A Sustainable Palladium-Intercalated Montmorillonite Clay Catalytic System for Imine Hydrogenation under Mild Conditions

Article abstract of DOI:10.1002/cplu.202000760

A series of palladium nanoparticles (Pd NPs) intercalated montmorillonite clay catalysts is reported for hydrogenation of 3-diphenyl prop-2-en-1-imine under mild reaction conditions. Pd/clay catalyst was prepared by a simple wet-impregnation method, and t

Full text of DOI:10.1002/cplu.202000760

A Multidisciplinary Journal Centering on Chemistry
Accepted Article
Title: A Sustainable Palladium Intercalated Montmorillonite Clay
Catalytic System for Imine Hydrogenation under Mild Conditions
Authors: Unnati Gupta, Krishnapriya R, and Rakesh K Sharma
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01/2020

10.1002/cplu.202000760
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A Sustainable Palladium Intercalated Montmorillonite Clay
Catalytic System for Imine Hydrogenation under Mild Conditions
Unnati Gupta,[a] Dr. R. Krishnapriya [a] and Prof. Rakesh K Sharma *[a]
[a]
Unnati Gupta, Dr. R. Krishnapriya and Prof. Rakesh K Sharma*
Sustainable Materials and Catalysis Research Laboratory (SMCRL), Department of Chemistry
Indian Institute of Technology Jodhpur
Jodhpur, Rajasthan India-342037
rks@iitj.ac.in.
Supporting information for this article is given via a link at the end of the document
Abstract: Herein, we report a series of palladium nanoparticles (Pd
NPs) intercalated montmorillonite clay catalysts for hydrogenation of
3-diphenyl prop-2-en-1-imine under mild reaction conditions. Pd/Clay
catalyst was prepared by a simple wet-impregnation method, and the
physicochemical properties were characterized extensively by various
techniques including N2 adsorption, XRD, TEM, XPS and TPR etc.,
which showed the intercalation of active Pd NPs between the clay
layers. Effect of reaction conditions such as catalyst loading, reaction
time, temperature and H2 pressure is explored, thereby a plausible
mechanism is proposed. The optimum amount of 6 wt% Pd/Clay
catalyst displayed significant catalytic activity to yield 3-phenyl propyl
aniline with 100 % conversion and selectivity under 5 bar pressure
for shorter reaction period of 3.5 h at 100 °C. The developed catalytic
system unveiled excellent reusability over five cycles and hence
paved the way for industrial applications.
and affinity to agglomeration, there is a growing demand to use
palladium catalyst with minimum loading, high turnover number
(TON) and excellent recyclability. To achieve this, the idea to
immobilize Pd nanoparticles (NPs) on suitable support is
receiving significant consideration.[8]
The performance of supported metal catalyst depends mainly on
the nature of support material, metal-support interaction and also
the metal particle size.[9] Moreover, the support should be capable
of reducing the metal agglomeration by controlling its size without
passivation of the active surface.[10] Therefore, various supports
such as zeolites[11], carbon nanotube[12], activated carbon[13]
,
silica[14] etc. have been reported. Most of these supports have low
thermal stability, require excess organic solvents and complex
preparation procedure and that adds extra cost to the
[11]
immobilization process.
Accordingly, the development of a
heterogeneous optimized catalyst system that could accomplish
organic transformations at ambient reaction conditions using
benign solvent systems is highly preferred.
Introduction
Recently, clay has emerged as a ubiquitous support material
owing to its lamellar structure and mechanical/thermal stability.[15]
The low-cost, ordered layered structure, unique swelling,
intercalation and high ion exchange property make this clay a
suitable candidate to use as a promoter, catalyst precursor and
also as a good support.[16] The outstanding feature of clay is that
H2O and other organic/inorganic entities can enter between unit
layers resulting in expansion of lattice in c-direction.[17] These
layers are functionalized by cation exchange by isomorphous
substitution, which causes charge imbalance. This is balanced by
interlayer compensatory hydratory cations such as Na+, Ca2+ and
Mg2+, which on further exchange introduces the catalytically
active materials to layers.[18] Moreover, acid treatment with dilute
acids like HNO3 resulted in the relocation of Al in interlayer
spacing which acts as brønsted sites, thus modifying acidic
centres.[19]
Considering ever-growing environmental cognizance,
a
sustainable green chemistry route to synthesize fine chemicals is
challenging and appeals to significant attention worldwide.[1]
Homogeneous catalysts offer unique advantages in terms of high
activity and selectivity; however, the catalyst recovery process for
an industrial scale requires laborious purification, which is
economically unviable.[2] Moreover, the catalyst recovery process
often generates a large amount of leftover chemicals which is
detrimental from the perspective of green chemistry.[3] Therefore,
heterogeneous catalysts are preferred as they allow the isolation
of catalytic sites resulting in facile separation and catalyst reuse.[4]
The catalytic hydrogenation of imines is
a clean and
environmentally benign methodology. The resultant amines are
necessary fine-chemicals that serve as building blocks for various
[5]
agrochemical and pharmaceutical products.
Several metal-
In recent years’ various clay minerals have been used as a robust
support for palladium to carryout important organic
transformations considering its low cost, simple preparation
method and remarkable reusability.[20] For instance, palladium
based bifunctional metal-acid catalyst supported on K-10 acidic
clay was used to transform HMF into 2,5-dimethyl furan (2,5-
DMF)[21], while Sreekumar et al. used clay-supported Pd catalyst
for the regioselective arylation of 3,4-ethylenedioxythiophene.[22]
Montmorillonite clay play a key role in the Pd catalysed furfural
hydrogenation to give 1,2-pentanediol (1,2-PeDO) with 66%
selectivity[23]. Another significant work reported is the Pd-
supported halloysite nanoclay (PdM@Hal) for efficient methane
oxidation.[24] Apart from these reports, the applicability of clay for
palladium-based catalyst can be understood from the enormous
based heterogeneous catalysts including platinum group metals
(PGM), first-row transition metals, alkali and alkaline earth metals
have been reported for hydrogenation reactions.[6]. Considering
the cost-effectiveness first-row transition metals are preferred, but
they require high H2 pressure and reaction temperature to break
the metal-hydrogen (M-H) bond. In addition, the 3d metals
tendency to engage in radical chemistry make them a less
attractive candidate. In the case of alkali metals, the absence of
vacant d orbital leads to severe substrate activation and low
product selectivity.[5a] In this regard, the noble metal palladium
offers a viable solution as it has weak Pd-H bond strength,
availability of vacant d orbitals and can dissolve a large amount
of hydrogen in its crystal lattice.[7] However, due to its high cost
1
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Scheme 1. Schematic representation of Pd NPs intercalated clay synthesis and proposed working mechanism of the interaction of Pd NPs with adsorbed H2 and
substrate imine.
