Chemoselective hydrogenation of cinnamaldehyde over a tailored oxygen-vacancy-rich Pd@ZrO2catalyst

Article abstract of DOI:10.1039/d0nj05595f

Selective hydrogenation of cinnamaldehyde to hydrocinnamaldehyde is captivating due to its industrial relevance. Herein, a two-step synthesis method was adopted to develop oxygen vacancies in Pd@ZrO₂catalysts. The oxygen vacancies were developed in Pd@ZrO₂catalysts during impregnation of Pd which was confirmed by XPS and HR-TEM analyses. The characterization results revealed that there was a synergistic role of oxygen vacancies and nano-sized active Pd metals in Pd@ZrO₂catalysts that assisted in achieving selectivity for hydrocinnamaldehyde which has been discussed in this study. We also studied the effects of different reaction parameters which revealed that 4 wt% Pd loading in a Pd@ZrO₂catalyst provided enough active sites for complete conversion of CAL. Additionally, 100 °C temperature and 10 bar H₂pressure provided enough energy for effective collisions and activation of reactants and catalysts to form the desired product in a reaction time of 9 h. Therefore, a defect-rich 4-Pd@ZrO₂catalyst demonstrated complete CAL conversion with 86% yield towards HCAL which is the best result amongst various Pd@ZrO₂catalysts with different Pd loading investigated for the hydrogenation of cinnamaldehyde. Moreover, a plausible mechanism was proposed to support the chemoselective hydrogenation of cinnamaldehyde over a 4-Pd@ZrO₂catalyst. Along with high catalytic performance, the 4-Pd@ZrO₂catalyst also showed impressive recyclability performance for up to six recycles. Thus, the oxygen-vacancy-rich Pd@ZrO₂can be considered as an efficient catalyst for the chemoselective hydrogenation of cinnamaldehyde.

Full text of DOI:10.1039/d0nj05595f

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Chemoselective hydrogenation of
cinnamaldehyde over a tailored
oxygen-vacancy-rich Pd@ZrO2 catalyst†
Cite this: New J. Chem., 2021,
45, 5659
Komal N. Patil,a Divya Prasad,a Jayesh T. Bhanushali,a Bhalchandra Kakade,b
a
Arvind H. Jadhav *a and Bhari Mallanna Nagaraja
*
Selective hydrogenation of cinnamaldehyde to hydrocinnamaldehyde is captivating due to its industrial
relevance. Herein, a two-step synthesis method was adopted to develop oxygen vacancies in Pd@ZrO2
catalysts. The oxygen vacancies were developed in Pd@ZrO2 catalysts during impregnation of Pd which
was confirmed by XPS and HR-TEM analyses. The characterization results revealed that there was a
synergistic role of oxygen vacancies and nano-sized active Pd metals in Pd@ZrO2 catalysts that assisted
in achieving selectivity for hydrocinnamaldehyde which has been discussed in this study. We also
studied the effects of different reaction parameters which revealed that 4 wt% Pd loading in a Pd@ZrO2
catalyst provided enough active sites for complete conversion of CAL. Additionally, 100 1C temperature
and 10 bar H2 pressure provided enough energy for effective collisions and activation of reactants and
catalysts to form the desired product in a reaction time of 9 h. Therefore, a defect-rich 4-Pd@ZrO2
catalyst demonstrated complete CAL conversion with 86% yield towards HCAL which is the best result
amongst various Pd@ZrO2 catalysts with different Pd loading investigated for the hydrogenation of
cinnamaldehyde. Moreover, a plausible mechanism was proposed to support the chemoselective
hydrogenation of cinnamaldehyde over a 4-Pd@ZrO2 catalyst. Along with high catalytic performance,
the 4-Pd@ZrO2 catalyst also showed impressive recyclability performance for up to six recycles. Thus,
the oxygen-vacancy-rich Pd@ZrO2 can be considered as an efficient catalyst for the chemoselective
hydrogenation of cinnamaldehyde.
Received 16th November 2020,
Accepted 21st February 2021
DOI: 10.1039/d0nj05595f
rsc.li/njc
quantities of a reducing agent.1 Under this perspective, catalysis
has turned out to be the foundation stone of chemical industries
1. Introduction
Catalytic hydrogenation reactions have attracted research interest when carrying out hydrogenation reactions using molecular
due to their process versatility and green credentials compared to hydrogen.2 The constant development in the field of catalysis in
a typical chemical reduction process that involves stoichiometric compliance with green chemistry principles to perform these
reactions under moderate reaction conditions permitting high
activity and selectivity is of high priority these days.3 Among all
the aspects of catalysis, chemoselectivity is highly targeted to
a Centre for Nano and Material Science (CNMS), JAIN (Deemed-to-be University),
Jain Global Campus, Bangalore-562112, India.
achieve atom efficiency by focusing on single product formation
E-mail: j.arvind@jainuniversity.ac.in, jadhav.ah@gmail.com,
and thus minimizing waste generation.4 In this perspective,
bm.nagaraja@jainuniversity.ac.in, nagarajabhari@gmail.com
b Department of Chemistry, SRM Research Institute, SRM Institute of Science and
Technology, SRM Nagar, Kattankulathur-603203, Chennai, Tamil Nadu, India
† Electronic supplementary information (ESI) available: Methods and materials,
characterization methods and techniques, spectroscopic data of the otained
product and by-product, general reaction pathway for typical hydrogenation of
cinnamaldehyde and applications of HCAL in different engineering aspects
chemoselective hydrogenation of a,b-unsaturated aldehydes,
classically cinnamaldehyde, using H2 can be considered as a clean
process with close to 100% atom efficiency.5 Selective hydro-
genation of cinnamaldehyde is of utmost importance amongst
several industrial hydrogenation processes and has attracted
(Scheme S1), (a) synthesis of ZrO2 support by a hydrothermal synthesis method extraordinary research interest because it provides both scientific
and industrial significance.2,6
Typically, hydrogenation of cinnamaldehyde may lead to the
formation of cinnamyl alcohol (COL) (selective hydrogenation of
and (b) synthesis of Pd@ZrO2 catalysts by a wet-impregnation method (Scheme
S2), (a) general illustration of depth analysis of XPS and TEM/EDX for elemental
analysis and (b) TEM/EDX results obtained at the centre of the particle and at the
edge of the particle of 4-Pd@ZrO2 catalyst (Scheme S3), and comparative study of
CQO), hydrocinnamaldehyde (HCAL) (selective hydrogenation
of CQC) and/or hydrocinnamyl alcohol (HCOL) (hydrogenation
catalytic hydrogenation of cinnamaldehyde over 4-Pd@ZrO2 with the literature
(Table S1). See DOI: 10.1039/d0nj05595f
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of both CQC and CQO).1 All of these products have their own modification, ZrO2 can also act as an active metal and directly
importance and applications in commercial sectors.1,2 The final participates in the hydrogenation reactions.9,14 One of the
product, HCOL, can be obtained irrespective of the type of strategies to modify the surface properties of ZrO2 is to create
catalyst employed in the cinnamaldehyde hydrogenation reaction. oxygen vacancies which are related to the existence of surface
However, the challenge lies in designing a suitable catalytic Zr(4Àx)+ sub-oxides.15,16 Literature studies clearly manifest that
system to selectively hydrogenate the CQC or CQO bond of oxygen vacancies can be generated either by introducing
CAL to obtain HCAL or COL, respectively.3 The emphasis herein dopant (La, Mg, Pd and Mn) or by reducing support in the
lies in selectively yielding HCAL as it forms an important class presence of H2.15,16 The latter one is quite difficult as ZrO2 falls
of molecules having considerable industrial relevance.2 The under the category of non-reducible oxides and requires high
general reaction pathway followed by the hydrogenation of temperature and pressure.15,16 This is true for ZrO2 bulk oxides
cinnamaldehyde in combination with certain industrial and not for the smaller crystallites.15,16 There are also findings
applications of HCAL is represented in Scheme S1 (ESI†).
on the reduction of ZrO2 and the resulting oxygen-vacancy
With regards to this, literature studies reveal that versatile creation by heating ZrO2 in the presence of water.15 This
catalytic systems have been explored which involve the participation procedure is believed to promote the formation of unstable
of active metals in combination with surface-modified supports.3,6 surface ÀOH groups which further leads to the formation of
There are catalysts based on transition metals, noble metals, as oxygen defects.15 Accordingly, a lot of research has been done
well as the combination of both, which are supported on to synthesize oxygen-vacancy-rich nano-structured ZrO2 which
various supports such as CeO2, CNT, graphene, SiO2 and TiO2 can be reduced at lower temperature and pressure to bring
for the hydrogenation of cinnamaldehyde. Researchers have about constructive changes in the physical and chemical properties
mainly worked on the surface modification of these catalysts to of ZrO2 oxide.15,16
chemoselectively obtain cinnamyl alcohol or hydrocinnamaldehyde
With relevance to the above context, the present work
as a product.3,5,6 Various transition-metal-based catalysts such features an oxygen-vacancy-rich Pd@ZrO2 catalyst that offers
as Ni, Co, and Cu with suitable supports have been employed in certain prominent merits in comparison with the reported
the hydrogenation of cinnamaldehyde.6 These catalysts with works. Precisely, the oxygen vacancies were developed in the
certain surface modifications were found to be highly active in ZrO2 support of the Pd@ZrO2 catalyst by a simple, low-
the hydrogenation of the CQO group of CAL to yield COL.2,3,5 temperature-assisted synthetic method. The synthesized
Moreover, noble-metal-based catalysts such as Pt, Pd, Rh, and oxygen-vacancy-rich Pd@ZrO2 catalyst was characterized using
Au with appropriate surface modifications have also proven to various analytical, spectroscopic and microscopic techniques.
be highly active towards the hydrogenation of the CQO group The XPS and TEM analyses endorsed the successful formation
of CAL to yield COL.2,6–9 There are limited studies that have of a nano-sized Pd@ZrO2 catalyst with sufficient oxygen vacancies.
selectively obtained HCAL as the major product by employing The catalytic performance of the Pd@ZrO2 catalyst towards the
hybrid catalysts such as Au-Cu, Pt-Ni, Pd-Ni, and Ni-Ir.1,6,10–13 chemoselective hydrogenation of cinnamaldehyde was then
Altogether, these literature studies reflect that, hydrogenation of investigated. The highly controlled nature of the 4-Pd@ZrO2
CQC is favoured over the CQO bond as it is thermodynamically catalyst led to 100% conversion of CAL with 86% yield towards
and kinetically stable.1,2 Also, noble-metal-based catalysts have HCAL at low H2 pressure (10 bar) in comparison with the
been reported to yield COL.4 It is difficult to achieve controlled literature studies. We have also put in efforts to demonstrate
hydrogenation of CQC without affecting CQO over a noble- the effects of varying temperature, pressure, active metal
metal-based catalyst without making use of any poisons.4 loading, catalyst loading and different solvents on the conversion
Most of the available studies on the selective production of and selectivity of the 4-Pd@ZrO2-catalysed hydrogenation of
HCAL over noble metals have encountered moderate activities cinnamaldehyde. All the conclusions drawn from the catalytic
at relatively higher pressure, as not much focus has been devoted activities have been very well correlated with the characterization
towards the surface modification of supports that can alter the results. Additionally,
a plausible mechanism has been
physical and chemical properties of the active metal in the constructed keeping in mind the role of oxygen vacancies,
catalyst to favour the formation of HCAL.2,4 Among various ZrO2 surface modification and Pd active metal towards the
noble metals, palladium (Pd) is considered as an appropriate chemoselective hydrogenation of cinnamaldehyde. The recyclability
candidate that can hydrogenate CQC present in conjugation studies performed with the 4-Pd@ZrO2 catalyst demonstrated
with CQO.1,6 However, most of the reported Pd-based catalysts consistency in the catalytic activity for six recycles without
displayed moderate catalytic performance towards CAL to HCAL. disturbing the physicochemical properties of the catalyst.
It was found that a suitable support with surface modification is However, in the present work, the authors have developed a
desirable to tune and control its electronic properties to achieve noble-metal-based Pd@ZrO2 catalyst for CAL hydrogenation.
selectivity towards saturated aldehydes.1,6,9
Taking into consideration the cost of noble metals, the authors
Among various support materials that have been widely will attempt in further work to replace the noble-metal-based
studied, ZrO2 was found to be an efficient candidate due to catalysts with the cost-effective non-noble-metal-based catalysts
its moderate surface area, and high thermal stability and for CAL hydrogenation. However, they strongly believe that the
conductivity.9,14 ZrO2 is known to behave as an inert support present work offers considerable advantages in comparison
in the hydrogenation reactions.9,14 With suitable surface with other reported studies.
