Sonochemical oxidation of vanillyl alcohol to vanillin in the presence of a cobalt oxide catalyst under mild conditions

Article abstract of DOI:10.1016/j.ultsonch.2016.11.015

The heterogeneous oxidation of vanillyl alcohol to vanillin was investigated on new grounds under eco-friendly conditions in the presence of hydrogen peroxide as an oxidant and water as solvent, coupled with low frequency ultrasonic irradiation. The sono-Fenton-like-assisted vanillyl alcohol oxidation was performed with a high-surface area nanostructured spinel cobalt oxide catalyst exhibiting small crystallites size. The catalytic reaction was also carried out under conventional heating conditions for comparison purposes. The influence of the reaction parameters, namely catalyst loading and hydrogen peroxide concentration was studied with the aim of determining the optimum yield and selectivity to the desired vanillin product. The chemical effects of ultrasound (ability to generate hydroxyl radicals) along with increased mass transfer appeared to be key prerequisites for enhancing the efficiency of the process, while decreasing the overall energy consumption.

Full text of DOI:10.1016/j.ultsonch.2016.11.015

Accepted Manuscript
Sonochemical oxidation of vanillyl alcohol to vanillin in the presence of a cobalt
oxide catalyst under mild conditions
Ronan Behling, Grégory Chatel, Sabine Valange
PII:
S1350-4177(16)30383-2
DOI:
http://dx.doi.org/10.1016/j.ultsonch.2016.11.015
ULTSON 3429
Reference:
Ultrasonics Sonochemistry
To appear in:
Received Date:
Revised Date:
Accepted Date:
23 August 2016
2 November 2016
8 November 2016
Please cite this article as: R. Behling, G. Chatel, S. Valange, Sonochemical oxidation of vanillyl alcohol to vanillin
in the presence of a cobalt oxide catalyst under mild conditions, Ultrasonics Sonochemistry (2016), doi: http://
dx.doi.org/10.1016/j.ultsonch.2016.11.015
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Sonochemical oxidation of vanillyl alcohol to vanillin in the presence of a cobalt oxide
catalyst under mild conditions
Ronan Behling a, Grégory Chatel a,b,*, Sabine Valange a,*
a Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), Université de Poitiers, CNRS,
ENSIP, B1, 1 rue Marcel Doré, F-86073 Poitiers Cedex 9, France.
b
Current address: Laboratoire de Chimie Moléculaire et Environnement (LCME), Université
Savoie Mont Blanc, 73376 Le Bourget du Lac Cedex (France)
* Corresponding authors:
E-mail addresses: gregory.chatel@univ-smb.fr (G. Chatel), sabine.valange@univ-poitiers.fr (S.
Valange)

Abstract
The heterogeneous selective oxidation of vanillyl alcohol into vanillin was investigated
on new grounds under eco-friendly conditions in the presence of hydrogen peroxide as an
oxidant and water as solvent, coupled with low frequency ultrasonic irradiation. The
sono-Fenton-like-assisted vanillyl alcohol oxidation was performed with a high-surface
area nanostructured spinel cobalt oxide catalyst exhibiting small crystallites size. The
catalytic reaction was also carried out under conventional heating conditions for
comparison purposes. The influence of the reaction parameters, namely catalyst loading
and hydrogen peroxide concentration was studied with the aim of determining the
optimum yield and selectivity to the desired vanillin product. The chemical effects of
ultrasound (ability to generate hydroxyl radicals) along with increased mass transfer
appeared to be key prerequisites for enhancing the efficiency of the process, while
decreasing the overall energy consumption.
Keywords: Selective oxidation; Sonochemistry; Ultrasound; Hydrogen peroxide; Cobalt oxide
catalyst.

1. Introduction
Liquid phase aerobic oxidation of lignin derived sub structure compounds such as
monomeric platform alcohols is being explored extensively to prepare functionalized
aromatics for the production of a variety of fine chemicals. Among them, the most studied
substrates are veratryl, vanillyl, p-sinapyl, p-coumaryl, coniferyl alcohols and 2-phenoxy-
1-phenylethanol [1]. The selective catalyzed oxidation of such aromatic alcohols to the
corresponding carbonyl compounds is indeed worthy of interest for plentiful industrial
applications. Vanillyl alcohol (VA) can be selectively oxidized to vanillin, which has a
wide range of applications in food and perfumes or as a platform chemical for
pharmaceuticals production [2]. Vanillyl-alcohol oxidase and laccase-catalyzed reactions
have been reported for the selective production of vanillin [3,4]. Direct routes to vanillin
are also well explored through conventional soxhlet extraction, microwave or ultrasound-
assisted extraction from vanilla pods [5,6,7].
Conventionally, oxidation of vanillyl alcohol is performed using homogeneous
catalysts, including metalloporphyrin, cobalt(salen) and methyltrioxo rhenium complexes,
as well as simple metal salt-based catalysts [8]. Heterogeneous catalysts are however
preferred over their homogeneous analogs from an environmentally point of view, thereby
promoting the atom-efficient synthesis of fine chemicals. The heterogeneous selective
oxidation of vanillyl alcohol using molecular O2 as a primary oxidant has been widely
studied. The group of Rode has deeply investigated the use of cobalt-based catalysts, such
as Co3O4 and Mn-Co mixed oxides (supported or not) for the selective oxidation of VA
and similar platform monoaromatic substrates under pressures until 4 MPa and
temperatures around 150-160 °C [9]. Alkali-mediated aerobic oxidation of VA into
vanillin has also been investigated with cobalt-based zeolitic imidazolate framework
catalysts, as well as with Pt and Pd immobilized on various carriers (carbon, MOFs, TiO2,
SiO2 and MgO) [1].
The heterogeneously-catalyzed photo-oxidation of aromatic alcohols to aldehydes
under sunlight and UV-Vis light was recently comprehensively reviewed [10,11]. Such
strategy may lead to relatively short reaction times, at mild conditions without addition of
any extra chemicals. The photo-production of vanillin was reported in aqueous medium in
the presence of a variety of TiO2 powders using air or oxygen as a primary oxidant [12].

