effect of alkylaluminum activators on ethylene trimerization based on 2,5-dmp/cr(iii)/tce catalyst...

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* E-mail: [email protected]; Tel.: 0086-022-60602936 Received January 21, 2011; revised and accepted February 28, 2011. Project supported by the Program for New Century Excellent Talents in University (No. NCET-07-0142), the Program for New Century Excellent

Talents in Heilongjiang Provincial University (No. NCET-06-010), the National Natural Science Foundation of China (No. 20972025) and the Sci-ence Foundation of Tianjin University of Science & Technology (No. 20090420).

Chin. J. Chem. 2011, 29, 1149—1153 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1149

Effect of Alkylaluminum Activators on Ethylene Trimerization Based on 2,5-DMP/Cr(III)/TCE Catalyst System

Jiang, Tao*,a,b(姜涛) Ji, Rongxina(纪荣欣) Chen, Hongxiaa(陈洪侠) Cao, Chenganga(曹晨刚) Mao, Guoliangb(毛国梁) Ning, Yingnanb(宁英男)

a College of Material Science and Chemical Engineering, Tianjin University of Science & Technology, Tianjin 300457, China

b Department of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing, Heilongjiang 163318, China

Ethylene trimerization toward 1-hexene catalytic system with 2,5-dimethylpyrrole (2,5-DMP)/Cr(III)/alkyl-aluminum/tetrachloroethane (TCE) was investigated. The effects of various cocatalysts on catalytic activity and product selectivity were discussed. The results showed that triethylaluminum (TEA), trimethylaluminum (TMA), tri-n-hexylaluminum (TNHA) and tri-isobutylaluminum (TIBA) were all effective cocatalysts for ethylene trimeri-zation toward 1-hexene. 2,5-DMP/Cr(III)/TEA/TCE catalytic system afforded the best results for ethylene trimeri-zation, while reducing the level of by-product formation. Some specific interaction modes of alkylaluminum with active Cr species in the catalytic cycle were proposed to explain the effect of cocatalyst on catalytic activity and 1-hexene selectivity.

Keywords ethylene, oligomerization, cocatalyst, coordination modes

Introduction

Selective olefin oligomerization systems have been reported for chromium, titanium, and tantalum with the chromium based catalysts being the most numerous, active and selective.1-4 Considerable academic and in-dustrial efforts have been devoted to the study of Phil-lips ethylene trimerization catalyst since the precursor work of Manyik.5 Much research are on variations of the ligand structures, reaction parameters and electron donor. Phillips Petroleum Company, Mitsubishi Chemi-cal Corporation and Sumitomo Chemical Company re-ported an unprecedented ethylene trimerization reaction that yielded 1-hexene in a greater than 90% overall se-lectivity (while concurrently producing less than 2 wt% PE), using 2,5-dimethylpyrrole (2,5-DMP)/Cr(III)/TEA/ halide catalytic system.6-10 The first process of ethylene trimerization for the selective production of 1-hexene was commercialized in 2003 by Chevron-Phillips. Al-kylaluminum other than triethylaluminum (TEA) have been widely used in titanium-based Ziegler-Natta cata-lyst, metallocene and late transition metal catalyst sys-tems as cocatalyst for olefin polymerization. But there is no comprehensive report on their use as cocatalysts for ethylene trimerization catalytic systems. We describe herein a systematic study of cocatalyst on the catalytic performance of 2,5-DMP/Cr(III)/TCE catalytic systems.

Some specific interaction modes of alkylaluminum with active Cr species in the catalytic cycle were proposed.

Experimental

Materials

2,5-DMP, chromium(III)tris(2-ethylhexanoate) [Cr(2-EH)3], trimethylaluminum (TMA), TEA, tri-n- hexylaluminum (TNHA), triisobutylaluminum (TIBA) and methylaluminoxane (MAO) were purchased from Aldrich. 2,5-DMP, Cr(2-EH)3, TMA, TEA, TNHA and TIBA were diluted to a cyclohexane solution of 1.0 mol/L before use. Polymerization grade ethylene was obtained from Daqing Petro-Chemical Ltd. (China). Cyclohexane, hexane, tetrachloroethane (TCE) and ethanol were dehydrated and degassed before use. All other chemicals were obtained commercially and used as received.

