Vapor phase esterification of levulinic acid catalyzed by Γ-Al2O3 supported molybdenum phosphate catalysts

Article abstract of DOI:10.1016/j.mcat.2018.01.011

A comprehensive comparison of the catalytic activities of alumina supported molybdenum phosphate (MoPO) catalysts with different loadings of MoPO was investigated in the vapor phase esterification of levulinic acid (LA) with ethanol (EtOH) to synthesize ethyl levulinate (EL). The catalysts were characterized by various techniques to find the surface and structural properties. The esterification reaction parameters such as MoPO loading on Al₂O₃, molar ratio of EtOH/LA, reaction temperature and reaction time were optimized. Raman and FT-IR studies confirmed the formation of pyrophosphate phase on alumina. XRD results revealed the presence of well dispersed MoPO species over alumina support. The esterification activity of these catalysts mainly depends on the number of acidic sites and dispersion of MoPO species.

Full text of DOI:10.1016/j.mcat.2018.01.011

MolecularCatalysisxxx(xxxx)xxx–xxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Vapor phase esterification of levulinic acid catalyzed by γ-Al2O3 supported
molybdenum phosphate catalysts
Sailaja Gadamsetti, Nagaraju Mathangi, Sk. Hussain, Vijay Kumar Velisoju,
Komandur V.R. Chary⁎
Catalysis Division, Indian Institute of Chemical Technology, Hyderabad, 500 007, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Molybdenum phosphate
Alumina
Levulinic acid
Ethyl levulinate
A comprehensive comparison of the catalytic activities of alumina supported molybdenum phosphate (MoPO)
catalysts with different loadings of MoPO was investigated in the vapor phase esterification of levulinic acid (LA)
with ethanol (EtOH) to synthesize ethyl levulinate (EL). The catalysts were characterized by various techniques
to find the surface and structural properties. The esterification reaction parameters such as MoPO loading on
Al2O3, molar ratio of EtOH/LA, reaction temperature and reaction time were optimized. Raman and FT-IR
studies confirmed the formation of pyrophosphate phase on alumina. XRD results revealed the presence of well
dispersed MoPO species over alumina support. The esterification activity of these catalysts mainly depends on
the number of acidic sites and dispersion of MoPO species.
1. Introduction
an excellent octane booster for gasoline, used as cold flow improvers in
biodiesel and used as diesel miscible bio-fuel in diesel car engines
Depletion of reserves of fossil fuels and increasing global energy
consumption leads to the development of alternative renewable re-
sources. In this context, a great deal of research is currently devoted
worldwide in pursuit of lignocellulosic biomass and its derivatives to
produce energy, chemicals and fuels [1–5]. Ethyl levulinate (EL), one of
the current biodiesel sources, can be obtained by catalytic ethanolysis
of renewable levulinic acid (LA).
Levulinic acid has drawn a significant attention as a renewable,
most promising platform molecule for its two active groups such as a
ketone and a carboxylic acid and it can be easily derived by acid
treatment of abundantly available bio resources like cellulose. LA is a
versatile building block in various synthesis routes for production of
value added chemicals such as acrylic acid, γ-valerolactone, α-angelica
lactone, 3-hydroxypropionic acid, levulinate esters and 1, 4-pentane-
diol [6,7]. Among these explorations, esterification of LA with EtOH has
received a great deal of attention as green process for ethyl levulinate
production.
[8–11].
The origin of green chemistry principles and due to environmental
problems, efforts have recently made in order to design recyclable solid
acid catalysts to replace toxic and corrosive homogenous acids. Thus,
cleaner technology could be facilitating by use of environment friendly
heterogeneous catalysts. In this area of research, Nandiwale et al. stu-
died the liquid phase esterification of LA with EtOH using dodeca-
tungstophosphoricacid on desilicated H-ZSM-5 and achieved 94% LA
conversion and 100% EL selectivity [12]. Amberlyst −15 and sulph-
ated SnO2 showed high conversion of LA among the catalysts tested for
liquid phase esterification of LA with EtOH by Fernandez et al. which
was probably due to strong acidity [13]. In addition, various types of
solid acid catalysts such as sulfated Si-doped ZrO2 [14], sulfonic me-
sostructured silicas [15] have been investigated for esterification of LA
with EtOH. The majority of the past studies have been reported in the
liquid phase under batch conditions. Whereas, the present study deals
with the solid acid catalyzed (MoPO/Al2O3) levulinic acid conversion to
ethyl levulinate in the vapor conditions at atmospheric pressure.
