(703c) First-Principles Grand-Canonical Simulations of Water Adsorption in Proton-Exchanged Zeolites Using a Highly Parallelizable Algorithm

Water is a ubiquitous solvent or an unavoidable impurity in many important engineering and biological processes. For reactions catalyzed by microporous materials, quantifying the amount or the structure of adsorbed water clusters at reaction conditions is prerequisite to understanding the relevant reaction mechanisms. Computational studies, such as those based on static electronic structure methods or first-principles molecular dynamics in the canonical ensemble, relies on preloading the framework with an often arbitrary amount of water molecules in order to examine their effects. We present here first-principles calculations in the grand-canonical ensemble of water adsorption onto proton-exchanged zeolites with different Si/Al ratios (Si/Al = 31--95). These simulations use pre-sampling with inexpensive force fields and a new parallelization algorithm to improve simulation efficiency and to bypass the scaling limit of the underlying electronic-structure method. These developments enable us to predict water adsorption under specified thermophysical conditions. Under almost all conditions examined (T = 373 -- 573 K, p = 10 -- 100 kPa), we observe an appreciable amount of water adsorption, and the zeolitic protons are delocalized by participating in the hydrogen-bonding chains formed by adsorbed water molecules.