Dehydration of glucose/fructose to 5-hydroxymethylfurfural (5-HMF) over an easily recyclable sulfated titania (SO42-/TiO2) catalyst

Article abstract of DOI:10.1039/d0nj04151c

A new SO42-/TiO2 catalyst was developed via sol-gel hydrolysis followed by the wetness impregnation method. The Lewis (L) and Br?nsted (B) site proportion in the catalyst was tuned by variation of the sulfate ion concentration in the catalyst. The dehydration of glucose/fructose to 5-hydroxymethylfurfural (5-HMF) was evaluated in a batch reactor in the presence of organic and/or aqueous medium at different temperatures (100-180 °C) and reaction times (1-8 h). The 0.5 M SO42-/TiO2 catalyst was found to be very efficient, which demonstrated 100% conversion of glucose/fructose with a maximum 5-HMF yield of ～37%, and ～75%, respectively, at 150 °C and after 6 h of reaction. Results showed that the proportion of Lewis and/or Br?nsted acidic strength of the catalyst played a crucial role in the yield of 5-HMF. The presence of water in the reaction medium demonstrates a negative effect on the 5-HMF yield and glucose/fructose conversion. Further, a new catalyst regeneration strategy was developed and it has been established that the 0.5 M SO42-/TiO2 catalyst can be regenerated completely and can be reused several times with constant activity and 5-HMF yield. A kinetic study of the glucose dehydration reaction over a 0.5 M SO42-/TiO2 catalyst suggested a second-order kinetic model with an activation energy of 195.4 kJ mol-1. This journal is

