Catalytic etherification of 5-hydroxymethylfurfural into 5-ethoxymethyfurfural over sulfated bimetallic SO42?/Al-Zr/KIT-6, a Lewis/Br?nsted acid hybrid catalyst

Article abstract of DOI:10.1016/j.mcat.2020.111176

A series of mesoporous composites of SO₄^2?/Al-Zr/KIT-6, with varied ratios of ZrO₂ to Al₂O₃ and degrees of sulfation have been synthesized as an efficient, clean, facile and environmental-friendly nano-catalyst for the selective etherification of biomass-derived 5 hydroxymethylfurfural (HMF) to produce 5-ethoxymethylfurfural (EMF) as a biofuel candidate in a one pot process. Structure/property relationships of the catalyst have been determined and optimized in terms of EMF yield and HMF conversion. The resulting catalyst has been further tuned through tailored reaction conditions to produce an EMF yield of 89.8 % and HMF conversion of 99 %.

Full text of DOI:10.1016/j.mcat.2020.111176

Molecular Catalysis 496 (2020) 111176
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Catalytic etherification of 5-hydroxymethylfurfural into 5-ethoxymethy-
furfural over sulfated bimetallic SO24ꢀ /Al-Zr/KIT-6, a Lewis/Brønsted acid
hybrid catalyst
,
Hamid Hafizia, Gavin Walkera, Javed Iqbala, J.J. Leahya, Maurice N. Collinsb *
a Department of Chemical and Environmental Sciences, Bernal Institute, University of Limerick, Limerick, Ireland
b School of Engineering, Stokes Laboratories, Bernal Institute, University of Limerick, V94T9PX, Limerick, Ireland
A R T I C L E I N F O
A B S T R A C T
A series of mesoporous composites of SO24ꢀ /Al-Zr/KIT-6, with varied ratios of ZrO2 to Al2O3 and degrees of
sulfation have been synthesized as an efficient, clean, facile and environmental-friendly nano-catalyst for the
selective etherification of biomass-derived 5 hydroxymethylfurfural (HMF) to produce 5-ethoxymethylfurfural
(EMF) as a biofuel candidate in a one pot process. Structure/property relationships of the catalyst have been
determined and optimized in terms of EMF yield and HMF conversion. The resulting catalyst has been further
tuned through tailored reaction conditions to produce an EMF yield of 89.8 % and HMF conversion of 99 %.
Keywords:
HMF etherification
5-Ethoxymethylfurfural
Lewis/Brønsted acid hybrid
Sulfated bimetallic
KIT-6
Mesoporous silica
1. Introduction
been proposed for EMF production but fructose has been extensively
used as the starting material via the utilisation of various Brønsted and
Biomass is considered the most abundant renewable and sustainable
carbon source. It is a promising alternative to fossil fuels for may in-
dustrial applications and products [1,2]. Due to ever diminishing fossil
resources in combination with ever increasing energy demands, as well
as increasing environmental awareness, researchers are focusing atten-
tion on obtaining biofuels and value-added chemicals through
chemo-catalytic conversion of biomass and its derivatives [3]. 5-hydrox-
ymethylfurfural (HMF), which is achieved via acid- catalyzed dehy-
dration of sugars (e.g., glucose and fructose), is regarded as a versatile,
biomass-derived platform that can be further converted into wide vari-
ety of green fuels and sustainably derived chemicals [4,5]. Amongst
various high-value compounds derived from HMF, a great deal of recent
attention has been focused on the production of 5-ethoxymethyfurfural
(EMF) [6,7]. It is believed that, EMF, due to its outstanding physico-
chemical properties is a potentially promising liquid biofuel and an
excellent additive for diesel engines [8]. EMF, which can be produced
through etherification of HMF with ethanol, has a high octane number
with a high energy density of 8.7 kWhLꢀ 1, 29 % greater than that of
ethanol, roughly equivalent to conventional gasoline (8.8 kWhLꢀ 1) and
close to that of diesel (9.7 kWhLꢀ 1). It exhibits high stability under
oxidative conditions [9]. Various biomass-derived feedstocks such as
sucrose and inulin [10–12], glucose [13,14] and corn stover [15] have
Lewis acid catalysts. Some examples include, 64 % EMF, achieved using
an ethanol–DMSO solvent system catalyzed by phosphotungstic acid
(HPW) [16], SO3H-functionalized polymers produced a 72.8 % yield of
EMF at 110 ◦C after 10 h [17], BF3⋅(Et)2O/AlCl3⋅6H2O gave a yield of 55
% EMF [18], a hydrogen sulfate ionic liquid co- solvent produced a yield
of 83 % [19] while heteropolyacid supported on K-10 clay solid acid
catalysts were reported to produce EMF with a 61.5 % yield [20].
