Tandem Catalysis for Hydrogenation of CO and CO2 to Lower Olefins with Bifunctional Catalysts Composed of Spinel Oxide and SAPO-34
Abstract: Transformation of syngas (H2/CO) and hydrogenation of CO2 into lower olefins are attractive routes for chemical utilization of various carbon resources and CO2, but both suffer from limited product selectivity. Tandem catalysis that integrates the activation of CO or CO2 to an intermediate and the subsequent controllable C−C bond formation to form lower olefins offers a promising approach. Here, we report the hydrogenation of both CO and CO2 over bifunctional catalysts composed of a spinel binary metal oxide and SAPO-34. ZnAl2O4/SAPO-34 and ZnGa2O4/SAPO-34 are found to be highly selective for the synthesis of lower olefins from both CO and CO2. Our studies reveal that the oxygen vacancy site on metal oxides plays a pivotal role in the adsorption and activation of CO or CO2, while the −Zn−O− domain accounts for H2 activation. We demonstrate that methanol and dimethyl ether formed on metal oxide are the reaction intermediates, which are subsequently converted to lower olefins by the Brønsted acid sites in zeolite. The hydrogenation of CO and CO2 on metal oxide surfaces proceeds via the same formate and methoxide species. We elucidate that the water-gas shift reaction on oxide surfaces is responsible for CO2 formation during syngas conversion. The co-feeding of CO2 in syngas offers a useful strategy to inhibit CO2 formation.