Journal of Catalysis

The methylation reaction of ethene with methanol over the Brønsted acidic ZSM-5 catalyst is one of theprototype reactions within zeolite catalysis for which experimental kinetic data is available. It is one ofthe premier reactions within the methanol-to-olefins process and has been the subject of extensive the-oretical testing to predict the reaction rates. Herein, we apply, for the first time, first principle moleculardynamics methods to determine the intrinsic reaction kinetics taking into account the full configurationalentropy. As chemical reactions are rare events, enhanced sampling methods are necessary to obtain suf-ficient sampling of the configurational space at the activated region. A plethora of methods is availablewhich depend on specific choices like the selection of collective variables along which the dynamics isenhanced. Herein, a thorough first principle molecular dynamics study is presented to determine thereaction kinetics via various enhanced MD techniques on an exemplary reaction within zeolite catalysisfor which reference theoretical and experimental data are available.

Zirconium terephthalate UiO-66 type metal organic frameworks (MOFs) are known to be highly active, stable and reusable catalysts for the esterification of carboxylic acids with alcohols. Moreover, when defects are present in the structure of these MOFs, coordinatively unsaturated Zr ions with Lewis acid properties are created, which increase the catalytic activity of the resulting defective solids. In the present work, molecular modeling techniques combined with new experimental data on various defective hydrated and dehydrated materials allow to unravel the nature and role of defective active sites in the Fischer esterification and the role of coordinated water molecules to provide additional Brønsted sites. Periodic models of UiO-66 and UiO-66-NH2 catalysts have been used to unravel the reaction mechanism on hydrated and dehydrated materials. Various adsorption modes of water and methanol are investigated. The proposed mechanisms are in line with experimental observations that amino groups yield a reduction in the reaction barriers, although they have a passive role in modulating the electronic structure of the material. Water has a beneficial role on the reaction cycle by providing extra Brønsted sites and by providing stabilization for various intermediates through hydrogen bonds.

Catalytic cracking of alkenes takes place at elevated temperatures in the order of 773–833 K. In this work, the nature of the reactive intermediates at typical reaction conditions is studied in H-ZSM-5 using a complementary set of modeling tools. Ab initio static and molecular dynamics simulations are performed on different C4single bond C5 alkene cracking intermediates to identify the reactive species in terms of temperature. At 323 K, the prevalent intermediates are linear alkoxides, alkene π-complexes and tertiary carbenium ions. At a typical cracking temperature of 773 K, however, both secondary and tertiary alkoxides are unlikely to exist in the zeolite channels. Instead, more stable carbenium ion intermediates are found. Branched tertiary carbenium ions are very stable, while linear carbenium ions are predicted to be metastable at high temperature. Our findings confirm that carbenium ions, rather than alkoxides, are reactive intermediates in catalytic alkene cracking at 773 K.