HYDROGENATION T h e papers on pages 1189 t o 1215 were presented before t h e Midwest Regional Meeting of t h e American Chemical Society, a t Purdue University, Lafayette, Zncl.
ROLE OF THE CATALYST HOMER ADKINS University of Wisconsin, Madison, Wis.
HE mechanism by which catalysts bring about the reaction of hydrogen with a compound is not a matter upon which anyone should attempt to speak with finality. Catalytic reactions are no more and no less mysterious than are the so-called noncatalytic reactions, and there is no apparent reason for believing t h a t the types of reaction involved in catalysis are different from those in the ordinary run of chemical transformations. The following discussion will outline a concept of the role played by the catalyst in hydrogenation. The hypothesis suggested rationalizes a multitude of observations, many of which are otherwise anomalous. Chemists are prone t o postulate that chemical reactions proceed through the shift of electrons and protons or through the intermediate formation of molecular compounds, ions, or radicals. Apparently the conclusion of a given chemist with respect to the mechanism of a particular reaction is as much a characteristic of his mind as it is of the objective facts upon which all chemists can agree. Therefore, no attempt will be made to enter into the fine details of the mechanism of catalysis. It will be assumed that we are concerned with the “ordinary” chemical reactions of hydrogen, catalyst, and hydrogen acceptor with one another. A catalyst is here defined as a substance that accelerates or causes a reaction to take place. Catalysis is concerned with the influence of one molecule upon the behavior of another molecule. This definition assumes that two molecules, such as those of hydrogen and ethylene, do not react with each other except under the influence of a third substance, such as nickel. The catalysts for hydrogenation apparently function by combining with the hydrogen and with the compound to be hydrogenated (hydrogen acceptor). The result of this combination with the catalyst is that the hydrogen and hydrogen acceptor react with each other. Simply expressed, we may say that molecules of hydrogen and molecules of ethylene are inert toward each other, but that hydrogen attached to nickel may react with ethylene attached to nickel to give ethane. The ethane then leaves the nickel, permitting the metal to react with more ethylene and hydrogen and so repeat the process of hydrogenation.
Characteristics of an Effective Catalyst What are the characteristics of a “good” or effective catalyst upon the basis of this simple concept of the role of the catalyst? First, a good catalyst must be stable under reac-
tion conditions, and many of the things that one does in preparing and using a catalyst are connected with stabilizing i t against change, Probably many so-called promoters merely tend toward stabilizing the catalyst rather than enhancing its activity. Certain catalysts, especially those used in catalytic oxidation, such as copper, silver, vanadium oxide, and molybdenum-iron oxide, are constantly renewing their surfaces by alternate oxidation-reduction. Such catalysts may have a long life, for they are going through the process of change and renewal that is characteristic of living things. A change in experimental conditions may render a catalyst useless because it can no longer maintain the active form. For example, copper chromite, an excellent catalyst for hydrogenation in the liquid phase, in many cases is not so satisfactory in the gas phase. This is because the divalent copper in the active catalyst is more readily reduced by hydrogen if the copper chromite is not wet. Similarly, in the dehydrogenation of alcohols by copper much better results are obtained if a stream of hydrogen maintains a considerable excess of that product of the reaction. An excess of water facilitates the dehydration of alcohols over alumina, whereas an excess of ammonia is advantageous in dehydrogenating amines. These various reagents probably improve the processes because they maintain the catalyst in the proper state of oxidation or because they prevent the accumulation of by-products on the active surface. A good catalyst for hydrogenation must combine several rather distinct characteristics or abilities in addition to maintaining its active state under reaction conditions (1, 6) : 1. It must adsorb and activate hydrogen. 2. It must adsorb and activate the hydrogen acceptor. 3. It must hold them in the proper ratio and space relationship. 4. It must desorb the desired product. This short paper will not discuss the precise meanings of “adsorb” and “activate”. Most physical-chemical investigations of catalysis have been concerned with ascertaining the nature of the chemical and physical forces involved in the adsorption and activation processes ( I O , I S , 14). The problem of why certain substances react and others do not, and why certain compounds are unstable while others are stable is obviously of fundamental importance, but no more so for catalytic reactions than for those considered to be noncatalytic. For the present we may say that by “adsorption” 1189
