pubs.acs.org/Langmuir © 2010 American Chemical Society
Temperature-Induced Uptake of CO2 and Formation of Carbamates in Mesocaged Silica Modified with n-Propylamines Zoltan Bacsik,† Rambabu Atluri,‡ Alfonso E. Garcia-Bennett,‡ and Niklas Hedin*,† †
Department of Materials and Environmental Chemistry, Berzelii Center EXSELENT on Porous Materials, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden, and ‡Nanotechnology and Functional Materials, Department of Engineering Sciences, The A˚ngstr€ om Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden Received January 12, 2010. Revised Manuscript Received February 21, 2010
Adsorption-mediated CO2 separation can reduce the cost of carbon capture and storage. The reduction in cost requires adsorbents with high capacities for CO2 sorption and high CO2-over-N2 selectivity. Amine-modified sorbents are promising candidates for carbon capture. To investigate the details of CO2 adsorption in such materials, we studied mesocaged (cubic, Pm3n symmetry) silica adsorbents with tethered propylamines using Fourier transform infrared (FTIR) spectroscopy and volumetric uptake experiments. The degree of heterogeneity in these coatings was varied by either cosynthesizing or postsynthetically introducing the propylamine modification. In situ FTIR spectroscopy revealed the presence of both physisorbed and chemisorbed CO2 in the materials. We present direct molecular evidence for physisorption using FTIR spectroscopy in mesoporous silica sorbents modified with propylamines. Physisorption reduced the CO2-over-N2 selectivity in amine-rich sorbents. Samples with homogeneous coatings showed typical CO2 adsorption trends and large quantities of IR-observable physisorbed CO2. The uptake of CO2 in mesocaged materials with heterogeneous propylamine coatings was higher at high temperatures than at low temperatures. At higher temperatures and low pressures, the postsynthetically modified materials adsorbed more CO2 than did the extracted ones, even though the surface area after modification was clearly reduced and the coverage of primary amine groups was lower. The principal mode of CO2 uptake in postsynthetically modified mesoporous silica was chemisorption. The chemisorbed moieties were present mainly as carbamate-ammonium ion pairs, resulting from the quantitative transformation of primary amine groups during CO2 adsorption as established by NIR spectroscopy. The heterogeneity in the coatings promoted the formation of these ion pairs. The average propylamine-propylamine distance must be small to allow the formation of carbamate-propylammonium ion pairs.
1. Introduction Carbon capture and storage (CCS) can reduce the CO2 levels in the atmosphere.1 The high cost of CCS currently prohibits the economical separation of CO2 from flue gases using current technologies.2 The main costs associated with CCS are in the CO2separation step. Adsorption technologies are used for a variety of gas-separation processes3 and are under investigation for potential use in CO2 capture. Gases can be separated with the help of adsorbents using three different principles: equilibrium, kinetic partitioning, and molecular sieving.4 Here, we study a partitioning process between the solid and gaseous phases that is enhanced by porous solid bases composed of mesoporous silica coated with propylamine groups. (Mesoporous materials have pores with sizes between 2 and 50 nm, according to IUPAC.) Mesoporous silica, modified with amines, adsorbs CO2 over N2 more preferentially than what is expected from the differences in the sublimation and boiling points of CO2 and N2. Modifications of the silica surface can be made in several ways. Chemical tethering as well as physical adsorption or filling may be employed to achieve surface modification. Surface coatings and pore fillings *Corresponding author. E-mail:
[email protected]. (1) Baxter, J.; et al. Energy Environ. Sci. 2009, 2, 559–588. (2) Ho, M. T.; Allinson, G. W.; Wiley, D. E. Ind. Eng. Chem. Res. 2008, 47, 4883–4890. (3) Ruthven, D. M.; Shamsuzzaman, F.; Knaebel, K. S. In Pressure Swing Adsorption; John Wiley and Sons: New York, 1994. (4) Ruthven, D. M. In Principles of Adsorption and Adsorption Processes; John Wiley & Sons: New York, 1984. (5) Huang, H. Y.; Yang, R. T.; Chinn, D.; Munson, C. L. Ind. Eng. Chem. Res. 2003, 42, 2427–2433.
