A non-noble bimetallic alloy in the highly selective electrochemical synthesis of the biofuel 2,5-dimethylfuran from 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c8gc03689f

The conversion of HMF to DMF has received great attention due to its potential as a gasoline alternative. Electrosynthesis is a powerful green tool due to its significant advantages and provides an efficient and environmentally friendly route. In this work, the process uses water as the hydrogen source in the electro-catalytic reduction of 5-hydromethylfurfural (HMF) for the synthesis of 2,5-dimethyl-furan (DMF). The performance of a non-noble CuNi bimetallic electrode was investigated in the electro-catalytic hydrogenation of HMF. The electrodes prepared were characterized using XRD, SEM, XPS, and BET techniques. 88.0% faradaic efficiency of DMF and 91.1% conversion selectivity of DMF were achieved. The CuNi bimetallic electrode also displayed a good stability during the reduction of HMF to DMF.

Full text of DOI:10.1039/c8gc03689f

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Journal Name
ARTICLE
Non-noble bimetallic alloy in the high selective electrochemical
synthesis of biofuel 2, 5-dimethyl-furan from 5-
hydroxymethylfurfural
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
a
Yan-Ru Zhang, Bing-Xin Wang, Lei Qin, Qiang Li * and Yong-Ming Fan *
DOI: 10.1039/x0xx00000x
Conversion of HMF to DMF has received great attention for its potential as gasoline alternative. Electrosynthesis is a
powerful green tool for its significant advantages and provides an efficient and environmentally friendly route. In this work,
the process use water as the hydrogen source in the electro-catalytic reduction of 5-hydromethyl furfural (HMF) for the
synthesis of 2, 5-dimethyl-furan (DMF). The performance of a non-noble CuNi bimetallic electrode was investigated in the
electro-catalytic hydrogenation of HMF. The electrodes prepared were characterized using XRD, SEM, XPS, as well as BET
techniques. 88.0% faradic efficiency of DMF and 91.1% conversion selectivity of DMF was achieved. The CuNi bimetallic
electrode also displayed a good stability during the reduction of HMF to DMF.
www.rsc.org/
catalysis, which is usually carried out in moderate reaction
conditions, is an environmentally friendly process for the
synthesis of chemicals with high reaction selectivity.20-21 The
electro-catalytic hydrogenation under room-temperature and
atmospheric-pressure was first introduced by Nilges and
Schröder22 over single metal electrodes with proton as
hydrogen donor under acid condition. DMF, 2, 5-
bis(hydroxymethyl)furan (BHMF), and 5-methyl-furfuryl alcohol
Introduction
In the view of irreversible consumption of fossil fuel and
the serious concerns with global climate, lignocellulosic
biomass, as a renewable and sustainable resource, has emerged
as a leading contender. Many efforts have been devoted to
chemical and biological conversion of lignocellulosic biomass
into biofuels and chemicals.1-4 5-hydroxymethylfurfural (HMF)
has been regarded as an important biomass intermediate and
can be obtained by the dehydration of hexoses including most
predominantly fructose and glucose.5-9 HMF contains two
functionalities groups (-OH, -C=O) attached to a furan ring, and
it can be converted into various products, such as 2, 5-
dimethylfuran (DMF),10 5-ethoxymethyl-furfural,11 and 2, 5-
furandicarboxylic acid.12 DMF has been specially attractive
among them for it is potentially the gasoline alternative due to
(
MFA) have been achieved with the selectivity of 35.6%, 33.8%,
and 11.1% respectively over copper electrode in the mixture of
equal amount of water and ethanol containing 0.5 M H SO
2
4
.
