Synthesis of Synthetic Lubricants by Trimerization of 1-Decene and 1-Dodecene with Homogeneous Chromium Catalysts

Article abstract of DOI:10.1002/1615-4169(20011231)343:8<814::AID-ADSC814>3.0.CO;2-H

The Cr-catalyzed trimerization of 1-decene and 1-dodecene is a highly selective method to synthesize C₃₀ and C₃₆ olefins. After hydrogenation, these products display very attractive viscosity indices (VI's). Due to the high trimer se

Full text of DOI:10.1002/1615-4169(20011231)343:8<814::AID-ADSC814>3.0.CO;2-H

FULL PAPER
Synthesis of Synthetic Lubricants by Trimerization
of 1-Decene and 1-Dodecene
with Homogeneous Chromium Catalysts
Peter Wasserscheid,a,* Siegfried Grimm,a Randolf D. KoÈhn,b Matthias Haufec
a Institut fuÈ r Technische und Makromolekulare Chemie der RWTH Aachen, Worringer Weg 1, 52074 Aachen, Germany
Fax: (+49) 241-888-8177, e-mail: Wasserscheidp@itmc.rwth-aachen.de
b Department of Chemistry, University of Bath, Bath BA2 7AY, UK
c Matthias Haufe, Continental AG, D-30165 Hannover
Received August 22, 2001; Accepted October 1, 2001
Abstract: The Cr-catalyzed trimerization of 1-dec- C20, C24 or C40+ oligomers from the product prior to a
ene and 1-dodecene is a highly selective method to potential use of these products in synthetic lubri-
synthesize C30 and C36 olefins. After hydrogenation, cants for automotive applications.
these products display very attractive viscosity in-
dices (VI's). Due to the high trimer selectivity of the
reaction no additional work-up is needed to separate
Keywords: alkene; chromium; lubricants; trimeri-
zation; viscosity
catalytic systems also convert mixtures of ethylene
and 1-olefins into mixtures of cotrimerization prod-
ucts but show no activity for the trimerization of high-
er 1-olefins alone (> C3).[5]
Introduction
Modern synthetic lubricants for high purpose uses
have to fulfill many requirements. Among others, in-
dustry asks for low pour points, low vapor pressure
and good lubricating properties at reasonable prices.
Of particular interest are low viscosities to lower fric-
tion and a high viscosity index (VI). The VI, calculated
by an empirical formula, reflects the dependence of
the viscosity on the temperature: the less significant
this dependency the higher is the VI.[1]
These requirements are fulfilled by C30±C40 al-
kanes with a certain degree of branching.[1] The
synthesis of these compounds is carried out indus-
trially by cationic oligomerization (e. g., with BF3/
methanol as catalyst[2]) using 1-decene as feedstock
followed by hydrogenation of the obtained oligomers.
However, the products obtained in this process con-
tain besides C30 and C40 oligomers significant
amounts of dimers and pentamers. Both side prod-
ucts decrease the lubricant's quality. Dimer products
add significant vapor pressure and the presence of
pentamers increases the pour point of the material.
Another decrease of the VI arises from skeleton iso-
merization that is observed as a side reaction of the
cationic oligomerization.
Results and Discussion
In contrast, we report here that Cr complexes with
N,N',N''-trialkyltriazacyclohexane ligands of the gen-
eral formula (R3TAC)CrCl3 (as shown in Figure 1) are
highly selective catalysts for the trimerization of 1-de-
cene and 1-dodecene after activation with MAO.
These Cr complexes can be synthesized, for example,
by reaction of CrCl3 with R3TAC in presence of cataly-
tic amounts of Zn and have been characterized by UV-
Vis spectroscopy, 2H NMR spectroscopy and X-ray
analysis.[6,7]
It is well known that homogeneous Cr catalysts
formed by in situ reaction of a Cr(III) salt, pyrrole
and alkylaluminum species are able to trimerize
ethylene to 1-hexene with high selectivity.[3,4] These
Figure 1. Cr complexes of general formula (R3TAC)CrCl3 as
used for the trimerization of 1-decene and 1-dodecene
814
Ó WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2001
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Adv. Synth. Catal. 2001, 343, No. 8

FULL PAPER
Ligand Influence
is sensitive towards elevated temperatures. Typically,
the oligomerization reactions are carried out at 0 °C
in aromatic or aliphatic hydrocarbon solvents (e. g.,
toluene or heptane). Another option is to use the feed-
stock as solvent. However, in this case the limited so-
lubility of the catalyst in the feedstock causes certain
restrictions.
