Supported H4SiW12O40 catalysts for α-pinene isomerization

Article abstract of DOI:10.2478/s11532-012-0039-9

The heterogeneous isomerization of α-pinene was studied at 100, 130 and 160°C using 10% supported H₄SiW₁₂O₄₀ (SiW) on SiO₂, TiO₂ and HZSM-5. The effect of the reaction temperature and the concentratio

Full text of DOI:10.2478/s11532-012-0039-9

Cent. Eur. J. Chem. • 10(4) • 2012 • 1208-1217
DOI: 10.2478/s11532-012-0039-9
Central European Journal of Chemistry
Supported H4SiW12O40 catalysts
for α-pinene isomerization
Research Article
József-Zsolt Szücs-Balázs1, Maria Coroş1*,
Adrian Woiczechowski-Pop2, Gabriela Blăniţă3, Mircea Vlassa2
1Institute of Chemistry Research Raluca Ripan,
Babeş-Bolyai University, 400294 Cluj-Napoca, Romania
2Faculty of Chemistry and Chemical Engineering,
Babeş-Bolyai University, 400028 Cluj-Napoca, Romania
3National Institute of Research and Development of Isotopic
and Molecular Technologies, 400293 Cluj-Napoca, Romania
Received 9 December 2011; Accepted 21 February 2012
Abstract:
The heterogeneous isomerization of α-pinene was studied at 100, 130 and 160°C using 10% supported H4SiW12O40 (SiW) on SiO2,
TiO2 and HZSM-5. The effect of the reaction temperature and the concentration of the catalyst over the conversion of α-pinene
and the selectivity of camphene, limonene and terpinolene had been monitored. The recovery, reuse and leaching effect of the catalysts
were studied. Simple, environmentally friendly and economic methods for the synthesis of camphene and limonene were developed
and optimized.
Keywords: Heteropolyacids • Monoterpenes • Catalytic activity • Isomerization • Selectivity
© Versita Sp. z o.o.
1. Introduction
camphene,theindustrialprocessisbasedonaweakacidic
TiO2 catalyst [3]. In order to reduce energy consumption
Monoterpenes are essential ingredients of flavor and to increase selectivity toward camphene, many solid
and fragrance, perfumery, cosmetics, fine chemicals catalysts were studied: sulfated zirconia [4-6], activated
and pharmaceutical industries. The most important clays [7-11], modified molecular sieves [12-14] and
constituent of the monoterpene group is α-pinene which zeolites [15-18]. Each solid catalyst leads to a complex
contains a labile bicyclo[3.1.1]heptene carbon skeleton mixture of terpenes, when usually, camphene, the major
and frequently participates in ring-opening reactions and product, is obtained with a selectivity of 30-50%.
rearrangements, producing various acyclic, mono- and
The literature data suggests that the bicyclic/
bicyclic terpenes [1]. In many cases, heterogeneous monocyclicterpenesratioisdependentonseveralfactors,
acid catalysis is used for the conversion of α-pinene into such as: the reaction temperature and the acid strength.
valuable products such as: camphene, limonene and Severino et al. [19] studied the α-pinene isomerization
terpinolene.
using zeolites and concluded that, on the Lewis sites,
The main products of α-pinene isomerization in the camphene and bicyclic products were preferentially
presence of the solid acidic catalysts are camphene and formed, while on the Brønsted sites, monocyclic terpenes
limonene. Camphene is an important intermediate in were favored. Recently, in addition to these catalysts, a
the synthesis of camphor; limonene can be transformed new family of acids on solid supports, the heteropolyacids
into other monocyclic terpenes such as: terpinolene, family, were successfully used in α-pinene isomerization
α-, γ-terpinene, heavier dimeric and polymeric products [20-22]. Heteropolyacids (HPAs), especially the Keggin
[2]. Conventionally, for the isomerization of α-pinene to series, are very attractive for researchers because of
* E-mail: m_coros@yahoo.com
1208

