Brønsted acid catalysts are deactivated by the presence of water in liquid phase reactions.  With the pervasiveness of water in biomass valorization reactions, the development of water tolerant acid catalysts is an important step in the path to economically viable production of fuels from biomass. Modification of a catalyst surface with hydrophobic groups is thought to improve tolerance to water by inhibiting the access of water to the active site. However, these hydrophobic groups can have undesirable side effects. Hydrophobic groups modify the solvation environment of the active site; if kinetically relevant reactants, intermediates and transition states are solvated to different degrees by the new environment, the rate of reaction will be altered.  We have successfully synthesized increasingly hydrophobic alkyl-modified SBA-15-supported propylsulfonic acid catalysts and confirmed the increased hydrophobicity of the modified catalysts relative to the parent sulfonic acid grafted silica by solution calorimetry.<br/> <br/>We have observed that alkyl groups (Me, Pr, Hex) have a detrimental effect on catalyst activity towards esterification of acetic acid, relative to the unmodified catalyst.  Furthermore, we have shown that modification with alkyl groups does not improve water tolerance.  Despite the presence of hydrophobic alkyl groups, water is able to access the active site and subsequently deactivate the catalyst. The hydrophobicity of the alcohol (solvent) does not appear to influence this behavior.  Activation enthalpies demonstrate there is a net destabilization of the catalytic proton relative to the rate determining transition state when solvated by alkyl groups.  In the presence of 3 M water, the influence of the alkyl groups disappears, with all catalysts exhibiting the same activity and activation enthalpy.  We have demonstrated the negative influence of surface hydrophobicity in heterogeneous Brønsted acid catalysts. Differential solvation of kinetically relevant species leads to a reduction in catalyst activity. Furthermore, the surface hydrophobicity does not serve its intended purpose of improving water tolerance.

Hydrophobic modification of grafted sulfonic acid/silica catalysts does improve their tolerance to water during esterification

256th National Meeting (2018)

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