Functionalization of Delaminated Zeolite ITQ-6 for the Adsorption of

Jul 9, 2009 - To obtain information on the surface energetics of CO2 adsorption on selected samples, isotherms were taken in the temperature range fro...
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Functionalization of Delaminated Zeolite ITQ-6 for the Adsorption of Carbon Dioxide  Arnost Zukal,* Irene Dominguez, Jana Mayerova, and Jirı´ Cejka J. Heyrovsk y Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i. Dolejskova 3, CZ-182 23 Prague 8, Czech Republic Received April 2, 2009. Revised Manuscript Received June 12, 2009 Novel functionalized adsorbents for CO2 separation were synthesized by grafting 3-aminopropyl, 3-(methylamino) propyl, or 3-(phenylamino)propyl ligands in the delaminated zeolite ITQ-6. On the basis of the texture parameters determined from nitrogen adsorption isotherms recorded at 77 K and the results of chemical analysis, physicochemical properties of functionalized ITQ-6 were evaluated and compared with those of mesoporous SBA-15 silica functionalized with the same ligands. To examine carbon dioxide adsorption on functionalized materials, adsorption isotherms at 293 K were measured. To obtain information on the surface energetics of CO2 adsorption on selected samples, isotherms were taken in the temperature range from 273 to 333 K and adsorption isosteres were calculated. Isosteric adsorption heats determined from the slope of adsorption isosteres proved that all of the 3-aminopropyl ligands in ITQ-6 take part in CO2 adsorption. It was found that in the whole region of CO2 pressures the efficiency of the amine ligand, defined as the number of adsorbed CO2 molecules per one amine ligand, is higher for functionalized ITQ-6 than for functionalized SBA-15 silica.

Introduction A lot of attention has been recently paid to carbon dioxide (CO2) adsorption because the high amount of this gas being emitted into the atmosphere is a serious problem. In the last years, much effort has been directed toward finding the optimum technologies of CO2 capture and sequestration.1 For a long time, aqueous solutions of alkanolamines were the most widely used for the separation of CO2 by absorption.2 In spite of the efficiency of gas-liquid systems, a low concentration of amines present in the aqueous phase, together with high energy consumption, corrosion problems, and solvent regeneration, is an important limitation when using absorption systems for industrial applications.3 Undoubtedly, a solution for these restrictions would be the use of solid sorbents. The CO2 adsorption properties of a number of microporous and mesoporous *E-mail: [email protected],cz. (1) Special Issue on the Capture of Carbon Dioxide from Industrial Sources: Technological Developments and Future Opportunities : Ind. Eng. Chem. Res. 2006, 45, 2413. (2) Rinker, E. B.; Ashour, S. S.; Sandall, O. C. Ind. Eng. Chem. Res. 2000, 39, 4346. (3) Xu, X.; Song, C.; Miller, B. G.; Scaroni, A. W. Fuel Process. Technol. 2005, 86, 1457. (4) Siporin, S. E.; McClaine, B. C.; Davis, R. J. Langmuir 2003, 19, 4707. (5) Walton, K. S.; Abney, M. B.; LeVan, M. D. Microporous Mesoporous Mater. 2006, 91, 78. (6) Plant, D. F.; Maurin, G.; Deroche, I.; Gaberova, L.; Llewellyn, P. L. Chem. Phys. Lett. 2006, 426, 387. (7) Plant, D. F.; Maurin, G.; Deroche, I.; Llewellyn, P. L. Microporous Mesoporous Mater. 2007, 99, 70.  (8) Pawlesa, J.; Zukal, A.; Cejka, J. Adsorption 2007, 13, 2413. (9) Zhang, J.; Singh, R.; Webley, P. A. Microporous Mesoporous Mater. 2008, 111, 478. (10) Ghoufi, A.; Gaberova, L.; Rouquerol, J.; Vincent, D.; Llewellyn, P. L.; Maurin, G. Microporous Mesoporous Mater. 2009, 119, 117.  (11) Pulido, A.; Nachtigall, P.; Zukal, A.; Dominguez, I.; Cejka, J. J. Phys. Chem. C 2009, 113, 2928. (12) Millward, A. L.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 17998. (13) Llewellyn, P. L.; Bourrelly, S.; Serre, C.; Vimont, A.; Daturi, M.; Hamon, L.; De Weireld, G.; Chang, J.-S.; Hong, D.-Y.; Hwang, Y. K.; Jhung, S. H.; Ferey, G. Langmuir 2008, 24, 7245. (14) Yong, Z.; Mata, V.; Rodrigues, A. E. Adsorption 2001, 7, 41. (15) Siriwardane, R. V.; Shen, M.-S.; Fisher, E. P.; Poston, J. A. Energy Fuels 2001, 15, 279.

