Mechanistic studies of base-catalysed lignin depolymerisation in dimethyl carbonate

Article abstract of DOI:10.1039/c7gc03110f

The depleting fossil reservoirs have stimulated global research initiatives on renewable lignin feedstocks as sustainable alternatives to petroleum-derived aromatics. Base-catalysed depolymerisation (BCD) is regarded as an economical and efficient approach for the valorisation of technical lignins. The major limiting factor encountered during this process is the re-condensation of the formed phenolic products, which results in lower monomer yields. To diminish these side reactions, we selected alkali earth metal catalysts in dimethyl carbonate (DMC) to produce methylated phenol derivatives as the final products. Herein, we demonstrate for the first time a base-promoted depolymerisation process affording low-molecular weight oils in high yields (52-67 wt%) wherein the employed bases are used in truly catalytic quantities (with catalyst loadings of around 5 mol%). The general applicability of this methodology was proved on four different lignin samples (1 Kraft, 3 organosolv) using caesium carbonate and lithium tert-butoxide as catalysts. The 2D NMR studies on the post-reaction lignin samples showed a similar degradation of the major lignin linkages for both bases. A difference in the reduction of phenolic moieties was revealed by quantitative ³¹P NMR analysis. Furthermore, GPC analysis demonstrated a significant shift towards lower mass fragments for the Cs₂CO₃-catalysed lignin degradation. A detailed GC-MS analysis for these samples identified a range of methoxy capped-monomeric degradation products. The scope of this reaction system was further expanded to lignocellulosic biomass such as milled beechwood chips, which notably showed similar product distributions. Based on the correlation of the experimental observations for extracted lignin samples and model compound studies, a mechanistic pathway for the Cs₂CO₃-catalysed system was suggested. DFT calculations provided reaction pathways for the observed cleavage products.

Full text of DOI:10.1039/c7gc03110f

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Heterogeneous catalytic oxidation for lignin valorization into valuable
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ARTICLE
Mechanistic studies of base-catalysed lignin depolymerisation in
dimethyl carbonate
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a,b
b
Saumya Dabral, Julien Engel, Jakob Mottweiler, Stephanie S. M. Spoehrle, Ciaran W. Lahive
a
and Carsten Bolm*
DOI: 10.1039/x0xx00000x
The depleting fossil reservoirs have stimulated global research initiatives on the renewable feedstocks of lignin as
sustainable alternatives for petroleum-derived aromatics. Base-catalysed depolymerisation (BCD) is regarded as an
economical and efficient approach for the valorisation of technical lignins. The major limiting factor encountered during
this process is the re-condensation of the formed phenolic products, which results in lower monomer yields. To diminish
these side reactions, we selected alkali earth metal catalysts in dimethyl carbonate (DMC) to produce methylated phenol
derivatives as the final products. Herein we demonstrate for the first time a base-promoted depolymerisation process
affording low-molecular weight oils in high yields (52-67 wt%) wherein the employed bases are used in truly catalytic
quantities (with catalyst loadings of around 5 mol %). The general applicability of this methodology was proven on four
different lignin samples (1 Kraft, 3 Organosolv) using caesium carbonate and lithium tert-butoxide as catalysts. The 2D
NMR on the post reaction lignin samples showed a similar degradation of the major lignin linkages for both bases. A
www.rsc.org/
3
1
difference for the reduction of phenolic moieties was revealed by quantitative P NMR analysis. Furthermore, GPC
analysis demonstrated a significant shift towards lower mass fragments for the Cs CO -catalysed lignin degradation. A
2
3
detailed GC-MS analysis for these samples identified a range of methoxy capped-monomeric degradation products. The
scope of this reaction system was further expanded onto lignocellulosic biomass such as milled beechwood chips, which
notably showed similar product distributions. Based on the correlation of the experimental observations for extracted
2 3
lignin samples and model compound studies, a mechanistic pathway for the Cs CO -catalysed system was suggested. DFT
calculations provided reaction pathways for the observed cleavage products.
Introduction
Lignin is a renewable biopolymer rich in aromatics which
represents a considerable portion (roughly 30% on a weight
basis and 40% on an energy basis) of the lignocellulosic
1
biomass. Despite being widely available as a waste product
from the pulp and paper industry, lignin is by far the least
utilised fraction in lignocellulose in comparison to cellulose
2
and hemicelluloses. The structural diversity and complexity of
this three-dimensional, amorphous lignin biopolymer
attributes to its overall recalcitrance. This represents a major
hindrance for obtaining high-value chemicals in an integrated
biorefinery. Thus, the lignin fractions are most often relegated
Scheme 1 Base-catalysed cleavage of lignin in DMC.
2
,3
to low-value uses and burnt to get energy. Since in the
forthcoming years biorefineries will process considerable
amounts of renewable feedstocks, it becomes important to
find efficient valorisation routes and upgrade this readily
available resource to make the overall process more
2
,3a
economical.