Figure 3 shows kinetic investigations of the 1-dode-
cene trimerization with different R3TAC ligands. In all
cases an induction period is observed at the begin-
ning of the reaction. Nevertheless, the main reactivity
occurs within the first five hours of the reaction. Later
the oligomerization activity slows very much down
even if in some cases the level of conversion is still
far from completion. Obviously, the Cr catalyst deac-
tivates after a certain reaction time. However, ligand
Exemplified for the trimerization of 1-dodecene, the
influence of the applied R3TAC ligand on the reactiv-
ity of the catalytic system is presented in Table 1. The
experiments reveal a significant dependence of cata-
lytic activity and trimerization/isomerization ratio on
the nature of the alkyl substituent R.
After reaction, some unconverted 1-dodecene, line-
ar C12 olefins with an internal double bond (products
of olefin isomerization) and C36 olefins were detected
by GC in the reaction mixture. In all cases, the
amount of C24 dimers and higher oligomers (C48+
)
was found to be very low (< 0.5 mass % of the overall
reaction mixture for all catalytic runs at 0 °C) indi-
cating a very high trimer selectivity of the oligo-
merization reaction. Schematically, the reactivity
of the catalytic 1-dodecene transformation with
[(R3TAC)CrCl3]/MAO can be described as shown in
Figure 2.
With regard to the trimerization/isomerization ra-
tio, it is noteworthy that a higher degree of branching
of the alkyl group R in the ꢀ-position enhances the un-
desired isomerization reaction (entry 4 in Table 1).
Moreover, it is important to mention that the catalyst
Figure 3. Trimerization of 1-dodecene ± kinetic investiga-
tions using different R3TAC ligands. [(Do)3TAC: ±^±; (1,5-
Me2Hex)3:±&±; (2-Et-Hex)3TAC:±~±; (1,1-Me2Un)3TAC:±+±;
(MeBz)3TAC:±´±].
Figure 2. Reactivity of 1-dodecene using [(R3TAC)CrCl3]/
MAO as catalytic system.
Table 1. Trimerization of 1-dodecene ± influence of the alkyl group R of the R3TAC-ligands on catalytic reactivity and selec-
tivity.
Entry
Ligand
Conversion [%]
S(C36) [%][a]
S(iso-C12) [%][b]
TON[c]
1
2
3
4
5
(Do)3TAC[d]
97.7
76.5
80.5
85.8
43.9
81.0
85.7
93.0
6.5
19.0
14.3
7.0
93.5
31.0
724
677
1026
50
(1,5-Me2Hex)3TAC[d]
(2-Et-Hex)3TAC[d]
(1,1-Me2Un)3TAC[d]
(MeBz)3TAC[d]
69.0
283
General conditions: about 10 mL 1-dodecene, 10 mL toluene, 0.05 mmol [(R3TAC)CrCl3], 5 mmol MAO, 20 h, 0 °C.
[a] Selectivity to C36 products calculated as amount of C36 in all products;
[b] Selectivity of linear, internal C12 olefins calculated as amount of linear, internal C12 olefins in all products,
[c] Turnover number calculated as mol 1-dodecene converted in oligomerization/mol Cr catalyst.
[d] Ligand structures:
Adv. Synth. Catal. 2001, 343, 814±818
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asc.wiley-vch.de
decomposition can be excluded since no degradation trimer products should be obtained after hydrogena-
products could be observes by GC analysis. A possible tion according to the mechanism displayed in Fig-
deactivation mechanism by product inhibition has ure 4. In fact, four main products are found in the tri-
been investigated as well. However, experiments mer fraction after hydrogenation which account for
with added trimer product from other catalytic runs more than 95% of all trimers (the side products may
at the start of the reaction could not confirm this hy- be explained by incomplete hydrogenation). Depend-
pothesis.
ing on the Cr complex under investigation the distri-
bution of the four regioisomers can vary significantly.
However, to date no clear assignment of the different
GC peaks to individual regioisomers has been possi-
ble.
Mechanistic Aspects
The high selectivity of the trimerization reaction is
based on the special mechanism of the reaction (as
shown in Figure 4, here with 1-decene as feed-
stock).[7] The catalytic reaction proceeds following a
metallacycle mechanism with five- and seven-mem-
bered Cr metallacycles being the key-intermediates.
