Synthesis Of Pd Nanoparticles Supported On Nitrogen-doped Carbon And Their Catalytic Applications In Hydrogenation Reactions

In this paper, two kinds of N-doped mesoporous carbon materials were synthesized with different methods from different precursors, and then well-dispared, uniform Pd nanoparticles were loaded on these two kinds of carbon by a simple ultrasound assistant method. The catalytic activities of these two Pd catalysts in hydrogenation and hydrogenation of model moleculer of lignin were studied.Ionic liquid is of high boiling point, high thermal stability and low vapour pressure, so it is an excellent precursor of carbon-based materials. Herein, functionalized nitrile-containing ionic liquid, N-butyl-3-methylpyridium dicyanamide (3-MBP-dca) was taken as the carbon source as well as the nitrogen source, and silica spheres was taken as templates, after calcination in N2flow, N-doped carbon CN0.132was prepared. The nitrogen content of CN0.132is up to12wt%, the graphitization degree of CN0.132is deep, and the surface area of CN0.132is tunable. Pd nanoparticles were loaded on the CN0.132by ultrasound assistant and because the doping of electron-rich N atoms, the ratio of Pd0is as high as71%. The Pd@CN0.132exhibited excellent activity in hydrodeoxygenation of vanillin, model molecular of lignin, under mild reaction conditions, for example, under mild reaction conditions both conversion and selectivity can reach to100%. The catalyst is of good hydrophily, making it disperse well in reaction solvent water during the reaction process, which is better for the contacting of substrate molecule and catalytic active sites. Besides, the interaction between N atoms and Pd can prevent the Pd from being oxidized and leach, keep high stability and reusability, and in fact, catalyst activity of Pd@CN0.132did not change significantly after7recycles.To expand the application of Pd@CN0.132, we tested the activity of it in selective hydrogenation of phenol to cyclohexanone. It is found that under mild reaction conditions (eg.45℃,0.1MPa H2), the conversion of phenol over Pd@CN0.132can reach100%, with99%selectivity of cyclohexanone, which is one time higher than commercial Pd@C and others. For the selective hydrogenation of substituted phenols, Pd@CN0.132also exhibits good activity, and the substituted phenols can be selectively hydrogened to the corresponding substituted cyclohexanone. The N-doped carbon (CN-x) derived from ILs with different cations were prepared and textural properties including surface area, pore size and pore volume, as well as N species of them were found to be different. And the ratio of Pd0is different for different Pd@CN-x, which leads to the catalytic difference of Pd@CN-x in selective hydrogenation of phenol.Considering the cost of ionic liquid is high, inexpensive, harmless, and naturally available glucosamine hydrochloride was choosen as both carbon and nitrogen precursor to synthesize N-doped hydrotherm carbon (CN0.077) by hydrothermal method followed by a post synthesis heating treatment. N content of CN0.077is around7wt%and the surface area is324m2/g. Pd nanoparticles were then trapped in CN0.077by a simple ultrasonic-assisted deposition method, which served as high active and recyclable catalyst for the selective hydrogenation of phenol to cyclohexanone and hydrodeoxygenation of vanillin. Hydrogenation of aromatic nucleus is a chanllenge in hydrogenation, in order to investigate the activity and application range of Pd@CN0.077, hydrogenation of benzoic acid to cyclohexanecarboxylic acid was choosen as model reaction. Under mild reaction conditions (85℃,0.1MPa H2),100%yield of cyclohexanecarboxylic acid was obtained over Pd@CN0.077after24hours while the yield was only10%-20%with other catalysts including Pd@C, Pd@metal oxide and so on. For comparision, Pd@HTC synthesized with glucose precursor showed only14%yield, suggesting the N-doping can promote the activity significantly. Further study indicated that solvent has great effect on the hydrogenation, for instance, the highest100%conversion was obtained in water and in other protic solvent like ethanol, isopropanol or acetic acid, the conversion was lower, however in aprotic polar solvent including dimethyl sulfoxide, N, N-methylformamide,1,4-dioxane and toluene, the reaction can hardly undergo. In addition, the influences of H2pressure and reaction temperature were also investigated and pseudo-first-order was an acceptable approximation of the reaction kinetics. The apparent activation energy of the reaction was approximatly obtained as53.5kJ/mol.In summary, two kinds of N-doped mesoporous carbon were synthesized with functionalized nitrile-containing ionic liquid N-butyl-3-methylpyridium dicycnamide precursor and biomass-derived glucosamine hydro chloride precursor, respectively. The Pd@CN0.132and Pd@CN0.077prepared by ultrasound assistant exhibited superior performance in hydrodeoxygenation of vanillin and hydrogenation of phenol and benzoic acid, revealing great potential of.N-doped carbon in catalytic hydrogenation.