Novelties of eclectically engineered sulfated zirconia and carbon molecular
sieve catalysts in cyclisation of citronellal to isopulegol
G. D. Yadav* and J. J. Nair
Chemical Engineering Division, University Department of Chemical Technology, University of Mumbai (formerly
Received (in Cambridge, UK), 2nd September 1998, Accepted 28th September 1998
Sulfated zirconia (S-ZrO
2
) is a well-known solid superacid
Zirconium oxychloride, 25% ammonia solution, polyvinyl
alcohol, toluene and 98% sulfuric acid were obtained from S.D.
Fine Chemicals Ltd. Citronellal containing about 14% iso-
pulegol was obtained from Arofine Industries Ltd.
catalyst used in various reactions of commercial importance
such as isomerisation, alkylation and acylation, nitration,
etc. The selectivity towards the formation of isopulegol, a
potential intermediate in the synthesis of menthol, can be
drastically increased by using carbon molecular sieve (CMS)
The catalyst was prepared using the conventional precipita-
9
tion method. 100 g of zirconium oxychloride were dissolved in
with S-ZrO
2
.
distilled water. The solution was then filtered. This solution and
2
5% aqueous ammonia were added dropwise simultaneously in
Isopulegol is an important intermediate for the manufacture of
menthol, used extensively in pharmaceuticals, cosmetics,
toothpastes, chewing gum, and other toilet goods as well as in
a beaker with constant stirring while a white precipitate of
zirconium hydroxide was obtained at pH 9–10. After complete
precipitation it was digested in the vessel for 6 h. The precipitate
was filtered through a Buchner funnel and washed thoroughly
with distilled water until free of ammonia and chloride ions. The
filtered precipitate was dried in an oven at 120 °C for 24 h. The
dried catalyst was then crushed to make a fine powder, which
1
cigarettes. Isopulegol is manufactured from the cyclisation of
2
citronellal. Bogert and Hasselstrom have reported the use of
UV in the cyclisation reaction. Activities and selectivities of
acid clinoptilolite, mordenite, and faujasite zeolites in the
isomerisation of citronellal in n-hexane, chloroform and
dichloromethane as solvents have been investigated.3 The
activities of the acidic zeolites for the isomerisation of
citronellal were found to be in the following order: HY(max.) >
HCC > (clinoptilolite) > HMCP (clinoptilolite + mordenite) >
HX at 84 °C in dichloroethane, and the selectivities to
isopulegol were HCC (90%) > HMCP (85%) > HY (80%) >
HMP (72%) at 80% conversion level. The activity in these
studies is related to the total amount of Brønsted acid sites of the
catalysts and only a fraction of these sites, located mainly on the
external surface of the crystal, were accessible to the reactants
due to the diffusional resistance. It was found that the selectivity
increases to the isopulegol ether as the accessibility to the acidic
centres increases. There are several reports whereby Cu–Cr and
2 4 2 4
was treated with 0.5 M H SO . 15 ml of 0.5 M H SO was
required for 1 g of the catalyst. The catalyst so prepared was
dried in an oven at 120 °C for 24 h followed by calcination at
230 to 650 °C.
To 10 g of the above prepared catalyst 7.2 ml polyvinyl
alcohol (PVA) solution (2 g PVA dissolved in 25 ml distilled
water) was added dropwise until it was just wet. It was mixed
well to get a uniform coating. It was dried at 100 °C for 1 h and
calcined at different temperatures. This catalyst is referred to as
2
S-ZrO /CMS catalyst.
In another method, S-ZrO
2
was initially soaked with different
solvents such as benzene, cyclohexane, carbon tetrachloride,
hexane till wetness. This was coated with the same amount of
PVA solution as was done without wetting the catalyst with
solvent. These were calcined at different temperatures. Eight
different catalysts were prepared as shown in Table 1 where the
4
5
Cu–Cr–Mn, tris(triphenylphosphine)rhodium chloride and
6
micellar catalysts have been employed to catalyse the cyclisa-
tion reaction. Several Lewis acids as catalyst for the preparation
nomenclature S-ZrO
with benzene followed by coating with CMS.
2 2
/Benzene/CMS refers to S-ZrO soaked
7
of l-isopulegol from
D
-citronellal have been used. Dean and
Whittaker8 have studied this reaction with superacids (e.g.
FSO H/SO ) to observe that the cyclisation follows the same
All experiments were conducted in a 100 ml fully baffled
glass reactor of 5 cm internal diameter. The reactant and solvent
were charged to the reactor and the temperature was raised to 95
3
2
path as the normal acids, yielding isopulegol and neoisopule-
gol.
We present here the efficacy of a novel shape selective
catalyst synergistically produced from sulfated zirconia (S-
2
2
23
°C. 0.5 g (2.13 3 10 g cm ) of the desired catalyst was then
added to the reactor under constant stirring.
ZrO
citronellal to isopulegol. S-ZrO
superacidic catalyst used in various reactions. The activity of
this catalyst is superior to many other solid acid catalysts. But
one of the major drawbacks of S-ZrO
selective catalyst. Hence it cannot be employed in reactions
where selectivity is of utmost importance. However, S-ZrO
when combined with other materials can produce the desired
shape selective catalyst. In this respect, carbon molecular sieve
2
) and carbon molecular sieve (CMS) in the cyclisation of
2
is a very well-known solid
Table 1 Activities of catalysts for cyclisation of citronellal
Selectivity
Conversion for
Time/ of citronellal isopulegol
2
is that it is not a shape
2
Catalyst
ZrO [230–350]
min (%)
(%)
2
90
10
05
30
20
20
30
20
0
—
46
35
65
52
61
53
58
(
CMS) can be used in combination with S-ZrO
composite shape selective catalyst. The selectivity engineering
aspects of catalysts are embodied in this CMS–S-ZrO
2
to get a
S-ZrO
S-ZrO
S-ZrO
S-ZrO
S-ZrO
S-ZrO
S-ZrO
2
2
2
2
2
2
2
[230–350]
[650]
[230–350]/CMS
[230–350]/Benzene/CMS
[230–350]/Cyclohexane/CMS
96
95
91
95
96
88
95
2
composite media where one acts as a siever and the inside core
as the true catalyst. The isomerisation of citronellal was
considered to be interesting in view of the fact that the reaction
has been studied by others using zeolites and there are several
products generated depending on the type of carbocation and
hence on the type and strength of acidic sites.
[230–350]/CCl
[230–350]/Hexane/CMS
4
/CMS
Solvent: toluene = 15 g, reactant: citronellal = 5 g, temperature: 95 °C.
Values inside square brackets indicate the calcination temperature.
Chem. Commun., 1998, 2369–2370
2369
Yadav
