Speaker
Description
The Brønsted acidity of zeolites has been addressed numerous times as zeolite science progressed in last 50 years, however, the question of acidity of particular zeolitic framework was not yet resolved either theoretically or experimentally. The theoretical issues stems from the fact that electrostatic part of the total energy does not converge for charged cells without introducing compensating background charge. As a result the deprotonation energies obtained from periodic models suffer from large errors depending on cells size and framework density. It is possible to avoid periodic formalism by using cluster models of increasing size, however, the convergence is rather slow and is not easily extrapolated to "infinite" limit. In our contribution we investigated five different zeolitic frameworks and calculated deprotonation energies in periodic and cluster formalism. We used simple electrostatic embedding model to extrapolate deprotonation energies of finite cluster models to crystal size limit. The results indicate that variability in calculated deprotonation energies is rather low. This begs the question whether the often calculated deprotonation energies are a "true" descriptor of the zeolite acidity or whether secondary effects are the ones that lead the material behavior (ie. material topology to accommodate reactants, transition states and products in Brønsted acid catalyzed reactions).