From the latter the trimer product olefin is elimi-
nated. In contrast, the five-membered metallacycle
does not allow olefin elimination. Therefore no for-
mation of dimer products is observed. Since elimina-
tion from the seven-membered metallacycle is signif-
icantly faster than additional 1-decene insertion, the
amount of tetramers and higher oligomers is very
low as well.
Viscosity of the Products
The viscosity and even more important the tem-
perature dependence of the viscosity (VI) is critical
for a potential use of the trimerization products in
synthetic lubricants (after hydrogenation with Pd on
carbon as catalyst). All trimerization samples
showed Newtonian behavior in the viscosity determi-
nation. Selected results for the kinetic viscosities and
the VI's (determined after ISO 2909) are presented in
Table 2.
From the viscosity and VI determination the fol-
lowing statements can be derived: a) As expected,
C12 trimers (C36 products) show higher absolute vis-
cosity at 40 °C and 100 °C and higher VIs than C10 tri-
mers (C30 products). b) All samples generated by Cr-
catalyzed trimerization display equal or better VI's
than commercial PAO 4 and PAO 6 samples (pro-
duced via the BF3/co-catalyst system) even if abso-
lute viscosities at 40 °C are lower in most cases (en-
tries 4 8). c) Different alkyl groups R at the R3TAC
ligand seem to have only little effect on the VI of the
product. However, aromatic substituents seem to
have an enhancing effect on the VI of hydrogenated
trimerization products (comparison of entries 6, 7
with entry 8). d) The use of Co-MAO activation in-
stead of MAO activation seem to enhance the VI of
the hydrogenated trimerization products. e) Pour
points of all hydrogenated C30 products are between
42 °C and 45 °C, pour point of the hydrogenated
C36 product (entry 3) is 30 °C.
The olefins formed are either internal (RHC=CHR)
or vinylidenes (H2C=CR2). No 1-olefins are observed
in the trimer fraction. Since comparative experiments
reveal that the Cr catalyst only converts 1-olefins it
becomes understandable that no consecutive cotri-
merization is found.
No skeleton isomerization of feedstock or product
could be observed. Consequently, only four different
Besides the attractive viscosity indices, it is ex-
pected that a significant technical advantage of the
Cr-catalyzed trimerization of 1-decene and 1-dode-
cene vs. the well-known BF3-catalyzed oligomeriza-
tion[2] arises from the high trimer selectivity of the
reaction. Both dimers and pentamers decrease the
quality of the product for a potential use in synthetic
lubricants due to high volatility (C20, C24) and/or
high pour point (C50+), respectively. Therefore, these
components have to be either removed (dimers by
distillation) or restricted in the conventional BF3 pro-
cess.
Figure 4. Mechanism of the Cr-catalyzed trimerization of 1-
decene.
816
Adv. Synth. Catal. 2001, 343, 814±818

FULL PAPER
Table 2. Kinetic viscosities and VIs (after ISO 2909) of C30 and C36 alkanes obtained by C10 and C12 trimerization with
[(R3TAC)CrCl3]/MAO followed by hydrogenation in comparison to commercial PAO 4 and PAO 6.[a]
Entry
Ligand[b]
Co-catalyst
Product
Kinetic viscosity 40 °C/cSt
Kinetic viscosity 100 °C/cSt VI
1
2
3
4
5
6
7
8
PAO 4
PAO 6
C36
C30
C30
C30
C30
C30
17.5
31.1
18.3
12.2
13.8
11.8
13.0
12.1
4.0
5.9
4.3
3.2
3.4
3.1
3.3
3.2
124
137
150
126
125
130
131
137
(1,5-Me2Hex)3TAC
(1,5-Me2Hex)3TAC
(Oc)3TAC
(1,5-Me2Hex)3TAC
(2-Et-Hex)3TAC
(Bz)3TAC
MAO
MAO
MAO
Co-MAO
Co-MAO
Co-MAO
[a] Trimerization conditions for entries 3 8: 10 mL 1-alkene feedstock, 10 mL toluene, 0.05 mmol [(R3TAC)CrCl3], 5 mmol
MAO or Co-MAO , 20 h , 0 °C; activity (turnover number) 1-decene trimerization/activity 1-dodecene trimerization (under
identical conditions) = 1.5 1.7 (depending on the Cr complex used); activity (turnover number) MAO activation/Co-MAO
activation (under identical conditions) = 1.7 1.9 (depending on the Cr complex used).
