Selective grafting of titanium on periodic nanoporous silica materials

Abstract

A highly-ordered periodic nanoporous silica (PNS) has been synthesized using a commercially available non-ionic poly(ethylene oxide) alkyl ether surfactant under acidic hydrothermal conditions. The periodic arrangement has been obtained of mesoscopically ordered pores with a two-dimensional hexagonal p6mm pore structure. Ti was then incorporated on this substrate by the chemical grafting of Ti(IV)(triethanolaminato)-isopropoxide on the OH terminated surface of PNS, using two different paths: the conventional one, where Ti is incorporated in the whole solid surface, and a second path in which titanium incorporation has been selectively directed inside the PNS pore channels. This latter procedure leads to better titanium dispersion (UV–Vis). The surface of the solids was rendered hydrophobic by silylation with hexamethyldisilazane. The samples were tested in the catalytic epoxidation of 1-octene with ethylbenzene hydroperoxide (EBHP). It was found that silylated samples record higher conversion and selectivity than their non-silylated counterparts as a consequence of the hydrophobicity of the catalyst surface. Moreover, the selective incorporation of the titanium inside the pore channels of the PNS substrate leads a better catalytic performance, due to the high dispersion of Ti centres achieved with this preparation procedure. The epoxide yield with this silylated catalyst was nearly quantitative with respect to the EBHP consumed.