Valorization of Glycerol in Propanediols Production by Catalytic Processes

In this chapter a review on hydrogenolysis reactions, promoted by enzymatic, homogeneous and heterogeneous catalysts, with particular emphasis on the different reaction mechanisms, is proposed, having the aim to provide a bridge between catalysis and glycerol deriving applications. The selective hydrogenolysis of glycerol to propylene and ethylene glycols is one of the most attractive routes since it is a clean and economically competitive process that allows formation of different valuable products, such as 1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO). Glycerol can be converted into 1,3-propanediol in a two-step, enzyme-catalyzed reaction using bacterial strains attaining to the groups citrobacter, enterobacter, ilyobacter, klebsiella, lactobacillus, pelobacter and clostridium. In all cases, in the first step, an enzyme dehydratase catalyzes the conversion of glycerol into 3-hydroxypropanal (3-HPA) then reduced to 1,3-PDO. The 1,3-PDO is not further metabolized and, as a result, accumulates in the medium. Generally, the hydrogenolysis of glycerol by homogeneous catalysts leads to a variety of by-products such as propanol or ethers and to a mixture of 1,2- and 1,3-PDO. Typical catalysts for this application are metals of the platinum group either in complexes having iodide and carbonyl ligands or compounds with phosphorous, arsenic or antimony containing ligands. Another possible way to obtain propanediols from glycerol is the direct hydrogenolysis of glycerol by classical heterogeneous hydrogenation catalysts. Different metal catalysts were, so far, widely employed: Cu, Ru, Rh, Pt and Pd supported on a wide range of qualitatively different carriers and sometimes in presence of Brønsted acids or bases as co-catalysts. With respect to heterogeneous catalysis, in the last years, two innovative catalytic processes affording propylene glycols, in absence of added hydrogen, were developed. In the first one the hydrogen being produced by steam reforming of glycerol in reaction conditions while, in the latter, the catalytic process allows to obtain the hydrogen necessary for the hydrogenolysis reaction directly from the reaction solvent (2-propanol). Finally, an accurate investigation on the possible intimate reaction mechanism is presented.