Highly active and low moisture sensitive supported thallium oxide catalysts for Friedel-Crafts-type benzylation and acylation reactions: Strong thallium oxide-support interactions

Article abstract of DOI:10.1006/jcat.2001.3245

Liquid-phase benzylation of benzene by benzyl chloride (at 80°C) over basic TlO_x (without support) and supported TlO_x prepared using different low and high surface area commonly used catalyst carriers has been investigated. The catal

Full text of DOI:10.1006/jcat.2001.3245

Journal of Catalysis 201, 225–235 (2001)
doi:10.1006/jcat.2001.3245, available online at http://www.idealibrary.com on
Highly Active and Low Moisture Sensitive Supported Thallium Oxide
Catalysts for Friedel–Crafts-Type Benzylation and Acylation
Reactions: Strong Thallium Oxide–Support Interactions
1
Vasant R. Choudhary and Suman K. Jana
Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
Received January 3, 2001; revised April 3, 2001; accepted April 3, 2001
lyst, corrosion, high toxicity, etc. Development of reusable
�
Liquid-phase benzylation of benzene by benzyl chloride (at 80 C) solid acid catalysts having high activity in the Friedel–Crafts
over basic TlOx (without support) and supported TlOx prepared us- reaction is, therefore, of great practical importance. Hence,
ing different low and high surface area commonly used catalyst
carriers has been investigated. The catalysts have been character-
ized by their surface area and also by XRD, FTIR, and XPS. Strong
TlOx–support interactions (which are chemical in nature) have been
observed for the catalysts prepared using high surface area (HS),
supports, such as Si–MCM-41, silica gel (HS), silica–alumina (HS),
alumina (HS), and zirconia (HS), which have surface hydroxyl
groups. These catalysts are found to be inactive for benzene ben-
zylation or benzoylation reaction. However, the TlOx supported on
efforts are made to replace the homogeneous acid cata-
lysts by solid acid catalysts, such as heteropolyacids salts
(
(
2, 3), sulfated ZrO2 or Fe2O3 (4–6), sulfated Al2O3–ZrO2
or TiO2) (7), acid-treated clays(8), Fe/MCM-41(9), cation-
exchanged clays (10, 11), ZnCl2, CuCl2, MgCl2, or FeCl3
supported on montmorillonite K-10 (12, 13), HY, H-beta,
and H–ZSM-5 zeolites (14–17), and Ga- or Fe-substituted
H–ZSM-5 (18), for benzylation (2–4, 9–12, 18) and benzoy-
sintered low surface area (LS) macroporous catalyst carriers [viz. lation (2, 3, 5–8, 13, 15–17) of aromatic compounds.
zirconia (LS), silica (LS), and silica–alumina (LS)], which have no
Acidic catalysts are, in general, moisture sensitive and
surface hydroxyl groups, showed high benzene benzylation activity. hence demand a moisture-free reaction mixture for these
TlOx/zirconia (LS) is a highly promising catalyst for both the ben-
zylation and acylation of benzene and other aromatic compounds
and it shows high activity, even in the presence of moisture. The ac-
tivity of this catalyst for the benzylation of benzene and substituted
benzenes is in the following order: benzene > toluene > p-xylene >
supported GaCl3 and InCl3 catalysts, showing high activ-
anisole. The induction period for the benzylation and acylation re-
actions over the catalyst, however, depends strongly on the moisture
present in the catalyst and/or in the reaction mixture. The induc-
tion period is drastically reduced by the HCl gas pretreatment of
reactions. It is, therefore, of great practical interest to have
a solid catalyst having high activity but little or no moisture
sensitivity for the Friedel–Crafts-type reactions. Recently,
we (19) reported on montmorillonite K-10 or Si–MCM-41
ity and only little moisture sensitivity in the benzylation
of benzene, which is otherwise relatively difficult to ac-
complish because of the absence of any electron-donating
the catalyst before the benzylation or acylation reaction. Only the group in the aromatic substrate. Very recently, we (20) ob-
catalyst that contains Tl2O3 shows activity for the benzylation and served that Si–MCM-41 supported Ga2O3 and In2O3 cata-
acylation reactions. A redox mechanism for these reactions over the lysts also show very high activity in the benzylation of ben-
supported TlOx catalysts has been proposed.
