Clean synthesis of alkyl levulinates from levulinic acid over one pot synthesized WO3-SBA-16 catalyst

Article abstract of DOI:10.1016/j.molcata.2016.10.032

The present work highlights the application of solid acid catalyst to produce alkyl levulinate from levulinic acid in continuous mode under vapor phase conditions. In this context, tungsten oxide incorporated SBA-16 catalysts were prepared by one pot direct synthesis method and evaluated for the titled reaction. Under optimized reaction conditions, 3 wt% WO₃-SBA-16 catalyst delivered complete conversion of levulinic acid with 95% selectivity towards ethyl levulinate. The synthesized catalysts were characterized to know the physico-chemical features by various techniques, namely, X-ray diffraction, N₂ physisorption, temperature programmed reduction of hydrogen (H₂-TPR), temperature programmed desorption of ammonia (NH₃-TPD), DR-UV–vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The characterization results suggest that, the superior catalytic activity can be ascribed due to the enhanced acidity of SBA-16 obtained through incorporation of tungsten oxide and easy of accessibility for the dispersed active sites through uniform pore channels. The constant catalytic activity in 10 h time on study shows the sturdiness of the catalyst and the spent catalyst can be regenerated several times. Moreover, various alkyl levulinates (methyl, n-propyl, and n-butyl) were synthesized with more than 90% selectivity over this catalyst.

Full text of DOI:10.1016/j.molcata.2016.10.032

Accepted Manuscript
Title: Clean synthesis of alkyl levulinates from levulinic acid
over one pot synthesized WO3-SBA-16 catalyst
Author: Enumula Siva Sankar Gurram Venkata Ramesh Babu
Chada Raji Reddy Burri David Raju Kamaraju Seetha Rama
Rao
PII:
S1381-1169(16)30455-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.10.032
MOLCAA 10094
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
23-7-2016
29-9-2016
29-10-2016
Please cite this article as: Enumula Siva Sankar, Gurram Venkata Ramesh Babu, Chada
Raji Reddy, Burri David Raju, Kamaraju Seetha Rama Rao, Clean synthesis of alkyl
levulinates from levulinic acid over one pot synthesized WO3-SBA-16 catalyst, Journal
of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.10.032
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Clean synthesis of alkyl levulinates from levulinic acid over one pot
synthesized WO3-SBA-16 catalyst
Enumula Siva Sankar, Gurram Venkata Ramesh Babu , Chada Raji Reddy, Burri
David Raju, Kamaraju Seetha Rama Rao *
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,
Hyderabad, India-5000071. E-mail: ksramarao@iict.res.in & ksramarao.iict@gov.in
Fax: +91-40-27160921; Tel: +91-40-27193163
Graphical abstract
Esterification of Levulinic acid with alcohols over one pot synthesized WO3-SBA-16 catalyst in
continuous process at atmospheric pressure.
1

Highlights
 Retainment of mesoporous nature after incorporation of WO3 during the preparation of
SBA-16
 Doping WO3 into SBA-16 increases the acidity
 Among the catalysts, 3 weight % WO3-SBA-16 is optimum for levulinic acid
esterification
 3 weight% WO3-SBA-16 catalyst exhibits steady activity during time on stream
operation for 10 cycles
Abstract:
The present work highlights the application of solid acid catalyst to produce alkyl levulinate
from levulinic acid in continuous mode under vapor phase conditions. In this context, tungsten
oxide incorporated SBA-16 catalysts were prepared by one pot direct synthesis method and
evaluated for the titled reaction. Under optimized reaction conditions, 3 weight % WO3-SBA-16
catalyst delivered complete conversion of levulinic acid with 95% selectivity towards ethyl
levulinate. The synthesized catalysts were characterized to know the physico-chemical features
by various techniques, namely, X-ray diffraction, N2 physisorption, temperature programmed
reduction of hydrogen (H2-TPR), temperature programmed desorption of ammonia (NH3-TPD),
DR-UV-visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy, transmission
electron microscopy (TEM) and scanning electron microscopy (SEM). The characterization
results suggest that, the superior catalytic activity can be ascribed due to the enhanced acidity of
SBA-16 obtained through incorporation of tungsten oxide and easy of accessibility for the
dispersed active sites through uniform pore channels. The constant catalytic activity in 10 h time
2

