Characterisation of nanocrystalline sulfated titania modified with transition metals and aluminum as solid acids for esterification

Abstract

TiO₂, transition metal (Cr, Mn, Fe, Co, Ni, Cu, and Zn)– and aluminum-doped TiO₂ nanoparticles were prepared by the sol–gel technique. The sulfated catalysts were prepared by the impregnation method with H₂SO₄ solution. The catalysts were characterised by X-ray diffraction (XRD), BET surface area measurement, diffuse reflectance spectroscopy (DRS), and Fourier transform infrared (FTIR) techniques. The sulfate content of the metal–incorporated samples was considerably higher than for SO₄^2-/TiO₂. The metal doping brought about a considerable reduction in the extent of sulfate loss from the catalyst surface due to the wide dispersion on the surface. The SO₄^2-/TiO₂ and SO₄^2-/M/TiO₂ nanoparticles have smaller crystalline size and higher surface area than pure TiO₂. Sulfation increases the acidity of pure titania. Addition of metal cations also changes the acidity of sulfated titania, which may be related to the interaction of the metal ions with titania and their incorporation into the lattice. The sulfation and addition of transition metals to nanocrystalline TiO₂ enhances the extent of esterification of phthalic anhydride with octan-2-ol. The SO₄^2-/Al/TiO₂ catalyst has the highest activity towards esterification.