application in numerous coupling reaction such as Suzuki-
Miyaura,[25] Ullmann coupling,[26] Sonogashira Reaction,[27] Heck–
Mizoroki reactions[28] etc. Moreover, we have also studied the
effective utilization of Pd NPs[29] and Ni/Co intercalated
montmorillonite clay[30] as green catalysts for the chemo selective
hydrogenation of squalene into squalane and for the conversion
of microalgae oil to diesel-grade hydrocarbons. In this
continuation of our research of developing green synthesis
protocols, we herein propose the use of montmorillonite clay as a
robust support for immobilization of Pd NPs and its application
potential for catalytic hydrogenation of imines. The influence of
reaction conditions (catalyst loading, H2 pressure, reaction time,
temperature) and recycling on the structural modification, activity
and selectivity of the desired product is investigated in detail. The
study also includes extensive characterization of prepared
catalyst that quantifies its synthetic efficiency and is expected to
have a profound impact in the field of sustainable catalysis.
The natural clay is composed of montmorillonite where octahedral
AlO6 sheets are sandwiched between two tetrahedral SiO4 sheet
as shown in Scheme 1. The interlayer space is occupied by K+
and Ca2+ ions coordinated by H2O molecule. The Pd intercalation
in natural clay was achieved by a simple wet impregnation method
in dilute nitric acid. Generally, during intercalation, swelling of clay
occurs which facilitates the cation exchange leading to the
effective incorporation of Pd2+ ions between the clay interlayers.
The incorporated Pd2+ ions on calcination under hydrogen
atmosphere lead to catalytically active Pd0 NPs.[31] The detailed
mechanism of intercalation is illustrated in Scheme 1. Accordingly,
natural clay-based catalysts loaded with 2, 4, 6 and 10 wt% Pd
were prepared and named as 2% Pd/Clay, 4% Pd/Clay, 6%
Pd/Clay and 10% Pd/Clay respectively. To understand the phase
formation, these catalysts were characterized by X-ray diffraction
(XRD) technique. The obtained XRD patterns of all synthesized
catalyst and also the pristine clay is shown in Fig. 1.
The resultant XRD pattern (Fig. 1a) revealed the crystalline
nature of the clay with characteristic peaks of montmorillonite and
quartz content (Fig. S1) of the supporting information.[29] The
broad diffraction peaks centered at 2θ values of 40°, 46.81°, 68°
and 82° correspond to the (111), (200), (220), (311) crystal planes
of face centered cubic (fcc) structure of metallic Pd in accordance
with JCPDS No 05-0681.[32] It is observed that as the Pd loading
in clay increases, the intensity of (111) plane increase gradually
( Fig. 1 b, c, d and e). This indicates the preferential growth of
(111) plane.[33] From the XRD pattern as shown in Fig. 1, neither
peak shifting of d(001) plane of clay nor its disappearance was
Results and Discussion
The characterization studies of Pd intercalated clay catalyst and
their significance in hydrogenation studies were started primarily
with its preparation using natural clay. The general formula of clay
used in this study is K3Ca1Ti0.6Al12Si40Fe6HxOy and was calculated
by collective X-ray fluorescence (XRF) spectrometry and
scanning electron microscopy coupled with energy dispersive X-
ray (SEM/EDX) spectroscopy reported in our previous report.[29]
2
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Figure 1. XRD patterns of Clay and Pd/Clay catalysts catalyst loaded with
different wt %Pd.
Figure 2. TEM images of Pd loaded clay with 2 wt%, 6 wt% and 10wt% (a-c)
and their SAED pattern respectively (d-f).
observed as earlier reported in literatures.[33-34] This may be due
to spacing irregularity of clay layers upon intercalation resulting in
presence of Pd particles on the clay layers. Additionally, the
intensity of (001), (002) and (003) planes of the montmorillonite
clay is decreased significantly upon metal intercalation (see Fig.
S1 b, c and d), which indicated the effective metal intercalation
occurs during wet impregnation aided by dilute nitric acid.
Further, the intercalation, surface morphology and elemental
distribution of Pd over the clay surface were confirmed by
transmission electron microscopy (TEM) as shown in Fig. 2. From
the Fig. 2 a, b and c (for 2% Pd/Clay, 6% Pd/Clay and 10%
Pd/Clay), spherical NPs of Pd with size around 2−20 nm were
found and the NPs are well-dispersed on the clay support. The
interplanar d-spacing of 2.6Ǻ and 2.3Ǻ which corresponds to the
Pd(111) plane for 2% Pd/Clay and 10% Pd/Clay respectively. [35]
Additionally, the detailed morphological studies and intercalation
properties of clay and Pd incorporated clay (for 2% Pd/Clay, 6%
Pd/Clay and 10% Pd/Clay) can be understood from TEM image
(Fig. 3). The layered structure of pristine clay of distinct layers can
be evidently seen and marked in Fig. 3(a). Furthermore, Fig. 3
(b), (c), (d) demonstrates that Pd intercalation has not
significantly affected the clay-layer structure.
