Selective Ethylene Oligomerization - American Chemical Society

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Selective Ethylene Oligomerization Orson L. Sydora*

Organometallics Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 02/18/19. For personal use only.

Research & Technology, Chevron Phillips Chemical Company LP, Kingwood, Texas 77339, United States ABSTRACT: Ethylene oligomerization to produce 1-alkenes is a cornerstone of organometallic research. The original αolefin commercial catalyst system, triethylaluminum, has grown to become one of the highest volume organometallics. This tutorial covers the developmental arc of ethylene oligomerization research and explains the ongoing technological shift from full-range (C4−C30) to selective (C6, C8) catalytic systems. Catalyst design determines product selectivity, and the differing mechanisms underpinning catalyst performance, linear versus metallacyclic, are covered in detail. Despite the field’s maturity, there are still significant opportunities for exploration and discovery which are discussed at the conclusion.

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INTRODUCTION

NONSELECTIVE ETHYLENE OLIGOMERIZATION Commercial Origins and Implementation. The original business driver for α-olefin production was the detergent market. Branched alkylbenzenesulfonates (alkyl group derived from propylene tetramer)3 were introduced into washing detergents in the United States beginning in the late 1940s. These compounds were unexpectedly resistant to biodegradation and dramatically polluted major waterways in the 1960s, forming large foam fields.1 A rapid technology fix was needed, and linear alkylbenzenesulfonates (alkyl group derived from C14−C16 1-alkenes) were identified as a safe and effective

Ethylene oligomerization is a massive research field that began over a half a century ago and continues to be actively pursued today. The commercial implementation of several catalyst technologies has produced a suite of intermediates, 1-alkenes, also referred to as linear α-olefins (LAOs) or normal α-olefins (NAOs), whose derivatives have a significant effect on the daily lives of all global citizens.1 The chain length determines the key application for each α-olefin. Shorter chain lengths (C4−C8) are used to modify the rheological melt and solid resin properties of polyethylene. High-density polyethylene (HDPE) grades contain 1−2% inserted α-olefin (usually 1hexene), while linear low-density polyethylene (LLDPE) grades can incorporate up to 10% comonomer.2 Longer chain length 1-alkenes provide lipophilic moieties key to micellular formation in surfactants. Again, specific chain length ranges are crucial for each application. Water-soluble surfactants such as hand soaps use C12−C16 based α-olefin sulfonates, while oil-soluble heavy-duty lubricant detergents favor longer chain C16−C24 based linear alkyl benzenesulfonates. Intermediate-length 1-alkenes (C10−C12) are oligomerized, hydrogenated, and distilled to produce high-performance lubrication base stocks. This tutorial will cover the scientific advancements that have shaped past and future design trends for commercial ethylene oligomerization processes. The catalyst architecture forms each technology’s foundation, dictating both the plant process and downstream separation scheme. Metal−carbon bonds are ubiquitous in ethylene oligomerizations, and many classic organometallic studies originated from investigations of ethylene oligomerization and closely related ethylene polymerization catalysts. In this tutorial, I will endeavor to include relevant discoveries, mechanistic proposals, and key catalyst structural features. © XXXX American Chemical Society

Scheme 1. Initiation Steps for Triethylaluminum-Catalyzed Ethylene Oligomerizationa

a

RDS denotes the rate-determining step.

alternative. There was no commercial process available to manufacture the desired α-olefins at the time; however, the catalyst technology did exist. Gellert, during his research at the Mülheim laboratories with Nobel Prize awardee Karl Ziegler, observed that lithium aluminum hydride on heating with ethylene produced α-olefins.4 Further work showed that simple alkylaluminums were also competent for ethylene oligomerization.5 Ziegler named the ethylene oligomerization the Auf baureaktion, which translates to construction or building reaction. The Aufbau chemistry is elegant in its simplicity and a triumph of main-group organometallic chemistry. Triethylaluminum (TEA) slowly oligomerizes ethylene under forcing Received: October 31, 2018

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DOI: 10.1021/acs.organomet.8b00799 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2. Cossee-Type Ethylene Oligomerization Mechanism

bar).10 The precatalyst nickel hydride is formed reductively in situ by combining a nickel(II) salt, sodium borohydride, and a P,O-ligand in the presence of ethylene (Figure 1A).11,12 Shell

conditions utilizing high temperature and ethylene concentration to produce a broad distribution of even-carbonnumbered oligomers. The reaction rate is described by the following: rate = kobs[Al2Et6]1/2[CH2CH2].6 Neat triethylaluminum is a dimer at room temperature, with two ethyl groups bridging through two-electron−three-center bonds [Et2Al(μ-Et)2AlEt2]. The dimer must first dissociate to provide a vacant orbital for ethylene insertion (Scheme 1). The dimer equilibrates with its monomer as the temperature is increased; for example, 20% of neat TEA exists as a monomer at 200 °C.7 Dilution pushes the equilibrium further to the right, and a 1 mol % solution of TEA in hydrocarbon at 200 °C contains 90% monomer. Ethylene does not bind triethylaluminum to form a discrete intermediate; rather, it undergoes insertion through a concerted asymmetric transition state with partial negative charge building on the aluminum and partial positive charge on the incipient β-carbon center.8 Chain termination can be driven by either thermally induced β-H elimination or transmetalation to a nickel catalyst (often referred to as the nickel effect), which induces α-olefin product formation and generates the corresponding aluminum hydride. Ethylene readily inserts into the Al−H bond, restarting the cycle. Trialkylaluminums do not have a propensity to polymerize ethylene, given their slow insertion rates and relatively high termination rates; thus, reactor fouling is nominal. Most αolefins are still generated using Ziegler’s original catalyst system: a testament to the operability, low catalyst cost, and reliability of the process.9 The Gulf and Ethyl Chemical Companies received licenses in the 1960s granting them access to the Ziegler Aufbau catalyst technology.1 Each company chose a different process strategy to control the α-olefin fraction distribution. Gulf Chemical, whose α-olefin production technology is now operated by Chevron Phillips Chemical Company LP, was the first company to commercialize a catalytic ethylene oligomerization process. The initial unit started up in 1966, producing 120 million pounds of α-olefins annually at Cedar Bayou, TX. Today over 2 billion pounds of α-olefins are produced at that same location. Ethyl Chemical Company, whose α-olefin production technology is now operated by Ineos, commercialized a stoichiometric process in 1971. This process will be reviewed later in Poisson Distribution. A drawback of TEA-based ethylene oligomerization technology is the high reactor pressure (>200 bar) required to drive the reaction to reasonable rates. Transition-metalcatalyzed ethylene oligomerization permits operation at substantially lower pressures. Two metals (Ni, Zr) are used in commercial LAO processes. The Shell Higher Olefin Process (SHOP) began operation in 1977, using a nickel catalyst to oligomerize ethylene at moderate pressure (90

