Gallium-containing polymer brush film as efficient supported Lewis acid catalyst in a glass microreactor

Article abstract of DOI:10.3762/bjoc.9.194

Polystyrene sulfonate polymer brushes, grown on the interior of the microchannels in a microreactor, have been used for the anchoring of gallium as a Lewis acid catalyst. Initially, gallium-containing polymer brushes were grown on a flat silicon oxide surface and were characterized by FTIR, ellipsometry, and X-ray photoelectron spectroscopy (XPS). XPS revealed the presence of one gallium per 2-3 styrene sulfonate groups of the polymer brushes. The catalytic activity of the Lewis acid-functionalized brushes in a microreactor was demonstrated for the dehydration of oximes, using cinnamaldehyde oxime as a model substrate, and for the formation of oxazoles by ring closure of ortho-hydroxy oximes. The catalytic activity of the microreactor could be maintained by periodic reactivation by treatment with GaCl₃.

Full text of DOI:10.3762/bjoc.9.194

Gallium-containing polymer brush film as efficient
supported Lewis acid catalyst in a glass microreactor
Rajesh Munirathinam1, Roberto Ricciardi1, Richard J. M. Egberink1,
Jurriaan Huskens1, Michael Holtkamp2, Herbert Wormeester3, Uwe Karst2
and Willem Verboom*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1698–1704.
1Laboratory of Molecular Nanofabrication, MESA+ Institute for
Nanotechnology, University of Twente, P.O. Box 217, 7500 AE
Enschede, The Netherlands, 2University of Münster, Institute of
Inorganic and Analytical Chemistry, Corrensstr. 28/30, 48149
Münster, Germany, and 3Laboratory of Physics of Interfaces and
Nanomaterials, MESA+ Institute for Nanotechnology, University of
Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
doi:10.3762/bjoc.9.194
Received: 05 June 2013
Accepted: 16 July 2013
Published: 16 August 2013
This article is part of the Thematic Series "Chemistry in flow systems III".
Guest Editor: A. Kirschning
Email:
Willem Verboom* - w.verboom@utwente.nl
© 2013 Munirathinam et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
dehydration of oximes; flow chemistry; gallium; microreactors; Lewis
acid catalysis; polymer brushes
Abstract
Polystyrene sulfonate polymer brushes, grown on the interior of the microchannels in a microreactor, have been used for the
anchoring of gallium as a Lewis acid catalyst. Initially, gallium-containing polymer brushes were grown on a flat silicon oxide
surface and were characterized by FTIR, ellipsometry, and X-ray photoelectron spectroscopy (XPS). XPS revealed the presence of
one gallium per 2–3 styrene sulfonate groups of the polymer brushes. The catalytic activity of the Lewis acid-functionalized
brushes in a microreactor was demonstrated for the dehydration of oximes, using cinnamaldehyde oxime as a model substrate, and
for the formation of oxazoles by ring closure of ortho-hydroxy oximes. The catalytic activity of the microreactor could be main-
tained by periodic reactivation by treatment with GaCl3.
Introduction
Heterogeneous catalysis plays a crucial role in organic syn- microreactor is gaining growing attention, owing to its advan-
thesis both in industry and academia. In the present situation, tages such as increased surface-to-volume ratio, faster heat and
microreactors offer a number of benefits over classical setups mass transfer, only small amounts of reagents are handled,
[1-3]. Especially, heterogeneous catalysis in a continuous-flow when compared to conventional laboratory equipment [4].
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Heterogeneous catalysis in microreactors is carried out using anchoring of gallium as a Lewis acid catalyst making use of
two approaches, viz. a) with a packed-bed microreactor, where polystyrene sulfonate (PSS) polymer brushes. The choice for
the catalyst is attached to a polymeric material enclosed in the gallium was inspired by the successful application of solid
microchannel [5], and b) when the catalyst is covalently supported gallium triflate to catalyze the Strecker reaction by
connected to the inner walls of a glass microreactor [6]. Wiles and Watts [13]. The dehydration of cinnamaldehyde
Although the former approach has advantages such as high oxime [18] was used as a model reaction to study the catalytic
catalyst loading and easy fabrication of the catalytic device by activity of a microreactor with gallium immobilized onto its
filling the channels with functional catalytic particles, however, channel walls (Scheme 1).
heat transfer limitations and pressure drop developing along the
microchannel are serious drawbacks [7].
