One-pot sol-gel synthesis of a phosphated TiO2 catalyst for conversion of monosaccharide, disaccharides, and polysaccharides to 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c9nj01677e

Catalytic conversion of biomass or biomass-derived carbohydrates into 5-hydroxymethylfurfural (HMF) is an important reaction for the synthesis of bio-based polymers, fuels, and other industrially useful products. In this study, phosphated titania (P-TiO₂) catalysts with different phosphoric acid content were prepared through a simple one-pot sol-gel method and characterized by BET, XRD, FT-IR, NH₃-TPD, py-FT-IR, and XPS techniques. The catalyst characterization results revealed the incorporation of phosphorus into the TiO₂ framework in the form of a Ti-O-P bond. The P-TiO₂ catalysts were applied to the conversion of glucose (≥10 wt%) into HMF in a biphasic water/THF reaction medium at 175 °C. Under optimized reaction conditions, 98% glucose conversion and 53% HMF yield were obtained over a 15P-TiO₂ catalyst, and the catalyst was reused for several cycles with consistent activity and selectivity. The presence of both Br?nsted and Lewis acid sites, high BET surface area and pore volume, and high acidity could account for the high catalytic activity and selectivity. Besides, the 15P-TiO₂ catalyst was also demonstrated to be active for the conversion of disaccharides (sucrose and cellobiose), polysaccharides (starch and microcrystalline cellulose) and industrial grade sugar syrups into HMF with reasonable yield.

Full text of DOI:10.1039/c9nj01677e

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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One-pot sol-gel synthesis of phosphated TiO catalyst for conversion of
DOI: 10.1039/C9NJ01677E
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monosaccharide, disaccharides, and polysaccharides to 5-
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hydroxymethylfurfural
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Kasanneni Tirumala Venkateswara Rao*, Sadra Souzanchi, Zhongshun Yuan,
Chunbao (Charles) Xu*
Department of Chemical and Biochemical Engineering, Western University, London,
Ontario, Canada N6A 5B9
*Corresponding Author: tvenkate@uwo.ca, cxu6@uwo.ca
Abstract
Catalytic conversion of biomass or biomass-derived carbohydrates into 5-
hydroxymethylfurfural (HMF) is an important reaction for the synthesis of bio-based
polymers, fuels, and other industrial useful products. In this study, phosphated titania (P-
TiO2) catalysts with different phosphoric acid content were prepared through a simple one-
pot sol-gel method and characterized by BET, XRD, FT-IR, NH3-TPD, py-FT-IR, and XPS
techniques. The catalyst characterization results revealed that the incorporation of phosphorus
into the framework of TiO2 in the form of a Ti-O-P bond. The P-TiO2 catalysts were applied
in the conversion of glucose (≥ 10 wt%) into HMF in a biphasic water/THF reaction medium
o
at 175 C. Under optimized reaction conditions, 98% glucose conversion, and 53% HMF
yield was obtained over a 15P-TiO2 catalyst, and the catalyst was reused for several cycles
with consistent activity and selectivity. The presence of both Bronsted and Lewis acid sites,
high BET surface area and pore volume, and high acidity could account for the high catalytic
activity and selectivity. Besides, the 15P-TiO2 catalyst was also demonstrated to be active for
the conversion of disaccharides (sucrose and cellobiose), polysaccharides (starch and
microcrystalline cellulose) and industrial grade sugar syrups into HMF with reasonable yield.
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Keywords: Glucose, Disaccharides, Polysaccharides, 5-HMF, PhosphateDOdI: 10t.1i0t3Vai9en/wCiaA9rN,ticJle01O6n7li7nEe
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Heterogeneous catalysis
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1. Introduction
Over the past few years, the world witnessed tremendous progress in the utilization of bio-
renewable feedstocks for sustainable production of fuels and bio-based chemicals.1 In this
context, 5-hydroxymethylfurfural (HMF) is one of the top ten value added bio-based
chemical identified by the US Department of Energy (DOE).2 HMF has a wide range of
applications in different sectors, including fine chemicals,3 polymer monomers,4 biofuels,5
and liquid alkanes.6 Therefore, the synthesis of HMF from renewable resources is currently a
hot area of research. Inexpensive and highly abundant glucose is an ideal feedstock for HMF
production.7,8 However, the conversion of glucose to HMF involves a tandem process in
which first, glucose is isomerized to fructose followed by dehydration of fructose to form
HMF.8 The isomerization step is typically facilitated by enzymes, base or Lewis acid catalyst,
and the Bronsted acid can promote the dehydration step.9 According to a recent report, many
bifunctional catalysts have been developed, allowing the direct conversion of glucose to
HMF.10
The pioneering work of Zhao et al. the first report on one-pot dehydration of glucose
catalyzed by CrCl2, was a breakthrough for HMF production.11 However, the chromium salts
are soluble in the reaction medium, which led to challenging for the catalyst separation and
subsequent recycling. Moreover, the chromium salts are toxic, and this process is not
environmentally benign for the sustainable production of HMF from glucose. Alternatively,
many bifunctional heterogeneous solid acid catalysts have been developed for the selective
conversion of glucose to HMF in a biphasic water and organic reaction medium.12 Examples
of solid acid catalysts include metal oxides,13-15 zeolites,16,17 metal phophates,18-24 mesoporous
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silica,25,26 and carbonaceous solid materials.27, 28 All these solid acid catalysts would show
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excellent catalytic performance for HMF synthesis, but they have their demerits.
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TiO2 based solid catalysts are well known as photocatalytic materials and also can be used as
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water tolerant Lewis acid catalyst for different catalytic reactions, including Mukaiyama-
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aldol
and MPV reduction reactions.31 Recently, the TiO2 based solid acid catalysts have
been used for the dehydration of glucose to 5-HMF.32,33 Nakajima et al. reported phosphated
titania (P-TiO2) catalyst for glucose dehydration reaction at 120 oC for 2 h, and achieved 99%
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glucose conversion and 81% HMF yield, respectively.
