Preparation And Applications Of Highly Active And Hydrothermally Stable Nickel Catalysts

Water is cheap, safe and widely available and may be used as an environmentally benign solvent. However, the hydrothermal stability of common supported catalysts was poor and catalytic activities might be significantly decreased in water. In order to be more environmentally benign, it is a trend to use water as a green solvent, instead of the traditional organic solvents. In fact, using water as a solvent in hydrogenation reactions develops fast in recent years, and some industrial processes have been developed. In addition, some hydrogenation reactions generate water as a by-product. Thus, it is highly desirable to develop the highly active and hydrothermally stable hydrogenation catalysts.Nickel is abundant and cheap. It usually exhibits the high activity for various hydrogenation reactions. Raney Ni and Ni/Al2O3 catalysts are widely used in industry; especially the latter are often used in the fixed bed reactors. However, the Ni/Al2O3 catalysts are not highly hydrothermally stable, which deactivated easily in water.Based on the highly active Ni/Al2O3 catalysts prepared previously in our group, SiO2 was added by the co-precipitation method in this work for the preparation of 60%Ni/AlSiO catalysts. The effect of SiO2 loading on the activity and hydrothermal stability was investigated. Excellent results were achieved by optimizing the ratio of SiO2/Al2O3. In addition, based on the optimized catalysts Ni/AlSiO, LaO (La2O3 or La(OH)3) was added by the sequential precipitation method for the preparation of 60%Ni/LaAlSiO catalysts. The effect of LaO loading on the activity and hydrothermal stability was investigated. After the careful optimization, the catalysts Ni/LaAlSiO were prepared, which exhibited even better activity and hydrothermal stability than the Ni/AlSiO catalysts.Techniques of microcalorimetric adsorption and infrared spectroscopy were used to study the bonding strengths and adsorption structures of reactants and products on the catalyst surfaces, and were correlated with the reactivities of catalysts. In this way, the micro-mechanisms of related catalytic reactions could be deeply understood.The main results obtained in this work are summarized below.(1) The 60%Ni/AlSiO catalysts were prepared by the co-precipitation method, in which AlSiO were the composite supports with different ratios of Al2O3/SiO2. It was found that the hydrothermal stability of the Ni/Al2O3 and Ni/SiO2 was poor, but the catalyst 60%Ni/AlSiO-4 with the Al2O3/SiO2 mass ratio of 4 in the support exhibited the high hydrothermal stability. The addition of proper amount of SiO2 effectively retarded the hydration of Al2O3, and prevented the growth of supported Ni particles. The surface area and active Ni surface area of the catalyst remained high (315 and 60 m2/g, respectively) after the hydrothermal treatment at 363 K for 8 h, and it maintained the high activity and stability for the hydrogenation of glucose in aqueous solution.(2) The highly active 60%Ni/LaAlSiO (NiLaO/NiAlSiO) catalysts were prepared by the sequential precipitation. The addition of La increased the surface basicity, the dispersion of supported Ni and the hydrothermal stability of the catalysts. The surface area and active Ni surface area of the catalysts remained high (e. g.,243 and 63 m2/g, respectively) after the hydrothermal treatment at 423 K for 8 h. The hydrothermally treated catalysts maintained the high activity and stability for the hydrodeoxygenation of N,N-dimethylformamide (DMF) to trimethylamine (TMA). The FTIR results showed that the DMF and TMA might be dissociatively adsorbed on Ni supported on the acidic and basic surfaces, respectively, leading to the decreased selectivity to TMA. The 60%Ni/LaAlSiO catalysts with the proper acidity and basicity could be prepared by adjusting the ratio of NiLaO and NiAlSiO, which exhibited the high activity and selectivity for the hydrodeoxygenation of DMF to TMA.(3) The Ni/AlSiO, Ni/LaAlSiO and Ni/LaO catalysts were compared for the amination of isopropanol with ammonia. It was found that the isopropanol and isopropylamine were strongly adsorbed on the Ni/AlSiO (with the initial heats of 155 and 125 kJ/mol, respectively). The addition of La decreased the surface acidity and increased the surface basicity of the catalysts, leading to the decreased heats for the adsorption of isopropanol and isopropylamine on Ni (with the initial heats of 148 and 113 kJ/mol, respectively). The initial heats for the adsorption of isopropanol and isopropylamine on Ni/LaO were low (98 and 56 kJ/mol, respectively), indicating that the adsorption heats of isopropanol on Ni was an important factor for the activity of amination of isopropanol (unfavorable if the heats were too high or too low). On the other hand, the decrease of heats for the adsorption of isopropylamine was favorable for desorption of isopropylamine, leading to the higher selectivity of isopropylamine. Under the optimized conditions, both the conversion of isopropanol and selectivity to isopropylamine could be higher than 96% over the Ni/LaAlSiO catalysts.(4) The Ni/AlSiO, Ni/LaAlSiO and Ni/LaO catalysts were also compared for the condensation of isopropanol or acetone with isopropylamine to produce diisopropylamine. It was found that the condensation reaction between acetone and isopropylamine was extremely fast, indicating that isopropanol might be first dehydrogenated to acetone, which was then condensed with isopropylamine during the reaction of isopropanol with isopropylamine. Both the conversion of isopropanol and selectivity to diisopropylamine for the reaction of isopropanol with isopropylamine on the catalysts Ni/AlSiO were slightly higher than that on Ni/LaAlSiO, indicating that the surface acidity of catalysts on the reaction could improve the conversion and selectivity. With the excess of isopropylamine, both the conversion of isopropanol and selectivity to diisopropylamine could be higher than 96% over the catalyst Ni/LaAlSiO.