Lewis acidity induced heteropoly tungustate catalysts for the synthesis of 5-ethoxymethyl furfural from fructose and 5-hydroxymethylfurfural

Article abstract of DOI:10.1016/j.mcat.2018.01.034

Heteropoly tungstate with tantalum ions in its secondary structure were prepared and subsequently dispersed on tin oxide. The prepared materials physical and chemical properties were estimated by different spectroscopic methods Characterization results indicate that the stable Keggin ion of tantalum heteropoly tungstate was well preserved on support. New Lewis acidic sites were generated with the presence of Ta ions in heteropoly tungstate. These samples were tested for their catalytic performance towards conversion of fructose to 5-ethoxy methyl furfural (EMF) and selective etherification of 5-hydroxymethylfurfural (HMF) with ethanol. The catalyst with 30 wt% of active component on SnO₂ exhibited highest HMF etherification activity with 90% of 5-ethoxymethylfurfural yield with in 45 min. The catalysts also able to converted fructose into EMF in one-pot with a yield of 68%. The etherification activity over these catalysts was studied under the influence of different reaction parameters such as reaction temperature, reaction time, catalyst weight and reactants mole ratio.

Full text of DOI:10.1016/j.mcat.2018.01.034

Molecular Catalysis 448 (2018) 108–115
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Lewis acidity induced heteropoly tungustate catalysts for the synthesis of 5-
ethoxymethyl furfural from fructose and 5-hydroxymethylfurfural
⁎
P. Krishna Kumari, B. Srinivasa Rao, D. Padmakar, Nayeem Pasha, N. Lingaiah
Catalysis Laboratory, Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, 500 007, Telangana, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Heteropoly tungstate with tantalum ions in its secondary structure were prepared and subsequently dispersed on
tin oxide. The prepared materials physical and chemical properties were estimated by different spectroscopic
methods Characterization results indicate that the stable Keggin ion of tantalum heteropoly tungstate was well
preserved on support. New Lewis acidic sites were generated with the presence of Ta ions in heteropoly tung-
state. These samples were tested for their catalytic performance towards conversion of fructose to 5-ethoxy
methyl furfural (EMF) and selective etherification of 5-hydroxymethylfurfural (HMF) with ethanol. The catalyst
5
-Hydroxymethylfurfural
Fructose
-Ethoxymethylfurfural
5
Etherification
Tungstophosphoric acid
2
with 30 wt% of active component on SnO exhibited highest HMF etherification activity with 90% of 5-ethox-
ymethylfurfural yield with in 45 min. The catalysts also able to converted fructose into EMF in one-pot with a
yield of 68%. The etherification activity over these catalysts was studied under the influence of different reaction
parameters such as reaction temperature, reaction time, catalyst weight and reactants mole ratio.
1. Introduction
certain cases it enhances the fuel properties [11,12]. In addition, EMF is
able to convert to 5-ethoxymethylfurfuryl alcohol by hydrogenation.
The world energy system is mostly dependent on the non renewable
This hydrogenated product is more miscible with diesel and has ana-
logous combustion properties as that of ethanol [13]. Therefore, EMF
synthesis from renewable sources has gained considerable interest.
EMF can be prepared using different substrates like 5-chlor-
omethylfurfural by nucleophilic substitution with ethanol [14], direct
etherification of carbohydrates like fructose into EMF with alcohol in
one-pot reaction as shown in Scheme 1. The etherification and direct
conversion of fructose to EMF are acid catalyzed reactions. Therefore it
is reasonable to carry fructose dehydration to HMF and its etherifica-
tion into EMF in one-pot reaction. Etherification of HMF to EMF reac-
tion was conceded using acidic catalysts like ionic liquids, organic-in-
fossil fuel resources, particularly petroleum, coal and natural gas [1].
The gradual depletion of these energy sources, concerns regarding
Green House Gases (GHG) emissions in the atmosphere and high de-
mand for energy has become severe concern for researchers [2]. In
order to overcome the problems of fossil fuels, researchers’ attention
was focused on finding alternative sustainable and eco-friendly energy
sources [3]. Among the renewable resources, biomass is the only sus-
tainable source to produce fuels, chemicals and carbon-based materials
[
4]. Biomass is broadly available in nature and different primary
compounds like carbohydrates can be derived [5]. Carbohydrates are
desirable resources for manufacture of bio fuels and valuable chemicals.