Full text of DOI:10.1039/d0nj04151c

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Dehydration of glucose/fructose to 5-Hydroxymethylfurfural (5-HMF) over an easily
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recyclable sulfated titania (SO /TiO ) catalyst
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Richa Tomer and Prakash Biswas*
Department of Chemical Engineering, Indian Institute of Technology Roorkee,
Roorkee 247667, Uttarakhand, India.
*Corresponding Author: Tel.: (+91)-1332-28-5820; Fax: (+1)-1332-27-6535
E-mail: prakash.biswas@ch.iitr.ac.in; prakashbiswas@gmail.com
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DOI: 10.1039/D0NJ04151C
Abstract
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A new SO /TiO catalyst was developed by sol-gel hydrolysis followed by the wetness
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impregnation method. The Lewis (L) and Brønsted (B) site’s proportion in the catalyst was
tuned with the variation of sulfate ion concentration in the catalyst. The dehydration of
glucose/fructose to 5-hydroxymethylfurfural (5-HMF) was evaluated in a batch reactor in the
presence of organic and or/aqueous medium at different temperatures (100-180 ℃) and reaction
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time (1- 8 h). The 0.5 M SO /TiO catalyst was found to be very efficient, which demonstrated
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00% conversion of glucose/fructose with a maximum 5-HMF yield of ~37%, and ~75%,
respectively, at 150 ℃ and after 6 h of reaction. Results depicted that the proportion of Lewis
and/or Brønsted acidic strength of catalyst played a crucial role in the yield of 5-HMF. The
presence of water in the reaction medium demonstrates a negative effect on the 5-HMF yield
and glucose/fructose conversion. Further, a new catalyst regeneration strategy was developed
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and it has been established that 0.5 M SO /TiO catalyst can be regenerated completely and
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can be reused several times with constant activity and 5-HMF yield. Kinetic study of glucose
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dehydration reaction over 0.5 M SO /TiO catalyst suggested a second-order kinetic model
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with an activation energy of 195.4 kJ mol-1.
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Keywords: Glucose dehydration, 5-HMF, SO /TiO catalysts, DMSO, catalyst regeneration.
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1
. Introduction
In very recent, the production of valuable chemicals, fuels, and fuel additives from
renewable sources such as biomass has received significant attention because of the depletion
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of fossil fuel resources and most importantly the environmental consequences. The cellulose
fraction (~35-50%) of biomass is the potential source of glucose, which can be a promising
candidate for the production of various high-priced finished products and intermediates such
as 5-hydroxymethylfurfural (5-HMF), formic acid (FA), levulinic acid (LA), lactic acid, acetic
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acid, gluconic acid, etc. Among all these chemicals, 5-HMF has been identified as the most
valuable chemical intermediate and it also has been regarded as a top ten building block
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chemical globally. 5-HMF can be a feedstock of fine chemicals and biofuel additives such as
levulinic acid, 2,5-diformylfuran (DFF), dihydroxymethylfuran (DMF), and 2,5-
furandicarboxylic acid (FDCA), etc.2,4 5-HMF can also be reduced catalytically to a high
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calorific (31.5 MJ L ) biofuel 2, 5-dimethylfuran (DMF). At present, the production cost of
5-HMF and the byproducts formation limits its availability. Therefore, a process for the
production of 5-HMF from renewable carbohydrates with low energy input is highly desirable.
Various recent studies reported very high yield (98.6%) of 5-HMF from fructose,6
although fructose is not recommended as a model feedstock due to its inflated price and less
availability. As a result, low cost and abundantly available glucose is a more preferred
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feedstock for the production of 5-HMF. The formation of 5-HMF from glucose is a two-step
process i.e. isomerization of glucose to fructose followed by the dehydration fructose to 5-
HMF with the elimination of three water molecules in presence of a catalyst. It has been shown
that the presence of both the Brønsted and Lewis acid sites in the catalyst is essential for the
formation of 5-HMF via glucose isomerization and dehydration steps, respectively.8
Various previous reports presented heterogeneous glucose dehydration in the presence
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of different reaction media such as aqueous, organic,
biphasic, ionic liquids, and hot
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compressed water. Aqueous medium manifested a detrimental effect on the yield of 5-HMF
DOI: 10.1039/D0NJ04151C
and the organic solvents such as dimethylsulfoxide (DMSO), methyl isobutyl ketone (MIBK),
tetrahydrofuran (THF), dimethylformamide (DMF) are generally reported as good for glucose
dehydration. Among the organic solvents, DMSO is reported as one of the most suitable ones
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because of its better dehydration ability, favorable solvation property, and suppression of
side reactions ability.17,18 Moreover, many studies reported an efficient 5-HMF yield in ionic
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liquid as a solvent. The elevated price of ionic liquids and its ineffective product separation
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ability make it less attractive. As a promising alternative, recently, many studies focused on
the biphasic reaction medium.21 However, high extraction efficiency and high solvent
recyclability is the key issue in the biphasic medium.22
Solid acid catalysts with a tunable Lewis and Brønsted acid sites,23 zeolites,24,25
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13,18,28
transition metals9
heteropolyacids, metal oxides, sulfated and phosphate metal oxides,
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have been evaluated for the synthesis of 5-HMF via glucose dehydration. Yan et al. reported
a 5-HMF yield of 47.5% with 97.2% conversion of glucose in the DMSO solvent in presence
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of SO /ZrO -Al O catalyst. The basic strength of the catalyst increased with increasing the
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alumina content, which enhanced glucose isomerization to fructose. However, the stability of
this catalyst was an issue. Hydrotalcite in combination with Amberlyst-15 was tested for
glucose dehydration in presence of various organic solvents including water, DMF, acetonitrile
(
MeCN), DMSO, and dimethylacetamide (DMA).29 The basic hydrotalcite is served for
isomerization and acidic Amberlst-15 served for dehydration reaction, respectively. The
presence of both the catalysts was essential for the selective conversion of glucose to 5-HMF.
A maximum 5-HMF selectivity to 61% was obtained in the DMSO medium, and the selectivity
was nil in the aqueous medium because of the deactivation of Amberlyst-15 in water. Qu et
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al. used [CO HMIM] BF ionic liquid as a catalyst in the DMSO medium and obtained a very
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good yield (67.4%) of 5-HMF at 180 ℃ in N atmosphere, due to its stabilizing effect of the
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ionic liquid. However, the extraction of 5-HMF from the reaction mixture was difficult. Otomo
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et al. disclosed a 55% yield of 5-HMF with ꞵ -zeolite at 180 ℃ in a water-DMSO-THF
medium after 3 h. It has reported that water and DMSO favored the dissolution of glucose
molecules, DMSO suppressed the side reactions of fructose and THF used for the extraction of
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-HMF from the water medium during the reaction. Liu et al. varied the Al/B molar ratio in
an Al -B catalyst and obtained a maximum 41.4% yield of 5-HMF in the DMSO medium
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at the Al: B molar ratio of 5:5. It has been shown that the surface area and acidity increased
with increasing the boron amount in the catalyst and it was highest at the Al: B ratio of 5:5.
The surface area and acidity of the catalyst affected the 5-HMF selectivity and glucose
conversion significantly. However, catalyst leaching was reported as a major problem. Lu et
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al. reported that partially hydroxylated AlF was extremely useful for the production of 5-
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HMF due to the presence of both the Brønsted and Lewis acid centers and reported a 57.3%
yield of 5-HMF with 95.5% conversion of glucose in the DMSO medium at 140 ℃ and after
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0 h of reaction. Shahangi et al. synthesized fibrous nanosphere Al-KCC-1 silica catalyst and
obtained a 39% yield of 5-HMF with 97.8% conversion of glucose in the DMSO solvent at
o
170 C. Textural properties and the acidity of the catalyst are reported as the primary factors
for higher activity and good selectivity to 5-HMF. DMSO was reported as a better solvent due
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to its more polarity. He et al. reported 34% selectivity to 5-HMF at the complete conversion
of glucose in DMSO. Highly porous SiO -ZrO based amide hollow fiber demonstrated
