12-Molybdophosphoric acid anchored on aminopropylsilanized magnetic graphene oxide nanosheets (Fe3O4/GrOSi(CH2)3–NH2/H3PMo12O40): a novel magnetically recoverable solid catalyst for H2O2-mediated oxidation of benzylic alcohols under solvent-free conditions

Abstract

In this work, 12-molybdophosphoric acid (H₃PMo₁₂O₄₀, HPMo) was chemically anchored onto the surface of aminosilanized magnetic graphene oxide (Fe₃O₄/GrOSi(CH₂)₃–NH₂) and was characterized using different physicochemical techniques, such as powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, energy-dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), BET specific surface area analysis and magnetic measurements. The results demonstrated the successful loading of HPMo (∼31.5 wt%) on the surface of magnetic aminosilanized graphene oxide. XRD patterns, N₂ adsorption–desorption isotherms and SEM images confirm the mesostructure of the sample. FT-IR and EDX spectra indicate the presence of the PMo₁₂O₄₀³⁻ polyanions in the nanocomposite. The as-prepared Fe₃O₄/GrOSi(CH₂)₃–NH₂/HPMo nanocomposite has a specific surface area of 76.36 m² g⁻¹ that is much higher than that of pure HPMo. The selective oxidation of benzyl alcohol to benzaldehyde was initially studied as a benchmark reaction to evaluate the catalytic performance of the Fe₃O₄/GrOSi(CH₂)₃–NH₂/HPMo catalyst. Then, the oxidation of a variety of substituted primary and secondary activated benzylic alcohols was evaluated with H₂O₂ under solvent-free conditions. Under the optimized conditions, all alcohols were converted into the corresponding aldehydes and ketones with very high selectivity (≥99%) in moderate to excellent yields (60–96%). The high catalytic performance of the nanocomposite was ascribed to its higher specific surface area and more efficient electron transfer, probably due to the presence of GrO nanosheets. The nanocomposite catalyst is readily recovered from the reaction mixture by a usual magnet and reused at least four times without any observable change in structure and catalytic activity.