The catalytic performance of sulphonated cross-linked polystyrene beads in the formation of isobornyl acetate

Abstract

The use of three sulphonated cross-linked polystyrene resins (Dowex 50WX8, Amberlyst 15 and Purolite MN500) is reported as heterogeneous Bronsted acid catalysts in the solvent-free liquid-phase hydro-acyloxy-addition reaction of acetic acid ( 1a) to camphene ( 2), giving the pine-fragrance isobornyl acetate ( 3a). As a homogeneous reference catalyst p-toluene sulphonic acid monohydrate was employed. Prior to use, the hygroscopic resins were dried in vacuo, followed by swelling with glacial acetic acid vapour. Since Karl Fischer titrations showed that the drying procedure did not remove all adsorbed water, acetic anhydride was used to eliminate traces of residual water. Ion-exchange reactions provided the number of sulphonic acid groups per gram of resin, while the accessibility of the acid groups was assessed from acid–base titrations and SEM/EDAX. The number of resin beads to be suspended to achieve a desired number of acid sites determined the rate of the reaction. It was demonstrated that transport within the pores of the beads did not affect the rate of the reaction. The conformation of the polystyrene chains determines the accessibility of the acid groups as well as the presence of liquids as dimethylsulfoxide that are able to establish hydrogen bonds with the acid sites. With both the resin catalysts and the homogeneous reference catalyst, the rate of the reaction increased upon addition of small aliquots of water to the reaction mixture. A rise of the proton mobility near the sulphonic acid moieties rationalises the increase of the rate of the reaction by addition of water. The specific absorption of water in the resins also decreased the agglomeration of the sulphonic acid groups of the resins and the shielding of the acid groups by the polystyrene chains, which proceeds with the more flexible (less cross-linked) Dowex 50WX8. The other two resins gave similar reaction rate constants as p-toluene sulphonic acid monohydrate. Since the conversion of camphene ( 2) completely stopped after separation of the resin beads from the reaction mixture, the three different types of resin beads can be effectively applied as heterogeneous solid acid catalysts. By application of a fixed-bed reactor and re-circulating the reactants, the equilibrium is quantitatively shifted to the product isobornyl acetate ( 3a).