Pore Confinement Effects In Zeolites Studied By Quantum Chemical Calculations

Environmentally friendly zeilite catalysts have been extensively developed and applied in the chemical and petrochemical industries due to their controllable internal void spaces and tunable acidity. The catalytic properties of the zeolites are correlated with both acidic strength and zeolite framework confinement. The latter effect is mainly derived from long-range electrostatic and van der Waals (vdW) interactions. In this work, we employ quantum chemical calculations to study the effects of acid strength and framework confinement of zeolites on the activity of catalytic reactions and the reaction mechanism. Some interesting results are obtained.(1) The performance of different exchange-correlation functionals for the description of the interaction of pyridine with different cluster models of ZSM-5zeolite has been assessed. Theoretical calculations indicate that upon increasing the cluster model from8T to128T, the adsorption energy of pyridine in ZSM-5zeolite increases gradually and reaches convergence at a72T cluster model. On the basis of the72T cluster model, the pyridine adsorption energy calculated with different functionals is further examined. Compared to the conventional functionals (B3LYP and M06-2X), the B97D functional, which takes into account the dispersion correction, provides calculated results that agree well with experimental data. The present results indicate that the B97D function is suitable for studying long-range interactions in weakly interacting systems.(2) The effects of the zeolite framework on the protonation reactions of ethene, propene and isobutene over MOR zeolite have been theoretically investigated. The results based on the8-MR and12-MR models indicate that when the size of adsorbate molecule is small compared with the zeolite pore, the pore confinement effect would be weak and the electronic contribution plays a leading role in the reaction. However, with increasing the size of the reactant molecules, the confinement effect will be predominant compared with the intrinsic acid strength. The confinement effects includes two aspects:when the size of hydrocarbon fragments is preferentially fitted into the channels or cages of zeolite, the zeolite framework gives an additional stabilizing effect on the confined species, on the other hand, this stabilizing effect from the zeolite framework will become less predominant if the size of reactant molecule is too bulky and the destabilizing effect from steric constraint be increased remarkably.