Dependence of Water Adsorption on Aluminum Content in the BEA (Polymorph A) Zeolite Using Monte Carlo Simulations

The effect of Al content on TIP5P-Ew water vapor adsorption in the BEA(A) zeolite was systematically investigated using Monte Carlo simulations. Several BEA(A) structures containing Al mole percents ranging from 0 to 16.67% were generated and charge-balanced with H+ cations. Water vapor isotherms were calculated using grand canonical Monte Carlo simulations at 15, 20, and 25 °C over a range of pressures below the model saturation pressure for each respective temperature. A three-site Langmuir model was then fit to the adsorption data to determine the isosteric heats of adsorption. These heats exhibited a rapid convergence to the heat of vaporization [ΔH̅vap(T)] of the water model as Al content was increased and water–water interactions began to dominate the adsorption thermodynamics with water condensing into the zeolite pores. The structure of adsorbed water was also analyzed via hydrogen-bond geometries (O–O distances, O–H distances, H–O···O angles, and the number of hydrogen bonds per water molecule) and Ow(water)–Al and Ow(water)–H+ density profiles. Hydrogen-bond geometries were characterized for the total water–zeolite system and by source [water–water, water–zeolite (Si–O), and water–zeolite (Al–O)]. Structural analysis revealed differences in hydrogen-bond geometries between high and low Al% systems with rapid convergence starting at moderate Al content. A comparison of these geometries between high and low pressures revealed a stronger loading dependence than Al%. Density profiles revealed no change in binding distance between water and Al/H+ sites. However, these peaks decreased in intensity as water loading increased with Al content and water–water adsorption sites became dominant.