Highly Efficient Cleavage of Ether Bonds in Lignin Models by Transfer Hydrogenolysis over Dual-Functional Ruthenium/Montmorillonite

Article abstract of DOI:10.1002/cssc.202000978

Cleavage of ether bonds is a crucial but challenging step for lignin valorization. To efficiently realize this transformation, the development of robust catalysts or catalytic systems is required. In this study, montmorillonite (MMT)-supported Ru (denoted as Ru/MMT) is fabricated as a dual-functional heterogeneous catalyst to cleave various types of ether bonds through transfer hydrogenolysis without using any additional acids or bases. The prepared Ru/MMT material is found to efficiently catalyze the cleavage of various lignin models and lignin-derived phenols; cyclohexanes (fuels) and cyclohexanols (key intermediates) are the main products. The synergistic effect between electron-enriched Ru and the acidic sites on MMT contributes to the excellent performance of Ru/MMT. Systematic studies reveal that the reaction proceeds through two possible reaction pathways, including the direct cleavage of ether bonds and the formation of intermediates with one hydrogenated benzene ring, for all examined types of ether bonds, namely, 4-O-5, α-O-4, and β-O-4.

Full text of DOI:10.1002/cssc.202000978

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Highly Efficient Cleavage of Ether Bonds in Lignin Models via
Transfer Hydrogenolysis over Dual-Functional Ru/Montmorillonite
Authors: Zhimin Xue, Haitao Yu, Jing He, Yibin Zhang, Xue Lan,
Rundong Liu, Luyao Zhang, and Tiancheng Mu
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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To be cited as: ChemSusChem 10.1002/cssc.202000978
Link to VoR: https://doi.org/10.1002/cssc.202000978
01/2020

10.1002/cssc.202000978
ChemSusChem
FULL PAPER
Highly Efficient Cleavage of Ether Bonds in Lignin Models via
Transfer Hydrogenolysis over Dual-Functional
Ru/Montmorillonite
Zhimin Xue,*[a] Haitao Yu[a], Jing He[a], Yibin Zhang[a], Xue Lan[b], Rundong Liu[a], Luyao Zhang[a], and
Tiancheng Mu*[b]
Dedication ((optional))
[a]
[b]
Dr. Z. Xue, Mr H. Yu, Prof. J. He, Mr Y. Zhang, Mr R. Liu, Miss L. Zhang
Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology
Beijing Forestry University
Beijing 100083, China
E-mail: zmxue@bjfu.edu.cn
Prof. T. Mu, Miss X. Lan
Department of Chemistry
Renmin University of China
Beijing 100872
E-mail: tcmu@ruc.edu.cn
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: Cleavage of ether bonds is the crucial but challenging
step for lignin valorization. To efficiently realize this transformation
significantly relays on the development of robust catalysts or
catalytic systems. Herein, montmorillonite (MMT)-supported Ru
in the great potential risk especially when the reactions are
conducted under high temperature. To avoid the use of high-
pressure H2, catalytic transfer hydrogenolysis (CTH) using
alcohols as the alternative hydrogen resources has been
recognized as a highly important lignin-first biorefinery approach
for the reductive catalytic fractionation of lignocellulosic
biomass.[6] For the hydrogenolytic cleavage of the ether bonds in
lignin/lignin models, several catalytic systems have been
constructed up to date. For example, employing isopropanol as
the hydrogen resource, the ether bonds in several lignin model
compounds could be cleaved over supported metals (i.e., Ru,
Pd, Pt, ReOx),[7] RANEY® Ni,[8] or bimetallic Pd/Ni.[9] Meanwhile,
Cu-based catalysts could catalyze CTH of lignin and its model
compounds using methanol as the hydrogen resource.[10]
Furthermore, the CTH process could directly cleave the ether
bonds in lignin to form small-molecular compounds over α-
molybdenum carbide[11] or RANEY® Ni.[12] More interestingly,
the alcoholic groups in lignin could be directly used as the
hydrogen resource to convert lignin and its model compounds.[13]
In spite of these important achievements, robust catalysts are
still highly desired for transfer hydrogenolytic cleavage of the
ether bonds in lignin or its typical models.
(denoted as Ru/MMT) was fabricated as
a
dual-functional
heterogeneous catalyst to cleave various types of ether bonds via
transfer hydrogenolysis without using any additional acids or bases.