�c 2001 Academic Press
zene with benzyl chloride; these catalysts were also found
Key Words: benzylation of benzene; benzylation of aromatic com- to be highly active in the benzoylation of benzene and other
pounds; benzoylation of aromatic compounds; TlOx/zirconia (LS).
aromatic compounds. The supported In2O3 and InCl3 cata-
lysts showed higher benzene benzylation activity than that
showed by supported Ga2O3 and GaCl3 catalysts. Since Tl
is the next group IIIA element, it is interesting to study
INTRODUCTION
the activity of the supported TlOx and TlCl catalysts in the
Friedel–Crafts-type reactions catalyzed by homogeneous
Friedel–Crafts-type reactions.
acid catalysts are commonly used in organic synthesis (1).
Our preliminary investigation indicated that TlCl de-
However, the commonly used homogeneous acid catalysts
posited on Si–MCM-41 or montmorillonite K-10 is almost
(
viz., AlCl3, BF3, and H2SO4) pose several problems, such as
inactive for the benzene benzylation with benzyl chloride
and the benzylation activity of supported TlOx (which is
basic in nature) varies drastically depending upon the na-
ture and/or surface area of the support used. The present
investigation was, therefore, undertaken to study TlOx
difficulty in separation and recovery, disposal of spent cata-
1
To whom correspondence should be addressed. E-mail: vrc@ ems.ncl.
res.in. Fax: (+91) 20-5893041/5893355.
225
0021-9517/01 $35.00
Copyright �c 2001 by Academic Press
All rights of reproduction in any form reserved.

2
26
CHOUDHARY AND JANA
(
catalyst)–support interactions, which strongly influence the crystalline phases by XRD using a Phillips diffractome-
the catalyticactivityofsupported TlOx (viz., TlOx deposited ter (1730 series) and CuK� radiation and also by FTIR (in
on different supports with or without surface hydroxyls) in Nujol medium) using a Perkin–Elmer FTIR (1600 series)
the Friedel–Crafts-type benzylation and acylation reac- and XPS using a VG-Scientific ESCA-3 MK II electron
tions.
spectrometer.
The liquid-phase benzylation and benzoylation reactions
over the catalysts were carried out in a magnetically stirred
glass reactor (capacity: 25 cm ) fitted with a reflux con-
EXPERIMENTAL
3
The supported TlOx catalysts, loading of metal oxide denser, having low dead volume, mercury thermometer,
based on Tl2O) = 2–40 wt% (Table 1), were prepared by and arrangement for continuously bubbling moisture-free
(
3
� 1
impregnation of different supports [viz., sintered low sur- N (30 cm min ) through the liquid reaction mixture at
2
face area macroporous silica (LS) (SS-5231 having pore the following reaction conditions by the procedure given
3
� 1
volume (pv) = 0.25 cm g ), silica–alumina (LS) (SA-5205 earlier (19, 20): reaction mixture = 13 ml of moisture-free
3
� 1
having pv = 0.35 cm g ), and zirconia (LS) (SZ-5564 hav- aromatic substrate and 1.0 ml of benzyl or benzoyl chloride,
3
� 1
ing pv = 0.15 cm g ), obtained from Norton Co., USA, amount of catalyst = 0.1 g (for benzylation) or 0.4 g (for
�
and high surface area silica gel (HS), silica–alumina (HS), benzoylation), and temperature = 80 or 110 C. In all the
Si–MCM-41 (HS), alumina (HS), and zirconia (HS)] in the cases, the product formed was mainly mono-benzylated or
form of fine particles with thallous nitrate from its aqueous acylated one, and there was no formation of polybenzyl or
solution by the incipient wetness technique, drying, and cal- polyacyl chloride. The amount of benzylated or acylated
�
cination in air at 450 C for 4.5 h. The TlOx (without support) product formed was equivalent (within 2–5% error) to the
was obtained by calcination of powdered thallous acetate at amount of benzylating or acylating agent consumed in the
�
4
50 C for 4.5 h. All low surface area supports are highly sin- reaction.
tered macroporous materials, having no surface hydroxyl
groups.
The catalysts were characterized by their surface area at 30 C for 12 h and the benzene saturated with water at
To study the effect of moisture present in the catalyst
and in the reaction mixture, the catalyst stored over water
�
�
using a surface area analyzer (Quantachrome, USA) and 30 C, respectively, were used for the reaction.
