INDUSTRIAL CATALYSIS1 B. B. CORSON Mellon In~titute,~ Pittsburgh, Pennsylvania
CATALYSIS is the speeding up or slowing down of
a chemical reaction by a substance-a catalyst-that can be recovered unchanged when the reaction is over. Catalysis is a high-sounding word which explains something that we don't understand.
A pompous word will stand you instead For that which will not go into the head.
In March, 1835, Berzelius reported to the Swedish Academy of Sciences that there was a new force in the world. Since the Greeks did not have a word for it, he invented one-~araXGsr~from ~ a r a(down) and Xi; = e m (to loosen). Berzelius thought of catalysis as the decomposition of chemical compounds by the catalytic force, as analysis was the decomposition of chemical compounds by the force of chemical affinity. A catalyst is a stimulant which arouses the dormant passions of the molecules so that they are raring to go places. This is essentially Berzelius' definition of 111 years ago. Not only does a catalyst stimulate reactions hut it also directs reactions. It is a combination of the whip and the reins. A catalyst is a substance that promotes union hetween substances, or separates them, without becoming involved in the reaction-like a clergyman in a wedding ceremony or a judge in a divorce hearing. The clergyman unites two people by the bonds of holy matrimony, and this union is practically impossible without his assistance; the clergyman can be recovered unchanged after the ceremony, and he can bring about the union of many couples. ENZYMES, BACTERIA, AND MOULDS oneof the oldest industrial catalytic processes is the conversion of sugar to ethyl alcohol and carbon dioxide: C611%Oa + 2CnHsOH
+ 2CO2
sugar is converted to alcohol with a 95 per cent efficiency. There are small amounts of several by-products formed, such as glycerin, succinic acid, and higher alcohols called fuse1 oils. The industrial operation of vinous fermentation goes back into antiquity. Although the ancients did not know exactly what they were doing--ethyl alcohol was not separated from wine and recognized as such until the 1700's--they manufactured it and consumed i t for several thousand years. All sortsof queer reactions are catalyzed by enzymes, bacteria, and moulds. A bacterium called clostridium acetobutylicum ferments starch to a mixture of butyl alcohol and acetone. 3CeHnO6
-
2C.HoOH
+ CHaCOCHa+ 7CO2 + 4Hs + H%O
This process was developed during World War I to produce acetone which was needed in the manufacture of smokeless powder. The by-product butyl alcohol, like all new chemicals and new-born babies, was absolutely useless. Nevertheless, it was not thrown away because chemists hate to throw away a perfectly good chemical, even when it is worthless. It was stored in bamls and tanks, and it was not long before butyl alcohol had many uses, and acetone was demoted to play second fiddle and become a by-product in the manufacture of butyl alcohol. A mould called mucor stalonijer oxidizes acetates to oxalates, and another mould, penicillium purpurogenum, oxidizes glucose to citric acid. Moulds can synthesize fats, complex aliphatic acids, and sterols. The ~ o i s o nexhaled bv damw wall wawer containine arseGcal pigments is broducid by rugged mould; penicillium brevicaule, which specializes on damp mixtures of arsenic and old wall WWr to manufacture himethyl arsine, (CH&As.
a
INORGANIC
This important industrial reaction is caused by the catalytic effect of an enzyme contained in yeast. Not only is the sugar converted to alcohol, but the alcoholic solution is nicely carbonated a t the same time. Grape
The subject matter of this paper is primarily concerned with inorganic catalysts, such as nickel, platinum, alumina, silica, vanadium pentoxide, aluminum chloride, hydrofluoric acid, and nitric oxide. The mention of enzymes, bacteria, and moulds is just a reminder that there are many industrial processes which depend on the catalytic effects produced by living or-
I presented before the ~ i ~ h tsummer h conference of the New England Association of Chemistry Teachers at Middlebury College, Middlehury, Yermont, August 25, 1946. ' Multlplo Pelloashlp on Tar Synthetics, Koppers Company, Inc.
