Metal-Free Photochemical Degradation of Lignin-Derived Aryl Ethers and Lignin by Autologous Radicals through Ionic Liquid Induction

Article abstract of DOI:10.1002/cssc.201901796

The degradation of lignin into aromatic products is very important, but harsh conditions and metal-based catalysts are commonly needed to cleave the inert bonds. Herein, an efficient self-initiated radical photochemical degradation for lignin-derived aryl ethers through ionic liquids (ILs) induction is demonstrated. The C?C/C?O bonds can be cleaved efficiently through free-radical-mediated reaction in the binary-ILs system 1-propenyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide [PMim][NTf₂] and the Br?nsted acid 1-propylsulfonic-3-methylimidazolium trifluoromethanesulfonate ([PrSO₃HMim][OTf]) under ambient conditions. [PMim][NTf₂] initiates the reaction by promoting the cleavage of the C_β?H bond, and [PrSO₃HMim][OTf] catalyzes the subsequent C?O?C bond fragmentation. Furthermore, alkyl, hydroxyl, and peroxy radicals are detected, which suggests degradation based on a photochemical free-radical process. Additionally, alkali lignin could also be degraded in the IL system. This work sheds light on sustainable biomass utilization through a self-initiated radical photochemical strategy under metal-free and mild conditions.

Full text of DOI:10.1002/cssc.201901796

Accepted Article
Title: Metal-Free Photochemical Degradation of Lignin-Derived Aryl
Ethers and Lignin by Autologous Radicals via Ionic Liquids
Induction
Authors: Ying Kang, Xingmei Lu, Guangjin Zhang, Xiaoqian Yao, Jiayu
Xin, Shaoqi Yang, Yongqing Yang, Junli Xu, Mi Feng, and
Suojiang Zhang
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To be cited as: ChemSusChem 10.1002/cssc.201901796
Link to VoR: http://dx.doi.org/10.1002/cssc.201901796
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10.1002/cssc.201901796
ChemSusChem
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Metal-Free Photochemical Degradation of Lignin-Derived Aryl
Ethers and Lignin by Autologous Radicals via Ionic Liquids
Induction
Ying Kang,[a][b] Xingmei Lu,*[a][b] Guangjin Zhang,[a] Xiaoqian Yao,[a] Jiayu Xin,[a][b] Shaoqi Yang,[a][b]
Yongqing Yang,[a][b] Junli Xu,[a] Mi Feng,[a][b] Suojiang Zhang*[a][b]
Abstract: Degradation of lignin into aromatic products is very
meaningful, but harsh conditions and metal-based catalysts are
commonly needed to cleave the inert bonds. We herein demonstrate
an efficient self-initiated radical photochemical degradation for lignin-
derived aryl ethers via ionic liquids (ILs) induction. The C-C/C-O
bonds could be efficiently cleaved through free radicals-mediated
reaction in the binary ILs system of [PMim][NTf2] and the brønsted
acid [PrSO3HMim][OTf] at ambient conditions. [PMim][NTf2] initiates
the reaction by promoting the cleavage of Cβ-H bond, and
[PrSO3HMim][OTf] catalyzes the subsequent C-O-C bond
fragmentation. Furthermore, alkyl, hydroxyl and peroxy radicals were
developed to effectively exploit the aromaticity of lignin and
lignin-derived compounds. However, these processes still suffer
from harsh and energy-intensive conditions issues, in which high
gas pressure and elevated temperature are required to break
the inert bonds in lignin, for example, the bond dissociation
energy of C-O and C-C linkages are 293 kJ/mol and 314-494
kJ/mol respectively[3a], resulting in high energy consumption[8].
Consequently, developing effective elegant approaches to
degradation of lignin under mild conditions are highly desired.
For the objective of mild lignin valorization, photocatalytic
degradation, which has some attractive advantages such as the
milder conditions, the avoidance of strong oxidants or reducing
agents and the efficient reaction sequences minimizing side
reactions[9], has been considered as a superior alternative to
detected, which suggested the degradation was based on
a
photochemical free radical process. Additionally, alkali lignin could
also be degraded in the ILs system. This work sheds light on the
sustainable biomass utilization by self-initiated radical photochemical
strategy under metal-free and mild condition.
conventional strategies[10]
.
Among all the photocatalysts,
semiconductor, like titanium dioxide (TiO2), remains the most
widely used[11]. The photocatalysis with TiO2 to carry out lignin
degradation is based on free radicals production process, that is,
when the light irradiates TiO2, the energy of which is equal to or
higher than TiO2 energy band (3.2 eV for anatase phase), an
electron is excited and leaves behind a hole, subsequently
Introduction
▪
▪
-
yielding OH or O2 radical to oxidize target lignin linkages[12]
.
