Synthesis of nopol via Prins condensation of β-pinene and paraformaldehyde catalyzed by sulfated zirconia

Article abstract of DOI:10.1016/j.apcata.2010.10.005

The present work describes the novel application of sulfated zirconia (SZ) solid acid catalysts for the synthesis of nopol via Prins condensation of β-pinene and paraformaldehyde. SZ catalysts with different percentages of sulfur loadings have been synthesized and characterized using various physico-chemical techniques like PXRD, FT-IR, surface area analysis and NH ₃-TPD studies. The influences of various reaction parameters such as sulfur loading, reaction temperature, molar ratio of reactants, reaction time, solvent effect and reusability of the catalyst have been investigated. SZ catalyst synthesized by impregnation of 2N sulfuric acid solution over Zr(OH)₄ was found to be a highly selective catalyst for β-pinene conversion (>99%) with ～99% selectivity to nopol. The catalyst could be reused up to five cycles with minor loss in catalytic activity for β-pinene conversion, while the nopol selectivity remains unaffected.

Full text of DOI:10.1016/j.apcata.2010.10.005

Applied Catalysis A: General 390 (2010) 158–165
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis of nopol via Prins condensation of ␤-pinene and paraformaldehyde
catalyzed by sulfated zirconia
∗
Sumit V. Jadhav, Krishna Mohan Jinka, Hari C. Bajaj
Discipline of Inorganic Materials & Catalysis, Central Salt & Marine Chemicals Research Institute (CSMCRI), Council of Scientific and Industrial Research (CSIR), G.B. Marg, Bhavnagar
364021, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2010
Received in revised form 4 October 2010
Accepted 5 October 2010
Available online 26 October 2010
The present work describes the novel application of sulfated zirconia (SZ) solid acid catalysts for the
synthesis of nopol via Prins condensation of ␤-pinene and paraformaldehyde. SZ catalysts with different
percentages of sulfur loadings have been synthesized and characterized using various physico-chemical
techniques like PXRD, FT-IR, surface area analysis and NH3–TPD studies. The influences of various reaction
parameters such as sulfur loading, reaction temperature, molar ratio of reactants, reaction time, solvent
effect and reusability of the catalyst have been investigated. SZ catalyst synthesized by impregnation of
Keywords:
Solid acid
Sulfated zirconia
Prins condensation
2
N sulfuric acid solution over Zr(OH)4 was found to be a highly selective catalyst for ␤-pinene conversion
>99%) with ∼99% selectivity to nopol. The catalyst could be reused up to five cycles with minor loss in
catalytic activity for ␤-pinene conversion, while the nopol selectivity remains unaffected.
© 2010 Elsevier B.V. All rights reserved.
(
␤
-pinene
Paraformaldehyde
Nopol
1
. Introduction
fragrances as well as in agrochemical industries for the synthesis
of pesticides and household products [7]. Nopol has been con-
ventionally synthesized by Prins condensation of ␤-pinene and
paraformaldehyde. The general methods of obtaining nopol [8] are:
(i) the reaction of ␤-pinene with paraformaldehyde in the pres-
ence of a small quantity of homogeneous catalyst such as zinc
There is a need for more environmentally acceptable processes
in the chemical industry [1]. The trend towards green chemistry
2] or sustainable technology necessitates a paradigm shift from
[
traditional concepts of process efficiency, that focus exclusively on
product yield, to one that assigns economic value to eliminating
waste and avoiding the use of toxic and/or hazardous substances.
Solid, heterogeneous catalysts have advantages of ease of
recovery and recycling and are readily amenable to continuous
processing. In particular, solid acid catalysts are, in principle, appli-
cable to a plethora of acid catalyzed processes in organic synthesis
◦
chloride, at 115–120 C for several hours, yields about 57% nopol;
(ii) reacting a mixture of ␤-pinene and paraformaldehyde in an
◦
acetic acid solution at 120 C, yields nopyl acetate, which is then
saponified to nopol and (iii) autoclaving paraformaldehyde and ␤-
◦
pinene at 150–230 C for several hours yields quantitative amounts
of pure nopol. However, apart from the numerous limitations of the
homogeneous catalyst systems in terms of their corrosive nature,
difficulty in separation, post-synthesis disposal and recovery, the
formation of monocyclic isomers and other side products along
with nopol are of concern and need to be addressed. Heterogeniza-
tion of homogeneous acids by supporting them on high surface area
solids such as graphite, Al O , SiO , ZrO , zeolites and clays, are the
[
3]. The incorporation of super acidity in solids has attracted con-
siderable attraction [4]. Among these, sulfated zirconia (SZ) had
found several applications and holds great promise in a number
of reactions of industrial importance [5]. SZ has been widely used
to catalyze reactions such as isomerization, alkylation, acylation,
esterification, condensation, etherification, nitration, cyclization,
etc. Thus, SZ and its modified versions form an important class
of catalysts, as is evident from the voluminous reports citing the
catalytic utility of SZ catalysts [6].
