Electrochemical reductive amination of furfural-based biomass intermediates

Article abstract of DOI:10.1039/c6gc01541g

Electrochemical reductive amination uses water as the hydrogen source without requiring other chemicals as the reducing agent, providing reaction conditions that are more environmentally benign. The catalytic abilities and efficiencies of various metal electrodes (Ag, Cu, Pt, Sn, Zn) were investigated for the electrochemical reductive amination of 5-hydroxymethylfurfural (HMF), a key intermediate for biomass conversion, with methylamine (CH₃NH₂). For each metal electrode, the potentials necessary to initiate reductive amination as well as potential dependent Faradaic efficiency (FE) and selectivity were investigated systematically. Based on the results, reduction conditions and a high surface area Ag electrode that can achieve a FE and a selectivity nearing 100% were reported. Furthermore, reductive amination of HMF derivatives such as 5-methylfurfural (5-MF), 2,5-diformylfuran (DFF), and 5-formyl-2-furancarboxylic acid (FFCA) using methylamine as well as reductive amination of HMF using ethanolamine (NH₂CH₂CH₂OH) were conducted to establish electrochemical reductive amination as a general route for the reductive amination of furfural-based biomass intermediates.

Full text of DOI:10.1039/c6gc01541g

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Electrochemical reductive amination of
furfural-based biomass intermediates†
Cite this: DOI: 10.1039/c6gc01541g
Received 4th June 2016,
Accepted 13th September 2016
John J. Roylance and Kyoung-Shin Choi*
DOI: 10.1039/c6gc01541g
www.rsc.org/greenchem
1
–4,14–17
Electrochemical reductive amination uses water as the hydrogen cellulosic matter.
Among the many reactions HMF may
2
–13,17–24
source without requiring other chemicals as the reducing agent, undergo (e.g. oxidation, reduction, etherification),
providing reaction conditions that are more environmentally reductive amination, which adds an amine group to a hydro-
benign. The catalytic abilities and efficiencies of various metal carbon framework, enables the synthesis of more diverse
electrodes (Ag, Cu, Pt, Sn, Zn) were investigated for the electro- biomass-driven compounds. For example, aminated HMF
chemical reductive amination of 5-hydroxymethylfurfural (HMF), a derivatives having an amine group on one end and carboxylic
key intermediate for biomass conversion, with methylamine acid on the other can be used as a monomer for the formation
2
5
(
3 2
CH NH ). For each metal electrode, the potentials necessary to of polyamides such as nylon. Also, various aminomethyl-
initiate reductive amination as well as potential dependent hydroxymethylfuran derivatives produced from furfural-based
Faradaic efficiency (FE) and selectivity were investigated systemati- molecules show a wide range of pharmaceutical activities (e.g.
cally. Based on the results, reduction conditions and a high surface anticancer therapies, diuretics, antihypertensive, and anti-
2
6–30
area Ag electrode that can achieve a FE and a selectivity nearing muscarinic agents).
00% were reported. Furthermore, reductive amination of HMF tical routes for the production of aminated furfural-based
derivatives such as 5-methylfurfural (5-MF), 2,5-diformylfuran molecules will be highly beneficial.
DFF), and 5-formyl-2-furancarboxylic acid (FFCA) using methyl- The reductive amination of furfurals involves the conversion
amine as well as reductive amination of HMF using ethanolamine of the formyl group to an amine group, which is commonly
Therefore, developing facile and prac-
1
(
3
0–32
(
NH
2
CH
2
CH
2
OH) were conducted to establish electrochemical accomplished in two steps as illustrated in Scheme 1.
In
reductive amination as a general route for the reductive amination this scheme the aldehyde is first converted to an aldimine (Step
of furfural-based biomass intermediates.
1 in Scheme 1) by reaction with ammonia or a primary amine.
Formation of the aldimine has traditionally utilized a concen-
trated or liquid amine source and nonaqueous solvents.
With the ever increasing concern over fossil fuel supply, which
has traditionally been the feedstock to produce a diverse range
of goods including fuels, polymers, organic solvents, and even
pharmaceutical compounds, the research and development of
biomass-based alternatives for fuels and commodity chemicals
33–36
However, aldimine can also be formed in aqueous media under
pH conditions where a large portion of ammonia or amine is
37,38
present in its unprotonated form.
The stability of aldimine
is pH dependent and lowering the pH can cause the hydrolysis
1
–13
has gained significant attention.
