Paired electrocatalytic hydrogenation and oxidation of 5-(hydroxymethyl)furfural for efficient production of biomass-derived monomers

Article abstract of DOI:10.1039/c9gc02264c

Electrochemical conversion of biomass-derived compounds is a promising route for sustainable chemical production. Herein, we report unprecedentedly high efficiency for conversion of 5-(hydroxymethyl)furfural (HMF) to biobased monomers by pairing HMF reduction and oxidation half-reactions in one electrochemical cell. Electrocatalytic hydrogenation of HMF to 2,5-bis(hydroxymethyl)furan (BHMF) was achieved under mild conditions using carbon-supported Ag nanoparticles (Ag/C) as the cathode catalyst. The competition between Ag-catalyzed HMF hydrogenation to BHMF and undesired HMF hydrodimerization and hydrogen evolution reactions was sensitive to cathode potential. Also, the carbon support material in Ag/C was active for HMF reduction at strongly cathodic potentials, leading to additional hydrodimerization and low BHMF selectivity. Accordingly, precise control of the cathode potential was implemented to achieve high BHMF selectivity and efficiency. In contrast, the selectivity of HMF oxidation facilitated by a homogeneous electrocatalyst, 4-acetamido-TEMPO (ACT, TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl), together with an inexpensive carbon felt electrode, was insensitive to anode potential. Thus, it was feasible to conduct HMF hydrogenation to BHMF and oxidation to 2,5-furandicarboxylic acid (FDCA) in a single divided cell operated under cathode potential control. Electrocatalytic HMF conversion in the paired cell achieved high yields of BHMF and FDCA (85% and 98%, respectively) and a combined electron efficiency of 187%, corresponding to a nearly two-fold enhancement compared to the unpaired cells.

Full text of DOI:10.1039/c9gc02264c

Green Chemistry
View Article Online
View Journal | View Issue
PAPER
Paired electrocatalytic hydrogenation and oxidation
of 5-(hydroxymethyl)furfural for efficient
production of biomass-derived monomers†
Cite this: Green Chem., 2019, 21,
6210
a,b
Xiaotong H. Chadderdon, a David J. Chadderdon, a Toni Pfennig,
a,c
Brent H. Shanks a,b and Wenzhen Li
*
Electrochemical conversion of biomass-derived compounds is a promising route for sustainable chemical
production. Herein, we report unprecedentedly high efficiency for conversion of 5-(hydroxymethyl)fur-
fural (HMF) to biobased monomers by pairing HMF reduction and oxidation half-reactions in one electro-
chemical cell. Electrocatalytic hydrogenation of HMF to 2,5-bis(hydroxymethyl)furan (BHMF) was
achieved under mild conditions using carbon-supported Ag nanoparticles (Ag/C) as the cathode catalyst.
The competition between Ag-catalyzed HMF hydrogenation to BHMF and undesired HMF hydrodimeriza-
tion and hydrogen evolution reactions was sensitive to cathode potential. Also, the carbon support
material in Ag/C was active for HMF reduction at strongly cathodic potentials, leading to additional hydro-
dimerization and low BHMF selectivity. Accordingly, precise control of the cathode potential was
implemented to achieve high BHMF selectivity and efficiency. In contrast, the selectivity of HMF oxidation
facilitated by a homogeneous electrocatalyst, 4-acetamido-TEMPO (ACT, TEMPO = 2,2,6,6-tetramethyl-
piperidine-1-oxyl), together with an inexpensive carbon felt electrode, was insensitive to anode potential.
Thus, it was feasible to conduct HMF hydrogenation to BHMF and oxidation to 2,5-furandicarboxylic acid
(FDCA) in a single divided cell operated under cathode potential control. Electrocatalytic HMF conversion in
the paired cell achieved high yields of BHMF and FDCA (85% and 98%, respectively) and a combined elec-
tron efficiency of 187%, corresponding to a nearly two-fold enhancement compared to the unpaired cells.
Received 3rd July 2019,
Accepted 20th October 2019
DOI: 10.1039/c9gc02264c
rsc.li/greenchem
number of processes and production volumes.8 Factors limit-
ing its widespread application are poor catalyst activity and
1. Introduction
Organic electrosynthesis has emerged as a promising method- reaction rates, as well as high energy demands.9
ology for environmentally-friendly chemical production.1 In
Electrocatalytic hydrogenation (ECH) of biomass-derived
electroorganic reactions, electrons serve as an inherently clean molecules has received significant attention in recent
reagent to replace stoichiometric oxidants or reductants, and years.10–16 ECH is analogous to thermal catalytic hydrogen-
thereby eliminate toxic waste and byproducts.2 The driving ation, with the key difference that surface-adsorbed hydrogen
force of electrode reactions can be directly manipulated by atoms are generated electrochemically from water or proton
controlling the potential, which may enable very high selecti- reduction, rather than from H2 dissociation.17 In this way, the
vity for desired molecular transformations.3–5 Moreover, the large kinetic barriers for H2 dissociation are avoided, therefore
electrochemical cells may potentially be powered by electricity allowing ECH to proceed at mild temperatures and pressures
from renewable sources,6,7 reducing the overall carbon foot- and without the need for conventional hydrogenation catalysts
print. Despite these advantages, the industrial electrosynthesis (e.g., Pt, Pd, Ni). However, it can be challenging to obtain high
of organic molecules has been very limited in terms of selectivity and efficiency for the desired transformation
because multiple reaction pathways and competing reactions
may exist.14 The electrocatalytic hydrogen evolution reaction
aDepartment of Chemical and Biological Engineering, Iowa State University,
Ames 50011, USA. E-mail: wzli@iastate.edu
bNSF Engineering Research Center for Biorenewable Chemicals (CBiRC),
Iowa State University, Ames 50011, USA
cUS Department of Energy Ames Laboratory, Ames 50011, USA
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc02264c
(HER) is operable on metal electrodes at cathodic potentials,
and consumes adsorbed hydrogen atoms in competition with
ECH. Additionally, carbonyl-containing substrates may undergo
direct electroreduction and hydrodimerization reactions.18
Electrochemical half-reactions occur in pairs, yet many
commercialized electrochemical processes only utilize one of
6210 | Green Chem., 2019, 21, 6210–6219
This journal is © The Royal Society of Chemistry 2019

View Article Online
Green Chemistry
Paper
the two electrodes for generation of desired products.19 For We elucidated the contribution of the carbon black (CB)
example, electrochemical reductions are typically paired with support material in Ag/C to the observed product selectivity;
water oxidation as a benign counter reaction. However, water undesired HMF hydrodimerization occurred on CB at cathode
oxidation has sluggish kinetics and the oxygen gas produced is potentials more negative than −1.2 V vs. Ag/AgCl. For Ag/C,
not valuable.20 Therefore, substantial gains in terms of econ- operating at potentials more positive than −1.4 V vs. Ag/AgCl
omic feasibility and energy efficiency can be made by pairing alleviated the impact of HMF reduction on CB; the optimal
two half-reactions that generate desired products in a single BHMF selectivity and faradaic efficiency was obtained at
electrochemical cell.21–23 In this way, the theoretical maximum −1.3 V vs. Ag/AgCl. For the anode reaction, a homogeneous
electron efficiency is 200%, two times greater than in conven- electrocatalyst, 4-acetamido-TEMPO (ACT, TEMPO = 2,2,6,6-
tional unpaired cells. Moreover, the paired electrolysis tetramethylpiperidine-1-oxyl), enabled indirect electrochemical
approach may lower capital and operation costs by reducing HMF oxidation to FDCA with nearly 100% faradaic efficiency.
