Mechanisms of Furfural Reduction on Metal Electrodes: Distinguishing Pathways for Selective Hydrogenation of Bioderived Oxygenates

Article abstract of DOI:10.1021/jacs.7b06331

Electrochemical reduction of biomass-derived platform molecules is an emerging route for the sustainable production of fuels and chemicals. However, understanding gaps between reaction conditions, underlying mechanisms, and product selectivity have limited the rational design of active, stable, and selective catalyst systems. In this work, the mechanisms of electrochemical reduction of furfural, an important biobased platform molecule and model for aldehyde reduction, are explored through a combination of voltammetry, preparative electrolysis, thiol-electrode modifications, and kinetic isotope studies. It is demonstrated that two distinct mechanisms are operable on metallic Cu electrodes in acidic electrolytes: (i) electrocatalytic hydrogenation (ECH) and (ii) direct electroreduction. The contributions of each mechanism to the observed product distribution are clarified by evaluating the requirement for direct chemical interactions with the electrode surface and the role of adsorbed hydrogen. Further analysis reveals that hydrogenation and hydrogenolysis products are generated by parallel ECH pathways. Understanding the underlying mechanisms enables the manipulation of furfural reduction by rationally tuning the electrode potential, electrolyte pH, and furfural concentration to promote selective formation of important biobased polymer precursors and fuels.

Full text of DOI:10.1021/jacs.7b06331

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Article
Mechanisms of Furfural Reduction on Metal Electrodes: Distinguishing
Pathways for Selective Hydrogenation of Bioderived Oxygenates
Xiaotong H. Chadderdon, David J Chadderdon, John E Matthiesen,
Yang Qiu, Jack M. Carraher, Jean-Philippe Tessonnier, and Wenzhen Li
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06331 • Publication Date (Web): 13 Sep 2017
Downloaded from http://pubs.acs.org on September 17, 2017
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Mechanisms of Furfural Reduction on Metal Electrodes: Distin-
guishing Pathways for Selective Hydrogenation of Bioderived Oxy-
genates
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Xiaotong H. Chadderdon,a,b,‡ David J. Chadderdon,a,b,‡ John E. Matthiesen,a,b,c Yang Qiu,a,b Jack
M. Carraher,a,c Jean-Philippe Tessonnier,a,b,c Wenzhen Lia,b,*
aDepartment of Chemical and Biological Engineering, Iowa State University, 618 Bissell Road, Ames, Iowa 50011,
United States
bUS Department of Energy Ames Laboratory, 2408 Pammel Drive, Ames, Iowa 50011, United States
cNSF Engineering Research Center for Biorenewable Chemicals (CBiRC), 617 Bissell Road, Ames, Iowa 50011, United
States
Supporting Information Placeholder
carbonyl groups, which are prevalent in important biobased
platform chemicals, may yield alcohols by hydrogenation with
two Hads, or alkyls through hydrogenation and hydrogenolysis
reactions with four Hads (Scheme 1). However, ECH is often in
competition with the hydrogen evolution reaction (HER),
which consumes Hads through Tafel or Heyrovsky reactions
and lowers the faradaic efficiency to ECH.5
ABSTRACT: Electrochemical reduction of biomass-derived
platform chemicals is an emerging route for the sustainable
production of fuels and chemicals. However, understanding
gaps between reaction conditions, underlying mechanisms,
and product selectivity have limited the rational design of ac-
tive, stable, and selective catalyst systems. In this work, the
mechanisms of electrochemical reduction of furfural, an im-
portant biobased platform molecule and model for aldehyde
reduction, are explored through a combination of voltamme-
try, preparative electrolysis, thiol-electrode modifications,
and kinetic isotope studies. It is demonstrated that two dis-
tinct mechanisms are operable on metallic Cu electrodes in
acidic electrolytes: (i) electrocatalytic hydrogenation (ECH)
and (ii) direct electroreduction. The contributions of each
mechanism to the observed product distribution are clarified
by evaluating the requirement for direct chemical interactions
with the electrode surface and the role of adsorbed hydrogen.
Further analysis reveals that hydrogenation and hydrogenoly-
sis products are generated by parallel ECH pathways. Under-
standing the underlying mechanisms enables the manipula-
tion of furfural reduction by rationally tuning the electrode
potential, electrolyte pH, and furfural concentration to pro-
mote selective formation of important biobased polymer pre-
cursors and fuels.
Scheme 1. Proposed pathways of electrochemical re-
duction of carbonyls in acidic electrolytes.
+
H
2O
ECH
R'
R"
4Hads
2Hads
OH
R"
O
2Hads
R'
R'
R"
e-
e-
H+
,
H+
,
H
OH
O OH
R" R"
Dim
R'
R'
Electroreduction
Volmer
R"
R'
e-
H+
Hads
reaction:
A significant challenge of performing selective electrocat-
alytic reductions is the coexistence of direct electroreduction
routes (Scheme 1), in which carbonyls participate in electron
transfer at the electrode and protonations occur in solution.6
The reaction with one H+/e- pair generates a radical interme-
diate (C•–OH), which may either dimerize through C–C cou-
pling with a second radical, or be further converted by another
H+/e- to yield the alcohol product.6 Alternatively, equal
amounts of aldehydes and alcohols may be formed by the dis-
proportionation of C•–OH. The preference for ECH or electro-
reduction routes is largely determined by the relative poten-
tials required for Hads and C•–OH formation. As a result, ECH
is strongly preferred on low hydrogen overpotential electrodes
(e.g. platinum-group metals), whereas electroreduction is pre-
ferred on high hydrogen overpotential electrodes such as Pb,
Hg, Cd, and graphite.1 However, the two routes may be in
INTRODUCTION
Electrocatalytic hydrogenation (ECH) is emerging as an
environmentally-friendly approach for the selective reduction
of multifunctional chemicals. ECH is analogous to conven-
tional thermocatalytic hydrogenation, with the key difference
that adsorbed hydrogen (Hads) is electrochemically generated
in situ on the electrode surface by proton or water reduction
(Volmer reaction) rather than through the dissociation of mo-
lecular H2.1 In this way, the kinetic barriers for H2 activation
are avoided and hydrogenations can be performed without the
need for external H2 supply and at mild conditions.2-4 ECH of
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competition on electrodes with intermediate hydrogen over-
tential Pb electrode indicate that Hads is required for hydro-
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potentials (e.g. Ni, Co, Fe, Cu, Ag and Au), which is a compli-
cating factor for mechanistic studies. In particular, it is not
straightforward to determine the pathway of alcohol for-
mation, which may occur through either ECH or electroreduc-
tion routes.
