A surface study of zirconia-based solid acids by Laser Raman spectroscopy of adsorbed pyridine

Abstract

Sulfated and phosphated zirconias were synthesized by calcination at 600°C of Degussa ZrO 2 impregnated with aqueous solutions of (NH 4) 2SO 4 and (NH 4) 2HPO 4 up to 5 wt.% sulfate and phosphate species in the eventual solids. Solids thus produced were characterized by X-ray diffractometry, Fourier transform infrared and laser Raman spectroscopies, surface area measurements, and laser Raman spectra of adsorbed pyridine. The results showed that phosphation had hardly any detectable effect on the crystalline bulk structure (predominantly monoclinic), surface area (43 m 2 g −1) or acid sites (hydrogen-bond-donor OH groups) of the zirconia. The same is shown to apply to sulfation, although only with regard to the bulk structure and surface area. Regarding the surface acid properties, sulfation is shown to produce strong acidity by generating Brønsted (proton-donor OH moieties) and Lewis (coordinatively-unsaturated Zr 4+) acid sites. The present results are in line with results from a number of previous studies. Those studies attributed the emerging Lewis acidity to an electron depleting effect of sulfate ligands on metal sites, and the Brønsted acidity to water molecules adsorbed nondissociatively on Lewis acid sites and/or dissociatively to form hydroxysulfates capable of donating protons.