeneration.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the NSF through Grant CBET-1054405.

CONCLUSIONS AND FUTURE DIRECTIONS Flue gas from power production results in the emission of significant amounts of CO2, which can be separated and captured using a variety of liquid and solid sorbents. The most common liquid absorption process requires costly regeneration and produces hazardous byproducts, so this critical review focuses instead on the use of solid carbon adsorbents for postcombustion CO2 capture. Pyrogenic carbon materials have potential for use in industrial applications because of their low cost of production and good regeneration. Even without activation, biomass-derived pyrogenic carbon is capable of physically adsorbing significant CO2. The pyrolysis process gives materials extensive surface area, which provides more active sites for CO2 bonding through physical adsorption. Further, basic functional groups tend to line the surface, which enhances electrostatic attraction. Activated carbon materials have been extensively researched as the conditions of activation, incorporation of solvents, and use of different biomass sources contribute to the addition or removal of a variety of functional groups. The CO2 sorption capacity is comparable to that of

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