Modified zirconia solid acid catalysts for organic synthesis and transformations

Article abstract of DOI:10.1016/j.molcata.2004.09.003

The sulfate, molybdate and tungstate promoted ZrO₂ catalysts were investigated by X-ray diffraction, NH₃-TPD, and Raman spectroscopy and were evaluated for various organic synthesis and transformation reactions. All catalysts exhibit good catalytic activity for synthesis of diphenylureas, coumarines and 1,5-benzodiazepines, acylation of alcohols, phenols and amines, and protection of carbonyl compounds. A series of sulfate, molybdate and tungstate promoted ZrO₂ catalysts were prepared by wet impregnation method. To incorporate these promoters to Zr(OH)₄, sulfuric acid, ammonium heptamolybdate, and ammonium metatungstate were used as precursors, respectively. Further, a Pt promoted Mo-ZrO₂ catalyst was also prepared separately by impregnating with hexachloroplatinic acid. The surface and bulk properties of various promoted ZrO₂ catalysts were investigated by means of X-ray powder diffraction, BET surface area, ammonia-TPD, and Raman spectroscopy techniques. The unpromoted ZrO₂ when calcined at 873 K exists in the crystalline form with monoclinic phase dominating over the tetragonal phase. Incorporation of various promoters into Zr(OH)₄ shows a strong influence on the bulk and the surface properties. Addition of promoters enhanced the tetragonal zirconia phase and the surface acidity. In the case of Pt/Mo-ZrO₂ catalyst, a complete tetragonal ZrO₂ phase is observed. The ammonia-TPD results indicate that the impregnated sulfate ions show a strong influence on the acidity of ZrO₂, which is followed by molybdate. The prepared catalysts were evaluated for various organic synthesis and transformation reactions in the liquid phase. All catalysts exhibit good catalytic activity for synthesis of diphenylureas, coumarines and 1,5-benzodiazepines, acylation of alcohols, phenols and amines, and protection of carbonyl compounds. In particular, the sulfate and molybdate promoted catalysts exhibited excellent catalytic activity.

Full text of DOI:10.1016/j.molcata.2004.09.003

Journal of Molecular Catalysis A: Chemical 225 (2005) 71–78
Modified zirconia solid acid catalysts for organic
synthesis and transformations
Benjaram M. Reddy∗, Pavani M. Sreekanth, Vangala R. Reddy
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Hyderabad – 500007, India
Received 8 July 2004; received in revised form 1 September 2004; accepted 1 September 2004
Available online 6 October 2004
Abstract
A series of sulfate, molybdate and tungstate promoted ZrO2 catalysts were prepared by wet impregnation method. To incorporate these
promoters to Zr(OH)4, sulfuric acid, ammonium heptamolybdate, and ammonium metatungstate were used as precursors, respectively. Further,
a Pt promoted Mo-ZrO2 catalyst was also prepared separately by impregnating with hexachloroplatinic acid. The surface and bulk properties
of various promoted ZrO2 catalysts were investigated by means of X-ray powder diffraction, BET surface area, ammonia-TPD, and Raman
spectroscopy techniques. The unpromoted ZrO2 when calcined at 873 K exists in the crystalline form with monoclinic phase dominating
over the tetragonal phase. Incorporation of various promoters into Zr(OH)4 shows a strong influence on the bulk and the surface properties.
Addition of promoters enhanced the tetragonal zirconia phase and the surface acidity. In the case of Pt/Mo-ZrO2 catalyst, a complete tetragonal
ZrO2 phase is observed. The ammonia-TPD results indicate that the impregnated sulfate ions show a strong influence on the acidity of ZrO2,
which is followed by molybdate. The prepared catalysts were evaluated for various organic synthesis and transformation reactions in the liquid
phase. All catalysts exhibit good catalytic activity for synthesis of diphenylureas, coumarines and 1,5-benzodiazepines, acylation of alcohols,
phenols and amines, and protection of carbonyl compounds. In particular, the sulfate and molybdate promoted catalysts exhibited excellent
catalytic activity.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Zirconium oxide; Solid superacid; Sulfate; Molybdate; Tungstate; Diphenylureas; Coumarines; 1,5-Benzodiazepines; Tetrahydropyranylation;
Acylation
1. Introduction
tinct advantage of solid acid catalysts such as non-toxicity,
non-corrosiveness, ease of handling, less expensive, and easy
to recover and reuse [1–6]. In this direction, various solid acid
systems were introduced which include hetropolyacids, ion
exchange resigns (Amberlyst and Nafion-H), zeolites, and
clays. The main disadvantage associated with hetropolyacids
is that they are fairly soluble in polar solvents and loose their
activity at higher temperatures by loosing structural integrity.
