CuZrO3 nanoparticles catalyst in aerobic oxidation of vanillyl alcohol

Article abstract of DOI:10.1039/c6ra26370d

A highly crystalline, mesoporous and perovskite type CuZrO₃ nanoparticles catalyst was prepared via a simple and facile one pot solvent evaporation method. The crystal planes (112) and (211) of CuZrO₃ were evidenced in HRTEM and SAED imagery by their lattice parameters, 2.83 and 3.15 ?. Also, the crystallite size of synthesized catalyst was measured in a range of 7-10 nm. The catalyst was superior as it possessed many physiochemical properties to perform a liquid phase aerobic oxidation of vanillyl alcohol to selectively produce vanillin under base free conditions. Catalytic activity of the synthesized perovskite type CuZrO₃ catalyst was substantially improved by the presence of surface oxygen vacancies speculated in O 1s spectra. Furthermore, a O₂-TPD technique was applied to understand the role of surface oxygen vacancy in enhancing catalytic activity. Moreover, numerous partial and full lattice dislocated grain boundaries were observed as a form of structural defects between different planes in HRTEM imagery. The redox capability of this superior catalyst was significantly enriched by the high content of Zr loading and confirmed by low temperature reduction in H₂-TPR analysis. Excellent catalytic behavior of CuZrO₃ (Cu:2Zr) catalyst (91% conversion and 76% selectivity to vanillin) in liquid phase aerobic oxidation was well correlated with the structural and chemical properties of the catalyst. Moreover, the catalyst was stable for four consecutive oxidation reactions without appreciable loss of catalytic activity.

Full text of DOI:10.1039/c6ra26370d

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CuZrO3 nanoparticles catalyst in aerobic oxidation
of vanillyl alcohol
Cite this: RSC Adv., 2017, 7, 9914
*
Subrata Saha and Sharifah Bee Abd Hamid
A highly crystalline, mesoporous and perovskite type CuZrO3 nanoparticles catalyst was prepared via a simple
and facile one pot solvent evaporation method. The crystal planes (112) and (211) of CuZrO3 were evidenced in
HRTEM and SAED imagery by their lattice parameters, 2.83 and 3.15 A˚. Also, the crystallite size of synthesized
catalyst was measured in a range of 7–10 nm. The catalyst was superior as it possessed many physiochemical
properties to perform a liquid phase aerobic oxidation of vanillyl alcohol to selectively produce vanillin under
base free conditions. Catalytic activity of the synthesized perovskite type CuZrO3 catalyst was substantially
improved by the presence of surface oxygen vacancies speculated in O 1s spectra. Furthermore, a O2-TPD
technique was applied to understand the role of surface oxygen vacancy in enhancing catalytic activity.
Moreover, numerous partial and full lattice dislocated grain boundaries were observed as a form of
structural defects between different planes in HRTEM imagery. The redox capability of this superior catalyst
was significantly enriched by the high content of Zr loading and confirmed by low temperature reduction
in H2-TPR analysis. Excellent catalytic behavior of CuZrO3 (Cu:2Zr) catalyst (91% conversion and 76%
selectivity to vanillin) in liquid phase aerobic oxidation was well correlated with the structural and chemical
properties of the catalyst. Moreover, the catalyst was stable for four consecutive oxidation reactions
without appreciable loss of catalytic activity.
Received 5th November 2016
Accepted 16th January 2017
DOI: 10.1039/c6ra26370d
www.rsc.org/advances
Ma et al. summarized the homogeneous catalysts investi-
gated for lignin valorization.16 Also, another group of
Introduction
researchers reviewed using unconventional media such as ionic
liquids for lignin reactions.17 However, using ionic liquids or
homogeneous catalysts has disadvantages of product separa-
tion and low selectivity although they offer good yields. Behling
outlined in detail heterogeneous catalysts up to the present for
oxidizing lignin and its model compounds.13 Numerous
heterogeneous catalytic systems were investigated for such
applications. Recently, Ajay jha et al. reported that three cata-
lytic combinations of Co3O4, Mn–Co mixed oxide, and reduced
graphene oxide promoted a Co–Mn catalyst for aerobic oxida-
tion of vanillyl alcohol.18–20 This Co3O4 catalyst was found to be
inactive without presence of the base NaOH. Using NaOH for
oxidizing the alcohol caused formation of a hydroxyl bridged
catalyst leading towards catalyst deactivation and production of
large scale inorganic waste. Two other catalysts showed
moderate catalytic activity with 62% conversion. Mariom et al.
reported CoTiO3 catalyst using H2O2 resulted in catalytic activity
of 66% with a 6 h reaction time.9 Recently, Balachandra et al.
