AlR3

Article pubs.acs.org/Organometallics

Mechanism of Ethylene Dimerization Catalyzed by Ti(OR′)4/AlR3 James A. Suttil and David S. McGuinness* School of Chemistry, University of Tasmania, Private Bag 75, Hobart 7001, Australia S Supporting Information *

ABSTRACT: Ti-alkoxide-based catalysts in combination with AlEt3 are responsible for the production of a significant proportion of the world's 1-butene supply, via the dimerization of ethylene. A metallacycle mechanism is normally presumed to operate with this system. However, despite its importance, the catalyst is not mechanistically well understood. The mechanism of dimerization has been studied through a series of C2H4/C2D4 co-oligomerization experiments and comparison of theoretical and experimental mass spectra. The results obtained show that the textbook metallacycle mechanism is most likely not responsible for dimerization with this catalyst. Instead, an excellent fit between the theoretical and experimental mass spectra is obtained when a conventional Cossee-type mechanism (insertion/β-hydride elimination) is modeled. The formation of both the primary product 1-butene and the secondary reaction products (ethylene/1-butene co-dimers) is best explained by this mechanism.

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INTRODUCTION The oligomerization of ethylene to form linear alpha olefins (LAOs) is a field of research that continues to receive much attention from both industry and academia. While the majority of ethylene oligomerization is carried out using full-range processes that are governed by either Schulz−Flory or Poisson distributions of LAOs,1 such processes struggle to meet the increasing market demand for short-chained LAOs such as 1butene, 1-hexene, and 1-octene, which are high-value comonomers for the production of linear low-density polyethylene. This imbalance between supply and market demand has resulted in a greater emphasis on developing and understanding selective, or on-purpose, oligomerization technologies (ethylene dimerization, trimerization, and tetramerization), which avoid production of excess higher LAOs.2−7 One such technology is the IFP Energies Nouvelles/Sabic Alphabutol process for the dimerization of ethylene to 1butene.2 This process was developed from Ziegler and Martin’s initial discovery that combinations of triethylaluminum and titanium or zirconium alkoxides readily convert ethylene to 1butene with high selectivity.8 A similar catalyst system, Ti(OAr)4/AlEt3, was also employed to develop the Toso 1butene process,9 and more recently mixed-ligand aryloxytitanium complexes have been investigated.10,11 Catalyst activities approaching 1 × 106 h−1 (TOF) with 1-butene selectivity of 93% have been achieved with the optimal system, Ti(OBu)4/ AlEt3.2 Given the high activity and selectivity of the system, it is unsurprising that there are currently 30 Alphabutol plants operating worldwide, providing around 25% (708 000 tons per annum) of the world’s 1-butene supply.1,12 Given the industrial importance of these selective oligomerization technologies, it is of equally high importance to understand the mechanistic basis for this selectivity. Such an understanding is vital for further catalyst development, in addition to being of significant fundamental interest. For this reason, mechanistic studies of ethylene trimerization and © XXXX American Chemical Society

tetramerization have been of much interest over recent years.13−33 In contrast, mechanistic studies of the Ti(OR′)4/ AlR3 dimerization system are rare; the highest volume and most mature of the selective oligomerization systems seems to be the least well understood. The high selectivity of the system has often been attributed to a metallacyclic mechanism for ethylene dimerization (Scheme 1).34 Such a process involves the Scheme 1. Metallacycle Mechanism for Ethylene Dimerization

coordination of two ethylene molecules to a divalent titanium species, whereupon these ethylene units undergo an oxidative addition generating a titanocyclopentane, which decomposes via a beta-hydride transfer (either in a stepwise process or by a concerted mechanism) to yield 1-butene. This mechanism can also be successfully invoked to explain the assortment of C6 byproducts that are generated in the process, which come about due to co-dimerization of ethylene and 1-butene. While such a mechanism has now become generally accepted in the academic literature and texts,1,2,34−38 direct evidence supporting such a mechanism is somewhat sparse. Studies by Rothwell on titanium aryloxide compounds have explicitly shown they can support metallacyclic species, although 1-butene is not generated by these complexes.39,40 Work by Whitesides and coReceived: September 2, 2012

