Synthesis of CoFeO mixed oxides via an alginate gelation process as efficient heterogeneous catalysts for lignin depolymerization in water

Article abstract of DOI:10.1039/c8cy00576a

A catalytic oxidative fragmentation of a lignin dimer and polymer extracted from wheat straw was successfully performed under eco-friendly conditions: 10% O₂/N₂ as the oxidizing agent, water as the solvent (pH ≈ 7), and Co₃O₄, Fe₂O₃ and CoFeO mixed oxides as heterogeneous catalysts and at temperatures of T = 150 °C and 200 °C. These catalysts unexpectedly showed tunable selectivity that directly depends on the composition of the selected bimetallic nanoparticles. High selectivity for benzoic acid and alkylbenzene (above 50%) was observed over Co₅₀-Fe₅₀ at 200 °C. Under similar conditions, the conversion of wheat organosolv lignin over Co₅₀-Fe₅₀ at 150 °C for 4 h yielded up to 50 wt% of monomeric species (based on dry lignin) and up to 19% of aromatic molecules with high selectivity to aromatic aldehydes (syringaldehyde and vanillin), up to 60%. An important fraction of water-soluble oligomers, with low molecular weights, was also formed during the catalytic treatment. The oxide nanomaterials were readily separated from the residual lignin during the recyclability test. The yield and the product distribution can be tuned by choosing the oxidation parameters: temperature, reaction time, oxygen partial pressure, solvent and catalyst charges.

Full text of DOI:10.1039/c8cy00576a

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0.1039/C8CY00576A.
Volume 6 Number 1 7 January 2016 Pages 1–308
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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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Synthesis of CoFeO Mixed Oxide via Alginate Gelation Process as an Efficient
Heterogeneous Catalyst for Lignin Depolymerization in Water
a,b
c,d
a
a,e
a
a,c*
Received 00th January 20xx,
Accepted 00th January 20xx
L. Hdidou, K. Khallouk, A. Solhy, B. Manoun, A. Oukarroum, and A. Barakat
The catalytic oxidative fragmentation of dimer and lignin polymer extracted from wheat straw was successfully performed
under eco-friendly conditions: 10%O /N as oxidizing agent, water as solvent (pH ≈ 7), Co , Fe and CoFeO mixed
DOI: 10.1039/x0xx00000x
2
2
3
O
4
2 3
O
oxides as heterogeneous catalysts, and at temperatures towards: T = 150°C and 200 °C. These catalysts showed
unexpectedly tunable selectivity that directly depends on the composition of the selected bimetallic nanoparticles. High
selectivity for benzoic acid and alkybenzene (above 50%) was observed over Co50-Fe50 at 200°C . Under similar conditions
the conversion of wheat organosolv lignin over Co50-Fe50 at 150°C for 4 hrs yielded up to 50 wt% of monomeric species
www.rsc.org/
(based on dry lignin) and up to 19% of armotic molecules with high selectivety to aromtic aldehyde (Syringaldehyde and
Vanillin) up to 60%. An important fraction of water-soluble oligomers, with low molecular weights, was also formed during
the catalytic treatment. The nanomaterials oxides were readily separated from lignin residual during the recyclability test.
The yield and the product distribution can be tuned by choosing the oxidation parameters: temperature, reaction time,
oxygen partial pressure, solvent and catalyst charges.
technologies such as delignification/bleaching, effluents treatment
and aromatic compounds fabrication. Many homogeneous and
heterogeneous catalysts have been used for the oxidation of both
Introduction
Lignin, that is a major component of plants, is one of the most
abundant sources of organic carbon on the planet. It is a cross-
linked polyphenolic material, with complex, irregular and very
stable structure, and therefore difficult to convert into monomeric
products. Consequently, more than 95 wt% of the lignin production
lignin and lignin model compounds. For example, lignins have been
16
2 3
oxidized under alkaline conditions, in the presence of Pd/γ-Al O
or copper (II) and cobalt (II) salts as catalysts
17, 18
to give aromatic
aldehydes. The polyoxometalates have been used as
multifunctional catalysts in the conversion of kraft lignin into
(
estimated at over 70 million tons/year) is used for the generation
19
monomeric products or for the delignification of lignocellulosic
of heat and power, and only a small amount is converted into
chemicals of industrial interest . Nowadays, with declining fossil
sources, the lignin is recognized as a suitable renewable raw
20-23
pulps
. The aerobic oxidative cleavage of lignins was effectively
1
, 2
24-26
performed with metal/bromide catalysts in acetic acid
.