The lignin structure is composed of a randomly branched
network of electron rich phenylpropanols (coumaryl, coniferyl,
sinapyl alcohols) that are connected to each other through a
4
variety of stable C-C and etheral C-O linkages. Amongst these,
the β-O-4 linkages (Scheme 1) are the most predominant alkyl-
aryl ether interconnecting bonds. A controlled and selective
2
-4
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Table 1 Characterisation of lignin samples by GPC and 2D-HSQC NMR.
a
b
c
b
Lignin source
Mw (Da)
S, G, H (%)
β
-O-4-OH
Linkages (per 100 C9 units)
d
β
-O-4-OR
β
-5
β-β
OS-O
OS-BW
2610
3103
15220
7106
85, 14, trace
73, 27, trace
27
20
37
5
6
4
3
5
5.1
6
3.2
2
OS-C
86, 14, trace
1.4
0
Aldrich-Kraft
trace, 100, trace
1
1
a
b
Determined by GPC (DMF/LiCl) using RI detector with respect to polystyrene standards. Determined by 2D-HSQC NMR by comparing the aliphatic and
β-O-4 linkages with
capped a-OH units.
c
aromatic signal intensities (Electronic Supplementary Information 2.2 a). Amounts of
d
-OH. Amounts of
β
-O-4 linkages with free
α
3
Table 2 Quantitative P NMR measurements of the aliphatic, phenolic, and carboxylic hydroxyl groups in lignin samples.
1
a
Lignin source
OH (mmol/g)
Aliphatic OH
2.37
5-substituted OH
guaiacyl OH (G)
p-hydroxyphenyl OH (H) total phenolic OH
COOH
0.12
0.01
0.07
0.20
OS-O
OS-BW
1.06
1.24
1.70
0.90
0.56
0.69
0.88
1.77
0.05
0.01
0.03
0.17
1.66
1.94
2.63
2.84
2.38
OS-C
1.90
Aldrich-Kraft
1.79
a
Lignin samples phosphitylated with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (Electronic Supplementary Information 2.2.b)
lignin depolymerisation strategy targeting these β-O-4 linkages monomer yields. To obtain higher yields for the monomer rich
is a key way for producing a variety of high-value, low oil fractions at higher temperature and pressure conditions,
5
molecular weight aromatics. Numerous publications in recent Lercher and Toledano independently reported boric acid and
5
,6
10j,l
years been devoted to such strategies involving oxidative,
phenols as suitable capping agents.
exploited the versatile chemical nature of DMC at elevated
-O-4 model compounds and extracted temperatures (> 90 °C) in the presence of bases, and reported
lignin samples. Most of the literature on BCD highlight the a BCD strategy for a range of lignin -O-4 model compounds
advantage of using cheap and readily available reagents obtaining methoxy-capped phenolic and benzoic acid
Alternatively, we have
7
5c,8
9
10-11
reductive, redox-neutral,
degradation of lignin
acid- and base-catalysed
β
β
1
0
10p
(
NaOH, KOH, Ca(OH)
2
, LiOH, liq. NH
3
etc.). Lately, in order to derivatives as products.
Recently, Barrett et al. also
make processes more economically viable, there have been demonstrated the intricate role of a MeOH/DMC solvent
attempts to perform the degradation reactions using solid system which helped to increase the aromatic yields by O-
bases that are frequently combined with other metal methylation of the phenolic intermediates for their Cu20PMO-
1
1
7i
catalysts. Along those lines a recent article by Chaudhary and catalysed reductive lignin disassembly pathway.
Dhepe compared the efficiency for lignin depolymerisation Predicting the reaction behaviour of real lignin based on
1
using a variety of heterogeneous bases. The highest yields results from lignin model compound studies is often not
1g
4
b,c
for the low molecular weight products (monomers to trimers, reliable because they frequently differ from each other.
1%) were attained using NaX zeolites at 250 °C with a Considering these implications, the current study focuses on
5
reaction time of 1 h. It is noteworthy, however, that the base the BCD of four lignin polymers, aiming to increase the yields
was employed in stoichiometric amounts and a significantly of the oil fractions with a higher composition of methoxy-
decreased yields for low molecular weight products capped low molecular weight products. After having
(
monomers to trimers) was observed (51% to 34%) in the first established that the same types of products are formed from
1
1g
recycle run.
lignin as in model compound-based studies, efforts were put in
The use of stoichiometric amounts of base in BCD relative to to studying the mechanistic aspects of the reaction. We herein
monomer units present in lignin is a common theme in propose reaction pathways leading to the formation of the
1
0b-c,j-k,m,q
literature.
wt% relative to the weight of solvent and not lignin.
example, one standard reaction condition entails 10%
The catalyst loadings are generally given in observed capped-methoxy-aromatics.
1
0b-l,n,q
For
Results and Discussion
[
w(lignin)/w(solvent)] lignin loading and 2% to 4%
Characterisation of lignins
[
w(base)/w(solvent)] catalyst loading using a base such as
1
0c,i-k,q
NaOH.
