Zirconia-modified superacid UDCaT-5: An efficient and versatile catalyst for alkylation reactions under solvent-free conditions

Article abstract of DOI:10.1080/00397910802139387

UDCaT-5, a modified version of zirconia, efficiently catalyzes alkylation of phenols with alcohols under environmentally safe, heterogeneous reaction conditions with high selectivity and in excellent yields. The high content present in UDCaT-5 with preservation of tetragonal phase of zirconia was responsible for the superactivity. Several phenolic compounds carrying either electron-sulfer releasing or electron-withdrawing substituents in the ortho, meta, and para positions afforded high yields of the products. Copyright Taylor & Francis Group, LLC.

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Zirconia-Modified Superacid
UDCaT-5: An Efficient and
Versatile Catalyst for Alkylation
Reactions under Solvent-Free
Conditions
Ganapati D. Yadav a & Ganesh S. Pathre a
a Department of Chemical Engineering , University
Institute of Chemical Technology, University of
Mumbai , Mumbai, India
Published online: 09 Sep 2008.
To cite this article: Ganapati D. Yadav & Ganesh S. Pathre (2008) Zirconia-Modified
Superacid UDCaT-5: An Efficient and Versatile Catalyst for Alkylation Reactions
under Solvent-Free Conditions, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 38:15, 2684-2691, DOI:
10.1080/00397910802139387
To link to this article: http://dx.doi.org/10.1080/00397910802139387
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Synthetic Communications1, 38: 2684–2691, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802139387
Zirconia-Modified Superacid UDCaT-5: An
Efficient and Versatile Catalyst for Alkylation
Reactions under Solvent-Free Conditions
Ganapati D. Yadav and Ganesh S. Pathre
Department of Chemical Engineering, University Institute of Chemical
Technology, University of Mumbai, Mumbai, India
Abstract: UDCaT-5, a modified version of zirconia, efficiently catalyzes
alkylation of phenols with alcohols under environmentally safe, heterogeneous
reaction conditions with high selectivity and in excellent yields. The high content
present in UDCaT-5 with preservation of tetragonal phase of zirconia was
responsible for the superactivity. Several phenolic compounds carrying either
electron-sulfer releasing or electron-withdrawing substituents in the ortho, meta,
and para positions afforded high yields of the products.
The alkylation of phenols with alcohols and alkenes using acid catalyst is
a well-established method in organic synthesis to prepare alkylphenols,
which have wide applications as raw materials for the manufacture of a
variety of resins, durable surface coatings, varnishes, wire enamels,
printing inks, surface-active agents, rubber chemicals, antioxidants,
fungicides, petroleum additives, ultraviolet absorbers, and heat stabilizers
for polymeric materials.[1,2] The most commonly used catalysts for this
purpose are sulfuric acid, polyphosphoric acid, anhydrous AlCl3, and
BF3, which are homogeneous and hazardous catalysts. Heterogeneous
catalysts are powerful methods for realizing environmentally benign
organic reactions.[3] These solids are easier to handle because they hold
the acidity internally, are readily separable from the products by simple
filtration, and are recyclable. Moreover, constraining a reaction to the
surface of a solid habitually allows use of milder conditions and increases
Received September 21, 2007
Address correspondence to Ganapati D. Yadav, Department of Chemical
Engineering, University Institute of Chemical Technology, University of
Mumbai, Matunga, Mumbai 400 019, India. E-mail: gdyadav@yahoo.com
2684

Zirconia-Modified Superacid UDCaT-5
2685
its selectivity. Thus, the solid-acid-catalyzed alkylation reaction of phe-
nols with alcohols is an environmentally friendly and strategic alternative
to homogeneous and hazardous catalytic processes.
Zirconia, when modified with sulfate ions, forms a highly acidic or
superacidic material that exhibits better catalytic activity than 100%
sulfuric acid in many reactions. It has been widely used to catalyze
hydrocarbon isomerization, methanol conversion to hydrocarbons, alky-
lation, acylation, esterifcation, etherification, condensation, nitration,
cyclization, and so on.[4] Recently, a number of novel solid catalysts which
are modified version of zirconia have been developed for alkylation reac-
tions of organic compounds in our laboratory.[5] The catalytic activity of
sulfated zirconia is mainly dependent on the precursor, type of sulfating
agent, and calcination temperature. All previous literature suggests that
S-ZrO2 has been prepared so far with a maximum 4% w=w of sulfur with
preservation of tetragonal phase of zirconia. Above this value, the tetra-
gonal phase of the zirconia is strongly affected, which is responsible for
the catalytic activity. We have reported, for the first time, a preparation
of sulfated zirconia with high sulfur content (9%), possessing better
activity and also preservation of tetragonal phase by using chlorosulfonic
acid as a new source for the sulfate ion.[6] This new catalyst is designated as
UDCaT-5. Many industrially important acid-catalyzed reactions have
been catalyzed such as isomerization of n-hexane, esterification, conden-
sation, and Claisen rearrangement using UDCaT-5.[6] Thus the stability
and robustness of the catalyst have been ascertained in the presence of
HCl and water respectively, formed as co-products in some of these reac-
tions. This prompted us to use this catalyst for alkylation reactions of
phenols by using alcohols where water is formed as co-product. The use
of alcohol as an alkylating agent gives many advantages over alkenes such
as elimination of coke formation due to oligomerization of alkenes, sub-
sequent longer life of catalyst, and no by-product formation. When alcohol
is used as alkylation agent, it gives alkene and water. This water, formed
in-situ, suppresses coke formation and results in prolonged catalytic
activity. Besides, its low cost, easy availability, and elimination of handling
and transport problems gives it the upper hand over alkenes. In the present
investigation, we report that UDCaT-5 serves as an excellent catalyst for
the alkylation of phenol compounds carrying either electron-releasing or
electron-withdrawing substituents in the ortho, meta, and para positions
with alcohols under environmentally friendly conditions (Figure 1).
The activity of UDCaT-5 was compared with conventional sulphated
zirconia in isopropylation of p-cresol, and it was found that the activity
of UDCaT-5 is more than that of conventional sulphated zirconia.
Table 1 compares the activity and selectivity of sulphated zirconia and
UDCaT-5 in alkylation of p-cresol with isoproapnol.