Ethylene trimerization

Ethylene trimerization was carried out in a 200 mL autoclave. After evacuation and flushing with nitrogen three times, then with ethylene two times, the autoclave was charged with 100 mL solvent and magnetically stirred under ambient ethylene atmosphere. When the desired reaction temperature was established, quantita-

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tive alkylaluminum cocatalyst, 2,5-DMP ligand, TCE and Cr(2-EH)3 were injected into the reactor, respec-tively. Typically 30 min later, the reaction solution was quickly cooled to 0 ℃ and then quenched by adding ethanol/HCl (10 wt%). The catalytic activity was calcu-lated from the increase of product weight.

Characterization of product

A small sample was washed by deionized water in order to remove alcohol, Cr(III), alkylaluminum and chlorohydric acid. The organic product was dried over anhydrous sodium carbonate and then analyzed by GC-FID using an HP-5890 with an HP-1 capillary col-umn (30 m×0.25 mm), working at 35 ℃ (10 min) and then heated at 10 ℃/min until reaching 280 ℃ (re-mained for 10 min).

Results and discussion

The results obtained from 2,5-DMP/Cr(III)/cocata-lyst/TCE catalytic systems for ethylene trimerization were shown in the following tables. The effects of Al/Cr molar ratio on catalytic properties and selectivity to 1-hexene were investigated. The reference experiment without cocatalyst was designed and the result showed no catalytic activity. It indicated that the catalytically active species were generated from the reaction of Cr(III) and alkylaluminum cocatalyst.

TEA as cocatalyst for ethylene trimerization

The effects of TEA/Cr molar ratio on catalytic activ-

ity and product selectivity were listed in Table 1. It showed a high selectivity of 92.36 wt% to 1-hexene and a lower concentration of by-product. With the increas-ing of Al/Cr molar ratio, the selectivity to 1-hexene was decreased and the catalytic activity obviously decreased from 13.80×105 to 4.10×105 g/(mol Cr•h). 1-Octene and 1-decene were also formed in liquid product, while solid product formation is negligible.

TMA as cocatalyst for ethylene trimerization

The data in Table 2 showed that TMA has a different trend in affecting 1-hexene selectivity and catalytic ac-tivity. With the increasing of Al/Cr molar ratio, the 1-hexene selectivity was decreased and the catalytic activity increased from 0.99×105 to 4.77×105 g/(mol Cr•h). At the same time, the amount of polymer de-creased. TMA can give a high yield of 1-hexene with a selectivity of above 94.02 wt%, which is higher than that obtained form TEA. The formed 1-butylene, 1-octene and 1-decene are less than those obtained from TEA.

TNHA as cocatalyst for ethylene trimerization

Table 3 showed clearly that the maximal selectivity to 1-hexene with TNHA as cocatalyst is only 75.54 wt%, the selectivity toward 1-octene is the same as that ob-tained from TEA as cocatalyst, while the 1-butylene, 1-decene and the solid product formation are higher than those obtained from TEA as cocatalyst.

Table 1 Results of ethylene trimerization with 2,5-DMP/Cr(III)/TEA/TCE

Product selectivity/wt% Run Al/Cra Activityb

1-C4＝ 1-C6

＝ 1-C8＝ 1-C10

＝ C10＋ 1-C6

＝

＋1-C8＝ Polymer

1 0 no activity — — — — — — —

2 15 13.80 0.47 92.36 0.41 0.72 5.99 92.77 0.05

3 20 11.22 0.63 82.83 0.53 2.28 13.57 83.36 0.16

4 25 5.74 1.09 77.32 0.49 2.73 18.30 77.81 0.06

5 30 7.21 1.07 75.65 0.49 3.10 19.54 76.14 0.15

6 40 4.10 2.14 67.45 0.50 3.51 26.22 67.95 0.18 a Molar ratio. b Unit: 105 g/(mol Cr•h); reaction conditions: solvent: cyclohexane; reaction pressure: 2.0 MPa; temperature: 80 ; ℃

Cr(III)∶2,5-DMP＝1∶3 (molar ratio); reaction time: 30 min.

Table 2 Results of ethylene trimerization with 2,5-DMP/Cr(III)/TMA/TCE


1-C4＝ 1-C6

＝ 1-C8＝ 1-C10

＝ C10＋ 1-C6

＝

＋1-C8＝ Polymer

1 0 no activity — — — — — — —

2 10 0.99 0.23 94.02 0.30 0.27 0.07 94.32 5.14

3 20 1.83 0.28 84.54 0.38 1.67 9.14 84.92 3.99

4 30 3.52 0.21 84.20 0.36 1.73 11.64 84.56 1.86

5 60 4.68 0.24 78.76 0.36 2.28 17.82 79.12 0.54



Effect of Alkylaluminum Activators on Ethylene Trimerization

Chin. J. Chem. 2011, 29, 1149—1153 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 1151

