(101e) Does Hydrophobic Modification of Solid Acid Catalysts Promote Water Tolerance during Condensed Phase Catalytic Reactions of Oxygenates?

Water is ubiquitous in biomass valorization reactions, oftentimes to the detriment of activity and selectivity of catalysts. In particular, the effectiveness of acid catalysts is strongly reduced, even in the presence of small amounts of water. Hydrophobicity has shown to improve water tolerance in acid catalysts. Hydrophobic surface area surrounding the active site is presumed to impede the ability of water to solvate the active site, and therefore prevent deactivation of the catalyst.

We have synthesized a series of hydrophobically modified SBA-15 supported propylsulfonic acid catalysts to systematically study the impact hydrophobic surface area has on acid catalyst activity in the presence of water. We introduced hydrophobicity in the form of straight-chain alkyl modifiers. The length of alkyl chains was varied from methyl to hexyl. We utilized the catalysts for the esterification of acetic acid in excess methanol and butanol. The influence of water was studied by varying the concentration of water between 0 M and 4 M. We analyzed the effect catalyst hydrophobicity had on effective activation barriers using the Eyring equation and considered the influence of non-ideal thermodynamics on reactants and transition states (solvent effects) utilizing COSMO-RS calculated activity coefficients for acetic acid, methanol, and butanol. Solution calorimetry was used to quantify the hdyrophobocity of the modified sulfonic acid catalysts. A structure-function relationship between hydrophobicity and catalyst hydrophobicity was assembled calorimetry and reaction studies.

In the absence of water, increased hydrophobicity reduced the activity of catalysts in both methanol and butanol. We suspect the hydrophobic surface groups affect activity through a combination of reduced interactions between SO₃H/SO₃^- and surface OH groups, stabilization of reactants and destabilization the transition state. In the presence of water, all catalysts were reduced to the same activity in their respective solvents. Equal activities indicated the hydrophobic surface area was ineffective in preventing active site solvation by water above 3 M water. We suspect the presence of water causes the alkyl modifiers to lie parallel to the support surface, reducing their interaction with the bulk solution and leading to the exposure of the active site.

Catalyst hydrophobicity was probed/investigated/determined/other using solution calorimetry. The heat of wetting of various probe molecules was used to indicate how hydrophobic a catalyst surface was. Water failed to wet the surface of the most hydrophobic catalysts, and thus was an inappropriate probe molecule to indicate the degree of catalyst hydrophobicity. Alcohols, methanol and n-butanol, were successfully used to demonstrate varying degrees of catalyst hydrophobicity.