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Phenomenological Inferences on the Kinetics of a Mechanically Activated Knoevenagel Condensation: Understanding the “Snowball” Kinetic Effect in Ball Milling

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Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali, Università degli Studi di Cagliari, via Marengo 2, 09123 Cagliari, Italy
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School of Chemistry and Chemical Engineering, Queen’s University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, UK
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Author to whom correspondence should be addressed.
Academic Editors: Vjekoslav Štrukil and Matej Baláž
Molecules 2019, 24(19), 3600; https://doi.org/10.3390/molecules24193600
Received: 31 August 2019 / Revised: 30 September 2019 / Accepted: 3 October 2019 / Published: 7 October 2019
(This article belongs to the Special Issue Recent Development of Mechanochemical Synthesis)
We focus on understanding the kinetics of a mechanically activated Knoevenagel condensation conducted in a ball mill, that is characterized by sigmoidal kinetics and the formation of a rubber-like cohesive intermediate state coating the milling ball. The previously described experimental findings are explained using a phenomenological kinetic model. It is assumed that reactants transform into products already at the very first collision of the ball with the wall of the jar. The portion of reactants that are transformed into products during each oscillation is taken to be a fraction of the amount of material that is trapped between the ball and the wall of the jar. This quantity is greater when the reaction mixture transforms from its initial powder form to the rubber-like cohesive coating on the ball. Further, the amount of reactants processed in each collision varies proportionally with the total area of the layer coating the ball. The total area of this coating layer is predicted to vary with the third power of time, thus accounting for the observed dramatic increase of the reaction rate. Supporting experiments, performed using a polyvinyl acetate adhesive as a nonreactive but cohesive material, confirm that the coating around the ball grows with the third power of time. View Full-Text
Keywords: mechanochemistry; ball milling; Knoevenagel condensation; kinetics mechanochemistry; ball milling; Knoevenagel condensation; kinetics
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Carta, M.; James, S.L.; Delogu, F. Phenomenological Inferences on the Kinetics of a Mechanically Activated Knoevenagel Condensation: Understanding the “Snowball” Kinetic Effect in Ball Milling. Molecules 2019, 24, 3600.

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