Photocatalytic Oxidation of Α-C?H Bonds in Unsaturated Hydrocarbons through a Radical Pathway Induced by a Molecular Cocatalyst

Article abstract of DOI:10.1002/cssc.201900394

To improve the photocatalytic oxidation of α-C?H bonds in unsaturated hydrocarbons, N-hydroxyphthalimide (NHPI) was used as a molecular cocatalyst with CdS as the photoabsorber. Compared with previously reported photocatalysts involving solid cocatalysts, metal-free NHPI offers better sustainability in addition to the significantly enhanced performance as cocatalyst. The photogenerated holes were transferred into the more active phthalimide-N-oxyl radical (PINO) by reacting with NHPI. In this way, α-C?H bond oxidation was significantly improved through the activation by PINO; even for the sluggish toluene oxidation, the apparent quantum efficiency was as high as 36.5 %. The effects of substrates/NHPI concentration ratio, reaction temperature, and time as well as the reaction intermediates were comprehensively studied. It was possible to identify ketones/aldehydes as the primary products, and overoxidation was controlled by adjusting the substrates/NHPI concentration ratio and reaction time. Thus, the radical path induced by the NHPI–PINO redox pair is an efficient alternative to boost the sluggish photocatalytic oxidation of α-C?H bonds.

Full text of DOI:10.1002/cssc.201900394

Accepted Article
Title: Photocatalytic oxidation of α-C-H bonds in unsaturated
hydrocarbons via a radical pathway induced by a molecular
cocatalyst
Authors: Guixia Zhao, Bin Hu, Wilma Busser, Baoxiang Peng, and
Martin Muhler
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Photocatalytic oxidation of α-C-H bonds in unsaturated
hydrocarbons via a radical pathway induced by a molecular
cocatalyst
a
a,b
a
a,b
a,b
Guixia Zhao,* Bin Hu, G. Wilma Busser, Baoxiang Peng, and Martin Muhler*
Abstract: To improve the photocatalytic oxidation of α-C-H bonds in
unsaturated hydrocarbons, N-hydroxyphthalimide (NHPI) was used
as molecular cocatalyst with CdS as photoabsorber. Compared with
previously reported photocatalysts involving solid cocatalysts, metal-
free NHPI offers better sustainability in addition to the significantly
enhanced performance as cocatalyst. The photogenerated holes
were transferred into the more active phthalimide-N-oxyl radical
environmentally friendly process for the oxidation of
hydrocarbons under ambient conditions.
[
8]
It is challenging to explore new approaches to boost the
3
photocatalytic oxidation of C(sp )−H bonds under mild conditions
using visible light. Up to now, suitable visible light-responsive
photocatalysts include Bi
2 6 2 3
WO , CdS, Fe O due to the required
valence band (VB) position to trigger oxidation by the
[
9]
(PINO) by reacting with NHPI. In this way, α-C-H bond oxidation was
photogenerated holes thermodynamically. Previous studies on
these photocatalysts indicated that the oxidation ability of the
photogenerated holes is limited by the sluggish kinetics of C-H
cleaving and the fast recombination of electron-hole pairs,
requiring alternative pathways to activate the primary C-H bond
significantly improved via the activation by PINO and even for the
sluggish toluene oxidation, the apparent quantum efficiency was as
high as 36.5%. The effects of substrates/NHPI concentration ratio,
reaction temperature and time, as well as the reaction intermediates
were comprehensively studied. It was possible to identify
ketones/aldehydes as the primary products, and over-oxidation was
controlled by adjusting the substrates/NHPI concentration ratio and
reaction time. Thus, the radical path induced by the NHPI-PINO
redox pair is an efficient alternative to boost the sluggish
photocatalytic oxidation of α-C-H bonds.
[10]
via photocatalysis.
Extracting the photogenerated holes to
form more active intermediates participating in the oxidation
process would boost the whole photoreaction as confirmed in
[
11]
[12]
Simon et al.’s and Bi et al.’s work. The fast hole trapping by
the molecular cocatalyst to form a radical intermediate not only
enhances the charge carrier separation and prolongs the life-
time of intermediates, but also facilitates C-H cleaving via a
radical pathway.
