Ethylene oligomerization in zeolite-grafted Cr(III)-diphosphinoamine catalysts using triisobutylaluminium as cocatalyst: Change from dimerization to trimerization due to confinement effect

Article abstract of DOI:10.1016/j.apcata.2017.07.021

Zeolite-supported Cr(III)-diphosphinoamine (Cr(III)-PNP) catalysts were prepared through grafting PNP on HY and NaY zeolites followed by complexing with CrCl₃(THF)₃ for ethylene oligomerization. The structure of supported Cr(III)-PNP catalysts was characterized by scanning electron microscopy, X-ray diffraction, nitrogen adsorption and desorption, thermogravimetric analyses and Fourier transform infrared, and the influence of the supported pattern on reactivity for ethylene oligomerization were investigated. The results revealed that the complex of Cr(III)-PNP was grafted on silicon hydroxyls in the pore channel of HY zeolite to decrease pore size but to maintain pore structure. Comparing with homogeneous Cr(III)-PNP producing 1-butene as main product, HY-supported catalyst had higher activity and selectivity toward 1-hexene increased from 4.07% to 73.24% using triisobutylaluminium as cocatalyst. The increase is attributed to confinement effect of the pore channel, which increases the stability of the chromacycloheptane intermediate to 1-hexene. The confinement effect for ethylene oligomerization was revealed in experiment.

Full text of DOI:10.1016/j.apcata.2017.07.021

AppliedCatalysisA,General544(2017)154–160
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Feature Article
Ethylene oligomerization in zeolite-grafted Cr(III)-diphosphinoamine
catalysts using triisobutylaluminium as cocatalyst: Change from
dimerization to trimerization due to confinement effect
Huaiqi Shaoa, Jingjing Wanga, Ruofei Wanga, Liwu Songa, Xiaoyan Guob, Tao Jianga,⁎
a
Tianjin Key Laboratory of Marine Resources and Chemistry, College of Chemical Engineering and Materials Science, Tianjin University of Science & Technology, Tianjin
300457, PR China
b
Key Laboratory of Pollution Processes and Environmental Criteria (Nankai University), Ministry of Education, Tianjin Key Laboratory of Environmental Remediation and
Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300457, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Zeolite-supported Cr(III)-diphosphinoamine (Cr(III)-PNP) catalysts were prepared through grafting PNP on HY
and NaY zeolites followed by complexing with CrCl3(THF)3 for ethylene oligomerization. The structure of
supported Cr(III)-PNP catalysts was characterized by scanning electron microscopy, X-ray diffraction, nitrogen
adsorption and desorption, thermogravimetric analyses and Fourier transform infrared, and the influence of the
supported pattern on reactivity for ethylene oligomerization were investigated. The results revealed that the
complex of Cr(III)-PNP was grafted on silicon hydroxyls in the pore channel of HY zeolite to decrease pore size
but to maintain pore structure. Comparing with homogeneous Cr(III)-PNP producing 1-butene as main product,
HY-supported catalyst had higher activity and selectivity toward 1-hexene increased from 4.07% to 73.24%
using triisobutylaluminium as cocatalyst. The increase is attributed to confinement effect of the pore channel,
which increases the stability of the chromacycloheptane intermediate to 1-hexene. The confinement effect for
ethylene oligomerization was revealed in experiment.
Ethylene oligomerization
Confinement effect
Ethylene trimerization
Heterogeneous catalyst
1. Introduction
(90%) [6]. A few years later, by removing ortho-substitution from the
aryl groups of PNP, researchers from Sasol discovered a catalytic system
Linear α-olefins, such as 1-hexene and 1-octene, are important in-
termediates for producing linear low-density polyethylene (LLDPE) as
co-monomers [1,2]. The present technologies for producing linear α-
olefins are mostly based on ethylene oligomerization, resulting in a
mathematical product distribution like Schulz-Flory or Poisson, which
require purification through fractional distillation [3]. Thus, the con-
tinually increasing demands for 1-hexene and 1-octene have lead to
interest in more-selective oligomerization technologies.
of selective tetramerization of ethylene to 1-octene (70%), and a me-
chanism of chromacyclononane intermediate was postulated [7,8].
