Free radical reaction promoted by ionic liquid: A route for metal-free oxidation depolymerization of lignin model compound and lignin

Article abstract of DOI:10.1039/c4cc10394g

Ionic liquid 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BnMIm][NTf₂]) can promote the generation of the OOH free radical and thereby efficiently transformed the β-O-4 lignin model compound 2-phenoxyacetophenone into benzoic acid and phenol using O₂ as the oxidant. Furthermore, the IL-based metal-free catalytic system can also depolymerize other lignin model compounds and organosolv lignin effectively. This journal is

Full text of DOI:10.1039/c4cc10394g

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Y. Yang, H. Fan,
J. Song, Q. Meng, H. Zhou, L. Wu, G. Yang and Prof. B. Han, Chem. Commun., 2015, DOI:
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

ChemComm
Page 1 of 4Journal Name
Dynamic Article Links ►
View Article Online
DOI: 10.1039/C4CC10394G
COMMUNICATION
Free radical reaction promoted by ionic liquid : a route for metalꢀfree
oxidation depolymerization of lignin model compound and lignin
Yingying Yang, Honglei Fan*, Jinliang Song, Qinglei Meng, Huacong Zhou, Lingqiao Wu, Guanying
Yang, Buxing Han*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
reaction media to substitute volatile organic solvents is very
19
promising.
The transformation of lignin or lignin model
Ionic
liquid
1ꢀBenzylꢀ3ꢀmethylimidazolium
([BnMIm][NTf2]) can
4, 19ꢀ20
compounds in ILs has become an interesting topic.
50 Moreover, ILs have some obvious advantages for lignin
valorization because their functions can be designed, 21 and they
are excellent solvents for gases, 28 liquids, and lignin.
bis(trifluoromethylsulfonyl)imide
10 promote the generation of ·OOH free radical, and thereby
efficiently transformed the βꢀOꢀ4 lignin model compound 2ꢀ
phenoxyacetophenone into benzoic acid and phenol using O2
as the oxidant. Furthermore, the ILꢀbased metalꢀfree
catalytic system can also depolymerize other lignin model
15 compounds and organosolv lignin effectively.
In this work, we report the first example that IL can promote
the generation of OOH free radicals during the transformation of
55 lignin, and this activation mode leads to producing benzoic acid
and phenol very efficiently from βꢀOꢀ4 lignin model compound
2ꢀphenoxyacetophenone(1) under metalꢀfree condition using O2
as the oxidant with catalytic amount of H3PO4 (Scheme 1). The
ILꢀbased catalytic system could also depolymerize other lignin
60 model compounds and organosolv lignin. Besides metalꢀfree
catalysis, this new route has some other attractive advantages. For
example, the use of base and volatile organic solvent is avoided;
high yields of the products can be isolated easily without tedious
postꢀtreatment; the IL used can be recycled and reused without
65 loss of the yields of the products.
Lignocellulosic biomass,
a promising alternative to fossil
resource, has attracted much attention in recent years. 1 As a main
component of lignocellulosic biomass, lignin is considered to be
2
the unique renewable aromatic resource and can be used to
20 produce useful aromatic compounds. Due to the structural
complexity and variability, model compounds with βꢀOꢀ4
linkage, which represents the most common substructure in
lignin, are often adopted to disclose the chemical issues in lignin
3
4
valorization. The transformations of lignin model compounds
5
6
7
25 involve catalytic cracking,
hydrolysis,
reduction
and
O
O
oxidation. 8 Catalysts used in these reactions involve homogenous
[BnMIm][NTf2]
H3PO4, O2, H2O
O
HO
9
10
OH
or heterogeneous nickel catalysts,
vanadium complex,
As one of the important
transformations, oxidation process is usually catalyzed by
+
11
12
ruthenium complex
and so on.
1
3
2
8
Scheme 1. Oxidation depolymerization of lignin model compound 1
30 different transitionꢀmetalꢀbased catalysts.
