Metal-doped mesoporous ZrO2catalyzed chemoselective synthesis of allylic alcohols from Meerwein-Ponndorf-Verley reduction of α,β-unsaturated aldehydes

Article abstract of DOI:10.1039/d1nj00936b

Meerwein-Ponndorf-Verley reduction (MPVr) is a sustainable route for the chemoselective transformation of α,β-unsaturated aldehydes. However, tailoring ZrO2 catalysts for improved surface-active sites and maximum performance in the MPV reaction is still a challenge. Here, we synthesized mesoporous zirconia (ZrO2) and metal-doped zirconia (M_ZrO2, M = Cr, Mn, Fe, and Ni). The incorporation of metal dopants into zirconia's crystal framework alters its physico-chemical properties such as surface area and total acidity-basicity. The prepared catalysts were evaluated in the MPVr using 2-propanol as a hydrogen donor under mild reaction conditions. The catalysts' remarkable reactivity depends mainly on their surface mesostructure's intrinsic properties rather than the specific surface area. Cr_ZrO2, which is stable and sustainable, presented superior activity and 100% selectivity to unsaturated alcohols. The synergistic effect between Cr and Zr species in the binary oxide facilitated the Lewis acidity-induced performance of the Cr_ZrO2 catalyst. Our work presents the first innovative application of a well-designed mesoporous Cr_ZrO2 in the green synthesis of unsaturated alcohols with exceptional reactivity. This journal is

Full text of DOI:10.1039/d1nj00936b

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Metal-doped mesoporous ZrO2 catalyzed
chemoselective synthesis of allylic alcohols
from Meerwein–Ponndorf–Verley reduction
of a,b-unsaturated aldehydes†
Cite this: New J. Chem., 2021,
45, 7878
Christianah Aarinola Akinnawo, Ndzondelelo Bingwa and Reinout Meijboom
*
Meerwein–Ponndorf–Verley reduction (MPVr) is a sustainable route for the chemoselective transforma-
tion of a,b-unsaturated aldehydes. However, tailoring ZrO2 catalysts for improved surface-active sites
and maximum performance in the MPV reaction is still a challenge. Here, we synthesized mesoporous
zirconia (ZrO2) and metal-doped zirconia (M_ZrO2, M = Cr, Mn, Fe, and Ni). The incorporation of metal
dopants into zirconia’s crystal framework alters its physico-chemical properties such as surface area and
total acidity-basicity. The prepared catalysts were evaluated in the MPVr using 2-propanol as a hydrogen
donor under mild reaction conditions. The catalysts’ remarkable reactivity depends mainly on their
surface mesostructure’s intrinsic properties rather than the specific surface area. Cr_ZrO2, which is
stable and sustainable, presented superior activity and 100% selectivity to unsaturated alcohols. The
synergistic effect between Cr and Zr species in the binary oxide facilitated the Lewis acidity-induced
performance of the Cr_ZrO2 catalyst. Our work presents the first innovative application of a well-
designed mesoporous Cr_ZrO2 in the green synthesis of unsaturated alcohols with exceptional
reactivity.
Received 24th February 2021,
Accepted 8th April 2021
DOI: 10.1039/d1nj00936b
rsc.li/njc
readily available secondary alcohol as a hydrogen donor instead
of high-pressure molecular hydrogen or hazardous reduction
Introduction
The chemoselective reduction of a,b-unsaturated aldehydes to reagents such as LiAlH4 and NaBH4.6 The MPV among several
their corresponding allylic alcohols is one of the significant reduction routes for functional group conversion has the
chemical transformations in synthetic organic chemistry. The following prevailing advantages: (i) easy to handle hydrogen
unsaturated allylic alcohol (UAA) produced through this pro- donor without the requirement of heavy gas containment, (ii)
cess is widely utilized as the primary feedstock in food and cheap and environmentally friendly source of hydrogen (iii)
perfumery industries and intermediates in pharmaceutical enhanced selectivity under mild reaction conditions (iv) safer
industries.1,2 However, the reaction is classically carried out process (v) minimized waste (vi) reduced cost, e.g., mainte-
using gaseous hydrogen in the presence of noble metals as nance, separation, and other production logistics (vii) econom-
catalysts with significant limitations such as high-pressure ical and environmentally sustainable process.7,8 However, to
requirements and low selectivity to UAA.3 The high-pressure force the equilibrium reaction towards the formation of UAA,
involved in such a chemical process requires expensive equip- the MPV reduction requires excess sacrificial H-donating
ment and an elaborate experimental set-up with associated molecule,9 which generates by-product.10,11 The product
safety risks.4,5 The problems associated with these classical separation through distillation is a tall task owing to the close
methods could be avoided in the Meerwein–Ponndorf–Verley boiling points of the a,b-unsaturated aldehydes or ketones, the
(MPV) reduction.
UAAs, and the sacrificial alcohol. Hence, efforts should be
The MPV is an alternative means of selective reduction of directed towards attaining 100% selectivity and product yield
unsaturated carbonyls at atmospheric pressure using safe and and recycling the by-product generated from the sacrificial
hydride donor.11
Research Centre for Synthesis and Catalysis, Department of Chemical Sciences,
The MPV reaction is highly chemoselective to the reduction
of CQO bond in the a,b-unsaturated aldehydes but, the hydro-
University of Johannesburg, P. O. Box 524, Auckland Park, Johannesburg 2006,
South Africa. E-mail: rmeijboom@uj.ac.za; Fax: +27 11 559 2819;
genation of conjugated CQC is preferentially favorable thermo-
Tel: +27 11 559 2367
dynamically and kinetically over the CQO bond.3,12 There is an
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00936b
ongoing research effort to develop an efficient catalyst system
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mainly for the chemoselective hydrogenation of CQO in the They found out that the catalytic performance in dehydrogena-
liquid phase at atmospheric pressure. The MPV reaction’s tion of propane with CO2 was influenced by the enhanced
generally accepted reaction mechanism involves a cyclic six- surface morphology, acidity and redox property of the Cr–ZrO2
membered transition state (a determining step in the reaction catalysts with different Cr doping percentage. Also, in the report
rate) when both reactants simultaneously coordinate to the of Wu et al. the catalytic activity of the Cr2O3–ZrO2 prepared
same Lewis acidic sites (Scheme 2). Various metal oxides hydrothermally was attributed to the presence of Cr6+ species.32
exhibiting either Lewis acidic or Brønsted acidic sites, or both
From the above literature survey coupled with the crucial
acidic sites are active in the MPV reaction. However, designing need for sustainable and environmentally benign catalytic
and developing a selective and efficient catalyst with desirable hydrogenation processes,11 it would be interesting to develop
concentration and strength of acid sites has become an essential novel mesoporous zirconia-based catalyst systems with
issue in selective hydrogenation of a,b-unsaturated aldehydes. increased acidic sites for the transfer hydrogenation process under
Despite several studies, hydrogenation of a,b-unsaturated alde- mild reaction conditions with high selectivity to unsaturated allylic
hydes in the absence of additives and/or co-solvent, with complete alcohol. Also, a stable catalytic MPV process without the use of
selectivity to UAA at atmospheric pressure, remains a challenge additives is imperative for the green and clean synthesis of UAA.