reported preparation of nano Fe3O4@APTES@Ni (OH)2 catalyst
with 73% conversion in a 8 h reaction time.21 Also, another
group of researchers reported a carbon supported iron oxide
catalyst with 86% selectivity for vanillyl alcohol oxidation.22
Though a few catalysts showed promising catalytic activity,
longer reaction times using such oxidation reactions is an
unfriendly economic process. Therefore, a genuine need exists
Catalytic upgradation of lignocellulosic biomass is a recent
great challenge due to its complexity associated with structure
and recalcitrance to transformation.1 Extensive research has
been explored in past decades for valorization of carbon-based
biomass constituents.2–7 Lignin is one of the less used biomass
constituents due to its polymeric complex structure which
limits potential applications of it in a wide range of applica-
tions. In order to create sustainable bio reneries, it is very
crucial to develop a suitable catalyst system to transform the
lignin fraction of biomass into selective value added products
because 30% carbon and 40% of the energy content of ligno-
cellulosic biomass constituents are lignin.8
Since lignin contains a large concentration of hydroxyl
groups, it is viable to oxidize or cause oxidative polymerization
of targeted products with environmental friendly oxidants such
as H2O2 or O2. Vanillyl alcohol is oen considered to be one of
the model compounds representing lignin since its structure is
similar to one of the building block units of lignin.9–13 Selective
oxidative products of vanillyl alcohol are important platform
chemicals in pharmaceutical and perfume industries and it is
also used as a food grade avor.14,15
Nanotechnology and Catalysis Research Center (NANOCAT), University of Malaya,
Kuala Lumpur 50603, Malaysia. E-mail: sharifahbee@um.edu.my; Fax: +60
379676956; Tel: +60 379676959
9914 | RSC Adv., 2017, 7, 9914–9925
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for a catalyst with high yields under mild reaction conditions dissolved in 30 mL tetrahydrofuran in a 100 mL Schenk ask.
with base free reactions and a simplied synthesis protocol. Aer cupper(II) acetate was completely dissolved, 1.15 g (4 mmol)
Mixed metal oxide catalysts are used with a wide range of of zirconium butoxide {Zr(dmae)4} was added dropwise followed
potential applications owing to their electronic properties and by addition of a few drops of triuoroacetic acid to lower the pH
unique redox properties.23 Cooperative oxide-based catalysts (<3.5). Then, the reaction mixture was kept for 4 h to complete the
showed superior catalytic activity to their counterparts with single reaction and the color of the solution turned from blue to light
phase. Cu-based mixed oxide catalysts showed promising catalytic blue. The solvent was removed from the reaction mixture under
activity in various oxidation reactions such as oxidation of the vacuum. The material was ground to a powder and followed by
volatile organic compounds ethanol and toluene, plus CO oxida- calcination at 500 C in air at a heating rate of 2 C minꢁ1. The
tion24–27 and also oxidation of lignin.13 Perovskite structured prepared catalyst was labeled as catalyst A (Cu : Zr). Two other
oxides especially have been widely and effectively used in hydro- catalysts labeled as Catalyst B (Cu:2Zr) and C (2Cu:Zr) were also
genations of hydrocarbons, and also for oxidation purposes such synthesized following a similar synthesis technique by changing
as CO oxidation, ammonia oxidations, and catalytic combus- the concentration of zirconium butoxide (8 mmol) and copper
tion.28–36 A large number of literature references previously re- acetate (8 mmol) respectively, in each catalytic system. Further-
ported the intimate relation of defect chemistry introduced by more, a catalyst with composition of (Cu:2Zr) was also prepared
perovskite type oxide catalysts to their catalytic activity.37–42 Thus, without addition of triuoroacetic acid.
ꢀ
ꢀ
catalytic activity of a perovskite structured mixed oxide catalyst
was notably improved by creating a large concentration of surface
defect oxygens in its structure, which beneted the oxidation
process. Therefore, in this study we designed a perovskite type
CuZrO3 catalyst prepared via a one pot simple and facile solvent
evaporation method. Also, to enrich the concentration of surface
oxygen vacant sites in the catalyst, we introduced triuoroacetic
acid (TFA) to the synthesis protocol. Emy et al. reported addition
of TFA in a TiO2 lattice during calcination notably improved the
concentration of surface oxygen vacancies by charge neutrality.43
Ali Ehsan et al. previously reported a CuZrO3–CuO composite
phase on thin lms.44 Moreover, catalytic activity of the CuZrO3
catalysts was tested in liquid phase oxidation of vanillyl alcohol
using an environmentally benign oxygen source. To the best of
our knowledge, that is the pioneer report for using a perovskite
type CuZrO3 catalyst to oxidize vanillyl alcohol using green
oxidant air. Various characterization techniques such as TG, XRD,
RAMAN, XPS, FESEM, HRTEM, SAED, H2-TPR, and O2-TPD were
employed to examine formation of the CuZrO3 phase, physi-
ochemical and textural properties such as the phase, crystallinity,
electronic state, morphology, and redox properties and their
inuence in the catalytic action in the vanillyl alcohol oxidation.
In addition, reusability of the CuZrO3 catalyst was also studied in
a vanillyl alcohol aerobic oxidation.
Characterization of CuZrO3 catalyst
Thermal analysis was carried out in a ceramic crucible on
a PerkinElmer TGA 4000 Thermogravimetric Analyzer con-
nected with a computer interface. Pure air with a ow rate of ꢀ30
cm3 minꢁ1 was passed continuously at a heating rate of 10 C
minꢁ1 under atmospheric pressure. The X-ray diffractograms of
prepared catalysts were recorded on a D8 Advance X-ray
ꢀ
diffraction-Bruker AXS using Cu Ka radiation (d ¼ 1.54 A) in
range of Bragg angle 5ꢀ to 90ꢀ with a step size of 0.026/min.