To prevent this there are some attempts to immobilize them
in silica or activated carbon matrix, which however limits the
accessibility and efficiency of the catalysts. Ion exchange re-
signs pose various problems like poor thermal stability and
low specific surface area. Styrine based resins like Amberlyst
isstableupto393–413 Kanditsacidstrengthandsurfacearea
are not very high (H0 = −2.2, SA = 0.4 m2 g−1). Nafion-H
exhibits better thermal stability (533–553 K) and acidity than
Acid catalysts play a predominant role in organic syn-
thesis and transformations. Many organic reactions such as
alkylation, acylation, isomerization, nitration, esterification,
and rearrangements like pinacol, Beckman, etc. are accom-
plished by acid catalysts. All these acid catalyzed reactions
are mostly carried out by employing conventional mineral
acids like H2SO4, HNO3, and HF or Lewis acids such as
AlCl3 and BF3. In view of environmental and economical
reasons, there is an ongoing effort to replace the conventional
catalysts with newer solid acids. This is mainly due to the dis-
∗
Corresponding author. Tel.: +91 40 2716 0123; fax: +91 40 2716 0921.
E-mail addresses: mreddyb@yahoo.com, bmreddy@iict.res.in
(B.M. Reddy).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.09.003

72
B.M. Reddy et al. / Journal of Molecular Catalysis A: Chemical 225 (2005) 71–78
Amberlyst but possesses less surface area (H0 = −12 to −13,
SA = 0.02 m2 g−1) [5]. Although clays and zeolites are quite
reliable, activities of these materials are much lower than the
conventional homogeneous acids due to pore blocking and
hydration. In view of these reasons there is an ongoing effort
to develop stronger solid acid systems which are water toler-
ant, stable at high temperatures and suitable for both liquid
and vapor phase conditions. Metal oxide based catalysts of-
fer several advantages over zeolite and clay-based catalysts.
These are active over a wide range of temperatures and more
resistant to thermal excursions.
rately and to which the powdered Zr(OH)4 was added. The
excess water was evaporated on a water-bath with vigorous
stirring, oven dried at 383 K for 12 h, and calcined at 923 K
for 4 h in air atmosphere. To prepare Pt/Mo-ZrO2 catalyst
(0.5 wt.% Pt), the powdered Mo-ZrO2 sample was added to
the 5% solution of hexachloroplatinic acid (H2PtCl6·xH2O).
The excess water was evaporated on a water-bath and the re-
sulting sample was oven dried at 393 K for 12 h and calcined
at 973 K for 4 h.
2.2. Catalyst characterization
Among the solid superacid catalysts reported, sulfated zir-
coniagainedmuchattentionduetoitshighactivitytocatalyze
many reactions even at low temperatures [10–12]. However,
it gets deactivated rapidly at higher temperatures and in re-
ducing atmosphere by forming H2S and SOx. Also it forms
sulfuric acid at high temperatures if there is water in the re-
action medium leading to the down stream contamination.