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1190
we mean “the formation of a compound”, without inquiring as t o the precise nature of the forces which hold together catalyst and hydrogen or catalyst and hydrogen acceptor. ?Then we say that the hydrogen or hydrogen acceptor is “activated” by the catalyst, we are stating that the combination of hydrogen acceptor with the catalyst reacts with hydrogen attached to catalyst while the hydrogen and hydrogen acceptor would not react if they were not each combined with the catalyst. Competitive Reactions The present paper is primarily concerned with several types of Observations, which may be rationalized in terms of the sequence of reactions on the catalyst outlined above. First let us consider an example of the results of a variation in the proportion of the hydrogen and hydrogen acceptor on the surface of the catalyst, as described by Craxford, for the Fischer-Tropsch synthesis of hydrocarbons (4). I n this process carbon monoxide and hydrogen a t atmospheric pressure are passed over a cobalt, iron, or nickel catalyst held a t about 200” C. During the first few hours after the catalyst is put into service there is a large amount of hydrogen on the catalyst, and methane is the chief product. After a time the amount of hydrogen on the catalyst is much less than in the early stages, and hydrocarbons containing many carbon atoms in each molecule are produced. T h a t is, during the early stages with an abundance of hydrogen on the catalyst, there is no opportunity for carbons to combine with one another to form long chains, for each carbon (as cobalt carbide) is adjacent t o adsorbed hydrogen so that methane is the main product. After the first few hours of use there is less hydrogen on the catalyst and therefore more chance for synthesis by interaction of adjacent carbon atoms. I n the above instance the change in the proportion of products is due to the change in the surface of the catalyst so that after a time hydrogen is held in lesser amounts than by the newly prepared catalyst. The proportion of the products from the hydrogenation of a given compound may also be rather profoundly modified by a variation in the pressure of hydrogen. For example, a t 120 atmospheres, a-oximinoacetoacetic ester gives mainly a pyrazine, formula I, when hydrogenated over nickel a t 80” C.; a t 320 atmospheres the hydrogenation gives a-amino-8hydroxybutyric ester, formula 11: N
CHs-8 EtOzC-C
I
‘C-CO2Et
It
CHaCHOHCHIiHzCOzEt
C-CH,
“/ I
I1
The formation of the pyrazine depends upon the reaction of two molecules of the oximino ester. At the higher pressure there would be more hydrogen on the catalyst and therefore less probability that two molecules of the oximino ester would be near enough to each other on the surface of the catalyst so that interaction would be possible. A slow hydrogenation of an ester, due to a low pressure of hydrogen or other cause, is likely to give considerable amounts of a high-molecular-weight ester. For instance, octadecyl stearate will be produced by the hydrogenation of ethyl stearate. This result may be rationalized if i t is assumed that stearaldehyde is an intermediate step in the hydrogenation. If the aldehyde is not quickly hydrogenated, two molecules may interact according to the Tischtschenko reaction to give octadecyl stearate:
+
--
C17HssCOzEt Hz SCI~H~SCHO
+
CirH3sCHO EtOH CI,H~SCO&H&I~H~S
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It may thus be said that a low concentration of hydrogen on the surface of the catalyst favors synthetic reactions which involve two or more molecules of hydrogen acceptor. Such a low concentration of hydrogen is advantageous in the FischerTropsch process, which is therefore carried out a t a low pressure of hydrogen. A high pressure of hydrogen is advantageous in hydrogenations where condensation with the production of high-molecular-weight compounds is not desired. The importance of having the right proportion of the two reactants upon the surface of the catalyst is also shown in the hydrogenation of acetylene on platinum. Farkas and Farkas observed that the rate of hydrogenation was decreased by an increase in the pressure of acetylene from 70 to 150 mm., while an increase in the pressure of hydrogen increased the rate of hydrogenation ( 5 ) . It is obvious that there was a deficiency in the amount of hydrogen on the platinum catalyst, and that too much acetylene was absorbed. Effect of Pressure The hypothesis outlined above enables one to rationalize many observations on the relation of pressure of hydrogen to the rate of hydrogenation. For example, platinum and palladium are active a t pressures near atmospheric, copper chromite requires pressures of 50 to 300 atmospheres, and nickel is somewhat intermediate in its pressure requirements. These observations are understandable if it is assumed that platinum and palladium take up enough hydrogen a t atmospheric pressure in proportion to the amount of hydrogen acceptor