Synthesised Sulfonic Solid Acid Catalysts for Liquid Phase Reactions

The move to use heterogeneous solid acid catalysts as new, cleaner, renewable catalysts for liquid phase reactions is at the forefront of scientific research, with the focus on the ability to tailor and improve the physical and chemical properties of the catalysts in order to tailor their acidity to catalytic activity for specific liquid phase reactions. The aim of this project was to determine the role of the support in imparting acidity and catalytic activity in liquid phase reactions to three different types of supported solid acid catalyst; polystyrene, silica and a fluorinated hydrocarbon polymer. For the sulfonic acids supported on silica, two main synthesis routes (grafted and co-condensed) with the use of two alternative tethers (propyl, phenyl and also an additional non acidic tether) were compared. Of the synthesised materials, structural properties were characterised and compared using several techniques. Nitrogen adsorption was used in order to ascertain the pore size and distribution, X-ray diffraction to determine the long range order, elemental analysis to determine the relative sulfur content and X-ray phototelectron spectroscopy to analyse the environment of the sulfur in order to establish successful acidification. Surface acidities of all three supported sulfonic acids were better characterised and compared using ammonia adsorption calorimetry. The extent of the adsorption and molar enthalpies of ammonia adsorption (ΔHadsn) were interpreted in terms of abundance, accessibility and strength of surface acid sites. Catalytic activities were measured and compared using the isomerization of α-pinene liquid phase reaction. In brief, the main findings of this project indicate that the commercially available fluorinated polymer Nafion® resin demonstrated the highest acid strength and much higher specific catalytic activities compared to the other supported sulfonic acids. The acid strengths on polymeric and silica supports were similar, with poor catalytic activities. The routes used in order to synthesise, characterise and acidify the supported sulfonic acids were successful

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 catalyst