Integrating Hydrogen Production with Aqueous Selective Semi-Dehydrogenation of Tetrahydroisoquinolines over a Ni2P Bifunctional Electrode

Article abstract of DOI:10.1002/anie.201903327

Exploring an alternative anodic reaction to produce value-added chemicals with high selectivity, especially integrated with promoted hydrogen generation, is desirable. Herein, a selective semi-dehydrogenation of tetrahydroisoquinolines (THIQs) is demonstrated to replace the oxygen evolution reaction (OER) for boosting H₂ evolution reaction (HER) in water over a Ni₂P nanosheet electrode. The value-added semi-dehydrogenation products, dihydroisoquinolines (DHIQs), can be selectively obtained with high yields at the anode. The controllable semi-dehydrogenation is attributed to the in situ formed Ni^II/Ni^III redox active species. Such a strategy can deliver a variety of DHIQs bearing electron-withdrawing/donating groups in good yields and excellent selectivities, and can be applied to gram-scale synthesis. A two-electrode Ni₂P bifunctional electrolyzer can produce both H₂ and DHIQs with robust stability and high Faradaic efficiencies at a much lower cell voltage than that of overall water splitting.

Full text of DOI:10.1002/anie.201903327

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Accepted Article
Title: Integrating Hydrogen Production with Aqueous Selective Semi-
Dehydrogenation of Tetrahydroisoquinolines over a Ni2P
Bifunctional Electrode
Authors: Chenqi Huang, Yi Huang, Cuibo Liu, Yifu Yu, and Bin Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903327
Angew. Chem. 10.1002/ange.201903327
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10.1002/anie.201903327
Angewandte Chemie International Edition
COMMUNICATION
Integrating Hydrogen Production with Aqueous Selective Semi-
Dehydrogenation of Tetrahydroisoquinolines over a Ni2P
Bifunctional Electrode
Chenqi Huang†, Yi Huang†, Cuibo Liu, Yifu Yu, and Bin Zhang*
Abstract: Exploring an alternative anodic reaction with accelerating
kinetics to produce value-added chemicals with high selectivity,
especially integrated with promoted hydrogen (H2) generation, is
and photocatalytic strategies usually lead to the complete
dehydrogenation products, isoquinolines (IQs).[6] The semi-
dehydrogenation products (DHIQs) are more difficult to be
obtained, especially via a sustainable route, such as a very
popular electrochemical dehydrogenation.[7] Therefore, the
controllable electrooxidation from THIQs to DHIQs with high
selectivity at low potential over a low cost electrocatalysts,
especially coupled with efficient HER, will be economically
attractive, but remains highly challenging.
Herein, we demonstrate an efficient strategy to promote H2
production by replacing OER with selective semi-
dehydrogenation of THIQs over a Ni2P anode (Figure 1). The in
situ formed NiII/NiIII redox active species in water are found to be
key factors for the controllable semi-dehydrogenation of THIQs
into DHIQs. Such anodic semi-dehydrogenation process can
selectively produce gram-scale DHIQs with electron-withdrawing
or donating groups in good yields. Additionally, the bifunctional
highly desirable. Here,
a selective semi-dehydrogenation of
tetrahydroisoquinolines (THIQs) is demonstrated to replace oxygen
evolution reaction (OER) for boosting H2 evolution reaction (HER) in
water over a Ni2P nanosheet electrode. The value-added semi-
dehydrogenation products, dihydroisoquinolines (DHIQs), can be
selectively obtained with high yields at anode. The controllable semi-
dehydrogenation is attributed to the in situ formed NiII/NiIII redox
active species. Such strategy can deliver a variety of DHIQs bearing
electron-withdrawing/donating groups in good yields and excellent
selectivities, and can be facilely applied to gram-scale synthesis.
Furthermore, a two-electrode Ni2P bifunctional electrolyzer can
produce both H2 and DHIQs with robust stability and high Faradaic
efficiencies at a much lower cell voltage than that of overall water
splitting.
Ni2P electrodes can be assembled into
a two-electrode
electrolyzer for boosting HER by coupling with the semi-
dehydrogenation of THIQs at a smaller voltage than that of
overall water splitting.
