Highly selective electrocatalytic oxidation of benzyl C-H using water as safe and sustainable oxygen source

Article abstract of DOI:10.1039/d0gc01871f

The selective oxidation of C-H bond is critical for feedstock manufacturing in chemical industry. Current strategies typically involve the use of oxygen or peroxide as the oxidation reagent under high temperature, which sets severe challenges in production sustainability and industrial safety. Herein, we demonstrate an environmental-friendly and safe electrocatalytic strategy for the selective oxidation of benzyl group to ketones at ambient conditions, while using water as the sole oxygen source. Water addition reduces the onset potential of anodic C-H oxidation, and produces 1-tetralone with satisfying conversion and excellent ketone to alcohol ratio. Layered MnO2 catalysts (with rich oxygen vacancies) further adjust the water affinity and facilitate the oxidation, leading to a significantly improved faradaic efficiency. This journal is

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DOI: 10.1039/D0GC01871F
ARTICLE
Highly Selective Electrocatalytic Oxidation of Benzyl C−H
Using Water as Safe and Sustainable Oxygen Source
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
b
a
Yuxia Sun, Xiaoshan Li, Miao Yang, Wentao Xu, Jin Xie, and Mengning Ding *
DOI: 10.1039/x0xx00000x
Selective oxidation of C-H bond is critical for feedstock manufacturing in chemical industry. Current strategies typically
involve the use of oxygen or peroxide as oxidation reagent under high temperature, which set severe challenges in
production sustainability and industrial safety. Here, we demonstrate an environmental-friendly and safe electrocatalytic
strategy for the selective oxidation of benzyl group to ketones at ambient conditions, while using water as the sole oxygen
source. Water addition reduces the onset potential of anodic C-H oxidation, and produces 1-Tetralone with satisfying
2
conversion and excellent ketone to alcohol ratio (one/ol~100%). Layered MnO catalysts (with rich oxygen vacancies)
further adjust the water affinity and facilitate the oxidation, leading to a significantly improved faradic efficiency.
recent studies. Pioneering works since 1983[11] adopted indirect
oxidation to lower the applied potential using electrochemical
Introduction
mediators.
Typical
mediators
include
modified
Efficient and selective oxidation of C−H bond in organic
substances is of essence in synthetic chemistry, and plays a
crucial role in manufacturing of feedstocks in current structure
of chemical industry.[1] Typical oxidation can produce various
NHPI[12]/TEMPO[13]/DDQ,[14] imidazoles,[15] nitrate[16] and
halogens,[13] offering an efficient indirect C−H oxidation
pathway.[17] Specifically, t-BuOOH or O
2
often function as the
terminal oxidants in these systems. Moreover, these
homogeneous systems lead to inevitable challenges in the
subsequent separation and recycling. Direct electro-oxidations
of the C−H bonds produce carbon centered radicals and/or
cations.[18] As a high-energy intermediate, its following attack
products such as aldehydes and ketones, two essential
intermediates for the demand of high value-added products
including fragrances, agrochemicals, and pharmaceuticals.[2]
However, the high bond energy and low polarity of C−H bond
present significant challenge for its activation in controlled and
2 3
by the oxygen-containing reagent (such as H O, CH OH, and
mild means. Harsh operating conditions, such as high
temperature,[3] high oxygen pressure,[4] acidic solvents,[5]
HOAc) can theoretically lead to C-O formation, and therefore
offers a simple and efficient pathway for C−H oxidation to
form aldehydes, ethers, or ester products.[18b-c] Through
electrochemical approach, the oxidation state of the product
might be easily manipulated by the precise setting of anodic
potential, offering digital control over oxidation selectivity.[9f]
Additionally, the target oxidation at anode can be paired with
proper cathodic production of desired substances,[9c, 19] giving
added value and potential atomic economy to the overall
electrolytic systems. However, strategies to lower the required
anodic potential and improve the efficiency is in urgent need.
Herein, we demonstrate a direct and highly selective electro-
stoichiometric additives,[6] as well as strong oxidants[7] (O
2
,
2 2
H O , t-BuOOH, etc.) are often adopted. In addition,
intermediate of alkyl hydroperoxide are inevitably included in
this process, and the combination of high temperature, strong
chemical oxidants and flammable organic substances raises
particular concerns of potential explosion in large-scale
production.
