Studies in the Catalytic Synthesis of Pyridines

Abstract

A study has been made of the catalytic synthesis of pyridines from tetrahydrofurfuryl alcohol and ammonia over metal catalysts supported on alumina. Product analysis, synthesis end study of possible intermediates and a kinetic investigation of the reaction have been carried out. The reaction products have been shown to consist of pyridine, piperidine, 2,3~dihydro-—4H-pyran, x-tripiperideine, 3—picoline, 3-ethylpyridine and higher boiling pyridines. Reactions of 2,3-dihydro-4H-pyran, 2-methylenetetrahydrofuran, prolinol and tetrahydrofurfurylamine have been studied in order to establish the sequence of the dehydration and ammonolysis reactions. The results of these studies indicate that tetrahydropyridine is produced by a surface reaction between chemisorbed tetrahydrofurfuryl alcohol and ammonia molecules. The kinetic study of the reaction was carried out using a palladium on alumina catalyst in the temperature range 235 - 350°. A mechanistic scheme has been worked out and mathematical equations which adequately predict reaction behaviour as a function of temperature, contact time and process time have been deduced. Tetrahydropyridine is formed as an intermediate (yields of up to 40 % were obtained) and it is the dehydrogenation of this compound that is the slow stage of the reaction. The exclusive formation of 3-substituted pyridines in the reaction is explained by alkylation of the intermediate 1.4.5.6-tetrahydropyridine. Catalyst studies have shown that an acidic catalyst was necessary for the dehydration and ring expansion reaction and that inclusion of transitional metals improved the yield of the heterocyclic bases which were produced. It has been established that inclusion of oxygen with the feed to the catalytic converter improved the conversion to pure pyridine. New details about the mechanism of the complex reactions that occur at the catalyst surface have been revealed by this investigation. This has been accomplished by a kinetic analysis using mathematical equations which describe and predict the reactions which take place during the conversion of tetrahydrofurfuryl alcohol and ammonia to pyridine over an alumina catalyst. This information has been used to suggest modifications of the reaction conditions and catalysts to improve the yield of pure pyridine.