(584al) DFT and Microkinetic Study of the Mechanism of Propene Metathesis on MoOx/SiO2 and MoOx/(AlOy-SiO2) Catalysts

Heterogeneously catalyzed olefin metathesis is an important route to making olefins in the chemical industry and is an emerging route to upcycle plastics. This chemistry enables reassembling C=C bonds whereby propene can be transformed to ethylene and 2-butene (or vice versa). However, the precise active site requirements, reaction mechanism, and molecular structure-function relationships of the catalysts still remain unclear. Thus, this contribution focuses on studying the energetics of propene metathesis on silica supported molybdena catalyst (MoO_x/SiO₂), a model catalyst with isolated MoO₄ clusters, and a bilayer MoO_x/AlO_y/SiO₂ with isolated MoO_x grafted on to isolated AlO₄ structures on silica.

A cluster model of dehydrated SiO₂ taken from Handzlik et al. was used to create MoO₄/SiO₂ (by grafting MoO_x on adjacent silanols) and MoO₄/AlO₄/SiO₂ catalysts by grafting AlO_x species on silanols, then adding MoOx on to this cluster. The activated forms of these catalysts, containing metal-carbene active sites, were used to study the mechanism of the reaction by performing density functional theory (DFT) calculations using Vienna Ab Initio Simulation Package (VASP). The molybdate cluster in MoO₄/SiO₂ and MoO₄/AlO₄/SiO₂ has two oxo (Mo=O) groups, activation of which will result in the metal carbene (metal-alkylidene) species. DFT calculations of the energetics of the mechanisms shows that: (1) cycloaddition of Mo=CH₂ (or Mo=CHCH₃) results in a four-membered metallocycle intermediate that then undergoes cycloreversion to produce an olefin (ethylene or 2-butene respectively) and forming the complementary Mo-alkylidene species. (2) The relative free energy of transition and intermediate states are significantly stabilized for the activated MoO₄/AlO₄/SiO₂ metathesis pathway compared to MoO₄/SiO₂ pathway, indicating higher activity for the former. This contribution will discuss the catalyst structure of bilayer catalysts, activation of the resulting site, the chemistry of olefin metathesis on these sites, the associated DFT energetics, and the results of the microkinetic model calibrated to reaction kinetics experiments.