Follow-up work focused on reaction kinetics, chromium species, ligands
and cocatalyst to investigate reaction mechanism and to improve the
reactivity and selectivity toward α-olefins [9–12]. Among them, a
number of heterogeneous catalyst systems for ethylene trimerization
and tetramerization were reported, and then the confinement effect was
investigated to improve the reactivity [13–16].
Manyik et al. discovered the selective trimerization of ethylene to 1-
hexene using homogeneous chromium 2-ethylhexanoate activated by
partially hydrolyzed triisobutylaluminium [4], and then Briggs im-
proved this system by the addition of dimethoxyethane with selectivity
for ethylene oligomerization to 1-hexene (74%) [5]. A mechanism in-
volving chromacycloheptane intermediate was proposed and accepted
as mechanism of selective ethylene trimerization [4,5]. In 2002, the
diphosphinoamine (PNP) ligands were reported as very successful ar-
chitecture in combination with a soluble chromium source and me-
thylaluminoxane for the selective trimerization of ethylene to 1-hexene
The confinement effect in microporous catalyst has been widely
accepted [17,18], but influence of this effect on reaction performance
for ethylene oligomerization has been rarely studied and is still unclear
[14–16]. Monol et al. reported a heterogeneous trimerization catalyst
based on Cr[N(SiMe3)2]3 and isobutylaluminoxane supported on silica,
which gave an overall 1-hexene selectivity of 74.2% and an activity of
3170 g/mmol Cr per hour, and unsupported catalyst had the activity
decreased to about 1/800 [13]. Toulhoat and co-authors used grand
canonical Monte Carlo simulations to investigate the confinement effect
within zeolitic structures on the activity and selectivity of ethylene
⁎
Corresponding author.
E-mail address: jiangtao@tust.edu.cn (T. Jiang).
http://dx.doi.org/10.1016/j.apcata.2017.07.021
Received 10 May 2017; Received in revised form 10 July 2017; Accepted 13 July 2017
Availableonline14July2017
0926-860X/©2017ElsevierB.V.Allrightsreserved.

H. Shao et al.
AppliedCatalysisA,General544(2017)154–160
oligomerization, using a (η5-C5H4CMe2C6H5)TiCl3 catalyst for ethylene
trimerization, which was confined in zeolite through Van der Waals'
force. Their result predicted that selectivity toward 1-hexene decreased
and selectivity toward 1-octene increased with increasing pore size, and
the suitable pore size for producing 1-octene was approximately 1 nm,
in which pore the chromacyclononane was more stable [14].
In our previous work, Cr(acac)3/PNP were supported on MAO-
treated HY and NaY to investigate confinement effect for ethylene oli-
gomerization. The results showed that NaY-supported Cr(acac)3/PNP
catalyst had weak confinement effect on increasing selectivity toward
1-octene, and detailed reason was not revealed yet, because the catalyst
structure was difficult to be analyzed clearly[15,16]. In this work, we
prepared zeolite-supported Cr(III)-PNP catalysts through grafting Cr
(III)-PNP on HY and NaY zeolites to obtain catalysts with clear structure
for investigating confinement effect. The structure of these supported
catalysts was characterized and reaction performance for ethylene oli-
gomerization was presented.
added dropwise in 1 h. The solution was warmed up to room tem-
perature and further stirred for 2 h. The mixture was filtered and the
solid was washed with a mixture of toluene (20 mL) and ether (20 mL).
The solution was merged and solvent was removed under reduced
pressure. The residue was recrystallized with ethanol to obtain a white
powder assigned as PNP. 31P NMR(CDCl3):62.5(s). 1H NMR(CDCl3):
δ = 0.22 (t, 2H, 3J(H,H) = 8.3 Hz; eCH2Si), 1.08–1.24 (m, 9H; e
(CH3)3), 1.42 (m, 2H; eCH2eCH2Si), 3.20 (m, 2H; eCH2N), 3.60 (m,
6H; eSi(OCH2e)3, 7.26–7.40 (m, 20H; ePPh2).
HY zeolite was pretreated at 100 °C for 10 h at −0.095 MPa of
pressure then stored in glove box until required. HY (5.0 g) and the PNP
(1.0 g) were charged in a flask in a glove box, and the sealed flask was
removed from glove box. Toluene (50 mL) was added to the flask. The
stirred mixture was heated up to 110 °C and then refluxed for 12 h. The
solid was washed with toluene (3 × 30 mL) and dried under vacuum to
give the HYPNP.
