Efficient microwave synthesis of novel aromatic esters catalyzed by zirconia and its modified forms: A kinetic study

Article abstract of DOI:10.1039/c5ra20430e

A series of solid acids such as ZrO₂, 5%Mo(vi)/ZrO₂, 10%Mo(vi)/ZrO₂, 20%Mo(vi)/ZrO₂ and SO₄^2-/ZrO₂ were prepared. These solid acids were characterized by BET, NH₃-TPD/n-butylamine back titration, powder-XRD, FT-IR spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ICP-OES techniques. The catalytic performance of these solid acids was evaluated in the synthesis of novel aromatic esters by the assistance of microwave irradiation and the catalytic activity of these solid acids was compared with pTsOH. The results clearly indicated that the zirconia based solid acids are efficient green catalysts for esterification reactions, which gave a maximum yield of the ester in a shorter reaction time and comparable catalytic activity of the pTsOH Br?nsted acid. Kinetic studies were carried out to calculate the temperature coefficients (1.66 and 1.56) and energy of activation (66.82 kJ mol^-1 and 58.93 kJ mol^-1) for SO₄^2-/ZrO₂ and pTsOH respectively. Pre-adsorption studies revealed that the reaction follows the Langmuir-Hinshelwood mechanism. The SO₄^2-/ZrO₂ solid acid catalyst was found to be reactivable and reusable.

Full text of DOI:10.1039/c5ra20430e

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Page 1 of 35
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Efficient Microwave synthesis of novel Aromatic Esters catalyzed by Zirconia
and its modified forms: Kinetic study
N. Thimmaraju1,2, S. Z. Mohamed Shamshuddin*1,2, S. R. Pratap1,2 and Raja Kapanaiah3
1Chemistry Research Laboratory, HMS Institute of Technology, Tumkurꢀ572104, Karnataka,
India.
2Research and Development Center, Bharathiar University, Coimbatore, India.
3Chemisty Department, Bangalore university, Bangalore, India.
∗Corresponding author: Eꢀmail address: mohamed.shamshuddin@gmail.com
Phone no: +91ꢀ9844718742
Abstract
A series of solid acids such as ZrO2, 5% Mo(VI)/ZrO2, 10% Mo(VI)/ZrO2, 20% Mo(VI)/ZrO2
2ꢀ
and SO4 /ZrO2 were prepared. These solid acids were characterized by BET, NH3ꢀTPD / nꢀ
butylamine back titration, powderꢀXRD, FTꢀIR spectroscopy, scanning electron microscopy
(SEM), transmission electron microscopy (TEM) and ICPꢀOES techniques. The catalytic
performance of these solid acids was evaluated in the synthesis of novel aromatic esters by the
assistance of microwave irradiation and the catalytic activity of these solid acids was compared
with pTsOH. The results clearly indicated that the zirconia based solid acids are efficient green
catalysts for esterification reactions, which gave maximum yield of ester in shorter reaction time
and comparable to the catalytic activity of pTsOH Brönsted acid. Kinetic studies were carried
out to calculate the temperature coefficients (1.66 and 1.56) and energy of activation (66.82
2ꢀ
KJmolꢀ1 and 58.93 KJmolꢀ1) for SO4 /ZrO2 and pTsOH respectively. Preꢀadsorption studies
2ꢀ
revealed that the reaction follows the LangmuirꢀHinshelwood mechanism. SO4 /ZrO2 solid acid
catalyst was found to be reactivable and reusable.
1

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Keywords: Esterification; Kinetic studies; Temperature coefficient; Energy of activation;
LangmuirꢀHinshelwood; Zirconia; Modified zirconia.
1. Introduction
Many organic transformations are assisted by heterogeneous solid acid catalysts such as
metal oxides, mixed metal oxides, doped metal oxides, zeolites, resins, clays, etc., 1ꢀ4. These
solid acids can replace liquid acid catalysts like H2SO4, HCl, AlCl3, BF3, H3PO4, SnCl4, ZnCl2,
etc., 5ꢀ6. The main advantages of solid acid catalysts are that they are ecoꢀfriendly and green
because, solid acids are nonꢀtoxic, non corrosive, easy to recover and reusable for several times
and also reduce or even eliminate the production of hazardous materials 7ꢀ8
.
Metal oxide based solid acids like zirconia (ZrO2) as well as modified forms of zirconia is
widely employed as catalysts in a number of acid catalyzed reactions. Such as,
transesterification, esterification, alkylation, acetylation, hydrocarbon isomerization, synthesis of
2ꢀ
heterocyclic compounds, etc., 7ꢀ14. It has been reported that Mo(VI) and SO4 ions impregnated
on ZrO2 can increase the number of catalytically active sites of zirconia and also increases the
more active tetragonal phase of zirconia which is even catalytically active at higher temperatures.
Microwave (MW) irradiation assisted organic reactions have been widely used in small scale
as well as large scale organic synthesis, because polar organic molecules selectively absorb MW
energy 15ꢀ18. MW irradiated organic synthesis gives higher yield of the products in shorter
reaction times when compared to conventional thermal reactions. Due to shorter reaction times
the catalyst could be preserved from decomposition and also increased catalytic efficiency could
19
be expected . Moreover, microwave heating accelerate the chemical reactions under increased
pressure conditions because solvents can rapidly get superheated to temperatures >100 °C above
its boiling point when irradiated under microwave conditions in a sealed vessel 20ꢀ21
.