the number of reactors or processing steps required. The ACT-mediated HMF oxidation was conducted using an in-
Nevertheless, there are challenges arising from mismatched expensive carbon felt anode and mild electrolyte conditions
optimal current densities for the two half-reactions, chemical (pH 9.2 buffer, same as catholyte). Electrocatalytic HMF con-
incompatibilities, and crossover issues.24
version in a paired electrochemical cell achieved a combined
Utilizing renewable feedstocks is a pillar of green chem- electron efficiency to BHMF and FDCA of 187%, which corres-
istry.25 One promising renewable molecule is 5-(hydroxy- ponds to a nearly two-fold enhancement compared to the
methyl)furfural (HMF), which is accessible from biomass unpaired cells. The individual yields for BHMF and FDCA
through the dehydration of fructose or glucose, and is (85% and 98%, respectively) were similar to those in unpaired
an important platform chemical that can be diversified cells, indicating that the two half-reactions were compatible
into
a
variety of value-added chemicals and fuels.26–28 and proceeded without major complications or adverse effects.
Electrocatalytic conversion has been recognized as a promising This approach satisfies many principles of green chemistry
method for either HMF hydrogenation or oxidation to bio- and demonstrates the feasibility of paired electrosynthesis for
based monomers (Scheme 1).29 Specifically, the selective biomass conversion.
hydrogenation of HMF generates 2,5-bis(hydroxymethyl)furan
(BHMF), which is an important precursor for production of
polyesters and polyurethane foams.30 Selective oxidation of
2. Results and discussion
HMF generates 2,5-furandicarboxylic acid (FDCA). FDCA is a
2.1 Ag/C synthesis and characterizations
feedstock for production of polyethylene 2,5-furandicarboxy-
Silver nanoparticles (Ag NPs) were synthesized by reducing
silver nitrate in oleylamine and oleic acid according to litera-
ture.35 Ag/C was prepared by depositing Ag NPs onto Vulcan
XC-72R carbon black (CB). Transmission electron microscopy
(TEM) images of the Ag/C catalyst are shown in Fig. 1. The
particle size histogram (ESI, Fig. S5†) showed that deposited
Ag NPs ranged from approximately 2.0 to 6.0 nm with a mean
particle diameter of 3.9 nm. X-ray diffraction (XRD) showed
that the Ag NPs were crystalline with a face-centered cubic (fcc)
structure, and average crystallite size of approximately 2.3 nm
(ESI, Fig. S6†). XPS analysis of the Ag/C catalyst showed charac-
teristic binding energy peaks for Ag3d5/2 (368.2 eV) and
Ag3d3/2 (374.2 eV), indicating Ag NPs were in the metallic Ag0
state (ESI, Fig. S7†). Raman spectra of the Ag/C catalyst was
late (PEF), a biobased alternative to petroleum-derived poly-
ethylene terephthalate.31
It was shown in two separate reports that electrocatalytic
hydrogenation of HMF to BHMF and redox-mediated oxidation
of HMF to FDCA can both be conducted in mildly basic elec-
trolytes (i.e. pH 9.2) with good selectivity.13,32 Such conditions
are well-suited for electrochemical conversion of biomass-
derived chemicals including HMF, as those chemicals are typi-
cally unstable in very acidic or basic environments. Moreover,
there is an opportunity to integrate the two HMF half-reactions
in a paired electrochemical cell without additional issues
related to electrolyte or HMF crossover. The overall cell reac-
tion consumes only HMF, water, and electricity, and generate
no waste products. Despite these benefits, there are only a few
reports of paired electrocatalytic conversion of HMF in
literature.33,34
Herein, we demonstrate the paired electrocatalytic conver-
sion of HMF to BHMF and FDCA. A self-synthesized Ag/C cata-
lyst facilitated HMF hydrogenation to BHMF with enhanced
faradaic efficiency compared to a polycrystalline Ag electrode.
Scheme 1 Paired electrosynthesis of BHMF and FDCA from HMF.
Fig. 1 TEM images of as-prepared Ag/C catalyst.
Green Chem., 2019, 21, 6210–6219 | 6211
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Green Chemistry
consistent with the spectra of Vulcan XC-72R carbon black HMF was approximately two for Ag-pc and Ag/C and one for
(ESI, Fig. S8†). The Ag loading on the CB support was esti- CB (ESI, Fig. S10†). This indicates that different HMF
mated by thermal gravimetric analysis (TGA) to be 9.1% (ESI, reduction mechanisms may be operable for Ag-based electro-
Fig. S9†).
des than for CB electrodes. Fig. 2 also shows voltammograms
measured in electrolytes without HMF, for which the reduction
waves can be assigned to the hydrogen evolution reaction
(HER). The HER current was negligible at potentials close to
the onset of HMF reduction, but increased substantially at
potentials more negative than about −1.4 V. This corresponds
to the potential range at which a second reduction wave
initiated for electrolytes containing HMF, suggesting that HMF
reduction and HER may proceed concurrently at very negative
potentials. The second reduction wave was less notable for CB,
reflecting the lower HER activity for CB compared to Ag-based
electrodes.
2.2 Electrochemical reduction of HMF
The Ag/C catalyst was initially evaluated for HMF reduction in
a borate buffer (pH 9.2) electrolyte by cyclic voltammetry. All
potentials herein are reported with respect to a Ag/AgCl refer-
ence electrode. Working electrodes were prepared by drop-
casting an ink dispersion of Ag/C onto a glassy carbon disk
electrode. Additionally, CB-modified glassy carbon and poly-
crystalline silver (Ag-pc) disk electrodes were tested for com-
parison. The electrochemically-active surface area (ECSA) for
Ag was determined by oxidative stripping voltammetry of
under-potential deposited lead (PbUPD).36 The amount of Ag/C
drop-cast onto the glassy carbon electrode was chosen to
give similar Ag ECSA as the Ag-pc disk electrode (details in
section 4.5).