Molybdenum is usually supported on TiO2, SiO2, Al2O3, and ZrO2 to
improve catalytic activity and mechanical strength. The surface struc-
tures of these supported molybdenum catalysts are quite different from
that of bulk molybdenum catalyst. Molybdenum based catalysts used in
both environmental catalysis and in the petrochemical industry such as
Due to low toxicity, flash point stability, lubricity, and moderate
flow properties under low temperature conditions, ethyl levulinate has
been marked as a promising chemical platform for utilization in various
applications in fragrance and flavoring industry. Ethyl levulinate have
shown better solubility in gasoline than other alkyl levulinates such as
butyl levulinate and methyl levulinate. Therefore, EL is considered as
⁎
Corresponding author.
E-mail address: charykvr@gmail.com (K.V.R. Chary).
https://doi.org/10.1016/j.mcat.2018.01.011
Received 27 September 2017; Received in revised form 5 January 2018; Accepted 10 January 2018
2468-8231/©2018PublishedbyElsevierB.V.
Pleasecitethisarticleas:Gadamsetti,S.,MolecularCatalysis(2018),https://doi.org/10.1016/j.mcat.2018.01.011

S. Gadamsetti et al.
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combustion of soot, hydrodesulphurization reactions, dehydrogenation
of alkanes, oxidation of methanol, and olefins metathesis [16–18].
Notably, from the application point of view, metal phosphates are the
most interesting species for stabilization of metal in different oxidation
states and also for its acidic properties. This acidic behavior can be
attributed to surface PeOH groups and coordinately unsaturated metal
ions exposed on the surface coupled to M]O double bonds. Mo-
lybdenum phosphorus oxides (MoPO) are mainly reported as new
cathode material for lithium and sodium batteries [19]. Alumina-sup-
ported molybdenum oxide catalysts are familiar in various catalytic
processes such as esterification of acetic acid, benzylation of anisole and
isomerization of α-pinene [20,21]. Recently, Mitran et al. have reported
that alumina-supported molybdenum was able to catalyze the ester-
ification reaction and showing selectivity of 100% for n-butyl acetate at
81% conversion of acetic acid [22]. Oxidative dehydrogenation reac-
tion was carried out with alumina supported molybdenum catalysts
which were modified by vanadium and phosphorous. In this work,
Haddad et al. observed that there is an increase in conversion of ethane
when molybdenum based catalysts are modified with phosphorous
[23]. To the best of our knowledge, metal phosphate catalysts have
been never employed earlier for esterification reactions.
KBr pellets were collected on an IR (Model: GC-FT-IR Nicolet 670)
spectrometer. The Raman spectra of the catalyst samples were done
with a Horbia Jobin Yvon Lab Ram HR spectrometer. The nitrogen
adsorption−desorption isotherms were collected from Autosorb-1
(Quanta chrome instruments), and the specific surface area, pore size
distribution studies (PSD) of the synthesized catalysts calculated by the
Brunauer−Emmett−Teller (BET) method. Degree of crystallinity was
observed via X-ray powder diffraction (XRD, Bruker D8 ADVANCE,
Germany) Ultima-IV X-ray diffractometer (M/s. Rigaku Corporation,
Japan). TPD experiments were conducted on AutoChem 2910 instru-
ment. In a typical TPD experiment, 100 mg of sample was supported on
quartz wool in U shaped tube and pre heated at 420 °C for 1 h and
saturated with highly pure anhydrous ammonia with a mixture of 10%
NH3–He at 80 °C for 1 h. After adsorption of NH3, the sample was flu-
shed with He flow (50 mL min−1) at 100 °C for 1 h to remove physi-
sorbed ammonia. TPD experiment was performed from ambient tem-
perature to 800 °C at a heating rate of 10°/min. The amount of NH3
desorbed was calculated using GRAMS/32 software.
2.3. Catalytic reaction
The aim of this work is to evaluate the catalytic properties of the
alumina supported MoPO catalysts and its contribution to esterification
of LA with EtOH to ethyl levulinate (Scheme 1). The reaction para-
meters such as loadings of MoPO on alumina, molar ratio between
EtOH and LA, reaction temperature and reaction time were optimized.
The catalysts were characterized by N2-sorption analysis, Fourier
transformed infrared spectroscopy (FT-IR), temperature programmed
desorption of ammonia (NH3-TPD), Raman spectroscopy and powder X-
ray diffraction (XRD) methods.