Arata and Hino reported that the solid superacids could be
synthesized by incorporating WO3 or MO3 into Zr- or Ti-
hydroxides under certain preparation conditions [13,14]. Ac-
tivity of these oxides depends on various parameters such
as nature of precursors, precipitation procedure, concentra-
tion of dopant, and calcination temperature [8]. In the case of
sulfated catalysts, nature of sulfating agent and sulfation pro-
cedure also play a crucial role [15]. Variation in any of these
parameters can drastically affect the resultant catalytic ac-
tivity of these materials. Many research groups have studied
these systems and explored them in petroleum industry, par-
ticularly for isomerization of light alkanes [16–18]. However,
these catalysts are not exploited in the areas of fine chemi-
cals synthesis. Therefore, a comprehensive investigation was
undertaken on promoted zirconia catalysts for organic syn-
thesis and transformation reactions in the liquid phase. In this
study sulfate, molybdate and tungstate promoted zirconia cat-
alysts were prepared and characterized by various physico-
chemical techniques namely, X-ray diffraction, BET surface
area, ammonia-TPD, and Raman spectroscopy. The prepared
catalysts were evaluated for various commercially important
organic reactions.
The powder X-ray diffraction patterns of various sam-
ples have been recorded on a Siemens D-5000 diffractometer
by using Cu K␣ radiation source and scintillation counter
detector. The XRD phases present in the samples were iden-
tified with the help of JCPDS data files. A conventional all
glass volumetric high vacuum (up to 1× 10−6 Torr) system
was used for BET surface area measurements. The BET sur-
face areas were measured by nitrogen physisorption at liquid
nitrogen temperature by taking 0.162 nm2 as the area of cross
section of N2 molecule. Raman spectra were recorded at am-
bient temperature on a DILOR XY spectrometer equipped
with a CCD detector. The spectra were recorded in the range
of 4000–100 cm−1 and a spectral resolution of 2 cm−1 using
the 514.5 nm exciting line from an argon ion laser (Spectra
Physics, USA).
The temperature programmed desorption (TPD) measure-
ments were carried on an AutoChem 2910 instrument (Mi-
cromeritics, USA). A thermal conductivity detector was used
for continuous monitoring of desorbed ammonia and the ar-
eas under the peaks were integrated using GRAMS/32 soft-
ware. Prior to TPD studies, the samples were pretreated at
473 K for 1 h in a flow of ultra pure helium gas (40 ml min−1).
After pretreatment, the sample was saturated with 10% ultra
pure anhydrous ammonia gas (balance He, 75 ml min−1) at
353 Kfor2 handsubsequentlyflushedwithHe(60 ml min−1
)
at 373 K for 2 h to remove the physisorbed ammonia. The
heating rate for the TPD measurements, from ambient to
1023 K, was 10 K min−1. All flow rates mentioned are at nor-
mal temperature and pressure (NTP).
2. Experimental section
2.3. Catalyst evaluation
2.1. Catalyst preparation
All chemicals and solvents used in this study were com-
mercially available and used without further purification. All
the reactions were carried out in the liquid phase batch mode.
These were carried out by taking a mixture of reactants,
catalyst and a suitable solvent (some reactions were car-
ried under solvent free conditions) in a round bottom flask
provided with a condenser and stirred/refluxed for appro-
priate times. Completion of the reaction was monitored by
TLC. After completion of the reaction, catalyst was recov-
ered by simple filtration and reused. The products were recov-
ered from the filtrate, concentrated on a rotatory evaporator
and chromatographed on a silica gel column to offered pure
Zirconium hydroxide was prepared from an aqueous solu-
tion of ZrOCl2 (Aldrich, GR Grade) by adding dilute NH4OH
drop-wise from a burette up to pH 8. The obtained precipitate
was washed several times until free from chloride ions and
dried at 393 K for 48 h. To prepare sulfated ZrO2, a 0.5 M sul-
furic acid solution (30 ml) was pored into the finely powdered
Zr(OH)4, dried at 393 K for 12 h and calcined at 923 K for 4 h.
To impregnate molybdate and tungstate promoters (6 mol%),
the desired quantity of ammonium heptamolybdate and am-
monium metatungstate were dissolved in excess water sepa-

B.M. Reddy et al. / Journal of Molecular Catalysis A: Chemical 225 (2005) 71–78
73
products. The isolated products were analyzed by 1H NMR,
mass spectroscopy methods and compared with authentic
samples.