and requires more research attention.4,13
The surface properties of zirconia, a major determining factor of
Conventionally, the MPV reaction is carried out in the homo- its catalytic activity, may depend on the method of synthesis.33
geneous system using metal (Al, Zr) alkoxides as catalysts.14,15 Several methods of preparing mesoporous zirconia have been
They suffer deactivation, which is peculiar to homogeneous reported, but the inverse micelles soft-templated technique is
catalysts due to the difficulty separating them from the end poorly represented. The inverse surfactant micelle approach
products; hence, they are non-reusable. Besides, most homo- enables the control of the surfactant-transition metal interactions,
geneous systems require base additives for their activation.11 hydrolysis, and condensation of inorganic sols.34,35 Transition
These challenges encountered during the utilization of homo- metal oxides prepared using the inverse micelles approach are
geneous catalysts have paved the way for increased research to known for their exceptional structural properties such as large
design sustainable heterogeneous catalysts. Heterogeneous cat- surface area (SBET), porosity, and crystallinity. The large surface
alysts offer the advantage of the easy recovery of product and area avails the active sites for better catalytic activity than other
catalyst as well as possible reuse of the catalyst system, among porous materials with low SBET
.
which mesoporous metal oxides were preferred. Hence, in this study, a triblock copolymer P-123 was
Metal oxide such as ZrO2,16–18 CeO2–ZrO2,4 MgO–ZrO2,19 employed as a structure-directing agent in the inverse micelles
Al2O3, Ga2O3 and In2O3 modified ZrO2,16 SiO2, ZrO2 and ZrO2/ approach34,35 for the synthesis of mesoporous zirconia and
SBA-15,20,21 ZrO2–La2O3/MCM-41,22 g-Al2O3, TiO2, and TiO2/g- metal-doped zirconia. The mesoporous zirconia-based materials’
Al2O323 as an active phase has shown excellent catalytic activities catalytic potential was investigated as active phases in the MPV
in MPV reactions. The catalytic performance of metal oxides reduction of selected aromatic aldehydes to their corresponding
depends on the surface’s structural features or environments, alcohols (Scheme 1) without any additives or co-solvent. We
influencing their adsorption strength and activation of the optimized the MPV process variables. A deep insight into the
adsorbates.24,25 Among a wide range of heterogeneous catalytic effect of cation dopant on the physicochemical properties of
species utilized in MPV reduction, zirconia showed the most ZrO2, including crystal phases, structural morphology, surface
promising performance.26 The potential catalytic activity of area, and acidity-basicity, was evaluated. Furthermore, the rela-
zirconia has increased its application in catalysis. It is known tivity of these properties to its catalytic performance in the MPV
for its acidic properties,27 high thermal stability, and corrosion process was also revealed. Elucidation of the activity was based
resistance.7,28 To further emphasize this fact, Alvarez-Rodriguez on the observed pseudo-first-order rate constants (kobs) and
et al. pointed out that the zirconia catalyst is more effective than calculated conversion Equations S1-3. Also, we show that all the
other conventional catalysts such as alumina or silica to produce synthesized catalysts exhibit excellent selectivity to the unsaturated
unsaturated allylic alcohol from citral and cinnamaldehyde.29 allylic alcohol in 2-propanol as H-donor at 80 1C, 450 rpm, and
The surface active sites of zirconia has been verified to improve atmospheric pressure. To the best of our knowledge, this is the
by addition of dopant.30 Xie et al. prepared Cr–ZrO2 using acid– first work that applied mesoporous Cr_ZrO2 prepared via inverse
base pair pathway with evaporation-induced self-assembly.31 micelle for the MPV process. The mesoporous Cr_ZrO2 showed
Scheme 1 MPV reduction of a,b-unsaturated aldehydes to their corresponding unsaturated allylic alcohol over the mesoporous zirconia-based catalyst.
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considerable conversion with 100% selectivity to the unsaturated placed on a carbon-coated Cu-grid then allowed to dry before the
alcohols in reducing citral as a model reaction and other alde- TEM analysis. The prepared samples’ surface morphology was
hydes. The synthesized Cr_ZrO2 is a sustainable catalyst for the identified on a Tescan Vega 3 LMH scanning electron micro-
Meerwein–Ponndorf–Verley process.
scope (SEM) using a scattering electron detector with a high
voltage of 20.0 kV. Prior to analysis, the samples were placed on
an aluminum stub and carbon-coated in an Agar Turbo carbon
coater. The quantity of dopant Mn+ was verified with energy-
disperse X-ray spectroscopy (EDX). The distribution of the metal
species was identified by elemental mapping on SEM. Also, the
dopant content in the solid samples was measured using a
Spectro Acros ICP-OES spectrometer. The Fourier transform
infrared spectroscopy (FTIR) spectra of the samples were
recorded on a Bruker FTIR Alpha spectrometer in the 4000–
400 cmÀ1 region. The samples were mixed with KBr, and the
analysis was performed in the transmission mode under ambi-
ent conditions. The NH3/CO2 temperature-programmed desorp-
tion (TPD) studies to determine the materials’ acidity or basicity
were performed on a Micromeritics AutoChem II. About 0.2 g of
the sample was loaded in a quartz tube reactor. The loaded
sample surface was degassed in a He gas flow at 200 1C for 1 h
before the TPD measurement. We used a mixture of NH3 or CO2
and helium in the ratio of 10: 90 as the probe gas at a flow rate of
50 ml minÀ1. Measurements were performed in the temperature
range of 30–550 1C at a temperature ramp of 10 1C minÀ1 and
3 1C minÀ1 for TPD-NH3 and TPD-CO2, respectively. For identifying
the Lewis and Brønsted acid sites on the samples, adsorbed
pyridine FTIR analysis was carried out. Before the analysis,
B0.03 g of the sample was activated by degassing under gaseous
nitrogen at 300 1C for an hour and cooled to room temperature.
After that, the activated catalyst was contacted with pyridine
(200 ml) at 120 1C for 30 min. Subsequently, the physisorbed
pyridine was evacuated under vacuum at ambient temperature
for an hour,36 and the sample was analyzed on a Bruker FTIR
Alpha spectrometer. The H2-TPR (hydrogen-temperature pro-
grammed reduction) analysis was conducted on the same Micro-
meritics Autochem II. Approximately 30 mg of the catalyst was
loaded in the quartz tube reactor and pretreated under Argon
flow at 200 1C for 1 h to ensure the catalyst surface is clean before
2. Experimental
2.1. Materials
Nitric acid (HNO3) (69–70%) was purchased from Rochelle Chemicals
(RSA). 1-Butanol (99.8%), ethanol (99.9%), poly(ethylene glycol)-
block-poly(propylene glycol)-block-poly(ethylene glycol) (PEO20-
PPO70-PEO20 or Pluronic P-123), zirconium(IV) butoxide solution
(80% in 1-butanol), manganese(II) nitrate tetrahydrate (97%),
nickel(II) nitrate hexahydrate (99%), cinnamaldehyde (99%), citral
(mixture of cis- and trans-isomers) (96%), benzaldehyde (99%),
crotonaldehyde (99%), furfural (99%), 2-propanol (99.5%), decane
(99%) were all purchased from Sigma-Aldrich. Ferric nitrate nona-
hydrate was purchased from SRL chemicals, and chromic(III)
nitrate nonahydrate (98%) was purchased from UNIVAR, SAAR
CHEM pty. All chemicals were of analytical grade and used as
received without further purification.
