Efficient biomass pretreatment using ionic liquids derived from lignin and hemicellulose

Article abstract of DOI:10.1073/pnas.1405685111

Ionic liquids (ILs), solvents composed entirely of paired ions, have been used in a variety of process chemistry and renewable energy applications. Imidazolium-based ILs effectively dissolve biomass and represent a remarkable platform for biomass pretreat

Full text of DOI:10.1073/pnas.1405685111

Efficient biomass pretreatment using ionic liquids
derived from lignin and hemicellulose
a,b,c
a
a,d
a,d
e,f
a,b,c
Aaron M. Socha
, Ramakrishnan Parthasarathi , Jian Shi , Sivakumar Pattathil , Dorian Whyte
,
a,d
a,d
d
e
e,f
Maxime Bergeron , Anthe George , Kim Tran , Vitalie Stavila , Sivasankari Venkatachalam , Michael G. Hahn
,
a,d
a,d,1
Blake A. Simmons , and Seema Singh
a
b
c
Deconstruction Division, Joint BioEnergy Institute, Emeryville, CA 94608; Center for Sustainable Energy and Department of Chemistry and Chemical
d
Technology, Bronx Community College, City University of New York, Bronx, NY 10453; Biological and Materials Science Center, Sandia National Laboratories,
Livermore, CA 94551; Complex Carbohydrate Research Center, University of Georgia, Athens, GA 30602; and The BioEnergy Science Center, Oak Ridge
National Laboratory, Oak Ridge, TN 37831
e
f
Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved July 9, 2014 (received for review March 27, 2014)
Ionic liquids (ILs), solvents composed entirely of paired ions, have
been used in a variety of process chemistry and renewable energy
applications. Imidazolium-based ILs effectively dissolve biomass
and represent a remarkable platform for biomass pretreatment.
Although efficient, imidazolium cations are expensive and thus
limited in their large-scale industrial deployment. To replace
imidazolium-based ILs with those derived from renewable sources,
we synthesized a series of tertiary amine-based ILs from aromatic
aldehydes derived from lignin and hemicellulose, the major by-
products of lignocellulosic biofuel production. Compositional
analysis of switchgrass pretreated with ILs derived from vanillin,
p-anisaldehyde, and furfural confirmed their efficacy. Enzymatic
hydrolysis of pretreated switchgrass allowed for direct compari-
son of sugar yields and lignin removal between biomass-derived
ILs and 1-ethyl-3-methylimidazolium acetate. Although the rate of
cellulose hydrolysis for switchgrass pretreated with biomass-
derived ILs was slightly slower than that of 1-ethyl-3-methylimida-
zolium acetate, 90–95% glucose and 70–75% xylose yields were
obtained for these samples after 72-h incubation. Molecular mod-
eling was used to compare IL solvent parameters with experimen-
tally obtained compositional analysis data. Effective pretreatment
of lignocellulose was further investigated by powder X-ray diffrac-
tion and glycome profiling of switchgrass cell walls. These studies
showed different cellulose structural changes and differences in
hemicellulose epitopes between switchgrass pretreatments with
the aforementioned ILs. Our concept of deriving ILs from lignocel-
lulosic biomass shows significant potential for the realization of
a “closed-loop” process for future lignocellulosic biorefineries and
has far-reaching economic impacts for other IL-based process tech-
nology currently using ILs synthesized from petroleum sources.
solvents is the processing of lignocellulosic biomass, and sub-
sequent fermentation, to produce specialty and commodity
chemicals including advanced biofuels (12–15).
Lignin and polysaccharides found in plant cell walls represent
the two largest components of biomass on Earth, and it has been
estimated that the United States has ∼1.3 billion tons of ligno-
cellulosic biomass available per year (16). Lignin is a heteroge-
neous polymer that constitutes 20–30% of dry biomass in woody
plants (17) and 15–20% in grasses (18). The monomeric com-
position of lignin comprises three primary phenylpropane units,
p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, al-
though this varies between species and the methods used for its
extraction. Cellulose, a crystalline polymer of D-glucose, is the
principal component of biomass, accounting for ∼40–50% of the
dry biomass in woody plants and grasses (18, 19). Hemicellulose
is a heterogeneous polymer of pentose and hexose sugars, and
the composition of the hemicellulosic wall fraction varies sig-
nificantly by species. Hemicelluloses account for ∼25% of the
dry biomass of woody plants (20) and ∼30% of grasses (18).