2686
G. D. Yadav and G. S. Pathre
Figure 1. Reaction Scheme.
UDCaT-5 was prepared by adding aqueous ammonia solution to
zirconium oxychloride (ZrOCl2Á8H2O) solution at pH of 9–10. The pre-
cipitated zirconium hydroxide so obtained was washed with deionized
water until a neutral filtrate was obtained. The absence of chlorine ion
was detected by AgNO3 tests. A material balance on Cl before and after
precipitation and washing shows no retention on the solid. Zirconium
hydroxide was dried in an oven for 24 h at 100 ꢀC and was crushed to
100-mesh size. Zr(OH)4 was then immersed in 15 cm3=g of 0.5 M solution
of chlorosulfonic acid and ethylene dichloride. All materials were
immersed for 5 min in the solution and then (without allowing moisture
absorption) were kept in an oven. Heating was started to evaporate the
solvent and then raised to a temperature of 120 ꢀC after about 30 min.
These materials were kept in an oven at 120 ꢀC for 24 h and calcined at
650 ꢀC for 3 h to get the active catalysts UDCaT-5.
Table 1. Activity of S-ZrO2 and UDCaT-5 for alkylation of phenol with
isopropanol
Conv. (%)
Yield (%)
Substrate Alcohol Time (h) S-ZrO2 UDCaT-5 S-ZrO2 UDCaT-5
0.5
41
55
62
66

Zirconia-Modified Superacid UDCaT-5
2687
Both O-and C-alkylation products are formed in alkylation of phenol.
The selectivity of O- and C-alkylation is dependent on the temperature,
nature of acidic sites, molar ratio, pore size of catalyst, and so forth. High
temperature favors C-alkylation,[7] whereas at lower temperature O-alky-
lation dominates.[8,9] The objective of this work was to prepare alkyl phe-
nols, especially mono-alkylated phenol, because of their wide applications
and importance. After some experimentation with respect to the catalyst
quantity, molar ratios, and temperature of reactants, we developed the
conditions that generally give the mono-C-alkylated phenol in good
yields. The conditions employed were 5:1 ratio of the phenol and alcohol
with catalyst loading of 0.05 g=cm3 of total volume using UDCaT-5 as
catalyst. The conversion of phenols was highest at this molar ratio. It
could be attributed to the presence of excess phenol, because of which
the activity of alkene increases, and it easily undergoes the alkylation
reaction. The other reason to select this molar ratio was to avoid the effect
of water generated in situ on catalyst activity and by-product formation.
The reactions were studied at two different temperatures depending on
reactivity of reactants to avoid undesired products. We also examined
the recycle ability of catalyst for some of the reactions. The catalyst used
was refluxed with toluene, filtered, dried at 120 ꢀC, and reused. It gave
excellent reusability. The scope and generality of this catalyst is illustrated
with respect to various phenols, and the results are presented in Table 2.[10]
In conclusion, the alkylation of various kinds of phenolic compounds
carrying either electron-releasing or electron-withdrawing substituents in
the ortho, meta, and para positions can be carried out using UDCaT-5 as
a strong solid acid catalyst under solvent-free conditions. This hetero-
geneous UDCaT-5 catalyst system has the following advantages: (1) high
catalytic activity and stability, (2) wide applicability for various phenols,
(3) non-polluting and recyclable catalyst, (4) solvent-free reaction con-
dition, and (5) shorter reaction time and low temperature as compared
to other superacids, such as clay and zeolites, where temperature required
is very high.
ACKNOWLEDGMENTS
G. D. Y. acknowledges the financial support from the Darbari Seth
Professorship Endowment. G. S. P. is thankful to Council of Scientific
and Industrial Research (CSIR), New Delhi, for awarding a junior
research fellowship during this work. G. D. Y. acknowledges the hospi-
tality of Purdue University, West Lafayette, for the invitation as a
Distinguished Visiting Scholar under its Asian Initiative, which allowed
him to spend time on creative pursuits.

2688
G. D. Yadav and G. S. Pathre
Table 2. Alkylation of phenols with alcohols to produce alkylphenols in the
presence of UDCaT-5
Selectivity (%)
Entry
1.
Substrate
Alcohol Time (h) Conv. (%) Yield (%)
o
p
2
1
1
1
94b
96a
92a
96a
91
92
88
83
70
30
2.
65
68
65
35
32
35
3
4
5
6
1
2
93>b
87
90
70
30
—
88b
100
(Continued)

Zirconia-Modified Superacid UDCaT-5
2689
Table 2. Continued
Selectivity (%)
Entry
Substrate
Alcohol Time (h) Conv. (%) Yield (%)
o
p
7
1
96a
86
100
—
8
1
1
1
92a
95a
87a
86
81
85
100
100
100
—
—
—
9
10
11
12
2
1
92b
79
82
80
93
20
94a
7
(Continued)

2690
G. D. Yadav and G. S. Pathre
Table 2. Continued
Selectivity (%)
Entry
13
Substrate
Alcohol Time (h) Conv. (%) Yield (%)
o
p
1
2
93a
85
88
94
6
14
15
81b
68
32
—
1.5
90a
88
100
aTemperature: 140 ꢀꢀC.
bTemperature: 180 C.
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Zirconia-Modified Superacid UDCaT-5
2691
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