Selenium-Modified Microgels as Bio-Inspired Oxidation Catalysts

Article abstract of DOI:10.1002/anie.201901161

Active colloidal catalysts inspired by glutathione peroxidase (GPx) were synthesized by integration of catalytically active selenium (Se) moieties into aqueous microgels. A diselenide crosslinker (Se X-linker) was successfully synthesized and incorporated into microgels through precipitation polymerization, along with the conventional crosslinker N,N′-methylenebis(acrylamide) (BIS). Diselenide bonds within the microgels were cleaved through oxidation by H₂O₂ and converted to seleninic acid whilst maintaining the intact microgel microstructure. Through this approach catalytically active microgels with variable amounts of seleninic acid were synthesized. Remarkably, the microgels exhibited higher catalytic activity and selectivity at low reaction temperatures than the molecular Se catalyst in a model oxidation reaction of acrolein to acrylic acid and methyl acrylate.

Full text of DOI:10.1002/anie.201901161

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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International Edition
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Accepted Article
Title: Selenium-Modified Microgels as Bio-Inspired Oxidation Catalysts
Authors: Kok Hui Tan, Wenjing Xu, Simon Stefka, Dan Demco, Tetiana
Kahrandiuk, Volodymyr Ivasiv, Roman Nebesnyi, Vladislav
Petrovskii, Igor Potemkin, and Andrij Pich
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901161
Angew. Chem. 10.1002/ange.201901161
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10.1002/anie.201901161
Angewandte Chemie International Edition
COMMUNICATION
Selenium-Modified Microgels as Bio-Inspired Oxidation Catalysts
Kok H. Tan, Wenjing Xu, Simon Stefka, Dan E. Demco, Tetiana Kharandiuk, Volodymyr Ivasiv, Roman Nebesnyi,
Vladislav S. Petrovskii, Igor I. Potemkin, and Andrij Pich*
pressure,[12] electric potential,[13] and light.[14] Ow ing to their
Abstract: Active colloidal catalysts inspired by the glutathione
biocompatibility and chemical diversity, microgels are utilized in
biological applications ranging from controlled drug delivery,[15]
peroxidase (GPx) were synthesized by integration of catalytically
active selenium (Se) moieties into aqueous microgels. Diselenide
crosslinker (Se X-linker) w as successfully synthesized and
incorporated into microgels through precipitation polymerization,
along w ith conventional crosslinker N,N´-Methylenebis(acrylamide)
(BIS). Diselenide bonds inside the microgels were cleaved through
oxidation by H2O2 and converted to seleninic acid whilst maintaining
the microgel microstructure intact. Using this approach catalytically
active microgels with variable amounts of seleninic acid were
synthesized. Remarkably, the microgels exhibited higher catalytic
activity and selectivity at low reaction temperatures compared with
the molecular Se catalyst in a model oxidation reaction of acrolein to
acrylic acid and methyl acrylate.
scaffolding
for
cell
proliferation,[16]
biosensors,[17]
to
biocatalysts.[18]
Poly(N-vinylcaprolactam) (PVCL) microgels have attracted much
attention because of their chemical stability, bio-compatibility,
and stimuli-responsiveness. PVCL microgels exhibit volume
phase transition temperature (VPTT) around 32 °C that is close
to physiological temperature. The temperature-triggered
sw elling/desw elling behavior of PVCL microgels allow s for the
controlling of their mesh size and solvent permeability. Therefore
PVCL microgels have been discussed as promising materials in
catalysis,[19] biology or biomedical applications.[20]
In particular, the application of microgels in catalysis may
open attractive possibilities for the design of new interactive
catalyst systems. Several studies reported that microgels can
allow guest molecules to diffuse into the porous polymeric
netw ork during sw elling, w here the molecules w ill then be
trapped inside the microstructure during collapsing of the
netw ork. Subsequent sw elling of the microgels releases the
guest molecules.[21] Furthermore, computer simulations have
demonstrated that polymeric microgels reduce the surface
tension of interface betw een tw o immiscible liquids by spreading
at the interface promoting the mixing of incompatible liquids
