Factors affecting the activity and selectivity of niobia-based gold catalysts in liquid phase glycerol oxidation

Abstract

Abstract The present study aims at the elucidation of the influence of niobia crystallinity, gold deposition method and the presence of copper dopant on the activity and selectivity of niobia supported gold catalysts in liquid phase oxidation of glycerol. The role of niobium pentoxide crystallinity was estimated with the use of gold catalysts supported on three different niobia supports but prepared by the same gold loading method, while the influence of gold deposition method and the presence of copper promoter were studied using mono- (Au, Cu) and bimetallic (AuCu) catalysts supported on the same niobia support but synthesized using different metal loading methods. The activity of the catalysts in glycerol oxidation was tested under different reaction conditions, i.e. different reaction time and various concentrations of sodium hydroxide as a base. Detailed analysis of relationship between the properties of catalysts and their activity/selectivity in glycerol oxidation considered in this study showed that niobia crystallinity had negligible influence on the activity of gold catalysts containing small metal nanoparticles with the size of ca. 5 nm. Furthermore, it was documented that copper species in the bimetallic AuCu catalyst play double roles. Firstly, copper acts as a structural promoter which enhances the dispersion of the active phase, and thus enables high activity in glycerol oxidation. Secondly, copper itself promotes the C C bond cleavage during glycerol oxidation. It was found that the bimetallic catalyst was much more selective to glycolic and formic acids than the monometallic gold catalyst, despite the fact that both catalysts contained gold particles of similar average size of ca. 5 nm. This observation allowed drawing the conclusion that the changes in selectivity of the bimetallic AuCu sample did not result from different size of gold nanoparticles, but were more likely to be related to the presence of copper dopant which improved the catalyst ability to C C bond cleavage.