FTIR Spectroscopic Study of Titanium-Containing Mesoporous Silicate

The surface acidity of different mesoporous titanium-silicates, such as well-organized hexagonally packed Ti-MMM, Ti-MMM-2, Ti-SBA-15, and amorphous ...
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Langmuir 2005, 21, 10545-10554

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FTIR Spectroscopic Study of Titanium-Containing Mesoporous Silicate Materials Natalia N. Trukhan,* Alexander A. Panchenko, and Emil Roduner Institute of Physical Chemistry, Stuttgart University, Pfaffenwaldring 55, D-70569 Stuttgart, Germany

Maxim S. Mel’gunov and Oxana A. Kholdeeva Boreskov Institute of Catalysis, Acad. Lavrentiev Av. 5, 630090 Novosibirsk, Russia

Julita Mrowiec-Białon´ Institute of Chemical Engineering, Polish Academy of Sciences, Bałtycka 5, 44-100 Gliwice, Poland

Andrzej B. Jarze¸ bski Department of Chemical Engineering, Silesian University of Technology, M. Strzody 7, 44-100 Gliwice, Poland Received June 2, 2005. In Final Form: August 10, 2005 The surface acidity of different mesoporous titanium-silicates, such as well-organized hexagonally packed Ti-MMM, Ti-MMM-2, Ti-SBA-15, and amorphous TiO2-SiO2 mixed oxides (aerogels and xerogels), was studied by means of FTIR spectroscopy of CO adsorbed at 80 K and CD3CN adsorbed at 293 K. The surface hydroxyl groups of mesoporous titanium-silicates with 2-7 wt % Ti revealed a Bro¨nsted acidity slightly higher to that of pure silicate. TiO2-SiO2 xerogels revealed the highest Bro¨nsted acidity among the titanium-silicates studied. CO adsorption revealed two additional sites on the surface in comparison to pure silicate, characterized by ν(CO) from 2185 (high pressure) to 2178 (low pressure) cm-1 and from 2174 (high pressure) to 2170 (low pressure) cm-1. These bands are due to CO adsorbed on isolated titanium cations in the silica surrounding or having one Ti4+ cation in their second coordination sphere and due to CO interactions with Ti-OH groups, respectively. CD3CN adsorption similarly revealed the existence of two additional sites, which were not detected for pure silicate: at 2289 cm-1 due to CD3CN interaction with titanol groups and from 2306 (low pressure) to 2300 (high pressure) cm-1 due to acetonitrile interaction with isolated framework titanium cations with probably one Ti4+ cation in their second coordination shell. The spectroscopic results are compared with computational data obtained on cluster models of titaniumsilicate with different titanium content. According to the IR data, the Ti accessibility on the surfaces for mesoporous titanium-silicates with similar Ti loading (2 wt %) was found to fall in the order TiO2-SiO2 aerogel ∼ TiO2-SiO2 xerogel > Ti-MMM ∼ Ti-MMM-2 > Ti-SBA-15. This order (except TiO2-SiO2 xerogel) correlates with the catalytic activity found previously for titanium-silicates in 2,3,6-trimethylphenol oxidation with H2O2.

1. Introduction Titanium-containing molecular sieves attract much attention as catalysts for selective oxidations of various organic substrates with H2O2 or tert-butyl hydroperoxide.1-6 Developed by “Enichem”, microporous titaniumsilicalite, TS-1, has been established to be a highly selective and efficient heterogeneous catalyst for H2O2-based oxidations, including epoxidation, ammoximation, and hydroxylation; however, its applications are limited to molecules smaller than 6 Å due to its rather small pore * To whom correspondence should be addressed. Tel.: (+49) 711685-4489. Fax: (+49) 711-685-4495. E-mail: n.trukhan@ ipc.uni-stuttgart.de. (1) Notari, B. Adv. Catal. 1996, 41, 253. (2) Corma, A. Chem. Rev. 1997, 97, 2373. (3) Clerici, M. G. Top. Catal. 2000, 13, 373. (4) Gao, X.; Wachs, I. E. Catal. Today 1999, 51, 233 and references therein. (5) Trong On, D.; Desplantier-Giscard, D.; Danumah, C.; Kaliaguine, S. Appl. Catal. A: Gen. 2001, 222, 299. (6) Perego, C.; Carati, A.; Ingallina, P.; Mantegazza, M. A.; Bellussi, G. Appl. Catal. A: Gen. 2001, 221, 63 and references therein.

size (5.3 × 5.6 Å).1,3,6 Following the discovery of TS-1 in the early 1980s, a large scientific effort has been made on the synthesis and characterization of various large-pore titanium-containing molecular sieves.2,5,7-9 Both structurally organized mesoporous molecular sieves, such as Ti-MCM-41,2,5 Ti-MCM-48,10,11 Ti-HMS,12,13 Ti-TUD1,14 Ti-MSU-1,15 Ti-SBA-1,16 Ti-MMM,17-19 Ti-SBA15,20,21 Ti-MMM-2,22 and amorphous TiO2-SiO2 mixed (7) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (8) Dusi, M.; Mallat, T.; Baiker, A. Catal. Rev.-Sci. Eng. 2000, 42, 213. (9) Ziolek, M. Catal. Today 2004, 90, 145 and references therein (10) Koyano, K. A.; Tatsumi, T. Chem. Commun. 1996, 145. (11) Zhang, W.; Pinnavaia, T. J. Catal. Lett. 1996, 38, 261. (12) Tanev, P. T.; Chibwe, M.; Pinnnavaia, T. J. Nature 1994, 368, 321. (13) Zhang, W.; Fro¨ba, M.; Wang, J.; Tanev, P. T.; Wong, J.; Pinnavaia, T. J. J. Am. Chem. Soc. 1996, 118, 9164. (14) Shan, Z., Jansen, J. C., Marchese, L., Maschmeyer, T. Microporous Mesoporous Mater. 2001, 48, 181. (15) Bagshaw, S. A.; Pouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (16) Ji, D.; Ren, T.; Yan, L.; Suo, J. Mater. Lett. 2003, 57, 4474.