Electro-oxidation represents a promising alternative, greener
and more environmentally-compatible methodology[8] under
ambient conditions, with electrons used as inherently clean
reactants to achieve target oxidation.[9] For electrochemical
C−H oxidation, overcoming the anodic overpotential[10] and
searching for green oxygen source[10a, c] has been the focus of
oxidative conversion of benzyl C−H,
using water as co-
solvent and confirmed sole oxygen source. Electrooxidation of
Tetralin (THN) to 1-Tetralone (TONE) was investigated as a
O
a.Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210023, China
E-mail: mding@nju.edu.cn
CH3CN-H2O, Bu4NClO4
+
H2
+
H2O
Modified C-Pt, Undivided cell
b.
State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of
Advanced Organic Materials, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210023, China
Scheme 1. Electro-oxidation of tetralin and simultaneous hydrogen generation using
Electronic
Supplementary
Information
(ESI)
available:
See
water as the oxygen source.
DOI: 10.1039/x0xx00000x
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ARTICLE
Journal Name
-
-
-
-
-
-
4
4
4
4
4
5
3
2
2
1
1
5
.0x10
Results and discussion
View Article Online
DOI: 10.1039/D0GC01871F
CH CN
3
(
a)
Electro-catalytic C−H oxidation was carried out in an
undivided cell with tetralin as the model substrate. An
undivided cell setup[10a, 10c] was chosen over divided cell with
proton exchange membrane step in our electrocatalytic
synthesis, as a 10 times larger internal electrical resistance
(measured by potentiostat) and half of the yield was observed
for the latter. Control experiment (isolated cathodic reduction
under the same conditions) also confirms that both substance
THN and anodic product TONE will not be reduced on the
cathode side. Systematic voltammetry investigations on the
anode oxidation of tetralin were first conducted for initial
reactivity evaluation. Figure
Voltammograms (CV) of tetralin at different reaction
conditions. Acetonenitrile (CH CN) was employed as solvent
due to its large electrochemical window,[20] and its good ability
to dissolve both tetralin and water. Bu NClO was used as
.5x10
.0x10
.5x10
.0x10
.0x10
THN in CH CN
3
THN in CH CN-H O
3
2
THN+NaOAc in CH CN-H O
3
2
Ep/2=1.39V
0
.0
Ep/2=1.60V
0
.5
1.0
1.5
E (V) vs Fc/Fc
2.0
+
1
shows the Cyclic
(
b)
-
-
-
-
-
5
5
5
5
5
7
6
4
3
1
.5x10
.0x10
.5x10
.0x10
.5x10
3
4
4
-
-
-
5
5
5
electrolyte for optimal conversion (see Table 1). As shown in
Figure 1a, anodic currents from tetralin was observed
(negligible current was observed for CH CN background),
3
3
2
1
.0x10
.0x10
.0x10
indicating its electrochemical oxidation in this system. With the
potential increasing, THN lose electrons to carbon radical or
carbon cation, which could dehydrogenate to 1,2-
dihydronaphthalene and naphthalene, verified by GC-MS (Fig.
p/2
0
2
4
6
8
C
/C
H2O THN
0
100
200
300
400
S1). The half peak potential (E ) for the tetralin oxidation was
CH2O/CTHN
+
about 1.60 V (vs. Fc/Fc ). With addition of H O, the oxidation
2
Figure 1. (a) CV of THN in CH
3
CN (blue, Ep/2=1.60 V), CH
3
CN-H
2
O (red, Ep/2=1.39 V) and
current shifted to a more negative potential (Figure 1a), with
corresponding Ep/2, the potential at which the current is equal to
CH
CH
3
CN-H
CN-H
2
O system with addition of NaOAc (green). (b) The anodic current of THN in
O system with different amount of H O. Data points correspond to the anodic
3
2
2
current at 1.6 V from corresponding CV scans with different amount of water. Inset one-half of the mass-transfer-limiting current, decreased to 1.39
depicts the enlarged area in low concentration region. Electrolyte (Bu
solvent (25 mL), electrode (glassy carbon), the potential was calibrated by Fc/Fc .
4
NClO
4
, 0.1 M),
V (vs. Fc/Fc+).
+
Table 1. Electro-oxidation of THN under different reaction conditionsa
proof-of-concept reaction. This half-reaction is paired with a
cathodic hydrogen evolution to afford additional green energy
production and overall atomic economy. The addition of water
significantly reduces the onset potential of benzyl C−H bond
oxidation, which can be further improved by the application of
Conv.
