Magnetically separable sulfated zirconia as highly active acidic catalysts for selective synthesis of ethyl levulinate from furfuryl alcohol

Article abstract of DOI:10.1039/c6gc02466a

Magnetically separable sulfated zirconia catalysts were prepared by a two-step approach. Coating of zirconia around the particles helps to increase the number of sites needed for sulfate ion loading and hence enhances the acidity of the catalyst. Different molar concentrations of chlorosulfonic acid were used for sulfonation. The prepared catalysts were used for selective synthesis of ethyl levulinate using renewable substrates: furfuryl alcohol and ethanol. Ethyl levulinate has many applications in different industries including as a potential blending component in biodiesel. The catalyst could be easily separated by the use of a magnet. The influence of different parameters was investigated to reach the optimum yield of ethyl levulinate. Detailed kinetics were established for scaling up purposes. The catalyst is robust and reusable.

Full text of DOI:10.1039/c6gc02466a

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Pleas eG dr eo e nn o Ct ha de jmu si st tmr yargins
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DOI: 10.1039/C6GC02466A
Green Chemistry
ARTICLE
Magnetically separable sulfated zirconia as highly active acidic
catalyst for selective synthesis of ethyl levulinate from furfuryl
alcohol
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Manishkumar S. Tiwari, Anil B. GawadeandGanapati D. Yadav *
www.rsc.org/
Magnetically separable sulfated zirconia catalysts were prepared by a two-step approach. Coating of zirconia
around the particles helps to increase the number of sites needed for the sulfate ion loading and hence
enhances the acidity of catalyst. Different molar concentration of chlorosulfonic acid was used for
sulfonation.The prepared catalysts were used for selective synthesis of ethyl levulinate using renewable
substrates: furfuryl alcohol and ethanol. Ethyl lavuliante has many applications in different industries
including a potenital blending componenet in biodiesel. The catalyst could be easily separated by the use of
magnet. Influence of different parameters was investigated to reach the optimum yield of ethyl levulinate. The
detailed kinetics was established for scale up purposes. The catalyst is robust and reusable.
5
value added chemicals derived from the biomass. There are
Introduction
several synthetic processes of levulinic acid production from
carbohydrates (i.e. glucose, fructose, sucrose) and utilization
Biomass derived chemicals have several applicationsin many
important industries such as pharmaceutical, fine chemical
of biomass such as starch, wood, agricultural waste and grain
1
-3
sorghum using homogeneous and heterogeneous catalysis has
intermediate and fuel additive, etc. One of the major and
6
been reported.
green parts of utilization of biomass is in the field of biofuel
4
Alkyl levulinate esters have potential uses in flavour and
fragrance industries and can be effectively used as blending
production eitheras intermediate or as additive. Continuous
efforts are expended to synthesize these fuel additives using
direct biomass or derived chemicals. However, efficient
methods and economical way of synthesis are still lagging.
The catalytic conversion of biomass/derivatives would be a
good option to achieve economical processes. Carbohydrates
and the derived molecules have a great potential to be
converted into fuels and chemicals as they are abundant and
5
-7
components in biodiesel and plasticizers, solvents, etc.
Usually these esters are synthesized by levulinic acid
esterification using different acid catalysts with alcohols, but
the high cost of levulinic acid makes the processes
6
uneconomical. Alternatively, several acid catalysed processes
have been reported to obtain alkyl levulinates with different
biomass resources such as HMF, cellulose, carbohydrates,
1
cost effective. The potential uses of levulinic acid and its
7
-9
and furfuryl alcohol with corresponding alkyl alcohols. The
basic conclusion with the use of acid catalysts in all processes
is that the amount and acidic strength of catalyst plays a vital
role to achieve better yields of alkyl levulinate. Therefore,
the right combination of a suitable feedstock and catalyst is
desirable to achieve this goal. The oversupply of furfuryl
derivatives in several industries make it one of the important
Department of Chemical Engineering, Institute of Chemical Technology, Matunga,
Mumbai-400 019, India; E-mail: gdyadav@yahoo.com, Fax: +91-22-3361-1002, +91-22-
3
361-1020; Tel: +91-22-3361-1001.
Electronic Supplementary
DOI: 10.1039/x0xx00000x
Information
(ESI)
available:].
See
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Pleas eG dr eo e nn o Ct ha de jmu si st tmr yargins
Page 2 of 18
ARTICLE
Green chemistry
make it very attractive to be used in VsiyewntAhrteicsleisOnlionef
alcohol (FAL) and its underutilization in the chemical
market, makes it a promising molecule to be converted into a
high value added products such as fuel additive i.e. alkyl
2
3, 24
DOI: 10.1039/C6GC02466A
heterogeneous catalysts.
There are several methods to prepare a magnetically
separable solid acid catalyst, either by attaching sulfonic acid
group to magnetic particle or to the mesoporous material
6
levulinates from low value added product FAL. Several
homogeneous catalysts such as mineral acids (H SO , HF and
2
4
2
3-26
H PO ) and heterogeneous solid acids and super acids
coated on the magnetic particles.
However, the acidity
3
4
catalysts, namely, zeolites, mesoporous aluminosilicates Al-
TUD-1, mixed acids, sulfated zirconia, sulfonic resins,
sulfonic acid containing ionic liquids, ion exchange resins,
and organic–inorganic hybrid acids were used for the
possessed by these catalysts is not high as compared to the
metal oxide supported acidic catalysts; also they are highly
moisture sensitive vis-a-vis sulfated metal oxides catalysts.
So far no detailed reports have been published on zirconia
coated on hydrothermally prepared magnetic particles and
their sulfonationusing chlorosulfonic acid to get the
magnetically separable sulfated zirconia.
6
,9-16
efficient synthesis of alkyl levulinate.
However, several
disadvantages associated with these catalysts such as catalyst
separation, reusability, thermal stability, activity loss, mass
transfer resistance and high cost still present formidable
challenges to be overcome. Thus, designing cheaper and
environmental benign catalysts is the solution.
In this work, we have successfully synthesized sulfated
zirconia coated on Fe O4 particle and its application to
3
synthesis of high valuable ethyl levulinate from furfuryl
alcohol. The catalyst was fully characterized by several
characterization techniques, before and after reuse. Detailed
kinetic study has been reported first time. The process is
green and novel.