It was found that the prepared Ru/MMT could efficiently catalyze the
cleavage of various lignin models and lignin-derived phenols, and
cyclohexanes (fuels) and cyclohexanols (key intermediates) were
the main products. The synergistic effect between electron-enriched
Ru and the acidic sites on MMT contributed to the excellent
performance of Ru/MMT. Systematic studies revealed that the
reaction was conducted through two possible reaction pathways,
including the direct cleavage of ether bonds and the formation of
intermediates with one hydrogenated benzene ring, for all the
examined types of ether bonds, i.e., 4-O-5, α-O-4, and β-O-4.
Introduction
To alleviate excessive reliance on the diminishing fossil
resources (i.e., coal, petroleum, and natural gas), utilization of
renewable and abundantly available lignocellulose has been
recognized as a promising alternative way to produce the
essential fuels and chemicals.[1] In this regard, transformation of
lignin (the major component of lignocellulose) and its derived
chemicals has gained increasing interest in recent years.[2]
Generally, the cleavage of ether bonds is the critical step to
covert lignin (including lignin derivatives hereafter) into
renewable fuels and valuable chemicals owing to the abundant
existence of various ether bonds in lignin.[2a,3]
Of the developed strategies (i.e., oxidation, hydrogenolysis,
and hydrolysis) to cleave the ether bonds in lignin,
hydrogenolysis has attracted significant attention owing to its
high atom-economy.[4] The hydrogenolytic cleavage process
often involves the use of high-pressure H2,[5] which would result
Fabrication of efficient catalysts using naturally occurring
materials is the important content of green chemistry.
Montmorillonite (MMT), a type of naturally abundant clay, is a
negatively charged two-dimensional silicate material, which is
neutralized by interlayer cations (e.g., Na+, K+, Ca2+) with the
high ion-exchange ability.[14] Generally, the layered structure of
MMT is beneficial for the preparation of size-controllable
supported metal catalysts.[15] More importantly, the acidic sites in
MMT could interact with the O atom in ether bonds,[16] which
could potentially promote the hydrogenolytic cleavage of the
ether bonds. Therefore, MMT-supported metals could be highly
promising catalysts for transfer hydrogenolytic cleavage of the
ether bonds in lignin.
Considering the unique property of MMT, herein, we
prepared MMT-supported Ru nanoparticles (denoted as
Ru/MMT) as a dual-functional heterogeneous catalyst to cleave
1
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the ether bonds in various lignin derivatives with cyclohexanes
(fuels) and cyclohexanols (key raw materials and intermediates)
as the target products via transfer hydrogenolysis. It was
observed that the prepared Ru/MMT possessed excellent
activity for the titled reaction without using any additional
additives (homogeneous acids or bases), and various
entry 3), indicating the key role of Ru0 to cleave the ether bond
in diphenyl ether. In comparison, Ru/SiO2 and Ru/TiO2 was
prepared to catalyze the transfer hydrogenolytic cleavage of
diphenyl ether. Although Ru/SiO2 and Ru/TiO2 showed moderate
activity for the reaction (Table 1, entries 4 and 5), the generated
products (cyclohexane, benzene, cyclohexanol, and phenol)
cyclohexanes and cyclohexanols could be successfully achieved. were complicated. Additionally, it was observed that diphenyl
ether could be converted over commercial Ru/C (Table 1, entry
6), but the activity of Ru/C was still lower than the Ru/MMT.
These results verified that the prepared Ru/MMT was a superior
Results and Discussion
heterogeneous catalyst for transfer hydrogenolytic cleavage of
diphenyl ether, and the reasons for the excellent performance of
Ru/MMT would be discussed in the following sections. In
another aspect, ethanol was examined as the hydrogen
resource for the reaction. Unfortunately, the conversion of
Fabrication and characterization of Ru/MMT
The desired Ru/MMT was fabricated through a two-step process,
including ion exchange of Ru3+ with the interlayer cations in
MMT and subsequent reduction of the above Ru3+-exchanged
MMT in H2/Ar at 300 oC, and the detailed route was described in
the experimental section. As examined by SEM technique
(Figure 1A), Ru nanoparticles were successfully supported on
the MMT with an average size of 1.8 nm without any
aggregation (Figure 1B). Elemental distribution mapping (Figure
S1) indicated that the Ru element was dispersed uniformly in
Ru/MMT. Meanwhile, Ru/MMT had similar XRD pattern with the
MMT (Figure 1C), suggesting that supporting Ru particles did
not change the structure of MMT, and there was no obvious
peaks for Ru particles in the XRD pattern of Ru/MMT, implying
the high dispersion of Ru particles without aggregation, which
was consistent with the TEM result (Figure 1A). Furthermore,
XPS spectra indicated most of the exchanged Ru3+ ions were
reduced to Ru0 (Figure 1D). Additionally, the content of Ru
element was 3.46 wt% determined by ICP-AES (VISTA-MPX).
