Investigation of Porous Silica Supported Mixed-Amine Sorbents for

Prospective post-combustion CO2 capture sorbents were prepared by the .... the porous silica surface can condense with APTES, in forming Si–O–Si n...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/EF

Investigation of Porous Silica Supported Mixed-Amine Sorbents for Post-Combustion CO2 Capture D. J. Fauth,* M. L. Gray,* and H. W. Pennline U.S. Department of Energy, National Energy Technology Laboratory (NETL), 626 Cochrans Mill Road, P.O. Box 10940, Pittsburgh, Pennsylvania 15236, United States

H. M. Krutka,* S. Sjostrom, and A. M. Ault ADA Environmental Solutions, 8100 South Park Way, Unit B, Littleton, Colorado 80120, United States ABSTRACT: Prospective post-combustion CO2 capture sorbents were prepared by the immobilization of a low-molecularweight, branched polyethyleneimine (PEI) and 3-(aminopropyl)triethoxysilane (APTES) within a commercially available porous PQ Corporation CS-2129 silica support to investigate (i) CO2 adsorption properties of the supported mixed-amine (PEI +APTES) sorbents in both pure CO2 environments and simulated flue gas conditions, (ii) their thermal and hydrolytic stability over numerous adsorption and desorption cycles, and (iii) their equilibrium and kinetic adsorption behavior. Initial CO2 adsorption−desorption measurements via thermogravimetric analysis (TGA) were conducted in pure CO2 to measure dry, nearequilibrium CO2 adsorption capacities, together in calculating amine efficiencies, which was recognized in being a meaningful criterion in evaluating sorbent performance for selecting the “most favorable” mixed-amine (PEI+APTES) composition. The asprepared materials containing various weight ratios of PEI to APTES showed less uptake of CO2, relative to the supported PEIonly impregnated material under investigated TGA experimental conditions. Nitrogen adsorption−desorption isotherms in evaluating the physical properties of the synthesized mixed-amine (PEI+APTES) samples showed reduced values specific to surface area, and total pore volume largely predictable from the successful incorporation of PEI multilayers into the structure of the porous silica matrix, together with unreacted APTES moieties remaining behind after material synthesis. Breakthrough curves produced by (PEI-15-APTES-35)-PQCS2129, (PEI-25-APTES-25)-PQCS2129, (PEI-35-APTES-15)-PQCS2129, and (PEI-50)PQCS2129 showed mean near-equilibrium CO2 adsorption capacities of 1.81 ± 0.17, 2.43 ± 0.26, 2.44 ± 0.19, and 2.44 ± 0.45 mol CO2/kg of sorbent, respectively, over multiple CO2 adsorption−desorption cycles utilizing a 10% CO2, 8% H2O (balance, He stream) at 60 °C and 1.01 bar for adsorption; followed by regeneration in a He stream containing 90 vol% water vapor at 105 °C. From these studies, (PEI-25-APTES-25)-PQCS2129 and (PEI-35-APTES-15)-PQCS2129 exhibited a higher CO2 capturing efficiency (absorbed amount of CO2 per gram of PEI), relative to (PEI-50)-PQCS2129, indicating the PEI/APTES interface (i.e., interaction between layers of surface alkyl chains associated with APTES and PEI) is perhaps contributing to improving the deposition/dispersion of PEI, thereby decreasing the diffusion resistance with regard to CO2 entering into the bulk of the PEI multilayers. Conversely, the lower amine efficiency of (PEI-50)-PQCS2129 can be ascribed to the possible clustering of the PEI molecules from the higher PEI loading, resulting in a decrease of accessible amine sites and creating a higher diffusional resistance in connection with CO2 molecules penetrating into the majority of layers of PEI. Near-equilibrium CO2 adsorption measurements of (PEI-25-APTES-25)-PQCS2129 in utilizing the laboratory-scale, fixed-bed flow reactor system located at ADAES (Littleton, CO) displayed ranges of 2.70−3.45 mol CO2/kg sorbent at 40 °C under different CO2 partial pressures. The (PEI25-APTES-25)-PQCS2129 material showed a relatively stable performance over many adsorption−desorption cycles (i.e., >250 cycles) under humidified simulated flue gas conditions, along with a higher amine efficiency relative to the (PEI-50)-PQCS2129 sample (“PEI-only” sample). In fitting the experimental data of ADA-ES, the Langmuir isotherm model was determined to be an acceptable representation of the observed thermodynamics.

■

INTRODUCTION Direct capture of anthropogenic carbon dioxide (CO2) from fossil-fuel-based electricity generation utilities is of considerable importance, because of the apprehension from both the scientific community and public relating to the ever increasing CO2 concentration in the atmosphere.1−3 For many decades, the United States and international countries have generated substantial amounts of CO2 from coal-fired power plants that are emitted into the Earth’s atmosphere without any regulatory framework in place.4 Comprehensive carbon capture and storage (CCS) programs5 that initiate research, development, © 2012 American Chemical Society

and demonstration activities relating to the capture, compression, transport, and long-term storage (i.e., sequestration) of CO2 from large-point stationary sources have undertaken various solutions to potentially mitigate CO2 emissions, thus allowing nations such as China, the United States, and India to continue to utilize their vast fossil fuel resources and existing energy infrastructure. However, retrofitting the existing fleet of Received: October 13, 2011 Revised: February 28, 2012 Published: February 29, 2012 2483

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496

Energy & Fuels

Article

Figure 1. Temperature swing CO2 capture process at a coal-fired power plant.

in the aqueous solution can be increased beyond 30 wt % MEA equivalent. A comprehensive study7 performed by NETL concluded that coal-burning power generation sources using a CCS technology, as a retrofit or new installation, may not exceed a 35% increase in the cost of electricity (COE) over existing rates, based on DOE goals. This benchmark has created new imperatives for the development of cost-effective and efficient CCS technologies to mitigate CO2 emissions from fossil-based power plants. Today, MEA-based CO2 capture processes are expensive and increases the COE by over 80%,6 thus the need to develop more-efficient CO2 capture processes. Solid sorbents have the potential to reduce the energy requirements of the CO2 capture process by eliminating aqueous solutions and their attendant energy requirements (especially the heat associated with vaporization) while providing adequate capture and regeneration rates. The materials developed can be used in a process similar to that shown in Figure 1 (including the process conditions). Note that the greater the partial pressure of CO2 during regeneration, the lower the CO2 compression costs, which is another key component of the CO2 capture and sequestration energy penalty. Research activities worldwide have mainly focused on the development and evaluation of different solid sorbents for postcombustion CO2 capture. Comprehensive reviews of sorbent properties and general characteristics have been compiled.8 While many different physical adsorbents have been evaluated, such as zeolites9 or carbonaceous10−13 materials, such sorbents are often characterized by low working capacities under CO2 capture process conditions. Metal organic frameworks (MOFs) are often cited as materials that can be tuned for specific CO2 capture, but are in an early stage of development.14−17 Progress relating to chemical adsorbents is also extensive. Chemical adsorbents exhibit a larger heat of reaction; however, in many cases, they have greater CO2 working capacities and superior selectivity toward CO2 under the process conditions shown in Figure 1. Supported carbonates, usually sodium or potassium carbonate, have been studied for use in the postcombustion temperature range of 40−60 °C.18,19 While supported carbonates can be produced to exhibit equilibrium CO2 capacities greater than that of physical adsorbents, their heat of reaction (∼130 kJ/mol CO2 to release the CO2 and H2O to convert sodium bicarbonate back to sodium carbonate during regeneration) is greater than that of amines (∼60−80 kJ/mol).18

