MAO

Applied Catalysis A: General 246 (2003) 11–16

Ethylene oligomerization by hydrazone Ni(II) complexes/MAO Liyi Chen, Junxian Hou, Wen-Hua Sun∗ State Key Laboratory of Engineering Plastics and Center for Molecular Sciences, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, PR China Received 26 October 2002; received in revised form 9 December 2002; accepted 9 December 2002

Abstract The ethylene oligomerization was investigated by using new hydrazone nickel complexes Ni(NˆO)2 Cl2 (2a, NˆO = 4,5-diazafluorene-9-one-benzoylhydrazone; 2b, NˆO = 4,5-diazafluorene-9-one-4-nitrobenzoylhydrazone; 2c, NˆO = 4,5diazafluorene-9-one-3-nitrobenzoylhydrazone) with methylaluminoxane (MAO) in toluene. Those catalytic systems mainly assisted the dimerization of ethylene with good catalytic activity (105 to 104 g mol−1 h−1 ) at ambient pressure. The reaction conditions, such as the ratios of Al/Ni, reaction temperature and reaction time, were investigated. The best catalytic activity for the three complexes was observed for complex 2a without nitro-substituent on its aryl ring. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ethylene oligomerization; Homogeneous catalysts; Hydrazone nickel(II) complexes

1. Introduction Ethylene oligomerization is a versatile process used to convert ethylene into value-added useful fine-chemicals, such as environmentally friendly fuels, feedstock for linear low-density polyethylene, lubricants, plastics, surfactants and detergents. The commercially practiced Shell High Olefin Process (SHOP) based on the pioneering research of Keim produced olefins in the range of C4 –C30 at the rate of more than one million tons every year [1]. The SHOP catalyst employed the catalyst of nickel complex bearing PˆO bidentate ligands (Scheme 1) and gave high selectivity, with 99% linear olefin containing 98% ␣-olefin [2]. However, it was reported that the SHOP process is performed at 80–120 ◦ C and 70–140 bar [3]; such reaction conditions were sometimes harsh. ∗ Corresponding author. Tel.: +86-10-6255-7955; fax: +86-10-6256-6383. E-mail address: [email protected] (W.-H. Sun).

Recently, cationic nickel ␣-diimine catalysts were reported by Brookhart’s group [4,5]. Following the report, independently from Brookhart’s and Gibson’s groups, 2,6-bis(imino)pyridine Fe(II) and Co(II) catalysts were reported with very high activities [6,7]. It seems promising to use late transition metal complexes as the promising next generation catalyst for ethylene oligomerization or polymerization at mild conditions. It is supposed that late transition metal complexes have a strong propensity of undergoing ␤-hydrogen elimination process on the central metal, which would induce ethylene oligomerization [8]. Adjustments of both steric and electronic environment around the central metal through tailoring ligands could improve the catalytic activity and olefins’ distribution [1]. Previously, we have designed series of catalysts, which contain PˆN, NˆNˆN and NˆN moieties, for ethylene oligomerizations [9–12]. In order to make comparisons among different ligands affecting the catalytic system, we designed hydrazone ligands bearing NˆO

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(02)00660-9

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L. Chen et al. / Applied Catalysis A: General 246 (2003) 11–16

tron ionization (EI), ESI or MALDI-TOF. Element analyses were performed on Flash EA1112 or Carlo Erba1106 (Scheme 2). Scheme 1. Model of SHOP catalysts.

bidentate according to the SHOP model catalysts. Each ligand contains a non-coordinating N atom bearing a lone electron pair as a method of adjusting the electronic effect. It was found that the hydrazone nickel(II) complexes 2 could perform ethylene oligomerization by using methylaluminoxane (MAO) as co-catalyst under ambient atmosphere. Several reaction parameters influencing the catalytic activity and selectivity have been evaluated.

2. Experimental parts We used Schlenk techniques to operate all moisturesensitive manipulations. Toluene was newly distilled over sodium under a dry nitrogen atmosphere. All the other chemical reagents were commercially purchased and used without further purification. The complexes were prepared as explained in the reference [13]. 1 H-NMR spectra were obtained on the BRUKER DMX 300. Mass spectra were obtained using either fast atom bombardment (FAB), elec-

