Indian Journal of Chemical Technology Vol. 12, March 2005, pp. 232-243

Catalytic Hydrogenation Jaime Wisniak*

Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel 84105

Development of catalytic hydrogenation is one of the most significant chapters in theoretical and applied chemistry, which led to the opening of a whole series of new industries, particularly in the petrochemical area. The mechanism for a catalytic reaction involving the presence of an intermediate complex formed by the catalysts and one of the reagents, which eventually led to our present understanding of the phenomenon was suggested by Paul Sabatier. For his achievements in the development of catalytic processes Sabatier was awarded the 1912 Nobel Prize of chemistry, together with Victor Grignard.

Catalytic hydrogenationa new technique, was con-tributed to science, by Paul Sabatier (1854-1941). The basic work of P Sabatier in this fundamental scientific and industrial subject forms the basis of our modern theories about catalysis and catalysts, as well as of the processes for the thermal and catalytic cracking of the heavy fractions of petroleum, isomerisation and po-lymerization of hydrocarbons, hydroforming, synthe-sis of ammonia, methane, methanol, a very large number of intermediates and fine chemicals, hydro-genation of liquid fats, dye intermediates, and the Fischer-Tropsch process for the manufacture of syn-thetic fuels.

Here, first the work of P Sabatier that led to the discovery of catalytic hydrogenation and the postula-tion of a mechanism for heterogeneous reactions, is being described and then some details about the life and career of P Sabatier, that will shed light on the road that led him to the Nobel Prize is being given. Inorganic chemistry

During his doctoral thesis Sabatier prepared so-dium monosulphide (Na2S) anhydrous and hydrosul-phides in the pure state (NaSH, NaSH.2H20, NaSH.3H2O); he established the formula of a hy-drated potassium hydrosulphide and showed that a number of alkaline polysulphides that had been de-scribed as definite chemical species were actually mixtures containing free sulphur. He developed an original method for the preparation of the sulphides of calcium, barium, and strontium in the pure state based on passing a stream of hydrogen over the correspond-ing carbonates heated to live red (about 500C). He

described for the first time a method for the prepara-tion of aluminum sulphide pure and crystallized, by reacting hydrogen sulphide with alumina heated to the temperature of red in a carbon boat3.

After moving to Toulouse he continued his studies of sulphur and sulphides. Carl Wilhelm Scheele (1742-1786) had shown in 1777 that alkaline and al-kaline-earth polysulphides treated with a diluted acid did not liberate hydrogen sulphide, like they did with the corresponding sulphides, but generated an oily liquid having an unpleasant smell, from which no compound of definite composition could be separated. Berthollet4 and Louis-Jacques Thnard (1777-1857)5,6, among others, had tried to determine the composition of this substance that seemed to be com-posed by a mixture of hydrogen polysulphides, ac-companied by hydrogen sulphide and sulphur, but the analysis of these products had proven very difficult because they decomposed easily in contact with many substances, particularly glass. Sabatier solved the problem by distilling the oil under vacuum and isolat-ing a liquid having a composition very close to hy-drogen disulphide, H2S2, which he named persulphure dhydrogne (hydrogen persulphide). The errors in relation to the theoretical composition were due to the presence of a small amount of dissolved sulphur. Sa-batier studied the properties of the persulphide, in par-ticular its ability to decompose violently under the action of light or the presence of substances that re-acted with it forming unstable combinations (eg, wa-ter, alcohols, ethers and alkaline sulphides). In the presence of water it formed a rather unstable form of amorphous sulphur, insoluble in carbon disulphide, while the action of ether led to the formation of crys-talline variety of sulphur known as soufre nacre (pearl sulphur) or soufre de Gerne3.

*E-mail: wisniak@bgumail.bgu.ac.il

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Afterwards, he developed a new preparation method for silicon disulphide SiS2, which had been synthetised before by Frmy: Treatment of crystalline silicon heated to red with hydrogen sulphide. He found the concurrent transport of amorphous and crystalline silicon, observation that led him to assume the simultaneous formation of a sublimable sub-sulphide of silicon, SiS, stable only at high tempera-ture, and decomposing slowly on cooling. In fact, rapid cooling of the vapours generated by the reaction allowed him to isolate this metastable sub-sulphide at room temperature and study its properties3.

By reacting hydrogen sulphide with boron at the red temperature he was able to develop a new method of preparation of boron sulphide, B2S3, which he iso-lated in two amorphous forms, one white and opaque, the other transparent and vitreous, and in a crystalline form obtained by sublimation at 200C of the white amorphous sulphide. The formation of a volatile bo-ron sulphide was accompanied by that of sub-sulphide, to which Sabatier assigned the formula B4S, and that of a hydrosulphide of probable composition B(SH)3. He also obtained the sub-sulphide by the ac-tion of hydrogen at red temperature on the normal sulphide, and described its properties and its com-pounds3.

He then went on to study selenides, He isolated for the first time a silicon selenide, SiSe2, having a metal-lic aspect, unstable under the action of water, and a yellow boron selenide, B2Se3, sublimable, and de-stroyed by water. In addition, he recognized the for-mation of a sub-selenide of boron, non-volatile3.

From 1881 onwards he studied different hydrates of metallic chlorides, determining their heats of hy-dration, stability, the possibility of their dehydration under cold, and their reaction with cold concentrated hydrogen chloride. In particular, he studied the hy-drates of ferric chloride and cupric chloride and the conditions for their formation and dehydration. He showed that the absorption of hydrogen chloride by a solution of cupric chloride decreased the solubility of this salt yielding crystals of hydrated cupric chloride that dissolved under the action of an additional amount of hydrogen chloride, leading to the formation of complex hydrochlorides. Sabatier was one of the first to use spectroscopy of absorption to study hy-drates, particularly those of cupric bromide7. Led by the indications of the absorption spectra of solutions of cupric bromide in hydrogen bromide, he succeeded in isolating a complex bromhydrate, which crystal-

lized into black crystals. His discoveries in this sub-ject led to the development of the technique for de-tecting cupric compounds in the presence of hydrogen bromide: they yield a purple coloration, easily ob-servable and having a characteristic wave length in the visible absorption spectra3.

