Nanosized sulfated zinc ferrite as catalyst for the synthesis of nopol and other fine chemicals

Article abstract of DOI:10.1016/j.cattod.2012.01.028

A nanosized highly ordered mesoporous zinc ferrite (ZnFe₂O ₄; ZF) was synthesized via co-precipitation method, further sulfated with ammonium sulfate solution to obtain sulfated ZF (SZF) and have been used for the synthesis of nopol by Prins condensation of β-pinene and paraformaldehyde. The NH₃-TPD and pyridine sorption DRIFT-IR studies revealed the significant enhancement in Lewis acidic sites of the zinc ferrite on sulfatation. The influence of various reaction parameters such as reaction temperature, effect of substrate stoichiometry and catalyst loading has been investigated. It gave 70% conversion of β-pinene with 88% selectivity to nopol. The spent catalyst was regenerated and reused successfully up to four cycles with slight loss in catalytic activity. The nanosized SZF catalyst was found to be highly active towards several other commercially important acid catalyzed reactions such as isomerization, acetalization and aldol condensation.

Full text of DOI:10.1016/j.cattod.2012.01.028

Catalysis Today 198 (2012) 98–105
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Catalysis Today
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Nanosized sulfated zinc ferrite as catalyst for the synthesis of nopol and other
fine chemicals
Sumit V. Jadhav, Krishna Mohan Jinka, Hari C. Bajaj∗
Discipline of Inorganic Materials & Catalysis, Central Salt & Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), G.B. Marg,
Bhavnagar 364002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A nanosized highly ordered mesoporous zinc ferrite (ZnFe2O4; ZF) was synthesized via co-precipitation
method, further sulfated with ammonium sulfate solution to obtain sulfated ZF (SZF) and have been used
for the synthesis of nopol by Prins condensation of ␤-pinene and paraformaldehyde. The NH3-TPD and
pyridine sorption DRIFT-IR studies revealed the significant enhancement in Lewis acidic sites of the zinc
ferrite on sulfatation. The influence of various reaction parameters such as reaction temperature, effect
of substrate stoichiometry and catalyst loading has been investigated. It gave 70% conversion of ␤-pinene
with 88% selectivity to nopol. The spent catalyst was regenerated and reused successfully up to four cycles
with slight loss in catalytic activity. The nanosized SZF catalyst was found to be highly active towards
several other commercially important acid catalyzed reactions such as isomerization, acetalization and
aldol condensation.
Received 7 December 2011
Received in revised form 17 January 2012
Accepted 19 January 2012
Available online 3 March 2012
Keywords:
ZnFe2O4
Sulfated zinc ferrite
Solid acid catalyst
Nopol
© 2012 Elsevier B.V. All rights reserved.
Fine chemicals
1. Introduction
Nopol [2-(7,7-dimethyl-4-bicyclo[3.1.1]hept-3-enyl)ethanol],
is an optically active bicyclic primary alcohol, useful in the agro-
To perform reactions with very high yield, reduction of by-
products and limited use of toxic solvents is a challenge of modern
research in organic chemistry. In this facet, heterogeneous catal-
ysis serves as a fundamental tool as it offers several advantages
including lower energy requirements, easy separation, increased
selectivity and elimination of the toxic substances [1]. Variety of
inorganic compounds such as zeolites, clays, supported heteropoly-
acids and metal oxides are employed for the development of ideal
solid acid catalyst [2].
chemical industry for the synthesis of pesticides and other fine
chemicals including soap perfumes [7]. The conventional syn-
thesis of nopol involves Prins condensation reaction of ␤-pinene
and paraformaldehyde using either zinc chloride as a catalyst at
115–120 ◦C for several hours, or acetic acid as a catalyst at 120 ◦C to
yield nopyl acetate, which is saponified to nopol; or by autoclaving
a mixture of ␤-pinene and paraformaldehyde at 150–230 ◦C for
several hours [8]. However, numerous limitations are associated
with the homogeneous catalyst systems in terms of their corrosive
nature, difficulty in separation, post-synthesis disposal of effluents
and recovery of reaction products [9].