Cyclic voltammetry revealed that Ag/C, Ag-pc, and CB elec-
trodes were active for HMF reduction (Fig. 2). The onset poten-
tials for HMF reduction were −1.03 V, −1.05 V, and −1.21 V for
Ag/C, Ag-pc, and CB, respectively, as defined herein as the
potential at which background-corrected current density
reached −0.1 mA cm−2. This suggests that the CB support
material present in Ag/C may participate in HMF reduction at
potentials more negative than −1.21 V. The peak currents den-
sities for Ag/C and Ag-pc (−6.8 and −7.0 mA cm−2, respectively)
were approximately two-fold higher than for CB (−3.3 mA
cm−2). Koutecký–Levich analysis of the reduction waves indi-
cated that the electron transfer number (n) with respect to
HMF reduction was studied using potential-controlled elec-
trolysis over the range of −1.15 V to −1.5 V. Electrodes were
prepared by drop-casting an ink dispersion of Ag/C onto a
carbon paper substrate. The amount of Ag/C loaded onto the
carbon paper was chosen to give similar Ag ECSA as the Ag-pc
electrode, as determined by PbUPD stripping voltammetry
(details in section 4.8). HMF reduction products for Ag/C and
Ag-pc were evaluated in terms of selectivity and faradaic
efficiency. The main HMF reduction products were BHMF
from hydrogenation and BHH (5,5′-bis(hydroxymethyl)hydro-
furoin) from hydrodimerization (Scheme 2). The product dis-
tribution was highly dependent on the cathode potential
(Fig. 3). BHMF selectivity increased and BHH decreased with
more negative potentials over the range of −1.15 V to −1.3 V.
BHMF selectivity was notably higher for Ag/C than Ag-pc over
this potential range; however, it decreased at more negative
potentials for Ag/C, corresponding to increased formation of
BHH and unidentified products. In contrast, BHMF selectivity
continued to rise with more negative potentials for Ag-pc.
Although, HER was active at those potentials and the faradaic
efficiency for BHMF was lower (e.g. 73.7% at −1.5 V for Ag-pc).
As a result, the optimal BHMF generation in terms of faradaic
efficiency was obtained using Ag/C at −1.3 V, for which BHMF
efficiency reached 96.2%. Stability tests were conducted by
reusing one Ag/C electrode for four consecutive trials. HMF
conversion rate and BHMF efficiency decreased slightly after
the first trial but remained stable for the remaining trials (ESI,
Fig. S11†). The initial drop in performance may be due to Ag
particle agglomeration; larger Ag NPs were observed by TEM
following the stability test (ESI, Fig. S12†). The ICP-AES ana-
lysis of the electrolyte solution showed that Ag leaching was
Fig. 2 Cyclic voltammograms for (a) polycrystalline Ag (Ag-pc), (b)
carbon supported Ag nanoparticles (Ag/C), and (c) Vulcan XC-72R
carbon black (CB). Solid lines with 20.0 mM HMF, broken lines without
HMF.
Scheme 2 HMF reduction to BHMF and BHH.
6212 | Green Chem., 2019, 21, 6210–6219
This journal is © The Royal Society of Chemistry 2019

View Article Online
Green Chemistry
Paper
spectra, which were not identified or quantified in this work.
As previously discussed, the Koutecký–Levich analysis indi-
cated that HMF reduction on CB electrodes proceeds mainly
by a one-electron transfer process. Therefore, we hypothesize
that the unidentified species are dimer or oligomer byproducts
of HMF hydrodimerization reactions, which consume one elec-
tron per HMF, rather than products of hydrogenation or hydro-
genolysis reactions as suggested in an earlier report.37 These
results show that CB electrodes were active for HMF reduction
and generated substantial amounts of BHH and unidentified
products starting around −1.3 V. On this basis, we attribute
the lower BHMF and total selectivities for Ag/C compared to
Ag-pc at −1.4 V and −1.5 V (Fig. 3) to the activity of the CB
support material present in Ag/C.
Regarding the mechanism of HMF hydrogenation to BHMF
in basic or neutral media, there is uncertainty whether the
hydrogen source is water or surface adsorbed hydrogen
(H*),11,13,29,37 as depicted in eqn (1) and (2), respectively.
Fig. 3 Electrochemical reduction of HMF at various potentials. (a) HMF
conversion and product selectivity and (b) faradaic efficiency and total
charge transferred in external circuit for Ag/C and (c and d) corres-
ponding results for Ag-pc. Conditions: pH 9.2 electrolyte, 20.0 mM
HMF, 30 minutes reaction time.
HMF þ 2H2O þ 2eꢀ ! BHMF þ 2OHꢀ
HMF þ 2H* ! BHMF
ð1Þ
ð2Þ
The latter pathway is known as electrocatalytic hydrogen-
ation (ECH).17 The electrochemical formation of H* (i.e.
Volmer step) occurs by water reduction in basic or neutral
media (eqn (3)),38 in which * designates a surface site.
negligible (i.e. less than detection limit of 0.6 ppb Ag in elec-
trolyte) after two hours of HMF reduction.
HMF reduction was conducted using CB-modified carbon
paper electrodes to decouple the contributions of Ag NPs and
the CB support material to the observed product selectivities
for Ag/C. HMF reduction products were detected at −1.2 V for
CB electrodes, however HMF conversion was insufficient (i.e.
<1%) for quantitative product analysis. Fig. 4 shows that HMF
conversion rates were enhanced at more negative potentials
and reached 50% at −1.5 V, which is comparable to the
values for Ag/C and Ag-pc (i.e., 49% and 52%, respectively at
−1.5 V). In sharp contrast to Ag-based electrodes, BHH was
the major detected product for CB electrodes, and very little
BHMF was generated (i.e. <3% selectivity). Also, HER was a
minor contribution to the total charge passed (i.e. ∼2% fara-
daic efficiency at −1.5 V), reflecting the poor HER activity of
CB.
H2O þ * þ eꢀ ! H* þ OHꢀ
ð3Þ
ECH reactions involve strong interactions between the elec-
trode surface and reactants, and are therefore highly depen-
dent on the nature of the electrode material.39 Kwon et al.
hypothesized that HMF hydrogenation to BHMF in neutral
media occurs directly by water molecules (eqn (1)) on the basis
of the nearly identical onset potentials observed for a wide
range of metallic electrodes.37 On the other hand, Roylance
et al. studied HMF reduction on Ag-based electrodes in basic
media (i.e. pH 9.2) and reported that H* was likely involved in
BHMF formation within the potential region where HER, and
therefore H* formation, were possible.13 Our viewpoint is that
ECH was the major BHMF generation pathway for Ag-based
electrodes under the conditions reported herein. This is self-
consistent with the low BHMF selectivity we report for CB elec-
trodes (Fig. 4), as carbon is known to have very weak H*
adsorption,40 and is therefore not expected to facilitate ECH
reactions. However, it should be noted that BHMF was a major
product for Ag/C and Ag-pc electrodes even within the poten-
tial region where HER current was negligible (i.e. −1.15 V ≥ E
≥ −1.3 V). A recent density functional theory (DFT) study of Ag
cathodes indicated that the lowest energy pathway for HER via
water reduction is the Volmer–Heyrovsky sequence, and the
Heyrovsky step is rate-limiting.41 Therefore, it is very plausible
that HMF hydrogenation by H* (i.e. ECH) can proceed within a
potential region where the Volmer step is facile but HER is
kinetically-limited by the Heyrovsky step.
