Highly selective liquid-phase aerobic oxidation of vanillyl alcohol to vanillin on cobalt oxide (Co3O4) nanoparticles

Article abstract of DOI:10.1039/c3nj00508a

Spinel Co₃O₄ nanoparticles prepared by solution phase method having particle size in the range of 12-20 nm exhibited excellent activity for the liquid-phase aerobic oxidation of vanillyl alcohol with 80% conversion and 98% selectivity to vanillin. Our catalyst could be reused three times without appreciable loss in activity. The catalytic activity of the Co₃O₄ nanoparticles was found to be similar to its homogenous precursor (cobalt acetate) and greater than the commercial Co ₃O₄ oxide. The detailed characterization results of morphology, size and structure of the prepared Co₃O₄ nanoparticles obtained by XRD, FT-IR, H₂-TPR, HR-TEM and cyclic voltammetry technique were used to understand the roles of various Co species in directing the selectivity towards vanillin.

Full text of DOI:10.1039/c3nj00508a

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Highly selective liquid-phase aerobic oxidation of
vanillyl alcohol to vanillin on cobalt oxide (Co O )
nanoparticles†
3
4
Cite this: NewJ.Chem., 2013,
3
7, 2669
Ajay Jha and Chandrashekhar V. Rode*
Spinel Co
2–20 nm exhibited excellent activity for the liquid-phase aerobic oxidation of vanillyl alcohol with 80%
conversion and 98% selectivity to vanillin. Our catalyst could be reused three times without appreciable
loss in activity. The catalytic activity of the Co nanoparticles was found to be similar to its homo-
genous precursor (cobalt acetate) and greater than the commercial Co oxide. The detailed character-
ization results of morphology, size and structure of the prepared Co nanoparticles obtained by XRD,
FT-IR, H -TPR, HR-TEM and cyclic voltammetry technique were used to understand the roles of various
Co species in directing the selectivity towards vanillin.
3 4
O nanoparticles prepared by solution phase method having particle size in the range of
1
Received (in Porto Alegre, Brazil)
3th May 2013,
1
3 4
O
Accepted 4th June 2013
3 4
O
O
4
DOI: 10.1039/c3nj00508a
3
2
www.rsc.org/njc
from an environmental point of view. Several noble metal-based
heterogeneous catalysts such as Au/CeO , Pt/C, Ru/TiO etc. have
1
Introduction
2
2
A great deal of research is focused on designing heterogeneous been explored for this purpose and such reactions are mediated
1
0–13
catalysts for liquid-phase aerobic oxidation for developing by the use of alkali.
Although noble metal catalysts exhibit
ecologically sustainable processes for fine chemicals. Liquid- high activity for oxidation reactions, they can be expensive,
phase aerobic oxidations provide a green route as aerial O
is one minimizing their potential for commercial applications.14
of the cheapest and greenest oxidants available and water is the Oxides of other transition metals, such as copper, cobalt,
1
,2
2
3
–5
only by-product formed.
A major challenge for liquid-phase manganese, and chromium, are also known to be effective
air oxidation is the activation of molecular oxygen under mild catalysts for oxidation reactions.
conditions to achieve the highest selectivity to the desired
Among these metals, cobalt is the most attractive because of
products. Although homogeneously catalyzed liquid-phase air its strong capability for dioxygen activation (through metal–O2
oxidations are commonly practiced in industry using organo- adduct formation) and because it is more cost effective com-
metallic complexes as catalysts, the major drawbacks of such pared to noble metals. As compared to their bulk counterparts,
oxidations are: (i) poor selectivity to the desired product, a variety generally, nanostructured materials are more efficient as cata-
of oxygenated products are formed because of the formation of lysts due to increased number of corner and edge atoms with
free radical intermediates; (ii) fast deactivation of homogeneous decreasing the crystal domains, leading to higher adsorption
1
5,16
catalysts due to formation of m-oxo dimers; (iii) contamination rates and activation of the reactants.
of the product due to trace metal impurities during catalyst undertaken to further explore and understand the mechanism
separation protocol.
Hence, this work was
6
–9
These problems can be overcome by by which nanostructured materials govern their performance,
designing a suitable heterogeneous catalyst because they offer especially their role in directing the selectivity pattern in a
many advantages over their homogenous counterparts such as consecutive oxidation reaction. For this purpose, we prepared
easy handling, separation and reusability. Therefore, development Co O nanoparticles by solution-phase method and investi-
3
4
of functionalized heterogeneous catalysts is highly desirable gated its catalytic performance for aerobic oxidation of vanillyl
alcohol in the liquid phase (Scheme 1). Vanillin has major
applications in the food and perfumery sectors because of its
Chemical Engineering & Process Development Division, CSIR-National Chemical
flavour and it also finds use in medicinal applications serving
Laboratory, Pune 411008, India. E-mail: cv.rode@ncl.res.in;
1
7
as a platform chemical for pharmaceutical production.
