Computational study for water sorption in AlPO(4)-5 and AlPO(4)-11 molecular sieves.

The unusual water adsorption behavior in aluminophosphate molecular sieves AlPO(4)-5 and AlPO(4)-11 was studied using canonical Monte Carlo and periodic density functional theory (DFT) calculation. The number of adsorbed water molecules per cavity ranging from 1 to 12 were located inside the molecular sieves by canonical Monte Carlo simulation methods using a "compass" forcefield. The DFT calculations were done for optimizing each structure with and without adsorbed water molecules employing generalized gradient approximation with the Perdew-Burke-Ernzerhof exchange-correction functional. Both classical and quantum mechanical calculations have exhibited hydrogen bonding between adsorbed water molecules inside the main 12-membered ring. The Al-O-P angles were observed to decrease after adsorbing water molecules in geometry optimized AlPO(4)-5 and AlPO(4)-11 molecular sieves. DFT calculations illustrate that the initial loading of water in the large cavity is due to the mild acidity in the framework but the isobaric increase in loading is due to the abundant hydrogen bonding between adsorbed water at higher water loading.