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Esters are a group of organic compounds which are widely used in the synthesis of food
additives, drugs, cosmetics, preservative solvents, perfumes, pharmaceuticals, plasticizers,
lubricants, solvents, detergents, soaps, reaction intermediates, etc., 22ꢀ29. Different approaches
have been employed in both laboratory and commercial scale to prepare esters. The process of
esterification may be enormously accelerated by the addition of corrosive reagents like sulfuric
acid, hydrochloric acid (HCl), thionyl chloride, trimethyl silane (TMS) diazomethane, etc., 26ꢀ31
.
The main goal of this research work was to obtain suitable solid acid catalysts for
esterification of aromatic carboxylic acids with aliphatic alcohols. Catalytic materials such as
2ꢀ
zirconia, 5%Mo(VI)/ZrO2, 10%Mo(VI)/ZrO2, 20%Mo(VI)/ZrO2 and SO4 /ZrO2 were prepared
and characterized for their surface area, surface acidity, crystalinity, functional group,
morphology, particle size and elemental analysis. The catalytic performance of the prepared
catalytic materials was evaluated in the synthesis of some novel esters by the esterification of
halogenated carboxylic acids with propanol under MW irradiation condition. The obtained
1
13
reaction products were characterized by LCMS, H NMR and
C NMR. Optimization of
reaction conditions were carried out by varying the reaction time, temperature, molar ratio of
reactants and amount of the catalyst. Kinetic study was taken up on a particular esterification
2ꢀ
reaction to find temperature coefficient and energy of activation of SO4 /ZrO2 and pTsOH.
Reaction mechanism was also proposed by preꢀadsorption study.
2. Experimental
2.1.1. Chemicals
Chemicals such as zirconyl nitrate octahydrate, ammonium molybdate, pTsOH and
H2SO4 were supplied by M/S LOBA Chemie Ltd., India. Aromatic halogenated carboxylic acids
and propanol were supplied by either AlfaꢀAesar or Sigma Aldrich or Leonid.
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2.1.2. Preparation of solid acids
i. Preparation of hydrated ZrO2
Precipitation method was used to prepare hydrated zirconia (Zr(OH)4. 100 g of zirconyl
nitrate was dissolved in 250 mL of distilled water and the resulting solution was heated at 80 °C
for 10 min. To this solution, aqueous ammonia (1:1) was added drop wise with constant stirring.
The obtained precipitate was filtered, washed with distilled water and dried in hot air oven at 120
°C for 12 h.
ii.
Preparation of Mo(VI)/ZrO2
5% Mo(VI)/ZrO2(5MZ), 10% Mo(VI)/ZrO2(10MZ) and 20% Mo(VI)/ZrO2(20MZ) were
prepared by impregnation method. Typically, 10MZ was prepared as follows: Zirconyl nitrate
(9.125 g) and ammonium molybdate (0.736 g) were made into a paste with 10 mL of distilled
water. The resulting paste was dried at 120 °C for 12 h in hot air oven and the obtained solid was
finely powdered.
iii.
Preparation of sulfated zirconia (SZ)
2ꢀ
Sulfated zirconia (SO4 /ZrO2) was prepared as follows: Previously prepared Zr(OH)4
was made into a paste with 6 mL of 3 M H2SO4. The resulting paste was dried at 120 °C for 12
h in a hot air oven and the obtained solid was finely powdered.
All the finely powdered solid acid catalytic materials were calcinated at 550 °C in a
muffle furnace for 5 h.
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2.2. Characterization of solid acids
Specific surface area of solid acid catalytic materials was determined by BET method. NOVA
1000 Quanta chrome highꢀspeed gas sorption analyzer instrument was used to measure the
specific surface area of all the prepared catalysts. The surface acidity of the solid acids were
determined by NH3ꢀTPD method using Plus Chemisorb 2705 (Micromeritics) as well as by nꢀ
butyl amine back titration technique using dry benzene solvent. PXRD patterns of these solid
acids was obtained from X’pert Pro Philips diffractometer equipped with a Ni filtered CuꢀKα
radiation with λ=1.5418 Å using a graphite crystal monochromator with a scanning range 25oꢀ
65°. The FTꢀIR spectras were recorded on with 4 cmꢀ1 resolution using a Nicolet IR200 FTꢀIR
Spectrophotometer in the range 400ꢀ4000 cmꢀ1 using KBr as standard reference. SEM images of
all the prepared catalysts were obtained using a JEOL JXAꢀ8530F microscope and TEM images
of SZ was performed on Philips CM200 electron microscope at an acceleration voltage of 20ꢀ200
kV. The amount of molybdenum present in all Mo(VI)/ZrO2 samples were determined by ICPꢀ
OES (Inductively Coupled PlasmaꢀOptical Emission Spectrometer) analysis using ThermoꢀiCAP
6000 Series analyser.
2.3. Catalytic activity studies of solid acids
The esterification of various halogenated carboxylic acids with nꢀpropanol or isoꢀpropanol in
the presence of solid acids was carried out in a pressurized reaction tube (10 mL) equipped with
a magnetic stirrer, which was then irradiated at a particular temperature (85ꢀ145 °C) for a definet
period of time (10ꢀ70 min) in a self tuning single mode Labmate® microwave synthesizer. After
the completion of the reaction, the reaction mixture was cooled rapidly to room temperature by
passing compressed nitrogen through the microwave cavity for 5 min. The catalyst was
recovered by filtration and the organic layer was dried and diluted with 15 mL of ethyl acetate.
