Selective nitration of phenol over nanosized tungsten oxide supported on sulfated SnO2 as a solid acid catalyst

Abstract

WO3 loaded on nano-crystalline sulfated tin oxide solid acid catalyst has been synthesized and studied for selective nitration of phenol. The catalysts were characterized by various analytical and spectroscopic techniques including thermal analysis, powder X-ray diffraction (XRD), N2 adsorption and FTIR spectra. The surface acidity was measured by no aqueous potentiometric titration and FTIR spectra of chemically adsorbed pyridine. The acidity measurements showed that the samples contain both Lewis and Brønsted acid sites; moreover, the total acidity increases with the rise of WO3 content up to 30 wt.% (monolayer coverage) and decreases thereafter. The results also showed the possibilities of interaction of WO3 with sulfate and/or SnO2 surface that led to some changes in the surface structure of the catalyst. The catalytic activity well correlates with Brønsted acid sites of these catalysts. The effect of the reaction parameters, i.e. reactants molar ratio, reaction temperature and time was also investigated. The catalyst was reused for five times with excellent percentage conversion and high selectivity towards ortho isomer. The possible reaction mechanism for the formation of ortho and para isomers was also discussed. © 2008 Elsevier B.V. All rights reserved.