Isomerization of styrene oxide to phenylacetaldehyde over supported phosphotungstic heteropoly acid

Article abstract of DOI:10.1016/j.apcata.2010.06.005

Silica-supported H₃PW₁₂O₄₀ (PW), the strongest heteropoly acid in the Keggin series, is an efficient, environmentally friendly heterogeneous catalyst for the liquid-phase isomerization of styrene oxide into phenylacetaldehyde, an industrially important intermediate for fine chemical synthesis. The reaction occurs in cyclohexane as a solvent under mild conditions at 25-70 °C with low catalyst loadings and without PW leaching in solution. At 60 °C, the yield of phenylacetaldehyde reaches 92% at 97% styrene oxide conversion, with a catalyst turnover number of 19 600. The catalyst can be recovered and reused.

Full text of DOI:10.1016/j.apcata.2010.06.005

Applied Catalysis A: General 383 (2010) 217–220
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Isomerization of styrene oxide to phenylacetaldehyde over supported
phosphotungstic heteropoly acid
Vinícius V. Costaa, Kelly A. da Silva Rochaa,1, Ivan V. Kozhevnikovb, Elena V. Gusevskayaa,∗
a Departamento de Química, Universidade Federal de Minas Gerais, Av Antonio Carlos 6627, 31270-901 Belo Horizonte, MG, Brazil
b Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Silica-supported H3PW12O40 (PW), the strongest heteropoly acid in the Keggin series, is an efficient,
environmentally friendly heterogeneous catalyst for the liquid-phase isomerization of styrene oxide
into phenylacetaldehyde, an industrially important intermediate for fine chemical synthesis. The reac-
tion occurs in cyclohexane as a solvent under mild conditions at 25–70 ◦C with low catalyst loadings
and without PW leaching in solution. At 60 ◦C, the yield of phenylacetaldehyde reaches 92% at 97%
styrene oxide conversion, with a catalyst turnover number of 19 600. The catalyst can be recovered and
reused.
Received 16 March 2010
Received in revised form 31 May 2010
Accepted 3 June 2010
Available online 11 June 2010
Keywords:
Isomerization
Styrene oxide
Phenylacetaldehyde
Acid catalysis
Heteropoly acid
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
this reaction. In most of these works, the reactions require very
high catalyst to substrate ratios (10–60 wt%) to reach a reasonable
Epoxides are
a
versatile raw material used for the syn-
yield. Although important advances have been made in the field,
the development of alternative catalytic processes for the synthesis
of phenylacetaldehyde from styrene oxide is still of interest in the
fine chemistry.
thesis of various chemical products. For example, the
isomerization of styrene oxide represents an industrial route to ␤-
phenylacetaldehyde, which is practically useful for the production
of drugs, perfumes, artificial sweeteners, and agricultural chemi-
cals such as insecticides, fungicides and herbicides [1]. Epoxides are
highly reactive compounds known to undergo easily ring opening
giving alcohols, diols, aldehydes, ketones, etc. These reactions
usually give a mixture of products and can be seriously compli-
cated by substrate polymerization. The selective transformation of
epoxides to aldehydes is a particularly problematic reaction due
to a competitive aldolization of aldehyde products. The formation
of high-boiling molecules in these systems often prejudices the
catalyst performance and leads to catalyst deactivation by coke
deposition.
Heterogeneous acid catalysis by heteropoly acids (HPAs) has
attracted much interest because of its potential of great eco-
nomic rewards and green benefits [8]. The majority of catalytic
applications use the most stable and easy available Keggin HPAs
comprising heteropoly anions of the formula [XM12O40]
n−, where
X is the heteroatom (P5+, Si4+, etc.) and M the addendum atom
(Mo6+, W6+, etc.). HPAs possess stronger acidity than conventional
solid acid catalysts such as acidic ion exchange resins, oxides
and zeolites. Due to their stronger acidity, HPAs often exhibit
higher catalytic activities compared to the conventional cata-
lysts. The acid strength of Keggin HPAs decreases in the order:
H3PW12O40 > H4SiW12O40 > H3PMo12O40 > H4SiMo12O40, and usu-
ally H3PW12O40 is the catalyst of choice because of its stronger
acidity [8]. Previously, we have reported HPAs to be versatile cata-
lysts for isomerization of various compounds, e.g., ␣-pinene oxide
[9,10].
Being a difficult industrial problem [1], the isomerization of
styrene oxide into phenylacetaldehyde has been investigated
by several research groups. Various solid acid materials, such as
zeolites [2–6] and Nafion-H [7] have been applied as catalysts for
In the present work, we report the application of silica-
supported H3PW12O40 (PW) as an efficient and recyclable solid
acid catalyst for the rearrangement of styrene oxide into pheny-
lacetaldehyde. To our knowledge, no attempt to use HPA catalysts
for this reaction has been made so far.
∗
Corresponding author. Tel.: +55 31 34095741; fax: +55 31 34095700.
E-mail address: elena@ufmg.br (E.V. Gusevskaya).
Present address: Departamento de Ciências Naturais, Universidade Federal de
1
São João del Rei, 36301-160 São João del Rei, MG, Brazil.
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.06.005

