Cleavage of the lignin β-O-4 ether bond: Via a dehydroxylation-hydrogenation strategy over a NiMo sulfide catalyst

Article abstract of DOI:10.1039/c6gc01456a

The efficient cleavage of lignin β-O-4 ether bonds to produce aromatics is a challenging and attractive topic. Recently a growing number of studies have revealed that the initial oxidation of C_αHOH to C_αO can decrease the β-O-4 bond dissociation energy (BDE) from 274.0 kJ mol^-1 to 227.8 kJ mol^-1, and thus the β-O-4 bond is more readily cleaved in the subsequent transfer hydrogenation, or acidolysis. Here we show that the first reaction step, except in the above-mentioned pre-oxidation methods, can be a C_α-OH bond dehydroxylation to form a radical intermediate on the acid-redox site of a NiMo sulfide catalyst. The formation of a C_α radical greatly decreases the C_β-OPh BDE from 274.0 kJ mol^-1 to 66.9 kJ mol^-1 thereby facilitating its cleavage to styrene, phenols and ethers with H₂ and an alcohol solvent. This is supported by control experiments using several reaction intermediates as reactants, analysis of product generation and by radical trap with TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) as well as by density functional theory (DFT) calculations. The dehydroxylation-hydrogenation reaction is conducted under non-oxidative conditions, which are beneficial for stabilizing phenol products.

Full text of DOI:10.1039/c6gc01456a

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ARTICLE
Cleavage of lignin β-O-4 ether bond via dehydroxylation-
hydrogenation strategy over a NiMo sulfide catalyst
Chaofeng Zhang,a,b Jianmin Lu,a Xiaochen Zhang,a Katherine MacArthur,c Marc Heggen,c Hongji
Lia,b and Feng Wanga*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Efficient cleavage of lignin β-O-4 ether bonds to produce aromatics is a challenging and attractive topic. Recently a growing
number of studies reveal the initial oxidation of CαHOH to Cα=O can decrease the β-O-4 bond dissociation energy (BDE)
www.rsc.org/
1
1
from 274.0 kJ·mol− to 227.8 kJ·mol− , and thus the β-O-4 bond is more readily cleaved in the subsequent transfer
hydrogenation, or acidolysis. Here we show that the first reaction step, except in the above-mentioned pre-oxidation
methods, can be a Cα-OH bond dehydroxylation to form a radical intermediate on the acid-redox site of a NiMo sulfide
1
1
catalyst. The formation of a Cα radical greatly decreases the Cβ-OPh BDE from 274.0 kJ·mol− to 66.9 kJ·mol− thereby
facilitating its cleavage to styrene, phenols and ethers with H2 and alcohol solvent. This is supported by control
experiments using several reaction intermediates as reactants, analysis of product generation and by radical trap with
TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) as well as by density functional theory (DFT) calculations. The
dehydroxylation-hydrogenation reaction is conducted under non-oxidative condition, which is beneficial for stabilizing
phenols products.
and then convert β-O-4-Ketone to other products.7a, 7b, 8c-e In
1. Introduction
HO
lignin β−O-4 structure
HO
C
γ
Compared with other biomass targets, lignin has received a
great amount of attention as it can potentially be used to
prepare functionalized aromatic fine chemicals.1 The key issue
for the lignin transformation lies in the development of
efficient catalysts and strategies to selectively cleave the stable
and ubiquitous ether bonds, whilst leaving the aromatic
benzene rings unconverted.2
C
α
Cβ-OPh bond
Cβ
O
4
C
O
MeO
OMe
OH
O
O
O
O
The β-O-4 ether bond structure (Figure 1) accounts for 40-
60% interunit linkage in lignin, and is the targeted bond to
break during lignin degradation.1a It also becomes the targeted
activation bond in the hydrogenolysis,3 oxidation,4 acidolysis,5
alcoholysis,6 redox-neutral7 and tandem methods.8
Experimental studies combined with DFT calculations have
shown that the substitutions at the Cα atom have a great effect
on the bond dissociation energy (BDE) of the Cβ-OPh bond and
the activation of β-O-4 lignin linkage.7a-d, 8c, 9
β−O-4-Alcohol
BDE: 274.0 kJ.mol-1
β−O-4-Ketone
BDE: 227.8 kJ.mol-1
β−O-4-Olefin
BDE: 336.6 kJ.mol-1
Figure 1. The structure of lignin β-O-4 linkage and transformation intermediates.
other reports, the β-O-4-Alcohol tends to dehydrate and
generate a β-O-4-Olefin.7e, 7f, 10 However, the BDE of Cβ-OPh
ether bond in β-O-4-Olefin is about 61 kJ·mol− and 108
1
kJ·mol−1 higher than the Cβ-OPh ether bonds in β-O-4-Alcohol
and β-O-4-ketone (Figure 1). The different catalytic routes
produce different intermediaries, which can cause variations in
the Cβ-OPh ether bond cleavage. While, the dominating factor
for the Cβ-OPh bond cleavage should not only contain the
target bond BDE of the original substrate but also one of the
1
The Cβ-OPh bond of β-O-4-Ketone has about 40 kJ·mol−
lower dissociation energy than that of β-O-4-Alcohol, causing
some studies to first convert CαHOH to Cα=O (β-O-4-Ketone),
activated intermediates’ phase.5c, 7c, In an ideal route for
7f
cleavage of the β-O-4 bond, an activated intermediate is
generated after a series of activation steps with low activation
energy, whose target bond has a lower energy and easily
breaks in the subsequent transformations. The unique
properties of catalysts should play dominative roles in the
activation route control.
