We have developed a united atom force field able to accurately describe the adsorption properties of linear alkanes in the sodium form of FAU-type zeolites. This force field successfully reproduces experimental adsorption properties of n-alkanes over a wide range of sodium cation densities, temperatures, and pressures. The force field reproduces the sodium positions in dehydrated FAU-type zeolites known from crystallography, and it predicts how the sodium cations redistribute when n-alkanes adsorb. The cations in the sodalite cages are significantly more sensitive to the n-alkane loading than those in the supercages. We provide a simple expression that adequately describes the n-alkane Henry coefficient and adsorption enthalpy as a function of sodium density and temperature at low coverage. This expression affords an adequate substitute for complex configurational-bias Monte Carlo simulations. The applicability of the force field is by no means limited to low pressure and pure adsorbates, for it also successfully reproduces the adsorption from binary mixtures at high pressure.

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Understanding the role of sodium during adsorption: a force field for alkanes in sodium-exchanged faujasites.

Understanding cage effects in the n-alkane conversion on zeolites

Adsorption of alkanes, aromatics, and other organic molecules in liquid phase and the dense vapor phase was studied on FAU and MFI zeolites at temperatures ranging from 25 to 300 °C. On faujasites,...

Adsorption of alkanes and other organic molecules in liquid phase and in the dense vapor phase : Influence of polarity, zeolite topology, and external fluid density and pressure

The liquid-phase adsorption of C5−C24 linear alkanes in zeolite 5A at room temperature was studied using the configurational bias Monte Carlo molecular modeling technique and the batch experimental method. Up to C16, a highly discontinuous variation of the saturation capacity with the carbon number occurred. Whereas the shortest chains remain as a whole within the cage, longer chains are distributed in a more bent configuration over adjacent cages or even adopt highly coiled configurations within one cage. For alkanes greater than C16, a sharp, unexpected drop in adsorption capacity with a minimum at C18−C19 was observed experimentally. The almost complete exclusion of these alkanes was not observed in the molecular simulation under equilibrium conditions and is explained by an extremely slow diffusion of C18−C19 because of “cage nestling”, i.e., an imperturbable adsorption of tightly fitting highly coiled n-alkanes in the first supercage they enter. The competitive adsorption and counterdiffusion of alka...

Fred G. Krautz

Papers

Equilibrium adsorption of liquid phase normal paraffins on type 5A molecular sieves