2
3
2
2
common practices currently adapted by industries to overcome the
drawbacks associated with homogeneous catalysis systems; how-
ever the leaching problem in this type of catalyst is a matter of
concern [9].
Several catalytic systems have been reported for Prins conden-
sation of ␤-pinene with formaldehyde for synthesis of nopol; these
include mesoporous iron phosphate [10], metal supported MCM-
Nopol [2-(7,7-dimethyl-4-bicyclo[3.1.1]hept-3-enyl)ethanol] is
an optically active bicyclic primary alcohol that is used in soap
4
1 mesoporous molecular sieves [11], SnCl grafted on MCM-41 by
∗ Corresponding author. Tel.: +91 278 2471793; fax: +91 278 2567562.
E-mail address: hcbajaj@csmcri.org (H.C. Bajaj).
4
chemical vapour deposition (CVD) [12], ZnCl2 impregnated Indian
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.10.005

S.V. Jadhav et al. / Applied Catalysis A: General 390 (2010) 158–165
159
These samples are hereafter denoted as SZ-xNCalcn, where x = 1.0,
1.5 and 2.0 represents the concentration of sulfuric acid added dur-
ing the sulfatation. For comparative studies, as synthesized catalyst
SZ-1NAs prepared using 1.0 N sulfuric acid was used.
OH
SO
Acetonitrile, Reflux
2-/ZrO
4 2
+
HCHO
β-pinene
Paraformladehyde
Nopol
2.3. Catalyst characterizations
Scheme 1. Prins condensation of ␤-pinene and paraformaldehyde to nopol over
sulfated zirconia.
The crystallinity of SZ after calcination was measured with
an X-ray powder diffractometer (Rigaku) using Cu K␣ radiation
◦
ꢀ = 1.54056 A˚ ). The samples were scanned in 2ꢁ range of 2–80
(
Montmorillonite [13] and double metal cyanide (DMC) complexes
◦
−1
at a scanning rate of 3 min
.
[
14]. Selvaraj and Choe [15] recently reported application of Sn-
SBA-15 as catalyst for synthesis of nopol with 99% conversion and
5% selectivity to nopol. Among several solid acid catalysts reported
To investigate the nature of bonding of sulfate ions with zirco-
nia surface, we performed FT-IR spectroscopic studies in the range
9
−
1
4
000–400 cm as KBr pellets (Perkin Elmer). To investigate the
so far, sulfated zirconia was never looked into for this kind of
condensation reaction, although it contains a potentially unique
distribution of Brönsted and Lewis acid sites, as is evident from
the XPS and IR spectroscopic studies, indicating the presence of
a chelating bidentate complex like structure in which the sulfate
species coordinated to ZrO2 [16].
To the best of our knowledge, there are no reports in the open
literature on the heterogeneous catalytic reaction for the Prins con-
densation of ␤-pinene over SZ catalysts. In continuation of our
recent studies to develop modified zirconia as an efficient hetero-
geneous catalyst for the synthesis of fine chemicals [17], herein
we report the selective synthesis of nopol via Prins condensation
of ␤-pinene with paraformaldehyde using SZ catalyst in the liquid
phase reaction conditions (Scheme 1). Various reaction parameters
such as effect of sulfatation, calcination temperature of SZ cata-
lyst, amount of catalyst, reaction temperature, reaction time, ratio
of reactant (␤-pinene:paraformaldehyde) and effect of solvents in
order to obtain optimal reactions conditions have been studied;
further, the catalyst reusability studies were also taken up.
Lewis and Brönsted acidic sites in the SZ catalyst, pyridine adsorbed
DRIFT spectra were recorded.
The BET surface area, total pore volume and pore size distribu-
tion were measured by nitrogen adsorption–desorption method at
liquid N2 temperature (77 K) (Micromeritics-ASAP 2020). The cal-
◦
cined samples were degassed under vacuum at 200 C for 4 h, prior
to adsorption measurement to evacuate the physisorbed moisture.