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The unreacted carboxylic acid was removed by washing with 5 mL of saturated sodium
bicarbonate solution. The organic layer was dried over anhydrous sodium sulphate and
1
evaporated in vacuum to get the desired product. Obtained product was characterized by H
NMR, 13C NMR (Varian) and LCMS (Agilent).
The reaction parameters such as reaction time, reaction temperature, molar ratio of the
reactants and amount of the solid acid catalyst were varied for optimization and to get more yield
of esters. Kinetic study and preꢀadsorption studies were also carried out for a particular
esterification reaction.
2.4. Reusability of solid acids
In order to study the reusability of the solid acid catalysts, the used solid acid was filtered
from the reaction mixture, washed with ethyl acetate, dried at 120 °C for 1 h and calcined at 550
°C for 1 h. Then, the reꢀactivated solid acid was used for esterification reaction under optimized
reaction conditions with fresh reaction mixture for five consecutive reaction cycles.
3. Results and discussion
3.1. Characterization of solid acids
The BET surface area (SA), total surface acidity (TSA) as well as acid site distribution
obtained by NH3ꢀTPD method of ZrO2, 5MZ, 10MZ, 20MZ and SZ are listed in Table 1.
The surface area of solid acids was found to follow the order:
ZrO2 < 5MZ < 20MZ < 10MZ < SZ
The surface area of zirconia was found to be least when compared to other solid acids. The
surface area of zirconia enhances when incorporated with either Mo(VI) or SO42ꢀ ions. Compared
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to all the prepared solid acids, SZ showed highest surface area which may be due to cracking of
2ꢀ
1
ZrO2 into fine particles when treated with SO4 ions . Among MZ catalytic materials, 10MZ
was having higher specific surface area than either 5MZ or 20MZ. It was observed that the
surface area increases with an increase in the concentration of Mo(VI) ions which may be due to
an increase in the stabilizing effect of Mo(VI) ions on ZrO2. When the concentration of Mo(VI)
ions was increased beyond 10% the surface area was found to decrease which may be due to the
blockage of zirconia pore structure by excess Mo(VI) ions. A similar observation has been
reported by A. H. Zapien et al 32.
Table 1. Physicoꢀchemical properties of solid acid catalytic materials used in the present
work.
Acid site distribution (mmol/g)
Weak Medium Strong Very
strong
BET
surface
area (m2/g)
42.9
% of
Mo(VI)
ꢀ
Catalyst
TSA
ZrO2
5MZ
10MZ
20MZ
SZ
0.03
0.40
0.20
0.16
0.18
ꢀ
ꢀ
ꢀ
0.43 (0.45)
0.90 (0.89)
1.02 (1.03)
0.95 (0.93)
1.31 (1.35)
79.7
ꢀ
ꢀ
ꢀ
ꢀ
0.70
0.86
0.77
0.87
ꢀ
4.96
9.87
20.04
ꢀ
88.1
ꢀ
ꢀ
83.6
161.8
0.44
Note: The TSA values obtained by nꢀbutyl amine back titration technique are shown in the
parenthesis.
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The TSA values of ZrO2, 5MZ, 10MZ, 20MZ and SZ determined by NH3ꢀTPD/nꢀbutyl
amine back titration technique are presented in Table 1. The TSA of the solid acids was found to
follow the order:
ZrO2 < 5MZ < 20MZ < 10MZ < SZ
2ꢀ
The TSA values indicate that the impregnation of either Mo(VI) or SO4 ions has a
strong impact on the acidic properties of zirconia.
When acid sites distribution of pure zirconia, MZ and SZ were compared, it was observed
that pure zirconia consists of ‘weak’ and ‘moderate’ acid sites, MZ consists ‘moderate’ and ‘strong’ acid
sites whereas SZ consists of ‘strong’ and ‘very strong’ acid sites. This type of acid sites distribution is
in good agreement with the acid site distribution reported in the literature 33ꢀ35
.
The PXRD patterns of solid acids are shown in Fig. 1. As could be seen from the figure,
both tetragonal and monoclinic phases are observed for pure zirconia. In case of 5MZ, 10MZ,
20MZ and SZ, the intensity of the peaks corresponding to monoclinic phase was found to
2ꢀ
decrease. This shows that Mo(VI) or SO4 ions strongly influences phase modification of
zirconia (monoclinic to the metastable tetragonal). For 10MZ, it is interesting to note that PXRD
peaks corresponding to monoclinic phase is very low and major peaks corresponding to
tetragonal phase are observed. In case of 5MZ and 20MZ, low intense peaks corresponding to
monoclinic phase were also observed. In case of SZ, only tetragonal phase of zirconia could be
2ꢀ
observed. The incorporation of SO4 ions may delay the formation of crystalinity of ZrO2
crystallites thus stabilizes it into metastable tetragonal phase.
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M
M
a
M
T
M
M
T
M
M
T
M
b
T
T
c
T
T
d
e
30
40
θ (degree)
50
60
2
Figure 1. Powder XRD patterns of (a) ZrO2, (b) 5MZ, (c) 10MZ, (d) 20MZ and (e) SZ. [where,
M indicates monoclinic and T indicates tetragonal phase].
Interestingly, a triangular correlation ship was found to exist between the surface area,
TSA and crystalinity of the solid acid catalysts. This observation indicates that the tetragonal
crystalline phase of zirconia could be responsible for higher surface area as well as higher
surface acidity of zirconia catalysts 36ꢀ37
.
