Evolution Process and Controlled Synthesis of Humins with 5-Hydroxymethylfurfural (HMF) as Model Molecule

Article abstract of DOI:10.1002/cssc.201902799

Elucidation of the chemical structure and formation mechanism of humins is a requisite to further improve the efficiency of acid-catalyzed biomass conversion. Through a low-temperature approach, the key intermediates resulting in the formation of 5-hydroxymethylfurfural (HMF)-derived humins were captured, revealing multiple elementary reactions such as etherification, esterification, aldol condensation, and acetalization. Through humin characterization, it was found out that the aldol condensation moiety between aldehyde group and levulinic acid is critical to justify the characteristic IR peaks (1620 and 1710 cm^?1) and aromatic fragments from pyrolysis GC–MS. Based on the investigations by means of HPLC–MS/MS, IR, pyrolysis GC–MS, and SEM, the structural models of humins at different temperatures were proposed, which are comprised of the elementary reaction types confirmed by the key intermediates. Humin structures with varying content of aldol condensation could be controllably synthesized under different reaction conditions (temperature and time), demonstrating the evolution process of HMF-derived humins.

Full text of DOI:10.1002/cssc.201902799

Accepted Article
Title: Evolution process and controlled synthesis of humins with 5-
hydroxymethylfurfural (HMF) as model molecule
Authors: Li Liu
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ChemSusChem
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RESEARCH ARTICLE
Evolution process and controlled synthesis of humins with 5-
hydroxymethylfurfural (HMF) as model molecule
[a]
[a]
[ab]
Haiyan Shen, Haozhe Shan, and Li Liu*
Abstract: Elucidation of the chemical structure and formation
mechanism of humins are requisite to further improve the efficiency
of acid-catalyzed biomass conversion. By the low-temperature
approach, the key intermediates resulting in the formation of HMF-
derived humins were captured, revealing multiple elementary
reactions such as etherification, esterification, aldol condensation,
and acetalization. Through humins characterizations, it was found
out that the aldol condensation moiety between aldehyde group and
to speculate the structure and mechanism of humins from
various feedstocks including cellulose, carbohydrates and HMF.
A big part of the literature report supports the notion that humins
formation could be ascribed to HMF. Moreover, compared with
cellulose or carbohydrate-derived humins, HMF-derived humins
are structurally less complicated. Herein, HMF was chosen as
the model molecule to simplify the humins investigation. By
singling out HMF model, the reaction complexity caused by the
additional constituents in the system could be greatly reduced,
thus enhancing the chances to identify the key intermediates
during the formation of humins.
-
LA is critical to justify the characteristic IR peaks (1620 and 1710 cm
1
)
and aromatic fragments from pyrolysis GC-MS. Based on the
investigations by means of HPLC-MS/MS, IR, pyrolysis GC-MS and
SEM, the structural models of humins at different temperatures were
proposed, which are comprised of the elementary reaction types
confirmed by the key intermediates. Our findings highlight the
humins structures with varying content of aldol condensation could
be controllably synthesized from different temperatures and times,
demonstrating the evolution process of HMF-derived humins.
Humins from hydrothermal treatment of biomass in the
absence of catalyst are usually called hydrothermal carbon
(
HTC) or hydrochar, which have been more thoroughly
[
5]
characterized than those from acid-catalyzed conversion.
Though close attentions have been paid to the formation and
structure of HTC, a unified conclusion has not been drawn either.
[6]
Matsumura and co-worker investigated hydrothermal treatment
of HMF under both subcritical and supercritical conditions,
whereas HTC was only observed under the subcritical
conditions. Under the supercritical conditions, HMF decomposed
rapidly and produced no HTC. While under the subcritical
conditions, HMF proceeded through two reaction pathways
concurrently: decomposition and polymerization. By means of
FT-IR spectroscopy and Raman spectrometry, they proposed
that HMF and aromatic liquid products from decomposition (e.g.,
Introduction
In view of diminishing fossil resources and increasing concern
about environmental protection, great attention has been turned
to produce fuels and chemicals from renewable biomass
[
1]
resources. One of the promising routes is acid-catalyzed
hydrolysis of cellulose to glucose, followed by dehydration of
glucose to 5-hydromethylfurfural (HMF, 1), and then rehydration
1,2,4-benzenetriol, 1,4-benzenediol) polymerized to form water-
[
2]
of HMF to levulinic acid (LA). Both HMF and LA are valuable
insoluble HTC, characteristic of cyclic and aromatic structures of
both furan and benzene ring but lacking aliphatic fragments.
