Depolymerization of alkaline lignin over mesoporous KF/: γ -Al2O3

Article abstract of DOI:10.1039/d0nj03063e

As a kind of biomass resource, lignin is the only natural renewable resource that can provide abundant aromatic compounds, which is undoubtedly considered as the best candidate to produce value-added chemicals by lignin depolymerization. Thus, it is of great significance to develop approaches that are able to accommodate the complex structure of lignin. Herein, an efficient depolymerization method is implemented in methanol using a mesoporous solid superbase KF/γ-Al2O3 catalyst. Research shows that the depolymerization of lignin is not obvious below the critical temperature of methanol, and the conversion of lignin increases significantly above the critical temperature, which can reach 60.6% at 270 °C. Phenols and methoxy-substituted monocyclic aromatic compounds are predominant in the liquid products acquired by gas chromatography/mass spectrometry. Furthermore, oligomers were analyzed by time-of-flight mass spectrometry equipped with electrospray ionization. It is found that the molecular mass of the oligomers varied between 200 and 600 Da, the degree of unsaturation is mainly 4 and 8 with 1-2 aromatic rings, and the carbon number is found to be within the range of 15-30. It is speculated that the oligomers are mainly monophenyl or biphenyl compounds containing branched chains.

Full text of DOI:10.1039/d0nj03063e

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DOI: 10.1039/D0NJ03063E
Depolymerization of alkaline lignin over mesopores KF-Al2O3
Yinshuang Guan, Wei Zhao, Kaishuai Liu, Tiantian Guo, Dingkai Wang, Mingyu Cui, Shuiyuan Fu, Xing Fan,
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Xianyong Wei
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Key Laboratory of Coal Processing and Efficient Utilization, China University of Mining & Technology, Xuzhou
221116, Jiangsu, People’s Republic of China
Abstract
As a kind of biomass resources, lignin is the only natural renewable resource that can provide abundant
aromatic compounds, which is undoubtedly considered as the best candidate to produce value-added chemicals by
lignin depolymerization. Thus, it is of great significance to make approaches that are able to accommodate the
complex structure of lignin. Herein, an efficient depolymerization method is implemented in methanol using
mesopore solid superbase KF-Al2O3 catalyst. Research shows that the depolymerization of lignin is not obvious
below critical temperature of methanol, and the conversion of lignin increases significantly above critical
temperature, which can reach 60.6% at 270 °C. Phenols and methoxy-substituted monocyclic aromatic compounds
are predominant in the liquid products acquired by gas chromatograph/mass spectrometer. Furthermore, the
oligomers were analyzed by time-of-flight mass spectrometry equipped with electrospray ionization. It is found that
the molecular mass of oligomers distributes between 200 and 600 Da, the degree of unsaturation is mainly 4 and 8
with 1 to 2 aromatic rings, and the carbon number is found to be within the range of 15 and 30. It is speculated that
the oligomers are mainly monophenyl or biphenyl compounds containing branched chains.
Keywords: lignin, depolymerization, KF-Al2O3, solid superbase
 Corresponding author
E- mail address: zhaow1965@163.com.

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also take up a considerable proportion D(O20I:-1305.1%03)9/iDn0NliJg0n3i0n63E
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1. Introduction
structure.
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It is estimated that 5 × 1011 tons of lignin is available
annually, making it the only renewable resource that can provide
aromatic polymers.1 Lignin is usually synthesized by
polymerization of 4-hydroxyphenyl propane (H), guaiacyl
propane (Guaiacyl, G) and lilac propane (Syringyl, S),2 as shown
in Fig. 1. At present, there is no consensus on the exact structure
of lignin. Different sources, preparation methods and some other
factors lead to the diversity and complexity of lignin structure.3
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Fig. 2 Connection modes between lignin structural units.
Although the basic structural units and linkages of lignin
have been known, the precise structure of lignin cannot be
defined because of its randomness and complexity. In addition,
many hydroxyl groups in lignin contribute to the formation of
hydrogen bonds, which greatly increased the difficulty of lignin
depolymerization.4-8 However, lignin is expected to be used as a
raw material for the production of high-value fine chemicals such
as aromatic hydrocarbons and phenolic compounds by
depolymerization. The key is to develop suitable routes to break
the linkage of adjacent structural units.
Fig. 1 Basic structural units of lignin.
Lignin is a kind of natural amorphous, three-dimensional
polymer compound composed of abundant aromatics, mainly
contains C, H, O and small amounts of other elements. Ethereal
C-O linkages (-O-4, -O-4, 4-O-5) are the main way of bonding
connections between lignin structural units, accounting for about
60-75%. Especially, -O-4 linkages are the most prominent ether
interconnecting bonds (roughly 50% of all connections). In
addition, as shown in Fig. 2, C-C linkages (，， etc.)
Numerous studies in recent years have been focused on the
catalyst for lignin depolymerization, which can be classified into
metal catalysts, acid and base catalysts according to the active
component. The metal catalyst is mainly composed of noble