Lignin is
representing 15 – 30% by weight and 40% by energy of
lignocellulosic biomass[1]. It has
rich aromatic structure
a
unique and abundant natural biopolymer,
Unfortunately, as demonstrated in previous researches, the
band gap of conventional TiO2 is large and recombination of
a
electron-hole pairs is fast, limiting its photocatalytic efficiency[9c]
.
primarily derived from methoxylated hydroxycinnamyl alcohol
building blocks and therefore can serve as a potentially valuable
source of renewable fuels and aromatic compounds[2]. However,
until now the large amount of lignin is chemically under-exploited
and only less than 5% of lignin is basically used as low-grade
fuels or concrete additives[3]. The heterogeneous and highly
cross-linked recalcitrant structure impedes the upgradation of
lignin to value-added fine chemicals[4]. Meanwhile, lignin-derived
compounds containing β-aryl ether bond, the most abundant
In order to improve these constraints, a variety of metal-based
catalysts, such as secondary semiconductors[13] and noble metal
nanoparticles[14], are introduced to TiO2 by facilitating electron
transfer and decreasing the recombination rate[15]. Nevertheless,
metals in doped photocatalysts suffer from leaching issues, and
as such, the photocatalysis efficiency degradation and the
generation of harmful metal waste remain grand challenges,
limiting the application of metal-based photocatalysts. A key step
to address this issue is to develop metal-free photochemical
routes, like the development of metal-free catalysts based on
polymeric carbon nitride[10b], which could yield free radicals to
oxidize target organic species by producing electrons and holes
after photons illumination. Obviously, the generation of free
radicals, primarily contributing to the cleavage of lignin linkages,
is the pivotal step for photochemical degradation method.
Interestingly, lignin could generate free radicals from itself after
exposing to UV irradiation and atmospheric oxygen[16]. However,
the process is very slow at ambient temperature. The finding
stimulated us to develop an effective metal-free route to induce
and accelerate free radical generation from lignin, which is also
meaningful for photochemical process.
linkage of native lignin[5], are often adopted to study lignin
6]
conversion process[4b,
. The existing strategies, including
hydrogenolysis, catalytic cracking and oxidation[7], have been
[a]
Dr. Y. Kang, Prof. Dr. X. Lu, Prof. Dr. G. Zhang, Prof. Dr. X. Yao,
Dr. S. Yang, Dr. Y. Yang, Prof. Dr.J. Xin, Prof. Dr. J. Xu, Dr. M.
Feng, Prof. Dr. S. Zhang
Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key
Laboratory of Green Process and Engineering, State Key
Laboratory of Multiphase Complex Systems, Institute of Process
Engineering, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: xmlu@ipe.ac.cn, sjzhang@ipe.ac.cn
[b]
Dr. Y. Kang, Prof. Dr. X. Lu, Dr. S. Yang, Dr. Y. Yang, Prof. Dr.J.
Xin, Prof. Dr. M. Feng, Prof. Dr. S. Zhang
Recently, the use of ionic liquids (ILs) provides a promising
alternative to meet these requirements. IL, an important type of
high-efficient designable media having attractive properties
such as high chemical stability and high solubility of
School of Chemistry and Chemical Engineering, University of
Chinese Academy of Sciences
Beijing 100190 (P. R. China)
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10.1002/cssc.201901796
ChemSusChem
FULL PAPER
substrates[17], have shown great practical application in lignin
upgradation acting not only as ideal solvent[18] but also as
catalyst for bond cleavage[19]. In particular, the designable ILs
has been exploited to promote free radicals generation and
demonstrate considerable potential for metal-free lignin
degradation. For example, the ILs containing anion [NTf2]- with
strong electronegativity could contribute to the delocalization of
electron pairs, which facilitate the formation of ·OOH and
thereby promoted lignin transformation, producing desired
products in encouraging yields[20]. Hence, the application of this
type of ILs in lignin photochemical degradation is promising,
which remains great potential for accelerating lignin degradation
by facilitating formation of free radicals under ambient conditions.
On the basis of the information and the experimental design
above, we propose the metal-free photochemical strategy for the
effective degradation of lignin-derived aryl ethers by autologous
radicals via ionic liquids (ILs) induction. In our proposed process,
the binary ILs system, namely [PMim][NTf2] and the brønsted
acid [PrSO3HMim][OTf], is highly efficient for C-C/C-O bonds
cleavage of lignin-derived aryl ethers to obtain desired aromatic
of 1 and formation of products through C-C/C-O bonds cleavage.
