ZSM-5 supported iron catalysts for Fischer–Tropsch production of light olefin
Introduction
With the increase in the cost of petroleum products due to depletion of oil reserves, alternative methods for synthesizing hydrocarbon fuels such as the Fischer–Tropsch Synthesis (FTS) have again received considerable attention. FTS is an important technology in the production of liquid fuels and chemicals from syngas derived from the gasification of coal, reforming of natural gas or other carbon-containing materials. Several metal-based catalysts can be used for the FTS; however, only iron and cobalt catalysts appear to be economically feasible in an industrial application. Cobalt-based catalysts mainly yield straight-chain hydrocarbons while iron-based catalysts are more useful when the H2/CO ratio is low (due to the water–gas-shift (WGS) activity of Fe) or for the production of alkenes, oxygenates, and branched hydrocarbons, depending upon promoters and process conditions employed. Also, iron-based composite catalysts prepared by physical mixture of the iron component and solid acid component such as zeolites have been studied to understand the influence of the nature of zeolite on the activity and product distribution. However, studies aiming at maximizing the yield of light olefins [1], [2] are scarce.
Many inorganic oxides such as Al2O3, SiO2, TiO2 and ZrO2 have been used as supports for improving the structural stability of iron-based catalysts. However, Fe can easily react with the supports to form metal complexes and is often difficult to reduce when highly dispersed on refractory oxides. The development of highly active supported Fe catalysts, however, would be useful since it could reduce catalyst density and encourage catalyst application in fluidized or slurry reactor. Zeolites are interesting supports for this purpose. Additionally, most of the studies reported have attempted to improve catalyst performance by promoting with additives such as K, Mn, Cr, Ru and Pd. Cu is usually present in bulk Fe catalysts as a reduction promoter. It facilitates reduction of iron oxide to metallic iron during hydrogen activation by lowering the reduction temperature which, in turn, reduces sintering of the metallic iron that is formed. Cu is more interesting than many other transition metals as it does not alloy with Fe. This suggests that there is less chance that the desired Fe catalytic properties would be altered [3].
In the present work, we report olefin synthesis from syngas over the catalysts containing Fe–Cu–K and supported on ZSM-5 of different Si/Al ratios. These composite catalysts were prepared by the conventional impregnation technique. The objective of this work is to find the influence of the Si/Al ratio on the physico-chemical properties such as the pore structure and the acidity of the ZSM-5 and on its performance in the FTS from syngas for light olefins.
Section snippets
Catalyst preparation
The iron-based catalysts for FTS from syngas were prepared by the conventional slurry-impregnation method of the proton-type ZSM-5 (supplied by Zeolyst with Si/Al ratio of 25, 40 and 140). Aqueous iron nitrate, copper nitrate and potassium carbonate solutions, with required composition, were mixed thoroughly in the slurry of ZSM-5. The impregnation on the ZSM-5 was carried out with continuous stirring for 12 h at room temperature. The samples were then dried in a rotary evaporator before
Results and discussion
The overall FTS reactions can be simplified as the combination of FTS reactions including paraffin and olefin formation as well as WGS reaction.Paraffin formation: nCO + (2n + 1)H2 ⇔ CnH2n + 2 + nH2OOlefin formation: nCO + 2nH2 ⇔ CnH2n + nH2OWGS reaction: CO + H2O ⇔ CO2 + H2
In the cases of iron-based catalysts, active sites for the above reactions are still not clear due to the fact that the iron-based catalysts starting from their metal precursors have complex phase transformation during the reduction (forming iron
Conclusions
The Si/Al ratio in ZSM-5 plays a critical role in controlling the activity and olefin selectivity in the FT synthesis using composite catalysts. The composite catalyst prepared by ZSM-5 with Si/Al ratio of 25 offers higher CO conversion and olefin selectivity in the C2–C4 range products. At higher Si/Al ratio (greater than 25), the reducibility of Fe species and the density of weak acid sites are lower, thus reducing the CO conversion and olefin selectivity, respectively. The facile formation
Acknowledgements
The authors would like to acknowledge the funding from the Korea Ministry of Knowledge Economy (MKE) through “Project of next-generation novel technology development” of ITEP. P.S. Sai Prasad thanks Korea Federation of Science and Technology (KOFST) for the award of the visiting research fellowship under Brain Pool program.
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