D. Lesthaeghe

Methylation reactions of naphthalenic species over the acidic microporous zeolite with chabazite topology have been investigated by means of two-layered ab initio computations. Large cluster results combined with van der Waals contributions provide thermodynamic and kinetic results of successive methylation steps. The growth of fused bicyclic species is important as these can act as hydrocarbon pool species within the methanol-to-olefin (MTO) process, but ultimately leads to the deactivation of the catalyst. The influence of the confined space of the zeolite pore on the resulting transition state or product shape selectivity is investigated in detail.

Silicalite-1 zeolite exhibits a characteristic pentasil framework vibration around 540−550 cm−1. In the initial stages of zeolite synthesis, however, this band is observed at much higher wavenumbers: literature shows this vibration to depend on particle size and to shift over 100 cm−1 with increasing condensation. In this work, the pentasil vibration frequency was derived from theoretical molecular dynamics simulations to obtain the correct IR band assignments for important nanoparticles. The IR spectroscopic fingerprint of oligomeric five-ring containing precursors proposed in the literature was computed and compared with experimental data. Our theoretical results show that, while isolated five-membered rings show characteristic vibrational bands around 650 cm−1, the combination of five-membered rings in the full MFI-type structure readily generates the bathochromic shift to the typical pentasil vibration around 550 cm−1. As opposed to what was previously believed, the IR band does not shift gradually as nanoparticle size increases, but it is highly dependent on the specific way structural units are added. The most important feature is the appearance of an additional band when double five-membered rings are included, which allows for a clear distinction between the key stages of early zeolite nucleation. Furthermore, the combination of the simulated spectra with the experimental observation of this spectral feature in nanoparticles extracted from silicalite-1 clear solutions supports their structured nature. The theoretical insights on the dependency of pentasil vibrations with the degree of condensation offer valuable support toward future investigations on the genesis of a zeolite crystal.

Over the past 30 years, the methanol-to-olefin (MTO) process on acidic zeolites has been subject to a vast number of studies from both industrial and academic researchers, leading to numerous controversies regarding the most probable reaction mechanism. Improvement of computational facilities during the past decade led to a sudden boost of theoretical contributions that, when considered individually, all seemed to provide reasonable evidence for partial pathways of the commonly proposed direct mechanisms. Not only the reactions suggested by experimental studies were investigated, but in addition novel potential routes were discovered by theoreticians as well. However, when all of the individual reactions scattered throughout the literature were recently combined, theoretically obtained rate coefficients turned out to show the exact opposite, that is, the complete failure of the direct mechanisms to produce ethylene from methanol only. In this paper, we give a detailed overview of the theoretical contributions that initially supported the direct mechanism proposal, but which finally culminated in its demise.

The kind of olefins obtained from methanol in zeolites is strongly dependent on specific combinations of the intermediate organic hydrocarbon-pool species and zeolite topology (see picture). If the cage is too large, neutral species are favored over reactive cations. If the cage is too small, transition-state-shape selectivity poses severe limitations on the reactivity of bulkier species.