10.1021/la0514516 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/27/2005

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oxides (aerogels and xerogels)8,23-25 have been found to catalyze selective oxidation of bulky organic molecules. Recently, we reported the catalytic studies of selective oxidations of several organic substrates using 30% aqueous H2O2 as oxidant and well-organized mesoporous titaniumsilicate materials Ti-MMM (MCM-41 type structure), TiMMM-2, Ti-SBA-15, and amorphous TiO2-SiO2 aerogels and xerogels as catalysts.17-20,22-25 The optimization of catalytic activities, yields, and selectivities, as well as the investigation of catalyst recycling and suggestions for the mechanisms of the catalytic reactions, were reported. It was noted that acidic properties of Ti-containing silica materials strongly depend on the preparation method, synthesis conditions, and chemical composition,26 which are in turn related to the Ti-O-Si connectivities, the degree of surface hydroxylation, and titanium distribution in a silicate matrix. Several studies have been published concerning the acidic properties of microporous TS-126-31 and TiO2-SiO2 mixed oxides27,32-35 using IR spectroscopy of adsorbed probe molecules. A detailed IR study of the acidic properties of mesoporous titaniumsilicates with adsorbed probe molecules has not been done yet. Usually, weakly (CO, N2), intermediately (CD3CN), or strongly (NH3, pyridine) interacting bases are used as probe molecules.36-40 Weakly interacting probe molecules are recommended, because they are much more specific than strongly interacting probes. Hence, weak bases, for instance carbon monoxide, are to be preferred against the (17) Trukhan, N. N.; Derevyankin, A. Yu.; Shmakov, A. N.; Paukshtis, E. A.; Kholdeeva, O. A.; Romannikov, V. N. Microporous Mesoporous Mater. 2001, 44-45, 603. (18) Trukhan, N. N.; Romannikov, V. N.; Paukshtis, E. A.; Shmakov, A. N.; Kholdeeva, O. A. J. Catal. 2001, 202, 110. (19) Kholdeeva, O. A.; Derevyankin, A. Yu.; Shmakov, A. N.; Trukhan, N. N.; Paukshtis, E. A.; Tuel, A.; Romannikov, V. N. J. Mol. Catal. A, Chem. 2000, 158, 417. (20) Trukhan, N. N.; Romannikov, V. N.; Shmakov, A. N.; Vanina, M. P.; Paukshtis, E. A.; Bukhtiyarov, V. I.; Kriventsov, V. V.; Danilov, I. Yu.; Kholdeeva, O. A. Microporous Mesoporous Mater. 2003, 59, 73. (21) Tuel, A.; Hubert-Pfalzgraf, L. G. J. Catal. 2003, 217, 343. (22) Kholdeeva, O. A.; Mel’gunov, M. S.; Shmakov, A. N.; Trukhan, N. N.; Kriventsov, V. V.; Zaikovskii, V. I.; Malyshev, M. E.; Romannikov, V. N. Catal. Today 2004, 91-92, 205. (23) Kholdeeva, O. A.; Trukhan, N. N.; Vanina, M. P.; Romannikov, V. N.; Parmon, V. N.; Paukshtis, E. A.; Mrowiec-Biało’n, J.; Jarze¸ bski, A. B. Catal. Today 2002, 75, 203. (24) Mrowiec-Białon´, J.; Jarze¸ bski, A. B.; Kholdeeva, O. A.; Trukhan, N. N.; Zaikovski, V. I.; Kriventsov, V. V.; Olejniczak, Z. Appl. Catal. A: Gen. 2004, 273, 47. (25) Trukhan, N. N.; Kholdeeva, O. A. Kinet. Catal. 2003, 44, 378. (26) Manoilova, O. V.; Dakka, J.; Sheldon, R. A.; Tsyganenko, A. Stud. Surf. Sci. Catal. 1995, 94, 163. (27) Armaroli, T.; Milella, F.; Notary, B.; Willey, R. J.; Busca, G. Top. Catal. 2001, 15, 63. (28) Bonino, F.; Damin, A.; Bordiga, S.; Lamberti, C.; Zecchina, A. Langmuir 2003, 19, 2155. (29) Bolis, V.; Bordiga, S.; Lamberti, C.; Zecchina, A.; Carati, A.; Rivetti, F.; Spano´, G.; Petrini, G. Langmuir 1999, 15, 5753. (30) Bolis, V.; Bordiga, S.; Lamberti, C.; Zecchina, A.; Carati, A.; Rivetti, F.; Spano´, G.; Petrini, G. Microporous Mesoporous Mater. 1999, 30, 67. (31) Bordiga, S.; Damin, A.; Bonino, F.; Zecchina, A.; Spano`, G.; Rivetti, F.; Bolis, V.; Prestipino, C.; Lamberti, C. J. Phys. Chem. B 2002, 106, 9892. (32) Hadjiivanov, K.; Reddy, B. M.; Kno¨zinger, H. Appl. Catal. A: Gen. 1999, 188, 355. (33) Liu, Z.; Tabora, J.; Davis, R. J. J. Catal. 1994, 149, 117. (34) Odenbrand, C. U. I.; Andersson, S. L. T.; Andersson, L. A. H.; Brandin, J. G. M.; Busca, G. J. Catal. 1990, 125, 541. (35) Fernandez, A.; Leyrer, J.; Gonza´lez-Elipe, A. R.; Munuera, G.; Kno¨zinger, H. J. Catal. 1988, 112, 489. (36) Kiselev, A. V.; Lygin, V. I. Infrared spectra of surface compounds; John Wiley & Sons: New York, 1975. (37) Kno¨zinger, H.; Huber, S. J. Chem. Soc., Faraday Trans. 1998, 94, 2047. (38) Zecchina, A.; Lamberti, C.; Bordiga, S. Catal. Today 1998, 41, 169. (39) Coluccia, S.; Marchese, L.; Martra, G. Microporous Mesoporous Mater. 1999, 30, 43. (40) Busca, G. Catal. Today 1998, 41, 191.

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classical probe molecules pyridine and ammonia for the characterization of solid acids. The detailed criteria for the selection of probe molecules have been formulated in ref 37. The acidic catalyst surface may have protonic (Bro¨nsted sites) and aprotonic (Lewis) sites. In titaniumsilicates, protonic Bro¨nsted sites are typically surface OH groups and aprotonic Lewis sites are framework Ti at the surface (and possibly some other cations such as Na+, K+, Al3+, if present in the catalyst). A basic (H-bond acceptor) probe molecule B will interact with hydroxy groups via H-bonding

OHs + B h OHs ‚‚‚B which results in a weakening of the O-H bond and in a shift (∆νOH) of the O-H stretching frequency to lower values. The larger |∆νOH|, the higher the acidic strength of the Bro¨nsted sites. In the case of aprotonic Lewis sites Mx+, the adsorption of the base B leads to a Lewis acidbase adduct:

Mx+ + B h Mx+ r B In this case, the perturbation of the base B leads to a modification of the bond energies and normal-mode frequencies. The higher the metal cation acidity, the higher the shift of adsorbed molecules stretching frequencies. In addition, FTIR spectroscopy of adsorbed probe molecules is a potentially useful technique to investigate the titanium accessibility in porous titanium-silicates.20 However, the accessibility of catalytically active Ti sites on the titaniumsilicate surfaces has not been sufficiently studied. In the present work, we performed an investigation of acid properties of various mesoporous titanium-silicates, such as Ti-MMM, Ti-MMM-2, Ti-SBA-15, and TiO2SiO2 mixed oxides prepared by different methods, by using FTIR spectroscopy. It revealed the structure, surface concentration, and accessibility of titanium centers active in catalytic oxidation. Two probe molecules, CO and CD3CN, having a basic character were used for determination of the strength of Bro¨nsted and Lewis acid centers. FTIR studies of the weakly interacting carbon monoxide and acetonitrile, which interacts in a stronger manner, provide complementary information about the nature of the active sites. To understand the effect of the local environment on the strength of the acid sites, IR frequencies of the adsorbed molecules were calculated for cluster models of titanium-silicate with different titanium content. 2. Experimental Section 2.1. Samples and Experimental Methods. Ti-MMM materials were synthesized by hydrothermal treatment under weakly alkaline conditions in the presence of C16H33N(CH3)3Br.17 Ti-MMM-222 and Ti-SBA-1520 materials were prepared by hydrothermal synthesis under moderately acidic conditions using C16H33N(CH3)3Br and the neutral triblock copolymer Pluronic EO20PO70EO20 (P123), respectively. TiO2SiO2 mixed oxides (aerogels and xerogels) were synthesized by the sol-gel method as described previously.23 For IR transmission measurements, the samples were prepared by powder pressing into self-supporting wafers under a pressure of ∼300 bar. The typical density of the wafers obtained was 6-20 mg/cm2. The wafers were calcined in situ in the IR cell by increasing the temperature to 623 K (5 K/min) under vacuum (∼2 × 10-5 mbar) and keeping at this temperature for 2 h. FTIR spectra were recorded on a Nicolet Magna 560 spectrometer with a resolution of 2 cm-1, accumulating 200 scans. The IR cell was equipped with CaF2 windows. CO adsorption was carried out at 80 K and equilibrium pressures of PCO ) 0.1-40 mbar. For CD3CN