The combined selectivities for BHH and BHMF were low
(i.e. 32–35%), indicating that other products were generated
from HMF reduction with CB electrodes. We detected several
1
unknown products in the HPLC chromatographs and H NMR
Fig. 4 Electrochemical reduction of HMF at various potentials with CB
electrode including (a) HMF conversion and product selectivity and (b)
faradaic efficiency and total charge transferred in external circuit.
Conditions: pH 9.2 electrolyte, 20.0 mM HMF, 30 minutes reaction time.
We found that the selectivity of HMF reduction to BHMF or
BHH was dependent on the cathode potential for Ag-based
This journal is © The Royal Society of Chemistry 2019
Green Chem., 2019, 21, 6210–6219 | 6213

View Article Online
Paper
Green Chemistry
Fig. 5 HMF conversion and product selectivity for the electrochemical
reduction of HMF with Ag/C electrodes at various initial HMF concen-
trations. Conditions: pH 9.2 electrolyte, E = −1.2 V, 30 minutes reaction
time.
Fig. 6 Logarithm of current density versus potential for Ag/C and Ag-
pc electrodes.
and 2.51 cm2 for Ag/C and Ag-pc, respectively). This suggests
that Ag/C may have intrinsically higher catalytic activity for
HMF reduction than bulk, polycrystalline Ag; however, defini-
tive elucidation of nanoscale or particle size effects is beyond
the scope of this work. At potentials more negative than about
−1.15 V, the plots of log( j ) vs. E (Fig. 6) for both electrodes
deviated from Tafel-like behavior, indicative of mass-transport
control. Almost completely mass-transport-limited behavior
(i.e. a current “plateau”) was observed from about −1.30 V to
−1.50 V, which corresponds to the optimal potential range for
BHMF formation (Fig. 3). We previously showed that BHMF
was favored at low HMF concentrations (Fig. 5), so we hypoth-
esize that the higher BHMF selectivity observed at more nega-
tive potentials was at least partially derived from lower local
HMF concentrations at the electrode surface resulting from
mass-transport limitations. In the same way, the enhanced
BHMF selectivity for Ag/C compared to Ag-pc at mild poten-
tials (i.e. −1.15 V and −1.2 V, Fig. 3) may be due to the higher
current densities for Ag/C compared to Ag-pc, which would
result in more significant mass-transport limitations and
lower local HMF concentrations.
electrodes; BHMF selectivity increased at more negative poten-
tials down to −1.3 V for Ag/C and −1.5 V for Ag-pc (Fig. 3). The
electrochemical Volmer step (eqn (3)) is accelerated at more
negative potentials, so one explanation for the selectivity trend
is that BHMF formation was promoted by higher H* avail-
ability. Additional experiments were performed with different
initial HMF concentrations to gain more insight regarding
BHMF and BHH selectivity. Ag-pc electrodes were used instead
of Ag/C to avoid contributions of the CB support material to
the observed product selectivity. Fig. 5 shows that BHMF
selectivity at −1.2 V decreased from 81% to 41% when the
HMF concentration was increased from 5.0 mM to 50 mM. It
is likely that higher concentrations led to increased surface
coverage of HMF, and that BHMF formation via ECH was less
favorable due to insufficient availability of H* relative to
adsorbed HMF. Accordingly, another reasonable explanation
for the higher BHMF selectivity observed at more negative
potentials (Fig. 3) would be that higher degrees of HMF con-
version were achieved at those potentials, and therefore lower
bulk HMF concentrations were present. However, we con-
ducted an extended electrolysis at −1.2 V and found that
BHMF selectivity (56%) was still notably lower than at −1.3 V
2.3 ACT-mediated HMF oxidation
(i.e. 85%), even though similar HMF conversion was reached In electrochemical cells, the two electrodes are constrained to
(ESI, Table S1†).
have equal, but opposite, current flow. As a result, a major
As heterogeneous process, electrochemical HMF challenge of conducting paired electrolysis is that the operat-
a
reduction may be subject to external mass-transport limit- ing potentials for the two half-reactions cannot be indepen-
ations. In this way, the HMF concentration at the electrode dently controlled via the external circuit. We found that the
surface may be significantly lower than in the bulk electrolyte, selectivity and efficiency of Ag-catalyzed HMF hydrogenation
which could impact the selectivity of HMF reduction. Quasi- were very sensitive to the cathode potential (Fig. 3). Therefore,
steady-state current densities were measured over a wide in order to successfully pair HMF oxidation to FDCA and
potential range using conditions (i.e., reactor geometry, elec- hydrogenation to BHMF in a single cell, it is necessary to find
trode size, stirring rate) identical to electrolysis experiments. a method to facilitate selective oxidation of HMF that is insen-
Fig. 6 shows the logarithm of current density versus potential sitive to anode potential. Unfortunately, it has been shown
(log( j ) vs. E) for Ag/C and Ag-pc. For both electrodes, Tafel-like that the selectivity of electrocatalytic HMF oxidation on
behaviour (i.e. linear log( j ) vs. E relationship) was observed carbon-supported metal catalysts is highly dependent on
within the potential range of approximately −1.05 to −1.15 V, anode potential.43
indicating that HMF reduction rate was limited by charge-
An alternative approach is to use a homogeneous electroca-
transfer kinetics.42 Within this potential region, HMF talyst to facilitate indirect electrochemical HMF oxidation.32
reduction currents were 2–3 times higher on Ag/C than Ag-pc, Organic nitroxyl radical catalysts, such as TEMPO and its
even though the Ag ECSA values were very similar (i.e. 2.52 cm2 derivatives, are widely used for selective oxidation of
6214 | Green Chem., 2019, 21, 6210–6219
This journal is © The Royal Society of Chemistry 2019

View Article Online
Green Chemistry
Paper
Fig. 7 (a) Cyclic voltammogram on
a glassy carbon electrode for
different electrolyte mixtures. ACT and HMF concentrations were
1.0 mM each. (b) Schematic of the ACT-mediated electrochemical oxi-
dation of HMF.
Fig. 8 (a) Possible reaction pathways for HMF oxidation to FDCA. (b)
Conversion of HMF and selectivity of oxidation products for ACT-
mediated HMF oxidation at various potentials. Conditions: Carbon felt
electrode, pH 9.2 electrolyte with 1.0 mM ACT and 10.0 mM HMF.
Reaction time was varied to obtain the 36.1 C charge transferred for
each experiment. (c) Conversion of HMF and yield of oxidation products
during the electrochemical oxidation of HMF at 0.7 V for electrolyte
containing 1.0 mM ACT and 10.0 mM HMF.
alcohols.44,45 In particular, 4-acetamido-TEMPO (ACT) has
been identified as a very promising homogeneous electrocata-
lyst for alcohol oxidation, owing to its superior activity and
lower cost compared to other TEMPO derivitatives.46 Fig. 7a
shows the cyclic voltammogram for ACT with a glassy carbon
electrode in borate buffer electrolyte (pH 9.2). ACT exhibited
reversible one-electron oxidation/reduction waves; ACT was oxi-
dized to an oxoammonium cation (ACT+) on the anodic sweep
and subsequently reduced back to a nitroxyl radical on the
cathodic sweep. After addition of HMF to the ACT-containing
electrolyte, the anodic current was increased. This is attributed
to the regeneration of ACT following the reaction between
HMF and ACT+ (Fig. 7b). The regeneration of ACT may occur
either by the reoxidation of the ACT hydroxylamine (ACTH) or
by the comproportionation of ACT+ and ACTH.47 The cathodic
wave disappeared in the presence of HMF because ACT+ was
consumed during the HMF oxidation reaction, and therefore
not present to be electrochemically reduced. No HMF oxi-
dation current was observed in electrolyte without ACT,
demonstrating that non-mediated HMF oxidation was not
operable under these conditions.