In 1896, he observed that the reaction of all copper compounds with a nitro sulphuric solution (nitrosul-phuric acid), obtained by dissolving nitric acid in sul-phuric acid, yielded an intense blue purple solution due to the reduction of the nitric acid to a new acid, which he named nitrosodisulphonic acid (today: nitro-sisulphonic acid). By studying the absorption spectra he established that the colouration was not due to the copper but corresponded to the new acid formed. He also found that this reduction could be obtained with the help of other metallic reducing agents or with or-ganic substances. He then proceeded to the direct syn-thesis of nitrosisulphonic dark blue, by the reaction between nitric oxide, oxygen, and sulphur dioxide, in the presence of a small amount of water. He proved that nitrosisulphonic acid could produce several me-tallic salts, particularly a blue cupric salt and a pink ferric salt3. Heterogeneous reactions

What makes Sabatiers discoveries even more sen-sational is the simplicity of the equipment he built for his studies of heterogeneous reactions: The reactor consisted of a glass tube filled with catalyst and con-nected to a oxygen generator, a mechanism for adding the reagents, and a receptacle for collecting the reac-tion products. The hydrogen generator, developed by Deville, operated continuously, and was based on the reaction between diluted hydrogen chloride over granulated zinc. The hydrogen generated was washed by passing it through caustic soda, sulphuric acid, and through tubes filled with copper turnings heated to about 500C. The reaction tube had a diameter of 14-18 mm and length 60 to 100 cm and was positioned within a bed of fine sand to assure constant tempera-ture. The reactor was heated by either a gas burner or an electrical heater8.

In 1890 Mond, Langer, and Quincke announced that by the direct action of carbon monoxide on very finely divided nickel, prepared by the reduction from its oxide, they had obtained nickel carbonyl, Ni(CO)4, a volatile compound resulting from the fixation of CO on the metal. They also reported that reduced iron yielded a similar compound9. Their procedure was

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very simple: It involved passing a current of CO over finely divided metallic nickel at a temperature be-tween 350 and 450C, yielding CO2 gas and a solid mixture of a black amorphous powder of nickel and carbon. The composition of the powder varied widely with the temperature employed and still more with the time the reaction was carried on. In this manner they obtained a product containing as much as 85 mass percent of carbon and 15% nickel. When a finely di-vided nickel was obtained, for example, by reducing nickel oxide by hydrogen at about 400C was allowed to cool in a slow current of carbon monoxide, the gas was readily absorbed as soon as the temperature de-scended to about 100C. If the current of CO was con-tinued or was replaced by one of inert gases (such as carbon dioxide, nitrogen, hydrogen, or even air), a mixture of gases was obtained which contained up-wards of 30% mass of nickel-carbon oxide. Analysis of the gas indicated that it corresponded to the for-mula Ni(CO)4.

After Berthelot and Mond in 1891, independently, had also succeeded in making iron carbonyl, Fe(CO)510,11, Sabatier and his doctoral student Ser-endens speculated about the possibility that other un-saturated gaseous molecules such as nitric oxide, ni-trous oxide, nitrogen peroxide, acetylene, and ethyl-ene could also be fixed on nickel or on reduced iron, giving well-defined, stable, and volatile products comparable to nickel carbonyl. They first tried unsuc-cessfully to fix nitric oxide (NO) on nickel, cobalt, iron, and copper; at high temperatures nitric oxide was reduced to nitrogen, with formation of NiO, CoO, FeO, and CuO. Similar results were obtained with N2O, but when passing vapours of NO2 over copper freshly reduced from its oxide, they observed that at room temperature (25 to 30C) there was a regular fixation leading to a definite compound, solid, black, unstable, and nonvolatile, of nitrated copper, Cu2NO2. In all cases, the experimental technique involved re-ducing with a current of hydrogen the finely divided metal oxide placed inside a heated glass tube and then passing the unsaturated gas through the tube. Sabatier and Senderens made a detailed study of the properties of the new compound: It was not altered by dry, cold air, but when heated in the presence of pure dry nitro-gen, it decomposed at about 90C releasing NO2, NO, and N2. Carrying the reaction in a Faraday tube (a closed bent tube cooled at one end) it was possible to obtain liquid NO2. Nitrated copper was decomposed violently by water (or humid air) with release of NO;

it was not decomposed by cold hydrogen but on heat-ing to about 180C, it generated ammonia and ammo-nium nitrite. Reduced cobalt, reduced nickel, and re-duced iron gave similar reactions, but the products were less stable12-15.

Sabatier and Senderens decided now to repeat their experiments, this time with ethylene and acetylene. Then, in 1896 they learned that Moissan and Moureu had recently tried the fixation of acetylene on the same metals16. They had passed a current of acetylene on slivers of iron, nickel, or cobalt freshly reduced from their oxides by hydrogen and chilled in this gas, and observed a brilliant incandescence. The high tem-perature thus produced decomposed the greater part of the acetylene into hydrogen and a large amount of carbon, which would eventually block the tube. The remaining acetylene was converted into liquid hydro-carbons (such as benzene and styrene), which closely resembled those obtained by Berthelot by heating acetylene to dull redness inside a bell inverted over mercury. According to Moissan and Moureu cette reaction est due un phnomne physique (this reac-tions is due to a physical effect): reduced iron, nickel, or cobalt being extremely porous, absorbed the acety-lene with production of enough heat to cause its spon-taneous destruction. The reaction being endothermic, incandescence was reached and maintained as long as the acetylene entered. The incandescence also deter-mined the polymerization of the acetylene into liquid products. However, Moissan and Moureu neglected to analyze the free gas, which they judged to consist of hydrogen, and examined the liquids only sufficiently to recognize the presence of benzene. Similar results were obtained with platinum black12.