Very recently, Hattori et al.
synthesized photo-assisted phosphorylation of anatase TiO2 with phosphoric acid for HMF
synthesis in a water/2-sec-butylphenol system.34 Although these catalysts were efficient for
the production of HMF, high catalyst loading is required and they are applicable only to a
dilute solution of glucose. In another report, the P-TiO2 catalyst was prepared via a two-step
process for conversion of glucose in a bi-phasic reaction medium, obtained HMF yields in the
range of 81 to 90.5% at glucose conversion of 98%.35,36 Even though this catalyst can show
good catalytic performance, the difficulty of isolating the HMF from the reaction mixture and
time-consuming process of preparing the catalyst are its significant drawbacks.
In the present study, the P-TiO2 catalyst is synthesized via a simple one-pot sol-gel method,
in which the phosphorylation reaction takes place by the addition of phosphoric acid over Ti-
alkoxide precursor, forming a titanium phosphate alcohol-gel, which would subsequently be
dried and calcined. The textural and catalytic properties of the P-TiO2 catalyst are
investigated for dehydration of mono, di, and polysaccharides into HMF in a biphasic
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H2O/THF solvent system at 175 C for 2 h. The effects of the main reaction parameters,
including the reaction time, temperature, catalyst loading, and reusability of the catalyst are
also studied. In addition, the effects of different bifunctional solid acid catalysts on selective
dehydration of glucose to HMF is also investigated.
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2. Experimental
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2.1 Materials
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D-(+)-glucose (>99.5%), D-(-)-fructose (>99%) and 5-hydroxymethylfurfural (99%), formic
acid, levulinic acid for preparing HPLC standard solutions as well as cellobiose, sucrose,
microcrystalline cellulose, titanium iso-propoxide (97%), zirconium (IV) n-propoxide (70
wt% in 1-propanol), silicotungstic acid (STA), phosphotungstic acid (TPA) and isopropanol
were purchased from Sigma-Aldrich, Canada. HPLC grade water, acetonitrile, and
triethylamine, for preparing the mobile phase for HPLC analysis, as well as
tetrahydrofuran(THF), n-butanol, phosphoric acid (85%), and nitric acid (70%) were
purchased from Caledon Laboratory Chemicals and used as received.
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2.2. Catalyst preparation
The phosphated TiO2 (P-TiO2) catalysts were prepared via a one-pot sol-gel method adopted
from a procedure reported in the literature for the preparation of sulfated ZrO2 catalyst.37 For
a typical synthesis of 15 mol% phosphoric acid loading on TiO2 (15P-TiO2) as follows, 70%
(v/v) nitric acid (1.805 mL) was added to a stirred solution of titanium isopropoxide (12.875
mL) in isopropanol (6.75 mL) followed by dropwise addition of 85% phosphoric acid
(0.9223 g). Then, a solution of iso-propanol (12 mL) in deionized water (3 mL) was added
dropwise to the former solution to form a phosphate/Ti-alkoxide-alcohol gel. After resting for
1.5 hours to settle, the solvent was evaporated, and then the gel was dried in the open air at
90oC for 4 hours. The obtained solid material was calcined in air at 300oC for 4 hours.
Similarly, other catalysts were also prepared by varying the H3PO4 content, and these
catalysts were denoted as 10P-TiO2 and 20P-TiO2. The zirconium-n-propoxide solution was
replaced by titanium isopropoxide for the synthesis of the 15P-ZrO2 catalyst.
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The above one-pot procedure was also applied to prepare TiO2 supported heterDoOpIo: 1l0y.10a3c9/iCd9sNJ01677E
(TPA and STA) catalysts. The TPA or STA replaced the phosphoric acid solution in the
above process for the synthesis of 0.5TPA/TiO2 and 0.5STA/TiO2 catalysts.
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For comparison, the 15P-TiO2-imp catalyst was also prepared via two-step processes. First,
Ti-alkoxide -alcohol gel was synthesized by following the above sol-gel process except
adding H3PO4. The resulting dry titanium hydroxide gel was then immersed in an aqueous
solution of phosphoric acid, followed by stirring at room temperature for 4h. The excess
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water is evaporated, followed by oven dried at 90 C for 12 h. Finally, the 15P-TiO2-imp
catalyst was obtained after calcination at 300 oC for 4h.
2.3 Catalyst characterization
Powder X-ray diffraction (XRD) patterns of the catalysts were measured using an X-ray
diffractometer (Rigaku RINT 2500, Tokyo, Japan) using Ni-filtered Cu Kα radiation (λ =
1.5406 Å) with a scan speed of 2° min−1 and a scan range of 2–80° at 30 kV and 15 mA. N2
physisorption was performed using Quadrasorb SI automated surface area and pore size
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analyzer. Prior to N2 adsorption measurement, the sample was degassed at 200 C until a
stable static vacuum of less than 510-3 Torr was achieved. Subsequently, N2 adsorption was
performed at 196 oC. The surface area of the catalysts was measured using multi-point BET
in the relative pressure range of 0.05 – 0.3. The total pore volume was estimated at a relative
pressure of 0.99.
Fourier transform infrared (FT-IR) spectra of the catalysts were recorded on a PerkinElmer
IR spectrometer using the KBr disc method. The nature of acid sites (Bronsted and Lewis) of
the catalysts was determined by pyridine adsorbed FT-IR spectroscopy. Before the pyridine
adsorption, the catalysts samples were dried at 150 oC for 1 h. To the dried samples (~50 mg
each) in a glass vial, 0.1 mL of pyridine was added directly. Then, the excess pyridine was
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removed by heating the sample at 120 C for 1 h. After cooling the sample to room
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temperature, FT-IR spectra of pyridine adsorbed catalysts samples were recorded. X-ray
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photo electron spectroscopy (XPS) measurements were conducted on an Axis Nova
spectrometer using a monochromatic Al K (alpha) source (15 mA, 14 kV). The XPS
instrument was calibrated using Au as standard. For energy calibration, the carbon 1s
photoelectron lines were used. The carbon 1s binding energy was taken as 285 eV. The
Kratos charge neutralizer system was used on all specimens.
