Preparation of low carbon olefins on a core-shell K-Fe5C2@ZSM-5 catalyst by Fischer-Tropsch synthesis.

Abstract

In this study, a core–shell catalyst based on Fe5C2@ZSM-5 (ZSM-5 capped Fe5C2 as active phase) is prepared by the coating-carbonization method for Fischer–Tropsch synthesis (FTS). Further, the designed ZSM-5 zeolites are utilized to screen the low carbon hydrocarbons from the products generated on the iron carbide active centre, and for catalytic disassembly of the long-chain hydrocarbons into low carbon olefins. Prior to utilization, the physical–chemical properties of the prepared catalysts are systematically characterized by various techniques of X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Fourier transform infrared (FT-IR), and scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) observations, in addition to the effects of coating-carbonization, molecular sieve coating amount, and K-doping on core–shell iron-based catalysts. Next, the performance of Fischer–Tropsch synthesis is investigated in a micro-fixed bed reactor. The results manifest that, comparing with Fe5C2 and a supported Fe/ZSM-5 catalyst prepared by the traditional impregnation method, the core–shell Fe5C2@ZSM-5 catalysts show higher CO conversion rate, reaction activity and selectivity to low-carbon olefins. Comparatively, the Fe5C2@ZSM-5C catalyst prepared by carbonization after the coating method exhibited more surface area, smaller average pore size, and more reactive active sites, resulting in the improvement of screening of high carbon hydrocarbons and the enhancement of selectivity to low carbon olefins, in comparison to those prepared by the carbonization-coating method. In conclusion, the K-doping catalyst had significantly improved the reactive activity of the core–shell Fe5C2@ZSM-5 catalyst and the selectivity to low carbon olefins, while the CO conversion on K–Fe5C2@ZSM-20C still remained good.