2.2. Synthesis of catalysts
The procedure already reported was followed for the synthesis
of ZrO2.34 Briefly, 15.34 g (0.040 mol) of zirconium butoxide was
added to a solution containing 25.04 g (0.336 mol) 1-butanol,
4.0 g (6.8 Â 10À4 mol) of P-123, and 4.0 g (0.064 mol) HNO3. The
mixture was stirred overnight, and the clear gel was dried in an
oven at 120 1C for 4 h. The yellow glassy thin flakes obtained
were calcined in air at 350 1C for 5 h with a heating rate of
2 1C minÀ1. The metal-doped zirconia was synthesized by
adding the metal dopant (Cr, Mn, Fe, and Ni) precursor to
zirconium butoxide in a molar ratio of 1 : 4. As in the case of pure
ZrO2, the same thermal treatment was applied (Scheme S1, ESI†).
The samples are tagged M_ZrO2, M = metal dopant.
2.3. Characterization of catalysts
Powder X-ray diffraction (p-XRD) analyses were carried out on a each test. After the pretreatment, H2/Ar (10 : 90) was passed over
Philips XPERT-PRO diffractometer system operating with Cu Ka1, the catalyst at a 50 ml minÀ1 flow rate. The measurements were
Ka2, and Ni Kb radiation (l = 1.5406, 1.54443 and 1.39225 Å, performed within the ambient temperature to 800 1C with a
respectively) at 25 1C. Both low and wide 2y range (i.e., 4–901) 10 1C minÀ1 ramping rate. The prepared samples’ thermal
angle diffraction patterns with a step of 0.1701 were measured. stability test was performed on a PerkinElmer STA 6000 thermo-
The Debye–Scherrer equation (eqn (S1), ESI†) was used to gravimetric analyzer (TGA). The degradation study temperature
calculate the mean crystallite size. Micrometric ASAP 2460 was varied from 25–900 1C with a ramping rate of 10 1C minÀ1
sorption system gave the nitrogen sorption measurements. under air at a 20 ml minÀ1 flow rate. The UV-vis spectra of the
The samples were firstly degassed under flowing nitrogen at samples were obtained on a microplate reader (PowerWave
100 1C for 18 h and under vacuum for 10 h at the same HT.Biotek microplate reader). Before obtaining the UV-vis
temperature before the experiments to remove any physisorbed absorption spectra, about 30 mg of the solid sample was
moisture. The surface areas were calculated using the Brunauer– sonicated in 2 ml methanol and decanted. After that, the super-
Emmett–Teller (BET) method. Transmission electron micro- natant was analyzed using a 24-well plate.
scopy (TEM) for the confirmation of the mesostructure was
achieved on a JEOL Jem-2100F electron microscope with an
2.4. Evaluation of catalytic performance
accelerating voltage of 200 kV. The pore diameter was measured The liquid phase MPV reduction experiments were performed
using ImageJ software. A 10 mg of the catalyst was sonicated in on a carousel reaction station multi-reactor (Radley Discovery
1 ml of methanol for 1 h, and a drop of the suspension was Technologies) with twelve 50 ml vials. The 50 ml reactor vial
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was charged with 0.4 g M-ZrO2, 2.50 mmol of aldehyde, solution of binary MxOy–ZrO2. This claim is supported by the
1.00 mmol (200 ml) decane as an internal standard, and gradual shift of the peak at 30.41 (101) of ZrO2 towards a higher 2y
130 mmol (10 ml) 2-propanol. Followed by reflux at 80 1C and degree. No identifiable peak is associated with the dopants, which
stirring at 450 rpm with a 16.5 mm crossbar stirrer. After is an indication that the dopants are well incorporated into the
filtration, the filtrate was analyzed on a Shimadzu GC-2010 ZrO2 matrix and high dispersion of the dopant species. The
with flame ionization detector (FID) using a capillary column incorporation of cation into the crystal framework of zirconia
(Restek RTX-5; 30 m, 0.25 mm ID, thickness 0.25 mm) in N2 significantly influenced its crystallinity. The tetragonal structure
carrier gas. The injection port and FID temperature were with reduced peak intensity remains in the presence of Mn and
maintained at 200 1C and 350 1C, respectively. The products Fe, while we observed crystal distortion in the case of Ni and Cr
were further confirmed by a Shimadzu GC-MS QP-2010 using dopants. The dopant species weakened the t-ZrO2 peaks in the
the same capillary column with the injection temperature at case of Mn and Fe and were significantly destroyed in Ni and Cr,
200 1C. The ion source and interface temperatures were 200 1C forming disordered ZrO2. This observation implies the degree of
and 250 1C, respectively. For the GC FID and MS, the column dopant incorporation and distribution and Mn+–Zr4+ interaction.
oven temperature program started at 40 1C (hold 2 min), then The observed broader and weaker diffraction peaks in Mn_ZrO2
programmed at 20 1C minÀ1 to 280 1C (hold 5 min); the total and Fe_ZrO2 explain the occurrence of higher surface area in
analytical time was 19 min (details in Section S1.3.2, ESI†). The correlation with the host ZrO2. Also, the crystallite size of t-ZrO2
catalysts were screened with the MPV reduction of citral as a (5.50 nm) decreased upon doping with Mn_ZrO2 (1.37 nm) and
model reaction. The catalyst exhibiting the best activity was chosen Fe_ZrO2 (2.59 nm). The decrease in the crystallite size possibly
for the transfer-hydrogenation of selected a,b-unsaturated alde- contributed to expanding the surface area, as depicted in Table 1.
hydes. The substrate conversion, product selectivity, and normal-
The experiments carried out to confirm the surface structure
ized activity were calculated (eqn (S2)–(S5), ESI†). Furthermore, the and porosity of pure and doped ZrO2 using nitrogen sorption
observed kobs for each experiment were calculated using Kinetic analysis shown in Table 1 revealed that the pure ZrO2 exhibited
studio version 2.08 software. For the recyclability study, the catalyst pores with an average diameter of 2.53 nm within the meso
was pretreated by calcining at 350 1C/5 h before reuse.
