Combined lignin defunctionalisation and synthesis gas formation by acceptorless dehydrogenative decarbonylation

Article abstract of DOI:10.1039/d0gc01209b

The valorization of lignin, consisting of various phenylpropanoids building blocks, is hampered by its highly functionalized nature. The absence of the γ-carbinol group in an unnatural C2 β-O-4 motif compared to the native lignin C3 β-O-4 motif provides great opportunities for developing new valorization routes. Thus efficient defunctionalisation approaches that transform the C3 β-O-4 motif into a simplified C2 β-O-4 motif are of interest. Based on a study with a series of model compounds, we established a feasible application of an iridium-catalysed acceptorless dehydrogenative decarbonylation method to efficiently remove the γ-carbinol group in a single step. This defunctionalisation generates valuable synthesis gas, which can be collected as a reaction product. By this direct catalytic transformation, a yield of ～70percent could be achieved for a C3 β-O-4 model compound that was protected from undergoing retro-aldol cleavage by alkoxylation of the benzylic secondary alcohol in the α position. A phenylcoumaran model compound containing a γ-carbinol group as well as a benzylic primary alcohol also proved to be reactive under dehydrogenative decarbonylation conditions, which can further contribute to the reduction of the structural complexity of lignin. Notably, the liberation of synthesis gas was confirmed and the signals for the defunctionalized C2 β-O-4 motif were observed when this dehydrogenative decarbonylation approach was applied on organosolv lignins. This selective defunctionalized lignin in conjunction with the formation of synthesis gas has the potential to enhance the development of profitable and sustainable biorefineries.

Full text of DOI:10.1039/d0gc01209b

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ARTICLE
Combined lignin defunctionalisation and synthesis gas formation
by acceptorless dehydrogenative decarbonylation
Zhenlei Zhanga, Douwe S. Zijlstraa, Ciaran W. Lahivea, and Peter. J. Deussa*
Received 00th January 20xx,
Accepted 00th January 20xx
The valorization of lignin, consisting of various phenylpropanoids building blocks, is hampered by its highly functionalized
nature. The absence of the γ-carbinol group in an unnatural C2 β-O-4 motif compared to the native lignin C3 β-O-4 motif,
renders great opportunities for developing new valorization routes. Thus efficient defunctionalisation approaches that
transform the C3 β-O-4 motif into a simplified C2 β-O-4 motif are of interest. Based on a study with a series of model
compounds, we established a feasible application of an iridium-catalysed acceptorless dehydrogenative decarbonylation
method to efficiently remove the γ-carbinol group in one single step. This defunctionalisation generates valuable synthesis
gas, which can be collected as reaction product. By this direct catalytic transformation, a yield of ~70% could be achieved
for a C3 β-O-4 model compound that was protected from undergoing retro-aldol cleavage by alkoxylation of the benzylic
secondary alcohol in the α position. A phenylcoumaran model compound containing a -carbinol group as well as a benzylic
primary alcohol also proved to be reactive under the dehydrogenative decarbonylation condition, which can further
contribute to the reduction of the structural complexity of lignin. Notably, the liberation of synthesis gas was confirmed and
the signals for the defunctionalized C2 β-O-4 motif were observed when this dehydrogenative decarbonylation approach
was applied on organosolv lignins. This selective defunctionalized lignin in conjunction with the formation of synthesis gas
has the potential to add the development of a profitable and sustainable biorefineries.
DOI: 10.1039/x0xx00000x
β-O-4 compounds (
motif without γ-carbinol group and easier to access
synthetically (Fig. 1a).24-27 The application of the
-type linkage
motif was shown to avoid side reactions such as the retro-aldol
reaction, which is a typical side reaction for the -type linkage
B-type) that are defunctionalized β-O-4
Introduction
a
B
Lignin, an amorphous cross-linked aromatic biopolymer,
represents a major structural component of lignocellulosic
biomass (up to 40%).1, 2 Efficient lignin valorisation is of vital
importance for the implementation of profitable and
sustainable biorefinery schemes for biomass.3-6 Due to its rich
aromatic structure, lignin has been identified as a renewable
source for a range of key aromatic chemicals that are currently
derived from petrochemical routes or as a the aromatic
component in resins and polymers.7-10 To this end, many
elegant selective catalytic chemical reactions have developed to
adjust the chemical structure of lignin or deconstruct it for
targeted aromatic chemicals.11-17 As the β-O-4 linking motif
accounts for over 50% of the linking motifs that connect the
aromatic units in lignin, β-O-4 model compounds are most
commonly applied for the development of catalytic
methodology in order to avoid the structural complexity of
extracted polymeric lignin.18-23 There are two main types of
model compounds representing the β-O-4 linking motif
currently being applied in such studies; 1) C3 β-O-4 compounds
A
motif,
especially
under
oxidative
depolymerisation
conditions.21, 28-30 Thus, the use of
B-type linkage motif allows
for some innovative elegant approaches to valorise lignin that
are not accessible for the polymeric lignin that contains A-type
β-O-4 motifs.