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materials, namely, zeolites,4-11 metal-organic frameworks,12,13 various carbon-based14-16 and metal oxide17 adsorbents, hydrotalcite-like compounds,18 etc., have already been investigated. In the last years, a great deal of attention has been paid to the CO2 adsorption on various amine-functionalized materials. The aminemodified mesoporous sorbents take advantage of high selectivity to CO2 capture together with a high mass transfer and high diffusion rate of gases within the mesoporous solid. Immobilized amine solid sorbents based on polymers are already used for this purpose in aircraft, submarine, and spacecraft technologies.19 Silica,20,21 activated carbon,22 and mesoporous materials (MCM41,22-25 MCM-48,20 SBA-12,19,26 SBA-15,27-31 SBA-16,32,33 and (16) Ottiger, S.; Pini, R.; Storti, G.; Mazzotti, M. Langmuir 2008, 24, 9531. (17) Gaffney, T. R.; Golden, T. C.; Mayorga, S. G.; Brzozowski, J. R.; Talyer, F. W. U.S. Patent 5,917,136, 1999. (18) Reijers, H. Th. J.; Valster-Schiermeier, S. E. A.; Cobden, P. D.; van den Brink, R. W. Ind. Eng. Chem. Res. 2006, 45, 2522.  (19) Zelenak, V.; Badanicova, M.; Halamova, D.; Cejka, J.; Zukal, A.; Murafa, N.; Goerigk, G. Chem. Eng. J. 2008, 144, 336. (20) Leal, O.; Bolı´ var, C.; Ovalles, C.; Garcı´ a, J. J.; Espidel, Y. Inorg. Chim. Acta 1995, 240, 183. (21) Huang, H. Y.; Yang, R. T.; Chinn, D.; Munson, C. L. Ind. Eng. Chem. Res. 2003, 42, 2427. (22) Franchi, R. S.; Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2005, 44, 8007. (23) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Microporous Mesoporous Mater. 2003, 62, 29. (24) Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2006, 45, 3248. (25) Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2007, 46, 446. (26) Zelenak, V.; Halamova, D.; Gaberova, L.; Bloch, E.; Llewellyn, P. Microporous Mesoporous Mater. 2008, 116, 358. (27) Chang, A. C. C.; Chuang, S. S. C.; Gray, M.; Soong, Y. Energy Fuels 2003, 17, 468. (28) Hiyoshi, N.; Yogo, K.; Yashima, T. Microporous Mesoporous Mater. 2005, 84, 357. (29) Sujandi; Park, S.-E.; Han, D.-S.; Han, S.-C.; Jin, M.-J.; Ohsuna, T. Chem. Commun. 2006, 4131. (30) Liu, X.; Zhou, L.; Fu, X.; Sun, Y.; Su, W.; Zhou, Y. Microporous Mesoporous Mater. 2007, 62, 1101. (31) Yue, M. B.; Sun, L. B.; Cao, Y.; Wang, Z. J.; Wang, Y.; Yu, Q.; Zhu, J. H. Microporous Mesoporous Mater. 2008, 114, 74. (32) Kn€ofel, C.; Descarpentries, J.; Benzaouia, A.; Zelenak, V.; Mornet, S.; Llewellyn, P. L.; Hornebecq, V. Microporous Mesoporous Mater. 2007, 99, 79. (33) Wei, J.; Shi, J.; Pan, H.; Zhao, W.; Ye, Q.; Shi, Y. Microporous Mesoporous Mater. 2008, 116, 349.

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hexagonal silica34) have been modified with different organic amines by using either impregnation or grafting methods. The interaction between the basic surface and acidic CO2 molecules is thought to result in the formation of surface ammonium carbamates under anhydrous conditions and in the formation of ammonium bicarbonate and carbonate species in the presence of water.26,35 Despite considerable progress in the development of advanced methods for the one-pot synthesis of functionalized mesoporous silica,36 approaches based on postsynthesis functionalization remain popular because of the fact that separation of the silica synthesis and functionalization steps allows independent and more straightforward control of the pore periodicity, particle size, and particle morphology.37 The grafting has the advantage that, under the synthesis conditions used, the porous structure of the starting silica support is usually retained. (Aminopropyl) trialkoxysilanes reacting with free silanol groups on the pore surface are among the most frequently used precursors for the grafting of amino groups. However, in some cases, after amine modification, the pore size inadequately limits the accessibility of the CO2 molecules to the amino groups. The lower limit for the pore size of the mesoporous silica support to be used in the preparation of efficient amine-based sorbents is approximately 3.5 nm.19 Below this limit, the amine adsorption sites inside the pores are hardly accessible for CO2 molecules because of the limited rate of their penetration into the pores. In the present work, three different amine ligands have been grafted in the delaminated zeolite ITQ-6. This new zeolitic material is produced by delamination of a layered precursor of ferrierite.38 ITQ-6 is characterized by high thermal and hydrothermal stability; it presents a large external surface area with a negligible volume of micropores. The 29Si MAS NMR spectra revealed a high ratio of Q3/Q4-type silicon atoms where Q3 atoms would mainly correspond to terminal (SiO)3SiOH groups. The spectra also show the formation of Q2 geminal groups at the vertexes of the ITQ-6 layers. The formation of Q3 and Q2 silicon atoms is consistent with a large amount of silanol groups present in ITQ-6. A large accessible external surface with a high concentration of silanol groups makes ITQ-6 a very promising material as a support for any type of active species. Indeed, ITQ-6 has been successfully used for enzyme immobilization39 and also as a support for vanadium oxide catalysts in the oxidative dehydrogenation of propane.40 To our best knowledge, this is the first report on aminefunctionalized ITQ-6. In order to show the efficiency of this material for CO2 separation, adsorption properties of the functionalized ITQ-6 are compared with those of the amine-functionalized mesoporous SBA-15 silica, which proved a high adsorption capacity for CO2.27-31 The evolution of the porous structure of SBA-15 depends on the synthesis temperature and the method for the removal of the triblock copolymer template from the as-synthesized material. Materials prepared at low or intermediate temperatures are characterized by the presence of (34) Knowles, G. P.; Delaney, S. W.; Chaffee, A. L. Ind. Eng. Chem. Res. 2006, 45, 2626. (35) Knowles, G. P.; Graham, J. V.; Delaney, S. W.; Chaffee, A. L. Fuel Process. Technol. 2005, 86, 1435. (36) Hoffmann, F.; Cornelius, M.; Morell, J.; Fr€oba, M. Angew. Chem., Int. Ed. 2006, 45, 3216. (37) Ritter, H.; Nieminen, M.; Karppinen, M.; Br€uhwiler, D. Microporous Mesoporous Mater. 2009, 121, 79. (38) Corma, A.; Diaz, U.; Domine, M. E.; Fornes, V. J. Am. Chem. Soc. 2000, 122, 2804. (39) Corma, A.; Fornes, V.; Rey, F. Adv. Mater. 2002, 14, 71. (40) Solsona, B.; Lopez Nieto, J. M.; Diaz, U. Microporous Mesoporous Mater. 2006, 94, 339.

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microporosity; more severe hydrothermal treatment (temperatures higher than 393 K) leads to the formation of secondary porosity with diameters larger than 1.5 nm.41 On the other hand, the template removal by calcination facilitates shrinking during calcination, leading to a decrease in the micropore volume.42 Because ITQ-6 practically does not contain micropores, SBA-15 used for the grafting of amine ligands was prepared under conditions preventing micropore formation.