Figure 1. (A) Commercial SHOP catalyst preparation. (B, C) Structurally characterized complexes proposed as models for SHOP intermediates (R = CF3).

has not disclosed the exact structure of the SHOP ligand, but most reported systems contain a bidentate monoanionic κ2-PO ligand forming a five-membered chelate (Figure 1B).13−15 The process runs under three-phase conditions: (1) the nickel catalyst dissolved in the polar solvent 1,4-butanediol, (2) nonpolar liquid LAO product, and (3) gaseous ethylene. This design permits recycling of the catalyst, solvent, and ethylene after simple liquid fractionation from the immiscible product. The intermediate and heavier α-olefins are further processed by metathesis and hydroformylation to produce alcohols, which are alternative surfactant feedstocks. The α-SABLIN and Idemitsu processes use a zirconium metal source (i.e., ZrCl4), an alkylaluminum cocatalyst (i.e., Et2AlCl), and Lewis base modifier (i.e., 1,4-dioxane) to adjust the distribution. Highmolecular-weight polymer formation has not been fully mitigated in either Zr system, and commercialization has been limited to two relatively small plants. General Oligomerization Mechanism. The general mechanism of metal-catalyzed ethylene oligomerization was proposed by Cossee (Scheme 2).16 Ethylene insertion into a metal hydride (i.e., SHOP) or metal−alkyl bond (i.e., TEA) initiates chain growth. Ethylene insertion into each subsequent 1° M−C bond (kp) causes chain propagation. Chain termination is executed by classical terminal β-H elimination or induced by β-chain transfer to ethylene, forming the desired 1-alkenes (kt). Significant off-cycle reactions include 1,2insertion of α-olefins followed by β-H elimination to form vinylidenes, chain walking followed by β-H elimination to form internal olefins, and 2,1-insertion followed by chain walking and/or β-H elimination to also form internal olefins.17 Metal hydride intermediates have been isolated and characterized for aluminum- and nickel-based systems. A nickel hydride complex (Figure 1C), [{Ph2PCH2C(CF3)2O}NiH(PCy3)], was structurally characterized and found to insert ethylene to form the corresponding nickel ethyl complex, although further insertion was not observed. These complexes were proposed as models for intermediates in the SHOP B


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Figure 2. Representative Poisson distributions where χ = λke−λ/k! starting at k = 0. The carbon number equals 2k + 2, and the distribution is renormalized to exclude unreacted ethylene.

process.18 [Et2AlH]3, a trimer in the liquid phase at room temperature with bridging hydrides, was isolated as an intermediate in the synthesis of TEA using the Ziegler direct method.19 Olefin insertion into trimeric [iBu2AlH]3 has also been studied extensively as a model system for the Aufbau process.20 Olefin hydroalumination favors the formation of primary Al−CH 2 R′ bonds with olefin insertion rates decreasing in the following order: CH2CH2 > RCHCH2 > R2CCH2 ≫ RCHCHR.8 A similar trend is observed for carboalumination, albeit at slower rates; therefore, high operating ethylene concentrations (pressures) are used to prevent side reactions with generated 1-alkenes. α-Olefin insertion is not perfectly regioselective; for example, at room temperature 97% of 1-alkene undergoes 1,2-insertion with [iBu2AlH]3 while 3% undergoes 2,1-insertion to form a secondary aluminum alkyl. The sterically hindered branched aluminum alkyl will not undergo further insertion and eliminates to generate an internal olefin or regenerate the 1alkene. Poisson Distribution. Stoichiometric ethylene oligomerizations can be modeled by a Poisson distribution which depends solely on the rate of occurrence. Each ethylene insertion must occur independently without chain-end influence for this model to be valid. For TEA, the rate of occurrence (λ) corresponds to the molar ratio of ethylene to each individual growing chain or 1/3 [AlEt3 ]. 1 The distribution broadens dramatically as the stoichiometric ratio increases (Figure 2). Chain termination rates must be minimized to achieve stoichiometric behavior. This is commercially practiced by separating the process into two separate steps (Scheme 3, top). Ethylene oligomerization is performed at lower temperatures (99%) almost exclusively. The high termination rates required to yield butene-only catalysts also churns large amounts of ethylene through nonproductive exchange. Thus, the main product of low K value ethylene oligomerization catalysts is ethylene.25 This activity is usually not accounted for in reported turnover numbers, therefore drastically underestimating the true catalytic activity of most ethylene dimerization catalysts. It also leads one to suspect that researchers have synthesized many incredibly active ethylene oligomerization catalysts but never noticed their activity due to

