In catalytic applications of surface modified mesoporous silica materials, distinguishing and quantifying different types of functional groups on the surface is crucial to enable accurate evaluation of catalytic activity and possible cooperativity amongst mixed functional groups.  One well-studied example is the grafting of thiol moieties and their subsequent oxidation to sulfonic acids by oxidation.  In order to precisely evaluate the efficiency of active sites of these grafted sulfonic acid catalysts, it is critical to quantitatively determine the different types of grafted surface functional groups (-SH, -SO<sub>3</sub>H, -OH, -S–S-) that exist on the catalyst surface after oxidation of grafted thiol groups.  We investigated sulfonic acid groups supported on SBA-15 mesoporous silica as a promising organic-inorganic hybrid solid acid catalyst for biomass-relevant conversion chemistry, and developed a robust technique for sulfur quantification and speciation after oxidation of grafted thiol groups.  We combined a SH-selective titration (Ellman’s test), acid-base titration, and ICP-OES analysis to quantitatively monitor S speciation at each step of the preparation steps of these catalysts.  Influence of grafting and oxidation conditions on S speciation on the catalyst were investigated, and the presence of partially oxidized S on the surface was quantified.  Despite the loss of a fraction of grafted S, the prepared sulfonic acid-functionalized SBA-15 catalysts exhibited comparable catalytic activity per H<sup>+</sup> with homogeneous and commercial heterogeneous sulfonic acid catalysts for esterification.  Re-grafting of SH or grafting of alkyl (or aryl) moieties groups after oxidation to examine the potential cooperative interaction for esterification and the impact of their hydrophobic character on the kinetic influence of water during esterification.  We demonstrate these ‘outer-sphere’ ligand effects can be used to modulate the concentration of water near to the grafted sulfonic acid influencing the impact of water on reaction kinetics and confirm the extent of hydrophobicity with immersion calorimetry.

Solid-supported sulfonic acid catalysts: Influence of functional group conversion on cooperativity and kinetic influence of water during biomass conversion

251st National Meeting (2016)

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