(Table 1, entries 2 to 3). While the results without [NTf2]-, like
[AMim][OTf], exerted obvious decrease on boosting the reaction
and gave less than 15% yields of products under comparable
conditions (Table 1, entries 4 to 6), suggesting that the bonds
cleavage is likely [NTf2]- assisted for the strong electronegativity
as reported for disruption of lignin-derived compounds[20]
.
However, the reaction in the absence of BAIL gives only a low
yield of aromatic products (Table 1, entry 7), we hypothesize
that the low yields could be due to the fact that the hydrogen
attached to the carbon linking to two nitrogens (NCHN) was
acidic[8].
To validate the hypothesis, an evaluation of the brønsted
acids was conducted (Table 1, entries 8-11). All the BAILs
contain the same cation, with a SO3H group covalently linked
through a propyl chain to the imidazolium cation (Figure S1).
Classic mineral acid phosphoric acid (H3PO4), the commonly
used acid in lignin degradation, was used as a reference. The
presence of BAILs is beneficial for the photochemical
degradation. The benzoic acid and phenol yields reached up to
products with more than 90% yields in the homogeneous system. 78.3±1.4% and 55.5±1.8% with minor catalytic amounts of
The alkyl, hydroxyl and peroxy free radicals are detected,
confirming that the process is based on a free radical reaction
mechanism. In the experiment, the results show that
[PMim][NTf2] initiates the reaction by promoting the cleavage of
Cβ-H bond, and [PrSO3HMim][OTf] catalyzes the subsequent C-
O-C bond fragmentation. Meanwhile, the ILs could dissolve
[PrHSO3Mim][OTf] (3 wt%). While other BAILs, like
[PrHSO3Mim][H2PO4], gave a much lower products yields. In
order to draw a correlation between catalytic activity and the
acids strength, the acid strength of all BAILs and H3PO4 were
measured. The Hammett acidity functions (H0) of the BAILs and
H3PO4 were spectrophotometrically determined (Figure S2) and
further calculated by UV/Vis spectroscopy (Table S1). The value
of H0 demonstrates that the acid strength of BAILs decrease as
lignin to provide
a homogeneous reaction system, which
significantly accelerate the transmission of free radicals and
therefore facilitate the subsequent degradation. In a word, this
research is considered to propose a metal-free photochemical
route for transforming lignin into renewable aromatics under
ambient conditions.
follows:
[PrSO3HMim][Cl] ＞ [PrSO3HMim][H2PO4]. In general, the
catalytic activity is closely related to acid strength of BAILs[21]
[PrSO3HMim][OTf]
＞
[PrSO3HMim][BF4]
＞
.
With BAILs in system, high acid strength is beneficial for the
reaction, the conversion of 1 displayed a significantly increasing
Table 1. Photocatalytic degradation of 1 in various ILs systema.
Results and Discussion
Influence of ionic liquids on the photochemical degradation
of 2-phenoxyacetophenone (1). The β-O-4 linkage has been
reported to constitute 45% to 60% of native lignin linkages.
Therefore, β-O-4 lignin dimmer, 2-phenoxyacetophenone (1),
was selected for benzylic photochemical degradation. Here, we
report the application of binary ILs system for the homogeneous
photochemical degradation of 1 under atmospheric oxygen,
room temperature and metal-free conditions. The ionic structure
of ILs have been reported to be influential for the lignin
degradation process[20]. Hence, several families of neutral ILs
with various anions ([NTf2]-, [BF4]-, [PF6]-, [OTf]-) and cations
([CPMim]+, [PMim]+, [BMim]+, [EMim]+, [AMim]+) were used to
investigate their effects on the process (shown in Table 1,
entries 1-6). The structures of these ILs were shown in Fig S1.
With [PMim][NTf2], the degradation of 1 achieves 87.1±2.7%
conversion for 120 min, affording benzoic acid and phenol with
yields up to 78.3±1.4% and 55.5±1.8%, respectively (Table 1,
entry 1). The anions of ILs significantly influenced the
photochemical degradation process. The presence of ILs with
[NTf2]- is beneficial for the reaction, which facilitates degradation
Yield (%)
Benzoic acid
Conv.
