Facile one-pot tandem reductive amination of aldehydes from nitroarenes over a hierarchical ZSM-5 zeolite containing palladium nanoparticles

Article abstract of DOI:10.1039/c6nj02262f

In the present work, palladium nanoparticles supported on hierarchical ZSM-5 zeolite (Pd/H-hierarchical ZSM-5) were evaluated as a novel acid-metal bi-functional heterogeneous catalyst for direct one-pot reductive amination of nitroarenes, using aldehyde in the presence of NaBH₄ and water as a mild reducing agent and green solvent, respectively. The structural and morphological properties of the prepared catalyst were investigated by XRD, BET, FT-IR, NH₃-TPD, SEM, TEM, XPS, DRS-UV, and ICP-AES techniques. The reaction was carried out at room temperature, in a short reaction time, without producing any by-products. The stability of the catalyst was good, and it could be reused six times without much loss of activity in the reductive amination reaction.

Full text of DOI:10.1039/c6nj02262f

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ARTICLE
Facile one-pot tandem reductive amination of aldehydes from
nitroarenes over a hierarchical ZSM-5 zeolite containing palladium
nanoparticles
Received 00th January 20xx,
Accepted 00th January 20xx
a*
b
DOI: 10.1039/x0xx00000x
Roozbeh J. Kalbasi and Omid Mazaheri
www.rsc.org/njc
In the present work, palladium nanoparticles supported on hierarchical ZSM-5 zeolite (Pd/H-hierarchical ZSM-5) was
evaluated as a novel acid-metal bi-functional heterogeneous catalyst for direct one-pot reductive amination of nitroarenes
using aldehyde in the presence of NaBH
4
and water as a mild reducing agent and green solvent, respectively. The
-TPD, SEM,
structural and morphological properties of the prepared catalyst were investigated by XRD, BET, FT-IR, NH
3
TEM, XPS, DRS-UV and ICP-AES techniques. The reaction was carried out at room temperature, in short reaction time
without producing any by-products. The stability of the catalyst was good and it could be reused six times without much
loss of activity in reductive amination reaction.
KEYWORDS: Tandem reductive amination; Hierarchical ZSM-5 zeolite; Palladium nanoparticles; Heterogeneous catalyst;
Acid-metal bifunctional catalyst.
6
compounds. In this regards, one of the economical and clean
1
. Introduction
pathways is direct reductive amination of aldehydes and
ketones using nitroarenes instead of amines. In recent years,
Amines are basically valuable compounds, which have been
widely used in dyes, rubber materials, agricultural chemicals,
herbicides, chelating agents, pharmaceuticals, surfactant
few studies have been reported on these types of three-step
reactions ,which start by using nitroarenes and usually need
7
-13
1
several hours to perform reaction.
In this method, after
textile auxiliaries, polymers. There are several methods for
reduction of nitroarenes to amines, the carbonyl compounds
react with amines to produce imines. Finally, the C=N bond of
the imine is hydrogenated with hydride compounds or
the preparation of substituted amines: (a) using alkyl halides
or sulfonates as alkylating agents, alkylation of ammonia,
primary or secondary amines; (b) reduction of nitrogen-
containing functional groups such as nitro, carboxamide
derivatives, cyano, and azide and (c) reaction of aldehydes or
ketones with ammonia, primary or secondary amines in
presence of different reducing agents which for this
transformation, the two synthetic methods were commonly
2
molecular hydrogen (H ) to yield the secondary and tertiary
1
0, 12,13
amines. (Scheme 1).
Bi-functional catalysts can be used to perform one-pot
cascade reductive amination reaction with nitroarenes in three
steps, which have to have metallic and acidic character. In this
respect, recently a variety of catalytic systems have been
2
-4
used: direct and indirect methods.
7
,11
developed
which in the midst of them, heterogeneous
Direct Reductive amination represents one of the most
convenient and versatile methods of amine synthesis that
offers significant advantages over other amine syntheses, like
facilitating the separation steps, the mild reaction conditions,
wide commercial availability of substrates and increasing
catalysts have many benefits like recovery capabilities and
7
easy separation in comparison to homogeneous ones.
,8,11,14,15
Recently, various metal nanoparticles have been used as
hydride transfer catalysts in the reduction systems, for
1
6-19
5
instance: Pt, Ni, Cu, and Pd.
However, an effective control
yields. Also, the operational simplicity and reducing the use of
of particle size and a uniform dispersion of nanoparticles in
catalytic applications are typically anticipated. Nonetheless,
nanoparticles usually aggregate to yield bulk-like materials
that extremely reduce the activity of catalysts and selectivity.
Accordingly, they must be immobilized in the pores of
reagents adds to the values of this method as a practical
alternative to the reductive amination of carbonyl
2
0
heterogeneous supports such as ordered mesoporous silica
or embedded in matrices such as macromolecular organic
2
1-23
ligands or polymers.
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Journal Name
NO2
NH2
O
N
HN
H
Pd/H-hierarchical ZSM-5
Pd/H-hierarchical ZSM-5
Pd/H-hierarchical ZSM-5
NaBH4
NaBH4
H
NaBH4
Scheme 1. Reaction steps and intermediates proposed in the direct one-pot reductive amination of aldehydes from nitroarenes.
It has been proven that in the direct reductive amination of (TEM) observations were performed with on a Philips CM30
carbonyl compounds with nitrobenzene, the acidic properties electron microscope at an accelerating voltage of 300.00 kV.
2
4,25
of supports can be effective to boost the reaction.