Using fructose and other C6-based carbohydrates as a feedstock is
desirable for the one-pot synthesis of EMF in terms of cost-effectiveness
but some technical hurdles remain for commercial scale up including
low product yield, relatively long reaction times, high temperatures and
high catalyst loadings. Key to improvement of performance is the se-
lective production of EMF from HMF (Scheme 1).
Among solid acid catalysts, silica-based mesoporous materials due to
their high specific surface area, high thermal/mechanical stability,
tunable mesopore diameter and ease of functionalization for the incor-
poration of transition metal ions and non-metallic groups into silica
framework have been widely investigated to develop efficient catalysts
for a wide variety of reactions. For instance, the synthesis of EMF from
HMF catalyzed by MCM-41-HPW achieved a yield of 83.4 % [21] and
H4SiW12O40/MCM-41 yielded 77.3 % HMF via an etherification reaction
[22]. Al-MCM-41 and SBA-15-supported zirconia have produced 100 %
* Corresponding author.
E-mail address: maurice.collins@ul.ie (M.N. Collins).
https://doi.org/10.1016/j.mcat.2020.111176
Received 17 July 2020; Received in revised form 18 August 2020; Accepted 19 August 2020
Available online 3 September 2020
2468-8231/© 2020 Elsevier B.V. All rights reserved.

H. Hafizi et al.
Molecular Catalysis 496 (2020) 111176
acids. They have been chosen for three main reasons; (I) It is known that
Al/Zr bimetallic catalyst has a synergistic effect allowing the bimetallic
catalyst superior performance and efficiency compared to their mono-
metallic counterparts [29–31]. (II) To improve the strength of the active
tetragonal phase of ZrO2 which is optimal crystalline phase for superior
catalytic activity [32] and (III) for tunability of the Lewis and Brønsted
acid sites [33]. These catalysts have been prepared through in-situ
incorporation of varying metal loadings in the KIT-6 matrix followed
by sulfation by impregnation in aqueous sulfuric acid of varying
molarity. The catalytic activities of the catalysts were evaluated for the
selective etherification of HMF with ethanol for the production of EMF.
2. Experimental
2.1. Chemicals and reagents
Tetraethylorthosilicate (TEOS, 98 %), Pluronic P123 (EO20PO70
-
EO20), hydrochloric acid solution (37 wt%), sulphuric acid (98 %), 1-
butanol, 5-hydroxymethylfurfural (≥ 99 %), 5-ethoxymethyfurfural
(97 %), ethanol absolute, zirconyl chloride octahydrate and
aluminium chloride hexahydrate were each obtained from Sigma-
Aldrich, Ireland.
Scheme 1. Schematic illustration of the production of EMF from HMF
and fructose.
2.2. Synthesis of AlxZry-KIT-6 and SO24ꢀ /AlxZry-KIT-6
2.2.1. Synthesis of AlxZry-KIT-6
A series of bimetallic KIT-6 containing Al and Zr with different Alx/
Zry ratios (x/y = 5/1, 1/1, 1/5 denoted as A5Z1-K, A1Z1-K, A1Z5-K)
where x and y represent the mole percentages of the Al and Zr, respec-
tively, with constant nSi/(nAl + nZr) molar ratio of 10 were one-pot
synthesized via a sol-gel approach. In a typical synthesis of AxZy-KIT-
6, Pluronic P123 (4.0 g), as a structure-directing agent, was dissolved in
deionized water (144 g) and concentrated hydrochloric acid (7.9 g)
under continuous stirring at 35 ◦C. 4.0 g of n-butanol was added drop-
wise to the solution followed by constant stirring for 1 h. Subsequently,
TEOS (8.6 g) and appropriate amounts of AlCl3⋅6H2O and ZrOCl2⋅8H2O
were added. The resulting mixture was kept under vigorous stirring for
24 h at the same temperature and then transferred to a polypropylene
bottle. Mixtures were hydrothermally treated at 100 ◦C for 24 h under
static conditions. The white product was collected by filtration washed
with deionized water and ethanol, dried at 60 ◦C overnight and finally
calcined at 550 ◦C for 6 h in air.