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2. Experimental section
2.1. Synthesis of ZrO2 support and impregnation of Pd over
pressure with H2 gas. The gas was continuously passed into
the system and the H2 pressure was maintained with a pressure
controller. The reaction was initiated by stirring at a speed of
600 rpm to neglect the external mass transfer limitation and the
system was heated to the required temperature. Zero reaction
time was noted as soon as the system was placed in the heating
unit. The hydrogenation reaction was allowed to run for the
desired time period and the temperature, pressure and agita-
tion speed were maintained at constant values throughout the
reaction cycle. After completion of the reaction, the H2 gas inlet
was closed following which stirring and heating were switched
off. The reactor was allowed to cool to room temperature
naturally. Then, the system was slowly depressurized by releas-
ing excess hydrogen. The reaction mixture was filtered using
Whatman filter paper to separate the catalyst. The reaction
progress and the product formation for the Pd@ZrO2-catalyzed
hydrogenation of CAL were monitored and confirmed by ana-
lyzing the reaction mixture and the authentic samples of
reactant as well as products using a gas chromatograph (GC-
7820 A; M/S Agilent, USA) equipped with a flame ionization
detector (FID) having a capillary column (HP-5, 19091J-413) of
30 m length, 0.32 mm inner diameter and 0.25 mm film
thickness. The products from the reaction mixture were sepa-
rated by HPLC technique and the obtained products, HCAL
the as-synthesized ZrO2 support
Zirconia (ZrO2) was synthesized using the hydrothermal synth-
esis method reported elsewhere.16 Precisely, 10 mL of 0.5 M
ZrO(NO3)2 solution was added to 60 mL of ethylene glycol
under continuous stirring for around 15 min. Furthermore,
the mixture was transferred into a 100 mL Teflon-lined auto-
clave which was tightly sealed and then heated at 180 1C for
16 h. After completion of the reaction, the autoclave was cooled
to room temperature (30 1C) naturally. The obtained brown
precipitate was collected and alternatively washed using DI
water and ethanol by centrifuging at 4000 rpm for 5 min several
times and dried at 60 1C for 6 h. Subsequently, the obtained
material was annealed at 450 1C for 3 h at a ramping rate of
4 1C minÀ1 to remove all the organic debris and produce ZrO2
support.
Furthermore, Pd as the active metal was supported on ZrO2
by a standard wet impregnation method.17 In particular, a
certain amount (based on the wt% of Pd to be loaded on the
ZrO2 support) of palladium salt (Pd(NO3)2.2H2O) was dissolved
in 60 mL of DI water and accordingly, the appropriate amount
of synthesized ZrO2 support was then introduced into the
aqueous solution of Pd precursor under vigorous stirring to
ensure homogeneous dispersion of Pd metal onto the ZrO2
support matrix. The entire solution was kept under stirring at
90 1C until all the water evaporated and a complete dry powder
residue was left over. The resultant powder was then calcined at
450 1C for 3 h at a ramping rate of 4 1C minÀ1 to obtain the
desired catalyst in its oxide form which is henceforth
designated as PdO@ZrO2. Prior to use in the reaction, the
catalyst was freshly reduced in a H2 atmosphere at a flow rate
of 30 mL minÀ1 at 100 1C for 3 h, which was the actual catalyst
hereafter designated as Pd@ZrO2. Various Pd@ZrO2 catalysts
with different Pd loading were also synthesized in a similar
manner by using appropriate quantities of Pd precursor. The
final catalysts thus obtained are designated as 1-Pd@ZrO2, 2-
Pd@ZrO2, 3-Pd@ZrO2, 4-Pd@ZrO2, 5-Pd@ZrO2 and 6-Pd@ZrO2,
respectively, wherein the numerical values denote the amount of
Pd loading in wt%. The entire synthesis procedure for the
Pd@ZrO2 catalysts is represented in Scheme S2 (ESI†).
(major) and HCOL (minor), were confirmed by 1H NMR and 13
C
NMR spectroscopic techniques and GC-MS as well as elemental
analysis.
Spectral data of the major product, hydrocinnamaldehyde
(HCAL) [3-phenylpropanal] (conversion-100%, selectivity-86%,
yield-86%): According to the general procedure, the separated
compound, HCAL, was obtained as a colorless liquid; b.p. 97–
98 1C;
1H NMR (400 MHz, CDCl3): d (ppm) = 9.810 (t, 1H), 7.32 (t,
2H), 7.24–7.19 (m, 3H), 2.89 (t, 2H), 2.69 (s, 2H). 13C NMR (100
MHz, CDCl3) d 201.32, 140.48, 128.48, 128.28, 126.18,45, 28.11;
+
HRMS calculated for C9H10O [M + H] : 134.07; found: 135.08;
elemental analysis: C, 80.54; H, 7.52; O, 11.93.
Spectral data of the minor product, hydrocinnamyl alcohol
(HCOL) [3-phenylpropanol] (conversion-100%, selectivity-14%,
yield-14%): According to the general procedure, the separated
compound, HCOL, was obtained as a pale yellow liquid; b.p.
B235 1C;
1H NMR (400 MHz, CDCl3): d (ppm) = 7.31 (t, 2H), 7.25–7.18
(m, 3H), 3.50 (t, 2H), 1.81 (m, 2H), 2.63 (t, 2H), 1.71 (s, 1H). 13
C
2.2. Selective hydrogenation of cinnamaldehyde (CAL) over
Pd@ZrO2 catalysts
NMR (100 MHz, CDCl3) d 140.89, 128.41, 128.41, 125.81, 128.42,
The catalytic activity of Pd@ZrO2 catalysts towards the selective 128.41, 32.11, 34.21, 61.59; HRMS calculated for C9H12O [M +
+
hydrogenation of CAL was performed in an indigenous 100 mL H] : 136.09; found: 137.0987; elemental analysis: C, 79.35; H,
stainless-steel high-pressure reactor coupled with a magnetic 8.89; O, 11.76.
stirrer and an automated temperature controller. Typically, an
appropriate amount of pre-reduced catalyst (at 100 1C for 3 h)
was transferred into the autoclave without exposing it to the
atmosphere. The reactor was then charged with the reactant
3. Results and discussion
cinnamaldehyde (CAL, 1 g) and 10 mL of a suitable solvent. The As discussed, Pd was loaded on ZrO2 supports to generate and
reactor was sealed tightly and the system was purged with pure enhance oxygen vacancies which are believed to alter the
H2 gas three times to ensure the complete removal of residual electronic properties of Pd that might be responsible for the
air and finally, the system was pressurized to the desired chemoselective nature of Pd@ZrO2 catalysts.13 Based on this
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conceptual strategy, it is necessary to provide appropriate corresponding to the [111], [200] and [220] planes,
evidence for the successful synthesis of ZrO2 supports and respectively.21 This exemplifies the presence of Pd with a
Pd@ZrO2 catalysts with different Pd loading. Thus, the materials face-centered cubic crystal structure (ICDD file no. 05-0681).21
were characterized by various advanced analytical, spectroscopic Furthermore, a short, low-intensity diffraction peak observed at
and microscopic techniques to determine their phases as well as a 2y value of B38.61 infers the presence of PdO.22 This could
analyze their morphological and physicochemical properties. possibly be due to the oxidation of Pd to PdO upon exposure to
These results may perhaps provide an understanding of an air atmosphere during analysis. Additionally, the peaks
the characteristics of Pd@ZrO2 catalysts that could aid in centered at 2y values of B28.21, 32.21 and 561 were also
the chemoselective hydrogenation of cinnamaldehyde to detected which correspond to [À111], [111] and [130] diffraction
hydrocinnamaldehyde.
planes, respectively.20 These characteristic diffraction peaks
belong to the monoclinic phase of the ZrO2 support which
correlates with the ICDD file no. 37-1484.20 This implies that a
certain percentage of the pure tetragonal phase in the ZrO2
support has been transformed into the monoclinic phase during
the Pd impregnation process.23 The cause for this transition at
moderately low temperatures (65–400 1C) can be attributed to
the presence of chemisorbed water molecules on the oxygen
vacancies of the ZrO2 lattice initiated as an aqueous Pd precursor
solution employed during the Pd impregnation process.23 This is
in good agreement with the mechanism anticipated for this
transition by Guo et al.23,24 This transformation initiated due to
Pd impregnation is supposed to stabilize the tetragonal phase of
the ZrO2 support in the Pd@ZrO2 catalyst.25 The percentage
composition of tetragonal and monoclinic phases in various
Pd@ZrO2 catalysts with different Pd loading was estimated with
the help of the Rietveld method.15 The calculated parameters are
listed and summarized in Fig. 1(b). The obtained XRD results
revealed that the percentage of the tetragonal phase was
observed to decrease with the increase in the Pd loading.
Additionally, a slight shift was observed in the position of
all the peaks that could possibly be due to the presence of
lattice defects and hence the occurrence of zirconium in sub-
oxide forms.
3.1. Catalyst characterization
3.1.1. XRD analysis. XRD analysis was performed to
obtain information on the crystallinity and phase purity of
the as-synthesized materials.18 Fig. 1(a) shows the XRD patterns
of the ZrO2 support and the Pd@ZrO2 catalysts with different
Pd loading. It is well reported that pure ZrO2 can occur in either
of the three phases: monoclinic, tetragonal or cubic, or it can
exist as a mixed phase.19 The existence of different phases in
ZrO2 is strongly dependent on the procedure followed during the
synthesis.20 The XRD patterns of the ZrO2 support synthesized
by the solvothermal synthesis method in the present work
displayed sharp diffraction peaks that emphasize the crystalline
nature of the support. The peaks positioned at 30.21, 35.11, 50.61,
60.11, 63.01 and 74.71 in the XRD patterns of the ZrO2 support
correspond to the respective [101], [110], [112], [211] and [220]
planes. These planes are in good correlation with the characteristic
peaks of ZrO2 with a pure tetragonal phase and well match the
International Center for Diffraction Data (ICDD) file no. 80-0965.20
The average crystallite size (D) of the tetragonal ZrO2 nanoparticles
was calculated for the dominant tetragonal diffraction peak at
[101] using the Debye Scherrer formula and was found to be
around 10.2 nm.20
Subsequently, the crystallite size of various Pd@ZrO2
catalysts with different Pd loading was calculated for the
intense peak at [111] using the Debye Scherrer formula.20
The calculated data displayed in Fig. 1(b) clearly show that
there is an increase in the crystallite size of the Pd@ZrO2
catalysts with an increase in Pd loading. With the increase in
Pd loading, there could be a possibility of agglomeration of the
Pd nanoparticles on the ZrO2 support which resulted in an
Similarly, the XRD analysis of the various Pd@ZrO2 catalysts
with different Pd loading was also performed after freshly
reducing the catalysts in the H2 atmosphere at 100 1C for 3 h.
The XRD patterns of various Pd@ZrO2 catalysts with Pd loading
displayed in Fig. 1(a) show the diffraction peaks corresponding
to the tetragonal phase of the ZrO2 support according to the
ICDD file no. 80-0965.20 Additionally, three peaks were also
observed at 2y values of B40.31, 46.751 and 65.941
Fig. 1 (a) XRD patterns of ZrO2 support and Pd@ZrO2 catalysts with different wt% Pd loading (where t-ZrO2 corresponds to tetragonal ZrO2, and m-ZrO2
corresponds to monoclinic ZrO2) and (b) summary of crystallite size and calculated phase content.