(%)
Yield of
TONE (%)
Yield of TOL
(%)
Entry
Electrolyte
Additive
1
2
3
4
5
6
7
8
9
LiClO
Bu NClO
NaClO
KClO
4
-
-
-
-
31.8
89.4
34.6
26.6
≥99
7.4
28.1
3.0
17.1
9.5
4
4
2
oxygen vacancy-rich layered MnO as electrocatalyst. The
4
14.8
cation intermediate generated at anode can then be attacked by
water molecules to form C−O bond and finally provide ketone
products, with high conversion and high ketone to alcohol ratio.
This water-assisted approach operates at room temperature and
ambient pressure, and more importantly excludes the use of
mediators or chemical oxidants such as gaseous oxygen or
hydrogen peroxide (therefore prevent formation of potentially
explosive alkyl hydroperoxide intermediate). It therefore
provides a sustainable approach for alkane oxidation, with a
green, abundant, and safe oxygen source, which significantly
reduce the cost and potential hazards for application in
industrial production. Systematic investigations were conducted
to elucidate the reaction mechanism and the electro-kinetics.
Functional approaches were further explored to optimize the
catalytic performance of electrodes via tailoring water affinity
of the electrode materials.
4
16.2
9.7
63.1(61.6b)
33.1
n.d.
24.4
24.6
19.4
24.3
6.2
Bu NClO
4
4
4
4
4
4
4
4
4
NaOAc
Bu NClO
Na
2
CO
3
96.3
83.6
86.9
97.3
69.3
98.2
Bu NClO
NaOH
NH OA
13.7
4
Bu NClO
20.9
c
Bu NClO
4
Na
HClO
Na PO
2
SO
4
27.3
1
1
0
1
Bu
Bu
4
4
NClO
NClO
4
4
14.7
4
3
4
62.9
n.d.
Reaction conditions: a 0.65 mmol tetralin, RVC-Pt, 25 mL solvent CH
3
2
CN-H O
+
(90:10), 6 h, additive (1 mmol), applied potential: 1.75 V vs Fc/Fc ; Q = 589 C
for entry 5; TONE and TOL are the abbreviation for 1-tetralone and 1-tetralol.
Conversion and selectivity were determined by GC using dodecane as the
internal standard, “n. d.” means not detected. b Isolated yield.
2
| J. Name., 2012, 00, 1-3
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Green Chemistry
Journal Name
ARTICLE
oxidation of THN. This CV characteristic excluded theViepwoAsrstiicbleleOrnolinlee
To confirm the products within a short time, typically, 35 C of NaOAc as redox mediator participating inDOthI:e10fo.1r0m3a9t/iDo0nGoCf0T18H71NF
of charge at various potentials were passed and the products radical and cations, in which case one enlarged anodic peak with
were analyzed and quantified (See Figure S2) by gas same onset potential of mediator oxidation (instead of two separate
chromatograph−mass spectrometry (GC-MS). Tetralone CV peaks) should be observed. However, because the oxidation of
formation was negligible when the potential was lower than acetate ion appears to occur at a lower potential compared to
+
1
.15 V vs Fc/Fc , along with fairly low faradic efficiency. oxidation of THN, it is likely that NaOAc is simultaneously
When bulk electrolysis was adopted in CH
Entry 1) and CH CN-H O (Table 1, Entry 2) systems, yield of oxidation not detected). To address this issue, we further tested
tetralone increased from 15.9% to 28.1% after the addition of Na PO as additive, which is a similar Bronsted base commonly
3
CN (Table S1, consumed during reaction (in small scale, product of acetate
3
2
3
4
water, which is consistent with the CV results. Different used in organic synthesis that should promote the reaction in a
electrolytes were explored, and the highest performance was similar way, but is more stable under anodic conditions (stability
achieved with Bu
also investigated by both CV and bulk electrolysis and optimal were obtained when replacing NaOAc with Na
CH CN-H O (90:10) was screened out as shown in Figure 1b confirmed the role of Lewis base additives, and showed that the
The corresponding CVs for data in Fig. 1b are shown in Figure possible oxidation of acetate ions did not largely affect the reaction
4
NClO
4
. Varying concentrations of water were confirmed by CV shown in Figure S4b). Similar yield and selectivity
3
PO , which further
4
3
2
(
S3) and Table S1 Entry 2-4, respectively. At the first stage of outcome. The slight decomposition of acetate ions might bring down
water addition, the anodic current presented an irregular the overall Faradaic efficiency (FE), yet it is observed that the FE
