Synthesis Of 2D Cobalt(iron)-based Nitrides And Catalytic Hydrogenation Of Nitroaromatics By Molten Salt Method

Along with the rapid development of industrialization,environmental pollution issue is increasingly serious.For example,the discharge of numerous organic pollutants results in the increased water pollution.Among them,nitroaromatic compounds with the characteristic of high toxicity,and difficult to be naturally degraded,will cause serious environmental pollution after spewed into water.Aminoaromatic compounds are the corresponding reduction products of nitroaromatic compounds,which not only feature low toxicity and easy to be degraded,but also have high medical values.Therefore,it is of great significance to convert nitro compounds into the corresponding amino compounds.The high cost and limited reserves of the traditional precious metal-based catalytic materials have prompted researchers to develop inexpensive non-precious-metal catalytic materials.Cobalt（iron）-based nitrides have received widespread attention due to their platinum-like electronic structure and good performance in the catalytic hydrogenation of nitroaromatic compounds.The catalytic performance of catalytic materials is closely related to the morphology and composition.Early research pointed out that cobalt（iron）-based nanoparticles can be used to catalyze the hydrogenation of nitroaromatic compounds.Two-dimensional（2D）nanomaterials are advantageous in catalytic applications because of their large accessible specific surface.Therefore,the design and synthesis of two-dimensional cobalt（iron）-based nitrides could be conducive to improving the catalytic activity.Molten salt system is provided with space confinement effect and high yields to synthesize nanomaterials.Based on the above characteristics of the molten salt system,this thesis is devoted to preparing two-dimensional cobalt（iron）-based nitrides by the molten salt method,exploring the formation process of the two-dimensional nanosheets,and optimizing the catalytic performance.The thesis mainly contains three aspects of contents:1、Considering iron-based nitrides with good catalytic performance and magnetic separation,2D iron nitride nanosheets have been designed and synthesized in Li Cl-KCl system by using ferbam as Fe source and melamine as carbon source.After heat treatment of the above reactants which were mixed fully by grinding and further covered by the salts,the iron nitride with a flaky morphology was obtained.The reduction of 4-NP to 4-AP was used as a model to evaluate its catalytic performance.The results indicate that the iron nitride material can effectively reduce 4-NP（0.2 m M）to 4-AP at room temperature,with a conversion rate of nearly 100%within 6 min.Besides,the catalyst can be easily separated from the system and reused,by virtue of its magnetism.However,its catalytic performance is not very desirable,due to the intrinsic activity of iron nitride and the rather thick lamellae structure.2、Cobalt nitrides show unique advantages in catalytic hydrogenation of aromatic nitro compound,which is also magnetic.Hence,we foucs on the synthesis of cobalt nitrides by drawing on our experience from the synthesis of iron nitrides.According to the characteristics of the synthesis,we changed the binary molten salt system（Li Cl-KCl）to the ternary molten salt system（NaCl-KCl-Mg Cl₂）.Using Co（NO₃）₂·6H₂O as the Co source and 2-methylimidazole（2-MIM）as the carbon source,2D Co_5.47N-MgO@C thin nanosheets were successfully synthesized by a stepwise heating treatment method.The results point out that the primary ZIF-67 small particles are formed after grinding,and they gradually fuse to produce a two-dimensional structure during the heat treatment process due to the"spacing"effect of the salts.The performance test indicates that 4-NP（0.2 m M）can be efficiently converted to 4-AP in only 225 s at room temperature.Co_5.47N-MgO@C shows the improved performance compared with that（360 s）of iron nitrides.The Co_5.47N-MgO@C sample is magnetic and can be easily separated by an external magnetic field.There is no obvious change in performance after ten times of repeated use.Co_5.47N-MgO@C has excellent activity and stability.3、Due to the"complementary"characteristics of the d-electron orbitals of the early transition metal and the late transition metal,combining the above two could further optimize the d-electron structure and enhance the catalytic ability.Based on this,we introduced Mo into the above synthesis system that is the molten salt synthesis of cobalt nitride-based materials,and Co-Mo₁₆N₇-MgO nanosheets material was obtained.The performance test indicates that the Co-Mo₁₆N₇-MgO can achieve nearly 100%reduction of 4-NP（0.2 m M）in 90 s.In comparison with the cobalt nitride-based material(Co_5.47N-MgO@C,225 s),the catalytic performance of Co-Mo₁₆N₇-MgO has been greatly enhanced.The above results manifest that the molten salt system provides an effective way to synthesize multi-element metal-based catalytic materials.