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observation of trimerized products in THF provided a clue to
the formation of trimers by further condensation reactions.
Similarly, the ESI-MS spectra of the products after reacting
fructose in MIBK, GVL, and NMP also illustrated the formation
of oligomers derived from the degradative condensation of
fructose, for which the signals at m/z 301 to 384 were mainly
ascribed to the di- or trimerized species after releasing AA or
FA molecules from fructose in MIBK (Figure 1c), the signals at
m/z 223 and 319 were assigned to dimerized species [2Fru–
solvents, theoretical calculations of the IR spectra of each tau-
tomer were performed by using Gaussian 09 software at the
B3LYP/6-31+G(d,p) level of theory. Considering the overlap-
ping of IR signals between a- and b-configured tautomers,
only the characteristic IR bands attributed to b-furanose, b-pyr-
anose, and open-chain fructose are shown in Table S3 in the
[21]
Supporting Information. Because of the exclusive attribution,
ꢀ
1
the IR band at 778 cm , which was assigned to the vibration
ꢀ
1
of C ꢀOꢀH in open-chain fructose, and 1084 cm which was
1
+
+
LA–C H O+Na] and [2Fru–FA–H O+Na] after releasing LA
assigned to the vibrations of C ꢀC , C ꢀOꢀH, and C ꢀOꢀH in
2
4
2
4
5
5
1
or FA molecules from fructose in GVL (Figure 1d), and the
pyranose, could be used to represent the characteristic signals
of open-chain fructose and fructopyranose, respectively.
+
signal at m/z 221 corresponded to [2Fru–LA–FA+Na] derived
ꢀ
1
from the condensation of degraded fructose fragment after
the release of FA or LA in NMP (Figure 1e). Additionally, the ap-
Although the band at 1725 cm , which was assigned to the
C=O stretching vibration, was also a characteristic IR band for
+
[28]
pearance of signals assigned to [Fru+GVL+Na] (m/z 303;
open-chain fructose, it was not an ideal characteristic signal
+
Figure 1d) and [Fru+NMP+Na] (m/z 302; Figure 1e) was ex-
on account of its overlap with the IR bands of solvents (i.e.,
MIBK, GVL, or NMP) that contained a C=O group.
plained by the possible aggregation of charged ions between
[
25]
fructose and solvent molecules after ionization. Notably, no
signals assigned to the products from the degradative conden-
The variations of the intensity of bands at 778 (I778) and
ꢀ
1
1084 cm (I1084) in various solvents are plotted in Figure 2b
[21]
sation of fructose were observed in DMSO (Figure 1 f). Instead,
and d and Figure S5b–S8b in the Supporting Information,
+
the appearance of signals corresponding to [2Fru-nH O+Na]
wherein the increase of the intensity of the band at an earlier
stage was indicative of the dissolution of fructose because
fructofuranose (crystalline fructose) in solvent would be trans-
ferred into fructopyranose via open-chain fructose to reach an
equilibrium tautomer distribution. After complete dissolution,
the intensity variations in the curves would further imply con-
figuration changes during the tautomerization process. From
the in situ ATR-IR spectra, we observed different dissolution
and tautomerization times for fructose in different solvents
2
(
n=1–6) species supported the mechanism of difructose anhy-
dride (DFA)-mediated fructose-to-HMF dehydration in DMSO
[
26]
because of the highly stable existence of caramel-like [2Fru–
+
[17b,27a,b]
2
H O+Na] (m/z 347) species.
However, it is interesting
2
+
to note that the [2Fru–H O+Na] (m/z 365) species was
2
found to be the main DFA species in DIO, THF, GVL, and NMP.
The different existence forms of the DFAs in various solvents
might be indicative of different conversion paths of fructose at
the initial reaction stage, which deserves a deeper study in our
future work.
[21]
(Table S4 in the Supporting Information). Fructose dissolved
quickly in GVL (15 min), NMP (5 min), or DMSO (3 min), but
slowly in DIO (30 min), THF (50 min), or MIBK (30 min). Mean-
while, faster dissolution favored a faster tautomerization equi-
librium. It was noted that a longer dissolution time (ꢁ30 min)
in DIO, THF, or MIBK favored the formation of shorter-chain
carboxylic acids (i.e., FA or AA) and oligomers, but the shorter
dissolution times (ꢂ15 min) in GVL, NMP, or DMSO favored the
formation of longer-chain products (i.e., LA or HMF) with or
without oligomers. Nevertheless, the same dissolution times in
DIO and MIBK led to different products (FA vs. AA), and the
same tautomerization equilibrium times in GVL and DMSO also
led to different products (LA vs. HMF), thereby indicating that
the time for dissolution and tautomerization equilibrium was
not the key factor that determined product evolution in the in-
itial stage of fructose conversion.
Significantly, we have also detected the formation of oligo-
mers as tetra-, penta-, hexa-, hepta-, and nonamers in the ESI/
MS spectra of the reaction mixture after reacting fructose in
the presence of HCl in various solvents (Figure S3 in the Sup-
[
21]
porting Information). Therefore, the di- and trimers formed
in the initial reaction stage of fructose conversion were sup-
posed to act as “precursors” or “cores” to trigger subsequent
oligomerization, typically cross-condensation with HMF or
other highly active intermediates during acid-catalyzed fruc-
tose dehydration. Considering this point, suppression of the
degradative condensation pathway in the initial reaction stage
of fructose dehydration is of particular importance to inhibit
the formation of humins.
Notably, we found that the formation of a special initial
product to accompany oligomers was related to the main fruc-
tose tautomer existing in a given solvent during both the dis-
solution and tautomerization processes. In DIO (Figure 2b), the
faster increase of I778 than that of I1084 was indicative of the
faster formation of open-chain fructose and slower formation
of fructopyranose, thereby revealing the main existence of
fructofuranose or open-chain fructose in DIO during the disso-
lution stage. After dissolution, the decrease of I778 illustrated
the gradual decrease of the open-chain fructose tautomer and
the nearly unchanged I1084 suggested a balanced content of
pyranose tautomer after fructose dissolution. Therefore, it was
In situ ATR-IR spectra for the initial fructose conversion in
typical solvents
To probe the effects of solvent on the formation of oligomers
through degradative condensation in the initial stage of fruc-
tose conversion, the in situ ATR-IR spectra of the reaction mix-
tures during fructose conversion at 1208C were monitored.
The raw spectra are shown in Figure S4 in the Supporting In-
[
21]
formation, whereas the spectra after deducting the solvent
bands to exclude solvent disturbance are exhibited in Fig-
ure 2a and c and Figure S5a–S8a in the Supporting Informa-
[
21]
tion. To assist recognizing the tautomers of fructose in the
ChemSusChem 2019, 12, 1 – 13
5
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ
Fu, Xing