Pretreatment of lignocellulosic biomass is a first step in either
chemical or enzymatic depolymerization of the holocellulosic
content of biomass. Approaches include steam explosion, high
temperature treatment with addition of dilute acid or alkali; these
have been reviewed extensively (21, 22). Although several meth-
ods can provide high yields of glucose and xylose, downstream
Significance
Ionic liquids (ILs) have unique properties applicable to a variety
of industrial processes. Nearly universal solvating capabilities,
low vapor pressures, and high thermal stabilities make these
compounds ideal substitutes for a wide range of organic sol-
vents. To date, the best performing ILs are derived from non-
renewable sources such as petroleum or natural gas. Due to
their potential for large-scale deployment, ILs derived from
inexpensive, renewable reagents are highly desirable. Herein,
we describe a process for synthesizing ILs from materials derived
from lignin and hemicellulose, major components of terrestrial
plant biomass. With respect to overall sugar yield, experimental
evaluation of these compounds showed that they perform
comparably to traditional ILs in biomass pretreatment.
renewable chemicals bioenergy
| lignocellulose conversion
|
|
saccharification green chemistry
|
oom temperature ionic liquids (ILs) are commonly defined
as molten salts with melting points less than 100 °C, and
R
many ILs are considered environmentally friendly solvents for
a variety of industrial applications. Their ionic, noncoordinating
nature allows ILs to dissolve combinations of organic and in-
organic compounds, facilitating diverse types of chemical trans-
formation and separation processes (1, 2). ILs are often
immiscible with organic solvents and thus provide nonaqueous
alternatives for biphasic reaction systems, such as those involving
homogeneous catalysts (3). In many cases, ILs are considered for
Author contributions: A.M.S., R.P., J.S., S.P., M.B., M.G.H., B.A.S., and S.S. designed re-
search; A.M.S., R.P., J.S., S.P., D.W., M.B., A.G., K.T., V.S., and S.V. performed research;
A.M.S., R.P., J.S., S.P., and M.B. contributed new reagents/analytic tools; A.M.S., R.P., J.S.,
S.P., D.W., M.B., A.G., M.G.H., B.A.S., and S.S. analyzed data; and A.M.S., R.P., J.S., S.P.,
M.G.H., B.A.S., and S.S. wrote the paper.
“
green chemistry” due to their low vapor pressures, high thermal
stabilities, and relative nontoxicity. As such, they are emerging as
important materials for drug delivery (4), lubrication (5), and
electrolytes (6), including those for lithium ion (7) and lithium
sulfur batteries (8). ILs have also found utility as heat transfer
media for solar thermal systems (9), carbon capture (10), and
biodiesel production (11). Among the industrial applications
with highest potential volume requirements of these remarkable
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.
1
To whom correspondence should be addressed. Email: seesing@sandia.gov.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1405685111/-/DCSupplemental.
www.pnas.org/cgi/doi/10.1073/pnas.1405685111
PNAS
|
Published online August 18, 2014
|
E3587–E3595

fermentation of these sugars are often confounded by toxic by-
products (23). Due to their ability to selectively remove lignin
and hemicellulose from biomass, effectively providing pure cel-
lulose for enzymatic hydrolysis, certain ILs are exceptional pre-
treatment solvents (24–26). The most studied ILs are currently
considered too expensive for large-scale biomass processing,
typically require multistep syntheses, and are primarily derived
from nonrenewable resources. For example, imidazole and pyr-
idine, two of the best cation moieties for biomass pretreatment,
are prepared industrially by the Radziszewski (27) and Chichi-
babin (28) condensation reactions, respectively. The starting
materials for these syntheses include glyoxal and acetaldehyde,
both of which are produced from ethylene obtained from pe-
troleum cracking and/or hydraulic fracturing.
itself results in a small aromatics stream from lignin breakdown
and could be potentially converted into renewable ILs (32).
We herein describe the first (to our knowledge) synthesis and
evaluation of ILs from these lignin- and hemicellulose-derived
compounds. Specifically, reductive amination chemistry was used
to produce tertiary amines that were protonated with phosphoric
acid to form the desired ILs. Dihydrogen phosphate containing
ILs (1–3), were prepared from furfural, vanillin, and p-anisaldehyde,
respectively. Combinations of computational and experimental
methods were used to compare these compounds to 1-ethyl-
3-methylimidazolium acetate, [C mim][OAc] (4), an IL that
2
has been well studied owing to its efficacy in biomass pre-
treatment (Fig. 2).