The literature contains numerous examples of Lewis acid catal-
ysis [8-11]. However, only a limited number of papers are
known dealing with heterogeneous Lewis acid catalysis, where
Scheme 1: Gallium-catalyzed dehydration of cinnamaldehyde oxime
(1).
a Lewis acid is tethered onto a solid surface like silica or gold
[12,13]. Furthermore, to the best of our knowledge, there are no
examples of the immobilization of a Lewis acid to a micro-
reactor channel wall.
Results and Discussion
The catalytic polymer brush layer was first developed on a flat
Polymer brushes have proven to provide a unique platform in silicon oxide surface in order to optimize the reaction condi-
supported catalysis [14,15]. Previously, we have described the tions before attempting the functionalization of a microreactor.
successful immobilization and evaluation of catalysts (e.g., Polystyrene sulfonate (PSS) polymer brushes [19] were synthe-
basic organocatalyst [6], metallic nanoparticles [16], and enzy- sized according to the procedure summarized in Scheme 2.
matic catalyst [17]) to the microchannel walls using polymer First, a monolayer of atom transfer radical polymerization
brushes. As a part of this program, herein, we report the (ATRP) initiator was covalently anchored on silicon oxide
Scheme 2: General scheme for anchoring of initiator, ATRP of styrene sulfonate, activation, and reaction with gallium chloride.
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substrates [20]. Then, a solution of styrene sulfonate in tion times were varied by changing the flow rates between
methanol/water (1:3) in the presence of 2-2’-bipyridyl and 1 and 26 µL·min−1. The reaction showed nearly complete
CuBr, was used to grow the PSS polymer brushes by means of conversion at a residence time of 13 min. Under similar reac-
ATRP. After activation of the polymer brushes with 1 M HCl, tion conditions, after activating the PSS polymer brushes with
they were incubated with a 100 mM solution of GaCl3 in aceto- 1 M HCl, no conversion was observed in the absence of
nitrile.
gallium, proving that gallium is the catalytically active species.
Under our conditions the reaction proceeded much faster when
Analysis of the surfaces by transmission FTIR spectroscopy compared with the lab scale: the gallium(III) triflate (5 mol %)
after the polymerization step revealed the incorporation of catalyzed dehydration of 1 in refluxing acetonitrile reported in
sulfonic acid moieties, as it exhibited symmetric and asym- literature [18] took 2 h to give 2 in 90% yield.
metric stretching vibrations at 1125 and 1011 cm−1, respective-
ly [21]. The intensity of these peaks remained the same, after A kinetic analysis of the dehydration of oxime 1 was performed
activation with HCl and upon treatment with GaCl3, indicating by carrying out the reaction at different concentrations of 1
that the sulfonic acid moieties are still intact. Proof for the func- (10–25 µM, Figure 1). The experimental data were fitted to a
tionalization with gallium, however, was obtained by XPS spec- first-order rate equation, giving an observed rate constant, kobs,
troscopy. The atomic composition of the polymer brushes after of (11 ± 2) × 10−3 s−1. The values of the rate constants at
anchoring of gallium was found to be C:O:S:Ga = 8:3.2:0.9:0.4, different oxime concentrations were the same, within experi-
while no traces of chlorine were detected. From this, it was mental error.
concluded that the polymer brushes contained on average one
gallium per 2–3 styrene sulfonate groups. The thickness of the
polymer brushes was determined by ellipsometry being about
77 nm in the dry state for a polymerization time of 1.5 h.
Swelling studies on gallium-containing polymer brushes were
performed in water, acetonitrile, and ethanol. Table 1 shows
that the polymer brushes swell to the same extent in water as
well as in organic solvents as acetonitrile and ethanol.
Table 1: Thickness of gallium-functionalized PSS polymer brushes on
a flat silicon oxide surface.
thickness (nm)
Figure 1: Gallium-catalyzed formation of nitrile 2 at 90 °C and 5 atm
pressure.
dry state
water
77 ± 2
91 ± 1
95 ± 1
96 ± 1
acetonitrile
ethanol
The activation energy of the dehydration of oxime 1 was deter-
mined by calculating the kobs values at different temperatures
ranging from 70–90 °C (Supporting Information, Figure 1),
The same procedure used for the flat silicon oxide substrates with increments of five degrees. From the slope of the
was followed to immobilize gallium onto the interior of a glass Arrhenius plot (Figure 2), the resulting activation energy was
microreactor with channel dimensions of 150 µm in width and calculated to be 6.55 kJ·mol−1.
depth, and having an internal volume of 13 µL. The PSS
polymer brushes were grown and functionalized with gallium The substrate scope of the dehydration of oximes was extended
on the microchannel interior in the continuous flow-mode.
using the same reaction conditions as above (Table 2).