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SZ
ZrO2
1646
1118
1391
3400
20 MZ
848
10 MZ
5 MZ
500 1000 1500 2000 2500 3000 3500 4000
-1
Wavenumber (cm )
Figure 2. The FTꢀIR spectrum of ZrO2, 5 MZ, 10 MZ, 20 MZ and SZ
The FTꢀIR spectra of all the prepared solid acids are given in Fig. 2. The bands at 1391
cmꢀ1 and 1118 cmꢀ1 can be due to ZrꢀOH bending and ZrꢀOꢀZr stretching modes respectively,
which confirms the formation of ZrO2 phases. The broad band at ~1646 cmꢀ1 is attributed to the
vibrations of acidic –OH and the band at ~3400 cmꢀ1 stretching mode with hydrogen bonding are
38
present in all the catalysts . In case of MZ samples the peak at ~848 cmꢀ1 is due to MoꢀOꢀMo
stretching mode of vibration for MoO3 39. This indicates that MoO3 is strongly dispersed on the
surface of ZrO2 wherein peak at ~ 1118 cmꢀ1 gets disappeared.
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Figure 3. SEM images of ZrO2, 5MZ, 10 MZ, 20MZ and SZ.
SEM micrographs of all the prepared catalysts are presented in Fig. 3. All catalysts
having irregular edge shaped particles with variable sizes. In micrometer range, the surface of
all the samples appears to be very porous and some aggregates formed by very small particles
and no agglomerates are found in all the prepared catalysts, which shows good compatibility
with poly matrix.
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Figur 4. TEM images of SZ nanoparticles (FFT pattern is shown in the inset).
The transmission electron microscopic (TEM) image of SZ nanoparticles is shown in Fig.
4. As seen from the figure, the nanoparticles are composed of spherical particles of dimension in
the range of about 6ꢀ15 nm. The boundary between the particles (low contrast) was not
detectable in the TEM of the SZ which may be due to the interaction between the neighbouring
particles. Further, from TEM images, black spots are uniformly distributed throughout the
sample and this can be attributed to the presence of SZ nanoparticles.
The ICPꢀOES results indicates that 5 MZ, 10 MZ and 20 MZ consisted of 4.96%, 9.87%
and 20.04% of Mo(VI) respectively.
3.2. Catalytic activity studies of solid acids (in esterification)
To begin with, in order to optimize the reaction conditions, liquid phase esterification
reactions between 3ꢀfluoro benzoic acid (3FBA) and nꢀpropanol (PrOH) over the solid acids
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such as ZrO2, 5MZ, 10MZ, 20MZ and SZ catalysts, as well as homogeneous Brönsted acids such
pTsOH and H2SO4 were carried out (Scheme 1). In general, all the solid acids used in the present
work were active in this reaction. In the absence of any solid acid catalyst negligible amount
(5.3%) of the esterified product was observed. This indicates that the esterification is a catalyzed
reaction.
O
OH
O
O
R
Acid Catalyst
MW heating
O
R OH
+
+
H
H
X
X
where, X = F, Cl, Br and I; R = nꢀpropyl and isoꢀpropyl group.
Scheme 1. Esterification of halogenated aromatic carboxylic acids with propanol in presence of
ZrO2, MZ, SZ, pTsOH and H2SO4 acid catalysts.
In order to obtain highest possible yield of the reaction product, esterification of 3FBA
with PrOH was carried out and optimized by varying reaction parameters such as, nature of solid
acids, reaction time, reaction temperature, molarꢀratio of the reactants (3FBA:PrOH) and weight
of the solid acid catalyst.
3.2.1. Effect of nature of solid acids on the yield of P3FB
To find out a facile catalytic material, esterification of 3FBA with PrOH was carried out
in presence 0.05 g of different solid acids such as ZrO2, 5MZ, 10MZ, 20MZ and SZ and
compared with homogeneous Brönsted acids namely H2SO4 and pꢀtoluenesulfonic acid
(pTsOH). The reactions were carried out at 130 ◦C for 30 min and the results are presented in the
Fig. 5. The catalytic activity was found to be significantly influenced by the dispersion of
2ꢀ
Mo(VI) or SO4 ions on ZrO2. The yield of desired reaction product i.e., propyl 3ꢀfluoro
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benzoate (P3FB) over different solid acid catalysts was found to be in the order: H2SO4 ≈ pꢀ
TsOH ≈ SZ > 10MZ > 20MZ > 5MZ > ZrO2
80
60
40
20
0
ZrO2
5MZ
10MZ
20MZ
SZ
pTsOH H2SO4
Acid catalysts
Figure 5. Effect of nature of catalyst on esterification of 3FBA with PrOH. [Reaction conditions:
0.05 g of catalyst at 130 °C for 30 min under MW irradiation, 3FBA = 0.5 g and PrOH = 2.7
mL].
Among all the solid acids, SZ showed highest yield of (propyl 3ꢀfluro benzoate) P3FB
ester, which can be due to the presence of ‘strong’ and ‘very strong’ acidic sites in SZ. However,
MZ catalysts showed poor yield of P3FB compared to SZ which can be due to the presence of
‘moderate’ and ‘strong’ acid sites in MZ. This clearly indicates that the esterification of aromatic
carboxylic acid with an alcohol is highly influenced by the strength of acid sites on the catalyst.