Although many
metalꢀcatalyzed methods are effective, the reserves of metal
catalysts are limited in the earth and these catalysts also generate
harmful metal waste leading to increased cost and energy
consumption in the postꢀtreatment. Obviously, it is highly
35 desirable to explore metalꢀfree routes using O2 or H2O2 as the
oxidant in lignin valorization. Recently, a metalꢀfree oxidation of
benzylic ketone to produce veratric acid and guaiacol was
We investigated the impacts of various ILs on the reaction
70 shown in Scheme 1, and analytic results by high performance
liquid chromatography (HPLC) are presented in Table 1. The
structures of ILs are given in Scheme S1. We found that
[BnMIm][NTf2] + H3PO4 was very effective for the oxidation of
lignin model compound 1 to produce benzoic acid and phenol in
75 high yields. According to Table 1, the neutral ILꢀ[BnMIm][NTf2]
could promote the reaction (entry 1). Interestingly, addition of
minor amounts of phosphoric acid could increase yields of the
products significantly (entry 2), suggesting that the acid is
favorable to the reaction. Therefore, we studied the efficiency of
13
reported. The reaction was conducted in organic solvent using
H2O2 as the oxidant in the presence of NaOH. Inorganic acid was
40 used to neutralize the NaOH after reaction to recover the
products.
In the reported transformations of lignin model compounds,
volatile organic solvents are often used as reaction media. 10a, 14
Recently, ionic liquids (ILs) with low volatility have received
80 acidic
ILs
1ꢀbutylꢀ3ꢀmethylimidazolium
hydrosulfate
([BMIm][HSO4])
and 1ꢀhexylꢀ3ꢀmethylimidazolium
15
45 much interest in various fields such as in organic synthesis,
dihydrophosphate ([HMIm][H2PO4]). But the yields of the
catalysis, 16 separation, 17 and biology. 18 Especially, using ILs as
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

ChemComm
Page 2 of 4
View Article Online
DOI: 10.1039/C4CC10394G
products were low (entries 3 to 4), indicating that the anion of
[BnMIm][NTf2] played a decisive role in the oxidation reaction.
added water, the system was diluted, resulting in lower
conversion and yields. Little products were generated without
To verify this argument, 1ꢀbutylꢀ3ꢀmethylimidazolium 50 adding water because phosphoric acid (85%) contained a small
bis(trifluoromethylsulfonyl)imide ([BMIm][NTf2]) and 1ꢀethylꢀ3ꢀ
amount of water. The results above reveal that water is necessary
in the reaction. The dependence of the conversion of 1 and the
yields of products on the pressure of oxygen is shown in Figure
1c. The reaction cannot proceed in the nitrogen atmosphere
5
methylimidazolium
bis(trifluoromethylsulfonyl)imide
([EMIm][NTf2]), which also have [NTf2]ꢀ anion, were used for
the reaction (entry 5 and 6). High yields of benzoic acid and
phenol were also obtained. For comparison, the common neutral 55 without oxygen, which means that oxygen is crucial for the
IL 1ꢀbutylꢀ3ꢀmethylimidazolium chloride ([BMIm][Cl]) and
reaction. The conversion increased rapidly as the oxygen pressure
increased. The yields of benzoic acid and phenol reached a
plateau when the oxygen pressure reached 1.0 MPa. After that,
the yields dropped evidently. The main reason is that the side
10 organic solvent toluene were also tested in the presence of H3PO4
(entry 7 and 8), and very poor conversion and yields were
obtained. The results demonstrate that H3PO4 cannot induce the
occurrence of the reaction but play the cooperative roles with 60 reactions occurred more considerably at higher oxygen pressure.
[NTf2]ꢀꢀIL pushing the proceeding of the reaction
The reusability of [BnMIm][NTf2] was tested and the results are
illustrated in Figure 1d. Obviously, the loss of the yields of
benzoic acid and phenol was not notable in the four cycles,
showing the excellent stability of the IL in the reaction system.
15 Table 1. Oxidation of lignin model compound 1 with different ILsa
entry
Solvent
Conversion(%) Y(2)(%)c
Y(3)(%)d
1b
2
3
4
5
6
7
8
[BnMIm][NTf2]
[BnMIm][NTf2]
[BMIm][HSO4]
[HMIm][H2PO4]
[BMIm][NTf2]
[EMIm][NTf2]
[BMIm][Cl]
40
100
<1
29
89
<1
4.8
84
78
2.6
<1
7
84
<1
3
30
100
100
50
68
65
2
Toluene
20
<1
a reaction conditions: reactant 1 0.212 g, IL 1 g, 85% H3PO4 16 ꢁL, H2O
46 ꢁL, O2 1.0 MPa, temperature, 403 K, reaction time 3 hours. b without
H3PO4; c Y(2) = Yield of benzoic acid; d Y(3)=Yield of phenol.