power plants with current carbon capture technologies is perceived as an extremely costly proposition. The barriers to CCS include both technological and economical challenges; the development and full-scale commercial deployment of efficient and cost-effective CO2 capture and sequestration technologies is recognized as being necessary in mitigating CO2 emissions, protecting our environment for future generations. To date, available commercial processes for selective CO2 adsorption mostly utilize technologies based on chemical absorption by aqueous alkanolamines, including monoethanolamine (MEA) or mixtures of primary (e.g., MEA), secondary (e.g., diethanolamine (DEA)), and tertiary (e.g., methyldiethanolamine (MDEA)) amines. Although MEA or mixed aqueous alkanolamine solvents are used industrially to remove CO2 from natural gas, it has yet to be translated into a viable commercial process for post-combustion CO2 capture. In this chemical absorption process, an aqueous MEA solution contacts the flue gas in an absorber; and, upon absorption, the primary and secondary amines react rapidly with CO2 to form carbamic acid (H2NCO2H), which reacts with a second MEA molecule to form a carbamate salt. The addition of a purely physical solvent (e.g., water) aptly increases the CO2 absorption capacity. However, the formation of the carbamate ions is typically associated with a relatively high heat of adsorption, making the cost of regenerating the primary and secondary amines extremely high. Tertiary amines lack the N− H bond required to form the carbamate ion. However, in aqueous solutions, tertiary amines promote the hydrolysis of CO2 to form bicarbonate and protonated amines. The CO2laden solutions are subsequently heated in a separate regenerator to release CO2. A generic temperature swing CO2 capture process that could be utilized within a coal-fired power plant is illustrated in Figure 1; the typical process conditions (temperature and CO2 partial pressure) are included for background. The process conditions provided in Figure 1 are similar for most post-combustion CO2 capture technologies, including liquid and solid amine capture processes. For aqueous aminebased CO2 capture, the sensible heating and the latent heat of evaporation are significant components of the large energy penalty. Even with the implementation of a rich/lean heat exchanger to reduce the sensible heat input required, the solvent thermal regeneration energy is estimated to be ∼3550 kJ/kg CO2 (1530 Btu/lb CO2).6 However, it is possible that, with further development, the effective concentration of amine 2484

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496

Energy & Fuels

Article

Figure 2. Reaction diagram illustrating the post-grafting of 3-aminopropyl groups, along with physical impregnation of PEI in the porous PQ CS2129 silica substrate at ca. 80 °C. Solvent: anhydrous ethanol. Introduction of spatially isolated 3-aminopropyl and silanol groups.

as high adsorption capacity, fast kinetics, enhanced selectivity toward CO2, excellent thermal and hydrolytic stability, oxidative stability, and moderate regeneration conditions. This present work reports on the collaborative efforts at the U.S. Department of Energy, the National Energy Technology Laboratory (NETL), and ADA Environmental Solutions (ADA-ES) in materials development and testing of potential CO2 capture sorbents comprised of a large-pore-volume silica matrix (with adequate surface area) immobilized with various weight ratios of 3-(aminopropyl)triethoxysilane (APTES) and polyethylenimine (PEI). The commercially porous silica matrix has rapid adsorption (due to relatively large pore diameters), excellent selectivity, and good mechanical stability. Furthermore, the abundant hydroxyl groups present within the porous silica surface can condense with APTES, in forming Si−O−Si networks. This one-step approach in functionalizing porous silica surfaces using PEI and APTES together in anhydrous ethanol will provide, in a simplistic way, the insertion of spatially spaced site-isolated amines and silanol groups.43 Figure 2 illustrates the plausible reaction for post-grafting of 3aminopropyl groups onto the porous silica matrix together with the simultaneous addition of PEI. By providing a sufficient number of silanol groups in forming covalently attached aminosilane layers coupled with PEI incorporation within the interior pores of the silica matrix, the formulated samples were examined with the expectation in enhancing the sorbent’s CO2 adsorptive performance, improving their thermal and hydrolytic stability, and their equilibrium, and kinetic behavior. The immobilization and covalent attachment of PEI, with APTES as a chemical linker within a porous silica matrix is anticipated to provide superior CO2 sorption performance over hundreds of thermal swing adsorption/regeneration cycles. Development of a practical, one-step, scalable synthesis, suitable for the production of commercial quantities of these sorbents is particularly attractive in advancing key process performance and design issues related to post-combustion CO2 capture.

Scores of academic, governmental, and private organizations have implemented research strategies dedicated to developing, testing, and progressing materials characterized by amine functional groups supported on a solid substrate.20−24 The multilayer’s of amine confined within a porous support behaves comparable to its liquid alkanolamine counterparts, in that they take up appreciable quantities of CO2 reversibly in utilizing pressure, temperature, or vacuum swing adsorption processes. Impregnating a high-surface-area, large-pore-volume substrate, such as MCM-41,25,26 SBA-15,27 and KIT-6,28 with polyethylenimine (PEI) exhibits moderate-to-high equilibrium CO2 capacities (in a range of 2−4 mol/kg sorbent under 100% CO2 and lower pressures).29−33 Song et al. introduced the molecular basket concept, in loading easily controlled quantities of a CO2philic polymer/oligomer onto the mesoporous molecular sieve MCM-41 and studied their CO2 adsorption properties.25,26 Sayari, et al. has considerably investigated the CO2 adsorption properties (i.e., thermal and hydrolytic stability) of aminegrafted pore-expanded MCM-41 mesoporous silica, in addition to PEI-impregnated PE-MCM-41 samples.34−37 Exceptional stability was demonstrated, although the temperature was not varied during the stability evaluation (i.e., only the CO2 partial pressure was varied to promote adsorption and regeneration). Note that water vapor was necessary during the regeneration step for the sorbents to remain stable over the 700+ cycles completed.36,37 Another approach in stabilizing amine functional groups was undertaken by Jones, et al., where hyperbranched aminosilicas were employed.38,39 Jones et al. demonstrated that primary and tertiary amines were stable under oxidative conditions.40 Although grafted and hyperbranched amines demonstrated improvements over sorbents where amines are simply impregnated within an inorganic substrate, there is reservation that, as the complexity of the sorbents increases, the number of required processing steps in sorbent processing, and their associated costs, will also continue to increase. Ma et al. describes the estimated cost in utilizing mesoporous silica molecular sieves MCM-41 or SBA-15 as the inorganic support will account for >90% of the absolute sorbent preparation cost, resulting in a cost greater than $700/kg.41 Furthermore, studies have also reported that PEI may not be completely stable on the surface of the support.42 Because of the required scale for CO2 capture, amine-functionalized and/ or amine-impregnated sorbents are required to be produced economically, along with establishing themselves as being environmentally benign during large-scale processing. Ideally, such materials must retain the ordered structure of the support during synthesis, along with possessing favorable attributes such

