Kinetic studies on the pyrrole-Cr-based Chevron-Phillips ethylene trimerization catalyst system

Article abstract of DOI:10.1016/j.apcata.2014.04.006

An extensive experimental kinetic study of ethylene trimerization was carried out over the Chevron-Phillips homogeneous catalyst system [Cr(2-ethylhexanoate)₃ (Cr(2-EH)₃)/2,5-dimethylpyrrole (DMP)/co-catalyst/halide] in terms of the effects from organic halide additives, type of co-catalysts, temperature, ethylene pressure, Cr concentration, and ligand/Cr molar ratio, etc. Ten kinds of organic halides and four kinds of organometallic co-catalysts were tested in this work. The 1,1,2,2- tetrachloroethane (TCE) and triethylaluminum (TEA) were found to be the best candidates for this catalyst system. The molar ratios of DMP/Cr, TCE/Cr and TEA/Cr showed a strong impact on the catalytic performance of ethylene trimerization. A detailed catalytic kinetic behavior and the kinetic simulation were also performed for the first time over this catalyst system. It could be extrapolated that the dependency of the trimerization reaction rate on ethylene concentration and catalyst concentration is second-order and first-order, respectively. The apparent activation energy of this reaction is 99.1 kJ/mol at the temperature range from 65 °C to 95 °C. And the apparent reaction rate equation is achieved as follows:.R=5.23×10¹⁸exp-99. 1RTCca1.12Ce1.95.

