Efficient Electrocatalytic Reduction of Furfural to Furfuryl Alcohol in a Microchannel Flow Reactor

Article abstract of DOI:10.1021/acs.oprd.8b00428

Furfural is considered to be an essential biobased platform molecule. Recently, its electrocatalytic hydrogenation is regarded as a more environmentally friendly process compared to traditional catalytic hydrogenation. In this study, a new, continuous-flow approach enabling furfural electrocatalytic reduction was developed. In an undivided multichannel electrochemical flow reactor at ambient temperature and pressure in basic reaction conditions, the yield of furfuryl alcohol reached up to 90% in only 10 min residence time. Interestingly, the faradaic efficiency was 90%, showing a good effectiveness of the consumed electrons in the generation of the targeted compound. Furthermore, the innovation lies in the direct electrolysis using the green solvent ethanol without the need for membrane separation or catalyst modification, which offers further proof for continuous and sustainable production in industry.

Full text of DOI:10.1021/acs.oprd.8b00428

Full text access provided via ACS AuthorChoice
Communication
Efficient Electrocatalytic Reduction of Furfural to
Furfuryl Alcohol in a Microchannel Flow Reactor
Yiran Cao, and Timothy Noël
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00428 • Publication Date (Web): 08 Feb 2019
Downloaded from http://pubs.acs.org on February 11, 2019
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Efficient Electrocatalytic Reduction of Furfural to
Furfuryl Alcohol in a Microchannel Flow Reactor
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Yiran Cao and Timothy Noël*
Department of Chemical Engineering and Chemistry, Sustainable Process Engineering, Micro
Flow Chemistry & Synthetic Methodology, Eindhoven University of Technology, De Rondom
70 (Helix, STO 1.37), 5612 AP Eindhoven, The Netherlands.
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ABSTRACT
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Furfural is considered to be an essential bio-based platform molecule. Recently, its
electrocatalytic hydrogenation is regarded as a more environmentally friendly train of thought
compared to traditional catalytic hydrogenation. In this study, a new, continuous-flow approach
enabling furfural electrocatalytic reduction was developed. In an undivided multi-channel
electrochemical flow reactor at ambient temperature and pressure in basic reaction conditions,
the yield of furfuryl alcohol reached up to 90% in only 10 minutes residence time. Interestingly,
the faradaic efficiency was 90%, showing a good effectiveness of the consumed electrons in the
generation of the targeted compound. Furthermore, the innovation lies in the direct electrolysis
using the green solvent ethanol without the need for membrane separation or catalyst
modification, which offers future proof for continuous and sustainable production in industry.
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KEYWORDS
Electrochemistry – Flow Chemistry – Furfural – Furfuryl Alcohol – Microreactor – Flow
Electrochemistry
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INTRODUCTION
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Furfural is one of the most significant industrial bio-based chemicals, which is obtained via
dehydration of sugars, as obtained from a variety of agricultural products, such as corncobs and
wheat bran.1 It possesses extensive application prospects as a renewable, non-petroleum-based
platform molecule.1–6 Consequently, furfuryl alcohol is widely used as a monomer for the
synthesis of furan resins.5 These polymers are used in thermoset polymer matrix composites,
cements, adhesives and coatings. It is also recognized as a valuable intermediate in the fields of
agriculture, medicine and dyes.6 Compared to conventional high-pressure hydrogenation
processes, the electrolysis of furfural offers not only a promotion for agricultural waste
utilization,7 but also constitutes an energy-saving green process.8 Li’s group recently reported an
interesting electrocatalytic reduction of furfural to furfuryl alcohol in a divided cell using a
cation exchange membrane.9 Recently, Koper et al. reviewed the state of the art for furfural
electroreduction.10 The majority of the electrochemical reduction reactions were performed
either with noble metal catalysts in batch electrochemical cells or required long residence times
in flow-through electrochemical cells.11 Arguably, the remaining problems lie in the
discontinuity of the batch process, the complexity of the utilized electrodes, and the limitation of
the use of divided cells.