range with a large BET surface area (SBET) 206 m2 gÀ1. The BET
surface area of the M_ZrO2 catalysts depends on the final
catalyst’s crystallinity, which is a function of the dopant’s
nature. The SBET of pure ZrO2 (206 m2
modified with Mn (221 m2
decreased in the case of Cr (190 m2 gÀ1) and Ni (193 m2 gÀ1
g
À1) increased when
3. Results
3.1. Surface properties of the M_ZrO2 catalysts
g
À1) and Fe (223 m2 À1) but
g
)
Fig. 1 illustrates the wide-angle XRD patterns of pure ZrO2 and
M_ZrO2 samples. The diffractogram of pure ZrO2 correspond to
a tetragonal crystal phase t-ZrO2 (JCPDS-89-7710), indexed at
2y = 30.41 (101), 35.31 (110), 50.41 (112), 60.11 (211) and 73.81
(220). No extra peaks were detected, which confirms the purity
of the synthesized t-ZrO2. Upon doping t-ZrO2 with a metal
atom, an isomorphous substitution was observed. Suggesting that
some surface Zr atoms in the M_ZrO2 samples are substituted
with the dopant atoms and the formation of a homogeneous solid
dopants. The pore sizes are within the range of 2.5–3.6 nm, a
characteristic feature of mesoporous oxides. The metal
dopants’ addition distinctly enlarged the pore size 2.5–3.6 nm
and the pore volume 0.1 to 0.26 cm3 gÀ1. The mesostructuring
is associated with the metal–metal grain boundary adhesion/
expansion during the formation of oxo-metal clusters at the
stage of sol condensation, diffusion of volatile species, and
subsequent removal of the surfactant template. The degree of
grain segregation is directly proportional to the porosity of the
material.37–39 Also, the larger the porosity, the slower the grain
growth, as depicted in the correlation of the pore size and the
crystallite size (Table 1). The observed physisorption isotherms
for all the catalysts in Fig. 2a are typical of Type IV compared
with the IUPAC classification reported by Thommes et al.40,41
This further indicates that all the catalysts are mesoporous
materials with thin capillary pores. The ZrO2 catalysts exhibited
hysteresis loops P/P0 typical of H2 type indicating the occur-
rence of cavitation controlled evaporation; this depicts that the
materials possess a heterogeneous pore network with the neck
size (W) distribution much more narrow than the size distribu-
tion of the cavities (Wc), that is, W o Wc.40–42 The pore size
distribution of pure zirconia and M_ZrO2 are shown in Fig. 2b.
All the catalysts showed unimodal pore size distribution, with
the M_ZrO2 exhibiting a narrower pore distribution compared
to the pure ZrO2 catalyst. Significantly, the corresponding BET
surface area, pore-size distribution, and pore volume is an
indication that the catalysts are mesoporous with high surface
Fig. 1 Wide-angle pXRD patterns of the pure ZrO2 and M_ZrO2 catalysts.
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Table 1 The structural properties of the synthesized mesoporous catalysts
Dopant content
(wt%)
Pore diameter
(nm)
Entry Meso
a
b
BET surface area (m2
g
À1) c
d
Pore volume (cm3
g
À1) Crystallite size (nm) Crystal structure
1
2
3
4
5
6
ZrO2
—
—
206
190
221
223
193
133
2.53
3.04
3.63
2.73
2.79
2.89
2.57
3.06
3.62
2.64
2.59
2.48
0.10
0.14
0.26
0.14
0.14
0.10
5.50
NA
1.37
2.59
NA
Tetragonal
Amorphous
Tetragonal
Tetragonal
Amorphous
—
Cr_ZrO2
Mn_ZrO2
Fe_ZrO2
Ni_ZrO2
Spent Cr_ZrO2
7.1
8.3
6.8
9.7
—
8.2
7.4
6.9
6.1
8.1
NA
Dopant content determined by EDX = a, ICP-OES = b, pore diameter determined by N2 sorption analysis = c, TEM = d, NA: not applicable.
Fig. 2 (a) N2 adsorption–desorption isotherms (b) pore size distribution of pure ZrO2 and M_ZrO2 catalysts.
area, and the surface mesostructure of ZrO2 could be tailored by
adding foreign atomic specie.
The thermal stability of the catalysts (Fig. 4a) suggests that
the catalysts are thermally stable. The dopant species enhance
Moreover, the transmission electron microscopy (TEM) and the thermal stability of the host ZrO2 (8%), except Mn_ZrO2,
high transmission (HRTEM) images of the zirconia systems as which shows an approximately similar weight loss of 8.3%. The
displayed in Fig. 3a, b, d, e and Fig. S1, S2 (ESI†) reveal that they 5% degradation between 30–257 1C is attributed to the removal
are made of nanosized particles with intraparticle voids that of adsorbed surface and bulk water molecules, while above
were preserved upon the addition of different metal ions. 257 1C could be classified as degradation due to the decom-
The TEM images (Fig. 3a, d and Fig. S1, S2, ESI†) show that position of the organic surfactant residue. Above 683 1C, the
the pores are well distributed in the ZrO2 matrix, supporting spectra seemingly flatten out, suggesting minimal or no decom-
the evidence of the presence of pore and the pore enlargement position and the inorganic material’s stability. The samples are
upon doping as presented by the N2 sorption experiment also stable in the catalytic characterization and application
(Table 1 and Fig. 2b). A similar observation was reported in temperature within this study’s scope.
the work of Xie et al.31
The catalysts’ hydrogen consumption temperatures (Fig. 4b)
The surface morphologies of the catalysts are shown in and the minimum temperatures (Table 2) suggest the catalysts’
Fig. 3c, f, and Fig. S3 (ESI†). Modification of the t-ZrO2 surface reducibility. The pure zirconia showed a poor hydrogen uptake
morphology upon the introduction of dopants is insignificant; with a small peak around 652 1C, corresponding to the
this is due to the homogeneity of the M_ZrO2 solid structures, reduction of bulk lattice oxygen of zirconia. We found that
as observed in the XRD patterns. The EDX mapping (Fig. 3g and doping enhanced the reducibility of ZrO2, with a significant
Fig. S4, ESI†) reveals the uniform distribution of the dopant shift in its reduction peak to lower temperatures; this depicts the rate
species in the ZrO2 matrix. The SEM-EDX analyses (Fig. 3h and of the redox reaction. However, the degree of reducibility is depen-
Fig. S5, ESI†) confirm the metal constituents of the synthesized dent on the kind of doping species. The reduction peaks between
ZrO2 systems and indicate the actual weight percentage of the 261–360 1C likely represent the reduction of the metal dopant
dopants incorporated.
species: Cr3+ - Cr2+, Fe3+ - Fe2+, Mn2+ - Mn0, Ni2+ - Ni0.
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Fig. 3 (a–c) TEM, HRTEM, and SEM images of the meso-ZrO2, respectively; (d–f) TEM, HRTEM, and SEM images of the Cr_ZrO2, respectively; (g) EDX
mapping of Cr_ZrO2 showing the metal distribution, and (h) EDX of Cr_ZrO2 indicating the weight percentage of the Cr content.