Fig. 1. A typical High Performance Liquid Chromatography (HPLC) Chro-
matogram of HMF and EMF.
conversion of HMF with 68 % and 76 % yields of EMF, respectively [23].
While Al-TUD mesoporous silica utilised as a solid acid catalyst for
production of EMF achieved 88 % conversion of HMF with a yield of 81
◦
% after 24 h at 140 C [24]. Among the mesoporous silicas, the most
promising catalyst is KIT-6 (Korean Institute of Technology) which has
tunable pore size and pore wall thickness providing better thermal sta-
bility [25,26]. Its interconnected cubic Ia3d mesostructure channels
permit better mass diffusion with a consequent increase in available
active sites for more efficient Lewis and Bronsted catalysis [27,28].
In this study, we explore the synthesis and characterization of a new,
sustainable, and recyclable bi-functional heterogeneous acid catalyst,
SO24ꢀ /Al-Zr/KIT-6 (SAZ-K) in which both ZrO2 and Al2O3 act as Lewis
2.2.2. Sulfation of AlxZry-KIT-6
The Sulfation procedure was carried out according to a previous
Fig. 2. SEM images of A1Z1-K (A) and (B), A5Z1-K (C) and (D), A1Z5-K (E) and (F), S0.5A1Z5-K (G) and (H).
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H. Hafizi et al.
Molecular Catalysis 496 (2020) 111176
Fig. 3. TEM images of A1Z1-K (A) and (B), A5Z1-K (C) and (D), A1Z5-K (E) and S0.5A1Z5-K (F).
report [34]. All bimetallic mesoporous KIT-6 were sulfated using SO42ꢀ
loadings by impregnation of samples with H2SO4 (aq) of molarity of 0.25,
0.5, 0.75 and 1 M at room temperature. For this propose, 2 g of AxZy--
KIT-6 were added to a 50 mL beaker containing 20 mL of H2SO4 solu-
tion. The slurry was stirred for 12 h followed by sonication. The
suspension was filtered by centrifugation, dried at 100 ◦C overnight to
get a fluffy powder and finally calcined at 550 ◦C for 3 h. These catalysts
are denoted as SmAxZy-K where m represents the molarity of H2SO4
solution.
2.3. Catalyst characterization
X-ray diffraction (XRD) patterns of catalysts with Cu Kα radiation (λ
=0.154 nm) were obtained on a PANalytical Empyrean diffractometer,
operated at 40 kV and 40 mA. The Fourier transform infrared (FTIR)
spectra of the catalysts after adsorption of pyridine were recorded on a
Nicolet Nexus FTIR spectrometer equipped with an attenuated total
reflectance accessory (ATR) over the range 1400–1700 cmꢀ 1. The Si/Al
+ Zr molar ratio and amount of sulfur loaded in the KIT-6 molecular
sieves were analysed and determined by inductively coupled plasma -
optical emission spectrometry (ICP-OES), (Agilent ICP-5100-OES). The
surface topography and chemical composition of the catalysts were
recorded using a field-emission electron microscope (FE-SEM, Hitachi
SU8030, Japan). The morphology of the catalysts was investigated by
high-resolution transmission electron microscopy (HRTEM, JEOL JEM-
2011 F electron microscope) at 200 kV. The preparation of samples
was carried out by drop-casting a suspension of catalysts in ethanol on
TEM Cu grids and then dried in air. The Brunauer–Emmet–Teller
equation (BET) specific surface areas of the samples were calculated
using the multipoint method and pore size distribution of the samples
were calculated from the desorption branches of the isotherms by
Fig. 4. Wide-angle X-ray powder diffraction patterns of A1Z1K (a), A5Z1-K (b),
A1Z5-K (c) and S0.5A1Z5-K (d).
applying the Barrett ꢀ Joynerꢀ Halenda (BJH) method on a Quantach-
rome Autosorb-1 Instruments. Samples were vacuum degassed at 200 ◦C
overnight prior to analysis.