2.4 Paired HMF hydrogenation and oxidation
Simultaneous conversion of HMF to BHMF and FDCA was
achieved in a paired electrochemical cell. The cathode poten-
tial was controlled at −1.3 V to minimize undesired hydrodi-
merization and hydrogen evolution reactions at the Ag/C elec-
trode. ACT-mediated HMF oxidation at the anode effectively
served as the counter reaction, as its potential was not con-
trolled. The cathode and anode electrolytes were separated by
an anion-exchange membrane. The initial amount of HMF in
the cathode electrolyte was three times greater than for the
anode, accounting for the stoichiometry shown in eqn (4)–(6).
Cathode : HMF þ 2H2O þ 2eꢀ ! BHMF þ 2OHꢀ
Anode : HMF þ 6OHꢀ ! FDCA þ 6eꢀ þ 4H2O
Overall : 4HMF þ 2H2O ! 3BHMF þ FDCA
ð4Þ
ð5Þ
ð6Þ
ACT-mediated HMF oxidation was conducted in an H-type
cell using a carbon felt anode. In this system, HMF is oxidized
through the non-electrochemical reaction with ACT+ in solu-
tion; therefore, HMF product selectivity is not directly depen-
dent on the anode potential. Fig. 8a shows that HMF oxidation
can proceed by two pathways, both leading to FDCA.
Fig. 9a shows that the individual yields for BHMF and
Accordingly, the selectivity of ACT-mediated HMF oxidation to FDCA reached 85% and 98%, respectively, after 72.2 C of
FDCA is mainly determined by the overall extent of reaction. charge was passed in the paired cell. The yields were very
This was demonstrated by performing the reaction at three similar to those achieved in separate (i.e. unpaired) cells,
different anode potentials while controlling the total amount which are also shown in Fig. 9a. These results show that the
of charge passed for each experiment to obtain the same two HMF half-reactions were compatible and proceeded
extent of reaction (i.e. ∼50%). In this way, HMF oxidation without severe complications or adverse effects when paired in
product distribution was largely unaffected as anode potential a single cell. A key feature of the paired electrolysis is that each
was varied between 0.7 V and 0.9 V (Fig. 8b). High HMF transferred electron participates in the generation of two
conversion and selectivity to FDCA was achieved after ACT- desired products (i.e. BHMF and FDCA). Accordingly, the com-
mediated HMF oxidation at 0.7 V was run to completion (i.e. bined electron efficiency to BHMF and FDCA was 187% for the
72.2 C of charge passed), as shown in Fig. 8c. The apparent paired cell, a nearly two-fold enhancement compared to the
reaction sequence was HMF → 2,5-diformylfuran (DFF) → unpaired cells (Fig. 9b). This is one of the first demonstrations
5-formyl-2-furoic acid (FFCA) → FDCA. The final yield and of paired HMF electrolysis and to the best of our knowledge is
faradaic efficiency for FDCA were about 97% and 98%, the highest reported combined electron efficiency for HMF
respectively.
conversion (ESI, Table S2†).
This journal is © The Royal Society of Chemistry 2019
Green Chem., 2019, 21, 6210–6219 | 6215

View Article Online
Paper
Green Chemistry
purchased from Fisher Scientific. Silver(I) nitrate (99.5%) and
lead(II) nitrate (99%) were purchased from Acros Organics. 2,5-
Bis(hydroxymethyl)furan (98%) was purchased from Ark
Pharm, Inc. 5-Formyl-2-furoic acid (FFCA, 99%) was purchased
from TCI. Silver foil (99.998%, 0.1 mm thickness) and carbon
felt (99.0%, 3.18 mm thickness) were obtained from Alfa
Aesar. Deionized water (18.2 MΩ cm) obtained from a
Barnstead E-Pure™ purification system was used to prepare all
electrolytes.
Fig. 9 (a) Yield and (b) electron efficiency of HMF conversion to BHMF
and FDCA for unpaired and paired electrochemical cells. Charge passed
was 72.2 C in each reaction. Yields for each product in the paired cell
were calculated individually with respect to the corresponding half-
reactions. Unpaired cathode reaction: Ag/C cathode at −1.3 V, 20.0 mL
of catholyte containing 20.0 mM HMF. Unpaired anode reaction: carbon
felt anode at 0.7 V, 12.5 mL of anolyte containing 1.0 mM ACT and
10.0 mM HMF. Paired cell: Ag/C cathode at −1.3 V, carbon felt anode
uncontrolled potential, 20.0 mL of catholyte containing 20.0 mM HMF
and 12.5 mL of anolyte containing 1.0 mM ACT and 10.0 mM HMF.
4.2 Catalyst synthesis
Ag nanoparticles (Ag NPs) were synthesized using a procedure
adapted from literature.35 Briefly, silver nitrate (1.5 mmol) was
dissolved in oleylamine (30 mL) and oleic acid (1.0 mL). The
solution was stirred at 350 rpm with a magnetic stir bar and
kept under nitrogen atmosphere. The solution was heated to
60 °C and held for 5 minutes to ensure the silver precursor
was completely dissolved. The solution temperature was then
ramped to 120 °C and held for 2 hours. The solution was
cooled to approximately 25 °C and 30 mL of acetone was
added. Then, the solution was divided into six tubes and
additional acetone (25 mL) was added to each tube. The mix-
tures were centrifuged for 8 minutes at 8500 rpm and then the
liquid was decanted. The Ag NPs were washed two more times
with acetone by centrifugation (8 minutes at 8500 rpm). Ag/C
was prepared by depositing Ag NPs onto carbon black (Vulcan
XC-72R, Cabot). Separately, Ag NPs were re-dispersed in
hexane (8.0 mg mL−1) and carbon black was dispersed in a
1 : 1 hexane/acetone solution (1.0 mg mL−1). For a typical syn-
thesis, 2.0 mL of the Ag NPs dispersion was added drop-wise
into 64.0 mL of carbon black dispersion under ultrasonication.
The mixture was kept under ultrasonication for 1 hour. Finally,
the Ag/C catalyst was recovered by vacuum filtration and dried
overnight in a vacuum oven at 30 °C.
