(100d) Polyol Hydrodeoxygenation over Molybdenum Carbide Catalysts: From Surface Science to Liquid-Phase Reactor Environments

Producing valuable chemicals from renewable biomass derivatives such as glycerol continues to be of high importance for improving the environmental sustainability of the chemical industry. Here, we studied the hydrodeoxygenation (HDO) of glycerol over b-Mo₂C catalysts under both ultra-high vacuum and liquid phase reactor environments. Under ultra-high vacuum conditions, b-Mo₂C(0001) is highly active for the full deoxygenation of glycerol to propylene; however, the catalyst slowly deactivates as the removal of oxygen from the catalyst surface is challenging. These computational results agree with experiments from the Chen group from Columbia University. Surprisingly, under reactor environments in liquid 1,4-dioxane at 180°C and 500 psi H₂, the Pagan-Torres group at UPRM observed a stable conversion of glycerol to 1-propanol and 1,2-propanediol over Mo₂C powder catalysts in flow reactor studies. To explain experimental observations, we performed a careful computational investigation of various active sites and reaction mechanisms over b-Mo₂C(0001) and b-Mo₂C(111) surface models. As expected, at higher glycerol chemical potentials the Mo₂C surfaces are oxygen and hydroxy covered. In case of b-Mo₂C(0001), the surface is so oxophilic that only acid sites are available for catalysis which leads to a stable but somewhat lower catalytic activity (and different selectivity) than experimentally observed. Interestingly, for b-Mo₂C(111) the surface becomes bifunctional with both metallic and acidic sites and a CSTR model for this site predicts not only a high catalytic activity but also a product distribution that is in excellent agreement with experiments.