fluctuation (Figure 1b blue region) until the water was only increased from 27% with acetate to 30% with phosphate,
accumulated to 96.8 µL (Cwater:CTHN = 8:1). This can be indicating that the OER was a more predominant side reaction
attributed to the hydration of electrolyte ions, which restricted responsible for the overall low FE at high potential (also see Figure
the freedom of added water molecules. As the amount of water S4a). Further investigation suggested that the relatively low carbon
kept increasing, the correlation between water content and the balance was caused by over-oxidation products soluble in aqueous
anodic current became positively linear (Figure 1b red region). phase (diketones, naphthoquinone, or even carbon oxides)
This indicates the occurrence of chemical reaction between C+ Hydrogen evolution reaction (HER) was utilized as cathodic
intermediate (from anodic oxidation of THN, see Scheme 2) reaction in this system. As an alternative to fossil fuels,
and H O, and the increased amount of water promoted the rate hydrogen fuel plays an important role in clean energy
of the EC process. The anodic current reached to steady state technologies.[22] As H
O is designed to be the oxygen source in
with further addition of water over 2500 µL (Cwater:CTHN the anodic process, HER is a natural choice for paired
10:1, Figure 1b yellow region). Presumably the formation of electrolysis, to give an overall atomic economy with
C+ intermediate became a restricted factor in the total EC simultaneous production of hydrogen gas. The overall reaction
process under this condition. can therefore be viewed as an improvement over
.
2
2
=
2
Considerable works have revealed that the presence of additives electrocatalytic water splitting, while maintaining HER for
can benefit the catalytic performance.[15, 20b, 21] To further improve hydrogen production the economically less favorable OER is
the reaction efficiency, series of additives were investigated, and the replaced with the anodic production of high value chemical
results were summarized in Table 1 (Entry 5-10). No obvious trend feedstock. For the cathodic HER, platinum holds the best
was presented from the Lewis acid/base perspective, nonetheless the
addition of NaOAc significantly promoted the conversion, raising
the yield of TONE from 28.1% to 63.1%. 1-Tetralone was detected
as the almost only product in organic phase after extraction by
EtOAc, giving an overall ketone to alcohol ratio of ~100%, without
the detection of 1-tetralol (TOL). Similarly, anodic current in CV
also demonstrate an enhancement with the addition of NaOAc.
100
TONE
TOL
THN
8
6
0
0
3 2
Compared with THN in CH CN-H O (Figure 1a, red curve) and
NaOAc in CH CN-H O (Figure S4a, violet curve), the anodic
3
2
current of THN (Figure 1a, green curve) was generally enhanced
after addition of NaOAc, indicating the possible benefit of Bronsted
base addition to stabilize the cation intermediate formed at anode
40
2
0
0
2
and promote the following H O attack (see plausible reaction
pathway in Supplementary Scheme S1). Using this identical set-up,
TONE was prepared on 0.5-g scale in 53.2% yield from chemically
pure THN. It should be noted that the CV curve of NaOAc in
0
50 100 150 200 250 300 350
Time (min)
3 2
CH CN-H O (Figure S4a) presents a relatively small anodic peak at
E= ~1.2 V, presumably corresponds to acetate ion oxidation through
the Kolbe reaction. This peak overlapped with the first small peak in
CV of THN in CH CN-H
3 2
O with NaOAc (also see Figure S4a), potential: 1.75 V vs Fc/Fc+.
Figure 2. Content of THN, TOL, TONE during the electro-chemical oxidation. Reaction
conditions: 0.65 mmol tetralin, RVC-Pt, 25 mL solvent:CH CN-H O (90:10), 6 h, applied
3
2
which is clearly separated from the second peak corresponding to the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Green Chemistry
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ARTICLE
Journal Name
OH
O
View Article Online
CH3CN-H2O, 0.1M Bu4NClO4
CH3CN-H2O, 0.1M Bu4NClO4
performance and therefore was employed in this study, with a
high faradic efficiency of H evolution (89.2% ± 1.8%)
determined by gas chromatography (Figure S5).
(1)
DOI: 10.1039/D0GC01871F
or
n.d.
2
OH
O
O
(2)
n.d.
