Solvent free synthesis of coumarins using environment friendly solid acid catalysts

Article abstract of DOI:10.1016/j.apcata.2012.12.024

Solid acid catalysts, ZrPW (Zirconium(IV) Phosphotungstate) and 12-TPA/ZrO₂ [12-Tungstophosphoric acid (12-TPA) supported onto ZrO₂] have been synthesized. The catalysts have been characterized for chemical stability, elemental analysis by ICP-AES, TGA, FTIR, SEM, EDX, XRD, surface area (BET) and surface acidity (NH₃-TPD). The performance of these materials as solid acid catalysts has been explored by studying Pechmann condensation as a model reaction, wherein phenols have been treated with methyl acetoacetate to give coumarins, under solvent free conditions using conventional heating as well as microwave heating. Catalytic activity of both the solid acid catalysts have been compared and correlated with surface properties of the materials.

Full text of DOI:10.1016/j.apcata.2012.12.024

Applied Catalysis A: General 453 (2013) 219–226
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Solvent free synthesis of coumarins using environment friendly solid
acid catalysts
Shrinivas Ghodke, Uma Chudasama∗
Applied Chemistry Department, Faculty of Technology and Engineering, The M. S. University of Baroda, Vadodara – 390001, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Solid acid catalysts, ZrPW (Zirconium(IV) Phosphotungstate) and 12-TPA/ZrO2 [12-Tungstophosphoric
acid (12-TPA) supported onto ZrO2] have been synthesized. The catalysts have been characterized for
chemical stability, elemental analysis by ICP-AES, TGA, FTIR, SEM, EDX, XRD, surface area (BET) and
surface acidity (NH3-TPD). The performance of these materials as solid acid catalysts has been explored by
studying Pechmann condensation as a model reaction, wherein phenols have been treated with methyl
acetoacetate to give coumarins, under solvent free conditions using conventional heating as well as
microwave heating. Catalytic activity of both the solid acid catalysts have been compared and correlated
with surface properties of the materials.
Received 19 June 2012
Received in revised form
12 December 2012
Accepted 14 December 2012
Available online xxx
Keywords:
Solid acid catalyst
Zirconium(IV) Phosphotungstate
12-TPA supported solid acid catalyst
Coumarin derivatives
© 2012 Elsevier B.V. All rights reserved.
Pechmann condensation
Microwave-assisted synthesis
1. Introduction
A number of solid acid catalysts have been reported for syn-
thesis of coumarin derivatives. Hoefnagel et al. have reported the
Coumarins are used as additives in food and cosmetics, optical
brightening agents, dispersed fluorescent and laser dyes [1], exhibit
useful and diverse biological activity and also serve as anticoagulant
agents [2]. Synthetic routes to coumarins include Pechmann con-
densation, Perkin, Knoevenagel and Reformatsky reactions and by
flash vacuum pyrolysis [3]. Pechmann condensation has been the
most widely applied method for coumarin synthesis, since it pro-
ceeds from simple starting materials (phenol and ␤-keto ester) and
gives good yields of coumarins with substitution in either pyrone or
benzene ring or in both. However, Pechmann condensation utilizes
various catalysts, such as sulfuric acid, trifluoroacetic acid, phos-
phorous pentoxide, ZrCl4, TiCl4, and ionic liquids, which require
long reaction times, corrodes reactor, creates by-products and salt
waste due to acid neutralization [4]. Use of heterogeneous solid acid
catalysts would provide safer operating conditions, ease of product
work up, reduction in equipment corrosion and minimization of
waste production and above all the catalysts can be regenerated and
reused. Therefore, there is a global effort to replace the conventional
liquid acid catalysts by heterogeneous solid acid catalysts.
use of Amberlyst – 15, H-Beta and Filtrol as solid acid catalysts
for synthesis of coumarin derivatives via Pechmann condensation
and observed that, these catalysts though require short reaction
times, require high reaction temperature and solvent medium for
removal of azeotropic water [5]. When phosphotungstic acid was
used, reaction time is short with good yields of coumarin, however
the major disadvantage is that the catalyst cannot be regenerated
and reused and requires the use of solvent [4]. Further, Laufer et al.
have reported the use of Nafion resin supported over silica compos-
ite materials for synthesizing coumarin derivatives that gives good
yields with short reaction times, but the catalyst regeneration and
reuse studies have not been reported [6]. Palaniappan and Shekhar
have reported the use of polyaniline supported catalytic systems
for synthesizing 7-hydroxy-4-methyl coumarin under solvent free
conditions but with poor yields of coumarin [7]. Gunnewegh et al.
have reported Zeolite catalyzed synthesis of coumarin derivatives
which requires short reaction times with regeneration and reuse of
catalyst, however with requirement of solvent and gives low yields
of coumarin derivatives [8]. Thus the need for an ideal solid acid
catalyst is on.
