Designing Nitrogen-doped Carbon Based Catalysts For Typical Hydrogenation Reactions

In this paper,we designed different nitrogen-doped carbon(CN)based catalytic systems depending on typical catalytic hydrogenation reactions.Specifically,employing naturally available biomass or biomass derivatives as the carbon precursors,a series of CN based materials were prepared through hard-temple or"leavening" methods,Employing different metals as the active components,we prepared a series of CN based metal supported catalysts with small and uniform metal particles,which showed excellent catalytic performance in typical hydrogenation reactions such as C=C,-C=O or arene hydrogenations.Firstly,utilizing bio-derived glucosamine hydrochloride as both the carbon and nitrogen sources and MgO as the original hard template,we prepared a nitrogen-doped flower-like porous carbon(NFPC)through hydrolysis,high-temperature carbonization and template removal processes.It is found that the as-obtained NFPC possesses abundant mesopores with a high specific surface area(SBET)of 411 m2 g-1,and the nitrogen content can reach up to 9 wt.%.Interestingly,the final NFPC product has a flower-like structure that is totally independent of the original MgO morphology and the kind of carbon precursors.With this observation,detailed investigations showed that the in situ formed Mg(OH)2 nanoplates from the hydrolysis of MgO were responsible for the directed synthesis of the final flower-like porous carbons.Through a traditional deposition-precipitation method,we prepared a Ru/NHPC catalyst with well distributed small Ru particles(2.7 nm),while the particle size of Ru in Ru/FPC(FPC is without nitrogen doping)around 4.4 nm.When the catalysts were sent to the hydrogenation of aromatic ring,it is found that Ru/NHPC showed a high TOF of 8752 h-1 in the solvent-free hydrogenation of methyl-benzene(100?,1.4 MPa H2),which is about six times as high as that of Ru/FPC;While in the hydrogenation of benzoic acid in aqueous media(55 0C,1 MPa H2),Ru/NHPC achieved a TOF of 69.8 h-1,much higher that of Ru/FPC(26.3 h-1).Combining characterizations,we concluded that the high activity of Ru/NFPC may result from the doped nitrogen in NFPC interacting with Ru particles,this thus on the one hand leads to higher content of active Ru0 site,on the other hand contributes to much smaller Ru particles size together with strengthened anchoring.Then,without MgO removal,the CN based composite with mixed carbon and MgO(CN@MgO)was directly utilized as a following catalyst support.This thus combines the basicity of MgO and many excellent properties of CN.Through a simple ultrasound assisted reduction method,palladium nanoparticles were deposited on the CN@MgO support,obtaining a bifunctional Pd/CN@MgO catalyst with both bacisity and hydrogenation ability.Characterizations showed that the Pd nanoparticles is uniformly distributed,presenting a small average particle size of 2.2nm.We later found that the MgO was actually coated by CN component,this kind of structure was later proven to be highly advantageous for ensuring the hydrothermal stability of MgO because of CN coating.Further,the hydrophilic property of the surface CN coating further allows the good dispersions of Pd/CN@MgO in water.With such improved features,Pd/CN@MgO achieved high performance in the tandem aldol condensation of acetone and furfural followed by hydrogenations(Aldol condition:0.1 MPa N2,80?,3 h;Hydrogenation condition:1.7 MPa H2,120?,3 h),achieving 99%of furfural conversion and 95%of targeted product selectivity,which is far more active than those of the comparison catalysts Pd/CN,Pd/MgO ?Pd/(CN+MgO).Finally,we reported the fabrication of a N-doped hierarchical porous carbon(NHPC)applying a simple modified "leavening" method,which employs cellulose as the carbon source,(NH4)2C2O4 as both nitrogen source and foaming agent and NaHCO3 as the activating agent.This support was then used to support Ni nanoparticles to obtain a Ni/NHPC catalyst.The catalyst showed uniformly distributed small Ni nanoparticles(6.5 run)with a high SBET of 788 m2 g-1,which well retained the hierarchical porous structure of the original NHPC.Then it was applied in vanillin hydrodeoxygenation,where the activity in different solvent was investigated.The results showed that ethyl acetate was the desired solvent,which well avoids the large amounts of polymerized products observed in water.Experimental results(?)showed that Ni/NHPC catalyst achieved full vanillin conversion and 98%of deoxygenated product selectivity under 2 MPa H2 at 120? for 3h,which is significantly superior to that of Ni/HPC.Furthermore,Ni/NHPC demonstrated to be very promising for efficient oxygen removal for a wide range of aromatic substrates with carbonyl or-OH at benzylic position,showing a direct hydrogenolysis or hydrogenation-hydrogenolysis reaction pathway.As for the high activity of Ni/NHPC,we concluded that the modified electronic state of nickel interacted with nitrogen dopant and enhanced mass transfer contributed by the hierarchical porous structure of NHPC may be reasons.In conclusion,depending on the typical hydrogenation reactions,we designed a series of CN based catalysts,accomplishing efficient activation of the corresponding substrates.The possible reasons for the enhanced catalytic hydrogenation activities of CN supported metal catalysts were preliminarily investigated,expecting to be of value for further extending the wide applications of CN based catalysts.