Cleavage of lignin C-O bonds over a heterogeneous rhenium catalyst through hydrogen transfer reactions

Article abstract of DOI:10.1039/c9gc01710k

Hydrogenolysis is one of the most popular strategies applied in the depolymerization of lignin for the production of aromatic chemicals. Currently, this strategy is mainly conducted under high hydrogen pressure, which can pose safety risks and is not sustainable and economical. Herein, we reported that heterogeneous rhenium oxide supported on active carbon (ReO_x/AC) exhibited excellent activity in the selective cleavage of lignin C-O bonds in isopropanol. High yields of monophenols (up to 99.0%) from various lignin model compounds and aromatic liquid oils (>50%) from lignin feedstock were obtained under mild conditions in the absence of H₂. The characterization of the catalyst by X-ray absorption fine structure, X-ray photoelectron spectroscopy and H₂-temperature-programed reduction suggested that the activity of ReO_x/AC could be attributed to the presence of Re^IV-VI. The interaction between the surface oxygen groups of the active carbon and rhenium oxide could also play an important role in the cleavage of the C-O bonds. Notably, an ReO_x/AC-catalyzed C-O bond cleavage pathway beyond a typical deoxydehydration mechanism was disclosed. More importantly, 2D-HSQC-NMR and GPC characterizations showed that ReO_x/AC exhibited high activity not only in β-O-4 cleavage, but also in the deconstruction of more resistant β-5 and β-β linkages in lignin without destroying the aromatic ring. This study paves the way for the development of rhenium-based catalysts for the controlled reductive valorization of realistic lignin materials through a hydrogen transfer pathway.

Full text of DOI:10.1039/c9gc01710k

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DOI: 10.1039/C9GC01710K
ARTICLE
Cleavage of Lignin C-O Bonds over a Heterogeneous Rhenium
Catalyst through Hydrogen Transfer Reactions
a
a
a
b
a
a
a
a,c
Bo Zhang, Zaojuan Qi, Xinxin Li, Jianwei Ji, Leilei Zhang, Hua Wang, Xiaoyan Liu, Changzhi Li*
Received 00th January 20xx,
Accepted 00th January 20xx
Hydrogenolysis is one of the most popular strategies applied in depolymerization of lignin for the production of aromatic
chemicals. Currently, such strategy is mainly conducted under high hydrogen pressure, which are either at the risk of safety
or not sustainable and economical. Herein, we report that heterogeneous rhenium oxide supported on active carbon
DOI: 10.1039/x0xx00000x
(
ReO
to 99.0 %) from various lignin model compounds and aromatic liquid oil (>50%) from lignin feedstock were obtained
respectively under mild condition in the absence of H . Characterization of the catalyst by X-ray absorption fine structure,
X-ray photoelectron spectroscopy and H -temperature-programed reduction suggests that the activity of ReO /AC can be
attributed to the presence of ReIV-VI. The interaction between the surface oxygens groups of active carbon and rhenium oxide
could also play an important role for the cleavage of C-O bonds. Notably, ReO /AC-catalyzed C-O bond cleavage pathway
beyond typical deoxydehydration mechanism was disclosed. More importantly, 2D-HSQC-NMR and GPC characterization
showed that ReO /AC exhibited high activity not only in β-O-4 cleavage, but also in deconstruction of more resistant linkages
x
/AC) exhibits excellent activity in selective cleavage of lignin C-O bonds in isopropanol. High yields of monophenols (up
2
2
x
x
x
of β-5 and β-β in lignin without destroying the aromatic ring. This study paves the way towards the development of rhenium-
based catalyst for the reductive valorization of realistic lignin material in a controlled through hydrogen transfer pathway.
avoiding side reactions such as over-hydrogenation and over-
oxidation under low-severity condition. In this aspect, several
breakthroughs have recently been achieved. For instance,
1
. Introduction
In light of the accelerating depletion of fossil fuels and
Luterbacher and co-workers reported that the diol protection
reagents such as formaldehyde can stabilize α,γ-diol group of
lignin and hence promote high-selective monomer production.6
Wang et al. reported solar energy-driven conversion of native
increasing demand for energy, lignocellulose, the most
abundant biomass, is generally regarded to be a sustainable
feedstock for energy, chemicals and biofuels. As one of the
major components of lignocellulose, lignin is by far the only
largest renewable source that supply aromatic chemicals owing
to its high abundance and inherent aromatic structure. Thus,
the effective depolymerization of lignin into aromatics would
greatly improve the economics of the overall biorefinery
process. Currently, both heterogeneous and homogeneous
catalytic processes for reductive or oxidative lignin
depolymerization strategies have been developed. However, in
most cases, high pressure (3-5 MPa) of molecular hydrogen or
oxygen atmosphere is required for lignin depolymerization at
high temperatures (> 250 °C), and low conversions in
conjunction with broad product distributions are usually
obtained. One major challenge reflects in the selective cleaving
of the aryl C-O linkages between primary aromatic units whilst
1
7
lignin at room temperature. Acidolysis using trifilic acid or
formic acid is also confirmed to be a promising route to obtain
2
8
aromatic monomers from lignin. Very recently, the lignin-first
strategy also shows promise in depolymerization of lignin to
afford high yield of aromatic chemicals without pretreatment of
3
9
lignocellulose. Despite of these progress, in many biorefinery
processes, including paper industry, great amount of lignin is
generated as a waste stream. Development of a new catalytic
system for the depolymerization of lignin to aromatic chemicals
in an efficient way is still desirable.