Sabatier made some discreet inquiries whether Moissan and Moureu would be continuing these ex-periments, and after learning they would not, he and Senderens repeated the experiments but using ethyl-ene instead of acetylene, a hydrocarbon less violent in its reactions. The procedure the followed was similar to the one used before: They directed a current of eth-ylene upon slivers of reduced nickel and noticed that this time no reaction occurred at room temperature. Raising the temperature progressively, a brilliant in-candescence of the metal took place at about 300C, which disappeared in a voluminous deposit of black carbon, proving the destruction of ethylene. At around 300C they, too, observed a blockage of the tube con-taining finely divided nickel, and the production of free hydrogen. The gases released did not react

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significantly with an aqueous solution of bromine or with an ammonia solution of cupric chloride, which showed that they did not contain measurable amounts of ethylenic or acetylenic hydrocarbons. But they also found methane to be almost pure. Sabatier and Senderens concluded that an unstable combination between nickel and ethylene had formed, analogous to nickel carbonyl, which doubled itself into carbon, methane, and nickel, C2H4 C + CH4, which could then repeat an identical process17. Pursuing this idea, the two tried the reduction of the finely divided nickel oxide at temperatures below 300C, cooling the re-duced nickel first in a current of hydrogen and then of ethylene. After washing the gases leaving the reactor with bromine to absorb any traces of ethylene, they discovered that is was a mixture of ethane, formne (a mixture of equal volumes of ethane and hydrogen), and hydrogen, with traces of hydrocarbons. Raising the temperature above 325C decomposed the ethane into methane and carbon and the formne into carbon and free hydrogen. The overall ratio between ethane and hydrogen formed varied with temperature, at 325C it was 75% by volume ethane and 25% hydro-gen and at around 390C it was essentially pure for-mene. Hydrogen could only be formed from hydro-genation of ethylene, and this hydrogenation had been provoked by the presence of nickel. In other words, it seemed that reduced nickel had the property of hy-drogenating ethylene. To test this possibility they re-peated their experiments, this time directing a mixture of equal volumes of ethylene and hydrogen upon a bundle of thin slivers of freshly reduced nickel, slightly heated at temperatures from 30 to 40C. A considerable increase in temperature was observed. The results confirmed their expectations: only one half of the volume of practically pure ethane was ob-tained, and the reaction continued indefinitely without the necessity of heating and without an appreciable modification of the metal. At a higher temperature (150 to 180C) the reaction was still very rapid and a catalyst bed of a few centimeters of metallic slivers was sufficient to accomplish it12. This result, they be-lieved, doit tre certainment attribue la formation temporaire dune combinaison directe et spcifique du nickel et de lthylne (ought certainly to be attrib-uted to the temporary formation of a direct and spe-cific combination of nickel and ethylene)18. Cobalt, iron, copper, and platinum black gave similar but less intense results. It was not only the easy hydrogenation of ethylene and acetylene that was extraordinary in

their results, but also the use of metals outside the platinum family for catalysts. This result was com-pletely unexpected19.

The following year Sabatier and Senderens discov-ered that nickel is also capable of hydrogenating acetylene, but that reaction could be initiated at room temperature20. Acetylene was first hydrogenated to ethylene and then to ethane, depending on if the react-ing mixtures contained the same or double the volume of hydrogen. The reaction was very exothermic and the temperature within the reactor could reach 100 to 150C, depending on the length of the tube. Again, reduced cobalt, iron, and copper, as well as finely di-vided platinum sponge or black, slightly heated, led to an analogous but less energetic reaction. With freshly reduced copper, for example, the composition of the exiting gas varied with the temperature; at about 130C, it contained 11% volume of ethylene and 178% ethane, and at 150C, 331% ethylene, 20% eth-ane, and 184% of higher unsaturated hydrocarbons21.

Further work examined in more detail the hydro-genation of ethylene and acetylene using other met-als22,23.

After having studied in depth the hydrogenation of unsaturated hydrocarbons Sabatier and Senderens turned to the next challenging problem: Hydrogena-tion of benzene. This reaction had been attempted by Berthelot using his universal agent of hydrogenation, a concentrated solution of hydrogen iodide in a sealed tube heated to 250C (under these conditions hydro-gen iodide decomposes into hydrogen and iodine), but instead of cyclohexane, which boils at 81C, he had only obtained its isomer, methylcyclopentane, which boils at 69C. Instead, Sabatier and Senderens tested the possibility of using reduced nickel and excess hy-drogen at 200C. The gaseous mixture issuing from the reactor was sent to a U-tube surrounded by ice, within which the vapours of cyclohexane were ex-pected to condense to a liquid product. After boiling the benzene for a rather short time, they noticed that the tube became clogged by colourless crystals, which they assumed to be benzene, solidifying at 4C, whereas cyclohexane was reported in the literature to crystallize at 11C. On opening the U-tube they de-tected, instead of the odour of the original benzene, the special intermediate odour between that of chloro-form and that of rose, which belongs to cyclohexane: It was from cyclohexane obtained practically pure at the first attempt, the fusion of which is in reality 65Cthat hour was one of the greatest joy in my

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life. The transformation of benzene had been com-plete14,24.

In the following three years, Sabatier and Sender-ens hydrogenated unsaturated ethylenic or acetylenic carbides into saturated carbides, nitro compounds and nitriles into amines, aldehydes and ketones into the corresponding alcohols, unsaturated hydrocarbons of cyclic nuclei, homologous with benzene (polyphenyls, naphthalene, anthracene, etc) into the corresponding saturated hydrocarbons, the phenols into the cyclo-hexanic alcohols, and aniline into cylohexylamine. They also found that they could produce the major types of natural petroleum by modifying conditions of the hydrogenation of ethylene1,14. They also realized the synthesis of methane by hydrogenating carbon dioxide or carbon monoxide25,26.

