Suspending Ion Electrocatalysts in Charged Metal–Organic Frameworks to Improve the Conductivity and Selectivity in Electroorganic Synthesis

Article abstract of DOI:10.1002/asia.201900640

Electroorganic synthesis is an environmentally friendly alternative to traditional synthetic methods; however, the application of this strategy is heavily hindered by low product selectivity. Metal–organic frameworks (MOFs) exhibit high selectivity in numerous catalytic reactions; however, poor conductivity heavily limits the application of MOFs in electroorganic synthesis. To realize the electrocatalytic application of MOFs in selective electroorganic synthesis, a practically applicable strategy by suspending ion electrocatalysts in charged MOFs is herein reported. This approach could markedly improve the product selectivity in electroorganic synthesis. In the electrocatalytic oxidative self-coupling of benzylamine experiments, the imine product selectivity is markedly improved from 61.3 to 94.9 %, when the MOF-based electrocatalyst is used instead of the corresponding homogeneous electrocatalyst under the identical conditions. Therefore, this work opens a new route to improve the product selectivity in electroorganic synthesis.

Full text of DOI:10.1002/asia.201900640

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Suspending Ion Electrocatalysts in Charged Metal-Organic
Frameworks to Improve the Conductivity and Selectivity in
Electroorganic Synthesis
Authors: Chuan-De Wu, Wei-Wei Guo, Chi Zhang, Ji-Jie Ye, Zi-Kun
Liu, and Kai Chen
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Suspending Ion Electrocatalysts in Charged Metal-Organic
Frameworks to Improve the Conductivity and Selectivity in
Electroorganic Synthesis
Wei-Wei Guo,[a] Chi Zhang,[a] Ji-Jie Ye,[a] Zi-Kun Liu,[a] Kai Chen,[a] and Chuan-De Wu*[a]
Dedicated to Professor Jin-Shun Huang on the occasion of his 80th birthday
Abstract: Electroorganic synthesis is an environmentally friendly
alternative to traditional synthetic methods; however, the application
of this strategy is heavily hindered by low product selectivity. Metal-
organic frameworks (MOFs) exhibit high selectivity in numerous
catalytic reactions; however, poor conductivity heavily limited the
application of MOFs in electroorganic synthesis. To realize
electrocatalytic application of MOFs in selective electroorganic
synthesis, we report a practically applicable strategy by suspending
ion electrocatalysts in charged MOFs, which could markedly improve
product selectivity in electroorganic synthesis. In electrocatalytic
oxidative self-coupling of benzylamine experiments, the imine
product selectivity is markedly improved from 61.3 to 94.9%, when
the MOF-based electrocatalyst is used instead of the corresponding
homogeneous electrocatalyst under the identical conditions.
Therefore, this work opens a new window to improve the product
selectivity in electroorganic synthesis.
materials constructed from multidentate organic ligands and
metal ions/clusters, featuring high porosity, high surface areas,
and tunable pore sizes and shapes.[24-26] MOFs have been
emerged as a powerful platform to integrate variety of functional
molecular entities in the pores/matrices for applications in
different fields.[27-43] The tunable pore nature and constituents of
MOFs endow systematically introduce different redox-active and
auxiliary centers to tune the catalytic properties for highly
efficient and selective catalysis.[32-43] MOFs have also been
appeared in electrocatalysis, such as oxygen reduction
reaction,[44,45] hydrogen evolution reaction,[46,47] electroreduction
[48,49]
of CO2
and electrooxidation of ethanol.[50] However, the
inherent low conductivity of most MOFs heavily deteriorated
their electrocatalytic properties, especially in electroorganic
synthesis.
Several approaches, trying to improve the electrical
conductivity of MOFs, have been developed in recent years.[51,52]
One strategy was to construct two-dimensional (2D) MOFs by
forming planar π-d conjugated sheets, which exhibit comparable
conductivity with graphite.[53] However, this strategy could not be
applied to three-dimensional (3D) MOFs, because their building
moieties are uniformly isolated in the 3D pore matrices. Another
attempt was to impregnate graphene and carbon nanotubes into
MOFs to generate composite conductive materials.[54] However,
this strategy could not fundamentally solve the inherent low
conductivity issue of MOFs. Attempts were also made to
improve the electrical conductivity of MOFs by using softer
sulfur-based organic linkers to combine with selected metal
ions.[55] The application of this method is heavily hindered by the
complex synthetic procedures and the scope of MOFs. An
alternative approach was to engineer long-range delocalization
through installation of closely spaced mixed-valence
functionalities to promote inter-valence charge transfer.[56] It is
unfortunate that direct synthesis of mixed-valence MOFs
remains a challenge.