The importance of catalysts is testified by its applications, use in
most biological reactions,[1] industrial syntheses[2] as w ell as
environmental protection.[3] Heterogeneous catalysts cover the
majority of catalysis applications, providing the advantage to
ease the processing and separation after the reactions. How ever,
the heterogeneity contributes to the inferiority in catalytic activity
and, therefore, tremendous research efforts w ere devoted
recently by merging heterogeneous and homogeneous catalysts
by immobilizing catalysts onto colloidal supports. Hence,
providing high catalytic efficiency (activity and selectivity) and
easy separation for catalyst recycling and re-use.[4] Up until
recently, the majority of colloidal supports used in
heterogeneous catalysts w ere metal oxides,[5] zeolites[6] or
polymers.[7]
w ithin the polymeric netw ork.[22] The microgels act as
a
“compatibilizer” and could be useful for the development of
stimuli responsive systems for interfacial catalysis.[23]
Considering these extraordinary properties, microgels have also
been used as functional carriers for catalytically-active
Microgels are porous particles consisting of a crosslinked
polymeric netw ork, forming colloidally-stable dispersions that
sw ell in w ater and other solvents.[8] Microgels can be tailored to
possess unique properties enabling them to respond to external
stimuli such as temperature,[9] ionic strength,[10] pH,[11]
nanoparticles,[24]
organocatalysts.[26]
organometallic
catalysts,[25]
and
Since the discovery of selenium functions in biological
systems,[27] much attention has been devoted to elucidating the
importance of selenium in selenoenzymes.[28] Glutathione
peroxidase (GPx), an enzyme w ith antioxidation properties, can
be found in most mammals.[29] The unique structure of the
selenocysteine as the active center w ithin the protein structure
enables the catalytic oxidation/reduction reaction to be carried
out.[30] Decades of research has been carried out to produce
materials that mimic the glutathione peroxidase (GPx).[31] For the
efficient mimicking of the protein structure, a three-dimensional
(3D) porous structure is required, w hich is responsible for the
[*]
K. H. Tan, S. Stefka, W. Xu, Prof. D. E. Demco, Prof. I. I. Potemkin,
Prof. A. Pich
DWI Leibniz Institute for Interactive Materials e.V.,
RWTH Aachen University
Forckenbeckstraße 50, 52074 Aachen, Germany
E-mail: pich@dwi.rwth-aachen.de
T. Kharandiuk, Dr. R. Nebesnyi, Dr. V. Ivasiv
Technology of Organic Products Department, Lviv Polytechnic
National University, Stepana Bandery str. 12, 79013, Ukraine
Prof. D. E. Demco
Technical University of Cluj-Napoca, Department of Physics and
Chemistry, RO-400027 Cluj-Napoca, Romania
V. S. Petrovskii, Prof. I. I. Potemkin
Physics Department, Lomonosov MoscowState University,
Leninskie Gory 1-2, Moscow 119991, Russian Federation
Prof. I. I. Potemkin
National Research South Ural State University, Chelyabinsk
454080, Russian Federation
fixation
of
the active Se-groups
and
provides
a
compartmentalized environment to promote the diffusion of
reagents for the catalytic reactions to occur. Several polymer
structures containing selenium e.g., supramolecular micelles,[32]
dendrimers,[33] hyperbranched polymers,[34] and nanogels[35]
have been discussed in the literature. How ever, the majority of
the developed systems utilize the redox properties of the
selenium atom for drug delivery applications. The use of Se-
modified functional polymer architectures as bio-inspired
catalysts has not been intensively explored so far.
Prof. A. Pich
Aachen Maastricht Institute for Biobased Materials (AMIBM),
Maastricht University, Urmonderbaan 22, 6167 RD Geleen, The
Netherlands
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
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10.1002/anie.201901161
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COMMUNICATION
a
b
294 cm-1
SeSe
889 cm-1
Se=O
4000
c
3200
2400
1600
800
3200
2400
1600
800
0
Wavenumber (cm-1
)
Raman shift (cm-1
)
d
306 ppm (SeSe)
1333 ppm (SeOOH)
400 350 300 250 200 150
Chemical shift (ppm)
1400
1350
1300
1250
Chemical shift (ppm)
Figure 1. (a) IR spectra and (b) Raman spectra of bis(11-hydroxyundecyl)
diselenide before (red) and after (green) oxidation by H2O2; 77Se NMR of
bis(11-acryloyloxyundecyl) diselenide (c) before and (d) after oxidation by
H2O2.
