Toward More Complete Descriptors of Reactivity in Catalysis by Solid Acids

Abstract

Density functional theory and classical electrostatics are used to develop reactivity descriptors for catalysis by solid acids. Acid strength, as deprotonation energies (DPE), reflects the charge reorganization required to disrupt covalent OH bonds in inorganic acids and the electrostatic forces that resist the separation of protons from conjugate anions. Both charge reorganization (covalent) and electrostatic (ionic) components vary monotonically with DPE on solid acids with different heteroatoms within a given type of oxide framework, but their relative contributions differ among different acid types. Ion-pair transition states recover predominantly the ionic part of the DPE, and the extent to which they recover each component is a unique property of a transition state and thus of an acid-catalyzed reaction, independent of the acid strength or type. These fractional recoveries, together with the ionic and covalent DPE components, a unique property of a solid acid, provide a general and complete descriptor of reactivity, which we illustrate here for diverse reactions (proton shuttling, H2O elimination, methyl shift, ring contraction) on several types of solid acids (Mo- and W-based polyoxometalate clusters with S, P, Si, Al, and Co central atoms and MFI type heterosilicates with Al, Ga, Fe, and B heteroatoms). For protons confined within small voids of heterosilicates, the transition state stabilization and reactivity depend additionally on van der Waals interactions that are unrelated to acid strength.