J.Z. Szücs-Balázs et al.
its numerous opportunities on many heterocatalytic (100-160°C). The reaction products were analyzed
systems, having a major economic impact and green by
gas
chromatography
(Hewlett
Packard
benefits [23,24].
5890/II) with flame ionization detector (FID), using
In this work, we report the isomerization of a SUPELCOWAXTM 10 (60 m × 0.32 mm) capillary
α-pinene in heterogeneous catalysis, using supported column. H2 was used as the carrier gas with a flow of
HPAc catalysts. The Keggin heteropolyacid used for 0.2 mL min-1 and the temperature was programmed
these studies was H4SiW12O40, the most commonly from 60 to 210°C at a rate of 4°C min-1 increase.
encountered heteropolyacid. SiO2, TiO2 and HZSM-5 The identification of reaction products was realized by
zeolite were used as solid supports.
comparison of retention time with standards and GC-
MS. The mass spectra were recorded with a Hewlett
Packard GC-MSD 5890 series II- 5972 spectrometer,
with DB-5MS (30 m/0.25 mm) capillary column.
2. Experimental procedure
2.1. Chemicals
All chemicals used in this study were purchased from
commercial sources. α-Pinene was 97% pure and
3. Results and discussion
before use was distillated, the other chemicals were 3.1. Catalysts characterization
used without further purification.
Nitrogen adsorption data were evaluated using the BET
method. Table 1 presents the surface area and pore
volume measurement of the catalyst and support. The
specific surface areas and the specific pore volumes
slightly decrease when the deposition of the catalysts
on solid surfaces take place.
Nitrogen physisorption isotherms were measured
at 77 K to verify the porosity of H4SiW12O40/SiO2
catalyst. The sample exhibits a type II isotherm with a
H2 hysteresis loop (Fig. 1a). The pore size distribution
(Fig. 1b) (Dollimore-Heal method) indicates the
completion of mesopore filling (99.71% of the volume
of pores). A narrow distribution of the pore diameter
2.2. Catalyst preparation
A series of SiW acid catalysts were prepared by
impregnation with loading 10 wt% SiW/solid support.
A 5 mL alcoholic solution of 0.27 g SiW was added to
2.5 g of solid support and the mixture was allowed to
stir at room temperature for 24 h. After the removal
of methanol by vacuum distillation, the samples were
dried in an oven, under atmospheric pressure at 100°C
for 2 h [29].
2.3. Catalyst characterization studies
Surface areas and pore volumes of the catalysts were is observed, 92.8% of pores being between 3.7 and
obtained by nitrogen absorption isotherms, at liquid 8.6 nm. The specific surface area is 412 m2 g-1
nitrogen temperature, after sample degassing, under (p/p0 = 0.03 - 0.3), with 0.62 ml/g specific pore volume
vacuum at 220°C, for 3 h, using a Sorptomatic 1990 at p/p0 = 0.95.
instrument (Thermo Electron) up to p/p0 ~0.95. The
Fig. 2a shows the isotherm curves for nitrogen
estimated specific surface areas of the samples were physisorption at 77K in the case of the H4SiW12O40/TiO2
calculated using the Brunauer-Emmett-Teller method in catalyst. The specific surface area is 8 m2 g-1 and the
the p/p0 range of 0.03-0.3, set so that the correlation specific pore volume is 0.006 mL g-1. Fig. 2b confirms
factors to be 0.9999.
the mesoporosity of the sample (94.27% mesopores).
Nitrogen adsorption/desorption isotherms of
Powder XRD patterns were measured with a Bruker
D8 Advance Powder diffractometer at 40 kV and 40 mA, H4SiW12O40/HZSM-5 measured at 77K are presented
equipped with an incident beam Ge 111 monochromator
using CuKα radiation (λ = 1.540598Å).
Table 1. Some characteristics of the catalysts and their support
materials.
b
Support/Catalyst
SBETa(m2 g-1) Vpore (mL g-1)
2.4. Catalytic test
SiO2 (1)
489
9
0.73
0.006
0.21
The prepared catalysts were tested in isomerization
reactions of α-pinene. The reactions were carried out
at normal atmospheric pressure in a three-necked
flask equipped with an efficient magnetic stirrer, reflux
condenser and a temperature controller. Typically,
20 g α-pinene and 0.2 g catalyst were loaded in the
reactor, stirred and heated up to working temperature
TiO2 (2)
HZSM-5 (3)
321
412
8
H4SiW12O40/SiO2 (4)
H4SiW12O40/TiO2 (5)
H4SiW12O40/HZSM-5 (6)
0.62
0.006
0.17
259
aspecific surface area, bpore specific volume
1209

