Selective synthesis of 6,8-di-t-butylated flavan over Zn-Al containing mesoporous silica catalysts

Article abstract of DOI:10.1039/c2dt31409f

We demonstrate a much green synthesis method for highly selective synthesis of 6,8-di-t-butylated flavan (6,8-DTBF) by liquid phase alkylation of 2,4-di-t-butylphenol (2,4-DTBP) with cinnamyl alcohol (Cin-OH) over mesoporous Zn-Al-MCM-41 catalysts synthesized under direct basic hydrothermal method. The main alkylated product, 6,8-DTBF is importantly used as an intermediate in the manufacture of biosynthetic organic compounds. The recyclable mesoporous Zn-Al-MCM-41 catalysts have also been reused in this reaction to study their catalytic activities. The influences of various reaction parameters such as temperature, time, ratios of reactant (2,4-DTBP-to-Cin-OH) have been extensively investigated for the synthesis of 6,8-DTBF. In addition, dimethyl sulfoxide (DMSO) has also been used as a solvent in this catalytic reaction. The mesoporous Zn-Al-MCM-41(75) gives excellent catalytic activity with 6,8-DTBF selectivity (86.0%) and 2,4-DTBP conversion (63.1%), and these catalytic results have also compared with that obtained using other mesoporous and microporous catalysts. On the basis of catalytic activity obtained by using the all catalysts, the Zn-Al-MCM-41(75) catalyst is found to be a highly active, recyclable and eco-friendly heterogeneous catalyst in the liquid-phase alkylation of 2,4-DTBP. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt31409f

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PAPER
Selective synthesis of 6,8-di-t-butylated flavan over Zn–Al containing
mesoporous silica catalysts†
a
b
a
c
d
c
M. Selvaraj,* P. K. Sinha, D.-W. Park, Il Kim, S. Kawi and C. S. Ha
Received 29th June 2012, Accepted 28th August 2012
DOI: 10.1039/c2dt31409f
We demonstrate a much green synthesis method for highly selective synthesis of 6,8-di-t-butylated flavan
6,8-DTBF) by liquid phase alkylation of 2,4-di-t-butylphenol (2,4-DTBP) with cinnamyl alcohol (Cin-
(
OH) over mesoporous Zn–Al–MCM-41 catalysts synthesized under direct basic hydrothermal method.
The main alkylated product, 6,8-DTBF is importantly used as an intermediate in the manufacture of
biosynthetic organic compounds. The recyclable mesoporous Zn–Al–MCM-41 catalysts have also been
reused in this reaction to study their catalytic activities. The influences of various reaction parameters
such as temperature, time, ratios of reactant (2,4-DTBP-to-Cin-OH) have been extensively investigated for
the synthesis of 6,8-DTBF. In addition, dimethyl sulfoxide (DMSO) has also been used as a solvent in
this catalytic reaction. The mesoporous Zn–Al–MCM-41(75) gives excellent catalytic activity with 6,8-
DTBF selectivity (86.0%) and 2,4-DTBP conversion (63.1%), and these catalytic results have also
compared with that obtained using other mesoporous and microporous catalysts. On the basis of catalytic
activity obtained by using the all catalysts, the Zn–Al–MCM-41(75) catalyst is found to be a highly
active, recyclable and eco-friendly heterogeneous catalyst in the liquid-phase alkylation of 2,4-DTBP.
by heterogeneous solid acid ones, such as clay minerals and zeo-
lites. Therefore, some solid acid catalysts are used in the alkyl-
ation of aromatics. Particularly, microporous zeolites are used in
the alkylation of 2,4-DTBP with Cin-OH, however, they give a
less yield of flavan (<10%) because of their small pore sizes that
1
. Introduction
Friedel–Crafts alkylation is an important reaction for attaching
alkyl chains to aromatic rings. The alkylation is traditionally per-
formed, with alkyl halides or alcohols as the alkylating regents
using Lewis acid catalysts such as AlCl , FeCl and ZnCl .
1
2
3
3
2
are unsuitable in this catalytic reaction. The microporous zeo-
However, the separation of homogeneous catalysts is not easy
after the reaction, and that catalytic system causes large amounts
of acid wastes. The use of homogeneous catalysts gives rise to
many problems such as a high corrosiveness, tedious work-up,
requirement of stoichiometric quantities, several undesirable side
products. The homogeneous catalysts cannot be reused and thus
are highly non-ecofriendly. The major goals of ‘green chemistry’
are to increase the product selectivity, to maximize the use of
starting materials, and to replace hazardous and stoichiometric
reagents with eco-friendly catalysts in order to facilitate an easy
separation of the final reaction mixture, including recovery of
the catalyst, and also reusable. Active research has been
addressed for substituting the traditional homogeneous catalysts
lites produce several byproducts with flavan product, and com-
plicated and expensive processes might be needed to separate the
2
flavan products.