In the course of develop-
36
of the aldimine back to the aldehyde.
ing biomass-based alternatives, raw biomass is processed to a
number of intermediates. 5-Hydroxymethylfurfural (HMF) is
one such intermediate that has been the subject of consider-
able attention and is regarded as a “sleeping giant” of building
block chemistry, being listed among the top 10 biomass-based
The second step of reductive amination is reduction of the
aldimine to an amine by the hydrogenation of the CvN bond
4
platform chemicals by the US Department of Energy. HMF is
produced by the dehydration of hexoses, such as glucose in
Department of Chemistry, University of Wisconsin – Madison, 1101 University Ave,
Madison, WI 53706, USA. E-mail: kschoi@chem.wisc.edu
Scheme 1 The general reductive amination pathway. Step 1: formation
†
Electronic supplementary information (ESI) available: NMR spectra of the ald-
of an aldimine by reaction of an aldehyde and a primary amine. Step 2:
imine solutions, XRD of Aggd, SEM images of Aggd, Ag, Cu, Sn, and Zn electrodes
before and after electrochemical reductive amination, LSVs of other furfural-
based aldimines. See DOI: 10.1039/c6gc01541g
2
reduction either (a) by hydrogenation using H as the hydrogen source
+
2
or (b) by hydrogenation using H from H O as the hydrogen source
accompanied by an electron source.
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(Step 2 in Scheme 1). This process has commonly been
achieved by using H over RANEY® Ni or precious metal cata-
2
2
9,32,34,39,40
lysts (e.g. Au, Ir, Pd, Pt, Rh, and Ru).
Recent efforts
have been made to utilize inexpensive alternatives, such as
3
1
FeNi alloy catalysts. However, for this method H , which is a
2
Scheme 2 Electrochemical reductive amination of HMF. Reversible for-
valuable fuel that must be generated from other primary
energy sources, needs to be consumed. Hydrogenation can
mation of an aldimine by reaction of the formyl group in HMF with
+
3 2
CH NH in Step 1 followed by electrochemical hydrogenation using H
also be achieved by using a hydride (e.g. sodium borohydride, from H O as the hydrogen source in Step 2.
2
sodium cyanoborohydride, and sodium triacetoxyboro-
3
0,39,41
hydride)
water as the hydrogen source.
require the consumption of reducing agents, significant solu- to 11 ensures a significant portion of the amine to be present
tion cleanup, and disposal of waste that can be toxic.
as a base form (0.5 M) to react with HMF to form aldimine. An
or by using Zn powder as a reducing agent and
3
6,38
+
These reactions, however, Since the pK of CH NH is ∼10.6, adjusting the solution pH
a
3
3
Electrochemical reductive amination can offer an attractive NMR spectrum of 20 mM HMF in a methylamine solution (pH
alternative approach by using water as the hydrogen source 11.0) shows that all HMF in this solution is converted to ald-
without requiring other chemicals as the reducing agent, pro- imine (Fig. S1a†). When the pH was lowered to 9.0 or 7.0, the
viding reaction conditions that are more environmentally acid–base equilibrium of the amine was shifted, resulting in
benign. However, few studies have been reported previously for hydrolysis of the aldimine to the aldehyde. As a result, HMF
the electrochemical reductive amination of substrates includ- was recovered to near completion by pH 7.0 (Fig. S1b and c†).
ing cyclohexanone and derivatives, acetone, hexanal, and 2,5-
The reduction of the aldimine to the amine product,
These studies commonly used mercury or 2-hydroxymethyl-5-(methylaminomethyl)furan
(HMMAMF)
3
7,42–45
hexanedione.
lead electrodes which are known to have high overpotentials (Step 2 in Scheme 2), was first examined by performing linear
for the hydrogen evolution reaction, the main competing reac- sweep voltammetry (LSV) using Pt, Cu, Sn, Zn, and Ag metal
tion for electrochemical reductive amination. No systematic electrodes using methylamine solution with and without
attempts to identify the optimum substrate types for catalytic 20 mM HMF (Fig. 1). The cathodic current generated in the
electrodes in terms of increasing Faradaic efficiency (FE) or absence of HMF can be attributed to the reduction of water to
minimizing the overpotential necessary for reductive amina- H2 as there is no other species that is reductively active.
tion have been made to date. It would be highly beneficial to Changes to the current profile upon addition of HMF, particu-
identify optimum non-toxic catalytic electrodes and operating larly a shift in the current onset to the positive direction, can
conditions to perform reductive amination in aqueous media
without a significant loss of FE to water reduction.