Full text of DOI:10.1016/j.apcata.2014.04.006

Accepted Manuscript
Title: Kinetic Studies on the Pyrrole-Cr-based
Chevron-Phillips Ethylene Trimerization Catalyst System
Author: Siyang Tang Zhen Liu Xiaowei Yan Ning Li Ruihua
Cheng Xuelian He Boping Liu
PII:
S0926-860X(14)00211-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.04.006
APCATA 14779
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
22-1-2014
1-4-2014
3-4-2014
Please cite this article as: S. Tang, Z. Liu, X. Yan, N. Li, R. Cheng,
X. He, B. Liu, Kinetic Studies on the Pyrrole-Cr-based Chevron-Phillips
Ethylene Trimerization Catalyst System, Applied Catalysis A, General (2014),
http://dx.doi.org/10.1016/j.apcata.2014.04.006
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Highlights
> Kinetic studies on the Chevron-Phillips ethylene trimerization catalyst system> The 1st order reaction
dependency to catalyst concentration> The 2nd order reaction dependency to ethylene concentration>
o
Experimentally derived average activation energy of 99.1 kJ/mol from 65 to 95 C > The apparent reaction
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rate equation is
>
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Article submitted to Applied Catalysis A: General
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Kinetic Studies on the Pyrrole-Cr-based Chevron-Phillips Ethylene Trimerization
Catalyst System
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Siyang Tang, Zhen Liu, Xiaowei Yan, Ning Li, Ruihua Cheng, Xuelian He, Boping Liu*
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Meilong
Road 130, Shanghai 200237, China
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dodoastang@163.com; liuzhen@ecust.edu.cn; bby0123@163.com; lining1989924@126.com;
rhcheng@ecust.edu.cn; hexl@ecust.edu.cn; boping@ecust.edu.cn
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*Corresponding Author:
Tel/Fax: +86-21-64253627. E-mail: boping@ecust.edu.cn
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Abstract
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An extensive experimental kinetic study of ethylene trimerization was carried out over the
Chevron-Phillips homogeneous catalyst system [Cr(2-ethylhexanoate)3 (Cr(2-EH)3)/2,5-dimethylpyrrole
(DMP)/co-catalyst/halide] in terms of the effects from organic halide additives, type of co-catalysts,
temperature, ethylene pressure, Cr concentration, and ligand/Cr molar ratio, etc.. Ten kinds of organic halides
and four kinds of organometallic co-catalysts were tested in this work. The tetrachloromethane (TCE) and
triethylaluminum (TEA) were found to be the best candidates for this catalyst system. The molar ratios of
DMP/Cr, TCE/Cr and TEA/Cr showed a strong impact on the catalytic performance of ethylene trimerization.
A detailed catalytic kinetic behavior and the kinetic simulation were also performed for the first time over
this catalyst system. It could be extrapolated that the dependency of the trimerization reaction rate on
ethylene concentration and catalyst concentration is second-order and first-order, respectively. The apparent
activation energy of this reaction is 99.1 kJ/mol at the temperature range from 65 oC to 95 oC. And the
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apparent reaction rate equation is achieved as follows:
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Keywords: Kinetic study; Ethylene trimerization; 1-Hexene; Pyrrole-Cr based catalyst;
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1. Introduction
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Linear α-olefins (LAOs) as useful organic chemical intermediates can be used to produce
high-performance polyolefins, detergents, higher fatty alcohol, lubricants, oil additives, surfactants and other
products. Particularly, 1-hexene as an important comonomer is mainly used to produce polyethylene products
with better performance compared with traditional 1-butene-based PE products. Because of its large market
demand and value-added potential, 1-hexene has become one of the most needed LAOs. The traditional
technology to manufacture 1-hexene was mainly through ethylene non-selective oligomerization, paraffinic
wax thermal cracking and lighter alkanes steam cracking, which require a subsequent complicated separation
process owing to the production of α-olefins normally with a Schulz-Flory or Poisson distributions.[1, 2] In
recent years, selective ethylene trimerization is commercialized successfully to match the large market
demand of 1-hexene.[3] Comparing to the traditional technologies, selective ethylene trimerization, with
advantages of high atomic efficiency and a simple reaction procedure, has attracted comprehensive interests
to match the increasing demand of highly valuable 1-hexene.[3, 4]
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The first ethylene trimerization was reported by Manyik et al. in 1967[5], in which 1-hexene was detected
as an in-situ formed co-monomer in their Cr-based ethylene polymerization system[6]. In 1989, Briggs and
co-workers further found that the selectivity of 1-hexene can be improved to 74% by adding
1,2-dimethoxyethane (DME) to the same Cr-based catalyst system.[7] Hereafter, many investigations were
carried out to improve the selectivity and activity of ethylene trimerization[8], particularly the synthesis of
new homogenous Cr-based catalysts with various organic ligands[2, 3], such as pyrrole ligands[9], maleimide
ligands[10], sulfide ligands[11], cyclopentadienyl ligands[12, 13], boratabenzenyl ligands[14] and some
phosphide ligands[15].
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Among all the reported selective ethylene trimerization catalytic systems, the combined system of
[Cr(III)compound/pyrrole/alkylaluminum] as a remarkable homogeneous catalyst system for ethylene
trimerization has attracted a lot of attentions.[16, 17] This catalytic system has been further modified via
addition of various Lewis bases to improve the catalytic performances of the system, such as amines,
phosphides, ethers and halide.[9] Especially, chloride-contained compounds as the Lewis base additives have
been reported and well defined to be effective in modifying catalytic activity and selectivity of this
homogeneous catalyst system for ethylene trimerization [9, 18]. Accordingly, this selective catalyst system of
ethylene trimerization to 1-hexene was developed by Chevron-Phillips Company in 1990s, yielding 1-hexene
in an overall selectivity higher than 90%.[3] After various improvements, the production of 1-hexene was
finally commercialized by Chevron-Phillips Company at Qatar in 2003, which is the first commercial
ethylene trimerization process in the world [19].
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Due to the importance of the Chevron-Phillips catalytic system, many experimental and theoretical works
about this famous catalytic system were carried out to investigate the mechanism of selective ethylene
trimerization.[3, 20, 21] The selective ethylene trimerization to 1-hexene was explained by a metallacycle
mechanism which was proposed initially by Manyik et al.[6] and modified later by Briggs[7](SC. 1).
Furthermore, several experimental and electron paramagnetic resonance (EPR) works reported that the
catalytic system follows the Cr(I)/Cr(III) redox cycle for trimerization,[22-25] while density functional
theory (DFT) work suggested that ethylene selective trimerization might follow a Cr(II)/Cr(IV) redox
cycle.[26]
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Scheme1 Metallacycle mechanism of ethylene trimerization proposed by Manyik and Briggs[6] [7]
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Since the kinetic study facilitates the commercial production and the mechanism understanding, some
particular attentions were paid on the ethylene trimerization catalytic systems.[6, 27-30] As shown in Table 1,
a few kinetic investigations were carried out over different ethylene selective oligomerization catalytic
systems.
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Table 1 Overview of the reported experimental kinetic results of ethylene oligomerization
Reaction
product order
Reaction
order
Ea
Catalyst systema
(kJ/mol)
[catalyst]
[C2H4]
Cr(2-EH)3/DMP/AlEt3/Cl compoundb[18]
Cr(2-EH)3/PIBAOc[6]
CrCl3(thf)3/Ar2P(Me)NPAr2/MAOd[29]
CrCl3(thf)3/Ph2PN(iPr)P(Ph)N(iPr)H/AlEt3[27]
TiCl3/Me3SiC5H3Me2C6H3Me2/MAO[31]
C6
C6
C6
C6
C6
-
2
2
2
1
1
-
-
Near 0
-
-
1
1
52.6
27.7
Cr(acac)3/Ph2P(THN)NPPh2/MMAO-3Ae[28]
C6/C8
1
1.57
64.6
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Cr(acac)3/Ph2P (iPr)NPPh2/MAOf[30]
C6/C8
0.6
1.71
23.1
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a Studies were conducted using different paremeters. b Ar: ortho-methoxyphenyl, DMP: 2,5-Me2-pyrrole. c
PIBAO: pily(isobutylaluminum oxide). d thf: tetrahydrofuran; MAO: methylaluminoxane. e acac:
acetylacetonate; THN: 1,2,3,4-tetrahydronaphthyl PNP; MMAO-3A: modified methyaluminoxane, type 3A.
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As shown in Table 1, there existed two controversial opinions regarding to ethylene concentration effect in
Cr-catalyzed ethylene trimerization systems in the literatures. On one hand, ethylene concentration is 2nd
order to the reaction, which is supported by Manyik et al.[6], Yang et al.[18], and Wass et al.[29] with
experimental data over three different chromium-based catalyst systems, [Cr(2-EH)3/DMP/AlEt3/Cl
compound],[Cr(2-EH)3/PIBAO] and [CrCl3(thf)3/Ar2P(Me)NPAr2/MAO], respectively. On the other hand,
ethylene concentration is 1st order to the reaction, which is supported by experimental investigations for the
[CrCl3(thf)3/Ph2PN(iPr)P(Ph)N(iPr)H/AlEt3] and [Cr(acac)3/Ph2P(THN)NPPh2/MMAO-3Ae] catalyst systems
[27, 28] and DFT calculations for the Chevron-Phillips catalyst system and other ethylene selective
trimerization catalyst systems [26, 32-35].
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As mentioned-above, for the important commercial Chevron-Phillips ethylene trimerization catalytic
system, neither detailed catalytic reaction behavior investigation nor systematic kinetic study over this
catalytic system has been reported yet. The dependence of the catalyst concentration and the apparent
activation energy over this catalyst system are still unclear, and the ethylene concentration dependence of this
catalytic system is still controversial. Manyik’s and Yang’s groups reported that ethylene concentration was
2nd order to the reaction by the experimental results, while it means that the rate-determining step is
coordination of two ethylene molecules to the chromium active site.[6, 18] On the contrary, the DFT
calculations by Rensburg et al. showed that the rate-determining step of ethylene trimerization was
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metallacycle growth from metallacyclopentane to metallacycloheptane, which suggested a reaction rate of 1st
order to the ethylene concentration.[26] Therefore, in this work, we carried out an extensive kinetic study
over the Chevron-Phillips ethylene trimerization catalytic system [Cr(2-EH)3/DMP/co-catalyst/halide]. The
influence of the catalyst components molar ratios on reaction kinetics and the optimal process parameters
over this catalyst system were investigated in detail. A reaction rate equation was developed through a kinetic
simulation over this catalytic system, and the effects of process conditions on product selectivity and activity
were illustrated.
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2. Experiments and simulations
2.1. Raw materials
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Cr(2-ethylhexanoate)3 (Cr(2-EH)3) was prepared according to Steele’s patent.[36] 2,5-Dimethylpyrrole
(DMP), dichloromethane (DCM), 1,2-dichloroethane (DCE), 1,1,2-trichloroethane (1,1,2-TCA),
1,1,1-trichloroethane (1,1,1-TCA), 1,1,2,2-tetrachloroethane (TCE), 1,1,2,2-tetrachloroethene (also known as
perchloroethylene(PCE)), tetrachloromethane (also known as carbon tetrachloride (CTC)), hexachloroethane
(also known as perchloroethanes (PCA)) and 1,1,2,2-tetrabromoethane (TBE) purchased from Aladdin were
of analytical grade and were purified using CaH2 under high purity N2 flowing at room temperature or at
boiling points of the specific compounds. Triethylaluminum (TEA), triisobutylaluminum (TIBA), ethyl
aluminum sesquichloride (EASC) and diethylzinc (DEZ) were purchased from Aldrich and were used
without purification. n-Heptane as solvent was refluxed with Na slices and freshly distilled before usage.
Silver molecular sieves and Q5 reactant catalyst (13 wt% of Cu(II) oxide on alumina) purchased from Aldrich
(St. Louis, MO) were used as oxygen scavenger for gas purification. Nitrogen was further purified by passing
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through columns of silver molecular sieves and 13X molecular sieves before usage. Ethylene monomer
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which was of a polymer grade with purity over 99.5%, was further purified by subsequently passing through
columns of 4A molecular sieves, Q5 reactant catalyst, and 13X molecular sieves.
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2.2. Ethylene trimerization
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As the flow chart shown in Fig. 1, all the experiments were carried in a 300 mL pressure reactor which is