2.4. Etherification of HMF into EMF and product analysis
In a typical etherification reaction, HMF (126.1 mg, 1 mmol),
SmAxZyK (50 mg) and 5 mL of ethanol were placed in an Ace pressure
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H. Hafizi et al.
Molecular Catalysis 496 (2020) 111176
Table 1
The main elements composition of samples.
Samples
nSi/n[Ma]
nSi/n(Al+Zr)
Sulfur content (wt %)
A1Z1-K
A5Z1-K
A1Z5-K
10
10
10
10
10
10
10
11.3
8.2
–
–
8.9
–
S
0.25A1Z5-K
8.1
1.4
1.8
2.1
2.2
S0.5A1Z5-K
0.75A1Z5-K
S1A1Z5-K
10.1
9.3
S
9.4
[a] Theoretical Si/(Al + Zr) ratio.
Fig. 6. FT-IR spectra after pyridine desorption at 200 ◦C of A1Z1K (a), A5Z1-K
(b), A1Z5-K (c), S0.25A1Z5-K (d), S0.5A1Z5-K (e), S0.75A1Z5-K (f) and S1A1Z5-K
(g)s.
tube (Sigma-Aldrich, 15 mL, 10.2 cm long). The tube was then sealed
and kept in an oil bath for a given time and temperature whilst being
stirred magnetically at 600 rpm. The reaction start time was recorded
when the tube was immersed in the oil bath at the desired temperature.
After completion of the reaction, the reaction tube was transferred to an
ice–water bath to cool, the catalyst was then separated from the reaction
mixture by centrifugation and the mixture was diluted before analysis.
The product was analysed by high performance liquid chromatography
(HPLC) (Fig. 1), using an Agilent (Agilent Technologies, Waldbronn,
Germany) 1260 Infinity II HPLC system equipped with a quaternary
pump G1311B, a diode array detector G1315D set at wavelength of 280
nm. The separation was carried out at 30 ◦C using a NUCLEODUR 100ꢀ 5
Fig. 5. N2 adsorption–desorption isotherms and pore size distributions distri-
bution (inset) of samples.
C18 reverse-phase column (150 mm × 4.6 mm, 5 μm) from Macher-
Table 2
Physicochemical properties of samples.
ey–Nagel. The mobile phase consisted of a 0.1 wt.% acetic acid aqueous
solution and acetonitrile (V:V = 85:15) with a flow rate of 0.5 mL.minꢀ 1
.
Sample
Surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (A˚)
As shown in Fig. 1, HMF and EMF were well separated under the
analytical conditions, and their retention times were 4.0 and 21.5 min,
respectively. The HPLC was calibrated with known standards.
EMF yield, HMF conversion and product selectivity were defined as
follows:
A1Z1-K
768.10
726.34
707.18
497.84
0.79
0.74
0.69
0.48
31.77
31.60
21.06
18.76
A5Z1-K
A1Z5-K
S0.5A1Z5-K
m oles of EMF produced
initial m oles ofHMF
EMF yield (%) = [
] × 100
m oles ofHMF in products
initial m oles ofHMF
HMF conversion (%) = [1 ꢀ
HMF selectivity (%) = [
] × 100
EMF yield
HMF conversion
] × 100
3. Results and discussion
3.1. Catalyst characterization
3.1.1. SEM analysis
The morphology of the catalysts tested in this work, A5Z1-K, A1Z1-K,
A1Z5-K and S0.5A1Z5-K, with Si/(Al + Zr) atomic ratio of 10, was char-
acterized by SEM (Fig. 2). These SEM images demonstrate that the
sample with Al/Zr ratio of 1:1 (Fig. 2A and B) consists of interconnected
Scheme 2. Schematic representation of the Bronsted and Lewis acid sites in the
SO24ꢀ /Al-Zr/KIT-6 catalyst.
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H. Hafizi et al.
Molecular Catalysis 496 (2020) 111176
Fig. 7. Effect of different Al/Zr ratios on the etherification of HMF (126.1 mg, 1 mmol) in ethanol (5 mL) after a reaction time of 1.5 h at 120 ◦C, using 50 mg of the
catalyst with a degree of sulfation of 0.5.
ionic radii of the tetrahedral Zr4+, Si4+ and Al3+ in the framework so that
the substitution of Si4+ by metal ions leads to changes in the bond angles
and this is exacerbated at high zirconium loadings [[35]]. However,
mesopores are clearly observed even after impregnation with H2SO4
solution (Fig. 3F).