It will be
hydrocarbon conversion, if such
a
promising and sustainable approach for
cocatalyst and the
Introduction
a
corresponding radical intermediate can be coupled as redox pair
bridging hole trapping and C-H activation without using metal-
containing cocatalysts. N-Hydroxyphthalimide (NHPI) is
frequently used as catalyst in the thermal catalytic oxidation of
With the well-established research on the photocatalytic water
2
splitting and CO reduction based on the oxidation of sacrificial
[1]
organic reagents such as alcohols, amines, more attention has
been paid to the organic oxidation half-reaction, aiming to
achieve the directed conversion of organics, including alcohol
[13]
alkanes, often in the presence of transition-metal complexes.
Therefore, we decided to use the photogenerated holes to
oxidize NHPI into N-oxyl radicals (PINO) and further to initiate C-
H activation via a radical pathway, based on cycling reactions
catalyzed by the molecular redox pair. In such a configuration,
the heterogeneous reaction could be, to some extent, converted
into a homogeneous reaction, which may be more effective. CdS
was chosen as a photocatalyst with a band gap of 2.4 eV and
valence band edge position at 2.2 eV. Toluene was chosen as
the simplest alkyl aromatic compound, which can be oxidized
into benzaldehyde, benzyl alcohol and benzoic acid, functioning
as important intermediates for commercial pharmaceuticals,
plasticizers or dyes. Here, we report the activation of saturated
C-H bonds in toluene and α-C-H bonds in unsaturated
hydrocarbons over synthesized CdS nanoparticles in the
presence of NHPI. The correlations between the structure of the
photocatalysts, NHPI molecules and the photocatalytic activity
as well as the possible mechanism are established.
[
2]
[3]
[4]
oxidation, amine oxidation, ether oxidation and even the
[5]
2
direct oxidation of aliphatic and aromatic C-H bonds with O as
oxidant. Among such oxidation processes, the activation of the
inert C-H bonds in hydrocarbons is much more difficult than
[5a, 5c, 6]
other organic oxidation reactions.
The industrial oxidation
of p-xylene by ionic cobalt, manganese and bromide compounds
[7]
with air in acetic acid known as AMOCO process typically
needs harsh conditions such as high pressure and high
temperature, resulting in the accumulation of toxic chemical
waste and catalyst deactivation. Thus, utilizing photogenerated
holes and electrons to activate C-H bonds and O
2
over
appropriate semiconductors can provide an economically and
[
a]
b]
Dr. G. Zhao, B. Hu, Dr. G. W. Busser, Dr. B. Peng, Prof. Dr. Muhler
Laboratory of Industrial Chemistry, Faculty of Chemistry and
Biochemistry, Ruhr-Universität Bochum, Universitätsstrasse 150,
44780 Bochum, Germany
E-mail: guixia.zhao@techem.rub.de, muhler@techem.rub.de
B. Hu, Dr. B. Peng, Prof. Dr. Muhler
[
Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-
Results and Discussion
36, D-45470 Mülheim an der Ruhr, Germany
We first tried to oxidize toluene over the different CdS samples.
As shown in Figure. 1, only less than 1 μmol of toluene can be
Supporting information for this article is given via a link at the end of
the document.
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ChemSusChem
10.1002/cssc.201900394
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converted in 6 h with sufficient selectivity. When adding NHPI,
all the CdS samples showed a strongly improved activity, among
which CdS-500 was the most active. CdS annealed at high
temperature with high crystallinity generally showed higher
performance, although at lower sintering temperatures the
higher specific surface area is also a positive factor. The band
structure of the CdS samples was analyzed by UV-Vis
spectroscopy and VB measurements, as shown in Figure S1
and Figure S2.
photocatalytic pathway. It is important to note that after the
catalytic reaction, the NHPI amount is almost unchanged
according to the GC results, implying NHPI can be considered
as an efficient cocatalyst, but not as an oxidant.
To avoid mass transfer limitations, we compared the oxidation
performance using different toluene concentrations (Figure 2).