218
V.V. Costa et al. / Applied Catalysis A: General 383 (2010) 217–220
2. Experimental
All chemicals were purchased from commercial sources and
used as received. H3PW12O40 hydrate and styrene oxide were from
Aldrich and Aerosil 300 silica from Degussa.
The catalyst, 20 wt% H3PW12O40/SiO2 (PW/SiO2), was prepared
by impregnating Aerosil 300 (SBET, 300 m2 g−1) with an aqueous
PW solution and pre-treated under static vacuum at 0.2–0.3 Torr at
130 ◦C for 1.5 h, as described elsewhere [11]. Tungsten and phos-
phorus content was measured by inductively coupled plasma (ICP
atomic emission spectroscopy, Spectro Ciros CCD). The BET sur-
face area and porosity of the catalyst were obtained from nitrogen
adsorption which was measured at −196 ◦C (Micromeritics ASAP
2000). 31P MAS NMR spectra were measured on a Bruker Avance
DSX 400 instrument at room temperature, 4 kHz spinning rate,
using 85% H3PO4 as an external reference. XRD pattern of the cata-
lyst was recorded on Rigaku Geigerflex-3034 diffractometer using
CuK␣ radiation.
Scheme 1. Acid-catalyzed isomerization of styrene oxide.
adsorption [14]. It has been found that bulk PW, 20%PW/SiO2 and
pure SiO2 possess initial heats of pyridine adsorption of 135, 58
and 28 kJ mol−1, respectively, in cyclohexane at 50 ◦C [14]. There-
fore, the 20%PW/SiO2 catalyst has quite a strong acidity, although
weaker than that of the bulk PW due to HPA interaction with the
support [8,12]. However, the bulk PW has the drawback of a very
low surface area (<5 m2 g−1) [8,12].
In
(0.07–0.15 mol L−1
2 mmol, internal
a
typical reaction run,
1.4–3.0 mmol), dodecane (0.10 mol L−1
standard), and PW/SiO2 (1–13 mg,
a mixture of styrene oxide
3.2. Isomerization of styrene oxide
,
,
The results on the isomerization of styrene oxide (1) in the
presence of silica-supported PW are presented in Table 1. All exper-
iments were performed using cyclohexane as a solvent. Due to a
high tendency of styrene oxide toward polymerization under acidic
conditions it was not possible to work in solvent-free systems. In
a blank reaction, with no catalyst added, no conversion of the sub-
strate was observed (run 1). On the other hand, styrene oxide is
highly reactive in the presence of PW. With only 0.08 wt% of the
PW/SiO2 catalyst, a nearly complete conversion was achieved in
30 min at 25 ◦C resulting in the formation of only one GC detectable
product identified as phenylacetaldehyde (2) (run 2, Scheme 1).
However, the selectivity to phenylacetaldehyde was only 50%, with
the rest attributed to oligomers, which were not detectable by GC.
Further, our main effort was aimed to control the substrate
oligomerization by choosing appropriate reaction conditions and to
improve the yield of the aldehyde. Decreasing the catalyst amount
to 0.025 wt%, which corresponded to a substrate to PW molar ratio
of nearly 10 000, did not result in a better selectivity (run 3). The
reaction was nearly complete in 1 h; however, a half of the sub-
