Catalytic conversion of lignin: Studies on oxidation of lignin model phenolic monomer over CoMCM-41 and CoMCM-48 catalysts

Article abstract of DOI:10.14233/ajchem.2018.21098

The present study focussed on investigation of the potentiality of cobalt containing mesoporous material (M41S) as a robust heterogeneous catalyst for the oxidation of lignin model phenolic compound. Cobalt containing MCM-41 and MCM-48 were prepared under hydrothermal condition and characterized by various spectroscopic and analytical techniques. Formation of well ordered hexagonal and cubic mesopore structures of MCMs, containing cobalt in the silicate framework was inferred from XRD, TG-DTA and N2 adsorption-desorption studies. DR UV-visible spectral analysis revealed existence of Co(II) and Co(III) in the tetrahedral framework. The oxidative ability of CoMCMs were studied for the lignin model phenolic monomer, 1-(4-hydroxy-3-methoxyphenoxy)-ethanol, under mild conditions using environmentally benign H2O2 as the oxidant. The catalytic results showed that under optimum reaction conditions, apocynol undergoes selective oxidation yielding 2-methoxybenzoquinone and acetovanillone. CoMCM-41 was found to be more selective towards acetovanillone, on the contrary, CoMCM-48 was better catalyst for 2-methoxybenzoquinone yield.

Full text of DOI:10.14233/ajchem.2018.21098

Asian Journal of Chemistry; Vol. 30, No. 2 (2018), 434-442
ASIAN JOURNAL OF CHEMISTRY
https://doi.org/10.14233/ajchem.2018.21098
Catalytic Conversion of Lignin: Studies on Oxidation of Lignin Model
Phenolic Monomer over CoMCM-41 and CoMCM-48 Catalysts
*
R. SADUAL, S. SAHOO and S.K. BADAMALI
Department of Chemistry, Utkal University, Vani Vihar, Bhubaneswar-751 004, India
*Corresponding author: Tel/Fax: +91 674 2567734, E-mail: skbuche@utkaluniversity.ac.in
Received: 14 October 2017;
Accepted: 21 November 2017;
Published online: 31 December 2017;
AJC-18719
The present study focussed on investigation of the potentiality of cobalt containing mesoporous material (M41S) as a robust heterogeneous
catalyst for the oxidation of lignin model phenolic compound. Cobalt containing MCM-41 and MCM-48 were prepared under hydrothermal
condition and characterized by various spectroscopic and analytical techniques. Formation of well ordered hexagonal and cubic mesopore
structures of MCMs, containing cobalt in the silicate framework was inferred from XRD, TG-DTA and N2 adsorption-desorption studies.
DR UV-visible spectral analysis revealed existence of Co(II) and Co(III) in the tetrahedral framework. The oxidative ability of CoMCMs
were studied for the lignin model phenolic monomer, 1-(4-hydroxy-3-methoxyphenoxy)-ethanol, under mild conditions using
environmentally benign H2O2 as the oxidant. The catalytic results showed that under optimum reaction conditions, apocynol undergoes
selective oxidation yielding 2-methoxybenzoquinone and acetovanillone. CoMCM-41 was found to be more selective towards acetovanillone,
on the contrary, CoMCM-48 was better catalyst for 2-methoxybenzoquinone yield.
Keywords: Lignin, Apocynol, Mesoporous, Methoxybenzoquinine, Acetovanillone.
quinones were selectively (about 80 %) obtained over a reaction
INTRODUCTION
period of 2 h. Similarly, 2-methoxybenzoquinone was obtained
Lignin is an integral component of plants and is most
abundant aromatic biopolymer in earth which can serve as a
renewable resource of chemical feedstock. It is suggested that
catalytic oxidation of lignin or lignin derived fragments is
interesting as it is expected to yield functionalized chemicals
which can be employed in fine chemical synthesis [1]. In this
subject, several reports have been appeared in literature largely
focused on catalytic reactions of lignin model compounds
which could provide useful information for the development
of selective and high yield processes of lignin oxidation namely
the conversion of lignin to aromatic monomers catalyzed by
triflic acid [2], depolymerization of oxidized lignin to aromatics
by formic acid [3], oxidation of phenolic and non-phenolic
compounds in presence of metal chlorides in ionic liquid
medium [4] are some of the significant achievements in the
area of lignin valorization. Among the lignin model phenolic
monomers, apocynol, 1-(4-hydroxy-3-methoxyphenoxy)-
ethanol is commonly studied over many catalysts. Recently,
much attention is focussed on the catalytic oxidation of lignin
derived fragments to fine chemicals. Wozniak et al. [5] have
studied the liquid phase oxidation of apocynol over potassium
nitrosodisulfonate (Fremy’s salt) and corresponding benzo-
as the common product from the catalytic oxidation of vanillyl
alcohol, resembling a guaiacyl unit of lignin with maximum
selectivity of 50 % by using Co(salen) catalysts and oxygen
as oxidant in liquid phase [6]. On the other hand, Canevali
and co-workers [7] have successfully carried out the liquid
phase oxidation of apocynol in presence of Co(salen) catalyst
under high pressure and 2-methoxybenzoquinone was selec-
tively obtained. The disadvantages of the reported results are
the choice of chlorinated solvents, reaction duration, use of
auxiliary bases and apply of high pressure, above all separation
of the catalyst remains the challenge. Crestini et al. [8] have
attempted to overcome these difficulties by performing the
reactions over metalloporphyrin supported on montmorillonite
catalysts, with the objective of developing a truly heterogeneous
catalyst with improved stability. The major difficulty with the
study was the reaction being non-selective. In order to address
these issues previously we have demonstrated that lignin model
phenolic compounds can selectively be transformed to useful
chemicals [9,10] over mesoporous SBA-15 heterogeneous
catalyst systems.
