Article pubs.acs.org/Organometallics
Effect of Cocatalysts and Solvent on Selective Ethylene Oligomerization Shaneesh Vadake Kulangara,† Daniel Haveman,† Bala Vidjayacoumar,‡ Ilia Korobkov,§ Sandro Gambarotta,‡ and Rob Duchateau*,† †
Department of Chemical Engineering and Chemistry, University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ‡ Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada § X-ray Core Facility, Faculty of Science, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada S Supporting Information *
ABSTRACT: Ethylene oligomerization activities of chromium catalysts stabilized by different dipyrrole-based ancillary ligands, [(Ph2C(C4H4N)2)] (2), [Ph2C(C4H4N)(C5H6N)] (3), [(Et) 2 C(C 4 H 4 N) 2 ] (4), and [(C 6 H 5 )(C 5 H 4 N)C(C4H4N)(C5H6N)] (5), have been investigated using different activation methods, and the results have been compared with the commercial Chevron−Phillips ethylene trimerization system. Upon activation with triethylaluminum (TEA), chromium catalysts stabilized by dipyrrole-based ligands 2−5 showed a lower activity and selectivity compared to the Chevron−Phillips trimerization system based on 2,5-dimethylpyrrole (1) as the ancillary ligand. However, unprecedented increases in both activity and selectivity have been observed by carrying out the oligomerization in methylcyclohexane using depleted-methylaluminoxane (DMAO) along with triisobutylaluminum (TIBA) (1:2 ratio) as cocatalyst system under mild conditions, even for the Chevron−Phillips system itself. Well-defined chromium complexes, [(Ph2C(C4H3N)2)Cr(Cl)(THF)3] (6) and {[Ph2C(C4H3N)(C5H6N]Cr(THF)(μ-Cl)}2 (7), have been synthesized and fully characterized. Upon activating with MAO, catalyst 7 produced a statistical distribution of oligomers, whereas under identical oligomerization conditions catalyst 6/MAO was found to be inactive. The use of MeAlCl2 as cocatalyst to activate 7 resulted in the switching of the catalyst’s behavior from producing a statistical distribution of LAOs to the selective trimerization of ethylene to 1-hexene. The addition of dialkylzinc along with MAO resulted in an unprecedented activity increase.
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company.4 Reagan and co-workers discovered that the combination of chromium(III) alkanoanate, such as chromium(III) 2-ethylhexanoate (Cr(III)2-EH), with 2,5-dimethylpyrrole and triethylaluminum (TEA) in an aliphatic hydrocarbon solvent such as cyclohexane produced 1-hexene with high selectivity and activity. Following this discovery, several new chromium-based ethylene trimerization and even tetramerzation catalysts containing different ligand systems have been reported.5,6 Among these catalysts, the Chevron−Phillips pyrrolyl-based chromium system occupies even today the prime position as it represents the first successful commercial trimerization system. Subsequently, Mitsubishi has significantly improved the performance of the same catalyst via precise control of the process and by slightly modifying the catalyst composition.7
INTRODUCTION
Oligomerization of ethylene to higher linear α-olefins (LAOs) continues to be an area of high research interest due to the wide use of these α-olefins as comonomers (C4−C8), plasticizers (C6−C10), synthetic lubricants (C8−C18), and detergents (C10−C18).1 A typical transition-metal-catalyzed oligomerization of ethylene usually results in the production of a statistical distribution of LAOs. The fractionation of these mixtures into LAOs of specific chain length is energy intensive, whereas the market demand for comonomer grade LAOs is considerable. Thus, much effort from both industrial and academic communities has been devoted to search for more efficient catalysts that can produce terminal alkenes of specific lengths. Ethylene dimerization catalysts date back to the discovery by Ziegler of the so-called “nickel effect”.2 Most selective dimerization catalysts, producing 1-butene, are still based on nickel.3 The first selective ethylene trimerization system producing 1-hexene with more than 90% selectivity was reported by researchers of Chevron−Phillips petroleum © XXXX American Chemical Society
Received: November 24, 2014
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DOI: 10.1021/om501013m Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
General Oligomerization Procedure. All oligomerizations were performed in a steel 250 mL Büchi reactor equipped with a double wall heating/cooling jacket. The reactor was dried in an oven at 120 °C for a 2 h period to each run and then evacuated for 0.5 h and rinsed with argon three times. After that, the reactor was charged with toluene and the desired amount of cocatalyst. After the solution was stirred for 10 min at the set temperature using a thermostat bath, it was saturated with ethylene. The reactor was temporarily depressurized to allow injection of the catalyst solution into the reactor under argon flow, after which the reactor was immediately repressurized to the desired set point. After 30 min reaction time and cooling to 0 °C, the reaction mixture was depressurized, and a mixture of ethanol and diluted hydrochloric acid was subsequently injected to quench the reaction. The polymer was separated by filtration and dried at 60 °C