Effects of Acidity and Pore Size Constraints on Supported Niobium Oxide Catalysts for the Selective Formation of Glycerol Monolaurate

Abstract

Supported niobium oxide catalysts with 5-30wt.% Nb2O5 were prepared by grafting niobium ethoxide onto MCM-41 and hydrous zirconia. The supported samples contain Brønsted-acid sites, whereas bulk niobium oxide has predominantly Lewis-acid sites. In the esterification of glycerol with lauric acid, good activity and a high glycerol monolaurate selectivity was achieved when the active niobium oxide phase was coated within the pore channels of an MCM-41 support, which has a mean pore diameter of 2.2nm. Phosphated Nb2O5/MCM-41 showed an even higher activity without any penalty in selectivity, enabling monolaurate yields of 89-90%. Glycerol monolaurate is an important ingredient in the food, pharmaceutical, and cosmetic industries. A key feature of these pore confined catalysts is the sustained high selectivity to monolaurate even at high conversions. In contrast, niobium oxide supported on wide-pored hydrous zirconia showed lower selectivity. The catalysts are active in the solventless esterification of a number of alcohols and acids and can be easily recycled for subsequent batch reactions, making them attractive as green catalysts for sustainable processes. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.