Current research efforts are being directed to develop eco-
friendly heterogeneous catalytic routes for the synthesis of fine
chemicals. Several heterogeneous catalysts have been reported for
the synthesis of nopol, such as metal supported MCM-41 meso-
porous molecular sieves [10], ZnCl2 impregnated montmorillonite
[7], mesoporous iron phosphate [11], SnCl4 grafted on MCM-41 by
chemical vapor deposition (CVD) [12], Sn-SBA-15 [13], Sn-MCM-41
[14], Sn and Zn loaded MCM-41 [15]. In continuation of our previous
studies on sulfated zirconia catalyzed synthesis of nopol [16], here-
with we report the use of nanosized sulfated zinc ferrite catalyst for
the synthesis of nopol and several other commercially important
fine chemicals (Scheme 1).
Ferrospinel materials having general formula M2+[Fe3+2]O4, are
well known for their catalytic properties [3]. Depending upon the
location of metal ions in the tetrahedral and octahedral sites, fer-
rospinel can be normal M2+[Fe3+2]O4, inverse Fe3+[M2+Fe3+]O4, or
mixed spinel wherein the divalent cations are distributed between
both sites. The cation location and distribution in zinc ferrite signif-
icantly affects its acid–basic properties [4]. Metal oxide materials,
impregnated with sulfur compounds showed remarkable enhance-
ment in their catalytic activity [5]. From the systematic studies
of sulfated metal oxide catalysts it was revealed that, sulfatation
increased number of weak and strong acid sites with simultaneous
decrease in the number of weak and strong base sites at the catalyst
surface [6]. Nevertheless, data on the catalytic applications of SZF
catalyst towards organic transformations is scanty.
The SZF catalyst showed good catalytic activity towards pro-
tection reaction of carbonyl compounds through formation of
corresponding acetals/ketals. Further the catalyst was evaluated for
isomerization of ␣-pinene oxide to campholenic aldehyde and cross
∗
Corresponding author. Tel.: +91 278 2471793; fax: +91 278 2567562.
E-mail address: hcbajaj@csmcri.org (H.C. Bajaj).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2012.01.028

S.V. Jadhav et al. / Catalysis Today 198 (2012) 98–105
99
by mounting acetone-dispersed samples on lacey carbon formvar
coated Cu grids.
FT-IR spectroscopic studies were performed in the range
of 4000–400 cm−1 as KBr pellets (Perkin Elmer). The pyridine
adsorbed DRIFT spectra were recorded to investigate the distribu-
tion of Lewis and Brönsted acidic sites of ZF and SZF catalysts.
The BET surface area of the catalyst was measured by nitro-
gen adsorption–desorption method at liquid N2 temperature
(Micromeritics-ASAP 2020). The calcined samples were degassed
under vacuum at 200 ◦C for 4 h, prior to adsorption measurement.
The acidic and basic properties of the catalysts were investi-
gated with TPD analysis using NH3 and CO2 as probe molecules
respectively. For the TPD measurements ∼0.3 g of each catalyst was
charged into a tubular quartz reactor of the TPD. The catalyst was
pretreated at 200 ◦C for 1 h under a flow of helium (40 ml/min).
Then the samples were saturated with 10% ultrapure adsorbate
gas (NH3 or CO2, balance He; 75 ml/min) at 50 ◦C for 1 h. The
physisorbed probe molecule was removed by evacuating the cat-
alyst sample at 50 ◦C for 1 h. Furnace temperature was increased
from 50 ◦C to 850 ◦C at the heating rate of 5 ◦C/min and the desorbed
NH3 or CO2 were analyzed.
Scheme 1. Prins condensation reaction of ␤-pinene and paraformaldehyde.
aldol condensation reaction of 1-heptanal and benzaldehyde to
produce jasminaldehyde. The catalyst also gave promising results
for Claisen–Schmidt condensation reaction for the synthesis of
flavanone and chalcones. All the synthesized compounds are of
industrial importance and find several applications, either directly
or as synthetic intermediates [17–20].
