Nanocrystalline ZSM-5 based bi-functional catalyst for two step and three step tandem reactions

Article abstract of DOI:10.1039/c4ra16697c

A Pd nanoparticle decorated nanocrystalline ZSM-5 catalyst was prepared for one-pot tandem reactions. The catalyst was characterized by the complementary combination of X-ray diffraction, N₂-adsorption, electron microscopy, and NH₃-temperature programmed desorption techniques. The catalyst was investigated in the one-pot tandem conversion of benzyl alcohol to 1,3-diphenyl-3-(phenylamino)propan-1-one, (E)-chalcone, and 2,3-dihydro-1,5-benzothiazepine. 1,3-Diphenyl-3-(phenylamino)propan-1-one and (E)-chalcone were prepared by two step tandem reactions from benzyl alcohol, whereas 2,3-dihydro-1,5-benzothiazepine was prepared by a three step tandem reaction from benzyl alcohol. A recycling study shows that no significant decrease in the catalytic activity was observed even after three recycles. To the best of our knowledge, this is the first report which deals with such a simple route to prepare 2,3-dihydro-1,5-benzothiazepines in a one-pot tandem methodology from benzyl alcohol.

Full text of DOI:10.1039/c4ra16697c

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Nanocrystalline ZSM-5 based bi-functional catalyst
for two step and three step tandem reactions
Cite this: RSC Adv., 2015, 5, 25998
a
Rajendra Srivastava, Bhaskar Sarmaha and Biswarup Satpatib
*
A Pd nanoparticle decorated nanocrystalline ZSM-5 catalyst was prepared for one-pot tandem reactions. The
catalyst was characterized by the complementary combination of X-ray diffraction, N2-adsorption, electron
microscopy, and NH3-temperature programmed desorption techniques. The catalyst was investigated in
the one-pot tandem conversion of benzyl alcohol to 1,3-diphenyl-3-(phenylamino)propan-1-one, (E)-
chalcone, and 2,3-dihydro-1,5-benzothiazepine. 1,3-Diphenyl-3-(phenylamino)propan-1-one and (E)-
chalcone were prepared by two step tandem reactions from benzyl alcohol, whereas 2,3-dihydro-1,5-
benzothiazepine was prepared by a three step tandem reaction from benzyl alcohol. A recycling study
shows that no significant decrease in the catalytic activity was observed even after three recycles. To the
best of our knowledge, this is the first report which deals with such a simple route to prepare 2,3-dihydro-
1,5-benzothiazepines in a one-pot tandem methodology from benzyl alcohol.
Received 19th December 2014
Accepted 4th March 2015
DOI: 10.1039/c4ra16697c
www.rsc.org/advances
zeolite is not suitable for the development of multi-functional
catalyst because it doesn't have large external surface area and
1. Introduction
surface silanol groups, which are important to incorporate cata-
lytic functionality. Surface silanol groups present at large external
surface of nanocrystalline/hierarchical zeolites are required for the
development of multi-functional catalysts. Several pioneers in
zeolite science have developed synthesis strategy to prepare
hierarchical/mesoporous/nanocrystalline zeolites.33–38 Top-down
and bottom-up approaches were used to prepare such zeolite
materials.39 Our research group also developed several organic
structure directing agents based on piperidine, imidazole, and 1,4-
diazabicyclo[2.2.2]octane that acted as structure directing agents
for the synthesis of zeolites of different framework structure.40–44
The objective of this study was to design and develop nano-
crystalline ZSM-5 based bi-functional catalysts for two steps and
three steps tandem reactions. Pd nanoparticles were supported
on the surface of acidic Nano-ZSM-5 to obtain an efficient bi-
functional catalyst. Catalyst was investigated in the one-pot
synthesis of 1,3-diphenyl-3-(phenylamino)propan-1-one (here-
aer represented as DPO), (E)-chalcone, and 2,3-dihydro-1,5-
benzothiazepines (hereaer represented as DBT). To the best of
our knowledge, this is the rst report, which deals with the
synthesis of 2,3-dihydro-1,5-benzothiazepines in one-pot tandem
methodology. In conventional practice, three steps and three
catalysts are needed to obtain 2,3-dihydro-1,5-benzothiazepines.
Development of heterogeneous catalytic processes for ne and
specialty chemicals is an important inter-disciplinary research
objective for industrialists and academicians.1–8 Several multi-
functional heterogeneous catalysts that catalyze more than one
organic transformation have been reported.9–16 A unique green
chemical approach for increasing the efficiency of a catalytic
process is to carry out more than one catalytic transformation in
a ‘one-pot’ process without isolating any of the intermediates.