[
3]
platform chemicals. During the process of acid-catalyzed
conversion, dark-brown insoluble by-products (known as humins)
were unavoidably formed. Most of the researches have been
focused on the synthesis and formation pathway of water-
soluble products (such as HMF, LA, furfural, formic acid). Only
little attention has been paid to the formation and structure of
humins, which however compete with desired products, restrain
the activity of catalyst and hinder the recycling of catalyst and
[7]
Titirici and co-workers believed that the morphologies and
chemical structures of HTC synthesized from hexoses at 180 °C
for 24 h could be directly related to that from HMF. Based on the
similarities with that of HMF-derived humins, they proposed
hydrothermal carbonization of glucose proceeds only through
two main reaction pathways: from hexoses to HMF and from
HMF to carbon. Further, with the help of SEM, elemental
[
2c, 4]
separation of products.
To further improve the efficiency of
13
analysis and solid-state MAS C NMR investigations, Titirici et
acid-catalyzed biomass conversion, unambiguous elucidation of
the chemical structure and formation mechanism of humins are
requisite.
[8]
al.
developed the mechanistic model for HTC. During
hydrothermal treatment, glucose first dehydrates to form HMF,
[8b]
LA and formic acid. Then the acidic compounds act as in situ
In the past few decades, several groups have made efforts
catalysts to promote further dehydration of glucose into HMF,
which subsequently undergoes various polymerization-
polycondensation reactions to give polyfuranic compounds. If
the process temperature is higher than 180 °C ， these
polyfuranic substances react further through intramolecular
condensation, dehydration and decarboxylation reaction to form
more aromatic materials with less oxygenous functional groups.
[
[
a]
b]
H. Shen, H. Shan, Prof. Dr. L. Liu
Dalian University of Technology
Dalian 116024 (China)
lliu@dlut.edu.cn
Prof. Dr. L. Liu
The Key Laboratory of Biomass Energy and Materials of Jiangsu
Province
Nanjing 210042 (China)
The structural insights obtained for HTC are nonetheless
valuable because similar characteristics are involved in humins
under acidic catalysis. In parallel with the investigations of HTC,
humins formed during acid-catalyzed HMF conversion have
Supporting information for this article is given via a link at the end of
the document.
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ChemSusChem
10.1002/cssc.201902799
RESEARCH ARTICLE
[
9]
been studied in detail. Horvat et al. was one of the first to
investigate the acid-catalysed humins, whereas they proposed
2
and reached 70% after 10 min. Apparently, chemical reactions
take place for HMF in the presence of acidic catalyst, even at
low temperatures of 20 ℃ or 4 ℃, which could account for the
formation of humins.
,5-dioxo-6-hydroxyhexanal (DHH) intermediate that could
polymerize to form humins subsequently. However, DHH has
[
10]
not been detected experimentally. Sumerskii et al. suggested
that the humins consist of 60% of furan rings and 20% of
aliphatic fragments. For hexoses, humins formation occurs via
HMF which enters into polycondensation processes by the
mechanism of electrophilic substitution, with the formation of
[11]
ether or acetal bonds. Lund and co-workers
adopted IR
analyses to demonstrate that furan ring and hydroxymethyl
group of HMF are present in HMF-derived humins, but the
carbonyl group is not. On the basis of HMF conversion pathway
[
9]
incorporating DHH intermediate suggested earlier by Horvat,
they proposed a model where humins growth occurs via aldol
[12]
addition/condensation of HMF with DHH. van Zandvoort et al.
proposed a polyfuranic molecular structure wherein HMF, LA
and DHH undergo nucleophilic addition to the α and β carbons
on HMF. Their structure model of humins took into account both
DHH mechanism and aldol condensation mechanism suggested
[
9a,9b]
[11a,11b]
by Horvat
and Lund,
respectively. The discrepancy
lies in that Lund suggested LA and formic acid may not
participate in the formation of humins, while van Zandvoort et al.
inferred aldol condensation of HMF with the ketone group of LA
is a plausible way to include LA in the humins structure.