HYPNP (4.0 g) was suspended in toluene (10 mL), and chloro-
trimethylsilane (20.0 mL) was added dropwise at 30 °C under nitrogen.
The mixture was kept for 6 h until filtration. The solid was washed
three times with toluene (3 × 30 mL) and then dried under vacuum to
obtain HYPT.
2. Material and methods
2.1. Materials
3.0 g of HYPT was charged in a flask and suspended in toluene
(10 mL) and then a solution of CrCl3(THF)3 (0.37 g, 1.0 mmol) in to-
luene (40 mL) was added dropwise in 30 min under nitrogen. The
mixture was heated at 50 °C for 12 h until filtration. The solid was
washed three times with 30 mL of toluene and then dried under vacuum
at 50 °C to obtain HYCr. NaY-supported Cr(III)-PNP catalyst was pre-
pared using similar procedure and assigned as NaYCr.
Tris(tetrahydrofuran) chromium trichloride (CrCl3(THF)3), 3-ami-
nopropyltriethoxysilane, chlorodiphenyl phosphine and chloro-
trimethylsilane were purchased from J & K Co. HY and NaY zeolites
were
purchased
from
Nankai
University
Catalyst
Co.
Methylaluminoxane (MAO) (toluene solution of 1.4 mol L−1), triethy-
laluminium (TEA) and triisobutylaluminium (TIBA) (toluene solution of
1.1 mol L−1
) were purchased from Aldrich. Polymerization-grade
2.3. Characterization of supports and catalysts
ethylene was obtained from Tianjin Summit Specialty Gases Co.
Cyclohexane and toluene were dried and degassed prior to use. All
other chemicals were obtained commercially and used as received.
Scanning electron microscopy (SEM) images were obtained using a
Hitachi SU1510 instrument operated at an accelerating voltage of 5 kV.
X-ray diffraction (XRD) patterns were recorded using
a Rigaku
2.2. Preparation of supported catalysts
RINT2000 diffractometer operated at 40 kV and 40 mA, using CuKα
radiation. The chromium content of the supported catalyst was de-
termined using a Agilent 7500 inductively coupled plasma mass spec-
trometer (ICP-MS). Nitrogen adsorption and desorption isotherms were
measured at −196 °C using a Quantachrome autosorb (iQ) apparatus.
Samples were first degassed at 300 °C for 4 h (zeolite) or at 60 °C for
12 h (supported catalyst). Specific surface areas (SBET) were calculated
using the Brunauer-Emmett-Teller (BET) method. Pore size distribu-
tions and pore diameters (DP) were calculated using the non local
density functional theory method (NLDFT). The total pore volumes (VP)
were determined from the adsorption volume at a value of P/P0 of
0.995. Micropore surface areas (Smicro) and volumes (Vmicro) were cal-
culated using the t-plot method. Thermogravimetric analyses were
performed using a STA449F5 system (TG-DSC, Netzsch) operated at a
Preparation process of zeolite-supported Cr(III)-PNP catalyst was
shown in Scheme 1.
(Ph2P)2N(CH2)3Si(OCH2CH3)3 was synthesized referring to the
method described in the literature [19]. 3-Aminopropyltriethoxysilane
(7.0 mL, 30.0 mmol), triethylamine (9.2 mL, 66.0 mmol) were dis-
solved in 120 mL of toluene in a two-neck flask under nitrogen. The
solution was cooled to −40 °C and Ph2PCl (11.2 mL, 60.0 mmol) was
heating rate of 10 °C min−1 up to 700 °C under
a flow of N2
(60 mL min−1). Fourier transform infrared (FTIR) spectra were re-
corded using a Bruker Tensor 27 instrument. The sample was mixed
with KBr powder and then was pressed to a disc in glove box. The
sample disc was stored under nitrogen until characterization.
