JEROME HOLLANDER1 and LEONARD SPIALTER Chemistry Research Branch, Aeronautical Research Laboratory, Wright-Patterson AFB, Ohio
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great abundance of carbon dioxide in the atmosphere and its ready availability from a vast variety of materials on the earth should make it of great interest as an organic raw material. It is most commonly formed during the oxidation of carbonaceous materials to obtain energy. The.reverse of this process, that is the transformation of various forms of energy with carbon dioxide into chemical potential energy, would lead to reduced forms of carbon since carbon dioxide displays the highest oxidized state of carbon. The reduction of carbon dioxide by plants during photosynthesis has long been recognized and has been the subject of a vast amount of study (1, 2, 3, 4). Effective reduction resulting from addition of organometallies to carbon dioxide is frequently performed (5). Reduction of carbon dioxide to carbon has also been known for some time (6) but, at present, this has no preparative value. Work on the reduction of carbon dioxide to carbon monoxide and subsequent catalytic hydrogenation of the latter has led to the development of the well-known Fischer-Tropsch synthesis which is being used commercially for the preparation of a variety of products, including alcohols, aldehydes, ketones, fatty acids, and some saturated and unsaturated aliphatic hydrocarbons (7, 8). This review will not include these topics because of the voluminous and adequate coverage elsewhere and because of the complex products obtained. However, a survey of the literature shows that a variety of simple products have been obtained from carbon dioxide by various simple and direct methods of reduction, both inorganic chemical and physical. At least five articles deal with the reduction to formic acid, six to formaldehyde, eleven to methanol, and two to methane. The reductions to hydrogen cyanide, oxalate, and carbohydrates have each been reported a t least once. A large number of articles deal with the reduction to carbon monoxide. Chemical reduction has been carried out using carbon, catalytic hydrogenation, metals, complex metal hydrides, and several miscellaneous reagents. Physical reduction has been achieved electrochemically and under the influence of several types of radiation. It is the purpose of this paper to review these latter studies and t o indicate areas for future experimentation. REDUCTION BY CARBON
Much of the early work on the reduction of carbon dioxide involved the use of carbon. The reduction of Present address: Graduate School, Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina. 446
carbon dioxide by carbon yields carbon monoxide in all cases. Quinn and Jones, in their American Chemical Society monograph (5), review most of the work done prior to 1936. Tsukhanova has studied the reduction by passing carbon dioxide gas preheated to 600'-700' through a tube of electrode carbon at 800'-1400° (10). These references, outside of the many patents in which such reduction is an incidental step, appear to cover fairly thoroughly the reduction reaction with carbon. CATALYTIC HYDROGENATION
Catalytic hydrogenation has been one of the most widely studied methods of reduction of carbon dioxide. Carbon monoxide, methane and higher hydrocarbons, and alcohols, particularly methanol, have been obtained by various catalytic hydrogenation methods. Carbon monoxide has been produced over a variety of catalysts: including carbon, platinum, platinum-iridium alloy,. copper, copper chromite, and alloys of copper and iron,. cobalt and iron, and copper, cobalt, and zinc (11, 12).. A patent describes the production of a very pure gradeof carbon monoxide by the passage of carbon dioxideover Group VI metal oxides which have been previously reduced by hydrogen (IS). The use of alkaline earth oxides (14), a mixture of aluminum oxide and molyb-. denum oxide (15), and metallic manganese (16) has also been reported. Bahr (15) describes the formation of methane by the use of a molybdenum oxide-thorium oxide catalyst, and paraffin hydrocarbons by an alu-. minum oxide-molybdenum oxide catalyst. A study of the reduction to methane over a nickel-thoria cata-lyst at 350° showed that ethane and ethylene were formed and were decomposed to methane (17).. Theconversion of carbon dioxide to higher hydrocarbons. at atmospheric pressure has been effected by catalysts of cobalt, iron, copper, nickel, aluminum, and thorium impregnated with potassium carbonate (18). Small scale (25 millimoles) and high yield (81%-86% based on C02) catalytic reduction of carbon dioxide to meth-. an01 has been described by Ipatieff and Monroe (15) and Tolbert (20). The former used a copper-alumina catalyst. Tolbert found optimum conditions at 460. atmospheres pressure and a temperature of 285' with a catalyst of KCu AI2O3. This method requires special equipment and special techniques for the handling of. small amounts of volatile organic compounds. Carbon14-labeled methanol has been prepared by thiS method. Another method for the preparation of C-labeled methanol involves the conversion of carbon dioxide to. potassium carbonate, which is reduced to potassium JOURNAL OF CHEMICAL EDUCATION.