The NH –temperature desorption measurements were per-
3
◦
◦
−1
formed in the temperature range 50–800 C at 10 C min ramp. A
thermal conductivity detector was used for continuous monitoring
of the desorbed ammonia. The areas under the peaks were inte-
grated (Micromeritics-Autochem 2920). Prior to TPD studies, the
◦
samples were pre-treated at 200 C for 1 h in a flow of ultra pure
He gas (40 mL/min). Then samples were saturated with 10% ultra
◦
pure anhydrous ammonia gas (balance He; 75 mL/min) at 50 C for
1
◦
h and subsequently flushed with He (60 mL/min) at 70 C for 2 h
to remove the physisorbed ammonia.
2.4. Catalytic studies
2
. Materials and methods
The liquid phase Prins condensation reaction of ␤-pinene with
paraformaldehyde was carried out in a batch reactor (50 mL) in
the presence of SZ solid acid catalyst. Required amounts of the
desired reactants viz., ␤-pinene and paraformaldehyde in the pres-
ence of a solvent (e.g. acetonitrile) with an appropriate amount
2
.1. Materials
Zirconium oxychloride and aqueous ammonia (28–30 wt%)
were purchased from Loba fine chemicals Ltd., India. ␤-pinene and
paraformaldehyde were procured from Sigma–Aldrich Chemical
Inc., USA. Analytical grade acetonitrile and toluene were obtained
from Rankem Ltd., India. All the chemicals were used as received
without further purification. Millipore water was used in catalyst
preparation.
◦
of catalyst (pre-activated at 450 C for 2 h) were taken in a two-
necked round-bottom flask fitted with an efficient coiled condenser
with continuous circulation of cold water. Reaction mixtures were
heated at a desired temperature using a PID temperature controller
under continuous magnetic stirring.
After the reaction, the reaction mixture was cooled and
centrifuged. The organic products were analyzed on a gas chro-
matograph (Varian, GC–450), equipped with a Factor-4 capillary
column (30 m long and 0.32 mm internal diameter) employed with
a flame ionization detector. The oven temperature was varied from
2.2. Catalyst synthesis
SZ catalysts with varied sulfur contents were synthesized by
a two-step precipitation and impregnation method. Zirconium
hydroxide was precipitated from an aqueous solution of zirco-
nium oxychloride by the addition of NH OH. A 0.45 M solution
◦
◦
−
1
8
0 to 220 C programmed at the ramp rate of 10 C min . Product
identification was done using a gas chromatograph–mass spec-
trometer (Shimadzu GCMS-QP-2010) with GC oven programmed
4
of ZrOCl ·8H O (600 ml) was hydrolyzed by drop wise addition of
◦
2
2
in the temperature range 40–200 C with helium as a carrier gas
aqueous ammonia (28–30 wt% NH ) with vigorous mechanical stir-
3
and MS in EI mode with 70 eV ion source. The calibration of GC
peak areas was carried out by taking solutions of known compo-
sitions with dodecane as an internal standard. The conversion and
selectivity are calculated as per the reported formulas.
ring, until the pH of the solution reached 8; then the mixture was
stirred for another 30 min. The Zr(OH)4 precipitate was then sepa-
rated by filtration and washed thoroughly with distilled water until
◦
chloride free (AgNO3 test). The sample was dried at 120 C for 24 h
and then ground into fine powder.
Conversion (mol%)
Sulfatation was done by incipient wetness impregnation
method. Subsequently, ca. 1 g of dried Zr(OH)4 sample was treated
with 10 mL sulfuric acid having desirable concentrations (1.0, 1.5
and 2.0 N), stirred for 30 min, filtered and washed gently with dis-
tilled water. The sample was dried at 100 C for 12 h, followed by
calcination at desired temperatures for 3.5 h in static air, cooled
to room temperature and stored in desiccators before being used.
= initial
mol% of ␤-pinene − final mol% of ␤-pinene
× 100
× 100
initial mol% of ␤-pinene
◦
GC peak area of nopol
Selectivity of nopol = ꢀ
GC peak area of all products

1
60
S.V. Jadhav et al. / Applied Catalysis A: General 390 (2010) 158–165
Fig. 2. N2 adsorption–desorption isotherm and pore size distribution of SZ-2NCalcn
catalyst.
Fig. 1. Effects on the crystallinity of sulfated zirconia catalysts as a function of
calcination temperature and sulfate loading.
