Mesoporous titanosilicates

The review of Notari (33) covers the synthesis methodologies of titanium silicate molecular sieves available up to 1996. The reviews of Corma (279) and subsequently of Biz and Occelli (280) describe the synthesis of mesoporous molecular sieves. An informative article on the preparation of TS-1 was reported recently by Perego et al. (68). In this section we list some of the recent developments in the synthesis of micro and mesoporous titanosilicate molecular sieves. 

A. K. Sinha, S. Sedan, S. Tsubota, and M. Haruta, A three-dimensional mesoporous titanosilicate support for gold nanoparticles Vapour-phase epoxidation of propene with high conversion, Angew. Chem. Int. Ed. 43(12), 1546-1548 (2004). 

All mesoporous titanosilicates obtained by the Ti-isopropoxide grafting exhibit the same XRD patterns and the intensity remained almost constant as those for their pure silica analogues as shown in Figure 1. These confirm that structural integrity of the mesoporous materials remained intact after TIPOT grafting treatment. 

Ti, V and Sn-modified mesoporous silicates were reported to be active in a number of liquid phase oxidation reactions. Ti-containing samples were used for the selective oxidation of large organic molecules in the presence of te/t-butyl hydroperoxide (TBHP) or dilute H2O2 [71,136,137,139-141,147,186,237]. Typical data shown in Table 5 indicate that both Ti-MCM-41 and Ti-HMS are efficient cat ysts for the epoxidation of bulky olefins such as a-terpineol and norbomene in the presence of TBHP or H2O2. Comparison with H-B indicates that the accessibility of active sites plays a critical role in the liquid phase oxidation of organic molecules. Mesoporous titanosilicates also exhibited remarkable activity in the hydroxylation of 2,6-di-rerr-butyl phenol (2,6 DTBP) [142,147] and the oxidation of cyclododecanol [147], naphthol [147] aniline [237] and chloroaniline [186]. However, they were disappointingly poor catalysts for the liquid phase oxidation of n-hexane and aliphatic primary amines, as well as the ammoximation of cyclohexanone [147,238]. 

Corma[7] has prepared Ti-MCM-41 by direct hydrothermal synthesis. Ti-MCM-41 is the first example of ordered mesoporous titanosilicates, and catalytic results exhibited good properties for the oxidation of bulky reactants under mild conditions. 

O. Franke, J. Rathousky, G. Schulz-Ekloff, J. Starek, A. Zukal, New mesoporous titanosilicate molecular sieve," in J. Weitkamp, H. G. Karge, H. Pfeifer, W. Hdlderich (Eds.), Zeolites and Related Microporous Materials State of the Art 1994, Stud. Surf. Sci. Catal. 84 (1994) p. 77. 

J. Q. Lu, X. Zhang, J. J. Bravo-Suarez, S. Tsubota, J. Gaudet, S. T. Oyama, Kinetics of propylene epoxidation using H2 and O2 over a gold/mesoporous titanosilicate catalyst, Catal. Today 123 (2007) 189. 

C. Qi, T. Akita, M. Okumura, M. Haruta, Effect of surface chemical properties and texture of mesoporous titanosilicates on direct vapor-phase epoxidation of propylene over Au catalysts at high reaction temperature, Appl. Catal. A Gen. 253 (2003) 75. 

A. K. Sinha, S. Seelan, M. Okumura, T. Akita, S. Tsubota, M. Haruta, Three-dimensional mesoporous titanosilicates prepared by modified sol-gel method Ideal gold catalyst supports for enhanced propene epoxidation, /. Phys. Chem. B 109 (2005) 3956. 

Mesoporous Ti-MCM-41 support was prepared by hydrothermal crystallization according to literature procedure [23]. Disordered mesoporous titanosilicate Ti-Meso was synthesized by modified sol-gel method and hydrothermal crystallization (at 100°C, 5 days) and using cetyltrimethyl ammonium bromide as template following the procedure similar to that for the synthesis of disordered mesoporous silica [24]. A modified sol-gel method without hydrothermal crystallization was used to prepare mesoporous titanosilicates, TiO-SiO(l) and TiO-SiO(2) [25]. Ti grafting on Ti-MCM-41 support was carried out according to literature procedure [26]. 

UV-Vis spectra of the titanium containing MCM-41 and disordered mesoporous titanosilicate samples are shown in Fig. 2. The UV-Vis analysis of these samples show a band near 220 nm range due to tetrahedrally coordinated Ti. Generally a shoulder at -330 nm is expected in the spectrum if the sample contains some bulk titania, but such a shoulder could not be observed. Absorption band at 260-270 nm has been generally attributed to the presence of Ti atoms in 5- and 6- fold coordinations, which are most likely generated through hydration of the tetrahedrally coordinated sites [24]. Ti/Ti-MCM-41 and TiO-SiO(2) samples with higher Ti content than that for the other titanosilicate samples studied here show broader UV-Vis bands with some red-shift. 

Table 1 summarizes the surface properties of the various mesoporous titanosilicate samples. The BET surface areas of these titanosilicates (850-1250 mg ) was typical for that shown by mesoporous materials. The BET surface area and BJH average pore diameter is found to decrease after Ti grafting onto Ti-MCM-41 sample. The postsynthesis grafted Ti is expected to react with the surface silanol groups of the walls in a random fashion in the most accessible sites near the pore mouth and wider pores. As a consequence there is clear decrease in pore size after titanium grafting. The pore size distribution is also found to become narrower around the average pore diameter. The BET surface areas of disordered mesoporous materials is found to be lower than that of the ordered MCM-41 type materials. BJH average pore diameter of mesoporous titanosilicates TiO-SiO(l) and TiO-SiO(2) prepared by modified sol-gel method is much lower than that of disorderd mesoporous Ti-Meso and ordered Ti-MCM-41 samples crystallized hydrothermally. But all these samples show a narrow pore size distribution. All the samples exhibit isotherms of type IV, typical of mesoporous materials, with a H2 hysteresis loop (Fig. not shown). 

What is very promising is the possibility to synthesize t irge-pore titanium-containing molecular sieves. The crux of the synthesis of these mesoporous sieves is to avoid the hydrolysis of the titanium component (such as tetrabutyl orthotitanate) to large titania particles. The performed syntheses of mesoporous titanosilicates have proved that the hydrolysis of TBOT to larger titania particles can be prevented. 

The performed syntheses of mesoporous titanosilicates have proved that the hydrolysis of the titanium component to larger titania particles can be prevented. 

Bravo-Suarez, J., Lu, J., DaUos, C., et al. (2007). Kinetic Study of Propylene Epoxidation with Ho and Oo over a Gold/Mesoporous Titanosilicate Catalyst, J. Phys. Chem. C, 111, pp. 17427-17436. 

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