High activity of Mn-MgAl hydrotalcite in heterogeneously catalyzed liquid-phase selective oxidation of alkylaromatics to benzylic ketones with 1 atm of molecular oxygen

Article abstract of DOI:10.1016/j.jcat.2007.02.004

Mn-MgAl hydrotalcite, surface-enriched with Mn, was prepared by thermal decomposition of MgAl hydrotalcite followed by reconstruction of its structure in the presence of Mn(II) cations and atmospheric CO₂ as a guest inorganic anion through contacting the decomposed mass with aqueous Mn(II) nitrate solution. The Mn-MgAl hydrotalcite showed very high catalytic activity, stability, and reusability in the liquid-phase selective oxidation of a wide range of alkylaromatics to their corresponding benzylic ketones, using atmospheric pressure of molecular O₂ as a sole oxidant under solvent-free and mild reaction conditions. The catalytic performance of Mn-MgAl hydrotalcite was found to be much higher than that of Mn-containing hydrotalcites prepared by different methods or MgAl hydrotalcite catalysts containing various other transition metals and synthesized through similar procedures.

Full text of DOI:10.1016/j.jcat.2007.02.004

Journal of Catalysis 247 (2007) 214–222
www.elsevier.com/locate/jcat
High activity of Mn-MgAl hydrotalcite in heterogeneously catalyzed
liquid-phase selective oxidation of alkylaromatics to benzylic ketones with
1 atm of molecular oxygen
Suman K. Jana a, Yoshihiro Kubota b, Takashi Tatsumi a,c,∗
a
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-Ku, Yokohama 226-0583, Japan
b
Division of Materials Science and Chemical Engineering, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku,
Yokohama 240-8501, Japan
c
Japan Science & Technology Corporation, Japan
Received 7 December 2006; revised 1 February 2007; accepted 2 February 2007
Available online 8 March 2007
Abstract
Mn-MgAl hydrotalcite, surface-enriched with Mn, was prepared by thermal decomposition of MgAl hydrotalcite followed by reconstruction
of its structure in the presence of Mn(II) cations and atmospheric CO as a guest inorganic anion through contacting the decomposed mass
2
with aqueous Mn(II) nitrate solution. The Mn-MgAl hydrotalcite showed very high catalytic activity, stability, and reusability in the liquid-phase
selective oxidation of a wide range of alkylaromatics to their corresponding benzylic ketones, using atmospheric pressure of molecular O as a
2
sole oxidant under solvent-free and mild reaction conditions. The catalytic performance of Mn-MgAl hydrotalcite was found to be much higher
than that of Mn-containing hydrotalcites prepared by different methods or MgAl hydrotalcite catalysts containing various other transition metals
and synthesized through similar procedures.
© 2007 Published by Elsevier Inc.
Keywords: Oxidation; Alkylaromatics; Benzylic ketones; Molecular oxygen; Mn-MgAl hydrotalcite
1. Introduction
mixture is difficult. Recently, there has been increased interest
in the development of clean and economical catalytic processes
for the synthesis of value-added product ketones by benzylic
oxidation of alkylaromatics [5–22].
Aromatic ketones are important chemical intermediates in
the pharmaceutical, fragrance, flavor, dye, and agrochemical
industries [1,2]. Conventionally, these are produced through
electrophilic acylation of aromatic compounds catalyzed by
homogeneous Lewis acids (e.g., AlCl3, BF3, FeCl3, ZnCl2,
SnCl4, TiCl4) or strong protonic acids (e.g., H2SO4, HF), which
leads to the formation of a large volume of toxic and corrosive
wastes [3]. In the past, alternative ways were sought to produce
benzylic ketones by oxidizing the methylene group of alkylaro-
matics using stoichiometric quantities of KMnO4 as an oxidiz-
ing agent [4]. However, in the above stoichiometric oxidation,
the volume of waste produced is also very large; moreover, the
separation of reactants and products from the liquid reaction
The current industrial production of benzylic ketones is
based on the oxidation of alkylbenzenes with molecular oxy-
gen using cobalt cycloalkanecarboxylate or cobalt acetate as
catalyst in acetic acid media [7]. However, this method suffers
from the corrosive nature of the solvent and a homogeneous fea-
ture of the catalyst. Hence, the use of heterogeneous catalysts
in the liquid-phase oxidation, particularly of neat substrate, is
a subject of considerable interest from the standpoint of envi-
ronmental consciousness, because losses of both solvent and
catalyst on separation can lead to unacceptable levels of waste.
A number of transition metal-containing heterogeneous
solids based on Ru, Cu, Fe, Co, and Mn has been reported
for the liquid-phase oxidation of alkylaromatics to ketones by
molecular O2 [15–22]. However, these earlier-reported solid
*
Corresponding author. Fax: +81 45 924 5282.
E-mail address: ttatsumi@cat.res.titech.ac.jp (T. Tatsumi).