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increase in the crystallite size of the Pd@ZrO2 catalysts.26 solvent plays a dual role of behaving as a complexing as well as
This increasing trend in the crystallite size with respect to the a capping agent in the synthesis of nanoparticles.27 Ethylene
increase in the Pd loading is clearly observed in Fig. 1(b).
glycol as a complexing agent has a tendency to produce
3.1.2. FE-SEM analysis. In order to obtain insight into the homometallic and heterometallic glycolates by alcoholysis of
morphology, the molecular surface structures of as-synthesized ligands existing in the metal precursors.28 In the present study,
materials were characterized using a field-emission scanning zirconium nitrate in the presence of ethylene glycol underwent
electron microscopy (FE-SEM) technique.18 ZrO2 supports, as pre-alcoholysis and resulted in the formation of zirconium
well as various Pd@ZrO2 catalysts with different Pd loading, glycolate which precipitated out from the solution due to its
were analyzed at various magnifications, the FE-SEM results of lower solubility.28 A continuous increase of the zirconium
which are shown in Fig. 2. The FE-SEM images of ZrO2 support glycolate (monomer) concentration led to the beginning of
shown in Fig. 2(a) consist of
a
rod-like morphology. the crystallization process.29 This step is in accordance with
We presume that the ZrO2 nanoparticles synthesized in the the theory proposed by La Mer et al.29 Once the concentration
presence of ethylene glycol under solvothermal conditions led crossed the saturation limit, fast busting of nucleation began
to the formation of rod-like nanostructures having average which produced zirconium glycolate nuclei in large numbers.29
uniform length and thickness. According to one of the reports The surface area kept on increasing due to the newly formed
by Sahar and Masoud, ethylene glycol which is widely used as a nuclei.29 To compensate for this, a reduction in bulk surface
energy occurred by forming new crystal lattices.29 This eventually
led to a reduction in the Gibbs energy which turned out to be a
reason for the continuous growth.29 The zirconium glycolate
nuclei grew large at the cost of the zirconium glycolate monomer
until its concentration dropped below the saturation level.29
The final shape of the zirconium glycolate particles will be
determined by the surface energies of the planes.29 The planes
in the crystal with higher surface energy will grow more rapidly
to stabilize and lower its surface energy.29 The ethylene glycol
bound strongly to a specific plane with higher surface energy
which lowered the surface energy of that plane and facilitated
the growth of zirconium glycolate along a one-dimensional
direction resulting in a rod-like morphology.29 Finally, the
calcination of zirconium glycolate led to the formation of a
ZrO2 support with a rod-like morphology, the surface of which
appeared to be smooth throughout the analysis, which could
specifically be observed at a higher magnification of 100 nm as
shown in Fig. 2(a).
The FE-SEM analysis was also performed for all the various
Pd@ZrO2 catalysts with different Pd loading. The results for all
Pd@ZrO2 catalysts are displayed in Fig. 2(b–g). Initially, with
1-Pd@ZrO2, 2-Pd@ZrO2 and 3-Pd@ZrO2 catalysts, we could
observe a rod-like structure and the surface appeared smooth.
However, on observing the FE-SEM images of 4-Pd@ZrO2,
5-Pd@ZrO2 and 6-Pd@ZrO2 catalysts, the surface of the
Pd@ZrO2 appeared to be rough and cracked in comparison
with the surface of the ZrO2 support. The tetragonal phase of
ZrO2 becomes unstable due to the presence of chemisorbed
water molecules on the oxygen vacancies of the ZrO2 lattice
initiated during the Pd impregnation process. Thus, some parts
of the unstable tetragonal phase may get transformed into a
monoclinic phase during the calcination process of the
PdO@ZrO2 precursor in order to reduce the stress and stabilize
the tetragonal phase of ZrO2. This transformation of phase may
lead to the development of cracks on the surface of the
catalyst.30 The FE-SEM results undoubtedly portrayed retention
in the surface morphology for all Pd@ZrO2 catalysts with
different Pd loading. However, with the increase in the Pd
loading, there was a gradual increase in the transformation of
the tetragonal phase into a monoclinic phase. The presence of
Fig. 2 FE-SEM analysis of (a) the ZrO2 support and (b–g) various Pd@ZrO2
catalysts with different Pd loading.
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the monoclinic phase along with the tetragonal phase in the inset of Fig. 3(a) confirms the even distribution of the Zr and O
Pd@ZrO2 catalysts with different wt% Pd loading can be well- elements on the surface of the ZrO2 support. Similarly, the
corroborated with the results obtained in the XRD analysis.
elemental analysis of the various Pd@ZrO2 catalysts with
The elemental composition of the synthesized ZrO2 support different Pd loading was also performed which is shown in
and the various Pd@ZrO2 catalysts with different Pd loading Fig. 3(b–g). The EDX graphs of 1-Pd@ZrO2, 2-Pd@ZrO2 and
was quantitatively analyzed using EDX and elemental mapping 3-Pd@ZrO2 catalysts displayed the presence of Zr, O and Pd
analysis and the results are shown in Fig. 3(a–g) and the inset of elements in agreement with their respective stoichiometry.
Fig. 3(a–g), respectively. The EDX graph of the ZrO2 support Similarly, the expected stoichiometry of Zr, Pd and O was also
showed the presence of only Zr and O elements which signifies observed in 4-Pd@ZrO2, 5-Pd@ZrO2 and 6-Pd@ZrO2 catalysts.
that the ZrO2 support has been successfully synthesized with The presence of the expected elements confirmed the successful
high purity. Additionally, the elemental mapping shown in the impregnation of Pd on the ZrO2 support. Moreover, the
Fig. 3 EDX analysis of (a) ZrO2 support and (b–g) various Pd@ZrO2 catalysts with different Pd loading and the elemental mapping of (inset of a) ZrO2
support and (inset of b–g) various Pd@ZrO2 catalysts with different Pd loading.
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elemental mapping shown in the inset of Fig. 3(b–g) further ZrO2.20,31,32 Additionally, lattice fringes with 0.27 nm spacing
elucidated the uniform dispersion of all the elements on were also found which correspond to the [110] plane of the
the surface of the various Pd@ZrO2 catalysts with different tetragonal ZrO2.20,33 Thus, the HR-TEM analysis confirmed the
Pd loading.
predominant presence of the tetragonal phase in the ZrO2
3.1.3. TEM and HR-TEM analyses. Controlled experiments support. This discussion is in line with that of the XRD data
discussed in the initial activity studies revealed that the 4-Pd@ZrO2 wherein the dominant diffraction peak of the tetragonal ZrO2
catalyst displayed outstanding results and hence TEM and XPS relates to the [101] plane. SAED patterns were also acquired for
analyses were performed only for the 4-Pd@ZrO2 catalyst in the tetragonal ZrO2 which displayed the presence of small spots
order to evaluate the presence and enhancement of oxygen making up the concentric rings, a characteristic pattern
vacancies. TEM and HR-TEM techniques are very crucial in observed for polycrystalline materials.34 Each concentric ring
understanding the catalyst morphology, assessment of size and corresponds to the diffraction plane which is identified by
particle size distribution of nanoparticles and determination of measuring the distance between two bright spots in each ring
lattice fringes.28 Due to this, the ZrO2 support and 4-Pd@ZrO2 and is shown in Fig. 4(C-1). The above elucidation is in
catalyst were analyzed using TEM and HR-TEM and the results correlation with the literature.34
are displayed in Fig. 4. The TEM image of the ZrO2 support
The TEM image of the ZrO2 support in the 4-Pd@ZrO2
shown in Fig. 4(A-1) clearly displayed a rod-like morphology catalyst shown in Fig. 4(A-2) signifies the presence of larger
which is in good accordance with the morphology obtained crystallites as compared to the ZrO2 support with a pure
through the FE-SEM analysis. The particle size distribution plot tetragonal phase. Its corresponding histogram of particle size
revealed that the average particle size of the ZrO2 support is distribution showed the presence of particles of sizes ranging
9.5 nm which is in good agreement with the ZrO2 crystallite size from 8 to 16 nm with an average particle size of 10.5 nm. This
calculated from the XRD analysis. The histogram of particle value also correlates with the crystallite size calculated using
size distribution showed that the size of the ZrO2 nanoparticles the XRD data. The HR-TEM and SAED analyses were also
ranged from 8 to 14 nm with an average particle size of 9.5 nm. performed for the 4-Pd@ZrO2 catalyst which resulted in the
This value is in good agreement with the crystallite size value patterns shown in Fig. 4(B-2,3) and (C-2,3), respectively.
calculated using the Debye-Sherrer equation.
The inter-planar d-spacing was calculated which showed the
The high-resolution TEM image of the ZrO2 support presence of the monoclinic phase along with the tetragonal
illustrated in Fig. 4(B-1) commonly showed a lattice spacing phase. The d-spacing of 0.28 nm and 0.34 nm corresponds to
of 0.3 nm, which resembles the [101] plane of the tetragonal the [111] and [À111] planes of the monoclinic phase of the ZrO2
Fig. 4 TEM analysis of (A-1) the ZrO2 support, (A-2) ZrO2 in the 4-Pd@ZrO2 catalyst and (A-3) Pd in the 4-Pd@ZrO2 catalyst; HR-TEM analysis of (B-1) the
ZrO2 support, (B-2) ZrO2 in the 4-Pd@ZrO2 catalyst and (B-3) Pd in the 4-Pd@ZrO2 catalyst; and SAED patterns of (C-1) the ZrO2 support, (C-2) ZrO2 in
the 4-Pd@ZrO2 catalyst and (C-3) Pd in the 4-Pd@ZrO2 catalyst.
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support.20,34–37 The presence of ZrO2 as a mixed phase presence of Zr(4Àx)+ ions.44 Literature reports articulate that the
(tetragonal + monoclinic) in the 4-Pd@ZrO2 catalyst has also electrons present in the lattice after the formation of oxygen
been confirmed by the XRD analysis. The SAED patterns vacancies could possibly be utilized in the reduction of nearby
show diffraction spots forming concentric rings each of which Zr+4 ions and this probably could be the reason for the presence
is indexed to the respective planes as shown in Fig. 4(C-2). of Zr(4Àx)+ species.46 The O 1s spectra with a single peak after
There is an occurrence of a diffraction spot relating to the deconvolution displayed three peaks corresponding to the
[111] plane of the monoclinic phase which additionally sup- presence of three different oxygen species.44 A peak centered at
ports the presence of the monoclinic phase in the 4-Pd@ZrO2 530.4 eV corresponds to the presence of lattice oxygen.46 A peak
catalyst.38
at 528.3 eV represents the presence of chemisorbed oxygen
The TEM image of Pd in the 4-Pd@ZrO2 catalyst is also (OHÀ) species and a peak at 532.5 eV could be related to low
displayed in Fig. 4(A-3) which demonstrated a spherical-shaped coordinated oxygen species (OÀ1).46 The interpretation of O 1s
Pd over the ZrO2 support. The corresponding histogram of deconvoluted spectra is significant to understand and clarify the
particle size distribution showed the presence of Pd particles concept of oxygen vacancies and thus it is discussed separately
with sizes ranging from 2 to 14 nm with an average particle size in Section 3.2. Finally, the carbon peak C 1s could be ascribed to
of 7 nm. The inter-planar d-spacing of 0.23 nm and 0.19 nm some residual carbon detected from the sample or the hydro-
was obtained as shown in Fig. 4(B-3) which could be related to carbon from the XPS instrument itself.47
the respective [111] and [200] planes of the face-centered cubic
Furthermore, the XPS analysis of the 4-Pd@ZrO2 catalyst was
Pd nanoparticles.39,40 The SAED patterns displayed concentric also studied in detail and the obtained survey spectra,
rings which confirm the polycrystalline nature of the spectra for Pd 3d, Zr 3d, C 1s and O 1s and their respective
material.41 The concentric ring corresponding to the respective deconvoluted spectra, are represented in Fig. 5(e–j). The survey
planes of Pd is displayed in Fig. 4(C-3) which can also be spectrum indicated the presence of Pd, Zr, O and C elements.
correlated with the XRD analysis. The TEM-EDX analysis was The deconvoluted spectrum of Pd 3d showed two peaks
also performed for both the ZrO2 support and the 4-Pd@ZrO2 centered at 333.20 eV and 346.6 eV which corresponds to Pd
catalyst which confirmed the presence of the respective 3d3/2 and Pd 3d5/2, respectively.48 These peaks can be assigned
elements in the material. The TEM-EDX analysis was helpful to the presence of Pd.48 Furthermore, as the literature reports
in the indirect assessment of oxygen vacancies in the material indicated that the Zr 3p3 peak overlaps the Pd 3d5/2, the Zr 3p3
and hence it has been discussed in brief in Section 3.2.