Different methodologies have been explored to process abundant and
cheaper biomass to chemicals/fuels [6].
Production of different bulk chemicals is possible from biomass and
among different chemicals 5-hydroxymethylfurfural (HMF) is con-
sidered as potential substitute for petroleum derived building blocks
3 3
organic hybrid catalyst [15], H PW12O40 [16] and AlCl [17]. The
solubility of these catalysts in reaction mixture and/or polar solvents
becomes difficult to separate the product from catalyst. In this contest
there is a need to develop heterogeneous catalyst for one-pot conver-
sion of fructose to EMF. Recently few researchers has studied hetero-
geneous catalysts such as MCM-41 supported HPW [18], silica coated
[
7]. It is considered as a useful platform molecule to produce fine
magnetic Fe
alysts for both HMF and fructose etherification [19]. Recently Xiao fang
Liu et al. used sulphated porous coordinated polymer (MIL 101- SO H)
3 4
O nano particles supported tungstophosphoric acid cat-
chemicals and fuel/fuel additives. HMF can be transformed to fine
chemicals by hydrogenation, oxidation, esterification and etherification
3
[
8–10]. Among the wide range of possibilities, HMF etherification to 5-
as a catalyst towards EMF synthesis from carbohydrates [20]. Lanza-
fame and co-workers were used a mesoporous silica based catalysts for
the translation of HMF to EMF [21]. Arenesulfonic acid-modified SBA-
ethoxymethylfurfural (EMF) is important approach to synthesis fuel
additives. EMF has energy density as diesel, high cetane number and in
⁎
Corresponding author.
E-mail address: nakkalingaiah@iict.res.in (N. Lingaiah).
https://doi.org/10.1016/j.mcat.2018.01.034
Received 21 August 2017; Received in revised form 26 December 2017; Accepted 29 January 2018
2468-8231/ © 2018 Elsevier B.V. All rights reserved.

P.K. Kumari et al.
Molecular Catalysis 448 (2018) 108–115
DIGILAB (USA) IR spectrometer. Pyridine adsorbed FT-IR spectroscopy
was used to measure the nature of the acid sites (Bronsted and Lewis) of
the catalysts. Diffuse reflectance infrared Fourier Transform (DRIFT)
mode was adopted to record the spectra. Initially the catalyst surface
was cleaned under vacuum at 200 °C for 3 h. Later dry pyridine was
spread in to the sample and the physisorbed pyridine was driven off by
heating the sample at 120 °C for 1 h. Pyridine-adsorbed spectra of
samples was recorded after bring the sample to room temperature.
Horiba Jobin-Yvon Lab Ram HR spectrometer with a 17 mW in-
ternal He–Ne laser source with 632.8 nm of excitation wavelength was
used. The powder samples of catalyst dispersed on a glass slide and the
spectra was recorded by focusing at different position. The spectra were
Scheme 1. Synthesis of EMF from fructose and HMF.
−1
recorded in the range of 200–1200 cm
.
1
5 catalysts were tested for the conversion of fructose to EMF selec-
Temperature-programmed desorption of ammonia was carried out
on BELCAT-II (Belsorb, Japan) instrument. About 0.05 g of oven-dried
sample was taken and pretreated at 300 °C for 1 h by passing pure he-
lium gas (99.9%, 30 mL/min). After pretreatment, the sample was sa-
tively [22]. Barbera et al. reported the conversion of HMF to EMF over
zirconia and sulphated zirconia supported on SBA-15 catalysts [23].
Even though all of these catalysts reported high HMF conversion, re-
quires lengthened reaction times. Moreover, these catalysts took long
reaction time to yield reasonable amount of EMF when reaction carried
from fructose.