TABLE 1
Results of the Benzylation of Benzene with Benzyl Chloride over TlOx and Supported
�
TlOx (Loading of TlOx = 20 wt%) Catalysts at 80 C
Time for 50 and 90%
conversion of benzyl
chloride (min)
2
� 1
)
Surface area (m g
TON
Apparent
Induction for half
reaction rate
Colour of
TlOx or
period
(min)
reaction constant, ka �
�
1
3
� 1
Catalyst
the catalyst Support supported TlOx
Substrate
50%
29.3
90%
40.7
(min
)
10 (min
)
TlOx (without support)
Dark grey
TlOx/silica–alumina (HS) Light grey
TlOx/silica–alumina (LS) Dark grey
—
255
<0.05
280
1180
0.5
207
0.2
51
Dry benzene
Dry benzene
Dry benzene
Dry benzene
Dry benzene
24.8
—
0.3
—
3.2
—
153.6
—
No reaction for 2.5 h
3.3 10.0
No reaction for 2.5 h
No reaction for 2.5 h
28.0
—
248.0
—
TlOx/silica gel (HS)
TlOx/Si–MCM-41 (HS)
Light grey
Very light
grey
49
—
—
—
TlOx/silica (LS)
Grey
Off white
Grey
Dark grey
Dark grey
Dark grey
0.2
151
51
0.1
0.1
0.1
0.3
116
29
0.3
0.3
0.3
Dry benzene
Dry benzene
Dry benzene
Dry benzene
Moist benzene
Moist benzene
9.0
19.0
4.0
—
—
0.1
0.4
0.6
10.3
—
—
38.5
26.4
24.3
163.3
—
—
291.5
267.8
274.2
TlOx/alumina (HS)
TlOx/zirconia (HS)
TlOx/zirconia (LS)
TlOx/zirconia (LS)
No reaction for 2.5 h
No reaction for 2.5 h
2.4
3.5
3.8
8.2
9.8
10.0
b
b
TlOx/zirconia (LS)
a
(
moist)
c
TlOx/zirconia (LS)
Very light
grey
0.1
0.2
Dry benzene
No reaction for 1.5 h
—
—
—
Note. Reaction conditions: reaction mixture = 13 ml of moisture-free substrate and 1.0 ml of benzyl chloride; amount of catalyst = 0.1 g of
supported TlOx and 0.02 g of TlOx.
a
b
c
�
Catalyst was stored over water in desicator at 30 C for 12 h.
�
Benzene saturated with water at 30 C (0.37 mol% water in benzene).
Catalyst was prepared by decomposing thallous nitrate deposited on zirconia (LS) under a N2 atmosphere at 450 C for 4.5 h.
�

TlOx FOR BENZYLATION AND ACYLATION REACTIONS
227
To study the effect of HCl gas pretreatment to the cata-
lyst on its performance in the benzylation or benzoylation
reaction, a HCl(gas)–N2 mixture (20 mol% HCl) was bub-
bled through a mixture of the aromatic substrate (which is
to be benzylated or benzoylated) and catalyst under stir-
ring at the reaction temperature for a period of 5 min.
The substrate–catalyst mixture was flushed with pure N2
3
� 1
(
30 cm min ) for 30 min to remove physically adsorbed
or absorbed HCl in the reaction mixture and then the re-
action was started by injecting benzyl chloride or benzoyl
chloride in the reaction mixture.
RESULTS
Catalyst Characterization
Color and surface area of catalysts. Data on the color
and surface area of the TlOx catalysts with and without
catalyst support are given in Table 1.
The color of catalysts varies from off white/light grey to
dark grey depending upon the catalyst support, more par-
ticularly its surface area. In general, the color of catalyst
supported on the high surface area support was lighter than
that on the low surface area support. The catalyst color is
changed from grey to dark grey with increasing loading of
TlOx from 20 to 40 wt% on zirconia (HS).
It is interesting to note that the surface area of supported
TlOx is lower than that of the support itself for the high
surface area supports, but it ishigher for the lowsurface area
supports (Table 1). There is a reduction in the surface area
after deposition of TlOx on the high surface area supports.
The reduction is, however, drastic for the silica gel and it is
more drastic for the Si–MCM-41.
XRD. XRD spectra of the TlOx catalysts with and with-
out the different supports are presented in Figs. 1 and 2.
The XRD spectra (Fig. 1) show that there is no presence
of Tl2O3 phase in the TlOx (20 wt% )/zirconia (HS) but the
Tl2O3 phase appears when the TlOx loading on the support
is increased from 20 to 40 wt% (Figs. 1b, c). However, even
at 20 wt% loading of TlOx, the presence of Tl2O3 phase at
a larger concentration is observed for the TlOx (20 wt% )/
zirconia (LS) catalyst (Fig. 1d).