To optimize the oxidative cleavage of the diselenide bond,
bis(11-hydroxyundecyl) diselenide w as chosen as the model
molecule. Upon oxidation by H2O2, diselenide bonds w ere
cleaved and the compound w as characterized by IR, Raman,
and 77Se NMR spectroscopy. A new strong absorbance band at
-1
-1
Scheme 1. Selenium modified microgels as a catalyst for the oxidation of
aldehydes followed by the synthetic routes of diselenide crosslinker (Se X-
linker) and the incorporation of Se X-linker to the PVCL microgels
simultaneously with BIS as the permanent crosslinker. The microgels can be
oxidized by H2O2 resulting the transformation of diselenide groups into
seleninic acid groups.
889 cm (Figure 1a), and a moderate signal at 859 cm (Figure
1b) w ere observed in IR and Raman spectra, respectively,
corresponding to the Se=O stretching, proving the diselenide
was cleaved and oxidized to seleninic acid. Also, the diselenide
signal at 294 cm-1 disappeared completely after oxidation
(Figure 1b). From 77Se-NMR show n in Figure 1c and 1d, the
peak shifted from 306 ppm to 1333 ppm after oxidation,
Herein, w e attempt to take advantage of the
microenvironment provided by the 3D polymer netw ork structure
of the PVCL microgels to introduce selenium functional groups
as the catalytic centers in order to achieve GPx-like activity. For
this purpose, w e first performed a three-step synthesis to obtain
the diselenide crosslinker (Scheme 1). The diselenide
crosslinker w as then incorporated into PVCL microgels by
B1.5 Se0.0
B1.5 Se1.0
B1.5 Se2.0
SeSe
B1.5 Se3.0
B1.5 Se5.0
B0.0 Se2.0
conventional precipitation polymerization together w ith
a
permanent crosslinker (BIS). After cleavage of the diselenide
bonds by H2O2, microgels modified w ith seleninic acid w ere
obtained (Scheme 1).
400
350
300
250
200
Raman shift (cm-1
)
The synthesized diselenide crosslinker w as characterized by
IR and Raman spectroscopy as w ell as 1H and 77Se NMR
spectroscopy. IR spectra proved that after conjugation of the 11-
bromoundecanol onto diselenide, an increase in hydroxyl group
signal intensity at an absorbance band of 3431 cm-1 and 3371
cm-1 w as observed (Figure S1 of Supporting Information). In
Raman spectra, the symmetrical stretching vibration of
diselenide (SeSe) bond gives rise to a strong polarized band.
The diselenide characteristic band can be observed at 294 cm-1
(Figures 1b and S2 of Supporting Information). After acrylation
of bis(11-hydroxyundecyl) diselenide, new absorbance bands in
3500
3000
2500
2000
1500
1000
500
0
Raman shift (cm-1
)
Figure 2. Raman spectra of Se-modified microgels with a constant amount of
(BIS) crosslinker and a variable amount of diselenide crosslinker (0.0 to 5.0
mol%). Microgels synthesized without (BIS) but with 2.0 mol% diselenide
crosslinker were synthesized as a control sample. The intensities of the peak
at 2928 cm-1 were normalized. In the inset, the enhanced spectra showed the
-1
signal related to the diselenide bond at around 290 cm-1
.
IR spectra can be observed at 1726 cm , 1192 cm-1 and 984 cm-
1 corresponding to carbonyl group stretching, C-O-C stretching in
ester and to the CH=CH2 out-of-plane deformation (Figure S1 of
Supporting Information).
indicating the diselenide w as cleaved and converted to seleninic
acid. The peak at 1316 ppm could be from the partial amount of
selenium derivative w ith a low er oxidation state. 1H NMR spectra
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10.1002/anie.201901161
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COMMUNICATION
of the diselenide crosslinker synthesis and after oxidation by 1
wt% H2O2 can be found in the Supporting Information (Figures
S3-S6).
crosslinks show ed only a slight reduction of the hydrodynamic
radius after oxidation (Figure 3b). Such behavior can be
explained by
a presence of hydrophobic alkyl groups in
Poly(N-vinylcaprolactam) (PVCL) microgels w ith diselenide
groups w ere synthesized by precipitation polymerization in
aqueous media at 70°C (Scheme 1). A small amount of DMSO
(10 v/v%) w as added to w ater to improve the solubility of the
hydrophobic diselenide crosslinker during the polymerization
process. Higher amounts of DMSO used in the reaction mixture
led to the inhibition of microgel formation, probably due to the
fact that grow ing PV CL chains lose the ability to precipitate due
to the shift of low er critical solution temperature (LCST) to higher
values.[36] The microgels containing diselenide crosslinker w ere
obtained in the form of colloidally stable dispersions. Series of
selenium-containing PVCL microgels w ith different diselenide
amounts w ere synthesized and some important characteristics
are show n in Table 1. Raman spectra of the series of selenium-
containing PVCL microgels w ere measured to characterize the
incorporation of diselenide crosslinker into the microgels. The
highest amount of incorporated SeSe w as obtained in B1.5
Se5.0 (Figure 2).
