Nitration of anisol in dispersions of mesoporous oxides impregnated with heteropolyacids

Journal article, 2012

Amorphous and long range ordered cubic and hexagonal silica impregnated with heteropolyacids was investigated as catalyst for nitration of anisol using HNO3 as nitrating agent. The use of HNO3 together with a solid acid catalyst is an environmentally attractive alternative to the conventional procedure that employs HNO3 together with H2SO4. The aqueous nitric acid was loaded in the pores of the mesoporous material and the HNO3 filled particles were subsequently dispersed in anisol. The reaction took place at the interface between the two liquid phases, i.e., at the pore openings. It was shown that ordered mesoporous silica is more efficient than amorphous silica and that silica with cubic geometry is the most efficient. Two heteropolyacids of Keggin type, phosphotungstic acid and phosphomolybdic acid, were tested. One Wells–Dawson type of phosphotungstic acid was included as well as cesium exchanged heteropolyacid acids of Keggin type. The best yield was obtained with mesoporous cubic silica impregnated with cesium exchanged phosphotungstic acid. The amount of heteropolyacid used in the impregnation turned out to be critical. Above a certain loading the activity went down, probably due to clogging of the pores. The preferred catalyst could also be reused with retained activity.