Organometallic Chemistry and Catalysis in Industry George W. Parshall and Richard E. Putscher Central Research & Development Department. E.I. duPont de Nemwrs & Company, Experimental Station. Wilmington, DE 19898 The period from 1950to 1977 was one of the most remarkable in the history of chemistry for the development of hoth chemical science and chemical technology. These years encompassed a virtual explosion in our knowledge of organometallic chemistry and in the use of this chemistry in catalytic processes. A whole new technology of organometallic catalysis, especially for olefin polymerization, blossomed (1). Nobel prizes were awarded to Karl Ziegler, Giulio Natta, Geoffrey Wilkinson, and E. 0. Fischer for their contrihutions to this area of science and technology. As illustrated in the figure, the volume and value of products arising from catalytic processes began meteoric growth in the 1950's. While the largest components are based on conventional heterogeneous catalysis, the value of products derived from organometallic catalysis and soluble (homogeneous) catalysts is also quite impressive. Traditionally, heterogeneous (solid) catalysts have been used for production of large-scale commodity chemicals such as methanol and ammonia and in the production of high octane gasoline from petroleum. Homogeneous catalysts, which are soluble in the reaction medium, are commonly used in the production of high purity, high value chemicals. The catalysts used for production of polypropylene and high density polyethylene are often hybrid species in which an organometallic functional group is hound to a solid support. The distinctions between these classes of catalysts are rather arbitrary because similar chemical reactions often take place in the coordination sphere of a catalytic metal ion whether i t is present in solution or adsorbed on a solid surface. Homogeneous and heterogeneous catalysis hoth began t o he used in the chemical industry about 1910,hut as shown in the figure, heterogeneous catalysis grew steadily while the use of soluble catalysts was minor until the 1950's. A major difference was that many large scale hydrogenation reactions were found to proceed well with solid catalysts. These reactions included the synthesis of methanol, ammonia, and aniline. In contrast, the applications of homogeneous catalysis were largely limited to reactions of acetylene, an expensive starting material. In spite of its cost, however, acetylene was used to produce organic chemicals such as acetaldehyde, acrylonitrile, and vinyl monomers (1). Catalysis underwent a spurt of growth during World War 11. The Allied forces gained a substantial advantage in aircraft performance through use of high octane gasoline ohtained by "cracking" of petroleum with heterogeneous catalysts (2). On the Axis side, German tanks operated on diesel fuel produced from coal-based "synthesis gas" by the Fischer-Tropsch process (3). Synthesis gas, a mixture of CO and hydrogen, was reacted over a metallic catalyst to produce a mixture of hydrocarbons akin to fuel oil. German industry also explored soluble catalysts to make a great variety of organic chemicals from CO and acetylene. In the years following the war, a number of factors worked together to generate the impetus for the burst of new technology mentioned in the introduction. On the "market pull" side, a large expansion in the number of automobiles created a great demand for high quality gasoline, thus stimulating broad use of catalytic cracking and new catalytic reforming techniques in petroleum refining. Similarly, the growth of the new synthetic polymer industry created a demand for
new, highly pure chemical intermediates. The purity requirements favored the use of soluble catalysts, which are frequently more selective for the formation of a single product than are heterogeneous catalysts. The "market pull" for new chemical processes was comdemented hv the "technoloev -..~ u s h "in the form of new organometallic chemistry. For example, alkylaluminum chemistry developed by Ziegler and his co-workers at the Institut fiir Kohlenforschung a t Mulheim (4) was a major factor in the evolution of organometallic catalysts for the synthesis of polyolefins. They observed that ethylene inserted readily into an AI-H or an AI-C bond to produce lowmolecular-weight polymers
In a remarkable chance discovery they found that traces of transition metal compounds (especially titanium chlorides) modified the reaction to produce a high-molecular-weight polymer (5).The new polymer was different from conventional ~olvethvlene " . in that i t was denser and higher meltine. This difference arose from the fact that the chains were linear, in contrast to the branched chains produced hy free radical polymerization of ethylene under high pressure. Polypropylene produced with related catalysts in Giulio
l 3 o M h of catalysis in the 20th cemury: dollar value of worldwide prcduct stream.
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Natta's laboratory also had unusuallv useful properties hecause the polyme; chains were ordered with ail the pendant methyl groups on the same side of the extended chain. From these discoveries, as well as work on chromium-based catalysts at Phillips Petroleum, grew a major industry, the catalytic production of the thermoplastic polyolefins. An equally interesting and significant event a t the time was the develooment of the Wacker orocess for ~roduction of acetaldehyde from ethylene (6). '?his innovation began the obsolescence of acetylene as a starting material for the production of organic chemicals. The Danish pharmacist Zeise had discovered in 1827 that olatinum(I1) chlorides coordinate with ethylene to form st&le comile&es, hut an American chemist, F. C. Phillips, found in 1894 that analogous palladium(I1) chlorides oxidized ethylene to acetaldehyde, C,H, H,O + Pd2+ CH,CHO + Pdo + 2 Ht
.