Studies dealing with the aqueous phase selective oxidation of VA using a
heterogeneous catalyst together with H2O2 as oxidant have however been scarcely
reported [1]. Crestini et al. immobilized a methyltrioxorhenium complex on low-cost
polymeric supports for the hydrogen peroxide selective oxidation of the side chain
hydroxyl group of VA with acetic acid as reaction solvent [13]. Such MTO-based
catalysts are however toxic for both the environment and human health [14]. Very
recently, the liquid phase oxidation of VA into vanillin with H2O2 was investigated with
the use of a cobalt titanate catalyst in various organic solvents at 85 °C [15]. The hydrogen
peroxide microwave-assisted oxidation of VA was explored in another study with a
lanthanum-containing SBA-15 mesoporous silica in acetonitrile [16].
To the best of our knowledge, no data on the combined use of water as solvent and
hydrogen peroxide as O-donor for such heterogeneously catalyzed reaction was reported
so far. The present study falls within this context with the aim of investigating the
hydrogen peroxide oxidation of vanillyl alcohol into vanillin with a non-noble metal based
solid catalyst under more eco-friendly conditions (ambient pressure, low temperature and
aqueous medium) coupled with low frequency ultrasonic irradiation (US), as
unconventional activation route (Scheme 1).
Scheme 1: Selective oxidation of vanillyl alcohol into vanillin under ultrasonic and mild
conditions in the presence of Co-based heterogeneous catalyst.
In order to evaluate the effects of US on the reaction and the associated oxidation
mechanisms, comparative studies were carried out under conventional heating (CH). For
that purpose, a nanostructured spinel cobalt oxide (Co3O4) was synthesized via
a
controlled co-precipitation route and fully characterized before and after reaction by
several complementary techniques, such as XRD, nitrogen adsorption, TG-DTA,
elemental analysis, TPR, scanning and transmission electron microscopy and XPS.

Moreover, sonochemistry could offer new strategies in heterogeneous catalyzed-
oxidation reactions through intense mechanical, thermal and chemical effects generally
responsible of an increase of reaction rates, changes in reaction mechanisms,
emulsification effects, crystallization, precipitation, erosion, etc [17,18]. The formation,
growth and the sudden collapse of gaseous microbubbles in the liquid phase, due to the
cavitation phenomenon, create locally high temperatures and pressures that can initiate
high-energy radical mechanisms and generate physical effects [19,20,21]. Even if the full
mechanism has not been elucidated yet, it is usually accepted that low frequencies (20–80
kHz) preferentially lead to physical effects, while high frequencies (150–2,000 kHz) favor
the production of radicals. However, it is known that the production of HOꢀ radicals can
be improved in water at low frequency in the presence of catalysts through sono-Fenton-
like processes [22]. In the present work, the ability of ultrasound to generate in situ
hydrogen peroxide and hence hydroxyl radicals (chemical effects) and the increase of
mass transfer between the Co3O4 catalyst and the substrate (physical effects) will be
discussed.
2. Experimental
2.1. Synthesis of the catalyst
The nanostructured spinel Co3O4 catalyst was prepared by a controlled
homogeneous co-precipitation route adapted from the synthesis procedure described by
Mate et al [23]. Cobalt nitrate hexahydrate was used as Co precursor along with potassium
carbonate as precipitating agent. The precipitation step was carried out at a fixed pH of
8.5. Briefly, a 0.03 mol L-1 solution of Co(NO3)2.6H2O and a 0.06 mol L-1 solution of
K2CO3 were first prepared. The controlled simultaneous addition of 400 mL of each
solution to a round bottom flask containing 100 mL of distilled water at 70 °C under
constant stirring resulted in the formation of a purple precipitate. The obtained slurry was
further aged at 70 °C during 16 h. The resulting solid was recovered by filtration,
thoroughly washed with distilled water until all potassium ions have been removed and
dried at 100 °C before being calcined at 300 °C for 5 h in a Nabertherm controller P320
muffle oven (heating rate: 2°C min-1).