(%)
Entry
ILs system
Phenol
55.5±1.8
49.6±2.0
42.6±1.7
5.1±1.1
6.8±0.7
6.6±2.1
2.0±0.8
22.2±2.7
7.3±0.5
9.5±1.1
14.7±3.0
1
2
3
4
5
[PMim][NTf2]
[BMim][NTf2]
[EMim][NTf2]
[AMim][OTf]
[CPMim][BF4]
[BMim][PF6]
87.1±2.7
82.6±2.2
85.1±1.8
56.9±2.7
72.1±1.8
29.9±2.6
32.3±3.4
76.6±1.4
58.7±1.6
50.8±1.9
43.7±1.7
78.3±1.4
70.6±1.8
73.9±2.5
12.9±1.4
5.0±1.6
13.1±2.5
17.5±1.3
42.4±2.6
30.5±1.1
29.6±3.3
28.0±2.2
6
7*
8†
9†
10†
11†
[PMim][NTf2]
[PrHSO3Mim][BF4]
[PrHSO3Mim][Cl]
[PrHSO3Mim][H2PO4]
H3PO4
aConditions: 1 (10 mg), 500 mg of IL, [PrHSO3Mim][OTf] 3% (weight, 0.028
mmol), H2O 1% (weight), atmospheric air, room temperature, 120 min, UV
irradiation.
*
1 (10 mg), 500 mg of [PMim][NTf2], H2O 1% (weight), atmospheric air, room
temperature, 120 min, UV irradiation. 1 (10 mg), 500 mg of [PMim][NTf2],
BAILs/ H3PO4 (0.028 mmol), H2O 1% (weight), atmospheric air, room
temperature, 120 min, UV irradiation.
†
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10.1002/cssc.201901796
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trend with the decrease of H0, and [PrSO3HMim][OTf] led to the
highest yield. Meanwhile, H3PO4 gave low conversion and yields
of products, probably relevant to the relatively low acid strength
(Table S1).
temperature (the system automatically heats up to 45 C), the
reaction took 150 min to achieve conversion of 98.4±1.0%.

However, when raising temperature to 100 C, only 15 minutes
were needed to afford 1 conversion with up to 97.7±2.2%,
revealing that the photochemical degradation was sensitive to
high temperature. Nevertheless, excessive temperature also
exerted a slight decline in products yields, probably because of
the repolymerization at high temperature[8].
Influence
of
experimental
conditions
on
the
photochemical degradation of 1. Light irradiation is
indispensable for the degradation, as no conversion of 1 occur
under dark conditions (Table S2, entry 1). The degradation
under UV light irradiation gave the best performance. Besides,
poor product yields were detected under argon atmosphere with
a pronouncedly decreased conversion (8.5±2.7%), confirming
the necessity of atmospheric oxygen, and the removal of active
oxygen suppressed the C-C/C-O bond cleavage of 1 (Table S2,
entry 4). In addition, amount of [PrSO3HMim][OTf] also have
great effects on the reaction activity (Figure 1a). A 3 wt%
addition of [PrSO3HMim][OTf] gave the highest product yields,
beyond which a slightly decreasing trend was observed. The
decline in yields could be attributed to the side reactions that
Reaction kinetics. In order to have more insights into the
reaction dynamics, we monitored the reaction at different
temperature as a function of time (Figure 2). The time-on-course
process conducted at room temperature was measured (Figure
2a). Under UV light irradiation, 1 was converted with the
formation of benzoic acid and phenol. 98.4±1.0% of
1
conversion was achieved after 150 min of illumination, and
yields of benzoic acid and phenol reached 93.3±4.7% and
68.8±3.1%, respectively. A slightly decline trend of product
yields was observed beyond 150 min, which might be caused by
esterification or further oxidation[20]. The experiments at other
temperatures were also monitored to obtain the kinetic
parameters (Figure 2b, c, d). The photochemical degradation
were accelerated by large amount of [PrSO3HMim][OTf][22]
.
Water is also beneficial for the reaction as an increasing trend of
products yields was obtained with growing addition of water,
Figure 1b. However, slight decrease in conversion was exerted
with water addition beyond 3 wt%, which might be attributed to
(a)
(b)
the diluted system[20]
.
As illustrated in Table S3, elevated temperature significantly
accelerates the reaction. The reaction time that was needed to
achieve around 98.0% conversion at high temperature was
much shorter in comparison to that at low temperature. At room
(a)
(c)
(d)
(e)
(b)
Figure 2. Time-on-course process at different temperatures and kinetics on
the photochemical degradation of 1. (a) at room temperature (automatically
heating up to 45 C); (b) at 60 C; (c) at 80 C; (d) at 100 C; (e) Kinetics on
the photochemical degradation of 1 at room temperature to 100 C. Reaction
conditions: 1 (10 mg), [PMim][NTf2] 500 mg, [PrHSO3Mim][OTf] 3% (weight),
H2O 3% (weight), atmospheric air, UV irradiation.
Figure 1. Effects of [PrHSO3Mim][OTf] and water amount on the photocatalyic
degradation. Conditions: 1 (10mg), 500 mg of [PMim][NTf2], atmospheric air,
room temperature, 120 min, UV irradiation, (a) H2O 1% (weight), varying
weight ratio of [PrSO3HMim][OTf], (b) [PrHSO3Mim][OTf] 3% (weight), varying
weight ratio of H2O.
