Chain Propagation Mechanism of Fischer–Tropsch Synthesis: Experimental Evidence by Aldehyde, Alcohol and Alkene Addition

Liping Zhou,Yongwang Li,Yong Yang,Junhu Gao,Xu Hao

doi:10.3390/reactions2020012

Abstract

Fischer–Tropsch synthesis (FTS) produces hundreds of hydrocarbons and oxygenates by simple reactants (CO + H2) and the detailed chain propagation mechanism is still in dispute. An industrial iron-based catalyst was used to further clarify the mechanism by adding aldehyde, alcohol and alkene species into a fixed-bed tubular reactor. The added species were investigated in H2 and syngas atmospheres, respectively. 1-alkene in the H2 atmosphere presented an obvious hydrogenolysis, in which the produced C1 species participated in C–C bond formation simultaneously. Co-feeding Cn alkene with syngas showed remarkable Cn+1 alcohol selectivity compared to the normal FTS reaction. In addition, the carbonyl group of aldehyde was extremely unstable over the iron-based catalyst and could easily be hydrogenated to an alcohol hydroxyl group, which could even undergo dehydration for hydrocarbon species formation. Experimental data confirmed that both heavier alkenes and alcohols added can be converted to chain growth intermediates and then undergo monomer insertion for chain propagation. These results provide strong evidence that the chain propagation in the FTS reaction is simultaneously controlled by the surface carbide mechanism and the CO insertion mechanism, with surface CHx species and CO as monomers, respectively. The study is of guiding significance for FTS mechanism understanding and kinetic modeling.

Highlights

Fischer–Tropsch synthesis (FTS) has attracted extensive attention because it is a feasible technology for the conversion of coal, biomass and natural gas into liquid fuels [1,2]
In the present work we mainly focused on the analysis of the reaction behavior of aldehyde, alcohol and alkene and the discrimination of chain growth mechanisms in FTS
By summarizing the experimental results shown in previous sections, we propose the aldehyde formed in the FTS process is an intermediate product of the CO insertion that the aldehyde formed in the FTS process is an intermediate product of the CO insertion mechanism, which is very unstable in an H2 atmosphere and could be hydrogenated mechanism, which is very unstable in an H2 atmosphere and could be hydrogenated to alcohol products

Summary

Introduction

Fischer–Tropsch synthesis (FTS) has attracted extensive attention because it is a feasible technology for the conversion of coal, biomass and natural gas (via gasification or steam reforming to syngas) into liquid fuels [1,2]. The fundamental understanding of the chain growth mechanism is still a developing field. Three independent chain growth assumptions have been proposed (summarized in Scheme 1): the surface carbide mechanism [3], the CO insertion mechanism [4] and the surface enol mechanism [5]. The surface carbide mechanism (Scheme 1a) implies that FTS chain propagation is performed by oxygen-eliminated monomers (CHx , formed by direct or H-assisted CO dissociation [6,7,8,9]) and the chain terminates for linear alkane and alkene formation by hydrogenation or β-hydride elimination of the intermediates. Hydrogen could react either in a molecular state or by dissociative adsorption, but here we have emphasized the dissociative adsorption in Scheme 1 because hydrogen dissociation has a low activation barrier [10]

Methods

Results

Discussion

Conclusion