Lignin oxidation by MnO2 under the irradiation of blue light

Article abstract of DOI:10.1039/c8gc03498b

Traditionally, conventional heat has been required for a large proportion of the oxidation and degradation process to utilise lignin from biomass. A photocatalysis system which is considered as a novel and green strategy for chemical reactions has been ap

Full text of DOI:10.1039/c8gc03498b

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ARTICLE
Lignin oxidation by MnO under the irradiation of blue light
2
a§
a
a§
a
a
Jinhuo Dai, Antonio F. Patti, * Gavin N. Styles, Sepa Nanayakkara, Leone Spiccia, Francesco
Received 00th January 20xx,
Accepted 00th January 20xx
b
c
a
Arena, Cristina Italiano, Kei Saito *
Traditionally, conventional heat has been required for a large proportion of the oxidation and degradation process to
utilise lignin from biomass. A photocatalysis system which is considered as a novel and green strategy for chemical
DOI: 10.1039/x0xx00000x
www.rsc.org/
reactions has been applied and photo-MnO
2
catalytic lignin oxidation method has been developed. In this study, we
which is readily
investigated a promising photocatalytic heterogeneous system for lignin oxidation. Recyclable MnO
2
available, and blue light which is harzardless light are used in this system. 1-Phenylethanol was used as a model compound
to study the suitable conditions for this system. After optimizing the reaction conditions, organosolv lignin, kraft lignin, and
alkali lignin were applied to this system and showed the successful oxidation of lignins and further degradation.
Introduction
In recent decades, with the increasing demand of renewable
materials for chemical feedstock all around the world,
lignocellulosic biomass, including lignin, cellulose and
hemicellulose, have attracted much attention due to the
abundant reserve from nature. Cellulose has been widely used
in the paper and pulp industry; however, lignin is still a by-
product and considered as a waste existing in the black liquor
1-5
from some paper making processes. The most common way
to utilize this “waste” is burn it to supply energy for the
process. Lignin is the most abundant renewable source of
aromatic units, and has the potential to be used in the
production of phenolic compounds as well as bio-based
polymers.6 The challenge for lignin depolymerisation is
always that there is no defined structure for lignin; however, it
is well known that there are several typical linkages in lignin
including β-O-4, 4-O-5, 5-5, β-β, and α-O-4 bonds.3
Fig. 1: β-O-4 linkages in lignin
different linkage composition compare to other lignin samples.
Particularly, the percentage of β-O-4 bonds decreased due to
the lignin degradation that occurred during the industrial
process. Das et al.
,7
14
investigated formic-acid-induced
hydrolytic lignin oxidation and depolymerisation with different
lignin samples. Their results showed that their system can
depolymerise all lignin including native lignin. The laboratory
isolated lignin, which had higher β-O-4 bonds content than
native lignin was used to demonstrate more efficient
depolymerisation and to prove their mechanism of β-O-4
bonds cleavage. It has been reported that the oxidation of the
secondary benzylic alcohol into a ketone promotes the
depolymerisation of most lignin via the cleavage of β-O-4
, 7-9
Of these, the β-O-4 linkage, which contains primary alcohol
γ α
at the C position and a secondary alcohol at the C position, is
7
-10
the most abundant in most types of lignin (Fig. 1). Research
carried out by Constant et al.12 and Crestini et al.13
demonstrated that the technical lignins, such as kraft lignin,
soda lignin and wheat straw organosolv lignin, showed
1, 8-15
linkages.
a.School of Chemistry, Monash University, Wellington Rd., Clayton, 3800,
Victoria, Australia
The bond dissociation energy of the C-OPh bond is
decreased after oxidation (Fig.1).
oxidation has a significant role in the process of lignin
utilization. Numerous strategies have been reported for the
degradation of lignin into smaller molecules after the oxidation
b.
7-10
Dipartimento di Ingegneria Elettronica, Chimica e Ingegneria Industriale,
Thus, this selective lignin
Università degli Studi di Messina, I-98166 Messina, Italy
CNR Institute of Advanced Technology for Energy "Nicola Giordano", Via
c.
Salita S. Lucia 5, 98126 Messina, Italy
§
These authors contributed equally to this research work
§
Electronic Supplementary Information (ESI) available: [details of any
16
17
of β-O-4 linkages. Rahimi et al. , Ma et al. and Lancefield et
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
18
al.
TEMPO/Fe(NO
benzoquinone)/tBuONO/O
have
reported
methods
using
2
TEMPO/O ,
3
)
3
,
and
DDQ(2.3-dichloro-5,6-dicyano-1,4
systems for the oxidation of β-O-4
2
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ARTICLE
bonds in lignin. These strategies can selectively oxidise the
Journal Name
MnO
2
is known to exist in a wide variety of structural forms
View Article Online
DOI: 10.1039/C8GC03498B
secondary alcohol in β-O-4 bonds in lignin into ketones. and these forms have a significant bearing on the photo-
34
However, conventional heating is always required for all the catalytic behaviour of these materials. These often contain
reactions. With the consideration of using renewable and significant amounts of other ions as incorporated into their
pollution-free energy, photo-catalysis has now presented itself structure. These structures are composed of a manganese
as a preferable choice for benzilic alcohol oxidation over these metal centre with oxygens coordinated in an octahedral
19-21
40
conventional treatments, as it can utilize solar energy.
