From useless humins by-product to Nb@graphite-like carbon catalysts highly efficient in HMF synthesis

Article abstract of DOI:10.1016/j.apcata.2021.118130

Highly dispersed supported NbOx species were prepared via a deposition precipitation-carbonization (DPC)-like method. As precursors for niobium species and carrier ammonium niobate(V) oxalate hydrate and humins were used. Characterization of the resulted catalysts indicated bi-functional acid-base niobium species anchored onto a highly hydrophobic graphite-like carbon structure. These catalysts were investigated in the one-pot conversion of glucose to 5-hydroxymethylfurfural (HMF) in a biphasic system consisting of a mixture of a 20 wt% NaCl aqueous solution phase and an organic extracting phase (methyl-isobutyl-ketone (MIBK), 2-tert-butylphenol (TBP) or 2-sec-butylphenol (SBP)). The optimization conditions afforded the highest yield to HMF (96 %) for the GHNb1.2 catalyst (with 2.5 wt%Nb), the base/acid sites ratio of 1.76, in biphasic TBP/water system, at 180 °C after 8 h.

Full text of DOI:10.1016/j.apcata.2021.118130

Applied Catalysis A, General 618 (2021) 118130
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
From useless humins by-product to Nb@graphite-like carbon catalysts
highly efficient in HMF synthesis
Magdi El Fergania, Natalia Candua, Madalina Tudorachea, Cristina Bucurb, Nora Djelalc,
,
,
Pascal Grangerc *, Simona M. Comana *
a Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, Regina Elisabeta Blvd., No. 4-12, Bucharest, 030016,
Romania
b
˘
National Institute of Materials Physics, 405 Atomiștilor Str., Magurele, Ilfov, Romania
c
´
Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unite de Catalyse et Chimie du Solide, F-59000, Lille, France
A R T I C L E I N F O
A B S T R A C T
Keywords:
Highly dispersed supported NbOx species were prepared via a deposition precipitation-carbonization (DPC)-like
method. As precursors for niobium species and carrier ammonium niobate(V) oxalate hydrate and humins were
used. Characterization of the resulted catalysts indicated bi-functional acid-base niobium species anchored onto a
highly hydrophobic graphite-like carbon structure. These catalysts were investigated in the one-pot conversion of
glucose to 5-hydroxymethylfurfural (HMF) in a biphasic system consisting of a mixture of a 20 wt% NaCl aqueous
solution phase and an organic extracting phase (methyl-isobutyl-ketone (MIBK), 2-tert-butylphenol (TBP) or 2-
sec-butylphenol (SBP)). The optimization conditions afforded the highest yield to HMF (96 %) for the GHNb1.2
catalyst (with 2.5 wt%Nb), the base/acid sites ratio of 1.76, in biphasic TBP/water system, at 180 ◦C after 8 h.
Glucose
5-hydroxymethylfurfural (HMF)
Humins valorization
Nb-based humins catalysts
1. Introduction
important efficiency losses in the biorefinery operations [12]. Therefore,
in order to improve the efficiency of the acid-catalyzed conversion of
The environnemental concerns and close associated regulations
generated consistent research efforts aiming the development of new
processes using biomass as a renewable source for the chemicals and
energy production [1–7]. Such processes require the development of
integrated conversion facilities, called biorefineries, where renewable
materials are converted to compounds analogous to current oil refineries
[8] in an economical, ethical and environmentally friendly way. More-
over, the complete and efficient valorization of non-edible feedstocks
ensures the transition from a linear economy to a circular economy that is
the key of the sustainable development.
sugars and to enhance the economic value of the process, the formation
of humins should be eradicate. This is also an important challenge for
the total selective synthesis of HMF or LA using both acidic homoge-
neous and heterogeneous catalysts [13–16]. Such an objective also re-
quires different reaction methodologies such as aqueous/bi/multiphase
organic solvent systems [17,18]. However, since the humins formation
is thermodynamically favored it is quite difficult a complete elimination
of these [19].
Currently, humins are used for low-value applications such as the
production of energy for biorefineries [20]. However, a sustainable
economy development should focus on their conversion to chemicals or
to the production of functional materials but studies focusing on such
applications are still scarce. Humins structure is obviously spherical
consisting of a condensed, hydrophobic core and a less dense, hydro-
philic shell and it depends on the synthesis conditions [20–28]. Around
60 wt% corresponds to carbon [29] in an aromatic environment con-
sisting of essentially furanic moieties linked by ether, acetal bonds or
aliphatic chains. The nature of the oxygen-containing functional groups,
such as carboxyl, carbonyl, and hydroxyl and their distribution depend
The acid-catalyzed dehydration of carbohydrates for the production
of renewable bulk chemicals, such as furfural, hydroxymethylfurfural
(HMF) or levulinic acid (LA), is leading to large amounts of so-called
humins (ie, carbonaceous, insoluble by-products) that are typically
formed by cross-polymerization reactions of HMF and several sugar-
dehydration intermediates [9–11]. Many other commercial processes
based on the acid-catalyzed conversion are also seriously hampered by
the extensive formation of humins by-products that leads to great losses
of the feed (around 10–50 % carbon loss) and, in consequence, to
* Corresponding authors.
E-mail addresses: pascal.granger@univ-lille.fr (P. Granger), simona.coman@chimie.unibuc.ro (S.M. Coman).
https://doi.org/10.1016/j.apcata.2021.118130
Received 29 December 2020; Received in revised form 23 February 2021; Accepted 28 March 2021
Available online 3 April 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

M. El Fergani et al.
Applied Catalysis A, General 618 (2021) 118130
on the acid catalyzed conversion of the carbohydrates process [21,22,
30,31]. Despite this complexity, the humins structure recommended
these as an important source of chemicals. Seshan and co-workers [20]
proposed these as feedstock for hydrogen or synthesis gas production
while Wang et al. [32] for the synthesis of alkyl phenolics and higher
oligomers in the presence of Ru-based catalysts. Thermal pyrolysis of
humins was also reported as a potential route to convert these into easily
transportable fuels with a high energy density or into low molecular
weight compounds from which high added value bulk chemicals, such as
acetic and formic acid can be isolated [33]. To produce functional ma-
terials like composites, humins can be introduced into a polyfurfuryl
alcohol network [34,35]. Other envisaged valorizations include: their
utilization as functional carbon materials for soil improvement or CO2
sequestration [36], the adsorption of metal ions, phenols [37,38] or
sulfonated and even as a “green catalyst” for hydrolysis, esterification
and condensation reactions [39,40]. Interestingly enough, the perfor-
mances of such catalysts were superior to sulfonated amorphous glassy
carbon, activated carbon or natural graphite [39,41,42]. They may also
2. Experimental section
2.1. Catalysts synthesis
2.1.1. Humins
Humins (GH) were prepared by heating an aqueous solution con-
taining D-glucose (1.0 M) and H2SO4 (0.01 M) in a steel autoclave at
200 ◦C for 12 h. The obtained humins were then isolated by filtration,
washed with water, dried for 12 h at 80 ◦C, and grounded. After an
additional Soxhlet extraction with water for 24 h, the samples were
dried for 24 h at 80 ◦C. Following the described preparation conditions,
humins were obtained with an yield of 30 %.
