Distinctions of hydroxylamine formation and decomposition in cyclohexanone ammoximation over microporous titanosilicates

A comprehensive investigation has been carried out to study hydroxylamine formation and decomposition in the ammoximation of cyclohexanone over three typical microporous titanosilicates, Ti-MOR, Ti-MWW, and TS-1. Cyclohexanone ammoximation involved the formation and decomposition of hydroxylamine intermediate as well as noncatalytic oximation. Ti-MOR and TS-1 were found to be extremely effective catalysts for hydroxylamine formation by ammonia oxidation with hydrogen peroxide; meanwhile, they showed low catalytic activity for hydroxylamine decomposition. Thus, Ti-MOR and TS-1 were capable of achieving high hydroxylamine yields such as 82.7–95.1% and cyclohexanone conversion of >97%. Serving as an effective ammoximation catalyst, however, Ti-MWW was less efficient for hydroxylamine formation because of its high catalytic activity for the decomposition of hydroxylamine. Ion exchange with alkali metal cations has been carried out on titanosilicates to remove the hydroxyl-related acid sites. Introduction of alkali cations by ion exchange removed the acid sites of titanosilicates selectively, resulting in enhanced activity for hydroxylamine formation and ammoximation without altering the hydroxylamine decomposition, and the exchange then enhanced their catalytic activity significantly for both hydroxylamine formation and ammoximation.