Table 3 Results of ethylene trimerization with 2,5-DMP/Cr(III)/TNHA/TCE


1-C4＝ 1-C6

＝ 1-C8＝ 1-C10

＝ C10＋ 1-C6

＝

＋1-C8＝ Polymer

1 0 no activity — — — — — — —

2 10 6.71 1.80 75.54 0.48 3.09 18.86 76.02 0.22

3 15 5.87 1.28 73.06 0.46 2.98 21.79 73.52 0.43

4 20 6.17 2.23 67.39 0.70 3.82 25.74 68.09 0.12

5 25 4.41 2.77 62.60 0.48 3.66 29.66 63.08 0.83



TIBA as cocatalyst for ethylene trimerization

The data in Table 4 showed that with the increasing of TIBA/Cr molar ratio, the selectivity to 1-hexene ob-viously decreased from 84.78 wt% to 41.44 wt%. The 1-butylene and solid product formation are obviously more than those obtained from TNHA as cocatalyst.

MAO as cocatalyst for ethylene trimerization

We also investigated MAO as cocatalyst in 2,5- DMP/Cr(III)/TCE system, and the results are shown in Table 5. When using MAO as cocatalyst, the catalytic activity and product selectivity to 1-hexene are much less than those obtained from alkylaluminum as cocata-lyst, although it needs much high Al/Cr molar ratio. But the selectivity to 1-octene is much higher than that ob-tained from alkylaluminum as cocatalyst. What’s more, the polymer formation is much more than that obtained from alkylaluminum as cocatalyst. The polymer forma-tion is undesirable as it not only will lead to lower

1-hexene yields, but can cause reactor fouling. It is probably attributed to a different degree of ion-pair formation between the cationic active chromium center and the weakly coordinating aluminum-based anion formed from the original alkylaluminum.

Function of alkylaluminum in ethylene trimerization

The function of cocatalyst in catalytic system can be realized from the dependencies of catalytic activity and product selectivity.11,12 The optimum molar ratio value of cocatalyst/Cr(III) is related to the formation of the active Cr species in catalytic reaction systems. The re-quirement of a relatively large amount of cocatalyst can be justified by the facts that the formation of pyrrolide anion from pyrrole and activation of stable 6-coordinated Cr(III) complex are associated with the chemical actions of cocatalyst. The removal of 2-ethyl-hexanoate ligands from Cr(III) complex should be fa-cilitated by the Lewis acid property of cocatalyst.13

Table 4 Results of ethylene trimerization with 2,5-DMP/Cr(III)/TIBA/TCE


1-C4＝ 1-C6

＝ 1-C8＝ 1-C10

＝ C10＋ 1-C6

＝

＋1-C8＝ Polymer

1 0 no activity — — — — — — —

2 30 1.12 0.54 84.78 0.57 0.57 11.26 85.35 2.28

3 40 0.52 3.55 68.69 Trace 0.81 20.59 68.69 6.36

4 50 1.36 8.44 73.67 0.42 1.34 12.93 74.09 3.20

5 60 0.85 6.43 55.58 1.07 0.00 30.90 56.65 6.02



Table 5 Results of ethylene trimerization with 2,5-DMP/Cr(III)/MAO/TCE


1-C4＝ 1-C6

＝ 1-C8＝ 1-C10

＝ C10＋ 1-C6

＝

＋1-C8＝ polymer

1 0 no activity — — — — — — —

2 15 0.02 21.86 17.72 0 0 60.42 17.72 —

3 150 0.94 9.08 55.16 7.58 0 12.38 62.74 16.67 a Molar ratio. b Unit: 105 g/(mol Cr•h); reaction conditions: solvent: cyclohexane; reaction pressure: 2.0 MPa; temperature: 80 ; ℃


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It was obvious that the highest catalytic activity to 1-hexene was obtained with TEA as cocatalyst, but the selectivities to 1-hexene obtained from TEA, TNHA and TIBA systems were lower than that obtained from TMA system. The mechanism for ethylene trimerization follows the standard Cossee-Arlman coordination/ migratory insertion mechanism.14,15 The high selectivity to 1-hexene can be explained by the mechanism pro-posed by Briggs et al. which was supported by Verhov-nik.16,17