on study shows the sturdiness of the catalyst and the spent catalyst can be regenerated several
times. Moreover, various alkyl levulinates (methyl, n-propyl, and n-butyl) were synthesized with
more than 90% selectivity over this catalyst.
Key words: Levulinic acid, Esterification, Tungsten oxide, SBA-16
1. Introduction
At present the World’s energy demand is alarming to cater the needs of upcoming
generations, because the petro industry is producing fuels and chemicals from the fossil
resources in an uninterrupted way. In this scenario, the production of bio based fuels has
attracted much attention to overcome the reliance on depleting fossil resources [1-2]. However,
the production of bio-fuels from starch and edible oil was associated with complications such as
their feedstock availability would be competitive with food supply and undesirable effects on
land requirements creating food versus fuel problem. Now the present ongoing research activities
have been switched over to the production of bio-fuels from renewable, abundant, non-food
lignocellulosic biomass [3-4]. The lignocellulosic biomass (composed of 40–50% cellulose, 16–
33% hemicelluloses and 15–30% lignin) identified as source of carbon raw materials for the
synthesis of variety of compounds [5]. The carbohydrate fractions of lignocellulosic biomass are
hemicelluloses and cellulose which can be converted into fuels, fuel additives and chemicals in
multiple steps [6-8]. In those conversions, esterification of levulinic acid is a promising route to
synthesize alkyl levulinates such as methyl, ethyl, propyl, and n-butyl levulinates. In this
particular conversion, the reactant, levulinic acid (LA) was identified as one of the top most
platform chemical that can be obtained from lignocellulosic biomass and the products alkyl
levulinates were found to be having potential applications in transportation fuel sector,
3

production of fine chemicals, as plasticizers, as precursors in flavoring and fragrance industries
[9-11]. Among these alkyl levulinates, ethyl levulinate (EL) has been enticed due to its
miscibility with gasoline and diesel fuels with attributing modifications such as high lubricity,
stable flash point, decrease in sulfur content, flow properties under cold conditions. EL can be
added up to 5 wt% as an additive in the diesel fuel directly. Recent studies suggest that EL can
be used as blending agent for biodiesel. Upon the addition of 20 vol % EL to bio-diesel derived
from cottonseed oils, the fuel properties of the bio-diesel were improved [12-13]. Owing to
versatile applications of EL, its synthesis would be fascinating. Conventionally, the esterification
of LA with alcohols was carried out by using mineral acids creating environmental issues,
recyclability problems, and need of neutralization after the reaction, made these processes as
insignificant. Several contemporary works have been reported to develop greener approaches for
the production of EL over solid acid catalysts such as supported or modified hetero poly acids,
sulfated metal oxides, and zeolites with inspiring activities [14-18]. However, most of these
reactions are employed in batch conditions. Synthesis of EL in continuous process is
advantageous with regard to avoiding neutralization after the reaction, ease of catalyst
separation, and reaction continuation for longer hours. Tungsten oxide, based catalysts have been
reported in numerous acid catalyzed reactions like isomerization of alkenes,
hydrodesulphurization and hydro cracking of heavy fractions in the petroleum chemistry [19-20].
Mesoporous silica (SBA-16) having 3D cubic arrangement with cage like structure possessing
unique characteristics like thermally stable pore walls, high surface areas and pore volumes to
disperse the active species, has been regarded as a suitable support for various applications
including catalytic conversions of biomass derived platform chemicals to value added products
[21-24]. The doped mesoporous silica, SBA-16 materials are emerging as the desired
4

physicochemical properties of the catalysts can be achieved in one pot direct synthesis thereby
decreasing the multistep process in the catalysts synthesis [25-26]. In our previous work aiming
for the synthesis of alkyl levulinates over ZrO2/SBA-15 catalyst, formation of gamma-
valerolactone was observed with alcohols like ethanol, propanol, and butanol. The formation of
gamma-valerolactone via catalytic transfer hydrogenation (MPV reduction) decreases the
selectivity to alkyl levulinates such as ethyl, propyl, and butyl levulinates [27]. Hence, in order to
obtain maximum selectivity to alkyl levulinates development of catalysts which do not promote
the catalytic transfer hydrogenation of levulinic acid with alcohols like ethanol, propanol and
butanol is necessary. In this scenario an attempt has been made to prepare tungsten oxide
incorporated SBA-16 catalysts with various loadings by means of one pot direct synthesis
method for the selective synthesis of alkyl levulinates from levulinic acid in a continuous process
at atmospheric pressure in a fixed bed reactor.
2. Experimental
2.1. Catalyst preparation
SBA-16 mesoporous silica and tungsten oxide doped SBA-16 materials were prepared
according to literature described elsewhere by using Pluronic F127 triblock polymer
(EO106PO70EO106, called F127, M/s. Sigma Aldrich chemicals, USA) as structure directing agent,
tetra ethyl ortho silicate (TEOS, M/s. Sigma-Aldrich chemicals, USA) as silica source, sodium
tungstate as tungsten oxide precursor and n-butanol as co-surfactant [26,28]. In a characteristic
synthesis 3.5 g of F127 dissolved in 175 mL of 0.4M HCl solution at 45 °C, then 13 mL of n-
butanol was added with stirring. After 1 h stirring, 16.7 g of TEOS and requisite amount of
sodium tungstate were added to the mixture and the stirring was continued for 20 h. Then it is
kept under static hydrothermal treatment at 100 °C for 24 h. It was filtered and dried at 100 °C
5

