Reductive fractionation of woody biomass into lignin monomers and cellulose by tandem metal triflate and Pd/C catalysis

Article abstract of DOI:10.1039/c6gc02962k

A catalytic process for the upgrading of woody biomass into mono-aromatics, hemi-cellulose sugars and a solid cellulose-rich carbohydrate residue is presented. Lignin fragments are extracted from the lignocellulosic matrix by cleavage of ester and ether linkages between lignin and carbohydrates by the catalytic action of homogeneous Lewis acid metal triflates in methanol. The released lignin fragments are converted into lignin monomers by the combined catalytic action of Pd/C and metal triflates in hydrogen. The mechanism of ether bond cleavage is investigated by lignin dimer models (benzyl phenyl ether, guaiacylglycerol-β-guaiacyl ether, 2-phenylethyl phenyl ether and 2-phenoxy-1-phenylethanol). Metal triflates are involved in cleaving not only ester and ether linkages between lignin and the carbohydrates but also β-O-4 ether linkages within the aromatic lignin structure. Metal triflates are more active for β-O-4 ether bond cleavage than Pd/C. On the other hand, Pd/C is required for cleaving α-O-4, 4-O-5 and β-β linkages. Insight into the synergy between Pd/C and metal triflates allowed optimizing the reductive fractionation process. Under optimized conditions, 55 wt% mono-aromatics-mainly alkylmethoxyphenols-can be obtained from the lignin fraction (23.8 wt%) of birch wood in a reaction system comprising birch wood, methanol and small amounts of Pd/C and Al(III)-triflate as catalysts. The promise of scale-up of this process is demonstrated.

Full text of DOI:10.1039/c6gc02962k

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G. Chatel et al.
Heterogeneous catalytic oxidation for lignin valorization into valuable
chemicals: what results? What limitations? What trends?
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Reductive fractionation of woody biomass into lignin monomers
and cellulose by tandem metal triflate and Pd/C catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
Xiaoming Huang, Olivia M. Morales Gonzalez, Jiadong Zhu, Tamás I. Korányi, Michael D.
b
, a
Boot, and Emiel J.M. Hensen*
DOI: 10.1039/x0xx00000x
A catalytic process for the upgrading of woody biomass into mono-aromatics, hemi-cellulose sugars and a solid cellulose-
rich carbohydrate residue is presented. Lignin fragments are extracted from the lignocellulosic matrix by cleavage of ester
and ether linkages between lignin and carbohydrates by the catalytic action of homogeneous Lewis acid metal triflates in
methanol. The released lignin fragments are converted into lignin monomers by the combined catalytic action of Pd/C and
metal triflates in hydrogen. The mechanism of ether bond cleavage is investigated by lignin dimer models (benzyl phenyl
ether, guaiacylglycerol-β-guaiacyl ether, 2-phenylethyl phenyl ether and 2-phenoxy-1-phenylethanol). Metal triflates are
involved in cleaving not only ester and ether linkages between lignin and the carbohydrates but also β-O-4 ether linkages
within the aromatic lignin structure. Metal triflates are more active for β-O-4 ether bond cleavage than Pd/C. On the other
hand, Pd/C is required for cleaving α-O-4, 4-O-5 and β-β linkages. Insight into the synergy between Pd/C and metal triflates
allowed optimizing the reductive fractionation process. Under optimized conditions, 55 wt% mono-aromatics - mainly
alkylmethoxyphenols - can be obtained from the lignin fraction (23.8 wt%) of birch wood in a reaction system comprising
birch wood, methanol and small amounts of Pd/C and Al(III)-triflate as catalysts. The promise of scale-up of this process is
demonstrated.
www.rsc.org/
6
, 9
Introduction
lignin as solid residue.
A significant problem in converting
technical lignin is its much lower reactivity as compared to the
1
0-12
Biorefining is gaining in importance as a technology to increase original lignin structure contained in the lignocellulose matrix.
energy security, to mitigate emissions of carbon dioxide, and to During pre-treatment, lignin’s relatively labile β-O-4 ether linkages
1
increase rural development. Widespread production of biomass- can be easily cleaved and the resulting fragments condense into
derived fuels and chemicals will require cost-effective processes for more recalcitrant structures. As a consequence, technical lignin
breaking down the principle biomass constituents cellulose, contains more C-C bonds than the original lignin. The condensation
2
13
hemicellulose and lignin into their monomers. One-pot conversion reactions typically involve phenolic fragments, formaldehyde and
of whole biomass results in a complex mixtures of aromatics, sugar-derived dehydration products such as 5-
alkanes and alcohols, which limits applications as fuels and hydroxymethylfurfural and furfural. The problem of
1
4,
15
3
-5
chemicals. In order to improve conversion efficiency and product condensation of lignin fragments is more significant in the presence
selectivity, biomass is usually pre-treated by physical, chemical or of acid or base catalysts. The low reactivity of technical lignin
biological methods. Pre-treatment of lignocellulosic biomass is one obtained by Kraft and soda pulping and organosolv methods has
6
, 7
of the most costly operations in its valorization.
Biological been emphasized in recent work. Detailed characterization revealed
1
6
approaches are time-consuming, while mechanical operations are considerable condensation of lignin during extraction. As a
usually energy- and capital-intensive. Chemical pretreatment with consequence, the depolymerization of lignin requires high
alkali, ozone, peroxide or organic solvents is typically targeted at temperature (>250 °C) to increase the rate of (thermal) C-C bond
1
7, 18,19
the extraction of lignin from the whole biomass with the purpose of cleavage.
8
, 9
enhancing enzymatic degradability of cellulose.
mineral acids, usually H SO and HCl, is widely practiced to
hydrolyze (hemi-)cellulose into fermentable sugars, leaving behind hydrocracking, hydrogenolysis, oxidation, and hydrolysis have been
Treatment with
2
4
Common approaches to depolymerize lignin such as pyrolysis,
1, 20-23
well documented in recent reviews.
Most of such thermo-
catalytic processes are non-selective and give rise to mixtures of
tens or even hundreds of different compounds. The complexity of
these product mixtures hinders application of lignin as feedstock for
producing high-value chemicals. As an example, we refer to our
own work in which technical lignin was catalytically converted in
ethanol over a Cu-Mg-Al mixed oxide catalyst into lignin monomers
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in high yield (typically ~60 wt% for soda and organosolv lignins and these and similar other efforts,
Journal Name
1
0, 26, 27
efficient conversion of lignin
8
6 wt% for Kraft lignin). High product yield was obtained under into useful chemicals remains a significant challenge.
1
3, 24, 25
severe conditions (T = 380 °C, supercritical ethanol).
Despite
Figure 1. Tandem catalytic system for the reductive fractionation of woody biomass into alkylmethoxyphenols and sugars over Pd/C and
metal triflate in methanol.
