A highly reduced graphene oxide/ZrOx–MnCO3 or –Mn2O3 nanocomposite as an efficient catalyst for selective aerial oxidation of benzylic alcohols

Abstract

Highly reduced graphene oxide (HRG) nanocomposites of manganese carbonate doped with (1%) zirconia (ZrO_x) nanoparticles [ZrO_x(1%)–MnCO₃/(X%)HRG (where X = 0–7)] were prepared employing a facile co-precipitation method in which the percentage of HRG was varied. The resulting nanocomposite was calcined at 300 °C. Further calcination of the catalyst at 500 °C resulted in the conversion of manganese carbonate to manganese oxide [ZrO_x(1%)–Mn₂O₃/(X%)HRG]. The effect of the inclusion of HRG on the catalytic activity along with its comparative performance between carbonates and their respective oxides was studied for the liquid-phase selective oxidation of benzylic alcohols into corresponding aldehydes using molecular oxygen as the eco-friendly oxidizing agent without adding any external additives or bases. The influence of different parameters such as different percentages of HRG, reaction times, calcination temperatures, catalyst dosages and reaction temperatures have also been systematically studied in order to optimize the catalyst composition and reaction conditions. The inclusion of HRG as a dopant remarkably enhanced the catalytic efficiency of ZrO_x–MnCO₃ nanocatalysts for the aerobic oxidation of alcohols. The as-prepared catalysts were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (XRD), thermal gravimetric analysis (TGA), Brunauer–Emmett–Teller (BET) surface area analysis, Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR). The catalyst with composition ZrO_x(1%)–MnCO₃/(1%)HRG obtained by calcination at 300 °C exhibited excellent specific activity (60.0 mmol g⁻¹ h⁻¹) with 100% benzyl alcohol conversion and more than 99% product selectivity within an extremely short time (4 min). The same catalyst is employed for the oxidation of a wide range of substituted benzylic and aliphatic alcohols. The catalyst i.e. ZrO_x(1%)–MnCO₃/(1%)HRG calcined at 300 °C yielded corresponding aldehydes with complete convertibility and selectivity in short reaction times under mild conditions whereas the as-prepared catalyst exhibited high selectivity for aromatic alcohols over aliphatic alcohols. The catalyst was recycled and reused at least five times without any obvious loss in its activity or selectivity.