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metals and other metals, such as Pt/C,9,10 Ni/C,11 Ru/C,12
exhibits good catalytic performance and high aromatic product
selectivity.21 The moderately acidic SiO2-Al2O3 catalyst is also
proved to be effective on the hydrolysis of lignin. At 250 °C,
about 30% of the aromatic monomer compound yield can be
obtained.1 The active components of solid acid are mainly
organic acids and molecular sieves. Organic acids are unstable
and easy to lose active components. Most of molecular sieves are
moderately acidic and also have some other problems such as
easy coking. Compared with acid catalysts, base catalysts can
significantly inhibit the formation of coke during the
thermochemical conversion of biomass,22 and solid base catalysts
can be repeatedly used by calcination. At present, there are few
reports on the use of solid superbase catalysts to catalyze the
depolymerization of lignin, so it is indispensable to develop solid
superbase catalysis to effectively depolymerize lignin into low
molecular mass aromatic products.
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Ru/TiO2,13 Ru/-A12O3,14 Al-SBA-15 supported Ni, Pd, Pt, Ru,15
etc. The depolymerization of lignin over metal catalyst generally
requires the introduction of hydrogen or hydrogen supply
solvents to generate hydrogen radicals under the action over the
catalyst, which attack C-O-C and C-C bond to achieve
depolymerization of lignin. Furthermore, hydrogenation on the
lignin structural unit branch or the benzene ring can be achieved
to increase the stability of the depolymerized products. Hydro-
depolymerization can realize the maximum utilization of lignin
to a greater extent, but it also has the problems of harsh conditions
and high cost.
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Acid- and base-catalysts mainly achieve depolymerization
of lignin by destroying C-O-C bonds. Most of them are
homogeneous catalysts such as H2SO4,16 HI,17 Co-Zn-beta
zeolite,18 NaOH, KOH, Ca(OH)2, LiOH and K2CO3.19,20
Although homogeneous acid- and base-catalysts have good
depolymerization effects, they also exist difficulties on separation
and recycling and are often accompanied by equipment corrosion.
Heterogeneous catalysts have been recognized for their
recyclability and high catalytic activity. Solid acids such as
HZSM-5 are also used in lignin depolymerization, which also
The solid base catalysts currently used are mainly alkali
metal oxides such as CaO, MgO, and BaO. Some researchers
have studied the depolymerization of pine lignin at 250 °C over
MgO catalyst, the conversion yield can reach more than 90% and
obtain 13% phenol yield.23 It is also reported that the
depolymerization of soda lignin in supercritical ethanol using

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CuMgAlOx catalyst24 gives a monomer yield of 23 wt% without
coking, and aromatics are the main products. Some scholars have
found that zeolite molecular sieves (NaX, NaY, NaP) have more
significant catalytic effects than traditional solid base catalysts
(CaO, MgO, HT, HAP), among which NaX can achieve yield
(51%) of the low molecular product.25
shows that KF/-Al2O3 catalyst is a super-base catalyst, which
has important applications in the field of organic synthesis. It has
the advantages of simple preparation, controllable pore size, etc.
28, 29 Although KF/-Al2O3 catalyst is simple to prepare with high
catalytic activity, there is no consensus on its catalytic
mechanism. And also, the active component which plays a high
catalytic role on lignin depolymerization is uncertain. Thus, in
our work, the preparation of supported mesoporous solid
superbase (KF/-Al2O3) is provided and then used as a catalyst to
propose the lignin depolymerization into value-added chemicals.
And also, the catalytic mechanism of lignin depolymerization
over KF/-Al2O3 is investigated.
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The study found that the lignin depolymerization
performance is closely related to the alkalinity of the catalyst. The
stronger the alkalinity is, the more completely the lignin
depolymerization will gain.26 Additionally, the lignin
depolymerization performance is also related to the property of
the catalyst itself, which determines the bio-oil content and main
components after the depolymerization. Different catalysts can
lead to different product distribution.27 Although the traditional
solid base catalyst has strong basicity, but factors such as specific
surface area and pore diameter also lead to unsatisfactory effect
of catalytic depolymerization. The supported catalyst can
maintain strong alkalinity and can also achieve adsorption and
depolymerization of lignin macromolecules by increasing
specific surface area and pore size. In summary, the preparation
of supported mesoporous solid superbase catalyzed lignin
depolymerization is of great significance. Catalyst screening
2. Experimental
2.1 Materials
Alkaline lignin used in this study was purchased from TCI
Chemical Reagent. The lignin sample was dried at 120 °C for 12
h before reaction. Other chemical reagents such as ethanol,
methanol,
Al(NO3)3·9H2O,
KF·2H2O
and
cetyltrimethylammonium bromide (CTAB) were purchased from
Sinopharm Chemical Reagent Co.,Ltd.