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JOURNAL OF CHEMICAL EDUCATION
He showed that a trace of hydrogen chloride or hydrogen sulfide also deactivated the platinum and that this contaminated platinum could not be reactivated by heating. This is an example of permanent poisoning. Simple physical adsorption of reactants on the surface of a solid is not the whole story. Silica gel has an enormous surface (one-third of an acre per pound of silica gel), and it is an excellent adsorbent, but i t is a poor catalyst per se, although it makes active catalytic combinations with other catalysts. Not only must the catalyst adsorb the reactant molecuks in the correct proportions and in the right positions, but once reaction has taken place, perhaps by rolling through successive intermediate formations, the catalyst must push off or desorb the product molecules to make room for fresh reactant molecules. Otherwise the catalytic surface is soon smothered with layers of product which makes it impossible for fresh molecules to reach the surface to be activated thereby, for a catalytic surface cannot work a t a distance; there must be actual physical contact with it. ganisms-a field for the biochemist. Some of the industrial reactions catalyzed by inorganic catalysts are hydrogenation, dehydrogenation, aromatieation, hydration, dehydration, esterification, hydrolysis, amination, alkylation, dealkylation, polymerization, halogenation, sulfonation, nitration, desulfurization, cracking, and isomerization.
IPATIEFF AND SABATIER, PIONEERS IN CATALYSIS
Modem industrial catalysis is based on the pioneer work of two professors of organic chemistry, Vladirnir Ipatieff and Paul Sabatier. They started the systematic study of catalysis about 50 years ago-Ipatieff in Russia and Sabatier in France. Ipatieff was interested in high-pressure catalytic reactions, and he is the father of MECHANISM OF CATALYSIS high-pressure technique. Sabatier studied catalytic Our purpose is to describe industrial applications of reactions a t ordinary pressure and considered that high pressure was not only dangerous hut unnecessary. catalysis and not to explain the why and whereforethe so-called mechanism--of catalysis. Many explana- And 50 years ago, high pressure was very dangerous to tions of catalysis have been advanced, and we still fool with because the pressure vessels were weak and don't know much about it. These theories, like hind- poorly made, but today high pressure in industrial sight, by picking and choosing the facts that fit, can catalysis is commonplace and safe. Just to emphasize how young is modern industrial explain the past with the greatest of ease but predict the future with halting steps. For more than 100 years catalysis: Professor Ipatieff is still active in chemistry it has been believed that catalysts promote the forma- a t the age of 79. He is an American citizen and lives a t the Pearson Hotel in Chicago. He is a professor a t tion of transitory intermediate compounds-anything from real chemical compounds to adsorption com- Northwestern University and research director of the plexes-and, by so doing, accelerate the formation of the Universal Oil Products Company. Professor Sabatier, end'products. Even as i t is easier for us to climb a hill recipient of many honors and Nobel Prize winner, died by gradually walking up rather than in a single gigantic in 1941 a t the age of 87. Catalysis would seem to he a jump, so it is easier for a chemical reaction to achieve very healthy occupation. itself through a series of easy intermediate steps rather than in one breath-takin~s w o o p t h e installment ~ l a n SULFURIC ACID Sulfuric acid is one of our really old chemicals. It has in nature. More than 100 years ago Michael Faraday believed been known and used for about 800 years. The chamber that the catalytic effect of platinum on the combustion process, by which about one-third of our sulfuric acid of hvdrown bv oxvmn was due to the adsorntion of the is made, is a catalytic process, and it was in commercial gase"s onYtheplatinum surface. He showed that clean operation, in more or less its present fonn, prior to our platinum caused a mixture of hydrogen and oxygen to Revolutionary War. bum, but that a small amount of grease or carbon monThe first sensible directions for making sulfuric acid oxide killed the catalytic effect of the platinum. He were given by Basilius Valentinus who lived in the late showed that the contaminated platinum could be re- 1600's. Ward, in 1740, manufactured sulfuric acid by activated by heating, which drove off the adsorbed burning brimstone with saltpeter in 50-gallon glass grease and carbon monoxide which fouled the active vessels, and his product was called "oil of vitriol made surface. This poisoning of platinum by grease and by the bell." In 1746 Dr. Roebuck substituted a lead carbon monoxide is an example of temporary poisoning. chamber for the 50-gallon glass-reaction vessel. An iron
FEBRUARY, 1947