[b] Ligand structures:
General Procedure for the Synthesis of R3TAC
Ligands
Conclusions
In summary, we have shown that a highly selective
trimerization of 1-decene and 1-dodecene can be car-
ried out with catalytic systems of the general type
[(R3TAC)CrCl3]/MAO (R = alkyl, aryl). The reaction
conditions are important in order to avoid thermal
decomposition of the catalyst and to ensure complete
solubility of the active catalyst. After hydrogenation,
the products may be interesting components of syn-
thetic lubricants since they display appropriate vis-
cosity and attractive VI's.
Paraformaldehyde was added to a solution of freshly distilled
amine in toluene at room temperature and the reaction mix-
ture was heated to reflux for 30 min. The formed water was
removed in an azeotropic distillation with the toluene sol-
vent. The crude product was dried under vacuum and used
without further purification in the complexation step.
N,N',N''-Tris(1,5-dimethylhexyl)triazacyclohexane: 2-Ami-
no-5-methylheptane (3.911 g, 30.26 mmol) was reacted with
paraformaldehyde (0.956 g, 30.26 mmol) in 15 mL toluene.
N,N',N¸-Tris(1,5-dimethylhexyl)triazacyclohexane was iso-
lated; yield: 3.85 g (91%). 1H NMR (200 MHz, CDCl3): d =
3.47 (br. s, 6 H, N±CH2±N), 2.65 (br. s, 3 H, N±CH±), 1.0±1.6
(21 H), 0.96 (d, 6.2 Hz, 9 H), 0.81 (d, 6.4 Hz, 18 H); 13C NMR
(CDCl3): d = 15.9, 22.4, 22.5, 24.1, 27.8, 34.2, 39.1, 54.4, 67.7.
N,N',N''-Tris(2-ethylhexyl)triazacyclohexane: 2-Ethylhex-
ylamine (2.04 g, 15.78 mmol) was reacted with paraformal-
dehyde (0.499 g, 15.78 mmol) in 7.5 mL toluene. N,N',N''-
Tris(2-ethylhexyl)triazacyclohexane was isolated as a clear
and colorless oil; yield: 1.9 g (85%). 1H NMR (300 MHz,
CDCl3): d = 3.19 (br, 6 H, N±CH2±N), 2.19 (d, 6 H, N±CH2±),
1.2 1.27 (m, 24 H, ±CH2±), 0.76 0.82 (two t, 18 H, ±CH3);
13C NMR: (100 MHz, CDCl3): d = 11.0, 14.4, 23.4, 24.9, 29.2,
31.7, 37.6, 57.0, 75.5.
N,N',N''-Tris(S-methylbenzyl)triazacyclohexane: S-1-Phenyl-
ethylamine (21.88 g, 181 mmol) was reacted with paraformal-
dehyde (5.17 g, 172 mmol) in 30 mL of toluene. Recrystalliza-
tion from hexane afforded N,N',N''-tris(S-methylbenzyl)-
triazacyclohexane; yield: 19.0 g (83%). 1H NMR (200 MHz,
CDCl3): d = 7.17 7.29 (m, 15 H, Ph), 3.72 (q, J = 6.7 Hz, 3 H, N±
CH±), 3.39 (br, 6 H, N±CH2±N), 1.29 (d, 6.7 Hz, 9 H, Me);
13C NMR (CDCl3): d = 20.1, 59.4, 70.0, 126.7, 127.3, 128.1, 144.3.
Experimental Section
General Remarks
All reactions were carried out under argon atmosphere
using standard Schlenk techniques. Solvents were dried
and deoxygenated by conventional methods prior to use. 1-
Olefins were purchased from Fluka, distilled and stored un-
der argon. MAO and Co-MAO (10% iso-butylmethylalumox-
ane in heptane) solutions were used as purchased from WIT-
CO. N,N',N''-Tribenzyltriazacyclohexane was purchased
from Aldrich. N,N',N''-Tris(octyl)triazacyclohexane and
N,N',N''-tris(dodecyl)triazacyclohexane and their chro-
mium complexes were prepared as described earlier.[7a]
Adv. Synth. Catal. 2001, 343, 814±818
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asc.wiley-vch.de
Synthesis of Cr Complexes [(R3TAC)CrCl3] with
R= benzyl, n-octyl, n-dodecyl, 1,5-dimethylhexyl,
2-ethylhexyl, S-methylbenzyl
Viscosity and VI Determination
Isolation of pure trimer oligomers was carried out by remov-
ing all C10 or C12 monomers under vacuum from the isolated
organic layer (see above). All samples were hydrogenated
(Pd on carbon; 50 bar H2; 25 °C; 16 h) prior to the viscosity
determination. All viscosity measurements and the VI deter-
mination was carried out at bp/Sunbury, UK according to
ISO 2909.