2.4. Ethylene oligomerization and products analysis
Ethylene oligomerization was performed in a 0.5-L autoclave. After
evacuation and flushing with N2 (three times) and then ethylene
(twice), the autoclave was charged with cyclohexane (0.1 L), which was
stirred mechanically under ambient ethylene atmosphere. When the
desired reaction temperature was reached, cocatalyst (MAO, or TEA, or
TIBA) and the supported catalyst suspended in cyclohexane (5 mL)
were injected into the reactor. Typically, after 30 min the mixture was
cooled rapidly to −20 °C and slowly vented, and then the reaction was
quenched through the addition of a solution of HCl in EtOH (10 wt.%).
The catalyst activity was calculated from the increase in product mass.
Scheme 1. Preparation of Cr(III)-PNP catalysts supported on HY and NaY zeolites.
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H. Shao et al.
AppliedCatalysisA,General544(2017)154–160
Fig. 1. Morphologies of HY(a), HYCr(b), NaY(c) and
NaYCr(d).
All experimental were repeated at least two times, and activity error
and selectivity error of main product less than 10% were accepted.
The product was sealed and analyzed as quickly as possible through
gas chromatography (GC) using a Agilent 7890A instrument equipped
with an HP-5 capillary column. The sample was maintained at 35 °C for
10 min, heated at 10 °C min−1 until the temperature reached 280 °C,
and then maintained at that temperature for 10 min.
3. Results and discussion
3.1. Characterization of the catalyst structure
Fig. 1 displays the morphologies of the zeolites and the zeolite-
supported Cr(III)-PNP catalysts. After supporting chromium on HY
zeolite, the particle size and the pattern of outside surface are kept
almostly constant, accompanying with breaking and recombination of a
small amount of particles. The results suggest that the chromium is
mainly loaded into the pore channel so that the pattern of outside
surface is kept. Comparing with HYCr catalyst, the particles of NaYCr
catalyst present agglomeration.
XRD patterns of the zeolites and the zeolite-supported Cr(III)-PNP
catalysts are presented in Fig. 2. After grafting Cr(III)-PNP on HY and
NaY, the crystal structures of the zeolites are maintained, as evidenced
by the absence of changes in diffracted angles and intensities [20]. A
small shoulder peak at 15.3° can be attributed to {331} crystal plane of
CrCl3 crystal for HYCr and NaYCr (JCPDS card no.32-0279), although
other characteristic peaks of CrCl3 crystal are overlapped by diffracted
peaks of the zeolites. Comparing with HYCr catalyst, stronger peak at
15.3° of CrCl3 is presented on NaYCr catalyst.
The different supported patterns of Cr(III)-PNP supported on HY and
NaY are also evident from the different changes in the pore structures.
After grafting Cr(III)-PNP, the obviously different pore structures are
presented on HYCr and NaYCr (see Table 1 and Fig. 3). Relative to its
support, HYCr decreases in its values of Smicro, Vmicro and DP of
546.975 m2/g, 0.202 cm3/g and 0.35 nm, respectively, suggesting main
change is presented in the micro pore. The shape of isotherms is
maintained but the adsorption volume is obviously decreased (see
Fig. 3), and the distribution of pore size becomes narrower. Such results
Fig. 2. XRD patterns of zeolites and zeolite-supported catalysts.
suggest that the loading mainly fills into the micropores, because hy-
droxyls, reacting with silicon hydroxyl of PNP ligand, mainly locate in
the pore channel. Differently, smaller change of Smicro and Vmicro and no
change of pore size are presented on NaYCr catalyst. In combination
with the results from XRD, it can be determined that chromium is
mainly loaded on the outside of the particle and in the pore of NaY,
although NaYCr has nearly the same amount of chromium loaded,
comparing with HYCr. It is possible that stronger acidity of HY zeolite
promotes the grafting reaction between PNP and hydroxyl to increase
the loading of PNP, which evidence is supported by results of nitrogen
distribution on HYCr and NaYCr from EDS results, shown in Fig. A1 in
the supplementary. The HCl formed in the treating process by using
chlorotrimethylsilane can destroy the PNP ligand, and more likely, it
can form salt with N in PNP to decrease the ability complexed with
CrCl3(THF)3. Fortunately, the amount of formed HCl was small (see Fig.
