Highly tunable selectivity to benzaldehyde over Pd/ZrO2 catalysts in Oppenauer oxidation of benzyl alcohol using acetone as H-acceptor

Article abstract of DOI:10.1016/j.apcata.2021.118022

The development of novel bifunctional catalysts with high selectivity is the key to achieving the goals of sustainable chemical synthesis. Here, we report the synthesis of mesoporous zirconia and transition metal-doped zirconia by inverse micelle approach. Also, meso-zirconia supported Pd nanocatalysts via the deposition-precipitation method. Highly crystalline Pd/ZrO₂ nanoparticles were formed, as evidenced in the pXRD and TEM analyses. The acid-base quantification was investigated using TPD-NH₃ and TPD-CO₂. Interestingly, the integration of the intrinsic chemical properties of the multi-component catalyst is significant in tailoring the catalytic activities. Interactions between the adsorbates and the moderate acid-base pair sites rather than a single dominating acid or base site mediate the higher selectivity for the aldehyde product. The 100 % chemoselectivity to benzaldehyde is ascribed to the strong synergy between Pd-Zr, which generated the moderate acid-base property. The Pd/ZrO₂ is reusable, with selectivity retained after ten cycles.

Full text of DOI:10.1016/j.apcata.2021.118022

Applied Catalysis A, General 613 (2021) 118022
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Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Highly tunable selectivity to benzaldehyde over Pd/ZrO2 catalysts in
Oppenauer oxidation of benzyl alcohol using acetone as H-acceptor
Christianah Aarinola Akinnawo, Dimakatso Jeannett Maheso, Ndzondelelo Bingwa,
Reinout Meijboom*
Research Centre for Synthesis and Catalysis, Department of Chemical Sciences, University of Johannesburg, P. O. Box 524, Auckland Park, Johannesburg 2006, South
Africa
A R T I C L E I N F O
A B S T R A C T
Keywords:
The development of novel bifunctional catalysts with high selectivity is the key to achieving the goals of sus-
tainable chemical synthesis. Here, we report the synthesis of mesoporous zirconia and transition metal-doped
zirconia by inverse micelle approach. Also, meso-zirconia supported Pd nanocatalysts via the deposition-
precipitation method. Highly crystalline Pd/ZrO2 nanoparticles were formed, as evidenced in the pXRD and
TEM analyses. The acid-base quantification was investigated using TPD-NH3 and TPD-CO2. Interestingly, the
integration of the intrinsic chemical properties of the multi-component catalyst is significant in tailoring the
catalytic activities. Interactions between the adsorbates and the moderate acid-base pair sites rather than a single
dominating acid or base site mediate the higher selectivity for the aldehyde product. The 100 % chemoselectivity
to benzaldehyde is ascribed to the strong synergy between Pd-Zr, which generated the moderate acid-base
property. The Pd/ZrO2 is reusable, with selectivity retained after ten cycles.
Mesoporous zirconia
Pd nanoparticles
Oppenauer oxidation
Acid-base sites
Selectivity
Aldol condensation
1. Introduction
importance, the challenge is that it requires additives, the use of
expensive noble metal catalysts, high temperature, and pressure [8].
The synthesis of aromatic aldehydes from selective oxidation of
benzyl alcohol is one of the most valuable synthetic processes highly
significant in the chemical industries as well as in academia [1]. Alde-
hydes are important intermediates in the synthesis of cosmetics, food
flavors, pharmaceuticals, agrochemicals, plastic additives, and dyestuffs
[2,3]. Diverse methods of oxidation of benzyl alcohol have been rec-
ommended; nevertheless, conventional methods involving the addition
of stoichiometric amounts of heavy metal reagents, expensive and toxic
oxidants are widely used. The synthetic process also involves the use of
solvents that are harmful to the environment. Moreover, such a chemical
transformation involves high temperature and pressure. Also, in the
process, unwanted by-products are generated in large quantities [4,5].
From the perspective of environmental sustainability and process
economy, it is more desirable to oxidize alcohol utilizing reagents that
are cheaper, less toxic, and environmentally friendlier than their con-
ventional counterparts such as permanganate, dichromate, and hypo-
chlorites [6,7]. Conversely, environmentally friendly catalytic oxidation
methods have been developed including aerobic oxidation protocols
over transition metal catalysts. Despite its environmental and economic
Hence, there is a need for improvement of an alternative facile route for
the oxidative transformation of -CH2-OH into -CH = O under mild re-
action conditions.
An alternative to aerobic oxidation is the chemoselective oxidation
of alcohols that employs sacrificial carbonyl compound such as acetone
as a hydrogen acceptor, through a reaction mechanism termed Oppe-
nauer oxidation [9]. The Oppenauer oxidation is the inverse of the
Meerwein-Ponndorf-Verley (MPV) reduction [10]. The mechanism is
predominantly via elimination and addition of hydride ion between the
alcoholate and the carbonyl compound. The mechanism proceeds over a
six-membered cyclic transition state which is regarded as the
rate-determining step (Scheme 1). Acetone, a cheap and readily avail-
able commercial solvent is still regarded as less toxic when compared to
other oxidants such as permanganate, dichromate, and hypochlorites
[11]. From both the environmental and economical point of view, the
Oppenauer oxidation of alcohols in acetone is very appealing [12].
Several studies on these processes reported the use of homogeneous
catalytic systems; however, they require stoichiometric amounts of base
or acid additives, extensive synthesis of ligands, suffer from air
* Corresponding author.
E-mail address: rmeijboom@uj.ac.za (R. Meijboom).
https://doi.org/10.1016/j.apcata.2021.118022
Received 12 August 2020; Received in revised form 21 January 2021; Accepted 25 January 2021
Available online 29 January 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

C.A. Akinnawo et al.