2. Experimental
2.1. Materials
FeCl3·6H2O and ZnCl2 were purchased from S.D. Fine Chemi-
cals, India. ␤-Pinene was procured from Sigma–Aldrich Chemical
Inc., USA. Other organic substrates and solvents were of analyti-
cal grade obtained from various commercial suppliers and were
used without further purification. Millipore water was used in the
catalyst preparation.
2.5. Catalytic studies
All the catalytic reactions were performed at ambient pressure
in liquid phase. In a typical run, requisite quantity of catalyst (pre-
activated at 200 ◦C for 1 h) and calculated quantities of substrates
were charged in an oven dried double-necked round bottom (RB)
glass flask either in presence of suitable solvent or under solvent-
free conditions. One neck of the flask was connected to a reflux
condenser and the other was blocked with a silicon rubber septum.
Water at ∼15 ◦C was circulated through the condenser and the flask
was then kept in an oil bath equipped with temperature controller
and magnetic stirrer.
In a typical Prins condensation reaction, ␤-pinene (5 mmol),
paraformaldehyde (15 mmol) in toluene (4 ml) with 0.14 g pre-
activated SZF catalyst was refluxed at 95 ◦C in the RB flask with
continuous stirring. The aliquots of reaction mixture were col-
lected periodically, cooled and centrifuged to separate the catalyst.
The reaction products were analyzed by gas chromatograph (Var-
ian GC-450), equipped with Factor-4 capillary column (30 m long
and 0.32 mm internal diameter) and flame ionization detector. The
oven temperature was increased from 80 ◦C to 220 ◦C at the rate of
10 ◦C/min. High purity nitrogen gas was used (30 ml/h flow rate) as
a carrier gas for the analysis.
2.2. Preparation of zinc ferrites
Nanosized ZF catalyst was prepared by a co-precipitation
method. Zinc chloride (1.775 g) and iron (III) chloride (4.225 g)
were separately dissolved in distilled water (125 ml) and heated
to 70 ◦C. In another beaker 1250 ml of NaOH solution (0.6 N) was
heated to 70 ◦C. The metal precursor solutions were then added
into NaOH solution as quickly as possible with vigorous stirring.
The resulting solution was stirred vigorously at 70 ◦C for 1 h and
aged overnight at room temperature. The precipitate was filtered
and washed extensively with warm distilled water until free from
chloride ions (AgNO3 test). The solid product was dried at 175 ◦C
for 16 h, and finally calcined at 470 ◦C for 3 h to yield the spinel cat-
alyst. The as synthesized sample dried at 175 ◦C and calcined ferrite
sample, hereafter denoted as ZF-175 and ZF-470 respectively.
2.3. Sulfatation of ZF catalysts
Product identification was done on gas chromatograph–mass
spectrometer (GCMS, Shimadzu – QP-2010) with GC oven pro-
grammed in the temperature range of 40–200 ◦C with helium as
a carrier gas, and MS in EI mode with 70 eV ion source. GC peak
areas were calibrated by taking solutions of known compositions
with dodecane and tetradecane as an internal standard. The con-
version and selectivity were calculated as per the formulas (i) and
(ii) respectively:
SZF catalysts were prepared by impregnating calcined ZF sam-
ple using 0.3 M aqueous ammonium sulfate ((NH4)2SO4) solution
[21]. Sulfatation was done by vigorously stirring 1 g of ZnFe2O4 in
20 ml of ammonium sulfate solution for 4 h. The resulting solid was
filtered and washed with distilled water followed by acetone. The
impregnated sample was dried overnight at 100 ◦C, and then it was
calcined at 470 ◦C for 1 h to get the SZF catalyst. The sulfated zinc
ferrite sample calcined at 470 ◦C is hereafter denoted as SZF-470.
(Ci − Cf)Substrate
%
%
ConversionSubstrate
=
× 100
× 100
(i)
(Ci)Substrate
2.4. Catalyst characterizations
(Cf)Product
(Ci − Cf)Substrate
SelectivityProduct
=
(ii)
Catalysts were characterized by powder X-ray diffraction
(PXRD), Fourier transform infrared spectroscopy (FT-IR), TEM anal-
ysis, N2 sorption and temperature programmed desorption (TPD)
studies using NH3 and CO2 as probe molecules.
where Ci and Cf are initial and final concentration of the respective
species in the reaction mixture.