3.1.3. XRD analysis
Wide-angle XRD patterns of the AxZy-K and S0.5A1Z5-K samples
within a range of 10–90◦ are depicted in Fig. 4. All XRD patterns display
a broad diffraction peak at approximately 2θ = 23◦ which is charac-
teristic of amorphous silica and a peak at approximately 2θ = 47◦,
typical of the crystalline phase of tetragonal ZrO2. A secondary
diffraction peak for tetragonal ZrO2 is detected at 2θ = 27◦ for A5Z1-K
and A1Z5-K samples. The intensity of the peak related to tetragonal ZrO2
in A1Z5-K is higher than that of the A5Z1-K sample, and this suggest that
increasing the amount of Al2O3, improves the dispersion of ZrO2 [33].
For A1Z1-K no characteristic peak was observed at 27◦ (Fig. 4a), indi-
cating that most of the Zr4+ and Al3+ ions have been incorporated into
the silica framework or are highly dispersed within the silica pore wall
surface which agrees well with the TEM findings. For A5Z1-K, besides
tetragonal ZrO2, a small peak corresponding to the monoclinic ZrO2 can
be seen at approximately 2θ = 25. Although in the presence of Al2O3, a
Fig. 8. Effect of sulfate loading on the etherification of HMF (126.1 mg, 1
monoclinic phase is more desirable thermodynamically, it has been
mmol) in ethanol (5 mL) after a reaction time of 1.5 h at 120 ◦C, using 50 mg of
proven that the sulfated monoclinic zirconia is less catalytically active
the SmA1Z5-K catalyst.
than the tetragonal phase [36]. A reduction of crystallinity in the
sulfated sample, is most probably because of the generation of an
short-rod-like particles in a fiber-like morphology while the sample with
Al/Zr ratio 5:1 is composed of non-uniform-sized and aggregated small
irregular particles with a smooth surface (Fig. 2C and D). In the case of
the catalyst containing the highest amount of Zr4+, Zr/Al = 5, the par-
ticles are partially spherical in shape (Fig. 2E and F). It can be seen the
aggregation of A1Z5-K increases after the H2SO4 treatment and this leads
to little differences in particle morphology, but sphericity of the particles
is reduced (Fig. 2G and H).
amorphous bulk sulfated zirconia species (Fig. 4d) [37].
3.1.4. ICP analysis
As summarized in Table 1, by decreasing in the amount of m, in the
SmA1Z5-K catalyst, the sulfur content was reduced from 2.2 to 1.5 wt%,
thus increasing the amount of SO42ꢀ species. The Si/Al + Zr ratios present
in the samples differ from those in the initial synthesis gel. With the
highest deviation from the initial synthesis gel of Si/(Al + Zr) with a
ratio of 10, while the maximum obtained Si/(Al + Zr) ratio was 11.3 in
the (A1Z1-K) sample.
3.1.2. TEM analysis
The mesoporous nature and the internal morphology of the catalysts
was confirmed by TEM analysis as shown in Fig. 3. TEM images (Fig. 3A
and B) indicate the 3D cubic ordered nature of the mesoporous structure
with continuous walls, which confirms that sample A1Z1-K with an Al-Zr
molar ratio of 1:1, keeps its original pore structure after the introduction
of metals into KIT-6. The A5Z1-K sample shows both ordered and
disordered mesopores, however the irregular domains are predominant
(Fig. 3C and D). While Fig. 3E, shows that the ordered framework of
A1Z5-K was severely damaged and this was attributed to differences in
3.1.5. BET analysis
The surface properties of unsulfated AxZy-K and sulfated S0.5A1Z5-K
samples were evaluated by nitrogen adsorption–desorption isotherms,
as shown in Fig. 5. Surface areas, pore volumes, and the average pore
size of each catalyst is listed in Table 2. N2 adsorption–desorption iso-
therms indicate typical type IV characteristic curves (according to
IUPAC) with H1-type hysteresis loops. Compared with those of the A1Z1-
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H. Hafizi et al.
Molecular Catalysis 496 (2020) 111176
Fig. 9. Effect of reaction temperature on the etherification of HMF (126.1 mg, 1 mmol) in ethanol (5 mL) after a reaction time of 1.5 h, using 50 mg of the S0.5A1Z5-
K catalyst.