Figure 1. The yields of humins from HMF at different temperatures under
acidic catalysis.
[13]
Recently, Vlachos and co-workers
applied ATR-FTIR
spectroscopy, SEM and Dynamic Light Scattering (DLS)
experiments to postulate that initially soluble oligomeric humins
form via aldol condensation and/or etherification reactions in
which HMF reacts with DHH-like intermediates or other HMF
molecules, followed by nucleophilic attack as proposed by van
Identification of the intermediates
In order to elucidate the chemical reactions to form humins,
capture of the key intermediates could definitely provide direct
evidence. For higher temperature such as 170 ℃, over 70% of
HMF turned into humins solid after 10 min, so that the
intermediates are transformed into humins too fast and the
remaining intermediates in solution are too minimal to
characterize. On the other hand, for lower temperature such as
[12]
Zandvoort et al.
Though a lot of excellent researches have been conducted
in this field, the formation mechanism of humins is still not
clearly understood yet. On account that the current researches
are mainly targeted at the static structure of humins, the
dynamic process of how humins are formed from solution to
solid has not been described exactly and remains challenging up
to date. The previous investigations were mostly undertaken at
high temperature ranging from 120 ℃ to 450 ℃, while it could
be difficult to identify the key intermediates for they are prone to
oligomerize rapidly into humins at high temperatures. Herein, the
humins formed from HMF were investigated thoroughly by low-
temperature approach, whereby the structural evolution process
of humins has been demonstrated for the first time.
4
℃, the conversion of HMF is too slow to accumulate enough
intermediates, resulting in that it took over 14 days to obtain the
samples rich of intermediates. To keep a balance between the
concentration of intermediates and reaction period, the chemical
reactions of HMF was monitored at 20 ℃, in the presence of
1
acidic catalyst. The H NMR spectrum shows a new set of peaks
at 9.64, 7.22, 6.58 and 4.64 ppm in coexistence with HMF
(
Figure S1), which is distinctively different from those of HMF.
Through column chromatography, the new set of peaks were
ascribed to 5,5’-oxy(bis-methylene)-2-furaldehyde (OBMF, 2),
the dimer of HMF through self-etherification reaction. In fact, the
etherification of HMF into OBMF was very easy to occur under
[14]
acidic condition. Galkin et al.
discovered the presence of
Results and Discussion
OBMF during the storage of HMF ascribed to acidic impurities.
1
Besides OBMF, H NMR spectrum shows the presence of LA
The process from HMF to humins solid
and formic acid, which are usually obtained at higher
temperature (Figure S1). Similar to OBMF, the content of LA and
formic acid in samples stored at 20 ℃ were higher than 4 ℃.
In our study, the color of HMF mixture in the presence of
H SO changed from dark brown to black during storage, until
2 4
1
the emergence of humins solid. As shown in Figure 1, the
reaction mixture lost its fluidity after 14 days and 4 days at 4 ℃
and 20 ℃, respectively, indicating humins were formed much
faster at 20 ℃ than 4 ℃. When the temperature was increased
to 170 ℃, the yield of humins ascended more quickly than 20 ℃,
In addition to H NMR, the reaction mixture after certain
intervals was dissolved in CH CN and analysed by HPLC-
3
MS/MS, revealing new intermediates of 3 and 4, besides 1 (HMF)
and 2 (OBMF). Through column chromatography, 3 and 4 were
characterized to be HMF formate and HMF levulinate,
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ChemSusChem
10.1002/cssc.201902799
RESEARCH ARTICLE
respectively, which were further confirmed by comparison with
the standard compounds. The intermediates of HMF formate
and HMF levulinate indicated that HMF can undergo
esterification under acidic catalysis.