equipped with stirrer, thermocouple, gas inlet (pressure regulating value, Brooks SLA5860S ethylene mass
flow meter), gas outlet port, cooling system and heating system. Before conducting an experiment, the
autoclave was heated to 90 oC and purged with high purity N2 several times to eliminate traces of water and
oxygen. The reactor was then allowed to cool down to lower temperature and the 50 mL of n-heptane, a
certain amount of TEC, TEA, DMP, and another 50 mL of n-heptane were transferred to the evacuated
reactor in turn by means of the pressure differential to the ethylene inert atmosphere. Then the reactor was
heated to the desired temperature and the stirring speed was set at a desired value. In the next step, the
Cr(2-EH)3 was transferred in to reactor and the reactor was pressurized with ethylene to start the experiment.
The ethylene-supply pressure was controlled by a pressure regulator, which held the pressure constant over
the entire run time yielding a semi-batch ethylene trimerization system. All reactions were performed at a
constant reaction temperature. After the reaction time, ethylene inlet valve should be shut and the cooling
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liquid (-15 C ethanol) inlet valve should be opened to cool reactor body to 15 C. The liquid and
polyethylene products should be transferred with the solvent after opening the autoclave. The mass balance of
the experiment was determined by quantification and characterization followed by comparison with the
ethylene-uptake data.
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Fig. 1 Schematic of kinetic investigation apparatus with a 300 mL semi-batch-type autoclave
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2.3. Characterization of catalysts and products
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Inductively-coupled plasma (ICP) characterization of raw materials was carried out to determine addition
amount of the materials.
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Mass spectroscopy (HP5971 Mass selective detector) was used for the identification of reaction products.
Gas chromatography (GC2060) with Supelco SPB-35 capillary column was used for liquid product analysis,
where heptane solutions of standard olefins were used for the quantitative analysis of the reaction products.
Polymeric material was separated from reaction products, weighted after drying at 100 °C for 24 h and
analyzed with a high temperature Polymer Laboratories PL220 Gel Permeation Chromatograph (GPC) at 135
°C using 1,2-dichlorobenzene as solvent to characterize the molecular weight (Mw) and molecular weight
distribution (MWD) of the polyethylene.
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2.4. Calculation of ethylene solubility
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Solubility calculations of ethylene in n-heptane and 1-hexene were calculated with a simple flash model
which was established with the software of Aspen Plus. The Peng-Robision state equation was selected as the
state equation according to Walsh[30] and Dornik.[37] Regardless of the feed solution composition, the liquid
composition was certain after the balance at a fixed pressure and temperature.
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3. Results and discussions
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In the following sections, we begin with a discussion on the selection of the co-catalyst and halide and the
determination of the optimal molar ratio of TEA/Cr, DMP/Cr, and TCE/Cr. Then, the effects of ethylene
pressure, catalyst concentration, and reaction temperature are discussed to explore the reaction kinetics
behaviors. It is generally accepted that the limitation of mass-transfer can be safely neglected during a
homogenous reaction[27], and therefore the ethylene flow could be directly translated into the ethylene
consumption as an important time characteristic for the experimental evaluation of the kinetics of the
catalytic ethylene trimerization. For clarity, the filling process of the reactor headspace and the rapid mass
transfer (gas phase ethylene to dissolved ethylene) in the beginning of each individual experiment are
generally omitted from the illustrations. However, the ethylene consumption rate curve includes both the
ethylene that is converted to product and the ethylene that is required for saturation of the newly formed
product.
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3.1. Effects of different halides and co-catalysts
3.1.1. Effects of different halide compounds on ethylene trimerization
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Due to the halide compounds of the different types showed a broad range of effects on the performance of
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the catalytic system[3, 18], ten kinds of halides were considered for the homogeneous Chevron-Phillips
ethylene trimerization catalyst system in order to figure out which one is the best candidate. The effects of
various halide-contained organic compounds on the activity and selectivity of ethylene trimerization at 2.5
MPa 95 oC are compared in Table 2.
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As shown in Table 2, the selectivity and activity varied with different types of chloro compounds. Basically,
the 1-C6 selectivity and activity increased with the number of the chloro substitution in the saturated alkanes
(Entry 1-5, 7) which can be explained by the theory of Yang[18]. However, the activity and the selectivity of
1-hexene in Entry 8 were less than those in Entry 5, which did not comply with the chloro substitution law as
expected. Yang et al.[18] reported that it might be a specific interactions of the chloral compound with the
active Cr species and TEA, and Luo et al.[38] and Yang et al.[18] also proposed that there might be a weak
coordination between chloro groups and the active Cr species. But the details are still unclear up to now.
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Table 2 Effects of different halides and co-catalysts on ethylene trimerization
Selectivity
Mass contained (g)
C6
Activity
Entrya
Halide
Co-catalyst
(%)
C8 C10 C10+ Total C6 1-C6
(kg/molCr h)
C4
1
2
DCM
DCE
TEA
TEA
TEA
TEA
TEA
TEA
TEA
TEA
TEA
TIBA
EASC
DEZ
1.2 3.5 0.2 0.5 0.1
1.1 3.4 0.2 0.4 0.1
5.5 64.3
5.3 64.1
81.8
78.6
88.4
96.1
98.1
85.1
96.9
93.5
98.4
90.1
-
2265
2199
4270
5592
11134
2046
14442
8363
4393
4174
331
3
1,1,2-TCA
1,1,1-TCA
TCE
1.0 7.6 0.4 1.0 0.4 10.3 73.1
0.3 12.0 0.4 0.8 0.1 13.5 88.9
0.3 24.4 0.6 1.5 0.2 26.9 90.6
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PCE
1.0 3.3 0.1 0.4 0.2
5.0 66.2
7
CTC
0.3 31.2 0.8 2.4 0.1 35.0 89.4
0.3 17.5 0.6 1.6 0.3 20.2 86.5
0.2 9.6 0.3 0.5 0.1 10.6 90.0
0.4 8.1 0.4 0.9 0.3 10.1 80.2
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PCA
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TBE
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TCE
TCE
-
-
-
-
-
-
-
-
-
-
0.8
-
-
-
TCE
-
-
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a Reaction conditions: catalyst concentration, 2,42×10-5 mol/L; reaction time, 60 min; ethylene pressure,
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2.5 MPa; solvent, heptane, 100mL; reaction temperature, 95 oC; components’ molar ratio,
Cr/DMP/co-catalyst/Halide=1:5:140:15.
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The unsaturated alkene PCE (Entry 6) was also investigated for comparison with the saturated alkane TCE
(Entry 5). As shown in Table 2, the activity of the catalytic system by adding PCE was much lower than that
of TCE. This might be a result of the improper coordination between chloro groups and Cr species as the Luo
and Yang’s proposed.[18, 38]
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The different halogens effects (chlorine, bromine and iodine) were also investigated in this work. Due to
the poor solubility of iodine compounds in the heptane, the ethylene trimerization with the iodine compounds
addition showed no activity. The effect of TBE is listed in Table 2 Entry 9. The element type of the halogen
substantially affects the reaction activity while it shows minor influence on the selectivity. When the bromide
compound was used instead of the chloride compound, the activity decreased from 11134 kg/molCr×h to
4393 kg/molCr×h. This phenomenon may be caused by the steric hindrance and the coordination ability of
the compounds with different halogen elements. Since the radius of bromine atom is much larger than the
chlorine atom, an ethylene molecule is more difficult to coordinate with the metal-bromine center.
Additionally, the bromine coordination capability with the metal center is stronger than that of the
chloride[39], while the coordination between the metal center and the halogen should not be too strong to
break the dissociation equilibrium for ethylene trimerization. That may lead to a difficult dissociation of the
bromine from the metal center.
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3.1.2. Effects of different co-catalysts on the ethylene trimerization
According to the literatures [18, 30, 40], co-catalysts play an important role to active the reaction, and it is
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also an important factor for the catalytic performance in ethylene selective oligomerization catalyst systems.
Four kinds of organometallic co-catalysts (TEA, TIBA, EASC and DEZ) were evaluated in order to find the
proper co-catalyst with the best performance.
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As illustrated in Table 2 Entry 12, the catalytic systems with DEZ as the co-catalyst showed no activity for
ethylene trimerization. It suggested that DEZ was not the ideal co-catalyst for ethylene trimerization. The
activity of the catalytic system with EASC was also quite low. When TIBA was used instead of the TEA, the
activity and 1-hexene selectivity decreased from 11134 kg/molCr×h and 88.9% to 4393 kg/molCr×h and
72.3%, respectively. The polymers of the reaction co-catalyzed by TEA and TIBA were analyzed by GPC.
The Mw and the MWD of the polymer co-catalyzed by TIBA were 1.78 × 106 and 2.16, respectively, while
the Mw and MWD of the polymer co-catalyzed by TEA were 1.1 × 104 and 4.88, respectively. The influence
of the co-catalyst of aluminium alkyl can be examined from the two aspects, reducibility (TEA>
TIBA>EASC) and steric effect (TIBA> EASC > TEA). According to the experimental results, the ideal
co-catalyst for this catalyst system is TEA with a strong reducibility and a small steric effect.
As discussed above, the optimum co-catalyst and halide compound for this catalytic system is TEA and
TCE, respectively. And the following works were based on the catalytic system composed of TEA, TCE,
DMP and chromium source for the Chevron-Phillips catalytic system.
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3.2. Influence of catalytic components on the reaction kinetics
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Since the catalytic components in terms of different molar ratios of the catalysts system affect the kinetic
behavior of the ethylene trimerization, many investigations of catalytic components’ molar ratio of a
combining catalyst system were carried out under different experimental conditions.[3, 21] Therefore, the
proper molar ratios of TCE/Cr, TEA/Cr and DMP/Cr should be first figured out on the basis of the optimized
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Chevron-Phillips catalytic system before the kinetic simulation.
3.2.1. Effect of TCE/Cr molar ratio
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Fig. 2-I illustrates the ethylene consumption curves for the reactions catalyzed by the combined
[(Cr(2-EH)3)/DMP/TEA/TCE] catalyst system with TCE/Cr molar ratios changing from 5 to 25. As shown in
Fig. 2-I, three kinds of kinetic behaviors were observed along with the increasing of TCE/Cr molar ratio. The
initial ethylene consumption rate increased with the TEC/Cr molar ratio. The ethylene consumption rate held
at a quite low value at a low TCE/Cr molar ratio of 5. The second kinetic behavior appeared at a TCE/Cr
molar ratio of 10, where the ethylene consumption rate increased gradually then became constant during the
reaction. When the TCE/Cr molar ratio is 15 or above 15, the ethylene consumption rate curves fluctuated in
a similar way with an obvious fall with deactivation after a quick rising. However, the higher the TCE/Cr
molar ratio, the higher the initial ethylene consumption rate and the higher the deactivation rate during the
reaction process. These behaviors might be caused by the over-coordination between the chloride atoms and
the metal center as suggested by Yang.[18]
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Fig. 2 Ethylene consumption rates at different catalytic components’ molar ratios, (Reaction conditions:
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catalyst concentration, 2,42×10-5 mol/L; reaction time, 60 min; ethylene pressure, 2.5 MPa; solvent, heptane,
o
100mL; reaction temperature, 95 C. I, catalytic components’ molar ratio, Cr/DMP/TEA =1:5:140. II,
catalytic components’ molar ratio, Cr/DMP/TCE =1:3:15. III, catalytic components’ molar ratio,
Page 16 of 41
16