Photo-catalysis has been widely used in different research
areas. It was originally applied for the decomposition of water in the cathode material of dry-cell batteries. However, the
into hydrogen and oxygen by using a TiO photo-anode in most active catalytic form for the oxidation of alcohols is
972.22 Since then, the basic mechanisms of photo-catalysis reported to be δ-MnO
. It has a layered structure, with sheets
have been studied and published by numerous research made from manganese-oxygen octahedra, separated by alkali
geometry.
The most commonly used form, γ-MnO
2
is generally found
2
1
2
groups.2
0-27
or other ions, and water. Gritter et al.
39
38
also studied the
Photo-catalysis has also been introduced into lignin oxidation of allylic alcohols and benzyl alcohols, in which
chemistry to depolymerise lignin.2 Nguyen et al. developed several forms of manganese oxide were investigated in the
5
27
3
9-41
a lignin depolymerisation approach using ([4-AcNH-TEMPO]BF
as a recyclable oxidant with a photo catalyst. The C−O bond ability of MnO
could be cleaved by utilizing the photo-catalyst from 370 nm to 570 nm, covering a significant part of the
Ir(ppy) (dtbbpy)]PF to selectively oxidise the secondary visible spectrum.
alcohol to a ketone in β-O-4 model compounds. Tanaka et al.
reported that in a lignin solution with suspended TiO , lignin the selective oxidation of a broader range of alcohols has
was first adsorbed onto the surface of the TiO , which was received little attention, particularly from the perspective of
4
) oxidation of such alcohols. Zhou et al.
studied the catalytic
2
for water splitting in the wavelength range
[
2
6
28
2
The application of visible light in the presence of MnO for
2
2
then irradiated by UV (λ>310nm). Lignin was then desorbed synthetic organic chemistry and controlled transformations.
and depolymerised with the subsequent illumination. However, Consequently, the aim of this study was to explore the effect
carboxylic acids and aldehydes were detected as products due of blue light on the oxidation of benzilic alcohols with a focus
2 2
to the aromatic ring opening during the reaction, which on the δ-MnO form. Three other forms of MnO (α, β, γ) were
44
indicates that this system cannot be used to generate phenolic also prepared by reported methods and first tested against a
compounds from lignin. Ksibi et al.29 reported the photo- reference model compound, 1-phenylethanol. The different
degradation of soluble lignin from black liquor using a UV/TiO
2
2
forms of MnO can be readily characterized by X-ray diffraction
photo-catalytic technique. The reaction was carried out at a and were compared with published spectra reported by
45
temperature of 20°C under the irradiation of UV (λ>290nm). Villalobos et al. The X-ray diffraction spectrum of the δ-MnO
2
Products including vanillin, coniferylic alcohol, syringaldehyde, sample is shown in Figure 2.
30
vanillic acid, were detected by GC-MS. Machado et al.
Herein, we present a very efficient lignin oxidation
reported a combination of TiO and H O with oxygen bubbling approach using δ-MnO as a photo-catalyst under the
2
2
2
2
under the irradiation of UV/visible light. In their study, the irradiation of blue light (λ = 470 nm) with oxygen bubbling
degradation of phenolic groups occurred by electron transfer through. 1-phenylethanol and a range of other alcohols,
between the phenolate anion and the ground state oxygen. including lignin model compounds, were used to test this
The hydroxylation of the lignin aromatic structure was also catalyst system. After optimising the reaction conditions,
reported.
TiO is generally considered to be an excellent photo-
original lignin samples were also tested using the same system.
2
catalyst for lignin oxidation. However, it should be noted that
only the ultraviolet light photons, which account for only 4% of
solar radiation, can replace its valence band electrons due to a
band-gap energy of 3.2eV.27 Unsurprisingly, as the field has
developed, a large range of new and efficient catalysts have
emerged in the quest for high energy yield efficiency.3
Results and discussion
Characterisation of δ-MnO
2
Synthesized MnO
2
2
was characterised using XRD, N -
physisorption and temperature programmed reduction (H
TPR) tests to confirm its form and analyse its surface area and
particle diameter. The XRD pattern of the synthesized MnO
2
-
1-35
x
MnO catalysts and doped analogues have also been
2
extensively studied and applied in water treatment specifically
to oxidise and remove organic pollutants. Under milder
conditions, selective oxidation has also been reported. The
selective oxidation of alcohols to the corresponding carbonyl
sample (Fig. 2) shows four main reflections at ca. 12°, 25°, 37°
and 66.5° (indexed to JCPDS 80-1098), which are typical of the
45
2
δ-MnO phase.
compounds has been known for some time and widely used in
industrial reactions.3
6-37
Many studies on the selective aerobic
oxidation of benzyl alcohol to benzaldehyde have involved the
use of MnO materials including combinations that incorporate
x
a variety of promoters and dopants that improve redox
behaviour.38
2
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5
20
35
50
65
80
2
θ (°)
2
73
373
473
573
673
773
873
Temperature (K)
Fig.2: XRD pattern of the δ-MnO
2
sample
Fig. 4: H
2
-TPR of the δ-MnO
2
sample.
6
Oxidation reactions of alcohols
In order to establish optimum conditions for this photo-
catalysis system, 1-phenylethanol was used as a model
compound. H-NMR was used to confirm the conversion of 1-
4
2
0
1
phenylethanol to acetophenone by comparing the signals for
the terminal methyl group of both compounds, which showed
a doublet around 1.41 ppm for 1-phenylethanol and a singlet
at 2.59 ppm for the oxidation product (Supporting Information
Fig. S1, Fig. S3). The recovered mass yield of reacted products
was always about 90%-95%. The percentage yield of the
corresponding ketone products was calculated using the
conversion and the recovered mass of the product. All the
conversions for all the reactions have been listed in Table S1
0.0
0.2
0.4
0.6
0.8
1.0
p/p0
2 2
Fig. 3: N -physical adsorption isotherm of δ-MnO sample.