Since Scanning Electron Microscope (SEM) studies were performed
zeolites are good candidates among the variety of the supports on Hitachi S4160 FESEM. The Pd amount content of the
to be used in this reaction. Despite that, because of diffusion catalyst was appraised estimated by inductively coupled
restriction in its microporous framework, the results indicated plasma atomic emission spectrometry (ICP-AES) using perkin-
26
a clogging and poisoning. During the past decade, significant Elmer optima 4300.
efforts have been devoted to developing methods that The products were characterized by C NMR and H NMR
13
1
introduce mesoporosity in zeolite materials with shape spectra (Bruker DRX-500 Avance spectrometer at 125.47 MHz
selective properties by different methods that decreasing and 500.13, respectively). Melting points were measured with
accessibility limitations and the diffusion of larger molecules in an electro thermal 9100 apparatus. All the products were
2
7
1
biomass promotion. The conflation of mesopores in the known compounds and they were characterized by H NMR,
1
3
zeolite materials can decrease pore blockage due large
C NMR and FT-IR.
molecules adsorbing to the surface and thus enhance 2.2. Catalyst Preparation
2
8,29
activation of the support.
2
.2.1. Preparation of KIT-6. Firstly, pursuant to the reported
To address this issue, we will introduce synthesis and
application of dispersed palladium nanoparticles incorporated
in ZSM-5 zeolite with hierarchy structure as a novel acid-metal
bi-functional heterogeneous catalyst. Furthermore, the
catalyst was used effectively for the direct one-pot reductive
amination of nitroarenes compounds using aldehyde in the
procedure, high quality ordered mesoporous KIT-6 was
30
prepared to generate the hierarchical zeolite. The large pore
3D (Ia3d) cubic mesostructure, designated as KIT-6, was
prepared using tetraethylorthosilicate (TEOS) and Pluronic
P123 (EO PO EO ) template as the silica precursor and a
2
0
70
20
structure directing agent, respectively. The detailed synthesis
procedures are explained as follows: 6 g of n-butanol (0.161
mol) and 6 g (1.03 mmol) of P123 were dissolved in 270 g (15
mol) of distilled water and 11.4 g (0.115 mol) of concentrated
hydrochloric acid (37 wt. % HCl). Then after, 12.9 g (0.061 mol)
of TEOS was added to this mixture. The mixture was stirred for
presence of a small amount of NaBH as a reducing agent and
4
water as a green solvent at room temperature without any by-
products.
2
. EXPERIMENTAL
2
4 h at 318 K in order to form the mesostructure product.
Finally, the reaction mixture was heated for 24 h at 370 K for
The samples were analyzed using the X-ray powder diffraction hydrothermal treatment under static conditions. The solid
XRD) of the catalyst was performed on a Bruker D8Advance X- product filtered, washed with deionized water and dried at
2.1. Catalyst Characterization
(
ray diffractometer using nickel-filtered Cu Kα radiation at 40 373 K. Subsequently, the samples were calcined for 6 h at 823
kV and 20 mA. FT-IR spectroscopy (using a Perkin-Elmer 65 in K to remove the organic template.
-
1
KBr matrix in the range of 4000-400 cm ). The BJH pore size
distribution, BET specific surface areas and MP-plot of the Hierarchical zeolite prepared by KIT-6 as an indirect template
2.2.2. Preparation of hierarchical ZSM-5 zeolite.
3
1
samples were determined by adsorption-desorption of (silica source) via the method described in recent literature.
nitrogen at liquid nitrogen temperature using a Series BEL In brief, solution of sodium aluminate and
SORP 18. Temperature-programmed desorption of NH (NH
tetrapropylammonium hydroxide (TPAOH, 40 wt.%) were
TPD) was recorded on FINESORB-3010 equipped with a impregnated into 1 g of KIT-6 powder. The synthesis gel
thermal conductivity detector. The DRS
UV-vis (Al :60SiO :4Na O:4800H O:18TPAOH) was obtained as the
3
3
-
O
3
2
2
2
2
Spectrophotometer was recorded with JASCO spectrometer, final molar ratio. The mixture was stirred for 12 h at 293 K.
V-670 from 190 to 2700 nm. Moreover, X-ray photoelectron Then, the aged synthesis mixture was transferred into a
spectrum (XPS) was carried out recorded on ESCA SSX-100 Teflon-lined stainless steel autoclave followed by reaction for
(
Shimadzu) using a non-monochromatized Mg Kα X-ray as the 48 h at 448 K. Then, the solid product was filtered, washed
excitation source. In order to get detailed information on the with deionized water and dried at room temperature at 393 K
size of Pd nanoparticles Transmission Electron Microscope overnight. In the end, the sample was calcined for 6 h at 823 K
2
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to remove the TPAOH template, and the hierarchical ZSM-5 by
zeolite was obtained.
ion-exchange
method.
Furthermore,
palladium
nanoparticles have been reduced by hydrazine hydrate and
2
.2.3. Preparation of acidic hierarchical zeolite (H- immobilized on hierarchical zeolite. Subsequently, the Pd/H-
hierarchical ZSM-5). The acid form of hierarchical zeolite was hierarchical ZSM-5 zeolite with excellent properties was
prepared by ion-exchange method. In short, 1 g of hierarchical fabricated by this simple method for the reductive amination
of nitro aromatic compounds in water medium and in the
zeolite was mixed up with 100 mL aqueous solutions of NH4Cl
1 M) in the reflux condition for 48 h, then the mixture was
4
presence of low amount of NaBH as a reducing agent.
(
washed with deionized water, dried for 12 h at 383 K and
finally was calcined for 6 h at 823 K to prepare H-hierarchical
ZSM-5.