Keggin type heteropolyacids (HPA) are commonly used as acid
catalysts and these can be made heterogeneous by exchange of its
protons (H ) with metal ions. These metal containing HPAs exhibit
both Lewis and Brønsted acidic sites [24,25]. We have been working on
metal exchanged HPAs for different acid catalyzed reactions [26,27]. In
the current work, tungstophosphoric acid (TPA) was modified by ex-
changing its protons with Ta ion in order to generate Lewis acidity and
turated with anhydrous ammonia (10% NH
3
-90% He mixture gas) at
100 °C for 1 h and was flushed with He gas at the same temperature to
remove physiosorbed ammonia. Then, the temperature programming
was carried out from 100 to 800 °C with a temperature increment of
3
10 °C/min. The amount of NH evolved was calculated using the cali-
+
brated thermal conductivity detector of the instrument.
The catalyst surface morphology of was observed by field emission
scanning electron microscope (FE-SEM) of JEOL JSM-7610F equipped
with an energy-dispersive X-ray spectroscopy. The sample powder was
randomly deposited on carbon tape placed on a stub. Before imaging,
the prepared sample was underwent gold metallization in order to
improve picture resolution. FESEM pictures were taken at accelerating
voltage of 2.00 KV with Secondary Electron Image probe (SEI) and the
Gentle Beam (GB) mode.
2
dispersing the modified TPA on SnO support. These catalysts were
studied for the selective HMF etherification to EMF and also for one-pot
conversion of fructose into EMF. The catalysts characteristics were
derived from different techniques and used to explain the catalytic
activity.
Transmission electron microscope (TEM; Philips Tecnai FEI F20,
operating at 200 kV) was used to observe the morphology and particle
size of the catalysts.
2
. Experimental
2.1. Catalyst preparation
2.3. Reaction of HMF to EMF and products analysis
All the chemicals of AR grade were used in the study. Tantalum
exchanged TPA (TaTPA) dispersed on tin oxide catalysts was prepared
in two steps [28]. Firstly, TaTPA was prepared by the exchange protons
of TPA with Ta ions. The calculated amount of TPA was completely
dissolved in water and to this calculated amount of tantalum chloride
HMF etherification was performed in a 15 mL sealed tube. In a usual
procedure, HMF (0.126 g) was dissolved in ethanol (2 g) and 3.8 wt% of
catalyst were taken in the tube. The reaction tube was kept in oil bath at
desired temperature and stirred magnetically at 300 rpm. After com-
pletion of the reaction or desired time, the reactor tube was quickly
removed from heating and the reaction mixture was cooled to room
temperature. The sample was diluted by adding ethanol and subjected
to centrifuge to separate the catalyst. The products were estimated by
separating them on innowax capillary column of gas chromatograph
(Shimadzu, 2010) equipped with flame ionization detector Products
were also identified by GC–MS (Shimadzu, GCMS-QP2010S) analysis.
(
5
TaCl ) dissolved aqueous solution of was added slowly with stirring.
The resulting solution was stirred further for 1 h at 80 °C. The excess
water was removed by rota evaporator to dryness. The samples were
dried at 120 °C in an oven. Finally the catalyst samples were treated at
300 °C in air for 2 h.
2
Secondly, the prepared TaTPA was dispersed on SnO by impreg-
nation method. The measured quantity of TaTPA dissolved in water and
slowly added to SnO . Similar procedure as described above was fol-
2
lowed to obtain final catalysts. Catalysts with different TaTPA loading
from 15 to 35 wt% were prepared. The catalysts are represented as x %
2.4. One-pot synthesis of EMF from fructose and products analysis
TaTPA/SnO
2
. Where x represents the weight percentage of TaTPA
(
x = 5, 10, 15, 20, 25, 30, 35).
One-pot EMF synthesis from fructose was carried similarly as
mentioned in the above Section 2.3. In this case 0.180 g of fructose,
6 mL of ethanol and 100 mg of catalyst were charged in to sealed tube.
The products were analyzed by using HPLC system (HITACHI) with
binary 2130 pumps, a manual sampler, and 2490 refractive index de-
tector maintained at 50 °C. The products were separated in sugar
column, maintained at 60 °C using water as mobile phase with a flow
rate of 0.8 mL/min. Calibration was carried by using standard 5-HMF,
fructose, EMF and EL solutions. The samples were diluted with a known
volume of millipore water before analysis to prevent the over loading of
the column. All the experiments were done in triplicates and reported
within standard deviations of triplicates within 2.0%.