o
diphenyl ether was very low even at 180 C (Table 1, entries 7
and 8) due to the much higher reduction potential of ethanol
than isopropanol, which was consistent with some earlier
works.[17]
Table 1. Transfer hydrogenolytic cleavage of diphenyl ether over various
catalysts.[a]
Yield of products (%)[b]
Conversion
Entry
Catalyst
(%)[b]
1
2
None
MMT
0
0
0
0
0
0
0
0
0
0
3
Ru/MMT
Ru/SiO2
Ru/TiO2
Ru/C
100
44.7
63.2
86.9
0
0
53.7
15.2
28.3
38.7
0
45.8
17.7
31.2
42.6
0
0
4[c]
5[c]
6[c]
7[d]
8[d,e]
5.8
2.6
4.3
0
2.2
0
0
Ru/MMT
Ru/MMT
0
1.9
0
0.7
0.8
0
[a] Reaction conditions: diphenyl ether, 1 mmol; isopropanol, 5.0 g; reaction
temperature, 150 °C; reaction time, 6 h; the catalyst amount, 100 mg (3.5 mol%
of Ru). [b] The conversions and yields were achieved by GC using n-
dodecane as the internal standard, and the yields were based on the amount
of C6 ring. [c] The amount of Ru/SiO2 (4.85 wt% Ru), Ru/TiO2 (4.73 wt% Ru),
and Ru/C (5 wt% Ru) were 71, 73, 69 mg, respectively. [d] Ethanol (5.0 g) was
used to replace isopropanol. [e] Reaction temperature was 180 oC.
Optimization
of
reaction
conditions
for
transfer
hydrogenolytic cleavage of diphenyl ether
Generally, reaction temperatures have significant impact on the
reaction efficiency. As shown in Figure 2A, the conversion of
diphenyl ether increased with the increasing of reaction
temperatures from 120 to 160 oC, and the reaction could be
completed at 150 oC. Meanwhile, the product distribution also
changed with the change of reaction temperature. For example,
at 120 oC, four main products, i.e., cyclohexane, benzene,
cyclohexanol, and phenol, were obtained with the yields of 10.2,
11.4, 18.4, and 2.1%, respectively. With the increase of reaction
Figure 1. Characterization of the Ru/MMT. (A) TEM image, (B) Size
distribution of Ru particles, (C) XRD patterns, and (D) XPS of Ru 3p.
Catalyst screening for the transfer hydrogenolytic cleavage
of diphenyl ether
o
temperatures, benzene and phenol would disappear at 140 C.
Diphenyl ether was initially employed as a model reactant to
evaluate the catalytic performance of the prepared Ru/MMT
using isopropanol as the hydrogen resource at 150 oC (Table 1).
There was no reaction occurring in the absence of any catalyst
(Table 1, entry 1) or using MMT as the catalyst (Table 1, entry 2).
To our delight, the Ru/MMT could completely convert the
diphenyl ether to form cyclohexane and cyclohexanol (Table 1,
Additionally, at all the examined temperatures, cyclohexyl phenyl
ether was detected as the reaction intermediate, and its amount
decreased with the increasing reaction temperature. On the
basis of the above results, we chose 150 oC as the optimal
temperature for subsequent experiments.
2
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Figure 2. Effect of reaction temperature (A), catalyst usage (B), and reaction time (C). Reaction conditions: diphenyl ether, 1 mmol; isopropanol, 5.0 g; reaction
temperature, 150 oC for B and C; reaction time, 6 h for A anad B; the catalyst amount, 100 mg (3.5 mol% of Ru) for A and C. In these figures, 1 was the
conversion of diphenyl ether, and 2 to 6 represented the yields of benzene, cyclohexane, cyclohexanol, phenol, and cyclohexyl phenyl ether, respectively.