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Although heterogeneous catalysts have been widely used
Journal Name
In the hydrogenation of β-O-4-A (Figure 2B), the NiMo
in the catalytic hydrogenation and alcoholysis lignin sulfide catalyst selectively catalyzed the cleavage of the Cβ-OPh
processes,11 research pays more attention to the conversion bond, and no products with aromatic ring hydrogenated were
promotion than the mechanism of ether bond cleavage,2b, 3k, 7c detected. The main products with Cβ-OPh bond cleavage
which usually references to the concept of homogeneous during the earlier stage of reaction (<0.5 hour) were styrene,
catalysis.7a, 7b, 7d, 7e, 12 In order to further optimize the lignin phenol,
(1-methoxyethyl)benzene
and
(2,2-
conversion over heterogeneous catalysts, new understanding dimethoxyethyl)benzene. Although a 69% conversion was
about the mechanism of ether bonds cleavage is necessary, obtained after 1 hour at 180 oC, the main product (35%) of β-
which is conducive to the exploration of new strategies and O-4-A transformation was β-O-4-E without ether bond
catalysts.
cleavage. Compared with β-O-4-K conversion with Cβ-OPh
In this work, we employed as-synthesized NiMo and other bond cleavage (14%), the β-O-4-A conversion with Cβ-OPh
MMo sulfide catalysts (M=Mn, Fe, Co) in the hydrogenation of bond cleavage (33%) was higher at the same reaction
lignin β-O-4 model compounds.11k, 11l, 13 After optimization of condition in 1 hour.
the reaction condition, the NiMo sulfide catalyst achieved
To further confirm the β-O-4-A with CαH-OH structure was
preferential cleavage of ether bonds and preserved the more easily transformed than the β-O-4-K with Cα=O structure
aromatic rings. Conversions of different β-O-4 model over the NiMo sulfide catalyst, we carried out the control
compounds showed that the efficiency of Cβ-OPh bond experiment with β-O-4 model compounds with CαH-OH and
cleavage was not controlled by BDE value of the substrate Cβ- Cα=O group in one reactor. In the competitive conversion of β-
OPh bond. For instance, the β-O-4 lignin model compound O-4-A-OCH3 and β-O-4-K (Scheme 2A), the products of
with CαHOH transformed more easily than the one with Cα=O conversion of β-O-4-K were small amount of PhC2H5, PhOH and
over NiMo sulfide catalyst. This differs from previous reports.2b, PhCOCH3 (Figure S11). The quantitative relationships among
8c, 8d
Based on the control experiments, product generation the converted β-O-4-A-OCH3 and β-O-4-K with Cβ-OPh bond
analysis and DFT calculation results, we found that the cleavage, PhOH and PhCOCH3 was shown in Scheme 2A. The
substrate activation routes play a crucial role in the ether bond
cleavage efficiency and products distribution.
In contrast to the previously reported strategies beginning
with dehydrogenation,8c-e we propose a new dehydroxylation-
hydrogenation strategy to achieve preferential cleavage of the
ether bond in β-O-4-Alcohol. The method begins with a
dehydroxylation reaction of the Cα-OH in β-O-4-Alcohol which
generates a possible intermediate PhCH·CH2OPh. This process
+
contains a carbocation (PhCHδ CH2OPh) generation and its
reduction. Then the cleavage of Cβ-OPh ether bond in the
radical intermediate occurs. DFT calculation results show that
Scheme 1. The β-O-4 linkage model compounds.
this process can decrease the BDE value of Cβ-OPh bond in the
1
CH2OPh intermediate from 274.0 kJ·mol− to 66.9
Conversion
PhCH2CH3
PhCH
·
(A)
1
kJ·mol−
.
PhCH=CH
PhOH
2
100
80
60
40
20
0
100
80
60
40
20
0
PhCH(OH)CH
PhCOCH
PhCH(OCH
PhC(OCH
3
3
3
)CH
CH
2
OCH
3
2. Results and discussion
3)
2
2
OPh
2.1 Cleavage of β-O-4 ether linkages over the NiMo sulfide catalyst
The β-O-4 ether bond is the primary targeted bond to break
during lignin degradation.1a In this work (Scheme 1), we
choose 2-phenoxy-1-phenylethanone (β-O-4-K), 2-phenoxy-1-
phenylethanol (β-O-4-A), (2-phenoxyethyl)benzene (β-O-4-H),
(2-phenoxyvinyl)benzene
(β-O-4-O),
(1-methoxy-2-
1
2
3
4
5
6
phenoxyethyl)benzene (β-O-4-E) and their analogues as β-O-4
linkage model compounds and the possible intermediates.
In the hydrogenation of β-O-4-K (Figure 2A), the NiMo
sulfide catalyst preferentially catalyzed the cleavage of Cβ-OPh
bond over the hydrogenation of C=O bond. The main products
were acetophenone and phenol, and no aromatic rings were
hydrogenated. Small amounts of ethylbenzene and (1-
methoxyethyl)benzene generated in the products as the
reaction proceeded. The reaction offered 99% conversion of β-
O-4-K in 6 hours.
Reaction time (h)
2 | J. Name., 2012, 00, 1-3
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ARTICLE
Conversion
PhCH CH
PhCH
β
β
2
CH(OCH
-O-4-H
-O-4-E
3)2
preferred to catalyze the hydrogenative cleavage of Cβ-OPh
bond connecting with a CαH-OH structure.
(B)
2
5
PhCH=CH
2
100
100
80
60
40
20
0
PhOH
PhCH(OCH3)CH3
The high rate of conversion and aromatic products with
ether bond cleavage showed that the NiMo sulfide was an
efficient catalyst for the selective conversion of β-O-4 model
compounds. The generation of β-O-4-E will be discussed in the
following section about the possible reaction route (Scheme 6).
Approaches for decreasing the β-O-4-E were also explored
(Figure S15). Furthermore, when TEMPO as a shielding reagent
of radical route was added in the hydrogenation of β-O-4-A
(Scheme S3), conversion was not obvious (<1%). When
reaction carried out under Ar condition (Scheme S4), only 1%
Cβ-OPh bond in β-O-4-A cleaved. These results indicated the
reaction probably occurred through a radical route,14 and the
acid sites (Figure S10) of the NiMo sulfide alone could not
catalyze the transformation of β-O-4-A via dehydration-
hydrolysis route.5a, 5c
80
60
40
20
Styrene
0
0
1
2
3
4
5
Reaction time (h)
Figure 2. The conversion of β-O-4-K (A) and β-O-4-A (B) over the NiMo sulfide catalyst.
Reaction condition: substrate 0.2 mmol, catalyst 20 mg, methanol 2.0 mL, H2 1.0 MPa,
180 oC.