2
.2. Catalysts characterization
Rigaku Miniflex diffractometer was used to measure the X-ray dif-
fraction patterns of the catalysts. Cu Kα radiation of 1.5406A° at 40 Kv
and 30 mA and secondary graphite monochromatic was used to record
the patterns. The measurements were carried in the 2θ range of 10–80°.
The BET surface areas were determined by using the N
sorption-desorption studies at liquid N
Instruments, Japan. Before analysis the samples were degassed at
2
physi-
2
temperature using BEL Sorb2
2
00 °C for 2 h
The FT-IR spectra were measured using the KBr disc method on a
109

P.K. Kumari et al.
Molecular Catalysis 448 (2018) 108–115
Fig. 1. XRD patterns of TaTPA/SnO
2
catalysts. (a) 5% TaTPA/SnO (b) 10% TaTPA/SnO
2
(e) 25% TaTPA/Sno (f) 30% TaTPA/SnO
2
2
2
(
(
c) 15% TaTPA/SnO
g) 35% TaTPA/SnO
2
2
(d) 20% TaTPA/SnO
(©) TPA (•) SnO
2
2
.
3. Results and discussion
3.1. Catalyst characterization
Fig. 1 displays X-ray diffractograms of the catalysts. The patterns
related SnO are predominant in all the catalysts. The diffraction lines
at 2θ values of 26.1, 33.5, 51.4, 54.4, 61.5, and 65.6 suggest the pre-
sence of tetragonal structure of SnO [29,30]. The patterns related to
Keggin ion of TaTPA were also observed at 2θ value of 10, 25.2, 29.3
and 34.6 for the catalysts with above 20 wt% of TaTPA on SnO [31].
The absence of Keggin ion characteristic patterns suggests the existence
of TaTPA is highly dispersed on the surface. Crystalline phase of Keggin
TaTPA are present when its content is ≥20 wt% suggests the attain-
ment of bulk nature of TaTPA on support.
2
Fig. 3. Pyridine adsorbed FT-IR spectra of TaTPA/SnO
0% TaTPA/SnO (c) 15% TaTPA/SnO (d) 20% TaTPA/SnO
30% TaTPA/SnO2 (g) 35% TaTPA/SnO2.
2
catalysts. (a) 5% TaTPA/SnO
2
(b)
(f)
1
2
2
2
(e) 25% TaTPA/Sno
2
2
2
bands at 1005 and 991 cm−1 were observed, which were related to
symmetric and asymmetric stretching vibrations of W = Ot of TPA
Keggin structure. The bending mode of the bridging W–O–W bonds of
the intact Keggin structure gives a Raman band at 220 cm
−1
and this
band was noticed for the present samples also [33]. The Raman analysis
indicate that with the increase in TaTPA loading on SnO , the band
intensities related to SnO were decreased and at the same time intense
bands related to heteropoly acid were noticed. These results reveal that
TaTPA Keggin ion was well preserved on the SnO support. The Raman
Fig. 2 shows the Raman spectral patterns of the catalysts. The
2
Raman spectrum of pure SnO
6
2
displayed primarily three bands at 474,
30 and 770 cm . The existence of these bands in catalysts confirms
the tetragonal structure of SnO and this observation is in concurrence
with XRD measurements [32]. In addition to these bands two more
2
−
1
2
2
spectral information was in good conformity with the results made from
FT-IR and powder XRD techniques.
FT-IR spectra of pyridine adsorbed TaTPA/SnO
in Fig. 3. In generally TPA shows typical bands at 1485 and 1530 cm
corresponding to Brønsted acid sites. TaTPA supported on SnO cata-
2
catalysts are shown
−1
2
−1
lysts displayed intense bands at 1445, 1480, 1530 and 1590 cm . The
−
1
−1
bands at 1445 cm
and 1580–1590 cm
were assigned to pyridine
−
1
adsorption on Lewis acidic sites and the band at 1485 cm
is a com-
bined band originating from pyridine bonded to both the Brønsted and
Lewis acid sites [34,35]. The results point out that Lewis acid sites were
generated after the replace of the protons of TPA with Ta metal ion.
