Catalysis by calcined Zn2+/Al3+ layered double hydroxides in Friedel-Crafts alkylation of benzene with benzyl chloride

Article abstract of DOI:10.1246/bcsj.78.937

Amorphous oxides formed upon the calcination of Zn₂Al layered double hydroxide chloride or carbonate intercalates showed high catalytic efficiency in the benzylation of benzene with benzyl chloride. The IR spectra of pyridine adsorbed on amorph

Full text of DOI:10.1246/bcsj.78.937

Short Articles
Bull. Chem. Soc. Jpn., 78, 937–939 (2005)
937
2þ
3þ
Catalysis by Calcined Zn /Al
Layered Double Hydroxides
in Friedel–Crafts Alkylation
of Benzene with Benzyl Chloride
ꢀ
Hiroshi Shimada and Tomokazu Ogoshi
Department of Chemistry, School of Science,
Tokai University, 1117 Kitakaname,
Hiratsuka, Kanagawa 259-1292
Received September 24, 2004; E-mail: hshima@keyaki.cc.u-
tokai.ac.jp
Amorphous oxides formed upon the calcination of
Zn2Al layered double hydroxide chloride or carbonate inter-
calates showed high catalytic efficiency in the benzylation of
benzene with benzyl chloride. The IR spectra of pyridine ad-
sorbed on amorphous oxides suggest that the benzylation ac-
tivity is due to the formation of zinc chloride on the surface.
2
Fig. 1. XRD patterns of Zn Al LDH chloride intercalates
ꢂ
ꢂ
ꢂ
calcined at (a) 150 C, (b) 350 C, and (c) 500 C for
1 h in air. : LDH; : ZnO; : ZnAl O .
2
4
Table 1. Benzylation of Benzene with Benzyl Chloride at
Room Temperature over Mixed Oxides Formed by the
Thermal Decomposition of Zn2Al LDH Chloride and
aÞ
Carbonate Intercalates
Layered double hydroxides (LDHs) are non-stoichiometric
compounds composed of positively charged brucite-like layers
of divalent and trivalent metal hydroxides with intercalated
anions and water molecules. LDHs are expressed by the gen-
Ph CH yield/%
2
2
Calcination Phases detected
bÞ
LDH
Reaction time/min
1
ꢂ
temp. / C
by XRD
5
30
60
Chloride
150
LDH
2
17
73
14
85
17
83
17
85
xþ
mꢁ
eral formula [M(II)ð1ꢁxÞM(III)x(OH)2] [A x=m]nH2O, where
M(II) is a divalent metal ion, M(III) is a trivalent metal ion,
3
5
50
00
Amorphous ZnO 67
ZnO + ZnAl2O4 12
Amorphous ZnO 64
mꢁ
A
is an anion with valency m, and x is a constant, generally
between 0.21 and 0.33.
Carbonate
300
a) Reaction condition: catalyst 0.5 g, benzyl chloride 20
3
Mixed oxides obtained by the thermal decomposition of
LDH carbonate intercalates have high basicity, and act as cat-
mmol, benzene 60 cm . b) Calcination was carried out in air
for 1 h.
2
þ
alysts for base-catalyzed reactions, especially for the Mg
/
3þ
1
Al system. The nature of the anions in the interlayer, which
decomposes during a thermal treatment, plays an important
role in the acid–base properties of the oxides derived from
LDHs. For instance, the oxides obtained from the thermal de-
the layered structure survives up to this temperature. The
ꢂ
XRD pattern of the 350 C sample reveals that the layered
structure has collapsed, and that a highly amorphous mixed ox-
4
ide phase with a ZnO-type structure has been produced. The
ꢂ
2þ
3þ
composition of Mg /Al LDH chloride intercalates possess
XRD pattern of the 500 C sample indicates the appearance
of crystalline ZnO and ZnAl2O4 spinel phases. The XRD pat-
terns of samples obtained by the calcination of carbonate inter-
2,3
acidic sites in addition to basic sites. However, the function
of thermally decomposed materials as acid catalysts in liquid-
phase organic reactions is still not completely understood. In
this paper, we report that the amorphous mixed oxide formed
by the thermal decomposition of Zn2Al LDH is exceedingly
active in the benzylation of benzene with benzyl chloride at
room temperature.
ꢂ
calates indicated that calcination at 300 C produced amor-
ꢂ
phous ZnO-type mixed oxides, and that above 600 C, the
amorphous oxide transformed to crystalline ZnO and ZnAl2O4
spinel phases (data not shown).
Table 1 gives the results of the benzylation of benzene with
benzyl chloride at room temperature over mixed oxides
formed by the thermal decomposition of Zn2Al LDH chloride
and carbonate intercalates. The crystalline LDH chloride inter-
calates and crystalline mixed oxides obtained by calcining at
The present work aims to clarify the function of the amor-
phous mixed oxide as a solid acid catalyst in liquid-phase
Friedel–Crafts alkylation. To this end, calcination was perform-
ed for both chloride and carbonate intercalates, and the catalytic
properties of these materials were then examined in the benzyl-
ation of benzene to diphenylmethane at room temperature.
The XRD patterns of Zn2Al LDH chloride intercalates cal-
ꢂ
500 C showed low activity, but the activity was drastically in-
ꢂ
creased for calcination at 350 C to give an amorphous oxide.
Benzylation using the amorphous oxide yielded diphenyl-
methane in 83% after 60 min. The amorphous oxide obtained
by calcining the LDH carbonate intercalates showed a similar-
ꢂ
ꢂ
ꢂ
cined at 150 C, 350 C, and 500 C for 1 h in air are shown in
ꢂ
Fig. 1. The XRD pattern of the 150 C sample indicates that
Published on the web May 6, 2005; DOI 10.1246/bcsj.78.937

