Ti-Containing Mesoporous Material

There are two main techniques for the incorporation of atomically dispersed titanium into a mesoporous silica framework. In the cocondensation method a titanium source is added during the preparation of the silica material framework and formation proceeds via simultaneous condensation of both titanium and silicon precursors, resulting in titanium incorporation throughout the material. Conversely, postmodification involves the grafting of a titanium-alkoxide precursor to the pore surface by condensation with surface hydroxyls of a preformed silica material. This forms a surface-modified titanium silicate. 

Related materials have been used for other reactions such as the partial oxidation of methane (where the redox capacity is important) and the production of fatty acid methyl ester (FAME) biodiesel (where the acid/base properties of the catalysts are of value). 

CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, Florida, 85th edn, 2005. 

Ruckenstein, Interactions and surface phenomena in supported metal catalysts, in Strong Metal Support Interactions, ed. R. T. K. Baker, 

Tauster and J. A. Dumesic, A.C.S. miposium series 298, American Chemical Society, Washington DC, 1986, p. 152. 

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One-pot synthesis of campholenic aldehyde from a-pinenc has been achieved using t-butyl hydroperoxide and a Ti-containing mesoporous material of the I IMS-type. [80]. Selectivity to campholenic aldehyde was 82.4 % under the following. Conditions a-pincnc (5 mmol), t-butyl hydroperoxide (5 mmol, dried over MgSO,i), Ti-llMS (0.1 g) in acetonitrile (30 ml) stirred 24 h at 75 °C. 

In parallel with this, a range of Ti-containing mesoporous materials were prepared to catalyse the + alkene reaction. These materials... 

Shioya, Y., Ikeue, K., Ogawa, M., and Anpo, M. (2003) Synthesis of transparent Ti-containing mesoporous silica thin film materials and their unique photocatalytic activity for the reduction of C02 with H20. Applied Catalysis A General, 254 (2), 251-259. 

This strongly suggests that the organically functionalized Ti-substituted mesoporous materials are much more hydrophobic than non-functionalized materials. This trend was also observed for organo-group containing mesoporous materials which showed the stability towards water and mechanical pressure [7]. In TGA, the difference in the amount of water adsorbed was not noticeable between the phenyl and the methyl substituents. 

In conclusion, the epoxidation of propylene with bulky oxidants (such as cumene or TBHP) can be successfully achieved using titanium-containing mesoporous materials as catalysts. The catalytic chemistry of the active sites can be controlled via the synthesis conditions and postsynthesis modifications. The hydrophobicity of the catalyst is of great importance to achieve a highly selective catalyst. The Ti-MCM-41-based heterogeneous catalyst has demonstrated excellent performance in the commercial process for PO manufacture. 

Here, we give an example for the synthesis of Ti-containing mesoporous silicas. This kind of catalyst has abilities for the selective oxidation of olefin and other unsaturated compounds, such as the epoxidation of 1-hexene, cyclohexene, and styrene [61,62]. The ratio of Si/Ti, structure, and hydrophobic nature of the material are the three most important factors in the catalytic activity. Therefore, a full understanding of the synthesis is necessary. 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

Fig. 4 shows the IR spectra of various Ti-mesoporous materials obtained by the solvent evaporation method. These samples were synthesized using a C22TMaC1 surfactant and methanol solvent. The IR spectra of Ti containing MCM-41 exhibited an absorption band near 970 cm"1, which was also found for the purely siliceous MCM-41 samples in the Fig. 4. 

Several conclusions may be drawn from these data. (1) Weakly ordered mesoporous structures with high surface areas were obtained after the surfactant template was removed from TEOS-treated phosphomolybdate salts by solvent extraction. (2) IR and XRD data confirm that the silicate was indeed incorporated in the salt structures and changed these structures. Finally the Mo-O-Si bond was formed. (3) In contrast to the formation of the Mo-O-Si, the Mo-O-Ti bond is difficult to form in the same reaction conditions. So we only obtained a silica-containing mesoporous PMA. A significant breakthrough may be the appearance of good catalytic activity with the synthesis of the porous phosphomolybdic acid. The applications of the material in catalytic reaction are also being studied in our future work. 

For redox catalysis, efforts have been spent on preparing transition metal modified mesoporous materials. These materials are capable of extending the catalytic oxidation chemistry to large molecules. The selective catalytic activity has also been demonstrated, for example, in the oxidation of aromatic compounds by using titanium-containing mesoporous silica (Ti-MCM-41 and Ti-HMS). ... 

We choose Ti-containing materials as an example to discuss heteratom-containing silica material and nonsilica mesoporous materials, because Ti-containing materials are useful catalysts and were studied extensively. 

Shortly after the initial foray into the use of microporous titanosilicates as the highly dispersed Ti-supports for propylene epoxidation, interest shifted to meso-porous titanosilicates. Mesoporous Ti-containing materials are similar to micro-porous materials in that they offer highly dispersed Ti centers and reasonably well-defined tetrahedral Ti sites incorporated in a silicious framework. Moreover, the existence of a mesoporous pore system of sufficient dimensions to incorporate Au species in the range of 2 nm allows for Au entities to access essentially the entirety of the support surface area and enhances transport of reactants and products to and from the sites. 

While MCM-41- and 48-based materials dominate as the primary mesoporous materials explored for gas-phase propylene epoxidation, a recent article examines the reactivity of Au deposited on Ti-TUD containing 3 mol% Ti [57]. Ti-TUD consists of a sponge-like structure with an average pore size of about 13 run. Although the specific surface area of this material is less than that of MCM-41 or MCM-48, the larger pore system allowed for essentially all of the deposited Au to have access to the pore system. A maximum rate of 53.7 gpo kgcat 470 °C... 

Titanium-based catalysts, would seem particularly attractive candidates, but the pore size of e.g. TS-1, is much too small to admit even a monosaccharide. Recently a number of synthetic approaches towards mesoporous titanium containing catalysts of the MCM-41 type have appeared in the literature . In the present paper we will deseribe the use of Ti-MCM-41 materials in the oxidation of the model mono- and disaccharides methyl a-D-glucopyranoside, sucrose and a,a-trehalose, and we will discuss the effect of the zeolite synthesis on the effectiveness in these reactions. Several preparative approaches of Ti-MCM-41 have been compared in the oxidation of these model carbohydrates. 

Micro- and mesoporous crystalline titanium silicas, such as Ti silicalite TS-1 and the Ti-containing MCM-41 zeolite, represent another important family of modem materials and find versatile application as catalysts for oxidation reactions and in absorption technologies. These materials are prepared under hydrothermal conditions using tetraalkylammonium hydroxide, titanium tetraalkoxide, and tetraethyl orthosilicate as preeursors. Only limited amounts of Ti can enter the Si02 framework and the actual bonding situation at Ti is not yet fully understood. Several data suggest that Ti substitution for Si in the lattice positions retaining the tetrahedral coordination is important for catalysis. 

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