Sulfonic acid-functionalized carbon coated red mud as an efficient catalyst for the direct production of 5-HMF from D-glucose under microwave irradiation

Article abstract of DOI:10.1016/j.apcata.2021.118237

Though suitable for the fructose conversion to 5-hydroxymethylfurfural (5-HMF), sulfonated carbon catalysts are inefficient for the direct glucose transformation to 5-HMF due to the lack of appropriate Lewis acids. Here, we have reported an efficient and inexpensive catalyst by suitably modifying red mud (RM), a by-product from the aluminum industry. The AD-1:1/SO₃H catalyst produced by the acid (HCl) treatment, carbon coating, and SO₃H grafting on RM exhibited enhanced surface area, mesoporous characteristics, and suitable Lewis and Bronsted acid sites. The XRD, FTIR, and XPS analysis suggested Fe₂(SO₄)₃, Fe₂O₃, and various carbon functionalities as the major active components in the AD-1:1/SO₃H catalyst. The NH₃-TPD analysis revealed an appreciable acid site density of 6.8 mmol g^?1. Under microwave heating at 180 °C, 30 min, and 90:10 DMSO/water weight percentage ratio, the catalyst produced a D-glucose conversion and 5-HMF yield of 93.05 % and 51.5 %, respectively.

Full text of DOI:10.1016/j.apcata.2021.118237

Applied Catalysis A, General 622 (2021) 118237
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Sulfonic acid-functionalized carbon coated red mud as an efficient catalyst
for the direct production of 5-HMF from D-glucose under
microwave irradiation
Bikashbindu Das, Kaustubha Mohanty*
Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, 781039, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Though suitable for the fructose conversion to 5-hydroxymethylfurfural (5-HMF), sulfonated carbon catalysts are
inefficient for the direct glucose transformation to 5-HMF due to the lack of appropriate Lewis acids. Here, we
have reported an efficient and inexpensive catalyst by suitably modifying red mud (RM), a by-product from the
aluminum industry. The AD-1:1/SO3H catalyst produced by the acid (HCl) treatment, carbon coating, and SO3H
grafting on RM exhibited enhanced surface area, mesoporous characteristics, and suitable Lewis and Bronsted
acid sites. The XRD, FTIR, and XPS analysis suggested Fe2(SO4)3, Fe2O3, and various carbon functionalities as the
major active components in the AD-1:1/SO3H catalyst. The NH3-TPD analysis revealed an appreciable acid site
density of 6.8 mmol gꢀ 1. Under microwave heating at 180 ◦C, 30 min, and 90:10 DMSO/water weight percentage
ratio, the catalyst produced a D-glucose conversion and 5-HMF yield of 93.05 % and 51.5 %, respectively.
Red mud
Sulfonated catalyst
5 HMF
Microwave
D-glucose
Industry waste
1. Introduction
thermodynamically favorable protonation of the C-2-OH group, result-
ing in a stabilized furan ring and accelerated dehydration reaction [8].
The growing demand for energy and chemicals with the depleting
fossil resources has thrown a significant challenge for sustainable
alternative development. Concerning this, lignocellulosic biomass has
emerged as a potential substitute for delivering alternative fuels as well
as other platform chemicals [1–3]. The US department of energy (DOE)
has categorized 5-HMF under the top ten biomass-derived promising
platform compounds for fuel and fine chemical applications [4]. The
hydroxyl, furan, and methyl functional groups of 5-HMF make it suitable
for the synthesis of multiple derivatives of it with applications in fuel,
polymers, and other important chemicals. For instance, 2,5-dimethyl-
furan, the hydrogenation product, and 2,5-furandicarboxylic acid, the
oxidation product of 5-HMF, are used as fuel and in the production of
polymers, respectively [5,6]. Another important derivative is lactic acid,
with substantial application in food, pharmaceutical, and polymer in-
dustries [7]. 5-HMF can be produced from different monomer sugars
such as glucose and fructose with a higher yield from the latter one but
less economically due to its higher cost. Contrarily, the low cost and easy
availability of glucose make it a suitable starting material for the pro-
duction of 5-HMF [8].
However, for glucose, the above pathway is thermodynamically less
favorable. The 5-HMF synthesis from glucose is reported to occur by the
isomerization to fructose first, preferably by a Lewis acid. The fructose is
then dehydrated to 5-HMF by a Bronsted acid [8,9]. Various catalysts
with suitable Lewis and Bronsted acidic characteristics both in homo-
geneous and heterogeneous forms have been tested for the 5-HMF syn-
thesis from sugars [10–16]. Drawbacks such as separation difficulty,
reactor corrosion, and in some cases, the high cost of homogeneous
catalysts hinder their commercialization. Heterogeneous catalytic sys-
tems could avoid the above drawbacks, due to which they are the focus
of current catalysis research. TiO2 and ZrO2 as bifunctional catalysts
with acid-base properties have been used for promoting the 5-HMF
synthesis from glucose [10]. Other bifunctional catalysts such as
2SZ@SBA-15-SO3H-NH2,
UiO-66-NH2-SO3H-2/C3N4@PDA,
and
nitrogen-doped carbonaceous catalyst (NCC) were reported to have
appreciable catalytic activity for the 5-HMF production from various
carbohydrates [17–19]. For instance, the 2SZ@SBA-15-SO3H-NH2
catalyst prepared with zirconia grafted sulfated SBA-15 was further
functionalized with SO3H, and NH2 groups could produce 42.2 % 5-HMF
yield from cellulose under an ionic liquid ([BMIM]Cl) medium at 120 ◦C
The high 5-HMF yield from fructose is facilitated by the
* Corresponding author.
E-mail address: kmohanty@iitg.ac.in (K. Mohanty).
https://doi.org/10.1016/j.apcata.2021.118237
Received 17 April 2021; Received in revised form 6 May 2021; Accepted 28 May 2021
Available online 31 May 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

B. Das and K. Mohanty
Applied Catalysis A, General 622 (2021) 118237
[18]. In another study by Zhang et al., a cascading enzymatic and
chemical reaction was reported for the conversion of cellulose to 5-HMF
[20]. The enzymatic catalyst consisting of cellulase immobilized on
Fe3O4 nanoparticles encapsulated SBA-15, and the chemical catalyst
consisting of sulfated zirconium dioxide grafted SBA-15 could produce
43.6 % 5-HMF yield from cellulose [20]. Functionalized organic resins
Amberlyst and Nafion were reported to have significant activity for the
5-HMF synthesis from fructose but less effective for converting glucose
to 5-HMF [21,22]. It is important to note that these monometallic or
mixed oxide catalysts could promote the 5-HMF synthesis from various
substrates to a reasonable extent. However, in many cases, their high
cost, lower hydrothermal stability, and low efficiency due to less
accessible active sites could limit their large-scale application.
found in our earlier study [37]. The RM-derived catalyst prepared by Sn
doping and sulfate functionalization produced an appreciable result for
the D-glucose conversion to 5-HMF under microwave irradiation [37].