A recent major step forward has been the direct conversion of organosolv lignins and wood sawdust in a 1,4-dioxane/H O solvent
2
the lignin fragments removed from whole woody biomass before mixture. Lignin monomer yields from these feedstocks were
3
5
condensation. For instance, Yan et al. reported that birch wood unsatisfactory. Related to these “lignin-first” approaches, we have
lignin can be hydrogenated to alkylmethoxyphenols in 46 wt% yield recently demonstrated that metal triflates act as efficient co-
3
6
in a 1:1 (v/v) dioxane/water solvent mixture over carbon-supported catalysts in the delignification of biomass. In combination with
2
8
noble metal (Pt, Ru, Pd and Rh) catalysts at 200 °C for 4 h. Li et al. Pd/C and methanol as the solvent
a narrow stream (3-7
reported the direct catalytic conversion of woody biomass into diols compounds) of lignin-derived alkylmethoxyphenols is obtained
and alkylmethoxyphenols in water over a carbon-supported Ni-W C under mild conditions. Cellulose is retrieved as a separate fraction
2
5
catalyst at 235 °C for 4 h. Song et al. upgraded birch wood lignin with a composition comparable to that of cellulose obtained by
into alkylmethoxyphenols in alcohols over nickel catalysts and Kraft pulping of birch wood. We have already discussed mechanistic
5
reported a lignin conversion of about 50 %. Ferrini et al. discussed aspects about the role of metal triflates in liberating lignin
a catalytic biorefining method that converts lignin from woody fragments from lignocellulosic biomass. By use of model
biomass into bio-oil rich in phenolic compounds and (hemi- compounds, we demonstrated that these homogeneous catalysts
)
cellulose-rich pulp over a Raney Ni catalyst in a H O/2-propanol are effective in catalysing cleavage of γ-ester and phenyl ether and
2
mixture. The obtained bio-oil was further upgraded using the same glycoside bonds between lignin and (hemi-)cellulose. The resulting
1
8
catalyst in 2-propanol. The group of Abu Omar used a bimetallic lignin fragments can then be disassembled into aromatic monomers
Zn/Pd/C catalyst to convert lignin in lignocellulosic biomass into two by Pd-catalyzed hydrogenolysis reactions.
alkylmethoxyphenols in methanol. A yield of 52 wt% of lignin
monomers was obtained from birch hardwood after reaction at 225
In this work, we focus on the synergistic effect of metal triflates
29, 19
°
C for 12 h.
Sels and co-workers reported that birch wood in the Pd-catalyzed deconstruction of lignin fragments. For this
sawdust was efficiently delignified through simultaneous solvolysis purpose, we employ benzyl phenyl ether (BPE), guaiacylglycerol-β-
and catalytic hydrogenolysis in the presence of Ru/C or Pd/C in guaiacyl ether (GG), 2-phenylethyl phenyl ether (PPE-H) and 2-
methanol under a H atmosphere at elevated temperature. The phenoxy-1-phenylethanol (PPE-OH) as model reactants. Plausible
2
main products were a carbohydrate pulp and a lignin oil containing mechanisms for the conversion of these model lignin compounds as
6
, 30
more than 50 % phenolic monomers.
The addition of acids such well as the depolymerization of wood lignin will be discussed. We
31
as H PO improved the overall efficiency of the process.
also varied the type of metal triflates and supported metal catalysts
as well as the solvent to identify optimum conditions for woody
3
4
Earlier, we demonstrated that Lewis acids (metal acetates, biomass upgrading. In so doing, we were able to optimize the
metal chlorides and metal triflates) can be used as catalyst to tandem process of birch wood towards high aromatic monomers
depolymerize a soda lignin in supercritical ethanol. Metal triflates yield under mild conditions (aromatic monomers yield of 55 wt% at
are promising water-tolerant Lewis acids catalysts for lignin T = 180 ° C for t = 2 h). Finally, we also demonstrate that this
3
2-34
conversion.
heterogeneous Ru or homogeneous Ir catalyst is effective in similar mono-aromatics yield.
A combination of a 10 mol% triflic acid with a process can be scaled-up to convert 100 g biomass with nearly
2
7
depolymerizing organosolv lignin in 1,4-dioxane solvent. A similar
catalytic system involving metal triflate and a homogeneous Rh
catalyst was employed for the depolymerization of various
2
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Results and discussion
Table 1. Composition of birch wood biomass and residue and product oil mixture after reductive fraction (all values are in mg).
a
b
Birch wood
-
Reductive fractionation
Pd/C
Yb(III)-triflate
1360
1082.6
817.4
238.7
0.0
Pd/C
Catalyst
Pd/C
1790
Al(III)-triflate
1190
975.8
772.8
187.7
0.0
c
Solids
2000
1276
800.0
422.0
6.0
d
Sugars
1249.4
801.9
407.9
2.0
Glucan
Xylan
Arabinan
Galactan
Mannan
14.0
26.0
8.0
9.9
1.6
1.4
23.8
4.0
21.8
19.5
0.0
Rhamnan
0.0
e
Lignin
475
402.0
73.0
5.5
319.4
281.2
38.2
17.2
204.0
120.5
109.9
10.6
91.2
83.4
7.8
Klason
Acid soluble
Ash
22.1
14.4
103.1
810
Unknown
243.5
-
137.9
640
Product oil
210
f
Methylated sugars
-
6
64
205
352
g
Phenolics
-
203
212
Unknown
-
140
232
246
a
b
Notes: Birch wood sample subjected to Soxhlet extraction with water followed by ethanol. Reaction conditions: 2000 mg extracted birch
c
2
wood sawdust (125-300 μm), 200 mg 5 wt% Pd/C, 0.0322 mmol metal triflate, 40 ml methanol, 30 bar H , stirring speed: 500 rpm, 2 h.
Solids analysis done by Celignis Limited Nexus Innovation Centre, University of Limerick, Ireland (www.celignis.com); mass of the residue
d
excluding Pd/C catalyst and moisture. Based on monomeric sugars detected after two-stage acid hydrolysis following a standard
3
7 e
f
g
protocol. Based on the of acid-soluble and -insoluble (Klason) lignin. Methylated sugars are detected and quantified by GC-FID. Based
on the total lignin content.
The catalytic process
unconverted in these experiments as evident from the unchanged
glucan content. Hemi-cellulose is hardly degraded by the action of
Figure 1 illustrates the overall concept of the tandem catalytic Pd/C, but significantly removed from the solid matrix when metal
system for the conversion of lignin from woody biomass. Metal triflates are present. Metal triflates strongly promote the reductive
triflates are effective in the extraction of lignin from the fractionation of lignocellulose. The lignin monomers yield with only
lignocellulosic matrix by cleaving ether and ester lignin- Pd/C was limited to 14 wt%. Addition of Yb(III)- or Al(III)-triflate to
carbohydrate intralinkages. These lignin fragments are then cleaved the reaction strongly increased the removal of lignin from the
by Pd-catalyzed reactions involving molecular hydrogen. In an biomass and, at the same time, increased the lignin monomers
experiment using Pd/C and Yb(III)-triflate at 180 °C for 2 h in 30 bar yield. The use of Al(III)-triflate led to a higher lignin monomers yield,
H
2
, the C9 lignin monomers yield was 43 wt% based on lignin but also degraded slightly more cellulose. Without Pd/C, mainly
content in birch wood. The main products were 4-n-propylsyringol lignin fragments were obtained when Yb(III)-triflate was the catalyst
3
6
(
PS-H), 4-n-propanolsyringol (PS-OH) and 4-n-methoxy propyl with a low monomers yield of 7 wt%. These results demonstrate
syringol (PS-OCH ) and their corresponding guaiacyl counterparts, that metal triflates and Pd/C work in tandem in delignifying
namely PG-H, PG-OH and PG-OCH Table 1 shows a detailed lignocellulosic biomass and converting the resulting fragments to
3
3
.