Experimental Section Materials. Silica Cab-O-Sil M5 (Cabot), alumina (boehmite, Catapal B), ammonium fluoride (NH4F; 98 wt %, Aldrich), hydrofluoric acid (HF; 48 wt %, Aldrich), 4-amino-2,2,6,6-tetramethylpiperidine (Fluka, 98 wt %), a cetyltrimethylammonium hydroxide solution (CTMAOH; Aldrich, 25 wt %, 50% exchanged Br/OH), a tetrapropylammonium hydroxide solution (TPAOH; Aldrich, 40 wt %, 30% exchanged Br/OH), tetraethoxysilane (TEOS; 98%, Aldrich), triblock copolymer Pluronic P123 (EO20PO70EO20; BASF/Aldrich), hydrochloric acid (HCl; 37 wt %, Aldrich), toluene (Fluka, puriss., H2O < 50 ppm), (3-aminopropyl)trimethoxysilane (APTMS; 97 wt %, Aldrich), [3-(methylamino)propyl]trimethoxysilane (MAPTMS; 98 wt %, Aldrich), and [3-(phenylamino)propyl]trimethoxysilane (PAPTMS; 98 wt %, Aldrich) were used without purification. Synthesis of ITQ-6. Delaminated material ITQ-6 was synthesized as reported in ref 38. In order to prepare precursor PREFER of the ferrierite-type structure, 10 g of silica Cab-O-Sil M5, 2.3 g of alumina Catapal B, 9.2 g of NH4F, 3.34 g of HF, 26 g of 4-amino2,2,6,6-tetramethylpiperidine, and 27.9 g of distilled water were stirred vigorously for 90 min at room temperature. Crystallization of the layered precursor PREFER was done at 448 K during 5 days in a static poly(tetrafluoroethylene)-lined stainless steel autoclave under autogenous pressure. After that time, the resulting material was recovered by filtration, exhaustively washed with distilled water, and dried at 333 K. A sample of ferrierite zeolite (FER) was prepared by calcination of a portion of the PREFER precursor in air at 853 K. The ITQ-6 sample was prepared from a PREFER precursor as follows: 10 g of the lamellar precursor was dispersed in 40 mL of distilled water. Next, 200 mL of a CTMAOH solution and 60 mL of a TPAOH solution were added. The resultant mixture was vigorously stirred at 368 K overnight. Delamination of such a prepared expanded sample PREITQ-6 was performed by placing the slurry in an ultrasound bath (50 W and 40 kHz) for 1 h, maintaining a pH of 12.5 and a temperature of 323 K. Then, by the addition of HCl (6 M), the pH was decreased to 3.0, leading to flocculation of the delaminated material. This material was recovered by centrifugation, washed out exhaustively with distilled water, filtered, and dried at 353 K overnight. Finally, the solid was calcined in air at 853 K for 3 h, yielding the delaminated ITQ-6 zeolite. Synthesis of SBA-15. A siliceous SBA-15 mesoporous molecular sieve was synthesized using Pluronic P123 as a structuredirecting agent and TEOS as a silica precursor.43 A total of 24 g of Pluronic P123 was dissolved at 308 K in a mixture of 126 mL of HCl and 774 mL of distilled water. Afterward, 51 g of TEOS was added and the mixture was vigorously stirred for 2 min and subsequently aged under static conditions at 308 K for 24 h and at 368 K for 48 h. The resulting solid was recovered by filtration, extensively washed out with distilled water and ethanol, and dried at 373 K overnight. The template was removed by calcination at 813 K for 8 h (temperature ramp of 1 K min-1). (41) Galarneau, A.; Cambon, N.; Di Renzo, F.; Ryoo, R.; Choi, M.; Fajula, F. New J. Chem. 2003, 27, 73. (42) Yang, C.-H.; Zibrowius, B.; Schmidt, W.; Sch€uth, F. Chem. Mater. 2004, 16, 2918.   (43) Zukal, A.; Siklov a, H.; Cejka, J. Langmuir 2008, 24, 9837.

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Zukal et al. Table 1. Physicochemical Properties of Prepared Materials

sample code

SBET, m2 g-1 VME, cm3 g-1 DME, nm VMI, cm3 g-1 SRs, m2 g-1 m,a mmol g-1 n,b nm-2 σ,c nm2 ϑ,d % VPRED, cm3 g-1

ITQ-6 580.4 0.318 9.5 0.066 382.1 ITQ-6/AP 102.6 0.118 9.5 1.26 1.41 0.71 42 0.224 ITQ-6/MAP 121.9 0.129 9.3 1.13 1.28 0.78 57 0.208 ITQ-6/PAP 58.3 0.102 9.8 0.85 0.99 1.00 72 0.184 ITQ-6/AP/C 375.8 0.275 9.5 0.061 212.0 SBA-15 882.9 (790.1) 0.932 (0.810) 6.2 865.6 SBA-15/AP1 561.7 0.785 5.8 1.1 0.80 1.25 24 0.784 SBA-15/AP2 459.9 0.677 5.6 2.0 1.54 0.65 46 0.663 SBA-15/MAP 445.3 0.572 5.6 2.5 2.08 0.48 94 0.511 e 307.2 0.472 4.8 1.3 1.07 0.93 77 0.475 SBA-15/PAP SBA-15/AP2/C 471.1 0.767 5.9 466.3 a The amine content is defined as the amount of ligands in mmol per 1 g of adsorbent. b The number of ligands is per 1 nm2 of the support. c The surface area of the support is per one amine ligand. d The surface coverage. e The BET surface area and mesopore volume of the starting support SBA-15 are given in parentheses.