1
2
Cr/TCE/TEA =1:15:140.)
3
The detailed results of the oligomerization products are shown in Table 3 (Entry 5, 13-16). The activity
first increased with the TCE/Cr molar ratio and reached a maximum at the TCE/Cr molar ratio of 15, and
then it slightly decreased after the molar ratio of 15. It can be also found that the selectivity for 1-hexene is
low for the catalytic system with low activities. However, the 1-hexene selectivity varies little when the
TCE/Cr ratio is above 15. This observation has significant consequences with the respect to a weak
coordination between chloro groups and Cr species. With a consideration of the activity of the catalytic
system and the selectivity for ethylene trimerization to 1-hexene, the optimal molar ratio of TCE/Cr is found
to be 15.
4
5
6
7
8
9
10
11
12
Table 3 Effects of catalytic components’ molar ratios on ethylene trimerization
Mass contained (g)
Selectivity (%)
Entrya TEA/Cr DMP/Cr TCE/Cr
Activity (kg/molCr h)
C4
C6
C8 C10 C10+ Total
C6
1-C6
13
14
5
140
140
140
140
140
20
5
5
5
5
5
3
3
3
3
3
1
3
6
9
5
0.2 2.7 0.1 0.2 0.0
0.4 20.8 0.6 1.4 0.3
0.3 24.4 0.6 1.5 0.2
0.3 24.1 0.6 1.4 0.3
0.3 19.9 0.4 0.9 0.2
0.1 8.0 0.1 0.1 0.0
0.2 9.8 0.1 0.2 0.1
0.4 14.1 0.6 0.9 0.2
0.4 17.9 0.4 1.0 0.2
0.4 20.6 0.6 1.5 0.2
0.2 8.9 0.2 0.5 0.1
0.2 18.0 0.4 1.1 0.2
0.2 21.6 0.7 1.7 0.6
0.3 18.4 0.5 1.2 0.4
3.2
84.4
88.9
90.6
90.4
91.8
95.3
94.2
87.8
89.9
88.3
89.4
90.3
87.1
88.5
91.2
96.6
98.1
97.4
98.0
90.9
90.2
92.3
94.3
90.3
95.3
96.5
96.6
96.2
1302
9682
11133
11025
8946
3488
4295
6641
8247
9623
4091
8223
10251
8605
10
15
20
25
15
15
15
15
15
15
15
15
15
23.4
26.9
26.7
21.7
8.4
15
16
17
18
19
20
21
22
23
24
25
60
10.4
16.1
20.0
23.3
9.9
100
140
180
140
140
140
140
19.9
24.8
20.8
13
a Reaction conditions: catalyst concentration, 2,42×10-5 mol/L; reaction time, 60 min; ethylene pressure,
Page 17 of 41
17