0021-9517/$ – see front matter © 2007 Published by Elsevier Inc.
doi:10.1016/j.jcat.2007.02.004

S.K. Jana et al. / Journal of Catalysis 247 (2007) 214–222
215
Scheme 1.
catalysts suffer from limitations in the alkylaromatic oxidation
reaction. The catalytic activity of Ru- and Cu-based heteroge-
neous solids is restricted only to doubly activated alkylaromat-
ics, these catalysts being unable to oxidize ethylbenzene [15,
17]. Although Fe- and Co-containing solids are active for eth-
ylbenzene oxidation, the Fe porphyrin shows low acetophenone
selectivity and inferior catalytic stability [18], and Co-based
catalysts suffer from their poor reusability [19,20].
ent methods and various other transition metals containing hy-
drotalcites synthesized by adopting the “memory effect” under
identical experimental conditions. The reusability and stability
of Mn-MgAl hydrotalcite for ethylbenzene oxidation is also in-
vestigated.
2. Experimental
In contrast, the Mn-containing catalyst exhibits high activity
in the ethylbenzene oxidation only after a long reaction period
or at high oxygen pressure [21,22]. Among the various het-
erogeneous solids reported earlier, the catalytic performance of
MnO−4 -exchanged MgAl hydrotalcite for the oxidation of eth-
ylbenzene (which is difficult to oxidize due to the absence of an
electron-donating aromatic ring-activating group under solvent-
free reaction conditions) is interesting [22]. Choudhary et al.
reported that KMnO4, commonly used for oxidations, could be
transformed into an active heterogeneous catalyst for oxidation
of ethylbenzene to acetophenone by immobilization of MnO4−
anions at the anion-exchange sites of highly basic MgAl hydro-
talcite.
It is well known that the interlayer space of MgAl hydrotal-
cite, which is responsible for the accommodation of exchange-
able anions, is very narrow (about 4 Å). Because MnO−4 anion
occupies the small interlayer space of hydrotalcite in MnO−4 -
exchanged MgAl hydrotalcite, it is expected that the catalyt-
ically active Mn sites are only partially accessible even to a
reactant as small as ethylbenzene. The low catalytic activity
of MnO−4 -exchanged MgAl hydrotalcite in the ethylbenzene
oxidation (TON, about 46 mol mol−1) even at high oxygen
pressure reported by Choudhary et al. seems to be the result
of restricted accessibility of ethylbenzene through its narrow
interlayer space [22]. Thus, to obtain maximum efficiency of
Mn-containing hydrotalcite for the oxidation of a wide range
of alkylaromatics by molecular oxygen, it is very important to
heterogenize catalytically active Mn species into the hydrotal-
cite matrix in which most of the active sites will be accessible
to even much larger reactants than ethylbenzene.
Mn-MgAl hydrotalcites (prepared from MgAl hydrotalcites
with Mg/Al molar ratios of 2.0–5.0 by providing Mn inputs
of 0.25–1.0 mmol per g of hydrotalcite) were synthesized by
adopting the “memory effect” of hydrotalcite [23,24]. In a typ-
ical procedure, a thermally decomposed hydrotalcite mass (cal-
cined at 450 ◦C for 15 h) was reconstructed into the hydrotalcite
structure along with Mn(II) cations by contacting with aqueous
Mn(II) nitrate solution in the presence of atmospheric CO2 (as
a source of CO23− anion) under stirring at room temperature for
up to 12 h.
MgAl hydrotalcites with CO23− as a guest inorganic anion
were prepared by a modified version of the coprecipitation pro-
cedure reported in the literature [25]. In a typical method, a
mixed aqueous solution of Mg2+ and Al3+ nitrates (with Mg/Al
molar ratios of 2.0–5.0) were added dropwise to an aqueous
Na2CO3 solution at 70 ◦C under vigorous stirring. The pH of
the solution was adjusted to 10.5 0.1 by adding aqueous
NaOH solution, and the resulting gel-like material was aged at
90 ◦C for 12 h. The resultant slurry was then cooled to room
temperature and separated by filtration, followed by washing
with distilled water several times and drying at 100 ◦C for 20 h.
MnAl hydrotalcite with a Mn/Al molar ratio of 5.0 was syn-
thesized by using CO23− or NO−3 as a guest inorganic anion in a
procedure similar to that described above for the preparation of
MgAl hydrotalcite.
MnO−4 -exchanged MgAl hydrotalcite with a MnO−4 loading
of 0.4 mmol per g of hydrotalcite was prepared by adopting the
“memory effect” of hydrotalcite under stringent N2 atmosphere
at 80 ◦C for 5 days, to avoid the contact with atmospheric CO2
during reconstruction, but otherwise similarly to the procedure
reported previously [22]. Different transition metals, such as
Ni-, Zn-, Cu-, Co-, Fe-, and Cr-containing MgAl hydrotalcites
(prepared from MgAl hydrotalcite with a Mg/Al molar ratio of
5.0 by providing a metal input of 1.0 mmol per g of hydro-
talcite) were synthesized following the procedure adopted for
Mn-MgAl hydrotalcite. Mn/MgAl hydrotalcite or Mn/calcined
MgAl hydrotalcite with a Mg/Al molar ratio of 5.0 was pre-
pared by the conventional incipient wetness impregnation tech-
nique using an acetonic solution of Mn(II) nitrate.
