Production of 5-HMF from glucose using TiO2-ZrO2 catalysts: Effect of the sol-gel synthesis additive

Article abstract of DOI:10.1016/j.catcom.2019.105723

Different TiO₂-ZrO₂ (50/50 wt%) mixed metal oxide catalysts were synthesized by the sol-gel method after varying the synthesis additive (e.g., CH₃COOH, HNO₃, NH₄OH and NaOH) to study its effect on the production yield of 5-HMF from the conversion of glucose at 175 °C and 30 bar (Ar) (batch reaction system). Various characterization techniques such as BET, XRD, FTIR, ¹HNMR, NH₃-TPD, TPO, SEM and TEM were employed to study the physicochemical properties of the synthesized materials and to correlate them with the reaction product yields. Highest HMF yield (76%) was obtained over the TiO₂-ZrO₂-C₂H₄O₂ catalyst in a biphasic reaction system with a glucose concentration of 2 wt% and a glucose/catalyst mass ratio of 2.5. From the NH₃-TPD results, highest amount of total acid sites was observed for the TiO₂-ZrO₂ (50/50 wt%) prepared by the sol-gel method using the acetic acid additive, and acidity value was 3 and 2.5 times higher than that of TiO₂ and ZrO₂ alone, respectively. It was concluded that total acid sites density is an important parameter to obtain high 5-HMF product yields. A higher number density of sites can be obtained using the complexing additive of acetic acid in the Sol-Gel method among the other additives investigated.

Full text of DOI:10.1016/j.catcom.2019.105723

Accepted Manuscript
Production of 5-HMF from glucose using TiO2-ZrO2 catalysts:
Effect of the sol-gel synthesis additive
Adib A. Silahua-Pavón, Claudia G. Espinosa-González, Filiberto
Ortiz-Chi, J. Gpe Pacheco-Sosa, H. Pérez-Vidal, J. Carlos
Arévalo-Pérez, Srinivas Godavarthi, J.G. Torres-Torres
PII:
DOI:
Article Number:
Reference:
S1566-7367(19)30185-2
https://doi.org/10.1016/j.catcom.2019.105723
105723
CATCOM 105723
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
14 January 2019
3 June 2019
5 June 2019
Please cite this article as: A.A. Silahua-Pavón, C.G. Espinosa-González, F. Ortiz-Chi,
et al., Production of 5-HMF from glucose using TiO2-ZrO2 catalysts: Effect of the
sol-gel synthesis additive, Catalysis Communications, https://doi.org/10.1016/
j.catcom.2019.105723
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ACCEPTED MANUSCRIPT
Production of 5-HMF from Glucose using TiO2-ZrO2 catalysts: Effect of
the sol-gel synthesis additive
Adib. A. Silahua-Pavón1, Claudia G. Espinosa-González2, Filiberto Ortiz-Chi2, J. Gpe.
Pacheco-Sosa1, H. Pérez-Vidal1, J. Carlos Arévalo-Pérez1, Srinivas Godavarthi2 and J. G.
Torres-Torres1,*
1Universidad Juárez Autónoma de Tabasco, Centro de Investigación de Ciencia y Tecnología
Aplicada de Tabasco (CICTAT), DACB, Laboratorio de Nanomateriales Catalíticos
Aplicados al Desarrollo de Fuentes de Energía y Remediación Ambiental, Km.1 carretera
Cunduacán-Jalpa de Méndez, C.P. 86690, Cunduacán (Tabasco), México.
2Cátedras-CONACyT- Universidad Juárez Autónoma de Tabasco, Centro de Investigación
de Ciencia y Tecnología Aplicada de Tabasco (CICTAT), DACB, Laboratorio de
Nanomateriales Catalíticos Aplicados al Desarrollo de Fuentes de Energía y Remediación
Ambiental, Km.1 carretera Cunduacán-Jalpa de Méndez, C.P. 86690, Cunduacán
(Tabasco), México.
*To whom the correspondence should be addressed: gilberto.torres@ujat.mx
Tel: +52 19143360300; Fax: +52 19143360928

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Abstract
Different TiO2-ZrO2 (50/50 wt%) mixed metal oxide catalysts were synthesized by the sol-
gel method after varying the synthesis additive (e.g., CH3COOH, HNO3, NH4OH and
NaOH) to study its effect on the production yield of 5-HMF from the conversion of glucose
at 175 oC and 30 bar (Ar) (batch reaction system). Various characterization techniques such
as BET, XRD, FTIR, 1HNMR, NH3-TPD, TPO, SEM and TEM were employed to study the
physicochemical properties of the synthesized materials and to correlate them with the
reaction product yields. Highest HMF yield (76%) was obtained over the TiO2-ZrO2-
C2H4O2 catalyst in a biphasic reaction system with a glucose concentration of 2 wt% and a
glucose/catalyst mass ratio of 2.5. From the NH3-TPD results, highest amount of total acid
sites was observed for the TiO2-ZrO2 (50/50 wt%) prepared by the sol-gel method using the
acetic acid additive, and acidity value was 3 and 2.5 times higher than that of TiO2 and ZrO2
alone, respectively. It was concluded that total acid sites density is an important parameter
to obtain high 5-HMF product yields. A higher number density of sites can be obtained
using the complexing additive of acetic acid in the Sol-Gel method among the other
additives investigated.
Keywords: 5-HMF; Glucose conversion; TiO2-ZrO2; Additive; Sol-gel.

