Utilization of SAPO-18 or SAPO-35 in the bifunctional catalyst for the direct conversion of syngas to light olefins

Abstract

SAPO-18 and SAPO-35 were synthesized and utilized as the zeotype in the bifunctional catalyst for the STO process, respectively. SEM and Ar physisorption proved that SAPO-18 displayed abundant outer cages, and facilitated the diffusion of the reactant and products. NH₃-TPD revealed the adequate acid strength of SAPO-18, thus ZnCrO_x + SAPO-18 bifunctional catalyst showed high selectivity to light olefins during the whole stage of the STO process. 19.9% CO conversion and 68.6% light olefins selectivity (free of CO₂) was achieved over ZnCrO_x + SAPO-18(0.048) at 653 K, 1.0 MPa, GHSV = 6000 mL g⁻¹ h⁻¹. The catalytic performance was stable after 6000 minutes of reaction because of the good diffusibility of SAPO-18. GC-MS and TG demonstrated that the ZnCrO_x + SAPO-35 bifunctional catalyst deactivated very quickly because of the severe formation of the heavy coke deposits, which should be attributed to the acidic properties of SAPO-35 and the poor diffusibility originating from its 2-dimensional channel system. Although the ZnCrO_x + SAPO-35 bifunctional catalyst exhibited high CO conversion and light olefins selectivity at the early stage of the STO process as well, its catalytic performance was unsustainable.