Bifunctional Au-Sn-SiO2 catalysts promote the direct upgrading of glycerol to lactates

Valuable alkyl lactates can be obtained from (waste) glycerol, through a two-step process that entails (i) the oxidation to dihydroxyacetone (DHA) and (ii) a rearrangement of DHA with an alcohol. While the latter reaction is effectively catalyzed by Sn-based heterogeneous catalysts, the former reaction can be carried out with Au-based catalysts. The overall process, however, is penalized by a limited selectivity of supported gold nanoparticles in the first reaction, which strongly restrains the overall lactate yield. To avoid transitional purification steps, it appears interesting to run the process as a cascade reaction, in one step, and ideally with bifunctional catalysts. The preparation of such catalysts, however, remains a challenge. Here, bifunctional catalysts are prepared in one step, through a straightforward aerosol-assisted sol-gel route. The catalysts feature small Au nanoparticles (3-4 nm) embedded at the surface of mesoporous Sn-doped silica microspheres. The preparation successfully leads to insert both active sites in their most active forms, and in close proximity. With the bifunctional catalysts, the selectivity for the final product of the cascade reaction (methyl lactate) is higher than the DHA selectivity when only the first reaction is carried out. This highlights a beneficial substrate channeling effect which helps avoiding side reactions. Interestingly, the bifunctional catalysts also markedly outcompeted mechanical mixtures of the corresponding Au- and Sn-based catalysts. Thus, the spatial proximity between the two active sites in bifunctional catalysts is identified as a key to stir the cascade reaction towards high yield.