With increasing the temperature to 170 ℃, these active
intermediates could also be detected in the reaction mixture,
albeit at a far low concentration, which indicated the formation
mechanisms of humins are similar under low temperature and
hydrothermal condition (170 ℃). It was assumed that through
these elementary reactions, humins oligomers started to grow in
the solution until precipitation as humins solid, which could be
further supported by means of IR and pyrolysis GC-MS in the
subsequent sections. The previous literature mainly focus on the
humins growing process ever since its appearance in solid,
whereas the key step from the feedstock to the active
intermediates in solution growing towards humins in solid is
regretably missing all this time. Luckily, the formation process
was slowed down by the low-temperature approach. Thereby
the key intermediates resulting in the formation of humins was
captured, offering opportunity to reveal the structure of humins,
so as to the successive growth mechanism of humins.
H
O
O
O
O
H
3
O
H
(b1) +HCOOH
O
O
H
O
O
H
O
O
O
O
2
(
a)
(b2)
O
O
H
4
HO
O
+
HMF
+LA
1
HMF
(
d) +
(c)
2HMF
LA
+LA
+
O
O
H
FT-IR study on the structural evolution of humins
O
HO
O
OH
O
O
As reported in the literature, the IR spectra of humins
derived from HMF exhibit two characteristic absorption peaks
O
O
O
O
5
O
-1
positioned at around 1710 and 1620 cm , which are the most
evident feature of IR spectra. There have been many arguments
on the functional groups corresponding to these two peaks until
6
O
(
a) Etherification
b) Esterification
(c) Aldol condensation
d) Acetalization
O
(
H
[
11,15,16]
(
now.
And how these functional groups are formed through
chemical reactions has not been explained unambiguously so
far.
Figure 2. The elementary reaction types detected through HPLC-MS/MS
analysis.
Interestingly, in our system, it was found that the IR spectra
of the humins change regularly with varying temperatures. For
the humins formed at 4 ℃, the IR spectrum shows only a strong
Furthermore, the intermediate 5 was characterized to be
E)-6-[5-(hydroxymethyl)furan-2-yl]-hex-4-oxo-5-enoic acid, the
-1
(
absorption peak appeared at 1674 cm , which could be ascribed
to the HC=O group of HMF moiety (Figure 3a). Among the
elementary reactions, this indicated that the furanics were
mainly connected through etherification and acetalization into
poly-furanics. Therefore, the aldehyde groups were consumed
much less than –OH groups, resulting in solely strong stretching
vibrations of HC=O on the IR spectra, whereas the peak at 1716
aldol condensation product of HMF and LA, which was also
confirmed by comparing with the standard compound. Herein,
LA provides the α-H for the aldol condensation with HMF to
occur. Patil et al. also proposed aldol condensation reactions as
important reactions for HMF-humins, except that they believed
that DHH provides the α-H required for the aldol condensation
[
11a]
-1
-1
reaction.
cm is very weak and the peak at 1624 cm is negligible.
Ananikov et al. proposed higher molecular weight
compounds of acetalization with three to ten furan cores during
HMF aging in the presence of acidic imputities based on the
For the humins formed at 20 ℃, the IR spectrum at the
early stage was quite similar to that at 4 ℃ (Figure 3b), whereas
-1
the HC=O stretching vibrations at 1674 cm was solely strong.
[
14]
-1
MS/MS data.
