Selective conversion of glycerol into lactic acid by Pt, Au and Pt-Au-catalyzed reactions

Lactic acid (2-hydroxypropanoic acid) has been recognized as one of the most important platform molecules for food and pharmaceutical industry. [1,2] Its market demand grows rapidly due to its application in the synthesis of poly (lactic acid), a biodegradable and biocompatible polymer. [3] The large-scale production of lactic acid is based on the fermentation of aqueous glucose under anaerobic conditions with a strict control of the pH-level in order for the bacteria to function optimally. [4] Alternative attractive heterogeneous synthesis processes for lactic acid are the conversion of dihydroxyacetone, glyceraldehyde [5,6] sugar [7], cellulose [8] and glycerol. [9-11] We present here the successful conversion of glycerol to lactic acid by Pt, Au and Pt-Au-nanoparticles (NPs) obtained by metal vapor synthesis [12] and supported onto Ketjenblack, which is a high surface area (ca. 1400 m2/g) carbon. We investigated the role of the structural features of mono- and bimetallic heterogeneous catalysts on their catalytic performance for the conversion of glycerol into lactic acid in the presence of either oxygen (air) or ethylene. The presence of a hydrogen acceptor is fundamental for the NPs-catalyzed conversion of glycerol into glyceraldehyde (Scheme 1, step a). This latter compound is then successively transformed via dehydration (Scheme 1, step b) and a base-catalyzed intramolecular Canizzaro reaction into lactic acid (Scheme 1, step c)). As a result, we found that ethylene gave significantly higher lactate selectivity compared to oxygen (i.e. >95% (ethylene) vs 78% (oxygen)).