3. Results and discussion
3.1. Influence of promoters
The XRD patterns of zirconia and promoted zirconia cat-
alysts are shown in Fig. 1. The corresponding phase com-
position and crystallite size measurements are presented in
Table 1. The hydrous zirconia sample calcined at 923 K is in
a poorly crystalline form with a mixture of monoclinic and
tetragonal phases. A strong influence on the phase modifi-
cation from thermodynamically more stable monoclinic to
the metastable tetragonal phase can be observed when var-
ious dopents are incorporated into the Zr(OH)4 structure. It
is well known from the literature that the tetragonal phase
of zirconia is more active in catalysis [9]. Generally, the
tetragonal phase of zirconia can be stabilized by incorporat-
ing the promoters like lanthanum, cerium, and yttrium into
the zirconia lattice [19]. The present investigation reveals
that all the incorporated promoters stabilize the tetragonal
phase at ambient conditions. The incorporation of various
promoters into the zirconia system may limit the formation
of large ZrO2 crystallites with the consequent stabilization
of tetragonal phase [19,20]. It can be observed from XRD
results (Fig. 1 and Table 1) that the sulfated zirconia shows
smaller crystallite size and more tetragonal phase when com-
pared to molybdated and tungstated catalysts. A complete
tetragonal phase can be observed in the case of Pt/Mo-ZrO2
catalyst, whereas in the case of Mo-ZrO2, and W-ZrO2 cat-
alysts the tetragonal phase dominates over the monoclinic
phase. A through examination of the XRD lines further in-
dicates that the molybdate shows relatively strong influence
than tungstate on the monoclinic to tetragonal phase trans-
formation of ZrO2. Specific surface areas of all the promoted
catalysts are higher than that of ZrO2 (Table 1). Among all
these catalysts, sulfated zirconia shows higher specific sur-
face area followed by Pt/Mo-ZrO2, Mo-ZrO2, and W-ZrO2,
respectively. The loss in the surface area of W-ZrO2 sample
may be due to penetration of the doped W-oxide into the pores
of the zirconia. Where as in the case of Mo-ZrO2 and Pt/Mo-
ZrO2 samples the observed large specific surface areas could
Fig. 1. X-ray powder diffraction patterns of pure and promoted zirconia
samplescalcinedat923 K. (᭹)Characteristiclinesduetotetragonalzirconia;
(ꢀ) characteristic lines due to monoclinic zirconia.
be explained as due to the formation of Mo O Zr linkages
resulting in the formation of porous materials [21]. These
results are in agreement with the earlier published reports
where sulfation inhibited the crystal growth and increased the
surface area and stabilized the tetragonal phase of zirconia
[7,8,12,22].
The ammonia-TPD results (Table 1) revealed that there
are at least two types of different acid sites on all the inves-
tigated samples. The total amount of ammonia desorbed in
the case of sulfate and Pt/Mo-, Mo-promoted samples was
observed to be higher than that of W-promoted sample. It ap-
pears from the NH3-TPD results that sulfate and molybdate
promoters exhibit a strong influence on the surface acidity of
the zirconia.
Ramanspectraofunpromotedandpromotedzirconiasam-
ples are shown in Fig. 2. Normally, crystalline zirconia shows
characteristic Raman bands in the range 150–700 cm−1 [23].
The spectrum of the ZrO2 calcined at 873 K exhibit the Ra-
man bands pertaining to a mixture of monoclinic (180, 188,
221, 331, 380, 476, and 637 cm−1) and tetragonal (148, 290,
Table 1
BET surface area, total acidity, and amount of monoclinic and tetragonal phases of ZrO2 and their crystallite size in various catalysts
Entry
Catalyst
BET SA (m2 g−1
)
NH3 desorbed (ml g−1
)
Monoclinic ZrO2
Amount (%)
Tetragonal ZrO2
Amount (%)
Size (nm)
Size (nm)
1
2
3
4
5
ZrO2
42
100
94
35
98
5
16
11
9
76
20
16
32
–
11.2
07.3
08.5
11.9
–
24
80
84
68
100
13.0
12.3
14.3
16.2
12.8
SO42−/ZrO2
Mo-ZrO2
W-ZrO2
Pt/Mo-ZrO2
11

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B.M. Reddy et al. / Journal of Molecular Catalysis A: Chemical 225 (2005) 71–78
corresponding to Zr(MoO4)2 are not observed. Here too the
absence of all characteristic bands due to crystalline MoO3
suggests that the impregnated molybdenum oxide strongly
interacted with the zirconia.
