13 June 1980, Volume 208, Number 4449 SCI ENCE Organometallic Chemistr3yTin Homogeneous Cata13Ksis George W Pa,rshall In most large-scale processes in the chemical industry catalysts are used to accelerate the desired reactions, both to increase productivity and to reduce the loss of starting materials in undesirable side reactions Most commonly, the catalysts are insoluble metal oxides or metals, so-called heterogeneous catalysts However, in an increasing number of processes, the catalyst is a transition metal complex that is soluble in the reac- interrelation betweera organometallic chemistry and catalys,is has stimulated extensive organometzallic research in both university and inidustrial laboratories The fundamental research that led to Nobel prizes for Karl Ziegler and Giulio Natta in 1963 and for Geoffrey Wilkinson and E F'ischer in 1973 has often underlain comme rcial development of processes similar tto those listed in Table Summary Many reactions catalyzed by soluble transition metail compounds proceed by way of organometallic intermediates, even though the ori ginal catalyst may be a simple salt This generality is illustrated for three industrial s!yntheses of acetic acid that use homogeneous catalysts Some developments in orgariometallic chemistry that may extend the utility of homogeneous catalysis are photo activation of catalysts and the recognition of the importance of metallacyclic interm ediates use of more economical starting materials or feedstocks From the first commercial use of a soluble transition metal catalyst in 1913 until about 1950, nearly all industrial applications of homogeneous catalysis were based on reactions of acetylene Much of the acetic acid made in this period was produced by a two-step process in which acetylene was hydrated to acetaldehyde, which was then oxidized to acetic acid (first reac- tion in Fig 1) Both steps were catalyzed by soluble metal salts This process had the disadvantage that acetylene is a hazardous chemical that requires large quantities of energy for its manufacture These characteristics provided a major incentive to replace acetylene with safer, more economical feedstocks Research in Germany from 1940 to 1960 showed that olefins, especially ethylene, could be substituted for acetylene in the production of many industrial chemicals Although much of this research was empirical, it provided the basis for much of homogeneous catalysis as we know it today Wacker-Chemie developed an elegant process for the catalytic oxidation of ethylene to acetaldehyde (second reaction in Fig 1), an ace- tic acid precursor as described above This process began commercial operation about 1960 and is still used extensively The same German research also laid the basis for utilization of synthesis gas, a readily available mixture of carbon monoxide and hydrogen, as a feedstock for the chemical industry A major virtue of synthesis gas is that it can be made I will illustrate here tthe role of organotion medium These homogeneous catalysts are used in about two dozen signifi- metallic compounds in catalysis in terms cant processes in the American chemical of three processes for the producindustry (I) Some of the varied appli- tion of acetic acid (CEi3COOH) (Fig 1) cations are listed in Table The total All of these processes proceed through a from many carbon sources-natural gas, production by homogeneous catalytic series of organometalIlic intermediates, petroleum, coal, or biological wastes processes in 1977 was nearly 10 million even though the catal)yst precursor add- This versatility commends it as a feedmetric tons, worth over $5 billion These ed to the reaction vess el is a simple metal stock for the future just as it did in the totals are small compared to those asso- salt with no C-M bo nds Because the petroleum-starved Germany of World ciated with the heterogeneous processes catalysts are soluble itn the reaction me- War II Both Badische Anilin und Soda of the petroleum industry but have pro- dia, it has been possilble to study these Fabrik (BASF) and Monsanto have deprocesses by the c(onventional tech- veloped processes to convert methanol vided a major incentive for research Many of these catalytic reactions pro- niques of physical o rganic chemistry derived from synthesis gas to acetic acceed by way of organometallic inter- Consequently, much more is known id by catalytic carbonylation reactions mediates (2), compounds that contain di- about the mechanism!s and reaction in- (third reaction in Fig 1) These processrect metal-carbon (M-C) bonds Such or- termediates in these acetic acid syn- es appear extremely useful in the energy ganometallic species occur even with theses than for classi4cal heterogeneous The author is in the Central Research and Develheterogeneous metallic catalysts or with catalytic processes Department, E I du Pont de Nemours & The three acetic ac id preparations of opment soluble catalysts based on simple salts Company Experimental Station, Wilmington, Delasuch as rhodium chloride, RhCl3 This Fig also illustrate al trend toward the ware 19898 SCIENCE, VOL 208, 13 JUNE 1980 0036-8075/80/0613-1221$01.00/0 Copyright K) 1980 AAAS 1221 and feedstock context of the next two decades These three processes are described individually below with emphasis on the nature of the organometallic chemistry that underlies the technology -02 -, CH3C00H cule by interaction with the metal ion illustrates a major characteristic of cata0 CH3CHO H2C=CH2 + 02 CH3COOH lytic reactions The mixing of molecular orbitals of the organic compound with those of the metal complex provides a CO + 2H2 )- CH3OH CH3COOH for reactions of the Fig The basic reactions in acetic acid syn- low-energy