FIG. 1. XRD spectra of (a) zirconia (HS), (b) TlOx (20% )/zirconia
HS), (c) TlOx (40% )/zirconia (HS), (d) TlOx (20% )/zirconia (LS), and
(
The XRD spectra in Fig. 2 show that, for the same (e) TlOx obtained from Tl (I) acetate by decomposition in air [Tl O
loading of TlOx, there is little or no presence of the
2
3
phase (᭹)].
Tl2O3 phase in TlOx (20 wt% )/Si–MCM-41 (Fig. 2a) and
TlOx (20 wt% )/silica–alumina (HS) (Fig. 2c). However, for
the catalyst supported on the low surface area supports
Because of the deposition of TlOx on the above supports,
the IR spectra are influenced as follows:
[
(
TlOx (20 wt% )/silica (LS) (Fig. 2b) or silica–alumina (LS)
Fig. 2d)], the Tl2O3 phase is observed distinctly.
—
There is a significant decrease in the intensity of IR
For the TlOx (20 wt% )/Si–MCM-41, no characteristic
� 1
peak at around 960 cm , which is assigned for terminal
Si–OH (21).
�
�
XRD peak (at 2� = 1 –2 ) is observed, indicating a struc-
tural collapse of the Si–MCM-41.
—
There is a shift in the IR peaks [assigned for Si–O–Si
�
1
FTIR. FTIR spectra of the Si–MCM-41, silica gel (HS), (21)] at around 450 and 1080 cm and also a change in the
and silica-alumina (HS) with or without TlOx deposited on IR band at 700–900 cm , which is also assigned for Si–O–Si
them are shown in Fig. 3. (21).
�
1

2
28
CHOUDHARY AND JANA
chloride converted per mole of TlOx per unit time. The
apparent reaction rate constant (ka) for the benzylation of
benzene by benzyl chloride (with excess of benzene) was
estimated from the linear plots (Fig. 4) according to the
first-order rate expression,
log[1/(1 � x)] = (ka/2.303) � (t � t0),
[1]
FIG. 2. XRD spectra of (a) TlOx (20% )/Si–MCM-41, (b) TlOx
(
(
20% )/silica (LS), (c) TlOx (20% )/silica–alumina (HS), and (d) TlOx
20% )/silica–alumina (LS) [Tl2O3 phase (᭹)].
XPS. XPS data for the zirconia-supported TlOx cata-
lysts are presented in Table 2.
The surface Tl/Zr ratio is increased by five- to six fold
with increasing TlOx loading on the high surface zirconia
from 20 to 40 wt% . However, even for the lower loading,
the Tl/Zr ratio is much higher when TlOx is deposited on
the low surface area zirconia (Table 2).
Benzylation Reactions
The TlOx catalysts with and without different commonly
used high and low surface area supports are compared in
Table 1 for their performance in the benzylation of benzene
�
(
at 80 C). The catalysts that showed little or no benzene
benzylation activity also did not show any activity for the
�
�
benzoylation of benzene (at 80 C) or toluene (at 110 C)
for a long period (3 h). Turnover number (TON) for half
the benzene benzylation reaction (i.e., at 50% conversion
0
FIG. 3. IR spectra of Si–MCM-41 (a), TlOx/Si–MCM-41 (a ), silica
0
gel (b), TlOx/silica gel (b ), silica–alumina (HS) (c), and TlOx/silica–
0
of benzyl chloride) was estimated as the moles of benzyl alumina (c ).

TlOx FOR BENZYLATION AND ACYLATION REACTIONS
229
TABLE 2
XPS Data for Zirconia-Supported TlOx Catalysts
Binding energy (BE) (eV)
BE [Tl (4 f7/2)]–
BE [Tl (4 f5/2)]
BE [Zr (3d5/2)]–
BE [Zr(3d3/2)]
Surface
Tl/Zr ratio
Catalyst
Tl (4 f7/2)
Zr (3d5/2)
O (1s)
TlOx (20)/zirconia (HS)
TlOx (40)/zirconia (HS)
TlOx (20)/zirconia (LS)
119.1
119.1
119.4
4.4
4.5
4.1
182.8
182.2
183.0
2.0
2.0
2.5
531.1
530.9
531.8
0.04
0.22
0.43
where x is the fractional conversion of benzyl chloride and the best performance (lowest induction period and highest
t and t0 are the time and induction period, respectively, of catalytic activity, TON).
the reaction.