A1 in the supplementary), so the unfavorable effect on the complexing
performance was small. In the supported process no extra acid was
added to avoid the condensation between PNP molecules. Chromium is
presented as Cr(III) according XPS result (binding energy of Cr3p/2 is
577.6 eV, see Fig. A2 in the supplementary). The distance between two
chromium molecules is larger than the pore size. In fact the distance is
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H. Shao et al.
AppliedCatalysisA,General544(2017)154–160
Table 1
Properties of zeolites and zeolite-supported catalysts.
Samples
Content of Cr
(wt.%)
SBET
Smicro
VP
Vmicro
DP
(nm)
Distance between two chromiuma
(nm)
(m2/g)
(m2/g)
(cm3/g)
(cm3/g1)
HY
NaY
HYCr
NaYCr
–
–
0.80
0.81
1011.068
1016.794
444.216
773.915
970.785
1000.499
423.810
766.643
0.453
0.429
0.265
0.361
0.367
0.376
0.165
0.294
1.17
1.11
0.82
1.11
–
–
2.2
2.9
a
Calculated according with content of chromium and specific surface area.
larger than the above size due to the existence of the crystal CrCl3, so
the chromium molecules should be located in a different cage of the
zeolite.
Such results show that earlier deactivation is presented on HYCr. It is
possible that the formed PE hinders the pore channel for ethylene
transfer.
The interaction between the support and Cr(III)-PNP is investigated
by using TG-DSC. Fig. 4 displays the thermograms of CrCl3(THF)3, the
PNP and the zeolite-supported catalysts. According to weight loss for
CrCl3(THF)3, the peaks at 142 °C and 293 °C can be attributed to de-
composition of THF and Cr-Cl bonds, respectively. For the PNP ligand,
the exothermic peak at 75 °C without weight loss suggests that is peak
of crystal transformation of the PNP. A large weight loss at 374 °C is
caused by the decomposition of other bonds besides SieO bonds in the
PNP. For HYPNP, the exothermic peak at 102 °C can be caused by vo-
latilization of residual toluene, and the exothermic peak at 422 °C is
attributed to decomposition of the PNP, which temperature is raised by
48 °C, comparing with the free PNP ligand. This temperature raise re-
veals the existence of chemical bond between the PNP and the zeolite.
In the TG-DSC trace for HYCr, the exothermic peaks at 123 °C, 266 °C
and 413 °C can be attributed to decomposition of THF, break of Cr-N
bonds of Cr(III)-PNP and followed decomposition of the PNP. By com-
parison, the broad peak at 307 °C for NaYCr catalyst can attributed to
combination of break of Cr-N bonds, break of Cr-Cl of the CrCl3 and
decomposition of the PNP. Such results mean that different supported
state is presented on HYCr and NaYCr catalysts.
Next, FTIR spectroscopy is used to characterize the interactions
between Cr(III)-PNP and the support (Fig. 5). For the PNP ligand, the
peaks at 1166 cm−1, 1077 cm−1 and 865 cm−1 can be attributed to
stretching vibrations of CeN bond, SieOeC bond and SieC bond. Re-
lative to the signals in the FTIR spectrum of HY, the vibrations of the
inner SieO tetrahedra of HYCr shift from 1043 and 804 cm−1 to 1046
and 804 cm−1, respectively, and the bending vibration of dicyclo fra-
mework shifts from 580 cm−1 to 584 cm−1. These little shifts suggest
that supporting of Cr-PNP has little effect on the structures of HY [21].
The peak at 856 cm−1 can be attributed to bending vibration of CeOeC
in THF [22]. Similar results are presented on NaYCr.
Table 2 presents the reactivities and product distributions of
homogeneous CrCl3(THF)3/PNP and the zeolite-supported Cr(III)-PNP
catalysts for ethylene oligomerization. For CrCl3(THF)3/PNP catalyst
with MAO as cocatalyst, the 1-octene is main product in agreement
with the Blamm’s result with (Ph2P)2N(CH2)3Si(OCH3)3 as ligand [23].