Heterogeneous catalysts are supposed to play a key role
in development of environment friendly processes and
technology. Among the various solid acid and super acid
catalysts, sulfated metal oxide catalysts are more interesting
because of their high acidity and excellent thermal stability
Experimental section
and
reusability,
leading
to
different
reactor
Materials
1
7
configurations. Various metal oxides such as ZrO , TiO and
2
2
Zirconium oxychloride, aq. ammonia solution,
chlorosulfonic acid and furfuryl alcohol were bought from SD
Fine Chem. Ltd. Mumbai, India. Ferric chloride, ferrous chloride
and ethanol were purchased from Thomas Baker, Mumbai.
Catalyst Synthesis
SnO , mesoporous materials, etc. have been used for this
2
purpose, among which sulfated zirconia (SZ) has been
reported to retain more amount of sulfur as compared to other
1
8
supports and resulting in highly acidic catalyst. Sulfated
zirconia was used in several industrially important reactions
such as esterification, Friedel-Craft reactions, condensation,
The synthesis of the catalyst was divided in two parts. First,
the preparation of the magnetic particles (F) and coating of
the zirconium hydroxide on the particle was achieved to get
oligomerisation,
and many more usual acid-catalysed
1
7-22
reactions.
We have developed a novel approach of
zirconium hydroxide coated magnetic particle (Zr (OH) F)
4
preparation of high acidic sulfated zirconia named as
and second was the acid treatment to get the magnetically
separable solid acid catalyst. The synthesis of the
magnetically separable sulfated zirconia (SZF) is illustrated
in Figure 1.
1
8
19
UDCaT-5 and FLSZ with the retention of tetragonal phase
of zirconia having more than 4% of sulfur loading by using
chlorosulfonic acid as thesulfating agent. The use of
chlorosulfonic acid results in the high acidic value and
complete phase transformation of zirconia was successfully
avoided which helps to get the high activity as compared to
those using sulfuric acid treatment. Use of magnetically
separable catalyst has attracted overwhelming attention in
recent years because of the ease of separation from the
reaction mass using simple magnetic field. Also the
nonreactive nature, cheap source and ease of preparation
The magnetic nanoparticles were synthesized by
27
hydrothermal method as reported by Deng et al. It was
reported as a cheap, simple and one step method to get the
monodispersed magnetic particles. The synthesis of magnetic
particle (F) is as follow: 25 ml of ethylene glycol was taken
in beaker and to this FeCl .6H O (2.5 mmol, 0.68 g) and
3
2
NaOAc.3H O (13 mmol, 1.8 g) were added and stirred for 30
2
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ARTICLE
min to form homogeneous mixture and then transferred to the
Teflon lined bomb reactor (100 ml).
chloride. The material was quickly transferred toVieowvAernticlaetO1nl2in0e
DOI: 10.1039/C6GC02466A
°C and kept for 24 h. It was calcined at 650°C for 3 h. Three
different catalysts so prepared are designated as 0.5M-SZF,
1
M-SZF and 2M-SZF, respectively. The sulfonated Fe O
3 4
(
SF) particle was prepared by immersing the prepared
magnetic particle in 15 ml/g 1 M chlorosulfonic acid in
ethylene chloride and rest of the procedure was the same as
described above. UDCaT-5, FLSZ and Cs-DTP/K-10
1
8, 19,
catalysts was prepared by the methods reported earlier.
2
8
Zirconium hydroxide coated magnetic particle was also
calcined at 650°C to prepare ZrO @Fe O particle noted as
2
3
4
ZF.
Characterization of catalysts
2
8, 29
Catalysts were characterized by various techniques
are provided in Supplementary Information.
Reaction procedure
and details
The reactions were conducted in stainless steel 100 ml
autoclave with four blades- pitched turbine impeller and PID-
controlled heating arrangement (Amar Equipments Mumbai).
The calculated amount of reactants was fed to the reactor
along with the internal standard (3 % v/v) and desired amount
of catalyst. The desired temperature was set by using a digital
temperature setup. In a particular reaction, 0.048 mol furfuryl
alcohol, 0.72 mol ethanol and 0.3 mL n-dodecane (internal
standard) were taken with catalyst loading of 7.5 g/L. Once
the temperature reached the set value, the reaction mixture
was stirred for 1 min and initial sample was collected.
Samples were collected at specified time intervals for
analysis at the desired stirring speed. The catalyst particles
present in collected samples were magnetically separated by
external magnet and the liquid was transferred to another vial
and then analysed by GC (Chemito-1000) having a BP-1
capillary column and FID detector.
Fig.1:Synthesis of magnetically separable sulfated
zirconia (SZF)
The reactor was then transferred to muffle furnace and heated
o
to 200°C for 24 h. The reactor was cooled down to 30 C,
magnetic particles collected, washed with ethanol to remove
unreacted material and dried at 100°C for 12 h. The magnetic
particles were then subjected to zirconium coating by using
zirconium oxychloride as precursor. One g magnetic particles
were sonicated to get dispersed material in 50 ml deionized
water. Aqueous solution of zirconium oxychloride and
ammonia was then added drop wise to the dispersed magnetic
particles. The pH was maintained around 9-10 and stirred for
3
h. The solid material was collected by using external
magnet and washed several times with deionised water to get
Results and Discussion
the Zr(OH)4 coated magnetic particle (Zr(OH) -F).The
4
BET surface area analysis
AgNO test was used to confirm the complete removal of
3
The nitrogen adsorption-desorption isotherm of sulfated
catalyst and zirconia coated magnetic particle was recorded
to get surface area (S.A.) and pore volume (P.V.) of the
prepared sample (Table 1). Zirconium oxide coated magnetic
particles (ZF) and the sulfated catalysts show that the pore
diameters in the mesoporous material range (i.e. between 2
nm to 50 nm). The hysteresis loop shown in Figure 2 further
confirms that the prepared catalysts are mesoporous in nature.
The micropore volumes of all samples are almost negligible,
chloride ions. The synthesized particles were dried at 120°C
for 24 h.
