ETHYLENE OLIGOMERIZATION OVER CATALYST SYSTEMS
129
CCl4 over the temperature range of 40–80
sure of 3.1 MPa and examined the activity and selectivity sure of 1.3 MPa.
of the catalysts.
°
С
at a presꢀ of the modifier M over the range of 40–80
°
С
at a presꢀ
Since the primary aim of the research concerns the
feasibility of obtaining ꢀolefins, we set about finding
α
optimal conditions for ethylene oligomerization.
EXPERIMENTAL
There are two reports in the literature on the feasiꢀ
bility of ethylene conversion in the presence of the
Manipulations with solvents and components of
the catalyst system were carried out in standard
Schlenk flasks under argon. Toluene was purified and
dried according to standard procedures [15]. PyH,
(Ph2P)2NСy, Cr(acac)3, AlEt3, AlEtCl2, MAO, and
CCl4 purchased from Aldrich were used without furꢀ
ther purification. To make a catalyst, a toluene soluꢀ
tion of a Cr(III) coordination compound and the
ligand with a total volume of 20 mL was prepared in
one Schlenk flask, using the components in the stoꢀ
ichiometric molar ratio. The color of the solution did
not change. A cocatalyst solution was prepared in
another flask, and toluene was added to bring the volꢀ
ume to 20 mL.
Cr(acac)3
catalyst systems. In the latter case, it was shown that
the polymerization of ethylene at 50 , an ethylene
–AlEt2Cl [16] and Cr(acac)3–MAO [17]
°C
pressure of 0.1 MPa, and an Al : Cr molar ratio of
50 proceeds at a low rate (279 gPE (gCr h)–1) to give
polyethylene with a broad bimodal molecular weight
distribution (MWD). WhenAlEt3 or AliBu3 (triisobuꢀ
tylaluminum) was used instead of MAO with an Al : Cr
molar ratio of 10, neither oligoꢀ nor polymerization of
ethylene was observed to any extent.
This fact noted in the literature seems rather surꢀ
prising, and we have decided to conduct research using
the Cr(acac)3–AlEt3 system at higher pressures and
temperatures. Even the first experiments showed that
pressure elevation in the range of 1–3 MPa activates
the Cr(acac)3–AlEt3 catalyst at an Al : Cr ratio of 10
and causes the formation of PE. It was found that at an
optimal molar ratio of Al : Cr = 20, the reaction prodꢀ
uct mixture contains a significant amount (up to
When the modifier was used, a solution of the
Cr(III) coordination compound in 10 mL of toluene
was prepared in one Schlenk flask, and 10 mL of a
solution containing the ligand and the modifier in
given molar ratios, in another flask. The color of the
solution did not change. A toluene solution of the
cocatalyst with a total volume of 20 mL was prepared
in the third Schlenk flask.
70 wt %) of the higher ꢀolefins butaneꢀ1, hexeneꢀ1,
α
and octeneꢀ1 along with polyethylene (table).
The ethylene conversion process was run in a temꢀ
peratureꢀcontrolled 0.1ꢀL stainless steel reactor. The
reaction temperature was maintained with a thermoꢀ
stat feeding the heat transfer fluid to the reactor jacket.
Before each experimental run, the reactor was evacuꢀ
ated for 30 min at the reaction temperature and filled
with ethylene to 0.6 MPa; then, 20 mL of toluene,
20 mL of the complex and ligand solution (in the case
of using the modifier, 10 mL of the complex solution
first and 10 mL of the ligand and modifier solution
next), and 20 mL of cocatalyst solution were succesꢀ
sively injected with a special syringe. The pressure was
adjusted to the working level. Ethylene was fed continꢀ
uously to the reaction zone. After 30 min, the reactor
was cooled and excess pressure was released to the
atmosphere. The resulting products consisting of the
liquid and solid phases were sampled for analysis.
Based on the published data for the Cr(acac)3–
AlEt3–2,5ꢀDMP catalyst system [18] and taking into
account that the basicity and geometrical dimensions
of the ligands can affect the selectivity of catalyst sysꢀ
tems, we decided to use PyH as a ligand that has a
lower basicity and a smaller geometric size.
Unlike the case of the twoꢀcomponent catalyst sysꢀ
tem CS1, 4 wt % polymer and 96 wt % higher ꢀoleꢀ
α
fins (of which hexeneꢀ1 makes 86 wt %) are produced
in the presence of PyH (CS2) as the temperature is
lowered to 40 . Lowering the molar ratio to Al : Cr =
2 at 80 led to an increase in the proportion of the
°
С
°С
polymer (to 25 wt %) and a decrease in the proportion
of hexeneꢀ1 (to 66 wt %). The attempt to modify CS2
by introducing the modifier CCl4 (CS3) resulted in an
increase in the polymer yield (above 91 wt %).
Taking into consideration that ethylene absorption
and the formation of various polymerization and oliꢀ
gomerization products take place in Cr(acac)3–AlEt3
catalyst system under the selected conditions, as well
as the published data [16] on the transformation of
ethylene on the Cr(acac)3–AlEt2Cl catalyst, we
decided to use AlEtCl2 as a cocatalyst, which is a
Quantitative analysis of the liquid phase for
ꢀolefins
α
was carried out on a Shimadzu GCꢀ2010 Plus highꢀ
performance gas chromatograph with a mass detector
and a HP5 column of 50 m length. Polyethylene was
washed, dried, and weighed.
RESULTS AND DISCUSSION
milder reducing agent in comparison with AlEt3
.
In this study we investigated the catalytic features
The replacement of the cocatalyst AlEt3 in CS1 by
of ethylene conversion on the Cr(асас)3–AlR3–L–M less severe AlEtCl2 (CS4) leads to the formation of
system, where AlR3 is triethylaluminum (AlEt3), exclusively PE; no oligomerization products were
dichloroethylaluminum (AlEtCl2), or methylalumoxꢀ found in the liquid phase. In the case of PyH as a ligand
ane (MAO); L is pyrrole (PyH) or bis(diphenylphosꢀ (CS5), a decrease in the polymer yield (to 93 wt %) and
phino)cyclohexylamine ((Ph2P)2NCy); and M is tetꢀ the formation of buteneꢀ1 in trace amounts and about
rachloromethane (CCl4), in the absence and presence 5 wt % hexeneꢀ1 were observed.
PETROLEUM CHEMISTRY Vol. 54
No. 2
2014
Vilms