Results
Imidazolium and other cations containing aromatic moieties
have been shown to improve IL dissolution of wood (29–32).
With this in mind, we turned our attention to the use of lignin-
and hemicellulose-derived compounds as potential raw materials
for IL synthesis. These polymers represent inexpensive and
abundant waste streams from a variety of biomass-processing
industries including textiles, pulp/paper, and biofuels. Chemical
processes to produce ILs directly from biomass could lower costs
by using large-volume, low-cost waste streams (Fig. 1). Critical to
development of a “closed-loop” biorefinery is the controlled
depolymerization of lignin and hemicellulose. Depending on the
methods used, it is possible to direct depolymerization toward
desired product streams, such as aromatic aldehydes, acids, and
alcohols (33–35). For example, classic oxidative methods in-
volving CuSO4 with NaOH yield hydroxyl- and methoxyl-
substituted aromatic aldehydes (36), particularly vanillin and
syringaldehyde. A similar catalytic system, using CuSO4 with
quaternary ammonium and imidazolium dimethylphosphate ILs,
has been shown to convert ∼30% of lignin to aldehydes (37).
Pyrolysis of lignin primarily produces aldehydes and phenols
Synthesis of ILs. Reductive amination of aldehydes derived from
lignin and hemicellulose proceeded in excellent yields. A solu-
tion of the aldehyde was treated with diethylamine and sodium
triacetoxyborohydride in 1,2-dichloroethane (40). A two-step
acid/base workup provided the desired tertiary amine product
without requiring any additional purification.
Furfural was first selected for reductive amination, as it is
readily obtained from the acid-catalyzed dehydration of pentose
sugars commonly found in hemicellulose. Its tertiary amine de-
rivative, N-ethyl-N-(furanyl-2-methyl)ethanamine, was obtained
in 82% yield. Vanillin and p-anisaldehyde were selected as lignin-
derived aldehydes, and reductive amination provided their ter-
tiary amine derivatives, 4-((diethylamino)methyl)-2-methoxyphenol
and N-ethyl-N-(4-methoxybenzyl)ethanamine in 87% and 94%
yield, respectively. The resulting amines were converted to
ILs [FurEt
2
NH][H
2
PO
PO
4
4 2 2 4
] (1), [VanEt NH][H PO ] (2), and
[p-AnisEt NH][H
2
2
] (3) via stoichiometric addition of phos-
phoric acid in nearly quantitative yields (Fig. 3).
In all cases, H NMR showed deshielding of methylene and
1
(
33). Biological treatment of lignin by fungi of the genus
methyl protons of the newly formed alkyl ammonium dihydrogen
phosphate ILs, compared with those observed in the respective
tertiary amines. For example, the methylene protons of the ter-
tiary amine derived from furfural migrated downfield from
δ = 2.41 ppm to δ = 2.79 ppm (q, 4H, J = 8 Hz) and from δ =3.56
Pleurotus has been shown to produce p-anisaldehyde as the
dominant aromatic metabolite found in the culture broth (38).
Aldehydes such as furfural and hydroxymethylfurfural can be
derived in high yields from cellulose, hemicellulose, and un-
treated biomass (39). IL pretreatment of lignocellulosic biomass
2
ppm to δ = 4.09 ppm (s, 2H) upon conversion to [FurEt NH]
Switchgrass
Poplar
Eucalyptus
Feedstocks
Corn stover
The “Closed Loop” Biorefinery
Pine
Rice straw
IL Pretreatment
FermentaƟon to Fuels
H
OH
and Chemicals
H
O
HO
HO
H
H
OH
EnzymaƟc SaccharificaƟon
H
OH
Liquids (Monomeric Sugars)
Solids (Lignin/Hemicellulose)
OH
H
OH
H
OCH3
OH
OHO
H
HO
O
H
H
H
OH
H
O
HO
OHO
H
OCH3
HO
H
H
O
O
H
O
OH
HO
OCH3
O
O
O
HO
HO
OH
H
O
H
N+
Cations
O
R'
N+
O
O
R
= H, CH2OH
OCH3
O
R'
R'
OCH3
R' = H, OH, OCH3
R
OCH3
OH
Conversion to ILs
O
O
S
O
O
Anions
DepolymerizaƟon
HO
P
O- HO
O-
& Conversion
O-
O- etc.