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earlier literature that alkoxyltitanium/trialkylaluminum systems could operate via a Cossee mechanism.2,36 These findings, combined with the fact that the oligomer distribution can be just as easily ratified by a Cossee mechanism (with a high rate of chain transfer relative to chain propagation), suggest that this alternative warrants further investigation. In a 2004 investigation on an ethylene trimerization system, Bercaw and co-workers demonstrated that the metallacyclic and Cossee mechanisms could be distinguished on the basis of analysis of the isotopomer distribution resulting from trimerization of a 1:1 mixture of ethylene and perdeuteroethylene.22 It was shown that, as the metallacyclic mechanism leads to no H/D scrambling, only C6H12, C6H8D4, C6H4D8, and C6D12 isotopomers should be observed. Conversely, for a Cossee-based mechanism H/D scrambling results from βhydride chain transfer, which would yield isotopomers containing odd numbers of hydrogen and deuterium. By employing a well-defined chromium catalyst they demonstrated that a metallacyclic mechanism was most likely in effect. A further study in 2007 extended this experiment and employed a Ni-based SHOP (full-range oligomerization) catalyst; analysis of the isotopomers in this case correlated well with the predicted Cossee mechanism.23 Since its inception, this simple experiment has been employed in mechanistic investigations for various tri/tetramerization24,32,43 and oligomerization catalysts.53−56 In principle this experiment should provide a straightforward means of elucidating the mode of 1-butene formation with Ti(OR′)4/AlR3. Indeed, a little known study from 1982 has previously undertaken this experiment employing Ti(OBu)4/AlMe3 as the catalyst system.57 Contrary to the expected distribution from their proposed synchronous coupling mechanism to form 1-butene (no H/D scrambling), a distribution consisting of H/D scrambled isotopomers was reported, which is more consistent with a Cossee mechanism. However, as reported below, we have found that the complex mass spectral fragmentation pattern of 1-butene complicates this analysis significantly. The distributions predicted in this 1982 study seem overly simplistic, and insufficient experimental procedures are reported to account for this. As a result, there is considerable doubt over the conclusions that can be drawn. On the question of the mechanism of this catalyst, the evidence to date is conflicting, and a definitive answer is overdue given the importance of this catalyst. Herein we report a detailed study of the mechanism of dimerization with this system, which for the first time provides strong evidence that a metallacycle mechanism is not operative and fully supports a Cossee mechanism.

workers utilizing preformed titanacyclopentanes has previously demonstrated their propensity to either decompose to generate 1-butene (Scheme 2, pathway a) or revert to ethylene via a Scheme 2. (a) Metallacycle Decomposition to Form 1Butene, and (b) Cβ−Cβ′ Bond Cleavage Forming Ethylene

carbon−carbon bond cleavage (Scheme 2, pathway b), although this work did highlight the stability of the metallacyclopentane ring.41,42 Follow-up studies by Chauvin and coworkers confirmed this result when Ti(OBu)4 was reacted with 1,4-dilithiobutane (expected to yield a titanacyclopentane species in the first instance).34 Additionally, our research group has recently published results showing titanium alkoxide and aryloxide compounds can oligomerize ethylene, yielding products consistent with an Alphabutol-type catalyst, via a metallacyclic mechanism.43 Several other academic studies have also implied a titanacyclic intermediate in ethylene dimerization.44−47 Conversely, early studies involving platinum(II) metallacycles have shown that five-membered platinacyclic species are more long-lived when compared to their seven-membered counterparts. It has been proposed that the rigidity of the fivemembered ring inhibits it from adopting the required platinum/β-hydrogen dihedral angle needed for elimination of 1-butene, while the seven-membered ring has the flexibility required for facile elimination of 1-hexene.48,49 Such a trend has been similarly demonstrated for chromium metallacyclic species,50 has been suggested by several theoretical studies of Ti complexes,29−31 and is also proposed to give rise to the selectivity of numerous ethylene tri- and tetramerization catalysts.3−6 Interestingly, ethylene dimerization catalysts based upon other transition metals such as Ni35,36,51,52 have been suggested in the literature to oligomerize via a Cossee− Arlman mechanism (Scheme 3). It has also been suggested in Scheme 3. Cossee Mechanism for Ethylene Dimerization

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RESULTS AND DISCUSSION Analysis of 1-Butene. Our initial work in elucidating the mechanism was focused on the standard system, Ti(OBu)4/ AlEt3. Ethylene oligomerization with this catalyst (Table 1) Table 1. Ethylene Oligomerization with Ti(OBu)4/AlR3a entry

co-catalyst

TON

1-butene (%)

3-methylpent-1-ene (%)

3-methylpentane (%)

1-hexene (%)

2-ethylbut-1-ene (%)

polymer (%)

1 2b 3 4c

AlEt3 AlEt3 AlMe3 AlEt3

696 530 399 186

78.4 54.5 75.2 94.9

7.0 4.7 7.0 0.9

0.3 2.4 0.3 0.7

1.4 2.1 0.9 0.7

12.3 8.7 14.7 2.8

0.6 27.4 1.9

a 0.15 mmol of catalyst, 3 equiv of cocatalyst, 20 mL of toluene, 55 °C, 20 bar ethylene pressure, 30 min run time. bRun employed 15 equiv of cocatalyst. cRun performed at 0 °C and 1 bar ethylene pressure.

