One-step green synthesis of composition-tunable Pt-Cu alloy nanowire networks with high catalytic activity for 4-nitrophenol reduction

Article abstract of DOI:10.1039/C8DT03810D

Controlling the structure, morphology, and composition of noble metals is of great significance to improve the catalytic activity and stability of catalysts. Herein, we have successfully synthesized self-interconnecting Pt-Cu alloy nanowire networks (NWNs) with controllable compositions via the co-reduction of the metal precursors potassium chloroplatinate (K₂PtCl₆) and CuCl₂ with sodium borohydride (NaBH₄). Owing to the hydrogen bubbles formed by NaBH₄ hydrolysis and oxidation as a dynamic template, the facile strategy was carried out without any organic solvent, capping agent, polymer, or special experimental device, ensuring that the surfaces of NWNs were definitely “clean”. The performance of the as-prepared Pt-Cu alloy NWNs for the reduction of 4-NP was dramatically improved compared with that of pure Pt NWNs and the commercial Pt/C catalyst. Particularly, the PtCu NWNs with a Pt/Cu atomic ratio of 1?:?1 exhibited excellent catalytic activity and reusability for the reduction of toxic 4-NP. The reaction rate constant and activity factor of the PtCu NWNs reached 1.339 × 10^?2 s^?1 and 66.95 s^?1 g^?1, respectively, which were dramatically better than those of pure Pt NWNs (11.5-fold) and commercial Pt/C (13-fold). The superior catalytic activity and reusability can mainly be attributed to the clean surface, the synergistic effect of Cu and Pt atoms and the self-interconnecting nanowire network structure.

Full text of DOI:10.1039/C8DT03810D

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Reactivity improvement by phenolation of wheat
straw lignin isolated from a biorefinery process
Cite this:DOI: 10.1039/c8nj05016c
ab
b
a
a
b
Fangda Zhang,
Xiao Jiang,
Jian Lin,* Guangjie Zhao, Hou-min Chang*
b
and Hasan Jameel
Phenolation can effectively improve the reactivity of lignin by directly increasing the phenolic hydroxyl
groups and reactive sites for adhesive applications. Wheat straw residue isolated from an autohydrolysis/
refining/enzymatic hydrolysis process was phenolated to improve the reactivity of lignin. The optimum
phenolation conditions were determined to be a lignin/phenol (L/P) ratio of 1/3 with a 10% acid charge
at 120 1C for 2 hours. Phenolated lignin resulted in 32 wt% of phenol incorporated onto lignin and a
significant decrease in molecular weight. A comprehensive characterization indicated that all lignin
0
0
0
0
Received 1st November 2018,
Accepted 23rd December 2018
substructures (b-O-4 , b-5 /a-O-4 , b-b , ferulate, p-coumarate and p-benzoate) were reacted along
with a hydrolysis of over 50% of polysaccharides during phenolation, resulting in a decrease in aliphatic
hydroxyl groups and an increase in the phenolic hydroxyl groups. Phenol was incorporated onto both
the sidechains and aromatic nuclei. These improvements in lignin reactivity will enable it to be used in
adhesives with higher substitution rates.
DOI: 10.1039/c8nj05016c
rsc.li/njc
1
3
10
14
11,14
woodmeal, kraft lignin, soda lignin, organosolv lignin,
Introduction
10
15
hardwood biorefinery residue, and lignosulfonate. In early
With a growing concern about sustainability, great efforts have studies, it was concluded that phenolation occurred at the C
been made to produce valuable chemicals, fuels and materials position of the lignin sidechain accompanying the cleavage of
a
1
3,16
from bio-renewable biomass as alternatives to fossil fuel based aryl ether linkages.
Recently, Podschun et al. reported that
resources. Biorefinery is one of the most promising processes that phenolation of organosolv lignin occurred both at the C
a
1
1
has gained increasing attention owing to its simplicity and position and at the C
g
position. They further investigated
effectiveness for converting cellulose and hemicellulose into the relationship of raw lignin structural properties and phenol
1,2
biosugars for further upgrading into higher value products.
incorporation and concluded that the amount of phenol inte-
However, lignin enriched biorefinery residues are generated as grated onto lignin (or the increase of reactive sites) depended
1
4
byproducts and are burnt for energy recovery. The pursuit of a on the quantity of aliphatic hydroxyl groups in the raw lignin.
value-added application of lignin-derived profit streams is essen- Our group also investigated the phenolation of pine kraft lignin
3,4
10
tial to improve the overall biorefinery economics. Lignin is a and sweetgum biorefinery lignin. The incorporation of phenol
cross-linked aromatic polymer with phenylpropane units linked through a phenolation reaction onto pine kraft lignin and
by carbon–carbon or ether bonds with both phenolic and alipha- sweetgum biorefinery lignin were 30 wt% and 60 wt%, respec-
1
0
tic hydroxyl groups. Due to the structural similarity, researchers tively, with lignin fragmentation. Furthermore, we found
have investigated the potential of the lignins from biorefinery several additional pathways by which phenol can be introduced
processes as a phenol substituent in phenol formaldehyde onto lignin. It can be concluded that phenol is incorporated
5–8
adhesives. Compared to phenol, lignin has fewer reactive sites into lignin through multiple pathways depending upon the
9
in the aromatic nuclei to form cross-links. Thus, the replacement lignin substructures in terms of biomass species, isolation pro-
level of phenol by lignin is limited because the adhesive bonding cesses and chemical structures. Moreover, phenolation can
performance decreases when lignin usage is increased.
be integrated with a conventional phenol formaldehyde pre-
Phenolation has been proven to be an effective approach paration process without having to isolate phenolated lignin.