Electrocatalytic water splitting represents a promising way to
produce clean hydrogen (H2).[1] The sluggish kinetics of oxygen
evolution reaction (OER), however, often limits the overall water
splitting
efficiency.[2]
Although
some
well-designed
a
b
R
1
-2H
-2H
electrocatalysts have been developed to promote the kinetic of
OER,[3] a large potential is still required to match hydrogen
evolution reaction (HER), leading to low energy conversion
efficiency. An alternative strategy that replacing the anodic OER
with the electrooxidation of thermodynamically more favorable
species is of increasing importance to boost HER.[4] A myriad of
value-added chemicals, rather than O2, are produced at anode.
Nevertheless, in most cases, the fully electrooxidized products
are obtained.[4a] Thus, using electrooxidative power to drive
controllable transformations of chemicals into corresponding
semi-oxidized products with high selectivity and industrial
practicability is highly challenging but more significant.
R
R2
R2
2
e-
e-
R1
R1
NH
N
N
√
×
-
+
1
2
3
c
H2
2
H+
1
1 μm
Cathode
Anode
Figure 1. (a) Electrocatalytic semi-dehydrogenation of THIQs 1 coupled with
HER in water. (b) Dehydrogenation reaction of 1 to form DHIQs 2 and IQs 3.
(c) Scanning electron microscopy (SEM) image of Ni2P nanosheet electrode.
Dihydroisoquinolines (DHIQs) display
a wide range of
bioactivities on anti-tumor, anti-fungal, vasodilation and
nonoamine oxidase inhibition, which demonstrate highly
potential applications in pharmaceutical industry.[5] The
The Ni2P porous nanosheet electrode was synthesized by a
facile phosphidation of porous nickel oxide precursors (Figure
S1). The scanning electron microscopy (SEM) images (Figure
1c and Figure S2) show that porous nanosheet arrays with open
network like structure are grown on nickel foam (Figure S3),
which can be further observed from transmission electron
microscopy (TEM) image (Figure S4a). The high-resolution TEM
(HRTEM) image (Figure S4b) displays well-resolved lattice
fringes with interplanar distances of 0.170 and 0.292 nm, which
are attributed to the (300) and (110) planes of Ni2P, respectively.
The X-ray diffraction (XRD) pattern (Figure S5) reveals that all
the diffraction peaks are indexed to Ni2P (JCPDS No. 03-0953)
and Ni. The X-ray photoelectron spectroscopy (XPS) spectra
further show the characteristic Niδ+ of Ni2P in as-prepared
dehydrogenation of tetrahydroisoquinolines (THIQs) is
a
potential way to synthesize the DHIQs, but the typical thermal-
[*]
C. Huang,[†] Dr. Y. Huang,[†] Dr. C. Liu, Prof. Y. Yu, Prof. B. Zhang
Department of Chemistry, Institute of Molecular Plus, School of
Science, Tianjin University, Tianjin 300072, China
E-mail: bzhang@tju.edu.cn
Prof. B. Zhang
Tianjin Key Laboratory of Molecular Optoelectronic Sciences,
Collaborative Innovation Center of Chemical Science and
Engineering, Tianjin 300072, China
[†]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201903327
Angewandte Chemie International Edition
COMMUNICATION
electrode (Figure S6). These results suggest the successful
synthesis of Ni2P nanosheet electrode.
S7). The high-resolution XPS spectra show the partial surface
oxidation of Ni2P after semi-dehydrogenation test (Figure S8),
which suggests that the oxidized Ni species on the surface of
Ni2P may be the actual active sites for electrocatalytic semi-
dehydrogenation.
The electrocatalytic semi-dehydrogenation of THIQs was
carried out in 1.0 M KOH aqueous solution. The 1,2,3,4-
tetrahydroisoquinoline 1a is selected as a model substrate.
