Direct Conversion of Syngas into Light Olefins with Low CO2 Emission

Direct conversion of syngas into light olefins over bifunctional catalysts has made significant progress; the C2=–C4= selectivity in hydrocarbons reaches >80%. Nevertheless, a relatively harsh reaction condition (>380 °C, 1.0 MPa) led to producing large amounts of CO2 (>40%) and gave a low olefin/paraffin (O/P) ratio (<10) as a result of significant promotion of water–gas shift (WGS) reaction and overhydrogenation of olefins. In this context, attempts are made here to develop a highly active low-temperature composite catalyst. It was found that a zinc–cerium–zirconium solid solution (ZnxCe2–yZryO4) and a SAPO-34 mixture showed CO conversion, light olefin selectivity in hydrocarbons, and O/P ratios of about 7%, 83%, and 23, respectively, at 300 °C and 1 atm. More interestingly, this catalyst showed CH4 selectivity and CO2 emission lower than 5 and 6%, respectively. A combination of experimental, in situ spectroscopy, and theoretical calculation results reveals that doping Ce in ZnxZr2.0O4 greatly inhibits the WGS reaction by increasing the formation energy barrier of carboxylate intermediate species, but increases surface oxygen vacancy concentration of the composite through formation of a solid solution, and as a consequence, improving the catalytic activity for conversion of syngas at mild conditions by enhancing the interaction of CO with the catalyst, which elongates the C–O bond of the HCO* species.