The doping of Sn was beneficial in terms of enhancing the stability of the
RM-based catalyst, which was attributed to the synergistic interaction of
Sn with other components of RM. In this study, to extend the catalytic
utilization of the waste RM and to address the specific issue of the low
efficiency of various earlier reported carbonaceous catalysts for the
D-glucose transformation to 5-HMF, we prepared a suitable sulfonated
carbonaceous catalyst from RM. The RM-derived carbonaceous catalyst
reported here avoids any use of external metals, which could lower the
additional cost of preparing the metal-carbon-based catalysts using
expensive external metal precursors for the 5-HMF synthesis process.
The novel sulfonic functionalized carbon coated RM catalyst devel-
oped here consisted of appropriate Lewis and Bronsted acid sites, effi-
ciently converting D-glucose to 5-HMF under microwave irradiation.
Metal-carbon acidic catalysts application for the direct 5-HMF produc-
tion from glucose is scarce. Rather, an appreciable quantity of research
on the use of sulfonated carbon catalyst, which lacks Lewis acid sites
were reported for the fructose transformation to 5-HMF. The utilization
of the hugely generated waste RM enables the circular economy,
simultaneously creating immense benefits for society through reducing
its quantity and hazardousness. The use of microwave heating is bene-
ficial in shorter reaction times and high efficiency than conventional
heating for chemical synthesis. The physicochemical characteristics of
the catalysts were studied through a series of advanced characterization
techniques. The 5-HMF synthesis process was optimized by studying
important parameters such as solvent types, reaction temperature,
catalyst dosage, and reaction time. Catalyst reusability test was per-
formed to reveal the stability of the RM based catalyst.
Recently, carbon-based solid acid catalysts have been successfully
explored for processes such as esterification, hydrolysis, and dehydra-
tion [23–27]. The highlighting characteristics of these carbon-based
catalysts were high stability, easy synthesis, low cost, and the pres-
ence of strong protonic acid sites. Notably, the SO3H functionalized
carbon materials are found to have acid density equivalent to concen-
trated H2SO4 [28]. Factors such as the nature of the substrates, tem-
perature, and the preparation procedure could affect the final properties
of the carbon catalyst. Usually, carbon particles formed by the incom-
plete carbonization of sugars produced rigid carbon materials of small
polycyclic aromatic carbons, the sulfonation of which could produce a
high density of sulfonated active sites [28]. The sulfonation of carbon
catalysts could be done through an in-situ or post-grafting approach. The
post-grafting approach, which is carried out by treating the
pre-synthesized carbon support with sulfonating agents to create cova-
lently bonded SO3H, preserves the core carbon structure and is widely
used [28]. It enables the synthesis of material with different surface
chemistry and a degree of SO3H grafting. The SO3H grafted carbon
catalysts have been used for fructose dehydration, where the Bronsted
acid sites produced an appreciable 5-HMF yield [29–33]. However, the
SO3H functionalized carbon catalysts are not suitable for the direct
transformation of glucose to 5-HMF as they lack the Lewis acid sites
required for the initial glucose isomerization to fructose. For instance,
the sulfonated carbon catalyst reported by Li and co-workers produced
complete fructose conversion and 70.1 % 5-HMF yield. However, a
reduced 5-HMF yield of 0.1 % was observed for the glucose substrate
under identical parametric conditions, suggesting a low catalytic effi-
ciency [33]. 5-HMF synthesis directly from glucose through incorpo-
rating active Lewis acids in the carbon-based catalyst is an important
and sustainable approach, the focus on which is very limited until now.
Li and co-workers reported 25.5 % HMF yield from glucose at 160 ◦C and
2 h under a THF/NaCl saturated H2O biphasic system using niobia/-
carbon composite prepared by hydrothermal technique [27]. The
incorporation of Nb on carbon enhanced the Lewis acid density, leading
to a high selectivity towards 5-HMF. Tyagi et al. reported Fe, Cr, K, Zn,
Cu, and Al immobilized sulfonated carbon for the ionic liquid mediated
cellulose transformation to 5-HMF [26]. A relatively low 5-HMF yield of
36.33 % was observed for the sulfonated carbon, whereas an appreciable
HMF yield of 49.02 % was reported by using the Cr immobilized sul-
fonated carbon. It is important to note that the immobilization of metal
ions increased the overall efficiency of the sulfonated carbon for the
5-HMF synthesis. However, considering the high reaction time and extra
cost associated with the use of various metals, the economy, and sus-
tainability of such catalysts could raise several uncertainties on their
future commercialization. Hence, finding the potential low-cost mate-
rials and their effective utilization as catalysts for chemical processes
always remains indispensable research for scientists. Red mud (RM),
which is an aluminum industry waste, is hugely produced (> 120 million
tons/year), has high pH (~13), and consisted of rare radioactive com-
pounds, hence is considered a hazardous waste [34–36]. Nonetheless,
the composition of RM, particularly the Fe, Al, Si, and Ti, could actively
contribute to the glucose isomerization to fructose and the overall
5-HMF yield during the D-glucose conversion to 5-HMF, which was
2. Experimental
2.1. Materials
RM was procured from National Aluminum Company Limited,
Damonjodi, Odisha, India. Dried RM was ground to uniform particles of
150 μm before use. Other chemicals such as H2SO4 (98 %), D-glucose
(>99 %), HCl (37 %), ethanol (>99.5 %), and fructose (>99 %) were
purchased from Merck Millipore, India. 5-HMF (>98 %), levoglucosan
(99 %), lactic acid (98 %), and levulinic acid (>98 %) were obtained
from Alfa Aesar India. Millipore water was used for the experiments
throughout the study.
2.2. Preparation of acid-activated red mud (AARM)
The acid-activated RM (AARM) was prepared by dispersing 10 g of
RM in 50 mL of water and 100 mL of 6 M HCl solution. The solution was
kept on stirring at 100 ◦C for 1 h. Aqueous NH3 solution was then slowly
added to the mixture to bring the pH close to 8. It resulted in the for-
mation of precipitates, which were separated from the liquid phase by
centrifugation. Excess ammonia on the solid precipitates was removed
by washing several times with warm water. The AARM was finally ob-
tained by drying the washed sample at 100 ◦C overnight.
