Influence of Adsorption of Electrically Neutral Reactants on the Formation of Electrical Double Layers in Solid Acid Catalysts and Its Relation to Catalytic Activity

Electrokinetic potential (zeta, ζ) of two strong insoluble Brønsted acids (Cs_2.5H_0.5PW₁₂O₄₀ and sulfonated carbon) was measured in water in the presence of small electrically neutral molecules (ethyl acetate, cellobiose). The strongly negative ζ potential, which originates from the dissociation of surface acid sites, is altered when adsorption of neutral species occurs. Superacidic Cs_2.5H_0.5PW₁₂O₄₀ showed strengthening of ζ when adsorbing hydrophobic substance (ester); together with the adsorption isotherm showing capillary condensation, this suggests displacement of hydrated protons from pores toward the outer particle surface, leading to a denser proton diffuse layer. A connection of this fact is established with the high catalytic activity of the solid polyoxometalate in comparison with homogeneous catalysis (based on literature data). Nonsuperacidic sulfonated carbon showed weakening of ζ upon adsorption of both hydrophobic and hydrophilic compounds, which suggests suppression of dissociation of the sulfonic groups. Poisoning of sulfonated carbons by dissociation-suppressing adsorbed intermediates is demonstrated in the hydrolysis of cellulose (based on literature data). The obtained results suggest a strong involvement of the diffuse layer of solid acids in acid-catalyzed reactions, as well as showing the importance of considering the influence of reactant adsorption on the double-layer configuration of solid acid catalysts.