During the analysis, the voltage and the current density were
maintained at 40 kV and 40 mA respectively, at ambient
temperature. Surface morphology analysis was performed by
eld emission scanning microscopy (FESEM Quanta FEI 200F)
and elemental composition analysis was measured by energy
dispersive X-ray spectroscopy (INCA Soware) attached to the
FESEM. Nitrogen adsorption desorption measurements were
carried out on a Micrometric ASAP 2010 instrument at ꢁ196 ꢀC.
ꢀ
The catalysts were rst degassed at 150 C for 5 h before anal-
ysis. Lattice spacing and selected area electron diffraction
(SAED) patterns were collected using high-resolution trans-
mission electron microscopy (HR-TEM, JEM 2100-F) at
a constant voltage of 200 keV.
Temperature programmed reductions and oxidations were
performed on a Micrometrics Chemisorb 1100 series instru-
ment. Before analysis, surface moisture from 0.2 g catalyst was
Experimental
ꢀ
removed via pretreatment at 180 C for 1 h by N2 gas (20 mL
minꢁ1). Then, the analysis was carried out under process
All chemicals were purchased from various commercial sources
and used without further purication. These include copper(II)
acetate monohydrate (Sigma-Aldrich, 98%) zirconium(IV) butoxide
(80 wt% in 1-butanol), vanillyl alcohol (Sigma-Aldrich, 98%),
vanillin (Sigma-Aldrich, 97%), vanillic acid (Sigma-Aldrich, 97%),
guaiacol (Sigma-Aldrich, 98%) hydrogen peroxide (30% in H2O)
(Sigma-Aldrich solution 30% in water), and acetic acid (Merck,
96%), tetrahydrofuran (Sigma-Aldrich, 99.8%), and NaOH (Merck).
ꢀ
conditions until 900 C under gas ow of 5% H2 with 95% N2
mixtures at a heating rate of 10 ꢀC minꢁ1 and was held for
10 min. The reduced sample also was further re-oxidized to
investigate the oxidation steps of the catalyst at the same
temperature programme. ICP-MS analysis of the reaction
mixture was performed before and aer a catalytic reaction
following the method ASTM-D5185 using a PerkinElmer Nex-
ION 350S series.
Catalyst preparation
Catalytic activity measurement
Three different catalysts with three different ratios of Cu and Zr
precursors were synthesized by following a simple one pot solu- Liquid phase aerobic oxidation was performed in a high pressure
tion method. First, copper(II) acetate (0.79 g, 4 mmol) was 200 mL autoclave reactor supplied by Parr Co. In a typical aerobic
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oxidation reaction, 3 mmol vanillyl alcohol was dissolved in 60 mL
acetonitrile with a catalyst loading of 0.0025 g cmꢁ3 while the
reactor temperature and pressure were kept at 120 ꢀC and 21 bar
air pressure, respectively. The oxidative products were collected by
a syringe lter aer every half an hour interval and analyzed by an
Agilent Technology HPLC Chromatography 1100 series instru-
ment connected to a UV detector and computer interface. A
mixture of solvent (85% water + 15% acetonitrile) with 1% acetic
acid was used as the mobile phase with a Zobrax RP-C18 column.
The products and substrate were successfully detected by a UV
detector at ƛmax ¼ 270 nm. The column temperature was stable at
28 ꢀC with a ow rate of 1 mL minꢁ1
.
Results and discussions
Catalyst characterizations
Fig. 2 The XRD pattern of catalyst A (Cu:Zr), catalyst B (Cu:2Zr),
catalyst C (2Cu:Zr) and spent catalyst B (Cu:2Zr).
Thermal stability. There were two distinctive weight losses
detected with TG/DTG analysis of the synthesized catalyst B
(Cu:2Zr) before calcination (Fig. 1). The continuous weight loss of
13.43% with a peak maximum at 280 ꢀC can be ascribed to loss of
surface moisture and crystallite water from bulk CuZrO3 catalyst.
The maximum weight loss of 46.30% with peak maximum at
330 ꢀC possibly corresponds to removal of an acetate organic
group present in the metal precursors as CO2 and water. Two
minor weight losses of 1.13% and 0.56% at 440 C and 550 C
might be due to elimination of further impurities from the cata-
lyst. The amount of residue of 38.52% was constant until 700 ꢀC
indicating thermal stability of the catalyst at higher temperatures.
Crystal phase and crystallinity. The chemical formula and
the phase were detected by XRD diffractogram analysis based
on Xpert High score plus an analysis database (Fig. 2). Careful
observation of the recorded XRD pattern suggests that the
synthesized crystal was identied as CuZrO3 phase in accor-
dance with the JCPDS number (ICSD 00-043-0953). However,
presence of a minor phase CuO was also detected which
matched with the database containing Terorite (ICSD 98-002-
6715). The peak positions at 2 theta ¼ 24.33, 28.29, 31.58, 34.34,
41.01, 49.47, 50.41, 54.41, 55.56, 58.23, 60.01, and 61.72
˚
respectively, with d spacing (A) 3.67, 3.15, 2.83, 2.61, 2.20, 1.84,
1.81, 1.69, 1.65, 1.58, 1.54, and 1.50 respectively, exactly corre-
spond to miller indices (012), (112), (211), (013), (023), (040),
(321), (322), (313), (401), (134), and (402) planes of CuZrO3,
respectively. Also, the peak positions at 2 theta ¼ 35.61 and
ꢀ
ꢀ
ꢁ
38.81, respectively can be ascribed to the planes of (111) and
(111) of CuO. Hence, the catalyst was synthesized as a highly
crystalline perovskite shaped CuZrO3 catalyst with a minor
phase of CuO and with no other impurities. In addition, the
˚
orthorhombic shape having the cell parameters (a ¼ 6.4465 A,
˚
˚
b ¼ 7.4008 A, and c ¼ 8.3128 A) varied from the typical cubic
˚
shape of a perovskite structure (a ¼ 3.905 A). This fact indicates
displacement of the B cation (Zr) introducing distortions in the
domain of the perovskite crystal.45 This phenomenon would
imply appreciable surface defects in the material which is one of
Fig. 3 Raman analysis of synthesized catalyst A (Cu:Zr), catalyst B
Fig. 1 Thermogravimetric analysis of synthesized catalyst B (Cu:2Zr).