Various inorganic and organic acids were tested (Table S1) in
20 the reaction. The results showed that strong acids such as
hydrochloric acid and sulfuric acid accelerated the conversion of
1, but the selectivity for phenol and benzoic acid was low. On the
other hand, weak acids like acetic acid and tungstic acid had low
activity for the reaction. As a result, phosphoric acid showed best
65
Figure 1. Effects of reaction parameters on the conversion and yields
with the typical reaction condition: 0.212 g reactant 1, 1 g IL, variational
amounts of 85% H3PO4 aqueous solution (a), water (b) and oxygen (c)
temperature, 403 K. (d) The reuse of [BnMIm][NTf2] at the condition in
6b
25 performance for the reaction. Just as reported, acids are often
used to catalyze lignin model compounds to produce useful
chemicals through the protonation, nucleophilic attack and
cracking reaction steps. Strong acids tend to cause more side
reactions, while weak acids show very low activity. Therefore,
30 the catalytic system with phosphoric acid was further studied.
The effects of various reaction parameters were studied in
[BnMIm][NTf2] and the results are presented in Figure 1,
together with those to test the reusability of the IL. Figure 1a
demonstrates the variation of the conversion and yields with the
35 amount of phosphoric acid. It can be known from the figure that
small amount of phosphoric acid could enhance the reaction very
effectively. The conversion increased continuously with the
amounts of the acid. The yields of benzoic acid and phenol
increased firstly, and then decreased as the amount of phosphoric
40 acid was more than 16 ꢁL. At the same time, the side reactions,
such as esterification of benzoic acid and phenol as well as
further oxidation of the products, were also be accelerated,
resulting in lower yields of the desired products at higher
phosphoric acid concentration. The dependence of amount of
45 water on the reaction was illustrated in Figure 1b. With
increasing amount of water, the conversion of the reactant and the
yields of the products increased at the beginning. But with more
70 Entry 2 of Table 1.
It is interesting to study the activation process of oxygen and
the role of the [BnMIm][NTf2] in the reaction. It was reported
that ethers can be oxidized and form contact charge transfer
complexes (CCTCs) through heating without light and any
75 additional initiator. 25 UVꢀVis spectroscopy can be used to study
the formation of CCTCs by the change of the maximum
absorption wavelength (λmax) of functional groups with and
25
without oxygen. We carried out the UVꢀvis experiments and
the results are shown in Figure 2A. With nitrogen of 1.0 MPa at
80 353 K for 12 hours, the λmax of 2ꢀphenoxyacetophenone was 243
nm. But with oxygen of 1.0 MPa under the same condition, the
λmax moved to 282 nm. The red shift of the wavelength can be
ascribed to that 2ꢀphenoxyacetophenone(1) formed CCTC with
26
oxygen via Van der Waals’ force. The interaction increases
85 delocalization of electrons of the complex, and eventually
reduces the energy between the ground state and the excited state
of the CCTC. 25 As we know from literature, after formation of
the CCTC, the complex is not stable and tends to dissociate into
oxygen free radical with partial negative charge, and the adjacent

Page 3 of 4
ChemComm
View Article Online
DOI: 10.1039/C4CC10394G
–CH2 group can transfer hydrogen to the negative oxygen radical
to form OOH free radical. There are two benzene rings in 1, and
reaction, which further decomposed into these gases. The
speculation was also confirmed by the detection of trace amount
of formic acid by high performance liquid chromatography
(HPLC) (Figure S3).
it can be deduced that the oxygen would interact with the one
27
conjugated with carbonyl group.
To further verify the
5
speculation, we studied the oxidation of other two lignin model
compounds (Scheme S2) diphenyl ether (without ꢀCH2 group)
and benzyl phenyl ether (with ꢀCH2 group). At the condition
shown in entry 2 of Table 1, no reaction occurred for diphenyl
ether, but benzyl phenyl ether was completely converted. The
55
Based on the above experimental results and the knowledge in
literature, we proposed a plausible mechanism of the reaction
which is schematically shown in Scheme 2, and is discussed in
the following. Firstly, 2ꢀphenoxyacetophenone(1) forms CCTC
with oxygen by the interaction between oxygen and the benzene
10 results also provide the information about the activation mode of 60 ring conjugated with carbonyl group. Then, the CCTC can be
O2 in our reaction system (Scheme 1). Due to the strong
electronegativity of heteroatoms contained in [NTf2]ꢀ, the anion is
beneficial to the delocalization of electron pairs, which
accelerates the formation of OOH free radicals. Eventually, the
15 oxidation depolymerization reaction is promoted significantly.