Fig. 10. Effect of reaction time on the etherification of HMF (126.1 mg, 1 mmol) in ethanol (5 mL) at 120 ◦C, using 50 mg of the S0.5A1Z5-K catalyst.
K and A5Z1-K samples, the N2 isotherms of the A1Z5-K and S0.5A1Z5-K
have a degree of shape change wherein the hysteresis inflection is less
sharp, and this is attributed to less and uniformity of pore size, which is
consistent with TEM observations (see Fig. 3). This phenomenon arises
3.1.6. Pyridine FTIR analysis
FT-IR spectra of SmAxZy-K samples with different Al/Zr ratios and
degree of sulfonation after pyridine desorption are shown in Fig. 6. Two
bands observed at 1447 and 1598 cmꢀ 1 indicate the presence of Lewis
acid sites, including Zr4+, Al3+ and Si4+ (Scheme 2) in the samples. For
non-sulfated samples (AxZy-K) (Fig. 6a, b and c), the intensity of peaks
related to these two bands is enhanced by increasing the Al content.
Among the sulfated samples, S0.5A1Z5-K (Fig. 6e) shows the highest in-
tensity at 1598 cmꢀ 1. When pyridine interacts with surface Brønsted
sites, the presence of the pyridinium species is evidenced by adsorption
bands at 1544 and 1637 cmꢀ 1 and the intensity of these peaks are higher
in the sulfated samples due to the presence of SO24ꢀ ions. Moreover, a
band at 1490 cmꢀ 1 can be attributed to pyridine interaction with both
Lewis and Brønsted acid sites [39,40]. It is clear the intensity of the
B/L band in sulfated samples dramatically increases compared with that
when M+ (Al3+ and Zr4+) substitutes Si4+, as the bond length of Al
̶O̶ Si
and Zr ̶O̶ Si is increased relative to Si ̶O̶ Si bonds which in turn modifies
bond angles (Scheme 2) and therefore BET parameters. Furthermore, at
lower P/P0 relative pressure an opening of the hysteresis loop is
observed, which indicates a reduction of the pore size [38]. A1Z1-K has
the highest surface area, pore size diameter and pore volume of 768.10
m2/g, 31.77 A˚and 0.79 cm3/g, respectively. Additionally, the values of
these parameters obtained after impregnation using H2SO4 solution
were decreased due to the immobilization of SO24ꢀ groups within pores
and perhaps mesoporous framework damage arising from the sulfona-
tion process. This is in agreement with the TEM and XRD results.
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H. Hafizi et al.
Molecular Catalysis 496 (2020) 111176
Fig. 11. Effect of S0.5A1Z5-K catalyst dosage on the etherification of HMF (126.1 mg, 1 mmol) in ethanol (5 mL) at 120 ◦C after a reaction time of 2.5 h.
in non-sulfated samples. Therefore, different B/L ratios can be tuned
Table 3
through varying metal ratios and degrees of sulfation.
Comparison of catalytic activity of the SO24ꢀ /Al1-Zr5/KIT-6 with other modified
silica-based solid acid catalysts.
3.2. Investigation of different parameters on the etherification of HMF to
EMF using SmAxZyK
Catalyst
Temperature
Time
(h)
HMF
EMF
Yield
(%)
Ref.
(◦C)
conversion
(%)
3.2.1. Effect of different Al/Zr ratios
40 wt.% MCM-
41-HPW
100
90
12
2
96.1
83.4
[21]
[22]
Etherification of HMF with ethanol was carried out using sulfated
Al2O3 and ZrO2 incorporated in KIT-6 channels as a catalyst. In order to
investigate and obtain the optimum ratio of Al/Zr loading, S0.5A5Z1-K,
H4SiW12O40
MCM-41
/
92.0
77.3
Al-MCM-41
140
140
140
100
5
100
100
88
68
[23]
[23]
[24]
[42]
S0.5A1Z1-K and S0.5A1Z5-K catalysts were assessed whilst the concen-
tration of SO24ꢀ , temperature and time were kept constant at 0.5 M, 120
◦C and 1.5 h, respectively. Fig. 7 shows the overall conversion of HMF
catalyzed by S0.5A1Z1-K was ~ 86.6 %, which was the highest of all these
catalysts. The yield increases as the concentration of zirconium in-
creases. Or in other words, the yield reduces as the Al content increases.
