Selective catalytic degradation of a lignin model compound into phenol over transition metal sulfates

Article abstract of DOI:10.1039/c9ra09706f

Transition metal salts were employed as the catalysts to improve the selective degradation of the α-O-4 lignin model compound (benzyl phenyl ether (BPE)) in the solvothermal system. The results concluded that most of the transition metal salts could enhan

Full text of DOI:10.1039/c9ra09706f

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Selective catalytic degradation of a lignin model
compound into phenol over transition metal
sulfates†
Cite this: RSC Adv., 2020, 10, 3013
Min-ya Wu, Jian-tao Lin, Zhuang-qin Xu, Tian-ci Hua, Yuan-cai Lv, Yi-fan Liu,
*
Rui-han Pei, Qiong Wu and Ming-hua Liu
Transition metal salts were employed as the catalysts to improve the selective degradation of the a-O-4
lignin model compound (benzyl phenyl ether (BPE)) in the solvothermal system. The results concluded
that most of the transition metal salts could enhance BPE degradation. Among which, NiSO4$6H2O
exhibited the highest performance on BPE degradation (90.8%) for 5 h and phenol selectivity (53%) for
4 h at 200 ꢀC. In addition, the GC-MS analysis indicated that the intermediates during BPE degradation
included a series of aromatic compounds, such as phenol, benzyl methyl ether and benzyl alcohol.
Furthermore, the mechanisms for BPE degradation and phenol selectivity in the NiSO4$6H2O system
involved the synergetic effects between the acid catalysis and coordination catalysis, which caused the
effective and selective cleavage of the C–O bonds.
Received 20th November 2019
Accepted 9th January 2020
DOI: 10.1039/c9ra09706f
rsc.li/rsc-advances
cleavage of the C–O bonds in benzyl phenyl ether (BPE) under
catalytic transfer hydrogenation conditions using 2-propanol as
the H-donor and Pd/Fe3O4 as catalyst. Roberts et al.14 studied
the inuence of alkali carbonates on BPE degradation efficiency
reactions between 270 ꢀC and 370 ꢀC and they found that
toluene and 2,4-benzyl phenol were primary productions by
formation of a cation-BPE adduct.
However, most lignin depolymerization processes still
suffered obvious disadvantage of low conversion and the
degradation products were uncontrollable. The single catalyst
could not meet the dual needs of further improving the degra-
dation efficiency and achieving higher product selectivity.
Therefore, the method of multiple catalysts combined to
degrade lignin has been widely investigated. Recently, transi-
tion metal salts have been more widely used as primary catalysts
or synergistic catalysts due to their more reactive chemical
properties in biomass conversion. Shu et al.15 proposed to use
Introduction
As a natural renewable aromatic polymer, lignin can be depo-
lymerized into small molecule compounds as high value-added
chemicals under certain conditions.1 Lignin has a complex
three-dimensional spatial structure, composing of three phenyl-
propanoid monomers with a large amount of C–O and C–C
bonds.2,3 Meanwhile, it contains methoxyl, phenolic hydroxyl,
alcohol hydroxyl and other characteristic functional groups.
However, due to its unique complexity, lignin is prevented from
further conversion into value-added chemicals.4,5 Consequently,
most of lignin is only used as a waste product or low quality fuel.
The C–O ether bonds are the most important linkages in the
structure of lignin.6 The most inter-unit linkages in lignin are b-
O-5, a-O-4 and 4-O-5 ether bonds,7 in which the a-O-4 is the
most vibrant due to the lowest bond dissociation energy of the
aliphatic C–O bond.8,9 At present, lignin related model
compounds, such as a-O-4 benzyl phenyl ether (BPE), have been
investigated to disclose selective degradation of lignin into
small aromatics because of the structural complex and variable
of lignin. So far, a great number of homogenous and hetero-
geneous catalysts have been applied extensively for cleaving the
C–O ether bonds by preserving the aromatic ring function-
ality.10–12 For example, Paone et al.13 achieved the efficient
CrCl3 cooperated with Pd/C to depolymerize lignin, and ach-
16
´
ieved highly controllable product distribution. Tribulova et al.