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The total acidity of the catalysts was measured by NH3-TPD. The NH3-TPD measurement
was carried out on a ChemBET Pulsar TPR/TPD automated chemisorption analyzer. In a
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typical experiment, about 0.1 g of the sample was pre-treated at 300 C for 1 h under a flow
of helium (99.9%, 120 mLmin-1). After pre-treatment, the sample was saturated with
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anhydrous ammonia at 100 C for 10 min and subsequently flushed with He at the same
temperature to remove any physisorbed ammonia. Then, the TPD analysis was carried out
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from ambient temperature to 800 C with a heating rate of 10 C min-1, and the desorbed
ammonia was measured by a thermal conductivity detector.
2.4 Catalytic activity tests
The catalytic dehydration of glucose was carried out in a custom-made micro scale reactor. In
a typical experimental run, the microscale reactor was charged with glucose (300 mg),
catalyst (75 mg), and solvent (12 mL of the organic phase and 3 mL of H2O saturated with
NaCl). Nitrogen was used for purging air inside the reactor and maintaining certain pressure
to prevent boiling. The reaction mixture inside the reactor was stirred vigorously using a
magnetic stirrer and heated for the given time and temperature using an oil batch. The starting
time of the reaction was taken when the temperature of the oil bath set to the desired point.
Upon completion of the reaction, the reaction mixture was rapidly quenched to room
temperature, the catalyst was separated by centrifugation, and the supernatant liquid of both
phases was collected for analysis. Both the aqueous and the organic phases were analyzed
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using high-performance liquid chromatography (HPLC) (Waters 2690 HPLC) equipped VwiewitAhrticle Online
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refractive index detector (Waters 410) and UV detectors (Waters 484) using an XBridge
Amide, 3.5 µm, 4.6 x 250 mm analytical column. 80/20 acetonitrile/water with 0.2%
triethylamine was used as mobile phase with a flow rate of 0.6 mL/min. The column
temperature was maintained at 35 oC. Glucose conversion, HMF, fructose, and Levulinc acid
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(LA) yields were calculated according to the following equations.
moles of glucose reacted
Glucose conversion (%) =
(1)
× 100
initial moles of glucose
moles of HMF produced
HMF yield (%)
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(3)
(4)
× 100
× 100
× 100
Initial number of moles of glucose
moles of fructose produced
Fructose yield (%)
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Initial number of moles of glucose
moles of levulinic acid produced
=
Initial number of moles of glucose
Levulinic acid yield (%)
3. Results and Discussion
3.1 Catalyst Characterizations
3.1.1 N2 Physisorption
The BET surface area and total pore volume of the catalyst can play a significant role in
heterogeneous catalysis. As shown in Table 1, the BET surface area of pure TiO2 catalyst
exhibit a value 143 m2/g, and the total pore volume of the catalyst is 0.14 cc/g. However, the
surface modification of TiO2 with phosphoric acid via a one-pot sol-gel process results in
substantial improvement in both BET surface area and total pore volume of the catalyst
comparing to the bare/plain TiO2 catalyst. For example, the BET surface area and total pore
volume of the 10P-TiO2 catalyst are determined to be 181 m2/g, and 0.22 cc/g, respectively.
The textural properties of the catalyst are further improved by increasing the phosphoric acid
loading and attained a maximum for 15P-TiO2 catalyst (BET = 247 m2/g, Vt = 0.31 cc/g). A
further increase of phosphoric acid loading to 20 mol% led to the reduction of both specific
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surface area and pore volume of the catalyst, which can be enlightened byDOthI:e10.1p0a3r9t/Cia9lNJ01677E
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blockage of pores with the excess amount of phosphoric acid.35
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Entry
Catalyst
BET surface
area (m2/g)
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Total pore
volume (cc/g)
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Total acidity B/L ratio
(mmol/g)
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(I1538/I1440
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TiO2
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15P-TiO2
20P-TiO2
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0.285
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15P-TiO2-500
15P-TiO2-600
The reason for the enhancement of both specific surface area and pore volume of the P-TiO2
catalyst is explained by the stabilization effect of phosphate anion against the sintering by
degeneration of Ti-O-Ti and the formation of Ti-O-P bond.35 For comparison, the 15P-TiO2-
imp catalyst was also prepared via two-step processes and the surface area and pore volume
of the catalyst was measured. The obtained results demonstrated that the one-pot synthesis of
the 15P-TiO2 catalyst is more advantageous in terms of the short time of preparation and
better textural properties compared to the 15P-TiO2-imp catalyst.
Furthermore, the calcination temperature of the catalyst is also could also influence
the textural properties of the catalyst. In this study, the 15P-TiO2 catalyst was subjected to
calcined at different temperatures in the range of 300 to 600oC and their corresponding BET
surface area, and total pore volume values are presented in Table 1. It can be observed that
the higher calcination temperature can significantly reduce both the BET surface area and the
pore volume of the catalyst. For instance, the BET surface area of the 15P-TiO2 catalyst
calcined at 300oC is determined to be 247 m2/g, whereas the surface area of the same catalyst
calcined at 600 C is reduced to 80 m2/g. The shrinkage of pore-walls or increase in
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crystallinity of the catalyst might be the reason for lower BET surface area values, and these
textural properties play a crucial role in the catalytic performance of the solid catalyst.
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3.1.2 XRD
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Powder X-ray diffraction analysis is used to analyze the crystal structure of the solid
materials. Fig. 1 shows the XRD pattern of pure TiO2 and P-TiO2 catalyst samples. The XRD
pattern of the pure TiO2 catalyst can display the broad crystalline peaks at 2θ values of 26o,
38o, 48o, and 55o, and these peaks are well matched with the anatase structure of TiO2.38.
Similarly, the P-TiO2 catalysts with different phosphoric acid contents exhibit the similar
XRD pattern as that of the unmodified TiO2 catalyst, demonstrating that the surface
modification of TiO2 with phosphoric acid did not affect the anatase structure of TiO2. The
anatase phase of P-TiO2 catalyst did not change even after thermal treatment of 15P-TiO2
catalyst at various temperatures in the range of 300 to 600oC, representing the thermal
stability of the catalyst. Furthermore, the 15P-TiO2-imp catalyst can exhibit similar XRD
pattern as that of 15P-TiO2 catalyst; however, the intensity of the crystalline peaks was
significantly low, which indicates that partly amorphous nature of the catalyst. Thus, the
XRD results demonstrate the advantage of the one-pot procedure for the synthesis of P-TiO2
catalysts in terms of shorter time to synthesize without compromising its structural properties.