Kwon and coworkers23-24 studied the electro-catalytic
hydrogenation of HMF to DMF on solid metal electrodes in
neutral or acidic medium and found that the overpotential for
hydrogenation of HMF was lower under acidic condition than
neutral media. More recently, a metallic oxide, ZrO , was
2
synthesized and used in the conversion of HMF into DMF with
yields of as high as 30.7%.10
its high energy density, high boiling point and low solubility in
water compared with ethanol.13-14
Recently, interest has considerably arisen in bimetal or
alloy catalysts owing to their superior catalytic hydrogenation
efficiency compared with single metal catalysts.25-27 The
efficient and low-cost alloys catalysts have been developed and
applied in chemical synthesis. For example, Cu and its alloys,
Production of DMF from HMF by catalytic hydrogenation
has been studied15-18 under severe conditions like high
temperature and pressure with H , formic acid and alcohols as
2
hydrogen donors. Although DMF can be synthesized in high
selectivity in the conventional catalytic thermochemical
process, the concerns about the economy and safety of the
2
8
such as copper-doped porous metal oxide (Cu/PMO), alumina-
2
9
30
31
cannot be avoided.19 Electro-
2 3
supported copper-nickel (CuNi/γ-Al O ), CuCo and Cu-ZnO,
HMF hydrogenation using H
2
exhibit fairly high catalytic activity and stability for chemically
synthesis of DMF from HMF. A typical inexpensive catalyst, CuNi
alloy has been widely used in many fields such as methanol
synthesis32 and dehydrogenation of alcohol to produce
hydrogen for the application in fuel cells.33-35 It has also been
reported that NiCu has been used in the chemical
hydrodeoxygenation of 5-hydroxymethylfurfural at high
pressure and obtained positive results.36
*
corresponding authors:
a.
Key Laboratory of Lignocellulosic Chemistry, College of Material Science and
Technology Beijing Forestry University, Beijing, 100083, China. E-mail:
fanym@bjfu.edu.cn; Tel: +86-185-15301003.
College of Science, Beijing Forestry University, Beijing 100083, China. E-mail:
liqiang@bjfu.edu.cn; Tel: +86-137-18679671.
b.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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However, CuNi has yet found to be published in the electro- Table 1. BET areas of as-prepared electrodes (FigurVeiewSAr2ticleinOntlhinee
Supporting Information).
DOI: 10.1039/C8GC03689F
catalytic synthesis of DMF from HMF. Based on the
performance of CuNi alloy in the field of thermochemical
synthesis, we believe that it can be potentially used in the
electro-chemical synthesis of DMF from HMF. Herein, we report
2
-1
Electrode code
Cu foil
Composition
Cu Ni
BET area (m g )
1
0
1
92
83
Cuhs
CuNi
Cu Ni0
1
a
bimetallic CuNi electrode for the electro-catalytic
hydrogenation of HMF into DMF in sulfate buffer solution (pH
.0). The structure of the bimetallic electrode is characterized,
Cu0.82 Ni0.17
2
of the surface area.
The XRD measurements of the as-prepared electrodes
were performed and the diffractograms are depicted in Figure
the performance of electro-catalyst of HMF is evaluated and the
electro-catalytic reaction mechanism of HMF over bimetallic
electrode for the synthesis of DMF is discussed.
2
. Intense metallic Cu diffraction patterns can be seen in all the
samples. The Cuhs (0 s in Figure 2) only displays the Cu
diffraction patterns at 43.3° (111) and 50.4° (200), while new
diffraction peaks arise with electroplating of Ni on Cuhs. It can
be observed that the diffraction peaks of Ni at 44.5° (111) and
Results and discussion
Structural Characterization of the as-prepared electrodes.
SEM images of Cu foil, high-surface-area Cu electrode 51.8° (200)27 are gradually intensified with the electro-
hereafter referred to as Cuhs) and CuNi electrodes (the sample deposition time, which corresponds to the deposition progress
(
of Ni deposition time of 70 s) are showed in Figure 1. The of Ni. The right phase offset of the two Cu diffraction patterns
roughness of Cu foil can be seen in Figure 1 A. It can be inferred can be attributed to the formation of CuNi alloy.38 The change
that the displacement of Cu2+ ions is kinetically fast and quickly of the diffraction patterns suggests the increasing electro-
depleted near the surface of Cu foil, which results in the deposition of Ni on the copper foil and the formation of CuNi
formation of the dendritic fractal morphology (Figure 1 B) due alloys (Cu3.8Ni, PDF 09-0205; Cu0.81Ni0.19, PDF 47-1406). A small
to the limitation of the diffusion of Cu2+ ions. The special diffraction peak appeared after 50 s at 37.2° (111) of the
morphology is beneficial for increasing surface area of the samples indicates the presence of a NiO in the alloy. On the
electrode while maintaining good electrical continuity between contrary, the intensity of diffraction peaks of Cu
2
O at 36.5° (111)
Cu and Cu crystals.37 Figure 1 C shows that the dendritic and 42.4° (200) decreases and gradually disappear with
morphology is maintained after the deposition of Ni. Elemental electrodeposition time, which suggests the reduction of Cu
distribution of CuNi electrode is studied with energy-dispersive and the formation of CuNi alloy.