1
0–13
to improve lignin’s reactivity.
It has been investigated on After phenolation, the acid will be neutralized and formalde-
hyde can be introduced into the same reactor for further resin
synthesis.
a
Beijing Key Laboratory of Wood Science and Engineering, Beijing Forestry University,
Beijing 100083, China. E-mail: linjian0702@bjfu.edu.cn
Wheat straw (Triticum aestivum L.), as a massive existing
agriculture residue, has gained great interest for its use as
renewable feedstock for biorefining. In the present study,
b
Department of Forest Biomaterials, North Carolina State University, Campus Box 8005,
Raleigh, North Carolina 27695, USA. E-mail: hchang@ncsu.edu
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we investigated the phenolation of wheat straw lignin-rich
residue isolated from a biorefinery process involving auto-
hydrolysis, mechanical refining and enzymatic hydrolysis. It
is well known that lignin structures differ greatly in wood and
non-wood biomass; non-wood lignin contains p-hydroxyphenyl-
propane (H) units as well as ferulate (FA), p-coumarate (pCA)
and p-benzoate (PB) units in addition to the normal guaiacyl-
(G) and syringyl-propane (S) units in hardwoods. Thus, pheno-
lation of wheat straw lignin may be different from phenolation
of hardwood lignin. The optimum conditions for phenolation
of wheat straw lignin including the lignin/phenol (L/P) ratio,
acid charge and reaction temperature were investigated with
regard to their effect on the phenolated lignin yield, molecular
weight and its solubility in methanol/acetone (1 : 6 v : v, a good
lignin solvent). The raw and phenolated lignins were fully
characterized to elaborate on the correlation between raw lignin
structures and phenol incorporation.
Fig. 1 Phenolated lignin yield under different phenolation conditions.
acid charge and reaction temperature on the phenolated lignin
yield were investigated, as shown in Fig. 1. It can clearly be seen
that the acid charge was very critical to both the yields of P-AER
and P-AER-S. The total yields of P-AER ranged from 94% to
Results and discussion
Chemical composition of AER
1
01% (based on AER) with a large amount of P-AER-I (42–51%)
The chemical composition of extractive-free wheat straw, the
autohydrolysis pulp (AH pulp) and the subsequent enzymatic
hydrolysis residue (AER) are shown in Table 1. The original
wheat straw contains 23.2% lignin, 63.9% polysaccharides, 12.9%
others. Autohydrolysis removed 83% (100 ꢀ 3.7 ꢁ 100/21.4) of
xylan and 31% (100 ꢀ 15.9 ꢁ 100/23.2) of lignin, whereas the
glucan was relatively stable as only around 14% was lost. These
at a 5% acid charge. A high acid charge (10%) improved the
yields of P-AER (112–123%) and P-AER-S (81–97%) and reduced
the yield of P-AER-I (26–31%). It can be concluded that the high
acid charge facilitated the phenol incorporation, lignin frag-
mentation as well as the hydrolysis of the polysaccharides. The
effect of the L/P ratio seemed to be insignificant when the
phenolation were performed at 110 1C with a 10% acid charge
as the yields of P-AER, P-AER-S and P-AER-I were almost
identical. However, the yields of P-AER and P-AER-S increased
with the increasing L/P ratio when the reaction temperature
was 120 1C at a 10% acid charge. This counterintuitive pheno-
menon was also observed for the phenolation of sweetgum
biorefinery lignin in our previous study. It might be because
the acid was diluted when more phenol was used since the
phenol acted as both the reactant and solvent and that acid
charge was based on the weight of lignin alone. As for the
reaction temperature, it was obvious that a higher tempe-
rature always assisted the phenolation reaction in terms of
increasing the yields of P-AER and P-AER-S and diminishing
that of P-AER-I. Concerning the objectives of phenolation,
the irrefutable optimum phenolation conditions for a wheat
straw biorefinery were an L/P ratio of 1/3 with a 10% acid
charge at 120 1C for 2 hours, since it gave the highest pheno-
lation yields (123% of P-AER and 97% of P-AER-S) and the
lowest yield of insoluble part (26% PAER-I). Thus, phenolated
lignin under the above conditions was selected for further
characterization.
1
7
results are consistent with our earlier results. It is also note-
worthy that autohydrolysis removes twice as much lignin during
the pretreatment from wheat straw as compared with hard-
1
0,17
woods.
