Lignin depolymerization to monophenolic compounds in a flow-through system

Article abstract of DOI:10.1039/c7gc02731a

A reductive lignocellulose fractionation in a flow-through system in which pulping and transfer hydrogenolysis steps were separated in time and space has been developed. Without the hydrogenolysis step or addition of trapping agents to the pulping, it is possible to obtain partially depolymerized lignin (21 wt% monophenolic compounds) that is prone to further processing. By applying a transfer hydrogenolysis step 37 wt% yield of lignin derived monophenolic compounds was obtained. Pulp generated in the process was enzymatically hydrolyzed to glucose in 87 wt% yield without prior purification.

Full text of DOI:10.1039/c7gc02731a

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Heterogeneous catalytic oxidation for lignin valorization into valuable
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Lignin depolymerization to monophenolic compounds in a flow-
through system
Ivan Kumaniaev,†a Elena Subboꢀna,†a Jonas Sävmarker,*b Mats Larhed,*b Maxim V. Galkin,*a and
Joseph S. M. Samec*a
0Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A reductive lignocellulose fractionation in a flow-through system
in which pulping and transfer hydrogenolysis steps were
separated in time and space has been developed. Without the
hydrogenolysis step or addition of trapping agents to the pulping,
it is possible to obtain partially depolymerized lignin (21 wt%
monophenolic compounds) that is prone for further processing. By
applying a transfer hydrogenolysis step 37 wt% yield of lignin
derived monophenolic compounds was obtained. Pulp generated
in the process was enzymatically hydrolyzed to glucose in 87 wt%
yield without prior purification.
Broad context
There are two important issues that cause growing concern for humanity–energy
and the environment. The demand for sustainable chemicals, materials, and low
carbon footprint fuels, on one hand, and the necessity to supply steadily growing
population with these products as well as enough energy, on the other, are
making the scientific community to look for an alternative to fossil carbon
resources. Lignocellulosic biomass is the primary reservoir of fixed renewable
carbon on the planet. The implementation of
a profitable and sustainable
biorefinery relies on the involvement of all biomass components into the
valorization process in one or another way. It requires cost-effective process for
breaking down main lignocellulose polymers–cellulose, hemicellulose, and lignin–
into their constituent monomers. The success of the petrochemical industry that
comes from its co-production of inexpensive high volume fuels and lesser
amounts of more profitable chemicals can become guidance for biorefineries. In
analogy, where high volume carbohydrates could be used for production of
energy and fuels and lignin could be utilized in high octane gasoline additives, fine
chemical, and polymer production. Currently, with the focus given to
carbohydrates, lignin is an unused source of aromatics that is hard to process.
Development of early step effective depolymerization method for lignin is critical
for the accelerated growth and deployment of the advanced second generation
biorefinery that convert lignocellulosic biomass into liquid transportation fuels.
