Sol-gel synthesised WO sub(3) nanoparticles supported on mesoporous silica for liquid phase nitration of aromatics

A series of WO sub(3)/SiO sub(2) catalysts have been prepared by sol-gel method using ammonium metatungstate and ethyl silicate-40 (ES-40) as WO sub(3) and SiO sub(2) precursors respectively. The sol-gel method has led to the formation of WO sub(3) nano-particles of 2-5 nm well dispersed on mesoporous silica along with some WO sub(3) agglomerates. Formation of monoclinic WO sub(3) was seen on the catalysts above 5 wt% WO sub(3) loading by XRD analysis. Silica has shown very high surface area of 606 m super(2)/g which decreased gradually upto 368 m super(2)/g with 20 wt% WO sub(3) loading. 20 wt% WO sub(3)/SiO sub(2) catalyst has shown maximum acidity (0.56 mmol NH sub(3)/g) with presence of both Lewis and Bronsted acidity. UV-vis DRS analysis showed formation of polytungstate species along with WO sub(3) on silica surface. The prepared catalysts were used for liquid phase nitration of aromatics using 70% nitric acid as nitrating agent without using any sulfuric acid. Very high conversion (99%) was obtained for p-cresol nitration with very high selectivity (99%) for 2-nitro p-cresol. The water formed during the reaction was removed azeotropically using ethylene dichloride as solvent. In case of o-xylene 74% conversion was obtained with 54% selectivity for 4-nitro o-xylene. The effect of different WO sub(3) precursors on nitration efficiency was studied using sodium tungstate and tungstic acid as precursors. However, ammonium metatungstate showed the highest acidity. Sodium tungstate showed formation of dimer of sodium tungstate which did not show any acidity and hence no activity for nitration. The mechanism for nitration using WO sub(3)/SiO sub(2) has been proposed based on polarisation of water on tungsten centre generating Bronsted acidity which can further generate nitronium ion giving subsequently nitration of the aromatic ring.