H
OH
O
Fig. 1. Hypothetical process flow for a closed-loop biorefinery using ILs derived from lignocellulosic biomass.
E3588
|
www.pnas.org/cgi/doi/10.1073/pnas.1405685111
Socha et al.

total solid recovery of 56.7%. When comparing the monomeric
sugar yields between [VanEt NH][H PO ] and [p-AnisEt NH]
2
2
4
2
2 4
[H PO ], it appears that the latter is a more promising candidate,
as it provided 73.2% of the recovered biomass as glucan or xylan,
compared with these combined sugar yields from [VanEt NH]
PO ] pretreatment, being ∼65.7%.
Compositional analysis of switchgrass pretreated with
FurEt NH][H PO ] showed 62.8% solid recovery, of which 52.5%
2
[H
2
4
[
2
2
4
was glucan and 17.8% was xylan. Significant xylan removal (48.8%)
and lignin removal (20%) was observed for furfural-derived IL.
Fig. 2. Lignin- and hemicellulose-derived ILs evaluated in this study.
FurEt NH][H PO ] (1), [VanEt NH][H PO ] (2), and [p-AnisEt NH][H PO
3) were prepared from furfural, vanillin, and p-anisaldehyde, respectively. IL 4
is [C mim][OAc].
[
(
2
2
4
2
2
4
2
2
4
]
Performance of Biomass-Derived ILs for Biomass Pretreatment. From
the compositional analysis, it was clear that 91–95% and 49–85%
of the samples’ glucan and xylan, respectively, were recovered in
the solids. To liberate monomeric sugars (glucose and xylose) for
downstream fermentation, enzymatic saccharification of this mate-
rial was performed using a mixture of cellulase and endoxylanase
2
[
H PO ]. A similar trend was observed for the equivalent pro-
2 4
tons of the methyl groups, shifting from δ = 0.99 ppm in the
amine to δ = 1.19 ppm (t, 6H, J = 8 Hz) in the IL. Upon for-
mation of [FurEt
enzymes. As expected, pretreatment with [C mim][OAc] provided
1
3
2
2
NH][H
2
PO
4
] from the tertiary amine,
C
>90% of the theoretical glucose and >70% of the theoretical xylose
NMR revealed shielding effects on methyl carbon atoms, which
shifted upfield from δ = 11.9 in the amine to δ = 9.7 ppm in the
resulting IL. A slight shielding of the methylene carbons in the α
position were also observed when converting the amine δ = 45.9
ppm (2C) and δ = 54.8 ppm (1C) to the IL δ = 45.3 ppm (2C)
and δ = 54.6 ppm (1C). Similar chemical shift trends were ob-
served for [VanEt NH][H PO ] and [p-AnisEt NH][H PO ],
yields after a 24-h incubation with enzymes. Based on the compo-
sitional analysis, it was not surprising that [VanEt NH][H PO
gave only 50% glucose and <40% xylose yields at this time point.
Pretreatment with ILs [FurEt NH][H PO ] and [p-AnisEt NH]
PO ] gave >80% glucose and ∼60% xylose yields after the 24-h
2
2
4
]
2
2
4
2
[
H
2
4
enzymatic saccharification period, and when the reaction was
extended to 72 h, both compounds provided 90–95% glucose
and 70–75% xylose yields. Although the hydrolysis rates were
2
2
4
2
2
4
compared with their respective amines derived from vanillin and
p-anisaldehyde. In total, these results compared well to model
tertiary amines (41).
In addition to being among the most abundant representative
aldehydes of chemically and biologically depolymerized lignin,
2 2 4 2 2 4
respectively, [VanEt NH][H PO ] and [p-AnisEt NH][H PO ]
were also selected based on their subtle differences in polarity
and substituent effects. The polarity of an IL has been correlated
with its ability to solubilize both lignin (42, 43) and cellulose (44–
slower for biomass pretreated with [FurEt
2
NH][H
2 4
PO ] and
[
[
p-AnisEt NH][H PO ] ILs, they compared very well to [C mim]
2
2
4
2
OAc] in terms of overall sugar yields (Fig. 4).