2.3. Preparation of the carbon coated AARM catalyst
Unless otherwise stated, the carbon coated AARM was prepared by a
simple wet-impregnation followed by the thermal carbonization tech-
nique. AARM to D-glucose (as carbon precursor) weight ratios of 1:0.5,
1:1, 1:1.5, and 1:2 were mixed in water and stirred at atmospheric
temperature for 3 h. Following this, the temperature was increased to 75
◦C to evaporate water from the mixture. The process was continued until
most of the water was evaporated, and the mixture turned into a gel-like
substance. The sample was oven-dried at 100 ◦C for 15 h. The dried
sample was carbonized under Ar atmosphere at 300 ◦C for 3 h to obtain
2

B. Das and K. Mohanty
Applied Catalysis A, General 622 (2021) 118237
the carbon coated AARM catalyst. The catalysts prepared by impreg-
nating different quantities of D-glucose on AARM were named AD-X:Y,
where A, D, X, and Y stands for AARM, D-glucose, the weight of
AARM, and weight of D-glucose, respectively.
2.7. Experimental procedure for the microwave synthesis of 5-HMF
The production of 5-HMF from D-glucose was performed on a CEM-
Discover microwave reactor under the DMSO/water solvent mixture.
The required amount of D-glucose, DMSO, water, and catalyst was mixed
inside a glass vessel (10 mL) by an in-built magnetic stirring facility of
the microwave reactor. The glass vessel was designed to withstand the
temperature of the reaction and the auto-generated pressure at the re-
action temperature. The temperature of the reactant mixture was raised
to the desired value at a fixed microwave power of 120 W. Once the
reaction was completed, and the microwave temperature was cooled
down to the atmospheric level, the product was taken out of the glass
vessel and centrifuged to separate the liquid phase from the solid.
2.4. Preparation of SO3H functionalized AD-XY catalysts
The preparation procedure of SO3H functionalized AD-XY catalysts
was selected based on a thorough study on the previously reported work
[28], which demonstrated the effect of temperature, time, and concen-
tration of H2SO4 on the final characteristics of the catalyst. H2SO4 (98 %)
and AD-X:Y in the weight ratio 12:1 was thoroughly mixed and kept in a
100 mL Teflon lined hydrothermal autoclave (4598: Parr, USA) at 150
◦C for 12 h. The mixture was taken out of the autoclave once the tem-
perature was reduced to atmospheric level and filtered to separate the
remaining H2SO4 from the solid sample. Following this, the recovered
solid part was washed thoroughly with warm water and filtered until the
pH of the permeate water reached near neutral. The washed solid part
was then dried at 80 ◦C under vacuum overnight to get the SO3H
functionalized AD-XY catalyst (AD-X:Y/SO3H).
2.8. Product analysis
The product analysis was performed on a Shimadzu High perfor-
mance liquid chromatography (HPLC) instrument. The system was
connected with an autosampler and a Reprogel H+ column for the
analysis. A mobile phase of 0.09 M H2SO4 at 1 mL minꢀ 1 flow rate was
used during the analysis. The conversion and yield were calculated by
the following equations.
2.5. Preparation of SO3H functionalized carbon-silica catalysts (SiD-1:1/
SO3H)
(
)
D ꢀ glucose rem ained (m oles)
D ꢀ glucose fed (m oles)
Conversion (%) = 1 ꢀ
× 100
(1)
Sulfonic acid functionalized carbon-SiO2 catalyst (SiD-1:1/SO3H)
was prepared by following the procedure as described for the prepara-
tion of AD-1:1/SO3H catalyst in sections 2.3 and 2.4. For the SiD-1:1/
SO3H preparation, acid-activated red mud (AARM) was replaced by
SiO2. The comparison between the catalytic activity of SiD-1:1/SO3H
and AD-1:1/SO3H catalyst is presented in Fig. 1 in the results and dis-
cussion section.
(
)
M produced (m oles)
Yield (M) (%) =
× 100
(2)
D ꢀ glucose fed (m oles)
where M stands for products such as 5-HMF, lactic acid (LCTA), fructose
(FCT), furfural (FU), levulinic acid (LA), and levoglucosan (LGS).
2.9. Reusability test of catalyst
2.6. Catalyst characterization
The reusability test was performed by recovering the spent catalyst
from the liquid products by centrifugation (Remi: R-24). It was followed
by the washing of the recovered catalyst by ethanol and drying it at 110
◦C for 10 h. The dried catalyst without any further modifications was
applied for the successive batch of reactions.
X-ray diffraction analysis (XRD) was carried out on a Rigaku Smar-
tLab X-ray diffractometer between the 2θ range of 10◦ to 80◦. The in-
strument was operated with Cu Kα radiation of 0.154 nm at112 mA and
40 kV. Surface functional groups were determined by the Fourier
transformed infrared spectrometer (FTIR) analyzer (Perkin Elmer
Spectrum two) between the wavenumber 4000 to 400 cmꢀ 1
.
Surface morphology and elemental composition analysis were stud-
ied on a FESEM apparatus (Zeiss:Gemini-300) and FESEM-EDX analyzer
(Zeiss:Sigma), respectively. N2 adsorption-desorption isotherm was ob-
tained by using a surface area analyzer (Quantacrome-Autosorb-IQ MP)
after degassing the sample at 170 ◦C for 4 h.
3. Results and discussion
3.1. Catalysts screening based on 5-HMF yield
The prepared catalysts from RM were applied for the conversion of D-
glucose under microwave heating, and based on the yield of the desired
product 5-HMF, the catalyst that showed the best performance was
figured out and used for the successive reactions. From the initial
assessment, the raw RM catalyst was found to be less effective for the 5-
HMF production from D-glucose (Fig. 1(a)). Though a reasonable D-
glucose conversion (53 %) could be achieved by RM catalyst, particu-
larly because of the metal elements such as Al, Ti, Si, and Fe present in it,
the 5-HMF yield was very low (3.25 %). It strongly suggested that these
metal elements, which could act as Lewis acids during the D-glucose
transformation to 5-HMF without appropriate Bronsted acid sites, were
not capable of producing a reasonable 5-HMF yield. The acid activation
of RM exclusively performed to remove the leachable Na and Ca from
RM led to a significant increase in surface area as well (Table 1). It
avoided the further contamination of the reaction product and produced
catalytic support with an enhanced surface area (219.6 m2 gꢀ 1, Table 1)
for the impregnation of D-glucose. The catalytic activity of AARM was
very similar to that of RM, which indicated that the catalyst with higher
surface area and Lewis acid sites without Bronsted acid sites was still
inefficient for the production of 5-HMF. The AD-1:1 catalyst developed
by impregnating D-glucose on AARM with an equal weight ratio fol-
lowed by thermal treatment at 300 ◦C produced a reasonably higher D-
X-ray photoelectron spectroscopy (XPS) study was performed by the
NEXSA surface analysis spectrometer (Thermo Scientific, USA). The
analysis was carried out with a micro-focused (400 μm, 12000 V, 72 W)
monochromatic Al-K
α radiation (1486.6 eV) and a multi-channel
detector.