diselenide crosslinker. Before oxidation, the alkyl double spacer
is strained connecting PVCL subchains. We can imagine that
the hydrophobic nature of the crosslinkers does not play a role in
this state: they cannot aggregate w ith each other and
hydrophobic units of PVCL w hich w ould be accompanied by
high elastic stress. On the contrary, after oxidation (i) tw o
spacers of each cleaved crosslinker get more degrees of the
freedom, (ii) the microgel becomes softer (less total number of
cross-linkers) and the spacers can aggregate w ith each other
and hydrophobic units of the PVCL w ithout strong elastic penalty
of the netw ork forming hydrophobic domains w hich result is
compaction of the microgels. The aggregation of the alkyl chains
w ith each other and PVCL chains is demonstrated in Figure S7
and Movie 1 using all-atomistic molecular dynamics simulations.
Optical images of microgel dispersions w ere captured before
and after degradation of the microgels. It can be seen that
microgels w ith only BIS crosslinks remain turbid after oxidation
(Figure 3d) but in the case of microgel sample B0.0 Se2.0, the
solution turned from turbid to clear ( Figure 3f). This indicates
that the dispersed microgels w ere degraded into w ater soluble
PVCL chains.
Table 1. Selenium containing PVCL microgels with a fixed amount of BIS
with variable amounts of incorporated diselenide crosslinker.
Sample
VCL/g
1.0
BIS/mol%a
1.5
Se X-linker/mol%a
Yield/%b
b
c
a
Before Ox
After Ox
Before Ox
After Ox
Before Ox
After Ox
B1.5 Se0.0
B1.5 Se1.0
B1.5 Se2.0
B1.5 Se3.0
B1.5 Se5.0
B0.0 Se2.0
0.0
1.0
2.0
3.0
5.0
2.0
82
76
84
65
69
73
1.0
1.5
1.0
1.5
1
10
100
RH (nm)
1000
1
10
100
RH (nm)
1000
1
10
100
RH (nm)
1000
1.0
1.5
e
f
d
1.0
1.5
1.0
0.0
a Ratio of the mol of each crosslinker over the mol of VCL & b Dried weight
of lyophilized microgels over the VCL input
B1.5 Se0.0
B1.5 Se2.0
B0.0 Se2.0
g
i
Next, the oxidation of diselenide bonds in microgels w as
investigated. Three selected microgels (B1.5 Se0.0, B1.5 Se2.0,
and B0.0 Se2.0) w ere incubated w ith 1.0 w t% H2O2 overnight.
The degraded microgels w ere characterized by dynamic light
scattering, Raman spectroscopy, and electron microscopy.
400
h
350
300
Raman Shift
250
200
B1.5 Se0.0 Ox
B1.5 Se2.0 Ox
B0.0 Se2.0 Ox
j
Figure
3 depicts the probability distribution curves of the
selected microgels. For microgels containing only BIS as the
crosslinker, negligible changes in the hydrodynamic radius after
treatment w ith H2O2 w ere observed (Figure 3a). How ever, a
significant reduction of the hydrodynamic radius can be detected
for microgel sample B0.0 Se2.0 containing only diselenide
crosslinks (Figure 3c). These results are consistent w ith results
presented in Figure 1 and prove that upon oxidation by H2O2,
diselenide bonds w ere cleaved and oxidized to seleninic acid.
This leads to the change of the netw ork topology and
degradation of microgels to w ater-soluble PVCL chains as
reflected in the strong reduction of the hydrodynamic radius. As
expected, microgel sample B1.5 Se2.0, w hich contains both
cleavable diselenide crosslinks and non-cleavable BIS
400
350
300
Raman Shift
250
200
Figure 3. Probability distribution curves generated by Inverse Laplace
Transform of g2(t) exponential correlation function of selenium containing
PVCL microgels before and after oxidation by H2O2 (a) B1.5 Se0.0, (b) B1.5
Se2.0, (c) B0.0 Se2.0. The photos (left: before oxidation; right: after oxidation)
of (d), (e), and (f) correspond to (a), (b), and (c), respectively. Raman spectra
of BIS 1.5% SeSe 2.0% (g) before and (h) after oxidation by H2O2. TEM
images of B1.5 Se2.0 (i) before and (j) after oxidation by H2O2 (scale bar is
1000 nm).