Preparation of Functionalized Adsorbents. Previous to the amine incorporation, 1 g of ITQ-6 or SBA-15 was dehydrated by heating under vacuum at 473 K for 3 h and dispersed in 50 mL of dry toluene. After the addition of 1.0-4.0 mmol of the respective (aminopropyl)trimethoxysilane (APTMS, MAPTMS, or PAPTMS), the suspension was refluxed in an inert atmosphere for 20 h. Finally, the solid was filtered off, washed with toluene and hexane, and dried at 333 K overnight. To remove organic ligands, small portions of ITQ-6 and SBA15 samples containing 3-aminopropyl ligands were heated in air at 813 K for 2 h. Methods. The nitrogen content in functionalized adsorbents was determined by elemental analysis using a CHN 2400 (PerkinElmer) instrument. FTIR spectra were recorded with a Nicolet Protege 460 apparatus using a technique of self-supported wafers; before IR measurement, the samples were activated inside the cell at 723 K for 3 h. X-ray diffractograms (XRD) of ITQ-6 and its corresponding samples were obtained with a Bruker D8 diffractometer equipped with a graphite monochromator and positionsensitive detector (Vantec-1) using Cu KR radiation (at 40 kV and 30 mA) in Bragg-Brentano geometry. The particle morphology of ITQ-6 and SBA-15 was evaluated by scanning electron microscopy (SEM) images using a JEOL JSM-5500LV instrument. Adsorption Measurements. Adsorption isotherms of nitrogen at 77 K and CO2 in the temperature range from 273 to 333 K were determined using an ASAP 2020 (Micromeritics) static volumetric apparatus. In order to attain sufficient accuracy in the accumulation of the adsorption data, the ASAP 2020 was equipped with pressure transducers covering the 1, 10, and 1000 Torr values. Before adsorption experiments, the sample was outgassed under a turbomolecular pump vacuum using a special heating program, allowing a slow removal of most preadsorbed water at low temperatures. This was done to avoid potential structural damage to the sample due to hydrothermal alternation. Starting at ambient temperature, the samples without organic ligands were outgassed at 383 K until the residual pressure of 0.5 Pa was obtained. After further heating at 383 K for 1 h, the temperature was increased until the temperature of 623 K was achieved. This temperature was maintained for 8 h. The samples containing organic ligands were outgassed in both steps at lower temperatures. The temperature of the first step was 363 K. In the second step, the temperature of 393 K was maintained overnight. The homemade thermostat maintaining the temperature of the sample with an accuracy of (0.01 K was used for the measurement of CO2 adsorption. Adsorption isotherms at 293 K were measured on all of the samples under study. With selected samples ITQ-6, ITQ-6/AP, SBA-15, SBA-15/AP1, and SBA-15/AP2, the isotherms at 273, 293, 313, and 333 K were taken. (The exact temperature of each measurement was determined using a platinum resistance thermometer.) Because adsorption isotherms of CO2 were measured on the same sample immediately after the nitrogen adsorption measurement, the degas procedure was 10316 DOI: 10.1021/la901156z

Figure 1. XRD patterns of the lamellar precursor PREFER (a), the expanded material PREITQ-6 (b), the delaminated zeolite ITQ-6 (c), and the ferrierite (d). performed at 393 K for 12 h under a turbomolecular pump vacuum. These conditions were also applied when the CO2 measurement was repeated at another temperature.

Results and Discussion All prepared samples are listed in Table 1. The starting materials are denoted as ITQ-6 or SBA-15. The functionalized samples are labeled ITQ-6/xxx or SBA-15/xxx, where xxx denotes the abbreviations AP, MAP, and PAP of 3-aminopropyl, 3(methylamino)propyl, and 3-(phenylamino)propyl ligands, respectively. Samples labeled SBA-15/AP1 and SBA-15/AP2 differ in the content of 3-aminopropyl ligands. Functionalized samples ITQ-6/AP and SBA-15/AP2, which were heated to remove aminopropyl ligands, are denoted as ITQ6/AP/C and SBA-15/AP2/C. Characterization of the Support ITQ-6 and Adsorbents Prepared from It. The direct calcination of the layered precursor PREFER leads to FER with a Si/Al ratio g 22.44 To prepare ITQ-6, the precursor PREFER was subjected to a swollen step, yielding expanded material PREITQ-6, which was further ultrasonically treated. The XRD patterns of the precursor PREFER, expanded material PREITQ-6, ITQ-6, and FER are shown in Figure 1. As can be seen from a comparison of the XRD patterns corresponding to FER and ITQ-6, no considerable changes in the (44) Schreyeck, L.; Caullet, P.; Mougenel, J. C.; Guth, J. L.; Marler, B. Microporous Mater. 1996, 6, 259.

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Figure 2. SEM images of PREFER.

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Figure 4. IR spectra in the region of hydroxyl stretching vibrations for ITQ-6 (a) and ferrierite (b).

Figure 3. SEM images of ITQ-6.

intensity of the diffraction lines assigned to planes 0kl occur; nevertheless, the h00 diffraction lines have strongly decreased for ITQ-6, indicating a considerable loss of order along the a axis. This evidences that the resultant material is formed mostly from monolayers of the layered precursor PREFER. The layer separation is also confirmed by means of SEM investigation. SEM images of PREFER and ITQ-6 (Figures 2 and 3) clearly show the change in the particle morphology due to the delamination of the layered precursor. It can be observed that relatively large particles of the layered precursor are transformed into much smaller particles of ITQ-6. IR spectroscopy reveals a high concentration of silanol groups on the external surface of ITQ-6. Indeed, in the Fourier transform infrared (FTIR) spectrum of this material (Figure 4, spectrum a), a high-intensity band associated with the presence of silanol groups (∼3740 cm-1) can be observed in contrast to the same band in the spectra of FER displaying a lower intensity (Figure 4, spectrum b). This is consistent with the signal of (SiO)3SiOH moieties on the external surface of the delaminated material. The textural parameters of the support ITQ-6 and corresponding functionalized adsorbents were evaluated from nitrogen isotherms (Figure 5). The isotherm on the starting ITQ-6 reveals type H3 hysteresis, which is observed with nonrigid aggregates of (45) Lowell, S.; Shields, J. E.; Thomas, M. A.; Thommes, M. Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density, 1st ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2004; Chapter 4.