peak was also deconvoluted which is shown as two peaks.48
3.1.4. XPS analysis. The hydrogenation reaction is a surface A low-intensity shoulder peak obtained after the deconvolution
phenomenon and thus it is important to understand the surface of Zr 3p3 peak at 336.9 eV is attributed to the presence of Pd+2
chemistry of the catalytic material.17 The XPS technique is which could be due to the oxidation of Pd to PdO due to the
considered as an effective tool for the surface analysis of the exposure of the catalyst to the atmosphere during analysis.48
material to obtain detailed information on its elemental compo- The deconvoluted Zr 3d spectra showed four peaks similar to
sition, chemical state, as well as the electronic state.18 Both the that of the ZrO2 support. However, a slight shift in the binding
ZrO2 support and the 4-Pd@ZrO2 catalyst were subjected to XPS energy was observed which could be due to the increase in the
analysis. Survey spectra, spectra for Zr 3d, O 1s and C 1s, were defect concentration.44 The peaks at 182.53 eV and 184.93 eV
obtained for the ZrO2 support. The survey spectra confirmed could be assigned to Zr 3d3/2 and Zr 3d5/2, respectively.44
the presence of the required components in the ZrO2 support. The difference in the binding energy of these two peaks was
The obtained spectra were deconvoluted and the peak fitting was found to be 2.4 eV which yet again confirmed the presence of
performed using CASA XPS software.42 The survey spectra and all Zr+4 species which is consistent with the literature.16 The low
the deconvoluted graphs are displayed in Fig. 5(a–d). The XPS binding energy peak centered at 180.23 eV corresponds to the
spectra of Zr 3d showed four peaks after deconvolution which presence of Zr(4Àx)+ ions.43 The shoulder peak is observed at the
could be related to the presence of Zr in different oxidation base of Zr 3d3/2 with a binding energy of 182.7 eV which is
states.43 Various literature reports suggest that the binding characterized for the presence of oxygen deficiency due to the
energy for Zr3d3/2 and Zr3d5/2 occurs at 182.0 and 184.4 eV, under-coordinated Zr sites in the material.44 The deconvoluted
respectively.20 However, in the present study, two peaks for Zr O 1s spectra displayed three peaks evincing the presence of
3d3/2 and Zr 3d5/2 were observed at the binding energy of three different oxygen species the same as in the case of the
184.5 and 186.9 eV, respectively. The binding energy for the Zr ZrO2 support. The detailed analysis is discussed in Section 3.2.
3d peak was observed to be shifted to a higher binding energy
3.1.5. BET surface area and pore size distribution analysis.
which could possibly be due to the presence of oxygen The specific surface area of the as-synthesized ZrO2 support
vacancies.44 The difference in the binding energy was 2.4 eV and the various Pd@ZrO2 catalysts with different Pd loading was
which confirmed the presence of Zr+4 species.20 This is in evaluated precisely by BET analysis by employing a nitrogen
accordance with the literature.20,45 The shoulder peak observed adsorption–desorption technique at liquid N2 temperature
at the base of Zr 3d3/2 with a binding energy of 182.7 eV could be (À77 K). Prior to the analysis, the materials were degassed at
ascribed to oxygen deficiency which could be because of the 100 1C for 3 h in order to remove all adsorbed water and other
under-coordinated Zr sites in the ZrO2 support.44 An additional gases from the surface of the material. The BET plots obtained
peak at a low binding energy of 180.5 eV corresponds to the by employing this technique provided useful information on the
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Fig. 5 XPS of ZrO2 support: (a) survey spectra, (b) C 1s spectra, (c) O 1s spectra and (d) Zr 3d spectra; XPS of the 4-Pd@ZrO2 catalyst: (e) survey spectra, (f)
C1s spectra, (g) O 1s spectra, (h) Zr 3d spectra, (i) Pd 3d spectra and (j) Zr 3p3 spectra.
surface porosity and pore diameter of the materials.20 different Pd loading. The specific surface area of the ZrO2
Fig. 6(a and b) represent the respective BET and BJH plots of support was found to be around 37 m2 gÀ1. The graph displayed
the ZrO2 support and the various Pd@ZrO2 catalysts with a type IV isotherm with H1 hysteresis loop which is a typical
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and thus blockage of pores with Pd particles, and these could
also be the reasons for the decrease in the surface area of the
catalysts with increasing Pd loading as supported by the
literature.26
The pore size distribution and pore diameter of the ZrO2
support and the various Pd@ZrO2 catalysts with different
Pd loading were acquired by employing the BJH method.
The mean pore diameter calculated for the ZrO2 support and
the various Pd@ZrO2 catalysts with different Pd loading is
displayed in Table 1, which confirms the mesoporous nature
of these materials. Furthermore, the ZrO2 support and various
Pd@ZrO2 catalysts with different Pd loading showed an
increase in pore diameter with an increase in Pd loading.
3.1.6. H2-TPR and NH2-TPD analyses. Various Pd@ZrO2
catalysts with different Pd loading were analyzed using
H2-temperature programmed reduction (TPR) in order to
determine the most effective reduction condition to reduce
PdO to active Pd0.23 This technique could provide information
on the extent of the interaction of the Pd metal with the ZrO2
support.18 H2-TPR profiles of the ZrO2 support and various
Pd@ZrO2 catalysts with different Pd loading are shown in
Fig. 7(a). The H2-TPR profile of the ZrO2 support synthesized
in the present work displayed a single, broad peak centered at
581 1C which is ascribed to the reduction of ZrO2.54 Various
literature reports articulate that it is difficult to reduce pure
ZrO2 nanocrystals and the tentative reduction temperature of
Zr+4 is above 500 1C.54 The H2-TPR profile of the 1-Pd@ZrO2
catalyst suggests the presence of an intense peak at a lower
temperature of 94 1C which is attributed to the reduction of
superficial PdO present on the catalyst surface.55,56 A minor
broad peak at 362 1C corresponds to the reduction of the PdO
present on the catalyst surface in the bulk phase.57 One broad
peak at a higher temperature of 628 1C was also evidenced
which might be due to the reduction of ZrO2 present at the
metal–support interface.54 Similar kinds of H2-TPR profiles
were observed for the 2-Pd@ZrO2, 3-Pd@ZrO2 and 4-Pd@ZrO2
catalysts. In the H2-TPR profile of most of the reported
Pd-based catalyst systems, a negative peak in the temperature
range of 50 1C to 120 1C is usually detected which indicates the
presence of b-Pd-hydride. The b-Pd-hydride species is formed
due to the decomposition of the weakly adsorbed hydrogen
from the Pd surface at room temperature. The presence of this
negative peak corresponding to b-Pd-hydride indicates the
existence of larger Pd particles on the catalyst surface.56,58
However, the peak in this region was not observed in the
reduction profiles of the Pd@ZrO2 catalysts till 4 wt% Pd
loading which suggests the presence of nano-sized Pd particles
in these catalysts.59 These results are well supported by the data
obtained from the XRD and BET analyses.
Fig. 6 BET plots of (a) ZrO2 support and various Pd@ZrO2 catalysts with
different Pd loading and BJH plots of (b) ZrO2 support and various
Pd@ZrO2 catalysts with different Pd loading.
characteristic of mesoporous materials.49 Hence, it can be said
that the ZrO2 support is porous in nature and can be utilized as a
good support material which is also supported by one of the
reports by Deshmane et al.14
The specific surface area of the various Pd@ZrO2 catalysts
with different Pd loading was also estimated and the obtained
results are summarized in Table 1. All the graphs reflected a
type IV isotherm with H1 hysteresis loop similar to that of the
ZrO2 support. The impregnation of Pd on the ZrO2 support
possibly introduced additional pores leading to the development
in the porosity of the final Pd@ZrO2 catalyst. The increase in the
pore volume for 1 wt% Pd@ZrO2 in comparison with that of the
ZrO2 support complements the results obtained and supports
the discussion. A similar trend in the results has also been
observed in the literature.50–53 With further increase in Pd
loading (2, 3, 4, 5 and 6 wt%), the surface area gradually
decreased. In agreement with the decrease in the surface area,
the pore volume was also observed to decrease. It is known that
with a larger crystallite size, the surface area tends to lower
down.53 Hence, a decrease in the specific surface area with
respect to the increasing Pd loading can also be correlated with
the crystallite size of the catalysts as calculated using the [111]
plane of the XRD diffractogram. Moreover, an increase in the Pd
loading from 2 wt% to 6 wt% might have led to agglomeration
Table 1 Textural properties of ZrO2 support and various Pd@ZrO2 cata-
lysts with different Pd loading
Pore
BET surface Pore volumea diametera Acidityb
Entry Catalysts
areaa (m2
g
À1) (cm3 gÀ1
)
(nm)
(mmol gÀ1
)
1.
2.
3.
4.
5.
6.
7.
ZrO2 support 36.6
1-Pd@ZrO2 82.7
2-Pd@ZrO2 85.6
3-Pd@ZrO2 75.6
4-Pd@ZrO2 44.1
5-Pd@ZrO2 38.3
6-Pd@ZrO2 36.0
0.059
0.094
0.097
0.094
0.076
0.066
0.063
3.33
3.35
3.38
3.42
3.52
3.54
3.55
0.52
1.0
4.2
6.6
8.6
12.6
18.4
Surprisingly, a negative peak corresponding to b-Pd-hydride
was observed in the H2-TPR profiles of the 5-Pd@ZrO2 and
6-Pd@ZrO2 catalysts at 120 1C.56,58 The appearance of this
negative peak infers the presence of larger PdO particles in
the 5 wt% and 6 wt% Pd-loaded Pd@ZrO2 catalysts.56,58
Additionally, peaks resulting from the reduction of bulk PdO
and ZrO2 were observed at 383 1C and 598 1C, respectively.54,57
a
b
Calculated from BET. Calculated from NH3-TPD analysis from the
summation of areas under the peaks.
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Fig. 7 (a) H2-TPR profiles of ZrO2 support and various Pd@ZrO2 catalysts with different Pd loading and (b) NH3-TPD profiles of ZrO2 support and various
Pd@ZrO2 catalysts with different Pd loading.
The presence of larger Pd crystallites in the 5-Pd@ZrO2 and various Pd@ZrO2 catalysts with different Pd loading.62
6-Pd@ZrO2 catalysts is also evidenced by the XRD data Precisely, a peak centered at 198 1C corresponds to the presence
represented in Table 1. The above discussion is in alignment of weak acidic sites obtained from the desorption of hydrogen-
with the reported literature.56–58 Based on the H2-TPR analysis, bonded NH3.62 This particular peak can also be attributed to
100 1C was chosen as the reduction temperature for the various the presence of Brønsted acidic sites (Zr-OH) as the bonding
Pd@ZrO2 catalysts with different Pd loading to preferably between NH3 and Brønsted acid sites is weaker and can easily
reduce PdO into Pd0 and activate the Pd@ZrO2 catalysts.
be broken down at lower temperatures.62 The appearance of a
Moreover, the extent of the acidic and basic sites on the peak at 415 1C confirmed the presence of moderately acidic
surface of the metal oxides is essentially important in order to sites.62 Finally, a peak centered at 680 1C relates to the presence
understand and predict the performance of a catalyst in the of strong acidic sites.62 With respect to the increase in the
hydrogenation reactions.60 The surface acidity of the Pd@ZrO2 Pd loading, the number of acidic sites was also observed to
catalysts was quantified by TPD analysis employing NH3 gas as increase. A similar result for the increase in acidity with respect
a probe molecule.60 The NH3-TPD profiles of the ZrO2 support to the increase in the Pd loading has also been reported by
as well as the various Pd@ZrO2 catalysts with different Pd Srinivas et al.63 The conclusion can be drawn from the above
loading are displayed in Fig. 7(b). Before the interpretation of discussion that, impregnation of Pd on the ZrO2 support
graphs, it is necessary to understand the classification of acidic drastically enhanced the acidic character of the Pd@ZrO2
sites on the basis of the desorption of basic NH3 molecules with catalysts with different Pd loading which is essentially an
respect to the change in the temperature.61 Peaks observed important factor to trigger the hydrogenation reaction in the
below 300 1C correspond to the weak acidic sites, whereas the forward direction.61
desorption of NH3 molecules from moderate acidic sites occurs
between 300 1C and 450 1C and strong acid sites adsorbed NH3 have been widely employed with H2 as a probe molecule,
that are released above 450 1C.61 especially for Pd-based catalysts.63 This technique assists to gain
3.1.7. H2-chemisorption studies. Chemisorption techniques
Taking this into consideration, the NH3-TPD graphs of all information on the number of accessible active sites, the surface
the materials were elucidated. The graph of the ZrO2 support area of active sites, the metal particle size and also the percent
showed the presence of one small peak in the region below metal dispersion, prominently for noble-metal-based hetero-
300 1C which corresponds to the presence of weak acidic geneous catalysts.18 Thus, to acquire all this information for
sites.60 Furthermore, two small peaks in the region between the various Pd@ZrO2 catalysts with different Pd loading, the
300 1C and 450 1C were observed at 325 1C and 410 1C, materials were subjected to H2 chemisorption analysis and the
respectively.60 These peaks were obtained due to the desorption results calculated are presented in Table 2.
of the weakly bonded NH3 molecules by hydrogen bonding
As could be observed, an increase in the Pd loading notably
which corresponds to the moderate acidic sites and not due to affected the Pd particle size as well as the extent of Pd
the desorption of ammonia from the surface of the acidic sites dispersion. With the increase in the Pd loading, a gradual
of the ZrO2 support.60 The total acidity of the ZrO2 support decrease in the metal dispersion was evidenced which might
calculated by measuring the area of the ammonia desorption possibly be due to agglomeration.52 This trend can also be
peaks was calculated to be 0.516 mmol gÀ1 which suggested that evidenced in the present case wherein, the increase in
the ZrO2 support has a very low acidic nature.
