Protection Group Effects During α,γ-Diol Lignin Stabilization Promote High-Selectivity Monomer Production

Article abstract of DOI:10.1002/anie.201710838

Protection groups were introduced during biomass pretreatment to stabilize lignin's α,γ-diol group during its extraction and prevent its condensation. Acetaldehyde and propionaldehyde stabilized the α,γ-diol without any aromatic ring alkylation, which significantly increased final product selectivity. The subsequent hydrogenolysis catalyzed by Pd/C generated lignin monomers at near-theoretical yields based on Klason lignin (48 % from birch, 20 % from spruce, 70 % from high-syringyl transgenic poplar), and with high selectivity to a single 4-n-propanolsyringol product (80 %) in the case of the poplar. Unlike direct hydrogenation of native wood, hydrogenolysis of protected lignin with Ni/C also led to high selectivity to this single product (78 %), paving the way to high-selectivity lignin upgrading with base metal catalysts. The use of extracted lignin facilitated valorization of polysaccharides, leading to high yields of all three major biomass polymers to a single major product.

Full text of DOI:10.1002/anie.201710838

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Accepted Article
Title: Protection Group Effects during α,γ-Diol Lignin Stabilization
Promote High-Selectivity Monomer Production
Authors: Wu Lan, Masoud Talebi Amiri, Christopher M. Hunston, and
Jeremy Luterbacher
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10.1002/anie.201710838
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Protection Group Effects during α,γ-Diol Lignin Stabilization Promote
High-Selectivity Monomer Production
Wu Lan, Masoud Talebi Amiri, Christopher M. Hunston, Jeremy S. Luterbacher*
Abstract: Protection groups were introduced during biomass pretreatment to
stabilize lignin’s α,γ-diol group during its extraction and prevent its
condensation. Acetaldehyde and propionaldehyde stabilized the α,γ-diol
without any aromatic ring alkylation, which significantly increased final
product selectivity. The subsequent hydrogenolysis catalyzed by Pd/C,
generated lignin monomers at near-theoretical yields based on Klason lignin
(48% from birch, 20% from spruce, 70% from high-syringyl transgenic
poplar) with – in the case of the poplar – high selectivity to a single 4-n-
We recently proposed a method enabling efficient lignin extraction
and depolymerization that was compatible with the traditional upgrading
of biomass polysaccharides[8]. Formaldehyde added during lignin
extraction reacted with the α,γ-diol on the side chain of lignin by forming
a 1,3-dioxane structure. This protection group completely suppressed
undesirable repolymerization, providing high monomer yield (47-78%)
in the following hydrogenolysis with a precious metal catalyst. However,
alkylation of aromatic rings by formaldehyde and unselective
propanolsyringol product (80%). Unlike direct hydrogenation of native wood, hydrogenolysis complicated the product mixture leading to the
hydrogenolysis of protected lignin with Ni/C also led to high selectivity to
this single product (78%), paving the way to high-selectivity lignin
upgrading with base metal catalysts. The use of extracted lignin facilitated
valorization of biomass’s polysaccharides, leading to high yields of all three
major biomass polymers to a single product.
production of 6-10 major monomers. Here, we report for the first time
that by modifying the protection group on α,γ-diol protected lignin, high
yields and selectivities can be achieved to only two products (4-n-
propanolguaiacol (POHG) and 4-n-propanolsyringol (POHS)).
Furthermore, when a transgenic high-syringyl poplar is used, only a
single major product (POHS) can be obtained.
We investigated the effect of adding different diol protection
reagents during the lignin extraction process on the final lignin monomer
yield after hydrogenolysis (Figure 1 and Table S1). As a control, we
performed the same extraction without a protection reagent in triplicate
and the average yield was 14 wt% with a 95% confidence interval
between 13-15 wt%. Therefore, any experiment resulting in a yield value
outside of this range was very likely due to the effect of protection
reagents. The first reagents that we tried were aldehydes that form cyclic
acetal with the α,γ-diol of the lignin side chain. All aldehydes except for
hydroxymethylfurfural led to a statistically significant improvement in
monomer yield. In the case of acetaldehyde and propionaldehyde, the
monomer yields were 37 and 42 wt%, respectively, which were
comparable to that obtained with formaldehyde (46 wt%). Interestingly,
no alkylated aromatic monomers were detected, resulting in higher
selectivity to ethylsyringol and ethylguaiacol. Ketal groups, especially
acetonide, are some of the most common diol protection groups in
carbohydrate chemistry. Therefore, acetone, butanone and other related
reagents that protect 1,2 and 1,3-diol were also tested. In all of these
experiments, some protective effect was detected during lignin
extraction, resulting in moderate but statistically significant monomer
yield improvements after hydrogenolysis. We also tested carbonate and
boronate protection groups that were known to react with alcohol groups
under alkaline conditions. The use of dimethyl carbonate and
phenylboronic acid as protection reagents resulted in monomer yields of
16 and 27 wt%, respectively (above the yield of the control experiment,
see Section 2.2 in the SI). This study demonstrated that while all diol
protection groups were effective in stabilizing lignin to some degree,
aldehydes were the most effective. Both acetaldehyde and
propionaldehyde showed especially remarkable activity in protecting the
α,γ-diol position and preventing condensation during lignin extraction
and pretreatment conditions.
