Molecular Modeling of Binary Liquid-Phase Adsorption of Aromatics in Silicalite Shaji Chempath and Randall Q. Snurr Dept. of Chemical Engineering and Center for Catalysis and Surface Science, Northwestern University, Evanston, IL 60208 John J. Low UOP Research Center, Des Plaines, IL 60017 Adsorption in the zeolite silicalite from binary liquid mixtures of p-xylene, m-xylene, and toluene was investigated using grand canonical Monte Carlo (GCMC) simulations. The results obtained agree well with experimental excess adsorption isotherms from the literature. The agreement is very good when the zeolite is modeled using the PARA form of the silicalite structure, but the results obtained with the native ORTHO structure are in some cases even qualitatively wrong. This supports the previous suggestion that the structure of silicalite undergoes a transition from ORTHO to PARA upon adsorption of aromatic molecules. Molecular-level details of the energetics and siting within the zeolite provide insights into the macroscopic behavior. The simulated single-component and binary results were used to test ideal adsorbed solution theory for these systems. © 2004 American Institute of Chemical Engineers AIChE J, 50: 463– 469, 2004 Keywords: GCMC, liquid phase adsorption, silicalite, xylene, toluene Introduction Grand canonical Monte Carlo (GCMC) simulations have been used successfully in recent years to study adsorption of gases in microporous materials such as zeolites (Fuchs and Cheetham, 2001). In theory, adsorption from a liquid phase can also be studied using the same simulation techniques. How- ever, since the simulations must be done near full loading of the zeolite channels, more aggressive biasing techniques are required to ensure convergence in a reasonable time. From an experimental point of view also, adsorption from the liquid phase poses some problems. It is difficult to obtain the absolute amount of sorbate molecules adsorbed in a zeolite, and, usu- ally, an excess isotherm is measured and reported instead. Because of these difficulties, almost all molecular simulations of adsorption in zeolites have been for gas-phase systems. There are many industrial processes where multicomponent liquid mixtures need to be separated. Adsorption in a micro- porous solid can be an attractive alternative to distillation, especially when the relative volatility of the compounds in the mixture is close to one (Ruthven, 1984). An important example is the separation of p-xylene and m-xylene, where p-xylene is the desired compound because it is widely used in the produc- tion of polyesters. Usually, fractional crystallization or adsorp- tion into a zeolite is used to separate p-xylene from its isomers or other alkyl benzenes. Adsorption based processes use tech- niques such as simulated moving bed (SMB), as in the Sorbex process (Broughton, 1984), which operates in the liquid phase. Recently, there has been interest in the use of MFI zeolite membranes for such separations (Min et al., 2003; Hedlund et al., 2002; Sakai et al., 2001; Xomeritakis et al., 2001; Gump et al., 2001). It should be noted that membrane based separations depend both on adsorption and diffusion characteristics of the compounds in the membrane. In this article we show that GCMC simulations can be used to estimate the adsorption from a liquid phase into a zeolitic phase. Comparisons are made to experimental data from the literature for binary aromatic mix- tures in the zeolite silicalite. Information about the siting of molecules inside the zeolite channels for single component Correspondence concerning this article should be addressed to R. Q. Snurr at snurr@northwestern.edu. © 2004 American Institute of Chemical Engineers AIChE Journal 463 February 2004 Vol. 50, No. 2