In our system, simple acetalization
Then the peak at 1674 cm was weakened gradually,
intermediates of HMF were not detected by MS/MS. However,
the intermediate 6 was proposed to be acetalization product of
HMF levulinate, verifying that acetalization can proceed under
acidic catalysis.
concomitant with a relative increase of two peaks at 1716 and
-1
1624 cm . Through comparison of the IR spectra with HMF and
-1
intermediates (Figure 3c), it was deferred that 1624 cm could
be attributed to the C=C group from aldol condensation moieties
-1
between aldehyde group and LA. In addition, 1716 cm could be
ascribed to the carbonyl group conjugated with C=C from the
aforementioned aldol condensation moieties, together with the
ester groups from the moieties of HMF formate and HMF
levulinate. When increasing the temperature from 4 ℃ to 20 ℃,
the aldol condensation between aldehyde group and LA was
promoted, due to higher LA content. Thereby, the intensities of
Therefore, as Figure 2 shows, the HPLC-MS/MS results
indicated that multiple elementary reactions were involved in the
acidic catalysis of HMF, such as etherification, esterification,
aldol condensation, and acetalization. These findings confirm
the possibility that the rehydration product of HMF, i.e. LA and
formic acid, are involved in the formation of humins, which
differs from Lund et al.’s assumption about the non-involvement
-1
two peaks at 1716 and 1624 cm ascend and get closer to that
[11]
of LA and formic acid in the humins structure.
-1
at 1674 cm .
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RESEARCH ARTICLE
In order to verify the IR characterizations, additional LA was
added into the reaction mixture. Consequently, the peak at 1674
-1
cm decreased at 4 ℃ (Figure 3a) and disappeared at 20 ℃
-
1
(
Figure 3b), while the two peaks at 1716 and 1624 cm
increased. Apparently, the aldehyde groups can be indeed
consumed by the additional LA into aldol condensation moieties
incorporated in the humins structure. In contrast, aldol
condensation between aldehyde group and formic acid could not
occur for formic acid does not contain an α-H. Therefore, with
-1
additional formic acid, the peak at 1674 cm didn’t change
significantly.
When the temperature was further increased to 170 ℃, the
IR spectra after 3 min and 10 min were similar to those at 4 ℃
for 20 days and at 20 ℃ for 20 days, respectively, showing the
-1
peak at 1672 cm decreased gradually. After 30 min, the IR
-1
spectra exhibit only two peaks at 1710 and 1628 cm , with the
-1
peak at 1672 cm disappeared completely (Figure 3d), indicating
the aldehyde groups were totally consumed by the aldol
condensation with LA. This was due to the enhanced amount of
LA produced from HMF at higher temperature. Thus, on the
basis of the IR characterizations, the humins structures with
varying content of aldol condensation could be obtained from
different temperatures and times, which were in good correlation
with the amount of LA in the system, demonstrating the
continuous evolution process of HMF-humins.
Pyrolysis GC-MS study of humins
To gain further insights of humins structure, analytical
pyrolysis GC-MS was performed on humin samples. It can be
seen from Figure 4, pyrolysis of humins produces various
furanics fragments dominantly (e.g. 2-methylfuran, 2,5-
dimethylfuran,
5-methyl-2-furancarboxaldehyde,
2,5-
furandicarboxaldehyde, HMF), confirming the poly-furanic nature
of humins.
Figure 4. A typical pyrogram of humins from HMF (at 20℃ for 2 days).
Figure 3. FT-IR analyses of humins from HMF at (a) 4 ℃, (b) 20 ℃ and (d)
170 ℃, with (c) blank controls of HMF and intermediates.
Besides furanics fragments, it is worthy to note that
aromatic compounds such as arenes and phenolics were
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ChemSusChem
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RESEARCH ARTICLE
[10, 15]
produced in the pyrolysis process.
As discussed before,
SEM study on the morphology evolution of humins
hydrothermal treatment of HMF in sub/supercritical water would
generate 1,2,4-benzenetriol (BTO), whereas it is difficult for BTO
production at relatively low temperature. As a matter of fact, we
didn’t detect any intermediates of aromatics such as BTO by
HPLC-MS/MS in the temperature range of 4-170 ℃, suggesting
that the aromatics might be formed by the secondary reactions
of the fragments under the pyrolysis condition.
