N. Zwettler et al. / Reactive & Functional Polymers 88 (2015) 47–54
49
of acetic anhydride was prepared in a dry round bottle and cooled
to 0 °C using a stirred ice bath. We then slowly added 3.6 mL of
100% H2SO4 under constant stirring using a dropping funnel. A
clear solution of acetyl sulfate was obtained. Due to the instability
of the sulfonating agent, it was prepared immediately before use.
For the sulfonation reaction, 10.00 g of PPS was placed into a
three-neck bottle equipped with a gas inlet and a reflux cooler
under a dry argon atmosphere. Subsequently, the acetyl sulfate
solution was added to the polymer. The mixture was then heated
to 90 °C and constantly stirred under reflux for 6 h or 18 h. The
reaction was stopped by the addition of 50 mL of methanol. The
resulting clear orange-brown solution containing the desired poly-
mer as precipitate was then neutralized to pH 6 using a 1.0 M solu-
tion of NaOH in H2O. Reactivation with HCl and washing were then
performed, similar to the other sulfonation method. Finally, 9.7 g
and 9.4 g of light brown pellets were obtained for the 6 and 18 h
reaction times, respectively. Prolonged reaction times did not lead
to further sulfonation, presumably due to the decomposition of the
labile sulfonating agent.
We developed a method for the fabrication of porous PPS sub-
strates based on the porogen leaching method originally developed
for the production of porous tissue engineering scaffolds [22]. PPS
powder (RytonÒ V-1) was mixed with table salt (NaCl) at a mass
ratio of 20%:80%. A cylindrical steel press form was filled and then
pressed using a close fitting piston. The sample was then pressed
for 2 min at room temperature with an external pressure of
40 kN. The complete press form containing the raw sample was
then transferred to a preheated oven where it remained at 280 °C
for 60 min. The hot sample was pressed again with a steady pres-
sure of 40 kN and was then left in the press while it cooled to room
temperature. The cooled sample was extracted from the form and
washed with 2 Â 200 mL of demineralized water for 15 min each
to remove the majority of the salt. The remaining salt was then
leeched overnight by stirring in 250 mL of demineralized water
at 90 °C. The samples were then dried and studied by computed
tomography (CT). A portion of the samples was then treated with
100% H2SO4 for 1 h at room temperature or at 0 °C. The reactions
were then stopped using cold ethanol, and the samples were
washed and dried. A catalysis test was performed at 170 °C over-
night using the batch catalysis method, as described below.
measurements were performed on a Bruker Optics ALPHA FT-IR
spectrometer.
2.4. Catalytic tests
Batch catalysis was performed by adding 5 mL of 99.9% EtOH
and either 1.0 g of pellet catalyst or one porous substrate to stain-
less steel tubes (tube volume 38 mL), which were sealed at both
ends and placed in a preheated oven at the indicated temperature
for the indicated duration. Liquid was then extracted and stored,
and the catalysts were washed with 99.9% EtOH and dried over-
night before use in further catalysis experiments.
Continuous flow catalysis was performed by pumping 1 mL/min
99.9% EtOH through a heated steel tube (volume 38 mL) containing
7.0 g of pellet catalyst retained by glass wool. Before collecting the
samples, the system was allowed to equilibrate at 35 bar and at the
set temperature for 1 h with a continuous flow of 1.0 mL/min 99.9%
EtOH to achieve stable conditions. The pump used was an Azura
P2.1S from Knauer, Germany. Temperatures were measured using
four thermocouples and logged using a TC-08 and PicoLog Ver.
5.20.3 data logging system from Pico Technology Ltd, UK. A graphi-
cal illustration of the setup is provided in Supplementary Fig. 1.
For gas chromatography (GC), 200 lL of liquid was evaporated
at room temperature in a clean gas sample bag filled with 1 L of
N2. When the liquid was completely evaporated, the contents of
the bag were pumped through a GC for 2 min before closing the
GC and analyzing the gas content. The GC used was a 7890B GC
System from Agilent Technologies, and the gas content was sepa-
rated on a DB-1 capillary column before detection using a flame
ionization detector (FID). The integrated peak areas and effective
carbon numbers were used to determine the concentration of each
compound. Liquid from stock solutions/gases of diethyl ether,
ethylene, ethanol, methanol, acetic anhydride and 1,2-dichlor-
oethane was used to determine the peaks corresponding to the
compounds.
3. Results
3.1. Sulfonation of PPS
2.3. Characterization
PPS pellets were sulfonated using two different methods, one
method based on SO3 and one method based on acetyl sulfate.
Unless stated otherwise, we used samples that were sulfonated
for 1 h and 6 h, respectively. CT, SEM and EDX were then used to
analyze the resulting pellets along with non-treated PPS and an
AmberlystÒ-15 control sample. AmberlystÒ-15 is a commercial
ion exchange resin composed of a sulfonated cross-linked styr-
ene–divinylbenzene resin with known solid acid catalytic proper-
ties; it is included in this study as a positive control.
Acid density was determined using a previously published pro-
tocol [23]. Briefly, 1 g of polymer was stirred with 100 mL of 1.0 M
NaCl over 4 h to substitute Na+ for H+. Next, three 20 mL aliquots of
the released acid were titrated with 0.1 M NaOH containing phe-
nolphthalein as an indicator. The standard deviation is indicated.
Computed tomography (CT): High-resolution 3D X-ray images
with a resolution of 5
lm per voxel were obtained using a
SkyScan 1172 CT scanner from Bruker, Belgium. All of the image
l
Using titration with sodium hydroxide, the acid density was
determined to be 0.42 mmol/g ( 0.01) for SO3-sulfonated PPS after
1 h. After 2, 3 and 4 h of reaction time, the acid density was deter-
mined to be constant within the error margins (2 h: 0.93 mmol/g
( 0.03); 3 h: 0.94 mmol/g ( 0.03); and 4 h: 0.93 mmol/g ( 0.03)).
This result could indicate a surface-saturation of the sulfonic acid
groups at prolonged reaction times. After 18 h of reaction time,
acetyl sulfate PPS exhibited an acid density of 0.11 mmol/g
( 0.03). We found the acid density of AmberlystÒ-15 to be
4.41 mmol/g ( 0.03), which is in good agreement with previously
published values [23]. Non-sulfonated PPS did not possess a
detectable acid functionality.
analyses and segmentations were primarily performed using the
open-source software ImageJ [24] with the algorithms developed
under the BoneJ package [25]. The open-source 3D Slicer software
was used for 3D visualization and analysis of the reconstructed
data [26].
Scanning electron microscopy (SEM): SEM was performed using
an Ultra55 microscope from Zeiss, Germany. Samples were placed
on aluminum pin stubs and sputter coated with 10 nm gold parti-
cles before investigation. Images were collected using secondary
electrons. The working distance, voltage and magnification are
indicated in individual figures. Energy-dispersive X-ray spec-
troscopy (EDX) was performed using an X-Max 50 mm2 detector
from Oxford, UK. The standard deviation is given in parenthesis.
Infrared spectroscopy (IR): IR was performed in a diamond cell
using a Nicolet iS50 FTIR from Thermo Fisher (USA). ATR-IR
The CT images (Supplementary Fig. 2) showed that the pellets
were variable in size, but the average diameter was approximately
a few millimeters. Internally, the pellets were composed of com-
pressed powder grains, as indicated by the manufacturer, and the