Furthermore, the influence of the catalyst usage on the
reaction efficiency was examined (Figure 2B). It was found that
the conversion of diphenyl ether increased with the increase of
catalyst amounts from 20 to 100 mg (0.7 to 3.5 mol% Ru), and
diphenyl ether could be completely transformed into
cyclohexane and cyclohexanol with a catalyst usage of 100 mg
(3.5 mol% Ru). Meanwhile, small amounts of benzene and
phenol would be detected with a low catalyst usage (20 and 40
mg), and they disappeared with the high catalyst usage (> 40
mg) because the catalytic activity of Ru/MMT was enhanced
with more catalyst.
Figure 3. (A) Time-conversion plots for transfer hydrogenolytic cleavage of
diphenyl ether over Ru/MMT, and (B) Reusability of Ru/MMT. Reaction
Reaction time was another important parameter to affect the
product distribution. As shown in Figure 2C, the conversion of
diphenyl ether steadily increased with the prolonging reaction
time, and the reaction was completed in 6 h. Meanwhile, same
tendency was observed for the yields of cyclohexane and
cyclohexanol. Additionally, cyclohexyl phenyl ether was
observed in the whole reaction process, and nearly all of it was
converted with a reaction time of 6 h.
conditions: diphenyl ether, 1 mmol; isopropanol, 5.0 g; reaction temperature,
150 oC; reaction time, 6 h; Ru/MMT, 100 mg (3.5 mol% of Ru).
Scope of the lignin-based derivatives
Encouraged by the above-achieved excellent results, Ru/MMT
was attempted to catalyze the transfer hydrogenolytic cleavage
of different types of ether bonds in lignin, including 4-O-5, α-O-4,
and β-O-4 (Table 2). First, the compounds containing the 4-O-5
ether bond (i.e., diphenyl ether, 4,4’-dimethyl diphenyl ether, and
4,4’-dihydroxy diphenyl ether) could be efficiently converted into
the corresponding cyclohexanes and cyclohexanols over
Ru/MMT (Table 2, entries 1-3) although the 4-O-5 type has been
considered as the most stubborn ether bond. Thereinto, a higher
Heterogeneity and reusability of Ru/MMT
The heterogeneous nature of Ru/MMT in the catalytic process
was evaluated by a control experiment, in which the Ru/MMT
was removed when the reaction was carried out at 150 oC for 2
h, and then the reaction continued to be conducted for another 6
h without the existence of Ru/MMT. Obviously, no further
conversion of diphenyl ether occurred (Figure 3A), and the
distribution of products was also unchanged (Figure S2),
implying that Ru/MMT was a heterogeneous catalyst under the
reaction conditions. Meanwhile, based on the result of ICP, the
concentration of Ru in the reaction mixture was extremely low (<
1.0 ppm), indicating the negligible leaching of Ru species into
the reaction solutions.
In another aspect, the reusability of Ru/MMT was
determined in the reaction process to convert diphenyl ether
(Figure 3B). It was observed that the conversion of diphenyl
ether and the distribution of products showed no significant
change after Ru/MMT was reused for eight cycles. As
characterized by TEM (Figure S3), the morphology of the
Ru/MMT and the size distribution of Ru particles showed no
obvious change after Ru/MMT was reused for eight catalytic
cycles. These above results suggested the good stability of
Ru/MMT under the employed reaction conditions.
o
temperature (160 C) was needed for the complete conversion
of 4,4’-dimethyl diphenyl ether (Table 2, entry 3). Second, the β-
O-4 ether bond, the most abundant linkage in lignin, could also
be successfully cleaved over Ru/MMT (Table 2, entries 4-7). For
example, the β-O-4 ether bond in phenethyl phenyl ether could
be fully cleaved to form ethylbenzene and cyclohexanol as the
main products (Table 2, entry 4). 2-Phenoxyacetophenone and
α-phenethyl alcohol phenyl ether, two typical models containing
β-O-4 ether bond, could also be converted to generate the
corresponding chemicals, but the reaction temperature should
o
be increased to 180 C with a reaction time of 12 h (Table 2,
entries 5 and 6). More interestingly, the compound containing
the methoxy ortho-substituted β-O-4 (2-(2-methoxyphenoxy)-1-
o
phenylethanol), which could be completely converted at 160 C
with a reaction time of 12 h, possessed higher reactivity (Table 2,
entry 7) because ortho-substituted methoxy group could assist
the cleavage of β-ether bond.[7c] Third, the α-O-4 ether bond
(e.g., benzyl phenyl ether and 4-benzyloxyphenol) could also be
cleaved over Ru/MMT at 160 oC with a reaction time of 6 h
(Table 2, entries 8 and 9).