(A-type) that provide a matching chemical representation of the
β-O-4 motif most commonly found in native lignin and 2) the C2
a. Department of Chemical Engineering (ENTEG), University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands, p.j.deuss@rug.nl
Electronic Supplementary Information (ESI) available: This includes details on
analytical procedures, data on set vs the measured T of reactions, the synthetic
procedure for PC, procedures for the extraction of lignin and supplementary analytic
GCMS and NMR data. See DOI: 10.1039/x0xx00000x
Fig. 1 (a) The two types of β-O-4 linkage motifs commonly represented in model
compounds used in chemical methodology development for lignin valorisation
and b) multistep defunctionalisation of alcohol incorporated organosolv lignin via
methylation, oxidation and decarbonylation demonstrated in our previous work.31
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DOI: 10.1039/D0GC01209B
Fig. 2 (a) Overview of the primary alcohol dehydrogenation and hydrogen shuttling for in situ stabilisation of depolymerisation products of lignin developed by
Bruijnincx et al.30 (b) Dehydrogenative decarbonylation of a series of simple primary alcohols demonstrated by Madsen et al.40 (c) Direct dehydrogenative
decarbonylation to defunctionalize a high alcohol incorporated lignin demonstrated in this work.
For example, Bergman and Ellman et al demonstrated a provide
hydrogen neutral cleavage method based on -type dimeric dehydrogenative decarbonylation step that is compatible with
model compounds as well as a -type synthetic polymer to free phenols that are inherently present in lignin, is attractive.
afford valuable acetophenone.13 However, James et al reported Recent work by Bruijnincx et al demonstrated the application of
that this procedure was ineffective for -type model an acceptorless dehydrogenation method to depolymerize
a shorter route, the application of a single
B
B
A
compounds due to chelation of the two hydroxyl groups to the lignin allowing for simplified product mixtures (Fig. 2a).30 This
catalyst.32 With respect to this discrepancy, the efficient lignin methodology relied on the dehydrogenation of the γ-hydroxy
defunctionalisation to remove the γ-carbinol functionality and group in β-O-4 structure, subsequent retro-aldol (Cα-Cβ) bond
thus transform an A-type into a
the application of such an alternative depolymerisation by conversion to the corresponding carboxylic acid or alcohol.
technology developed for -type model compounds. Acceptorless decarbonylation itself has also been reported to
B-type structure, can allow for cleavage and finally in-situ stabilisation of the formed aldehyde
B
Additionally, the defunctionalisation changes other polymer be an effective approach for in situ stabilization of the lignin
properties such as stability towards condensation18 and acidolysis intermediate aldehydes, removing the aldehyde
39
solubility in organic solvents due to the removal of hydroxyl functionality to avoid undesired repolymerisation.11,
groups.33 Previously, we have demonstrated that the However,
a
coupled
acceptorless
dehydrogenative
transformation of A- to B-type structures by stepwise catalytic decarbonylation has not yet been applied for lignin valorisation.
modification that removed the γ-carbinol functionality over For this, we were inspired by the works of the group of Madsen
several steps (Fig. 1b).31 This stepwise approach relied on the and Sadow40-42 who achieved acceptorless dehydrogenative
selective oxidation of the γ-hydroxy group to the aldehyde and decarbonylation reactions on a variety of primary alcohols by
a subsequent decarbonylation step. To allow selective oxidation iridium or rhodium-based homogenous catalysts, importantly in
and prevent decomposition via retro-aldol, alkoxylated A-type conjunction with the release of valuable syngas (Fig. 2b).
structures were used as starting point.29-31 Such alkoxylated Syngas or synthesis gas, constituting of CO and H2, is an
structures are readily accessible by organosolv extraction with important feedstock for both the chemical and energy
alcohol under specific conditions.33-35 Nonetheless, the results industries; the syngas introduces a great opportunity for
obtained from the application of this stepwise methodology on sustainable energy developments.43 The production of syngas
ethanol extracted lignin, showed a considerable loss of β-O-4 from biomass is considered to be an attractive and sustainable
linkages during the catalytic oxidation step, demonstrating that route. Biomass-derived syngas can be typically obtained by
there is significant room for improvement. In addition, the gasification of various lignocellulose feedstock or industrial
oxidation method did not tolerate the presence of phenol wastes from the pulp and paper industry.44 However, harsh
moieties in lignin; this meant that an additional phenol temperatures ranging from 600 to 900 °C are applied in
methylation step using environmentally undesirable MeI was gasification technology with the formation of large amounts of
necessary.36-38 In order to overcome these drawbacks and biochar as side-product.45 In addition, undesirable
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contaminants, such as the formed tars, nitrogen and sulphur methoxybenzene (BE1
based compounds, exist in the raw biomass gasification derived promising 81% selectivity (Fig. 3a).
syngas, imposing necessity for further cleanup.46 In contrast, a
much milder temperature is required for the production of
syngas by acceptorless dehydrogenative decarbonylation
catalytic reaction (around 160 °C). Conceptually, by targeting
primary alcohol groups in lignin, much cleaner syngas with
relatively stable syngas composition would also be expected in
ARTICLE
)
after 36 hours, although with
a
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DOI: 10.1039/D0GC01209B
100
90
80
70
60
50
40
30
20
10
0
a) at 165 °C*
Conversion 12h
Yield 12h
spite of lignin source variability.