strate was again transformed into high-boiling products. On the
other hand, with a decrease in the initial concentration of the sub-
strate, the selectivity for 2 increased to nearly 60% (cf. runs 4 and
3). This can be explained by a higher reaction order in substrate for
oligomerization compared to the desired isomerization of styrene
oxide. We did not observe significant changes in selectivity in the
course of these runs, at least after ca. 60% conversion of the sub-
strate. The reactions were too fast to be monitored by GC at lower
conversions.
0.08–0.006 wt%) in cyclohexane (20 mL) was intensely stirred
in a glass reactor under air at a specified temperature. The reaction
progress was followed by gas chromatography (GC, Shimadzu 17,
Carbowax 20M). At appropriate time intervals, aliquots were taken
and analyzed by GC. The GC mass balance was based on the sub-
strate charged. Any difference in mass balance was attributed to
oligomers, which were not GC determinable. Phenylacetaldehyde
was identified by GC–MS (Shimadzu QP2010-PLUS, 70 eV) and also
by GC co-injections with an authentic sample.
3. Results and discussion
3.1. Catalyst characterization
Silica-supported PW catalysts with different PW loadings have
been characterized in detail previously (for a review see [8,12]).
The catalyst, 20%PW/SiO2, that was used in this work had a BET
surface area of 200 m2 g−1, a total pore volume of 0.53 m3 g−1 and
an average pore diameter of 107 Å. The integrity of PW Keggin
structure was confirmed by 31P MAS NMR. In agreement with the
previous report [13], the catalyst showed a well-known peak at
−15 ppm characteristic of H3PW12O40. From XRD, the catalyst was
amorphous; it did not exhibit PW crystalline phase, which also
agrees with the previous study [13]. The PW crystalline phase on
silica has been observed only at PW loadings above 20% [8,12].
The catalyst acidity under the conditions relevant to the present
study has been characterized by microcalorimetry of pyridine
Table 1
Isomerization of styrene oxide (1) into phenylacetaldehyde (2) catalyzed by 20 wt% H3PW12O40/SiO2 in cyclohexane.
Run
[1] (mol L−1
)
Catalyst (×102 wt%)
T (◦C)
Time (min)
Conversion (%)
Selectivity for 2 (%)
TONa
1
2
3
4
5
6
7
8
0.15
0.15
0.15
0.07
0.07
0.07
0.07
0.07
0.07
0.07
0.07
None
8.0
2.5
2.5
2.5
0.6
0.6
0.6
0.6
0.6
0.6
25
25
25
25
15
25
40
60
70
60
60
240
30
60
0
98
98
99
95
90
81
83
80
97
95
0
50
50
59
37
50
70
85
87
95
95
–
3300
10 600
5000
60
200
300
300
180
120
90
4800
18 200
16 300
16 800
16 100
19 600
19 200
9
10b
11b,c
90
a
Turnover number: the number of substrate molecules converted per molecule of PW.
The substrate was charged in two equal portions: the first one at the beginning of the reaction and the second one 30 min later, after the first portion had been almost
b
completely consumed.
c
After run 10, the catalyst was separated by centrifugation, washed with hexane and reused in run 11.