With increasing substrate concentration, the oxidized toluene
amount increased as well. In the case of 18 mmol toluene in 3
3
mL CH CN, the consumed amount of toluene is about 170 μmol
in 6 h irradiation, although the conversion is quite low. However,
the increased oxidation rate is accompanied by decreased
selectivity, especially when the toluene concentration is higher
than 1 mol/L. In this case the acid fraction is higher, indicating
that toluene is more prone to be over-oxidized. For higher
toluene concentrations the concentration of produced aldehyde
is also much higher, obviously favoring the consecutive
oxidation of the aldehyde into the acid. The optimized selectivity
3
is about 75%, when 2 mmol toluene in 3 mL CH CN is used.
Figure 1. a) Toluene oxidation over different CdS samples with (left Y axis) /
without (right Y axis) NHPI; Conditions: 50 mg catalyst, 0.2 mmol toluene, 3
o
mL CH
3
CN, 1 bar O
2
, 60 C with/without 10 mg NHPI, 6 h, 300 W Xe lamp,
visible light; b) XRD patterns of the different CdS samples.
Table 1. Physical properties of the CdS samples.
S
m²/g)
BET
Band gap
(eV)
VB edge
(eV)
CB edge
(eV)
Sample
(
CdS-precip
CdS-100
CdS-200
CdS-300
CdS-400
CdS-500
CdS-600
CdS-ref
71.5
68.2
66.0
64.3
57.3
42.3
21.6
11.9
2.17
2.15
2.13
2.10
2.10
2.22
2.26
2.32
1.30
1.45
1.46
1.44
1.51
1.63
1.57
1.39
-0.87
-0.70
-0.67
-0.66
-0.59
-0.59
-0.69
-0.93
Figure 2. a) Toluene oxidation performance with different substrate
concentrations over CdS-500 in the presence of NHPI; Condition: 50 mg
o
catalyst, 3 mL CH
3
CN, 1 bar O
2
, 60 C, 10 mg NHPI, 6 h, 300 W Xe lamp,
visible light; b) Long-time oxidation of toluene over CdS-500, Conditions: 100
o
mg catalyst, 1 mmol toluene, 6 mL CH
W Xe lamp, visible light.
3 2
CN, 1 bar O , 60 C, 20 mg NHPI, 300
Long-term evaluation indicates that the rate of the oxidation
process seems to be accelerated during the long-time irradiation,
which may be due to the increased over-oxidation of
benzaldehyde to benzoic acid. To investigate the mechanism of
the NHPI-triggered oxidation, different scavengers for
intermediates were added to the reaction suspension (Entries 7-
10 in Table 2). It turned out that the addition of benzoquinone as
superoxide radical scavenger, ammonium oxalate as a hole
scavenger and tert-butyl alcohol as radical scavenger entirely
quenched the oxidation activity. Especially when adding
benzoquinone, conversion was lowered from 13% to 1.62%,
while the addition of sodium azide did not affect conversion,
clearly indicating that the reactive oxygen species is the
superoxide radical, but not singlet oxygen. In addition, the
oxidation is also dependent on the hole and radical
intermediates, which should be related to the NHPI cocatalyst.
The trade-off between crystallinity, specific surface area and
VB edge is supposed to result in the highest conversion over
CdS-500 (Table 1). It is possible to deduce that the high VB
edge plays the most important role in the oxidation among the
various factors. We further checked the role of NHPI during the
oxidation by several control experiments as shown in Table 2.
The combination of CdS-500 and NHPI reached a conversion of
1
3% under light, while without CdS-500 under light, the
conversion was only 1.1% (Entry 3, Table 2), indicating that
CdS-500 is necessary. In the dark, almost no conversion
occurred over NHPI with or without CdS-500 (Entries 4 and 6,
Table 2). When using trifluorotoluene as solvent, a similar
increase in conversion from 0.05% without NHPI to 1.1% with
NHPI was observed, although the concentration of NHPI was
just one tenth of that in acetonitrile due to the limited solubility.
These control experiments confirm that NHPI is able to improve
toluene oxidation not by thermal catalysis but via
a
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FULL PAPER
[
a]
Table 2. Toluene oxidation control experiments.