—The TlOx/zirconia (LS) shows high benzene benzyla-
From the comparison in Table 1, the following important tion activity, even when moisture at the level of its satura-
observations can be made:
tion is present in the substrate and/or in the catalyst. How-
ever, because of the presence of moisture in the reaction
mixture, the reaction induction period is increased from 0.1
to 0.6 min.
—
The chemically similar supports with low surface area
provided a much more active supported TlOx catalyst for
the benzene benzylation reaction. TlOx when supported on
the high surface area catalyst carriers showed almost no
catalytic activity in the reaction.
—
When the TlOx/zirconia (LS) catalyst was prepared by
calcination under N2 instead of air, it showed no activity for
the benzene benzylation.
—
The TlOx without support also shows high catalytic
activity but the reaction induction period for this catalyst is
much larger.
Results showing the influence of TlOx loading on the
reaction rate constant (ka) for the zirconia (LS), zirconia
—
Among the TlOx catalysts supported on the low sur- (HS), and silica gel (HS) supported TlOx catalysts are pre-
face area catalyst carriers, the TlOx/zirconia (LS) showed sented in Fig. 5.
�
FIG. 4. First-order reaction plots (according to Eq. [1]) for the benzylation of benzene (at 80 C) over different supported TlOx catalysts.

2
30
CHOUDHARY AND JANA
�
FIG. 5. Variation of apparent reaction rate constant (ka) for the benzylation of benzene (at 80 C) over TlOx/zirconia (LS), TlOx/zirconia (HS),
and TlOx/silica gel (HS) with the loading of TlOx on the different supports.
For the zirconia (LS) supported TlOx catalyst, the does not show any catalytic activity, even at the TlOx
reaction rate constant is increased almost linearly with loading as high as 60 wt% .
the TlOx loading. However, the zirconia (HS) supported
Figure 6 shows the effect of the time of reflux of the
TlOx catalyst shows catalytic activity only above the TlOx unsupported TlO catalyst (with dry benzene) and also of
x
loading of 20 wt% but the silica gel (HS) supported TlOx the HCl (gas) pretreatment to the catalyst, before starting
FIG. 6. Effect of time of reflux with benzene and HCl pretreatment of the catalyst, before starting the reaction, on the induction period (t0) and
�
conversion of benzyl chloride in the benzylation of benzene (at 80 C) over TlOx obtained from Tl (I) acatate [reflux time: (a) 0.5 h and (b) 4 h; (c) HCl
pretreated catalyst].

TlOx FOR BENZYLATION AND ACYLATION REACTIONS
231
FIG. 7. Effect of the presence of different substituent groups in the aromatic (benzene) nucleus on its benzylation with benzyl chloride over the
�
TlOx (20% )/zirconia (LS) at 80 C (reaction mixture = 13 ml of benzene or substituted benzene + 1 ml of benzyl chloride + 0.1 g catalyst).
the benzylation reaction, on the induction period in the decreased due to the presence of electron-donating groups,
benzene benzylation reaction.
such as methyl and methoxy groups, in the aromatic
The induction period is decreased markedly (from 24.8 compound.
to 9.3 min) with increasing reflux time (from 0.5 to 4 h)
(
Fig. 6a, b). It is also decreased drastically due to the pre- Acylation Reactions
treatment by HCl gas of the catalyst (Fig. 6a, c). It is in-
teresting to note here that although the induction period is
strongly influenced by the reflux period and the HCl pre-
treatment of the catalyst, there is only a very small change
in the reaction rate constant. Thus, in all the cases, after the
induction period, the reaction proceeds at almost the same
rate.
The high and low surface area zirconia-supported TlOx
catalysts are compared for their performance in benzene
�
�
benzoylation (at 80 C) and toluene benzoylation (at 110 C)
reactions in Table 3.
Like the benzylation reaction, the TlOx (20 wt% )/
zirconia (LS) showed much higher benzene or toluene ben-
zoylation activity than that of the TlOx (40 wt% )/zirconia
Results showing the influence of different substituent
groups attached to an aromatic benzene nucleus on
the conversion of benzyl chloride in the benzylation of
(
HS). The reaction induction period for the former is
also smaller. The TlOx (20 wt% )/zirconia (HS) showed
no activity in both the benzene and toluene benzoylation
reactions.
Like the benzene benzylation, the induction period in
the benzene benzoylation reaction over TlOx/zirconia (LS)
catalyst is decreased very markedly due to the HCl pretreat-
ment of the catalyst before the reaction (Fig. 8). The rate of
benzoylation after the induction period for both cases (i.e.,
with and without the HCl pretreatment) remained almost
the same.
