Sulfonated poly(styrene-co-divinylbenzene) ion-exchange resins: acidities and catalytic activities in aqueous reactions

A series of sulfonated poly(styrene-co-divinylbenzene) ion-exchange resins with varying levels of sulfonation has been prepared. The acidities of these resins in the hydrated form have been characterised microcalorimetrically by titration with aqueous NaOH solution. The resultant molar enthalpies of neutralisation have been compared with acid strengths of the same resins measured in the absence of water by ammonia adsorption microcalorimetry, and catalytic activities in two test reactions, the dehydration of 1-hexanol and the hydration of propene. The molar enthalpies of neutralisation of hydrated resins with aqueous NaOH have been shown to increase with increasing sulfonation level. This has been explained in terms of an increasing concentration of sulfonic acid in the internal gel solution, generating stronger acid sites and higher specific catalytic activities. A similar trend of increasing acid strengths has been observed in anhydrous resins, but this has been explained in terms of structural features such as di-substitution of sulfonic acid groups on styrene units, giving rise to intrinsically stronger acid sites.