In this contribution, we subsequently evaluated a combination
of carefully selected lignin model compounds to show the
potential of using the iridium-based acceptorless
Conversion 24h
Yield 24h
Conversion 36h
Yield 36h
dehydrogenative
decarbonylation40
for
the
lignin
defunctionalisation (Fig. 2c). In particular, we aimed for
application to lignins with high proportions β-O-4 linkages in
order to allow for selective defunctionalisation of
A-type
structures to -type structures, allowing further β-O-4 targeted
B
mesitylene
DEC
1,4-dioxane
GVL
diglyme
catalytic valorisation. For this purpose, different lignin model
compounds were applied to verify that the reaction is
compatible with phenols and indeed releases syngas for
100 b) in DEC
100
90
80
70
60
50
40
30
20
10
0
c) in 1,4-dioxane
90
80
70
60
50
40
30
20
10
collection. Then α-alkoxylated A-type model compounds were
used to evaluate that the defunctionalisation is possible without
degradation via retro-aldol reactions. Finally, α-alkoxylated
walnut lignins were used to demonstrate the formation of
defunctionalized β-O-4 linkages using this methodology.
0
165 °C*
180 °C*
Temperature
190 °C*
165 °C*
180 °C*
Temperature
190 °C*
Results and discussion
Fig. 3 Overview of the iridium catalysed dehydrogenative decarbonylation of 4-
methoxybenzenepropanol (BP1) to 1-methoxy-4-ethylbenzene (BE1). a) Solvent
screen at 165 °C*, b) temperature screen in diethyl carbonate, c) temperature
screen in 1,4-dioxane. Reaction conditions: 15 mg substrate, with a 1 : 2 : 4 ratio
of [Ir(cod)Cl]2 : rac-BINAP : LiCl (2.5 mol% [Ir(cod)Cl]2, other components were
adjusted to the [Ir(cod)Cl]2 amount to keep the ratio), performed under N2
atmosphere in 2 mL solvent. Yield and conversion were determined by GC-FID
using n-octadecane as internal standard. *Set temperature of the heating block,
Dehydrogenative decarbonylation of 4-
methoxybenzenepropanol (BP1)
Targeting the defunctionalisation of lignin, we previously
reported sequential oxidation of the γ-cabinol group followed
by decarbonylation to release CO. We aimed to simplify this
process by performing these two key transformations in the
sequence in one step (steps 3 and 4, Fig. 1b). Additionally, we
saw the opportunity for the release of syngas by acceptorless
dehydrogenation and decarbonylation in conjunction with the
targeted defunctionalisation of the native β-O-4 motif at the γ
position. Of the reported catalytic systems that can perform
both dehydrogenation and decarbonylation of primary alcohols
without being deactivated by the formation of syngas, we found
the one reported by Madsen et al. consisting of an commercial
iridium precursors and BINAP the most convenient to use.40 In
this catalytic system, [Ir(coe)2Cl]2 or [Ir(cod)Cl]2 serves as metal
precursor with rac-BINAP as ligand, LiCl as additive in
mesitylene. In addition, the catalytic solution is saturated by
water (150 ppm), which leads to overall improved performance
although the mechanism for this remains uncertain. This
catalyst was reported to achieve good product yields for both
benzylic and non-benzylic primary alcohols.40 Initially, we tested
the measured corresponding inside temperature is shown in Table S1
.
We were keen to move away from mesitylene as it is not a
suitable solvent for dissolving most types of lignin. Therefore,
we screened a set of alternative solvents that are more polar
and are known to have higher capability for dissolution of lignin
(Fig. 3a). To our delight the use of diethyl carbonate, considered
to be a green organic solvent,47, 48 had a significant positive
effect on the reaction rate, reaching 35% yield with 60%
selectivity after 12 hours at 165 °C (Fig. 3a). Notably,
substantially cleaner GCMS spectra were obtained by using
diethyl carbonate instead of mesitylene (Fig. S1 and S2a). The
dehydrogenation intermediate 4-methoxybenzenepropanal
(
BPA) was also observed in the spectra, though the amount was
very low (Fig. S1), therefore we suspected the presence of some
higher molecular weight species that were not visible by GC to
explain the poor carbon balance. The yield of BE1 then could be
increased to 42% by leaving the reaction for 36 hours although
the selectivity decreased to 44%. At the same time, it was
noteworthy that the inside temperature of the reaction was
lower than 165 °C that was used a set temperature for the
heating block, which corresponds to the boiling point of
this
decarbonylation procedure on 4-methoxybenzenepropanol
BP1), which mimics the presence of a γ-carbinol group in lignin
reported
iridium-catalysed
dehydrogenative
(
(Fig. 3). However, the reaction with the reported conditions
procedure proceeded sluggishly. The reaction with BP1 in
mesitylene at 165 °C only yielded 23% of 1-ethyl-4-
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mesitylene that was used in the reported procedure. Therefore,
the inside temperature was measured and found to be only 139
°C. One reason for this difference was the partially refluxing of
the solvent (boiling point of diethyl carbonate = 126 °C), as the
reaction vessel was only heated at the bottom. Thus, the
differences between actual inside temperature and set
temperature are listed for solvents with relatively low boiling
point (Table S1). Another such solvent that was tested was 1,4-
dioxane. Although 1,4-dioxane may impose potential risks for
human health and the environment,47, 49 it has still shown to be
one of the best lignin extraction mediums due to its ability to
dissolve many types of lignin as well as its stability during the
lignin solvolysis process.33, 50 By changing the solvent to 1,4-
dioxane, the yield to BE1 was greatly improved to 82%, along
with a satisfactory conversion of 83% conversion at 165 °C
(inside 130 °C) after 36 hours (Fig. 3a). Some significantly higher
boiling point polar solvents were also tested. However, also in
diglyme selectivity remained below 40% at all times and in GVL
tremendous side reactions were observed leading to very poor
selectivity (Fig. S2d).