The tubular structure observed in Fig. 3d may be attributed to the
presences of traces of halloysite content which is not evident from
the XRD analysis.[20a] Moreover, Pd NPs were found to be present
inside and outside of the clay layers, which confirms the
successful intercalation of Pd onto the clay.[36] The Selected Area
Electron Diffraction (SAED) patterns for the samples (Fig. 2 d, e
Figure 3. TEM images of clay (a) 2 wt% Pd (b) 6 wt% Pd (c) 10 wt% Pd (d)
loaded clay
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Figure 4. (a) XPS Survey scan of 6 wt% Pd/Clay catalyst, XPS core level spectra of 6 wt% Pd/Clay catalyst showing (b) Si 2p (c) Pd 3d (d) O 1s (e) Fe 2p.
and f) showed bright small spots forming rings of Pd (111), Pd
(200) and Pd (220) planes which revealed the polycrystalline
nature of catalyst.[37] The presence of active metal Pd and the
other metals from the clay such as Fe, Si, O, Ca and K were
identified from the EDS spectra of the catalyst and were given in
Fig. S2.
catalyst were shown in Fig. 5. For pure Montmorillonite clay
sample, no reduction peak was found below 450 ℃. However, all
the Pd loaded clay catalysts exhibited one strong negative peak
at 65-72 °C, which is characteristic of the decomposition of the β-
PdH phase formed at RT by adsorption of hydrogen on the Pd
based catalyst as illustrated by equation:[42]
The surface elemental composition and oxidation state of active
metals in clay catalyst were investigated by using X-ray
photoelectron spectroscopy (XPS) as shown in Fig. 4. XPS
survey scan (Fig. 4a) revealed the presence of Pd, Fe, Si, Ca, K
and O species in the catalyst which is corroborated with the EDS
data. The C 1s peak obtained at 284.8 eV was used for spectral
calibration. Core level spectra of palladium in Fig. 4c revealed two
peaks at binding energy 335.2 and 340.48 eV which corresponds
to 3d5/2 and 3d3/2 electronic states of Pd(0) respectively.[38] No
other peaks for Pd2+ were observed for the sample. The core
PdO+ H2 →βPdH→Pd(s)+ H2
spectra for Fe (Fig. 4e) showed two peaks at 710.552 and 723.80
[39]
eV respectively which were assigned as Fe 2p3/2 and Fe 2p1/2
.
A satellite peak of Fe(II) 2p3/2 is found at binding energy of 714.05
eV. The Si 2p spectra (Fig. 4b) present a single band around
102.6 eV which is characteristic of silicon bonded to carbon and/or
oxygen.[40] The O 1s core spectra displayed a single strong peak
around 532 eV in Fig. 4d which is related to the silicon oxygen
bonding (Si-O-Si) present in silica and shows good agreement
with clay minerals studies.[41]
To understand the reducibility of the synthesized catalyst
temperature programmed reduction (H2-TPR) analysis was
carried out and the TPR profiles for clay and Pd loaded clay
Figure 5. H2 TPR Profile of clay and unhydrogenated Pd/Clay catalysts with
different wt% Pd
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Intensity and position of this negative peak depend on the size of
supported Pd metal particles. It can be observed from TPR profile
that as the palladium loading is increased, the fraction of
palladium increases resulting in more pronounced negative peak.
H2 consumption between 100 and 200 °C is clearly observed for
Pd 2% and Pd 4% resulting from the reduction of small nanosized
PdO species and is supported from TEM analysis.[43] Reduction
process in high temperature range of 250–550 °C range is due to
the reduction of PdO particles interacting strongly with the surface
of the support.
according to the BDDT classification, and confirm the
mesoporous layered nature of the clay.[44] . Compared to the
pristine clay sample, there is decrease in the N2 adsorption
capacity of palladium loaded clay and the associated surface area
from 44.547m2/g in clay to 16.519m2/g in 6 wt% Pd/Clay. This
could be ascribed to the space occupied by metallic Pd on the
clay surface. Further, the appreciable increase in the average
pore size of the Pd incorporated clay (from 25.25 Ǻ (2 wt%
Pd/Clay) to 165.7 Ǻ (10 wt% Pd/Clay )) as compared to pristine
clay (15.83 Ǻ) as shown in Table S1 indicates that the occurrence
of Pd NPs in interlayered clay.[36] A huge increase of the average
pore size is observed for 10 wt% Pd/Clay. This is because, with
increase in metal loading, the smaller pores are closed, while
larger pore size are only available for physisorption.
N2 adsorption-desorption isotherms, adsorption plots and BJH
pore distribution curve at 77 K of the initial montmorillonite and
the metal intercalated clays are shown in Fig. S3, which displayed
typical type IV N2 isotherms with an H3 type hysteresis loop
Table 1: Catalytic hydrogenation of 3-diphenyl prop-2-en-1-imine with prepared catalyst[a]
Entry
Catalyst
Reaction
time [h]
Temperat
ure
Pressure
[bar]
Conversion
[%]
Selectivi
ty [b] A%
Selectivity[c] B%
Substrate to
Metal ratio
TOF [h-1]
[°C]
1
2% Pd/clay
2% Pd/clay
2% Pd/clay
2% Pd/clay
2% Pd/clay
4% Pd/clay
4% Pd/clay
4% Pd/clay
4% Pd/clay
6% Pd/clay
6% Pd/clay
6% Pd/clay
6% Pd/clay
6% Pd/clay
6% Pd/clay
6% Pd/clay
6% Pd/clay
10% Pd/clay
6% Pd/SiO2
6% Pd/Al2O3
5
100
100
100
70
15
15
15
15
15
15
15
15
15
15
12
10
12
7
84 ± 0.9
97 ± 0.8
100
84 ± 0.9
97 ± 0.8
100
-
1: 4 x 10-3
1: 4 x 10-3
1: 4 x 10-3
1: 4 x 10-3
1: 4 x 10-3
1: 9 x 10-3
1: 9 x 10-3
1: 9 x 10-3
1: 9 x 10-3
1: 11 x 10-3
1: 11 x 10-3
1: 11 x 10-3
1: 11 x 10-3
1: 11 x 10-3
1: 11 x 10-3
1: 11 x 10-3
1: 11 x 10-3
1: 18 x 10-3
1: 1 x 10-3
1: 12 x 10-3
39.23
28.26
19.25
18.98
16.49
29.61
20.91
13.07
10.45
17.62
17.62
17.62
29.38
29.38
25.18
27.24
26.46
10.60
14.19
15.20
2
8
-
3
12
12
12
3
-
4
98 ± 0.6
87 ± 0.5
84 ± 1
100
98 ± 0.6
85 ± 0.2
84 ± 1
100
-
5
50
2 ± 0.3
6
100
100
100
100
100
100
100
100
100
100
100
50
-
7
5
-
8
8
100
100
-
9
10
5
100
100
-
10
11
12
13
14
15
16
17
18
19
20
100
100
-
5
100
100
-
5
100
100
-
3
100
100
-
3
100
100
-
3.5
3
5
100
100
-
5
96 ± 0.5
96 ± 0.4
100
92 ± 0.7
90 ± 0.1
100
3 ± 0.5
6 ± 0.2
-
3
5
5
100
100
100
15
5
5
88 ± 0.5
89 ± 0.6
80 ± 0.5
86 ± 0.5
7 ± 0.6
3 ± 0.4
5
5
[a] Reaction was performed with 0.5g Schiff base and 0.05g prepared catalyst with 10 ml methanol under varied temperature and pressure condition
[b] A = N-(3-phenylpropyl) aniline, [c] = N,3-diphenyl propan-1-amine
