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

[Display omitted] • The reaction rate or product selectivity for C 6 H 10 transformation varies with solvent applied. • C 6 H 10 epoxidation follows an Eley-Rideal mechanism, while C 6 H 10 O hydration follows a Langmuir-Hinshelwood mechanism. • The modulation of the C 6 H 10 concentration around Ti- by solvent affects epoxidation rate. • The difference in epoxidation rates is more kinetic control than being thermodynamically influenced. 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 (CH 3 CN, (CH 3) 2 CO, CH 3 OH, etc.) affect the C 6 H 10 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 C 6 H 10 epoxidation most likely following the Eley-Rideal mechanism and C 6 H 10 O hydration following a Langmuir-Hinshelwood mechanism, and both nucleophile and carbonium were found to be involved in the transition state (c.a. SN 2 process) of hydration. However, in the epoxidation reaction of C 6 H 10 with CH 3 CN and (CH 3) 2 CO as solvents, the reaction order of [C 6 H 10 ] is subtly different and the epoxidation rate increases with increasing volumetric fraction of CH 3 CN in the binary mixture, which indicates that the solvent affects the concentration of dissolved C 6 H 10 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. [ABSTRACT FROM AUTHOR]