Applied Catalysis A, General 613 (2021) 118022
sensitivity, and are difficult to separate and reuse [13]. The aluminum
alkoxide catalyst conventionally employed in this synthetic process
suffers low reactivity and deactivation due to difficult separation from
the homogeneous reaction mixture [10]. Hence, there is a need for the
development of a highly stable and more efficient heterogeneous cata-
lytic system to circumvent some of the inherent challenges that confront
the homogeneous system.
method. To the best of our knowledge, this paper presents the first
application of Pd NPs supported on mesoporous ZrO2 in Oppenauer
oxidation of benzyl alcohol. The synergistic effect of the metal-Zr
interaction on the product distribution in catalytic Oppenauer oxida-
tion of benzyl alcohol with acetone under mild reaction conditions is
reported. Preliminary investigation of the Oppenauer oxidation oper-
ating parameters was achieved over pure zirconia. The optimized pa-
rameters were utilized in the catalytic evaluation of the as-prepared
catalysts. Two possible reaction pathways were evident: transfer dehy-
drogenation of benzyl alcohol for benzaldehyde production and aldol
condensation of acetone. The selectivity to either benzaldehyde or aldol
products is discussed in terms of tailored concentration of Lewis acid and
base sites on the zirconia catalysts, as a consequence of the synergy
between the Pd-ZrO2, CeO2-ZrO2 interface, and Pd-Zr, Pd-Ce, Ce-Zr
intermetallic alloy active sites. Our results reveal that the Pd-ZrO2
could be a promising catalyst for Oppenauer oxidation processes in the
view of stability and effectiveness.
Several reports on the utilization of solid solution (multicomponent
material exhibiting a single crystal phase of the host metal oxide) of
mesoporous zirconia-based catalysts in aldehyde production from cat-
alytic oxidation of benzyl alcohol have been reported [9,14,15]. How-
ever, the efficacy of cation dopants in the crystal lattice of ZrO2 in
Oppenauer oxidation is sparsely reported. Generally, metal-doped ox-
ides exhibit improved catalytic activity compared to their undoped
counterparts due to their enhanced intrinsic properties [16]. The
incorporation of cations into the zirconia lattice possibly leads to some
crystal defects, depending on the valence electron difference between
the dopant and the host oxide. The high energy defects lead to the
generation of active sites, which in turn serve as activation sites for the
reacting species at the molecular level on the surface of the catalyst.
Also, the selectivity of such catalysts could be promoted by depositing
metal nanoparticles on the surface [17]. The strong metal-support
interaction facilitates enhanced electron density for desirable activa-
tion of the substrate molecules.
2. Experimental section
2.1. Materials
All reagents purchased were used without further purification. Milli
Q (18 MΩ cm) water was used in the preparation of aqueous solutions.
The 1-butanol (99.8 %), poly(ethylene glycol)-block-poly(propylene
glycol)-block-poly(ethylene glycol) (Pluronic P-123 or PEO20-PPO70-
PEO20), zirconium(IV) butoxide solution (80 % in 1-butanol), nickel
(II) nitrate hexahydrate (99 %), manganese(II) nitrate tetrahydrate (97
%), cerium(III) nitrate hexahydrate (99 %), tetraethoxysilane (99 %),
decane (99 %), and benzyl alcohol (99.8 %) were purchased from Sigma-
Aldrich. Palladium acetate (47 %) and iron(III) nitrate nonahydrate
were purchased from Sisco Research Laboratories (SRL) Pty Ltd, India,
anhydrous acetone (99.5 %) from Glassworld South Africa. Chromium
(III) nitrate nonahydrate (98 %) was purchased from UNIVAR, SAAR
CHEM pty. Nitric acid (HNO3) (69–70 %) was purchased from Rochelle
Chemicals (RSA).
Palladium nanoparticles have been widely reported to be highly
active in the selective oxidation of benzyl alcohol to benzaldehyde, this
has prompted various recent research reports on Pd-based catalysts [1,
18]. Luque et al. [19] employed Fe doped SBA-15 supported Pd NPs in
aerobic benzyl alcohol oxidation. They observed conversions higher
than 80 % and high selectivity to benzaldehyde. The Pd NPs supported
on nitrogen-doped carbon nanotubes Pd/N-CNT developed by Wang
et al. [20] gave 90 % selectivity to benzaldehyde in aerobic oxidation of
benzyl alcohol. Nevertheless, the support plays a key role in increasing
palladium dispersion as demonstrated in the report by Parlett et al. [21]
Zirconia is shown to be promising support for Pd NPs with excellent
catalytic activity for ethylene glycol electro-oxidation [22]. The stability
of the active Pd sites is crucial for selective alcohol oxidation [1].
Furthermore, the design and development of novel multicomponent
catalysts with high activity and selectivity, specifically the latter, is the
key to achieving the goals of green chemical synthesis. Moreover, the
increasing emphasis on the improvements in the atomic efficiency of the
feedstocks and the need for making chemical reactions green, demand
more attention to the design and development of highly selective cata-
lysts [18,23,24]. Among the reported research on catalytic dehydroge-
nation of benzyl alcohol via Oppenauer oxidation, only a few directed
efforts to the systematic suppression of side products and tuning the
selectivity for aldehyde only over Pd supported Zr-based catalysts. This
prompts us to systematically investigate the key role(s) influencing the
selective Oppenauer oxidation of benzylic alcohols to unsaturated al-
dehydes, as this is relevant commercially in industrial chemical
processes.
2.2. Catalysts synthesis
2.2.1. Synthesis of mesoporous zirconia-based catalysts
The surfactant-assisted, inverse micelles protocol detailed in [26]
was used in the preparation of pure ZrO2. In a typical synthesis, 0.040
mol of zirconium butoxide was dissolved in a solution containing 0.336
mol 1-butanol, 6.8 × 10ꢀ 4 mol P-123, and 0.064 mol HNO3. The solution
was covered with a parafilm M and stirred for 24 h at room temperature.
The 1-butanol was removed through evaporation in an oven at 120 ◦C
for 4 h. After evaporation, calcination of the obtained yellow glassy thin
flakes directly to 350 ◦C for 5 h at a 2 ◦C min heating rate was performed.