NH3-Temperature programmed desorption (NH3-TPD) study was
carried out by using a chemisorption instrument (MicrotracBEL: BEL-
CAT II-84). The catalyst previously degassed at 150 ◦C for 1 h was
exposed to NH3 gas at 50 mL minꢀ 1 at 50 ◦C and 60 min. The NH3
desorption analysis was recorded with the help of a thermal conductivity
◦
◦
detector (TCD) in the temperature range between 50 C–800 C. The
infrared spectroscopic analysis of pyridine adsorbed catalyst was per-
formed by an attenuated total reflectance (ATR)-FTIR technique to
determine the Bronsted and Lewis acid sites. The catalyst was degassed
under vacuum at 250 ◦C for 4 h. Once the catalyst was cooled down to
atmospheric temperature, it was adsorbed with neat pyridine for 15 h
under an inert atmosphere. The unadsorbed pyridine was then removed
by heating at 100 ◦C under vacuum. The PY-ATR-FTIR analysis was
carried out in the wavenumber between 1400 cmꢀ 1 to 1700 cmꢀ 1
.
Atomic absorption spectroscopy (AAS) analysis was performed on a
Varian Spectra AA 220 FS spectrometer.
3

B. Das and K. Mohanty
Applied Catalysis A, General 622 (2021) 118237
Fig. 1. Activity of different catalyst for the 5-HMF production from D-glucose under microwave heating. Time = 30 min, temperature = 180 ◦C, catalyst/D-glucose
weight ratio = 0.6, DMSO/water = 90:10 (weight% ratio).
glucose conversion (78.2 %) and 5-HMF yield (15.75 %) than RM and
AARM. It has been observed that the incomplete carbonization of carbon
precursors, including D-glucose, could retain few oxygen functional
D-glucose conversion and 5-HMF yield.
The SO3H functionalized AD:X:Y catalysts (AD-X:Y/SO3H) prepared
as mentioned in Section 2, were applied for the 5-HMF production
process. It can be observed that a higher D-glucose conversion and 5-
HMF yield were obtained when the D-glucose (carbon precursor)
amount increased from 50 % to 100 % with respect to the weight of the
AARM (Fig. 1(b)). At the AARM/D-glucose weight ratio of 1:0.5 (AD-
1:0.5/SO3H), the conversion reached 87.5 %, and the 5-HMF yield was
40.75 %. The conversion and yield increased to 93.05 % and 51.5 %,
with an increase in the D-glucose quantity on AARM to 100 % (AD-1:1/
SO3H). It indicated that there could be more carbon species and the
–
–
groups such as phenolic OH, and COOH, which have Bronsted acid
characteristics [30]. The enhanced catalytic activity of the AD-1:1
catalyst could be ascribed to the enhanced Bronsted sites because of
–
–
the COOH and phenolic OH groups in this case as well. The presence
of these functional groups was confirmed from FTIR analysis (Fig. 3)
presented in the upcoming sections. A notable increase in D-glucose
conversion to 93.05 % and 5-HMF yield to 51.5 % was observed after
functionalization of the SO3H groups on the AD-1:1 catalyst. The
AD-1:1/SO3H catalyst exhibited appropriate Lewis acid sites in the form
of Fe, Si, Al, and Ti and Bronsted acid sites from SO3H, OH, and COOH
groups, which enhanced the efficiency of the 5-HMF production process.
The detailed catalyst characterization identifying the Bronsted and
Lewis acid sites and the total acid site density is discussed in the
following sections. Previously, several authors also reported the
enhanced catalytic activity of the carbonaceous catalyst after SO3H
functionalization, ascribed to the increased Bronsted acid density [30,
33]. The comparison between the catalytic activity of SiD-1:1/SO3H and
AD-1:1/SO3H catalyst presented in Fig. 1(a) confirmed that the
AD-1:1/SO3H had higher catalytic activity than the SiD-1:1/SO3H
catalyst. The D-glucose conversion and 5-HMF yield obtained using the
AD-1:1/SO3H catalyst were 93.05 % and 51.5 %, respectively, whereas,
using the SiD-1:1/SO3H catalyst, the corresponding value of conversion
and yield were 74.6 % and 29.8 %, respectively. It suggested the
contribution of different elements in red mud such as Fe, Al, Ti, and Si
towards enhancing the efficiency of the AD-1:1/SO3H catalyst for the
–
–
acidic COOH and phenolic OH sites present on the catalyst at higher
D-glucose loading, leading to a higher catalytic activity. As discussed
–
previously, in addition to the -SO3H acid sites, the COOH and poly-
–
cyclic aromatic OH groups in the sulfonated carbon catalysts formed
after incomplete carbonization could also contribute to the activity of
the catalyst for the production of 5-HMF [30]. However, there was no
significant variation in catalytic activity found with further increase in
D-glucose quantity beyond 100 %. It suggested that the AD-1:1/SO3H
catalyst prepared with a D-glucose to AARM weight ratio of 1:1 followed
by carbonization and SO3H functionalization was best suitable for the
D-glucose transformation to 5-HMF.
3.2. Characterization study
The XRD analysis of RM and AARM indicated peaks at 2θ of 33.05◦,
35.5◦, 40.8◦, 49.6◦, 54.05◦ and 62.75◦, attributed to the (104), (110),
(113), (024), (116), and (214) planes of hematite (Fe2O3) [34,35]
Fig. 2. XRD analysis of (a) AD-X:Y and (b) AD-X:Y/SO3H catalysts.
4

B. Das and K. Mohanty
Applied Catalysis A, General 622 (2021) 118237
Fig. 3. FTIR spectra of (a) RM, AARM and AD-X:Y and (b) AD-X:Y/SO3H catalysts.