2.2 The effect of the functional group at the Cα atom
In the hydrogenation of lignin model compounds, the reaction
route plays a crucial role in the bonds selective cleavage and
products distribution, which is controlled by substrate
structure and catalyst properties.2b, 7b, 7c, 8c, 8d, 14
The functional group attached to the Cα atom has a
remarkable effect on the BDE value of Cβ-OPh ether bond in β-
O-4 model compounds (Table S1). The alkyl-OPh bond energy
1
1
of β-O-4-A is 274.0 kJ·mol− , 46.2 kJ·mol− higher than the one
in the β-O-4-K. It is 275.3 kJ·mol−1 for the alkyl-OPh bond in the
β-O-4-H. Additionally, the BDE of the alkenyl-OPh ether bond
1
in the β-O-4-O increases to 336.6 kJ·mol− which is nearly 110
1
kJ·mol− higher than the alkyl-OPh bond in the β-O-4-K. Based
on the ether bond BDE energy order (β-O-4-K < β-O-4-A ≈ β-O-
4-H < β-O-4-O), the two-step method is usually employed in
the conversion of the β-O-4 lignin model compounds, which
contains a preoxidizing step to oxidize CαH-OH to Cα=O before
a subsequent transfer hydrogenation or acidolysis.8c-e
The conversion of lignin β-O-4 model compounds over the
NiMo sulfide catalyst is not consistent with the BDE energy
order (Table 1). Even though the BDE of Cβ-OPh ether bond in
1
β- O-4-K is 46.2 kJ·mol− lower than the one in β-O-4-A, the
Scheme 2. The competitive conversion of β-O-4 model compounds with CαH-OH and
conversion of Cβ-OPh bond in β-O-4-A (33%) is almost two and
Cα=O group in one reactor. Reaction condition: each model compound 0.2 mmol, NiMo
sulfide 20 mg, methanol 2.0 mL, H2 1.0 MPa, 180 oC, 1 h. The n(substrate) stands for the a half times of the one of Cβ-OPh bond in β-O-4-K (14%). No
amounts of converted substrate with Cβ-OPh bond cleavage.
ether bonds cleave during the conversion of β-O-4-H, despite it
having the approximate Cβ-OPh bond BDE as β-O-4-A. The β-O-
4-O has a highest ether bond BDE (336.3 kJ·mol-1) and it offers
amounts of PhOH was 6.2 times of PhCOCH3, which showed
that the conversion of β-O-4-A-OCH3 with Cβ-OPh bond
cleavage to PhOH was 5.2 times of β-O-4-K. Furthermore, the
cleavage of Cβ-OPh bond in the competitive conversion of β-O-
4-A-OCH3 and β-O-4-K under Ar condition was not obvious
(Figure S12). In the completive reaction of β-O-4-A and β-O-4-
K-OCH3 (Scheme 2B), the Cβ-OPh bond conversion difference
reduced to 1.8 times. These results showed that NiMo sulfide
a 37% conversion with ether bond cleavage. These results
show the effect of substrate structure on the conversion of ether
bond cleavage in lignin β-O-4 model compounds, and the
dominative factor of the conversion is not the BDE value of
target ether bond in the original substrate.
Table 1. The difference of the β-O-4 lignin model compounds conversion over the NiMo sulfide catalyst. a
Products distribution (%)b
BDE
(kJ·mol−
Conv.
(%)
Entry
Substrate
1
)
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69
(33)c
20
1
2
3
274.0
227.8
275.3
336.6
8
<1
0
10
<1
0
11
25
0
13
1
0
40
0
4
0
1
0
0
0
52
0
33
0
<1
0
(14)c
No
0
0
(0)c
37
4
8
10
20
4
0
58
0
0
(37)c
a
b
Reaction conditions: substrate 0.2 mmol, NiMo sulfide catalyst 20 mg, methanol 2.0 mL, H2 1.0 MPa, 180 oC, 1 h. The distribution ratio of each product was
calculated basing on the total molar amount of GC products with and without Cβ-OPh ether bond cleavage. c Cβ-OPh ether bond cleavage conversion.
2.3 The possible routes for the β-O-4-A conversion over the NiMo
sulfide catalyst based on the styrene generation
One reason for the discrepancy between Cβ-OPh bonds
conversions and their BDE value could be that β-O-4-A has
undergone a conversion to an active intermediate,7c whose
target ether bond has a lower energy that makes the
conversion process more competitive.
We detected 7% yield of styrene in products (arrowed in
Figure 2B), which was seldom detected in earlier studies.7e, 11b
The styrene generation in the β-O-4-A conversion implies a
different reaction route. Regarding styrene as the target
product (Scheme S1), we propose the possible routes of β-O-4-
A transformation (Scheme 3). (1) β-O-4-A loses a H2O and
generates β-O-4-O,15 then the β-O-4-O transforms to styrene
and phenol in the subsequent direct hydrogenation, or the
Scheme 3. The routes for styrene generation in β-O-4-A conversion over the NiMo
styrene generation is a acidolysis-hydrogenation-dehydration
process with a PhCH2CHO intermediate.5a-c (2) Cleavage of Cβ-
sulfide catalyst.
OPh bond in β-O-4-A occurs, and then the intermediate
(PhCH(OH)CH2·) transform to styrene directly after losing a
hydroxy radical at Cα or via a PhCH(OH)CH3 intermediate.
Alternatively, the β-O-4-A loses a hydroxyl group at Cα, then
the intermediate (PhCH·CH2OPh) transforms to the styrene,
phenol and other products after the cleavage of Cβ-OPh ether
bond. (3) β-O-4-A hydrogenolysis generates a β-O-4-H, then
the subtraction of two hydrogen atoms from β-O-4-H results in
a β-O-4-O, which is subsequently converted into styrene and
phenol. (4) The Cβ-OPh ether bond in β-O-4-H cleaves, and
then the generated radical intermediate (PhCH2CH2·) loses an
H· radical to generate the styrene.11b (5) β-O-4-A transforms to
Comparing with the active hydrogen of Cα-Hα and CαH-OHO in
β-O-4-A (PhCαH(OH)CβH2OPh), the hydrogen of Cβ-Hβ is a
relative inert one. During the previous reports about the first
4d, 7b, 7d,
step of β-O-4-A activation, the target bond is CαHO-HO
7f
7c, 7e
or Cα-Hα
not the Cβ-Hβ bond. Furthermore, the β-O-4-H
conversion over NiMo sulfide catalyst is not obvious (Table 1,
Entry 3), and there is only 1% β-O-4-H in the β-O-4-A
conversion products (Table 1, Entry 1). The hydrogenolysis of
the hydroxyl group at Cα atom of β-O-4-A to β-O-4-H over the
NiMo sulfide catalyst occurs slowly. These results indicate that
Routes 3-6 are not the main route for the β-O-4-A conversion
over the NiMo sulfide catalyst.
β-O-4-K via
a
dehydrogenation process,11h then the
In Route 1, the activated β-O-4-A loses a H2O and
generates a β-O-4-O. When β-O-4-O was used as the substrate
(Table 1, Entry 4), a 37% conversion was obtained in 1 hour at
180 oC and the main products generated with Cβ-OPh ether
hydrogenated product acetophenone transforms to the
styrene. (6) Cβ-OPh bond of the dehydrogenation intermediate
(PhC·(OH)CH2OPh or PhCH(OH)CH·OPh) cleaves, and the
generated species transforms to PhCOCH3 or be hydrogenated
to PhCH(OH)CH3, then PhCH(OH)CH3 transforms to styrene.
