Abstract

Rhenium oxides as well as other rhenium compounds are known to display a broad spectrum of catalytic activity,^25 being resistant to attack by acids under nonoxidizing conditions, and showing a remarkable ability to resist poisoning which is a serious limiting factor in many commonly used catalysts. Rhenium catalysts are cheaper than the platinum metals with the exception of palladium. One of the most remarkable properties of the rhenium catalyst is the ability to catalyze the hydrogenation of carboxylic acids and amides to the corresponding alcohols and amines respectively. The carboxyl group (-COOH) is notoriously difficult to reduce catalytically and most of the commonly used catalysts are ineffective in catalyzing the hydrogenation of carboxylic acids. Either hydrogenolysis of the carbon-oxygen bond occurs or the carboxylic acid remains inert. It is for this reason that low molecular weight carboxylic acids are often used as inert solvents in catalytic hydrogenations. This investigation was undertaken to study the effects of various parameters upon product formation in the hydrogenation of carboxylic acids using the catalyst formed from the reduction of rhenium heptoxide i situ. Acetic acid was used throughout the investigation as the representative carboxylic acid. The parameters studied were pressures ranging from 2000 to 3000 psig, temperatures ranging from 115° to 175°, reaction time varied over the range of 0 to 24 hours, the effects on product formation from repeated use of the catalyst, and agitation of reactants during the reaction with emphasis on the optimization of these parameters. In several series of reactions repeated reuse of the catalyst showed that rhenium catalyst can be reused at least 8 times while still achieving greater than 50 per cent reduction. Relative decrease in catalyst activity appears to be greater when temperature and pressure of the hydrogenation are high compared to the range. Experimental data shows that the concentration of ethyl acetate in the product mixture as a function of time passes through a maximum between 1.0 and 1.5 hours. Beyond this maximum there is a relatively rapid decrease in ester concentration. A change in initial hydrogen pressure from 2000 to 3000 psig does not appreciably change the product composition in the reaction mixture while a modest increase, i.e., a temperature increase of 25°, was observed to improve yields of the alcohol by a very significant amount. Moreover, the investigation shows that for a reaction time of 5 hours the "optimum" conditions for reaction are a temperature of 175°, a catalyst to substrate ratio of 1.0 g Re_2O_7/50 g AcOH, and an initial hydrogen pressure of 3000 psig while reactions carried out for only 1.5 hours require a catalyst to substrate ratio of from 1.0 to 2.5 g Re_2O_7/50 g AcOH to effect a quantitative reduction optimally. Whether or not the reaction system was agitated during the warm-up period seemed to have little effect on the composition of the product mixture, however, it was observed that partial reduction takes place during the initial heating period before agitation of the reactor begins. The results of this work have shown that "optimum" conditions for quantitative reduction of acetic acid to the corresponding alcohol are the mildest yet reported and clearly confirm the superiority of the rhenium catalyst over other catalysts in the hydrogenation of carboxylic acids.

Degree

College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry

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