Minimizing the Size of Palladium Nanoparticles Immobilized within the Channels of Ionic Liquid-Derived Magnetically Separable Heteroatom-Doped Mesoporous Carbon for Aerobic Oxidation of Alcohols

An efficient and well-designed nanocasting procedure has been developed for the synthesis of a series of nitrogen and sulfur codoped magnetically separable ordered mesoporous carbons using an imidazolium-based ionic liquid as the carbon source, guanine as the nitrogen source, FeCl3 as the iron source, and SBA-15 as the hard template. The role of different percentage of the guanine as the nitrogen source on the uniformity of the mesochannels and physicochemical properties of the prepared materials was fully investigated using various techniques including N2 sorption analysis, TEM, FESEM, XRD, AAS, XPS, Raman spectroscopy, CHN, VSM, elemental mapping, and FT-IR. It was observed that large amounts of small and monodispersed iron oxide nanoparticles could be embedded within the carbon rods without any channel blocking, a considerable loss of surface area, and aggregation of nanoparticles. It was also found that both the nitrogen content and surface area of the material have a crucial synergistic effect on the production of small and monodispersed Pd nanoparticles within the carbon pores. The smaller size and more uniform distribution of the Pd nanoparticles produced in the Pd@Fe-GIOMC-40 (in which 40 wt % guanine was employed) resulted in improved catalytic activity in the aerobic oxidation of benzyl alcohol compared to other samples. This catalyst demonstrated outstanding activity in direct and selective oxidation of primary alcohols into the corresponding carboxylic acids under aqueous conditions using very low amounts of catalyst. Importantly, it was found that its activity was completely switchable since it provided a useful procedure for the oxidation of primary alcohols into aldehydes under controlled reaction conditions (in toluene as a solvent instead of water). The catalyst could be simply separated from the reaction mixture using an external magnet and reused in both aqueous and organic reaction media for the next six successive runs with no considerable loss of activity.