In this regard, we report here the synthesis of Mn-MgAl hy-
drotalcite, surface-enriched with Mn, by adopting the “memory
effect” of hydrotalcite and its high oxidation activity for a wide
range of alkylaromatics to their corresponding benzylic ketones
by atmospheric pressure of molecular oxygen as the sole ox-
idant in the absence of any solvents and reducing reagents
(Scheme 1). The catalytic activity of Mn-MgAl hydrotalcite
in the ethylbenzene oxidation is compared with Mn-containing
hydrotalcite/hydrotalcite-derived catalysts prepared by differ-

216
S.K. Jana et al. / Journal of Catalysis 247 (2007) 214–222
Before catalytic use, all of the solid samples were treated
further at 200 ◦C for 12 h. The phase(s) present in the differ-
ent solid materials was confirmed by X-ray diffraction (XRD)
(using a Mac Science M3X Model 1030 instrument equipped
with a CuKα radiations), surface area was measured by the
N2 adsorption–desorption method (using a Bell Japan Bel-
sorp 28SA sorptometer), elemental analysis was performed on
a Perkin-Elmer 240C elemental analyzer, surface composition
was determined by a Shimadzu ESCA-3200 electron spectrom-
eter, and the carbon analysis was conducted by a Perkin-Elmer
2400 Series II CHNS/O analyzer. The basicity of hydrotal-
cite and hydrotalcite-derived materials was measured by titrat-
ing them with nonaqueous benzoic acid using phenolphthalein
(pKa = 9.3) as an indicator.
Fig. 1. X-ray diffraction patterns of Mn-MgAl hydrotalcite (prepared from
MgAl hydrotalcite having Mg/Al molar ratio of 5.0 by providing Mn input of
1.0 mmol g of hydrotalcite) obtained at different reconstruction time.
H2O2 decomposition over the different hydrotalcite and
hydrotalcite-derived catalysts was studied in a magnetically
stirred glass reactor (capacity: 50 cm3) at 60 ◦C by introduc-
ing 1 ml of 31% aqueous H2O2 solution in a reactor containing
0.1 g of powdered catalyst and 20 ml of distilled water and
measuring quantitatively the amount of O2 evolved in the H2O2
−1
1
2
decomposition process (H2O2 = H2O + O2) as a function of
reaction time.
Catalytic oxidation of alkylaromatics in the absence of sol-
vent over various Mn-based solids was carried out in a magnet-
ically stirred glass reactor (capacity: 50 cm3), equipped with
a mercury thermometer and a reflux condenser. A continu-
ous flow of oxygen (flow rate: 5.0 cm3 min−1) was bubbled
through a vigorously stirred liquid reaction mixture containing
100 mmol of alkylaromatics and the required amount of solid
catalyst, in the form of fine powder, at 135 ◦C for up to 5 h. The
reaction products (identified by comparing with the authentic
samples) and unconverted alkylaromatics present in the reac-
tion mixture were quantified by a gas chromatograph with a
flame ionization detector using the internal standard method.
TON was calculated based on mol of alkylaromatics converted
per mol of Mn. The amount of ethylbenzene hydroperoxide in
the reaction mixture was estimated by the standard iodometric
titration method.
Fig. 2. X-ray diffraction patterns of MnAl hydrotalcite prepared in the presence
−
2−
of NO or CO
as a guest inorganic anion and Mn/MgAl hydrotalcite or
3
3
Mn/calcined MgAl hydrotalcite (!, MgO phase; ", MnCO phase).
3
of atmospheric CO2 as a source of CO23− guest inorganic an-
ion for the incorporation of Mn into the MgAl hydrotalcite
material (Figs. 1 and 2). The presence of CO23− in the inter-
layer space of hydrotalcite is important for obtaining the sam-
ple with high basicity, which is beneficial for exhibiting the
high alkylaromatic oxidation activity and/or ketone selectivity
of the hydrotalcite-based catalysts [22]. The absence of MnCO3
phase formation in the Mn-MgAl hydrotalcite prepared by the
postsynthesis method seems to be due to the fact that the recon-
struction of MgAl hydrotalcite leading to the incorporation of
Mn into the hydrotalcite material is much faster than the forma-
tion of MnCO3 from the reaction between aqueous Mn nitrate
and atmospheric CO2 under the present synthesis conditions.
This seems to be due mainly to the presence of CO2 in the aque-
ous phase at very low concentrations, as well as its high affinity
to act as an interlayer anion for the reconstruction of hydrotal-
cite phase from its thermally decomposed mass.
3. Results and discussion
3.1. Characterization of catalyst
XRD patterns of Mn-MgAl hydrotalcite (prepared from
MgAl hydrotalcite with a Mg/Al molar ratio of 5 by provid-
ing Mn input of 1.0 mmol per g of hydrotalcite) obtained after
various periods of reconstruction are presented in Fig. 1. XRD
patterns of MnAl hydrotalcite prepared in the presence of CO32−
or NO−3 as a guest inorganic anion, Mn/MgAl hydrotalcite, and
Mn/calcined MgAl hydrotalcite are shown in Fig. 2. Figs. 1
and 2 show that by adopting the “memory effect” of hydrotal-
cite, Mn can be successfully incorporated into the hydrotalcite
samples. In addition, the undesirable MnCO3 phase observed
in MnAl hydrotalcite prepared using CO23− as a guest inor-
ganic anion by the conventional coprecipitation method was
not formed by adopting the postsynthesis method and the use
Fig. 3 shows the changes in metal content as a function of re-
construction period during the incorporation of Mn into MgAl
hydrotalcite (with a Mg/Al ratio of 5 mol mol−1). With increas-
ing reconstruction time, the amount of Mn increased at the
expense of Mg. A slight decrease in the Al content is also ob-
served. This finding suggests that during reconstruction, a part

S.K. Jana et al. / Journal of Catalysis 247 (2007) 214–222
217
mostly over the surfaces of magnesium–aluminum mixed ox-
ides. It is important to note that, in addition to the above phe-
nomenon, another possible reason for the decreased surface
area of the hydrotalcite material is simple rehydration, at least
to some extent, as reported by Figueras et al. [26].