In this study, we investigated and compared the catalytic
ability of various metal electrodes (Pt, Cu, Sn, Zn, and Ag) for
the reductive amination of HMF with methylamine. These
electrodes are non-precious and non-toxic while being known
to be poorly catalytic for water reduction except for Pt, which
was chosen to serve as a control electrode. For each metal elec-
trode, potentials necessary to initiate reductive amination as
well as potential dependent FE were investigated systemati-
cally. We discovered that Ag and Zn electrodes show an excep-
tional ability for the reductive amination of HMF. Based on
these results, we report optimum electrodes and reduction
conditions that can achieve a FE and a selectivity nearing
1
00%. We also demonstrate the reductive amination of HMF
derivatives such as 5-methylfurfural (5-MF), 2,5-diformylfuran
DFF), and 5-formyl-2-furancarboxylic acid (FFCA) using
methylamine as well as the reductive amination of HMF using
ethanolamine (NH CH CH OH) to establish electrochemical
(
2
2
2
reductive amination as a general route for the reductive amin-
ation of furfural-based biomass intermediates.
A 0.7 M aqueous methylamine buffer solution (pH 11.0)
containing 20 mM HMF was prepared for electrochemical
reductive amination. Aldimine formation is pH sensitive
because amines go through base dissociation reaction in
aqueous media and only the unprotonated base form of the
Fig. 1 LSVs of (a) Pt, (b) Cu, (c) Zn, (d) Sn, and (e) Ag electrodes in 0.7 M
methylamine buffer solution (pH 11.0) with (red) and without (black)
amine can undergo aldimine formation (Step 1 in Scheme 2). 0.02 M HMF (scan rate 5 mV s− ).
1
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be considered an indication that the reductive amination of Table 1 Results obtained from the electrochemical reductive amination
a
of HMF by Cu, Zn, Sn, Ag, and Aggd electrodes at various potentials
HMF is favored over water reduction.
The LSVs obtained for a Pt electrode with and without HMF
are shown in Fig. 1a. When HMF was added, the cathodic
onset was shifted to the negative direction with a decrease in Elec. E (V) vs. FE
current density. This suggests that the reduction of aldimine is
Average
current
Rate of
HMMAMF
formation
−2
Selectivity density −
2
(μmol cm min−1
)
mat. Ag/AgCl (%) (%)
(mA cm
)
not favored over water reduction on Pt and aldimine adsorp-
tion on the Pt surface suppresses water reduction by blocking
Cu
−1.2
−1.3
84
71
39
26
96
65
68
69
9.02
14.1
34.2
46.0
0.789
1.04
1.38
1.22
−
1.4
1.5
a portion of the active sites for H
2
evolution.
−
When Cu was used, a slight onset shift to the positive direc-
tion was observed, indicating that aldimine reduction is more Zn
favored than water reduction on Cu (Fig. 1b). However, catho-
dic current in the higher overpotential region (E < −1.41 V vs.
Ag/AgCl, −0.56 V vs. RHE), where both aldimine and water
reduction can occur at a considerable rate, was less than the
cathodic current observed without HMF. This suggests that in
this potential region, aldimine reduction is slower than water
−1.3
82
85
9.83
21.6
28.6
35.9
0.831
2.14
2.77
3.34
−1.4
−1.5
−1.6
95 >99
94 >99
90 >99
Sn
−1.3
70
72
7.08
21.4
27.9
37.0
0.515
2.08
2.41
2.96
−
1.4
1.5
94 >99
83 >99
77 >99
−
−1.6
reduction and interferes with water reduction. Consequently,
the reduction of both aldimine and water results in a smaller
current than water reduction alone.
Ag
−1.1
79 >99
83 >99
68 >99
51 >99
6.81
12.2
18.8
41.0
0.558
1.05
1.33
2.17
−
1.2
−1.3
1.4
−
When Zn was used, the most dramatic shift in cathodic
current onset to the positive direction (by 250 mV) was
observed upon the addition of HMF (Fig. 1c). Also, the catho-
dic current is significantly higher for all potential regions
when HMF is present. This shows that Zn is particularly cata-
lytic for the reductive amination of HMF.