The reduction peaks demonstrated in the region of 430–486 1C are depicted in Fig. 5 and 6, respectively. The total acidity and
and above 600 1C are typical of surface and bulk reduction of basicity, along with their density, are summarized in Table 2.
lattice oxygen of zirconia, respectively. The Cr_ZrO2 demon- The total acidity and basicity were obtained from the peak area
strated the superior reduction capacity of surface interaction of NH3 and CO2 desorption, respectively. The acid or base sites
with hydrogen at the lowest temperature of 261 1C. This is likely density was derived by dividing the total acidity or basicity by
due to the strong synergistic interaction between the Cr3+ and the surface area (Table 1). The NH3/CO2 desorption peak
Zr4+ (CrxOy–ZrO2 solid solution). The H2-TPR data supported the around 200 1C represents the acid/base sites of a weak strength,
superior catalytic activity of Cr–Zr active phase species for the H from 200 1C to 350 1C depicts medium strength acid/base sites,
abstraction-release mechanism in the MPV dehydrogenation– and above 400 1C corresponds to strong acid/base sites.43
hydrogenation reaction.
The surface basic properties of all the catalysts are depicted in
We investigated the surface acid–base properties of the Table 2 and Fig. 5. A similar chair-like CO2-TPD profile was
prepared catalysts by NH3- and CO2-TPD analyses. The spectra reported for ZrO244 and Cu/ZrO2/CaO.45 The base concentrations
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Fig. 4 (a) TGA spectra, (b) H2-TPR profiles of the ZrO2, and M_ZrO2 catalysts.
Table 2 Surface acidity, basicity, and reducibility of ZrO2 and M_ZrO2 catalysts
Total acidity
Acidic site density (mmol Total basicity
Basic site density (mmol Ratio
TH consumption
(1C)
2
Entry Catalyst (mmolNH gÀ1
)
mÀ2
)
(mmolCO gÀ1
)
mÀ2
)
acid : base
3
2
1
2
3
4
5
ZrO2
0.32
0.0016
0.0035
0.0021
0.0018
0.0027
0.86
0.26
1.12
0.93
0.95
0.0042
0.0014
0.0051
0.0042
0.0049
0.37
2.54
0.42
0.44
0.55
652
261
270
331
360
Cr_ZrO2 0.66
Mn_ZrO2 0.47
Fe_ZrO2 0.41
Ni_ZrO2 0.52
of the prepared catalysts ranged from 0.3–1.1 mmolCO
g
À1. The proton-donor tendency, the more strongly it binds with the base
2
basic sites distribution of the catalysts illustrated in Fig. 5 (NH3), and the higher the required NH3 desorption temperature.
depicted that all the catalysts exhibited basic sites of both weak Hence, Cr_ZrO2 possesses stronger electrophilic active sites (acid
and strong strength, although with different peak intensities. sites) needed for the selective adsorption of citral via the
The base concentration of the pure ZrO2 (0.9 mmolCO
g
À1) was CQO bond.
2
approximately similar in the presence of Fe (0.9 mmolCO gÀ1
)
)
Comparatively, the NH3- and CO2-TPD data revealed that all
the catalysts exhibit both surface acidic and basic sites. How-
2
but slightly increased upon doping with Mn (1.1 mmolCO gÀ1
2
and Ni (1.0 mmolCO
g
À1). An exception occurred in the case of ever, these active sites are not equivalent, as observed in the
2
Cr_ZrO2; the Cr species significantly decreased the base concen- data derived from the acid to base ratio (Table 2); the dom-
tration of pure ZrO2 to 0.3 mmolCO
À1. The catalysts’ basic inance in terms of strength, total concentration, and density
g
2
density showed a similar trend, which was also confirmed by the depends on the metal–metal interaction nature. Meanwhile,
reduction in the peak intensity representing the weak strength Cr_ZrO2 presents more acid sites density and strength than the
basic sites on the pure ZrO2 in the case of Cr_ ZrO2.
pure ZrO2, which possibly originates from the interaction of the
Fig. 6 shows the distribution of the acidic sites of weak to Cr–Zr species at the atomic level. The tuning of the surface
strong strength in the meso-ZrO2. Upon doping, the acidic sites acid–base character of ZrO2 was achieved by incorporating
underwent modulation. In the presence of Mn, Fe, and Ni, only metal ions, which facilitated the understanding of the effect
two broad peaks representing weak and medium strength acid of acid/base sites of the catalysts on the MPV reduction of
sites were observed. Whereas the Cr_ZrO2 appears unique, aldehydes.
which gave a more prominent shoulder desorption peak on
To further understand the surface components, types, and
the high-temperature side at ca 436 1C, suggesting a stronger structures of the acid sites of the catalytic materials, investiga-
surface acidic site. A similar trend of NH3 desorption over Cr tion of the surface functionality and nature of acid sites was
doped ZrO2 was reported.31 The Cr_ZrO2 (0.7 mmolNH3 gÀ1
)
achieved through FTIR (Fig. 7a and b) and pyridine-adsorbed
possessed the highest concentration of acidity among the catalysts FTIR (Fig. 7c and d) spectroscopy methods, respectively. In
(Table 2). The NH3 desorption results suggest the proton-donating Fig. 7a, the broad absorption band around 500–823 cmÀ1 is
capacity of the surface acid site on the catalysts. The stronger the typical of Zr–O–Zr vibration in the tetragonal structure, and
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1622 cmÀ1 suggests the bending hydroxyl group vibrations,
while the broad and strong peak at 3435 cmÀ1 is a reflection of
physically adsorbed moisture on the surface, hence showing
the O–H stretching of water.47
Upon doping, the peak intensity at 3435 cmÀ1 decreased,
indicating a decline in hydroxy groups and hydrophilic property
of ZrO2.48 Like pXRD patterns (Fig. 1), the peak intensity at 500–
823 cmÀ1 representing the t-ZrO2 decreased upon the substitu-
tion of Mn+ into ZrO2. Besides, the shifting of the bands towards
the lower wavenumber was observed (for instance, 591 to
519 cmÀ1), which is most significant in Cr_ZrO2. This shifting
is due to variation in the bond length when Mn+ ions replace
Zr4+ ions. Hence, it confirmed the successful incorporation of
the metal ions into the ZrO2 lattice. In Cr_ZrO2 catalyst, small
peaks at 1210 and 1740 cmÀ1 are observed, attributed to the
formation of CrxOy,49 due to the strong interaction between
Cr–Zr. This could be responsible for the generation of more
active sites on the surface of Cr_ZrO2. It is observed in Fig. 7b
that the peak at 1740 cmÀ1 corresponding to the Cr species
disappeared after five catalytic cycles, which suggests that the
Cr_ZrO2 perhaps undergoes a surface structural transformation
in the course of further thermal pretreatments during reuse.