The surface areas of the catalysts are presented in Table 1. The bulk
2
SnO
2
and TaTPA has a surface area of 15 and 5 m /g respectively. The
Table 1
The physico-chemical properties of the catalysts.
2
Catalyst
S
BET (m /g)
Acidity (mmol/g)
SnO
TaTPA
2
15
5
0.064
1.351
0.123
0.134
0.142
0.344
0.405
0.415
5
1
1
2
2
3
% TaTPA/SnO
2
16
17
18
23
21
19
0% TaTPA/SnO
5% TaTPA/SnO
0% TaTPA/SnO
5% TaTPA/SnO
0% TaTPA/SnO
2
2
2
2
2
Fig. 2. Raman spectra of TaTPA/SnO
2
catalysts. (a) 5% TaTPA/SnO (b) 10% TaTPA/
2
SnO
SnO
2
2
(c) 15% TaTPA/SnO
(g) 35% TaTPA/SnO2.
2
(d) 20% TaTPA/SnO (e) 25% TaTPA/Sno (f) 30% TaTPA/
2
2
110

P.K. Kumari et al.
Molecular Catalysis 448 (2018) 108–115
etherification of HMF was evaluated and the results are presented in
Table 2. The support SnO showed about 2% conversion of HMF. Blank
2
reaction was also performed and there was no conversion of HMF.
Unsupported TaTPA was also tested and it gave near 100% conversion
with 88.3% EMF yield. Although high HMF conversion and selectivity
towards EMF was observed for TaTPA, it was partially dissolved in
reaction mixture. In this regard, to make this catalyst heterogeneous
TaTPA was supported on support SnO
increase in TaTPA loading on SnO
conversion and EMF yield was noticed. The HMF conversion was 35%
with 30% EMF yield was observed for 5 wt% TaTPA/SnO catalyst at
20 °C after 45 min. The catalysts with 30 wt% TaTPA on SnO able to
-TPD results in-
2
. The results suggest that with
2
a substantial increase in HMF
2
1
2
convert HMF completely with 90% EMF yield. NH
3
dicated the increase in acidity as the content of TaTPA increases on
support and the 30 wt% TaTPA/SnO catalyst showed maximum
2
acidity. The HMF etherification and EMF yields also followed the same
trend as that of acidity profile of the catalysts. A linear correlation was
observed between catalysts acidity and HMF etherification activity.
Bing Liu et al. reported high 92.9% EMF yield from HMF catalyzed
3 3
by AlCl which is not recyclable [17]. Silica supported SO H catalyst
showed 83.8% of EMF yields from HMF at high reaction time of 10 h
[37] and K-10 clay-HPW catalyst exhibited 91.5% of EMF yield after
10 h of reaction [38]. Although considerable EMF yields were reported,
these catalysts took long reaction times. When compared with other
Fig. 4. Temperature programmed desorption of NH
3
patterns of TaTPA/SnO
(d) 15% TaTPA/SnO
2
(h) 35% TaTPA/SnO (i) TaTPA.
2
catalysts.
(e) 20%
(
a) SnO
2
(b) 5% TaTPA/SnO
2
(c) 10% TaTPA/SnO
(g) 30% TaTPA/SnO
2
2
TaTPA/SnO
2
(f) 25% TaTPA/Sno
2
2
catalytic systems, present catalyst 30 wt% TaTPA/SnO
EMF yields with in 45 min. The efficiency of the TaTPA/SnO
HMF etherification with high EMF yields might be due to the presence
of Lewis acidic sites which were generated with the exchange of Ta ions
in TPA.
2
showed 90% of
towards
surface areas of the supported catalysts were increased after the dis-
persion of TaTPA on SnO up to 20 wt% and thereafter a marginal
decrease was noticed. The increase in surface might be related to the
well dispersed TaTPA Keggin clusters on SnO
TPD of ammonia was carried to know the acidic strength distribu-
tion and total acidity and the profiles of TaTPA/SnO catalysts are
2
2
2
.