The phase purity of the calcined zinc ferrite catalysts was eval-
uated using PXRD (Rigaku, MiniFlex II) using Cu K␣ radiation
The best catalyst, SZF-470 (as identified in Prins conden-
sation reaction) was further investigated for several other
catalytic reactions such as, isomerization of ␣-pinene oxide to
campholenic aldehyde, synthesis of jasminaldehyde by cross
aldol condensation reaction of 1-heptanal with benzaldehyde,
◦
(ꢀ = 1.54178 A) in 2ꢁ range of 2–80 at a scanning rate of 3◦ min−1
.
˚
Transmission electronic microscope (TEM) images were col-
lected using a JEOL TEM 2100 microscope. Samples were prepared

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S.V. Jadhav et al. / Catalysis Today 198 (2012) 98–105
Scheme 2. Organic reactions performed using ZF-470 and SZF-470 catalysts.
acetalization of ethylene glycol with carbonyl compounds to pro-
duce corresponding acetals/ketals, and synthesis of chalcone and
flavanone by Claisen–Schmidt condensation reaction using 2-
hydroxyacetophenone and benzaldehyde (Scheme 2). However
only primary investigations are carried out for these catalytic reac-
tions and detailed studies for optimizing reaction parameters are
in progress.
3. Results and discussion
3.1. Characterization of the ferrite catalysts
The X-ray diffraction studies for ZF revealed the well-matched
pattern with characteristic reflections for spinel phase confirming
the structural purity of the samples (Fig. 1). The atomic ratio of
Fe/Zn in ZF determined with ICP-AES was found to be 2.02, in good
agreement with the theoretical value of 2.0. PXRD pattern of SZF
also showed the characteristic peaks for spinel phase, indicating
no change in the crystallinity of the parent ferrite materials after
sulfatation. Furthermore, no characteristic peak of ammonium sul-
fate or sulfur containing salt was observed in the SZF catalyst. It
clearly revealed that sulfur was not in the form of any sulfur con-
taining salts such as ammonium sulfate, ammonium sulfide and
ammonium persulfate [22].
Fig. 1. PXRD pattern of ZF and SZF catalyst.
sulfates over the metal oxide surfaces [23]. The FT-IR spectra (Fig. 3)
2−
of calcined SZF catalyst depict the IR bands of SO4 group in the
region of 900–1200 cm−1, with peaks at 972–980, 1042–1058, and
1118–1135 cm−1 (characteristics of inorganic chelating bidentate
sulfate) and are assigned to asymmetric and symmetric stretch-
The TEM image of the calcined ZF at 475 ◦C (Fig. 2) clearly
showed the presence of ZF particles (average particle size of
7–10 nm) with well-ordered cubic morphology. The consistent lat-
tice orientation of the particles reveals the highly ordered and
crystalline nature of ZF particles.
ing frequencies of partially ionized S O double bonds and S
O
bonds (Structure A, Fig. 3) [24]. The elemental compositions
along with the corresponding BET surface areas derived from N2
adsorption–desorption studies of ZF and SZF samples are presented
in Table 1. The pristine ZF material had surface area of 68 m2/g
which increased to 81 m2/g after sulfatation.
It has been reported that, S6+ species presumably exists in
the form of inorganic chelating bidentate complexes or organic

S.V. Jadhav et al. / Catalysis Today 198 (2012) 98–105
101
Fig. 2. TEM micrographs of (a) zinc ferrite catalyst and (b) focussed morphology of particles of ZF. (c) Lattice view and (d) SAED pattern of ZF catalyst.
Table 1
Textural and compositional properties of ZF and SZF samples.
Catalyst
Surface area (m2/g)
S content (wt%)
Total acidity (mmol/g)
L/B ratioa
ZF-470
SZF-175
SZF-470
68
76
81
–
0.72
0.63
0.257
0.511
0.555
1.80
1.23
1.86
a
Ratio of Lewis to Brönsted acid sites.