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TiO2
10 P-TiO2
15 P-TiO2
20 P-TiO2
15 P-TiO2-400
15 P-TiO2-500
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15 P-TiO2-600
15 P-TiO2-imp
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2-Theta (degree)
Fig. 1. XRD pattern of different TiO2 catalysts
3.1.3 FT-IR
The FT-IR spectra of TiO2 and P-TiO2 catalysts are shown in Fig. 2. The FT-IR spectra of all
catalyst samples display two distinguished FT-IR peaks, one at 3400-3500 cm-1 and another
peak at 1620 cm-1. These two peaks could be assigned to the O-H stretching, and bending
vibrations of the surface adsorbed water and hydroxyl groups.39 All P-TiO2 catalysts exhibit a
one strong FT-IR peak at 1080 cm-1 and this band could be assigned to the formation of Ti-O-
P bond, and no such type of FT-IR peak was detected for pure TiO2 catalyst, demonstrating
that the phosphorous was incorporated into the TiO2 matrix .39 On the other hand, the absence
FT-IR peak in the region of 1300-1400 cm-1 reveals that the phosphorus did not exist as PO4
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or polyphosphoric acid attached to the surface of TiO2.40. Furthermore, no extra distinguished
FT-IR peaks were observed when the 15P-TiO2 catalyst (Fig. S1) was calcined at various
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temperatures, representing the thermal stability of the catalyst. Thus, the FT-IR results fuVritehweArrticle Online
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support the XRD results where the anatase structure of TiO2 is thermally stable.
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15P-TiO2
20P-TiO2
15P-TiO2-imp
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Wavenumber (cm-1)
Fig. 2. FT-IR spectra of different TiO2 catalysts
3.1.4 NH3-TPD
The acidity of the catalyst could play a significant role, especially in the catalytic conversion
of sugars into 5-HMF.18, 35 In this study, the acidity of the catalysts was measured by NH3-
TPD, and the TPD profile of TiO2, 15P-TiO2, and 15P-TiO2-imp catalysts are shown in Fig.
3a. The total acidity values are presented in Table 1. The NH3-TPD profile of pure TiO2
catalyst mainly displays a high-intensity peak at the high-temperature region, demonstrating
that the presence of strong acid sites on the catalyst surface. The total acidity of pure TiO2
catalyst is estimated to be 0.87 mmol/g. However, the acidity of TiO2 catalyst is substantially
improved by the treatment of TiO2 surface with phosphoric acid through either one-pot sol-
gel or two-step impregnation method. A broad distribution of acid sites was exposed on the
surface of the catalyst as evidenced by strong and broad NH3-desorption peak was appeared
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between 150 - 800oC representing that the presence of weak, medium and strong acid sites.
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However, the total acidity of the 15P-TiO2 catalyst was significantly higher than that of the
15P-TiO2-imp catalyst (Table 1). The difference in total acidity between these two catalysts
can be explicated based on BET results, where the high surface area of 15P-TiO2 catalyst
(Table 1) had a more number of accessible acid sites to probe molecule compared to the 15P-
TiO2-imp catalyst.
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Furthermore, the effect of phosphate loading on the acidity of TiO2 catalysts was also
examined. The NH3-TPD profile is shown in Fig. S2. Three different loadings of phosphoric
acid (10, 15 and 20 mol%) on TiO2 catalyst was examined. However, the 15P-TiO2 catalyst
can display the relatively broad distribution of acid sites, and the total acidity of the catalyst
(Table 1) was significantly higher than that of other two catalysts (10P-TiO2 and 20P-TiO2).
On the other hand, the acidity of the catalyst could also be influenced by the calcination
temperature of the catalyst and which in turn will influence the catalytic dehydration of
glucose to 5-HMF. In this study, the highly active 15P-TiO2 catalyst was calcined at a
different temperature in the range of 300 - 600oC, and their corresponding NH3-TPD pattern
is depicted in Fig. S2. As the calcination temperature is increased from 300 oC to 600 oC, the
total acidity of the catalyst was significantly reduced from 1.71 mmol/g to 0.36 mmol/g,
respectively. As shown in Fig. S2, the intensity of NH3-desorption peak is decreased with the
increase of calcination temperature of the catalyst; probably the Bronsted acid sites could
play a significant role as they were decreased with thermal treatment of the catalyst.
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TiO2
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15 P-TiO2
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Temperature (oC)
Fig. 3. NH3-TPD pattern of TiO2, 15P-TiO2, and 15P-TiO2-imp catalysts
3.1.5 py-FTIR
Pyridine FT-IR spectroscopy was used to characterize the nature of acid sites on the P-TiO2
catalyst surface. The pyridine FT-IR spectra of pure TiO2 and 15P-TiO2 catalyst are shown in
Fig. 4. Pure TiO2 catalyst exhibit two types of FT-IR bands in the region of 1400 - 1700 cm-1.
The first peak at 1440 cm-1 could characterize the presence of Lewis acid sites and the second
peak at 1480 cm-1 is ascribed to the pyridine molecule coordinates with both Bronsted and
Lewis acid sites.41 However, after phosphoric acid treatment, for example, the 15P-TiO2
catalyst, besides those two peaks (1440 and 1480 cm-1), a new peak would appear at 1538 cm-
1
This peak could characterize the pyridine molecule interacted with the Bronsted acid
.
sites.17,35,41 Moreover, the pyridine FT-IR peaks for the phosphoric acid treated TiO2 is more
intense compared to the pure TiO2, indicating the strong acidity of 15P-TiO2 catalyst and
these results are in line with NH3-TPD results. Furthermore, these results also could
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demonstrate that the 15P-TiO2 catalyst contains two types of acid functionalities in contrast
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to pure TiO2, which will help to improve the catalytic activity and selectivity. The intensity of
Bronsted (B) and Lewis (L) acid sites obtained from the absorbance at 1538 and 1440 cm-1,
respectively, are used to calculate the B/L ratios and are presented in Table 1. The B/L ratio
increased with the increase of phosphoric acid content and a maximum obtained for the 15P-
TiO2 catalyst prepared via one-step sol-gel method. Additionally, the calcination temperature
of the 15P-TiO2 catalyst also could influence the Bronsted and Lewis acidic sites. The B/L
ratio decreases with the increasing the calcination temperature from 300 to 600 oC, and these
results are in line with NH3-TPD measurements.