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2.2 Catalyst preparation
2.3 Catalyst characterization
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1.6 g of CTAB was dissolved in ethanol/water (1:1, ~150
mL), and then the mixture was stirred until CTAB was
completely dissolved. After that, 22.5 g of Al(NO3)3•9H2O was
added into the mixed solution and then kept stirring at 60 °C until
the solution was clear and transparent. The diluted ammonia
water was added by dropwise to adjust the solution pH to 9-10,
and then the reaction was kept for 12 h. After cooling to room
temperature and standing still, the supernatant was decanted, the
white precipitate as-prepared was washed several times with
deionized water by filtering and then placed in a freeze-drying
oven to get a white solid. Finally, the obtained samples were
calcined at 350 °C for 2 h in a tube furnace, then calcined at
550 °C for 6 h with a heating rate of 10 °C/min. With the removal
of CTAB, carrier -Al2O3 can be obtained. Next, a certain amount
of KF•2H2O and -Al2O3 were put in a mortar followed by the
addition of deionized water (5 mL), ground thoroughly and then
dried in freeze dryer at -56 oC. At last, the precursor was calcined
according to the same procedure above to give the final catalyst
KF/-Al2O3.
X-ray diffraction (XRD) patterns of the catalyst were
measured by D8-ADVANCE X-ray diffractometer from Bruker
in a 2θ range from 4° to 80°. The temperature programmed
desorption of CO2 (CO2-TPD) was carried on automated
chemisorption flow analyzer (Quantachrome, USA) instrument
in range from 120ꢀ°C to 800ꢀ°C. Specific surface area and average
pore diameter was analyzed on an Autosorb-1 instrument
(Quantachrome) via N2 absorption and desorption. The micro
structure of the catalyst was determined using a field emission
scanning electron microscope (SEM, SU8220).
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2.4 Lignin depolymerization experiments
The lignin was first dried under vacuum at 120 °C for 12 h,
and the depolymerization reaction was conducted in a 100 mL
autoclave reactor. In this experiment, lignin (0.3 g), anhydrous
methanol (30 mL), and KF/-Al2O3 (0.1 g) were added into the
micro-reactor to react at a certain temperature. After the reaction,
the mixture was quickly cooled to room temperature and the
product was taken out. The reaction mixture was ultrasonically
extracted for 20 min in methanol and separated by filtration, and
this procedure was repeated at least 10 times to obtain the liquid
products and residues. Then the liquid product was concentrated

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in a rotary evaporator to remove solvent. Product yield was
TOF-MS was run in positive ion mode with a scanning mass
range from m/z 200 to 3000. Nitrogen was used as both drying
gas and nebulizer gas. The source voltage and the nebulizer
pressure were set to 4.0 kV and 30 psi, respectively. MS data were
processed by Agilent MassHunter WorkStation and the
molecular formulas were assigned to a maximum of 50 C, 100 H,
10 O, 6 N and 2 S atoms. The solid products were conducted on
a Fourier transform infrared spectrometer (FTIR, Nicolet Magna,
USA, IR-560) to research the changes in lignin before and after
depolymerization.
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calculated based on the following equations:
m0 - mR
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YR (%) =
× 100
m0
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mL
m0
YL (% ) =
× 100
00
YG (%) = 1 - YR - YL
where YL，YR and YG depict the yield of the liquid products,
residual lignin and gas products respectively. And m0, mR and mL
represent the mass of the lignin feedstocks, dried residual lignin
and the concentrated liquid product, respectively.
2.5 Product analysis
The liquid products were detected by gas chromatography
mass spectrometer (GC/MS, Agilent, USA, 7890/5973) and
orbitrap mass spectrometer (Orbitrap MS, Q Extractive). The
GC/MS is equipped with a capillary column coated with HP-5MS
(60 m × 0.25 mm × 0.25 mm) and the ion source is operated in
electron impact mode (70 eV). Helium is the carrier gas with a
flow rate of 1.0 mL min-1 and a split ratio of 20: 1. The oven
temperature raised from 60 °C to 90 °C at a rate of 5 °C min-1 and
was kept at 90 °C for 1 min, then raised to 240 °C at 10 °C min−1
and held at 240 °C for 2 min, and eventually increased to 300 °C.
The data acquired were analyzed by ChemStation (Agilent
Technologies, USA) software. Compounds were identified by
matching mass spectra with NIST11 library database. The ESI-
3. Results and discussion
3.1 Catalyst characterization
The XRD patterns of -Al2O3, KF/-Al2O3 are shown in Fig.
3. The patterns show that the -Al2O3 has characteristic
diffraction peaks at 37.1°, 47.2° and 67.1°. For the KF/-Al2O3
catalyst, the peaks with 2 =18.7°, 31.7°, 36.7°, 43.5°, 54.7°and
62.7° were assigned to the K3AlF6 phase. Typical diffraction
peaks at 12.7°, 27.3°, 32.7° and 45.3° belong to the K2CO3 phase.
The characteristic diffraction peak of the K2CO3 phase may be
caused by the reaction of K2O or KOH with CO2 in the air.