X-Ray Diffraction of Untreated and Pretreated Biomass. The pro-
portions of crystalline cellulose (cellulose I) to amorphous cel-
lulose (cellulose II) found in pretreated switchgrass samples
were determined by powder X-ray diffraction. Because the bio-
mass used in this study contained lignin and hemicellulose,
crystallinity index (CrI) values were interpreted as relative
comparisons. Untreated switchgrass had a CrI of 0.56 and
showed major reflections at 22.5° and 15.7° 2θ, characteristic of
4
6). Hydroxyl and methoxyl groups activate aromatic ring sys-
tems, and therefore [VanEt NH][H PO ] and [p-AnisEt NH]
H PO ] were used to dissect the role of these substituents on the
2
2
4
2
[
2
4
relative acidity of the cation and relate these effects to biomass
pretreatment.
the cellulose I polymorph, corresponding to [002] and combined
ꢀ
[
101] + [101] lattice places, respectively (48). A CrI of 0.22 ac-
Compositional Analysis of Untreated and Pretreated Switchgrass.
Compositional analysis of cellulose, hemicellulose, and lignin
was performed directly on the untreated switchgrass, and glucan
and lignin values are in good agreement with previously pub-
lished values (18, 47). The raw biomass contains 34.7% glucan,
companied by distortion in peak shape and shifts in position to
21° 2θ for the [002] lattice place and near disappearance of the
∼
ꢀ
combined [101] + [101] lattice places were observed for switchgrass
pretreated with [C mim][OAc]. These changes are characteristic
of the transition from cellulose I to cellulose II, and are commonly
observed in biomass pretreated with [C mim][OAc] (49). Al-
though [p-AnisEt NH][H PO ]-pretreated switchgrass showed a
2
2
1.8% xylan, and 19.3% lignin by mass (Table 1). The remaining
2
biomass is likely a combination of cell wall components not
detected by our methods, such as nonlignin phenolics, proteins,
and arabinoglactans.
2
2
4
2 2 4
slightly lower CrI (0.47), [FurEt NH][H PO ] showed a CrI of 0.57
and all of the samples pretreated with biomass-derived ILs had 2θ
Switchgrass was pretreated at 160 °C with ILs synthesized from
peak shapes and positions similar to the untreated switchgrass
2
lignin and hemicellulose as well as our gold standard, [C mim]
(SI Text, section S3). The results show that cellulose decrystalli-
[
OAc], and subsequent compositional analysis was performed on
zation, as measured by CrI, is not a prerequisite for efficient
enzymatic saccharification after pretreatment with [p-AnisEt NH]
H PO ] and [FurEt NH][H PO ]. Cellulose accessibility, therefore,
biomass regenerated from reactions using water as the anti-
solvent. As expected, pretreatment increased the percent by
mass of cellulose through the solubilization of lignin and hemi-
2
[
2
4
2
2
4
cellulose. The benchmark IL for biomass pretreatment, [C
2
mim]
[
OAc] provided results consistent with previous studies, showing
5
8.0% biomass recovery, of which 55.2% was glucan, 20.8% was
xylan, and 15.8% was lignin. This equates to 44.8% xylan and
5
2.4% lignin removal, respectively.
2 2 4
Of the lignin-based ILs tested, [VanEt NH][H PO ] provided
the highest solid recovery (77.9%) but performed poorest in terms
of xylan and lignin removal, with 33.9% and 3.9%, respectively. It
is hypothesized that because [VanEt NH][H PO ] is the most
2
2
4
polar IL, it is least suitable for lignin removal. The least polar
2 2 4
biomass-derived IL, [p-AnisEt NH][H PO ], showed far greater
xylan and lignin removal, 51.4% and 43.0%, respectively, with
Fig. 3. Two-step synthesis to produce ILs from biomass-derived aldehydes.
Socha et al.
PNAS
|
Published online August 18, 2014
|
E3589

Table 1. Composition analysis of IL-pretreated and untreated switchgrass
IL pretreatment
Solid recovery, %
Glucan, %
Xylan, %
Arabinan, %
Lignin, %
Xylan removal, %
Lignin removal, %
1
2
3
4
[FurEt
[VanEt
[p-AnisEt
[C mim][OAc]
2
NH][H
NH][H
NH][H
2
PO
PO
PO
4
]
62.8 ± 1.1
77.9 ± 0.9
56.7 ± 1.7
58.0 ± 1.2
N/A
52.5 ± 0.7
41.9 ± 0.4
54.5 ± 0.9
55.2 ± 0.5
34.7 ± 1.3
17.8 ± 0.6
23.8 ± 0.3
18.7 ± 0.3
20.8 ± 1.2
21.8 ± 0.5
2.7 ± 0.1
3.1 ± 0.1
3.0 ± 0.1
3.9 ± 0.1
2.6 ± 0.4
24.6 ± 0.6
23.8 ± 0.3
19.4 ± 0.9
15.8 ± 0.1
19.3 ± 1.5
48.8
33.9
51.4
44.8
N/A
20.0
3.9
43.0
52.4
N/A
2
2
4
]
2
2
4
]
2
Untreated switchgrass
Note: ± indicates SD.
