Ab Initio Simulation of the Acid Sites at the External Surface of Zeolite Beta
Résumé
The acidity at the external surface of protonic zeolites, due to the finite size of crystallites, is strongly questioned for decades. Using density functional theory calculations, we propose atomistic models for the external surface of zeolite Beta, showing that bridging Si-(OH)-Al groups are still existing at the pore mouth, in what we call open micropores (pores emerging at the external surface). However, at the outermost surface (no emerging micropores), water molecules adsorbed on aluminum atoms (Al-(H2O)) prevail. The local structure of those surface Al atoms depends on the temperature and water partial pressure. A detailed vibrational study of adsorbed CO helps for the assignment of the different sites and reveals a generalized vibrational Stark effect. Proton transfer ability was quantified by adsorption of isobutene. Carbenium ion appear to be stabilized on the bridging Si-(OH)-Al groups located on the open micropores of the external surface in a similar way as in the bulk of zeolite Beta. By contrast, on the outermost surface is not able to stabilize carbenium ion , and promotes the existence of alkoxides. This work brings new atomic scale´s insights into the concept of pore mouth catalysis and provides a molecular architecture of potential active sites located in open micropores.