Aggd
−1.0
−1.1
85 >99
>99 >99
91 >99
83 >99
59 >99
3.86
18.5
26.7
33.0
48.8
0.339
1.91
2.53
2.83
2.96
−1.2
−1.3
−1.4
a
Reported values are averages obtained from three or more measure-
For the case of Sn and Ag, a shift in the cathodic onset of
ments. Reaction conditions: 0.7 M methylamine buffer (pH 11.0) con2-
2
00 mV to the positive direction was observed when HMF was taining 0.02 M HMF for 20 C passed at room temperature with 3.0 cm
electrodes (geometric area).
added. Also, a well-defined diffusion-limited reduction peak
for aldimine was observed before water reduction, which
clearly confirms that HMF reduction is favored over water
reduction on the Sn and Ag surface (Fig. 1d and e). In terms of for aldimine reduction gradually decreases as water reduction
the onset potential for aldimine reduction, the Ag electrode becomes more favorable.
showed the best performance as its onset for HMF reduction
occurred at as early as −0.97 V vs. Ag/AgCl (−0.12 V vs. RHE).
The Zn, Sn, and Ag electrodes showed high FE for aldimine
reduction in a very wide potential region because there is a
Using the LSVs as a guide for potential regions of interest, wide window of potential where HMF reduction can occur
reductive amination under constant potential was performed to before water reduction initiates. The most efficient condition
investigate the effect of the potential on amine formation and for reductive amination in terms of FE was achieved by Zn at
FE. Results for HMF reductive amination with Ag, Cu, Sn, and −1.4 V vs. Ag/AgCl (−0.55 V vs. RHE) with 95% FE. In fact, the
Zn can be seen in Table 1. The FE and selectivity of each reac- FE of Zn remained near 90% to E = −1.6 V vs. Ag/AgCl (−0.75 V
tion were calculated using the following equations, where F is vs. RHE), at which point H
the Faraday constant (96 485 C mol ) and n is the number of according to the LSVs (Fig. 1e). However, in terms of the over-
evolution becomes prevalent
2
−1
electrons required for the conversion of the aldimine to an potential required, Zn was not as good as Ag. Although the
amine, which is 2 for all substrates except for DFF, where n = 4.
maximum FE achieved by Ag was slightly less at 83%, it was
achieved at −1.2 V vs. Ag/AgCl where Zn generates a negligible
current for aldimine reduction. As the potential becomes more
negative, the FE for aldimine reduction by all metals com-
monly decreased because water reduction becomes more
favorable.
mol of amine formed
Total charge passed ðCÞ=ðF ꢀ nÞ
FEð%Þ ¼
ꢀ 100%
ð1Þ
mol of amine formed
mol of HMF consumed
Selectivityð%Þ ¼
ꢀ 100%
ð2Þ
The selectivity of the aldimine reduction was approximately
100% for most metals and potential conditions. Cu is the
The Pt electrode did not produce any HMF-related notable exception, where the selectivity decreased when
reduction products, suggesting that all the cathodic current moving to more negative potentials. Judging from the NMR
generated was associated with water reduction. The Cu elec- results, the main reason for the decrease in selectivity
trode shows a FE of 84% for aldimine reduction at −1.2 V vs. appeared to be the formation of dimers and oligomers of
Ag/AgCl but as the potential becomes more negative, the FE aminated HMF, although the exact structures of these side-
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products were difficult to identify. The electrode dependence achieving the highest FE at the lowest overpotential. While
of aldimine reduction shown in Fig. 1 and Table 1 suggests Aggd is highly catalytic for the reduction of HMF of BHMF,
that aldimine reduction occurs via inner sphere electron trans- since all HMF is present as aldimine in the amine buffer solu-
2
0
fer like HMF reduction to 2,5-bis(hydroxymethyl)furan. This tion (pH 11.0) used in this study, BHMF production cannot
means that the initial step of aldimine reduction involves the compete with reductive amination, ensuring a high FE for ald-
formation of surface-bound intermediates, which can be imine reduction.
released as aminated HMF by completing the final reduction
The LSVs of the Aggd electrode for water and aldimine
2
0
step. If an increase in the potential applied to Cu facilitates reduction are shown in Fig. 2b in comparison with those of the
the formation of surface bound intermediates more favorably plain Ag electrode. The current onset for water reduction is −1.2
than the completion of the final reduction step, the coverage V vs. Ag/AgCl (−0.35 V vs. RHE) for both Ag and Aggd, though
of the surface-bound intermediates will increase as the applied the current increases at a greater rate moving to more negative
potential increases and the chances for the interaction potentials for Aggd, likely due to the increased surface area. In
between the surface-bound species, and thus the formation of the presence of HMF, while Ag shows the shift of onset to the
dimers or oligomers of aminated HMF, can increase. For the positive direction by 200 mV, Aggd shows the shift of onset by
case of Sn and Zn, the formation of dimers and oligomers of 300 mV, providing a wide window of overpotential where only
amine was observed only in the low potential region (−1.3 V aldimine can be reduced. The onset shift observed for Aggd by
vs. Ag/AgCl). All other potentials yielded HMMAMF exclusively adding HMF is even greater than that of Zn, which suggests
as a conversion product of HMF. This suggests that for these that Aggd is particularly catalytic for aldimine reduction.