2.1. Preparation of ligands Synthesis of 4,5-diazafluorene-9-one (dafo) was synthesized according to the literature method [14]. The condensation reaction of dafo with arylhydrine gave the ligands for coordinating with nickel chloride. 2.1.1. Synthesis of 4,5-diazafluorene-9-onebenzoylhydrazone (1a) 4,5-Diazafluorene-9-one (0.36 g, 2 mmol) was added to a solution of benzoylhydrine (0.27 g, 2 mmol) in absolute ethanol (30 ml). After the addition of p-toluene sulfonic acid (catalytic eq.), the solution was refluxed for 6 h. The ethanol was partly removed on vacuum line and the remainder was kept in a cool place over night. Yellow crystalline materials came out; the product was filtered, washed with ethanol and dried in a vacuum oven overnight. Yield: 85%. 1 H-NMR (CD SOCD ): δ = 7.50–7.69 (m, 5H), 8.03 3 3 (d, J = 6 Hz, 2H), 8.245 (d, J = 9 Hz, 1H), 8.545 (d, J = 9 Hz, 1H), 8.74–8.77 (t, 2H), 12.07 (NH). EI MS—m/z: 300 [M]+ . C18 H12 N4 O·CH3 CH2 OH calculated: C (69.35%), H (5.24%), N (16.17%); found: C (69.24%), H (5.16%), N (16.49%).

Scheme 2. Synthesis of complexes.

L. Chen et al. / Applied Catalysis A: General 246 (2003) 11–16

2.1.2. Synthesis of 4,5-diazafluorene-9-one-4nitrobenzoylhydrazone (1b) The procedure was the same as that of 2.1.2 and golden yellow powder is obtained. Yield: 92%. 1 H-NMR (CD SOCD ): δ = 7.50–7.60 (m, 2H), 3 3 8.26–8.28 (d, 3H), 8.415 (d, J = 9 Hz, 2H), 8.61 (d, J = 6 Hz, 1H), 8.76–8.79 (t, 2H), 12.33 (NH). FAB MS—m/z: 344 [M − H]+ . C18 H11 N5 O3 calculated: C (62.61%), H (3.21%), N (20.28%); found: C (62.64%), H (3.18%), N (20.08%). 2.1.3. Synthesis of 4,5-diazafluorene-9-one-3nitrobenzoylhydrazone (1c) The procedure was the same as that of 2.1.2, and yellow-greenish powder is obtained. Yield: 94%. 1 H-NMR (CD SOCD ): δ = 7.50–7.60 (m, 2H), 3 3 7.88–7.93 (t, 1H), 8.245 (s, 1H), 8.47–8.52 (t, 2H), 8.61 (d, J = 6 Hz, 1H), 8.76–8.79 (t, 2H), 8.84 (s, 1H), 12.34 (NH). MALDI-TOF MS—m/z: 346 [M + H]+ . C18 H11 N5 O3 calculated: C (62.61%), H (3.21%), N (20.28%); found: C (62.45%), H (3.14%), N (20.05%). 2.2. Preparation of complexes 2.2.1. Synthesis of 4,5-diazafluorene-9-onebenzoylhydrazone nickel(II) complex (2a) 4,5-Diazafluorene-9-one-benzoylhydrazone (60 mg, 0.2 mmol) was dissolved in hot ethanol (20 ml) at reflux. A ethanol solution (5 ml) of NiCl2 ·6H2 O (24 mg, 0.1 mmol) was dropwise added. The product was refluxed for 24 h and yellow-greenish precipitate came out. The product was filtered, washed with hot ethanol and dried in a vacuum oven. Yield: 53%. ESI MS—m/z: 695 [M − Cl]+ , 660 [M − 2Cl]+ . C36 H24 N8 O2 NiCl2 ·H2 O calculated: C (57.79%), H (3.50%), N (14.98%); found: C (57.82%), H (3.41%), N (14.73%). 2.2.2. Synthesis of 4,5-diazafluorene-9one-4-nitrobenzoylhydrazone nickel(II) complex (2b) NiCl2 ·6H2 O (48 mg, 0.2 mmol) was dissolved in ethanol (10 ml), and then the solution was dropwise added to a suspension of 4,5-diazafluorene-9-one-4nitrobenzoylhydrazone (138 mg, 0.4 mmol) in refluxing ethanol (100 ml). The mixture was refluxed for 24 h. During this period, the mixture first became an