Traditionally, vanillin was synthesized from guaiacol through
Fax: +91 20 2590 2621; Tel: +91 20 2590 2349
Electronic supplementary information (ESI) available. See DOI: 10.1039/
†
1
8,19
c3nj00508a
(i) the nitrose, and (ii) the glyoxylic method.
(Scheme 2).
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and an Xcelerator detector. Temperature programmed reduction
(TPR) experiments were carried on a Micromeritics Chemisorb
2
720 instrument. For this purpose, 0.02 g of the catalyst was
À1
placed in a quartz tube and treated with He gas (25 mL min
)
at 200 1C for 1 h. A gas mixture of 5% hydrogen in argon was
then passed through the quartz reactor at 50 1C for 1 h. The
temperature was raised from room temperature to 900 1C at a
À1
heating rate of 10 1C min and held at 900 1C for 10 min.
Scheme 1 Aerobic oxidation products of vanillyl alcohol.
2.2 Catalytic activity measurement
3
All the catalytic oxidation reactions were carried out in a 300 cm
capacity high-pressure Hastelloy reactor supplied by Parr
Instruments Co., U.S.A. The reactor was connected to an air
reservoir held at a pressure higher than that of the reactor.
Thermo Scientific HPLC model AS3000 liquid chromatograph
equipped with an ultraviolet detector was used for the analysis.
HPLC analysis was performed on a 25 cm RP-18 column. The
products and reactant were detected using a UV detector at
lmax = 254 nm. Aqueous methanol (35%) was used as a mobile
phase at a column temperature of 35 1C and a flow rate of
À1
0.7 mL min
.
In a typical experiment, 0.5 g of NaOH and 70 mL of iso-
Scheme 2 Traditional methods of vanillin synthesis. (i) Nitrose and (ii) glyoxylic
propanol were heated in a flask with a reflux condenser until
the NaOH dissolved completely. This NaOH solution was
charged with 0.5 g of vanillyl alcohol and 0.1 g catalyst into a
methods.
Though these methods gave higher selectivity to vanillin, the
formation of undesirable side products (nitrile and p-amino-
dimethyl aniline) in the case of the nitrose method, and the use
of toxic oxidants (CuO, PbO
method, and lower product yields (57%) are the serious draw-
3
8
00 mL Parr autoclave. The reaction mixture was heated to
0 1C. After the desired temperature was attained, the reactor
was pressurized with 6.8 bar air. Then the reaction was started
by agitating the contents at 1000 rpm and was continued for
2 2
and MnO ) in case of the glyoxylic
6
h. After 6 h, the reactor was cooled to room temperature and
2
0
backs of these methods.
Here, we report the synthesis and characterization of Co
the unabsorbed air was vented out. Then the contents of the
reactor were discharged and the final volume was recorded.
3 4
O
nanoparticles and their use as catalysts for the aerobic liquid-
phase oxidation of vanillyl alcohol to vanillin. The reusability of
the catalyst was also demonstrated by the thermal regeneration
steps during the catalyst recycle study. To the best our knowl-
edge, ours is the first report on highly selective Co O nano-
3 4
particles for the liquid-phase aerobic oxidation of vanillyl
alcohol to vanillin.
3
Results and discussion
3
.1 Structural properties of Co
XRD diffractograms of the uncalcined and calcined samples of
the prepared nano Co and commercial Co are displayed
3 4
O nanoparticles
3
O
4
3 4
O
in Fig. 1. The as-synthesized material showed broad and asym-
metric peaks at 2y B17.2, 33.6 and 39.31, which are character-
2
2
istic of the cobalt hydroxycarbonate phase. However, the
diffractogram of the calcined sample was completely different
from the as-synthesized sample. The broad XRD diffraction
2
Experimental
3 4
2.1 Synthesis and characterization of Co O nanoparticles
Co O nanoparticles were prepared by solution-phase method peaks observed for calcined Co O oxide at 2y B19, 31.4, 36.9,
3
4
3 4
2
1
as described earlier. In a typical synthesis, 4.98 g of cobalt 44.9, 59.5 and 65.41, were ascribed to the cubic phase of the
acetate tetrahydrate was dissolved in 60 mL of ethylene glycol Co (JCPDS No. 74-1657). The average crystallite size of Co
and the solution was gradually heated to 160 1C. To this, nanoparticles estimated from the full width at half maximum
00 mL of aqueous 0.2 M Na CO solution was added dropwise of the 311 diffraction peak by applying the Scherrer equation
and the slurry was further aged for 1 h under nitrogen atmo- was about 14 nm, which was much smaller (six times) than that
sphere. The resulting solid was filtered, thoroughly washed observed for the commercial Co (86 nm). The broader XRD
with distilled water until neutralization, and dried overnight at differaction peaks observed for our Co O oxide indicate the
3
O
4
3 4
O
2
2
3
3 4
O
3
4
100 1C, then calcined at 450 1C for 4 h.
nanocrystalline nature of the prepared catalyst.