4-Nitrobenzaldehyde oxime (3, Table 2, entry 1) resulted in a
The dehydration of cinnamaldehyde oxime (1, 25 µM in aceto- conversion of 62% within 13 min reaction time, while for the
nitrile) was used as a model reaction to study the catalytic batch reaction, using gallium(III) triflate as a catalyst, 16 h at
activity of the gallium-functionalized microreactor, at 90 °C and 120 °C was needed to give the nitrile 8 in 82% [18]. Anthracen-
5 atm pressure, generated using a back pressure regulator in 9-carbaldehyde oxime (5, Table 2, entry 3) showed a relatively
continuous flow. The conversion of the reagent was monitored poor conversion, which is ascribed to the steric hindrance of the
online by in-line UV–vis detection, measuring the decrease in molecule in reaching the catalytically active sites within the
the extinction of the solution of oxime 1 at 286 nm. The reac- polymer brushes. In literature [18] this reaction, using
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Beilstein J. Org. Chem. 2013, 9, 1698–1704.
The dehydration of 1 (25 µM in acetonitrile), under similar
conditions as mentioned above, was used to test the stability of
the catalytic layer in the microreactor. When the catalytic
microreactor was continuously run for 50 h with a residence
time of 13 min (flow rate of 1 µL·min−1), the conversions
remained nearly quantitative. However, with a shorter resi-
dence time of 78 s (flow rate of 10 µL·min−1), the conversions
(maximum about 55%) gradually decreased. Total reflection
X-ray fluorescence (TXRF) analysis of one of the samples with
residence times of 78 s and 13 min, showed the presence of
1.0 ppm and 0.2 ppm of gallium, respectively. Nevertheless, the
catalytic activity could be easily reactivated to the original value
(error 2–3%) by treating the microreactor with 100 mM GaCl3
in acetonitrile overnight in the continuous flow mode
(0.1 µL·min−1) (Figure 3). This proves that, at a flow rate of
1 µL·min−1, our catalytic system could be continuously used for
Figure 2: Arrhenius plot for the dehydration of cinnamaldehyde oxime
(1).
gallium(III) triflate, required a reaction time of 8 h in refluxing at least two days without any noticeable decrease in the
acetonitrile to obtain the corresponding nitrile 10 in 87% yield. catalytic activity. If necessary, especially using higher flow
In case of 2-hydroxy-1-naphthaldehyde oxime (6) [22] (Table 2, rates, the catalytic system can be easily reactivated by treat-
entry 4) and salicylaldehyde oxime (7) [18] (Table 2, entry 5) ment with GaCl3. To estimate the amount of gallium present in
dehydration happened between the ortho-hydroxy group and the the polymer brushes, the catalytically active microreactor was
oxime to give ring closure to the corresponding oxazoles 11 and deactivated by flowing 1 M HCl with a flow rate of 1 µL·min−1
12, respectively, in very good conversions.
and subsequent, thorough rinsing with water and acetonitrile.
Table 2: Dehydration of oximes in the gallium-functionalized catalytic microreactora.
entry
substrate
product
conversionb
62
1 [18]
3
4
8
9
2 [23]
3 [18]
47
19
10
5
4 [22]
5 [18]
95
90
11
12
6
7
aAll reactions were performed using 25 µM substrate in acetonitrile at 90 °C, 5 atm pressure, using a back pressure regulator, and a residence time of
13 min. bConversions were determined using online UV–vis spectroscopy by following the change in the extinction of the substrate at a specific wave-
length.
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Beilstein J. Org. Chem. 2013, 9, 1698–1704.
TXRF analysis of the solution showed the presence of rinsing with ethanol and acetone, was stored in a vacuum desic-
3.41 ± 0.01 µg of gallium. When the catalytic microreactor was cator. Methanol and ethanol (VWR, analytical reagent grade)
activated and deactivated following the above mentioned were used without further purification, water was purified with
protocol, the amount of gallium detected remained the same the Milli-Q pulse (MILLIPORE, R = 18.2 MΩ cm) ultra-pure
within the error limit.
water system, dry toluene was from an encapsulated solvent
purification system (MB-SPS-800), and acetonitrile (for
analysis EMSURE® ACS, Reag. Ph Eur, Merck).
Methods
FTIR spectra were recorded using a Nicolet 6700 FTIR spec-
trometer. Ellipsometry measurements were performed with a
Spectroscopic Ellipsometer M-2000X (J.A. Woolam Co., Inc.)
with light reflected at 70° and a spot size of 2 mm diameter.
Over a wavelength range of 340–1000 nm, with spectral resolu-
tion of about 2 nm both Psi and Delta were recorded as well as
the intensity and amount of depolarization of the reflected light.