Incorporation of platinum has often been reported to in-
duce significant changes in the nature and acidity of pro-
moted ZrO2 catalysts [7]. In particular, Pt addition makes
the catalyst bifunctional with acidic and noble metal prop-
erties together. In the case of platinum promoted Mo-ZrO2
catalyst only tetragonal phase is observed. In the presence
of hydrogen, platinum prevents rapid deactivation of the cat-
alyst by supplying hydrogen to acid sites in isomerization
reactions [27]. There are some reports, which predict that the
¨
impregnated platinum creates Bronsted acid sites by hydro-
gen spillover from metallic platinum particles [28]. Hattori
et al. reported that heating the platinum promoted tungstated
¨
zirconia in the presence of hydrogen creates Bronsted acid
Fig. 2. Raman spectra of pure and promoted zirconia samples in the range
of 140–1060 cm−1: (Z) ZrO2; (SZ) SO42−/ZrO2; (MZ) Mo-ZrO2; (WZ)
W-ZrO2.
sites at the expense of Lewis acid sites [29]. In view of
its significance, a Pt promoted Mo-ZrO2 catalyst was pre-
pared in this investigation. Indeed, this catalyst exhibits ex-
cellent catalytic property in line with other promoted ZrO2
catalysts.
311, 454, and 647 cm−1) phases and the lines due to tetrag-
onal phase are less intense than the lines due to monoclinic
phase [16,24]. Where as in the spectra of sulfate, molyb-
date and tungstate promoted catalysts bands representing the
tetragonal phase are intense. The Pt/Mo-ZrO2 and Mo-ZrO2
catalysts exhibited a similar spectrum. In the case of sul-
fated zirconia catalyst the band at 1032 cm−1 with a shoulder
can be attributed to the hydrated sulfated groups [25]. Crys-
talline WO3 shows characteristic Raman bands at 807, 715,
and 274 cm−1 [16]. Absences of these bands indicate that mi-
crocrystalline WO3 is not formed on the surface of W-ZrO2
catalyst. These results agree well with the XRD results where
no independent peaks due to crystalline WO3 are observed.
The band at 1020 cm−1 assigned to W O stretching mode is
normally observed for mono- and polytungstate species. The
presence of shoulder peaks at high frequency region indicates
the presence of geometrically different WOx species on the
surface [24]. These may be octahydrally and tetrahydrally co-
ordinated WOx species. The well-crystallized MoO3 shows
characteristic Raman bands at 998 cm−1 (Mo O), 821 and
668 cm−1 (Mo O Mo) [25]. The Mo-ZrO2 sample treated
923 K exhibits two additional bands at 746 and 943 cm−1
which suggest the formation Zr(MoO4)2 compound between
zirconia and molybdenum [21,26]. However, the XRD peaks
3.2. Catalytic activity for synthesis of coumarins
Coumarins belong to the benzopyrone class of compounds
that are present in many natural products. These compounds
have great biological importance. Many coumarin deriva-
tives act as antibiotics. Generally, coumarines are prepared
by the Pechmann reaction, that is, condensation of phenols
with malic acid or with ␤-keto ester in which sulfuric acid is
used as the catalyst [30]. Recently many alternatives to sul-
furic acid were reported which include zeolites, clays, and
sulfonic acid resins [31,32]. However, most of the reported
catalysts suffer from various drawbacks such as rapid catalyst
deactivation and long reaction times, and some of the reagents
used are expensive. To overcome these difficulties, we em-
ployed the promoted ZrO2 catalysts for the synthesis of sub-
stituted coumarins from resorcinol and substituted resorci-
nols with ethyl acetoacetate and ethyl-␣-methylacetoacetate
(Scheme 1) [33]. All catalysts exhibited good to excellent
yields for this reaction. The results obtained with W-ZrO2
catalyst are summarized in Table 2. From entries 1–4 the reac-
tion was carried out with ethyl acetoacetate, and from entries
5–8 the reaction was carried out with ethyl-␣-methylaceto-
acetate.