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To validate the hypothesis, we used electron spin resonance
(ESR) to identify radicals generated in the reaction. The ESR
spectrum of 1-ILs reaction system, which was irradiated with
UV-light irradiation (or in dark) and trapped with 50 mM DMPO,
was shown in Figure 4. As expected, two typical radicals, ·R
and ·OOR free radicals, could be labeled through determining
the hyperfine splitting constants (A factors). The spin adduct
conformed to the first-order reaction law from room temperature
to 100 C with an apparent activation energy (Ea) of 48.1 ± 5.4
kJ/mol (Figure 2e).

Stability and recyclability of the ILs. Subsequently, the
photostability of [PMim][NTf2] was also tested. Figure S3 showed
1
the comparison of H and 13C NMR for IL before and after UV-
(
AN =1.39 mT, AβH =1.04 mT) could be reasonably assigned to
light irradiation. The [PMim][NTf2] irradiated by UV-light for 150
min showed almost the same NMR spectra as that of fresh IL,
indicating that no significant decomposition of the IL occured
during UV irradiation. The 13C NMR spectra of the IL after UV
irradiation present signals of strength assigned to the [NTf2] unit,
a quartet peak around 125 ppm, which show almost the same
level as the IL before irradiation. This phenomenon indicates
that almost no [NTf2] is lost during the irradiation. Meanwhile, we
examined the recyclability of [PMim][NTf2] in the photochemical
degradation. As shown in Figure 3, [PMim][NTf2] could be
reused at least 5 times without notable loss of efficiency,
confirming the good stability of the IL in the reaction.
the peroxy radical adduct[23], which were labeled as radical b
(Scheme 1). The spectrum also showed A factors of
AN =1.61-
1.68 mT and AβH =2.14-2.15 mT, which could be assigned to
the formation of ·R[24], labeled as radical a. Furthermore, it was
interesting to notice the appearance of the A factors typical for
hydroxyl radical ( AN
=1.45 mT,
AβH
=1.33-1.37 mT)[25]
, which
was labeled as radical c. Negligible ESR signals were observed
under dark conditions.
Figure 3. The reuse of [PMim][NTf2] at room temperature for 150 min. Other
conditions were the same as those of Figure 2 (a).
Figure 4. ESR spectrum of 1-ILs reaction system at room temperature with
UV-light irradiation or in dark.
Mechanism of the photochemical degradation. Our
previous work showed that the binary ILs system, [PMim][NTf2]
and [PrHSO3Mim][OTf], exhibited high performance for the
reaction. Density functional theory (DFT) calculations indicated
that the interaction of 1 with [NTf2]- could lower the bond
dissociation energy of Cβ-H2 bond (Figure S4). Therefore, the
presence of ionic liquid [PMim][NTf2] could be favorable for
cleavage of the Cβ-H2 bond, and then promote the formation of
alkyl radical. Meanwhile, the imidazolium based BAIL,
[PrHSO3Mim][OTf], could provide H+ as well as facilitate the
cleavage of C-O-C bonds by interacting with lignin moieties via
protonated ‘O’[8], consistent with the experimental results that
very low yields of products were obtained without BAIL.
On the basis of the above described experimental results,
combined with experimental phenomena, we envisioned the
mechanism shown in Scheme 1. It had been reported that the
ether parts in polymer were selectively attacked and form
radicals during photodegraded process[26]. Firstly, assisted by
anion [NTf2]-, 1 generated alkyl radical a on the β-C atom linking
to ether bond with UV-light irradiation. Then free radical a
reacted with atmospheric oxygen to form the peroxy radical b.
Radical b subsequently abstracted protons from H2O to generate
hydroperoxides d and hydroxyl radical c, which further reacted
with 1 to produce H2O and radical a. The hydroperoxides d,
attacked by H+, BAIL and water, was further transformed to
benzoic acid and phenol. No formic acid was detected in the
products which might be converted to CO2 during the UV
irradiation[20]. The conjecture was proved by the FT-IR spectrum
of the reaction system conducted for 80 minutes, which showed
the peak appearing at 2323 cm-2 for CO2 (Figure S5). The FT-IR
spectrum of the reaction without adding 1 was also detected as
a reference (Figure S5). Furthermore, Figure S6 showed the
ESR detections of pure [PMim][NTf2] or pure [PrHSO3Mim][OTf]
with UV-light irradiation, no free radical signal was observed.
The result reveals that ionic liquid [PMim][NTf2] and
[PrHSO3Mim][OTf] cannot generate free radicals but promote
the reaction.