6
3
analysis of the birch wood composition, the solid residue and monomers. We next demonstrate that metal triflates also affect the
product oil composition after upgrading by Pd/C only and by a performance of Pd-catalyzed hydrogenolysis reactions.
combination of Pd/C and metal triflates. Cellulose remains nearly
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Model compound studies
in woody biomass, respectively. BPE is often used as model
3
8-40
compound in screening studies.
Although it does not contain
With the purpose of understanding better the role of metal hydroxyl and methoxy groups usually encountered in lignin, its
triflates in Pd-catalyzed deconstruction of lignin fragments, we benefit is the relative facile product identification and
employed benzyl phenyl ether (BPE) and guaiacylglycerol-β-guaiacyl quantification in evaluating catalyst performance in ether linkage
ether (GG) as models for α-O-4 and β-O-4 ether lignin intralinkages
80
60
40
20
0
1
1
20
00
(
a)
(b)
Others
80
60
40
20
0
BPE
k
/
C
te
te
ifla
tr
-
n
ifla
la
d
B
P
tr
)
-
I)
II
II
(
I
(
b
b
Y
Y
+
C
/
d
P
Figure 2. Reaction of (a) benzyl phenyl ether (BPE) and (b) guaiacylglycerol-β-guaiacyl ether (GG) in methanol over Pd/C and/or Yb(III)-
triflate catalysts at 160 °C for 2 h in 30 bar H
2
.
2
0
disruption and aromatic ring hydrogenation. GG is the most cleavage activity of Yb(III)-triflate is suppressed in the presence of
suitable model compound for the most abundant β-O-4 (40-65%) Pd/C. The reason is that Pd/C is active in the removal of the α-OH
2
3
ether linkage in native lignin. Therefore and also because it is group. This converts the β-O-4 ether bond into an ether bond which
commercially available, GG has been frequently used in mechanistic cannot be cleaved by the metal triflate. As we will show later, this
1
0, 26, 27, 41-43
studies of lignin deconstruction.
these two dimer compounds were performed at 160 °C for 2 h in towards high aromatic monomers yield.
methanol and 30 bar H . Blank experiments (without catalyst) were
compared to catalytic experiments with either Pd/C or Yb(III)-
Model reactions using result is important in further optimizing the tandem catalyst system
2
It is worthwhile to mention that in the tandem system ring-
triflate or a combination of the two. Fig. 2 shows the product yield saturated products are obtained, very different from the
obtained in these experiments. Without a catalyst, the ether bonds experiments with only Pd/C or the metal triflates. The enhanced
are relatively stable under the given reaction conditions. Pd/C is hydrogenation activity of Pd/C in the presence of Yb(III)-triflate is
very active in cleaving the α-O-4 ether bond: complete ether bond confirmed in an experiment with guaiacol as the reactant. The use
conversion was achieved within 2 h. However, Pd is much less of Pd/C results in a conversion of 61 % to ring-saturated products.
active in cleaving the β-O-4 ether bond in GG and the product of In the presence of Yb(III)-triflate, full conversion was achieved
ether bond cleavage, guaiacol, is only obtained in 18 % yield. The under the same conditions. Accordingly, we conclude that there is
substantial activity difference stems from competitive adsorption synergy between Pd/C and Yb(III)-triflate in aromatic ring
between the ether bond and the α-OH and γ-OH groups of the hydrogenation. On the other hand, no synergy was observed with
1
9
reactant. Yb(III)-triflate is nearly inactive in cleaving the α-O-4 respect to ether bond cleavage. Synergy in aromatic ring
ether bond of BPE, yet very active in rupture of the β-O-4 ether hydrogenation between Lewis acids and Pd/C has been described in
4
4, 45
bond in GG. This difference is caused by the strong activation of the recent literature.
For instance, Zhou and co-workers have
and ZnCl on
This Pd/C improve aromatic ring hydrogenation activity. Our own
ether bond upon coordination of a Lewis acid to oxygen atoms of shown that the presence of Lewis acids such as AlCl
3
2
1
9, 42, 44, 45
44
OH groups in α- and γ-positions of the ether bond.
explains the higher activity of Yb(III)-triflate in cleaving the β-O-4 results evidenced that the presence of propanol side chains on
ether bond in GG as compared with the α-O-4 ether bond in BPE. A aromatic rings hinders their hydrogenation, presumably due to
36
combination of Pd/C and Yb(III)-triflate results in similar BPE competitive adsorption for Pd surface sites. The presence of a
conversion and products as Pd/C, suggesting that in this case the hydroxyl-group-rich substrate (i.e., the biomass itself and derived
metal triflate does not affect the performance of Pd/C. Fig. 2b sugars) also appears to suppress hydrogenation of aromatic rings.
shows the yield of guaiacol and thereof-derived products obtained This is demonstrated by comparing the hydrogenation of guaiacol in
by conversion of GG. Contrary to the results with BPE, the rate of the presence and absence of glucose. Model reactions were
ether bond conversion in GG is significantly lower in the tandem performed at 160 °C for 2 h in methanol using Pd/C as the catalyst.
catalyst system compared with Yb(III)-triflate. Thus, the ether bond The results attest to the lower rate of aromatic ring hydrogenation
4
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of guaiacol in the presence of glucose (Figure S1). These findings bond linkage. This is cumbersome with GG as side-reactions such as
also explain the absence of ring hydrogenation of aromatics in the etherification and hydrogenation occur. Thus, we carried out
conversion of woody biomass. We speculate that the synergy reactions with two other model compounds 2-phenoxy-1-
between Pd/C and metal triflate in aromatic ring hydrogenation is phenylethanol (PPE-OH) and 2-phenylethyl phenyl ether (PPE-H).
due to the reduced amount of hydroxyl groups, owing to increased The former compound contains an OH group in the position β to
rate of etherification with methanol catalyzed by metal triflate.
the ether bond, while the latter does not. The reactions were
conducted at 200 °C for 4 h in presence of 30 bar H . Consistent
The distinct difference observed in the way the two catalysts with the BPE and GG results, Pd/C is much more active in cleaving
2
convert the model compounds prompted us to more systematically the
investigate the importance of aliphatic OH groups close to the ether
ether
bond
in
PPE-H
than
in
PPE-OH.
Figure 3. Catalytic results for the reaction of (a) 2-phenylethyl phenyl ether and (b) 2-phenoxy-1-phenylethanol in methanol solvent over
Pd/C and/or Yb(III)-triflate at 200 °C for 4 h (the product quantification for the model lignin compound was based on the GC area
normalization method.)
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(a)
(b)
(c)
(d)
linkage as illustrated in Figure 4. Pd/C is very active in cleaving the
ether linkage in PPE-H by hydrogenolysis (Figure 4a). However, in
the presence of metal triflate, the Lewis acid will coordinate to the
O
O
O
O
H
H
H
OH
Me(OTf)n
H
H
H
H
Me(OTf)n
H
H
OH
H
Pd
Pd
Pd
Pd
Pd
4
6
ether bond, preventing its cleavage as evident from experiments
X= 78.2 %
X= 53.7%
X= 37.1%
X= 75.3%
Figure 4. Proposed mechanism for the cleavage of β-O-4 ether with BPE and PPE-H as the model reactant (Figure 4b). The
linkage using (a) PPE-H by Pd/C; (b) PPE-H by Pd/C+ metal triflate; preference of ether linkages that coordinate to the metal triflates
(
c) PPE-OH by Pd/C and (d) PPE-OH by Pd/C + metal triflate. (
X
=
over the metal surface results in a decreased rate of ether bond
cleavage. When PPE-OH is the reactant, the ether bond conversion
rate is slower due to the competitive adsorption of the α-OH and
ether bonds on the Pd active sites (Figure 4c). We argue that this
competitive adsorption for sites on the metal surface is partially
suppressed by the strong coordination of the metal triflate to the α-
OH group (Figure 4d). This interaction increases the ether bond
cleavage rate, as it contributes to weakening of the ether linkage.
