ON THE NATURE AND FORMATION OF THE ACTIVE-SITES IN RE2O7 METATHESIS CATALYSTS SUPPORTED ON BORATED ALUMINA

Re2O7 catalysts on borated aluminas have been investigated with a view to correlating the structure of the active site and its activity in the methathesis of methyl oleate. Modification of alumina with boria results in much more active metathesis catalysts. Infrared spectroscopy was used for the characterization, pyridine adsorption measurements for determining the Lewis acid and Brønsted acid sites, and temperature-programmed IR measurements to follow the reactions occurring during calcination of the supports and catalysts. At a loading up to 5 wt% B2O3, an alumina phase is present together with a boria phase. Both phases contain surface hydroxyls. At higher B2O3 loadings (>5 wt%) the alumina hydroxyls are completely covered by boria. The amount of boron hydroxyls decreases with increasing B2O3 loading, due to the formation of linear boria polymers. Boria binds to the surface via the alumina hydroxyls. Upon Re2O7 loading of nonborated alumina, the ReO4 groups react first with Lewis acid sites, onto which they are strongly bonded. Above a Re2O7 loading of 3 wt% surface hydroxyls are also substituted by ReO4 groups, resulting in an increase in catalytic activity. When the borated supports are loaded with Re2O7, the ReO4 groups are also first bonded to the Lewis acid sites. During calcination these ReO4 groups substitute surface hydroxyls preferably on alumina hydroxyls. The substitution of the boron hydroxyls only takes place at a calcination time of at least 2 h at 823 K. At high borate loadings (>10 wt%) the reaction of ReO4 groups with boron hydroxyls competes with the condensation reaction of two neighbouring boron hydroxyls. Taking into account that a ReO4 group which has substituted an acidic OH group on the support is the precursor of an active site, the increase in activity of Re2O7 catalysts by modification of the alumina support with boria can be ascribed to two effects, namely, the reduction of the bonding strength of Lewis acid sites with ReO4, making the ReO4-OH substitution reaction possible during calcination even at low rhenium loadings, and the formation of acidic surface hydroxyls. © 1994 Academic Press, Inc.