2.1.2. Niobium deposition
Catalysts with 1.2 (0.03 mol%, 2.5 wt%) (GHNb1.2) and 1.7 mmols
Nb (0.04 mol%, 3.5 wt%) (GHNb1.7) were prepared by a deposition
precipitation-carbonization (DPC) method reported in literature [52,53]
adapted for the use of humins, as following: urea (CON2H4, 0.3 g) and
ammonium niobate(V) oxalate hydrate (C4H4NNbO9xH2O, 0.36 or
0.51 g) were added to a slurry of 4.0 g of humins in 250 mL dionized
water. The mixture was heated in an autoclave at 200 ◦C for 12 h. The
separated solid was then calcined at 450 ◦C, for 4 h, in a N2 flow of
serve as support for iron oxides where demonstrated
a
microwave-assisted selective behavior for the oxidation of isoeugenol
[43].
On the other side, the production of 5-hydroxymethylfurfural (HMF)
from biomass received a special interest because it may serve as an in-
termediate for a large number of value added chemicals. For instance,
HMF could replace terephthalic acid (TA) for the production of poly-
esters after the oxidation to 2,5-furandicarboxylic acid (FDCA) or it can
be converted into 2,5-dimethylfuran (DMF) as a liquid biofuel [44]. The
transformation of glucose to HMF is a two steps process which involves
the isomerisation of glucose to fructose and the subsequent dehydration
of fructose to HMF. This process requires the concerted participation of
both the Lewis acid, for glucose isomerization to fructose, and Brønsted
acid sites, for fructose dehydration to HMF. In this respect, deal-
uminated zeolites [45] and heteroatomic inserted Lewis acid, as
bifunctional zeolites [18] with an improved stability in hot water,
emerged as effective catalysts for the direct transformation of glucose to
HMF.
30 cm3 minꢀ 1
.
2.2. Catalysts characterization
The resulted catalysts were exhaustively characterized by: X-ray
diffraction (XRD), dynamic light scattering (DLS), thermogravimetric-
differential thermal analysis (TG-DTA), IR diffuse reflectance spectros-
copy with Fourier transform (DRIFT), NH3- and CO2-temperature pro-
grammed desorption (NH3-, CO2-TPD), X-ray photoelectron
spectroscopy (XPS) and Scanning Electron Microscopy (SEM) coupled
with Energy-dispersive X-ray spectroscopy (EDX) for elemental analysis.
Powder XRD patterns were collected at room temperature using a
Shimadzu XRD-7000 apparatus with the Cu Kα monochromatic radia-
tion of 1.5406 Å, 40 kV, 40 mA at a scanning rate of 0.1 2θ minꢀ 1, in the
2θ range of 5◦–80◦. DLS measurements were done by using a Master-
size2000 apparatus from Malvern Instruments. TG-DTA analyses were
recorded using a Shimadzu apparatus in a Pt crucible. The heating rate
was of 5◦ and 10 ◦C minꢀ 1, respectively, starting from room temperature
till 900 ◦C under a nitrogen flow of 10 mL minꢀ 1. DRIFT spectra were
recorded with a Thermo spectrometer 4700 (400 scans with a resolution
of 4 cmꢀ 1) in the range of 600–4000 cmꢀ 1. CO2- and NH3-TPD mea-
surements were carried out using the AutoChem II 2920 station. The
samples (100ꢀ 200 mg), placed in a U-shaped quartz reactor with an
inner diameter of 0.5 cm, were pre-treated under He (Purity 5.0) at 80 ◦C
for 1 h, and then exposed to a flow of CO2 or a flow of 1 vol% NH3 in
helium, for 1 h. After that, the samples were purged with a flow of He
(50 mL minꢀ 1) for 20 min at 25 ◦C in order to remove the weakly
adsorbed species. TPD was then started, with a heating rate of 10 ◦C
minꢀ 1 till 500 ◦C where was maintained for 30 min. The desorbed
products were analyzed by GC-TCD chromatography. The desorbed
CO2/NH3, expressed as mmoles of CO2/NH3 per gram of catalyst, was
determined using a calibration curve. XPS measurements were per-
formed at normal angle emission in a Specs setup, by using Al Kα
Niobium compounds are highly stable in water and display
remarkable catalytic acid properties as either bulk or component of the
catalysts. Bulk niobia was shown to be active for the dehydration of
glucose to HMF in the aqueous phase with a HMF yield of 20 % [46].
However, highly dispersed Nb species into a zeolite matrix [47] display
a higher catalytic efficiency in glucose dehydration to HMF in a biphasic
water/MIBK solvent [18] leading at 180 ◦C and after 12 h to selectivities
to MHF of 84.3 % for a conversion of glucose of 97.4 %. With smaller
yields (< 40 %) the izomerisation of glucose to fructose can also be
catalyzed by bases [48–50]. Accordingly, solid catalysts with combined
Brønsted acid and base sites emerged as efficient materials for the direct
dehydration of glucose to HMF [15,51].
Carbon-loaded oxide composites demonstrated enhanced hydro-
thermal stability [52]. Moreover, in the particular case of the biphasic
systems, carbon-loaded oxide composites are located in one of the
phases, as a function of their hydrophobic/hydrophilic character, con-
trolling in this way the catalyst’s performance [53].
Herein humins by-product obtained from the dehydration of glucose
was employed as carrier to prepare a niobium-based carbonaceous solid
catalyst. The preparation methodology may induce structural modifi-
cations of the humins carrier with the formation of highly hydrophobic
graphite-like carbon structure and acid-base functionalities. The cata-
lytic performances of these humins-derived Nb-based catalysts were
investigated in the direct dehydration reaction of glucose to HMF.
monochromated radiation (h
300 W (12 kV/25 mA) power. A flood gun with electron acceleration at
1 eV and electron current of 100 A was used to avoid charge effects.