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orange solution, then the precipitate came out as orange powder. The product was filtered, washed with hot ethanol and dried in a vacuum oven. Yield: 80%. MALDI-TOF MS—m/z: 928 [M + 6H2 O]+ , 910 [M + 5H2 O]+ , 892 [M + 4H2 O]+ , 874 [M + 3H2 O]+ , 856 [M + 2H2 O]+ . C36 H22 N10 O6 NiCl2 ·4H2 O calculated: C (48.46%), H (3.39%), N (15.70%); found: C (48.53%), H (2.74%), N (15.23%). 2.2.3. Synthesis of 4,5-diazafluorene-9one-3-nitrobenzoylhydrazone nickel(II) complex (2c) The procedure was the same as that of 2.2.2 and an earthy yellow powder is obtained. Yield: 20%. MALDI-TOF MS—m/z: 749 [M − 2Cl]+ C36 H22 N10 O6 NiCl2 calculated: C (52.72%), H (2.70%), N (17.08%); found: C (52.53%), H (2.96%), N (16.94%). 2.3. Ethylene oligomerization 2.3.1. Preparation of active species from nickel catalysts precursor Typically 5 ␮mol of nickel(II) complex was added into a Schlenk tube, and then 5 ml newly distilled toluene under dry nitrogen was charged into the tube with magnetic stirring. Methylaluminoxane (MAO) toluene solution (1.4 M) of desired amount was injected into the suspension above. Immediately the color of the solution was changed, and the active catalytic species in toluene solution was available. 2.3.2. Oligomerization of ethylene The oligomerization reaction was carried out in a 100 ml three-neck flask. After evacuation and flushing with nitrogen three times, then with ethylene two times, the flask was charged with 25 ml toluene and magnetically stirred under ambient ethylene atmosphere. When the desired reaction temperature was established by oil bath, the solution of nickel catalyst was injected into the reactor. Typically 30 min later, the reaction solution was quickly cooled down to −20 ◦ C and then quenched by adding 6 M HCl. Finally, 0.5 ml n-pentane was added as an internal standard. Oligomers were analyzed by SHIMADZU GCMSQP505A with a DB-5MS column (30 m × 0.25 mm). The program was set the initial temperature 40 ◦ C

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(hold 2 min) and finishing temperature 220 ◦ C (hold 10 min) with a heating rate of 10 ◦ C/min.

3. Results and discussion 3.1. The effect of Al/Ni ratios Just as with the SHOP catalytic system, the catalysts 2 show an electronic conjugated system between coordinated atoms. Adapting Al/Ni ratios, the catalytic system obtained the moderate to high catalytic activities for ethylene oligomerization. This may be attributed to the hemilibility of the NˆO bidentate, which can be flexible in different circumstances, resulting in space for ethylene coordination and insertion. Due to electron-withdrawing nitro-group in two precursors 2b and 2c, ethylene is firmly coordinated on metal

center and migratory insertion is also hindered. Therefore, catalyst 2a performed the best activities among them. The detailed results are collected in Tables 1 and 2. The data in the tables indicate that their catalytic activities did not always rise with increasing Al/Ni ratio; the same phenomena were observed in our previous works [11,12]. Take complex 2a as a typical example: the catalytic activity reached its highest point 1.59 × 105 g (ethylene) mol−1 (Ni) h−1 with the ratio of Al/Ni in 1000/1. Subsequently, its activity dropped; 5.18 × 104 g (ethylene) mol−1 (Ni) h−1 was obtained with the Al/Ni ratio of 1500/1. One possible reason was that a threshold amount of MAO as co-catalyst was needed to effectively activate the catalyst precursor, but large amounts of MAO contained too many trialkylaluminum impurities, which might cause such degradation.

Table 1 The effect of Al/Ni ratios on activity and selectivity of complex 2a Entry

1 2 3d 4 5

Cata

2a 2a 2a 2a 2a

Al/Mb (mol/mol)

500/1 800/1 1000/1 1200/1 1500/1

Activity (g mol−1 h−1 )

1.23 1.46 1.59 1.06 5.18

× × × × ×

105 105 105 105 104

Product distribution (%)

␣-Olefinc (%)

C4

C6

C8

␣-C4

␣-C6

␣-C8

87.0 83.4 65.4 86.6 98.4

11.5 14.8 27.0 12.6 1.6

1.5 1.8 7.6 0.8 –

100 100 100 100 100

6.9 6.1 32.3 – –

16.2 87.8 5.6 – –

Catalyst, entry 3 10 ␮mol, and the others 5 ␮mol each. Al/M, the molar ratio of MAO and Ni catalyst. c The percentage of ␣-olefin in its analogue olefins. d Reaction time, entry 3 1 h, others 30 min. a

b

Table 2 The effect of Al/Ni ratios on activity and selectivity of complexes 2b and 2c Entry

Cata

Al/Mb (mol/mol)

Activity (g mol−1 h−1 )

1 2 3 4 5 6 7 8

2b 2b 2b 2b 2c 2c 2c 2c

500/1 800/1 1000/1 1500/1 800/1 1000/1 1200/1 1500/1

3.16 7.79 5.58 5.14 1.28 2.00 1.03 1.18

× × × × × × × ×

104 104 104 104 104 104 104 104

Reaction time 30 min. a Catalyst 5 ␮mol each. b Al/M, the molar ratio of MAO and Ni catalyst. c The percentage of ␣-olefin in its analogue olefins.