for 12 h and then dried solid was calcined at 550 °C for 6 h in air flow. The resultant solid was
labeled as XWS where X indicates the weight percentage of tungsten oxide incorporated in SBA-
16 and tungsten oxide alone was prepared in a conventional precipitation method. For
comparison purpose 3 wt% WO3/SBA-16 catalyst was prepared by impregnation method and
labeled as 3WS-I.
2.2. Characterization of catalysts
The X-ray diffraction patterns were recorded on an Ultima-IV (M/s. Rigaku Corporation,
Japan) operated at 40 kV and 40 mA equipped with nickel-filtered Cu Kα radiation
(λ =1.54056 Å). The BET surface areas, pore volume and pore sizes were determined by using
the N2 physisorption-desorption studies (M/s. Quantachrome Instruments, USA, samples were
degassed at 150 °C for 2 h) by nitrogen adsorption at liquid N2 temperature. The UV–Vis spectra
were recorded on a UV-2000, (M/s Shimadzu, Japan) spectrophotometer with BaSO4 as a
reference material. The temperature programmed desorption of ammonia (NH3-TPD) was
performed as follows, Approximately, 50 mg of catalyst sample was placed in a quartz reactor
and pretreated in a flow of helium (30 mL min-1) at 400 °C for 2 h. Subsequently, the catalyst
sample was saturated with 10% NH3/He (30 mL min-1) at 80 °C for 30 min. then the helium
was allowed to flow for 30 min at 100 °C for complete removal of physisorbed ammonia.
Helium flow was continued while increasing the temperature to 500 °C at a ramp of 10 °C min−1
and the desorbed NH3 was monitored with on-line connected thermal conductivity detector
(TCD) equipped GC. Temperature programmed reduction of hydrogen (H2-TPR) was performed
on a homemade TPR system. Typically, About 50 mg of the catalyst was placed in a quartz
reactor and pre-treated in Helium flow (30 ml min-1) at 300 °C for 1 h. Later, the catalyst was
treated with 5% H2–Ar mixture gas (30 ml min-1) while increasing the temperature up to 1000 °C
6

with a temperature ramp of 10 °C min-1 and it was monitored using a thermal conductivity
detector. The FTIR spectra were recorded on a spectrum GX spectrometer (M/s. Perkin-Elmer,
Germany) in the scan range of 1400–400 cm-1. The pyridine adsorption studies were carried out
in the diffuse reflectance infra red Fourier transform (DRIFT) mode. In the typical experiment
the catalyst was degassed under vacuum at 200 °C for 3 h followed by suspending dry pyridine.
Then, the excess pyridine was removed by heating the sample at 120 °C for1 h. After cooling the
sample to room temperature, FT-IR spectra of the pyridine-adsorbed samples were recorded.
SEM images of the catalysts were recorded on a scanning electron microscope (M/s. JEOL,
Switzerland) and TEM images were obtained on a JEM 2000EXII apparatus (M/s. JEOL,
Switzerland) respectively. Prior to TEM analysis, the catalyst sample was ultrasonicated in
ethanol and a drop was placed onto the carbon coated copper grid, the solvent was then dried in
an air oven at 80 °C for 6 h.
2.3. Catalytic activity
The catalytic activity tests were conducted in fixed bed down flow reactor (14 mm id and
300 mm length) at atmospheric conditions. In a typical experiment about 0.5 g of the catalyst
was mixed with same amount of quartz particles and sandwiched between two plugs at the centre
of the reactor. Before the catalytic run, the catalyst was preheated in N2 flow (30 mL min−1) at
250 °C for 1 h. The liquid feed with required molar ratio of levulinic acid and alcohol was fed at
a flow rate of 1 ml/h using syringe feed pump (M/s. B. Braun, Germany). The product mixture
collected from an ice cooled trap was analyzed by FID equipped GC-17A (M/s. Shimadzu
Instruments, Japan) with EB–5 capillary column (30 m x 0.53 mm x 5.0 µm) and confirmed by
GC-MS, QP-2010 (M/s. Shimadzu Instruments, Japan) with EB–5 MS capillary column (30 m x
0.25 mm x 0.25 µm).
7