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1. Introduction
The imminent need of oil production and global energy demand stimulate significant
efforts to identify carbon renewable sources for the production of fuels and traditional
chemical products from sustainable origin, mainly related to biomass [1]. Glucose, the most
abundant monosaccharide and the cheapest hexose, appears as a promising raw material to
produce 5-hydroxymethylfurfural (5-HMF), which is a prospective platform molecule for the
synthesis of polymers and biofuels [2]. The reaction of the production of 5-HMF from
glucose can be promoted by a variety of bifunctional catalysts with acid-base characteristics
such as TiO2 [3], ZrO2 [4] and mixed metal oxides [5]. Conversion of glucose to 5-HMF is
believed to follow the mechanism of the isomerization of glucose to fructose (basic sites)
followed by dehydration of fructose to 5-HMF (acid sites) as reported by Watanabe et al.
2-
[6]. In addition, Osatiashtiani et al. [7] showed that on the SO4 /ZrO2 glucose is first
isomerized by Lewis acid and base sites of ZrO2, and the formed fructose was subsequently
2-
dehydrated to 5-HMF with Brønsted acid sites offerred by SO4 groups followed
rehydration to organic acids (formic acid and levulinic acid) in low concentrations.
by
However, the use of heterogeneous Lewis acid catalysts can be effective for the HMF
production from glucose in water or an organic solvent using TiO2 as reported by Noma et
al. [8]. Thus, it is important to know the nature of the acid sites formed on the surface of
these catalysts. Regarding titania (TiO2) and zirconia (ZrO2), it has been reported that they
both have redox and acid-base properties, which makes them a good choice for being
catalysts and catalytic supports. In addition, the catalytic properties of these oxides can be
improved by considering both Ti and Zr metals in a mixed oxide matrix. This improvement
might be the result of the generation of new surface catalytic sites due to the strong
interaction between the individual oxides, which brings forth a mixed oxide with broad

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surface acid-base properties and high thermal stability [9]. For example, Zhang et al. [10]
prepared sulfated titania-zirconia (SO4/TiO2-ZrO2)in a co-precipitation method and obtained
a 26% yield of 5-HMF after 2 h in a biphasic system of (1:1) n-butanol : H2O. However, He
et al. [5] achieved 51.3% yield of 5-HMF after 4 h in a biphasic system of (1:5) H2O : THF
using phosphotungstic acidified titanium-zirconium (HPA/TiO2-ZrO2), which was also
prepared by a co-precipitation method. In another example, using TiO2-ZrO2 with Amberlyst
70 as co-catalyst prepared in sol-gel, Atanda et al. [11] obtained 81% yield to 5-HMF in 3 h
in a biphasic system (4:1) THF : H2O. It is clear from these examples that modifying TiO2-
ZrO2 is not so helpful in achieving higher yields. It is important to note that even though the
use of a co-catalyst helps in improving the product yield, the cost of 5-HMF production
increases drastically. On the other hand, sol-gel method is a suitable procedure to produce
mixed metal oxides with a homogeneous distribution of metal atoms [12]. However, strict
control over the synthesis parameters such as temperature, type of additive, solvent, pH, etc.
is necessary to obtain materials with different catalytic properties.
For the sol-gel method, the effect of synthesis additive has been reported using different
additives, namely: Complexing acetic acid (CH3COOH); condensation catalyst Ammonium
hydroxide (NH4OH), and Hydrolysis catalyst nitric acid (HNO3) [13]. However, these studies
focused to observe the modification in the textural and acid-base properties of the supports
alone. To the best of our knowledge, the influence of sol-gel synthesis additive for TiO2-
ZrO2 mixed metal oxides on the conversion of glucose to 5-HMF product is not available in
the literature.
This work was focused on the effect of the sol-gel synthesis additive towards TiO2-ZrO2
on their catalytic properties in the 5-HMF production from glucose. For this purpose, TiO2-