3.1. Catalyst characterization
The expected crystalline structure of H-hierarchical ZSM-5 and
Pd/H-hierarchical ZSM-5 were confirmed by powder XRD
2
.2.4. Preparation of palladium nanoparticles supported measurements in Fig. 1S (ESI†) and Fig. 1, respectively. The
on hierarchical zeolites (Pd/H-hierarchical ZSM-5). Pd/H- major phases of H-hierarchical ZSM-5 show reflections at 2θ
hierarchical ZSM-5 was prepared through in situ reduction of values of 7-9˚ and 23-25˚, which completely agree with the
Pd ions inside the channels of H-mZSM-5, according to our
3
2
previous report. The preparation of Pd/H-hierarchical ZSM-5
was carried out as follows:
H-hierarchical ZSM-5 (0.1 g) and 10 mL of an aqueous acidic
2
solution (CHCl = 0.09 M) of Pd(OAc) (0.0088 g, 0.0392 mmol)
were placed in a round bottom flask. The mixture was heated
to for 5 h at 353 K while being stirred. Next, 0.6 mL (2.47
2 4 2
mmol) aqueous solution of N H •H O (80 vol.%) was added to
the mixture drop by drop in 20–25 min. Afterward, the
solution was stirred for 1 h at 333 K. After that, it was filtered
and washed sequentially with methanol and chloroform to
remove excess N H •H O and was dried at room temperature
2
4
2
to yield Pd/H-hierarchical ZSM-5 zeolite. The Pd content of the
catalyst was estimated by decomposing the accurate amount
of the catalyst by hydrochloric acid, nitric acid, fluoric acid and Figure 1. XRD pattern of Pd/H-hierarchical ZSM-5 (2θ = 5-90).
-1
perchloric acid. ICP-AES estimated 0.332 mmolg Pd content
3
3
of Pd/H-hierarchical ZSM-5.
characteristic peaks of MFI structure. In addition, the wide-
angle XRD pattern of the Pd/H-hierarchical ZSM-5 (2θ = 5-90◦)
2.3. General procedure for direct reductive amination of
◦
◦
◦
Fig. 1) show peaks of 2θ values 39.92 , 46.54 , 67.90 and
(
aldehydes from nitroarenes. A mixture of nitrobenzene (2
mmol) and benzaldehyde (2 mmol) in water (3 mL) was placed
in a round bottom flask and stirred for 1 min at room
temperature. Afterward, to the resulting mixture, Pd/H-
◦
2.0 , which are assigned to the corresponding (111), (200) and
8
(
220) and (311) indices of the face-centered cubic (fcc) lattice
3
4,35
of metallic Pd.
The XRD results imply that after the
immobilized of palladium nanoparticles on hierarchical
zeolites, zeolite structure has not changed, indicating a
successful synthesis of the Pd/H-hierarchical ZSM-5 (Fig. 1).
The FT-IR spectra were carried out to identify of pure H-
hierarchical ZSM-5 and Pd/H-hierarchical ZSM-5, which are
shown in Fig. 2S (ESI†) and Fig. 2. A broad band at around 3440
4
hierarchical ZSM-5 (0.04 g) and NaBH (6 mmol) were added
and the mixture was stirred at room temperature until TLC
showed the complete disappearance of the benzaldehyde.
Then, the reaction mixture was quenched with water (10 mL)
and the product was extracted with diethylether (2 × 10 mL).
After they finished, the organic phase was dried over
-1
cm is observed in both samples which represent the O-H
2 4
anhydrous Na SO , filtered and concentrated. In the end, the
stretching vibration of the adsorbed water molecules and
hydroxyl groups of zeolite surface. Additionally, the band
products were obtained very pure just by extract with
diethylether in the majority of the reactions. In a few cases,
-1
around ~3650 cm is generally attributed to the bending
column chromatography was used to obtain the pure product. vibration of Si-OH-Al groups that shown to be acidic (more
The product was identified with a melting point, FT-IR specifically Bronsted acid sites) and in agreement with prior
1
spectroscopy techniques, HNMR and CNMR.
13
36,37
-1
reports.
adsorbed water, which is principally close to related
The band at about 1630 cm can be attributed to
3
8,39
-1
The peaks around 1220 cm (external asymmetric
reports.
3
. RESULTS AND DISCUSSION
-1
stretch), 1150-1050 cm (internal asymmetric stretch), ~800
-1
cm-1 (symmetric stretch) and 450 cm (T–O bend) are
As a first step towards the accomplishment of the synthesis of
the hierarchical zeolite, high quality ordered mesoporous KIT-6
silica was prepared by sol-gel technique. After that, KIT-6 was
used as an indirect template for the synthesis of zeolite.
Moreover, the acid form of hierarchical zeolite was prepared
characteristic of highly siliceous materials. In the FT-IR spectra
of H-hierarchical ZSM-5 and Pd/H-hierarchical ZSM-5, the new
-1
band at around 550 cm corresponds to the double five rings
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Journal Name
2
Si–O–Si or Al–O–Si groups) of the typical structure of MFI-type with a high BET surface area (223 m g ), pore size (4.6 nm)
-1
(
3
1
3
-1
zeolites. Both the FT-IR spectra and the XRD can represent and pore volume (0.26 cm g ) (Table 1). In addition, the co-
that MFI-type zeolite (H-hierarchical ZSM-5) has been presence of micropores is suggested by -Plot and MP-Plot
successfully synthesized. methods (Table 1). Apparently these results demonstrate the
t
-
Furthermore, the ratio of the intensities of 550 and 450 cm formation of a new hierarchical zeolitic structure with an
bands was used to figure out the (IR) crystallinity of the acceptable porosity and surface area. Based on the results, H-
1
prepared samples. The results roll up in Table 1.
hierarchical ZSM-5 illustrates both mesoporous and
microporous structures.
