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Catalytic Ethylene Dimerization and Oligomerization Speiser Et Al Acc. Chem. Res. 2005, 38, 784-793 reaktion)2 of ethylene, nickel salts could modify the nature Catalytic Ethylene Dimerization of the products from R-olefins to 1-butene. This phenom- and Oligomerization: Recent enon became known in the literature as ªthe nickel effectº1,3 and led to the discovery of the ªZiegler catalysisº4 Developments with Nickel and to the remarkable chemistry developed by Wilke and 5 others over decades. The selective synthesis of C4-C20 Complexes Containing linear R-olefins has become a topic of considerable P,N-Chelating Ligands interest in both academia and industry owing to their growing demand most notably as comonomers with ² ,² FREDY SPEISER, PIERRE BRAUNSTEIN,* AND ethylene [C4-C8 to yield branched linear low-density LUCIEN SAUSSINE³ polyethylene (LLDPE) with impressive rheological and Laboratoire de Chimie de Coordination (UMR 7513 CNRS), mechanical properties6], for the synthesis of poly-R-olefins Universite Louis Pasteur, 4 rue Blaise Pascal, F-67070 and synthetic lubricants (C ), as additives for high-density Strasbourg CeÂdex, France, and Institut Franc¸ais du PeÂtrole, 10 Direction Catalyse et SeÂparation, IFP-Lyon, BP 3, F-69390 polyethylene production and for the production of plas- 7-9 Vernaison, France ticizers (C6-C10) and surfactants (C12-C20). The annual 8 Received February 14, 2005 worldwide consumption of polyolefins is close to 10 tons. Because the demand for linear R-olefins is growing faster - × 6 ABSTRACT in the C4 C10 range (a ca. 2.5 10 tons/year market) than Catalytic ethylene oligomerization represents a topic of consider- in the C12+ range, the selective formation from ethylene able current academic and industrial interest, in particular for the of specific shorter chain R-olefins, which could circumvent R - production of linear -olefins in the C4 C10 range, whose demand the typical, broad Schulz-Flory distributions observed in is growing fast. Identifying and fine-tuning the parameters that influence the activity and selectivity of metal catalysts constitute ethylene oligomerization, is therefore a topic of major major challenges at the interface between ligand design, coordina- fundamental and applied importance. tion/organometallic chemistry, and homogeneous catalysis. In this R-Olefins can be produced by four main processes that Account, we show how comparative studies aiming at modulating operate either by elimination reactions (i-iii) or by chain- the coordinating properties of functional ligands for a metal, such 10 as nickel, which is used in industrial processes, lead to beneficial growth reactions (iv): (i) the cracking of paraffins, (ii) effects in catalytic ethylene oligomerization. the dehydrogenation of paraffins, (iii) the dehydration of alcohols, and (iv) the oligomerization of ethylene. The cracking of paraffins still finds widespread application for 1. Introduction the synthesis of ethylene, but generally the first three One of the most important breakthroughs in organome- processes no longer play a central role for the preparation R tallic chemistry resulted from the discovery by Ziegler et of -olefins. Ethylene is a readily available feedstock, and 1 its oligomerization represents the main source for R-ole- al. that, in the AlR3-catalyzed ªgrowth reactionº (Aufbau- fins in industry.11 In contrast to the products from processes i-iii, ethylene oligomerization products possess Fredy Speiser was born in 1971 and received his ªDiplomº at the Rheinisch exclusively an even number of carbon atoms and repre- Westfa¨lische Technische Hochschule (RWTH) Aachen (Germany) in 1999. He sent commercially most valuable products.12-14 The pro- prepared his Ph.D. at the Universite´ Louis Pasteur in Strasbourg and the Institut Franc¸ais du Pe´trole, Rueil Malmaison, under the supervision of Drs. P. Braunstein duction of R-olefins was initially realized by the ªstoichi- and L. Saussine on the oligomerization of ethylene catalyzed by new nickel ometricº Alfen process,2,8 whereby the chain-growth complexes containing P,N ligands. He then joined the Beiersdorf AG in Hamburg reaction on aluminum was followed by the liberation of in 2002 and is currently a Postdoctoral Research Associate in the group of R. Ko¨hn, University of Bath, England. the product in a separate reactor at higher temperatures. A one step catalytic process was developed by Gulf (now Pierre Braunstein was born in 1947 and graduated from the Ecole Supe´rieure de Chimie de Mulhouse (1969) and obtained his Dr. Ing. (1971) and Doctorat d©Etat Chevron Phillips), where both reactions occur simulta- (1974) from the Universite´ Louis Pasteur (ULP) in Strasbourg. He is Director of neously in the same reactor. The Ethyl process (now BP Research with the CNRS and the Director