To probe the nature of acidic sites over the catalyst surface,
pyridine sorption DRIFT-IR analysis was performed (Fig. 4). The as-
synthesized sample (ZF-175) showed peaks at 1600 and 1540 cm−1
corresponding to Brönsted acid sites (surface –OH), and peak at
1440 cm−1 corresponding to Lewis acid sites. A peak at 1490 cm−1
corresponding to combined Lewis and Brönsted sites was also
observed. The calcined sample (ZF-470) showed prominent peaks
at 1445 cm−1 indicating the presence of strong Lewis acid sites,
wherein only a small hump at ∼1600–1640 cm−1 due to Brönsted
sites was observed [25]. This indicates that as synthesized (ZF-175)
sample has surface hydroxyl groups there by resulting in higher
Brönsted acid sites whereas calcined samples, ZF-470 and SZF-470
were in pure spinel phase with prominent Lewis acidic sites [26].
To probe the distribution of Lewis and Brönsted acidic sites,
NH3-TPD profiles of ZF and SZF were collected (Fig. 5). The as-
synthesized sample (ZF-175) showed a prominent broad peak at
350 ◦C attributed to Brönsted acidic sites with medium strength,
arising due to the surface –OH groups; however a small hump at
100 ◦C indicates the presence of Lewis acidic sites. On calcination
to obtain spinel phase (ZF-470) there was decrease in the num-
ber of Brönsted acidic sites due to the loss of terminal –OH species
Fig. 3. FT-IR spectrum of ZF and SZF catalyst.

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S.V. Jadhav et al. / Catalysis Today 198 (2012) 98–105
Fig. 4. DRIFT spectrum of pyridine adsorbed ZF and SZF catalyst.
Fig. 6. CO2-TPD profiles of ZF and SZF catalyst.
Brönsted acid sites. While studying the effect of sulfatation, it was
revealed that sulfatation of ZF catalyst enhanced the catalytic per-
formance, thereby improving the ␤-pinene conversion up to 70%
with 88% selectivity to nopol (Table 2, entry 5). The other prod-
ucts formed in addition to nopol include camphene, pinocarveol,
pinocarvone, myrtenal, myrtenol, nopyl acetate and nopadiene as
detected by GC–MS analysis of the reaction products. The mass data
showed standard fragmentation pattern corresponding to nopol
(m/z: 166, 122, 105, 91, 79, 41), pinocarveol (m/z: 152, 135, 119,
109, 92, 83, 55, 41), pinocarvone (m/z: 150, 122, 108, 91, 81, 69,
53), myrtenal (m/z: 150, 135, 107, 93, 91, 79, 41), myrtenol (m/z:
152, 119, 108, 93, 91, 79, 41), nopyl acetate (m/z: 148, 133, 105, 91,
79, 41) and nopadiene (m/z: 148, 133, 105, 91, 79, 41).
3.3. Solvent effect
Fig. 5. NH3-TPD profiles of ZF and SZF catalyst.
To study the effect of solvent on catalytic activity of SZF, the
Prins condensation reaction was conducted in different solvents
using ␤-pinene to paraformaldehyde molar ratio 1:3 (Table 3). Very
low conversion (9%) of ␤-pinene was observed in protic solvents
such as methanol (Table 3, entry 1). These results are in agree-
ment with previous reports [30], ascribed to weak interaction of
protic solvents with reactant or poor solubility of paraformalde-
hyde in alcohols. In case of apolar–aprotic solvents such as hexane,
ethyl acetate and toluene, increase in the ␤-pinene conversion as
well as nopol selectivity was observed (Table 3, entries 2, 3, and 6).
