Acid strengths and catalytic activities of sulfonic acid on polymeric and silica supports

The acidic and catalytic properties of sulfonic acids supported on polystyrene, on silica (via propyl and phenyl tethers) and on a fluorinated hydrocarbon polymer (Nafion) are compared. Surface acidities are characterised using ammonia adsorption calorimetry under flow conditions in which pulses of ammonia are introduced to the sample from a flowing carrier stream. The extent of adsorption and molar enthalpies of ammonia adsorption (ΔHads°) are interpreted in terms of the abundance, accessibility and strength of surface acid sites. Catalytic activities are measured for the isomerisation of α-pinene. The Nafion catalysts show the highest ΔHads°(NH3) and the highest catalytic activities. Although both silica-supported and polystyrene-supported sulfonic acids show lower specific activities and lower ΔHads°(NH3) values, the correlation between activity and ΔHads°(NH3) is relatively poor for these supported forms of the acid. It appears that while ΔHads°(NH3) is certainly sensitive to the strength of acid groups on which ammonia is adsorbed, it can only be used to compare acid strengths in a meaningful way for structurally similar catalysts