use the mol C12/mol C10 ratio to calculate the K value, since these are relatively abundant nonvolatile liquid fractions with GC peaks well separated from reaction solvents. 1-Butene, a gas under ambient conditions, is particularly challenging to reliably quantify and is often reported using extrapolated values. Britovsek has recently proposed a numerical modeling approach using “first-order linear homogenous recurrence relations with a constant coefficient”.23,24 This methodology uses multiple data points to fit a K value and allows multiple distributions from different catalysts or mechanisms to be modeled simultaneously. A few comments regarding Schulz−Flory distributions are in order. The Schulz−Flory distribution can be reported in molar or weight percentages, which show different trends (Figure 3). Molar fraction graphs exponentially decrease as the oligomer number increases. A plot of ln mol % versus oligomer number D


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Figure 4. Precatalyst (FePDI) ligand substituent steric effects on K value.

low K values. Catalysts with K > 0.90 produce mostly waxes and low-molecular-weight polyethylene. Theoretically, a catalyst operating at K = 1 would produce all lengths of oligomers and polymers in equal molar amounts. Catalysts operating at K < 0.4 still produce more than 50 wt % 1-butene, while catalysts with an K > 0.8 convert over a third of the ethylene to wax products; therefore, most commercial development has focused on 0.4 < K < 0.8. Each full-range commercial process has its nuances, but the designers’ ultimate goal was to maximize the high-value fractions between C6 and C20 and minimize the C4 and C20+ wax fractions that compete with traditional large-volume oil refinery streams. Schulz−Flory Distribution Manipulation. For a given catalyst, the Schulz−Flory distribution can be modified by two major factors: temperature and ethylene concentration. Ethylene insertion rates generally increase with temperature following Arrhenius behavior but are eventually tempered by thermally induced catalyst deactivation. Catalyst deactivation rates can be sensitive to many factors, including metal type, ligand set, solvent, and ethylene concentration.26 Higher reaction temperatures lead to lower K values, as the rate of chain termination increases more rapidly with temperature in comparison to the chain growth rate.27 Propagation rates often show a first-order dependence on ethylene concentration. Catalysts that chain-terminate through chain transfer to ethylene do not show an ethylene-dependent K value, since both chain propagation and chain termination have the same ethylene dependence. Ligand architecture can be modified to control the Schulz− Flory distribution. The best-known example of distribution control through ligand modification is Brookhart and Gibson’s well-studied iron pyridine diimine (FePDI) system.28 (PDI)FeCl2 complexes with aryl groups substituted at the 2,6positions produce highly active polyethylene catalysts when they are activated with MMAO.29,30 Bulky substituents permit high-molecular-weight polyethylene formation, a historic first for late transition metals, by protecting the growing iron alkyl chain from termination induced by chain transfer to ethylene or the kinetically indistinguishable β-hydride elimination followed with olefin displacement by an ethylene and insertion pathway. Inversely, reduction from two to a single ortho substituent per aryl ring creates a sufficiently open active site favoring α-olefin production.31 K value control is possible through simple modification of the R group, with the K value increasing as expected in the order Me < Et < iPr (Figure 4). A related “pendant donor diimine” iron system, Fe(PDD), shows similar steric trends and exquisite control over the K value, spanning a range of 0.4−0.8.32 A K value change of 0.01, which can noticeably change product distributions, corresponds to a mere 0.02 kcal/mol difference between the propagation and termination activation energies.33

Paramagnetic catalyst systems can be challenging to study mechanistically due to limited information from broadened NMR peaks. Recently, significant progress has been made investigating potential active structures in Fe(PDI) chemistry. Chirik has demonstrated that discrete bis(imino)pyridine iron(II) alkyl cations are competent for ethylene polymerization.34 Interestingly, both the four-coordinate base-free (iPrPDI)FeIIR+ species and the structurally characterized fivecoordinate cationic tethered iron(II) alkene complex, [(MePDI)Fe(OC(Ph)2(η2-C3H5))]+, have high-spin ground states (S = 2).35 These results show that olefin coordination does not necessarily induce a spin state change and suggests that the catalytic cycle may stay on the high-spin-state manifold, although the role of the tether must first be untangled. This mechanism would be quite unusual, since highly active catalytic systems normally require spin crossover for at least part of the catalytic cycle.36 Deviant Behavior. Schulz−Flory distributions can deviate from idealized behavior for a variety of reasons. Released αolefins can re-enter the catalytic cycle via 1,2-insertion to form 2,2′-disubstituted metal alkyl intermediates. These sterically encumbered metal alkyls disfavor further ethylene insertion and chain-terminate by β-H elimination to form vinylidenes. Lighter α-olefins are formed in higher concentrations early in batch reactions but are incorporated independently of the growing linear metal alkyl chain length. As a result, lighter olefins are depleted at a faster rate in comparison to heavier αolefins, resulting in an asymmetric skew to the Schulz−Flory distribution. This has been termed “K-value drift” in the literature.37 Britovsek recently proposed that chromium bis(benzimidazolemethyl)amine systems show an alternating α-olefin distribution due to concurrent single- and doubleinsertion pathways.24 The authors propose that the tridentate catalyst system follows two related pathways containing metallacyclic intermediates. One route follows a classical single ethylene insertion mechanism, and a second mechanism invokes ligand dissociation to form a bidentate catalyst, allowing a second ethylene to coordinate and insert, forming double-ethylene-insertion products. A catalytic ethylene oligomerization system that produces a Schulz−Flory distribution has been modified to produce a Poisson distribution by shutting down β-H elimination via proper selection of a chain transfer agent (CTA). This approach was first applied to α-olefin synthesis by Gibson using ZnEt2 as the CTA for the Fe(PDI) system.38 This process is now referred to as catalyzed chain growth (CCG) of ethylene.39 α-Olefins were released from the β-H elimination inert ZnR2 through judicious application of the nickel effect in the presence of ethylene to regenerate ZnEt2. It was not possible to perform ethylene oligomerization and α-olefin formation in the same step due to the incompatibility of the E


