Solvent tunes the selectivity of alkenes epoxidation over Ti-Beta Zeolite: A systematic kinetic assessment on elementary steps, kinetically relevant and reaction barriers

Abstract

Alkene epoxidation is a very important reaction process for the selective generation of chiral products, usually performed in some solvent(s) with different polarity, solubility and proton properties. These physical properties are entangled and do not explain very well whether they are mainly against population or stability of the reaction intermediates during catalysis. Here, we investigate how different solvents with known polarity properties (CH3CN, (CH3)2CO, CH3OH, etc.) affect the C6H10 transformation over Ti-Beta catalysts. Combining kinetic measurements, thermodynamic investigations and isotope experiments, the solvent was found not to change the reaction path or mechanism, with C6H10 epoxidation most likely following the Eley-Rideal mechanism and C6H10O hydration following a Langmuir-Hinshelwood mechanism, and both nucleophile and carbonium were found to be involved in the transition state (c.a. SN2 process) of hydration. However, in the epoxidation reaction of C6H10 with CH3CN and (CH3)2CO as solvents, the reaction order of [C6H10] is subtly different and the epoxidation rate increases with increasing volumetric fraction of CH3CN in the binary mixture, which indicates that the solvent affects the concentration of dissolved C6H10 surrounding the active sites and leads to the corresponding differences in activity and product selectivity. Nevertheless, the measured apparent reaction barrier (energy difference between transition state and MASIs) spans the elementary step of solvent modification, thus allowing for the observation of negligible solvent thermodynamic modification. Collectively, these findings provide clear evidence of solvent effects on alkene epoxidation and can be easily extended to some other reactions in solvents.