1
2
2.5 MPa; solvent, heptane, 100mL; reaction temperature, 95 oC; the co-catalyst of TEA; halide of TCE.
3
3.2.2. Effect of TEA/Cr molar ratio
4
5
Fig. 2-II shows the ethylene consumption curves for the reactions catalyzed by the combined
[(Cr(2-EH)3)/DMP/TEA/TCE] catalyst system with TEA/Cr molar ratios varying from 0 to 180. As
illustrated in Fig. 2-II, the ethylene consumption rate increased with increasing of the TEA/Cr molar ratio
increasing, while the catalytic system showed no activity for ethylene trimerization in the absence of
co-catalyst (TEA/Cr molar ratio = 0). The ethylene consumption rate experienced a rapid decrease at the
TEA/Cr molar ratio of 20 during the reaction time, while it slightly and slowly climbed up during the entire
reaction time at the TEA/Cr molar ratio of 60. When the molar ratio of TEA/Cr is above 60 (Fig. 2-II d,e,f),
the reaction kinetics featured with higher initial ethylene consumption rate followed by higher stationary
reaction rate with only slight deactivation during the reaction time.
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
As shown in Table 3 (Entry 17-21), the activity increased with the TEA/Cr molar ratio increasing. At the
TEA/Cr molar ratio of 20, the 1-hexene selectivity is the highest while the selectivity of 1-hexene firstly
dropped and then increased with the molar ratio increasing at the molar ratio of TEA/Cr above 20. It reached
another peak at TEA/Cr molar ratio of 140 with the 1-hexene selectivity of 84.8%. Moreover, the C10 product
was increasing with 1-hexene activity increasing. It was proposed that the probability of 1-hexene insertion
into metallacyclic species increases because of the increased concentration of 1-hexene, which could generate
much higher olefins. As a result of the insertion competition between 1-hexene and ethylene into
metallacyclic species, the 1-hexene selectivity at TEA/Cr molar ratio of 180 is less than that of 140.
As reported in the literature [18, 30, 40], partial TEA is used to remove the impurity of the system and the
Page 18 of 41
18