3
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Table 2. Transfer hydrogenolytic cleavage of various ether bonds in lignin models over Ru/MMT.[a]
Entry
1
Substrate
Temperature (oC)
Conversion (%)[b]
Yield of products (%)[b]
150
100
2
160
99
3
150
97
4
150
180
180
100
5[c]
99
6[c]
99
7[c]
160
99
8
9
160
160
100
100
[a] Reaction conditions: substrate, 1 mmol; isopropanol, 5.0 g; Ru/MMT, 100 mg (3.5 mol% of Ru); reaction time, 6 h. [b] The conversions and yields were
achieved by GC using n-dodecane as the internal standard, and the yields were based on the amount of C6 ring. [c] The reaction time was 12 h.
4
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o
As well-known, lignin bio-oil contains many phenols, in
which the benzene ring was generally substituted by methoxy
group. Therefore, the possibility of transfer hydrogenolytic
cleavage of Ar-O-CH3 in various phenols over Ru/MMT was
examined (Table 3). Initially, phenol was easily transformed into
cyclohexanol with small amount of cyclohexane at 120 oC with a
reaction time of 6 h (Table 3, entry 1). In contrast, phenyl methyl
at 150 C with a reaction time of 6 h (Table 3, entries 3 and 4).
The difference for these two substrates was that the amount of
the methoxy-substituted products from guaiacol (Table 3, entry 3)
was much lower than that from 4-methoxyphenol (Table 3, entry
4), indicating that the ortho-substituted substrate was more
reactive, which was consistent with the above finding in
cleavage of the 4-O-5 and β-O-4 ether bonds. However, when
there were two ortho-substituted methoxy groups (2,6-
dimethoxyphenol), the reactivity would be significantly
decreased owing to the steric hindrance effect (Table 3, entry 5).
Even at 180 oC, too much methoxy-substituted products were
obtained although the substrate was completely converted. To
our surprise, eugenol could also be converted into the
corresponding propyl-substituted products (Table 3, entry 6) with
a reaction temperature of 180 oC.
o
ether needed a much higher temperature (170 C) to achieve a
complete conversion (Table 3, entry 2) with the
methoxycyclohexane yield of 16.5%, and its yield was still 11.9%
even when the reaction time was prolonged to 12 h, indicating
the difficulty to completely cleave this type of ether bonds
(cyclohexyl-O-CH3). Subsequently, the reactivity of various
substituted phenols was detected. Both guaiacol (with one ortho-
substituted methoxy group) and 4-methoxyphenol (with one
para-substituted methoxy group) could be completely converted
Table 3. Transfer hydrogenation of phenols in various lignin bio-oil over Ru/MMT.[a]
Entry
Substrate
Temperature (oC)
Cononversion (%)[b]
Yield of products (%)[b]
1
120
100
2
3
170
150
99
100
4
5
150
180
98
85
6
180
93
[a] Reaction conditions: substrate, 1 mmol; isopropanol, 5.0 g; Ru/MMT, 100 mg (3.5 mol% of Ru); reaction time, 6 h. [b] The conversions and yields were
achieved by GC using n-dodecane as the internal standard, and the yields were based on the amount of C6 ring.
All these above results in Tables 2 and 3 indicated the
superior activity of Ru/MMT in the cleavage of various ether
bonds in lignin models and the conversion of various methoxy-
substituted phenols via transfer hydrogenolysis.
5
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reaction process, implying cyclohexyl phenyl ether was the
reaction intermediate. The C-O bond in the cyclohexyl phenyl
ether would be cleaved to generate cyclohexane and phenol
(not detected because of its rapid conversion rate at 150 oC)
owing to the lower bonding energy of the C(sp3)-O bond
compared to the C(sp2)-O bond.[7c,20] In another aspect, benzene
would be detected at lower reaction temperature (Figure 2A),
indicating the occurrence of the direct cleavage of the C-O bond
in diphenyl ether. More importantly, the yield of benzene was
higher than that of cyclohexane, while almost no benzene could
be generated through the former pathways. Therefore, the
cleavage of the 4-O-5 ether bond proceeded through two
reaction pathways (Scheme 1A), and we deduced that the later
pathway was major. In addition, the final yield of cyclohexane
was slightly higher than that of cyclohexanol because
cyclohexanol could be further converted into cyclohexane under
the reaction conditions.