Homogeneous Brönsted acids (H2SO4 or pTsOH) showed comparatively higher yield than other
solid acids in the present study. However, use of ecoꢀfriendly heterogeneous catalysts is
advantageous over homogeneous acids due to easy separation and reꢀusability. Under the present
reaction conditions, the catalytic activity of SZ was comparable to the catalytic activity of a
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conventional homogeneous Brönsted acids and this clearly demonstrates that the efficiency of SZ
heterogeneous catalyst is suitable for the replacement for homogeneous catalysts such as sulfuric
acid or pTsOH.
SZ and pTsOH were selected as catalysts for further optimization of reaction conditions
and for kinetic study.
Further, the yield of P3FB was found to increase over different solid acids in the order:
ZrO2<5MZ<20MZ<10MZ<SZ. This order clearly indicates that, there is correlation between the
catalytic activity and their physicꢀchemical properties like surface area, total surface acidity and
tetragonal phase of the catalyst.
3.2.2. Effect of MW irradiation on the yield of P3FB
In order to study the effect of MW irradiation on the yield of P3FB, esterification of
3FBA with PrOH was carried out with and without MW irradiation and the results are given in
table 2a and 2b. The reactions were carried out at different reaction times (10ꢀ70 min) and also
by varying reaction temperature (90ꢀ130 °C). It was observed that the yield of P3FB was higher
in the reactions were carried out in presence of MW irradiation when compared to the yield of
P3FB obtained without MW irradiation (thermal conventional method). A maximum yield of
P3FB (98.5%) was obtained at 130 °C when the reaction was carried out for 60 min under MW
irradiation.
However, A maximum yield of P3FB (38.7%) was obtained at 130 °C when the reaction
was carried out for 60 min without MW irradiation.
This study indicates that the esterification reaction under MW irradiation is much more
effective than the conventional thermal method.
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Time (min)
SZ
catalyst
10
20
30
40
50
60
70
Yield of
38.4
(4.3)
69.5
(8.1)
85.2
93.6
97.3
98.5
98.4
P3FB (%)
(15.3)
(21.5)
(25.8)
(29.9)
(35.4)
Table 2a. Effect of MW irradiation and reaction time on esterification of 3FBA with PrOH.
[Reaction conditions: 0.05 g of catalyst at 130 °C, 3FBA = 0.5 g, PrOH = 2.7 mL].
Temperature ( °C)
110
SZ
catalyst
90
100
120
130
Yield of
59.1 (9.8) 76.8 (15.4) 87.1 (22.5)
92.3 (26.7) 98.5 (38.7)
P3FB (%)
Table 2b. Effect of MW irradiation and reaction temperature on esterification of 3FBA with
PrOH. [Reaction conditions: 0.05 g of catalyst for 60 min, 3FBA = 0.5 g and PrOH = 2.7 mL].
Note: The yield of P3FB by thermal conventional method are shown in the parenthesis.
3.2.3. Effect of reaction time on the yield of P3FB
In order to study the effect of reaction time on the yield of the P3FB, the esterification
reactions of 0.5 g of 3FBA with 2.7 mL PrOH were carried out in the time range of 10ꢀ70 min
over 0.05 g of catalyst at 130 °C and the results are given in Fig. 6. It was observed that the yield
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(%) of P3FB increases with an increase in the reaction time and a maximum yield was observed
at 60 min and stabilizes thereafter over both SZ and pTsOH catalysts. Hence, reaction time of 60
min was chosen to optimize the other reaction conditions.
100
SZ
pTsOH
80
60
40
20
0
10
20
30
40
50
60
70
Time (min)
Figure 6. Effect of reaction time on esterification of 3FBA with PrOH. [Reaction conditions:
0.05 g of catalyst at 130 °C under MW irradiation, 3FBA = 0.5 g, PrOH = 2.7 mL].
3.2.4. Effect of reaction temperature on the yield of P3FB
The esterification reactions of 3FBA with PrOH were carried out in a temperature range
of 85ꢀ145 °C over SZ and pTsOH for 60 min and the results are shown in Fig. 7. The results
clearly indicate that the yield (%) of P3FB increases with an increase in the reaction temperature
and 130 °C was found to be an optimized reaction temperature for the current reaction. The yield
of ester was stabilized when the temperature was increased beyond 130 °C. This may be due to
the fact that shorter reaction time requires higher temperature to obtain highest possible yield of
o
ester. Therefore, 130 C was chosen as the optimum reaction temperature to obtain highest
possible yield (%) of P3FB ester.
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SZ
pTsOH
100
90
80
70
60
90
100
110
120
130
140
Temperature ( °C)
Figure 7. Effect of reaction temperature on esterification of 3FBA with PrOH. [Reaction
conditions: 0.05 g of catalyst for 60 min under MW irradiation, 3FBA = 0.5 g and PrOH = 2.7
mL].
3.2.5. Effect of molar ratio of the reactants (3FBA:PrOH) on the yield of P3FB
In order to study the effect of molar ratio of the reactants i.e., 3FBA to PrOH,
esterification was carried out at different molar ratios ranging from 1:5 to 1:20 over SZ and
pTsOH catalysts and the results are shown in Fig. 8. The yield of P3FB increases with an
increase in the molar ratio of 3FBA to PrOH from 1:5 to 1:10. But when the molar ratio was
increased beyond 1:10, the yield of desired ester was found to decrease. The reason can be
attributed as follows:
Theoretically, esterification reaction requires one mole of alcohol for one mole of carboxylic
acid; however, in practice, higher amount of alcohol acts as a solvent as well as it may shift the
equilibrium to the right side (Scheme 1) resulting in the formation of the ester. In the present
study, alcohol (PrOH) acts both as a reactant as well as a solvent. Higher molar ratio of PrOH
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(more than 10 molar ratio) decreased the yield of ester due to solvent effect of propanol. Hence,
1:10 molar ratio of 3FBA to PrOH was chosen for further optimization of reaction conditions.