Supported H4SiW12O40 catalysts for α-pinene isomerization
Figure 1. (a) Nitrogen adsorption/desorption isotherms at 77 K; (b) Pore size distribution of H4SiW12O40/SiO2 catalyst.
Figure 2. (a) Nitrogen adsorption/desorption isotherms at 77 K; (b) Pore size distribution of H4SiW12O40/TiO2 catalyst.
Figure 3. (a) Nitrogen adsorption/desorption isotherms at 77 K; (b) Pore size distribution of H4SiW12O40/HZSM-5 catalyst.
1210

J.Z. Szücs-Balázs et al.
in Fig. 3a. The sample exhibits a type II isotherm, with
slight hysteresis. The pore size distribution, presented
in Fig. 3b, confirms the mesoporosity of the catalyst.
The physisorption data estimates that the specific
surface area is 259 m2 g-1 (p/p0 = 0.03 - 0.3) and the
specific pore volume is 0.17 mL g-1 at p/p0 = 0.95. Adding
H4SiW12O40 onto the zeolite kept the isotherm type, but
it may be observed a decrease of specific surface and
pore volume.
The PXRD patterns for SiO2, TiO2 and HZSM-5
supported H4SiW12O40 (Fig. 4) display no reflections
corresponding to the SiW Keggin structure. These
results indicate that HPA was highly dispersed at 10%
loading on the supports and are in accordance with
literature data [25].
3.2. Isomerization of α-pinene
Catalytic activity and selectivity of the mentioned
supported SiW catalysts for the isomerization of
α-pinene were studied in liquid phase. Scheme 1
presents two parallel routes for the isomerization
of α-pinene, resulting tricyclic terpenes (tricyclene),
bicyclic terpenes (fenchene, camphene) and monocyclic
terpenes (α-, γ- terpinene, limonene and terpinolene).
The double bond in the terpene skeleton is protonated
by an electrophilic attack when the pinanyl carbocation
is obtained. This intermediate is further involved in a
Wagner-Meerwein rearrangement, with the formation of
isobornyl and p-menthenyl carbocations [26-28]. These
two carbocations represent the key for the formation of
the complex mixture of isomerization products.
The catalytic activity of SiO2 (1), TiO2 (2) and
HZSM-5 (3) in the isomerization of α-pinene (1%
catalyst, 160°C) was tested. In the case of SiO2 (1)
and TiO2 (2) α-pinene remained unreacted even after
a long time at this high temperature. An exception was
observed in the case of HZSM-5 (3), which proved to
act itself as a catalyst. Table 2 highlights the distribution
of isomerization products, using the supports and the
supported catalysts. Camphene and limonene proved to
be the main products of isomerization reactions.
In the presence of catalyst 6, the isomerization
reaction of α-pinene did not take place under the same
conditions as in the case of catalysts 4 and 5 (5 hours,
100°C). When the reaction temperature was increased
up to 160°C for 2 hours (Table 2), the results obtained
were good. As presented in this table, when the
isomerization reaction was performed using 4 and 5 as
catalysts, a low conversion of α-pinene was obtained.
Our purpose was to optimize these reactions in order to
obtain comparable utile conversions. So, our strategy
was focused on extending the reaction time, without
any modifications of the temperature and the catalyst
PXRD patterns for: (a) SiW, SiW10%/SiO , SiO ; (b) SiW,
SiW10%/TiO2, TiO2; (c) SiW, SiW10%/HZSM-5, HZSM-5,
Figure 4.
2
2
1211