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Table 1. Physicochemical and Catalytic Characteristics of the Mesoporous Titanium-Silicates Studied catalysts (wt % Ti)

ABETa (m2/g)

dpb (nm)

VMec (cm3/g)

Vµd (cm3/g)

λmaxe (nm)

TMP convn/ TMBQ yieldf (%)

TiO2-SiO2 aerogel23 (1.68) TiO2-SiO2 aerogel23 (6.52) Ti-MMM18 (2.31) Ti-MMM-222 (2.22) TiO2-SiO2 xerogel23 (1.44) TiO2-SiO2 xerogel23 (7.29) Ti-SBA-1520 (2.05) Ti-SBA-1520 (7.17) MMM-2

770 651 1289g 1198g 737 519 603g 558g 1059g

15.5 15.2 3.45 3.20 2.96 2.87 10.6 10.9 3.20

3.03 2.41 0.90 0.74 0.21 0.17 1.34 1.10 0.65

0.21 0.15 -

229 258 226 211 216 238 248, sh 296 271, sh 300 -

99/95 92/76 96/77 100/80 40/33 24/19 20/8 10/5 -

a BET specific surface area. b Mesopore diameter. c Mesopore volume. d Micropore volume. e In DR-UV spectrum. f Catalytic results in the oxidation of 2,3,6-trimethylphenol (TMP) to 2,3,5-trimethyl-1,4-benzoquinone (TMBQ) with H2O2 in CH3CN: TMP conversion and TMBQ yield after 25 min of the reaction.18,20,22,23 g AMe + Aext, AMe, specific mesopore surface area; Aext, specific external surface area.

adsorption, the spectra were run at 293 K and equilibrium pressures PCD3CN ) 0.1-5 mbar and after 5 mbar adsorption followed evacuation during 5, 20, and 60 min. In the IR experiments, CD3CN was used instead of CH3CN to avoid the spectroscopic complication due to Fermi resonance between the ν(CN) vibration and the combination mode of δ(CH3) + ν(CC).41 2.2. Theoretical Approach. In the present work, a cluster model is used to simulate the Ti sites in mesoporous titaniumsilicates in order to study interactions of the adjacent OH groups and titanium with CO and CH3CN molecules. All calculations reported here were performed using the Gaussian 03 software.42 Density functional theory as implemented in the B3LYP hybrid exchange-correlation scheme was used to include some effects of electron correlation. The B3LYP functional employs a threeparameter linear combination of Hartree-Fock exchange, the 1988 Becke density gradient correction to exchange,43 and the LYP correction to correlation of Lee, Yang, and Parr.44 A LANL2DZ effective core potential was used to represent titanium and a 6-311+G(d,p) basis set for all other atoms. In a widely accepted model, a mesoporous MCM-41 system is considered to consist of hexagonally arranged cylindrical pores with amorphous silica walls.2,7 Aerogels and xerogels have amorphous structures.45 It is well-known that mesoporous titanium-silicates are characterized by a more hydrophilic surface in comparison to microporous TS-1.1,2,46,47 According to XANES and EXAFS analyses, tripodally bound HO-Ti(OSi)3 active centers predominate on the surface of the Ti-MCM-41 catalysts over tetrapodal Ti(OSi)4 centers, which are usually considered for TS-1 zeolite.48-50 We used typical values for angles and bond lengths for the amorphous titania-silica oxide materials and mesoporous Ti-MCM-41 for construction of the clus(41) Angell, C. L.; Howell, M. V. J. Chem. Phys. 1969, 73, 2551. (42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.04; Gaussian, Inc.: Pittsburgh, PA, 2003. (43) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (44) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (45) Iler, R. K. The chemistry of silica: Solubility, polymerisation, colloid and surface properties and biochemistry of silica; Wiley: New York, 1979; Vol. 2. (46) Vayssilov, G. N. Catal. Rev.-Sci. Eng. 1997, 39, 209. (47) Drago, R. S.; Dias, S. C.; McGilvray, J. M.; Mateus, A. L. M. L. J. Phys. Chem. B 1998, 102, 1508. (48) Thomas, J. M.; Sankar, G. Acc. Chem. Res. 2001, 34, 571. (49) Gleeson, D.; Sankar, G.; Catlow, C. R. A.; Thomas, J. M.; Bordiga, G. S.; Zecchina, A.; Spano, C. L. Phys. Chem. Chem. Phys. 2000, 2, 4812. (50) Sankar, G.; Thomas, J. M.; Catlow, C. R. A.; Barker, C. M.; Gleeson, D.; Kaltsoyannis, N. J. Phys. Chem. B 2001, 105, 9028.

ters.4,24,48,51 The clusters TiOH(OSi(OH)3)X+1(OTi(OH)3)2-X and SiOH(OSi(OH)3)Y(OTi(OH)3)3-Y, where X ) 0-2 and Y ) 0-3, were considered. To have a more accurate description of the active site, OH terminations were used for Si and Ti atoms neighboring the central atom (Ti or Si). In all geometry optimizations, the H atoms of the nine peripheral OH groups were fixed while the other atoms in the cluster were allowed to relax. The CO molecule was attached by the C atom to the hydrogen atom of the central OH group or to titanium. The CH3CN molecule was attached by the N atom to the titanium center.

3. Results 3.1. Sample Characterization. Recently we reported the catalytic studies of selective oxidations of various organic substrates, such as cyclohexene, methyl phenyl sulfide, and 2,3,6-trimethylphenol (TMP), using aqueous 30% H2O2 as oxidant and well-organized mesoporous silicate materials Ti-MMM, Ti-MMM-2, Ti-SBA-15, and amorphous TiO2-SiO2 mixed oxides (aerogels and xerogels) as catalysts.17-20,22,24,25 All the catalysts were characterized previously with different physicochemical techniques.17-20,22,24,25 In Table 1 we present briefly the most important physicochemical and catalytic properties of these mesoporous titanium-silicates for the present discussion. 3.2. FTIR Study of the Surface Hydroxyl Groups. FTIR spectra of mesoporous titanium-silicates (Figure 1) are characterized by the sharp band at 3745 cm-1 corresponding to isolated Si-OH (Ti-OH) groups and the broad band at ∼3550 cm-1, which is attributed to hydrogen-bonded silanol (titanol) groups.26-28,36,37 Among the titanium-silicates, only TiO2-SiO2 xerogels show significant differences in the spectra, in particular a less intensive maximum of the band at 3745 cm-1 and a broader band at lower frequency. The mesoporous titaniumsilicates with high titanium loading (∼7 wt % Ti) have lower amounts of isolated silanol (titanol) groups in comparison to titanium-silicates with ∼2 wt % Ti and pure silicate MMM-2 (compare spectra 3 and 4; 7, 8, and 9 in Figure 1). In the range from 2000 to 1600 cm-1 the framework overtone vibrations were detected. 3.3. Low-Temperature CO Adsorption. After CO adsorption at 80 K in the O-H stretching region for the mesoporous titanium-silicates (Figure 2) and mesoporous pure silicate (Figure 3) the 3745 cm-1 band is converted into two broader bands. Their maxima are located at 36653645 cm-1 (with a shoulder at 3604-3591 cm-1) and 35273503 cm-1. The red-shifts of the band at 3745 cm-1 for titanium-silicates with ∼7 wt % Ti (except TiO2-SiO2 xerogels, where ∆νOH ) -100 cm-1 for both samples with 1.44 and 7.29 wt % Ti) are slightly higher (-90 ÷ -91 (51) Liu, Z.; Davis, R. J. J. Phys. Chem. 1994, 98, 1253.

