PNNL

Abstract

The ability of Brønsted acidic sites to transfer protons to sorbed molecules in the confines of zeolite pores, i.e., their acid strength, is one of the most critical parameters required for predicting chemical reactivity in sorption and catalysis. Water has been proposed to profoundly change this process by forming clusters at Brønsted acidic sites that may form hydrated hydronium ions. In this work, the acid strength of Brønsted protons in water filled zeolites is examined by ab initio molecular dynamics combined with enhanced sampling based on well-tempered metadynamics and a recently developed set of collective variables, which revolutionizes the study of acid-base reactions in condensed media. Specifically, we investigate how the relative excess energy of Brønsted acid sites (BAS) changes as a function of the number of water molecules n (n=1-8) and zeolite pore size. Here, we show that at low water content (1-2 water/BAS) the acidic protons prefer to be shared between the oxygens of the zeolites and the water, whereas raising the water content (n>2) invariably leads to solvation of the protons within a confined water cluster. By decomposing the free energy, we observe that the evolutions of the relative acid strength of these confined protons has a universal behavior, independent of the nature of the zeolite. At low water loadings the standard free energy of acid base interactions is dominated by an enthalpic driver associated with the acid strength of the BAS as induced by the zeolite lattice topology. Conversely, an entropic term, which grows linearly with the concentration of waters present within the pores, favors proton solvation and is independent of the pore size/shape. We find that exclusive and localized water cluster formation, at zeolite BAS, is ubiquitously observed in all cases regardless of pore size or shape.