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TiO2
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Wavenumber (cm-1)
Fig. 4. Pyridine FT-IR spectra of TiO2 and 15P-TiO2 catalysts
3.1.6 XPS
XPS is used to determine the oxidation states of individual elements and surface atomic
composition of the catalyst sample. Fig. 5 shows the XPS spectra of the 15P-T iO2 catalyst.
The survey scan spectra of 15P-TiO2 catalyst show the presence of P, Ti, and O elements.
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The deconvoluted XPS spectra of Ti 2p exhibits three binding energy peaks at 457.5, 459.9
and
O 1s
(a)
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(b)
Ti 2p
Ti 2p
P 2p
P 2p
(d)
O 1s
(c)
Fig. 5. XPS spectra of 15P-TiO2 catalyst (a) survey scan spectra; (b) Ti 2p, and (c) P 2p, and
(d) O 1s
464.2 eV, respectively. The first two peaks are ascribed to Ti 2p3/2 indicates that Ti exists in
+4 oxidation state and also demonstrates that the Ti ions are in two different environments.42
The first peak at binding energy value of 457.5 eV can be assigned to Ti ions in an octahedral
coordination with oxygen, and the second peak (459.9 eV) could be assigned to Ti ions are in
a tetrahedral environment.43 The third peak at higher binding energy (464.2 eV) value is
ascribed to Ti 2p1/2, which is originated due to spin-orbit splitting of Ti4+. The XPS spectra of
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P 2p exhibits a single broad peak at 133.1 eV, suggesting that phosphorus exists in the
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pentavalent-oxidation state (P5+).44 The absence of binding energy peak nearby 128.6 eV
indicates that phosphorus is not directly linked in the form of Ti-P bond.45 On the other hand,
the deconvoluted spectra of O 1s show two different binding energy peaks, indicating that the
presence of two different types of oxygen atoms. The first peak at 529.3 eV could
characterize the Ti-O in TiO2, and another peak at 532.6 eV corresponds to the hydroxyl
groups in a 15P-TiO2 catalyst.43 The presence of the third type of oxygen bonded with
phosphorus (P-O) cannot be seen as this peak is probably overlapped by the other two types
of oxygen binding energy peaks in the 15P-TiO2 catalyst.
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3.2 Catalytic Activity
Initially, the TiO2 based solid acid catalysts were evaluated for the selective
dehydration of glucose to HMF under the following reaction conditions; glucose/catalyst 4:1
(w/w), NaCl (0.525 g), H2O (3 mL), THF (12 mL), 175 oC, 2 h. The catalytic activity results
are shown in Fig. 6. The reaction products are mainly HMF, fructose, and organic phase
soluble humins. The bare/plain TiO2 catalyst was active for the conversion of glucose to
HMF and 27% HMF yield at 86% glucose conversion was obtained. Besides HMF, fructose
was also detected. On the other hand, the phosphated TiO2 catalysts prepared through one-pot
sol-gel method (15P-TiO2), or two-step impregnation method (15P-TiO2-imp) exhibited
much high catalytic activity and selectivity compared to unmodified TiO2 catalyst. These two
phosphated titania catalysts produced 53% and 43% HMF yields respectively, at glucose
conversion of 98% and 93%. No fructose is detected under these reaction conditions indicates
that fructose is immediately converted into HMF. The difference in the catalytic activity and
selectivity between 15P-TiO2 and 15P-TiO2-imp catalysts is explained based on BET surface
area, py- FT-IR, and NH3-TPD results. The presence of high BET surface area, strong
Bronsted and Lewis acid sites, and high concentration of total acid sites on the catalyst
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surface, which account for its high catalytic activity and selectivity. Generally, leDvOuIl:i1n0.i1c03Vai9e/cwCiA9drNticJle01O6n7li7nEe
and formic acid are the by-products in the dehydration reaction of glucose to HMF.35 In the
present study, these products are not detected by HPLC analysis. It indicates that the acid
sites on the catalyst surface are not strong enough to rehydrate the HMF under the present
experimental condition. By conducting a control experiment with pure HMF in a biphasic
reaction medium under the same dehydration reaction conditions, no levulinic and formic
acid was detected, signifying that rehydration of HMF is negligible with the 15P-TiO2
catalyst. This result supported by a recent study of Jimenez-Morales et al. where HMF was
not rehydrated over mesoporous tantalum phosphate catalyst.19
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The phosphate content on TiO2 could also influence the selective dehydration of
glucose to HMF in a biphasic H2O/THF reaction medium. In this study, three different
loadings (10, 15, and 20 mol%) of phosphoric acid on TiO2 catalysts were synthesized and
evaluated their catalytic properties for dehydration of glucose. As shown in Fig. 7, the HMF
yield increased with the increase of phosphoric acid content up to 15 mol% and a further
increase of phosphoric acid loading to 20 mol% led to low HMF yield, despite the complete
conversion of glucose. Thus, the optimal loading of phosphoric acid for this reaction is 15
mol %.
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HMF yield (%)
Fructose yield (%)
TiO2 15P-TiO125P-TiO2-imp
Catalyst
Fig. 6. Dehydration of glucose over TiO2 based solid acid catalysts. Other reaction
conditions: glucose/catalyst 4:1 w/w, NaCl 0.525 g, water 3 mL, THF 12 mL, 175oC, 2h.
100
80
Glucose conversion (%)
60
HMF yield (%)
Fructose yield (%)
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Phosphate loading (mol%)
Fig. 7. Effects of phosphate loading on TiO2 for dehydration of glucose. Other reaction
conditions: glucose/catalyst 4:1 w/w, NaCl 0.525 g, water 3 mL, THF 12 mL, 175oC, 2h.