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Fig. 4 N2 adsorption/desorption isotherms (a) and pore
distribution (b) of -Al2O3 and KF/-Al2O3.
Fig. 3 XRD patterns of -Al2O3 and KF/-Al2O3.
The BET surface area, pore sizes and average pore volumes
Several fluorine-containing substances such as KF, K3AlF6
of -Al2O3 and KF/-Al2O3 are listed in Table 1. The specific
surface area of KF/-Al2O3 decreased from 309.157 m2g-1 of -
Al2O3 to 147.645 m2g-1, which may be due to the loading of KF.
and AlF3 can be detected by XRD, it may be caused by the
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reaction of KF with the carrier. Possible reactions are as follows:
12KF + Al2O3 + 3H2O → 2K3AlF6 + 6KOH
12KF + Al2O3 → 3K2O + 2K3AlF6
Table 1 The specific surface area and pore structure properties
of the catalysts.
BET surface
Pore volume
Pore size
Therefore, there may exist variable mixtures of F-, AlF6
,
3-
Sample
area（m2g-1）
（cm3g-1）
（nm）
309.157
147.645
0.618
0.260
9.542
6.535
-Al2O3
CO32-, OH- and O2- on the KF/-Al2O3 catalyst, which leading to
the high catalytic performance and strong basicity of the KF/-
Al2O3 catalyst.
KF/-Al2O3
The SEM images of KF/-Al2O3 (a) and KF/-Al2O3 (b-c)
were shown in Fig. 5. Remarkable pore structure can be seen from
the images. During the calcination process with high temperature,
the decomposition of CTAB can help the samples form more pore
structures and contribute to the formation of mesopores. The pore
structure of the catalyst (b-c) can be clearly seen from the picture,
which is beneficial to the reaction of lignin depolymerization.
As shown in Fig. 4a, both -Al2O3 and KF/-Al2O3 display
the conventional type IV isotherm shape and typical H3
hysteresis loop in each curve, which proves that the samples
contain mesoporous structure. Wide pore size distribution can be
seen from Fig.4b. Both of them possess micropores and
mesopores and mainly have mesoporous structure. The average
pore diameter of the support and KF/-Al2O3 are 9 nm and 6 nm
respectively, larger than the pore size of the general -Al2O3,
which is beneficial to the adsorption of lignin macromolecules.
Fig. 5 SEM images of -Al2O3 (a) and KF/-Al2O3 (b-c).

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3.2 Analysis of the products from lignin depolymerization
CO2-TPD was conducted to characterize the alkalinity of the
catalysts, and the results were shown in Fig. 6. The desorption
temperature indicates the strength of the basic sites. It can be seen
from the figure that the carrier only has a desorption peak at
171 °C, which represents the weakly basic site of -Al2O3. There
are weak and strong acid sites in KF/-Al2O3 and the adsorbed
CO2 on the sites was released around 185, 457, and 617 °C,
respectively, during the temperature programmed desorption of
CO2. The relative peak areas around 457 and 617 °C are much
larger than that around 185 °C, indicating that strong acid sites
are predominant in KF/-Al2O3. Combined with the
characterization results, it can be found that the alkalinity
enhancement of KF/-Al2O3 was attributed to the presence of KF
and the formation of new species between KF and -Al2O3. It is
precisely due to the strong basicity of the KF/-Al2O3 catalyst that
promotes the depolymerization reaction.
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As the FTIR spectra of raw material and its depolymerized
residues showed in Fig. 7 and Table 2, they all have wide
absorption bands around 3438 cm-1, which is caused by the O-H
stretching. The C-H stretching vibration of methyl and methylene
groups are indicated by the peaks at 2934 cm-1 and 2838 cm-1.
The absorption peaks at 1600 cm-1, 1512 cm-1 and 1463 cm-1 are
the stretching vibrations of the aromatic ring skeleton, indicating
that the lignin and the residues contain large amounts of benzene
ring structure. The peaks around 1267 cm-1, 1133 cm-1, and 1033
cm-1 should be attributed to the stretching vibrations of C-O,
which are mainly caused by ether bonds or methoxy groups in
lignin and residues. The deformation vibration of the C-H bonds
ranges from 610 to 700 cm-1.
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Table 2 Parameters of peaks from lignin sample’s infrared
spectrum.
Center (cm-1)
Peak
Assignment
1
2
3
4
5
3438
stretching vibration of O-H
asymmetric stretching vibration of CH3
2934
asymmetric stretching vibration of CH2
2838
1600, 1512, 1463
1267, 1133, 1033
stretching vibration of C=C in aromatic rings
asymmetric stretching vibration of C-O-C
Compare the FTIR spectra of lignin before and after
Fig. 6 CO2-TPD spectra of -Al2O3 and KF/-Al2O3.
depolymerization, it can be found that there is no significant
change of the absorption peaks around 2800-3500 cm-1 and 600-