Meanwhile, the experimental results showed that ultraviolet
light irradiation and air had an important role in promoting 1
photochemical degradation. Actually, Hon et al.[16] has reported
a
lignin photochemical degradation process based on a
photochemical free-radical-mediated mechanism (exposed to
UV light and atmospheric oxygen). Free radicals, such as alkyl
radical and peroxy radical, were generated when lignin was
irradiated with UV-light in the atmosphere, which could further
lead to chain scission in lignin structure. Therefore, we deduce
that the photochemical degradation of 1 is a free radical reaction
related to alkyl radical (·R) and peroxy radical (·OOR).
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the implementation prospect of the photochemical system to C-
O bond cleavage in lignin α-O-4 linkages.
Subsequently, attention was paid to the photochemical
degradation of alkali lignin. 2D (HSQC) NMR was employed to
analyze the change in lignin interunit linkages (Figure 5) and
functional groups in the MTBE soluble oily products (Figure S9).
As shown in Figure 5, the signals observed at the characteristic
regions of C/H 100-140/6-8 and 50-90/2-6 ppm correspond to
the aromatic and aliphatic lignin substructures[8, 27]. In the
aliphatic regions, the signals at C/H 72.0/4.8, 63.8/3.2 and
71.5/3.8-4.2 ppm correspond to Aα, Bγ and Cβ respectively in the
lignin sample. Nonetheless, these signals were absent in the
degraded lignin, manifesting that lignin undergoes degradation
during the photochemical process. Additionally, in the aromatic
regions, the cross-signals observed at C/H 111.2/74, 115.4-
115.7/6.6-6.9 ppm linked to G2, G5 and G6, and the signals at
C/H 104.1/6.6 ppm linked to S2/6 in lignin. While in the degraded
lignin sample, the absence of signals for G2 again confirms that
lignin is degraded. Meanwhile, in oily products, the signal for aryl
methoxy groups were also observed by the cross-signal at C/H
56.3/3.7 ppm, which were in accordance with the products
containing –OCH3 group identified by GC-MS (Figure S10).
However, due to the complex structure of alkali lignin, the
selectivity for products still remains a challenge using this
method.
Scheme 1. Possible free radical mechanism for photochemical degradation of
1.
Photodegradation of other lignin-derived aryl ethers and
alkali lignin. To investigate the generality of the photochemical
system, we set out to delve the degradation of another β-O-4
lignin-derived
aryl
ether,
2-(2-methoxyphenoxy)-1-(4-
methoxyphenyl) ethanone (2), and the results are shown in
Table 2. As could be concluded from Table 2, the IL based
system was also verified to be effective for photochemical
degradation of 2, with anisic acid, guaiacol and p-acetanisole as
main products. As expected, the conversion of 2 in the IL system
showed an obvious trend towards higher value with prolonged
irradiation time, which increases to nearly 99.0% after 120 min.
Meanwhile, the reaction for 120 min gives the yields of anisic
acid up to 72.8±1.6%.
Aliphatic regions
Aromatic regions
Lignin
Lignin
Table 2. Photocatalytic degradation of 2b.
Degraded lignin
Degraded lignin
Yield (%)
Time
(min)
Entry
Conv. (%)
Anisic
acid
Guaiacol
P-acetanisole
1
2
3
4
5
30
60
90
120
150
44.3±2.8
76.2±1.6
91.5±2.3
98.9±0.5
99.0±0.2
35.1±1.9
56.3±1.7
72.2±3.2
72.8±1.6
68.1±1.5
21.0±1.3
36.9±1.7
25.8±1.1
24.0±2.1
22.8±2.3
5.6±1.4
21.0±1.6
21.2±1.2
20.1±1.3
18.7±2.5
[PMim][NTf2]
Lignin
bConditions: reactant (10 mg), 500 mg of [PMim][NTf2], [PrHSO3Mim][OTf] 3%
(weight, 0.028 mmol), H2O 3% (weight), atmospheric air, room temperature,
UV irradiation.
γ
γ
5
γ
γ
2
6
5
2
6
β
α
β
α
β
α
β
4
α
In the subsequent experiments we turned our attention
towards the photochemical degradation of 2-phenoxy-1-
phenylethanol (3), as shown in Figure S7. From the GC-MS
analysis, 3 first converted to ketone (1), and then underwent
similar cleavage of C-C/ C-O to 1. Further, attention was paid to
the photochemical degradation of α-O-4 types of lignin-derived
aryl ethers, benzyl phenyl ether (4) and 4–(benzyloxy)phenol (5),
as shown in Figure S8. From the results, aromatic aldehydes
originating from oxidative processes were detected, indicating
A (β-O-4)
B (β-5)
C (β-β)
G
S
Figure 5. HSQC NMR spectra (ppm) of lignin and degraded lignin samples. The
photochemical degradation of alkali lignin (100 mg) was conducted at the same
conditions as those of entry 5, table 2.