The ratio of Cβ-OPh bond cleavage in β-O-4-A conversion is
higher than β-O-4-K and β-O-4-H (Table 1, Entries 1-3). No
acetophenone as the representative product of the β-O-4-K or
PhC·(OH)CH2OPh hydrogenolysis is generated in β-O-4-A
conversion, which indicates the main β-O-4-A conversion
process does not contain the dehydrogenation step at Cα
atom.
bond cleavage, which presented
a potential application
method (dehydration-hydrogenation).7e, 7f, 15 When the β-O-4-
A was used as the substrate (Table 1, Entry 1), an approximate
conversion with ether bond cleavage (33%) was obtained.
However, the main ether product of β-O-4-O conversion was
(2,2-dimethoxyethyl)benzene, which was only a minor ether
product in the β-O-4-A conversion (Table 1, Entries 1 and 4).
The main ether product, also the byproduct without ether
bond cleavage, β-O-4-E in the β-O-4-A conversion was not
detected in the β-O-4-O conversion. Furthermore, the
generation of β-O-4-O from β-O-4-A via a dehydration process
was not obvious under Ar condition (Scheme S4). These
differences indicated that Route 1 was not the main route of
β-O-4-A conversion over the NiMo sulfide catalyst.
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The BDE value of Cβ-OPh ether bond in β-O-4-O was 336.3 activation of the CαHO-HO
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4d, 7b, 7d, 7f
7c, 7e
or Cα-Hα
usually occurs
1
KJ·mol− , which was higher than Cβ-OPh bonds in β-O-4-A, β-O- firstly before the cleavage of Cβ-OPh bond. Taking into account
4-H, β-O-4-K and the CβO-Ph bond in β-O-4-O (Scheme 3). that β-O-4-A with CαH-OH structure is more easily transformed
However, it did give the highest conversion. This anomaly than β-O-4-K with Cα=O structure over the NiMo sulfide
raises another question: whether the Cβ-OPh bond in the β-O- catalyst, new reaction route should exist. As discussed before,
4-O cleaved directly or the β-O-4-O conversion occurred via an the Cα-OH transformation is critical for cleavage of Cβ-OPh
intermediate with lower BDE ether bond. This question is bond, we should discuss the potential routes with Cα-OH bond
discussed in the following part about ether generation from β- cleavage.
O-4-O (Scheme 6 and Figure 3).
Routes 2b and 2c both begin with Cα-OH bond cleavage,
and the cleavage occurs by homolysis in Route 2b and
heterolysis in Route 2c. For the Cα-OH homolysis in Route 2b,
2.4 The Route 2 for β-O-4-A conversion
1
1
In route 2, based on the cleavage order of the Cα-OH and Cβ- the BDE of the Cα-OH is 370.8 kJ·mol− , which is 96.8 kJ·mol−
OPh bonds, there are three plausible routes for the β-O-4-A higher than the Cβ-OPh bond in the same molecule. This makes
conversion to styrene (Scheme 4): (2a) Cβ-OPh ether bond in β- Route 2b more complex than Route 2a and it has a Cα-OH bond
O-4-A cleaves, and then the intermediate (PhCH(OH)CH2·) with a higher BDE to cleave.
transform to styrene. (2b) β-O-4-A loses a hydroxy radical and
·
In route 2c, β-O-4-A loses a hydroxy group and generates a
+
generates a radical intermediate (PhCH
CH2OPh), after which carbocation (PhCH+CH2OPh or PhCHδ CH2OPh) intermediate,
Cβ-OPh ether bond cleaves and generates a styrene. (2c) β-O- which involves acid-catalysed benzyl hydroxy heterolysis
4-A loses a hydroxy group and generates a carbocation dehydroxylation and can occur easily at acid sites.3k, 5a, 5b Based
+
intermediate (PhCH+CH2OPh or PhCHδ CH2OPh),3k, 5a, 5b which on the NH3-TPD result (Figure S10), the NiMo sulfide catalyst
then transforms to the styrene after some activation.
contains weak and medium strong acid sites, and the NH3
As shown in Table 1, the Cβ-OPh bond BDEs in β-O-4-A and absorption capacity is 90.2 μmol·g−1. Different from benzyl
β-O-4-H are approximate. However, when an H atom replaces hydroxy heterolysis with strong protonic acid to generate a
the OH group at Cα, the β-O-4-H shows no conversion, but β-O- free carbocation (PhCH+CH2OPh),5a,
5b
weak and medium
4-A conversion with ether bond cleavage is about 33%. The strong acid sites of NiMo sulfide trend to achieve the
obvious difference shows the effect of Cα-OH group on the Cβ- heterolysis dehydroxylation via a adsorbed intermediate of
+
-
OPh bond cleavage. In the heterogeneous catalysis process, Cδ ···δ OH–Acid site.3k For the heterolysis dehydroxylation of β-
substrate adsorption is the origin of the substrate activation.
O-4-A over the acid sites of NiMo sulfide, trace amount of β-O-
4-O as an acid-catalyzed product is detected during β-O-4-A
conversion under Ar condition (Scheme S4). Styrene is
detected in 1-phenethyl alcohol conversion (Figure S13B).
Additionally, the addition of KOH can decrease the β-O-4-A
conversion with ether bond cleavage to only 2% under H2
condition (Scheme S4).
In later steps of Route 2c, the ether bond cleavage of
+
adsorbed PhCHδ CH2OPh can occur directly and generate a
+
PhCHδ CH2· (Route 2c-1), which has two unstable structures
suggesting its generation should be difficult. In other
transformations, the adsorbed PhCHδ+CH2OPh can lose an H+
at Cβ to β-O-4-O (Routes 2c-2 or 1).5a However, as discussed
earlier, even though the dehydration-hydrogenation route
(Route 1) is an efficient route, it is not the main route here.
Furthermore, if conditions permit, the adsorbed
Scheme 4. The route 2 for β-O-4-A conversion with different cleavage order of Cβ-OPh
ether bond and Cα-OH bond.
