Controlled Preparation And Hydrogenation Performance Study Of Carbon Loaded Metal Catalysts

The conversion of biomass lignin-derived phenolic compounds into value-added chemicals is of wide interest.Non-precious metal Co-based catalysts have shown some catalytic activity for the hydrogenation of phenols.Charcoal materials have high potential as carriers for metal nanoparticles.However,conventional carbon materials usually contain chemically inert surfaces and microporous structures that are not conducive to mass transfer in the catalytic process.Heteroatom-doped carbon materials,especially nitrogen-doped carbon materials,can effectively improve the structure and properties of carbon materials and enhance the dispersion and stability of metal nanoparticles（NPs）,thus significantly enhancing the catalytic performance of catalysts.In this paper,different new Co-based nitrogen-doped carbon materials were designed and constructed using a template-based strategy,and the properties and structures of a series of Co-based nitrogen-doped carbon materials catalysts were investigated by various characterisation tools.DFT calculations were performed to further reveal the hydrogenation mechanism of phenol and eugenol.The main studies include:（1）A nitrogen-doped porous carbon material loaded with Co nanoparticles catalyst（Co/NPCN）was constructed and used for the reaction of selective hydrogenation of phenol to cyclohexanol using the zeolitic imidazole skeleton material ZIF-67 as the precursor and graphitic phase carbon nitride（g-C₃N₄）as the templating agent.The results showed that the introduction of an appropriate amount of g-C₃N₄ reduced the size of Co nanoparticles,increased the specific surface area and pore volume of Co/NPCN,and further promoted the formation of Co⁰.The Co/NPCN catalyst achieved complete conversion of phenol and 100%cyclohexanol selectivity at a reaction temperature of 120°C,a reaction time of 3 h and 2 MPa H₂.Furthermore,the Co/NPCN catalyst showed good stability and substrate scalability in the phenol hydrogenation reaction.In addition,the reaction temperature and reaction time had a significant effect on the phenol hydrogenation reaction.The DFT calculations showed that the parallel adsorption conformation of phenol was preferred and favoured the highly selective hydrogenation of phenol to cyclohexanol.（2）A one-step pyrolysis method was used to prepare Co/NPC catalysts with different pyrolysis temperatures using dicyandiamide and cobalt nitrate hexahydrate as precursors for the selective hydrodeoxygenation of eugenol to propylcyclohexane.The results showed that the Co crystallinity increased gradually with the increase of the pyrolysis temperature.The best performance of Co/NPC catalyst was achieved when the pyrolysis temperature was 900°C.At a reaction temperature of 300°C,a hydrogen pressure of 3 MPa,a liquid-time air velocity of 12h^-1 and a hydrogen-oil volume ratio of 500:1,the conversion of eugenol reached 100%and the selectivity of propylcyclohexane reached 92.1%.Increasing the pyrolysis temperature to1000°C resulted in severe aggregation of the metal particles of Co/NPC leading to a decrease in catalyst activity.It was shown that at a pyrolysis temperature of 900°C,the crystallinity of cobalt on the surface of the Co/NPC catalyst was better and the Co nanoparticles were more uniformly dispersed.（3）A Co-based nitrogen-doped porous carbon nanotube Co/NCNT catalyst with high specific surface area can be prepared for the selective hydrodeoxygenation of eugenol to propylcyclohexane using a triazine polymer（PAF-53）as a template agent.The results showed that Co nanoparticles were embedded into the nitrogen-doped carbon nanotubes.The introduction of a certain mass ratio of PAF-53 with dicyandiamide can reduce the size of Co nanoparticles,increase the specific surface area and pore capacity of the Co/NCNT catalyst,and promote the formation of a large number of sparse and porous carbon nanotubes.The Co/NCNT-1:4-900 catalyst allowed for a 100%conversion of eugenol and 98.3%selectivity of propylcyclohexane.Furthermore,the pyrolysis temperature affects the structure,properties and catalytic performance of the Co/NCNT catalysts,with 900°C being the preferred pyrolysis temperature.（4）The reaction temperature and liquid-time air velocity significantly influenced the performance of the Co/NCNT catalyst in the hydrodeoxygenation of eugenol,and the better reaction conditions for the hydrodeoxygenation of eugenol were T=300°C,P=3 MPa,LHSV=12h^-1 and H₂/Oil=500.The stability of the catalyst was also investigated.The conversion of eugenol was maintained at 100%and the selectivity of propylcyclohexane only decreased to a certain extent during 108 h of continuous reaction time,indicating that the overall stability of the catalyst was good.（5）DFT calculations of the adsorption energies of eugenol and some of the products on the cobalt-carbon nitrogen surface were carried out.The results showed that the parallel adsorption energy of eugenol on the Co（111）-CN surface（-3.08 e V）was higher than its vertical adsorption energy（-0.57 e V）and also higher than the parallel adsorption energy of eugenol on the Co（111）surface（-0.76 e V）,indicating that the parallel adsorption energy of eugenol was always dominant on the Co（111）-CN surface.The introduction of CN enhances the interaction between Co（111）-CN and eugenol adsorption and improves the adsorption of eugenol on the cobalt-carbon nitrogen surface,and also reveals that the adsorption state on the cobalt-carbon nitrogen surface has an important influence on the hydrodeoxygenation of eugenol.Combined with the experimental data,it is further shown that on the cobalt-carbon nitrogen surface,eugenol can selectively cleave the C_aryl-OCH₃ bond before the aromatic ring is hydrogenated,after which the aromatic ring is first saturated and hydrogenated and then deoxygenated to make propylcyclohexane.