3. Results and discussion
3.1. Catalyst characterization
The powder X-ray diffraction patterns of the SBA-16 and WO3-SBA-16 samples were
shown in Figure 1. From low angle XRD patterns, it can be observed that SBA-16, and tungsten
oxide incorporated SBA-16 samples have retained their 3D pore arrangement, by exhibiting
three diffraction peaks at 2θ values around 0.8°, 1.0° and 1.1° corresponding to the (110), (200)
and (211) planes respectively, of regular mesoporous 3D cubic pore (Im3m ) structure [28,29].
The d
and unit cell parameter (ao) values presented in Table 1 indicates the evenness of
(110)
mesoporous 3D cubic structure even after tungsten oxide incorporation into SBA-16. However,
there is a shift of (110) reflection to lower angles in the low angle XRD patterns of the tungsten
oxide incorporation which causes increase in the unit cell parameters of the WO3-SBA-16
samples. It can be attributed to the incorporation of tungsten having higher atomic radius
compared to the Si+4 frame work of SBA-16. These results suggest the tungsten oxide
incorporation into SBA-16 frame work [30, 31, 34]. Wide angle XRD patterns (Figure 1 inset)
SBA-16 and WO3-SBA-16 samples show a broad peak in the 2θ region of 15-30° which is
characteristic peak for amorphous silica. No crystalline peaks of WO3 are found in the tungsten
oxide incorporated SBA-16 samples designating the existence of amorphous form or highly
dispersed WO3 species in the SBA-16 [26, 32]. From the EDX analysis, the presence of tungsten
can be observed and the respective weight percentages are presented in Table 2.
The N2 adsorption-desorption isotherms and pore size distributions of the samples were
depicted in Figure 2. From the N2 sorption isotherms, all the samples, i.e., SBA-16 and tungsten
oxide incorporated SBA-16 display type IV isotherm with H2-type hysteresis loop representing
8

the mesoporous materials with cage like pore structure [26, 28, 33]. For all the samples, capillary
condensation was observed within the P/P0 range of 0.5-0.8 which is characteristic feature for
῀
the mesoporous materials. This clearly confirms the intactness of mesoporous structure of the
SBA-16 even after tungsten oxide incorporation. The shift in inflection point of isotherms
towards higher relative pressure in tungsten oxide incorporated SBA-16 samples compared to
SBA-16 was observed due to the incorporation of WO3 on the walls of mesopores of SBA-16
[34]. Increase in pore diameter of the WO3-SBA-16 samples can be noticed from the NLDFT
calculations. The pore size distributions of the samples (Figure.2 inset) exhibit a bimodal pore
size distribution with an average pore size of ῀ 4-5 nm which is characteristic feature of cage type
mesopores. [26, 35]. The textural and structural parameters of the samples are presented in Table
1. The surface area and pore volumes of the tungsten oxide incorporated samples decreases with
increase in the amount of tungsten oxide. Even though, a gradual or slight decrease in surface
area per gram catalyst exists, surface area per gram support remains more or less same with
increase in WO3 loading which can be attributed to the incorporation of WO3 in the SBA-16
framework.
SEM and TEM images of the SBA-16 and 3WS samples were depicted in Figure 3. The
morphology of the SBA-16 and 3WS samples show agglomerated spheres like morphology
typical of reported SBA-16 materials [36, 37]. It can be observed that 3WS sample is also
maintaining the sphere like morphology even after tungsten oxide incorporation in SBA-16.
TEM images display the well ordered cubic pore structure in the SBA-16 and tungsten oxide
incorporation in SBA-16 sample [38, 39]. The pore channel width of the SBA-16 and 3WS are ~
9.1 nm and ~10.5 nm respectively. The results from the X-ray diffraction, N2 physisorption and
9