Such tandem or cascade reactions need to be carefully planned to
ensure that the correct sequence of reactions is followed in the
right order to achieve the target product.17–21 The design of a
suitable multi-functional catalyst is the key for a successful
tandem reaction. For the development of a multi-functional
catalyst, it is important to choose a suitable support material
that itself possess some catalytic activity and also has the ability
to incorporate more catalytic functionalities. Our research is
based on the development of ionic liquid and zeolite based
catalytic process.22–28 It is possible to develop ionic liquid and
zeolite based multi-functional catalysts but due to their easy
recovery and efficient recycling, zeolite based bi-functional cata-
lysts were developed for tandem reactions in this study.
Zeolites play important role in catalysis, adsorption, and
separation processes, because of well-dened porous crystalline
structures, unique channel systems of molecular dimension, and
tunable chemical compositions.29–32 Conventional microporous
2. Experimental
2.1. Catalyst synthesis
aDepartment of Chemistry, Indian Institute of Technology Ropar, Rupnagar-140001,
India. E-mail: rajendra@iitrpr.ac.in; Fax: +91-1881-223395; Tel: +91-1881-242175
bSurface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF,
Bidhannagar, Kolkata 700 064, India
Tetraethylorthosilicate (TEOS), sodium aluminate (53 wt% Al2O3,
43 wt% Na2O), tetrapropylammonium hydroxide (TPAOH) (1 M
aqueous solution) and propyltriethoxy silane (PrTES) were used
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in the synthesis of zeolite. Nano-ZSM-5 with Si/Al ¼ 20 (input) Material was centrifuged and dried in vacuum to obtain Pd
was prepared using molar composition of 90TEOS/10 PrTES/2.5 nanoparticles supported on Nano-ZSM-5. In this study, three
Al2O3/3.3 Na2O/25 TPAOH/2500 H2O by following the reported different catalysts denoted as Pd (0.5%)/Nano-ZSM-5, Pd
procedure.45,46 In a typical synthesis of nanocrystalline zeolite (1%)/Nano-ZSM-5, and Pd (2%)/Nano-ZSM-5 were prepared. For
(Nano-ZSM-5), 1.44 g sodium aluminate (53 wt% Al2O3, 43 wt% comparative study, Pd (1%)/ZSM-5 was also prepared. Amount
Na2O) was dissolved in 75 mL distilled water (Solution A). 6.39 g of Pd incorporated in the resultant materials was conrmed
PrTES was mixed with 75 mL of aqueous TPAOH solution using ICP analysis, which is given in Table 1.
(Solution B). Solution A and Solution B were mixed, and the
resultant solution was stirred for 15 minutes at ambient
temperature, until it became a clear solution. 57.39 g TEOS was
added into the resultant solution and stirring was continued for
6 h. This mixture was transferred to a Teon-lined autoclave, and
hydrothermally treated at 443 K for 4 days under static condition.
The nal product was ltered, washed with distilled water, and
dried at 373 K. Material was calcined at 823 K for 6 h under
owing air.
5 g of obtained zeolite was then added to 200 mL, 1 M
aqueous solution of NH4NO3 and stirred at 343 K for 6 h. The
resultant product was ltered and washed with distilled water
several times. The ion-exchange process was repeated thrice.
The nal product was then dried and calcined at 823 K for 6 h.
For comparative study, conventional ZSM-5 with Si/Al ¼ 20
(input) was prepared using molar composition 100TEOS/2.5
Al2O3/3.3 Na2O/25 TPAOH/2500 H2O. Bulk Si/Al ratios of the
calcined Nano-ZSM-5 and ZSM-5 are given in Table 1.
0.5 g of Nano-ZSM-5 was added into 3 mL of toluene solution
containing desired amount of Pd(OAc)2 (for example; for 1 wt%
sample, 10.5 mg of Pd(OAc)2 was added). Aer stirring for 2 h at
ambient temperature, the solid was centrifuged, washed with
toluene, and dried in oven. Pd2+ species adsorbed on the
resultant Nano-ZSM-5 was reduced with desired amount of
NaBH4 (for example; for 1 wt% sample, 7 mg of NaBH4 was
added) in a 10 mL solution of toluene and ethanol (v/v ¼ 20 : 1).