Ionic conduction has been realized widespread applications
in the electrolytes of batteries and fuel cells, gas sensors and
biomaterials chemistry.[57] It has been demonstrated that
introducing ionic liquids into MOFs would result in highly
conductive materials.[58] We have shown that confining metal
cations in the anionic pores of ionic MOFs would endow the
catalytic sites freely mobile, which are easily accessed to
reactant molecules in heterogeneous catalysis.[59,60] If
suspended ion catalysts (SICs) are used as heterogeneous
electrocatalysts, the freely mobile ionic active sites should be
easily reached to working electrodes induced by electric field. By
tuning the pore sizes, shapes and properties of ionic MOFs, the
electrochemical selectivity in electroorganic synthesis would be
Introduction
Organic electrochemistry has been emerged as an
environmentally friendly, atom-economic and sustainable
alternative to traditional synthetic methods, thanks to the use of
clean redox power in place of stoichiometric amount of oxidants
or reductants.[1-3] The reactivity of electrochemical reactions is
systematically tunable by changing the applied potentials, which
makes certain complex molecules easily accessed under mild
conditions.[4] Over the past decade, electrochemistry has been
an effective strategy for the synthesis of high value-added
chemicals by forming C-C,[5,6] C-N,[7,8] C-O,[9-11] C-S,[12-14] C-P,[15]
and C-Hal bonds.[16,17] Some significant redox reactions were
also easily realized by electrochemical reactions, such as
ammonia synthesis[18-20] and CO2 reduction.[21,22] However, there
remain some issues that hinder the practical applications of this
strategy in organic synthesis, including low catalyst turnovers,
low product selectivity, and low stability and conductivity of
working electrodes.[2,3,23]
Metal-organic frameworks (MOFs) are a class of porous
[a]
Wei-Wei Guo, Chi Zhang, Ji-Jie Ye, Zi-Kun Liu, Kai Chen, and Prof.
Dr. Chuan-De Wu
State Key Laboratory of Silicon Materials
Department of Chemistry
Zhejiang University
Hangzhou, 310027, P. R. China
E-mail: cdwu@zju.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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10.1002/asia.201900640
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systematically improved, controlled by the special pore
(Figure 1e). The BET surface area of post-modified MIL-101(Cr)-
HIMBr is of 2163.71 m2/g, which is slightly smaller than that of
the pristine MIL-101(Cr) (2526.74 m2/g). DFT analysis of the
pore size distributions showed that the pore sizes of MIL-
101(Cr)-HIMBr are also slightly smaller than those of the pristine
MIL-101(Cr) (Figure 1f and Table S1). These results should be
ascribed to grafting of imidazolium moieties on the framework of
MIL-101(Cr)-HIMBr. It is worth noting that there remains very
large pore space in MIL-101(Cr)-HIMBr, which is ready for
subsequent introduction of anionic electrocatalysts inside the
cationic pores through ion exchange.
microenvironment. When
a suspended ion electrocatalyst
TEMPO-SO3@MIL-101(Cr)-HIM (HIM = hexane imidazolium),
consisting of cationic framework MIL-101(Cr)-HIM and
-
suspended anionic active sites TEMPO-SO3 in the charged
pores, was immobilized on working electrode, it demonstrates
high catalytic efficiency and selectivity in electro-oxidative self-
coupling of benzylamines, which is much superior to the
corresponding homogenous molecular catalyst.