Selectivity (%)
Entry
Light
Catalyst
Additive
Conversion (%)
aldehyde
alcohol
acid
1
2
3
4
5
6
7
8
9
+
+
+
-
-
-
+
+
+
+
CdS-500 + NHPI 10mg
CdS-500
-
-
-
-
-
13
0.29
1.1
0
0
0.49
1.62
4.19
4.97
12.2
52.5
100
72.7
-
5.9
0
9.25
-
-
0
27.1
6.56
6.31
9.21
41.6
0
18.05
-
-
0
7.4
22.08
29.93
4.1
NHPI 10mg
NHPI 10mg
CdS-500
-
CdS-500 + NHPI 10mg
CdS-500 + NHPI 10mg
CdS-500 + NHPI 10mg
CdS-500 + NHPI 10mg
CdS-500 + NHPI 10mg
-
100
65.52
71.36
63.76
86.69
Benzoquinone
Ammonium oxalate
tert-butanol
Sodium azide
1
0
o
[a] Conditions: 0.2 mmol toluene, 3 mL CH
3
CN, 1 bar O
2
, 60 C, 6 h, 50 mg CdS-500 (if used under control conditions), 300 W Xe lamp with visible light (if
used under control conditions), 0.2 mmol additives (if used under control conditions).
further confirm the presence of radical intermediates, we
conducted the catalytic reaction under irradiation in Ar with a
suspension containing toluene in CH CN solution, the CdS-500
3
catalyst and NHPI. GC-MS analysis of the products (Figure S4)
showed that 2-(phenylmethoxy)isoindoline-1,3-dione and
bibenzyl were formed, indicating that PINO abstracted the α-H of
toluene, leading to the formation of benzyl radicals.
The temperature effect in photocatalytic oxidation is also
studied. Although for pure photocatalysis, the reaction initiated
by photoexcitation is independent of temperature, which was
confirmed by the performance evaluation at different
temperatures over CdS-500 sample without NHPI. The toluene
oxidation activity at different temperatures is indeed almost the
same (not shown). In contrast, as shown in Figure 4a, toluene
oxidation is increasing with temperature in the presence of NHPI,
implying that the NHPI-assisted reaction process is temperature-
dependent. When using trifluorotoluene as solvent, a similar
temperature effect was found, but the selectivity of benzyl
aldehyde was much higher (Figure S5), probably due to the
lower concentration of NHPI and the low dispersion of the NIPO
intermediate in such a solvent with low polarity.
Figure 3. EPR measurements using NHPI in CH
3
CN under different conditions
(from top to bottom): NHPI under visible light; NHPI in the dark; NHPI with
CdS suspension under visible light and NHPI with CdS suspension in the dark.
We further used in situ EPR spectroscopy to directly detect
the intermediates in the presence of NHPI (Figure 3). In the
absence of CdS, pure NHPI with and without excitation showed
no signals, implying that NHPI cannot be excited on its own. In
the presence of CdS, signals in the dark and under visible light-
irradiation are different, with triplet signals emerging when CdS
and irradiation are combined. The g value of the triplet signal is
2
.0078, suggesting the formation of phthalimide-N-oxyl radicals
[13a]
when CdS, NHPI and visible light are present.
The EPR
results indicate that only when CdS is excited, NHPI is
converted to PINO, excluding the direct excitation of NHPI by
[14]
visible light. According to Baciocchi et al. , the PINO signal is
[
5d]
Figure 4. a) Toluene oxidation at different temperatures in the presence of
NHPI over CdS-500, conditions: catalyst 50 mg, 0.2 mmol toluene, 3 mL
stable on the millisecond time scale, and Zhang et al.
also
identified PINO as a long-lived N-oxyl radical. Thus, the active
intermediate has a much longer lifetime than the photogenerated
hole, which contributes to the significantly improved
performance in toluene oxidation in the presence of NHPI. To
CH
3 2
CN, 1 bar O , 10 mg NHPI, visible light, 6h; b) Toluene oxidation with
different amounts of NHPI over CdS-500, conditions: catalyst 50 mg, 0.2 mmol
toluene, 3 mL CH CN, 1 bar O , 14 C, 300 W Xe lamp, visible light, 6 h.