2.2. Catalyst characterizations
X-ray diffraction (XRD) patterns were recorded in the 2q range of
5–60ꢀ with a scan speed of 2ꢀ minꢁ1 on a PANalytical X'PERT
PRO diffractometer using Cu Ka radiation (l ¼ 0.1542 nm,
40 kV, 40 mA) and a proportional counter detector. Nitrogen
adsorption measurements were performed at 77 K by Quan-
tachrome Instruments, Autosorb-IQ volumetric adsorption
analyzer. Sample was out-gassed at 573 K for 3 h in the degas
port of the adsorption apparatus. The specic surface area of
the material was calculated from the adsorption data points
obtained at P/P0 between 0.05 and 0.3 using the Brunauer–
Emmett–Teller (BET) equation. The pore diameter was esti-
mated using the non-localized density functional theory
method (DFT). Scanning electron microscopy (SEM) measure-
ments were carried out on a JEOL JSM-6610LV to investigate the
morphology of the zeolites. In depth structural analysis were
carried out using FEI, TF30-ST transmission electron micro-
scope (TEM) operating at 300 kV equipped with a GATAN Orius
CCD camera. The TEM is also equipped with a scanning unit
and a high-angle annular dark eld (HAADF) detector from
Fischione (model 3000). The compositional analysis was per-
formed using energy dispersive X-ray (EDX, EDAX Inc.) spec-
troscopy attachment on the TF30. The samples were dispersed
in ethanol using ultrasonic bath, and dispersed sample was
mounted on a carbon coated Cu grid, dried, and used for TEM
Table 1 Physico-chemical properties of various catalysts investigated in this study
Physical properties
Total surface area SBET
External surface area
Total pore volume
Sample
(m2 gꢁ1
)
(m2 gꢁ1
)
(cm3 gꢁ1
)
ZSM-5
Nano-ZSM-5
Pd (1%)/Nano-ZSM-5
Pd (1%)/ZSM-5
Pd (0.5%)/Nano-ZSM-5
Pd (2%)/Nano-ZSM-5
297
502
446
258
478
437
52
253
218
29
240
206
0.21
0.46
0.38
0.18
0.41
0.36
Chemical properties
Sample
Bulk Si/Al
ratio
Pd content
wt%
Total acid sites NH3 desorbed
Weak acid sites
Strong acid sites
(mmol gꢁ1
)
(mmol gꢁ1
)
(mmol gꢁ1
)
ZSM-5
Nano-ZSM-5
Pd (1%)/Nano-ZSM-5
Pd (0.5%)/Nano-ZSM-5
Pd (2%)/Nano-ZSM-5
Pd (1%)/ZSM-5
20.8
21.6
22.1
—
—
21.0
—
—
0.94
0.46
1.90
0.95
0.77
0.66
0.65
0.66
0.63
0.75
0.39
0.38
0.38
0.38
0.36
0.38
0.38
0.28
0.27
0.28
0.27
0.37
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measurement. For NH3-temperature programmed desorption n-dodecane (0.5 mmol, internal standard), and toluene (5 mL)
techniques (TPD) experiments, the catalyst sample was pre- were heated at 358 K for 9 h, during which oxygen was charged
treated in He (50 cm3 minꢁ1) at 873 K for 1 h. Aer cooling down into the reaction mixture at ow rate of 10 mL minꢁ1. Aer the
to 373 K, ammonia (partial pressure 100 Torr) was passed reaction, the oxygen ow was stopped and the reaction mixture
through the samples for 1 h. Then, the sample was subse- was brought to ambient temperature, and then aniline (1 mmol,
quently ushed by He stream (50 cm3 minꢁ1) at 373 K for 1 h to 0.093 g) and acetophenone (1 mmol, 0.12 g) was charged into
remove physisorbed ammonia. TPD experiments were carried the mixture rapidly. The mixture was le at ambient tempera-
out in the range of 373–1073 K at a heating rate of 10 K minꢁ1
The ammonia concentration in the effluent was monitored by catalyst was removed by centrifugation, the reaction products
.
ture (approximately 303 K) for 7 h. Aer the reaction, solid
using a gold-plated, lament thermal conductivity detector.
were analyzed by GC and GC-MS.
The one-pot tandem oxidation–condensation reaction for
the synthesis of (E)-chalcone was carried out in a glass ask
equipped with a condenser. In a typical run, benzyl alcohol
2.3. Procedure of catalytic reactions
2.3.1. Oxidation of benzyl alcohol to benzaldehyde. In a (1 mmol), catalyst (100 mg), n-dodecane (0.5 mmol, internal
typical synthesis, benzyl alcohol (1 mmol), catalyst (100 mg), standard), and toluene (5 mL) were heated at 358 K for 9 h,
n-dodecane (0.5 mmol, internal standard) and toluene (5 mL) during which oxygen was charged into the reaction mixture at
were magnetically stirred (at 500 rpm) and heated at 358 K for ow rate of 10 mL minꢁ1. Aer the reaction, the oxygen ow was
9 h, during which oxygen was charged into the reaction mixture stopped and toluene was removed by distillation. Acetophenone
at ow rate of 10 mL minꢁ1. Catalyst was centrifuged and the
reaction mixture was analyzed using gas chromatography (GC)
(Yonglin 6100; BP-5; 30 m ꢂ 0.25 mm ꢂ 0.25 mm). Reactant
conversion and product selectivity were obtained using GC.
Product was conrmed using GC-MS (Shimadzu GCMS-QP 2010
Ultra; Rtx-5 Sil Ms; 30 m ꢂ 0.25 mm ꢂ 0.25 mm). Product was
also conrmed by the authentic samples obtained from Aldrich.