The catalyst loadings relative to lignin are
In this study, four lignin samples namely three organosolv
lignins, isolated from cherry (OS-C), oak (OS-O), beechwood
therefore usually between 20 wt% and 40 wt%. The majority of
BCD reactions require elevated temperatures from 200 °C to
(OS-BW), and commercially available Kraft lignin from Sigma
3
40 °C and are often accompanied by high pressures.
Aldrich (370959) were evaluated under our BCD reaction
conditions in DMC. Since the isolation process of a lignin
sample significantly influences the physical and chemical
properties of the polymer including the identity and ratio of
Furthermore, these conditions promote secondary reactions
between the reactive phenolic and aldehyde/ketone
intermediates hence leading to an overall decrease in
2
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Fig. 1 2D-HSQC NMR spectra (in DMSO-
d
6
) of organosolv, cherry, oak, beechwood and Aldrich-Kraft lignin; before the reaction; after the reaction with Cs
-O-4′ aryl ether linkages (b) resinol substructures formed by
′, -O- ’, and -O- ′ linkages; (c)
-O-4′ linkages.
2 3
CO at
1
80 °C; after the reaction with LiO
phenylcoumaran substructures formed by
t
-Bu at 180 °C; (a)
β
β
–
β
α
γ
γ
α
β
-5′ and
α
the chemical linkages, the lignin samples were characterised These methods were used to determine the abundance of
3
by 2D-HSQC NMR, GPC and P NMR [after derivatisation with
1
β-O-4, β-5 and β-β linkages as well as to quantify the total
the phosphorylating agent 2-chloro-4,4,5,5-tetramethyl-1,3,2- amounts of aliphatic hydroxyl groups present in the individual
dioxaphospholane (TMDP)] measurements prior to usage.
lignin samples. The data from these experiments are outlined
in Tables 1 and 2.
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Lignin Degradation Studies
DMC catalysed by both the employed bases as shown in
Scheme 3. Reports in literature suggest that the various
phenolic-OH groups can undergo base promoted
The lignin samples were subjected to BCD studies using 10
wt% [w(base)/w(lignin)] of either of the two bases (Cs
LiO -Bu) at 180 °C for 8 h (Cs CO ) or 12 h (LiO -Bu). Depending
on the lignin source this corresponded to a catalyst loading of
to 6 mol% for Cs CO and 23 to 26 mol% for LiO -Bu. The
resulting reaction mixtures were analysed by 2D-HSQC NMR,
2 3
CO or
7
h,13
methylation.
Additionally, the cleavage of the
t
2
3
t
interconnecting bonds could also result in the decreased peak
intensities in the defined regions of the spectra. Therefore,
follow-up experiments focused on determining any shifts in
the average molecular weights of the residual lignin mixtures
after BCD reactions.
5
2
3
t
3
1
quantitative P NMR and GPC. Lower molecular weight
fractions (oil) were separated from the residual lignins by
solvent extraction using ethyl acetate and analysed by GC-MS
to identify and individually quantify the capped-aromatic
monomer products.
2
D NMR Studies
Preliminary studies to determine the changes in lignin
1
13
structures mainly focused on their two-dimensional H- C
correlation NMR spectroscopy (HSQC) before and after the
BCD reactions. In particular, the aliphatic oxygenated side
chain regions ( C
showed a major decrease in the regions corresponding to the
-aryl-ether ( -O-4) and phenylcoumaran ( -5) bonds. The
resinol ) linkages seemed to remain unaltered for both
the Cs CO and LiO -Bu-catalysed BCD processes in all four
(Fig. 1). A follow up reaction using model
compound 1a further confirmed the low reactivity of the
resinol ( ) linkages for our reaction system (Scheme 2). This
δ / δH 50.0-90.0/2.5-5.8) of the HSQC spectra
β
β
β
(β-β
t
2
3
lignin samples
β-β
could in turn be attributed to the lack of aliphatic hydroxy
groups, which were later proven during the mechanistic
studies to have a considerable influence on the overall
degradation pathway.
Cs
2 3
CO (0.05 equiv)
DMC
O
O
180 °C, 8 h
O
H
H
O
LiOt-Bu (0.05 equiv)
DMC
O
O
1a
180 °C, 12 h
Scheme 2 Reactivity of
β
-
β
model compound 1a in the BCD reactions.
3
1
Fig. 2 Quantitative P NMR spectra of lignins;
t
A
reaction with LiO -Bu; after the reaction with Cs
before the reaction;
CO
B after the
C
2
3
.
3
Table 3 Reduction in hydroxyl groups of lignins quantified by P NMR using
1
3
1
Quantitative P NMR Studies
cyclohexanol as internal standard.