Thermogravimetric analysis (TGA) revealed the thermal stability
of prepared catalyst and is shown in Fig. S4. TGA profile of
processed clay show weight loss due the removal of the
physisorbed water (below 100 °C), loss of the strongly bonded
water molecules existing in the first coordination sphere of the
interlayer ions (100-500 °C) and the loss of structural hydroxyl
groups (500-900 °C).[45] TGA analysis of palladium intercalated
catalyst revealed that catalysts with higher metal loadings (Pd 6
wt% and Pd 10 wt%), were stable up to 900 °C with slight initial
weight reduction, which could be ascribed to loss of water
molecules.[46] However, the significant weight loss was observed
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with less metal loading particularly for Pd 2% and Pd 4%, which
indicates that catalyst with higher metal loadings are more
thermally stable. The maximum weight loss (%) was observed in
4 wt% Pd/Clay probably owing to the weakening of bonds in the
clay in this case. The FT-IR spectrum of clay and palladium
intercalated clay is recorded and is shown as in Fig. S5 and it
exhibited the presence of O−H bonds at 3627 cm−1 (OH
stretching) and 3408 cm−1 (H-O-H stretching) corresponds to the
hydroxy groups of clay and interlayer water molecules A peak at
1629 cm−1 was detected for O−H bending. The presence of silica
is represented by broad and intense peak detected at 1018 cm−1
(Si-O-Si stretching).[47] Broadly, quartz presence is identified in the
wavenumber region from 1200 – 500 cm-1. The peak intensities
due to O-H bonds were reduced after wet impregnation due to
dihydroxylation caused by nitric acid.
1, exp no.15). When the time is further reduced to 3 h, nearly
complete conversion was obtained but with the presence of side
product represented by Selectivity B% (Table 1, exp no. 16).
Moreover, the control experiment for the SiO2 and Al2O3 loaded
with 6% Pd/Clay was performed and the complete conversion was
not obtained as shown in Table 1, entry 19 and 20 respectively.
This clearly indicates the advantage of clay as a support in
comparison to SiO2 and Al2O3. From Table 1, it is understood that
the proposed catalyst is cost effective and exhibits excellent
reactivity towards imine hydrogenation. Here, the intrinsic
properties of the clay support have played a significant role in
improving the catalytic performance. During the hydrogenation
process, firstly the molecular H2 binds with the Pd surface
resulting in its dissociation to form H atoms that occupy the
octahedral holes within the Pd surface. These adsorbed H atoms
interact with the substrate imine, where Pd intercalated clay
Optimization of reaction parameters
The effects of reaction parameters (catalyst loading, reaction time, provides the working surface for hydrogenation reaction. This is
temperature and H2 pressure) were studied to optimize the
hydrogenation of 3-diphenyl prop-2-en-1-imine to 3-
phenylpropylaniline as shown in Table 1. In order to identify the
appropriate amount of palladium, the influence of different
catalytic loading (2, 4, 6 and 10 wt%) was investigated. Increasing
the metal loading leads to increase in conversion efficiency. It can
be seen from Table 1, in case of lower metal loading of 2 wt%
Pd/Clay, complete conversion was only achieved at the extended
reaction of 12 h (Table 1, exp no. 3). In comparison, at higher
metal loading (4 wt% Pd/Clay), it occurs within a shorter reaction
period of 5 h (Table 1, exp no. 7). This is attributed to the increase
in the available active sites with an increase in palladium
content.[21] The influence of variation in reaction time on
conversion of 3-diphenyl prop-2-en-1-imine was also studied. It
was found that in case of 2 wt% Pd/Clay, conversion of 3-diphenyl
prop-2-en-1-imine was raised from 84.66% to 97.84% with
reaction time increased from 5 h to 8 h (Table 1, exp no 1, 2) at
100 °C and 15 bar pressure. Interestingly, increasing the reaction
time further to 12 h results in the complete conversion (Table 1,
exp no. 3). To understand the impact of temperature on
conversion and selectivity of the reaction, the temperature was
varied in range 50 -100 °C. It was observed that with a change in
temperature, both the conversion and selectivity gets affected.
For 2 wt% Pd/Clay, it has been seen that complete conversion
occurs at 100 °C with reaction time of 12 h, but as the temperature
is decreased to 70 °C, there is a decrease in conversion to
98.60% (Table 1, exp no. 4). When the temperature is further
decreased to 50 °C the conversion value decreases to 87.94%
with a decrease in selectivity towards 3-phenylpropylaniline
denoted by selectivity A% (Table 1, exp no. 5). Another important
variable that plays a crucial role in catalytic activity in liquid-phase
hydrogenation reaction is H2 pressure. As the H2 pressure
increases, the dissolved concentration of H2 in reaction mixture
rises resulting in the high conversion values.[48] Thus,
maintenance of optimum pressure is a key to obtain high yield
product. For 6 wt% Pd/Clay catalyst, complete conversion is
achieved despite the pressure is decreased from 15 bar to 10 bar
(Table 1, exp no. 10-12). To further investigate the optimized
reaction conditions, H2 pressure and reaction time were
decreased simultaneously and their effect on the conversion and
selectivity was examined at 100 °C. To our delight, for 6 wt%
Pd/Clay, complete hydrogenation of 3-diphenyl prop-2-en-1-imine
occurs with 3-phenylpropylaniline as the sole product, when the
H2 pressure was as low as 5 bar with reaction time 3.5 h (Table
according to the hydrogenation studies based on Horiuti–Polanyi
mechanism,[49] which suggest C=C bonds adsorption on the
catalyst form two M-C chemical bonds, which leads to a classical
three-centered transition state (C, M, H). Further identification of
the reaction products, calculation of conversion and selectivity
were done by comparing GC chromatograms and NMR data of
imines and product (Fig. S6, S7). A comparison between
conversion and yields of as prepared Pd/Clay catalyst and
previously reported transition metal catalyst is given (Table S2).