Recently, the green catalytic reaction processes have been
importantly focusing with minimal environmental threats and
maximal economical benefits. A great demand in fine chemical
industries begins to develop a suitable heterogeneous catalyst
with a large number of Brønsted acid sites for mild reaction con-
ditions without employing toxic materials. In 1992, the highly
ordered mesoporous silicate materials discovered with high
surface area, large pore size, and large pore volume have
3
attracted a great interest for their catalytic applications.
Although the mesoporous MCM-41 materials offer great advan-
tages for the conversion of bulky molecules in the studies of cat-
alysis, pure mesoporous silica materials have not been used as
catalysts or catalyst supports in industry yet due to the lack of
acidity and poor hydrothermal stability. Furthermore, these
mesoporous materials are easily destroyed in boiling water and
steam as a result of the fast hydrolysis of the relatively thin amor-
a
School of Chemical and Biomolecular Engineering, Pusan National
University, Busan 609-735, Korea. E-mail: chems@pnu.edu;
Fax: +82 51 512 8563; Tel: +82 51 510 2397
b
Waste Management Division, Bhabha Atomic Research Centre (BARC),
Trombay, Mumbai 400085, India
c
4
Department of Polymer Science and Engineering, Pusan National
phous silica pore walls. Thus, huge amounts of Al-ions into the
University, Busan 609-735, Korea
mesoporous silica walls are incorporated to improve the surface
d
Department of Chemical and Biomolecular Engineering, National
5–7
acidity and hydrothermal stabilities.
Generally, mesoporous
University of Singapore, Singapore 119260, Singapore
Al–MCM-41 materials have lower content of acid sites as com-
pared to zeolites. Because of its uniformly hexagonal pore size,
†
1
Electronic supplementary information (ESI) available. See DOI:
0.1039/c2dt31409f
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mesoporous Al–MCM-41 is a promising catalyst in the pro-
duction of bulky fine chemicals like 6,8-DTBF. Al-incorporated
0.25 mm × 0.3 μm). Additionally, the products were further
confirmed using gas chromatography combined mass spectro-
scopy (GC-MS). 1,3-Di-t-butylbenzene and toluene were used
as an internal standard for the quantitative analysis of the
products.
2
into HMS, ethane- and benzene-mesoporous silicas have also
been used for the synthesis of 6,8-DTBF, and the catalysts are
2
,8–10
produced up to 75% selectivity of 6,8-DTBF.
various mesoporous catalysts have also been used in alkylation
of aromatics.
Additionally,
The high and low metal content containing mesoporous cata-
lysts, such as Zn–Al–MCM-41(75) and Zn–Al–MCM-41(380),
respectively, were reused in the alkylation of 2,4-DTBP to study
their catalytic stabilities. In a typical experimental procedure, the
Zn–Al–MCM-41(75) catalyst used in a catalytic run was separ-
ated from the reaction mixture, washed with dichloromethane
several times and dried at 393 K. Finally, the Zn–Al–MCM-41-
(75) catalyst was calcined at 473 K for 6 h in air to remove the
adsorbed species (like products and unreacted reactants) and
again reused for further catalytic runs. The recyclable Zn–Al–
MCM-41(75) has been regenerated before applying to each run.
A similar procedure was used for recycling studies of other cata-
lysts like Zn–Al–MCM-41(380). After completion of the reac-
tion, the catalyst was filtered and analyzed by ICP-AES to
determine the percentage of Zn and Al, and the conversion of
2,4-DTBP and selectivity of 6,8-DTBF were calculated with the
standard formulas obtained by analyzing the results of GC and
GC-MS.
11–14
Selvaraj and his research group have reported the details of
the synthesis and characterization of mesoporous Zn–Al–
MCM-41 catalysts which have been used as the alkylation cata-
lysts for the synthesis of fine chemicals along with ‘a goal of
15–19
green chemistry’.
To the best of our knowledge, the meso-
porous Zn–Al–MCM-41 molecular sieves have not been used as
the green catalysts for the synthesis of 6,8-DTBF. Herein we dis-
close the report of selective synthesis of 6,8-DTBF using Zn–
Al–MCM-41 catalysts under mild reaction conditions. The cata-
lytic results of Zn–Al–MCM-41, for the selective synthesis of
6
,8-DTBF, have also been correlated and compared with other
solid catalysts, such as Al–MCM-41(21), Zn–MCM-41(21),
USY, Hβ, H-ZSM-5 and H-mordenite, including a strong acid
catalyst, 1 N H SO .