2.2. Selective oxidation reaction
2.2.1. Oxidation of vanillyl alcohol under silent conditions
0.2 g (1.3 mmol) of vanillyl alcohol were dissolved in distilled water at 75 °C. After
dissolution, the catalyst (4 mg, 2 wt%) was added, followed by the addition under magnetic
stirring (1000 rpm) of 1–4 molar equivalents of 30 wt% aqueous hydrogen peroxide solution
(135–540 µL), so that the total volume was 4 mL.
2.2.2. Ultrasound-assisted oxidation of vanillyl alcohol
0.2 g (1.3 mmol) of vanillyl alcohol were dissolved in distilled water at 75 °C. After its
dissolution, the catalyst (4 mg, 2 wt%) was added, followed by the addition of 1–4 molar
equivalents of 30 wt% aqueous hydrogen peroxide solution (135–540 µL) so that the total
volume was 4 mL under ultrasonic irradiation. Ultrasound was generated by a Digital Sonifier®
S-250D from Branson (power of standby P0 = 27.0 W, nominal electric power of the generator
Pelec = 8.2 W). A 3.2 mm diameter tapered microtip probe operating at a frequency of 19.95 kHz
was used. The volume acoustic power of this system was Pacous.vol = 0.25 W.mL-1 in water
–
(determined by calorimetry measurements) [24] and an average radical formation rate of v(I3 )
=1.94x10-6 mol s-1 in 4 mL of water (determined by dosimetry method from 0.1 mol L-1 solution)
[25]. The reaction medium (vanillyl alcohol, catalyst, H2O2 and water) was inserted in a rounded
cylindrical glass reactor (17 mm in interior diameter, 102 mm in height) and the temperature was
not controlled. The ultrasonic probe was directly immersed in the reaction medium. Energy
consumption was measured with a wattmeter (Perel®).
2.2.3. Determination of vanillyl alcohol conversion and vanillin yield
At the end of the reaction, the aqueous solution was diluted, filtered (Fisherbrand®
PTFE membrane filter with a pore size of 0.45 µm) and analyzed by a Shimadzu® HPLC
LC-20AD (injection volume: 10 µL) equipped with a C18 column (Interchrom UP50DB-
250/046 C18-ODB 5 µm 250x4.6mm HP) using a Waters® 2410 refractive index
detector, and a 10/90 acetonitrile/water (V/V) mobile phase with a rate of 0.6 mL min-1.
The catalytic performance of the nanostructured spinel Co3O4 catalyst was
evaluated in terms of conversion of vanillin alcohol (%) and yield/selectivity (%) to
vanillin product, according to Eqs. (1–3):

ꢀꢁꢂꢃꢄꢅꢆꢇꢁꢂꢈ % = ꢋꢌꢍꢎꢍꢏꢐꢈꢑꢒꢐꢓꢔꢈꢒꢕꢈꢖꢗꢘꢈꢙꢓꢑꢏꢍꢌꢍꢌꢚꢈꢑꢒꢐꢓꢔꢈꢒꢕꢈꢖꢗ ꢈ× ꢈ100%
ꢉ ꢊ
(1)
ꢋꢌꢍꢎꢍꢏꢐꢈꢑꢒꢐꢓꢔꢈꢒꢕꢈꢖꢗ
ꢛꢜꢂꢇꢝꢝꢇꢂꢈꢞꢇꢄꢝꢟꢈ % = ꢠꢒꢐꢓꢔꢈꢒꢕꢈꢕꢒꢡꢑꢓꢢꢈꢣꢏꢌꢍꢐꢐꢍꢌ ꢈ× ꢈ100%
(2)
(3)
ꢉ ꢊ
ꢋꢌꢍꢎꢍꢏꢐꢈꢑꢒꢐꢓꢔꢈꢒꢕꢈꢖꢗ
ꢛꢜꢂꢇꢝꢝꢇꢂꢈꢆꢄꢝꢄꢤꢥꢇꢃꢇꢥꢞꢈ % = ꢖꢏꢌꢍꢐꢐꢍꢌꢈꢦꢍꢓꢐꢢ ꢈ× ꢈ100%
ꢉ ꢊ
ꢧꢒꢌꢣꢓꢡꢔꢍꢒꢌ
3. Results and discussion
3.1. Catalyst characterization
Fig. 1 shows the XRD pattern of the calcined Co3O4 catalyst recorded with a cobalt
anode. All diffraction lines can be readily indexed according to the reference sample (JCPDS
card 98-002-8158) for Co3O4 having a cubic spinel structure (Fd-3m space group). The XRD
pattern indeed confirmed the presence of the (111), (022), (113), (222), (004), (224) and (115) hkl
reflections at 2θ = 22.11°, 36.50°, 43.09°, 45.10°, 52.59°, 65.70° and 70.23°, respectively. No
additional XRD lines corresponding to other phases and/or unreacted cobalt nitrate were
observed. The average crystallite size of the calcined Co3O4 was estimated from the full width at
half maximum of the (111), (113) and (115) XRD line by applying the Scherrer equation. A mean
value of 6.3 nm was obtained which was far lower than that observed for the commercial Co3O4
sample (86 nm). Such small size is to be related to the broadening of the XRD lines, which
emphasizes the nanocrystalline nature of the spinel Co3O4 catalyst.
Fig. 1. XRD patterns of the calcined nanostructured Co3O4 sample and the reference Co3O4.