Comparison of XRD and ICP results reveals that the incor-
poration of Mn leading to the formation of Mn-MgAl hydro-
talcite occurred very rapidly, even within 10 min of the re-
construction process (Figs. 1 and 3). However, the XRD peak
intensities related to the hydrotalcite material increased grad-
ually with increasing reconstruction time even after 10 min of
reconstruction. This seems to be due to the continuous progress
of reconstruction of MgAl hydrotalcite from its thermally de-
composed mass even after complete incorporation of Mn into
the hydrotalcite sample. This is expected because, compared
with the amount of Mn used in the preparation of Mn-MgAl
hydrotalcite, a large amount of Mg is present in the parent ma-
terial. Thus, even after incorporation of the total amount of Mn,
the replacement of equivalent amount of Mg into the hydrotal-
cite material means that a large amount of Mg is still available
and seems to be gradually transformed into MgAl hydrotalcite
material. This may be the reason for the progressive increase
in the XRD peak intensities even after the complete incorpo-
ration of Mn into the MgAl hydrotalcite material. The gradual
increases in the amount of carbon or, in other words, the amount
of CO23− (used mostly for the reconstruction of hydrotalcite
from its thermally decomposed mass) with an increase in recon-
struction time also support the above reaction pathway (Fig. 4).
However, even after 12 h of reconstruction, the amount of CO32−
in Mn-MgAl hydrotalcite did not reach its theoretical value.
This finding indicates that the reconstructed material contains
Fig. 3. Changes in the metal content in Mn-MgAl hydrotalcite (prepared from
MgAl hydrotalcite having Mg/Al molar ratio of 5.0 by providing Mn input of
1.0 mmol g of hydrotalcite) as a function of reconstruction time during the
−1
incorporation of Mn into MgAl hydrotalcite adopting the ‘memory effect.’
of the Mg in MgAl hydrotalcite is replaced by Mn to form
Mn-MgAl hydrotalcite. XPS analysis of the bulk and surface
Mn compositions confirmed that in the Mn-MgAl hydrotalcite,
Mn exists mostly on the surfaces of the hydrotalcite (Table 1).
During the reconstruction process, Mg species present on the
surfaces of MgAl hydrotalcite likely will be replaced by Mn,
resulting in surface-enriched Mn in Mn-MgAl hydrotalcite.
The unusually drastic decrease in the BET surface area of
magnesium–aluminum mixed oxide on contact with aqueous
Mn(II) nitrate solution (from 134 to 31 m2 g−1) suggests that
during reconstruction of hydrotalcite from its thermally decom-
posed mass, the surface of the precursor material is covered by
a dense layer, which seems to be Mn-MgAl hydrotalcite grown
Table 1
Physicochemical properties of various Mn-containing hydrotalcite-based basic solids
Catalysts
Bulk
Surface
BET surface
area (m g
Basic sites
−1 i
(µmol g )
t
for H O
1/2 2 2
2
−1
Mn/[(Mn + Mg + Al)
or (Mn + Al)] molar
ratio
Mn/(Mn + Mg + Al)
)
decomposition
(min)
j
molar ratio
a
b
c
d
e
Mn-MgAl hydrotalcite (Mg/Al = 1.75)
Mn-MgAl hydrotalcite (Mg/Al = 2.69)
Mn-MgAl hydrotalcite (Mg/Al = 4.57)
Mn-MgAl hydrotalcite (Mg/Al = 4.80)
Mn-MgAl hydrotalcite (Mg/Al = 4.88)
0.081
0.075
0.070
0.035
0.018
0.065
0.040
0.847
0.203
0.210
0.214
0.109
0.056
–
29
34
31
37
44
90
139
67
41
58
7.2
5.1
2.7
2.9
3.0
3.0
2.6
5.0
117
115
116
104
110
61
f
Mn/MgAl hydrotalcite (Mg/Al = 5)
f
Mn/calcined MgAl hydrotalcite (Mg/Al = 5)
–
2−
g
MnAl hydrotalcite (Mn/Al = 5.07, CO
)
–
3
−
h
MnAl hydrotalcite (Mn/Al = 5, NO
)
0.831
–
–
–
–
–
51
38
93
20
129
106
20.4
2.1
3.0
3
−
MnO -exchanged MgAl hydrotalcite
4
MgAl hydrotalcite (Mg/Al = 5)
a
b
c
d
e
f
−1
Mn loading 1.14 mmol g of solid.
Mn loading 1.18 mmol g of solid.
Mn loading 1.19 mmol g of solid.
Mn loading 0.59 mmol g of solid.
−1
−1
−1
−1
Mn loading 0.28 mmol g of solid.
−1
Mn loading 1.0 mmol g of solid.
2−
−
g
Prepared in the presence of CO as a guest inorganic anion.
3
h
Prepared in the presence of NO as a guest inorganic anion.
3
i
j
0.15 g hydrotalcite, suspended in 2 ml phenolphthalein indicator solution, is titrated with 0.01 M benzoic acid.