2
4
2. Experimental
3
. Results and discussion
The diverse mesoporous catalysts, such as MCM-41, Zn–Al–
MCM-41 catalysts (n /(n + n ) = 75, 151, 228, 304 and
Si Zn
Al
3
.1. Physico-chemical characterization of Zn–Al–MCM-41
3
2
80), Al–MCM-41 (n /n = 21) and Zn–MCM-41 (n /n
1), were synthesized under direct hydrothermal method and
=
Si Al
Si Zn
The diverse mesoporous catalysts, such as Zn–Al–MCM-41(75),
Zn–Al–MCM-41(151), Zn–Al–MCM-41(228), Zn–Al–MCM-
41(304) and Zn–Al–MCM-41(380), Al–MCM-41(21), Zn–MCM-
41(21) and MCM-41, synthesized by direct hydrothermal
method, have been characterized using the relevant instrumental
characterized according to our previous published pro-
15,16,20,21
cedure.
PQ); H-mordenite (n /n = 20, PQ) and H-ZSM-5 (n /n
Hβ (n /n = 20, Strem); HY (n /n = 2.9,
Si Al
Si Al
=
Si Al
Si Al
3
0, PQ) were obtained from commercial sources. For the
1
5,16,20,21
removal of moisture from the inner surface of pores, these cata-
lysts were then calcined at 200 °C in air for 6 h before catalytic
reaction. 1 N H SO was also used as an acid catalyst in this cat-
techniques according to the published method.
Based
on the physico-chemical characterization results, it can be
observed from the XRD patterns (Fig. 1S†) of a variety of meso-
porous catalysts, such as Zn–Al–MCM-41, Al–MCM-41(21)
and Zn–MCM-41(21) that the structural properties, such as
2
4
alytic reaction.
The alkylation of 2,4-DTBP (Aldrich) with Cin-OH (Aldrich)
was carried out under vigorous stirring in a thermostated glass
vessel reactor. In a typical experimental procedure, 1.0 mmol
d-spacing value (d100) and unit cell parameter (a ), decrease with
o
increasing the metal-ions content (Table 1). FT-IR studies, as
shown in Fig. 2S,† predict that zinc- and aluminum-ions are
incorporated into the hexagonal mesoporous MCM-41 materials;
the infrared wavenumber of the antisymmetric Si–O–Si vibration
bands (1096 cm ) in Zn–Al–MCM 41 is higher than that of
Al–MCM-41 and Zn–MCM-41 (1083 or 1085 cm− ), and this
shift should be attributed to the increase of the mean Si–O dis-
tance in the surface wall caused by the incorporation of the small
ionic radii of silicon by the larger ionic radii of zinc. The zinc-
and aluminum-ions content on the silica surface, before and after
catalytic reaction of 2,4-DTBP, have been determined by
ICP-AES (Table 1). The textural properties, such as surface area
(SABET), pore diameter (d ) and pore volume (V ) of Zn–Al–
MCM-41(75), are smaller than that of other Zn–Al–MCM-41
(Table 1), but the total and Brønsted–Lewis acid sites in Zn–Al–
MCM-41(75), as shown in Table 1S† and Fig. 3S,† are higher
than that in other Zn–Al–MCM-41, Al–MCM-41(21) and Zn–
MCM-41(21) due to increasing the number of acid sites by the
2
,4-DTBP (206 mg), 1.0 mmol Cin-OH (134 mg) and 50.0 mL
isooctane as a solvent were taken in the glass vessel reactor. The
reaction mixture was stirred under constant stirring, and sub-
sequently the reaction temperature was slowly raised to 363 K
and then 200 mg of Zn–Al–MCM-41(75) catalyst was added.
After a reaction time of 24 h as well as reaching the reactor
temperature to room temperature, the catalyst was filtered and
extracted with dichloromethane to recover the adsorbed reaction
products. The alkylation of 2,4-DTBP was also carried under the
same reaction conditions using a variety of mesoporous and
microporous catalysts, and an acid catalyst, 1 N H SO . To find
−1
1
16
2
4
an optimal condition over Zn–Al–MCM-41(75), the catalytic
reaction was further carried out with different reaction conditions
such as time, temperature, stoichiometric molar ratios of reac-
tants (2,4-DTBP-to-Cin-OH). Additionally, this reaction was
also carried out with DMSO solvent.