The Complete EASE v.4.64 software package (J.A. Woolam
Co., Inc.); was used to both control the instrument as well as for
data analysis and modeling. For swelling experiments an in-situ
homemade glass cell with an inner volume of about 70 mL was
used. The 5 mm thick glass windows were positioned perpen-
dicular to the light beam and were transparent in the employed
wavelength range. For all measurements, the data were
corrected for residual polarization by the windows. X-ray
Figure 3: Conversion of cinnamaldehyde oxime (1, 25 µM in acetoni-
trile) by continuously running the catalytic microreactor: (a) for 50 h
with 13 min residence time, (b) for 12 h with 78 s residence time, (c)
also for 12 h with 78 s residence time but after reactivating the micro-
reactor with GaCl3.
Conclusion
In conclusion, to the best of our knowledge, we described the photoelectron spectroscopy (XPS) on the gallium-functional-
first example of the application of polystyrene sulfonate-based ized silicon oxide wafers were obtained on a Quantera Scan-
polymer brushes to anchor gallium as a Lewis acid catalyst onto ning X-ray Multiprobe instrument, equipped with a monochro-
the microchannel interior of a microreactor and proved its matic Al Kα X-ray source producing approximately 25 W of
catalytic activity for the dehydration of oximes to the corres- X-ray power. XPS-data were collected from a surface area of
ponding nitriles and the ring closure of ortho-hydroxy oximes 1000 μm × 300 μm with a pass energy of 224 eV and a step
to the corresponding oxazoles. In general, our catalytic system energy of 0.8 eV for survey scan and 0.4 for high resolution
showed faster conversions for most substrates than using lab scans. For quantitative analysis, high resolution scans were
scale conditions. XPS data, obtained on a flat silicon oxide used. Total reflection X-ray fluorescence analysis (TXRF) was
surface, showed that on average one gallium per 2–3 styrene carried out on a S2-PICOFOX instrument (Bruker AXS, Berlin,
sulfonate groups of the PSS polymer brushes was incorporated. Germany) with an air-cooled molybdenum anode for X-ray
Upon slow deactivation the catalytic activity of the micro- generation. The excitation settings were 50 kV and 750 mA and
reactor can be easily reactivated to its initial value by flowing quartz glass disks were used as sample carriers. The analysis
through a solution of GaCl3. We believe that this approach has a was performed by signal integration over 500 seconds. For the
wider scope and can be used to anchor other Lewis acids in a determination, the signal of gallium (Kα1 = 9.251 keV) was
microreactor to study a range of Lewis acid-catalyzed reactions quantified by using strontium (Kα1 = 9.251 keV) as internal
in an efficient way.
standard ([Sr] = 10.0 µg/mL). Quantification was performed by
the Bruker Spectra software (version 6.1.5.0) and based on the
known concentration of the internal standard.
Experimental
Materials
The chemicals and solvents were purchased from Sigma- Set up of the flow microreactor
Aldrich unless otherwise stated and were used without purifica- All microreactor experiments were performed in a setup as
tion unless specified. Single-side-polished silicon wafers were described in reference [16]. A back pressure regulator (Future
purchased from OKMETIC with (100) orientation. 3-(5’- Chemistry) was connected to the outlet of the microreactor.
Trichlorosilylpentyl) 2-bromo-2-methylpropionate was synthe- Glass microreactors with a residual volume of 13 µL and
sized following a literature procedure [20]. CuBr was purified dimensions of 150 µm depth and 150 µm width were purchased
by washing with glacial acetic acid and, after filtration, by from Micronit Microfluidics (Enschede, The Netherlands).
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Beilstein J. Org. Chem. 2013, 9, 1698–1704.
Synthesis of the catalytic polymer coating
cinnamonitrile (2) in acetonitrile at 286 nm are 35640 and
Immobilization of the trichlorosilane initiator and the polymer 17800 L·mol−1·cm−1, respectively. The kobs values were calcu-
brushes synthesis on the silicon oxide surface [19,20] and the lated by fitting the experimental data with the following equa-
microchannels [16] were carried out following literature pro- tion: [2] = [1]0 · (1 − exp (−kobs·t)) using least-squares fit. The
cedures.
experimental errors in these measurements are ±4%.