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Figures 4b and S13 depict the overall catalytic results obtained
for the methanol-w ater mixture. All selenium-containing
In the case of microgel sample B1.5 Se2.0, the turbidity w as
partially reduced after oxidation due to the reduction of the
scattering intensity originating from reduced density of polymer
chains w ithin microgels (Figure 3e). The hydrodynamic radii of
microgel samples before and after oxidation w ith H2O2 are
summarized in Table 2. The hydrodynamic radii of microgels
measured at 50°C are smaller due to the fact that microgels
collapse above their VPTT. These experimental results
microgels and Se-X-linker show
a much higher selectivity
tow ards methyl acrylate compared w ith acrylic acid (ester/acid
ratio is 0.9-0.95).
a
Table 2. Hydrodynamic radii (RH) of microgels before and after oxidation by
H2O2.
Before oxidation/RH
20 °C (nm) 50 °C (nm)
After oxidation/RH
20 °C (nm) 50 °C (nm)
Sample
b
c
100
80
60
40
20
0
100
80
60
40
20
0
Y
AA
Y
Y
MA
AA
B1.5 Se0.0
B1.5 Se2.0
B0.0 Se2.0
286
192
131
105
87
284
179
32
102
91
40
32
Se(X)
0
1
2
3
5
Se(X)
0
1
2
3
5
indicate that the incorporation of Se into microgels is not
influencing the temperature-responsiveness of microgels in
aqueous solution but rather the transition temperature.
d
e
100
80
60
40
20
0
100
80
60
40
20
0
Raman spectra for the selected microgels w ere measured
X
Y
Y
X
Y
acrolein
MA
acrolein
AA
after the oxidation process and compared w ith those collected
before oxidation (Figure 3). The diselenide absorbance band
observed reduced significantly show ing the diselenide bonds in
the microgels netw orkw ere cleaved (Figure 3g & 3h). 77Se NMR
of oxidized B0.0 Se2.0 depicts the typical seleninic acid peak at
1050 ppm (Figure S8). STEM images of microgel sample B1.5
Se2.0 show ed core-shell morphology before (Figure S9a) and
after oxidation (Figure S9b). This can be explained by the higher
reactivity of BIS[37] compared w ith VCL and diselenide
crosslinker. Therefore, a microgel core contains more crosslinks
and a denser netw ork structure compared to the shell. According
to TEM images (Figure 3i & 3j), the microgels B1.5 Se2.0 before
oxidation have an average diameter of 268 nm and after
oxidation an average diameter of 236 nm. The reduced microgel
sizes are due to the drying effect during TEM sample
preparation. How ever, the structure of the microgels remains
intact. TEM images of microgel samples B1.5 Se0.0 and B0.0
Se2.0 before and after oxidation are show n in Figures S10 and
S11.
AA
0
5
10
15
20
25
0
5
10
15
20
25
Time (hour)
Time (hour)
Figure 4. a) The reaction scheme of the oxidation of acrolein by H2O2
catalyzed by selenium containing PVCL microgels resulting in acrylic acid (AA)
and methyl acrylate (MA). Oxidation of acrolein catalyzed by the selenium
crosslinker (Se(X)) and the series of selenium containing PVCL microgels with
BIS 1.5 mol% after 8 hours reaction yielded b) methyl acrylate (MA) and
acrylic acid (AA) in methanol-H2O medium and c) acrylic acid in dioxane-H2O
medium. The catalytic yield after 24 hours reaction d) and e) correspond to b)
and c), respectively. Y is the yield and Xacrolein is the conversion of acrolein.
As such, Se-microgels can be used for the selective synthesis of
methyl acrylate as a primary product. Comparatively, in an
oxidation reaction of unsaturated aldehydes w ith H2O2 as the
oxidant and H2SeO3 as the catalyst in methanol medium under
similar reaction conditions, the ester/acid ratio is 0.3-0.5,[39]
which indicates probable differences in the reaction mechanism
betw een selenous acid and seleninic acid groups in the microgel.