The collected products were analyzed with authentic samples
using an Agilent 6890 gas chromatograph equipped with a flame
ionization detector (FID) and an OV-1 capillary column (30 m ×
p
p
16
introduction of zinc ions. SEM studies confirm that the all cal-
cined Zn–Al–MCM-41 catalysts have either micellar rod-like
14198 | Dalton Trans., 2012, 41, 14197–14203
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Table 1 Physico-chemical properties of Zn–Al–MCM-41 catalysts prepared by basic hydrothermal method
a
a
SABET (m g−1)
2
(cm g−1)
3
Catalysts
Zn content (wt%)
Al content (wt%)
d
100 (Å)
a
0
(Å)
D
p
(Å)
V
p
w
t (Å)
Zn–Al–MCM-41(75)
Zn–Al–MCM-41(151)
Zn–Al–MCM-41(228)
Zn–Al–MCM-41(304)
Zn–Al–MCM-41(380)
0.123
0.060
0.040
0.031
0.020
0.122
0.020
0.220
0.123
0.079
0.060
0.049
0.219
0.049
0.499
—
37.91
38.41
38.91
39.98
41.09
37.85
41.00
38.41
37.9
43.77
44.35
44.92
46.15
47.44
43.70
46.32
44.35
43.76
41.45
820
867
912
970
1071
810
1018
830
635
22.8
25.6
27.0
29.3
32.2
22.9
31.5
27.5
27.3
29.3
0.852
0.893
0.944
0.963
0.983
0.852
0.982
0.866
0.611
0.675
20.97
18.75
17.90
16.85
15.24
20.80
14.82
16.85
16.46
12.15
b
Zn–Al–MCM-41(75)
b
Zn–Al–MCM-41(380)
Al–MCM-41(21)
Zn–MCM-41(21)
MCM-41
0.242
—
—
35.90
1179
a
b
The results were obtained by ICP-AES. The results were obtained for the recyclable catalysts used in 4th run.
1
6
hexagonal shape or spherical edge (Fig. 4S†). The uniform
pore channels with its hexagonal symmetry can be clearly ident-
ified by the TEM images of Zn–Al–MCM-41(75), indicating
that the all mesoporous Zn–Al–MCM-41 catalysts synthesized
under basic hydrothermal method have uniform mesostructures
of 2,4-DTBP, and subsequently 6-t-Butyl-2-phenyl-2,3-dihydro-
[4H]benzopyran or 6-t-butylated flavan (6-TBF, Product 3) also
forms as a secondary byproduct by the dealkyation of 6,8-DTBF
or alkylation of 4-TBP with Cin-OH.
1
6
(Fig. 5S†) as inferred by the XRD results.
3
.3. Selectivity of 6,8-di-t-butylated flavan
The alkylation of 2,4-DTBP with Cin-OH (Scheme 1) for the
highly selective synthesis of 6,8-DTBF was carried out by using
the reaction conditions noted in Table 2 over mesoporous and
microporous solid acid catalysts.
3
.2. Alkylation of 2,4-di-t-butylphenol with cinnamyl alcohol
The liquid phase alkylation of 2,4-DTBP has been conducted
over mesoporous and microporous solid catalysts with
In 1995, Armengol et al. applied the mesoporous aluminosili-
cate MCM-41 as a convenient solid acid catalyst in the liquid
phase alkylation of 2,4-DTBP with Cin-OH, and that catalyst is
produced 35% of 6,8-DTBF yield with 25% of 4-TBP yield as a
1
: 1 mmol ratio of 2,4-DTBP to Cin-OH at 363 K for 24 h, as
shown in Scheme 1. In this alkylation reaction 6,8-di-t-butyl-2-
phenyl-2,3-dihydro[4H]benzopyran or 6,8-di-t-butylated flavan
(
6,8-DTBF, Product 1) forms as a major product by an intramole-
cular cyclization of the primary cinnamyl phenol intermediate
Product 4). Additionally, the 4-t-butylphenol (4-TBP, Product 2)
2
main byproduct. Pauly et al. reported that in the similar catalytic
system the 6,8-DTBF selectivity is 71.8% and 74.2% over Al–
(
8
MCM-41 and Al–HMS, respectively. Aluminum-containing
as a primary byproduct produces by acid catalyzed dealkylation
mesoporous ethane- and benzene-silicas produce 25.3–27.5% of
9,10
6
,8-DTBF selectivity with 51.3% of 4-TBP selectivity.
From
the above literature results, we found that monometallic meso-
porous silicas are produced up to 75% selectivity of 6,8-DTBF
due to increasing the selectivity of dealkylated products whereas
aluminum containing organosilicas lead two times less selec-
tivity of 6,8-DTBF as compared to the selectivity of primary
cinnamylphenol. To improve the selectivity of 6,8-DTBF, the
mesoporous Zn–Al–MCM-41 catalysts have been used in this
catalytic reaction. In order to increase the conversion of 2,4-
DTBP as in the series of the catalysts: Zn–Al–MCM-41(75) >
Zn–Al–MCM-41(151) > Al–MCM-41(21) > Zn–Al–MCM-41-
(228) > Zn–MCM-41(21) > Zn–Al–MCM-41(304) > Zn–Al–
MCM-41(380) > 1 N H SO > Hβ(20) > HY(2.6) > H-morde-
2
4
nite(20) ≥ H-ZSM-5(15). It has been observed that the meso-
porous Zn–Al–MCM-41(75) shows that its high catalytic
activity has been enhanced in the production of 6,8-DTBF with
no diffusional constraint, as 2,4-DTBP conversion (63.1%) and
6
,8-DTBF selectivity (86.0%) in Zn–Al–MCM-41(75) are
higher than that of other Zn–Al–MCM-41 (Table 2) due to the
higher number of Brønsted acid sites on the surface of pore walls
15,16
caused by the increasing amounts of Zn-ions.