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700 cm-1, and the changes are mainly concentrated in the range
addition of catalyst can produce more phenol. Similarly, the main
changes in liquid products FTIR spectra are still concentrated in
the range of 1000 cm-1 to 1600 cm-1. These peaks in this range
are mainly assigned to C-O-C stretching vibration, and the
absorption peaks of liquid products obtained by depolymerization
with or without catalyst have strong intensity in this region. The
main reason for the absorption peaks at 1000 cm-1 to 1300 cm-1
in liquid products is the linkage between benzene ring side chain
and a methoxy group, which are the basic molecular structures of
lignin. The peaks of the aromatic hydrocarbon at 1450 cm-1 and
1632 cm-1 show higher strength, indicating the enrichment of
aromatic compounds in liquid products. Additionally, the
absorption peaks intensities of the lignin depolymerized liquid
product over KF/-Al2O3 are stronger than those without catalyst,
which means the catalyst can promote lignin depolymerization.
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of 1000 cm-1 to 1600 cm-1, which can be attributed to benzene
rings and C-O stretching respectively. Absorption peaks of lignin
residue with no catalyst in the range of 1000 cm-1 to 1600 cm-1
are more sharper and stronger than that of the bands of
depolymerization residue over KF/-Al2O3 catalyst, indicating
that the addition of the catalyst can reduce the contents of ether
bonds and benzene rings, and break the ether bond in the lignin,
thereby promoting the lignin depolymerization.
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Fig. 7 FTIR spectra of lignin and its depolymerized residues at
250 °C.
FTIR spectra of lignin and its depolymerized liquid products
(with and without catalyst) in the range of 400-4000 cm-1 are
shown in Fig. 8. It can be seen that the main characteristic peaks
of the liquid product are significantly increased, and the peaks
intensity also have a stark increase. It is clear that the bands of
liquid product over KF/-Al2O3 at 3438 cm-1 related to the
formation of hydrogen bonds is wider than that of no catalyst.
Combined with the structure of lignin, it is shown that the
Fig. 8 FTIR spectra of lignin and its depolymerized liquid
products at 250 °C.
The liquid products of lignin depolymerization at different
temperatures were detected by GC/MS, and a close correlation

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was observed between the liquid products distribution and the
reaction temperature (for further details, see supplemental
information). The total ion chromatogram (TIC) of the liquid
products from lignin depolymerization with no catalyst are
displayed in Fig. 9a. As shown in Fig. 9, temperature has a great
influence on the products distribution. Under the condition of no
catalyst, the compound species of lignin depolymerization are
less in the range of 190-230 °C, and the relative content of 2-
methoxyphenol is higher. The product types around 250-270 °C
increase sharply, and the relative content of 2-methoxyphenol at
270 °C decrease significantly. The depolymerization products
over KF/-Al2O3 at different temperatures are also different from
those without the catalyst. As shown in Fig. 9b, in the range of
190-230 °C, the product types are further less than those of no
catalyst and the content of 2-methoxyphenol increases further. At
250 °C, 2-methoxyphenol began to diminish remarkably, because
that an increase of other compounds from depolymerize of lignin
under this condition results in a decrease in the relative amount
of 2-methoxyphenol. The conversion of lignin below the critical
temperature of methanol is relatively low, and there is a sudden
change at the critical temperature with the conversion greatly
improved. As illustrated in Figs. S6-S10 along with Table S5–S8,
a total of 191 organic compounds were dDeOteIc:t1e0d.10in39t/hDe0NliJq0u3i0d63E
products from lignin depolymerization, which can be classified
into phenols, non-phenolic aromatics, aliphatic compounds and
others.
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Fig. 9 TICs of the liquid products from lignin depolymerization
with no catalyst or over KF/-Al2O3 at 250 °C.
Fig. 10 shows the distribution of group components in the
liquid products from lignin depolymerization with and without
catalyst (a) at 250 °C and distributions at different temperature
over KF/-Al2O3 (b). Lignin is the only renewable resource in
nature that can provide aromatic compounds. The liquid products
from lignin depolymerization with no catalyst or over KF/-
Al2O3 are mainly aromatic compounds, and these products can be
classified into phenols, non-phenolic aromatic compounds,

