Direct Conversion of Syngas into Light Olefins with Low CO 2 Emission

Direct conversion of syngas into light olefins over bifunctional catalysts has made significant progress; the C 2 = –C 4 = selectivity in hydrocarbons reaches >80%. Nevertheless, a relatively harsh reaction condition (>380 °C, 1.0 MPa) led to producing large amounts of CO 2 (>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 (Zn x Ce 2– y Zr y O 4 ) 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 CH 4 selectivity and CO 2 emission lower than 5 and 6%, respectively. A combination of experimental, in situ spectroscopy, and theoretical calculation results reveals that doping Ce in Zn x Zr 2.0 O 4 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.