Structure–Activity Relationships in Tungsten Oxide Superacid Catalysts: Insights from Isopropanol Conversion and Reaction Mechanisms

Tungsten oxide-based superacid catalysts (WO₃, SO₄²⁻/WO₃, GO/WO₃, and GO/SO₄²⁻/WO₃) were synthesized via a green, sustainable route using sugarcane bagasse-derived graphene oxide and ammonium persulfate as an eco-friendly sulfate precursor, and applied to the catalytic conversion of isopropanol as a diagnostic probe reaction for surface acid–base characterization. Structural and textural properties were thoroughly investigated by XRD, FE-SEM, FTIR, BET surface analysis, TGA, and non-aqueous potentiometric titration. XRD confirmed the monoclinic WO₃ phase across all samples, while potentiometric titration revealed a dramatic enhancement in surface acid strength upon sulfation, with the initial electrode potential rising from + 18 mV for bare WO₃ to + 530 mV for SO₄²⁻/WO₃ and + 257 mV for the ternary GO/SO₄²⁻/WO₃ composite (GSW), both qualifying as superacid materials. Catalytic performance at 100 °C demonstrated that GSW achieves the highest IPA conversion (97.8%) and propene selectivity (98.4%), substantially outperforming pure WO₃ (42.0% conversion, 85.2% selectivity) and benchmark catalysts from the literature at comparable or higher operating temperatures. The superior activity of GSW is attributed to a synergistic combination of sulfate-derived Brønsted superacidity, GO-mediated dispersion of WO₃ nanoparticles, enhanced pore accessibility, and additional surface acidity contributed by GO oxygenated functional groups. GSW also demonstrated satisfactory recyclability over six consecutive runs with retained structural integrity of the active acid sites. To provide atomic-level mechanistic insight, Density Functional Theory (DFT) calculations were performed on a WO_x cluster model. The dehydration pathway proceeds via an E2-like elimination with a rate-determining C–O bond cleavage barrier of 34.1 kcal/mol, whereas the competing dehydrogenation pathway is kinetically obstructed by a substantially higher H₂ recombination barrier of ~ 53.0 kcal/mol. This decisive ~ 19 kcal/mol kinetic advantage conclusively rationalizes the experimentally observed propene selectivity and establishes kinetic control as the governing principle of the catalytic performance.