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of step 2) were immediately detected. Given that step 2 is faster
than 1, step 1 should be the limiting process during the
conversion of HMF into a mixture of HMFCA and FFCA.
Therefore, the decrease in HMF concentration was plotted to
represent the reaction rate of step 1. As shown in Figure 7, FDCA
Conflicts of interest
There are no conflicts to declare.
DOI: 10.1039/D0CC02206C
Notes and references
production in step 3 (rate = 1.22 10–9 M s–1) was much slower
×
than HMF consumption (rate = 4.58 10–8 M s–1) in step 1. Since
×
the concentration of two reactants are the same, the
comparison between their reaction rates can serve as a proxy
for the reaction rate constant – assuming similar potentials for
activation. Therefore, step 3 was identified as the rate-limiting
process during HMF oxidation. These results are further
supported by the linear sweep voltammetry (LSV) analysis
(Figure S4). As mentioned earlier, the onset potential for
HMFCA oxidation (step 2) is lower than that of step 1,
suggesting that step 2 is thermodynamically more efficient.
With similar onset potentials, FFCA oxidation (step 3) showed
smaller current than HMF oxidation in step 1, implying that step
3 is kinetically slower than step 1. Therefore, future efforts
should be on the development of catalysts that are highly
efficient in FFCA oxidation.
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Figure 7 The comparison of reaction rates of 0.5 mM HMF oxidation (black)
and 0.5 mM FFCA oxidation (red), under an applied potential of 1.55 V vs.
RHE for 40 min.
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In this work, a Ni(II) modified COF, TpBpy-Ni@FTO, was
prepared and used as an electrocatalyst for HMF oxidation in
alkaline solution. The HMF oxidation pathway was identified
following HMFHMFCAFFCAFDCA. Upon
a constant
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potential applied, a high conversion of HMF (96%) was
achieved, with FFCA (34% yield) and FDCA (58% yield) as
products. The last step, oxidation of FFCA into FDCA was
identified as the rate-determining step. While the FDCA yield is
not ideal, the TpBpy-Ni@FTO provides
a platform for
optimization of COF catalysts for biomass conversion.
This material is based upon work supported by the National
Science Foundation under Grant No. 1551964. We thank Dr.
Mehdi Ashraf-Khorassani for his help with HPLC. We thank Dr.
Xu Feng and the Surface Analysis Laboratory at Virginia Tech for
the XPS analysis supported by the National Science Foundation
under Grant No. CHE-1531834.
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4 | J. Name., 2012, 00, 1-3
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