Tuning silica acidity with ammonium fluoride: Boosting iridium catalyst performance in furfural hydrogenation

This work reports a strategy to enhance iridium-catalyzed furfural hydrogenation by tuning the acidity of amorphous silica supports via ammonium fluoride treatment. The NH4F treatment induced controlled silicon leaching, which increased the pore volume and generated additional Lewis acid sites, thereby promoting superior metal dispersion. Among the catalysts, Ir-SiO-1.0F achieved nearly complete furfural conversion (>95%) with ∼100% selectivity toward furfuryl alcohol, clearly outperforming both unmodified and over-fluorinated systems. Structure–activity correlations revealed that the optimal Ir dispersion (∼40%, particle size ∼3 nm) and a balanced Ir0/Ir4+ ratio are decisive for high selectivity. XPS and NH3-TPD measurements demonstrated that acidity and Ir oxidation states act synergistically to favor carbonyl hydrogenation while suppressing ring saturation. Density functional theory (DFT) calculations confirmed the preferential adsorption of furfural on IrO2. Dual-site top (perpendicular) adsorption of furfural on the IrO2 surface enables its selective hydrogenation to furfuryl alcohol, with hydrogen dissociating on metallic iridium. These findings establish NH4F-treated silica as a versatile support design and provide a clear framework for tailoring catalysts for the selective upgrading of biomass.