ν =1486.7 eV) of an X-ray gun, operating at
μ
Photoelectron energy was recorded in normal emission by using a
Phoibos 150 analyzer, operating with pass energy of 30 eV. The XP
spectra were fitted by using Voigt profiles combined with their primitive
functions, for inelastic backgrounds. The Gaussian width of all lines and
thresholds do not differ considerably from one spectrum to another,
being always in the expected range of 2 eV. Scanning Electron Micro-
scopy (SEM) analysis was carried out on a Hitachi S-4700 Cold Field
Emission Gun Scanning Electron Microscope operating at an
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M. El Fergani et al.
Applied Catalysis A, General 618 (2021) 118130
acceleration voltage of 15 kV. Energy-dispersive X-ray spectroscopy
(EDX) was performed for elemental analysis. Oxford EDS system was
employed for these analyzes.
at 35ꢀ 50◦ becomes visible, irrespective of the loading of niobia. Then,
after calcination, the narrower diffraction line accompanied by a shift to
higher 2θ corresponds to the formation of a graphite-like carbon struc-
ture that is also in concordance with previous reports [55]. No diffrac-
tion lines characteristic to niobia (ie, TT- [JCPDS card 00ꢀ 028-0317] or
T-phase [JCPDS card 00ꢀ 030-0873]), were identified in these patterns
suggesting either an amorphous phase of niobia or a high dispersion of
the niobium oxide crystalline particles.
2.3. Catalytic tests
The activity tests were carried out in the batch mode as follows:
0.05 g of catalyst was added to a solution of 0.18 g (1.0 mmol) glucose in
5 mL water. After closing, the reactor was heated at 160ꢀ 180 ◦C, under
stirring (1000 rpm), for 6ꢀ 12 h. Additional catalytic tests were made in
the same conditions but in a biphasic solvent formed from 3.5 mL
aqueous solution of 20 % NaCl and 1.5 mL organic solvent. 2-Sec-butyl-
phenol (SBP), 2-tert-butylphenol (TBP) and methyl isobutyl ketone
(MIBK) were used as organic solvents.
3.2. DLS measurements
DLS measurements confirmed changes in the size distribution of the
catalysts as a function of the composition (Fig. 2). They also indicate that
the presence of niobium induced a clearer bimodal size distribution of
the dispersed particles that corresponds to the modification of the sur-
face properties (wettability) by the impregnation-deposition conditions.
2.4. Catalytic recycling
3.3. TG-DTA measurements
After the first catalytic cycle the used catalyst was recovered by
centrifugation, washed three times with water, and several times with
water/methanol. The washed catalysts were dried at 80 ◦C for 48 h, and
calcined at 450 ◦C for 4 h in a flow of N2 (30 cm3. minꢀ 1) prior to the
next cycle.
Thermogravimetric/differential (TG-DTA) thermal analyses of the
humins support and catalytic samples showed an endothermic effect at
around 50ꢀ 60 ◦C that is associated to an weight loss assigned to phys-
ically adsorbing water (Figures S1) and a major weight loss of pristine
humins Figures S1) occurring as an exothermic effect, at 430 ◦C. This
one can be attributed to different processes as dehydration, decarbox-
ylation, and decarbonylation of humins resulting in the formation of a
relative stable polycyclic aromatic structure that is also in accordance to
previous literature reports [35,56]. Besides these, a third exothermic
peak at 573 ◦C accompanying a further weight loss of pristine humins,
can be associated to a further decomposition of oxygenated groups with
the formation of a more stable carbon network (Figures S1).
2.5. Products analysis
After the reaction, the catalyst was separated by centrifugation and
the reaction products were analysed in both phases by HPLC chroma-
tography, in the following conditions: mobile phase: acetonitrile/
water = 75/25; flow rate: 1 mL/min; detectors: diode-array detection
(DAD), at 254 and 210 nm, and differential refractometer (RID); col-
umn: Zorbax Carbohydrate, 4.6 × 250 mm, 5 μm. Retention times: HMF:
The absence of any loss of mass for the GHNb1.2 and GHNb1.7
samples till 430 ◦C is the consequence of the calcination these samples.
However, further heating of these samples led to a weight loss with an
exothermic effect with a maximum at around 500 ◦C.
DAD, 254 nm: 2.86 min; RID: 3.09 min; levulinic acid: DAD, 210 nm:
7.6 min; RID: 7.9 min; fructose: DAD, 210 nm: 8.4 min; RID: 8.9 min;
glucose: RID: 9.7 min, 10.1 min and 10.4 min.
In conclusion, these measurements confirmed the thermal stability of
the polycyclic aromatic structure at temperatures of 160ꢀ 180 ◦C, ie
under the reaction conditions and are in a good concordance to the XRD
measurements.
3. Results and discussion
3.1. XRD characterization
Fig. 1 illustrates the XRD patterns of the two humins-derived cata-
lysts, namely GHNb1.2 and GHNb1.7. The broad diffraction line at
15ꢀ 30◦ is assigned to amorphous carbon composed of polycyclic aro-
matic carbon sheets oriented in a random manner [54]. After the
deposition-precipitation of the niobium precursor a weak diffraction line
3.4. SEM-EDS analysis
SEM images recorded on as-synthesized humins and calcined in ni-
trogen at 450 ◦C do not reveal strong alteration of the texture and
morphology (Fig. 3). Spherical interconnected particles appear in
agreement with previous observations [56].
SEM coupled with EDX analysis was performed on nobium-based
samples. SEM images and elemental mapping are reported in Fig. 4 on
GHNb1.7 sample before (GHNb1.7n.c.) and after calcination (GHNb1.7)
and on calcined GHNb1.2. Similarly to previous observations, the
presence of spherical particles still predominates emphasizing the fact
that carburization during calcination in inert atmosphere should not
occur significantly. Such assertion seems in rather good agreement with
TG analysis which only shows significant weight loss above 430 ◦C.
Nevertheless, in this temperature conditions elemental analysis clearly
indicates a lower density of oxygen on calcined samples like due to the
partial removal of oxygenates functionalities. Previous investigations
pointed out possible rearrangement of the polymer network as well as
aromatization of the structure.
Regarding the distribution of NbOx species, it is worthwhile to note
the absence of large and intense spots which could reflect the occurrence
of strong aggregation process. As observed, niobium seems to be
randomly distributed irrespective of the composition. Interestingly,
some Nb-riched zones appear distinctly on GHNb1.7 which coincides
with the spherical structures. Such observations are less pronounced on
GHNb1.2 despite some heterogeneities are discernible. A questioning
Fig. 1. XRD patterns of the humins and humins-derived Nb catalysts. A-
humins; B -GHNb1.7 n.c. (non-calcined); C - GHNb1.7; D - GHNb1.2.
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M. El Fergani et al.