formate by hydrogen in the presence of palladium black. The formate is esterified by heating with methyl sulfate, and the ester converted into methanol by hydrogenolysis a t atmospheric pressure over a copper-chromite catalyst (81, 22). The over-all yield in this method (on a 3.5 millimole scale) is greater than 73%, but a 100'% dilution with unlabeled material is inherent in the method if unlabeled methyl sulfate is used. The production of higher alcohols in good yield by hydrogenation of carbon dioxide under pressure in the presence of a catalyst of manganese, iron, copper, chromium, cadmium, and their mixtures has also been reported (23.9). REDUCTION BY METALS
Very little work seems to have been done on the reduction of carbon dioxide by metals or by the action of metals and acid. Carbon dioxide is reduced by heating with metals such as aluminum and magnesium to carbon (15). The reduction can be controlled in some cases such as in the reaction of tin and carbon dioxide to yield tin (IV) oxide and carbon monoxide (9). By treating carbon dioxide with metallic sodium or potassium it is possible to carry the reduction down to salts of oxalic acid (9). There are two reports of the reduction of carbon dioxide by metallic magnesium in aqueous solutions (84, 25). Under suitable conditions formaldehyde is produced, its amount being increased by the presence of a weak base. When this reduction is carried out in the presence of ammonia or ammonium carbonate, hexamethylene-tetramine results. If the reaction mixture is heated with sodium hydroxide, "an odor" of methyl amine is noticeable. The convenient large scale reduction of calcium carbonate with zinc powder to yield pure carbon monoxide in quantitative yields has been described (26, 27, 28). COMPLEX METAL HYDRIDES
Before the advent of lithium aluminum hydride and related complex metal hydrides, the preparation of organic compounds by reduction of carbon dioxide was not a practical laboratory procedure. By use of standard high-vacuum techniques and apparatus, reductions with lithium aluminum hydride can now be carried out in the laboratory to give good yields of methanol, formaldehyde, and formic acid. A monograph on lithium aluminum hydride and its use in organic chemistry has been written (24). It has been suggested (30) that the reduction of carbon dioxide may proceed first to a formate, then to the methylene glycol salt, and finally to the alcoholate. Thus, at least three reactions seem possible, each of which represents a definite degree of reduction. Hydrolysis or alcoholysis of the salts would give formic acid, formaldehyde, or methanol. Nystrom (31) describes the reduction of carbon dioxide to methanol by a solution of lithium aluminum hydride in a nonvolatile solvent; the methanol being recovered after alcoholysis of the resulting complex with a high boiling alcohol. The yield reported, based on BaC03 from which COz was generated, was 81y0 of redistilled material of high purity. Several other workers have studied this reaction (X-$4). An intensive study of the preparation of carbon-14labeled methanol is described VOLUME 35, NO. 9, SEPTEMBER,1958
using diethylcarbitol as the solvent for the LiAIHl and n-butylcarbitol for alcoholysis of the complex (51). The use of tetrahydro-furfuroxytetrahydro-pyran as the solvent for the LiAlH, and tetrahydro-furfuryl alcohol for alcoholysis due to the elimination of ethanol and diethyl ether as byproducts (32) seems to be an improvement. By condensing carbon dioxide (generated from barium carbonate) on a frozen solution of lithium aluminum hydride in tetrahydrofuran and allowing the temperature to rise to O0C., formaldehyde is obtained in 65% yield (based on the BaCO,), after hydrolysis of the reaction mixture with methanol and dilute acid (35-51). Carbon-14-labeled formaldehyde has been prepared by this method (36). The reduction of carbon dioxide to formic acid in good yield is described using a high ratio of carbon dioxide to lithium aluminum hydride ($7). Formaldehyde was identified as a byproduct in all of the reductions to formic acid, and when more concentrated solutions were used methanol was also isolated. The over-all yield of formic acid was 87T0-88% if the formaldehyde was oxidized with barium peroxide. Carbon dioxide has been reduced to formic acid in 69%-88% yield by passage of the gas into an ether solution of lithium borohydride at 0°C.