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aminosilane grafting increased the porosity of the catalyst. The presence of both the Lewis and
Brønsted acidic sites on the catalyst was reported as the most influencing factor for high 5-
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HMF selectivity and glucose conversion. Feng et al. showed a 25.8% yield of 5-HMF over
+
sulfated zirconia modified with H zeolites in DMSO and the yield increased to 61.6% when
NaCl-H O-DMSO was used as a solvent. The inclusion of water as a solvent increased glucose
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dissolution, which enhanced 5-HMF yield because of the salting-out effect of saturated NaCl
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solution. Halide ion acted as a base and nucleophile to facilitate the dehydration of fructose to
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-HMF.32
The primary objective of this study was to develop an acceptable catalyst and process
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for glucose and fructose dehydration to 5-HMF in the DMSO medium. In this study, anatase
TiO was synthesized by the sol-gel hydrolysis method and this TiO was functionalized with
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the sulfate group by wetness impregnation technique. The sulfated titania catalyst was tested
for dehydration of glucose to 5-HMF in the DMSO solvent and the results were compared
with fructose dehydration. The effect of various reaction parameters including temperature,
time, catalyst amount, water to DMSO ratio was evaluated. Results demonstrated that among
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all other 0.5 M SO /TiO catalyst was very effective for selective conversion of glucose and
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fructose to 5-HMF. The catalyst reusability and stability were also explored, and the results
were analyzed in light of the fresh and used catalysts characterization data, respectively. A
new catalyst regeneration strategy is developed and the results suggested that the sulfated
catalyst can be regenerated completely after each cycle and can be used in several cycles with
constant activity and 5-HMF yield.
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. Experimental
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.1 Catalyst synthesis
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33,34
Anatase TiO and SO /TiO were synthesized by the sol-gel hydrolysis technique
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with some modification. In a typical TiO synthesis, titanium isopropoxide, [Ti(OPr) ] (98%
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Avra Synthesis, India), and isopropanol (99%, Thomas Baker, India) at the volume ratio of 1:2
were mixed in a beaker and the excess amounts of water (~100 mL, hydrolysis ratio of 31.7)
was added dropwise to the mixture under constant stirring for 2 h. The obtained white
suspension was kept at room temperature overnight for aging. The resulting solution was
filtered and the solids in the filtrate were washed thoroughly by deionized water to remove the
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alkoxide accompanied by overnight drying at 80 ℃ in a hot air oven. The light, fluffy and white
powder obtained after drying was calcined at three different temperatures i.e. at 400, 500, and
600 ℃, respectively, for 3 h in presence of air to obtain the anatase TiO . The anatase form of
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TiO was more intense for the TiO calcined at 500 ℃.
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To induce the Brønsted acid sites to the anatase TiO powder, sulfation was performed
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by the wetness impregnation method. In this process, the required amounts of TiO powder
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was added to 1 M H SO [98%, Thomas Baker, India] solution (~50 mL) and stirred for 2 h at
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room temperature. The resulting slurry was kept in a hot-air oven maintained at 80 ℃ for
overnight to evaporate excess water. Further, the dried powder obtained was grounded in a
mortar and pestle followed by calcination for 3 h at 500 ℃ in air. The resulting solid dry catalyst
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powder was labeled as 1 M SO /TiO . Similarly, the catalyst with different sulfate
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concentrations i.e. 0.75 M SO /TiO , 0.50 M SO /TiO and 0.25 M SO /TiO were
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prepared only by varying the H SO concentration in the impregnation step.
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2.2 Catalyst characterization
The characteristics of the catalysts were determined by several characterization
techniques. The textural properties i.e. surface area and pore-size distribution of the catalysts
were analyzed in an ASAP-2060 instrument (Micromeritics, USA). The N adsorption-
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desorption isotherms were obtained at the liquid nitrogen temperature of ˗196 ℃. The surface
area was determined by the Brunauer-Emmett-Teller (BET) procedure and the pore size was
measured by following the Barret-Joyner-Halenda (BJH) technique. Before each analysis, the
degassing of the catalyst powder was performed for 2 h at 200 ℃ under vacuum.
The X-ray diffraction (XRD) study of the catalyst was performed in a Bruker
diffractometer (D8-Advance, Germany) with Cu-Kα radiation (λ = 1.5406 Å) generated at 40
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kV and 40 mA, respectively. The data were collected at the 2θ range of 5 to 90° with a step of
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.02° and a scanning rate of 10° min .
Thermogravimetric analysis (TGA) was performed in an EXSTAR TG/DTG 6300
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instrument (Japan) under the nitrogen flow of 200 mL min . The powder catalyst was heated
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from room temperature to 900 ℃ at the rate of 10 ℃ min .
The surface morphology was determined by Field Emission Scanning electron
microscope (FE-SEM) by Carl Zeiss (Carl Zeiss Ultra Plus Model, Germany) equipped with
energy dispersive X-ray spectra (EDX). Powder catalyst was dispersed on a sample holder
followed by gold coating by using a sputter coater (Edwards S150). Further, the SEM images
were captured at an acceleration voltage of 20 kV under a vacuum. The morphology of the
2
catalysts was also recorded in a TEM (Tecnai G 20 S-Twin, FEI Model, USA). The TEM
instrument was coupled with an energy dispersive spectroscopy (EDX) electron mapping.
Energy-dispersive X-ray spectroscopy (EDX) was used to quantify the surface elemental
composition. Prior to TEM analysis, the catalyst powder was disseminated in alcohol with
ultra-sonication for 15 minutes and put on the copper grid.
The total acidic strength of the catalysts was measured by NH temperature-
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programmed desorption (NH -TPD) analysis in a Micromeritics chemisorb-2750 instrument
3
(
Mircomeritics instrument corporation, USA) equipped with a thermal conductivity detector.
In a typical NH -TPD analysis, approximately 300-400 mg of sample was loaded in a U-tube
3
quartz reactor and degassed the sample at 200 ℃ for 2 h under the flow of helium (20 mL
-1
min ), accompanied by cooling to atmospheric temperature. Further, the degassed sample was
saturated with NH by purging the mixture gas (10% NH -He) over the catalyst sample for 45
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min at 20 mL min . After NH saturation, the surplus ammonia was removed by flushing the
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tube with pure helium (20 mL min ) for 1 h. Further, the catalyst was heated up to 1000 ℃ at
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a rate of 10 ℃ min . The amounts of desorbed NH was monitored by using a thermal
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conductivity detector connected with chemisoft-tpx software.
To analyze the chemical state of titania and sulfur on the solid surface, X-ray
photoelectron spectroscopy (XPS) was performed in PHI 5000 Versa Proble-III scanning XPS
microscopy (USA) with Al-K-α line excitation source. The recorded spectra were calibrated
by the characteristic binding energy of C 1s at 284.0 eV.
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1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fourier transform infrared spectroscopy (FT-IR) was carried out in Thermo Nicolet
6
700 FT-IR spectrometer (USA) to determine the functional group attached to TiO and sulfate
2
functionalized TiO . Pellets of the dried sample were made with KBr and scanned in the ranges
2
-1
o
of 400-4000 cm at 25 C.
Pyridine FTIR was recorded on a Bruker Tensor II spectrophotometer (Bruker,
Germany). In a typical procedure, 50 mg of catalyst sample was dried at 120°C for 1 h and
o
cooled to 25 C under vacuum. Then the vacuum dried sample was saturated in 0.1 mL of
pyridine in a vial and kept for 1 h to complete the pyridine adsorption process, followed by
evaporation of excess pyridine at 120°C for 1 h in an oven. Typically, 95 mg of KBr was mixed
with 5 mg of pyridine treated sample and pressed into a self-supporting wafer of 13 mm
diameter, and the weight of the pallet was measured. Further, the wafer was mounted in the IR
-1
-1
cell and scanned in the ranges of 500-4000 cm with 32 scans at a resolution of 4 cm . The
concentration of Lewis and Brønsted acid sites on the catalyst was calculated based on the
method described elsewhere.7,35
2
.3 Catalytic test
The performance of different catalysts (TiO , 0.25 M SO /TiO , 0.5 M SO /TiO
2,
2