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The electrocatalytic reduction is an intriguing example which enables the hydrogenation of
molecules using adsorbed hydrogen at the electrode which originates from H+ or H2O via the
Volmer reaction (Scheme 1).12 Hence, the reaction can be carried out at very mild reaction
conditions (room temperature, atmospheric pressure, no H2 needed) and does not require
expensive first or second row transition metal-based catalysts. However, despite these apparent
advantages in favor of electrocatalytic reductions, there are some challenges with regard to non-
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productive competitive reactions (Scheme 1). One is the occurrence of hydrogen evolution where
adsorbed hydrogen is consumed via the so-called Tafel and Heyrovsky reactions,13 which lowers
the Faradaic efficiency of the transformation significantly.14 Another competitive transformation
is the electroreduction of the carbonyl moiety to generate the respective radical. These radicals
can consume another electron to generate the targeted furfuryl alcohol or can dimerize to
generate undesired diols. Hence, efficient electrocatalytic reduction transformations require a
careful balancing of all the different parameters (e.g. electrode material, reaction conditions) to
favor the desired transformation over these unproductive pathways.
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Scheme 1. Desired electrocatalytic hydrogenation of furfural towards furfuryl alcohol versus
undesired reactions.
Herein, we report a continuous-flow direct electrolysis approach for electroreduction of furfural
to address the above mentioned issues using an electrochemical flow reactor developed in our
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group (Figure 1).15 Key in the design of this electrochemical reactor is its flexible reactor volume,
enabling two different operation modes, i.e. a serial (volume ranging from 88 μL/channel up to
704 μL) or a parallel mode (numbering-up or parallel screening of reaction conditions) (Figure
2).16 In this reactor, we screened both basic and acidic reaction conditions and our results
indicate that a basic environment is optimal not only to obtain a high yield of the targeted
furfuryl alcohol but also to obtain high Faradaic efficiencies.
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Figure 1. Continuous multi-channel electrochemical flow reactor, which allows for a rapid
screening of key reaction conditions in parallel.
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Figure 2. Serial mode (left) or parallel mode (right) operation of the electrochemical flow
reactor
EXPERIMENTAL SECTION
1. General Information
For the chemicals, furfural (99%), furfuryl alcohol (98%), 3-furancarboxaldehyde (97%), furan-
3-methanol (99%), potassium hydroxide pellets (85%), potassium ethoxide (95%), sodium
hydroxide pellets (98%), tetrabutylammonium bromide (99%), were purchased from Sigma-
Aldrich and used as received. Hydrochloric acid (37%), ethanol (99.5%), acetonitrile (99%) were
bought from VWR. Sulfuric acid (98%) was obtained from Merck. Ammoeng 110
Poly[oxy(methyl-1,2-ethanediyl)],a-[2-(diethylmethylammonio)ethyl]-w-hydroxy-,chloride (1:1),
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95% was gained from Iolitec. 1 M HCl and 0.5 M H2SO4 solution were prepared by dilution with
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deionized water (18.2 MΩ.cm).
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2. Flow setup
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For the flow set-up, micro syringe pumps (Fusion 720, Chemyx) were used to infuse the liquid
solutions. Before each set of experiments, stock solutions were prepared according to the
different conditions as described herein. The liquids were taken up into disposable plastic
syringes (10 mL, BD Discardit II) and mounted on the syringe pumps. Microfluidic connections
between the electrochemical flow reactor and the syringes consisted from perfluoroalkoxy
capillary tubing (ID 0.75 mm or 1.58 mm, PFA) and were attained from APT. Microfluidic
connectors (LT-115X), ferrules (P-259X) were procured from IDEX. Electric power (0-30 V, 0-
5 A) was delivered by a LABPS3005D power supply. Electrode material (i.e. stainless steel
316L, copper Cu-DHP and graphite) were bought at Salomon's Metalen B.V. The reactor body
(reinforced iron plates and PTFE supporting containers) were manufactured by Equipment &
Prototype Center at Eindhoven University of Technology. It should be noted that the electrodes
did not display any corrosion under the given reaction conditions.15,17
3. Voltammogram
Prior to each experiment, a voltammogram was acquired which determines the suitable
electrochemical reaction conditions. 18 As shown in Figure 3, a plateau appeared at 2.80 - 3.00
V in acidic electrolyte, while it situated at 2.10 - 2.30 V in basic electrolyte. Therefore, 2.90 V
and 2.20 V were respectively chosen for subsequent optimization.
