Perovskites as New Support Materials for the Iron Based Fischer-Tropsch Synthesis

Khasu, Motlokoa

Perovskites as New Support Materials for the Iron Based Fischer-Tropsch Synthesis

Doctoral Thesis

2022

Abstract

In the Fischer-Tropsch process, valuable products consisting mainly of long chain hydrocarbons and water as by-product are synthesized from the basic starting materials hydrogen and carbon monoxide, derived from sources such as coal, gas or biomass [1]. Iron catalysts and cobalt based systems are the preferred catalysts for the commercial Fischer-Tropsch synthesis (FTS) employed at PetroSA and Sasol in South Africa [2]. The use of iron catalysts is not only influenced by the low price of the active material compared Co and Ru, but also by higher activity and a wide window of FT operating conditions. Though the product selectivity of iron-based FT synthesis can, to a certain extent, be controlled by changing the process temperature, promoters are added to improve selectivity and activity of the catalyst by modifying structural and/or electronic properties of the active phase [3]. Potassium is the most used among alkali promoters in iron-catalyzed FTS because of its Lewis acid nature [4–6]. Alkali promoters simultaneously promote dissociative CO chemisorption and inhibit chemisorption of H2 by a charge transfer to the surface of the iron based active phase, resulting in a higher C/H ratio on the catalyst surface and faster carburization rates, longer chain products and higher olefin content [5,7,8]. The addition of promoters is beneficial up to a certain amount, above which it becomes detrimental to the overall activity of the catalyst due to coverage of active sites and/or excessive coking [9–11]. Its mobility under reaction conditions is another challenge as it results in a highly dynamic system. The goal of the here proposed study is to investigate the possibility of using a new class of supports (perovskites) that do not only provide high surface area and enhance the dispersion of the active phase but also adopt the role currently played by promoting elements without suffering from the disadvantages reported above (mobility on catalyst, blocking active sites, etc.) [12]. Perovskites are mixed-metal oxides that have attracted much scientific attention due to their low price, adaptability and thermal stability. They generally have the formula ABO3 or A2BO4 (A and B are cations of different sizes, and O is the anion that binds them), exhibiting a range of stoichiometries and crystal structures. Their structural features make it possible to accommodate around 90% of the metallic elements from the periodic table in positions A and/or B, without destroying the matrix structure. ‘A' can be a rare earth alkali (La, Ce, Pr) or alkaline earth metals (Cs, K, Ca) of a larger diameter than ‘B' which is normally a transition metal (Co, Fe, Cu, Mn, Cr) [13,14]. This study aims to incorporate selectivity and activity promoters into the perovskite structure, anchor iron particles on the perovskite and investigate the reducibility and activity/selectivity of the obtained model catalyst under Fischer-Tropsch conditions. The model catalysts are developed by preparing a series of LaAlO3 perovskites with partial substitution of lanthanum with 10 atom-% potassium, and 0, 10, 20, 60, and 100 atom-% substitution of aluminum with manganese. The performance of iron supported on these materials is compared with the one on LaAlO3 support with potassium promotion via impregnation (2, 1 and 0.5 wt.-%). EDS-STEM and XANES results confirm the successful incorporation of potassium and manganese into the matrix of the LaAlO3. In situ studies highlight the effect of potassium on the activation of the oxidic iron precursor. The iron supported on the potassium incorporated perovskite exhibits a lower reduction temperature and a faster subsequent formation of Hägg carbide (χ-Fe2C5). It is well reported that iron carbide is the active phase for the FischerTropsch synthesis [15–17]. The Fischer-Tropsch activity of the iron supported on the unmodified LaAlO3 perovskite support compares well with common supports such as SiO2, Al2O3, ZrO2 and TiO2 in terms of CO conversion and selectivity towards FTS products. The incorporation of low concentrations of manganese into the perovskite support results in a slight decrease in CO conversion but no significant change of the product selectivity was observed. With a further increase in manganese concentration, CO conversion decreases accompanied by an increase in C1 and C2–C4 product selectivity. The same phenomenon is observed with these high manganese containing samples when the perovskite support is further modified with potassium. Incorporation of potassium into the LaAlO3 and LaMn0.2Al0.8O3 perovskites results in a substantial increase in CO conversion, beyond 70%. At a CO conversion comparable to the promoter free perovskite catalyst, the novel materials show a comparable CH4 selectivity with lower CO2 formation, resulting in a lower overall undesired C1 product fraction suggesting a decoupling of the potassium enhancement of the FTS activity versus the enhancement of the water gas shift reaction. Additionally, these materials also show good stability under FT reaction conditions as no significant change is observed in the perovskite structure post reaction. It can therefore be confirmed that perovskites can double as both the support and electronic promoter via the incorporation of the known promoter elements within their structure, and their electronic effects can surpass those of conventional promoters.

Keywords

Chemical Engineering

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