Heteropoly acid as a novel efficient catalyst for fries rearrangement

Article abstract of DOI:10.1039/b202148j

Heteropoly acid H₃PW₁₂O₄₀ is a very efficient and environmentally benign catalyst for the Fries rearrangement of phenyl acetate in homogeneous or heterogeneous liquid-phase systems at 100-150°C.

Full text of DOI:10.1039/b202148j

Heteropoly acid as a novel efficient catalyst for Fries rearrangement
Elena F. Kozhevnikova, Eric G. Derouane and Ivan V. Kozhevnikov*
Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool,
UK L69 7ZD. E-mail: kozhev@liverpool .ac.uk
Received (in Cambridge, UK) 28th February 2002, Accepted 17th April 2002
First published as an Advance Article on the web 26th April 2002
Heteropoly acid H3PW12O40 is a very efficient and envir-
onmentally benign catalyst for the Fries rearrangement of
phenyl acetate in homogeneous or heterogeneous liquid-
phase systems at 100–150 °C.
composed of heteropoly anions and protons as the counter-
cations. They are stronger than many conventional solid acids
such as mixed oxides, zeolites, etc. The Keggin-type HPAs
typically represented by the formula H8 2 x [XM12O40], where
X is the heteroatom (e.g. P or Si), x is its oxidation state, and M
is the addenda atom (usually Mo6+ or W6+), are the most
important for catalysis.5–7 They have been widely used as acid
and oxidation catalysts for organic synthesis and found several
industrial applications.5–7 A few studies on the use of HPAs for
Friedel–Crafts acylation have been published,5,8 and only one
recent communication on the Fries reaction9 reporting that
silica-supported HPAs catalyse the rearrangement of phenyl
acetate in the gas phase at 200 °C with 11% conversion to 2- and
4-hydroxyacetophenone (2HAP and 4HAP).
The Friedel–Crafts aromatic acylation and related Fries re-
arrangement are the most important routes for the synthesis of
aromatic ketones that are intermediates in manufacturing fine
and speciality chemicals as well as pharmaceuticals.1 The Fries
rearrangement of aryl esters, e.g. phenyl acetate (eqn. (1)
Here we report on a very efficient and environmentally
benign catalysis by the strongest heteropoly acid H3PW12O40
(PW) for the Fries rearrangement of phenyl acetate (PhOAc)
[eqn. (1)] in liquid phase. The reaction occurs at 100–150 °C to
yield 2HAP, 4HAP, 4-acetoxyacetophenone (4AAP), and
phenol (Table 1). One of important advantages of HPA, as
compared to zeolites or mineral acids (e.g. H2SO4), is that the
reaction can be carried out both homogeneously and heteroge-
neously. The homogeneous process occurs in polar media, for
example, in neat PhOAc or polar organic solvents like
nitrobenzene (PhNO2) or o-dichlorobenzene (entries 1–5) that
are commonly used for Fries reaction. All these media will
easily dissolve PW at elevated temperatures (ca. 100 °C). On
the other hand, when using nonpolar solvents, such as higher
alkanes (e.g. dodecane) that will not dissolve HPA, the reaction
proceeds heterogeneously over solid HPA catalysts (entries
7–13). In the latter case, supported HPA, preferably on silica, is
the catalyst of choice, as bulk HPA possesses a low surface area
(1–5 m2 g21).5–7 The HPA catalysts are easily separated from
(1)
to yield hydroxyacetophenones involves acylium ion inter-
mediates that are generated from the ester by interaction with an
acid catalyst. Present industrial practice requires a stoichio-
metric amount of soluble Lewis acids (e.g. AlCl3) or mineral
acids (e.g. HF) as catalysts, which results in substantial amount
of waste and corrosion problems.2 In view of the increasingly
strict environmental legislation, the application of heteroge-
neous catalysis has become attractive. In the last couple of
decades, considerable effort has been put into developing
heterogeneously catalysed Friedel–Crafts chemistry using solid
acid catalysts such as zeolites, clays, Nafion-H, etc.,2 zeolites
being the most studied catalysts for both the acylation2,3 and
Fries rearrangement.2,4
Heteropoly acids (HPAs) are another type of promising solid
acid catalysts for aromatic acylation. HPAs are Brønsted acids
Table 1 Fries rearrangement of phenyl acetate catalysed by H3PW12O40 (2 h)a
Selectivity, %
Solvent
(PhOAc, wt%)
Conversion TOF
1
b
21
Catalyst (wt%)
T °C
(%)
(t ⁄ , min) min
PhOH 2HAP 4HAP 4AAP
2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
PW (0.60)
PW (3.0)
PW (3.0)
PW (0.60)
PW (0.60)
H2SO4 (1.4)
PW (0.60)
40% PW/SiO2 (1.5)
40% PW/SiO2 (1.5)e
40% PW/SiO2 (7.5)
10% PW/SiO2 (6.0)
10% PW/SiO2 (6.0)f
40% PW/SiO2 (3.3)g
H-Beta (1.3)gh
Cs2.5H0.5PWO40 (0.67)
PhOAc (100)
PhOAc (100)
PhNO2 (25)
PhNO2 (25)
PhNO2 (50)
150
150
150
130
100
130
130
130
130
130
130
130
160
160
130
5.5
19.2
45.8
21.0
10.5
12.8
3.1
31 (2)
22 (2)
13 (2)
19 (3)
6 (10)
49
52
52
46
55
67
69
62
51
77
66
41
66
38
49
5.2
5.7
12
7.8
5.1
9.4
8.0
10
11
12
5.6
15
24
18
10
40
28
12
27
29
16
2.3
22
32
PhNO2 (25)
0.1 (8)
~ 30c (8)
5d (5)
7.6
0
Dodecane (25)
Dodecane (25)
Dodecane (25)
Dodecane (25)
Dodecane (25)
Dodecane (25)
Dodecane (36)
Dodecane (36)
PhNO2 (25)
8.3
6.7
6.0
5.0
5.0
9.6
4.4
8.2
6.4
4.4
3d (6)
18.0
11.8
8.5
18.0
9.3
5d (5)
6.0
17d (2)
5d (5)
8.0
16
41
14
24
41
14
11
32
12d (3)
0.1 (120)
30d (5)
8.7
6.1
a The reaction with PW is homogeneous in PhOAc and PhNO2 and heterogeneous in dodecane. b Turnover frequency corresponding to reaction halftime (t ⁄ ).
1
2
c Assuming PW surface area of 5 m2g21, Keggin heteropoly anion cross section of 100 Å2 and three accessible H+ per Keggin anion. d Assuming that all
H+ in PW are accessible; the true TOF may be higher. e Reuse of the run given in entry 8. f Acetic anhydride (1.5 wt %) added. g 5 h. h Si/Al = 11; 1.4 mmol
g21 H+; from Ref. 4a.
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This journal is © The Royal Society of Chemistry 2002