This attributes to the differences in the raw lignin
structures in hardwood and non-wood biomass. Plausible expla-
1
7
nations were discussed in the literature. After the subsequent
refining and enzymatic hydrolysis, most of the polysaccharides
in wheat straw are removed (B88% of glucan, 97% of xylan)
resulting in a lignin-rich residue (AER, 20% yield based on the
raw wheat straw).
1
0
Effects of phenolation conditions
The phenolation process of AER includes two reactions, namely
the phenolation of lignin and the hydrolysis of polysaccharides
in AER. In other words, this research aims to maximize the
yields of total phenolated lignin (P-AER) and especially the
soluble fraction (P-AER-S) with the minimum amount of acid
and phenol. The effects of the lignin/phenol (L/P) ratio,
Table 1 Chemical composition of wheat straw, AH pulp and AER (based
on 100 g of OD wheat straw)
The chemical compositions of wheat straw, AER and
phenolated lignins are shown in Table 2. Only B21% of the
AER, namely AER-S, was soluble when it was extracted with a
methanol/acetone mixture. The soluble proportion is less com-
pared to what it is in the biorefinery lignin isolated from
a
b
c
d
e
Yield AIL ASL TL
Glucan Xylan MS TS
Other
WS
100
21.7 1.5
23.2 38.5
15.9 33.2
13.9 4.4
21.1 4.3 63.9 12.9
AH pulp 56.4 15.5 0.4
AER
3.7
0.7
0.8 37.7 2.8
nd 5.4 1.2
20.4 13.7 0.2
1
0
a
b
c
sweetgum (30%). The AER-S is composed of 90.4% lignin
with trace sugars (4.8%), whereas the AER-I (79% of AER)
Acid insoluble lignin/klason lignin. Acid soluble lignin. Total lignin.
d
e
Minor sugars. Total sugars; nd: not detected or below 0.1.
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Table 2 Chemical composition of wheat straw, AER and phenolated
lignins
a
b
c
d
e
AIL
ASL
TL
Glucan Xylan MS
TS
Others
WS
AER
AER-S
AER-I
21.7
67.1
89.2
61.3
1.5 23.2 38.5
0.8 67.9 21.8
21.1
3.5
1.3
3.4
nd
4.3
1
0.2
0.4
nd
0.8
63.9 12.9
26.3
4.8
5.8
4.8
8.9
8.0
5.6
1.2 90.4
0.8 62.1 25.2
0.1
2.8 42.4 46.6
3.3
29.0
0.1
P-AER-S 80.3 11.6 91.9
P-AER-I 39.6
0.4
47.8
a
b
c
Acid insoluble lignin/klason lignin. Acid soluble lignin. Total lignin.
d
e
Minor sugars. Total sugars; nd: not detected.
contains 62.1% lignin, 29.0% sugars and 6.6% ash. Since the
methanol/acetone solvent used is a good lignin solvent, the low
yield of AER-S indicates that the lignin left behind after the
extraction (AER-I) is of a much higher molecular weight and/or Fig. 2 Normalized GPC chromatograms of AER-S and P-AER-S.
linked to carbohydrates through chemical bonds as a lignin-
carbohydrate complex (LCC).
Molecular weight
The phenolation of AER resulted in the incorporation of a
Due to the solubility limitation, only the soluble fraction of
lignin can be characterized. The molecular weight distributions
of AER-S and P-AER-S were determined by GPC, as shown in
Fig. 2 and Table 4. The AER-S had a weight average molecular
substantial amount of phenol onto lignin and in the depoly-
1
0
merization of both lignin and carbohydrates in AER. In the
present case with AER from wheat straw, a total of 123 g of
phenolated AER (P-AER) was recovered from 100 g of AER after
phenolation. Of the 123 g of P-AER, 97 g was soluble in
methanol/acetone (P-AER-S, 79%) whereas only 26 g was insolu-
ble (P-AER-I, 21%). The P-AER-S contains 91.9% lignin whereas
the P-AER-I contains only 42.4% lignin along with 47.8% carbo-
hydrates. Taking the yield into consideration, it can be con-
cluded that phenolation led to the incorporation of 32.3 g of
phenol per 100 g of AER (97 g ꢁ 91.9% + 26 g ꢁ 42.4% ꢀ 100 g ꢁ
ꢀ1
w
weight (M ) around 2500 g mol and a number average mole-
ꢀ
1
cular weight (M ) around 1100 g mol . Since the AER-S only
n
represents 28% of lignin in AER (Table 3), it can be safely
assumed that the lignin in the insoluble fraction of AER had a
much higher molecular weight. The M
1
n
and M
000 and 3200 g mol , respectively. It can be seen from Fig. 2
that the molecular weights of P-AER-S and AER-S were similar,
though P-AER-S had slightly higher M . The P-AER-S had a
w
of P-AER-S were
ꢀ
1