9916 | RSC Adv., 2017, 7, 9914–9925
(Cu:2Zr) and catalyst C (2Cu:Zr).
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Fig. 4 N2 adsorption–desorption curve for (a) catalyst A (Cu:Zr); (b) catalyst B (Cu:2Zr); (c) catalyst C (2Cu:Zr); (d) spent catalyst B (Cu:2Zr).
the clues to the signicant catalytic activity of the perovskite structural distortion associated with the formation of the
type mixed CuZrO3 catalyst.
perovskite structure CuZrO3.48
The reused catalyst B (Cu:2Zr) was also characterized by XRD
Textural property. In order to attribute textural properties of
to detect any change in the phase, chemical composition, and the catalysts to catalytic activity, a BET N2 adsorption–desorp-
crystallinity. There was no change observed in the phase and tion isotherm was employed (Fig. 4); it demonstrated a type (III)
chemical composition as the recorded XRD ngerprints for type isotherm and slit shaped pore distribution. One of the key
spent catalyst showed the same pattern as the fresh catalyst barriers to obtaining high catalytic activity of a perovskite type
based on peak location at Bragg angle. However, the crystallinity oxide is low surface area.49 Textural properties such as pore size,
was slightly reduced relative to the fresh catalyst aer per- pore diameter, and BET surface area (Table 1) of a synthesized
forming the catalytic reaction.
CuZrO3 catalyst clearly suggest that the catalyst via this facile
Raman analysis. The Raman spectrum observed in Fig. 3 solution method has moderate surface area with mesoporosity
shows two distinctive patterns of Zr–O bonds and Cu–O bonds. in the structure. Also, it is worthwhile to note that higher
Raman shi of Zr–O bonds was present at 146 cmꢁ1, 181 cmꢁ1
,
loading of ZrO2 in catalyst B (Cu:2Zr) contributed to enrichment
and 472 cmꢁ1, respectively.46 The presence of Raman active of the surface area (40.83 m2 gꢁ1) whereas it resulted in
modes of Cu–O bond was observed at 297 cmꢁ1 (Ag), 344 cmꢁ1 a decrement (20%) in the case of low Zr loaded catalysts A
(Bg), and 629 cmꢁ1 (Bg), respectively.47 In the Raman spectrum (Cu:Zr) and C (2Cu:Zr) in the bulk phase of the CuZrO3 catalyst.
at 616 to 635 cmꢁ1, peak shiing was observed. In bulk (Cu:2Zr) The enrichment by 2 fold magnitude in the BET surface area of
catalyst, the Raman peak was noticeably shied to the le catalyst B (Cu:2Zr) was reasonably the key parameter to be
relative to bulk (Cu:Zr) catalyst and was attributed to the larger
concentration of Zr–O bonds in the CuZrO3 catalyst structure.
To the contrary, the Raman peak of bulk (2Cu:Zr) catalyst with
higher loading of Cu slightly shied to the right and was
attributed to the larger concentration of Cu–O bonds. It is
worthwhile to mention that shorter bond lengths of Cu–O
facilitates a blue Raman shi as a result of a stress and strain
phenomenon.43 Furthermore, it was noticed that a larger stoi-
chiometric ratio of Cu in the CuZrO3 catalyst (Cu:2Zr) induces
structure defects and thus results in blue Raman shiing as
well. In addition, the broadening of the peak illustrates
Table 1 Textural properties derived from N2 adsorption–desorption
isotherms
c
Catalyst
SBETa (m2 gꢁ1
)
Vtb (cm3 gꢁ1
)
DBJH (nm)
A (Cu:Zr)
B (Cu:2Zr)
C (2Cu:Zr)
31.52
40.83
20.00
0.1578
0.1387
0.1325
20.02
13.59
26.49
a
b
c
BET surface area. The total pore volume. BJH average pore
diameter.
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Fig. 5 Topography of CuZrO3 catalysts. (a) Catalyst A (Cu:Zr); (b) catalyst B (Cu:2Zr); (c) catalyst C (2Cu:Zr); (d) spect catalyst B.
observed (Fig. 15). Moreover, it was found that textural prop- measured from the observed images and determined to be in
erties of the spent catalyst remained almost unchanged aer the range of 7 to 10 nm (Fig. 7c). The selective area electron
the oxidation reaction.
diffraction pattern (SAED) was applied to further support the
Morphology. Morphology of the fresh and spent catalysts lattice parameters (d spacing) of the CuZrO3 catalyst (Cu:2Zr)
was studied by FESEM under various resolutions as shown in (Fig. 6d). Diffused rings with bright spots in the SAED pattern
Fig. 5. Images revealed that the catalyst is an agglomerated revealed that the catalyst was polycrystalline. In addition, the
akes type with a rough surface. The uneven surface is believed bright spots observed in the SAED pattern correspond to the
to perform superior catalytic activity as it facilitates chemi- plane index of (112), (211), and (013) of a CuZrO3 nanocrystal-
sorption of the reactants that take part in the selective oxidation line catalyst.