This aspect of the mechanism also explains why addition of metal
triflate significantly increases the lignin monomers yield during
sawdust wood upgrading.
ether bond conversion).
As expected, the amount of products derived from the phenyl and
benzyl parts of the reactant were very similar. Yb(III)-triflate alone
did not convert PPE- H, in keeping with the result obtained with
BPE. On the other hand, the metal triflate is very active in ether
bond cleavage of PPE-OH due to the presence of the α-OH group.
We observed only one product in this case, namely 1-phenyl-1,2-
dimethoxyethane deriving from the trans-etherification of the α-OH
group and ether linkage by methanol. No products from the phenyl
part were observed by GC-MS, indicating that phenolic
intermediates rapidly underwent secondary reactions. When Pd/C
was used in combination with Yb(III)-triflate, the conversion of the
PPE-H ether linkage decreased. The opposite was observed when
PPE-OH was used as substrate, that is, Yb(III)-triflate promoted the
Pd-catalyzed ether bond cleavage.
Different metal triflates
In order to explore whether Yb can be replaced by the other
less scarce metals, divalent (Ni and Cu), trivalent (Al, Sc and La) and
tetravalent (Hf) triflates were evaluated for woody biomass
conversion. All of the tested metal triflates significantly increase
Based on these model lignin compound investigations, we
propose a reaction mechanism for the cleavage of the β-O-4 ether
Table 2. Influence of metal on the tandem-catalyzed reductive depolymerization of lignocellulosic biomass in methanol at 180 °C.
Lignin monomers yield
wt% (mg)
Methylated
C5 sugars
Total
yield
(%)
Residue mass
Entry
Catalyst
Co-catalyst
a
(
mg)
(mg)
1
h
2 h
1
2
3
4
5
6
7
8
9
5wt% Pd/C
5wt% Pd/C
5wt% Pd/C
5wt% Pd/C
5wt% Pd/C
5wt% Pd/C
5wt% Pd/C
5wt% Pd/C
5wt% Pd/C
5wt% Pd/C
5wt% Pd/C
-
1790
1340
1360
1190
1360
1250
1390
1110
1690
1540
1470
10
32
33
42
39
37
32
40
14
20
9
14 (64)
37 (176)
34 (163)
45 (212)
43 (203)
39 (187)
36 (171)
42 (200)
22 (104)
28 (134)
17 (82)
6
93
Ni(II)-triflate
Cu(II)-triflate
Al(III)-triflate
Yb(III)-triflate
Sc(III)-triflate
La(III)-triflate
Hf(IV)-triflate
194
162
352
205
259
190
345
31
86
84
88
88
85
88
83
91
87
83
ZnCl
AlCl
Yb(III)-triflate
2
1
1
0
1
3
64
b
106
Conditions: 2000 mg extracted birch wood sawdust (125-300 μm), 200 mg 5 wt% Pd/C, 0.0322 mmol co-catalyst, 40 ml methanol, 30 bar
a
b
2 2 2
H , stirring speed: 500 rpm. mass of the residue excluding Pd/C catalyst and moisture. 30 bar N was added instead of H .
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EDGE ARTICLE
Table 3. Lignin monomers yield after reaction in methanol at 180 °C lower lignin monomer yield (Table 2 entry 11). This result shows
over the Pd/C and Al(III)-triflate catalysts as a function of the Pd/Al
molar ratio.
that transfer hydrogenation reactions are not effective in the
current process.
Lignin monomers
Al(III)-
triflate
Residue
Pd/Al molar
ratio
yield wt%
(a)
0
1 h
2
h
Entry
6
5
4
3
2
1
0
0
0
0
0
0
0
Mass (mg)
a
(
mg)
0
h
1 h
2 h
h
1
2
3
4
No triflates
0
1790
1645
1180
828
N/A
12
10
15
39
46
42
14
23
44
48
41
9
3
5
15
30
45
23
1.5
1
43
5
156
45
a
Note: heating time (30 min), the amount of 5 wt% Pd/C is fixed as 200 mg
(
0.0940 mmol Pd).
the C9 lignin monomers yield as compared with the Pd/C-only
reference (Table 2 entry 1). The promoting effect is higher for tri-
and tetravalent metal triflates than for divalent metals, which is
most likely related to the increasing Lewis acidity with increasing
cation charge of the transition metal. Quantitative determination of
the Lewis acidity of metal triflates is challenging, especially when a
coordinating solvent is involved. Lawton ranked the Lewis acidity of
metal triflates on the basis of pyridine IR in an ionic liquid
0
0.3
0.75
1.5
3.0
4.5
Pd/Al molar ratio
(
b)
PG-H
PS-H
PG-OH
PS-OH
PG-OCH3
PS-OCH3
3
2
2
00
50
00
Others
(
[bmim][OTf]) and found that trivalent (Al, Sc, La, In) and tetravalent
Zr and Hf) metal triflates are much stronger Lewis acids than
(
4
7
150
00
divalent metal triflates.
These Lewis acidity differences
qualitatively explain the activity differences in woody biomass
conversion. Among the metal triflates tested here, Al(III)-triflate
and Hf(IV)-triflate gave the highest lignin monomers yield, slightly
higher than the monomers yield obtained with Yb(III)-triflate (Table
1
5
0
0
1
). Concomitantly, the methylated C5 sugar yield with these two
metal triflates was higher, showing a higher degree of removal of
hemi-cellulose from the biomass matrix. In lignocellulose, lignin is
typically bound to hemi-cellulose. High monomer yield was already
obtained after 1 h reaction. Prolonging the reaction time to 2 h
resulted only in a slightly increase of the monomer yield. We argue
that this is due to rapid conversion of the predominant β-O-4 ether
bonds, followed by slower cleavage of other ether bonds. As the β-
O-4 ether bonds are the most abundant lignin intralinkages, the
maximum lignin monomer yield should mainly relate to the β-O-4
ether bond content of the woody biomass. Conventional Lewis
0
.3
0.75
1.5
3.0
4.5
Pd/Al molar ratio
Figure 5. Influence of Pd/Al molar ratio on (a) lignin monomers yield
different reaction times) and (b) product distribution after reaction
in methanol for 2 h. Conditions: 2000 mg birch wood, 15 mg
0.0316 mmol) Al(III)-triflate, 40 ml methanol, 180 °C, 30 bar H
(
(
2
.
Optimization catalyst ratio in tandem Pd/C-Al(III)-triflate system
acids such as AlCl
3 2
and ZnCl were much less effective in
Using the optimal Al(III)-triflate catalyst, we then varied the ratio
between the Pd/C metal and Lewis acid functions. In the first set of
experiments, we fixed the amount of 5 wt% Pd/C to 200 mg (0.0940
mmol Pd) and varied the amount of Al(III)-triflate between 5 mg
and 45 mg so as to obtain molar Pd/Al ratios of 1, 2, 3 and 9. At the
deconstructing lignocellulose. Hydrolysis of these Lewis acids by
water generated in etherification and hydrogenolysis reactions will
likely contribute to the lower efficiency of these Lewis acids. Based
on these results, the preferred Lewis acid catalyst is Al(III)-triflate.