2
O
X-ray spectroscopy (EDS) and element mapping analysis. The
result indicates that the CuNi electrode contains Cu 82.1%, Ni were investigated with XPS analysis, and the results are
7.3% and trace amount of oxygen (Figure 1 D). The result in displayed in Figure 3. The general XPS survey (Figure 3 A) of the
Figure 1 E showed that Ni elements were mainly deposited on bimetallic electrode demonstrates that the element in
the protruding area of Cu dendrites. electrode is mainly composed of copper, nickel and oxygen. The
The surface chemical states of CuNi electrode (sample 70 s)
1
BET surface area of the three electrode is compared in oxygen element can be attributed to the surface oxidation upon
Table 1. The specific surface area of Cuhs remarkably is increased air exposure. Figure 3 B demonstrates the Cu 2p1/2 and Cu 2p3/2
by the formation of dendritic fractal morphology. However, it spin-orbital splitting, at the binding energies of 932.1 and 952.4
2
+
decreases a little after electrodeposition of Ni on the Cuhs eV, which suggests that the high valence state Cu species are
+
0 39
surface, which may be caused by the deposition of Ni elements effectively reduced to Cu or/and Cu . However these two
0
on some of the concave area of dendrites resulting the decrease spices cannot be identified by XPS because Cu and Cu
2
O have
Figure 1. SEM images of (A) Cu, (B) Cuhs and (C) CuNi electrodes (sample 70 s); (D) EDS spectrum of CuNi electrode and (E) EDS elemental
mapping of CuNi.
2
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DOI: 10.1039/C8GC03689F
Figure 3. XPS spectra of Cu–Ni bimetallic electrodes after electro-
deposition time of 70 s (wide range of binding energies (A), Cu 2p (B),
Cu LMM (C), Ni 2p (D), and O 1s (E))
Figure 2. XRD patterns of the CuNi bimetallic electrodes. The inset
o
o
shows the in zoomed weak signals (2θ of 35 -43 ).
composition on the Cu foil surface. For Ni and CuNi electrodes
in the system without HMF, the onset potential on Ni electrode
was more positive, indicating that Ni is more likely to bond to
hydrogen than Cu and is favourite for the hydrogen evolution.
The onset potential can be seen to be shifted in the positive
direction (Figure 4 B) for all the as-prepared electrodes after the
addition of HMF, indicating that HMF is more easily reduced
than water. A distinctive cathodic curve for HMF reduction was
observed on both Cuhs and CuNi electrodes, which suggests a
better performance of the alloy in the reduction of HMF than
mono Cu and Ni electrode. A more positive onset potential for
CuNi electrode is seen in Figure 4 B in comparison with Cuhs
electrode, implying CuNi electrode is more catalytic for the
reduction of HMF.
almost the same binding energy of Cu 2p1/2 and Cu 2p3/2.40 The
result of CuLMM XAES analysis confirms that the peak at 568.2
0
41
eV in Figure 3 C is attributed to Cu , indicating that CuNi
0
electrode contains Cu rather than Cu
2
O, which agrees with the
results of XRD analysis (Figure 2). A peak of the Ni 2p3/2 has been
observed at 852.6 eV (Figure 3 D), which is attributed to Ni
metal. The peaks at 856.2 and 873.6 eV in combination of the
satellite peaks at 861.2 and 879.9 eV can be assigned the
satellite peaks at 861.2 and 879.9 eV can be assigned to Ni .
