Theory Of Acidity Of Mcm-22 Based On The Template Effect

Aluminum distribution in zeolite is a subject of interest as that is characteristic of the Br?nsted site location and hence the catalytic properties of the material. Different experimental techniques involving adsorption and desorption of bases, as well as NMR MAS and IR spectroscopy, have been used to characterize the position of Al atoms, but seldom have given satisfied information. Recent studies show that the distribution of Al sites is not random, but is controlled by the template used in the synthesis process. The Al sites tend to locate near the positive charge introduced by the template. In this way, the template has not only the effect of directing the synthesis toward a particular structure but is also responsible for the Al distribution in the as-made zeolite.In this work, the preferential location of Al in zeolites was investigated by studying the interaction between the zeolite, template and Al distribution. Monte Carlo (MC), molecular mechanics (MM) and molecular dynamics (MD) were employed to predict the location and orientation of template molecules in the zeolite channel. Density functional theory (DFT) in quantum mechanics was used for the study of Al preferential location and, in particular, the role of the template in the Al distribution. To this aim we have chosen the MCM-22 zeolite, which processes two independent channel systems, and whose template is hexamethyleneimine (HMI).The result of MC, MM and MD show that the HMI molecules can stably dock in the supercages, the crossing windows and the sinusoidal channels of the zeolite. By energy minimization for the guest-host non-bonding interactions between the templates and the framework, which including VDW and Columbic interactions, we draw the conclusion that the interactions are mostly contributed by VDW attractions. In the DFT study, geometry optimizations were performed and the substitution energies for Al replacement to Si were calculated. After calculations on the interaction between zeolite cluster model and template, we obtained the preferable sites for Al replacement was T1>T4>T8>T5>T2, whereas the less preferable site is T2. Those T sites in the supercages include T1, T4, T5 are more preferable for the Al substitution, as such the more chance for the Br?nsted acid sites formation, than those in sinusoidal channels include T8, T3 and T2. We can predict more B-acid is on the outside surface of MCM-22 zeolite, which is in well agreement with the experiment.