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Langmuir 2007, 23, 13164-13168
Hydroxyl Species in Large-Pore Phenylene-Bridged Periodic Mesoporous Organosilica B. Camarota,† B. Onida,† Y. Goto,‡ S. Inagaki,‡ and E. Garrone*,† Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Torino, INSTM, Torino Politecnico Unit, and Toyota Central R&D Laboratories, Inc., Nagakute, Aichi, 480-1192, Japan ReceiVed July 25, 2007. In Final Form: September 24, 2007 Silanol species in phenylene-bridged periodic mesoporous organosilica (PMO), templated via tri-block copolymer Pluronic P123 and thus characterized by large pores and amorphous wall structure, have been characterized by means of FT-IR spectroscopy. Investigation has been carried out on both the naked sample outgassed at different temperatures and the sample when interacting with molecular probes able to form H-bonding (ammonia and carbon monoxide). After outgassing at 773 K, the material shows both isolated silanols and silanols engaged in “intraframework” H-bonding with the π-cloud of structural aromatic rings. Interaction with ammonia showed that a fraction of these species is inaccessible, being probably located inside the pore walls. Thermal treatment above 673 K causes the appearance of SiO3(OH) species formed as a consequence of the cleavage of some Si-C bonds. The presence of hydroxyls slightly more acidic than isolated silanols has been evidenced: these are interpreted as perturbed geminal species.
Introduction Periodic mesoporous organosilicas (PMOs) are hybrid materials synthesized from organosilane precursors and surfactant templates, capable of being further processed so to acquire properties as functional materials, adsorbents, and catalysts.1-5 PMOs modified with acidic sulfonic groups have been prepared and tested in a number of acid-catalyzed reaction.6-9 In particular, phenylene-bridged PMOs functionalized with sulfonic groups show activity in reactions such as condensation of phenol with acetone to form biphenol,10 isopropylation of phenol and dimerization of 2-phenylpropene,11 and Friedel-Crafts acylation.12 These materials are attracting increasing attention, one reason being the high mechanical and hydrothermal stability of their hybrid matrix.13 * To whom correspondence should be addressed. E-mail: edoardo.
[email protected]. Phone: +39-0115644661. Fax: +39-0115644699. † Politecnico di Torino. ‡ Toyota Central R&D Laboratories, Inc. (1) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302-3308. (2) Asefa, T.; MacLachan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867-871. (3) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611-9614. (4) Hatton, B.; Landskron, K.; Whitnall, W.; Perovic, D.; Ozin, G. A. Acc. Chem. Res. 2005, 38, 305-312. (5) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. J. Nanosci. Nanotechnol. 2006, 6, 265-288. (6) Hamoudi, S.; Kaliaguine, S. Microporous Mesoporous Mater. 2003, 59, 195-204. (7) Hamoudi, S.; Royer, S.; Kaliaguine, S. Microporous Mesoporous Mater. 2004, 71, 17-25. (8) Nakajima, K.; Tomita, I.; Hara, M.; Hayashi, S.; Domen, K.; Kondo, J. N. Catal. Today 2006, 116, 151-156. (9) Yuan, X. D.; Lee, H. I.; Kim, J. W.; Yie, J. E.; Kim, J. M. Chem. Lett. 2003, 32, 650-651. (10) Yang, Q. H.; Liu, J.; Yang, J.; Kapoor, M. P.; Inagaki, S.; Li, C. J. Catal. 2004, 228, 265-272. (11) Rac, B.; Hegyes, P.; Forgo, P.; Molnar, A. Appl. Catal. A-Gen. 2006, 299, 193-201. (12) Kapoor, M. P.; Kasama, Y.; Yanagi, M.; Yokoyama, T.; Inagaki, S.; Shimada, T.; Nanbu, H.; Juneja, L. R. Microporous Mesoporous Mater. 2007, 101, 231-239. (13) Burleigh, M. C.; Markowitz, M. A.; Jayasundera, S.; Spector, M. S.; Thomas, C. W.; Gaber, B. P. J. Phys. Chem. B 2003, 107, 12628-12634.
The activity of solid catalysts is dramatically influenced by their surface properties, which in turn strongly depend on nature and abundance of surface species, such as hydroxyls. In previous studies, the surface of phenylene-bridged PMOs, either with crystal-like wall structures14 or partially ordered wall structures,15 prepared in basic media using alkyltrimethylammonium as a templating surfactant, has been investigated by means of IR spectroscopy and ab initio computations.15,16 In the present work, the study is extended to a large-pore phenylene-bridged PMO, prepared in acidic conditions in presence of Pluronic P123 tri-block copolymer as surfactant, exhibiting no molecular order in the pore walls.17 Attention has been focused on the identification and the characterization of hydroxyl species, studied by infrared spectroscopy following both the effect of outgassing treatments at increasing temperature and the adsorption of molecular probes capable of H-bonding interactions (ammonia and carbon monoxide). Experimental Section The synthesis of the phenylene-bridged large-pore hexagonal mesoporous organosilica (hereafter denoted as P-Ph-HMM, P referring to Pluronic surfactant) was described in detail in ref 17.17 XRD analysis of the surfactant-free material revealed a well-defined pattern with three peaks in the low-angle region which are typical of a two-dimensional hexagonal symmetry with a cell parameter a of 10.7 nm. Nitrogen adsorption isotherms allowed to calculate the BET surface area, 938 m2 g-1, and the BJH pore diameter, 6.76 nm. Because those values are in agreement with those reported in literature, spectra were not reported. For FT-IR measurements, the powder was pressed into a thin self-supporting wafer (1.2 ton cm-2), then placed into a quartz cell allowing thermal treatments in controlled atmosphere. Before adsorption of probe molecules, samples were outgassed at regular (14) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304307. (15) Onida, B.; Camarota, B.; Ugliengo, P.; Goto, Y.; Inagaki, S.; Garrone, E. J. Phys. Chem. B 2005, 109, 21732-21736. (16) Onida, B.; Borello, L.; Busco, C.; Ugliengo, P.; Goto, Y.; Inagaki, S.; Garrone, E. J. Phys. Chem. B 2005, 109, 11961-11966. (17) Goto, Y.; Inagaki, S. Chem. Commun. 2002, 2410-2411.
10.1021/la702252j CCC: $37.00 © 2007 American Chemical Society Published on Web 11/17/2007
Hydroxyl Species in Mesoporous Organosilica
Figure 1. IR spectrum of P-Ph-HMM outgassed at 773 K in the O-H stretching region. intervals between room temperature and 773 K for about 1 h. Spectra were recorded using a Bruker FTIR Equinox 55 spectrometer, equipped with a MCT cryodetector, at a resolution of 2 cm-1. Adsorption of gases was carried out by connecting the IR cell to a vacuum frame (residual pressure