Conclusions
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10.1002/cssc.201901796
ChemSusChem
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couple. After a suitable time, the reactants were allowed to stop by
cooling to room temperature. The products were extracted by methyl
tertiary butyl ether (MTBE) from the reaction mixture. All the experiments
were carried out for three times.
In summary, we report an efficient self-initiated radical
photochemical strategy for degradation of lignin-derived aryl
ethers via ionic liquids induction under metal-free condition. The
C-C/C-O bonds could be effectively cleaved through free
radicals-mediated reaction with high yields of products. The IL
[PMim][NTf2] could promote the cleavage of the Cβ-H bond, and
the BAIL [PrSO3HMim][OTf] catalyzes the subsequent C-O-C
bond fragmentation. The mechanism is based on a free radical-
mediated process, involving in the alkyl, hydroxyl and peroxy
free radicals. In addition, alkali lignin could also be degraded
using this strategy. Our strategy rendered the photochemical
cleavage of C-C/C-O bonds effective under ambient conditions
while obviating the need for participation of metal. Moreover, the
self-initiated radical strategy opens an avenue for the metal-free
photochemical degradation of other polymer containing ether
linkages.
The recycling experiments were performed as follows. The resulting
mixture after products extraction was washed by 5 mL of water for three

times, which was then dried at 70 C for 24 h. Then the recycled IL was
used in the subsequent reactions.
Analytical methods
(1)
Quantification
by
Ultra-high
performance
liquid
chromatography (UPLC). UPLC was applied to quantify the reactants
and products in the reaction. UPLC spectra of the samples were detected
on Waters UPLC-QTOF equipped with a UV detector (at 225 nm) and a
C18+ column (2.1×100 mm). Column temperature was maintained at 298
K and the separation was conducted by gradient elution with a flow rate
of 0.2 mL/min. Acetonitrile (CH3CN) and 0.01 mol/L sodium acetate
(NaOAc) solution, pH 5.4-5.8 adjusted by acetic acid, were used as the
mobile phase. The gradient program was as follows: 1-2 min
(CH3CN:NaOAc(0.01mol/L) = 5:95), 2-4min (CH3CN:NaOAc(0.01mol/L)=
20:80),
4-6min
(CH3CN:NaOAc(0.01mol/L)=30:70),
6-15min
15-19min
Experimental Section
(CH3CN:NaOAc(0.01mol/L)=50:50),
(CH3CN:NaOAc(0.01mol/L)=95:5).
Reagents and Materials. Methyl tertiary butyl ether (>99%), p-
nitroaniline(>99%) are purchased from Shanghai Aladdin Reagent Co.
Ltd. Acetonitrile (>99.9%), phosphoric acid (H3PO4, 85% aqueous
solution), phenol (99%), benzoic acid (99%), sodium acetate (>99%)
were obtained from XiLong Scientific Co., Ltd. Acetic acid (>99%) are
provided by Beijing Chemical Works. 2-phenoxyacetophenone was
provided by Shanghai SHUYA Pharmaceutical Technology Co., Ltd. 2 -
(2-methoxyphenoxy)-1-(4-methoxyphenyl)ethanone was purchased from
Bidepharm technology Co., Ltd. 2-phenoxy-1-phenylethanol (97%) was
provided by Ark Pharm, Inc. benzyl phenyl ether (97%) was purchased
from Alfa Aesar Reagent Co., Ltd. 4–(benzyloxy)phenol (98%) was
provided from Aladdin Reagent Co., Ltd. Benzaldehyde (99%), benzyl
alcohol (99%), hydroquinone (99%), 4-methoxyacetophenone (99%),
anisic acid (99%) and guaiacol (99%) were purchased from Innochem
Technology Co., Ltd. The ILs 1-propenyl-3-methylimidazolium
The conversion of lignin-derived aryl ethers and yields of products
were calculated as follows:
Conversion (%) = (C0-Clignin)×100/ C0
Yield (%) = Cproducts×100/ C0
(1)
(2)
Where C0 represents the initial concentration of lignin-derived aryl
ethers; Clignin represents the concentration of lignin-derived aryl ethers at
sampling time; and Cproducts represents the concentration of the products,
respectively.
(2) Gas chromatography
–
mass spectrometry (GC-MS).