X-Ray diffractograms (XRD) of the catalysts were recorded in
the 2y range of 10–801 (scan rate of 5.31 min ) on a PANalytical preparation of Co
The various cobalt oxide species possibly formed during the
À1
3
O
4
spinel were postulated from the tempera-
PXRD Model X-Pert PRO-1712, using Ni-filtered Cu Ka radiation ture-programmed reduction analysis (TPR). The TPR profiles
l = 0.154 nm) as a source (current intensity, 30 mA; voltage, 40 kV) of Co showed a two step reduction process initiated at
(
3 4
O
2
670 New J. Chem., 2013, 37, 2669--2674
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dependent on the morphology of Co
3
O
4
. As reported earlier, in
case of the nanoparticles, the reduction of CoO to Co metal
started at 400 1C and was completed at 550 1C, while for
nanotube morphology, it started from 500 1C and completed
at 650 1C. However in our case, CoO reduction started at 350 1C
and was completed at around 650 1C. This observation clearly
3 4
shows that the prepared Co O existed with mixed nanocom-
posite morphology giving an unsymmetrical reduction peak as
observed during reduction of CoO to metallic Co (Fig. 2).
HR-TEM images (discussed later, Fig. 4) also suggest the mixed
nanocomposite morphology of the prepared Co O spinel.
3
4
Formation of Co O spinel was also evident from the FT-IR
3
4
characterization. Fig. 3 shows the FTIR spectra of the as-
synthesized material (cobalt hydroxycarbonate) and Co
nanoparticles. As compared with the cobalt hydroxyl carbonate,
the IR spectrum of the Co nanoparticles displayed two sharp
3 4
O
3 4
O
À1
Fig. 1 XRD patterns of (a) the as-synthesized cobalt hydroxy carbonate, (b) bands at 563 and 667 cm , originating from the stretching
Co nanoparticles, and (c) commercial Co
3
O
4
3
O
4
.
vibration of the metal–oxygen bonds. On the basis of earlier
2
5,26
À1
3+
reports,
the former band at 563 cm was attributed to Co
À1
2+
in an octahedral hole and the latter band at 667 cm for Co
in a tetrahedral hole. The fingerprint of IR absorption at 563
À1
and 667 cm for the Co
3
O
4
spinel was fully developed after
calcination at 450 1C.
3 4
Fig. 4 shows HR-TEM images of Co O
nanoparticles synthe-
sized by the solution phase method and subsequent calcination
at 450 1C. The as-synthesized material had a fully developed
rod-like morphology (Fig. 4a), while the calcined nanoparticles
showed a mixed morphology which included cubic, tubular and
truncated polyhedron shapes (Fig. 4b). These cobalt oxide
nanoparticles were arranged in chain-like fashion with particle
size ranging from 12–20 nm, which matched well with the
crystallite size calculated from XRD. The fringe patterns of the
prepared Co
in Fig. 4c. According to this, the dominant exposed planes were
111) with a lattice space of 0.46 nm. The other exposed planes
observed were (220) and (311) with lattice spaces of 0.28 and
.24 nm, respectively. Fig. 4d shows the cross-section view near
3 4
O nanoparticles obtained by HR-TEM are shown
(
Fig. 2
2 3 4
H -TPR profile of Co O nanoparticles.
0
approximately 200 1C as shown in Fig. 2. The first reduction the (110) orientation exhibiting a rectangular shape with a long
peak was very sharp and symmetrical and the temperature
maximum was centred at about 325 1C. This peak was asso-
3 4
ciated with the reduction of trivalent cobalt oxide (Co O ) to
divalent cobalt oxide (CoO) (eqn (1)). The second reduction
peak was broad and unsymmetrical with the temperature
maxima centred at about 548 1C. This could be attributed to
the subsequent reduction of divalent cobalt oxide (CoO) to
2
3,24
metallic cobalt (Co1).