■

EXPERIMENTAL SECTION

Chemicals and Synthesis of Sorbents. Methanol was obtained from Sigma−Aldrich, USA and used without further purification. A high-purity silica substrate designated as CS-2129 was procured from PQ Corporation, USA. Low-molecular-weight branched polyethylenimine (Aldrich PEI, ethylenediamine branched, number-average molecular weight of Mn ≈ 600, as determined by gel permeation chromatography (GPC)), and 3-aminopropyltriethoxysilane (APTES), were purchased from Sigma−Aldrich USA. PEI and APTES were chosen for their availability and low cost. Important parameters dealing with materials development (i.e., high basicity, high N/C ratio, and 2485

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496

Energy & Fuels

Article

N2 (100 cm3 min−1), along with increasing the temperature to 105 °C. The loss of sample mass, representing the expulsion of CO2 from the sample was continuously recorded for an additional 120 min. To help quantify the mass of organic amine groups deposited onto the substrate in reality, the materials were subjected to thermogravimetric analysis (TGA), utilizing a Perkin−Elmer Pyris 1 thermogravimetric analyzer. Four milligrams of each of the investigated five supported mixed-amine (PEI+APTES) materials, together with the asreceived silica substrate, were separately charged into a platinum sample pan and heated in a pure N2 atmosphere (75 cm3 min−1) under a multiple temperature-programmed segment. Experimentally, a load temperature of 25 °C was preserved for 1 min, followed by the sample being heated to 65 at a rate of 5 °C min−1 and held isothermally for 20 min. After the expired time, the temperature was then increased to 100 °C and held for an additional 5 min. The final segment concluded in having the sample heated from 100 °C to a final temperature of 650 °C. The resulting weight loss profile in heating the mixed-amine (PEI +APTES) sorbents in an atmosphere of N2 to 650 °C provided support for quantifying the combination of PEI and APTES deposited/grafted onto the silica substrate, as illustrated in Figures 5 and 6. Thermal stability studies for mixed-amine (PEI+APTES), and “PEI only” samples were performed on a Perkin−Elmer Pyris Diamond differential scanning calorimeter. DSC experiments were conducted utilizing hermetically sealed aluminum pans with a pin-sized hole in the top. Samples were scanned under controlled heating rate at 10 °C min−1. ADA-ES. A Perkin−Elmer Pyris 1 TGA was employed to conduct CO2 uptake measurements at ADA-ES (Littleton, CO) under a range of several temperatures and CO2 partial pressures, utilizing the (PEI25-APTES-25)-PQCS2129 sorbent prepared at NETL. The CO2 partial pressure within the TGA assembly was controlled using CO2/N2 gas blends. A Hiden Analytical mass spectrometer was used downstream of the TGA exhaust to confirm the gas concentrations. A small quantity of moisture was introduced into the gas stream by passing a portion of the sample gas through a heat-traced bubbler during testing in Littleton, CO. Previous studies indicated completely dry conditions may lead to loss of amine reactivity.14,17 Moisture levels were calculated at ca. 1% by volume, minimizing its effect on the weight change of the sorbent. [Note: TGA experiments conducted at ADA-ES headquarters in Littleton, CO (elevation 1643 m) were operated at less than standard atmospheric pressure; therefore, even with 100% CO2, the atmospheric pressure was calculated to be 0.81 bar.] In a typical experiment, five milligrams of the mixed-amine (PEI +APTES) sorbent was initially pretreated in an atmosphere of pure N2 to 120 °C to remove residual solvent, adsorbed CO2 and moisture. After 120 min, the gas flow was then switched to the reacting gas, containing N2 (except when pure CO2 was used), CO2, and trace quantity of H2O, while the temperature was held at 120 °C The mass gain (or loss) of the sorbent was recorded, for this case, at intervals of 2 s. Gas flow conditions within the furnace were maintained for 90 min allowing the sorbent to approach equilibrium, after which the furnace target temperature was decreased to 110 °C. Again, the condition was held for an additional 90 min, after which the temperature was reduced to 100 °C. This specific procedure was repeated for target temperatures of 90, 80, 70, 60, 50, and 40 °C. Partial pressure measurements for the reactant gas (CO2) were studied at 0.04, 0.08, 0.50, and 0.81 bar. CO2 sorption isotherms were obtained at nine different temperatures from 40 and 120 °C at pressures up to 0.81 bar by ADA-ES in fitting the experimental data with the Langmuir isotherm, hence gaining a better understanding of the adsorption phenomena. Acquisition of relevant parameters associated with the Langmuir isotherm was derived. Fixed-Bed Flow Systems. NETL. The near-equilibrium CO2 capacity performance of the mixed-amine (PEI+APTES) sorbents was examined through breakthrough experiments. A schematic of the NETL fixed-bed flow system is shown in Figure 3, with Table 4 summarizing the experimental specifics. The experiments were conducted at 60 °C and 1.01 bar of

high molecular weight (>500 g/mol)) were considered in selecting the polymeric branched PEI. APTES was introduced with PEI in formulating amine-functionalized materials containing spatially distributed 3-aminopropyl- and silanol groups with the expectation of improving the deposition/dispersion of PEI, along with possibly tailoring these materials in being more “hydrophobic”. The hydrophobic character of the formulated materials was likely to increase, because of the effective inclusion of moderate concentrations of covalently bound amino-functional groups. For this study, supported mixed-amine (PEI+APTES) materials were prepared by a wet impregnation technique. In a typical preparation, the required amounts of PEI and APTES were dissolved in methanol, the dissolution of which was aided by mechanically stirring within a round-bottom flask. A predetermined mass of the silica support was then added to the solution and placed onto a rotary evaporator producing a slurry containing the organic constituents and inorganic silica. The flask carrying the resulting slurry was continuously turned at a high rpm rate under various stages of reduced pressure. The slurry was agitated for an additional 40−60 min above 80 °C until the solvent was drawn off. Physical properties of the PQ CS-2129 silica substrate are presented in

Table 1. Properties of Commercial PQ Corporation CS2129 Silica property

value/comment

specific surface area specific pore volume average pore diameter median particle size pore size distribution

319 m2/g 2.52 mL/g 316 Å 107 μm unimodal

Table 1. The flask containing the final product was then removed from the rotary evaporator for product characterization. The as-prepared, supported amine-functionalized materials were denoted as (PEI-x)PQCS2129, (PEI-x-APTES-y)-PQCS2129, and, (APTES-y)PQCS2129, where x and y represent the weight percent of PEI (or APTES) or the combined weight percentage of PEI and APTES before preparation (i.e., in solution) of the individual sorbents. The identification of the sorbents and the respective description is provided in Table 2. The purity of gaseous CO2, N2, and He was >99.8%, as determined by NETL and ADA-ES.