�
corresponding substituted benzenes (at 80 C) over the
TlOx (20 wt% )/zirconia (LS) catalyst are presented in
Fig. 7. The first-order rate constants (ka) and indu-
ction period for the benzylation reactions are as
follows:
Substrate: Benzene Toluene p-Xylene Anisole
3
ka � 10
min 1):
�
(
291.5
Induction
period (min): 0.1
267.8
0.3
253.0
0.9
203.8
1.2
The TlOx (20 wt% )/zirconia (LS) and TlOx (20 wt% )/Si–
MCM-41 catalysts are compared with the earlier reported
catalysts for their performance in the benzene benzylation,
The results indicate that, unlike the conventional acid benzene benzoylation, and toluene benzoylation reactions
catalyst (1), the benzylation activity of the catalyst is under similar conditions in Table 4.

2
32
CHOUDHARY AND JANA
TABLE 3
Results of the Benzoylation with Benzoyl Chloride of Benzene and Toluene over Supported TlOx Catalysts
Time (min) required for 50
and 80% conversion of
acylating agent
Induction
period (min)
TON for half
reaction (min 1)
�
Catalyst
50%
80%
�
Benzoylation of benzene at 80 C
TlOx (20)/zirconia (LS)
TlOx (20)/zirconia (HS)
TlOx (40)/zirconia (HS)
9.2
—
11.5
158.0
—
0.15
—
0.06
No reaction for 2.5 h
196.0
—
�
Benzoylation of toluene at 110 C
TlOx (20)/zirconia (LS)
TlOx (20)/zirconia (HS)
TlOx (40)/zirconia (HS)
2.3
—
4.5
17.4
96.5
1.32
—
0.36
No reaction for 2.5 h
32.0
165.0
Note. Reaction mixture = 13 ml of benzene or toluene and 1.0 ml of benzoyl chloride; amount of catalyst = 0.4 g.
DISCUSSION
MCM-41 and silica gel (Table 4), showed very high ben-
zene benzylation activity (20). The observed strong influ-
ence of the catalyst support on the activity of the supported
TlOx catalysts clearly shows strong TlOx–support interac-
Influence of Catalyst Support: Strong
TlOx–Support Interactions
A comparison of the supported TlOx catalysts for their tions. Strong metal–support interactions are well known
performance in the benzene benzylation (Table 1) shows and are observed for many supported metals (22, 23). How-
that TlOx deposited on the low surface area supports has ever, the information on the strong metal oxide–support
high catalytic activity in the reaction but that deposited interactions, which are mainly chemical in nature, is scarce;
on the high surface area supports has almost no activity these have been observed for a few supported metal oxide
for the reaction. In contrast, the Ga2O3 and In2O3 cata- (Li–MgO, La–MgO, and La–CaO) catalysts (24–26). In the
lysts deposited on high surface area supports, such as Si– present case, since TlOx is basic, it can interact chemically
FIG. 8. Effect of HCl pretreatment of the catalyst, before starting the reaction, on the induction period (t0) and conversion of benzoyl chloride in
�
the benzoylation of benzene (at 80 C) over TlOx/zirconia (LS) catalyst [(a) without HCl pretreatment and (b) with HCl pretreatment].

TlOx FOR BENZYLATION AND ACYLATION REACTIONS
233
TABLE 4
Comparison of the Present Catalysts with the Earlier Ones for Their Activity in the Benzylation (with Benzyl Chloride)
�
�
�
of Benzene (at 80 C) and Benzoylation (with Benzoyl Chloride) of Benzene (at 80 C) and Toluene (at 110 C)
Conversion of benzyl or benzoyl chloride
and reaction time required
Catalyst/(C6H5CH2Cl/or C6H5COCl)
Catalyst
wt ratio
Conversion (% )
Time (min)
Ref.