found that low molecular products, such as monosaccharides,
were formed when transition metal sulfates were used to
hydrolyze cellulose, which improved the cellulose sensitivity to
oxidation. Zakzeski et al.17 studied the oxidative degradation of
lignin in ionic liquids with varies transition metal salts as co-
catalysts. The results showed the positive impact of the metal
salts on the catalytic degradation of lignin. Besides, the catalytic
activity of metal salts could be enhance remarkably by the ionic
liquid under mild conditions.
Fujian Provincial Engineering Research Center of Rural Waste Recycling Technology,
College of Environment & Resources, Fuzhou University, No. 2 Xueyuan Road,
Shangjie Town, Minhou County, Fuzhou, Fujian, 350116, China. E-mail:
mhliu2000@fzu.edu.cn
In addition, the effect of solvent for cleaving C–O bonds of
aryl ethers were also concerned.18–20 Methanol has obvious
solvent thermal effect and benign hydrogen supply, which can
† Electronic supplementary information (ESI) available: GC-MS and MS spectra.
See DOI: 10.1039/c9ra09706f
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produce H+ and other free radicals under mild conditions. areas with products solutions of known concentrations of these
Besides, it is cheap and bio-renewable. Importantly, it can compounds. In addition, the recovered catalyst was character-
stabilize the monocyclic radicals generated in the degradation ized by X-ray diffraction (XRD, Miniex 600, Rigaku Co., Japan)
process, resulting in generating more monomer aromatic spectrum for recovered catalyst was obtained using Cu Ka
groups and reducing the role of polymerization.21,22
radiation at 40 kV and 40 mA.
In this study, in order to investigate the effect of transition
The primary products from this process were divided into
metal sulfates on catalytic cleavage of C–O bond and products gaseous fraction, volatile products, nonvolatile products and
selectivity, and conrm the possibility of catalytic degradation residual solid. The weight of the gaseous fraction was negli-
of lignin as primary catalyst or cooperate with others. The gible. The yield and selectivity of degradation products were
transition metal sulfates were employed as homogenous cata- evaluated by the aromatic ring balance and according to the
lyst to degrade BPE in methanol. The conditions such as following equations (eqn (1) and (2)) based on the HPLC and
temperature and time for BPE degradation and phenol selec- GC-MS results.
tivity were studied. In addition, the intermediates during BPE
BPE degradation efficiency ¼ ((1 ꢁ n/n0)) ꢂ 100%
(1)
degradation were detected and the synergistic mechanism for
the selective cleavage of the C–O bonds were proposed. This
research would be a benecial reference for the future efficient
and product-controlled depolymerization of lignin by using the
cooperation effect of multiple catalysts.
Degradation products selectivity ¼ (nn/(2(n0 ꢁ n))) ꢂ 100% (2)
n: the residual mole of BPE; n0: the initial mole of BPE; nn: the
mole of degradation products.
Experimental section
Degradation of benzyl phenyl ether
Results and discussion
Benzyl phenyl ether degradation with different catalysts
Catalysts played a vital role in the BPE degradation.14 To study
the effects of different transition metal on the BPE degradation,
11 transition metal compounds were chosen as the catalyst and
the results were shown in Fig. 1 and Table S1.† As presented in
Fig. 1a, in the control system, BPE degradation efficiency was
rather low (3.6%) in the absence of the catalysts, suggesting that
the possible effect of methanol on BPE degradation was negli-
gible. Whereas, the rapid decline of BPE was observed in various
transition metal sulfates systems, suggesting that transition
metal sulfates could effectively accelerate catalytic degradation
of BPE. As reported in previous studies, the Lewis acid catalyst
could effectively degrade the lignin and model compounds
through the cleavage of C–O linkages.23–25 Herein, the hydrogen
The degradation of BPE was conducted 0.3000 g BPE, 1.4 mmol
transition metal salts, and 20 mL methanol were separately
poured into a 50 mL reactor equipped with PPL coating in
a glove box (LM1000S, Dellix Industry Co. Ltd, China). N2 was
kept in the headspace of the test tube before sealing the Teon
coated rubber cap with aluminium crimp. The autoclave was
heated to given temperature (rate of 7 ꢀC minꢁ1). When the
reaction was nished, the mixture was cooled down to room
temperature during 30 min using owing water. At selected
time intervals, 1 mL of liquid sample was taken out for analysis.