Furthermore, the calcination temperature of the catalyst could also influence the HMF
selectivity.18,35 In this study, the highly active 15P-TiO2 catalyst was subjected to calcined at
various temperatures in the range of 300 to 600 oC, and their corresponding catalytic activity
results for dehydration of glucose at 175oC for 2h are summarized in Fig S3. It can be
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observed that the HMF yield decreased with the increase of calcination temperature of the
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catalyst. As high as 98% glucose conversion and 53% HMF yield was obtained using 15P-
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TiO2 catalyst calcined at 300 C. As such, 15P-TiO2 (300oC calcination) was chosen as the
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best catalyst to optimize the effects of other reaction parameters such as reaction temperature
and time.
The reaction temperature had played a crucial role in the selective conversion of
glucose to HMF.18,19 As shown in Fig. S4, the glucose conversion, and HMF yield increased
with the increase of reaction temperature from 155 to 175oC. Initially, at 155oC, the glucose
o
conversion and HMF yields were 86.5% and 34%, respectively, whereas at 175 C these
values are increased to > 98% and 53%, respectively. However, by further increasing the
o
reaction temperature to 185 C, the HMF yield was declined to 45%, probably due to the
degradation of HMF at elevated temperatures cannot be ruled out.18,35 In addition to HMF,
0
fructose is also obtained at a lower reaction temperature below 175 C, signifying that the
o
reaction proceeds through intermediate fructose. However, it was not detected at 175 C
0
perhaps completely converted into HMF. Therefore, 175 C reaction temperature was
optimum for the selective dehydration of glucose to HMF over the 15P-TiO2 catalyst. In
further studies, the influence of reaction time on glucose dehydration was studied in the
presence of a 15P-TiO2 catalyst at 175oC. The catalytic activity results are illustrated in Fig.
8. The reaction profile shows that both glucose conversion and HMF yield increased with the
progress of the reaction time and maximum HMF yield (53%) was obtained after 2 h. Despite
the complete conversion of glucose, the HMF yield decreased to 46% with further extending
the reaction time to 2.5 h. From these results, the best reaction time for HMF production with
the 15P-TiO2 catalyst at 175 oC was approx. 2 h.
For the industrial scale synthesis of HMF, it is inevitable to study the effects of initial
glucose concentration on selective production of HMF over 15P-TiO2 catalyst under the best
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reaction conditions by far (glucose/catalyst 4:1 (w/w), NaCl (0.525 g), H2O (3 mL), THF (12
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o
mL), 175 C, 2 h). Three different levels of initial glucose concentrations (10 wt%, 15 wt%,
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and 20 wt%) in water were examined, and the catalytic activity results are presented in Fig. 9.
It can be observed that, as the initial glucose concentration increased from 10 to 20 wt%, the
HMF yield decreased from 53.5 to 33%, although the almost complete conversion of glucose
was achieved under all the reaction conditions. The reason for lower HMF selectivity at a
higher initial glucose concentration could be attributed to the condensation polymerization
reactions between the initially formed HMF with glucose or other reactive intermediates to
form humin substances that cannot be avoided during the reaction.46 Hence, these results
evinced that the dilute solution of glucose is more promising in terms of high HMF yield and
also can avoid the catalyst deactivation to a large extent by the accumulation of humin
substances.
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(%)
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Reaction time (h)
Fig. 8. Influence of reaction time on dehydration of glucose over the 15P-TiO2 catalyst.
Other reaction conditions: glucose/catalyst 4:1 w/w, NaCl 0.525 g, water 3 mL, THF 12 mL,
175oC.
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Initial glucose concentration (wt%)
Fig. 9. Effects of initial glucose concentration on dehydration of glucose over 15P-TiO2
catalyst. Other reaction conditions: glucose/catalyst 4:1 w/w, NaCl 0.525 g, water 3
mL, THF 12 mL, 175oC, 2h.
The most important advantage of heterogeneous catalysis is the catalyst reusability. In
this study, the recyclability of the 15P-TiO2 catalyst was studied according to a known
procedure in the literature.35,47 After the reaction, the 15P-TiO2 catalyst was simply separated
from the reaction mixture by centrifugation and washed several times with water and
o
methanol followed by oven dried at 100 C for overnight. The retrieved catalyst was then
directly used for the next cycle under the same reaction conditions. As shown in Fig.10, the
HMF yield is decreased with the increase of the usage cycle, and the difference in HMF yield
for the initial and fourth cycle was approximately 13%. The decrease in catalytic selectivity
after consecutive cycles is due to the partial deactivation of the catalyst surface caused by the
deposition of organic matter or leaching of the catalyst. The ICP analysis of the reaction
solution found that little amount of phosphorous content (< 1 ppm), indicating that slight loss
of a catalyst during the reaction. After the 4th cycle, the spent catalyst was characterized by
FT-IR (Fig. S5) and XPS analysis (Fig. S6). No additional peaks were found in the FT-IR
spectra of the spent catalyst against the native catalyst. The XPS results of spent catalyst
reveal that the presence of P, Ti, and O elements on the catalyst surface, and these results
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successive recycles. The surface atomic composition of fresh and spent 15P-TiO2 catalyst is
presented in Table S1, quantified based on the XPS peak areas of individual elements on the
surface of the catalyst. These results demonstrate that a slight decrease in the content of
phosphorous and titania on the surface of the catalyst; however, the stability of the structure
is still maintained even after consecutive usage cycles. The spent catalyst was regenerated by
the thermal treatment in the air for 4 h and reused for the 5th catalytic cycle. The regenerated
catalyst displays the slightly improved catalytic selectivity compared to the third and fourth
catalytic cycles. In order to find the difference in catalytic selectivity between the spent and
regenerated catalyst, these catalysts were characterized by NH3-TPD. The NH3-TPD pattern
of fresh, spent, and regenerated 15P-TiO2 catalyst is depicted in Fig. S7. The NH3-TPD
pattern clearly shows that the difference in intensity of the NH3 desorption peak among these
catalysts. The total acidity of the regenerated catalyst (15P-TiO2-R, 1.33 mmol/g) is
significantly higher than the spent catalyst (15P-TiO2-S, 0.96 mmol/g) and lower than that of
a native catalyst (15P-TiO2-F, 1.71 mmol/g). The spent catalyst after the fourth cycle had a
less number of acid sites and exhibited a lower catalytic selectivity compared regenerated
catalyst. After heat treatment, the spent catalyst is almost regained its original acidity and
improved its catalytic selectivity. These results would signify that the acidity of the catalyst
would play a prominent role in the selective dehydration of glucose to HMF.