3 2
o
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ChemSusChem
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Figure 5. Equations of all the possible reactions for toluene oxidation in the presence of CdS and NHPI as cocatalyst.
the obtained distributions of the oxidized products, it can be
concluded that the primary product in the photocatalytic
oxidation is benzaldehyde both in the presence of NHPI and in
its absence.
Table 3. Photocatalytic oxidation of α-C-H bonds in unsaturated
hydrocarbons with CdS-500 and NHPI.
[
a]
To determine the efficiency of photocatalytic toluene oxidation
in the presence of NHPI, the apparent quantum efficiency was
derived under irradiation with monochromatic light (420 nm),
which turned out to be 36.5%. We also applied the catalytic
system for C-H oxidation of other unsaturated hydrocarbons
(
Table 3). Expectedly, all the reactants with a R-C=C-C-R
structure were oxidized to ketones which is always the main
product compared with alcohols and acids. It is also noted that
oxidation preferably occurs at the α-position (Entries 2-4, Table
3). For the dimethyl reactants (Entries 5 and 6, Table 3), the
main products originate from the selective oxidation of a single
methyl group.
Based on these results, it is concluded that under visible light
irradiation, the photo-generated holes are trapped by NHPI
forming PINO radicals, which are then able to oxidize toluene
into cationic radicals, while molecular oxygen is reduced by
photogenerated electrons forming superoxide radical species.
[
5a, 5c]
Benzaldehyde is finally formed as primary product.
The
surface reactions are described by Equations 1-4, and 5a-5b
[
5d]
(
Figure 5).
formed, accompanied by the formation of equivalent amounts of
and benzaldehyde according to the electron-hole balance
Equations 5b-8b), although we did not detect any H by
Alternatively, a small amount of benzyl alcohol is
[
a] Reaction conditions: substrate 0.2 mmol, CdS-500 50 mg, NHPI 10 mg,
o
3 2
CH CN 3 mL, O 1 bar, 60 C, visible light, 6 h.
2 2
H O
(
2 2
O
The amount of NHPI also plays an important role in the
oxidation performance (Figure 4b): when the NHPI concentration
is high enough, the catalytic activity is no longer increasing
linearly but the selectivity to benzaldehyde is improved. From all
spectrophotometric methods in the complex reaction system.
Probably, benzyl alcohol would be further oxidized into
benzaldehyde by H O (Equation 9b). It is not possible to avoid
2 2
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ChemSusChem
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o
the formation of the over-oxidized product, i.e., benzoic acid, to
a small extent according to Equations 7c-10c.
surface area was calculated according to the BET method in the p/p
0
=
.05-0.25 range of the adsorption branch. Electron spin resonance (ESR)
spectra were obtained using a Bruker spectrometer (A300) in the X-band
at room temperature in air with a field modulation of 100 kHz and the
microwave frequency maintained at 9.401 GHz. 10 mg NHPI
Conclusions
(
with/without 50 mg CdS) was dissolved in 3 mL acetonitrile, and then the
suspension was enclosed in an EPR tube in air. The EPR signals were
collected in the dark and under illumination with a Xe lamp light source
(300 W, visible light).
We added NHPI successfully as molecular cocatalyst to CdS
nanoparticles to achieve the photocatalytic activation of α-C-H
bonds in unsaturated hydrocarbons. The catalytic results
indicated that with the aid of NHPI, α-C-H bonds in unsaturated
hydrocarbons can be primarily oxidized into aldehyde or ketone
groups. Even for the sluggish toluene oxidation, the apparent
quantum efficiency was high as 36.5%. Mechanistic
investigations proved that the photogenerated holes can be
trapped by NHPI, forming the PINO radical, which then triggers
the C-H cleavage in a radical pathway. Simultaneously, the
PINO radicals are recovered into NHPI. In this way, the
NHPI/PINO redox pair is able to couple photogenerated trapping
and C-H activation. Thus, utilizing a radical pathway to trigger
the photocatalytic α-C-H oxidation with metal-free molecular
cocatalysts is a promising and sustainable alternative.
