Transition metal-containing mixed oxides catalysts derived from LDH precursors for short-chain hydrocarbons oxidation

摘要

MMgAlO mixed oxide catalysts (M = Mn, Fe, Co, Ni, Cu, Zn, Ag and Pd) obtained from layered double hydroxide (LDH) precursors calcined at 1023K were used in the complete oxidation of methane and the oxidative dehydrogenation of propane. The catalysts were characterized by XRD, N2 adsorption, EDX and H2-TPR experiments. In the catalytic complete oxidation of methane, the Pd-based catalyst has been proved the most active, but the less stable catalyst. Other mixed oxides containing transition metals have been tested and their catalytic activity followed the order: MgAIO ≈ FeMgAlO < NiMgAlO < ZnMgAlO < MnMgAlO < CoMgAlO < AgMgAlO < CuMgAlO. Total conversion was achieved at 858 K with CuMgAlO the most active and highly stable catalyst also able to perform the complete oxidation of ethane, propane and propene. It has been shown that highly reducible metal oxide species play an important role in the catalytic combustion of methane. In the oxidative dehydrogenation of propane in the temperature range from 723 K to 873 K the propene selectivity passed through a maximum for the Mn- and Fe-containing catalysts and decreased continuously for the other transition metal-containing catalysts to the benefit of CO_x for CuMgAlO and of cracking products for CoMgAlO, NiMgAlO and ZnMgAlO. Considering the propene yield, the catalysts can be ranked in the following order: CoMgAlO MnMgAlO > NiMgAlO > ZnMgAlO > FeMgAlO > CuMgAlO. No straight correlation between the H2-TPR reducibility and the catalytic performances of the samples was found in the oxidative dehydrogenation of propane into propene. The effects of the contact time and of the propane-to-oxygen molar ratio on the catalytic performances of the most active CoMgAlO, MnMgAlO and NiMgAlO mixed oxides have been investigated.