Applied Catalysis A, General 618 (2021) 118130
Fig. 2. Dispersed particle size distribution of the humins-based samples: humins (red), GHNb1.7n.c (black), GHNb1.2 (blue) and GHNb1.7 (green) (For interpre-
tation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Fig. 3. SEM images recorded on as-synthesized humins (GH) and GH calcined in nitrogen at 450 ◦C.
point arise from these observations related to local formation of highly
hydrophobic graphite-like carbon structure which could strengthen the
interaction with oxidic Nb species.
765 cmꢀ 1, characteristic to substituted furans, and that at 800 cm-1
accompanied by a small shift at 813 cmꢀ 1, indicates both structural
modifications of the pristine humins towards a more polyaromatic
structure.
3.5. DRIFT spectra
3.6. XPS analysis
The DRIFT spectra of the humins and Nb-based samples are shown in
Fig. 5. The DRIFT spectra of humins and GHNb1.7n.c. are very similar
The XPS analysis is reflected by deconvoluted spectra (Figures S2-S5)
and binding energies listed in Table S1. Thus, the deconvoluted spectra
of the C1s level led to components assigned to C1s-sp2 hybridized C
showing a broad band at around 3250 cmꢀ 1, assigned to C O stretching
–
of alcohols groups, and a second one at around 2950 cmꢀ 1, assigned to
–
– –
O C,
(C = C, 284.5–284.6 eV), hydroxyl/epoxy/ether (C OH/C
–
aliphatic C H stretch (Fig. 5A). These bands are seen as shoulders due
–
285.5–285.9 eV), carbonyl (C O, 286.7–287.2 eV), and carboxyl
–
–
to the presence of the OH groups. Differently, the spectra corre-
–
–
(HO
C O, 288.9 eV) functionalities. Deconvolution of the O1s level
–
sponding to calcined GHNb1.2 and GHNb1.7 show two additional bands
–
–
corresponded to carboxyl (HO
C
–
O, 530.2–531.2 eV), carbonyl
at 3640 and 3059 cm-1 while the large band at 3250 cm-1 is narrower
–
–
–
(C
OH, 532.7–533.4 eV) and epoxy/
and shifted to 3323 cmꢀ 1 (Fig. 5A). These bands were attributed to OH
O, 5–31.7––532.2 eV), hydroxyl (C
–
ꢀ 1
ether (C O C, 533.7–534.6 eV) functionalities [58]. The binding en-
–
groups arisen from the presence of Nb(V) OH species (3640 cm
)
ergy of the Nb 3d5/2 level (207.0–207.1 eV) corresponded to Nb(V)
species (Figures S3-S5 and Table S1). Table 1 presents the surface XPS
composition of investigated catalysts. It indicates a decrease in the
surface oxygen after the calcination step that also paralleled a slight
increase in the surface niobium. Interesting enough, the concentration of
surface niobium is quite similar for samples prepared from different
loadings of the niobium and with a different analytic content (ie 0.03
and 0.04 mol%Nb for GHNb1.2 and GHNb1.7, respectively). Also, the
XPS infirmed the presence of the Nb(IV) species that is also in agreement
with the DRIFT analysis. However, the SEM analysis suggests a somehow
different distribution on the surface of the calcined catalyst. For
GHNb1.2, niobium is merely homogeneously (Fig. 4C) distributed while
for the GHNb1.7 some Nb-riched areas were detected (Fig. 4B). No any
large niobium oxide aggregate was detected.
induced during the calcination step, and connected to the humins sur-
–
face through C OH groups. This also corresponds to diminution of the
population of the humins surface ꢀ OH groups (3250 cmꢀ 1). However,
irrespective of the Nb-based sample, no additional bands at
3555–3602 cmꢀ 1 were observed, confirming the absence of Nb (IV)
species in these materials.
In the 1800ꢀ 400 cmꢀ 1 range (Fig. 5B) the recorded spectra display
the C O stretching (1700 cmꢀ 1) resulting from the presence of acid,
–
–
–
–
aldehyde, and ketone groups, conjugated with C C ascribed to
substituted furan rings. Then, the band at 1600 cm-1 can be assigned to a
ꢀ 1
–
–
–
C stretch, while the band at 1025 cm to a C O stretch of the furan
C
ring deformation [27]. Bands at around 800 and 765 cmꢀ 1 are charac-
–
teristic to the C H out-of-plane deformation and to substituted furans
[11,27].
The DRIFT spectra of GHNb1.2 and GHNb1.7 showed, in addition to
the bands recorded for of the humins (however with major differences in
the relative intensities (Fig. 5B)) new bands located at 1700 cmꢀ 1
3.7. CO2- and NH3-TPD measurements
–
–
–
–
(assigned to C O stretching conjugated with C O) and 1440 and
CO2- and NH3-TPD analyses revealed the acid-base properties of both
pristine humins and GHNb samples confirming the presence of both acid
and basic sites but with a different relative population (Table 2) that
–
873 cmꢀ 1 (assigned to stretching vibration of the C C and the presence
–
of the niobyl Nb = O species [57]). The intensification of the band at
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M. El Fergani et al.
Applied Catalysis A, General 618 (2021) 118130
Fig. 4. SEM images and elemental analysis recorded on GHNb1.7n.c. (A), GHNb1.7 (B) and on calcined GHNb1.2 (C).
depends on both the presence of niobium and the calcination conditions.
Thus, the humins impregnation with the niobium precursor in the
presence of urea led to an increased concentration of the base sites while
the pristine concentration of acid sites remains constant (ie, GHNb1.7n.
c. sample, entry 2). The calcination results in a high decreases of the acid
sites concentration while the total concentration of base sites remained
apparently un-affected (ie, GHNb1.7n.c., entry 2 versus GHNb1.7, entry
4). However, the nature of the base sites in the calcined sample seems to
be different by comparison with those existing in the pristine humins.
These new stronger base sites may originate from the ammonium
niobate (V) oxalate hydrate complex impregnation/deposition step but
studies have been reported for humins prepared from monomeric sugars.
These indicate that pristine humins possess a furan-rich structure with
ether and (hemi) acetal linkages [59]. However, under basic conditions,
as those utilized for the impregnation-deposition of niobium species, an
arene-rich structure could be generate at the expense of the furan con-
tent [24]. Finally, after calcination of niobium deposited onto humins
supplementary structural modifications may undergo toward the for-
mation of a graphite-like carbon structure, as XRD demonstrated in this
study. This can also be accompanied by the formation of novel base sites
onto the humins surface leading to catalysts presenting acid-base
bi-functionality.
–
–
also from the OH groups arisen from the Nb(V) OH species, in
accordance to the DRIFT results. The acidity can be generated by
different species, such as the Nb = O, NbO-H(acid) or organic acid groups
still existent on the carrier surface.