3. Conclusions
This work demonstrated that HMF can be efficiently converted
to two important biobased polymer precursors, BHMF and
FDCA, in a paired electrochemical cell. Electrocatalytic hydro-
genation of HMF to BHMF was achieved using self-prepared
Ag/C as the cathode catalyst. The selectivity and efficiency for
BHMF formation were dependent on cathode potential and
bulk HMF concentration. We also showed that the carbon
support material in Ag/C was active for HMF reduction at
cathodic potentials more negative than about −1.2 V, leading
to hydrodimerization to BHH and low BHMF selectivity. A key
feature of this work was the application of ACT as a homo-
geneous electrocatalyst to facilitate indirect HMF oxidation at
the anode. The selectivity of ACT-mediated HMF oxidation was
not dependent on anode potential, which enabled us to suc-
cessfully pair HMF hydrogenation and oxidation half-reactions
in a single divided cell operated under cathode potential
control. Electrocatalytic HMF conversion in the paired cell
achieved high yields for BHMF and FDCA (85% and 98%,
respectively) and a combined electron efficiency of 187%,
corresponding to a nearly two-fold enhancement compared to
the unpaired cells. This approach shows the potential benefits
of using paired electrochemical cells for the sustainable pro-
duction of chemicals.
4.3 Catalyst characterizations
X-ray diffraction (XRD) patterns were collected with a Rigaku
Ultima IV system operated with a Cu Kα source (λ = 1.5406 Å)
at 40 kV and 44 mA and equipped with a diffracted beam
monochromator (carbon). The average crystal size was esti-
mated from the Ag (220) diffraction peak according to the
Scherrer equation,48 as shown in eqn (7):
Kλ
β cos θ
τ ¼
ð7Þ
in which τ is the mean size of the ordered crystalline domains,
K is a dimensionless shape factor (0.9), λ is the wavelength, β
is the line broadening at half the maximum intensity (FWHM)
in radians, and θ is the Bragg angle. The particle size distri-
4. Experimental
4.1 Chemicals and materials
Sodium hydroxide (97%), 4-acetamido-TEMPO (ACT, 98%), bution and morphology of Ag/C were characterized using a FEI
oleylamine (70%), oleic acid (90%), 5-(hydroxymethyl)furfural Tecnai G2-F20 200 kV instrument. XPS was carried out on a
(HMF, 99%), 2,5-furandicarboxylic acid (FDCA, 97%), 5-hydro- Kratos Amicus/ESCA 3400 X-ray photoelectron spectrometer
xymethyl-2-furancarboxylic acid (HFCA), and 2,5-diformylfuran with Mg Kα X-ray (1253.7 eV photon energy). All spectra were
(DFF, 97%) were purchased from Sigma Aldrich. Acetone calibrated with the C 1s peak at 284.8 eV as reported in our
(99.8%), hexanes (99.9%), 2-propanol (99.9%), boric acid recent work.49 Thermal gravimetric analysis (TGA) was per-
(100%) and buffer standard solutions (pH 7.0 and 10.0) were formed with a TA Instruments Discovery Thermal Gravimetric
6216 | Green Chem., 2019, 21, 6210–6219
This journal is © The Royal Society of Chemistry 2019

View Article Online
Green Chemistry
Paper
Analyzer using a temperature ramp of 10 °C min−1 and an air current density values at −1.5 V. The electron transfer number
flow of 100 mL min−1. The Ag loading for the Ag/C catalyst was for HMF reduction, n, was extracted from the slope of the
determined as the weight percent remaining after TGA. Koutecký–Levich plot, defined by eqn (8):
Inductively coupled plasma-optical emission spectroscopy
Slope ¼ ð0:201nFD2=3
υ
ꢀ1=6CÞꢀ1
ð8Þ
(ICP-OES, Perkin Elmer Optima 8000 instrument) was used to
determine the Ag concentration in the cathode electrolyte after
reaction. Raman spectra were collected with an XploRA Plus
confocal Raman microscope (Horiba Scientific, Edison, NJ)
using a 532 nm excitation source at 1.25 mW and a 50× (N.A. =
0.5) working distance objective. Spectra were collected for 60
seconds with three accumulations for five different spots on
the sample.
in which F is the Faraday constant (96 485.3 C mol−1), D is the
diffusion coefficient of HMF in water (9.169 × 10−6 cm2 s−1),50
υ is the kinematic viscosity of water (0.01 cm2 s−1), and C is the
HMF concentration (20.0 mM).
4.6 Electrolysis of HMF
Electrocatalytic hydrogenation was performed in an H-type
cell. Anode and cathode chambers were separated with an
anion-exchange membrane (Tokuyama Corp., A201). The
cathode electrolyte was purged with argon gas (99.999%,
Airgas, Inc.) throughout the reaction to remove dissolved O2
and evolved H2. Electrolysis was conducted using 20.0 mL of
electrolyte containing HMF (typically 20.0 mM) in the cathode.
The cathode electrolyte was stirred with a PTFE-coated mag-
netic bar (size: 7/8″ × 3/16″) at 500 rpm. A graphite rod counter
electrode was used as the anode.
ACT-mediated HMF oxidation was performed in an H-type
cell with a similar configuration. The anode electrode was
carbon felt with dimensions of 1.5 cm by 1.0 cm exposed to
the electrolyte. Ag-pc served as the counter electrode to facili-
tate the hydrogen evolution reaction. Carbon felt electrodes
were washed with acetonitrile to remove organic residues and
then rinsed with deionized water before each use. Unless
noted otherwise, the anode electrolyte volume was 12.5 mL
and contained 10.0 mM HMF and 1.0 mM ACT. The anode
electrolyte was purged with argon gas (99.999%, Airgas, Inc.)
throughout the reaction, and was stirred with a small PTFE-
coated stir bar at 500 rpm.
4.4 Electrochemical measurements
Electrochemical tests were performed with a BioLogic SP-300
electrochemical workstation. The reference electrode was a
single-junction Ag/AgCl (Pine Research Instrumentation) for
all experiments. Solution resistance was determined by poten-
tiostatic electrochemical impedance spectroscopy and com-
pensated at 90% by the electrochemical workstation. All
current density values are reported with respect to geometric
surface area. The electrolyte was a sodium borate buffer (0.5
M) prepared from boric acid and sodium hydroxide, adjusted
to pH 9.2, as measured by a pH probe (Hanna HI98103).
4.5 Cyclic voltammetry and Koutecký–Levich analysis
Cyclic voltammetry (CV) for HMF reduction was performed for
a polycrystalline silver (Ag-pc) disk electrode (5.0 mm dia-
meter, Pine Research Instrumentation) and for glassy carbon
disk electrodes (GCE, 5.0 mm diameter, Pine Research
Instrumentation) modified with Ag/C or carbon black (CB).
CVs for ACT-mediated HMF oxidation were performed on a
bare GCE. Before use, the disk electrodes were polished with
an alumina suspension (0.3 µm, Allied High Tech Products,
Inc.) on a microcloth polishing disk (Buehler) and cleaned
with deionized water in an ultrasonic bath. Ink was prepared
by dispersing Ag/C or CB in a solution of isopropanol and de-
The paired HMF electrolysis was performed in an H-type
cell that combined the cathode configuration previously
described for HMF hydrogenation and the anode configur-
ation described for HMF oxidation. The paired cell was oper-
ated by controlling the cathode potential to be −1.3 V.
ionized water (1 : 1 v/v) at a concentration of 1.0 mg mL−1
.