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aliphatic compounds and the others. As shown in Fig. 10a,
Da. Interestingly, the obtained results for molecular mass
distribution are similar between the products with catalyst and no
catalyst, but it was found that but the relative content of
components with less than 400 Da of molecular mass increased
and decreased in the range of 400-600 Da using catalyst,
indicating that the lignin depolymerization over KF/-Al2O3
resulted in an overall decrease in the relative content of
compounds with higher molecular mass.
phenols are the most abundant products followed by non-
phenolic aromatic compounds, aliphatic and other compounds are
relatively less, accounting for about 10%. It is notably that
although the proportion of each classification product is similar
under the same reaction condition, the yields of the liquid
products from KF/-Al2O3-catalysed depolymerization of lignin
are higher than those without catalyst.
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Fig.10 Distribution of group components in the liquid products
from lignin depolymerization with no catalyst or over KF/-
Al2O3 at 250 °C (a) and distributions at different temperature
over KF/-Al2O3 (b).
Fig. 11 Molecular mass distributions of the liquid products from
lignin depolymerization with no catalyst or over KF/-Al2O3 at
250 °C.
GC/MS is not good at analyzing compounds with relatively
higher molecular mass and more aromatic rings, which limits its
measurement range. Hence, in this research, the products have to
be detected other methods. ESI is a soft MS ionization method to
ionize compounds with high to medium polarity while keep the
molecular structure undestroyed.30 Thus, the liquid products from
lignin depolymerization was further detected by ESI-TOF-MS,
the results are shown in Fig. 11. The molecular mass mainly
distributes from 300 to 600 Da and concentrate among 400-500
The unsaturation and cyclic structures of the compounds can
be represented by double bond equivalent (DBE) and the iso-
abundance plots of DBE versus carbon number for compounds
(with relative abundance represented as the area of bubble) are
shown in Fig. 12. It is suggested that the carbon number of the
oligomer compounds in liquid products mostly distribute
between 15 and 30, the DBE value was mainly 4 and 8, and there
were also some compounds having an unsaturation of 12 and 16.
The DBE value among the products differs by 4, which may be

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the difference in one aromatic ring structure. Under the addition
of the KF/-Al2O3, the relative abundance of the compounds with
8 DBE value decreased, the value less than 8 increased, indicating
that non-aromatics and aromatics with 1-2 rings are the major
compounds in liquid products detected by Orbitrap MS.
(MALDI-TOF-MS, Microflex LRF, BrukerDDOaIl:t1o0n.1ic0s3,9G/De0rmNJa0n3y0)63E
analysis. Fig. 13 depicts the mass distribution chromatograms for
the lignin before and after the depolymerization over KF/-Al2O3
under the given reaction conditions. As shown in Fig. 13, there
was a stark decrease in the value of the molecular weight for the
sample treated with KF/-Al2O3, in comparison with the
corresponding raw lignin. Before the reaction, the molecular
weight of the lignin sample distributed between 3000-24000 Da
evenly. However, after the depolymerization, the mass of the
lignin residue concentrated between 3000-9000 Da and the
intensity became stronger. This significant decrease in mass can
be attributed to the high catalytic activity of the catalyst for
depolymerization of the alkaline lignin.
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Fig. 12 Iso-abundance plots of DBE versus carbon number of
liquid products from lignin depolymerization with no catalyst or
over KF/-Al2O3 at 250 °C.
Combined with lignin structure, it can be seen from ESI-
TOF-MS analysis that the oligomer products contain aromatic
ring structures, and most aromatics have one or two ring
structures. After the addition of KF/-Al2O3, the monophenyl ring
products increased significantly. The production of oligomers
may be caused by the cleavage of lignin macromolecules or self-
condensation from depolymerization products, and the catalyst
can promote the conversion of lignin while reduce the content of
oligomers with higher molecular mass.
Fig. 13 Mass spectra of lignin and its depolymerized residue
between 3000-24000 Da at 250 °C.
3.3 Analysis of factors affecting lignin depolymerization
As shown in Fig. 14a, the conversion of lignin did not
increase significantly as the temperature rising with no catalyst,
and the YL was below 20% at different temperatures. It was
accompanied by severe coking and residues sticking to the inner
The relative change in the molecular weight of the alkaline
lignin and residue samples were examined by matrix-assisted
laser desorption/ ionization time of flight mass spectrometry