Replacing the hydrogen atmosphere by nitrogen resulted in a much
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highest Pd/Al ratio (5 mg Al(III)-triflate, Table 3, entry 2), the of 3 only 6 compounds were observed in such amount. The larger
monomers yield is significantly higher than the Pd-only case. number of products is the result of etherification of different
Increasing the amount of Al(III)-triflate to 15 mg (Pd/Al ratio = 3, hydroxyl groups, among which the phenolic OH groups, due to the
Table 3 entry 3) results in a strong increase of the lignin monomers high amount of metal triflate. In addition, some aliphatic ketones
yield to 44 wt% after reaction at 180 °C for 2 h. At Pd/Al ratio = 2 such as 2,5-hexanedione and esters such as the methyl ester of
(
Table 3, entry 4), the same monomers yield was already reached levulinic acid were observed in higher amounts than in the other
upon heating to 180 °C. After 2 h, the monomers yield amounted to experiments. This points to more extensive degradation of cellulose
8 wt%. At the highest Pd/Al ratio of 1 (Table 3, entry 5), the in this case at low Pd/Al ratio. Consistent with this, only 156 mg
4
highest lignin monomers yield of 46 wt% was also reached during solid residue remained after 2 h reaction, showing that in this case
heating and it slightly decreased when prolonging the reaction not only hemi-cellulose but also a large part of cellulose was
time. For longer reaction times, a larger variety of aromatic converted. Because the presence of these additional products is
products were obtained. By GC-MS, we could detect 12 monomeric undesired as their separation is not straightforward, the amount of
compounds in amounts greater than 2 mg, whereas at a Pd/Al ratio metal triflates should be limited during the reaction.
Scheme 1. Plausible scenario for the tandem catalyzed reductive depolymerization of wood lignin into phenolic monomers over Pd/C and
Al(III)-triflate at different Pd/Al ratios.
Using an amount of 15 mg Al(III)-triflate (0.0316 mmol), which is
PG-OH (182 g/mol) GG (320 g/mol)
0
sufficient to break the lignin-carbohydrate intralinkages but small
0
0
1
3
4
.3
.75
.5
.0
.5
enough to prevent release and degradation of glucose, we then
varied the amount of Pd/C from 0 mg to 300 mg (Pd/Al ratios from
0
to 4.5). The product distribution and molecular weight
distribution of the product mixture are shown in Figure 5 and Figure
, respectively. Without Pd/C, the amount of lignin monomers is
6
low (Figure 5a) and the product mixture contains mainly dimers and
oligomers, as evident from the gel permeation chromatogram
shown in Figure 6. Addition of a small amount of Pd/C (20 mg, Pd/Al
ratio= 0.3) was already effective in converting most of these
oligomers and a significant fraction of the dimers to monomers
1
00
1000
10000
(
Figure 5b and Figure 6). The lignin monomers yield for Pd/Al = 0.3
was 55 wt% after reaction at 180 °C for 2 h. Two products were
obtained in high yield, namely 160 mg PS-OCH and 40 mg PG-OCH
Molecular weight (g/mol)
3
3
Figure 6. Influence of the Pd/Al molar ratio on the molecular weight (the lignin content of the feedstock was 476 mg). Increasing the
distribution of the product mixture determined by GPC. Reaction amount of Pd/C led to enhanced depolymerization of the lignin
conditions: 2000 mg birch wood, 15 mg (0.0316 mmol) Al(III)- oligomers, as the broad peak due to higher-molecular-weight
2
triflate, 40 ml methanol, 180 °C, 30 bar H . The maxima observed in compounds decreased (Figure 6). This confirms that Pd/C is active
GPC chromatograms of solutions of PG-OH (monomer model) and in cleaving ether bonds that cannot be cleaved by metal triflates. It
GG (dimer model) are indicated.
is also seen that the amount of dimers does not change appreciably
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ARTICLE
for Pd/Al > 0.3, indicative of the recalcitrance of the C-C ether bonds. The model compound results show how the positive
intralinkages in these dimers. These C-C intralinkages are either effect of metal triflates in the presence of α-OH groups changes to a
from the native lignin structure, or retained after the cleavage of negative effect once these hydroxyl groups are removed. A time
2
0, 48, 49 49
the recalcitrant ether linkages such as β-5 and β-β.
and this course study for the conversion of the PPE-OH model compound in
does not contribute to monomers formation. Thus, the use of very the tandem Pd/C and Yb(III)-triflate system confirms that the α-OH
small amounts of Pd/C and Al(III)-triflate is sufficient to reach group are completely removed during heating to 200 °C (t = 30
maximum lignin monomers yield. With increasing Pd/Al ratio, the min) before ether bond cleavage commences (Figure 7).
3 3
product distribution shifts from PS-OCH and PG-OCH to PS-OH, Accordingly, the amount of Pd/C should also be limited. Optimum
PG-OH, PS-H, and PG-H. Moreover, despite enhanced Pd/Al ratios are in the 0.3-3 range.
depolymerization of oligomers, the lignin monomers yield is seen to
decrease with increasing Pd/Al ratio (Figure 5). These two trends
Thus, the role of the metal triflates as a homogeneous catalyst
are caused by competitive reactions catalyzed by Pd/C and Al(III)- is two-fold. Metal triflates enhance delignification of the solid
3
0
triflate as shown in Figure 1. Pd/C catalyzes the removal of γ-OH biomass as demonstrated before, and they rapidly decrease the
groups by hydrogenolysis, while Al(III)-triflate catalyzes the molecular weight of the released lignin backbone by cleaving β-O-4
etherification of OH groups by methanol solvent. Therefore, the ether linkages. The smaller fragments are more accessible for Pd/C.
increasing hydrodeoxygenation activity and associated removal of Pd/C is also active in cleaving more recalcitrant ether linkages as
γ-OH groups when more Pd/C is added results in the different well as removing α-OH and γ-OH groups. In this way, the ether
product mixture. Consistent with this, comparison of the data at linkages of the lignin structure can be effectively cleaved by metal
short and long reaction times (Figure 5a) shows that at the initial triflate via trans-etherification, followed by Pd/C catalyzed
stages of the reaction a smaller amount of Pd/C is preferred. Using hydrogenolysis reactions.
too much Pd/C appears to hinder the catalytic action of Al(III)-
triflate, which can be explained in two manners. First, the high
activity of Pd/C in the removal of α-OH and γ-OH groups close to
ether bonds decreases the ability of metal triflates to cleave ether
bonds. Second, part of the metal triflate might be immobilized on
the activated carbon, for instance, by interaction with surface
hydroxyl groups. Such interactions have also been proposed to be
100
80
60
2
relevant in the ZnCl and Pd/C catalyzed depolymerization of
1
9, 45
Benzyl derivatives
Phenyl derivatives
lignocellulose.