(eqn (2))
Co
3
O
4
+ H - 3CoO + H
2
2
O
(1)
(2)
2 2
3CoO + 3H - 3Co + 3H O
The amount of hydrogen consumed at low temperature
À1
À1
(
325 1C) was about one third (3.83 mmol g ) of that for the
higher temperature (548 1C, 11.8 mmol g ). The ratio of
hydrogen consumed between the 1st and 2nd peaks was close
to 1 : 3, which is consistent with the stoichiometry of Co O
3 4
reduction. The reduction of CoO to metallic Co was strongly Fig. 3 FT-IR spectra for (a) cobalt hydroxycarbonate, and (b) Co O nanoparticles.
3 4
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Fig. 5 CV curve in 0.1 M KOH solution of (a) prepared Co
Co
3
O
4
and (b) commercial
3
4
O .
3
+
that the exposed surface area of Co in Co
nano Co sample than in the commercial Co
area of the oxidation–reduction peak for the commercial Co
3
O
4
was higher in our
3
O
4
3 4
O
sample. The
3 4
O
sample was very much less, which was attributed to its higher
crystallite size.
3 4
3.2 Catalytic performance of Co O nanoparticles
The comparison of the activity and selectivity of different
catalysts for the liquid-phase air oxidation of vanillyl alcohol
is shown in Table 1. All the reactions were performed under
optimum reaction conditions in alkaline medium except entry 4.
The role of base is probably to deprotonate the phenolic –OH to
form the phenoxy anion, which is the reactive species and
favours coordination to the cobalt(III) species, increasing the
3 4
Fig. 4 (a–e) HR-TEM images of Co O nanoparticles and (f) SAED pattern of
3 4
Co O nanoparticles.
2
8
edge parallel to (001). These 001 planes were constructed from activity. Oxidation of vanillyl alcohol in an alkaline medium
220) planes. Selected area ED analysis was also carried out to without catalyst did not show any activity. However, oxidation
understand the crystallographic orientation of the Co
nano- of vanillyl alcohol in the presence of catalyst and a base led to
(
3 4
O
cystallites. Fig. 4f displayed the ED patterns of the cube present the selective formation of vanillin (98%), with 80% conversion
in the Co O nanocomposite. The lattice constants obtained of vanillyl alcohol. This could be possible by active participa-
3
4
3
+
from the ED patterns are in excellent agreement with those tion of the Co species. As observed in the HR-TEM images of
from the XRD analysis.
the material, the presence of (110) plane was revealed. According
could be approximately to Xie et al., (110) planes of Co O are a characteristic of Co
3 4
estimated by the Co /Co redox profiles obtained by cyclic species, which is the active component for the oxidation reaction.
voltammetry (CV). CV measurements were performed at a scan A comparable oxidation activity of the homogeneous precursor
rate of 50 mV s over the potential range of 0–0.6 V in 0.1M was also observed (entry 1, Table 1), but it suffers a major
3
+
The active sites present in Co
3
O
4
3
+
4+
À1
KOH solution by using a quasi reference electrode. As can be problem of difficult recovery and then recyclability. A control
seen in Fig. 5, during the positive scanning a broad hump was experiment with commercial Co
3
O
4
gave a much lower conver-
sion (45%) with almost the same selectivity pattern as shown in
3 4
Table 1 (entry 3). The lower activity of the commercial Co O
3
+
observed, which could be associated with the oxidation of Co
on the surface of Co
3
O
4
.
Corresponding to the oxidation process, a similar broad
hump was observed at the cathode near 0.45 V, which was
Table 1 Catalyst screening for liquid-phase aerobic oxidation of vanillyl alcohol
4
+
3+ 27
attributed to the reduction of Co to Co . The broad peak
may be due to the mixed morphology of Co , as it was also
Selectivity (%)
3 4
O
confirmed by H -TPR and HR-TEM measurements.
Entry Catalyst
Conversion (%) Vanillin Vanillic acid
2
These results are characteristic of the surface confined redox
couple as shown by the reaction in Fig. 5. Morphology and
a
1
2
3
4
Co(ac)
Co
Commercial Co
2
82
96
98
98
88
4
2
2
3
O
4
nanoparticles 80
45
nanoparticles 25
crystallite size of the Co
3
+
O
4
has a significant influence on the
3
O
4
b
3
Co
3
O
4
12
exposed number of Co species. From the integrated area of
3
+
a
b
the Co oxidation peaks shown in Fig. 5, it could be evaluated
Homogeneous catalyst. Reaction without NaOH.
2
672 New J. Chem., 2013, 37, 2669--2674
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was due to its higher crystallite size (86 nm), which would offer
less Co(III) active species for the oxidation, as is also supported
by its CV measurement. In addition, the surface area of
2
À1
nanosize Co O was found to be 87 m
g , about 17 times
3
4
2
À1
higher than that of the commercial Co O sample (5 m g ).