After the optimization of reaction conditions, time course of
the THN electro-oxidation was investigated. As shown in
Figure 2, an initial increase of TOL with the consumption of
THN, and then a decrease along with the formation of TONE
was observed. These results manifested that the oxidation
process went through the TOL intermediate. The nuclear
or
1.75 V, Undivided cell
Route A
OH
- 2e- - H+
+H O/ OH-
2
(3)
×
1
magnetic resonance ( H NMR) spectrum of the sample taking
Route B excluded
from the reaction system at 30 min also identified the existence Scheme 2. (1-2) Control experiments with or without applied voltage using 1,2-
dihydronaphthalene, the intermediate in route B, as the substrate. (3) The possible
of TOL (Figure S6). However, when the electro-oxidation was
started with TOL substrate, different product distributions were
route A and B in the process of THN electro-oxidation.
observed depending on the involvement of additive. A rapid
18O
generation of tetralone was presented when NaOAc was
presented. For systems free of NaOAc additive, dehydration
products (1,2-dihydronaphthalene and naphthalene) were obviously
accumulated and retained as byproducts (see Supplementary Figure
S7). This revealed another mechanistic benefit of NaOAc additive,
facilitating the target oxidation over dehydration of TOL
intermediate by promoting H-abstraction of intermediate as a
suitable base (see Scheme S1). Note that the electro-catalytic
functionalization of C-H in N-containing heterocycles have been
extensively reported, via dehydrogenation of the heterocycle C+
CH3CN-H218O, Bu4NClO4, NaOAc
Undivided cell
(a)
(b)
18
O
H
2
18O
16
O
16O
2
H
Mass of tetralone
CN-18O-
3
Figure 3. Mass spectra of TONE for the electro-oxidation of THN in (a) CH
CN-water, respectively
intermediate to cyclic imine, followed by addition of other agents at enriched water (b) CH
the double bond.[23] A similar vinyl intermediate pathway might also
apply during the electro-oxidation of THN to give an alternative
3
mechanism (The dehydrogenation of THN first produced intercalated into the C-H bond in THN is derived from water
dihydronaphthalene, then went through the addition reaction with molecule.
H
2
O to form TOL and TONE, as illustrated in route B, Scheme 2).
However, neither 1-tetralol nor 1-tetralone was detected when generated in situ during the anodic process, such as H
dihydronaphthalene was adopted as the starting substrate (Scheme 2) water oxidation or oxygen molecule from oxygen evolution
in the same CH CN-H
O system with or without applied voltage. reaction (OER). H was added in this system to probe its
We further investigated the possibility of other intermediates
2 2
O
from
2 2
O
3
2
Therefore, different from the N-contained heterocycle compounds, involvement during the electro-oxidation of THN. The results
dehydrogenation-addition pathway does not occur in the electro- revealed that trace tetralone was obtained, with or without
oxidation of THN, which instead might go through the direct attack applied voltage (see Scheme S2), therefore excluding H
2
O
2
as
+
of C intermediate by the H
2
O molecules
.
an intermediate oxidant in our approach. Besides, the overhead
To further confirm the origin of the oxygen atoms in the ketone gas component in the cell was taken into gas chromatography
products, isotope trace measurements and electro-kinetic studies for quantitative analysis, production of H was obviously
were conducted. Air was another possible source of oxygen and has observed while the O generation was negligible. Another test
been extensively applied as oxidant in the reported catalytic was conducted by bubbling O gas into the dry solvent (relacing
oxidation.[24] However, when system was thoroughly purged water with saturated oxygen in the reaction), to evaluate the
with Ar to exclude the dissolved oxygen, similar amount of water to O intermediate (via OER) pathway. For this
tetralone was obtained during electrolysis (as shown in Figure condition, TONE was present after reaction but the yield was
S8). This result indicates that O
in air does not necessarily significant lower than that in the standard CH CN-H O system.
contribute to the C-H oxidation. Additionally, oxygen isotope Furthermore, to investigate the pH-dependence for further
(18
O) tracer technique was carried out to elucidate the source of mechanistic insight, the CV scans of reaction system under
2
2
2
2
2
3
2
oxygen. The isotopic distributions of tetralone was obtained different pH were collected (see Fig. S9). In alkaline electrolyte
from the GC-MS spectrum of tetralone synthesized from (where the pH value is adjusted by NaOH), the anodic onset
electro-oxidation
with
H
2
18O-enriched
solvent.