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wall of the reactor, which caused cleaning difficultly. With the
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addition of the KF/-Al2O3, YL was significantly improved and YR
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decreased, which can be clearly observed in Fig. 14b. And also,
during the experiment, the coking problem reduced, along with a
visible color change of the liquid mixture. The YR decreased and
the YL increased while the YG also increased slowly with the
temperature rising. The changes of YR, YL and YG are not obvious
before 230 °C, whereas extremely obvious at 250 °C and 270 °C.
The most likely reason can be credited to the critical temperature
of the solvent methanol (240 °C). When the temperature reached
this point, the viscosity and diffusion coefficient of the solvent
are close to that of the gas, thereby resulting in the large solubility
of lignin in methanol and accordingly increasing the product
yields. Conversely, when the temperature was below 230 °C, the
reaction system did not reach the critical temperature, hence
leading to the low product yields. In summary, temperature has a
great influence on lignin depolymerization in methanol. The
increase of temperature is beneficial to the lignin
depolymerization, but at the same time the gas products will
inevitably increase.
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Fig. 14 Effect of temperature on lignin depolymerization (a. no
catalyst; b. with KF/-Al2O3).
Fig.15 shows the effect of temperature on the molecular
mass distribution of liquid products. At different temperatures
(190-270 °C), the molecular mass distribution trend of the liquid
products obtained by lignin depolymerization is approximately
the same, the molecular mass mainly distributes between 300-600
Da, and the oligomers with less than 300 Da and greater than 600
Da of molecular mass are relatively few. In the range of 200-300
Da, the relative content of liquid products depolymerized below
the critical temperature of methanol (190-230 °C) decreases with
increasing temperature, yet increases above the critical
temperature (250-270 °C). In the range of 400-600 Da, the change
of the relative content of molecules at different temperatures is
opposite to that in the range of 200-300 Da. Below the critical
temperature, only a few small-molecule lignin fragments and
larger-molecule lignin fragments which are more easily reacted
will be dissolved in methanol, and the increase of temperature
will cause the larger-molecular lignin fragments gradually
dissolving in methanol. Therefore, below the critical temperature,

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the higher the temperature is, the lower will the relative content
of oligomers (200-300 Da) with smaller molecular mass gets, but
the relative content of oligomers (400-600 Da) with larger
molecular mass increases. Above the critical temperature, the
bonds between the lignin molecules begin to break a lot, making
the oligomers with larger molecular mass continue to
depolymerize and eventually cause the phenomenon that the
molecular mass distribution is opposite above and below the
critical temperature. In summary, when the reaction temperature
is above the critical temperature of methanol, the rise in
temperature is beneficial to the lignin depolymerization reaction
(from 250 °C to 270 °C), but it's not that the higher the
temperature, the better the result will get.
on lignin depolymerization was not apparent in the absence of
catalyst. The changes of YR, YL and YG were obvious from 1 h to
2 h, but not obvious in the range of 2~8 h. This is because the
reaction is incomplete within 1 h and completed 2 h later. When
the reaction time was 4 h and 6 h, the YL increased very slowly,
and the YG was basically unchanged. At 8 h, the YR increased and
the YL decreased. Based on this series of studies, it can be
concluded that the lignin depolymerization in methanol did not
need long time, the appropriate reaction time for
depolymerization is between 2-4 h. Besides, polymerization may
occur when depolymerization was underway.
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Fig. 16 Effect of time on lignin depolymerization (a. no catalyst;
b. with KF/-Al2O3).
Fig. 17 shows the effect of reaction time on the molecular
mass distribution of liquid products at 250 °C. It can be seen
that there is no obvious change regularity. From 1 h to 2 h, the
relative molecular content of 300-500 Da decreases over time.
Compounds larger than 500 Da increase, and the molecular
mass in the 4-8 h range does not change significantly.
Fig. 15 Molecular mass distributions of the liquid products from
lignin depolymerization at different temperatures over KF/-
Al2O3.
Since the lignin conversion was greatly improved above the
critical temperature of methanol, the follow-up reactions studying
the effect of the time on lignin depolymerization were carried out
at 250 °C. It can be seen from Fig. 16 that the influence of time

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the distribution was more dispersed. It can be seen that when the
reaction time is 4 h, the monobenzene ring products are more
concentrated, and the diphenyl ring products and polybenzene
rings are less, which proves that the favorable reaction time is 2-
4 h.
Fig. 17 Molecular mass distributions of the liquid products from
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lignin depolymerization at different reaction time over KF/-
Al2O3 at 250 °C.
Further analysis of the effect of time on the reaction is shown
in Fig. 18, it displays the oligomer DBE and carbon number
distribution obtained at different reaction times. Overall, the
oligomers' DBE are mainly concentrated in 4 and 8, while there
are oligomers with DBE of 12, 16 and 20, and the carbon number
is mainly concentrated in 15-30. It can be seen from the figure
that the difference in the DBE value between oligomers is mainly
4, indicating that different oligomers may differ by one benzene
ring, and the oligomers are mainly dominated by aromatic
compounds. At 1 h, the DBE of the oligomer has no regularity,
only the compound with a DBE of 4 is relatively obvious. At 2 h,
there are more oligomers with a DBE of 4, and the distribution
area is more dispersed. Among 2-6 h, with the time increasing,
the dispersion region of oligomer with DBE 4 becomes
concentrated, the oligomer with DBE 8 increases, and the
compounds with a DBE of 12 or more do not change much. At 6-
8 h, the oligomers with a DBE of 4 and 8 increased suddenly, and
Fig. 18 Iso-abundance plots of DBE versus carbon number of
liquid products from lignin depolymerization at different
reaction time over KF/-Al2O3 at 250 °C.
The effect of solid-liquid ratio on lignin depolymerization
was elevated under the condition of a known amount of KF/-
Al2O3 catalyst, provided in Fig. 19. With a certain amount of
solvent, as the lignin amount increased, YG was basically
unchanged, YR decreased firstly and then increased, and YL
changed contrary to the YR. Combined the results above, it can be
concluded that the amount of lignin that can be catalyzed