Tetravalent Metal Acid (TMA) salts are inorganic
cation
exchangers
possessing
general
formula
M(IV)
(HXO4)2·nH2O[M(IV) = Zr, Ti, Sn, Ce, Th, etc. X = P, W, Mo, As,
Sb, etc.] where, H+ of the structural hydroxyl groups are respon-
sible for cation exchange, due to which TMA salts indicate good
∗
Corresponding author. Tel.: +91 265 2434188x415; fax: +91 265 2423898.
E-mail addresses: shrini86@hotmail.com (S. Ghodke), uvcres@gmail.com,
uvc50@yahoo.com (U. Chudasama).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.12.024

220
S. Ghodke, U. Chudasama / Applied Catalysis A: General 453 (2013) 219–226
potential for application as solid acid catalysts, the acidic sites
being bronsted acid sites in nature [9,10]. Since, TMA salts are syn-
thesized by sol–gel method, TMA salts with varying water content,
composition and crystallinity can be obtained by varying several
parameters such as mole ratio of reactants M:X (M = tetravalent
metal, X = polyvalent anion), temperature of mixing, mode of
mixing (metal salt solution to anion salt solution or vice versa),
pH and rate of mixing. Variation in any of these parameters yields
materials with different characteristics. The preparation procedure
thus affects the structural hydroxyl groups, which is reflected in
their performance as solid acid catalysts. Thus, the area of study of
TMA salts is evergreen and much remains to be investigated.
From our laboratory, use of TMA salts as solid acid catalysts
has been explored for esterification [11–13], cyclodehydration [14],
ketalisation of ketones [15], hydration of nitriles [16] and cyclode-
hyration of 1,n-diols [17]. Synthesis of coumarin derivatives have
been earlier reported by us using TMA salts as solid acid catalysts
[18].
(IEC)/protonating ability, varying several parameters such as mole
ratio of reactants, temperature, mode of mixing (metal salt solu-
tion to anion salt solution or vice versa), pH and rate of mixing.
Several sets of materials were prepared varying conditions in each
case using IEC as the indicative tool. The optimized parameters for
synthesis of ZrPW have been presented in Table 1. We describe
herewith the synthesis of ZrPW at optimized condition. An aqueous
mixture of NaH2PO4·2H2O (0.1 M, 50 ml) and Na2WO4·2H2O (0.1 M,
50 ml) was added dropwise (flow rate 1 ml min−1) to an aqueous
solution of ZrOCl2·8H2O (0.2 M, 50 ml) with continuous stirring for
an hour at 70 ◦C. The gelatinous precipitates obtained was filtered,
washed with double distilled water and dried at room temperature.
The material was then broken down to the desired particle size
(30-60 mesh) by grinding and sieving and acid treated by method
reported earlier [18]. This material was used for all studies.
2.2. Synthesis of 12-TPA/ZrO2
It has been earlier observed by us that, mixed materials of
the class of tetravalent bimetallic acid (TBMA) salts i.e. Zirconium
Titanium Phosphate (ZTP), used as solid acid catalyst, exhibits
enhanced catalytic activity [13]. It was therefore thought of inter-
est to explore the potential utility of mixed material of the class
of Tetravalent Metal Bianionic Acid (TMBA) salts containing two
different anions and a cation as solid acid catalyst with an aim to
enhance surface acidity.
Heteropoly acids (HPAs) have proved to be the alternative for
traditional acid catalysts due to both strong acidity and appropri-
ate redox properties. The major disadvantage of HPAs, as catalyst
lies in their low thermal stability, low surface area (1–10 m2/g) and
separation problems from reaction mixture. HPAs can be made eco-
friendly, insoluble solid acids, with high thermal stability and high
surface area by supporting them onto suitable supports. The sup-
port provides an opportunity for HPAs to be dispersed over a large
surface area which increases catalytic activity [19].
The use of microwave (MW) irradiation has been employed for
a number of organic transformations to reduce the reaction time,
rate enhancement and to increase selectivity and yields. The use of
solid acid catalysts (SACs) for MW assisted synthesis of coumarin
is scarce. Tyagi et al. have reported MW assisted solvent free syn-
thesis of coumarin derivatives using sulfated zirconia which is also
associated with the difficulty of leaching of sulfate ion during the
course of reaction depriving reusability [20].