Results and Discussion
The pristine MOF MIL-101(Cr) was synthesized according to the
literature, which was further reacted with aluminium chloride and
methoxyacetyl chloride in nitromethane to generate
chloromethylated product MIL-101(Cr)-CH2Cl (Scheme 1).[61-63]
MIL-101(Cr)-HIMBr was synthesized by reaction of MIL-101(Cr)-
CH2Cl with imidazole, and subsequent with 1-bromohexane.[64,65]
The structural identity of as-synthesized MIL-101(Cr) and post-
modified MIL-101(Cr)-HIMBr was characterized by monitoring
the PXRD patterns (Figure S1). All samples display good
crystallinity and the diffraction peaks match well with those of the
simulated, which proved the same structural identity of MIL-
101(Cr) and MIL-101(Cr)-HIMBr. In the FT-IR spectrum of MIL-
101(Cr)-HIMBr, the peaks at 1660, 1330, 1250 and 1104 cm-1
are attributed to the stretching vibrations of imidazolium moieties
(Figure S2).[63] The band at 1584 cm-1 corresponds to the
stretching vibration of C=N bonds in imidazolium, which
confirmed successfully anchoring of imidazolium moieties on the
framework of MIL-101(Cr)-HIMBr. Additionally, the bands at
2929 and 3135 cm-1 are derived from the stretching mode of
aromatic C-H and the aliphatic C-H on imidazolium, respectively,
which are other evidences to prove the existence of imidazolium
moieties in MIL-101(Cr)-HIMBr. SEM and TEM images show
that the octahedral morphology of MIL-101(Cr) is well reserved
for the post-modified MIL-101(Cr)-HIMBr (Figure 1a and 1b).
Elemental mapping images show that Br and N elements are
well distributed in the post-modified crystalline solid MIL-
101(Cr)-HIMBr (Figure 1c and 1d).
Figure 1. SEM and TEM (insert) images (a and b), and elemental mapping
images (c and d) for MIL-101(Cr) (left column) and MIL-101(Cr)-HIMBr (right
column), (e) N2 physical adsorption-desorption isotherms of MIL-101(Cr), MIL-
101(Cr)-HIMBr and TEMPO-SO3@MIL-101(Cr)-HIM, and (f) their pore size
distributions, calculated by DFT method.
Electrochemical
impedance
spectroscopy
(EIS)
measurements were conducted to investigate the interfacial
electron transfer capability between cationic MIL-101(Cr)-HIMBr
and electrode surface by using the redox probe [Fe(CN)6]3-/4-
(Figure 2a). The working electrode was prepared by pipetting a
suspension of MIL-101(Cr)-HIMBr in PVDF NMP and H2O
solution onto a glassy carbon electrode. The EIS of MIL-101(Cr)-
HIMBr exhibits a high-frequency semicircle and a low-frequency
tail, which is the typical impedance plot for ionic materials. The
semicircle is given by the bulk electrical properties of MIL-
101(Cr)-HIMBr relating to the electron transfer limited process,
and the tail corresponds to the diffusion at the interface between
electrode and electrolyte.[66] Compared with that for the pristine
MIL-101(Cr) (166 Ω), the semicircle diameter (108 Ω), equal to
Scheme 1. Schematic representation of the synthetic procedures for MIL-
101(Cr)-HIMBr.
N2 physical adsorption-desorption isotherms of MIL-101(Cr)
and MIL-101(Cr)-HIMBr are the typical type I N2 sorption
isotherms, featuring characteristics of microporous materials
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the electron transfer resistance (Ret) in the Nyquist diagram, in
the high-frequency region for MIL-101(Cr)-HIMBr is evidently
reduced. The decrease in the diameter of the semicircle
indicates the decrease in the resistance of electron transfer of
[Fe(CN)6]3-/4- redox couple between the solution and the
electrode, demonstrating that ionic MIL-101(Cr)-HIMBr formed
high electron conduction pathways between the electrode and
electrolyte. These results could be attributed to the electrostatic
attraction between the positively charged framework and
[Fe(CN)6]3-/4- anions, which could accelerate the anion transfer
and improve the diffusion of [Fe(CN)6]3-/4- toward the electrode
surface for fast electron transfer. Compared with the MIL-
101(Cr) coated electrode, the MIL-101(Cr)-HIMBr coated
electrode presents larger anodic and cathodic currents in the
cyclic voltammogram (CV) curve, indicating that cooperation of
ionic MIL-101(Cr)-HIMBr could accelerate electron transfer,
which is in accordance with the EIS results (Figure 2b). The
markedly improved electrical conductivity and interfacial charge
transfer capability of ionic MIL-101(Cr)-HIMBr would be able to
We have demonstrated that suspending redox-active metal
cations inside the anionic pores of ionic MOFs would result in
SICs, in which the active sites are freely mobile to significantly
improve the catalytic efficiency.[59,60] The endogenous Br- anions
inside the cationic pores of MIL-101(Cr)-HIMBr should be
systematically replaceable with different anionic redox-active
moieties to develop electrochemical SICs by ion exchange.