genation and hydrogenolysis reactions, namely by the ECH
mechanisms. A pathway study reveals that further reduction
of FA to MF is not a significant contribution to the MF ob-
served during the electrolysis of furfural, and instead those
products are likely formed by parallel reactions. Finally, the
reactions are manipulated by applying knowledge of the un-
derlying mechanisms and tuning reaction conditions to pro-
mote selective formation of important chemicals for biobased
polymers and fuels synthesis. The techniques used in this
work to distinguish different mechanisms are not uniquely ap-
plicable to furfural reduction, and can potentially be extended
to study other important electrochemical reductions.
Biomass-derived oxygenates have the potential to replace
fossil resources as feedstocks for the sustainable production of
fuels and chemicals.7-8 Currently, there is significant interest
in the electrochemical conversion of furanic compounds such
as furfural and 5-hydroxymethylfurfural (HMF), owing to their
versatility as platform chemicals for production of polymers,
fine chemicals, and biofuels, and their availability from bio-
mass by the acid-catalyzed dehydration of pentose and hexose
sugars, respectively.9-10 In particular, the selective hydrogena-
tion of the aldehyde groups in furfural or HMF generates fur-
furyl alcohol (FA) or 2,5-bis(hydroxymethyl)furan (BHMF),
respectively (Scheme 2), which are precursors for production
of polymers, resins, and chemicals.11 Selective hydrogenolysis
generates 2-methylfuran (MF) or 2,5-dimethylfuran (DMF),
which are potential liquid transportation fuels.12-13 It has been
demonstrated that the selectivity to hydrogenation or hydro-
genolysis products is influenced by the nature of the catalysts,
applied potential or current density, electrolyte pH, and initial
reactant concentration.14-20 However, much of the recent liter-
ature has focused on proof-of-concepts, and there remain un-
derstanding gaps between reaction conditions, underlying
mechanisms, and observed product selectivity. Notably, it re-
mains unclear whether hydrogenations occur by Hads at the
electrode surface (ECH) or by H+ in solution (electroreduc-
tion).9, 18
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METHODS
General considerations. Furfural (99%), furfuryl alcohol
(98%), 2-methylfuran (99%), 3-mercaptopropionic acid
(99%), 2-mercaptobenzothiazole (99%), 12-mercaptododeca-
noic acid (96%), sulfuric acid-d3 (95–98%, 99.5 atom% D),
deuterium oxide (D2O, 99.9 atom% D), and sodium sulfate
(99%), were purchased from Sigma Aldrich. Acetonitrile
(CH3CN, 99.9% “HPLC grade”), 2-propanol (99.9%), hydro-
chloric acid (37%), sulfuric acid (98%), and buffer standard so-
lutions (pH 4.00 and 7.00) were purchased from Fisher Scien-
tific. Acetonitrile-d3 (99.8 atom% D) was obtained from Cam-
bridge Isotope Laboratories, Inc. Water was deionized (18.2
MΩ • cm) with a Barnstead E-Pure™ purification system and
used to prepare all electrolytes. Typical electrolytes were 0.5
M sulfuric acid (pH 0.5) or 0.5 M sulfate solutions (pH 1.4–3.0)
with 25% v/v CH3CN cosolvent. Electrolytes in deuterated sol-
vents were prepared with D2SO4, adjusted to the desired pD
according to the widely accepted formula21: pD = pH* + 0.40,
in which pH* is the reading measured in D2O solution with a
pH meter calibrated in conventional aqueous buffers.
Electrochemical measurements. Electrochemical meas-
urements were performed with a Bio-Logic SP-300 electro-
chemical workstation. A double-junction Ag/AgCl (Pine Re-
search Instrumentation) and a graphite rod (Pine Research In-
strumentation) were used as the reference and counter elec-
trodes, respectively. The reference electrode was calibrated
against a reversible hydrogen electrode (RHE, eDAQ) and all
potentials herein are reported on the RHE reference scale.
Electrolyte pH was measured with a handheld pH meter
(Hanna HI98103) calibrated in standard aqueous buffer solu-
tions. Solution resistance between working and reference
electrodes was measured by potentiostatic electrochemical
impedance spectroscopy and compensated (85%) by the elec-
trochemical workstation.
Cyclic voltammetry was performed on a Cu rotating disk
electrode (RDE, 5.0 mm diameter, Pine Research Instrumen-
tation) at 2500 rpm. The RDE was polished with an alumina
suspension (0.3 µm, Allied High Tech Products, Inc.) on a mi-
crocloth polishing disk (Buehler) and cleaned with DI water
in an ultrasonic bath before each use. The electrolyte was
purged with nitrogen gas before and during measurements.
Cyclic voltammograms were collected using a 50 mV s-1 sweep
rate.
Preparative electrolysis was performed in an H-cell reactor
with anode and cathode chambers separated by a Nafion® 212
proton exchange membrane (PEM). Cathode electrolyte was
purged with argon gas (99.999%, Airgas, Inc.) throughout the
reaction to remove dissolved gases and evolved H2, and to
Scheme 2. Key hydrogenation and hydrogenolysis
products of furfural and HMF.