9
38 Bull. Chem. Soc. Jpn., 78, No. 5 (2005)
Ó 2005 The Chemical Society of Japan
Fig. 2. Diphenylmethane yield vs reaction time plots for
the benzylation at room temperature over (a) Zn2Al
ꢂ
LDH carbonate calcined at 300 C, (b) LDH chloride
ꢂ
calcined at 350 C.
ly high activity to those obtained from the chloride interca-
lates. However, in the case of a reaction using carbonate de-
rived amorphous oxide, an induction period was observed
for the formation of the diphenylmethane (Fig. 2). The induc-
tion period indicates that the catalysts are activated during the
initial reaction period.
Fig. 3. IR spectra of pyridine adsorbed on (a) Zn2Al LDH
ꢂ
carbonate calcined at 300 C, (b) LDH chloride calcined
ꢂ
at 350 C, and (c) benzyl chloride treated LDH carbonate
ꢂ
The nature of the acid sites for the oxides obtained by cal-
cining Zn2Al LDH chlorides or carbonate intercalates was
characterized by the FT-IR spectra of the adsorbed probe
molecule pyridine. The FT-IR spectrum of pyridine adsorbed
on an amorphous ZnO-type mixed oxide obtained from an
LDH chloride intercalates exhibited sharp bands at 1448 and
calcined at 300 C.
ꢁ
1
1
modes of pyridine coordinatively bound to Lewis acid sites.
608 cm (Fig. 3(b)). These bands result from the vibrational
5
2
Pinnavaia and co-workers assumed that, upon calcination,
the interlayer chloride replaces the hydroxy groups in the bru-
cite-like layers. The generation of Lewis acidity in the amor-
phous mixed oxide derived from the chloride likely originates
from the formation of a zinc chloride phase. On the other
hand, an amorphous oxide derived from LDH carbonate does
not indicate the presence of coordinatively bound pyridine
(Fig. 3(a)). Thus, the amorphous oxide derived from the car-
bonate essentially did not include Lewis acid sites. However,
high activity was observed in benzylation. It is presumed that
the Lewis acid sites of the oxide derived from LDH carbonate
were created by substitution of the oxygen on the surface of
the amorphous oxide with chloride, which probably originated
from HCl produced by hydrolysis from the small amount of
water contained in the benzyl chloride system. To confirm this,
we also investigated the FT-IR spectra of the adsorbed pyri-
dine on amorphous oxide after a treatment with benzyl chlo-
ride. The benzyl chloride treatment was performed at room
temperature by suspending 0.5 g of amorphous oxide in cyclo-
Fig. 4. Pore-size distribution of the (a) Zn2Al LDH carbon-
ꢂ
ꢂ
ate calcined at 300 C, (b) chloride calcined at 350 C.
Amorphous mixed oxides with the ZnO-type structure, ob-
tained by calcining Zn2Al LDH chloride or carbonate, effi-
ciently catalyzed benzylation, possibly due to the formation
of zinc chloride. Clark and co-workers6 have reported the
use of a montmorillonite supported zinc chloride, ‘‘clayzic’’,
in Friedel–Crafts alkylation. They concluded that clayzic owes
its remarkable Friedel–Crafts activity to the presence of high
local concentrations of zinc ions in structural mesopores.
Figure 4 shows pore-size distribution curves for the BJH de-
,7
3
hexane (20 cm ) containing benzyl chloride (1 mmol). The FT-
IR spectrum obtained after the benzyl chloride treatment of
amorphous oxide from the LDH carbonate intercalate exhibit-
ꢁ1
ꢁ1
ed sharp peaks at 1448 cm and 1608 cm consistent with
pyridine coordinated to Lewis acid sites (Fig. 3(c)).

H. Shimada et al.
Bull. Chem. Soc. Jpn., 78, No. 5 (2005)
939
sorption (dV=dD) of pore volumes for the Zn2Al LDH chloride
and carbonate derived amorphous mixed oxides. The pore-size
distribution curves show that Zn2Al LDH derived mixed
oxides possess porosity, principally in the mesopore range.
We believe that, as with ‘‘clayzic’’, the Friedel–Crafts activity
in our system is closely related to the presence of mesopores.
In benzylation using the Zn2Al LDH derived amorphous
oxide, dissolved zinc ions were not detected in the reaction
mixture. Thus, the amorphous oxides were proven to catalyze
the benzylation as insoluble heterogeneous catalysts.
FT-IR Measurement of Adsorbed Pyridine. The adsorption
of pyridine was performed by exposing the sample to saturated
pyridine vapor at room temperature, which was then outgassed
at room temperature under 0.13 Pa. FT-IR was performed at room
temperature using a KBr pellet.
References
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1991).
2
F. Cavani, F. Trifiro, and A. Vaccari, Catal. Today, 11, 173
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Experimental
3þ
3
2þ
2þ
3þ
Synthesis of LDH. Zn /Al LDHs (Zn /Al atomic
ꢁ
2ꢁ
ratio = 2) containing Cl or CO3 as interlayer anions, were
prepared at room temperature by coprecipitation according to a
procedure of Ramkumar and co-workers.4
4
5
3
Reaction. Benzyl chloride (20 mmol), benzene (60 cm ), and
ꢂ
LDH (0.5 g), which was heat-treated at 150–500 C in air for 1 h,
6
and P. London, J. Chem. Soc., Chem. Commun., 1989, 1353.
J. H. Clark, A. P. Kybett, D. J. Macquarrie, S. J. Barlow,
were mixed together and stirred at room temperature for a speci-
fied time. The reaction products were determined by gas chroma-
tography.
7
J. H. Clark, S. R. Cullen, S. J. Barlow, and T. W. Bastock,
J. Chem. Soc., Perkin Trans. 2, 1994, 1117.