Estudio de sistemas níquel-magnesia. Aplicación a la hidrogenación de dinitrilos

Abstract

Hydrogenation of nitriles is one of the most widely used methods to obtain amines commercially. One interesting industrial process is the hydrogenation of 1,6-hexanedinitrile (adiponitrile) to obtain 1,6-hexanediamine which is used as a precursor in the preparation of Nylon-6,6, and also to obtain 6-aminohexanenitrile, which is used in the preparation of caprolactam (precursor to Nylon-6). The industrial interest of these processes is clearly confirmed by the great number of registered world patents in the last years, all supported by chemical industries such as BASF, Rhodia, Du Pont, DSM, etc. The selectivity in the hydrogenation of dinitriles seems to be related to the morphology and basicity of the catalysts. The aim of this work is the study of new catalytic systems of nickel-magnesia with octahedral morphology and moderate basicity that allow us to obtain selectively from dinitriles: monoamine and/or diamine, decreasing the formation of condensation products and graphitic residual which are responsible for catalyst deactivation. In order to carry out this work the following partial objectives have been developed. 1.- Synthesis and characterization of NiO-MgO samples with homogeneous particles of octahedral morphology, from the decomposition of nickel nitrate hexahidrate and using different sources of magnesia with the purpose of controlling the basicity of the final systems. To obtain NiO-MgO with the desired particle size and morphology, different preparative paths have been designed. Also, the factors that affect the structure and the surface properties as well as the reducibility of the NiO-MgO systems, have been studied. 2.- Preparation and characterization of nickel-magnesia catalysts. Studies of the catalytic activity for the dinitrile hydrogenation in the gas phase at atmospheric pressure. Comparison with bulk nickel catalysts. Tests of catalytic activity for a model dinitrile as the 1,4-butanodinitrile (succinonitrile) have been carried out. The best catalytic systems have been later used for the hydrogenation of 1,6-hexanedinitrile (adiponitrile) which is a dinitrile with more industrial interest. These activity results were compared with those obtained for an usual industrial hydrogenation catalyst, Raney-Ni. The Ni-MgO systems, bulk Ni and Raney-Ni catalysts have been subjected to a study of their evolution during a long time of reaction in order to correlate the activity and lifetime of each catalyst with the structural modifications observed after reaction. The preparation and characterization of the catalytic precursors and their corresponding catalysts, as well as their application to the dinitriles hydrogenation allow us to make the following conclusions: ? The controlled decomposition of nickel nitrate hexahidrate to obtain Ni3(NO3)2(OH)4 as a single phase is a way to prepare, after calcination, NiO-MgO systems with homogeneous particle size and octahedral morphology. ? It is possible to synthesize NiO/MgO systems with different surface characteristics and also with different interaction degrees between the nickel oxide and magnesia phases. ? The formation of a NiO-MgO solid solution leads to a strong interaction between the oxides. When this interaction increases, the reduction of nickel oxide decreases. However, during the reduction process of this solid solution phase there is also a lower sintering effect and, therefore, higher metallic areas and similar particle sizes that those obtained by using a Raney-Ni commercial catalyst, are obtained. ? Octahedral morphology seems to be favoured when there is a lower interaction between the NiO-MgO phases. ? It is possible to obtain selectively 6-aminohexanenitrile (80%) and 1,6-hexanediamine (100%) by using Ni-MgO catalysts. Ni-MgO catalysts allow us to obtain selectively and during a long period of time the product of more interest, 6-aminohexanonitrile. This high selectivity towards 6-aminohexanenitrile is related to the presence of octahedral morphology. However, a key point will be to choose the optimum reaction conditions in order to obtain the desired products, maintaining the maximum surface and metallic area values.