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Figure 5. Nitrogen adsorption isotherms at 77 K on ITQ-6 (O, offset by 50 cm3 g-1 STP), ITQ-6/AP/C (], offset by 50 cm3 g-1 STP), ITQ-6/AP (3), ITQ-6/MAP (0, offset by 50 cm3 g-1 STP), and ITQ-6/PAP (4, offset by 100 cm3 g-1 STP). Solid symbols denote desorption.

platelike particles.45 It is in accordance with a model of the ITQ-6 structure, in which individual zeolitic sheets of the precursor PREFER are ordered as a “house of cards”, prioritizing the edgeto-face orientation as opposed to the face-to-face one characteristic for PREFER.39 This arrangement introduces the meso- and macroporosity of delaminated ITQ-6. The Brunauer-Emmett-Teller (BET) specific surface area SBET was evaluated using adsorption data in a relative pressure range from 0.05 to 0.25. The mesopore volume VME and mean mesopore diameter DME were calculated from the desorption branch of the hysteresis loop using the Barrett-Joyner-Halenda algorithm. These textural parameters are summarized in Table 1. To obtain more detailed information on the texture of ITQ-6, the nitrogen isotherm was processed by means of the Rs plot using standard nitrogen adsorption data for the characterization of nanoporous silicas (Figure 6).46 Because the support ITQ-6 (46) Jaroniec, M.; Kruk, M.; Olivier, J. P. Langmuir 1999, 15, 5410.

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Figure 6. Rs plot for samples ITQ-6 (3), ITQ-6/AP/C (0), SBA-15 (O, offset by 100 cm3 g-1 STP), and SBA-15/AP2/C (4, offset by 100 cm3 g-1 STP).

contains micropores, the linear fit does not pass through the origin. To obtain the micropore volume VMI, the intercept on the adsorption axis of the extrapolated linear fit was converted to liquid volume. The linear part of the Rs plot corresponds to the formation of a monolayer and the beginning of multilayer adsorption; therefore, from the slope of the linear fit, the surface area SRs can be calculated. Determined micropore volume VMI and surface area SRs are given in Table 1. The value of VMI equal to 0.066 cm3 g-1 shows that the volume of the micropores present in ITQ-6 is rather small. Because micropores are filled with nitrogen molecules at equilibrium pressure below the lower limit of the BET equation validity, the determined BET monolayer capacity comprises not only the capacity of the monolayer but also the amount of nitrogen filling the micropore volume. Therefore, the surface area SBET is higher than the surface area SRs. The textural parameters combined with the results of chemical analysis enable us to obtain more detailed insight into the material properties of functionalized adsorbents. The amounts m of amine ligands in mmol per 1 g of the adsorbent, which were calculated from the nitrogen content determined by elemental analysis, are listed in Table 1. On the basis of these data, the average surface density n of amine ligands (i.e., the number of ligands per 1 nm2 of the ITQ-6 surface) was determined; the reciprocal value σ = 1/n denotes the average surface area of the support per one amine ligand. The found values of n and σ are listed in Table 1. The values of σ make it possible to calculate the surface coverage of ITQ-6 with amine ligands using their cross-sectional area. The cross-sectional area of the 3-aminopropyl ligand ωAP = 0.30 nm2.28 The values ωMAP = 0.45 nm2 and ωPAP = 0.72 nm2 for 3-(methylamino)propyl and 3-(phenylamino)propyl ligands, respectively, were assessed from their structure formulas. The coverage ϑAP of the support surface with the AP ligands expressed in percent is provided by the fraction ϑAP ¼ 100mAP NL ωAP =mAP NL σAP ¼ 100ωAP =σAP

ð1Þ

where mAP is the content of the AP ligands, ωAP their crosssectional area, σAP the surface area per one AP ligand, and NL Avogadro’s number. The values of ϑAP and similarly calculated values of ϑMAP and ϑPAP are given in Table 1. A large surface area of ITQ-6 attaining 580.4 m2 g-1, together with a high concentration of silanol groups, shows that this material could act as an optimum support for the immobilization of amine ligands. With functionalized adsorbents prepared from 10318 DOI: 10.1021/la901156z

it, the amount m of ligands decreases with an increase in the ligand size from 1.26 to 0.85 mmol g-1 from ITQ-6/AP to ITQ-6/PAP. Consequently, the surface density of the ligands decreases from 1.41 AP ligand per nm2 to 0.99 PAP ligand per nm2. With an increase in the ligand size, the coverage ϑ of the ITQ-6 surface increases from 42% to 72% in the order ϑAP < ϑMAP < ϑPAP. Analysis of the nitrogen isotherms on functionalized adsorbents based on the Rs plot reveals that they do not contain any micropores. A striking feature of the structure of these adsorbents is a remarkable decrease in the surface area SBET and mesopore volume VME compared with the same parameters of the ITQ-6 support. With the sample ITQ-6/PAP, the surface area SBET and mesopore volume VME drop to 58.3 m2 g-1 and 0.102 cm3 g-1, respectively. The volume of the ligands can be roughly estimated from their content m and the density of liquid propylamine, Nmethylpropylamine, or N-phenylpropylamine. A predicted mesopore volume VPRED of the functionalized adsorbents can then be calculated by subtraction of the ligand volume from the mesopore volume of the support ITQ-6. A comparison of predicted volume VPRED and experimentally determined volume VME in Table 1 shows that the experimentally determined mesopore volume VME of the adsorbents ITQ-6/AP, ITQ-6/ MAP, and ITQ-6/PAP is smaller than the predicted volume VPRED. The found difference in the predicted and determined mesopore volumes could be a consequence of a breakdown of the ITQ6 structure or the pore plugging by the organic ligands. In order to assess the influence of functionalization on the structure of ITQ-6, the sample ITQ-6/AP/C was prepared by ligand removal from the sample ITQ-6/AP by oxidation in air at 813 K for 2 h. The textural parameters of the sample ITQ-6/AP/C were evaluated from the nitrogen adsorption data in the same way as that for the sample ITQ-6. The nitrogen adsorption isotherm in Figure 5 reveals that the H3 type of hysteresis is preserved; this fact indicates maintenance of the delaminated structure. The surface area SBET and mesopore volume VME are due to the grafting and subsequent burning off of the ligands somewhat lower than the same parameters of the sample ITQ-6 (Table 1). The micropore volume VMI determined from the Rs plot (Figure 6 and Table 1) is practically unchanged. Hence, it can be concluded that ITQ-6 is a stable support because treatment of the sample ITQ-6/AP/C has caused only a small decrease in the pore volume and some smoothing of the surface of the ITQ-6 layers. These changes of the structure parameters (especially the decrease in the surface area) could be related to the silica that remains in the surface of the material as a consequence of removal by oxidation in air of the ligands containing Si. It will be shown below that all of the grafted amine ligands are accessible for adsorption of CO2 molecules. From this, it follows that the pore plugging could be ruled out. We can suppose that because of the randomly stacked ITQ-6 layers the simple calculation of the ligand volume based on the approximation of the ligand density by the density of corresponding liquid amines is not possible. Characterization of the Support SBA-15 and Adsorbents Prepared from It. Nitrogen adsorption isotherms at 77 K recorded on the parent SBA-15 silica, functionalized samples, and sample SBA-15/AP2/C, from which the ligands were removed by heating, are shown in the Supporting Information. All isotherms are characterized by the H1 hysteresis loop being typical of the SBA-15 mesoporous structure. This evidences that the SBA-15 porous structure is preserved in all of the functionalized samples. Structural parameters of all samples under study in Table 1 were evaluated in the same way as that for ITQ-6 Langmuir 2009, 25(17), 10314–10321

