Communication
Green Chemistry
opening of DMF to HD is shown in Fig. 4b as an example.30 not involve hydrogenolysis, both require only 4 electrons and 4
Consequently, one carbonyl oxygen of HD is from the furan protons. The relationship between the reduction processes of
oxygen and the other carbonyl oxygen is from water.
these three reactions are demonstrated in Fig. S3.†
A key remaining question in proposing a mechanism for
In summary, we demonstrated that Zn electrodes have the
the reduction of HMF to HD is when the hydrogenolysis of unique ability to electrochemically convert HMF to HD in
HMF occurs (e.g. before, during, or after Clemmensen acidic aqueous media under ambient condition using only
reduction). In order to examine whether the hydrogenolysis water as the H source. The overall reaction is composed of
step of HMF is critical for reductive ring opening, we per- hydrogenolysis and Clemmensen reduction coupled with
formed the reduction of 5-methylfurfural (5-MF) on Zn using furan ring opening. FE and selectivity as high as 72.4% and
the same reduction conditions (Fig. 3b). 5-MF is an analogous 81.6%, respectively, were achieved for HMF reduction at −1.2 V
compound of HMF having a furan ring and a formyl group, vs. Ag/AgCl in a pH 2 solution, which was an optimal condition
but without an alcohol group. The result shows that the same to suppress H2 evolution and the conversion of HMF to BHMF.
HD was obtained as the only detectable HMF reduction Comparing the reduction of HMF and 5-MF on Zn, a plausible
product (FE = 38.3%). The fact that the absence of the alcohol mechanism for the formation of HD and HHD were proposed.
group does not affect the reductive ring-opening process of the The unique reduction pathway that can directly convert HMF
furan ring suggests that the hydrogenolysis of HMF to 5-MF is or 5-MF to HD reported in this study will serve as a new route
an independent step from reductive ring opening and it most to valorize biomass intermediates.
likely occurs before reductive ring opening.
By combining Clemmensen reduction and ring-opening
mechanisms, a predictive model for the reduction of 5-MF to
HD on the Zn surface is proposed in Fig. 4c. The resemblance
Acknowledgements
of the predictive model to Clemmensen reduction and ring- This study was supported by University of Wisconsin-Madison
opening mechanisms can be easily seen by comparing Fig. 4c and the U.S. Department of Energy, Office of Science, Office
with Fig. 4a and b, where Steps 1, 2, 3, and 7 are associated of Basic Energy Sciences, Division of Chemical Sciences,
with Clemmensen reduction while steps 4–6 are associated Geosciences, and Biosciences under Award Number
with ring opening. As in the case of the conversion of DMF to DE-SC0008707.
HD, one carbonyl oxygen of HD is from the furan oxygen of
5-MF and the other carbonyl oxygen is from water while the
formyl group of 5-MF is converted to the terminal alkyl group
by Clemmensen reduction. In the conventional Clemmensen
Notes and references
reduction Zn serves as the electron source and is oxidized to
Zn2+. However, in our electrochemical cell, electrons are pro-
vided from the half reaction occurring at the anode (water oxi-
dation). Therefore, the catalytic surface of Zn is not oxidized
during the reduction process. Scanning electron microscopy
(SEM) images of the Zn electrode before and after the HMF
reduction are shown in Fig. S2.† After HMF reduction, the Zn
surface was roughened, suggesting that the Zn surface was
restructured during the catalytic reaction by HMF–Zn inter-
actions. However, after initial restructuring, no further altera-
tion of the Zn surface was observed when the reduction was
prolonged.
A plausible mechanism for the reduction of HMF to HD is
also shown in Fig. S3† where hydrogenolysis of HMF precedes
ring opening and the remaining reduction steps are exactly the
same as those shown for the reduction of 5-MF. As mentioned
earlier, HMF reduction also produced a small amount of
HHD, which was not detected from reduction of 5-MF. This
can be explained by the possibility that a small fraction of
HMF goes through reductive ring opening without going
through hydrogenolysis first. A plausible mechanism for this
pathway is also shown in Fig. S3.† In this case, the final ring-
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