Langmuir 2010, 26(12), 10013–10024
have demonstrated the highly selective uptake of CO2.5-11 The longevity of the physically modified mesoporous silicas is questionable because of the potential loss of amines.11 The n-propylamine modification has been particularly popular,5,12-16 probably because of its high basicity and resulting favorable interactions with CO2.17 When applying chemical modifications to mesoporous silica with n-propylamines, several chemical approaches may be employed. The surface may be modified postsynthetically and 3-aminopropyl triethoxysilane (APES) may be introduced in a separate step,18 after calcination or removal of (6) Knowles, G. P.; Graham, J. V.; Delaney, S. W.; Chaffee, A. L. Fuel. Process. Technol. 2005, 86, 1435–1448. (7) Knofel, C.; Descarpentries, J.; Benzaouia, A.; Zelenak, V.; Mornet, S.; Llewellyn, P. L.; Hornebecq, V. Microporous Mesoporous Mater. 2007, 99, 79–85. (8) Khatri, R. A.; Chuang, S. S. C.; Soong, Y.; Gray, M. Ind. Eng. Chem. Res. 2005, 44, 3702–3708. (9) Franchi, R. S.; Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2005, 44, 8007–8013. (10) Chen, C.; Yang, S. T.; Ahn, W. S.; Ryoo, R. Chem. Commun. 2009, 3627– 3629. (11) Hicks, J. C.; Drese, J. H.; Fauth, D. J.; Gray, M. L.; Qi, G.; Jones, C. W. J. Am. Chem. Soc. 2008, 130, 2902–2903. (12) Serna-Guerrero, R.; Dana, E.; Sayari, A. Ind. Eng. Chem. Res. 2008, 47, 9406–9412. (13) Gray, M. L.; Soong, Y.; Champagne, K. J.; Pennline, H.; Baltrus, J. P.; Stevens, R. W.; Khatri, R.; Chuang, S. S. C.; Filburn, T. Fuel Process. Technol. 2005, 86, 1449–1455. (14) Gray, M. L.; Soong, Y.; Champagne, K. J.; Pennline, H.; Baltrus, J. P.; Stevens, R. W.; Khatri, R.; Chuang, S. S. C. Int. J. Environ. Technol. Manage. 2004, 4, 82–88. (15) Hiyoshi, N.; Yogo, K.; Yashima, T. Chem. Lett. 2004, 33, 510–511. (16) Choi, S.; Drese, J. H.; Jones, C. W. ChemSusChem 2009, 2, 796–854. (17) Zelenak, V.; Halamova, D.; Gaberova, L.; Bloch, E.; Llewellyn, P. Microporous Mesoporous Mater. 2008, 116, 358–364. (18) Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12, 1403–1419.
Published on Web 03/10/2010
DOI: 10.1021/la1001495
10013
Article
surfactants via solvent extraction. Alternatively, recently developed synthesis routes have taken advantage of the favorable electrostatic interactions between anionic surfactants and APES during the synthesis of a mesostructured solid to afford (after solvent extraction of the surfactant) homogeneous coatings of the propylamine moieties.19-21 A high level of control is achievable in tailoring the structural details of mesoporous silica materials. By varying the reaction conditions, synthesis pH, temperature, and the identities of the organic or inorganic reactants, silicas or organosilicas may be formed with controlled size and structural arrangements. Mesocaged silica materials with cubic Pm3n symmetry are a particular example of such structures. Such solids may potentially be used for gas separation because of the small interconnecting window apertures between cages. The structural details may be fine tuned using certain synthesis protocols.22 The first mesostructured solid prepared with this symmetry was SBA-1.23 Here, we show that amine-rich sorbents with Pm3n symmetry adsorbed CO2 differently, depending on whether the frameworks were modified postsynthetically or in situ. The pore structures of calcined mesocaged samples were visualized using 3D electrostatic potential maps reconstructed from the structure factors obtained by electron crystallography. A simple model of a 3D electrostatic potential map for a Pm3n