Economical utilization of lignin as a value-added product at a
commercial scale is a key to make a lignocellulosic biorefinery
cost effective. The ability to deconstruct lignin first from the
intact biomass revolutionizes the conventional concept of the
biorefinery by capturing high-value products from lignin as the
first step and enhancing downstream of the cellulosic residue
(improving ethanol production, etc.).1-3
The most prevalent lignin depolymerization target is the
ether bond of the β-O-4′ motifs. There are a number of
different strategies available where some are focused on the
in-situ stabilization1 of the monophenolic compounds released
during pulping2-14 and the rest are focused on obtaining lignin
that structurally is close to a native one or, at least, have an
equivalent of the β-O-4′ motif preserved in its structure.15-19
These strategies allow to obtain monophenolic products in
near theoretical yields to produce 45 to 55 mol % of
monophenolic compounds, along with enzyme-digestible
pulps. The dominant lignin depolymerization strategy is the
hydrogenolysis of lignin during the biomass fractionation
process (reductive catalytic fractionation or catalytic upstream
biorefining). Such processes usually involve mixing
a
heterogeneous metal catalyst with a solid biomass in a batch
reactor. These reductive catalytic fractionation methods are
complicated by the catalyst recovery, requirement of hydrogen
gas or other additives, mass transfer problems, and pulp
contamination by the catalyst; these issues limit and restrict
implementation of the method.20
A process that would not require an additional trapping
agent for stabilization of generated monophenolic compounds,
no addition of hydrogen gas or hydrogen source,21 where
biomass intercalation and hydrogenolysis steps are separated
time- and space-wise but still in the same flow through system
would avoid most of the mentioned drawbacks.22
We report here an application of a flow-through system23
for biomass fractionation where the lignin released during the
solvolysis step is mainly fragmented to monophenolic
compounds, dimers, and trimers and to lesser extent higher
oligomers. The hemicellulose fraction is used as an internal
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raised from 160 to 220 °C and the temperature of the reactor
with Pd/C kept constant (180 °C), the yield of monophenolic
compounds changed from 17 to 24 wt% with a maximum of
27 wt% yield at 200 °C (Table 1, entries 2–4). The increase of
temperature improves delignification where higher
temperature signifies repolymerization reactions and
therefore decreases the yield.29 A similar effect was observed
for the temperature of the reactor with Pd/C, where both too
high and too low temperatures impaired productivity (Table 1,
Entry 3, 5, and 6). Optimal flow rate was found to be
0.3 mL∙min-1 (31 wt% yield), where lowering the rate
(0.1 mL∙min-1) leads to over hydrogenation of the
monophenolic compounds (Table 1, Entry 7–9).
Scheme 1. Schematic representation of the flow through system used in the study;
Back Pressure Regulator (BPR) pressure for optimized reaction conditions.
reducing agent and hydrogen donor. Application of a transfer
hydrogenolysis step increased the yield of monophenolic
compounds to 83% of the theoretical maximum.
The system used consists of a percolation reactor filled
with woody biomass and a fixed catalytic bed reactor filled
with Pd/C (Scheme 1).24 For biomass delignification,
organosolv pulping conditions were chosen. When continuous
removal of the pulping media take place an addition of
Brønsted or Lewis acids is advantageous.25 Therefore, a
methanol-water solution of phosphoric acid (0.02 M) was
used. The influence of water to methanol ratio, and the
influence of concentration and the nature of an acid on the
biomass components during the pulping process have been
studied elsewhere.26-28 The results from the optimization of
the pulping conditions are in a good agreement with those
reported previously (ESI, Tables S1–S6, see Ref. 26–28).
To increase the permeability of the biomass pulping
solution, dewaxed biomass is commonly used.30 Consequently,
one set of samples was dewaxed prior to experiment to
investigate the effects of waxes on the delignification and
catalyst activity. Interestingly, lower yield of monophenolic
compounds was obtained when dewaxed biomass was used
(Table 1, Entry 10 and ESI 3.1.1 and Table S6). The effect of
wood meal pre-soaking was also tested. When the biomass
was left in the pulping solution over night at RT, the yield of
monophenolic compounds raised to 37 wt% (Table 1,
Entry 11). The solvent mixture in the current continuous flow
system may operate in different phases due to temperature,
flow rate, and pressure that may vary within the system. This
would be of great interest to study in the future.
To obtain monophenolic compounds in high yields, the
effect of temperature, flow rate, and process time on the
fractionation of birch wood and the following reduction step
were examined. When the temperature at which birch wood
was subjected to percolation with MeOH-H2O (7:3, v/v) was
Additional studies were performed (NMR, SEC, GC-MS,
etc.) to gain further insight on the process of the lignin
depolymerization using the optimized process conditions.
Besides the possibility to control the pulping and catalytic
parameters separately, the system allowed both rapid heating
and cooling of the reaction media. Thereby, we could study
the nature of lignin directly after pulping.
Table 1. Influence of the process parameters on the lignin depolymerization.