Supported H4SiW12O40 catalysts for α-pinene isomerization
Scheme 1. Acid-catalyzed isomerization of α-pinene.
Table 2. Distribution of the products for isomerization of α-pinene over the catalyst (20 g α-pinene, 0.2 g catalyst).
Catalyst
Composition (% mole)
α-Pinene
Tricyclene
Fenchene
Camphene
Terpinenes
Limonene
p-Cymene
Terpinolene
Others
4a
5a
6a
6b
3b
26.6
74.6
95.9
3.9
0.5
0.4
0.2
2.6
1.4
3.1
1.6
0.3
6.2
2.0
31.5
10.6
1.5
6.1
1.6
0
22.8
7.3
1.5
0.5
0.3
0.5
0.2
7.2
2.0
0.7
1.4
0.8
5.6
4.1
0.8
0.2
37.3
9.0
14.4
3.2
18.7
4.4
10.8
1.6
74.1
aReaction temperature 100°C, reaction time 5 hours
bReaction temperature 160°C, reaction time 2 hours
Table 3. Conversion of α-pinene and selectivity to camphene, limonene and terpinolene (20 g α-pinene, 0.2 g catalyst).
Catalyst
Reaction time (h) Conversion (%)
Selectivity (%)
Limonene
Camphene
Terpinolene
4a
5a
6b
3b
8
36
2
81
83
96
70
43.7
46.1
38.7
38.0
28.3
28.6
19.5
20.0
9.1
6.4
11.6
8.0
24
aReaction temperature 100°C
bReaction temperature 160°C
concentration. After the work-up, the conversion of The selectivity of the main products was almost the same
α-pinene and the selectivity to camphene, limonene in the case of the catalysts based on the oxide supports
and terpinolene were calculated and the results were (4 and 5). Significant differences could be highlighted
presented in Table 3. Concerning the conversion, the with respect to the reaction time until a favorable
best results could be observed when catalyst 6 was conversion was reached. So, a good utile conversion
used. In the case of catalysts 4 (reaction time 8 h) and was obtained 4.5 times earlier when catalyst 4 was
5 (reaction time 36 h), good results were obtained with used. This phenomenon can be attributed to the major
respect to the selectivity of camphene and limonene.
differences between the specific surfaces of those two
Conversion of α-pinene and selectivity of products catalysts. Catalyst 6 represents a special case: the main
were calculated using the following equations:
products, camphene and limonene, being obtained less
selective than when catalysts 4 and 5 were used. When
compared the catalytic activity of 3 and 6, important
differences could be also observed in the reaction time
of the isomerization of α-pinene. The results presented
in Table 3 concluded that the high catalytic activity of
6 was caused by the presence of the heteropolyacid
1212

J.Z. Szücs-Balázs et al.
H4SiW12O40, which did not change the behavior and the was 88% and the selectivity of camphene and limonene
selectivity of the zeolite.
was 56%, 7% respectively. Recently, Atalay and Gündüz
In literature, studies of supported heteropolyacids [22] reported some results for the isomerization reaction
(PW) catalyzed isomerization of α-pinene were focused of α-pinene in liquid phase using a non-conventional
on the reaction in liquid or gas phase. When sillica- support, a natural zeolite, clinoptilolite. The reaction was
supported PW (0.6 wt% PW20%/SiO2) was used as a conducted in a nitrogen atmosphere and at a constant
catalyst [29], best results were obtained at 100°C, in temperature of 125°C. A 2.3 wt% PW4.25%/zeolite was
liquid phase. After 1h, the conversion of α-pinene was used and after 3 h the conversion was almost total. The
90%, selectivity of camphene 50% and of limonene selectivity for camphene proved to be 39.3% and for
28%.
Studies concerning theTiO2-supported PW (PW15%/
limonene 12.6%.
Our results are similar to those reported until now,
TiO2) effect on the isomerization reaction were also even better with respect to limonene. We proved that
reported [30]. The reaction was accomplished in gas can be achieved good results with minimum costs. Our
phase at 200°C, 20 mL min-1 flow rate, 10 h time on supported SiW catalysts can be simply obtained in mild
nitrogen stream. In this case, the conversion of α-pinene conditions without any additional precautions.
(a) Concentration of the major products in the reaction mixture depending on the reaction time for H SiW O /SiO catalyst
Figure 5.
4
12
2
(α-pinene/catalyst ratio of 100/1, at 100°C); (b) Concentration of the minor products in the reaction mixture depending4o0 n the reaction
time for H4SiW12O40/SiO2 catalyst (α-pinene/catalyst ratio of 100/1, at 100°C)
Figure 6. Effect of the reaction temperature on the selectivity of
camphene, limonene and terpinolene (α-pinene/catalyst:
100/1, catalyst H4SiW12O40/SiO2)
Figure 7. Effect of the reaction temperature on the selectivity of
camphene, limonene and terpinolene (α-pinene/catalyst:
100/1, H4SiW12O40/TiO2 as a catalyst).
1213