2
3.2.2. Effect of catalyst weight
displayed in Fig. 4. The catalysts acidity calculated from TPD are ta-
bulated in Table 1. Three types of desorption peaks in the range of
2
As the 30 wt% TaTPA/SnO catalyst showed better activity, further
the reaction conditions were optimized on this catalyst. The catalyst
weight influence on HMF etherification to EMF was studied and the
outcome from this is shown in Fig. 7. The catalyst weight main influ-
ence was on the yield of EMF. A substantial increase in conversion of
HMF was observed with incremental increase in catalyst weight from
2
00–650 °C were observed. The catalysts with low content of TaTPA on
support showed one desorption peak at 200 °C corresponds to weak
acidic sites. Apart from this low temperature peak, a strong desorption
peak in between 450 and 500 °C was also observed. This peak origi-
nated from the sites possessing moderate to strong acidity. As the
TaTPA content increased a shift in the high temperature desorption
peak to further high temperature. This guides us that the strong acidic
sites were formed as the amount of TaTPA increased on support. Bulk
TaTPA showed sharp desorption peak at 575 °C related to the sites of
strong acidity. The total acidity of catalysts increased with incremental
increase in TaTPA content from 5 to 30 wt% on support [36]. There was
no much variation in acidity beyond 30 wt% TaTPA on the support.
Field emission scanning electron microscope pictures were dis-
1
.9 to 7.6 wt% due to the available of more number of active sites. As
the catalyst weight increased the yield of EMF also increased and about
0.2% EMF yield was obtained at a catalyst amount of 3.8 wt%. When
9
the catalyst amount increased further to 7.6 wt% HMF conversion in-
creased up to 100% but the yield for EMF was decreased to 87%. The
decrease of EMF yield might be owed to the formation of ethyl levuli-
nate (EL). The reason is that with increasing catalyst weight the
availability of catalytically active sites also increased which also pro-
mote the further conversion of EMF into byproduct EL. The catalyst
weight of 3.8 wt% is optimum for this reaction.
2
played in Fig. 5. The images of 10, 20, 30, 35%TaTPA/SnO catalysts
were shown for the sake of clarity to observe the variation in the pic-
tures. The SEM images of the catalysts reveal that the active component
3.2.3. Effect of reaction temperature
TaTPA was uniformly dispersed on SnO
2
. The TaTPA clusters present on
The etherification of HMF was carried in the temperature range of
0–140 °C and the results projected in Fig. 8. HMF conversion was only
8.2% at 80 °C, and reached almost 100% conversion when the tem-
support were in below 100 nm size. At higher loading agglomerated
8
4
TaTPA particles were seen particularly in the case of 35%TaTPA/SnO
catalyst.
2
perature increased up to 140 °C. The EMF yield also gradually increased
from 44.2 to 90.2% with variation in temperature from 80 to 120 °C and
marginally decreased to 82.8% at 140 °C. These results point out that
EMF was not stable at high reaction temperatures and converted to EL.
Our results were consistent with the results reported by Balakrishnan
and co-workers [39]. Ethyl levulinate could be formed via two routes
including direct ethanolysis of EMF and ring-opening hydrolysis of HMF
followed by esterification. The formation of EL from EMF is evidenced
as the formation of EL increased with reaction temperature. Thus,
Transition electron microscope images of the catalyst samples are
presented in Fig. 6. TEM images of 20, 30, 35%TaTPA/SnO catalysts
morphology was captured. The images indicate that the particle size of
catalysts increased with increase in TaTPA content on SnO . The par-
ticle size was below 50 nm for the samples up to 30 wt% TaTPA/SnO
Further increase in TaTPA content to 35% the particle size was in-
creased. The TEM images were in support of the morphology observed
from FESEM.
2
2
2
.
120 °C is an appropriate reaction temperature for EMF formation se-
3
3
.2. Activity measurements
lectively over TaTPA/SnO catalyst.
2
.2.1. Etherification of HMF over TaTPA/SnO
The catalytic performance of TaTPA/SnO
2
catalysts
catalysts for the
3.2.4. The role of reaction time
2
The consequence of reaction time on the HMF etherificaion to EMF
111

P.K. Kumari et al.