The metal-doped zirconia was prepared by adding the required amount
of dopant precursor with zirconium butoxide (1:4 M ratio) into the
P-123 acidic solution. Thereafter, the resulting solution was thermally
treated as that of the pure ZrO2. The samples are represented as M_ZrO2,
M = metal dopant.
In this contribution, we report the design of pure and metal (M = Mn,
Ni, Fe, and Ce) doped zirconia catalysts using a sol-gel, inverse micelle
approach. Also, Pd nanoparticles were deposited on mesoporous zirco-
nia and dopant modified zirconia using the deposition precipitation
Scheme 1. Oppenauer oxidation mechanism over heterogeneous zirconia system [25].
2

C.A. Akinnawo et al.
Applied Catalysis A, General 613 (2021) 118022
2.2.2. Deposition of Pd nanoparticles
2.4. Catalytic evaluation of prepared catalysts in Oppenauer oxidation of
alcohols
The Pd nanoparticles were deposited on the various mesoporous
supports (SiO2, pure and metal-doped ZrO2) by
a deposition-
precipitation method. An aqueous solution of palladium acetate
(0.086 g in 10 mL deionized water) was added dropwise into a 100 mL
aqueous suspension containing 2 g of the support. Then heated at 80 ◦C
for 1 h after urea addition. After stirring at 80 ◦C for 1 h, it was allowed
to cool to room temperature, and 10 M excess of NaBH4 was added
dropwise to the metal-support suspension and allowed to stir for 18 h.
The suspension was washed thrice with an excess amount of deionized
water and separated by centrifugation at 4500 rpm for 0.5 h. The sup-
ported Pd catalyst was dried in a vacuum oven at 70 ◦C overnight.
The liquid phase Oppenauer oxidation experiments were performed
on a carousel reaction station multi-reactor (Radley Discovery Tech-
nologies) having twelve 50 mL reaction vials. The substrate, acetone,
and catalysts were added to the reaction vessel in the following pro-
portion, substrate:catalyst amount 1:1 mass ratio, substrate:acetone
1:100 M ratio. The mixture with the required amount of decane as in-
ternal standard was heated with a stirring rate of 450 rpm using a 16.5
mm crossbar stirrer at a temperature of 50 ◦C. The agitation rate was
chosen after a preliminary study. The upper part of the vial was kept at 5
◦C using a temperature-controlled condenser to prevent the loss of
acetone via evaporation.
2.3. Characterization of prepared catalysts
The filtered liquid products were analyzed using a Shimadzu GC-
2010 equipped with a flame ionization detector (FID), capillary col-
The strength and concentration of both the acid and base sites on the
samples were examined using temperature-programmed desorption
(NH3- and CO2-TPD) methods on a Micromeritics AutoChem II chemi-
sorption analyzer. The catalyst, approximately 0.4 g was loaded in a
quartz tube reactor and pretreated in helium gas flow at 200 ◦C for 1 h
(50 mL minꢀ 1) before the experiment. A mixture of NH3/He or CO2/He
in the ratio of 1:9 was used for the measurement in the temperature
umn (Rxi-5Sil; 30 m, 0.2 mm ID, thickness 0.25 μm), and N2 as the
carrier gas. The injection port and FID temperatures were 200 ◦C and
350 ◦C, respectively. Also, on a Shimadzu GC–MS QP-2010 operating
with similar capillary column and injection temperature. For both GC-
FID and MS, the column oven temperature program started 40 ◦C
(hold 2 min), then programmed at 20 ◦C minꢀ 1 to 280 ◦C (hold 5 min)
resulting in 19 min analytical time (see details in SI pg. S11).
range of 30ꢀ 550 ◦C at a ramping rate of 10 ◦C min-1 or 3 ◦C minꢀ 1
,
The process variables were optimized such as the concentration of
benzyl alcohol, catalyst loading, reaction temperature, and time over
pure zirconia. The optimized reaction conditions were followed in
examining the activity of as-prepared M_ZrO2, Pd/ZrO2, and Pd/M_ZrO2
catalysts. The recyclability tests were performed under optimized re-
action conditions. The solid catalyst was separated by centrifugation,
washed with acetone five times, and dried under a vacuum at 80 ◦C for
12 h before subsequent use. The substrate conversion and product
selectivity of the catalysts were calculated (Eqs. 2 and 3).
respectively. The measured flow rate for NH3-TPD and CO2-TPD was 25
mL STP minꢀ 1 and 50 mL STP minꢀ 1, respectively.
The sample (10 mg) was dispersed in 1 mL methanol and sonicated
for 1 h, then a drop of the suspension was placed onto a carbon-coated
Cu grid. The images were captured on a JEOL Jem-2100 F transmission
electron microscope (TEM) operating with an accelerating voltage of
200 kV. The particle sizes of the Pd supported catalysts were evaluated
using the ImageJ software and size distribution curve.
The samples for the scanning electron microscope (SEM) were
carbon-coated in an Agar Turbo carbon coater before analysis. The
prepared samples were captured on a Tescan Vega 3 LMH Scanning
electron microscope (SEM) using a scattering electron detector with a
voltage of 20.0 kV. The distribution of the metal species was identified
by elemental mapping on SEM. The metal Mn+ component of the pre-
pared catalysts and their corresponding weight percentage were verified
with Oxford energy-disperse X-ray spectroscopy EDXS. Also, the Pd
loading (wt%) in the catalytic samples was determined using the Spectro
Acros ICP-OES instrument.
benzyl alcoholt
benzyl alcoholin
Conversion (%) =
× 100
(2)
product(x)
product(1) + product(2) + product(3)
Selectivity (%) =
× 100
(3)
Where benzyl alcoholt = mol of benzyl alcohol at time t, benzyl alcoholin
= initial mol of benzyl alcohol, product(x) = mol of (1) or (2) or (3)
produced after time t, (1) = benzaldehyde, (2) = β-hydroxyketone, and
(3) = benzylideneacetone.