3
. Result and discussion
3.1. Characterization of sulfated zirconia catalyst
The PXRD of SZ catalysts after calcination at a designated tem-
perature typically showed the presence of corresponding peaks for
tetragonal and monoclinic phases, in accordance with the litera-
ture [18]. The presence of tetragonal phase was observed initially
◦
and on further increase in calcination temperatures (>650 C), mon-
oclinic phase started to appear. Co-existence of both tetragonal
and monoclinic phases was observed at calcination temperature
◦
of 750–850 C (Fig. 1) and the phase transition from tetragonal to
monoclinic increased predominantly on increasing the calcination
temperature. Furthermore, we have studied the effect of sulfur con-
tent on the crystalline phases of the SZ. As is evident from Fig. 1,
the sample synthesized using 1.0 N H SO4 showed peaks corre-
2
sponding to tetragonal phase only, whereas further increase in
concentration of H SO4 used for impregnation resulted in partial
2
development of monoclinic phase coexisting with major tetrago-
nal phase, as evident from PXRD patterns of SZ samples, in good
agreement with reported literature [19].
Fig. 3. FT-IR spectrum of sulfated zirconia catalysts.
The sulfur contents obtained for various SZ samples by elemen-
tal analysis are depicted in Table 1, along with the corresponding
BET surface areas derived from N2 adsorption/desorption stud-
ies. The sulfur content in SZ-1NCalcn, SZ-1.5NCalcn and SZ-2NCalcn
was 1.6%, 2.3% and 2.9% respectively. Furthermore, there was an
increase in BET surface area in the SZ samples compared to parent
ZrO2 as reported earlier [20]. All the SZ catalysts typically showed
the adsorption–desorptionisotherm of type IVhavingH2hysteresis
confirming the mesoporous nature of SZ samples (Fig. 2) Larger size
mesopores were observed at higher relative pressure values (P/Po),
making acid sites accessible to reactant molecules [21]. Increasing
the sulfur content resulted in decrease in the surface areas of SZ cat-
alysts (Table 1). In sulfated zirconia, during calcination the sulfur
layer anchored on the surface stabilizes the zirconia. The sintering
phenomenon is hampered thus resulting in a decrease in surface
area and an increase in pore volume [22]. The variation in the pore
volume and pore size (Fig. 2) may be due to formation of mesopores;
the SZ-2NCalcn has extended P/Po in the mesoporous region com-
pared to the SZ-1NCalcn. Similar kinds of phenomenon have been
reported in the literature by other researchers where on increas-
ing the sulfur content resulted in an increase in the pore diameter
[23,24].
FT-IR analysis of SZ catalyst revealed the presence of sulfate
groups and the mode of binding with zirconia surface. The peaks
−
1
2−
around 1200–900 cm (Fig. 3) can be assigned to the SO4 group;
−
1
in particular, the peaks at 1240, 1141, 1072, 1044, and 995 cm
,
can be attributed to characteristic inorganic chelating bidentate
sulfate, and are assigned to asymmetric and symmetric stretch-
ing frequencies of partially ionized S O double bonds and S
O
bonds [25]. The ionic structure of SO42 group in the presence of
−
Table 1
Textural properties of the prepared sulfated zirconia catalysts.
◦
2
3
˚
Catalyst
Strength of H2SO4 (N)
Calcination temperature ( C)
Surface area (m /g)
Pore volume (cm /g)
Pore size (A)
Sulfur content (wt%)
SZ-1NCalcn
SZ-1.5NCalcn
SZ-2NCalcn
1.0
1.5
2.0
650
650
650
103
96
93
0.116
0.131
0.134
50
51
56
1.6
2.3
2.9

S.V. Jadhav et al. / Applied Catalysis A: General 390 (2010) 158–165
161
Fig. 5. NH3–TPD profiles of sulfated zirconia catalysts prepared with various sulfur
Fig. 4. DRIFT spectrum of pyridine adsorbed on pure ZrO2, Zr(OH)4 and SZ-xN
x = 1.0, 1.5, and 2.0) samples prepared with various sulfur contents.
contents.
(
On the basis of NH –TPD analysis, catalytic activity of SZ-xN
3
towards selectivity to nopol can be explained. As described ear-
lier significant presence of Brönsted acid sites in SZ-1NCalcn, may
be responsible for comparative low selectivity to nopol, as Brön-
sted acid sites are mainly responsible for isomerization of ␤-pinene
into ␣-pinene and camphene. For SZ-2NCalcn sample, appreciable
increase in nopol selectivity was observed ascribed to the presence
of large number of Lewis acid sites along with a few strong Brönsted
acid sites. These studies clearly suggested that synergic effects of
weak Lewis and strong Brönsted acid sites present in SZ are respon-
sible for the high conversions of ␤-pinene with good selectivity to
nopol.
adsorbed water molecules is responsible for the Brönsted acidity in
SZ catalysts [24].
