Titanium-containing molecular sieves

A very comprehensive review has been published on TS-1 and other titanium-containing molecular sieves [26]. TS-1 was synthesized by the hydrothermal crystallization of a gel obtained from Si(OC2H5)4 and Ti(OC2H5)4 [10, 27] (Enichem method, hereafter named method A). The incorporation of Ti into the framework of... 

Post-synthesis modifications have been successful in preparing titanium-containing molecular sieves active in oxidation. The method employed for the post-synthesis incorporation of Ti to the zeolite beta was also applied to the incorporation of Ti into MOR and FAU [60]. 

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

It is well known that the catalytic activity in case of titanium containing molecular sieves originates from the framework titanium [7-9], The results ootained on hydroxylation of phenol with H2O2 over TS-NU-1 samples are given in Table 4. It was observed that conversion values for TS-NU-1 series are found to be nearly the same. 

Saxton, R. J. Zajacek, J. G. Crocco, G. L. Epoxidation of Olefins with Hydrogen Peroxide Catalyzed by Titanium-Containing Molecular Sieve Isomorphous to Zeohte b. 1993-172404 5412122, 19931223, 1995. 

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. 

Titanium-Containing Molecular Sieves by Secondary Synthesis. . 203... 

The crucial problem in the synthesis of titanium-containing molecular sieves is to find proper conditions for obtaining isolated tetrahedrally coordinated titanium species. In fact, in the presence of even small amounts of water, the titanium compounds used in the reaction mixture undergo hydrolysis very quickly and then polymerize and precipitate in the form of hydrated Ti02. The large amount of work performed on TS-1 allows some of the critical parameters of the synthesis procedure to be explained, though the overall mechanism is not yet fully understood. 

Synthesis and Characterization of Other Titanium-Containing Molecular Sieves... 

A family of titanium-containing molecular sieves with pentasil-type framework structures is represented by titanivun-silicalite-3 (TS-3) [74], characterized by a framework topology similar to that of TS-2 but with a different degree of stacking faults. In fact, as found by Perego et al. for the boron-substituted MFI/MEL molecular sieves, the frequency of stacking faults may be controlled by choosing the appropriate pair of tetraalkylammonium ions (e.g., TMAOH/TPAOH, TMAOH/TBAOH, TEAOH/TBAOH) [72]. 

Perego, C., Carati, A., Ingallina, R, et al. (2001). Production of Titanium Containing Molecular Sieves and their Application in Catalysis, Appl. Catal. A Gen., 221, pp. 63-72. 

---