Table 2. NETL Classification of Amine (PEI or APTES) and Mixed-Amine (PEI+APTES) Sorbents sorbent classification

description

(APTES-50)-PQCS2129 (PEI-15-APTES-35)-PQCS2129 (PEI-25-APTES-25)-PQCS2129 (PEI-35-APTES-15)-PQCS2129 (PEI-50)-PQCS2129

“APTES only” sample mixed-amine sample,“PEI+APTES” mixed-amine sample,“PEI+APTES” mixed-amine sample,“PEI+APTES” “PEI only” sample

Thermogravimetric Analysis. NETL. CO2 uptake measurements for the synthesized materials were experimentally obtained using a Thermo Scientific ThermMax 300 thermogravimetric analyzer. In a typical experiment, a 30-mg sample was charged within a microbalance quartz sample bowl. Prior to CO2 adsorption measurements, the sample was pretreated in flowing N2 to 105 °C (flow rate: 100 cm3 min−1) at a heating rate of 5 °C min−1 and held isothermally for 120 min, to remove residual solvent, preadsorbed CO2, and moisture. After 120 min, the sample temperature was then cooled to 60 °C before the feed gas was switched to a pure dry CO2 stream. Near-equilibrium CO2 adsorption capacities were calculated based on the mass gain after 120 min in CO2 with reference to the initial sample mass after nitrogen (105 °C) activation. After CO2 sorption, the reactant gas was exchanged to a flow of pure 2486

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496

Energy & Fuels

Article

containing the mixed-amine (PEI+APTES) sorbent was quickly switched off-line and purged with helium to flush out any residual reactant gas, followed by switching back online with a high-purity helium stream for a short period of time (i.e., ca. 30 min) at 60 °C and 1.01 bar of pressure. During this set of experiments for the investigated mixed-amine (PEI+APTES) sorbents, eight adsorption−desorption cycles were performed. The CO2 adsorption progression consisted initially of two adsorption cycles under dry 10% CO2 in helium, followed by four additional cycles with a humidified reactant gas stream, immediately followed by two additional cycles identical to the first two cycles (i.e., 10% CO2, in helium). Regeneration of the mixed-amine (PEI+APTES) sorbent after each stage of CO2 adsorption was conducted by purging the sorbent bed in an atmosphere of dry or humidified helium after the target temperature was increased to 105 °C by an external wraparound heating belt. Equilibrium CO2 capacities were calculated from the breakthrough curves for each adsorption−desorption cycle. The run-to-run CO2 adsorption capacity performance (% loss in capacity) was evaluated, comparing the difference between consecutive adsorption-desorption cycles. After regeneration for 1 h, the column was then cooled under helium to an ambient temperature. ADA-ES. A schematic of the ADA-ES fixed-bed flow system is illustrated in Figure 4. The system, with exception of the heated water bubbler and CO2 analyzer, was fully enclosed and heated utilizing a finned heater, to avoid the condensation of liquid water. A CO2 analyzer was in continuous service during the breakthrough tests measuring the CO2 content in the effluent stream employing a NDIR sensor. A programmable logic controller (PLC) was used to operate a series of solenoid valves for controlling the flow of gases entering and exiting the fixed-bed flow system. In a typical experiment, there were several basic steps:

Figure 3. Schematic of the NETL laboratory-scale fixed-bed flow system. pressure utilizing either a dry or humidified simulated flue gas stream composed of 10% CO2, helium balance stream. In a typical experiment, ∼1.0 g of the silica-supported mixed-amine (PEI+APTES) sample was charged into a stainless steel column 301.6 mm in length and 7.1 mm in inner diameter. The sorbent was then heated to 105 °C overnight under a high-purity helium stream (100 cm3 min−1), to remove preadsorbed moisture, CO2, and/or residual solvent. The following day, the Omnistar mass spectrometry (MS) system was calibrated with the reactor off-line utilizing a stream of 10% CO2 in helium (or a humidified 10% CO2, helium balance stream containing 8 vol % water vapor). After charging and activating the sample bed overnight, the mixed-amine (PEI+APTES) sample was isolated and the simulated flue gas stream was permitted to flow through the fixed-bed flow system for an appreciable time (i.e., 30 min), ensuring that steady-state conditions have been reached. With the CO2 concentration being reasonably stable, the reactor containing the mixed-amine (PEI+APTES) sorbent was brought online. The gas residence time through the reactor was of the order of 1 s. The Omnistar MS system was employed to continuously monitor the effluent gas stream, expressed in terms of percent by volume of CO2. CO2 was considered broken through the sorption bed when its content reached 2 ppm by volume in the effluent stream. The sorption bed was considered saturated by CO2 when the concentration of gas in the effluent was identical to the inlet feed concentration. After saturation with CO 2 , the reactor

(1) initial purge (only completed for the first cycle) under N2 at 100 °C;

(2) baseline, during which simulated flue gas bypassed the sorbent, to allow for an initial measurement of the CO2 concentration;

(3) adsorption, where the simulated flue gas was passed through

the fixed bed of sorbent until the CO2 concentration reached the baseline level; (4) regeneration, moist N2 at 100 °C continued until no CO2 was measured from the effluent; and

Figure 4. Schematic of the ADA-ES sorbent screening test unit. 2487

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496

Energy & Fuels

Article

Figure 5. Thermogravimetric analysis (TGA) profiles of mixed-amine (PEI+APTES) sorbents with various PEI and APTES loading. Sorbents referenced to PQ CS-2129 support. Gas flow rate of pure N2 stream: 75 cm3 min−1.

Figure 6. DTA profiles of mixed-amine (PEI+APTES) sorbents with various PEI and APTES loading. Gas flow rate of pure N2 stream: 75 cm3 min−1. and Cin and Ceff are the associated concentrations. Integrating the equation from t = 0 to equilibrium time (t) when Cout reaches Cin provides the equilibrium CO2 adsorption capacity of the sorbent (q) (expressed in units of mmol CO2/g sorbent).

(5) cooling, which providing time for the sorbent to reach the adsorption temperature.

For long-term cyclic studies, steps 2−5 were repeated using the automated system for the designated number of cycles. The simulated flue gas (with a composition of 12% CO2, 4% O2, in N2) was initially saturated to 9 vol % water vapor by passing the flow gas through a heated water bubbler before entering the enclosed fixed-bed flow system. The temperature, gas composition, moisture level, and time for the key experimental steps are provided in Table 4. A separate series of experiments were also conducted without any sorbent material being present within the fixed-bed flow system, (i.e., blank experiment). Several adsorption and regeneration cycles were conducted to determine effects of free CO2 and free N2 remaining in the system tubing for computing equilibrium CO2 capacities through integration of measured adsorption profiles.