buggy loaded with brimstone and saltpeter was wheeled into the lead chamber and the door was closed. The charge was kindled and burned, and the sulfuric acid collected on the lead floor. During the next 50 years larger and larger chambers were built, more air was used in burning the sulfur, s t e m was blown into the chamber, and, finally, the process was made continuous. Around 1870 Glover and Gay-Lussac towers were incorporated into the process, the function of the former being to enrich the gas with nitric oxide, which is the oxidation catalyst, the function of the latter being to remove nitric oxide from the exit gas before it leaves the plant. The nitric oxide catalyst is an oxygen carrier, and this catalysis is an example of homogeneous catalysis, since the reactants (sulfur dioxide, oxygen, and steam) and the catalyst (nitric oxide) are all present in a single gaseous phase. Many explanations have been proposed for this catalytic reaction, but the mechanism is not yet clear, in spite of the age of the chamber process and the enormous investment involved. Numerous more or less imaginary intermediates have been proposed, for example, HNSOs H2NSOr, H2N2SOa,eta. A simple explanation is that nitric oxide picks up oxygen to form nitrogen dioxide and that sulfur dioxide hijacks the nitrogen dioxide and grabs the oxygen. Evidently sulfur dioxide prefers to steal its oxygen rather than get its own. This mechanism was proposed by Peligot in 1844, and it is similar to one by Berzelius (1835) who believed that the intermediate oxide was nitrogen trioxide (N20s). Fortunately for the chamber process, nitric oxide has never become discouraged because i t can never seem to keep its oxygen; it immediately goes after another oxygen and keeps the process going.
I n the contact process a solid oxidation catalyst is used. A mixture of sulfur dioxide and air is passed through a bed of platinum or vanadinum pentoxide catalyst a t 40G50O0C., whereby the sulfur dioxide is oxidized to sulfur trioxide which is subsequently hydrated by water to produce sulfuric acid or dissolved in sulfuric acid to produce oleum. This process is an example of heterogeneous catalysis, the reactants and catalyst being in more than one phase, the reactants (sulfur dioxide and oxygen) being gaseous whereas the catalyst is solid. The rate of oxidation of sulfur dioxide is very much accelerated by the presence of these oxidation catalysts and, if the feed gas is clean, these catalysts will operate continuously for ten years or so. However, they are very easily mined by a variety of poisons, such as arsenic, sulfuric acid, and even dust. The result is that most of the machinery in a contact sulfuric acid plant is devoted to the purification of the feed gas. In 1944 there were about 90 chamber plants and 90
contact plants. The chamber plants produced 3.2million tons of acid and the contact plants produced 5.3-million tons, or 8.5-million tons in all, which means about half a barrel of sulfuric acid for every man, woman, and child in the United States. The contact process had to travel a long road of failure before i t was a commercial success. An 1831 English patent to Phillips, a vinegar manufaeturer, describes oxidizing sulfur dioxide by air over a platinum catalyst and dissolving the resulting sulfur trioxide in water to produce sulfuric acid. This process, which now produces 5,300,000 tons of sulfuric acid per year, was a complete flop in the beginning. I n fact, the process continued to flop for 70 years and chemists tried in vain to make it work. Finally, i t was realized that the feed gas should be carefully purified to remove all catalyst poisons and that an excess of oxygen should be present during the reaction because the oxidation of sulfur dioxide is an equilibrium reaction, but there was still one more difficulty-the absorption of sulfur trioxide in water to make sulfuric acid. This would seem an easy thing to do, but on a large scale i t was very di5cult. A dense white fog of sulfuric acid persisted in escaping from the absorber tower and this represented a loss of product and a loss of countryside. The story is that the operator of one of the early pilot plants took a nap while his boss was out to lunch. Perhaps he made a habit of it, but this t i e something happened. The water supply to the absorber tower was shut off, and the concentration of the acid in the tower rose and rose until it was approximately 98 per cent. About this time the boss returned from lunch and he immediately knew that something was radically wrong, because the absorber tower was no longer smoking like a chimney with wasted sulfuric acid fog. He checked the catalytic converter and found that sulfur trioxide was being produced a t the usual rate. He finally had to accept the fact that the sulfur trioxide was being completely absorbed in the tower, as i t should be, but as it never had been before. The outcome of the nap was the perfection of a process. Sulfur trioxide has ever since been absorbed in 97-98 per cent sulfuric acid, and the concentration of the acid in the absorber is maintained a t this value by feeding in water or dilute acid. The moral of the contact process is that inventors should not be too smart-Mr. Phillips was 70 years too soon, a patent runs only 17 years--operaton should not be too bright, and bosses should not be too strict. HARDENING OF FATS