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Figure 5. Single-reactor three-catalyst ethylene oligomerization catalytic system producing a Poisson distribution. Reprinted with permission from ref 40.

nickel and iron systems. Cariou has recently made CCG onepot catalytic by substituting nickel for a second iron catalyst, (BiPy)2FeEt2 (BiPy = 2,2′-bipyridine), which acts as both the CTA between Fe(PDI) and ZnR2 to regenerate ZnEt2 and the chain termination catalyst to generate α-olefins by β-H elimination (Figure 5).40

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SELECTIVE ETHYLENE OLIGOMERIZATION Commercial Drivers. The α-olefins industry is undergoing a seismic shift in production caused by disruptive selective ethylene oligomerization technologies. Markets for the lighter 1-alkenes (C4−C10) barely existed in the 1960s. The widespread proliferation of polyethylene, particularly highdensity (HDPE) and linear low-density (LLDPE) polyethylene grades, has spurred global demand for the three α-olefin comonomers (C4, C6, and C8). Most 1-butene is supplied from distillation of refinery streams, but 1-hexene and 1-octene are supplied almost exclusively from ethylene oligomerization. Chevron Phillips Chemical Company LP commercialized the first ethylene trimerization process in 2003 and now operates three units, producing almost a billion pounds of 1-hexene each year.41 Sasol rapidly developed a 1-hexene and 1-octene process that started up in 2014.9 The major developments in this field have been driven by catalyst design, which is sustained through a strong mechanistic understanding based on organometallic chemistry. This discussion will focus on chromium-based systems, but the reader is encouraged to reflect on Hessen’s beautifully designed Ti systems containing hemilabile cyclopentadienyl-arene ligands.42 Metallacyclic Mechanism. The key to the unique selectivity of the ethylene trimerization and tetramerization reactions is the metallacyclic mechanism (Figure 6). The chromacyclopentane intermediate formed by oxidative coupling of two coordinated ethylene molecules was originally proposed by Manyik (Union Carbide) in 1977. He observed that a polyethylene catalyst system composed of basic chromium(III) 2-ethylhexanoate activated by partially hydrolyzed triisobutylaluminum produced similar amounts of polyethylene and 1-hexene (up to 50%).43 Interestingly, it was reported that uranium was the only other metal tested that showed trimerization activity similar to that of chromium! I am not aware of any further reports exploring uranium-based ethylene trimerization catalysts. Manyik foreshadowed future developments in the field with the following observation based on additive studies: “Dimethoxyethane (DME)...may have increased hexene-1 production”. Indeed in 1989, Briggs, also at Union Carbide, reported improved 1-hexene selectivity (up to 72%) through further addition of dimethoxyethane to the catalyst system and proposed a chromacycloheptane as another

Figure 6. General metallacyclic ethylene trimerization catalytic cycle (ligand and counterion not shown).

key intermediate in the process.44 The role of DME in the catalytic cycle was recently probed using DFT calculations. Liu proposed that DME acts as a bidentate ligand, suppressing polyethylene formation through capture of unligated cationic chromium species while promoting 1-hexene formation by lowering activation barriers for the ethylene trimerization catalytic cycle.45 The unique metallacyclic mechanism was experimentally verified using an elegant isotopic labeling study.46 Agapie and Bercaw followed the ethylene trimerization product distribution of a 1:1 mixture of C2H4 and C2D4 catalyzed by a welldefined Cr precursor and discrete activator (Scheme 4).47 The isotopologue distribution (1:3:3:1 C6D12:C6D8H4:C6D4H8:C6H12) did not show H/D scrambling, consistent with that expected for a metallacyclic mechanism, and clearly contrasted with the isotopologue distribution experimentally generated using a SHOP catalyst (Cossee mechanism). Further probing with cis- and transethylene-d2 showed only terminal CHD formation, supporting rapid reductive elimination or a 3,7-hydride shift with no product 2,1-reinsertion. Trimerization with 1,1-dideuterioethylene confirmed that ethylene insertion to form the chromacycloheptane was irreversible.48 This methodology F


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Organometallics Scheme 4. Proposed Catalyst Activation Mechanisma

a

PNP = PNPOMe = Ar2PN(Me)PAr2, where Ar = 2-MeO-C6H4. Cationic species are charge-balanced by the weakly coordinating anion [B(C6H3(CF3)2)4]−.

