Accurate Adsorption Thermodynamics of Small Alkanes in Zeolites. Ab initio Theory and Experiment for H-Chabazite

Heats of adsorption of methane, ethane, and propane in H-chabazite (Si/Al = 14.4) have been measured and entropies have been derived from adsorption isotherms. For these systems quantum chemical ab initio calculations of Gibbs free energies have been performed. The deviations from the experimental values for methane, ethane, and propane are below 3 kJ/mol for the enthalpy, and the Gibbs free energy. A hybrid high-level (MP2/CBS): low-level (DFT+dispersion) method is used to determine adsorption structures and energies. Vibrational entropies and thermal enthalpy contributions are obtained from vibrational partition functions for the DFT+dispersion potential energy surface. Anharmonic corrections have been evaluated for each normal mode separately. One-dimensional Schrodinger equations are solved for potentials obtained by (curvilinear) distortions of the normal modes using a representation in internal coordinates.