Pd loading from 1 wt% to 6 wt% decreased the metal dispersion
Similarly, the various Pd@ZrO2 catalysts with different Pd with a comparative increase in the Pd particle size. This trend is
loading were also analyzed using NH3-TPD for quantifying the in good correlation with the literature.26 The increase in the
presence of acidic sites and the results are displayed in Pd particle size with the increase in the Pd loading should
Fig. 7(b). The obtained result reveals the presence of three undoubtedly lead to a decrease in the surface area of the
broad and intense peaks respectively, in all three regions of the catalysts.26 This observation is in line with the BET analysis
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Table 2 H2-chemisorption measurements for various Pd@ZrO2 catalysts
with different Pd loading
in peak positions as can be seen in Fig. S4 (ESI†). This
interpretation correlates with that in one of the literature
studies by D’souza et al.65
Pd metal
Pd metal
Pd particle dispersion surface area H2 uptake
3.1.9. Catalytic activity screening of the CAL hydrogenation
reaction with the prepared Pd@ZrO2 catalysts. After confirming
the successful synthesis of oxygen-vacancy-rich Pd@ZrO2
catalysts by employing various spectroscopic and analytical
techniques, the catalysts were tested for their activity and
selectivity towards the hydrogenation of cinnamaldehyde.
Initially, certain controlled experiments were performed in
order to get insight into the behavior of Pd@ZrO2 catalysts
for the selective hydrogenation of cinnamaldehyde. The outcome
of the reactions performed is tabulated in Table 3. To begin with,
the cinnamaldehyde (CAL) hydrogenation reaction was screened
in the absence of a catalyst and solvent at 120 1C and 10 bar
H2 pressure (Table 3, entry 1). Insignificant conversion and
selectivity towards hydrocinnamaldehyde (HCAL) were evidenced.
However, good selectivity was obtained towards hydrocinnamyl
alcohol (HCOL; a complete hydrogenated product) which is
supposed to form regardless of the type of catalyst or reaction
parameters employed for the reaction.66 This reaction demon-
strated that in the absence of both the catalyst and the solvent, the
reaction did not run and these two parameters are immensely
important to selectively drive the reaction and avoid the formation
of the completely hydrogenated product (HCOL). Furthermore,
with the aim to investigate the impact of varying the Pd loading on
the selective hydrogenation of CAL, the reactions were performed
by charging 1 g CAL, using 10 mL IPA, loading different wt%
Pd on Pd@ZrO2 at 120 1C, and using 10 bar H2 pressure for a time
period of 9 h.
Preliminarily, with 1 wt% Pd@ZrO2 catalysts, a noteworthy
conversion of 95.1% was observed which accentuated that Pd in
its zero oxidation state acts as an active metal and has a
tendency to readily dissociate molecular hydrogen and adsorb
onto its surface (Table 3, entry 2). This phenomenon has a vital
impact on the conversion of CAL. However, the selectivity of
only 60.3% and 30.7% towards HCAL and HCOL, respectively,
could be achieved with 1 wt% Pd@ZrO2 catalysts. The remaining
9% selectivity was accredited to COL. Performing reactions with
2 wt% Pd@ZrO2 catalysts did not bring much alteration in the
conversion (98.8%) and selectivity (63.5%) towards HCAL
(Table 3, entry 3). The remaining two products, HCOL and
COL, were also obtained in 32.5% and 4% selectivity, respec-
tively. On the other hand, by employing 3 wt% Pd@ZrO2
catalysts, an increase in the selectivity towards the desired
product HCAL (72.0%) was observed with a conversion of
95.6% (Table 3, entry 4). The selectivity of the remaining two
products was reduced to 25.4% and 2.6%, respectively.
Availability of more dispersed active sites on the catalyst surface
could have led to the increase in the selectivity of the desired
product which has been confirmed by the H2 chemisorption
studies.
Entry Catalysts
size (nm)
(%)
(m2 gÀ1
)
(mmol gÀ1
)
1
2
3
4
5
6
1-Pd@ZrO2 1.27
2-Pd@ZrO2 1.32
3-Pd@ZrO2 1.33
4-Pd@ZrO2 1.49
5-Pd@ZrO2 2.18
6-Pd@ZrO2 3.13
89
86
82
72
50
35
392
380
376
335
229
160
0.2
0.4
0.6
0.7
0.6
0.5
wherein the surface area was observed to decrease with the
increase in Pd content (Table 1).
3.1.8. FT-IR spectroscopic studies. FT-IR analysis was per-
formed to confirm the existence of the metal oxide framework
and different functional groups in the synthesized materials.45
Fig. 8 represents the FT-IR spectrum of the ZrO2 support which
displays a broad peak at 3396 cmÀ1 that corresponds to the
–OH stretching vibration from the surface hydroxyl groups.64
Additionally, a peak at 1635 cmÀ1 and 1385 cmÀ1 can be
attributed to the bending vibrations of –OH and vibrations
due to the non-bridging –OH groups, respectively.62 The
presence of peaks at 658 and 582 cmÀ1 is due to the vibrations
of the tetragonal Zr-O bond as also confirmed in one of the
reports by Jayakumar et al.64 In the FT-IR spectrum of the
various Pd@ZrO2 catalysts with different Pd loading, similar
parent peaks corresponding to the presence of the ZrO2 support
were evidenced. Additionally, a peak observed at 740 cmÀ1
corresponded to the vibration of m-ZrO2.45 Moreover, we also
observed the characteristic peaks in the fingerprint region
between 700 cmÀ1 and 450 cmÀ1 which confirmed the presence
of the M-O framework.65 The spectra obtained for all the
Pd@ZrO2 catalysts were similar to each other with regularity
Furthermore, 100% conversion was achieved with 4 wt%
Pd@ZrO2 catalysts with 86% selectivity towards HCAL (Table 3,
entry 5). The only by-product obtained using the 4-Pd@ZrO2
catalyst was HCOL with 14% selectivity. This trend suggested
Fig. 8 FT-IR spectra of ZrO2 support and various Pd@ZrO2 catalysts with
different Pd loading.
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Table 3 Catalytic activity screening of the CAL hydrogenation reaction with the prepared Pd@ZrO2 catalystsa
Selectivity (%)
Entry
Reaction parameters
Conversion (%)
HCAL
HCOL
Yield (%) HCALb
1
2
3
4
5
6
7
8
Solvent and catalyst free
1-Pd@ZrO2
2-Pd@ZrO2
3-Pd@ZrO2
4-Pd@ZrO2
5-Pd@ZrO2
6-Pd@ZrO2
With ZrO2
5
95.1
98.7
95.6
100
100
100
19.3
68
3.2
60.3
63.5
72
86
67.1
47
96.4
36
50.6
65
40.8
3
1.9
10
60
0.2
59.2
63
69
86
67.1
47
18.6
24.5
48.0
2.9
39.9
0.1
1.7
30.7
32.5
25.4
14
27.9
24
3.6
42
30
35
21
35
5.8
18
9
With PdO
10
11
12
13
14
15
With Pd0
95
Catalyst free, with IPA solvent
With catalyst, IPA solvent free
N2 atmosphere (10 bar)
Atm. Pressure (H2)
At R.T.
14.9
97.9
2.5
86.7
15
15
a
b
Reaction conditions: 4-Pd@ZrO2, 1g CAL, 10 mL IPA, 10 bar H2, temperature, and 9 h. Calculated using GC.
that Pd as an active metal played a major role in enhancing the selectivity towards HCAL. Furthermore, reactions were also
conversion of CAL. Also, Pd in association with ZrO2 support performed in the presence of ZrO2 support which showed low
gradually improved the selectivity and hence the yield of HCAL. conversion and low yield towards HCAL (Table 3, entry 8).
Additionally, reactions with 5 and 6 wt% Pd@ZrO2 catalysts Furthermore, to investigate whether the hydrogenation of
were also performed in an attempt to obtain higher yields. cinnamaldehyde proceeds in the presence of Pd+2 or metallic
Nevertheless, a drop in the selectivity towards HCAL was Pd, the reaction was performed in the presence of PdO as well as
observed though there was consistency in the conversion of metallic Pd and the results were compared. In the presence of
almost 100%. With 5 wt% Pd@ZrO2 catalysts, 67.1% selectivity PdO catalyst (Pd in +2 state), 68% conversion and 36% selectivity
was achieved for HCAL (Table 3, entry 6). HCOL and COL were were observed which were unexpectedly pretty good results
obtained in 27.9% and 5% selectivity, respectively. With 6 wt% (Table 3, entry 9). It is known that PdO is much less capable of
Pd@ZrO2 catalysts, selectivity for HCAL dropped to 47%. HCOL adsorbing and thus dissociated the molecular hydrogen into
and COL were obtained in 24% and 29% selectivity, respectively atomic hydrogen which can then be utilized in the hydrogenation
(Table 3, entry 7). From this particular discussion, it could be of the reactant. However, in the present scenario, there can be a
inferred that 4 wt% Pd@ZrO2 catalysts could possibly provide possibility of in situ reductions of PdO during the reaction
enough active sites for complete conversion of CAL. Furthermore, course as the reaction was performed at 120 1C by supplying H2
an increase in the Pd loading decreased the selectivity and externally. This could be the reason for achieving the high
increased the formation of HCOL. The excess of active sites conversion of CAL over PdO as a catalyst. When the reaction
present in the 5-Pd@ZrO2 and 6-Pd@ZrO2 catalysts led to the was performed in the presence of metallic Pd, 95% conversion
further hydrogenation of HCAL which resulted in the formation of was achieved by selectively yielding 50.6% HCAL (Table 3,
the complete hydrogenated product HCOL and a marginal entry 10). The results obtained confirmed that the high hydro-
amount of COL was also detected.66 Additionally, the high acidic genating capacity of Pd as an active metal in its zero state is
nature of the catalysts with 5 and 6 wt% Pd loading can also be responsible for achieving the high conversion of CAL. However,
one of the reasons for the formation of the by-products in the absence of ZrO2 support, the achieved selectivity was
(Table 1).66 Furthermore, the unsatisfying results obtained using comparatively low. These outcomes convincingly illustrate that
5-Pd@ZrO2 and 6-Pd@ZrO2 as the catalysts could also be due to both the active metal Pd and the ZrO2 support act synergistically
the agglomerated Pd particles on the ZrO2 support as a result of for achieving high conversion and selectivity towards HCAL.
less dispersion as supported by the H2 chemisorption data.
Furthermore, few reactions were also performed to reveal
Overall, the literature shows the same trend and thus supports the individual role of the solvent and catalyst in the selective
the conclusion drawn from the effects of the Pd loading study.12 hydrogenation of CAL. The reaction performed in the presence
From these initial screenings, it was clear that the 4-Pd@ZrO2 of a solvent and the absence of a catalyst showed only 14.9%
catalyst in the presence of IPA solvent led to high conversion and conversion which indicated that the solvent could not bring
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about effective conversion single-handedly. However, moderate effects of different solvents followed by the effects of
selectivity of 65% was observed for HCAL which showed the temperature and pressure on the CAL hydrogenation over the
probable role of a solvent in obtaining selectivity (Table 3, entry 11). 4-Pd@ZrO2 catalyst which has been discussed further in detail.