TEA contains ethyl, while the alkyl group for TMA, TNHA and TIBA are —CH3—, n-hexyl and iso-butyl, respectively. The reactivity of the Me—Al bond is ap-parently higher than Et—Al, Hexyl—Al and iso-Bu—Al bonds. So the conversion of Cr(Ⅲ) into active site should be in the order of TMA＞TEA＞TNHA＞TIBA. The ability to transfer Cr(III) to the stable cation type active site is mainly related to them.18 When TNHA, TIBA and MAO were used as cocatalysts, more 1-butylene and polymer can be obtained. It is possible that the larger variety of unsaturated Cr(II) species, pos-sibly not all Cr centers are accessible to the ligand, therefore a pure trimerization catalyst may not be ob-tained. The remaining uncoordinated sites will act most probably either as spectator species or polymerization active sites. The steric hindrance of their alkyl groups is larger than TEA, so the rate of further insertion of eth-ylene to yield 1-hexene is lower than that of elimination of 1-butene.

It is believed that the monomeric TEA is advanta-geous to the formation of the active chromium species due to its better Lewis acid property,19 which prefers dimeric form with two ethylene bridge bonds and two Al Lewis acid sites of specific arrangement. Chemical roles of TEA in the ethylene trimerization reaction are associated with Lewis acid property for the removal of carboxylate ligands from Cr(III) to provide open sites for the reactant coordination and H＋ abstraction from the N-H moiety of pyrrole compound to form pyrrolide ligand (Scheme 1 a).

Scheme 1 Proposed coordination structure of TEA with 2,5-DMP (a) and Cr(2-EH)3 (b)

C2H5

C2

AlC2H5

N

CH3

Al

Et

Et

Et

R2

C O

O

C7H15

R1

Cr

::

a

3

b

H5

CH3

TEA activation of the Cr(III) catalyst precursor is believed to involve cleavage of a Cr—O bond through interaction of Al with the carbonyl oxygen of an ethyl-hexanoate fragment14 (Scheme 1 b). Scheme 2 shows the possible effective interaction mode that TEA coor-dinated both with Cr(III) and ligand, then was promoted by TCE to form active Cr catalytic species.

Scheme 2 Proposed interaction modes of components in ethyl-ene trimerization

C

Cl

Cl

C

H H

Cl Cl

Cr O breakCr

R1R1

R2 R2

R1

R2CH2 CH2

Compete insertion

Cr

R2 R2

R2

TNHA and TIBA were also effective activators in 2,5-DMP/Cr(III) catalytic systems. The selectivity to-ward 1-hexene with TIBA as cocatalyst is larger than that with TNHA as cocatalyst, but the catalytic activity is opposite. The butyl in TIBA inhibits the trimerization activity at the expense of oligomerization or polymeri-zation activity.20 The scavenger effect of TIBA in-creases the stability of the moisture-sensitive Cr trimerization site, which is more than TNHA. With TNHA, decrease in 1-hexene selectivity may be deter-mined by the significant enhancement of the polymeri-zation activity.21

Usually the residual AlMe3 in commercial MAO so-lution (prepared from partial hydrolysis of AlMe3) is 10%—30%. It is an active ingredient in both metal reduction and M—Cl activation. It is well known that TMA in MAO is often necessary for MAO to be cata-lytically active.22 The presence of free TMA in MAO is likely to play a dual role, first the methylation of pre-cursor chromium precatalyst and second the activation of MAO itself for interaction with methylated chro-mium species and subsequent ion-pair formation. If the MAO is less active, this will lead to an increase in PE formation.23 In addition, it will also lead to a lower production of 1-hexene as the absolute number of species active in trimerization is less. MAO has a three-dimensional cage structure in which four-coordi-nate Al atoms are bridged by three-coordinate oxygen atoms, and a Me group is introduced to the open coor-dination site on Cr to yield a distorted octahedral com-plex with the β-H agostic interaction to the Me group. It appears that the use of MAO, which either contains aluminum hydride species or is able to generate them, affects both productivity and selectivity of 1-hexene.

Conclusions

The effects of various cocatalysts in 2,5-DMP/ Cr(III)/TCE catalytic systems on catalytic activity and selectivity to 1-hexene have been investigated. The re-sults showed that 2,5-DMP/Cr(III)/TEA/TCE catalytic system afforded the preferred catalytic activity and se-lectivity to 1-hexene. It indicates that the structure of alkylaluminum cocatalyst has an important effect on the property of catalytic system. Some specific interaction modes of alkylaluminum with active Cr species in the catalytic cycle were proposed.

Effect of Alkylaluminum Activators on Ethylene Trimerization

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(E1101211 Pan, B.)

effect of alkylaluminum activators on ethylene trimerization based on 2,5-dmp/cr(iii)/tce catalyst...

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