Active Site Oxidation State. The formation of the key chromacyclopentane intermediate necessitates an n + 2 metal oxidation state change as electrons occupying the metal-based d orbitals are transferred to the coordinated ethylenes’ unoccupied π* orbitals. Two weaker C−C π bonds are broken with formation of two stronger M−C σ bonds and a C−C σ bond driving metallacycle formation. Early on there was a healthy debate in the literature over the exact assignment of the metal redox couple, as chromium can readily access multiple oxidation states.54,55 Direct measurement of the active species oxidation state is particularly challenging due to a number of factors, including low concentration of the active species,56 measurement under relevant ethylene concentrations,57 and inability to monitor all potentially relevant oxidation states under identical conditions with a single instrument.58 Still, the majority of experimental and computational studies support a Cr(I)/Cr(III) cycle.59 A number of interesting synthetic and catalytic studies reinforce this assignment. Hanton and Wass contemporally demonstrated that a discrete Cr(I) species with a weakly coordinating anion, [Cr(CO)4(PNP)][Al(OC(CF3)3)4] (PNP = Ph2PN(iPr)PPh2), is an active ethylene tri-/tetramerization catalyst when trialkylaluminums are used to initiate catalysis by CO abstraction.60,61 Theopold has shown that the neutral Cr(I) species [(i-Pr2Ph)2nacnacCr]2(μ-η2:η2-N2) ((i-Pr2Ph)2 nacnac = 2,4-pentane-N,N′-bis(2,6-diisopropylphenyl)diketiminate) is a competent precatalyst for ethylene trimerization, as is the corresponding monomeric Cr(III) chromacyclopentadiene, further supporting the competency of a Cr(I)/Cr(III) redox cycle.62 The catalytic turnovers are quite low, which is consistent with the requirement of a cationic system for high activity. Gambarotta has shown that a discrete, monomeric Cr(I) pyrollide precatalyst leads to selective ethylene trimerization, while Cr(II) pyrollides produce polyethylene, which was supported by EPR studies on in situ activated Cr/ pyrrole/alkyluminum systems.63,64 Bercaw has established the most definitive model system, supporting a cationic Cr(I)/ Cr(III) reaction channel. A bromochromium(III) biphenyldiyl complex ligated by PNPOMe was activated by NaBArF4 in the presence of ethylene, producing both the ethylene trimerization product, 1-hexene, and the catalyst initiation byproduct, vinylbiphenyl (Scheme 4).48 Formation of vinylbiphenyl is consistent with ethylene coordination followed by insertion to

was utilized by Sasol researchers to confirm the metallacyclic mechanism of the ethylene tetramerization reaction.49 Catalyst Preparation and Evaluation. Catalyst activation is often achieved in situ by mixing a chromium source, a ligand, and activator prior to addition of ethylene.50 Cr(acac)3 is somewhat soluble in aromatic solvents and is a commonly used chromium source. Ligand choice has a major effect on catalyst performance and will be discussed in more detail later, but most ligands are bidentate or tridentate with neutral donors. Common activators are methylaluminoxane derivatives and/or alkylaluminums; however, large stoichiometric excesses (>300 equiv) are usually required. Discrete activators such as [H(OEt2)2]+B[C6H3(CF3)2]4− and Na+B[C6H3(CF3)2]4− have been successfully employed but require tedious isolation of arylated chromium ligand complexes LCrPh3 and LCrPh2Cl, respectively.51,52 Precomplexation is critical for ligands susceptible to degradation or rearrangement by alkylaluminums.53 CrCl3(THF)3 is the most commonly used chromium source for precomplexation, given the lability of the THF molecules. I encourage the reader to delve into the detailed catalytic studies referenced throughout Selective Ethylene Oligomerization and offer here some general guidelines. It is critical to compare catalytic examples under identical reaction conditions, but unfortunately it is not always possible. Reported catalytic activity is very sensitive to reaction time, chromium concentration, and ethylene concentration. Catalyst activity can appear remarkably high at short reaction times but significantly lower at longer reaction times due to rapid catalyst deactivation. Catalyst productivity measured over a time period capturing most of the catalyst activity (i.e., 30 min) is a better metric of catalyst performance. It is well-known that catalyst productivity increases at lower chromium concentrations. This is a beneficial effect, but lowering the chromium concentration below a critical point reduces ethylene conversion and reactor efficiency. Catalyst evaluations at low ethylene concentrations (i.e., 1 atm) can yield false negative screens (see Ethylene Tetramerization) and generally provide low-value data. Small amounts of polymer (milligrams) in laboratory batch units can lead to severe operational difficulties in continuous units. Finally, most studies report catalyst productivities but ignore cocatalyst productivity, which is equally if not more important for catalyst economics. G


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cyclopentane expands the ring by two carbons to form a chromacycloheptane. This is the point in the mechanism where ligand design dictates whether the catalyst will produce only 1-hexene or expand further to produce 1-octene in addition to 1-hexene. Increasing sterics around the active site destabilizes the chromacycloheptane, forcing 1-hexene elimination by either β-H elimination/reductive elimination or a 3,7-H-shift.71 The vast majority of ethylene trimerization precatalysts use tridentate ligand coordination to encumber the active site. Notable examples include Wass’ PNPOMe system,72 Wasserscheid’s easily accessible SNS ligands,73 Exxon Mobil’s NN-bidentate ligands that transform into tridentate ligands through cyclometalation of one o-CH bond (CNN),74 and Köhn’s standalone facially coordinated 1,3,5-triazacyclohexane (NNN) system (Figure 7).75 Alternatively, we have shown that

form a biphenyldiyl chromacycloheptane species which undergoes either β-H elimination/reductive elimination or a 3,7-H shift to release vinylbiphenyl and generate a Cr(I) site. Consistent with Theopold’s model systems, treatment of the neutral complex Cr(PNPOMe)(o,o′-biphenyldiyl)Br with ethylene produced vinylbiphenyl but not 1-hexene. This further supports the need for an electrophilic cationic chromium center to drive catalytic activity. Metallacyclopentane: A Key Intermediate. Formation of the chromacyclopentane by coordination of two ethylene molecules and oxidative cyclization was proposed early on, but the initial ethylene coordination steps were assumed to be kinetically irrelevant with nominal activation barriers.65 My collaborators and I have recently studied the ethylene consumption and 1-hexene formation kinetics using highpressure NMR spectroscopy. This first of its kind operando kinetic study on a selective ethylene trimerization catalyst, [N(diisopropylphosphino)-N′-(3,5-dimethylphenyl)benzamidine](THF)CrCl3 activated with MMAO, concluded that at least one of the first two ethylene coordination steps in chromacyclopentane formation is reversible (Scheme 5).66 All Scheme 5. Best-Fit Kinetic Model Detailing Reversible Ethylene Coordination Steps for a (P,N)Cr/MMAO Ethylene Trimerization Systema