Liquid vegetable and animal oils, such as soybean, cottonseed, peanut, corn, fish, and whale, are mixtures of unsaturated glycerides which are unsuited for use as shortening, salad dressing, and for the manufacture of soap and candles because of odor, taste, or poor keeping quality. However, these liquid fats can be hydrogeuated to solid and semisolid fats which are suitable for the above purposes. The filling up of the double bonds is illustrated by the following equation. Actually,
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JOURNAL OF CHEMICAL EDUCATION
natural butter so hard that it has to pay ten cents per pound as a handicap for the addition of yellow color in order to be fair to the dairyman. Vested interests get strange dispensations'from the law. If the Japanese (38) CHs(CHl)rCH=CH(CHWOO , H silk raisers had a really smart lobbyist in Washington, ~ H 8 ( c H 2 ) ~ ~ = ~ ~ ( ~ ~ z ) r ~ ~ ~ Jnylon ~ z could be legislated out of existence on the basis that it is unfair to the Oriental worms. (glyceride of oleic acid, a liquid fat)
hydrogenation is stopped before all the double bonds have been Wed. CH8(CHa),CH==CH(CH~)~C0OCH~
4
-
CH~(CHJ~CH~-CH~CHWOOCHI
CH8(CH2)~CHrCHdCH&C0O t.H + 3H10 CH8(CH2),CH*-CHdCH&COO HI (glyceride of stearic acid, a solid fat)
This hydrogenation process is called the hardening of fat because the addition of hydrogen raises the melting ~ o i n of t the fat. It is a large industry which started in the eady 1900's, very soon after the discovery of catalytic hydrogenation. About 1,000,000,000 pounds of hydrogenated oils are manufactured in this country per year. A mixture of liquid fat and finely powdered nickel is heated a t 100° to 200°C. in a pressure vessel with stirring, and hydrogen under a Pressure of 200 pounds or so is bubbled through the mixture until the desired degree of hydrogenation is reached. These hydrogenated fats can be made to have the same composition and properties as natural fats. BY the addition of vitamins, color, etc., a synthetic fat can be madewhich has ~racticallythe Same properties and nutritive value as butter. Synthetic butter pushes
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AMMONIA (FIXATION OF NITROGEN)
Ammonia is the starting point in making nitric acid, explosives, and fertilizer. Although the atomic bombers claim that they can take care of everything in the future, World Wars I and I1 required enormous amounts of ammonia. The Germans postponed their unsuccessful 1914 attemptto take over the world until the German General Staff was sure that the first synthetic ammonia plant was a success. Previously, explosives had been made from saltpeter (NaNOa) which came from Chile. the Germans f e a ~ that d the British Navy would look with a bilious eye a t any such trading with Chile during war time, and they planned to make their nitric by the oxidation of synthetic ~ f world t war ~ 1~all industrialized nations built synthetic ammonia plants and were no longer dependent on Chile saltpeter, which practically mined Chile whose economy was based on saltpeter. Chile has whole deserts saltpeter. Cpmmercial &raction of this m a terial started in the early 18001s,and from 1880 to 1914 the Chilean nitrate industry, controlled by English and German capital, held a world monopoly on &xed nitrogen, and the income from it ran the Chilean Government. Ammonia was discovered by Priestley in 1774; it was first catalytically synthesized from the elements by Faraday in 1825, and later by Ramsay and Young in 1884. Le Chatelier accomplished the same synthesis in 1901, and he used pressure as do the modern processes. Unfortunately, Le Chatelier'spressure equipment blew up, and he decided to leave well enough alone. Thus i t fell to Haber and his coworkers, who started in 1904, to be the first to carry the catalytic synthesis to commercial success. The first commercial plant started production in 1913. There are several processes in use now, with operating pressures varying from 1500 to 15,000 pounds per square inch. The reaction temperature is about 500°C., and the catalyst is a fused iron oxide containing small amounts of other oxides, such as 0.5 per cent of alumina and 0.5 per cent of potassium carbonate. The feed gas is a mixture of one volume of nitrogen and three volumes of hydrogen. is obtained by the liquefaction of air. Hydrogen is obtained from water gas and coke-oven gas, and by the electrolysis of water and the decomposition of natural gas. The ammonia catalyst is an example of a promoted catalyst. A promoter is a substance which, added in small amounts to a catalyst, produces a catalyst of higher activity than correspondmg to the sum of the catalytic activities of the two components. A promoter The classical example of is. at catalyst i ~ ~for a catalyst.
