Effect of PVA Concentration on Structure and Performance of Precipitated Iron-Based Catalyst for Fischer-Tropsch Synthesis

Abstract

Fe 2 O 3 catalysts were prepared by co-precipitation method with the assistance of polyvinyl alcohol (PVA) for Fischer-Tropsch synthesis. Effects of PVA concentration on structure and performance of the catalyst were investigated in a fixed reactor at 230-310 °C, 1.5 MPa, 2000 h -1 , and syngas H 2 /CO = 2.0. The catalysts were characterized by N 2 adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), H 2 or CO temperature-programmed reduction (TPR) and H 2 temperature-programmed desorption (H 2 -TPD), Fourier transform infrared (FTIR) spectra and thermogravimetric analysis (TGA). It was found that there was strong interaction between Fe and PVA, which controlled the structure of the catalyst. Among the catalysts investigated, the catalyst prepared with 15 wt.% PVA exhibited better catalytic performance due to the dispersion of iron oxides and the formation of the more active phase on the catalyst. Meanwhile, this catalyst showed the high selectivity to heavy hydrocarbons and satisfactory thermal stability.Keywords: Fischer-Tropsch synthesis, iron-based catalyst, PVA concentration, structure, stability IntroductionFischer-Tropsch synthesis (FTS), an efficient technology to convert syngas into liquid fuels and chemicals, has attracted great interest due to the shrinking of the petroleum resource in the past decades.1 Iron-based catalysts have attracted considerable focus, thanks to their low cost, high activity and excellent water-gas-shift reactivity, which match coal gasification with low H 2 /CO ratio. 2 Remarkably, three principal challenges are posed for iron-based FTS catalyst: activity, selectivity and stability. 3 It is well known that the activity, selectivity and stability of FTS catalysts rely strongly on the texture and surface properties of the resultant catalysts. 4 Most of all, the pore size of catalyst is one of the key factors, which has significant effect on the mass transfer of reactants and products, 5 the re-adsorption of the α-alkene, and the chemisorption ratio of H 2 on the surface active sites exposed. 6 Therefore, these factors are vital for the performance of catalyst in the FTS. For instance, smaller pore size of catalyst can afford high Brunauer, Emmett and Teller (BET) surface area and dispersion of the catalyst. However, diffusion effects of products were limited, resulting in light hydrocarbons as the main product for excessive hydrogenation. In contrast, the larger pore size of catalyst can facilitate the diffusion of products, and thus products of heavy hydrocarbons are obtained.7 Tao et al. 8 proposed that the FTS catalytic activity and product selectivity rely strongly on the pore size distribution of the catalysts. Xiong et al. 9 found that CO conversion increased and then decreased with the pore size in the range studied. Sun and co-workers 10,11 reported that the FTS catalytic performances were closely correlated to the pore sizes of the mesoporous zirconia.In general, the preparation process has a significant e...