pathway molecule the overall organic Although theses of successive "feedstock eras." reaction could occur in the absence of Acetylene Hydration the catalyst, its rate is greatly enhanced by reduction of its normal activation enIn 1916, under pressure of feedstock on the process was done before the de- ergy If the catalyst favors only one of shortages brought about by World War I, velopment of modern kinetic and spec- several possible reactions of the organic plants in Germany and Canada began to troscopic techniques compound, the catalyst improves the produce acetic acid from acetylene (Fig Figure illustrates one reasonable yield as well as the rate of the reaction 1) (3) This technology was widely used proposal (4) for the mechanism of this for 50 years process The sequence of reactions is deThe catalysis of acetylene hydration picted in Tolman's cyclic convention (5) Acetaldehyde Oxidation by mercury salts was discovered in 1881 to emphasize the cyclic nature of the catand was used as a laboratory synthesis of alytic reaction In the initial step, startThe catalytic reactions of acetylene, acetaldehyde for many years The com- ing from the top of the cycle, an acety- olefins, and carbon monoxide usually inmercial process was a scaled-up version lene molecule coordinates to Hg2+ This volve prior coordination of the organic of the laboratory method in many ways coordination involves mixing of the vr reactant to the metal catalyst through CAcetylene was blown through a hot, di- and 7r* acetylenic molecular orbitals M bonds In contrast to these organolute sulfuric acid solution that contained with both filled and unfilled orbitals of metallic processes, another major famsmall amounts of mercury and iron salts the metal ion Overall, the interaction ily of catalytic reactions is not based on as catalysts The acetylene was added at produces a fairly strong bond with some C-M bonds in the intermediates The such a rate that unchanged acetylene net transfer of electron density from the most common of these are Co2+-cataswept the acetaldehyde product out of organic molecule to the metal This de- lyzed air oxidations of organic comthe solution before it could undergo un- pletion of electron density on the coordi- pounds The oxidation of acetaldehyde desirable side reactions The yield was nated acetylene in structure makes to acetic acid is typical high and the process was simple, two acetylene susceptible to attack by nucleThe oxidation of acetaldehyde is the characteristics that kept this technology ophilic reagents such as water Reaction second step in the production of acetic with water and dissociation of HI forms acid from acetylene or ethylene (6) A economically viable until recently The chemistry of this seemingly a 2-hydroxyvinylmercury complex similar reaction has also been used to simple process is quite complex Even (structure 2) Cleavage of the labile C- oxidize grain alcohol to acetic acid, a though the nominal catalyst mercuric Hg bond by acid regenerates the Hg2+ process that may become significant if sulfate (HgSO4) is a classical inorganic ion for another catalytic cycle The vinyl natural products again become major salt, organometallic intermediates partic- alcohol liberated in this step rearranges feedstocks for the organic chemicals ipate in the catalytic cycle The nature of to its more stable isomer, acetaldehyde, industry Typically, acetaldehyde that the catalysis is not as well defined as one under the usual reaction conditions contains a trace of the acetate of Co2` is would like because most of the research The activation of the acetylene mole- heated with air or oxygen at 65°C and atmospheric pressure The yield of acetic acid approaches 90 percent, and the technology is relatively simple Table Catalytic processes based on organometallic chemistry The chemistry consists of two basic 1977 U.S steps (7): production Catalytic process (X 103 02 0-CH3C 0metric tons) 'Soluble organometallic catalysts b_ 2CH3Ca CH3C Acetic acid from CO and H2 or ethylene 590 OH 'H Alcohols from CO-H2 and olefins 780 Long-chain olefins from ethylene 177 The first step, oxidation of acetaldehyde Adiponitrile from hydrogen cyanide and butadiene 200 Chloroprene from chlorine and butadiene 166 to peracetic acid, is a free-radical proPolybutadiene, polyolefins 845 cess that does not involve the metal ion Surface-supported organometallics except perhaps as an initiator The metal Polyethylene (high density) 1,140 ion catalyzes the second step in which Polypropylene 1,230 Total organometallic the peracetic acid oxidizes another mole5,128 Soluble nonorganometallic catalysts cule of acetaldehyde The chemistry of Acetic acid from butane oxidation this step is less well understood One of 986 Terephthalic acid fromp-xylene oxidation 2,277 the many reactions in the catalytic cycle Adipic acid from cyclohexane oxidation 698 is the oxidation of the cobalt ion: Polyester polymerization 1,655 Polyurethane polymerization 1,500 Total nonorganometallic CH3O3C + Co2+- CH3C ! + C03 + OH 7,116 "OOH~~~~~0 Total organometallic and homogeneous 12,244 1222 HC OCH + H20 - CH3CHO SCIENCE, VOL 208 Ka CH3Co Hg HC CH / H* r ^ H H+~~~~ Fig A likely mechanism for the catalysis of acetylene hydration by Hg2+ within the coordination sphere of a metal 2Cu Pd2+4C2H4 ion Curiously, almost every step in this 2Cu2+ CH2 scheme is controversial in spite of carePCU22vt &1Pd2+ ful study with modem kinetic and spec\ H20 Pd° troscopic techniques One major aspect of this scheme, the CH_ activation of the olefin by coordination H\ ZOH CH2OH to a metal ion, is unchallenged As in C-Pd+ acetylene hydration, the mixing of ethylene v and ir* orbitals with the orbitals of CH3 'lK, CHOH h