electron microscopic images suggest the maintenance of 3D mesoporous cubic arrangement in
the SBA-16 and tungsten oxide incorporated SBA-16 samples.
Diffuse reflectance UV-visible spectra of WO3-SBA-16 samples and bulk WO3 were
presented in Figure 4. WO3-SBA-16 samples show two absorption bands at around 218 nm and
265 nm. The ligand to metal charge transfer in isolated [WO4] tetrahedral species results the
absorption band at 218 nm, indicating the incorporation of tungsten oxide species in the frame
work of SBA-16 [30, 40-42]. The absorption band around 265 nm is due to the charge transfer
from O2- to W6+ in an octahedrally coordinated tungsten oxide species or isolated tungsten
species [41-42]. The absorption band at around 377 nm in the bulk WO3 designates the presence
of tungsten oxide crystallites, which is absent in the WO3-SBA-16 samples, clearly manifesting
the absence of crystalline WO3 in these samples. These results are parallel with the wide angle
XRD patterns of the WO3-SBA-16 samples.
FT-IR spectra of SBA-16 and WO3-SBA-16 samples were shown in Figure 5. All the
samples display characteristic vibration bands at around 468, 806 and 1084 cm-1, representing
typical vibration bands for Si-O-Si structural units [43, 44]. The vibration band at 1084 cm-1 with
a shoulder at 1245 cm-1 can be ascribed to asymmetric stretching and the band at 806 cm-1 is due
to the symmetric stretching modes of Si-O-Si units [43, 44]. The typical vibration band at 968
cm-1 is due to the stretching vibration mode of Si-O-W in the tungsten oxide incorporated SBA-
16 samples. As well, the vibration band at 959 cm-1 in pure SBA-16 can be attributed to the Si-O
stretching of Si-OH groups. The introduction of transition metal atoms in the frame work of
silica can change the Si-O stretching vibration mode [43]. The slight shift of the vibration band
at 959 cm-1 towards higher wave number in the tungsten oxide incorporated SBA-16 samples
indicates the presence of tungsten in the frame work of SBA-16 silica [45, 46].
10

The temperature programmed reduction (H2-TPR) patterns of tungsten oxide
incorporated SBA-16 samples were presented in Figure 6. These patterns show that bulk WO3
possesses three reduction peaks at around 650, 730 and 880 °C. The high temperature reduction
peaks indicate the reduction of tungsten oxide species in tetrahedral coordination and the smaller
reduction peak at 650 °C is due to the reduction of supported tungsten oxide crystallites [47-49].
The tungsten oxide incorporated SBA-16 samples show slightly higher reduction temperatures
than the bulk tungsten oxide. The 5WS sample exhibits two reduction peaks at 750 and 890 °C.
The TPR patterns of 1WS and 3WS show a shift in the reduction temperature to ~915 °C
representing the high dispersion of tungsten oxide species [47]. The increase in the reduction
temperature may be due to the reduction of highly dispersed tetrahedrally coordinated tungsten
species or due to the reduction of frame work tungsten oxide species which are interacted with
the silica [49, 50, 26].
The temperature programmed desorption of ammonia (NH3-TPD) profiles of tungsten
oxide incorporated SBA-16 samples were presented in Figure 7 and the respective total acidity
values are summarized in Table.2. The acidic sites distribution can be categorized depending
upon the desorption temperature of ammonia as weak (< 250 °C) moderate (250-400 °C) and
strong acidic sites (> 400 °C) [51]. From the NH3-TPD profile, the SBA-16 doesn’t show any
appreciable desorption of ammonia. Whereas the tungsten oxide incorporated SBA-16 samples
show desorption peak in the temperature region of 140-280 °C representing the presence of more
population of weak acidic sites in the samples. It can be observed that, the incorporation of
tungsten oxide into the frame work of SBA-16 leads to the development of higher number of
weak acidic sites. The total acidity of the samples is increasing with the increase in amount of
tungsten oxide incorporation. Whenever high amount of tungsten oxide incorporated in the frame
11

work the total acidity decreases due to the formation of crystalline tungsten oxide [26, 34, 47]. In
the present investigation, the increase in WO3 loading is clear indication of the presence of WO3
within the frame work.
The pyridine adsorbed FT-IR analysis was performed to access the type of acidic sites
present on the catalyst surface and their respective patterns were presented in Figure 8. The pure
SBA-16 sample possess two adsorption bands at 1446 and 1595 cm-1 representing the presence
of coordinately bonded pyridine to the surface Lewis acidic sites [53, 54]. In addition to these
two bands the tungsten oxide incorporated SBA-16 samples exhibit the bands at 1548 and 1630
cm-1 showing the presence of Brønsted acidic sites and one more band at 1489 cm-1 result due to
the combination of Lewis and Brønsted acidic sites on the catalyst surface [47, 53, 54]. It can be
observed that, the incorporation of tungsten oxide in the SBA-16 tends to increase the surface
Lewis acidic sites as well as surface Brønsted acidic sites thereby increasing the total acidity of
the tungsten oxide incorporated SBA-16. It is obvious from the figure that tungsten oxide
incorporated SBA-16 samples exhibit a higher number of Lewis acidic sites compared to
Brønsted acidic sites.
3.2. Catalytic activity studies
3.2.1. Effect of WO3 loading
The influence of tungsten oxide loading on the esterification of levulinic acid was studied
in order to know the optimum loading of tungsten oxide and the results were presented in Figure
9. The esterfication of LA with ethanol (EtOH) over pristine SBA-16 and WO3 results lower
conversions of LA 39% and 73% with selectivity’s to ethyl levulinate (EL) are 74% and 72%
respectively. The incorporation tungsten oxide into the frame work of mesoporous SBA-16
12