adsorbed, whereas copper chromite does not have sufficient hydrogen on its surface except a t relatively high pressures. With a given catalyst the effectiveness of increased pressure of hydrogen varies with the particular hydrogen acceptor involved. For example, higher pressures of hydrogen are much more important with esters such as ethyl trimethyl acetate or diethyl camphorate than with straight-chain esters which have no branching on the carbon atom alpha to the carbethoxy group. This observation is understandable in terms of the picture of the reaction process sketched above. I n order for reaction to take place, hydrogen must be adsorbed on “active centers” of the catalyst sufficiently close to the carbethoxy group for reaction. The branched-chain esters will tend to cover a larger area of the catalyst than the straightchain esters. Therefore a higher pressure of hydrogen must be used to overcome the shielding effect of the branched chains on active centers of the catalyst adjacent to the carbethoxy group. Active Centers The commonly accepted picture of the surface of the catalyst is one in which active centers for adsorption are distributed over the surface (10, 1S, 14). The active centers presumably consist of atoms whose valence forces are not entirely satisfied by other atoms in the surface of the catalyst. These active centers vary in activity. For example, a given center may be sufficiently active to combine with ethylene but not with hydrogen, whereas another center may be so strong that it will hold hydrogen as well as ethylene. The activity of these centers may change with the use of the catalyst, as has been illustrated in the case of the catalyst for the Fischer-Tropsch process. The number of centers of a given degree of activity available per unit area of the catalyst is often small. For example, Almquist and Black (3) concluded that in the hydrogenation of nitrogen to ammonia only one in two thousand atoms of iron in the catalyst mass was active. The number and activity of the active centers in a catalyst are determined in part by the particular procedure used in t h e preparation of the catalyst.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
There are no experimental methods which show conclusively that the spacing of the active centers on the catalyst determines the rate or direction of the reactions brought about b y the catalyst. By inference there are many facts which suggest that the space relations on the catalyst surface are of primary importance. The variation in relative reactivity among organic compounds with variation in the size and shape of molecules shows conclusively that steric factors may determine the speed of reaction and the proportion of products. I n fact, one finds differences between the behavior of geometrical isomers in catalytic hydrogenation. If variation in the configuration of the hydrogen acceptor plays a role in the catalytic reaction, it is but reasonable to conclude that a similar type of variation in the catalyst will also be a factor in the process. The soundness of this conclusion is borne out by the fact that d-quartz will preferentially dehydrate one of the enantiomorphs of 2-butanol ( 1 2 ) . The probable importance of the spacing of active centers of a catalyst in determining the course of reaction was shown several years ago, particularly clearly in the case of titania ( 2 ) . Two titania catalysts were prepared, one from tetraethyl and the other from tetrabutyl titanate. Both the titanates were repeatedly distilled and were free from electrolytes. The titania catalysts were compared against ethanol which gives over titania a mixture of ethylene and ethane. One titania gave twice as much ethylene in proportion to the ethane as did the other. The difference in the behavior of the catalysts can hardly be ascribed to anything else than the difference in the size of the alkyl groups which were removed during the process of preparing the titanias from the ethyl and butyl titanates. Selectivity One of the most striking facts about catalytic hydrogenation is the selectivity shown by the catalyst and the hydrogen acceptor. For example, nickel is more active against carbon-tocarbon than i t is toward carbon-to-oxygen double bonds, whereas copper chromite is more active toward carbon-tooxygen than toward carbon-to-carbon double bonds. However, both catalysts will cause the hydrogenation of both types of unsaturation so that the difference between the catalysts is quantitative rather than qualitative. For example, the ring in ethyl P-phenylpropionate is hydrogenated over nickel at 200" C. to give ethyl P-cyclohexylpropionate. Over copper chromite at 250" C. the carbethoxy group of ethyl P-phenylpropionate is hydrogenated and y-phenylpropyl alcohol is produced. At temperatures above 330' C. both types of hydrogenation occur over either catalyst. But a third type of reaction ensues so that the product of the hydrogenation is largely the hydrocarbon propylcyclohexane: C~HF,CH,CH,CO~E~ 2 CeH5CH,CH,C02Et %C*4, 250"
~ C~HIICH,CH,COZE~ i ~ -+
C.
C~H~CH~CH,CH,OH
Ki -or CuCrzOd 330" C.