Recently, perovskite-type oxide has been proposed as
a
3,
4
5, 6
material for production of transport fuels , materials
and
heterogeneous catalyst for the alkaline oxidation of lignin to
2
, 7
chemicals . Lignin could be processed into commercial products
via either biological or thermochemical processes. Among the
chemical processes used for the valorization of this natural
polymer, oxidative degradation to useful low molecular weight
seems to be very promising. It is worth noting that the oxidation of
lignins to aromatic aldehydes such as vanillin, syringaldehyde and p-
hydroxybenzaldehyde has been much studied and is commercially
7
aromatic aldehyde . Crestini et al. studied the ability of the
hydrogen peroxide as oxidant for the lignin degradation, using
27
28
catalytic systems based on methyltrioxorhenium or porphyrins
.
Most of catalytic oxidation studies were performed in acidic or
alkaline aqueous media or in organic solvents. With respect to the
green chemistry principles, the ideal oxidation system should use
oxygen as an oxidant, stable solids as heterogeneous catalysts and
8
-10
important . Different systems and methods were used for the
lignin oxidative degradation among which we mention: air in
29,30
water as a solvent
.
7
11, 12
alkaline solution at high temperatures , nitro aromatics
electrochemical
aromatic/electrochemical oxidation . More recently, the catalytic
oxidation with oxygen and hydrogen peroxide was suggested as an
efficient method for lignin degradation in industrial pulping
,
In view of these findings, in this study we developed an effective
method for the oxidative depolymerization of lignin model
compounds, using molecular oxygen, metallic oxides as
heterogeneous catalysts, and water at pH = 7 as solvent. The
screening of Fe-Co catalysts, with outstanding textural properties,
based on lignin model depolymerization was prepared by an
original method. In this study, we report also for the first time on
the oxidative fragmentation and depolymerization of extracted
lignin polymer, performed under eco-friendly conditions (water,
1
3,
14
oxidation
and
combined
nitro
1
5
1
50 °C, 10 bar, 4 h). Organosolv lignin was oxidized with oxygen in
the presence of Co-Fe mixed oxides as heterogeneous catalysts.
Experimental section
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Starting materials
Organosolv lignin: The Organosolv lignin was extracted from wheat
3
1
straw according to a procedure described previously .The
average-molecular weights (evaluated by size-exclusion
chromatography) was 3180 g/mol for the Organosolv lignin.
Nanostructured
OXIDES
NANOPAR Ti dI CLES
Mixed ox es
Calcination
2
+
Alginate
Coba
lt@Co2+
Alginate@ FI r
e
on
OXIDES
GELLATION
Gellation (Xyrogel beads)
«
BEADS »
LIGNINES
Oxidation
Cleavage of
ether bonds
WATER
ALGINATES
Catalytic reaction
WHEAT STRAW
Fig.1: Iron-Cobalt oxide nanomaterials synthesis and oxidative catalytic depolymerisation of lignin developed in this study
analysis was performed using a multi-detection system consisting of
a pump (model 510, Waters), autosampler (U6K injector, Waters),
two Polymer Laboratories (PLgel Mixed D, 5μ) column and UV-
detector (model 920, Waters). Separation in THF was performed by
was injecting 100 μL of 0.1 % of acetylated samples into thermostatically
Lignin model compound synthesis:
-(benzyloxy)-3-methoxybenzyaldehyde
4
lignin
model
synthesized according to Fig.S1. 4g of vanillin was dissolved in 20ml controlled PLgel columns (40°C). The flow rate was 1 ml/min. Molar
3
32, 33
of pyridine at 5°C. 3 cm of benzoyl chloride was added under mass evaluation was based on elution polystyrene standards
.
3
magnetic stirring. After 30 min, 50 cm of hexane was injected to
the reaction mixture. The precipitate was faltered and crystallised Cobalt-Iron oxide preparation and characterization
from ethyl acetate and hexane, to give 4-(benzyloxy)-3-
Cobalt-iron oxides with different Co/Fe ratios were prepared via
alginate gelling method. 2wt% sodium alginate aqueous solution
was prepared under à medium stirring at room temperature for
around 3h. 60 ml of the obtained solution was added dropwise into
methoxybenzyaldehyde in 92%.
Characterization of lignin
2
+
3+
2+
Thioacidolysis: Lignin polymer was degraded by thioacidolysis to a 100 ml of homogeneous metal solution containing Co , Fe , Fe
3
2
2+
2+
estimate the nature of the aromatic units, according to reference
.
and/or a mixture of Co /Fe with a total concentration of 0.15M.
