Chemistry Reference
In-Depth Information
surfactant, water, dimethylamine (DMA,
40 wt%, Aldrich), tetraethyl ortosilicate
(TEOS, 98 wt%, Aldrich) as the silica
source, and aluminium (molar ratio of Si/
Al
¼
15, 60 and infinite) was incorporated
during synthesis using aluminium isoprop-
oxide (AIP, 99 wt%, Aldrich) as the alu-
minium source. The surfactant was first
dissolved in water and DMA. The mixture
of TEOS and AIP was added to this solu-
tion to form the MCM-41 supports. The
final mixture was subjected to an ageing
procedure at 105
8
C for 48 hours. Finally,
the resulting material was filtered and
calcined at 550
8
C for 5 hours in static
conditions.
The SBA-15 supports were prepared
according to a direct synthesis procedure
previously published in the literature
[21,25-27]
in acidic medium at 40
8
C using pluronic
EO
20
PO
7
EO
20
(P123, Aldrich) as surfac-
tant, hydrochloric acid (HCl, 35 wt%,
Sharlau) and tetraethyl ortosilicate (TEOS,
98 wt%, Aldrich) as the silica source. The
surfactant was first dissolved in a solution of
HCl (pH
¼
1.5), then TEOS was added and
the mixture was kept at 40
Characterization of Supports and Catalysts
Nitrogen adsorption-desorption isotherms
at 77 K were obtained with a Micromeritics
Tristar 2050 apparatus. The samples were
previously out-gassed under vacuum at
200
C for 2 hours. Surface areas were
calculated by means of the BET equation
whereas mean pore size was obtained from
the maximum of BJH pore size distribution.
8
Polymerization and Polymer
Characterization
Polymerizations were performed at 70
8
Cin
a 1 litter stirred-glass reactor. The ethylene
flow rate was followed by a mass-flow indi-
cator in order to keep the reactor pressure
at 5 bar during the polymerization. Differ-
ent amounts of 1-hexene were injected into
the reactor with a syringe at the beginning
of the polymerization. In these reactions,
tri-isobutylamunimum (TIBA, 30 wt% in
heptane, Witco) was added as scavenger in
an Al
(TIBA)
/Zr molar ratio of 400. After
30 minutes, the polymerization was stopped
by depressurization and quenched by addi-
tion of acidified (HCl) methanol. The poly-
mer obtained was separated by filtration
and dried under atmospheric pressure at
70
8
C.
Molecular weight averages and distri-
butions were determined with a Waters
ALLIANCE GPCV 2000 gel permeation
chromatograph (GPC) equipped with a
refractometer, a viscosimeter and three
Styragel HT type columns (HT3, HT4
and HT6) with exclusion limit 1
C for 20 h. In
order to make silica SBA-15 supports with
different pore sizes 1,3,5-trimethylbenzene
(TMB, 98 %wt, Aldrich) and n-decane
(98 wt%, Fluka) were used as swelling
agents. After crystallization, they were
aged at 105
8
C for 24 hours. Finally, the
resulting material was filtered and calcined
at 500
8
C for 5 hours under dry air flow.
Heterogeneous catalysts were prepared
by impregnating commercial and synthe-
sized supports with a mixture (Al
(MAO)
/
Zr
¼
170 mol/mol) of methylaluminoxane
(MAO 30 wt% in toluene, Witco) and
a solution of bis(butylcyclopentadienyl)
zirconium dichloride ((nBuCp)
2
ZrCl
2
,
Crompton) in dry toluene (99 wt%, Schar-
lab) under inert nitrogen atmosphere using
Schlenk techniques and a glove box. The
ratio between the volume of impregnating
solution and support pore volume was three.
The reaction was performed at room tem-
perature, in a stirred vessel during 3 hours.
The supported catalyst was dried under
nitrogen flow and stored in a glove-box.
8
10
7
for
polystyrene. 1,2,4-Trichlorobenzene was
used as solvent, at a flow rate of 1 cm
3
min
1
.
The analyses were performed at 145
8
C.
The columns were calibrated with standard
narrow molar mass distribution polystyr-
enes and with linear low density polyethy-
lenes and polypropylenes.
Polymer melting points (T
m
), crystal-
lization temperatures (
T
c
) and crystalli-
nities were determined in a METTLER
TOLEDO DSC822 differential scanning
calorimeter (DSC), using a heating rate of
10
Cmin
1
in the temperature range 23-
160
8
C. The heating cycle was performed
twice, but only the results of the second
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