structure templated by C16TMABr is shown in Figure 1. The black-and-white map indicates the silica wall density such that the black surface indicates areas of high silica density and the white surface indicates areas at the center of the pore space. As shown, every lattice consists of two types of cages: (A) spherical and (B) ellipsoidal, with connecting windows that open onto the neighboring cages (B-B or B-A). For full descriptions of the fine structure of the various mesocaged materials described in this study, please refer to ref 22. Porous materials can adsorb CO2 physically or chemically, and FTIR spectroscopy is an excellent tool for evaluating the specific adsorption processes that take place. Different versions of this technique can be used to study the adsorption of CO2. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and the transmission IR technique have different advantages. Transmission spectra are best for quantitative analysis. The relative intensities of the bands in the spectra are quantitatively related to concentration. However, DRIFTS is advantageous when bands are saturated and the sample can be diluted with KBr or when a self-supporting pellet cannot be made. For silica or organosilica materials, samples are semitransparent to IR radiation above 1200 cm-1. Transparency allows studies of both physisorbed and chemisorbed CO2 using the quantitative transmission FTIR technique. IR spectroscopy can reveal the nature of CO2 physisorption in different materials.24-30 Its implementation relies on the fact that (19) Araki, S.; Doi, H.; Sano, Y.; Tanaka, S.; Miyake, Y. J. Colloid Interface Sci. 2009, 339, 382–389. (20) Kim, S. N.; Son, W. J.; Choi, J. S.; Ahn, W. S. Microporous Mesoporous Mater. 2008, 115, 497–503. (21) Garcia-Bennett, A. E.; Terasaki, O.; Che, S.; Tatsumi, T. Chem. Mater. 2004, 16, 813–821. (22) Atluri, R.; Bacsik, Z.; Hedin, N.; Garcia-Bennett, A. E. Submitted for publication. (23) Huo, Q. S.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147– 1160. (24) Ward, J. W.; Habgood, H. W. J. Phys. Chem. 1966, 70, 1178–1182. (25) Delaval, Y.; Delara, E. C. J. Chem. Soc., Faraday Trans. 1 1981, 77, 869–877. (26) Forster, H.; Schumann, M. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1149– 1158. (27) Ramis, G.; Busca, G.; Lorenzelli, V. Mater. Chem. Phys. 1991, 29, 425–435. (28) Montanari, T.; Busca, G. Vib. Spectrosc. 2008, 46, 45–51. (29) Stevens, R. W.; Siriwardane, R. V.; Logan, J. Energy Fuels 2008, 22, 3070– 3079. (30) Goodman, A. L. Energy Fuels 2009, 23, 1101–1106.
10014 DOI: 10.1021/la1001495
Bacsik et al.
Figure 1. Typical scanning electron microscopy (SEM) image of a cubic mesocaged material prepared using the C16TEABr surfactant (top); a representative transmission electron microscopy image recorded along the [100] direction of a particle of cubic mesocaged material prepared using the C16TMABr surfactant (middle); and the corresponding 3D electrostatic potential density model of a Pm3n mesocaged material derived from electron crystallographic data and EM investigations (bottom). The surface cages are denoted type B, and the corner cages are type A. Cageconnecting windows are present between B-B and B-A cages, as shown.
the infrared bands of gaseous versus adsorbed CO2 contain key differences. The vibrational frequencies of physisorbed CO2 are similar to those of gaseous CO2 because the molecules are linear in both cases. Unlike gaseous CO2, adsorbed CO2 molecules cannot rotate freely, so the rotational fine structure of the (rovibrational) Langmuir 2010, 26(12), 10013–10024
Bacsik et al.