The removal of polar hydroxy group can affect the absorption
of β-O-4-H onto the NiMo sulfide catalyst. However, taking
into account the good conversion of β-O-4-O, which also
contains no polar group at the Cα atom, we can see that the
promotion of absorption by the polar group at Cα atom is
limited in the cleavage of Cβ-OPh bond. The effect of the Cα-OH
can be attributed to how critical the Cα-OH transformation is
for Cβ-OPh bond conversion11h over NiMo sulfide catalyst.
In route 2a, the Cβ-OPh bond breaks directly without Cα-OH
transformation. This is similar to the conversion of β-O-4-K
with acetophenone and phenol as the main products. While, β-
O-4-A with higher ether bond BDE transforms more easily than
β-O-4-K, it indicates that Route 2a may not be an efficient
route for the β-O-4-A conversion over the NiMo sulfide
catalyst. In addition, there is no catalysis system achieving β-O-
4-A conversion via the direct cleavage of Cβ-OPh bond, and the
+
PhCHδ CH2OPh can transform to an adsorbed radical
intermediate (PhCH·CH2OPh) after obtaining an electron from
the catalyst redox cycle (Route 2c-3). The BDE of Cβ-OPh ether
1
bond in PhCH
(Table S1).
·
CH2OPh can be reduced to only 66.9 kJ·mol−
Referring to the enthalpy change of heterolysis
dehydroxylation (PhCH(OH)CH2OPh + H+ → PhCH+CH2OPh +
1 5a
)
H2O; ꢀ
(PhCH(OH)CH2OPh → PhCH
H= 49.4 kJ·mol−
and direct dehydroxylation
H= 370 kJ·mol−1),
·
CH2OPh + HO·; ꢀ
Route 2c-3 containing carbocation generation and reduction to
PhCH·CH2OPh could be regarded as the optimization of Route
2b (Scheme S2). This is especially feasible when considering
the fact that adsorbed PhCH·CH2OPh should have a lower
energy than its gas phase. This provides that Route 2c-3 has
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the potential to promote the cleavage of Cβ-OPh bond.
activity alone. The β-O-4-A conversion over NiMo sulfide is not
a simple acid-catalysed process to PhCH2CHO with a free
carbocation (PhCH+CH2OPh) intermediate.5a-c Given the result
that the main product (Entry 1), also ether product of
2.5 The analysis of ether compounds generation for understanding
β-O-4-A conversion
Besides the analysis of styrene generation, analysis of ethers PhCH2CHO conversion (Scheme S7), is PhCH2CH(OCH3)2, which
generation is also important to improve our understanding is different from β-O-4-A conversion with PhCH(OCH3)CH3 as
about the main reaction route. As shown in Scheme S5, for the the main ether product of Cβ-OPh cleavage. The β-O-4-A
ether compound generation from alcohols, there are two conversion over NiMo sulfide is not an acidolysis-etherification
mechanisms, with either a carbocation or radical intermediate. or acidolysis-hydrogenation process, which produce ether
In order to confirm the generation route of ether compounds and PhCH=CH2 with PhCH2CHO as the key
compounds, various control experiments were carried out. intermediate (Scheme 6A).5b
Firstly, we checked the β-O-4-A conversion under the Ar
Furthermore, the different conversions of β-O-4-A under
atmosphere, where only 1% conversion was obtained (Scheme H2 (Table 1, Entry 1) and Ar (Scheme 5, Entry 1) demonstrates
how critical H2 is in β-O-4-A conversion and ether generation.
Entry
Products distribution:
There is no β-O-4-E but small amount of other ethers in the β-
O-4-A conversion under Ar, which indicates that the
generation of the ether compounds, especially for the β-O-4-E,
probably occurs via a radical intermediate under H2 over the
NiMo sulfide catalyst (Scheme 6A). For the β-O-4-E generation,
its precursor should be the adsorbed PhCH·CH2OPh.
Furthermore, β-O-4-E conversion is only 4% at 1 h (Scheme 5,
Entry 2), which is much lower than the β-O-4-A conversion
(Table 1, Entry 1). The most ether products with Cβ-OPh bond
cleavage in β-O-4-A conversion should not generate from β-O-
4-E but from the plausible radical intermediate (Scheme 6A).
In order to further confirm the main route of β-O-4-A
conversion and the cleavage order of Cα-OH and Cβ-OPh bonds
during the ether products generation (Scheme 6A), the first
analysis about the β-O-4-O conversion without Cα-OH might be
conducive to understand the β-O-4-A conversion (Scheme 6B).
There is a high percentage of PhCH2CH(OCH3)2 in β-O-4-O
conversion products (Table 1, Entry 4). For its generation,
when styrene was used as the substrate (Scheme S6), there
were only PhCH2CH3, PhCH(OCH3)CH3 and some condensation
compounds in the products, but no PhCH2CH(OCH3)2. So, the
PhCH2CH(OCH3)2 is not generated from styrene. In addition, no
β-O-4-O conversion is detected under Ar condition (Scheme 5,
Entry 3, 4). This result indicates the acid sites on the NiMo
sulfide catalyst alone cannot promote the β-O-4-O
transformation to PhCH2CHO, which is a typical acid-catalyzed
OH
OH
O
O
Ar
(1)
+
+
+
+
O
31%
14%
19%
14%
Conv. 1%
O
O
O
4%
18%
O
OH
O
H2
Ar
O
(2)
+
+
+
Conv. 4%
O
48%
24%
8%
19%
(3)
(4)
no reaction
no reaction
O
Ar
H2O
Scheme 5. The conversion of the β-O-4 model compounds. Reaction conditions:
substrate 0.2 mmol, catalyst 20 mg, methanol 2.0 mL, 1.0 MPa H2 or 1.0 MPa Ar, 180 oC,
1 h. In the reaction under Ar condition, the catalyst is pre-treated at 180 oC for 1 h in
the methanol under 1.0 MPa H2. The amount of water added in Entry 4 is 200 μL.
HO
O
Acid site
O
(A)
C
H
O
H2, CH3OH
Ar
CH OH X
3
Very slow
Ar
CH3OH
H2, CH3OH
OH
(Acid site)
O
OH
O
C -OPh bond
O
O
(Minor)
cleavage
C
H2
+
H2
H2, CH3OH
CH3OH
O
(Plausible radical intermediate)
-H2O (Acid site)
Very slow
(Major)
product and the precursor of PhCH2CH(OCH3)2 even under Ar
condition (Scheme S7).5a,
O
X
H2O
(Acid site) X
CHO
(Main product is not consistent)
5b
The ether compounds should
(Main product is not consistent)
generate via a radical route over the NiMo sulfide.