O
OH
O
O
O
+4H
-H
+2H
2O
furfuryl alcohol
_(FA)_
furfural
2-methylfuran
_(MF)_
H
(C5
4O2
)
O
OH
O
O
O
+6H
+2H
HO
HO
-2H2O
HMF
2,5-dimethylfuran
_(DMF)_
2,5-bis(hydroxymethyl)furan
_(BHMF)_
H
6O3
(C6
)
Herein, we investigate the mechanisms for electrochemi-
cal reduction of the aldehyde functionality of furfural in acidic
electrolytes in order to elucidate the interplay between ECH
and electroreduction routes, and their effects on observed
product selectivies. Cu was selected as the electrode material
because of its unique ability to generate hydrogenolysis prod-
ucts with high selectivity.14 We demonstrate that both the
ECH and electroreduction mechanisms are operable on Cu,
leading to formation of FA, MF, and the dimer product hydro-
furoin. Electrochemical measurements on electrodes modified
with organothiol self-assembled monolayers (SAMs) provide
strong evidence that hydrogenation and hydrogenolysis reac-
tions require direct chemical interaction with the electrode
surface, whereas hydrofuroin formation is relatively insensi-
tive to the nature of surface. Moreover, observed H/D isotope
effects, behavior under proton mass-transport-limited condi-
tions, and a comparative study with a high hydrogen overpo-
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strip the volatile product MF into a two-chamber cold trap
terminated alkanethiols with different chain lengths (C3 and
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filled with CH3CN and cooled to –10 °C. Cu foils (0.127 mm
thick, 99.9%, Alfa Aesar) were used as electrodes for prepara-
tive electrolysis immediately after pretreatment by a cleaning
sequence of 2-propanol, deionized water, dilute hydrochloric
acid, and deionized water. Pb foils (0.76 mm thick, 99.8%, Alfa
Aesar) were mechanically polished with 600 grit sandpaper
and cleaned with laboratory tissue. Unless noted otherwise,
the exposed geometric surface area of electrodes was main-
tained at 5.0 cm2, accounting for both the front and back sides.
Preparative electrolysis was performed in 20 ml of aqueous
electrolyte containing 0.5 M sulfuric acid (approximately pH
0.5) or 0.5 M sulfate solutions (pH 1.4–3.0) with 25% v/v
CH3CN cosolvent and stirred at 1000 rpm with a magnetic stir
bar. For studying the effect of furfural initial concentration, a
short reaction time of 30 min was used to make comparisons
at low degrees of conversion and relatively constant bulk fur-
fural concentrations. All reactions were performed at room
temperature (23 ± 1 °C).
Product analysis. After preparative electrolysis reactions,
liquid aliquots were collected from the reactor chamber and
cold trap and diluted in water or CH3CN for analysis by high
pressure liquid chromatography (HPLC). Hydrogen gas was
quantified with a gas chromatograph connected to the outlet
of the cold trap. Hydrofuroin (1,2-di(furan-2-yl)ethane-1,2-
diol) was identified by 1H NMR and mass spectrometry (MS),
and quantified by HPLC. Two isomers of hydrofuroin are re-
ported together for simplicity. Product analysis details and
calculations of selectivity and faradaic efficiency are included
in Supporting Information.
C12), namely 3-mercaptopropionic acid (MPA) and 12-mercap-
tododecanoic acid (MDA) and the heterocyclic 2-mercapto-
benzothiazole (MBT). Electron tunneling resistance has an ex-
ponential dependence on tunneling distance, which is deter-
mined by the molecular length of the SAMs.32 As a result,
outer-sphere ET processes are suppressed on surfaces blocked
by long-chain linear alkanethiols (e.g. MDA, C12) SAMs,
whereas they are essentially unaffected on surfaces blocked by
short-chains (e.g. MPA, C3).24 Furthermore, SAMs of conju-
gated molecules like MBT may be particularly effective at
blocking surface sites and inner-sphere ET, while still allowing
electron tunneling (outer-sphere ET).33-36
HER can serve as a probe reaction to evaluate the quality
of the SAMs and their effectiveness to inhibit ET.37-38 Cyclic
voltammetry revealed a significant decrease in HER and dou-
ble-layer charging (capacitive) currents on thiol-modified Cu
electrodes (Figure 1a), indicating that the SAMs inhibited ac-
cess to electrochemically-active Cu surface sites. The degree
of current suppression was in the order of MPA < MBT ≪
MDA, with MDA nearly completely blocking both faradaic
and non-faradaic processes. Longer-chain thiols, such as
MDA, assemble into complete and impenetrable SAMs owing
to their stronger van der Waals intermolecular attractions,39
and as noted above their SAMs are of sufficient thickness to
inhibit electron tunneling. Layers of MPA and MBT would be
expected to completely suppress inner-sphere reactions, so
the fact that HER current was observed suggests that those
films are incomplete or contain pinhole defects which allow
access to some active metal sites. MBT was more effective at
inhibiting HER than MPA, likely owing to the stronger
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RESULTS AND DISCUSSION
Distinguishing ECH and electroreduction mechanisms.
The first challenge in bridging the understanding gaps be-
tween mechanisms, reaction conditions, and product selectiv-
ity for the electrochemical reduction of carbonyl groups is to
clarify which mechanisms are responsible for hydrogenation,
hydrogenolysis, and hydrodimerization products. Electrodes
modified with SAMs of organothiols were employed to exper-
imentally determine the nature of electrode processes respon-
sible for formation of each product. ECH involves strong in-
teraction of reacting species with the electrode surface and re-
quires Hads as the hydrogen source. The electron transfer (ET)
for Hads formation is defined as an inner-sphere electrode pro-
cess, and as such is highly dependent on the electrode surface
properties.5 However, for the electroreduction mechanism,
protonations occur in solution, and the heterogeneous ET
may proceed by either inner-sphere or outer-sphere processes.
Outer-sphere reactions do not require strong interactions be-
tween reactants and the electrode surface and ET occurs by
electron tunneling.22-23 Therefore outer-sphere reactions are
generally less dependent on the nature of the electrode mate-
rial. Organic SAMs inhibit inner-sphere ET and ECH reactions
by preventing free diffusion of electroactive species and their
direct access to the electrode surface, but have only a small
effect on outer-sphere ET rates if the layers are sufficiently
thin for facile electron tunneling.24-26 For the electroreduction
pathway, the reduction of furfural is expected to be unaffected
by such SAM-modifications if it occurs by an outer-sphere ET
process.