The prepared magnetically separable material Zr(OH) -F was
4
then subjected to the chlorosulfonic treatment to get the
active sulfated zirconia coated on magnetic particle (SZF)
catalyst. The procedure is as follow: zirconium hydroxide
coated magnetic particle was sulfonated by immersing the
3
material in 15 cm /g of three different molar concentration
(
0.5M, 1M and 2M) of chlorosulfonic acid in ethylene
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Green chemistry
which further confirms mesoporous nature of catalyst
samples. As reported earlier; with increase in sulfur loading,
Fe O , respectively (Figure 3(a)).The coating of zirconia
3 4
View Article Online
DOI: 10.1039/C6GC02466A
results in a marginal decrease in intensity of the above peaks
which is due the coating of the zirconia on the surface of the
magnetic particle (Figure 3). Zirconia coated catalyst shows
the tetragonal phase of zirconia along with the peaks related
to the magnetic particles. The sulfonation of the zirconia
coated material results in further decrease in the intensity of
the magnetic particles related peaks. Here, it is also seen that
monoclinic phase is present along with the tetragonal phase
of zirconia due to the high sulfur loading on material. It is
also reported that above 600°C pure zirconia completely
transforms in to monoclinic phase from tetragonal phase;
2
the surface area first increases (119 m /g) up to 4 % w/w
2
sulfate content and then suddenly decreases to 71 m /g at
5.6% w/w sulfate content which is attributed to the sulfate ion
1
8,22
migration from the surface to the bulk phase.
The surface
area of ZF decreases after sulfonation. The decrease in
surface area and pore volume of the sulfated catalyst
confirms the successful grafting of sulfate ions on the
zirconia coated magnetic particle. Further catalysts treated
with different concentration of chlorosulfonic acid show that
on increasing the concentration of chlorosulfonic acid it
results in decrease in surface area and pore volume. It
indicates that more sulfate ions have been moved to the bulk
phase of the catalyst. The magnetic particles after zirconia
coating show typical mesoporous nature due to the zirconia
on the surface. Even though the sulfonation of these particles
decreases the overall surface area and pore volume of the
catalyst, it does not affect the mesoporous nature (Figure
1
7
however, the existence of the tetragonal phase as well as the
monoclinic phase of the zirconia is due to the doping of
1
7-19
sulfate ions as reported earlier.
Zirconia coated magnetic
particles (ZF) are crystalline in nature while the crystallinity
of the ZF decreases after the sulfonation with chlorosulfonic
acid.Crystallinity of ZF is 76% as calculated which decreases
to 56% after sulfonation. This loss in crystallinity is due to
the phase transformation of zirconia from tetragonal to
monoclinic after sulfonation and hence it shows low
crystallinity as compared to ZF. The sizes of the crystallites
calculated using Scherer equation were in the range of 8-17
nm with average crystallite size of 12 nm for ZF. The crystal
size increases slightly after sulfonation and found to be in
range of 10 -20 nm with average crystallite size of 15 nm.
The used catalysts and virgin catalyst show almost identical
XRD graphs which confirms that there was no alteration in
the morphology of the catalyst after the reaction and hence
catalyst is reusable.
2).Reused catalyst shows slight increase in the surface area
and pore volume which may be due to leaching of a few
sulfate ions.
Fig. 2: N adsorption-desorption isotherms of (a) ZF, (b) 1M-
2
SZF, (c) Reused 1M-SZF (d) 2M-SZF
X-ray powder diffraction (XRD)
XRD patterns of ZF and sulfated catalyst (1M-SZF) have
been recorded. The magnetic particles show distinct peaks at
2θ values of 30.2, 35.6, 43.3, 57.84, and 63.1 which are
ascribed to (220), (311), (400), (511) and (440) planes of
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Fig.3 XRD of different catalysts (a) Fe
3
O
4
particles, (b) ZF, (c)
retain by the 1M-SZF after the calcination temVpieewraAtrtuicrleeO(nil.inee.
1
M-SZF, (d) Reused 1M-SZF .
DOI: 10.1039/C6GC02466A
650 °C)
Table 1: Surface area pore volume and pore diameter properties
of different catalysts
Surface
Pore
Pore
Sr.
No
Catalyst
area
diameter Volume
2
3
(
m /g)
(nm)
(cm /g)
1
2
2
3
3
ZF
51.6
31.9
13.7
7.9
17.2
20.9
22.1
24.9
22.8
0.23
0.16
0.06
0.05
0.07
0.5M-SZF
1M-SZF
2M-SZF
Fig 4FT-IR spectra of catalysts (a) Fe O particles, (b) ZF,
Reused 1M-SZF
16.26
3
4
(
c) 1M-SZF, (d) Reused 1M-SZF.
Fourier transforms infrared spectroscopy (FT-IR)
The nature of surface retained sulfur and acidic sites
generated on the surface was studied by using FTIR
spectroscopy (Figure 4). For the magnetic particles, the
(
Figure 5).The DSC-TGA profile of Zr (OH) -F shows two
4
stages of weight loss; first loss below 200°C and second
between 200 and 550°C. The first weight loss is assigned to
the removal of surface adsorbed water while the second loss
corresponds to the terminal hydroxyl group removal which
-
1
vibration band at ν = 580 cm corresponds to Fe-O vibration.
The zirconia coated magnetic particle shows a characteristic
3
1
are bonded to zirconia surface (Figure5 a). Thereafter, it
shows very less decrease in weight, of about 2.5% and hence
it indicates that the structure is stable (up to 700°C).
Exothermic peak seen at 530 °C can be assigned to the
-
1
peak between 450 and750 cm related to zirconium oxide
and to magnetic particles present in the material. The
sulfated catalyst (1M-SZF) shows the characteristic peaks at
-
1
1
320, 1280, 1124.47, 1009.65, and 972 cm related to the
3
1
crystallization phenomena. This peak is due to amorphous
phase getting transformed in to the tetragonal phase of
-
zirconia coordinated bidentate sulfate ion (-SO ). The peak at
4
-
1
1445
cm
corresponding
to
the
presence
of
zirconia. The DSC-TGA profile of the sulfated ZrOH -F
4
polynulclearsulfates on the surface of the catalyst was not
noticed in the IR spectra which confirms the absence of the
shows a similar pattern of weight loss as seen in ZrOH -F up
4
to 600 °C. This loss in weight is because of removal of
moisture and other adsorbed species. Endothermic peak
around seen at 210 °C is due to the removal of solvent
2
-
-1
S2O7 ions. The IR peaks at 1637 cm is due to the presence
3
0
of water molecule associated with sulfate group. These
molecules are supposed to be present due to the
decomposition of chlorosulfonic acid during calcination of
catalyst and hence it confirms the absence of chlorosulfonic
acid in the catalyst and retention of sulfate ion on the surface
3
1
ethylene chloride in the sulfonation process. The amount of
sulfur retained by the material after calcination was
calculated by TGA analysis in the temperature range of 650
°
C to 700°C since the weight loss was mainly due to the
-
1
of the catalyst. The band at 3450 cm is due to the vibration
caused by the presence of the free OH group. The fresh
catalyst and used catalyst show no difference in the spectra
except a small decrease in intensity after reuse.