In the present endeavor, a mixed material Zirconium Phospho-
tungstate (ZrPW) of the class of TMBA salts containing two different
anions and a cation, has been synthesized by sol–gel route. Another
catalyst 12-Tungstophosphoric acid (12-TPA) supported on ZrO2
(12-TPA/ZrO2) possessing same component (i.e. Zr, P, and W), has
been synthesized by process of anchoring and calcination [21]. Both
the catalysts have been characterized for chemical stability, ele-
mental analysis by ICP-AES, TGA, FTIR, SEM, EDX, XRD, surface area
(BET) and surface acidity (NH3-TPD) and their utility as solid acid
catalysts has been explored by studying Pechmann condensation as
a model reaction, wherein phenols have been treated with methyl
acetoacetate to give coumarins, under solvent free conditions using
conventional heating as well as microwave heating. Catalytic activ-
ity of both catalysts has been compared and correlated with surface
properties of the materials.
10 g of ZrOCl2·8H2O was dissolved in 100 ml double distilled
water followed by dropwise addition of liq. NH3 (25%) with vig-
orous stirring. The pH of the solution was adjusted to 9.5. White
precipitates obtained, was filtered and washed with conductivity
water till removal of adhering ions which was first dried at 120 ◦C
for 3 h, and then calcined at 550 ◦C for 5 h. For the preparation of 12-
TPA supported catalyst, a series of aqueous solution of 10–30 wt.%
of 12-TPA per gram of precalcined ZrO2 was used, and the mixture
was stirred for 36 h. The excess water was removed at 70 ◦C under
vacuum. The resulting solid was dried at 120 ◦C for 3 h, followed by
grinding to get a fine powder. This material was used for all studies.
2.3. Catalyst characterization
The solubility of ZrPW in various media such as acids (HCl, H2SO4
and HNO3), bases (NaOH and KOH) and organic solvents (ethanol,
benzene, acetone and acetic acid) was studied by taking 0.5 g of
ZrPW in 50 ml of the particular medium and allowed to stand for
24 h. The change in color, weight, solubility, etc. was observed. Ele-
mental analysis was performed on ICP-AES spectrometer (Thermo
Scientific iCAP 6000 series). FTIR spectra was recorded using KBr
pellet on Shimadzu (Model 8400S). Thermal analysis (TGA) was
carried out on a Shimadzu (Model TGA 50) thermal analyzer at
a heating rate of 10 ◦C min−1. X-ray diffractogram (2Â = 10–80◦)
was obtained on X-ray diffractometer (Brucker AXS D8) with
Cu-K␣ radiation with nickel filter. SEM and EDX of the sample
were scanned on Jeol JSM-5610-SLV scanning electron micro-
scope. Surface area measurement (by BET method) was carried
out on Micromeritics Gemini at −196 ◦C using nitrogen adsorp-
tion isotherms. Surface acidity was determined on Micromeritics
Chemisorb 2720, by a temperature programmed desorption of
ammonia. Ammonia was chemisorbed at 120 ◦C and then desorp-
tion was carried out upto 700 ◦C at a heating rate of 10 ◦Cmin−1
.
2.4. Catalytic activity: Pechmann condensation
In a typical reaction, methyl acetoacetate (MA) (15 mmol) (sub-
strate as well as solvent) and phenols (10 mmol) [resorcinol (R),
pyrogallol (Py), phloroglucinol (Ph), hydroquinone (Hq) and p-
nitrophenol (pNp)] was stirred with catalyst (0.20 g) in 50 ml two
necked round bottom flask at 130 ◦C for 8 h. Using same conditions,
the reactions were subjected to microwave irradiation (250 W) at
130 ◦C for 30 min. In both cases, after completion of reaction, the
mixture got solidified within an hour on cooling.
2. Experimental
2.1. Synthesis of ZrPW
The resulting solidified mixture (both cases) was dissolved
in ethyl acetate (2 ml), and catalyst separated by filtration using
a Buchner funnel. The filtrate, distilled under vacuum, yielded
the crude product, which was purified by recrystallization. All
ZrPW was synthesized by sol–gel method, the main objec-
tive being to obtain a material with high ion exchange capacity

S. Ghodke, U. Chudasama / Applied Catalysis A: General 453 (2013) 219–226
221
Table 1
Optimization of parameters for synthesis of ZrPW.
