Computation of the Heat and Entropy of Adsorption in Proximity of Inflection Points

The adsorption of different heptane isomers in MFI- and MEL-type zeolites is studied to investigate the performance of molecular simulation for computing the heat and entropy of adsorption as a function of loading. It is shown that none of the conventional methods are capable of computing the heat or entropy of adsorption of bulky molecules such as 2,4-dimethylpentane around the inflection point of the adsorption isotherm. The “Energy Slope” method is introduced which outperforms the present techniques at loadings near and above the inflection point. There is a quantitative agreement between the heats of adsorption computed with the energy slope method for a butane/isobutane mixture and the ones computed by the method based on energy/particle fluctuations in the grand-canonical ensemble. This verifies that the energy slope method is a valuable and robust tool for computing the heat and entropy of adsorption for a wide range of hydrocarbon lengths and loadings.