Esterification of levulinic acid with EtOH experiments were per-
formed in a down flow fixed bed quartz reactor operating at atmo-
spheric pressure. In a typical experiment, 0.3 g of the catalyst along
with equal weight of glass beads were charged into the reactor and were
supported on a glass wool bed and flushed at 250 °C under N2 flow for
1 h with a GHSV of 1.27 mL gcat−1 s−1. A different ratio of LA and EtOH
were injected by means of B-Braun syringe pump and was vaporized in
a preheating zone provided on the top of the catalyst bed. The reaction
was carried out in the range of 250–300 °C. The products were collected
periodically every hour under ice cooled trap and analyzed using an HP
6890 gas chromatograph equipped with FID using an HP-5 column and
confirmed by HP GC 5973 MSD instrument using a HP-1MS column.
2. Experimental section
2.1. Catalyst preparation
The procedure for the preparation of molybdenum phosphate
(MoPO) is described elsewhere [24,25]. A 250 mL round bottom flask
fitted with a reflux condenser was first charged with 7.5 g of MoO3
(99.5%, Aldrich) and 22.5 cm3 85% H3PO4 (331 mmol, Aldrich) at
approximately 180 °C. The mixture was stirred for1.5 h and allowed to
cool down to room temperature. To this solution, 200 cm3 of 15.8 M
HNO3 (Fisher) was added and refluxed further for 12 h. Upon cooling,
the white Mo containing material was precipitated from the solution.
The target solid part was filtered off from the liquid by vacuum-filtra-
tion washed with acetone and dried in air. This prepared material was
calcined in a furnace at 550 °C (5 °C/min) for 6 h.
A series of MoPO/Al2O3catalysts with different MoPO loadings in
the range of 5–50 wt% were prepared by wet impregnation method.γ-
Al2O3, used as the support was procured from Engelhard Corporation.
The supported MoPO/Al2O3 catalysts were prepared by impregnation
of a solution containing MoPO on Al2O3 support at 60 °C. These Al2O3
supported MoPO samples were dried at 110 °C for 16 h and subse-
quently calcined at 550 °C for 6 h in a muffle furnace. The as-prepared
catalyst samples were denoted as x wt% MoPO/Al2O3, where x denotes
the MoPO weight content on γ-Al2O3 support.
3. Results and discussion
3.1. Nitrogen adsorption-desorption analysis
The N2 adsorption-desorption analysis was carried out to measure
the textural properties of alumina supported molybdenum phosphate
(MoPO) catalysts (Fig. 1). The surface area, pore volume and average
pore diameter of the samples were also given in Table 1. Pure alumina
has BET surface area of 288 m2 g−1. It is evident from Table 1 that
MoPO loading influence the structural properties of MoPO/Al2O3 cat-
alysts. The surface area of supported catalysts decreased from 251 to
57 m2/g with the increase in MoPO loading from 5 to 50 wt% on Al2O3.
There is a sudden drop of surface area observed from 20 wt% to 30 wt%
MoPO loadings (Table 1) which is mostly due to the blockage of pores
by MoPO species. All catalysts exhibited an average peak pore dia-
meters in the range 30–70 A° (Table 1).
3.2. Raman spectroscopy
The Raman spectra of representative samples are provided in Fig. 2
in the region of 400–1200 cm−1 that was maintained in an ambient
environment. The appearance of band in the region of 900–1000 cm−1
2.2. Catalyst characterization
is observed at higher MoPO loading (> 30 wt%). This band can be as-
Fourier transform infrared (FT-IR) spectra of the samples pressed in
3−
signed to asymmetric stretching frequency of PO4
group which
confirms the formation of molybdenum phosphate phase on alumina.
The other band at 825 cm−1 is exhibited by higher MoPO loading of
50 wt% due to the stretching frequency of MoeOeMo bond. These
findings confirm the formation of polymolybdate structure over the
surface of Al2O3 at higher MoPO loadings due to agglomeration of
MoPO species [26].
Scheme 1. Esterification of LA with EtOH.
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Fig. 1. Adsorption-desorption isotherms and PSD curves of various MoPO/Al catalysts. a) Pure Al2O3, b) 10 wt% MoPO/Al2O3, c) 20 wt% MoPO/Al2O3, d) 40 wt%
MoPO/Al2O3 and e) 50 wt% MoPO/Al2O3.