Flow rate
Entry
1b
Time (h)
T1 (°C)
T2 (°C)
Yield (wt%)a
(mL∙min-1)
By monitoring the reaction, we found that the
delignification of the biomass takes place within the first
90 min (ESI, Fig S12, S13). However, in the lignin reaction with
Pd/C, it is apparent that the product desorption from the
catalyst bed leads to a delay in the release of lignin products
from the second reactor. In fact, it takes approximately 180
min for the lignin products to be discharged from the reactor.
Previously Rinaldi’s group has demonstrated that lignin
fragments released at the beginning of the pulping process
display bimodal distribution of monophenolic compounds and
oligomeric fragments up to trimers, whereas the lignin
fragments obtained after prolonged reaction time represents
higher oligomers with a considerable fraction greater than
3 kDa.6 Notably, apparent Mw distribution of lignin fragments
released in the present process changes insignificantly with the
time passed from the beginning of the pulping (Fig 2, t1 and
t2), where the main part of the lignin belongs to monophenolic
compounds and dimer fractions and the rest belongs to higher
oligomers. At the end of pulping there is an increase in
apparent molecular weight of the released lignin fragments
(from 823–t1 to 1340 Da–t3). However, this part accounts for
only 5 wt% of the total lignin. These results clearly show that
the lignin depolymerization to a large extent takes place during
Organosolv pulping (no Pd/C)
200 0.3
1-3
-
21 ± 2.8c
Organosolv pulping followed by transfer hydrogenolysis (Pd/C)
2
3
3
3
180
200
220
200
200
200
200
200
200
200
180
180
180
200
160
180
180
180
180
180
0.2
0.2
0.2
0.2
0.2
0.3
0.5
0.1
0.3
0.3
17
27
4
3
24
5
3
22
6
3
22
7
3
31
8
3
21
9
3-6
3
0
10b,d
11b,e
29
3
39/37 ± 0.6/1.5c
General conditions: non-dewaxed oven-dry birch wood meal (0.150 g, 2 wt%
residual moister), Pd/C 5 wt% (0.150 g), 2.8 g∙L-1 H3PO4 in MeOH-H2O 7:3 v/v.
a Monophenolic compounds yield estimated from 1H NMR data (ESI 4.2.1),
represented as wt% of total lignin (ESI 3.2.2); b Impregnated wood meal;
c Average of 4 (entry 1) and 3 (entry 11) repetitions. Monophenolic compounds
yield estimated from GC-MS data. (ESI 4.3) For Entry 1 : 4-(3-hydroxyprop-1-en-1-
yl)-2,6-dimethoxyphenol (1a), 2,6-dimethoxy-4-(3-methoxyprop-1-en-1-yl)phenol
(2a), and 2-methoxy-4-(3-methoxyprop-1-en-1-yl)phenol (2b); For Entry 11: 4-(3-
hydroxypropyl)-2,6-dimethoxyphenol (3a), 4-(3-hydroxypropyl)-2-methoxyphenol
(3b),
2,6-dimethoxy-4-(3-methoxypropyl)phenol
(4a),
2-methoxy-4-(3-
methoxypropyl)phenol (4b), 2,6-dimethoxy-4-propylphenol (5a), and 2-methoxy-
4-propylphenol (5b); d Dewaxed wood meal; e Non-dewaxed wood meal.
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the solvolysis and partially during the transfer hydrogenolysis
step (vide infra, Fig 3).
Analysis of the obtained hydrolyzed lignin by 2-D NMR
spectroscopy (HSQC, HMBC) revealed a decrease in the
occurrence of native ether bonds (Fig 4) and the appearance of
alkene functionalities. The signals of the new alkene
functionalities were ascribed to three major lignin
monophenolic compounds (1a, 2a, and 2b; Table 1, Entry 1;
Fig 4) where the most abundant 2,6-dimethoxy-4-(3-
methoxyprop-1-en-1-yl)phenol (2a) was generated in up to
15 wt% yield. Identification and quantification of the
monophenolic compounds are described in the electronic
supplementary information (ESI 4.2 and 4.3).