Parameters optimized
Concentration
No
Concentration (M)
Volume (ml)
Mole ratio
Zr:P:W
Temp 2 ◦
C
Stirring time (h)
Aging time (h)
IEC (meq/g)
Zr
P
W
Zr
50
P
W
1
2
3
4
5
6
7
8
0.1
0.2
0.2
0.2
0.1
0.1
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.1
0.1
0.2
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.1
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
1:1:1
2:1:1
2:1:2
2:2:1
1:2:2
2:1:1
4:1:1
2:1:1
2:1:1
2:1:1
2:1:1
2:1:1
30
30
30
30
30
30
30
30
30
30
70
70
1
1
1
1
1
1
1
1
1
1
1
2
15
15
15
15
15
15
15
1
3
5
0
0
0.56
0.98
0.44
0.89
0.38
1.08
0.75
0.90
0.92
0.92
1.20
1.18
50
50
50
50
100
100
50
Volume
Aging Time
9
50
50
10
11a
12
Temperature
Stirring time
50
50
a
Optimum condition for synthesis of ZrPW.
symmetric –OH stretching. A medium band around ∼1635 cm−1 is
Table 2
Solubility data of ZrPW (maximum tolerable limits).
attributed to P
O H bending. These bands indicate the presence
Chemical media
Maximum tolerable limits
of structural –OH protons (bronsted sites) contained in ZrPW. A
band in the region ∼1083 cm−1 is attributed to the presence of P
O
Acids
Bases
Organic solvents
18N H2SO4, 16N HNO3, 11.3N HCl
5N NaOH, 5N KOH
Ethanol, benzene, toluene, acetone and acetic acid
stretching. FTIR spectrum of 12-TPA/ZrO2 (Fig. 2) exhibits peaks
at ∼3450 cm−1, ∼1635 cm−1, ∼1083 cm−1, ∼987 cm−1, which cor-
responds to asymmetric and symmetric –OH stretching, P
bending, P O stretching and W O stretching respectively.
O H
synthesized coumarin derivatives were characterized for IR spec-
troscopy. Melting point (Tables 6 and 7) was compared with the
corresponding reported melting point [22]. Reaction parameters
such as, mole ratio of reactants, amount of catalyst, reaction time
and reaction temperature were optimized.
For regeneration and reusability, the catalyst was refluxed in
ethanol for half hour, washed with double distilled water and dried
at room temperature. ZrPW was acid treated as described in catalyst
synthesis whereas 12-TPA/ZrO2 was dried in oven at 150 ◦C for 2 h
and used.
TGA thermogram of ZrPW (Fig. 3) shows 19.78% weight loss
in the temperature range of 40–150 ◦C which corresponds to loss
of surface moisture and hydrated water. A 7.25% weight loss in
the temperature range of 550–600 ◦C is attributed to condensation
of structural hydroxyl groups. TGA thermogram of 12-TPA/ZrO2
(Fig. 3) exhibits 0.9% weight loss in the temperature range of
30–180 ◦C which corresponds to the loss of surface moisture. SEM
images (Figs. 4 and 5) of ZrPW and 12-TPA/ZrO2 exhibits irregular
morphology for both the catalysts.
EDX graph for ZrPW (Fig. 6) shows atomic % of Zr, P and W is
found to be 60.79%, 18.53% and 20.67% respectively. EDX graph for
12-TPA/ZrO2 (Fig. 7) shows atomic % of Zr, W and O is found to be
28.75%, 3.44% and 67.81% respectively. Absence of sharp peaks in X-
ray diffractogram of ZrPW (Fig. 8) reveals the amorphous nature of
ZrPW, while X-ray diffractogram pattern of 12-TPA/ZrO2 (Fig. 8)
shows intense and well defined characteristic diffraction peaks
3. Results and discussion
3.1. Catalyst characterization
A study on the solubility of ZrPW shows that it is stable in
acid and organic solvent media. It is however not so stable in
base medium. The maximum tolerable limits are presented in
Table 2. Elemental analysis performed by ICP-AES for ZrPW shows
Zr = 26.87%, P = 5.67% and W = 25.37%. The ratio of Zr:P:W was found
to be 2:1:1. FTIR spectrum of ZrPW (Fig. 1) exhibits broad band
in the region ∼3400 cm−1 which is attributed to asymmetric and
˚
with d values of 2.84, 2.54, 1.81, and 1.54 A (JCPDS data card no.
17-923). Surface area (BET method) for ZrPW and 12-TPA/ZrO2 was
found to be 80.96 m2/g and 33.90 m2/g respectively.
ZrPW exhibits broad desorption peaks (Fig. 9) at higher tem-
perature (at ∼240 ◦C and ∼525 ◦C) compared to 12-TPA/ZrO2 (at
Fig. 1. FT-IR spectrum of ZrPW.
Fig. 2. FT-IR spectrum of 12-TPA/ZrO2.

