Probing the Acid Strength of Brønsted Acidic Zeolites with Acetonitrile:  An Atomistic and Quantum Chemical Study

Computational chemistry methods have been used to study factors contributing to the Brønsted acidity of seven high-silica zeolites. For each structure type, one representative Brønsted site was chosen based on a systematic minimization protocol. Calculations of the ν(OH) stretching frequencies and the gradient norm of the electric potential at the proton site were performed. From these values, the sequence in order of decreasing intrinsic acidity, i.e., proton affinity, in the absence of any adsorbed base molecule, was established as follows: MFI > MOR > MTW > CHA > FER > TON > FAU. In the second phase of the calculations, an acetonitrile molecule was introduced into each zeolite using the Monte Carlo docking method, followed by DFT cluster optimizations. Acidity was related to the strength of the O−H···N hydrogen bond, which was characterized by a number of geometric parameters, and by adsorption energy. From the DFT calculations, the ranking of zeolites in order of decreasing hydrogen bond strength was MFI > FAU > MOR > MTW > CHA > FER > TON. The enhanced H-bond strength in FAU, relative to the other zeolites, is interpreted in terms of the large free volume of its pore system, which allows the acetonitrile to interact with the acidic site with minimal steric hindrance. Conversely, MFI maintains its position as the most acidic zeolite, despite the fact that acetonitrile adsorbs less favorably than in large-pore zeolites. In discussing the acidity actually encountered by a molecule within a given zeolite, it is therefore necessary to consider not only the intrinsic acidity of the Brønsted proton but also constraints due to the particular pore topology, which strongly influence the geometry by which a basic molecule is able to interact with the acid site.