Aer ltered through 0.22 mm glass ber lters (Tianjin Branch
billion Lung Experimental Equipment Co., Ltd, China), the
residual BPE concentration and the generated phenol concen-
tration were monitored. Used catalyst was washed thrice with
ꢀ
methanol. Then it was dried at 40 C until a constant weight.
The system without catalyst was set the control. All experiments
were conducted in triplicates.
Analytical methods
The BPE concentration was monitored by high-performance
liquid chromatography (HPLC, Agilent, USA), which equipped
with an UV detector set at 254 nm; and column temperature
40 ꢀC and 80% methanol (v/v), 20% ultra water (v/v) was used as
mobile phase at a ow rate of 1.0 mL minꢁ1. The products
during BPE degradation process were determined by gas chro-
matography coupled to concentration spectrometry (GC-MS,
Agilent 7890B + 5977, USA), which equipped with a HP-5ms
column (30 m ꢂ 0.25 mm ꢂ 0.25 mm), and ion source was EI.
ꢀ
The oven temperature was programmed as 50 ꢁC1 hold 2 min,
ꢀ
ꢀ
Fig. 1 (a and b) Effect of transition metal salts on BPE degradation
efficiency and phenol selectivity. Condition: 300 mg BPE, 1.4 mmol
transition metal salts, 20 mL methanol, 200 ꢀC, 5 h. (c and d) Hydrogen
ion concentration (c(H+)) of catalyst systems. Condition: 1.4 mmol
transition metal salts, 20 mL methanol, 200 ꢀC, 5 h.
and then ramped up to 280 C with 10 C min and hold for
ꢀ
another 5 min. The injector kept at 300 C in spit mode (5 : 1)
with helium as the carrier gas. The column ow rate was 1
mL minꢁ1. The residual concentration of the reactant BPE as
well as the products were determined by calibrating the peak
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ion concentration (c(H+)) detection results (Fig. 1c) showed that According to Roberts, et al.'s investigation, alkali carbonates
these systems with different transition metal sulfates main- can promote the ether bonds cleaved by forming the cation-BPE
tained more hydrogen ion than that of control during BPE adduct.14 However, due to the lower protons concentration in
degradation, which kept systems more acidic and conrmed the the system, the generated phenol would react with benzyl
essence of Lewis acid catalyst. Thus, as Lewis acids, the tran- alcohol and form molecular compounds, causing a decline of
sition metal sulfates could enhance the BPE degradation. In the phenol yield.14 On the other hand, the anion could signi-
order to conrm this, the catalytic activities of traditional cant affect the reactivity of the catalyst by charging the density
proton acids as H2SO4 was investigated. More than 90% on the metal center.29 During the cellulose decomposition,
2ꢁ
degradation efficiency and less than 20% of phenol selectivity SO4 in Lewis acid was found to be an excellent hydrogen
could be achieved. It demonstrated clearly that lignin model bonding acceptor and nucleophilic reagent.30 Hence, the higher
compound could be efficiently degraded through acid catalysis phenols selectivity and BPE degradation was mainly contrib-
of metal sulfates. Meanwhile, the product distribution were uted to the synergic effect between Ni2+ and SO42ꢁ. Based on the
signicantly depended on other element.
above results, NiSO4$6H2O was an excellent catalyst for BPE
As shown in Fig. 1a, these transition metal sulfates showed degradation.