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the -site hydrogen is the main reaction pathway.31 Therefore,
the cleavage of the -O-4 bond is mainly achieved by capturing
the -hydrogen. After losing the -hydrogen, carbon radical is
formed, the electron in -carbon transferred to the oxygen atom
to produce oxygen radical, and form a double bond between -
carbon and -carbon. The oxygen radical would form hydroxyl
group or methoxy group with H• and •CH3. The obtained
phenolic compounds will further react under alkaline condition,
undergo the loss of hydrogen, the transfer of radical, and finally
react with some free radicals from methanol to get the final
products. After the hydroxyl group loses a hydrogen atom, the
electron on the oxygen transfers to the benzene ring, generating
carbon radical in the ortho-position of the hydroxyl group, and
then the generating carbon radical combines with the radicals
produced by solvent methanol such as •CH3, •CH2CH3,
•CH(CH3)2 to form alkylphenols through alkylation reaction. On
the other hand, the GC/MS analysis of the obtained
depolymerization products show that the carbon-carbon bond or
carbon-oxygen bond at different positions on the side chain of the
resulting phenylpropane structural compound would also be
broken to produce some new different compounds, such as 2-
methoxy-4-methylphenol, 4-allyl-2-methoxyphenol and so on.
depolymerization was constant when the amount of the catalyst
is certain. An appropriate increase in the amount of lignin can
stimulate the conversion, whereas excessive lignin could result in
the insufficiency of catalyst to break the ether bonds in lignin.
The suitable solid-liquid ratio is 0.4: 30 (g: mL).
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Fig. 19 Effect of different solid-liquid ratio on lignin
depolymerization over KF/-Al2O3.
3.4 Reaction mechanism
The cleavage route of C-O-C in lignin catalyzed by solid
base is shown in Fig. 20. First, methanol can generate free
radicals such as •CH3, •OCH3, OH•, H•, •CH2CH3 and
•
CH(CH3)2 under alkaline condition. There are two main ways to
break the -O-4 bond. One is to break the ether bond through
electron transfer by capturing the hydrogen atom in  position,
and the other is to capture the  hydrogen atom and make the
cleavage of ether bond through carbon ion rearrangement. The
dissociation energy of the -hydrogen is 86 kcal•mol-1, and 94
kcal•mol-1 of the -hydrogen. Theoretically, the -hydrogen is
more easily dissociated than the -hydrogen, and the
experimental results also indicate that the reaction occurring at

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h and the solid-liquid ratio is 0.4: 30 (g: mL). The temperature
made a significant impact on lignin conversion, critical
temperature of methanol is the demarcation point. A rising in
temperature will increase the yields and the types of the liquid
products, accompanied with changes in relative content, but also
due to the formation of additional gas products correspondingly.
The resulting depolymerization liquid products detected by
GC/MS and ESI-TOF-MS were mainly phenols, non-phenolic
monocyclic aromatics as well as small amounts of aliphatic
compounds, and the DBE value of products decreased
significantly, demonstrating high catalytic activity of the KF/-
Al2O3 in promoting the cleavage of lignin into low molecular
mass products.
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Fig. 20 The cleavage mechanism of -O-4 bond under alkaline
condition with methanol.
4. Conclusions
An active solid superbase catalyst KF/-Al2O3 with high
specific surface area and mesoporous structure has been
developed. A series of lignin depolymerization reactions were
conducted over the prepared catalyst, aiming to investigating the
influences of the KF/-Al2O3. Compared with the condition of no
catalyst, the presence of the catalyst can greatly reduce the coking
of lignin during the reaction process and increase the conversion
of lignin and product yields, highlighting the important role of
KF/-Al2O3 catalyst. In this work, it has been found that the
favorable reaction time for depolymerization is between 2 and 4
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Fundamental Research
Funds for the Central Universities (China University of Mining
and Technology, 2017XKZD10), and Priority Academic
Program Development of Jiangsu Higher Education Institutions.

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[7] G. Y. Chen, Y. L. Wang, F. Liu, B. B. Yan, Z. J. Cheng and
W. C. Ma, Comparison of combustion kinetics of the
biomass hydrolysis residue with raw biomass materials,
Energy Fuels, 2020, 34, 1193-1201.
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DOI: 10.1039/D0NJ03063E
Graphical abstract
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A KF/-Al2O3 catalyst prepared is employed in lignin depolymerization into low-
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molecular weight compounds, such as phenols and aliphatic compounds.
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