Time required for half H O decomposition (half of 1 ml of 31 wt% H O by 0.1 g catalyst) at 60 C.
◦
2
2
2 2

218
S.K. Jana et al. / Journal of Catalysis 247 (2007) 214–222
decomposition is decreased, or, in other words, the H2O2 de-
composition activity is increased (Table 1). An increased Mn
content in Mn-MgAl hydrotalcites (prepared from MgAl hydro-
talcites with similar Mg/Al ratios) does not affect the basicity
but does slightly increase the H2O2 decomposition activity.
3.2. Alkylaromatics oxidation by molecular O2
To develop highly active Mn-containing hydrotalcite-based
heterogeneous solids for the oxidation of ethylbenzene (which
is difficult to oxidize due to the absence of an electron-donating
aromatic ring-activating group), we studied ethylbenzene oxi-
dation with molecular oxygen on hydrotalcite and hydrotalcite-
derived catalysts (Table 2). Generally, acetophenone is the ma-
jor product, along with minor quantities of benzaldehyde and
1-phenylethanol and traces of ethylbenzene hydroperoxide in
the ethylbenzene oxidation over Mn-based solids.
2−
Fig. 4. Changes in the amount of CO anion in Mn-MgAl hydrotalcite (pre-
3
pared from MgAl hydrotalcite having Mg/Al molar ratio of 5.0 by providing
Mn input of 1.0 mmol g of hydrotalcite) as a function of reconstruction time
−1
Among the various Mn-containing hydrotalcite and hydro-
talcite-derived catalysts used in the present study, the Mn-MgAl
hydrotalcites are most efficient for the oxidation of ethylben-
zene to acetophenone with atmospheric pressure of molecu-
lar oxygen even in the absence of any solvents and reducing
reagents (entries 1–5). Mn-MgAl hydrotalcite with a Mg/Al
molar ratio close to 5 shows the best performance. MnAl hy-
drotalcite prepared by the conventional co-precipitation method
using CO23− or NO−3 as a guest inorganic anion is also ac-
tive for ethylbenzene oxidation reaction; however, the TON
over both of these catalysts is low compared with that of the
Mn-MgAl hydrotalcites (entries 10 and 11). It is interesting to
note that among the different Mn-containing hydrotalcite and
hydrotalcite-derived catalysts, MnAl hydrotalcite prepared in
the presence of NO−3 as a guest inorganic anion shows lower
acetophenone selectivity. This is due to the presence of ethyl-
benzene hydroperoxide in the reaction mixture, which appar-
ently is related to the lower basicity of the catalyst (Table 1).
The MnO−4 -exchanged MgAl hydrotalcite synthesized follow-
ing a procedure similar to that reported by Choudhary et al.
(entry 12) also exhibits lower catalytic activity compared with
the Mn-MgAl hydrotalcite. The TON of Mn-MgAl hydrotal-
cite is about 4.5 times higher than that of the MnO−4 -exchanged
MgAl hydrotalcite in the ethylbenzene oxidation under similar
experimental conditions. Mn/MgAl hydrotalcite, Mn/calcined
MgAl hydrotalcite, and MgAl hydrotalcite without Mn show
very low ethylbenzene oxidation activity (entries 8, 9, and 13).
The ethylbenzene oxidation activity of Mn-MgAl hydrotal-
cites with similar Mn contents (i.e., close to 1.18 mmol per g of
hydrotalcite solid) but different Mg/Al molar ratios (Mg/Al =
1.75 to 4.57 mol mol−1) varies significantly depending on the
Mg/Al ratio (entries 1–3). The higher the Mg/Al ratio (in other
words, the higher the basicity), the higher the ethylbenzene
oxidation activity. Choudhary et al. observed a similar effect
for MnO−4 -exchanged MgAl hydrotalcite for ethylbenzene ox-
idation. The beneficial effect of basicity in the alkylaromatic
oxidation reaction is quite well known; aqueous alkali has been
widely used industrially for the transformation of alkylbenzene
to alkylbenzene hydroperoxide by molecular oxygen [27]. It is
important here to note that the catalytic activity of Mn-MgAl
during its preparation by adopting the ‘memory effect.’
appreciable amounts of mixed-oxide material and suggests the
formation of a thick layer of Mn-MgAl hydrotalcite over the
magnesium–aluminum-type mixed oxides.
Based on the above findings, we propose that the formation
of Mn-MgAl hydrotalcite by contacting magnesium–aluminum
mixed oxide with aqueous Mn(II) cations proceeds through
a dissolution–recrystallization mechanism. In the presence of
aqueous acidic Mn(II) nitrate solution (pH about 4.5), Mg(II)
cations in the mixed oxides tend to dissolve, while Mg(II)
cations in the solution tend to form the hydrotalcite phase, prob-
ably due to self-organization as the motivating force, similar to
that observed in the conventional coprecipitation method. The
copresence of Mn(II) cation reasonably resulted in the incor-
poration of Mn(II) at the Mg(II) site. It is important here to
note that in the initial course of reconstruction (particularly
<5 min), the amount of Mn accumulated in the solid mater-
ial was found to be slightly greater than the decrease in the
amount of Mg, suggesting the presence of additional Mn(II)
cations as absorbed or adsorbed species on the basic surfaces of
MgAl hydrotalcite and/or hydrotalcite-derived solids (Fig. 3).