rather different performance on BPE degradation and phenol
selectivity. The maximum degradation efficiency (90.4%)
Effect of reaction temperature on benzyl phenyl ether
degradation
occurred to NiSO4$6H2O system, following CuSO4$5H2O
(65.7%), FeSO4$7H2O (57.2%), ZnSO4$7H2O (43.6%), Cr2(-
SO4)3$6H2O (38.5%), CoSO4$7H2O (28.3%), and the minimum
degradation efficiency was observed in MnSO4$H2O system
(18.7%). Meanwhile, the similar results happened to the phenol
selectivity, suggesting that NiSO4$6H2O exhibited the best
performance on BPE degradation and phenol selectivity. Aer
treatment (200 ꢀC, 5 h), the hydrogen ion concentration (Table
S1†) in these systems without BPE were as follows: FeSO4$7H2O
(6.92 mmol Lꢁ1) > CuSO4$5H2O (2.40 mol Lꢁ1) > NiSO4$6H2O
(1.29 mmol Lꢁ1) > Cr2(SO4)3$6H2O (1.15 mmol Lꢁ1) > ZnSO4-
$7H2O (0.98 mmol Lꢁ1) > CoSO4$7H2O (0.19 mmol Lꢁ1) >
MnSO4$H2O (0.03 mmol Lꢁ1). According to the results in Fig. 1a
and c, it could be found that the systems with higher hydrogen
ion concentration exhibited better performance. However, it
seemed no signicant positive correlation between the BPE
degradation efficiency and nal hydrogen ion concentration,
which meant that besides the hydrogen ion concentration, the
transition metal cations might play a key role in enhancing BPE
degradation and phenol production. As previously reported by
others, the difference among these system were probably
attributed to the strengths of the substrate–catalyst interactions
in presence of different metal centers.26,27 According to Paul
et al.'s investigation, during the pyrolysis of cellulose by metal
salts, the mechanism mainly involved both acid and ionic
catalysis.28 In addition, they also found that the acidity or
alkalinity of the metal salt was less important than the ionic
catalysis. Considering the highest electronegativity of Ni atom
among these transition metal, it could be inferred that NiSO4-
$6H2O could produce more coordination points besides acidic
centers and increase the reaction activities through the
substrate–catalyst interactions during the process. Further-
more, the effect of anion on BPE degradation was further
investigated, NiCl2$6H2O, NiNO3$6H2O and NiCO3 were also
investigated with NiSO4$6H2O as catalyst. As displayed in
Fig. 1b, both BPE degradation efficiency and phenol generation
for NiCl2$6H2O, NiNO3$6H2O and NiCO3 were inferior to those
for NiSO4$6H2O. It can be explained by two reasons. On one
hand, as depicted in Fig. 1d, the hydrogen ion concentration
value of NiCO3 system was near zero, which resulting in supe-
rior BPE degradation efficiency but inferior phenol selectivity.
During lignin decomposition, temperature was one of the most
important factors and it played a vital position in activating the
catalyst and fracturing the C–O and C–C bonds.10 Herein, the
BPE degradation and phenol generation at varied reaction
temperature (180–220 ꢀC) were monitored and the results were
shown in Fig. 2. It was noticed that BPE degradation process
was highly dependent on temperature. The degradation effi-
ciency dramatically increased as the rise of temperature before
200 ꢀC (from 19.6 to 93.5%). Subsequently, BPE degradation
efficiency kept steady when the reaction temperature was over
200 ꢀC. Similarly, the phenol selectivity also exhibited rapid
ꢀ
ꢀ
increasing (from 180 C to 200 C). Once the temperature was
over 200 ꢀC, the phenol selectivity signicant decline (from 48.7
to 44.4%). All the results suggested that the phenol selectivity
was more sensitive to temperature than BPE degradation effi-
ciency. According to previous reports, much higher temperature
partly caused the carbonization of organic compounds and
accelerated the condensation reaction among degradation
products, which resulted in the decline of the phenol
selectivity.31
Fig. 2 Effect of reaction temperature on BPE degradation efficiency
and phenol selectivity. Condition: 300 mg BPE, 1.4 mmol NiSO4-
$6H2O, 20 mL methanol, 5 h.