Due to limited fossil reserves and increasing environmental concerns
over petroleum use, converting renewable resources such as
lignocellulosic biomass into chemicals, materials, and fuels has attracted
substantial attention in recent years[1]. Lignin, a heteropolymer of
phenylpropanoid units, is one of the three major components in
lignocellulosic biomass and is the most abundant natural source of
aromatic molecules[2]. For this reason, lignin is often presented as a
future feedstock for aromatic chemicals[3]. However, few commercial
processes or biorefinery schemes are being developed for lignin
compared to other bio-polymers like cellulose or hemicellulose. These
polysaccharides can be upgraded at high yields (~70-100%) to simple
sugars such as glucose and xylose using relatively simple processes[4]
.
In contrast, there are limited practical processes that can convert lignin
at high yields and high selectivity to a narrow set of products, while
remaining compatible with sugar production.
This production of sugars often involves biorefinery fractionation
processes that use organic solvents[5] or water[6] at high temperature with
mineral acids to effectively remove lignin (and hemicellulose). But the
resulting
lignin
systematically
suffers
from
irreversible
repolymerization that limits its ability to be upgraded at high yields.
Several studies have developed so-called “lignin first” extraction
methods, where native wood is treated in the presence of
a
heterogeneous metal catalyst that allows for the rapid hydrogenation of
lignin intermediates before they condense[7]. However, these methods
introduce catalyst separation issues from unreacted wood and usually
result in at least partial conversion of the hemicellulose fraction of
biomass, which limits their integration into current biorefinery schemes.
[*]
Dr. W. Lan, C. M. Hunston, M. T. Amiri, Prof. J. S. Luterbacher
Laboratory of Sustainable and Catalytic Processing, Institute of
Chemical Sciences and Engineering. École polytechnique fédérale
de Lausanne (EPFL). CH-1015 Lausanne (Switzerland)
E-mail: jeremy.luterbacher@epfl.ch
Supporting information for this article is given via a link at the end of
the document
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Figure 1. Hydrogenolysis of birch lignin extracted with the addition of different protection reagents in dioxane. Extraction condition, 80 °C, 5
hours; Hydrogenolysis condition, Ru/C, 250 °C, 40 bar of H2, 15 hours. Monomer yields are given based on klason lignin content (see Table S1 for
details)
To better understand the reaction mechanism, we performed
reactions using the common lignin dimeric model compound
veratrylglycerol-β-guaiacyl ether (VG) and acet- or propionaldehyde.
The yields of ethylidene acetal and propylidene acetal protected VG
were 90% and 86%, respectively, after a 30-min reaction, slightly lower
than that of methylene acetal protected VG (92%). The formation of the
acetal protected structures were confirmed by NMR (Figure 2, A and B).
According to the HSQC spectra, the major compound of the mixture was
the acetal protected VG with trace amounts of unreacted VG. A detailed
inspection of the HSQC and COSY spectra revealed no peaks
originating from –CH2OH/–or CH2CH2OH in their corresponding region
(green dash line), indicating that no hydroxyethylation or
hydroxypropylation occurred during the reaction. After a subsequent
hydrogenolysis reaction, no products with additional ethyl/propyl side
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Figure 2. HSQC spectra of the product mixture resulting from the reaction between veratrylglycerol-β-guaiacyl ether and (A) acetaldehyde, (B)
propionaldehyde, or isolated birch lignin extracted in the presence of (C) acetaldehyde or (D) propionaldehyde. The contour areas were integrated
to calculate the S:G ratio and interunit linkage distributions. S:G equals (S2/6)/2:(G2); AAα, Aα, plus Cα equals 100%. The green dashed line
indicates the region of hydroxypropyl and hydroxyethyl correlations.