Pyridine is a sensitive probe molecule for the classification
of Lewis acid and Brønsted acid sites. As depicted in Fig. S5.7c
(ESI†), the pyridine-IR bands at 957, 1400, and 1615 cmÀ1 are
typical of the Lewis acid site. Two different acidic strengths due
to the Lewis acid sites are shown in ZrO2, Fe_ZrO2, and
Mn_ZrO2, whereas Lewis acidity of three different strengths
was observed for both Ni_ZrO2 and Cr_ZrO2. The higher the
assumed frequency of the IR bands, the stronger the acidity of
the sites.50 The IR band at 1538 cmÀ1 suggests Brønsted acid
site while 1485 cmÀ1 indicates C–C oscillation of pyridine
aromatic ring chemisorbed on both Brønsted and Lewis acid
sites.51 The ZrO2 catalyst showed no peak typical of the
Brønsted acid site, but two characteristic bands (1400 and
1615 cmÀ1) associated with pyridinium ions coordinately
bonded to Lewis acid sites.52 These two adsorption bands were
retained with increased intensity upon the addition of Mn and
Fe species. More broad bands for Lewis acid, Brønsted acid,
and a combination of Lewis acid and Brønsted acid sites were
found when ZrO2 was doped with Ni and Cr, accompanied by
an increased intensity on Cr_ZrO2 catalyst. The pyridine-
adsorbed IR reveals that the total acidity obtained from the
NH3-TPD data has a larger Lewis to Brønsted acid ratio, which
is highest in the Cr_ZrO2 and the main factor that governs its
catalytic activity in this study. The results indicate the possibi-
lity of tuning the active sites of ZrO2 by adding foreign atomic
species.
Fig. 5 CO2-TPD profiles of the ZrO2 and M_ZrO2 catalysts.
Fig. 6 NH3-TPD profiles of the ZrO2 and M_ZrO2 catalysts.
The UV-vis absorption spectra of undoped and metal-doped
ZrO2 are shown in Fig. 8. The results indicate that the undoped
ZrO2 and M_ZrO2 (M = Fe, Mn, and Ni) have no absorption
1534 cmÀ1 suggests the stretching vibration of the Zr–O bond. peaks in the visible wavelength region of 300 to 700 nm.
The strong bands around 1394 and 3143 cmÀ1 are ascribed to However, after Cr doping, a new absorption peak appears at
the C–O–C and C–H stretching vibrations of P-123, respec- around 360 nm; this is attributed to the band-gap transition of
tively.46 This agrees well with the TG analysis (Fig. 4a), indicat- ZrO2 due to Cr3+ ions.53,54 The Cr-3d electronic configuration
ing residual surfactant is present in the catalysts. The IR peak at results in the appearance of some localized states in the host
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Fig. 7 FTIR spectra of the (a) ZrO2 and M_ZrO2, (b) fresh and spent Cr_ZrO2, and Pyridine-adsorbed FTIR spectra of the distribution of the acid sites of
(c) ZrO2 and M_ZrO2, (d) fresh and spent Cr_ZrO2, recorded at room temperature. L (Lewis acid site), B (Brønsted acid site), and L + B (Lewis + Brønsted
acid site).
band-gap and makes electron transfer easier than the undoped conversion of citral. The catalytic activities’ observed trend follows
system. This agrees well with the H2-TPR data (Fig. 4b) of the order Cr_ZrO2 4 ZrO2 4 Mn_ZrO2 4 Fe_ZrO2 4 Ni_ZrO2.
Cr_ZrO2. All these issues explain the reason for the generation The correlation of the surface area, acidity, and basicity with the
of more Lewis acid sites on Cr_ZrO2 compared to the undoped catalytic activity in MPV reduction of citral is displayed in Fig. 9.
system.
Generally, the surface area controls the catalyst activity (high
surface area, high catalytic activity). This is not the case in our
proposed catalytic systems, as catalysts with higher surface area
Mn_ZrO2 and Fe_ZrO2 gave low conversion of citral. Instead, we
3.2. Surface performance of the M_ZrO2 catalysts in MPV
reduction of citral
3.2.1. Dopants vs. catalytic reactivity. The MPV reduction found that the catalysts’ catalytic activity in the MPV reduction of
of citral with 2-propanol was selected as a model reaction to citral is governed mainly by the kind of metal dopant, acidic site
examine the activity of the prepared pure and metal-doped density, and reducibility. As shown in Table 2 and Fig. 9, the catalyst
mesoporous zirconia (M_ZrO2) catalysts. Table 3 indicates their Cr_ZrO2 with the highest acidity (acid:base ratio) presented the
respective performance. Interestingly, this work’s catalytic systems highest activity, whereas catalysts Mn_ZrO2, Fe_ZrO2, and Ni_ZrO2
gave 100% selectivity to the unsaturated allylic alcohol (nerol + with higher basicity compared to that of Cr_ZrO2 and the pure ZrO2
geraniol). The pure ZrO2 presented 62.6% citral conversion. decreased the activity of the host ZrO2. It could be deduced from the
Comparison with the pure ZrO2, the effect of metal dopant in results that surface-active acid sites, Lewis acid in particular, play an
terms of activity enhancement was only observed when ZrO2 was essential role in the MPV reduction of citral. Also, the superior
doped with Cr species. The Cr_ZrO2 gave optimal activity of 76.4% reduction capacity of Cr_ZrO2 favors its performance.
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Fig. 9 Correlation of the surface properties with catalytic activity in MPV
Fig. 8 UV-vis spectra of the mesoporous ZrO2 and M_ZrO2.
reduction of citral.
cycles. Moreover, the comparison of the Cr content before
(8.2 wt%) and after five use (8.1%) as quantified on the ICP/OES
showed no leached Cr species. The leaching test (Fig. 10a) and
the ICP results (Table 1) confirmed the homogeneity of the
Cr_ZrO2 solid structure. Hence, the mesoporous Cr_ZrO2 is
catalytically stable and reusable with retained activity.
3.2.2 Recyclability, leaching test, and characterization of
Cr_ZrO2 catalyst after reuse. Developing a recyclable catalyst
without decomposing due to long-term reuse has become a
critical factor in achieving a sustainable catalytic system. As
shown in Fig. 10a, the reduction of citral hardly proceeds after
removing the catalyst, indicating no residual active component in
the reaction liquid. The recyclability test (Fig. 10b) reveals that the
Cr_ZrO2 is catalytically stable and reusable with excellent selectiv-
ity to UAA although, a slight variation in the activity during the 3rd
and 4th reaction cycle was observed. The characterization of the
spent gave more insights into the possible transformation in the
catalyst during reuse. The FTIR (Fig. 7b) shows that the crystal
phases remained, the nitrogen sorption results (Table 1 and
Fig. S5.7a, b, ESI†) show a significant decrease in the SBET and
pore size ascribed to the sintering effect due to repeated thermal
treatment during the regeneration process. The adsorbed pyridine
experiment (Fig. 7d) reveals a decline in the Lewis acid sites and a
significant loss of the Brønsted acid site; this suggests that the
Cr_ZrO2 catalyst undergoes surface restructuring during catalyst
pretreatment before reuse. Hence, the effect of Brønsted acidity
could be negligible in this study.