4
Rhenium (Re) is a group 7 transition metal that is less
electronegative than typical transition metals such as
ruthenium, rhodium, and palladium. The wide range of
oxidation states of Re suggests the potential for the discovery
5
10
of new reactivity features of Re in chemical reactions. Recently,
it has emerged as a promising metal in the field of biomass
conversion. It has been found to promote the
deoxydehydration of polyols and alcohols to olefins owing to its
specific ability to form diolate through the coordination of diol
a.CAS Key Laboratory of Science and Technology on Applied Catalysis, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan
Road, Dalian, 116023, China.
E-mail: licz@dicp.ac.cn
Shaanxi Key Laboratory of Catalysis, School of Chemistry and Environment
b.
Science, Shaanxi University of Technology, 1 Dongyihuan Road, Hanzhong,
11
with Re species. This is of interest because lignin contains a
7
23001, China
c.
large number of hydroxyl groups whilst rhenium has
outstanding capacity to remove the hydroxyl via
Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian,
16023, China.
1
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
12
deoxydehydration reaction. Therefore, Kühn and co-workers
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ARTICLE
Journal Name
first reported the Re(VII) complex CH ReO (MTO)-catalyzed collected, then evaporated at 30 °C to remove solvent, affording
3 3
View Article Online
1
3
DOI: 10.1039/C9GC01710K
cleavage of β-O-4 compounds in the absence of redox species.
Furthermore, the depolymerization of lignin over Re through and a 50.2% yield from organosolv wheat lignin (OWL). The
hydrogenation has been demonstrated by our own group.
More recently, we found that ReO /AC can catalyze monomeric products were quantified by the internal standard
hydrogenolysis of lignin α-O-4 model compounds under 3 MPa method (standard: mesitylene) using GC-FID (HP-5 column, 30
a 54.0% yield of oil products from organosolv poplar lignin (OPL)
2 7
O
14
liquid oil constants volume of 2 mL with isopropanol, and the
x
15
2
H atmosphere. However, this catalytic system exhibited m × 0.32 mm × 0.25 µm). The molecular weight distributions of
much poorer performance with regard to the conversion of β- the liquid oils were determined by matrix assisted laser
O-4 model compounds, the most abundant linkage in native desorption/ionization time-of-flight MS (MALDI-TOF-MS). The
4
lignin. On the other hand, hydrogen itself is a clean energy, and oil yields were calculated according to the equation:
hydrogenolysis of lignin with H
pressure needs special equipment, which increases the cost and weight of original lignin)
may cause safety issues. Therefore, new catalytic system for the Catalyst preparation: The MO
cleavage of β-O-4 linkages in lignin without H is highly desirable. Mo) catalysts were prepared by impregnating the AC with an
Compared with hydrogenation reactions involving H , transfer aqueous solution of NH ReO (NH O,
hydrogenation by using hydrogen-donating solvent such as (NH Mo O and NH VO
In the present work, the MO
2
at high temperature and high Oil yield
（
wt%
）
=(the weight of produced oily product)/(the
100%
/AC (AC, NORIT, M = Re, W, V,
×
x
2
2
4
4
,
4 6 2 12 2
) H W O40·H
o
4
)
6
7
O
24·4H
2
4
3
. After drying at 110 C for 12h,
/AC was calcined in nitrogen at 500 C for 3 h. The
16-17
o
alcohols is safer and easier to handle.
x
selective cleavage of C-O bonds in β-O-4 model compounds loading amount Re, W, Mo and V was 1.18%, 0.72%, 0.63%,
Scheme 1) and lignin over ReO /AC without H was 0.40%, respectively, which is determined by ICP-AES.
(
x
2
demonstrated in isopropanol through hydrogen transfer
mechanism. High yields of aromatics (up to 99%) from model
compounds and liquid oils (>50%) from isolated lignin were
obtained respectively in isopropanol. The reaction pathway and
active species were extensively studied as well.