reaction of vanillyl alcohol to vanillin. However, the FESEM
Additionally, the EDX spectrum of synthesized CuZrO3
micrographs of the used catalyst demonstrated a plane surface catalysts with different metal loadings was acquired to conrm
which possibly indicates lower adsorption of reactants to the the elemental composition (Fig. 7). The observed percentage of
surface (Fig. 5d). atomicity tentatively conrms the elemental ratio of Cu and Zr
Furthermore, high resolution TEM equipped with SAED was loaded in the catalyst synthesis protocol.
used to observe presence of the CuZrO3 crystal phase in terms of
Oxidation state and surface analysis. Surface elemental
˚
d spacing (A) for catalyst B (Cu:2Zr) (Fig. 6). Image (Fig. 6b) of composition and oxidation state of the synthesized CuZrO3
the catalyst B (Cu:2Zr) conrms the miller planes of (112) and catalyst (Cu:2Zr) were investigated using X-ray Photoelectron
(211) of the CuZrO3 nanocrystalline domain with d spacing of Spectrum (XPS) as an analytical tool (Fig. 8). The XPS wide
˚
˚
2.83 A and 3.15 A, respectively. In addition, the image shows spectrum of the catalyst further conrmed presence of Cu, Zr,
presence of numerous grain boundaries between the lattice and O with no other impurities of the peaks located at binding
planes accommodated by the lattice dislocations (Fig. 6b). energy 933.2 eV, 181.9 eV, and 529.2 eV for Cu 3d, Zr 3d, and O
Grain boundaries dislocations observed in CuZrO3 catalyst 1s spin orbitals, respectively (Fig. 8a). Also, the surface
consist of two types: full dislocation (labelled as F) and partial elemental composition of CuZrO3 (Cu:2Zr) catalyst is listed in
dislocation (labelled as T) which lead to stalking faults Table 2. To obtain further information on the chemical state of
(Fig. 6e).50 It was postulated that grain boundaries in the form of the components of the catalyst, high resolution spectra of
surface defects can increase the catalytic activity substantially particular interesting areas were examined and shown in Fig. 8.
because it acts as a trapping site in the catalyst.51–54 Moreover, There were two distinguishable peaks positioned at 181.81 eV
the average crystallite sizes of the CuZrO3 catalyst (Cu:2Zr) were and 184.18 eV attributed by doublet terms of Zr 3d3/2 and Zr 3d5/2
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Fig. 6 HRTEM images of mixed oxide CuZrO3 (Cu:2Zr) catalyst (a) overview of CuZrO3 (b) measured lattice spacing. (c) Measured particle sizes.
(d) SAED pattern (e) observed grain boundaries. (Full dislocation labeled as F and partial dislocation as T).
Fig. 7 EDX spectra of [a] catalyst A (Cu:Zr); [b] catalyst B (Cu:2Zr); [c] catalyst C (2Cu:Zr).
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Fig. 8 XPS spectra of mixed oxide CuZrO3 catalyst (Cu:2Zr). (a) Survey spectrum of CuZrO3 (b) Zr 2p (c) O 1s (d) Cu 2p.
Table 2 Surface elemental atomicity in XPS analysis for catalyst B
(Cu:2Zr)
in Zr 3d spectra due to spin orbital coupling (Fig. 8b). Moreover,
the energy difference between the two spin orbitals by 2.4 eV
further conrms presence of the Zr4+ oxidation state which is in
good agreement with previously reported values of ZrO2.55 The
photoelectrons split from the spin orbital of Cu 3d3/2 and Cu
3d5/2 at 933.8 eV and 953.88 eV, respectively evidently corre-
spond to the Cu2+ oxidation state (Fig. 8d).56 Furthermore, the
presence of a satellite peak in between 940–950 eV in Cu 2p spin
spectra ascertained the Cu2+ chemical state.57 Two peaks were
observed at binding energies 529.67 eV and 531 eV in a O 1s spin
orbital using nonlinear de Gaussian measurements (Fig. 8c).
The peak at low binding energy 529.67 eV corresponds to the
O2ꢁ chemical state which represents Cu–O and Zr–O linkages
present in the catalyst framework. The higher binding energy
peaks at 531 eV can be attributed to weakly held oxygen species
(Oꢁ, O2ꢁ) at the surface compared to lattice oxygen species
(O2ꢁ) which is in good agreement with previously reported
results by Roberts et al.58 Deng et al. also reported presence of
oxygen vacancy on the surface of a perovskite type Cu doped
Co based oxide catalyst based on the higher binding energy
peak of O 1s spectra in between 531 to 532 eV.59 A similar
observation was also previously reported for a La-based tran-
sitional perovskite catalyst.41 Thus, based on similar observa-
tions in this work, the presence of oxygen vacancy in the
synthesized perovskite type CuZrO3 catalyst was supported.