(B)
H2
X
Regarding PhCH2CH(OCH3)2 as the target product in the β-
O-4-O transformation, based on the order of Cβ-OPh bond
cleavage and Cβ-OCH3 bond generation, two radical routes are
proposed (Scheme 6B). (Route E1) Cβ-OPh ether bond in β-O-4-
CH3OH
O
O
Acid site
(Minor)
O
X
CH3OH + H2O
+
O
O
H2
O
O
O cleaves directly and generates a PhCH=CH· radical, which
CH3OH
(Route E1)
then reacts with methanol or methoxyl radical to the ethers
under H2. (Route E2) One methoxyl radical should have
connected with the activated β-O-4-O and generates a radical
H2
(Major)
CH3O
(Route E2)
CH3OH
OCH3
Scheme 6. The routes of ether products generation in the conversion of β-O-4-A (A) or
β-O-4-O (B) over the NiMo sulfide catalyst.
intermediate (PhCH·CH(OCH3)OPh) before the cleavage of Cβ-
OPh ether bond, then the generated PhCH=CH(OCH3) reacts
with methanol or methoxyl radical to PhCH2CH(OCH3)2.
DFT calculation is employed to compare these two radical
routes. The direct cleavage of Cβ-OPh bond in β-O-4-O has a
5, Entry 1). The change of NH3 absorption capacity for the
NiMo sulfide catalyst is inconspicuous after its treatment in
the methanol under H2 (Figure S10), which indicates that the
weak and medium strong acid sites of NiMo sulfide show low
1
336.6 kJ·mol− BDE in Route E1. When the Cβ of β-O-4-O
6 | J. Name., 2012, 00, 1-3
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combines with a methoxyl radical in Route E2, the Cβ-OPh cleavage of Cα-OH bond before the subsequent cleavage of Cβ-
bond BDE in PhCH·CH(OCH3)OPh decreases obviously to 41.2 OPh bond.
1
kJ·mol− (Table S1), which is also much lower than the CβO-Ph
1
2.6 The potential Route 2c-3 for β-O-4-A transformation over
bond (199.6 kJ·mol−
) in the same PhCH·CH(OCH3)OPh
NiMo sulfide catalyst
intermediate. As shown in Figure 3A, the enthalpy change of
1
direct cleavage of Cβ-OPh bond in β-O-4-O is 336.6 kJ·mol−
,
Based on the analysis of styrene and ethers generation, Route
and the enthalpy change of PhCH·CH(OCH3)OPh generation 2c-3 is the potential main route of β-O-4-A conversion over the
1
from β-O-4-O and CH3O· is -133 kJ·mol− . The obvious NiMo sulfide catalyst (Figure 4). (1) The adsorbed β-O-4-A
difference between two reaction enthalpy changes shows that firstly loses a hydroxy group and generates a carbocation
+
the preponderant reaction route is not Route E1 but Route E2 intermediate (PhCHδ CH2OPh) at the acid site of the NiMo
+
(Scheme 6B). The much lower BDE of the Cβ-OPh ether bond in sulfide catalyst. (2) The PhCHδ CH2OPh then transforms to the
PhCH
·
CH(OCH3)OPh intermediate can also explain the fact that free radical intermediate (PhCH·CH2OPh) by obtaining an
electron from the catalyst redox cycle. (3) Because of the
lower BDE of the Cβ-OPh ether bond in the PhCH·CH2OPh
(A)
400
300
200
100
0
intermediate, the Cβ-OPh bond cleaves and the generated
radical species reacts with activated H2 or methanol to various
arenes, phenol and ether products. (4) As a side reaction, the
PhCH·CH2OPh can also react with activated methanol and
generates a β-O-4-E without Cβ-OPh bond cleavage.
Route E1
Route E2
-100
-200
2.7 The promotion effects for the β-O-4-A conversion
In the Cβ-OPh ether bond selective cleavage, besides the
influence factors of the bonds BDE and adjacent substituent
groups’ structure, the NiMo sulfide heterogeneous catalyst
characterizations should also play a crucial role in substrate
activation and catalytic reaction route determination.
(B)
400
300
200
100
0
Based on the potential reaction mechanism (Figure 4),
there is a cooperation between the acid-redox site and
hydrogenation site. This cooperation causes the NiMo sulfide
catalyst to prefer Cα-OH the mechanism that generates a
-100
-200
PhCH·CH2OPh intermediate for the following Cβ-OPh bond
Reaction coordinate
cleavage over catalysing β-O-4-A hydrogenolysis to the inert β-
O-4-H.
Figure 3. Enthalpy profile of the β-O-4-O transformation in the gas phase.
As shown in Figure 5A, there are MoS2 phase and NiS phase
peaks existence in the XRD pattern of the NiMo sulfide catalyst.
Besides these peaks, there are peaks attributed to Ni0.7Mo3S4,
β-O-4-O with highest Cβ-OPh ether bond BDE can be selectively
and efficiently transformed.
Furthermore, styrene also generates in the β-O-4-O
conversion. Referencing to the PhCH2CH(OCH3)2 generation via
Route E2, when an H· radical replaces the CH3O· radical to
attack the Cβ atom (Figure 3B), the possible intermediate
PhCH·CH2OPh has a much lower Cβ-OPh bond BDE (66.9
OH
O
+
+
O
+
O
+
H2, CH3OH
1
1
kJ·mol− ) than β-O-4-O (336.6 kJ·mol− ) or β-O-4-A (274.0
kJ·mol−1). And the enthalpy change of PhCH·CH(OCH3)OPh
generation from β-O-4-O and H· is -186 kJ·mol−1, which is
obviously lower than the direct cleavage of Cβ-OPh bond in β-
O-4-O. These results suggest that the PhCH·CH2OPh could be a
key intermediate in the β-O-4-O conversion to styrene.
(Ⅲ)
Acid site
Redox site
Hydrogenation site
(Ⅱ)
(Ⅰ)
NiMo sulfide catalyst
During β-O-4-A transformation, as discussed above, β-O-4-
E generates via the PhCH·CH2OPh radical intermediate, and
other ether products with Cβ-OPh bond cleavage generates
from the plausible radical intermediate, whose structure
correlates with the cleavage order of Cα-OH and Cβ-OPh bonds
(Scheme 6A). The obvious change of ether bond BDE among β-
O-4-A, β-O-4-O and PhCH·CH2OPh suggests the possibility of
PhCH·CH2OPh as the intermediate in the β-O-4-A
transformation to ether compounds and styrene over NiMo
catalyst. The NiMo sulfide catalyst prefers to catalyze the
Dehydroxylation (Ⅰ and Ⅱ )
Hydrogenation (Ⅲ)
Figure 4. The potential Route 2c-3 for β-O-4-A conversion over the NiMo sulfide
catalyst.