In this study, the catalytic behavior of the niobia/humins catalysts
was investigated in the one-pot conversion of the glucose to HMF in a
biphasic system consisting of a combination of glucose dehydration
under aqueous conditions and an in situ extraction of the HMF in an
organic phase [18,44,60,61]. With this purpose, different phases as
methyl-isobutyl-ketone (MIBK), 2-tert-butylphenol (TBP) and 2-sec-bu-
tylphenol (SBP) were used as an extraction solvent. Previously work
conducted in our group, showed that highly dispersed Nb species into a
zeolite matrix [47] display a high catalytic efficiency in glucose dehy-
dration to HMF (selectivities to MHF of 84.3 %, for a conversion of
glucose of 97.4 %, at 180 ◦C and after 12 h) in a biphasic water/MIBK
solvent [18]. However, other reports indicate 2-sec-butylphenol (SBP)
as an efficient extraction solvent, its advantage being generate by higher
For the final catalysts the base/acid sites ratio increased from 1.76
(GHNb1.2) to 4.25 (GHNb1.7), which may corresponds to the formation
of Nb-riched zones for the last sample.
3.8. Catalytic behavior of Nb-based humins catalysts
The valorization of humins as materials, including catalysts, is still at
the proof-of-principle stage. Their structure is still incomplete under-
stood and, as a specific example, a limited number of characterization
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M. El Fergani et al.
Applied Catalysis A, General 618 (2021) 118130
Fig. 5. DRIFT spectra of humins (black), GHNb1.7n.c. (red), GHNb1.7 (green) and GHNb1.2 (blue) samples. Ranges of 4000-2500 cmꢀ 1 and 1800-400 cmꢀ 1 are
magnified in (A) and (B) figures (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
HMF yields compared to other organic solvents such as tetrahydrofuran
Table 1
(THF), methyl isobutyl ketone (MIBK), and 2-butanol [62,63].
Surface XPS composition of investigated samples.
In this study, results collected from a series of catalytic experiments
Samples
C, at %
O, % at
Nb, % at
O/C
Nb/O
highlighted some very important features. Reactions carried out at
140ꢀ 180 ◦C for 6ꢀ 12 h led to the very good yield to HMF, with a
maximum for a temperature of 180 ◦C and a reaction time of 8 h, irre-
spective of the extracting solvent (ie, MIBK, TBP and SBP) or the catalyst
nature. However, the presence of NaCl in the aqueous phase induces an
important difference between an inefficient and a highly efficient cata-
lytic system (Table 3).
Humins
81.4
78.8
81.7
81.4
18.6
19.3
16.0
16.3
–
0.23
0.24
0.20
0.20
–
GHNb1.7 n.c
GHNb1.2
GHNb1.7
1.9
2.4
2.3
0.10
0.15
0.14
Table 2
As Table 3 shows, in the presence of the calcined humins no HMF was
detected into reaction products (entry 1, Table 3) that might be attrib-
uted to the graphite-like carbon structure of calcined humins with a low
concentration of weak surface acid/base functional groups. The HMF
production in pure water or TBP, as monophasic solvent, is also not a
viable option for the HMF synthesis (GHNb1.2 catalyst, in monophasic
solvent, entries 2 and 3, Table 3), presumably due to the slow rate of
HMF formation and fast rehydration to furan ring opening products,
such as levulinic acid (LA) and formic acid [64,65].
Acid-base sites concentration determined through NH3- and CO2-TPD.
Base sites concentration
Acid sites
Base/
acid
(mmols CO2/gcat
)
concentration
(mmols NH3/
Entry
Sample
<100 ◦
C
>
Total
ratio
cat) < 100 ◦
C
150 ◦
C
g
1
2
Humins
GHNb1.7
n.c.
0.042
0.042
0.022
0.049
0.0004
0.033
0.0224
0.0820
0.53
1.95
3
4
GHNb1.2
GHNb1.7
0.021
0.020
0.021
0.028
0.016
0.057
0.0370
0.0850
1.76
4.25
Replacing the monophasic solvent with a bi-phasic TBP/water, the
HMF starts to accumulate (entry 4, Table 3) and a yield of 16 % HMF was
6

M. El Fergani et al.
Applied Catalysis A, General 618 (2021) 118130
In the second catalytic cycle the initial yield to HMF decreased until
< 60 % (for GHNb1.2 catalyst, reaction conditions similar to entry 5,
Table 3). This high decreases may be due to the structural modifications
of the carbonous carrier as an effect of the methanol washing solvent. In
agree with literature [69], adopting different washing solvent in the
humins-based catalysts recycle might result in the different recycling
performances, as the solubilities of the polycyclic aromatic hydrocar-
bons in the different washing solvents are varied. Further study about
the effect of washing solvent on the stability will be conducted in our
following work.
Table 3
Catalytic results in glucose dehydration to HMF in biphasic systems.
Entry
Catalyst
Solvent
Yield HMF, (%)
1
Calcined GH
GHNb1.2
GHNb1.2
GHNb1.2
GHNb1.2
GHNb1.2
GHNb1.2
GHNb1.7
GHNb1.7
GHNb1.7
TBP/water
Water
0
2
0
3
TBP
0
4a
5
TBP/water
TBP/water
SBP/water
MIBK/water
TBP/water
SBP/water
MIBK/water
16
96
38
15
85
24
14
6
7
8
9
10
4. Conclusions
Reaction conditions: glucose 0.18 g (1 mmol), 50 mg of catalyst, solvent [3.5 mL
H2O (NaCl 20 %) +1.5 mL extracting solvent], reaction temperature: 180 ◦C,
reaction time: 8 h, 1000 rpm.
In summary, in this paper we report a novel active and selective
catalyst for the one-pot catalytic transformation of glucose to HMF,
prepared by the deposition precipitation-carbonization (DPC) of
ammonium niobate(V) oxalate hydrate over humins carrier produced by
glucose dehydration. After calcination resulted catalysts with highly
dispersed niobia particles anchored onto the graphite-like carbon car-
rier. This preparation procedure afforded an unexpected highly efficient
catalyst for the HMF synthesis. The distribution of niobium species on
the carrier surface makes the difference between an efficient and an less-
efficient catalyst. Thus, for the same concentration of surface niobium
(ie, 2.4at% from XPS analysis) deposited on, a relatively more homo-
a
No NaCl in aqueous phase.
determined in reaction products. However, as already reported [66], the
presence of NaCl in the aqueous phase plays an important role in
improving the partition coefficient of HMF. Indeed, in bi-phasic
TBP/water system and 20 %NaCl in the aqueous phase, the yield to
HMF highly increased from 16 % to 96 % (entry 5, Table 3).