Because the size of TEMPO-SO3- is much smaller than the pore
window dimensions of MIL-101(Cr)-HIMBr, the anionic catalyst
should easily diffuse into the pore space by ion exchange.
-
Treatment of MIL-101(Cr)-HIMBr with BF4 and subsequent with
-
TEMPO-SO3 resulted in a SIC material TEMPO-SO3@MIL-
101(Cr)-HIM, in which the extra-framework Br- anions were
almost fully exchanged by TEMPO-SO3- anions as confirmed by
EDX spectroscopy, elemental mapping images and HPLC
analysis (Figures S3-S6 and Table S2). Because the
coordination ability of sulfonate is very weak, anionic TEMPO-
-
SO3 should be freely mobile but not able to leach out of the
cationic pores confined by the strong electrostatic interaction
improve the electrocatalytic efficiency in electroorganic synthesis. between cationic host and anionic guests. PXRD patterns
showed that the fundamental structure of TEMPO-SO3@MIL-
101(Cr)-HIM was retained during ion exchange process (Figure
S8). FT-IR spectrum of TEMPO-SO3@MIL-101(Cr)-HIM shows
the typical stretching vibration bands at 1220 and 1197 cm-1 for
-
S=O bond in TEMPO-SO3 , which further confirmed
-
encapsulation of TEMPO-SO3 in the cationic pores (Figure
S9).[67] N2 physical adsorption-desorption isotherms of an
activated solid sample of TEMPO-SO3@MIL-101(Cr)-HIM show
that it takes up 730 cm3 g-1 N2 at 77 K and 1 bar, resulting in a
BET surface area of 1791.3 m2 g-1 with the micropore widths in
the range of 0.7-2.3 nm (Figure 1e and 1f). Compared with those
of the pristine MIL-101(Cr)-HIMBr, the BET surface area and
pore sizes are slightly decreased due to encapsulation of
TEMPO-SO3- anions inside the cationic pores.
Imines and their derivatives are a class of highly reactive
and important intermediates for preparation of heterocyclic
chemicals, pharmaceuticals and fine chemicals.[68] Imines were
traditionally synthesized by condensation of amines and
carbonyl compounds. However, this approach requires
dehydrating agents and activated aldehydes catalyzed by Lewis
acids, which is not an environmentally friendly approach.[69] As
an alternative strategy, electrosynthesis is a simple and green
synthetic approach, which does not require additional
contaminative agents. We therefore selected electro-oxidative
self-coupling of benzylamine as a model reaction to evaluate the
mobility, accessibility, stability and activity of suspended ion
electrocatalyst TEMPO-SO3- in TEMPO-SO3@MIL-101(Cr)-HIM.
The electrochemical behaviors of TEMPO-SO3@MIL-
101(Cr)-HIM were investigated by monitoring the CV curves at
room temperature in acetonitrile containing 0.1 M Bu4NBF4
supporting electrolyte. The potentials reported herein are
referenced to Ag/AgCl (in 3 M KCl) reference electrode, which is
Figure 2. (a) Electrochemical impedance spectra of MIL-101(Cr) and MIL-
101(Cr)-HIMBr in 0.1 M KCl solution containing 5 mM [Fe(CN)6]3-/4- at room
temperature. (b) Cyclic voltammograms of MIL-101(Cr) and MIL-101(Cr)-
HIMBr in 5 mM [Fe(CN)6]3-/4- in the presence of 0.1 M KCl supporting
electrolyte at a scan rate of 20 mV s-1 with platinum wire as the counter
electrode and Ag/AgCl as the reference electrode.