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Figure 2. FTIR difference spectra of adsorbed CO at nominally 80 K on the Ti-MMM-2 (2.22% Ti) (O-H stretching region): (1) 0.1 mbar, (2) 0.5 mbar, (3) 1 mbar, (4) 5 mbar, (5) 10 mbar, (6) 20 mbar, and (7) 40 mbar CO. The spectrum of Ti-MMM-2 before interaction with adsorbate molecules (Figure 1) was used for background subtraction. The band near 3516 cm-1 and the sharp feature above 3750 cm-1 are artifacts (see the text).

Figure 3. FTIR difference spectra of adsorbed CO at nominally 80 K on the silicate MMM-2 (O-H stretching region): (1) 0.1 mbar, (2) 0.5 mbar, (3) 1 mbar, (4) 10 mbar, (5) 20 mbar, and (6) 40 mbar CO. The spectrum of MMM-2 before interaction with adsorbate molecules (Figure 1) was used for background subtraction. Figure 1. FTIR spectra at nominally 80 K of porous titaniumsilicates in the O-H stretching region before interaction with CO or CD3CN: (1) TiO2-SiO2 xerogel (7.29% Ti), (2) TiO2SiO2 xerogel (1.44% Ti), (3) TiO2-SiO2 aerogel (6.52% Ti), (4) TiO2-SiO2 aerogel (1.68% Ti), (5) Ti-MMM-2 (2.22% Ti), (6) Ti-MMM (2.31% Ti), (7) Ti-SBA-15 (7.17% Ti), (8) Ti-SBA15 (2.05% Ti), and (9) silicate MMM-2. (The 3745 cm-1 bands (4 and 9) are cut off in intensity. There are offsets for spectra in the Y axis.)

cm-1) than ∆νOH for titanium-silicates with ∼2 wt % Ti (-82 ÷ -87 cm-1) and pure silicate MMM-2 (-79 cm-1) (Table 2). The second band, which appears after CO adsorption at lower frequency, 3527-3503 cm-1, was already observed for titanium-silicates (Figure 2, spectrum 1) and pure silicate (Figure 3, spectrum 1) at the lowest CO pressure. After the subsequent increase of CO pressure, the intensity of this band does not change significantly. A similar band (3522 cm-1) was detected for MMM-2 after addition of 0.1-0.5 mbar He in the IR-cell (Figure 4). Helium is not expected to adsorb at these pressures and temperature. Therefore, the appearance of this band at 3527-3503 cm-1 is an artifact that can be explained by a change of sample temperature when gas is added to the evacuated IR-cell, changing the heat conductivity and permitting full temperature equilibration between the cold sample holder and the sample. Such a difference in temperature results in a slight shift of the

narrow 3745 cm-1 band and in an intensity change in the hydrogen-bonded region of the initial spectra, which we use as backgrounds. This effect was not observed for the room-temperature CD3CN adsorption. To improve the thermal contact of the sample and the cooled part of the IR cell for experiments with low-temperature CO adsorption, some authors added a small quantity (about 0.7 mbar) of He in the sample containing volume before CO addition.26 In this case, the CO adsorption was carried out in the presence of He gas. In the carbonyl stretching region of the mesoporous titanium-silicates, five types of bands can be observed after spectral deconvolution (Figure 5): 2112, 2137, 2156, from 2174 (low pressure) to 2170 (high pressure), and from 2185 (low pressure) to 2178 (high pressure) cm-1. For the pure silicate MMM-2, only three bands appear after CO adsorption (at 2112, 2137, and 2156 cm-1). Neither the band at 2174-2170 cm-1 nor those at 21852178 cm-1 were observed on the MMM-2 silicate (Figure 6). 3.4. Room-Temperature CD3CN Adsorption. For the CD3CN adsorption, only one sharp band is observed in the O-H stretching region for the mesoporous titanium-silicates and the silicate MMM-2 (Figure 7). After CD3CN adsorption, the O-H stretching band at 3745 cm-1 is red-shifted by 310-355 cm-1 and ∆νOH is different for

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Table 2. ∆νOH Shifts for CO and CD3CN Adsorptions catalysts (wt % Ti)

νOH(CO)a (cm-1)

∆νOH(CO)b (cm-1)

νOH(CD3CN)c (cm-1)

∆νOH(CD3CN)d (cm-1)

TiO2-SiO2 xerogel (7.29) TiO2-SiO2 aerogel (6.52) Ti-SBA-15 (7.17) TiO2-SiO2 xerogel (1.44) TiO2-SiO2 aerogel (1.68) Ti-MMM-2 (2.22) Ti-MMM (2.31) Ti-SBA-15 (2.05) MMM-2

3645 3655 3654 3645 3659 3658 3660 3663 3666

-100 -90 -91 -100 -86 -87 -85 -82 -79

3390 3418 3420 3404 3427 3423 3427 3435 3430

-355 -327 -325 -341 -318 -320 -318 -310 -315

a The frequency after CO adsorption. b ∆ν c d OH(CO) ) νOH(CO) - 3745. The frequency after CD3CN adsorption. ∆νOH(CD3CN) ) νOH(CD3CN) - 3745.