The nitrogen sorption experiments were performed on a Micro-
metrics ASAP 2460 sorption system to study the BET surface area and
pore structure. The samples were degassed at 100 ◦C under nitrogen for
18 h and under vacuum for 10 h at the same temperature before the
experiments for the removal of any physisorbed moisture. The surface
areas were calculated using the Brunauer-Emmett-Teller (BET) method.
The X-ray powder diffraction (pXRD) patterns were collected on a
3. Results
3.1. Acidity-basicity characterization of the as-synthesized ZrO2 based
catalysts
A significant change in the acid-base properties of ZrO2 upon the
incorporation of metal dopants and the loading of Pd nanoparticles was
observed. The total acidity and basicity of the surface of the synthesized
materials were determined through NH3-TPD and CO2-TPD experi-
ments, respectively (Table 1). The peak area as shown in (Figs. 1a and
S1) implies the acidic strength of the catalysts. A broad peak of ammonia
desorption in the region of 109ꢀ 283 ◦C was observed in all the
Rigaku SmartLab diffractometer system operating with Cu Kα1 radiation
(λ =1.5406 Å) at 25 ◦C. The 2θ diffraction patterns measurement was
taken in the range 5ꢀ 90◦ angle with a step of 0.020◦. The crystallite size
was calculated using the Debye-Scherrer formula:
kλ
βCos θ
D =
(1)
Where k (shape factor) = 0.89, λ = the wavelength of the Cu-K
α
radi-
ations, β = the full width half maximum, and θ = angle derived from 2θ
value of peak with maximum intensity in the XRD diffractogram.
Furthermore, the reducibility of the catalysts was investigated with
H2 temperature-programmed reduction (H2-TPR) using a Micromeritics
Autochem II. About 30 mg of the catalytic sample was loaded in a quartz
tube reactor and pretreated under Argon flow at 200 ◦C for 1 h before
each test. The material was probed by passing 10 % H2/Ar through the
catalyst bed at a flow rate of 50 mL minꢀ 1. The measurements were
performed within the range of 25ꢀ 800 ◦C at 10 ◦C min-1 ramping rate.
Table 1
The surface acidity and basicity of the as-synthesized ZrO2 based catalysts.
Entry
Meso
Total acidity
Total basicity
Ratio (acidity/
basicity)
ꢀ 1
(mmol
g
)
(mmolCO2 gꢀ 1
)
NH3
1
2
3
ZrO2
0.80
0.38
0.69
0.63
1.43
1.04
1.27
0.27
0.66
Ce_ZrO2
Pd/
Ce_ZrO2
Pd/ZrO2
4
0.42
0.36
1.17
3

C.A. Akinnawo et al.
Applied Catalysis A, General 613 (2021) 118022
Fig. 1. (a) NH3-TPD and (b) CO2-TPD profiles of the as-synthesized mesoporous ZrO2 based catalysts.
materials. This is an indication of weak and medium surface acidic sites.
Also, a strong acidic site with a broad peak around 450 ◦C was observed
of a new peak at 287 ◦C representing medium base sites in Pd/Ce_ZrO2
could be attributed to the Pd-Ce interaction. The intensity of these peaks
significantly decreased compared to that of the support Ce_ZrO2. The
basic site concentration on Ce_ZrO2 decreased to 1.04 mmolCO2 gꢀ 1 after
the deposition of Pd nanoparticles. Nevertheless, the base concentration
of Pd/Ce_ZrO2 1.04 mmolCO2 gꢀ 1 is higher than that of the pure ZrO2
0.80 mmolCO2 gꢀ 1 and Pd/ZrO2 0.36 mmolCO2 g-1. It could be deduced
that the interaction between Ce-Zr dominates that of other metal-metal
interactions (Pd-Zr and Pd-Ce) in the case of the Pd/Ce_ZrO2 catalyst.
The results of the NH3-TPD and CO2-TPD indicate that all the as-
prepared catalysts possess both acidic and basic sites of different
strengths. The influence of foreign atomic species in the framework of
zirconia on its acidic and basic sites was evident. The acidic sites
decreased upon the incorporation of metal species (Ce and Pd). How-
ever, basic sites depend on the kind of dominant metal-metal interac-
tion. For instance, the catalysts containing Ce even in the presence of Pd
gave more basic sites than that of pure ZrO2, while the catalyst con-
taining only Pd showed less basic sites. Also, it could be said that the
strength of the adsorption sites depends on the kind of foreign metal
specie and interaction with the host ZrO2.
in the Cr_ZrO2 sample (Fig. S1). The Cr_ZrO2 sample presented the
ꢀ 1
highest total acidity of 1.11 mmol
g
(Table S1), which was higher
NH3
than that of the pure ZrO2 (0.80 mmolNH3
g
ꢀ 1) similar to previous
studies [27]. The total acidity of the as-prepared catalysts is within the
range of 0.38–1.11 mmolNH3 gꢀ 1 (Table S1). The Ce_ZrO2 sample
showed the lowest amount of weak acidic sites 0.38 mmolNH3 g-1 with a
broad peak around 230 ◦C. However, there was a significant increase in
the acid sites when Pd nanoparticles were supported on Ce_ZrO2 but not
as much as that of pure zirconia. In the case of Pd/ZrO2, the total acidity
of pure ZrO2 was reduced by about 50 %, from 0.80ꢀ 0.42 mmolNH3 g-1
after the loading of Pd nanoparticles. Upon loading with Pd nano-
particles, the main peak shifted to lower temperatures. This could be
ascribed to a partial blanketing of the ZrO2 medium acidic sites by the Pd
nanoparticles, generating weak acidic sites [27]. The NH3-TPD profile of
other synthesized mesoporous M_ZrO2 as shown in Fig. S1 in the sup-
plementary information suggests that the dopant-induced acidity de-
creases across the period.