■

RESULTS AND DISCUSSION In this present work, a number of prospective CO2 capture sorbents were formulated, in which a commercially available, low-cost silica substrate was impregnated depositing various weight ratios of polyethylenimine (PEI) and 3-(aminopropyl)triethoxysilane (APTES) by means of a wet impregnation technique. Impregnation of a porous silica matrix with “CO2phillic” polymeric PEI is accredited in significantly improving CO2 adsorptive properties, as exemplified by “molecular basket”, MB-based sorbents.25,26 However, previous laboratory-scale fixed-bed flow system investigations at NETL, utilizing a porous silica substrate impregnated with different weight percentages of PEI, revealed both a significant uptake of water and a considerable loss of PEI resulting in noticeable sorbent degradation during multiple CO2 adsorption−desorption cycles under simulated regeneration streams containing 90 vol % water vapor in helium at 105 °C and 1.01 bar of pressure. For these reasons, a single-step material synthesis was devised, entailing “hydrophobic” APTES modification of the silica

The CO2 adsorption capacity was estimated at instantaneous time t (given in minutes) as

q=

1 m

∫ (Q inCinQ outCout) dt

where m is the mass of sorbent (in grams); Qin and Qout are the influent and effluent flow rates, respectively (in units of cm3 min−1); 2488

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496

Energy & Fuels

Article

support coupled with immobilization of PEI with the expectation in enhancing the sorbent’s CO2 adsorption performance, potentially improving their thermal and hydrolytic stability, and advancing their equilibrium and kinetic behavior. The immobilization and covalent attachment of PEI, with APTES as a chemical linker onto the silica support is anticipated to provide improved CO2 sorption performance over many hundreds of thermal swing adsorption/regeneration cycles. NETL Analyses. Thermogravimetric Analysis Testing of Mixed-Amine (PEI+APTES) Sorbents. The estimated quantities of PEI-, APTES-, and intermixed amino-containing entities, PEI, and APTES deposited on the PQ CS-2129 silica support were determined by heating the individual samples in pure nitrogen. The resulting weight loss profiles clearly show two major weight loss regions, as illustrated in Figures 5 and 6. For the “APTES only”, mixed-amine “PEI+APTES”, and “PEI only” samples, the first weight loss region appears at temperatures of 250 cycles). Although the testing conditions at ADA-ES were similar to those at NETL, some conditions varied slightly (see Table 4, Summary of Experimental ConditionsNETL and ADA-ES Fixed-Bed Flow System). The near-equilibrium CO2 adsorption capacities obtained from the long-term cyclic testing are shown in Figure 10. Similar to the evaluation at NETL, the fixed-bed tests completed by ADA-ES revealed an initial decrease in nearequilibrium CO2 adsorption capacity. However, beyond the

Figure 10. Multiple adsorption−desorption testing of the (PEI-25APTES-25)-PQCS2129 in a simulated humidified flue gas stream containing 12% CO2, 4% O2, balance N2, with 9 vol % water vapor. Adsorption temperature = 55 °C at 1.01 bar. Desorption temperature = 100 °C in nitrogen.

initial 50 adsorption−desorption cycles of the total 250 completed, the near-equilibrium CO2 uptake for the (PEI-25APTES-25)-PQCS2129 sorbent seems to be constant. This present result infers the (PEI-25-APTES-25)-PQCS2129 sample may be a potential candidate upon increasing its adsorption capacity with further testing employing multiple adsorption−desorption cycles by a combination of pressure/ temperature swing operation. Future investigations being considered by NETL and ADA-ES to determine if the initial loss in CO2 capacity can be avoided will require solutions entailing modifications to the sorbent synthesis process specifically involving (i) different aminosilane coupling agents, (ii) different solvent systems, and (iii) different synthesis conditions (e.g., reaction time and reaction temperature). As a final note, in keeping compression costs as low as possible in a viable, commercial-scale CO2 capture system, it is highly desirable to regenerate the sorbent under the highest possible CO2 partial pressure. Each option for increasing the working capacity will have associated costs. It is outside the scope of this work to determine optimal CO2 capture/ regeneration options. However, because solid, amine-functionalized and amine-impregnated sorbents are evaluated at increasing scales and complexity and the equipment/process options are taken into greater consideration, such evaluations become imperative. The data provided in Figure 11 illustrates 2492

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496

Energy & Fuels

Article

In this contribution, ADA-ES evaluated the (PEI-25-APTES25)-PQCS2129 sorbent in measuring the near-equilibrium CO2 sorption capacity at various CO2 partial pressures and temperatures. Specifically, near-equilibrium CO2 capacity measurements at temperatures between 40 °C through 120 °C in 10 °C increments at four specific partial pressures (0.04, 0.081, 0.5, and 0.81 bar) were completed. The near-equilibrium CO2 adsorption isotherms formulated for the (PEI-25-APTES25)-PQCS2129 sorbent within the temperature range of 40− 120 °C are displayed in Figure 11. The data points in Figure 11 were measured using TGA, calculated from eq 8 (discussed below), or calculated using linear interpolation. Note that linear interpolation was only used in the region where the isotherms were relatively linear (e.g., ≥0.4 bar CO2). The uptake of CO2 evolved rapidly at the lower spectrum of temperatures (40−70 °C), as illustrated by the filled symbols and lower CO2 partial pressures (0.08−0.15 bar). The resulting decrease in nearequilibrium CO2 sorption capacity, as a function of temperature, alludes to the exothermic nature of the adsorption process, whereas increasing the gaseous CO2 concentration leads to an increase in near-equilibrium adsorption of CO2 for the (PEI-25-APTES-25)-PQCS2129 sorbent. The near-equilibrium CO2 sorption capacities obtained for the (PEI-25APTES-25)-PQCS2129 sorbent at 40, 50, 60, 70, 80, 90, 100, 110, and 120 °C at 0.81 bar were 3.45, 3.30, 3.15, 3.02, 2.87, 2.70, 2.49, 2.26, and 1.99 mol CO2/kg sorbent, respectively (as tabulated in Table 5). The data described as “calculated” in Table 5 were derived using eq 8, while the points described as “interpolated” were derived using linear interpolation in the applicable regions (e.g., 0.4 bar CO2). The near-equilibrium CO2 capacity of the (PEI-25-APTES25)-PQCS2129 sorbent can be aptly described by a Langmuir adsorption isotherm, which is an expression of the partitioning between the gas phase and the adsorbed species, as a function of applied pressure. The Langmuir isotherm model is based on the assumptions of monolayer coverage of adsorbate occurring over homogeneous sites and a saturation point is reached where no further adsorption can occur. Utilizing a thermodynamic approach, derivation of the semiempirical Langmuir isotherm is based on the equilibrium equation described below; the absorption of CO2 from the bulk gas surrounding the sorbent particle taking place can be represented by the equation

Figure 11. CO2 sorption isotherms for (PEI-25-APTES-25)PQCS2129 sorbent at different temperatures and CO2 partial pressures. Experimental data is fitted to the Langmuir isotherm. PCO2 = 0.04−0.81 bar.

how regeneration at higher CO2 partial pressures will lead to a decrease in the overall CO2 working capacity compared to regeneration with steam or an inert purge gas (i.e., the equilibrium CO2 capacity measured in fixed-bed systems). For example, if CO2 capture occurred at 50 °C and 0.081 bar partial pressure and regeneration occurred at 120 °C and 0.81 bar partial pressure, the working capacity would be the difference between the equilibrium CO2 capacities at the respective adsorption and regeneration conditions (i.e., 2.7 mol/kg − 2.0 mol/kg = 0.7 mol/kg). Thermogravimetric Analysis Testing of (PEI-25-APTES-25)PQCS2129 Sorbent. For single and multicomponent systems, equilibrium data obtained by adsorption isotherms defines the relative amounts a specific material can adsorb at a defined pressure. In addition, acquiring a series of adsorption isotherms at defined temperatures permits the calculation of the (isosteric) enthalpy of adsorption, which is an essential parameter in evaluating the sorbent performance as it cycles between adsorption and desorption.