is likely affected by other factors such as lignin/hemicellulose
content, porosity, and particle size (50). (See SI Text, section S3
and Fig. S1 for full experimental details.)
with the same anion demonstrated that cation acidity is also
important for cellulose dissolution (54). In the case of ILs
[VanEt NH][H PO ] and [p-AnisEt NH][H PO ], one would
2 2 4 2 2 4
expect the differential substitution of the electron-donating
Solvation Parameters for Biomass-Derived ILs. Kamlet–Taft solvent
parameters have been used to measure the ability of a solvent to
donate a hydrogen bond (α), and accept a hydrogen bond (β)
groups to affect the N atom’s affinity for the H PO4 proton.
3
Molecular modeling was performed to estimate these effects.
Table 2 shows the calculated interaction energies (IEs) cor-
rected with basis set superposition error correction and IL sol-
vent parameters for ILs investigated here. The IEs for lignin-
derived ILs are considerably higher in energy than [C mim]
2
[OAc]. Notably, all three biomass-derived ILs also have higher β
(51). It has been shown that basicity (β) correlates well with an
IL’s ability to dissolve lignocellulose (52), and that net basicity
correlates with an IL’s ability to dissolve cellulose (46, 53). A
recent experimental study on a range of cations in combination
values compared with [C mim][OAc]. The optimized geometries
2
of ILs from biomass-derived aromatic aldehydes are shown in
Fig. 5. In general, it can be seen from the IL geometries that the
most stable conformation arises from interactions of an oxygen
atom of the anion with the hydrogen atom on the nitrogen in the
cation. Elongation of the N—H bond (from 1.1 to 1.5 Å) is noted
in comparison with the isolated cation N—H bond distance (1.03
Å). Due to the significant elongation of the N—H bond in
1
00
A
8
6
4
2
0
0
0
0
0
[
2 2 4
FurEt NH][H PO ], the hydrogen atom migrates to the oxygen
atom of the anion, allowing strong intermolecular interactions
between the IL cation and the anion.
1
2
3
4
The presence of a para hydroxyl group and a meta methoxyl
groups in [VanEt
[p-AnisEt NH][H
2
NH][H
2 4
PO ] versus a para methoxyl group in
Raw SG
2
2
PO ], showed only slight differences in IEs and
4
proton affinities, but significantly influenced the calculated solvent
parameters. Comparison of the experimental results with the
calculations of Kamlet–Taft solvent parameters of these ILs shows
that effective pretreatment requires an IL with high hydrogen
bond basicity and high net basicity. From Table 2, it can be seen
that net basicity of [FurEt NH][H PO ] and [p-AnisEt NH]
0
24
48
72
2
2
4
2
Time, h
[H
paratively lower proton affinities of the biomass-derived ILs
compared with [C mim][OAc] could also provide hydrogen-
2 4 2
PO ] ILs are higher than that of [C mim][OAc]. The com-
100
B
2
bonding interactions with lignin, and delignification has been
shown to significantly improve saccharification kinetics and yields
+
8
6
4
2
0
0
0
0
0
(
2
42). A computational comparison showed that the [FurEt NH]
cation had a stronger IE (28.8 kcal/mol) with a lignin model com-
pound compared with the [C
model predicts that the proton of the [FurEt NH] cation can form
+
2
mim] cation (21.4 kcal/mol). The
+
2
hydrogen bonds with ether linkages and hydroxyl oxygen atoms of
lignin. (See SI Text, section S1 and Fig. S2, for full experimental
details.) Combined, the compositional analysis and computational
data from biomass-derived ILs suggest that high basicity and
high net basicity are critical parameters required for efficient
pretreatment.
