Epoxidation of olefins over supported bimetallic catalysts using oxygen as oxidant and carbon dioxide as moderator

Abstract

The conversion of olefins, into value-added products such as epoxides is of academic and industrial interest. The epoxides have various industrial applications although they are usually produced using environmentally unfriendly processes. This study presents a green approach for production of epoxides from readily available olefins catalyzed by a heterogeneous gold and silver-based polyoxometalate (Au-Ag@POM). The Au-Ag@POM catalyst was characterized using different techniques such as Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), Zetasizer, powder X-ray diffraction (PXRD), Transmission electron microscope (TEM) and Inductively coupled plasma-mass spectroscopy (ICP-MS). The catalyzed epoxidation process was carried out in a Schlenk line setup connected to round bottomed flasks through which oxygen and carbon dioxide where bubbled and the products were analyzed using GC-MS and GC-FID against biphenyl as the internal standard. The optimal reaction conditions were determined using response surface methodology with norbonene as the model substrate, tert-butyl hydroperoxide (TBHP) as the initiator, carbon dioxide (CO2) as promoter and oxygen (O2) as the oxidant. Under optimal condition of catalyst load (0.075 g), reaction time (24 hours), O2 (1 atm), CO2 pressure (1 atm), and reaction temperature (80 ℃), the process achieved a norbonene conversion of 71.5 %, norbonene oxide selectivity of 98.4 % and yield of 70.4 %. When the Au-Ag@POM catalyst was tested on other olefins such styrene, cyclooctene, cyclohexene and 1-hexene, the catalytic system produced the desired epoxide in 79.9 %, 41.9 %, 14.8 % and 14.4 % yield respectively. On the other hand, when that catalyst was tested in different solvent systems of acetonitrile, in absence of a solvent, 1,2-dichloroethane, acetonitrile 1,2-dichloroethane ratio of 1:15, the epoxide yields produced were 19.84 %, 17.2 %, 25.6 %, and 70.4 % respectively. The Au-Ag@POM catalyst efficiently produced epoxides under mild, with high selectivity and yields, especially for norbonene and styrene. It showed reusability over multiple cycles and adaptability to different olefins with variable reaction times. Future work should expand substrate scope, explore greener oxidants, and assess economic and environmental viability for industrial application.