Over the period, MCMs have emerged as selective hetero-
geneous catalytic materials having tremendous potential in

Vol. 30, No. 2 (2018)
Studies on Oxidation of Lignin Model Phenolic Monomer over CoMCM-41 and CoMCM-48 Catalysts 435
the field of catalysis. Two of the stable members of this family,
MCM-41 and MCM-48 have been studied well in the field of
catalysis. MCM-48 with its three dimensional pore system
has certain advantages for catalytic applications [11-13], as it
could allow faster diffusion through the three dimensional
channels and make this material less resistant to pore blocking
[14]. Similarly, MCM-41 has found to be most studied meso-
porous material over last two decades as versatile shape
selective catalyst, apart from being an excellent support and
adsorbent material [15,16]. In particular, the unidimensional
tuneable regular pore channel (2-10 nm), large surface area
(~ 1000 m2 g-1), good thermal stability as well as a high density
of surface silanol (hydroxyl) groups (~ 30-40 %), made this
mesoporous material as a potential catalytic system involving
bulky organic substrate [17,18]. It has been demonstrated that
CoMCM-41 and CoMCM-48 shows a good catalytic ability
for oxidation of variety of organic substrates including Fischer-
Tropsch synthesis [19-21].
We have attempted to screen the oxidative ability of either
purely siliceous [22] or cobalt modified mesoporous solids
[23] toward lignin model phenolic compounds. With an aim to
extend the application of these catalysts for renewable biomass
utilization, we have undertaken a study to explore and assess
the potential of CoMCM-41 and CoMCM-48 towards selective
oxidation of lignin based model molecule.A series of CoMCMs
with varying silicon-to-cobalt (molar) ratios by the hydrothermal
synthesis route and systematically characterized by using various
analytical and spectroscopic techniques. The effect of cobalt
content on the textural properties of MCMs has also been inves-
tigated. The oxidative ability and stability of the catalyst has
been evaluated by performing the liquid phase oxidation of
apocynol, an important lignin model phenolic monomer using
a clean and environment tolerant oxidant, hydrogen peroxide,
under mild reaction conditions.
as-synthesized samples were calcined at 823 K for 8 h in flow
of nitrogen followed by 2 h in air.
Preparation of CoMCM-48 mesoporous molecular sieve:
A series of CoMCM-48 samples were prepared by hydrothermal
synthesis route in a basic medium with cetyltrimethylammonium
bromide (CTAB) as the surfactant, tetraethyl orthosilicate (TEOS)
as a source of silica and cobalt chloride as cobalt precursors as
per the following procedure. First, cobalt containing surfactant
solution was prepared by the addition of a solution of cobalt
chloride in water to the aqueous solution of CTAB and stirred for
30 min. In another beaker a mixture containing sodium hydroxide
and tetraethyl orthosilicate was prepared and stirred for 5 min.
This mixture was then added to the surfactant solution, imme-
diately a blue coloured gel was obtained. The resulting gel was
then transferred to aTeflon-lined stainless steel autoclave and aged
at 373 K for 72 h. The solid product was recovered by filtration
and repeatedly washed with deionized water, followed by drying
at 343 K overnight. The synthesized gel had a chemical (molar)
composition of 1.0CTAB: 1.6TEOS: 0.8NaOH: 97H2O: (0.0125-
0.05)CoCl2·6H2O.The resulting sample is designated as as-synthe-
sized CoMCM-48. The as-synthesized samples were calcined at
823 K for 3 h in a flow of nitrogen followed by 3 h in air.
Preparation of apocynol: Apocynol was synthesized as
per the procedure outlined elsewhere [24] and characterized
by melting point, 1H, 13C NMR, GC and GC-MS studies.
Characterization of the catalysts: The synthesized mate-
rials were characterized by various spectroscopic and analytical
techniques. Powder X-ray diffraction (XRD) patterns were
recorded using Brucker D8 diffractometer with CuKα radiation.
Diffuse reflectance ultraviolet and visible (DRUV-vis) spectra
were recorded in the range 200-800 nm on Shimadzu 2450 using
BaSO4 as the reference. Fourier transform infrared (FT-IR) spectra
were recorded in a Perkin-Elmer Paragon-500 spectrometer
in KBr medium. The cobalt content in the synthesized sample
was measured by atomic absorption spectroscopy (AAS, Shimadzu
AA-6300). BET-surface area and pore size were measured by
using ASAP 2020, Micromeritics analytical system.
EXPERIMENTAL
Preparation of CoMCM-41 mesoporous molecular
sieve: A series of CoMCM-41 mesoporous molecular sieves
were synthesized by hydrothermal method in a basic medium
with the molar ratio of 1.0CTAB: 2.2Na2SiO3: 6.4EtOH: 84·4H2O:
xCoCl2·6H2O, where x is 0.022, 0.036 and 0.075, respectively.
In a typical synthesis; cetyltrimethylammonium bromide (CTAB)
was used as the surfactant, sodium silicate as a source of silica
and cobalt chloride as cobalt precursor. First cobalt containing
surfactant solution was prepared by the addition of a solution
of cobalt chloride in water to the aqueous solution of CTAB
and stirred for 30 min. To this mixture ethanol was added and
stirred for another 30 min. In another beaker, an aqueous solu-
tion of sodium silicate was prepared and stirred for 30 min.
