A SURFACE STUDY OF ZIRCONIA-BASED SOLID ACIDS BY LASER RAMAN-SPECTROSCOPY OF ADSORBED PYRIDINE

Sulfated and phosphated zirconias were synthesized by calcination at 6 00 degrees C of Degussa ZrO2 impregnated with aqueous solutions of (NH 4)(2)SO4 and (NH4)(2)HPO4 up to 5 wt.% sulfate and phosphate species i n the eventual solids. Solids thus produced were characterized by X-ra y diffractometry, Fourier transform infrared and laser Raman spectrosc opies, surface area measurements, and laser Raman spectra of adsorbed pyridine. The results showed that phosphation had hardly any detectabl e effect on the crystalline bulk structure (predominantly monoclinic), surface area (43 m(2) g(-1)) or acid sites (hydrogen-bond-donor OH gr oups) of the zirconia. The same is shown to apply to sulfation, althou gh only with regard to the bulk structure and surface area. Regarding the surface acid properties, sulfation is shown to produce strong acid ity by generating Bronsted (proton-donor OH moieties) and Lewis (coord inatively-unsaturated Zr4+) acid sites. The present results are in lin e with results from a number of previous studies. Those studies attrib uted the emerging Lewis acidity to an electron depleting effect of sul fate ligands on metal sites, and the Bronsted acidity to water molecul es adsorbed nondissociatively on Lewis acid sites and/or dissociativel y to form hydroxysulfates capable of donating protons.