The degradation of the lignin linkages as observed in the 2D-
NMR studies prompted us to further investigate its
implications for the functional groups still present in the lignin
structures. Thus, a series of product analyses were carried out
%
reduction in -OH
5- substitutes OH
Base
lignin
OS-O
OS-BW
OS-C
aliphatic- OH
guaiacyl OH
84
89
92
96
98
99
97
99
48
68
64
93
98
97
98
99
73
74
74
93
97
98
97
94
3
1
using quantitative P NMR spectroscopy in the presence of a
LiOt-Bu
1
2
known amount of an internal standard (Fig. 2, Table 3). As
seen in Fig. 2, for both bases an overall reduction in the
amounts of aliphatic, phenolic, and carboxylic -OHs in the
lignin samples was observed. However, quantification studies
Kraft
OS-O
OS-BW
OS-C
2 3
Cs CO
revealed that the Cs
2
CO
3
-catalysed reactions underwent a
Kraft
complete removal of these functional groups (Table 3 and
Table S1). One likely reason for the decrease in aliphatic -OH
groups can be credited to the transesterification reaction with
4
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Fig. 3 GPC measurements for the cherry, oak, beechwood and Kraft lignin samples: before reaction (no catalyst); after reaction with Cs
2 3
distribution with respect to polystyrene standards calibration; Right: elugram.
2
mmol), 8 h, 180 °C; after reaction with LiOt-Bu (10 wt%, 0.125 mmol), 12 h, 180 °C; after reaction with Cs CO (5 wt%, 0.015 mmol), 8 h, 180 °C. Left: mass
CO
3
(10 wt%, 0.030
Table 4 Average molecular weight change before and after the reactions.
Lignins
Mw (Da)
before the reaction
5 wt% Cs
2
CO
3
10 wt% Cs
253
2
CO
3
10 wt% LiOt-Bu
3235
Organosolv-cherry
Organosolv-oak
15220
2610
3103
7106
2436
2022
2008
2192
212
2506
Organosolv-beechwood
Aldrich-Kraft
383
1898
1696
5790
Gel Permeation Chromatography Analysis
ether linkages resulting from the more invasive pre-
treatment conditions. Furthermore, reactions with
The relative change in the weight average molecular
2 3
reduced amounts of Cs CO (5 wt%) proved to be less
w
weight (M ) for the lignin samples preceding the BCD
effective as observed in the GPC (pink lines) thereby
emphasising the effect of base concentration on the
product outcome. The influence of the base loading did
not become apparent through previous HSQC
measurements considering that the degradation of the
alkyl-aryl ether linkages was already observed with 5
was determined by GPC analysis. Fig. 3 depicts the mass
distribution chromatograms and the elugrams for the
lignins both before and after the BCD with 10 wt%
[
w(base)/w(lignin)] of Cs
given reaction conditions. As reflected in Table 4, there
was a stark decrease in the M values for all the samples
treated with Cs CO (10 wt%, Fig. 3, red lines), in
comparison to the corresponding LiO -Bu-catalysed
2 3
CO and LiOt-Bu under the
w
1
0p
2 3
wt% of Cs CO and 1.2 wt% of LiOt-Bu, illustrating the
2
3
importance of various analytical techniques for the
characterisation of lignin depolymerisation.
t
reactions (Fig. 3, blue lines). For the organosolv lignins,
the decrease in mass was more significant in comparison
to the Kraft lignin. This can be attributed to the rather
condensed structure of Kraft lignin with less alkyl-aryl
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Table
5
Quantification of selected methoxy-capped aromatic monomers
afforded the lowest low-molecular weight oil fraction
identified in the oils obtained after the depolymerisation of lignin samples
(52 wt%). Table 5 summarises quantification results of
(
[
OS-OL, OS-CL, OS-BWL, KL = 100 mg) at 180 °C for 8 h with 10 wt%
the major mono-aromatic cleavage products (2a,b and
w(base)/w(lignin)] of Cs
2 3
CO
in DMC (5 mL).
3
a,b) that we expected to find based on our model-
a
Capped
OS-OL OS-CL OS-BWL
KL
KL
based studies (Scheme 3). The product mixtures from
oak, cherry, and beechwood lignin contained the capped
syringaldehyde ester (3b) in major quantities while the
capped guaiacyl ester yields remained within the
monomers
wt% of starting material
0.03
0.02
0.05
0.3
0.2
-
expected product ratio (2a,b / 3a,b), corresponding well
to the amount of S and G units in the lignin starting
material (compare Tables 1 and 5). Because the Kraft
lignin from Sigma Aldrich (#370959) almost exclusively
contains G units, the S containing units were only
observed in trace quantities. Additionally, various
capped liner esters were observed in the oil fraction of
the lignin samples (see Electronic Supplementary
Information). Considering that the cost of the reagents
can be a limiting factor for reaction scale up with regards
to biorefinery application, we also performed the BCD
reaction with a widely available Kraft lignin and KOH as
base (Table 5, last column). To our delight, similar
selectivities for the aromatic products were observed
0.2
-
0.1
0.05
0
.6
0.9
4.0
1.1
3.2
1.8
1.5
3.4
-
-
with comparable yields as to the Cs
reactions.