Supported H4SiW12O40 catalysts for α-pinene isomerization
Typical kinetic curves for H4SiW12O40/SiO2 catalyst H4SiW12O40/HZSM-5. These studies were based on a
are presented in Fig. 5.
ratioof100/1α-pinene/catalyst.Thecatalyticactivitywas
According to Fig. 5a, the concentration of α-pinene testedat100, 130and160°Candsignificantconversions
decreases, while the concentration of the main products were obtained only at 160°C. At this temperature,
(camphene and limonene) increases after 12 hours to after 2 hours, the conversion of α-pinene was 96%.
38.4% and 21.8%, respectively. Terpinolene reaches The selectivity of reaction products was monitored and
also a concentration of 7.4%. Other isomerization the results were 39% for camphene, 20% for limonene
products, Fig. 5b, are obtained in traces (<4%).
and 12% for terpinolene. Another compound, terpinene,
was obtained with high selectivity (15%).
3.2.1. The effect of temperature
Temperature plays an important role in the selectivity of 3.2.2. The effect of catalyst concentration
terpene products. Fig. 6 reveals the effects of reaction The effect of catalyst concentration, for supported SiW
temperature on the selectivity of camphene, limonene with the loading of 10% on SiO2 (Fig. 8) was studied.
and terpinolene using H4SiW12O40/SiO2 as a catalyst. The reactions were performed at 130°C, using different
For these studies, the α-pinene/catalyst ratio was α-pinene/catalyst ratios, as follows: 100/1, 100/0.5 and
adjusted to 100/1, and the isomerization processes 100/0.25.
were achieved at 100, 130 and 160°C.
The selectivity of camphene decreases by
High temperatures led to a decreased selectivity increasing the ratio α-pinene/catalyst (Fig. 8). In the
with respect to camphene, limonene and terpinolene. case of terpinolene it can be noticed that the selectivity
Best results were obtained at 100°C for camphene enhances with the increasing of the α-pinene/catalyst
and limonene, and 130°C for terpinolene. It is worth ratio. When analyzing the selectivity of the limonene,
mentioning that at 160°C fair selectivity could also be interesting results are observed. A maximum selectivity
observed for α- and γ- terpinene, 18.9% and 8.6%, is obtained when the ratio is 100/0.5. When adding more
respectively. Concerning the reaction time required catalyst in the synthesis (100/1), a fair selectivity is not
for conversions higher than 80% it can be stated that achieved. This phenomenon can be explained by the
it significantly decreases as the temperature increases: weak stability of the limonene in these conditions and
after 8 hours at 100°C a conversion of 81% was the formation of other terpenoidic derivatives, such as
obtained, after 30 minutes at 130°C a total conversion α- and γ-terpinene. The isomerization reaction time to
was obtained, and at 160°C a total conversion was obtain useful conversions was important for our studies.
already observed after 5 minutes.
Four days were needed for a desired conversion when
Interesting results could be highlighted when using less catalyst was used but only a few minutes when
H4SiW12O40/TiO2 as a catalyst. A graphical picture more catalyst was added (100/1).
including the effect of the temperature on the selectivity
H4SiW12O40/TiO2 proved to be less active
is shown in Fig. 7.
than the catalyst loaded on SiO2: the reaction time
When TiO2 was used as a support, the highest being prolonged and the conversion of α-pinene
selectivity of camphene was obtained at a lower unsatisfactory. Therefore, different concentrations
temperature (see Fig. 7). The selectivity of camphene of catalyst and temperatures were needed. Very good
decreased when the reaction was performed at higher results were obtained using 2% catalyst at 130°C
temperatures. Similar results are observed in the case for only 1 hour, the conversion being 98%. This catalyst
of limonene, but more obvious variation of selectivity is disadvantage yielded substantial changes inin the
defined; at 130°C the selectivity level remains almost composition of the obtained mixture: fenchene and
constant, but at 160°C the selectivity drops to 1%. tricyclene were obtained with relatively high selectivity
According to literature data, this phenomenon can be (9.7%, 10.1% respectively), camphene was obtained
explained by the fact that during the reaction limonene with a relatively low selectivity (34.7%) and limonene
reacts further obtaining other products [31]. The proof appeared only in traces. The most important change
of this affirmation is the formation of α- and γ-terpinene was observed for p-cymene, synthesized with high
with a selectivity of 24.5%. In case of terpinolene the selectivity (20.1%).
highestselectivityisachievedat130°C. Asstatedabove,
Another catalyst taken into account for our studies
the reaction time for a total conversion decreases when concerning the effect of the concentration of catalyst
higher temperatures are used, so, the best conversion over the conversion of α-pinene and the selectivity
is obtained after 30 minutes at 160°C.
of terpenoidic compounds was represented by
The effect of temperature over the activity H4SiW12O40/HZSM-5. The results of these studies
and selectivity of the catalyst was also studied for performed at 160°C are presented in Fig. 9.
1214