–
(Fig. S1). It suggested a dominant surface composition of Fe2O3 in RM
and AARM. A relative loss in crystallinity of the Fe2O3 phases could be
seen in the AARM catalyst, which was due to the acid activation with
HCl. Besides Fe2O3, Al(OH)3, SiO2, and TiO2 were observed with the
peak at 2θ of 18.5◦ (010), 26.6◦ (101), and 27.5◦ (110), respectively [35,
38,39]. After D-glucose impregnation followed by carbonization at 300
◦C, the AD-X:Y based catalysts produced a new XRD spectrum in the 2θ
range of 15◦ to 30◦ ascribed to the amorphous structure of carbon (Fig. 2
(a)). All the other phases found in RM and AARM, such as Fe2O3, SiO2,
and TiO2, remained intact at their respective 2θ positions. The AD-X:
Y/SO3H catalysts produced after SO3H functionalization showed addi-
tional XRD peaks of Fe2(SO4)3 at 20.2◦ (104), 21.4◦ (110), 39.7◦ (033),
and 50.6◦ (226) [40] (Fig. 2(b)). It is possible that during the treatment
of AD-X:Y catalysts with concentrated H2SO4, few Fe could get convert
to their sulfates, due to which the Fe2(SO4)3 peaks were observed. The
Fe2O3 peaks were observed in the AD-X:Y/SO3H based catalysts at 35.5◦,
40.8◦, 53.6◦, and 62.75◦, however, with relatively low intensity. It could
be ascribed to the carbon species formation on the catalyst surface after
the carbonization of D-glucose and conversion of a few amount of Fe2O3
to Fe2(SO4)3 that decreased the relative concentration of Fe2O3. The XPS
study discussed in the impending sections also confirmed the Fe2(SO4)3
species in the catalyst after sulfonation. An identical phenomenon was
also reported previously by Daizo et al., where the formation of
Fe2(SO4)3 was observed after sulfonation of carbon-based Fe2O3 catalyst
[41]. The slightly soluble nature of the Fe2(SO4)3 in water is expected to
leach some of the compounds from the catalyst matrix during the
washing of the catalyst. However, following the sulfonation, the catalyst
was washed thoroughly with warm water until the pH reached near
neutral. Therefore, further leaching of Fe2(SO4)3 during the 5-HMF
synthesis reaction was very unlikely to occur. This was also confirmed
from the XRD, FTIR, and FESEM-EDX study of the catalyst discussed in
the upcoming paragraphs that suggested very similar characteristics
between the fresh and the reused catalyst.
CH3, respectively [42]. The H2SO4 treated AD-X:Y catalysts (AD-X:
Y/SO3H) exhibited all the functional groups of carbon, as observed in
the AD-X:Y catalysts. Additional IR peaks produced in the AD-X:Y/SO3H
catalysts related to the functional groups of sulfur in SO3H at 1040 cmꢀ 1
[30] (Fig. 3(b)). Moreover, IR spectrum at 1195 cmꢀ 1 and a shoulder
spectrum at 980 cmꢀ 1 corresponded to the bidentate sulfate ions coor-
dinated with Fe [43]. It confirmed the presence of both SO3H and SO4ꢀ 2
groups in the catalyst matrix.
The XPS analysis of the AD-1:1/SO3H catalyst confirmed Fe, Al, Ti,
Si, O, C, and S in the catalyst matrix (Fig. 4). The Fe2p XPS spectrum at
711.4 eV and 724.9 eV indicated the presence of Fe+3 particles in the
catalyst [44]. The C1s XPS spectra of AD-1:1/SO3H consisted of peaks at
2
3
–
–
–
the 284.6 eV and 289 eV assigned to the sp (C C)/sp (C C) carbon
and COOH, respectively [45]. The well-recognized XPS spectra of sulfur
groups with peaks at 164 eV and 169 eV of binding energy corresponded
to the sulfur in SH/-C-S-C- and SO3H/SOꢀ4 2 groups. The intensity of the
latter was higher, suggesting a larger percentage of sulfur in the
SO3H/SOꢀ4 2 groups [33,45]. It confirmed the presence of both sulfonic
and sulfate groups in the catalyst matrix as well. The O1s spectrum at
532.6 eV in the catalyst was assigned to the oxygen atom in SO3H/SO4ꢀ 2
[33]. Moreover, the XPS analysis confirmed the presence of Al2p, Si2p,
and Ti2p, spectrum at 75 eV, 104 eV, and 458.7 eV, respectively as well.
The morphology of RM is based on small and aggregated particles
with varying sizes and structures. The catalyst exhibited well-defined
pores between the particles (Fig. S2(a)). The acid (HCl) activation of
RM led to particles with relatively smaller sizes with amorphous nature
(Fig. S2(b)). The morphology of the AD-1:1/SO3H catalyst produced
after sulfonation of AD-1:1 showed a rigid composite-like appearance
(Fig. 5(a–c)). The carbon coating and SO3H functionalization reduced
the relative numbers of pores in the AD-1:1/SO3H than the AARM. The
elemental composition of AD-1:1/SO3H catalyst obtained from the
FESEM-EDX analysis is presented in Fig. 5(d). The study confirmed the
presence of C, O, S, Si, Fe, Ti, and Al in the AD-1:1/SO3H catalyst.
The N2 adsorption-desorption isotherm (Fig. 6) of RM, AARM, and
AD-1:1/SO3H indicated a type IV isotherm with a distinct hysteresis
loop. The occurrence of the hysteresis loop was due to the capillary
condensation at a P/P0 of 0.6 to 0.85, typical for a mesoporous material.
Enhanced N2 adsorption in this pressure range signified the increased
pore size as well. RM had a low surface area of 10.24 m2 gꢀ 1 (Table 1).
The average pore size and total pore volume of RM were 16.19 nm and
0.0414, respectively. The acid treatment of RM produced a surface area
of 219.6 m2 gꢀ 1, which was almost 21.5 times higher than the RM. It was
ascribed to the Na and Ca removal from the RM surface that increased
the number of pores and pore volume to a reasonable extent. The AD-
1:1/SO3H catalyst showed an appreciable surface area of 69.23 m2 gꢀ 1
with an average pore size of 6.84 that remained in the mesoporous
range. The carbon deposition followed by SO3H functionalization could
The surface functional groups in the prepared catalysts were iden-
tified by FTIR analysis (Fig. 3). The stretching vibration of Fe-O was
observed in the FTIR spectra of RM at 460 cmꢀ 1 and 540 cmꢀ 1 [35]
(Fig. 3(a)). Other functional groups in RM such as Si-O-Si and Al-O were
identified at 1002 cmꢀ 1 and 670 cmꢀ 1, respectively [35]. The spectrum
close to 1400 cmꢀ 1 in RM was due to the C O stretching vibration in
–
–
COꢀ3 2. A broad IR peak in the range of 3300 to 3550 cmꢀ 1 was attributed
–
to the OH groups stretching vibration in RM. The IR peaks in AARM
remained almost similar to RM. In AD-X:Y catalysts, additional IR peaks
related to the carbon and its functional groups were observed (Fig. 3(a)).
At 760 cmꢀ 1 and 1715 cmꢀ 1, the two shoulder IR spectrum could be
–
–
ascribed to the C H bending vibration and the C O groups of COOH,
–
respectively [42]. Further, IR peaks at 1607 cmꢀ 1 and 2930 cmꢀ 1 cor-
responded to the skeletal vibration of phenyl groups and vibration in the
5

B. Das and K. Mohanty
Applied Catalysis A, General 622 (2021) 118237
Fig. 4. Deconvoluted XPS spectra (a) Fe2P, (b) Ti2p, (c) Al2p, (d) Si2p, (e) C1s, (f) O1s, (g) S2p and (h) XPS survey of the AD-1:1/SO3H catalyst.
