Solvent Effects and Reaction Intermediate

The role of the solvent is complex polarity, solubility of reactants and products, diffusion and counter-diffusion effects, and also interaction with the active centers [88]. Using a triphase system (solid-liquid-liquid) in the absence of any cosolvent, a considerable increase in the conversion of various water-immiscible organic compounds (toluene, anisole, benzyl alcohol, etc.) can be achieved [89]. 

A similar retardation effect of cosolvent was reported previously for benzene oxidation [90, 91]. The solvent may compete with reactant for diffusion in the channels and adsorption at the active sites of TS-1 catalyst. The activity Ti-beta for 1-hexene and cydohexanol oxidations is highest in acetonitrile, which is a polar, nonprotic solvent [92]. This is in contrast with the observed enhancement of the activity of TS-1 by methanol and protic solvents [68]. These differences have been 

10 mmol substrate and 10 mmol H202 (31% aqueous solution). 

When 1 -hexene was oxidized over Ti-beta, the conversion in acetonitrile solvent was about twice as high as that attained in methanol. However, for cyclohexene oxidation, methanol as solvent was much superior to acetonitrile, which conflicts with the results reported by Corma et al. [92], but is consistent with our previous findings [55], 

It has long been known that acetonitrile should be the solvent of choice for the oxidation of 1 -hexene over Ti-beta and Ti-MWW while methanol is preferred by TS-1. In contrast, for cyclohexene oxidation, methanol is favorable for Ti-beta, whereas acetonitrile is the best for TS-1 and Ti-MWW. The effect of hydrophilicity/hydropho-bicity is demonstrated by a series of catalytic results, but this can not interpret the solvent effect on the oxidation of cydohexene over Ti-beta. 

It has also been shown that the catalytic activity can be enhanced when appropriate amounts of mixed solvents are present in the oxidation system, compared to a single solvent [98]. In particular, converse results are obtained when the same titanosilicate is used for catalyzing the oxidation of 1-hexene and cyclohexene. Thus, further insight into the solvent effect is required. 

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