To verify this hypothesis, analytical pyrolysis GC/MS was
performed on four intermediates (2-5) as blank controls under
the same conditions. According to the pyrolysis results on
intermediates, the aromatics selectivities pyrolyzed for OBMF,
HMF
formate,
HMF
levulinate
and
(E)-6-[5-
(
hydroxymethyl)furan-2-yl]-hex-4-oxo-5-enoic acid are 1.68, 0, 0
and 19.88%, respectively. Thereby, our hypothesis of aromatics
was testified, while the aromatics could be actually generated by
the secondary reactions of the fragments under the pyrolysis
condition. It further indicated the remarkable effect of C=C from
aldol condensation moieties on the aromatics selectivity after
pyrolysis. As illustrated by IR analyses, the amount of C=C bond
produced by aldol condensation between aldehyde group and
LA increased with increasing temperature and time.
Consequently, as Figure 5 shows, the selectivity of aromatics for
the pyrolyzed products increased with the rise of temperature
and time, which was in good agreement with the IR results.
Figure 6. SEM images of humins for HMF stored at 4℃ for (a) 2 days, (b) 9
days, (c) 20 days, 20℃ for (d) 2 days, (e) 9 days, (f) 20 days, and 170℃ for
(g) 10 min, (h) 30 min, (i) 3 h.
Figure 6 presents SEM images of humins that were formed
at 4 °C, 20 °C and 170 ℃. Surprisingly, the humins obtained at
4°C exhibit a layered structure after 20 days (Figure 6a-c). It was
speculated that the linkage of HMF mainly through etherification
and acetalization at 4 ℃ produces poly-furanics structure which
intends to stack closely into layered structure. For the humins
obtained at 20°C, the initial stage also shows a layered structure
but loosely packed (Figure 6d-e), and then agglomerated
particles at micrometer scale prevail after 20 days (Figure 6f).
Presumably, with more aldol condensation between aldehyde
group and LA occurred together with esterification, the higher
involvement of LA into the poly-furanics hinders the highly
ordered stacking orientation, leading to more agglomerated
particles coexistent with less layered structure. When the
reaction temperature was further increased to 170 ℃, as the
reaction rate was too fast, bigger clusters were formed (Figure
6g-i).
Under the traditional hydrothermal conditions, the humins
solid obtained were mostly dispersed or agglomerated spherical
[
11b, 12, 15]
Figure 5. Aromatics selectivities for humins pyrolyzed products at different
temperatures and reaction times.
particles.
Herein, when increasing temperature and
prolonging reaction time, a series of morphologies vary from
layered structure, coexistent layered structure and agglomerated
particles, agglomerated particles to big clusters, thus verifying
the molecular-scale evolution process of humins from
perspectives at micrometer scale.
More specifically, it was assumed that Diels-Alder
cycloaddition reactions occur between furanics and C=C of aldol
condensation moieties under pyrolysis conditions, leading to
aromatic fragments. This was consistent with the literature
reports. Huber et al. synthesized aromatics by Diels-Alder
reactions with furans and olefins pyrolyzed over ZSM-5
Proposed structure models of humins
On the basis of the aforementioned investigations by means
of HPLC-MS/MS, IR, pyrolysis GC-MS and SEM, the structure
models of humins containing the typical linkages were proposed
[17]
2 2 3
catalyst. Heeres et al. used HZSM-5 (SIO /Al O =50) catalyst
in humins pyrolysis to obtain humins oil such as benzene,
toluene and xylene, assumably through Diels-Alder reactions as
(
Figure 7), which are comprised of the elementary reaction types
[
18]
well.
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ChemSusChem
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RESEARCH ARTICLE
confirmed by the key intermediates. Etherification and Conclusion
acetalization were the major reaction types to form humins at
4
℃ (Figure 7a), while the humins structures at 20 ℃ (Figure 7b)
For the first time, low-temperature approach was adopted to
demonstrate the growth mechanism of HMF-derived humins.
The key intermediates were identified in our study, which were
ascribed to etherification, esterification, acetalization and aldol
condensation. Through humins characterizations, it was found
out that the aldol condensation moiety between aldehyde group
and LA is critical to justify the characteristic IR peaks (at 1620
and 1710 cm-1) and aromatic fragments from pyrolysis GC-MS.