All three cases of the specific surface area, pore diameter
(calculated by BJH) and pore volume of Pd/H-hierarchical ZSM-
5
are lower than those of H-hierarchical ZSM-5 (Table 1),
according to this results, palladium nanoparticles distributed
on the outer surface and their incorporation inside the (micro
and meso) pores of the hierarchical zeolites. Nonetheless, this
isotherm is still similar to type IV that is the characteristic of a
mesoporous material.
Fig 4S (ESI†) and Fig. 4 shows the pore size distribution
curves (using BJH method) of the synthesized zeolite. As can
be seen, the samples of H-hierarchical ZSM-5 and Pd/H-
hierarchical ZSM-5 have a narrow range pore size distribution
with a pore diameter of about ~4 nm displaying the existence
of a mesoporous structure in these zeolites. After the
incorporation of palladium nanoparticles, the pore size
distribution has slightly changed that indicates the distribution
of palladium nanoparticles on the surface of zeolites.
Figure 2. FT-IR spectrum of Pd/H-hierarchical ZSM-5. (× Impurity: carbonyl
bond of acetone, which was used for rinse off the silicon wafer while
preparation of KBr pellet).
High IR crystallinity matters to a high degree of ordering of
the aluminosilicate (Al–O–Si) phase in a manner similar to that
4
0
in ZSM-5 zeolite.
MP-Plot methods and t-Plot corroborate the presence of
The BET specific surface areas, the pore sizes and the pore
volumes of both samples were computed by BET, BJH, MP-Plot
microporosity of H-hierarchical ZSM-5 zeolite structures as
well (Table 1, Fig. 5S(ESI†)). These microporous structures are
still visible after the incorporation of palladium nanoparticles
and
isotherms for these samples are shown in Fig. 3S (ESI†) and Fig.
. Moreover, the corresponding pore size distribution curves
micro and meso pores) are drawn in Fig. 4-5S (ESI†) and Figs.
-5 for H-hierarchical ZSM-5 and Pd/H-hierarchical ZSM-5,
2
t-Plot methods (Table 1). The N adsorption–desorption
4
2-44
(Fig. 5).
3
The aforementioned results indicate that H-hierarchical
(
ZSM-5 and Pd/H-hierarchical ZSM-5 still show a hierarchical
structure with a suitable surface area, pore diameter and pore
volume, which make it reasonable to act as a catalyst.
4
respectively. As shown, according to the IUPAC nomenclature,
two samples demonstrate isotherms of type IV, which are the
typical characteristics of mesostructure materials (Fig. 3 and 3S
According to the TPD results, the numbers of acid sites
-1
-1
were 0.96 mmol g and 0.90 mmol g for H-hierarchical ZSM-5
and Pd/H-hierarchical ZSM-5, respectively (Table 1), which
indicates that despite of distribution palladium nanoparticles
on H-hierarchical ZSM-5, the acidic properties of Pd/H-
hierarchical ZSM-5 are still maintained.
4
1
(ESI†)).
2
Figure 3. N adsorption/desorption isotherm of Pd/H-hierarchical ZSM-5.
Figure 4. Pore size distribution of Pd/H-hierarchical ZSM-5 obtained by BJH
As shown, the H-hierarchical ZSM-5 can be applicable as a
suitable supply for the deposition of palladium nanoparticles
method.
4
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Table 1. Physicochemical properties of H-hierarchical ZSM-5 and Pd/H-hierarchical ZSM-5 samples obtained from N
2
adsorption.
NH
3
S
2
m g )
BET
-1
V
mesopore
-1
D
mesopore
a
D
mesopore
b
S
micropore
-1
S
micropore
-1
V
micropore
D IR crystallinity
micropore
Sample
chemisorbed
a
3
a
2
c
2
d
3
-1
c
c
e
(
(cm g )
(nm)
4.60
4.1
(nm)
4.80
4.20
(m g )
(m g )
(cm g )
(nm)
(%)
f
(
mmol/g)
H-hierarchical
ZSM-5
2
1
23
0.26
269
293
0.10
0.7
0.7
99
94
0.96
Pd/H-hierarchical
ZSM-5
54
0.23
236
234
0.07
0.90
a
b
c
Calculated by BET method. Mean pore diameter determined by using BJH method from the adsorption branch of the isotherm curves. Calculated by MP-
d
e
Plot method. The micropore surface area was estimated by the t-plot analysis using the adsorption branch of the isotherm curves. IR crystallinity defined
4
0
-1
f
3
as (I550/I450)/0.72×100% with I550 and I450 the intensities of the bands at 550 and 450 cm , respectively. NH TPD.
According to the literature results, the UV-vis spectrum of
Pd(OAc) revealing a peak at 400 nm refer to the existence of
2
2
+ 45
Pd . As mentioned in Section 2, we used hydrazine hydrate
as a reduction agent to prepare Pd nanoparticle supported on
2
+
hierarchical zeolite from Pd /H-hierarchical ZSM-5. In spite of
a fact that we just mentioned, there were not any peaks at 400
nm in the UV-vis spectrum of Pd/H-hierarchical ZSM-5, which
2
+
illustrated a complete reduction of Pd to Pd nanoparticles.