MEK and acetonitrile being dipolar solvents gave low conversion
(7% in the case of MEK, whereas acetonitrile gave 40%) of ␤-pinene
with good selectivity to nopol (Table 3, entries 4 and 5). The reason-
able conversion in acetonitrile may be attributed to stabilization
of polar carbocation by interaction of positively charged species
with the nitrogen lone pair of electrons, thereby favoring the ␤-
pinene conversion. This assumption is supported with the fact that,
under the same experimental conditions on increasing the ace-
tonitrile amount, the ␤-pinene conversion drops, may be due to
competition of acetonitrile with paraformaldehyde species for their
interaction with Zn2+ sites, thereby blocking the catalytically active
sites leading to decrease in ␤-pinene conversion [31]. On the basis
of observed catalytic activity toluene was identified as best solvent
for maximum ␤-pinene conversion and nopol selectivity (Table 3,
entry 6).
which may have been converted into oxide bridges as supported
with FT-IR spectra [27]. The strong acid sites pertaining to sul-
fate binding especially around 440–460 ◦C corresponding to stable
Lewis site were observed in SZF-470 [28]. A desorption peak at
700 ◦C observed in SZF-470 was supposed to be originated due to
decomposition of sulfur at elevated temperatures [29].
To investigate the effect of sulfatation on the basic properties
of ZF catalyst, TPD analysis using CO2 as probe molecule was per-
formed (Fig. 6). The weak and strong basic sites at respectively
100 ◦C and 350 ◦C observed for calcined sample (ZF-470) were
supposed to be originated due to surface oxygen of the ZnFe2O4
catalyst. On sulfatation, significant decrease in basic sites was
observed with simultaneous increase in the acid sites for SZF-470
sample (Fig. 5). These results indicate that the base sites of the cat-
alyst were converted into acid sites due to the inductive effect of
S
O bond in the sulfated ZnFe2O4 catalyst [5,26].
3.2. Screening of catalyst for Prins condensation
The catalyst ZF-175, ZF-470 and SZF-470 were evaluated for
Prins condensation reaction of ␤-pinene and paraformaldehyde for
synthesis of nopol. The ZF-175 catalyst resulted in 30% conversion
of ␤-pinene with 64% selectivity to nopol (Table 2, entry 1), wherein
the ZF-470 catalyst gave 38% ␤-pinene conversion with 84% nopol
selectivity (Table 2, entry 2). The low values of ␤-pinene conver-
sion and nopol selectivity with ZF-175 could be attributed to excess
The amount of toluene was varied using 1:3 molar ratio of ␤-
pinene to paraformaldehyde at 95 ◦C. With substrate to solvent
ratio of 1:6, 70% ␤-pinene conversion and 88% nopol selectivity was

S.V. Jadhav et al. / Catalysis Today 198 (2012) 98–105
103
Table 2
The catalytic activity of ferrite catalysts toward Prins condensation reaction.
Entry
Catalyst
Time (h)
␤-P:PFa ratio
Temperature (◦C)
Conversionb (%)
Selectivity (%)
1
2
3
4
5
6
7
8
9
ZF-175
ZF-470
12
12
12
12
12
12
12
12
12
12
12
1:3
1:3
1:2
1: 4
1:3
1:3
1:3
1:3
1:3
1:3
1:3
95
95
95
95
95
65
80
110
95
95
95
30
38
44
68
70
25
55
72
56
72
63
64
84
86
88
88
83
84
57
88
87
42
SZF-470
SZF-470
SZF-470
SZF-470
SZF-470
SZF-470
SZF-470c
SZF-470d
SZF-470e
10
11
Catalyst recycling experiments
12
13
14
15
SZF-470 (Run 1)
SZF-470 (Run 2)
SZF-470 (Run 3)
SZF-470 (Run 4)
12
12
12
12
1:3
1:3
1:3
1:3
95
95
95
95
70
69
67
64
88
87
86
87
a
␤-Pinene to paraformaldehyde molar ratio.
Reaction conditions: ␤-pinene (5 mmol); catalyst (0.14 g); Toluene (4 ml).
Catalyst quantity: 0.12 g.
Catalyst quantity: 0.16 g.