View Article Online
DOI: 10.1039/D0GC01209B
Entry
Solvent
Time Set Temperature* (°C)
Conversion
Yield
1
2
diethyl carbonate
diethyl carbonate
1,4-dioxane
24h
36h
24h
36h
24h
36h
24h
36h
24h
36h
24h
36h
165
165
165
165
180
180
180
180
190
190
190
190
3
4
1,4-dioxane
5
diethyl carbonate
diethyl carbonate
1,4-dioxane
6
7
8
1,4-dioxane
9
diethyl carbonate
diethyl carbonate
1,4-dioxane
10
11
12
1,4-dioxane
0
10 20 30 40 50 60 70 80 90100
Fig.
4
Overview of the dehydrogenative decarbonylation of 4-hydroxy-3-
to 2-methoxy-4-ethylphenol EP2). Reaction
methoxybenzenepropanol (BP2
)
(
Based on the preliminary solvent screen at 165 °C, diethyl
carbonate and 1,4-dioxane were selected for further
investigation at increased temperature. Diethyl carbonate as
solvent at 180 °C (inside 145 °C) lead to a 14% increase in yield
at 36 hours compared to the reaction performed at 165 °C,
however, the selectively was only 55%, with still no apparent
identifiable side products (Fig. 3b). A further increase of the
reaction temperature to 190 °C (inside 148 °C) led to full
conversion of BP1 as well as remarkably improved selectivity.
Within 12 hours, the yield of BE1 reached 86% and maintained
relatively constant despite an increase in time, indicating that
the product was stable in solution. In 1,4-dioxane, the yield
after 36 hours was enhanced from 82% to 93% by increasing the
temperature from 180 °C to 190 °C (inside temperatures of 138
and 142 °C, respectively). (Fig. 3c). Overall, these stepwise
screens show the possibility to apply the dehydrogenative
decarbonylation of primary alcohols in 1,4-dioxane and diethyl
carbonate, with elevated temperatures being beneficial for
both yield and selectivity.
conditions: 15 mg substrate, [Ir(cod)Cl]2 2.5 mol%, 4 : 2 : 1 LiCl : rac-BINAP :
[Ir(cod)Cl]2, 165-190 °C in 2 mL solvent. Yield and conversion were determined by
GC-FID using n-ocatadecane as internal standard. *Set temperature of the heating
block, the measured corresponding inside temperature is shown in Table S1
.
Reactions were performed at 165, 180 and 195°C and sampled
at 24 and 36 hours. Compared with non-phenolic model
compound BP1, much better conversion of BP2 and yield of 4-
ethylphenol (EP2) was already observed at 165 °C in both
diethyl carbonate and 1,4-dioxane (Fig. 4, entry 1-4). Increasing
the temperature to 180 °C and 190 °C, full conversion and >98%
selectivity were achieved for both solvents within 24 hours (Fig.
4, entry 5-12). The results clearly show that this phenolic model
BP2 is actually more selectively converted and at higher rates
compared to its non-phenolic counterpart. The catalysts might
benefit from an increase of acidity of the solution, which might
be one of the reasons the requirement of the H2O addition as
an additive. This defunctionalisation approach thus allows for
the combination of the dehydrogenation and decarbonylation
steps, as well as the omission of the toxic phenol methylation
step.
Dehydrogenative decarbonylation of 4-hydroxy-3-
methoxybenzenepropanol (BP2)
Next, we aimed to confirm the release of valuable syngas. For
this purpose, gas evolution was monitored for the reaction with
BP2 in diethyl carbonate at 190 °C. 1 mmol starting material was
reacted in a flask with condensers, connected with a water-
filled burette for the quantification of gas formation monitored
over time. As shown in Fig 5., the volume of syngas steadily
increased as a function of time. Finally, 38.3 mL syngas was
produced at about 30 hours, which is equivalent to 1.55 mmol
syngas calculated using the ideal gas equation of state. This
equates to an 82% yield of syngas being produced with 100%
selectivity for EP2 as detected by GC-FID analysis, with
theoretically maximum yields being calculated as 40.7 mL of
syngas. Furthermore, the CO/H2 ratio analysed by GC-TCD was
1.1 : 1, indicating the successful implementation of acceptorless
dehydrogenative and decarbonylation without the requirement
of a sacrificial agent. Interestingly, a small amount of CO2 was
The presence of phenol groups in lignin has proven to be
detrimental to many strategies that have taken an oxidative
approach to the γ-cabinol group in lignin as it frequently results
in an insoluble and intractable material.31, 38 To prevent this
problem, an additional methylation step using TBAF/MeI was
required to convert phenols prior to selective primary alcohol
38
oxidation (step 2, Fig. 1b).31,
Nevertheless, this
environmentally unfriendly methylation step can be avoided
when the lignin defunctionalisation catalyst can tolerate phenol
moieties. Therefore, 4-hydroxy-3-methoxybenzenepropanol
(
BP2), containing a phenol was used as model compound for the
dehydrogenative decarbonylation catalyst in both 1,4-dioxane
and diethyl carbonate.
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also detected in the gas phase, which could derive from the
water-gas shift reaction between the liberated CO and the
added water from the catalyst preparation. Additionally, the
steady continued gas formation indicated that the catalyst
appears to be relatively stable.