Molecular Catalysis 448 (2018) 108–115
Fig. 5. FE-SEM images of (a) 10%TaTPA/SnO
2
(b) 20%TaTPA/SnO
2
(c) 30%TaTPA/SnO
2
(d) 35%TaTPA/SnO
2
catalysts.
was studied and the observations are depicted in Fig. 9. The conversion
of HMF and EMF yield improved with increase in reaction time. The
HMF conversion was only 53.7% with 42% EMF yield after 15 min of
reaction time. As reaction time increased to 30 min both HMF conver-
sion and EMF yield were increased. When the reaction time was 45 min
the conversion of HMF reached to 99.5% with 90.2% EMF yield. Fur-
ther increase in time over 45 min led to complete conversion and at the
same time the yield of EMF decreased. This aspect is related to etha-
nolysis of product EMF to EL. Therefore, the optimal reaction time is
characteristic of TaTPA/SnO
as well as the reaction products.
2
catalyst can favor to recover the catalyst
3.2.7. One-pot synthesis of EMF from fructose and ethanol
Although a high yield of EMF was obtained from HMF etherifica-
tion, one-pot synthesis of EMF from fructose is a most attractive pro-
cess, As the dehydration of fructose into HMF and the etherification of
HMF into EMF both are acid-catalyzed reactions, it is rational to merge
the two consecutive reactions into a one-pot reaction. Thus, EMF
synthesis from fructose was carried and the results are shown in Fig. 12.
It was noted that fructose conversion and EMF yield increased gradually
as the reaction proceeded. The conversion of fructose was only 34%
after 2 h with 20% EMF yield. As the reaction time increased to 4 h the
fructose conversion reached to 65.6% with 41.5% EMF yield. Further
enhancement in reaction time to 8 h the conversion of fructose reached
to 100% with 68% of EMF yield. Comparison of EMF yields from
fructose with that from HMF, the EMF yield from fructose was lower
than that from HMF. The results reiterate that the dehydration of
fructose into HMF might inversely promote the rehydration of HMF into
levulinic acid and the alcoholysis of HMF into EL, which also lowered
EMF yield. As the reaction time increased from 2 to 8 h, the yield of EL
was also increased from 0.5 to 16.5%. The decrease in EMF yield was
due to the formation of EL.
45 min for selective formation of EMF.
3.2.5. Effect of HMF to ethanol mole ratio
Effect of mole ratio of reactants on the etherification reaction was
carried and the results are presented in Fig. 10. It is a key parameter to
achieve high HMF conversion and EMF yield. The reaction was con-
ceded with HMF to ethanol mole ratios ranging from 1:21 to 1:65. The
conversion and yield increased with increase in the amount of ethanol.
The maximum HMF conversion and EMF yield attained at a mole ratio
of 1:43 and a marginal decreased in the conversion was observed above
this ratio. The excessive amount of alcohol may dilute the reactant and
blocking the active sites on the catalyst surface thereby decreasing the
selective etherification of HMF. The EMF yield also decreased at higher
ethanol concentrations due to the formation of other side products like
DEF (5-(diethoxymethyl)-2-furanmethanol).
Comparing the results of earlier catalysts like Ar-SO H-SBA-15 [22],
3
MCM-41 supported on TPA [18] and MIL-101-based sulfated porous
3.2.6. Reusability
coordination polymers [20] for one-pot synthesis of EMF from fructose
The reusability of catalyst is an essential characteristic in the de-
3
with present catalyst, Ar-SO H-SBA-15 catalyst exhibited EMF yield of
velopment of heterogeneous catalysts. After completion of the reaction,
the catalyst was separated by centrifugation from the reaction mixture
and washed with ethanol. Then, the recovered catalyst was dried in an
oven at 80 °C for 2 h and used for next reaction cycle under the same
63.4% at 116 °C with 8.3 vol.% of DMSO in 4 h of reaction time.
However, the use of DMSO becomes difficult in separation of reaction
products by conventional processes such as distillation, due to the
sensitivity of EMF to high temperatures. MCM-41 supported on TPA
catalysts reported 42.9% EMF yield in 24 h. MIL-101-based sulfated
porous coordination polymers showed 67.7% of EMF yields at 130 °C
conditions. As shown in Fig. 11, TaTPA/SnO
2
catalyst exhibited con-
stant activity during reuse. Furthermore, the intrinsic heterogeneous
112

P.K. Kumari et al.