Catalytic photoactivity
Photocatalytic oxidation of unsaturated hydrocarbons was performed in a
home-made closed reactor filled with O by injecting the suspension
2
mixture into the reactor. The suspension mixture was composed of
acetonitrile, unsaturated hydrocarbons as substrates, with or without
NHPI. Different substrate/acetonitrile amounts were used as mentioned
o
in the captions. The reactor was thermostated at 15-80 C with a cycling
pump and a condenser tube. Subsequently, the mixture in the closed
reactor was irradiated with visible light (wavelength > 400 nm) using a
Xenon light source (300 W, MAX-303) with a long-pass filter. After
reaction, the products in solution were quantified by GC (ZBWax-Plus
column with He carrier gas) using a FID. Control experiments were
carried out by adding different scavengers to the reaction system.
The apparent quantum yield (AQY) was determined with monochromatic
light (420 nm) obtained with a bandpass filter (ASA XBPA420, FWHM
Experimental Section
1
0nm). The light density was determined by a photometer (PM100D,
2
Catalyst preparation
THORLABS), and the irradiated area was ca. 6.15 cm . The AQY for
toluene oxidation was calculated as follows:
The CdS samples were prepared by precipitation followed by sintering in
AQY(%) = (naldehydes + 2nalcohol + 2nacid)/nphotons * 100% = (naldehydes
2nalcohol + 2nacid)/(iphotonsst) * 100%
where naldehydes is the number of produced aldehyde molecules, nphotons is
the number of incident photons, iphotons is the incident light intensity
measured photometrically, s is the irradiated area and t is the irradiation
time.
+
Ar. Typically, 0.958 g cadmium chloride (CdCl
2
·xH
2
O) was dissolved in
2+
200 mL of deionized water to form a Cd solution. 0.65 g sodium sulfide
2-
(
2 2
Na S·9H O) was dissolved in 50 mL of deionized water to form a S
2-
2+
solution. The S solution was then slowly dropped into the Cd solution
in 90 min, and then aged for 2 h with vigorous stirring. The resulting
o
suspension was filtrated and washed with water and finally dried at 60 C
to obtain the CdS-precip sample. The CdS-precip sample was further
o
sintered under Ar at different temperatures (100-600 C) for 4 h to obtain
Acknowledgements
the CdS-X samples (x represents the temperature). For comparison,
commercial CdS (Strem chemicals) in micro-meter scale was used and
labeled as CdS-ref.
The authors acknowledge funding from the Alexander von
Humboldt Foundation, the Max Planck Society (Max Planck
Fellowship, IMPRS RECHARGE) and DFG (SFB/TRR 247).
Catalyst characterization
Powder X-ray diffraction (XRD) patterns were carried out using a
Keywords: toluene oxidation • N-hydroxyphthalimide •
PANalytical theta-theta powder diffractometer with Cu
α
K radiation.
photocatalytic oxidation • radical mechanism • CdS
Transmission electron microscopy (TEM) images were recorded with a
field emission transmission electron microscope (JEM2100F, JEOL Co.,
Japan) operating at 200 kV. The UV-Vis diffuse reflectance spectra were
obtained using a PerkinElmer Lambda 650 spectrophotometer and a
Harrick Praying Mantis diffuse reflectance accessory with a resolution of
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o
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FULL PAPER
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ChemSusChem
10.1002/cssc.201900394
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Entry for the Table of Contents
FULL PAPER
Toluene oxidation is triggered
indirectly with enhanced efficiency by
the radical intermediate PINO, which
originates from NHPI oxidation by
photogenerated holes in CdS. The
NHPI-PINO redox pair offers a
sustainable cocatalyst alternative to
boost photocatalytic oxidation of α-C-
H bonds.
Guixia Zhao*, Bin Hu, G. Wilma
Busser, Baoxiang Peng, Martin
Muhler*
Page No. – Page No.
Photocatalytic oxidation of α-C-H
bonds in unsaturated hydrocarbons
via a radical pathway induced by a
molecular cocatalyst
This article is protected by copyright. All rights reserved.