Abstract

The purpose of this study was to characterize the catalytic activity of rhenium trioxide and rhenium trichloride as hydrogenation catalysts. Work was also undertaken to further study the catalytic properties of rhenium heptoxide. The activity of this latter compound had received some study in a previous project. A complete review of the literature has been made on the organic and inorganic chemistry of rhenium trioxide and rhenium trichloride. A survey of the literature on catalytic hydrogenation and the chemistry of rhenium heptoxide has also been included. Rhenium trioxide was prepared by three similar methods. The first two involved the formation of a complex between rhenium heptoxide and anhydrous dioxane. The complex was isolated and subsequently decomposed by gentle heating to give the pure trioxide. The third method employed tetrahydropyran as a complexing agent. The complex was isolated and decomposed in the same manner as previously indicated. All three methods appeared to give the trioxide in high purity, and little difference was apparent in their catalytic activities. Rhenium heptoxide and rhenium trichloride were commercially available, and thus required no special preparation. Both were generally reduced in situ to the active catalyst. In three instances rhenium heptoxicid was reduced ex situ. It was found that the in situ derived catalysts were generally more active than those derived ex situ. All reductions were carried out in a high pressure hydrogenation vessel. The reduction products were analyzed in a gas chromatograph, by refractive indicies, distillation and/or chemical extraction. Catalysts were analyzed by dissolving in concentrated nitric acid or a 30% hydrogen peroxide and ammonia solution. The resulting perrhenate was precipitated from the solution with tetraphenylarsonium chloride. Generally the analytical data obtained was not of sufficient accuracy to determine the exact chemical structure of the catalyst. The activities of the catalysts were determined by performing a large variety of hydrogenations. Results indicated that the heptoxide and trioxide derived catalysts were generally very similar in their catalytic activities. The trichloride proved to be slightly lower in its activity than the oxides. The catalysts used in this study generally required slightly more drastic conditions for the reduction of a carbonyl group than was generally necessary for a rhenium derived catalyst. The reduction of cyclohexanone was catalyzed at 123° with rhenium tioxide, while the heptoxide and trichloride required temperatures of ca. 150°. The olefinic compounds such as 1-hexene were reduced at temperatures of 95-100°, while styrene required slightly higher conditions due to its conjugation with the benzene ring. Nitro compounds and benzenoid compounds were found the most difficult to reduce with the trioxide and trichloride derived catalysts. Nitrobenzene yielded aniline only under temperatures of 226-275° depending on the catalyst used. Benzene yielded slight reduction with the trichloride catalyst at 200°. The trioxide, heptoxide and trichloride catalysts were outstanding in their ability to reduce the carboxylic acids. In most cases reduction could be effected 150-160°. The catalysts were superior to any reported in the literature for the reduction of the cargoxyl group. Most rhenium catalysts exhibit this high activity toward the carboxyl group to some extent. The oxide catalysts were also found to possess an extremely high activity toward the hydrogenation of amides and anilides. The reduction generally took place at ca. 225° giving a good yield of the primary amine in most cases tried. These catalysts compared very favorably with the better catalysts reported in the literature.

Degree

College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry

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