(
Supporting Information). For all the oxidations of model
2 2
The δ-MnO sample featured a N -physisorption isotherm
1
compounds, the H-NMR spectroscopy showed there were no
side reactions occurring, as there were no other products
detected except the relevant aldehyde/ketone.
typical of macroporous solids, accounting for a Brunauer–
2
Emmett–Teller (BET) surface area value of 43±2 m /g (Fig.3).
Accordingly, a rather featureless desorption branch indicated
3
an intra-particle porosity of 0.18 cm /g, and an average pore
diameter of 18 nm. The particle size of δ-MnO
which was analysed by the optical microscope (Fig.S14). For
the following reactions, ca. 5-hold excess of MnO was applied
to carry out the catalytic reaction from these results.
The H -TPR profile of the δ-MnO material, shown in Fig. 4
shows two reduction peaks at 335 and 376°C, respectively,
2
was ca. 4 μm,
2
2
2
indicating a considerably harder reducibility in comparison to Scheme 1: Screening reaction with 1-phenylethanol.
poorly crystalline MnO
corresponds to a H /Mn ratio of 0.9±0.05, in agreement with
the final reduction of the MnO material (i.e., MnO1.9±0.05) to
MnO, while the relative intensity of the reduction peaks
suggests the intermediate formation or presence of Mn
species. In a separate study of manganese oxide molecular
sieves, Tang et al.48 included a H
-TPR of their material and
suggested that the lower temperature reduction peak may be
associated with the reduction of MnO –Mn , while the
higher temperature peak is associated with further reduction
of Mn to MnO. As a result, the synthesized MnO was
confirmed as δ-MnO
samples.47 The overall consumption
2
2
Table 1: Initial investigation of oxidation of 1-phenylethanol
2
Entry
Catalyst
Time
Conditions
Yield
2 3
O
1
2
3
4
5
6
MnO
MnO
MnO
2
2
2
(10 mmol)
(10 mmol)
(15 mmol)
20 h
20 h
20 h
20 h
3 h
Ambient
55%
57%
71%
12%
92%
92%
o
40 C, O
2
2
Ambient
Ambient
MnO
2
(5 mmol)
(10 mmol)
(10 mmol)
2
3 4
O
MnO
2
2
UV (λ=254 nm), O
2
2
MnO
3 h
UV (λ=365 nm), O
O
3 4
2
2
.
Note: Petroleum benzine was used as the solvents for all the reactions.
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Journal Name
Screening reactions were carried out to test several Table 3: Screening reaction of oxidation of 1-phenylethanol with four types of MnO
2
.
View Article Online
DOI: 10.1039/C8GC03498B
conditions. Initial reactions were conducted using water as a
solvent under ambient conditions, however no reaction was
observed. Table 1 shows that the oxidation of 1-phenylethanol
occurred when using petroleum benzine as a solvent at
ambient conditions with 55% yield.
Entry
Wavelength
470 nm
Time
2 h
Notes
Yield
1
2
3
4
δ-MnO
2
71%
28%
35%
25%
470 nm
2 h
α-MnO
β-MnO
γ-MnO
2
470 nm
2 h
2
470 nm
2 h
2
Increasing the reaction temperature to 40°C and bubbling
oxygen through the reaction only marginally increased the
Note: Petroleum benzine was used as the solvent for all the reacitons.
yield by 2%. The dosage of MnO
MnO dosage could significantly increase the yield from 12%
with 5 mmol of MnO to 71% with 15 mmol of MnO . When
both UV (λ=365 nm) (Table 1, Entry ) and UV (λ=254 nm)
Table 1, Entry ) irradiation and oxygen were applied for the
reaction resulting the yield increased to 92% with 3 hours. The
hypothesis is that MnO could absorb photons leading the
2
was also investigated. Higher
The yield of acetophenone after the reaction decreased to
8% (Table 2, Entry 5) and 57% (Table 1, Entry 2) without
oxygen bubbled through or blue light respectively. This
indicated that both the irradiation and oxygen flow are very
important for the regeneration of manganese (II) to
manganese (IV) under the irradiation of blue light.
2
4
2
2
5
(
6
2
The recyclability of the δ-MnO
yield of 24% (Table 2, Entry 6) using the recycled MnO
without further treatment. The reactivation of δ-MnO was
achieved by heating the recovered δ-MnO in a furnace at
30°C for 15 min in air (Table 2, Entry 7). A significant weight
2
was tested and only gave a
reduction of Mn (IV) to Mn (II), which resulted in the
occurrence of the reaction.2
2
6-30
2
Based on the results above, we continued to optimize the
conditions for this oxidation reaction. The standard conditions
2
2
for Table 2 are 10mmol of δ-MnO
phenylethanol and oxygen bubbled through, unless otherwise
stated. Decreasing the amount of δ-MnO from 10mmol to 5
2
, 1.64 mmol of 1-
loss (13% to 30%) in the catalyst was noted as a result of the
furnace reactivation. This weight loss has been previously
2
33
observed by Arena et al. and was attributed to adsorbed
mmol lead to a drop in yield from 92% (Table 2, Entry 1) to 25%
under 365 nm UV (Table 2, Entry 2).