FT-IR spectroscopy of adsorbed pyridine is a useful tech-
nique commonly used for discernment of Brönsted and Lewis
acid sites. The adsorbed pyridine probe molecules tend to couple
with aprotic (Lewis) and/or protonic (Brönsted) catalytic cen-
ters through the nitrogen lone-pair electrons and hence can be
detected by monitoring their ring vibrations [26]. The pyridine
adsorbed DRIFT spectrum of SZ samples (Fig. 4) showed bands
−1
in the 1700–1400 cm region. The characteristic peaks for pyri-
dinium ion (Brönsted sites) and covalently bonded pyridine (Lewis
−
1
−1
and 1445 cm , respectively, were observed
sites) at 1541 cm
in SZ catalyst. Also, peaks at 1636 cm
−1
−1
and 1612 cm corre-
3.2. Catalytic activity studies of Prins condensation of ˇ-pinene
with paraformaldehyde
−1
sponding to Brönsted sites and 1489 cm representing the total
acidic sites were present in the pyridine adsorbed SZ sample
[
27,28].
To investigate the acid strength of the catalyst, we collected
The catalytic activity of the SZ samples was investigated for Prins
condensation of ␤-pinene and paraformaldehyde for the synthesis
of nopol. Prior to the use of SZ as catalyst for the Prins condensa-
tion reaction, the potential catalytic activity values of zirconium
hydroxide (Zr(OH)4) and zirconia (ZrO2) for the title reaction was
also investigated. In the cases of Zr(OH)4 and ZrO2 catalysed Prins
condensation reaction (Table 2, entries 1–2), only 39% conversion
of ␤-pinene with 63% nopol selectivity was observed with Zr(OH)4.
NH –TPD profiles of the calcined SZ samples by adsorption of
NH followed by temperature programmed desorption. The results
were in good agreement with the reported data [29] where both
the Lewis and Brönsted sites were actively present. The initial
broad low temperature desorption peak around 110 C can be
3
3
◦
attributed to the NH3 adsorbed on to the weak Lewis sites. Two
broad medium temperature peaks in the range 200–450 C ascribed
◦
◦
Calcination of pure Zr(OH)4 at 650 C resulted in formation of ZrO2
to NH3 coordinated to acid sites with medium strength. A distinct
high temperature peak at ca. 600 C (Fig. 5), suggesting the presence
of very strong Brönsted acid sites, in accordance with earlier stud-
ies by Riemer et al. [30]; they showed that these very strong acid
sites or superacidity found in SZ are mostly due to the presence of
Brönsted acid sites, as confirmed with 1H MAS NMR. These studies
with coexisting tetragonal and monoclinic phases. As can be seen
from the pyridine adsorbed FT-IR spectrum for the pure ZrO2 sam-
◦
−
1
ple, vibrational bands at 1455 and 1607 cm associated with Lewis
acid sites were observed [28]. The Prins condensation of ␤-pinene
with paraformaldehyde carried out in acetonitrile under identical
experimental conditions using ZrO2 resulted in 22% conversion of
␤-pinene with 92% nopol selectivity.
were later supported by Babou et al. [31]. The NH –TPD profile of
3
◦
one of the SZ sample studied at an elevated temperature ca. 800 C
revealed a broad high temperature desorption peak (not shown
Table 2
in the figure) assigned to decomposition of SO42− species [20]. The
Distribution of Lewis and Brönsted acidity in sulfated zirconia catalysts detected
with NH –TPD analysis.
NH –TPD results signify that, sulfatation treatment leads to uneven
3
3
distribution of acid sites in SZ-xN, resulting in a broad distribution
of acid sites with weak, medium and strong acid strengths (Table 2).
The results observed in the case of SZ-2NCalcn and SZ-1NCalcn were
in agreement with those of the findings of Chen et al. [20], where the
amounts of Brönsted acid sites decreased in comparison to those of
the Lewis acid sites along with the decrease in B/L ratio. The vari-
ation observed in case of SZ-1.5NCalcn may be due to dispersion of
sulfate ion on the surface of zirconia [20].
Catalyst
Acid amounta (mmol/g)
Brönsted
Lewis
Total
SZ-1NCalcn
SZ-1.5NCalcn
SZ-2NCalcn
0.41
0.12 (0.17)
0.27
0.55
0.73 (0.67)
0.91
0.96
b
b
0.85 (0.84)b
1.18
a
Data obtained from NH –TPD analysis.
Repeated TPD analysis result of SZ-1.5NCalcn data is given in parentheses.
3
b

1
62
S.V. Jadhav et al. / Applied Catalysis A: General 390 (2010) 158–165
Table 3
Condensation of ␤-pinene with paraformaldehyde over various sulfated zirconia catalysts.
