Eco-Friendly and Solvent-Less Mechanochemical Synthesis of ZrO₂–MnCO₃/N-Doped Graphene Nanocomposites: A Highly Efficacious Catalyst for Base-Free Aerobic Oxidation of Various Types of Alcohols

• Main Authors: Mufsir Kuniyil, J. V. Shanmukha Kumar, Syed Farooq Adil, Mohamed E. Assal, Mohammed Rafi Shaik, Mujeeb Khan, Abdulrahman Al-Warthan, Mohammed Rafiq H. Siddiqui, Aslam Khan, Muhammad Bilal, Hafiz M. N. Iqbal, Waheed A. Al-Masry
• Format: Article
• Language: English
• Published: MDPI AG 2020-10-01
• Series: Catalysts
• Subjects: ZrO₂; MnCO₃; N-doped graphene; nanocomposites; catalyst; alcohols
• Online Access: https://www.mdpi.com/2073-4344/10/10/1136
• ISSN: 2073-4344

In recent years, the development of green mechanochemical processes for the synthesis of new catalysts with higher catalytic efficacy and selectivity has received manifest interest. In continuation of our previous study, in which graphene oxide (GRO) and highly reduced graphene oxide (HRG) based nanocomposites were prepared and assessed, herein, we have explored a facile and solvent-less mechanochemical approach for the synthesis of N-doped graphene (NDG)/mixed metal oxide (MnCO₃–ZrO₂) ((X%)NDG/MnCO₃–ZrO₂), as the (X%)NDG/MnCO₃–ZrO₂ nano-composite was synthesized using physical grinding of separately synthesized NDG and pre-calcined (300 °C) MnCO₃–ZrO₂ via green milling method. The structures of the prepared materials were characterized in detail using X-ray powder diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-Ray Analysis (EDX), Fourier-transform infrared spectroscopy (FTIR), Raman, Thermogravimetric analysis (TGA), and N₂ adsorption-desorption isotherm analysis. Besides, the obtained nanocomposites were employed as heterogeneous oxidation catalyst for the alcohol oxidation using green oxidant O₂ without involving any surfactants or bases. The reaction factors were systematically studied during the oxidation of benzyl alcohol (PhCH₂OH) as the model reactant to benzaldehyde (PhCHO). The NDG/MnCO₃–ZrO₂ exhibits premium specific activity (66.7 mmol·g⁻¹·h⁻¹) with 100% conversion of PhCH₂OH and > 99.9% selectivity to PhCHO after only 6 min. The mechanochemically prepared NDG based nanocomposite exhibited notable improvement in the catalytic efficacy as well as the surface area compared to the pristine MnCO₃–ZrO₂. Under the optimal circumstances, the NDG/MnCO₃–ZrO₂ catalyst could selectively catalyze the aerobic oxidation of a broad array of alcohols to carbonyls with full convertibility without over-oxidized side products like acids. The NDG/MnCO₃–ZrO₂ catalyst were efficiently reused for six subsequent recycling reactions with a marginal decline in performance and selectivity.