The N2 adsorption-desorption isotherm of the Co3O4 sample and its corresponding pore
size distribution are shown in Fig. 2. According to the IUPAC classification, the isotherm is of
Type IVa and is attributed to a mesoporous material [26]. The capillary condensation is
accompanied by hysteresis that starts to occur for pores wider than ∼ 4 nm [26]. The shape of the
isotherm exhibits a long adsorption plateau typical of the capillary condensation in the
mesopores. The Type H1 hysteresis loop (IUPAC) is readily attributed to the presence of uniform
mesopores with a narrow range, in accordance with the shape of the pore size distribution. The
calcined Co3O4 nanoparticles exhibited a specific surface area of 155 m2 g-1 coupled to a pore
volume of 0.41 cm3 g-1 and a mean pore diameter of 10.1 nm.
Fig. 2. N2 adsorption-desorption isotherm for the calcined nanostructured Co3O4 and the
corresponding pore size distribution obtained from the adsorption branch (inset).
Representative SEM and HRTEM micrographs of the calcined Co3O4 sample are
displayed in Fig. 3. Scanning electron microscopy images (Fig. 3a and b) showed that the Co3O4
sample was relatively porous and built up of aggregates consisting of primary particles that
occurred non-isolated while forming a loose network. SEM EDS mapping of the calcined Co3O4
catalyst revealed that the Co and O atoms were homogeneously distributed throughout the surface
of the nanoparticles (Fig. S1 ESI). The light blue area in the carbon mapping corresponds to the
carbon tape on which the sample was deposited prior to analysis. HRTEM analyses provided
additional convincing evidence for the nanostructuration of the Co3O4 sample. The size and
morphology of the cobalt oxide particles were better depicted in Fig. 3c and d. TEM micrographs

revealed that the nanoparticles displayed a cubic morphology while being arranged in a roughly
nanorod-like loosely structure. These particles exhibited a very small size varying between 5 and
10 nm, in agreement with the average crystallite size calculated by XRD (6.3 nm). Such results
may also be correlated with the data obtained from nitrogen sorption analysis. The small size of
the Co3O4 nanoparticles coupled to their porous arrangement obviously account for their high
developed surface area. The way they are arranged generates void (small cavities) whose mean
diameter corresponds to the mean pore size determined by N2 adsorption-desorption
measurements.
Fig. 3. Representative SEM (a-b) and HRTEM (c-d) micrographs of the calcined nanostructured
Co3O4 sample.
The formation of the spinel Co3O4 was also duly confirmed by X-ray photoelectron
spectroscopy (XPS). Such analysis was used to monitor the oxidation state and the relative
percentage of the cobalt species in the Co3O4 catalyst. As can be seen from Fig. S2 (ESI), two
XPS signals were observed due to the Co 2p1/2 and Co 2p3/2 core level peaks at binding energies
of 794.6 and 779.6 eV, respectively. A difference of 15 eV was observed corresponding to Co2+
and Co3+ species [27]. Both Co 2p1/2 and Co 2p3/2 peaks could be deconvoluted into three peaks.

Binding energies of 779.7, 781.0 and 782.3 eV were determined for the Co 2p3/2 peak, ascribed to
Co3+ and Co2+ species. Moreover, the surface composition calculated from XPS indicated that the
atomic percentages of Co3+ and Co2+ were 67 and 33%, hence confirming the proper formation of
the spinel oxide. Additionally, low intensity satellite peaks characteristic of Co3O4 were also
present [28].
The reducibility of the nanostructured spinel Co3O4 was evaluated by temperature-
programmed reduction. The H2-TPR profile was characterized by a two-step reduction process
resulted from a progressive reduction of the Co3O4 nanoparticles (Fig. S3 ESI). The first
reduction peak was very sharp, symmetrical and centered at a temperature of 289 °C. This low-
temperature peak corresponds to the reduction of Co3O4 to divalent cobalt oxide (CoO), in
accordance with literature data [29,30]. While being also almost symmetrical, the second
reduction peak was broader with a temperature maximum located at 449 °C. This peak was
associated to the subsequent reduction of CoO to metallic cobalt Co0. The observed maximum
reduction temperature for both peaks was however far lower than that mentioned in the literature
for similar cobalt oxides. The reduction of Co3O4 to CoO (and hence CoO to Co0) was reported to
be strongly dependent on the morphology and/or on the crystallites size of the initial Co3O4
particles [29,31]. In the case of Co3O4 nanoparticles, the reduction process into metallic cobalt
started at 400°C and ended at 550°C, while for Co3O4 nanotubes it only started at 500°C and was
complete at 650°C. In our case, the lower maximum temperatures of the reduction peaks were
obviously related to the small crystallite size of the calcined nanostructured Co3O4 particles. All
together, these observations (symmetrical shape, starting and ending reduction temperature
values) are best corroborated with the morphological characteristics of the prepared Co3O4
catalyst.
3.2. Selective oxidation of vanillyl alcohol over Co3O4
3.2.1. Optimization of experimental conditions
The activity of the nanostructured spinel Co3O4 catalyst was evaluated in the selective
oxidation of vanillyl alcohol in terms of VA conversion (%) and yield to vanillin (%) by varying
the experimental conditions under ultrasound activation during 15 min. Preliminary results
indicated that the highest yields were generally obtained at this reaction time, according to the