Fig. 5. (a, b, and c) FESEM images of AD-1:1/SO3H, (d) FESEM-EDX spectra of AD-1:1/SO3H, and (e) FESEM-EDX spectra of AD-1:1/SO3H after fourth cycle.
occupy some of the vacant pores, due to which a relative decrease in the
surface area of AD-1:1/SO3H as compared to AARM was observed.
The Bronsted and Lewis acid sites in the AD-1:1 and AD-1:1/SO3H
catalysts were identified by the Py-ATR-FTIR techniques (Fig. 7(a)). The
Py-ATR-FTIR peak at 1452 cmꢀ 1 was attributed to the Lewis acids in
both AD-1:1 and AD-1:1/SO3H catalyst [42]. In the AD-1:1 catalyst, the
6

B. Das and K. Mohanty
Applied Catalysis A, General 622 (2021) 118237
Fig. 6. N2 adsorption-desorption isotherm of (a) RM, (b), AARM, and (c) AD-1:1/SO3H.
and strong acid sites can be observed from the relatively broader and
intense NH3 desorption peaks in the range between 290 ◦C–800 ◦C. The
total acid site density calculated from the NH3-TPD analysis was 5.1
mmol gꢀ 1 and 6.8 mmol gꢀ 1 for the AD-1:1 and AD-1/SO3H catalyst
(Table 2), respectively. It confirmed a higher number of acid sites in the
AD-1:1/SO3H catalyst owing to the presence of SO3H groups after
sulfonation.
Table 1
BET analysis of catalysts.
Catalyst
Surface area (m2
Average pore size
(nm)
Total pore volume (cm3
gꢀ 1
)
gꢀ 1
)
RM
10.24
219.6
69.23
16.19
6.68
6.84
0.0414
0.3669
0.1184
AARM
AD-1:1/
SO3H
3.3. Reaction parametric study
Bronsted acids coordinated pyridinium ion was observed at 1552 cmꢀ 1
[42]. The Bronsted acids could be assigned to the polycyclic aromatic
The effectiveness of the AD-1:1/SO3H catalyst for the microwave
production of 5-HMF from D-glucose at different reaction parametric
conditions was studied. The solvent types could affect the efficiency of
the 5-HMF synthesis process to a reasonable extent [8]. Due to its
nontoxic nature, DMSO has been significantly used as a single organic
solvent or in combination with H2O for the 5-HMF synthesis process [8,
17,47]. An appropriate combination of DMSO and water has been
proved to enhance the 5-HMF yield by maintaining a proper balance
between the undesired rapid dehydration and rehydration of 5-HMF [8,
47]. In this study, the reactions were performed in different weight
percentage ratios of DMSO and water. It can be seen in Fig. 8(a)) that
–
–
OH and COOH groups in the AD-1:1 catalyst formed after the
incomplete carbonization of D-glucose [27]. The peak corresponded to
the Bronsted acid sites with higher intensity than AD-1:1 was detected in
the AD-1:1/SO3H catalyst at a higher frequency of 1580 cmꢀ 1 [46]. It
suggested a relative improvement in the Bronsted acids in the
AD-1:1/SO3H catalyst produced after sulfonation of AD-1:1 catalyst.
Besides this, a shoulder IR spectrum at 1488 cmꢀ 1 detected in the
AD-1:1/SO3H catalyst was the characteristic of pyridine adsorbed on
both Lewis and Bronsted acids [46]. The ratio between Bronsted to
Lewis acid density was calculated by integrating the corresponding IR
peaks (Table 2).
Table 2
The acid site density and strength of AD-1:1 and AD-1:1/SO3H
catalyst were analyzed by the NH3-TPD analysis (Fig. 7(b)). The NH3-
TPD profile indicated broad and small intensity peaks in the region
between 90 ◦C–280 ◦C assigned to the NH3 desorption from weak acid
sites in both AD-1:1 and AD-1:1/SO3H catalysts. Additional sharp NH3
desorption peaks at 425 ◦C and 670 ◦C in the AD-1:1 catalyst suggested
medium and strong acid sites. In the AD-1:1/SO3H catalyst, the medium
Acid sites analysis of catalysts.
Catalyst
Total acid density mmol gꢀ 1 (From
NH3-TPD)
Bronsted/Lewis acid ratio
(B/L)
AD-1:1
AD-1:1/
SO3H
5.1
6.8
1.23
2.25
Fig. 7. (a) Py-ATR-FTIR spectra and (b) NH3-TPD profile of AD-1:1 and AD-1:1/SO3H catalysts.
7

B. Das and K. Mohanty
Applied Catalysis A, General 622 (2021) 118237
under pure DMSO solvent, D-glucose conversion of 97.1 % and 5-HMF
yield.
◦
yield of 41 % could be achieved at 180 C and 30 min. DMSO/water
The temperature had a remarkable influence on the 5-HMF pro-
duction process (Fig. 8(b)). At 160 ◦C, 53.4 % conversion of D-glucose
and 27.3 % yield of 5-HMF yield were observed. The D-glucose con-
version increased continuously up to 190 ◦C and reached close to 100 %.
The improved catalytic efficiency at higher temperatures was attributed
to an adequate activation energy availability for 5-HMF production from
D-glucose through the isomerization and dehydration pathway. How-
ever, the positive effect of temperature on the 5-HMF yield was seen up
to 180 ◦C only, at which it reached 51.5 %, and beyond that, a
decreasing trend was observed. A decrease in the FCT yield from 4.3 %
percentage weight ratio of 90:10 enhanced the 5-HMF yield to 51.5 %.
However, a further increment in the water amount was unable to pro-
duce any positive effect on the 5-HMF yield. DMSO itself helps disrupt
the intermolecular hydrogen bonding in glucose and stabilizes its fur-
anic form, acting as an excellent dehydration medium [8]. However, the
strong dehydrating nature of DMSO could drive the undesired and rapid
dehydration of D-glucose towards levoglucosan. Hence, water up to a
certain percentage along with DMSO inhibits the rapid D-glucose
dehydration to levoglucosan. Nevertheless, higher water quantity could
also accelerate the rehydration of 5-HMF to LA and decrease the overall
5-HMF yield. In our system, a levoglucosan yield of 12.8 % was observed
under pure DMSO solvent. Water with a DMSO to water weight percent
ratio of 90:10 decreased the levoglucosan to 8.3 %, and the trend
continued with the increased water percentage. Though DMSO/water at
90:10 produced an enhanced 5-HMF yield, higher water quantity
beyond that did not contribute positively towards more 5-HMF pro-
duction. An increased amount of LA was observed at higher water
content, suggesting more rehydration reactions of 5-HMF. Besides this, a
continuous increase in FCT yield and an inconsistent variation in LCTA
and FU yield was observed with increased water quantity. Previously,
few authors also reported similar observations on 5-HMF synthesis
DMSO and water solvent mixture [8,47,48]. The study suggested a
DMSO/water weight ratio of 90:10 most appropriate for the 5-HMF
◦
◦
at 160 C to 0 % at 190 C was observed as well. With the increased
temperature, the formation of LGS, LCTA, and FU increased. It indicated
an upsurge in the unwanted reactions such as rapid dehydration and
retro-aldol processes that transformed D-glucose, FCT, and 5-HMF to-
wards these undesired products at higher temperatures (Scheme 1).
