Mixed metal oxide catalysts for the selective oxidation of ethylbenzene to acetophenone

Article abstract of DOI:10.1016/j.cclet.2010.10.035

MgAl and MgMAl oxides (M = Co, Ni and Cu) with a Mg:M:Al molar ratio = 4:1:1 were synthesized from the calcination of their corresponding layered double hydroxide (LDHs) precursors. Their catalytic activities were examined for the oxidation of ethylbenzene using tert-butylhydroperoxide (TBHP) as an oxidant. The oxidized product was mainly acetophenone. The catalytic activities were in the order of MgCuAl > MgNiAl ～ NiAl ～ MgCoAl ～ CoAl > CuAl > MgAl oxides. Reusability studies show that the catalysts are stable under the reaction conditions.

Full text of DOI:10.1016/j.cclet.2010.10.035

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 401–404
www.elsevier.com/locate/cclet
Mixed metal oxide catalysts for the selective oxidation of
ethylbenzene to acetophenone
*
Warangkana Kanjina, Wimonrat Trakarnpruk
Program of Petrochemistry, Faculty of Science and Center of Excellence for Petroleum, Petrochemicals, and Advanced Materials,
Chulalongkorn University, Bangkok 10330, Thailand
Received 12 July 2010
Available online 15 January 2011
Abstract
MgAl and MgMAl oxides (M = Co, Ni and Cu) with a Mg:M:Al molar ratio = 4:1:1 were synthesized from the calcination of
their corresponding layered double hydroxide (LDHs) precursors. Their catalytic activities were examined for the oxidation of
ethylbenzene using tert-butylhydroperoxide (TBHP) as an oxidant. The oxidized product was mainly acetophenone. The catalytic
activities were in the order of MgCuAl > MgNiAl ꢀ NiAl ꢀ MgCoAl ꢀ CoAl > CuAl > MgAl oxides. Reusability studies show
that the catalysts are stable under the reaction conditions.
#
2010 Wimonrat Trakarnpruk. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Ethylbenzene; Oxidation; Mixed metal oxides
Acetophenone is used as a component of perfumes and as an intermediate for the manufacture of pharmaceuticals,
resins and alcohols. Its current industrial production is based on the oxidation of ethylbenzene with molecular oxygen
in acetic acid using cobalt acetate as the catalyst [1]. However, oxidation of ethylbenzene by homogeneous catalysts
gives rise to problems of catalyst recovery and recycling [2,3]. It is therefore of great practical interest to develop a
more efficient, reusable, and environmentally friendly catalyst. Toward this end, permanganate or dichromate can be
replaced by cleaner oxygen- or peroxide-based oxidants. Indeed, heterogeneous catalysts using O [4–6], H O [7] or
2
2 2
TBHP [8,9] have been reported. TBHP should be regarded as being preferable to molecular oxygen as molecular
oxygen-organic mixtures sometimes ignite [10].
The catalysts to be employed herein involve high surface area mixed transition metal oxides. These may have
oxidative or reductive properties, and can be obtained by controlled thermal decomposition of their layered double
x+
hydroxides (LDHs), (M(II)1 M(III) (OH) ) (A
xÀ
) ÁmH O, for which M(II) and M(III) are divalent and trivalent
Àx
metal cations, and A is an n-valent anion.
x
2
nÀx/n
2
In this work we report the solvent-free, liquid phase oxidation of ethylbenzene catalyzed by the above
heterogeneous catalysts using the clean oxidant, TBHP. The activities of the various catalyst systems are compared.
*
Corresponding author.
E-mail address: wimonrat.t@chula.ac.th (W. Trakarnpruk).
1001-8417/$– see front matter # 2010 Wimonrat Trakarnpruk. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.10.035

402
W. Kanjina, W. Trakarnpruk / Chinese Chemical Letters 22 (2011) 401–404
1
. Experimental
A mixture consisting of Mg(NO ) Á6H O, Al(NO ) Á9H O and metal nitrate was prepared, having a Mg:M:Al
3
2
2
3 3
2
molar ratio of 4:1:1, from their 0.06 mol/L solutions, and the mixture was heated to 65 8C. To this a mixed solution of
.6 mol/L NaOH and 0.06 mol/L Na CO was added dropwise with constant and uniform stirring to maintain a
0
2
3
constant pH of 10. The precipitate was maintained at 65 8C for 18 h. The precipitate was then filtered, washed and
dried at 110 8C for 12 h and calcined at 500 8C for 5 h.
Catalytic tests were carried out in a magnetically stirred stainless steel reactor. Ethylbenzene (1.2 mL, 10 mmol)
and the catalyst (0.2 g) were added, followed by 70% aqueous TBHP as the oxidant in an amount necessary to yield the
desired EB/TBHP molar ratio. The reaction times and temperatures were varied. The catalysts were separated by
centrifugation and the products were analyzed by GC. For the reusability tests, the catalysts were removed from the
reaction mixtures, washed with acetone, and calcined at 500 8C for 5 h. Fresh ethylbenzene was then employed in the
repeated testings.
2
. Results and discussion
The analyzed Mg:M:Al atomic ratios (by ICP), BET surface areas and phases (by XRD) of the mixed metal oxides
are shown in Table 1. LDH precursors were found to have been transformed into their corresponding oxides, or into
spinel phases [11] with other metal oxides in some cases [12,13]. The activities of all catalysts were tested under the
same reaction conditions and compared to simple metal oxides (MgO, NiO, Co O and CuO). The main product from
3
4
the oxidation of ethylbenzene with TBHP was acetophenone with only small amounts of other products (Eq. (1)).
Without the catalyst or the oxidant, no reaction occurred.
O
O
O
OH
H
+
OH
+
+
(1)
ethylbenzene
EB)
benzaldehyde
(BZ)
acetophenone
(AP)
1-phenylethanol
(PE)
benzoic acid
(BA)
(
In Table 1 the mixed oxides obtained from this work (entries 1–7) show higher activities than those of the simple
metal oxides (entries 8–11). The catalytic activities of the oxides were found to follow the order
MgCuAl > MgNiAl ꢀ NiAl ꢀ MgCoAl ꢀ CoAl > CuAl > MgAl. The enhanced activity of the MgCuAl oxide
relative to the CuAl or MgAl oxides indicated a synergistic effect in the former metal combination. A similar result
was reported in the oxidation of 1-phenylethanol [14]. In addition, the presence of Mg has been found to contribute to
improved dispersion of Cu at the surface of the catalyst [15,16].
Table 1
Oxidation of ethylbenzene (catalyst 0.2 g, EB:TBHP molar ratio = 1:2, 130 8C, 12 h).
Entry
Catalyst
Mg:M:Al
analyzed
BET surface
2
area (m /g)
XRD phase
Conversion (%)
Selectivity to
acetophenone (%)
1
2
3
4
5
6
7
8
9
MgAl
NiAl
4.3:1
4.8:1
4.6:1
4.9:1
2.4:1:1
2.1:0.8:1
3:1:1
–
306
114
105
90
130
174
150
–
Spinel
Spinel
50
75
74
66
74
73
80
15
38
40
43
98
98
98
95
92
92
92
98
98
98
95
CoAl
3 4
Spinel, Co O
CuAl
Spinel, CuO
MgNiAl
MgCoAl
MgCuAl
MgO
Spinel
Spinel
Spinel, CuO
–
–
–
–
NiO
–
–
1
0
1
Co
3
O
4
–
–
1
CuO
–
–
Other products detected: 1-phenylethanol, benzaldehyde (and benzoic acid for MgMAl oxides).

