Unravelling Some of the Key Transformations in the Hydrothermal Liquefaction of Lignin

Article abstract of DOI:10.1002/cssc.201700528

Using both experimental and computational methods, focusing on intermediates and model compounds, some of the main features of the reaction mechanisms that operate during the hydrothermal processing of lignin were elucidated. Key reaction pathways and their connection to different structural features of lignin were proposed. Under neutral conditions, subcritical water was demonstrated to act as a bifunctional acid/base catalyst for the dissection of lignin structures. In a complex web of mutually dependent interactions, guaiacyl units within lignin were shown to significantly affect overall lignin reactivity.

Full text of DOI:10.1002/cssc.201700528

Accepted Article
Title: Unravelling Some of the Key Transformations in the
Hydrothermal Liquefaction of Lignin
Authors: Matthew Yuk-Yu Lui, Bun Chan, Alexander Yuen, Anthony
Masters, Alejandro Montoya, and Thomas Maschmeyer
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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Link to VoR: http://dx.doi.org/10.1002/cssc.201700528
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Unravelling Some of the Key Transformations in the
Hydrothermal Liquefaction of Lignin
[
a]
[b]
[a]
[a]
[c]
Matthew Y. Lui, Bun Chan, Alexander K. L. Yuen, Anthony F. Masters, Alejandro Montoya and
[
a]
Thomas Maschmeyer*
Abstract: Using both experimental and computational methods,
focusing on intermediates and model compounds, we elucidate
some of the main features of the reaction mechanisms that operate
during the hydrothermal processing of lignin. We propose key
reaction pathways and their connection to different structural
features of lignin. Under neutral conditions, subcritical water was
demonstrated to act as a bifunctional acid/base catalyst for the
dissection of lignin structures. In a complex web of mutually
dependent interactions, guaiacyl units within lignin were shown to
significantly affect overall lignin reactivity.
conditions when linkages such as β-O-4 and α-O-4 are broken
down through free radical chemistry and react further in
[10]
pericyclic reactions.
The hydrothermal conversion of benzyl phenyl ether, a
simple model compound reminiscent of α-O-4 linkages, has
[
11]
[12]
been studied by Siskin/Katritzky
and Lercher
. They
demonstrated that the ether hydrolyzed to benzyl alcohol and
phenol. Furthermore, it was proposed that the stable benzyl
cation intermediate, arising from benzyl alcohol, abstracted a
[
11]
hydride to give toluene or alkylated the phenol fragment.
In
another study, Barbier and co-workers examined the
hydrothermal conversion of 4-(benzyloxy)phenol as a model for
Hydrothermal liquefaction (HTL) is becoming an important
[13]
the α-O-4 linkage at 370–390 °C.
The conversion of
[
1]
process to convert biomass into fuel and chemicals. This
process involves the heating of biomass feedstock in water at
moderate to high temperature (250–500 °C) and pressure (5–25
4-(benzyloxy)phenol was similar to that of benzyl phenyl ether:
The ether compound was hydrolyzed to generate hydroquinone
and benzyl alcohol, hydroquinone then underwent ring
benzylation.
[
2]
MPa).
A base catalyst is often added to improve the
performance of the conversion process. HTL is superior to many
other biomass conversion processes in terms of the relatively
low oxygen content of the resulting liquids. Thus, e.g. bio-oils,
produced from the HTL process are of significantly higher
energy density than pyrolysis oils. It is particularly attractive for
the conversion of wet biomass since, unlike gasification or
pyrolysis treatments, pre-drying of feedstock is unnecessary.
Vanillin (benzaldehyde) structural units (Figure 1) have
been found in lignin samples as end-groups and vanillin is
[14]
[15]
recovered in large amounts following the basic extraction and
.[16,17]
hydrothermal conversion of lignin.
Vanillin is therefore
proposed to be a key intermediate. Besides vanillin units,
coniferaldehyde (cinnamaldehyde) units (Figure 1) are also
[15]
commonly found as end-groups in lignin.
A NMR study by
Besides the liquid bio-oils, gaseous products (mostly CO
2
and
Capanema et al. showed that spruce milled wood lignin contains
units/100 Ar of coniferaldehyde and 5 units/100 Ar of vanillin
minor amounts of, e.g. CH and H ) are also formed as a result
4
2
4
[
3]
of HTL.