Table 5. Calculated and Measured Equilibrium CO2 Adsorption Capacity for the (PEI-25-APTES-25)-PQCS2129 Sorbent at Various Temperatures and CO2 Partial Pressures CO2 Capacity (mmol/g) source

PCO2 (bar)

40 °C

50 °C

60 °C

70 °C

80 °C

90 °C

100 °C

110 °C

120 °C

assumed calculated calculated calculated measured measured calculated calculated calculated calculated interpolated measured interpolated measured

0.00 0.005 0.010 0.020 0.040 0.081 0.100 0.150 0.200 0.300 0.400 0.500 0.655 0.810

0.00 1.06 1.62 2.20 2.70 2.86 2.96 3.11 3.18 3.27 3.31 3.35 3.40 3.45

0.00 0.49 0.86 1.36 2.45 2.66 2.74 2.91 3.00 3.09 3.16 3.22 3.26 3.30

0.00 0.40 0.70 1.15 2.21 2.44 2.56 2.73 2.83 2.93 3.02 3.10 3.13 3.15

0.00 0.30 0.54 0.92 1.94 2.22 2.33 2.52 2.63 2.75 2.84 2.94 2.98 3.02

0.00 0.22 0.40 0.71 1.64 1.98 2.11 2.31 2.43 2.56 2.64 2.72 2.80 2.87

0.00 0.15 0.28 0.51 1.31 1.65 1.78 2.01 2.15 2.30 2.39 2.49 2.59 2.70

0.00 0.07 0.14 0.26 0.80 1.24 1.38 1.62 1.77 1.96 2.09 2.22 2.36 2.49

0.00 0.01 0.03 0.05 0.21 0.67 0.78 1.00 1.16 1.39 1.66 1.93 2.10 2.26

0.00 0.00 0.00 0.00 0.00 0.16 0.19 0.28 0.35 0.48 1.03 1.57 1.78 1.99

2493

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496

Energy & Fuels S + A ↔ SA

Article

Keq =

[SA] [S][A]

Table 6. Constants of Langmuir Model for Measured Equilibrium CO2 Uptake of (PEI-25-APTES-25)-PQCS2129 Sorbent at 0.81 bar

(4)

where A represents the gas-phase CO2 molecules, S and SA are the vacant adsorption sites and occupied surface sites, respectively, and Keq is a thermodynamic equilibrium constant. In this case, the mass balance is [Stotal ] = [S] + [SA]

(5)

where [Stotal] is the total amount of surface adsorption sites. Combining eqs 4 and 5, along with assuming no changes to [SA], with respect to time, one obtains [SA] =

([Stotal ]/Keq)[A] (1 + [A]/Keq)

(7)

where qe is the equilibrium capacity of CO2 (defined as θ × qm, where qm is the Langmuir constant related to maximum adsorption capacity of CO2 and θ the fractional surface coverage (0 < θ < 1)), b is the Langmuir affinity constant related to the free energy of CO2 adsorption on the sorbent, and Ce the equilibrium CO2 concentration. Rearranging the above equation in terms of fractional coverage (θ = qe/qm) leads to θ=

bPCO2 1 + bPCO2

(8)

where PCO2 refers to the partial pressure of carbon dioxide. Assumptions specific to the Langmuir isotherm model were taken into account during evaluation of the (PEI-25-APTES25)-PQCS2129 sorbent. The enthalpy of adsorption (ΔHads) yields significant information related to the mechanism and properties of adsorption. The Langmuir isotherm model allows for the evaluation of (isosteric) heat of adsorption. In taking the van’t Hoff relation, which can be written as ⎛ ΔHads ⎞ ⎟ b = b0 exp⎜ − ⎝ RT ⎠

qm (g/dm3)

correlation coefficient, r2

40 50 60 70 80 90 100 110 120

2.09 1.69 1.37 1.08 0.85 0.60 0.32 0.09 0.02

3.44 3.32 3.20 3.09 2.94 2.80 2.73 3.28 6.24

0.996 0.996 0.995 0.995 0.996 0.995 0.998 0.994 0.080

combustion CO2 capture. The coefficient of determination values (R2) were found in the 0.998−0.994 range, with the exception of the 120 °C entry, confirming the belief that the CO2 Langmuir adsorption isotherms provided a reasonable representation to the measured CO2 adsorption equilibria. The constant qm, which represents the maximum CO2 adsorption capacity, remained constant at the temperatures studied, which is consistent with the assumptions taken for the Langmuir isotherm model. The affinity constant parameter of the Langmuir isotherm is associated with the enthalpy of adsorption, inferring that the (PEI-25-APTES-25)-PQCS2129 sorbent has a higher CO2 affinity at lower temperatures. To provide a better understanding of the adsorption properties of the (PEI-25-APTES-25)-PQCS2129 sorbent, the isosteric heat of adsorption was calculated from the CO2 adsorption isotherms at a fractional coverage of θ = 2.6 mol CO2/kg of sorbent, employing the Clausius−Clapeyron equation. The Clausius−Clapeyron equation gives

qmbCe 1 + bCe

b (mmol/cm3)

(6)

which can be rearranged to qe =

temperature (°C)

⎛ TT ⎞ ⎛ p ⎞ ΔHads = Q st = R ⎜ 1 2 ⎟ ln⎜⎜ 2 ⎟⎟ ⎝ T2 − T1 ⎠ ⎝ p1 ⎠

(11)

The constructed plot of ln(p) versus 1/T for the (PEI-25APTES-25)-PQCS2129 sorbent is shown in Figure 12. The (isosteric) heat of adsorption was obtained from the slope of the line (−ΔHads/R), yielding a value of 56 kJ/mol CO2. This calculated value was consistent with other experimental values reported in the literature for various PEI-modified silica

(9)

Henry’s law constant can be expressed as ⎛ ΔHads ⎞ ⎟ bH = qmb = qmb0 exp⎜ − ⎝ RT ⎠

(10)

The isosteric heat of adsorption at zero coverage can be experimentally obtained in utilizing the values of bH at multiple temperatures. The constants of the Langmuir adsorption isotherm provided in Table 6 were obtained in fitting the measured uptake of CO2 at each specific temperature to the Langmuir equation. In calculating the fractional coverage, we assumed 100% CO2 concentration at 40 °C corresponded to full coverage of the available chemical adsorption sites. This was assumed because the CO2 uptake at temperatures lower than 40 °C was prohibitively slow and even under days of exposure to a CO2containing atmosphere, the sorbent did not demonstrate high CO2 uptake at lower temperatures. Therefore, the assumption of full surface coverage at 40 °C is only used for comparison under the experimental conditions of interest for post-

Figure 12. Plot of ln(p) versus 1/T for the (PEI-25-APTES-25)PQCS2129 sorbent (−ΔHabs calculation). The coefficient of determination (R2) value, relative to the treadline, equals 0.997. 2494

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496

Energy & Fuels

Article

adsorbents.47 The value is also lower, with respect to the heat of adsorption/reaction associated with a 30% liquid MEA solution, which has been reported as ca. 86 kJ/mol CO2. The overprediction of the near-equilibrium CO2 capacity for the Langmuir isotherms at the higher partial pressures may allude to (i) some physical absorption of CO2 or (ii) the existence of a multistep adsorption mechanism occurring during the uptake of CO2.

opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the organizations that provided both financial and technical support for this project. Support for the ADA-ES portion of this work was provided by the U.S. Department of Energy National Energy Technology Laboratory (DE-NT0005649), with additional cost share from American Electric Power, Ameren, EPRI, Luminant, North American Power Group, Southern Company, and Xcel Energy. The authors also want to gratefully acknowledge Drs. Brian Kail, Sonna Hammache, Sheila Hedges, and James Hoffman for their active participation related to the planning, scheduling, and implementation of NETL experimental fixed-bed flow system and thermal analysis (e.g., TGA, DSC) testing.