A solution of styrene sulfonate (1.25 g, 6.0 mmol) and 2,2’- Substrate scope
bipyridyl (140 mg, 0.9 mmol) in a 3:1 mixture of methanol and The dehydration reaction with different oxime substrates
water (12 mL) was degassed using the freeze-pump-thaw (Table 2, entries 1–5) was carried out in a catalytic micro-
method (in a sealed Schlenk vessel). (The above-mentioned reactor at 90 °C, 5 atm pressure, with a residence time of
solution was frozen by immersion in liquid nitrogen. When the 13 min. A substrate concentration of 25 µM was used for all the
solvent was completely frozen the flask is kept under high substrates. The conversions were determined using online
vacuum for 5 min, with the flask still immersed in the liquid UV–vis spectroscopy by following the change in the extinction
nitrogen. The flask was then closed and brought to room of a substrate specific wavelength. In case of 2-hydroxy-1-
temperature until the solvent has completely melted. This naphthaldehyde oxime (6, Table 2, entry 4) and salicylaldehyde
process was repeated three times and after the last cycle the oxime (7, Table 2, entry 5), the conversions were determined by
flask was filled with nitrogen.) CuBr (57 mg, 0.40 mmol) was following the decrease in the absorbance at 353 nm (ε353 =
added to this solution. To dissolve all solids, the mixture was 8360 L·mol−1·cm−1) and 310 nm (ε310 = 4820 L·mol−1·cm−1),
stirred for 30 min under a continuous flow of nitrogen. After- respectively; the corresponding oxazoles 11 and 12 showed no
wards an initiator coated silicon wafer was placed in a Schlenk absorbance in that region. With 4-nitrobenzaldehyde oxime (3,
tube and the flask sealed with a septum. The tube was filled Table 2, entry 1) and 2-naphthaldehyde oxime (4, Table 2, entry
with argon and the monomer solution was syringed inside. For 2), the conversions were determined by following the decrease
the polymerization in the device, the same solution was in the absorbance at 303 nm (ε303 = 20560 L·mol−1·cm−1) and
syringed through the microchannel till the device was 283 nm (ε283 = 20360 L·mol−1·cm−1), respectively, using the
completely filled. The solution was kept in contact with the formula that was applied for 1, as the corresponding nitriles 8
silicon wafer and with the microchannel at a flow rate of and 9 showed molar absorptivities (ε) of 2200 and
0.1 µL·min−1 for 1.5 h. After the polymerization, the silicon 7160 L·mol−1·cm−1, respectively, in that region. In case of
wafer and the microchannel were rinsed with water and anthracen-9-carbaldehyde oxime (5, Table 2, entry 3), the
methanol, and dried with a stream of nitrogen. In the next step conversions were determined by following the increase in the
the silicon wafers were soaked in a 1 M solution of HCl. The absorbance at 403 nm (ε403 = 2600 L·mol−1·cm−1); the
same solution was flowed with a flow rate of 0.1 µL·min−1 corresponding product 10 has a molar absorptivity (ε) of
through the microreactor. After 2 h they were rinsed with water, 13400 L·mol−1·cm−1 in that region.
acetone, and acetonitrile, and subsequently dried with a stream
of nitrogen. For the preparation of the gallium-based Lewis acid On-line UV–vis detection
bearing polymer brushes, all samples were first incubated The conversion of the oximes was followed using online
overnight in a 100 mM solution of GaCl3 in acetonitrile. The UV–vis spectrometry as described in reference [16].
same solutions were flowed at a flow rate of 0.1 µL·min−1
through the microreactor. Subsequently, the silicon wafer and
Supporting Information
microchannel were rinsed with acetonitrile.
Supporting Information File 1
Kinetic study
Conversion of cinnamaldehyde oxime (1, 25 µM in
acetonitrile) catalyzed by gallium in a microreactor at
different temperatures.
The dehydration of cinnamaldehyde oxime (1, 10–25 µM) was
carried out in acetonitrile at 90 °C under 5 atm pressure.
Cinnamaldehyde oxime (1) and cinnamonitrile (2) exhibit the
absorption maximum in acetonitrile at 286 nm and 271 nm, res-
pectively. The conversion of cinnamaldehyde oxime (1) to
cinnamonitrile (2) was calculated based on the decrease in the
absorption at 286 nm using the formula: [2] = (ε286 (1) · [1]0 −
Aobserved) / (ε286 (1) − ε286 (2)) (Aobserved = absorbance
measured experimentally, while carrying out the reaction). The
molar absorptivities (ε) of cinnamaldehyde oxime (1) and
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-194-S1.pdf]
Acknowledgements
We acknowledge the financial support from NanoNext NL
(RM) and the Netherlands Organisation for Scientific Research
(NWO) (RR).
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23.Desai, D. G.; Swami, S. S.; Mahale, G. D. Synth. Commun. 2000, 30,
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