2.3.2. Synthesis of 1,3-diphenyl-3-(phenylamino)propan-1-
one. Aniline (4 mmol, 0.37 g), benzaldehyde (4 mmol, 0.42 g),
acetophenone (4 mmol, 0.48 g), toluene (5 mL) and catalyst
(50 mg) were stirred at ambient temperature (approx. 303 K) for
7 h. Aer the reaction, product was separated by column
chromatography using 5% ethyl acetate/hexane solvent system.
Product,
1,3-diphenyl-3-(phenylamino)propan-1-one
was
1
conrmed using H NMR and GC-MS.
1H NMR (CDCl3): 7.91–7.90 (m, 2H), 7.57–7.07 (m, 10H),
6.67–6.55 (m, 3H), 5.00 (dd, 1H), 4.55 (br, 1H), 3.50 (dd, 1H),
3.42 (dd, 1H).
2.3.3. Synthesis of (E)-chalcone. Benzaldehyde (0.16 g,
1.5 mmol), acetophenone (0.18 g, 1.5 mmol), n-dodecane
(0.5 mmol, internal standard), and catalyst (50 mg) were mixed
and stirred for 7 h at 323 K. Aer the reaction, catalyst was
separated and the reaction mixture was analyzed using GC.
Products were conrmed using GC-MS. Product was also
conrmed by the authentic sample obtained from Aldrich.
2.3.4. Synthesis of 2,3-dihydro-benzothiazepine. E-Chal-
cone (0.21 g, 1 mmol), 2-aminothiophenol (0.15 g, 1.2 mmol),
catalyst (50 mg) and 8 mL toluene were charged into a 50 mL
round-bottom ask equipped with a magnetic stirrer and
condenser. The reaction was conducted at 383 K for 2 h. Aer
the reaction, product was separated by column chromatography
using 5% ethyl acetate/hexane solvent system. Product, dihydro-
1,5-benzothiazepines was conrmed using 1H NMR and GC-MS.
1H NMR (CDCl3): 8.25 (d, 2H), 7.65–7.75 (m, 5H), 7.30–7.55
(m, 7H), 5.20 (dd, 1H), 3.50 (dd, 1H), 3.25 (t, 1H).
2.3.5. One-pot two-steps tandem reactions. The one-pot
tandem oxidation–condensation reaction for the synthesis of
DPO was carried out in a glass ask equipped with a condenser.
Fig. 1 (a) XRD patterns of various catalysts investigated in this study.
(b) N2-adsorption isotherms of various catalysts investigated in this
In a typical run, benzyl alcohol (1 mmol), catalyst (100 mg), study. Inset shows pore size distribution.
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equipped with a condenser. In a typical run, benzyl alcohol
(1 mmol), catalyst (100 mg), n-dodecane (0.5 mmol, internal
standard), and toluene (5 mL) were heated at 358 K for 9 h,
during which oxygen was charged into the reaction mixture at
ow rate of 10 mL minꢁ1. Aer the reaction, the oxygen ow was
stopped and toluene was removed by distillation. Acetophenone
(0.12 g, 1.0 mmol) was charged into the mixture rapidly. Reaction
mixture was stirred for 7 h at 373 K. Aer 7 h, 2-aminothiophenol
(0.15 g, 1.2 mmol) and 8 mL toluene were charged into the
mixture rapidly. The reaction was conducted at 383 K for 2 h.
Aer the reaction, solid catalyst was removed by centrifugation,
the reaction products were analyzed by GC and GC-MS.
Fig. 2 SEM micrographs of (a) Nano-ZSM-5 and (b) Pd (1%)/Nano-
ZSM-5.
(0.12 g, 1.0 mmol) was charged into the mixture rapidly. Reac-
tion mixture was further stirred for 7 h at 373 K. Aer the
reaction, solid catalyst was removed by centrifugation, the
reaction products were analyzed by GC and GC-MS.
3. Results and discussion
3.1. Physico-chemical characterization
2.3.6. One-pot three steps tandem reaction for the
synthesis of dihydro-1,5-benzothiazepine. One-pot, three steps
tandem oxidation–condensation reactions for the synthesis of
dihydro-1,5-benzothiazepine was carried out in a glass ask
Nano-ZSM-5 and ZSM-5 exhibited MFI framework structure
with high phase purity, which was conrmed by XRD (Fig. 1a).
XRD pattern of Nano-ZSM-5 was broad when compared to
ZSM-5, which conrms that the material is nanocrystalline in
nature. XRD pattern of Pd (1%)/Nano-ZSM-5 was found to be
similar to Nano-ZSM-5 (Fig. 1b). No separate phase corre-
sponding to Pd nanoparticles was observed in the XRD pattern,
which conrms that Pd nanoparticles are uniformly dispersed
in Nano-ZSM-5 matrix. Textural properties of the material were
investigated by nitrogen adsorption–desorption measurements
(Fig. 1b). Nano-ZSM-5 and Pd (1%)/Nano-ZSM-5 exhibited type-
IV isotherms similar to that of mesoporous materials (Fig. 1b).