Differently, 1-butene and 1-hexene are main products using TEA and
TIBA as cocatalysts, and meanwhile, the activities of ethylene oligo-
merization are rapidly decreased. The products distribution means that
oligomerization acts up on the metallacyclic mechanism [5]. Com-
paring with CrCl3(THF)3/PNP catalyst with MAO as cocatalyst, HYCr
and NaYCr catalysts present lower activities and broader product dis-
tributions. In contrast, the activities are rapid decreased by using TEA
and TIBA as cocatalysts, while 1-hexene becomes main product for
TIBA, and 1-hexene and 1-octene become main products for TEA. In
general, the influence of activity affected by supported process may be
attributed to: (i) change of the state of the active sites after supporting;
(ii) the steric hindrance of pore wall on accessibility of cocatalyst to the
active sites; (iii) restriction of ethylene access to the active sites, thereby
hindering the ethylene insertion [24]. Among them, the MAO with
large volume cannot enter into small pore of zeolite, so that chromium
is not enough activated, causing activities decreased and product dis-
tributions changed for HYCr and NaYCr [25]. HYCr has more selectivity
toward 1-hexene than NaYCr, because of the difference of the active site
distribution (see Figs. 2 and 4), and CrCl3 is active less.
The catalytic performances of CrCl3(THF)3/PNP for ethylene oli-
gomerization in the presence of zeolite were presented in Table 2 (see
entry 2, 4 and 6). In the presence of zeolite, the CrCl3(THF)3/PNP using
MAO, TEA or TIBA as cocatalysts presents similar activity with homo-
geneous CrCl3(THF)3/PNP catalyst. This result means that performance
of the alkylaluminium and aluminoxane is presented slight change. It is
possible that the amount of zeolite is too small (about 30 mg, equal to
amount of HYCr in entry 10) and does not lead obvious modification of
alkylaluminium. The selectivity toward 1-octene using MAO as coca-
talyst increases obviously, and 1-hexene becomes main product using
TEA and TIBA as cocatalysts in the presence of HY zeolite. Change of
main product might be caused by the confinement effect in accordance
with Toulhoat’s calculating results.
3.2. Reaction performances
The influences of reaction time on the activity of ethylene oligo-
merization on CrCl3(THF)3/PNP and HYCr catalysts presented in Fig.
A3 in the supplementary show that the activities decrease after 30 min
on homogeneous Cr(III)/PNP and after 20 min on HYCr, respectively.
According to Toulhoat’s result, the zeolite having pore size of
Fig. 3. Isotherms(a) and pore size distributions(b) of zeolites
and zeolite-supported catalysts.
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AppliedCatalysisA,General544(2017)154–160
Fig. 4. Thermogravimetric curves of CrCl3(THF)3(a), PNP(b), HYPNP(c), HYCr(d) and NaYCr(e).
approximately 1 nm would increase the production of 1-octene due to
the confinement effect of pore channel [14]. However, MAO as most
effective cocatalyst for usual Cr-PNP catalyst system has kinetic dia-
meter of greater than 1 nm, so confinement effect could not be checked
in our present work (entry 7 and 8). In this work, ethylene trimerization
is presented over HYCr (entry 10 and 12), instead of ethylene dimer-
ization over homogeneous CrCl3(THF)3/PNP catalyst (entry 5). The
oligomerization reaction, changed from dimerization to trimerization,
may be caused by change of the active state or confinement effect.
Because the chromium active site is far enough from bonding position
of the PNP on the support though Si-O-Si bond, so the change of the
active state might be insignificant, so the confinement effect is likely to
be crucial. The pore size of HYCr is 0.82 nm, so the ethylene trimer-
ization to 1-hexene becomes main reaction. Such results are according
with calculated regularity by Toulhoat.
The reactivity of in situ HYPT and CrCl3(THF)3 was investigated for
ethylene oligomerization (entry 13), similar with proposed method by
Toulhoat [14]. Comparing with homogeneous CrCl3(THF)3/PNP, the
similar activity is presented, and 1-hexene becomes main product. In
the presence of porous HYPT, CrCl3(THF)3 can coordinate with PNP in
the pore to catalyze ethylene trimerization. Comparing with HYCr
catalyst, insufficiently confinement effect is presented, because of in-
sufficient formation of Cr-PNP complex, being consistent with our
previous work [15]. Jabri et al. investigated the Cr-Al species of PNP/
CrCl3 with AlMe3 cocatalyst through X-ray Crystallography [10]. Their
results showed that new divalent catalyst precursor was formed and
subsequently activated by MAO for ethylene oligomerization to pro-
duce 1-hexene and 1-octene. In our work, the divalent catalyst with
similar structure is difficult to be formed due to the immobilization of
the PNP ligand. Mononuclear chromium active site activated by TIBA is
formed and catalyzes ethylene trimerization to 1-hexene (entry 10 and
12 in Table 2).