Zukal et al.

Article Table 2. Adsorption of CO2 at 293 K 3

-1

a (cm g

η

STP)

sample code

p = 7.5 Torr

p = 90 Torr

p = 750 Torr

ITQ-6 ITQ-6/AP ITQ-6/MAP ITQ-6/PAP SBA-15 SBA-15/AP1 SBA-15/AP2 SBA-15/MAP SBA-15/PAP

1.59 8.69 5.69 0.37 0.35 1.78 6.54 5.40 0.24

7.71 15.11 10.88 3.25 3.43 5.59 12.21 10.67 2.20

24.45 26.84 21.14 11.05 20.83 18.91 23.68 22.37 11.20

Figure 7. Adsorption isotherms of CO2 at 293 K on ITQ-6 (O), ITQ-6/AP (4), ITQ-6/MAP (0), and ITQ-6/PAP (]).

materials. The transformation of nitrogen isotherms on SBA-15 and SBA-15/AP2/C into Rs plots (Figure 6) shows that these silicas practically do not contain micropores. Therefore, surface areas SRs calculated from the slope of the Rs plots are close to the BET surface areas SBET. Inspection of Table 1 reveals that owing to the functionalization the surface area 882.9 m2 g-1 of the SBA-15 support decreases; the minimum value of 307.2 m2 g-1 is achieved with the sample SBA-15/PAP. The mesopore volume of the support decreases from 0.932 to 0.472 cm3 g-1 for the sample SBA-15/ PAP with the most bulky PAP ligand. The mesopore diameter 6.2 nm of SBA-15 is reduced to 4.8 nm of SBA-15/PAP. Similarly to the ITQ-6 materials, the results of the chemical analysis of the functionalized SBA-15 samples were used for evaluation of the average surface density n of amine ligands, average surface area σ of the support per one amine ligand, and surface coverage ϑ (Table 1). The surface coverage of the samples SBA-15/AP2 and SBA-15/PAP is close to that of the samples ITQ-6/AP and ITQ-6/ PAP. In contrast to the sample ITQ-6/MAP, the surface coverage of the sample SBA-15/MAP achieves nearly 100%. The predicted mesopore volume VPRED of the functionalized SBA-15 silicas was calculated in the same way as that in the case of ITQ-6 adsorbents. The calculated values of VPRED (Table 1) are close to the mesopore volume VME. Thus, it appears that for the inner surface of a well-organized mesopore structure use of the density of liquid amines instead of the ligand density is possible. It is also evident that pore plugging by amine ligands does not occur. The surface area 471.1 m2 g-1 of the sample SBA-15/AP2/C is significantly lower than that of the support SBA-15. On the other (47) Imperor-Clerc, M.; Davidson, P.; Davidson, A. J. Am. Chem. Soc. 2000, 122, 11925.

Langmuir 2009, 25(17), 10314–10321

p = 7.5 Torr

p = 90 Torr

p = 750 Torr

0.31 0.22 0.02

0.53 0.43 0.17

0.95 0.83 0.59

0.07 0.14 0.10 0.01

0.23 0.27 0.19 0.07

0.76 0.53 0.40 0.39

Figure 8. Adsorption isotherms of CO2 at 293 K on SBA-15 (O), SBA-15/AP1 (3), SBA-15/AP2 (4), SBA-15/MAP (0), and SBA15/PAP (]).

hand, the decrease in the mesopore volume and mesopore diameter is not so pronounced. Several recent studies43,47,48 have indicated that the cylindrical mesopores are surrounded by a corona, which may represent some surface corrugations of the pore walls.48 The corona consists of silica of a reduced mean density, while the pore walls (i.e., the outer region of the corona) are composed of a dense silica; however, the surface corrugations do not contribute to the micropore volume. The reason why the surface area of the sample SBA-15/AP2/C became lower than the surface area of the starting silica SBA-15 can be considered as a consequence of the densification of the corona because of the silica remaining on the inner mesopore pore surface after removal of the ligands containing Si. A small decrease in the mesopore volume indicates that the mesopore structure is essentially preserved. Adsorption of CO2. Adsorption isotherms of CO2 at 293 K on supports ITQ-6 and SBA-15 and functionalized adsorbents are displayed in Figures 7 and 8. The determination of Henry’s constant was unreliable because of the very high steepness of the CO2 isotherms on adsorbents functionalized with AP and MAP ligands. Therefore, the adsorption ability of materials under study in a low-pressure region was characterized by means of amounts adsorbed at 7.5 Torr calculated by interpolation of the isotherms; the obtained data are listed in Table 2. Besides that, the amounts adsorbed at two higher pressures were determined (Table 2). Because capture of CO2 from flue gas requires adsorption at a partial pressure of around (48) Shenderovich, I. G.; Buntkowsky, G.; Schreiber, A.; Gedat, E.; Sharif, S.; Albrecht, J.; Golubev, N. S.; Findenegg, G. H.; Limbach, H.-H. J. Phys. Chem. B 2003, 107, 11924.