Figure 1. (a) Cyclic voltammograms in 0.5 M H2SO4 with or
without the addition of 0.25 mM MPA, MBT, or MDA and (b)
with 0.05 M furfural on a Cu RDE. (c) Observed production
rates during preparative electrolysis of furfural on Cu, Cu-
MPA, Cu-MBT, and Cu-MDA electrodes. Conditions: 0.05 M
furfural and 0.25 mM of the indicated organothiol, pH 0.5
electrolyte, 1 h duration, E = –0.55 V.
Three organothiols that readily form compact SAMs on
Cu27-31 were selected to modify the electrodes: two carboxyl-
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intermolecular attractions of its aromatic structure.40 The
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2
3
4
same trend of current suppression was observed in the pres-
ence of furfural (Figure 1b), demonstrating that the thiol-mod-
ified electrodes also reduced the overall rate of furfural reduc-
tion.
Constant-potential preparative electrolysis of furfural con-
ducted in 0.5 M H2SO4 electrolyte confirmed that both furfu-
ral reduction and HER were inhibited by the SAMs. Moreover,
the distribution of furfural reduction products was very sensi-
tive to surface modifications, as shown in Figure 1c. FA and MF
production rates were severely suppressed on thiol-modified
electrodes; for example, FA decreased from 64.5 to 2.7 μmol h-
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1
and MF decreased from 399.0 to <0.1 μmol h-1 on Cu-MBT
compared to unmodified Cu. Similar results were obtained in
a pH 3.0 electrolyte, in which FA and MF were both sup-
pressed on SAM-modified electrodes (Figure S2). Only trace
amounts of furfural reduction products or H2 were detected
on the MDA-Cu electrode, confirming that C12 thiol SAMs
block nearly all the electrode reactions. These results indicate
that hydrogenation and hydrogenolysis pathways to FA and
MF require direct interaction of reactants with the Cu surface.
In contrast, hydrofuroin formation rate was remarkably un-
changed on MPA-Cu (decreased 24%) and MBT-Cu (increased
6.1%) compared to the unmodified Cu, strongly suggesting
that the first e- transfer of the electroreduction pathway may
occur through outer-sphere ET, which does not require spe-
cific adsorption of furfural or H.
Electrochemical experiments performed with H/D isotop-
ically substituted solvents and electrolytes were performed to
further elucidate the mechanisms of furfural hydrogenation
and hydrogenolysis. Kinetic isotopic effects (KIE) have been
observed for HER on many electrode materials, including Cu,
where the kinetics (i.e. exchange current density) of HER are
greater than deuterium evolution reaction (DER).41 This is ex-
emplified by the cathodic shift between the HER/DER waves
(e.g. 87 mV at 10 mA cm-2) in deuterated electrolytes, shown
in Figure 2a. A normal KIE is also expected for the ECH mech-
anism, which shares a common step with HER (Volmer reac-
tion) and also involves reactions with Hads. The voltammo-
grams in Figure 2b show that furfural reduction experiences a
KIE, and that its magnitude is similar to HER (e.g. 98 mV shift
at 10 mA cm-2). Production rates for MF and FA determined by
preparative electrolysis at –0.55 V decreased by 95.6% and
21.5% in pD 0.5 electrolyte, respectively, compared to pH 0.5
(Figure 2c). We rationalize that the relatively minor decrease
in FA rate compared to MF is attributed to a selectivity change
from MF to FA under deuterated conditions. Analogous ex-
periments performed at pH 3.0, a condition at which FA is the
major product, showed that the production rates for both MF
and FA were significantly decreased (47.1% and 76.6%, respec-
tively) in D-electrolyte compared to H-electrolyte. In contrast,
the rate of hydrofuroin production actually increased in D-
electrolyte compared to H-electrolyte at both pH (or pD) con-
ditions, which may be a result of an equilibrium-shift for a pre-
ceding homogeneous reaction (e.g. protonation) due to ther-
modynamic isotope effects.42 These results are consistent with
the behavior expected if MF and FA are from ECH reactions
and hydrofuroin is from electroreduction, and therefore are in
agreement with the study on thiol-modified electrodes.
Figure 2. (a) Cyclic voltammograms in 0.5 M H2SO4/H2O (pH
0.5) or D2SO4/D2O (pD 0.5) and (b) with 0.05 M furfural on a
Cu RDE. (c) Observed production rates during preparative
electrolysis of furfural on Cu electrodes in H- or D- electro-
lytes. Conditions: 0.05 M furfural, 1 h duration, E = –0.55 V.
ECH and electroreduction mechanisms can be further dis-
tinguished by studying furfural reduction under a special case
of mass-transport-limited conditions. The rates of reactions
which consume protons at the electrode/electrolyte interface
(e.g. HER and ECH) can become completely mass-transport-
limited in mildly acidic conditions at moderate current densi-
ties, owing to the low bulk proton concentrations. This is
shown by the rotating-disk voltammogram in Figure 3a, in
which the rate of HER plateaus to a mass-transport-limited
current density (jl,HER) of –20.9 mA cm-2 in a pH 3.0 electrolyte.
Similarly, ECH reactions consume interfacial protons through
the Volmer reaction (H+ + e– → Hads). Consequently, if furfural
reduction proceeds solely by ECH, it would also be subject to
proton transport limitations and its rate would be limited ac-
cordingly. In contrast, if furfural reduction proceeds only by
electroreduction, in which protonations occur in the bulk so-
lution, its rate would not be limited by proton transport to the
electrode surface. This has been demonstrated for the similar
case of benzaldehyde electroreduction, for which the mass-
transport-limited current was proportional to substrate con-
centration, even in very mild acidic conditions (i.e. pH 5.2).43
Preparative electrolysis results (Figure 3b) show that H2, FA,
MF, and hydrofuroin formation accounted for 11.6%, 44.8%,
26.9%, and 4.9% of the total current at –0.75 V, a potential
within the transport-limited region under electrolysis condi-
tions (Figure S4). Assuming that FA and MF are from ECH re-
actions and hydrofuroin is from electroreduction, this would
indicate that the vast majority of the total current should be
subject to proton transport limitations (i.e. 83.3% to H2, FA,
and MF), while a minor fraction would not (i.e. 4.9% to hydro-
furoin). Figure 3a shows that mass-transport-limited furfural
reduction currents were remarkably similar to jl,HER, and in-
creased only slightly as furfural concentration was increased
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(e.g. +5.2% difference with 0.04 M furfural). Therefore, the
comparison between mass-transport-limited currents for HER
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2
3
4
5
6
7
and furfural reduction is very consistent with the proposal
that FA and MF are products of ECH and hydrofuroin is from
electroreduction. Moreover, Figure 3b shows that the product
distribution was nearly identical at lower potentials (e.g. –0.55
V), which is evidence that these mechanisms are applicable
within the potential range of interest.
