Design of bifunctional bimetallic Fischer–Tropsch synthesis (FTS) catalysts: acid functionalization of TiO2 support for enhanced product selectivity

Abstract

This study investigates the functionalization of TiO₂via sulfation to enhance the performance of Fischer–Tropsch synthesis (FTS) catalysts for hydrocarbon production from syngas. Acid-functionalized TiO₂ serves as the support material and is impregnated with iron–cobalt bimetallic nanoparticles as the active phase. This catalyst structure aims to introduce bifunctionality that sustains chain growth and promotes isomerization and hydrocracking by the acid sites during FTS. The introduction of Brønsted and Lewis acid sites is evaluated, and the effects of calcination post-functionalization are quantified in the resulting C₅₊ selectivity and CO conversion. XRD analysis reveals that the functionalization process favors the stabilization of the anatase phase of TiO₂ as a function of SO₄²⁻ ions, resulting in large (>100 nm) particles due to agglomeration. The support's surface chemistry is significantly altered, with the functionalization leading to an oxygen-deficient surface and subsequent calcination removing sulfur-containing compounds, as shown by XPS. The acid site density for the S-doped support is determined to be 4.57 × 10⁻³ mmol_{acid sites} m⁻² post-calcination at 300 °C. Pyridine-DRIFTS indicates a reduction in Brønsted acid species (e.g., surface S–OH) after calcination, with the Brønsted to Lewis acidity (B/L) decreasing from 1.68 pre-calcination to 1.27 post-calcination. FTS reaction testing was performed at 250 °C and 300 psi(g). FTS results show that acid functionalization of TiO₂ followed by a calcination step induces desirable structural and surface chemistry changes on the support which favor the formation of C₂–C₄ hydrocarbons, while maximizing catalyst performance, reaching a CO conversion of 29.0% ± 4.95 and CH₄ selectivity of 3.03% ± 0.33.