View Article Online
DOI: 10.1039/D0GC01209B
Entry Starting material
Solvent
Time Set Temperature*
Conversion
Yield
1
2
A-OEt
A-OEt
A-OEt
A-OEt
A-OEt
A-OBu
A-OBu
A-OBu
A-OBu
A-OBu
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
24h
48h
24h
48h
48h
24h
48h
190 °C
190 °C
200 °C
200 °C
210 °C
200 °C
200 °C
200 °C
210 °C
210 °C
3
4
5
6
7
8
diethyl carbonate 48h
diethyl carbonate 24h
diethyl carbonate 48h
9
10
0
10 20 30 40 50 60 70 80 90100
Fig. 6 Catalytic deheydorgenative decarbonylation A-OEt or A-OBu to B-OEt or B-
OBu. Reaction conditions: 15 mg substrate, [Ir(cod)Cl]2 5 mol%, 4 : 2 : 1 LiCl : rac-
BINAP : [Ir(cod)Cl]2, as catalyst; 190-210 °C in 1 mL solvent. Yield and conversion
were determined by GC-FID using n-decane as external standard; *Set
temperature of the heating block, the measured corresponding inside
temperature is shown in Table S1.
Fig. 5 Syngas formation over time using 4-hydroxy-3-methoxybenzenepropanol
(
BP2) as model compound. Reaction conditions: 1 mmol substrate, [Ir(cod)Cl]2 2.5
mol%, rac-BINAP 5 mol%, LiCl 10 mol%, 190 °C (Setting temperature of the oil
bath) in 2 mL solvent. Yield and conversion were determined by GC-FID using n-
ocatadecane as external standard.
led to further improvement, achieving 46 % yield at 24 hours
and 59% yield at 48 hours (Fig. 6, entry 3 and 4). Yet, there was
no further improvement of the product yield at 210 °C (inside
150 °C, Fig. 6, entry 5). In these reactions, the main side
products were identified as veratrylglycol-β-guaiacyl ether
Catalytic defunctionalisation of A-OEt and A-OBu via
dehydrogenative decarbonylation
With the successful demonstration of acceptorless catalytic
dehydrogenative decarbonylation on benzenepropanol model
compounds, the α-ethoxylated β-O-4 motif A-OEt was tested as
the starting material in the same catalytic system. Compared
with the native β-O-4 motif A-OH, the α-ethoxylated A-OEt
motif avoids decomposition via retro-aldol reaction29, 30 under
these catalytic conditions and thus allows for selective
modification on the γ position of the motif followed by
defunctionalisation.31 Furthermore, this motif is readily
accessible by extraction of lignin with ethanol.31, 33 When A-OEt
in 1,4-dioxane was heated to 190 °C in the presence of 5 mol%
of [Ir(cod)Cl]2, only 27% yield was observed with full conversion
of A-OEt (Fig. 6, entry 1). When the reaction time was doubled
to 48 hours, the yield only increased by 8% reaching 35% (Fig. 6,
entry 2). An increase in the temperature to 200 °C (inside 146
°C)
(
SP1), 1,2-dimethoxy-4-ethylbenzene (SP2) and guaicol (SP3),
see Scheme 1 and Fig. S3. There were two possible routes for
the formation of these side products (Scheme 1). Either the
desired defunctionalized β-O-4 motif B-OEt eliminated ethanol
affording SP, which can undergo hydrogenation resulting in SP1
and further hydrogenolysis to SP2 and SP3 or the starting β-O-
4 motif A-OEt was directly cleaved by hydrogenolysis to get
guaiacol
(
SP3
)
and SP4
,
which gets converted by
SP4 was not detected
dehydrogenative decarbonylation to SP2
.
by GC-MS, but this route cannot be excluded as the lack of its
presence might be obscured due to the high activity of
dehydrogenative decarbonylation catalyst. Both routes require
hydrogen, but this is present due to its formation in the
reaction.
Scheme 1. Possible side reactions during deheydorgenative decarbonylation of A-OEt or A-OBu
.
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In addition, we tested this catalyst on the α-butoxylated model (see ESI S3.0 for details).18 After conducting the
View Article Online
compound A-OBu, which represents the β-O-4 motif obtained dehydrogenative decarbonylation reaDctOioI:n10.1039/D0GC01209B
with
5 mol%
51
in lignin from n-butanol extraction instead of ethanol.34,
[Ir(cod)Cl]2 for 48 hours, most of PC was converted (Fig. 7).
Reacting the A-OBu at 200 °C for 24 hours in 1,4-dioxane gave However, the expected compound PC0 was not identified by
57% yield of B-OBu, which was comparable to 59% yield GC-MS. Instead, PC2 was detected by GC-MS to be the main
obtained for A-OEt (Fig. 6, entry 6). An even higher yield of 70% product with PC1 as minor product (Fig. 7 and S6). PC1 is
was obtained by doubling the reaction time to 48 hours (Fig. 6, thought to form via a ring-opening reaction that forms a phenol
entry 7). In order to show its compatibility for a greener solvent, and
this reaction was also performed in diethyl carbonate at 200 °C hydrogenation (Fig. 7). Similar to the β-O-4 model reaction, the
(inside 154°C), however, the yield dropped dramatically to only exchange reaction between diethyl carbonate and -carbinol
a stilbene. The stilbene then can afford PC2 by

22% even after 48 hours (Fig. 6, entry 8). The GC spectra of this group was identified as well as shown by the identification of
mixture showed two new signals, which were identified as two PC3 (Fig. 7 and S7). Therefore, under the dehydrogenative
diastereomers of a A-OBu analogue on which ethyl carbonate decarbonylation reaction conditions, other structural motifs in
got attached on the
 position in conjunction with ethanol lignin such as the γ-carbinol of β-5 linkage motifs and possible
liberation (Fig. S4 and S5). This side reaction accounted for the benzylic primary alcohols are verified to be able to contribute
low yield of the B-OBu in diethyl carbonate. However, due to to the formation of syngas as well via the removal of these
the reversible nature of this side reaction, it could be avoided primary alcohol groups, leading to even further
to a certain extent by increasing reaction temperature. At 210 defunctionalisation of lignin structures.