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Figure 4. Comparison of the preparative electrolysis of furfural
in pH 0.5 or pH 3.0 electrolytes on Pb or Cu electrodes. Con-
ditions: 1 h duration; E = –0.55 V.
Reaction pathways. It has been previously implied that
MF is a secondary product of furfural reduction on Cu, with
FA formed as an intermediate under electrochemical condi-
tions.14, 16, 19 This would suggest that ECH product selectivity
can be tuned by varying the extent of reaction, with MF pre-
ferred at high conversion after subsequent hydrogenolysis of
FA. In clear contradiction, we found that FA and MF selectiv-
ities were remarkably constant with respect to reaction dura-
tion and degree of furfural conversion (Figure 5a), which sug-
gests that the two products are mainly generated from paral-
lel, not consecutive reactions, on Cu electrodes.
The cyclic voltammogram of FA was nearly identical to
that of the blank electrolyte, which suggests that HER was the
dominant electrode reaction in the presence of FA (Figure 5b).
Preparative electrolysis of FA resulted in ca. 100% efficiency to
HER at –0.55 V at both pH 0.5 and 3.0 on the Cu electrode,
whereas only a negligible amount of MF was detected corre-
sponding to <1% efficiency (Figure 5c). An initial FA concen-
tration of 0.01 M was chosen to represent typical bulk concen-
trations present during furfural reduction. These results indi-
cate that hydrogenolysis of FA is very slow under these condi-
tions; therefore, the large amount of MF produced during fur-
fural reduction is generated in parallel with FA, rather than
from the decoupled sequential reduction of FA product.
Insight into feasible pathways for MF formation which by-
pass the FA intermediate was obtained by Shi et al. who ex-
plored the catalytic conversion of furfural to FA and MF on
Cu(111) surfaces on the basis of density functional theory
(DFT) calculations.47 They found that H addition or decompo-
sition pathways of the preferred monohydrogenated alkoxyl
intermediate (C4H3O–CH2O) had similar energy barriers (1.17
and 1.18 eV, respectively) and thermodynamic free energy
changes (0.24 and 0.33 eV, respectively), and therefore parallel
routes to FA or MF through this intermediate should be com-
petitive. Comprehensive DFT modeling of furfural conversion
under electrochemical conditions and in situ spectroscopy
should be focuses of future work to confirm the pathway.
Figure 3. (a) Cyclic voltammograms at pH 3.0 with various fur-
fural concentrations recorded on a Cu RDE. Baseline correc-
tion was performed to remove the contribution of double-
layer charging currents. (b) Faradaic efficiency to furfural re-
duction products and H2 measured from preparative electrol-
ysis. Conditions: pH 3.0 electrolyte, 0.05 M furfural, 1 h dura-
tion.
Additional mechanistic insight may be gained by compar-
ing Cu to a different electrode material on which direct elec-
troreduction mechanisms are known to dominate. Pb elec-
trodes are commonly used to perform electroreductions with-
out competition from interfering reactions (e.g. HER or elec-
trocatalytic reactions), owing to their weak hydrogen adsorp-
tion properties.44 In fact, the onset potential for furfural re-
duction on Pb is more than 400 mV more positive than HER
(Figure S5), indicating that Hads does not participate in furfu-
ral reduction mechanisms. Hydrofuroin was the major de-
tected product from preparative electrolysis at –0.55 V on Pb,
corresponding to faradaic efficiencies of 34% and 38% at pH
0.5 and pH 3.0, respectively (Figure 4). On the other hand, no
H2 or MF, and very little FA (<2% faradaic efficiency), were
detected under these conditions. A resinous precipitate in the
reactor and unidentified peaks in the product chromato-
graphs were observed which were not identified or quantified
in this work, but likely contributed to the low total faradaic
efficiencies to detected products. Such products have been
previously observed during electrochemical furfural reduc-
tion,16, 45 and may be other dimer or oligomer byproducts of
electroreduction reactions.46 Figure 4 also shows that the
products observed on Pb were distinctly different than Cu, on
which FA and MF were the main products. This result is fur-
ther evidence that FA and MF formation requires Hads and oc-
curs through ECH mechanisms on Cu.
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can occur by an outer-sphere process, and is not sensitive to
the presence of surface adsorbates.
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The onset potential of furfural reduction on Cu is about –
0.35 V, as shown by the increased cathodic current in the pres-
ence of furfural compared to HER (Figure 6a). Preparative
electrolysis was performed at constant potentials ranging
from –0.45 V to –0.65 V. Figure 6b shows that FA and MF were
the dominant furfural reduction products over the evaluated
potential range, indicating that there was sufficient Hads on
the electrode surface for the ECH mechanism to dominate un-
der these conditions (i.e. pH 0.5, initial furfural concentration
0.05 M). Accordingly, hydrofuroin formation via the electro-
reduction mechanism was minor, with typically less than 2%
faradaic efficiency. The total current density increased at more
negative potentials, however, faradaic efficiency to furfural re-
duction decreased significantly, corresponding to an increase
in H2 production. The rate of the Volmer reaction (eq 1) is en-
hanced at more negative potentials, resulting in a greater total
availability of Hads for HER or ECH. However, HER will out-
pace ECH at more negative potentials because the rate con-
stant of the Heyrovsky reaction (eq 2) increases exponentially
with potential, whereas the reaction between furfural adsorb-
ates and Hads is non-electrochemical and the rate constant is
potential-independent. Cathode potential also influenced the
ECH selectivity, with the preference for MF more pronounced
at increasingly negative potentials, as seen qualitatively by the
relative faradaic efficiencies of MF and FA in Figure 6b. Elec-
trode potential regulates the charge transfer kinetics, surface
states, and stability of adsorbed species, and therefore can
have significant influence on the selectivity of complex multi-
step reactions.49
The influence of initial furfural concentration was evalu-
ated over the range of 0.01 to 0.2 M in a pH 0.5 electrolyte.
