Post-synthesis Treatment of TS-1 with TPAOH: Effect of Hydrophobicity on the Liquid-Phase Oxidation of Furfural to Maleic Acid

A series of modified TS-1 catalysts was prepared by post-synthesis hydrothermal treatment of a parent TS-1 zeolite with solutions of different concentrations of tetrapropylammonium hydroxide (TPAOH). The treatment results in an advantageous improvement of the catalytic activity for the liquid oxidation of furfural with H2O2 to produce maleic acid. The highest yield of maleic acid (83%) was obtained when using 0.025 M TPAOH solution; while the maximum yield of the untreated TS-1 was 70%. The catalysts were thoroughly characterised by XRD, UV–Vis, ICP–OES, XPS, TEM, N2-adsorption, DRIFT of chemisorbed deuterated acetonitrile and TEM, in order to elucidate the origin of the catalytic improvement. The characterisation studies allowed us to conclude that, besides the well-known creation of mesopores within the zeolite primary particles, the TPAOH treatment also results in the increase of the hydrophobicity balance of the channels and cavities of the zeolites (via silanols removal). Both properties have a relevant effect on the improvement of the catalytic properties.