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PhCH(OCH
PhCH=CHOPh
3)CH2OPh
(A)100
100
80
60
40
20
PhCH2CH2OPh
PhCH
PhCH(OCH
PhOH
PhCH=CH
PhCH2CH3
2
CH(OCH3)2
80
60
40
20
0
3)CH3
2
1
2
3
4
5
Reaction Time (h)
(B)100
100
80
60
40
20
Figure 5. The x-ray diffraction pattern (A), annular dark-field scanning transmission
electron microscopy image (B) of the NiMo sulphide catalysts. (C-E) show the element
maps of Ni, Mo and S from energy dispersive x-ray analysis with the compound image
of all three elements in (F). The bar inserting the image stands for 10 nm.
80
60
40
20
0
which is one of the Ni doped heterometallic cubane-type Mo
clusters. Elements mapping results (Figure 5C-F) shows the Mo
and S are uniformly distributed, and the Ni species appears
aggregation to about 10 nm in some zones, which is
corresponding to the NiS phase and Ni0.7Mo3S4 phase in the
XRD pattern of the NiMo sulfide catalyst. Based on the NH3-
TPD result (Figure S10), the NiMo sulfide catalyst contains
weak and medium strong acid sites, and its NH3 absorption
capacity is 90.2 μmol·g−1. For the acid sites, they could be
attributed to the coordinative unsaturated Mo or Ni ion
centres.16
1
2
3
4
5
Reaction Time (h)
Figure 6. The conversion of the β-O-4-A over the NiS (A) and MoS2 (B) catalyst. Reaction
condition: β-O-4-A 0.2 mmol, catalyst 20 mg, methanol 2.0 mL, H2 1.0 MPa, 180 oC. The
height of the histogram stands for the conversion.
NiMo sulfide catalyst can obviously restrain the β-O-4-H
generation (Figure 2B). Based on the characterization of NiMo
sulfide (Figure 5), the introducing Ni species can dilute or
separate the surface Mo centers and generate new NiMo
cluster17 with hydrogenation ability. Referring to the catalytic
performance of NiS and MoS2 in the β-O-4-A transformation,
the central metal of the redox site could be the Mo, and the
hydrogenation site could be the MoSx or NiMo cluster.
When NiS was used as the catalyst, the cleavage of Cβ-OPh
bond was not obvious (Figure 6A), and β-O-4-A mainly reacted
with the alcohol solvent to the etherification product of β-O-4-
E. During the β-O-4-A transformation catalyzed by MoS2
(Figure 6B), besides the etherification product of β-O-4-E and
products with Cβ-OPh bond cleavage, lots of β-O-4-H
generated in this process. Although the β-O-4-H was a by-
product, its generation suggested the existence of the
Although the introducing Ni species can restrain the
hydrogenolysis of β-O-4-A to β-O-4-H over the NiMo sulfide,
the etherification product of β-O-4-E increases. To decrease β-
O-4-E, different solvents and introducing metals are screened.
When the non-alcohol solvents such as n-dodecane, 1,4-
dioxane, acetonitrile, and N-methyl pyrrolidone were used,
low conversions and high ratio of β-O-4-H were obtained
(Scheme S8). When ethanol was used as the solvent (Figure
S15A), the reaction offered >99% conversion and about 80%
products yield with ether bond cleavage in 2 h. When
isopropanol was used as solvent (Figure S15B), >99%
conversion and 88% products (phenol and ethylbenzene) yield
were obtained in 5 h.
intermediate PhCH·CH2OPh, which preferred to react with the
active H to β-O-4-H rather than cleave the Cβ-OPh bond over
·
the MoS2 catalyst. Compared with MoS2 catalyst performance,
Ni species introducing in the
For the effect of the solvent on β-O-4-A conversion and β-
O-4-H generation, as we proposed in Figure 4, the
PhCH·CH2OPh is
a
key intermediate, and the high
concentration of active hydrogen species on the NiMo sulfide
surface should be favorable for the transformation of
PhCH·CH2OPh to β-O-4-H. A competitive adsorption among
8 | J. Name., 2012, 00, 1-3
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solvent, substrate (hydroxyl group of β-O-4-A) and H2 molecule control-regulate catalysis system, the characterizations of
may exist at the surface of the NiMo sulfide.6 N-containing catalyst play
crucial role. Compared with efficient
a
solvent (acetonitrile, and N-methyl pyrrolidone) have strong homogeneous catalysts that usually contain single-site
absorption effect, and their basic hydrolysis products14 can catalytic center, heterogeneous catalysts can integrate
affect the acid sites of NiMo sulfide, which cause low multiple catalytic centers with different catalytic ability via
conversions. When n-dodecane and 1,4-dioxane with lower ingenious preparation process. If these catalytic centers are
absorption effect than alcohol are used, H2 absorption on Mo wisely located in a catalysis micro surrounding, they can
center can compete with β-O-4-A absorption6b and the provide more possibility. The more possibility are necessary for
concentration of active hydrogen species on the NiMo sulfide the lignin transformation. As shown in Figure 5, further work
surface should be higher, which causes low conversions and about the optimization of NiMo sulfide catalysts preparation
high ratio of β-O-4-H. Combined the catalytic ability of NiMo need be done. In addition, study about the accurate structure
sulfide with competitive adsorption between alcohol and H2, of catalytic centers will be useful in screening catalysts for
the alcohol solvent is a good choice to restrain the generation lignin conversion.
of β-O-4-H.
For the ratio of β-O-4-E in β-O-4-A conversion products,
3. Conclusions
one tendency exists that the steric hindrance of the alcohol
solvent molecular is favorable for reducing the ratio of β-O-4-E
(Figure 2B and Figure S15). However, when tertiary butanol is
used as the solvent (Figure S15C), the β-O-4-A conversion with
Cβ-OPh cleavage is 18% in 1 h. Butanol contains a tertiary
alcohol group with steric bulk and can be used as radical
scavenger, which can affect the activation of substrate and H2.
Furthermore, referring to the concept to reduce the
generation of β-O-4-H via competitive adsorption between
solvent and H2, if β-O-4-A model compound contains a CγH2-
OH structure to compete with solvent alcohol to absorb on the
catalyst, the generation of ether product without Cβ-OPh
cleavage can be restrained. The experiments prove this
In summary, we present an efficient hydrogenation route for
Cβ-OPh bond cleavage in the β-O-4 lignin model compounds.