2
2
4
2
4
2

2
0
.75 M SO /TiO 1 M SO /TiO ) for the dehydration of glucose/fructose to 5-
4 2, 4 2
hydroxymethylfurfural (5-HMF) was evaluated in a 100 mL round bottom glass reactor
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equipped with a condenser and a magnetic stirrer (Digital Hot Plate, Tarsons, India) connected
with a temperature controller and thermocouples. Thermocouples were used to measure the
temperature of the oil bath as well as the reaction mixture. In the vertical glass condenser (400
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
o
mm), cold water (~25 C) was continuously circulated at a volumetric flow rate of 200±5 mL
-1
min through its spiral circulating path during the reaction. The predetermined amount of
reactant (10 wt.% of solvent) and the catalyst was added to the reactor placed in an oil bath.
Glass reactor was placed in the oil bath after achieving the desired reaction temperature. The
reaction was performed in the temperature span of 100-180 ℃. The effect of reaction time (1-
6
h), catalyst amount (0.15-1 g), and solvent (water/DMSO) were studied. All the reactions
were allowed to proceed for a specified time and after the reaction, the glass reactor was
quenched in cold water to stop the reaction immediately and centrifuged at 8000 rpm to
separate the catalyst. For all those reactions, where only water was used as a solvent, inside the
reactor, a little higher pressure than ambient might be possible during the reaction. However,
since the condenser top was open to the atmosphere, and proper cooling was maintained at the
top of the reactor, it was assumed that the reactions were operated at atmospheric pressure.
Product composition was identified and quantified in high-pressure liquid
chromatography (HPLC, WATERS, 515, USA) furnished with a binary pump, HPX 87-H
column (Bio-Rad Aminex, 4.6 mm × 350 mm, USA), and a RI detector (WATERS 2414,
USA). During product analysis, the temperature of the HPLC column was adjusted with the
help of a column heater. The detector and column temperatures were maintained at 35 ºC and
6
0 ºC, respectively. Before each analysis, dissolved oxygen present in the mobile phase (0.005
M H SO ) was removed while stirring under vacuum. The flow rate of the mobile phase was
2
4
-1
maintained at 0.55 mL min and the injection volume in the HPLC was 20 µL. The product
liquid was filtered with a syringe filter (0.22 µm) before each analysis. Standard calibration
curves prepared for the individual compound was used to calculate the molar concentration of
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reactant as well as products. Conversion and product yield were calculated by using the
following equations.
Conversion (%) = [(moles converted / moles fed) × 100]
Yield (%) = [(moles of product / moles of reactant fed) × 100]
(1)
(2)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The retentate obtained after the separation of the catalyst from the reaction mixture by
centrifuge were analyzed in an Ion chromatography (882 IC Plus-Anion, Metrohm, USA)
equipped with Metrosep A Supp 5–250/4 ion-exchanged column and suppressed CD (J003)
detector. The mobile phase was sodium carbonate (3.2 mmol/L) and sodium bicarbonate (1
mmol/L). The flow rate of the mobile phase was kept at 0.7 mL/min and the analysis time was
40 minutes, respectively.
3
. Results and discussion
3.1 Catalyst characterization
.1.1 BET
The obtained isotherms of catalysts are shown in Fig. 1. Distinctive type- IV isotherms
3
with prominent H3 type hysteresis loop with a sharp descent at a comparative pressure of 0.7
P/Po < 1 are observed. This result established a regular mesoporous structure of catalysts.36–38
<
2
-
The obtained pore volume, average pore size, and surface area of TiO , and SO /TiO are
2
4
2
2
-1
summarized in Table 1. The surface area of TiO was highest (65 m g ) and after incorporation
2
2
-
of SO , it was decreased significantly, due to the aggregation of small crystallites and the
4
blockage of pores by sulfate species.36,38–40 Huang et al. reported a ~ 60% reduction of the
41
3
8
surface area of commercial TiO after the treatment of 0.5 (M) H SO . Nakhate et al. observed
2
2
4
4
0
2-
the reduction of surface area after sulfation of TiO . Lam et al. prepared the SO /SnO
2
2
4
catalyst and reported the reduction of surface area after sulfation. However, the previously
reported surface area for sulfated TiO was different as compared to the values reported in
2
Table 1. This variation was might be due to the use of different catalyst synthesis techniques.
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3
3
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The pore volume of the catalysts was also followed a similar trend as surface area. TiO
3
-1
3
-
showed the highest pore volume of 0.24 cm g with respect to sulfated TiO (0.03-0.2 cm g
2
1) (Table 1). The pore volume of the catalyst dropped because of the incorporation of sulfate
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
39
on TiO . The obtained pore size of the catalyst was in the range of 12.6 - 18.2 nm.
2
(
A)
(
B)
0
.002
.001
140
TiO2
0
.25 M SO 2-/TiO2
4
1
1
20
00
2-
0
.50 M SO4 /TiO2
0.75 M SO 2-/TiO2
4
0
2-
1
M SO4 /TiO2
8
0
60
0.000
0
200
400
600
800
1000
1200
Pore Diameter (Å)
4
2
0
0
0
0
.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/Po)
Fig. 1 (A) Nitrogen adsorption-desorption isotherm (B) Pore size distribution of catalyst.
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Table 1 Physio-chemical properties of catalysts
BET
Pore
surface
Lewis (L) and
Average
Acidic strength
Pore size
(nm)
Brønsted (B) sites strength
-1
Catalyst
volume
crystallite
(mmol g cat.)
-1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
(mmol g cat.)
area
3
-1
(
cm g )
size (nm)
weak
2
-1
(
m g )
medium
0.035
Total
0.06
0.14
L- sites
0.039
B-sites
0.056
0.063
B/L ratio
TiO2
65.0
58.0
24.7
15.7
7.5
0.24
0.20
0.09
0.06
0.03
12.6
12.9
13.2
12.8
18.2
14.0
18.3
20.3
21.9
22.8
0.027
0.098
0.031
0.010
0.008
1.43
2.99
8.81
5.51
4.41
2
0
0
0
.25 M SO /TiO
0.042
0.021
4
2
2
2
2