obtained kinetic profiles that will be discussed later on (Fig. 7). The influence of the catalyst
loading and hydrogen peroxide concentration on the catalytic properties was first investigated. In
the absence of Co3O4 catalyst, no vanillin was formed even if vanillyl alcohol conversion was
observed, probably due to some substrate degradation (Fig. 4). The optimum catalyst loading was
determined to be 2wt% of Co3O4 related to the substrate. When higher amounts of catalyst were
used in the reaction, the conversion increased until a plateau of 55%, whereas vanillin yields
decreased. This is due to the overoxidation of vanillin that once formed, competes with the
substrate for the oxidizing species available in the media. Such overoxidation issues will be
discussed in the following sections.
Fig. 4. Influence of the catalyst loading on the vanillyl alcohol oxidation (Experimental
conditions: Co3O4 as catalyst, 2 eq. H2O2, total volume of 4 mL (water), 20 kHz ultrasound, 75
°C, 15 min).
Interestingly, we observed that traces of vanillin are formed in the presence of the catalyst
without H2O2 addition (Fig. 5). This can be explained by the fact that ultrasound irradiation in
water itself can generate in situ hydrogen peroxide and hence hydroxyl radicals [32], leading to
some vanillin formation in the presence of the catalyst. Note that this effect was not observed in
the absence of catalyst. In addition, increasing the amount of H2O2 added into the reaction over 2
equivalents resulted in higher VA conversions but lower selectivities to vanillin due to the
substrate and/or vanillin degradation/overoxidation. It was also noticed that the portionwise
addition of H2O2 in order to better control its consumption and reactivity in the reaction medium

did not improve the catalytic results under our conditions in the presence of the Co3O4 catalyst
(Fig. S4 ESI). This observation will be explained later on, in view of the understanding of the
reactivity of this catalytic system under the investigated aqueous conditions.
By increasing the amount of catalyst or oxidant, the conversion of vanillyl alcohol
increased, while the selectivity decreased. Gusevskaya et al. studied the selective oxidation of
isoeugenol into vanillin using a vanadium homogeneous catalyst (n-Bu4NVO3) [33]. The authors
explained that as soon as a primary oxidation product undergoes further oxidation or degradation,
the maximum yield is limited and depends on the rate in which the substrate and primary product
get oxidized. Based on the rate constants k1 (first oxidation) and k2 (overoxidation) as well as on
the reaction order, they calculated the maximum yield attained for the primary oxidation product.
Thus, when k1 = k2 in a first order reaction, the maximum yield to the primary product is
theorically 37% and only 25% when the primary product gets oxidized twice as fast as the
substrate. The yield reaches 95% when the substrate reacts 100 times faster than the primary
product [33]. In our case, neither vanillic acid nor hydroquinone was detected in the reaction
medium. However, we observed the formation of few amounts of formic acid. The degradation
pathway and the nature of the secondary oxidation products will be discussed in the last section.
Fig. 5. Influence of the amount of oxidant on the vanillyl alcohol oxidation. (Experimental
conditions: 2wt% Co3O4, 30% H2O2 as oxidant, total volume of 4 mL (water), 20 kHz ultrasound,
75 °C, 15 min).

Along with the determination of the consumption of vanillyl alcohol as a function of time,
we also determine that of vanillin as substrate. Fig. 6 clearly shows that both reaction rates were
similar under these sonochemical conditions, thereby limiting the maximum yield to vanillin that
can be reached.
In other words, high yields to vanillin are hard to be achieved if vanillin and vanillyl
alcohol have similar oxidation rate constants. Under our optimal conditions with 2wt% catalyst
and 2 molar equivalents of H2O2 (30% aqueous solution), a maximum selectivity to vanillin of
50% corresponding to a yield of 19% could be attained (Table 1, Entry 4).
Fig. 6. Oxidation of vanillyl alcohol and vanillin as substrates under ultrasound (Experimental
conditions: 2 wt% Co3O4, 2 eq. 30% H2O2, total volume of 4 mL (water), 20 kHz ultrasound,
75°C).
The stability of the nanostructured Co3O4 catalyst was also evaluated in order to verify the
absence of competitive homogeneous pathway during the oxidation reaction. For that purpose,
the catalyst was separated from the aqueous medium at the end of the reaction and the filtrate
solution was analyzed by ICP-OES to determine whether cobalt leaching occurred or not. The
amount of cobalt in solution was shown to be negligible (<1.0wt%) for both activation systems.
Moreover, the filtrate solution was not able to catalyze the reaction in a second run, thereby
confirming that the oxidation process takes place through a heterogeneous pathway.