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Figure 3: Voltammogram of furfural at a residence time of 10 min in acidic (A) or basic (B)
reaction conditions.
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RESULTS AND DISCUSSION
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We commenced our investigations towards the electroreduction of furfural in 0.5 M sulfuric acid
in combination with acetonitrile using a copper/stainless steel electrode pair (Table 1, Entry 1).
However, only a yield of 8% was obtained for the targeted furfuryl alcohol. When switching to a
more protic solvent, such as ethanol, an increase in yield towards 29% was witnessed (Table 1,
Entry 2). Interestingly, a further improvement was observed in the presence of hydrochloric acid
(Table 1, Entries 3-4). In contrast, changing the electrode material did not result into a further
increase in yield (Table 1, Entries 5-8), indicating that the combination of stainless steel/copper
is optimal. Finally, we also checked the need for supporting electrolytes which are required to
minimize the Ohmic drop in electrochemical processes. It is often claimed that microreactors do
not require any supporting electrolytes due to small inter-electrode gap.19,20 However, in our case,
small amounts of electrolyte (0.05 M), such as the ionic liquid Ammoeng 110 or n-Bu4NBr, were
beneficial to increase the overall yield.
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Table 1: Optimization for furfuryl alcohol production in acidic environmenta
Working
Electrodec
Counter
Entry
Electrolyted
Solvent
Supporting Electrolyte
Yield (%)b
Electrode
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4
5
6
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8
H2SO4
H2SO4
HCl
Copper
Copper
Stainless Steel
Stainless Steel
Stainless Steel
Stainless Steel
Stainless Steel
Stainless Steel
Graphite
CH3CN
EtOH
CH3CN
EtOH
EtOH
EtOH
EtOH
EtOH
--
--
--
--
--
--
--
--
8
29
33
41
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22
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Copper
HCl
Copper
HCl
Stainless Steel
Graphite
Copper
HCl
HCl
HCl
Copper
Copper
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HCl
HCl
Copper
Copper
Stainless Steel
Stainless Steel
EtOH
EtOH
Ammoeng 110
n-Bu4NBr
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[a] Reaction conditions: 0.1 M furfural, 0.1 M H+, 0.05 M supporting electrolyte, 10 % H2O, flow rate 0.075 mL/min, residence time 10
min. [b] GC-yield using GC-FID with external standard calibration. [c] The SAE international stainless steel type is 316L, the copper type
is Cu-DHP. [d] The molar concentration of HCl is 0.1 mol/L , the molar concentration of H2SO4 is 0.05 mol/L in stock solution.
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Next, we turned our attention to basic reaction conditions using potassium ethoxide in
ethanol.21,22 Interestingly, a substantial increase in the reaction yield was observed (Table 2).
Varying the electrode material showed that the best results were obtained with a copper/graphite
couple (Table 2, Entry 3) yielding the targeted furfuryl alcohol in 90 %. Other bases, such as
NaOH and KOH, did not lead to any further improvement (Table 2, Entries 6 and 7).
Table 2: Optimization for furfuryl alcohol production in basic environmenta
Working
Electrodec
Counter
Entry
Electrolyte
Yield (%)b
Electrode
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2
3
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7
C2H5OK
C2H5OK
C2H5OK
C2H5OK
C2H5OK
NaOH
Graphite
Stainless Steel
Copper
Graphite
Graphite
25
46
90
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10
41
78
Graphite
Copper
Stainless Steel
Copper
Copper
Copper
Graphite
KOH
Copper
Graphite
[a] Reaction conditions: 0.1 M furfural, 0.05 M electrolyte, ethanol solvent, flow rate 0.075
mL/min, residence time 10 min. [b] GC-yield using GC-FID with external standard
calibration. [c] The SAE international stainless steel type is 316L, the copper type is Cu-DHP.