°C (inside 159°C) a yield of 61% of B-OBu was achieved at 24
Dehydrogenative decarbonylation of walnut organosolv
lignins (LA-OEt and LA-OBu)
hours and 65% yield was obtained at 48 hours (Fig. 6, entry 9
and 10). In summary, this dehydrogenative decarbonylation
catalytic system could work satisfactorily for α-ethoxylated and
butoxylated β-O-4 motifs at 200-210 °C which corresponds to
an inside temperature of 146-150 °C in 1,4-dioxane and 154-
159°C in diethyl carbonate.
Based on the successful application of this catalytic
methodology on a series of representative model compounds,
we investigated the effectiveness of the defunctionalisation on
isolated lignin. For this purpose, organosolv lignins with high
levels of β-O-4 motifs as well as with high α-ethoxy or α-butoxy
incorporation were used as starting materials. According to the
2D-HSQC NMR spectra shown in Fig. 8 a and c, the obtained α-
Dehydorgenative decarbonylation of other primary alcohol
containing lignin structures
Considering the presence of other structures in lignin that protected lignins (LA-OR) consisted of mainly β-O-4 α-OR
contain primary alcohols, we further examined the structure, R = Et, Bu (see ESI S4.0 and Table S2 for details). No
effectiveness of dehydrogenative decarbonylation on
a
further phenol methylation step, as required in our previous
51
phenylcoumaran model compound (PC) shown in Fig. 7 that procedure31,
, was performed as the dehydrogenative
contains a -carbinol as found in the lignin β-5 motif as well as a decarbonylation catalyst was confirmed to be compatible with
benzylic primary alcohol. This model compound was phenol groups in model studies. When dehydrogenative
synthesized via the direct reduction of a previously reported decarbonylation was performed on α-ethoxylated lignin (LA-
intermediate structure reported for the synthesis of β-5 models OEt), at the set temperature of 205 °C in diethyl carbonate for
Fig. 7 Dehydrogenative decarbonylation of the phenyl coumaran model compound (PC). Reaction conditions: 10 mg substrate, [Ir(cod)Cl]2 5 mol%, 4 : 2 : 1 LiCl : rac-
BINAP : [Ir(cod)Cl]2, heating block set at 210 °C in 1 mL solvent.
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48 hours, an obvious product signal was observed in 2D-HSQC achieved by taking both LB-OEt and LC structure VinietwoAraticclceoOunnlinte.
NMR spectra, and there was a good match of the product Likewise, α-butoxylated lignin underwenDtOthI:e10d.1e0h3y9/dDr0oGgCe0n1a2t0iv9eB
spectra with those of the defunctionalized β-O-4 model decarbonylation reaction under the same procedures. There
compound. The β-signal of the desired defunctioanlized lignin was an obvious shift of the β signal in the LB-OBu structure as
(
LB-OEt) was clearly identified (Fig. 8b). To have a better well (Fig. 8d). Similarly, LC and LD structures were found in the
understanding on the effectiveness of the acceptorless product. Especially, this dehydrogenative decarbonylation
dehydrogenative decarbonylation on lignin, a tentatively semi- methodology remained effective when the stating lignin was
quantification was carried out based on the 2D-HSQC spectra of extended to a pentanolsolv lignin (LA-OPn), showing the
lignin (Table S2, calculation detailed see ESI section 4.4). The possibility of using different alcohols to protect the α-hydroxy
defunctionalized linkage, LB-OEt, was obtained in about 15% group during the extraction stage (Fig. S14 and S15) prior to the
yield and 16% selectivity based on the conversion of the starting application in iridium-based dehydrogenative decarbonylation.
β-O-4 α-OEt. In accordance with our model compound
The release of syngas from lignins by dehydrogenative
decarbonylation
dehydrogenative decarbonylation study, derivatives of
veratrylglycol-β-guaiacyl ether SP1 and 1,2-dimethoxy-4-
(
)
ethylbenzene (SP2) seem to be part of the cause (Scheme 1).