for 18 h under reduced pressure before the molecular weight was determined. Synthesis of [(Ph2C(C4H3N)2)Cr(Cl)(THF)3] (6). A solution of ligand 3, [Ph2C(C4H4N)2], (0.298 g, 1.0 mmol) in THF (7 mL) was treated with KH (0.088 g, 2.2 mmol) and stirred overnight to give a colorless solution. Addition of CrCl3(THF)3 (0.375 g, 1.0 mmol) to the above solution changed the color to brown and the stirring was continued for an additional 6 h. The solvent was completely removed in vacuum, and the residue was redissolved in THF (5 mL). The mixture was then centrifuged to discard the insoluble colorless solid, and the solution was reduced to half of its original volume and kept in the freezer at −30 °C for 2 days to yield X-ray quality brown crystals of the product. Yield (0.320 g, 0.53 mmol, 53%). Anal. Calcd for C33H40N2O3ClCr: C, 66.05; H, 6.68; N, 4.65. Found: C, 66.29; H, 6.74; N, 4.80. [μeff = 3.82 μB]. Synthesis of {[Ph2C(C4H3N)(C5H6N]Cr(THF)(μ-Cl)}2 (7). A solution of ligand 2, [Ph2C(C4H4N)(C5H6N], (0.312 g, 1.0 mmol) in THF (10 mL) was treated with KH (0.044 g, 1.1 mmol) and stirred overnight to give a light yellow solution. Addition of CrCl2(THF)2 (0.267 g, 1.0 mmol) to the above solution changed the color to blue and the stirring was continued for an additional 12 h. The solvent was completely removed in vacuum, and the residue was redissolved in THF (5 mL). The mixture was then centrifuged to discard the insoluble colorless white solid. The solution was reduced to half of its original volume and layered with n-hexane to yield X-ray quality pale blue crystals of the product. Yield (0.263 g, 0.28 mmol, 56%). Anal. Calcd for C52H54N4O2Cl2Cr2: C, 66.31; H, 5.78; N, 5.95. Found: C, 66.59; H, 5.72; N, 5.98.[μeff = 4.96 μB].
Even though the pyrrolyl-based chromium catalyst system has been known for decades, questions such as how the ancillary ligand system determines the remarkably high selectivity and activity, as well as which oxidation state of the chromium is responsible for the selective ethylene trimerization rather than polymerization, have remained unanswered for a long time. Active ethylene trimerization catalysts are typically generated in situ by mixing a metal precursor, the ancillary ligand, and a cocatalyst. The individual activation steps and cocatalysts do vary from system to system. It has been proposed that the cocatalyst reduces the ligated chromium metal at the early stage of activation, and hence, the composition of the final products (selective or nonselective oligomeric products and polyethylene) will largely depend on the type of the active species formed after the activation step. Selective ethylene trimerization is believed to proceed via a metallacyclic redox/ring expansion process.8 Different redox couples such as Cr(I)/Cr(III),9 Cr(II)/Cr(IV),10 and even Cr(III)/Cr(V)11 have been proposed to be responsible for selective ethylene oligomerization. Recent work on aluminumpyrrolyl chromium catalysts12a has indicated that the Cr(I)/ Cr(III) redox couple is the most likely one to be responsible for the selectivity. The successful role played by the pyrrolide anion is 2-fold. It retains one aluminate function in a zwitter-ionic type of structure and dissociates on demand to vacate empty coordination sites around chromium In this paper, the ethylene oligomerization capabilities of chromium catalysts supported by various dipyrrole-based ancillary ligands have been evaluated, and the results have been compared with the Chevron−Phillips trimerization system. The rationale behind this ligand modification was advised by the necessity to slow the ligand dissociation dynamism in the hope to improve catalyst lifetime. The effect of experimental parameters such as the nature of the cocatalyst, type of solvent, and reaction temperature on the catalyst activity and selectivity have been studied.
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EXPERIMENTAL SECTION General Procedures. All air and/or water sensitive reactions were performed under a nitrogen atmosphere, in oven-dried flasks using standard Schlenk techniques. Anhydrous reaction solvents were obtained by means of a multiple column purification system. 2,5-Dimethypyrrole (ligand 1) was obtained from Sigma-Aldrich and was used without further purification. CrCl2(THF)2, CrCl3(THF)3, and the ligands 2−5 were prepared according to published procedures.13,14 Elemental analyses were carried out using a PerkinElmer 2400 CHN analyzer. Data for single crystal X-ray structure determination were obtained with a Bruker diffractometer equipped with a 1K Smart CCD area detector. Molecular weights and molecular weight distributions of the polyethylenes were determined by means of high-temperature SEC on a PLGPC210, equipped with a refractive index detector and a 3 × PLgel 10 μm MIXED-B column set, at 160 °C with 1,2,4trichlorobenzene as solvent. BHT and Irganox have been used as antioxidants. The molecular weights of the polyethylenes produced were referenced to linear polyethylene standards. Results of the oligomerization reactions were assessed by 1H NMR spectroscopy for activity and by GC-MS for reaction mixture composition. Gas chromatography of oligomerization products was conducted on a Varian 450-GC equipped with an auto sampler.