This trend is in line with zirconia phase transitions obtained via XRD
results. All catalysts show conversion of HMF and EMF yield in the range
of 69.6–86.6 % and 27.9–66.5 % after 1.5 h, respectively, obtaining the
maximum yield and selectivity values of 66.5 % and 80.8 % using 50 mg
of S0.5A1Z5-K catalyst. The activity of the catalysts follows the following
ZrO2/SBA-15
Al-TUD
5
76
24
11
81
Fe3O4@SiO2-
HPW
89.5
76.9
Silica-SO3H
Silica sulfuric
acid
100
75
10
24
96.5
100
83.8
36
[43]
[44]
Fe3O4@SiO2-
SO3H
100
120
10
98.8
99
89.3
89.8
[45]
SO24ꢀ /Al1-Zr5/
KIT-6
2.5
This
work
Fig. 12. Reusability of S0.5A1Z5-K catalyst. Reaction conditions: HMF: 1 mmol, Ethanol: 5 mL, Catalyst: 75 mg, Temp.: 120 ◦C, Time: 2.5 h.
7

H. Hafizi et al.
Molecular Catalysis 496 (2020) 111176
order S0.5A1Z5-K > S0.5A1Z1-K > S0.5A5Z1-K. Therefore, the catalyst with
a Zr/Al molar ratio of 5:1 was the optimum ratio and was utilised for
subsequent evaluations.
etherification reaction.
3.2.7. Recyclability of catalyst
Reusability is one of the most important factors related to hetero-
geneous acid catalysts for variety of practical catalytic applications. For
the evaluation of reusability of the catalyst, S0.5A1Z5-K was used in five
reaction cycles under optimal conditions (i.e. 75 mg of catalyst, 2.5 h
and 120 ◦C). The catalyst was recovered via centrifugation, washed with
3.2.2. Effect of sulfate loading
To study the effect of of SO2ꢀ 4 loadings, the etherification reaction of
HMF was conducted with S0.25A1Z5-K, S0.5A1Z5-K, S0.75A1Z5-K and
S1A1Z5-K catalysts. The results obtained are shown in Fig. 8. It was found
that as the concentration of the sulfate is increased from 0.25 to 0.5 M
the conversion and yield increase in the range of 68.2–82.3 % and
45.4–66.5 % after 1.5 h at 120 ◦C, respectively. This is presumably due
to increasing levels of acidic active sites on the catalytic surface due to
rising acidic concentrations. However, no real change in yield and
conversion was observed thereafter. However, higher degrees of sulfa-
tion, did show drop offs in the percentage yield and conversion, this
confirms that the addition of a certain quantity sulfate can reduce
crystallinity of the tetragonal ZrO2 phase, agreeing with XRD results
(Fig. 4). These results indicate that S0.5A1Z5-K is the optimal sulfated
catalyst. So, it was used to consider the effect of reaction conditions on
the catalyst performance.
◦
ethanol and acetone, dried at 200 C and then the spent catalyst was
reused in another etherification reaction under the same conditions.
From the results shown in Fig. 12, it can be seen that there is a slight
reduction in yield of EMF observed for the first cycle (82.7 %) compared
with the fresh catalyst. While after that the performance of the catalyst
was retained constant in the third catalytic use. But in the fourth cycle,
the EMF yield dropped to 66.1 %. We attribute this to the loss of acid
sites during their repeated use, caused by elution of sulfate anions
(SO24ꢀ ) into the reaction mixture or it may be attributed to a partial
active-site obstruction by intermediates, by-product species or carbon
deposits during the reaction [46,47].