This mixture was then added to the surfactant solution, which
resulted in a blue coloured gel. The resulting gel solution
was then stirred for another 30 min and then transferred to a
Teflon-lined stainless steel autoclave and aged at 373 K for
72 h. The solid product was recovered by filtration and washed
repeatedly with deionized water, followed by drying at 343 K
overnight. The resulting sample was designated as as-
synthesized CoMCM-41(x), where x stands for the Si/Co molar
ratio in synthesis gel from 100, 60 and 30 respectively. The
Catalytic activity studies: In a typical catalytic reaction,
apocynol (1.0 mmol, 168 mg), acetonitrile (5 mL), the catalyst
(100 mg) and 35 % aqueous H2O2 (3.0 mmol, 0.4 mL) were
placed in a round bottomed flask and allowed to react with
varying reaction temperature and duration. Prior to the recycling
experiments the spent catalyst was activated by heating at 773
K for 6 h in flowing air. Recycling tests were conducted for
five times following typical reaction conditions.
GC analyses were carried out on aVarian-450 instrument,
equipped with aVF-1ms column (15 m × 0.25 mm × 0.39 mm
film thickness) and flame ionization detector (FID) using
nitrogen as the carrier gas. The initial column injector was set
to 473 K with an initial column temperature of 333 K, raised
to 423 K with a ramp rate of 288 K min-1 and then to 533 K at
283 K min-1. Substrate conversion and product yield were
determined using an external standard method, according to
the method reported earlier [24].
RESULTS AND DISCUSSION
Catalyst characterization: The colour of the as-synthe-
sized CoMCM-48 as well as CoMCM-41was light pink in

436 Sadual et al.
Asian J. Chem.
colour, however, after calcination the sample turned to pale
blackish colour in case of CoMCM-48 and blue colour in case
of CoMCM-41. Usually such a change in colour is correlated
with variation in the oxidation state and coordination of cobalt.
Blue colour of the CoMCM-41 sample was assigned to the
tetrahedral Co(II) [25] and pale blackish is assigned to the
mixture of both Co(II) and Co(III) [26], means the possible
formation of mixed spinel oxide Co3O4. The cobalt content in
the calcined CoMCM-48 material was found to be 1.14-3.63
µmol per gram of catalyst and that for CoMCM-41 was 0.9-
3.13 for various silicon-to-cobalt molar ratio in the gel.
Powder X-ray diffraction: The low angle XRD patterns
of the calcined CoMCM-48 and CoMCM-41 are shown in
Figs. 1 and 2. In case of siliceous MCM-48 sample (Fig. 1a),
strong reflections observed in between 2θ = 2 to 3° in addition
to several weak reflection in the range of 2θ = 3 to 7° indicates
the formation of ordered mesoporous structure [27]. Similar
patterns in the small angle XRD patterns were noticed after
incorporation with cobalt (Fig. 1b), demonstrating the preser-
vation of the cubic mesoporous structure of the parent material.
Similarly, it has been shown from the Fig. 2a that, MCM-41
exhibited a high-intensity reflection related to the (100) plane
at 2θ = 2.1° and have small and weak diffraction peaks at 2θ
value ranging from 3.4 to 6° corresponding for the (110), (200),
(210) planes respectively, which is consistent with the charac-
teristic diffraction patterns of the hexagonal mesoporous mole-
cular sieve [28]. This indicates the formation of a highly ordered
structure of a hexagonal pore arrays. In cobalt incorporated
MCM-41 Fig. 2b, four reflection peaks, characteristic of the
hexagonal mesoporous structure, were clearly seen although
the intensity is less when compared with pure MCM-41. In
both cases it has been observed that the unit cell parameter
(ao) of cobalt modified samples was shifted to higher values
as compare to that of the pure siliceous samples. This could
be mainly due to the large atomic size of Co2+ (0.72 Å) as
compared with Si4+ (atomic radius of 0.40 Å), leading to a
longer Co-O distance. Moreover, these observations suggest
the successful incorporation of cobalt into the silica framework
[29]. The hexagonal unit cell parameter (ao), calculated by the
formula a0 = 2d100/√3 and the cubic unit cell parameter (ao),
2000
1500
1000
500
0
1200
(a)
(b)
1000
800
600
400
200
0
1.8
2.8
3.8
4.8
(°)
5.8
6.8
7.8
1.8
2.8
3.8
4.8
5.8
6.8
7.8
2
θ
2θ
(°)
Fig. 1. XRD patterns of calcined MCM-48 (a) and CoMCM-48 (b)
(a)
(b)
1.75
2.75
3.75
4.75
(°)
5.75
6.75
7.75
1.75
2.75
3.75
4.75
5.75
6.75
7.75
2
θ
2θ (°)
Fig. 2. XRD patterns of calcined MCM-41 (a) and CoMCM-41(b)

Vol. 30, No. 2 (2018)
Studies on Oxidation of Lignin Model Phenolic Monomer over CoMCM-41 and CoMCM-48 Catalysts 437
calculated by the formula a0 = 2d211/√6 and d-spacing calculated
by the formula 2dsin θ = nλ are listed in Table-1. The wide-
angle XRD patterns of the CoMCM-48 and CoMCM-41 samples
are shown in Figs. 3 and 4, respectively. A broad diffraction
peak between 57-60° has appeared in case of CoMCM-48,
which corresponds to (511) reflection of nano-crystallite Co3O4
(according to card no. JCPDS 78-1970) highly dispersed in
the molecular sieve frame work of MCM-48. However, absence
of peaks corresponding to oxides of cobalt in case of CoMCM-
41 indicates either the crystalline size of metal oxides is below
the lower limit for XRD detectability or an amorphous metal
oxide highly dispersed in the molecular sieve framework of
MCM-41 is formed [30].