Figure 3. Reaction mechanism for the hydrogenolysis of form-/acet-/propionaldehyde stabilized lignin. When using formaldehyde during lignin
extraction, alkylation on the aromatic rings occurred and the corresponding products were generated. When using Ru/C as the catalyst,
propanolphenolic monomers were largely converted to ethylphenolic monomers and demethoxylation of syringyl units occurred; when using Pd/C
as the catalyst, these reactions were mostly suppressed and the major products were propanolphenolic monomers.
chains on the aromatic ring were detected (Figure S3), confirming the
results obtained with native lignin in birch (Figure 1). Lignin extracted
from birch in the presence of both acet- and propionaldehyde were also
prepared for NMR characterization (Figure 2 C and D) and confirmed
the structure observed with the model compound. Contours
corresponding to each structure were integrated to calculate the S/G ratio
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and the percentage of different inter-linkages. Results showed that more
than 90% of the β-O-4 structure was protected by an ethylidene or
propylidene acetal group, which was consistent with the model
compound study.
and/or time of hydrogenolysis to suppress this demethoxylation and
CH2OH cleavage (Figure 4). When decreasing the reaction time from 15
h to 5 h, a 73% monomer yield was obtained. Additionally, the POHS
and POHG products were favored at a lower temperature: 60%
selectivity to POHS was reached at 200 °C for 15 h over Ru/C and the
fraction of the syringyl products increased to 88%.
To further increase the selectivity to POHS/POHG, Pd/C was used
as the hydrogenolysis catalyst due to its low alcohol hydrogenation
activity[10]. By running hydrogenolysis at 200 °C for 20 h, a yield of 70%
monomers was obtained with 80% selectivity to a single POHS product
(89% to POHS+POHG) (Figure 4), which is a potential precursor of
adhesives and polymers. HSQC spectra of the hydrogenolysis product
mixtures further confirmed the monomers structure, and demonstrated
full conversion of β-O-4 linkages of the lignin polymer (Figure S5). This
result is in agreement with previous work by Sels and colleagues, which
also reported a high selectivity to POHG and POHS when treating birch
sawdust mixed with Pd/C in MeOH to directly upgrade native lignin[7a]
.
Here, we have achieved similar results with extracted lignin, as well as
significantly higher selectivities to a single product (POHS) when using
F5H-poplar. Ni has often been proposed as a non-noble metal
hydrogenolysis catalyst for lignin, producing mainly propylphenolic
monomers during direct hydrogenolysis of untreated wood. When F5H-
poplar was used during direct hydrogenolysis of untreated wood with
Ni/C in MeOH, a relatively low product selectivity was obtained (79%
monomer yield with a 59% selectivity to PS and only 29% to POHS).
Interestingly, when propionaldehyde-protected extracted lignin was
used, a selectivity of 78% to POHS was achieved with an only slightly
diminished yield of total monomers (72%) (Figure 4). The acetal
protection group is likely causing
hydrogenation over Ni, but further study of the mechanism is still
underway.
a suppression of alcohol
Figure 4. Monomer yields (on the basis of klason lignin) and product
distribution resulting from the hydrogenolysis of propylidene acetal
protected lignin extracted from F5H poplar (see Table S2 for details).
*Direct hydrogenolysis. The molecules corresponding to each color are
given in Figure 1.
The absence of aromatic ring alkylation when acet- or
propionaldehyde were used as protection reagents instead of
formaldehyde reduced the number of products by half, allowing for high
selectivity lignin upgrading (Figure 3). We (and others) had previously
shown that hybrid poplar in which the ferulate 5-hydroxylase (F5H)
gene was overexpressed contained a much higher proportion of syringyl
units in the lignin (more than 95%)[9], resulting in higher content of
cleavable β-O-4 ethers[8]. We therefore used F5H poplar as a feedstock
to get higher monomer yields and a higher product selectivity to syringyl
monomers. The results of hydrogenolysis of lignin extracted with the
addition of propionaldehyde are shown in Figure 4 (and Table S2).
Using the same extraction conditions, a 54% monomer yield was
obtained with propionaldehyde compared to 75% for formaldehyde.
With formaldehyde, long extraction time benefited lignin extraction.