[18]
subunits respectively.
Lignin constitutes 25–35% of the organic matrix of
lignocellulosic biomass, adding strength and rigidity to cell
[4]
walls. The complex structure of lignin contains aromatic units
connected by linkages such as β-O-4, α-O-4, 4-O-5, 5-5, β-5
and β-β cross-links. Of these, the β-O-4 linkage predominates,
Figure 1. Coniferaldehyde (left) and vanillin (right) end-groups in lignin.
[5]
accounting for over 50% of the cross-links. Functional groups
[
6]
[7]
To understand the conversion of coniferaldehyde units
under hydrothermal conditions, cinnamaldehyde 1 and 3,4-
dimethoxycinnamaldehyde 2 (Scheme 1), which act as model
compounds of coniferaldehyde units, were heated to 350 ºC with
such as benzylic alcohols as well as methoxy-groups are also
present on the aryl rings. Additionally, lignin is the most readily
available source of renewable aromatics and is a major by-
[
8]
product of the paper and pulp industries.
The chemistry of lignin under hydrothermal conditions is
2
water or aqueous NaOH (0.1 M) solution under N for 10 min, in
[
2,9]
a 316 stainless steel reactor using rapid heat transfer via a
fluidized sand bath followed by a fast quench. The 3,4-
dimethoxy groups of 2 represent the guaiacyl subunits found in
coniferaldehyde. Under neutral conditions, benzaldehyde (70%)
was the major product obtained from the conversion of 1. The
comparatively less well understood
than that under pyrolysis
[
a]
Dr. M. Y. Lui, Dr. A. K. L. Yuen, Prof. A. F. Masters, Prof. T.
Maschmeyer
Laboratory of Advanced Catalysis for Sustainability,
School of Chemistry, The University of Sydney, 2006, Australia
E-mail: thomas.maschmeyer@sydney.edu.au
Dr. B. Chan
Graduate School of Engineering
Nagasaki University
Bunkyo 1-14, Nagasaki 852-8521, Japan
Dr. A. Montoya
School of Chemical and Biomolecular Engineering,
The University of Sydney, 2006, Australia
conversion of 2 was quantitative, giving 90% yield of
veratraldehyde. Cinnamaldehyde is known to hydrolyze to
benzaldehyde and acetaldehyde in the presence of base
[
[
b]
c]
[
19]
[20]
catalysts such as NaOH and Al-Mg hydrotalcite, and it has
been suggested that these products are formed through retro-
aldol condensation. DFT calculations at the B-PW91/6-31+G(d)
level show that the hydrations of 1 and 2 are both associated
–
1
with reasonable barriers of ca 120 kJ mol , indicating viable
conversion to alcohols 3 and 4 (Scheme 1). Under subcritical
Supporting information for this article is given via a link at the end of
the document.
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conditions, the self-ionization of water is increased, leading to
is followed by dehydrogenation of the carboxylate with a barrier
−
+
–1
greater concentrations of OH and H , both of which are likely to
be involved in the catalysis of the hydrolysis process. In our
of 122 kJ mol . The water-assisted decarboxylation is
–
1
associated with a barrier of 173 kJ mol , and this completes the
overall reaction. Thus, hydroxide does appear to provide a
significantly lower-energy pathway for the generation of veratrole
−
proposed mechanism, Michael addition of OH to 1 and 2, with
+
carbonyl group activation by H , followed by keto–enol
–
1
tautomerization gives the hydration products 3 and 4. These
species then undergo retro-aldol reactions generating the
benzaldehyde derivatives (5 and 6) and acetaldehyde.
with overall reaction barriers of 407 kJ mol for the
–
1
uncatalyzed process and 108 kJ mol for the catalytic one.
Scheme 1. Conversion of coniferaldehyde units under neutral hydrothermal
conditions.
Benzaldehyde is found to be the major product from 1 in
both neutral and basic media (from hydration and subsequent
retro-aldol reaction). Under basic conditions, benzyl alcohol and
benzoic acid were also observed in addition to the major product.
Previously, Strauss and co-workers reported that benzaldehyde
decomposes in 0.1 M NaOH at 290 ºC in 60 min, undergoing a
Scheme 2. Conversion of coniferaldehyde and vanillin units under basic
hydrothermal conditions.
