Aristid
V. Grosse
WAR PRODUCTION BOARD, WASHINGTON, D. C.
Fundamentally, when chemical substances are associated together and one or more of them react, two possibilities should b e distinguished: In the first case one or several chemical substances react without interference or influence from the other chemical substances present. In the second case these other substances, which are not consumed . i n the reaction, influence the chemical behavior of the reacting substances. They may increase or decrease the velocity of the reaction and they may also change its direction. Obviously, reactions according t o the first possibility form the content of classical chemistry. Reactions according t o the second possibility should form the subject matter of what the author proposes to call “catalytic chemistry“. The objective of catalytic chemistry i s t o collect, systematize, correlate, explain, and, where possible, predict the catalytic behavior of inorganic and organic substances, The explanations and predictions should be based o n the known or determinable chemical roperties of the reactants and catalysts invo ved, under the physical reaction conditions. Various deductions are made from these basic viewpoints.
P
A Two-Coil Dobbr Cracking U n i t Built by
Universal O i l
Products
Company
At present, textbooks of inorganic and organic chemistry and chemical discussions talk about the “phenomenon of catalysis” and the application of this phenomenon by the chemist as a means of overcoming chemical inertia and lack of reactivity. OstwaldJs physicochemical definition of catalysis has induced physical chemists for the last decade t o attempt a “general explanation” of this phenomenon in terms of physical concepts. The state of our understanding and insight into this field is closely analogous to that prevailing a t the beginning of the nineteenth century in regard t o organic chemistry. At that time only a small number of compounds were isolated from organic sources, and it was believed that the vis vitalis, a mysterious force of life, was necessary to produce them. After Woehler’s synthesis of urea, these viewpoints quickly disappeared, and new discoveries soon led to the new concept that all these mysterious compounds are but compounds of czrbon and form a new branch of chemistry, which we now all know a3 “organic chemistry”. I n a similar way, it is useless now t o look for a “general explanation” of the phenomenon of catalysis. From the material on hand we may conclude that we are confronted here with a new field of chemistry, which we propose to call “catalytic chemistry”. It is a logical expansion of the fields
ITHIN the last decade a large number of new catalytic reactions have been discovered and developed into important industrial processes. Taking the petroleum and rubber industries as an example, one need only be reminded of the following catalytic achievements: polymerization of mono-olefins to aviation gasoline (Ipatieff), of diolefins to Buna and Buna S (Tshunkur), of acetylene t o vinylacetylene and further t o neoprene (Kieuwland and Carothers) ; dehydrogenation of low-molecular paraffins to corresponding mono- and diolefins (butadiene, isoprene) , of naphthenes to aromatics, of ethylbenzene to styrene; cyclization of paraffins, olefins (mono- and di-) , or acetylenes containing a row of six or more carbon atoms to aromatics (toluene) ; alkyhtion of paraffins with olefins to higher paraffins; hydrogenation of carbon monoxide to hydrocarbons (Fisher-Tropsch) ; cracking of gas oils to aviation and motor gasoline (Houdry) ; and condensation of alcohol to butadiene (Lebedew). It is safe to say that, for every new noncatalytic reaction, a t least three catalytic ones are being discovered and industrially developed a t present. I n contrast to this amazing development in the applied field, our theoretical and scientific concepts are lagging far behind. This is due to our antiquated attitude toward catalysis. 762
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
of classical inorganic and organic chemistry into a new region, and we should recognize it as such. Fundamentally, when chemical substances are associated together and one or more of them react, two possibilities should be distinguished: I n the iirst case one or several chemical substances react without interference or influence from the other chemical substances present. I n the second case these other substances, which are not consumed in the reaction, influence the chemical behavior of the reacting substances. They may increase or decrease the velocity of the reaction and they may also change its direction. Obviously, reactions according to the first possibility form the content of classical chemistry. Reactions according to the second possibility should form the subject matter of catalytic chemistry.