Scheme 1.

B.M. Reddy et al. / Journal of Molecular Catalysis A: Chemical 225 (2005) 71–78
75
Table 2
W-ZrO2 solid acid catalyzed synthesis of coumarins
Table 4
Pt/Mo-ZrO2 solid acid catalyzed protection of carbonyl compounds
Entry
Substrate
Product yield
Entry
Substrate
Product yield
1
2
3
4
5
6
7
8
Resorcinol
80
70
65
56
67
65
60
50
1
2
3
4
5
6
7
8
Cyclopentanone
Cyclohexanone
Cycloheptanone
Acetophenone
98
93
84
87
40
80
65
95
5-Methyl resorcinol
2-Hydroxy resorcinol
5-Hydroxy resorcinol
Resorcinol
5-Methyl resorcinol
2-Hydroxy resorcinol
5-Hydroxy resorcinol
Cinnamaldehyde
4-Methoxy benzaldehyde
4-Nitro benzaldehyde
4-Chloro benzaldehyde
Note: From entries 1–4 the reaction is carried out with ethyl acetoacetate;
from entries 5–8 the reaction is carried out with ethyl-␣-methylacetoacetate.
3.4. Protection of carbonyl compounds
The carbonyl group protection is a commonly used reac-
tion in organic chemistry. In most of the multistep organic
synthesis processes carbonyl group should be protected from
the nucleophilic attack until its electrophillic property is uti-
lized. The frequently used processes for the carbonyl group
protection are acetalyzation or thio-acetalyzation [37]. We
have carried out the selective protection of various aldehydes
and ketones with ethane-1,2-diol as protecting group by using
various promoted ZrO2 catalysts (Scheme 3). In all the exper-
iments 1,3-dioxolane was obtained exclusively in good yields
and in short reaction times. All catalysts exhibited excellent
product yields. In particular, the Pt/Mo-ZrO2 catalyst exhib-
ited remarkable activity for the carbonyl group protection
as presented in Table 4. It also converted the ␣,␤-unsaturated
carbonyl compounds into oxolanes without effecting the dou-
ble bond [38].
Scheme 2.
Table 3
Synthesis of diphenylureas catalyzed by Mo-ZrO2 solid acid catalyst
Entry
Substrate
Product yield
1
2
3
4
5
6
Aniline
70
75
58
72
70
65
Methylaniline
Methoxyaniline
Hydroxyaniline
Chloroaniline
Fluoroaniline
3.3. Synthesis of diphenylureas
3.5. Catalytic activity for esterification reactions
In recent years diphenylureas gained tremendous interest
towards their synthesis due to their diverse applications in
various domains like tranquilizing and antidiabetic drugs, an-
tioxidants in gasoline, and corrosion inhibitors. The conven-
tional method for the synthesis of diphenylureas is based on
phosgene, carbonates, carbamates or carboxylicacid deriva-
tives [34,35]. However, phosgene and isocynates are highly
toxic and expensive to handle. Therefore, there is an ongo-
ing interest in the catalytic synthesis of ureas via phosgene
free routes. The promoted ZrO2 catalysts were employed to
perform this reaction with non-toxic methyl acetoacetate as
carboxylating agent (Scheme 2) [36]. This reaction was car-
ried out at 453 K with substituted anilines and ␤-keto ester
by using 1,2-dichlorobenzene as the solvent. All promoted
catalysts exhibited good catalytic activity. The results of the
experiments carried out with Mo-ZrO2 catalyst and with var-
ious substituted diphenylamines are summarized in Table 3.