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ZrO2 was prepared by the sol-gel method and modified using various additives, namely:
CH3COOH, HNO3, NaOH and NH4OH.
2. Experimental
2.1. Synthesis of TiO2-ZrO2 materials
The synthesis of zirconium oxide (ZrO2), titanium oxide (TiO2) and TiO2-ZrO2 (50/50
wt%) was carried out using the sol-gel method (hydrolysis of zirconium butoxide, Sigma-
Aldrich (Zr (OC4H9)4) 80 wt% in 1-butanol) and titanium butoxide (Sigma-Aldrich (Ti
(OC4H9)4) 80% wt in 1-butanol) with ultra-pure water and 1-butanol (Sigma-Aldrich,
CH3(CH2)3OH) 98%) as solvent medium). The alkoxide/water and water/alcohol molar ratios
were 1:16 and 1: 8, respectively. In a typical synthesis, the butoxide/butanol mixture was
added to a glass flask and the water was slowly added dropwise for approximately 3 h. The
pH of the water was adjusted to 3 for acetic acid [CH3COOH] and nitric acid [HNO3]
additives, and to 9 for ammonium hydroxide [NH4OH] and sodium hydroxide [NaOH]
additives, and then left in constant agitation for 24 h. After the gel was formed, it was
subjected to the extraction of liquids in an oven at 70 °C for 48 h. And finally, the materials
were calcined at 500 ºC for 12 h with a ramp of 2 ºC/min. The materials were labeled
according to the type of additive used in the sol-gel synthesis as follows: Mixed oxides TiO2-
ZrO2 with CH3COOH, NH4OH, HNO3 and NAOH as: TZ5050-C3H4O2, TZ5050-NH4OH,
TZ5050-HNO3 and TZ5050-NaOH, and reference samples of TiO2 and ZrO2 with
CH3COOH as Ti-C3H4O2 and Zr-C3H4O2, respectively.
2.2. Catalysts characterization
2.2.1. Textural analysis

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The determination of specific surface area, pore size and pore volume of the catalysts
was carried out by the N2 physisorption technique using the BET method. Measurements
were performed using Micromeritics Tristar 3020 II apparatus at 77 K. In typical
measurement, 0.1 g of solid was weighed and degassed in N2 gas atmosphere (1 bar) during
2 h at 300 ° C to remove adsorbed water and carbon dioxide gases.
2.2.2. X-ray diffraction (XRD)
XRD analysis was carried using Bruker Advance D8 X-ray diffractometer at the
following conditions: radiation source of Cu Kα at λ=1.790307 Å; 30 kV and 15 mA. Data
were collected in the 20 - 80° 2θ range.
2.2.3. Fourier-transform infrared spectroscopy (FTIR)
FT-IR spectra of solid materials prepared were collected at room temperature and
atmospheric pressure by a Shimadzu IRAffinity-1 spectrometer in the frequency range 4000–
400 cm−1. In situ FTIR-pyridine adsorption was carried out using a Nicolet 170 SX FTIR
equipment. The solid material was pressed into thin wafer and then placed in a glass Pyrex
cell with CaF2 windows coupled to a vacuum line (1 × 10-3 Torr) to be evacuated in situ at
400 °C for 30 min. Subsequently, the temperature was decreased to room temperature and
the pyridine was introduced into the cell until adsorption equilibrium was achieved. The
excess pyridine was evacuated at the same temperature. Afterwards, the adsorbed pyridine
was desorbed from room temperature to 120 °C and spectra were recorded at several
desorption temperatures.
2.2.4. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM)
SEM micrographs of the samples were recorded using a scanning electron microscope
(JEOL, model JSM-6010LA). TEM analysis was performed using a JEM-2100 (JEOL,
Japan) operating at 200 kV accelerating voltage. The powder was sonicated in isopropanol

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to ensure a homogeneous dispersion. A small drop was deposited on the holey carbon films
on a 200-mesh copper grid, which was introduced into the TEM analysis chamber after
complete solvent evaporation.
2.2.5. Temperature Programmed Desorption of NH3 (NH3-TPD)
NH3-TPD experiments were carried out in a Bel Japan Belcat-B device equipped with a
thermal conductivity detector (TCD). Sample was first heated at a heating rate of 10 °C
min−1 up to 500 °C for 0.5 h under helium (P = 1 bar). Then, the sample was cooled down
to 70 °C in a flow of He (50 mL min−1), followed by NH3/He adsorption for 30 min.
Following adsorption the sample was flushed in He flow (50 mL min−1) for 0.5 h at 70 °C
to remove weakly adsorbed NH3. Then, ammonia was desorbed upon heating to 500 °C in
helium flow with a heating rate of 2ºC/min, while the evolved gases were analyzed by TCD.
The quantification of the acid sites was made by deconvolution (gaussian peak shape) of the
area under the ammonia desorption curve from the obtained signal.
2.2.6. Temperature programmed oxidation (TPO)
The TPO study was carried out in a Bel Japan Belcat-B apparatus using a thermal
conductivity detector (TCD) and 0.1 g of catalyst sample. The flow rate of the 5%/O2/95%
He gas mixture was 10 mL min−1 and the heating rate was 10 ◦C min−1. Finally, the TPO
traces were recorded from room temperature to 500 ◦C.
2.3. Catalytic evaluation (Glucose yield to 5-HMF, Conversion and Selectivity)
The glucose conversion reaction to form 5-HMF was carried out in a 50 ml Teflon
stainless-steel autoclave equipped with a magnetic stirring system. The reaction was
performed with 0.6 g of D-Glucose (Sigma-Adrich 99.5%) in a 30 ml biphasic solution
(75:25, THF: H2O), with a glucose/catalyst mass ratio of 2.5. Finally, an amount of 1.2 g of

