Structure of Titanium Species and Activity

Although the identification of tetrahedrally coordinated, tetra- and tripodal Ti4+ ions on the surface of titanosilicates, as the likely active sites in reactions that require Lewis acidity, seems convincing, the structure and role of the sites active in catalytic oxidation, presumably oxo-titanium species, formed by the interaction of H202 (or H2 + 02) with these surface Ti ions, are not clear. In recent years, this problem has been investigated by FTIR (133), Raman (39,40), XANES (46-48), electronic (54-57), and EPR (51-54) spectroscopies. This is one of the areas in which major progress has been made since the reviews of Notari (33) and Vayssilov (34). Zecchina et al. (153) recently summarized some of the salient features of this progress. 

If the tetra- and tripodal Ti structures and the titanium oxo species derived from these structures in the presence of ROOH (R = H, alkyl) are involved as active sites and reaction intermediates, the next step beyond their identification is to seek correlations between the structure and concentrations of these titanium oxo species and catalytic activity and selectivity. Clerici and Ingallina (204) were the first to propose the Ti(02H) group as the active site of alkene epoxidation by... 

Both fractions were characterised by NMR spectroscopy and mass spectrometry. Fraction A mainly consists of silsesquioxane a7b3 (Fig. 9.5) [38], while fraction B is a mixture of different silsesquioxanes, mostly incompletely condensed species, with the main species assigned to silsesquioxane structure 6i>2 (Fig. 9.5). Finally, both fractions were reacted with a titanium alkoxide and tested for catalytic activity in the epoxidation of 1-octene as a function of the reaction time and the results compared with those of HTE lead (all three catalysts are homogeneous) (Fig. 9.6). 

Study of these new catalysts is intensive. Small molecular-weight distribution was demonstrated by Petrova (112) and by Baulin et al. (113). In addition, polymer substrates have been used (114-116) in order to increase lifetime and activity. As shown by Suzuki (36), stabilization is caused by inhibition of reduction by polymeric ligands. Karol (117, 118) described the reaction of chromocene with silica to form highly active catalysts sensitive to hydrogen. An unknown role is played by the structure mt—CH2—CH2—mt which is formed with ethylene and reduced forms of titanium (119). For soluble systems, it has been shown that the mt—CH2—CH2—mt structure is formed in a biomolecular reaction with /3-hydrogen transfer (120). It was considered that this slow, but unavoidable, reaction is the reason for changes in activity during reaction and that the only way to avoid it is to prevent bimolecular reaction of two alkylated species. 

A number of models for catalytic active sites have been proposed to explain the isospecific polymerisation of propylene in the presence of TiC -based catalysts [68]. It was Natta who proposed [366], for the first time, that the steric control in isospecific propylene polymerisation is caused by the structure of the active species located on the borders of crystal layers of violet titanium trichloride. Arlman and Cossee [277] suggested that the isospecific active sites are located on lateral crystal surfaces, which in violet titanium trichloride such as oc-TiCh correspond to (110) planes. Titanium atoms present on the above-defined fracture surface have a vacant octahedral site bonded to five chlorine atoms (Figure 3.26) [1,68]. 

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