Molecular Catalysis 448 (2018) 108–115
2 2 2
Fig. 6. TEM images of (a) 20%TaTPA/SnO (b) 30%TaTPA/SnO (c) 35%TaTPA/SnO catalysts.
Table 2
2
Etherification of HMF to EMF over TaTPA/SnO catalysts and their acidity values.
Catalyst
HMF conversion (%)
Yield (%)
EMF
EL
Without catalyst
0
2.0
0
0
0
11.7
1
2.5
5.8
5.9
8.4
9.0
14.7
SnO
TaTPA
2
1.8
88.3
30
46.4
68
76.1
85.5
90.2
82
100
35.0
52.5
76.0
83.0
94.5
99.5
100
5
1
1
2
2
3
3
% TaTPA/SnO
2
0% TaTPA/SnO
5% TaTPA/SnO
0% TaTPA/SnO
5% TaTPA/SnO
0% TaTPA/SnO
5% TaTPA/SnO
2
2
2
2
2
2
Reaction conditions: HMF (0.126 g), ethanol (2 g), catalyst weight (3.8 wt%), reaction
temperature (120 °C), time (45 min).
after 15 h. However, these catalysts showed high EMF yields at long
Fig. 7. Effect of catalyst weight on etherification of HMF. Reaction conditions: HMF
reaction times. While the present TaTPA/SnO
catalytic activity with 67.5% of EMF yield from fructose in ethanol at
20 °C within a reaction time of 8 h. The efficient activity of this cat-
alyst compared to MCM-41 supported on TPA is due the presence of
2
catalyst exhibited higher
(0.126 g), Ethanol (2 g), Reaction temperature (120 °C), Time (45 min).
1
113

P.K. Kumari et al.
Molecular Catalysis 448 (2018) 108–115
Fig. 8. Effect of reaction temperature on etherification of HMF.
2
Fig. 11. Reusability of TaTPA/SnO catalyst.
Reaction conditions: HMF (0.126 g), Ethanol (2 g), Catalyst weight (3.8 wt%), Time
Reaction conditions: HMF (0.126 g), Ethanol (2 g), Catalyst weight (3.8 wt%), Reaction
(45 min).
temperature (120 °C), Time (45 min).
Fig. 9. Influence of reaction time on etherification HMF.
Reaction conditions: HMF (0.126 g), Ethanol (2 g), Catalyst weight (3.8 wt%), Reaction
temperature (120 °C).
Fig. 12. Activity profiles during one-pot synthesis of EMF from fructose over 30%TaTPA/
SnO2 catalyst.
Reaction conditions: Fructose (0.180 g), Ethanol (6 mL), Catalyst weight (100 mg),
Reaction temperature (120 °C).
Lewis acidic sites generated by the presence of Ta ions.
4. Conclusions
2
Ta exchanged TPA supported on SnO catalysts was prepared with
retention of Keggin ion structure of TaTPA. Lewis acidic sites were
generated by the exchange of TPA protons with Ta ions. Etherification
of HMF depends on the acid strength distribution and total acidity of
the catalysts. The total acidity of the catalysts was related to the content
2 2
of TaTPA on SnO . The optimum loading of TaTPA on SnO was 30 wt%
for obtaining high etherification activity with 90% EMF yield at 120 °C
with in 45 min. This catalyst also showed high activity towards the one-
pot synthesis of EMF from fructose with 68% yield at 120 °C with in 8 h.
The catalyst was stable and showed consistent activity upon repeated
use. The HMF etherification activity depended on reaction temperature,
time, catalyst weight and HMF to ethyl alcohol mole ratio.
Fig. 10. Effect of HMF to ethanol mole ratio on etherification of HMF.
Reaction conditions: HMF (0.126 g), Catalyst weight (3.8 wt%), Reaction temperature
Acknowledgements
(120 °C), Time (45 min).
P. Krishna Kumari thanks to University Grants Commission (UGC),
India for financial support in the form of Junior Research Fellowship.
The authors thank Council of Scientific & Industrial Research (CSIR) for
114

P.K. Kumari et al.
Molecular Catalysis 448 (2018) 108–115
the financial support in form of Catalysis for Specialty Chemicals (CSC-
1977–1984.
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