From Scheme 1, it can be observed that the expansion of
interlayer spacing in clay allows the formation of Pd NPs with
highly faceted Pd (111) plane which is supported by XRD analysis
(Fig. 1). The result obtained is corroborated to Zheng and
coworkers.[50] According to their report, the hydrogenation
reaction by Pd nanocrystals is facet-dependent catalysis reaction.
To further substantiate the reactivity of catalyst, a study of imine-
mixed catalyst was performed. In this regard, an equal amount of
2-diphenyl prop-2-en-1-imine and highly reactive 6 wt% Pd/Clay
catalyst were mixed in the presence of methanol at 10 bar H2
pressure for 2 h at RT and then methanol was air-dried at 80 °C
overnight. The XRD patterns of imine-mixed clay catalyst (Fig. 6a)
shows weakly intense characteristic Pd (111) peak compared to
the pristine catalyst, indicating the strong imine adsorption on the
Pd surface at RT.
This signifies the considerable role of Pd (111) plane in
determining catalytic activity.[51] This strong metal-substrate
interaction enables the activation of C=N and C=C bonds for
effective hydrogenation. The IR spectrum of imine-mixed 6 wt%
Pd/Clay (Fig. 6b) shows the narrowing of IR bands to give sharper
peaks which again confirm the strong adsorption of imines on Pd
surface.
Reusability and metal leaching studies
After demonstrating the reactivity of catalyst, reusability was
ascertained to determine the stability of the catalyst.[52] To do the
reusability test, the catalyst was recovered after the
hydrogenation reaction. The obtained catalyst were washed with
methanol three times to extract the whole reaction products. The
remaining catalyst obtained was dried at 60 °C and tested for
further hydrogenation reaction under optimized conditions up to 5
cycles. The comparative XRD patterns of 6 wt% Pd/Clay catalyst
are shown in (Fig. S8) for fresh catalyst and reused catalyst after
3 and 5 cycles. There was no significant difference in the XRD
patterns of fresh and reuse catalysts.
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exhibited 100% conversion of the imine with 100% selectivity of
3-phenylpropylaniline without any side products. The catalyst is
stable and can be reused over five cycles. Owing to its cost
effectiveness, simple synthesis procedure and high catalytic
activity, this study would help in crafting rational approaches to
explore other hydrogenation reactions by bridging the two
disciplines of metal NPs and clay systems.
Experimental Section
Materials
Natural montmorillonite clay was obtained from a native source in
Rajasthan, India. Palladium nitrate hydrate (99.9 %) was procured from
Alfa Aesar. Extra dry methanol, Aniline and Nitric acid were purchased
from Acros organics Fisher Scientific. Trans Cinnamaldehyde was
obtained from Spectrochem. Ethanol was obtained from Changshu
Hongsheng Fine Chemical Co.Ltd.
Figure 6. (a) XRD spectra of 6 wt% Pd/Clay, imine-mixed 6 wt% Pd/Clay and
its hydrogenated product. (b) FTIR spectra of prepared imine, 6 wt% Pd/Clay
and 6 wt% Pd/Clay/imine.
Preparation of schiff base
Metal leaching experiments were also conducted for 6%Pd/Clay
catalyst. For these, the hydrotreatment reaction was carried out
with 5g substrate and 500 mg catalyst using 100 mL methanol (at
100 °C, 5 bar H2 pressure at 3.5 h). Upon completion of the
reaction, the liquid product was separated from catalyst through
centrifugation. These are analysed by ICP-OES measurement for
the palladium leaching content. The remaining catalyst was
washed with methanol, dried and was used in subsequent five
cycles. During each cycle, the amount of the catalyst and the
amount of palladium in the product were analysed and which is
given in Table S3 in the Supporting Information. The amount of
Pd leaching in the separated product after each cycle was found
to be less than 0.5 ppm. Consequently, a small drop in the
conversion values was obtained after the second cycle. After the
third cycle, the conversion value is decreased to 94%. which is
probably due to the catalyst lost during the centrifugation process
and metal leaching after the hydrogenation reaction. The actual
metal loading and leaching of metal is examined by ICP-OES
analysis and the obtained results is shown in Table S3,Table S4
and Table S5. These experiment results indicates the excellent
stability as well as efficiency of the Pd/Clay catalysts for the
hydrogenation of imine under mild reaction conditions.
Schiff base, 3-diphenyl prop-2-en-1-imine is synthesized by condensation
of Equimolar concentration of Trans Cinnamaldehyde and Aniline (0.01
mol) in ethanol (5ml), and the resultant mixture was gently stirred at
ambient temperature. The progress of the reaction was monitored by TLC.
The resultant mixture was left undisturbed at RT for half an hour until small
yellowish crystals start growing. It was then carefully set in an ice bath to
complete the crystallization process. The chilled solution was then filtered
through vacuum filtration to isolate the pure crystals, and the crystals were
washed with cold ethanol solvent.
Preparation of catalysts
A series of palladium loaded (2, 4, 6, 10 wt%) on to clay catalysts were
prepared following the procedure reported earlier.19 Briefly, the clay was
sequentially treated with 1N HNO3 and 1N NH4OH for 12 h each at RT.
The resulting clay was washed thoroughly with deionized water and dried
overnight in an oven at 120 ℃ to be utilized as the solid support. Palladium
metal was incorporated by using the wet impregnation method. The
required amount of Palladium(II) nitrate hydrate in 3N HNO3 (10 mL) was
impregnated onto clay support (1g), and the mixture was dried at 120℃
with incessant stirring until it forms a thick paste. The powdered material
was calcined at 500 ℃ for 5 h (flow rate: 2 mL/min) under a nitrogen
atmosphere, succeeded by reduction of the metal at 300 ℃ for 3 h (flow
rate: 2 mL/min) under a hydrogen atmosphere. The resulting solid was
finely grinded to form a fine powder.