3
2
decomposition of sulfates and there is no weight loss seen
in the case of the zirconia coated magnetic particle. The
weight loss between 650 and 700°C is about 11% which
indicates that such high sulfur content of the catalyst was
present even after 650°C calcination temperature. Hence it is
TGA analysis
The TGA analysis of the samples helps to check the
thermal stability and to find out the amount of the sulfur
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Green chemistry
3
3
concluded that the catalyst possesses a good amount of sulfur
al. A strong desorption peak of ammonia at 630 °C is also
View Article Online
DOI: 10.1039/C6GC02466A
needed for high activity of the catalyst.
seen, which is characteristic of strong Lewis acidic sites.
0
Thus, the catalyst possesses a good amount of the strong
Lewis acidic sites. The increase in molar concentration of
chlorosulfonic acid results in enhancement of overall acidity
of catalyst (Table 3). When concentration of chlorosulfonic
acid is increased from 0.5 to 2 M the total strength of the
acidic sites increase from the 0.72 to 1.11mmol/g. The
sulfonation of magnetic particles results in very week acidic
sites generation and the overall acidity possessed by SF is
only 0.23 mmol/g. Hence, the incorporation of zirconia on
the magnetic particles helps to increase more sites for the
loading of the sulfate ions and results in increase in the
1
00
8
20
40
60
80
9
96
94
92
90
88
1
1
1
1
1
00
20
40
60
80
(a)
86
0
100 200 300 400 500 600 700
Temperature (°C)
overall acidity. The NH –TPD profiles of two well-known
3
catalysts, i.e.UDCaT-5 and FLSZ are not shown here but we
have included the acidic strength of both catalysts for the
comparison in Table 2. By comparing the overall acidity
possessed by the two catalysts with the newly prepared
magnetically separable catalyst, it is concluded that the
enhancement in the acidity of SZF catalyst is due to the
additional sites provided by zirconiaandwhich is due to the
0
0
0
0
0
1
00
2
4
6
8
95
90
85
80
75
70
100
120
140
160
180
200
core-shell structure formed around the Fe O4 particles.
3
Hence the magnetic particles not only provided the ease of
separation but also increased the acidity of catalyst as
(
b)
confirmed by NH -TPD.Reused catalyst shows almost same
3
65
desorption peaks with slight decrease in the acidity (Table 2).
0
100 200 300 400 500 600 700
Temperature (°C)
Fig.5 DSC-TGA analysis of (a) Zr(OH) -F, (b) Sulfated
4
Zr(OH) -F
4
NH -TPD
3
The NH –TPD characterization of different samples was
3
done from 100 to 650°C and the total acidity was calculated
in terms of mmol of ammonia (Table 2, Figure 6). ZF shows
a very low acidic strength which may be due to the
amphoteric nature of zirconia along with magnetic particles.
Different ammonia desorption peaks in the range of 150 -
400°C (weak acidic sites), 400-550°C (moderate acidic sites)
and > 550°C (strong acidic sites), could be found easily in
the magnetically separable zirconia catalyst after sulfonation.
The sulfate groups present in the material generate strong
Lewis and Bronsted acidic sites. The ammonia desorption
peak at 627°C in the sulfated material is due to the presence
of strongest Lewis acidic sites as reported by Barthos et
Fig. 6: NH - TPD patterns of different catalysts (a) ZF, (b)
3
SF,(c) 0.5M-SZF (d) 1 M-SZF (e) 2 M-SZF (f) Reused 1M-
SZF.
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Table 2: Acid strength distribution of different catalysts
ARTICLE
View Article Online
DOI: 10.1039/C6GC02466A
Acidity (mmol/g)
Moderate/Strong
No
Catalyst
Weak
0.2
Total
0.2
1
2
3
4
5
6
7
ZF
-----
0.12
0.54
0.74
1.01
0.23
0.34
0.73
SF
0.11
0.18
0.20
0.10
0.30
0.26
0.16
0.23
0.72
0.94
1.11
0.53
0.60
0.89
0.5M-SZF
1M-SZF
2M-SZF
UDCaT-5
FLSZ
st
8
9
1 reused 1M-SZF
nd
2
reused 1M-SZF
0.17
0.16
0.67
0.69
0.84
0.85
th
1
1
0
1
4 reused 1M-SZF
th
8 reused 1M-SZF
0.15
0.64
0.79
SEM analysis
The SEM images 1M-SZF is shown in Figure 7.The
formation of small and large sulfated particles can be easily
seen in the images. The particles are well distributed with
irregular shapes and agglomerations of particles can be easily
seen.The similarity in the external morphology of both the
reused and virgin catalysts confirms that the structure
remained intact during the reaction and after the regeneration.
Hence the catalyst is robust and reusable.
The sulfonation does not affect the external morphology.The
average particle size are in the range of 12-20 nm. The
diffraction pattern of ZF (Figure 8 (a)) confirms the
crystalline nature of material as the same in XRD, while
after sulfonation the crystallinity decreases as evidenced in
both TEM (Figure 8 (d)) and XRD. The well-defined patterns
corresponds to different phases of zirconia and magnetic
particles. The loss of crystallinity is due to the high loading
of sulfur (here, ~10%), which is reported to be the main
1
7, 22
reason for the loss in crystallinity of catalyst.
(
a)
(b)
Fig.7: SEM Images of (a, b) 1M-SZF (c) Reused 1M-SZF
TEM analysis
The TEM images of ZF and 1M-SZF reveal that there is a coating
of the zirconia around the magnetic particles and thus the
magnetic property is introduced in the catalysts (Figure 8).