DOI: 10.1021/la901156z

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0.12*760 Torr, the amounts adsorbed at 90 Torr were determined. The amounts adsorbed at 750 Torr (maximum pressure achieved in the adsorption apparatus) were evaluated to characterize CO2 adsorption at high pressure. Both isotherms and interpolated data show distinct differences in the adsorption properties of studied ITQ-6 and SBA-15 materials. Small amounts of CO2 adsorbed on ITQ-6 and SBA15 at 7.5 Torr indicate that these supports do not interact very strongly with CO2 because the surface hydroxyl groups are not able to induce sufficiently strong interactions and real adsorption sites are missing. In contrast to parent supports, CO2 is adsorbed on functionalized adsorbents under dry conditions through the formation of ammonium carbamate, in which two nitrogen atoms of amine ligands are involved.28 The overall reaction between CO2 and primary (AP) or secondary (MAP and PAP) amine ligands can be expressed as CO2 þ 2R1 R2 NH f R1 R2 NH2 þ þ R1 R2 NCO2 where R1 = H for primary amines and R1/R2 = alkyl/aryl for secondary amines. The formation of carbamate requires a break of the amine N-H bond and trapping of a proton by a neighboring amine group. The described process depends on the ability of the amine ligand to split or bind the proton. The electron-donating methyl group is supposed to make the MAP ligand a stronger base than the primary amine AP ligand. However, a steric hindrance effect has to be taken into account. The result of the combination of electron-donating and steric effects is that the secondary amines are not generally stronger bases than the primary amines and therefore the AP and MAP ligands are of approximately equal basicity (pKb ≈ 4).26 In the PAP ligand, an aromatic ring connected to a nitrogen atom makes this ligand less basic than AP or MAP ligands. This is due to the fact that a lone electron pair on the nitrogen atom of the PAP ligand is delocalized into the aromatic π system. Therefore, the stabilizing overlap on the PAP ligand makes the lone electron pair less active, and this ligand is a weaker base (pKb ≈ 9).26 The interpolated adsorption data in Table 2 corresponds to the basicity of amine ligands. The sample ITQ-6/AP is characterized by the steepest isotherm with an amount of adsorbed CO2 of 8.69 cm3 g-1 STP at 7.5 Torr. The amounts adsorbed on this adsorbent at 90 and 750 Torr also attain the maximum value in comparison with other materials. The adsorbed amounts of CO2 on samples SBA-15/AP1 and SBA-15/AP2 at 7.5 Torr increase with an increase in the content of the AP ligands from 1.78 to 6.54 cm3 g-1 STP; at 90 Torr, amounts adsorbed increase from 5.59 to 12.21 cm3 g-1 STP. Similarly to ITQ-6/AP, the maximum adsorption at 750 Torr is observed on the sample SBA-15/AP2. Although the AP and MAP ligands are of approximately equal basicity, the observed difference in the amounts of CO2 adsorbed at 7.5 and 90 Torr is clearly a consequence of steric hindrance and a lower accessibility of the lone electron pair of the MAP ligand. Last but not least, a weaker basicity of the PAP ligand results in a reduced steepness of CO2 isotherms on ITQ-6/PAP and SBA-15/ PAP. It is worth noting that the amounts of CO2 adsorbed on supports ITQ-6 and SBA-15 at 750 Torr are comparable with the adsorption on the most effective adsorbents ITQ-6/AP and SBA-15/AP2. Therefore, it appears that adsorption on the bare surface of supports can play an important role at higher pressures of CO2. Adsorption isotherms of CO2 on samples ITQ-6, ITQ-6/ AP, SBA-15, SBA-15/AP1, and SBA-15/AP2 were measured at 10320 DOI: 10.1021/la901156z

Figure 9. Isosteric heat of CO2 adsorption of ITQ-6 (b), ITQ-6/ AP (2), SBA-15 (O), SBA-15/AP1 (3), and SBA-15/AP2 (4).

temperatures of 273, 293, 313, and 333 K, respectively. Adsorption isosteres calculated from isotherms using a polynomial interpolation procedure were linear in coordinates log p vs 1/T. Isosteric adsorption heats qst were then determined from the slope of adsorption isosteres using the equation ½Dðlog pÞ=Dð1=TÞa ¼ - qst =2:303R

ð2Þ

where a is the amount adsorbed and R the gas constant. Dependences of isosteric adsorption heats qst on the amount of CO2 adsorbed are shown in Figure 9. Low values of qst for ITQ-6 and SBA-15 indicate a weak interaction of the CO2 molecule with the bare support surface. On the other hand, the formation of ammonium carbamate in samples containing AP ligands gives rise to a pronounced increase in the isosteric heat of adsorption. The approximately constant value of qst ≈ 47 kJ mol-1 at the onset of CO2 adsorption on ITQ-6/AP (i.e., in the region of a < 7 cm3 g-1 STP) indicates the formation of ammonium carbamate on regularly ordered ligands. Isosteric adsorption heat of CO2 on ITQ-6/AP is higher that that on the ITQ-6 support in the whole range of the amounts adsorbed. Only when CO2 adsorption on ITQ-6/AP approaches 30 cm3 g-1, does the isosteric adsorption heat decrease close to the values obtained for the support ITQ-6. This suggests that all of the AP ligands participate in the adsorption process and plugging of the porous structure does not occur. Isosteric adsorption heats for SBA-15/ AP1 and SBA-15/AP2 are in accordance with the amine content. The isosteric heat of the sample SBA-15/AP1 is lower than that of SBA-15/AP2, and for a ≈ 20 cm3 g-1 STP, it corresponds to qst found for the support SBA-15. Isosteric heat of CO2 adsorption on SBA-15/AP2 attaining 65 kJ mol-1 for a < 5 cm3/g STP indicates a strong energetic heterogeneity of adsorption sites. This effect is probably a consequence of the surface corrugations of the pore walls of the SBA-15 support, which cause irregular localization of the AP ligands on the surface. The efficiency of the amine ligands in the adsorption of CO2 on a functionalized adsorbent can be expressed as a ratio η of adsorbed CO2 and amine content, both in mmol/g: η ¼

adsorbed CO2 ðmmol g -1 Þ amine content ðmmol g -1 Þ

ð3Þ

The adsorbed amounts interpolated for pressures 7.5, 90, and 750 Torr and converted into efficiencies η are listed in Table 2. Langmuir 2009, 25(17), 10314–10321

Zukal et al.