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RESULTS AND DISCUSSION Ethylene Oligomerization Using in Situ Prepared Chromium Catalysts. Different types of dipyrrole-based ancillary ligands (Scheme 1) were synthesized according to known literature procedures.14 In order to compare the effect of ligand modification on the activity and the selectivity, the ethylene oligomerization runs were carried out by in situ mixing Cr(acac)3 and the cocatalyst with the reference ligand 2,5dimethylpyrrole (1) and the dipyrrole-based ligands (2-5). TEA was used as cocatalyst and the catalytic runs were carried out in methylcyclohexane (MeCy) at 120 °C, similar as was described for the Chevron−Phillips system. For the preliminary screening all the oligomerizations were carried out for 30 min. During the activation process a chlorine-containing reagent, hexachloroethane (HCE), was also added as a promoter.15 Yang et al. reported that chlorine-containing compounds can B
DOI: 10.1021/om501013m Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
hydrocarbon solvent and elevated temperature are crucial for these catalysts to become active. Recent work on selective ethylene oligomerization has also highlighted that the type of activator may have a profound influence on the oligomerization selectivity and activity.16 This prompted us to use different aluminoxane-based cocatalysts such as methylaluminoxane (MAO), modified methylaluminoxane (MMAO), and trimethylaluminum (TMA)-depleted methylaluminoxane (DMAO) in two different solvents. The in situ activation of Cr(acac)3 with MAO, MMAO and DMAO in the presence of ligand 3 in toluene resulted in a statistic distribution of oligomers (α ≈ 0.6) along with a considerable amount of polyethylene. Use of MAO or MMAO as cocatalysts with ligand 3 as ancillary ligand in MeCy resulted in trace amounts of polyethylene. Interestingly, when experiments were carried out in MeCy using DMAO as cocatalyst, a switch in the catalytic behavior was observed. The catalyst produced a large amount of polyethylene (Mw = 1.2 × 105 g/ mol, PDI = 2.3) along with small amounts of oligomers. Although only a side product under these conditions, the oligomers showed excellent selectivity toward 1-hexene (95.4%). The switching of the catalyst behavior upon changing the polymerization solvent was previously reported in the case of chromium-based oligomerization catalysts supported by aluminum-pyrrolyl and amidophoshine ligands.12,17 A possible explanation for the switch in catalytic behavior was ascribed to poisoning of the catalytically active Cr(I) species by coordination of toluene to form a stable μ6-arene complex.5,10 The high-temperature 13C NMR (C2D2Cl4) analysis results of the polymer produced with 3/DMAO (entry 4, Table 2) revealed that the polymer is significantly branched and it contains methyl (9.9 methyl branch/1000 carbon), ethyl (1.0 ethyl branch/1000 carbon), and butyl branches (2.2 butyl branch/1000 carbon; see the Supporting Information for the 13 C NMR spectrum). A similar observation was also reported by Manyik et al. in the case of TEA-activated Phillips trimerization catalysts, where in situ produced 1-hexene was incorporated into the polymer chain to form butyl branches.18 Indeed, this branching is also reflected in a rather low melting temperature of the polymer (Tm = 122.2 °C). The formation of ethyl and butyl branches can be ascribed to the incorporation of in situ generated 1-butene and 1-hexene, respectively, into the polymer chain. However, the origin of a significant number of methyl branching in the polymer chain is rather surprising. A possible explanation for this might be a chain walking
Scheme 1. Chemical Structure of the Ligands Investigated
act as a Lewis acid to break down the dimeric TEA to monomeric TEA, which is advantageous to the formation of the active chromium species due to its stronger reducing power. Ethylene oligomerization results of the in situ formed catalysts, obtained by mixing ligands 1−5 upon with Cr(acac)3, TEA and HCE, are given in Table 1. As it can be seen from Table 1 and Figure 1, the ethylene oligomerization activities of the catalyst systems containing the ligands 2−5 were considerably lower than the original system containing ligand 1; however, the selectivity toward 1-hexene was comparable in all cases (Cr/ligand = 1:3) except for the system stabilized by ligand 4. This system did not show selective trimerization behavior at Cr/ligand ratios of 1:1.5 nor 1:3, suggesting that the presence of a phenyl group is crucial to obtain selectivity in the case of dipyrrole ligands. Varying the metal to ligand ratio from 1:3 to 1:1.5 in the case of ligands 2− 5 enhanced the oligomerization activity; however, the selectivity dropped, indicating that there is a delicate balance between selectivity and activity depending on the metal to ligand ratio. The chromium system stabilized by ligand 5, containing one pyridyl and one phenyl group on the carbon atom connecting the two pyrrole moieties, showed higher 1-C6= selectivity both at Cr/ligand ratios of 1:1.5 and 1:3. The polyethylene obtained with ligand 1−5/Cr(acac)3 in all cases showed an Mw