Qualitative analysis was proceeded by gas chromatography mass
spectrometry (GC-MS). The GC-MS analyses were performed on a
SHIMADZU GCMS-QP2020 instrument equipped with an Rtx-5MS
capillary column (30m*0.25mm), with helium as carrier gas. An injection
volume of 1 μL was used with an injection temperature of 290 C. The
initial temperature of GC oven was 35 C, then raised to 280 C with a
rate of 10 C/min, where it was held for 10 minutes. The total run time
was 34.5 minutes.
bis((trifluoromethyl)sulfonyl)
imide
[PMim][NTf2],
1-butyl-3-
methylimidazolium bis((trifluoromethyl)sulfonyl) imide [BMim][NTf2], 1-
propylronitrile-3-methylimidazolium tetrafluoroborate [CPMim][BF4], 1-
Allyl-3-methylimidazolium trifluoromethanesulfonate [AMim]OTf, 1-butyl-
3-methylimidazolium hexafluorophosphate [BMim][PF6], 1-propylsulfonic-
3-methylimidazolium chloride [PrSO3HMim]Cl, 1-propylsulfonic-3-
methylimidazolium dihydrogen phosphate [PrSO3HMim][H2PO4], 1-
(3) NMR measurements. The 1H spectra were recorded on Bruker
Avance III HD 600 operating at 600 MHz. 13C NMR spectra were
recorded on the same instrument at an operating frequency of 150 MHz.
the spectra were detected at room temperature. The heteronuclear single
quantum coherence (HSQC) NMR spectra were recorded on the same
instrument. For each test, approximately 50 mg lignin sample or 20 mg
oily products were dissolved in 0.5 mL of DMSO-d6 in an NMR tube,
respectively. The spectra were measured under the following acquisition
parameters: spectral width of 11 ppm in F2 (1H) with 1024 data points
and 190 ppm in F1 (13C) with 512 data points; 64 scans and 1 s interscan
delay. Bruker’s Topspin 3.5 pl4 software was used for volume integration
of contours in HSQC spectra.
propylsulfonic-3-methylimidazolium
[PrSO3HMim][OTf] and
trifluoromethanesulfonate
1-propylsulfonic-3-methylimidazolium
tetrafluoroborate [PrSO3HMim][BF4] were purchased from Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences. The ILs
were vacuum dried at 70 C for 24 h before used. Alkali lignin was
provided by Green Value SA.
Helium (>99%) was provided by Beijing Beiwen Special Gases Factory.
(4) Electron spin resonance (ESR). ESR was performed on a JEOL
JES-FA 200 ESR spectrometer at 298 K. The typical spectrometer
parameters included: microwave frequency, 9.051 GHz; microwave
power, 0.998 mW; center field, 3234 G; sweep width, 10 mT; sweep time,
1 min; time constant, 100 ms; modulation frequency 100 kHz; modulation
amplitude, 1000.
The sample with 50 mM DMPO was shaken for several minutes. Then
a certain amount of the mixtures were taken out and the ESR spectra
were recorded in dark and under UV-light irradiation respectively at room
temperature.
Light source. A Xenotest apparatus AULTT CEL-HXUV300 was used
to simulate UV and visible light. The ultraviolet reflector UVREF was
used to obtain UV, the cut-off filter UVIRCUT400 was used to simulate
visible light. The samples were positioned in irradiation area of the lamp
and irradiated in air.
Photochemical Reaction. The typical procedure of photochemical
degradation experiments is as follows. A certain amount of ionic liquids,
brønsted acidic ionic liquids, water, lignin-derived aryl ethers or alkali
lignin were added into a quartz reaction vial and stirred. The reactor was
exposed to the required gas (Ar or air) and placed under light illumination
with a stirring of 1000 rpm. The radiation energy of light generated was
100 mW/cm2. The autogenous temperature was measured by thermal
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10.1002/cssc.201901796
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FULL PAPER
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Acknowledgements
This research was financially supported by National Basic
Research Program of China (973 Program, 2015CB251401),
National Natural Science Foundation of China (No. 21878292,
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Keywords: metal-free • photochemical • ionic liquids • lignin •
autologous radicals
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UV-light on lignin: an efficient self-
initiated radical photocatalytic strategy
for degradation of lignin-derived aryl
ethers and lignin via ionic liquids
induction is developed.
Ying Kang, Xingmei Lu,* Guangjin
Zhang, Xiaoqian Yao,Jiayu Xin, Shaoqi
Yang, Yongqing Yang, Junli Xu, Mi
Feng, Suojiang Zhang*
Page No. – Page No.
In the homogeneous system, the C-
C/C-O bonds could be efficiently
cleaved through free radicals-
mediated reaction and afford aromatic
products in high yields at metal-free
conditions.
Metal-Free Photochemical
Degradation of Lignin-Derived Aryl
Ethers and Lignin by Autologous
Radicals via Ionic Liquids Induction
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