67.9% = 32.3 g) and the hydrolysis of B52% of sugar ((100 g ꢁ
26.3% ꢀ 97 g ꢁ 0.1% ꢀ 26 g ꢁ 47.8%)/(100 g ꢁ 26.3%) ꢁ 100% =
52.3%). The material balance of lignin and carbohydrates during
w
wider molecular weight distribution compared to AER-S. It indi-
cated that there were more lignin moieties with low molecular
weights in P-AER-S, which is evidence of lignin fragmentation
considering the high yield of P-AER-S (97% based on AER). It is
worthwhile to emphasize that the values of the molecular
weights presented are relative as they were estimated by using
polystyrene standards. As P-AER-S represents 89% of the lignin
in AER (Table 3), its relatively low molecular weight clearly
indicated that a substantial depolymerization of lignin in AER
occurred during phenolation. These results are consistent with
phenolation is shown in Table 3. As can be seen in the table,
9% of lignin is recovered as P-AER-S, indicating that the lignin
underwent significant fragmentation during phenolation. The
8
ꢀ1
incorporated 32.3 g of phenol is equivalent to 5.1–6.2 mmol g
of lignin in AER. These data are lower than those of sweetgum
ꢀ1
P-AER of 6.4–8.0 mmol g of lignin but are higher than those of
ꢀ
1
10
pine kraft lignin of 3.6–4.5 mmol g of lignin. Such variations
are attributable to the differences in lignin characteristic struc-
tures, i.e., the contents of inter-unit linkages, the reactivity
difference between lignin units (G/S/H), etc.
1
0
those of sweetgum P-AER reported previously.
Aliphatic and phenolic hydroxyls groups
The aliphatic and phenolic hydroxyl (Ph-OH) contents in AER-S
3
1
Table 3 Material balance of lignin and carbohydrates before and after and P-AER-S were determined by P NMR, as shown in Table 4.
phenolation
It is important to point out that AER-S only represents 28% of
lignin in AER and the insoluble lignin (AER-I) is presumed
Total lignin %
Total carbohydrates %
to have fewer free phenolic hydroxyls as the lignin units were
a
b
a
b
Yield %
As it is
% of AER
As it is
% of AER
18
still linked through aryl ethers.
The aliphatic hydroxyl
ꢀ1
AER
AER-S
AER-I
100
21
67.9
90.4
62.1
91.9
42.4
100
28
72
89
11
26.3
4.8
29.0
0.1
100
3.8
87.1
0
(aliphatic-OH) content in AER-S was 5.19 mmol g and equi-
valent to B100 aliphatic-OHs per 100 C9 units, assuming the
79
97
26
1
9
c
c
C9 unit weight of wheat straw lignin was 201 g per C9. The
aliphatic-OH content in the raw wheat straw lignin was esti-
mated to be 67–70 per 100 C9 units with a significant amount of
P-AER-S
P-AER-I
47.8
47.3
a
b
Values are based on starting solids. Values are based on the com-
c
19–21
ponent in AER. Assuming phenol only incorporated onto P-AER-S.
acylation on g-OH with acetates and p-coumarate (pCA).
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Table 4 Molecular weight, aliphatic and phenolic hydroxyl groups of AER-S and P-AER-S
a
n
a
w
a
b
b
b
b
b
b
M
M
PDI
Aliphatic-OH
5-subst.-OH
G-OH
H-OH
Total Ph-OH
RS
AER-S
P-AER-S
1100
1000
2500
3200
2.3
3.3
5.19
1.49
1.26
1.38
0.89
1.00
0.31
2.67
2.46
5.05
1.51
6.34
a
ꢀ1
b
ꢀ1
Unit is g mol
.
Unit is mmol g lignin, corrected for carbohydrates.
Compared to raw wheat straw lignin, there was an increase in depicted the information of guaiacyl (G)/syringyl (S)/p-hydroxy-
aliphatic-OHs of 30 per 100 C9 units. Such an increase may be phenyl (H) units as well as ferulate (FA), p-coumarate (pCA) and
attributed to the deacylation at C
enzymatic hydrolysis treatment.
hydroxyls groups in AER-S was 2.46 mmol g with 1.26 mmol g
of 5-substituted hydroxyl (5-subst.-OH) groups, 0.89 mmol g of conjugated form. After autohydrolysis and enzymatic hydrolysis,
g
during autohydrolysis and p-benzoate (PB) units. As can be seen in Fig. 3, AER-S was enriched
The total free phenolic in G and S units along with minor amount of H units. Tricin (T), a
1
8
ꢀ1
ꢀ1
type of flavonoid, is commonly found in grasses in an ether-free or
ꢀ
1
ꢀ
1
uncondensed G-hydroxyl (G-OH) and 0.31 mmol g p-hydroxy- tricin still existed in AER-S indicating the applied biorefinery
phenyl (H-OH) groups.
treatment was not able to remove tricin completely.