(38). Carbon14carbon dioxide yields carbon-14-labeled formic acid on similar treatment as aqueous sodium formateC-14. Methanol was identified among the reaction products, hut no formaldehyde could be detected. MISCELLANEOUS CHEMICAL REDUCTIONS
An early report (50) stated that the reduction of carbonates with hydrogen peroxide yielded formaldehyde. I t was later shown (51) that the formaldehyde rame from the oxidation of the barbitnric acid which was used as a preservative in the hydrogen peroxide. The formation of hydrogen cyanide is described by absorption of carbon dioxide on permutoid aminosiloxenes (e.g.,Si60a(NH&); carbaminosiloxenes being formed (39). The reduction of the carbon dioxide was effected by slow oxidation of the Si-Si bonds in the boundary space by addition of small quantities of oxygen over a period of ,several days a t room temperature. Hydrogen cyanide was formed, with oxygen being taken up by the silicon compounds. No reduction of carbon dioxide occurred without oxygen unless the "carbaminosiloxene" complex was irradiated with visible light. A British patent describes a process in which a gaseous mixture composed of 90% carbon dioxide and 10% hydrogen is subjected to the action of a reaction mixtnre containing distilled water, white phosphorus, pulverized quicklime, copper chips, and carbonated magnesia (40). The entire reaction mixture is activated by blowing ozone through it. The hydrogen and carbon monoxide mixtnre which forms is heated to 100'-150' and passed over sulfur a t 130°. The gases are then subjected to a pressure of 3 0 4 0 atmospheres. It is reported that the resulting liquid can be used as a synthetic petroleum. ELECTROCHEMICAL REDUCTION
The electrolytic reduction of carbon dioxide dissolved under pressure has been described (41). The carbon dioxide is reduced quantitatively to formic acid
using electrodes of copper which have been plated with zinc and amalgamated. High concentrations of formic acid cannot be obtained by the method. The reduction of carbon dioxide on mercury cathodes has also been reported (43). Polarization curves recorded with solutions of potassium chloride, ammonium chloride, sodium hydrogen carbonate, and tetramethyl ammonium bromide, through which purified carbon dioxide was passed contmuously, exhibited sudden increases of current a t cathodic potentials similar to those a t which polarographic currents due to carbon dioxide began to increase, but they were appreciably lower than the potentials a t which the current began to increase when carbon dioxide was replaced by nitrogen. Electrolysis experiments carried out yielded formic acid as the only detectable reaction product and in yields often approaching 100yo current efficiency. REDUCTION UNDER THE INFLUENCE OF RADIATION