3.3. Substrate scope of mesoporous Cr_ZrO2 catalyst
The chromium doped zirconia exhibited the best catalytic activity
in the reduction of citral with 76.4% conversion, Table 3. Hence,
the catalytic scope of Cr_ZrO2 in MPV reduction is extended to
some unsaturated aldehydes to form their corresponding unsatu-
rated alcohols Table 4. The Cr_ZrO2 is most active in the MPV
reduction of furfural. The appreciable reactivity suggests that the
mesoporous Cr_ZrO2 acid catalyst is highly active in the MPV
process and 100% selective to unsaturated allylic alcohols.
4. Discussion
Herein, a series of transition metal-doped mesoporous ZrO2
Nevertheless, the TEM image (Fig. S7c and d, ESI†) shows (M_ZrO2, M = Cr, Mn, Fe, and Ni) catalysts were designed for
the stability of the mesostructure after 5 consecutive catalytic the MPV reduction of aldehydes. Interestingly, the tunability of
Table 3 Catalytic performance of the synthesized catalysts for MPV reduction of citral with 2-propanol to form nerol + geraniol
Entry
Catalyst
Conversion (%)
Selectivity (%)
Amount converted (mmol)
Activitynormalized* (mmol gÀ1) Â 102
kobs (hÀ1
)
1
2
3
4
5
ZrO2
62.6
76.4
55.8
41.1
26.8
499
499
499
499
499
1.57
1.91
1.39
1.03
0.67
393
478
348
258
168
0.24 Æ 0.03
0.40 Æ 0.04
0.23 Æ 0.02
0.20 Æ 0.03
0.06 Æ 0.05
Cr_ZrO2
Mn_ZrO2
Fe_ZrO2
Ni_ZrO2
Reaction condition: 2-propanol:citral = 52 molar ratio, 1.0 mmol decane, 0.4 g catalyst, stirring rate = 450 rpm, T = 80 1C, t = 10 h. * Activitynormalized
obtained by normalizing the amount converted at 10 h to the mass of the catalyst used in each run.
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Fig. 10 (a) Leaching test and (b) recyclability of Cr_ZrO2 catalyst in MPV reduction of citral. Reaction condition: 2-propanol:citral = 52 molar ratio,
1.0 mmol decane, 0.4 g catalyst, stirring rate = 450 rpm, T = 80 1C, t = 6 h.
the surface properties of the resulting catalysts is governed by control condensation of the oxo-clusters was achieved by form-
the metal dopant’s nature. The SBET decreases from 206 to 189 ing NOx species from the nitric acid’s thermal decomposition.34
and 193 m2 gÀ1 upon doping with Cr and Ni, respectively. An
The increase in the porosity (pore diameter and pore
improvement in SBET from 206–223 m2 gÀ1 was observed when volume) upon doping and SBET in the case of Mn_ZrO2 and
doped with Mn and Fe (221 and 223 m2 gÀ1, respectively). Also, Fe_ZrO2 could be due to the metal–metal interactions during
upon doping, there was an enlargement of pore diameter from condensation of the inorganic sols and the condition of the
2.53–3.63 nm and increased pore volume from 0.10–0.26 cm3 gÀ1 reaction media. A similar phenomenon was explained by
(Table 1). The mesostructure properties of the synthesized pure Grosso et al.,55 that the mesostructuring occurs during the
zirconia ZrO2 and the metal-doped zirconia M_ZrO2 are typical of formation of surfactant-templated inorganic materials by eva-
type IV hysteresis loops as shown by the BET isotherms (Fig. 2a). poration. The chemical composition of the film governs the
This indicates the successful design of a mesopore structure meso-organization. Also, it depends on relative vapor pressure
of the materials via a sol–gel approach. This study took advan- in the environment, the evaporation conditions, and the
tage of the structure-directing ability of P-123 in the inverse chemical conditions in the initial solution. The changes in
micelles system, which serves as the nanoreactors. Also, a the structure and crystallography of ZrO2 resulting from doping
Table 4 MPV reduction of a,b-unsaturated aldehydes over mesoporous Cr-ZrO2 acid catalyst
Entry
Substrate
kobs (hÀ1
)
Activitynormalized* (mmol mÀ2
g
À1) Â 10À5
Conversion (%)
56.31
Selectivity (%)
100
1
0.05 Æ 0.05
0.28 Æ 0.06
741
2
3
790
60.09
76.40
100
100
0.40 Æ 0.04
1006
4
5
0.24 Æ 0.07
0.49 Æ 0.02
581
44.14
98.17
100
100
1292
Reaction condition: 2.5 mmol substrate, 1.0 mmol decane, 130 mmol 2-propanol, 0.4 g catalyst, stirring rate 450 rpm, temperature 80 1C, and
reaction time 10 h. *Activitynormalized obtained by normalizing the amount converted at 10 h to the SBET
.
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species modified the concentration of active phases involved with a higher concentration of Lewis acid sites having weak,
in the catalyzed reaction on the surface of the ZrO2 based medium, and strong strength. The Lewis acid sites are possibly
catalysts.
generated by the concerted metal ions (Cr3+ and Zr4+) acting as
The NH3-TPD and CO2-TPD experiments showed that the the electron-acceptor. Also, the metal–metal synergy influenced
synthesized catalysts possess acid–base properties that could the hydrogen consumption, with the Cr–Zr catalyst showing
be tuned. The acid/base strength and density are related to the superior reducibility (H-abstraction capacity).
nature of the metal dopant. The acid density decreases in this
According to Stavale et al., the electronic structure and
order Cr_ZrO2 4 Ni_ZrO2 4 Mn_ZrO2 4 Fe_ZrO2 4 ZrO2. On chemical properties of oxide materials, and their catalytic
the other hand, the basicity of the M_ZrO2 gave this trend activities, could be tailored by doping with metal.56 The
Mn_ZrO2 4 Ni_ZrO2 4 Fe_ZrO2 4 ZrO2 4 Cr_ZrO2, which approach takes advantage of the metal dopants’ tendency to
could be related to neither the electron density nor ionic exchange electrons with the host oxide and surface-bound
charge. These results show that the presence of metal dopant adsorbates. It has also been reported that dopant-modified
in the ZrO2 framework possibly tunes the active acid–base sites, metal oxides exhibit improved catalytic performance than their
resulting mainly from the metal–metal synergy between the pure oxides.57 In our case, the MPV process is catalytic driven;
metal dopant and the host ZrO2. It was revealed that the no activity was observed in the blank reaction. The catalytic
acid:base in ratio 3 : 1 is required for the chemoselective activity of the materials in the MPV reduction of a,b-
transfer hydrogenation of citral via the MPV system. To further unsaturated aldehydes is dependent on the concentration of
understand the kinds of acid sites on the solid catalysts, the the surface acidic sites. Among the transition metal dopants
pyridine-adsorbed experiments indicated that the metal–metal incorporated, the catalytic activity of pure ZrO2 (62.6%) in
synergy generated both surface Lewis and Brønsted acid sites terms of citral conversion in the model reaction was only
Scheme 2 Proposed mechanism for the chemoselective MPV reduction of a,b-unsaturated aldehydes to their corresponding unsaturated allylic alcohol
over the Lewis acid sites on mesoporous M_ZrO2 catalyst.