1
2
3
4
5
6
7
8
rest TEA takes part into the activation of the catalyst. As for this catalytic system, it plays a role of
de-protonation to take a proton from DMP, making it much more possible to coordinate with the
electron-deficient chromium centers. While some vacant sites are formed by 2-EH group dropping from the
chromium center under the effect of TEA. It is easier for the ligand or ethylene to coordinate with the
chromium atom. Accordingly, the impurities might not be completely removed from the reaction at a low
TEA/Cr molar ratio of 20, leading to an obvious fall of the ethylene consumption rate during the reaction
period. And the significant increase of the reaction activity with the molar ratio of TEA/Cr increasing might
be a consequence of more chromium centers reduced to form more active sites by TEA.
9
3.2.3. Effect of DMP/Cr molar ratio
10
11
12
13
14
15
16
17
18
19
20
21
The ratio of DMP/Cr is one of the most important factors in this catalyst system. Fig. 2-III shows the
ethylene consumption curves for the reactions catalyzed by the combined [(Cr(2-EH)3)/DMP/TEA/TCE]
catalyst system with DMP/Cr molar ratios ranging from 1 to 9. The initial ethylene consumption rate
increased with the DMP/Cr molar ratio and reached a maximum value at DMP/Cr molar ratio of 5. At a molar
ratio of 1, the ethylene consumption rate is dropping rapidly during the reaction time, which results in a low
activity. However, at a little higher DMP/Cr molar ratio of 3, the ethylene consumption rate declined much
slower. At the DMP/Cr molar ratio of 5, the catalyst always shows the highest ethylene consumption rate
during the whole reaction process. When the molar ratio of DMP/Cr is above 5, the ethylene consumption
rates experienced a similar rapid rise followed by a steady and obvious decreasing with much lower average
consumption rates.
As illustrated in Table 3 (Entry 5, 22-25), both 1-hexene selectivity and activity first increased then
decreased with the increasing of DMP/Cr molar ratio. And both the C6 selectivity and the activity reached a
Page 19 of 41
19

1
2
3
4
5
6
7
8
9
maximum at the molar ratio of 5, while the 1-hexene selectivity reached a maximum as well. The lower the
DMP/Cr molar ratio is, the lower activity is. That may be a result of the insufficient DMP coordination to the
chromium center. Similarly, the over-coordination of the DMP would also lead to a lower activity. At an even
higher molar ratio of DMP/Cr (more than 5), the steric hindrance derived from over-coordination of DMP to
each chromium center might become the main disadvantage for ethylene coordination.[22, 23, 41] As a
consequence, a proper association-dissociation interaction between the DMP ligands and Cr center exists in
the solution only with an appropriate molar ratio of 5, and it would mostly promote the activity apparently.
After considering the catalytic components’ molar ratio as above, the optimal molar ratios of TCE/Cr,
TEA/Cr and DMP/Cr for this catalytic system were observed at 15, 140 and 5, respectively.
10
3.3. The influence of catalyst concentration
11
12
13
14
15
16
17
18
19
20
21
Fig. 3-I illustrates the ethylene consumption curves for the reactions catalyzed by the combined
[(Cr(2-EH)3)/DMP/TEA/TCE] catalyst system with the Cr concentration ranged from 0.81×10-5 to 3.23 ×10-5
mol/L at a constant DMP/TEA/TCE/Cr molar ratio of 5:140:15:1. The ethylene consumption rate differed
with the catalyst concentration during the reaction time, and two kinetic behaviors were observed. In General,
the initial ethylene consumption rate increased with the increasing of chromium concentration. One kinetic
behavior appeared at a Cr concentration lower than 2.00×10-5 mol/L as shown in Fig. 3-I (curves a, b, c, and
d), where the ethylene consumption rate kept constant or decreased slightly after a significant rise. The other
kinetic behavior appeared at a Cr concentration higher than 2.42×10-5 mol/L as shown in Fig. 3-I (curves f
and e), where a higher initial ethylene consumption rate is followed by a notable decrease. This kinetic
phenomenon, which is similar with other ethylene oligomerization systems [27, 30, 31], may be caused by
critical amount of the active sites related with the solubility of catalyst components in the reaction solvent at
Page 20 of 41
20

1
2
certain temperature and mass transfer limitation as well.
3
4
Fig. 3 Effects of Cr concentration on ethylene trimerization, (Reaction conditions: reaction time, 60 min;
ethylene pressure, 2.5 MPa; solvent, heptane, 100mL; reaction temperature, 95 oC; components’ molar ratio,
Cr/DMP/TCE/TEA =1:5:15:140.) I, ethylene consumption rates at different Cr concentration. II, relationship
of average reaction rate (lgR) and average chromium concentration (lgCCr).
5
6
7
8
9
10
11
Page 21 of 41
21

1
Table 4 Effects of parameters process on ethylene trimerization
Selectivity
(%)
Ethylene
Pressure
(Mpa)
Mass contained (g)
Cr(2-EH)3
(×10-6 mol)
Activity
Entrya
T(℃)
(kg/molCr h)
C4
C6
C8 C10 C10+ Total
C6
1-C6
26
27
28
29
5
0.81
1.20
1.61
2.00
2.42
3.23
2.42
2.42
2.42
2.42
2.42
2.42
2.42
2.42
2.42
2.42
2.42
2.42
2.5
2.5
2.5
2.5
2.5
2.5
1.0
1.5
2.0
3.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
95
95
95
95
95
95
95
95
95
95
65
75
80
85
90
100
105
110
0.2 8.8
-
-
-
8.9
98.0 99.0
11070
11433
10952
10704
11133
8557
1874
3340
7055
15161
1715
2839
3760
5413
7533
8781
8562
7331
0.3 12.2 0.4 0.8 0.1
0.4 16.4 0.3 0.5 0.1
0.4 19.0 0.6 1.4 0.2
0.3 24.4 0.6 1.5 0.2
0.3 25.0 0.6 1.5 0.2
0.2 4.3 0.0 0.0 0.0
0.2 7.9 0.0 0.0 0.0
0.3 15.9 0.3 0.7 0.0
0.5 31.1 1.3 3.3 0.5
13.7 89.0 97.8
17.7 90.9 98.1
21.4 88.7 97.5
26.9 90.6 98.1
27.6 90.5 97.6
30
31
32
33
34
35
36
37
38
39
40
41
42
4.5
8.1
94.7 96.6
97.1 96.9
17.1 93.0 97.9
36.7 84.8 96.8
0.2 2.1 0.6 1.2
-
4.2
6.9
9.1
50.4 88.0
73.2 88.1
88.6 92.4
0.3 5.0 0.7 0.9 0.0
0.3 8.1 0.2 0.5 0.1
0.3 11.8 0.4 0.7 0.2
0.3 17.1 0.3 0.6 0.1
0.3 19.5 0.4 1.1 0.7
0.3 19.2 0.4 0.9 0.2
0.2 16.3 0.4 0.8 0.1
13.1 89.9 94.8
18.2 94.0 96.2
21.3 91.8 97.3
20.7 92.4 97.2
17.7 92.1 97.4
2
3
a Reaction conditions: reaction time, 60 min; solvent, heptane, 100mL; Cr/DMP/TCE/TEA =1:5:15:140.
4
As shown in Table 4 (Entry 5, 26-30), the catalytic system with low Cr concentration below 2.42×10-5
mol/L showed a almost constant high activity basically above 104 kg/molCrh for ethylene trimerization.
However, when the Cr concentration further increase to 3.23×10-5 mol/L, the activity falls sharply to 8557
kg/molCrh . The C6 selectivity decreased and bottomed at 88.7% at the Cr concentration of 2.00×10-5 mol/L,
while the 1-hexene selectivity was almost constant at Cr concentration range (0.81×10-5 - 3.23×10-5 mol/L).
5
6
7
8
+
9
Particularly, trace C8 was produced at the Cr concentration of 0.81×10-5 mol/L.
10
11
Since the activity is linearly proportional to chromium concentration below the Cr concentration of
2.42×10-5 mol/L, the ethylene trimerization can be simply considered as a first order to Cr concentration.
Page 22 of 41
22