Figure 4. FTIR difference spectra after He addition at nominally 80 K on the silicate MMM-2 (O-H stretching region): (1) 0.1 mbar and (2) 0.5 mbar He. The spectrum of MMM-2 before interaction with adsorbate molecules (Figure 1) was used for background subtraction. These spectra reveal that the sharp feature at 3750 cm-1 and the broad band at 3522 cm-1 are artifacts of temperature equilibration.

the various titanium-silicates (Table 2). Titaniumsilicates with a high titanium loading (∼7 wt % Ti) have a larger ∆νOH than samples with lower wt % Ti. Similar to the CO adsorption, after the CD3CN adsorption, ∆νOH for the pure silicate MMM-2 (-315 cm-1) is lower than for titanium-silicate Ti-MMM-2 with 2 wt % Ti (-320 cm-1). Again, the maximal ∆νOH is observed for the xerogel samples. In the CtN stretching region for mesoporous titaniumsilicates, four types of bands at 2262, 2273, 2289, and from 2306 (low pressure) to 2300 (high pressure) cm-1 can be distinguished after spectral deconvolution (Figure 8). For the Ti-free MMM-2, the two last bands are not observed (Figure 9). 3.5. Computational Results. Figure 10 shows optimized TiOH(CO)(OSi(OH)3)3 and TiOH(CH3CN)(OSi(OH)3)3 clusters with Ti-OH‚‚‚CO and Ti‚‚‚NCCH3 interactions. The calculated geometries and frequencies of the optimized TiOH(CO)(OSi(OH)3)X+1(OTi(OH)3)2-X, SiOH(CO)(OSi(OH)3)Y(OTi(OH)3)3-Y, and TiOH(CH3CN)(OSi(OH)3)X+1(OTi(OH)3)2-X clusters, where X ) 0-2 and Y ) 0-3, upon CO and CH3CN adsorption are given in Tables 3 and 4. The calculated geometry features of the optimized clusters in terms of the first and second shells distances of Ti-O (1.768-1.832 Å, first shell distances), Ti-Ti (3.490-3.537 Å, second shell distances), and Ti-Si (3.344-3.384 Å, second shell distances) and Ti-O-Si angles (152.2-156.4°) are in good agreement with the experimental data from EXAFS analysis.4,24,48,51 CO interacts with Si-OH and Ti-OH centers via its C atom and CH3CN with Ti(IV) via N atom, as shown in Figure

Figure 5. (a) FTIR difference spectra of adsorbed CO at nominally 80 K on the TiO2-SiO2 aerogel (6.52% Ti) (carbonyl stretching region): (1) 0.1 mbar, (2) 0.5 mbar, (3) 1 mbar, (4) 5 mbar, (5) 10 mbar, (6) 20 mbar, and (7) 40 mbar CO. (b) Spectrum at 1 mbar CO with deconvolution. The spectrum of TiO2-SiO2 aerogel (6.52% Ti) before interaction with adsorbate molecules (Figure 1) was used for background subtraction. (There are offsets for spectra in the Y axis.)

10. Ti‚‚‚NCCH3, Ti-O-H‚‚‚CO, and Si-O-H‚‚‚CO distances as well as ∆νCN, ∆νOH, ∆νCO are listed in Tables 3 and 4.

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Figure 6. FTIR difference spectra of adsorbed CO at nominally 80 K on the silicate MMM-2 (carbonyl stretching region): (1) 0.1 mbar, (2) 0.5 mbar, (3) 1 mbar, (4) 10 mbar, (5) 20 mbar, and (6) 40 mbar CO. The spectrum of MMM-2 before interaction with adsorbate molecules (Figure 1) was used for background subtraction. (There are offsets for spectra in the Y axis.)

Figure 7. FTIR difference spectra of adsorbed CD3CN at 293 K on the Ti-MMM (2.31% Ti) (O-H stretching region): (1) 0.1 mbar, (2) 0.5 mbar, (3) 1 mbar, (4) 2 mbar, (5) 5 mbar CD3CN, (6) outgassing for 5 min, (7) outgassing for 20 min, and (8) outgassing for 60 min after recording spectrum 5. The spectrum of Ti-MMM before interaction with adsorbate molecules (Figure 1) was used for background subtraction.

4. Discussion

Figure 8. (a) FTIR difference spectra of adsorbed CD3CN at 293 K on the TiO2-SiO2 xerogel (7.29% Ti) (CtN stretching region): (1) outgassing for 60 min, (2) outgassing for 20 min, (3) outgassing for 5 min, (4) 0.1 mbar, (5) 0.5 mbar CD3CN, (6) 1 mbar, (7) 2 mbar, and (8) 5 mbar. (b) Spectrum at 0.5 mbar CD3CN with deconvolution. The spectrum of TiO2-SiO2 xerogel (7.29% Ti) before interaction with adsorbate molecules (Figure 1) was used for background subtraction. (There are offsets for spectra in the Y axis.)

4.1. Surface Hydroxyl Groups. The IR spectra of mesoporous titanium-silicates before interaction with adsorbate molecules are characterized by at least two components at 3745 and ∼3550 cm-1 (Figure 1). According to literature, these bands are attributed to isolated SiOH (and possibly Ti-OH) groups and hydrogen-bonded silanol (titanol) groups, respectively.26-28,36,37 TiO2-SiO2 xerogels show significant differences in the IR spectra, in particular, smaller amounts of isolated silanol groups and more hydrogen-bonded silanol groups. This is probably due to the presence of micropores in their structures, which leads to the increased formation of hydrogen-bonded silanol groups due to the high surface curvature in the pores.45 The titanium-silicates with high titanium content (∼7 wt % Ti) in comparison to catalysts with 2 wt % Ti have a lower amount of isolated silanol groups and according to N2 adsorption analysis a lower surface area (Table 1).

4.2. Acid Sites on the Surface of Mesoporous Titanium-Silicates. 4.2.1. Bro¨ nsted Acidic Sites. The Bro¨nsted acidity of zeolites can be characterized by the shift of the absorbance band due to their interaction with probe molecules in the O-H stretching region of the spectra.36-40 In the case of mesoporous titanium-silicates, the band with the maximum at 3745 cm-1 is perturbed due to interactions with probe molecules. According to the CO adsorption, TiO2-SiO2 xerogels (1.44 and 7.29 wt % Ti) have the highest ∆νOH values (-100 cm-1) and therefore the highest Bro¨nsted acidity among the titanium-silicates studied (Table 2). For the other titaniumsilicates studied, the samples with higher titanium loading (∼7 wt % Ti) are characterized by a stronger Bro¨nsted acidity (∆νOH ) -90 ÷ -91 cm-1) than titanium-silicates with 2 wt % Ti (∆νOH ) -82 ÷ -87 cm-1). Titaniumsilicate Ti-MMM-2 with 2 wt % Ti (∆νOH ) -87 cm-1) reveals a Bro¨nsted acidity slightly higher than that of the

FTIR Ti-Containing Mesoporous Silicate

Figure 9. FTIR difference spectra of adsorbed CD3CN at 293 K on the silicate MMM-2 (CtN stretching region): (1) outgassing for 60 min, (2) outgassing for 20 min, (3) outgassing for 5 min, (4) 0.1 mbar, (5) 0.5 mbar, (6) 1 mbar, (7) 2 mbar, and (8) 5 mbar CD3CN. The spectrum of MMM-2 before interaction with adsorbate molecules (Figure 1) was used for background subtraction. (There are offsets for spectra in the Y axis.)