3
4
Secondly, the lattice fringe patterns of nano Co
presence of the (111) plane, which was absent in the commer-
cial Co , in which ‘d’ spacing of 0.28 nm was characteristic of
3 4
O showed the
3 4
O
the (220) plane (ESI,† S4). Hence both the factors viz. higher
surface area as well as a dominant (111) plane exposing the
3
+
active Co sites of nano Co O , are responsible for its higher
3
4
specific activity. The TOF calculated for Co O nanoparticles
3
4
Fig. 7 Recycling study of vanillyl alcohol oxidation.
À1
À1
was 6.4 h while that for commercial Co O was 1.2 h . Under
3
4
the same reaction conditions, oxidation of vanillyl alcohol in
the absence of a base leads to the lower conversion of vanillyl under vacuum at 100 1C for 2 h and then reused after calcina-
alcohol (25%) with 88% selectivity to vanillin and 12% vanillic tion at 450 1C for 1 h. The procedure was followed for three
acid, as shown in Table 1 (entry 4). It seems that in the subsequent oxidation experiments, and the results are shown
oxidation of the vanillyl alcohol, the catalytic role of cobalt in in Fig. 7. The conversion of vanillyl alcohol was found to
the absence of base is weak but not absent because abstraction decrease from 80 to 69% after the 3rd recycle study, however,
of the b-proton from the vanillyl alcohol depends on the selectivity to vanillin remained constant (98%). XRD and
2
9
formation of the superoxo-Co(III) species.
The effect of reaction time on vanillyl alcohol conversion that the particle size of the catalyst remained intact while the
and selectivity to vanillin was also studied for Co
nanocata- morphology of the Co was changed from mixed morphology,
HR-TEM image of the used catalyst after the 3rd recycle showed
3
O
4
3 4
O
lyst and the results are shown in Fig. 6. The conversion of which included cubic, tubular and truncated polyhedron, to
vanillyl alcohol increased from 49 to 80% with a consistent roughly spherical shape (ESI,† S1and S2). The decrease in the
selectivity (98%) to vanillin. While an increase in reaction time catalytic activity of the used catalyst might be due to the change
from 2 to 6 h, conversion became low (86%) with a decrease in in the morphology, which leads to the change in the crystal
selectivity to vanillin from 98 to 94% in 8 h. The decrease in planes of the catalyst, which are responsible for exposing the
3
0
conversion and selectivity to vanillin might be due to the active sites on the surface. The selectivity of the product did
formation of polymeric materials over the catalyst surface as not change during recycling of the catalyst. This implies that
observed by the increase in the catalyst weight after the the morphology affects the reaction rate significantly but has
reaction.
no influence on the reaction pathway.
In order to study the stability of this heterogeneous catalyst,
In order to eliminate the possibility of a homogeneously
its recycle experiments were carried out in the following way: catalyzed reaction due to the dissolution of cobalt oxide, a
after the first oxidation run with fresh Co O catalyst, it was leaching test was performed as follows. The nano Co
O
3 4
3
4
filtered out and washed several times with methanol, dried catalyst was separated from the reaction mixture by simple
filtration after a partial conversion of 45% in 2 h, and the
filtrate was then continued for further reaction under similar
conditions (6.8 bar air pressure at 80 1C), which did not give any
conversion. The absence of any cobalt species in the filtrate was
also confirmed by inductively coupled plasma spectrometry.
These results clearly support the fact that the oxidation of
vanillyl alcohol, catalyzed by Co O , is truly heterogeneous in
3
4
nature.
4
Conclusions
3 4
In this work, Co O nanoparticles were prepared by a solution
phase method and its catalytic activity for liquid-phase aerobic
oxidation of vanillyl alcohol was evaluated. Comparison
between commercial and prepared Co O for vanillyl alcohol
3
4
oxidation showed that the prepared Co O was a better catalyst
3
4
for vanillyl alcohol oxidation. The higher activity of the pre-
pared Co was due to its smaller crystallite size (14 nm),
higher surface area (87 m g ) and higher exposure of Co
3 4
O
Fig. 6 Effect of time on conversion and selectivity. Reaction conditions: molar
ratio of NaOH/vanillyl alcohol, 3.8; Co nanocatalyst (0.1 g), 353 K, iso-propyl
alcohol (70 ml), O -pressure (6.8 bar).
2
À1
3+
3
O
4
2
species over the Co
3
O
4
surface as confirmed by CV measurement.
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4
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This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013