This absorption or adsorption of acidic Mn (II) species on the
basic material seems to occur in the early stage of dissolution.
The slight decrease in the Al content observed during the prepa-
ration of Mn-MgAl hydrotalcite by the reconstruction process
seems to be due to the partial dissolution of basic Al species
in contact with aqueous acidic Mn(II) nitrate solution. Similar
effects were observed for the incorporation of Mn into MgAl
hydrotalcites with Mg/Al molar ratios of 2 and 3.
For various Mn-MgAl hydrotalcites with similar Mn load-
ings but different Mg/Al molar ratios, the number of basic sites,
as measured by the nonaqueous benzoic acid titration method,
is increased with an increasing Mg/Al ratio (Table 1). Simi-
lar effects of Mg/Al ratios in the Mn-MgAl hydrotalcites on
the H2O2 decomposition activity (at 60 ◦C) are also observed.
This is expected because the H2O2 decomposition activity of
hydrotalcite is due mostly to its basicity. Thus, with increas-
ing basic sites of Mn-MgAl hydrotalcite, the t1/2 for H2O2

S.K. Jana et al. / Journal of Catalysis 247 (2007) 214–222
219
Table 2
Results on ethylbenzene oxidation over various Mn-containing basic solid catalysts [reaction conditions: ethylbenzene = 100 mmol, catalyst = 0.4 mmol based on
−1
◦
Mn, oxygen flow = 5 ml min , temperature = 135 C, and reaction time = 5 h]
Entry
Catalysts
Conversion
(%)
Ketone selectivity
(%)
TON
(mol mol
−1 j
)
a
1
2
3
4
5
6
7
8
9
Mn-MgAl hydrotalcite (Mg/Al = 1.75)
Mn-MgAl hydrotalcite (Mg/Al = 2.69)
Mn-MgAl hydrotalcite (Mg/Al = 4.57)
Mn-MgAl hydrotalcite (Mg/Al = 4.80)
Mn-MgAl hydrotalcite (Mg/Al = 4.88)
Mn-MgAl hydrotalcite (Mg/Al = 4.88)
Mn-MgAl hydrotalcite (Mg/Al = 4.88)
37.4
41.1
47.3
49.1
50.3
51.0
0.9
96.3
96.0
96.5
96.9
96.7
96.5
98.6
95.2
95.1
94.7
81.3
95.8
93.5
102.7
118.2
122.7
125.6
127.5
–
b
c
d
e
e,f
e,g
h
Mn/MgAl hydrotalcite (Mg/Al = 5)
0.7
0.6
–
–
h
Mn/calcined MgAl hydrotalcite (Mg/Al = 5)
2−
i
10
11
12
13
MnAl hydrotalcite (Mn/Al = 5.07, CO
)
26.1
23.5
10.7
0.4
2.8
2.5
26.8
100
3
−
i
MnAl hydrotalcite (Mn/Al = 5, NO
)
3
−
MnO -exchanged MgAl hydrotalcite (Mg/Al = 5)
4
MgAl hydrotalcite (Mg/Al = 5)
100
a
−1
−1
Mn loading 1.14 mmol g of solid.
b
c
d
e
f
Mn loading 1.18 mmol g of solid.
−1
Mn loading 1.19 mmol g of solid.
−1
Mn loading 0.59 mmol g of solid.
−1
Mn loading 0.28 mmol g of solid.
Fourth reuse of the catalyst.
g
h
i
Presence of 5.0 mmol of hydroquinone.
−1
Mn loading 1.0 mmol g of solid.
Amount of catalyst = 9.3 mmol Mn.
j
TON is calculated based on total amount of Mn present in the catalyst.
hydrotalcite prepared with MgAl hydrotalcite with a Mg/Al
molar ratio of ca. 5 increases slowly with decreasing Mn load-
ing, even though the amount of Mn used for the reactions is
kept constant at 0.4 mmol (entries 3–5). Comparing the physic-
ochemical properties of the above catalysts suggest that the
higher catalytic performance (based on TON) of the catalyst
with lower Mn loading seems to be due in part to the larger por-
tion of the surface-exposed catalytically active Mn species as
a result of the formation of a relatively thin layer of Mn-MgAl
hydrotalcite over the magnesium–aluminum mixed oxide. This
is supported by the small decrease in the BET surface area of
Mn-MgAl hydrotalcite with lower Mn loading compared with
that with higher Mn loading (Table 1). Among the different Mn-
MgAl hydrotalcites with Mg/Al molar ratios of 1.75–4.88 and
Mn content of 0.28–1.19 mmol per g of hydrotalcite, the cat-
alyst with the Mg/Al ratio of 4.88 mol mol−1 and Mn loading
of 0.28 mmol per g of hydrotalcite shows the highest activity in
the oxidation of ethylbenzene to acetophenone.
The foregoing findings demonstrate that among the vari-
ous Mn-containing hydrotalcite and hydrotalcite-based solids,
the Mn-MgAl hydrotalcites prepared by adopting the “mem-
ory effect” of hydrotalcite are highly efficient catalysts for the
ethylbenzene oxidation reaction using atmospheric pressure of
molecular oxygen as the sole oxidant. The catalytic activity or-
der of different Mn-containing hydrotalcite-based solids used
in the present study is as follows: Mn-MgAl hydrotalcite ꢀ
MnO−4 -exchanged MgAl hydrotalcite ꢀ MnAl hydrotalcite ꢀ
Mn/MgAl hydrotalcite or Mn/calcined MgAl hydrotalcite.