T H U S catalytic chemistry deals with the reactions of chemical substances in the presence of other chemical substances. The latter do not appear in the end products of the reaction, but do change the reaction velocity and may change its direction. In other words, it deals with the influence one chemical substance exerts on the reactions of one or many other substances without going into a stable combination with any one of them under the reaction conditions. I n contrast to noncatalytic or ordinary chemistry, every chemical substance in catalytic chemistry may play a triple role. Thus it may be catalyzed to react, under conditions
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where it would not react by itself; it may be a catalyst itself and promote the reaction of one or more substances; and it may prevent or “poison” the catalytic activity of another substance (or substances). Theoretically, and in many cases practically, any given chemical reaction may be catalyzed by a large number of different substsnces. Thus the field- or scope of catalytic chemistry is potentially much greater than that of classical organic and inorganic chemistry. I n line with this the future systematic handbooks of catalytic chemistry will be many times as bulky as our present day Gmelin, Mellor, or Beilstein! The general relation between inorganic and organic chemistry to catalytic chemistry may be illustrated schematically as in Figure 1. Present explanations of catalysis are adequate in some cases and fail in most others. Past attempts to develop a systematic catalytic science have never progressed far, from a chemist’s point of view, because of their failure t o recognize the importance of the catalytic factor of a given substance, in a given reaction, in comparing it with other substances and in attempting t o classify it. Defining the catslytic factor, f, as the ratio of the rates, IC, of the catalyzed and noncatalyzed reaction, f = k,,t./k,oneat. According t o Ostwald’s definition, a catalyst is “a substance that changes the rate of a chemical reaction”. Since even very small changes in reaction rates may be measured, we have practically an
. * * The photograph shows a section of a government unit for the production of synthetic GRrubber, using grain alcohol as the source of butadiene. It i s part o f an integrated plant, built and operated for the Rubber Reserve Company b Carbide and Carbon Chemicals Corporation and United States Rubber Company. The piping in d e foreground is supported on steel salvaged from the Brooklyn elevated street railway1 approximately 3 0 0 tons of steel were obtained for this purpose. Some 400 miles of pi ing are used in the construction of the process units. The photogra h on the Contents Page of t l i s issue (see page 3 o f the advertising section) shows one of t i e butadiene unrts.
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Figures
I , 2, and 3
Vol. 35, No. 7
unliniitccl nunibe:. vi chcinical substances t hnt mn)- act as c:it:ily.sts for s given reaction. They will range from some that will just ineasurably chmge itsr:*te to others that will increase it by a factor of a million a n d eyen much more. 0 b vi o tis ly, those substnnces that will change it by hrge factors are of imn:ediate interest and primary concern to the catalytic chemist. In most of these cases there is R strong chcrniciil interaction between thecatalystnnd the catalyzed moleculcs, and in a great many of them this interaction can beclearly established arid esplnined on thc basis of the knonn chemicnl propertics of t h e substances involved. On the other hand, it would be futilc ill the present stste of our kriomlcdge to try to explain small changes in reaction rates and try to establish a relstiLii Letwen the chemical propertiesof the reactants and nn ineffective catalyst-Le., a substancewhich will accelerate their reaction rate to only a small extent.
I N D U S T R I A L A N D 1 M a f N EERX NIG C H E M I S T R Y
July, 1943
T H E objective of catalytic chemistry is to collect, systematize, correlate, explain, and, where possible, predict the catalytic behavior of organic and inorganic substances. The explanations and predictions should be based on the known or determinable chemical properties of the reactants and catalysts involved, under the physical reaction conditions. Before analyzing concrete examples of catalytic reactions, it will be appropriate to discuss them generally from the physicochemical viewpoint. Physical chemistry teaches that the relation between reaction rate k and the energy required to overcome the chemical at absolute inertia of molecules, or the activation energy, EBCt., temperature T is expressed by the well known Arrhenius equation: k =
ae-Eaot./RT
I n vie% of the exponential nature of this equation, relativcly small changes in activation energy will cause large differences in reaction rates. Increases in reaction rate for given values of activation energy decrease a t loo”, 300°, and 1000” K. (assuming no change in a constants) are given in Table I. For example, a 10 per cent lowering of activation energy E of 50,000 calories a t 300’ K. will increase the reaction rate 4400 fold, whereas even a 1per cent lowering will more than double it. TABLEI.