The chemical composition of Pd/H-hierarchical ZSM-5 was
also analysed by XPS (Fig. 7). The XPS results of Pd
nanoparticles dispersed in H-hierarchical ZSM-5 zeolite for Pd
3d spectrum with the binding energies of Pd 3d5/2 and Pd 3d3/2
lying at about 335.3 and 340.7 eV, respectively (Fig. 7). Which
indicate Pd nanoparticles are stable as a metallic state in H-
2
+
hierarchical ZSM-5 zeolite (the XPS spectrum of Pd ions
appears two peaks at about 337 and 342 eV refer to Pd 3d5/2
4
6
0
Figure 5. Pore size distribution of Pd/H-hierarchical ZSM-5 obtained by MP- and 3d3/2 ) in comparison to the standard binding energy (Pd
4
with Pd 3d5/2 of about 335 eV and Pd3/2 of about 340 eV ). In
7
Plot method.
this context, it could be concluded that the Pd peaks in H-
0
Fig. 6 presents the result of the UV-vis spectrum of Pd/H- hierarchical ZSM-5 shifted to higher binding energy toward Pd
2
hierarchical ZSM-5. After reduction of Pd /H-hierarchical ZSM- standard binding energy. As we know, the situation of Pd 3d
+
0
by hydrazine hydrate the UV-vis spectrum of Pd /H- peak is generally influenced by the local chemical/physical
5
hierarchical ZSM-5 was studied.
environment around Pd species besides the formal oxidation
state, and shifts to higher binding energy when the charge
2
1,47
density around it decreases.
Figure 6. UV-vis spectrum of Pd/H-hierarchical ZSM-5.
Figure 7. XPS spectrum of Pd3d of Pd/H-hierarchical ZSM-5.
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In other words, the existence of the acidic sites of the
Journal Name
Also, the TEM images of Pd/H-hierarchical ZSM-5 (Fig. 9b-c)
indicative of the places with darker contrast. Apparently, it
0
zeolite (with the electron withdrawing nature) around the Pd
species conduct toward a decrease in the charge density could be related to the presence of palladium nanoparticles.
37
Also, the small dark dots in the images with an average
0
around Pd and the slight increase in binding energy.
The full XPS spectrum of Pd/H-hierarchical ZSM-5 showed diameter of ~3-6 nm can be related to palladium nanoparticles
peaks of silicon, aluminum, carbon, oxygen and palladium. The and are located into the mesopores (Fig. 9d). As shown, the
presence of Al, Si and O confirm that ZSM-5 was obtained. mesopore structure still remains after the incorporation of Pd
Carbon has also been detected after the template removal nanoparticles in H-hierarchical ZSM-5 (Fig. 9b-c).
corresponds to remaining organic template of TPAOH inside
The SEM was conducted to identify the morphologies of H-
the zeolite pores (Fig. 8). Recent studies have surely shown hierarchical ZSM-5 and Pd/H-hierarchical ZSM-5 that are
that even after template removal, a small amount of template shown in Fig. 10. In this regard, H-hierarchical ZSM-5 has a
4
8-50
stays inside the small cages in the MFI type zeolite.
crystalline structure with small cubic regular particles and a
Moreover, calibration of XPS instrument is done by using the size about 3 μm (Fig. 10a). As shown, it can be concluded that
binding energy of the reference substance which always there is no difference in particle surface morphology between
causes present carbon pollution with a C1s binding energy of the H-hierarchical ZSM-5 and Pd/H-hierarchical ZSM-5 (Fig.
5
1,52
284 eV.
10b), indicating that the supporting of Pd nanoparticles takes
place inside the channels of H-hierarchical ZSM-5 which is not
detectable in the SEM images (these particles shown in the
TEM section). This observation is also supported by the
decrease in pore volume and in surface area as shown in Table
1. It’s also noteworthy to mention that, the crystalline
structure of the zeolite is not destroyed during the loading of
Pd nanoparticles on the H-hierarchical ZSM-5.
Figure 8. Full XPS spectrum of Pd/H-hierarchical ZSM-5.
In order to gain insights into the morphology of the catalyst,
TEM analyses were carried out on H-hierarchical ZSM-5 (a) and
Pd/H-hierarchical ZSM-5 (b-d), which depicted in Fig. 9. This
images are demonstrated the formation of the ordered
mesostructure in H-hierarchical ZSM-5 with pore diameter
about 6-7 nm and the distribution of the palladium
nanoparticles in the H-hierarchical ZSM-5 (Fig. 9 (a-c)).
Figure 10. Scanning electron microscopy (SEM) photographs of (a) H-
hierarchical ZSM-5 and (b) Pd/H-hierarchical ZSM-5.
3.2. Catalytic activity
Synthesized Hierarchical zeolite was characterized by
several methods in the previous section. Here we introduce
the application of this catalyst to the reductive amination
reaction because it has many specific characteristics contrast
to traditional zeolite. Over the past two decades, many studies
devoted to industrial and academic research groups to the
development of environmentally friendly synthesis.
Accordingly, using water as a reaction medium in transition
metal-catalyzed processes has merited increasing attention
and nowadays it is one of the most essential targets of
5
3,54
sustainable chemistry.
Water is an inexpensive, nontoxic
solvent, readily available, non-inflammable that provides
significant advantages over common organic solvents both
1
1,55
from an environmental and an economic standpoint.
Thus,
the effect of different parameters on the one-pot tandem
reductive amination of aldehydes with nitroarenes over Pd/H-
hierarchical ZSM-5 as acid-metal bifunctional catalyst was
surveyed at room temperature in water and the results are as
follows:
Figure 9. Transmission electron microscopy (TEM) of (a) H-hierarchical ZSM-
5
and (b-d) Pd/H-hierarchical ZSM-5.
