Comparison of hydrophilic and hydrophobic silicas as supports for titanium catalysts

Abstract

Siloxane bridges on the silica surface react with HCl leading to a more hydrophilic solid, which surprisingly shows a better performance as support for titanium centres than the parent silica. The use of a weaker electrophilic species, such as an acyl chloride, allows the modulation of the support properties in such a way that it can be prepared the best silica-supported titanium catalyst for epoxidation with diluted hydrogen peroxide. This result demonstrates that the hydrophobic character of the solid is not a sine qua non requirement to obtain efficient epoxidation catalysts. Although the reasons for this good behaviour are not fully understood, it seems that the type of surface titanium species is the key factor, and modifications of silanol density with the different treatments control the distribution of titanium species.