The nature of the catalysts and organic solvents plays an important
role in determining an effective extraction of HMF. Using the same
approach, ie a biphasic system consisting of methyl-isobutyl-ketone
(MIBK) as extraction solvent and aqueous NaCl (20 %) salt, Nb-based
humins catalysts produced HMF in much lower yields (15 %, on
GHNb1.2 catalyst) than previously reported Nb-based Beta zeolite cat-
alysts (82.1 % at 180 ◦C and 12 h) [18]. Moreover, these catalysts have
different degrees of hydrophilicity. Accordingly, the Nb-based Beta
zeolite catalyst was located in the aqueous phase while the GHNb1.2 one
was located in the organic phase indicating its high hydrophobicity.
Such a behavior led to significant differences in the glucose conversion
to HMF. This statement is also supported by the work of Datye and
co-workers [53] which showed that different applied preparation
methodologies lead to Nb-based carbonous carriers with different de-
grees of hydrophilicity, and hence with different locations in the
biphasic water/SBP system. Acording with this location their catalytic
behavior in the conversion of glucose to HMF was higly affected. A
comparison of the obtained results in this work with those reported by
Datye and co-workers [53], in similar biphasic systems, indicates the
superiority of our developed materials in terms of HMF yields. Clearly
enough, the differences highly depend on both the location of the
catalyst and the intrinsec catalytic features.
–
– –
geneous distribution (SEM analysis) as Nb OH and Nb O species
–
(DRIFT analysis) leads to an efficient catalyst while the agglomeration in
relatively rich areas determines an appreciable decrease of the HMF
yield. The relative base/acid sites ratio is influenced by the distribution
of niobium, the formation of Nb-rich particles corresponding to an
increased ratio.
The highest yield to HMF (96 %) was obtained with the GHNb1.2
(0.03 mol% Nb) catalyst in a bi-phasic TBP/water system, at 180 ◦C after
8 h.
In conclusion, these results confirm that by-products of renewable
biomass-based technologies, such as humins, may offer valuable solu-
tions for the production of important chemicals, such as HMF and, in this
way, to provide support for a sustainable economy.
CRediT authorship contribution statement
Magdi El Fergani: Methodology, investigation, validation of the
catalysts synthesis and catalytic tests. Natalia Candu: Investigation and
validation of part of catalysts characterization techniques. Madalina
Tudorache: Investigation and validation of the reaction products
identification and analysis. Cristina Bucur: Investigation and validation
of XPS measurements. Nora Djelal: Investigation and validation of SEM-
EDX measurements. Pascal Granger: Coordination for part of the cat-
alysts preparation and characterization research activity planning and
execution, writing-reviewing and editing, funding acquisition. Simona
M. Coman: Conceptualization, coordination for the research activity
planning and execution, writing original draft preparation, writing-
reviewing and editing, funding acquisition.
However, by changing the extraction solvent from MIBK to SBP, in
the presence of GHNb1.2 catalyst and under similar reaction conditions,
the yield in HMF increased to 57 %, after only 8 h (entry 6, Table 3). The
different reaction results are supported by the value of the partition
coefficient which highly increases for all organic solvents in the presence
of NaCl. However, its relative increase depends on the nature of the
organic solvent and is explained by a salting-out effect [67,68].
Working under the same experimental conditions in the presence of
the GHNb1.7 the trend was similar but the yields were smaller than
those obtained in the presence of the GHNb1.2 (entries 8–10, Table 3).
With GHNb1.7 the highest yield to HMF (85 %) was obtained in bi-
phasic TBP/water and 20 %NaCl in the aqueous phase, while in bi-
phasic SBP/water system and 20 %NaCl in the aqueous phase, the
yield to HMF reached only 24 %. These results confirm the importance of
the base/acid ratio in this reaction. As XPS measurements showed, the
concentration of superficial niobium is similar in both GHNb1.2 and
GHNb1.7 catalysts but the TPD measurements evidenced a much higher
base/acid sites ratio in the GHNb1.7 one (4.25). However, GHNb1.7 led
to lower yields to HMF. A high yield to HMF requires an optimum base/
acid sites ratio and as it has been demonstrated this condition is fulfilled
for GHNb1.2 with a ratio of 1.76.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
Simona M. Coman kindly acknowledges UEFISCDI for the financial
support (project PN-III-P4-ID-PCE-2016-0533, Nr. 116/2017). Magdi El
Fergani kindly acknowledges ERASMUS and the PICS “Nanocat”
N◦230460 for supporting and partially funding this work.
7

M. El Fergani et al.
Applied Catalysis A, General 618 (2021) 118130
[33] S. Agarwal, Dv. Es, H.J. Heeres, J. Anal. Appl. Pyrol. 123 (2017) 134–143, https://
doi.org/10.1016/j.jaap.2016.12.014.
Appendix A. Supplementary data
[34] J.M. Pin, N. Guigo, A. Mija, L. Vincent, N. Sbirrazzuoli, J.C.V.D. Waal, E.D. Jong,
ACS Sustain. Chem. Eng. 2 (9) (2014) 2182–2190, https://doi.org/10.1021/
sc5003769.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118130.
[35] A. MijaJan, J.C.V.D. Waal, J.M. Pin, N. Guigo, E.D. Jong, Constr. Build. Mater. 139
(2017) 594–601, https://doi.org/10.1016/j.conbuildmat.2016.11.019.
[36] D.J. Hayes, J. Ross, M.H.B. Hayes, S.W. Fitzpatrick, in: B. Kamm, P.R. Gruber,
M. Kamm (Eds.), Biorefineries - Industrial Processes and Products, Wiley, Germany,
2006, pp. 139–164.
References
¨
[1] K. Triantafyllidis, A. Lappas, M. Stocker, The Role of Catalysis for the Sustainable
Production of Bio-fuels and Bio-chemicals, Elsevier Ltd., Oxford, UK, 2013. ISBN:
978-0-444-56330-56339.
[37] Z. Liu, F.S. Zhang, J. Hazard. Mater. 167 (2009) 933–939, https://doi.org/
10.1016/j.jhazmat.2009.01.085.
[2] R. Luque, A.M. Balu, Producing Fuels and Fine Chemicals from Biomass using
Nanomaterials, CRC Press, Taylor & Francis Group, Boca Raton, 2014. ISBN (PDF):
978-1-4665-5340-5348.
[38] P. Regmi, J.L. Garcia Moscoso, S. Kumar, X. Cao, J. Mao, G. Schafran, J. Environ.
Manag. 109 (2012) 61–69, https://doi.org/10.1016/j.jenvman.2012.04.047.