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NaCl (Sigma-Aldrich 99.5% purity) was added. NaCl was used to separate the reactive
aqueous phase from the extractive organic phase, since NaCl increases the partition
coefficient in the biphasic system. The reaction was conducted under inert gas conditions
with 30 bar of Ar and at the temperature of 175 °C with a constant stirring at 1000 rpm,
using similar reaction conditions as reported [11]. Reaction samples from the organic
solution were taken every 15 min. The conversion of glucose, yield of 5-HMF and selectivity
of reaction to 5-HMF, fructose, formic acid and levulinic acid (FA+LA) was calculated using
the following Eqs. (1)-(4):
푚표푙푒푠 표푓 5-HMF 푝푟표푑푢푐푒푑
( )
퐻푀퐹 푦푖푒푙푑 % = 푚표푙푒푠 표푓 푠푡푎푟푡푖푛푔 푔푙푢푐표푠푒 푥 100%
[1]
푚표푙푒푠 표푓 푔푙푢푐표푠푒 푟푒푎푐푡푒푑
퐺푙푢푐표푠푒 퐶표푛푣푒푟푠푖표푛 (%) = 푚표푙푒푠 표푓 푠푡푎푟푡푖푛푔 푔푙푢푐표푠푒 푥 100 %
[2]
푚표푙푒푠 표푓 5-HMF 푝푟표푑푢푐푒푑
푆5-HMF(%) =
푥 100 %
[3]
[4]
푚표푙푒푠 표푓 푔푙푢푐표푠푒 푟푒푎푐푡푒푑
푚표푙푒푠 표푓 푖푛푡푒푟푚푒푑푖푒푠
푆퐹푟푢푐푡표푠푒, 퐹퐴+퐿퐴)(%) = 푚표푙푒푠 표푓 푠푡푎푟푡푖푛푔 푔푙푢푐표푠푒 푥 100 %
(
2.4. Reaction Products Analysis - High Performance Liquid Chromatography (HPLC)
The glucose and fructose were quantified using Shimadzu Prominence HPLC with a Bio-
Rad Aminex HPX-87H column made of stainless-steel and measuring 300 mm in height, 7.8
mm of internal diameter and with a RID–10 (refractive index) detector. The HPLC was
o
operated under the following conditions: oven temperature, 50 C; mobile phase, 5 mM
H2SO4; flow rate, 0.6 mL min-1; injection volume, 20 L. 5-HMF, formic acid and levulinic

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acid were detected using Shimadzu Prominence HPLC with a Restek Model FORCE C18
column made of stainless-steel (measuring 250 mm in height, 4.6 mm of internal diameter
and 5 μm in particle size) and with a Photodiode Array Detector (PDA) at the wavelength of
284 nm (5-HMF) and 210 (formic and levulinic acids). Mobile phase was methanol/water
with a ratio of 80/20. The operating conditions of the HPLC were: oven temperature of 30
°C, flow rate of 1 mL min-1 and an injection volume of 15 μL. The analysis was carried out
through the external standard method using 5-HMF (Sigma Aldrich, 99%), glucose (Sigma
Aldrich, 99.5%), fructose (Sigma Aldrich, 99%), formic acid (Sigma Aldrich, 95%) and
levulinic acid (Sigma Aldrich, 97%) as standard.
1
2.5. Analysis of the reaction by-products by HNMR Nuclear Magnetic Resonance
Spectroscopy
After the reaction, the reactor was cooled to room temperature, the catalyst was filtered
and the organic phase was separated by decantation. The collected organic phase was
evaporated at room temperature and the crude product was weighed, the products were then
1
measured by H NMR to determine the 5-HMF formation and other possible products
generated in the course of the glucose conversion reaction, such as formic acid (AF) and
levulinic acid (AL), by means of a Bruker NMR Spectrometer (Progress III 600 MHz), using
deuterated methanol (MeOD, 25 °C). The spectra were processed with the MestReNova
program(1H NMR (600 MHz, MeOD)).
3. Results and discussion
3.1. Textural properties