4. Conclusion
3.2.3. Effect of reaction temperature
EMF yield, HMF conversion and the selectivity of the product over
In the present study, a series of sulfated bimetallic SO24ꢀ /Al-Zr/KIT-6
with constant nSi/(nAl + nZr) = 10 but varied ratios of ZrO2 to Al2O3 and
degrees of sulfation was synthesized and characterized as an efficient
nano-catalyst for the synthesis of EMF by etherification of HMF. Results
show, the catalyst with the ratio of Zr/Al = 5, (A1Z5-K) exhibited the
highest catalytic performance which is attributed to the presence of
tetragonal ZrO2 crystalline phases in the solid acid catalyst. The results
further demonstrate that B/L ratios can be tuned through varying metal
ratios and degrees of sulfation to obtain an optimal sulfated S0.5A1Z5-K
catalyst for the etherification reaction with conversions and yields 99 %
S
0.5A1Z5-K catalyst were investigated by varying the reaction tempera-
tures in the 80–140 ◦C range after 1.5 h reaction time; the results are
shown in Fig. 9. When the reaction was carried out at 80 ◦C, the EMF
yield was about 6%, with 14.2 % conversion of HMF. Increasing tem-
perature from 80 to140 ◦C, leads to a rise in HMF conversion from
14.2–95.7%. However, when the reaction temperature increases from
120 to 140 ◦C EMF yield was reduced from 66.5–50.8%, due to the lower
stability of EMF and formation of by-products at higher reaction tem-
peratures in the presence of an acid based catalyst. Therefore, 120 ◦C is
selected as an optimal reaction temperature for the selective production
of EMF applying S0.5A1Z5-K catalyst.
◦
and 89.8 % recorded after 2.5 h at 120 C, respectively. Additionally,
reusability experiments demonstrate that catalytic activity of the
S
0.5A1Z5-K remains unchanged after the first and second reaction cycles.
3.2.4. Effect of reaction time
Finally, our results demonstrate that the SO24ꢀ /Al-Zr/KIT-6 catalyst is
competitive and efficient compared to other silica- based solid acid
catalysts and is thereby an excellent candidate for the etherification
reaction of HMF with obvious potential for many green chemistry syn-
thetic routes.
The influence of reaction time on the catalytic etherification of HMF
to EMF was assessed for the determination of the shortest residence time
to achieve highest yields of EMF with maximum conversion rates. The
observations are summarized in Fig. 10. The data reveals that EMF
yields increase with reaction time until 2.5 h to 79.3 % yield and a
selectivity of 81.8 % but longer than 2.5 h reaction times lead to a
reduction in yield and selectivity.
CRediT authorship contribution statement
Hamid Hafizi: Conceptualization, Methodology, Validation, Formal
analysis, Investigation, Writing - original draft. Gavin Walker: Re-
sources, Conceptualization, Formal analysis, Writing - original draft,
Supervision. Javed Iqbal: Validation, Investigation, Formal analysis,
Writing - original draft, Data curation, Methodology. J.J. Leahy: Vali-
dation, Investigation, Resources, Writing - review & editing. Maurice N.
Collins: Conceptualization, Methodology, Formal analysis, Supervision,
Resources, Writing - review & editing.
3.2.5. Effect of catalyst dosage
The impact of catalyst concentration (in the range of 25ꢀ 100 mg) on
the etherification reaction was then evaluated under the obtained
optimal conditions. As Fig. 11 shows, decreasing the loading of S0.5A1Z5-
K catalyst to 25 mg results in lowering the conversion of HMF to 85.8 %
and subsequent EMF yield to 41 % after 2.5 h reaction time, at 120 ◦C,
this is attributed to the reduction in catalytic sites available for reaction.
Increased catalyst loading afforded higher yields and conversion
amounts. While a maximum selectivity of EMF (89.8 %) with 99 %
conversion was achieved using 75 mg of the catalyst. This can be
explained based on increasing availability and number of catalytically
active sites. Further increasing the catalyst loading to 100 mg led to a
decrease in the yield to 71 % and this is attributed to the prevalence of
side reactions such as the formation of ethyl levulinate or humins [41].
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
3.2.6. Comparison with other modified silica-based solid acid catalysts
Our catalyst was also evaluated against other catalysts reported in
the literature for the synthesis of EMF. Based on the data provided in
Table 3, S0.5A1Z5-K catalyst under optimum condition including 75 mg
dosage, 120 ◦C and 2.5 h reaction time, shows superior catalytic activity
than other catalysts which suggest that a Lewis/Brønsted acid hybrid
system such as in our catalytic system has a synergistic effect on the
The authors wish to acknowledge the support provided by Electricity
Supply Board of Ireland (ESB) and the Faculty of Science and Engi-
neering at the University of Limerick for this project.
8

H. Hafizi et al.
Molecular Catalysis 496 (2020) 111176
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