Early reports (45-46) indicated that carbon dioxide could be reduced to formaldehyde under the influence of ultraviolet light. Spoehr reported that repetition of these earlier experiments upon the reduction of carbon dioxide by ultraviolet light failed to give a definite test for formaldehyde in any case (47). The entire polernic in this field which is related to photosynthesis is detailed by Baly (1). The reduction of carbon dioxide by electromagnetic oscillations has been described by Lorenz (48). Fibrous aluminum(II1) oxide was dried a t 117', placed in an ozonizer, and carbon monoxide passed through the apparatus a t the rate of 0.5 l./min. Waves of 10,000 hertzes were produced by a generator under these conditions. Carbon dioxide was reduced in a slightly acid aqueous solution, but not in solutions of bicarbonate. The distillate contained formaldehyde. The nonvolatile residue was a mixture of carbohydrates and their alcohol and acid derivatives. Crystals were obtained which resembled glucose but were optically inactive. The reduction of carbon dioxide in aqueous solutions by ionizing radiation has been described by Garrison ( 4 9 ) Aqueous solutions of carbon-14-labeled carbon dioxide, with and without addition of iron(I1) sulfate, were bombarded in the 40 Mev. helium ion beam of the 60411. cyclotron. After bombardment, carrier amounts of formic acid, formaldehyde, and methanol were added to the solution, recovered as solid derivatives and tested for carbon-14-activity. I n the presence of FeSO,, the carbon dioxide had been reduced to formic acid and formaldehyde in yields of 22% and 0.13%, respectively, based on total dissolved CO1. Without iron(I1) sulfate the yield was 0.14% for formic acid, with no formaldehyde. Considering the universal availability of carbon dioxide, it is somewhat surprising that more voluminous chemical literature is not available on its simple r e duction reactions. One particularly glaring omission is work on the reduction of carbon dioxide by the usual acid-metal combinations, such as iron, zinc, or tin with hydrochloric acid. The polarographic studies reported so far are only fragmentary. Powerful reducing agents such as chromous chloride and hydro-
gen iodide have not been evaluated as to suitable conditions for reaction. Many other interesting systems come readily to mind. The desirability of carrying on basic studies in this area seem self-evident and the many unexplored regions should attract research investigators in increasing numbers in the near future. LITERATURE CITED (1) BALY,E. C. C., "Photmynthesis," Methuen and Co., Ltd., London, 1940. (2) SPOEHR, H. A., "Phot~~ynthesi~," Chemical Catalog Ca., Inc., New York, 1926. T ~I., ~ , "Photosynthesis and Related Proc(3) R A B I N ~ W I E. esses." Interscience Publishers. Inc.. New York. in two ' volumes (three books), 1945-56. (4) CALVIN,M.,"The Path of Carbon in Photosynthesis," (The Peter C. Reilley Lectures in Chemistry, Vol. 2), University of Notre Dame, Notre Dame, Indiana, 1949. H , S., AND 0.REINMUTH, "Gripnard Reaction8 (5) K ~ R A S C M. of Nan-Metallic Suhetances," Prentiee-Hall, Ine., 1954. (6) MELLOR,J. W., "A Comprehensive Treatise on Inorganic and Theoretical Chemistry," Longmans, Green & Co., Ino., London, 1946 impression, Val. V, pp. 735-54. H. H.,N. GOLUMBIC, A N D R. B. ANDERSON, "The (7) STORCH, Fischer-Tropsch and Related Syntheses," John Wiley & Sons, Inc., New York, 1951. B. T., ET AL., "The Chemistry of Petroleum Hy(8) BROOKS, drocarbons," Reinhold Publishing Corp., New York, 1955, Vol. I, pp. 631-46. "Carbon Dioxide," (A.C.S. (9) QUINN,E.L.,AND C. L. JONES, Monograph #72) Reinhold Publishing Corp., New York, 1936, pp. 131-35. 0. A,, J . Phys. Chem. (L'. S. S. R.), 21, 653 (10) TSUKHANOVA, 11947). - , (11) PETERS,K., AND H. KUSTER,Z. Physik. Chem., A148, 284 110xn) < ...- - , (12) B ~ R H., , Ges. Abhandl. Kenntnis Kohle, 8, 217 (1929); Chem. Zalr. 1929 11, 3263; C. A,, 24, 5207 (1930). C. H.. U. 8. Patent 2.514.282. (13) . . HOLDER. . . . Julv. 4.. 1950: C. A. 44, ~ 8 0 7 (isso). 0~ W.. AND G. SCHILLER. German Pat. 730.516. (14) . KNOBLOCK. , ~ecember17,' 1942; C.A., 28, ~ 4 6 (1942). 2 (15) BAHR,H., Ges. Abhandl. Kenntnis Kohle, 12, 292 (1987). A. N., AND E. A. BROWN, J . Am. Chem. Sac., 60, (16) CAMPBELL, \ -
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