As we know, electron paramagnetic resonance (EPR) is the
most direct and effective way to detect the existence of free
radicals. Riboflavin is often chosen to generate OOH free radicals
oxidized into ROOH transition state, which is the key step in the
oxidation process. After the dissociation of CCTC into oxygen
free radical with partial negative charge and 1 with partial
positive charge, the CꢀH bond adjacent to etherꢀoxygen atom is
65 activated. On one hand, the lone pair electrons of oxygen atom in
oxygen free radical can activate the hydrogen atom by forming
hydrogen bond. On the other hand, the strong electronegativity of
[NTf2]ꢀ is also in favour of the delocalization of electrons
29
under the irradiation of ultraviolet light. In our experiment,
20 firstly, a standard system was used to generate and trap free 70 a result, the dissociated hydrogen atom combines with O2 to
between CꢀH bond, thus promoting the cleavage of CꢀH bond. As
ꢀ
radical consisting of 1 mM riboflavin in [BnMIm][NTf2] with
DMPO (dimethyl pyridine Nꢀoxide) as the spin trap, and the
spectra was obtained as depicted in Figure S1a. Then the EPR
detection of our reaction system was conducted using
25 [BnMIm][NTf2] as the reaction medium with DMPO added to the
reactor before the reaction, and it gave rise to identical spectrum
with the above experiment (g=2.01±0.002) (Figure 2B), which
proved the generation of OOH free radicals in our reaction
process. For comparison, another EPR detection was made using
form OOH free radical, and ROOH transition state is obtained.
Finally, under the cooperation attack of H+ and water, the
intermediate was transformed, releasing formic acid and
producing benzoic acid and phenol.
To verify the application of the reaction system, anther
common lignin model compound, 2ꢀphenyloxyꢀ1ꢀphenylethanol
was tested at 443 K with other conditions same as that in Entry 2
of Table 1. It was conversed completely after four hours, and the
yield of benzoic acid reached to 75%, phenol, benzaldehyde and
75
30 [BMIm][HSO4] as the reaction medium at the same condition, but 80 other products were also obtained. We also studied the oxidation
no signal was detected (Figure S1b). The results show that no free
radicals was generated in [BMIm][HSO4], which also reveals that
inorganic acids in the reaction system cannot promote the
generation of free radicals in [BMIm][HSO4]. In addition, Table
of organosolv lignin employing the ILꢀbased system. After
reaction of 20 hours, an orange oil fraction(75 wt%) was
obtained. The oil can be easily dissolved in organic solvents and
analysed by HPLC and GCꢀMS. The products contained benzoic
35 1 shows that no reaction occured in this IL (entry 3), suggesting 85 acid (43 wt%), benzaldehyde, and some other compounds. The
that formation of free radicals is crucial for the reaction.
above results indicate that the ILꢀbased system is also very
effective to the depolymerization of organosolv lignin.
O
O
O
HO
C
OH
+
O2
heat
HCOOH
O2
O
OH
C+
O
O
O
HOO
H2O
[BzMIM][NTf2]
O
Figure 2. A) UVꢀvis absorption spectra of 2ꢀphenoxyacetophenone with
N2(b) and O2(a). B) EPR spectrum of OOH free radical trapped by DMPO
40 in the reaction.
H+
O
δ+
O
H
Ο
Οδ−
It can be known from Scheme 1 that a carbon atom of the
reactant 1 lost in the reaction to produce benzoic acid and phenol.
The gas chromatography (GC) analysis of the collected gases
vented from the reactor after the reaction showed that the vented
45 gases consisted of carbon dioxide, hydrogen and carbon
monoxide. Figure S2 gives the GC spectra of the gaseous
products in the reaction shown in entry 2 of Table 1. Considering
that formic acid can be decomposed into carbon monoxide and
water or into carbon dioxide and hydrogen under heating, 22 we
50 speculated that the lost carbon formed formic acid firstly in the
HOO
[NTf2]ꢀ
O
O
OOH
Scheme 2. Possible mechanism of oxidation of lignin model compound 1.
90
In conclusion, we found that IL [BnMIm][NTf2] combined with a
small amount of H3PO4 is an excellent catalytic system and

ChemComm
Page 4 of 4
View Article Online
DOI: 10.1039/C4CC10394G
Matsuura, C. R. J. Stephenson, J. Am. Chem. Soc. 2014, 136, 1218;
d) X. Xu, Y. Li, Y. T. Gong, P. F. Zhang, H. R. Li, Y. Wang, J. Am.