The suggested reason for this occurrence is that due to the
heterogeneous nature of the system, the effective
reactants, products and solvent. Thermogravmetic analysis
TGA) was used to analyse the recovered δ-MnO after the
2
(
reaction. The TGA spectrum has been added in the supporting
information as Figure S8. Two weight loss peaks were
appeared from TGA and the first weight loss peak was related
to the loss of the absorbed solvent (10 to 25% weight loss)
while another weight loss peak around 200°C was related to
the absorbed substrate/product (3 to 5% weight loss). As a
concentration of MnO
than the total amount of MnO
required. Zhou et al. 41 proved that the photon from
wavelength 350 nm to 570 nm could meet the band gap
2
available for the reaction is much lower
, meaning that a large excess is
2
2
energy of the birnessite, which is a natural form of δ-MnO .
control reaction, MnO
petroleum benzine for 24 h before the reaction. The reaction
of oxidation of 1-phenylethanol using this pretreated MnO
2
was soaked and stirred in the solvent
This, along with the consideration of reducing the health
hazards posed by the use of UV light, a blue LED (λ=470 nm)
system was tested for the oxidation. The results in Table 2,
Entry 4, indicated that the blue light could also efficiently
2
under the standard condition resulted a yield of 88%, which is
slightly lower than the reaction without the solvent
pretreatment (93% of yield).
enhance the oxidation reaction, with yield reaching 93% when
1
the reaction was undertaken for
3
hours. H-NMR
spectroscopy(Fig. S1) and GC-MS (Fig. S2) spectra were shown
in supporting information. As such, blue light was selected as
the light source for further oxidation reactions. A control
2
Table 4. δ-MnO oxidation reactions for different alcohol classes.
Substrate
Wavelength
Time
Yield
reaction involving the amount of MnO
bubbled through was also carried out to further elucidate the
role of the irradiation of MnO in the reaction.
2
, but no oxygen
1-pentanol
2-pentanol
470nm
470nm
470nm
470nm
470nm
470nm
470nm
470nm
470nm
3h
3h
3h
3h
3h
3h
3h
3h
3h
26%
0%
2
Furfuryl alcohol
69%
64%
92%
0%
Benzyl alcohol
Cinnamyl alcohol
Table 2: Screening reaction of oxidation of 1-phenylethanol.
Diphenyl ether
Lignin model compound 1
Lignin model compounds 2
Lignin model compounds 3
89%
90%
89%
Entry
Wavelength
365 nm
365 nm
470 nm
470 nm
470 nm
470 nm
470 nm
Time
3 h
3 h
2 h
3 h
3 h
3 h
3 h
Notes
Yield
92%
25%
72%
93%
48%
24%
92%
1
2
3
4
5
6
7
Standard conditions
5 mmol MnO
10 mmol MnO
Standard conditions
Under air[a]
2
2
Note: Petroleum benzine was used as the solvents for all the reactions.
Recycled MnO
Furnace treated recycled
MnO
Note:[a] This reaction was performed under air without oxygen bubbled through.
2
2
[
b]Petroleum benzine was used as the solvents for all the reactions.
4
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OH
5
'
5
4
'
O
4
6
'
6
α
View Article Online
DOI: 10.1039/C8GC03498B
Table 5. δ-MnO
2
oxidation reactions of 1-phenylethanol with different solvents.
β
1
'
3'
3
1
2
'
2
Entrya
Solvent
Time
2h
Yield
72%
18%
64%
9.5%
1
1
2
3
4
Toluene
OH
OMe
OH
OMe
Chloroform
Acetonitrile
2h
5
'
5'
5
5
4'
β
O
4
MeO 6
'
4
'
O
4
MeO
α
α
6
6'
6
2h
β
3
γ
3
3
3
'
3'
1
1
'
1
BzO 1' 2' HO MeO
Petroleum Benzine/
3h
BzO
2'
MeO
2
2
OMe
OMe
1 mol equivalent of acetic acid
2
3
[
a] For all the entries, 1-phenylethanol was used as the substrate and the
wavelength is 470 nm. [b] Standard reaction conditions were used in all reactions
and involved the use of 10mmol of δ-MnO , 1.64 mmol of 1-phenylethanol
dissolved in the solvent and oxygen bubbled through at ambient temperature (20-
3°C), unless otherwise stated.
Fig. 5: Lignin model compounds.
2
All the above results indicated that the solvent has a minor
effect and the catalyst deactivation of recovered MnO was
2
2
mainly due to the adsorption of substrate/product that on the
surface. Consequently, these adsorbed substrate/product on
The conversion of lignin model 2 and 3 were based on the
ratio of signal of protons on the C before reaction to after the
reaction (Supporting Information Fig. S4). The yields of these
three lignin model compounds were calculated by conversion
times mass yield.
This showed the potential for lignin oxidation using δ-
2
MnO under mild conditions using blue light. The secondary
alcohol in β-O-4 bonds in lignin can be oxidised into ketone
bonds, which leads to the bond dissociation energy reduction
of the β-O-4 bonds. Thus, oxidised β-O-4 linkages can be
cleaved more readily.