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showed a relatively stable uctuation between 0–2 h. Aer-
wards, when the reaction time prolonged from 2 h to 2.5 h, the
hydrogen ion concentration increased to 23.40 mmol Lꢁ1
dramatically, and maintained increasing until the reaction time
was extended to 4 h, but rose to 1.29 mmol Lꢁ1 at 5 h. Mean-
while, the similar results happened to the BPE degradation
system. In both systems, it was observed that the hydrogen ion
concentration evidently increased during the process, which
demonstrated the essence of NiSO4$6H2O as a Lewis acid
catalyst.23–25 Nevertheless, the difference of hydrogen ion
concentration (Dc(H+)) during the whole process in both
systems was rather different. In the ltrate, the change of
hydrogen ion concentration was lower than that in control
system, which indicated that hydrogen ions participated in the
degradation process of BPE. According to previous studies, the
H+ played a very important role in BPE degradation.13
Benzyl phenyl ether, hydrogen ion concentration and catalyst
variation under different time
To study the effect of the reaction time on BPE degradation and
phenol selectivity, BPE degradation and pꢀhenol generation with
various time were monitored under 200 C (Fig. 3). As seen in
Fig. 3a, when the reaction time prolonged from 0.5 h to 5 h, BPE
degradation efficiency gradually increased from 5.6% to 90.8%.
However, the phenol selectivity varied signicantly with pro-
longed reaction time. It increased from 16.3% for 0.5 h to 53.1%
for 4 h, but dropped to 48.7% for 5 h, which meant that the
phenol might participate in the subsequent reaction. According
to Long et al.'s study, long reaction time was benecial to BPE
degradation but harmful to the phenol production.31
In order to further explore the catalytic mechanism of
NiSO4$6H2O, the hydrogen ion concentration during BPE
degradation was closely inspected at different reaction times
(Fig. 3b). In the control system, the hydrogen ion concentration
In order to further explore the mechanism of NiSO4$6H2O
for BPE degradation, the recovered catalysts were characterized
by XRD. As displayed in Fig. 3c, during 0–2 h, no new diffraction
peaks turned up, but the intensity of the (004) and (112) crystal
faces diffraction peaks of NiSO4$6H2O signicantly weakened,
indicating that NiSO4$6H2O participated in BPE degradation
process, but no signicant change in the structure occurred to
NiSO4$6H2O. Then, when the reaction time prolonged to 2.5 h,
the intensity of the diffraction peaks of NiSO4$6H2O were
weaker and a new diffraction peak at 26.7ꢀ appeared. Aer
conrmed by the PDF card (no. 21-0974), the new diffraction
peak was assigned to the (111) crystal face diffraction peak of
NiSO4$H2O, suggesting that NiSO4$6H2O lost crystal water
during the BPE degradation. Similarly, the crystal water loss
process of NiSO4$6H2O was also conrmed in the previous
reports.32 Besides, the corresponding hydrogen ion concentra-
tion showed a signicant increasing within 2.0 to 2.5 h, indi-
cating that the dehydration reaction of NiSO4$6H2O in
methanol caused a signicant increasing in the concentration
of hydrogen ions in the system. Besides, there were two
unknown diffraction peaks as a and b occurred at 5 h, which
were speculated to be NiSO4$xH2O (0 # x < 1), which meant
catalyst continued to lose water aer 2.5 h. According to Huang
et al.'s study, NiSO4$6H2O could achieve dehydration by co-
pyrolysis with methanol and the structure had been changed,
which further promoted the release of protons by methanol and
formed an acidic environment in the BPE degradation system.33
Therefore, the dehydrated catalyst played an important role in
the acid-catalyzed degradation of BPE.