.50 M SO /TiO
0.538
0.047
0.029
0.57
0.06
0.04
0.027
0.016
0.012
0.237
0.087
0.055
4
2
.75 M SO /TiO
4
2

1
M SO /TiO
4 2
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3
.1.2 X-ray diffraction (XRD)
XRD patterns of TiO and sulfated TiO are shown in Fig. 2. For TiO , major diffraction
2
2
2
peaks detected at the 2θ value of 25.3, 37.8, 48.2, 53.9, 55.1, 62.7, 68.9, 70.4, 75.3 and 82.6
represented (101), (112), (200), (105), (211), (204), (116), (220), (215) and (224) tetragonal
TiO anatase phase [JCPDS:21-1272].37,42,43 For sulfated TiO , additional peaks detected at the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
2
θ value of 16.2, 21.3, 22.2, 23.7, 27.6 and 28.2, corresponding to (020), (120), (101), (111),
3
9
(
121) and (200) crystal planes of TiO(SO ) [JCPDS: 032-1372]. The peak intensity
4
2
-
corresponding to TiO(SO ) was found to be increased with increasing the SO concentration
4
4
in the catalysts. These results demonstrated the interactions between TiO and sulfate species
2
incorporated. Moreover, it was also observed that the intensity of the major peaks at the 2θ of
25.3 correspondings to the anatase (101) crystal plane of TiO decreased significantly with
2
increasing the sulfate concentration in the catalyst. This might be due to the distribution of the
3
7
sulfate group on the surface of Titania. A similar kind of observation for the sulfated TiO
2
4
4
catalyst is also reported by Dalai et al. The obtained XRD patterns of sulfated TiO (Fig. 2)
2
demonstrated that with increasing the sulfate concentration in the catalysts, the tetragonal
structure of the anatase phase of TiO gradually transformed into the orthorhombic phase
2
[
JCPDS: 032-1372]. This phase transformation was prominently observed for the catalyst
treated with a higher H SO concentration (> 0.75 M). The average crystallite size of TiO and
2
4
2
sulfated TiO were calculated by using the Scherrer formula from the line width of their
2
relevant peaks detected at the 2θ value of 25.3, 37.9, 48.2, 53.9, 55.2, 62.7 and 16.2, 21.4, 22.1,
23.6, 25.3, 37.8, 48.2, 53.9, 55.2, 62.7, respectively. For TiO , the average crystallite size was
2
found to be 14.0 nm (Table 1). For the sulfated TiO these values were found to be increased,
2
and it was in the range of 18.3-22.5 nm. With increasing the sulfate ion concentration, these
values were also exhibited a marginal increasing trend. After impregnation and with increasing
the sulfate amount, the particles get agglomerated and aggregated due to the electrostatic
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interaction of sulfate ions. After the impregnation of SO42- ion on TiO , it replaced one O atom
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DOI: 10.1039/D0NJ04151C
2
4
2
in O-Ti-O framework which increased the atomic radius. As a result, the particle size, pore-
volume, and crystallite size of sulfated titania were increased. For sulfated titania, a similar
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
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9
0
1
2
3
4
5
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8
9
0
1
2
3
4
5
6
7
8
9
0
type of result was also found by Niu et al.42
1
M SO 2-/TiO
4 2
0.75 M SO42-/TiO2
2-
0
.50 M SO4 /TiO2
2
-
0.25 M SO4 /TiO2
TiO2
2
0
30
40
50
60
70
80
90
2
Theta
Fig. 2 XRD pattern of catalyst.
.1.3 Thermo-gravimetric analysis (TGA)
3
The thermal stability of the catalysts was determined by TGA and the results obtained
o
are shown in Fig. 3. For TiO , minor weight loss was detected at < 200 C, due to the removal
2
45
of moisture . For sulfated catalysts, two steps weight loss was detected. Initial weight loss at
<
200 ℃ was due to the dissipation of bound water and moisture, and another major weight
loss observed at higher temperatures (550-800 ℃) was due to the decomposition of sulfate.39,45–
48
The high-temperature wet loss was found to be increased from 1.4 to 32.5%, with increasing
the sulfate concentration in the catalyst (Fig. 3(A)-(E)). It was also observed that higher
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2
2
2
2
2
2
2
2
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3
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3
3
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temperature weight loss for sulfated TiO shifted gradually towards higher temperature side
2
with increasing the SO42- concentration. Gardy et al. reported two steps weight losses for the
39
titania nanocatalyst due to moisture and sulfate decomposition in the temperature range of 50-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
00 ℃ and 600-800 ℃, respectively. The observed weight loss due to sulfate decomposition
4
8
was ~47% at 600-800 ℃. Bai et al. explained the weight loss due to sulfate decomposition
at >500 ℃, which also collapsed the porous channel of sulfated TiO . Sulfated Titania catalyst
2
46
synthesized by hydrolysis of titanium oxysulfate was found to be stable up to 500 ℃. The
o
obtained TGA results in this study depicted that calcination of this catalyst at 550 C and higher
will affect the acidic strength of the catalyst due to sulfate decomposition.46
Loss of water
Decomposition of sulfate
(
E)
3
2.5 wt.%
(D)
1
M SO42-/TiO2
2
1.4 wt.%
0.75 M SO -/TiO2
2
(
C)
4
1
4 wt.%
2-
0
.50 M SO /TiO2
4
(B)
.25 M SO42-/TiO2
0
1
.4 wt.%
(
A)
1
TiO2
00
200
300
400
500
600
700
800
900
o
Temperature ( C)
Fig. 3 TGA profile of catalyst.
.1.4 Morphology
Surface morphology and elemental composition of TiO and sulfated TiO catalysts
3
2
2
determined by FE-SEM with EDX and TEM analysis, respectively. In the SEM image of TiO2
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(
Fig. 4(A)), uniformly distributed spherical particles were detected. After the addition of the
sulfate group in TiO , small spherical particles were found to be agglomerated [Fig. 4(B)] and
2
bigger clusters were formed at higher sulfate concentrations [> 0.5 (M) (Fig. 4(B)-(E)]. The
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
aggregation of particles might be due to an electrostatic attraction among the sulfate ions.39,49
Furthermore, FE-SEM equipped with EDX was used to analyze the elemental
compositional of the catalysts. The presence of titanium, oxygen, and sulfur in the catalysts
was confirmed by SEM-EDX as shown in Fig. (4(A)-(E)). TiO exhibited two major peaks at
2
around 4.5 keV and 5 keV, respectively. The intense peak obtained at 4.5 keV corresponding
5
0,51
to the bulk TiO and the less intense peak detected at 5 keV was assigned for surface TiO
2
2.
The peak corresponding to sulfur was visible at around 2.25 keV (Fig. 4(B)-(E)). In addition
to that, uniformly distributed sulfate on the catalyst was confirmed from the elemental mapping
2
-
2-
of 0.5 M SO /TiO catalyst (Fig. 4(E)). The obtained TEM and SAED images of 0.5 M SO
4
4
2
2
-
/TiO catalyst are shown in Fig. 4(F). In the TEM image of 0.5 M SO /TiO catalyst, similar
2
4
2
spherical morphology of the particles was observed as SEM. The measured particle sizes were
in the range of 15-20 nm (inserted histogram in Fig. 4(F)), which agreed very well with the
mean crystallite size (20.3 nm) deliberated by XRD. The polycrystalline structure of the
catalyst was confirmed from the SAED pattern obtained. As shown in Fig. 4(F), rings of the
bright spots were detected and each spot arises from the Bragg’s reflection from their crystallite
planes.
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2

Fig. 4 FE-SEM image of catalyst (A) TiO with EDX (B) 0.25 M SO /TiO with EDX (C)
2
4
2
2
2
0
.50 M SO /TiO with EDX and electron mapping of Ti, S, O (D) 0.75 M SO /TiO with
4 2 4 2
2

