CATALYTIC OXIDATION OF BENZENE TO MALEIC ACID WEIS & DOWNS, NEW YORK, N. Y. The process comprises the partial oxidation of benzene, in the vapor phase, by air, using vanadium oxide as a catalyst. It has been carried through the laboratory, experimental plant, and pilot plant stages and up until the time the inventors severed their connection with the interests to which the patent rights had been assigned, about 50,000 pounds of maleic acid had been produced and disposed of in the form of one or another of its derivatives. I t is interesting to consider in this connection that prior to the development of this process maleic acid was merely a laboratory curiosity. It was derived by tedious processes from certain plant or fruit juices and, if sold a t all, commanded prices approximating $50.00 per pound. Although maleic acid and its stereoisomer, fumaric acid, to which it is readily converted, have since the day of Kekule been objects of interest to research workers in pure chemistry owing to their stereoisomeric relations, yet it is doubtful whether all laboratory workers together ever had collectively as much as 10 pounds of maleic acid up to the time of the perfection of this new synthesis. The main chemical reaction takmg place in this synthesis may be graphically represented as follows:
There are also certain side reactions resulting in the formation of formaldehyde, benzoquinone, carbon monoxide and excess carbon dioxide and water. The reactions involved are highly exothermic; that repre-
N BENZENE TO MALBICACID VOL. 2, NO. 12 CATALYTIC O X ~ A T I O08
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sented by the preceding equation liberating 10,560 B. t. u. per pound of . benzene converted. Carbon dioxide and water over and above that shown in this equation are formed by the complete combustion of the benzene. When such complete combustion takes place there are 18,100 B. t. u. liberated per pound of benzene reacting. The instant removal of the heat of reaction is absolutely essential to obtain economical yields of maleic acid; otherwise complete combustion results and this reaction is not reversible. For this purpose even laboratory apparatus must be very carefully designed. The yields of maleic acid obtained in the ordinary hot tube apparatus used in high temperature synthesis in the past are very small. This difficulty is naturally magnified many fold when an attempt is made to transfer such a reaction from the laboratory-size reaction chamber producing a few grams of an organic product per day to a commercial scale where hundreds of pounds must be made. Without such commercial equipment, compounds like maleic acid would still be classed as rare chemicals. This difficulty was solved by the use of boiling mercury to maintain a temperature uniform over extended periods of time as well as uniform at all points throughout a reaction zone. With this equipment, any increase in the violence of the reaction liberating heat immediately results in boiling more mercury without change in temperature and the desired temperature is governed by the pressure arbitrarily applied t o the boiling mercury. Practical yields of maleic acid amounting to 75 parts of acid per 100 parts of benzene fed to the oxidizer0havebeen obtained and there is little doubt in the minds of the inventors that these yields are still capable of considerable improvement. Maleic acid is a white crystalline solid melting at 130.5'C. and decomposing on distillation into maleic anhydride and water. It is soluble in water to the extent of 79 parts per 100 parts of water a t 25'C. and is relatively insoluble in benzene and carbon tetrachloride. So far no uses have been developed for the acid per se except as a raw material for the synthesis of other products. It is extremely corrosive to metals but certain cheap alloys have been found to be satisfactorily resistant. Maleic anhydride obtained from the acid by distillation under suitable conditions is a white solid melting a t 52.6'C. and boiling at about 200°C. I t is soluble in organic solvents while with water it is hydrated to form maleic acid. By warming a solution of maleic acid with the addition of a small amount of a mineral acid it is transformed into its stereoisomer, fumaric acid, which separates, due to its sparing solubility. (0.7 part per 100 parts of water a t 25OC.) When hydrogenated, maleic and fnmaric acids are changed to succinic acid. This may be easily accomplished by catalytic reduction using hydrogen and nickel. By various other reactions it is possible to produce a
large number of the straight chain organic acids such as tartaric, propionic, asparic, lactic, acrylic, etc. By condensation reactions, tbiophene and pynol are readily obtainable. A whole series of new raw materials become commercial possibilities and the various possible ramifications are quite large.