(a)
(b)
10
20
30
40
50
60
TABLE-1
TEXTURAL PROPERTIES OF CoMCM-48 AND CoMCM-41
2θ (°)
Fig. 4. XRD patterns in higher 2θ range of calcined MCM-41 (a) and
Surface area
(m2 g-1)
Pore volume
(cm3 g-1)
Pore diameter
(nm)
Material
MCM-48
CoMCM-48
MCM-41
CoMCM-41 (b)
978
1014
1339
860
0.50
0.70
0.73
0.50
2.60
3.30
2.35
2.47
N2 adsorption-desorption studies: Fig. 5(a,b) shows the
nitrogen adsorption-desorption results of CoMCM-48(a) and
CoMCM-41(b) respectively. And the pore size distribution
curves were obtained from desorption branch using BJH
method and are shown inset. Textural properties of both the
samples are listed in Table-1. Both the samples show type IV
adsorption isotherms (IUPAC) belonging to a typical meso-
porous materials [31-33] with a sharp inflection between the
relative pressure (0.1 < p/po< 0.35), which is due to the capillary
condensation of nitrogen within the mesopores [34]. The sharp-
ness of this step reflects the uniform pore size in these materials
[35].As shown in the figure the isotherms of CoMCM-48 also
contain a H3 hysteresis loop at relative pressure of above 0.8,
indicating a partial loss in the ordering of the pore channel. It
has been observed in both cases that there is an increment of
pore size after incorporation of cobalt into the siliceous material
(not reproduced here), suggesting the possible distortion of
the silica framework. This may be due to the fact that Co-O
bond length is longer than that of Si-O, presumably which
leads to an increase of the pore size.
CoMCM-41
Co3O4
10
20
30
40
(°)
50
60
70
Diffuse Reflectance UV-visible spectroscopy (DRUV-
vis): The diffuse reflectance UV-visible spectra, recorded at
room temperature are used for the identification of metal ion
2
θ
Fig. 3. XRD patterns in higher 2θ range of calcined CoMCM-48 (32) (blue)
and CoMCM-48 (64) (pink)
500
350
(a)
(b)
300
400
250
0.05
0.06
0.05
0.04
0.03
0.02
0.01
0
300
200
100
0.04
200
0.03
0.02
150
100
50
0.01
0
10 20 30 40 50 60 70 80
Pore diameter (Å)
15 20 25 30 35 40 45 50
Pore diameter (Å)
0
0.2
0.4
0.6
0.8
1.0
0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p0)
Relative pressure (p/p0)
Fig. 5. N2 adsorption-desorption isotherms and pore size distributions curve (inset) of calcined CoMCM-48 (a) and CoMCM-41(b)

438 Sadual et al.
Asian J. Chem.
existence in the framework of metal-containing mesoporous
materials. Figs. 6 and 7 depicts the DRUV-visible spectra of
the calcined CoMCM-48 and CoMCM-41, respectively. For a
comparison, the spectrum of siliceous materials is also included.
The appearance of bands in between 300-800 nm in case of
cobalt containing materials, against a little absorbance noticed
for siliceous material in this region suggested the fact that
incorporation of cobalt in the silica structure. Both the calcined
cobalt containing samples exhibit absorption band in the region
of 220-285 nm that can be assigned to charge transfer transitions
from oxygen to Co(II) ion [36], possibly Co(II) ions are present
in a tetrahedral coordination. Furthermore, the observed bands
located in the 500-650 nm region (shown in inset) in case of
both the calcined samples correspond to the d-d transitions of
Co(II) ions [37] in tetrahedral geometry. Upon calcination a part
of Co(II) has been oxidized to Co(III), which shows a broad
band at 465 nm attributed to octahedral coordination of Co(III),
in case of cobalt containing MCM-48 material. Due to the
appearance of these bands, formation of mixed oxide of cobalt,
Co3O4 cannot be excluded. However no such evidence was
noticed in CoMCM-41 materials, indicate the presence of Co(II)
predominantly in tetrahedral environment in the matrix.
Fourier transform infrared (FT-IR) spectroscopy: FT-
IR spectra of both calcined CoMCM-48 and CoMCM-41
samples with different ratio of silicon to cobalt along with
siliceous samples are shown in Figs. 8 and 9 respectively. The
characteristic band for surfactant template, CTAB, at 2928,
2854 and 1490 cm-1 [38] disappeared in the calcined spectra.