With Mailhe, another doctoral student, Sabatier found that some metal oxides were catalysts not for hydrogenation and dehydrogenation, but instead for hydration and dehydration. While ordinary alcohols directed at 250C on reduced copper split into hydro-gen and aldehyde, the same vapour directed on fine alumina or thoria split into water and aldehyde27,28. They also observed that amorphous oxides were more active catalysts for dehydrogenation or dehydration than the crystalline oxides ones28,29. Calcination of the latter at temperatures higher than 500C led to a nota-ble agglomeration. The calcinations reduced the ac-tive surface by modifying the nature and distribution of the active centers. This was the first example of the sintering effects, which were later used to graduate the activity of catalysts. Catalysis

Catalysis is a phenomenon known from very an-cient times, although not its theory or characteristics. By the nineteenth century, enough experimental in-formation began to accumulate to call the attention of scientists. In 1811, Sigismund Constantin Kirchhoff (1764-1833) discovered that mineral acids upon heat-ing changed starch into dextrin and sugar, without themselves being modified by the reaction30. In 1833 Anselme Payen (1795-1871) and Jean-Franois Per-soz (1805-1868)31 found that the transformation of starch discovered by Kirchhoff was attributable to the action of a special substance, which they called dia-stase (amylase), which can be extracted from germi-nated barley by water and purified by repeated pre-cipitation with ethyl alcohol; they had also found that the activity is eliminated by heating to 100C. Payen

and Persoz studied in detail the transformation of starch into dextrin and then into sugar by the action of diastase and proved how starch, once rendered solu-ble, went from one tissue to another, as much as to accumulate again, as much as to bind in strong aggre-gation and participate in this form in the formation of cellular membranes in the tissue.

In 1823, Johann Wolfgang Dbereiner (1780-1849) obtained a spongy platinum material by calcinating ammonium chloroplatinate32. This material was shown to be able to absorb hydrogen at room condi-tions and on heating up to ignite a stream of air di-rected to it. Water was formed as a result without the spongy mass changing its aspect or its weight, and being capable of repeating the process after cooling. In those days, when there was no simple way to pro-duce fire, Dbereiners discovery led immediately to its application, the hydroplatinic lamp, also called briquet hydrogene (hydrogen lighter).

Platinum did not behave in this manner only when made as a sponge; it did also when finely divided as filings, wire, or turnings, as long as it was first heated slightly. Many experiments led to think that this activ-ity increased the more the platinum was divided; it even increased more if before calcination the aqueous solution of chloroplatinate was boiled with a little of sodium carbonate and sugar. The chloroplatinate was completely decomposed and the metal precipitated as a black powder. This powder was much more active than the sponge; it absorbed hydrogen rapidly and the smallest particle led to the instantaneous ignition of a mixture of hydrogen and air. Thnard then found that black platinum decomposed hydrogen peroxide rap-idly and with violence into oxygen and water, without absorbing the gas releasing or losing its activity33. By 1900 this property of platinum was found also in other metals (such as copper and iron), metallic oxides (such as manganese dioxide), carbon, certain acids, etc. The only difference with platinum was that these additional materials had to be heated, sometimes to red temperature34.

In his 1836 Annual Survey35, Jns Jacob Berzelius (1779-1848) summarized the findings of different scientists on the formation of ether from alcohols; on the enhanced conversion of starch to sugar by acids; the hastening of gas combustion by platinum, the stability of hydrogen peroxide in acid solution but its decomposition in the presence of alkali and such metals as manganese, silver, platinum, and gold; and the observation that the oxidation of alcohol to acetic

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acid was accomplished in the presence of finely divided platinum. In a brilliant stroke, he was able to understand that all these processes, although seemingly different, had a common denominator, which he called catalysis (either catalysis of inorganic reactions by metals or of biological reactions by enzymes). In the Annual Survey he wrote: In inorganic nature when compounds arise through the interaction of several substances, the available combining units strive for a state of better satisfaction. Thus, the substances endowed with strong affinities combine readily on the one hand, while those weakly endowed form combinations among themselves on the other. The agent causing the conversion of substances does not participate in the new compounds formed but remains unchanged, thus operating by means of an internal power, the nature of which is still unknown, although it was in this way that it revealed its existence. Thus, it is certain that substances, both simple and compounds, in solid form as well as in solution, have the property of exerting an effect on compound bodies which is quite different from ordinary affinity in that they promote the conversion without necessarily participating in the process. This is a new power to produce chemical activity belonging to both inorganic and organic natures. It will also make it easier for us to refer to it if it possesses a name of its own I shall call it the catalytic power of substances, and decomposition by means of this power catalysis( from the Greek kata-, "down" and lyein "loosen). Mechanism of catalysis

When Sabatier commenced his investigations on catalysis there were two theories of heterogeneous catalysis, a physical and a chemical one. In 1833, af-ter studying the data on the catalytic combination of hydrogen and oxygen on the surface of platinum, Faraday suggested a physical theory in which one or more of the reacting gases were condensed by attrac-tion on the surface of the metal. He wrote: The course of events when platinum acts upon and com-bines oxygen and hydrogen may be stated according to these principles as follows. From the influences of the circumstances mentioned, ie, the deficiency of elastic power and the attraction of the metal for the gases, the latter, when they are in association with the former, are so condensed as to be brought within the action of their mutual affinities at the existing tem-perature, the deficiency of their elastic power not only subjecting them more closely to the attractive influ-

ence of the metal, but also bringing them into more favourable states for union by abstracting a part of that power (upon which depends their elasticity) which elsewhere in the mass is opposing their combi-nation. The consequence of their combination is the production of the vapour of water and an elevation of temperature. But as the attraction of the platina for the water formed is not greater than for the gases, if so great (for the metal is scarcely hygrometric), the va-pour is quickly diffused through the remaining gases. The platina is not considered as causing combination of any particles with itself but only associating them closely around it and the compressed gases are as free to move from the platina being replaced by other par-ticles as a portion of dense air upon the surface of the globe or at the bottom of a deep mine is free to move by the slightest impulse into the upper and rarer parts of the atmosphere.

Berzelius argued that the catalytic force acted on the polarity of atoms through some phenomenon of temperature elevation.

The physical theory was supported by the work of Jacques Duclaux (1877-1978) and Moissan on the absorption of gases by finely divided metals36,37.

Wilhelm Ostwald (1853-1932; 1909 Nobel Prize for Chemistry) and others had also assumed that cata-lyzed gas reactions resulted from the absorption of gases in the cavities of the porous metal, where com-pression and local temperature elevation led to chemi-cal combination. Ostwald believed that a catalyst did not induce a reaction but rather accelerated it but not with formation of intermediate compounds. He argued it was necessary to prove that the succession of as-sumed reactions required less time than the direct re-action itself.