Furthermore, formation of oxygen vacancy on the catalyst
surface was further triggered via a charge compensation
Cu (%)
8.06
Zr (%)
25.41
O (%)
66.39
Table 3 Area percentage of oxygen species based on O 1s spectra in
catalyst B (Cu:2Zr)
Lattice oxygen (%)
60.30
Surface oxygen (%)
39.70
phenomenon, and also during the calcination process, as
a result of triuoroacetic acid addition during the synthesis
step for TiO2.43 A similar demonstration was observed in this
CuZrO3 catalyst. Moreover, the percentage of surface oxygen
vacancy and lattice oxygen was calculated based on the peak
area obtained in O 1s spectra (Table 3).
Redox analysis. The H2-TPR spectrum clearly revealed the
reduction potential of the synthesized catalyst (Fig. 9). A careful
observation of spectra in the range of 180 ꢀC to 400 ꢀC indicated
a broad reduction peak with a shoulder peak towards a lower
reduction temperature. The shoulder peak with a maximum ꢁat
200 ꢀC could be attributed to desorption of oxygen (Oꢁ, O2
)
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Fig. 9 H2-TPR profile of catalyst A (Cu:Zr), catalyst B (Cu:2Zr), catalyst
C (2Cu:Zr).
Fig. 10 O2-TPD profile of catalyst A (Cu:Zr), catalyst B (Cu:2Zr),
catalyst C (2Cu:Zr).
from oxygen vacancy sites on the catalyst's surface.60 This
oxidation step required low temperature to be reduced as they
are weakly held at the sites. Moreover, the most intense
shoulder peak of catalyst B (Cu:2Zr) implies a large concentra-
tion of desorbed surface oxygen species occurring at a low
temperature and which thus enhanced the catalytic activity
(Fig. 15). Moreover, the broad peak at higher temperature
corresponds to reduction of lattice oxygen (O2ꢁ). Moreover, the
catalyst with a higher loading of Cu in the preparation protocol
required large consumption of H2 as reected by peak broad-
ening. In addition, the consistent shiing of the broad peaks
towards high reduction temperatures in the order of catalyst B
(Cu:2Zr) < A (Cu:Zr) < C (2Cu:Zr) indicated the magnitude of the
interaction between Cu and ZrO2 in the catalyst framework.
Hence, it worthwhile to mention from the apparent shiing that
the ratio of Cu to 2Zr in the bulk phase of the catalyst favored
formation of the highest degree of a Cu–O–Zr phase. Conse-
quently, this electronic interaction in the Cu–O–Zr phase
induced improvement of the reducing capabilities of the cata-
lyst at a low reduction temperature.
To further evaluate catalyst activation and O2 desorption
behavior, O2-TPD was investigated (Fig. 10). We speculated that
oxidation of the catalyst took place in four distinctive steps
denoted by a, b, g, and d. The oxidation peak at the lower
temperature around 180–210 ꢀC was possibly due to the
desorption of physiosorbed (O2ꢁ) species at the present surface
oxygen vacant sites as suggested by O 1s XPS spectra. Hence, the
a peak in the TPD prole was attributed to O2ꢁ. Oxygen
desorption at (Oꢁ) species occurred at comparatively higher
temperatures (>400 ꢀC) than O2ꢁ species (<300 ꢀC). So, it was
reasonable to assign b and g peaks to Oꢁ species. The d desorp-
tion peak could be attributed to the lattice oxygen species as it
required a high temperature (>800 ꢀC). It is worthwhile
mentioning that the oxygen desorption peak at low temperature
for catalyst B with high content of Zr was greatly shied towards
a lower temperature. That was indicative of enhancement of the
oxygen mobility on the surface of the catalyst which facilitated
the redox ability. This phenomenon could considerably enhance
the catalytic activity observed for catalyst B (Fig. 15). In addition,
the oxygen uptake at lower temperatures for catalyst B was greater
(566 mmol gꢁ1) than catalyst C (522 mmol gꢁ1) and catalyst A (285
mmol gꢁ1). Based on the amount of oxygen desorption, we
conclude that catalyst B contains the highest degree of physi-
osorbed (O2ꢁ) species associated with surface defects at low
temperature which notably improved the catalytic activity.
Therefore, the oxygen adsorption occurs in the following
sequence based on the O2-TPD prole:
O2(ad) / O2ꢁ(ad) / Oꢁ(ad) / O2ꢁ(lattice)
Catalytic activity
The catalyst CuZrO3 (Cu:2Zr) was considered to optimize reaction
parameters such as catalyst loading, time, temperature, and
pressure in aerobic oxidation of vanillyl alcohol in base-free
conditions. Acetonitrile was chosen as the solvent as it showed
promising catalytic activity for other catalysts in an aerobic
oxidation of vanillyl alcohol.20 We prepared two other catalysts
labeled as catalyst A (Cu:Zr) and catalyst C (2Cu:Zr) which were
also experimented with at determined suitable reaction condi-
tions. A blank reaction was also carried out without the presence
of a catalyst and no conversion was observed. This implied that
the oxidation reaction does not proceed in absence of a catalyst.
The progress of the reaction was recorded aer every half an
hour time interval until the reaction time reached 2.5 hours and
the results are displayed in Fig. 11 (reaction conditions: 3 mmol
vanillyl alcohol, 21 bar air pressure, 0.0025 g cmꢁ3 catalyst B
(Cu:2Zr), 120 ꢀC). The catalytic activity was improved as ex-
pected at a precipitous rate with the increase of time until 2
hours of reaction time. The conversion was measured as 91%
conversion with 76% selectivity with a reaction time of 2 h.