◦
Moreover, at a temperature above 180 C, higher production of solid
humins was observed, suggesting the occurrence of undesired dehy-
dration and polymerization process. Therefore, the temperature of 180
◦C that produced the highest 5-HMF yield was considered the optimum
temperature here.
The reaction time was beneficial for the D-glucose conversion, due to
which a continuous increment in conversion was observed up to 40 min
(Fig. 8(c)). At the reaction time of 20 min, the AD-1:1/SO3H catalyst
showed 72.5 % of D-glucose conversion. The corresponding 5-HMF yield
Fig. 8. Reaction parametric study of D-glucose conversion to 5-HMF under microwave heating (a) effect of solvent (time = 30 min, temperature = 180 ◦C, catalyst/D-
glucose weight ratio = 0.6): (b) effect of reaction temperature (time = 30 min, catalyst/D-glucose weight ratio = 0.6, DMSO/water weight% ratio = 90:10): (c) effect
of reaction time (temperature = 180 ◦C, catalyst/D-glucose weight ratio = 0.6, DMSO/water weight% ratio = 90:10): (d) effect of catalyst dosage (time =30 min,
temperature = 180 ◦C, DMSO/water weight% ratio = 90:10). 5-HMF: 5-hydroxymethylfurfural, LA: levulinic acid, LGS: levoglucosan, FCT: fructose, LCTA: lactic
acid, FU: furfural.
8

B. Das and K. Mohanty
Applied Catalysis A, General 622 (2021) 118237
Scheme 1. Plausible pathway for the D-glucose transformation to 5-HMF and other products.
was 36.4 % only. The D-glucose conversion continuously increased, and
at 40 min, it reached almost 100 %. On the contrary, an increment in the
yield of 5-HMF was seen up to 30 min only, and beyond that, it started to
decrease. At prolonged reaction time, the undesired polymerization/
dehydration of fructose, D-glucose, and 5-HMF to humins, 5-HMF
rehydration to LA, and D-glucose retro-aldol reaction to LCTA could be
significant (Scheme 1). As shown in Fig. 8(c), with the increment in the
reaction time, the higher production of undesired LGS, LA, FU, and
LCTA and decrement in 5-HMF and FCT yield confirmed the above
phenomenon. Hence, 30 min of reaction time was more suitable for the
production of 5-HMF. Using conventional heating, under identical re-
action conditions, significantly low D-glucose conversion and 5-HMF
yield of 32.1 % and 14.8 %, respectively, were observed, suggesting
the low efficiency of conventional heating over microwave heating
(Fig. S3).
The effect of the AD-1:1/SO3H loading on the efficiency of the 5-HMF
synthesis process is presented in Fig. 8(d). An increment in the weight
ratio of AD-1:1/SO3H to D-glucose from 0.2 to 0.6 positively drove the D-
glucose conversion from 53.4 %–93.05 % and 5-HMF yield from 32.2 %–
51.5 %. A higher catalyst loading could produce a larger active site
quantity for the adsorption of reactant molecules and facilitate the
interaction between them for shifting the reaction towards the product
formation. However, no significant variation in the conversion of D-
glucose was seen at the AD-1:1/SO3H to D-glucose weight ratio beyond
0.6. On the contrary, a minimal decrease in 5-HMF yield to 48 %
occurred at the AD-1:1/SO3H to D-glucose weight ratio of 1. A notable
increase in the LA, LCTA, LGS, FU and solid humins yield was observed,
owing to the presence of higher active sites enabling an acceleration in
the rate of undesired reactions at higher catalytic loading.
Fig. 9. Reusability of AD-1:1/SO3H catalyst for microwave conversion of D-
glucose to 5-HMF (temperature = 180 ◦C, time = 30 min, catalyst/D-glucose
weight ratio = 0.6, DMSO/water weight% ratio = 90:10).
bidentate sulfate (SO4ꢀ 2) functional groups at 980 cmꢀ 1 and 1195 cmꢀ 1
were seen in the spent catalyst as compared to the fresh catalyst. The
elemental composition analysis performed by the FESEM-EDX technique
showed a relative increase in carbon content in the spent catalyst during
the reaction (Fig. 5(e)). It could be ascribed to the deposition of carbo-
naceous compounds such as solid humins on the catalyst surface during
the 5-HMF synthesis process. The increased carbon content led to a
relative decrease in the percentages of other elements in the FESEM-EDX
analysis. The NH3-TPD and Py-ATR-FTIR analysis confirmed similar
acidic characteristics between the fresh AD-1:1/SO3H and the fourth
used AD-1:1/SO3H (Fig. S4). A minor loss in the total acid density from
6.8 mmol gꢀ 1 in the fresh AD-1:1/SO3H to 6.5 mmol gꢀ 1 in the fourth
used AD-1:1/SO3H was observed (Table S2). Moreover, the atomic ab-
sorption spectroscopy (AAS) analysis of the reaction product confirmed
minimal leaching of metals such as Fe (2.7 ppm), Al (9.8 ppm), and Ti
(4.2 ppm) in the reaction product. The minor loss in catalytic activity
observed during the catalyst reuse could be due to loss in certain active
sites due to the carbon deposition on the surface of the catalyst and the
minor leaching of the metal species.
4. Reusability of AD-1:1/SO3H catalyst
The AD-1:1/SO3H was successively reused for the 5-HMF production
to reveal its catalytic stability. From Fig. 9, it can be seen that no sig-
nificant decrease in the D-glucose conversion and 5-HMF production was
seen up to the fourth cycle. The loss in 5-HMF yield from 51.5 % in the
first cycle to 48.3 % in the fourth cycle was insignificant. It indicated the
appreciable stability of the AD-1:1/SO3H catalyst for the 5-HMF pro-
duction from D-glucose under microwave heating.
