(52b) Functional Assessments of Solid Acid Strength: 2-Butanol C-O Bond Activation on Brønsted and Lewis Acid Sites

Solid acid catalysts are critical for green chemical transformations such as alcohol upgrading to sustainable aviation fuel and biomass depolymerization. These catalyst surfaces contain Brønsted and/or Lewis acid sites, and the strength of each site dictates the reaction rates, e.g., for alcohol dehydration. Previous studies have established a framework to predict solid Brønsted acid strength,^[1] but for Lewis acids, their strength is only understood in the homogeneous phase; this latter framework is incomplete for a structurally-complex heterogeneous surface with unknown active site coordination environments. Here, we establish both the Born-Haber thermochemical framework and experimental technique for assessing the Lewis acid strength of a solid acid catalyst,^[2] where we construct a scaling relation between 2-butanol intramolecular dehydration activation enthalpies and pyridine adsorption enthalpies—derived from kinetic results—as a function of the electronic chemical potential of metal-oxygen site pairs. Spanning a wide range from acidic (Al₂O₃) to basic (ZnO) metal oxides, 2-butanol C-O bond activation enthalpies increase by over 180 kJ mol^-1, coupled with pyridine adsorption enthalpies that increase by over 100 kJ mol^-1. From this scaling relation, we can estimate the 2-butanol dehydration rates for a wide range of temperatures of any solid acid catalyst simply from pyridine temperature-programmed desorption experiments. Using this scaling relation, we can also distinguish between Brønsted and Lewis acid site identities, because C-O scission transition state structures differ between the two acid sites: for 2-butanol dehydration, C-O scission reactions proceed through loosely-bound [H₂O···C₄H₉⁺···A^-]^‡ states on Brønsted acids and tightly-bound [CH₃CH(H···O-M)(CH₃)CH⁺···^-O(H)]^‡ states on Lewis acids. This difference manifests as distinct enthalpy-entropy compensation lines, with a lower isokinetic temperature for Brønsted acids versus Lewis acids.

[1] Macht, J.; Carr, R. T.; Iglesia, E. Journal of Catalysis 2009, 264, 54­–66.

[2] Broomhead, W. T.; Chin, Y.-H. ACS Catalysis 2024, 14, 2235–2245.