For 2-phenoxy-1-phenylethanol (β-O-4-A) conversion over the
NiMo sulfide catalyst, the efficient reaction route is not via Cβ-
OPh ether bond direct cleavage or Cα-OH dehydrogenation to
Cα=O before subsequent transformation. The β-O-4-A loses a
hydroxy group (Cα-OH) at weak and medium strong acid sites
of the catalyst. Then the PhCHδ+CH2OPh transforms to
PhCH·CH2OPh by obtaining an electron from the catalyst redox
cycle. Since the possible intermediate PhCH·CH2OPh has a
1
much lower Cβ-OPh bond BDE (66.9 kJ·mol− ) than β-O-4-A
1
(274.0 kJ·mol− ), the Cβ-OPh bond breaks easily and generates
thought.
When
2-(2-methoxyphenoxy)-1-(4-
styrene, ethylbenzene, phenol and various ether products
under a hydrogenation condition with alcohol solvent. The
cooperation between the acid-redox site and hydrogenation
site play a crucial role in this route. Although questions such as
etherification product ratio and low catalytic activity still exist
during the NiMo sulfide catalyst application, they can be
resolved by the concept of competitive adsorption via solvent
and substrate structure optimization. Finally, we believe that
our finding about the new dehydroxylation-hydrogenation
strategy will be useful in screening catalysts for lignin
conversion.
methoxyphenyl)propane-1,3-diol is used as substrate, the
NiMo sulfide can selectively catalyze the cleavage of Cβ-OPh
without aromatic rings hydrogenation, and no hydrogenolysis
or etherification product at Cα without Cβ-OPh cleavage is
detected (Scheme S9).
Besides solvent screening, we also changed the introducing
metal. When Mn, Fe and Co replaced the Ni in the MMo
sulfide catalysts, the catalytic performance changed obviously.
In the β-O-4-K conversion (Figure S17), when Mn replaced the
Ni, the MnMo sulfide catalyst could catalyze the
hydrogenation of carbonyl group and ether bond cleavage, and
lower β-O-4-K conversions were obtained over FeMo and
CoMo sulfide catalysts. In the β-O-4-A conversion (Figure S18),
the MMo sulfide catalysts (M=Mn, Fe, Co) offered lower
conversion than NiMo sulfide catalyst. Therefore, for the
catalysis performance of MMo sulfide catalysts, the promotion
of Ni introducing was obvious, which implied the catalytic
ability of NiMo cluster in selective cleavage of Cβ-OPh bond of
β-O-4-A.
Experimental Section
Lignin β-O-4 model compounds synthesis
The β-O-4 lignin model compounds 2-phenoxy-1-
phenylethanone (β-O-4-K),2b 2-phenoxy-1-phenylethanol (β-O-
4-A),2b 1-(4-methoxyphenyl)-2-phenoxyethanone (β-O-4-K-
OCH3),2b 1-(4-methoxyphenyl)-2-phenoxyethanol (β-O-4-A-
OCH3),2b
(2-phenoxyethyl)benzene
(β-O-4-H),3e
(2-
In the hydrogenation of β-O-4-A, the cooperation between
acid-redox sites and hydrogenation sites in NiMo sulfide makes
the dehydroxylation-hydrogenation strategy an efficient route,
and the advantage of this route makes the catalyst control the
main reaction direction. Meanwhile, the structure of the
substrate, the competitive adsorption (among substrate, H2
and solvent), as well as reaction condition (temperature,
additives etc.) can regulate the detailed routes, which offer
different conversions and products distributions. In this
phenoxyvinyl)benzene (β-O-4-O),5a (2-phenoxyethyl)benzene
(β-O-4-H),3e (1-methoxy-2-phenoxyethyl)benzene (β-O-4-E)7a
and 2-(2-methoxyphenoxy)-1-(4-methoxyphenyl)propane-1,3-
diol (model compound with CγH2-OH structure)6a,
18
are
synthesized according to the previous procedures. The
detailed procedure are provided in Supporting Information.
Catalysts preparation
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Journal Name
Yan. Catal Surv Asia, 2014, 18, 164-176; (g) S. Van den Bosch; W.
Schutyser; R. Vanholme; T. Driessen; S. F. Koelewijn; T. Renders;
B. De Meester; W. J. J. Huijgen; W. Dehaen; C. M. Courtin; B.
The (NH4)2MoS4 (5.2 g) was dispersed in a nickel chloride
solution (200 mL, 20 mmol). After stirring at 100 oC for 6 h, the
solid was collected by filtration, washed with distilled water
and ethanol, dried at 60 oC under vacuum, followed by
calcination in a horizontal furnace at 450 oC for 6 h in an Ar
Lagrain; W. Boerjan; B. F. Sels. Energy Environ. Sci., 2015, 8, 1748-
1763; (h) R. Ma; K. Cui; L. Yang; X. L. Ma; Y. D. Li. Chem. Commun.
,
2015, 51, 10299-10301.
1
flow (50 mL·min− ) at a 5 oC·min−1 heating rate from 25 oC. The
2 (a) C. Li; X. Zhao; A. Wang; G. W. Huber; T. Zhang. Chem. Rev.
,
final sample was designated as NiMo sulfide.
2015, 115, 11559-11624; (b) Z. Strassberger; A. H. Alberts; M. J.
Louwerse; S. Tanase; G. Rothenberg. Green Chem., 2013, 15, 768-
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3 (a) A. G. Sergeev; J. F. Hartwig. Science, 2011, 332, 439-443; (b) J.
Zhang; J. Teo; X. Chen; H. Asakura; T. Tanaka; K. Teramura; N.
The preparation procedures of MMo (M=Mn, Fe, Co)
sulfide catalyst were according to the NiMo catalyst, just used
Mn(OAc)2, FeSO4 and CoCl2 as the M2+ ion precursor. The MoS2
was obtained by treating the as-prepared (NH4)2MoS4 in a
horizontal furnace at 450 oC for 6 h in an Ar flow. NiS was
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Graphical abstract:
H2, CH3OH
(Ⅲ)
Acid site
Redox site
Hydrogenation site
(Ⅱ)
(Ⅰ)
NiMo sulfide catalyst
Dehydroxylation (Ⅰ and Ⅱ )
Hydrogenation (Ⅲ)
Herein, different from reported strategies beginning with dehydrogenation, we present an
efficient dehydroxylation-hydrogenation strategy in the lignin β-O-4 model compounds
transformation over a NiMo sulfide catalyst.