A theoretical study of the alkylation reaction of toluene with methanol catalyzed by the acidic Mordenite (Si/Al = 23) is reported. Cluster DFT as well as periodical structure DFT calculations have been performed. Full reaction energy diagrams of the elementary reaction steps that lead to the formation of the three xylene isomers are given. The use of periodical structure calculations allows one to account for zeolite framework electrostatic contributions and steric constraints that take place in zeolitic catalysts. Especially the steric constraint energy contribution has a significant effect on the energies and bond formation paths. The activation energy barrier of p-xylene formation is found to be approximately 20 kJ/mol lower than the corresponding values for the formation of its isomers. Computed host-guest binding energies according to the DFT method need a correction due to the absence of the dispersive interaction with the zeolite wall. Apparent activation energies obtained with this correction are in good agreement with experimental data.

The stability of the (100) MoS2 surface has been studied using periodic DFT calculations taking into account various parameters such as the temperature and the partial pressure ratios of H2 and H2S present in the surrounding atmosphere. It appears that the sulfur coverage of the surface is strongly dependent on the H2/H2S ratio and that under working conditions, the most stable surface does not contain any coordinately unsaturated sites (CUS). Direct comparisons with experimental literature data such as EXAFS or TPR measurements show a good agreement between calculations and these experiments. The second part of the study deals with the behavior of hydrogen on the surfaces. The endothermic dissociation always leads to Mo−H and S−H groups. This implies that hydrogen is not stable on the MoS2 surface unless at very high pressure or very low temperature. Furthermore, H2 dissociation on the surface will not lead to the formation of CUS.

A periodic density functional theory (DFT) study of the isomerization reactions of toluene and xylene catalyzed by acidic mordenite is reported. Monomolecular isomerization reactions have been considered and analyzed. The different reaction pathways have been discussed in detail. The use of periodic structure calculations allows consideration and analysis of zeolite electrostatic contributions and steric constraints that occur within zeolite micropores. Major differences in the details of protonation reaction pathways are found when periodic structures are used rather than small cluster models of the Brønsted acidic site. Complex relationships are found between zeolite topology and reaction pathways.