After phenolation, a significant decrease in aliphatic-OH and
After phenolation, the 2D HSQC spectra exhibited enormous
3
1
an increase in H-OH were observed by P NMR (Table 4). The differences from the starting lignin. As expected, the signals of
ꢀ1
aliphatic-OH decreased to 1.49 mmol g whereas the H-OH the lignin substructures (A, B, C, FA, pCA and PB) disappeared
ꢀ1
increased to 2.67 mmol g (more than 8 times that in AER-S). from their original chemical shifts. For the lignin substructures
The decrease in aliphatic-OH is due to the substitution of of A, B and C, phenolation rendered phenol incorporation onto
hydroxyls by phenol and b-elimination of the g-hydroxymethyl lignin and new signal emergence (Fig. 3) of substructures of 1,
1
0
10
group. The increase in H-OH is owing mainly to the incor- 2, 3, 4, 5, 6, 8, 9 and 10 (Fig. 5 and Table 6). The substructures
poration of phenol onto lignin with minor contributions from of 8, 9 and 10 indicated the occurrence of the b-elimination of
the hydrolysis of aryl ethers, since the raw wheat straw lignin the g-hydroxymethyl group as formaldehyde and the formation
2
1
only has B6% p-hydroxyphenyl units. The slight increases in of diphenylmethane structures. The ferulate (FA), p-coumarate
-subst.-OH and G-OH are attributable to the hydrolysis of aryl (pCA) and p-benzoate (PB) units have unsaturated sidechains
5
0
0
ether (such as b-O-4 ,b-5 ) and the formation of diphenyl- and reacted with phenol during phenolation, supporting by the
1
0
methane substructures. Compared with phenolated pine kraft evidence of signal disappearance (pCA
lignin and phenolated sweetgum AER, the P-AER-S of wheat and PB2,6, d /d 131.3/7.65). The signals of C2/6 and C3/5 of FA,
straw had a similar H-OH content with a lower yield.
a a C H
/FA , d /d 144.2/7.52
C
H
pCA and PB shifted from their original chemical shifts and
overlapped with the signal of the G and H units. Interestingly,
the signals of substructures 7 and 11 presented in phenolated
pine kraft lignin were absent in both AER-S and P-AER-S of wheat
straw. Substructure 7 was derived from the secoisolariciresinol
substructures, which are found only in softwood lignin. As for
substructure 11, it may be derived only from softwood lignin.
2
D HSQC NMR
2
D HSQC NMR spectra of AER-S and P-AER-S (Fig. 3) were
acquired to obtain structural information. The signal assign-
ments were confirmed with published literature and the
1
0,18,20–25
chemical shifts are listed in Table 5.
The color-coded
The signals of T
whereas the two signals remained at the resonance chemical
shifts of T2 and T . Since both the T and T positions are
6 8
and T of tricin disappeared after phenolation
substructures found in AER-S (Fig. 4) correspond to their
respective colored spectral contours. The prominent signals found
0
0
,6
3
6
8
in the side-chain region (d
C
/d
H
50–105/2.5–6.0 ppm) were b-aryl
0
0
0
highly activated towards electrophilic substitution, it is concei-
vable that these structures may have condensed with themselves,
lignin and/or phenol/formaldehyde, which would explain the
ether units (b-O-4 , A), phenylcoumaran units (b-5 /a-O-4 , B),
0
pinoresinol units (b-b , C) along with lignin methoxyls (–OMe).
0
The signal of g-acylated b-aryl ether (A ) also presented in AER-S
disappearance of signals T
and T . The signal intensity decreases in the S and G units
and increases in the H units were consistent with our previous
6 8
and T and the remaining signals
with low levels of resonance. The g-acylation of wheat straw
T
2
0
6
0
3
lignin was reported from 5.4% to 15.6% and mainly were esters
2
0,21
with p-coumarate (pCA) and acetate.
linkages upon autohydrolysis was reported, but some of them
Depletion of these
1
0
study.
1
8
do survive, which is consistent with their low-level resonance.
Interestingly, the dibenzodioxocin (DBDO, D) and cinnamyl
alcohol (I) presented in wheat straw lignin were not detect-
able in AER-S, indicating a structural modification occurred
Experimental
Materials
1
8,21
during the biorefinery process. The signals of polysaccharides Wheat straw (Triticum aestivum L.) was purchased at a home-
were detected in AER-S, such as b-D-(1-4) linked xylosyl units (X). improvement retailer, Lowe’s (Raleigh, NC, USA). The lignin was
These signals were attributed to the residual carbohydrates isolated as an enzymatic hydrolysis residue by our biorefinery
(
4.8%) in the form of lignin-carbohydrate complexes (LCC). process involving autohydrolysis followed by mechanical refining
2,17,26
The aromatic/unsaturated region (d
/d
C H
105–130/6.0–8.0 ppm) and enzymatic hydrolysis.
When the autohydrolysis–enzymatic
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Fig. 3 2D HSQC spectra of AER-S and P-AER-S.
hydrolysis residue (AER) was extracted with methanol/acetone to 180 1C and held for 40 min with a liquid to solid ratio of 4 : 1.