100
SZ
pTsOH
80
60
40
20
0
1:5
1:10
1:15
1:20
Molar ratio (3FBA:PrOH)
Figure 8. Effect of molar ratio of reactans on esterification of 3FBA with PrOH. [Reaction
conditions: 0.05 g of catalyst at 130 °C for 60 min under MW irradiation and 3FBA = 0.5 g].
3.2.6. Effect of amount of solid acid on the yield of P3FB
o
The effect of amount of SZ or pTsOH in esterification reaction was studied at 130 C by
using 3FBA to PrOH molar ratio of 1:10 and the results are shown in Fig. 9. The yield (%) of
P3FB increases when the weight of the catalyst was increased from 0.02 g to 0.05 g and an
excellent yield of P3FB was achieved over 0.05 g of SZ catalyst. Further, an increase in the
weight of SZ catalyst beyond 0.05 g stabilizes the yield of the ester.
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100
80
60
40
20
0
SZ
pTsOH
0.02
0.03
0.04
0.05
0.06
Amount of catalyst (g)
Figure 9. Effect of amount of catalyst on esterification of 3FBA with PrOH. [Reaction
conditions: temperature at 130 °C for 60 min usnder MW irradiation, 3FBA = 0.5 g and PrOH =
2.7 mL].
3.3. Kinetic studies
The kinetic studies were carried out over SZ and pTsOH catalysts. The kinetic study
experiments were conducted to obtain temperature coefficient and energy of activation of SZ
with pTsOH and mass transfer limitations during esterification of 3FBA with PrOH. The mass
transfer limitation was evaluated by changing the amount of catalytic material. A linear increase
in the yield of P3FB with an increase in the catalytic material was observed (Fig. 9). This shows
that the resistance to mass transfer is negligibly small between the liquid bulk of reactant and the
other outer surface of the catalytic materials. In fact in the absence of all transport limitations, the
rate of reaction is proportional to the amount of catalytic materials 40.
Plot of –ln[1ꢀyieldP3FB] against reaction time with varying the reaction temperature from
110 °C to 130 °C over SZ and pTsOH are shown in Fig. 10.
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4
3
2
1
0
SZ
10
20
30
40
50
60
Time (min)
5
4
3
2
1
pTsOH
10
20
30
40
50
60
Time (min)
Figure 10. Plot of firstꢀorder equation for the formation of P3FB over SZ and pTsOH at ■ꢀ 110
°C, ●ꢀ 120 °C and ▲ꢀ 130 °C.
The values of firstꢀorder rate constants for SZ and pTsOH at different reaction
temperatures are obtained from slopes of plots (Fig. 10). The values of energy of activation
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calculated from Arrhenius equation and the temperature coefficient calculated using equations 1
and 2 respectively and values are given in Table 3. Among SZ and pTsOH catalysts, SZ (66.82
KJmolꢀ1) was slightly higher energy of activation than pTsOHand (58.93 KJmolꢀ1). This clearly
indicates that SZ catalyst has comparable activity and can easily replace homogeneous pTsOH
Brönsted acid catalyst.
ꢇꢈ
ꢀꢁ = 2.303ꢂ ꢃꢄꢅ ꢆ ꢊ ꢋꢌꢉ ꢍ ꢌꢈ
ꢏ
ꢀꢀꢀꢀꢀ Eq. (1)
ꢀꢀꢀꢀꢀ Eq. (2)
ꢇꢉ ꢌꢈ ꢎ ꢌꢉ
ꢇꢈ
ꢐꢑꢒꢓꢑꢔꢁꢕꢖꢔꢑ ꢗꢄꢑꢘꢘꢙꢗꢙꢑꢚꢕ = ꢆ ꢊ
ꢇꢉ
where, R = gas constant, k1= rate constant at temperature T1 and k2= rate constant at temperature
T2.
Table 3. Reaction rate constant, energy of activation and temperature coefficients for the
formation of P3FB of SZ and pTsOH catalysts for esterification of 3FBA with PrOH.
Rate constant for
formation of P3FB (10-3
min-1)
Energy of activation
Catalyst
(Ea) for the formation Temperature co-
of P3FB
efficient (k2/k1)
(KJ mol-1)
120 °C
130 °C
SZ
pTsOH
45.06
51.10
74.84
79.94
66.82
58.93
1.66
1.56
3.4. Effect of reactant pre-adsorption
In order to study the catalytic reaction mechanism of esterification of 3FBA with PrOH,
reactant preꢀadsorption study was carried out 41. The important mechanistic question in
esterification catalyzed by solid acid catalyst is the involvement of one or two surfaceꢀbonded
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species corresponding to either EleyꢀRideal (ER) or LangmuirꢀHinshelwood (LH) type
mechanism. The ER mechanism involves the adsorption of only one of the reactants (either
3FBA or PrOH) onto the surface, after which the other reactant interacts with the adsorbed
species directly followed by desorption of the reaction product 42. In case of LH mechanism,
both reactants (3FBA and PrOH) first adsorb onto the surface before reaction could take place 43.