Figure 2a displays the linear sweep voltammetry (LSV) curves in
the presence and absence of 1a over a Ni2P anode. In the
absence of 1a, the onset potential is about 1.48 V vs. RHE
(reversible hydrogen electrode) and the obvious O2 bubbles
appeared after this potential. The oxidation peak centered at
about 1.40 V is attributed to NiII/NiIII.[8] After adding 1a, the
current density increases dramatically from about 1.32 V and O2
bubbles start to appear when the potential reaches to about 1.48
V. The products obtained at different reaction times are collected
at the constant potential of 1.35 V. The product 2a increases
with the time and no fully dehydrogenated product, isoquinoline,
is generated (Figure 2b). The reaction can be completed within 1
h. To investigate the effect of potential on the selectivity of
E / V vs. RHE
1.1 1.2 1.3 1.4 1.5 1.6
a
8
4
b
c
472
without 1a
with 1a
without 1a
with 1a
472
553
0
553
-4
0.65V
0.1 0.2 0.3 0.4 0.5 0.6
0.65V
E / V vs. Ag / AgCl
0.60V
0.55V
d
0.60V
0.55V
472
553
initial
0.50V
0.45V
adding 1a
0.50V
0.45V
5 s
0.40V
0.35V
0.30V
10 s
15 s
20 s
25 s
0.40V
0.35V
0.30V
initial
initial
400
800
600
800
400
600
400 500 600 700 800
Raman shift/ cm-1
Raman shift/ cm-1
Raman shift/ cm-1
e
f
1.5
1.0
0.5
0.0
GC with 1a
GC with 2a
GC with 1a and H2O
oxidative
dehydrogenation,
a
series
of
long-term
2a
2a·+
e-
e-
E1/2= 2.47 V
chronoamperometry at various potentials from 1.30 to 1.70 V
were carried out with 0.5 mmol 1a (Figure 2c). At 1.30 V, no
product can be collected. At the potentials from 1.35 to 1.45 V,
the products were collected after passing charge of about 96 C
(the theoretical charge for the semi-dehydrogenation of 1a to 2a).
2a with Faradaic efficiencies (FEs) of about 96% and
selectivities of about 99% are achieved. O2 bubbles can be
observed obviously after 1.50 V. When the potential is increased
to 1.70 V, the FEs and yields for 2a reduce to about 50% due to
OER, while the selectivity of 2a is still about 96%.
NH
N
GC with H2O
H2
Niδ+
1a
H+
Ⅲ
1a 1a·+
E1/2 = 1.29 V
Ni
+ 2H+
2H+
OOH
2a
- H+
O2/H2O
- e-
-e-
- H+
NH
•+
N
N
•
•
1.0
1.5
2.0
2.5
3.0
+
A
B
C
E / V vs. Ag/AgCl
Figure 3. (a) CV curves of a Ni2P anode in 0.1 M KOH with and without 1a. (b,
c) Potential-dependent in situ Raman spectra of a Ni2P anode collected
without 1a (b) and with 1a (c). (d) In situ Raman spectra collected under
chronoamperometry (I-t) test at 0.5 V vs. Ag/AgCl in 0.1 M KOH solution
without 1a (initial) and after the addition of 1a. (e) Proposed conversion
process of semi-dehydrogenation over a Ni2P anode. (f) CV curves (anodic
parts) in acetonitrile with glassy carbon (GC) as the working electrode.
a
180
135
90
45
0
b
100
80
60
40
20
0
Charge
0.5
0.4
0.3
0.2
0.1
0.0
0.5 mmol 1a
without 1a
O2
1a
2a
N
2a
To further get insight into the mechanism for electrocatalytic
semi-dehydrogenation of THIQs over a Ni2P anode, in situ
potential-dependent Raman spectroscopy was first measured.
The cyclic voltammetry (CV) was performed with and without 1a
(Figure 3a), and Raman spectra at different potentials were
collected (Figure 3b, c). The oxidation peak (0.4-0.5 V vs.