The obtained hydrogel beads were left in the metal solution
Size-exclusion chromatography of acetylated samples in THF: The overnight with a gentle stirring. The beads were then removed from
molecular-mass distribution of the initial and residual lignin has the solution, filtered and washed with distilled water three time to
been analyzed using size-exclusion chromatography (SEC). Prior to remove the adsorbed free cation in the surface of alginate beads.
2
+
2+)
chromatography, the lignin was submitted to acetylation in a Alginates@cations (Fe and Co were then dried at 40°C giving
mixture of acetic anhydride-pyridine (1:1) for 24 h at 40°C. The SEC
2
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xerogel beads. Alginates@cations xerogel beads were also
calcinated at 550°C for 4h giving nanostructured oxides (Fig.1).
Dimer and Polymer Lignin
Catalytic reaction:
Fe-Co mixed oxides
0
0
1
2
-200MPa
Extraction CH Cl2
Lignin monomers
2
-20% O /N2
2
50-200°C
or 4 hrs
Residue + Catalyst
Dioxane Extraction
GC/MS
Lignin Oligomers
Recovered
Catalyst
SEC/MALLS
Fig.2: Experimental process of Catalytic depolymerization of dimer and lignin
polymer
Results
Fig.3: X-ray patterns of the synthesized oxides: (a) Co
3
O
4
, (b) Co2.4Fe0.6
O
4
, (c)
and
Characterization of Cobalt-Iron oxides nanomaterials
Co1.8Fe1.2
O
4
, (d) Co0.8Fe1.2
O
3
/Co1.8Fe1.2
O
4
, (e) Co0.8Fe1.2
O
3
, (f) Co0.4Fe1.6O
3
(
2 3
g) α-Fe O
The composition of metallic oxides depending on Co/Fe atom ratios
in the gelling solution of alginates polymer. The coordination
The phase of the various samples were determined by powder X-ray
diffraction, using a Bruker D2 PHASER diffractometer, with the
Bragg–Brentano geometry, using CuKα radiation (λ=1.5418 Å) with
3
0
were observed by scanning electron microscopy SEM. The SEM was
coupled to an Energy Dispersive X-ray Spectroscopy EDS that give us
a chemical analysis of the materials surface.
2+
3+
2+
between Co , Fe , Fe and the alginate functional groups (OH,
COOH) provide nano-oxides nucleation and growth sites (Fig.1).
After the calcination step, the organic matter degrades releasing
the metal oxides nanoparticles. The oxide phases obtained depend
essentially on the Co/Fe atom ratios (Fig.3). Due to the thermal
0KV and 10 mA, The pattern was scanned through steps of
.010142 ° (2θ), between 10 and 80 °. The morphology and the size
2+
2+
treatment of alginate@Co and alginate@Fe xerogels, pure cubic
spinel Co O (Co ) (Fig.3-a) and hematite α-Fe O3 (Fe100) (Fig.3-g)
3
4
100
2
were produced. From XRD patterns (Fig.3-a) the peaks at 2θ values
9.0, 31.2, 36.7, 38.4, 44.8, 55.6, 59.2, 65.0 correspond to the
diffraction planes (111), (220), (311), (222), (400), (422), (511),
face-centred cubic structure
1
Catalytic depolymerisation of dimer and lignin polymer
3
Experiments were conducted in a reactor of 500 cm (PARR 5500, (440). This phase is assigned to Co
O
3
4
USA). The reactor was loaded with 50 mg of dimer and polymer belonging to Fd-3m space group. The insertion of iron ions in
lignin, 10 or 20 mg of catalyst, 30 mL of distilled water and O
(10% tetrahedral and octahedral sites allows the formation of cobalt
in N ) (100 and 200KPa). The mixture was heated for 2 or 4h at ferrite oxides Co2.4Fe0.6 (Co80-Fe20) (Fig.3-b) and Co1.8Fe1.2
Fe40) (Fig.3-c); both oxides have the same space group as for Co
Fig.2). The organic solution was dried over MgSO and evaporated When the iron concentration achieved 50% of the total solution
2
O
4
4
O (Co60-
2
1
50°C or 200°C, and then the mixture was extracted with CH
2
Cl
2
O .