222
S. Ghodke, U. Chudasama / Applied Catalysis A: General 453 (2013) 219–226
20
18
16
14
12
10
8
12-TPA/ZrO2
ZrPW
6
0
100 200 300 400 500 600 700 800 900
Temperature (ºC)
Fig. 3. TGA plot of ZrPW and 12-TPA/ZrO2.
Fig. 6. EDX image of fresh ZrPW.
Fig. 4. SEM image of ZrPW.
Fig. 7. EDX image of fresh 12-TPA/ZrO2.
6.05 mmol/g respectively, while surface acidity for 12-TPA/ZrO2
at 150 ◦C and 200 ◦C preheating temperature (Fig. 12) was found
to be 1.07 mmol/g and 0.89 mmol/g respectively. Decrease in sur-
face acidity with increasing preheating temperatures could be
attributed to condensation of structural hydroxyl groups as dis-
cussed above in thermal behavior of the material.
∼210 ◦C) (Fig. 10) indicating more as well as strong acid sites in
ZrPW. Surface acidity for ZrPW and 12-TPA/ZrO2 at 700 ◦C preheat-
ing temperature was found to be 3.93 mmol/g and 0.17 mmol/g
respectively. Surface acidity for ZrPW at 150 ◦C and 200 ◦C pre-
heating temperature (Fig. 11) was found to be 9.34 mmol/g and
Fig. 5. SEM image of 12-TPA/ZrO2.
Fig. 8. XRD pattern of ZrPW and 12-TPA/ZrO2.

S. Ghodke, U. Chudasama / Applied Catalysis A: General 453 (2013) 219–226
223
Fig. 12. NH3-TPD patterns of 12-TPA/ZrO2 at 150 ◦C and 200 ◦C preheating temper-
ature.
Fig. 9. NH3-TPD pattern of ZrPW at 700 ◦C preheating temperature.
Table 3
Optimization of reaction parameters for Pechmann condensation of resorcinol using
ZrPW.
Reactants
Mole ratio
Amount of
catalyst (g)
Reaction
time (h)
Reaction
temperature
(◦C)
% yield
R + MA
R + MA
R + MA
R + MA
R + MA
R + MA
R + MA
R + MA
R + MA
R + MA
R + MA
R + MA
R + MA
R + MA
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1.5
1:2
1.5:1
2:1
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.15
0.20
0.25
0.20
0.20
0.20
0.20
2
4
6
8
10
8
8
8
8
8
130
130
130
130
130
120
140
130
130
130
130
130
130
130
4.88
12.55
28.31
43.90
44.62
40.14
39.83
51.69
54.85
56.48
58.12
50.41
51.47
48.32
8
8
8
8
Bold text indicates optimum condition for Pechmann condensation of resorcinol
using ZrPW.
3.2. Catalytic activity: Pechmann condensation
Fig. 10. NH3-TPD pattern of 12-TPA/ZrO2 at 700 ◦C preheating temperature.
In case of Pechmann condensation of resorcinol by conventional
heating process using ZrPW, yield increases with reaction time until
equilibrium is reached within 8 h (Table 3). For the same reaction
time, yield increases with catalyst (ZrPW) amount, since number
of active sites per g of substrate increases. A maximum product
yield is obtained with reaction temperature 130 ◦C, beyond which
product degradation is observed. In the present study, the catalytic
activity increases as the wt.% of 12-TPA loaded onto ZrO2 increases
(Table 4). The material with 20 wt.% 12-TPA/ZrO2 gives good yields
of coumarin derivatives. Thus, for all studies 20 wt.% 12-TPA/ZrO2
has been used [23,24].
Table 4
Optimization of amount of 12-TPA loading on ZrO2 for Pechmann condensation of
resorcinol with methyl acetoacetate.
Reactants
Catalyst used
% yield
R + MA
R + MA
R + MA
10 wt.% 12-TPA/ZrO2
20 wt.% 12-TPA/ZrO2
30 wt.% 12-TPA/ZrO2
40.12%
57.01%
57.26%
Mole ratio of R:MA = 1:1.5; reaction temperature: 130 ◦C; reaction time: 8 h; amount
of catalyst: 0.20 g.
Bold text indicates optimum condition for amount of 12-TPA loading on ZrO2 for
Pechmann condensation of resorcinol with methyl acetoacetate.
Fig. 11. NH3-TPD patterns of ZrPW at 150 ◦C and 200 ◦C preheating temperature.

224
S. Ghodke, U. Chudasama / Applied Catalysis A: General 453 (2013) 219–226
Table 5
Optimization of reaction time for Pechmann condensation of phenols using ZrPW
under MW heating (250 W).