~~~~~~~
-
+
This observation, in itself, had no technological significance because palladium salts were too expensive t o use as stoichiometric oxidizing agents, hut Jurgen Smidt and his coworkers at Wacker-Chemie saw i t was a starting point for a catalvtic orocess. Thevfound a wav to reoxidize the reduced palladium species with air in situ; thus creating a catalytic process,
This process was rapidly adopted by industry because i t substituted cheap ethylene for the expensive acetylene previously used. The Wacker process was especiallv imoortant as an example of how academic scientific knowledge could he transformed into practical technology. As such, i t set the pattern for researchin the 1960's that-led to newcommercih technology in the 1970's. One major benefit of the 1960's work was the recognition of the rich and varied coordination chemistry of rhodium. Geoffrey Wilkinson and his students a t the Imperial College of Science and Technology showed that soluble complexes of rhodium were efficient catalysts for the hvdroeenation of olefins. One of these comolexes. R ~ C I ( P P ~ , ) ~customarily ,-~~ called Wilkinson's catalyst: Even more importantly, Wilkinson and others showed that complexes such as RhH(CO)(PPh3)3 catalyzed reactions of carbon monoxide such as hydroformylation (7).
Even though many of the reactions catalyzed by rhodium could he acromplished with catalysts hased on cheaper and New Homogeneous Catalytic Processes
Stan Up
Estimated 1983 US. Production (metrictons)
Du Porn
1971
307,000
Shell
1977
183,000
Celanese, Union Carbide Monsanto
1976
264.000
1970
522,000
Monsanto
1974
...
Company
Process
Adiponiirlle from HCN and butadlenea Linear aaleflnsfrom ethylenea Rh-catalyzed Hydrotormyiations Acetic Acid fromCO and methanola LOOPA by asymmetric hvdrwenatlon'
.Seidsl. W. C.: Tolman. C. A. Ann. N Y . Acad. ScI. 1983. 415.201
~ham~ae*.1974, (23 &I 70. ), *Prutm. R. L. Ann N Y . Acsd Scl. 1977,295,239. %om.
J. F.: CraWoc*. J.
n.:M n h M n A,:
PaulL. F. E. Chem. T&.
*Ref. 8
190
Journal of Chemical Education
1971.
1,800.
more abundant metals, the efficiency of the rhodium catalysts made them more attractive. For example, the hydroformylation of propylene t o give hutyraldehyde (a major intermediate in the production of plasticizers and lubricants) had long been carried out with cobalt catalysts such as Co2(CO)s, hut RhH(CO)(PPh3), showed many advantages. The cobalt catalysts require high pressures (-200 atm) and give about 70% yields of n-hutyraldehyde along with some branched isomer. In contrast, the rhodium catalysts operate a t low pressure (10-20 atm) and give over 90% yields of the linear product. The advantages are such that both Celanese and Union Carbide commercialized rhodium-based hvdrofomvlation processes in the mid-1970's (see table) ( 1 ) : The tahle lists five sienificant new homoeeneous catalvtic processes brought intoommercial productyon in the earfir to mid-1970's. These processes shared the characteristics of giving high yields under mild conditions and of heing hased on a good understanding of the underlying organometallic chemistry. The two Monsanto processes, like the hydroformylation processes, are based on rhodium catalysts, while the Du ~ o nand t Shell processes use soluhle nickel catalysts. The second Monsanto process, the synthesis of L-DOPA (a drug used in treatment of c ark ins on's disease), is not carried out on a large scale. Nevertheless, its introduction in 1974 was highly significant both because i t was the prototype use of homogeneous catalysis in synthesis of fine chemicals and because it is one of the most selective catalvtic reactions yet reported (8). I t involves hydrogenation of a prochiral olefin to eive specificallv one optical isomer of the . product, H
A ASH,.'CH,
A~CH=C, NHAc
/COOH NHAc
The success in formine the desired optical isomer in over 90% yield arises from use of a rhodium complex (related to Wilkinson's catalvst) in which the . ~hosohine lieands are . chiral, thus creating chiral environment around ihe rhodium ion.
