Role of Residual CO Molecules in OX–ZEO Relay Catalysis for Syngas Direct Conversion

The traditional Fischer–Tropsch synthesis generates products with a distribution that follows a theoretical restriction, suffering from low selectivity toward desired products. Recent use of metal oxide–zeolite bifunctional catalysts has allowed a break of such a restriction, accompanied by residual CO molecules. In these systems, CO conversion is usually low and the presence of residual CO molecules would accompany the diffusion of intermediates into the zeolite pores and the subsequent transformation. However, the relationship between CO and the selectivity is elusive. Herein, in a CoMn oxides/H-β zeolite system, we demonstrate that CO molecules could not only improve the strength of the acidity and increase the ratio of B acid sites to L acid sites of the zeolite but also help to stabilize the intermediate allylic carbonium ion. Hence, high selectivities of 73.5% toward iso-paraffins and 61.2% toward isobutane were achieved. Our results are informative for future efforts on the control over the selectivity of syngas direct conversion.