Additionally,
Zn–Al–MCM-41(75) produces higher selectivity of 6,8-DTBF
than Al–MCM-41, as found in the literatures, due to promotion
of the catalytic activity, which was caused by the introduction of
Scheme 1 Reaction products obtained by alkylation of 2,4-DTBP with
Cin-OH over Zn–Al–MCM-41 catalysts.
15,16
high Zn-ions on the surface of Al–MCM-41.
4-TBP
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a
Table 2 Alkylation of 2,4-DTBP with Cin-OH over various types of catalysts
Selectivity (%)
Catalysts
Conversion of 2,4-DTBP (%)
Yield of 6,8-DTBF (%)
1
2
3
4
Zn–Al–MCM-41(75)
Zn–Al–MCM-41(151)
Zn–Al–MCM-41(228)
Zn–Al–MCM-41(304)
Zn–Al–MCM-41(380)
HY (2.6)
63.1
52.2
43.4
34.2
25.3
7.5
9.5
5.4
4.8
21.3
62.9
61.8
49.3
40.1
5.0
54.3
39.2
27.4
17.8
10.7
6.1
8.1
4.0
3.6
9.1
53.9
45.2
36.7
12.2
<1
86.0
75.2
63.3
52.1
42.3
82.0
85.5
73.5
77.0
43.0
85.8
73.2
74.4
30.5
2
12.6
10.1
8.3
6.7
5.8
6.2
7.7
5.0
5.0
35.2
12.4
12.5
9.8
47.5
—
1.4
1.1
1.0
1.0
1.0
—
—
—
—
13.6
27.4
40.2
50.9
11.8
6.8
21.5
18.0c
3.6
Hβ (20)
H-mordenite (20)
HZSM-5(15)
—
b
4
1
N H
2
SO
18.2
1.8
14.3
1.1
22.0
—
d
e
Zn–Al–MCM-41(75)
Zn–Al–MCM-41(75)
Al–MCM-41(21)
Zn–MCM-41(21)
MCM-41
—
—
14.7
—
g
98
80
f
g
Fíltrate solution
10
<1
5
7
8
a
b
c
Reaction conditions: 200 mg of catalyst, 1 : 1 mmol ratio of 2,4-DTBP to Cin-OH, 50 ml of isooctane, T = 363 K, t = 24 h. 50 mg. Tri-t-
d
e
f
butylphenol. Dimethyl sulfoxide was used as a solvent. Dimethyl sulfoxide was used at 383 K. The results obtained from the hot-filtrate solution
after Zn–Al–MCM-41(75) catalyst removal. Unreacted reactants.
g
selectivity in Zn–Al–MCM-41(75) is also higher than that of
other Zn–Al–MCM-41. Al–MCM-41(21) catalyst has produced
higher 6,8-DTBF selectivity than Zn–MCM-41(21) due to
increasing Brønsted acid sites on the surface of pore walls
whereas the Zn–MCM-41(21) leads to increase the dealkylated
products due to acting Lewis solid acid catalyst, as shown in the
has been observed that the microporous catalysts are unsuitable
for the higher conversion of 2,4-DTBP by the alkylation of 2,4-
DTBP with Cin-OH.
An acid catalyst, 1 N H SO has been used in this catalytic
2
4
reaction and gives 2,4-DTBP conversion (21.3%) with a 6,8-
DTBF selectivity (43%) which is much less as compared
to solid acid mesoporous catalyst, Zn–Al–MCM-41(75), but the
1 N H SO gives 4-TBP selectivity (35.2%), which is higher as
22
literature. Based on the literature results, it can be observed
from the results of FT-IR-pyridine and ammonia-TPD that the
acid sites of the all microporous catalysts are almost similar
2
4
compared to other solid catalysts used in this reaction due to its
strong acidity (Table 2). DMSO has been used as a solvent in
this catalytic reaction under the same reaction condition over
Zn–Al–MCM-41(75) and gives 2,4-DTBP conversion (62.9%)
with 6,8-DTBF selectivity (85.8%), and its catalytic activity is
almost similar to isooctane solvent used as solvent in this cataly-
tic reaction (Table 2). The 6,8-DTBF selectivity (Table 2)
decreases when this catalytic reaction is carried out in the pres-
ence of DMSO at 383 K because of the increase of the bypro-
ducts selectivity, 4-TBP (12.5%) and 6-TBF (14.3%), as
23
except H-ZSM-5. Zeolite Hβ (pore size, 7.6 and 5.4 Å), which
has a 9.5% 2,4-DTBP conversion, is found to be more active
than HY (pore size, 7.4 Å), which has a 7.5% 2,4-DTBP conver-
sion. These catalytic results clearly show that both catalysts (Hβ
and HY) cannot be allowed the bulky reactants, 2,4-DTBP and
Cin-OH, to enter into their pore sizes. 2,4-DTBP conversion in
HY and Hβ is less than 10% due to less acid sites on their exter-
nal surfaces but their inner pore sizes have strong acid sites,
2
which could not help for this catalytic reaction. Although 2,4-
2
DTBP conversion is less, 6,8-DTBF selectivity in both catalysts