The influences of the reaction condition on catalytic reactivity over
HYCr are presented in Table 3. The catalytic activity increases initially
with increasing temperature and reaches a maximum at 45 °C, and then
decreases from 45 to 65 °C. The elevated temperature can decrease
ethylene solubility in solvent and increase deactivation rate of the ac-
tive sites, although the elevated temperature is expected to result in
overall higher propagation and transfer rates of ethylene. It is combi-
nation of these effects that is likely to account for the observed tem-
perature dependence of the activity. The change trend of selectivity
toward 1-hexene is according with change trend of the activity. It in-
dicates that the rate of chain transfer, particularly β–H elimination,
increases more than the rate of propagation below 45 °C [26]. The
productions of higher α-olefins (> C6) increase with increasing tem-
perature above 45 °C.
Improving ethylene pressure dramatically increases the catalytic
activity and the selectivity toward 1-octene, and decreases the
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AppliedCatalysisA,General544(2017)154–160
tetramerization using Cr(acac)3/PNP/MAO catalyst system [9]. How-
ever, reaction order on ethylene pressure is far less than 2 on HYCr
catalyst (about 0.17 calculated from Table 3). This result means that the
restriction of the pore channel on ethylene transfer is very obvious, and
ethylene transfer across pore channel may be rate-determining step
affecting the activity.
The catalytic activity and selectivity toward 1-octene first increase
with Al/Cr ratio and decrease thereafter. In contrast, the selectivity
toward 1-hexene reach a maximum at Al/Cr ratio of 400, meaning the
activated chromium state at this Al/Cr ratio is most suitable for ethy-
lene trimerization. The alkylaluminium can activate chromium sites to
form appropriate activated state, but excess amount of alkylaluminium
may interfere with the formation of the active chromium species by the
over-reduction of chromium species.
4. Conclusion
The complex of Cr(III)-PNP is mainly grafted into the pore channel
of HY zeolite and in pore channel and outside of pore of NaY zeolite due
to the difference of the acidity of zeolites. The different supported
pattern affects the reactivity of ethylene oligomerization. The selective
trimerization of ethylene is presented on HY-supported Cr(III)-PNP
catalyst using TIBA as cocatalyst, and broader product distribution is
presented on NaY-supported one. HY-supported Cr(III)-PNP catalyst has
higher activity for ethylene oligomerization than homogeneous one,
while main product becomes from 1-butene to 1-hexene. The change of
the selectivity can be attributed to the confinement effect of pore
channel of supported catalyst to increase stability of chromacyclo-
heptane intermediate for 1-hexene. The confinement effect for ethylene
trimerization in the agreement of Toulhoat’s theory is confirmed in
experiment. In the future, the more effort will be focused on the con-
struction of heterogeneous Cr-based catalyst with regular structure for
investigating confinement effect in ethylene tetramerization.
Fig. 5. FTIR spectra of PNP, zeolites and zeolite-supported catalysts.
selectivities toward 1-butene and 1-hexene. This result shows that the
elevated pressure leads to an increase of ethylene solubility, which
results in the increase of chain propagation rate and thus induces the
increase of catalytic activity and selectivity toward 1-octene [27]. The
dynamic regularity of ethylene oligomerization on the zeolite-sup-
ported Cr(III)-PNP is similar with normal Cr(III)/PNP catalyst for
ethylene tetramerization, on which catalyst the selectivity toward 1-
octene is increased with ethylene pressure [26]. The previous results
presented that the reaction rate was dependent on ethylene con-
centration in the order of 2 for ethylene trimerization over homo-
geneous Cr(2-EH)3 catalyst [4] and in the order of 1.57 for ethylene
Acknowledgments
This study was supported by the National Key Research and
Development Program of China (2017YFB0306700), the Natural
Science Foundation of Tianjin City (14JCYBJC23100, 15JCYBJC48100
and 16JCZDJC31600).
Table 2
Catalytic activity and selectivity of zeolite-supported catalysts.