3 4
(
4
under reduced pressure. The residues were dissolved in 1mL of concentration a transition zone was observed, both Co0.8Fe1.2
O
and
(Co50-Fe50) co-exist (Fig.3-d). (Fig.3-e and -f) refers to
(Co40-Fe60) and Co0.4Fe1.6 (Co20-Fe80) respectively,
3
CH Cl in the presence of internal standard GC-MS analyzed. For the Co1.8Fe1.2
O
O
4
2
2
residue (insoluble fraction in CH
2
Cl
2
)
,
water was removed and the Co0.8Fe1.2
3
O
3
solid residue was dissolved in dioxane (30mL) and filtered to these phases exhibit hexagonal structure whose main
eliminate the catalyst. The dioxane was evaporated under reduced crystallographic planes are: (012), (104), (110), (113), (024), (116),
pressure at 50°C and the residue was dissolved in dioxane (3 mL) (214) and (300). The compounds crystallize in R-3c space group. The
and freeze dried. The organic extract was dried under anhydrous large Full With at Half Maximum and low peak intensities reveal the
34
magnesium sulphate and concentrated at 40°C, using a rotating formation of nanostructured oxides
.
evaporator. The analysis of the organic phase was performed on a
Fig.4 shows the evolution of the unit volume for all samples. The
phase transition from cubic to hexagonal is well defined when
system GC/MS-QP 2010 Shimadzu, equipped with a SLB-5MS
capillary column (30 m x 0.25 mm x0.25 µm), using helium as a
carrier gas. The temperature was ramped at 10 °C/min from 80 to
3
+
Fe/(Co+Fe) = 0.5. In the case of the Co O the substitution of Co by
3
4
3
+
Fe in both octahedral and tetrahedral sites leads to a slight
increase in the lattice parameters and the unit volume since r(Fe )=
.55 > r (Co ) = 0.545 . Also the incorporation of Co in the
2
50 °C, where it was kept for 10 min.
3
+
3
+
35
2+
0
hematite lattice increases faintly the unit volume because
2
+
3+
35
r(Co )=0.65>r(Fe )=0.55 . These results could be confirmed from
the shift of the peaks to lower θ value.
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5
0.2 to 114 nm respectively. Alginate biopolymer acts as a
sacrificial bio-template. All samples were analyzed with energy
dispersive X-ray spectroscopy (EDS) to determine their elemental
composition. (Table 1) reveals the presence of Co and O elements
for Co100 nanoparticles, with increasing iron amount cobalt
percentage decrease until the pure Fe100 where only Fe and O were
detected.
Catalyst screening: Oxidative catalytic depolymerization of dimers
lignin model
Catalytic oxidation of lignin dimer was performed over different
synthesized catalysts (Co100, Co80-Fe20, Co50-Fe50, Co20-Fe80 and
Fe100). The performance of these catalysts was compared to
Pd/Al O and O without catalysts as a reference.
2
3
2
a
b
4 2 3
Fig. 4: Unit volume evolution of the as-synthesized Co3-xFexO , Co: α-Fe O
and α-Fe
2 3
O oxides
The pore structure parameters of oxides catalysts including specific
2
3
surface area (m /g) pore volume (cm /g) are shown in (Table 1). A
clear effect of Co/Fe atom ratios on SBET and pore volume was
observed. A good correlation was observed with particle size and
c
d
S
S
BET, as we know that particles with smaller size may possess larger
BET. In Table 1, it can be found that the Co50-Fe50 oxide has a
greater porosity and SBET than the Co50-Fe50 compared to other
nanomaterials oxides. Besides, the Co50-Fe50 oxide comprises a total
3
pore volume (0.045 cm /g) that is larger than that of the Fe100 (0.07
3
3
cm /g) and Co100 (0.07 cm /g). The SBET for the Co50-Fe50 was
2
2
calculated as 42.7 m /g compared to only 31.3 m /g for Co100
,
28.7
2
2
2
e
f
m /g for Co80-Fe20, 24.4 m /g for Fe100, and 18.8 m /g for Co20-Fe80
.
The increased pore volume and surface area of Co50-Fe50 oxide may
be attributed to the presence of two phases hematite and spinel
2
+
resulted by the incorporation of Co in the hematite lattice that
increased faintly the unit volume. The influence of Co/Fe atom
ratios on the morphology and the microstructure of Co-Fe mixed
oxides nanoparticles was investigated using SEM, and the images
are shown in Fig.5. Co-Fe synthesized oxides showed different
morphology and microstructure which depending on the calcination
temperature and the Co/Fe atom ratio. Spherical-like particles with
an average diameter of 39 nm were observed for Co100 (Fig.5-a).
The cubic spinel structure must give a cubic morphology owing to
the privileged growth along the axes <100>, however this shape
exhibit an isotropic evolution providing spherical particles, like this
Fig. 5: SEM images of the synthetized oxides: (a) Co
c) Co1.8Fe1.2 (c) Co0.8Fe1.2 /Co1.8Fe1.2 (d) Co0.8Fe1.2
Co0.4Fe1.6 and (f) α-Fe
3 4 4
O , (b) Co2.4Fe0.6O ,
(
O
4
,
O
3
O
4
,
3
O , (e)
O
3
2 3
O .