This relative inactivity of oxide catalysts toward alkene linkages (7) is so marked in the case of zinc chromite that the esters of the unsaturated acids such as oleic acid may be hydrogenated to unsaturated alcohols a t temperatures 250 " C. above that a t which nickel would induce rapid hydrogenation of the alkene linkage in the oleate. This selectivity in action, upon the basis of the hypothesis outlined above, is probably dependent upon preferential combination with the catalyst. That is, nickel tends to attach the hydrogen acceptor to itself a t alkene or benzenoid link-
1191
ages, whereas copper chromite shows a greater affinity for carbonyl groups as compared to carbon-to-carbon double bonds. The higher the temperature, the less selective the catalyst and the more the probability that the hydrogenation will go to the ultimate stage of the saturated hydrocarbons. Structure of Reactant I n considering the role of the catalyst, one should not ignore the possible variations in the structure of a given reactant. For example, a catalyst such as copper chromite which is rather inactive toward the benzenoid nucleus may bring about such a hydrogenation through a tautomeric form of the hydrogen acceptor. The ethyl ether of P-naphthol does not react with hydrogen over copper chromite a t 200" C. (9). However, @naphthol is readily converted to 1,2,3,4-tetrahydro-2-naphthol a t 200" C. over copper chromite. Presumably this is because naphthol may tautomerize to an unsaturated ketone, a type of compound which is rapidly hydrogenated over this catalyst : OH
0
OH
With nickel there is little difference in rate of hydrogenation between the naphthol and its ethers, since nickel is as effective against the benzenoid nucleus of the naphthol or its ether as it is against the unsaturated ketone of the tautomer. Equilibria and Rates In general, catalysts only serve to decrease the time required for the system to reach equilibrium, but instances are known where the concentrations of reactants attained from a catalyst are not the same as those that would be anticipated if the role of the catalyst were ignored. For example, Reid reported that he had obtained ethyl acetate in yields above 80 per cent by passing equimolecular amounts of acetic acid and alcohol over silica gel (15). These results were criticized because it was pointed out that the maximum concentration of ethyl acetate could be no more than 67 per cent in a system starting with a mole each of alcohol and acid. Reid's results have been fully confirmed, and the reason for the apparent excep tion to the prediction based on results in a homogeneous system is easily seen if the process of esterification over a solid catalyst is similar to that outlined above for hydrogenation. Alcohol and acetic acid passing over silica gel would each be adsorbed, reaction would occur between molecules of alcohol and acetic acid adsorbed on adjacent active points on the catalyst, and then the ethyl acetate and water so produced would be desorbed. If alcohol and acetic acid were irreversibly adsorbed and water and ethyl acetate were rapidly desorbed, then the yield of ester would be 100 per cent. No such catalyst is known or likely to be found, but it will be obvious that the yield of ester is determined not by the thermodynamics of the alcohol-acid-water-ester system, but simply by the relative adsorption by the catalyst of the reactants as contrasted to the products. An effective catalyst for the hydrolysis of an ester would be one which irreversibly adsorbed ester and water and rapidly desorbed alcohol and acid. Over various catalysts one might obtain yields of ester varying from 0 to 100 per cent, depending upon the characteristics of the catalyst involved. Poisoning The last step to be performed by a good catalyst is to give up the product a t the right time or stage. Sometimes it is possible to modify a catalyst so that it will desorb a product a t an intermediate stage of the reaction. For example, methanol is oxidized by air to give ultimately carbon dioxide:
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INDUSTRIAL AND ENGINEERING CHEMISTRY 0 HCHO 0 CHaOH 4
HCOiH
0
COZ
Over iron oxide at 370’ C. the reaction runs to completion, b u t if molybdenum oxide is incorporated with the iron oxide, the first product of oxidation] formaldehyde, is desorbed. The iron-molybdenum oxide catalyst thus makes possible a process by which methanol is oxidized almost quantitatively to formaldehyde (8). The failure of a hydrogenation catalyst to desorb a product sufficiently rapidly may result in poisoning-i. e., covering of the catalyst by the product-or interaction between m o l e cules of the desired product still adsorbed on the catalyst, There is also danger that if the desired product is not quickly desorbed, i t may react further with hydrogen as illustrated above in the hydrogenation of an ester to a hydrocarbon at 330’ C. Taylor believes t h a t the relative rates of dehydration and dehydrogenation of an alcohol over a zinc oxide catalyst are determined by the relative rates of desorption of water and hydrogen by the catalyst (19). There is a balance or competition between the adsorption of each of the compounds present-e. g., hydrogen, hydrogen acceptor, solvent, and products. Often a high pressure of hydrogen will minimize the poisoning effect of the products as well as their tendency to interact while on the surface of the catalyst, since it will increase the proportion of the surface covered by hydrogen, which thus replaces other adsorbates. I n a similar way high pressures of hydrogen minimize or eliminate the effect of small amounts of “poisons” present in the reaction mixture. It is probably for this reason that less care need be used in the purification of compounds for hydrogenation at pressures of 100 to 300 atmospheres than with platinum or palladium at 1t o 3 atmospheres. The solvent or reaction medium as well as the hydrogen, hydrogen acceptor, and product are no doubt adsorbed by the catalyst and so may play a role in determining the extent or course of the reactions. Solvents may be beneficial only because they facilitate the dispersion of the catalyst and the contact of the three essential materials, hydrogen, catalyst, and organic compound. However, in some cases a more specific role is played b y the solvent. For example, in the presence of ethanol only two of the three phenyl groups in triphenylmethane are hydrogenated, whereas in the presence of methylcyclohexane the hydrogenation goes to completion. I n the hydrogenation of amides and lignin, dioxane appears t o be particularly beneficial in facilitating reaction.