Furthermore, the humins structures with varying content of aldol
condensation could be controllably synthesized from different
temperatures and time, which were in good correlation with the
amount of LA in the system, demonstrating the continuous
evolution process of HMF-humins. Thereby, the missing step
from the feedstock in solution to humins in solid has finally been
completed, with the active intermediates revealed and the
structure discrepancies between low temperature and
hydrothermal conditions compared unambiguously. We believe
elucidation of the chemical structure and formation mechanism
of HMF-derived humins is deemed to provide new insights in the
field of biomass conversion.
and 170 ℃ (Figure 7c) contain more LA’s involvement through
aldol condensation and esterification, mainly due to the higher
LA content at higher temperatures.
Experimental Section
HMF was used as the model compound in this study. HMF and 50%
H
2
SO
HMF/H
4
were mixed up in a glass vial, wherein the molar ratio of
SO was 1/0.36. The mixture was stored at different
2
4
temperatures for a certain time. Then the reaction mixture can be
subjected to HPLC-MS/MS analysis or silica gel column chromatography
with MeOH/CH Cl (1/100 to 1/10, v/v) for separation of the intermediates.
2 2
On the other hand, the humins solid can be obtained by washing
thoroughly with deionized water until the liquid phase was neutral. Then
the humin samples were centrifuged and dried in desiccator for 2 days,
followed by characterisations.
The NMR spectra were recorded using Bruker Avance II 400 MHz
spectrometer. The scanning electron microscopy (SEM) images were
obtained on NOVA NanoSEM 450 field emission scanning electron
microscopy. The infrared spectra were recorded on Thermo Fisher 6700
infrared spectrometer.
The HPLC-MS/MS analyses of the reaction mixture were conducted
on Thermo Scientific LTQ Orbitrap XL mass with electrospray ionization
(ESI) in positive ion polarity mode. The applied spray voltage was 3500 V.
The capillary temperature and voltage was 350 °C and 35 V, respectively.
The separation by the HPLC method was carried out using Thermo
Fisher Accela 1250 LC equipped with a diode array detector and a
chromatographic column ZORBAX SB-C18 (150 × 4.6 mm, 5 μm). The
reaction mixture was analysed using gradient elution, with mobile phase
of acetonitrile/water and flow rate 0.8 mL/min.
Analytical pyrolysis GC-MS of humins was performed on a CDS
1500 equipped with Agilent 5975-6890N GC-MS. The pyrolysis program
was set to ramp from 60 °C to 550 °C at a rate of 20 °C/ms and
maintained at 550 °C for 10 s. After pyrolysis, the pyrolyzed products
were transferred into a capillary column (Angilent 19091J-433, 30 m ×
250 μm × 0.25 μm) with helium as the carrier gas (1.0 mL/min). The GC
temperature program was applied: initial temperature of 60 °C for 3 min,
followed by heating to 280 °C at a rate of 10 °C /min, and finally at
Figure 7. Proposed humin models for HMF at (a) 4 ℃, (b) 20 ℃ and (c)
170 ℃, which contain the typical linkages.
280 °C for 35 min. The electron multiplier voltage was set at 1964.7 V.
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10.1002/cssc.201902799
RESEARCH ARTICLE
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Keywords: biomass
• humins • 5-hydroxymethylfurfural •
reactive intermediates • aldol reaction
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ChemSusChem
10.1002/cssc.201902799
RESEARCH ARTICLE
Entry for the Table of Contents
RESEARCH ARTICLE
Haiyan Shen, Haozhe Shan, Li Liu*
Page No. – Page No.
Evolution process and controlled
synthesis of humins with 5-
hydroxymethylfurfural (HMF) as
model molecule
By the low-temperature approach, the key intermediates towards HMF-derived
humins were captured, revealing multiple elementary reactions such as
etherification, esterification, aldol condensation, and acetalization. Furthermore, the
humins structures with varying content of aldol condensation could be controllably
synthesized from different temperatures and times, demonstrating the evolution
process of HMF-derived humins.
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