Reasons for the excellent performance of Ru/MMT
As shown in Table 1, Ru/MMT possessed better performance
than commercial Ru/C for the reaction involving diphenyl ether
as the reactant (Table 1, entry 3 vs. 6). Moreover, it was
observed that the difference in catalytic activity between
Ru/MMT and Ru/C would be enhanced when more functional
groups were contained in the reactants. For example, 4,4’-
dihydroxy diphenyl ether could be completely converted over
Ru/MMT at 150 oC with a reaction time of 6 h (Table S1, entry 1),
o
while the conversion was only 46.8% over Ru/C even at 180 C
with a reaction time of 24 h (Table S1, entry 2). More importantly,
the ether bond in phenethyl phenyl ether could be fully cleaved
over Ru/MMT at 150 oC (Table S1, entry 3), while almost no
reaction was detected over Ru/C even at 180 oC (Table S1,
entry 4). These results strongly indicated that Ru/MMT was a
highly superior catalyst for the transfer hydrogenolytic cleavage
of the ether bonds in comparison with Ru/C.
Two reasons possibly resulted in the much better
performance of Ru/MMT than Ru/C. First, the property of Ru
particles played a key role for the reaction. As determined by
XPS (Figure 4), the binding energy of Ru 3p of Ru0 in Ru/MMT
(484.5 and 462.3 eV) was lower than that in Ru/C (485.6 and
463.4 eV), suggesting the generation of electron-enriched Ru0 in
Ru/MMT.[18] Electron-enriched Ru0 was beneficial for the
Second, phenethyl phenyl ether was employed to
investigate the reaction pathway of the cleavage of β-O-4 ether
bonds. In the reaction process, there were no 2-phenylethanol
and
2-cyclohexylethanol
yielded,
and
[2-
(cyclohexyloxy)ethyl]benzene and (2-cyclohexylethoxy)benzene
can be detected. Meanwhile, the yield of cyclohexane, which
should be generated from cyclohexanol conversion, was very
low. At a lower reaction temperature, the yield of ethylbenzene
was much higher than that of ethylcyclohexane (Table S2). From
these results, we could deduce that the cleavage of the β-O-4
ether bond in phenethyl phenyl ether occurred through the direct
cleavage of phenethyl phenyl ether or the formation of [2-
(cyclohexyloxy)ethyl]benzene and (2-cyclohexylethoxy)benzene
as the reaction intermediates (Scheme 1B).
formation of active
H (from isopropanol) for the transfer
hydrogenolysis. Second, the acidic sites could interact with the
O of the ether bonds (Scheme S1),[19] benefitting their cleavage.
Therefore, the acidity of Ru/MMT and Ru/C was determined by
NH3-TPD method. The results clearly suggested that the acidity
of Ru/MMT (0.25 mmol/g) was much higher than Ru/C (0.05
mmol/g), and thus there were more acidic sites in Ru/MMT to
interact with the ether bonds, which resulted in the excellent
performance of Ru/MMT. On the basis of the above discussions,
the synergistic effect between the electron-enriched Ru0 and the
stronger acidity significantly contributed to the excellent
performance of Ru/MMT for the transfer hydrogenolytic cleavage
of the ether bonds.
Third, the reaction pathway to cleave the α-O-4 ether bonds
was studied using benzyl phenyl ether as the model reactant. In
the reaction process, four main products, i.e., cyclohexanol,
cyclohexane, toluene, and methylcyclohexane, were achieved.
Meanwhile,
[(cyclohexyloxy)methyl]benzene
and
(cyclohexylmethoxy)benzene were detected, and their yields
increased first and then decreased until disappeared. Moreover,
the amount of cyclohexane was much lower than cyclohexanol
because cyclohexane was generated from the conversion of
cyclohexanol rather than the cleavage of the α-O-4 ether bond.
Additionally,
small
amount
of
benzyl
alcohol
and
cyclohexanemethanol could be generated. From these results,
we could deduce that the cleavage of the α-O-4 ether bond in
benzyl phenyl ether mainly occurred through the direct cleavage
of
phenethyl
phenyl
ether
or
the
formation
of
[(cyclohexyloxy)methyl]benzene
and
(cyclohexylmethoxy)benzene as the reaction intermediates
(Scheme 1C). In addition, small amount of side pathways also
occurred via the formation of benzyl alcohol and
cyclohexanemethanol (Scheme S2).