The counter electrode is graphite.
Given the importance of the use of acids or bases to effect the desired transformation, we next
turned our attention to scan a wide variety of different concentrations (Table 3). For this specific
experiment, our home-made electrochemical reactor design provided a clear and unique
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advantage as eight reaction conditions can be screened simultaneously in parallel in the eight
different channels. Hence, we rapidly found that 0.10 M HCl and 0.05 M C2H5OK provided
optimal results for electro-reduction of furfural.
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Table 3: Optimization of electrolyte via 8-channel simultaneously parallel reactions
Channela
Electrolyte
0.05 M HCl
0.07 M HCl
0.09 M HCl
0.10 M HCl
0.11 M HCl
0.13 M HCl
0.15 M HCl
0.17 M HCl
Yield (%)c
Channelb
Electrolyte
Yield (%)c
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0.01 M C2H5OK
0.02 M C2H5OK
0.03 M C2H5OK
0.04 M C2H5OK
0.05 M C2H5OK
0.06 M C2H5OK
0.07 M C2H5OK
0.08 M C2H5OK
38
50
62
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90
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83
[a] Reaction conditions: 0.1 M furfural, 2.90 V, 10% H2O, ethanol solvent, flow rate 0.075 mL/min, residence time 10
min. [b] Reaction conditions: 0.1 M furfural, 2.20 V, ethanol solvent, flow rate 0.075 mL/min, residence time 10 min.
[c] GC-yield using GC-FID with external standard calibration.
Having determined the relative importance of the reaction partners, we investigated the influence
of the reaction time (Figure 3). Interestingly, in both acidic and basic media, the optimal
residence time to reach full conversion is only 10 minutes. We believe this very short residence
time can be attributed to the large electrode surface-to-volume ratio and the short diffusion
distances observed in the electrochemical microreactor. Indeed, when we carried out the reaction
in a batch electrochemical cell, the yield dropped to 18% in acidic medium and 31% in basic
conditions and require 6 hours of reaction time.
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Figure 4: Influence of the residence time on the electro-reduction of furfural to furfuryl alcohol
in acidic and basic media.
Next, we tested the influence of constant current or constant potential electrolysis conditions
(Figure 5). Constant potential electrolysis offers a more stable and tunable selectivity,17 while
constant current electrolysis provides a more controlled and constant reaction conversion.23
However, for the electroreduction of furfural, almost similar behavior was observed under
these two operational regimes. This gives us a high level of flexibility: if higher selectivity is
required, one can use the constant potential conditions, while galvanostatic conditions are
preferred to establish an adequate chemical conversion in shorter time scales. However, it must
be noted that even though we did use potentiostatic reaction conditions, the flow processing
enables us the keep the conversion time very short as described above (tR = 10 min, see Figure
4).
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Potential (V)
(C)
Current (mA)
(D)
Figure 5: Constant-potential (A)(C) vs constant-current (B)(D) electrolysis.
Having the optimal reaction conditions for the electroreduction of 2-furfural in hand (Table 4,
Entries 1 and 2), we investigated if the conditions were also applicable to 3-furfural (Table 4,
Entries 5 and 6). Interestingly, the same trend was observed for 3-furfural with the most optimal
conversion and yield observed in basic media (Table 4, Entry 6). In addition, for all the different
conditions, we calculated the Faradaic efficiency: High Faradaic efficiencies were obtained for
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the basic reaction conditions (90-92%), while in acidic media very poor Faradaic efficiencies
were observed (18-19%). This can be attributed to the generation of higher amounts of hydrogen,
as the hydrogen evolution reaction competes more efficiently with the electrocatalytic
hydrogenation reaction in acidic reaction conditions. Interestingly, the corresponding batch
conditions were not only considerably slower, but also provided lower yields and Faradaic
efficiencies (Table 4, Entries 3 and 4). We surmise that the slower reaction times are due to mass
transfer limitations and lower electrode surface-to-volume ratios and give more time for the
undesired hydrogen formation reaction to occur.