is high, as the 6,8-DTBF selectivity (85.5%) in Hβ is higher than
that (82%) of HY. It can be also observed from the catalytic
results that the higher selectivity of Hβ might be attributed to its
outer surface acid sites, and hence a higher selectivity of 6,8-
DTBF is achieved over Hβ rather than HY zeolite. In contrast,
some microporous zeolites (H-mordenite and H-ZSM-5) were
also found to be much less active in the reaction. The 2,4-DTBP
molecule is too large to enter the pores of ZSM-5 (5.4 Å) and
H-mordenite (7.1 Å). Hence, the reaction should proceed only
on the outer surfaces of ZSM-5 and H-mordenite. Although the
reaction is suggested to be catalyzed by moderately acidic sites,
ZSM-5 gives slightly higher 6,8-DTBF selectivity than H-mor-
denite, as shown in Table 2. It might be also attributed to the
stronger acidic strength on ZSM-5. However, 2,4-DTBP conver-
sion in ZSM-5 is much less than that of H-mordenite, as shown
in Table 2. In all microporous catalysts the 6-TBF selectivity is
also much less than 8%. On the basis of the catalytic results, it
reported by Armengol et al. On the basis of the literatures, it
can be also observed that the catalytic activity of Zn–Al–
MCM-41(75) is higher as compared to other catalysts used for
2,8–10
this catalytic reaction.
On the basis of the performance of
the catalytic activity over different Zn–Al–MCM-41 mesoporous
catalysts, the well hexagonally ordered Zn–Al–MCM-41(75) is
found to be a promising heterogeneous catalyst in the liquid-
phase alkylation of 2,4-DTBP for the highly selective synthesis
of 6,8-DTBF. In conclusion it can be also confirmed that the
well ordered mesoporosity with huge amounts of Zn and Al
loadings plays an important catalytic role in the production of
6,8-DTBF with a higher selectivity.
3
.4. Effect of reaction temperature and time
Fig. 1 and 2 show the results of alkylation of 2,4-DTBP with
1 : 1 mmol ratio of 2,4-DTBP to Cin-OH at different reaction
14200 | Dalton Trans., 2012, 41, 14197–14203
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Fig. 1 Effect of reaction temperature over Zn–Al–MCM-41(75). Reac-
tion conditions: 200 mg of catalyst, 1 : 1 mmol ratio of 2,4-DTBP to
Cin-OH, 50 ml of isooctane and t = 24 h.
Fig. 3 Effect of 2,4-DTBP to Cin-OH ratios over Zn–Al–MCM-41-
(75). Reaction conditions: 200 mg of catalyst, 50 ml of isooctane, T =
3
63 K and t = 24 h.
(75) at 363 K for 48 h have not been changed that can be
measured by the studies of remarkably hydrothermal stability
16
and acidity measurements.
3
.5. Effect of 2,4-di-t-butylphenol to cinnamyl alcohol ratio
To find the best ratio of 2,4-DTBP to Cin-OH for the highly
selective synthesis of 6,8-DTBF, the liquid phase alkylation of
2
,4-DTBP has been carried out with different ratios of n2,4-DTBP/
nCin-OH at 363 K for 24 h over Zn–Al–MCM-41(75) as shown in
Fig. 3. When this reaction is carried out with 1 : 1 mmol ratio,
2
,4-DTBP conversion (63.1%) as well as 6,8-DTBF selectivity
(86.0%) can be observed because of the equilibrium of the reac-
tants with the greater chemisorption on the Brønsted acid sites of
catalyst surfaces. The decrease in 6,8-DTBF selectivity is
observed in other mmol ratios of 1 : 2 and 2 : 1 because of
increasing of the byproducts selectivity. Particularly, 4-TBP
selectivity (15.5%) in the ratio of 2 : 1 as well as a primary cin-
namylphenol (17.6%) in the ratio of 1 : 2 can be observed. It is
clearly found a correct reason in the decrease of 6,8-DTBF selec-
tivity, as the formation of byproducts increase by using unevenly
ratios of reactants whereas the surface acid sites of catalyst in the
ratio of 1 : 2 may be deactivated due to the formation of polymer-
Fig. 2 Effect of reaction time over Zn–Al–MCM-41(75). Reaction
conditions: 200 mg of catalyst, 1 : 1 mmol ratio of 2,4-DTBP to Cin-
OH, 50 ml of isooctane and T = 363 K.
