Communications
ring hydrogenation over Cu/g-Al2O3, the conversion was carried
out with 2 wt% DMF in pure THF solvent to yield 4% of
DMTHF after 17 h at the same reaction conditions, demonstrat-
ing that Cu is not active for furan ring hydrogenation. Hence,
we propose a pathway for DMTHF formation from BHMF.
Our studies of HMF conversion in the THF–H2O solvent
(THF/H2O=95:5 w/w) over Cu/g-Al2O3, show that BHMF and
MF are the primary products formed, whereas the concentra-
tions of DMF and MHMF remained relatively low throughout
the reaction process. The products DMTHF, 12HD, and 25HD
were undetected. Previous studies of HMF hydrogenation in bi-
phasic 1-butanol–H2O reaction systems and in monophasic
THF–H2O reaction systems suggest that degradation product
formation originates from BHMF.[20] The overall selectivity to
unidentified products increased when the solvent system was
changed from 1-butanol–H2O to water or to THF–H2O mix-
tures, suggesting the involvement of water in degradation re-
actions for HMF hydrogenation reactions.
Based on previous literature studies as well as our experi-
mental observations for HMF conversion in THF, we suggest
that Scheme S1 is initially a reaction network to describe our
reaction system. Accordingly, we have used this reaction
scheme to build a kinetic model to quantify the reaction kinet-
ics for HMF conversion processes. We then use sensitivity anal-
yses and the Akaike information criterion (see the Supporting
Information) to identify the key aspects of this reaction
scheme for HMF conversion over the Cu/g-Al2O3 catalyst. After
a series of refinements to Scheme S1, we find that the more
simplified Scheme 2 achieves the best model fit with the high-
est number of products and the least number of reaction path-
ways. Scheme 2 still preserves the aforementioned characteris-
tics of parallel versus consecutive reactions. MHMF hydroge-
nolysis to DMF (k8), although mechanistically feasible, has
a small effect on DMF production compared to BHMF hydroge-
nolysis to DMF (k4). The rate of 25HD production (k11) is com-
bined with the DMF decomposition step (k10), because the
small amount of 25HD that is produced throughout the reac-
tion does not affect the overall concentration profiles.
Figure 1. Concentration profiles of HMF, reaction intermediates, and prod-
ucts in (a) pure THF solvent and (b) THF–H2O mixture (THF/H2O=95:5 w/w)
at 300 psi H2 pressure, 448 K, 550 rpm stirring speed, and with Cu/g-Al2O3
catalyst (200 mg in pure THF and 500 mg in THF–H2O). Data points and lines
represent experimental data and model, respectively. The compounds quan-
&
&
&
&
tified are as follows: HMF ( , c), BHMF ( , c), MF ( , c), MHMF ( ,
&
&
&
c), DMF ( , c), 12HD ( , c), DMTHF ( , c).
droxymethylfuran (MHMF), and DMTHF, as well as the ring-
opened compounds 1,2-hexanediol (12HD) and 2,5-hexanediol
(25HD). Key features of the concentration profiles in Figure 1
were then assessed in an attempt to construct a simplified re-
action scheme for purposes of reaction kinetics modeling. The
primary intermediates formed from HMF were BHMF and MF,
which are one-step hydrogenation and hydrogenolysis prod-
ucts from HMF. Further conversions of BHMF and MF to MHMF
exhibit the rise and fall of an intermediate concentration pro-
file in a consecutive reaction. In contrast, neither the MHMF
profile nor the DMF profile versus time behave as BHMF and
MF in consecutive reactions. Instead, they embody the charac-
teristics of being parallel reactions. Specifically, in pure THF sol-
vent the concentration of DMF is always greater than that of
MHMF, indicating that DMF is not solely produced from MHMF,
as proposed for noble metal catalysts. Therefore, we suggest
that over the Cu/g-Al2O3 catalyst used in this study, DMF can
also be produced from BHMF. In support of this perspective,
the evolution of the selectivity profile is provided in Figure S1
(in the Supporting Information). Furthermore, significant
DMTHF production by furan ring hydrogenation was not ob-
served over our Cu-based catalyst in pure THF solvent, where-
as DMTHF production from HMF is prevalent with noble
metals, such as Pd or Pt[9,19] To further explore the absence of
The kinetic model developed for describing Scheme 2 is
summarized in Equations (1)–(7).
rHMF ¼ Àððk1 þ k2Þ½HMFÞ Â n
rBHMF ¼ ðk1½HMF À ðk3 þ k4 þ k5 þ k6Þ½BHMFÞ Â n ð2Þ
rMF ¼ ðk2½HMF À k7½MFÞ Â n
ð1Þ
ð3Þ
ð4Þ
ð5Þ
ð6Þ
ð7Þ
rDMTHF ¼ k5½BHMF Â n
rMHMF ¼ ðk3½BHMF À k9½MHMFÞ Â n
rDMF ¼ ðk4½BHMF À k10½DMFÞ Â n
r12HD ¼ k9½MHMF Â n
where ri represents the rate of consumption of species i, n is
the number of moles of active sites of the catalyst, [i] is the
concentration of species i, and kx is the apparent reaction rate
constant for reaction step x in Scheme 2. A first-order rate de-
ChemSusChem 2015, 8, 3983 – 3986
3984
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