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Initial
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Number of cycles
Fig. 10. Recyclability of 15P-TiO2 catalyst for dehydration of glucose to 5-HMF. Other
reaction conditions: glucose/catalyst 4:1 w/w, NaCl 0.525 g, water 3 mL, THF 12 mL, 175oC,
2h.
3.2.1 Mechanism of selective dehydration of glucose to HMF catalyzed by 15P-TiO2
catalyst
Based on the above catalytic activity and catalyst characterization results, and previous
literature on various bifunctional solid acid catalysts for the selective dehydration of glucose
to HMF,35, 48 a plausible reaction mechanism for the one-pot dehydration of glucose to HMF
in a biphasic reaction medium over 15P-TiO2 catalyst was proposed (Scheme 1). Since the
15P-TiO2 catalyst contains two types of acid functionalities, Lewis and Bronsted acid sites as
confirmed by pyridine FT-IR results and the fructose is a by-product in the dehydration of
glucose at the lower reaction temperature, signifies that the reaction proceeds through two
steps. First, isomerization of glucose to fructose by Lewis acid sites followed by dehydration
of generated fructose to HMF by Bronsted acid sites. Thus, the synergism in the two-stage
process results in higher selectivity of glucose dehydration to HMF. Besides, the rehydration
of HMF is inhibited in this reaction system and the carbon mass balance below 100% over a
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15P-TiO2 catalyst, which implies that HMF can starts secondary reactions via condensation
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with glucose to form humins that result in a high rate of glucose conversion.
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OH
H
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H
HO
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53
54
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OH
OH
O
H
OH
O
OH
Glucose
Fructose
5-HMF
OH
Ti4+
OH
P
O
O
O
Ti4+
Scheme 1. Plausible pathway for the selective dehydration of glucose to HMF over 15P-TiO2
catalyst
3.2.2 Comparison of catalytic activity of 15P-TiO2 catalyst with other solid acid catalysts
Heteropoly acid (HPA) catalysts are well known for acid catalyzed and oxidation reactions,
and they have also been used for the dehydration of fructose, glucose, and cellulose.49 These
catalysts are homogeneous in polar solvents, and efforts are being made to transform them
into heterogeneous catalysts by supporting them on various acidic or neutral supports.50 In
this study, different supported heteropoly acids like 0.5TPA/TiO2, and 0.5STA/TiO2 catalysts
were prepared and compared their catalytic performances with a 15P-TiO2 catalyst. As shown
in Table 2, almost complete conversion of glucose was achieved over TiO2 supported
heteropoly acid catalysts; however, the HMF yields were considerably low compared to the
15P-TiO2 catalyst. The difference in selectivity among these catalysts is originated due to the
difference in the structural properties of the catalysts and will be discussed below. On the
other hand, 97.8% glucose conversion and 45% HMF yield were obtained over phosphated
zirconia (15P-ZrO2) catalyst, which is much slightly higher than the supported heteropoly
acid catalysts and lower than the 15P-TiO2 catalyst. In order to find the difference in catalytic
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selectivity, these catalysts were characterized by N2-physisorption and DONI: 1H0.130-3VTi9eP/wCDA9rNticJle01O6n7li7nEe
measurements. It can be observed from Table 2 as expected, the acidic and textural
properties of the catalyst would play a crucial role in the selective synthesis of HMF from
glucose. In contrast, the 15P-ZrO2 catalyst displays low surface area, and pore volume but
exhibits a high catalytic selectivity compared to TiO2 supported heteropoly acid catalysts.
The NH3-TPD profile (Fig. 11) would reveal the difference in the distribution of acid sites
among these catalysts and shows that 15P-ZrO2 catalyst had comparatively more number of
strong acid sites as evidenced by strong NH3 desorption peak at the high-temperature region.
However, further studies will be needed to deeper understand the structure-activity
relationship of these catalysts is currently under examination.
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11
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Table 2: Comparison of catalytic activity different solid heterogeneous catalysts for
dehydration of glucose in a biphasic reaction medium
Catalyst
BET surface
Total pore
volume
(cc/g)
0.14
Total acidity
(mmol/g)
Glucose
conversion
(%)
HMF yield
(%)
area (m2/g)
0.5TPA/TiO2
0.5STA/TiO2
15P-ZrO2
228
188
82
1.51
0.85
1.24
1.71
100
40.17±3.2
36.12±2.7
45.05±2.6
53.5±2.0
0.10
0.04
0.33
98.9±0.57
97.8±1.5
98.5±0.63
15P-TiO2
247
Reaction conditions: glucose/catalyst 4:1 w/w, NaCl 0.525 g, water 3 mL, THF 12 mL,
175oC, 2h
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9
0.5 TPA/TiO2
0.5 STA/TiO2
15 P-TiO2
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60
15 P-ZrO2
100
200
300
400
500
600
700
800
Temperature (oC)
Fig. 11. NH3-TPD pattern of different solid acid catalysts
3.2.3 Comparison of catalytic activity of 15P-TiO2 catalyst with few other solid
heterogeneous catalysts reported in the literature
The catalytic performance of 15P-TiO2 catalyst for selective dehydration of glucose (10 wt%)
to 5-HMF in a biphasic reaction medium is compared with other heterogeneous bifunctional
solid acid catalysts reported in the literature, as shown in Table 3. The mesoporous tantalum
oxide and tantalum phosphate19 achieved HMF yields about 23% and 32.8% at glucose
51
conversion of 69% and 56.3% respectively, which is quite low as compared with the present
15P-TiO2 catalyst. Moreover, these catalysts would show high activity at low glucose/catalyst
weight ratio, whereas the present catalyst is highly active and selective at high
glucose/catalyst weight ratio. Even the present 15P-TiO2 catalyst was more active and
obtained high HMF yield than our previously reported tin phosphate catalyst at the same
reaction temperature .18 In another report, tin phosphate (SnPO) catalyst was achieved high
HMF yield even at low reaction temperature; however, the ionic liquids were used as the
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reaction media which are expensive and practically not beneficial for the large scale
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operations. The mesoporous silica-alumina (AlSiO) composite catalyst 52 has been reported to
be highly active with HMF yield of 63%; though the high catalyst dosage confines the
practical application. The combination of Sn-Beta zeolite and HCl catalyst system obtained a
similarly high HMF yield (56.88%) as that of the 15P-TiO2 catalyst. However, the difficulty
in recycling HCl would remain challenging for the environment,53 whereas the present 15P-
TiO2 catalyst can be easily recovered and reused for many catalytic cycles. Although all these
catalysts would show good catalytic performance, they have their demerits. On the other
hand, the present 15P-TiO2 catalyst can be easily prepared in a short time and requires low-
temperature activation and exhibits high catalytic activity and selectivity.