As potential presented a negative shift with higher pH. This result
− 0.0591pH, where
mass-to-charge ratio (m/z) of 146 (attributed to tetralone) was the pH effect comes from H on the product side in the overall
demonstrated in Figure 3a&3b, the dominant ion peak at a is consistent with Nernst Equation E = E
0
+
−
shifted to 148, when the electrolysis was conducted in a anodic reaction from THN to TONE (THN + H
2
O − 4e →
18O TONE + 4H , the overall oxidation involves ECEC steps, with
goes into the final product TONE. From these combined the transfer of four electrons and generation of four protons,
evidences, it can be concluded that the oxygen atom
although the very first electron transfer step
CH
3 2 2
CN-H 18O system. This result confirmed that 18O in H
+
4
| J. Name., 2012, 00, 1-3
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Page 5 of 8
Green Chemistry
Journal Name
a)
ARTICLE
TONE formation on concentration of water was VaiecwqAuritriceledO.nBlinye
(
Yield of TONE
ITONE
varying the concentration of water fromDOI
0
: 1.50.1
t
0
o
39/
1
D
.50G
M
C0
,
1871F
t
h
e
0.025
dependence of the partial current density for TONE (iTONE) on
the concentration of water with 26 mM THN was obtained
8
6
4
2
0
0
0
0
0
0.01875
0.0125
0.00625
0
(Figure 4b). An approximate linear dependence was obtained
from the ln(iTONE) vs ln(Cwater) plot. The results suggest that the
ln(iTONE) has a positive correlation with water concentration.
As the anodic current of THN to TONE was positively
related to the water concentration, it is also reasonable to infer
that the hydrophilicity of the anode materials may promote the
proximity of water molecules to anode surface, and therefore
facilitate the target electro-oxidation. To adjust the special
wetting behavior of the anodes surfaces, the carbon paper
substrate was teflonated to achieve hydrophobicity (signed as
pH
HClO
4
None NaOAc NaOH
(
b)-4.4
(c)
<10o
Yield of TONE
FE
90
o
6
4
3
0
5
0
-
-
-
-
-
-
4.6
4.8
5.0
5.2
5.4
5.6
1
70
o
hydrophobic
CP)
or
fenton-oxidation
treated
to
superhydrophilicity (signed as Fenton-hydrophilic CP, see
Supplementary Figure S10), respectively. The contact angle (CA)
was an commonly-used indicator to characterize the surface
15
0
wettability.[ The water CA decreased from 170° to <10° in the
28]
ydrophilic CP
-
1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
ln(CH2O/M)
c
CP
hilic
CP
phobi
Hydr
op
on-H
Hydro
Fent
sequence of hydrophobic CP, hydrophilic CP and Fenton-
hydrophilic CP, indicating the effective modification. These
different carbon papers were then adopted in the electro-
oxidation system. The TONE yield on Fenton-treated CP was
about 54.5% above that of hydrophobic CP, as shown in Figure
Anode
Figure 4. (a) Yield and partial anodic current of TONE at various pH adjusted by
different additives. The additives were marked by different colored-columns. (Acidic
environment was adjusted by different concentration of HClO
4
while basicity was
adjusted by NaOH. pH 9 was adjusted by NaOAc and NaOH respectively). (b) H
2
O
concentration (at 26 mM THNO) dependences of TONE partial current. (c) Yield and 4c. This corroboration further reveals the crucial role of water
faradaic efficiency for tetrolone adopting different modified anodes. Applied potential: in the electro-oxidation of THN.
+
1
.75 V vs Fc/Fc . Inset were the contact angle results (The carbon paper was abbreviate
as CP).
Table 2. Substrate scope under the electro-oxidation reaction conditionsa
does not include proton generation, see Figure S9d).
Interestingly, in the acidic electrolytes, the CV curves did not
demonstrate significant change with varying pH value. The
different CV behaviors under alkaline or acidic electrolytes
indicate that in our electrochemical oxidation of THN to TONE
in mixed organic-water solvent, the rate determining step in the
overall ECEC processes might be different in alkaline and
acidic conditions. Nonetheless, the pH dependence on anodic
current at constant potential was investigated in the electro-
oxidation, and no obvious dependence of anodic current on the
pH value was observed (as shown in Figure 4a). Since the pH
dependence is a signature characteristic of the OER,[25] this result
indicates that although dissolved oxygen molecules can participate in
the oxidation of THN, its OER generation is unlikely to be chief
pathway when water is present in the system. Moreover, certain
excellent OER catalyst such as NiFe-LDH[26] and Zn-Co-OOH[27]
gave inferior tetralone yield when utilized as the anode materials at
pH 9 (NaOAc as the additive, see Scheme S2), providing
Yield of
TONE (%)
Substrate
Product
Entry
O
1
2
84.9
70.1
OCH3
OCH3
O
O
O
O
3
4
5
77.1
75.2
60.6b
O
COCH3
COCH3
O
O
69.2b
6
O
O
7
8
55.3
30.1
O
O
2
additional evidence that O from OER was not the predominent
oxygen source in the reaction mechanism. These combined
results suggested that the oxygen atoms in this reaction are
O
9
28.3
34.7
2
directly from the H O molecules.