4
0
0
0
Scheme 1 illustrates a plausible scenario for the influence of the
ratio of the metal and Lewis acid functionalities in the tandem
reductive depolymerization of wood lignin. Al(III)-triflate is active in
catalysing the cleavage of ether and ester interlinkages between
lignin and carbohydrates, facilitating the release of lignin from the
lignocellulosic matrix. The metal triflate can also selectively cleave
β-O-4 lignin ether bonds, which contain both α-OH and γ-OH
groups. The β-O-4 ether linkage is the dominant lignin intralinkage,
usually making up between ca. 43 % and ca. 65 % of all intralinkages
2
0
50
100
150
200
250
Time (min)
Figure 7. Time course study for the conversion of the PPE-OH model
compound in the tandem Pd/C and Yb(III)-triflate system at 200 °C
for 4 h.
2
3
in soft- and hardwood, respectively. Metal triflates are more
active in cleavage of these bonds than Pd/C. On the other hand,
metal triflates cannot cleave ether bonds in the absence of an α-OH
group. Such ether linkages (α-O-4, 4-O-5 and β-β) are also common
in lignin, accounting for ca. 12-16 % and ca. 13-22 % in soft- and
Recycling metal triflates
The literature contains many instances in which metal triflates
50, 51
could be recycled without loss of activity.
We evaluated the
2
3
hardwood, respectively. In the tandem system, Pd/C is able to
cleave these bonds. This specificity for cleaving ether bonds
explains the benefit of combining metal triflates and Pd/C in the
reductive depolymerization of lignin fragments released from
biomass. The amount of metal triflate should be kept relatively low
to limit the release of glucose from cellulose and its further
degradation. The results show that using a small amount of metal
triflates does not impede its effectiveness in releasing lignin from
biomass and aiding in the depolymerization of the released lignin
fragments. The Pd/Al ratio is therefore a key parameter controlling
the activity and product selectivity. When the Pd/Al ratio is too high
possibility to recycle Al(III)-triflate in the reductive fractionation of
woody biomass. A partitioning work-up using ethyl acetate and
water solvent was applied in order to recover Al(III)-triflate from
the liquid products (Scheme 2). As metal triflates are better soluble
in water than in common organic solvents, they can be extracted
into the aqueous phase and recovered by evaporating water. Table
4
compares lignin monomer yields of a reference experiment and
recycle tests in which Al(III)-triflate was recovered twice and then
combined with fresh Pd/C catalyst, methanol and woody biomass.
The amount of recovered Al(III)-triflate could not be determined as
the solid also contained released sugars. The lignin monomer yield
(Pd/Al ratio > 3-4.5), the monomers yield is decreased, because
Pd/C removes α-OH groups that enable the metal triflates to cleave
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was 55 wt% after reaction at 180 °C for 2 h using the fresh catalysts. one-step experiment (Table 5). These initial experiments suggest
After recycling, the yield after 2 h decreases to 49 wt% and 37 wt%,
that a two-reactor approach is possible. In the first reactor, solid
biomass is contacted with a solution of metal triflates in methanol.
After addition of H , the released fragments are then taken to a
2
second reactor containing the Pd/C catalyst. The decreased contact
Table 4. Recycling of metal triflates in reductive fractionation of
time of the metal triflates with the Pd/C catalyst in the second
reactor will also decrease the loss of metal triflate due to
adsorption on the activated carbon surface.
a
woody biomass.
Lignin monomer yield (wt%)
Pd/Al molar
Entry
ratio
Scale-up of reduction fractionation process
0
h
1 h
2 h
1
2
3
2
45
34
23
47
43
29
52
49
37
As a first step towards scale-up of this process, we employed a 4 L
high-pressure autoclave to convert a larger amount of biomass. For
this purpose, we obtained from a local timber manufacturer
sawdust from oak hardwood and Douglas fir softwood. The sawdust
was dried, sieved to obtain particles in two size ranges (0 - 300 µm
and 300 -1000 µm), and used without pre-treatment in the
reductive fractionation process. The autoclave was charged with 2 L
methanol, 100 g sawdust, 1.5 g Pd/C and 0.75 g Al(III)-triflate and
st
b
1 recycling
nd
b
2
recycling
a
Conditions: 2000 mg birch wood, 200 mg fresh Pd/C catalyst, 40
mL methanol, 180 °C, 30 bar H
extraction in water (details see ref. ).
b
2
.
Metal triflates recycled by
36
2
Table 5. Lignin monomer and solid residue yields obtained from then pressurized to 30 bar with H . The lignin monomer yields
reactions at 180 °C for 2 h in methanol using birch wood.
Lignin monomer yield (wt%)
0.5 h 1 h 2 h
obtained after heating to 180 °C and prolonged reaction at this
temperature are listed in Table 6 (detailed composition of oak and
Douglas fir wood in the experimental section, Table 7). For oak
hardwood, the lignin monomer yield was already 40 wt% after
heating to the reaction temperature and only increased slightly
after 4 h. Nearly similar results were obtained with the smaller
particles, demonstrating the effectiveness of the approach. Similar
Entry
Catalyst
0
h
One-pot
Yb(III)-triflate + Pd/C
Yb(III)-triflate
Pd/C
21
n.a.
37
31
n.a.
36
37
n.a.
37
43
5
a
Two-step
36
to the results obtained for Scotch pine softwood, conversion of
36
Douglas fir softwood led to lower lignin monomer yield (17 wt%).
a
Reactions performed in two consecutive steps. In the first step, 20 The result that the lignin monomer yield did not increase much
mg Yb(III)-triflate was used as catalyst to convert 2000 mg birch when the reaction was prolonged is likely due to the relatively long
wood. After reaction, the reactor mixture was filtered, and the heating time (1 h) of the large autoclave. These initial results
filtrate was subjected to a second reaction in the presence of 200
evidence that the woody biomass reductive fractionation process
2
mg Pd/C and pressurizing to 30 bar H .
can be easily scaled up. An important aspect is that promising lignin
monomer yield was obtained without fine-milling and extractive
pre-treatment of the wood sawdust. This greatly contributes to
reducing the number of steps and the overall cost of the process.
indicating that not all Al(III)-triflate could be recovered by our work-
up procedure. Incomplete recovery may be due to inefficient
extraction into the aqueous phase and adsorption of metal triflate
on activated carbon support of the Pd catalyst. To verify the latter,
we determined by elemental analysis the uptake of Al(III)-triflate by
Pd/C in methanol (15 mg in 40 mL). In this way, we found that that
about 20-30 % of the metal triflate was adsorbed by the Pd/C
catalyst. Although these experiments show the principle possibility
of recovering and reusing metal triflates, the interaction of the
metal triflates with activated carbon leads to partial loss of the
homogeneous co-catalyst.
Table 6. Scale-up of reduction fractionation of oak and Douglas fir
sawdust at 180 °C for 4 h (Pd/Al molar ratio = 0.75).
