Synthesizing Ni-Al₂O₃ Catalysts In A Rotating Packed Bed And Their Performance Study

Nickel-based catalysts,a non-noble catalysts,are widely used in industry such as catalytic hydrogenation,catalytic reforming and catalytic cracking.Coprecipitation is one of the main methods for preparing nickel-based catalyst.However,due to the limited mixing and dispersing performance of traditional stirred reactor,nickel-based catalysts prepared by coprecipitation method possess large Niparticle size,heterogeneous particle size distribution and serious agglomeration,resulting in low activity and unstable performance.In order to solve the above problems,we first utilized a rotating packed bed（RPB）to intensify the mixing and dispersing of the preparation process of the Ni-Al₂O₃ catalyst.The reaction crystallization mechanism of Ni-Al₂O₃ catalyst prepared by coprecipitation method was studied.On this basis,the effect of mixing and dispersion in RPB on NiAl-LDH and Ni-Al₂O₃ catalyst was revealed.The influence of catalyst characteristics on the hydrogenation performance of maleic anhydride（MA）was studied by adjusting the nickel content and the thickness of NiAl-LDH.Finally,a new process for preparing ultrathin Ni-Al₂O₃ catalyst derived from NiAl-LDH with high Nicontents by RPB was developed.The research content and main results of this paper are as follows:（1）The reaction crystallization mechanism of Ni-Al₂O₃ catalyst prepared by coprecipitation method was revealed.Firstly,the reaction nucleation mechanism of NiAl-LDH synthesized by coprecipitation method was revealed by tracking the composition and the evolution law of solid-phase crystal,morphology and structure during the reaction nucleation process.It is found that the formation process of NiAl-LDH follows a two-step reaction mechanism.In the first step,OH^-group react with aluminum ions to form amorphous aluminum hydroxide colloid;in the second step,nickel ions and carbonate/nitrate assemble into amorphous aluminum hydroxide to form amorphous NiAl-LDH.The crystal growth of NiAl-LDH mainly follows Ostwald ripening mechanism.During the aging/crystallization process,increasing the temperature can promote the growth of NiAl-LDH.With prolonging aging/crystallization time,the lateral dimension and thickness of NiAl-LDH increase.When the crystallization time exceeds a certain degree,the NiAl-LDH reaches the equilibrium of precipitation and dissolution,and the particle sizes of NiAl-LDH are basically unchanged.Ni Al-LDH was converted into Ni-Al₂O₃ catalyst by topological transformation.In the topological transformation process,NiAl-LDH first loses physically adsorbed water and free water molecules,and the structure of NiAl-LDH does not collapse at this stage.When the temperature rises to about 300℃,NiAl-LDH loses the layer hydroxyl group,resulting in structure collapse and forming NiO/Al₂O₃.Further promoting the temperature,nickel ions are gradually reduced,forming Ni-Al₂O₃ catalyst with confined structure.（2）The effect of mixing and dispersion on NiAl-LDH and Ni-Al₂O₃ catalyst was revealed.It was found that enhancing mixing and dispersion can promote the nucleation of aluminum hydroxide and thus accelerate the reaction crystallization rate of NiAl-LDH,finally reducing the lateral dimension and thickness of NiAl-LDH as well as improving the particle dispersion of NiAl-LDH,which ultimately improve the specific surface area of the catalyst,reduce the particle size of nickel,and improve the dispersion of catalyst particles.（3）The effects of nickel content and the thickness of NiAl-LDH on catalyst properties and hydrogenation performance were studied.It was found that Ni-Al₂O₃ catalysts with higher nickel contents possess lower specific surface area,larger nickel particle size and higher reduction degree.With increasing the nickel contents,the specific surface area of active metal first increases and then decreases.The Ni-Al₂O₃ catalyst with 82%Ni has higher specific surface area of active metal.The hydrogenation performance of MA showed that the catalyst with 82%Ni content possesses higher MA conversion and hydrogenation rate.With increasing the thickness of NiAl-LDH,the specific surface area of NiAl-LDH decreases,resulting in the decrease in catalyst specific surface area.Increasing NiAl-LDH thickness leads to the increase in nickel particle size,the decrease in nickel dispersion,and the decrease in specific surface area of active metal of Ni-Al₂O₃ catalyst.Meanwhile,higher thickness of NiAl-LDH leads to the increase in metal support interaction in the catalyst,and thus the decrease in catalyst reduction degree,resulting in the decrease of MA hydrogenation performance.（4）A new process for preparing ultra-thin Ni-Al₂O₃ catalyst derived from NiAl-LDH with high nickel content by RPB was developed.By optimizing the process parameters（aging time,aging temperature,organic solvent treatment and rotation speed）,the optimal conditions were as follows:ethanol treatment,rotation speed of 1000 rpm,reaction/aging temperature of 60℃,aging time of 0.5 h.Under the optimal conditions,the thickness of NiAl-LDH is about 2 nm;the specific surface area is 328 m²/g;the nickel particle size is～7.0 nm;the nickel content is 79.6%;the hydrogen adsorption capacity is 7.8times that of the industrial catalyst（6.87 vs 0.88 cm³/g）.Compared with the catalyst synthesized by conventional stirred reactor,the particle size of Ni-Al₂O₃ synthesized by RPB is smaller（7.0 vs 8.9 nm）.The morphology and nickel particle size have no obvious change after 10 times magnification.The MA hydrogenation performance show that the ultrathin Ni-Al₂O₃ catalyst achieves the～100%conversion of MA and～100%succinic anhydride selectivity at room temperature,which reveals the broad prospect of Ni-Al₂O₃ catalyst prepared by RPB in hydrogenation field.