Cation Effects on the Brønsted Acidity of Solid Tungstosilicic Acid Clusters

Abstract

Rigorous kinetic assessments, pyridine chemical titration and desorption, together with density functional theory calculations establish the trends in the modulation of chemical identity, valence, site density, and strength of Brønsted acid sites by counter cations (Na⁺, K⁺, Cu²⁺, Mg²⁺, and Al³⁺) on Keggin-type polyoxometalate tungstosilicic acid clusters (H₄SiW₁₂O₄₀, POM). Monovalent cations (Na⁺ and K⁺) exchange protons and decrease the acid strength of the residual protons, as indicated by the deprotonation energy (DPE) that increases from 1100 to 1175 kJ mol^–1 with an increasing extent of proton exchange (decreasing the nominal H⁺-to-POM ratio). In contrast, di- and trivalent cations preferentially exchange protons in the form of hydroxides (Y^z+(OH)_m, Y^z+ = Cu²⁺, Mg²⁺, or Al³⁺, 0 < m < z), resulting in an average DPE value on both POM clusters and associated hydroxides ranging between 1100 and 1150 kJ mol^–1. A portion of these cations disperse on the silica support, generating Lewis acid sites. The exchanged cations modulate the charge within the W₁₂O₃₆ oxide shell, rather than the central SiO₄^4– tetrahedron, which mainly modifies the ionic component of DPE values. Monovalent cations with smaller electronegativities than di- and trivalent cations donate more electrons, which increases the electrostatic interaction of residual protons with conjugate POM^– anions and leads to higher DPE values (weaker acids). This study expands the library of Brønsted acidic catalysts with flexibility in tuning their acid strengths and densities, thus providing a series of samples for constructing structure–reactivity relationships and probing site electrostatic correlations on structurally constrained domains.