Conclusions A realization of the probable complexity of a typical organic reaction such as catalytic hydrogenation is advantageous. One is led to see something, like a living organism, that depends for its existence upon coordination and synchronization of a variety of relatively simple changes. The complexity is
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functional as well as statistical. Each step in the process is probably simple and conventional, but the combination of them seems mysterious. The good or useful catalyst is one that combines several rather unrelated properties or characteristics in a proper balance or proportion: The development of organic chemistry and of catalysis is largely an art and not a science. The practice of an a r t involves judgment, taste, experience, and intuition in addition to analytical and logical reasoning. Organic chemistry and catalysis lie in the region where on the one side there are certain phenomena physical in character, which may be treated by the methods of mathematics. However, on the other side are phenomena which become lifeless and lose their meaning when they are analyzed and simplified as is necessary before the methods of mathematics may be applied. I n the simplification and analysis used in the process of breaking down a complex phenomenon into its parts, one is likely, in the process of finding the parts, to lose sight of the fact t h a t complexity may constitute an essential ingredient not found in any of the parts. The whole may be much greater than or different from the sum of the parts, which often exist as parts only in the mind of the student ( l a ) . The only successful way of finding out whether a given compound or combination of compounds is a good catalyst for a particular reaction has been to try it out. There are no physical or chemical measurements which can be made a priori to distinguish the good from the poor catalyst. After a particular substance prepared in a certain way has been found to be the best catalyst available, this fact can usually be explained. But the good catalysts and methods were discovered and developed by Sabatier, Ipatieff, Skita, Pad, Willstatter, Patart, hlittasch, Adams, Lazier, and Raney, who made direct and empirical attacks upon the problem of the hydrogenation of organic compounds.
Literature Cited (1) Adkins, Homer, “Reactions of Hydrogen”, Madison, Univ. Wis. Press, 1937. (2) Adkins and Millington, J . Am. Chem. SOC.,51,2449 (1929). (3) Almquist and Black, Ibid., 48,2814 (1936). (4) Craxford, S. R..Trans. Faraday SOC.,35,946 (1939). (5) Farkas and Farkas, J. A m . Chem. SOC.,61,3396(1939). (6) Langmuir, Irving, Ibid., 39, 1848 (1917); Tram. Faraday SOC., 17,621 (1922). (7) Lazier and Vaughen, J . Am. Chem. SOC.,53, 3719 (1931); 54, 3080 (1932);Sauer and Adkins, Ibid., 59,1 (1937). (8) Meharg and Adkins, U.S. Patents 1,913,404-5 (June 13. 1933): Peterson and Adkins, J. Am. Chem. Soc., 53, 1512 (1931). (9) Musser and Adkins, Ibid., 60, 664 (1938). (10) Natl. Research Council, Comm. on Catalysis, Twelfth Rept.. New York, John Wiley & Sons, 1940. (11) Schwab and Rudolph, Naturwissenschaffen. 20, 363 (1932). (12) Smuts, Jan, Brit. Assoc. Advancement Sci., Rept., p. 9, 1931. (13) Taylor, H . S.,J . A m . Chem. SOC.,54, 602 (1932); 56, 1685 (1934). (14) Taylor, H. S.,Tmns. Electrochem. SOC.,71,375 (1937). (15) Tidwell and Reid, J. Am. Chem. SOC.,53,4353 (1931).