Conclusion
Characterization
In conclusion, a green and sustainable natural clay-based
catalytic system is developed via easy one-step wet impregnation
method and applied for hydrogenation of imines. Here, the clay
plays a vital role as the support for the immobilisation of Pd NPs
owing to its intrinsic properties. The interlayered structure of the
clay and effective intercalation was confirmed by various
characterization techniques such as XRD, TEM and BET analysis.
Among the different Pd loaded clay catalysts, 6 wt% Pd loading
showed the good stability and exhibits excellent high catalytic
performance for the hydrogenation of imines. This is attributed to
the dominating highly faceted Pd (111) plane in the interlayered
clay structure. Hydrotreatment studies revealed that reaction
parameters play a crucial role in determining activity and
selectivity. The obtained GC results established that under
optimized conditions of 5 bar H2 pressure for 3.5 h, 6 wt% Pd/Clay
X-ray diffraction (XRD) measurements were performed using D8 Advance
diffractometer (Bruker, U.S.A.), with Cu Kα (λ = 1.54Å) as the radiation
source at 40 kV voltage and 40 mA tube current for scanning diffraction
angle in the range of 5 to 90°. For morphological and elemental
composition studies, High-resolution Transmission Electron Microscope
(HR-TEM) analysis was carried out using TALOS F200S G2 instrument.
The hydrogen temperature-programmed reduction (H2-TPR) was
performed on a Chemisorption Analyzer (AMI-300, Altamira Instruments)
equipped with a thermal conductivity detector (TCD). For H2 TPR studies,
pre-treatment at rate 10 °C per min under helium for 1 hr is done until it
reaches 150 °C, then it is cooled to ambient temperature. A mixture of H2
(5%) + He (95%) was flowed at a rate of 30 mL/min to be used as reducing
agent and heated to 800 °C at 10 °C/min and maintained for 30 min and
TPR profile was monitored in a range of 30-800 °C. Autosorb-1Q-MP-XR
(Quantachrome Instruments, USA) was used for adsorption-desorption
measurement to perform surface area and porosity analysis. N2 gas was
7
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10.1002/cplu.202000760
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FULL PAPER
taken as carrier gas with 99.995% purity. The sample was degassed under
Keywords: • heterogeneous catalysis • hydrogenation • imines •
montmorillonite clays • Pd nanoparticles
a
vacuum at 180 °C for 3 h, which is followed by nitrogen
adsorption−desorption analysis at -196 °C. Thermogravimetric (TGA)
analysis was done on Simultaneous Thermal Analyzer (STA) – 6000 from
Perkin Elmer under nitrogen atmosphere at a flow rate of 19.8 mL/min and
pressure 3 bar with a heating rate of 10 °C min−1. Fourier transformed
infrared spectroscopy (FTIR) from Bruker (Vertex, 70V+PMA50) was used
for recording the FTIR spectrum from 400 cm-1 to 4000 cm-1 at room
temperature (RT) with 32 scan performance. The surface composition and
the elemental states were determined with X-ray photoelectron
spectroscope (XPS) by an Omicron NanoTechnology GmbH (Oxford
Instruments) equipped with the monochromatic Al Kα radiation. The
charge correction was done with reference to standard carbon 1s peak
attained at 284.1 eV. Nuclear Magnetic Resonance (NMR) spectra were
recorded on an 11.2 Tesla Bruker operating at 500 MHz in CDCl3. Gas
chromatography (GC) analysis was performed by using PerkinElmer
Clarus 580 gas chromatograph equipped with Elite-5 capillary column with
30 m length, 0.10 μm film thickness, 0.25 mm inner diameter and a
temperature limit of −60 to 325/350 °C.
References
[1]
a) M. Poyatos, F. Márquez, E. Peris, C. Claver, E. Fernandez, New J.
Chem. 2003, 27, 425-431; b) C.-J. Li, B. M. Trost, Proc. Natl. Acad. Sci.
U. S. A. 2008, 105, 13197; c) O. V. Kharissova, B. I. Kharisov, C. M. O.
González, Y. P. Méndez, I. López, R. Soc. Open Sci. 2019, 6, 191378;
d) J. H. Clark, Green Chem. 1999, 1, 1-8.
[2]
[3]
P. J. Deuss, K. Barta, J. G. de Vries, Catal. Sci. Technol. 2014, 4, 1174-
1196.
a) R. Nie, M. Chen, Y. Pei, B. Zhang, L. Qi, J. Chen, T. W. Goh, Z. Qi, Z.
Zhang, W. Huang, ACS Sustainable Chem. Eng. 2018, 7, 3356-3363; b)
S. S. Joshi, A. Bhatnagar, V. V. Ranade, in Ind. Catal. Processes Fine
Spec. Chem. (Eds.: S. S. Joshi, V. V. Ranade), Elsevier, Amsterdam,
2016, 317-392.
[4]
a) J. M. Fraile, N. García, C. I. Herrerías, M. Martín, J. A. Mayoral, ACS
Catal. 2011, 2, 56-64; b) D. Macquarrie, in Heterogenized Homogeneous
Catalysts for Fine Chemicals Production: Materials and Processes (Eds.:
P. Barbaro, F. Liguori), Springer Netherlands, Dordrecht, 2010, 1-35; c)
M. J. Climent, A. Corma, S. Iborra, Chem. Rev. 2011, 111, 1072-1133.
a) H. Bauer, M. Alonso, C. Färber, H. Elsen, J. Pahl, A. Causero, G.
Ballmann, F. De Proft, S. Harder, Nat. Catal. 2018, 1, 40-47; b) X. Jv, S.
Sun, Q. Zhang, M. Du, L. Wang, B. Wang, ACS Sustainable Chem. Eng.
2019, 8, 1618-1626.
