Adsorption Studies of Biomass-Derived Molecules in HY and NaY Zeolites By Quantum Mechanics/Molecular Mechanics Calculations

An embedded-cluster model and Density Functional Theory (DFT) are employed to investigate the adsorption energy of unsaturated aliphatic, aromatic and heterocyclic compounds in HY and NaY zeolites. The quantum mechanics/molecular mechanics (QM/MM) ONIOM method is used to model the catalytically active site and the surrounding zeolitic environment, and the results are calculated using the Gaussian 09 computational package and compared to those calculated by Nikbin et al.¹using a quantum cluster.

Our results show that 1) ethylene exhibits the weakest binding among all adsorbates considered in both zeolites, regardless of the cluster models and basis sets employed; 2) in HY zeolite, the quantum cluster calculations predict comparable binding energies of oxa-norbornene and p-xylene, while the QM/MM calculations predict significantly higher binding energy of oxa-norbornene than that of p-xylene; 3) in NaY zeolite, both the quantum cluster and the ONIOM scheme predict very similar binding energies of oxa-norbornene and p-xylene, regardless of the cluster models and basis sets used; 4) smaller clusters tend to underestimate the binding energy; 5) the QM/MM methodology is very sensitive to the choice of cluster size and basis set, as the use of different cluster models and basis sets can contribute to differences of more than 7 kcal/mol and 5 kcal/mol in binding energy, respectively; 6) the parallel configuration of ethylene and DMF co-adsorption in HY zeolite was found to have much higher adsorption energy than the vertical configuration.

Reference

1. Nikbin, N.; Caratzoulas, S.; Vlachos, D. G. (submitted to J. Catal)