However other characteristic peaks for the corresponding mate-
rials are found to present. It indicates the mesoporous structure
of the samples remains unchanged after calcination. A broad
band observed between 3800-3000 cm-1 can be assigned to
the O-H stretching vibrations of the surface silanols along with
an intense band at around 1645-1625 cm-1 from vibration of
adsorbed water molecules. The intensity of these peaks in case
of calcined spectra is a clear evidence of the hydrophilic nature
of the calcined materials. Absorption bands noticed at 1082,
808 and 460 cm-1 are attributed to the asymmetric, symmetric
stretching and bending vibration for Si–O–Si bonds [39]
respectively. These bands are found to be present both in the
parent siliceous as well as in all cobalt incorporated samples,
which indicate that silica framework was not affected after
modification, which correlates with the XRD results. Further-
more an additional band at around 965 cm-1 noticed in all the
(d)
(c)
(b)
(a)
400
500
600
700
Wavelength (nm)
(d)
(c)
(b)
(a)
4000
3500
3000
2500
2000
1500
1000
500
200
300
400
500
Wavelength (nm)
600
700
800
Wavenumber (cm–1)
Fig. 6. DRUV-visible absorption spectra of calcined: (a) MCM-48; (b)
CoMCM-48 (128); (c) CoMCM-48 (64) and (d) CoMCM-48 (32)
Fig. 8. FT-IR spectra of calcined: (a) MCM-48; (b) CoMCM-48 (32); (c)
CoMCM-48 (64) and (d) CoMCM-48(128)
(c)
(b)
(a)
500
600
700
Wavelength (nm)
(d)
(c)
(b)
(a)
4000
3500
3000
2500
2000
1500
1000
500
200
300
400
500
Wavelength (nm)
600
700
800
Wavenumber (cm–1)
Fig. 7. DRUV-visible absorption spectra of calcined: (a) MCM-41; (b)
CoMCM-41 (100); (c) CoMCM-41(60) and (d) CoMCM-41 (30)
Fig. 9. FT-IR spectra of calcined: (a) MCM-41; (b) CoMCM-41 (100) and
(c) CoMCM-41 (60)

Vol. 30, No. 2 (2018)
Studies on Oxidation of Lignin Model Phenolic Monomer over CoMCM-41 and CoMCM-48 Catalysts 439
TABLE-3
spectra of siliceous as well as in all cobalt incorporated samples
can be assigned to the stretching vibration of Si-O-Si and Si-O-
Co bond [40]. Interestingly, it has been observed that ν(O-H)
(Si-OH) intensity gradually increased with an increase in cobalt
content in case of CoMCM-41 (Fig. 9) sample and such a trend
is attributed to the possible prevention in the condensation
of silanol groups by the insertion of cobalt. However, ν(O-H)
(Si-OH) intensity was not affected as a function of cobalt
content in case of CoMCM-48 sample (Fig. 8).
Catalytic activity study: Catalytic activity of CoMCM-48
and CoMCM-41 was evaluated by investigating the oxidation
of apocynol, 1-(4-hydroxy-3-methoxyphenoxy)ethanol (1)
using H2O2 as the oxidant under varying reaction conditions,
Scheme-I and the results are summarized in Tables 2 and 3.
In order to understand the role of siliceous matrix alone, the
reaction was also studied over the siliceous MCM-48 and
MCM-41.
The results showed that upon oxidation apocynol pro-
duced both 2-methoxybenzoquinone (2) and acetovanillone
(3). It is worth mentioning that, benzoquinones are found
to be a key intermediates in various organic reactions [41]
and acetovanillone is the precursor for the production of fine
chemicals such as acetoveratron, veratric acid and veratric acid
chloride [42]. Reaction was studied over 15-180 min duration
REACTION PROFILE OF APOCYNOL OVER CoMCM-48
Product distri-
bution (%)
S.
No.
Duration Conversion
(min)
Catalyst*
(%)
(2)
65
67
39
40
16
30
36
38
41
40
30
(3)
16
-
-
-
13
4
-
-
-
1
2
3
4
5
6
7
7
8
9
MCM-48
MCM-48
MCM-48
MCM-48
CoMCM-48 (32)
CoMCM-48 (32)
CoMCM-48 (32)
CoMCM-48 (32)
CoMCM-48 (64)
CoMCM-48 (128)
15
30
60
180
15
30
95
94
94
> 99
87
88
85
86
87
87
60
180
180
180
180
-
-
10 Co(NO3)2.6H2O
93
Reaction conditions: 1.0 mmol apocynol, 3 mmol H2O2, 5 mL
acetonitrile, 100 mg catalyst at 313 K, duration 15-180 min.
a1 in acetonitrile; b1 in acetonitrile and H2O2
*The numbers in the parentheses indicates the silicon-to-cobalt molar
ratio in the gel.
and from the observed reactivity it is well evident that apocynol
is quite stable under the reaction condition irrespective of 3 h
of heating in the solvent (Table-2, entry 1). However, in presence
of hydrogen peroxide mixture of 2-methoxybenzoquinone,
acetovanillone and high molecular weight compounds (un-
identified) were obtained (entry 6). Interestingly the reaction
proceeded in a remarkable manner by using the siliceous either
MCM-41 or MCM-48 with a substrate conversion of > 80 %
within 15 min of reaction and selective towards 2-methoxy-
benzoquinone (Table-2, entry 2). Recently, we have observed
that mesoporous silicas can catalyse the reaction yielding
selectively methoxybenzoquinone [20]. Further we have
observed that apocynol (1) is susceptible to oxidation in
presence of H2O2 alone and the reaction proceeds in an uncont-
rolled and non-selective manner resulting in several high mole-
cular weight products (not identified). The tabulated results
of the reaction catalyzed by cobalt modified materials reveal
that the reaction becomes less selective in presence of CoMCM-
48 and CoMCM-41 as catalyst in general. However, the selectivity
towards 2-methoxybenzoquinone remains almost unchanged
with varying cobalt content in both cases. Interestingly, selec-
tivity towards acetovanillone was found to be enhanced with
varying cobalt content in case of CoMCM-41. On the other
hand, over CoMCM-48, selectively (2) was obtained despite an
increase in cobalt content. Reaction carried out over CoMCM-
TABLE-2
REACTION PROFILE OF APOCYNOL OVER CoMCM-41
Product distri-
bution (%)
S.
No.
Duration Conversion
Catalyst*
(min)
(%)
(2)
-
(3)
-
1a
2
3
Blank
MCM-41
MCM-41
MCM-41
MCM-41
Absent
CoMCM-41 (30)
CoMCM-41 (30)
CoMCM-41 (30)
180
15
30
60
180
15
15
30
60
-
84
96
98
> 99
80
53
85
87
90
89
84
65
77
80
84
22
28
31
40
43
44
40
19
4
-
-
15
6
18
47
52
46
34
4
5
6b
7
8
9
10 CoMCM-41 (30)
11 CoMCM-41 (60)
12 CoMCM-41 (100)
180
180
180
Reaction conditions: 1.0 mmol apocynol, 3 mmol H2O2, 5 mL
acetonitrile, 100 mg catalyst at 313 K, duration 15-180 min.
a1 in acetonitrile; b1 in acetonitrile and oxidant.
