ACTIVITY OF SUPPORTED RE2O7 CATALYSTS FOR THE METATHESIS OF METHYL OLEATE

Catalysts containing 3 wt.-% Re2O7 were prepared using a gamma-alumina support or silica-alumina supports with varying alumina contents and surface areas. The activities of these catalysts in the metathesis of methyl oleate were determined in batch experiments in which tetraethyltin was used as a catalyst promoter. Both the alumina content and the surface area of the catalyst turned out to have a substantial influence on the catalytic activity per unit weight of rhenium. The catalyst with a silica-alumina support containing 25 wt.-% alumina appeared to be most active, while the activity also increased with increasing surface area. Diluting a catalyst with extra support material resulted in a spectacular increase in its specific activity. It appeared that during calcination part of the rhenium was transferred from the original catalyst to the admixed support. This spreading behaviour is either ascribed to a solid-solid wetting process, probably only concerning the ReO4- tetrahedra bonded to silicon atoms, or to surface diffusion. An additional role of the extra support material is to remove impurities in the substrate. In accordance with these results, the turnover number of the non-diluted silica-alumina-supported catalysts appeared to increase with decreasing rhenium content. At low catalyst loadings the Si-Al bridging hydroxyl groups on the surface of the support are replaced by ReO4- -tetrahedra, resulting in active rhenium centres, while at higher rhenium loadings the hydroxyl groups attached to silicon atoms are also replaced by ReO4- tetrahedra. This latter type of rhenium centre is not active in metathesis. In contrast, the turnover number of the alumina-supported catalysts increased with increasing rhenium loading. This can be explained by the fact that on gamma-alumina the ReO4- tetrahedra first replace basic surface hydroxyl groups, resulting in inactive sites. At higher rhenium loadings acidic hydroxyl groups are also replaced by ReO4- tetrahedra, which does result in active rhenium centres.