NO2
NO2
As revealed in CV plots of THN with different H
2
O
O
1
0
concentration (Figure 1b), the anodic current increased as more
water was added until the steady state. To get more insight of
the electro-oxidation mechanism, systematic electro-kinetic
measurements were then conducted. From chronoamperometry
measurements, dependence of the partial current density for
N
N
Reaction conditions: a 0.65 mmol substrate, RVC-Pt, 25 mL solvent CH
CN-H
90:10), NaOAc (1 mmol), 1.75 V vs Fc/Fc ; isolated yield. GC yield using
dodecane as the internal standard.
3
2
O
+
b
(
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Journal Name
In order to testify the versatility of this electro-oxidation set- prepared catalyst materials, showing layered moVirepwhAortlicolegOynlionef
up, different kinds of componds with benzyl groups were MnO
with periodic lattice fringes of 4.8DO
utilized as substrates (Table 2). Under the identical conditions, matched well with the (001) and (200) lattice planes of MnO
the corresponding ketones were achieved, owing to the relative To increase the density of oxygen vacancies that promote the
stability of aromatic ring as well as electron-rich property of the generation of M-OOH on the oxide surfaces,[32] layered MnO
was
C=C bonds. Similarly, alcohols were not present in the reaction further calcinated in N (abbreviated as MnO -N ). As shown in
mixture. Moreover, the activity and selectivity for the electro Figure 5e, O1s spectra of both MnO and MnO -N displayed
2
Å
I: 1
a
0
n
.1
d
03
2
9
.
/
8
D0
Å
G
,
C
w
01
h
871F
i
c
h
2
.
2
2
2
2
2
2
2
oxidation of compounds with pyridyl and furan ring were three overlapped peaks, with binding energies at 529.2 eV,
testified, the activities were relatively poor, which might be 531.0 eV, and 533.0 eV, which can be attributed to lattice O (OI),
originated from the negative influence of the heteroatoms inside suface adsorbed O species/O vacancies (OII), and adsorbed water
the rings.
As previously discussed, lowering of the over potential is one and MnO
major objective in the electro-chemical C-H oxidation. To this was increased after calcination. The variation of Vo was further
end, heterogeneous catalysts have great benefit in simplified investigated by ESR spectra. The MnO samples both exhibited
molecules (OIII), respectively.[33] Comparision between MnO
2
2
-N manifested that the amount of O vacancies (OII)
2
2
operation and advanced reactivity/selectivity,[29] and search for a sharp signal with a symmetric distribution (Fig. 5c), in
suitable catalysts is critical to keep improving the reaction accordance with a Vo signal with g-factor ofꢀ2.003, a character
performance in the field of heterogenerous and electrochemical of electron trapping at oxygen vacancies.[34] Incomparison to
catalysis. Mn-OOH possesses good oxidizing activity and can pristine MnO
promote the activation of C−H bond and hydrogen substraction of treated MnO
ethylbenzene.[30] Therefore, manganese oxides were employed as successfully introduced. Furthermore, the increase of Mn(III)
the heterogeneous catalysts for the electro-oxidation of THN. content (indicated by 640.8 eV peak[35]
was observed after
2
, a stronger V
O
signal intensity was observed for
N
2
2
, suggesting that a greater amount of V
O
was
)
As water was involved in our system, the Mn-OOH group can treatment, increasing the surface density of unsaturated Mn site
be in situ generated on the oxides surface, facilitating the for M-OOH. Figure 5a&b show the CV of different catalysts in
electro-oxidation of THN. In addition, better electrical the electro-oxidation of THN. The application of MnO -N
2 2
conductivity can further improve the overall performance of the significantly elevated the anodic current, with 39.6 mA/cm2
2
catalysts. To this end, layered MnO
2
(see Supplementary Figure compared to the 10.0 mA/cm from CP electrode at 1.40 V vs