6
the MnO
the need to use excess amounts with respect to the substrate
in all the reactions. The recycled and reactivated δ-MnO was
2
surface inhibit its oxidation efficiency and explains
2
shown to achieve a quantitative yield to acetophenone (92%
yield) when re-used in the oxidation reaction. In addition,
there was no MnO
or manganese salts were observed after evaporating the
filtrate. The structure of the reactive MnO was confirmed by
XRD (Fig. S7) showing it was still δ-MnO
Next, four types of manganese dioxide, α-, β-, γ-, and δ-
2 2
solubilised during the process as no MnO
2
2
.
Changing the solvent to toluene or acetonitrile did not
significantly affect the yield of product for 1-phenylethanol
using 2 hours reaction time (Table 5). However, a change to
chloroform gave a significant yield loss. Chloroform is known
44
MnO
2
were synthesized by a reported thermo method. Table
3
presents the results of oxidation of 1-phenylethanol using
each of the different forms.
2
The reaction catalysed by δ-MnO showed a yield of 71%
3
8
to be slightly acidic. The presence of acid on the reaction was
separately tested by adding acetic acid to the reaction mixture.
The yield of acetophenone was significantly suppressed (9.5%
yield) compared to 93% yield under the same conditions, but
without added acid. The effect of the acid, coupled with the
order of reactivity of the various alcohols tested, suggests the
possible involvement of anionic species in the reaction
mechanism.
with 2 hours reaction time (Table 3, Entry 1), which is a higher
yield than was achieved for any other form of MnO . This
indicated δ-MnO has the highest catalytic ability for oxidizing
alcohols of the different forms of MnO
Based on the results of the various optimisation trials, it
was decided to use 10 mmol of δ-MnO to 1.64 mmol of
2
2
2
.
2
substrate with oxygen bubbled through under the irradiation
of blue light (λ=470 nm) as the standard condition for further
reactions. Some variability in the reactivity of the different
alcohol classes was noted in Table 4. A relatively low yield of
pentaldehyde (26%) was achieved for the oxidation of 1-
pentanol under the standard conditions chosen. Furthermore,
The reaction mechanism for the oxidation of simple
alcohols to aldehydes or ketones via metal oxide catalysts has
received limited attention and is not well understood. Arena et
38
al. suggested that the active oxygen species for alcohol
−
2−
oxidation could be either O
2
or O
2
based on other studies on
2
-pentanol did not give any reaction. “Benzylic” type alcohols
manganese oxide octahedral molecular sieves (OMS). Iyer et
(1-phenylethanol, furfuryl alcohol, benzylalcohol) and the one
42
al. proposed a mechanism for the photo oxidation of 2-
propanol to acetone on the surface of a manganese oxide OMS
conjugated alcohol (cinnamyl alcohol) in the test compounds
all gave good yields of oxidation products. Three β-O-4 lignin
model dimers (Fig. 5) were also synthesised to test the
reaction, which also showed good yields of oxidation products.
36
37
catalyst. Furthermore, Cao and Suib and Duan et al.
proposed that the oxygen released from the MnO
2
catalysts is
−
in the form of O
2
and originates from the movement of this
1
All the yields were calculated based on the H-NMR
species from the catalyst bulk structure to the surface upon
the absorption of light that weakens the MnO bonds. The
involvement of lattice oxygen has also been proposed in
spectroscopy. The conversion of lignin model 1 was based on
the ratio of signal of protons on the C before reaction to after
β
the reaction (Supporting Information Fig. S3).
2
49
another study by Makwana et al. , involving the oxidation of
benzyl alcohol with OMS MnO
2
.
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Journal Name
The results of oxidation for the lignin model compounds in
View Article Online
DOI: 10.1039/C8GC03498B
Table 4, indicates that this strategy showed high potential for
the oxidation of lignin itself.
Oxidation reactions of lignins
For these oxidation trials, three lignin sources were utilized;
organosolv lignin, kraft lignin, and alkali lignin. All three lignin
samples were analysed by 2D-NMR to give structural
information, shown in Fig. 7 (a), (b), and (c). Three typical
bonds, β-O-4 bonds, β-β bonds, and β-5 bonds were detected,
and are shown in Fig. 7. The full HSQC-NMR spectra with
integration (Fig. S9, Fig. S10, Fig. S11) and the content of β-O-4
bonds in the three lignin samples are listed (Table S2 and Table
S3) in the supporting information.
After characterisation of the lignin samples, δ-MnO
2
photo-
catalytic oxidations were carried out. The reaction mixture is
not fully homogeneous due to the solubility of lignin in
acetonitrile. The optimized conditions (200 mg of lignin, 10 ml
of acetonitrile, and 870 mg of MnO2, 3 hours with oxygen
bubbled through under the irradiation of blue light) were
applied to all these three samples.
Fig. 6: Infrared Spectra of MnO
2
oxidised lignin samples.
The oxygen species abstracts hydrogen from the alcohol
+
and this abstracted hydrogen (H) is further oxidised to H
17
forming acid sites on the surface of the catalyst. It is
-
conceivable that the O
2
species on the surface of the catalyst
would be suppressed by the presence of acid in the reaction Fig. 7: Different lignin linkages and 2D-NMR spectra of (a) organosolv lignin, (b) alkali
lignin, (c) kraft lignin, (d) oxidised organosolv lignin, (e) oxidised alkali lignin, and (f)
oxidised kraft lignin.
medium. Gas phase oxygen from air, which is bubbled through
the reaction medium, then acts to reoxidise the manganese (II)
to the manganese (IV).3
6-37
6
| J. Name., 2012, 00, 1-3
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increased from 12% prior to reaction. The molecular weight
View Article Online
DOI: 10.1039/C8GC03498B
n n
decreased from M = 1,400 to M = 400 with polydispersity
decreased from 12.9 to 2.5 (Fig. 8). This indicated that the
oxidised organosolv lignin was successfully depolymerised with
the molecular weight decreased. The signal in the red curve
from the retention time around 18 min significantly increased
compared to the black curve. This indicated there was a
significant generation of low molecular weight fractions after
depolymerisation. The GC-MS spectrum (Fig. S13) of ethyl-
acetate soluble fraction from the depolymerised organosolv
lignin showed that monomers including 4-hydroxy-3,5-
dimethoxybenzaldehyde, and oligomers were produced after
depolymerisation. However, these products were difficult to
isolate from the mixture.