William Charles Henry (1774-1836) in 182438 and August de la Rive (1801-1873) in 182839 proposed a chemical theory where intermediate compounds, for example, oxides of metals, were formed and decom-posed.

Sabatier did not accept this purely physical view of the function of the catalyst, remarking that if it was true then charcoal should be almost a universal cata-lyst, whereas it proved to be somewhat mediocre ex-cept for the formation of carbonyl chloride40. While finely divided metals were able to absorb substantial quantities of gas, these absorptions were somewhat specific, being characterized by a sort of selective affinity. Not only that, some catalytic reactions were extremely specific, for example, zinc oxide decom-

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posed formic acid into hydrogen and carbon dioxide, but at the same temperature titania gave carbon mon-oxide and water. The idea that absorption of gases facilitated their liquefaction could not be true since the easily absorbable hydrogen was very difficult to liquefy. The physical theory was unable to explain the development of high local pressure and temperature in the cases where the catalyst was held in suspension, and did not account for the specificity of catalysts and the remarkable diversity of effects they produced, de-pending on the particular metal or oxide used1,40.

Sabatier and his students observed that an extended surface was not necessarily a synonym of catalytic activity. For example, colloidal nickel, contrary to palladium and nickel, was essentially inert. The activ-ity of the catalyst depended particularly of the surface structure; this was the reason why it was preferable to use a chemical method of preparation instead of me-chanical ones. Sabatier demonstrated that the manner by which the reducible oxide was prepared played a critical role, and searched which were the best deriva-tives to prepare the reduced metal. He also showed that these metallic catalysts were extremely fragile and that they became stronger when using high reac-tion temperatures. Higher temperatures led to an irre-versible decrease of the catalytic activity; the activity was not recovered if the operating temperature was lowered8.

Sabatier then formulated a chemical theory of ca-talysis involving the formation of unstable chemical compounds as intermediate stages, which determined the direction and rate of the reaction. He assumed that in hydrogenation various nickel hydrides were in-volved, whose composition depended on the activity of the nickel. Carefully prepared nickel resulted in the very active NiH2, which would operate on benzene, while impure nickel or nickel prepared at too high temperature gave an impoverished hydride, Ni2H2, which is inactive with benzene, but works with the ethylenic carbides or nitrate derivatives. Sabatier ar-gued that the formation and decomposition of inter-mediate compounds usually corresponds to a diminu-tion of the Gibbs energy of the system. This reduction is accomplished in several steps and this process is frequently much easier than an immediate and direct decrease of Gibbs energy, just as the use of a staircase facilitates a descent1. He argued that while the pres-ence of catalyst might indeed lower the temperature required by a reaction, the catalysts greatest asset was in reacting with a molecular gas in order to pro-

vide a free ion for a reaction which simply would not occur otherwise. Thus, hydrogen peroxide solutions decompose relatively slowly in the cold the same as solutions of chromic acid do, but when the two solu-tions are mixed there is a rapid decomposition with brisk evolution of oxygen and appearance of an in-tense blue colouration (reciprocal catalysis). The col-our is due to the unstable combination 3H2O2.2H2CrO4, which can be isolated by shaking with ether and evaporating the ether1.

Sabatier also addressed himself to the problem of orientation and specificity of the catalyst and claimed that they strongly supported the chemical theory. He showed that different contact masses produced differ-ent reactions: At 350C amyl alcohol in the presence of copper yielded valeric aldehyde and hydrogen; at the same temperature but in the presence of some tho-ria as catalyst it yielded amylene and water. Chromic oxide acted both in oxidation and in dehydrogenation and dehydration reactions and heated alumina decom-posed alcohol into ethylene and steam, while metallic molybdenum and zinc oxide decomposed it into acet-aldehyde and hydrogen. He attributed the orientation of the reaction to the metallic or nonmetallic character of the catalyst or, in other words, to the intervention of different electrical contact forces. He understood that this differentiation was not valid in every case. He wrote: Jai trouv avec M Mailhe que les vapeurs dacide formique en prsence doxyde de zinc 250C, donnent de lanhydride carbonique et de lhydrogne, en presence doxyde titanique elles se dtruisent exclusivement en eau et oxyde de carbone Ici les deux oxides nont aucune dissemblance physi-que et lintervention daffinit chimique spciale sexerant la surface de ces catalyseurs est seule capabe dexpliquer une inversion aussi complete du phnomne (I have found with M Mailhe that va-pours of formic acid in the presence of zinc oxide at 250C yield carbon dioxide and hydrogen, in the presence of titania they decompose only into water and carbon monoxide. Here, the two oxides do not have any physical resemblance and the intervention of a special chemical affinity that is exerted at the sur-face of the catalyst is capable of explaining such a complete inversion of the phenomenon)8.

Sabatier postulated the formation of different in-termediate compounds, each with its own mode of decomposition, and he also clearly established that some organic reactions are reversible. In cases where the intermediate compounds could not be isolated, he

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assumed the formation of surface compounds, a phe-nomenon, which he named fixation, thus linking the physical, and the chemical theories of catalysis41. For example, in the catalytic oxidation of organic com-pounds with the aid or copper, or the decomposition of carbon monoxide on nickel, the intermediaries, copper oxide and nickel carbonyl, respectively, can be isolated and identified. On his studies of the hydro-genation of ethylene, Sabatier suggested the fixation of hydrogen by the nickel as the intermediary com-plex. In the case of acetylene, however, he remarked that it was adsorbed more energetically than hydro-gen, thus indicating the possibility of organometallic compounds playing a role in heterogeneous cataly-sis42. During his acceptance speech at Stockholm Sa-batier said43: Jadmets que lhydrogne agit sur le mtal en donnant trs rapidement sur sa surface une combinaison Lhydrure ainsi engendr est facilement et rapidement dissociable, et sil est mis en prsence de matires capables dutiliser de lhydrogne, il le leur cede, en rgnrant le mtal, qui recommence indfiniment le mme effect La distinction que jai faite entre plusieurs sortes deactivit du metal conduirait admettre quil existe plusieurs stades de combinaison (I admit that hydrogen acting on the metal produces rapidly on its surface a combination. The hydride thus generated is easily and rapidly dis-sociated; put in contact with substances capable of using hydrogen, gives it to them, regenerating the metal and restarting indefinitely the process)8.