Ag/AgCl) of NiII/NiIII can be clearly observed in the CV curve
without 1a, and at the same time, two peaks, attributed to the Ni-
O vibrations of NiOOH,[3b] start to appear at 472 and 553 cm-1,
and their intensity increase gradually with the potentials (Figure
3b). After the addition of 1a, the current is obviously enhanced,
as observed from CV curve (Figure 3a). But, the NiOOH peaks
do not appear until the potential reaches to 0.6 V (Figure 3c). In
addition, the NiOOH peaks without 1a can be weakened
gradually and disappear in about 20 s after adding 1a (Figure
3d). Considering the formation of NiOOH on the surface of Ni-
based electrocatalysts toward OER in an alkaline
electrolyte,[1a,8,9] it is reasonable to conclude that with 1a, NiIII-
OOH is firstly formed on the surface of Ni2P, and then quickly
captures the electrons from 1a to go back to NiII, meanwhile, the
1a is oxidized to 2a, so that the peaks of NiOOH are not
observed from in situ Raman spectra during the process of
organic transformation (Figure 3e). Therefore, NiII/NiIII, on the
surface of electrode, works as an efficient redox active species
N
1a
1.0 1.1 1.2 1.3 1.4 1.5 1.6
0.0
0.2
0.4
0.6
0.8
1.0
Time / h
E / V vs. RHE
c
d
100
Yield
FE
100
100
80
60
40
20
0
O2
80
60
40
20
0
2a
80
60
40
20
0
no
product
1.3
1.4
1.5
1.7
1
2
3
4
5
6
E / V vs. RHE
Cycles
Figure 2. (a) LSV curves of a Ni2P anode at a scan rate of 5 mV s-1 in 40 mL
of 1.0 M KOH with and without 0.5 mmol 1a. (b) Time-dependent evolution of
1a and its semi-dehydrogenation product 2a. (c) FEs and selectivities of 2a at
different potentials. (d) Cycle-dependent yields and FEs of 2a.
The durability of electrocatalytic semi-dehydrogenation over a
Ni2P anode was also investigated. After six cycle runs, no
apparent decrease of activity is observed and the yields and FEs
of 2a is ~96% (Figure 2d), reflecting the good stability of the
Ni2P anode for this electrocatalytic reaction. The Ni2P electrode
after stability test was characterized. The maintained nanosheet
arrays morphology can be observed from SEM images (Figure
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10.1002/anie.201903327
Angewandte Chemie International Edition
COMMUNICATION
to promote the conversion of 1a into 2a, similar to the reported
organic electrooxidation processes driven by TEMPO
mediator.[10] In addition, the radical process is possibly involved
in the electrocatalytic selective semi-dehydrogenation of 1a to
2a (Figure 3e and Figure S9).[6a, 11]
Impressively, the as-prepared Ni2P electrode is also highly
active for HER (Figure S12)[4h, 12] and can be assembled into a
two-electrode electrolyzer for both HER and semi-
dehydrogenation of THIQs, which show much better energy
efficiency compared to overall water splitting (Figure 4a). For
example, achieving the current density of 20 mA cm-2 requires a
300 mV smaller voltage than that of overall water splitting (inset
of Figure 4a). A long-term electrolysis at 1.50 V vs. counter
electrode (CE) was also carried out. The FEs of generated H2
and 2a reach to about 100% and 96%, respectively (Figure 4b).
The high selectivity of 2a can also be retained after the long-
term test. Additionally, no apparent decrease of FEs is observed
after six cycle runs, indicating the excellent durability. Note that
such assembled two-electrode electrolyzer for semi-
dehydrogenation reaction coupled with HER can be also driven
by a 1.5 V single-cell battery (Figure S13, 14) and conducted to
a gram scale synthesis of DHIQs with excellent yields (Figure
S15), suggesting a great potential for practical application.
The CV was also carried out in acetonitrile with glassy carbon
(GC) as the working electrode to analyze the electrooxidative
behaviour of organic substrate (for details see the Supporting
Information). As shown in Figure 3f, two main peaks at 1.29 and
2.47 V vs. Ag/AgCl, associated with the first and secondary
oxidation of 1a, respectively, are observed from CV curve
recorded in the solution of 1a (magenta line) compared to that of
in the solution of 2a (blue line). Upon the addition of a small
amount of water, the first peak remains, while the current density
sharply increases from the potential of 1.69 V because OER
starts to proceed at this potential, suggesting that the secondary
dehydrogenation of 1a would be more difficult than OER. That is
to say, combining with the results above of in situ Raman
spectra, the in situ formed NiII/NiIII can boost the conversion of
1a into 2a and the OER can happen to inhibit the generation of
full dehydrogenation products at high potentials, so that the high
selectivity of semi-dehydrogenation can be achieved (Figure 2c).
Furthermore, such electrocatalytic semi-dehydrogenation
strategy shows good compatibility with various THIQs with
electron-withdrawing or donating groups, and the corresponding
semi-dehydrogenation products can be obtained in high yields
(Table 1 and Figure S10). Besides, the developed method was
also applied to the dehydrogenation of tetrahydroquinoline (THQ)
(Figure S11), which also shows more thermodynamically
favourable than OER to boost the HER at cathode.