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FEBRUARY, 1947
promoter action is connected with %he Welsbach gas by phosphine, its activit.y being lowered by the presence mantle, in which the brightest light is emitted when of two parts of phosphine per 100,000,000 parts of feed the cotton mantle is impregnated with a chemical mix- gas and ruined by 20 parts of phosphine per 100,000,000 ture containing 99 per cent of thoria and one per cent parts of feed gas. Phosphine is a temporary poison if of ceria. its presence is discovered before much damage has been The ammonia formed in each pass through the cata- done to the catalyst, and purified feed gas will restore lyst bed is removed by condensation or water scrub- the activity of the catalyst. Dark spots appear on bing, and the unreacted gas is mixed with fresh hydrogen the gauze when phosphine is present and grow in size if and nitrogen equivalent to the ammonia formed in the phosphine persists. Torealize the sensitivity that is previous pass and recycled. able to detect two parts of phosphine per 100,000,000 parts of feed gas, let us imagine a machine able to scan N1+ 3H2 -,2NH, the population of the United States for five minutes A small side-stream of recycle gas is vented and thrown and give an answer, yes or no, if two undesirable people away to prevent accumulation of argon which is present are present in the 100,000,000 or so of population. in atmospheric nitrogen. What a machine that would he for a Hitler or a Stalii, During World War I1 the United States Government especially if the machine could also locate the undesirinvested $200,000,000 in ten synthetic ammonia plants. able individuals. The Government plants can produce 700,000 tons of fixed nitrogen per year, and the privately owned plants PHENOL can produce 862,000 tons, making a total capacity of Phenol or carbolic acid is used in the manufacture of 1,562,000 tons. The prewar annual supply was 626,000 phenolic plastics such as Bakelite. In 1940, 24,000,000 tons, of which 451,000 tons were domestically produced pounds of phenol were recovered from coal tar and 72,and 175,000 tons were imported. 000,000 pounds were made synthetically. Phenol is manufactured noncatalytically by the caustic fusion of NITRIC ACID benzene sulfonic acid, by the hydrolysis of chlorobenThe old method of making nitric acid was to distill zene, and by the air oxidation of benzene. There is one saltpeter with sulfuric acid. The Birkeland-Eyde proc- catalytic process, the Raschig process. ess, involving the oxidation of nitrogen in an electric In 1940 the Durez Company started production by arc, is operated on a small scale in Norway. Practically the Raschig process. Benzene is chlorinated by a mixall nitric acid nowadays is made by the oxidation of am- ture of hydrogen chloride and air in the presence of an monia. oxidation catalyst which oxidizes hydrogen chloride to A mixture of one volume of ammo,nia and seven vol- chlorine. The chlorohenzene is hydrolyzed to phenol umes of air is heated to 500°C. and passed through over a phosphate catalyst. The hydrogen chloride fine platinum or platinum-rhodium gauze. The oxida- from the chlorination and hydrolysis steps is recycled tion reaction is thereby catalyzed and about 95 per cent to the oxidation-chlorination stew. and the net result is of the ammonia is converted to nitric oxide. The reac- that benzene plus air gives phenol: tion is very fast, the gas being in contact with the catalyst about 0.0005 second. The nitric oxide is subsequently oxidized to nitrogen dioxide which is dissolved in water to make nitric acid. The heat of reaction makes the oxidation self-sustaining, and the excess heat is utilized to preheat the incoming feed gas. The gauze The idea is a combination of known catalytic steps, automatically maintains itself at 800' to 900°C. This but the engineering is difficult because the equipment is a very clean example of catalysis, and one square foot has to withstand the corrosive action of hot hydroof double gauze produces seven tons of nitric acid per chloric acid. The process was unsuccessful in Germany where it originated, but it was successful in this country day: The oxidation catalyst is very sensitive to poisoning due to superior chemical engineering technique. To be continued