Chem. Soc. 2012, 134, 16987.
a) B. Sedai, C. DíazꢀUrrutia, R. T. Baker, R. L. Wu, L. A. Silks, S. K.
Hanson, ACS Catal. 2013, 3, 3111; b) C. Crestini, P. Pro, V. Neri, R.
Saladino, Bioorgan. Med. Chem. 2005, 13, 2569; c) J. Zakzeski, P. C.
A. Bruijnincx, B. M. Weckhuysen, Green Chem. 2011, 13, 671.
a) A. G. Sergeev, J. F. Hartwig, Science 2011, 332, 439; b) Q. Song,
F. Wang, J. Xu, Chem. Commun. 2012, 48, 7019; c) J. Y. He, C.
Zhao, J. A. Lercher, J. Am. Chem. Soc. 2012, 134, 20768.
medium for transformation of lignin model compound (1) into
benzoic acid and phenol. A plausible mechanism is proposed
based on the analysis of UVꢀVis spectroscopy and electron
65
70
8
9
paramagnetic
resonance
(EPR).
Firstly,
2ꢀ
5
phenoxyacetophenone(1) forms CCTC with oxygen by the
interaction between oxygen and the benzene ring conjugated with
carbonyl group. Then, with the promotion of [BnMIm][NTf2],
OOH free radicals can be generated and CCTC can be oxidized
into ROOH. Finally, under the cooperation attack of H+ and water,
10 the intermediate was transformed, releasing formic acid and
producing benzoic acid and phenol. The results demonstrate for
the first time that ionic liquid can promote the generation of OOH
free radicals thereby accelerating the reactions. This metalꢀfree
route can also be applied for other lignin model compounds and
15 organosolv lignin. This new protocol to cleave the CꢀC/CꢀO
bonds in lignin has obvious advantages, such as metal free, highly
effecient, avoiding the use of base and volatile organic solvent,
and the IL can be easily recycled and reused. We believe that the
route of free radical reaction promoted by an IL opens a
20 promising way to design metalꢀfree catalytic systems.
10 a) S. K. Hanson, R. L. Wu, L. A. Silks, Angew. Chem. Int. Ed. 2012,
51, 3410; b) S. Son, F. D. Toste, Angew. Chem. Int. Ed. 2010, 49,
3791.
75 11 J. M. Nichols, L. M. Bishop, R. G. Bergman, J. A. Ellman, J. Am.
Chem. Soc. 2010, 132, 12554..
12 a) M. Chatterjee, T. Ishizaka, A. Suzuki, H. Kawanami, Chem.
Commun. 2013, 49, 4567; b) E. Feghali, T. Cantat, Chem. Commun.
2014, 50, 862; c) H. Ohta, H. Kobayashi, K. Hara, A. Fukuoka,
Chem. Commun. 2011, 47, 12209; d) R. Rinaldi, Angew Chem Int
Edit 2014, 53, 8559.
13 a) A. Rahimi, A. Azarpira, H. Kim, J. Ralph, S. S. Stahl, J Am Chem
Soc 2013, 135, 6415.
14 B. Sedai, C. DiazꢀUrrutia, R. T. Baker, R. L. Wu, L. A. Silks, S. K.
Hanson, ACS Catal. 2011, 1, 794.
15 a) T. Welton, Chem. Rev. 1999, 99, 2071; b) J. P. Hallett, T. Welton,
Chem. Rev. 2011, 111, 3508; c) T. Fischer, A. Sethi, T. Welton, J.
Woolf, Tetrahedron Lett. 1999, 40, 793.
80
85
90
Acknowledgements
16 a) Z. C. Zhang, Adv. Catal. 2006, 49, 153; (b) X. C.Kang, J. L.
Zhang, W. T.Shang, T. B. Wu, P. Zhang, B. X. Han, Z. H. Wu, G.
Mo, X. Q. Xing, J. Am. Chem. Soc. 2014, 136, 3768.
17 P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 2000, 39, 3772.
18 E. Reichert, R. Wintringer, D. A. Volmer, R. Hempelmann, Phys.
Chem. Chem. Phys. 2012, 14, 5214.
The authors thank the National Natural Science Foundation of China
(21173234, 21133009, U1232203, 21021003), and the Chinese Academy
of Sciences (KJCX2.YW.H30)
25 Notes and references
95 19 G. Chatel, R. D. Rogers, ACS Sustainable Chem. Eng. 2014, 2, 322.