1 : 6 v : v), only 21% was soluble (AER-S), and the rest, 79%, was The ramp-up time was about 90 min. After autohydrolysis, the
(
insoluble (AER-I).
remaining solids (autohydrolyzed wheat straw) were collected
and washed thoroughly with tap water, then disintegrated
using a disk refiner (148-2, Bauer, Springfield, OH, USA) at a
Autohydrolysis, refining and enzymatic hydrolysis
Autohydrolysis was performed in a pulping digester (M/K 0.13 mm (0.005 in.) gap. The collected autohydrolysis pulp
System Inc., Danvers, MA). Wheat straw was heated indirectly (AH pulp) was loaded into the valley beater (Valley Iron Works,
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Table 5 Chemical shift assignment of main substructures in wheat straw AER-S
Label
d
C
/d
H
Assignment
0
B
C
b
53.0/3.50
53.9/3.05
C
C
b
–H
–H
b
in b-5 substructures (B)
in pinoresinol substructures (C)
b
b
b
–
A
B
A
C
A
A
OMe
g
56.8/3.81
59.3/3.44 and 3.74
62.4/3.72
63.9/3.99 and 4.35
70.9/3.82 and 4.18
71.8/4.89
83.8/4.34
85.1/4.64
86.4/4.13
87.1/5.45
C–H in methoxyls
0
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
g
g
g
g
–H
–H
–H
–H
g
g
g
g
in b-O-4 substructures (A)
in b-5 substructures (B)
0
g
0
0
0
g
in g-acylated b-O-4 substructures (A )
in pinoresinol substructures (C)
g
0
0
a
a
b
a
b
a
–H
a
b
a
b
a
in b-O-4 substructures (A)
b(G/H)
–H
–H
–H
–H
in b-O-4 substructures linked to a G unit (A)
C
a
in pinoresinol substructures (C)
0
A
B
T
T
S
T
S
G
FA
pCA
b(S)
a
in b-O-4 substructures linked to a S unit (A)
0
in b-5 substructures (B)
8
93.9/6.41
98.8/6.23
8
–H
–H
8
in tricin (T)
in tricin (T)
6
6
6
2/6
103.7/6.70
104.1/7.31
106.8/7.21
110.5/6.95
111.1/7.29
114.2/6.26
115.2/6.95
115.1/6.77 and 118.6/6.79
122.4/7.12
127.1/7.18
130.0/7.46
131.3/7.65
144.2/7.52
72.3/3.15
2,6–H2,6 in etherified syringyl units (S)
2,6–H 2,6 in tricin (T)
0
2,6–H2,6 in syringyl units with C QO groups (S )
a
0
0
0
0
2
,6
0
2,6
2
2
–H
–H
2
in guiacyl units (G)
in ferulate (FA)
b
in p-coumarate (pCA) and ferulate (FA)
2
2
2
b
/FA
b
b
–H
3,5–H3,5 in p-coumarate (pCA)
pCA3,5
/G
FA
G
5
6
5
–H
–H
5
6 6
and C –H in guiacyl units (G)
6
6
6
in ferulate (FA)
H
pCA2,6
PB2,6
pCA
X
X
X
2,6
2,6–H2,6 in p-hydroxyphenyl units (H)
2,6–H2,6 in p-coumarate (pCA)
2,6–H2,6 in p-benzoates (PB)
a
/FA
a
a
–H
a
2
3
4
in p-coumarate (pCA) and ferulate (FA)
in b-D-(1-4) linked xylosyl units (X)
in b-D-(1-4) linked xylosyl units (X)
in b-D-(1-4) linked xylosyl units (X)
2
3
4
2
–H
3
–H
4
–H
74.1/3.29
75.5/3.52
Fig. 4 Substructures found in wheat straw lignin.
Appleton, WI, USA) and refined for 20 min according to the under vacuum and referred to as the autohydrolysis-enzymatic
TAPPI standard method (TAPPI, 2001). The purpose of refining hydrolysis residue (AER).
was to increase the enzymatic digestibility of the AH pulp. After
Composition analysis
refining, the pulp was subjected to enzymatic hydrolysis
at a substrate loading of 5% (w/v) at 55 1C for 72 h. The The raw wheat straw was extracted with benzene/ethanol
enzyme cocktail of Cellic CTec 2 and Cellic HTec2 (9 : 1, v : v) (2 : 1, v : v) in a Soxhlet apparatus for 8 hours prior to composi-
(Novozymes, USA) was used with an enzyme dosing of 5 FPU per tional analysis. The compositional analysis was performed
g OD substrate. After enzymatic hydrolysis, the solid residue according to the standard protocol provided by the National
2
7
was collected by filtration and washed thoroughly with a buffer Renewable Energy Laboratory. A sample of B300 mg was
solution followed by DI water. The washed solids were dried subjected to 72 wt% sulfuric acid digestion for 2 h, followed by
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10
Fig. 5 Plausible substructures found in phenolated lignin.