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Table 5 The SBET, the activity of the mesoporous Cr_ZrO2, and the MPV process conditions compared with literature
Entry Catalysts
SBET (m2 gÀ1
)
Substrate
Time (h) Conversion (%) Selectivity UAA % Reaction conditions
Ref.
1
2
3
4
5
6
7
8
Pt/ZrO2
ZrO2/Gaim 300
Zr-BEA
Zr-SBA-15
P-Zr 200
ME-Zr-200UW
ZrO2
42
Crotonaldehyde
Crotonaldehyde
Crotonaldehyde
Furfural
Furfural
Furfural
Benzaldehyde
Benzaldehyde
Citral
Citral
Cinnamaldehyde 24
Cinnamaldehyde 0.5
Benzaldehyde
Cinnamaldehyde
Citral
Crotonaldehyde
Furfural
8
8
—
6
24
24
8
8
5
3.3
37
26
15
54
55.3
67.6
33.7
88.5
91
90
24
59
24
42
63
30
85
4
86
72
41
79.9
83.6
99.2
99.1
74
30
98
30 1C, 0.414 MPa
130 1C, atm
200 1C, atm
90 1C, atm
100 1C, atm, 1000 rpm 18
100 1C, atm 1000 rpm 18
82 1C, atm
82 1C, atm
82 1C, atm
50 1C, 5 MPa, 500 rpm 29
82 1C, atm 1000 rpm
60 1C, 1.0 MPa
80 1C, atm 450 rpm
80 1C, atm 450 rpm
80 1C, atm 450 rpm
80 1C, atm 450 rpm
80 1C, atm 450 rpm
33
16
26
21
175
470
553
230
210
154
63
63
64
5%ZrO2/Si-MCM-41 776
9
Zr-beta zeolite
Ru/ZrO2
ZrSr-PN
490
102
191
274
189
189
189
189
189
10
11
12
13
14
15
16
17
7
4
Pt/ZrO2
99
Cr–ZrO2
Cr–ZrO2
Cr–ZrO2
Cr–ZrO2
Cr–ZrO2
4
4
4
4
4
100
100
100
100
100
This work
improved by Cr_ZrO2 (76.4%). The observed linear relationship conversion in 4 h and 98.2% in 10 h at 80 1C than P-Zr 200
between the surface acidity and activity of the synthesized (55.3%), ME-Zr-200UW (67.6%), and Zr-SBA-15 (54%) after 24, 24
catalysts suggests that the MPV reduction reaction is perhaps and 6 h, respectively. However, Pt/ZrO2 synthesized by Wei et al.4
governed by the extent of acidity induced by the electronic gave better activity than our catalyst in cinnamaldehyde
interaction between Cr and Zr. The experiment performed with reduction and Ru/ZrO2 in citral reduction but, this is due to
pure chromium oxide showed no activity after 24 h, this the H2 gas pressure used in their catalytic system.
suggests that the synergistic interaction might be responsible
for the enhanced activity in Cr_ZrO2. Also, the acid character of
5. Conclusion
the Cr_ZrO2 catalyst with the polarity of the citral molecule made it
possible for citral to preferably adsorb through the carbonyl group.
Herein, we have demonstrated that incorporating metal dopants
Hence, the transfer hydrogenation of the carbonyl to produce UAA
into zirconia’s crystal framework alters its physico-chemical proper-
is favored. A similar scenario in which the adsorption of citral on
ties such as surface area, mesopore structure, crystallinity, basicity,
the Lewis acid site is via the carbonyl was reported.58 The local
acidity, reducibility, and thermal stability. The reducibility and the
structure of the Lewis acid sites on zirconia catalyst was also
strength of the Lewis acid sites govern the activity of ZrO2 based
reported.59,60 In view of these and the findings of this study, a
catalysts in the MPV reduction of citral. Specifically, the Cr dopant
possible mechanism for MPV reduction of citral on the Lewis acid
weakens the crystallinity of ZrO2. However, it improves the redu-
sites of M_ZrO2 is proposed (Scheme 2).
cibility, acidity, and catalytic reactivity for MPV reduction of
Distinctively, despite the high acidic properties of the cata-
aldehydes. All the prepared zirconia-based catalysts in this work
lytic system, no secondary products were formed. Acidic cata-
showed a remarkable selectivity of 100% to UAA. The surface-
lysts frequently favor secondary reactions as either dehydration
induced performance of the Cr_ZrO2 is due to the enhanced active
of alcohol or aldehyde condensation.61,62 All the prepared
centers generated from the synergistic electronic interaction
catalysts in this work exhibited excellent selectivity to unsatu-
between CrOx and ZrO2. Hence, this work unveils that the reactivity
rated allylic alcohol as evidenced in the GC spectra (Fig. S8 and
of ZrO2 depends solely on the intrinsic properties of its’ surface
S9, ESI†) compared to their previously reported counterparts in
structure rather than the specific surface expanse. Also, the
Table 4; this is paramount to a sustainable catalytic process.
Cr_ZrO2 exhibited good stability and recyclability for at least five
Moreover, the MPV process in this work was carried out under
reaction cycles. We proposed a plausible mechanism of the MPV
milder reaction conditions in the absence of additives and
transformation over the Lewis acidic sites. This work presents the
gaseous hydrogen. The reactivity retained after five consecutive
first application of a well-designed mesoporous Cr_ZrO2 via an
runs evidence the sustainability of the Cr_ZrO2 catalyst.
inverse micelle approach in the MPV reduction of aldehydes with
Furthermore, the synthesized Cr_ZrO2 in this work showed
exceptional reactivity and efficient reusability. The green produc-
considerable reactivity compared to its counterparts in litera-
tion of UAA was successfully achieved under mild reaction condi-
ture Table 5. Our catalyst gave a maximum selectivity of 100%
tions without pressurized hydrogen gas. The Cr_ZrO2 is proposed
to the UAA under milder reaction conditions in the absence of
to be a potential sustainable catalyst for industrial applications.
H2 gas pressure. For instance, it is more reactive than the ZrSr-
PN catalyst in the MPV reduction of cinnamaldehyde; the ZrSr-
PN gave 24.0% conversion of cinnamaldehyde in 24 h while our
Cr_ZrO2 gave 60% conversion in 10 h. Also, in the MPV reduction
Conflicts of interest
of furfural, our Cr_ZrO2 showed higher activity of 85%
The authors declare no competing interest.
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20 Y. Kuwahara, W. Kaburagi, Y. Osada, T. Fujitani and
H. Yamashita, Catal. Today, 2017, 281, 418–428.
21 J. Iglesias, J. Melero, G. Morales, J. Moreno, Y. Segura,
Acknowledgements
We profoundly acknowledge the financial support by the
National Research Foundation/the World Academy of Science
NRF/TWAS UID: 105463, NRF (UID: 121039), and the University
of Johannesburg. Also, Shimadzu company for their instru-
ments and Spectrum UJ for the use of SEM and TEM
instruments.
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