Table 1
and at around 1650 cm−1 observed due to stretching and bending vi-
brations of surface eOH groups respectively. The peaks in the absorp-
tion range of 500–1000 cm−1 were assigned to AleO (782 and
691 cm−1) vibrations [27]. For alumina supported MoPO catalysts,
apart from main characteristic peaks of alumina, additional peaks were
observed at 1124, 928, 858 and 727 cm−1. The additional peak in-
tensities increase with increase in MoPO loading. The other two bands
Surface area, pore volume and pore diameter values of various MoPO/Al2O3
catalysts.
a
MoPO loadings
(wt%)
Surface Area
(m2/g)
Pore Volume
(cc/g)
Average pore
diameter (Å)
TOFEL
(s−1
)
0
5
288
251
228
211
122
113
57
0.48
0.35
0.29
0.19
0.11
0.09
0.04
66.9
55.6
51.5
39.4
37.4
33.8
33.6
–
0.0012
0.0009
0.0078
0.0014
0.0017
0.0052
at 1124 and 858 cm−1 are attributed to symmetric vibration (νs) and
10
20
30
40
50
−3
asymmetric vibration (νas) of PeO bonds in PO4
group. The bands
also observed at 727 and 928 cm−1 assigned to νs and νas of PeOeP
bonds respectively which confirms the formation of molybdenum pyr-
ophosphate on alumina.
a
TOFEL = rateEL/NH3 uptake; (Acid sites estimated from NH3-TPD are as-
sumed to be surface active sites for this reaction).
3.5. Temperature programmed desorption of NH3
3.3. Powder X-ray diffraction analysis
TPD is one of the important techniques to estimate the acidic
strength and amount of acidic sites of the catalysts, using ammonia as a
probe molecule. TPD profiles of pure alumina and MoPO/Al2O3 catalyst
were shown in Fig. 5 and total ammonia desorbed values were pre-
sented in Table 2. The TPD results of all catalyst samples show a broad
peak at low temperature, which corresponds to weak acidic region.
With an increase in MoPO loading from 5 to 20 wt%, the intensity of
additional peak in the moderate acidic region also observed till 20 wt%
MoPO loading. Further increase in MoPO loading, decrease in peak
intensity is observed in both week and moderate acidic region. From
Fig. 5, it is clearly seen that the area under desorption peaks for 20 wt%
MoPO/Al2O3 sample is much higher than other alumina supported
MoPO catalysts. Therefore, the total amount of acid sites and also
number of moderate acidic sites of 20 wt% MoPO/Al2O3 is high than
other loadings which are found to exhibit better activity compared to
other samples [28]. The TPD results demonstrated that the acidity of
the supported catalysts strongly influenced by the variation of MoPO
loading. Thus, the drop of acidity of higher loadings (> 30 wt% MoPO/
Al2O3) is caused by aggregation of MoPO species, which may block the
pores of alumina support. The catalytic activity results of alumina
supported MoPO catalysts are in good agreement with the total acidity
results acquired from TPD of NH3.
To examine the crystallinity of the catalytic samples, XRD analysis
was performed and the patterns are presented in Fig. 3. The pure MoPO
sample exhibits main reflection at 2θ = 22° and 28.1°. The XRD pattern
of γ −Al2O3 showed three reflections at 2θ = 37.2°, 45.6° and 66.9°
which coincide with database standard (JCPDS No. 10-0425). As the
metal phosphate loading increases on the alumina, the intensity of XRD
peaks at 2θ = 37.2°, 45.6° and 66.9° decreased. At higher loadings
(50 wt% MoPO/Al2O3), new peaks noticed due to agglomerisation of
MoPO species at 2θ = 22.2° and 28.1°. The absence of these peaks at
lower loadings indicates the well dispersion of MoPO species over
alumina. However, MoPO species crystalline size of lower loadings
might be less than 4 nm, which is beyond the detection range of the
XRD technique.
3.4. FT- IR spectroscopy
To further investigate the structural properties of the catalysts, the
FT-IR spectra of pure alumina support and different loading of MoPO
ranging from 5 to 50 wt% supported on alumina were shown in Fig. 4.
The FT-IR spectra of pure MoPO sample shows the IR bands at 3600,
1620,1251, 1124, 1025, 928, 858 cm−1 and 738 cm−1. In the FT-IR
spectrum of alumina, the broad absorption band at 3300–3700 cm−1
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825
998
(g)
948
(f)
(e)
(d)
(c)
(b)
(a)
700
750
800
850
900
950
1000
-1
Fig. 2. Raman spectroscopy profiles of various MoPO/Al catalysts. a) Pure Al2O3, b) 5 wt% MoPO/Al2O3, c) 10 wt% MoPO/Al2O3, d) 20 wt% MoPO/Al2O3, e) 30 wt%
MoPO/Al2O3, f) 40 wt% MoPO/Al2O3 and g) 50 wt% MoPO/Al2O3.