The major difference in the isotherm of Nano-ZSM-5/Pd (1%)/
Nano-ZSM-5 when compared to ZSM-5 is a distinct increase of
N2-adsorption in the region 0.4 < P/P0 < 0.95, which is inter-
preted as a capillary condensation in the inter-crystalline mes-
opore void spaces. Bi-modal pore size distribution was obtained
for Nano-ZSM-5 samples using non-localized DFT method,
which conrms the presence of ZSM-5 micropores and inter-
crystalline mesopores. Mesopores show a pore size distribu-
tion in the range of 2–10 nm in these materials. Textural
Fig. 3 (a and b) Low magnification and (c and d) high magnification
TEM images of Pd (1%)/Nano-ZSM-5. (e) STEM-HAADF image of Pd
(1%)/Nano-ZSM-5, (f) EDX spectrum from a region marked by red area
1 in (e) and (g) elemental maps taken from the area marked by an
orange box 2 indicating the locations of different atoms across the
structure.
Fig. 4 NH3-TPD profiles of ZSM-5 and Nano-ZSM-5.
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Scheme 1 One-pot tandem conversion of benzyl alcohol to 1,3-diphenyl-3-(phenylamino)propan-1-one, (E)-chalcone, and 2,3-dihydro-1,5-
benzothiazepine.
properties obtained from N2-adsorption study are summarized ZSM-5 particles. The elemental presence of Si, O, Al, and Pd
in Table 1.
are shown in EDX spectrum in Fig. 3f, which is from the rect-
Uniform spherical particles were observed for Nano-ZSM-5 angular area 1 marked in Fig. 3e. For a detailed distribution of
(Fig. 2a). Morphology of Pd (1%)/Nano-ZSM-5 was found to be atomic contents in the materials, elemental mapping was per-
similar to Nano-ZSM-5 (Fig. 2b). To obtain the in-depth infor- formed using STEM-HAADF-EDX technique (Fig. 3g). Based on
mation about nano-structure of bi-functional catalyst Pd the TEM imaging and elemental mapping, it can be concluded
(1%)/Nano-ZSM-5, TEM investigations were made. TEM images that Pd nanoparticles decorated Nano-ZSM-5 material was
clearly show that Pd nanoparticles are uniformly dispersed on successfully prepared.
the surface of Nano-ZSM-5 (Fig. 3a–c). Size of Pd nanoparticles
Acidity of the sample was investigated using NH3-TPD
was in the range of 5–10 nm as shown in the magnied TEM (Fig. 4). The total acidity decreases in the order: ZSM-5 z Pd
image in Fig. 3d from area shown in Fig. 3b. TEM images also (1%)/ZSM-5 > Nano-ZSM-5 z Pd (1%)/Nano-ZSM-5 (Table 1). In
conrmed that the large spherical Nano-ZSM-5 particles were the TPD proles of ZSM-5 and Nano-ZSM-5, two well-resolved
built with small zeolite nanocrystals of 15–20 nm (Fig. 3c). To symmetric peaks were observed. They dominate with
investigate the chemical composition of the dots and maximum at 460 K & 630 K for ZSM-5 and 455 K & 650 K for
surrounding matrix, Scanning TEM (STEM)-HAADF-EDX anal- Nano-ZSM-5. These peaks demonstrate the presence of weak
ysis was performed (Fig. 3e–g). In the HAADF image (Fig. 3e), acid sites and strong acid sites. In Nano-ZSM-5, the rst peak
the Z-contrast image shows Pd particles brighter than Nano- becomes smaller and the maxima of second peak shis to
Table 2 Catalytic activity data obtained for the oxidation of benzyl alcohol over various catalysts investigated in this studyd
S. no.
Catalyst
Catalyst amount
Atmosphere
Temp. (K)
Yielda (%)
Average TONc
1
2
3
4
5
6
7
8
Pd (1%)/Nano-ZSM-5
Pd (1%)/Nano-ZSM-5
Pd (1%)/Nano-ZSM-5
Pd (1%)/Nano-ZSM-5
Pd (1%)/Nano-ZSM-5
Pd (1%)/Nano-ZSM-5
Pd (1%)/Nano-ZSM-5
Pd (0.5%)/Nano-ZSM-5
Pd (2%)/Nano-ZSM-5
Pd (1%)/Nano-ZSM-5b
Pd (1%)/ZSM-5
100 mg
150 mg
50 mg
O2
O2
O2
Air
O2
O2
O2
O2
O2
O2
O2
358
358
358
383
313
333
383
358
358
358
358
80
98
49
43
8
46
96
44
96
81
55
91
74
111
49
9
52
109
102
57
92
63
100 mg
100 mg
100 mg
100 mg
100 mg
100 mg
100 mg
100 mg
9
10
11
a
b
c
Yield ¼ conversion obtained from GC (because benzaldehyde was obtained as only product). Data obtained aer third recycle. Average
d
TON ¼ turnover number [moles of benzyl alcohol converted per mole of active Pd]. Reaction condition: benzyl alcohol (1 mmol), time (9 h),
O2 ow (10 mL minꢁ1), toluene (5 mL), n-dodecane (0.5 mmol, internal standard).