Entry
Catalyst
Time (h)
Conversion (%)
Selectivity (%)
Nopol
Nopyl acetate
Others
1
2
3
4
5
6
7
8
9
0
1
2
3
Zr(OH)4a
ZrO2a
12
12
12
12
12
12
09
09
09
09
6
39
22
36
45
41
66
63
90
92
99
94
88
40
63
92
87
95
98
99
89
92
95
99
98
05
88
10
02
04
01
Minor
Minor
05
01
01
Minor
01
Minor
03
27
06
09
04
02
01
06
07
04
01
01
95
09
a
SZ-1NAs
SZ-1NCalcna
SZ-1.5NCalcn
a
a
SZ-2NCalcn
b
SZ-1NAs
SZ-1NCalcnb
SZ-1.5NCalcn
b
b
1
1
1
1
SZ-2NCalcn
SZ-2NCalcnc
d
SZ-2NCalcn
09
09
SZ-2NCalcne
a
◦
Reaction conditions: reaction was carried out under previously studied reaction conditions (see ref. [14]); 0.1 g of catalyst (pre-activated at 450 C for 2 h); ␤-
◦
pinene:paraformaldehyde = 4:8 mmol; reaction time = 12 h; reaction temperature = 80 C; solvent = acetonitrile, 5 mL.
b
◦
Reaction conditions: reaction was carried out under optimized reaction conditions. 0.15 g of catalyst (pre-activated at 450 C, except entry 7); ␤-
◦
pinene:paraformaldehyde = 6:18 mmol; reaction time = 9 h, reaction temperature = 80 C, ␤-pinene:acetonitrile molar ratio = 1:5.
c
Reaction was carried out in aforementioned optimized conditions, taking 0.2 g of catalyst.
Solvent-free reaction condition.
Reaction carried out using toluene as solvent.
d
e
The data obtained for Prins condensation of ␤-pinene and
formaldehyde, generated in situ by paraformaldehyde, over vari-
ous SZ catalysts was presented in Table 3. Nopol was the major
product, along with nopyl acetate and other isomerized products of
aforementioned hypothesis provides additional supports to the
results presented herein.
3.2.2. Effect of catalyst calcination temperature on Prins
␤
-pinene (␣-pinene, camphene), along with traces of pinocarvone
condensation reaction
and trans-pinacarveol. Sulfated zirconia catalyst showed relatively
high conversion (∼65%) with good selectivity for nopol (∼99%)
The synthesized SZ catalysts calcined at 650, 750 and 850 ◦C in
static air were cooled to RT gradually and then stored in vacuum
(
Table 3, entries 3–6) compared to Zr(OH) and ZrO under identical
desiccators. ␤-pinene conversions obtained were as 98%, 48% and
4
2
reaction conditions.
8% for catalysts calcined at temperatures of 650, 750 and 850 ◦C
respectively. The above observation can be correlated with crys-
tallite phase of SZ formed during calcinations as well as the sulfur
content of the catalyst. As reported the tetragonal phase of SZ cata-
3
.2.1. Effect of sulfate content of SZ catalyst on Prins
condensation reaction
SZ catalysts with different sulfur contents were prepared by con-
trolled impregnation of zirconium hydroxide with fixed quantities
of H SO . For SZ catalysts impregnated with H SO solutions of
1
were obtained respectively. The sulfur content of the SZ samples
can directly be correlated with conversion of ␤-pinene as well as
selectivity to nopol as relative amounts of Lewis and Brönsted sites
largely depend on the surface concentration of sulfates [32]. There
are numerous discrepancies regarding the exact structure of active
sites as well as their nature (Lewis to Brönsted type) [5,6]. The effect
of sulfate concentration of SZ in correlation to results obtained for
Prins condensation reaction has been discussed below:
◦
lyst is the catalytically most active phase, predominant at 650 C [5].
◦
When the calcination temperature was further increased to 750 C,
2
4
2
4
SZ mainly exists in monoclinic form, leading to significant decrease
.0 N, 1.5 N and 2.0 N, the sulfur contents of 1.6, 2.3 and 2.9 wt%
in the ␤-pinene conversion. Furthermore, if one increases the calci-
◦
nation temperature up to 850 C, the catalyst is almost catalytically
inactive. This may be due to the formation of catalytically inactive
monoclinic phase as well as decomposition of sulfate species from
the catalyst surface (evident from the FT-IR spectra of SZ catalyst).