Among tested catalysts maximum total yield of methyl
acrylate and acrylic acid (89.1 %) w as achieved w ith B1.5 Se2.0
microgel, w hich is much higher than for Se-X-linker (67.3 %) and
other Se-microgels. It should be noted that non-catalytic reaction
also occurs w ith acrylates yield up to 12 % as indicated by
performing the reaction w ith PVCL microgel w ithout Se. B1.5
Se1.0, B1.5 Se3.0, and B1.5 Se5.0 microgels demonstrate low er
methyl acrylate and acrylic acid yields compared w ith B1.5
Se2.0. In the case of the B1.5 Se1.0 microgel, this can be
caused by low er diffusion rates of the reagents as a result of the
higher viscosity of the reaction mixture (the highest amount of
microgel w as used due to the low est content of Se-units). In the
case of the B1.5 Se3.0 and B1.5 Se5.0 microgels low er yields
can be attributed to reduced reaction volume, as the reaction
primarily occurs inside of the microgel particles, and, considering
In the next step, the catalytic activity of selenium containing
microgels w as evaluated in the oxidation of acrolein as a model
reaction (Figure 4a). The oxidation reactions w ere performed in
methanol and dioxane at 50 °C up to 24 hours reaction time w ith
constant catalyst concentration (Se-unit concentration for
microgels and Se X-linker w as 0.0044 mol/L. For PVCL its mass
was the same as for Se 2.0 microgel) in all reactions. As proved
by dynamic light scattering, PVCL microgels lose their
temperature-responsive properties in MeOH and dioxane, but
remain fully sw ollen and colloidally stable (Supporting
Information, Figure S12).[38] The control reactions w ere
performed using the diselenide crosslinker and the PVCL
microgel w ithout Se. The oxidation of acrolein using Se-catalysts
led to the formation of both methyl acrylate and acrylic acid in
methanol solution and only acrylic acid in dioxane solution.
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10.1002/anie.201901161
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COMMUNICATION
that average Se concentration w as kept constant for all
microgels, higher Se content inside the microgel means low er
total volume of microgel particles.
modified microgels can be recycled and used efficiently in many
catalytic cycles.
In
conclusion, selenium-containing
microgels
w ere
Best results w ere achieved w hen the reaction time w as 8
hours (Figure 4d). Further increases in reaction time results in a
decrease of methyl acrylate yield and an increase of acrylic acid
yield. This can be caused by methyl acrylate hydrolysis to acrylic
acid as the concentration of w ater increases during the reaction
due to w ater formation as a by-product. Therefore, 8 hours of
reaction time can be assumed as optimum for methyl acrylate
synthesis.
successfully synthesized and characterized. The diselenide
groups w ithin the microgels w ere able to be oxidized and
cleaved into seleninic acid groups w hile maintaining the
microstructure of microgels.
Se-modified microgels w ere
successfully used as colloidal catalysts in the model reaction of
the oxidation of acrolein to acrylic acid and methyl acrylate. We
demonstrate that the selenium moieties in the microgels exhibit
high catalytic activity reflected in favorable catalytic yields and
selectivities. In addition, microgel samples B1.5 Se1.0 and B1.5
Se2.0 exhibit exceptional catalytic activity compared w ith the
diselenide crosslinker and other reported results. This study
indicates that Se-modified PVCL microgels could be potentially
used as efficient catalysts for various oxidation processes. In
addition, the use of Se-modified PVCL microgels as catalysts in
aqueous solutions w ill allow exploring the influence of their
temperature-induced sw elling-desw elling on the catalytic activity.