However, considering that propylidene acetal is less stable than
methylene acetal, a longer extraction time could favor the partial
cleavage of the protection groups and lead to some lignin
repolymerization. Therefore, we reduced the lignin extraction time to 2.5
h, which increased the monomer yield to 65 wt%. Under these conditions,
about 20% of guaiacyl monomers were obtained, which is higher than
what would be expected based on the native lignin (~5%). This
discrepancy was likely due to the demethoxylation of syringyl units
during hydrogenolysis over Ru/C. We also assumed that ethylphenols
were mainly produced from propanolphenols after cleaving of the –
CH2OH group over Ru/C (Figure 3). Model compound studies using
POHS and propylsyringol (PS) as reactants for hydrogenolysis
demonstrated the cleavage of –CH2OH and demethoxylation under the
same reaction conditions (Figure S4). We lowered the temperature
Figure 5. Monomer yields (on the basis of klason lignin) and product
resulting from the hydrogenolysis of lignin extracted from F5H poplar,
birch, and spruce with addition of acetaldehyde (AA) and
propionaldehyde (PA). The extraction conditions were 80 °C, 2.5 hours
for F5H poplar and birch, and 3.5 hours for spruce. The hydrogenolysis
was performed at 200 °C, with 40 bar of H2 for 15 hours with Pd/C (see
Table S2 for details). The molecules corresponding to each color are
given in Figure 1.
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We also performed lignin extraction with acet- and propionaldehyde
as well as hydrogenolysis with Pd/C with birch and spruce (Figure 5 and
S6). In all cases, more than 91% selectivity to POHS and POHG
monomers was achieved. Total monomer yields were those that are
typically expected for both wild type species based on their fraction of
inter-unit ether linkages: 48% and 20% for birch and spruce, respectively
(extraction yields were within 10% of the yield from direct
hydrogenolysis of untreated wood[11]). The relatively low yield from
spruce is consistent with the low β-aryl ether content in softwood lignin,
as reported elsewhere[11a, 11b]. Nevertheless, due to softwoods having
only guaiacyl linkages, the selectivity to the single POHG product was
91%.
Keywords: biomass • depolymerization • heterogeneous catalysis •
hydrogenolysis • lignin
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Besides lignin depolymerization, recovery of carbohydrates was
also evaluated. Unlike direct hydrogenolysis of native wood, which
causes irreversible conversion of some of the carbohydrates, this method
generated clean cellulose or acetal-protected carbohydrates (mostly
xylose). The latter could be easily deprotected by aqueous acidic
treatment (77% of xylose was obtained after 10 min at 120 °C in 6 wt%
H2SO4, Figure 6 and S7 A). The leftover solids after pretreatment could
undergo a typical enzymatic hydrolysis process without any further acid
treatment, generating an 80% yield of glucose (Figure S7 B). The high
enzymatic hydrolysis yield without any further acid treatment was a
significant improvement over our previous work with formaldehyde[8],
where the resulting cellulose required a harsh acid treatment to remove
formyl groups that caused significantly reduced digestibilities (~10%).
We suspect that any binding of propionaldehyde to cellulose was much
more easily reversible than that of formaldehyde, and was likely
removed even under the mild conditions (50°C, pH 5).
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Figure 6. Mass balance of polysaccharides (glucan and xylan) and
lignin fractions of biomass (Table S3, entry 14 and Figure S7).
In summary, we developed a high yield lignin extraction and
depolymerization process that is compatible with most biorefinery
schemes and can, depending on the biomass source, produce a single
product at high selectivity. To our knowledge, we are reporting the
highest selectivities obtained to a single product from hardwood lignin
depolymerization, and the instance of a non-precious metal being used
for high-selectivity lignin hydrogenolysis with molecular hydrogen.
Because lignin is extracted under typical pretreatment conditions, both
xylose and glucose can be produced at high yields as well, leading to
mostly single products from all three major biopolymers.
Acknowledgements
This work was supported by the Swiss Competence Center for Energy Research:
Biomass for a Swiss Energy Future through the Swiss Commission for Technology and
Innovation grant KTI.2014.0116. We thank J. Ralph, C. Chapple, and R. Meilan for
providing F5H transgenic poplar and M. Studer for providing spruce and birch wood.
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Entry for the Table of Contents
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Wu Lan, Masoud Talebi Amiri, Christopher
M. Hunston, Jeremy S. Luterbacher*
Page No. – Page No.
Protection Group Effects during α,γ-Diol
Lignin Stabilization Promote High-
Selectivity Monomer Production
Lignin extraction in the presence of acetaldehyde or propionaldehyde prevents lignin
condensation and facilitates the subsequent hydrogenation, generating 4-n-propanolphenolic
monomers at >90% selectivity with high monomer yields (>70%). Because lignin is
extracted under typical pretreatment conditions, both xylose and glucose can be produced at
high yields after acid hydrolysis of the liquor and enzymatic hydrolysis of the remaining
solids.
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