20 J. Zakzeski, A. L. Jongerius, B. M. Weckhuysen, Green Chem. 2010,
12, 1225.
21 E. Baciocchi, C. Chiappe, C. Fasciani, O. Lanzalunga, A. Lapi, Org.
Lett. 2010, 12, 5116.
100 22 T. M. McCollom, B. R. T. Simoneit, E. L. Shock, Energy Fuels 1999,
13, 401.
23 G. D. Yadav, G. George, J. Mol. Catal. A-Chem. 2008, 292, 54.
24 C. Zhao, W. J. Song, J. A. Lercher, ACS Catal. 2012, 2, 2714.
25 J. M. Zhu, W. X. Cao, X. D. Feng, Chinese Chem. Lett. 1997, 8, 869.
105 26 D. F. Evans, J. Chem. Soc. 1953, 345.
Beijing National Laboratory for Molecular Sciences (BNLMS), Institute
of
Beijing 100190, China
Tel. /Fax:
Chemistry,
Chinese
Academy
of
Sciences,
(+86)
10ꢀ62562821
30 hanbx@iccas.ac.cn (Buxing Han); hlfan@iccas.ac.cn (Honglei Fan).
† Electronic Supplementary Information (ESI) available: [GC analysis
spectra of gases collected after the reaction; structures of the ILs used;
oxidation of lignin model compound 1 with different acids; UVꢀvis
absorption study; structures of other lignin model compounds studied in
35 the work; EPR experiments and spectra; reuse of [BnMIm][NTf2].]. See
DOI:
27 G. X. Wang, R. X. Zhu, D. J. Zhang, C. B. Liu, Chinese J. Struc.
Chem. 2006, 25, 1402.
28 Z. G. Lei, C. N. Dai, B. H. Chen, Chem. Rev. 2014, 114, 1289.
29 V. Roubaud, S. Sankarapandi, P. Kuppusamy, P. Tordo, J. L. Zweier,
1
a) A. J. Ragauskas, G. T. Beckham, M. J. Biddy, R. Chandra, F.
Chen, M. F. Davis, B. H. Davison, R. A. Dixon, P. Gilna, M. Keller,
P. Langan, A. K. Naskar, J. N. Saddler, T. J. Tschaplinski, G. A.
Tuskan, C. E. Wyman, Science 2014, 344, 1246843; b) B. B. Zhang,
J. L. Song, G. Y. Yang, B. X. Han, Chem. Sci. 2014, 5, 4656.
M. Y. He, Y. H. Sun, B. X. Han, Angew. Chem. Int. Ed. 2013, 52,
9620.
40
45
50
55
60
110
Anal. Biochem. 1997, 247, 404.
2
3
4
F. S. Chakar, A. J. Ragauskas, Ind. Crop. Prod. 2004, 20, 131.
J. Zakzeski, P. C. A. Bruijnincx, A. L. Jongerius, B. M. Weckhuysen,
Chem. Rev. 2010, 110, 3552.
115
5
6
a) B. K. Avellar, W. G. Glasser, Biomass Bioenerg. 1998, 14, 205;
b) S. Chu, A. V. Subrahmanyam, G. W. Huber, Green Chem. 2013,
15, 125.
a) V. Molinari, C. Giordano, M. Antonietti, D. Esposito, J. Am.
Chem. Soc. 2014, 136, 1758. b) S. Y. Jia, B. J. Cox, X. W. Guo, Z. C.
Zhang, J. G. Ekerdt, Ind Eng Chem. Res. 2011, 50, 849; c) M. R.
Sturgeon, S. Kim, K. Lawrence, R. S. Paton, S. C. Chmely, M.
Nimlos, T. D. Foust, G. T. Beckham, ACS Sustainable Chem. Eng.
2014, 2, 472.
120
7
a) T. H. Parsell, B. C. Owen, I. Klein, T. M. Jarrell, C. L. Marcum, L.
J. Haupert, L. M. Amundson, H. I. Kenttämaa, F. Ribeiro, J. T.
Miller, M. M. AbuꢀOmar, Chem. Sci. 2013, 4, 806; b) Z.
Strassberger, A. H. Alberts, M. J. Louwerse, S. Tanase, G.
Rothenberg, Green Chem. 2013, 15, 768; c) J. D. Nguyen, B. S.