the heterogeneous system by filtration and could be reused
(entries 8 and 9). From the homogeneous systems, HPA can be
effectively separated without its neutralisation by extraction
with water and reused or utilised otherwise.
The heterogeneous catalysis in the PhOAc–dodecane media
was clearly proved by filtering off the catalyst from the reacting
system which completely terminated the reaction. In contrast,
filtration did not affect the reaction course in homogeneous
systems, e.g. PhOAc–PhNO2.
SiO2, it shows high TOF and is more selective to acetophenones
than HPA. The explanation of this may be that the less
hydrophilic Cs salt6 possesses stronger proton sites than the
solid PW or PW/SiO2 which, from TGA, retain 4–6 H2O
molecules per Keggin unit after pretreatment at 150 °C (see
Experimental†).
In conclusion, heteropoly acid H3PW12O40 is a very efficient
and environmentally friendly catalyst for the Fries rearrange-
ment of phenyl acetate in homogeneous or heterogeneous
liquid-phase systems.
Strong inhibition of HPA-catalysed process with reaction
products was observed both in homogeneous and heterogeneous
systems—which is not unexpected, as the same takes place with
other catalysts (AlCl3, zeolites, etc.). Addition of more HPA
catalyst allowed reaching a higher PhOAc conversion (cf.
entries 1 and 2, 8 and 10). Some catalyst deactivation was also
observed. For example, the 40% PW/SiO2 catalyst separated
after the reaction in PhOAc–dodecane 25+75 wt% system
showed in the second run ca. 80% of its initial activity (entry 9).
The catalyst after the first run was significantly coked (C
content ca. 13%) which probably caused catalyst deactiva-
tion.
The total selectivity of reaction (1) towards the sum of PhOH,
2HAP, 4HAP and 4AAP was found to be over 98%. Some
acetic anhydride was also formed. The homogeneous reaction is
more efficient than the heterogeneous one because it makes less
phenol and more acetophenones, the selectivity to the more
valuable para acetophenones, 4AAP and 4HAP, being also
higher. Addition of acetic anhydride (entry 12) improves the
selectivity towards acetophenones, as expected, although
slightly decreases the catalytic activity. The turnover fre-
quencies (TOF) corresponding to the reaction halftime were
calculated as the number of moles of PhOAc converted per mole
of total protons in the catalyst. Being a much stronger acid,5–7
HPA is almost 200 times more active than H2SO4 in
homogeneous reaction, as well as more selective to acet-
ophenones (cf. entries 4 and 6). In heterogeneous systems, HPA
is also two orders of magnitude more active than H-Beta zeolite
which is one of the best zeolite catalysts for this reaction (cf.
entries 13 and 14). (Note that in solid HPA catalysts only a small
part of H+ is accessible, thus the true TOF may be much higher.)
However, H-Beta shows a higher total selectivity to acet-
ophenones than HPA. It should be pointed out that HPA in
homogeneous systems gives a higher selectivity to para
acetophenones 4AAP and 4HAP than H-Beta. The efficiency of
solid HPA (at constant loading) increases in the order PW <
40% PW/SiO2 < 10% PW/SiO2 in which the number of
accessible proton sites increases (cf. entries 7, 8 and 11).
Insoluble salt Cs2.5H0.5PWO406 is an efficient solid catalyst for
the reaction in polar media such as PhNO2 (entry 15). Although
less active per unit weight than the homogeneous PW or PW/
The authors are grateful to EPSRC, UK for a quota
studentship (E.F.K.) and to J. Kaur for her help with the
experiments.
Notes and references
†
Experimental. Silica-supported PW catalysts were prepared by impreg-
nating Aerosil 300 silica (SBET = 300 m2g21) with a methanol solution of
PW.10 Cs2.5H0.5PW was prepared as described in Ref. 11. The catalysts PW,
40% PW/SiO2 and Cs2.5H0.5PW had BET surface areas of 5, 130 and 112
m2g21, respectively. Prior to the reaction, the catalysts were calcined at
150°C/0.1 torr for 1.5 h. The rearrangement of PhOAc was carried out in the
liquid phase (7.0 g of PhOAc + solvent) at 100–160 °C under nitrogen
atmosphere in a 25 ml glass reactor equipped with a condenser and a
magnetic stirrer. Decane (1 wt%) was added as a GC internal standard. To
monitor the reaction, 0.1 ml samples of the reaction mixture were taken
periodically, diluted to 1 ml with 1,2-dichloroethane and analysed by gas
chromatography (Varian 3380 chromatograph) using 30 m 3 0.25 mm BP1
capillary column.
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