[
22]
[
21]
Sawama and Sajiki
demonstrated previously that
Cannizzaro reaction to give benzoic acid and benzyl alcohol.
aliphatic and aromatic aldehydes can be converted to the
corresponding carboxylic acid in the presence of NaOH and
noble metal catalysts. In particular, benzyl alcohols were
converted to benzoic acids with Pd/C and Rh/C catalysts at low
temperatures (80–100 ºC). It was proposed that the hydroxide
ion of NaOH facilitated the formation of a geminal diol. The
When 2 was converted under similarly basic conditions, the
major products were vanillin, guaiacol and catechol. Vanillin was
likely to be formed by methyl group hydrolysis of veratraldehyde
6, which itself was produced by the retro-aldol process,
described above, being detected in trace amounts by GC-MS.
To better understand how catechol and guaiacol were
formed, the hydrothermal conversions of 3,4-disubstituted
benzoic acids were then generated as
a result of the
dehydrogenation of geminal diols. Our calculations support such
a proposal, i.e. that veratrole is generated from the hydration of
veratraldehyde, giving veratric acid, which thermally
decarboxylates. It is noteworthy that benzoic acid, which lacks
any alkoxy groups, does not readily decarboxylate under these
conditions. In addition, the proposed mechanism is also
consistent with our independent results showing thermal
decarboxylation of veratric acid at 350 ºC to veratrole, which
then undergoes rapid hydrolysis of one methoxy group affording
guaiacol.
benzaldehydes
(veratraldehyde and vanillin) were then
examined under neutral and basic conditions (350 ºC, 160–170
bar, 10 min). When veratraldehyde (6) was reacted under
neutral conditions, the methoxy groups on the aromatic ring
were hydrolyzed affording vanillin and isovanillin (Figure S1 in
Supporting Information). A trace amount of veratrole was
generated in the process. Conversely, when 6 was heated with
aqueous sodium hydroxide solution, a significantly greater
amount of veratrole as well as guaiacol, veratryl alcohol and
veratric acid were formed. It was evident that the base played an
important role in removing the carbonyl group from
veratraldehyde (6). In addition, the presence of veratryl alcohol
and 4-methylveratrole, formed by reduction of the starting
material, is consistent with a transfer hydrogenation process
occurring in-situ.
A set of reactions that unify all of these processes is
shown in Scheme 2. The theoretical viability of this mechanism
was examined using quantum chemical computations at the B-
PW91/6-31+G(d) level. The corresponding reaction profile is
shown in Figure 2: for the uncatalyzed decarbonylation, there is
–1
a high barrier of 375 kJ mol . The carbon monoxide could then
undergo an (uncatalyzed) water-gas shift reaction to generate
hydrogen and carbon dioxide. In comparison, the addition of a
Figure 2. DFT reaction profile for the conversion of veratraldehyde to veratrole.
Dotted lines represent transition structures connecting minima (bold solid
–
1
hydroxide to veratraldehyde has a barrier of 31 kJ mol , which
–1
lines). Relative energies (kJ mol ) are shown in brackets.
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dimethoxyphenylacetaldehyde and phenol were detected as
major products by GC. A possible reaction pathway is shown in
Scheme 4, consistent with the enol ether compounds having
been generated as a result of dehydration of the model
compound 8 by means of acid-catalyzed elimination. To verify
that the enol ether compounds are the intermediates that
generate phenol and (3,4-dimethoxyphenyl)acetaldehyde,
compound 9 (predominantly Z) was subjected to the same
hydrothermal treatment. As predicted, phenol and 3,4-
dimethoxyphenylacetaldehyde were formed. Proton transfer to 9
was facilitated by the increase in self-ionization of water at
350°C by more than 2 orders of magnitude and the resulting
intermediate was hydrolyzed to form the monomeric products.
The hydrogen generated in the dehydrogenation of
geminal diols could be utilized for the reduction of
veratraldehyde. In parallel a classical Cannizzaro reaction might
.
[21]
take place, as in the report of Strauss and co-workers.
The
presence of 4-methylveratrole however, suggested that
hydrogenolysis of veratryl alcohol occurred. In separate HTL
experiments, some veratryl alcohol dehydrogenated to
veratraldehyde, liberating reductive equivalents which were able
to hydrogenolyse a portion of the remaining veratryl alcohol to 4-
methylveratrole (and water). In these processes the stainless
steel reactor wall potentially acted as a catalyst for the
[
23]
(
de)hydrogenations and hydrogenolyses.