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spectrum obviously complicate the isotopomer analysis significantly, and their effects have been incorporated by employing a model whereby the probability of loss of H or D via EI ionization is proportional to the ratio of H and D in the isotopomer. This correction is detailed in the Supporting Information. A final factor that could potentially affect the isotopomer distribution is a kinetic isotope effect (KIE) related to a β-hydride transfer step. This would not lead to H/D scrambling, but could potentially skew the distribution toward less deuterated isotopomers. The results presented below, however, indicate that any KIE is small to absent. This is expected given the selectivity of this catalyst toward dimers, which suggests that product releasing β-hydride transfer is not a rate-determining step (if it were, further insertion leading to longer chain products would result). Rather olefin insertion (or metallacycle formation) is more likely to be rate limiting, which accounts for the selectivity to dimers. Our initial comparison of the theoretically derived distributions to the experimental data (Figure 2) showed no

yielded the expected high selectivity toward 1-butene as well as the typical ethylene/1-butene co-dimerization products 3methylpent-1-ene, 1-hexene, and 2-ethylbut-1-ene.2,35 In addition to these expected products, a small amount of 3methylpentane was detected by GC analysis; this most likely results from chain transfer between an alkyltitanium species and triethylaluminum followed by hydrolysis during reactor quenching. A small percentage of polymer was also formed, which increases significantly with increasing cocatalyst loading (cf. entries 1 and 2, Table 1). This effect is known,2 with excess cocatalyst reported to yield a catalytic species that generates high molecular weight polyethylene. For the reasons outlined below, we also tested a modified system in which the cocatalyst is trimethylaluminum. This led to a significant decrease in activity (Table 1, entry 3), which again is a known effect and has been attributed to the more stable dimeric structure of trimethylaluminum in solution, yielding a less effective activator.2 At the same time, a comparable selectivity profile to that resulting from activation with triethylaluminum is obtained. The selectivity of the catalyst to 1-butene is highest at 0 °C, although a marked decrease in activity also results. Replication of the experiment employing a 1:1 mixture of C2H4/C2D4 yielded isotopomers of the oligomerization products detailed above. Careful analysis of the H/D isotopomer distribution in the 1-butene formed, as determined by GC-MS, was undertaken. If 1-butene is formed via a metallacyclic mechanism, then no H/D scrambling should be evident, and as such, the isotopomers should consist of C4H8, C4H4D4, and C4D8 in a ratio of 1:2:1 (full details of how predicted distributions were obtained are given in the Supporting Information). Conversely, for a Cossee mechanism involving β-hydride transfer58 between oligomers, scrambling will occur and the expected theoretical distribution becomes 1:1:0:1:2:1:0:1:1 across the molecular weight range 56 to 64. In order to convert these theoretical isotopomer distributions into the actual expected mass spectral ion (m/z) ratios, a number of corrections can to be applied (natural abundance of D and 13C in ethylene, 1H impurity in C2D4). However, in this work we have discovered a much greater complicating factor, specific to 1-butene, which arises due to the fragmentation pattern of this compound under mass spectral ionization (EI). The mass spectrum of 1-butene over the range m/z 49−57 is shown graphically in Figure 1 and reveals significant contributions due to ions down to [M − 7]+, with the [M − 1]+ signal being particularly abundant.59 These contributions to the mass

Figure 2. Experimental and predicted mass spectrum of 1-butene produced from C2H4/C2D4 with Ti(OBu)4/AlEt3. Data normalized to mol. ion m/z 60.

definitive support for either the Cossee or metallacycle mechanism. It was noted that an overexpression of the molecular ion at m/z 56 ([M]+ for completely undeuterated 1-butene) was evident compared to either of the theoretical distributions. We subsequently found that commercial triethylaluminum contains ca. 5% butyl groups relative to ethyl, including the cocatalyst used in this study. As such, it seems likely that a portion of 1-butene formed is derived from (undeuterated) butyl group transfer to titanium, followed by elimination of 1-butene. The evidence for chain transfer between the catalyst and cocatalyst that is discussed above (formation of saturated products) supports this notion. This incorporation of an undeuterated butyl (and most likely also ethyl) functionality into a portion of the 1-butene skews the distribution away from either of the theoretical distributions to the point where deconvolution becomes fruitless, as there is no way of knowing how much incorporation has occurred. The most straightforward solution to the problem of chain transfer with triethylaluminum is to use trimethylaluminum as the cocatalyst instead. This would eliminate any incorporation of an even-numbered alkyl functionality into 1-butene that could result from alkylation of titanium and subsequent ethylene insertion/β-hydride elimination; conversely the only side-products that should result would be odd-numbered hydrocarbons. As shown in Table 1, activation with trimethylaluminum generates a very similar selectivity profile to triethylaluminum under the same conditions (cf. entries 1