Thus, the effect of reactant preꢀadsorption has been studied under optimized reaction conditions
such as 0.05 g of SZ at 130 °C for 30 min under MW irradiation with 0.5 g of 3FBA and 2.7 mL
of PrOH. 0.05 g of SZ was premixed with PrOH or both 3FBA and PrOH for 24 h at room
temperature and the reaction was performed under optimized reaction conditions after adding
remaining reactants. The obtained yield of P3FB verses time plots are shown in Fig. 11, where
the plot corresponding to without premixing of reactants is also presented for comparison.
Premixing study of the catalyst with 3FBA was not carried out because of its solid nature and
literature results revealed that the chemisorption of an alcohol is stronger than the acid 44.
100
80
60
40
20
0
0
10
20
30
40
50
60
70
Time (min)
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Figure 11. Effect of reactant preꢀadsorption on catalyst on esterification of 3FBA with PrOH.
[Reaction conditions: 0.05 g of SZ at 130 °C for 60 min under MW irradiation, 3FBA = 0.5 g
and PrOH = 2.7 mL.. (●) preꢀadsorption of PrOH, (■) no preꢀadsorption of reactants and (▲)
preꢀadsorption of both 3FBA and PrOH.
It can be observed that the highest yield of P3FB was obtained when the catalyst was
premixed with both 3FBA and PrOH where as lowest yield was obtained when premixed with
PrOH and moderate yield was obtained under nonꢀpremixing condition of reactant with the
catalyst. The results in Fig. 11 suggest that the reaction needs chemisorption of both 3FBA and
PrOH involving LH type mechanism.
A probable LR type mechanism for an acid catalyzed esterification of aromatic carboxylic
acids with aliphatic alcohols is shown in the Scheme 2. It is possible that an alcohol and an
aromatic carboxylic acid are preferentially adsorbed on the Bronsted acid sites of modified
zirconia during esterification. The hydroxyl group of alcohol is protonated by Brönsted acid on
the solid acid catalyst surface while the carboxylic acid on an adjacent site leading to the
formation of carbocation (‘A’ in Scheme 2). Deprotonation of alcoholic oxygen produces a
nucleophile, which attacks the carbocation to generate a tetrahedral intermediate (‘B’ in Scheme
2). The tetrahedral intermediate eliminates water to produce an ester45ꢀ46
.
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O
OH
X
X
R
O+
X
B+
B
+
+
R
OH
OH
B
+ OH
+
+
B
B
H
R
O+
C
O
R
O+
O+
B
H
B
B
H
(Bronsted acid
sites of solid acid)
(A)
X
X
X
O
B
O
B
+ OH
R
R
C
O
R
O
+
-H2O
OH2
O
+H+
B+
-H+
O
B+
B
B
(B)
O
O
R
B+
B+
+
X
(Regenerated
Bronsted acid
sites of solid acid)
(ester)
Scheme 2. Probable mechanism for an acid catalyzed esterification of a carboxylic acid with an
alcohol over Brӧnsted acid sites of acid catalysts.
3.4.1. Esterification of various halogenated aromatic carboxylic acid with propanol under
optimized reaction conditions
Esterification of various halogenated aromatic carboxylic acids with propanol were
carried out in the molar ratio of carboxylic acid : propanol = 1:10 over 0.05 g of SZ at 130 °C
and the results are presented in Table 4. Spectral data of the synthesized esters are given in
Appendixꢀ1.
Table 4. List of halogenated aromatic carboxylic acids, alcohols, reaction time, products and
yield of products.
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Entry
Carboxylic acids
Alcohols
Time
(min)
Esterified product Yield (%) of
esterified
product
1
O
O
O
O
OH
Cl
nꢀpropnol 60
O
O
O
O
O
O
O
O
93.5
Cl
Br
I
2
3
4
OH
Br
nꢀpropnol 60
nꢀpropnol 60
nꢀpropnol 60
83.6
81.5
98.5
OH
I
OH
F
F
F
5
O
OH
Iso
60
O
O
79.6
propanol
F
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6
7
O
O
OH
nꢀpropnol 60
O
O
O
O
99.2
Br
Br
OH
nꢀpropnol 60
98.7
I
I
[
Reaction conditions: reaction temperature = 130 °C, catalyst weight = 0.05 g of SZ, 1:10
(carboxylic acid: alcohol).
Appendix – 1
Spectral data of synthesized esters (Refer to table 4)
Propyl 2-chloro benzoate (Table 4, Entry 1): 1H NMR (CDCl3, 400 Hz): δ (ppm) 1.041 (t,
3H, J=7.6 Hz), 1.801 (sextate, 2H, J=7.6 Hz), 4.308 (t, 2H, J=6.4 Hz), 7.311 (t, 1H, J=8.0 Hz),
7.385ꢀ7.461 (m, 2H), 7.816 (d, 1H, J=7.2 Hz); 13C NMR (CDCl3, 400 Hz): δ (ppm) 10.524,
21.990, 67.149, 126.494, 130.585, 130.980, 131.259, 133.312, 133.552, 165.827; Calculated for
C10H11ClO2 [m+H.+: 199.04, found 199.1; Physical state: Colourless liquid.