*The numbers in the parentheses indicates the silicon-to-cobalt molar
ratio in the gel.
Catalyst, CH3CN, H2O2
15-180 min at 313 K
2-Methoxy benzoquinone (
2
)
Acetovanillone ( )
3
Apocynol ( )
1
Scheme-I: Reaction profile of oxidation of apocynol (1) over cobalt modified catalysts

440 Sadual et al.
Asian J. Chem.
48 from 15-180 min showed similar percentage of substrate
conversion along with a marginal increase in 2-methoxybenzo-
quinone selectivity. However, using CoMCM-41 as catalyst
both apocynol conversion and selectivity of products were
increased with increase of reaction duration. To evaluate the
role of Co(II) ions alone, reactions were conducted using cobalt
nitrate in homogeneous medium and achieved > 90 % substrate
conversion with selective formation of 2-methoxybenzoquinone
(Table-3, entry 10).
Although it is premature to comment upon the exact
mechanism of the reaction over cobalt modified materials,
based on the results obtained, assuming Co(II) as an active
species for such type of product profile, we propose a plausible
pathway for the formation of the products over the catalysts.
Cobalt(II) present in the catalyst, interacts with the hydrogen
peroxide yielding hydroxyl radical [43,44]. Subsequently, the
reaction proceeds in two possible pathways, the hydroxy
radical formed either abstracts a phenolic hydrogen atom from
the phenolic substrate (1) (path-1) and produces a reactive
phenoxy radical (4) or it abstracts a proton from the benzylic
carbon (path-2), forming the reactive radical (6). By the path-1,
(4) transformed to intermediate (5), through addition of another
hydroxyl radical. Intermediate 5, undergoes rearrangement to
form 2-methoxybenzoquinone (2). On the other hand, through
path-2, intermediate 6 undergoes deprotonation via another
hydroxy radical leading to the formation of acetovanillone (3)
(Scheme-II).
41 containing silanol groups as well as Co(II) yields mixture
of products 2 and 3. Increment of OH-groups with increase in
cobalt content in CoMCM-41 can lead to proton extraction
both from benzylic carbon as well as from phenolic substrate.
As a result both the products (2 and 3) are possible. On the
other hand, 2-methoxybenzoquinone was found as the oxida-
tion product from the reaction catalyzed by CoMCM-48 having
silanol groups along with Co(II) and Co(III). And also there is
a relatively lesser effect on OH-group intensity with increase
in cobalt content in CoMCM-48.
The results obtained from the present investigation indi-
cate the fact that the reaction proceeds simultaneously both in
homogeneous and heterogeneous medium. In order to verify
the heterogeneous nature of the catalyst, (i.e. leaching of cobalt
from the matrix during reaction) oxidation reaction of apocynol
was carried out with the treated catalyst under identical condi-
tions. In a typical reaction, the catalyst CoMCM-41 was stirred
with solvent and oxidant at 323 K for 3 h and filtered. Oxidation
of 1 was then carried out both with the residue and filtrate. To
the filtrate, substrate and oxidant were added and the reaction
was allowed to run for 3 h. In a separate experiment the residue
was used as catalyst and the reaction was carried out with the
addition of substrate, solvent and oxidant. The observed results
are tabulated in Table-4. It was noticed that, conversion and
selectivity towards 2-methoxybenzoquinone are close to that
obtained from the use of cobalt salt in homogeneous medium.
This significant activity of the filtrate indicated the leaching
out of cobalt species from the catalyst into the medium. For
further confirmation, the filtrate was analyzed by AAS for the
estimation of cobalt. The leached cobalt amounting to 0.33 ppm
(for CoMCM-48) and 0.30 ppm (for CoMCM-41), confirmed
the leaching of cobalt from the catalyst during reaction. The
It is quite understandable that hydrogen peroxide can be
activated either by cobalt [20] or silanol groups [44] to generate
hydroxyl radicals which catalyzes the reaction. Presence and
involvement of multiple active sites such as oxidant, cobalt
and silanol groups makes the reaction non-selective. CoMCM-
H2O2
+
Co(II)
Co(III)–OH
+
·OH
HO
CH3
CH3
O
O
H
H2O
HO
OCH3
OCH3
OCH3
OH
HO
CH3
Path-1
O
O
O
(4)
(5)
(2)
OH
+
HO
CH3
O
CH3
OCH3
Path-2
H2O
OH
(1)
OCH3
OCH3
OH
H2O
OH
(6)
OH
(3)
Scheme-II: Probable oxidation pathway of (1) over cobalt containing catalyst

Vol. 30, No. 2 (2018)
Studies on Oxidation of Lignin Model Phenolic Monomer over CoMCM-41 and CoMCM-48 Catalysts 441
TABLE-4
Conclusion
A series of cobalt containing mesoporous catalysts is prepared
OXIDATION OF APOCYNOL OVER
TREATED CoMCM-41 AND CoMCM-48
via direct hydrothermal synthetic route and evaluated catalytic
performance of catalyst for the oxidation of an important lignin
model phenolic monomer under varying reaction conditions. The
catalytic activity results showed that under the optimum reaction
condition apocynol undergoes selective oxidation, yielding 2-
methoxybenzoquinone and acetovanillone. It was observed that
with increase in the cobalt amount there is an increment in the
acetovanillone yield in case of CoMCM-41. On the other hand,
CoMCM-48 is more selective towards 2-methoxybenzoquinone
under the present reaction conditions. Cobalt(II) present in the
catalyst is suspected toberesponsibleforsuch activity.Theintensity
of silanol groups has increased with an increase in cobalt content
in CoMCM-41. It is correlated that Co(II), H2O2 and silanol groups
are responsible for the observed activity. Combining with FT-IR
results, it is assumed that presence of more silanol groups is
responsible for the reaction to proceed following path-2. Further,
leaching of cobalt from CoMCM-41 and CoMCM-48 into the
medium during the reaction was noticed during initial catalytic
run. Subsequently, the catalysts were recycled and reused for
five times without any significant loss in activity and selectivity
demonstrating the heterogeneous nature of catalyst in subsequent
runs.