3
. Results and Discussion
3.1 Optimization of the reaction conditions
β-O-4 is the most frequent linkage in almost all types of lignin,
and the chemical reactivity of this bond results in the
4
considerable resistance of lignin to chemical digestion. 2-(2-
methoxyphenoxy)-1-phenyl ethanol (1), a common lignin β-O-4
dimeric model compound, was initially studied to explore the
activity of the catalysts in isopropanol. First, the activity of
x
Scheme 1. Depolymerization of lignin catalyzed by ReO /AC
through hydrogen transfer reactions
ReO
oxides, including WO
ReO /AC displayed a substantially higher catalytic activity than
x
/AC was compared to other active carbon-supported metal
x
, MoO , and VO (entries 1-5), and
x
x
2
. Experimental
x
others, giving 83.0% yield of guaiacol and 81.6% yield of
ethylbenzene (entry 1). In sharp contrast, an experiment using
solely AC as the catalyst led to lower conversion and different
selectivity, providing 1-isopropoxy-2-phenoxyethyl benzene as
x
General procedure for ReO /AC catalyzed lignin model
compounds: The catalytic reaction was performed in a Parr
autoclave (Parr, 75 mL). Typically, the model compound (0.41
mmol), catalyst (1.6 mol% Re), the standard mesitylene (18 mg)
and isopropanol (15 mL) were placed to the autoclave (Parr, 75
18,9c
the main product, which was also detected in literature.
However, this catalyst provided very poor yields of
ethylbenzene and guaiacol (Table 1, entry 15, for details
regarding the products, see Figure S2). This is mainly attributed
to acidic catalysis, because the surface of AC has numerous
acidic functional groups due to nitric acidic treatment before
use, as demonstrated by X-ray photoelectron spectroscopy (XPS)
mL) and flushed by N
conducted at 200 °C with 100 psi N
2
for several times. The reaction vessel was
and stirred (800 rpm) for 3
2
h. After reaction, the gases were collected and the liquid phase
was analyzed by gas chromatography with a flame ionization
detector (GC-FID) employing the mesitylene as an internal
standard (HP-5 column, 30 m × 0.32 mm × 0.25 µm).
General procedure for conversion of lignin feedstocks: The
catalytic conversion of lignin feedstocks was carried out in a Parr
(Figure S4 and Table S1). With these results in hand, we
conducted subsequent reactions using ReO /AC as the catalyst
x
to optimize the reaction conditions. Table 1 shows that
decreasing amount of Re led to incomplete C-O bond cleavage
x
autoclave (75 mL). Typically, lignin (100 mg), ReO /AC (1.6 mol%
Re), and the solvent isopropanol (15 mL) were placed to the Parr
autoclave, followed by stirring at 800 rpm. When the reaction
was finished, the gases were collected and analyzed, and the
residue solid was washed with isopropanol, and then dried at
(
entries 1, 7 and 8). As an example, reducing the catalyst
amount to 0.1 mol% Re afforded a decreased yield of (75.8%)
and a much lower yield of (53.5%) (entry 8). Interestingly,
styrene ( ) was obtained in a yield of 13.8% in this experiment.
In the case of MeOH as solvent, despite relatively high
2
3
4
5
0 °C in preparation for two-dimensional heteronuclear single
quantum coherence nuclear magnetic resonance (2D-HSQC-
NMR) characterization. The liquid was vacuumed at 30 °C to
afford a liquid oil. Noting that lignin does not dissolve in ethyl
acetate, ethyl acetate was added (3×15 mL) in the oil to extract
the low molecular aromatic products. The extraction liquid was
conversion was also achieved, the total yield of
was lower than that using PrOH as solvent (entry 10 vs entry 1).
3
and
4
(54.7%)
i
Moreover, using THF as solvent under H
2
atmosphere gave
i
much poorer performance (entry 11) than that in PrOH. These
results suggested that ReO /AC exhibited high catalytic activity
x
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9GC01710K
ARTICLE
Table 1. Results of the ReO
x
/AC-catalyzed C-O bonds cleavage of lignin model compound 1 under different reaction conditions.a
Entry
Cat.
Metal amount
Solvent
Temp.
(oC)
200
200
200
200
200
200
200
200
200
200
200
120
150
180
200
200
t
Conv.