Fig.8 TEM images of (a,b) ZF, (c,d) 1M-SZF
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Green chemistry
Magnetic Properties of prepared catalyst
particles. Similarly, we have evaluated thVeieweAfrtficelcetOnlionef
DOI: 10.1039/C6GC02466A
To confirm the magnetic property of the catalyst, sulfated
catalyst was dispersed in methanol taken in vials to form
brown dispersion (Figure 9). This dispersed material was then
brought into the magnetic field which was applied by using
an external magnet or magnetic needle bar and within a few
seconds the sulfated magnetic particles (SF) were completely
gathered on the wall of the vial or on the magnetic needle as
shown in Figure 9, resulting in the clear and transparent
methanol solution. This confirms the magnetic nature of
catalyst.
concentration of chlorosulfonic acid on the magnitude of
magnetism of different samples. The time for accumulation
of 0.5M-SZF, 1M-SZF and 2M-SZF catalysts were 82 , 120
and 400 s, respectively. For better understanding the
concentration was increased up to 4M and almost negligible
magnetism was observed in the sample. Hence it is
concluded that increase in concentration of chlorosulfonic
acid introduces more sulfate ions which results in decrease in
magnitude of magnetization.
Efficacy of prepared catalysts
The prepared catalysts were employed for the synthesis of the
ethyl levulinate. Furfuryl alcohol conversion to ethyl
levulinate is a two-step process, in which first furfuryl
alcohol reacts with ethanol to form intermediate and this
intermediate is further converted into the ethyl levulinate
(
Scheme 1). Therefore, overall GC yield of the ethyl
levulinate with time was considered for screening different
catalysts which included 20% (w/w) Cs-DTP/K-10
(
Cs H PW O supported K-10), FLSZ (Fuel lean sulfated
2.5 0.5 12 40
zirconia), UDCaT-5 (modified mesoporous sulfated zirconia),
Sulfated Magnetic particle (SF) and different magnetically
separable sulfated catalysts (SZF) as described above. The
yield of ethyl levulinate was calculated after 2 h ((Figure10).
Fig.9 Magnetic separation of the 1M-SZF catalysts; (a)
dispersed in Methanol and (b, c) in the presence of magnetic
force.
Further, to see the effect of zirconia loading and degree of
sulfonation on the magnitude of magnetization of prepared
samples, 3 different reaction vials were used and dispersed a
calculated amount of different materials (i.e. magnetic
particle (F), ZF and 1M-SZF) in 5 ml of methanol. Then the
dispersed magnetic particle (F) were brought in contact of the
magnetic field and within 26 s all the particles gathered at the
side wall of reaction vials. The same procedure was
employed for the other two samples and the time of
accumulation of the materials was increased up to 43 s and
20% w/w Cs-DTP/K-10 was found to be the least active for
the reaction and gave 10% yield after 2 h while the sulfated
catalyst showed better activity. The magnetically separable
sulfated zirconia catalyst shows the highest activity as
compared to the other catalysts used in the system. 1 M-SZF
shows the 96% yield of the ethyl levulinate after 2 h with
100% conversion of furfuryl alcohol at 120°C.The increase in
concentration of chlorosulfonic acid (0.5M-1M) in the
treatment results in the high yield of the ethyl levulinate
(
Figure 10). However, as the concentration of the
120 s, respectively. The magnitude of magnetization of the
chlorosulfonic acid is increased from 1 to 2 M, there is hardly
any change in the overall yield of the ethyl levulinate. 2M-
SZF has more acidity, but as shown in surface area analysis
the large amount of sulfate ions are present in the bulk of the
catalyst and not on the surface and hence limits the access of
the catalytic sites for the reaction. The reaction seems to be
coated Fe O particle decreased after the coating of
3
4
nonmagnetic material on the Fe O particle and is due to
3
4
3
4
quenching of surface moments as explained by Xu et al.
Since the magnetic particles were coated by the non-
magnetic materials such as zirconia and sulfur group, similar
explanation can also be applied here. This results into
decrease in the magnitude of magnetism and hence the non-
magnetic material coated Fe O particles (ZF and SZF) takes
dependent
strongly on the amount of acidic sites
present,which is being already been described by different
3
4
6,8,11
authors.
more time for accumulation as compared to the Fe O4
3
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Green Chemistry
ARTICLE
View Article Online
DOI: 10.1039/C6GC02466A
O
OH
EtOH, -H O
O
O
O
k1
k2
+H O
O
2
2
O
Ethyl Levulinate
Furfuryl alcohol
Ethoxymethylfuran
Scheme 1: Ethanolysis of Furfuryl alcohol to ethyl levulinate
Agitation speed effect was studied (200 to 1000 rpm) to
witness that beyond 800 rpm, there was no effect on
conversion and yield (supplementary Figure S1). To be on
safer side we have chosen 900 rpm for the further study.
To confirm it, activity of various catalysts were correlated
with the NH -TPD analysis, which shows that the increase in
3
acid strength of the catalyst results in increases in the yield of
EL. The detail discussion about the amount of acidity
possessed by these catalysts were already described in NH3-
TPD section of characterization part. Table 2shows that the
total acidity of different catalysts is in following order: SF
1
00
80
60
40
20
0
(
(
least)<UDCaT-5<FLSZ<0.5M-SZF<1M-SZF
≈
2M-SZF
max). The activity of these catalysts is also in the same
order. Hence 1M-SZF catalyst was chosen for parameter
optimization to get the best possible yield of EL.
We have also compared the activity of different
heterogeneous catalyst for synthesis of EL from LA and FAL
(
Table3). In the synthesis of EL from LA, very high
temperature and long reaction time requires to get a good
yield of EL as compared to the process reported with FAL
and hence FAL is a better choice for synthesis.Further we
have also compared activity of different heterogonous
catalyst with our prepared catalyst. Long reaction time, high
temperature, high mole ratio (more than 1:30 of FAL to
ethanol) and high cost of catalyst are the major drawback.
With our catalyst we have obtained 96% yield of EL within
Catalyst
Fig. 10Effect of various catalysts on yield of EL. Furfuryl
alcohol 0.048mol, Ethanol 0.72 mol, catalyst loading 7.5 g/
L, temperature 120°C, speed of agitation 800rpm, total
volume 0.041 L and reaction time 120 min.