Figure 10. Optimized geometry of the TiOH(CO)(OSi(OH)3)3 and TiOH(CH3CN)(OSi(OH)3)3 clusters: (a) Ti-OH‚‚‚CO and (b) Ti‚‚‚NCCH3 interactions. For clarity hydrogen atoms on the terminal OH groups are not shown.

pure silicate MMM-2 (∆νOH ) -79 cm-1). In the case of catalysts with 7 wt % titanium loading, the surface TiOH groups can influence the surface acidity. However, the Bro¨nsted acidity of the present titanium-silicates is much weaker than the acidity of aluminum-containing zeolites; for instance, for H-ZSM-5 after CO adsorption,

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∆νOH ) -307 cm-1.52 It is interesting that different acidity of the surface Ti-OH hydroxyl groups was reported for TiO2/anatase53 and TiO2/rutile.54 For anatase, under CO equilibrium pressure a shift by about -115 cm-1 of the bands for the surface hydroxyl groups occurs (from 3675 to 3560 cm-1). Simultaneously, one additional absorption band appears in the carbonyl stretching region due to the CO H-bonded to the surface hydroxyl groups [ν(CO) at 2155 cm-1]. In the case of rutile, the red shift -85 cm-1 of the ν(OH) modes (from 3655 to 3570 cm-1) is observed in parallel with the band appearance at 2150 cm-1. For a TiO2-SiO2 mixed oxide with high Ti loading (Ti:Si molar ratio 1:1) prepared via the homogeneous precipitation method, a smaller shift was measured, namely, -78 cm-1.32 ∆νOH ) -90 cm-1 was detected for microporous titaniumsilicate TS-1 after CO adsorption.26 Therefore, different results about ∆νOH and conclusions about the Bro¨nsted acidity are published at this moment for TiO2 and titanium-silicates. Some authors concluded that no relevant Bro¨nsted acidity can be found on titaniumsilicates,27,28,55 and others expected new Bro¨nsted acid sites which are created when titania and silica form Ti-O-Si chemical bonds.33,56,57 Several models have been proposed to explain the generation of the acid sites in the titaniumsilicates.1,4 Although it is widely accepted that the Bro¨nsted acid sites are associated with the Ti-O-Si bridges, however, the exact location of the proton is still open to discussion. We obtained a slight increase of Bro¨nsted acidity for mesoporous titanium-silicates with 2-7 wt % of Ti loading in comparison to pure silicate. It is most reasonable to conclude that the acidic properties of TiO2 and Ti-containing silicate materials depend on the preparation method, synthesis conditions, sample pretreatment, structure of the final material, and the presence of admixtures, which are in turn related to the degree of surface hydroxylation and titanium distribution in a silicate matrix. Similar results were obtained after the CD3CN adsorption for the catalysts studied (Table 2). TiO2-SiO2 xerogels are characterized by the strongest Bro¨nsted sites (∆νOH ) -355 cm-1 for the sample with 7.29 wt % Ti and ∆νOH ) -341 cm-1 for the sample with 1.44 wt % Ti). Mesoporous titanium-silicates with a high titanium content (∼7 wt % Ti) possess stronger Bro¨nsted acidity (∆νOH ) -325 ÷ -327 cm-1) than samples with lower titanium content (∼2 wt % Ti) (∆νOH ) -310 ÷ -320 cm-1). The Bro¨nsted acidity for the pure silicate MMM-2 (∆νOH ) -315 cm-1) is slight lower to that of the titanium-silicate Ti-MMM-2 with 2 wt % Ti (∆νOH ) -320 cm-1). For microporous TS1, a comparable red shift was observed upon CD3CN dosage.28 The authors observed the total erosion of the band at 3735 cm-1 and a parallel growth of a broad band centered at about 3400 cm-1 (∆νOH about -335 cm-1). The H-bonded species, formed upon interaction of the catalysts surfaces with CO and CD3CN contribute to the IR spectra with a strong and very broad band, so it is very difficult to study the behavior of the component originally centered at ∼3550 cm-1. However, they are not more perturbed than the isolated ones, because neither for the CO adsorption nor for the CD3CN adsorption did we observe the formation of additional lower frequency bands. (52) Wakabayashi, F.; Kondo, J. N.; Domen, K.; Hirose, C. J. Phys. Chem. 1995, 99, 10573. (53) Hadjiivanov, K.; Lamotte, J.; Lavalley, J.-C. Langmuir 1997, 13, 3374. (54) Hadjiivanov, K. Appl. Surf. Sci. 1998, 135, 331. (55) Hu, S.; Willey, R. J.; Notari, B. J. Catal. 2003, 220, 240. (56) Kataoka, T.; Dumesic, J. A. J. Catal. 1988, 112, 66. (57) Molna´r, A Ä .; Barto´k, M.; Schneider, M.; Baiker, A. Catal. Lett. 1997, 43, 123.

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Table 3. Calculated Geometric and Frequencies Features of the Optimized TiOH(CO)(OSi(OH)3)X+1(OTi(OH)3)2-X and SiOH(CO)(OSi(OH)3)Y(OTi(OH)3)3-Y Clusters, Where X ) 0-2 and Y ) 0-3, upon CO Adsorption (H-bonded complexes)

a

∆νOH(CO) ) νOH(CO) - νOH. b ∆νCO ) νCO - 2211. c In the presence of the adsorbed CO molecule.

Table 4. Calculated Frequencies Features of the Optimized TiOH(CH3CN)(OSi(OH)3)X+1(OTi(OH)3)2-X Clusters, Where X ) 0-2, upon CH3CN Adsorption (Ti‚‚‚NCCH3 Interactions)

a

∆νCN ) νCN - 2362.

4.2.2. Lewis Acidic Sites. In the carbonyl stretching region of the mesoporous titanium-silicates, five types of bands are observed after spectral deconvolution (Figure 5b). The band at 2112 cm-1 is probably due to the 13CO (natural abundance in CO gas) interaction with silanol groups. The band at 2137 cm-1 is present in the spectra of all studied samples and its position is close to the gap between the rotational R and S bands of free CO in gas phase (2143 cm-1). It is associated, evidently, with CO molecules, which are physically adsorbed at sites without any specific interaction with the surface. The complexes of CO with silanol groups were detected at 2156 cm-1.

Adsorption of CO revealed the existence of two additional types of sites on the surface of titanium-silicates (Figure 5). None of the two bands was observed on the silicate MMM-2 (Figure 6). The first type is characterized by a CO band at 2185 cm-1 at low pressure (0.1 mbar), which shifts to 2178 cm-1 at saturation. The second less intensive band is detected at low pressure at 2174 cm-1 with a shift to 2170 cm-1 at higher pressure. The authors in ref 32 reported two different bands from 2199 (low pressure) to 2181 (high pressure) cm-1 and from 2177 (low pressure) to 2173 (high pressure) cm-1 on TiO2-SiO2 mixed oxide prepared via the homogeneous precipitation method (Ti: Si molar ratio 1:1) after low-temperature CO adsorption. The shift of the CO bands with coverage increase is due to static (adsorbate-adsorbate) interactions in this case. The static shift occurs due to interaction with the surface and is a measure of the ability of the adsorbate molecules to transmit negative charge to the neighboring adsorption sites.32 For the band at 2199-2181 cm-1, the static shift is relatively large and means that the adsorption sites cannot be ascribed to isolated Ti4+ framework cations. The authors imputed the band at 2199-2181 cm-1 to titanium cations having at least one additional Ti4+ cation in their second coordination sphere, and the other band at lower frequency to isolated Ti4+ cations. One CO band at 2182 cm-1 was observed at low temperature and CO pressure about 7 mbar on TS-1,26 which is characterized by high isolated titanium centers in silica surrounding.1,3,46 It was found that on the surface of both anatase and rutile there are also large fractions of Ti4+ sites. For lowtemperature CO adsorption on anatase, four different titanium(IV) sites were distinguished: R-sites (the strongest sites) at 2208-2206 cm-1; β-sites (β′ + β′′sthe sites