[39] M. Okamura, A. Takagaki, M. Toda, J.N. Kondo, K. Domen, T. Tatsumi, M. Hara,
S. Hayashi, Chem. Mater. 18 (13) (2006) 3039–3045, https://doi.org/10.1021/
cm0605623.
[3] V. Blay, L.F. Bobadilla, A. Cabrera-Garcia, Zeolites and Metal-Organic Frameworks:
From Lab to Industry, Amsterdam University Press, 2018. ISBN: 978-94-6298-556-
558.
[40] S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato, S. Hayashi, M. Hara,
J. Am. Chem. Soc. 130 (38) (2008) 12787–12793, https://doi.org/10.1021/
ja803983h.
[4] A. Balu, A.G. Nunez, Biomass and Biowastes – New Chemical Products From Old,
De Gruyter GmbH Publisher, 2020. ISBN-10: 3110537788; ISBN-13: 978-
3110537789.
[41] X. Qi, H. Guo, L. Li, R.L. Smith, Chem. Sus. Chem. 5 (11) (2012) 2215–2220,
https://doi.org/10.1002/cssc.201200363.
´
´
[5] R. Mariscal, P. Maireles-Torres, M. Ojeda, I. Sadaba, M. Lopez Granados, Energy
Environ. Sci. 9 (4) (2016) 1144–1189, https://doi.org/10.1039/C5EE02666K.
[6] F.D. Pileidis, M.M. Titirici, Chem. Sus. Chem. 9 (6) (2016) 562–582, https://doi.
org/10.1002/cssc.201501405.
[42] J. Yang, G. Li, L. Zhang, S. Zhang, Catalysts 8 (1) (2018) 14, https://doi.org/
10.3390/catal8010014.
´
[43] L. Filiciotto, A.M. Balu, A.A. Romero, E.R. Castellon, J.C.V.D. Waal, R. Luque,
[7] P. Sudarsanam, R. Zhong, S.V.d. Bosch, S.M. Coman, V.I. Parvulescu, B.F. Sels,
Chem. Soc. Rev. 47 (2018) 8349–8402, https://doi.org/10.1039/C8CS00410B.
[8] J.J. Bozell, G.R. Petersen, Green Chem. 12 (2010) 539–554, https://doi.org/
10.1039/B922014C.
Green Chem. 19 (18) (2017) 4423–4434, https://doi.org/10.1039/C7GC01405H.
[44] R.J.V. Putten, J.C.V.D. Waal, E.D. Jong, C.B. Rasrendra, H.J. Heeres, J.G.D. Vries,
Chem. Rev. 113 (2013) 1499–1597, https://doi.org/10.1021/cr300182k.
[45] R. Otomo, T. Yokoi, J.N. Kondo, T. Tatsumi, Appl. Catal. A Gen. 470 (2014)
318–326, https://doi.org/10.1016/j.apcata.2013.11.012.
[9] B.F.M. Kuster, H.S.V.D.Baan van der Baan, Carbohydr. Res. 54 (2) (1977) 165–176,
https://doi.org/10.1016/S0008-6215(00)84806-5.
[46] K. Nakajima, Y. Baba, R. Noma, M. Kitano, J.N. Kondo, S. Hayashi, M. Hara, J. Am.
Chem. Soc. 133 (2011) 4224–4227, https://doi.org/10.1021/ja110482r.
[47] M. El Fergani, N. Candu, S.M. Coman, V.I. Parvulescu, Molecules 22 (12) (2017)
2218, https://doi.org/10.3390/molecules22122218.
[10] R. Weingarten, J. Cho, R. Xing, W.C. Conner, G.W. Huber, Chem. Sus. Chem. 5
(2012) 1280–1290, https://doi.org/10.1002/cssc.201100717.
[11] S.K.R. Patil, J. Heltzel, C.R.F. Lund, Energy Fuels 26 (8) (2012) 5281–5293,
https://doi.org/10.1021/ef3007454.
[48] Q. Yang, M. Sherbahn, T. Runge, ACS Sustainable Chem. Eng. 4 (2016) 3526–3534,
https://doi.org/10.1021/acssuschemeng.6b00587.
[12] D.M. Alonso, S.G. Wettstein, J.Q. Bond, T.W. Root, J.A. Dumesic, Chem. Sus. Chem.
4 (2011) 1078–1081, https://doi.org/10.1002/cssc.201100256.
[13] H. Zhao, J.E. Holladay, H. Brown, Z.C. Zhang, Science 316 (2007) 1597–1600,
https://doi.org/10.1126/science.1141199.
[49] C. Liu, J.M. Carraher, J.L. Swedberg, C.R. Herndon, C.N. Fleitman, J.P. Tessonnier,
ACS Catal. 4 (2014) 4295–4298, https://doi.org/10.1021/cs501197w.
[50] S. Lima, A.S. Dias, Z. Lin, P. Brandao, P. Ferreira, M. Pillinger, J. Rocha, V.
C. Casilda, A.A. Valente, Appl. Catal. A Gen. 339 (2008) 21–27, https://doi.org/
10.1016/j.apcata.2007.12.030.
[14] C. Fan, H. Guan, H. Zhang, J. Wang, S. Wang, X. Wang, Biomass Bioenerg. 35
(2011) 2659–2665, https://doi.org/10.1016/j.biombioe.2011.03.004.
[15] H. Yan, Y. Yang, D. Tong, X. Xiang, C. Hu, Catal. Commun. 10 (2009) 1558–1563,
https://doi.org/10.1016/j.catcom.2009.04.020.
[51] P. Zhou, Z.H. Zhang, Catal. Sci. Technol. 6 (2016) 3694–3712, https://doi.org/
10.1039/C6CY00384B.
[16] F. Yang, Q. Liu, X. Bai, Y. Du, Bioresour. Technol. Rep. 102 (2011) 3424–3429,
https://doi.org/10.1016/j.biortech.2010.10.023.
[52] H.F. Xiong, H.N. Pham, A.K. Datye, J. Catal. 302 (2013) 93–100, https://doi.org/
10.1016/j.jcat.2013.03.007.
[17] L. Hu, G. Zhao, W. Hao, X. Tang, Y. Sun, L. Lin, S. Liu, RSC Adv. 2 (2012)
11184–11206, https://doi.org/10.1039/C2RA21811A.
[53] H. Xiong, T. Wang, B.H. Shanks, A.K. Datye, Catal. Lett. 143 (2013) 509–516,
https://doi.org/10.1007/s10562-013-1004-8.
[18] N. Candu, M. El Fergani, M. Verziu, B. Cojocaru, B. Jurca, N. Apostol,
C. Teodorescu, V.I. Parvulescu, S.M. Coman, Catal. Today 325 (2019) 109–116,
https://doi.org/10.1016/j.cattod.2018.08.004.
