RATE AND SELECTIVITY OF HYDROGENATION OF 2-NITRO-2'-HYDROXY-5'-METHYLAZOBENZE OVER NICKEL CATALYST IN AQUEOUS SOLUTIONS OF ALIPHATIC ALCOHOLS

Abstract

Elucidation of the causes and character of the composition and nature solvent influence on the kinetic regularities and selectivity of hydrogenation reactions of compounds containing several reactive groups is of theoretical and practical significance. In the present work, the process of 2-nitro-2'-hydroxy-5'-methylazobenzene catalytic hydrogenation in the liquid phase is considered in order to obtain the corresponding benzotriazole, an effective UV stabilizer of polymeric materials. The process was carried out on a skeletal nickel catalyst in aqueous solutions with different contents of aliphatic alcohols, both at atmospheric and elevated hydrogen pressures. It has been experimentally established that the shape of the hydrogenation kinetic curves is determined by the composition of the water-alcohol solvent and is transformed from the dependence with a maximum in media with low alcohol content to the stepped curve in concentrated solutions. The dependences of the reaction rate on the content of aliphatic alcohol are complex, and the maximum hydrogenation rates are observed in solvents with a content 0.6÷0.7 ppm. of alcohol. The change in the process rate is associated with a change in the solubility, value of adsorption and reactivity of the starting compound and hydrogen. These factors also determine the selectivity of the process due to their influence on the rates of individual steps in the 2-nitro-2'-hydroxy-5'-methylazobenzene conversion scheme, in particular, on the cyclization rate of the associative surface complex "nitroazobenzene∙H2". The cyclization product of the complex is an intermediate compound, benzotriazole N-oxide, the reduction of which leads to the formation of a substituted benzotriazole. It is shown that the maximum yield of the target product is 48÷3% at atmospheric hydrogen pressure, and increases to 73.5-0.5% at PН2 = 1 MPa.