(%)b
99.9
99.9
99.9
99.9
99.9
99.9
99.9
90.0
89.3
95.3
53.7
0.8
Yield (%)b
[
mol%]
1.6
1.0
1.6
1.6
1.9
1.6
0.9
0.1
1.6
1.6
1.6
1.6
1.6
1.6
0
(h)
3
3
3
3
3
6
3
3
1
3
3
3
3
3
6
6
2
3
4
3 + 4
81.6
69.0
76.9
70.0
72.9
82.1
76.4
68.4
64.8
54.7
29.7
0.6
1
2
3
4
5
6
7
8
9
ReO
x
/AC
/AC
/AC
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
MeOH
THF
83.0
63.4
69.9
59.6
79.4
84.1
76.2
75.8
44.1
93.6
34.5
0
81.6
69.0
76.9
70.0
72.9
82.1
76.4
53.5
44.1
7.9
0
WO
x
0
WO
x
0
MoO
VO /AC
/AC
/AC
x
/AC
0
x
0
ReO
ReO
ReO
ReO
ReO
ReO
ReO
ReO
ReO
AC
x
0
x
0
x
/AC
13.8
20.7
46.8
19.4
0.6
14.4
14.1
28.9
0
x
/AC
10
1
1c
12
13
14
15
16
x
/AC
/AC
/AC
x
10.3
0
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
x
x
/AC
37.0
80.3
69.3
0
25.8
63.2
29.4
0
5.5
19.9
57.7
31.4
0
x
/AC
43.6
2.5
--
0
0
aReaction conditions:
stainless-steel autoclave (Parr, 75 mL) with an initial N
1
(100 mg), isopropanol (15 mL), MO
pressure of 100 psi; Conversions and yields were determined by GC-FID
was the initial pressures.
x
/AC (M = Re, W, Mo, V) and mesitylene (18 mg) were added to a
b
2
c
with mesitylene as an internal standard; 100 psi H
2
in isopropanol system. A shorter reaction time and lower investigated (Scheme 2). After the reaction, the substrates were
reaction temperature also tended to produce styrene (entries 9, almost completely consumed, whilst the yields of the aromatics
1
2-14). We therefore propose that ethylbenzene might be varied. Electron-donating methoxyl group substitution on the
generated from styrene via deep-hydrogenation, which will be aryl ring (compounds 10 14 and 15) led to high yields of the
further discussed along with the time course experiments target products, while the substrates without substitution on
below. In all trials, the catalytic activity and the selectivity for the the o-terminus aryl ring (compounds and 13) gave relatively
8
,
,
5
desired products were greatly affected by the reaction lower yields of C-O cleavage products. In particular, compound
o
temperature. For instance, low temperature of 120 C almost
5 provided the lowest performance in C-O bond cleavage;
o
gave no conversion, while a higher temperature of 180 C instead, 1-phenethoxybenzene (
significantly increased the reaction rate (entries 12-14), and 200 C obtained, suggesting that the rate of hydrogenation is superior
was the optimized reaction temperature (entry 1). Moreover, a to hydrogenolysis reaction. Hence the substitution pattern of
7
) with a yield up to 56.9 % was
o
blank experiment failed to convert compound
Additional, acetone and H were found in the catalytic reaction, distribution. It is interesting to note that 12 was one of the
suggesting that isopropanol serves as in-situ hydrogen sources major products from the conversions of 10 13 and 14, implying
Figure S5-S6). that hydrolysis reaction occurred as well, which was different
from the reactions of and . Noting that the substrates (10
and 14) had additional methoxy group on the C-terminus aryl
To generalize this protocol, the conversion of a series of lignin ring compared to compounds and , this major difference
model compounds with different functional groups were in structure may induce the different reaction pathway. In
1 (entry 16). the ring plays an important role in determining the products
2
,
(
1
,
5
8
,
3.2 Expanding the substrate scope
1
3
1
,
5
8
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ARTICLE
addition, there was a possibility that 11 might be produced from obtained from ReO
Journal Name
x
/AC. This result, in combination with the
View Article Online
DOI: 10.1039/C9GC01710K
2 7
, however, catalytic experiment under the same conditions data generated over AC (Table 1, entry 12) and Re O alone
1
2
suggested that the reaction did not occur when conversion of (Table S2, entry 2), demonstrates a clear synergistic effect
1
1
1
2
0
8
. Thus compound 12 is a final product for the conversion of between AC and the Re species. Furthermore, the recycling
13 and 14. Moreover, highly substituted substrates 15 and experiment associated with incomplete conversions for lignin
,
also worked well under the same conditions, affording the
model compounds was also performed. The result showed that
the catalyst can be recycled for five times with a slight loss in
activity (Figure S8) due to leaching of Re species into the
surrounding solution according to ICP-AES results (1.18 wt% Re
in fresh catalyst vs 0.94 wt% in used catalyst, and 0.17% Re in
the reaction solution after reaction). It was also found that
ethylbenzene yield dropped fast whereas styrene yield
increased in recycle runs, suggesting lower hydrogenation
activity of the catalyst in recycle runs. Similar trend was
observed when recycle experiment was performed using
organsolv lignin as the substrate. For instance, the yield of 2,6-
corresponding C-O bond cleavage product guaiacol in 95.8%
and 96.7% yields, respectively, along with 16 (48.7%), 19
(23.1%), 17 (31.4%) and 20 (37.4%). Based on all above results,
one can conclude that ReO /AC promotes the cleavage of
various β-O-4 linkages with different functional groups in
isopropanol in the absence of hydrogen atmosphere.
