The Structure And Acidity Of H-AlMOR And H-FeMOR: A Computational Study

Zeolites are used as catalysts with a wide range of applications due to their Br nstedacidity and shape-selectivity. Incorporation of heteroatoms into the framework can change theacidity and pore structure of zeolites and then alter catalytic activity, selectivity and stability.Many experimental studies have investigated the structures and acidity of the isomorphouslysubstituted zeolite containing different framework atoms. However, X-ray analysis and evenmore sensitive neutron scattering techniques cannot distinguish between Si and Al or othermetal cations in the framework. Fortunately, the ab initio and DFT quantum mechanicalmethods provide an efficient way to predict the acidity and catalytic reactivity of zeolites, andeven detail microstructures and configurations that cannot be gained directly fromexperimental measurements.Density functional theory （DFT） and dispersion corrected density functional theory（DFT-D2） were employed to investigate the distribution of Al and Fe in the framework ofH-MOR and the strengths of acid sites by NH₃and pyridine adsorption. The main results aresummarized as:1. Compared with DFT, the DFT-D2method considering the van der Waals interactionsis adjusted with more accurate parameters, so this method describes a clear dispersiveinteraction between atoms and molecules in mordenite.2. DFT and DFT-D2were employed to investigate the distribution of Al/Fe and thelocation of protons in the framework of H-MOR.（1）Based on the substitution energies, the most favorable site for distribution of Al isT2and the proton preferentially located at O5site; the most favorable site for distribution ofFe is T1and the proton preferentially located at O6site. The results show that the location ofprotons plays an important role in the stability of the Al/Fe substitution site.（2）We can find that the energy differences for Al and Fe in different T sites are verysmall, which indicates the Al and Fe atoms might distribute in all kinds of four non-equivalentcrystallographic tetrahedral sites of MOR. 3. Minimum energy structure and adsorption energies for NH₃and pyridine adsorbed ateach crystallographic position of H-AlMOR and H-FeMOR were also determined.（1）It is shown that the Al-T2O5-NH₃site is the most stable location for H-AlMOR andthe Fe-T2O5-NH₃site is the most favored site for H-FeMOR.（2）We found that the Al-T1O6-py site is the most stable location for H-AlMOR andthe Fe-T2O3-py site is the most favored site for H-FeMOR.（3）The adsorption of NH₃and pyridine on the Lewis acid sites of H-AlMOR andH-FeMOR is weaker than on the Br nsted acid sites.（4）The structures of the optimized adsorption complexes show that the ammonia andpyridine molecules are protonated by the acidic proton of H-MOR.（5）Comparing with the adsorption of NH₃and pyridine in H-FeMOR, the adsorption inH-AlMOR is stronger than in the H-FeMOR. So the acidity of H-AlMOR is more stable thanH-FeMOR.