Figure 7. Select examples of notable ethylene trimerization ligands.

sufficiently bulky bidentate P,N-ligands can produce very active ethylene trimerization catalysts on activation by MMAO.76 All of the previous systems require weakly coordinating anions, usually introduced by methylaluminoxanes, to achieve high activity. In contrast, the only commercial selective 1-hexene catalyst can be activated by simple alkylaluminums.77 The chromium center is ligated by 2,5-dimethylpyrrole, which can coordinate with η1, η3, and η5 hapticity depending on the electronic requirements of the chromium center. The catalyst system is generated in situ, and the active species structure is still a subject of debate. Gambarotta has isolated and characterized a variety of chromium pyrrollide structures that may relate to active species structure.78 A DFT computational study by Budzelaar suggests that a Cr(I)/Cr(III) trimerization cycle is viable.79 Müller and Rosenthal have reported an alkylaluminum activated bidentate coordinated PNPNH/Cr catalyst that trimerizes ethylene at modest rates with long catalyst lifetimes.80 Ethylene Tetramerization. It had been speculated that 1octene formation would not be possible through a metallacyclic mechanism.81,82 Undeterred, Bollmann reported the first ethylene tetramerization catalyst in 2004.83 This feat was accomplished by converting Wass’ ethylene trimerization Cr/ PNPOMe catalyst from a tridentate to a bidentate coordinated catalyst via removal of the pendant methoxy groups attached to the phosphine aryl moieties. Catalyst evaluation at higher pressures was critical to success, since Wass had evaluated the same catalyst system earlier at 1 atm ethylene but did not observe catalytic activity.84 1-Octene-producing active catalysts have thereafter been confined to bidentate ligands, with the vast majority ligated by bisphosphines.85 The current upper limit for 1-octene to 1-hexene formation with high productivity

a

The initial reaction mechanism evolving the active species Crn(C2H4) has not been determined.

three of the ethylene coordination steps may be reversible, but only one reaction was more kinetically significant in this study. This reversibility explains the variability in ethylene reaction order dependence reported in the literature (values between first and second order) for ethylene trimerization catalysts, as the reaction order varies with starting reaction conditions, particularly precatalyst concentration [Cr]. The metallacyclopentane is quite stable and does not undergo degradation by β-H elimination at moderate temperatures, as the β-hydrogens cannot contort to form a β-agostic interaction due to ring constraints.67 Thus, ethylene dimerization to 1-butene does not follow a metallacyclic oligomerization reaction channel. Indeed, McGuinness has recently reconfirmed a Cossee mechanism for the Alphabutol process (Ti(OR)4/AlR′3), which had previously been assumed in the general literature to proceed via a titanocyclopentane intermediate.68,69 Ring Expansion. Ethylene coordination to the chromacyclopentane species is reversible on the basis of experimental and computational studies. Computational studies indicate that ethylene binding is relatively weak, as expected for a high-spin Cr(III) cationic system with essentially no π back-bonding to stabilize the complex.70 Ethylene insertion into the chromaH


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Figure 8. Ligand steric effects on ethylene trimerization and tetramerization selectivity. C6 and C8 fraction data from Roodt were extracted and renormalized for clarity.

angle PCN-ligand (∼66°) which produces an amount of 1octene similar to that of a larger PCCP-ligand (∼80°) under comparable conditions.85,95 We have recently shown that the 1-hexene to 1-octene selectivity ratio can be accurately predicted using DFT methods by calculating the relative free energy difference between the β-H transfer transition state to form 1-hexene from the chromacycloheptane and the competing ethylene migratory insertion transition state to form the metallacyclononane.89 Competitive Nonproductive Pathways. The selective ethylene tri-/tetramerization catalytic cycle is further complicated by numerous off-cycle pathways that can affect product purity and selectivity. The presence of significant amounts of impurities, particularly isomers with boiling points similar to those of the desired products, can require additional distillation columns, increasing the overall process economics. One significant pathway common to all ethylene tri-/tetramerization catalysts is the reincorporation of the product 1-alkenes into the cyclization pathway to produce cotrimerization products (C2 + C2 + C6 = C10; C2 + C2 + C8 = C12). This pathway becomes more significant as the product 1-alkenes build up in the reactor. Bercaw has shown for Cr/PNPOMe that the 1-alkenes preferentially incorporate through oxidative coupling with ethylene to form an intermediate chromacyclopentane, with the alkyl chain strongly favoring the β-position.96 The substituted asymmetric chromacyclopentane can then undergo further ethylene insertion from either side followed by elimination to produce internal linear alkenes and terminal linear and branched alkenes. Oxidative coupling of two 1alkenes or insertion of a 1-alkene into a chromacyclopentane is highly disfavored and not observed. The only established exception to this mechanistic observation for chromium is Köhn’s triazacyclohexane chromium catalysts, which are capable of trimerizing α-olefins through a metallacyclic mechanism.97 These results indicate a more sterically open pocket, although interestingly these facially coordinated tridentate systems are ethylene trimerization catalysts and do not form 1-octene. Bercaw has recently reported a very active Ti-phenoxyimine system that trimerizes α-olefins in a similar fashion.98 Another off-cycle pathway that disproportionately affects 1octene-producing catalysts stems from the chromium methylcyclopentyl hydride intermediate (Figure 9). This transient intermediate, which has not been observed spectroscopically but has been implicated kinetically, becomes more prevalent in

