Ammonia and water in zeolites: Effect of aluminum distribution on the heat of adsorption

Abstract

We have investigated MOR, MFI, and FAU zeolites to acquire a better understanding of the aluminum T-position effect on the heat of adsorption of ammonia and water at a microscopic level. We performed Monte Carlo simulations using RASPA code to obtain the heats of adsorption of water and ammonia in zeolites with one Al-substitution. We show that the values of heats of adsorption of water and ammonia on MOR do not vary much for the T1, T2, and T4 positions being around 41 kJ/mol and 31 kJ/mol respectively. However, the T3 position shows a noticeable variation for both molecules. When aluminum is in the T3 position ammonia with zeolite have the strongest interaction, inversely water and zeolite have the weakest interaction compared to the other T-sites. MFI-type zeolite is also sensitive in terms of Al-position, and more significantly in the case of ammonia adsorption. In MFI all favorable/unfavorable for water T-sites of MFI were also favorable/unfavorable for ammonia respectively. The heats of adsorption of ammonia and water as a function of the position of aluminum substitution in FAU (super-cage, sodalite cage, or hexagonal prism) have almost constant values with 40.5 kJ/mol for water and 28.3 kJ/mol for ammonia. Thus, FAU zeolite has been shown to be insensitive with regard to T-positions which are crystallographically identical in a highly symmetric spherical cage topology.