Anhydrous CrCl3 was suspended in toluene and 10 mg Zn
was added. A solution of 1 equivalent of R3TAC ligand in to-
luene was added and the reaction mixture was heated to re-
flux for 60 min. The product formation was observed by the
deep violet color of the reaction mixture. The solvent was re-
moved under vacuum and the crude product was dissolved
in CH2Cl2 for further purification by column chromatogra-
phy (column material: Silica 80 A; load 1 g crude product
per 200 mL silica). After chromatography a deep violet solu-
tion was obtained. After removal of the CH2Cl2 under va-
cuum the residue was stirred in n-pentane to obtain the
product as a crystalline powder.
[N,N',N''-Tris(1,1-dimethylundecyl)triazacyclohexane]CrCl3:
Paraformaldehyde (373 mg, 12.4 mmol) was added at 20 °C
to 1,1-dimethylundecylamine (2.6 g, 12.4 mmol) dissolved
in 13 mL toluene. To fully dissolve the aldehyde, the mixture
was heated shortly to 50 °C. After cooling down to 20 °C the
sample was stirred for 20 h. The solvent was removed and
the N-methylidine-1.1-dimethylundecylamine was dried un-
der vacuum; yield: 2.5 g (90%). 1H NMR (CDCl3): d = 0.82 (tr,
J = 6.7 Hz, 3 H), 1.07 (s, 6 H), 1.19 (br. s, 16 H), 1.43 (m, 2 H),
7.26 (d, J = 2.0 Hz, 2 H); 13C NMR (CDCl3): d =147.7, 60.6,
42.85, 31.79, 30.08, 29.52, 29.23, 26.32, 23.93, 22.55, 13.95.
The imine (1.61 g, 7.16 mmol, 2.8 equivalents) was dis-
solved in 10 mL toluene at 20 °C and 0.50 g of CrCl2(THF)
was added. The reaction mixture was stirred at 20 °C for
20 h. Hexachloroethane (0.30 g) was added and the mixture
was stirred for another 0.5 h. The solvent was removed un-
der vacuum and the crude product was dissolved in CH2Cl2
for further purification by column chromatography (column
material: Silica 80 A; load 1 g crude product per 200 mL sili-
ca). After chromatography a deep violet solution was ob-
tained. After removal of the CH2Cl2 under vacuum the resi-
due was stirred in n-pentane to obtain the product as a
crystalline powder; yield: 0.81 g (37% ).
Acknowledgements
P. W. and S. G. thank Prof. Willi Keim for his continuous inter-
est in this research and Drs. Mike D. Jones and Marc Howard
(both bp) for fruitful discussions. Determination of the VI's
and financial support by bp is thankfully acknowledged.
R. D. K. thanks the DFG for financial support.
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Catalytic Experiments
A Schlenk tube was charged with 0.05 mmol [(R3TAC)CrCl3]
and 10 mL of toluene. The complex dissolved partly under
gentle heating forming a violet colored solution. This reac-
tion mixture was cooled to 20 °C and 10 mL of the 1-olefin
and 0.5 mL of cyclohexane as internal standard were added.
The mixture was cooled to 50 °C and 5 mmol of MAO or
Co-MAO were added, respectively. Then the reaction was al-
lowed to reach 0 °C and this temperature was maintained
with a cryostat. To analyze a sample after a certain reaction
time, 0.5 mL of the reaction mixture were reacted with
0.5 mL of an aqueous HCl solution (2 M) at room tempera-
ture. After centrifugation, the aqueous layer was removed
and the organic layer was dried with Na2SO4. The organic
layer was then analyzed by gas chromatography using a Sie-
mens Sichromat machine with a 25 m HP Ultra column
(temperature program: 0 min iso, 12 °C/min, 50 280 °C;
evaporator temperature: 300 °C; carrier gas: 2.0 bar H2).
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818
Adv. Synth. Catal. 2001, 343, 814±818