0.194 V vs. standard hydrogen electrode (SHE). As shown in
Figure 3 (curve a), TEMPO-SO3@MIL-101(Cr)-HIM exhibits a
reversible oxidation wave at +0.858 V and a cathodic peak at
+0.830 V. When benzylamine was added into the solution, the
oxidation peak current increased dramatically, while the
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corresponding reduction peak current decreased (curve b). The
obvious increase of electron current indicates that electro-redox
catalysis occurred between TEMPO-SO3 and benzylamine
result revealed that TEMPO-SO3@MIL-101(Cr)-HIM exhibits
preferred sorption capability of reactant over product molecules,
which would selectively enrich substrate and release product
molecules to improve the electrochemical selectivity by
preventing formation of over-oxidized benzonitrile product. When
MIL-101(Cr)-HIMBF4 without TEMPO-SO3- anion inside the pore
space was used instead of TEMPO-SO3@MIL-101(Cr)-HIM
under the otherwise identical conditions, benzylamine was
almost not oxidized (Table 1, entry 4). This result indicates that
-
under the conditions.
-
the active sites are the suspended TEMPO-SO3 anions in the
pores of TEMPO-SO3@MIL-101(Cr)-HIM.
Table 1. Electrocatalytic properties of different electrocatalysts in oxidative
self-coupling of benzylamine.[a]
Entry
Electrocatalyst
Conv.(%)
>99.9
trace
Sel.(%)[b]
94.9
1
2
3
4
5
6
7
TEMPO-SO3@MIL-101(Cr)-HIM
TEMPO-SO3@MIL-101(Cr)-HIM
TEMPO-SO3H
[c]
-
Figure 3. Cyclic voltammograms of TEMPO-SO3@MIL-101(Cr)-HIM in
Bu4NBF4 CH3CN solution (curve a) and in the presence of benzylamine (curve
b) at a scan rate of 20 mV s-1 on the glassy carbon working electrode with
platinum wire as the counter electrode and Ag/AgCl as the reference electrode.
>99.9
trace
61.3
-
MIL-101(Cr)-HIMBF4
TEMPO-SO3H@MIL-101(Cr)
TEMPO@MIL-101(Cr)-HIMBF4
TEMPO-COO@MIL-101(Cr)-HIM
75.6
70.6
80.8
80.7
Electrocatalysis was conducted in an undivided cell with
working electrode as the anode, platinum wire as the cathode,
Bu4NBF4 as the electrolyte and CH3CN as the solvent. The
working electrode was prepared by pipetting a suspension of
TEMPO-SO3@MIL-101(Cr)-HIM in PVDF NMP and H2O solution
onto a carbon plate (1.0 × 1.0 cm2). The reaction was
electrolyzed at a constant potential of 1.0 V vs. Ag/AgCl for 3 h
at room temperature. As shown in Table 1, benzylamine was
almost fully oxidized (>99% conversion) and the selectivity of
oxidative self-coupling product imine is of 94.9% (Table 1, entry
1). Electricity is indispensable for the oxidation reaction as the
reaction was almost inactive when the voltage on working
electrode was switched off (Table 1, entry 2). When molecular
TEMPO-SO3H was used as a homogeneous catalyst and carbon
plate was used as working electrode, benzylamine was also
almost fully oxidized (>99% conversion) under the otherwise
identical conditions (Table 1, entry 3). However, the selectivity of
self-coupling product is of only 61.3%, accompanying formation
of large amount byproducts. These results demonstrate that
>99.9
89.6
[a] Reaction conditions: benzylamine (0.2 mmol), Bu4NBF4 acetonitrile
solution (0.1 M, 4 mL), working electrode consisting of 0.0032 mmol
-
TEMPO or TEMPO-SO3 , Pt cathode, and Ag/AgCl reference electrode,
room temperature, 3 h. [b] Yield and selectivity were determined by 1H
NMR in the presence of dibromomethane as an internal standard. The main
side product is benzonitrile. [c] Switching off the working voltage.
To make comparisons, we synthesized a series of control
catalysts, including encapsulating TEMPO-SO3H in neutral MIL-
101(Cr)
(denoted
as
TEMPO-SO3H@MIL-101(Cr)),
encapsulating neutral TEMPO in cationic MIL-101(Cr)-HIMBF4
(denoted as TEMPO@MIL-101(Cr)-HIMBF4) and encapsulating
anionic TEMPO-COO- in MIL-101(Cr)-HIM (denoted as TEMPO-
COO@MIL-101(Cr)-HIM) (Figures S10-S16). As shown in Table
1 (entries 5-7), the catalytic properties of these control catalysts
are much inferior to those of TEMPO-SO3@MIL-101(Cr)-HIM.
