Synthesis of light olefins from syngas catalyzed by supported iron-based catalysts on alumina

Abstract

The modified alumina-supported iron-based catalysts for the direct synthesis of light olefins from syngas were prepared by incipient wetness impregnation method and tested in a fixed-bed reactor. The catalysts were characterized by H2-TPR, NH3-TPD, TEM, N2-physical adsorption, mercury intrusion and XRD methods. And the effects of calcination temperature of alumina supporter, iron loadings, S precursor, S/Fe molar ratio, Na/Fe molar ratio and reaction conditions on the catalytic performance were investigated. Experimental results showed that the change in calcination temperature of alumina supporter had significant influences on the structure properties and catalytic performances of the catalysts, and when the calcination temperature of alumina supporter was 1200 ºC, the catalyst exhibited the optimal catalytic performance with a CO conversion of 93.5%, a CO2 selectivity of 24.7%, a C2−4 olefins selectivity of 34.1% and an olefins to paraffins ratio in C2–4 fraction of 6.7. Below 1200 ºC, strong metal-supporter interaction and surface acidity impair the catalytic performance of the catalysts; above 1200 ºC, the catalyst exhibited a poor stability. Moreover, all the S precursors were found with the ability to increase the light olefins selectivity but decrease the activity. The influence degree of these S precursors is following the order: Na2S > Na2S2O3 > Na2SO3 > Na2SO4. The synergistic effect of Na and S promoters can optimize the catalytic performance of the catalyst. Under the reaction conditions of 340 ºC, 2.0 MPa, and 4000 h−1, the 1200 ºC-calcined alumina supported iron-based catalyst with a Na/Fe molar ratio of 0.8 and a S/Fe molar ratio of 0.02 exhibited the best catalytic performance with a CO conversion of 90.3%, a CO2 selectivity of 27.9%, a light olefins selectivity of 47.8% and an olefins to paraffins ratio in C2–4 fraction of 9.2.