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slightly higher temperature (650 K). This indicates that the acid of DPO, (E)-chalcone, and DBT from benzaldehyde. Role of
sites in Nano-ZSM-5 are stronger than the acid sites present in these catalysts was investigated in the synthesis of DPO by
ZSM-5.
following the Mannich condensation of aniline with benzalde-
During the synthesis of Nano-ZSM-5, additive propyltriethox- hyde and acetophenone (Table 3). Signicantly low product
ysilane disfavoured the extended growth of large zeolite crystal, yield was obtained in the absence of catalyst under the opti-
which resulted in the formation of very small size Nano-ZSM-5. mized reaction condition. When ZSM-5 was used as catalyst,
Mesopores were created by the crystal packing of these zeolite only slight increase in the DPO yield was observed. Nano-ZSM-5
nanocrystals. Mesopore size distribution obtained from N2- produced very good DPO yield under mild reaction condition.
adsorption studies conrmed that they were in the range of 2–10 Pd (1%)/Nano-ZSM-5 produced similar DPO yield when
nm. Based on SEM, TEM and N2-adsorption studies, one can
conclude that Pd nanoparticles decorated spherical Nano-ZSM-5
was prepared, and the spherical Nano-ZSM-5 is composed of
Table 3 Catalytic activity data obtained in the condensation reactions
several zeolite nanoparticles of approximately 15–20 nm.
investigated in this study under optimized reaction conditiong
Synthesis of DPO
3.2. Catalytic investigation
The aim of this study was to develop zeolite based catalysts in S. no.
the one-pot tandem conversion of benzyl alcohol to DPO,
Catalyst
DPO yielda (%)
1
None
ZSM-5
Nano-ZSM-5
Pd (1%)/Nano-
ZSM-5
5.7
13.4
88
(E)-chalcone, and DBT (Scheme 1). DPO and (E)-chalcone can be
prepared in two steps synthesis protocol from benzyl alcohol,
2
3
whereas synthesis of DBT requires three steps from benzyl
alcohol. The rst step of these reactions is the oxidation of
benzyl alcohol to benzaldehyde. The rst step requires an
oxidation catalyst. Pd nanoparticles are known to catalyze the
oxidation of benzyl alcohol.47–49 Therefore, Pd nanoparticles
supported Nano-ZSM-5 and ZSM-5 based catalysts were
prepared and investigated in the oxidation of benzyl alcohol in
the rst step of the reaction (Table 2). In all the cases, benzal-
dehyde was obtained as selective product. Amount of catalyst,
reaction temperature, and reaction atmosphere play important
role in the oxidation of benzyl alcohol. Reaction proceeds well
in O2 atmosphere using 100 mg of catalyst per mmol of
substrate in 9 h at 358 K. Aer the reaction, catalyst was
removed and the reaction mixture was subjected to ICP anal-
ysis. Pd was not detected in the ICP analysis. Pd (1%)/Nano-
ZSM-5 was reused several times in the oxidation reaction of
benzyl alcohol. Aer each experiment, catalyst was washed with
toluene and dried in oven and then used in the next cycle.
Activity of Pd (1%)/Nano-ZSM-5 was remained the same even
aer three recycles (Table 2). Heterogeneity and Pd-leaching of
this catalyst was also examined by the ‘hot ltration’ test.
Catalyst was removed from the reaction mixture aer 4 h (49%
benzyl alcohol conversion) then the reaction was continued for
an additional 5 h gave a nal benzyl alcohol conversion of
49.6%. This experiment further conrmed that the Pd was not
leached during the reaction. Activity of Pd (1%)/Nano-ZSM-5
was found to be more when compared to Pd (1%)/ZSM-5.
These results show that highly dispersed Pd nanoparticles
were formed on the high surface area Nano-ZSM-5, which is
responsible for the high catalytic activity of Pd (1%)/Nano-ZSM-
5. Furthermore, enhanced diffusion provided by inter-
crystalline mesopores of Nano-ZSM-5 is also responsible for
the high activity of Pd (1%)/Nano-ZSM-5.
4
82
5
6
Pd (1%)/ZSM-5
12.6
85.7
Nano-ZSM-5f
Synthesis of E-chalcone
S. no.
Catalyst
(E)-Chalcone yieldb (%)
1
2
3
4
None
ZSM-5
Nano-ZSM-5
Pd (1%)/Nano-
ZSM-5
<1
6.8
30.5
29.1
5
6
7
8
9
Pd (1%)/ZSM-5
Nano-ZSM-5c
Nano-ZSM-5d
Nano-ZSM-5e
Pd (1%)/Nano-
ZSM-5e
5.3
55.8
60.5
84.5
82.3
10
Nano-ZSM-5e,f
85.1
Synthesis of DBT
S. no.
Catalyst
DBT yielda (%)
1
2
3
4
None
ZSM-5
Nano-ZSM-5
Pd (1%)/Nano-
ZSM-5
15.2
28.3
95
93
5
6
Pd (1%)/ZSM-5
24.7
93.4
Nano-ZSM-5f
a
b
Yield ¼ isolated yield. Yield ¼ conversion obtained from GC
c
(E-chalcone was obtained as only product). Catalyst (100 mg).
d
f
e
Temperature (373 K). Catalyst (100 mg), temperature (373 K).