3.2.2. Ultrasonic vs. conventional conditions
The difference of reactivity between the sonochemical oxidation of vanillyl alcohol and
the corresponding reaction under conventional heating (75 °C) is emphasized in Table 1. First,
we checked that almost no conversion was observed in the absence of both the catalyst and the
oxidant under ultrasound and silent conditions (Table 1, Entry 1). When H2O2 was added in the
absence of the Co3O4 catalyst, no vanillin was obtained (Table 1, Entry 2). On the contrary, when
the catalyst was added in the absence of any oxidant, no formation of vanillin occurred under
conventional heating, while only traces were detected under ultrasound due to the interaction of
the catalyst with few amounts of in-situ generated oxidative species (HOꢀ and H2O2) (Table 1,
Entry 3). The synergistic combination between Co3O4, hydrogen peroxide and ultrasonic
activation was clearly highlighted by comparison with the reaction carried out under silent
conditions (yield almost three times higher, Table 1, Entry 4). A commercial Co3O4 nanopowder
(Sigma-Aldrich) exhibiting a surface area of 32 m2 g-1 was also used for comparison purposes.
While being far less efficient than the nanostructured spinel cobalt oxide we synthesized, again
higher yield and selectivity to vanillin were obtained under ultrasonic irradiation than under silent
conditions (Table 1, Entry 5), thereby confirming the synergistic “catalyst/oxidant/US” effect.
Table 1
Screening of different experimental conditions for vanillyl alcohol oxidation.
Ultrasound (15 min)
Silent conditions (60 min)
Entry
Experimental conditionsa
Conv. Yield Select. Conv. Yield Select.
(%)
12
(%)
0
(%)
0
(%)
3
(%)
0
(%)
0
1
2
3
4
5
Blank (without H2O2 and Co3O4)
H2O2 (2 eq.), without Co3O4
Co3O4 (2 wt%), without H2O2
H2O2 (2 eq.), Co3O4 (2 wt%)
Same as entry 4 but with a
35
16
38
28
0
2
0
12
2
0
0
7
1
0
0
12
50
18
19
5
32
24
23
5
commercial Co3O4 (32 m2 g-1)
Green chemistry metrics for entry 4
EFb
12
PMIb
111
EFb
35
PMIb
302
a Experimental conditions: 1.3 mmol of vanillyl alcohol, total volume of 4 mL (water), 75 °C.
b E Factor (EF) and Process Mass Intensity (PMI) indexes defined in the ESI.

The kinetic profiles showed a maximum yield reached after 15 min under ultrasound
(19%) and after 60 min under conventional heating (7%). It has to be noted that the total
consumption of H2O2 was observed after 5 min under ultrasound and after 15 min under
conventional heating (Fig. S5 ESI). The VA oxidation kinetic profile under ultrasound is faster,
but the degradation of vanillin is following the same trend under these conditions compared to the
ones under conventional heating (Fig. 5 and Fig. S6 ESI).
Taking together, the catalytic results provided evidence that the reaction performed under
low frequency ultrasound is faster than that under conventional heating conditions (15 min vs. 60
min to reach the maximum yield in vanillin), more selective (50% vs. 22%) and more efficient
(19% vs. 7%). Moreover green metrics indicators such as the E Factor (EF) and the Process Mass
Intensity (PMI) clearly show the benefit of the ultrasonic-mediated oxidation reaction (Table 1).
Both the EF and PMI achieve a considerable decrease around 66% and 63% respectively. These
results highlight the efficiency of the innovative combination between the heterogeneous catalyst
(Co3O4), the sonochemical activation method (20 kHz) and the eco-friendly oxidant (H2O2) that
are simultaneously used under mild conditions (low temperature and ambient pressure).
Additionally, the real challenge here was to work in aqueous solution with the aim of developing
a more sustainable oxidation process conforming to the principles of Green Chemistry. In this
regard, we previously mentioned that only a few literature articles have described the catalytic
selective oxidation of vanillyl alcohol using water as solvent, in particular when H2O2 was used
as O-donor, underlining the fact that further developments are still needed in this major field.
In summary, our results show that the ultrasound irradiation can improve reaction yields
while decreasing reaction times, especially in water solution in the presence of H2O2 and a
heterogeneous catalyst compared to a conventional heating activation. Most importantly, the
evaluation of the energy consumption confirmed that the proposed ultrasound-based catalytic
system was eight times less energy consuming than that carried out under silent conditions (36 kJ
vs. 288 kJ). Hence, such sonochemical-assisted heterogeneously catalyzed reaction proved to be a
cost-effective oxidation process that should receive increasing level of interest in the future.

(a)
(b)
Fig. 7. Kinetic profiles of the selective oxidation of vanillyl alcohol a) under ultrasound and b)
under unconventional heating (Experimental conditions: 2 wt% Co3O4, 2 eq. 30% H2O2, total
volume of 4 mL (water), 75 °C).
In addition to the evaluation of the catalytic performances, we also investigated the
mechanistic aspects of the reaction in order to explain the synergistic effects observed under low
frequency ultrasound. First, it is clear that the reaction kinetics is increased by the physical effect
of ultrasound, in particular through mass transfer and micro-mixing, improving the contact
between the solid catalyst, the substrate and the oxidant within the solvent (heterogeneous
system). From Fig. 7 and Fig. S5 (ESI), we can observe that the reaction pursued after the
complete consumption of H2O2 in both ultrasonic and conventional heating conditions. While it is
well known that hydrogen peroxide can undergo Fenton-like reaction in the presence of cobalt
species [34], generating HOꢀ radicals that can assist the oxidation of VA into vanillin, our study
also suggested that another oxidizing species is involved in the reaction mechanism. To assess the
existence of chemical effects through the combined use of low frequency ultrasound and the
catalyst in the presence of H2O2, the amount of hydroxyl radicals generated in the reaction
medium was quantified by a potassium iodide dosimetry method. Compared to only irradiated
water, the production of HOꢀ was increased by a factor 10 in the presence of H2O2, and by a
factor 10 as well in the presence of the catalyst in water (Table 2), confirming the increased
production of hydroxyl under ultrasound/Co3O4 conditions through a sono-Fenton-like reaction.