2
h and with 1:15 mole ratio of ethanol and at 120°C. The
Effect of catalyst loading
results shows that prepared catalyst is better than different
reported heterogeneous catalyst in terms of activity and
selectivity of EL.
In the absence of both mass-transfer and intra-particle
resistances, increase in number of catalytic sites should
always result in a proportional increase in initial reaction rate.
Thus, the catalyst loading was changed from 2.5 to 10 g/L
Optimization of reaction Parameters
(
Figure11). The yield of EL was also increased with increase
Different parameters affecting the reaction were studied and
optimized for the highest possible yield under the specified
condition and are explained in detail.
in catalyst loading. However, an increase in the loading
beyond 7.5 g/L does not affect the overall yield of the EL as
the minimum sites needed for the reaction were provided. So
the 7.5 g/L was selected as optimum loading for subsequent
studies.
Effect of Speed of agitation
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Green chemistry
View Article Online
DOI: 10.1039/C6GC02466A
Table 3: Summary of the catalytic results obtained for the synthesis of ethyl levilunate (EL) from levulinic acid (LA)
and furfuryl alcohol (FAL) with ethanol over heterogeneous catalysts reported in literature
Substrate
R)
Mole ratio
(R: EtOH)
S. No.
Catalyst
T (°C)
X (%)
R
YEL (%)
t (h)
Ref.
(
1
2
3
4
5
6
7
8
9
.
.
.
.
.
.
.
.
.
LA
LA
SO /SnO
1:5
1:5
70
70
44
54
44
54
5
5
35
35
36
37
16
38
39
40
40
41
39
42
39
42
39
4
2
Amberlyst-15
40WD/S
LA
1:64
1:6
78
76
76
10
5
LA
H/BEA
78
40
40
LA
UDCaT-5
Zr-MOFs
HPA-ZrO2
1:20
1:15
1:51
1:30
1:30
1:6
160
78
98
98
3
LA
94
94
8
FAL
FAL
FAL
FAL
FAL
FAL
FAL
FAL
FAL
120
200
200
140
120
140
120
90
100
n.g
n.g.
100
100
100
100
140
100
48
6
2
-
SO4 /ZrO2
64.3
68.3
73
2.5
2.5
4
2
-
SO4 /TiO2
1
1
0
1
HZ-5
ZSM-5
1:51
1:57
1:51
1:57
1:51
85.8
80
6
1
1
1
1
2.
3.
4.
5.
Al-TUD-1
GO
24
6
95.5
90
Amberlyst-15
4
SO H/AC
120
89.6
6
3
[
BMIm-SH]
HSO ] IL
1
6.
FAL
~1:40
130
99
94
2
43
[
4
(
HSO -p) im]-
3 2
1
7.
FAL
1:34
110
100
95
3
6
[
HSO ] IL
4
1
8.
FAL
1M-SZF
1:15
120
100
96
2
This work
n.g.=not given
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Green Chemistry
ARTICLE
EL was noticed. Lower mole ratio between FAVLiewtAorticeltehOannlionel
1
00
DOI: 10.1039/C6GC02466A
results in polymerization of furfuryl alcohol in the presence
of strong acidic catalyst. It confirmed that the optimum mole
ratio of FAL to EL is 1:15 to get maximum yield of EL under
the specified reaction condition.
8
0
6
0
1
00
4
0
0
0
8
6
4
0
0
0
(a)
2
0
20
40
60
80
100
120
Time (min)
1
00
20
0
(a)
8
6
4
2
0
0
0
0
0
0
20
40
60
80
100
120
Time (min)
1
00
8
6
4
2
0
0
0
0
0
(b)
0
20
40
60
80
100
120
Time (min)
(b)
Fig 11:Effect of catalyst loading on (a) conversion of furfuryl
alcohol (FAL) and (b) yield of Ethyl levulinate (EL). Furfuryl
alcohol 0.048mol, ethanol 0.72 mol, temperature 120°C,
speed of agitation 900rpm, total volume 0.041 L.
0
20
40
60
80
100
120
Time (min)
Fig. 12: Effect of mole ratio of FAL to ethanol on (a)
conversion of furfuryl alcohol (FAL) and (b) yield of ethyl
levulinate (EL). Catalyst loading 7.5 g/L, temperature 120°C,
speed of agitation 900rpm, total volume 0.041 L.( ) 1:05, (
(
) 2.5 g/L, ( ) 5 g/L, ( ) 7.5 g/L, ( ) 10 g/L
×
Effect of the mole ratio
)
1:10, ( ) 1:15, ( )1:20
×
The moles of the ethanol with respect to FAL were varied in
such a way that the total volume remained constant and hence
the loading and other parameters remained the same (Figure
Effect of temperature
Under optimized reaction conditions (catalystloading 7.5 g/L,
mole ratio of furfuryl alcohol to ethanol 1:15 and speed 900
rpm), reaction temperature was varied from 100 to 130°C
12). The mole ratio of FAL to Ethanol was varied from 1:05
to 1:20 under the same reaction condition. The change in
mole ratio of ethanol with FAL enhances the rate of
formation of EL as it is changed from 1:05 to 1:15 beyond
which no distinct change in the rate and overall yield of the
(
Figure 13). In all cases the conversion of FAL is 100%, but
the conversion of intermediates to final product ethyl
levulinate is greatly dependent on the temperature. It also
confirms that the rate limiting stepis not the conversion of
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Green chemistry
FAL to intermediate (ethoxymethylfuran) but conversion of
intermediate to EL. The rate of formation of EL increases
with increase in temperature, but after 120°C there is no basic
difference in yield of EL after 2 h. Hence 120°C was
considered as optimum reaction temperature.
adsorbed products. Hence the additional regenerVaiteiwoAnrtimcleeOthnloinde
DOI: 10.1039/C6GC02466A
was employed. The recovered catalyst was further calcined at
500°C in order to get rid of any adsorbed material on the
surface. The same procedure was followed each time after the
reaction and catalyst reused for 8 times. The recovered
catalyst was made by approximately 5% (by weight) to the
original loading before the reaction. The reaction shows a
decrease in yield of ethyl levulinate from 96% to 92% after
first reuse which further decreases to 89% (second reuse) and
remains consistent as it gives 88.1% and 87.8% yield of EL
1
00
80
60
40
20
0
th
th
after 4 and 8 reuse, respectively. This decrease in
conversion can be correlated to decrease in acidity (Table 2,
entry 8, 9 and 10). Acidity of catalyst decreases significantly
nd
up to 2 reuse and remains almost constant for further
(a)
cycles.The decrease in acidity is may be due to leaching of
some active sulfate ions. The reused catalyst was further
0
20
40
60
80
100
120
characterized by XRD, FTIR, ASAP and NH -TPD. The
3
Time(min)
basic structure and morphology of catalyst was found to be
the same as shown in characterization and the acidity
possessed by the catalyst was slightly changed.