1
2
Further study about the reaction order dependency with the respect to catalyst concentration is discussed as
follow.
3
After exclusion of the effect from mass transfer in this catalytic ethylene trimerization system, the
homogeneous kinetic equation can be simplified as Eq. 1 (Where R is reaction rate, k is reaction rate constant,
Cca is concentration of catalyst activity center, Ce is concentration of ethylene in liquid phase).
Eq. 1
4
5
6
7
Herein, the average reaction rate and average Cr concentration were utilized in this study. As the ethylene
consumption rate changed over time, the average reaction rate (R) based on the activity over the reaction time
was used to fit the kinetic equation in this simulation. Due to that the real active site concentration in this
reaction is difficult to be determined, the chromium amount added was employed to calculate the apparent
kinetic constants, namely, all the chromium sites was considered as the potential active sites. Herein, the
logarithmic calculation was taken with the both sides of the kinetic Eq. 1, and the figure of plots of lgR-lgCCr
was shown in Fig. 3-II.
8
9
10
11
12
13
14
15
16
17
18
19
The average reaction activity here is not in accordance with the regular rule at the Cr concentration of
3.23×10-5 mol/L, and it might be explained by that it is over the limitation of Cr concentration in this catalytic
ethylene trimerization system. As a result, the experimental values at higher Cr concentration of 3.23×10-5
mol/L is not performed in the linear regression analysis in Fig. 3-II, and the slope of the plots is obtained (n =
1.12). The results allow us to conclude that the ethylene trimerization is a first-order reaction to Cr
concentrations, which is in agreement with other reports by Walsh [30], Carter [29] and Dornik [37].
20
21
3.4. The influence of ethylene pressure
Fig. 4-I illustrates the ethylene consumption curves for the reactions catalyzed by the combined
Page 23 of 41
23

1
2
[(Cr(2-EH)3)/DMP/TEA/TCE] catalyst system with the ethylene pressure varied from 1.0 MPa to 3.0 MPa at
isothermal conditions with a constant DMP/TEA/TCE/Cr molar ratio of 5:140:15:1 and the detailed results
are listed in Table 4 (Entry 5, 31-34). As shown in Fig. 4-I, ethylene pressure is a dominating factor
influencing the activity of ethylene trimerization. Basically, the ethylene consumption rate increased with the
pressure increasing, and it started at a relatively high value and decreased slightly during the reaction time.
However, ethylene consumption rate at ethylene pressure of 2.0 MPa or 2.5 MPa is easier to decrease
comparing to the other ethylene consumption rates (1.0 1.5 and 3.0 MPa). Moreover, the ethylene
consumption rate of a low pressure (1.0 or 1.5 MPa) was quite low, while it held constant at a higher value at
3.0 MPa.
3
4
5
6
7
8
9
10
11
Page 24 of 41
24

1
2
Fig. 4 Effects of ethylene pressure on ethylene trimerization, (Reaction conditions: catalyst concentration,
2,42×10-5 mol/L; reaction time, 60 min; solvent, heptane, 100 mL; reaction temperature, 95 oC; components’
molar ratio, Cr/DMP/TCE/TEA =1:5:15:140.) I, ethylene consumption rates at different ethylene pressure. II,
relationship of average reaction rates (lgR) and ethylene pressure (lgPe).
3
4
5
6
As shown in Table 4, the activity of the catalyst and the by-product amount increased with the increase of
ethylene pressure. As mentioned in both chain growth mechanism and metallacycle mechanism, the ethylene
oligomerization is initiated by ethylene molecules coordinating to the chromium active center. An increase of
ethylene pressure could lead to both increase of the ethylene concentration in liquid phase and the reaction
rate. In general, 1-hexene selectivity dropped with the rise of the ethylene pressure, and it reached to a
maximum of 94.1% at 1.5 MPa. The high 1-hexene activity also accompanied with a poorer C6 selectivity
because of more 1-hexene insertion to the metallacyclic species. As mentioned above, the high activity leads
to higher insertion of 1-hexene into the metallacyclic species, which results in the increase of higher olefins.
Although the high ethylene pressure leads to high ethylene trimerization activity, it is disfavored to 1-hexene
selectivity.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
To elucidate the ethylene pressure effects on ethylene trimerization by this combined catalytic system, a
detailed kinetic simulation was further conducted as follows. The ethylene concentration of liquid phase,
which was considered as ethylene equilibrium concentration under a certain pressure at a certain temperature,
was calculated by the P-R equation using the ASPEN software. The solubilities of ethylene in two different
solutions (n-heptane and 1-hexene) are almost consistent with Herry’s Law, while it suggested that the
solubilities were proportional to the gas partial pressure. Therefore, the ethylene partial pressure was taken
for the ethylene concentration under a certain temperature at a certain Cr concentration. And the reaction rate
Page 25 of 41
25