Lignin monomer yield (wt%)
Entry
Feedstock
a
b
b
2 h
c
0
h
1 h
4 h
1
2
Oak (300-1000 μm)
Oak (0-300 μm)
40
36
40
39
41
39
46
40
Recycling the Pd/C catalyst is a challenge because it ends up in
the solid cellulose-rich residue. Possible solutions such as the use of
Douglas fir (300-
1000 μm)
3
14
17
16
17
18, 48
a magnetic catalyst
or embedding the metal function in a cage
2
9
a
b
for separation have been proposed. An alternative solution is to
Heating time (1 h). Samples were taken during reaction via a
c
separate the lignin release step and hydrogenolysis steps. Initial
experiments show that nearly similar yield is obtained in this way
sampling valve. samples were taken after the reactor was cooled
down to room temperature. Conditions: 100 g dry wood substrate,
(
Table 5). We first treated birch wood with Yb(III)-triflate at 180 °C
in methanol for 2 h. The C9 lignin monomer yield after 2 h was only
wt%. The solid residue was then removed by filtration and the
filtrate was subjected to a second reaction in the presence of Pd/C
and H , leading to nearly similar C9 lignin monomer yield as in the
1
2
.5 g 5 wt% Pd/C, 0.75 g Al (III)-triflate, 2 L MeOH, 30 bar H , 180 °C,
stirring speed: 800 rpm.
5
Conclusions
2
1
0 | J. Name., 2012, 00, 1-3
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We describe a tandem catalyst system comprising Pd/C and metal obtained from houtzagerij Menten (www.houtzagerijmenten.nl).
triflates to remove lignin fragments from lignocellulosic biomass Detailed composition of these woods is provided in Table 7). 5 wt%
and convert these fragments in good yield to alkylmethoxyphenols, Pd/C and all the metal triflates were purchased from Sigma Aldrich.
leaving cellulose as a solid residue. Under optimized conditions (20 4-n-propanolguaiacol (PG-OH) was purchased from TCI. 4-n-
mg Pd/C, 15 mg Al(III)-triflate, 180 °C, 2 h), the monomers yield propylsyringol (PS-H) and 4-n-methoxy propyl syringol (PS-OCH
obtainable from birch wood is 55 wt% (based on a lignin content of were recovered from the reaction mixture (T = 180 °C, t = 2 h over
3.8 wt% in the biomass). This optimized lignin monomers yield is Pd/C and Yb(III)-triflate catalyst using birch wood sawdust). The
3
)
2
3
6
close to the theoretical maximum value, which is based on the reaction mixture was first subjected to a partitioning work-up to
typical ether content of lignin in lignocellulosic biomass in the 67 %- removed sugars and then separated using column chromatography
20
76 %. The solid residue is mainly composed of cellulose with small by choosing 1:9 (v/v) hexane/diethyl ether as eluent. These
amounts of hemi-cellulose and lignin (Table 1). The metal triflate is phenolic compounds were used to determine the weight response
effective in breaking the bonds between lignin and the factors relative to n-dodecane. Extra-dry absolute methanol was
carbohydrate part of biomass, which are typically comprised of purchased from Biosolve. Benzyl phenyl ether and 2-phenoxy-1-
ether and ester linkages. Lignin dimer model compound phenylethanol
were
purchased
from
Sigma
Aldrich.
experiments revealed that both metal triflates and Pd/C contribute Guaiacylglycerol-β-guaiacyl ether (>97 %) was purchased from TCI.
to the further depolymerization of the resulting fragments into 2-phenylethyl phenyl ether was purchased from Santa Cruz
aromatic monomers. Metal triflates catalyst can cleave (β-O-4) Biotechnology. All the other commercial chemicals are analytical
ether linkages within the aromatic lignin structure due to their grade and used without further purification.
interaction with vicinal OH groups that active the ether bond. Metal
triflates are more active for this reaction than Pd/C. Pd/C is able to Feedstock pretreatment
cleave a wider range of ether linkages such as α-O-4, 4-O-5 and β-β.
The Pd-to-metal triflate ratio is an important parameter to the Birch wood was first ball milled to small particles and then sieved to
overall efficiency of the reductive fraction process of woody obtain particle with sizes between 150 μm and 300 μm. The wood
3
biomass. Lower Pd/Al ratios resulted in PS/PG-OCH as dominant sawdust was subjected to Soxhlet extraction with water followed by
reaction products, while higher ratios led to formation of PS/PG-OH ethanol to remove extractives. After extraction, the feedstock
and PS/PG-H products. It is important to use a small amount of particles were dried at 105 °C overnight for catalytic reaction.
Pd/C, as otherwise rapid hydrodeoxygenation of aliphatic OH
groups occurs, which limits the synergetic action of metal triflates. Catalytic activity measurements
In addition to deoxygenation, Pd/C also saturates C-C double bonds
of small lignin fragments, which are formed from fragmentation In a typical reaction experiment, the autoclave was charged with a
reactions catalyzed by metal triflate. These insights allowed suspension of 2.0 g feedstock, 200 mg Pd/C (5 wt% Pd loading), 15
decreasing the amount of metal and metal triflate catalysts and the mg Al(III)-triflate and 30 μl n-dodecane internal standard in 40 ml
reaction temperature and time. A range of metal triflates are methanol. The reactor was sealed and purged with nitrogen and
effective with Al(III)-triflate providing the most promising results. hydrogen several times sequentially. After leak testing, the pressure
We also demonstrated that the process can be easily scaled up to brought to 30 bar with hydrogen and the reaction mixture was
convert 100 g woody biomass with nearly similar lignin monomer heated to reaction temperature under continuous stirring at 500
yield as obtained in a small-scale experiments. The mild conditions, rpm within 0.5 h. A sampling valve was installed, which allows
short reaction times and use of small amounts of catalyst make this taking liquid samples during the reaction. In some cases, about 0.4
tandem process very promising as a method to remove and ml solutions were taken out from autoclave during reaction and
upgrade lignin from second-generation biomass. Prospects of the analysed directly by GC-MS. After reaction, the heating oven was
resulting alkylmethoxyphenols are their use in the fragrance removed and the reactor was cooled to room temperature in an
industry (e.g., dihydroeugenol), their upgrading to alkylbenzenes or ice-water bath. After releasing the pressure, the autoclave was
alkylphenols by catalytic hydrodeoxygenation, or combining opened and the mixture was collected and combined with the
demethoxylation and dealkylation to obtain phenol, a valuable base solution obtained from washing the autoclave with methanol. The
chemical.
collected reaction mixture was filtered by using a filter crucible
under vacuum. The carbohydrate and Pd/C solid residue was left in
air to dry for one day to determine the amount of all solid residue.
In some cases, the solid residue was further subjected to a two-
stage acid hydrolysis to determine the (hemi-)cellulose, lignin, and
ash composition.
Experimental
Chemicals and materials
Birch wood was obtained from the Energy Research Center of
Netherlands (ECN). Oak and Douglas fir wood sawdust were
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Table 7. Detailed composition analysis results of the lignocellulosic biomass.
Woody
Klason
lignin
Acid soluble
Total sugars
56.75
Glucose Xylose Mannose Arabinose Galactose Rhamnose
Extractives Ash
biomass
lignin
2.98
0.50
oak
Douglas
fir
37.55
41.88
15.65
3.25
1.83
0.42
1.06
0.98
2.00
0.32
0.11
23.30
29.29
8.80
5.77
0.32
0.16
60. 45
12.15
A mini-autoclave with a volume of 12 ml was used for reactions small amounts), the RF of PS-OCH
with model compound. In a typical run 50 mg model compound yield of lignin monomers was calculated using following equation:
BPE and GG), 10 mg Pd/C, 5 mg Yb(III)-triflate and 30 μl n-
3
was used for quantification. The
(
dodecane internal standard were added to 5 ml methanol. After Yield of monomers (wt%) = (weight of monomers) / (weight of
sealing, purging and checking for leaks, the autoclave was heated to starting feedstock × lignin weight percentage in feedstock)×100%
160 °C and maintained at that temperature for 2 h. After reaction,
an aliquot of 1 ml was taken from the reaction mixture and directly The product quantification for the model lignin compound reactions
analysed by GC-MS without dilution following filtration with a 0.45 (BPE, PPE-H and PPE-OH) was based on the GC area normalization
μm syringe filter. In experiments involving PPE-H and PPE-OH as method. In the case of GG model lignin compound reaction, the
model compounds, 30 mg reactant was used, and the reaction experimentally determined weight response factor for guaiacol
temperature and time were 200 °C and 4 h, respectively.
relative to n-dodecane was used for product quantification.