x
3
.3 Probing the active species
To ascertain the nature of the active species, characterization
of the catalyst was conducted. XPS provided the binding energy
(BE) values of the surface Re 4f7/2 orbitals in ReO
x
/AC before
VII dimethoxy-4-propylphenol and 2-methoxy-4-propylphyenol
and after reaction (Figure 1). As references for BE of Re
and ReO (Re ) locate at 46.7 eV and 44.5 eV, respectively,
3
one can conclude that the BE of 45.8 eV for the fresh catalyst
corresponds to Re
2 7
O (Re )
VI
19 decreased in the second run, however the yields of 2,6-
dimethoxy-4-(prop-1-enyl) phenol and 2-mehtoxy-4-(prop-1-
VI-VII
species. The X-ray absorption near-edge enyl)phenol increased. These results indicated that the ability of
structure (XANES) results in Figure 2 confirmed this hypothesis. hydrogenation of the catalyst become weaker probably due to
-
that of ReO
4
, and the valence of Re was 6.7 according to the
the leaching of Re species during the reaction and carbon
deposition on the surface of the catalyst.23 Noting that the
11c
Re-L3 edge XANES white line area (Figure S7). This result is
likely attributed to partial reduction of rhenium oxide by carbon
products distribution shift from propyl chain to propenyl chain
in recycle runs (Fig S8B), the acid catalysed pathway producing
the propenyl chain is less affected by the above two factors. It
was also found that in the last two recycle runs (R4 and R5), the
yields of aromatic monomers slightly increased, which might be
caused by the depolymerization of the residue in last
experiment, since both the lignin residue and the catalyst are in
solid state and cannot be separated each other. These results
arise a question that this material might actually function as a
homogeneous catalysis through the leaching of Re into the
solvent. To address this point and confirm that it is
heterogeneous catalysis, several control experiments were
carried out. First, a hot filtration after removal of
during the thermal treatment under N
2
.20 Another possible
reason is an interaction between rhenium oxide and surface
oxygen groups of the carbonaceous materials through
electronic perturbation/chemisorption.21-22 Such an interaction
could modify the electronic density of the Re species and thus
increase the activity of the ReO
physical mixing of rhenium oxide and AC, which is supported by
the following experiments. When a mixture of AC and Re was
applied to the conversion of substrate , products and 4
x
/AC to a greater extent than
2 7
O
1
2
,
3
were obtained in yields of 33.3%, 20.6% and 8.8% (Table S2,
entry 3), respectively, which were much lower than that
the solid catalyst was performed, followed by
attempted conversion of an additional quantity
of substrate
1 under the same condition as in
Table 1, entry 1. The results showed that no
reaction occurred in the filtrate. In addition, a hot
filtration followed by the addition of fresh AC and
additional substrate after removal of the catalyst
was performed, leading to a 7.9% yield of
without any of product Comparative
experiments with different oxorhenium
compounds such as NH ReO , Re , ReO and
ReO also showed extremely low conversions
<11.0%) (Table S3). These above results
indicated that the active Re species were present
on the carbon support. -temperature-
programed reduction (TPR) also gave an
evidence on this hypothesis. H -TPR profiles of
ReO /AC, pure AC and a physical mixture of
2
3
.
4
4
2
O
7
3
2
(
H
2
2
x
Scheme 2. C-O bonds cleavage of various lignin model compounds catalyzed by ReO /AC.
x
4
| J. Name., 2012, 00, 1-3
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Journal Name
Re and AC are presented in Figure 3. The broad signals above
ARTICLE
2 7
O
View Article Online
DOI: 10.1039/C9GC01710K
o
00 C can be assigned to the partial reduction of AC,22 both AC
4
and the mixture of Re and AC have such signals. In addition,
2 7
O
the mixture sample generated a sharp signal at 278 °C. It should
be noted that two weak, broad signals at 260 °C and 355 °C
appeared in H
be assigned to the reduction of the surface oxide. The reduction
temperature of ReO /AC was significantly lower than that of the
physical mixture sample, indicating that ReO /AC is more readily
2 x
-TPR profile of ReO /AC sample, both signals can
x
x
reduced and thus there is an interaction between AC and
rhenium oxide. XPS characterization showed that the reactivity
of the catalysts correlated with the valence of the Re species.