3.6. Catalytic activity results
of MoPO/Al2O3 catalysts. A blank reaction (without catalyst) was also
carried out and negligible LA conversion was observed. Pure MoPO has
shown 11% of LA conversion with 34% selectivity of EL and 66% se-
lectivity of AAL probably due to less acidity confirmed by NH3-TPD,
whereas the conversion of LA was high for all alumina supported MoPO
catalysts irrespective of MoPO loadings. Higher LA conversion over
alumina supported MoPO than pure MoPO is attributed to the increase
in acidity and BET surface area of catalysts. From Fig. 6, one can notice
that the conversion of LA increased with an increase in MoPO loading
from 5 to 20 wt%. Decrease in LA conversion at higher MoPO loading
3.6.1. Effect of MoPO loading on alumina
A detailed study was investigated with an aim to explore the cata-
lytic properties of MoPO/Al2O3 catalysts for the esterification of LA
with EtOH. Usually, the vapor phase esterification of LA with EtOH
yields EL as a major product along with γ-valerolactone (GVL) and α-
angelica lactone (AAL) as minor products, which depend on molar ratio
of EtOH:LA, reaction temperature and reaction time. Fig. 6 shows de-
tailed view of ethanolysis of LA over pure alumina and various loadings
Fig. 3. Wide angle XRD patterns of various MoPO/Al catalysts. a) Pure Al2O3, b) 5 wt% MoPO/Al2O3, c) 10 wt% MoPO/Al2O3, d) 20 wt% MoPO/Al2O3, e) 30 wt%
MoPO/Al2O3, f) 40 wt% MoPO/Al2O3 and g) 50 wt% MoPO/Al2O3.
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Fig. 4. Infrared spectroscopy profiles of various MoPO/Al catalysts. a) Pure Al2O3, b) 5 wt% MoPO/Al2O3, c) 10 wt% MoPO/Al2O3, d) 20 wt% MoPO/Al2O3, e) 30 wt
% MoPO/Al2O3, f) 40 wt% MoPO/Al2O3 and g) 50 wt% MoPO/Al2O3.
(> 30 wt%) from 100% to 75% was observed. The selectivity of EL
increased from 70 to 99% with an increase in MoPO loading from 5 to
20 wt% and decreased to 82% at higher MoPO loadings. Higher LA
conversion to EL over 20 wt% MoPO/Al2O3 suggested that the acidic
strength and dispersion of MoPO could significantly affect the product
selectivity. It can be seen from Fig. 6, the decrease in the conversion of
LA may have been partly owing to aggregation of MoPO species and
also due to less acidity of the catalyst, which is clearly observed from
NH3-TPD. The catalytic experiments were also carried out with pure
alumina under similar reaction conditions, which showed 70% of LA
conversion and 40% of EL selectivity.
3.6.2. Effect of ethanol: levulinic acid molar ratio
The influence of EtOH: LA molar ratio was evaluated in the range of
2:1–10:1 using the catalyst 20 wt% MoPO/Al2O3 catalyst at 250 °C.
Ethanolysis of LA is a reversible reaction and is well-known from the Le
Chatelier’s principle that the large amount of EtOH favors the formation
of EL [29]. Increase in EtOH: LA ratio from 2:1 to 8:1, selectivity of EL
(f)
(d)
(e)
(c)
(b)
(a)
100
200
300
400
500
600
700
Fig. 5. TPD profiles of various MoPO/Al catalysts. a) 5 wt% MoPO/Al2O3, b) 10 wt% MoPO/Al2O3, c) 20 wt% MoPO/Al2O3, d) 30 wt% MoPO/Al2O3, e) 40 wt%
MoPO/Al2O3 and f) 50 wt% MoPO/Al2O3.
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Table 2
Results of TPD of ammonia of various loadings of MoPO/Al2O3 catalysts.