Fig. 8: SEC chromatograms of depolymerised lignin ethyl-acetate soluble fraction Conclusions
The products were analysed by IR to determine the
In this study, we presented a novel heterogeneous photo-
presence of newly introduced carbonyl frequencies, as
evidence of oxidation having occurred.5 The results of this
IR analysis are shown in Fig. 6. The black curves are the
samples before oxidation and the red curves are the samples
0-52
catalytic system to oxidise lignin using recyclable MnO at
2
ambient temperature. Blue light, was used as the light source
instead of UV light or any other conventional heating. Several
solvents were tested and found that acetonitrile can be used
for the lignin oxidation. Three different lignin samples were
successfully and efficiently oxidised and shown that they can
-
1
after oxidation. After oxidation, new peaks around 1700 cm
appeared (labelled with arrows), which are peaks typical of
C=O bonded to an aromatic ring.5
1-53
This demonstrated that
be partially depolymerised. The amount of MnO
the oxidation of phenols and lignin is still high (ca. 5-hold) and
requires heat 230°C for 15 min to recycle MnO and needs
2
requires for
all these lignin samples were successfully oxidised with δ-
MnO as a photo-catalyst.
2
Oxidised lignin samples were also analysed by 2D-NMR to
give further structural information. 2D-NMR spectra of
oxidised lignin are shown in Fig. 7 (d), (e), and (f), which
support the finding that β-O-4 bonds were oxidised in the
lignin during this process. When comparing lignin before the
reaction Fig. 7(a), (b), and (c) to after the reaction Fig. 7(d), (e),
2
further study to improve the catalytic activity of MnO
controlling its surface area and pore size in the future.
2
by
Experimental
and (f), the signal from A disappeared after oxidation. This Material:
α
suggested the secondary hydroxyl group at the C
oxidised to a ketone. In addition, the signal of A
oxidation. This indicated the β-O-4 bonds were successfully
oxidised without bond cleavage. In addition, the signal from β-
α
position was
shifted after
Organosolv lignin (M
corn cob from Stigmate maydis was purchased from Chemical
Point (Munich, Germany). Kraft lignin (M = 1,500 g/mol, Đ=7.7
by SEC) was isolated through the process described later from
black liquor supplied by Visy from its paper mill in Tumut,
NSW, Australia, which mainly uses Pinus radiata (softwood).
Alkali lignin (M = 1,100 g/mol, Đ = 9.1 by SEC) was purchased
n
from Sigma-Aldrich (Sydney, NSW, Australia). All these three
lignin samples were used without any further purification.
n
= 1,400 g/mol, Đ = 12.9 by SEC) from
β
n
5
bonds still existed after oxidation. The percentages of β-O-4
bonds before and after oxidation have been listed in Table S2
(
Supporting Information). A previously reported method was
18,55
used to calculate the ratio of linkages.
After oxidation, a depolymerisation strategy that has been
previously reported was applied for lignin depolymerisation.
MnSO4·
H
2
O and KMnO
Ethyl acetate, acetonitrile, dimethyl formamide (DMF),
chloroform (CDCl ), dimethyl sulfoxide (DMSO-d ), and
4
were purchased from Sigma-Aldrich.
54
Formic ionic liquid, 2-hydroxy ethylammonium formate, was
used for the depolymerisation of oxidised lignin. The proposed
3
6
5
4
mechanism has been reported as the bond dissociation
energy of C -OPh decreased after oxidation. The formate anion
will attack the other hydroxyl group in the oxidised β-O-4
bonds resulting in C -O bond cleavage.
methanol were purchased from Merck. All reagents were used
as purchased from the supplier without any further
purification.
β
β
After the reaction, ethyl acetate was used to extract the Instruments and measurements:
low molecular weight fraction of the product mixture. SEC was
1H-NMR and 2D-NMR spectroscopy were recorded on a Bruker
DRK-600 spectrometer operating at 600 MHz as solutions in
CDCl and DMSO-d . The molecular weight of isolated lignin
and depolymerised samples were measured by SEC performed
on a Tosoh EcosHLC-8320 Gel Permeation Chromatograph
used to analyse the molecular weight before (black curve) and
after (red curve) depolymerisation reactions (Fig. 8). Oxidised
lignin was successfully depolymerised with the generation of
ethyl acetate fraction yielding 51% of soluble products,
3
6
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ARTICLE
equipped with both refractive index (RI) and ultraviolet (UV) reaction, MnO
Journal Name
was removed by filtration. The mixture was
2
View Article Online
detectors (UV detection, λ = 280 nm) using Tosoh alpha 4000 then partitioned between water (30 mL)DaOnI:d10e.t1h03y9l/aCc8eGtCa0te34(928Bx
and 2000 columns. DMF (with 10 mM LiBr) was used as solvent 20 mL). The combined organic layers were washed with
to dissolve all the lignin samples as well as mobile phase with saturated brine (1 x 30 mL) and dried over anhydrous
flow rate of 1.0mL/min. UV detector in SEC chromatograms set magnesium sulfate. The solvent was removed in vacuo and the
1
at 280 nm was used to analyse all the depolymerised samples product was then analysed by H-NMR to determine the
and isolated lignin. Calibration curves were obtained using percentage yield of the secondary alcohol to a ketone. 1H NMR
polystyrene standards. The SR-02-B0040 LED assembly has 7 (400ΜHz, CDCl
LXML-PB01-0040 Blue (470 nm) LUXEON Rebel LEDs soldered 3Η).