After reaction, ReO
x
/AC displayed four signals at BEs of 45.8,
IV
4
3.5, 41.8 and 41.0 eV. Since the Re 4f7/2 peak of ReO
2
(Re )
appears at 42.5 eV while 44.5 eV is assigned to ReO
3
(ReVI),24 it
V
is reasonable to assign the 43.5 eV peak to Re , which was the
richest Re species in the catalyst (46.7%; Table S4) according to
semi-quantitative analysis. The peaks at 41.8 and 41.0 eV,
accounting for 10.7% and 7.2%, respectively, are assigned to
III
II
species Re and Re . According to the white-line area of the Re-
L3 edge XANES data, the estimated average valence was 5.6
Figure 1. Re 4f XPS patterns of ReO
x
/AC before and after reaction.
(
Figure S5). Noting that the Re valence of the fresh catalyst was
estimated 6.7, these results provided us a key information that
ReO /AC catalyst can be reduced by H (generated in situ from
x
2
isopropanol) to form different low valence Re species. While
these new species are not stable and are liable to be oxidized
back to higher valence forms when exposing to air even for a
short time (Figure S9). Previously, Kühn and co-workers
reported that methdioxorhenium (V) complex, formed via
reduction of methtrioxorhenium (VII) acted the active specie for
13
cleavage of C-O bond in lignin model compounds. Tomishige
IV
VI
reported that Re /Re species of the heterogeneous catalysts
ReO -Pd/CeO are the active sites in the deoxydehydration
Figure 2. Re L3 edge XANES spectroscopy.
x
2
reaction.11a, 11c Based on the above XPS and EXAFS results
accumulated and published results,11c, 13, 15, 25 it is reasonable to
propose that ReIV- VI is the active species that responsible for C-
O bond activation.
3.4 Exploring the catalytic reaction pathway
Monitoring the time course of the conversion of 2-(2-
Methoxyphenoxy)-1-phenyl ethanol in Figure 4 showed that the
concentrations of 2-(2-methoxyphenoxy)-1-phenylethanone
and (E)- and (Z)- 1-methoxy-2-(styryloxy)benzene isomers were
firstly increased, then quickly decreased after 1 h. In parallel, C-
2
Figure 3. H -TPR data of various catalysts.
O bond cleavage products increased with increasing the via a cascade hydrolysis, isomerization and hydrogenolysis
reaction time, indicating that dehydrogenation product (2-(2-
methoxyphenoxy)-1-phenylethanone) and dehydration
product ((E)- and (Z)- 1-methoxy-2-(styryloxy)benzene) might
be the plausible intermediates. Hence, two reaction pathways
reactions (Route
phenylethanone (22) was used as the substrate, the
deoxygenation product (47.1% yield) along with (38.3% yield)
and (20.2% yield) were generated (Scheme 3, equation (2)).
Taken these two comparative reactions and the above time
course experiments together, one can suppose that in the
1
in Scheme 4). If 2-phenoxy-1-
7
3
6
(
dehydrogenation and dehydration) probably exist in this
transformation. To verify this hypothesis, the following control
experiments were carried out. When the conversion of (E)-1-
transformation of compound
E)- and (Z)- 1-methoxy-2-(styryloxy)benzene
dehydrogenation product 2-(2-methoxyphenoxy)-1-
1
, both the dehydration species
methoxy-2-(styryloxy)benzene (21) was catalyzed by ReO
under the same conditions (Scheme 3, equation (1)), products
and were obtained in yields of 71.1% and 65.3%, respectively,
x
/AC
(
and
3
6
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DOI: 10.1039/C9GC01710K
Figure 4. Time course experiment catalyzed by ReO
transformation of the compound 1. Reaction conditions: lignin
model compound 1 (100 mg, 0.41 mmol), ReO /AC (1.6 mol%
Re), and internal standard mesitylene (18 mg) in isopropanol
15 mL) with N (100 psi) at 200 °C. Conversion and yields were
x
/AC in the
x
(
2
determined with GC-FID by an internal standard method.
Scheme 3. Control experiments used to determine the reaction pathway.
guaiacol (equation (4)). The above two reactions, including the
results in Figure 4, confirm that the presence of a hydrogen in
the C or C position is not necessary for the cleavage of β-O-4
α β
aryl ethers, and that a hydrogenolysis reaction occurs during the
transformation (Route 3 in Scheme 4). The isotopically-labeling
experiment further supports that both route 1 (dehydration)
phenylethanone were reactive intermediates. Furthermore,
after comparison the products distributions of two control
reactions (Scheme 3, equations (1) and (2)), one can easily
acknowledge that the corresponding vinyl ether should be the
major intermediate in the transformation of compound 1, and
the reaction starting from dehydration is preferred. The acidity
of the catalyst (originated from acidic groups of the AC surface)
may play an important role in catalyzing the dehydration
reaction and subsequent C-O cleavage to afford styrene and
guaiacol according to Table 1, entry 15).