2-
EDX (E) 1 M SO /TiO with EDX (F) HR-TEM image of 0.50 M SO /TiO with SAED
4
2
4
2
image and lattice fringes of TiO and particle size histogram.(All the FESEM images are taken
2
at 100 nm scale and 200.00 K X magnification. The EDAX scale is from 0 to 10 keV for all
the FESEM-EDAX images.)
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.1.5 Acidic strength
The comprehensive acidic strength of catalysts was measured by temperature-
o
programmed desorption (TPD) analysis. TPD was performed up to 1000 C (Fig. 5(A)),
however, the peak detected at 550 ℃ and higher were not considered for calculation because
these peaks represented sulfate decomposition as confirmed by TGA (Fig. 3). As shown in Fig.
5(A), for all the catalysts, ammonia desorption peaks were observed primarily in two different
temperature regions. Peaks obtained in the lower temperature region (50-300 ℃) represented
the weak acidic sites and the peaks detected at higher temperatures (> 300 ℃) attributed to
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
2
medium strength acidic sites. The total acidic strength of the catalyst was estimated based on
the total amount of ammonia desorbed and detected by TCD (Table 1). The total integral area
-1
of the NH desorption curve represented the amount of ammonia adsorbed (STP g ) at different
3
temperatures. It can be seen from Fig. 5(A) that sulfated catalyst displayed weak and medium
-1
strength acidic sites and the highest total acidic strength of 0.57 mmol g cat. was obtained for
2-
0
.5 M SO /TiO . On further increasing the sulfate loading (> 0.5 M), this value decreased
4
2
significantly. This decrease in acidic strength might be due to the formation of polysulfate.53
The gradual shifting of the TPD peak towards higher temperatures was also observed with
increasing the sulfate concentration in TiO . This temperature shift indicated some monotonic
2
increase in the average acidic strength of catalyst with increasing the sulfate concentration.44
2
-
Lewis and Brønsted acid sites on the TiO and SO /TiO catalyst characterized by
2
4
2
pyridine-FTIR (Fig. 5(B)). Pyridine absorption peaks detected at the wavenumber of 1443 cm-1
-1
and 1484 cm represented the Lewis and Brønsted acid sites, respectively, whereas the
-
1
1
absorbance peak at ~1535 cm demonstrated the combined Lewis and Brønsted acid sites.
-1
31
The vibrational band detected at ~1639 cm represented the hydrogen-bonded pyridine. The
ratio of Brønsted/Lewis acid strength on the catalyst surface was calculated by following a
3
5
standard method described elsewhere. The integral area of the respective absorbance peaks
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1
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1
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1
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
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detected was considered for calculation. The ratio of Brønsted/Lewis acidic strength on the
catalyst was found to be increased with sulfate concentration up to 0.5 (M), and the maximum
ratio of 8.81 was obtained (Table 1). Further increasing the sulfate loading on the catalyst, this
ratio decreased due to the morphological changes of the catalyst surface treated with
concentrated H SO acid. These results revealed a good agreement with the NH -TPD data.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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2
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9
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1
2
3
4
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7
8
9
0
2
4
3
The presence of both the Brønsted and Lewis acidic sites on TiO is also reported
2
5
4
earlier. Bezrodna et al. investigated pyridine-TiO surface interaction as a probe for surface
2
active center analysis and reported the presence of both the acidic sites on the TiO surface. It
2
was demonstrated that the availability of hydroxyls groups on the TiO surface was responsible
2
5
5
for the Brønsted acidity. Li et al. also confirmed the presence of both Brønsted and Lewis
sites on the TiO surface in their study on the synthesis and characterization of TiO
2
2
polymorphs. In this study, in the FTIR of TiO (Figure 7) the O-H, H-O-H, and Ti-O stretching
2
-1
-1
-1
were detected at the Web number of 3414.8 cm , 1635.5 cm , and 500-700 cm , respectively.
These results supported the presence of the O-H group on the TiO surface which was
2
responsible for Brønsted acidity.
(B)
TiO
2
(A)
Py-OH
2-
-
0.25 M SO
/TiO
1
M SO 2-/TiO
4
2
2
4
2
0.5 M SO /TiO
4
2
2-
0.75 M SO 2-/TiO
4
0.75 M SO
/TiO
4
2
2
2-
1 M SO /TiO
4
2
0.50 M SO42-/TiO
0.25 M SO42-/TiO
2
B + L sites
B sites
2
L sites
TiO2
1
00 200 300 400 500 600 700 800 9001000
1400
1450
1500
1550
1600
1650
1700
1750
o
-1
Temperature ( C)
Wavenumber (cm )
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
Fig. 5 (A) TPD profile of catalyst, (B) Pyridine-FTIR spectra of TiO , 0.25 M SO /TiO , 0.5
2
2

2
2
M SO /TiO , 0.75 M SO /TiO , and 1 M SO /TiO at room temperature.
4
2
4
2
4
2
3.1.6 X-ray photoelectron spectrum (XPS)
High-resolution XPS spectrum survey scans of TiO , and 0.5 M SO /TiO catalysts
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2

2
4
2
are shown in Fig. 6. As shown in Fig. 6(A), four significant XPS peaks respective to S 2p, C
s, Ti 2p, and O 1s are observed at 164.8 eV, 284.7 eV, 458 eV, and 531 eV binding energy,
1
respectively. In the entire spectra, C 1s peak was allied to the hydrocarbon species present in
the XPS instrument itself. However, the XPS spectra detected at the binding energy of 458.4
eV and 464.3 eV corresponded to Ti 2p and Ti 2p of Ti 2p of pure TiO . These characteristic
3
/2
1/2
2
5
6
XPS spectra represented the anatase phase of TiO as reported earlier. For TiO , the spin-
2
2
orbital splitting of 5.7 eV revealed a +4 oxidation state of titanium in the lattice.39,57,58 For 0.5
2
-
M SO /TiO catalyst, the peaks corresponding to Ti 2p and Ti 2p shifted slightly towards
4
2
1/2
3/2
higher binder energy i.e., at 459.2 eV, and 464.9 eV, respectively. This result indicated the
interaction between the sulfate group and Titania in the catalyst (Fig. 6(B)). However, the spin-
orbit splitting value was found to be unchanged (5.7 eV), which indicated the +4 oxidation
2
-
state of titanium in the 0.5 M SO /TiO catalyst. The major peak at the binding energy of
4
2
529.7 eV corresponded to the O-Ti-O bond,46,56 and the shoulder peak at the 531.1 eV might
be due to the O-H bond of the chemisorbed water molecule present on the catalyst
surface.36,37,49 For the sulfated catalyst, the peaks corresponding to O 1s shifted towards higher
binding energy (0.7-0.8 eV) (Fig. 6C). The peaks at 530.4 eV were due to the Ti-O-S bond,
4
6,56
and the second peak at 531.9 eV was due to the O-S bond of sulfate group and/or O-H
bond and chemisorbed water molecules, respectively.36,37,49 For all the catalysts, the XPS
spectra of S 2p were de-convoluted into two peaks by Gaussian fitting (Fig. 6(D)). Finally, the
peaks detected at the binding energy of ~169 eV for S 2p were de-convoluted into two different
peaks at 168.9 eV and 169.7 eV which represented S 2p and S 2p , respectively. This S 2p
1
/2
3/2
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1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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peaks confirmed the presence of sulfate group on TiO . Similar kind of metal sulfate
2
integrations is reported by Niu et al.42
(A)
(
B) Ti 2p
459.2eV
TiO2
.5 M SO 2 /TiO2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Ti 2p3/2
-
0
TiO2
4
0
.5 M SO42-/TiO
2
Ti 2p1/2
4
58.4eV
4
64.9eV
5.7eV
464.1eV
464
5
.7eV
700
600
500
400
300
200
100
0
468
466
462
460
458
456
454
Binding Energy (eV)
Binding energy (eV)
(C) O 1s
TiO2
530.4eV
(D) S 2p
0.5 M SO 2-/TiO
4
2
169.7eV
0
.5 M SO42-/TiO
2
1
68.9eV
531.9eV
529.7eV
S 2p1/2
S 2p3/2
531.1eV
174
173
172
171
170
169
168
167
166
5
37 536 535 534 533 532 531 530 529 528 527 526 525
Binding energy (eV)
Binding energy (eV)
2

