UC Berkeley

The influence of composition and structure on the reactivity of solid Brønsted acids is not well understood, in part, because of difficulties in separating contributions of acid strength and solvation by van der Waals interactions to the stability of transition states and reactive intermediates. This study examines the diversity of acid strength and solvation across solid acids and their consequences for the rates of methanol dehydration turnovers to form dimethyl ether (DME). The results form a framework based on fundamental principles for understanding the catalytically relevant differences of solid acids and their impact on the rate and selectivity of Brønsted acid catalyzed turnovers. A methodology is presented and assessed for the prediction of reactivity from the estimation of van der Waals interactions of transition states at all acid site locations in zeolites.

The acid strength of a Brønsted acid is defined as the deprotonation energy (DPE) required to heterolytically cleave the O-H bond. DPE values are not accessible to experiment on solids, but can be calculated from DFT. Methanol dehydration rate constants, which reflect the free energy differences between bimolecular DME formation transition states and either methanol monomers (first-order) or dimers (zero-order), decreased exponentially with increasing DFT-derived DPE values (decreasing acid strength) of MFI zeolites with different heteroatoms (Al, Ga, Fe and B). These results demonstrate that weaker acids decrease the reactivity of zeolites because more free energy is required to break the Brønsted O-H bond and form the necessary ion-pair transition state. The free energy differences between transition states and reactive intermediates are proportional to DPE values on zeolites and polyoxometalate clusters, indicating the ubiquitous influence of acid strength regardless of acid composition or structure.

The unique reactivity of zeolites compared with other solid acids is often attributed, non-rigorously and inappropriately, to differences in the acid strength of different structures. Methanol dehydration rate constants, however, are similar on MFI zeolites with different densities of Al heteroatoms and increase monotonically with van der Waals interaction energies of transition state surrogates on zeolites with a wide range of void sizes (FAU, SFH, BEA, MOR, MFI, MTT). Combined with the large influence of DPE on rate constants, these results indicate that the acid strength differences of aluminosilicates are negligible for catalysis and that the remarkable catalytic diversity of aluminosilicates instead reflects differences in the size and shape of voids that confine and stabilize transition states and reactive intermediates through van der Waals interactions.

The misconceptions surrounding the acid strength of zeolites are due, in part, to differences in DFT-derived DPE values in different zeolite frameworks or acid site locations. In particular, DPE values calculated (Periodic-DFT; RPBE/PAW) on zeolites (MFI, BEA, MOR, CHA, FAU, FER) for protons at all crystallographically unique O-sites differ markedly (up to 47 kJ mol^-1) depending on the location of protons and Al-atoms in their structures. DPE values appropriately averaged over all O-atoms at an Al location, however, are similar (within 13 kJ mol^-1) and reflect the negligible differences in acid strength for catalysis. These calculations demonstrate the pitfalls of DFT-derived DPE values and clarify their relationship to catalysis at temperatures above 0 K.

Deprotonation energies, appropriately averaged above 0 K, and van der Waals interaction surrogates, provide a general framework for understanding the reactivity differences of solid acids. van der Waals interaction energies of transition state surrogates at all crystallographically unique proton binding sites calculated from force-fields correlate with methanol dehydration rate constants on zeolites (FAU, SFH, BEA, MOR, MFI, MTT) and demonstrate that these calculations provide a useful method to narrow the selection of zeolites with enhanced reactivity or selectivity. Ray histograms, presented here for the first time, provide a representation of the shape and size of zeolite voids as a distribution of ray lengths and allow for the rapid comparison and selection of zeolites with voids of a certain shape/size. These findings open new opportunities for the rational design and selection of zeolites with enhanced reactivity and selectivity.