[54] J. Yang, H. Zhang, Z. Ao, S. Zhang, Catal. Commun. 123 (2019) 109–113, https://
doi.org/10.1016/j.catcom.2019.02.016.
[55] W.Y. Lou, Q. Guo, W.J. Chen, M.H. Zong, H. Wu, T.J. Smith, Chem. Sus. Chem 5 (8)
(2012) 1533–1541, https://doi.org/10.1002/cssc.201100811.
[19] G. Yang, E.A. Pidko, E.J.M. Hensen, J. Catal. 295 (2012) 122–132, https://doi.org/
10.1016/j.jcat.2012.08.002.
[56] J. Yang, X. Niu, H. Wub, H. Zhang, Z. Ao, S. Zhang, Waste Manage. 103 (2020)
407–415, https://doi.org/10.1016/j.wasman.2020.01.004.
[20] T.M.C. Hoang, E.R.H.V. Eck, W.P. Bula, J.G.E. Gardeniers, L. Lefferts, K. Seshan,
Green Chem. 17 (2015) 959–972, https://doi.org/10.1039/C4GC01324G.
[21] M. Sevilla, A.B. Fuertes, Carbon 47 (2009) 2281–2289, https://doi.org/10.1016/j.
carbon.2009.04.026.
[57] F.F.P. Medeiros, M.F.V. Moura, A.G.P. da Silva, C.P. Souza, K.K.P. Gomes, U.
U. Gomes, Brazilian J. of Chem. Eng. 23 (2006) 531–538. https://www.scielo.
br/pdf/bjce/v23n4/11.pdf.
[22] M. Sevilla, A.B. Fuertes, Chem. Eur. J. 15 (2009) 4195–4203, https://doi.org/
10.1002/chem.200802097.
[58] R. Al-Gaashania, A. Najjar, Y. Zakaria, S. Mansour, M.A. Atieh, Ceram. Int. 45
(2019) 14439–14448, https://doi.org/10.1016/j.ceramint.2019.04.165.
[59] I.V. Sumerskii, S.M. Krutov, M.Y. Zarubin, Russ. J. Appl. Chem. 83 (2010)
320–327, https://doi.org/10.1134/S1070427210020266.
[23] C. Yao, Y. Shin, L.Q. Wang, C.F. Windisch, W.D. Samuels, B.W. Arey, C. Wang, W.
M. Risen, G.J. Exarhos, J. Phys. Chem. C 111 (42) (2007) 15141–15145, https://
doi.org/10.1021/jp074188l.
´
[60] M.M. Recio, I.J. Morales, P.L. Arias, J.S. Gonzalez, P.M. Torres, ChemistrySelect. 2
[24] I.V. Zandvoort, E.J. Koers, M. Weingarth, P.C. Bruijnincx, M. Baldus, B.
M. Weckhuysen, Green Chem. 17 (2015) 4383–4392, https://doi.org/10.1039/
C5GC00327J.
(2017) 2444–2451, https://doi.org/10.1002/slct.201700097.
´
[61] E. Nikolla, Y. Roman-Leshkov, M. Moliner, M.E. Davis, ACS Catal. 1 (2011)
408–410, https://doi.org/10.1021/cs2000544.
[25] T.M.C. Hoang, L. Lefferts, K. Seshan, Chem. Sus. Chem. 6 (2013) 1651–1658,
https://doi.org/10.1002/cssc.201300446.
[62] Y.J.P. Torres, T. Wang, J.M.R. Gallo, B.H. Shanks, J.A. Dumesic, ACS Catal. 2
(2012) 930–934, https://doi.org/10.1021/cs300192z.
[63] Y.R. Leshkov, J.A. Dumesic, Top. Catal. 52 (2009) 297–303, https://doi.org/
10.1007/s11244-008-9166-0.
[26] S.K. Patil, C.R. Lund, Energy Fuels 25 (2011) 4745–4755, https://doi.org/
10.1021/ef2010157.
[27] I.V. Zandvoort, Y. Wang, C.B. Rasrendra, E.R.V. Eck, P.C. Bruijnincx, H.J. Heeres,
B.M. Weckhuysen, Chem. Sus. Chem. 6 (2013) 1745–1758, https://doi.org/
10.1002/cssc.201300332.
[64] X. Qi, M. Watanabe, T.M. Aida, R.L. Smith, Green Chem. 11 (2009) 1327–1331,
https://doi.org/10.1039/B905975J.
[65] F.S. Asghari, H. Yoshida, Ind. Eng. Chem. Res. 46 (2007) 7703–7710, https://doi.
org/10.1021/ie061673e.
[28] G. Tsilomelekis, M.J. Orella, Z. Lin, Z. Cheng, W. Zheng, V. Nikolakis, D.G. Vlachos,
Green Chem. 18 (2016) 1983–1993, https://doi.org/10.1039/C5GC01938A.
[29] I.V. Zandvoort, Y. Wang, C.B. Rasrendra, E.R.H.V. Eck, P.C.A. Bruijnincx, H.
J. Heeres, B.M. Weckhuysen, Chem. Sus. Chem. 6 (9) (2013) 1745–1758, https://
doi.org/10.1002/cssc.201300332.
[66] B. Saha, M.M.A. Omar, Green Chem. 16 (2014) 24–38, https://doi.org/10.1039/
C3GC41324A.
[67] E.O. Eisen, J. Joffe, J. Chem. Eng. Data 11 (1966) 480–484, https://doi.org/
10.1021/je60031a007.
[30] N. Baccile, G. Laurent, F. Babonneau, F. Fayon, M.M. Titirici, M. Antonietti,
J. Phys. Chem. C 113 (2009) 9644–9654, https://doi.org/10.1021/jp901582x.
[31] Y.Z. Mi, W.B. Hu, Y.M. Dan, Y.L. Liu, Mater. Lett. 62 (2008) 1194–1196, https://
doi.org/10.1016/j.matlet.2007.08.011.
[68] T.C. Tan, S. Aravinth, Fluid Phase Equilib. 163 (1999) 243–257, https://doi.org/
10.1016/S0378-3812(99)00231-9.
[69] J. Yang, X. Niu, H. Wub, H. Zhang, Z. Ao, S. Zhang, Waste Manag. 103 (2020)
407–415, https://doi.org/10.1016/j.wasman.2020.01.004.
[32] Y. Wang, S. Agarwal, A. Kloekhorst, H.J. Heeres, Chem. Sus. Chem. 9 (9) (2016)
951–961, https://doi.org/10.1002/cssc.201501371.
8