Effect of precursor on the catalytic performance of supported iron catalysts for the Fischer–Tropsch synthesis of lower olefins

• A more uniform distribution of iron particles was achieved using AIC as precursor.
• High coke levels were observed for catalysts with highly aggregated iron particles.
• Large iron particles fragmented due to carbon deposition during the FTO.

Lower olefins are traditionally produced from cracking of naphtha and other crude oil fractions. The Fischer–Tropsch-to-Olefins process (FTO) enables the direct synthesis of lower olefins from synthesis gas (CO + H2) derived from alternative feedstocks such as natural gas, coal or biomass. A catalyst suitable for this process must comply with different requirements: high selectivity for C2C4 olefins, low methane selectivity, high catalytic activity and excellent mechanical and chemical stability under demanding reaction conditions (high temperatures and low H2/CO ratios). These features have been reported for a catalyst consisting of iron-containing nanoparticles promoted with sodium and sulfur dispersed on a weakly interactive support. In this study, Na plus S promoted α-alumina supported catalysts with loadings of 1–20 wt% Fe have been prepared using different iron precursor salts to investigate their effects on catalytic performance. The catalysts prepared from iron nitrate or ammonium iron citrate both consisted of iron nanoparticles of 15-20 nm and displayed high selectivity to lower olefins (>50% C) in combination with low methane selectivity (<20% C) when tested under industrially relevant conditions (340 °C, 20 bar and H2/CO = 1 v/v). The catalyst synthesized with ammonium iron citrate exhibited a higher catalytic activity and a much lower rate of carbon lay-down (factor 4–6) compared to the sample prepared with iron nitrate. Tentatively, the differences in catalytic performance are attributed to a more uniform distribution of the iron particles observed when ammonium iron citrate was used as precursor. These results suggest that the extent of aggregation of iron (carbide) nanoparticles affects their catalytic performance.

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