Ab Initio Simulation of the Acid Sites at the External Surface of Zeolite Beta

Abstract

AbstractThe acidity at the external surface of protonic zeolites, because of the finite size of crystallites, has been questioned strongly for decades. We used density functional theory (DFT) calculations to propose atomistic models for the external surface of zeolite Beta, which show that bridging Si−(OH)−Al groups still exist at the pore mouth in what we call open micropores (pores that emerge at the external surface). However, at the outermost surface (no emerging micropores), water molecules adsorbed on Al 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 in the assignment of the different sites and reveals a generalized vibrational Stark effect. Proton‐transfer ability was quantified by the adsorption of isobutene. Carbenium ions 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, the outermost surface is not able to stabilize carbenium ions and promotes the existence of alkoxides. This work brings new atomic‐scale insights into the concept of pore‐mouth catalysis and provides the molecular architecture of potential active sites located in open micropores.