Nafion solution (5% w/w, Ion Power) was added to the ink to
achieve a Nafion loading of 10% w/w in the dry catalyst film.
The ink dispersion was mixed ultrasonically and then drop-
cast on the GCE. The volume of Ag/C ink drop-cast onto the
GCE was 8.0 μL, which was chosen to give similar Ag electro-
chemically-active surface area (ECSA) as the Ag-pc disk elec-
trode. Details of Ag ECSA determination are included in
section 4.9. The volume of CB ink drop-cast onto the GCE was
6.4 μL. The counter electrode was a platinum coil, separated
4.7 Quasi-steady-state current measurements
Quasi-steady-state current densities were measured by staircase
voltammetry in an H-type cell (pH 9.2, 20.0 mM HMF). Each
potential step was held for 15 seconds and the mean current
measured over the last 10 seconds of each step was reported.
4.8 Electrode preparation for electrolysis
from the main electrolyte with a fritted glass tube. The electro- Ag foil served as the Ag-pc electrode for electrolysis following a
chemical cell was purged with nitrogen gas before and during cleaning sequence of isopropanol, DI water, 0.5 M hydro-
measurements. Cyclic voltammograms were collected with a chloric acid and DI water. The Ag foil was masked off to
50 mV s−1 sweep rate.
expose a 1 cm by 1 cm region, giving a geometric area of 2 cm2
Linear sweep voltammograms for HMF reduction were col- after accounting for the front and back sides. The Ag/C and CB
lected at various electrode rotation rates (ω, RPM) using a electrodes for electrolysis were prepared by drop-casting ink
modulated speed rotator (Pine Research Instrumentation) for dispersions onto carbon paper. The carbon paper electrodes
Koutecký–Levich analysis. Koutecký–Levich plots (1/j versus were masked off to a geometric area of 2 cm2 in the same
ω−0.5
) were constructed using the background-corrected manner as the Ag foil. Ink was prepared by dispersing Ag/C or
This journal is © The Royal Society of Chemistry 2019
Green Chem., 2019, 21, 6210–6219 | 6217

View Article Online
Paper
Green Chemistry
CB in a solution of isopropanol and deionized water (9 : 1 v/v) Professorship; and is grateful to Ting-Han Lee, Hengzhou Liu,
at a concentration of 1.0 mg mL−1. Nafion (5% w/w, Ion Power) Prof. J. P. Tessonnier, and Prof. Eric Cochran from Iowa State
was added to the ink with a target loading with a catalyst to University for fruitful discussions. We are grateful to Dr Tao
Nafion ratio of 9 : 1 w/w. The ink dispersion was sonicated for Ma from the Sensitive Instrument Facility at Department of
3 minutes immediately before being drop-cast. The volume of Energy Ames Laboratory, Dr Yang Qiu, and Yifu Chen for
Ag/C ink drop-cast onto carbon paper was 129.6 μL, which was assistance with TEM. We thank Prof. Emily Smith and Jingzhe
chosen to give similar Ag ECSA as the Ag-pc electrode (ESI, Li of Iowa State University for collecting Raman spectra. We
Fig. S13 and 14†).
appreciate Dr Patrick Johnston, Dr Dapeng Jing, and Dr Scott
Schlorholtz from Iowa State University for conducting ICP-
OES, XPS and TGA measurements, respectively.
4.9 Determination of Ag ECSA
Stripping voltammetry of PbUPD was performed for Ag-pc and
Ag/C electrodes to estimate the Ag ECSA. The electrolyte was
0.5 M borate buffer solution containing 125 μM lead nitrate.
The electrolyte was purged with nitrogen before and during
experiments. For Ag/C, Pb UPD was conducted by holding
−0.55 V for 2 min. The PbUPD stripping voltammogram was
collected immediately after deposition by sweeping from the
deposition potential to 0.0 V at 20 mV s−1. The same procedure
was used for Ag-pc, except the deposition potential was
changed to −0.5 V to avoid bulk Pb deposition. The Ag ECSA
was determined using eqn (9):
References
1 B. A. Frontana-Uribe, R. D. Little, J. G. Ibanez, A. Palma
and R. Vasquez-Medrano, Green Chem., 2010, 12, 2099–2119.
2 E. J. Horn, B. R. Rosen and P. S. Baran, ACS Cent. Sci., 2016,
2, 302–308.
3 M. J. Janik, S. A. Wasileski, C. D. Taylor and M. Neurock, in
Fuel Cell Catalysis, ed. M. T. M. Koper, John Wiley & Sons,
Inc., Hoboken, New Jersey, 2009, ch. 4, pp. 93–128.
4 L. Xin, Z. Zhang, J. Qi, D. Chadderdon and W. Li, Appl.
Catal., B, 2012, 125, 85–94.
5 Z. Zhang, L. Xin, J. Qi, D. J. Chadderdon, K. Sun,
K. M. Warsko and W. Li, Appl. Catal., B, 2014, 147, 871–878.
6 B. H. Nguyen, R. J. Perkins, J. A. Smith and K. D. Moeller,
Beilstein J. Org. Chem., 2015, 11, 280–287.
7 B. H. Nguyen, A. Redden and K. D. Moeller, Green Chem.,
2014, 16, 69–72.
Qstrip
ECSA ¼
ð9Þ
q
in which Qstrip is the charge integrated from the anodic strip-
ping peak, and q is the charge density (0.26 mC cm−2) for
PbUPD on Ag from literature.36 Anodic stripping voltammo-
grams are shown in Fig. S13 in the ESI.† The Ag/C loadings on
GCE and carbon paper electrodes were chosen to give roughly
equivalent Ag ECSA as the Ag-pc disk and foil electrodes,
respectively. Table S3 in the ESI† provides a summary of PbUPD
stripping charges and calculated ECSA values for electrodes
used for voltammetry and electrolysis experiments.
8 F. Harnisch and C. Urban, Angew. Chem., Int. Ed., 2018, 57,
10016–10023.
9 D. S. P. Cardoso, B. Šljukić, D. M. F. Santos and
C. A. C. Sequeira, Org. Process Res. Dev., 2017, 21, 1213–1226.
10 Y. Kwon, E. de Jong, S. Raoufmoghaddam and
M. T. M. Koper, ChemSusChem, 2013, 6, 1659–1667.
11 Y. Kwon, Y. Y. Birdja, S. Raoufmoghaddam and
M. T. M. Koper, ChemSusChem, 2015, 8, 1745–1751.
12 F. J. Holzhäuser, J. Artz, S. Palkovits, D. Kreyenschulte,
J. Büchs and R. Palkovits, Green Chem., 2017, 19, 2390–2397.
13 J. J. Roylance, T. W. Kim and K.-S. Choi, ACS Catal., 2016, 6,
1840–1847.
