Nickel-based Catalyst For Hydrogenation Of Adiponitrile To 1,6-hexamethylenediamine Under No Alkali Conditions

1,6-Hexanediamine（HMDA）is a kind of vital chemical midbody and raw material for organic processing.Since both ends of the carbon chain skeleton have amino（-NH₂）groups,polymerization can take place to produce nylon fibers,polyurethane and polyamide resins and other materials.The production of 1,6-hexamethylene diamine is highly monopolized by foreign companies.The production capacity of 1,6-hexamethylenediamine in our country cannot meet our own demands,and there is an urgent need to improve the production process.Raney Ni has been chosen as the catalyst commonly in the current production,while its low mechanical strength,poor anti-toxicity,and inflammability cannot be ignored.In this paper,the supported Ni-based catalyst for the preparation of 1,6-hexamethylenediamine by liquid phase hydrogenation of adiponitrile（ADN）under no-alkali conditions is studied.And the size effect of the Ni nanoparticles and the impact of synergistic metals on the catalytic performance have been significantly focused.First,controlling the size of Ni nanoparticles through three different preparation methods with Al₂O₃ as the carrier:incipient wetness impregnation（Ni/Al₂O₃-I）,deposition-precipitation（Ni/Al₂O₃-D）,and co-precipitation（Ni/Al₂O₃-C）,catalysts with different particle sizes were obtained.The impact of size effect and surface structure of the catalyst on the liquid-phase hydrogenation of adiponitrile were studied.The results of XRD,BET,TEM,XPS and temperature-programmed experiments suggested that the precipitant contributes to the formation of fine-sized Ni particles.However,the size of the mesoporous pores in the Ni/Al₂O₃-C catalyst is narrow with most of Ni particles evenly embedded in the supporting materials,leading to the poor hydrogenation performance.On the contrary,average size of Ni particles in Ni/Al₂O₃-I is as large as 18.5 nm.However,the Ni particles are fully-exposed on the surface of alumina,the higher content of reduced Ni⁰ species and moderate acidic sites that are conducive to adsorb reactants have been formed easily in Ni/Al₂O₃-I.The Ni/Al₂O₃-I catalyst exhibits excellent low-temperature activity with the prominent conversion frequency（TOF）of 39.7 h^-1 at 60 ℃,but its stability and cycle performance are poor.Furthermore,by modifying the low-content Ni/Al₂O₃-I catalyst with a small amount of Cu element,the introduction of Cu species effectively improves the dispersibility of Ni nanoparticles,and forms bimetallic nano-alloy particles with part of Ni components.The Ni₁₅Cu₃/Al₂O₃ catalyst has no obvious deactivation after 5 cycles of reaction,and the morphology and chemical structure of the catalyst have not changed significantly.The introduction of copper species can effectively improve the stability of the catalyst while maintaining the low-temperature activity of the Ni-based catalyst.The TOF of the catalyst is 52.1 h^-1.Then,the article continued to explore the influence of the relative content of Ni-Cu on the catalytic performance.Incipient wetness impregnation was determined as the preparation method under the above premise,Ni_xCu₃/Al₂O₃ and Ni₂₀Cu_x/Al₂O₃catalyst were further prepared to study the catalytic performance,whose surface texture and chemical properties were characterized by XRD,XPS,H₂-TPD,N₂physical adsorption and other experiments.The results show that the Ni₂₀Cu₃/Al₂O₃catalyst has excellent catalytic activity and stable cycle performance due to its fine Ni particle dispersion,small Ni particle size,and high zero-valent Ni⁰ content.Under the rigorous condition of no ammonia and no alkali,the conversion rate of adiponitrile can reach 100%while the selectivity of hexamethylene diamine is 65%at 80 ℃ within 3 h.