The values of η at pressures of 7.5 and 90 Torr show the same tendencies as the corresponding amounts adsorbed a expressed in cm3 g-1 STP; thus, they corroborate the discussion of the steepness of CO2 isotherms vide supra. The efficiencies η achieved at 750 Torr need a special discussion. The value of η = 0.5 correspond to CO2 adsorption under optimum conditions, which would take place exclusively via the formation of ammonium carbamate on densely anchored ligands affording amine pairs. However, the experimentally determined efficiencies η > 0.5 clearly indicate that adsorption of CO2 occurs simultaneously on both amine ligands and the bare surface. The steepness of CO2 isotherms indicates the formation of ammonium carbamates at low pressures; on the other hand, at higher pressures, adsorption on the bare surface prevails. One of the ways in which to understand in more detail the mode of interaction of CO2 with adsorbents is the utilization of IR spectroscopy. The in situ IR study performed by Hiyoshi et al.28 has evidenced that the densely anchored ligands affording amine pairs are effective adsorption sites. On the basis of these results, we can expect that not only the amount of amine ligands but also their ordering on the surface of the support play a decisive role in CO2 adsorption. The comparison of the efficiency η at a pressure of 750 Torr for adsorbents ITQ-6/AP, SBA-15/AP1, and SBA-15/ AP2 reveals an interplay of the amine content m, the density of ligands expressed as parameter n, and the surface coverage ϑ. With the sample ITQ-6/AP, the efficiency η attains 0.95 because of densely ordered ligands (m = 1.26 mmol g-1 and n = 1.41 nm-2) and a relatively large fraction of the bare surface (ϑ = 42%). Because the sample SBA-15/AP1 (m = 1.1 mmol g-1) is characterized by a large fraction of the bare surface (ϑ = 24%), adsorption on this surface prevails and the efficiency η = 0.76 is apparently high. The sample SBA-15/AP2 contains a larger amount of amine ligands (m = 2.0 mmol g-1). Although the number of ligands per 1 nm2 (n = 1.54 nm-2) and the coverage of the surface by amino ligands (ϑ = 46%) are comparable with the same parameters of the sample ITQ-6/AP, the efficiency η = 0.53 is significantly lower than the efficiency η = 0.95 of CO2 adsorption on ITQ-6/AP. This difference indicates that the uncovered surface of the sample SBA-15/AP2 adsorbs CO2 only in a limited extent. (It can be seen that a comparison of the amounts adsorbed on the bare surface of supports SBA-15 and ITQ-6 shows a similar effect. At 750 Torr, SBA-15 adsorbs 0.023 cm3/m2 STP; on the contrary, ITQ-6 adsorbs 0.042 cm3/m2 STP at the same pressure.) The pairs of adsorbents containing MAP and PAP ligands show tendencies similar to those of samples ITQ-6/ AP and SBA-15/AP2. With samples ITQ-6/MAP and SBA-15/ MAP or samples ITQ-6/PAP and SBA-15/PAP, the efficiency η at a pressure of 750 Torr decreases with an increase in the surface coverage ϑ. The adsorption isotherms of CO2 on functionalized samples are displayed in Figure 10 in coordinates η vs p. Thus, transformed isotherms enable us to compare the adsorption properties of amine-grafted ITQ-6 and SBA-15 in the whole region of CO2 pressures. It can be seen that the efficiency η of CO2 adsorption for adsorbents prepared from ITQ-6 is higher than that of adsorbents prepared from SBA-15 with the same ligands. In the low-pressure region (p < 90 Torr), the maximum efficiency of

Langmuir 2009, 25(17), 10314–10321

Article

Figure 10. Adsorption isotherms of CO2 at 293 K on ITQ-6/AP (2), ITQ-6/MAP (9), and ITQ-6/PAP ([), SBA-15/AP1 (3), SBA-15/AP2 (4), SBA-15/MAP (0), and SBA-15/PAP (]).

CO2 adsorption on ITQ-6 with 3-aminopropyl ligands is more than 2 times higher than that found for an analogous adsorbent prepared from SBA-15 silica.

Conclusions Characterization of ITQ-6 proved that this delaminated zeolite possesses a large hydroxylated and accessible external surface, and therefore it can act as a support of the amine ligands. In the present work, 3-aminopropyl, 3-(methylamino)propyl, and 3(phenylamino)propyl ligands were grafted in ITQ-6. The adsorption properties of prepared materials toward CO2 were compared with those of the amine-functionalized mesoporous SBA-15 silica containing the same amine ligands. The experimentally determined efficiencies of amine ligands in ITQ-6 clearly indicate that adsorption of CO2 occurs simultaneously on both amine ligands and the bare surface. At low pressures of CO2, ammonium carbamates are formed; at higher pressures, adsorption on the bare surface prevails. The adsorption capacity of delaminated zeolite ITQ-6 and the efficiency of amine ligands, which is higher than that of a functionalized SBA-15 silica, prove to be advantages of the delaminated zeolite ITQ-6 as a support and open new possibilities for the synthesis of amine-functionalized adsorbents. Acknowledgment. This work was supported by the Grant Agency of the Czech Republic (Project 203/08/0604) and the EU (Project DeSSANS, SES6-CT-2005-020133). The work of I.D. was supported by EU (Project INDENS, MRTN-CT-2004005503). Supporting Information Available: Nitrogen adsorption isotherms at 77 K on the SBA-15 silica, functionalized samples, and sample SBA-15/AP2/C and a low-pressure region of adsorption isotherms of CO2 at 293 K on ITQ-6, ITQ-6/AP, ITQ-6/MAP, and ITQ-6/PAP. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la901156z

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