INCREASES IN THE
Temperature c. K. 100 173 300 -F7g; 1000
-
REACTION RATE FACTOR, kl/kz
Decrease in Aotivation Energy, A E 600 cal. 5000 C d . 50,000 cal. 12 8.6 X 1010 2 x 10“~ 4400 3 x loas 2.3 1.3 12 8 . 5 X 1010
A simple scheme illustrating the difference in mechanism between an ordinary or noncatalytic reaction and a catalytic one is given in Figure 2. I n the noncatalytic case the reacting molecule or molecules have to overcome the energy barrier, Eord.. I n the catalytic case the reactants first react with the catalyst, overcoming the energy barrier Eoah. 1; in the second step they react, under regeneration of the catalyst, overcomand are smaller than ing E,,t.2. Since both Eord., although their sum may be larger, the catalytic reaction rate will be much faster. Obviously its rate will be governed by the rate of the slowest step or the one requiring the greatest energy of activation. So far the temperature-independent factor, a, has been neglected. The value of a in a given reaction may be changed by a catalyst-for instance, by selective geometrical orientation of reacting groups. On the whole, since rate k changes linearly with a, the latter has a lesser significancethan activation energy E. K E E P I N G in mind the above-mentioned scheme, there are certain general possibilities confronting a given system of molecules, alone and in the presence of various catalysts. The general case is illustrated in Figure 3. On the left is the noncatalytic case where the reacting molecules have to overcome the highest barrier (E,,d.). For the catalytic reaction the number of possibilities is practically limitless. First there will be a large number of substances causing only a slight decrease in activation energy and a corresponding increase in rate; these substances are indicated by E,, E,, Es, E4, etc. Then there will be a lesser number of substances producing substantial decreases in the energy barrier, such as E,, E#,etc., or E,, Eb, and only a few, indicated by E., which will produce very high increases in rate. (The effect of factor a has been neglected for simplicity. In a number of cases cer-
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tain catalysts may affect i t favorably and thus change the sequence ot catalyst efficiency based on activation energy decreases alone.) Thus we see a whole gamut of catalytic possibilities. The chemical interaction between reactants and catalyst will increase as we go from left to right, whereas from right t o left the catalysis will assume a more physical aspect. Thus, in general terms, on the extreme right of the diagram the catalytic mechanism will go through the formation of intermediate stoichiometric compounds of greater or lesser stnbility; then as we move toward the left, it will gradually change to less stable and definite surface compounds, then to avtivated complexes, and finally be due to mere molecular deformations caused by molecular fields of force. Since theoretically the transition from enormous catalytic effects to none a t all is continuous, there is no sharp boundary between classical and catalytic chemistry. However, in practice such a boundary has already been established and following the present tradition there should be no difficulty in maintaining it in the future. A particular picture of a reaction and its various possible catalysts will depend on the physical reaction conditions, such as pressure, concentrations of reactants and catalyst, and particularly temperature. I n making comparisons of catalytic performance of various substances, two extremes must be differentiated: (a) The noncatalytic reaction proceeds at a fast rate under experimental conditions; and (b) the catalytic reaction proceeds a t a barely measurable rate only in the presence of the best catalysts. Obviously in the first case even potentially effective catalysts will not show up well, whereas in the second case medium and poor catalysts will not be noticed. Failure to distinguish between the catalytic factor of various substances in a given reaction, failure to take due consideration of the physical conditions of the reaction, and particularly failure to compare with the rate and course of the noncatalytic reaction have been a great deterrent in chemical attempts to elucidate and classify the phenomena of catalysis in the past; generally substances of enormously varying degree of efficiency were uncritically thrown together as “catalysts”. I n comparing the catalytic performance of various substances in heterogeneous catalysis, due consideration must be given to such physical factors as accessibility and availability of catalytic surface, its porosity, thermal and time stability, diffusion factors, crystalline or amorphous nature, and the effect of various substances on these factors, the size of crystals, etc. YPICAI, examples of catalytic reactions will illustrate to some extent the various points raised in the preceding paragraphs. First is the catalytic reaction which can be fully predicted on the basis of the chemical properties of the substances involved. Such is the Squibb reaction (8) or the catalytic decomposition of acetic acid to acetone over calcium carbonate. Acetone was originally produced by converting acetic acid with calcium carbonate or oxide into calcium acetate, according to Equation 1. This acetate has the property of decomposing at about 400’ C. into acetone, water, and carbon dioxide, following Equation 2 :
+
+ +
PCH,.COOH CaCOd-+ Ca(OOC.CH3)z Hz C02 (1) Ca(OOC.CH& ----f CaC03 CH3.C0.CH, (2)
+
Instead of this two-step chemical process, Squibb (8) proposed simply to carry out the reaction in a single catalytic step, using calcium carbonate as catalyst, in line with Equation 3.