Because the interaction between electrocatalytic active moieties
and pore matrices for TEMPO-SO3H@MIL-101(Cr) and
TEMPO@MIL-101(Cr)-HIMBF4 is very weak, the active sites
easily leach out of the pore space during catalysis. Because the
carboxylate group of TEMPO-COO- is easily coordinating to CrIII
metal nodes, the mobility of TEMPO-COO- moieties in TEMPO-
COO@MIL-101(Cr)-HIM is heavily confined, which are difficult to
access to the working electrode. In addition, the electrical
conductivity of neutral MIL-101(Cr) is much lower than that of
ionic MIL-101(Cr)-HIM. These inferior characters of control
-
suspending ion electrocatalyst TEMPO-SO3 in the cationic
pores of TEMPO-SO3@MIL-101(Cr)-HIM would significantly
improve the product selectivity, controlled by the unique pore
nature of the MOF. After the electrocatalytic reaction was
proceeded for 1.5 h, the catalysis was deliberately interrupted.
The TEMPO-SO3@MIL-101(Cr)-HIM coated working electrode
was subsequently immersed in acetonitrile to release the
included guest molecules inside the pore space. GC analysis
showed that the molecular ratio of released benzylamine
reactant and N-benzylidenebenzylamine product is of 3.3, even
though the conversion of benzylamine has been of 80%. This
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catalysts heavily deteriorated their electrocatalytic properties in
the electroorganic reaction. In contrast, suspending anion
electrocatalyst TEMPO-SO3 in the cationic pores of MIL-
Table 2. Electrochemically oxidative coupling of benzylamine and its
derivatives to the corresponding imines.[a,b]
-
101(Cr)-HIM endows the active sites freely mobile and easy
access to the working electrode surface, which is beneficial to
polarize and activate reactant molecules. Combined with these
superior characters, TEMPO-SO3@MIL-101(Cr)-HIM exhibits
high efficiency and selectivity in the electrocatalytic reaction.
Under the optimized electrocatalytic conditions, various
substituted amines were investigated to expand the scope of the
reaction substrates catalyzed by TEMPO-SO3@MIL-101(Cr)-
HIM. As shown in Table 2, the electronic properties of different
substituents had little effect on substrate conversions and
product selectivities with high product yields of 91.0-98.2%
(Table 2, 2a-2l). Due to steric hindrance, the conversion of 1-
naphthalenemethylamine is decreased with low imine product
yield of 71.0% (2m). Similarly, diphenylmethylamine (2n) was
obtained in low yield (73.7%), and the main side product is
diphenylmethanone. Additionally, TEMPO-SO3@MIL-101(Cr)-
HIM is also highly efficient in dehydrogenation of secondary
amines to form the corresponding imines, including 3,4-
dihydroisoquinoline (2o, 84.6% yield) and (E)-N-benzylidene-1-
phenylmethanamine (2a, 96.9%), under the identical
electrocatalytic conditions.
2a (96.9%)[c]
2b (91.7%)
2c (92.5%)
2d (98.2%)
2f (97.4%)
2i (91.0%)
2e (94.3%)
2h (93.9%)
2g (94.1%)
2j (96.4%)
2k (93.1%)
2l (90.0%)
2o (84.6%)
2m (71.0%)
2n (73.7%)
[a] Reaction conditions: benzylamine (0.2 mmol), Bu4NBF4 acetonitrile
solution (0.1 M, 4 mL), TEMPO-SO3@MIL-101(Cr)-HIM carbon plate anode
Immobilization of TEMPO-SO3@MIL-101(Cr)-HIM on the
surface of working electrode would simplify the work-up
procedures and realize recycling usage of the electrocatalyst.
Deferent to the active sites immobilized in traditional
-
(consisting of 0.0032 mmol TEMPO-SO3 ), Pt cathode, and Ag/AgCl reference
electrode, room temperature, 3 h. [b] Yield was determined by 1H NMR in the
presence of dibromomethane as an internal standard. [c] The yield was
resulted from dehydrogenation of dibenzylamine.
