Oxidation Sites in Framework Positions

The most relevant heterogeneous catalysts used in liquid phase oxidations are [63]  

Another option that sometimes enables immobilization of isolated metal ions stable to leaching, and avoidance of the formation of oligomers, is the synthesis of zeolites or zeotypes containing isolated metal ions in framework positions. In these the oxidation properties of the metal atoms are associated with the main characteristics of zeolites which involve shape-selective effects and unique adsorption properties which can be tuned in terms of their hydrophobicity-hydrophi-licity, enabling selection of the proportions of reactants with different polarities that will be adsorbed in the pores. Researchers at ENI succeeded in introducing Ti into silicalite producing the TS-1 redox molecular sieve oxidation catalyst [64]. TS-1 has an MFI structure formed by a bidimensional system of channels with 0.53 nm X 0.56 nm and 0.51 nm X 0.51 nm pore dimensions. The incorporation of Ti into the framework has been demonstrated by use of several techniques-XRD, UV-visible spectrophotometry, EXAFS-XANES a good review has been published by Vayssilov [65]. 

The presence of a silica framework with few defects makes TS-1 a highly hydrophobic material suitable for oxidations in the liquid phase with H2O2 as oxidant. Thus, TS-1 has proven to be successful in the oxidation of alcohols, epox-idation of linear olefins, hydroxylation of aromatics, ammoximation of cyclohexanone, oxidation of alkanes to alcohols and ketones, oxidation of amines, oxidation of sulfur-containing compounds, and oxidation of ethers [66-75]. 

Since then other zeolite structures have been synthesized including TS-2 [76], Ti-Beta [77,78], Ti-ZSM-48 [79], Ti-ZSM-12 [80], Ti-mordenite [81], Ti-UTD-1 [82], and Ti-ferrierite [83,84]. Ti has also been incorporated in mordenite. Beta, SSZ-33, Y, L, MAZ, UTD-1 and NU-1 structures by post-synthetic treatment [74,85-87] although they have some activity, the activity and selectivity of samples produced by post-synthetic procedures are lower than those obtained by direct synthesis [88]. 

Method of synthesis Chemical composition Si/Al Ti02 (%, w/w) Defects Turnover frequency (mol (mol Ti) h ) Epoxide selectivity 

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