Monte Carlo investigations of the water adsorption behavior in MFI type Zeolites for different Si/Al ratios with regard to heat storage applications
Adsorption processes are turning more and more important for heat transformation applications like thermally driven heat pumps and cooling cycles as well as heat storage. Standard adsorption materials have not been developed for these purposes. New materials are necessary in order to improve the adsorption characteristics for these applications. Molecular simulations are seen as a promising tool to investigate the influence of molecular structure on the adsorption characteristics and thus to provide means for future improvements of such materials for application in heat transformation. Here, Molecular simulations of adsorption isotherms are performed using the Sorption module within Cerius(2) from Accelrys Inc. Existing Force-Fields are examined for their use in Grand Canonical Monte Carlo Simulation (GCMC) of water adsorption in zeolites. The augmented Consistent Valence Force-Field (CVFF-aug) is currently the best available force-field for use with non-polerizable water models like SPC in zeolite frameworks as reported by Hill and Sauer (1994a). Hence, this force field is used to compare simulated adsorption isotherms with experimental data obtained at Fraunhofer ISE for different types of zeolites. The significance of the Al/Si ratio is investigated with regard to a material optimization for the use in beat transformation applications. Furthermore the role of the extra-framework cations is evaluated as different ions are placed inside the zeolite structure. In the first instance the SPC (Berendsen et al., 1981) water model is used in the simulation.