FTIR Ti-Containing Mesoporous Silicate

having the largest population), with β′-sites at 2192-2186 cm-1 and β′′-sites at 2179 cm-1; and very weak acid γ-sites at 2165 cm-1.53 The higher the metal cation acidity, the higher the stretching frequency of the adsorbed molecules. This implies that the β-sites possess lower acidity than the R ones. R-Sites were ascribed to 4-fold coordinated Ti4+ cations on the (110) surface. β′- And β′′-sites are believed to be located on the (101) and (100) anatase planes and represent pentacoordinated titanium. γ-Sites were assigned to pentacoordinated Ti4+ ions on the (001) basal anatase plane. Additionally, it was found that series of TiO2 samples (anatase phase) prepared at the different pH of precipitation are characterized by different Lewis acidity.58 The absorption band corresponding to the interaction of Ti4+ with CO molecules shifts from 2183 to 2190 cm-1 for different TiO2 samples. For rutile,54 one band is reported at 2193 cm-1 at low coverages with a shift to 2183 cm-1 at higher pressures/coverages as well as a high-frequency shoulder at about 2203 cm-1 with a shift to 2192 cm-1. According to the literature data, we ascribe the band at 2185-2178 cm-1 in our spectra to isolated titanium cations in the silica surrounding or having one Ti4+ cation in their second coordination sphere. The other band at 2174-2170 cm-1 with a very small coverage dependence is probably due to CO interactions with Ti-OH groups. After CD3CN adsorption four types of bands in the CtN stretching region can be distinguished for mesoporous titanium-silicates after spectral deconvolution (Figure 8b). It is possible to attribute the band at 2272 cm-1 to CD3CN interaction with Si-OH. The band at 2262 cm-1 is most probably due to physically adsorbed CD3CN, because its position is close to the CtN frequency of CD3CN in a liquid phase (2263 cm-1). Additionally, after CD3CN adsorption two bands at 2289 cm-1 and from 2306 (low pressure) to 2300 (high pressure) cm-1 can be distinguished in all studied mesoporous titanium silicates in comparison to pure silicate (Figure 9). For CD3CN adsorption on TS-1 (1.8 wt % Ti) only one band was detected at 2302 cm-1 at low coverages and at 2297 cm-1 at higher pressure. The authors assigned this band to a direct interaction of the CN group with isolated Ti(VI), which acts as a medium-strength Lewis site. In the case of mesoporous titanium-silicates studied, it is possible to attribute the band with the maximum at 2306-2300 cm-1 to acetonitrile interaction with isolated titanium cations in silicate matrix with probably one Ti4+ cation in their second coordination sphere and the band at 2289 cm-1 to CD3CN interaction with weaker acid sites, titanol groups. It is reasonable to assume that all studied mesoporous titanium-silicates (except Ti-SBA-15 catalysts) have not more than one Ti atom in the second coordination sphere, because according to the earlier DRUV data18,20,22,23 in all materials with Ti loadings of 2-7 wt % titanium is homogeneously dispersed, without anatase phase formation (Table 1). The Ti-SBA-15 catalyst with 7.17 wt % has the poorest Ti dispersion (λmax in DRS-UV is equal to 248 nm with a shoulder at 300 nm) and reveals the presence of anatase-like Ti-O-Ti agglomerates according to Raman spectroscopy data.20 But it is quite difficult to detect the difference between the titanium state in this sample by IR spectroscopy data using CO or CD3CN adsorption, because of the small intensity of the band for Ti-SBA-15 corresponding to the interaction of titanium species with adsorbate molecules (see below). (58) Kozlov, D. V.; Paukshtis, E. A.; Savinov, E. N. Appl. Catal. B: Environ. 2000, 24, L7-L12.

Langmuir, Vol. 21, No. 23, 2005 10553 Table 5. Surface-Normalized IR Absorbance (Q) Due to Ti‚‚‚CO and Ti‚‚‚NCCD3 Interactions for Titanium-Silicates after CO and CD3CN Adsorption catalysts (wt % Ti)

Q(Ti‚‚‚CO)a × 103 (1/m)

Q(Ti‚‚‚NCCD3)b × 103 (1/m)

TiO2-SiO2 aerogel (6.52) TiO2-SiO2 xerogel (7.29) Ti-SBA-15 (7.17) TiO2-SiO2 aerogel (1.68) TiO2-SiO2 xerogel (1.44) Ti-MMM (2.31) Ti-MMM-2 (2.22) Ti-SBA-15 (2.05)

2.29 1.98 0.21 0.43 0.38 0.24 0.22 0.07

3.35 3.93 0.21 1.95 1.48 1.16 1.34 0.22

a Q(Ti‚‚‚CO) ) A C(Ti‚‚‚CO)/A o BET (or AMe + Aext), where the Ti‚‚‚CO band is at 2185-2178 cm-1. b Q(Ti‚‚‚NCCD3) ) AoC(Ti‚‚‚ NCCD3)/ABET (or AMe + Aext), where the Ti‚‚‚NCCD3 band is at 2306-2300 cm-1.

4.3. Titanium Accessibility. The surface ion concentration can be estimated according to the formula A ) AodC, where A is the absorption band area (cm-1), Ao is the absorbance coefficient for the particular adsorbate and adsorbent (cm/µmol), d is the wafer thickness (mg/ cm2), and C is the ion surface concentration (mmol/g). Table 5 summarizes the calculated AoC(Ti) values divided by the surface area (m2/g) for all studied catalysts after the CO and CD3CN adsorptions. One can see that for the CO adsorption as well as for the CD3CN adsorption the surface-normalized IR absorbance due to Ti‚‚‚CO and Ti‚‚‚NCCD3 interactions (Q) was found to fall in the order for titanium-silicates with similar Ti loading (∼2 wt %): TiO2-SiO2 aerogel (1.68 wt % Ti) ∼ TiO2-SiO2 xerogel (1.44 wt % Ti) > Ti-MMM (2.22 wt % Ti) ∼ Ti-MMM-2 (2.31 wt % Ti) > Ti-SBA-15 (2.05 wt % Ti). Moreover, this order correlates with the catalytic activity for the present titanium-silicates in the TMP oxidation with H2O2 (Table 1).18,20,22,23 Only the TiO2-SiO2 xerogel displays a lower catalytic activity, despite its high titanium concentration on the surface. This is probably because of the micropores present in this material that cause diffusion limitation problems in the catalytic oxidation of such a large organic substrate like TMP. Despite its large pore size, Ti-SBA15 has a poor catalytic activity. We found that its low activity is due to the poor titanium dispersion (according to DR-UV and Raman spectroscopy) (Table 1) and larger wall-thickness (about 2.0 nm) in comparison with TiMMM and Ti-MMM-2 (wall-thickness is about 1.0 nm).20 The thick silicate wall makes a fraction of titanium sites unavailable for reactants and adsorbate molecules. The low surface concentration of titanium cations in Ti-SBA15 materials is confirmed by IR spectra of adsorbed molecules. Similar results were obtained for CH3CN adsorption on TiO2-SiO2 aerogels.27 The samples with 1-10 wt % Ti consist of an anatase-like core with a silica shell, where, however, small amounts of Ti cations are dissolved. It is interesting to note that in our experiments, despite large particles size (3 nm for aerogels and 4-5 nm for xerogels), TiO2-SiO2 mixed oxides have high titanium concentration on the surface, and according to DR-UV and Raman spectroscopy,23,24 there are no anatase particles in their structures. Therefore, it is possible to summarize that the titanium accessibility in the titanium-silicates depends on both the pore size and the silicate wall thickness. Figure 11 shows the dependence of Ti content in TiO2SiO2 aerogels versus Q(Ti‚‚‚CO) and Q(Ti‚‚‚NCCD3) calculated from the IR experiments. A similar behavior was observed for TiO2-SiO2 xerogels and Ti-SBA-15 materials with different Ti loadings. For CO adsorption, a linear plot is observed, whereas in the experiments with

