Oxidative coupling of methane over sodium-salt-promoted zirconia catalysts prepared by the mixed solution method

Abstract

Sodium-salt-promoted zirconia catalysts prepared from mixed solutions of zirconyl chloride and sodium salts (chloride, carbonate, nitrate or sulfate) are found to exhibit effective catalytic performances in the oxidative coupling of methane. The highest C2+ yield (16.6%) and ethylene-to-ethane ratio are obtained over the Na2CO3-added catalyst having an appropriate Na/Zr ratio; however, the active promoting species in this catalyst are actually identified to be Na+ and Cl−. The mixed solution method is considered to be an effective preparation method when proper precursor compounds of zirconia and promoters are chosen, giving a desirable interaction between them. The reaction and characterization results indicate that the chlorine ion plays a more prominent role than the sodium ion, and the X-ray photoelectron spectroscopy results show the presence of multiple chlorine species on the surface. It is suggested that the Cl− bound to Na+ is capable of activating both methane and ethane whereas the Cl− bound to Zr4+ is only capable of activating ethane. The sodium ion is considered to play an important role in stabilizing the chlorine ion.