2 3
CO -catalysed
total (2+3)
Reaction with 10 wt% [w(base)/w(lignin)] of KOH
4.23
4.92
4.45
2.15 1.7
a
Depolymerisation of Beechwood Chips
GC-MS Analysis
Based on the GPC results, reactions catalysed by 10 wt%
of Cs CO showed the highest reduction in molecular
weight with a M value in the range of aromatic
After the successful degradation of extracted lignin
samples, we decided to test the direct applicability of
our BCD system on wood. The reaction was performed
using a sample of pre-milled beechwood chips (100 mg)
2
3
w
2 3
with Cs CO (10 wt%) in DMC. Pleasingly an oil fraction
monomers and oligomers (200-1900 Da) for the four
lignin samples (Table 4). The identification and
quantification of these products was thus sought to be
done by GC-MS analysis of the low-molecular weight oil
fractions (52-67%) [w(oil)/w(initial lignin)] obtained after
its separation from the ethyl acetate insoluble residue of
the reaction mixture (for further details see Electronic
Supplementary Information section 2.4). A correlation
was observed in the yields of the low-molecular weight
oil fractions and the general solubility of the lignin
sample in the reaction solvent DMC. Organosolv cherry
lignin had the lowest solubility in DMC and accordingly
of 40 wt% was recovered while affording a deep brown
residue in 60 wt%. The oil fraction was then subject to
GC-MS analysis (Fig. 4) and gratifyingly we could identify
methoxy-capped monoaromatic products with a similar
product distribution as observed for the extracted lignin
samples. A control reaction carried out in the absence of
base did not afford any cleavage products thereby ruling
out any influence of the pre-milling conditions on the
product formation (see Electronic Supplementary
Information Fig. S9).
6
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Fig. 4 GC-FID trace of the product mixture obtained from the depolymerisation of milled beechwood chips (100 mg) using Cs
1
individual products, using n-octadecane as an internal standard.
2
CO
3
(10 wt %) in DMC (5 mL) at
80 °C for 8 h. Identification of the major peaks is based on the GC-MS data (full peak overview in Table S6). Quantification is based on the standardization of
OMe
OH
O
LiOt-Bu (0.05 equiv)
Cs
2
CO
3
(0.05 equiv)
DMC
O
OMe
OMe
MeO
MeO
MeO
O
DMC
OMe
+
OMe
OMe
OMe
OMe
1
2 h, 180 °C
HO
8 h, 180 °C
4
a: 75% (Z), 4b: 9% (E)
1b
2a: 60%
3a: 14%
O
Me
Me
O
OMe
OH
Me
Me
O
O
LiOt-Bu (0.05 equiv)
2 3
Cs CO (0.05 equiv)
O
OMe
OMe
MeO
MeO
DMC
DMC
OMe
+
O
OMe
OMe
12 h, 180 °C
HO
8 h, 180 °C
OMe
OMe
OMe
5
: 80%
+
1c
2a: 88%
3a: 16%
4
a: 15% (Z), 4b: 3% (E)
Me
OMe
OH
LiOt-Bu (0.05 equiv)
2 3
Cs CO (0.05 equiv)
MeO
O
DMC
O
DMC
OMe
OMe
OMe
OMe
12 h, 180 °C
OMe
OMe
8
h, 180 °C
HO
Me
6
a: 40% (Z), 6b: 17% (E)
2a: 30%
1
d
O
O
OMe
O
OMe
OMe
OH
OMe
O
OMe
O
LiOt-Bu (0.05 equiv)
O
2
Cs CO
3
(0.05 equiv)
DMC
O
DMC
OMe
OMe
OMe
OMe
OMe
MeO
: 99%
12 h, 180 °C
MeO
8 h, 180 °C
MeO
OMe
7
1e
7: 99%
OMe
OMe
OMe
OMe
OMe
OMe
MeO
LiOt-Bu (0.05 equiv)
O
Cs CO (0.05 equiv)
O
O
2
3
DMC
DMC
OMe
OMe
OMe
OMe
OMe
OMe
O
12 h, 180 °C
HO
8 h, 180 °C
O
MeO
O
O
8
: 99%
1f
8: 90%
OMe
OMe
LiOt-Bu (0.05 equiv)
Cs CO (0.05 equiv)
2 3
O
DMC
DMC
OMe
OMe
MeO
1
2 h, 180 °C
8 h, 180 °C
1
g
OMe
O
OMe
O
MeO
O
LiOt-Bu (0.05 equiv)
Cs CO (0.05 equiv)
OMe
+
MeO
MeO
2
3
OMe
OMe
DMC
O
O
O
DMC
OMe
OMe
OMe
12 h, 180 °C
8 h, 180 °C
OMe
a: 80% (Z), 4b: 10% (E)
4
2a: 62%
3a: 12%
O
OMe
1
h
Scheme 3 Base-catalysed degradation in lignin β-O-4 model compounds.
