Exploring untapped potential of the molecular precursor approach to control fluorine-doping in mesostructured titania for acid catalysis

Abstract

Employing molecular precursors with well-defined F/Ti ratios, this work presents a facile way to introduce fluorine inside the titania matrix in a controlled manner. A tunable library of new fluorinated Ti(IV) molecular precursors with F/Ti ratios varying in the range of 4.8–24 were synthesized, thoroughly characterized (single-crystal X-ray structures, IR, ¹H, ¹³C and ¹⁹F NMR, TGA), and studied for their hydrolysis and condensation characteristics. Hydrolysis of these precursors led to mesostructured fluorinated titania nanoparticles, in which the doping amount was broadly governed by the F-content of the precursors (XRF and XPS studies). The introduction of fluorine species in the TiO₂ matrix significantly enhances the acid properties, particularly the Brønsted acidity of the resulting materials, as indicated by NH₃ adsorption calorimetry and pyridine adsorption FTIR studies. This is supported by activity and selectivity changes in two model reactions upon fluorination of TiO₂: i) isopropanol conversion in the gas phase leading to formation of propene at the expense of acetone and ii) dihydroxy acetone conversion in the aqueous phase with enhanced formation of pyruvaldehyde and parallel suppression of Lewis acid-catalyzed lactic acid as well as base-catalyzed sugar formation.