The Quantum Confinement Effect on the Adsorption and Reaction of Aliphatic and Aromatic Hydrocarbons on ‘Nano Reactor’ ZSM-5 Zeolite: A Newly Developed Density Functional Theory (DFT) Investigation
B. Boekfa, S. Choomwattana, P. Maitarad, P. Limtrakul, J. Limtrakul
Kasetsart University, TH
Keywords: nanostructured Zeolites, confinement effect, aromatic adsorption, reaction mechanism, DFT
Abstract:The quantum confinement effect on the adsorption and reaction mechanism of aliphatic and aromatic hydrocarbons on H-ZSM-5 zeolite was studied for the first time with a newly developed functional, M06L. With this well calibrated method, one of the important interactions, such as the van der Waals interaction which is widely known to be vital in the description for the confinement effect from the zeolite framework, is taken into account. Full quantum calculations with 6-31G(d,p) basis set on the ZSM-5 models of from 5T, 12T, 34T, 46T and up to 128T were performed to simulate the nanoporous system. Methane, ethane, ethylene, benzene, and ethylbenzene are chosen to represent various adsorbates of saturated, unsaturated and aromatic hydrocarbons. The calculated adsorption energies of saturated compounds, -7.01 kcal/mol for methane and -11.12 kcal/mol for ethane, agree well with experimental estimates of -6.67 and -9.08 kcal/mol. Consequently, the use of the full DFT(M06L/6-31g(d,p) model seems to be a good approach to accurately predict the adsorption energy for other adsorbates The adsorption energy of ethylene, benzene and ethylbenzene are predicted to be -14.19, -17.53 and -24.56 kcal/mol, respectively, which is expected to be very close to experimental observations if available. The M06L method is also applied for the first time to systematically investigate the hydrogen exchange reactions of methane and ethane within the nano-reactor of zeolite. The concerted mechanism is proposed for the reaction. The calculated activation energy of methane is 35.34 kcal/mol, which is comparable with the experimental data (33.40 kcal/mol), whereas the energy of ethane is 33.03 kcal/mol. As expected, ethane requires less energy for the reaction to proceed due to the enhanced stability on the transition structure compared with the same reaction of methane. As a conclusion, the newly developed “M06L” density-functional method is a capable tool for benchmarking the adsorption and reaction of aliphatic and aromatic hydrocarbons on H-ZSM-5. The effect of nano-quantum confinement of the extended zeolite framework on adsorption complexes and reaction mechanisms has been clearly demonstrated not only to better stabilize the adsorption complexes achieving the observed values but also to lower their corresponding activation energies to approach experimental benchmarking.