■

CONCLUSIONS Adsorption processes are attractive, because of their low energy requirements, thus stimulating research activities in finding suitable solid sorbents for separating CO2 from flue gas streams generated from large point sources (i.e., coal-fired power plants). In a collaborative effort between the U.S. Department of Energy, the Office of Research and Development, the National Energy Technology Laboratory (NETL), and ADA Environmental Solutions (ADA-ES), near-equilibrium CO2 adsorption capacities adsorption capacities were independently and successfully measured using thermogravimetric analysis and fixed-bed flow systems. Post-combustion CO2 capture sorbents prepared by the physical impregnation of polyethyleneimine (PEI), together with the collective grafting of 3-(aminopropyl)triethoxysilane (APTES) onto a commercially manufactured PQ Corporation CS-2129 silica support, were investigated. Surface modification of the porous silica support by solutionphase silanization utilizing APTES in anhydrous ethanol at ca. 80 °C potentially emerges as a practical route in grafting spatially, site-isolated 3-aminopropyl groups within the interior pore channels of a silica matrix, herein improving its CO2 adsorption efficiency, and potentially enhancing its thermal and hydrolytic stability. The silane coupling agent APTES, introduced with PEI, improves the efficiency of amines, facilitating the diffusion of CO2 into the deeper layers of PEI. The development of a practical, one- or two-step scalable synthesis, suitable for the production of commercial quantities of sorbents possessing favorable attributes as high adsorption capacity, fast kinetics, enhanced selectivity toward CO2, excellent thermal, hydrolytic, and oxidative stability, and moderate regeneration conditions, is particularly attractive in advancing key process performance and design issues related to post-combustion CO2 capture.

■

■

REFERENCES

(1) IEA, Clean Coal Roadmaps to 2030, CCC/152, September 2009. (2) U.S. DOE NETL, Carbon Sequestration Technology Roadmap and Program Plan, April 2007. (3) IEA, CO2 ReductionProspects for Coal, CCC/26, December 1999. (4) IEA, Key World Energy Statistics, CCC/75, 2002. (5) JGCRI, Carbon Dioxide Capture and Geologic Storage, Global Energy Technology Strategy Plan, April 2006. (6) NETL, Cost and Performance Baseline for Fossil Energy Plants, Volume 1: Bituminous Coal and Natural Gas to Electricity, Revision 2, November 2010, www.netl.doe.gov, accessed January 2010. (7) Feeley, T. J., Fout, T. E., Jones, A. P. DOE’s Carbon Capture and Sequestration R&D Program, paper presented at the PowerGen Conference, Orlando, FL, December 2007. (8) Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources. ChemSusChem 2009, 2 (9), 796−854. (9) Siriwardane, R. V.; Shen, M. S.; Fisher, E. P.; Poston, J. A. Adsorption of CO2 on Molecular Sieves and Activated Carbon. Energy Fuels 2001, 15, 279−284. (10) Drage, T. C.; Arenillas, A.; Smith, K. M.; Pevida, C.; Piippo, S.; Snape, C. E. Preparation of Carbon Dioxide Adsorbents from the Chemical Activation of Urea−Formaldehyde and Melamine−Formaldehyde Resins. Fuel 2007, 86, 22−31. (11) Plaza, M. G.; Pevida, C.; Arenillas, A.; Rubiera, F.; Pis, J. J. CO2 Capture by Adsorption with Nitrogen Enriched Carbons. Fuel 2007, 86, 2204−2212. (12) Radosz, M.; Hu, X.; Krutkramelis, K.; Shen, Y. Flue-Gas Carbon Capture on Carbonaceous Sorbents: Toward a Low-Cost Multifunctional Carbon Filter for “Green” Energy Producers. Ind. Eng. Chem. Res. 2008, 47, 3783−3794. (13) Lu, C.; Bai, H.; Wu, B.; Su, F.; Hwang, J. F. Comparative Study of CO2 Capture by Carbon Nanotubes, Activated Carbons, and Zeolites. Energy Fuels 2008, 22, 3050−3056. (14) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Design and synthesis of an exceptionally stable and highly porous metal-organic framework. Nature 1999, 402, 276−279. (15) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; O’Keeffe, M.; Yaghi, O. M. Systematic design of pore sized and functionality in metalorganic frameworks and application in methane storage. Science 2002, 295, 469. (16) Gassensmith, J. J.; Furukawa, H.; Smaldone, R. A.; Forgan, R. S.; Botros, Y. Y.; Yaghi, O. M.; Fraser Stoddart, J Stong and Reversible Binding of Carbon Dioxide in a Green Metal-Organic Framework. J. Am. Chem. Soc. 2011, 133, 15312−15315. (17) Yazaydin, A. O.; Snurr, R. Q.; Park, T.-H.; Koh, K.; Lui, J.; LeVan, M. D.; Benin, A. I.; Jakubczak, P.; Lanuza, M.; Galloway, D. B.;

AUTHOR INFORMATION

Corresponding Author

*E-mails: [email protected] (D.J.F.), mac.gray@netl. doe.gov (M.L.G.), [email protected] (H.M.K.). Notes

Disclaimer. The U.S. Department of Energy, NETL and ADA-ES contributions to this report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and 2495