Fig 1. Reaction concentration profile for birch wood pulping with and without catalyst
present: (0.150 g), Pd/C 5wt% (0.150 g), 2.8 g∙L-1 H3PO4 in MeOH-H2O 7:3 v/v.
It is gratifying that lignin undergoes solvolysis under the
applied fractionation conditions and then the homolysis of the
β-O-4′ bonds starts from the free phenol groups–peeling
(Fig 5).31-34 Reducing sugars released from biomass can act as
an internal reducing agent to react with the radicals of synapyl
and coniferyl alcohols to yield corresponding alcohols (1a and
1b
)
as products.35-37 This homolytic pathway has been
extensively
studied (predominantly utilizing model
compounds).31-38 Thereby, the reaction allows lignin
depolymerization to monomers starting from the free phenolic
OH groups (of the β-O-4′ motif) and the reaction will maintain
only in a continuous sequence of monomers joined with β-O-4′
bonds. Therefore, we attribute the presence of higher
oligomers in the lignin obtained after pulping partially to the
inability of homolysis reaction to take place at β-O-4′ motifs
with other types of surroundings and to a lower extent to
repolymerization reactions of reactive radicals.38,39 It is evident
that the catalyst has an important role in transforming these
less available β-O-4′ bonds (Fig 3 and Fig 4).
Fig 2. SEC chromatograms: apparent Mw distribution for lignin obtained at different
time from solvolysis step: t1 from 0 to 15 min; t2 15-30 min and t3 from 30 to 60 min.
Conditions Table 1, Entry 1.
In contrast to direct biomass hydrogenolysis, our approach
is compatible with most polysaccharide depolymerization
techniques and does not need tedious pulp and catalyst
separation. Thorough analysis of all fractions obtained after
the process revealed that 92 wt% of cellulose was intact and
Fig 3. SEC chromatograms of lignin obtained after 3 h reaction with and without Pd/C.
Fig 4. Complete HSQC spectra of: Organosolv lignin (left) and organosolv lignin after Pd/C treatment (right). Structures found in the dissolved lignin: (A) β-O-4´, (C) and (D) methoxy
and hydroxy propenyl phenol fragments including compounds 1a, 2a, and 2b; F: compound 3a and 3b, E: compound 4a and 4b, and G: compound 5a and 5b. Reaction conditions:
non-dewaxed, flushed with solvent system (RT, 0.3 mL∙min-1, 10 min) and left overnight before reaction wood meal, 2.8 g∙L-1 H3PO4 in MeOH-H2O 7:3 v/v; preheater on; T1 = 200 °C;
T2 = 180 °C; flow rate 0.3 mL∙min–1; reaction time 3 h.
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Fig 5. Proposed mechanism of the β-O-4′ bond cleavage. Organosolv pulping step: the formation of coniferyl and sinapyl alcohols and consequently their ethers in the absence of
catalyst is attributed to homolysies reaction with following reduction of the radicals with reducing sugars (ESI Fig S32). Catalytic hydrogenation and hydrogenolysis step: cleavage
of the non-terminal β-O-4′ bonds and saturation of unsaturated intermediates. The yield of monophenolic compounds estimated from GC-MS data (ESI 4.3).
present in the solid residue. As a result, the treatment of birch
wood under mild enzymatic hydrolysis conditions gave glucose
Acknowledgements
in 95% yield, which corresponds to 87 wt% of the original
This work was supported by the Swedish Energy Agency. We
glucose content in the woody biomass (Table 2). In the present
would like to acknowledge Vanhälls Såg AB for providing
system the hemicellulose fraction is used as an internal
biomass samples and Magnus Fagrell for providing the
reducing agent and hydrogen donor (ESI 5). The yield of lignin
RedHeater continuous flow system.
derived monophenolic compounds is comparable to the best
methodologies previously reported (38-55%) as well as a
cellulose yield (74-94%).18,24,40
Notes and references
The present approach results in a very good yield of
1
2
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