On performing the reaction with a catalyst and without a From the above experiments, it is clear that the CAL hydro-
solvent, the conversion of around 98% was observed which genation performed over the 4-Pd@ZrO2 catalyst generated
represents the intrinsic role of the presence of a catalyst in the three products: hydrocinnamaldehyde (HCAL) as the major
reaction (Table 3, entry 12). Additionally, selectivity of 40.8% product, and hydrocinnamyl alcohol (HCOL) and cinnamyl
was observed for HCAL which is lower than that observed in alcohol as minor products which were confirmed by GC as well
the previous reaction in the presence of a solvent. All these as GC-MS analyses. Hereafter, the selectivity of HCAL and
observations may possibly suggest that the catalyst is solely HCOL will be taken into account. It is to be noted that the
responsible for achieving the conversion in the presence of H2. remaining minor percentage of selectivity is for COL.
It is worth highlighting that 100% conversion and 86%
3.2. Evaluation of oxygen vacancy enhancement in the
4-Pd@ZrO2 catalyst
selectivity were obtained towards HCAL in the presence of
the 4-Pd@ZrO2 catalyst and IPA solvent (Table 3, entry 5).
The result thus illustrates that both the catalyst and the solvent As the physicochemical properties of the metal oxides could be
are responsible for obtaining HCAL with high selectivity.
improved by the presence of oxygen vacancies, it is thus very
The 4-Pd@ZrO2 catalyst along with the IPA solvent is important to directly or indirectly investigate the oxygen vacancies
believed to enhance the selectivity towards HCAL as evidenced using various spectroscopic and microscopic techniques.13
from the above reactions. However, IPA is also known to act as a Therefore, a careful evaluation of the presence of oxygen vacancies
hydrogen donor in the transfer hydrogenation process.67 in the ZrO2 support and its enhancement in the 4-Pd@ZrO2
Hence, a reaction was performed without supplying H2 externally catalyst was carried out to support the concept of oxygen vacancies
under the following reaction conditions: 4-Pd@ZrO2, 1g CAL, controlling the selectivity of HCAL in the CAL hydrogenation.
10 mL IPA, 10 bar N2, 120 1C and 9 h reaction time to confirm the For this, XPS, FE-SEM-EDX and TEM-EDX analyses quantitatively
role of IPA in the present reaction. Low CAL conversion of 2.5% provided useful information. In XPS, deconvoluted O 1s spectra of
and HCAL selectivity of 3% were obtained in an N2 atmosphere the ZrO2 support displayed three peaks as shown in Fig. 9(a),
which emphasized that IPA was not capable of providing wherein a peak centered at 530.4 eV corresponds to the presence
hydrogen for the conversion of CAL (Table 3, entry 13).67 This of lattice oxygen.46 A peak at 528.3 eV represents the presence of
experiment indicated that IPA did not play the role of hydrogen chemisorbed oxygen (OHÀ) species.46 The peaks at higher binding
donor but only behaved as a solvent in the 4-Pd@ZrO2-catalyzed energy (531–532 eV) correspond to the weakly adsorbed oxygen
CAL hydrogenation. Few controlled experiments were performed species on the surface of the material.46 Ionization of these weakly
to also investigate the role of pressure and temperature in adsorbed oxygen species could permit compensation for some
the CAL hydrogenation over the 4-Pd@ZrO2 catalyst. Under defects in the subsurface of the metal oxides which can be
atmospheric pressure, the appreciable conversion was observed; referred to as low coordinated oxygen ions (O1À).46
however, negligible selectivity of 2% towards HCAL was
Precisely, a peak at 532.5 eV indirectly indicates the presence
evidenced (Table 3, entry 14). On performing the reaction at of oxygen vacancies as it is reported that the concentration of
room temperature, both conversion and selectivity were adsorbed oxygen species is related to the presence of oxygen
observed to be lower than 15% (Table 3, entry 15). These two vacancies.46 The percentage concentration of oxygen vacancies
reactions demonstrated the importance of pressure and in the ZrO2 support has been calculated by integrating the peak
temperature, respectively. Taking all these controlled experiments area using CASA XPS which showed the presence of 9.3% of
into consideration, several activity studies were performed with oxygen defects in the ZrO2 support.
A similar analysis
respect to the 4-Pd@ZrO2 catalyst loading, reaction time, and performed for the 4-Pd@ZrO2 catalyst conveyed that the
Fig. 9 Deconvoluted spectra of O 1s (a) for ZrO2 support and (b) 4-Pd@ZrO2 catalyst.
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Table 5 The atomic percentage of respective elements obtained by XPS
analysis and weight percentage of respective elements obtained by TEM
analysis
intensity of a peak at the binding energy of 531.8 eV for the
lattice oxygen (O2À) was observed to decrease as compared to
that for the ZrO2 support. In line with this, the intensity of a
peak centered at 532.5 eV increased which could be related to
the increase in the oxygen vacancies as can be seen in Fig. 9(b).
The percentage concentration of oxygen vacancies calculated
for 4-Pd@ZrO2 by integrating the peak area using CASA XPS
software was found to be 18.4% which is higher than that for
the ZrO2 support. Moreover, the electrons generated during
Atomic% of elements
Wt% of elements
TEM TEM
XPS
(boundary) (center)
Oxygen Zr(4Àx)+ Oxygen
species species vacancies species
Oxygen
Oxygen
species
Entry Catalysts
1. ZrO2 support 78.8
6.5
9.4
9.3
18.4
55.3
45.3
62.3
52.0
oxygen-vacancy creation could possibly be used to reduce the 2.
Zr4+ to its sub-oxides.46 The deconvoluted spectrum of Zr 3d
clearly depicts the presence of the sub-oxides of Zr4+ species.46
Thus, the presence of Zr(4Àx)+ reduced species is also evidence
for the existence of oxygen vacancies.46 The percentage (%)
concentration of Zr(4Àx)+ species was calculated for both the
ZrO2 support and the 4-Pd@ZrO2 catalyst which was found to
be 6.5% and 9.5%, respectively, which again complements
the oxygen-vacancy enhancement in the 4-Pd@ZrO2 catalyst.
4-Pd@ZrO2
catalyst
60.2
in the ZrO2 support at the surface. Thus, we observe less oxygen
species at the boundaries as compared to those in the central
bulk portion. Hence, we presume that the concept of calculating
the amount of oxygen species from the XPS and TEM-EDX data
and correlating them could be considered valid as the results are
in line with each other. The idea behind this analysis is
illustrated in Scheme S3 (ESI†).
The XPS analysis thus illustrated that the oxygen-vacancy
concentration is higher in the 4-Pd@ZrO2 catalyst than that in
the ZrO2 support which could help in enhancing the selectivity
towards HCAL. The percentage composition of the respective
elements and the binding energies of different oxygen species in
the ZrO2 support and 4-Pd@ZrO2 catalyst are presented in
Table 4. The above discussion is very well supported by the
literature.68–70
Moreover, it is known that XPS analysis can be utilized to
examine the surface stoichiometry of mainly a few atomic layers
at the surface, say up to 5–10 nm in depth.71 In order to
measure the bulk concentration of the elements in the material,
say up to 100 nm, TEM-EDX analysis could be an appropriate
spectroscopic technique.72 This complementary analysis can
indirectly provide useful quantitative information on the
presence of oxygen vacancies at the surface as well as in the
bulk.72,73 The results obtained by the quantitative analyses of
XPS and TEM-EDX are presented in Table 5. Precisely, the
computable analysis was performed at the boundary and at the
center part of the particle to determine the amount of oxygen
species present and are shown in Table 5. It could be seen that
the percentage of oxygen species at the boundary is less than that
in the interior part. Additionally, the literature studies support
the fact that the addition of metal leads to the formation of
oxygen vacancies. In the present work, impregnation of Pd on
the ZrO2 support surface is believed to enhance oxygen vacancies
3.3. Effects of various reaction parameters on the CAL
hydrogenation reaction with Pd@ZrO2 catalysts
3.3.1. Effects of 4-Pd@ZrO2 catalyst loading on the CAL
hydrogenation reaction. The activity of any catalytic reaction
depends on the amount of active surface sites made available
for the reaction.74 Hence, it is mandatory to study the effects of
the 4-Pd@ZrO2 catalyst loading on the hydrogenation of CAL.
In this study, the reactions were performed by employing
different wt% of the 4-Pd@ZrO2 catalyst under the same
reaction conditions as above. The reaction results are compiled
and shown in Fig. 10. To begin with, a neat reaction without
catalyst under optimized reaction conditions of temperature
(120 1C) and H2 pressure (10 bar) could convert only 15% CAL
with selectively producing 75% HCAL along with HCOL and
COL as minor products. This signifies that the presence of a
catalyst in the reaction is of utmost importance in order to
bring out the effective conversion of CAL.62 On the introduction
Table 4 Chemical composition of respective elements obtained by XPS
analysis and the binding energies of different oxygen species in the ZrO2
support and 4-Pd@ZrO2 catalyst
Chemical compo-
sition (%)
Samples
Binding energy (eV)
Lattice
Low
coordinated
oxygen Chemisorbed oxygen species
Zr
O
Pd
—
(O2À oxygen (OHÀ) (OÀ1
)
)
Fig. 10 Catalytic performance over different loading of the 4-Pd@ZrO2
ZrO2 support 21.2 78.8
530.4 528.3 532.5
catalyst.
4-Pd@ZrO2 28.81 60.2 10.99 531.1 528.5
532.5
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of the 1 wt% 4-Pd@ZrO2 catalyst in the reaction, a slight
increase in the conversion (31.3%) was observed with 36%
selectivity towards the major product HCAL. Introduction of
the 1 wt% 4-Pd@ZrO2 catalyst in the reaction did not provide
sufficient catalytic sites for the reactants to react and therefore
low conversion of CAL was obtained.74
Increasing the catalyst loading to 2.5 wt% gradually
increased the conversion and selectivity to 58% and 58%,
respectively. Furthermore, an increase in catalyst loading was
continued to 5 wt% which showed 100% conversion of CAL
with 86% selectivity towards HCAL and the calculated yield was
estimated to be 86% with only HCOL as the minor product.
Although the desired conversion was achieved, to further study
the effects of increasing the catalyst loading on selectivity, the
reaction with 7.5 wt% and 10 wt% of catalyst loading was also
performed. As expected, the conversion remained 100%;
however, there was a drop in the selectivity for HCAL and an
Fig. 11 Catalytic activity study over Pd@ZrO2 catalyst with different
reaction times (reaction conditions: 4-Pd@ZrO2, 1g CAL, 10 mL solvent,
120 1C temperature, 10 bar H2, and 9 h).
increase in the HCOL product formation with the presence of HCAL and COL are converted into HCOL, the rate of the latter
COL in minor quantities. The reason for this observation could getting converted into HCOL is greater.75 Therefore, in a 9 h
be attributed to the presence of excess of active sites on the reaction time, HCAL was obtained selectively from CAL hydro-
catalyst surface that might lead to further hydrogenation of genation and COL produced in the reaction was completely
HCAL to form the completely hydrogenated product HCOL.74 converted into HCOL. Hence, 9 h was fixed as an optimum
The similar cause for the formation of HCOL is reported by reaction time for obtaining high conversion and selectivity.
Kluson et al.74 The commendable activity observed with 5 wt% Thus, the study showed the time period of 9 h as the optimum
of the 4-Pd@ZrO2 catalyst loading could be attributed to the reaction time for cinnamaldehyde hydrogenation with respect to
presence and availability of sufficient active sites for the the 4-Pd@ZrO2 catalytic system.
reactant to react.74 The increase in the conversion of CAL up
3.3.3. Effects of different solvents on CAL hydrogenation
to 5 wt% loading suggests that the reaction follows first-order reaction. While studying the effects of different solvents on the
kinetics with respect to the catalyst loading. The trend observed catalytic activity towards hydrogenation reactions, few factors
in the catalyst loading study is also in accordance with the need to be taken into consideration such as solvent polarity,
available literature.74 In conclusion, the 5 wt% 4-Pd@ZrO2 easy removal from the reaction system and environmental
catalyst showed excellent conversion with 86% yield towards tolerability.74 Water, ethanol, isopropanol, dioxane and toluene
HCAL and thus was chosen as an optimized catalyst loading for were selected for studying their effects on CAL hydrogenation.
further reactions.
The reaction process that runs without solvent is always
3.3.2. Effects of reaction time on CAL hydrogenation preferred. Hence, initially, the reaction was performed in the
reaction. In addition to catalyst loading, reaction time is also absence of a solvent under optimized conditions that resulted
a crucial parameter to control the activity and selectivity of in 98% conversion with a moderate yield of HCAL (41%).
the Pd@ZrO2 catalyst for the hydrogenation of CAL.75 Hence, the Furthermore, to enhance the selectivity of the catalyst towards
reaction was performed by varying the reaction time and the HCAL, the reaction was screened by using different polar and
results are displayed in Fig. 11. Initially, the reaction was non-polar solvents and the results are displayed in Fig. 12.
conducted under optimized reaction conditions for an hour.