Model compound studies
the
α
-proton, γ-proton, or either one or both the
aliphatic hydroxyls substituted by methyl or methoxy
groups (Scheme 3). The idea was to compare the
The obtained results for the BCD on lignin samples and
wood gave rise to the question which reaction pathway
reactivity of these models (1c
model compound 1b. Model compound 1c which has
both of its -protons replaced by methyl groups resulted
in increased veratrole (2a) and dimethoxy benzoate (3a
-h) with the unsubstituted
is involved in the cleavage of the
subsequent capping of products. To answer this
question a range of lignin -O-4 model compounds were
β-O-4 bonds and the
γ
β
)
investigated having some of their pivotal sites such as
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Journal Name
yields for the Cs
2
CO
3
-catalysed reaction. However,
-proton formed a
complex mixture of products with the overall yields for
decreasing by half (30%). This loss in yield suggests a
possible involvement of the -proton in the cleavage
reactions for 1b. On the contrary, the reaction of both 1c
and 1d with LiO -Bu resulted in an overall decrease in
yields for the corresponding alkenes and possibly
due to the formation of an additional cyclic product such
as . Furthermore, substituting either one of the two
highlighting the necessity of both the positions to be
model 1d containing the substituted
α
free to initiate a successful cleavage and elimination
reaction. This was further confirmed by model 1g that
remained unreactive for both base catalysts. Based on
our observations on model compound studies (Scheme
4), the formation of dicarboxylated product 1h starting
from model 1b in DMC was speculated to be a common
2a
α
t
4
6
intermediate for the pathways involving both Cs
2 3
CO
and LiOt-Bu. Indeed, individual reactions of 1h with
5
these bases furnished comparable results as with 1b
aliphatic hydroxyls such as in model 1e and 1f restricted
affording slightly increased yields (Schemes 3 and 4).
the reactions for both bases to stop at the
monocarboxylated products
7
and
8
,
thereby
O
OMe
O
OMe
OMe
O
C
OMe
s
MeO
OMe
O
C
t
e
u
ro
OMe
O
OMe
OMe
O
O
+
O
3a
2a
1
b
OMe
OMe
OMe
OMe
HO
O
O
L
MeO
O
iO
t-
OMe
ro
B
OMe
u
u
te
MC
1h
OMe
4a/4b
Scheme 4 Reaction of the
LiOt-Bu.
2 3
β-O-4 model linkage under BCD conditions showing the reaction sequence and the point of divergence in reactivity for Cs CO and
Fig. 5 Reaction profiles: a) Cs
experimental data points, whereas the line is a modeled reaction. a) Black: dilignol (1b); Red: monocarboxylated dilignol (MC); Green: dicarboxylated dilignol
2 3
CO 180 °C in DMC with (β-O-4) model compound 1b. b) LiOt-Bu 180 °C in DMC with (β-O-4) model compound 1b. Dots show
(1h); Blue: veratrole (2a); Pink: 3,4-dimethoxy benzoate (3a). b) Black: dilignol (1b); Red: monocarboxylated dilignol (MC); Green: dicarboxylated dilignol (Ih);
Brown: cis/trans alkenes (4a/4b).
second carboxylation of the α-hydroxy group resulting in
common intermediate 1h (reaction rates of 1h = 0.15
Reaction Profile Study
−1
−1
To gain further insight into the degradation of the
β-O-4
min for Cs
the formation of 1h, the reaction pathways diverge. The
reaction with Cs CO promotes the cleavage of the C-O
bond resulting in veratrole (2a). The production of 2a
2 3
CO and 0.025 min for LiOt-Bu). Following
linkage under the Cs CO and LiOt-Bu-catalysed
2
3
-
reaction conditions, the consumption of 1b and the
formation of stable intermediates and products was
monitoring by HPLC analysis (Fig. 5, see Electronic
Supplementary Information section 4). From this
analysis, a series of reaction pathways were proposed,
and rate analysis provided the curve fits shown in
corresponding figures.
2
3
,
however, does not correlate directly with the
accruement of 3,4-dimethoxy benzoate (3a). A probable
intermediate
M is speculated to be formed which
undergoes a subsequent reacion to 3a. The detailed
reaction pathway leading to
the computational analysis.
M is discussed later during
The initial reactions were found to be the same for both
Cs
2 3
CO and LiOt-Bu. Carboxylation of the γ-hydroxy
Upon the formation of 1h in the reaction with LiOt-Bu,
the alkenes 4a and 4b are formed without any
−
1
group occurred first (reaction rates of MC = 0.69 min
−1
2 3
for Cs CO and 0.2 min for LiOt-Bu) followed by a
8
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ARTICLE
observable intermediates (rate of formation of 4a
/
4b
=
inhibition for the formation of veratrole by o-methoxy
10p
−1
0.005 min ).
groups was also observed in our previous study. The
overall change of the selectivity thus indicates that the
Computational Study
steric hindrance at the
the deprotonation.
α-carbon might be preventing
Based on the studies above, quantum chemical
calculations were considered necessary to find a
reasonable mechanism rationalising the observed
selectivities for the obtained cleavage products. Here
again dilignol 1b was chosen as representative substrate
1
5
Based on a recent computational study by Yu, we also
investigated a possible elimination of guaiacolate from
the monocarboxylated structure M9 resulting in an
oxirane intermediate (see Scheme 6). However, the
Gibbs energy of the transition state TS-9 of this reaction
and cesium methanolate (CsOMe) as
a model base
−1
−1
considering the depletion of Cs CO (0.05 equiv) over the
2
3
is 69.4 kcal mol relative to 1b and 32.2 kcal mol
relative to TS-1 Therefore, Yu's mechanism is highly
course of the reaction resulting in methanolate as by-
product.