1
00
8
6
4
2
0
0
0
0
0
Leaching test of the catalyst
In the literature it is reported that the sulfate ions tend to
leach in the reaction medium and hence catalysed the system
17-19
homogeneously.
Also, in reusability test we have seen a
decrease in acidity which may be due to the leaching of some
active sulfate ions. To confirm the heterogeneous nature and
the stability of prepared catalyst, hot filtration method was
used. The reaction was stopped after 40 min as at this point
all FAL is converted to the intermediate along with a little
EL. The catalyst was separated using a magnet and the clear
reaction mass was again placed in the reaction vessel. The
reaction was further continued for next 2 h at 120°C. After 2
h reaction mixture was analysed and it was found that there
was no further conversion of the intermediates. This confirms
that there is no leaching of the active sulfate ion in the
reaction mass or the amount of sulfate ions are very low and
hence it does not results in significant change in conversion.
(
b)
0
20
40
60
80
100
120
Time (min)
Fig. 13Effect of temperature on (a) conversion of furfuryl
alcohol (FAL) and (b) yield of ethyl levulinate (EL). Furfuryl
alcohol 0.048mol, ethanol 0.72 mol, catalyst loading 7.5 g/L,
speed of agitation 900rpm, total volume 0.041 L.( ) 100°C,
(
) 110°C, ( ) 120°C, ( )130°C
×
Catalyst reusability studies
The magnetically separable catalyst was separated using
external magnet, dried for 4 h at 120 °C and used again. No
regeneration was done. Makeup of fresh catalyst was done
for any loss. The yield of EL found to be decrease
significantly from 96% to 56% after 2 h. It may be due to the
blockage of pores or active sites caused by the different
Reaction mechanism and kinetic model
Apart from EMF none of the intermediates were observed
experimentally. Overall mechanism is based on the reported
1
1, 15
literature.
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Green Chemistry
ARTICLE
View Article Online
DOI: 10.1039/C6GC02466A
Scheme2. Proposed mechanism for the formation of ethyl levulinate from furfuryl alcohol
The most feasible path is the formation of alkoxy furfuryl
alcohol and its further conversion to the alkoxylevulinate in
From the above mechanism, it is clear that water is required
for the formation of final product EL.
1
5
the presence of water. Here we have used ethanol as a
source of alcohol. The concentration profiles at optimized
condition (Figure 14 and ESI Figure S2) of the analysis
implies that first there is a formation of intermediate
To confirm it, the two different sets of the experiment were
performed. Firstly, the reaction was conducted under
optimized conditions and stopped it after 40 min because at
this time the total FAL was converted into the intermediate
along with the EL (about 48 % yield). The catalyst was
filtered out and fresh catalyst was added in order to get rid of
the water adsorbed on the surface of catalyst. Further dried
molecular sieve was added to absorb the water present in the
reaction mixture and started the reaction again. In second
experiment, to see the effect of water on the rate, different
moles of water were added in the system. In each experiment
the above method was repeated as the initial addition of water
may result in formation of levulinic acid in the acidic
condition. In first experiment one equivalent water
(approximately equivalent to the moles of EMF remaining
after 40 min i.e. 0.025 mol) was added and started reaction.
In second experiment, two equivalent of water (i.e.0.05 mol)
(
identified as EMF by using GC-MS ) which is the fast step
as compared to the second step of formation of the ethyl
levulinate from the EMF (Scheme 1). The possible
mechanism of formation of the EL is shown in Scheme 2
6
, 8, 11, 14
based on the earlier reports.
FAL reacts with ethanol to
form the intermediate EMF (i), which is then converted to
another intermediate (ii). Further, it protonates to give cyclic
oxonium (iii). The electron pair transfer in cyclic oxonium
yields exocyclic oxonium compound (iv). The exocyclic
oxonium then undergoes the ring opening reaction in the
presence of water to form the intermediate species (v) which
isomerizes to give the desired product ethyl levulinate (vi).
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ARTICLE
Green chemistry
and in the third 4 equivalent of water (i.e.0.1 mol) were
added.
The results are shown in the Figure 15. The rateVoiefwfAortricmleaOtniloinne
DOI: 10.1039/C6GC02466A
of EL got affected by the removal of water and it gave only
6
0% yield in 2 h as compared to 98% with 1 equivalent
1
1
.4
.2
1
water. The increase in yield of EL even after removal of
water is due to the presence of trace of water which could not
be removed by above mentioned procedures. Further the
addition of water greatly enhances the formation of EL from
the 48% to almost 100% within 120 min for 2 equivalents of
water while it reached to 98% with one equivalent of water
within 2 h. The addition of more amount of water also
increases the rate of formation of EL. Hence, from this study,
it is clear that the water formed in the system is responsible
for the formation of EL from the intermediate as reported
0
0
0
0
.8
.6
.4
.2
0
0
20
40
60
80
100
120
14
earlier in the formation of butyl levulinate.
Time (min)
With above discussion, we further developed a mathematical
model for reaction. As shown in Scheme 2, formation of EL
from FAL is achieved through various steps. However, all the
intermediates have not been detected. Hence we can
summarize that overall reaction in terms of single
intermediate formed (i.e. EMF) is shown in Scheme 1. Initial
analysis showed that all species are weakly adsorbed.
Fig. 14
Concentration profiles of different products with
time. Furfuryl alcohol 0.048mol, Ethanol 0.72 mol, catalyst
loading 7.5 g/L, temperature 120°C, speed of agitation
900rpm, total volume 0.041L.
(
) Conc. of FAL, (
) Conc. of EMF, (
) Conc.
of EL
00
Consider A (FAL), B (Ethanol), C (EMF), D (EL) and W
(
water).
