Anharmonic Adsorption Free Energies Beyond GGA for Monomer Ethanol in H-ZSM-5.

Zeolites play a pivotal role as catalysts in biomass conversion, and understanding how alcohols interact with these materials is essential for designing next-generation catalysts that reduce dependence on nonrenewable fuels and enable a more sustainable energy landscape. Achieving this understanding requires accurate free-energy calculations. In this study, we present the anharmonic enthalpy, entropy, and vibrational free energy associated with ethanol adsorption on H-ZSM-5, computed at a hybrid DFT level of theory. We critically assess the need to go beyond both GGA functionals and the harmonic approximation to achieve chemically accurate vibrational free energies. The vibrational contributions to the free energy are obtained using a newly implemented strategy that combines the DOS-P method with a QM/QM molecular-dynamics scheme. This approach yields significant improvement in accuracy compared to the commonly used PBE + D2 functional. By combining the anharmonic vibrational free energies computed at the B3LYP + D2 level with electronic energies refined at the CCSD(T) level, we obtain an anharmonic adsorption enthalpy, ΔH, of -101.9 kJ mol-1, the entropic term, -TΔS, equal to 61.0 kJ mol-1, and a free energy of adsorption, ΔG, of -40.8 kJ mol-1 at 313 K. We performed a critical comparison between these values and the experimental data. Notably, the entropic term agrees well with the experimental value of 62.0 kJ mol-1. We also examine the significance of anharmonicity, finding a correction of approximately 1 kJ mol-1 for both the vibrational entropy and enthalpy, and about 3.7 kJ mol-1 for the ZPE.