To begin with, water, which is considered as a green solvent,
The reaction proceeded with 99% conversion and 51.4% selectivity was used as a solvent and the reaction was run under similar
towards HCAL. Furthermore, with an increase in the reaction reaction conditions. The present 4-Pd@ZrO2 catalytic system in
time to 3 h and then 9 h, the conversion remained the same. the presence of water yielded 43% HCAL with 100% conversion
The selectivity of 54% was obtained at 3 as well as 6 h reaction of CAL. However, HCOL was also obtained with a 35% yield.
time. When the reaction time was extended to 9 h, an increase Instead, the HCAL which is selectively obtained over the 4-Pd@ZrO2
in the selectivity was observed with an 86% yield of HCAL. catalyst undergoes further hydrogenation to form HCOL in
Interestingly, the only by-product detected was HCOL along with the presence of water.67 The enhanced adsorption and thus
the HCAL as the desired product. Furthermore, the reaction was activation of hydrophilic CQO group of HCAL over the catalyst
run for 12 h in which a decrease in the selectivity towards HCAL due to the presence of water could possibly be the reason for its
was observed with a slight increase in the selectivity towards further hydrogenation to obtain HCOL. This observation has
HCOL. This signifies that after 9 h of reaction time, HCAL also been highlighted in the report by Dai et al., wherein the
produced during the reaction course further hydrogenated to form researchers have achieved high selectivity towards COL over
HCOL, thus decreasing the yield of HCAL.75 It could be inferred PtFe/CNT in the presence of water as a solvent.67 This remark
from the above study that HCAL or COL could be produced from suggested that water as a solvent was unable to enhance the
the hydrogenation of CAL.75 Though, with progressing time, both catalytic activity of the 4-Pd@ZrO2 catalyst for the selective
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3.3.4. Effects of temperature variation on the CAL hydro-
genation reaction. It is well known that heat needs to be
supplied to most of the catalytic hydrogenation reactions in
order to provide enough activation energy so as to drive the
reaction in the forward direction.65 Hence, the temperature
parameter needs to be varied and investigated for its effect on
the catalytic hydrogenation of CAL. The temperature was varied
between room temperature and 140 1C and the reactions
were performed keeping the rest of the parameters constant.
The turnover frequency has also been calculated for the 4-Pd@ZrO2-
catalyzed hydrogenation of CAL at different temperatures using
the formula [moles of HCAL formed/(no. of active centers Â
reaction time)] and is displayed in Table 6. On performing the
reaction at room temperature, a conversion of 13.1% was
obtained with a selectivity of only 8% towards the desired
product HCAL (Table 6, entry 1). This result was quite under-
standable as no external energy in the form of heat was
Fig. 12 Catalytic activity study over a Pd@ZrO2 catalyst with different
solvents (reaction conditions: 4-Pd@ZrO2, 1g CAL, 10 mL solvent, 120 1C
temperature, 10 bar H2, and 9 h).
production of HCAL. Furthermore, ethanol was the choice of supplied to the system. Hence, a lower TOF of 8.1 hÀ1 was
solvent as it could easily be separated from the reaction mixture observed. However, at room temperature, the Pd@ZrO2 catalyst
due to its low boiling point. Performing reactions with ethanol could produce another product COL with 86% selectivity. When
did not alter the conversion (100%); however, a drastic drop the reaction was performed at 80 1C, not much improvement
in the selectivity towards HCAL was observed (18%). The in conversion (14.3%) and selectivity (14%) was observed.
selectivity of HCOL was also not enhanced in the presence of Similarly, a modest TOF was obtained (17.1 hÀ1) (Table 6, entry 2).
ethanol as a solvent. Hence, the reaction mixture was analyzed Increasing the reaction temperature to 100 1C provided enough
for the detection of other side-products which revealed the energy to activate both the catalyst and the reactant molecule to
formation of acetal. It is a known reaction wherein aldehyde react which resulted in the conversion of 99.7% with 69.3%
and alcohol react to form acetal.75 Moreover, acetal forms easily selectivity towards HCAL with an increase in TOF (436.5 hÀ1
with linear and shorter alcohol such as ethanol as compared to (Table 6, entry 3).
)
branched alcohol such as isopropanol, due to the steric
Furthermore, by increasing the temperature to 120 1C, 100%
hindrance effect.76 Therefore, the reaction was further tried conversion was achieved with an 86% yield of HCAL with a
with IPA as a solvent which interestingly led to 100% higher TOF (710 hÀ1) (Table 6, entry 4). The effective collisions
conversion with 86% yield towards HCAL. As discussed and of the reactant molecules with the active catalyst surface at this
proved in the controlled experiments, IPA does not act as a temperature could be the reason for the observed conversion
hydrogen donor and the reaction does not follow the transfer and selectivity.77 Additionally, the reaction was also performed
hydrogenation pathway.67 Most probably, IPA might stabilize at 140 1C which showed an increase in the formation of HCOL
the intermediates that lead to the formation of HCAL.
(19%) and COL (27%) as the by-products, thus decreasing the
Furthermore, to examine the effects of non-polar solvents on selectivity towards HCAL (54%) (Table 6, entry 5). There might
the catalytic activity of the present reaction, the reaction was be a possibility of partial deactivation of the catalyst due to side
also performed in the presence of toluene and dioxane as reactions that usually occur at higher temperatures leading to a
solvents. The conversion in the presence of toluene was 99% decrease in the selectivity of HCAL.78 Simultaneously, there was
and the selectivity towards the desired product was 58%, a decrease in the TOF (435.3 hÀ1). Hence, the temperature study
whereas in the presence of dioxane solvent, both conversion was performed up to 140 1C and the study suggested 120 1C as
and selectivity decreased to 42% and 41%, respectively. It an optimum temperature for obtaining maximum conversion
is reported that non-polar solvents produce saturated
aldehydes with high selectivity.65 However, moderate
selectivity was evidenced in the presence of both the non-
polar solvents with the 4-Pd@ZrO2 catalytic system.
Besides, dioxane and toluene solvents are mostly not preferred.
The 4Pd@ZrO2 catalytic system in the presence of IPA as a
solvent led to better conversion and selectivity towards HCAL.
Table 6 Catalytic activity study over the 4-Pd@ZrO2 catalyst at different
reaction temperaturesa
Selectivity (%)
HCAL HCOL HCALb (%) TOF (hÀ1
Conversion
Entry Temperature (%)
Yield
)
1
2
3
4
5
R.T.
80
100
120
140
13.1
14.3
99.4
100
7.8
6.1
2.5
16
14
18.7
1.0
2.0
59.2
86
52.8
8.1
Moreover, IPA being
a volatile solvent can be easily
14.1
59.6
86
17.1
436.5
710
separated and recovered from the reaction mixture which is a
crucial factor in terms of academic as well as industrial
approach. Therefore, IPA was chosen as a suitable solvent to
carry out the reaction in the presence of the 4-Pd@ZrO2
catalytic system.
100
53.5
435.3
a
Reaction conditions: 4-Pd@ZrO2, 1g CAL, 10 mL IPA, 10 bar H2,
b
temperature, and 9 h. Calculated by GC.
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and selectivity with respect to HCAL. Moreover, the study Furthermore, at 8 bar pressure, conversion reached 98% with
suggested that the 4-Pd@ZrO2-catalyzed hydrogenation of 62% selectivity (Table 7, entry 4). When the reaction was
cinnamaldehyde followed first-order kinetics with respect to performed by applying 10 bar pressure, 100% conversion was
the reaction temperature. This observation with respect to the achieved with 86% HCAL yield (Table 7, entry 5). Furthermore,
variation in reaction temperature is also supported by the by increasing the pressure to 12 and 14 bar, a drop in the
literature.77
selectivity towards HCAL was evidenced with a proportional
3.3.5. Effects of pressure variation on the CAL hydrogenation increase in the yield of HCOL (Table 7, entry 6,7). From this
reaction. Pressure plays a major role in many liquid-phase study, 10 bar was chosen as an optimized pressure for the
hydrogenation reactions. It is a known fact that pressure is reaction under investigation. Furthermore, change in the
essentially important in order to enhance the solubility of pressure led to the change in conversion and selectivity which
molecular hydrogen in the reaction system.79 Henry’s law suggested that the system followed first-order kinetics with
suggests the linear increase in hydrogen solubility with the respect to the change in pressure which can be correlated with
increase of pressure.79 Thus, the effects of variation in pressure the reports in the literature.76
were studied with respect to cinnamaldehyde hydrogenation.
3.3.6. Comparative study of 4-Pd@ZrO2 catalyst with
The study was performed at different H2 pressure ranging from reported catalysts in CAL hydrogenation reaction. The present
1, 4, 6, 8, 10, 12 and 14 bar in the same high-pressure reactor study not only demonstrates the catalytic performance of 4-Pd@ZrO2
under similar reaction conditions (1 g CAL and 5 wt% towards the chemoselective hydrogenation of cinnamaldehyde
4-Pd@ZrO2 catalyst in 10 mL IPA solvent at 120 1C for 9 h) but also briefly describes its comparative study with the most
and the obtained results are represented in Table 7. This recent catalytic systems available in the literature for this
reaction system under varying pressure was also considered reaction. The details of the literature studies employed for
for calculating TOF using the formula [moles of HCAL formed/ comparison are mentioned in Table S1 (ESI†). To begin with,
(no of active centers  reaction time)], which is shown in the Pd/C catalyst for cinnamaldehyde hydrogenation displayed
Table 7.
good conversion and selectivity towards HCAL at lower reaction
To begin with, the hydrogenation of cinnamaldehyde was temperature and pressure (Table S1, ESI,† entry 1).80 However,
performed under atmospheric pressure in continuous H2 flow. the drawback of this catalyst is that in order to obtain selectivity
A good conversion of around 87% was obtained. However, towards HCAL, it required the presence of additive (potassium
selectivity towards HCAL was negligible (2%) (Table 7, entry 1). acetate) which is not necessary for the present 4-Pd@ZrO2
A very less TOF (14 hÀ1) was obtained for this reaction system. catalytic system.80 Furthermore, comparing the catalytic
The less solubility and hence less availability of hydrogen on the performance of 4-Pd@ZrO2 with 1 wt% Pt/BN inferred an
catalyst surface can probably be the reason for this outstanding conversion of about 95% and selectivity of 85%
observation.79 The H2 pressure of 4 bar was then applied and (Table S1, ESI,† entry 2). However, 1 wt.% Pt/BN catalyst was
maintained for the same reaction which led to the conversion of more selective towards COL and not HCAL at high H2 pressure
95.2% and enhancement in the selectivity towards HCAL (54.3%) of 40 bar.78 Additionally, to control the hydrogenation capacity
with 426.5 hÀ1 TOF (Table 7, entry 2). An increase in the of noble-metal-based catalysts, transition metals are known to
selectivity, as well as TOF, could clearly be observed as soon as be used as a poison.12 Han et al. demonstrated controlled
the catalytic system was pressurized with H2. This emphasizes selectivity towards HCAL achieved by the addition of Ni to
the importance of pressure on the activity of the catalyst towards Pd/SBA-15 (Table S1, ESI,† entry 3).12 It was observed that the
the hydrogenation reaction. Furthermore, the reaction was Pd/SBA-15 catalyst alone led to around 50% conversion and
performed at 6 bar and 8 bar H2 pressure wherein further 50% selectivity.12 However, the addition of Ni to Pd/SBA-15
increase in conversion and selectivity was observed as expected. enhanced the reactivity and 88% selectivity was achieved
At 6 bar pressure, 92.5% conversion and 57.2% selectivity were towards HCAL.12 In this work, Ni was used as a poison to
observed with higher TOF (436.5 hÀ1) (Table 7, entry 3). achieve admirable activity towards the hydrogenation of
cinnamaldehyde and to target hydrocinnamaldehyde as a
major product.