Entry Catalyst
Cocatalyst Activity
(105 g/(mol Cr·h))
Productc (wt.%)
=
=
=
=
1-C4
C6
1-C6
(in C6)
Ccyclics
(in C6)
C8
1-C8
(in C8)
C10
1-C10
(in C10
> C10
)
1
2
3
4
5
6
7
8
PNP + CrCl3(THF)3
MAOa
59.0
1.07
3.61
15.98 41.52
21.76 58.00
52.02
31.33
–
55.02 95.10
69.41 96.46
5.33 42.12
2.22 11.17
22.6
3.00
–
4.46
–
2.75
33.25
32.42
–
0.03
37.71
0.28
8.85
PNP + CrCl3(THF)3 + HY MAOa
51.3
0.18
0.16
1.33
1.65
1.90
1.82
3.85
4.76
4.50
2.51
1.70
PNP + CrCl3(THF)3
PNP + CrCl3(THF)3 + HY TEAb
PNP + CrCl3(THF)3
TIBAb
TEAb
77.94 22.06 100.00
34.45 56.62 93.74
90.41 8.40
–
–
–
–
2.36
1.74
1.01
7.25
98.32
100.00
98.76
2.73 17.85
48.46
51.54
25.77
15.79
30.47
5.28
0.21
2.05
30.15
22.40
0.18 100.00
1.33 48.89
6.81 36.48
3.67 56.22
6.74 29.08
0.46 32.03
0.68 51.56
0.30 32.15
0.54 42.53
PNP + CrCl3(THF)3 + HY TIBAb
34.69 53.98 72.08
HYCr
NaYCr
HYCr
HYCr
NaYCr
HYCr
HYPT + CrCl3(THF)3
MAOa
MAOa
TEAa
5.23
3.86
7.97
8.08
6.03
28.34 69.84
28.94 68.32
49.03 86.49
75.00 97.65
39.49 97.23
26.37 78.34
31.29 72.34
36.26 82.47
16.43 89.36
16.09 95.67
9
10
11
12
13
TIBAa
TIBAa
TIBAb
TIBAb
13.48 82.34 64.30
20.60 54.01 54.03
3.60
60.57
16.00 71.88
a
Reaction conditions: 4.8 μmol of Cr-based catalyst; pressure: 4 MPa; Al/Cr mole ratio = 400; temperature: 35 °C; solvent: cyclohexane; reaction time: 30 min.
Reaction conditions: 4.8 μmol of Cr-based catalyst; pressure: 1 MPa; Al/Cr mole ratio = 400; temperature: 35 °C; solvent: cyclohexane; reaction time: 30 min.
Oligomers analyzed by GC, and a small amount of polyethylene is not listed.
b
c
159

H. Shao et al.
AppliedCatalysisA,General544(2017)154–160
Table 3
Influences of reaction condition on reaction activities and product distributions.
T (°C)
Pressure (MPa)
Al/Cr molar ratio
Activitya(105g/(mol Cr·h))
Product (wt.%)
=
=
=
1-C4
C6
1-C6 (in C6)
Ccyclics (in C6)
C8
1-C8 (in C8)
C10
> C10
45
55
65
35
35
35
35
35
35
4
4
4
3
2
4
4
4
4
400
400
400
400
400
500
300
200
100
9.76
4.30
2.64
4.52
4.23
4.19
4.26
3.02
2.10
5.46
4.87
3.25
9.14
11.23
6.32
5.15
4.30
3.77
80.83
67.08
53.64
76.34
79.82
52.13
53.12
65.34
67.89
99.26
88.64
92.65
85.63
73.68
92.34
98.14
35.67
40.21
0.56
4.79
3.24
13.62
20.15
4.35
1.03
45.23
55.68
11.81
18.63
20.16
10.64
8.34
14.76
27.03
22.77
16.34
96.26
70.62
82.26
76.34
66.89
90.26
98.32
72.87
78.67
1.82
2.63
4.67
1.02
0.23
3.64
0.29
0.27
0.16
0.08
6.79
18.28
2.86
0.38
23.15
14.41
7.32
11.84
a
Reaction conditions: 2.4 μmol of HYCr catalyst; cocatalyst: TIBA; solvent: cyclohexane; reaction time: 30 min.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
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