is approximately 75:25, and therefore all known ethylene tetramerization catalysts also produce appreciable amounts of 1-hexene.86,87 Qualitatively, 1-octene formation is favored for a less sterically encumbered catalytic pocket, while 1-hexene formation is favored for a more congested active site. This is most obvious for tridentate (1-hexene) versus bidentate (1hexene/1-octene) ligated systems (vide supra). The relationship can also be clearly shown by varying the ligand sterics within a catalyst family. Roodt has shown that increasing the steric bulk of the R group attached to the nitrogen backbone of the original PNP/Cr system increases hexene formation at the expense of octene formation.88 The alkyl group attached to the nitrogen points away from the metal center; thus, longer bulky groups have a greater effect on selectivity. We have shown that decreasing the steric bulk in proximity to the active site in phosphine monocyclic imine κ2-P,N-ligated chromium precatalysts significantly affects 1-octene yields (Figure 8).89 The proposed general mechanism of ethylene tetramerization builds off the ethylene trimerization metallacycloheptane intermediate with ethylene coordination followed by insertion and 1-octene elimination.90 Agapie recently published an isotopic labeling study confirming that the ethylene trimerization and tetramerization pathways share a common monomeric chromacycloheptane intermediate.91 A previously proposed binuclear mechanism for ethylene tetramerization via coupling of two chromacyclopentane intermediates was not supported by the same data.92 The high selectivity to 1-octene and 1-hexene versus higher oligomers suggests that the chromacyclononane is kinetically unstable and decomposes rapidly to form the 1-octene product before ethylene can intercept it and undergo further ring expansion. Intriguingly, Gibson has proposed a large-ring metallacyclic mechanism for a nonselective tridentate chromium system.93 The factors determining selective versus nonselective behavior are still largely empirical at this stage of catalyst development. Selectivity Prediction. Tool development to advance from empirical to quantitative prediction of product selectivities has become an active area of exploration. Kang has shown that the bite angle (∠P−Cr−P) correlates with product ratios within the PCCP/Cr catalyst family, where large bite angles favor 1-hexene formation and small bite angles favor 1-octene formation.94 However, this single parameter clearly does not predict the selectivity across different catalyst families. As an example, Hanton recently reported a small-biteI


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sophisticated phosphines providing catalyst thermal stability up to 100 °C for modified PNP-systems, although polymer formation was not fully mitigated.100 Second, 1-octene yields decrease rapidly with increasing temperature even when the ethylene concentration is held constant.86 This trend is consistent with metallacycloheptane decomposition rates responding to temperature more strongly than ethylene insertion rates. Even if current catalysts were stable at 140 °C, their extrapolated 1-octene production would be well below 10%. This leaves the current state of technology with the dilemma of handling a high-melting polymer (>130 °C) in a low-temperature (99%) for both fractions remains elusive for substantial 1-octene-yielding catalysts. Significant research has gone into ligand and metal complex design, but new activator research has been relatively paltry in comparison. Catalyst−activator combinations are selected empirically through tedious “trial and error” screening. Discrete complexes containing well-defined cation−anion pairs have been explored but are prohibitively expensive. The activator field is still dominated and held ransom by the ill-defined methylaluminoxane derivatives. Recent valiant efforts have attempted to clarify this challenging area.102,103 Now for the opportunities: only three metals, Cr, Ti, and Ta, have demonstrated selective ethylene oligomerization behavior. I suspect many other metals are capable of selective ethylene oligomerization: it is only a matter of choosing the proper ligands and activators! The general mechanisms for selective and nonselective ethylene oligomerization have been established, but there are many details and relationships to be explored. Selectivity in these highly active systems comes down to tenths of kcal/mol differences and are excellent testing zones for high-end calculations. Truncated ligand sets and limited spin state calculations will not suffice. Finally, the ligand sets and catalytic systems designed for ethylene oligomerization are sure to have applications far outside the field.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for O.L.S.: [email protected]. ORCID