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All samples showed (See Fig. S1, ESI) a type-IV isotherm (IUPAC classification)
indicating that the catalysts are mesoporous [14]. The hysteresis loop (H2-type) is commonly
associated with the presence of ink-bottle shaped pores, which could be due to inter- or intra-
particle porosity. Table 1 shows the effects of addition of the different additives on the
surface area of TiO2, ZrO2 and mixed oxides TZ5050 prepared by the sol-gel method. The
results of specific surface area obtained in the simple oxides Ti-C2H4O2 and Zr-C2H4O2,
used as reference, are 40 and 41 m2g-1, respectively. However, SSA values for the mixed
oxides are 234, 239, 257 and 289 m2g-1 for TZ5050-NH4OH, TZ5050-HNO3, TZ5050-NaOH
and TZ5050-C2H4O2, respectively. It is clear that highest SSA value was obtained for
TZ5050-C2H4O2, e.g. the catalyst synthesized with the complexing additive (CH3COOH).
Also, there was a considerable increase in the SSA from the individual oxides to the mixed
metal oxides, with an increase of  7 times.
Average pore size was calculated by the BJH method for the mesoporous solids and the
mean pore sizes determined are 5.2 and 7.6 nm for Ti-C2H4O2 and Zr-C2H4O2, respectively.
On the other hand, for the mixed metal oxides (TZ5050) the mean pore size varied between
3.1 and 3.6 nm, where the material with the smallest mean pore size was TZ5050-C2H4O2.
It is clear that the mean pore size is decreased for the mixed oxides with respect to the
single metal oxides. It has been reported [15] that these changes, i.e. the increase in the
SSA and decrease in mean pore size, occur due to competition between hydrolysis and
condensation reaction steps causing agglomeration of small particles, which agrees with the
obtained results by SEM (Fig. S5, ESI) . It is suggested that the formation of the
organometallic complex between acetate (CH3COO-) and the metal ion (Ti and Zr) during

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the gelation time is the main reason behind the achievement of the smallest and uniform solid
particles when acetic acid is used as additive.
3.2. X ray Diffraction (XRD) analyses
The diffraction patterns of TZ5050 samples with different additives, Ti- C2H4O2 and Zr-
C2H4O2 and treated at 500 °C are shown in Fig. S2 (ESI). Ti-C2H4O2 presents only the
anatase crystalline phase with its most abundant peak at ~ 25.3º 2θ related to the [101] plane
(JCPDS No. 86-1157) [16]. In the case of Zr-C2H4O2, the coexistence of two crystalline
phases was found, namely tetragonal at 2θ= ~ 30º (JCPDS No. 89-7710) and monoclinic ~
28° (JCPDS No. 83-0944) [17], where the tetragonal phase exists in greater abundance as
compared to the other phase. Regarding TZ5050, these patterns show very poor crystallinity
or amorphous form. This is in agreement with the XRD results reported by Anzures et al.
[18], and it can be concluded that TiO2–ZrO2 mixed metal oxides, prepared by the sol–gel
◦
method, do not crystallize at temperatures below 600 C. Two main broad diffraction peaks
between 2θ = 20º - 40º and 40º - 70º are observed, and these are the regions where the main
diffraction peaks of monoclinic/tetragonal phases of ZrO2 and anatase phase of TiO2 are
expected to form [19].
3.3. FTIR studies
Fig. S3 (ESI) shows the IR spectra of the materials prepared in this work. The FTIR
spectrum of the TiZr-NH4OH catalyst before the heat treatment (Fig. 3 (a)), shows a very
broad IR band at  3406 cm-1 related to the -OH stretching vibrational mode, and another
small band at 1631 cm-1 corresponding to the -OH vibrational mode [20]. The -OH groups

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are formed as a result of the dissociative adsorption of H2O molecules to reduce the
coordinative unsaturation of the surface sites [21]. The IR bands at 2961, 2382 and 1381 cm-
1 represent vibrational modes of -CH group due to the solvent of 1-butanol in reaction, and
probably residues of the unreacted butoxide [22]. Besides that, a small band is observed at
+
1442 cm-1 and it can be related to the NH4 resulted from the synthesis additive [23]. In the
case of the catalyst with the complexing additive before calcination, as shown in Fig. S3 (b)
(ESI), two IR bands at 1546 and 1449 cm-1 are assigned to the as-COO- and s-COO- mode,
respectively [24], and are related to the ligands of the acetate (CH3COO-). The separation
between these signals is = 97 cm-1 and indicates that CH3COO- ions act as a bidentate
ligand. According to Barboux et al. [25], the acetate may form a chelate attached to a metal
or a bridge between the two metals. IR bands at 1118, 1073 and 1032 cm-1 are the
characteristic ones of the butoxide groups (OBun). Also, the -OH band and -CH band are
observed at around 3232 and 2960-2863 cm-1 respectively. The broad IR band  700 cm-1 is
,
assigned to the -M-O vibrational mode, i.e. Ti-O-Ti, Zr-O-Zr and/or Ti-O-Zr. It is clear
from Fig. S3 (b) (ESI) that there is some kind of linkage between butoxide, acetate and
hydroxyl groups: (TiZr (OBun) (OAc)). In materials synthesized at 500 °C, shown in Fig.
S3 (c) (ESI), the decrease in -OH and -OH band intensities can be observed with respect
to the materials before calcination. The broad IR band between 300 and 900 cm-1
approximately, is observed for all TZ5050 materials as a result of the interaction M-O-M in
the Ti-O-Zr network [26]. The signals at 2353 and 1519 cm-1 are assigned to the stretch and
bending modes of -CH, respectively, and are observed only in the case of materials prepared
at pH=3. For materials synthesized at pH = 9, no traces of organic matter are observed after
the thermal treatment at 500 ºC.

