Use of Aqueous Slurries of Oxides for Ecocompatible Organic Synthesis
There is a continuous interest and need of developing environmentally benign synthetic routes. Increased public awareness and more rigid environmental legislation have elevated the drive towards sustainable development and ecocompatible syntheses. This thesis demonstrates the use of concentrated aqueous dispersions of mesoporous materials as a mean to perform effective ecocompatible organic syntheses between incompatible reactants. Mesoporous materials are highly attractive for this purpose since they can be precisely designed, functionalised, reused and have an overall low toxicity. An additional attractive feature of solid catalysts, such as mesoporous materials is the easy workup associated with their use. The use of mesoporous materials allows synthesis in aqueous reaction systems. Aqueous reaction systems have several benefits, such as low toxicity, low cost and high safety compared to flammable, carcinogenic or even explosive solvents.
Three different types of organic reactions have been studied: (1) oxidation; the conversion of cyclohexene into adipic acid (2) carbon-carbon coupling; Friedel-Crafts alkylation of sodium salicylate with 4-tert-butylbenzyl chloride and (3) nitration of anisol with aqueous nitric acid. For the oxidation reaction tungsten oxide, either used as the sole oxide material or as a mixed oxide with silica turned out to be very efficient and gave an almost quantitative yield of adipic acid. The proposed and designed reaction system minimize the emissions of the greenhouse gas NOx compared to the traditional industrial process. For the Friedel-Crafts carbon-carbon coupling reaction alumina, either used as the sole oxide material or as a mixed oxide had a high catalytic ability and gave an almost complete conversion of the substrate, sodium salicylate. The reaction system eliminates the use of stoichiometric amounts of acid. For the nitration reaction it was shown that silica or silica impregnated with heteropolyacid, in particular cesium exchanged phosphotungstic acid, enables nitration under mild and controlled conditions. The suggested reaction route eliminates the use of mineral acids, thus avoiding the problems associated with the liquid acid waste.