A DFT Study on Single Brønsted Acid Sites in Zeolite Beta and Their Interaction with Probe Molecules

Zeolites are subjects of intensive research, as they have vast industrial applications. However, exact nature of catalytic active sites remains elusive. With this research, we attempt to shed light on the Brønsted acid sites in zeolite beta modification A and correlate the widely used TPD methods of their identification with the theoretically determined strongest and weakest acid sites. A diverse set of probe molecules with a wide range of acid strength of their conjugated acids is applied to determine the heats of their adsorption over protonated zeolite beta samples. The molecules studied were acetonitrile, dimethyl sulfide, dimethyl sulfoxide, isothiazole, pyridine, tetrahydrofuran, and ammonia. It was found that the heat of adsorption of the probe molecules correlated with the acid strength of acid sites only partially. Only some of the strong acid sites were able to be identified, while the weakest acid site was often identified correctly. To conclude, the widely used method of zeolites acid sites studies delivers only partially correct results, and the conclusions from such TPD studies should be treated with caution.