36
result was reported by Givanneli et al . In the case of cobalt ferrite
Fig.5-b) and (Fig.5-c) polyhedral with octahedral-like morphology
(
was formed. The nanoparticles size (NPs) is around 57.3 and 41.8
nm respectively. The formation of the octahedron shape is due to
the growth rate along the <100> directions that is faster than along
3
7, 38
<
111>, that have lowest surface energy
and (Co20-Fe80 nanoparticles have
morphology following the same shape of hematite nano-oxide
Fig.5 - f) with a size distribution in the range of 110 to 216 nm and
. From (Fig.5- e and -f)
(
Co40-Fe60
)
)
a
flake-like
(
4
|
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Table 1: Nanostructured metal oxides properties
Catalysts
DRX
SEM
Morphology
EDX
BET
2
3
-1
Phase and structure
Hexagonal Hematite
Hexagonal Hematite
Hexagonal Hematite/
Cubic Spinel
Size (nm)
91
157.5
%Co
0
2.0 ± 1.7
%Fe
Surface m /g
Pore Vol. cm g
Fe100
Flake-like
Flake-like
31.7 ± 4.0
29.1 ± 4.4
24.4
18.8
0.021
0.011
Co20-Fe80
Co -Fe
50
_
5
0
31.6
57.3
39.0
6.9 ± 1.6
24.1 ± 2.4
16.4 ± 2.6 20.7 ± 2.3
37.5 ± 2,2
42.7
28.7
31.3
0.045
0.019
0.033
Co80-Fe20
Co100
Cubic Spinel
Cubic Spinel
Octahedral-like
Spherical-like
0
Table 2: Selectivity of major products resulted from catalytic oxidation of lignin dimer over different catalysts in water (pH=7), 100MPa of O
2
Selectivity of the products (%)
HO
O
OCH3
Catalysts
0%Pd-Al
OH
1
2
O
3
28.0
38.0
31.2
2.90
27.7
17.0
7.40
14.3
7.80
17.4
29.2
4.8
37.4
47.6
50.8
27.9
0.80
2.80
0.30
1.30
0.00
0.00
0.00
0.00
22.4
19.6
2.20
82.5
1.62
2.00
0.70
2.90
29.2
18.7
0.30
0.50
0.00
1.50
0.00
0.00
8.23
3.35
36.6
4.70
33.2
32.0
41.2
54.8
0.00
Co100
0.00
0.00
3.30
0.00
0.00
0.00
0.00
Co80-Fe20
Co50-Fe50
Co80-Fe20
Fe100
Co100 without O
without cata.
2
O
2
The mixed oxide Co1.8Fe1.2O / Co . Fe1.2O3 (Co50-Fe50) and Co O4
100
4
0 8
3
(
9
1
Co100) showed the highest conversion for lignin dimer 96.9%, and
9
0
0
6.7%, respectively compared to 88.8% obtained over (Fig.6).
0%Pd/Al 3. These results indicate that the combination of
8
2
O
heterogeneous catalyst with O2 oxidant greatly improved dimer
depolymerization and functional monomers production. In order to
investigate this synergistic, the effect of Co100 in the absence of O2
70
6
0
0
5
and O
conversion is estimated to 54.9% and 52.7%, respectively for Co100
in the absence of O and O without catalyst under the same
conditions. The higher conversion obtained with Co1.8Fe1.2O4/
Co compared to 10%Pd/Al is generally considered as a
benchmark material for lignin depolymerisation. Fig.7 show the
proposed mechanism of lignin dimer depolymerization, O could be
2
without catalyst has been tested. Fig.6 shows that the
40
2
2
30
.
8
Fe1.2
O
3
2
O
3
20
0
10
2
0
the main responsible of the first ether bond R-O-R’ cleavage giving
vanillin and benzalcohol products. Nevertheless, the other
compounds are formed due to the oxidation reaction with the
presence of different catalysts.
Fig.6: Lignin dimer conversion with different catalyst
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polymer at 200°C for 4h in the presence of 10% of oxygen (Exp. 2).