MoPO loadings (wt%) Tmax1 (°C) Desorbed NH3 (μmol/g) Tmax2 (°C) Desorbed NH3 (μmol/g) Tmax3 (°C) Desorbed NH3 (μmol/g) Total Desorbed NH3 (mmol/g)
5
77
65
63
80
80
–
314
448
522
310
232
–
167
157
143
287
361
152
64
547
506
679
588
525
469
111
161
206
108
87
0.49
0.81
1.02
0.52
0.36
0.11
10
20
30
40
50
201
263
104
37
28
79
Fig. 6. Catalytic activity studies of various loadings of MoPO catalysts. Reaction conditions: Reaction temperature 250 °C, Time: 2 h, N2 flow rate: 20 cc/min, Catalyst
wt.: 0.3 g, GHSV: 1.27 mL gcat−1 s−1, mole ratio of EtOH:LA = 8:1.
100
Conversion
Selectivity
95
90
85
80
75
70
65
60
2:1
4:1
6:1
8:1
10:1
Fig. 7. Effect of mole ratio of EtOH:LA on esterification of LA to EL. Reaction conditions: Reaction temperature 250 °C, cat.: 20MoPO/Al2O3, Time: 2 h, N2 flow rate:
20 cc/min, GHSV: 1.27 mL gcat−1 s−1, Catalyst wt.: 0.3 g.
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Conversion of LA (%)
Selectivity of EL (%)
Selectivity of GVL (%)
Selectivity of AAL (%)
100
80
60
40
20
0
240
250
260
270
280
290
300
Fig. 8. Influence of reaction temperature over 20 wt% MoPO/Al2O3 catalyst. Reaction conditions: Time: 2 h, N2 flow rate: 20 cc/min, Catalyst wt.: 0.3 g, mole ratio of
EtOH:LA = 8:1.
100
96
92
Con of LA
Sel of EL
88
Sel of GVL
Sel of AAL
84
4
3
2
1
0
0
10
20
30
40
50
60
70
80
Fig. 9. Time on stream studies of esterification of LA. Reaction conditions: cat.: 20MoPO/Al2O3, N2 flow rate: 20 cc/min, Catalyst wt.: 0.3 g, GHSV:
1.27 mL gcat−1 s−1, mole ratio of EtOH:LA = 8:1.
and conversion of LA was increased as shown in Fig. 7. The LA con-
version of 100% was achieved at EtOH: LA molar ratio of 8:1. Con-
version was not significantly altered with further increase in molar ratio
of EtOH to LA from 8:1 to 10:1 but decrease in the EL selectivity is
observed. GVL was also detected during the esterification of LA at molar
ratio of EtOH: LA = 10:1, since excess EtOH act as hydrogen source
trough Meerwein-Ponndorf-Verley (MPV) mechanism. Dehydration of
LA forms AAL and the formation of diethyl ether occurs due to inter
molecular dehydration of EtOH which found to be the main reasons for
the decrease in EL selectivity at higher molar ratio of EtOH to LA.
250–300 °C. Notably, an increase in temperature from 250 to 300 °C
had a negative effect on selectivity of EL. Formation of various un-
desired products such as AAL and GVL was observed with the increase
in temperature from 250 to 300 °C. Therefore, the optimal reaction
temperature to perform the ethanolysis of LA is found to be 250 °C,
which is near to the boiling point of LA.
3.6.4. Time on stream
The stability and catalytic performance of 20 wt% MoPO/Al2O3 has
been investigated for esterification of LA with EtOH as a function of
reaction time and the results presented in Fig. 9. The formation of EL
was high (99%) at the beginning of the reaction but exhibited a de-
creasing trend after 20 h of time and reached to 93% at 75 h. However,
formation of GVL and AAL was observed with increase in time. This
little deactivation can be explained in terms of carbon deposition
3.6.3. Effect of reaction temperature
The reaction was carried out at different experiments in order to
examine the effect of reaction temperature over the 20 wt% MoPO/
Al2O3 catalyst (Fig. 8). The reaction temperature varied over a range of
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(∼2 wt%) on the surface of 20 wt% MoPO/Al2O3 catalyst which is at-
tributed to the irreversible chemisorption of other undesired products
such as AAL and GVL [30]. The results demonstrated that the conver-
sion of LA is 85% and selectivity of EL is 93% even after 75 h indicating
the catalyst stability under the experimental conditions adopted for the
esterification LA with EtOH to EL. The used 20MoPO/Al2O3 catalyst
(after 75 h) was characterized using XRD and the results are compared
with the fresh form of the catalyst (Fig. S1). The results obtained in this
analysis indicated insignificant differences between fresh and used
forms of the 20MoPO/Al2O3 catalyst.
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