Influence of Si/Zr ratio on the formation of surface acidity in silica-zirconia aerogels

A series of silica-zirconia mixed oxides were prepared by sol-gel routes wherethe zirconia content was varied from 9 to 75 mol%. The bulk and surface properties were analysed and compared with those of a sample wherezirconia was deposited as an external layer on a silica surface thereby avoiding internal mixing of the component oxides. Spin-lattice relaxation measurements made by 29Si MAS NMR provide a useful estimation of the relative size of domains containing silica only and thus the degree of mixing. High levels of silica distribution throughout the mixed oxide prevented crystallisation of zirconia phases even at high Zr:Si ratios. The extent of mixing at the surface and throughout the bulk is discussed in relation to the generation of both Lewis and Brønsted acid sites. Increased levels of mixing and heteroatom linkages throughout the bulk did not necessarily result in an increase in the density of surface Brønsted acid sitesgenerated.