The broad peak of O1s at 532.3eV in Figure 2 E suggests the
presence of MOx (M = Ni, Cu). From the above analysis, we
can conclude that Cu and Ni with tiny amount of NiO coexist
on the surface of the composite electrode.
0
4
2
2
+
4
3
0
0
Linear sweep voltammetry (LSV) performance of as-prepared
electrodes
The catalytic behavior of the electrodes of Cu, Ni, Cuhs and
CuNi (sample 70 s) for the reduction of HMF was investigated
by LSV in an undivided three-electrode cell containing 0.2 M
sulfate buffer solution (pH 2.0). For electrochemical reduction
of HMF in aqueous media, water reduction to H is the major
2
competing reaction. Therefore, a good catalyst for the
reduction of HMF should facilitate the reaction between the
hydrogen atom and HMF rather than hydrogen atoms for H
2
evolution. In that Cu has been reported to be one of the active
electrode material for HMF hydrogenation among the metal
catalysts,44, 23 it was chosen as the basic catalysts in this work.
The current at the prepared cathodic electrodes in the
electrolysis of water was recorded in the absence of HMF, which
2
can be attributed to the generation of H (Figure 4 A). We note
that Cu and Ni electrodes shows different onset potentials for
hydrogen evolution. Ni is more active than Cu in the hydrogen
evolution, which is consistent with previous studies.24 Cu and
Cuhs demonstrate the same onset potentials, however, the
current on the Cuhs electrode is higher than Cu foil due to the
larger surface area (Table 1) by the formation of nanocrystal
Figure 4. LSVs characteristics of Cu (black), Ni (olive), Cuhs (blue), and
CuNi (red) (sample in 70 s) electrodes in 0.2 M sulfate buffer solution
-1
(pH 2.0) ( (A): control, (B): 2 g L of HMF, scan rate: 2 mV/s).
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-
1
Electro-catalytic reduction of HMF into DMF
electrolyte: 0.2 M sulfate buffer solution (pH 2.0) and HMF: 2 g L ;
View Article Online
DOI: 10.1039/C8GC03689F
voltage: -0.5 V ≥ E ≥ -1.0 V vs Ag/AgCl; Charge: 150 C.
HMF reduction in a divided cell was performed in 0.2 M
sulfate buffer solution (pH 2.0) containing 2 g L-1 HMF. The
(Figure 5 C, Table S 1 in the Supporting Information). This is
mainly contributed by both the higher surface area and nano
effect of the Cu crystal structure. The nano structure will
provide more effective binding sites for reactants and facilitate
the interfacial charge transfer for reduction processes.37
Bimetallic CuNi has been reported the synergistic distinct
characteristics and catalytic properties in the chemical
reaction.34 So far, its application in the electro-catalytic
production of DMF from HMF has yet found to be published. It
can be seen in Figure 5 D that, DMF FE 88.0% and selectivity
region of potential used in this work is chosen based on the
results of LSVs. After the reduction, the products in the
electrolyte were analyzed and quantified by GC. Results for
HMF reduction with Cu, Ni, Cuhs and CuNi electrodes (sample 70
s) are shown in Figure 5 and Figure S 9 in the Supporting
Information. The faradic efficiency and reduction selectivity of
products were calculated using the following equations,
moles of X formed
퐹퐸(%) =
푆(%) =
× 100%
(1)
(2)
total charge passed(퐶)/(F × n)
9
1.1 % (Table S 1) have been achieved on CuNi electrode, with
moles of X formed
× 100%
moles of HMF consumed
only a trace amount of BHMF (FE< 1%) produced. The effect of
Ni electro-deposition time on FE and selectivity of DMF is shown
in Figure S 3 in the Supporting Information. Because the highest
FE and selectivity of DMF was achieved at 70 s, the sample was
selected as the working electrode for the electro-catalysis
reduction of HMF into DMF in this study. The as-synthesized
CuNi electrode in this work even exhibit a better selectivity in
the electro-conversion of HMF to DMF than the mono noble
catalysts, such as Pt and Pd.22 The efficient bimetallic catalyst
from inexpensive metal makes conversion system more
commercially feasible.