3.2.3. Effect of reaction temperature
In order to optimize the reaction conditions, we investigated
the effect of reaction temperature on the conversion of ␤-pinene
with selectivity to nopol. A set of reactions performed at 60, 80 and
(
i) Maximum selectivity to nopol was observed in the case of cat-
alyst SZ-2NCalcn, which may be attributed to the presence of
Lewis acidic sites over the catalyst surface. As we switched to
SZ-1.5NCalcn and SZ-1NCalcn, a slight decrease in the selectiv-
ity to nopol was observed. The observations discussed before
◦
1
9
00 C, resulted in 21%, 97% and 98% conversion of ␤-pinene with
8%, 99% and 95% selectivity to nopol respectively using a substrate:
◦
solvent molar ratio of 1:5 (Fig. 6). Lower conversion at 60 C may be
associated with negligible in situ generation of formaldehyde from
paraformaldehyde in the reaction mixture [14] thereby dropping
are in accordance with NH –TPD analysis, wherein succes-
3
◦
the conversion. The reaction proceeds faster (ca. 6 h) at 100 C, but
sive increase in the concentration of Lewis type acid sites was
resulted in a decrease in the selectivity for nopol, due to formation
observed upon increasing the normality of impregnated H SO4
2
◦
of nopyl acetate as side product. The reaction carried out at 80 C
solution (Table 2).
gave better conversion of ␤-pinene as well as higher selectivity to
(
ii) The generation of Lewis type acid sites over the surface of
SZ catalyst can be explained in correlation with structural
models proposed by various researchers [5]. Sulfate treat-
ment of the hydrous zirconia results in the elimination of
terminal –OH species due to their substitution with bisulfate
species. Furthermore, the Lewis acidity enhancement can be
attributed to the increase in the electron-accepting properties
of three-coordinate zirconium cations via the inductive effect
◦
nopol, so all further studies were carried out at 80 C.
3.2.4. Effect of solvent on Prins condensation of ˇ-pinene and
paraformaldehyde
To begin with, we attempted the Prins condensation of ␤-pinene
with paraformaldehyde under solvent-free reaction conditions. It
resulted in 88% ␤-pinene conversion in 12 h, however the reaction
leads to the formation of isomerization products of ␤-pinene (␣-
pinene, camphene, ␣-terpinene) with nopol selectivity <5%. The
2
−
of SO4 anions, which withdraw electron density from zirco-
nium cations through the bridging oxygen atom [33,34]. The

S.V. Jadhav et al. / Applied Catalysis A: General 390 (2010) 158–165
163
Fig. 6. Optimization of Prins condensation reaction as a function of reaction tem-
perature and substrate:solvent ratio.
Fig. 8. Kinetic profile of Prins condensation reaction.
Fig. 7. Effect of ␤-pinene:paraformaldehyde molar ratio on ␤-pinene conversion
and nopol selectivity.
Fig. 9. Reusability of SZ-2NCalcn catalyst.
low selectivity to nopol under solvent-free conditions may be due
to the in situ minimal generation of formaldehyde.
Prins condensation reaction was studied (Table 4). The substrate:
solvent molar ratio significantly affects the ␤-pinene conversion.
The nopol selectivity remained almost constant but conversion of
␤-pinene decreased with the increase in the solvent quantity when
The polarity of the solvent was found to play an important role
in ␤-pinene conversion, as higher conversions of ␤-pinene with
good selectivity to nopol were observed in acetonitrile. In contrast,
when the reaction was carried out in toluene, formation of side
products predominated, with very low selectivity to nopol. Ace-
tonitrile due to its higher polarity (the dielectric constant value of
acetonitrile is 37.5 and that of toluene is 2.4) gave higher conver-
sion. Further the effect of ␤-pinene: acetonitrile molar ratio on the
◦
the reaction was conducted at 80 C for 9 h in acetonitrile by varying
the substrate: solvent molar ratio from 1:5 to 1:72. The dependency
of ␤-pinene conversion on the acetonitrile may be ascribed to the
fact that, using a higher acetonitrile to ␤-pinene molar ratio, ace-
tonitrile molecules start competing with formaldehyde species for
their interaction with active sites of the catalyst and thereby block-
Table 4
Effect of acetonitrile amount on product distribution.
Entry
Substrate:solvent (molar ratio)
Reaction time (h)
% Conversion
% Selectivity
Nopol
NA
Others
1
2
3
4
5
6
1:2.5
1:5
1:9
1:18
1:36
1:72
08
09
09
12
12
12
90
99
87
78
63
47
91
98
96
94
95
94
2
1
1
2
2
3
7
1
3
4
3
3
◦
Reaction conditions: ␤-pinene:paraformaldehyde = 4:8 mmol; catalyst (SZ-2NCalcn) = 0.1 g; temperature = 80 C; NA = nopyl acetate; others = mixture consisting of unequal
quantities of ␣-pinene, camphene, pinocarveol, pinacarvone, myrtenal, limonene, trans-caryophyllene.