Esters are the compounds of considerable interest due to
their diverse applications in many areas of chemical indus-
try. Esters are useful as solvents, artificial flavors, plasticiz-
ers, essences, and in perfumery industry. General methods
available for the preparation of esters include reaction be-
tween acid and alcohol, anhydride and alcohol, acid chlo-
ride and alcohol, and transesterification [37,39]. The con-
ventional catalyst used in esterification reactions is sulfuric
acid, which presents many drawbacks. Importantly it is toxic,
corrosive and causes environmental problems by producing
large amounts of byproducts. It is also difficult to recover
from the reaction mixture. Many alternatives were reported
in the literature for the preparation of esters including zeo-
lites, heteropolyacids, silica-supported sulfuric acid, and Ti-
loaded montmorillonite [40–42]. We have employed various
promoted ZrO2 catalysts for this reaction. As in the previous
cases, all catalysts exhibited excellent catalytic activity. In
Scheme 3.

76
B.M. Reddy et al. / Journal of Molecular Catalysis A: Chemical 225 (2005) 71–78
Scheme 4.
Table 5
Scheme 5.
Esterification of alcohols catalyzed by Mo-ZrO2 solid acid catalyst
Entry
Carboxylic acid
Alcohol
Yield
1
2
3
4
5
6
7
Acetic acid
Acetic acid
Benzoic acid
Acetic acid
Acetic acid
Oxalic acid
Malonic acid
Butanol
Isobutanol
Butanol
Ethylene glycol
Benzyl alcohol
Methanol
95
85
57
63
71
77
95
Scheme 6.
3.7. Acetylation of alcohols, phenols and amines
Acylation of alcohols is an important method for prepar-
ing organic esters, which are valuable intermediates for the
production of fine chemicals (Scheme 6). Acylation is fre-
quently used for the derivatization and characterization of al-
cohols and also for further transformations in organic chem-
istry [37]. Generally, acylation of alcohols can be effected
with compounds having activated acyl groups like acid anhy-
dridesandacylchlorides. Traditionallytheacylationreactions
are carried out in the presence of basic catalysts such as 4-
dimethylaminopyridine (DMAP), 4-pyrrolidinopyridine, try-
butyl phosphene, Lewis acids like AlCl3, FeCl3, BF3, in-
Methanol
this process various esters were synthesized in good yields
from both aliphatic, aromatic alcohols and acids [43]. Since
esterification is a reversible reaction, an excess of alcohol was
taken in the reaction to shift the equilibrium to right-hand side
(Scheme 4). The results obtained with Mo-ZrO2 catalyst are
presented in Table 5.
3.6. Tetrahydropyranylation of alcohols and phenols
¨
dium(III) chloride and with conventional Bronsted acids like
sulfuric acid, polyphosphoric acid, and with metal triflates
[49–51]. The conventional acylation reactions require Lewis
acid catalysts or amine base catalysts in stoichiometric quan-
tities. These catalysts form complexes with both the acylating
agents and the products thus restricting their reuse. The use
of strong mineral acids leads to several disadvantages as out-
lined earlier. Therefore, there is an ongoing effort to look for
efficient catalysts. We have employed promoted ZrO2 solid
acid catalysts for the direct acylation of various alcohols and
phenols with acetic anhydride into their acetates under mild
reaction conditions [52]. All catalysts effected the transfor-
mations smoothly at ambient conditions and the acetates were
produced in high purity and in excellent yields. Table 7 shows
the catalytic results with W-ZrO2 for the acylation of alcohols
and phenols.
Protection of hydroxyl group is an important and fre-
quently used process in organic synthesis mainly in carbo-
hydrate, peptide, and steroid chemistry [37]. Under acidic
conditions alcohols and phenols react with 3,4-dihydro-
2H-pyran (DHP) to give tetrahydropyranyl ethers. These
THP ethers are stable under a variety of reaction condi-
tions including reduction, oxidation, strongly acidic and ba-
sic media [44]. Many reagents were employed for this reac-
tion in the literature, which include hydrochloric acid, Al-
ZnCl2, HY-zeolite, Nafion-H, montmorillonite clay, boron
trifluride-diethyl ether complex, p-toluenesulfonate, sup-
ported heteropolyacids, and metal triflates [45–47]. Most of
the reagents employed are expensive and require stringent re-
action conditions. We have carried out these transformations
with less expensive, environmentally benign, and reusable
promoted ZrO2 catalysts under solvent free conditions at
room temperature [48]. Under these conditions tetrahydropy-
ranyl ethers were produced in excellent yields at less reaction
times. The catalytic results obtained with sulfated zirconia are
summarized in Table 6 (Scheme 5).