Metal-triflate recycling
Lignocellulosic Composition Analysis
The reaction mixture was treated by the work-up procedure All the wood samples were delivered to Celignis Limited Nexus
3
6
mentioned in our previous work (ref. ).. From the liquid fraction,
methanol was removed from the liquid products (methanol-soluble
lignin and sugar products) by evaporation. The residue was
separated by extraction with water and ethyl acetate. The water
soluble fraction will contain most of the initial metal triflate. Water
was removed from this solution by rotary evaporation under
vacuum. The resulting product was re-dissolved in 40 ml methanol
and used in a new reaction under standard conditions.
Innovation
Centre,
University
of
Limerick,
Ireland
(
www.celignis.com) for analysis. All the samples were analysed in
duplicate and take the mean value. The detailed analysis procedure
is shown as follows.
Acid insoluble lignin (Klason lignin)
Weight 300 mg of the sample into a pressure tube, then adding 72
%
2 4
H SO (3 ml) by means of an automatic titrator. The sample and
Scale-up of reductive fractionation
acid were then mixed thoroughly using a glass rod and the pressure
tube was transferred to a water bath, maintained at 30 °C, for a
period of 1 h. The sample/acid mixture was stirred every 5 min.
Subsequently 84 ml of water was added to achieve a 4 % acid
concentration and the pressure tube sealed. All of the pressure
tubes of an analytical batch (between 16 and 20) were then
transferred to an autoclave along with two/three pressure tubes
each containing 10 ml of a solution of a known sugar composition
Oak and Douglas fir wood sawdust were used to explore conversion
of larger amounts of woody biomass. The sawdust samples were
dried overnight in an oven at 105 °C and sieved to obtain particles
with sizes between 0-300 μm and 300-1000 μm for oak, and
between 300-1000 μm for Douglas fir. No extraction treatment was
employed for these samples.
2 4
to which 348 μl of 72% H SO was added. These additional tubes
Product analysis and characterization
are referred to as the sugar recovery solutions and were used to
determine the sugar losses associated with this secondary
hydrolysis step (121 °C for 60 min). The pressure tubes were
removed from the autoclave once the temperature fell below 80 °C
and allowed to cool to room temperature at which point the
hydrolysates were filtered (using vacuum suction), through filter
crucibles of known weight, and the resulting filtrate was stored.
Residual solids were washed from the tube using deionised water
until all the residue resided on the filter crucible. This was then
dried overnight at 105 °C and weighed to determine the Acid
Insoluble Residue (AIR) content. The filter crucible was then ashed
The liquid phase products were analysed by a Shimadzu 2000 GC-
MS system equipped with a RTX–1701 column (60 m × 0.25 mm ×
0.25 μm) and a flame ionization detector (FID) together with a mass
spectrometer (MS) detector. For lignin monomers, the product
peaks were identified using MS and by authentication with
reference compounds. All the quantitative analyses of liquid phase
products were based on GC-FID. Experimentally determined weight
response factors (RF) were used for calculation relative to
n-
dodecane, which was the internal standard. For some of the
commercially unavailable compounds (side-products obtained in
1
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6GC02962K
ARTICLE
to determine the acid-insoluble ash (AIA) content. The Klason lignin 4. K. Barta and P. C. Ford, Acc. Chem. Res., 2014, 47, 1503-1512.
content was determined as AIR minus AIA. The sum of Klason and
5
.
C. Z. Li, M. Y. Zheng, A. Q. Wang and T. Zhang, Energy Environ.
Sci., 2012, , 6383-6390.
acid soluble lignin was used for lignin monomer yield calculation.
5
Acid soluble lignin (ASL)
6. P. F. H. Harmsen, W. J. J. Huijgen, L. M. B. López and R. R. C.
Bakker, Literature Review of Physical and Chemical
Pretreatment Processes for Lignocellulosic Biomass, ECN-E--10-
The hydrolysate was placed in a 1 cm path-length (3 ml volume)
quartz cuvette and diluted with water until the UV-absorbance was
within a linear region. The spectrum of the sample was collected in
0
13, Wageningen UR, Food & Biobased Research, 2010.
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8
.
.
A. T. W. M. Hendriks and G. Zeeman, Bioresour. Technol., 2009,
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transmission mode using
a HP Agilent 8452A diode array
1
spectrophotometer. The absorbance at 205 nm was used to
-
determine the ASL content using an absorptivity constant of 110 M
J. L. Xu, X. M. Zhang and J. J. Cheng, Bioresour. Technol., 2012,
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1
-1
cm
1
Sugar analysis
9. V. Chaturvedi and P. Verma, 3 Biotech, 2013, 3, 415-431.
1
1
1
0. A. Rahimi, A. Ulbrich, J. J. Coon and S. S. Stahl, Nature, 2014,
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The hydrolysates obtained from the two-stage hydrolysis of the
carbohydrate residue were diluted 5 times with a solution
containing a known concentration of the internal standard fucose. A
series of sugars including arabinose, galactose, rhamnose, glucose,
xylose, and mannose were used for sugar quantification. Relative
response factors were determined via sugar standard samples
injected at regular intervals in the analytical sequence. Detailed
5
1. F. P. Bouxin, A. McVeigh, F. Tran, N. J. Westwood, M. C. Jarvis
and S. D. Jackson, Green Chem., 2015, 17, 1235-1242.
2. J.L. Wen, B.L. Xue, S.L. Sun and R.C. Sun, J. Chem. Technol.
Biotechnol., 2013, 88, 1663-1671.
3
7
description of the analysis procedure can be found in ref
http://www.celignis.com/package.php?value=1.
and
1
1
3. X. Huang, T. I. Korányi, M. D. Boot and E. J. M. Hensen, Green
Chem., 2015, 17, 4941-4950.
4. X. M. Zhang, P. Murria, Y. Jiang, W. H. Xiao, H. I. Kenttamaa, M.
Gel permeation chromatography (GPC)
M. Abu-Omar and N. S. Mosier, Green Chem., 2016, 18, 5219-
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229.
GPC analyses were performed using a Shimadzu apparatus
equipped with two columns in series (Mixed-C and Mixed-D, 15. P. Sannigrahi, A. J. Ragauskas and S. J. Miller, Bioenergy Res.
,
2
008, 1, 205-214.
polymer laboratories) and a UV-Vis detector at 254 nm. The column
was calibrated with polystyrene standards. Analyses were carried
out at 25 °C using THF as eluent at a flow rate of 1 ml/min. For the
lignin residue analysis, the sample was prepared at a concentration
of 2 mg/ml. All samples were filtered using 0.45 μm filter
membrane prior to injection.
1
1
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Eindhoven Energy Institute in collaboration with the Knowledge and
Innovation Community InnoEnergy of the European Institute of
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Table of Content
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