Study On Nickel-based Catalysts For Selective Hydrogenation Of Diene At Low Temperature

Olefins produced by steam cracking, catalytic cracking or thermal cracking processes are associated with few alkynes and dienes invariably. The presence of these compounds is undesirable and harmful to successive process, especially for olefin polymerization. It was a valid means to take selective hydrogenation of dienes to mono-olefins. It was a key method to develop a new hydrogenation catalyst with high activity and selectivity.In this paper, preparation rules and pretreatment conditions of Ni-based catalysts were studied and a Ni-based double promoter catalyst with high activity, mono-olefins selectivity and stabilization was designed and developed. This catalyst possesses good catalytic properties with high activity, selectivity and stability in diene hydrogenation reaction.At first, studies on preparation, characterization and properties of Ni catalysts were performed. Nickel-based catalysts prepared by impregnation were characterized by BET, XRD, H2-TPR and XPS methods to study the surface area, crystal structure, reduction temperature, the chemical valence of surface elements and the used catalyst property changes compared with fresh catalyst. Selective hydrogenation of isoprene was used as a probe reaction to study the effect of different treatment to support and preparation conditions on catalytic activity of Ni-based catalyst. Strong Metal-support interaction (SMSI) and the effect of this interaction on catalytic performance was studied. The results showed that (γ+ )-Al₂O₃ and (κ+θ)-Al₂O₃ supports obtained by calcination Al(OH)3 and the interaction between the different supports and NiO have great influence on catalytic properties. The catalysts with 15% nickel supported on (γ+ )-Al₂O₃ and 7.5% on (κ+θ)-Al₂O₃ have good catalytic properties.High nickel loading over (γ+ )-Al₂O₃ with large surface area, high metal dispersion and too many active sites lead to over-hydrogenation and selectivity decreased, however, carbide deposition on catalyst surface was suppressed to improve the stability by SMSI. Catalysts support on (κ+θ)-Al₂O₃ with weak interaction between metal and support have large sizes of Ni crystal with high Ni loading, which leads to the active component together sintering and surface carbon deposition, resulting in the decrease of activity and stability.To obtain high hydrogenation activity and selectivity of mono-olefins, 7.5% (κ+θ+ )-Al₂O₃ catalyst was studied by pre-curing treatment. In the optimal curing conditions, the selectivity of mono-olefins can be greatly improved(from 0 to 93%). By the discussion of poisoning mechanism with CS2,it was found that CS2 easily deactivate the catalyst at low temperature, besides metal sulfide formation, CS2 physical adsorption on the catalyst surface also leads to catalyst deactivation. The method of adding promoter to modify the single Ni catalyst by co-impregnation was applied with the function of the second metal Cu or Ce metal.The modified catalyst can be used to hold good stabilization to selective hydrogenation of isoprene. The introduction of Ce resulted in the higher activity and stability. With Cu modification, the catalytic activity decreased, however, the selectivity of mono-olefins catalyst increased significantly, and olefin products varied significantly compared with Ce and other promoters.Ni-Ce/Al₂O₃, Ni-Cu/Al₂O₃ and Ni-Ce-Cu/Al₂O₃ catalysts were characterized by H2-TPR, BET, XRD, XPS and other characterization methods to study the promoter modulation mechanism on catalytic performance. In the discussion of hydrogenation mechanism of promoters catalysts,it was found that the addition of Cu promoted the interaction between Cu and support and increased the reduction degree of Ni metal significantly, while the addition of Ce with the synergetic effect of two metals increased the degree of metal dispersion. As selective hydrogenation of diene on surface, the stability of catalyst, the stability of catalyst enhanced.To study the effect of interaction between metal and support on the catalytic properties, this paper used the most stable configuration -Al₂O₃ which is closer to (κ+θ+ )-Al₂O₃ as the support, to simulate the adsorption and deposition of Ni4 cluster on the -Al₂O₃(0001) surface with the Dmol3 module based on density functional theory (DFT). Nickel clusters tend to coordinate with the unsaturated surface oxygen atoms and aluminum atoms in bonding.The support weakened the binding energy between Ni atoms compared with pure Ni4 clusters, and reduced the H2 molecules adsorption energy on the nickel catalyst, indicating the dispersion effect of active metal by support. This work has calculated isoprene and H₂ molecules adsorption on catalyst Ni₄/ -Al₂O₃(0001) by DFT. The results would provide many theory data for the study of the reaction mechanics in selective hydrogenation and be helpful in understanding real heterogeneous catalytic process.