electrodes an increase in the applied potential expedites the
release of the aminated products more significantly and, there- for the reductive amination of HMF on Aggd is >99% at −1.1 V
fore, the probability to form dimers or oligomers decreases. vs. Ag/AgCl (−0.25 V vs. RHE), which is remarkable. The FE for
The ideal catalytic electrode should achieve a high FE for reductive amination decreases for E ≤ −1.2 V vs. Ag/AgCl as H
aldimine reduction, like Zn, but requires a lower overpotential, evolution initiates. As in the case of Ag, other than H2,
The constant potential reduction results show that the FE
2
like Ag. We have recently demonstrated that high surface area HMMAMF was the only reduction product produced by Aggd
dendritic Ag electrodes prepared by the galvanic displacement resulting in a selectivity of >99% for HMMAMF formation.
,
of Cu (referred to as Ag ) (Fig. 2a) are particularly catalytic for
The surface morphologies of all the electrodes before and
gd
the electrochemical reduction of HMF to 2,5-bis(hydroxy- after reductive amination are shown in Fig. S3.† No signs of
2
0
methyl)furan (BHMF) in a pH 9.2 borate buffer solution. The surface changes were observed for the Ag, Aggd, and Sn electro-
XRD of the Aggd electrode can be found in the ESI (Fig. S2†). des. For the Cu and Zn electrodes, minor surface restructuring
We discovered that the onset potentials for water reduction by was observed, which should be due to the interactions with
Ag and Aggd were similar but the onset potential for HMF aldimine and not due to dissolution as these metals were cath-
reduction on Aggd was significantly more positive than on the odically protected during the reduction process.
Ag electrode, providing a wide potential window where HMF
To further assess the viability of an electrochemical reduc-
can be reduced effectively before water reduction initiates. As a tive amination, the Aggd electrode was used to investigate the
result, a FE as high as 99% was obtained by the Ag electrode reductive amination of various HMF-based substrates, which
gd
2
0
for BHMF production.
included 5-MF, DFF, and FFCA (Fig. 3a–c). The reductive amin-
We postulate that the Aggd electrode may also be highly
catalytic for the reduction of the aldimine form of HMF,
Fig. 2 (a) SEM of a Aggd electrode and (b) LSVs of a Aggd electrode with
Fig. 3 Electrochemical reductive amination of (a) 5-MF, (b) DFF, and (c)
(red, solid) and without (black, solid) 0.02 M HMF compared with LSVs of a
2
FFCA with methylamine and (d) HMF with ethanolamine (NH EtOH). The
Ag electrode with (red, dashed) and without (black, dashed) 0.02 M HMF in
a 0.7 M methylamine buffer solution (pH 11.0) (scan rate 5 mV s ).
potentials used for reductive amination and FE for each case are also
shown.
−1
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ation of FFCA results in an artificial amino acid, which could Department of Energy through Grant DE-SC0008707. The
have implications in food and drug applications as well as authors thank Stephen R. Kubota for his help with revisions.
2
5,46
polymer synthesis.
The reductive amination product of
4
6
DFF could also have applications in polymer synthesis.
The results show that these substrates were efficiently con-
verted to the corresponding amines with high FEs (>90%). The
efficiency for the reductive amination of DFF appeared to be
low with an observed FE of 51%. However, this is likely due to
the low solubility of the resulting amine, as the product solu-
tion became cloudy and a small amount of precipitate could
be seen after reductive amination. Judging from the fact that
there are no other soluble DFF related reduction products
detected by NMR analysis and that water reduction is not
efficient at −1.2 V vs. Ag/AgCl, we believe that the actual FE
value for the reductive amination of DFF should be much
higher than the observed value.
We also tested the reductive amination of HMF using
ethanolamine instead of methylamine in a 0.7 M ethanol-
amine solution containing 20 mM HMF (Fig. 3d). The use of
ethanolamine results in the addition of an alcohol group as
well as an amine group, further diversifying the chemistry the
product can undergo. Our result shows that the reductive
amination of HMF with ethanolamine can be achieved with a
FE of 92%. LSVs obtained for all reactions shown in Fig. 3 can
be found in Fig. S4.†
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