Thesis by Jérôme Rey*, 2020 Yves Chauvin prize-winner
Zeolites are nanoporous solids widely used as acid catalysts for the conversion of hydrocarbon molecules. However, determining the rates of the elementary steps of reaction mechanisms represents a significant challenge, due to the large number of degrees of freedom in the reactants, intermediatesa and the transition statesb in the zeolite cavity. This challenge was overcome thanks to quantum calculation techniques.
In partnership with Comenius University in Bratislava, we used a constrainedc AIMDd method to precisely model isomerization and alkene cracking reactions, and quantify their rate constants. It turned out that AIMD surpassed traditional static methods for these reactions. It was thus possible to identify the reaction intermediates involved, as well as the key transition states (figure) for alkene isomerization reactions via tertiary(1) and secondary(2) carbocationse, and for their cracking via β-scission(3).
The research resulted in a unique understanding of the mechanisms at work alongside the detailed quantification of the associated rate constants, which directly depend on the free energy different between transition states and intermediates. These will be incorporated in kinetic models to predict the catalytic performances of zeolites on a macroscopic scale, in oil refining and biomass conversion.
*Thesis entitled "Isomerization and alkene cracking mechanisms and kinetics in the chabazite zeolite quantified via constrained ab initio molecular dynamics"
a - Products formed and then reconverted during the reaction steps
b - States theoretically present in the reaction process but not concretely observed
c - Making it possible to guide the evolution of the system according to a given reaction process
d - Ab initio molecular dynamics
e - Ion derived from an organic compound, with a positive electric charge on one or more carbon atoms
(1) J. Rey, A. Gomez, P. Raybaud, C. Chizallet, T. Bučko, J. Catal., 373, 361–373, 2019.
(2) J. Rey, P. Raybaud, C. Chizallet, T. Bučko, ACS Catalysis, 9, 9813−9828, 2019.
(3) J. Rey, C. Bignaud, P. Raybaud, T. Bučko, C. Chizallet, Angew. Chem., Int. Ed., 59,18938-18942, 2020.
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