The basic properties of all the mesoporous catalysts as revealed in
(Table 1 and Fig. 1b) are a function of the synergistic metal-metal
interaction [28]. The distribution of the basic sites over the surface of
the catalysts showed two prominent peaks at 80 ◦C and 419 ◦C for ZrO2;
78 ◦C and 376 ◦C for Pd/ZrO2 representing both weak and strong basic
sites, respectively. The shift to lower temperature observed in the CO2
desorption peaks of Pd/ZrO2 evidenced the formation of weaker basic
sites on the catalyst’s surface by the Pd nanoparticles and the reduced
peak intensity signifies a reduction in the concentration of the basic
sites. The results correlate well with the literature [29]. The surface base
concentration of pure mesoporous ZrO2 was 0.80 mmolCO2 gꢀ 1. How-
3.2. Textural characterization of the as-synthesized mesoporous ZrO2
based catalysts
The transmission electron (TEM) images showed the morphology
and the crystallinity of the prepared pure mesoporous ZrO2 and ZrO2
based catalysts (Figs. 2 and S2). In the case of ZrO2, a porous structure
was observed and the preservation of the pore structure upon deposition
of Pd nanoparticles and addition of dopant is evident (Figs. 2c and S2
respectively). The Pd nanoparticles, with an average size of 6.5 ± 1.0 nm
(Fig. 2e), are uniformly distributed on the host zirconia (Fig. 2c). Also,
enhanced crystallinity upon deposition of Pd nanoparticles was
observed (Fig. 2d), this supports the results from the pXRD analysis
showing increased peak intensity compared to the host. The high-
resolution electron micrograph indicates the distinct fringe patterns
which confirmed the formation of single-phase ZrO2 with the long-range
order in the structure (Fig. 2d).
ever, a significant decrease in the concentration of the base sites to 0.36
-1
mmol
g
was observed for Pd/ZrO2. When the pure mesoporous ZrO2
CO2
was doped with Ce, the concentration of the base sites significantly
ꢀ 1
increased to 1.43 mmol
g
, showing a more prominent strong basic
CO2
peak around 425 ◦C and a weak base around 84 ◦C. The two broad weak
and strong basic peaks shifted to higher temperatures indicating gen-
eration of stronger basic sites upon the incorporation of Ce into the Zr
matrix.
The scanning electron microscopy (SEM) images (Figs. 3 and S3)
showed that there was no occurrence of microscopical structural change
after the incorporation of metal dopants and deposition of Pd nano-
particles. The surface textural properties of ZrO2 could be said to be
stable upon the incorporation of another metal specie. The metal
composition on the catalysts is also evidenced in the SEM-EDX and their
corresponding weight percentage (Figs. 3c and S4c). The EDX elemental
mapping (Figs. 3d, and S5) also indicate that the Pd nanoparticles are
The higher amount of basic sites in Ce_ZrO2 is attributed to the
presence of Ce4+ which generates surface oxygen anions as basic sites in
the catalyst [29]. When Pd nanoparticles were deposited on the Ce_ZrO2
support, the distribution of the basic site over the surface of the
Pd/Ce_ZrO2 catalyst as shown by the CO2-TPD experiments (Fig. 1b)
presented three broad peaks representing weak, medium, and strong
base sites around 85 ◦C, 287 ◦C and 402 ◦C, respectively. The generation
4

C.A. Akinnawo et al.
Applied Catalysis A, General 613 (2021) 118022
Fig. 2. TEM and HRTEM micrographs of (a and b) pure ZrO2 and (c and d) Pd/ZrO2 samples and (e) Pd nanoparticle size distribution histogram for Pd/ZrO2 (200
particles counted at 100 nm scale bar).
well dispersed on the ZrO2 matrix due to strong interaction between the
Pd-Zr.
doping with a metal resulted in the pore volume stability and
enhancement from 0.10-0.26 cm3 gꢀ 1 whereas a slight decrease was
found when Pd nanoparticles were immobilized on the ZrO2 and
Ce_ZrO2 support. These suggest that the Pd nanoparticles are well
embedded in the ZrO2 support. The isotherms of all the catalysts (Figs. 4
and S6a and b) are typical of Type IV according to the IUPAC classifi-
cation [30,31] which is a characteristic feature of mesoporous materials.
The preservation of the pore network after metal incorporation was also
indicated in the TEM micrographs (Figs. 2 and S2).
Furthermore, the nitrogen sorption analysis revealed that the cata-
lysts exhibited high surface area SBET and mesopores with Type IV iso-
therms. The presence of foreign atomic species alters the structural
properties of the host ZrO2 (Tables 2 and S2). The BET surface area of the
host zirconia (206 m2 gꢀ 1) increased upon doping with Mn (221 m2 gꢀ 1
)
and Fe (223 m2 gꢀ 1). The surface area decreased in the presence of Cr
(193 m2 gꢀ 1), Ni (193 m2 gꢀ 1), and Ce (145 m2 gꢀ 1). The deposition of
Pd NPs also reduced the SBET of the supports with a drastic reduction
from 205 m2 gꢀ 1-73 m2 gꢀ 1 in the case of Pd/ZrO2 and 145 m2 gꢀ 1-132
m2 gꢀ 1 in Pd/Ce_ZrO2. Enlargement of the pore diameter from 2.58 to
3.63 nm upon the incorporation of metal dopants and deposition of Pd
nanoparticles on ZrO2 occurred (Table 2, Figs. 4b, and S6c). Besides,
The XRD diffractograms (Figs. 5 and S7) reveal the fact that ZrO2
nanoparticles with tetragonal symmetry were formed. The tetragonal
phase corresponding to JCPDS Card No. 79-1771 was indexed at peaks
30.4◦ (101), 35.3◦ (110), 50.7◦ (112), 60◦ (211), 63.1◦ (202), 74.8◦
(220). This is similar to previously reported results [32,33]. The intense
5

C.A. Akinnawo et al.
Applied Catalysis A, General 613 (2021) 118022
Fig. 3. SEM image (a) pure ZrO2, (b) Pd/ZrO2; EDX (c) Pd/ZrO2 and EDX elemental maps (d) Pd/ZrO2 respectively.
Table 2
The structural properties of the synthesized mesoporous ZrO2 based catalysts.