Identication of the intermediates and the pathway of benzyl
phenyl ether during the process
Furthermore, the variation of the degradation products was
detected and the results were displayed in Fig. S1 and Table S2.†
The corresponding spectra of MS were attach in appendix
(Fig. S2†). During BPE degradation, approximately ve types of
possible products were identied through the National Institute
of Standards and Technology (NIST11) library. Peak 4 was BPE
(RT ¼ 17.235 min), with molecular ion at 184 m/z and major
fragment ions at 91 m/z. The peaks (1–3) with retention time at
Fig. 3 (a) Effect of reaction time on BPE degradation; (b) the hydrogen
ion concentration during the process. (c) X-ray diffraction pattern for
recovered catalysts. Condition: 300 mg BPE, 1.4 mmol NiSO4$6H2O,
20 mL methanol, 200 ꢀC.
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9.810, 7.048 and 7.888 min were the primary productions of BPE
degradation, which involved phenol, benzyl methyl ether, benzyl
alcohol, respectively. The peak 5 (RT ¼ 18.800 min) with molec-
ular ion at 184 m/z was conrmed as 2-benzyl phenol, which was
formed through secondary reaction between the intermediates.
Based on the GC-MS, the yield and selectivity of specic
compounds (phenol, benzyl methyl ether and benzyl alcohol)
with time were illustrated in Fig. 4 and Table S3.† As shown in
Fig. 4, with the prolonging of the reaction time (0–5 h), the
yields of phenol, benzyl methyl ether and benzyl alcohol
increased and reached maximum (1.410, 0.905 and 0.330 mmol,
respectively) at 5 h, which was consistent with the results of GC-
MS chromatogram (Fig. S1†). Instead, the selectivity of the
specic compounds was rather different. For phenol, the
selectivity increased before 4 h, but declined at 5 h. The selec-
tivity of benzyl methyl ether began to decrease at 2.5 h. All the
results indicated that during BPE degradation, the intermedi- Fig. 5 The pathway of BPE degradation. Condition: 300 mg BPE,
1.4 mmol NiSO4$6H2O, 200 ꢀC.
ates also participated in some side reaction, resulting in the
decline of the benzyl methyl ether and phenol selectivity, which
was in accordance with the results in Fig. 3.
Proposed catalytic mechanism
Based on the intermediates, the pathway of BPE during the
reaction system was proposed. During the synergistic effect of
catalyst and heat, methanol and the lost crystal water was
transformed into the methoxyl group, hydroxyl group and
hydrogen ions, respectively, while the decomposition of BPE via
the cleavage of C–O bonds involved two ways. In route a (Fig. 5),
BPE was cleaved to phenoxy radicals (1) and benzyl radicals (2).
Aerwards, the phenoxy radicals (1) combined with the hydrogen
ions preferentially and the benzyl radicals (2) bonded to the
methoxyl group, leading to the formation of phenol (5) and
benzyl methyl ether (6). In route b, BPE was broken into benzoxy
radicals (3) and phenyl radicals (4). Then the benzyl radicals (3)
combined with the hydrogen ions to form benzyl alcohol and
phenyl radicals (4) bonded to hydroxyl group to generate phenol.
According to related research, Caromatic–O bond dissociation
energy was much higher than Calkyl–O.10 Thus, it could be
concluded that route a was the main pathway for BPE decom-
position. Meanwhile, the formation dealkylation of benzyl methyl
ether (6) was ascribed into the rearrangement reaction between
the phenol (5) and intermediates, which was similar to Yang
et al.'s ndings.34 In addition, benzyl methyl ether (6) were prone
to rearrangement reaction with phenol and formed a new C–C
bond, resulting the presence of 2-benzyl phenol (8).
Based on the degradation pathway and the structural analysis
of the catalyst, the main degradation mechanism for BPE was
proposed in Fig. 6. First, NiSO4$6H2O maintained acid
condition during the BPE degradation as acid catalyst
Fig. 4 The yield (a) and selectivity (b) of degradation products.