Product distribution (%)

␣-Olefinc (%)

C4

C6

C8

␣-C4

␣-C6

␣-C8

97.7 92.5 94.0 92.9 92.4 100 100 92.8

2.3 7.5 6.0 5.3 7.6 – – –

– – – 1.8 – – – 7.2

100 100 100 100 100 100 100 100

– 29.8 – 100 24.0 – – –

– – – – – – – –

L. Chen et al. / Applied Catalysis A: General 246 (2003) 11–16

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Table 3 The effect of temperature on activity and selectivity of complex 2a Entry

1 2 3 4 5

Cata

2a 2a 2a 2a 2a

Temperature (◦ C)

0 30 50 70 90

Activity (g mol−1 h−1 )

5.50 1.59 4.90 4.33 –

× × 105 × 104 × 104 104

Product distribution (%)

␣-Olefinb (%)

C4

C6

C8

␣-C4

␣-C6

␣-C8

84.5 65.4 86.7 78.8 –

15.5 27.0 13.1 21.2 –

– 7.6 0.2 – –

100 100 100 100 –

– 32.3 – 79.6 –

– 5.6 – – –

Al/M (the molar ratio of MAO and Ni catalyst) = 1000/1. Reaction time, entry 2 1 h, entry 3 40 min, others 30 min. a Catalyst, entry 2 10 ␮mol, and the others 5 ␮mol each. b The percentage of ␣-olefin in its analogue olefins.

In the term of the products’ distribution, the lower Al/Ni ratio seemed to produce more ␣-olefins for C6 and C8 , which is probably caused with the competition between the chain transfer and the chain propagation. The Al/Ni ratio of 1000/1 was also the optimum ratio for the production of C6 and C8 .

toluene and the deactivation of the catalyst. In addition, the trend of variations of ␣-olefins distribution is nearly the same as the trend of catalytic activity. 3.3. The effect of reaction time In the industrial processes, the lifetime of a catalyst plays an important role. Therefore, ethylene oligomerization by complex 2a proceeded at 50 ◦ C within different periods (Table 4). The hydrazone Ni(II) system 2a showed very similar catalytic dynamic performances to those observed for 2-(2-pyridyl)quinoxaline nickel complexes [11]. A deactivation of catalyst occurred about 10 min after initiating, however, its activities vary a little during the periods over 20–40 min. With the prolonged reaction time, products of C6 and C8 were increased while the amount of C4 decreased. The amount of C8 was raised more quickly compared to C6 in 20 min. The double bond immigration controlled by thermodynamics resulted in the decrease of ␣-olefins.

3.2. The effect of reaction temperature Further investigation of complex 2a was performed at different reaction temperatures (Table 3). Like the complexes in the 2-(2-pyridyl) quinoxaline [11] and 8-(diphenylphosphino) quinoline [12] systems, the catalytic activities of complex 2a were sensitive to reaction temperatures. Lower temperatures are generally favorable, however, the activity of 5.18 × 105 g (ethylene) mol−1 (Ni) h−1 was best at 30 ◦ C. The low temperature is unfavorable because it may hinder the formation of the active species. There was no activity for ethylene oligemerization obtained at 90 ◦ C, due to a decrease of ethylene solubility in

Table 4 The effect of reaction time on activity and selectivity of complex 2a Entry

1 2 3

Cata

2a 2a 2a

Time (min)

10 20 40

Activity (g mol−1 h−1 )

1.29 × 105 4.75 × 104 4.90 × 104

Product distribution (%)

␣-Olefinb (%)

C4

C6

C8

␣-C4

␣-C6

␣-C8

92.2 92.6 86.7

7.0 5.7 13.1

0.8 1.7 0.2

100 100 100

18.5 – –

– – –

Al/M (the molar ratio of MAO and Ni catalyst) = 1000/1. Temperature 50 ◦ C. a Catalyst 5 ␮mol each. b The percentage of ␣-olefin in its analogue olefins.

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4. Conclusions Ethylene oligomerization by hydrazone Ni(II) complexes with MAO as co-catalyst was investigated. The results demonstrated that those catalysts performed high or moderate catalytic activities. The nickel(II) complex 2a indicated higher catalytic activity compared with other two analogues and there was an optimum situation for the ethylene oligomerization catalyzed by complex 2a under normal pressure.

Acknowledgements We are grateful to the Chinese Academy of Sciences for financial support under the Fund of One Hundred Talents and Core Research for Engineering Innovation KGCX203-2, and National Natural Science Foundation of China No. 20272062. References [1] W. Keim, Angew. Chem. Int. Ed. Engl. 121 (1990) 235. [2] W. Keim, F.H. Kowaldt, R. Goddard, C. Krüger, Angew. Chem. Int. Ed. Engl. 17 (1978) 466.

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