�
Benzene benzylation (at 80 C)
TlOx/zirconia (LS)
TlOx/Si–MCM-41
Ga2O3/zirconia (LS)
In2O3/zirconia (LS)
Ga2O3/Si–MCM-41
Ga2O3/silica gel
In2O3/Si–MCM-41
GaCl3/Mont. K-10
InCl3/Mont. K-10
H–FeMFI
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
50
2.4
Present work
Present work
No reaction for 2.5 h
50
50
50
50
50
50
50
50
50
24
50
50
5.4
7.3
2.5
11
2.4
4.7
2
6.7
22.8
120
135
33
16
16
16
16
16
15
15
14
14
3
H–GaAlMFI
Nafion-H
Sulfated ZrO2
Sulfated Fe2O3–ZrO2
4
4
�
Benzene benzoylation (at 80 C)
TlOx/zirconia (LS)
TlOx/Si–MCM-41
In2O3/Si–MCM-41
H-Beta
0.33
0.33
0.33
0.33
50
165
Present work
Present work
No reaction for 2.5 h
50
54
286
1080
16
11
�
Toluene benzoylation (at 110 C)
83
TlOx/zirconia (LS)
TlOx/Si–MCM-41
In2O3/Si–MCM-41
Ga2O3/Si–MCM-41
H-Beta
0.33
0.33
0.33
0.33
0.33
0.5
103
Present work
Present work
No reaction for 2.5 h
83
83
83
50
148
205
1083
192
16
16
12
5
Sulfated ZrO2
with the weakly or strongly acidic surface hydroxyls of high support, its structure is collapsed with a drastic reduction
2
� 1
surface area supports.
The low surface area supports (Table 1) are highly sin- deposition of TlOx.
tered macroporous materials and hence have no surface
—The intensity of the IR peak at 960 cm 1 (assigned to
hydroxyl groups, whereas the high surface area supports terminal Si–OH) is reduced markedly due to the deposition
Table 1) are known to have appreciable amounts of sur- of TlOx on the Si–MCM-41, silica gel, and high surface area
in the surface area (from 1180 to 49 m g ) because of the
�
(
face hydroxyl groups. Even a highly crystalline material like silica–alumina (Fig. 3). This indicates the consumption of
mesoporous Si–MCM-41 has different types of surface hy- the surface hydroxyl groups in the chemical interactions
droxyls, as terminal Si–OH groups (27, 28). The Si–MCM- with the deposited TlOx.
4
1 sample used in the present study also has appreciable
—The XRD spectra of the TlOx (20 wt% ) supported on
amounts of surface hydroxyl groups (29). The silica gel sup- Si–MCM-41, silica–alumina (HS), and zirconia (HS) show
port also has large amounts of surface hydroxyl. The strong little or no presence of the Tl2O3 phase (Figs. 1b and 2a,
TlOx–support interactions in the present case are mostly c). A small Tl2O3 phase for the TlOx/zirconia (HS) is seen
due to the presence of surface hydroxyl groups in the high only when the TlOx loading is increased from 20 to 40 wt%
surface area supports. This hypothesis is supported by the (Fig. 1b, c). In contrast, for the TlOx/zirconia (LS), even
following observations:
for the TlOx loading of 20 wt% , a distinct Tl2O3 phase is
observed (Fig. 1d). It may be noted that only those sup-
—
For the high surface area supports, the surface area of ported TlOx catalysts are active in the reaction for which
the supported TlOx catalysts is much lower than that of the distinct XRD peaks showing the presence of Tl2O3 phase
corresponding support (Table 1). In the case of Si–MCM-41 are observed.

2
34
CHOUDHARY AND JANA
—
The surface Tl/Zr ratio is an order of magnitude in both the benzylation and benzoylation reactions. The in-
(
10 times) larger when TlOx is deposited on the low sur- fluence of moisture on the induction period is, therefore,
face area zirconia than on the high surface area zirconia attributed to a reduction due to the presence of moisture in
Table 2). Also, there is a large increase in the surface Tl/Zr the rate of catalyst activation by the HCl produced in the
ratio (from 0.04 to 0.22) with doubling of the TlOx loading initial reaction.
(
on the high surface area zirconia.
Catalyst activation by HCl during the induction period
mayresult from the removalofadsorbed water from the cat-
alyst (TlOx is basic in nature and hence adsorption of HCl
on it is stronger than that of water) or from the chemisorp-
tion of HCl on the catalyst modifying its surface active sites
or from both. After use in the reaction, the catalyst showed
the presence of a significant amount of chlorine. A further
detailed investigation is necessary for understanding cata-
lyst activation during the induction period.
The observations on the Tl2O3 phase and surface Tl/Zr
ratio for the high surface area zirconia for the two TlOx
loadings (20 and 40 wt% ) show that an appreciable fraction
of the TlOx loaded on the support is consumed in the TlOx–
support interactions.
The drastic reduction in the surface area of the Si–MCM-
4
1 and silica gel after the deposition of TlOx on them is
due to their structural collapse resulting from the reaction
between their surface hydroxyls and TlOx. The reduction
in the surface area for the silica–alumina (HS) is relatively
smaller and this may be due to its smaller number of surface
hydroxyls.