infrared bands is not present for adsorbed CO2. The absence of rovibrational fine structure compensates for the spectral contributions of the gaseous phase. For example, the band from the ν3 mode of CO2 appears at 2349 cm-1 in the gas phase. Physisorbed CO2 has the same vibrational modes at roughly the same wavenumber but without the presence of splitting from the rotational states. Here, we present a study of physisorbed CO2 on propylaminemodified silica in which the structures are characterized by FTIR spectroscopy. A low degree of physisorption is characteristic of mesoporous adsorbents with a high selectivity for CO2-over-N2 adsorption. Many solids adsorb CO2 chemically.28,29,31 The propylamine groups react with CO2, and chemisorption can dominate the adsorption of CO2 on porous silica solids modified with propylamine groups. The main aspects of CO2 amine chemistry are known in the liquid state,32,33 but one expects variation of this chemistry for amine groups tethered to solid supports. When CO2 and amines react, the resulting ammonium carbamate salts are thermally unstable and release CO2 upon heating. Battjes et al.34 showed that alkyl ammonium carbamates are the principal product of reactions between primary or secondary amines and CO2. Other authors have shown that when primary amines react with CO2, ammonium-carbamate ion pairs form at room temperature and under nonaqueous conditions. At lower temperatures (273 K), the dimeric form of carbamic acid has been observed.35,36 However, many details are clarified in the liquid phase; when the amine groups are tethered to solid supports, the chemistry can be different and needs to be understood to improve these adsorbents. The chemisorption of CO2 on porous silica materials modified by amine functional groups has been studied;5,31,37-41 however, the identity of some chemisorbed species and the influence of heterogeneities in the coating itself remain to be determined. CO2 adsorbs via a different mechanism on amine-rich solids in the presence of water. The carbamate formed during the reaction of CO2 with the (primary or secondary) amine group can further react with CO2 and H2O to form a bicarbonate group. In the presence of water, one mole of amines can theoretically chemisorb one mole of CO2. In the absence of water, this ratio is shifted to two moles of amine to one mole of CO2. Several authors have examined this type of enhanced CO2 capacity on amine-modified materials in the presence of water.9,12,31,42-45 (31) Hiyoshi, N.; Yogo, K.; Yashima, T. Microporous Mesoporous Mater. 2005, 84, 357–365. (32) Hoerr, C. W.; Harwood, H. J.; Ramarao, G. V. J. Org. Chem. 1944, 201. (33) Alauzun, J.; Besson, E.; Mehdi, A.; Reye, C.; Corriu, R. J. P. Chem. Mater. 2008, 20, 503–513. (34) Battjes, K. P.; Barolo, A. M.; Dreyfuss, P. J. Adhes. Sci. Technol. 1991, 5, 785–799. (35) Aresta, M.; Quaranta, E. Tetrahedron 1992, 48, 1515–1530. (36) Dibenedetto, A.; Aresta, M.; Fragale, C.; Narracci, M. Green Chem. 2002, 4, 439–443. (37) Khatri, R. A.; Chuang, S. S. C.; Soong, Y.; Gray, M. Energy Fuels 2006, 20, 1514–1520. (38) Chang, A. C. C.; Chuang, S. S. C.; Gray, M.; Soong, Y. Energy Fuels 2003, 17, 468–473. (39) Wang, X. X.; Schwartz, V.; Clark, J. C.; Ma, X. L.; Overbury, S. H.; Xu, X. C.; Song, C. S. J. Phys. Chem. C 2009, 113, 7260–7268. (40) Leal, O.; Bolivar, C.; Ovalles, C.; Garcia, J. J.; Espidel, Y. Inorg. Chim. Acta 1995, 240, 183–189. (41) Kn€ofel, C.; Martin, C.; Hornebecq, V.; Llewellyn, P. L. J. Phys. Chem. C 2009, 113, 21726–21734. (42) Belmabkhout, Y.; Sayari, A. Adsorption 2009, 15, 318–328. (43) Hiyoshi, N.; Yogo, K.; Yashima, T. J. Jpn. Pet. Inst. 2005, 48, 29–36. (44) Xu, X. C.; Song, C. S.; Miller, B. G.; Scaroni, A. W. Fuel Process. Technol. 2005, 86, 1457–1472. (45) Xu, X. C.; Song, C. S.; Miller, B. G.; Scaroni, A. W. Ind. Eng. Chem. Res. 2005, 44, 8113–8119.
Langmuir 2010, 26(12), 10013–10024
Article
We have studied the molecular details of the adsorption process of CO2 onto propylamine-coated mesocaged silica materials, focusing on how heterogeneities may affect the uptake and the CO2-over-N2 selectivity. A variety of mesocaged silica materials modified with propylamines, either in situ or postsynthetically, are presented, all of which possess the same space group symmetry (Pm3n) with only slight variations in mesocage size and porosity.22 The pressure and temperature dependencies of the CO2 uptake were investigated. Transmission IR spectroscopy was utilized to study the chemisorption and physisorption mechanisms of CO2 adsorption onto these solids.
