Effect of zeolite acidity and mesoporosity on the activity of Fischer–Tropsch Fe/ZSM-5 bifunctional catalysts

Abstract

The effect of acidic properties and mesoporosity created by desilication on ZSM-5 zeolites was investigated to evaluate changes in product selectivity during Fischer–Tropsch synthesis (FTS) over Fe-based bifunctional catalysts. The reaction was studied at 300°C, 10bar, and H2:CO=2:1, using a physical mixture of the iron-based FTS catalyst and zeolites with different Si/Al ratio. Hierarchical zeolites with micro- and mesoporosity were prepared by alkaline treatment. Higher acidity (lower Si/Al ratio) in the pristine zeolite was found to be responsible for cracking of heavy hydrocarbons and the formation of heavier aromatic products, while higher Si/Al ratio induced a higher selectivity towards gasoline-range products (isoparaffins and olefins). The presence of mesopores prompted a variable behavior depending on the final acidity of the resulting Fe-based hierarchical catalyst; zeolites that retained acidity led to a slight increase in C2-C4 paraffins due to overcracking of heavy molecules as a result of enhanced transport to the acidic sites through the mesopores. The products obtained with the Na+-containing alkali-treated zeolites with no acidity were similar to those obtained with the iron FTS catalyst alone, with most of the products in the paraffin range. Rather than improving the activity and selectivity to gasoline products, the better accessibility in the hierarchically-based Fe/ZSM-5 catalysts coupled to the preserved acidity of the zeolite induced the formation of aromatics and a higher amount of small paraffins (<C4+). This reinforces the idea of optimizing the catalyst towards a desired FTS hydrocarbon range through the control of its mesoporosity and acidity. Iron-based ZSM-5 catalysts with high Si/Al ratios and irrespective of the presence of mesoporosity shift the product distribution towards the formation of isoparaffins and lighter aromatics.