Development of Highly Active Silica-Supported Nickel Phosphide Catalysts for Direct Dehydrogenative Conversion of Methane to Higher Hydrocarbons

Abstract

The direct dehydrogenative conversion of methane (DCM) to higher hydrocarbons was investigated over silica-supported nickel phosphide catalysts (NixPy/SiO2) over 1023 K. NixPy/SiO2 catalysts were prepared by precipitation method to promote formation of nickel phosphide (Ni2P) as an active phase for the DCM reaction. Characterization studies of the NixPy/SiO2 catalysts with different P/Ni molar ratios were conducted by a X-ray diffraction analysis, a H2-temperature-programmed reduction spectrum, a scanning electron microscopy image, a X-ray absorption spectroscopy and a N2-adsorption measurement. Catalytic activity tests for the DCM reaction were conducted using a conventional fixed-bed reactor. Products of C2H4 (ethylene), C2H6 (ethane), C2H2 (acetylene), C3H6 (propylene), C6H6 (benzene), C7H8 (toluene), C10H8 (naphthalene) and H2 were analyzed by GC-TCD and GC-FID instruments. Different degrees of the Ni2P phase and character were observed for the NixPy/SiO2 catalysts from characterization studies. Data from characterization studies indicated that smaller and dispersed Ni2P particles were obtained by precipitation method as compared to that of impregnation method. NixPy/SiO2 with a molar ratio of P/Ni = 3.0 showed optimum catalytic performance with 3.28% of methane conversion, 1.93% of total product yield, and 60% of selectivity to hydrocarbons. The experimental results of the effects of reaction temperatures on the product distributions and activation energies indicated that the Ni2P phase successfully activated the C–H bond of methane and selectively converted to ethane. Ethane thermally converted to other higher hydrocarbons in the gas phase without the participation of the catalyst.