Orson L. Sydora: 0000-0001-5353-1436 Notes

The author declares no competing financial interest. J


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(7) Smith, M. B. The Monomer-Dimer Equilibria of Liquid Aluminum Alkyls. I. Triethylaluminum. J. Phys. Chem. 1967, 71, 364−370. (8) Eisch, J. J. Aluminum. In Comprehensive Organometallic Chemistry; Pergamon Press: Oxford, England, 1982. (9) Table 1 provides an instructive breakdown of 2012 production numbers by catalyst technology: Breuil, P.-A. R.; Magna, L.; OlivierBourbigou, H. Role of Homogeneous Catalysis in Oligomerization of Olefins: Focus on Selected Examples Based on Group 4 to Group 10 Transition Metal Complexes. Catal. Lett. 2015, 145, 173−192. (10) Freitas, E. R.; Gum, C. R. Shell’s Higher Olefins Process. Chem. Eng. Prog. 1979, 75, 73−76. (11) Keim, W. Oligomerization of Ethylene to α-Olefins: Discovery and Development of the Shell Higher Olefin Process (SHOP). Angew. Chem., Int. Ed. 2013, 52, 12492−12496. (12) Keim, W. Organometallic Complexes as Catalyst Precursors: Advantages and Disadvantages. J. Mol. Catal. 1989, 52, 19−25. (13) Keim, W.; Kowaldt, F. H.; Goddard, R.; Krüger, C. Novel Coordination of (Benzoylmethylene)-triphenylphosphorane in a Nickel Oligomerization Catalyst. Angew. Chem., Int. Ed. Engl. 1978, 17, 466−467. (14) Peuckert, M.; Keim, W. A New Nickel Complex for the Oligomerization of Ethylene. Organometallics 1983, 2, 594−597. (15) Keim, W.; Behr, A.; Gruber, B.; Hoffmann, B.; Kowaldt, F. H.; Kürschner, U.; Limbäcker, B.; Sistig, F. P. Reactions of Chelate Ylides with Nickel(0) Complexes. Organometallics 1986, 5, 2356−2359. (16) Cossee, P. Ziegler-Natta Catalysis I. Mechanism of Polymerization of α-Olefins with Ziegler-Natta Catalysts. J. Catal. 1964, 3, 80−88. (17) Gee, J. C.; Hickox, R. M. Kinetics of Elimination and Addition Reactions in Mixtures of Tri-n-Octyl Aluminum and 1-Dodecene. Organometallics 2007, 26, 93−101. (18) Müller, U.; Keim, W.; Krüger, C.; Betz, P. [{Ph2PCH2C(CF3)2O}NiH(PCy3)]: Support for a Nickel Hydride Mechanism in Ethene Oligomerization. Angew. Chem., Int. Ed. Engl. 1989, 28, 1011− 1012. (19) Ziegler, K.; Gellert, H.-G.; Lehmkuhl, H.; Pfohl, W.; Zosel, K. Metallorganische Verbindungen, XXVI Aluminiumtrialkyle und Dialkyl Aluminiumhydride aus Olefinen, Wasserstoff und Aluminium. Liebigs Ann. Chem. 1960, 629, 1−13. (20) Egger, K. W. 148. Reactions of Group 3 Metal Alkyls in the Gas Phase. Part 101): The Addition of Olefins to the Monomeric Diisobutylaluminumhydride. Helv. Chim. Acta 1972, 55, 1502−1509. (21) Schulz, G. V. Ü ber die Beziehung zwischen Reaktiongeschwidigkeit und Zusammensetzung des Reaktionsproduktes bei Makropolymerisationsvorgängen. 122. Mitteilung ü ber hochpolymere Verbindungen. Z. Phys. Chem. 1935, 30B, 379−398. (22) Flory, P. J. Molecular Size Distribution in Linear Condensation Polymer. J. Am. Chem. Soc. 1936, 58, 1877−1885. (23) Britovsek, G. J. P.; Malinowski, R.; McGuinness, D. S.; Nobbs, J. D.; Tomov, A. K.; Wadsley, A. W.; Young, C. T. Ethylene Oligomerization beyond Schulz-Flory Distributions. ACS Catal. 2015, 5, 6922−6925. (24) Tomov, A. K.; Nobbs, J. D.; Chirinos, J. J.; Saini, P. K.; Malinowski, R.; Ho, S. K. Y.; Young, C. T.; McGuinness, D. S.; White, A. J. P.; Elsegood, M. R. J.; Britovsek, G. J. P. Alternating α-Olefin Distributions via Single and Double Insertions in ChromiumCatalyzed Ethylene Oligomerization. Organometallics 2017, 36, 510−522. (25) Tomov, A. K.; Gibson, V. C.; Britovsek, G. J. P.; Long, R. J.; van Meurs, M.; Jones, D. J.; Tellmann, K. P.; Chirinos, J. J. Distinguishing Chain Growth Mechanisms in Metal-Catalyzed Olefin Oligomerization and Polymerization Systems: C2H4/C2D4 Cooligomerization/Polymerization Experiments Using Chromium, Iron, and Cobalt Catalysts. Organometallics 2009, 28, 7033−7040. (26) Crabtree, R. H. Deactivation in Homogeneous Transition Metal Catalysis: Causes, Avoidance, and Cure. Chem. Rev. 2015, 115, 127−150.

Orson Sydora was born in Calgary, Alberta, Canada, but grew up in the western half of the United States, residing in Colorado, Texas, and California. He received his Bachelor’s degree in Chemistry from Rice University in 1999, where he performed research on bismuth oxo alkoxide clusters with Professor Kenton Whitmire. He received a Ph.D. from Cornell University in 2004, studying first-row transitionmetal thiolates under the direction of Professor Peter Wolczanski. His postdoctoral research at the University of Chicago with Professor Richard Jordan focused on probing the mechanism of zirconocenecatalyzed olefin insertion using low-temperature NMR spectroscopy. He began his independent research career at Chevron Phillips Chemical Company in 2007 as a Research Chemist supporting the selective 1-hexene process while commercializing several catalyst and process improvements. He quickly found his passion was catalyst design and mechanism, so he initiated a selective hexene/octene catalyst research program. His team successfully scaled up the catalyst technology to a pilot plant and is now focused on commercialization. He became the α-olefins research Team Leader at Chevron Phillips Chemical Company in 2014, directing a group of 13 chemists, engineers, and technicians and was promoted to Research Fellow in December 2018.

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ACKNOWLEDGMENTS Chevron Phillips Chemical Company is acknowledged for their support of this work. I wish to thank some of my incredible ethylene oligomerization collaborators over the past decade, including Mr. Steve Hutchison, Eric Fernandez, and Ray Rios, Drs. Uriah Kilgore, Bruce Kreischer, Ronald Knudsen, Brooke Small, Steven Bischof, Jared Fern, Doo-Hyun Kwon, and Thilina Gunasekara, and Professors Michael Carney, Daniel Ess, James Carruthers, and Mahdi Abu-Omar.

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REFERENCES

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Selective Ethylene Oligomerization - American Chemical Society

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