3.8. Synthesis of 1,5-benzodiazepines
Benzodiazepines and their polycyclic derivatives are im-
portant class of bioactive compounds. They are known to
Table 6
Table 7
Sulfated ZrO2 catalyzed tetrahydropyranylation of alcohols and phenols
W-ZrO2 solid acid catalyzed acetylation of alcohols phenols and amines
Entry
Substrate
Product yield
Entry
Substrate
Product yield
Time (h)
1
2
3
4
5
6
7
8
Benzyl alcohol
3-Methoxybenzyl alcohol
Phenol
2-Nitrophenol
Geraneol
2-Butanol
4-Benzyloxyphenol
Cinnamyl alcohol
94
96
82
90
91
94
86
92
1
2
3
4
5
6
7
8
Methanol
Geraneol
95
97
92
90
92
89
95
98
4
2
4
3
3
3
1
1
4-Nitrobenzyl alcohol
4-Methyl phenol
Resorcinol
Cinnamyl alcohol
Benzylamine
Cyclohexylamine

B.M. Reddy et al. / Journal of Molecular Catalysis A: Chemical 225 (2005) 71–78
77
Scheme 7.
Table 8
spectrum of the W-ZrO2 catalyst exhibited shoulder peaks in
high frequency region, indicating the presence of geometri-
cally different WOx species. The Zr(MoO4)2 compound for-
mation was noted from Raman results in the case of Mo-ZrO2
catalyst. (7) The sulfate, molybdate, and tungstate promoted
zirconia catalysts have been employed to catalyze various
acid catalyzed organic synthesis and transformation reac-
tions. All the investigated catalysts showed unique catalytic
activities under very mild reaction conditions. Further stud-
ies are essential to determine the exact nature of the acid
sites present on these oxides. Although much about the gen-
eration of active sites is unresolved, the catalytic activity of
these materials is remarkable.
Sulfated zirconia catalyzed synthesis of 1,5-benzodiazepines
Entry
Diamine
Ketone
Product yield
1
2
3
4
5
o-Phenylenediamine
o-Phenylenediamine
o-Phenylenediamine
o-Phenylenediamine
4,5-Dimethyl-o-
phenylenediamine
4,5-Dimethyl-o-
phenylenediamine
CH3COCH3
CH3COPh
CH3COCH2CH3
CH3CH2COCH2CH3
CH3COCH3
94
96
91
84
94
6
CH3COPh
93
act as anti-convulsant, anti-inflammatory, analgesic, hyp-
notic, sedative, and anti-depressive agents [53]. Generally,
benzodiazepines are synthesized by the condensation of
o-phenylenediamines with ␣,␤-unsaturated carbonyl com-
poundsor␤-haloketonesorwithketones. Manyreagentshave
been tried for this reaction including polyphosphoric acid,
BF3-etherate, NaBH4, Yb(OTf)3, MgO/POCl3 and more re-
cently with acetic acid under microwave conditions [54–56].
In continuation to our efforts in finding new applications for
solid acid catalysts in the area of fine chemicals synthesis,
we have carried out this reaction with promoted zirconia cat-
alysts [57]. In the presence of promoted ZrO2 catalysts, o-
phenylenediamines reacted smoothly with various ketones
and produced the corresponding 1,5-benzodiazepine deriva-
tives in good to excellent yields under solvent free conditions
(Scheme 7). Enhanced reaction rates and selectivities were
obtained for all the reactions in the presence of sulfated zir-
conia catalyst. Results of this reaction are summarized in
Table 8.
Acknowledgment
P.M.S and V.R.R acknowledge the CSIR, New Delhi for
the research fellowships.
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