The very low catalytic activity of Mn/MgAl hydrotalcite and
Mn/calcined MgAl hydrotalcite (Table 2, entries 8 and 9) in
comparison with the high activity of MnAl hydrotalcite or Mn-
MgAl hydrotalcites (Table 2, entries 1–5 and 10–12) in the
oxidation of ethylbenzene indicates that the Mn species not
included in the brucite-like structure of hydrotalcite exhibit al-
most no activity for this reaction, because it is reasonable to
suppose that Mn is not incorporated in the brucite structure in
the supported catalysts. On the other hand, in MnAl hydrotal-
cite and Mn-MgAl hydrotalcite, Mn is present in association
with Al in the brucite-like structure, which seems to be respon-
sible for their high alkylaromatic oxidation activity. Meanwhile,
the comparatively lower activity (based on TON) of MnAl hy-
drotalcite prepared by the coprecipitation method compared
with Mn-MgAl hydrotalcite synthesized by adopting the “mem-
ory effect” for ethylbenzene oxidation seems to be due to the
presence of smaller numbers of accessible catalytically active
Mn species. Compared with the total amount of Mn present
in the MnAl hydrotalcite, a very small fraction of Mn species
available on the outer surfaces are accessible only for their cat-
alytic activities. On the other hand, in Mn-MgAl hydrotalcite,
most of the Mn species are present on the surface and thus are
accessible to the reactants in the oxidation reaction. The par-
ticipation of surface-exposed Mn species as the main catalytic
active centers is supported by the high oxidation activity of Mn-
MgAl hydrotalcite for various bulky alkylaromatics substrate
(Table 4). The low catalytic performance of MnO−4 -exchanged
MgAl hydrotalcite seems to be due mainly to its presence in the
narrow interlayer space of hydrotalcite, resulting in its restricted
accessibility to the reactant. Apart from this, the other possible
reason for the different catalytic performance of Mn-MgAl hy-
drotalcite and MnO−4 -exchanged MgAl hydrotalcite could be
the presence of different catalytically active species, that is, Mn
in different oxidation states.

220
S.K. Jana et al. / Journal of Catalysis 247 (2007) 214–222
To compare the catalytic activity of Mn-MgAl hydrotalcite
In addition, to confirm the advantage of the heterogeneous
nature of Mn-MgAl hydrotalcite in liquid-phase oxidation
process, we reused the Mn-MgAl hydrotalcite in ethylben-
zene oxidation several times, after its separation from the hot
liquid reaction mixture followed by washing with reactant eth-
ylbenzene while avoiding the loss of catalyst fine particles. No
significant changes in activity and acetophenone selectivity are
observed even after the fourth successive reuse of the catalyst in
the oxidation reaction (Table 2, entry 6). Inconsistent with the
ICP analysis of the filtrate, the Mn content in the used catalyst
is quite similar to that in the fresh catalyst. The XRD pattern of
the fresh catalyst is not significantly changed after the oxidation
reaction (results not shown). Moreover, the surface area of the
used catalyst is also found to be similar, 41 m2 g−1, compared
with 44 m2 g−1 for the fresh catalyst. The foregoing results in-
dicate that Mn-MgAl hydrotalcite is quite stable and reusable in
liquid-phase ethylbenzene oxidation under the present reaction
conditions.
The Mn-MgAl hydrotalcite showing the best catalytic per-
formance in the present study was also further evaluated for
the oxidation of a wide range of alkylaromatics to their corre-
sponding benzylic ketones using atmospheric pressure of mole-
cular oxygen as a sole oxidant at 135 ◦C under solvent-free
condition to investigate the scope and limitation of this oxida-
tion process. As shown in Table 4, alkylbenzene conversion is
high in most cases, even under solvent-free reaction conditions.
In all cases, the –CH2– group of alkylaromatics is selectively
converted to the –CO– group, indicating high chemoselectiv-
ity in the oxidation. Alkylbenzenes with different alkyl chain
lengths of C2 and C4 are transformed into the corresponding
benzylic ketones at moderate yield (about 50%) and conver-
sion is not affected significantly with the chain length of sub-
strate (entries 1, 2). The ethylbenzene derivatives possessing an
electron donating substituent such as –CH3 or –NH2 at the 4-
position afford the corresponding acetophenones in good yield
(62–65%) (entries 3, 4). The amine function is intact under
the present reaction conditions. The presence of the electron-
withdrawing substituent –NO2 group adversely affects the cor-
responding acetophenone yield (entry 5). Bulky alkylarenes,
such as diphenylmethane and 4-ethylbiphenyl, are also oxidized
to give the corresponding oxygenated products in very high
yield (70–79%) (entries 6, 7). In all cases, only the benzylic
C–H bonds of alkylaromatics are oxidized selectively to their
corresponding ketones.
with other transition metals containing hydrotalcites for eth-
ylbenzene oxidation, a number of layered double hydroxides
containing various transition metals (e.g., Ni, Zn, Cu, Co, Fe,
Cr) were prepared in a procedure similar to that used for the
incorporation of Mn into the MgAl hydrotalcite material, and
the activities were tested under similar experimental conditions
(Table 3). Of the various transition metal-based hydrotalcite
catalysts, Ni-MgAl hydrotalcite exhibits the best catalytic per-
formance in ethylbenzene oxidation; however, the activity of
Ni-MgAl hydrotalcite is be significantly lower than that of Mn-
MgAl hydrotalcite. The other transition metals containing hy-
drotalcite catalysts show significantly lower activities. Note that
the Mn-MgAl hydrotalcite exhibits slightly lower acetophenone
selectivity compared with the other transition metal-containing
MgAl hydrotalcite-based solid catalysts.