The fresh and spent catalyst was characterized by XRD, FTIR, and
FESEM-EDX techniques to analyze any change in their properties. The
XRD analysis of the fresh was found very similar to the spent catalyst
(Fig. 10(a)). All the active phases of the fresh AD-1:1/SO3H catalyst
remained intact in the fourth used AD-1:1/SO3H catalyst. The FTIR
analysis of the spent catalyst confirmed all the functional groups as
detected in the fresh catalyst (Fig. 10(b)). Particularly, no changes in the
5. Comparison of the AD-1:1/SO3H efficiency with previously
reported catalysts
The catalytic activity of the AD-1:1/SO3H for the production of 5-
HMF was compared with other synthetic functionalized carbon and
9

B. Das and K. Mohanty
Applied Catalysis A, General 622 (2021) 118237
Fig. 10. Comparison of (a) XRD and (b) FTIR analysis of fresh AD-1:1/SO3H and fourth used AD-1:1/SO3H catalyst.
carbon-metal catalysts (Table 3). Waste polyurethane (PU) derived
sulfonated carbon catalyst was used for the production of 5-HMF by Li
and coworkers [33]. A high 5-HMF yield of 70.1 % from fructose was
reported by the PU based catalyst under the ethanol/1,4-dioxane solvent
at 140 ◦C, and 2 h. However, the catalyst showed less activity for
transforming glucose to 5-HMF, due to which only 0.1 % yield of 5-HMF
was seen. Niobia carbon composite with 50 % of Nb2O5 (Nb/C-50)
prepared by the hydrothermal method was applied for the carbohy-
drates conversion to 5-HMF [27]. A glucose conversion of 43.9 % and
5-HMF yield of 25.5 % was achieved at 160 ◦C, and 2 h under the NaCl
saturated water and THF solvent. Yuan et al. developed sulfonated
carbon modified with Al2O3-TiO2 catalyst with Lewis and Bronsted acid
characteristics for the 5-HMF production from glucose [42]. The authors
reported an appreciable 5-HMF yield of 40.3 % at 130 ◦C, and 3 h under
DMSO solvent. At an extended reaction time of 5 h, 57.36 % of 5-HMF
yield was achieved. The catalyst stability test performed at 3 h reac-
tion time produced a 6.2 % loss in the 5-HMF yield during the fifth cycle
as compared to the first cycle. Sulfonated graphene quantum dots pre-
pared from carbon black was used for the conversion of carbohydrate to
5-HMF [49]. Glucose conversion of 45.1 % and 5-HMF yield of 19.5 %
was seen at 170 ◦C, and 2 h. Surprisingly, Nahavandi et al. [50] reported
a high 5-HMF yield of 93 % from glucose using sulfonated oxidized
activated carbon (SO3H-OAC) at 160 ◦C and 3 h under a reaction me-
dium of THF/H2O-NaCl. The 5-HMF yield as compared to other reported
carbonaceous catalyst was very high, however, was obtained in the
presence of NaCl and in relatively high reaction time. In another study
by Li et al. [51], an appreciable 5-HMF yield of 58.6 % was reported
using SO3H, NH2, and Cr(III) functionalized carbon nanotubes catalyst.
◦
The corresponding 5-HMF yield could be obtained at 130 C and at a
higher reaction time of 3 h. Comparing the catalytic activity of our
prepared AD-1:1/SO3H catalyst with other similar synthetic catalysts, it
is clear that AD-1:1/SO3H has comparable or even higher efficiency than
others. Only in few cases, the catalytic activity of AD-1:1/SO3H is
relatively low. However, the high cost of the raw materials and catalyst
preparation procedure used for those catalysts with relatively higher
activity could be considered
a disadvantage compared to the
AD-1:1/SO3H catalyst. It suggests the suitability of the AD-1:1/SO3H
catalyst for the economical and efficient 5-HMF production from
D-glucose.
6. Conclusion
Waste red mud modified by HCl treatment followed by carbon
coating and sulfonic group functionalization was used as an inexpensive
and efficient catalyst for the 5-HMF production from D-glucose. The AD-
1:1/SO3H catalyst prepared by the above procedure produced an
appreciable catalytic activity with 93.05 % D-glucose conversion and
51.5 % 5-HMF yield. The inherent Lewis acid sites in red mud combined
with the Bronsted acid sites generated after the carbon coating and SO3H
functionalization on red mud were the major reason for its higher cat-
alytic efficiency. The high efficiency of the microwave heating was
beneficial in enhancing the reaction rate compared to the conventional
Table 3
A comparison of AD-1:1/SO3H catalytic efficiency for the 5-HMF production from glucose with earlier reported functionalized carbon and carbon-metal catalysts.
Catalyst
process
Temperature
Time
Solvent
5-HMF
Catalyst stability
Ref.
(◦C)
(min)
yield (%)
PU-Cat
Glucose conversion
Glucose conversion
Glucose conversion
Glucose conversion
Glucose conversion
Glucose conversion
Glucose conversion
140
160
130
170
180
130
160
180
240
120
180
120
30
Ethanol/1,4-dioxane
(V/V = 70/30)
NaCl saturated water/
THF
0.1
Glucose substrate was not used for
stability test
[33]
[27]
[42]
[49]
[25]
[51]
[50]
Nb/C-50
25.5
40.3
19.5 %
43.8
58.6
93
Glucose substrate was not used for
stability test
Al2O3-TiO2 modified
sulfonated carbon
Sulfonated graphene
quantum dots
DMSO
6.2 % decrease in 5-HMF yield at
fifth cycle.
DMSO/Water (V/V =
70/30)
Glucose substrate was not used for
stability test
Sulfonated tobacco stem
carbon
GVL/water (V/V =
4.7:0.3)
5-HMF yield decreased to 36.3 %
at the fifth cycle
CNTs-SO3H-NH2-Cr (III)
180
180
30
BMIMCl
Glucose substrate was not used for
stability test
SO3H-OAC
NaCl Saturated THF/
H2O (V/V = 3)
DMSO:water (W/W =
90:10)
Catalyst activity decreased by 20
% during fifth use
AD-1:1/SO3H
Microwave glucose
conversion
51.5
Minor loss in 5-HMF yield to 48.3
% at the fourth cycle
Present
work
10

B. Das and K. Mohanty
Applied Catalysis A, General 622 (2021) 118237
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heating, due to which an appreciable 5-HMF yield (51.5 %) was ach-
ieved at 180 ◦C and at a low reaction time of 30 min. The time required
to achieve the 5-HMF yield was insignificant than the previously re-
ported conventional heating method. Water in DMSO (DMSO/water
weight percentage ratio = 90:10) inhibited the undesired D-glucose
dehydration to levoglucosan and helpful for the 5-HMF synthesis. The
deposition of solid humins blocked the active sites on the catalyst sur-
face, leading to a minor decrease in the catalytic activity during the
reuse of the catalyst. The decrease of D-glucose conversion and 5-HMF
yield during the fourth use of the catalyst was 93.05 %–91.6 % and
51.5 %–48.3 %, respectively. The AD-1:1/SO3H catalyst being devel-
oped from an inexpensive and abundant industrial by-product has the
economic advantages over the other synthetic metal-based catalysts and
has equal or higher catalytic efficiency (in few cases) for the production
of 5-HMF. Moreover, the utilization of such a hugely generated waste in
such a creative way helps enable the circular economy as well, hence
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Declaration of Competing Interest
The authors declare no conflict of interest.
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