Table 6 Chemical shift assignment of plausible substructures in pheno- mixture was quenched and dissolved with 20 ml of ethyl acetate
lated lignin
and then precipitated into a 10-fold amount of petroleum ether.
The precipitate was filtered out and washed with water until
d /d
C H
Assignment
neutral and vacuum-dried and referred to as the phenolated
lignin (P-AER).
3
3
3
3
3
4
4
5
5
6
0.5/1.89–2.25
4.4/3.76
5.8/3.16
8.2–42.1/2.70–3.24
9.5/3.69
3.8/4.54
8.3/4.21
0.9/4.02
7.6–62.0/3.20–3.90
4.1/3.46
C
C
C
C
C
C
C
C
b
a
b
b
a
a
a
a
–H
–H
–H
–H
–H
–H
–H
–H
b
a
b
b
a
a
a
a
in 6
in 8
in 4
in 1, 5/C
in 9
in 3 and 4
in 1, 5/C
in 6
The part of P-AER that can be soluble in a methanol/acetone
a
–H
a
in 2
in 2
(1 : 6 v : v) mixture was labeled as P-AER-S and the insoluble part
was marked as P-AER-I.
b
–H
b
Molecular weight determination
Methoxyl (R
1
) in 10
All lignin samples were acetylated before analysis to enhance
C
g
–H in 3
g
2
8
the lignin’s solubility. GPC of the samples was conducted on a
Shimadzu LC-20AD liquid chromatography system equipped
dilution to 4 wt% and autoclaving for 1.5 h at 121 1C. After with a diode array detector (SPD-20A) under the following
autoclaving, the suspension was cooled down to room tempera- conditions: Waters Styragel HR1 and Styragel HR5E columns,
ꢀ1
ture and filtered using a fine crucible. The collected residue was each 7.5 ꢁ 300 mm; a 1 mg ml in tetrahydrofuran (THF)
ꢀ
1
dried and weighed to measure the acid insoluble lignin. The eluent; a 0.7 ml min
flow rate at a 35 1C column oven
acid soluble lignin was measured by UV spectroscopy at 205 nm. temperature and a 280 nm sample detection. A series of
The monosaccharides in the filtrate were measured by an HPLC monodispersed polystyrene standards were used as calibration
system (Agilent 1200 series, Agilent, USA). The HPLC was standards.
equipped with a Shodex SP081 column (8 ꢁ 300 mm, Showa
3
1
P NMR analysis
Denko, Japan) and a refractive index detector. The column was
operated at 80 1C using Milli-Q water as the mobile phase and Around 40 mg of a vacuum dried lignin sample (record exact
the detector was operated at 50 1C.
weight for later calculation) was firstly charged into a NMR tube,
followed by the addition of a 500 ml CDCl /pyridine solution
1: 1.6 v : v). After the lignin sample was dissolved with conti-
3
Phenolation of lignin
(
ꢀ1
Phenolation was performed according to our previous study, nuous shaking, a 200 ml internal standard solution (9.23 mg ml
and the phenolation conditions including the l/p ratio, acid endo-N-hydroxy-5-norbornene-2,3-dicarboximide solution) and
charge and reaction temperature were investigated. The pheno- a 50 ml relaxation reagent solution (5.6 mg ml chromium(III)
lation of AER was performed by dissolving 1 g of AER in 3–5 g of acetylacetonate solution) was added. The solution employed for
phenol. The homogeneous mixture was treated with a defined the dissolution of the internal standard and relaxation reagent
1
0
ꢀ1
3
amount of 72% sulfuric acid at a defined temperature (T). After was also the CDCl /pyridine solution (1 : 1.6 v : v), as dis-
reacting at the defined temperature for 2 hours, the reaction cussed above. After that, 100 ml of the phosphitylating reagent
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(
namely, 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane) Notes and references
was charged into the NMR tube and mixed thoroughly prior
3
1
31
1 M. Ertas, Q. Han, H. Jameel and H.-m. Chang, Bioresour.
Technol., 2014, 152, 259–266.
to P NMR analysis. A P NMR spectrum was obtained by
using a Bruker 300 MHz spectrometer with a quad probe. The
contents of the hydroxyl groups were determined according to
ref. 29 and 30.
2
C. Huang, B. Jeuck, J. Du, Q. Yong, H. Chang, H. Jameel and
R. Phillips, Bioresour. Technol., 2016, 219, 600–607.
G. T. Beckham, C. W. Johnson, E. M. Karp, D. Salvach u´ a and
D. R. Vardon, Curr. Opin. Biotechnol., 2016, 42, 40–53.
3
2
D HSQC NMR
1
3
1
4 A. J. Ragauskas, G. T. Beckham, M. J. Biddy, R. Chandra,
F. Chen, M. F. Davis, B. H. Davison, R. A. Dixon, P. Gilna
and M. Keller, Science, 2014, 344, 1246843.
Two-dimensional (2D) C– H heteronuclear single quantum
coherence (HSQC) correlation NMR spectra were acquired
3
1
according to the procedures described by Capanema, et al.