To evaluate the catalytic stability of CuNi electrode, CuNi
was reused in the electro-catalytic reduction of HMF for five
cycles with the used bimetallic electrode cleaned with ultrapure
water (3 times) and dried in vacuum overnight. The results are
showed in Figure 6, which demonstrate that FE of DMF on
bimetallic CuNi electrode only changed slightly from 88.0% to
Where X is DMF, BHMF or MFA. F is the Faraday constant (96485
−1
C mol ), n is the number of electrons required for the
conversion of one HMF molecule to one corresponding
molecule of DMF, BHMF or MFA, which is 6, 2 or 4, respectively.
It can be seen that DMF is the dominant product on the
electrode of Cu, Cuhs and CuNi, while BHMF was the major
product on Ni electrode. In the case of Cu electrode (Figure 5
A), FE for the synthesis of DMF is 2.5% at −0.5 V vs Ag/AgCl. It
then increased to 34.6% at −0.9 V vs Ag/AgCl and declined to
2
1.2% at −1.0 V vs Ag/AgCl. Small FE values has been detected
for the synthesis of both BHMF and MFA at different voltage.
For Ni electrode, in Figure 5 B, the maximum of FE for
BHMF(26.0%) has been obtained at −0.6 V vs Ag/AgCl, which is
consistent with the literature.23 The Cuhs electrode displays a
pretty high faradic efficiency for the electrochemical synthesis
of DMF from HMF. More than 70% DMF FE and selectivity of
more than 80% can be observed throughout the potential
ranged from −0.7 V to −0.9 V vs Ag/AgCl, with little amount of
BHMF and MFA produced
8
3.5% after 5 runs, which demonstrate a good catalytic stability
for the electrochemical synthesis of DMF from HMF. However,
decrease of selectivity is observed from 88.9% to 72.5%, which
is due possibly to the surface oxidation of the CuNi electrode in
the cleaning operation between the runs.
Discussion on electro-catalytic reduction mechanism of HMF to
DMF
2
It can be observed from LSVs (Figure 4 A) that H evolution
Figure 5. Faradaic efficiency (column) and selectivity (line) of the
electro-catalytic reduction of HMF on the electrode of (A) Cu, (B) Ni, (C)
Cuhs and (D) CuNi electrodes (deposition time of 70 s). Conditions:
Figure 6. The electro-catalytic conversion selectivity and FE of DMF from
HMF on CuNi electrode (deposition time of 70 s). Conditions:
4
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electrolyte: 0.2 M sulfate buffer solution (pH 2.0) and HMF 2 g L ;
voltage: -0.8 V vs Ag/AgCl; Charge: 150 C.
This work was supported by the National NatuVireawlASrtcicielenOcneline
Foundation of China (No. 21671021) and State key research
project (2017YFD0600203).
DOI: 10.1039/C8GC03689F
on the CuNi electrode did not initiate until −0.6 V vs Ag/ AgCl.
Therefore, in the potential region of E >-0.6 V, the reduction of
HMF has little to do with the hydrogen evolution reactions Supporting Information
(
HER).24, 37 For −0.5 V ≥ E > −0.6 V vs Ag/AgCl, the production of DMF,
Experimental details on preparation, characterization of
electrodes and product analysis, electrochemical set-up, GC
spectra of products, LSVs, and electrochemical test results.
BHMF and MFA were found to be inefficiently performed, as shown
in Figure 5 D and Table S 1. According to the literature,37 the
reduction in this potential region could be conducted either
through electron transferring to HMF molecule and forming an
anionic intermediate followed by the reaction with H or through
simultaneous transferring of e and H to HMF to produce the BHMF
and DMF.
+
Notes and references
-
+
1
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4
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In the potential region of E< 0.6 V, H evolution on the CuNi
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be readily adsorbed (Hads) on Ni atoms on the surface of the
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The oxygen atoms of the chemical groups have been known to
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HMF to DMF with a series of reduction steps was proposed in Figure
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0
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20
2
1
Acknowledgments
2
492—2521.
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