Sabatier summarized his views in respect to the mechanism of catalytic action as follows1,40: As far as I am concerned, this idea of temporary unstable intermediate compounds has been the beacon light that has guided all my works on catalysis; its light may perhaps be dimmed by the glare of light as yet unsuspected which will rise in the better explored fields of chemical knowledge. Actually, such as it is, in spite of its imperfections and gaps, the theory ap-pears to us good because it is fertile and permits us in a useful way to foresee reactions.

We must note that Sabatier was the first to demon-strate, the reversibility of the reaction alco-holaldehydehydrogen, which gave place to a large number of thermodynamic and statistical investiga-tions on the Gibbs energy changes of organic reac-tions.

During the Second World War Irving Langmuirs (1881-1957; 1952 Nobel Prize for Chemistry) pub-lished a rival theory called theory of chemisorption,

according to which a gas adsorbed over a catalyst was fixed thanks to its unsaturated valences yielding a gas-metal compound of the type MxGy, where x is a function of the mass of the catalyst and y varies ac-cording to the specific surface and physical conditions of pressure and temperature44,45. This theory was con-trary to Sabatiers hypothesis of distinct, individual intermediates and allowed more importance to physi-cal conditions19. Although Langmuirs theory retained the concept of fixation of the reagents on the surface of the catalysts, it assigned a predominant importance to the physical conditions that Sabatier had purpos-edly ignored. For a time, Langmuirs theory of the fixation of a monomolecular layer on the catalyst gave it a certain advantage because it permitted to address quantitatively the problem of heterogeneous catalysis and played a considerable role in experimen-tal studies8. Langmuirs theory became particularly important because it permits a first quantitative analy-sis of the possible mechanism of a reaction.

Poisoning

The catalysts sensitivity to poisons, discovered by the work of Rudolph Knietsh (1854-1906) on cata-lysts used for the synthesis of sulphur trioxide (was shown to be a general phenomenon, which could be used to control catalytic reactions Sabatier, when comparing catalysts to ferments, described poisoning in the following words: Comme les ferments or-ganiss qui sont tus par des doses infinitsimales de certains toxiques, le ferment minral quest le mtal est tu par des traces de chlore, de brome, diode, de soufre, darsenic, soit quelles lui viennent par lhydrogne soit quelles lui soient apportes par la substance qui doit subir lhydrogenationle nickel un peu intoxiqu ne pourrait fournir quun premier hy-drure, comparable celui du cuivre, et capable dagir sur les groupes nitrs ou sur la double liason thyl-nique; seul le nickel sain pourrait fournir un hydrure capable dhydrogner le noyau aromatique (In the same way that organized ferments are killed by infini-tesimal amounts of certain toxins, the mineral ferment which is the metal is killed by traces of chlorine, bro-mine, iodine, sulphur, or arsenic that are carried by the hydrogen or by the substance to be hydrogen-atednickel, a little poisoned, can only provide a first hydride, similar to that of copper and capable of act-ing over the nitro groups or on the ethylenic double bond, only nickel cannot provide a hydride able to completely hydrogenate the aromatic ring)1,8.

Sabatier and Espil46 made the interesting discovery

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that after being used to hydrogenate nitrobenzene, nickel poisoned by chlorine recovers its ability to hy-drogenate benzene1.

Discussion on the life and scientific career of Sa-batier, in the following paras will give an overall view of the background that led to this major contribution. Life and career

Paul Sabatier (Fig. 1) was born on November 5, 1854, at Carcassonne, Southern France, one of the seven children of Alexis Sabatier and Pauline Guil-hem. He received his first education at the primary school and the local Lyce in Carcassonne. In 1868, he entered the Lyce at Toulouse and a year later joined the Collge Saint-Marie. In June 1872, after receiving his diplomas of bachelier s sciences (bachelor of sciences) and bachelier s letter (bache-lor of humanities), he departed for Paris to prepare for the entrance examinations to the Grand coles (cole Polytechnique and cole Normale Suprieure). Al-though he placed highly in the entrance competitions for both coles, he chose the latter, where his brother Thodore has preceded him19,47. At the cole Normale he took the courses given by Henry Sainte-Claire De-ville (1818-1881), Charles Friedel (1832-1899), Jean Gaston Darboux (1842-1917), and Pierre Auguste Bertin (1818-1884)8.

Three years later (1877), Sabatier received the li-cense de physique and the license de mathmatiques at the cole Normale and joined the local school at Nmes as professor of physics. In 1878, he was rec-ommended as normalien to the laboratory of Marcelin Berthelot (1827-1907), then at the Collge de France. At that time, Berthelot was the most outstanding chemist of France and was well connected to the high ranks of the academic and political establishment. Sabatier took advantage of his stay at the Collge de France to successfully prepare for his doctorate, re-ceiving the degree of Docteur s Sciences (Doctor of Sciences) in 1880 with a thesis on the thermochemis-try of sulphur and metallic sulphides48.

The fact that Sabatier had been raised at home on a religious and conservative atmosphere soon led to strong divergences of opinion with Berthelot on po-litical and philosophical grounds. This inflexible posi-tion explains why Berthelot forced Sabatier to use in his thesis the notation of equivalents (as seen in all the tables), instead of the atomic one. Nevertheless, at various points in his thesis Sabatier mentions that his experiments confirm Berthelots principles or predic-tions8,1.

These divergences affected profoundly Sabatiers career In 1880, after receiving his doctorate Berthelot led him to understand that he should look for a posi-tion in the provinces. In France of that time this statement was equivalent to academic exile. Berthelot was known to punish in this harsh manner those who criticized his theories and opposed his ideas. One of the most famous scientists thus penalized was Pierre Maurice Duhem (1861-1916)49. His choices to start an academic career were now limited to three universi-ties: Algiers, Bordeaux, or Lyon. Berthelot recom-mended Sabatier for the position at Bordeaux, where he stayed for one year as Matre de conferences in physics. Eventually Sabatiers personality and bril-liant scientific achievements overcame the ideological barrier and on December 1905 he became Doyen (dean) of the Faculty of Sciences at Toulouse, a posi-tion he occupied for the next twenty-five years.