shows greater selectivity to EL due to enhanced acidity obtained through the dispersed acidic
sites of incorporated tungsten oxide over larger surface areas of SBA-16. The conversions of LA
and selectivity to EL increases with increasing amount of tungsten oxide. The conversions of LA
over 1WS and 3WS are 92% and 100% with selectivity’s to EL are 78% and 95% respectively.
Further increase in the amount of tungsten oxide (5WS) results no considerable change in the
selectivity to EL. The acidity of 3WS catalyst is enough to catalyze the esterification of LA to
yield EL with maximum selectivity. The high catalytic performance of these catalysts signifies
the easy accessibility of the active acidic sites in the mesostructured tungsten oxide incorporated
SBA-16 catalysts. The formation of levulinate ester and angelica lactones may proceed through
the probable mechanism as shown in Scheme 1 and 2 respectively. As evident from the pyridine
adsorbed FT-IR analysis the tungsten oxide incorporated SBA-16 catalysts posses both the Lewis
acidic sites as well as Brønsted acidic sites which both can catalyze the esterification reaction
[55, 56]. In the Lewis acidic sites catalyzed esterification reaction (Path I), the adsorption of
carbonyl group of the carboxylic acid in LA on the Lewis acidic site takes place increasing the
electrophilicity at the carbonyl carbon. The Lewis acidity originates from the O=W=O of the
tungsten oxide incorporated SBA-16 [44, 47]. The nucleophilic attack of the oxygen atom of the
alcohol on the carbonyl carbon followed by the dehydration leads to the formation of levulinate
ester. In the Brønsted acidic sites catalyzed esterification reaction (Path II), the adsorption of
carbonyl group of the carboxylic acid of the LA on the Brønsted acidic sites which are originate
from W- OH of the tungsten oxide incorporated SBA-16 leads to the generation of electrophilic
carbonyl carbon [44, 47, 55]. The subsequent nucleophilic attack followed by dehydration step
forms the levulinate ester. In the present case, the most of the levulinate ester is produced by path
I as the number of Lewis acidic sites are dominant compared to Brønsted acidic sites. The
13

formation of angelica lactones takes place by either Lewis acidic sites or Brønsted acidic sites
[57, 58]. The adsorption of carbonyl group (keto) of LA instead of carbonyl group of carboxylic
acid in LA on Lewis acidic site or Brønsted acidic site creates an electrophilic centre at the
carbonyl carbon (keto). The intra molecular nucleophilic attack of the oxygen atom of the
carboxylic acid group in LA and subsequent dehydration leads to the formation of angelica
lactones.
3.2.2. Effect of molar ratio
The effect of molar ratio of LA to ethanol on the selectivity to EL was studied over 3WS
catalyst and results were depicted in Figure 10. The LA to ethanol molar ratio varied from 1:1 to
1:11. The selectivity towards EL increases with increase in molar ratio of LA to ethanol from 1:1
to 1:7 and reaches a maximum selectivity of 95% at a molar ratio of 1:7. Further increase in the
concentration of ethanol results gradual decrease in selectivity to EL from 95% to 87% (at molar
ratio of 1:11). These results can be ascribed as the esterification reaction of LA with ethanol is a
reversible reaction, high amount of alcohol is necessary to favor the forward reaction. However,
an excess amount of alcohol may dilute the reactant there by decreasing the selectivity to the EL
due to mass transfer limitation. Hence, it can be suggested that maximum selectivity to EL can
be achieved in this reaction with an optimum molar ratio of 1:7 (LA: ethanol).
3.2.3. Effect of temperature
The effect of temperature ranging from 250 °C (near to the boiling point of LA [27]) to
310 °C on the esterification of LA with ethanol was studied on 3WS catalyst and the results were
depicted in Figure 11. At all the temperatures the conversion of LA is found to be complete, with
a variation in the selectivity to EL. As the temperature increases from 250 °C to 310 °C, the
14