Pd content (wt%)
Entry
Meso
SBET (m2 gꢀ 1
)
Pore diameter (nm)
Pore volume (cm3 gꢀ 1
)
Crystallitec size (nm)
Crystal structure
a
b
1
2
3
4
ZrO2
–
–
206
145
132
73
2.58
2.91
2.89
3.59
0.10
0.11
0.09
0.08
5.32
1.87
8.83
8.86
Tetragonal
Tetragonal
Tetragonal
Tetragonal
Ce_ZrO2
Pd/Ce_ZrO2
Pd/ZrO2
–
–
0.60
1.79
1.10
1.80
Pd content determined by ICP-OES = a, EDX = b, BET surface area = SBET. Crystallite size = c, calculated using Debye-Scherrer formula (Eq. 1).
XRD lines observed for the doped ZrO2 (Fig. 5a) are broader, which can
be attributed to the lattice distortion of the tetragonal phase of ZrO2
caused by the incorporation of cation dopant. However, the reflections
in the case of Ni and Cr significantly disappeared, an indication of the
amorphous phase [34]. The absence of reflections of the Pd nano-
particles phase could be due to the low Pd percentage loading which is
below the detection limit of the diffractometer. More so, it depicts that
the Pd species are well dispersed on the ZrO2. Meanwhile, the TEM, EDX,
and SEM mapping further confirmed the presence of Pd nanoparticles.
The Pd supported catalysts showed higher crystallinity compared to the
pure ZrO2 and other M_ZrO2 evidenced in the calculated crystallite size
shown in Table 2 and the increased peak intensity (Fig. 5b). Increased
peak intensities depict enhanced crystallinity which could be ascribed to
the improvement in the structural arrangement of the ZrO2 framework
upon deposition of Pd nanoparticles. This phenomenon also explains the
occurrence of enlargement of pore and widened isotherm (Fig. 4). The
tetragonal phase of zirconia was modulated upon doping with transition
metals and after loading of Pd nanoparticles.
3.3. Reducibility characterization of ZrO2, M_ZrO2, and Pd/ZrO2
catalysts
The redox properties of the as-synthesized Zr-based catalysts were
examined using H2-TPR analysis and the profiles of the catalysts are
presented in (Figs. 6 and S8). There was no significant peak in the profile
of the pure ZrO2. This is typical of non-reducible oxide [29]. The
interaction between Zr and the transition metal dopant enhances the
reducibility of ZrO2 (Fig. S8). The Cr dopant in correlation with others
showed the lowest temperature reduction peak at 260 ◦C ascribed to the
reduction of bulk oxygen (Fig. S8). The presence of Pd tends to improve
the reducibility of the pure ZrO2, this is confirmed by the appearance of
a broad reduction peak at 518 ◦C (Fig. 6). The broad peak at 518 ◦C
6

C.A. Akinnawo et al.
Applied Catalysis A, General 613 (2021) 118022
Fig. 4. (a) N2 adsorption-desorption isotherms (b) pore size distribution of pure ZrO2 and Pd/ZrO2.
Fig. 5. Comparison of wide-angle powder XRD of (a) pure ZrO2 and M_ZrO2 (b) Pd/M_ZrO2 patterns.
Wang et al. [35] Also, the Pd/ZrO2 catalyst has a negative peak at 64 ◦C
indicating the presence of metallic Pd. In H2-TPR, Pd metal tends to
absorb a considerable amount of H2 at low temperature and subse-
quently; releases the adsorbed hydrogen with increase temperature
[35].
3.4. Catalytic evaluation of prepared catalysts in Oppenauer oxidation of
alcohol
The selective Oppenauer oxidation of benzyl alcohol over pure ZrO2
was used as the model reaction. The reaction was significantly influ-
enced by different process conditions such as the catalyst loading, con-
centration of benzyl alcohol, reaction temperature, and time. There was
no significant product formed from the blank experiment after 24 h. The
reaction parameters were optimized using an undoped ZrO2 catalyst.
3.4.1. The amount of benzyl alcohol converted is limited by the surface
active site on the catalyst
A benzyl alcohol concentration in the range of 1.0–3.0 mmol was
utilized in the catalytic reaction and shown in Figs. 7a and S9a. The rate
of agitation was maintained at 450 rpm at 50 ◦C using 0.10 g of the
prepared ZrO2 catalyst. There was not much influence of increasing the
concentration of benzyl alcohol on the amount converted. The amount
converted after 8 h remains approximately constant (~ 0.70 mmol)
when the concentration of benzyl alcohol in the catalytic system was
Fig. 6. H2-TPR profiles of mesoporous pure ZrO2 and (b) Pd/ZrO2.
evidenced the strong Pd-Zr interaction. Besides, it could be explained
that the reducibility of the ZrO2 support is made possible by the
H2-spillover from metallic Pd, a similar phenomenon was reported by
7

C.A. Akinnawo et al.
Applied Catalysis A, General 613 (2021) 118022
Fig. 7. Effect of (a) concentration of benzyl alcohol (b) catalyst loading (c) temperature and (d) reaction time on the catalytic activity of ZrO2 in the Oppenauer
oxidation of benzyl alcohol.
increased above 1.5 mmol. The indicates that in this system, a similar
amount of substrate was converted in each experiment. The amount of
benzyl alcohol molecules converted per time is limited to the available
surface active phase on the ZrO2 catalyst. This could be due to over-
saturation of the catalyst active sites, which possibly hinder uniform
migration of the benzyl alcohol molecules over the catalyst surface,
leading to a surface reaction where there is competitive adsorption for
reactive sites. A concentration of 1.0 mmol benzyl alcohol was chosen
for the subsequent reactions. The reaction performed using toluene in
addition to acetone gave about a 50 % decrease in conversion. This
implies that the ZrO2 catalyst is fairly active in the presence of toluene. A
100-molar equivalent of acetone only was used for the subsequent
reactions.
Oppenauer experiments. This experimental data confirm the trend
observed in the variation of concentration of benzyl alcohol with the
absolute amount converted (Fig. S9c), that the catalytic activity in this
system is governed by the active sites available for reaction within the
surface coverage of the catalyst.
