(289c) Molecular Simulation of Liquid Phase Adsorption of Chain Molecules in Zeolites

Molecular simulations have become an important tool for understanding adsorption in zeolites. However, most molecular simulations of adsorption have focused on adsorption from the gas phase. Liquid phase adsorption is important industrially and differs from gas phase adsorption mainly due to the high density of adsorbates inside the pores under liquid-phase conditions. This results in strong interactions among adsorbate molecules and can cause very non-ideal behavior. Although in principle, grand canonical Monte Carlo (GCMC) simulation of an adsorbed phase in contact with a liquid or gas phase is similar, there are several challenges in practice. The high density of adsorbates inside the zeolite makes conventional GCMC insertion and deletion moves inefficient, resulting in poor sampling. Recently, De Meyer et al. [1] have shown that through the use of techniques such as configurational biasing and identity swaps, which increase the sampling efficiency, GCMC simulations can be applied to liquid phase adsorption of n-alkanes up to C₁₄ in silicalite. The n-alkanes were modeled as chain molecules using a united atom forcefield.

In this work we have applied these techniques to study adsorption of n-alkanes in zeolite A (LTA-5A), zeolite X (NaX) and zeolite Y (NaY). Zeolites A, X and Y have "cage" structures, and calculating molecular conformations and ordering in these zeolites will provide a useful comparison with the "channel" structure of silicalite. Adsorption in zeolite A is important for the separation of linear C₁₀ to C₁₄ n-alkanes from kerosene to perform linear alkyl benzene (LAB) synthesis, en route to biodegradable detergents. Previous studies of n-alkanes in zeolite A have either been performed at low concentrations or completely neglected the zeolite cations. We perform simulations at conditions of liquid phase adsorption taking into account the role of cations during adsorption. Extending the work of De Meyer et al., we also studied the adsorption of linear alpha-olefin in silicalite and compared the isotherms with n-alkanes. Energy efficient separation of alkane/olefin mixtures is a holy grail of separations. Here we analyze the molecular-level conformations, siting and packing of the chains to understand how these factors affect the macroscopic adsorption and selectivity. In addition to simulations, a comparison of our results with experiments will be presented.

1. K. M. A. DeMeyer, S. Chempath, J. F. M. Denayer, J. A. Martens, R. Q. Snurr and G. V. Baron, "Packing effects in the liquid-phase adsorption of C5-C22 n-alkanes on ZSM-5.", J. Phys. Chem. B, 107, 10760-10766, 2003.