The results of these six reactions experiments are shown in (Table
3
). The distribution of major products identified in GC/MS for
different reactions is reported in (Table 3) and (Fig.8). The recovery
of lignin monomers was calculated from the aromatic identified
compounds using an internal standards, as well as the mass balance
was calculated by weighing of the residue after filtration and
separation of liquid/solid (Fig.8) and (Table 3), shown that, the
lignin conversion and selectivity to different molecules strongly
depended on the reaction conditions. Syringaldehyde (13), vanillin
(
9), acetosyringone (14), syringone (6), guaïacol (5) and benzoic
acids (11), were the major products, and various alkyl phenol (3)
and alky benzene (2) was also produced (Table 3).
OH
OH
O
Furfural
C1 + C2 Alkylphenol
1
3
C2 + C3 Alkylbenzene
2
O
O
O
O
OH
Fig.7
:
Proposed catalytic depolymerization and oxidation mechanism of
H3CO
H3CO
OCH3
OCH3
OH
OH
Phenol
OH
OH
Acetophenone
lignin dimer.
OH
Syringone
Vanillin
Guaiacol
8
4
OH-hydroxybenzaldehyde
9
5
6
7
Re-polymerization and self-condensation ability of reactive radicals
formed during lignin oxidation are one of the main faced issues
O
O
OH
O
O
3
9
HO
O
HO
O
.
However, the catalysts basic sites may inhibit the re-condensation
3
9
H3CO
H3CO
H3CO
OCH3 H3CO
OCH3
of this phenolic intermediates . The main products produced via
dimer lignin model depolymerization are: benzaldehyde,
benzalcohol, benzoic acid, and vanillin. The selectivity for each
compound depends essentially of the catalyst structures. The use of
Co50-Fe50 mixed oxides promotes benzoic acid formation with a high
selectivity of 82.5%. This compound may be produced from di-oxo-
benzaldehyde (Fig.7) or benzalcohol in which hydroxylmethyl group
OH
Acetovanillone
OH
OH
OH
OH
OH
Vanillin acid
Syringaldehyde
Acetosyringone
1
0
OH-hydroxybenzoic acid
12
13
14
11OH
O
O
O
O
O
O
194
H3CO
H3CO
OCH3 H3CO
H3CO
H3CO
H3CO
OH
OH
OH
OH
OH
OH
guaiacyl-3-hydroxylpropanal guaiacylpropanone guaiacyl-malonaldehyde Guaiacyl-acetaldehyde
Isoeugenol
reacts with O
2
to form benzaldehyde that its aldehyde group
Sinapyladehyde
1
9
20
1
6
17
18
oxidizes giving carboxylic group. Co100 has a moderate selectivity
15
3
8%, 17.4%, 19.6%, 18.7% toward benzaldehyde, benzalcohol, Fig 8: Monomeric products obtained from oxidative fragmentation of
lignins.
Benzoic acid and di-oxo-benzaldehyde, respectively. While Fe100
shows good selectivity for benzalcohol and vanillin. This
difference in term of selectivity is directly related to oxides phases,
specific surface area and volume pore.
a
The formation of alkyl benzene and alkyl phenol indicates clearly
the demethoxylation of lignin polymer; which produced methanol,
consecutive alkylations of the obtained phenol with methanol were
3
8
considered to produce these compounds
reported also that the complete cleavage of aryl-O-bonds in
phenols in the presence of ethanol over MoC1-x/CuMgAlO at 300°C
With Fe50-Co50 catalyst for 4h of reaction in water, a total yield of
. Yan et al; 2017,
Oxidative catalytic depolymerization of organosolv lignin polymer
The results of catalytic depolymerization of dimer lignin model over
Fe-Co oxides indicates clearly that Fe50-Co50 mixed oxide found to be
excellent heterogeneous catalysts for the oxidative fragmentation
of lignin polymer, with diluted oxygen, under relatively low
pressure. Therefore, in this part, catalytic oxidative fragmentation
and depolymerization of extracted lignin polymer was performed
under similar eco-friendly conditions. Fe50-Co50 catalyst was used in
different experiments to convert organosolv lignin in water at 200°C
for 4h in the presence of 10% of oxygen (Table 3, Exp. 1). The
oligomers (denoted as residual lignin) represented an important
fraction of the oxidation products (Fig.8). It is important to note
that lignin oligomers with low molecular weight, was completely
soluble in the reaction medium. The effect of the reaction time (2h
or 4h) and temperature (150°C or 200°C), catalyst charge (Table 3,
Exp.5 & 6) was also investigated in the oxidation of extracted wheat
straw lignin polymer, in the presence of Fe50-Co50 catalyst for 4h
z
4
1
.
identified aromatic compounds of 17.8% and 19.7% was observed
at 200°C and 150°C, respectively (Exp.1 & 2), with a selectivity to
syringaldehyde of 36.8% and 49.8%, respectively. Syringone (6),
guaïacol (5), vanillin (9), acetosyringone (14) and alkybenzene (2)
were also produced with selectivity varying from 6 to 18% which
depending in the temperature (Table 3). It is important to note that
Fe50-Co50 catalyst exhibited high catalytic activity, and the aromatic
monomers yield does not differ much for 150°C and 200°C.