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55
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Table 3: Comparison of catalytic performance of 15P-TiO2 catalyst with other heterogeneous
catalysts reported in the literature for the conversion of glucose to 5-HMF
Catalyst
Glucose/catalyst
weight ratio
Reaction
temperature
(oC)
Solvent
system
Glucose
conversion yield
HMF Reference
(%)
69
56.3
100
94.1
91.7
79
(%)
23
32.8
42
58.3
63.1
56.88
meso-Ta2O5
meso-TaPO
SnP-1
SnPO
AlSiO
Sn-Beta/HCl
15P-TiO2
3
3
2.5
2
1
-
175
170
175
120
160
180
175
H2O/MIBK
H2O/MIBK
H2O/THF
EMIMBr
H2O/THF
H2O/THF
H2O/THF
51
19
18
20
52
53
4
98.5
53.5 This work
3.2.4 Catalytic conversion of disaccharides, polysaccharides, and industrial grade sugar
syrups to HMF
With the successful conversion of glucose to HMF in a biphasic reaction medium over a
heterogeneous 15P-TiO2 catalyst at 175oC for 2h, the scope of the catalyst was further
extended to the dehydration of di and polysaccharides, and industrial grade sugar syrups to
HMF under the best reaction conditions. The disaccharides such as sucrose and cellobiose
were smoothly dehydrated to form HMF, resulted in 43% and 40% HMF yields (Table 4,
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entries 2 & 3), respectively. The 15P-TiO2 catalyst was also active for the dehydration of
5
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7
8
polysaccharides, including starch and cellulose in a one-pot process. The starch can easily
undergo dehydration to form HMF (Table 4, entry 4) compared to cellulose (Table 4, entry 5)
under identical reaction conditions. However, after extending the reaction time to 4h, the
conversion of cellulose was significantly increased from 43% to 80% and thereby improved
the HMF yield (Table 4, entry 5). The main reason for the difference in reactivity of these
two substrates over the 15P-TiO2 catalyst is attributed to the difference in structural
arrangements. The glucose units in the starch molecule are connected through α-1,4 or α-1,6
linkages, whereas in cellulose linked through unbranched β-1,4 manner.54 Furthermore, the
15P-TiO2 catalyst was also used for the conversion of industrial grade sugar syrups such as
high fructose corn syrup (HFCS) and glucose corn syrup (GCS). The HFCS mainly contains
fructose and glucose along with a small fraction of higher saccharides, and their
concentrations are reported elsewhere.55 The HFCS-90 containing 92% of fructose by dry
basis was smoothly converted into HMF, obtaining 50% yield at 175oC for 30 min. Also, the
15P-TiO2 catalyst was active for the dehydration of other grades of HFCS such as HFCS-55
and HFCS-42, achieving 48% and 45% HMF yields, respectively. On the other hand, the
GCS contains a high fraction of glucose (91.4 wt%) can easily undergo dehydration to form
HMF and resulted in 43.4% HMF yield under the best reaction conditions. From these above
results, one can conclude that the 15P-TiO2 is a versatile catalyst to convert different types of
carbohydrates into HMF with reasonable yields.
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Table 4. Catalytic dehydration of mono, di, and polysaccharides into 5-HMF over 15P-TiO2
catalyst
Entry
Substrate
Glucose
Sucrose
Cellobiose
Starch
Conversion (%)
HMF yield (%)
1
2
3
4
5
98.5
95
53.5
43
40
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31
32
33
34
35
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37
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40
41
42
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48
49
50
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52
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56
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58
59
60
94.3
93
39
Microcrystalline
Cellulose
HFCS-90
HFCS-55
HFCS-42
GCS
43, 80a
20, 33b
6
7
8
9
>99
>99
>99
99
50
48
45
43.4
Reaction conditions: substrate/catalyst 4:1 w/w, NaCl 0.525 g, water 3 mL, THF 12 mL,
175oC, 2h, b 4h
4. Conclusions
In conclusion, the phosphated titania (P-TiO2) catalyst prepared through a one-pot sol-gel
method was demonstrated as an efficient heterogeneous catalyst for the selective dehydration
of simple sugars, polysaccharides, and industrial grade sugar syrups to HMF by using a
biphasic reaction medium (water/THF). The catalyst characterization techniques revealed that
the phosphoric acid was not simply impregnated on the surface of TiO2, but it is actually in
the form of P-O-Ti bond. The experimental results showed that the catalytic activity of P-
TiO2 catalysts depended on the phosphate loading, calcination temperature, reaction
temperature, reaction time, and substrate concentration. The high catalytic activity of the
15P-TiO2 catalyst was attributed to the presence of strong Bronsted and Lewis acid sites, high
acidity, and exhibit large BET surface area and pore volume. Besides, the 15P-TiO2 catalyst
prepared through the one-pot sol-gel method exhibited superior catalytic activity compared to
the same catalyst prepared through a two-step impregnation method. Moreover, the 15P-TiO2
catalyst is heterogeneous and can be separated from the reaction mixture and can be reused
several times with consistent activity and selectivity. Furthermore, the preparation and
29