S11&S12) was prepared by a solid interface reaction[31]and adopted Fc/Fc . Bulk electrolysis of THN was further conducted to
as the anode catalyst. Figure 5d depicts the High-resolution evaluate the catalytic performance of MnO and MnO -N . A
Transmission Electron Microscopy (HRTEM) image of as- similar trend was presented, with the yield of TONE increased
+
2
2
2
(
a)
(b)
(c)
<10
o
0
.08
.06
.04
.02
.00
0.08
0.06
0.04
0.02
0.00
MnO -N2
90
o
2
CH CN-H O-THN (MnO -N )
MnO2
3
2
2
2
0
0
0
0
CH CN-H O-THN (MnO )
3
2
2
CH CN-H O-THN (CP)
3 2
VO
CH CN-THN (MnO -N )
3
2
2
CH CN-THN (CP)
3
CH CN-H O (MnO -N )
3
2
2
2
CH CN-H O (CP)
3
2
CH CN (MnO -N )
3
2
2
CH CN (CP)
3
0
.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
3
460
3480
3500
3520
3540
3560
E (V) vs Fc/Fc+
E (V) vs Fc/Fc+
Magnetic field (G)
(d)
(e)
(f)
Mn 2p3
Mn(IV)
O 1s
O(I)
0
0
0
0
0
0
.07
.06
.05
.04
.03
.02
2
MnO -N /CP 535 F/cm
O(II)
2
2
satellite
MnO2
Mn(III)
O(III)
2
234 F/cm
MnO -N2
MnO
2
/CP
2
10 nm
6
48 645 642 639
B.E. (eV)
536 534 532 530 528
B.E. (eV)
10
20
30
40
50
Scan rate (mv/s)
Figure 5. (a) LSV plots of THN in different system, with MnO
Bu NClO , 0.1 M), solvent CH CN-H
after N annealing
2
and N
2
-teated MnO
2
as catalysts deposited on CP. (b) LSV of THN in different system with CP as the anode. Electrolyte
+
(
4
4
3
2
O (90:10, 25 mL), the potentiall was calibrated by Fc/Fc . (c) Electron paramagnetic resonance (EPR) spectra of the MnO
2
powders and MnO
2
2
.
(d) HRTEM image of prepared MnO
2
catalyst. (e) Mn 2p3 and O1s XPS spetra of prepared MnO
2
and MnO
2
-N
2
catalysts; (f) The scan-rate dependence of current
+
density at E = -0.08 vs. Fc/Fc for the MnO
2
-N
2
and MnO
2
.
6
| J. Name., 2012, 00, 1-3
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ARTICLE
View Article Online
DOI: 10.1039/D0GC01871F
from 23.5% to 49.8% and 72.0% at 1.40 V adopting CP, MnO
and MnO -N
longer required, as MnO
2
oxidation, particularly with a green and safe oxygen source that
catalysts respectively. Notably, NaOAc was no will lead to low-cost and hazard-free industrial production.
can promote the alcohol (TOL in this
2
2
2
case) oxidation[
36]
via the Mars-van Krevelen redox
mechanism.[36a,
b]
Conflicts of interest
The required onset potential was further
reduced for another 350 mV with the application of MnO
catalysts, along with a significantly increased faradic efficiency
from 29.0% to 73.9%, showing the ability of MnO to promote
the electro-oxidation of THN. Electrochemical Surface Area Acknowledgements
ECSA) is a general alternative methodology[37] to characterize
2
There are no conflicts to declare.
2
(
We thank Prof. Dechen Jiang and Prof. Xu Cheng for their
inspirational comments and discussions during this work. We
thank the Key Laboratory of Mesoscopic Chemistry for the use
of GC-MS. Funding for this research was provided by
Fundamental Research Funds for the Central Universities in
China (020514380224), Natural Science Foundation of Jiangsu
the amounts of catalytic active sites. The capacitance of the
double layer Cdl is commomly used to estimate the effective
ECSA, assuming that the two quantities are linearly
proportional.[38] Therefore, the CV scans (See Figure S13) were
2 2 2
carried out for MnO and MnO -N , in the region where the
charging currents of the double layer are predominant. The
capacitance can be calculated from the scan rate dependence of
charging current density, as shown in Fig. 5f. In this system, the
Province
(BK20180321),
and
the
“Innovation
&
Entrepreneurship Talents Plan” of Jiangsu Province.
results revealed that Cdl of MnO
MnO . In this term, the activated ECSA by N
role in the superior activity of MnO
first reaction was completed, the reaction mixture was
transfered and replaced by a fresh substrate feed in the same
2 2
-N was about twice that of
2
2
treatment plays a Notes and references
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-N
2
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