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3.3.1. FTIR-pyridine adsorption
The results of the pyridine adsorption by FTIR are shown in Fig. 1. Pyridine adsorption
is a valuable method to determine the nature of acid sites. Absorption bands  1605, 1575,
1490 and 1444 cm−1 are characteristic of Lewis acid sites [27]. The Brønsted acid site can be
observed at 1540 cm−1 [28]. In the case of Ti-C2H4O2, Zr-C2H4O2 and TZ5050 oxides with
different additives show Lewis acid sites at 1603, 1573, 1490 and 1444 cm−1 IR bands (Fig.
1). In the case of TiO2, it has been reported [8] that the structure of anatase promotes the
formation to Lewis acid sites. In addition, Glorius et al. [29] reported that zirconia-tetragonal
can also form Lewis acid sites. These results agree with the analysis of XRD (Fig. S2, ESI).
On other hand, the Brønsted acid sites were not observed in any of the samples, simple or
mixed metal oxides. The formation of Brønsted acid sites was not reported so far for the
TiO2–ZrO2 solids under similar experimental conditions [30]. According to the IR intensity
shown for the Lewis acid sites of the present metal oxides, the behavior from the highest to
the lowest intensity is : TZ5050-C2H4O2> TZ5050-NaOH>TZ5050-HNO3>TZ5050-
NH4OH>Ti-C2H4O2> Zr-C2H4O2, where clearly the TZ5050-ACT catalyst showed the
highest IR intensity. Similarly, the same behavior was observed from the NH3-TPD analysis
(See Table 1).
3.4. TEM studies
TEM micrographs of TiZr5050-C2H4O2 and TiZr5050-NH4OH samples treated at 500 ºC
are presented in the Fig. S4 (ESI). Both samples showed small nanoparticles of spheroid
shape with an average particles size  6 nm. The electron diffraction of both samples showed
no sign of crystalline phase(s). TiO2-ZrO2 treated at 550ºC [31] showed similar results.

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Furthermore, this result is in agreement with the XRD analysis (Fig. S2, ESI) and confirm
that prepared materials are in amorphous phase.
3.5. Acidity measurements - NH3 TPD
Table 1 presents the amount (μmol/g) of total acid sites, acid sites density (μmol/m2) and
acid strength distribution calculated from NH3-TPD profiles of the catalysts synthesized by
sol-gel after using different additives. Measured total acid sites for Ti-C2H4O2 and Zr-
C2H4O2 are 695 and 700 μmol g-1, respectively. However, mixed metal oxides show
relatively high total amount of acid sites, namely: 1301, 1375, 1429 and 1959 μmol g-1 for
TZ5050-NH4OH, TZ5050-HNO3, TZ5050-NaOH and TZ5050-C2H4O2, respectively.
According to these results, highest number of surface acid sites on was observed for the
TZ5050-C2H4O2 catalyst. Therefore, it can be confirmed that when the SSA increases, the
number of surface acid sites increases. Density of acid sites for Ti-C2H4O2, Zr-C2H4O2,
TZ5050-NH4OH, TZ5050-HNO3, TZ5050-NaOH and TZ5050-C2H4O2 are: 17.4, 17.1, 5.6,
5.8, 5.6 and 6.8 μmol m-2, respectively. Density of acid sites is high for individual metal
oxides, i.e. Ti-C2H4O2 and Zr-C2H4O2 due to the relatively low specific surface area of these
samples when compared to that of mixed metal oxides.
Acid strength distribution for the present solids investigated is shown in Fig. 2. The
desorption temperature of ammonia depends on the bonding strength of the acid site with
adsorbed ammonia species, thus the surface acidity of the solid samples is classified as weak
(100-200 ◦C), medium (200-300 ◦C), strong (300-400 ◦C), and very strong (>400 oC) [32].
From the TPD-NH3 profiles shown in Fig. S6 (ESI), TZ5050-C2H4O2 displayed a maximum
desorption temperature at relatively lower temperature, i.e. 145 °C when compared to that of
all other samples. Table 1 shows that the greatest number of weak acid sites is 926 μmol g-

