Activated Carbon-Supported Ruthenium as a Catalyst for the Solvent- and Initiator-Free Aerobic Epoxidation of Limonene

Abstract

The activity and selectivity of ruthenium catalysts supported on various activated carbons have been investigated in the epoxidation of limonene using molecular oxygen, the greenest oxidizing agent, under solvent/reductant- and initiator-free reaction conditions. The catalysts were prepared via different methods including impregnation, sol immobilization, and cation exchange. Supported Ru catalysts with a metal particle size ranging from 1.8 to 21 nm were obtained. The catalyst prepared by cation exchange using Darco G60, with the highest mesopore surface area and pore volume and the lowest Ru particle size (1.8 nm), was found to give the best combination of limonene conversion of 35% and higher selectivity for epoxidation products (Sepox) of 57% at 80 °C and 3 bar oxygen pressure, compared to the total selectivity of 25.5% for allylic oxidation products (Sallyl). Other activated carbon supports, e.g., Darco MRX, Norit RB4C, and Norit RX3, were found to yield less effective catalysts. In addition, the sol immobilization method led to the highest conversion of limonene (40.4%) but with a lower total selectivity toward epoxidation products (Sepox) of 36.8% and that toward allylic oxidation products (Sallyl) of 33.6%. The textural parameters of carbon-supported Ru catalysts were determined by BET, TEM, SEM, TPR, and CO chemisorption. Highly dispersed Ru particles were found to promote the formation of limonene epoxides via the reaction of limonene hydroperoxide with limonene instead of hydroperoxide decomposition to allylic radicals. Finally, the appropriate reaction conditions to maximize the selectivity of the epoxides and the reusability of the catalyst were established.