J.Z. Szücs-Balázs et al.
As shown in the figure above, using 0.25% catalyst,
the highest selectivity for camphene and limonene
is obtained. Almost similar results as in the case of
H4SiW12O40/SiO2 were determined for terpinolene.
An interesting effect of the concentration of catalyst
can be observed for camphene. When using 0.5%
H4SiW12O40/HZSM-5, the selectivity decreases (35.4%),
but at higher concentration of the catalyst (1%),
the selectivity of camphene rises up to 38.7%.
Concerning the reaction time, the results were in
accordance with those for the catalysts presented
above. α-Pinene was highly converted (96%)
to camphene only after 2 hours when the ratio
α-pinene/catalyst was 100/1.
Figure 8. Effect of the catalyst concentration on the selectivity of
camphene, limonene and terpinolene at 130°C, using
H4SiW12O40/SiO2 catalyst.
3.2.3. Recovery and reuse of catalyst. Evaluating
catalyst leaching
One of the most challenging issues concerning
the advantage of using heterogeneous catalysis
for the isomerization of α-pinene is represented by
the possibility of recovery and reuse of the solid
catalyst. Another problem that arises is the leaching
of catalyst components that can cause its deactivation.
Herein, the study of the stability of supported H4SiW12O40
on SiO2, TiO2 and HZSM-5 is presented. The recovery
yields (94-98%) of solid catalyst were gravimetrically
determined after filtration at reaction temperature.
The results obtained after three recovery/reutilization
cycles for catalysts 4 and 6 are presented in Figs. 10
and 11. As shown, the catalytic activity of supported
SiW on SiO2 and HZSM-5 decreases with the increase
of reutilization cycles, but even after three cycles,
a promising utile conversion is obtained (around
75%). That sustains the stability of these catalysts
under reported reaction conditions. In the case of the
TiO2 based catalyst, only two reutilization cycles were
performed (Fig. 12). As expected, the catalytic activity
sharply decreased, due to the fact that the specific
surface of catalyst 5 was smaller than in the other two
cases.
Figure 9. Effect of the catalyst concentration on the selectivity
to
camphene,
limonene
and
terpinolene
(160°C, H4SiW12O40/HZSM-5).
The study of catalyst leaching was evaluated.
When the catalyst leached into a product phase,
the sample should exhibit some catalytic activity
[32]. The efficient method that allowed the evaluation
of leaching required the removal of the catalyst
by filtration, exclusively at reaction temperature,
and the reaction continued to run in its absence.
The kinetic curves of the reactions catalyzed by
supported SiW before and after its removal are
displayed in Figs. 10, 11 and 12. As it can be observed,
no catalyst leaching occurred, proving that the
isomerization reactions took place in heterogeneous
catalysis.
Figure 10. Kinetic curves for three recovery/reutilization cycles
and the effect of the SiW leaching for H4SiW12O40/SiO2
catalyst (α-pinene/catalyst ratio of 100/1, at 100°C).
1215

Supported H4SiW12O40 catalysts for α-pinene isomerization
Kinetic curves for three recovery/reutilization cycles
and the effect of the SiW leaching for H4SiW12O40/TiO2
catalyst (α-pinene/catalyst ratio of 100/1, at 130°C).
Figure 11. Kinetic curves for three recovery/reutilization cycles and Figure 12.
the effect of the SiW leaching for H4SiW12O40/HZSM-5
catalyst (α-pinene/catalyst ratio of 100/1, at 160°C).
4. Conclusion
HZSM-5 was studied. The effect of reaction temperature
and catalyst concentration in the isomerization process
Isomerization of α-pinene in the presence of the solid was also determined. We succeeded in the optimization
acidic catalysts became an important methodology of an efficient synthetic route to obtain simultaneously
in industry since camphene and limonene, essential camphene and limonene. No leaching effect could be
intermediates in organic chemistry, were obtained. observed with respect to all the three studied catalysts.
A simple, clean and economic methodology for the From the recovery/reuse cycles, one may conclude
preparation of supported catalysts was developed. The that catalysts 4 and 6 are stable under used reaction
XRDstudiesindicatedthatSiWwascompletelydispersed conditions. The isomerization method presented
on the supports. Here, the isomerization of α-pinene in here offers also advantages such as high yields and
heterogeneous catalysis using catalysts prepared from simple procedures for synthesis, recovery and reuse of
H4SiW12O40 (SiW) supported on different oxides and catalysts.
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