Figure 1. Mass spectrum of 1-butene in the region of the molecular ion as a result of electron impact ionization. Relative intensities shown in comparison to [M]+ = 100. C

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of the possible insertion geometries and elimination pathways (Scheme 4) can indeed account for the major product

and 3), albeit at somewhat reduced activity. Interestingly, at most only trace amounts of propene are formed, which can again be related back to the higher stability of the trimethylaluminum dimer, thus not favoring chain transfer reactions. Catalysis employing a 1:1 mixture of C2H4/C2D4 in combination with Ti(OBu)4/AlMe3, upon analysis by GC-MS, yields an excellent fit between the experimental data and the theoretical distribution for 1-butene formed via a Cossee mechanism (Figure 3a). The experimental distribution differs

Scheme 4. Co-dimerization of Ethylene and 1-Butene via a Metallacyclic Mechanism

distributions. Oxidative addition yielding an ethyl moiety at the C1 position (Scheme 4, pathway a) has the potential to undergo exocyclic β-hydride transfer to give 2-hexene or endocyclic β-hydride transfer resulting in either 1-hexene or 3hexene. However, it has been argued that due to unfavorable steric interactions between the titanium center and ethyl moiety, these oligomers should be present as minor products, a result that has been previously supported experimentally.34,45 Conversely, oxidative addition placing the ethyl moiety at the C2 position (Scheme 4, pathway b) gives two possible endocyclic β-hydride transfer pathways, which result in 3methylpent-1-ene and 2-ethylbut-1-ene. Alternatively, a Cossee mechanism can also be invoked to explain the product distribution. This is somewhat more complicated, as both the order and geometry of insertion govern the resultant product selectivity and isotopomer distribution. Starting from a titanium hydride, ethylene insertion yields an ethyl-titanium moiety. Insertion of 1-butene into this moiety, with subsequent β-hydride transfer, can result in either 2-ethylbut-1-ene (Scheme 5, pathway a) or 2-hexene

Figure 3. Experimental and predicted mass spectrum of 1-butene produced from C2H4/C2D4 with Ti(OBu)4/AlMe3: (a) Cossee mechanism and (b) metallacyclic mechanism.

Scheme 5. Co-dimerization of Ethylene and 1-Butene via a Cossee Mechanism

significantly from a metallacyclic distribution (Figure 3b), particularly for molecular ions at m/z 57, 61, and 63, where a negligent contribution would be expected to occur.60 These findings strongly disfavor the prevailing metallacycle mechanism and are wholly consistent with a conventional Cossee mechanism for 1-butene formation.61 The existence of an extremely close fit between the experimental and calculated distributions indicates that a number of approximations built into the model are accurate. First, a kinetic isotope effect related to 1-butene formation is small to absent. Second, the ratio of H:D in a given isotopomer is the controlling factor in determining the mass spectral fragmentation pattern of that species, without needing to introduce a kinetic isotope effect for this fragmentation process (Supporting Information). Nonetheless, this latter approximation can be eliminated entirely by considering the ethylene/1-butene co-dimerization products, which do not suffer from the complex fragmentation pattern of 1-butene. This analysis is discussed below. Analysis of the C6 Oligomers. The byproducts of the Ti(OR′)4/AlR3 system have been shown to consist of significant portions of 2-ethylbut-1-ene, 3-methylpent-1-ene, 1-hexene, and 2-hexene as well as other minor C6 constituents. In previous reports these oligomers have been rationalized by the co-dimerization of one unit of ethylene with one unit of 1butene by a metallacycle mechanism.34,35 Careful consideration

and 3-hexene (Scheme 5, pathway b) depending on the 1butene insertion geometry. If, however, 1-butene were to reinsert into a titanium hydride followed by ethylene insertion and β-hydride transfer, then 1-hexene (Scheme 5, pathway c) and 3-methylpent-1-ene (Scheme 5, pathway d) are generated. Given that such oligomers are predicted to form from the same dimerization mechanism and that they do not suffer from D