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tions practically any solid substance may measurably change the reaction rate and therefore be a ‘ ‘catalyst” and it would be futile for a catalytic chemist, a t the present state of our knowledge, to interpret the better or lesser catalytic performance of a given substance as a function of its chemical properties. At low temperatures this reaction is enormously catalyzed by noble metals, particularly platinum and palladium, The affinity of noble metals for both hydrogen and oxygen is well known, and the formation of unstable surface noble metal hydrides and oxides is well established. The easier oxidation or reduction, respectively, of these compounds by the reacting gases and their regeneration during the catalytic cycle as compared to direct reaction between the gases adequately explains t o the catalytic chemist the chemical mechanism of this reaction and the very much faster combination of the two gases in their presence. It does not follow that any hydrogenation catalysts will promote this reaction. The knowledge of their chemical properties, however, will permit a catalytic chemist to decide whether they are suitable or not. For instance, sodium and the other alkali metals, which readily form hydrides and may be used as high-temperature hydrogenation catalysts, will not be suitable for this reaction; for both oxygen and water, the reaction product, will convert them into stable compounds such as sodium oxide, sodium peroxide, sodium hydroxide, and others, which cannot be regenerated by hydrogen. Thus it will not be possible to establish a catalytic cycle.
TABLE11. CATALYTIC REACTIONS OF HYDROCARBONS KNOWN BEFORE 1932 Para5ns
Olefins
Nsphthenes
Aromatios
-
+-
A
Small-Scale Cracking Plant Used b y the U. S. Bureau o f Mines to Convert Medium O i l into Gasoline or Diesel Fuel
CaC03
2CH~,COOH+CH3.CO.CHa
+ Hz0 + COz
(3)
Since all carbonates may be converted into acetates by contacting them with acetic acid, it follows that any metal acetate which will decompose, under reaction conditions, into acetone and regenerate carbonate (or a metal oxide readily convertible into acetate) will act as a catalyst for this reaction. In line with this, barium, strontium, and zinc carbonates were found to be catalysts for this reaction, since their acetates on heating give acetone. A second example is catalysis by means of surface or absorption compounds. A typical case is the well known reaction: 2H2
+
0 2
+2H20
This reaction does not proceed alone with any noticeable velocity at room temperature. However, a t high temperatures it will proceed without a catalyst. Under such condi-
The third example is merely physical catalysis, correspondE3, etc., of Figure 3. The catalytic ing t o the region E,, EP, cracking of hydrocarbons, pioneered by Houdry within the last fifteen years, is typical. The noncatalytic or thermal cracking takes place a t a practical rate a t around 500” C. A large number of inorganic substances, particularly metal oxides, will catalyze the reaction and allow it t o proceed with sufficient speed a t about 450” C. and even lower. Some of these substances will also appreciably change the direction of cracking-i. e., the nature of reaction products. In view of what was said about the ‘‘physical region” of Figure 3, it is difficult a t present t o explain the good catalytic performance of a specific oxide mixture from its chemical nature. I n this and similar cases, such factors as the size and extent of internal surface, thermal stability, lack of crystalline compound formation, etc.-in other words, factors depending on the solid chemistry of the catalyst mixturebecome decisive in practice and not the chemical interrelation between the reactants and the solid catalyst. The fourth and last example illustrates the large number of different substances that may be used t o catalyze one particular reacBion and also the wide range of physical conditions under which it may be accomplished. A typical case is the alkylation of paraffins by olefins (6). Catalysts may be (a) various solid metal halides, such as aluminum chloride, aluminum bromide, zirconium tetrachloride, etc. (3), which form solid or liquid complexes, (b) gaseous boron fluoride and water, with or without nickel (b),and (c) acids like sulfuric (1, 6) or anhydrous hydrogen fluoride (4). The reaction takes place from temperatures below -50” (7) to about 400” C.