Product distribution (%)
Conversion
(%)
Entry
Catalyst*
(2)
32
40
29
40
(3)
1
2
3
4
CoMCM-41(F)
CoMCM-41(R)
CoMCM-48 (F)
CoMCM-48 (R)
90
90
90
93
-
42
-
-
Reaction conditions: 1.0 mmol apocynol, 3 mmol H2O2, 5 mL
acetonitrile, 100 mg catalyst at 313 K, duration 15-180 min.
*F-Filtrate, R-Residue
residue, containing the remaining cobalt was also active to catalyze
the reaction.
Recycling studies of both the catalyst was conducted for
five times following the similar reaction condition. The spent
catalyst was recovered from the reaction mixture by filtration
and recycled by washing with dichloromethane repeatedly
followed by activation i.e., by heating at 773 K for 6 h in flowing
air. Catalytic reactions were conducted over regenerated catalyst
and the catalysts were found to nearly constant for both substrate
conversion and product yield at least upto five runs (Figs. 10
and 11).
110
Apocynol conv.
2-Methoxybenzoquinone sel.
ACKNOWLEDGEMENTS
100
90
80
70
60
50
40
30
20
10
0
The financial support provided by Department of Science
and Technology under PURSE Scheme, Government of India
is gratefully acknowledged. The authors are thankful to Drs.
S.E. Dapurkar (Tata Chemicals Limited, Pune) andA. Sakthivel
(Delhi University) for providing instrumental facility during
catalyst characterization.
REFERENCES
1. J. Zakzeski, P.C.A. Bruijnincx, A.L. Jongerius and B.M. Weckhuysen,
Chem. Rev., 110, 3552 (2010);
https://doi.org/10.1021/cr900354u.
2. P.J. Deuss, M. Scott, F. Tran, N.J. Westwood, J.G. de Vries and K. Barta,
J. Am. Chem. Soc., 137, 7456 (2015);
1
2
3
4
5
Fig. 10. Recyclability test of CoMCM-48
https://doi.org/10.1021/jacs.5b03693.
3.
A. Rahimi, A. Ulbrich, J.J. Coon and S.S. Stahl, Nature, 515, 249 (2014);
https://doi.org/10.1038/nature13867.
130
120
110
100
90
Apocynol conversion
4. S. Jia, B.J. Cox, X. Guo, Z.C. Zhang and J.G. Ekerdt, Ind. Eng. Chem. Res.,
50, 849 (2011);
2-Methoxybenzoquinone selectivity
Acetovanillone selectivity
https://doi.org/10.1021/ie101884h.
5. J.C. Wozniak, D.R. Dimmel and E.W. Malcom, Institute of Paper Science
and Technology, Paper series No. 349, pp. 1-62 (1990).
6. D. Cedeno and J.J. Bozell, Tetrahedron Lett., 53, 2380 (2012);
https://doi.org/10.1016/j.tetlet.2012.02.093.
7. C. Canevali, M. Orlandi, L. Pardi, B. Rindone, R. Scotti, J. Sipila and
F. Morazzoni, J. Chem. Soc., Dalton Trans., 3007 (2002);
https://doi.org/10.1039/b203386k.
80
70
60
8. C. Crestini, A. Pastorini and P. Tagliatesta, J. Mol. Catal. Chem., 208, 195
(2004);
50
https://doi.org/10.1016/j.molcata.2003.07.015.
9. S.K. Badamali, R. Luque, J.H. Clark and S.W. Breeden, Catal. Commun.,
12, 993 (2011);
40
30
https://doi.org/10.1016/j.catcom.2011.02.025.
10. S.K. Badamali, R. Luque, J.H. Clark and S.W. Breeden, Catal. Commun.,
10, 1010 (2009);
20
10
https://doi.org/10.1016/j.catcom.2008.12.051.
11. K. Schumacher, M. Grün and K.K. Unger, Micropor. Mesopor. Mater.,
27, 201 (1999);
0
1st recycle 2nd recycle
3rd recycle 4th recycle
5th recycle
https://doi.org/10.1016/S1387-1811(98)00254-6.
Fig. 11. Recyclability test of CoMCM-41

442 Sadual et al.
Asian J. Chem.
12. W. Zhan, Y. Guo, Y. Wang, X. Liu, Y. Guo, Y. Wang, Z. Zhang and G.
Lu, J. Phys. Chem. B, 111, 12103 (2007);
29. S.C. Laha, P. Mukherjee, S.R. Sainkar and R. Kumar, J. Catal., 207, 213
(2002);
https://doi.org/10.1021/jp074521l.
https://doi.org/10.1006/jcat.2002.3516.
13. S. Gomez, L.J. Garces, J. Villegas, R. Ghosh, O. Giraldo and S.L. Suib,
J. Catal., 233, 60 (2005);
30. J. Panpranot, S. Kaewkun, P. Praserthdam and J.G. Goodwin Jr., Catal.
Lett., 91, 95 (2003);
https://doi.org/10.1016/j.jcat.2005.04.015.