Reactants Mole ratio Amount of
catalyst (g)
Reaction
time (min)
Reaction
temperature
(◦C)
% yield
R + MA
R + MA
R + MA
R + MA
1:1.5
1:1.5
1:1.5
1:1.5
0.20
0.20
0.20
0.20
10
20
30
40
130
130
130
130
32.12
49.83
61.82
61.90
Bold text indicates optimum reaction time for Pechmann condensation of phenols
using ZrPW under MW heating (250 W).
Under MW irradiation it is observed that, yield increases with
reaction time until equilibrium is reached within 30 min (Table 5).
It is observed that when the reaction time is further prolonged to
40 min, there is not much increase in % yield of coumarin deriva-
tive. Therefore, the reaction time 30 min was taken as optimum
reaction time under MW (Table 5). Further, higher yields obtained
under MW irradiation is probably due to the fact that the pheno-
lic substrates and methyl acetoacetate being polar molecules, are
microwave active and absorb the MW radiations rapidly and accel-
erate the rate of reaction. Formation of polar methanol (by-product)
also helps in absorption of MW radiation and thereby accelerating
the reaction [20].
Fig. 13. EDX image of spent ZrPW.
presented in Fig. 9 and 10. % yields of different coumarin deriva-
tives at optimized condition, under conventional heating as well
as by microwave heating using ZrPW and 12-TPA/ZrO2 have been
presented in Tables 6 and 7.
The reactivity of phloroglucinol with methyl acetoacetate was
observed to be higher than pyrogallol due to two hydroxyl groups at
meta-positions in phloroglucinol compared to one hydroxyl group
in pyrogallol [20]. Presence of meta-hydroxy group strongly acti-
vates the substrates due to resonance effect. Further, low yield
in case of p-nitrophenol may be due to the presence of electron
withdrawing group (–NO2) in p-nitrophenol.
Pechmann condensation proceeds through acid-catalyzed reac-
tions. Therefore, Pechmann reaction depends strongly on acidity
of catalysts. Number and nature of surface acid sites play a pre-
dominant role in evaluating and correlating catalytic activity [18].
In all cases, yields are higher for ZrPW which could be attributed
to high surface area as well as high surface acidity of ZrPW com-
pared to 12-TPA/ZrO2 as is also indicative in NH3-TPD patterns
Both the catalysts turn dark brown after each catalytic run dur-
ing the course of the reaction, probably due to the fact that reactant
molecules come onto surface of catalyst. Some molecules enter
into reaction to give product while a few of them get adsorbed on
surface, which is marked by change in color of catalysts. Reactant
molecules are weakly adsorbed is evident from catalysts regain-
ing its original color, when refluxed in ethanol as described in
experimental section. It is observed that after every regeneration
decreased yields are observed which is probably due to the deacti-
vation of catalysts because of substrate molecules getting adsorbed
on surface or also entering interstices of the catalyst material [18].
Further, EDX of spent ZrPW (Fig. 13) shows slight decrease in atomic
% of Zr and W is found to be 54.26% and 20.19% respectively, while
EDX of 12-TPA/ZrO2 (Fig. 14) shows decrease in atomic % of Zr and
Table 6
Pechmann condensation of phenols using ZrPW and 12-TPA/ZrO2 at optimum condition.
Reactants
R + MA
Catalyst used
% yield
Melting point (◦C)
185–186
Product formed
ZrPW
12-TPA/ZrO2
ZrPW
12-TPA/ZrO2
ZrPW
12-TPA/ZrO2
ZrPW
12-TPA/ZrO2
ZrPW
12-TPA/ZrO2
58.12
57.01
54.22
48.45
61.00
58.37
64.48
63.00
42.12
38.18
7-Hydroxy-4-methyl coumarin
Py + MA
Ph + MA
Hq + MA
pNp + MA
244–245
283–284
165–166
149–150
7,8-Dihydroxy-4-methyl coumarin
5,7-Dihydroxy-4-methyl coumarin
6-Hydroxy-4-methyl coumarin
6-Nitro-4-methyl coumarin
Mole ratio of phenol:MA = 1:1.5; reaction temperature: 130 ◦C; reaction time: 8 h; amount of catalyst: 0.20 g.
Table 7
Pechmann condensation of phenols using ZrPW and 12-TPA/ZrO2 at optimized conditions under MW heating (250 W).