Similarly, when vanillin was heated under neutral
conditions only a trace of guaiacol was generated. But heating
under basic conditions generated a variety of products, including
guaiacol, catechol and 4-methylguaiacol. The formation of these
products is consistent with the mechanism describing the
conversion of veratraldehyde (Scheme 2). In addition,
“
scrambling” of methoxy groups was also observed as
veratraldehyde, veratrole and 4-methylveratrole were formed.
Under basic conditions, methanol generated by hydrolytic
demethylation may methylate phenoxide groups. Alternatively,
direct methyl group transfer, e.g. from veratraldehyde to
guaiacolate, may also occur.
To understand how the β-O-4 linkage reacts under
hydrothermal conditions, model compound 7, which has been
Scheme 4. Conversion of β-O-4 model compound
hydrothermal conditions.
8 under neutral
[24]
used in other conversion studies,
was subjected to
hydrothermal treatment under both neutral and basic conditions.
Only trace products were generated when 7 was heated with
water at 350 ºC for 10 min. Conversely, 7 was fully converted to
give acetophenone and phenol under basic conditions (0.1 M
NaOH). Rather than hydrolysis to give styrene glycol and phenol,
the linkage was broken down by hydroxide-mediated elimination
in which the phenoxide acted as the eventual leaving group, with
the resultant enol tautomerizing to acetophenone. Indeed, our
DFT calculations indicate that the conversion to acetophenone is
thermodynamically favorable, with both steps shown in Scheme
Figure 3. DFT reaction profile for the acid-catalyzed decomposition of 7 (R =
Ph) and 8 (R = Ar = 3,4-dimethoxyphenyl). Relative energies (kJ mol ) are
shown in brackets.
–1
–1
3
being exothermic (by 68 and 77 kJ mol for the first and
second step, respectively). The first step involving proton
abstraction by hydroxide, has a low calculated reaction barrier of
To underpin these mechanistic interpretations, we have
performed DFT computations for these acid-catalyzed pathways
(for both 7 and 8, Figure 3). Together with the modeling for the
base-catalyzed reactions for 7, the calculations enable us to
elucidate the features that lead to the different reaction
outcomes. Whereas the steps involved in the base-catalyzed
reaction of 7 are all exothermic, the first step of the acid-
catalyzed reaction, namely protonation of 7, is endothermic (by
–1
20 kJ mol .
–1
69 kJ mol ). Thus, under neutral (equilibrium) conditions in
Scheme 3. Conversion of β-O-4 model compound 7 under basic hydrothermal
conditions.
which protonation is thermodynamically fairly unfavorable, the
acid-catalyzed reaction does not proceed.
The electron-donating alkoxy substituents on 8 facilitate
the formation of the carbocation intermediate, with the first step
being endothermic by only 35 kJ mol . This intermediate then
further undergoes a series of near-thermoneutral or exothermic
transformations to form the (3,4-dimethoxyphenyl)acetaldehyde
and phenol products. Overall, the difference in the reactivities of
To strengthen our conclusions we investigated the
–1
conversion of a β-O-4 compound (8) with guaiacyl instead of
[
23b]
phenyl subunits , which much more closely resembles a
typical lignin linkage. Under neutral HTL conditions, various
products were formed. Besides trace amount of the “base-
catalyzed” product 3,4-dimethoxyacetophenone, enol ether
7
and
8
can be attributed to the difference in the
thermodynamics of the initial protonation. We note that a similar
compounds
(compound
9
as
E/Z
isomers),
3,4-
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5
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Water, water everywhere and all the
bonds did shrink: A tandem
Matthew Y. Lui, Bun Chan, Alexander K.
L. Yuen, Anthony F. Masters, Alejandro
Montoya and Thomas Maschmeyer*
experimental/computational approach
to understand complex processes that
proceed during the hydrothermal
liquefaction of lignin is presented. The
choice of the right model compounds
to represent key linkage-motifs and
end-group functionalities is paramount
for achieving realistic insight into
these reaction cascades.
Page No. – Page No.
Unravelling Some of the Key
Transformations in the Hydrothermal
Liquefaction of Lignin
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