Hydrogenation of imine
[5]
[6]
All the hydroprocessing was carried out in a high pressure vessel equipped
with a steel container of capacity 250mL provided by Amar Equipments
Pvt. Ltd. The catalyst (50 mg), as prepared 3-diphenyl prop-2-en-1-imine
(500 mg) and dry methanol (10ml) were taken in the air-dried container. A
vacuum was created in the container, accompanied by purging the
hydrogen gas. The reaction temperature was raised from 25 ℃ at 5 ℃ min-
a) Y. Obora, T. Ohta, C. L. Stern, T. J. Marks, J. Am. Chem. Soc. 1997,
119, 3745-3755; b) H. Elsen, J. Langer, M. Wiesinger, S. Harder,
Organometallics 2020.
1
to attain the required temperature (50 - 100 ℃) prolonged for several
[7]
[8]
[9]
Q. A. Chen, Z. S. Ye, Y. Duan, Y. G. Zhou, Chem Soc Rev 2013, 42,
497-511.
hours for time- dependent studies at varied pressure. At the end of the
catalytic test, the reactor was allowed to attain RT and depressurized. The
mixture was centrifuged and the liquid fraction was separated. Further,
methanol (10 mL × 2) was added to the catalyst, stirred, centrifuged to
extract most of the product. Conversion and selectivity of reaction were
measured by GC equipped with FID detector. The product formation was
further supported by 1H NMR, 13C NMR spectroscopy and mass
spectroscopy.
P. Das, P. P. Sarmah, B. J. Borah, L. Saikia, D. K. Dutta, New J. Chem.
2016, 40, 2850-2855.
Y.-F. He, J.-T. Feng, Y.-Y. Du, D.-Q. Li, ACS Catal. 2012, 2, 1703-1710.
[10] P. P. Sarmah, D. K. Dutta, Green Chem. 2012, 14.
[11] C. Wang, L. Wang, J. Zhang, H. Wang, J. P. Lewis, F.-S. Xiao,
Am. Chem. Soc. 2016, 138, 7880-7883.
J.
[12] F. Yang, C. Chi, S. Dong, C. Wang, X. Jia, L. Ren, Y. Zhang, L. Zhang,
Y. Li, Catal. Today 2015, 256, 186-192.
Recycling tests
[13] E. J. García-Suárez, P. Lara, A. B. García, M. Ojeda, R. Luque, K.
Philippot, Appl. Catal., A 2013, 468, 59-67.
[14] S.-Y. Chen, L. Attanatho, T. Mochizuki, Q. Zheng, M. Toba, Y. Yoshimura,
P. Somwongsa, S. Lao-Ubol, Adv. Porous Mater. 2016, 4, 230-237.
[15] T. J. Pinnavaia, Science 1983, 220, 365-371.
To do the reusability test, the used catalyst for hydrogenation reaction was
recovered after the reaction by centrifugation. The obtained catalyst were
washed with methanol three times to extract the whole reaction products
and unreacted substrate if any. The obtained catalyst was dried overnight
at 60 °C in vacuum oven. These were and tested further for hydrogenation
reaction as explained above under the optimized reaction conditions up to
5 cycles.
[16] a) D. Dutta, D. K. Dutta, Appl. Catal., A 2014, 487, 158-164; b) T.
Hayakawa, M. Minase, K.-i. Fujita, M. Ogawa, , Ind. Eng. Chem. Res.
2016, 55, 6325-6330.
[17] D. Manikandan, D. Divakar, T. Sivakumar, Catal. Commun. 2007, 8,
1781-1786.
[18] a) B. S. Kumar, A. Dhakshinamoorthy, K. Pitchumani, Catal. Sci. Technol.
2014, 4, 2378-2396; b) G. Qiu, C. Huang, X. Sun, B. Chen, Green Chem.
2019, 21, 3930-3939.
Leaching Tests
The leaching tests were carried out in a Berghof HR-1000 high pressure
reactor equipped with a teflon container of 1000 mL capacity. The liquid
product obtained after the hydrogenation reaction of 5 g 3-diphenyl prop-
2-en-1-imine with 0.5g of 6% Pd/Clay catalyst at 100 °C, 5 b pressures for
3.5 hr was analyzed by ICP-OES.
[19] F. M. T. Luna, J. A. Cecilia, R. M. A. Saboya, D. Barrera, K. Sapag, E.
Rodriguez-Castellon, C. L. Cavalcante, Jr., Materials (Basel) 2018, 11.
[20] a) Y. Zheng, L. Wang, F. Zhong, G. Cai, Y. Xiao, L. Jiang, Ind. Eng. Chem.
Res. 2020, 59, 5636-5647; b) C. B. Molina, A. H. Pizarro, J. A. Casas,
J. J. Rodriguez, Appl. Catal., B 2014, 148-149, 330-338; c) S. Barama,
C. Dupeyrat-Batiot, M. Capron, E. Bordes-Richard, O. Bakhti-
Mohammedi, Catal. Today 2009, 141, 385-392; d) L. C. A. Oliveira, R. M.
Lago, J. D. Fabris, K. Sapag,
Appl. Clay Sci. 2008, 39, 218-222; e)
Acknowledgements
G. Ngnie, G. K. Dedzo, C. Detellier, Dalton Trans. 2016, 45, 9065-9072.
[21] A. B. Gawade, M. S. Tiwari, G. D. Yadav, ACS Sustainable Chem. Eng.
2016, 4, 4113-4123.
The authors express their gratitude toward DBT-PAN IIT Centre
for Bioenergy (BT/EB/PANIIT/2012) for financial assistance. We
also thank Centre for Advanced Scientific Equipment (CASE)-IIT
Jodhpur, Scientium Analyze Solutions for XPS, CeNS Bangalore
for TEM analysis. We also thank Mr. Bhagirath for helping in
calculations and proof reading.
[22] S. Sreekumar, S. Xavier, A. Govindan, R. E. Thampikannu, K. Vellayan,
B. González, Res. Chem. Intermed. 2020, 46, 4529-4542.
[23] N. S. Date, R. C. Chikate, H.-S. Roh, C. V. Rode, Catal. Today 2018,
309, 195-201.
[24] Y. H. Ahmad, A. T. Mohamed, K. A. Mahmoud, A. S. Aljaber, S. Y. Al-
Qaradawi, RSC Adv. 2019, 9, 32928-32935.
8
This article is protected by copyright. All rights reserved.