Propyl 2-bromo benzoate (Table 4, Entry 2): 1H NMR (CDCl3, 400 Hz): δ (ppm) 1.043 (t,
3H, J=7.6 Hz), 1.807 (sextate, 2H, J=7.6 Hz), 4.308 (t, 2H, J=6.4 Hz), 7.296ꢀ7.379 (m, 2H),
13
7.757 (dd, 1H, J1=7.6 Hz, J2=2.0 Hz), 7.780 (dd, 1H, J1=7.6 Hz, J2=2.0Hz); H NMR (CDCl3,
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400 Hz): δ (ppm) 10.547, 21.974, 67.234, 121.497, 127.083, 131.166, 132.328, 132.669,
134.249, 166.339; Calculated for C10H11BrO2 [m+H.+: 242.99, found 243.0; Physical state:
Colourless liquid.
Propyl 2-iodo benzoate (Table 4, Entry 3): 1H NMR (CDCl3, 400 Hz): δ (ppm) 1.043 (t, 3H,
J=7.6 Hz), 1.812 (sextate, 2H, J=7.2 Hz), 4.304 (t, 2H, J=6.4 Hz), 7.140 (t, 1H, J=7.6 Hz), 7.397
(t, 1H, J=7.6 Hz), 7.787 (d, 1H, J=7.2 Hz), 7.986 (d, 1H, J=8.0 Hz); 1H NMR (CDCl3, 400 Hz): δ
(ppm) 10.570, 21.145, 67.273, 93.901, 127.834, 130.786, 132.421, 135.620, 141.214, 166.648;
Calculated for C10H11IO2 [m+H.+: 290.98, found 291.0; Physical state: Colourless liquid.
Propyl 3-fluoro benzoate (Table 4, Entry 4): 1H NMR (CDCl3, 400 Hz): δ (ppm) 1.036 (t,
3H, J=7.2 Hz), 1.800 (sextate, 2H, J=7.2 Hz), 4.293 (t, 2H, J=6.4 Hz), 7.229ꢀ7.261 (m, 1H),
1
7.415 (q, 1H, J=8.0 Hz), 7.723 (d, 1H, J=9.2 Hz), 7.842 (d, 1H, J=7.6 Hz); H NMR (CDCl3,
400 Hz): δ (ppm) 10.392, 22.021, 66.839, 116.260, 116.484, 119.661, 119.878, 125.200,
125.223, 129.849, 129.926, 132.669, 132.746, 161.303, 163.759, 165.463; Calculated for
C10H11FO2 [m+H.+: 183.07, found 183.1; Physical state: Colourless liquid.
Isopropyl 3-fluoro benzoate (Table 4, Entry 5): 1H NMR (CDCl3, 400 Hz): δ (ppm) 1.375
(d, 6H, J=6.4 Hz), 5.254 (h, 1H, J=6.0 Hz), 7.232 (dd, 1H, J1=8.4 Hz, J2=2.0 Hz), 7.406 (q, 1H,
J=8.0 Hz), 7.712 (d, 1H, J=9.2 Hz), 7.830 (d, 1H, J=7.6 Hz); 1H NMR (CDCl3, 400 Hz): δ (ppm)
21.843, 68.838, 116.252, 116.477, 119.560, 119.777, 125.177, 129.787, 129.864, 133.087,
133.164, 161.295, 163.751, 164.921; C10H11FO2 [m+H.+: 183.07, found 183.1; Physical state:
Colourless liquid.
Propyl 3-bromo benzoate (Table 4, Entry 6): 1H NMR (CDCl3, 400 Hz): δ (ppm) 1.032 (t,
3H, J=7.2 Hz), 1.797 (sextate, 2H, J=7.2 Hz), 4.287 (t, 2H, J=7.2 Hz), 7.317 (t, 1H, J=8.0 Hz),
1
7.677 (d, 1H, J=8.0 Hz), 7.976 (d, 1H, J=7.2 Hz), 8.175 (s, 1H); H NMR (CDCl3, 400 Hz): δ
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(ppm) 10.439, 22.036, 66.917, 122.388, 128.090, 129.856, 132.452, 132.498, 135.690, 165.246;
Calculated for C10H11BrO2 [m+H.+: 242.99, found 243.0; Physical state: Colourless liquid.
Propyl 4-iodo benzoate (Table 4, Entry 7): 1H NMR (CDCl3, 400 Hz): δ (ppm) 1.023 (t, 3H,
J=7.6 Hz), 1.787 (sextate, 2H, J=7.6 Hz), 4.274 (t, 2H, J=6.8 Hz), 7.747 (d, 2H, J=8.4 Hz), 7.799
1
(d, 2H, J=8.4 Hz); H NMR (CDCl3, 400 Hz): δ (ppm) 10.454, 22.052, 66.746, 100.494,
129.996, 130.972, 137.650, 166.091; Calculated for C10H11IO2 [m+H.+: 290.98, found 291.0;
Physical state: Colourless liquid.
3.4.2. Effect of re-usability of solid acids in the esterification of 3FBA with PrOH
In order to study the reusability of solid acid catalysts, esterification of 3FBA with PrOH
o
was carried with 0.05 g of SZ at 130 C for 60 min under microwave irradiation. The solid acid
catalyst (SZ) was recovered after the reaction by filtration, washed with ethyl acetate, dried in an
o
o
oven at 120 C for 1 h and calcined at 550 C in a muffle furnace for 1 h. Thus thermally
regenerated solid acid catalyst was reꢀused in the next esterification reaction cycle. Such studies
were carried out for 5 consecutive reaction cycles and the results are presented in Fig. 12. It can
be seen from the figure that, not much change in the activity of reꢀactivated SZ was observed in
esterification reaction which indicates that the SZ solid acid can be effectively reꢀactivated and
reꢀused at least for 5 reaction cycles.
29