a
Tlme ol Change The burst of activitv in laree scale industrial use of homogeneous catalysis endkd in 1577. New plants continue t o he built, hut, with one significant exception mentioned helow. there has been little introduction of new technology. The causes for this lull in innovation are complex, hut all seem to be based on the relative maturity of th; chemical industry. Few new products of thescale of nylon or polyester are heing introduced: hence. there is little demand for new intermediates. The existing technology is very efficient, so there is little incentive to develoo new orocesses. Perhans most importantly, industry's investment in current plants is enormous. The raoacirv exist3 tu make all the chemiral intermediates needed in t i e near future. A new process must he uery good to justify scrapping an existing plant. Despite these negatlve influences on the growth of homogeneous catalysis as we have known it, the prospects for the furure are bright. This optimistic prognosis is based on opportunities presented LO the chemical indurtrv hv two maior factors: 1977-1985-A
changes to new feedstocks; remarkable grlwth in fine rhemirnls such as pharmaceuticals, agricultural chemicals, and rpecialty polymers. Both of these developments call for sophisticated new technology, much of which can he hased on organometallic chemistry and homogeneous catalysis. Future changes in feedstocks for the chemical industry
may result from limitations on the availability of cheap vetroleum and natural gas, the major starting materials for the production of organic chemicals. ~ 1 t h 0 G ~these h raw materials are abundant now, it seems clear that their cost will escalate significantly by the end of the century. Given the long lead time involved in developing new industrial process& now is the time to start research on processes based on enduring feedstock such as coal and biomass. Processes hased on synthesis gas, such as the Monsanto acetic acid process, have a great advantage because CO and Hz can be produced from coal or organic refuse. An excellent example of this trend is the sole major new process commercialized in the 1977-1985 period, Tennessee Eastman's coalhased acetic anhydride plant (9).In this plant, which started production in 1983, coal is used to generate synthesis gas. Hydrogen and CO are combined to form methanol, which is then reacted with more CO to produce acetic anhydride. The soluble catalyst in the final step resembles that used in the Monsanto acetic acid process. This plant is very significant as a nrototvoe in the switch to coal as a feedstock. ~ k secdid e major trend in homogeneous catalysis, its use in svuthesis of fine chemicals. is illustrated bv the Monsanto L-%PA process described above. This pro~ess,however, is not the only example nor are pharmaceuticals the only commercial outlet. A new generation of agricultural chemicals is far more selective and potent that those used in the past. These compounds are applied a t a rate of a few ounces per acre (rather than pounds per acre) and are much more complex chemically than their predecessors. For example, some potent new insecticides contain cyclopropane ring structures like that in the natural insecticide pyrethrin. At least
in initial nroduction processes. a soluble rhodium complex was used to generate [he cyclopiopane ring hy catalytic addition of the carbenoid fragment :CHCOOEt to an olefinic double bond (10). Similarly, homogeneous catalysis plays a key role in synthesis of 2,6-diethylaniline, which is an intermediate in manufacture of Lasso herbicide. An aluminum anilide complex is used to catalyze the ortho-ethylation of aniline (11): In summary, the burst of creative activity in commercial use of homogeneous catalysis from 1955 to 1977 has been followed by a time of change in which the character of research objectives has altered dramatically. Instead of designing huge plants to produce polymer intermediates and commoditv chemicals, much of the emphasis is now on sophisticated chemistry t o produce cornpiex organic molecules that will be exceedingly valuable even though the scale of production may he small. The opportunities for creative research in organometallic chemistry and homogeneous catalysis seem greater than ever. Literature Cried ,I Pnrrhnll.C: !V ' Ilt~mo;~~~~nuaCatsly~~~~', Wiicv-lnrcruicnrc Ycu Y ~ r k 1980. . M . s c l c ~ C C J i-nem h d b - 1981.,,1.655 ,\~,d+nuu R I 3 "The FVrhcr Tr~wnSynrheaa'': r s d c m l c New York. 1981
.I I I,
pp 19P269. Ziegler, K.: Halzkamp, E.; Bmil, H.: Martin: H. Angolu. Chem. 1955,67,426,541. Smidt. J.: Hafner. W.: Jire. R.: Ssdlmeier.. J.:. Sieber.. R.; . Ruttimer. R.;Koier. H. ~ n . & ~ . ' c h e mism; . 71.1'76. Bra-, C. K.; Wilkinson, G.J. Chom. Soe. A. 1910.2753. Knowloa, W.S. Acc.Chpm.Ras. 1983,16,106. Layman, P. L. Chem. En#. N P W1982, ~ (29 NovJ. 9. Anciaur. A. J.: Hubert.. A. J.:,N d r . A. F.:. Petiniot.. N.:. Tevwie. . . Ph. J. O m Chem. L980, b5.695. Sfmh, R. In "Newer Methoda of Preparative Organic Chemistry"; Foerst. W. Ed.; Academic: Nelv York, 1963; Vol2.
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