W. Kanjina, W. Trakarnpruk / Chinese Chemical Letters 22 (2011) 401–404
403
Table 2
Oxidation of ethylbenzene over MgCuAl oxide catalysts (0.2 g) at various conditions.
Entry
EB:TBHP
Temperature (8C)
Time (h)
Conversion (%)
Selectivity (%)
Acetophenone
a
b
c
1
2
3
4
5
6
7
1:2
1:2
1:2
1:2
1:1
1:3
1:3
80
100
130
150
130
130
130
12
12
12
12
12
12
18
64
71
80
77
73
87
90
91
91
92
89
91
92
89
5
3
3
3
5
4
2
3
3
2
3
3
1
3
1
3
3
5
1
3
6
a = phenylethanol, b = benzaldehyde, c = benzoic acid.
The effects of the quantities of TBHP, and the reaction temperatures and times on the efficiency of the ethylbenzene
oxidation were investigated over the most active MgCuAl oxide catalyst. The results are shown in Table 2. In entries 1–
4
, ethylbenzene conversion can be seen to increase with an increase in temperature from 80 to 130 8C, though a
decrease is observed afterwards. This could be ascribed to competitive thermal decomposition of TBHP, preventing its
subsequent involvement in the desired reaction. The selectivity for acetophenone showed a similar trend. In entries 3, 5
and 6, an increase in conversion from 73% to 87% was observed on increasing the ethylbenzene:TBHP ratio from 1:1
to 1:3. However, the selectivity towards acetophenone was not affected (91–92%). At a 1:1 ethylbenzene:TBHP ratio
the ethylbenzene conversion was 73% and the selectivity was 91%, both being higher than found for Mn/MCM-41
(
57.7% conversion and 82.2% selectivity) [16]. It was also found that an increase in time led to an enhancement of the
conversion to 90%, but with a decrease in acetophenone selectivity to 89% (entry 7). This is due to further oxidation of
acetophenone to other products [17].
With regards to reusability, it was found that the MgCuAl oxide catalyst showed a slight drop in activity (from 90%
to 89%) after the third run, with similar acetophenone selectivity (89%). The decrease in activity could be possibly due
to the decrease in active base sites from their coverage by polar products on the catalyst surface. After regeneration, the
structure of the mixed oxide catalyst remained unchanged, as revealed by its XRD pattern. For other catalysts, the
reusability tests also revealed only a 2–4% drop in activity.
In order to check whether the mechanism of the oxidation occurs via free-radicals, a free-radical scavenger
hydroquinone) was added to the reaction. The experiments with hydroquinone addition showed a decrease of
(
ethylbenzene conversion by about half for all mixed oxides, indicating that the oxidation occurs partly via a free-radical
pathway. Another pathway might involve activation of TBHP by coordination to the active site of the oxide catalyst.
Insertion of activated oxygen of co-coordinated TBHP into a C–H bond of the methylene group in ethylbenzene would
produce 1-phenylethanol. Subsequent abstraction of alcoholic OH hydrogen and the CH hydrogen of 1-phenylethanol
would yield acetophenone or benzaldehyde. Both of these can be oxidized further to benzoic acid.
3
. Conclusions
The mixed metal oxides can effectively catalyze the oxidation of ethylbenzene with TBHP under solvent-free
conditions. The MgCuAl oxide exhibits the highest catalytic activity. At 130 8C, 12 h and with a ethylbenzene:TBHP
ratio of 1:3, the conversion of ethylbenzene is 87% with 92% selectivity to acetophenone. The catalyst can be reused.
Acknowledgment
The authors appreciate support from the research grant ‘‘The 90th Anniversary of Chulalongkorn University Fund’’
from the Graduate School, Chulalongkorn University.
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