To rule out the possibility of homogeneous reaction caused
by leaching of active catalytic components in the oxidation of
ethylbenzene in the presence of Mn-MgAl hydrotalcite (with a
Mg/Al ratio of 4.88 mol mol−1 and a Mn loading of 0.28 mmol
per g of hydrotalcite), the solid catalyst was filtered off from
the hot liquid reaction mixture after 1 h of reaction, and the ox-
idation was continued with the resulting filtrate for another 4 h.
The ethylbenzene conversion and acetophenone selectivity of
10.3% and 98.1%, respectively, after 1 h of reaction are not
changed significantly even after additional 4 h of reaction un-
der the above conditions; indicating that the leached-out metal
ions (if any) are not responsible for the observed activity and/or
selectivity (results not shown). ICP analysis of the resulting fil-
trate obtained after 5 h of reaction indicates the presence of Mn
in traces (at only the ppb level).
Table 3
Catalytic results on ethylbenzene oxidation over various transition metal-
containing MgAl hydrotalcites [reaction conditions: ethylbenzene = 100 mmol,
−1
catalyst = 0.4 mmol based on transition metal, oxygen flow = 5 ml min
,
◦
temperature = 135 C, and time = 5 h]
Catalyst
Conversion (%)
Ketone selectivity (%)
Mn-MgAl hydrotalcite
Ni-MgAl hydrotalcite
Zn-MgAl hydrotalcite
Cu-MgAl hydrotalcite
Co-MgAl hydrotalcite
Fe-MgAl hydrotalcite
Cr-MgAl hydrotalcite
47.3
29.7
0.3
2.1
3.2
96.5
99.4
100
99.5
99.6
100
99.5
0.5
1.1
Table 4
Results on oxidation of various alkylaromatics catalyzed by Mn-MgAl hydrotalcite prepared from MgAl hydrotalcite having a Mg/Al molar ratio of 5 and Mn loading
−1
◦
of 0.28 mmol per g of solid [reaction conditions: alkylaromatics = 100 mmol, catalyst = 0.4 mmol based on Mn, oxygen flow = 5 ml min , temperature = 135 C,
and time = 5 h]
Entry
Alkylaromatics
Conversion (%)
Major product
Ketone selectivity (%)
1
2
3
4
5
6
7
Ethylbenzene
Butylbenzene
50.3
49.7
62.8
65.1
1.8
Acetophenone
Butyrophenone
4-Methylacotophenone
4-Aminoacetophenone
4-Nitroacetophenone
Benzophenone
96.7
94.1
97.0
95.1
96.8
95.1
92.7
4-Methylethylbenzene
4-Aminoethylbenzene
4-Nitroethylbenzene
Diphenylmethane
4-Phenylethylbenzene
70.1
78.5
4-Phenylacetophenone

S.K. Jana et al. / Journal of Catalysis 247 (2007) 214–222
221
Scheme 2.
Although the mechanism of oxygen activation and trans-
fer by the Mn-containing hydrotalcite-based basic catalyst for
alkylaromatic oxidation remains incompletely understood, the
finding of traces of ethylbenzene hydroperoxide in the reac-
tion mixture suggests that the reaction proceeds through the
formation of ethylbenzene hydroperoxide as an active interme-
diate. The formation of ethylbenzene hydroperoxide through
redox processes was confirmed by analyzing the presence of
Mn species in two different oxidation states, +2 and +3 (as ob-
served from XPS analysis), in the catalyst obtained after ethyl-
benzene oxidation. The transformation of ethylbenzene to eth-
ylbenzene hydroperoxide by free-radical pathway is confirmed
by the drastic decrease in ethylbenzene oxidation activity of
Mn-MgAl hydrotalcite on addition of hydroquinone as a free-
radical scavenger (Table 2, entry 7). Based on the four identified
products in the ethylbenzene oxidation reaction we speculate
that the ethylbenzene oxidation involves the following reactions
over the Mn-based hydrotalcite catalysts:
alytic activity in the oxidation of various alkylaromatics to their
corresponding benzylic ketones with atmospheric pressure of
molecular oxygen as the sole oxidant under solvent-free and
mild reaction conditions. The catalytic performance of Mn-
MgAl hydrotalcite postsynthesized by adopting the “memory
effect” was found to be much higher than that of MnAl hydro-
talcite prepared by a conventional coprecipitation technique or
MnO−4 -exchanged MgAl hydrotalcite synthesized by adopting
the “memory effect.” We have confirmed that the Mn-MgAl hy-
drotalcite is stable and reusable in the reaction.
Acknowledgment
S.K.J. acknowledges the Japan Society for the Promotion of
Science (JSPS) for a postdoctoral research fellowship as a for-
eign researcher.
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