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.