1
2
3
4
Raw SG
Glycome Profiling of Untreated and IL-Pretreated Switchgrass. Glycome
profiling of untreated switchgrass and switchgrass pretreated with
either [FurEt
OAc] was conducted to monitor changes in the overall com-
2 2 4 2 2 4 2
NH][H PO ], [p-AnisEt NH][H PO ], or [C mim]
0
24
48
72
[
Time, h
position and extractability of most major noncellulosic plant cell
wall glycans. This method of analysis was performed to differentiate
specific changes to biomass cell walls, and correlate those
changes with reduced recalcitrance as a function of pretreatment.
Fig. 4. Yields of glucose (A) and xylose (B) from untreated switchgrass (Raw
SG) and switchgrass pretreated with [FurEt NH][H PO ] (1), [VanEt NH]
PO ] (2), [p-AnisEt NH][H PO ] (3), and [C mim][OAc] (4).
2
2
4
2
[H
2
4
2
2
4
2
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Socha et al.

Table 2. Calculated IE, proton affinity, acidity (α), basicity (β), and net basicity values of ILs
evaluated in this study
Proton affinity (base),
Compound
IE, kcal/mol
kcal/mol
α, eV
β, eV
Net basicity
[
[
[
[
FurEt
VanEt
p-AnisEt
mim][OAc] (4)
2
NH][H
NH][H
NH][H
2
PO
PO
PO
4
] (1)
] (2)
] (3)
165.51
118.14
117.83
106.42
233
234
235
257
2.14
2.35
2.24
2.28
3.53
2.99
3.37
2.97
1.39
0.63
1.13
0.69
2
2
4
2
2
4
C
2
Fig. 6 shows glycome profiles of untreated and IL-pretreated
switchgrass. The profiles of the IL-pretreated switchgrass dif-
fered significantly from untreated switchgrass, emphasizing an
overall change in the cell wall structure and integrity due to the
solubilization and removal of lignin and lignin-associated car-
bohydrates during the pretreatment. The major changes in the
profiles are highlighted as yellow dotted rectangles. The most
dramatic changes in the glycome profiles were observed in the
untreated switchgrass, while still being abundant in the 4 M KOH
and 4 M KOHPC extracts. Loss of arabinogalactan epitopes was
observed with [C mim][OAc] pretreatment, but this loss was not as
2
extensive as was observed for the other two ILs. Finally, all homo-
galacturonan epitopes (recognized by the homogalacturonan
backbone-1 group of monoclonal antibodies) were completely
removed in all extracts of switchgrass pretreated with [C mim]
2
[OAc], as was the case with the other two ILs.
[
FurEt NH][H PO ]- and [p-AnisEt NH][H PO ]-pretreated
Pretreatment of switchgrass with any of the ILs also altered
the amounts of material recovered in each extraction step. A
marginal increase in the amount of carbohydrate materials re-
covered in the carbonate extracts was observed in the case of
biomass pretreated with [FurEt NH][H PO ] and [C mim]
2
2
4
2
2
4
biomass samples. Both of these ILs removed essentially all of the
arabinogalactan and homogalacturonan epitopes, and almost all
of the xylan epitopes recognized by the xylan-3 through -5 groups
of xylan-directed antibodies. The significant reduction observed
2
2
4
2
in these epitopes from [FurEt
2
NH][H
2
PO
4
]- and [p-AnisEt
2
NH]
[OAc]. In all IL-pretreated samples, relatively higher amounts of
carbohydrate materials were released in 1 M KOH extracts
compared with the untreated biomass. Correspondingly, notable
reductions were observed in all IL-pretreated samples in terms of
the amount of carbohydrates extracted with 4 M KOH. These
results suggest an enhanced extractability of hemicellulosic gly-
cans, which are the predominant alkali-extractable wall compo-
nents in IL-pretreated samples. Earlier studies have shown that
such increased hemicellulose extractability (particularly of
xylans) is indicative of reduced recalcitrance (55).
[
H PO ]-pretreated biomass samples may be due to two possible
2
4
reasons. First, pretreatment conditions facilitated the removal of
epitope structures from the corresponding glycans, and second,
pretreatment caused significant cleavage of these cell wall gly-
cans making their adsorption to the ELISA plates inefficient. It
is also possible that a combination of these effects occurred.