Fig. 6 (A) Survey scan XPS spectra of TiO , 0.5 M SO /TiO , (B) Ti 2p, (C) O 1s, and (d) S
2
4
2
2
p.
.1.7 Fourier transform infrared spectroscopy (FT-IR)
3
2

Fig. 7 presents the FT-IR spectra of TiO and 0.5 M SO /TiO catalysts. For TiO ,
2
4
2
2
-1
four major stretching vibrational bands were detected at the wavenumber of 3414.8 cm ,
-1
-1
-1
2
356.7 cm , 1635.5 cm , and 500-700 cm , respectively. A strong wide band at the
-1
wavenumber of 3414.8 cm was due to O-H stretching of water molecule adsorbed on the TiO
2
surface.37,39,42,48 The vibration stretching detected at 2356.7 cm-1 wavenumber was
39
corresponding to the O=C=O stretching of CO molecule adsorption from the air. However,
2
H-O-H symmetric vibration of adsorbed water molecules was detected at the wavenumber of
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635.5 cm . The band at 557 cm and 667.5 cm wavenumbers represented the characteristics
of vibrational stretching of the Ti-O bond.
2

In the FT-IR spectra of 0.5 M SO /TiO catalyst additional four peaks were detected
4
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
in the wavenumber range of 900-1300 cm (Fig. 7). The absorption bands detected in the
-1
59
region of 1130-1240 cm , were assigned to S=O vibration stretching. The strong band at
-1
-1
1047.8 cm and 987 cm were related to S-O stretching which was assigned to chelating
bidentate SO42- coordination with Ti .
4+ 59,60
These results are in good agreement with XPS
results which manifested a strong predominance of SO42- ions at the surface of the sulfated
catalyst.
TiO2
0
.5 M SO 2-/TiO2
4
O-H
O=C=O
S-O
S=O
Ti-O
H-O-H
4000
3500
3000
2500
2000
1500
1000
500
-
1
Wavenumber (cm )
2

Fig. 7 FTIR spectra of TiO , and 0.5 M SO /TiO at room temperature.
2
4
2
3
.2 Catalytic performance
Initially, the dehydration ability of TiO and 1 M SO /TiO catalyst was evaluated at
2

2
4
2
17
standard reaction conditions. To verify the homogeneous reaction, a blank run was performed
in absence of a catalyst, and it was demonstrated ~32.6% conversion of glucose, and the main
products including 5-hydroxymethylfurfural, levulinic acid, formic acid were not detected in
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2
2
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2
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the product mixture (Fig. 8). At this condition, glucose was converted to some other unknown
unidentified compounds. The HPLC chromatogram of the products is given in the
supplementary file (Fig. S1). However, in presence of a catalyst, the major reaction products
were 5-HMF, LA, FA, and black organic polymer might be humin.27 The black organic
polymer (soluble and insoluble) might be formed due to the condensation reaction of glucose,
fructose, 5-HMF, and other intermediates in the presence of high acidity of the catalyst.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
5
Dehydration of glucose followed the reaction path as shown in Fig. 9. In presence of TiO
2
catalysts, glucose conversion, and the yield of 5-HMF, LA, FA was 74.7%, 14%, 4.4%, and
2

5
%, respectively (Fig. 8). For 1 M SO /TiO catalyst, glucose conversion was further
4 2
increased to 98.4%, and the yield 5-HMF, LA, and FA was 31.8%, 4.8%, and 11.5%,
2