Unfortunately, only complete dehydrogenation product, quinoline,
was obtained.
a
50
40
30
20
10
0
b
100
without 1a
0.5 mmol 1a
H2
2a
without 1a
0.5 mmol 1a
80
60
40
20
0
1.8
1.6
1.4
10
20
j / mA cm-2
0.8 1.0 1.2 1.4 1.6 1.8 2.0
1
2
3
4
5
6
Cycles
E / V vs. CE
Figure 4. (a and inset) LSV curves and potential comparison for achieving
benchmark current densities (10 and 20 mA cm-2) over a Ni2P ‖ Ni2P
electrolyzer in 1.0 M KOH with and without 0.5 mmol 1a. (b) Cycle-dependent
FEs of a Ni2P ‖ Ni2P electrolyzer for both H2 and 2a production in 1.0 M KOH
solution with 0.5 mmol 1a.
Table 1. Substrate scope of the selective semi-dehydrogenation of THIQs
over a Ni2P anode.a
In summary, we have developed a facile but efficient strategy
to boost H2 production coupled with selective semi-
dehydrogenation of THIQs instead of OER in water over a Ni2P
anode. The controllable semi-dehydrogenation conversion of
THIQs into DHIQs is attributed to the in situ formed NiII/NiIII
redox active species to boost semi-dehydrogenation reaction.
Impressively, the bifunctional Ni2P electrodes can be assembled
into a two-electrode electrolyzer for both HER and semi-
dehydrogenation of THIQs, which requires a smaller voltage
than that of overall water splitting. This work opens an
economical and highly efficient route for the electrochemical
production of both hydrogen and DHIQs.
Ni P
2
R
R
2
2
R
R
+
H
1
1
2
1.0 M KOH, RT
NH
N
1
2
Substrate
Product
Substrate
Product
Br
Br
NH
N
NH
NH
NH
NH
N
1a
1b
1c
2a, 96%
1e
1f
2e, 92%
2f, 99%
2g, 84%
MeO
MeO
Cl
Cl
NH
NH
NH
N
N
N
2b, 93%
2c, 87%
MeO
MeO
MeO
MeO
N
O N
2
O N
2
1g
N
N
Acknowledgements
Me
2d, 98%
Ph
2h, 70%
Me
Ph
1h
1d
We do appreciate the National Natural Science Foundation of
China (Nos. 21871206 and 21422104) and China Postdoctoral
Science Foundation (Nos. 2018M630269 and 2019T120182) for
financial support.
aReaction conditions: THIQs (0.5 mmol), Ni2P anode (working area: 1 cm2), Pt
cathode, 1.0 M KOH (40 mL), room temperature (RT, 25 oC), 0.35 V vs.
Ag/AgCl, 1-3 h (passing charge of about 96 C). Yields are determined by gas
chromatograph. The products are indentified by NMR.
This article is protected by copyright. All rights reserved.

10.1002/anie.201903327
Angewandte Chemie International Edition
COMMUNICATION
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Keywords: Electrocatalysis • energy efficiency • hydrogen
evolution reaction • Raman spectroscopy • semi-
dehydrogenation
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10.1002/anie.201903327
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
A selective semi-dehydrogenation
of tetrahydroisoquinolines (THIQs)
instead of oxygen evolution
reaction is reported to boost H2
evolution reaction in water over a
Ni2P nanosheet electrode. Both
value-added semi-
dehydrogenation products and H2,
can be obtained at a much lower
cell voltage than that of overall
water splitting.
Chenqi Huang†, Yi Huang†, Cuibo Liu,
Yifu Yu, and Bin Zhang*
e-
e-
-
+
Page No. – Page No.
R2
NH
R1
Integrating Hydrogen Production
with Aqueous Selective Semi-
Dehydrogenation of
Tetrahydroisoquinolines over a Ni2P
Bifunctional Electrode
H2
H+
Niδ+
H+
Semi-Dehydrogenation
Ⅲ
Ni
R2
R1
OOH
N
cathode
anode
This article is protected by copyright. All rights reserved.