Synthesized Se-modified microgels can catalyze oxidation
reactions w ith high activity and selectivity at mild temperatures,
As it w as mentioned above in the case of dioxane-w ater
mixture the only desired product is acrylic acid (Figure 4c and
S13). But the maximum yield of acrylic acid (91.0 %) w as
achieved w ith B1.5 Se1.0 microgel, w hich is also much higher
than for Se X-linker (58.4 %). With B1.5 Se2.0 microgel a similar
acrylic acid yield w as achieved (79.1 %), but B1.5 Se3.0 and
B1.5 Se5.0 microgels have a much low er catalytic activity. In the
case of dioxane solution non-catalytic reaction (the reaction w ith
PVCL microgel w ithout Se) practically does not occur and only
traces of acrylic acid are formed after 8 hours of the reaction. It
is w orth mentioning that the oxidation of acrolein in dioxane
solution occurs w ith very high selectivity for acrylic acid – nearly
99 % (Figure 4e). As mentioned above acrylic acid yield after 8
hours of the reaction is 79.1 % w ith B1.5 Se2.0 microgel; further
increase of reaction time leads only to the minor increase of
acrylic acid yield (90.7 % after 24 hours). Based on the TOF
values (h-1) (Table S2), synthesized microgels and Se X-linker
can be placed in the follow ing ranges: Se 2.0 (50.6) > Se X-
linker (41.2) > Se 5.0 (27.0) > Se 3.0 (24.1) > Se 1.0 (22.4) for
methanol-w ater media; Se 1.0 (51.7) > Se 2.0 (44.9) > Se X-
linker (33.2) > Se 3.0 (7.1) > Se 5.0 (3.4) for dioxane-w ater
media.
There are also reported results of acrolein oxidation w ith
Au/ZnO catalyst and air as an oxidant under similar reaction
conditions.[40] Methyl acrylate yield w as 30 % at 8 hours and
84 % at 50 hours. Furthermore, acrolein oxidation reaction w as
performed in a gas phase w ith MoVTeNb as the catalyst.[41]
Methyl acrylate yield w as 78 %, w hich is low er than in our w ork,
and the reaction temperature w as much higher at 250 ºC.
Compared w ith these results on non-Se catalysts,[40-41] using
B1.5 Se2.0 microgel facilitates a similar methyl acrylate yield but
at a significantly low er reaction time and under milder reaction
temperatures. In preliminary experiments, w e could demonstrate
that selenium modified microgels can be used in a consecutive
oxidation run (Supporting Information, Figure S14) w ithout
considerable loss of their activity. By adding new reagents
(acrolein and hydrogen peroxide) to the system after first
catalytic cycle (24 hours), the formation of acrylic acid w as
observed w ith slightly low er yield (Figure S14). The low er acrylic
acid yield after second and third catalytic cycles are associated
w ith the presence of a high amount of acrylic acid in the reaction
volume (acrylic acid is constantly formed during the reaction, the
second and third cycles w ere performed w ithout preliminary
products separation). High acrylic acid concentration affects the
equilibrium of the reaction, the increase of reaction mixture
viscosity and leads to the deterioration of the diffusion process.
Nevertheless, selenium-modified microgels remain catalytically
active even after 72 h of reaction and w e believe that selenium-
which allow s for
a considerable reduction of the energy
consumption in technical processes. The biocompatibility of
microgels unleashes an attractive possibility for their use in
biomedical applications as redox-triggered drug carriers or
scavengers for reactive oxygen species (ROS) and cell
protection from oxidative stress.
Acknowledgments
The authors acknow ledge Volksw agen Stiftung for the project
A115859 funding. Authors thank SFB 985 “Functional Microgels
and Microgel Systems” for financial support. The financial
support of the Government of Russian Federation w ithin Act 211,
Contract 02.A03.21.0011, and the Russian Foundation for Basic
Research, project No. 19-03-00472, is gratefully acknow ledged.
The computer simulations w ere carried out using the equipment
of the shared research facilities of HPC computing resources at
Lomonosov Moscow State University. The authors also thank Dr.
Walter Tillmann for the FTIR and Raman spectroscopy
measurements and many thanks to Lindsey Weger for
proofreading the manuscript.
Conflict of interest
The authors declare no conflict of interest.
Keywords:microgels • selenium • redox chemistry •
organocatalysis
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Entry for the Table of Contents
COMMUNICATION
Excess H2O2
Kok H. Tan, Wenjing Xu, Simon
Stefka, Dan E. Demco, Tetiana
Kharandiuk, Volodymyr Ivasiv,
Roman Nebesnyi, Vladislav S.
Petrovskii, Igor I. Potemkin, and
Andrij Pich*
Precipitation
polymerization
H2O2
Selenium microgels: inspiredby glutathione peroxidase (GPx), a mimicking of GPx
approach carried out by incorporation of diselenide as crosslinks into PVCL microgels.
Treatment of diselenide with hydrogen peroxide led to degradation of diselenide into
seleninic acid, an oxidative functional group. The degraded microgels demonstratedhigh
catalytic efficiency and selectivity in oxidation of acrolein at mild temperatures.
Page No. – Page No.
Selenium-Modified Microgels as Bio-
Inspired Oxidation Catalysts
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