Table 2
–
Average formation of I3 under ultrasound (mol s-1) determined by KI dosimetry under
ultrasound.
-
Entry Time (min) Experimental conditionsa Formation rate of I3
1
2
3
4
5
6
5
15
5
H2O
2.4 x 10-6
1.1 x 10-6
1.1 x 10-5
0.8 x 10-5
3.7 x 10-5
1.3 x 10-5
H2O
H2O, H2O2 (2.6 mmol)
H2O, H2O2 (2.6 mmol)
H2O, Co3O4 (4 mg)
H2O, Co3O4 (4 mg)
15
5
15
a 0.1 mol.L-1 aqueous KI solution (4 mL), 75 °C, ultrasound (20 kHz, Pacous.vol = 0.25 W mL-1)
However, the results gathered in Table 1 (Entry 2) showed that the HOꢀ radicals formed
in-situ were not responsible alone for the selective oxidation of vanillyl alcohol into vanillin. In
addition, when the reaction was performed in acetonitrile a very low selectivity to vanillin (11%)
was obtained, reaching a rapid plateau in terms of conversion (Fig. S7 ESI). Because in this case,
no additional radical species were formed to pursue the reaction, no vanillin overoxidation was
observed in acetonitrile after complete consumption of hydrogen peroxide, due to the same
reasons.
Ali et al. investigated the selective oxidation of a lignin model compound through H2O2
and a cobalt titanate in the presence of various solvents [15]. In terms of mechanisms, they
proposed the formation of a hydroperoxyl group in interaction with the titanium within the cobalt
titanate catalyst. Among the literature mechanisms describing the decomposition of H2O2 through
oxygen vacancies on the surface of transition metal oxides, the authors mentioned that a bond
was formed between the catalyst oxygen surface vacancies and the oxygen stemming from H2O2
–
ꢀ
(Vsurf−O) [35,36]. Other authors reported the formation of a superoxide O2 radical species [37]
or suggested the formation of hydroperoxide, hydroxide or oxy species chemisorbed on the
surface of cobalt based catalysts [23,38,39]. In our case, we evidenced by XPS analyses of (i) the
catalyst itself, (ii) the catalyst reacting with H2O2 under conventional heating, and (iii) the
catalyst reacting with H2O2 under sonochemical activation, that the O1s component at around
530.8–531 eV contribution increased in both cases (Fig. 8). Those values of binding energies may
be assigned to a variety of oxygen species such as adsorbed O– or OH-like species, while peaks at

higher binding energies, ranging from 531.4 to 532 eV, could be assigned to subsurface O–
species [40]. Peaks located at lower binding energies, in our case ranging from 329.6 to 530 eV,
correspond to lattice oxygen. The ratio between oxygen surface species and lattice oxygen
(Osurface/Olattice) was shown to increase from 0.28 for the parent Co3O4 catalyst to 0.36 for Co3O4
after reaction with H2O2 under conventional heating and to 0.53 for Co3O4 that reacted with H2O2
under ultrasonic conditions.
(c)
(b)
(a)
Fig. 8. O1s X-ray photoelectron spectroscopy (XPS) of (a) Co3O4, (b) Co3O4 after treatment with
H2O2 under conventional heating and (c) Co3O4 after treatment with H2O2 under ultrasound.
This result suggests that such surface species might be the oxidizing species involved in
the selective VA oxidation reaction mechanism. Further investigations are currently being
conducted in our lab to better understand this mechanism.
3.2.3. Catalyst poisoning
At the end of both US- and CH-assisted vanillyl alcohol oxidations, a higher mass of
catalyst was collected than the one engaged for the reactions, suggesting the presence of a
carbonaceous deposit. TGA analyses of the recovered catalyst (by filtration) indicated a mass loss
of about 30 wt%, along with the initial mass loss corresponding to physisorbed water,
independently of the activation method used for the VA oxidation reaction (Fig. S8 ESI).
Scanning electron microscopy mapping images of both Co3O4 samples after reaction were then