3.4.3. Dependence of catalytic reactivity on temperature
The relationship between the reaction temperature and the catalyst
reactivity is shown in Fig. 7c. As expected, the activity of ZrO2 in
Oppenauer oxidation of benzyl alcohol increased with increasing reac-
tion temperature. This explains the fact that the Oppenauer reaction is
kinetically driven. The temperature effect on the prepared catalysts’
performance and selectivity to the desired products was investigated,
varying the temperature from 30 to 50 ◦C. Irrespective of the tempera-
ture used, aldol products were formed over pure ZrO2. However,
selectivity to benzylideneacetone increased with increasing tempera-
ture. The reaction temperature of the subsequent experiments was
limited to 50 ◦C considering the boiling point of acetone.
3.4.2. The reactivity increases with catalyst amount
The effect of the amount of catalyst was investigated in the domain of
0.05–0.20 g with other variables kept constant. It is observed in Figs. 7b
and S9b that the reactivity increased with increasing catalyst amount as
expected. The activity could be attributed to increased active sites on the
catalyst’s surface. A further indication of surface reaction. Moreover, the
kinetic control regime that is, a linear relationship lies between the
varied catalyst amount. Any of the amount used could be chosen as the
optimum amount hence, 0.10 g was chosen for the subsequent
3.4.4. The products distribution as the reaction time progresses
As shown in the time variation study Fig. 7d, the catalytic activity
increased with time and was found to be approximately constant beyond
14 h reaction time. At 50 ◦C, a considerable conversion of 56 % was
8

C.A. Akinnawo et al.
Applied Catalysis A, General 613 (2021) 118022
attained after 14 h. The selectivity for the products varied as the reaction
time progresses. The selectivity for β-hydroxyketone drastically
decreased from 93ꢀ 63% while that for benzylideneacetone appreciably
increased over time from 7 to 37%. However, the target product benz-
aldehyde was not detected over ZrO2. This could be due to over oxida-
tion or consumption of the benzaldehyde in aldol condensation with
acetone (Scheme 2).
benzaldehyde. The observed results may not be comparable with liter-
ature as related catalytic study in the oxidation of benzyl alcohol in
acetone is rare in literature.
However, an aldol condensation product β-hydroxyketone was
formed alongside benzaldehyde in the case of Pd/Ce_ZrO2, as shown in
Table 3 and Fig. 8a. This is attributed to the dominating basic sites as a
result of Ce-Zr interface synergy as depicted in Table 1 and Fig. 8b, with
the Ce-Zr catalyst showing the highest concentration of basic sites
(Fig. S9d). It could be deduced from the results summarized in Figs. 8b
and S9d that, moderate basicity and acidity is the major factor governing
selectivity to benzaldehyde in the Oppenauer oxidation of benzyl
alcohol with acetone in the proposed catalytic system. Therefore,
tailoring the surface acid-base adsorption sites of the catalytic system is
worthwhile.
3.5. Synergistic effect of M-Zr vs Pd-Zr interaction on product distribution
The activity of the Zr containing catalysts were examined in the
Oppenauer oxidation of benzyl alcohol at 50 ◦C for 8 h. The effect of
modification of zirconia via doping with transition metals or immobili-
zation of Pd nanoparticles was significant in the reaction pathway
determining the product distribution (Table 2, Table S3, Scheme 2, and
Fig. 8a). The Cr doped zirconia presented no activity due to a larger
amount of acid sites than that of the host (Table S3). Upon doping with
Mn, Fe, Ni, and Ce, the Oppenauer oxidation spontaneously extends to
aldol condensation of acetone with the synthesized benzaldehyde (Step
1B, Scheme 2). The Mn, Fe, and Ni-doped ZrO2 were selective to inter-
mediate product β-hydroxyketone of the aldol addition. However, over
Ce-ZrO2 and pure ZrO2, the reaction proceeded to the final product
benzylideneacetone. Komanoya et al. [36] showed from their experi-
ments that ZrO2 has large amounts of basic sites. We further showed by
3.6. Stability and reusability of Pd/ZrO2
The ability of heterogeneous catalysts to retain their activity and
selectivity after many reaction cycles portray the stability and sustain-
ability of such a catalyst. Fig. 9 presents the data from the stability and
reusability studies of Pd/ZrO2 for the Oppenauer oxidation of benzyl
alcohol to benzaldehyde. The reactivity of Pd/ZrO2 shows that the
catalyst has good stability for the subject reaction. The first catalytic run
gave 43 % benzyl alcohol conversion and 98 % benzaldehyde selectivity.
Thereafter, the selectivity to benzaldehyde increased to 100 % and was
retained throughout the subsequent reuse. However, after eight
consecutive catalytic cycles, the efficacy of the Pd/ZrO2 catalyst
declined, this could be ascribed to the reduction in the amount of the
catalyst in the system due to loss during washing, before the next cat-
alytic cycle.
testing a pure CeO2 with basic sites concentration of 2.28
μ
mol mꢀ 2
,
higher than that of pure ZrO2 (0.73
μ
mol mꢀ 2) as shown by the CO2-TPD
in the reaction, that the two aldol products were formed with a higher
yield (Table S3). Therefore, we conclude that basic sites are responsible
for the synthesis of aldol products.