However, the selectivity towards aromatic compounds and
particularly to syringaldehyde and vanillin depended on the
temperature. For 150°C and 200°C, the selectivity to syringaldehyde
and vanillin was much higher at 150°C than that obtained at 200°C.
For instance, with Fe50-Co50 at 150°C, the syringaldehyde and
vanillin represented about 50 wt% and 20%, respectively in the
aromatic fraction. The amount of the aromatic lignin monomers
increases also with increasing the reaction time, a total yield of
(
5
Table 3, Exp. 1 & 3). The effect of organic solvent by addition of
00 μl of methanol was tested also in the depolymerization of lignin
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1
(
3
0.4% and 19.7% was observed for 2h and 4h, respectively at 200°C In contrast, the lignin conversion into monomers, and particularly
Table 3, Exp.1 & 3), with a selectivity to syringaldehyde of about into syringaldehyde (13) and vanillin (9), is substantially higher in
5% for 2h and 4h, respectively. High selectivity of syringone (12%), reactions carried out in presence of Fe50-Co50 catalyst at 150°C or
guaïacol (15%), vanillin (15%) was also observed after 2h of reaction 200°C for 4h. For instance, the amount of aromatic products
(
(
Table 3, Exp.3) compared to only 7%, 6% and 12% obtained for 4h increases by a factor of 2.8 and 5.2 when the lignin oxidation was
Table 3, Exp.1). These results clearly indicate that aromatic lignin performed in the presence of 10 and 20 mg of Fe50-Co50
,
monomers increase with reaction time. However, at 200°C for 4h; respectively. The positive effect of the catalyst on the vanillin and
the syringone, syringaldehyde and guaïacol will undergo an syringaldehyde yield is also noticeable. Thus, it was 9.6 and 17.8%
extensive demethoxylation to generate aromatic acids, ketones and (based on dry lignin) for 10 and 20 mg Fe50-Co50, respectively. It is
alkybenzene (Table 3). In all experiments of this study, prior to important to note that the yield of the aromatic products reached
oxidation, the reactor was pressured up to 100 kPa with a mixture 20% when the lignin was treated in the presence of 20 mg of
2 2
10 % O - 90% N . In order to point out the effect of the partial catalyst at 150°C. We also note that only very low amount (3.4%) of
pressure of oxygen, oxidation tests were conducted under the same aromatic compounds was identified without Fe50-Co50 catalyst at
total pressure, but using oxygen-nitrogen mixtures with different 200°C for 4h (Table 3). The small amount of aromatic molecules
4
0
concentrations. As shown by the results given in Table 3, the effect produced without catalysts were also observed in other studies
.
of the initial concentration of oxygen on both aromatic monomer We are also studied the effect of organic solvent on the selectivity
yield and product distribution was considerable. Thus, the residual and yield of oxidative depolymerization of lignin (Table 3, Exp.6) by
lignin (oligomers) decreased with increasing of oxygen partial addition of 500 µl of methanol in the reaction. With methanol
pressure from 100 to 200 kPa (Table 3). In contrast, the total yield addition, a total yield of aromatic compounds of 16.6% was
of aromatic products decreased from 178 to 133 mg/g lignin with observed at 200°C (Table 3, Exp.6), with a high selectivity to
an increase in oxygen partial pressure from 100 to 200 kPa. A high alkylbenzene of 12%. These results confirm previous studies, which
selectivity of vanillin (6%) acid and benzoic acids (10%) were concluded the alkylation of phenol in the presence of methanol.
observed at 200 kPa compared to 1% and 5%, respectively This hypothesis confirmed by the reduction of char formation
produced at 100 KPa. A further increase in oxygen partial pressure (lignin residual), which decreased to 44%, compared to 53% without
at 200 KPa, considerably reduced the amount of the aromatic methanol with similar conditions. The organic solvent such as
oligomers, whereas the amount of non-aromatic products methanol and ethanol act as capping agent and formaldehyde
significantly increased. These results clearly indicate that, under an scavenger thereby suppressing char formation by reduction C-C
4
2
excess of oxygen, the initial oxidation products (aromatic species) linkage . Yoshikawa et al. 2014 reported from the model studies
were quickly oxidized into advanced degradation products. From that catechol methoxyphenol are converted to cresols and
these data presented in (Table 3) it is obvious that initially the alkybenzene over the iron oxide catalyst in the presence of 1-
4
0
aromatic aldehydes (vanillin and syringaldehyde) were selectively butanol
formed. Subsequently, the aldehydes reacted and the acids,
phenols, ketones and alkyl phenols became the dominant products.
