The preparation and catalytic properties of rhenium blacks obtained by reduction of Re(VII) in anhydrous ammonia and amines with alkali metals


The purpose of the investigation was to determine the catalytic activity of rhenium blacks obtained by reduction of rhenium salts in alkali metal-ammonia or amine systems. A review is presented on the chemical, physical and catalytic properties of rhenium, and on the rhenide ion, on the nature of the alkali metal-liquid ammonia system, and the reduction of metallic salts therein. The first reduction system studied was that obtained by dissolving sodium in liquid ammonia. The salts reduced in this system were KReO_4, Re_2O_7 and NH_4ReO_4. The reduction of KReO_4 was unsuccessful, and the heptoxide was difficult to handle. The product obtained by the reduction of NH_4ReO_4 was the best characterized. The lithium-ammonia system was also used to reduce NH_4ReO_4. One amine-alkali metal system was studied extensively. Lithium was used as the reducing agent and ethylamine as the solvent on NH_4ReO_4. A standard method of preparation was developed for each catalytst. It was found that the order of addition of the salt and alkali metal, the reaction ratio employed and the amount of solvent used apparently had no effect on the activity of the catalyst. It was found, however, that unless the alkali metal amide formed during the reduction process was removed completely, the catalysts were almost entirely inactive. For this purpose an acid extraction was employed. A spectrophotometric method, based upon the strong absorption of the hexachlororhenate ion at a wave length of 281 mμ, as well as the well recognized gravimetric method, based upon the insolubility of tetraphenylarsonium chloride, were used for the analysis of the catalysts. The spectrophotometric method was shown to be suitable for obtaining an approximate analysis, but lacked the precision necessary for an ultimate analysis. To determine the exact rhenium content of the material, the gravimetric method was therefore used. The valence state of the rhenium in the catalyst material was determined by oxidizing the catalyst in acidic potassium dichromate and then back titrating the excess oxidant. The analytical data indicated that all the catalysts were identical in composition; apparently a definite compound (ReO•2H_2O). The activities of the sodium-ammonia catalysts were evaluated by 69 reductions of organic substrates. The lithium-ammonia catalyst's activity was demonstrated in 24 hydrogenations, while the lithium-ethylamine catalyst was similarly used in a total of 55 reductions. Several catalysts prepared in miscellaneous systems were also evaluated. In all, a total of 161 hydrogenations were performed. The analysis of the reduction products was performed by means of gas chromatography except in the case of simple substrates used without solvent, for which refractive indices were used. In general the activity of the lithium-ethylamine catalyst was found to be somewhat superior to that of the catalysts prepared in ammonia. This was definitely the case in the reduction of the carbonyl group as contained in such substrates as butanone, acetone, and cyclohexanone. Acetone was reduced by the Li-EtNH_2 catalyst at about 55°C., butanone required 90°C., while cyclohexanone required a temperature of ca. 120°C. In contrast the Na-NH_3 catalyst required a temperature of 78°C. to reduce acetone, 130°C. to reduce butanone, and 160-170°c. to reduce cyclohexanone. In the case of the olefinic bond, except when conjugated to the benzenoid structure, the Li-NH_3 catalyst was the most effective. This castalyst reduced hexene-1 at about 100°C. and cyclohexene at 105°C. Styrene on the other hand was reduced by the Li-EtNH_2 catalyst in one experiment at room temperature while the Li-NH_3 catalyst required 70°C. and the Na-NH_3 catalyst 100°C. The Li-EtNH_2 and Li-NH_3 catalysts were about equally effective in the reduction of nitrobenzene. The temperature required was about 110°C. The Na-NH_3 catalysts in contrast required temperatures of about 175°C. The Na-NH_3 catalyst was used in an evaluation of the ease of reduction of a series of ketones. A very interesting relationship evolved from this study. It was observed that in all but two cases, the ketones containing an even number of carbon atoms in the principal chain reduced under significantly milder conditions than their odd numbered homologs. A number of hydrogenations were attempted on compounds containing a nitro group and another easily reducible group. In all cases, no reduction of the second group occurred until the nitro group was hydrogenated, inspite of the fact that the conditions for the reduction of the nitro group alone were more drastic than for the reduction of any of the other functions. This phenomenon has been observed in previous investigations in which selectivity of reduction, with a rhenium catalyst, involving the nitro group was attempted. A number of compounds containing both the carbonyl group and olefinic bond, i.e., allylacetone, 2-allylcyclohexanone, were hydrogenated in attempt to obtain selective reduction. The results clearly indicated that the rhenium catalysts reduce the olefinic bond preferentially to the carbonyl group. This was also demonstrated to be the case with the "Standard" catalysts. However, in one experiment in which a partial poisoning of the catalyst from pyridine occurred, the product obtained was 5-hexene-2-ol, indicating a reversal of ease of reduction of the two groups in this case. In the case of crotonaldehyde the carbonyl group was apparently more easily reduced than the olefinic bond by both the rhenium and "Standard" catalysts. On one occasion, however, the Li-EtNH_2 catalyst produced crotyl alcohol. The greatest worth of the catalyts of this investigation was undoubtly their ability to catalyze the reduction of carboxylic acids. The catalysts prepared in all three systems seemed to be equally active in this respect. It was observed in this connection that when the reduction was run without solvent, the product was a mixture of both the alcohol and ester. However, if water was used as a solvent, the reduction yielded in most cases only the alcohol. The Li-EtNH_2 catalyst was used in the reduction of a series of carboxylic acids. Included in the series were acetic, propionic, isobutyric, valeric, caproic, caprylic, capric, and lauric. All were reduced in the temperature range of 160-180°C., except acetic which was reduced at 145°C. This catalyst, therefore, is as effective in this reaction as any rhenium catalyst heretofore studied and much superior to any other catalyst reported in the literature.



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Chemistry and Biochemistry



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