and route
3
(hydrogenolysis) occurred since both
hydrogenolysis product 3-2D and dehydration product 3-D were
detected (equation (5) and Figure S12). Therefore, we propose
a reaction pathway that proceeds through three routes
(dehydration, dehydrogenation and hydrogenolysis), as
To obtain additional information with regarding to the summarized in Scheme 4.
hydrogenolysis process and to clarify the role of different
3
.5 Depolymerization of organosolv lignins catalyzed by
functional groups in β-O-4 aryl ethers, several comparative
reactions with various substrates were conducted. Substrate 23
x
ReO /AC
,
Based on the results obtained in model compounds
conversion, two organosolv lignins were extracted from wheat
in which the C
that it cannot undergo dehydration, was synthesized for
conversion over ReO /AC under the same conditions (equation
3)). Hydrogenolysis products 24 25 and were obtained in
yields of 10.9%, 54.1% and 29.0%, respectively. Furthermore,
substrate 27, without OH group in the C position (in contrast
β
position is blocked by two methyl groups such
2
6
straw and poplar. Fourier transform infrared spectroscopy
FTIR) characterization indicated that the isolated organosolv
x
(
(
,
6
poplar lignin (OPL) and organosolv wheat lignin (OWL) both had
a basic lignin skeleton (Figure S13and Table S6). The relative
ratios of the main linkages were semiquantitative determined
by 2D-HSQC NMR (Figure S14).
Catalytic depolymerization of the
two lignins over ReO /AC in
isopropanol in the absence of H
provided three products: oil, solid
residue and trace CO . In the case of
α
to substrate 1), is unable to undergo dehydrogenation, giving a
small amount of the hydrogenolysis products ethylbenzene and
27
x
2
2
OPL, an oil yield of 54.0 wt% and a
monomer yield of 11.4 wt% were
obtained, while OWL provided an oil
yield of 50.2 wt% with a monomer
yield of 5.3 wt% (Table 2). GC-MS
analysis confirmed the absence of
cycloalkanes in the oil products,
indicating that the catalyst was highly
selective in activating the C-O bond
linkages in lignin without destroying
aromatic rings. As the grass lignin OWL
Scheme 4. Proposed reaction pathways for ReO
x
/AC-catalysed in lignin β-O-4 model compounds.
6
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9GC01710K
ARTICLE
Table 2. Catalytic hydrogenolysis of two lignins over ReO
x
/AC.a
-
1 d
A
:
B
:
C
S
:
G
:
H
Afterc
M
w
[g mol ]
Oil
Monomer
[wt%][e]
11.4
Monomers
Residual
[wt%]
30.2
Lignin
beforeb
before
1707
after
710
618
[
%]b
[wt%]
54.0
50.2
in oil [%]
OPL
74.3:2.8:12.9
63.6:36.4:0
68.6:31.7:0
21.1
OWL
48.7:28.2:23.1 28.6:57.6:13.8 30.9:61.7:7.4 2403
5.3
10.6
26.7
aReaction conditions: lignin (100 mg), ReO
/AC (1.6 mol% Re), PrOH (15 mL), T = 200 °C, P = 100 psi N
i
, t = 8 h. Determined by 2D
b
x
2
c
d
HSQC NMR characterization of isolated lignin. Determined by 2D HSQC NMR characterization of oil products. Determined by gel
permeation chromatograph (GPC) after acetylation. Identified with GC-MS and quantified with GC-FID by an internal standard.
e
provided a lower oil yield, particularly a lower yield of and OWL, major phenolic compounds such as guaiacylpropane
monomers from OWL than that from OPL, indicating that (1.9% for OPL, 0.8% for OWL) and 4-propylsyringol (6.5% for
hardwood lignin OPL was easier to be deconstructed than grass OPL, 1.1% for OWL), which are attributed to guaiacyl (
G) and
lignin OWL. Three facts may cause the results: 1) The β-O-4 ( syringyl ( ) units, were obtained. Other monomers, including
A
)
S
proportion of OPL (74.3%) was higher than that of OWL (48.7%), methyl 3-(4-hydroxyphenyl)propanoate (0.4%) and ethyl 3-(4-
which is in agreement with Bouxin and co-workers report that hydroxyphenyl)acrylate (0.2%) were derived from p-coumarate
the yield and the nature of monomeric products were largely
(
pCA
)
units,
while
methyl
3-(4-hydroxy-3-
dependent on the proportion of β-O-4 linkage.2 2) OWL has methoxyphenyl)acrylate (0.5%) originated from ferulate (FA
6a
)
higher molecular weight (see GPC results, Table S7) and more units. The
stubborn C-C linkages such as phenylcoumaran ( , β-5, 28.2%) lignin oils were similar to those in the original lignins (Table 2).