The lignin samples were dissolved in DMSO-d with the concen-
tration ca. 20%. A special Shigemi NMR microtube was used to
reduce the amount of sample for the NMR experiments. The
D HSQC NMR spectra were acquired on a Bruker AVANCE
00 MHz spectrometer equipped with a 5 mm double reso-
nance broadband BBI inverse probe using a coupling con-
stant J C–H of 147 Hz. The HSQC experiment was performed
with a Bruker phase-sensitive gradient-edited HSQC pulse
sequence ‘hsqcetgpsi.2.’ The experimental parameters used
were 160 transients (scans per block) acquired using 1 K data
points in the F2 ( H) dimension for an acquisition time of
51 ms and 256 data points in F1 ( C) for an acquisition time
of 7.68 ms for a total of 16.5 h. The 2D data set was processed
with 1 K and 91 K data points using the Qsine function in both
dimensions.
5
6
7
8
9
S. J. Lee, H. J. Kim, E. J. Cho, Y. Song and H. J. Bae,
Int. J. Biol. Macromol., 2015, 72, 1056–1062.
W. Zhang, Y. Ma, C. Wang, S. Li, M. Zhang and F. Chu,
Ind. Crops Prod., 2013, 43, 326–333.
6
2
5
Y. Jin, X. Cheng and Z. Zheng, Bioresour. Technol., 2010, 101,
2046–2048.
W. Qiao, S. Li, G. Guo, S. Han, S. Ren and Y. Ma, J. Ind. Eng.
Chem., 2015, 21, 1417–1422.
A. Pizzi, Handbook of adhesive technology, 2003, vol. 2.
1
10 X. Jiang, J. Liu, X. Du, Z. Hu, H.-m. Chang and H. Jameel,
1
ACS Sustainable Chem. Eng., 2018, 6, 5504–5512.
1
3
11 J. Podschun, B. Saake and R. Lehnen, Eur. Polym. J., 2015,
7, 1–11.
1
6
12 G. J. Abarro, J. Podschun, L. J. Diaz, S. Ohashi,
B. Saake, R. Lehnen and H. Ishida, RSC Adv., 2016, 6,
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1
1
1
3 M. Funaoka, M. Matsubara, N. Seki and S. Fukatsu, Bio-
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Conclusions
Phenolation can be used to improve lignin’s reactivity by increas-
ing phenolic hydroxyl groups and reactive sites for adhesive
31
applications. P NMR revealed that phenolated lignin had
ꢀ
1
ꢀ1
aliphatic-OH of 1.49 mmol g and H-OH of 2.67 mmol g (more
than 8 times that in AER-S). The 2D HSQC indicated that all lignin
1013–1018.
1
6 H.-K. Ono and K. Sudo, in Lignin: Properties and
Materials, ed. W. G. Glasser and S. Sarkanen, American
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7 R. H. Narron, Q. Han, S. Park, H. M. Chang and H. Jameel,
Bioresour. Technol., 2017, 241, 857–867.
0
0
0
0
substructures (b-O-4 , b-5 /a-O-4 , b-b , ferulate, p-coumarate and
p-benzoate) in AER were reacted during phenolation, along with
the hydrolysis of over 50% of the residual carbohydrates
determined by composition analysis. The b-elimination of the
g-hydroxymethyl group as formaldehyde was also detected. The
released formaldehyde further reacted with phenol and lignin
to form diphenylmethane structures.
1
1
1
2
2
8 D. J. Yelle, P. Kaparaju, C. G. Hunt, K. Hirth, H. Kim,
J. Ralph and C. Felby, BioEnergy Res., 2013, 6, 211–221.
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Conflicts of interest
0 J. Zeng, G. L. Helms, X. Gao and S. Chen, J. Agric. Food
Chem., 2013, 61, 10848–10857.
1 J. C. Del R ´ı o, J. Rencoret, P. Prinsen, A. n. T. Mart ´ı nez,
J. Ralph and A. Guti ´e rrez, J. Agric. Food Chem., 2012, 60,
The authors declare no competing financial interest.
Acknowledgements
5922–5935.
This research project was supported financially in part by a 22 H. Kim and J. Ralph, Org. Biomol. Chem., 2010, 8, 576–591.
USDA grant through Domtar and in part by the Biomass to 23 A. Jensen, Y. Cabrera, C.-W. Hsieh, J. Nielsen, J. Ralph and
Biochemicals and Biomaterials Research Consortium and the
Fundamental Research Funds for the Central Universities 24 C. Huang, J. He, L. Du, D. Min and Q. Yong, J. Wood Chem.
FRFCU, Project No. 2017PT11). The authors are grateful to
Technol., 2016, 36, 157–172.
USDA, Domtar and members of the research Consortium 25 C. Huang, J. He, R. Narron, Y. Wang and Q. Yong, ACS
C. Felby, Holzforschung, 2017, 71, 461–469.
(
and FRFCU.
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