Interestingly, upon immobilization of Pd nanoparticles, 100 %
selectivity to benzaldehyde was observed. The Pd-Zr interface modifies
the electronic properties of zirconia as evidenced by the H2-TPR
experiment (Fig. 6). The Oppenauer oxidation terminates after the
benzaldehyde synthesis without an extension to the aldol condensation
pathway thus, enhancing the selectivity of the system for the production
of benzaldehyde (Step 1A, Scheme 2). Also, it could be said that the
presence of Pd nanoparticles governs the adsorption sites. It facilitates
the moderation of the amount of Lewis acid and base sites for the
selectivity to benzaldehyde (Figs. 8b and S9d). Moderate Lewis acid-
base sites are essential to facilitate a synergy between the Lewis acidic
and basic sites, which are required for tuning the selectivity towards
4. Discussion
The most important role of a catalyst is to favor the selectivity for the
most desired product in a multi-product reaction even at the expense of
the catalytic activity that is, substrate conversion [37]. In this case,
tunability of the product distribution in Oppenauer oxidation of benzyl
alcohol in acetone is achieved upon modification of zirconia with
metals. Enhanced selectivity to benzaldehyde was obtained over
Pd/ZrO2. We propose that the Pd-Zr intermetallic alloy site is respon-
sible for the enhanced selectivity. This hypothesis was supported by
Scheme 2. Illustrative schematic diagram of the possible reaction pathways: Step IA (Oppenauer oxidation of benzyl alcohol) and Step 1B (aldol condensation
of acetone).
9

C.A. Akinnawo et al.
Applied Catalysis A, General 613 (2021) 118022
Fig. 8. (a) Distribution of products and conversion of benzyl alcohol using acetone as H-donor over Zr-based catalysts (b) influence of surface acid:base ratio on
selectivity for benzaldehyde as a function of metal-Zr synergy.
Table 3
Comparison of the SBET and activity of the mesoporous zirconia-based catalysts.
Selectivity (%)
Entry
Catalyst
SBET (m2 gꢀ 1
)
Specific activity* (mꢀ 2g) × 10-3
Conversion (%)
(a)
(b)
(c)
1
2
3
4
5
6
7
8
9
ZrO2
206
190
221
223
193
145
132
73
204
0
42
0
0
82
0
18
0
Cr_ZrO2
Mn_ZrO2
Fe_ZrO2
Ni_ZrO2
Ce_ZrO2
Pd/Ce_ZrO2
Pd/ZrO2
Pd/SiO2
0
24
5
0
100
100
100
90
64
0
0
117
80
26
15
55
28
46
2
0
0
0
0
378
213
630
–
0
10
0
36
100
100
0
–
0
0
Products: (a) Benzaldehyde, (b) β-hydroxyketone, (c) benzylideneacetone.
Reaction condition: acetone:benzyl alcohol = 100 M ratio, 1.0 mmol decane, 0.10 g catalyst, stirring rate 450 rpm, temperature 50 ◦C for 8 h.
*
Specific activity (mꢀ 2 g) = conversion normalized by SBET
.
employing Pd supported on neutral SiO2 support in the Oppenauer
process, 100 % selectivity to benzaldehyde was obtained but with
modest activity. Therefore, the synergy between Pd-Zr is responsible for
both reasonable activity and selectivity to benzaldehyde. Komanoya
et al. [36] in their experiments showed that the metal center acts as a
Lewis acid site and facilitates the formation of a six-membered ring
transition state with alcohol and ketone to accomplish the hydride
transfer. The Pd nanoparticles moderate the concentration of Lewis
acidic sites, this is also in agreement with the report of Gilkey et al. [38]
that Lewis acid sites favor hydrogen transfer from alcohols to carbonyl
groups through a six-member transition state. Therefore, the Pd-ZrO2
interface served as the active site for benzyl alcohol Oppenauer oxida-
tion to benzaldehyde and prevents further aldol condensation between
the benzaldehyde and the acetone.
transition state (Scheme 1). Subsequently, isopropanol is formed, and
the benzyl alcohol converted to benzaldehyde (Step 1A, Scheme 2). The
synergy between ZrO2 and the Pd-Zr intermetallic alloy is advantageous
for benzaldehyde synthesis. The surface modification with Pd mediates
improved selectivity to benzaldehyde in the Oppenauer oxidation of
benzyl alcohol.
On the other hand, pure ZrO2 with more acidic:basic sites in the
presence of excess acetone activates the acetone to form an enolate
which recombines with the product benzaldehyde to form hydrox-
ylenolate and upon further hydration formed an aldol condensation
product (Step 1B, Scheme 2). To achieve the synthesis of benzaldehyde,
a synergy between almost equivalent Lewis acid and base sites on the
catalyst is required (Fig. 8a and b). Also, catalysts with high Lewis base:
acid concentration are selective to aldol products (β-hydroxyketone and
benzylideneacetone) while those with excess Lewis acid:base concen-
tration such as Cr_ZrO2 (Table S1) were not active in either of the two
possible reaction pathways (Oppenauer oxidation and aldol condensa-
tion). Oppenauer oxidation via the six-membered cyclic transition was
achieved over active Pd supported zirconia catalyst with a moderate
concentration of Lewis acidic-basic sites (Table 1 and Fig. 8a and b).
Moreover, the abstraction of hydrogen from the benzyl alcohol
occurred at the active Pd sites. The electronic environment as evidenced
by the H2-TPR (Fig. 6) suggests the Pd particle sites favor H-transfer, the
critical contributor to the significant promoting effect of Pd on the
selectivity for benzaldehyde. Chan-Thaw et al. [39] stated that the
deposition of Pd on a basic metal oxide facilitates H-abstraction from the
O-H. The Pd-induced hydrogen transferred from the O-H was available
for the reduction of the C = O of the acetone via a six-membered cyclic
10

C.A. Akinnawo et al.
Applied Catalysis A, General 613 (2021) 118022
Acknowledgments
The authors acknowledge the National Research Foundation of South
Africa: NRF/TWAS UID: 105463, NRF UID: 117997, 85386, and 85364,
and the University of Johannesburg for financial assistance. Also, the
Spectrum division of the University of Johannesburg and Shimadzu,
South Africa for their analytical instruments.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118022.
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CRediT authorship contribution statement
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Christianah Aarinola Akinnawo: Investigation, Data curation,
Writing - original draft. Dimakatso Jeannett Maheso: Investigation.
Ndzondelelo Bingwa: Conceptualization, Methodology, Writing - re-
view & editing, Supervision. Reinout Meijboom: Conceptualization,
Funding acquisition, Methodology, Project administration, Supervision,
Writing - review & editing.
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Declaration of Competing Interest
The authors report no declarations of interest.
11