July, 1943
INDUSTRIAL AND RNGINEERING CHEMISTRY
At these high temperatures (under pressure) no specific catalyst is required, and it becomes a pure thermal reaction ( 2 ) . The reaction may be carried out heterogeneously, using solid boron fluoride, nickel, water (or hydrogen fluoride) complexes, or liquid aluminum chloride or bromide complexes as catalysts. Again it may be carried out homogeneously in the liquid phase with hydrogen fluoride as catalyst. I n the latter case it is necessary to use an excess of olefin over paraffin. The olefin forms an alkyl fluoride which, in turn, dissolves both the paraffin to be alkylated and the catalyst (hydrogen fluoride). The mechanisms of these reactions, aside from the activation of the C-H bond, may be widely different, and no satisfactory explanation for any one of them has yet been proposed.
T H E S E examples show how the various types of catalysis (chemical and physical) form but parts of a continuous picture. The relation between various parts of this picture is established by the krrhenius equation. The writer hopes that a more complete discussion, in book form, of the enormous volume of catalytic facts available, in the light of concepts introduced here, may be possible after this war. The enormous potential size of this new chemical field has already been stressed, although now we usually think only of inorganic and organic reactants and inorganic catalysts. The scientific vista becomes breath-taking when we begin to include organic molecules as catalysts. The conversion of the highly inert molecules of nitrogen, a t room temperatures, into ammonia, nitrous acid, and nitric acid by Azotobacter bacteria is a catalytic marvel of the first order. No less so is the assimilation of carbon dioxide and elimination of oxygen by plants and the regulated oxidation of organic substances by oxygen to carbon dioxide in living bodies. I n condusion it may be worth while to emphasize how the development of the catalytic chemistry of a particular type of compounds, such as hydrocarbons, has enlarged our general knowledge of their chemistry. Let us recall their most important classical properties. The known chemistry of paref-
767
fins is particularly brief. All textbooks stress their lack of reactivity. Sulfuric acid does not sulfonate them, nitric acid acts only with difficulty, and only halogens like chlorine attack them vigorously; of course, they burn. Olefins are characterized by addition and polymerization reaction. The aromatics, in contrast t o paraffins and naphthenes, are sulfonated and nitrated. All catalytic reactions between various classes of hydrocarbons known before 1932 are marked in Table I1 by plus signs. Since then systematic research, a b n g the lines discussed in this article, has led to the discovery of all the other reactions, indicated by a minus sign, and some of them have already attained great industrial importance. Thus the volume of our knowledge of hydrocarbon chemistry has substantially increased, thanks to the development of their catalytic chemistry. There can be no doubt that other branches of chemistry could do likewise. Their task would be simplified if textbooks would begin to give consideration to the subject of catalytic chemistry. ACKNOWLEDGMENT
The writer had the privilege of being associated in his catalytic work over a period of years with some of the leading pioneers in this field-namely, V. N. Ipatieff (since 1926), H. Tropsch (since 1930), and in the last few years with E. Houdry and H. S. Taylor. To all of them he is indebted for many valuable discussions which gradually led him to his present viewpoint. LITERATURE CITED
Birch, S. F., Dunstan, A. E., Fidler, F. A., Pim, F. B., and Tait, T.,J . Inst. Petroleum Tech., 24, 303-20 (1938). Frey, F. E., and Hepp, H. J., IND.ENO.CHEX.,28, 1439 (1936). Grosse, A. V., and Ipatieff, V. N., J. Org. Chem., 1, 559 (1937). Grosse, A. V., and Linn, C., U. S. Patent 2,267,730 (1941). Ipatieff, V. N., and Grosse, A. V., J . Am. Chem. Soc., 5 7 , 1616 (1935). Morrell, J. C., U. S. Patent 2,169,809 (1939). Pines, H., Grosse, A. V., and Ipatieff, V. N., J . Am. Chem. Soc., 64, 33 (1942). Squibb, Ibid., 17, 187 (1895).
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