1
2
equation can be simplified as Eq. 2.
Eq. 2
3
The logarithmic calculation was taken over both sides of the Eq. 2, and the figure of plots of lgR and lgPe
is shown in Fig. 4-II.
4
5
The slope of the lgR-lgPe line shown in Fig. 4-II is 1.95, which suggested a second order of ethylene
trimerization rate to the ethylene pressure. In contrast to Wöhl’s[27] and Kuhlmann’s[28] experimental
conclusions and some theoretical conclusions[26], this kinetic study showed a 2nd order dependency with the
respect to ethylene concentration for this catalyst system, which were supported by several experimental
works[6, 29] based on different chromium-based catalyst systems. And this conclusion suggested that the rate
determining step should be the process of two ethylene molecular coordinating to the chromium active site to
form a metallacyclopentane.
6
7
8
9
10
11
12
3.5. The influence of reaction temperature
13
14
15
16
17
18
19
20
21
Fig. 5-I shows the ethylene consumption curves for the reactions catalyzed by the combined
o
o
[(Cr(2-EH)3)/DMP/TEA/TCE] catalyst system for different reaction temperatures from 65 C to 110 C at
isothermal conditions with a constant DMP/TEA/TCE/Cr molar ratio of 5:140:15:1. As shown in Fig. 5-I,
reaction temperature is a dominating factor influencing the activity and selectivity of ethylene trimerization.
Curves a, b and c in Fig. 5-I showed that ethylene consumption rates started at a low value and climbed
o
slowly along with the reaction time at low temperature (75, 80 and 85 C). However, the ethylene
consumption rates experienced a notable rise and a clear fall at a higher temperature as shown in Fig. 5-I
(curves d, e, f, and g). At a higher temperature of 110 oC, the ethylene consumption rate dropped significantly
during the reaction time.
Page 26 of 41
26

1
2
3
Fig. 5 Effects of reaction temperature on ethylene trimerization, (Reaction conditions: catalyst concentration,
2,42×10-5 mol/L; reaction time, 60 min; solvent, heptane, 100mL; ethylene pressure, 2.5 MPa; components’
molar ratio, Cr/DMP/TCE/TEA =1:5:15:140.) I, ethylene consumption rates at different reaction temperature.
4
5
Page 27 of 41
27

1
II, relationship of reaction rate constant (lnk) and reaction temperature (1/T).
2
3
As shown in Table 4 (Entry 5, 35-42), the relevance between activity and temperature can be divided into
o
4
two groups (65-95 C, 95-110 oC). At low temperature, the activity increased with the reaction temperature
5
increasing. However, at a higher temperature above 90 oC, the activity decreased clearly with the temperature
increasing, and it might be attributed to the deactivation of the active site or transformation of the active site
to a less active one.
6
7
8
As reported over the Chevron-Phillips ethylene trimerization system[30, 42], the active site is much more
stable and harder to deactivate at a low temperature, while the deactivation rate of active site is quicker at a
higher temperature. Walsh[30], Dornik[37], Wöhl[27] also reported that two types of active sites may exist in
the reaction, one of fast reaction rate, the other of slow reaction rate. At high temperature, the active site is
easy to be converted to the slow active center, which could lead to a low activity. Meanwhile, another
explanation was given by the mechanistic investigation of the influence of elevated temperatures on the
Ph2PN(iPr)P(Ph)N(iPr)H/AlEt3 system by Peitz[43]. At higher temperatures, a ligand rearrangement takes
place in the presence of AlEt3. Although this effect was not verified until now for the chromium-containing
catalyst system, an analogous process might be a possible explanation for the catalyst deactivation.
By interpreting the temperature dependency of the reaction rate/activity, an average solubility of ethylene
in liquid phase was expressed as Eq. 3, where Sethylene is the average solubility of ethylene in liquid phase,
mheptane is the amount of n-heptane added, mproduct is the quantity of the liquid product, Sethylene-heptane is the
solubility of ethylene in n-heptane, Sethylene-hexene is the ethylene solubility in the liquid products.
9
10
11
12
13
14
15
16
17
18
19
20
21
Eq. 3
Page 28 of 41
28

1
2
3
4
5
The ethylene solubility in n-heptane and 1-hexene are calculated by the Peng-Robision State Equation with
ASPEN PLUS software. In the respect that about 90% of liquid product is 1-hexene, the 1-hexene solubility
would be used to calculate the solubility of liquid products.
According to the Arrhenius equation, the reaction rate constant (k) can be expressed as Eq. 4, where A is
Pre-exponential factor, Ea is average activation energy.
6
Eq. 4
7
8
A natural logarithmic was taken with the both sides of the equation, and the relationship of lnk and 1/T was
shown in Fig. 5-II. Due to that the active sites may be easily deactivated or transferred to other type of active
sites at a high temperature, the higher temperature results are not taken into linear regression. In the range
9
o
10
11
12
13
14
from 65 to 95 C, the reaction rate follows the typical Arrhenius behavior with experimentally derived
average activation energy (Ea) of 99.1 kJ/mol and the pre-exponential factor (A) of 5.23×1018.
From all the above discussions, it can be seen that, the reaction orders with respect to catalyst and ethylene
concentrations differ with catalyst systems. So is it with the apparent activation energies. For this combined
[(Cr(2-EH)3)/DMP/TEA/TCE] catalytic system, the apparent reaction rate equation can be expressed as
15
follows:
.
16 4. Conclusions
17
18
19
20
In this work, a detailed kinetic study was carried out on the commercial Chevron-Phillips ethylene
trimerization catalyst system [Cr(2-EH)3/DMP/co-catalyst/halide] in terms of the effects from organic halide
additives, type of co-catalysts, reaction temperature, ethylene pressure, Cr concentration, and ligand/Cr molar
ratio, etc.. It was found that tetrachloromethane (TCE) and triethylaluminum (TEA) showed the best
Page 29 of 41
29