Reaction was performed under oxygen purged atmosphere.
b
c
d
e
Table 3
Effect of solvent on Prins condensation reaction of ␤-pinene and paraformaldehyde.a
Entry
Solvent
Polarity
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
Methanol
Hexane
Ethyl acetate
Methyl ethyl ketone
Acetonitrile
Toluene
Protic
09
33
42
07
40
70
62
73
81
89
86
89
88
84
Apolar aprotic
Apolar aprotic
Dipolar aprotic
Dipolar aprotic
Apolar aprotic
Dipolar aprotic
Di-chloroethane
a
Reaction conditions: ␤-pinene (5 mmol); paraformaldehyde (15 mmol); catalyst (0.14 g); solvent (4 ml); reflux for 12 h.
at 65, 80, 95 and 110 ◦C gave 25, 55, 70 and 72% ␤-pinene conver-
sion with 83, 84, 88 and 57% nopol selectivity respectively (Table 2,
entries 5–8). Low conversion up to 80 ◦C was probably due to insuffi-
cient in situ formation of formaldehyde from paraformaldehyde. As
the temperature increased to 110 ◦C ␤-pinene conversion increased
to 72% with slight drop in selectivity for nopol (57%). Therefore 95 ◦C
has been identified as optimal reaction temperature for further
studies.
3.5. Effect of substrate:paraformaldehyde ratio
The Prins condensation reaction was investigated with variable
molar ratio of ␤-pinene to paraformaldehyde (1:2 to 1:4) in toluene
at 95 ◦C using SZF-470 as catalyst, wherein 1:3 molar ratio gave
maximum conversion (Table 2). Only 44% conversion of ␤-pinene
was observed with 1:2 molar ratio of ␤-pinene to paraformalde-
hyde (Table 2, entry 3), ascribed to less availability of formaldehyde,
generated in situ from paraformaldehyde. Conversion increased
to 70% when molar ratio of substrates increased to 1:3 (Table 2,
entry 5), while further increase in paraformaldehyde amount did
not show any notable increase in ␤-pinene conversion. From the
observed results, it was clear that only ␤-pinene conversion was
affected with substrate to paraformaldehyde molar ratio, while
nopol selectivity remained almost unaffected.
Fig. 7. Effect of substrate:solvent ratio on ␤-pinene conversion (reaction conditions:
␤-pinene = 5 mmol; paraformaldehyde = 15 mmol; catalyst = 0.14 g; toluene = 4 ml;
reaction temperature = 95 ◦C).
observed. On increasing the toluene amount there was slight drop
in ␤-pinene conversion, while nopol selectivity remained almost
unaffected (Fig. 7).
3.6. Effect of catalyst amount
3.4. Effect of reaction temperature
The effect of catalyst concentration on the conversion and selec-
tivity was studied by varying the catalyst amount from 0.12 to 0.16 g
using 1:3 molar ratio of ␤-pinene and paraformaldehyde in toluene
at 95 ◦C. The ␤-pinene conversion increased with an increase in
The effect of reaction temperature on the Prins condensation
reaction was studied in the temperature range of 65–110 ◦C, by
keeping other experimental conditions same. Reactions performed

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S.V. Jadhav et al. / Catalysis Today 198 (2012) 98–105
Table 4
Comparative studies on the catalytic activity of ZF-470 and SZF-470 for various organic transformations.
Reaction
Product
Reaction conditions
% Conversion (% selectivity)
ZF-470
SZF-470
Prinsa
Nopol
Campholenic aldehyde
Jasminaldehyde
Flavanone/chalcone
1,4-Dioxa-spiro[4,5]decane
95 ◦C, 12 h
100 ◦C, 9 h
150 ◦C, 12 h
150 ◦C, 9 h
60 ◦C, 6 h
38 (84)
50 (58)
95 (68)
81 (70/30)
79 (99)
70 (88)
93 (64)
81 (55)
93 (72/28)
96 (99)
Isomerizationb
Cross aldolc
Claisen–Schmidtd
Acetalizatione
a
Solvent = toluene; ␤-pinene (5 mmol); paraformaldehyde (15 mmol); catalyst (0.14 g).
Solvent = toluene; ␣-pinene epoxide (5 mmol); catalyst (0.10 g).
