Study On The Preparation Of Cobalt Based Carbon-nitrogen Composites And Their Catalytic Performance In The Oxidation Of Cyclohexane

Catalytic oxidation of common alkanes into high value-added products is still an important task in today's industry.The selective catalytic oxidation of cyclohexane to KA oil is one of the important chemical conversion processes.KA oil?cyclohexanol and cyclohexanone?is widely used in industry and an important intermediate for the manufacture of nylon 6 and nylon 66.However,cyclohexane is structurally stable and difficult to activate,while its product KA oil possesses higher activity than its reactant cyclohexane,and is prone to excessive oxidation,so it is difficult to ensure high conversion and high selectivity to KA oil at the same time in the oxidation of cyclohexane.The industry generally maintains a low conversion rate to achieve high selectivity to KA oil.Therefore,finding a high-efficiency catalyst is of great significance for the catalytic oxidation of cyclohexane.As a new type of material,carbon-nitrogen materials are increasingly used in catalysis.The use of MOFs as a sacrificial template for the preparation of nitrogen-doped carbon materials with specific structures or synthesis of graphite-like phase g-C₃N₄ and related carbon-nitrogen compounds have become a research hotspot in the field of catalysis in recent times.In this work,ZIF-67 was used as a template to prepare nitrogen-doped graphite carbon materials by pyrolysis.The cobalt particles in ZIF-67 were effectively separated by coordinating dimethylimidazole?2-mIM?,which helped to minimize the aggregation of cobalt particles during the pyrolysis process and effectively improved the stability of the catalyst.The catalysts were characterized by XRD,SEM,TEM and BET.The results showed that the catalytic activity of the catalyst after pyrolysis was slightly improved.And the catalyst after pyrolysis maintained the original crystal structure compared with the original ZIF-67,which made the stability was greatly increased and the catalyst could be recycled at the same time.In order to investigate the effect of the morphology of cobalt-based carbon-nitrogen composites on the catalytic performance,ZIF-67 was controlled by introducing surfactant to obtain particles of different morphologies,and then a cobalt-based carbon-nitrogen composites with different morphologies was obtained by pyrolyzing the precursor ZIF-67.In this paper,the effects of different surfactant addition and solvent changes on the morphology and catalytic properties of the catalysts were investigated.When the additional amount of surfactant cetyltrimethylammonium bromide?CTAB?was 0.1wt%,H₂O as the solvent,uniform and small cubic particles were obtained,which have a best effect in catalytic oxidation of cyclohexane.The Co₃O₄/g-C₃N₄ material was synthesized by one-step pyrolysis.To uniformly precipitate the cobalt ion,the hydroxide ion was continuously ionized by the weakly alkaline substance urea.In the meanwhile,the generated of gas by the thermal decomposition process of urea left pores in the carbon nitride material,leading the cobalt particles encapsulated in the pores.The synthesis of the carrier and the loading of the cobalt particles were carried out in one step.The catalyst prepared by this method have a more uniform distribution of the active components than the conventional catalyst preparation method,which effectively prevented the agglomeration and loss of metal nanoparticles.It also avoided templating agent and environmental pollution.The catalyst preparation process was simple and short,and the raw materials were cheap,which greatly reduced the cost.