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496

Energy & Fuels

Article

Moisture and Adsorbent Regeneration Strategies. Adsorption 2010, 16, 567−575. (37) Sayari, A.; Belmabkhout, Y. Stabilization of Amine-Containing CO2 Adsorbents: Dramatic Effect of Water Vapor. J. Am. Chem. Soc. 2010, 132 (18), 6312−6314. (38) Hicks, J. C.; Drese, J. H.; Fauth, D. J.; Gray, M. L.; Qi, G.; Jones, C. W. Designing Adsorbents for CO2 Capture from Flue Gas− Hyperbranched Aminosilicas Capable of Capturing CO2 Reversibility. J. Am. Chem. Soc. 2008, 130, 2902−2903. (39) Li, W.; Choi, S.; Drese, J. H.; Hombostel, M.; Krishnan, G.; Eisenberger, P. M.; Jones, C. W. Steam-Stripping for Regeneration of Supported Amine-Based CO2 Adsorbents. ChemSusChem 2010, 3 (8), 899−903. (40) Bollini, P.; Choi, S.; Drese, J. H.; Jones, C. W. Oxidative Degradation of Aminosilica Adsorbents Relevant to Postcombustion CO2 Capture. Energy Fuels 2011, 25 (5), 2416−2425. (41) Wang, D. X.; Shalaby, C.; Ma, X. L.; Song, C. S. High-Capacity and Low-Cost Carbon-Based “Molecular Basket” Sorbent for CO2 Capture from Flue Gas. Energy Fuels 2011, 25, 456−458. (42) Drage, T. C.; Arenillas, A.; Smith, K. M.; Snape, C. E. Thermal Stability of Polyethylenimine based Carbon Dioxide Adsorbents and its Influence on Selection or Regeneration Strategies. Microporous Mesoporous Mater. 2008, 116, 506−512. (43) Sharma, K. K.; Asefa, T. Efficient Bifunctional Nanocatalysts by Simple Postgrafting of Spatially Isolated Catalytic Groups on Mesoporous Materials. Angew. Chem., Int. Ed. 2007, 46, 2879−2882. (44) Choi, S.; Gray, M. L.; Jones, C. W. Amine-Tethered Solid Adsorbents Coupling High Adsorption Capacity and Regenerability for CO2 Capture From Ambient Air. ChemSusChem 2011, 4, 628. (45) Wang, D.; Sentorun-Shalaby, C.; Ma, X.; Song, C. S. HighCapacity and Low-Cost Carbon-Based “Molecular Basket” Sorbent for CO2 Capture from Flue Gas. Energy Fuels 2011, 25, 456−458. (46) Sayari, A.; Heydari-Gorji, A.; Belmmabkhout, Y. Polyethylenimine-Impregnated Mesoporous Silica: Effect of Amine Loading and Surface Alkyl Chains on CO2 Adsorption. Langmuir 2011, 27, 12411− 12416. (47) Ebner, A. D.; Gray, M. L.; Chisholm, N. G.; Black, Q. T.; Mumford, D. D.; Nicholson, M. A.; Ritter, J. A. Suitability of a Solid Amine Sorbent for CO2 Capture by Pressure Swing Adsorption. Ind. Eng. Chem. Res. 2011, 50 (9), 5634−5641.

Low, J. J.; Willis, R. R. Screening of metal-organic frameworks for carbon dioxide capture from flue gas using a combined experimental and modeling approach. J. Am. Chem. Soc. 2009, 131, 18198−18199. (18) Nelson, T.; Coleman, L.; Anderson, M.; Herr, J.; Pavani, M. The Dry Carbonate Process: Carbon Dioxide Recovery from Power Plant Flue Gas. Presented at the Annual NETL CO2 Capture Technology for Existing R&D Meeting, Pittsburgh, PA, March 2009. (19) Shigemoto, N.; Yanagihara, T.; Sugiyama, S.; Hayashi, H. Material and Energy Consumption for CO2 Recovery from Moist Flue Gas Employing K2CO3-on-Activated Carbon and Its Evaluation for Practical Adaptation. Energy Fuels 2006, 20, 721−726. (20) Pirngruber, G. D.; Cassiano-Gaspar, S.; Louret, S.; Chaumonnot, A.; Delfort, B. Amines Immobilized on a Solid Support for Post-Combustion CO2 CaptureA Preliminary Analysis of the Performance in a VSA or TSA Process Based on the Adsorption Isotherms and Kinetic Data. Energy Procedia 2009, 1, 1335−1342. (21) Sjostrom, S.; Krutka, H. Evaluation of Solid Sorbents as a Retrofit Technology of CO2 Capture. Fuel 2010, 89, 1298−1306. (22) Gray, M. L.; Hoffman, J. S.; Hreha, D. C.; Fauth, D. J.; Hedges, S. W.; Champagne, K. J.; Pennline, H. W. Parametric Study of Solid Amine Sorbents for the Capture of Carbon Dioxide. Energy Fuels 2009, 23, 4840−4844. (23) Drage, T. C.; Smith, K. M.; Pevida, C.; Arenillas, A.; Snape, C. E. Development of Adsorbent Technologies for Post Combustion CO2 Capture. Energy Procedia 2009, 1, 881−884. (24) Birbara, P. J., Filburn, T. P., Nalette, T. A. U.S. Patent 6,364,938, 2006. (25) Xu, X. C.; Song, C. S.; Andresen, J. M.; Miller, B. C.; Scaroni, A. W. Novel Polyethyleimine-modified Mesoporous Molecular Sieve of MCM-41 Type as High-Capacity Adsorbent for CO2 Capture. Energy Fuels 2002, 16, 1463−1469. (26) Xu, X. C.; Andresen, J. M.; Song, C. S.; Miller, B. C.; Scaroni, A. W. Preparation and Characterization of Novel CO2 Molecular Basket Adsorbents based on Polymer-modified Mesoporous Sieve MCM-41. Microporous Mesoporous Mater. 2003, 62, 29−45. (27) Yue, M. B.; Sun, L. B.; Cao, Y.; Wang, Z. J.; Wang, Y.; Yu, Q.; Zhu, J. H. Promoting the CO2 adsorption in the amine-containing SBA-15 by hydroxyl group. Microporous Mesoporous Mater. 2008, 114, 74−81. (28) Son, W. J.; Chio, J. S.; Ahn, W. S. Adsorptive removal of carbon dioxide using polyethyleneimine-loaded mesoporous silica materials. Microporous Mesoporous Mater. 2008, 113, 31−40. (29) Birbara, P. J.; Filburn, T. P.; Nalette, T. A. (United Technologies Corporation). Regenerable Solid Amine Sorbent, U.S. Patent 5,876,488, 1999. (30) Siriwardane R. V. (The United States Department of Energy). Solid Sorbents for Removal of Carbon Dioxide from Gas Streams at Low Temperatures, U.S. Patent 6,908,497, 2005. (31) Gray, M. L.; Champagne, K. J.; Fauth, D.; Baltrus, J. P.; Pennline, H. Performance of Immobilized Tertiary Amine Solid Sorbents for the Capture of Carbon Dioxide. Int. J. Greenhouse Gas Control 2008, 2, 3−8. (32) Kim, S.; Ida, J.; Guliants, V. V.; Lin, J. Y. S. Tailoring Pore Properties of MCM-48 Silica for Selective Adsorption of CO2. J. Phys. Chem. B 2005, 109, 6287−6293. (33) Chen, Chao; Yang, Seung-Tae; Ahn, Wha-Seung; Ryoo, Ryong Amine-impregnated silica monolith with a hierarchical pore structure: enhancement of CO2 capture capacity. Chem. Commun. 2009, 3627− 3629. (34) Harlick, P. J. E.; Sayari, A. Applications of Pore-Expanded Mesoporous Silica. 3. Triamine Silane Grafting for Enhanced CO2 Adsorption. Ind. Eng. Chem. Res. 2006, 45, 3248. (35) Harlick, P. J. E.; Sayari, A. Applications of Pore-Expanded Mesoporous Silica. 5. Triamine Grafted Material with Exceptional CO2 Dynamic and Equilibrium Adsorption Performance. Ind. Eng. Chem. Res. 2007, 45, 446−458. (36) Serna-Guerrero, R.; Belmabkhout, Y.; Sayari, A. TriamineGrafted Pore-Expanded Mesoporous Silica for CO2 Capture: Effect of 2496

dx.doi.org/10.1021/ef201578a | Energy Fuels 2012, 26, 2483−2496