Reactants
R + MA
Catalyst used
% yield
Melting point (◦C)
185–186
Product formed
ZrPW
12-TPA/ZrO2
ZrPW
12-TPA/ZrO2
ZrPW
12-TPA/ZrO2
ZrPW
12-TPA/ZrO2
ZrPW
12-TPA/ZrO2
61.82
60.10
53.68
49.82
61.73
60.19
66.37
65.43
41.53
40.68
7-Hydroxy-4-methyl coumarin
Py + MA
Ph + MA
Hq + MA
pNp + MA
244–245
283–284
165–166
149–150
7,8-Dihydroxy-4-methyl coumarin
5,7-Dihydroxy-4-methyl coumarin
6-Hydroxy-4-methyl coumarin
6-Nitro-4-methyl coumarin
Mole ratio of phenol:MA = 1:1.5; reaction temperature: 130 ◦C; reaction time: 30 min; amount of catalyst: 0.20 g.

S. Ghodke, U. Chudasama / Applied Catalysis A: General 453 (2013) 219–226
225
Table 8
Results for reusability of catalyst in case of synthesis of 6-hydroxy-4-methyl
coumarin at optimum condition.
Catalyst used
Cycle no. %Yield
%Yield (MW
(conventional heating)
heating)
1
2
3
1
2
3
1
2
3
64.48
63.12
60.18
64.48
56.21
44.34
63.00
48.10
39.81
66.37
65.81
62.31
66.37
55.28
47.61
65.43
47.12
38.36
ZrPW (with regeneration/acid
treatment)
ZrPW (with out regeneration/acid
treatment)
12-TPA/ZrO2
3.3. Characterization of the products
The isolated products were characterized by melting point and
FT-IR spectroscopy.
Fig. 14. EDX image of spent 12-TPA/ZrO2.
3.3.1. 7-Hydroxy-4-methyl coumarin
Melting point: 185–186 ◦C; FT-IR (KBr): 3155 cm−1, 1678 cm−1
,
1227 cm−1, 1057 cm−1, 974 cm−1, 844 cm−1, 748 cm−1
.
W is found to be 26.12% and 2.81% respectively. Therefore, decrease
in yield of coumarin derivatives may be due to the leaching of Zr and
W. Results for reusability of both the catalysts in case of synthesis
of 6-Hydroxy-4-Methyl coumarin at optimum condition have been
presented in Table 8.
The Pechmann condensation proceeds through transesterifica-
tion followed by intramolecular hydroalkylation and dehydration
[25–27]. These three steps are acid catalyzed reactions. A possible
mechanism for the Pechmann condensation of phenols and ␤-keto
ester by solid acid catalysts is presented in Scheme 1, which is also
proposed by other workers [7].
3.3.2. 7,8-Dihydroxy-4-methyl coumarin
Melting point: 244–245 ◦C; FT-IR (KBr): 3420 cm−1, 1839 cm−1
,
,
1597 cm−1
,
1447 cm−1
,
1325 cm−1
,
1270 cm−1
,
1061 cm−1
901 cm−1, 784 cm−1, 515 cm−1
.
3.3.3. 5,7-Dihydroxy-4-methyl coumarin
Melting point: 283–284 ◦C; FT-IR (KBr): 3447 cm−1, 1865 cm−1
,
,
1660 cm−1
,
1618 cm−1
,
1530 cm−1
,
1456 cm−1
,
1160 cm−1
815 cm−1, 750 cm−1, 570 cm−1
.
O
C
R'
O
C
R'
O
O
H2C
CH2
C
C
H-O-
O
O--
H
OH
H
-H+
Catalyst
H3C
CH3
OH
Catalyst
HO
OH
Resorcinol
HO
OH
HO
O
C
R'
R'
O
O
OH
-R'OH
-H2O
O
O
O
C
HO
H-O-
OH
H2C
C
+
CH
H
C
OH
C
CH2
Catalyst
CH3
O
C
H3C
+OH2
CH3
HO
Scheme 1. Possible mechanism for the Pechmann condensation of phenols and ␤-keto ester by solid acid catalysts.

226
S. Ghodke, U. Chudasama / Applied Catalysis A: General 453 (2013) 219–226
3.3.4. 6-Hydroxy-4-methyl coumarin
References
Melting point: 165–166 ◦C; FT-IR (KBr): 3200 cm−1, 1820 cm−1
1597 cm−1, 1447 cm−1, 1053 cm−1, 832 cm−1, 725 cm−1, 575 cm−1
,
.
[1] D. Bogdal, J. Chem. Res. (1998) 468–469.
[2] Li Tong-Shuang, Z.H. Zhang, F. Yang, C.G. Fu, J. Chem. Res. (1998) 38–39.
