Titanosilicate molecular sieve

EPR spin Hamiltonian parameters (at 77 K) ofTi3+ in titanosilicate molecular sieves generated by... 

The catalytic activity of the titanosilicate molecular sieves, especially those of TS-1, TS-2, Ti-beta and Ti-MCM-41 has been investigated extensively... 

Titanosilicate molecular sieves, especially TS-1, are active in the hydroxylation of both alkanes and aromatic compounds (33,165) when H202 is used as... 

In contrast to their vanadosilicate analogues, the titanosilicate molecular sieves do not hydroxylate the terminal primary carbon in n-alkanes. Ramaswamy et al. (218,219) found that when n-hexane was hydroxylated under identical conditions in the presence of TS-2 or VS-2 (VS-2 is a vanadium analogue of TS-2), the distribution of products was as follows ... 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

Significant progress has been achieved in the preceding few years in the study of titanosilicate molecular sieves, especially TS-1, TS-2, Ti-beta, and Ti-MCM-41. In the dehydrated, pristine state most of the Ti4+ ions on the surfaces of these materials are tetrahedrally coordinated, being present in either one of two structures a tetrapodal (Ti(OSi)4) or a tripodal (Ti(OSi)3OH) structure. The former predominates in TS-1, TS-2, and Ti-beta, and the latter is prominent in Ti-MCM-41. The Ti ions are coordinatively unsaturated and act as Lewis acid sites that coordinatively bind molecules such as H20, NH3, CH3CN, and H202. Upon interaction with H202 or H2 + 02, the Ti ions form titanium oxo species. Spectroscopic techniques have been used to identify side-bound hydroperoxo species such as Ti(02H) and superoxo structures such as Ti(02 ) on these catalysts. 

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. 

Titanosilicates molecular sieves, especially TS-1, have been widely studied for the selective oxidation of a variety of organic substrates, using aqueous H202. ° Recently, there have been attempts to substitute aqueous H2O2 by a mixture of H2 and O2 in the presence of metals such as Pd, Pt, Au, etc. Selectivities of 99% for propylene oxide formation from propylene were observed by Haruta and co-workers over Au-containing catalysts. We had also found that the epoxide selectivity in the epoxidation... 

Preparation of Titanosilicate Molecular Sieves. TS-1 and TiMCM-41 were prepared according to the reported procedures. In the preparation of Pd(n)-TS-1 (n refers to the Pd content in wt.%), an aqueous solution (15 mL) containing the required amounts of [Pd(NH3)4]Cl2-H20 or Pd(CH3COO)2 (Aldrich) was added... 

V. N. Shetti, P. Manikandan, D. Srinivas, and P. Ratnasamy, Reactive oxygen species in epoxidation reactions over titanosilicate molecular sieves, J. Catal. 216, 461 67 (2003). 

D. Srinivas, P. Manikandan, S. C. Laha, R. Kumar, and P Ratnasamy, Reactive oxo-titanium species in titanosilicate molecular sieves EPR investigations and structure—Activity correlations, J. Catal. 217, 160-171 (2003). 

The use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

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. 

Zecchina A, Otero Arean C, Turnes Palomino G, Geobaldo F, Lamberti C, Spoto G and Bordiga S (1999), The vibrational spectroscopy of H2, N2, CO and NO adsorbed on the titanosilicate molecular sieve ETS-10 , Phys Chem Chem Phys, 1, 1649. 

Srinivas, D. R. Srivastava P. Ratnasamy. Transesterifications over titanosilicate molecular sieves. Catalysis Today 200, 96, 127—133. 

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