Figure 6c shows that the competition between furfural reduc-
tion and HER was highly dependent on furfural concentration,
evidenced by the decrease in FEHER from 80.0% to 9.9% over
the evaluated concentration range. Increasing bulk furfural
concentrations generally facilitated higher surface coverage of
furfural adsorbates, and consequently enhanced rates of ECH
reactions forming FA and MF. Conversely, the rate of HER was
decreased owing to the increased competition for available
Hads. Interestingly, the ECH rate actually decreased when fur-
fural concentration was further increased from 0.1 M to 0.2 M,
indicating that the electrode surface became oversaturated
with furfural-related adsorbates. In contrast, the rate of hydro-
furoin formation, which does not require Hads, increased
steadily over the range of furfural concentrations. Addition-
ally, the selectivity of ECH reactions was sensitive to furfural
concentration, as the molar ratio of MF to FA products de-
creased from ca. 6.2 at 0.01 M to 2.4 at 0.2 M furfural concen-
tration. Unidentified products were also observed after tests
in which greater hydrofuroin formation occurred (e.g. 0.1 and
0.2 M furfural), and likely contributed to the lower total fara-
daic efficiencies at those conditions, similarly to the previ-
ously mentioned results of furfural reduction on Pb elec-
trodes.
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Figure 5. (a) Conversion and selectivity over time during the
preparative electrolysis on Cu. Conditions: 0.05 M furfural, pH
0.5 electrolyte, E = –0.55 V. Each bar represents an independ-
ent experiment. (b) Cyclic voltammograms in 0.5 M H2SO4
with or without 0.01 M FA recorded on a Cu RDE. (c) Faradaic
efficiency to MF and H2 during the preparative electrolysis of
0.01 M FA performed on Cu. Conditions: pH 0.5 or 3.0 electro-
lyte, 1 h duration, E = –0.55 V.
Impact of reaction conditions. The efficiency and selectiv-
ity of furfural reduction can be manipulated by applying
knowledge of the underlying mechanisms to rationally choose
experimental conditions such as electrode potential, reactant
concentration, and electrolyte pH. The faradaic efficiency for
furfural reduction is determined by its kinetic competition
with HER. It is generally accepted that HER on Cu occurs by
the Volmer-Heyrovsky mechanism,5 shown in eqs 1–2 for
acidic electrolytes.
H+ + e– →Hads
(1)
Hads + H+ + e– →H2
(2)
The present work provides strong evidence that FA and MF
are produced by ECH reactions through parallel hydrogena-
tion and hydrogenolysis pathways. Accordingly, their for-
mation rates will be dependent on the surface coverages of
both Hads and furfural adsorbates.3 The surface coverage of
Hads is determined by the relative kinetics of its adsorption and
desorption (eqs 1–2), which are potential-dependent electro-
chemical reactions, as well as the non-electrochemical reac-
tion between furfural adsorbates and Hads. Therefore, the rate
of ECH will depend on potential and other factors which in-
fluence Hads and furfural adsorbate coverage, including furfu-
ral concentration and electrolyte pH. Hydrofuroin formation
Varying electrolyte pH had a significant effect on the se-
lectivity and efficiency of furfural reduction, as shown by the
preparative electrolysis results at –0.55 V in Figure 6d. The dif-
ferent pH conditions were compared at a constant potential
versus the pH-corrected RHE reference scale in order to ac-
count for the change in equilibrium reduction potentials with
via the electroreduction mechanism does not involve Hads
,
however its formation is also dependent on potential, sub-
strate concentration, and electrolyte pH.48 Furthermore, it
was shown in this work that the ET preceding dimerization
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Figure 6. (a) Cyclic voltammograms in 0.5 M H2SO4 with or without 0.05 M furfural on a Cu RDE. (b) Preparative electrolysis of
furfural at varying cathode potentials. Conditions: 0.05 M furfural, pH 0.5 electrolyte, 144 C charge transferred, electrode area was
2.5 cm2 for –0.6 and –0.65 V. (c) Preparative electrolysis of furfural with various initial furfural concentration. Conditions: pH 0.5
electrolyte, 30 min duration, E = –0.55 V (d) Preparative electrolysis of furfural with electrolyte pH 0.5–3.0. Conditions: 0.05 M
furfural, 1 h duration, E = –0.55 V.
electrolyte pH. Even so, the rates of HER and ECH decreased
notably with increasing electrolyte pH, which can be rational-
ized by considering mass transport effects. HER and ECH con-
sume protons at the electrode/electrolyte interface and can
become mass-transport-limited if proton transport from the
bulk solution is insufficient, as is the case in mildly acidic elec-
trolytes (e.g. pH 3.0, see Figure S4). In contrast, the rate of hy-
drofuroin formation, which does not require surface hydro-
gen, was not significantly affected by pH. The increased fara-
daic efficiency to hydrofuroin at higher pH can be attributed
to the lower relative contributions from ECH and HER. The
preference to FA or MF was very sensitive to changes in elec-
trolyte pH, which suggests that the electrocatalytic hydro-
genation and hydrogenolysis pathways may have different pH
dependences. MF was preferred at low pH, whereas FA was
the major product at pH 3.0 (FEFA = 62.9%, FA/MF molar ratio
= 2.2). It has been previously suggested that the ratio of hydro-
genation and hydrogenolysis products of ECH reactions is
sensitive to the presence of metal ions, such as Na+, in electro-
lytes.50 However, this effect is unlikely to explain the present
results because the same selectivity trends were observed
when H2SO4/Na2SO4 electrolytes were replaced with dilute
H2SO4 electrolytes (Figure S3). The increased selectivity to FA
at higher pH may be a result of the lower relative availability
of Hads compared to furfural adsorbates. As previously dis-
cussed, the rates of HER and ECH reactions decreased at
higher pH owing to limited availability of protons at the elec-
trode/electrolyte interface. It is plausible that the hydrogena-
tion pathway to FA, which requires two Hads, and the hydro-
genolysis pathway to MF in which four Hads participate could
be sensitive to the relative availability of Hads and furfural ad-
sorbates. This explanation would also be consistent with the
initial concentration study (Figure 6c), in which MF was more
favorable at lower furfural concentrations, which facilitate
higher coverage of Hads relative to furfural adsorbates. How-
ever, without direct analysis of surface species the exact nature
of this selectivity change remains a focus for future investiga-
tions.