Table 7 Catalytic activity study over the 4-Pd@ZrO2 catalyst at different
In one of the reports on a Pt noble-metal-based catalyst with
H2 pressuresa
Sn as a poison, 3 wt% Pt 0.05 wt% Sn/HPZSM-5 showed a good
conversion of 100% (Table S1, ESI,† entry 4).81 However, the
Selectivity (%)
HCAL HCOL HCALb (%) TOF (hÀ1
Conversion
Entry Pressure (bar) (%)
Yield
authors found that the catalyst was active to produce COL as a
major product by applying a high pressure of 20 bar.69 Apart
from Pd and Pt, Au has also been explored for the selective
hydrogenation process of cinnamaldehyde. 1.5 wt% Au/ZnO
was employed as a catalyst to achieve a high conversion of 95%
and outstanding selectivity of 100% (Table S1, ESI,† entry 5).82
However, the product obtained was COL and the catalyst hardly
yielded HCAL (5%).70 Liao et al. reported the synthesis of
Au–Cu/CeO2 by a direct anion exchange method and its
)
1
2
3
4
5
6
7
1
4
6
8
10
12
14
56.7
95.2
92.5
97.6
100
100
100
2
4.1
2
14
427
437
495.0
710
549.5
356.0
54.3 14.0
57.2 9.5
62
86
67.0 19.1
44 25
52
53
60
86
67
43.1
12
14
a
Reaction conditions: 4-Pd@ZrO2, 1 g CAL, 10 mL IPA, H2 pressure,
b
120 1C temperature, and 9 h. Calculated by GC.
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application in cinnamaldehyde hydrogenation resulted in proposed by Horiuti-Polyani et al.76 Likewise, as suggested by
moderate-to-good conversion and selectivity towards hydrocin- Huckel calculations, the interaction of the active metal surface
namaldehyde (Table S1, ESI,† entry 6).11 It is very well known with the conjugated CQC bond will be larger as compared to
that gold-catalyzed systems are highly selective towards the CQO bond if the d-bandwidth of the metal is narrower.76
cinnamyl alcohol and the catalyst properties need to be tuned The metal d-bandwidth of the Pd metal is lower than that of the
to switch its selectivity towards HCAL.11 However, high other noble metals (such as Pt, Ir and Os).76 Hence, CQC could
selectivity was obtained over Au–Cu/CeO2 at the expense of favorably be chemisorbed on the 4-Pd@ZrO2 catalyst surface.
CAL conversion. Another set of a gold-catalyzed system, Additionally, in the present scenario, the induced oxygen
Au/Mg-Al-CeO2, was explored for its application in CAL hydro- vacancies in the 4-Pd@ZrO2 catalyst are proficient at decreasing
genation by Tian et al. to selectively yield HCAL (Table S1, ESI,† the electron density on the Pd surface by pulling electrons
entry 7).13 The reaction conditions employed in the reported from the Pd metal surface.13 The back-bonding interaction of
studies were found to be quite similar to those of the present CQC with the Pd surface is thus favored over CQO as this
work. The reason for high activity was attributed to the phenomenon may increase the interaction of CQC with respect
presence of oxygen vacancies in the catalyst.13 However, the to the Pd metal surface.11
cause for the formation of oxygen vacancies was not taken
All these parameters predominantly lead to the adsorption
into discussion. Additionally, solely non-noble-metal-based of the olefinic double bond of CAL over the catalyst surface
supported catalysts have also been investigated for cinnamal- (Step 3).13 The different modes of interaction of CAL over the
dehyde hydrogenation. In one of the reports, the Ni/TiO2 4-Pd@ZrO2 catalyst surface are shown in Scheme 1(b). To
catalyst was synthesized by different methods and its activity confirm this adsorption mode of CAL over the present catalytic
towards cinnamaldehyde hydrogenation was compared.83 The system, FT-IR studies of CAL-adsorbed catalysts were
Ni/TiO2 catalyst synthesized by chemical reduction was found performed.13 The FT-IR spectra of CAL absorbed Pd@ZrO2
to display a good conversion of 91%; however, the catalyst was shown in Scheme 1(c) displayed two peaks, one at 1630 cmÀ1
selective towards COL (Table S1, ESI,† entry 8).83
ascribed to the conjugated CQC stretching vibration and the
Taking into consideration the recent literature studies for other at 1676 cmÀ1 attributed to the CQO stretching vibration
the selective hydrogenation of cinnamaldehyde, it could be of the CAL molecule. While for CAL, the CQC stretching
inferred that mostly Pd and Au-based catalytic systems have vibration is centered at 1625 cmÀ1 and the CQO stretching
substantiated decent activity to yield HCAL. However, most of vibration peak is observed at 1672 cmÀ1. In general, the
the catalytic systems are complex and reactions have been literature studies suggest that the shift in the IR vibration of
performed under pretty harsh reaction conditions. Furthermore, an adsorbed molecule reflects the degree of bond activation
the detailed information on the mechanistic approach is some- and its interaction with the catalytic surface. Additionally, the
where lacking. The authors in the present work describe a simple studies suggest that the change in the bond length may also
Pd@ZrO2 catalytic system for the selective CAL hydrogenation affect its corresponding vibration frequency.84 In the present
with an admirable activity and selectivity demonstrated under scenario, there is a possibility of a decrease in the bond length
moderate reaction conditions which are very well supported by of the CQO and CQC bonds due to their interaction with the
the characterization and experimental evidence.
catalyst surface, which makes the peak wave number shift to
3.3.7. Plausible reaction mechanism for the formation of higher values. The shift in the stretching vibration of both CQC
chemoselective hydrocinnamaldehyde (HCAL) in the presence and CQO bonds to a higher wavelength in the CAL-adsorbed
of an oxygen-vacancy-rich 4-Pd@ZrO2 catalyst. The present 4-Pd@ZrO2 suggests the interaction of the CAL molecule after
4-Pd@ZrO2 catalytic system proved highly efficient in the adsorption by orienting itself in such a way that the CQC bond
selective hydrogenation of CAL to HCAL. The activity study comes in close proximity of the catalyst surface, as shown in
revealed that ZrO2 as a support has a specific role to play in Step 4.13 Furthermore, the dissociated hydrogen atom present
achieving the desired product. Additionally, IPA solvent helps on the catalyst surface adds to the olefinic double bond (Step 5
to accelerate the activity and selectivity of the 4-Pd@ZrO2 and 6). Finally, the selectively hydrogenated product HCAL
catalyst towards CAL hydrogenation. Hence, an effort was made easily desorbs from the catalyst surface and is replaced by a
to propose a well-supported and accepted plausible mechanistic new CAL reactant molecule (Step 7). This quick release of HCAL
pathway for the present reaction occurring in the presence of the from the catalyst surface avoids further hydrogenation
4-Pd@ZrO2 catalyst represented in Scheme 1(a).
to HCOL.
3.3.8. Recyclability studies. Heterogeneous catalysts are
The reduced Pd in the 4-Pd@ZrO2 catalyst serves as a
medium to adsorb molecular hydrogen and desorb it into two recommended due to their efficient recyclability, as this factor
hydrogen atoms which remain adsorbed on the catalyst surface is important from an environmental as well as industrial
(dissociative adsorption) (Steps 1 and 2). The CAL molecule viewpoint.17,85–89 The present 4-Pd@ZrO2 catalytic system was
comes in the proximity of the catalyst surface (Step 3). The also tested for its efficient recyclability performance in CAL
interaction or the adsorption of the reactant molecule on the hydrogenation. The first catalytic cycle resulted in 100%
catalyst surface will predominantly decide the product to be conversion with 86% selectivity towards HCAL. The catalyst
formed. The adsorption of unsaturated aldehydes on metal obtained from the first reaction cycle was recovered by a simple
surfaces may occur through different adsorption modes as filtration process. The residue was washed three times
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Scheme 1 (a) Plausible reaction mechanism for the selective hydrogenation of CAL over the 4-Pd@ZrO2 catalyst, (b) different modes of interaction of
CAL with the 4-Pd@ZrO2 catalyst and (c) FT-IR of CAL adsorbed on the 4-Pd@ZrO2 catalyst.
alternatively with ethanol and water. The catalyst was dried at Fig. 13(a). Furthermore, the catalyst recovered from the sixth
80 1C for 12 h and prior to its reuse in the reaction, the catalyst recycle was subjected to various characterization techniques to
was freshly reduced in an H2 atmosphere at 100 1C for 3 h. study the structural and morphological changes taking place in
The stoichiometric amount of the reactant was taken with the catalyst.
respect to the amount of the recycled catalyst for the next
Initially, XRD analysis was performed for the identification
reaction cycle. The second catalytic cycle showed consistently of structural changes in the recycled catalyst and the result is
100% conversion with 82% selectivity. Furthermore, after the shown in Fig. 13(b). Interestingly, no phase destruction was
third cycle, the recycled catalyst was able to achieve 99% observed and the XRD patterns were similar to those of the
conversion with consistency in selectivity (82%). For the fourth fresh 4-Pd@ZrO2 catalyst. All the major peaks corresponding to
catalytic cycle, a conversion of 95% was evidenced with 80% the tetragonal (30.21, 35.11, 50.61, 60.11, 63.01 and 74.71) and
selectivity. Similarly, after the fifth consecutive catalytic cycle, monoclinic phases (241, 28.21, 32.21 and 561) of ZrO2 are
the appreciable activity with 95% conversion and 78% selectivity consistent in the XRD of the recycled catalyst. Additionally,
was demonstrated. The sixth reaction cycle decreased the peaks corresponding to PdO were also evidenced in the XRD of
conversion to 90% and the selectivity dropped to 70%. A drastic the recycled catalyst with a decrease in the intensity of the peaks.
drop in the activity of the catalyst was observed after the seventh The crystallite size calculated for the recycled catalyst was
recycle. Thus, the obtained results displayed that the catalyst 18.2 nm which suggested the formation of bigger crystallites in
could produce consistency in the conversion and selectivity for the recycled catalyst.74,75 Moreover, for investigating any
up to six recycles. The result of recyclability is shown in morphological changes in the reused catalyst, FE-SEM analysis
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Fig. 13 (a) Recyclability studies over the 4-Pd@ZrO2 catalyst, (b) XRD of the reused 4-Pd@ZrO2 catalyst, (c) FE-SEM analysis of the reused 4-Pd@ZrO2
catalyst and (d) EDX analysis of the reused 4-Pd@ZrO2 catalyst.
was performed. The rod-like morphology obtained in the fresh without appreciable loss in their activities and physicochemical
4-Pd@ZrO2 catalyst was consistent with the morphology of the properties.
reused catalyst which is shown in Fig. 13(c). However, the
particle size seemed to be slightly increased and was found in
Conflicts of interest
the range of 15–20 nm possibly due to the agglomeration of Pd
particles to some extent on the surface of the recycled catalyst.
This elucidated that the catalyst was structurally very stable even
The authors state that there are no conflicts to declare.
after six recycles. Moreover, the EDX analysis performed for the
recycled catalyst is shown in Fig. 13(d). The analysis confirmed
the presence of all the elements (Pd, Zr and O) in the required
Acknowledgements
stoichiometry in the recycled catalyst. No other elements were
observed which confirmed the purity of the recycled catalyst.
Thus, the 4-Pd@ZrO2 catalyst can be thought of as a highly
active, selective and stable catalytic system for the CAL hydro-
genation reaction.
The authors would like to acknowledge the CNMS, JAIN
(Deemed-to-be University) and Nano Mission, DST, Government
of India, for their financial support (SR/NM/NS-20/2014). They also
gratefully acknowledge the Centre for Nano and Material Sciences
(CNMS), JAIN (Deemed-to-be University), Bangalore, for the
partial funding support through the basic research grant of JAIN
(No. II(39)/17/005/2017SG).
4. Conclusion
The present work demonstrated a simple, two-step synthetic
protocol to develop oxygen vacancies in various Pd@ZrO2
catalysts with different Pd loading. We strongly believe that
the synthetic approach used in this study to enhance oxygen
vacancies in the ZrO2 support helped to modify the electronic
properties of Pd nanoparticles on the 4-Pd@ZrO2 catalyst,
which was confirmed using various characterization techniques.
Among the various Pd@ZrO2 catalysts with different Pd loading
tested for the chemoselective hydrogenation of cinnamaldehyde
(CAL), the 4-Pd@ZrO2 catalyst led to the excellent conversion of
100% with 86% selectivity towards hydrocinnamaldehyde
(HCAL) under milder reaction conditions. The proposed
plausible mechanism revealed that the oxygen-vacancy-rich
4-Pd@ZrO2 catalyst modified the electronic properties of Pd and
influenced the orientation of CAL such that the olefinic bond
was selectively hydrogenated to yield hydrocinnamaldehyde.
Furthermore, Pd@ZrO2 catalysts could be recycled up to six cycles
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