Importance of solvents in preparing highly active Ti-SBA-15 catalysts by grafting method

Abstract

The importance of solvents in preparing Ti-incorporated SBA-15 mesoporous silica (Ti-SBA-15) of highly catalytic activities in epoxidation of bulky alkenes using titanium alkoxides as grafting agents was disclosed. Solvents were found to markedly affect the coordination environment of Ti incorporated on SBA-15. Among the common solvents examined, best dispersion of tetrahedrally coordinated Ti(IV) on silica surface was obtained by grafting in 1-butanol and the second was in 1-hexanol. It is attributed to that the hydrolysis of titanium alkoxides in these alcohols is slower than in other studied solvents, including short-chain alcohols and non-polar hydrocarbons. Detailed studies on the hydrolysis rates of titanium alkoxide in different solvents were examined. In contrast, titanium alkoxides of different alkyl groups as titanium sources were found to have little influences on the structures and catalytic activities of resultant Ti-SBA-15 materials. Ti-SBA-15 of high Ti loading (ca. 4.2% Ti/Si ratio) prepared by grafting with TBOT in 1-butanol was found to have optimal catalytic activities in epoxidation of cycloalkenes, including cyclohexene and dicyclopentadiene, using tert-butylhydroperoxide as the oxidant.