Selective oxidation of methane to methanol over lignin-derived biochar supported transition metals catalysts

Continuous gas-phase selective oxidation of methane to methanol (SOMM) was a promising method for supplying liquid fuels but faced with huge challenges. Here, lignin-derived biochar (BC) supported transition metals including (Cu, Ni, Zn, Fe, and Co) catalysts were prepared and then applied for SOMM. The experiments demonstrated the continuous and simultaneous introduction of O2 and H2O were requirements to achieve methane-to-methanol. The methanol yields of various catalysts presented the following order: Cu/BC > Ni/BC > Zn/BC > Fe/BC > Co/BC at the first 1 h of reaction, while only Cu/BC maintained the catalytic activity for 18 h during SOMM on stream. Especially, Cu/BC obtained the highest methanol yield of 686.92 μmol/gCu/h under the optimal reaction conditions. The characterizations confirmed that the introduction of transition metals reduced the oxygen-containing functional groups of BC and improved the further carbonization of the BC to enlarge the pore volume of all catalysts, which promoted the adsorption of methane and the dispersion of active species. In addition, the activation process was the key for generating the active species. It demonstrated there were metallic Cu and Cu(II)-hydroxide species appeared in Cu/BC after activation process. The in-situ FTIR of Cu/BC identified that the synergistic effect between metallic Cu and Cu(II)-hydroxide species promoted the activation of the adsorbed CH4 to *CH3 species, which were further transformed to methanol.