Highly efficient solvent-free oxidation of cyclohexanol to cyclohexanone over nanocrystalline CaO–MgO binary metal-oxide catalysts

Abstract

The development of inexpensive solid-base CaO–MgO binary metal oxides (BMOs) as efficient and reusable heterogeneous catalysts was reported for the selective production of cyclohexanone (CyONE) from cyclohexanol (CyOH) oxidation under solvent-free conditions. These BMOs were prepared by ultra–diluted co-precipitation procedure and characterized by several sophisticated techniques, such as XRD, BET, CO2–TPD, FTIR, SEM, EDX, TEM, and ICP-OES. The crystallinity of the BMO samples was found from the XRD analysis and the estimated crystalline size range of the samples is 19.02–23.42 nm. The BET and CO2–TPD analysis of prepared samples gave information regarding the specific surface area and basicity (concentration and strength). The morphology and particle size of the 1CaO–1.5MgO sample were obtained from the SEM (61.8 nm) and TEM (55.2 nm) analyzes. Among all the catalysts, the 1CaO–1.5MgO catalyst displayed a higher conversion of CyOH (∼85%) with superior selectivity to CyONE product (∼90.97). The greater catalytic efficiency of the 1CaO–1.5MgO catalyst was attributed to the greater specific surface area, the smaller particle size of the MgO, the optimized composition of CaO–MgO (1:1.5), and higher basic strength along with a large number of basic sites. We have also optimized various reaction parameters including reaction temperature, time, catalyst weight, and solvent. Catalyst recyclability experiments were carried out to disclose that the catalyst has more robust, stable, and easily recyclable. The 1CaO–1.5MgO catalyst was very stable up to the 4th cycle. We also studied the spent catalyst characterization (XRD, BET, FTIR, SEM, TEM, and ICP-OES) to know the fundamental reason for the deactivation of the CM-4 catalyst.