1
64
S.V. Jadhav et al. / Applied Catalysis A: General 390 (2010) 158–165
Fig. 10. Proposed reaction mechanism for Prins condensation of ␤-pinene with paraformaldehyde over sulfated zirconia catalysts.
ing the catalytic sites which will result into decrease in conversion
3.2.7. Kinetic profile
of ␤-pinene.
The detailed kinetic profile of the reaction was investigated as
functions of reaction temperature and time (Fig. 8). The concentra-
tion of ␤-pinene decreases with time and reaches almost zero after
8
h, with a simultaneous increase in nopol concentration. The ini-
3
.2.5. Effect of substrate-to-paraformaldehyde molar ratio
After optimizing the reaction temperature, we began studies on
tial rate of the reaction calculated per gram of catalyst was found
to be 8.5 mmol/h, at 80 C.
◦
optimizing the relative ␤-pinene to paraformaldehyde ratio. The
molar ratio of reactants was varied as 2:1, 1:1, 1:2, 1:3 and 1:4; the
results are illustrated in Fig. 7. The low conversion of ␤-pinene was
observed in the case of 2:1 molar ratio of ␤-pinene: paraformalde-
hyde may be ascribed to nominal availability of formaldehyde,
whereas in the case of 1:2 molar ratio of ␤-pinene: paraformalde-
hyde 82% conversion was achieved in 9 h and 98% conversion with
3.3. Catalyst reusability studies
The reusability studies were performed using SZ-2Ncalcn cata-
lyst. The used catalyst was washed thrice with warm acetone and
dried at 120 C for 2 h in order to remove the adhered organics
◦
␤
-pinene: paraformaldehyde molar ratio to 1:3.
and unreacted paraformaldehyde; the dried catalyst was then ther-
mally regenerated at 450 C for 2 h. The catalyst regenerated after
◦
every cycle of reaction gave good catalytic activity up to 5 cycles
with slight decrease in the conversion of ␤-pinene, this may be due
to the catalyst handling loss although no change in nopol selectivity
was observed (Fig. 9).
3
.2.6. Effect of catalyst amount on ˇ-pinene conversion
The Prins condensation reaction was conducted with different
amounts of SZ (SZ-2NCalcn) catalyst using 1:3 molar ratio of ␤-
pinene: paraformaldehyde in the presence of 1.25 mL of acetonitrile
◦
at 80 C for 9 h. As the catalyst amount was amplified from 0.05 to
3.4. Reaction mechanism
0.1 g, the ␤-pinene conversion increased from 58% to 87% devoid of
any changes in nopol selectivity. When the catalyst amount was fur-
ther increased to 0.15 g, 97% ␤-pinene conversion was achieved in
Ever since the discovery of SZ catalyst, various models have been
proposed for explaining the surface structure and nature of catalyti-
cally active sites in SZ. Although the origin of superacidity in SZ has
been a matter of conflict, most of the proposed models take into
account the formation of Lewis as well as Brönsted acid sites. For-
9
h, while 0.2 g catalyst gave 94% conversion in 6 h. The nopol selec-
tivity remained unaffected with respect to catalyst to substrate
ratio.

S.V. Jadhav et al. / Applied Catalysis A: General 390 (2010) 158–165
165
mation of Lewis acid sites may be ascribed to the highly covalent
Acknowledgement
character of the adsorbed sulfates and formation of Brönsted sites
to a result of the interaction of water molecules with these sulfates
The authors are thankful to CSIR-Network project (NWP 010)
for financial support and to the Analytical science discipline of the
institute for instrumentation. KMJ acknowledges CSIR, New Delhi
for the award of senior research fellowship. SVJ wants to thank Mr.
J. Churchil, for his assistance in GC analysis. The authors want to
thank the editor and reviewers for their constructive comments.
[
5,6].
The mechanism for Prins condensation reaction of ␤-pinene
with paraformaldehyde over Lewis acidic catalysts had been
reported [13,14]. However, a reaction pathway via Lewis acidic
sites as well as Brönsted acidic sites is discussed here. The coor-
dinatively unsaturated Zr4+ ions in SZ are the probable Lewis acidic
sites which assist in the formation of carbocation at formaldehyde
centre. ␤-pinene then reacts with these cationic species, followed
by allylic proton transfer from ␤-pinene, which resulted in the for-
mation of unsaturated alcohol (nopol). A similar mechanism can be
postulated over the Brönsted acidic sites of SZ, except that the inter-
actions of formaldehyde species with hydrogen bonded hydroxyl
groups lead to formation of carbocations in the initial step of the
reaction. A tentative reaction mechanism over the active Lewis
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