Thereaer, the reaction progress was not signicant as only
enrichment of 2% in conversion was produced due to the
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Fig. 11 Effect of time on conversion and selectivity in aerobic oxida-
Fig. 13 Effect of temperature on conversion and selectivity in aerobic
tion. Reaction conditions: 3 mmol vanillyl alcohol, 60 mL acetoꢁn3itrile,
oxidation. Reaction conditions: 3 mmol vanillyl alcohol, 60 mL
21 bar air pressure, 120 ꢀC, catalyst mass (Cu:2Zr) 0.0025 g cm
.
acetonitrile, 21 bar air pressure, catalyst mass (Cu:2Zr) 0.0025 g cmꢁ3
2 h.
,
catalytic conversion to 91% from 63% was observed while the
catalyst loading was changed to 0.0025 g cmꢁ3 from 0.0012 g
cmꢁ3. However, further increments in the catalyst loading up to
0.0050 g cmꢁ3 was observed not to promote noticeable catalytic
conversion. This was due to the earlier mentioned fact of mass
transfer resistance.61 Noteworthy, the selectivity (76%) was not
affected by changes in the catalyst loading.
The apparent response in catalytic activity with respect to
temperature was investigated in range of 85 ꢀC to 140 ꢀC
(reaction condition: 3 mmol vanillyl alcohol, 21 bar air pressure,
0.0025 g cmꢁ3 catalyst B (Cu:2Zr), 2 h) (Fig. 13). The catalytic
conversion was substantially improved to 94% from 68% when
the temperature was elevated from 85 ꢀC to 140 ꢀC. However, the
selectivity slightly decreased by 5% from the initial selectivity
Fig. 12 Effect of catalyst loading (Cu:2Zr) on conversion and selec-
tivity in aerobic oxidation. Reaction conditions: 3 mmol vanillyl alcohol,
60 mL acetonitrile, 21 bar air pressure, 120 ꢀC, 2 h.
formation of 2-methoxy benzoquinone, which is a well-known
radical trapping agent that can decrease the rate of reaction.
Thus, 2 hours was chosen as the optimum reaction time. It is
noteworthy that vanillic acid (17%) and quinone (7%) were
found as the only detected side products by HPLC analysis. The
perovskite type CuZrO3 catalyst was outstanding for improving
catalytic conversion by aerobic oxidation of vanillyl alcohol as
compared with other previously reported literature references
under short reaction times.21,22
To examine the obvious inuence of catalyst loading, catalyst
(Cu:2Zr) mass in a range 0.0012 g cmꢁ3 to 0.0050 g cmꢁ3 was
considered (reaction conditions: 3 mmol vanillyl alcohol, 21 bar
air pressure, 120 ꢀC, 2 h) (Fig. 12). A steep improvement in the
Fig. 14 Effect of air pressure on conversion and selectivity in aerobic
oxidation. Reaction conditions: 3 mmol vanillyl alcohol, 60 mL
acetonitrile, 120 ꢀC, catalyst mass (Cu:2Zr) 0.0025 g cmꢁ3, 2 h.
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(79%) with that increment of temperature. This was possibly Interestingly, the synthesized catalysts showed more improved
due to next step-oxidation of the product vanillin to its corre- catalytic action than their counterpart single oxide catalysts. For
sponding acid at higher temperatures. It was worthwhile catalyst A (Cu:Zr), conversion (67%) with selectivity (75%) was
mentioning that char formation was observed at the high achieved. When catalyst B (Cu:2Zr) was used, the best conversion
temperature 140 ꢀC. Therefore, 120 ꢀC was considered the best (91%) with selectivity (76%) was obtained. The least conversion
temperature for liquid phase aerobic oxidation of vanillyl (43%) with selectivity (76%) was observed in the case of catalyst C
alcohol. The obtained reaction temperature 120 ꢀC with 91% (2Cu:Zr). It is worth noting that the selectivity towards vanillin in
conversion was comparatively lower than in other reports.20 the case of all prepared catalysts was almost the same because
This clearly evidenced that a perovskite type CuZrO3 catalyst the same XRD ngerprint was identied in the catalysts with
prepared via the solvent evaporation technique was superior in different ratios. Comparison of catalytic activity of synthesized
terms of catalytic activity at mild reaction conditions.
catalyst B (Cu:2Zr) with and without TFA indicated that intro-
Air pressure was found to be the vital determinant to obtain duction of TFA in the synthesis step induced formation of more
superior catalytic activity from 7 bar to 28 bar (reaction condi- surface oxygen vacancies as evident in O 1s XPS spectra and O2-
tion: 3 mmol vanillyl alcohol, 0.0025 g cmꢁ3 catalyst B (Cu:2Zr), TPD. Thus, catalytic conversion was substantially enriched when
120 ꢀC, 2 h) (Fig. 14). The pressure was changed from 7 bar to 14 the surface oxygen vacant sites were present in large concentra-
bar, and the conversion was enriched to 79% from 66%, but tions on the catalyst. Surface oxygen vacant sites could hold
a drop in the selectivity to 78% from 81% resulted for vanillin. various weakly bonded oxygen species on the surface which could
When the pressure was increased to 21 bar air pressure, excel- possibly accelerate oxygen mobility from the catalyst surface to
lent catalytic activity was measured at 91% conversion with 76% substrate. It is worth mentioning that the highest catalytic activity
selectivity to vanillin. Further rise in the air pressure was found was obtained by catalyst B (Cu:Zr) and was due to the high
to be detrimental to the selectivity (71%) but to the contrary, content of ZrO2. High loading of ZrO2 could cause a large
conversion was further improved up to 96%. The slight drop in concentration of acidic sites on the surface to strongly interact
the selectivity was associated with the over-oxidation of vanillin with Cu2+ ion on the surface. Consequently, formation of a high
to vanillic acid in presence of excess desorbed oxygen in the degree of Cu–O–Zr active phase occurred in the catalyst and it was
solvent. Based on the experimental data, 21 bar air pressure was presumed that Cu–O–Zr was the active phase for oxidation of
selected as the optimum pressure for liquid phase aerobic vanillyl alcohol. The catalytic activity of synthesized CuZrO3
oxidation of vanillyl alcohol.