and resinol ( , β-β, 23.1%) as determined by 2D HSQC NMR The overall monomer yields in the oils suggest that these oils
analysis (Figure S12-4). 3) As reported by Sels and co-workers, contained high boiling point products which cannot be detected
lignin rich in -units more readily undergoes depolymerization by GC-MS. Thus, both GPC and MALDI-TOF-MS analysis of the
due to more methoxy group. In our study, OPL had a higher oil products were performed. The GPC results of the oils showed
proportion of units (63.6%) compared with OWL (28.6%), a significant decrease in the molecular weight compared with
representing another reason why OPL underwent that of the original lignin samples (Table S7), suggesting that the
depolymerization to a greater extent than OWL. two lignins underwent depolymerization. This outcome is
S: G: H (coumaryl) ratios of the monomers in the
B
C
S
9
d
S
Careful analysis of the 2D HSQC NMR spectra of the lignin supported by the MALDI-TOF-MS data, which show numerous
feedstocks and lignin oils provided additional information of the products in the m/z range of 100 to 700 and 150 to 750 for OPL
depolymerization behaviours. In the linkage region of the OPL and OWL, respectively (Figure S17). Noting that the molecular
oil spectrum, the signals of
B
α
(δ
C
/δ
H
87.21/5.59 ppm) and
B
β
weights of typical
S
,
G
and
H
units such as syringylpropane (168
-
1
-1
(δ
C
/δ
H
53.30/3.59 ppm) related to phenylcoumaran linkages (β- g mol ), guaiacylethanol (210 g mol ) and 4-propylphenol (136
-1
-1
5
) are much weaker compared with those of the original OPL g mol ) are in the range of 100-300 g mol , the products
signal related to β-O-4 obtained from the OPL and OWL evidently contain monomers,
linkage greatly decreased, whereas the (δ /δ 84.89/4.71 dimers, trimers and tetramers, with the first three as the
ppm), (δ /δ 53.80/3.06 ppm) and (δ 71.07/4.22 ppm, dominant compounds.
/δ 71.07/3.88 ppm) signals from resinol linkages (β-β) were
sample (Figure S14-3). However,
A
C
α
C
H
C
β
C
H
C
γ
C H
/δ
δ
C
H
almost unchanged. These results indicate that β-O-4 and β-5
linkages were converted or modified to some extent over
4
. Conclusions
ReO
NMR spectra of the lignin oil shows that the peaks related to
and linkages clearly decreased, suggesting that β-O-4, β-5
x
/AC. With regard to OWL transformation, the 2D-HSQC-
This work demonstrated the cleavage of various lignin β-O-4
A
,
model compounds and realistic lignin materials to give aromatic
compounds over ReO /AC catalyst in isopropanol solvent
B
C
x
and β-β linkages were probably converted or modified.
Integration of the peak areas (linkages/100 C9 units) provided
us an estimation of the differences in interunit linkages
composition (Figure S16), which led to the same conclusion.
The oily products were identified and quantified by GC-MS
and the detailed distributions of phenolic monomers are
collected in Table S8. Following the reactions of both the OPL
through hydrogen-transfer pathway. High yields of
monophenols (up to 99.0%) were obtained from the model
compounds, and significant amounts of aromatic oils (50.2%-
5
4.0%) along with a 6.5% yield of 4-propylsyringol resulted from
the lignin feedstock. Characterization of the catalyst and a study
of the associated reaction mechanism showed a clear
IV-VI
synergistic effect between Re species and the AC, with Re
being the primary active species. A reaction pathway involving
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Dang, Z. Liu, K. M. Nicholas, F. C. Jentoft, ChemCatChem 2013, 5, 3567;
View Article Online
β-O-4 cleavage was proposed, in which isopropanol serves as a
hydrogen donor. These results suggest the potential application
of Re-based catalysts for the conversion of biomass.
(
e) L. Sandbrink, E. Klindtworth, H.-U. Islam, A. M. Beale, R. Palkovits,
DOI: 10.1039/C9GC01710K
ACS Catal. 2016, 6, 677-680.
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Conflicts of interest
There are no conflicts to declare.
Z. J. Qi, B. Zhang, J. W. Ji, X. X. Li, T. Dai, H. W. Guo, A. Q. Wang, L. C.
Lu, C. Z. Li, ChemPlusChem 2018, 83, 500.
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Sustainable Chem. Eng. 2019, 7, 208.
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A graphical and textual abstract for the table of contents
entry
One sentence abstract: ReO /AC exhibits high catalytic activity and selectivity for the
x
C-O bond cleavage of lignin via hydrogen transfer reactions in isopropanol.