6
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6NJ02262F
Journal Name
ARTICLE
a
2
Table 2. Effect of Pd(OAc) molar ratio on the catalytic activity.
c
Pd content of catalyst
Reaction time
(min)
Conversion of benzaldehyde
Yield (%)
Entry
2
Pd(OAc) (wt%)
b
c
(mmol/g catalyst)
(%)
amine
imine
45
2
4
0
5
65
55
1
4.2
8.1
0.167
100
98d
2
2
2
2
2
3
4
0.332
0.464
0.598
20
18
15
100
100
100
98d
98d
11.6
15
98d
a
Reaction conditions: benzaldehyde (2 mmol), nitrobenzene (2 mmol), Pd/H-hierarchical ZSM-5 (0.10 g), H
2
O (3 mL) and NaBH
4
(6 mmol) at room
b
c
d
temperature. Estimated by ICP-AES. Conversion of benzaldehyde and yield were analyzed by GC, and n-dodecane was used as the internal standard.
Isolated yield after work-up.
a
At the first screening of experiments, different amounts of Table 3. Effect of NaBH
Pd(OAc) were examined to recognize the effect of palladium
nanoparticles concentration on the reaction. So, while the
other parameters were constant, the amount of Pd(OAc) to
prepare Pd/H-hierarchical ZSM-5 was changed from 4.2 wt% to
5 wt% and then measured by the ICP-AES technique which
4
on the direct one-pot reductive amination.
2
Amount
of NaBH
mmol)
b
Reaction
Conversion of
Yield (%)
Entry
4
b
time (min) benzaldehyde (%)
2
(
amine imine
2
0
36
77
21
66
15
11
1
1
2
showed in Table 2. As can be seen in Table 2, the activity of
catalytic increased steadily between 4.2 and 8.1 wt%. By
60
increasing the amount of Pd(OAc)
2
. As the catalytic reaction
20
35
62
95
53
90
9
5
2
4
mechanism involves Pd nanoparticle mediated electron
-
4
transfer from BH ion to the nitro compounds (in the first
step) and to the imine intermediates (in the third step), the
-
amount of H sites on the catalyst surface are increased by
c
3
6
8
20
100
98
2
increasing palladium nanoparticles, and a larger amount of
hydrides can be transferred to the nitro compounds and then
c
5
6
4
20
100
98
2
imine groups through the catalyst. The excess amount of
Pd(OAc) cause the reaction time decrease with mild slope and
because of the expensive price of palladium acetate, we chose
.1wt% of Pd(OAc) as an optimum amount.
The effect of NaBH amount (as a hydride donor) on the
a
2
Reaction conditions: benzaldehyde (2 mmol), nitrobenzene (2 mmol),
b
2
Pd/H-hierarchical ZSM-5 (0.10 g) and H O (3 mL) at room temperature.
8
2
Conversion of benzaldehyde and yield were analyzed by GC, and n-
c
4
dodecane was used as the internal standard. Isolated yield after work-up
reductive amination was investigated in the presence of Pd/H-
hierarchical ZSM-5. The results revealed that there is a direct
Table 4. Effect of catalyst amount on the direct one-pot reductive amination
a
relationship between yield and the amount of NaBH
mmol (Table 3). With further increase in the NaBH
the yield of the reaction leveled off at 98% between 6 and 8
mmol NaBH . Fewer values were not enough for the reduction
4
up to 6 reaction.
amount,
Amount of
catalyst (g)
Reaction time
Yield of amine
4
Entry
1
b
(min)
20
(%)
c
4
74
0.03
of the amount of nitro and imine compounds and the excess
amount didn't have any effect on the reaction as well.
3
8
98
98
98
Therefore, the best value of NaBH
4
was 6 mmol.
2
3
0.04
0.05
20
20
The effect of amount of Pd/H-hierarchical ZSM-5 was
determined for the reductive amination reaction. It can be
seen that with an increase in the amount of the catalyst from
5
0.10
20
98
0
.03 g to 0.04 g, a considerable decrease in the reaction time
a
2
Reaction conditions: benzaldehyde (2 mmol), nitrobenzene (2 mmol), H O
was observed from 38 to 20 minutes (Table 4) which are due
to the availability of more acidic and metallic sites.
b
(
3 mL) and NaBH
4
(6 mmol) at room temperature. Isolated yield after work-
c
up. Yield was analyzed by GC, and n-dodecane was used as the internal
standard.
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As shown from this table, a higher amount of catalyst (0.05
g) had no any appreciable effect on the yield. It might be
mentioned that using low amounts of NaBH and catalysts in
4
this reaction shows the extraordinary catalytic activity of this
catalyst.
The reusability of the catalyst was tested by using Pd/H-
hierarchical ZSM-5 in recycling experiments (Chart 1). After
each cycle, the catalyst was filtered off, washed with water (10
mL) and ethanol (3 mL × 5 mL). Thereafter, it was dried in an
oven at 60 ˚C and reused in the reaction with a fresh reaction
mixture. It should be noted that after each run, some of the
catalysts were lost in the filtration process.
Herein, to solve this problem, after each run the amount of
remaining catalyst was determined and the molar ratio of the
reactants was changed according to the remaining amount of
the catalyst. Nonetheless, it’s noteworthy that there was a
very low amount of lost catalyst after each cycle.
The catalyst was reused up to 6 times without any
modification and 17% loss of activity was observed. It’s
noticeable that there was very low Pd leaching (2.7 %
measured by ICP-AES) during the reaction and the catalyst
exhibited high stability even after six runs (Chart 1).
Figure 11. XRD patterns of the used Pd/H-hierarchical ZSM-5 after the
fifth run of the recycle reaction.
It should be mentioned that in the presence of palladium
hydride, the efficiency of the catalyst doesn't detract since
palladium hydride is reversible in metallic form by slightly
heating. In addition, the existence of palladium hydride crystal
in reused catalyst indicates that this catalyst is very suitable for
hydride transfer.