selectivity to EL decreases due to the formation of angelica lactones through dehydration of LA.
These results suggest that the favorable temperature for this esterification of LA is 250 °C.
3.2.4. Influence of Time on stream
A time on stream study was conducted over 3WS catalyst under optimized reaction
conditions and the results were illustrated in Figure 12. The results indicate stable catalytic
activity of the 3WS catalyst during 10 h time on stream study. The conversion of LA is constant
during the time of study, while the selectivity to EL decreases slightly. For the sake of
comparison, a WO3/SBA-16 catalyst with 3 weight % WO3 prepared by impregnation method
(3WS-I) has been subjected for the time on stream study for the levulinc acid esterification with
ethanol and the results are shown in Figure 13. This catalyst experiences a loss in the conversion
of levulinic acid from 91 % (initial conversion) to 72 % at the end of 10th hour. Continuous
decrease in the selectivity towards ester (EL) has been observed over this catalyst. Thus 3WS
maintains steady activity compared to the catalyst with same WO3 loading prepared by
impregnation method. Interestingly, the spent 3WS catalyst was regenerated by performing
calcination in flow of air after completion of each cycle. The regenerated catalysts exhibit similar
catalytic activity in the esterification reaction of LA with ethanol during 10 h time on stream of
10 cycles (Figure 12). The 3WS could be regenerated for 10 cycles. The conversion of LA is
consistent while the selectivity to EL is slightly decreased from 88% to (initial selectivity at first
hour) to 77 % (10th hour) at the end of the 10th cycle. The slight decrease in the catalytic activity
of the catalyst may be due to the poisoning of catalyst surface by water molecules released
during the esterification reaction or due to the deposition of carbonaceous species over the
catalyst surface. The poisoning of catalyst surface by water molecules is obvious in the case of
catalysts containing conventional supports like hydrotalcites, magnesia etc. The hydrophobic
15

nature of the mesoporous silica helps to maintain the catalytic activity in water released catalytic
reactions [59]. In the present study, tungsten oxide incorporated mesoporous silica (SBA-16)
resists the water molecules to adsorb on the catalyst surface. Hence, there is no scope for the
poisoning of catalyst surface by water molecules released during the reaction. The spent catalyst
was characterized to know the reason for the slight decline in the catalytic activity of the 3WS
catalyst in the esterification reaction. It could be observed that there is no change in the small
angle and wide angle XRD patterns of the catalysts (results were not shown), indicating the
retention of mesoporous 3D structure of 3WS catalyst. The total acidity of the fresh 3WS catalyst
(0. 23 mmol NH3 g-1) decreases to (0.15 mmol NH3 g-1) after 10th cycle (10th hour) and it regains
to similar extent (0.19 mmol NH3 g-1) after calcination in flow of air. The decrease in acidity of
the 3WS catalyst after the reaction is due to the deposition of carbonaceous species over the
catalyst surface. The regaining acidity of the regenerated catalyst is due to the removal of
carbonaceous species during the calcination in flow of air. Further, CHNS analysis shows the
presence of carbon in the spent catalyst, supporting the deposition of carbonaceous species. From
these investigations, the slight decrease in the catalytic activity is due to the deposition of
carbonaceous species over the catalyst surface.
3.2.5. Influence of different alcohols
The catalytic performance of the 3WS catalyst in the esterfication of LA with different
alcohols was tested to know the efficiency of the catalyst and the results were illustrated in
Figure 14. The 3WS catalyst shows superior catalytic performance in the selective synthesis of
alkyl levulinates from LA with methanol (MeOH), ethanol (EtOH), n-propanol (PrOH), and n-
butanol (BuOH). The complete conversion of LA is observed with corresponding yields of the
alkyl levulinates are 96, 95, 94 and 94 % respectively. A decrease in the conversion of LA (93
16