Accordingly, we checked the existence of LD structure in
defunctionalized lignin by overlaying the product 2D-HSQC
NMR spectra with 4-ethylguaicol, the derivative of 1,2-
dimethoxy-4-ethylbenzene (SP2), which showed a good match
in the aliphatic region (Fig. S18). Furthermore, in order to
The volume and composition of the gas released were analysed
to confirm the formation of valuable syngas during
defuncationalization
of
lignin
by
dehydrogenative
decarbonylation. 200 mg of ethanosolv lignin generated 5.4 mL
syngas with a CO : H2 ratio of 4.2 : 1 after reacting in 1,4-dioxane
at 205 °C for 48 hours. We suspect that the higher content of
CO than that of H2 was due to the consumption of H2 for the
side reactions that analogue to those shown in scheme 1. Thus
some of the liberated H2 was likely consumed by hydrogenation
or hydrogenolysis of the β-O-4 linkage to afford LC or LD
structures (Fig. 8), which was consistent with the model
compounds study. To exclude that demethoxylation of the
aromatic rings contributes to synthesis gas formation we
confirm the presence of the LC structure, reported NMR data of
25
veratrylglycol-β-guaiacyl ether (SP1
)
was compared with the
2D-HSQC NMR spectra of the defunctionalized lignin. The
comparison indicated that the β signal in the LC structure
actually corresponded to Hα–Cβ (δH/δC 3.99–4.29/73.19–74.62
ppm) in the HSQC NMR spectra (Fig. 8b). In addition, there were
some unknown impurities originating from the starting lignin
which overlapped this signal region. Based on the linkages
calculated by HSQC NMR (Table S2), the LC structure, another
type of defunctionalized β-O-4 linkage, was obtained at a yield
calculated the S/G and - : -OMe ratio in the 2D-HSQC NMR
spectra and found that the ratios were not significantly changed
(Table S3). Interestingly, by changing the solvent to diethyl
carbonate, significantly higher amount of syngas, 18.9 mL, was
of about 38% and 40% selectivity. Thus for the
-ethoxylated
lignin, an overall 53% yield of the defunctionalized lignin was
Fig. 8 2D-HSQC NMR spectra (600 MHz, CDCl3) of the linkage region of a) α-ethoxylated lignin (starting lignin), b) defunctionalized α-ethoxylated lignin c) α-butoxylated lignin
(starting lignin), d) defunctionalized α-butoxylated lignin; defunctionalized lignins were obtained by dehydrogenative decarbonylation, reaction conditions: 200 mg LA-OEt or LA-
OBu with [Ir(cod)Cl]2 13.4 wt%, rac-BINAP 24.9 wt%, LiCl 3.4 wt% and a drop of water dissolved in 2.5 mL diethyl carbonate reacting at a set temperature of 205 °C for 48 hours.
This journal is © The Royal Society of Chemistry 20xx
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generated. However, in this case also 11.6 mL methane, the tolerate phenol groups, which lead to the omission of the toxic
View Article Online
product of dehydrogenative decarbonylation of ethanol was methylation step on lignin. MeaDnOwI:h1i0le.1,039t/Dh0isGC0d1i2r0e9cBt
observed. This could be from ethanol released from the lignin dehydrogenative decarbonylation defunctionalisation method
structure but decomposition of diethyl carbonate could not be is atom-economic and waste-free by producing synthesis gas as
excluded. As mentioned previously in the model compound another valuable product, offering great opportunity for further
studies, the diethyl carbonate could react with alcohols releasing the valorisation potential of lignin yet with a well
resulting in ethanol formation. Additionally, methane was preserved and less complex β-O-4 linkage structure.
observed in butanosolv lignin reaction in diethyl carbonate, Additionally, this study shows that the dehydrogenative
however in a much smaller amount of 9% compared to 17% for decarbonylation methodology can serve as a powerful tool to
ethanosolv lignin. This suggested that the methane was sustainably valorise alcohol containing substrates which are
generated from ethanol released from ethanol elimination from widely accessible from renewable biomass resources. Further
the β-O-4 motif as well as from exchange reactions between development of the catalyst and the reaction set-up are
alcohols and diethyl carbonate. No methane was detected in required and are ongoing towards more efficient and selective
the reactions with BP2 showing that diethyl carbonate conversion in lignin, in particularly to obtain improved
decomposition only occurs at the more elevated reaction selectivity in defunctionalized lignin structures.
temperatures used on
A-type model compounds and lignin.
Analogously, 4% propane was found in the gas phase in the
reaction using butanosolv in diethylcarbonate. No propane was
found in the gas released from the dehydrogenative
decarbonylation of ethanosolv or pentanosolv lignins,
confirming also that alcohols released by elimination from the
lignin structure are in part dehydrogenative decarbonylated to
give syngas and the corresponding alkanes. Nevertheless, these
results show the possibility to apply dehydrogenative
decarbonylation for lignin defunctionalisation with a joint
production of syngas.
Experimental
Chemicals
1,5-cyclooctadiene-iridium(I) chloride dimer ([Ir(cod)Cl]2) and
rac-BINAP were purchased from Fluorochem. Other chemicals
obtained from Acros, Sigma-Aldrich or Fluorochem were used
as received. Both 1,4-dioxane and diethyl carbonate utilised in
this work were acquired anhydrous under argon and used using
standard Schlenk techniques. The α-ethoxylated lignin C3 β-O-4
model compound A-OEt and α-butoxylated lignin C3 β-O-4
model compound A-OBu, were synthesized following the
51
published procedure.31,
The C2 model compounds 4-(1-
Conclusion
ethoxy-2-(2-methoxyphenoxy)ethyl)-1,2-dimethoxybenzene
B-OEt and 4-(1-butoxy-2-(2-methoxyphenoxy)ethyl)-1,2-
In this study, we report the first example of combined targeted
lignin defunctionalisation and biomass-derived synthesis gas
formation. This is achieved by iridium catalysed acceptorless
dehydrogenative decarbonylation. Its effectiveness relies on
the high activity as well as the high selectivity towards primary
alcohol groups in the lignin structure. Here, our key innovation
is the extension of the application of this catalytic methodology
for the first time to lignin model compounds with different
primary alcohol groups. For the model compounds with a
representative β-O-4 motif, 70% selectivity towards the
(
)
dimethoxybenzene (B-OBu) were synthesized as previously
reported.31 The synthesis of the phenylcoumaran model
compound was provided in the electronic supplementary
information (ESI, S3.0 for details). The α-ethoxylated lignin (LA-
OEt), α-butoxylated lignin (LA-OBu) and α-pentoxylated lignin
(
LA-OPn) were obtained following the similar procedure
provided in ESI (S4.0 for details).