respectively. An almost 2.2-fold increase of 5-HMF yield was observed with 1 M SO /TiO
4
2
catalyst as compared to pure TiO . The appearance of Brønsted acid centers after the addition
2
of sulfate ions in TiO was the primary reason for higher dehydration activity, which also
2
promoted the yield of 5-HMF. The presence of both Lewis and Brønsted acid sites on the
8
catalyst surface derived the glucose dehydration reaction as reported earlier. A decent yield
(
31.8%) of 5-HMF was higher as compared to the results reported in the previous literature in
the DMSO medium. Yan et al. reported a 19.2% 5-HMF yield at 95.2% glucose conversion
1
8
over sulfated zirconia catalyst at 140 ℃ and after 4 h. Feng et al. achieved a 25.8% yield of
2
-
+
28
5
-HMF with SO /ZrO modified H zeolite catalyst at 195 ℃ after 90 min. Since the sulfated
4
2
TiO performed better as compared to pure TiO in terms of glucose dehydration to 5-HMF,
2
2
therefore, the effect of sulfate concentration on TiO catalysts was investigated and the reaction
2
variables were optimized for higher 5-HMF yield, which is discussed in the following sections.
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00
9
6.6
Conversion
FRU
FA
9
0
80
70
LA
74.7
5
-HMF
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6
5
4
3
2
1
0
0
0
0
0
0
0
32.6
3
1.8
1
4
1
1.5
5
4.4
4.8
0
0
0
0
0
0
2
-
without catalyst
TiO2
1 M SO /TiO
4 2
Fig. 8 Performance of various catalysts on glucose dehydration to 5-HMF.
Reaction condition: 1 g glucose, 0.5 g catalyst, 10 mL DMSO, 150 ℃, 6 h.
Isomerization
Dehydration
Glucose
5
-Hydroxymethylfurfural
Fructose
Lewis acidic sites
Brønsted acidic sites
-3H O
2
Rehydration
+
H O
2
Humin
Levulinic acid + Formic acid
Fig. 9 Reaction pathways of glucose dehydration.
3.2.1 Effect of temperature and time on conversion of glucose to 5-HMF
The effect of reaction temperature on glucose conversion and product yields are shown
o
in Fig. 10(A). The temperature was deviated in the span of 100-180 C by keeping other
reaction parameters constant and the experimental data were collected after a run time of 6 h.
The temperature effect on glucose conversion was found to be very significant and it increased
o
o
from 11.7% at 100 C to 100% at 180 C. In the entire temperature domain, 5-HMF, LA, and
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1
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2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
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FA were found to be the major reaction products. As reported in the previous literature, 5-HMF
has formed via two-step dehydration of glucose i.e. glucose to fructose followed by fructose to
5-HMF. In this study, fructose was not detected, which indicated the faster dehydration of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
fructose in presence of 1 M SO /TiO catalyst having both Lewis and Brønsted acid sites.
4
2
o
The yield of 5-HMF passed through maxima at ~150 C, and at higher temperatures, 5-HMF
degraded to other side products such as FA and organic polymers. At higher temperatures
o
(
typically >150 C), the formation of the soluble and insoluble dark color compounds was
observed. Fig. 10(B) showing the gradual color variation of the reaction medium with
temperature. These organic polymers are formed due to the degradation of glucose as well as
23
other primary glucose dehydration products via side reactions. The highest 5-HMF yield of
~32% was obtained at 150 ℃.
Significant amounts of 5-HMF rehydration products such as LA and FA were also
formed and their concentrations varied with the variation of reaction temperature. The yield of
o
LA increased from 2.1% to 4.9% at 150 C and further it decreased at higher temperatures. Up
o
to 140 C, the yield of FA was negligible, after that the yield of FA was found to increase
linearly with simultaneous decreasing the yield of 5-HMF as well as LA. These results
demonstrated the formation of FA due to the rehydration of 5-HMF at higher temperatures 11.
o
The maximum FA yield of 38.7% was detected at 180 C. At higher temperatures, the color of
the reaction mixture became dark brown or black due to the more production of organic
polymers which also supported the rehydration of 5-HMF as well as condensation of glucose
to organic polymers at a higher temperature (Fig. 10(B)). The temperature effect study
o
suggested that higher reaction temperature (>150 C) favored the undesirable side reactions.
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(
A)
1
00
90
8
7
0
0
Conversion
-HMF
LA
FA
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
60
5
4
3
0
0
0
20
1
0
0
100
120
140
160
180
o
Temperature ( C)
Fig. 10 (A) Effect of reaction temperature on glucose dehydration to 5-HMF.
2-
Reaction condition: 1 g glucose, 0.5 g 1 M SO /TiO catalyst, 10 mL DMSO, 6 h
4
2
(
I)
(II)
(III)
(IV)
(V)
(VI)
Fig. 10 (B) Color of reaction mixture at temperature (I) 100 ℃ (II) 120 ℃ (III) 140 ℃ (IV) 150
℃
(V) 160 ℃ (VI) 180 ℃.
Fig. 11 shows the time effect on glucose conversion and the variation of product yield
at 150 ℃. Very high glucose conversion of ~80.5% was achieved even after 1 h, and it was >
o
9
8% after 5 h. These results suggested that the glucose dehydration rate was faster at 150 C.
27

New Journal of Chemistry
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2
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
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5
5
5
5
5
5
5
5
5
6
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DOI: 10.1039/D0NJ04151C
A similar kind of dehydration rate also reported by Teimouri et al. The 5-HMF yield was
found to be increased with time and the maximum yield of 32.8% was achieved after 6 h,
further it decreased at a slower rate. The formation of other rehydrated products such as FA,
LA, and organic polymers was the primary reason for declining the yield of 5-HMF. Similar
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
2
kinds of phenomena were also observed by several authors. Therefore, higher reaction time
was not favorable for a greater 5-HMF yield. The obtained LA and FA yield were in the range
of 1.5-4.9% and 6.9–13%, respectively.
1
00
9
8
0
0
Conversion
-HMF
5
LA
FA
3
2
0
0
10
0
1
2
3
4
5
6
7
8
Time (h)
Fig. 11 Effect of reaction time on glucose dehydration to 5-HMF.
2
Reaction condition: 1 g glucose, 0.5 g 1 M SO /TiO catalyst, 10 mL DMSO, 150 ℃.
4
2
3
.2.2 Effect of catalyst amount
The role of catalyst amounts (0.15-1 g) was evaluated at 150 C, and the products were
o
analyzed after a run time of 6 h. As shown in Fig. 12, initially with increasing the catalyst
amounts glucose conversion, as well as 5-HMF yield, increased due to increasing the
accessibility of more active sites. A maximum of 32.8% 5-HMF yield was obtained with 0.5 g
of catalyst and a further increase in catalyst amount yield decreased. The accessibility of more
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Page 29 of 49
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DOI: 10.1039/D0NJ04151C
active centers facilitated the rehydration of 5-HMF as well as other side reactions. Therefore,
0
.5 g catalyst was considered as optimum.
100
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
90
Conversion
-HMF
5
LA
FA
30
2
0
0
0
1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Catalyst weight (g)
Fig. 12 Effect of catalyst amount on glucose dehydration to 5-HMF.
2
Reaction condition: 1 g glucose, 1 M SO /TiO catalyst, 10 mL DMSO, 150 ℃, and 6 h.
4
2
3
.2.3 Sulfate loading effect on TiO2
The sulfate group was induced on titania to enrich the Brønsted acidic strength of the
catalyst. Various Titania catalyst was prepared by treating the catalyst sample with H SO of
2
4
o
various molarity [0.25 (M)-1(M)], and their performance was evaluated at 150 C. A maximum
of 98.3% glucose conversion and ~37% 5-HMF yield were achieved in presence of titania
catalyst treated with 0.5 (M) H SO (Fig. 13). On further increment of the sulfate concentration
2
4
(
> 0.5 (M)), no significant increase in glucose conversion and 5-HMF yield were identified. It
might be due to the saturation of the sulfate group on the titania surface beyond 0.5 (M).
29