.
disfavoured in our reaction system.
Thus, after an initiation period mainly methanolates acted as
a base in the reaction. The investigated reaction pathways
were limited to intermediates and transition states without
explicit coordination of the cesium ions. The main reason for
this approximation being the dramatically increased number
of different structures and possible oligomers that would
have to be taken into consideration for each step. Secondly,
the PCM solvation model would be inaccurate for the
description of the solvated species in this system preventing
O
TS-9 [69.4]
O
Ar'O
Ar
Ar
OMe
O
O
O
O
OMe
O
OMe
M9
M10
11
Scheme 6 Elimination of 11 According to Yu et al.
1
5
; Ar: 3,4-
dimethoxyphenyl; Ar’: 2-methoxyphenyl; [Relative Gibbs Energies to 1b
–
in kcal mol ].
1
a
reliable comparison of structures with different
The final step for the formation of 2a is the methylation
of 11 by dimethyl carbonate. The relative Gibbs energy
of the transition state of the methylation under Walden
coordination of the cesium cation. Also, based on Pearson’s
HSAB principle, it can be expected that the uncoordinated
intermediates and transition states provide
a good
−1
inversion at the methyl group is 24.5 kcal mol relative
to 11
correlation for the reactions with a cesium base owing to its
weak coordination to the deprotonated oxygen atoms of the
.
1
4
The formation of 3a is a result of the decomposition of
M2. The addition of a methanolate to the methoxy
carbonate group is endergonic. The resulting
substrate.
The model compound studies (Scheme 3) suggest the
involvement of both the - and -protons in the
formation of veratrole (2a) and 3,4-dimethoxy benzoate
3a). The decreased yield of 2a for model compound 1d
specifically indicates that these reaction products are
formed after a deprotonation at the -position. This
α
γ
–1
intermediate M3 has a Gibbs energy of 19.6 kcal mol
relative to M2. The transition state of this step has an
(
−1
energy of 33.8 kcal mol compared to M2. Elimination
of dimethyl carbonate from M3 results in a simultaneous
α
elimination of monomethoxy carbonate
(MMC,
deprotonation reaction, however, is most likely
occurring on the dicarboxylated dilignol 1h since, in
contrast to 1b, it has no free hydroxy groups, which are
more acidic than the aliphatic hydrogen atoms.
Moreover, the experimental results already indicate 1h
as an intermediate in the formation of 2a and 3a (see
decomposing into MeOH and CO ). The transition state
2
−
1
TS-4 of this step is only 2.0 kcal mol higher in energy
than M3. Moreover, the reaction is highly exergonic
since the Gibbs energy of the vinyl ketone M4 is
−
1
7
5
4.6 kcal mol lower than M3 and the energy is
−
5.0 kcal mol lower than M2. The vinyl ketone M4 can
1
Scheme 3; reaction of 1h with Cs
2 3
CO and Fig. 5a).
react with methanolate to give the enolate M5. The
The deprotonation at the α-carbon of 1h results in
intermediate M1 (Scheme 5). This intermediate is
addition reaction is endergonic with an energy
−
difference of 13.9 kcal mol and the corresponding
1
favoured with a Gibbs energy difference of 15.2 kcal
−
1
−1
transition state TS-5 having an energy of 37.2 kcal mol .
Addition of dimethyl carbonate to the carbon of M5 in
mol over M1b which is deprotonated at the γ-carbon.
The energy of M1 is 36.9 kcal mol relative to 1b. The
−1
α-
a Claisen condensation-type reaction (TS-6) results in
intermediate M6. Instead of an elimination of a
methanolate, a transfer of a methoxy group of the
dimethyl carbonate group to the carbonyl group is likely
to occur. The resulting intermediate M7 resembles the
intermediate of a Claisen condensation between 3,4-
dimethoxy benzoate and the enolate M8. Hence, a
retro-Claisen condensation-type step (TS-8) yields the
experimentally observed product 3a. The formation of
transition state of the deprotonation (TS-1) has a Gibbs
−1
energy of 37.2 kcal mol relative to 1b. The elimination
of guaiacolate (11) from M1 proceeds without a
significant barrier and gives enol ether M2. The
formation of veratrole from 1d (Scheme 3) can be
attributed to
However, the lower acidity of the hydrogen at the
position compared to the one in -position explains the
low yield for veratrole (2a). Furthermore, the absence of
in the product spectrum of 1d can be explained by
a deprotonation at the γ-position.