times and temperatures over Zn–Al–MCM-41(75). With increas-
ing temperature from 333 to 363 K for 24 h, 2,4-DTBP conver-
sion as well as 6,8-DTBF selectivity gradually increases, and
byproduct selectivity (4-TBP) also slightly increases by the acid
catalyzed dealkylation of 2,4-DTBP. Moreover, 2,4-DTBP con-
version as well as 6,8-DTBF selectivity increases with increasing
time from 6 to 24 h whereas the conversion and selectivity
decrease at 48 h due to increasing of the byproduct selectivity
2
ized products of cinnamyl alcohol. On the basis of the effect of
the ratios, it can be clearly noted that 1 : 1 mmol ratio of 2,4-
DTBP to Cin-OH is an optimum ratio for the highly selective
synthesis of 6,8-DTBF.
(
4-TBP and 6-TBF) by the acid catalyzed dealkylation of 2,4-
3
.6. Recyclability
2
DTBP or 6,8-DTBF at a longer time. At low reaction temp-
erature (<343 K) as well as reaction time (<12 h), the formation
of primary cinnamyl phenol increases due to the less chemisorp-
tion occurred between the reactants and the Brønsted acid sites
of catalytic surface. Based on the catalytic results, it is interesting
to note that the reaction temperature (363 K) and time (24 h) are
suitable to be a better conversion of 2,4-DTBP as well as selec-
tivity of 6,8-DTBF while the active sites of Zn–Al–MCM-41-
The bimetallic mesoporous catalysts, Zn–Al–MCM-41(75) and
Zn–Al–MCM-41(380), have been examined to determine their
catalytic stabilities as follows. Initially, these catalysts used in
the catalytic reaction usually suffer from the loss of catalytic
activities, and hence the catalysts need to be regenerated by
chemical treatments and calcination. At first the recycled cata-
lysts were washed several times with dichloromethane and dried
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reactions, as shown in Table 1. The conversion and selectivity in
Zn–Al–MCM-41(380) almost remain constant with each cycle
because the low active species can be homogeneously incorpor-
ated on the inner silica pore walls, as a better catalytic activity
found in Zn–Al–MCM-41(380). The metal-ions in Zn–Al–
MCM-41(380) have also been measured by ICP-ACS after the
recyclable reaction (Table 1). After collecting the results of four
runs over Zn–Al–MCM-41(75), the results of hot-catalyst fil-
tration experiments were also carried out twice at 363 K during
alkylation of 2,4-DTBP with Cin-OH. In this case, the filtrate
solution gives a very low 2,4-DTBP conversion (10%) with trace
amount of 6,8-DTBF yield, indicating that the alkylation of 2,4-
DTBP takes place on the surface of Zn–Al–MCM-41(75) and is
24,25
found as a true heterogeneous process.
This reaction was
also carried out using pure MCM-41 synthesized by basic hydro-
20
thermal method. In this case, conversion of 2,4-DTBP (5%)
forms with a trace amount of 6,8-DTBF selectivity (Table 2),
thus indicating that major activity is only due to both metal-ions,
Al- and Zn-ions, incorporated in the framework of MCM-41.
4
. Conclusions
The bimetallic mesoporous Zn–Al–MCM-41 catalysts have been
successfully used for the synthesis of 6,8-DTBF. The active sites
of these catalysts are highly efficient for the generation and intra-
molecular cyclization of cinnamylphenol intermediate to 6,8-
DTBF. Zn–Al–MCM-41(75) has higher catalytic activity in the
liquid-phase alkylation of 2,4-DTBP as compared to other meso-
porous and microporous catalysts, and an acid catalyst, 1 N
H SO , due to the higher number of Brønsted acid sites on the
2
4
surface pore walls. From the optimized conditions, it can be
found that the higher 6,8-DTBF selectivity and 2,4-DTBP con-
version are obtained at 363 K for 24 h using 1 : 1 mmol ratio of
2
,4-DTBP to Cin-OH with 50 ml isooctane. On the basis of all
catalytic studies, it is clearly found that the Zn–Al–MCM-41(75)
catalyst is a highly active, recyclable and promising hetero-
geneous catalyst for the selective synthesis of 6,8-DTBF.
Fig. 4 Effects of recyclability ((a) conversion and (b) yield and selec-
tivity) over various mesoporous solid catalysts. Reaction conditions:
2
00 mg of catalyst, 1 : 1 mmol ratio of 2,4-DTBP to Cin-OH, 50 ml of
Acknowledgements
isooctane, t = 24 h and T = 363 K.
