Adsorption of n-Alkanes in Faujasite Zeolites: Molecular Simulation Study and Experimental Measurements

Abstract

An application of a previously developed force field for the adsorption of hydrocarbons onto silicalite (Pascual et al. 2003) to linear alkane/sodium faujasite systems is reported. In order to extend this force field from siliceous to cationic zeolites, it is proposed that account be taken of the polarization component of the zeolite-molecule interaction energy. A first-order polarization term is explicitly considered for this purpose, using standard molecular polarizabilities. Polarization appears to amount to 30-40% of the zeolite-alkane interaction energy as a consequence of the strong electric field created by the sodium cation distribution and the negatively charged framework.This approach is compared with the experimental adsorption isotherms of ethane, propane, n-octane and n-decane in NaY taken from the literature and with original measurements of n-butane isotherms in NaY obtained by thermogravimetric methods. Henry constants and heats of adsorption at zero coverage of n-alkanes (n = 6-10) have also been compared with experimental measurements. Although no specific parameter was invoked for extending the force field, general agreement between the simulation results and experiment was satisfactory. Cation redistribution upon alkane adsorption was not observed in these simulations.

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…Wender et al 222 proposed a force field that includes polarization. Calero et al 121 introduce an effective polarization by fitting the Lennard-Jones interactions between the cations and the atoms of the alkane.…”

Isotherms of Fluids in Native and Defective Zeolite and Alumino-Phosphate Crystals: Monte-Carlo Simulations with “On-the-Fly”ab initioElectrostatic Potential

“…The molecular ratios and their evolution as a function of the fugacity are practically equivalent in 2 and 4. The material that shows the highest affinity for CO 2 slope of CO 2 /N 2 ratio in 1, CO 2 shows the highest affinity in 1. The highest CO 2 affinity in 1 is reached at 30 bar and then it lowers when CO 2 starts to condensate in the micropores.…”

“…Even at extremely low water fugacity (1 mbar), a water molecule binds to the solid: inspection of the conformations generated by Monte-Carlo simulations reveals that this water molecule is always found in the vicinity of Na + . Unless the driest conditions are met, it seems experimentally impossible to avoid presence of this water molecule and the stoichiometric formula of 4 is better considered as [H 2 (Fig. 6).…”

“…Molecular mechanics methods have shown their efficiency at complementing and further validating experimental adsorption data of fluids in microporous solids [1,2]. However, the determination of the forcefield parameters that are used to compute the guest-host potential energy can be a difficult and time consuming process, especially for the parameters describing the electrostatic interactions.…”

“…The CO 2 , N 2 and H 2 O isotherms in the native and defective crystal structures are determined and compared. The case of the CO 2 …”