2. Experimental Section Mesoporous Pm3n adsorbents were modified with APES by cosynthesis or postsynthesis methods. The amine-modified materials were characterized using different methods, and the capacity for CO2 uptake was studied under various conditions. 2.1. Materials. Mesostructured adsorbents with Pm3n mesocaged symmetry were synthesized using different surfactants as templating agents, as described by Atluri et al.22 Samples were prepared according to one of two published methods: under high pH conditions using anionic (N-lauroyl-L-glutamic acid, C12GluA) and cationic (cetyltrimethylammonium bromide, C16TMABr) surfactants or under acidic conditions using cationic (cetyltriethylammonium bromide, C16TEABr) and gemini dicationic surfactants ([CH3(CH2)15N(CH3)2(CH2)3N(CH3)3]Br2, C16-3-1). The extracted-1 and extracted-2 samples correspond to materials prepared using C16TMABr and C12GluA surfactants, respectively.46-48 The surfactants were removed via solvent extraction from the noncalcined Pm3n mesostructured solid by refluxing the solid in an ethanol/HCl (37%) solution in a ratio of 8:2 solid/ solvent by weight for 12 h. The material was then filtered, and the extraction was repeated before filtering and drying the solid under ambient conditions. The complete removal of surfactant was determined by thermogravimetric analysis (TGA) of the extracted samples. Postsynthesis modification was carried out using three mesocage silica absorbents with Pm3n symmetry. Postsynthesis-1 used the calcined adsorbent prepared under high pH conditions and with C16TMABr and APES. Postsynthesis-1 is structurally identical to extracted-1, except that the material was calcined and the amine coating was introduced postsynthetically. Postsynthesis-2 employed a Pm3n sorbent templated by cetyltriethyl ammonium bromide (C16TEABr) surfactants under acidic conditions,49 and postsynthesis-3 was templated by dicationic gemini surfactant C16-3-1 (also under acidic conditions).50 Carbon dioxide (>99.9%) was supplied by the Linde Gas Company (AGA) and was used as received.
2.1.1. Postsynthesis Modification of SBA-1 with 3-Aminopropyl Triethoxysilane. Modification of the mesoporous silica materials with n-propylamine groups was performed by tethering APES to the internal surfaces under autocatalytic reaction conditions. Prior to postmodification, the internal surfaces of the substrates were hydrated with water (50 mL per gram of solid) for 3 h and heated to 80 °C, and the materials were collected by filtration and washed with toluene. The water remaining in the wet solid substrate was removed during 3 h of azeotropic distillation (100 mL/g of toluene), and the distillation procedure was repeated twice to permit the complete removal of water from the (46) Atluri, R.; Sakamoto, Y.; Garcia-Bennett, A. E. Langmuir 2009, 25, 3189– 3195. (47) Garcia-Bennett, A. E.; Kupferschmidt, N.; Sakamoto, Y.; Che, S.; Terasaki, O. Angew. Chem., Int. Ed. 2005, 44, 5317–5322. (48) Garcia-Bennett, A. E.; Miyasaka, K.; Terasaki, O.; Che, S. Chem. Mater. 2004, 16, 3597–3605. (49) Sakamoto, Y.; Kaneda, M.; Terasaki, O.; Zhao, D. Y.; Kim, J. M.; Stucky, G.; Shin, H. J.; Ryoo, R. Nature 2000, 408, 449–453. (50) Garcia-Bennett, A. E.; Williamson, S.; Wright, P. A.; Shannon, I. J. J. Mater. Chem. 2002, 12, 3533–3540.
DOI: 10.1021/la1001495
10015
Article
Bacsik et al.
substrate. The dried substrate was again mixed with 100 mL of toluene, a specified quantity of APES was added to the solution, and heating was continued (80 °C) for 12 h. The total quantity of APES added to the silica substrate was held constant at 2 mols per every 1 mol of silica substrate (always a 5% molar excess relative to the estimated silane content). The functionalized silica material was filtered and heated again in fresh toluene for 1 h. The crude products were then collected, washed with several volumes of toluene, and dried at 40 °C for 24 h. The postsynthetically modified silica and the extracted silica had n-propylamine group surface coverages of between 1 and 3.1 groups/nm2, estimated through thermogravimetric analysis (TGA).
2.2. Brunauer-Emmett-Teller (BET) and Pore Analysis. Nitrogen adsorption-desorption isotherms were recorded at liquid-nitrogen temperature (77 K) for all porous materials using a Micrometrics ASAP2020 volumetric adsorption analyzer. The calcined samples were treated under near-vacuum conditions (degassed at pressure