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Table 4: Scope for furfural electroreduction in flow and comparison with batch reaction
conditions.
Reactor Type
Faradaic
Efficiency (%)
Entry
Substrate
Electrolyte
Yield (%)[e]
1[a]
2[b]
3[c]
4[d]
3[a]
4[b]
Flow
Flow
Batch
Batch
Flow
Flow
furfural (2-furaldehyde)
furfural (2-furaldehyde)
furfural (2-furaldehyde)
furfural (2-furaldehyde)
3-furfural (3-furaldehyde)
3-furfural (3-furaldehyde)
HCl
C2H5OK
HCl
55
90
18
31
40
78
19
90
12
60
18
92
C2H5OK
HCl
C2H5OK
[a] Reaction conditions: 0.1 M furfural, 2.90 V, 0.1 M electrolyte, 10% H2O, ethanol solvent, flow rate 0.075
mL/min, residence time 10 min. [b] Reaction conditions: 0.1 M furfural, 2.20 V, 0.05 M electrolyte, ethanol
solvent, flow rate 0.075 mL/min, residence time 10 min. [c] Reaction conditions: 0.1 M furfural, 2.90 V, 0.1 M
electrolyte, 10% H2O, ethanol solvent, residence time 300 min. [d] Reaction conditions: 0.1 M furfural, 2.20 V,
0.05 M electrolyte, ethanol solvent, residence time 300 min [e] GC-yield using GC-FID with external standard
calibration.
Finally, we investigated the steady state production of 2-furfuryl alcohol under these acidic and
basic reaction conditions. We took reaction samples every 5 min and analyzed the obtained yield
via GC-FID. The overall observed trend for both conditions showed a decline in yield over time.
This decline can be attributed to erosion of the electrode efficiency. It was noteworthy, however,
that the efficacy of the electrodes could be restored by incorporating an adequate cleaning and
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surface treatment step (Figure 6). This indicates that the reduced yield is caused by a deposition
on the electrode surface which can be washed away effectively.24 In order to keep the reaction
yield constant, we could place two reactors in parallel: one of them is in operation mode while
the other one is in cleaning mode. When the yield drops, the modes can be switched and a
“continuous” production of electrocatalytic reduction of furfural can be simulated.
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91.0
90.5
90.0
89.5
89.0
88.5
88.0
87.5
87.0
86.5
86.0
55.5
55.0
54.5
54.0
53.5
53.0
52.5
52.0
51.5
51.0
50.5
Basic
Acidic
60
10*
Duration (min)
20*
30*
40*
50*
60*
10
20
30
40
50
Figure 6: Two runs of stability test (60 min) for reactions (Table 4, Entries 1 and 2) with 10 min
residence time.
CONCLUSION
In conclusion, we have developed a continuous-flow protocol for the electrocatalytic reduction
of furfural. Both acidic and basic reaction conditions were evaluated. We found that the basic
reaction conditions delivered the highest yield for the targeted furfuryl alcohol and provided a
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high Faradaic efficiency. The reaction could be completed in only 10 minutes and required only
a minimum amount of supporting electrolyte, highlighting the benefits of the microreactor
environment. While an efficient process was established in flow, a reduced efficiency was
observed due to pollution of the electrodes. However, we found that a cleaning step could
alleviate this issue effectively.
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AUTHOR INFORMATION
Corresponding Author
*t.noel@tue.nl
Notes
The authors declare no competing financial interest
ACKNOWLEDGMENTS
The authors acknowledge the support from China Scholarship Council (CSC)
SUPPLEMENTARY MATERIAL
The Supplementary Material contains (i) design details of the electrochemical batch and flow
reactor; (ii) details on the calculation of the Faradaic Efficiency; (iii) GC results for the optimal
reaction conditions; (iv) A detailed procedure for the cleaning of our electrodes.
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