carried out with the aim of characterizing the catalyst environment. The comparison of Co, O,
and C EDS mapping of the used catalyst after the ultrasonic reaction revealed the presence of
carbon species admixed with the catalyst particles (Fig. 9), thereby confirming that they were
contaminated with a carbonaceous deposit during the oxidation reaction. The same behavior was
observed for the Co3O4 catalyst after the reaction performed under conventional heating (Fig. S9
ESI).
Fig. 9. EDS mapping of the recovered Co3O4 catalyst at the end of the ultrasound-assisted
reaction (Experimental conditions: 2 wt% Co3O4, 2 eq. 30% H2O2, 20 kHz, total volume of 4 mL
(water), 75 °C).
CHNS elemental analysis of the post-reaction catalyst under CH activation indicated a
total carbon + hydrogen content of about 25 wt%, in the same order as the mass loss determined
by TGA. In an attempt to remove the admixed carbonaceous species, the used catalyst was
further calcined at 300 °C under air for 2 h. However, a significant residual carbon + hydrogen
content (∼ 11 wt%) was still observed, suggesting that a higher calcination temperature is
required to eliminate this deposit.
GC-MS and MALDI-TOF analyses were further carried out, in order to identify the nature
of the carbonaceous deposit admixed with the Co3O4 catalyst. The catalyst was washed with
dichloromethane and first analyzed by GC-MS. Such analyses indicated the presence of traces of
vanillyl alcohol and vanillin (about 25 ppm). The analysis by MALDI-TOF of the

dichloromethane washed catalyst solution (Fig. S10 ESI) pointed out the presence of relatively
high molecular weight compounds adsorbed at the surface of the catalyst. According to these
analyses and to literature data, it is suggested that uncontrolled oligomerization of aromatics
compounds occurred in such oxidative reaction medium [41]. For instance, the Dakin oxidation is
a well-known reaction, in which an aromatic aldehyde such as vanillin, gets oxidized by H2O2 to
the corresponding hydroquinone [42], which can easily undergo further reactions through
polymerization. The oxidation products can rapidly form larger colored oligomers through further
oxidation reactions in water [41]. Despite the fact that we did not observe hydroquinone in our
reaction medium analyses, we did confirm the presence of high molecular weight compounds
through MALDI-TOF. Such adsorbed oligomers onto the surface of the catalyst contribute to its
poisoning, thereby hampering its recyclability. All these data demonstrate the complexity of the
heterogeneous oxidation reaction carried out with H2O as solvent and hydrogen peroxide as the
primary oxidant. The operational parameters (such as the nature of the solid catalyst, the mode of
addition of H2O2, sustainable biphasic systems) deserve careful attention, so as to increase the
yield to the desired product and to avoid undesirable oligomerization side reactions. Further
researches are currently being undertaken to this end in our lab in order to limit these recycling
issues, while increasing the catalytic efficiency.
4. Conclusions
In this paper, the challenge was to perform the catalytic oxidation of vanillyl alcohol to
vanillin under milder reaction conditions than those generally used in the literature. Working
under mild conditions (hydrogen peroxide, low temperature, ambient pressure) while promoting
the mass transfer in a heterogeneously catalyzed system, requires the employment of specific
activation methods, such as ultrasound irradiation. Ultrasound generated in aqueous medium
indeed favors the production of hydroxyl radicals through in situ formation of H2O2 (chemical
effects), as well as the increase of mass transfer between the Co3O4 catalyst and the organic
substrate (physical effects). Such sono-Fenton-like mediated selective oxidation proved to be
more efficient than the reaction carried out under conventional heating owing to synergistic
effects between the ultrasound, H2O2 and the solid catalyst. A vanillyl alcohol conversion of 38%
was reached after only 15 min of reaction under US conditions with a selectivity to vanillin of
50%. Even if further improvements are still needed, the novelty of this work relies on the

coupling of a non-noble metal based heterogeneous catalyst with a sonochemical activation
method for the oxidation of a lignin model compound under more eco-friendly conditions,
especially when the challenging use of water as solvent is concerned. The ultrasound-assisted
H2O2 catalytic oxidation of vanillyl alcohol into vanillin in water proved to be faster (4x), more
selective (2.3x) and more efficient (2.7x) than the corresponding reaction under silent conditions.
Additionally, a far decrease of the energy consumption of the reaction was observed under
ultrasound (36 kJ vs. 288 kJ). This preliminary work opens the door for further investigations in
heterogeneously catalyzed sono-Fenton-like-assisted reactions in order to develop greener
selective oxidation processes in the future.
Acknowledgements
The authors gratefully acknowledge the CAPES Foundation (Brazilian Ministry of Education) for
the awarding of the PhD scholarship (13342-13-4) to Ronan Behling through the Science without
Borders program. The authors are also very much indebted to the technical assistance from the
research support services of LNEC and IC2MP (University of Poitiers, France), in particular to E.
Beré, S. Pronier, S. Arrii-Clacens, C. Canaff and M. Tarighi for recording the SEM, TEM, XRD,
XPS and MALDI-TOF analyses, respectively.
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HIGHLIGHTS
Sonochemical selective oxidation of vanillyl alcohol in the presence of a cobalt oxide
catalyst under mild conditions
Ronan Behling, Gregory Chatel* and Sabine Valange*
1) Catalytic selective oxidation of vanillyl alcohol into vanillin under mild conditions.
2) Coupling of a solid catalyst with low frequency ultrasonic irradiation in aqueous
medium.
3) Nanostructured spinel Co3O4 prepared and characterized before and after reaction.
4) Reaction performed at ambient pressure, low temperature, with H2O2 as oxidant.
5) US-H2O2 process faster and more selective than that performed under silent conditions.