[3] B.M. Reddy, M. Patil, P. Lakshmanan, J. Mol. Catal. A Chem. 256 (2006) 290–294.
[4] B.C. Raju, T.H. Babu, J.M. Rao, Indan J. Chem. 48 B (2009) 120–123.
[5] A.J. Hoefnagel, E.A. Gunnewegh, R.S. Downing, H.V. Bekkum, J. Chem. Soc.,
Chem. Commun., (1995) 225-226.
[6] M.C. Laufer, H. Hausmann, W.F. Hoelderich, J. Catal. 218 (2003) 315–320.
[7] S. Palaniappan, R.C. Shekhar, J. Mol. Catal. A Chem. 209 (2004) 117–124.
[8] E.A. Gunnewegh, A.J. Hoefnagel, H. van Bekkum, J. Mol. Catal. A Chem. 100
(1995) 87–92.
[9] A. Bhaumik, S. Inagaki, J. Am. Chem. Soc. 123 (2001) 691–696.
[10] R.C.T. Slade, J.A. Knowels, D.J. Jones, J. Roziere, Solid State Ionics 96 (1997) 9–19.
[11] R. Joshi, H. Patel, U. Chudasama, Indian J. Chem. Technol. 15 (2008) 238–243.
[12] H. Patel, R. Joshi, U. Chudasama, Indian J. Chem. 47A (2008) 348–352.
[13] R. Thakkar, U. Chudasama, Green Chem. Lett Rev. 2 (2009) 61–69.
[14] S. Patel, U. Chudasama, P. Ganeshpure, React. Kinet. Catal. Lett. 76 (2002)
317–325.
3.3.5. 6-Nitro-4-methyl coumarin
Melting point: 149–150 ◦C; FT-IR (KBr): 2920 cm−1, 1788 cm−1
,
,
1567 cm−1
,
1440 cm−1
,
1325 cm−1
,
1250 cm−1
,
1061 cm−1
922 cm−1, 854 cm−1, 795 cm−1
.
4. Conclusions
The study reveals good performance of both the catalysts ZrPW
and 12-TPA/ZrO2 under conventional and MW assisted synthesis
of coumarins, the highlighting feature being reduction of reac-
tion time from several hours (8 h) to few minutes (30 min) under
solvent free synthesis and MW conditions, besides advantages of
operational simplicity, mild reaction conditions and environment
friendly nature of the catalysts. Though both the catalysts can be
regenerated and reused, ZrPW scores over 12-TPA/ZrO2 where,
yields go down considerably in subsequent cycles. Thus, ZrPW
has potential for commercialization especially under microwave-
assisted synthesis of coumarin derivatives.
[15] S. Patel, U. Chudasama, P. Ganeshpure, J. Mol. Catal. A Chem. 194 (2003)
267–271.
[16] S. Patel, U. Chudasama, P. Ganeshpure, Indian J. Chem. 42B (2003) 1168–1169.
[17] S. Patel, U. Chudasama, P. Ganeshpure, Green Chem. 3 (2001) 143–145.
[18] R. Joshi, U. Chudasama, J. Sci. Indian Res. 67 (2008) 1092–1097.
[19] A.E.R.S. Khder, Appl. Catal. A Gen. 343 (2008) 109–116.
[20] B. Tyagi, M.K. Mishra, V.J. Jasra, J. Mol. Catal. A Chem. 286 (2008) 41–46.
[21] B.M. Devassy, G.V. Shanbhag, F. Lefebvre, W. Bohringer, J. Fletcher, S.B. Hal-
ligudu, J. Mol. Catal. A Chem. 230 (2005) 113–119.
[22] R.S. Keri, K.M. Hosamani, H.R.S. Reddy, Catal. Lett. 131 (2009) 321–327.
[23] B.M. Devassy, S.B. Halligudi, S.G. Hegde, A.B. Halgeri, F. Lefebvre, Chem. Com-
mun. (2002) 1074–1075.
[24] Biju M. Devassy, G.V. Shanbhag, F. Lefebvre, S.B. Halligudi, J. Mol. Catal. A Chem.
210 (2004) 125–130.
[25] H. Valizadeha, A. Shockravi, Tetrahedron Lett. 46 (2005) 3501–3503.
[26] M.K. Potdar, S.S. Mohile, M.M. Salunke, Tetrahedron Lett. 42 (2001) 9187–9285.
[27] J.C. Rodriguez-Dominguez, G. Kirsch, Tetrahedron Lett. 47 (2006) 3279–3281.
Acknowledgment
The authors thank Gujarat Council on Science and Tech-
nology (GUJCOST), Gandhinagar, Gujarat for providing financial
assistance.