-
heterogeneous catalysts, the anionic TEMPO-SO3 active sites
On the basis of the results above, we proposed a plausible
catalytic mechanism for the electrochemically oxidative self-
are suspended in the cationic pores of ionic MIL-101(Cr)-HIM,
which are freely mobile. The heterogeneity and stability of the
working electrode should be the most concerned issues, which
would determine the long-term application capability of the
working electrode. Recycling experiments showed that the
working electrode is stable, which can be simply reused at least
six runs in electrochemically oxidative coupling of benzylamine
with basically retained catalytic properties (91.5% yield for the
sixth run) (Figure S17). HPLC analysis of the digested working
coupling of benzylamine (Scheme 2).[70-72] TEMPO-SO3 was
-
firstly oxidized at the anode via a single electron transfer
process to generate the corresponding active oxoammonium
TEMPO+-SO3 , which would oxidize benzylamine substrate to
-
result in a radical-cation intermediate A. Intermediate A is highly
reactive, which would easily react with another benzylamine
molecule via oxidative dehydrogenation of the amine to afford
benzylimine B, accompanying release of proton and ammonium
-
electrode showed that there is of 91.3% retained TEMPO-SO3
cations. Meanwhile, active species TEMPO+-SO3 was reduced
-
active sites inside the cationic pores of TEMPO-SO3@MIL-
101(Cr)-HIM. In contrast, the remained neutral TEMPO in
TEMPO@MIL-101(Cr)-HIMBF4 is of only 40.5% (Table S2).
PXRD pattern of the reused working electrode is almost identical
to that of as-synthesized one, which proved the structural
stability of TEMPO-SO3@MIL-101(Cr)-HIM in the electrocatalytic
reaction (Figure S18). The above results indicate that
to TEMPO-SO3 via one electron transfer. At the cathode, H+
-
ions were electrochemically reduced to generate H2 via
electrochemical reduction.
-
suspending anion electrocatalyst TEMPO-SO3 in the cationic
pores of MIL-101(Cr)-HIM would prevent leaching of active sites
by strong electrostatic interaction during electrocatalysis.
Compared with the corresponding homogeneous molecular
-
catalyst, suspending TEMPO-SO3 in the cationic pores of the
MOF immobilized on the working electrode would also prevent
degradation of the oxidative intermediates (oxoammonium
-
moieties) from TEMPO-SO3 on the counter electrode, and thus
Scheme 2. Proposed electrocatalytic cycles for oxidative coupling of
benzylamine
electrocatalyst.
to
form
imine
over
TEMPO-SO3@MIL-101(Cr)-HIM
to improve the faradic efficiency, stability and catalytic
efficiency.[3,23]
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mL). 100 uL of the resulting ink was pipetted onto a carbon plate (1.0 ×
1.0 cm2) as the working electrode. After air drying for 12 hours, the as-
prepared electrode was further treated at 80 °C in an oven for 30 min
before electrocatalysis.
Conclusions
We synthesized an imidazolium modified cationic MOF MIL-
101(Cr)-HIMBr by post-modification, which exhibited high
conductivity, compared with the pristine neutral MOF MIL-
Electrochemical measurements. Electrochemical measurements were
conducted with a CHI 660E electrochemical workstation (Shanghai
Chenhua, China) using a standard three-electrode system. To prepare
the working electrode, an as-prepared sample (20 mg) was ultrasonically
dispersed in a mixture of 1.5% PVDF NMP solution (0.25 mL) and H2O
(0.25 mL) for 15 min. 10 uL of the resulting ink was pipetted onto a
glassy carbon electrode (3 mm diameter). Cyclic voltammograms (CVs)
were recorded in an electrolyte of Bu4NBF4 in CH3CN (0.1 M, 10 mL)
using the glassy carbon working electrode, a Pt wire auxiliary electrode
and a Ag/AgCl (in 3 M KCl) reference electrode. Impedance spectra were
collected using excitation amplitude of 5 mV within the frequency range
spanning from 100 kHz to 0.01 Hz for the working electrode. Each
measurement was performed at open-circuit potential in 0.1 M KCl
aqueous solution (5 mL) containing 5 mM of [Fe(CN)6]3-/4- at room
temperature. All impedance spectra were fitted to equivalent circuits
using the ZView software.[73] All measurements were collected at least in
triplicate.