.
Table 3: Amounts and selectivity of products resulted from the Organosolv lignin catalytic oxidation over Fe50-Co50 catalyst
Catalytic conditions
Cata. (mg)
Exp.1
20
Exp.2
20
Exp.3
20
Exp.4
20
Exp.5
10
Exp.6
Exp.7
0
20
Lignin (mg)
Temp. (°C)
Time (h)
100
200
4
100
150
4
100
200
2
100
200
4
100
200
4
100
200
4
100
200
4
O (KPa)
Solvent
100
Water
53.5
28.7
17.8
100
Water
59.8
20.6
19.6
100
Water
73.3
14.3
11.4
200
Water
48.2
38.5
13.3
100
Water
65.1
22.3
9.6
100
Water/Methanol
44.4
100
Water
85.8
10.8
3.4
2
Oligomers (lignin residual) (% w/w)
Non-identified molecules (% w/w)
Aromatic molecules (% w/w)
Selectivity (%)
39.0
16.6
Alky benzene
Alkyl phenols
Acetophenone
Guaïacol
Syringone
OH-benzaldehyde
Vanillin
OH-benzoic acids
Acetovanillone
Vanillin acid
7.73
4.74
2.02
5.34
7.41
2.28
14.10
5.78
3.37
0.92
35.83
7.58
7.00
6.92
2.05
2.97
1.38
0.15
17.06
1.57
1.58
1.30
49.77
3.80
4.24
4.09
0.15
14.85
11.45
1.30
14.60
0.77
3.15
1.22
34.48
6.40
7.72
4.37
1.04
5.01
0.69
18.67
6.60
10.60
4.11
6.00
30.53
3.59
3.59
4.58
0.11
14.22
9.04
1.96
13.49
-
3.61
0.12
32.63
7.96
12.04
3.65
2.15
6.46
8.18
0.95
12.61
1.15
0.55
0.14
38.84
6.16
2.50
4.58
1.20
13.20
7.96
2.15
13.40
-
3.61
0.92
35.63
5.26
Syringaladehyde
Acetosyringone
Some comments may be drawn based on these results: (i) the lignin compounds, as well as the aromatic aldehydes yield go through a
conversion into monomeric products decreases by increasing the maximum for a temperature of about 150°C in the presence of Fe50
temperature and decreasing the time; (ii) the sum of aromatic Co50 catalyst; (iii) at 150°C the selectivity levels of syringaldehyde
-
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reached 50% (vi) The selectivity of alkyl benzene increased and mixed oxide has been found to be excellent heterogeneous
reached 12% in the presence of methanol; (v) The recovery yield of catalysts for the oxidative fragmentation of organosolv lignin in
aromatic compounds increases by increasing of catalyst charge; (iv) water, with diluted oxygen, under relatively low pressure. The
The selectivity of aromatic acids (benzoic and vanillin acids) product distribution strongly depended on the temperature,
increased for a oxygen pressure of 200KPa at 200°C.
reaction time, oxygen partial pressure and catalyst amount. For
instance, to obtain high yields in both monomeric compounds and
aromatic aldehydes (syringaldehyde and vanillin) with a selectivity
up to 60%, the following oxidation conditions should be employed:
Table 4: Catalyst recycling in the catalytic oxidation of lignin in water;
2
0mg of Fe50-Co50, 200 °C, 10 bar, 4 h
Run 1
Run 2
Run 3
100 mg of organosolv wheat straw lignin, 20 mg Co50-Fe50,
Aromatic molecules (% w/w)
Selectivity
17.8
18.2
16.7
temperature, 150°C, initial oxygen partial pressure, 100 kPa,
reaction time, 4h.
Alky Benzene
Guaïacol
Syringone
7.73
5.34
7.41
5.83
6.36
8.50
7.88
6.12
7.86
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This study was financially supported by the University
Mohamed VI Polytechnic (UM6P) of Benguerir and INRA
SupAgro Montpellier.
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CoFeO mixed oxide an efficient catalyst for lignin depolymerization in water
to functional phenols
Oxides
OXIDES
NANOPARTICLES
Depolymerization
Oxidation
Demethoxylation (-OCH3)
Alkylation
Functional Phenols
H O
2
Lignin
Oligomers hydrosolubles
Depolymerization