CONCLUSIONS
The electrochemical reduction of furfural in acidic electro-
lytes was explored with the goals of distinguishing mecha-
nisms of product formation and bridging understanding gaps
between mechanisms, reaction conditions, and product selec-
tivity. Electrodes modified with organothiol SAMs were uti-
lized to determine the requirement for direct reactant-elec-
trode interactions, and the nature of heterogeneous ET re-
sponsible for formation of hydrogenation, hydrogenolysis,
and hydrodimerization products. FA and MF formation were
inhibited on electrodes modified with MPA and MBT, which
indicates those processes require direct interaction with the
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electrode surface. In comparison, hydrofuroin formation was
versity. X.C. and D.C. are grateful to undergraduate research-
ers Hyunjin Moon, Aimee Pierce, Geng Sun, and Yuan Tan
from Iowa State University for their assistance.
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unaffected, which suggests that the first electron transferred
in the electroreduction mechanism, prior to dimerization, is
an outer-sphere process and is therefore insensitive to the
electrode surface properties or catalytic activity. Production
rates of FA and MF exhibited a strong H/D isotope effect,
which strongly suggests they are formed through the ECH
mechanism by a reaction with electrochemically adsorbed H
(or D). An investigation of furfural reduction under proton
mass-transport-limited conditions by RDE voltammetry con-
firmed that MF and FA formation consume protons at the
electrode/electrolyte interface, consistent with the ECH
mechanism. A comparison of the products formed on a Cu
electrode to those formed on a high hydrogen overpotential
Pb electrode provided additional evidence that MF and FA for-
mation requires Hads, through the ECH mechanisms. A path-
way study revealed that hydrogenation and hydrogenolysis
products, FA and MF, are formed mainly through parallel re-
actions, in which FA is not a major intermediate for MF for-
mation. Finally, understanding of the underlying mechanisms
enabled the manipulation of electrochemical furfural reduc-
tion by rationally tuning the electrode potential, electrolyte
pH, and furfural concentration to promote selective formation
of important chemicals for bio-based polymers and fuels pro-
duction. Collectively, these studies highlight the decisive role
that reaction conditions play in determining the selectivity of
ECH reactions of bioderived oxygenates to hydrogenation or
hydrogenolysis products, and the competition between ECH,
electroreduction, and HER pathways.
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ASSOCIATED CONTENT
Supporting Information
1
Experimental details, H NMR and MS characterizations for
hydrofuroin, supplemental voltammetry and preparative elec-
trolysis results. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*wzli@iastate.edu.
Author Contributions
All authors have given approval to the final version of the
manuscript. ‡ X.C. and D.C. contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This research was funded by NSF-CBET 1512126. W.L. acknowl-
edges the Iowa State University Start-Up Fund, Ames Labora-
tory Start-Up Fund, Iowa Energy Center, Bailey Research Ca-
reer Development Award, and Richard Seagrave professor-
ship. We thank Dr. Sarah Cady (ISU Chemical Instrumenta-
tion Facility) for training and assistance pertaining to the
AVIII-600 results included in this publication, fruitful discus-
sion with Prof. Kurt Hebert, Prof. Brent Shanks, and Dr. Toni
Pfennig from Iowa State University, and exchange of ideas
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Figure 1. (a) Cyclic voltammograms in 0.5 M H2SO4 with or without the addition of 0.25 mM MPA, MBT, or
MDA and (b) with 0.05 M furfural on a Cu RDE. (c) Observed production rates during preparative electrolysis
of furfural on Cu, Cu-MPA, Cu-MBT, and Cu-MDA electrodes. Conditions: 0.05 M furfural and 0.25 mM of the
indicated organothiol, pH 0.5 electrolyte, 1 h duration, E = –0.55 V.
172x201mm (300 x 300 DPI)
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Figure 1. (a) Cyclic voltammograms in 0.5 M H2SO4 with or without the addition of 0.25 mM MPA, MBT, or
MDA and (b) with 0.05 M furfural on a Cu RDE. (c) Observed production rates during preparative electrolysis
of furfural on Cu, Cu-MPA, Cu-MBT, and Cu-MDA electrodes. Conditions: 0.05 M furfural and 0.25 mM of the
indicated organothiol, pH 0.5 electrolyte, 1 h duration, E = –0.55 V.
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Figure 1. (a) Cyclic voltammograms in 0.5 M H2SO4 with or without the addition of 0.25 mM MPA, MBT, or
MDA and (b) with 0.05 M furfural on a Cu RDE. (c) Observed production rates during preparative electrolysis
of furfural on Cu, Cu-MPA, Cu-MBT, and Cu-MDA electrodes. Conditions: 0.05 M furfural and 0.25 mM of the
indicated organothiol, pH 0.5 electrolyte, 1 h duration, E = –0.55 V.
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Figure 2. (a) Cyclic voltammograms in 0.5 M H2SO4/H2O (pH 0.5) or D2SO4/D2O (pD 0.5) and (b) with
0.05 M furfural on a Cu RDE (c) Observed production rates during prepara-tive electrolysis of furfural on Cu
electrodes in H- or D- electrolytes. Conditions: 0.05 M furfural, 1 h duration, E = –0.55 V.
174x193mm (300 x 300 DPI)
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Figure 2. (a) Cyclic voltammograms in 0.5 M H2SO4/H2O (pH 0.5) or D2SO4/D2O (pD 0.5) and (b) with
0.05 M furfural on a Cu RDE (c) Observed production rates during prepara-tive electrolysis of furfural on Cu
electrodes in H- or D- electrolytes. Conditions: 0.05 M furfural, 1 h duration, E = –0.55 V.
