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The cluster approach was used to simulate the chemisorption of dialkylperidines on the surface of a vanadium oxide catalyst, involving active centers of various nature, and to assess the enthalpy of proton abstraction from the alkyl substituents. Under oxidative ammonolysis conditions, the order of the transformation of the substituents in unsymmetrical dialkylpyridines into the cyano group is determined by the enthalpy of their deprotonation.Oxidative ammonolysis of alkyl derivatives of benzene and pyridine is known as the most rational method of synthesis of nitriles of aromatic and pyridinecarboxylic acids, which are valuable intermediate products in the production of medicinals, polymers, and plant protectants [1].Kinetic studies and semiempirical calculations of the properties of alkylbenzenes and monomethylpyridines in the gas phase and in conditions simulating their chemisorption on the surface of an oxide catalyst made it possible to rank the compounds by reactivity and to find out that the rate constants of the transformation of alkyl groups into the cyano group correlate with the enthalpy of proton abstraction from the a-carbon atoms of the substituents [234]. On this basis, it can be suggested that initial reaction stages involve heterolytic C3H bond fission in alkyl groups. A similar mechanism has been suggested for propylene and lower paraffins [5].As judged from published data for oxidative ammonolysis of unsymmetrical dialkylpyridines on oxide catalysts, the 2-and 4-alkyl groups are more reactive than those in the 3 and 5 positions. For this reason, for example, the primary product of the transformation of 2,3-lutidine on a V3Sn3Fe oxide catalyst is 3-methylpyridine-2-carbonitrile. The 3-Me group reacts later, when the 2-CN group formed undergoes elimination. As a result, pyridine-2,3-dicarbonitrile is lacking among the reaction products, and the major product is nicotinonitrile [6]. The oxidative ammonolysis of 2,5-lutidine and 2-methyl-5-ethylpyridine on the same catalyst initially involves the 2-Me groups to form 5-methyl-and 5-ethylpicolinonitriles. Under the conditions used, transformations of the 5-alkyl groups in the subsequent stage were not attended with elimination of the 2-CN group; as a result, both 2,5-lutidine and 2-methyl-5-ethylpyridine gave up to 73% of pyridine-2,5-dicarbonitrile [7]. A kinetic study of the oxidative ammonolysis of 2-methyl-5-ethylpyridine on a V3Ti oxide catalyst showed that the ethyl substituent is much less reactive: Neither 2-methyl-5-vinylpyridine nor 2-methylpyridine-5-carbonitrile were found among the reaction products [8]. Compelling evidence for the reactivity effect of the position of the substituent in the pyridine ring was provided by the results of the oxidative ammonolysis of 2,3-, 2,5-, and 3,4-lutidines on a Cr 2 O 3 (5%)/g-Al 2 O 3 catalyst: At a low conversion (~25%) of the starting compounds, 3-methylpyridine-2-carbonitrile, 5-methylpyridine-2-carbonitrile, and 3-methylpyridine-4-carbonitrile, respectively, were obtained with a selectivity of hi...
Catalytic properties of vanadium-titanium-tin oxides catalysts of various compositions were studied in oxidative ammonolysis of 3-and 4-methylpyridines.Heterogeneous catalytic oxidation by atmospheric oxygen is the main way for oxidative processing of petrochemical and coking hydrocarbon raw materials into oxygen-containing organic compounds. The reaction of oxidative ammonolysis is a catalytic reaction of hydrocarbons with atmospheric oxygen and ammonia belong to promising industrial methods for synthesis of valuable nitrogen-containing organic half-products and monomers [1].The reactions of oxidation and oxidative ammonolysis of alkyl aromatic compounds are purposively studied at Bekturov Institute of Chemical Sciences Joint-Stock Company. Particular attention is being given to development of catalysts determining the degree and selectivity of conversion of raw materials into appropriate target products. For example, the catalytic activity of a number of multicomponent vanadium oxide catalysts modifi ed with oxides of Group IV and VI metals in oxidative ammonolysis of 3-and 4-methylpyridines has been studied. Catalysts developed for this process on the basis of vanadium, tin, and titanium oxides are highly effi cient: the oxidative ammonolysis of methylpyridines occurs with high conversion of starting substances [2]. The optimal conditions, in which target products can be obtained in >90% yield, have been determined for synthesis of pyridinecarboxylic acid nitriles. It has been found that, to achieve a high selectivity of nitrile formation, the process of oxidative ammonolysis on V-Ti-Sn-O oxides catalysts should be performed with 5-10 mol of ammonia per mole of starting substances, which raises the ammonia concentration in effl uent gases and makes the process more technologically intricate. The excess of ammonia and high temperature occasionally lead to a spontaneous rise in temperature and enhance oxidative destruction processes, which diminishes the formation selectivity of the target products. In addition, these catalysts operate at high temperatures (360-400°C), the conversion of the starting substances being low at moderate temperatures. Therefore, a search for new active and selective catalysts capable of operating at lower temperatures at a minimum amount of ammonia was conducted.It is known [3] that ammonia can be oxidized by several pathways, and the nature of a catalyst and the process conditions (temperature, composition of the ammonia-air mixture, contact duration, and other factors) affect the degree and selectivity of ammonia conversion. Previously, the composition of ammonia oxidation products formed on vanadium oxide catalysts modifi ed with titanium and tin oxides have been studied and it was found that ammonia is oxidized on the catalysts under study to give N 2 , N 2 O, and NO 2 . It was shown that, selecting the molar ratio between oxygen
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