Data obtained aer third recycle. Reaction condition: synthesis of
g
DPO: benzaldehyde (4 mmol), acetophenone (4 mmol), aniline
(4 mmol), catalyst (50 mg), toluene (5 mL), temperature (303 K),
time (7 h). Synthesis of (E)-chalcone: benzaldehyde (1.5 mmol),
acetophenone (1.5 mmol), catalyst (50 mg), temperature (323 K),
time (7 h). Synthesis of DBT: (E)-chalcone (1 mmol),
2-aminothiophenol (1.2 mmol), catalyst (50 mg), toluene (8 mL),
temperature (383 K), time (2 h).
Before the investigation of one-pot tandem reaction starting
with benzyl alcohol using bi-functional catalysts, it would be
interesting to obtain catalytic data using ZSM-5, Nano-ZSM-5,
Pd (1%)/Nano-ZSM-5, and Pd (1%)/ZSM-5 for the production
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Table 4 Catalytic activity data in the one-pot, two steps tandem reactions
Oxidation–Mannich condensation
Product yield (%)
Benzaldehyde
S. no.
Catalyst
Time (T1 + T2)
Benzyl alcohol conv. (%)
DPO
1
2
3
Nil
9 + 7
9 + 7
9 + 7
0
54.6
81.8
—
35.7
6.7
—
18.9
75.1
Pd (1%)/ZSM-5
Pd (1%)/Nano-
ZSM-5
Oxidation–aldol condensation
Product yield (%)
Benzaldehyde
S. no.
Catalyst
Time (T1 + T2)
Benzyl alcohol conv. (%)
E-Chalcone
1
2
3
Nil
9 + 7
9 + 7
9 + 7
0
55.3
81.4
—
45.9
7.8
—
9.4
73.6
Pd (1%)/ZSM-5
Pd (1%)/Nano-
ZSM-5
compared to Nano-ZSM-5. This clearly shows that Pd has no role the diffusion of reactant to the acid sites and diffusion of the
to play in the Mannich reaction. The role of these catalysts was product from the active sites, which is responsible for the high
investigated in the synthesis of (E)-chalcone by the aldol activity.
condensation of benzaldehyde and acetophenone (Table 3). In
The catalytic activity of Pd (1%)/Nano-ZSM-5 and Pd
the absence of catalyst, no reaction took place under the opti- (1%)/ZSM-5 in the one-pot tandem synthesis of DPO/(E)-chal-
mized reaction condition. ZSM-5 exhibited signicantly low cone from benzyl alcohol was studied. The results are listed in
activity when compared to Nano-ZSM-5. Activity of Nano-ZSM-5 Table 4. The one-pot synthesis of DPO was realized through
in the synthesis of (E)-chalcone was low when compared to DPO. the oxidation of benzyl alcohol followed by Mannich conden-
The activity of Pd (1%)/Nano-ZSM-5 was found to be similar to sation with aniline and acetophenone. The one-pot synthesis
Nano-ZSM-5. Activity of Nano-ZSM-5 in the synthesis of of (E)-chalcone was realized through the oxidation of benzyl
(E)-chalcone can be improved by performing the reaction at alcohol followed by aldol condensation with acetophenone.
373 K or using more amount of catalyst (Table 3, compare Benzaldehyde was produced as an intermediate and DPO/(E)-
entry no. 6–8 with entry 3). Furthermore, when (E)-chalcone was chalcone was the nal product. Reaction did not take place in
reacted with 2-aminothiophenol, it formed DBT in the presence the absence of catalyst. Furthermore, DPO/(E)-chalcone was
of various catalysts investigated in this study. Reaction pro- not formed using Nano-ZSM-5 catalyst in one-pot tandem
ceeded even in the absence of catalyst. However, product yield synthesis from benzyl alcohol. In one-pot tandem synthesis of
was very low. Product yield was improved when ZSM-5 was used DPO/(E)-chalcone, Pd (1%)/Nano-ZSM-5 was found to exhibit
as catalyst. The activity of Nano-ZSM-5 was found to be excep- very good catalytic activity. Excellent benzyl alcohol conversion
tionally high in the synthesis of DBT when compared to DPO and very good selectivity for DPO/(E)-chalcone was achieved
and (E)-chalcone (Table 3). The activity of Pd (1%)/Nano-ZSM-5 using Pd (1%)/Nano-ZSM-5 when compared to Pd (1%)/ZSM-5.
