Cleaner hydrothermal hydrogenolysis of glycerol to 1,2-propanediol over Cu/oxide catalysts without addition of external hydrogen

Abstract

The hydrogenolysis of glycerol is considered a sustainable process to produce 1,2-propanediol (1,2-PDO). However, the development of a cost-effective solid catalyst and a cleaner process minus the addition of external hydrogen remains a challenge. In the present work, a series of Cu/oxide (SiO2, MgO, Al2O3, and ZnO) catalysts were prepared and evaluated for the hydrothermal hydrogenolysis of glycerol. The relationships of structure-catalytic properties were probed by thermogravimetric analysis (TG), powder X-ray diffraction (XRD), nitrogen adsorption-desorption, temperature-programmed desorption of carbon dioxide (CO2-TPD), transmission electron microscopy (TEM), and X-ray photoelectron spectra (XPS). The catalytic activity of Cu/oxide catalysts was in the order of Cu/Al2O3<Cu/SiO2≈Cu/ZnO<Cu/MgO. The Cu/MgO catalysts were identified as highly active catalysts for the hydrothermal hydrogenolysis of glycerol under autogenic pressure without the addition of external hydrogen. When 1.0g Cu/MgO (Cu/MgO=0.5, molar ratio) catalyst and 50g 20wt% aqueous glycerol solution as the feedstock was loaded in a batch autoclave reactor, the conversion of glycerol reached 55% with a selectivity of 68% to 1,2-propanediol at 473K after 6h. Cu0 was identified as the active species both for the catalytic in situ aqueous-phase reforming of glycerol and for the hydrothermal hydrogenolysis of glycerol. The support basicity played a role in stabilizing the Cu nanoparticles on the support surface. During the catalytic reaction, part of Cu species leached from the MgO support into solvent and meanwhile part of Cu species on the support surface aggregated. The basicity and acidity of the solvent affected the behavior of the Cu/MgO catalysts. This work shows that hydrothermal hydrogenolysis process of glycerol is a facile process and the cost-effective Cu/MgO is worth further development for future industrial practice.