This research work was supported by Basic Science Research
Program through the National Research Foundation (NRF) of
Korea funded by the Ministry of Education, Science and Tech-
nology (2012-0007172).
at 393 K overnight. Further, the catalysts were calcined at 473 K
for 6 h in air for complete removal of the organics and unreacted
2
,4-DTBP molecules. The treated catalysts have been reused as
recyclable catalyst in this reaction, as shown in Fig. 4. Using the
above procedure, the recyclable catalyst has been regenerated
before applying to each run. The 2,4-DTBP conversion as well
as 6,8-DTBF selectivity slightly increase in the first two runs
References
1
Ullmann’s Encyclopedia of Industrial Chemistry, ed. H.-W. Voges,
B. Elvers, S. Hawkins and G. Schulz, VCH, Weinheim, 1991, vol. A19,
(Fig. 4). On the basis of first and second runs, it can be clearly
328.
observed that somewhat active sites on the inner pore surface of
Zn–Al–MCM-41(75) have been blocked by the non-framework
octahedral aluminum and zinc oxides that are completely
removed after washing with dichloromethane. After third and
fourth runs, the 2,4-DTBP conversion as well as 6,8-DTBF
selectivity remains constant, indicating that the incorporated or
substituted active species could not be leached on the mesopor-
ous silica matrix, which is in good agreement with ICP-AES
results of Zn–Al–MCM-41(75) obtained after the recyclable
2
3
4
E. Armengol, M. L. Cano, A. Corma, H. Garcia and M. T. Navarro,
J. Chem. Soc., Chem. Commun., 1995, 519.
C. T. Kresge, M. E. Leonowicz, M. E. Leonowicz, W. J. Roth,
J. C. Vartuli and J. S. Beck, Nature, 1992, 359, 710.
P. Van Der Voort, M. Baltes and E. F. Vansant, Catal. Today, 2001, 68,
119.
5 A. Corma, Chem. Rev., 1997, 97, 2373.
6
A. Corma, V. Forance, M. T. Navarro and J. Perez-Parients, J. Catal.,
994, 148, 569.
1
7
M. Selvaraj, A. Pandurangan, K. S. Seshadri, P. K. Sinha,
V. Krishnasamy and K. B. Lal, J. Mol. Catal. A: Chem., 2002, 186, 173.
14202 | Dalton Trans., 2012, 41, 14197–14203
This journal is © The Royal Society of Chemistry 2012

View Online
8
9
T. R. Pauly, Y. Liu, T. J. Pinnavaia, S. J. L. Billinge and T. P. Rieker,
J. Am. Chem. Soc., 1999, 121, 8835.
Q. Yang, Y. Li, L. Zhang, J. Yang, J. Liu and C. Li, J. Phys. Chem. B,
17 M. Selvaraj and P. K. Sinha, J. Mol. Catal. A: Chem., 2007, 264,
44.
18 M. Selvaraj and S. Kawi, J. Mol. Catal. A: Chem., 2007, 265,
250.
19 M. Selvaraj and S. Kawi, Microporous Mesoporous Mater., 2008, 109,
458.
2
004, 108, 7934.
0 Q. Yang, J. Yang, Z. Feng and Y. Li, J. Mater. Chem., 2005, 15, 4268.
1 S. K. Das, M. K. Bhunia, A. K. Sinha and A. Bhaumik, J. Phys. Chem.
1
1
C, 2009, 113, 8918.
2 P. K. Thallapally, C. A. Fernandez, R. K. Motkuri, S. K. Nune, J. Li and
20 M. Selvaraj, S. W. Song and S. Kawi, Microporous Mesoporous Mater.,
2008, 110, 472.
1
1
1
1
1
C. H. F. Peden, Dalton Trans., 2010, 39, 1692.
3 M. Fujiwara, A. Sakamoto, K. Shiokawa, A. K. Patra and A. Bhaumik,
Microporous Mesoporous Mater., 2011, 142, 381.
21 M. Selvaraj, PhD thesis, Anna University, Tamil Nadu, India, 2002.
22 E. A. Gunnewegh, S. S. Gopie and H. v. Bekkum, J. Mol. Catal. A:
Chem., 1996, 106, 151.
23 J. Medina-Valtieera, M. A. Sanchez, J. A. Montoya, J. Navarrete and
J. A. de los Reyes, Appl. Catal., A, 1997, 158, L1.
24 R. A. Sheldon, M. Wallau, I. W. C. E. Arends and U. Schuchardt, Acc.
Chem. Res., 1998, 31, 485.
4 J.-L. Niu, X.-Q. Hao, J.-F. Gong and M.-P. Song, Dalton Trans., 2011,
4
0, 5135.
5 M. Selvaraj, A. Pandurangan, K. S. Seshadri, P. K. Sinha and K. B. Lal,
Appl. Catal., A, 2003, 242, 347.
6 M. Selvaraj, B. R. Min, Y. G. Shul and T. G. Lee, Microporous Meso-
porous Mater., 2004, 74, 143.
25 J. Dedecek, N. Zilkova and J. Cejka, Microporous Mesoporous Mater.,
2001, 44–45, 259.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 14197–14203 | 14203