1
The adsorption of FAL and ethanol to the catalytic surface S
is given by:
8
6
4
2
0
0
0
0
0
K
A
A S AS
(
0)
2)
K
B
B S BS
(
The surface reaction of adsorbed species AS and BS gives the
intermediate CS as
k1
AS  BS
CS WS
'
k
1
(
3)
Further the reaction between CS and WS gives the desired
0
20
40
60
80
100
120
product DS
Time (min)
k2
CS WS
DS  S
'
k
2
(
4)
Fig. 15:Influence of the water content on the yield of ethyl
The desorption step of different products can be written as:
levulinate after 40min. Furfuryl alcohol 0.048mol, ethanol
1
0
.72 mol, water (set amount) catalyst loading 7.5 g/L, Speed
K
C
CS C  S
(
5)
of agitation 900 rpm, temperature 120°C, total volume 0.041
L.
1
K
D
DS D  S
(
6)
(
) Without water, (
) with 1eqivalent water, (
) with 4 equivalent water.
)
with 2 equivalent water, (
×
As discussed above the conversion of intermediate to the EL
is slow. Therefore considering it as a rate determining step,
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Green Chemistry
ARTICLE
Table 4 Kinetic parameters evaluation of the reaction
View Article Online
DOI: 10.1039/C6GC02466A
'
1
'
2
4
4
k1X10
k2 X10
k
4
k
4
X10
X10
Sr. No
T (°C)
2
−1 −1 −1
2
−1 −1 −1
(
L mol g s )
(L mol g s )
2
−1 −1 −1
2
−1 −1 −1
(
L mol g s )
(L mol g s )
1
2
.
.
100
110
1.9
0.1
0.005
0.012
0.0006
0.0021
3.36
0.19
3
4
.
.
120
130
5.14
7.1
0.36
0.63
0.022
0.040
0.0072
0.016
'
2

k K C K C  k K C  w

2
C
C
W
W
D
D

r2 
all other steps will be in equilibrium and the intermediates
concentration can be written as:
1
 K C  K C  K C  K C  K C
A
A
B
B
C
C
D
D
W
W
(
13)
Using above equation we have calculated the different
constants for the reaction. The values of the adsorption
constants were found to be very small hence the above
equation can be reduce to:
C  K C C
AS
A
A
S
(
(
7a)
7b)
C  K C C
BS
B
B
S
'
k C C
r  k C C k C C w
1
AS BS
1
12
A
B
12
C
W


C 
(14)
15)
CS
'
k C
WS
'
1
(
7c)


r  k C C k C w
2
22
C
W
22
D


(
The rate equation for the two steps can be written as can be
written as:
Where,
'
'
'
'
2
k  k K K ,k  k K K ,k  k K K ,k  k K
'
12
1
A
B
12
1
C
W
22
2
C
W
22
D
r  k C C k C C
1
1
AS BS
CS WS
1
2
S
'
2
S
Different rate constants were evaluated and listed in Table
r  k K C K C C k K C K C C
1
1
A
A
B
B
C
C
W
W
1
(
8)
3.We can see that rate constant value of first step, i.e.
formation of ethoxy methyl furfural is greater than that for
the second step. Hence the first step is fast as compared to
second step. Also, we can see the values of backward rate
constants are very small as compared to the forward reaction.
Hence we can conclude that reaction is irreversible. The
Arrhenius plots for different steps were plotted (ESI, Figure
S3, S4) to calculate activation energy. The activation energy
values for step 1 and step 2 were found to 13.1 kcal/mol,
'
r  k C C k C C
2
2
CS WS
DS
S
2
2
'
2
r  k K C K C C k K C C
2
2
C
C
W
W
S
D
D
S
2
(
(
9)
The total catalytic site balance can be given as
C  C C C C C
T
AS
BS
CS
WS
S
10)
From the above equations, we have
CT
1
8.4 kcal/mol for forward reactions and 20 kcal/mol and 33
C 
S
1
 K C  K C  K C  K C  K C
(
A
A
B
B
C
C
D
D
W
W
11)
kcal/mol for backward reactions, respectively. The values of
the energy confirm that the reaction is kinetically controlled.
From equation 8,9, 10 and 11 we have,
'

k K C K C k K C K C  w
1
A
A
B
B
C
C
W
W

1

r1 
1
 K C  K C  K C  K C  K C
A
A
B
B
C
C
D
D
W
W
(
12)
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Green chemistry, 2016, 00, 1-3 | 15
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Pleas eG dr eo e nn o Ct ha de jmu si st tmr yargins
Page 16 of 18
ARTICLE
Green chemistry
−
1
K
adsorption equilibrium constant for (Lmol )
Conclusion
View Article Online
DOI: 10.1039/C6GC02466A
−1
M
r
mole ratio of A to B
We have successfully synthesized magnetically separable
sulfated zirconia catalysts and compared their activity for
synthesis of ethyl levulinate from furfuryl alcohol and
ethanol. 1M-SZF possesses the highest acidity and activity as
it gives 96% yield of EL with 100% conversion of FAL with
−
1
rate of surface reaction (mol L s )
water
W
w
catalyst loading(g/L)
XA
fractional conversion of A
1:15 mole ratio in 2 h. The use of magnetic particles in the
Acronyms
synthesis of catalyst helps to get the high amount of acidity
apart from its easy separation. The catalyst is separated by
using magnetic field and is recycled for 8 times and found to
be active. The detail study was carried out to optimize the
reaction conditions to get an optimum yield of ethyl
levulinate. A kinetic model was developed and two different
rate constants were calculated. The activation energy values
for different steps confirm that reaction is kinetically
controlled. The overall process is clean and green.
UDCaT-5 University Department of Chemical Technology
catalyst -5
FLSZ
F
Fuel lean sulfated zirconia
Magnetic Particles (Fe O )
3
4
ZrOH -F ZrOH Coated magnetic particles
4
4
ZF
ZrO Coated magnetic particles
2
SZF
Magnetically separable sulfated zirconia
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ARTICLE
Green chemistry
View Article Online
DOI: 10.1039/C6GC02466A
GRAPHICAL ABSTRACT
Magnetically separable sulfated zirconia as highly active acidic
catalyst for selective synthesis of ethyl levulinate from furfuryl
alcohol
Manishkumar S. Tiwari, Anil B. Gawadeand Ganapati D. Yadav *
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