Condition: 300 mg BPE, 1.4 mmol NiSO4$6H2O, 20 mL methanol,
200 ꢀC.
Fig. 6 Catalytic mechanism of NiSO4$6H2O. Condition: 300 mg BPE,
1.4 mmol NiSO4$6H2O, 20 mL methanol, 200 ꢀC, 5 h.
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(Fig. 3b). During the co-pyrolysis process of NiSO4$6H2O and
methanol, NiSO4$6H2O gradually lost water (Fig. 3c). Mean-
while, the sp3d2 orbital hybridization occurred at nickel atom,
and lead to the catalyst formed an octahedral metastable
transition state (Fig. 6).32 In this structure, an empty track on
the nickel atoms tended to form Lewis acid site besides that of
S atom. Hence, NiSO4$6H2O had more than one acidic site,
which was benecial to the breakage of ether bonds at acid
condition. Simultaneously, the SO42ꢁ, which acted as an
excellent hydrogen bond donor and nucleophile, synergized
with Ni2+ to promote the cleavage of C–O bonds. This mech-
anism accorded well with most acid catalytic lignin depoly-
merization process.35,36
Conclusions
The transition metal sulfates could be used as catalysts to
effectively decompose the lignin model compounds. Among
which, NiSO4$6H2O exhibited excellent performance on the
degradation of benzyl phenyl ether (BPE) and the phenol
selectivity. Due to the synergetic effects between the acid
catalysis mechanism and the coordination catalysis mecha-
nism, NiSO4$6H2O effectively and selectively clove the C–O
bonds in BPE and formed a series of aromatic compounds, such
as phenol, benzyl methyl ether and benzyl alcohol. Conse-
quently, this study would provide theoretical and experimental
basis for the decomposition of lignin by metal salt base
catalysts.
In addition, NiSO4$6H2O had the potential as coordination
catalysis, which exhibited important inuence on not only BPE
degradation efficiency but also the selectivity of productions.
On the one hand, the sp3d2 orbital hybridization occurred at
nickel atom also formed coordination site, which were apt to
form stable complexes with the oxygen atoms on a-O-4 bonds
(Fig. 6). The stable complexes could effectively reduce the acti-
vation energy of ether bond and then resulted in the fracture of
C–O bonds in the BPE, resulting in high BPE degradation effi-
ciency. Hence, BPE was depolymerized into phenoxy radicals
and benzyl radicals in methanol (Fig. 5) through the acid
catalysis and coordination catalysis of NiSO4$6H2O. On the
other hand, Ni atom could weaken the O–H bond in methanol
through coordination, which promoted the release of protons
by methanol and accelerated hydrogenation reactions.14 Since
the hydrogen absorption capacity of phenoxy radicals was better
than that of benzyl radicals, the proton preferentially bonded
with phenoxy group to form phenol, leading to the increasing of
phenol.21 Meanwhile, the benzyl radicals combined with
methoxyl group to formed benzyl methyl ether. Aerwards, with
prolonged reaction time, the O atoms on the methoxyl group of
benzyl methyl ether exhibited partial positive charge due to the
coordination between Ni and O, which greatly enhanced the
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The work was supported by the Production and Study Project of
Cooperation in Universities and Colleges in Fujian Province
(No. 2019H6008) and Natural Science Foundation of China (No.
21577018).
Notes and references
´
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electrophilicity and reactivity of the methyl.37 Meanwhile, as an
2ꢁ
excellent nucleophile, the SO4
could attack the methyl,
caused the separation of the methyl from methoxyl group.
Aerwards, the initial position of the methyl was replaced by
protons and then formed phenol. Accordingly, as the reaction
time prolonged, the difference between the yields of phenol and
benzyl methyl ether increased (Fig. 4).
Besides, according to previous report, due to the attack of Ni
atom, the lost crystal water was broken away and formed
protons and a small amount of hydroxyl14 and then BPE could
be hydrolyzed to form phenol and benzyl alcohol10,38(Fig. 5). In
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