to a 40 mm round 7-Up CoolBase and run with 40V and a 1000
3
), 7.94 (δ, J = 7.6Ηz, 2Η), 7.48 (m, 3Η), 2.58 (s,
Procedure for UV (365nm) oxidation of 1-phenylethanol with
MnO
mA current draw. UV irradiations were performed using a
CL1000 M UV-crosslinker lamp (UVP, LLC, Australia) that
2
:
-1
provide polychromatic light centred at 365nm and 254 nm. A UV reactor was charged with 0.2 ml (d = 1.013 gmL , 202
The N
-physical adsorption isotherms (77K) were obtained mg, 1.64 mmol) of 1-phenyl ethanol and 10 ml of petroleum
using an ASAP 2010 static adsorption device (Micromeritics benzine. 870 mg, 10 mmol of MnO was then added to the
2
2
Instrument). Before measurement the sample was outgassed mixture. A UV light was suspended in the reactor, and oxygen
at 150°C for 6h. Surface area (SA), pore volume (PV) and bubbled through the solution. The reactor was then covered in
average diameter (APD) data were obtained from the foil and stirred without external heating for 5 hours. After the
2
elaboration of the isotherm by the standard BET and BJH reaction, MnO was recovered by filtration. The mixture was
methods, respectively. X-ray diffraction (XRD) analyses were then partitioned between water (30 mL) and ethyl acetate (2 x
performed using a Philips X-Pert diffractometer, operating 20 mL). The combined organic layers were washed with
with Ni β-filtered Cu Kα radiation (40 kV; 30 mA) at a scan rate saturated brine (1 x 20 mL) and dried over anhydrous
of 0.05°/min. Temperature programmed reduction (H
2
-TPR) magnesium sulfate. The solvent was removed in vacuo and the
1
measurements in the range of 293-1073K were carried out product was then analysed by H-NMR to determine the
using a linear quartz microreactor (dint=4 mm) heated at the percentage yield of the secondary alcohol to a ketone.
rate of 12K/min. The reactor was loaded with a catalyst sample
2
Procedure for 470nm oxidation of 1-phenylethanol with MnO :
2
of ca. 20 mg and fed with a 5% H /Ar carrier (F, 60 stpmL/min).
The H consumption was monitored by a TCD, after water A 100 mL round bottom flask was charged with 0.2 ml (d =
2
-
1
removal by a “chemical trap” containing Mn(ClO
4
)
2
.
1.013 gmL , 202 mg, 1.64 mmol) of 1-phenyl ethanol and 10
ml of petroleum benzine. Manganese dioxide was then added
(
Isolation of kraft lignin from black liquor:
870 mg, 10mmol) and the solution stirred vigorously at room
The procedure to isolate kraft lignin from black liquor has been temperature for 3 hours under irradiation with 470nm blue
reported before.5
1-52,54
Elemental analysis of carbon, hydrogen, light. The mixture was then partitioned between water (30 mL)
nitrogen and sulphur gave the following result (average over and ethyl acetate (2 x 20 mL). The combined organic layers
three measurement): Carbon 65.30%, Hydrogen 6.07%, were washed with saturated brine (1 x 30 mL) and dried over
Nitrogen <0.3% (detection limit), Sulphur 2.57%. M
g/mol, Đ=7.7
n
= 1,500 anhydrous magnesium sulfate. The solvent was removed in
1
vacuo and the product was then analysed by H-NMR to
determine the percentage yield of the secondary alcohol to a
ketone
Synthesis of δ-MnO
A 100 mL round bottom flask was filled with 70 ml of distilled
water and stir bar. MnSO O (1.69 g, 1 mol) and KMnO
1.58 g, 1 mol) were added into the round bottom flask. The
mixture was stirred for 2 hours at the temperature of 80°C. A 100 mL round bottom flask was charged with 0.2 ml (d =
2
:
Procedure for 365nm/254nm oxidation of 1-phenylethanol with δ-
MnO
4
H
2
4
2
:
(
-
1
After 2 hours, 0.5 g of NaOH was added into the mixture and 1.013 gmL , 202 mg, 1.64 mmol) of 1-phenyl ethanol and 10
was stirred for another 2 hours. After reaction, the brown ml of petroleum benzine. MnO was then added (870 mg,
residue, which is MnO , was filtered and washed with distilled 10mmol) and the solution stirred vigorously at room
2
2
water to remove all the unreacted chemicals. The filter cake temperature for 3 hours in a CL1000 M UV-crosslinker under
was then dried in the oven at the temperature of 100°C for irradiation with 365 nm or 254 nm lamps. The mixture was
over night. Power XRD was used to confirm the structure of then partitioned between water (30 mL) and ethyl acetate (2 x
the MnO2.