-
101(Cr). Anionic redox-active TEMPO-SO3 moieties were easily
integrated and suspended inside the cationic pores of ionic MIL-
101(Cr)-HIM by ion exchange, which served as an efficient
electrocatalyst for oxidative self-coupling of benzylamines.
Compared with the corresponding molecular catalyst, TEMPO-
SO3@MIL-101(Cr)-HIM exhibited superior electrocatalytic
selectivity in the electrochemical reaction. This work highlights a
general method for assembling suspended ion electrocatalysts
with superior characters of high catalyst turnovers, conductivity,
and electrochemical selectivity and stability. Because of the
versatility and molecular tunability of ionic MOFs, we believe that
the strategy reported in this work is generally applicable for
design, synthesis and applications of heterogeneous
electrocatalysts with distinct catalytic properties in electroorganic
synthesis.
A typical procedure for electrocatalytic oxidation of benzylamine. A
mixture of benzylamine (0.2 mmol) in 0.1 M Bu4NBF4 acetonitrile solution
(4 mL) was added into an undivided cell. The cell was equipped with the
as-prepared working electrode as anode, Pt plate as cathode and
Ag/AgCl (in 3 M KCl) as reference electrode. The reaction mixture was
stirred and electrolyzed at a constant potential of 1.0 V at room
temperature for 3 h. When the electrocatalytic reaction was finished, the
solution was extracted with EtOAc (3 × 10 mL). The combined organic
layer was dried over Na2SO4, and filtered. The solvent was removed with
a rotary evaporator. The yield was determined by 1H NMR in the
presence of dibromomethane as an internal standard.
Experimental Section
Materials and Methods. All chemicals were obtained from commercial
sources and were used without further purification. FT-IR spectra were
collected from KBr pellets on an FTS-40 spectrophotometer. Powder X-
ray diffraction (PXRD) data were recorded on
2550/PC for Cu Ka radiation (k 1.5406 Å). Scanning electron
a RIGAKU D/MAX
=
microscopy (SEM) images were recorded on a Hitachi S-4800 equipment.
Transmission electron microscopy (TEM) equipped with an energy
dispersive X-ray (EDX) detector was recorded on
a JEM 2100F
equipment, and the samples were deposited onto ultrathin carbon films
on carbon grids. H NMR and 13C NMR spectra were recorded on a 400
1
Acknowledgements
MHz Bruker spectrometer and the chemical shifts were reported relative
to internal standard TMS (0 ppm). A Micromeritics ASAP 2020 surface
area analyzer was used to measure N2 gas adsorption/desorption
isotherms. The electrochemical measurements were conducted with a
CHI 660E electrochemical workstation (Shanghai Chenhua, China) using
a standard three-electrode system.
We are grateful for the financial support of the National Natural
Science Foundation of China (grant nos. 21373180, 21525312
and 21872122).
Keywords: Conductivity
• Electrocatalysis • Metal-organic
A typical procedure for preparation of TEMPO-SO3@MIL-101(Cr)-
HIM. MIL-101(Cr)-HIMBr was prepared according to previously reported
procedures.[61-65] 200 mg MIL-101(Cr)-HIMBr was added into 20 mL 1 M
NaBF4 aqueous solution, which was stirred at room temperature for 8 h,
centrifuged, and washed with H2O. This procedure was repeated five
frameworks • Electroselectivity • Suspended ion catalysts
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Entry for the Table of Contents
FULL PAPER
Suspending ion electrocatalysts in
charged metal-organic frameworks
(MOFs) markedly improves the
conductivity and catalytic efficiency,
Wei-Wei Guo, Chi Zhang, Ji-Jie Ye, Zi-
Kun Liu, Kai Chen, Chuan-De Wu*
Page No. – Page No.
and
catalytic
selectivity
in
Suspending Ion Electrocatalysts in
Charged Metal-Organic Frameworks
to Improve the Conductivity and
electroorganic synthesis, compared
with the corresponding neutral MOFs
and
((Insert TOC Graphic here: max.
width: 5.5 cm; max. height: 5.0 cm))
homogeneous
molecular
Selectivity
Synthesis
in
Electroorganic
catalysts, respectively.
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