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1, or 47.3% of the total sites available on the surface for the TZ5050-C2H4O2 solid. This
value of weak acid sites is higher by at least 2 times when compared to that of any other
solid sample, and this might be a key point for the selective yield of 5-HMF observed.
3.6. Catalytic performance studies
Catalytic performance studies of the present metal oxides were focused on the yield of
5-HMF for the model glucose conversion reaction. The reactivity study was conducted with
THF/Water as the reaction medium. Fig. 3A shows the evolution of the reaction 5-HMF
product with time. It is observed that the production yield of 5-HMF with individual metal
oxides, i.e. Ti-C2H4O2 and Zr-C2H4O2, increases linearly, with reaction rates of 0.1619 and
0.1395 mmol min-1 g-1, respectively. However, the mixed metal oxides display an exponential
increase for the transformation to 5-HMF with a reaction rate between 0.2152- 0.2938 mmol
min-1 g-1 (See Table 2), where the TZ5050-C2H4O2 catalyst exhibits the highest reaction rate.
Table 2 also shows results of glucose conversion, yield to 5-HMF and selectivity to 5-HMF.
According to the glucose conversion (Table 2, Fig. 4b) results, the Ti-C2H4O2 and Zr-C2H4O2
catalysts resulted in 90 and 87% conversion, respectively. In the case of mixed oxides
TZ5050, a conversion of  99% is observed. This result might be the effect of the large
amount per gram basis of Lewis acid sites on these materials (Table 1). In the case of the
mixed metal oxides, the catalyst TZ5050-C2H4O2 provides the highest performance in the
reaction with a production yield of 76%, whereas the materials synthesized with NH4OH,
HNO3 and NaOH achieved yield values of 67, 63 and 61%, respectively. It is known that
acid-base properties are needed to produce 5-HMF, where the Lewis acid sites are more
responsible for the isomerization of glucose to fructose, whereas Brønsted acid sites are

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responsible for fructose dehydration. In addition, the presence of mannose compound was
investigated as a by-product of reaction. Mannose can be generated by an epimerization of
glucose by the presence of Lewis acid sites in water [33]. However, in this work the presence
of mannose was not found. This can be the result of a rapid and selective isomerization of
glucose to fructose with TiO2-ZrO2 catalysts, avoiding the formation of mannose.
Following the above discussion, the number of total surface acid sites coincides with the
reaction yield performance for the materials synthesized in this work. More precisely, the
simple metal oxides Ti-C2H4O2 and Zr-C2H4O2 shows the lowest number of total acid sites
and therefore exhibit the lowest yield to 5-HMF. Moreover, the mixed metal oxides TZ5050
show better yield as they have more surface acidic sites (1959 - 1301 μmol g-1). Also, the
catalyst with the highest number of acid sites, and especially with the highest proportion of
weak acid sites, i.e. TZ5050-C2H4O2, provide the highest yield. Therefore, it can be
confirmed that the total amount of surface acid sites and the percent of weak acid sites present
in the synthesized materials are key factors for a better reaction performance. Also, it is
confirmed that it is possible to obtain a greater number of surface acid sites using acetic acid
as complexing additive in the sol-gel synthesis of TiO2-ZrO2 materials.
Concerning the reaction by-products selectivity, Table 2 presents results for HMF,
fructose, levulinic acid (LA) and formic acid. It is noted here that glucose and reaction by-
products were analyzed by HPLC and a calibration curve was made in order to quantify their
concentration by samples taken during the reaction ( the times of response and the conditions
of analysis are shown in Fig. S7, ESI). The Ti-C2H4O2 and Zr-C2H4O2 catalysts showed the
lowest selectivity to 5-HMF, namely 61 and 63%, respectively. This is due to a non-complete
dehydration of fructose to 5-HMF, showing a selectivity to fructose of 8 and 11%,
respectively. In the case of TZ5050 catalysts, a lower selectivity to the simple oxides was

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observed, between 1-5%, where the TZ5050-C2H4O2 catalyst showed the lowest fructose
selectivity due to its greater selectivity to 5-HMF.
The presence of acids was analyzed in order to corroborate the existence of a rehydration
of 5-HMF to formic and levulinic acids. All the samples showed the presence of levulinic
and formic acids with a selectivity value between 22 and 28% likely due to the presence of
water in the reaction system.
In addition, to confirm the formation of by-products in the present reaction system,
analysis by 1H NMR of the TZ5050-C2H4O2 catalyst (see Fig. S8, ESI) was conducted. The
1H NMR spectrum (600 MHz, MeOD) showed the following displacements: the protons of
5-HMF were calculated from integrated values of: (CHOA, δ = 9.54 ppm), (CHB, δ = 7.42
ppm and CHA, δ = 6.61 ppm), and (CH 2 δ = 4.64 ppm). In the case of LA (CH2 δ = 2.78
ppm and CH2 δ = 2.55 ppm), (CH3 δ =2.19 ppm) and finally of FA (-COOH δ=8.12 ppm),
it was found that after the formation of 5-HMF, levulinic and formic acids are formed,
following the reaction pathway presented in Fig. S8 (ESI).
Recycling tests for the TZ5050-C2H4O2 catalyst stability are shown in Fig. 4. The
reaction was carried out at 175 ° C and 30 bar of Ar in the THF/H2O biphasic system, where
the yield to 5-HMF was quantified at the end of 1 h of reaction. In each recycle, the catalyst
was filtered and washed with methanol several times, and the solvent was evaporated at 120
°C, followed by a heat treatment at 400 °C for 1 h. The results obtained show that the yield
of 5-HMF is reduced to  60% after 3 consecutive cycles. This deactivation of catalyst might
be due to the stability of the material’s surface (amorphous characteristics of the material),
which was confirmed by XRD and TEM analyses. The decrease in the yield also could be
due to carbon deposits on the mixed metal oxide surface, as a result of the formation of