Sabatier remained faithful to Toulouse for all his life, turning down many offers of respectful positions elsewhere. In 1907, he was offered Henri Moissans (1852-1907; 1906 Nobel Prize for Chemistry) chair at the Sorbonne and that of Berthelot at the Collge de France (ironies of life!). Although he realized that all candidates for the Acadmie des Sciences were re-quired to be residents of Paris, he nevertheless chose

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to remain at Toulouse. He retired from his professor-ship in 1930 after nearly fifty years of uninterrupted service in the Faculty of Science.

In 1913, he became the first scientist elected to one of six chairs newly created by the Acadmie for pro-vincial members.

Sabatiers initial researches were in the field of in-organic and physical chemistry, within the framework of Berthelots laboratory. His thesis, which sought to complete the study of sulphides, included accounts of the preparation of various metal and nonmetal, espe-cially alkali and alkaline-earth sulphides and polysul-phides, with determinations of their heat of reaction and solution. As described below, Sabatier ap-proached the problem from a variety of possibilities. In the 1879-1897 period, he performed analyses of metallic and alkaline earth sulphides, chlorides, chromates, and notably of copper compounds7, the preparation of hydrogen disulphide by vacuum distil-lation, the isolation of selenides of boron and silicon, the definition of basic cupric salts containing four copper atoms, and preparation of the deep blue nitro-sodisulphonic acid and the basic mixed argentocupric salts. He studied the partition of a base between two acids, using the spectrophotometric change of colour-ation of chromates and dichromates, as an indicator of acidity50,51, and analyzed the velocity of transforma-tion of metaphosphoric acid52. From 1896 to 1899, he made some in-depth studies of the oxides of nitro-gen12,53-55 and of nitrosodisulphonic acid and its salts3,55-58.

Sabatier himself has described how his interest in the field of catalysis arose12. In 1890, after learning that Ludwig Mond (1839-1909), Carl Langer, and Friedrich Quincke had succeeded in preparing nickel carbonyl13, a volatile compound, by the direct action of carbon monoxide on finely divided nickel he de-cided to investigate the possibility that other incom-plete (unsaturated) gaseous molecules would behave in the same manner, giving well-defined, stable, and volatile products comparable to nickel carbonyl. In collaboration with his doctoral student Jean Baptiste Senderens (1856-1937) they succeeded in preparing nitrated copper, Cu2NO2, by reacting NO2 with re-duced copper at room temperature15.

Sabatier and Senderens were about to try to fix acetylene on several metals (copper, cobalt, iron, and nickel) when they learned that Moissan and Franois Charles Lon Moureu (1863-1929) had failed to achieve it16. Sabatier and Senderens tried instead the

less violent reaction of ethylene and reduced nickel and found that the reaction product was mainly ethane with a little of hydrogen. Discovery of this hydro-genation reaction opened a new world of chemical synthesis, which was intensively researched by Sabat-ier and his students. He promptly demonstrated the general applicability of his method to the hydrogena-tion of unsaturated and aromatic carbides, ketones, aldehydes, phenols, nitriles, nitrate derivatives, carbon monoxide and carbon dioxide, and liquid fats. He also showed that certain metallic oxides, particularly man-ganous oxide, behave analogously to metals in hydro-genation and dehydrogenation, although at slower rates; and that powdered oxides such as thoria, alu-mina, and silica possess hydration and dehydration properties. His work revealed also the general in-crease in catalytic activity arising from the dispersal of the active material on suitable supports.

Sabatiers discoveries lay at the root of most of the giant chemical industries of today (petroleum, petro-chemicals, chemical synthesis, synthetic fuels, fat hy-drogenation, etc), nevertheless, like Claude-Louis Berthollet (1748-1822) and Michael Faraday (1791-1867) before him, he pursued science only and did not seek the commercial benefits of his inventions, pat-enting very few of them.

Sabatier passed away in Toulouse on August 1941, at the age of 87.

The scientific work of Sabatier was very extensive; in addition to a large number of speeches, reports, and eight patents, he published over 250 scientific mem-oirs, his famous book La Catalyse en Chimie Or-ganique 1 (1913), as well as Leons lmentaires de Chimie Agricole2 (1890), and collaborated in major works such as Edmond Frmys (1814-1894) LEncyclopdie. Charles-Adolph Wrtzs (1817-1884) Dictionnaire de Chimie, and Moissanss book Chimie Minrale.

La Catalyse en Chimie Organique was first pub-lished in 1913; this book on catalysis marks a mile-stone in the evolution of modern chemistry and still continues to be quoted extensively. Honors and Positions

Sabatier received many honours for his contribu-tion to science, industry, and the Nation. He became Correspondent Member of the Acadmie and was nominated to the Lgion d'Honneur. He was awarded the degree of Doctor of Science, Honoris Causa, by several foreign universities and elected member or

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honorary member of many foreign scientific societies. In 1897, the Acadmie des Sciences awarded him the Lacaze prize and in 1905 the Jecker prize, jointly with Senderens. The Royal Society of London awarded him the Davy Medal (1915) and the Royal Medal (1918), the Royal Society of Arts the Albert Medal (1926), and the Franklin Institute the Franklin Medal (1933).

The maximum award came in 1912 when Sabatier was awarded the Nobel Prize of Chemistry for his method of hydrogenating organic compounds in the presence of finely divided metals. He shared the prize with Victor Grignard (1871-1935), who received it on account of his discovery of the so-called Grignard reagent.