catalyst could be due to presence of Cu+/Cu2+ and Zr3+/Zr4+ ion
The prepared catalysts A (Cu:Zr), B (Cu:2Zr), and C (2Cu:Zr) pairs generated by one electron transfer between these ions
were tested for oxidation of vanillyl alcohol at the obtained during O2 activation. These ions could form a metal superoxo
reaction conditions (Fig. 15). To rationalize, commercially avail- intermediate (M–O–O$) reactive species by oxygen reduction on
able pure CuO, ZrO2, and catalyst (Cu:2Zr) prepared without TFA the catalyst for the selective oxidation of alcohol.62 Also, high
were also used as catalysts at the same conditions as catalyst B dispersion of Cu on the ZrO2 in catalyst B (Cu:2Zr) minimizes the
(Cu:2Zr). Isolated CuO catalyst showed 4% conversion with 72% grain size to less than 10 nm and exposed numerous grain
selectivity and 6% conversion with 68% selectivity for ZrO2. And, boundaries which were observed in the HRTEM micrograph
catalysts synthesized without the addition of triuoroacetic acid (Fig. 6e). Furthermore, the order of catalytic performance (Cu:2Zr
showed catalytic conversion of 41% with 71% selectivity. > Cu:Zr > 2Cu:Zr) was in good agreement with that of the above-
Fig. 15 Effect of catalyst composition on conversion and selectivity in Fig. 16 Recycling study on conversion and selectivity in aerobic
aerobic oxidation. Reaction conditions: 3 mmol vanillyl alcohol, 21 bar oxidation. Reaction conditions: 3 mmol vanillyl alcohol, 21 bar air
air pressure, 60 mL acetonitrile, 120 ꢀC, catalyst mass 0.0025 g cmꢁ3
2 h.
,
pressure, 60 mL acetonitrile, 120 ꢀC, catalyst mass (Cu:2Zr) 0.0025 g
cmꢁ3, 2 h.
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Table 4 Comparison of current preparation methods with available methods in literatures for Cu–Zr mixed oxide
Methods
Applications
Problems of available methods
Advantages of current method
Sol–gel nanocasting
Co-precipitation
Steam reforming of methanol
Partial oxidation of methane
Methanol oxidation
Complicated
One pot
Facile
Superior crystallinity
Small crystallite size
Homogeneity of metals
Multiple steps
Separate phase
Low reducing ability
Thin lms
Chemical vapor deposition
Current method (solvent evaporation)
Low catalytic activity
Vanillyl alcohol oxidation
High catalytic activity
Stable phase
mentioned BET surface area as a large surface area could offer O–Zr linkage as an active phase by the highest interaction of Cu
exposure to many active sites to partake in the reaction. with the ZrO2 lattice thus leading to the improved redox prop-
Stability of the catalyst was also examined using the reused erty of the catalyst in a H2-TPR prole. The highest catalytic
catalyst for oxidation of vanillyl alcohol at the optimum reaction activity was measured at 91% conversion and 76% selectivity to
conditions (reaction conditions: 3 mmol vanillyl alcohol, vanillin for catalyst CuZrO3 (Cu:2Zr) in liquid phase aerobic
0.0025 g cmꢁ3 catalyst B (Cu:2Zr), 21 bar air pressure, 85 ꢀC, 4 h) oxidation of vanillyl alcohol under mild reaction conditions.
(Fig. 16). Aer catalytic reaction, the catalyst was recovered by
ltration and washed several times with acetonitrile to remove
Acknowledgements
desorbed oxygenated products on the surface. The spent cata-
ꢀ
lyst was dried at 80 C for 4 hours. The subsequent reactions
were carried out at the optimum reaction conditions following
the similar recovery procedure. There was a slight loss of the
catalytic activity in the recycled catalysts. However, the selec-
tivity remained almost unchanged at 76% for all consecutive
reactions. That indicated that similar reaction pathways were
governed with all recycling experiments. Crystal phase was well
preserved as in the fresh catalyst during a recycling procedure,
so there was no obvious change in the amount of selectivity.
But, the amount of conversion was decreased possibly owing to
transformation of uneven surface to a plain surface aer the
catalytic reaction (Fig. 5d) and blockage on the active sites by
un-desorbed oxygenated products.
The authors are cordially grateful to Universiti Malaya grant
GC001A-14AET for their nancial support to accomplish this
work. Also, the authors would like to thank Emy Marlina Sam-
sudin for assistance in analysis of Raman spectrum.
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