and 64 %) and yields of the alkyl levulinates (61 and 13 %) are noticed with isopropanol (2-
PrOH) and isobutanol (2-BuOH) respectively. The drop in activity with secondary alcohols may
be due to steric hindrance on hydroxyl group of the alcohol, which reduces the nucleophilic
attack of the hydroxyl group of alcohol on carbonyl carbon of the carboxylic acid group in LA.
4. Conclusions
In conclusion, tungsten oxide incorporated SBA-16 catalysts were successfully
synthesized in one pot direct synthesis method and applied to the esterification of LA with
alcohols in continuous process at atmospheric pressure. X-ray diffraction, N2 physisorption
suggest the retention of 3D mesoporous structure in the tungsten incorporated SBA-16 catalysts.
SEM and TEM techniques show the morphological aspects in the investigated catalysts. DR-UV-
vis spectroscopy, temperature programmed reduction, EDX analysis show the presence of
tungsten in the synthesized catalysts. The temperature programmed desorption of ammonia
demonstrated the acidity of the catalysts. Under optimized reaction conditions, 3WS catalyst
exhibits highest selectivity (95%) to EL with complete conversion of LA. The superior catalytic
performance of the catalyst can be assigned to the enhanced acidity of SBA-16 due to the
incorporation of tungsten oxide, possessing well dispersed accessible acidic sites through
uniform pore channels. The 3WS catalyst shows stable catalytic activity during the 10 hours time
on stream study for 10 cycles. The slight decline in the selectivity to EL is due to the deposition
of carbonaceous species over the catalyst surface. The selective formation of alkyl levulinates
(methyl, ethyl, n-propyl, and n-butyl, more than 90%) designates the excellence of the 3WS
catalyst in the esterification of LA with alcohols in continuous process.
17

Acknowledgements
The authors E.S.S, G.V.R.B and C.R.R gratefully thank University Grant Commission, New
Delhi, for financial support.
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22

Figure captions
Figure 1 Low angle XRD patterns of pristine SBA-16 and WO3- SBA-16 samples and in inset
figure wide angle XRD patterns of WO3- SBA-16 samples
Figure 2 N2 adsorption-desorption isotherms for pristine SBA-16 and WO3- SBA-16
samples and in inset figure pore size distribution of pristine SBA-16 and
WO3- SBA-16 samples
Figure 3 SEM images of (a) pristine SBA-16 (b) 3WS (c) EDX spectrum of 3WS, TEM
images of (c) pristine SBA-16 and (d) 3WS
Figure 4 UV-Vis-DRS spectra of WO3 and WO3- SBA-16 samples
Figure 5 FT-IR spectra of SBA-16 and WO3- SBA-16 samples
Figure 6 H2-TPR patterns of WO3 and WO3- SBA-16 samples
Figure 7 TPD profiles of SBA-16 and WO3- SBA-16 samples
Figure 8 Pyridine adsorbed FT-IR spectra of SBA-16 and WO3- SBA-16 samples
Figure 9 Effect of WO3 loading on esterification of LA with EtOH
Reaction conditions: Catalyst weight: 0.5 g, LA: EtOH =1:7 reaction temperature:
250 ºC, N2 flow: 30 mL min−1, Liquid feed flow: 1 mL h−1
Figure 10 Effect of molar ratio of LA to EtOH on esterification of LA with EtOH
Reaction conditions: Catalyst weight: 0.5 g, reaction temperature: 250 ºC,
N2 flow: 30 mL min−1, Liquid feed flow: 1 mL h−1
Figure 11 Effect of reaction temperature on esterification of LA with EtOH
Reaction conditions: Catalyst weight: 0.5 g, LA: EtOH =1:7, N2 flow: 30 mL min−1,
Liquid feed flow: 1 mL h−1
Figure 12 Influence of time on stream esterification of LA with EtOH
23

Reaction conditions: Catalyst weight: 0.5 g, LA: EtOH =1:7 reaction temperature:
250 ºC, N2 flow: 30 mL min−1, Liquid feed flow: 1 mL h−1
Figure 13 Influence of time on stream on esterification of LA with EtOH over 3WS-I catalyst.
Reaction conditions: Catalyst weight: 0.5 g, LA: EtOH =1:7 reaction temperature:
250 ºC, N2 flow: 30 mL min−1, Liquid feed flow: 1 mL h−1
Figure 14 Influence of various alcohols on esterification of LA
Reaction conditions: Catalyst weight: 0.5 g, LA: Alcohol =1:7 reaction temperature:
250 ºC, N2 flow: 30 mL min−1, Liquid feed flow: 1 mL h−1
Scheme 1 Plausible reaction pathway for the esterification of levulinic acid with alcohols over
WO3-SBA-16 catalyst
Scheme 2 Plausible reaction pathway for the formation of angelica lactones over WO3-SBA-16
catalyst
24

Figure-1
Figure 1

Figure-2
Figure 2

Figure-3
Figure 3

Figure-4
Figure 4

Figure-5
Figure 5