4.10 Product analysis
HMF hydrogenation and oxidation products were analyzed by
high-performance liquid chromatography (HPLC). Evolved H2
gas was quantified with a gas chromatography connected to
1
the outlet of the cathode chamber. BHH was identified by H
NMR and HSQC and quantified by HPLC. Two isomers of
BHH are reported together for simplicity. Details of product
analysis and calculations for selectivity, faradaic efficiency,
and combined electron efficiency are provided in the ESI.†
14 X. H. Chadderdon, D. J. Chadderdon, J. E. Matthiesen,
Y. Qiu, J. M. Carraher, J. P. Tessonnier and W. Li, J. Am.
Chem. Soc., 2017, 139, 14120–14128.
15 M. Suastegui, J. E. Matthiesen, J. M. Carraher,
N. Hernandez, N. R. Quiroz, A. Okerlund, E. W. Cochran,
Z. Shao and J.-P. Tessonnier, Angew. Chem., Int. Ed., 2016,
55, 2368–2373.
Conflicts of interest
The authors claim no conflicts of interest.
16 U. Sanyal, J. Lopez-Ruiz, A. B. Padmaperuma, J. Holladay
and O. Y. Gutiérrez, Org. Process Res. Dev., 2018, 22, 1590–
1598.
17 J. Lessard, in Organic Electrochemistry, ed. O. Hammerich
and B. Speiser, CRC Press, Boca Raton, FL, 5th edn, 2015,
pp. 1658–1664.
Acknowledgements
This research was funded by NSF-CBET 1512126 and Bailey
Research Career Development Award. W. L. acknowledges the
Iowa State University Start-Up Fund, Ames Laboratory Start-Up 18 P. Nilges and U. Schröder, Energy Environ. Sci., 2013, 6,
Fund, Iowa Energy Center, and Richard Seagrave
2925–2931.
6218 | Green Chem., 2019, 21, 6210–6219
This journal is © The Royal Society of Chemistry 2019

View Article Online
Green Chemistry
Paper
19 J. G. Ibanez, B. A. Frontana-Uribe and R. Vasquez-Medrano, 35 S. Peng, J. M. McMahon, G. C. Schatz, S. K. Gray and
J. Mex. Chem. Soc., 2016, 60, 247–260.
20 M. Tahir, L. Pan, F. Idrees, X. Zhang, L. Wang, J.-J. Zou and
Z. L. Wang, Nano Energy, 2017, 37, 136–157.
21 M. J. Llorente, B. H. Nguyen, C. P. Kubiak and
K. D. Moeller, J. Am. Chem. Soc., 2016, 138, 15110–15113.
22 R. S. Sherbo, R. S. Delima, V. A. Chiykowski, B. P. MacLeod
and C. P. Berlinguette, Nat. Catal., 2018, 1, 501–507.
23 P. Zhang, X. Sheng, X. Chen, Z. Fang, J. Jiang, M. Wang,
Y. Sun, Proc. Natl. Acad. Sci. U. S. A., 2010, 107, 14530–
14534.
36 E. Kirowa-Eisner, Y. Bonfil, D. Tzur and E. Gileadi,
J. Electroanal. Chem., 2003, 552, 171–183.
37 Y. Kwon, E. d. Jong, S. Raoufmoghaddam and
M. T. M. Koper, ChemSusChem, 2013, 6, 1659–1667.
38 B. E. Conway and B. V. Tilak, Electrochim. Acta, 2002, 47,
3571–3594.
F. Li, L. Fan, Y. Ren, B. Zhang, B. J. J. Timmer, 39 A. J. Bard, J. Am. Chem. Soc., 2010, 132, 7559–7567.
M. S. G. Ahlquist and L. Sun, Angew. Chem., Int. Ed., 2019, 40 Y. Zheng, Y. Jiao, L. H. Li, T. Xing, Y. Chen, M. Jaroniec and
58, 9155–9159.
S. Z. Qiao, ACS Nano, 2014, 5, 5290–5296.
24 G. Chamoulaud, D. Floner, C. Moinet, C. Lamy and 41 M. R. Singh, J. D. Goodpaster, A. Z. Weber, M. Head-
E. M. Belgsir, Electrochim. Acta, 2001, 46, 2757–2760.
25 R. A. Sheldon, Green Chem., 2014, 16, 950–963.
Gordon and A. T. Bell, Proc. Natl. Acad. Sci. U. S. A., 2017,
114, E8812–E8821.
26 J. N. Chheda, Y. Roman-Leshkov and J. A. Dumesic, Green 42 A. J. Bard and L. R. Faulkner, Electrochemical Methods:
Chem., 2007, 9, 342–350.
27 J. M. R. Gallo, D. M. Alonso, M. A. Mellmer and
J. A. Dumesic, Green Chem., 2013, 15, 85–90.
Fundamentals and Applications, John Wiley & Sons, Inc.,
New York, 2nd edn, 2001.
43 D. J. Chadderdon, L. Xin, J. Qi, Y. Qiu, P. Krishna,
K. L. More and W. Li, Green Chem., 2014, 16, 3778–3786.
28 Y. Jiang, X. Wang, Q. Cao, L. Dong, J. Guan and X. Mu, in
Sustainable Production of Bulk Chemicals, ed. M. Xian, 44 A. E. J. d. Nooy, A. C. Besemer and H. v. Bekkum, Synthesis,
Springer, 2016. 1996, 1153–1176.
29 Y. Kwon, K. J. P. Schouten, J. C. van der Waal, E. de Jong 45 D. J. Chadderdon, L.-P. Wu, Z. A. McGraw, M. Panthani and
and M. T. M. Koper, ACS Catal., 2016, 6, 6704–6717. W. Li, ChemElectroChem, 2019, 6, 3387–3392.
30 C. Moreaua, M. N. Belgacemb and A. Gandini, Top. Catal., 46 M. Rafiee, K. C. Miles and S. S. Stahl, J. Am. Chem. Soc.,
2004, 27, 11–30. 2015, 137, 14751–14757.
31 A. Gandini, A. J. D. Silvestre, C. P. Neto, A. F. Sousa and 47 R. Ciriminna, M. Ghahremani, B. Karimi and M. Pagliaro,
M. Gomes, J. Polym. Sci., Part A: Polym. Chem., 2009, 47,
295–298.
32 H. G. Cha and K.-S. Choi, Nat. Chem., 2015, 7, 328–333.
33 X. H. Chadderdon, PhD thesis, Iowa State University, 2018.
34 S. Li, X. Sun, Z. Yao, X. Zhong, Y. Cao, Y. Liang, Z. Wei,
ChemistryOpen, 2017, 6, 5–10.
48 C. Suryanarayana and M. G. Norton, X-Ray Diffraction,
Springer, Boston, MA, 1998.
49 K. Kim, Y. Chen, J.-I. Han, H. C. Yoon and W. Li, Green
Chem., 2019, 21, 3839–3845.
S. Deng, G. Zhuang, X. Li and J. Wang, Adv. Funct. Mater., 50 K. Yui, N. Yamazaki and T. Funazukuri, J. Chem. Eng. Data,
2019, 1904780.
2012, 58, 183–186.
This journal is © The Royal Society of Chemistry 2019
Green Chem., 2019, 21, 6210–6219 | 6219