14. K. Schumacher, P.I. Ravikovitch, A. Du Chesne, A.V. Neimark and
K.K. Unger, Langmuir, 16, 4648 (2000);
https://doi.org/10.1023/B:CATL.0000006323.17873.78.
31. P.J. Branton, P.G. Hall and K.S.W. Sing, J. Chem. Soc. Chem. Commun.,
1257 (1993);
https://doi.org/10.1021/la991595i.
https://doi.org/10.1039/c39930001257.
15. A. Corma, A. Martínez, V.M. Soria and J.B. Montón, J. Catal., 153, 25
(1995);
32. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity,
Academic Press: New York, edn 2 (1982).
https://doi.org/10.1006/jcat.1995.1104.
16. J. Rathouský, A. Zukal, O. Franke and G. Schulz-Ekloff, J. Chem. Soc.,
Faraday Trans., 91, 937 (1995);
33. J. Xu, Z. Luan, H. He, W. Zhou and L. Kevan, Chem. Mater., 10, 3690
(1998);
https://doi.org/10.1021/cm980440d.
https://doi.org/10.1039/FT9959100937.
17. A. Corma, Chem. Rev., 97, 2373 (1997);
34. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J.
Rouquerol and T. Siemieniewska, Pure Appl. Chem., 57, 603 (1985);
https://doi.org/10.1351/pac198557040603.
https://doi.org/10.1021/cr960406n.
18. D.T. On, D. Desplantier-Giscard, C. Danumah and S. Kaliaguine, Appl.
Catal. A, 222, 299 (2001);
35. U.S. Taralkar, P. Kalita, R. Kumar and P.N. Joshi, J. Appl. Catal. A, 358,
88 (2009);
https://doi.org/10.1016/S0926-860X(01)00842-0.
19. H. Li, S. Wang, F. Ling and J. Li, J. Mol. Catal. Chem., 244, 33 (2006);
https://doi.org/10.1016/j.molcata.2005.08.050.
20. S.S. Bhoware, K.R. Kamble andA.P. Singh, Catal. Lett., 133, 106 (2009);
https://doi.org/10.1007/s10562-009-0162-1.
21. S.V. Sirotin and I.F. Moskovskaya, Petrol. Chem., 49, 99 (2009);
https://doi.org/10.1134/S0965544109010186.
22. R. Sadual, S.K. Badamali, S.E. Dapurkar and R.K. Singh, World J. Nanosci.
Eng., 5, 88 (2015);
https://doi.org/10.1016/j.apcata.2009.02.001.
36. D. Kaucky, J. Dedecek and B. Wichterlova, Micropor. Mesopor. Mater.,
31, 75 (1999);
https://doi.org/10.1016/S1387-1811(99)00058-X.
37. S. Lim, D. Ciuparu, Y.H. Yang, G. Du, L.D. Pfefferle and G.L. Haller,
Micropor. Mesopor. Mater., 101, 200 (2007);
https://doi.org/10.1016/j.micromeso.2006.11.002.
38. Q. Zhao, J.Y. Chu, T.S. Jiang and H.B. Yin, Colloids Surf. A Physico-
chem. Eng. Asp., 301, 388 (2007);
https://doi.org/10.4236/wjnse.2015.53011.
https://doi.org/10.1016/j.colsurfa.2007.01.002.
23. R. Sadual, S.K. Badamali and R.K. Singh, Adv. Por. Mater., 2, 48 (2014);
https://doi.org/10.1166/apm.2014.1044.
24. S.K. Badamali, J.H. Clark and S.W. Breeden, Catal. Commun., 9, 2168
(2008);
https://doi.org/10.1016/j.catcom.2008.04.012.
25. F.A. Cotton, G. Wilkinson, C.A. Murillo and M. Bochmann, Advanced
Inorganic Chemistry, Wiley: New York, edn 6 (1999).
26. V.P. Shiralkar, C.H. Saldarriaga, J.O. Perez, A. Clearfield, M. Chen, R.G.
Anthony and J.A. Donohue, Zeolites, 9, 474 (1989);
https://doi.org/10.1016/0144-2449(89)90041-9.
27. V. Alfredsson and M.W. Anderson, Chem. Mater., 8, 1141 (1996);
https://doi.org/10.1021/cm950568k.
39. E.M. Flanigen, H. Khatami and H.A. Szymanski, eds.: E.M. Flanigen
and L.B. Sand, Molecular Sieve Zeolites,ACSAdvance Chemical Series
vol. 101, American Chemical Society, Wasington, DC, p. 201 (1971).
40. C.Y. Chen, H.X. Li and M.E. Davis, Micropor. Mater., 2, 17 (1993);
https://doi.org/10.1016/0927-6513(93)80058-3.
41. S. Patai and Z. Rappoport, The Chemistry of Quinonoid Compounds,
Wiley: New York, vol. 2 (1988).
42. H.R. Bjørsvik and L. Liguori, Org. Proc. Res. Dev., 6, 279 (2002);
https://doi.org/10.1021/op010087o.
43. R. Sheldon, eds.: R.A. Sheldon, and J.K. Kochi, Metal Catalysed Oxidation
of Organic Compound, Academic Press, New York, p. 373 (1981).
44. T. Baskaran, R. Kumaravel, J. Christopher and A. Sakthivel, RSC Adv.,
4, 11188 (2014);
28. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck,
Nature, 359, 710 (1992);
https://doi.org/10.1039/c3ra46703a.
https://doi.org/10.1038/359710a0.