174x193mm (300 x 300 DPI)
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Figure 2. (a) Cyclic voltammograms in 0.5 M H2SO4/H2O (pH 0.5) or D2SO4/D2O (pD 0.5) and (b) with
0.05 M furfural on a Cu RDE (c) Observed production rates during prepara-tive electrolysis of furfural on Cu
electrodes in H- or D- electrolytes. Conditions: 0.05 M furfural, 1 h duration, E = –0.55 V.
155x81mm (300 x 300 DPI)
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Figure 3. (a) Cyclic voltammograms at pH 3.0 with various furfural concentrations recorded on a Cu RDE.
Baseline correction was performed to remove the contribution of double-layer charging currents. (b)
Faradaic efficiency to furfural reduction products and H2 measured from preparative electrolysis. Conditions:
pH 3.0 electrolyte, 0.05 M furfural, 1 h duration.
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Figure 3. (a) Cyclic voltammograms at pH 3.0 with various furfural concentrations recorded on a Cu RDE.
Baseline correction was performed to remove the contribution of double-layer charging currents. (b)
Faradaic efficiency to furfural reduction products and H2 measured from preparative electrolysis. Conditions:
pH 3.0 electrolyte, 0.05 M furfural, 1 h duration.
172x173mm (300 x 300 DPI)
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Figure 4. Comparison of the preparative electrolysis of furfural in pH 0.5 or pH 3.0 electrolytes on Pb or Cu
electrodes. Conditions: 1 h duration; E = –0.55 V.
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Figure 5. (a) Conversion and selectivity over time during the preparative electrolysis on Cu. Conditions: 0.05
M furfural, pH 0.5 electrolyte, E = –0.55 V. Each bar represents an independent experiment. (b) Cyclic
voltammograms in 0.5 M H2SO4 with or without 0.01 M FA recorded on a Cu RDE. (c) Faradaic efficiency to
MF and H2 during the preparative electrolysis of 0.01 M FA performed on Cu. Conditions: pH 0.5 or 3.0
electrolyte, 1 h duration, E = –0.55 V.
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Figure 5. (a) Conversion and selectivity over time during the preparative electrolysis on Cu. Conditions: 0.05
M furfural, pH 0.5 electrolyte, E = –0.55 V. Each bar represents an independent experiment. (b) Cyclic
voltammograms in 0.5 M H2SO4 with or without 0.01 M FA recorded on a Cu RDE. (c) Faradaic efficiency to
MF and H2 during the preparative electrolysis of 0.01 M FA performed on Cu. Conditions: pH 0.5 or 3.0
electrolyte, 1 h duration, E = –0.55 V.
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Figure 5. (a) Conversion and selectivity over time during the preparative electrolysis on Cu. Conditions: 0.05
M furfural, pH 0.5 electrolyte, E = –0.55 V. Each bar represents an independent experiment. (b) Cyclic
voltammograms in 0.5 M H2SO4 with or without 0.01 M FA recorded on a Cu RDE. (c) Faradaic efficiency to
MF and H2 during the preparative electrolysis of 0.01 M FA performed on Cu. Conditions: pH 0.5 or 3.0
electrolyte, 1 h duration, E = –0.55 V.
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Figure 6. (a) Cyclic voltammograms in 0.5 M H2SO4 with or without 0.05 M furfural on a Cu RDE. (b)
Preparative electrolysis of furfural at varying cathode potentials. Conditions: 0.05 M furfural, pH 0.5
electrolyte, 144 C charge transferred, electrode area was 2.5 cm2 for –0.6 and –0.65 V. (c) Preparative
electrolysis of furfural with various initial furfural concentration. Conditions: pH 0.5 electrolyte, 30 min
duration, E = –0.55 V (d) Preparative electrolysis of furfural with electrolyte pH 0.5–3.0. Conditions: 0.05 M
furfural, 1 h duration, E = –0.55 V.
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Figure 6. (a) Cyclic voltammograms in 0.5 M H2SO4 with or without 0.05 M furfural on a Cu RDE. (b)
Preparative electrolysis of furfural at varying cathode potentials. Conditions: 0.05 M furfural, pH 0.5
electrolyte, 144 C charge transferred, electrode area was 2.5 cm2 for –0.6 and –0.65 V. (c) Preparative
electrolysis of furfural with various initial furfural concentration. Conditions: pH 0.5 electrolyte, 30 min
duration, E = –0.55 V (d) Preparative electrolysis of furfural with electrolyte pH 0.5–3.0. Conditions: 0.05 M
furfural, 1 h duration, E = –0.55 V.
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Figure 6. (a) Cyclic voltammograms in 0.5 M H2SO4 with or without 0.05 M furfural on a Cu RDE. (b)
Preparative electrolysis of furfural at varying cathode potentials. Conditions: 0.05 M furfural, pH 0.5
electrolyte, 144 C charge transferred, electrode area was 2.5 cm2 for –0.6 and –0.65 V. (c) Preparative
electrolysis of furfural with various initial furfural concentration. Conditions: pH 0.5 electrolyte, 30 min
duration, E = –0.55 V (d) Preparative electrolysis of furfural with electrolyte pH 0.5–3.0. Conditions: 0.05 M
furfural, 1 h duration, E = –0.55 V.
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Figure 6. (a) Cyclic voltammograms in 0.5 M H2SO4 with or without 0.05 M furfural on a Cu RDE. (b)
Preparative electrolysis of furfural at varying cathode potentials. Conditions: 0.05 M furfural, pH 0.5
electrolyte, 144 C charge transferred, electrode area was 2.5 cm2 for –0.6 and –0.65 V. (c) Preparative
electrolysis of furfural with various initial furfural concentration. Conditions: pH 0.5 electrolyte, 30 min
duration, E = –0.55 V (d) Preparative electrolysis of furfural with electrolyte pH 0.5–3.0. Conditions: 0.05 M
furfural, 1 h duration, E = –0.55 V.
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Scheme 1. Proposed pathways of electrochemical reduction of carbonyls in acidic electrolytes.
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Scheme 2. Key hydrogenation and hydrogenolysis products of furfural and HMF.
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Table of Contents
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