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the same mass spectral fragmentation issue that complicates 1butene, isotopomer analysis was also undertaken. As previous studies have shown, for C6 products formed via a metallacyle mechanism only the isotopomers C6H12, C6H8D4, C6H4D8, and C6D12 should form in a ratio of 1:3:3:1.22,23 However, isotopomers that result from a Cossee mechanism undergo H/D scrambling as discussed above. The distribution in this case is also dependent on the order of insertion during codimerization; insertion of 1-butene into a metal hydride followed by ethylene insertion yields a theoretical distribution that can be loosely termed as bimodal, whereas if ethylene insertion occurs prior to 1-butene insertion, then a more Gaussian-like distribution results (see Supporting Information for derivation). Isotopomer analysis of 1-hexene and 3-methylpent-1-ene, from the above detailed experiment with Ti(OBu)4/AlMe3 and C2H4/C2D4, revealed an excellent fit with the theoretical distributions predicted by a Cossee mechanism involving 1butene insertion and subsequent ethylene insertion (Figures 4

Figure 6. Experimental and predicted mass spectrum of 2-ethylbut-1ene produced from C2H4/C2D4 with Ti(OBu)4/AlMe3.

same mechanism; however the mechanism is most likely a Cossee mechanism rather than the favored metallacyclic mechanism. One point that warrants some discussion is the mode of formation of 1-hexene. While ethylene/1-butene co-dimerization is typically involked to account for this, 1-hexene can also result simply from three successive insertions of ethylene (trimerization). Both routes involve a common intermediate, a Ti-butyl species, into which ethylene inserts and 1-hexene is then eliminated. Interestingly, the results shown in Figure 4 reveal that 1-butene is primarily formed by reinsertion of scrambled 1-butene. A much poorer fit to the experimental data is obtained for the case in which 1-hexene is formed by three consecutive ethylene insertions (see Supporting Information), although the theoretical distributions are similar enough that a contribution from this route is quite possible. This conclusion is supported by an experiment in which 1-pentene was added to the reactor; 1-heptene was formed through co-dimerization of ethylene and pentene, in addition to the expected brached codimers. The result can be rationalized by considering the rate of β-hydride elimination from the Ti-butyl intermediate relative to the rate of ethylene insertion into the same intermediate. When the rate of β-hydride elimination is much faster than ethylene insertion (as is the case here, because the catalyst is selective for 1-butene), formation of H/D scrambled 1-butene will preferentially occur before further ethylene insertion. As such, the 1-hexene that does form for the most part incorporates scrambled 1-butene. In light of these results, it is also possible to speculate as to why 2-ethylbut-1-ene is the favored co-dimerization product. It is known that β-hydride elimination from an ethyl chain has a higher barrier than from longer chains (propyl and higher),62 due to stabilization of the product olefin by alkyl substitution.63 Thus, if 1-butene were to insert first (paths c and d, Scheme 5), rapid elimination of 1-butene (the reverse reaction) can be anticipated to largely outcompete insertion of ethylene, as we have discussed above. This is precisely the reason that the system is selective for 1-butene, although the near absence of 2butenes is somewhat puzzling given the amount of 3methylpent-1-ene formed. If ethylene inserts first (paths a and b, Scheme 5), then the Ti-ethyl may be more stable against the reverse reaction, and further insertion (ethylene or 1butene) occurs preferentially. The dominant formation of 2ethylbut-1-ene over internal linear hexenes most likely results from a favored 1-butene insertion geometry that is under steric control. Finally, such a situation also explains why H/D scrambling in the C2H4/C2D4 mixed monomers is not observed

Figure 4. Experimental and predicted mass spectrum of 1-hexene produced from C2H4/C2D4 with Ti(OBu)4/AlMe3.

Figure 5. Experimental and predicted mass spectrum of 3-methylpent1-ene produced from C2H4/C2D4 with Ti(OBu)4/AlMe3.

and 5). Additionally, analysis of the 2-ethylbut-1-ene formed in the reaction gives a strong correlation with the distribution predicted for a Cossee mechanism of ethylene insertion followed by subsequent 1-butene insertion (Figure 6). As the isotopomer distributions and resulting mass spectra of the codimers depend on both the order and geometry of insertion, these results also show that H/D scrambling does not occur following oligomer formation.60 In each of these cases, a metallacycle mechanism leads to a very poor fit to the experimental data (see Supporting Information). These results confirm that dimerization and co-dimerization do occur by the E

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(as it has been previously55), because reversible insertion and βhydride elimination of ethylene are not occurring.

bar of ethylene pressure, while the temperature was maintained, for a 30 min run time. Upon completion, the reactor was cooled to