SEM images of the catalyst recovered from the fifth run of
the recycle reaction are presented in Fig. 12, too. In contrast,
between these images and SEM images of the fresh catalysts
(Fig. 10), it’s indicated that the structure of the ZSM-5 zeolite is
well retained even after the fifth runs of the recycle reaction.
Since, the catalysts exhibit high stability over 6 runs which is
essential for the applications of catalyst.
Chart 1. The catalysts reusability and leaching of palladium for direct
one-pot reductive amination reaction.
Moreover, the XRD pattern of the used catalyst after five
Figure 12. Scanning electron microscopy (SEM) photographs of the
used Pd//H-hierarchical ZSM-5 catalyst after the fifth run of the
recycle reaction.
cycles showed that the structure of the catalyst is well retained
◦
Fig. 11). Also, the XRD pattern in 2θ = 45-75 shows the
(
◦
◦
reflections at 45 and 53 suggesting that during the reaction,
the palladium nanoparticles remain in the form of a crystal. On
the other hand, XRD analysis of reused Pd/H-hierarchical ZSM-
The catalytic activity of the Pd/H-hierarchical ZSM-5 in the
reductive amination was compared with H-hierarchical ZSM-5,
5
indicates the presence of palladium crystals to form both,
2
+
Pd /H-hierarchical ZSM-5, Pd/H-ZSM-5 (prepared based on
traditional ZSM-5) and without a catalyst. The results are
presented in Table 5. The result emphasizes the important role
of the acid-metal catalyst in this reaction. As shown, the
reaction efficiency without catalyst will be only 5% as long as
100 minutes (entry 1).
metallic and hydride. At room temperature, palladium
hydrides may contain two crystalline α and β, phases. The
existence of Palladium metal hydrides is the result of the very
high tendency of palladium nanoparticles to accept the
hydride ions and then conversion to palladium hydride. This
5
7
4
process is faster in the presence of NaBH .
8
| J. Name., 2012, 00, 1-3
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ARTICLE
a
Table 5. Effect of catalyst for the direct one-pot reductive amination.
Yield of
Entry
catalyst
Reaction time (min)
00
b
amine (%)
5
1
1
Without catalyst
2
0
5
0
5
5
20
15
2
3
H-hierarchical ZSM-5
2
2
+
Pd /H-hierarchical
ZSM-5
55
45
2
5
2
0
5
0
25
75
55
4
5
Pd/H-ZSM-5
Figure 13. Heterogeneity test for direct one-pot reductive amination.
Under the optimized reaction conditions, the scope of the
reaction was explored with structurally and electronically
diverse aldehydes with a wide range of nitroarenes. Reactions
were investigated at room temperature in water at different
times, which mentioned in Table 6. The results show that all
reactions are almost completed within 17 to 45 min at room
temperature (Table 6). The reaction rate goes up by the
decrease electron density at the nitro group with locating
electron-withdrawing group on the nitro aromatic ring (entry
Pd/H-hierarchical
ZSM-5
c
20
98
a
Reaction conditions: benzaldehyde (2 mmol), nitrobenzene (2 mmol),
4
2
catalyst (0.04 g), NaBH (6 mmol) and H O (3 mL) at room
b
temperature. Yield was analyzed by GC, and n-dodecane was used as
c
the internal standard. Isolated yield after work-up.
There is the important point that NaBH4 act as a mild
hydride donor agent, which is poor reagent for reducing nitro
groups. By using the H-hierarchical ZSM-5 catalyst (entry 2),
with increase acid sites of the zeolite the nitro group activated
and so the yield slightly increased. (20% yield in 55 minutes).
2
). On the other hand, electron-donating groups show the
reverse effect and cause the reaction rate decreased (entries 3
and 4). Actually, by decreasing the electron density of the nitro
groups, the tendency of nitro groups to accept the hydride
ions is increased. In contrast, the existence of electron-
donating groups on the benzaldehyde ring cause increases in
the reaction rate (entries 5 and 8). Actually, the electron-
donating groups stabilize the protonated carbonyl bond
intermediate and cause an increase in the reaction rate.
Additionally, the direct reductive amination from
nitroarene was performed on a 20-time scale-up (Table 6,
entry 9), which the results represent the high activity of the
catalyst.
2
+
Using Pd /H-hierarchical ZSM-5 (entry 3), the yield of nitro
benzene reduction was increased to 45% due to the role of Pd
ions as species to transfer hydride ions from NaBH to nitro
4
groups. Also, using Pd/H-ZSM-5 (traditional ZSM-5) (entry 4),
the reaction performance was increased to 75% because of the
role of palladium nanoparticles as species to transfer hydride
ions from NaBH4 to nitro groups. In order to prove the
efficiency of the hierarchical structure of Pd/H-hierarchical
ZSM-5 than traditional Pd/H-ZSM-5 zeolite, they compare with
each other that Pd/H-hierarchical ZSM-5 showed the higher
performance (entries 3 and 4). In fact the hierarchical
structure containing micro/mesoporous buildings cause easier
access of reactants to the active sites of the catalyst.
As shown in Scheme 2, this process is highly selective for
the preparation of amines from aldehydes in the presence of
ketones and only secondary amines of the aldehydes is
produced.
In catalyst study, one interesting point is heterogeneous
nature. In this context, heterogeneity test was performed. In
this regard, the catalyst was separated from the reaction
mixture at approximately 50% conversion of the starting
material through centrifugation.
O
O
NO
2
HN
HN
CH
3
H
Pd/H-hierarchical ZSM-5
5 ml H O at r.t.
