Surface features and catalytic activity of sulfated zirconia catalysts from hydrothermal precursors

ZrO2-SO4 (SZ) powders were prepared by calcining at 740, 820, and 900 K hydrous precursors, obtained by a hydrothermal precipitation route from ZrCl4 solutions at 334 K, and subsequently sulfated with H2SO4 (1 M). The samples were characterised as to phase composition (XRD, Raman spectroscopy), surface area (BET) and total sulfur content. Surface functionalities and acidity features were investigated by FTIR spectroscopy of the powders themselves and of probe molecules (carbon monoxide, pyridine) adsorbed thereon. By a revised Hammett-Bertolacini technique, both the density and strength of the (protonic) surface sites were obtained. XPS analyses of the samples were performed by elaboration of the sulfur 2p, oxygen 1s and zirconium 3d spectral regions. Catalytic tests were performed both in the gas phase, for the isomerisation of n-butane at 423 and 523 K, and in liquid media, for the esterification of benzoic acid to methylbenzoate. Correlations were obtained with the samples' calcination temperature, catalytic activity, distribution of Lewis/Brønsted acid sites, and spectral features of the XPS Zr 3d doublet.