10.1002/cplu.202000760
ChemPlusChem
FULL PAPER
[25] P. R. Boruah, P. S. Gehlot, A. Kumar, D. Sarma, Mol. Catal. 2018, 461,
54-59.
[26] K. Vellayan, B. González, R. Trujillano, M. A. Vicente, A. Gil, Appl. Clay
Sci. 2018, 160, 126-131.
[27] R. K. Loganathan, S. N. Ramachandra, Shekharappa, V. V. Sureshbabu,
ChemistrySelect 2017, 2, 8059-8062.
[28] A. V. Martínez, A. Leal-Duaso, J. I. García, J. A. Mayoral,
RSC Adv.
2015, 5, 59983-59990.
[29] V. K. Soni, R. K. Sharma, ChemCatChem 2016, 8, 1763-1768.
[30] V. K. Soni, P. R. Sharma, G. Choudhary, S. Pandey, R. K. Sharma, ,
ACS Sustainable Chem. Eng. 2017, 5, 5351-5359.
[31] M. Zeng, Y. Wang, Q. Liu, X. Yuan, S. Zuo, R. Feng, J. Yang, B. Wang,
C. Qi, Y. Lin, ACS Appl. Mater. Interfaces 2016, 8, 33157-33164.
[32] G. B. B. Varadwaj, S. Rana, K. Parida, The Journal of Physical Chemistry
C 2014, 118, 1640-1651.
[33] I. B. Adilina, N. Rinaldi, S. P. Simanungkalit, F. Aulia, F. Oemry, G. B. G.
Stenning, I. P. Silverwood, S. F. Parker, The Journal of Physical
Chemistry C 2019, 123, 21429-21439.
[34] a) Q.-Q. Hao, G.-W. Wang, Z.-T. Liu, J. Lu, Z.-W. Liu, Ind. Eng. Chem.
Res. 2010, 49, 9004-9011; b) D. Varade, K. Haraguchi, Langmuir 2013,
29, 1977-1984.
[35] a) Z. Wu, Q. Zhu, C. Shen, T. Tan, ACS Omega 2016, 1, 498-506; b) V.
Srivastava, Open Chem. 2018, 16, 605-613.
[36] D. Varade, K. Haraguchi, Phys. Chem. Chem. Phys. 2014, 16, 25770-
25774.
[37] W. Xu, H. Sun, B. Yu, G. Zhang, W. Zhang, Z. Gao, ACS Appl. Mater.
Interfaces 2014, 6, 20261-20268.
[38] a) L. Duan, R. Fu, B. Zhang, W. Shi, S. Chen, Y. Wan,
ACS
Catal. 2016, 6, 1062-1074; b) Y. Huang, S. Gao, T. Liu, J. Lü, X. Lin, H.
Li, R. Cao, ChemPlusChem 2012, 77, 106-112.
[39] T. Yamashita, P. Hayes, Appl. Surf. Sci. 2008, 254, 2441-2449.
[40] I. Santana, A. Pepe, W. Schreiner, S. Pellice, S. Ceré, Electrochim. Acta
2016, 203, 396-403.
[41] B. J. W. J. Theo Kloprogge, Clay Sci. 2018, 22, 85-94.
[42] a) A. Aznárez, A. Gil, S. A. Korili, RSC Adv. 2015, 5, 82296-82309; b) B.
Tyagi, C. D. Chudasama, R. V. Jasra, Spectrochim. Acta, Part A 2006,
64, 273-278.
[43] J. Oh, H. B. Bathula, J. H. Park, Y.-W. Suh, Commun. Chem. 2019, 2.
[44] a) M. Thommes, K. Kaneko, A. V. Neimark, J. P. Olivier, F. Rodriguez-
Reinoso, J. Rouquerol, K. S. W. Sing, Pure Appl. Chem. 2015, 87, 1051-
1069; b) S. Brunauer, L. S. Deming, W. E. Deming, E. Teller, J. Am.
Chem. Soc. 1940, 62, 1723-1732.
[45] a) V. S. Kshirsagar, A. C. Garade, R. B. Mane, K. R. Patil, A. Yamaguchi,
M. Shirai, C. V. Rode, Appl. Catal., A 2009, 370, 16-23; b) K. Motokura,
S. Matsunaga, H. Noda, A. Miyaji, T. Baba, ACS Catal. 2012, 2, 1942-
1946.
[46] D. Aldhayan, A. Aouissi, Bull. Chem. React. Eng. Catal. 2017, 12.
[47] S. Bentahar, M. A. Taleb, A. Sabour, A. Dbik, M. El Khomri, N. El
Messaoudi, A. Lacherai, R. Mamouni, Russ. J. Org. Chem. 2019, 55,
1423-1431.
[48] W. Gong, C. Chen, Y. Zhang, H. Zhou, H. Wang, H. Zhang, Y. Zhang, G.
Wang, H. Zhao, ACS Sustainable Chem. Eng. 2017, 5, 2172-2180.
[49] B. Mattson, W. Foster, J. Greimann, T. Hoette, N. Le, A. Mirich, S.
Wankum, A. Cabri, C. Reichenbacher, E. Schwanke, J. Chem. Educ.
2013, 90, 613-619.
[50] X. Zhao, Y. Zhao, G. Fu, N. Zheng, Chem. Commun. (Cambridge, U. K.)
2015, 51, 12016-12019.
[51] R. Kumar, B. S. Rana, R. Tiwari, D. Verma, R. Kumar, R. K. Joshi, M. O.
Garg, A. K. Sinha, Green Chem. 2010, 12, 2232-2239.
[52] M. Jeganathan, A. Dhakshinamoorthy, K. Pitchumani, , ACS Sustainable
Chem. Eng. 2014, 2, 781-787.
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Entry for the Table of Contents
A facile and straightforward synthesis protocol using natural clay as support was demonstrated for the preparation of cost-effective
Pd/Clay catalyst. Among the different loaded metal catalyst, 6 wt% Pd/Clay catalyst system displayed the maximum catalytic
performance for the hydrogenation of 3-diphenyl prop-2-en-1-imine with complete conversion and 100 % selectivity to yield 3-phenyl
propyl aniline under mild reaction conditions of 5 bar pressure under shorter duration of 3.5 h at 100 °C.
10
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