Solvent-free; 1-heptanal (5 mmol); benzaldehyde (25 mmol); catalyst (0.075 g).
Solvent-free; 2ꢀ-hydroxy acetophenone (5 mmol); benzaldehyde (10 mmol); catalyst (0.1 g).
Solvent-free; cyclohexanone (5 mmol); ethylene glycol (25 mmol); catalyst (0.1 g).
b
c
d
e
Fig. 8. Proposed reaction mechanism of Prins condensation reaction for nopol production over SZF-470 catalyst.
the catalyst loading (0.12 to 0.16 g) without affecting the nopol
selectivity (Table 2, entries 9 and 10).
these organic transformations. To investigate the effect of sulfata-
tion, the reactions were also carried out using ZF catalyst (catalyst
without sulfatation). Table 4 represents the comparison of catalytic
activity of as synthesized and sulfated zinc ferrite; from the data it is
evident that, sulfatation causes pronounce effect on catalytic activ-
ity of zinc ferrite samples, in terms of conversion and selectivity of
the desired substrate.
3.7. Reusability studies
In order to investigate the recyclability of the catalyst, the Prins
condensation reaction was performed at 95 ◦C, with ␤-pinene to
paraformaldehyde molar ratio 1:3. The catalyst SZF-470 obtained
after the first catalytic cycle was washed with warm acetone to
remove the adhered organics and refluxed with H2O2 at 80 ◦C for
3 h. Finally the catalyst was regenerated by heating at 200 ◦C for
2 h. The PXRD and elemental analysis of the regenerated catalyst
showed no significant changes in the textural and compositional
properties. It was successfully recycled up to four catalytic runs
with slight loss in conversion and selectivity (Table 2, entries 12–15).
As SZF was found to be highly effective for Prins con-
densation reaction, it was further used as a catalyst for the
protection of carbonyl compounds using ethylene glycol, iso-
merization of ␣-pinene epoxide to campholenic aldehyde, cross
aldol condensation of 1-heptanal and benzaldehyde to give
jasminaldehyde and Claisen–Schmidt condensation reaction of
benzaldehyde and acetophenone. All these reactions were per-
formed in solvent-free condition, except for isomerization reaction.
The conversion–selectivity data for the desired compounds of the
above mentioned reactions is summarized in Table 4. The catalytic
activity of the synthesized nanosized SZF was quite good towards
4. Reaction mechanism for SZF catalyzed synthesis of nopol
It was previously reported that, presence of Lewis acid sites are
favorable for nopol formation [31]. The proposed structure of the ZF
(Fig. 3, structure A) shows coordinatively unsaturated Zn2+ species
over the catalyst surface. The generation of Lewis acidic sites over
the catalyst surface could be attributed to the inductive effect of
2−
SO4 species through bridging oxygen atoms, thereby generating
electron deficient Zn2+ species. Paraformaldehyde interacts with
these Zn2+ species, to form carbocation via polarization of C O link-
age, thereby generating electrophillic center. ␤-Pinene then reacts
with the cationic species, followed by allylic proton transfer thus
completing the reaction cycle with nopol formation (Fig. 8). The
above postulated mechanism could be correlated with the sur-
face acid–base properties of the catalyst. As we increase the extent
of sulfur loading over ZnFe2O4 catalyst, increased inductive effect
of the sulfate species account for enhanced Lewis acidity thereby
increasing the ␤-pinene conversion. Formation of minor oxidation

S.V. Jadhav et al. / Catalysis Today 198 (2012) 98–105
105
products of ␤-pinene like pinocarveol, pinocarvone, myrtenol and
References
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Nanosized SZF catalyst was prepared through a simple and
versatile synthetic procedure. The SZF shows good catalytic
activity towards Prins condensation reaction of ␤-pinene and
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Acknowledgments
Authors gratefully acknowledge CSIR-New Delhi, for financial
assistance under network project NWP-0010. Analytical discipline
of the institute is acknowledged for analytical support. SVJ would
like to thank CSIR, New Delhi for award of senior research fellow-
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their constructive comments.
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