was found to be similar to that of Nano-ZSM-5 (Table 3). To the
Finally, one-pot synthesis of DBT was realized through the
best of our knowledge, we could nd only one report in which oxidation of benzyl alcohol followed by aldol condensation with
wide range of acid catalysts were reported for the synthesis of acetophenone and the reaction of (E)-chalcone with 2-amino-
DBT.50 The activity of Nano-ZSM-5 was found to be comparable thiophenol (Table 5). Benzaldehyde was produced as rst
or better when compared to these catalysts reported in the intermediate and (E)-chalcone was the second intermediate. At
literature.50 Recycling study conrmed that Nano-ZSM-5 was the end of the reaction, four products were obtained in the GC
found to be reusable and exhibited almost similar activity even analysis. They were remaining amounts of benzaldehyde and
aer three recycles (Table 3). Activity of Nano-ZSM-5 was found (E)-chalcone. Apart from the desired product DBT, one addi-
to be more than ZSM-5 in the acid catalyzed condensation tional product 2-phenylbenzothiazole was also obtained. 2-
reactions described above. This difference in the activity can be Phenylbenzothiazole was formed by the reaction of benzalde-
correlated to high surface area and inter-crystalline mesopores. hyde and 2-aminothiophenol. No reaction took place in the
In the case of Nano-ZSM-5, the acid sites located at the pore- absence of catalyst. In contrast to the synthesis of DBT from
mouth or little bit farther inside the channels (which may be benzaldehyde, in this one-pot tandem synthesis from benzyl
called subsurface centers) are the real centers of the reaction. alcohol, DBT was not formed in the presence of Nano-ZSM-5. In
Inter-crystalline mesopores present in the Nano-ZSM-5 facilitate one-pot tandem synthesis of DBT, Pd (1%)/Nano-ZSM-5
26004 | RSC Adv., 2015, 5, 25998–26006
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Table 5 Catalytic activity data in one-pot, three steps tandem reaction
Product yield (%)
Benzaldehyde
Time
(T1 + T2 + T3) (h)
Benzyl alcohol
conv. (%)
S. no.
Catalyst
(E)-Chalcone
DBT
Othera
1
2
3
4
Nil
9 + 7 + 2
9 + 7 + 2
9 + 7 + 2
9 + 7 + 2
0
—
—
—
67.0
4.3
—
Pd (1%)/Nano-ZSM-5
Pd (1%)/ZSM-5
Pd (1%)/Nano-ZSM-5b
80.6
55.1
81.2
0.8
16.8
1.0
1.4
5.4
1.9
11.4
28.6
10.9
67.4
a
b
2-Phenylbenzothiazole. Data obtained aer third recycle.
exhibited excellent benzyl alcohol conversion and good DBT products. Pd nanoparticles supported on Nano-ZSM-5 was
selectivity. found to be an efficient catalyst in these tandem reactions. High
Pd/ZSM-5 is widely investigated in organic synthesis, for surface area and surface silanol groups were favorable for the
example oxidation of volatile organic compounds, oxidation of dispersion of Pd nanoparticles on its surface to produce highly
benzyl alcohol, coupling reactions etc.51,52 In this study efficient bi-functional catalysts. Furthermore, inter-crystalline
Pd/Nano-ZSM-5 was investigated for the one-pot tandem reac- mesopores present in the Nano-ZSM-5 facilitate the diffusion
tions. Recently, an effort was made to prepare Pd nanoparticles of reactant to the acid sites and diffusion of the product from
supported organic–inorganic hybrid ZSM-5 based bi-functional the active sites, which is responsible for the high activity of Pd
catalysts, which catalyzed one-pot synthesis of benzylidene nanoparticles supported on Nano-ZSM-5 in the synthesis of
malononitrile from benzyl alcohol under soluble-base-free these chemicals. ICP analysis and recycling study conrmed
conditions, in which the benzyl alcohol was rst oxidized to that no signicant loss in the catalytic active Pd species and
intermediate benzaldehyde followed by Knoevenagel conden- catalytic activity were observed even aer three recycles.
sation with malononitrile.53 Activity of catalysts investigated in
this study for the oxidation of benzyl alcohol under O2 atmo-
Acknowledgements
sphere was comparable with the catalyst reported in the litera-
ture.53 It may be noted that Pd supported SiO2 exhibited very low
Authors thank Department of Science and Technology, New
Delhi for nancial assistance (DST grant SB/S1/PC-91/2012). BS
is grateful to UGC, New Delhi for JRF fellowship.
activity in the oxidation of benzyl alcohol.54 However, the cata-
lytic activity was improved by incorporating FeOx.54 Literature
reports suggest that bi-metallic catalysts such as Pd–Au is good
for the oxidation of benzyl alcohol under O2 atmosphere.55 In
this study, we provided a simple and straight forward route for
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