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humic compounds [34]. The latter is supported by the color change from white to dark
yellow color after 3 cycles.
The temperature programmed oxidation (TPO) was used to investigate the deactivation
of the TZ5050-C2H4O2 catalyst through the evolution of CO2 from the combustion of carbon
deposited on the surface of the catalyst [35]. In Fig. 4 (right-axis), the fresh catalyst shows
a carbon amount of 17.5 mol as a reference. It was observed that for each recycle, the
amount of carbon increased, up to  35.5 mol in the third recycle. This is due to carbon
deposition in the TiO2-ZrO2 active sites. This result corroborates the loss in activity for the
production of 5-HMF in the recycle tests over the TZ5050-C2H4O2 catalyst.
4. Conclusions
It was possible to prepare TZ5050 materials by the sol-gel method using different
synthesis additives. . Acetic acid was the additive that allowed the formation of materials
with better textural and surface structural properties. In relation to the results obtained for
the glucose transformation reaction, the catalyst TZ5050-C2H4O2 presented the highest yield
(ca. 76%) to 5-HMF. Furthermore, selectivity to the reaction by-products was analyzed and
1
the pathway of reaction was corroborated by HNMR analysis, where it was found that
besides 5-HMF, levulinic acid and formic acid are formed. However, this selectivity was low
over the TZ5050-C2H4O2 catalyst. Recycling tests indicated that the performance of the
catalyst was reduced to  60% after 3 cycles. This result generates the need to investigate
other calcination temperatures to improve the stability of the material.

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Acknowledgements
Adib Abiu Silahua Pavón thanks the National Council for Science and Technology
(CONACYT) for scholarship postgraduate studies (Ph. D) for financing the projects number
132648 and 1024 “Cátedras Program” and finally thanks the Universidad Juárez Autónoma
de Tabasco for the support with PFCE Project.
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Figure Captions
Figure 1. FTIR-Pyridine adsorption on a) Zr-C2H4O2, b)Ti-C2H4O2, c) TiZr5050-NH4OH,
d) TiZr5050-NaOH, e) TiZr5050-HNO3, and f) TiZr5050-C2H4O2 solids.

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Figure 2. Surface acid strength distribution over the Ti-C2H4O2, Zr-C2H4O2 and TiZr5050
(C2H4O2, NH4OH, HNO3 and NaOH) catalysts synthesized by the sol-gel method.
Figure 3. A) Reaction yield of 5-HMF product from glucose conversion and B) Glucose
conversion and selectivity to 5-HMF, fructose and organic acid (FA+LA), where: a) Zr-
C2H4O2, b)Ti-C2H4O2, c) TiZr5050-C2H4O2, d) TiZr5050-NH4OH, e) TiZr5050-HNO3 and
f) TiZr5050-NaOH catalyst. Experimental conditions: 0.6 g of glucose; 0.24 g of catalyst;
30 ml (THF/Water); reaction time of 1 h; reaction T=175 °C; 30 bar Ar.
Figure 4. Catalyst recyclability activity test and TPO analysis for carbon (μmol/g). () 5-
HMF yield from glucose conversion on TZ5050-C3H4O2 catalyst. Reaction conditions:
glucose/catalyst: 2.5 , 30 mL solvent (Water/THF, 1:4 v/v), 2.1 g NaCl, 60 min reaction time,
175 ºC reaction temperature, 30 bar Ar.

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Fig. 1

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Fig. 2.

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Fig. 3.

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Fig. 4.

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Table 1. Specific surface area (SBET), specific pore volume (Vp), mean pore size (Dp),
surface acidity strength distribution, amount of desorbed NH3 and acid sites density,
parameters obtained through BET and TPD-NH3 measurements.
Acid strength distribution
Acid sites
(µmol/g)
SBET
Vp
Dp
TPD-NH3
(µmol/g)
Catalysts
density
(m²/g) (cm³/g)
(nm)
Weak
Medium
Strong
Very strong
(>400 oC)
(µmol/m2)
(100-200 oC) (200-300 oC) (300-400 oC)
Ti-C2H4O2
Zr-C2H4O2
41
40
0.07
0.11
5.2
7.6
695
700
17.4
17.1
79.4
73.8
281.3
82.9
154.3
292.0
180.3
251.7
TZ5050-C2H4O2
TZ5050-NH4OH
TZ5050-HNO3
TZ5050-NaOH
289
234
239
257
0.22
0.24
0.29
0.29
3.1
3.3
3.6
3.4
1959
1301
1375
1429
6.8
5.6
5.8
5.6
925.5
437.8
339.7
331.5
518.5
404.4
440.0
551.5
330.8
275.7
305.9
304.7
183.3
183.2
289.8
242.1