A Study Of Amination Of Alcohols To Amines Over Supported Nickel Catalysts

Amines and their derivatives are important chemical raw materials, which are widely used in pesticides, pharmaceuticals, detergents, dyes and other fields. They are mainly synthesized in industry with two methods, the reduction amination and ammonolysis amination. The method of amination of alcohols is more selective, safer and less contaminative, and thus it becomes a research hotspot in the amine industry. Two types of catalysts are usually used for the amination of alcohols, i. e., the dehydration and dehydrogenation/hydrogenation ones. The latter type possesses the advantages for the reactions with mild conditions and high yields, among which metallic nickel usually exhibits the high activity and low cost.In this work, the highly active and hydrothermally stable catalyst 60%Ni/LaAlSiO was applied as a basis for the amination of n-propanol to n-propylamine, in comparison with the catalysts 60%Ni/AlSiO and 60%Ni/LaO. In addition, small amounts of copper were added into the 60%Ni/LaAlSiO to see the effects of copper on the amiantion of isopropanol to isopropylamine.The strengths of surface bonds and surface states of adsorbed reactants and products on the catalysts were determined by the techniques of microcalorimetric adsorption and infrared spectroscopy. The results were correlated with the catalytic behavior for the better understanding of mechanisms of related reactions.The main results obtained are summarized as follows: (1) The catalyst Ni/LaAlSiO possessed the high active site density and strong surface basicity, and thus the high activity for the dehydrogenation of n-propanol to propylaldehyde, favoring the activity for the amination of n-propanol to n-propylamine. Although the catalyst Ni/LaO had the strong surface basicity, its active site density was low, leading to the low activity for the amination of n-propanol. In contrast, the catalyst Ni/AlSiO possessed the high active site density, but the low surface basicity. Thus, the activity of amination of n-propanol on the Ni/AlSiO was not high enough. In addition, the experimental results showed that the heats for the adsorption of n-propanol, H2, NH3 and n-propylamine were decreased with the increase of surface basicity. Apparently, the decrease of heat for the adsorption of n-propylamine favored desorption of n-propylamine and led to the higher selectivity to n-propylamine. In fact, the results of infrared spectroscopy showed that the surface basicity inhibited the dissociative adsorption of n-propylamine on Ni. On the other hand, the low active site density (for example on the Ni/LaO) resulted in the incomplete hydrogenation of n-propylimine (a product from the condensed reaction between propylaldehyde and ammonia) that lowered the selectivity to n-propylamine. On the surface of Ni/AlSiO with relatively stronger acidity, the dissociatively adsorbed n-propylamine was expected to be easier to condense with propylaldehyde (formed from the dehydrogenation of n-propanol) for the formation of di-and tri-propylamines, leading to the decreased selectivity to n-propylamine.(2) Addition of small amounts of Cu (2-5%) into the Ni/LaAlSiO promoted the reduction of nickel, but increased the particle sizes of nickel, leading to the decrease of active surface area of nickel and thus the activity of the catalyst. In addition, the presence of Cu increased the heat for the adsorption of isopropanol and isopropylamine while decreased the heats for the adsorption of H2. Apparently, the increase of heat for the adsorption of isopropylamine adversed desorption of isopropylamine and favored the formation of diisopropylamine. Meanwhile, the decrease of heat for the adsorption of H2 might mean the decrease of hydrogenation activity of the catalyst, leading to the increase of selectivity to isopropylimine and the decrease of selectivity to isopropylamine.