V.V. Costa et al. / Applied Catalysis A: General 383 (2010) 217–220
219
Fig. 1. Isomerization of styrene oxide into phenylacetaldehyde on 20 wt%
H3PW12O40/SiO2 in cyclohexane (Table 1, runs 8 and 10). Both runs were performed
under the same conditions except that in run 10 the substrate was charged in two
equal portions.
At 15 ◦C, the reaction occurred much slower; however, the selec-
tivity for phenylacetaldehyde dropped to 37% (cf. runs 5 and 4). This
result could imply that the activation energy for the isomerization
is higher than that for oligomerization. Thus, we suggested that
the increase in temperature could favor the reaction selectivity. To
be able to monitor the reaction, we decreased the amount of cat-
alyst to 0.006 wt%, which corresponds to a substrate to PW molar
ratio of about 20 000 (run 6). To our surprise, the PW/SiO2 cata-
lyst even in such a small amount converted styrene oxide almost
completely in 5 h with a phenylacetaldehyde selectivity of 50%.
Then the same reaction was run at 40, 60 and 70 ◦C (runs 7–10).
The reaction rate increased significantly with temperature; how-
ever, the most encouraging was a remarkable improvement in
selectivity, which increased from 50% at 25 ◦C to nearly 90% at
70 ◦C.
It should be mentioned that in runs 6–9, where very small
amounts of the catalyst were used, the reaction became stagnated
after 85–90% conversion and could be completed only by addi-
tion of fresh catalyst. A possible reason for that could be catalyst
deactivation due to styrene oxide polymer deposited on the cat-
alyst surface. In order to control the polymerization and prevent
the catalyst deactivation, we decided to charge the substrate into
the reactor in portions. In run 10, a half of the substrate was added
at the beginning of the reaction and the second half 30 min later
after the first portion had been almost completely consumed. An
excellent selectivity of 95% was obtained at almost complete con-
version matching the best results on the reaction yield reported so
far.
Scheme 2. Mechanism of the acid-catalyzed isomerization of styrene oxide.
per min) of ca. 220 min−1. Furthermore, a real catalyst activity may
be higher, because only a part of proton sites are accessible for the
substrate.
The acid-catalyzed transformation of styrene oxide is likely to
occur via a carbenium-ion mechanism, which may be represented
in Scheme 2. Protonation of the oxygen atom in the substrate
induces the epoxy ring opening to form carbocation A, followed
by 1,2-shift of a hydrogen atom and proton loss to give phenylac-
etaldehyde. The selectivity of reaction depends on the direction of
ring cleavage. The neighboring aryl group favors the formation of
carbocation A by stabilizing positive charge on ␣-carbon atom.
Because the HPA is insoluble in non-polar solvents, such as
cyclohexane, neither HPA leaching nor any contribution of homo-
geneous catalysis was expected. To prove this, the catalyst was
filtered off after reaction completion. Then a fresh portion of sub-
strate was added to the supernatant and the reaction was allowed
to proceed further. No activity was observed in this experiment,
which indicated that PW did not leach from silica into the reaction
medium under the conditions used. The catalyst can be recycled.
After run 10, the catalyst was reused after washing with hexane
without any loss in its activity and selectivity (run 11).
The time courses for runs 8 and 10 performed at 60 ◦C (Table 1)
are shown in Fig. 1 to illustrate the advantage of maintaining a low
concentration of the substrate during the reaction. It is important
that almost 100% selectivity was observed in run 10 up to near 90%
conversion, with selectivity being decrease to 95% only at the very
end of the reaction. Thus, the formation of polymers was reduced
significantly, which benefited the catalysts stability—the rates of
the substrate conversion during the second half of the reaction, i.e.,
after ca. 50% conversion, were much higher in the run with the
portioned charge of the substrate (Fig. 1).
It is also important that the PW/SiO2 catalyst is active at a very
low catalyst/substrate weight ratio of 1:150. Most of solid acid cat-
alysts reported for this reaction so far are used in much higher
catalyst/substrate ratios, e.g., 1:0.6 or 1:7.5 for zeolite [6,4] and
1:4.8 for Nafion-H [7]. The result presented in run 10 corresponds to
a turnover number (the number of substrate molecules converted
per molecule of PW) of ca. 20 000 and average turnover frequency
(the number of substrate molecules converted per molecule of PW
4. Conclusions
We have found an efficient and environment-friendly solid acid
catalyst, H3PW12O40/SiO2, for the liquid-phase isomerization of
styrene oxide into the industrially important compound, pheny-
lacetaldehyde. The catalyst is easy to prepare; it is very active and
selective in small amounts exhibiting high turnover numbers (up
to 20 000), stable to leaching under reaction conditions, and can be
recovered and reused.
Acknowledgments
Financial support from CNPq, CAPES, FAPEMIG, and INCT-
Catálise (Brazil) is acknowledged.

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V.V. Costa et al. / Applied Catalysis A: General 383 (2010) 217–220
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