Figure 4. XPS spectra of Ru 3p in Ru/MMT (A) and Ru/C (B).
From above discussions, it was obvious that the direct
cleavage of the ether bonds and the formation of intermediates
with one hydrogenated benzene ring was the dominant pathway
for the three types of ether bonds contained in the examined
three model compounds. However, the reaction pathway would
be highly different when more functional groups (e.g., methoxy
and phenolic hydroxyl) were involved. Therefore, more work
should be done for substrates with more complex structures.
Discussions on reaction pathway
It is a highly important aspect to explore the reaction pathway for
the cleavage of different types of ether bonds.
First, diphenyl ether was selected as the model compound
to study the pathway to cleave the 4-O-5 ether bond. As
described in Figure 2C, the yield of cyclohexyl phenyl ether was
increased first and then decreased with the prolonging reaction
time, and no dicyclohexyl ether was generated in the whole
6
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Scheme 1. The possible reaction pathway for the cleavage of 4-O-5 (A), β-O-4 (B), and α-O-4 (C) ether bonds.
Experimental Section
Conclusion
Fabrication of Ru/MMT
In conclusion, Ru/MMT was successfully fabricated via simple
To prepare Ru/MMT, Ru3+-exchanged MMT should firstly be prepared. In
a typical route, MMT (20.0 g) was dispersed in water (800 mL).
Subsequently, aqueous solution of RuCl3 (50 mL containing ca. 1.8 g
RuCl3) was added into the above dispersion of MMT. The mixture was
stirred for about 48 h at room temperature. Finally, Ru3+-exchanged MMT
was achieved through filtration and freeze drying.
ion exchange of Ru3+ with the interlayer cations of MMT and
subsequent reduction of the exchanged Ru3+. The prepared
Ru/MMT could efficiently cleave various lignin derivatives
containing different types of ether bonds (i.e., 4-O-5, α-O-4, β-O-
4, and Ar-O-Me) by transfer hydrogenolysis using isopropanol as
Ru/MMT could be successfully obtained by the reduction of Ru3+
-
the
hydrogen
resource,
and
various
corresponding
exchanged MMT using H2/Ar mixture (The volume ratio of H2/Ar was
5/100.) at 300 oC for 5 h.
cyclohexanes and cyclohexanols could be achieved. Detailed
investigations indicated that the excellent performance of
Ru/MMT was resulted from the cooperation of electron-enriched
Ru and the acidic sites on MMT. Systematic studies revealed
that two possible reaction pathways, including the direct
cleavage of ether bonds and the formation of intermediates with
one hydrogenated benzene ring, occurred for all the examined
types of ether bonds, i.e., 4-O-5, α-O-4, and β-O-4. We believe
that transfer hydrogenolytic reaction is an efficient strategy to
cleave the ether bonds in lignin derivatives, and Ru/MMT has
great potential applications in this important transformation for
lignin valorization.
Transfer hydrogenolytic cleavage reactions
In a typical experiment, substrate (1 mmol), desired amount of catalyst,
and isopropanol (5.0 g) were charged into a stainless reactor of 22 mL
with a magnetic stirrer. After being sealed, the reaction mixture was
stirred at a desired temperature for the desired time. After the reaction,
the products were analyzed quantitatively using n-dodecane as the
internal standard by gas chromatography (GC, Agilent 7890 equipped
with an FID detector). The yields were obtained on the basis of the
amount of C6 ring in the used substrates.
Reusability of the Ru/MMT
7
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10.1002/cssc.202000978
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To examine the reusability of Ru/MMT, Ru/MMT was recovered by
centrifugation after each cycle, and washed with ethanol (4×5 mL). After
drying under vacuum at 50 oC overnight, the catalyst was reused for the
next catalytic cycle.
6698-6723.
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Keywords: Biomass conversion • transfer hydrogenolysis •
cleavage of ether bonds • lignin derivatives • synergistic effect
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Entry for the Table of Contents
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Montmorillonite (MMT) supported Ru particles (denoted as Ru/MMT) was fabricated as a dual-functional heterogeneous catalyst to
cleave various types of ether bonds via transfer hydrogenolysis. Without employing any additional acids or basses, various types of
ether bonds in lignin derivatives could be efficiently cleaved over Ru/MMT. The synergistic effect between electron-enriched Ru and
the acidic sites on MMT contributed to the excellent performance of Ru/MMT.
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