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Figure 11. Surface-normalized IR absorbance Q due to Ti‚‚ ‚CO and Ti‚‚‚NCCD3 interactions vs wt % Ti for TiO2-SiO2 aerogels: (1) from CD3CN adsorption, (Ti‚‚‚NCCD3, the band at 2306-2300 cm-1) and (2) from CO adsorption, (Ti‚‚‚CO, the band at 2185-2178 cm-1).

CD3CN adsorption for catalysts with higher titanium content (∼ 7 wt % Ti) a lower amount of surface titanium cations is detected from the IR spectra. This fact can be explained by the larger size of CD3CN molecules, which leads to steric problems that are not present in the adsorption of CO molecules. 4.4. DFT Calculations of the CO and CH3CN Adsorptions. To understand the nature of Bro¨nsted and Lewis acid sites, theoretical DFT calculations of the CO and CH3CN adsorption on titanium-silicate clusters have been performed. The calculated frequency shifts for the O-H and C-O stretching vibrations in Tables 3 and 4 are in agreement with experimental data. ∆νOH for isolated Ti in a silica surrounding (structure 1, Figure 10a) is slightly higher than for the pure silica cluster (structure 4) (Table 3). Subsequent substitution of Si atoms by Ti atoms in the second coordination sphere (structures 2, 3, 5-7) results in the reduction of ∆νOH in absolute value. This effect is more pronounced for Si-OH centers (structures 5-7). It is reasonable to assume that for mesoporous titanium-silicates with 2-7 wt % Ti there is a superposition of all such frequency shifts, which results in one broad absorption line in experimental data. The Ti-O-H angle increases with increasing number of Ti atoms in the second shell. This tendency is not observed for the Si-O-H angle. The TiO-H and SiO-H bond lengths after CO adsorption are the same for clusters with different Ti content and amount to 0.965-0.963 Å. The calculated SiOH‚‚‚CO distances increase with the increasing number of Ti atoms in the second shell from 2.233 to 2.299 Å. A similar but less pronounced effect is observed for TiOH‚‚‚CO distances (the increase from 2.207 up to 2.219 Å). The calculated ∆νCO are equal to 24-23 and 19-15 cm-1 for Ti-OH and Si-OH groups, respectively, and are independent from the number of titanium atoms in the second coordination sphere. The calculated ∆νCO for the Si-OH group is in good agreement with the experimental value of 2156 cm-1 (∆νCO ) 13 cm-1). The calculated ∆νCO for Ti-OH group is relatively close to the shift observed experimentally (∆νCO ) 31-27 cm-1). The adsorption of CO molecules on the central Ti atom was also studied. It was shown earlier that DFT is capable of effectively describing the adsorption properties of CO on titanium dioxide in different modifications.59-61 However, in our case adsorption of CO on Ti in different local environments (TiOH(OSi(OH)3)X+1(OTi(OH)3)2-X, where (59) Fahmi, A.; Minot, C. J. Organomet. Chem. 1994, 478, 64. (60) Sorescu, D. C.; Yates, Jr., J. T. J. Phys. Chem. B 1998, 102, 4556. (61) Sorescu, D. C.; Yates, Jr., J. T. J. Phys. Chem. B 2002, 106, 6184.

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X ) 0-2) resulted in Ti‚‚‚CO distances of 3.238, 3.830, and 3.990 Å for the structures with 0, 1, and 2 Ti atoms in the second coordination sphere, respectively. These distances are far too large in comparison to the calculated values for CO adsorption on a rutile(110) surface (2.3202.376 Å).60 As a result of such a weak interaction between titanium atom and CO molecule, the calculated ∆νCO values were very small (6-3 cm-1). A possible explanation is that dispersion forces that contribute to Ti‚‚‚CO interaction are underestimated by the B3LYP functional, as it is known for the CO adsorption on NiO(100)62,63 and MgO(001)63,64 surfaces. In the case of acetonitrile adsorption, the calculated CtN frequency shifts due to Ti‚‚‚NCCH3 interactions are in better agreement with experimental data. Acetonitrile interacts with Ti(IV) centers via its N atom, as shown in Figure 10b, where the optimized TiOH(CH3CN)(OSi(OH)3)3 cluster (structure 8) is presented. The calculated Ti‚‚‚NCCH3 distances decrease with increasing titanium number in the second coordination shell (Table 4). The calculated frequency shifts for the CtN stretching vibrations due to the acetonitrile interaction with Ti(IV) cations in different surroundings have a tendency to a small decrease with increasing titanium number in the second coordination sphere. According to calculations, the presence of additional titanium atoms in the second shell of Ti(IV) centers does not influence significantly the position of the band corresponding to Ti‚‚‚NCCH3 interactions. The calculated ∆νCN values describe rather well the experimental shifts for mesoporous titanium-silicates (43-37 cm-1). Conclusions IR data with adsorbed CO and CD3CN molecules on the surface of mesoporous TiO2-SiO2 aerogels, TiO2-SiO2 xerogels, Ti-MMM, Ti-MMM-2, and Ti-SBA-15 with 2-7 wt % Ti revealed similarities between the two probe molecules. CO and CD3CN adsorption demonstrated the same order of acidity and, with some exceptions, which may relate to the different spatial requirements, the same order of Ti quantity exposed to the catalysts surface. The surface hydroxyl groups of mesoporous titanium-silicates with 2-7 wt % Ti revealed a Bro¨nsted acidity slightly higher to that of pure silicate. The order of Ti accessibility provided by the IR data is the same as that of catalytic activity for mesoporous titanium-silicates in the H2O2-based oxidation of TMP. The titanium accessibility in the titanium-silicates depends on both the pore size and the silicate wall thickness. DFT calculations reproduce the shifts of vibrational frequencies well, except for that of the Ti‚‚‚CO interaction, giving significant support for the band assignment. FTIR of adsorbed CO and CD3CN is therefore very valuable for the characterization of titanium-silicate catalysts. Acknowledgment. Dr. N. N. Trukhan acknowledges the financial support from the Alexander von Humboldt Foundation. The authors thank Dr. I. Tkach and Dr. H. Dilger for their technical support. LA0514516 (62) Bredow, T. J. Phys. Chem. B 2002, 106, 7053. (63) Pacchioni, G.; Di Valentin, C.; Dominguez-Ariza, D.; Illas, F.; Bredow, T.; Klu¨ner, T.; Staemmler, V. J. Phys.: Condens. Matter. 2004, 16, S2497. (64) Damin, A.; Dovesi, R.; Zecchina, A.; Ugliengo, P. Surf. Sci. 2001, 479, 255.