NaBH
CH
3
H
2
4
The reaction progress in the filtrate was monitored (Fig.13).
No further reductive amination reaction occurred even at
addition times, representing that the nature of reaction
process is heterogeneous and there is not any progress for the
reaction in homogeneous phase.
0%
98%
Scheme 2. Selective reductive amination of aldehyde from nitroarenes.
According to the results, it can be found that the catalyst
afforded an average to excellent yields of the amine products.
Compared with the other studies on the tandem reductive
amination of aldehydes the significant advantages of our
7
-9,11-15,58-60
method are:
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a
Table 6. Direct one-pot reductive amination of aldehydes with nitroarenes over Pd/H-hierarchical ZSM-5.
O
NO2
R1
6
mmol NaBH4
H
3 ml H2O at r.t.
R1
+
R2
H
N
0
.04g of Pd/H-hierarchical ZSM-5
1
2
R2
b
Time
min)
-1
c
Entry
1
2
Product
Yield (%)
TON/TOF (h )
(
HN
1
98
98
20
17
148/443
148/521
2
3
92
30
138/277
4
5
96
96
35
10
144/248
144/867
6
7
8
97
96
95
27
35
15
45
146/325
144/248
143/572
135/181
d
9
90
a
Reaction conditions: aldehyde (2 mmol), nitro aromatic compound (2 mmol), NaBH
4
(6 mmol), Pd/H-hierarchical ZSM-5 (0.04 g) and H
2
O (3 mL) at room
b
c
d
temperature. Isolated yield after work-up. Turn-over number (mol product/mol Pd) and turn-over frequency (mol product/(mol Pd.h)). Scale-up
condition: benzaldehyde (40 mmol) , nitrobenzene (40 mmol) , catalyst (0.80 g), NaBH
4
(120 mmol) and H
2
O (150 mL) at room temperature.
1
0 | J. Name., 2012, 00, 1-3
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ARTICLE
•
The use of water as a green solvent;
7
8
9
Y. Yamane, X. Liu, A. Hamasaki, T. Ishida, M. Haruta, T.
Yokoyama, M. Tokunaga, Org. Lett. 2009, 11, 5162-5165
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•
•
•
•
Green and simple reaction system;
The fast reaction rate with high TOF and TON;
The use of very low amount of catalyst and Pd(OAc)
2
;
,
The reaction was performed under mild conditions at room
temperature;
1692-1695
10 E. Byun, B. Hong, K. A. De Castro, M. H. Lim, J. Org. Chem.
007, 72, 9815-9817
1
2
•
The use of heterogeneous catalyst and elimination of toxic
reagents;
1
Q. Zhang, S. S. Li, M. M. Zhu, Y. M. Liu, H. Y. He, Y. Cao,
Green Chem. 2016, 18, 2507-2513
Y. Z. Chen, Y. X. Zhou, H. Wang, J. Lu, T. Uchida, Q. Xu, S.
•
•
•
•
•
Wide substrate scope and generality;
1
2
The catalyst is highly selective and versatile;
Expensive and toxic ligands are not needed;
The high yields of the products;
H. Yu, H. L. Jiang, ACS Catal. 2015,
5
, 2062-2069
(8), 4846-4850.
M. M. D. Annaa, P. Mastrorilli, A. Rizzutia, C. Leonelli,
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and can be kept under an inert atmosphere for several
months.
4
64, 147-152.
16 A. Saha, B. Ranu, J. Org. Chem. 2008, 73, 6867-6870
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1
8
P. K. Mandal, J. S. McMurray, J. Org. Chem. 2007, 72,
6
•
The reactions are relatively more environmentally friendly
with easy and efficient recyclability of the catalyst.
These advantages make the present method to be
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9
8
considered as a convenient alternative method for the
reductive amination of aldehydes. Also, these advantages
make this methodology attractive for the development of
large-scale industrial synthesis.
1
2
1
R.J. Kalbasi, N. Mosaddegh, A. Abbaspourrad, Appl. Catal.
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2 (6),
4
. CONCLUSIONS
1
4
3,
In one word, we have developed a simple and highly efficient
and eco-friendly method for one-pot reductive amination
using a stable and recoverable catalyst that immobilized Pd on
hierarchical zeolite under mild conditions. The existence of
both acid and metal in the structure of Pd/H-hierarchical ZSM-
5
5
26 Y. Tao, H. Kanoh, K. Kaneko, J. Am. Chem. Soc. 2003, 125
,
6044-6045.
2
2
7
8
K. Na, M. Choi, R. Ryoo, Micropor. Mesopor. Mater. 2013,
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K. Egeblad, C.H. Christensen, M. Kustova, C.H.
5
as a bi-functional heterogeneous catalyst plays an important
1
role to conduct this tandem reaction in one-pot. Moreover,
this method can be used to produce a variety range of amines
in good to excellent yields. This catalyst exhibits high
activity/stability even after six cycles. It offers an economical
and clean method for the one-pot synthesis of amines from
nitrobenzenes and attractive for the development of large-
scale industrial synthesis.
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29 S. L. Orozco, A. Inayat, A. Schwab, T. Selvam, W.
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3
3
3
0
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2
R. J. Kalbasi, N. Mosaddegh, J. Inorg. Organomet. Polym.
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N. Chu, J. Yang , C. Li, J. Cui, Q. Zhao, X. Yin, J. Lu, J.
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New Journal of Chemistry
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DOI: 10.1039/C6NJ02262F
Table of Contents
Palladium nanoparticles embedded hierarchical zeolite acts as acid-metal bi-functional catalyst for efficient reductive
amination of aldehyde from nitroarenes.
graphical abstract