Thus, unlike Ga2O3 and In2O3, TlOx interacts strongly
with the high surface area supports, mostly through their
surface hydroxyl groups. Further studies are necessary for
understanding the nature of chemical interactions. Nev-
ertheless, when compared to the low surface area sup-
ports, the supported TlOx is more active than the supported
Ga2O3 and In2O3 catalysts for the benzene benzylation
Effect of Electron-Donating Substituent Groups
According to the classical mechanism of the Friedel–
Crafts-type acid-catalyzed benzylation or acylation reac-
tion, the benzylation or acylation of an aromatic com-
pound is easier if one or more electron-donating groups are
present in the aromatic ring (1). Surprisingly, in the present
case the activity of the TlOx/zirconia (LS) catalyst for ben-
zylation (under the same conditions) of the aromatic com-
pounds with or without electron-donating groups (CH3 and
CH3O) is opposite to that expected from the classical mech-
anism (Fig. 7). The first-order rate constant for the benzyla-
tion of benzene and substituted benzenes is in the following
order: benzene > methyl benzene > p-dimethylbenzene >
methoxy benzene. This shows that, for this catalyst, the re-
action mechanism is different from that for the classical
acid-catalyzed benzylation reactions.
(
Table 4). Also, like the supported Ga2O3 and In2O3 cata-
lysts (20), the activity of the TlOx/zirconia (LS) catalyst is
only little affected by the presence of moisture in the cata-
lyst or in the reaction mixture (Table 1). However, because
of its toxicity, the TlOx catalyst should be handled much
more carefully.
Effect of Moisture and HCl Pretreatment on the Reaction
Induction Period
Reaction Mechanism
The active supported TlOx catalysts were found to con-
The induction period for the benzene benzylation is re- tain Tl2O3, which is basic in nature. The standard reduc-
duced markedly because of increasing of reflux period and tion potential for Tl3 → Tl (E Tl3 / Tl
+
1+
�
1+
) is positive
+
3+
1+
consequently removal of the adsorbed moisture from the ( + 2.06 V), and hence the reduction of Tl to Tl is rel-
unsupported TlOx catalysts (Fig. 6a, b). It is also interest- atively easy. The benzylation and acylation reactions over
ing to note that the TlOx catalysts supported on the low these catalysts are, therefore, expected to follow a redox
surface area support (which are sintered macroporous ma- mechanism similar to that proposed earlier for the alkyla-
terials and hence do not contain significant amounts of tion (10) and acylation reactions (30), as follows:
adsorbed moisture) have a much lower induction period
3+
2
C6H5CH2(or CO)Cl + Tl
(
Table 1). These observations indicate that the adsorbed
+
1+
moisture present in the catalyst is responsible for the high
induction period for the unsupported TlOx; the removal of
adsorbed water from the catalyst results in a decrease in the
induction period.
→ 2C H CH (or CO)Cl + Tl
[2]
6
5
2
+
+
+
C6H5CH2(or CO)Cl → C6H5CH (or CO ) + Cl [3]
2
+
+
C H CH (or CO ) + ArH
6
5
2
The induction period of the unsupported TlOx catalyst is
reduced drastically because of the HCl pretreatment to the
catalyst before the benzene benzylation reaction (Fig. 6a, c).
A similar observation is also made for the benzoylation of
benzene over the TlOx/zirconia (LS) (Fig. 8). These obser-
vations reveal that, during the induction period, the catalyst
+
→
C6H5CH2(or CO)Ar + H
[4]
[5]
[6]
1+
�
3+
2Cl + Tl → 2Cl + Tl
+
�
H
+ Cl → HCl.
The TlOx/zirconia (LS) obtained by calcination in a N2
is activated by the HCl (which is a by-product) produced atmosphere contained Tl2O and no Tl2O3. This catalyst

TlOx FOR BENZYLATION AND ACYLATION REACTIONS
235
showed no activity for the benzene benzylation (Table 1),
ACKNOWLEDGMENTS
which is consistent with the above mechanism.
Suman K. Jana is grateful to CSIR, New Delhi, for the award of Junior
Research Fellowship. The authors are grateful to Dr. A. B. Mandale and
Dr. (Mrs.) A. Mitra for XPS and XRD data.
CONCLUSIONS
REFERENCES
The following important conclusions have been drawn
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2
3
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1
1
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. The TlOx/zirconia (LS) catalyst shows the following 16. Singh, A. P., Bhattacharya, D., and Sarma, S., J. Mol. Catal. A: Chem.
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1
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2
23 (2000).
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2
2
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2
9
2
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