A 29Si NMR investigation of the structure of amorphous silica—alumina supports

Abstract

Commercial alumina-based catalysts are often formulated with silica to provide either acidity in the support or structural stability. Hydrocracking, catalytic cracking, and mild hydrocracking catalysts generally have silica incorporated at concentrations of 20% or more; catalysts for hydrotreating, on the other hand, may have silica added at less than 5% by weight. A magic angle spinning nuclear magnetic resonance (MAS NMR) study is reported for a series of amorphous silica—aluminas using 29Si NMR to monitor local structure. The NMR of 29Si has distinctly different chemical shifts when SiO ligands are exchanged for AlO ligands, i.e. the distribution of chemical shifts can be used to determine the structure and the degree of homogeneity of SiO 2 distribution in γ-Al 2O 3 supports at SiO 2 concentrations as low as one percent. Results of such NMR measurements and correlations between method of preparation and the distribution of silica in and on alumina is presented. A particularly interesting observation is that the form of silica may play a significant role in determining the thermal stability of alumina as reflected in the retention of surface area and porosity under severe conditions and that this stability can be correlated with a given kind of silica observed by NMR.