Interesting enough, the road of both Sabatier and Grignard to the Nobel Prize of Chemistry was deter-mined by a twist of destiny. As mentioned before, Sabatier went to become a chemist and not an engi-neer because his acceptance to the cole Polytech-nique was arrived several days after that to the cole Normale Suprieure. In 1887, Grignard graduated with honours the lyce at Cherbourg. At that time the city of Paris offered scholarships to brilliant graduates from the secondary schools in the provinces, to pre-pare for the entrance examinations to one of her uni-versities. The Cherbourg lyce had received a promise that Grignard would be awarded one of those scholar-ships in order to prepare for the entrance examina-tions to the cole Normale Suprieure in Paris, to study mathematics. Unfortunately, because of the ex-penses involved in the preparation of the 1889 World Exposition (that would see the inauguration of the Eiffel Tower) no scholarships were offered at the time of Grignards graduation from high school.

Whoever took the decisions that affected the ca-reers of Sabatier and Grignard, could have hardly guessed the tremendous impact they would have in the development of modern organic chemistry. Epilogue

Sabatier ended his speech at the Nobel Prize award ceremony saying43: Theories cannot be indestructi-ble. They are only the plough, which the ploughman uses to draw his furrow and which he has every right to discard for another one, of improved design, after the harvest. To be this ploughman, to see my labours result in the furtherance of scientific progress, was the height of my ambition, and now the Swedish Acad-emy of Sciences has come, at this harvest, to add the

most brilliant of crowns. References

1 Sabatier P, La Catalyse en Chimie Organique, C, Branger, Paris,1913.

2 Sabatier P, Leons lmentaires de Chimie Agricole, Mason, Paris, 1890.

3 Champetier G, Bull Soc Chim Fr, 3 (1955) 469. 4 Berthollet C L, Ann Chim, 25 (1798) 233. 5 Thenard L J, Ann Chim, 85 (1812) 132. 6 Thenard L J, Ann Chim, 93 (1831) 79. 7 Sabatier P, Compt Rendus, 106 (1888) 1724; 107 (1888) 40. 8 Wojtkowiak B, Paul Sabatier Un Chimiste Ind?pendant

(1854-1941), Jonas Editeur, Argueil, 1989. 9 Mond L, Langer C & Quincke F, J Chem Soc, 57 (1890) 749.

10 Mond L & Quincke F, J Chem Soc, 57 (1890) 604. 11 Berthelot M, Compt Rendus, 112 (1891) 1343. 12 Sabatier P & Senderens J B, Compt Rendus, 114 (1892)

1429. 13 Mond L, Langer C & Quincke F, J Chem Soc, 57 (1890) 749. 14 Sabatier P, Ind Eng Chem, 18, (1926) 1005. 15 Sabatier P & Senderens, J B, Bull Soc Chim Fr, 9 (1893)

669. 16 Moissan H & Moureu, Compt Rendus, 122 (1896) 1240. 17 Sabatier P & Senderens J B, Compt Rendus, 124 (1897) 616. 18 Sabatier P & Senderens J B, Compt Rendus, 124 (1897)

1358. 19 Nye M J, Isis, 68 (1977) 375. 20 Sabatier P & Senderens J B, Compt Rendus, 128 (1899)

1173. 21 Sabatier P & Senderens J B, Compt Rendus, 130 (1900)

1559. 22 Sabatier P & Senderens J B, Compt Rendus, 130 (1900)

1761. 23 Sabatier P & Senderens J B, Compt Rendus, 131 (1900) 40. 24 Sabatier P & Senderens J B, Compt Rendus, 132 (1901) 210. 25 Sabatier P & Senderens J B, Compt Rendus, 134 (1902) 514. 26 Sabatier P & Senderens J B, Compt Rendus, 134 (1902) 689. 27 Sabatier P & Mailhe A, Ann Chim Phys, 20 (1910) 289. 28 Sabatier P & Mailhe A, Compt Rendus, 150 (1911) 823. 29 Sabatier P & Mailhe A, Compt Rendus, 152 (1911) 1212. 30 Kirchhoff S C, Bull Neusten Wiss Naturwiss, 10 (1811) 88. 31 Payen A & Persoz J F, Ann Chim, 53 (1933) 73. 32 Dbereiner J W, Edinburgh Phil J, 10 (1824), 153. 33 Thenard L J, Ann Chim, 9 (1818-1819) 441. 34 Bertrand G, Bull Chim Fr, 473 (1955) 475. 35 Berzelius J J, Jahresbericht 1836, 15, 237, 243. 36 Moissan H, Trait de Chimie Minrale, Masson, Paris, 1904. 37 Duclaux J, Compt Rendus, 152 (1911) 1176. 38 Henry W C, Phil Trans, 114 (1824) 266. 39 De la Rive A, Ann Chim Phys [2], 39 (1828) 297. 40 Sabatier P, Compt Rendus, 152 (1911) 1176. 41 Partington J R, Nature, 174 (1954) 859. 42 Rideal E K, J Chem Soc, Abstracts, (1951) 1640. 43 Anonymous, Nobel Lectures Chemistry 1901-1921, Novel

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Foundation, Elsevier, Amsterdam, 1966; pp 216-231. 44 Langmuir I, J Am Chem Soc, 38 (1916) 2221. 45 Langmuir I, J Am Chem Soc, 40 (1918) 1361. 46 Sabatier P & Espil L, Compt Rendus, 158 (1914) 668. 47 Chamichel C, Bull Soc Chim Fr, (1955) 466. 48 Sabatier P, Recherches Thermiques Sur les Sulphures, Thse

de Doctorat, #445, Sorbonne, Paris, 1880. 49 Wisniak J, Chem Educator, 5 (2000) 156. 50 Sabatier P, Compt Rendus, 106 (1888) 1724; 107 (1888) 40.

51 Sabatier P, Compt Rendus, 103 (1886) 49. 52 Sabatier P, Compt Rendus, 103 (1866) 138. 53 Sabatier P, Compt Rendus, 106 (1888), 63: 108 (1888) 738. 54 Sabatier P, Compt Rendus, 114 (1892) 1476. 55 Sabatier P, Compt Rendus, 115 (1892) 236. 56 Sabatier P, Compt Rendus, 114 (1892) 1429. 57 Sabatier P, Compt Rendus, 122 (1896) 1479. 58 Sabatier P, Compt Rendus, 122 (1896) 1537. 59 Sabatier P, Compt Rendus, 123 (1896) 255.