20 mL). The combined organic layers were washed with
saturated brine (1 x 30 mL) and dried over anhydrous
2
Procedure for oxidation of 1-phenylethanol with MnO :
magnesium sulfate. The solvent was removed in vacuo and the
1
A 100 mL round bottom flask was charged with 0.2 ml (d = product was then analysed by H-NMR to determine the
-
1
1
.013 gmL , 202 mg, 1.64 mmol) of 1-phenyl ethanol and 10 percentage yield of the secondary alcohol to a ketone.
ml of petroleum benzine. MnO was then added to the desired
quantity (eg. 870 mg, 10 mmol) and the solution stirred
vigorously at room temperature for 20 hours. After the
2
2
Procedure for 470nm Oxidation of Lignin with MnO :
8
| J. Name., 2012, 00, 1-3
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A 100 mL round bottom flask was charged with 200 mg of organic layer under vacuum to give low moleVciuewlaArrticwleeOinglihnet
lignin and 10 ml of acetonitrile. 870 mg (10mmol) of MnO
was fraction. Products were analysed by SEC.DOI: 10.1039/C8GC03498B
2
then added and the solution stirred vigorously at room
temperature for 3 hours under irradiation with 470 nm blue
Synthesis of α-, β-, γ-, and δ-Manganese Dioxide Catalysts:
lights. The mixture was then diluted (using methanol for The procedures used followed reported methods.44 In a typical
2 4
methanol soluble lignin, and DMF for methanol insoluble) and synthesis of the α-MnO nanorods, 2.5 g of KMnO and 1.05 g
the MnO filtered off. The solvent was removed in vacuo and of MnSO ·H O were mixed in distilled water (160 mL) and
2
4
2
the product was then analysed by FTIR and SEC.
magnetically stirred about 10 min to form a homogeneous
mixture. Then, the mixture was transferred into a Teflon-lined
stainless steel autoclave (400 mL) and heated at 160 °C for 12
Synthesis of 2-phenoxy-1-phenylethanol (lignin model 1):
A 250 ml round bottom flask was equipped with a reflux h. The product was collected, washed, and dried at 80 °C.
condenser and dropping funnel was charged with phenol Similarly, the β-MnO nanorods were obtained from the
0.520 g, 5.53 mmol) and stirred at room temperature. To this reaction of KMnO (1.40 g) and MnSO O (2.25 g) at 160 °C
solution, 2-bromoacetophenone (1.00 g, 5.02 mmol) in for 12 h. For the preparation of the γ-MnO nanorods,
acetone (50 ml) was added dropwise over 30 min at room MnSO O (6.75 g) and (NH (9.15 g) were well mixed
temperature. The resulting suspension was stirred at reflux for and hydrothermally treated at 90 °C for 24 h. The δ-MnO
hours, after which the suspension was filtered and nanorods were obtained by the chemical oxidation of
concentrated in vacuum. The crude product was purified by MnSO (0.55 g) and KMnO (3 g), which were
2
(
4
4 2
·H
2
4
·
H
2
4 2 2 8
) S O
2
4
4
·
H
2
O
4
recrystallisation from ethanol to give 2-phenoxy-1- hydrothermally heated at 240°C for 24 h. All the catalysts were
phenylethanone as white solid (1.050 g, 4.95 mmol) in 98% analyzed by Powder XRD to confirm the structures.
yield. To a solution of 2-phenoxy-1-phenylethanone (0.315 g,
1
.49 mmol) in ethanol, NaBH
4
(0.112 g, 2.98 mmol) was added
Conflicts of interest
There are no conflicts to declare.
at 0°C. The resulting solution was stirred at 0°C for 5 min. The
reaction mixture was partitioned between ethyl acetate (50
ml) and water (30 ml). The organic layer was washed with
2 4
saturated brine (30 ml, twice), dried over Na SO and
evaporated under reduced pressure to give 2-phenoxy-1- Acknowledgements
1
phenylethanol as a white solid (0.301 g in 94% yield). H-NMR:
This work is dedicated to the memory of the late Professor
Leone Spiccia whose work on the application of manganese
catalysts for water splitting under UV and visible light
irradiation inspired this study.
4
00 MHz, CDCl
t, 2H), 6.96 (t, 1H), 6.91 (d, 2H), 5.12 (dt, 1H), 4.10 (dd, 1H),
.99 (t, 1H), 2.76 (d, 1H) ESI: m/z 214.1, found 214.0.
3
) δ 7.45 (d, 2H), 7.38 (t, 2H), 7.33 (d, 1H), 7.27
(
3
Synthesis of lignin model 2:
-(4-Benzoyloxy-3,5-dimethoxyphenyl)-2-(2,6-
dimethoxyphenoxy)ethanone was synthesized following a
The financial support of the ARC Industrial Transformation
Research Hub - Bioprocessing Advanced Manufacturing
Initiative (BAMI), Monash University, PRESTO JST, JPMJPR1515,
Japan and the China Scholarship Council (CSC) is gratefully
acknowledged.
1
reported procedure.5 H-NMR: 400 MHz, CDCl
6 1
3
) δ 8.5 (d, 1H),
.13 (m, 2H), 7.60 (tt, 3H), 6.97 (t, 1H), 6.85 (ds, 2H), 6.75 (d,
8
2
6
H), 5.17 (m, 2H), 4.60 (m, 1H), 4.40 (m, 1H), 3.9 (s, 6H). 3.8 (s,
H).ESI: m/z: 454.16, found 454.0.
Synthesis of lignin model 3:
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