of the Coordination Chemistry Amoco) is a combination of stoichiometic and catalytic Laboratory (UMR 7513 CNRS) of the ULP. His main research interests concern chain-growth reactions.8,9 In contrast to the SHOP process the inorganic and organometallic chemistry of the transition- and main-group elements, where he has (co)authored over 350 scientific publications and review developed by Shell, which is based on organometallic articles. He is a member of various academies, and his most recent awards nickel complexes (see below),8,9,15 other systems for eth- include the Chini Memorial Lecture (Italian Chemical Society, 2003), the Nyholm ylene oligomerization or polymerization require transition Medal (Royal Society of Chemistry, 2003), and the Grand Prix de l'Institut Franc¸ais 12-14,16-18 du Pe´trole (French Acade´mie des Sciences, 2004). metals associated with aluminum cocatalysts (eq 1). Lucien Saussine was born in Nõˆmes (France) in 1948. He graduated from the Ecole Nationale Supe´rieure de Chimie in Paris and worked with Professor M. Julia on the use of organopalladium complexes in organic synthesis. He received his Ph.D. in 1974 from the University Paris VI and, shortly after, joined the ¸ homogeneous catalysis group at IFP (Institut Francais du Pe´trole) where he * To whom correspondence should be addressed. Fax: 33-3-90-24- developed several aspects of the chemistry of transition-metal peroxidic 13-22. E-mail: [email protected]. complexes. Since 1986, his research interests deal with the synthesis of new ² Universite Louis Pasteur. organometallic compounds and their catalytic application for olefin transformation. ³ Institut Franc¸ais du PeÂtrole. 784 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 38, NO. 10, 2005 10.1021/ar050040d CCC: $30.25 2005 American Chemical Society Published on Web 09/09/2005 Catalytic Ethylene Dimerization and Oligomerization Speiser et al. Scheme 1. Catalytic Ethylene Oligomerization Polymerization Both oligomerization and polymerization reactions can Scheme 2. Ethylene Oligomerization Involving Metallacyclic be represented by a general mechanism that involves Intermediates common organometallic elementary steps (Scheme 1). First, a coordinatively unsaturated species is generated, which is stabilized after ethylene insertion by a â-agostic interaction I.19 The coordinated ethylene in complex II inserts into the M-alkyl bond to give III. Finally, after (n - 1) further insertions leading to IV, the product molecule Va is eliminated. The exact mechanism that leads to the liberation of the olefin, chain transfer/â-H elimination, still deserves detailed studies.20-23 For values of n ranging from 1 to 40, oligomers are formed, and for larger values of n, polymers are eliminated (Scheme 1). Isomerization leading to internal olefins Vb is accounted for by species VI and VII. The reaction sequence II-IV depends strongly on the ethylene concentration (pressure) because the rate of chain transfer relative to chain isomerization increases operative with chromium as a metal, in the Phillips with the ethylene concentration.14,24 Brookhart and co- process (pyrrolide ligand), the SASOL mixed heteroatomic workers have noted that an increase in ethylene pressure systems (S,N,S ligands), and the BP system (P,N,P ligands) led to an increased selectivity for R-olefins and in the (pathway B in Scheme 2).29 Although expansion of a seven- Schulz-Flory constant R. Therefore, any change in the to nine-membered metallacycle intermediate was con- ethylene concentration, e.g., pressure, reaction tempera- sidered unlikely, a new MAO-activated chromium/diphos- ture, and ethylene solubility, will have a decisive influence phinoamine system was recently reported, which allows on the branching ratio in polymerization or on the R-olefin ethylene tetramerization with selectivities up to 70% for content in oligomerization reactions.14 1-octene.30 Reaction mechanisms, which resemble the Another reaction mechanism, shown in Scheme 2, can concerted coupling, are observed for transition metals apply to systems producing exclusively a single group of such as titanium,25,28,31 tantalum,32 nickel,33 or chrom- olefins such as, e.g., 1-butene or 1-hexene.25-28 ium.26,27,34-37 Here, two molecules of coordinated ethylene in VIII A significant difference between degenerate polymer- react by oxidative coupling to form a metallacyclopentane ization and concerted coupling is that only the former may IX. In the titanium-based Alphabutol process, an intra- also yield internal olefins (or branched polymers), whereas molecular hydrogen transfer and a reductive elimination the concerted coupling only gives terminal olefins. are observed at this stage, liberating 1-butene (pathway Numerous NiII catalysts tend to favor chain termination A in Scheme 2).25 Insertion
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