γ
-
α
3a and M8 is highly exergonic with an energy difference
3a
the formation of a different intermediate after the
elimination process. Additionally, a similar product
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Scheme 5 Reaction pathway from dicarboxylated Dilignol 1h to 3,4-dimethoxy benzoate (3a); Ar: 3,4-dimethoxyphenyl; Ar’: 2-methoxyphenyl; MMC
:
−
1
]
monomethoxy carbonate; [Gibbs Energies Relative to 1b in kcal mol
low-molecular weight oils. For all lignin sample, the low-
molecular weight oil and the ethyl acetate insoluble
fractions were quantified and characterised using HSQC
−
1
−1
of 92.8 kcal mol relative to 4a and 17.7 kcal mol
relative to M4 TS-5 is the transition state with the
highest Gibbs energy (35.1 kcal mol relative to M4
.
−
1
31
)
2D-NMR, P NMR, GPC, and GC-MS measurements. The
between M4 and 3a. To support the proposed reaction
pathway, we investigated a crucial intermediate: the
vinyl ketone M4 (Scheme 7). M4 was synthesised (see
Electronic Supplementary Information 5) and used as a
starting material in a reaction under the same conditions
as shown for 1b in Scheme 3.
2 3
oil fractions for the Cs CO -catalysed lignin degradation
were found to be within the range of 52-67 wt% of the
starting lignin material and contained a range of
methoxy capped aromatic monomers. Furthermore, it
was shown that for potential future biorefinery
applications the base can be changed to readily available
KOH affording similar monomer yields. The methodology
with the caesium salt was also effective for the direct
degradation of milled beechwood chips thereby further
expanding the scope on a more robust lignocellulosic
starting material. These results are well in agreement
with our model compound studies and hence prove the
system’s efficiency and applicability to the complex
lignin polymer. In our quest to understand the different
reaction pathway involved in the degradation using LiOt-
O
O
OMe
OMe
OMe
OMe
DMC
MeO
Cs CO (0.05 equiv)
2
3
1
80 °C, 8 h
M4
3a
4
0%
Scheme 7 Reaction of M4 with Cs
2
CO
3
in DMC.
The formation of 3a in 40% yield when employing M4 as
starting material confirms that there is a reaction
pathway between M4 and 3a. This is also in accordance
Bu and Cs CO in dimethyl carbonate, we further studied
2
3
a variety of lignin model compounds (1a-1h), which
with the proposed formation of an intermediate
M while
demonstrated the importance of the free aliphatic
studying the reaction profile for the decomposition of 1b
with caesium carbonate to 3a (Electronic Supplementary
hydroxy groups and the proton at α-carbon of the lignin
-O-4 linkage in the depolymerisation reaction.
β
Information section 4.3;
M = M4).
Monitoring the reaction of 1b for both bases helped to
elucidate the overall reaction pathway and the point of
divergence for the bases that leads to the different
product selectivities. This was further confirmed by the
quantum chemical calculations performed for the
Cs CO -catalysed cleavage reaction on model compound
Conclusion
We have developed a base-catalysed depolymerisation
strategy for various lignin samples including organosolv
and Kraft lignin in dimethyl carbonate using alkaline
earth metal catalysts. For the first time, a truly catalytic
base-catalysed depolymerisation process (catalyst
loadings of around 5 mol%) is demonstrated affording
2
3
1b. Ultimately, we could provide a rational reaction
pathway for our BCD system in dimethyl carbonate
1
0 | J. Name., 2012, 00, 1-3
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55, 8164
−
8215; (d) U. Weißbach, S. Dabral, L. Konnert,
based upon our extensive studies on both model
1
compound and real lignin samples.
7
C. Bolm and J. G. Hernández, Beilstein J. Org. Chem.,
017, 13, 1788–1795; (e) S. Gillet, M. Aguedo, L.
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200–4233.
2
,
Conflicts of interest
4
5
Selected oxidative degradation approach via benzylic
alcohol oxidation: (a) A. Wu, B. O. Patrick, E. Chung
and B. R. James, Dalton Trans., 2012, 41, 11093–
There are no conflicts to declare.
1
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Acknowledgements
(
This research was supported by the European
Commission (SuBiCat Initial Training Network, Call FP7-
PEOPLE-2013-ITN, grant no. 607044). S. D., S. S. M. S.,
and C. W. L. thank the international training network
SubiCat for their predoctoral stipends. J. M. is grateful to
the NRW Graduate School BrenaRo and the German
Academic Exchange Service (DAAD) for predoctoral
(
,
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E.
thanks
the
Deutsche
Forschungsgemeinschaft through the International
Research Training Group Seleca (IGRK 1628) and the IT
Center of RWTH Aachen University for providing the
computational resources. We thank Dr. N. Anders
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5
5
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2 | J. Name., 2012, 00, 1-3
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PleaseGdr oe en no Ct ha de jmu si st t mr yargins
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DOI: 10.1039/C7GC03110F
TOC
Text:
Various analytical techniques and DFT calculations have been
applied in studying base-catalysed lignin degradations in
dimethyl carbonate.
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Green Chemistry
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DOI: 10.1039/C7GC03110F
Various analytical techniques and DFT calculations
have been applied in studying base-catalysed
lignin degradations in dimethyl carbonate.