Dehydrogenative decarbonylation of model compounds
removal of the -carbinol group to the desired defunctionalized
A typical procedure for dehydrogenative decarbonylation
reaction is as follows: catalyst stock solution was premade by
dissolving 8.75 µmol [Ir(cod)Cl]2, 17.5 µmol rac-BINAP, 35 µmol
LiCl, a drop of water and 0.15 mmol n-octadecane in a 10 mL
volumetric flask, which was then sonicated for 10 minutes.
Model compound (e.g. 4-methoxybenzenepropanol or 4-
hydroxy-3-methoxybenzenepropanol 0.07 mmol) was weighed
out in a 20 mL pressure vial (Biotage) equipped with a magnetic
stirring bar. 2 mL catalyst stock solution was added to the vial,
and the vial was then sealed with a Biotage airtight cap, which
allowed for the reaction to be run under the pressure caused by
heating the reaction above the boiling point of the respective
solvent used. The vial was flushed with nitrogen for two minutes
via a needle connection to ensure a nitrogen atmosphere. The
mixture was then heated to the desired temperature in a
heating block. Before sampling, the vial was cooled in cold
water bath. A 0.2 mL sample was taken through the septum of
lignin structure was achieved. The reactions could be performed
in 1,4-dioxane which is highly compatible with many lignins.
Diethyl carbonate was shown to be a significantly greener
alternative, but also contributes to synthesis gas formation at
more
elevated
temperatures.
Furthermore,
the
dehydrogenative decarbonylation reaction was demonstrated
on organosolv lignin. The same defunctionalised β-O-4 motif
was clearly visible by 2D-HSQC NMR for an
-ethoxylated, -
butoxylated and -pentoxylated lignins, although the high

temperatures required to get efficient conversion lead to the
formation of some side-products that could be identified using
data from the model compound reactions. Nevertheless, this
defunctionalisation route shows promise. Compared with our
previous defunctionalisation strategy that required firstly
oxidation followed by
a
decarbonylation step, this
dehydrogenative decarbonylation methodology can not only
combine these transformations into one single step but also
8 | J. Name., 2012, 00, 1-3
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the vial cap using a syringe equipped with a thin needle. The
sample was diluted in 0.8 mL 1,4-dioxane and analysed by GC-
MS and GC-FID. After sampling, the vial was flushed with
nitrogen for 2 minutes and reheated to the appropriate
temperature. For the starting materials A-OEt and B-OEt, 0.04
mmol was used and n-decane was applied as external standard
while the other relative quantities and procedures were kept
similar. Regarding the -5 model compound, 29 µmol was used
for each reaction with 1,4-dinitrobenzen as internal standard.
After the reaction, it was analysed by both NMR and GC-MS.
Conflicts of interest
The authors declare no conflicts of interest.
View Article Online
DOI: 10.1039/D0GC01209B
Acknowledgements
For funding, Z.Z. acknowledges the China Scholarship Council
for funding (grant numbers 201704910922). The work
performed by C.W.L. has partly been conducted within the
framework of the Dutch TKI-BBEI project “CALIBRA”, reference
TEBE117014. The financial support of the Dutch Ministry of
Economic Affairs and Climate for this project is gratefully
acknowledged. Additionally, the authors would like to
acknowledge Johanna H. L. Kaelen and Lambert J. Deuss for
providing the walnut shells used for lignin extraction. Analytical
support was provided by Leon Rohrbach and Johan Kemmink.
Collection and quantification of syngas from the model
compound
To collect gas released from the reaction, 25 µmol [Ir(cod)Cl]2,
50 µmol rac-BINAP, 100 µmol LiCl and a drop of water was
dissolved in 2 mL diethyl carbonate in a 15 mL Schlenk tube
equipped with a magnetic stirring bar. After stirring for 10
minutes, 1 mmol 4-hydroxy-3-methoxybenzenepropanol was
added. A short condenser was then installed on the Schlenk
tube, which the condenser was further connected to a tube
installed with a long needle allowing the gas being able to go
out. Prior to heating the oil bath temperature to 190 °C, the
whole set-up was flushed with nitrogen for 2 minutes. Once the
flushing was completed, the liberated gas volume was
measured via an inverted burette placed inside a water bath.
Similarly, a blank reaction was performed without the addition
of starting material, and the volume increase due to heating was
noted when it became constant. The actual gas volume was
corrected by deducting the volume increase generated by the
blank reaction.
Abbreviations
[Ir(cod)Cl]2 = Bis(1,5-cyclooctadiene)diiridium(I) dichloride, rac-
BINAP = [2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl], cod =
1,5-Cyclooctadiene, ES = External standard, GC-FID = Gas
chromatography with flame ionization detector, GC-MS = Gas
chromatography with mass spectrometry detector, GC-TCD =
Gas chromatography with a thermal conductivity detector,
HSQC = Heteronuclear single quantum coherence spectroscopy,
IS = Internal standard, TBAF = Tetrabutylammonium fluoride
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Page 11 of 11
Green Chemistry
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DOI: 10.1039/D0GC01209B
TOC entry:
Iridium catalysed acceptorless dehydrogenative decarbonylation of organosolv lignin is exemplified
for the combined lignin defunctionalisation and liberation of synthesis gas.