Valorization of biomass derivatives: Keggin heteropolyacids supported on titania as catalysts in the suitable synthesis of 2-phenoxyethyl-2-furoate

Abstract

Titania modified by tungstophosphoric (TPA) and tungstosilicic (TSA) acid (30% w/w) were synthesized by the sol-gel method, using urea as low cost pore-forming agent and annealing at 500 °C for 2 h (TiO2/TPA and TiO2/TSA respectively). The obtained materials were characterized by 31P, 29Si, 1H nuclear magnetic resonance (31P, 29Si, 1H MAS-NMR), X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS), Raman spectroscopy (FT-Raman), acid strength by potentiometric titration with n-butylamine. Mesoporous materials were obtained, without important microporosity, as determined from N2 adsorption-desorption isotherms by the Brunauer-Emmett-Teller (BET) method. The XRD patterns of the modified samples exhibited only peaks of anatase phase. According 31P/29Si MAS-NMR and XPS studies, the main species present in the TiO2/TPA and TiO2/TSA samples are the Keggin anions by forming surface acid species, and probably surface complex between Keggin anion and titania as well. Solids were evaluated in the synthesis of 2-phenoxyethyl-2-furoate, by esterification of 2-furoic acid, a valuable product which can be obtained from biomass, with 2-phenoxyethanol, where TiO2/TPA sample showed the highest catalytic activity. The reaction temperature, molar ratio acid:alcohol and catalyst amount were studied as variables using TiO2/TPA sample. Results of catalytic activity and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT-FT-IR) measurements suggested that the reaction mechanism may involve a protonated intermediate of 2-furoic acid polarizing the C[dbnd]O bond of the acid, and leaving that can be easily attacked by 2-phenoxyethanol.