Pretreatment with [FurEt NH][H PO ] and [p-AnisEt NH]
2
2
4
2
2 4
[H PO ] also resulted in the loss of a substantial portion of the
most tightly bound xyloglucan epitopes (4 M KOHPC extract)
and a significant amount of the unsubstituted homoxylan epito-
pes recognized by the xylan-6 and -7 groups of antibodies. These
latter epitopes became more extractable after pretreatment as
indicated by the increased abundance of these xylan epitopes in
Glycome profiling revealed significant changes in both glycan
epitope content and extractability for IL-pretreated switchgrass
compared with untreated switchgrass. Overall, the effects of the
two biomass-derived ILs were different and more severe than
the oxalate and carbonate extracts of the [FurEt
2
NH][H
2
PO
4
]-
the effects of [C mim][OAc]. The observed differences suggest
2
and [p-AnisEt
2
NH][H
2
PO
4
]-pretreated samples.
a mechanism of action for [FurEt NH][H PO ] and [p-AnisEt NH]
2
2
4
2
Pretreatment of switchgrass with [C mim][OAc] also led to
2 4 2
[H PO ] that differs from that of [C mim][OAc]. (See SI Text,
2
a significant change in the glycome profile compared with the
untreated biomass, but the changes are clearly distinct from those
observed with the other two ILs. In particular, there was no ap-
parent loss of xylan epitopes, but rather some xylans became more
easily extractable and were observed in the oxalate and carbonate
extracts, in contrast to the untreated biomass where almost no
xylan epitopes were observed in these extracts. Enhanced extract-
ability of xyloglucan epitopes was observed in switchgrass pre-
section S2 and Table S1 for full experimental details.)
Effect of Biomass-Derived ILs on Lignin–Carbohydrate Associations.
Treatment of plant cell walls with chlorite cleaves and removes
lignin, extracting all lignin-associated polysaccharides. The chlorite
extracts from samples pretreated with [FurEt
NH][H
PO ] and
2
2 4
[p-AnisEt NH][H PO ] was virtually devoid of all glycan epito-
2 2 4
pes, suggesting a significant reduction in lignin–polysaccharide
treated with [C mim][OAc]. In these samples, epitopes recognized
associations in switchgrass subjected to pretreatment with these
2
by nearly all groups of xyloglucan-directed monoclonal antibodies
were abundantly present in 1 M KOH extracts, compared with
ILs. In contrast, the chlorite extracts from [C mim][OAc]-pre-
2
treated switchgrass, however, still contained significant amounts
Fig. 5. Optimized geometries of [FurEt
2
NH][H
2
PO
4
] (1), [VanEt
2
NH][H
2
PO
4
] (2), and [p-AnisEt
2
NH][H
2
4
PO ] (3) ILs evaluated in this study.
Socha et al.
PNAS
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|
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Fig. 6. Glycome profiling of untreated switchgrass (SG), and switchgrass pretreated with ILs [FurEt
[
(
2
NH][H
OAc] (4): Sequential cell wall extracts (Bottom) were subjected to ELISA screens with monoclonal antibodies for most major noncellulosic plant glycan classes
Right). The ELISA binding response values are represented as a color-coded heat map (Center), and the recovered masses of carbohydrate material resulting
2 4 2 2 4 2
PO ] (1), [p-AnisEt NH][H PO ] (3), and [C mim]
from each extraction step is represented with bar graphs (Top).
of xylans and pectic-arabinogalactan epitopes. Compositional
hemicellulose. Reductive amination of these aromatic aldehydes
followed by treatment with phosphoric acid provided three ILs in
excellent yields without the need for chromatographic purifica-
tion. Compositional analysis and sugar yields from enzymatic
analysis revealed that [C
of lignin (52.4%) compared with [FurEt
and [p-AnisEt NH][H PO ] (43.0%), suggesting that switchgrass
pretreated with [C
other cell wall associations. There seems to be no such associations biomass-derived ILs to [C
between lignin and xyloglucans in any of the ILs tested. Taken tion with [FurEt NH][H PO
together, the above results provide further evidence that both vided 90% and 96% of total possible glucose and 70% and 76%
FurEt NH][H PO ] and [p-AnisEt NH][H PO ] reduce biomass
of total possible xylose, respectively, after biomass pretreatment.
NH][H PO ] and
] also showed the highest β values, highest
2
mim][OAc] removes greater amounts
2
NH][H PO ] (20.0%)
2
4
2
2
4
mim][OAc] may retain polysaccharides through hydrolysis of pretreated switchgrass was used to compare these
mim][OAc]. Enzymatic saccharifica-
] and [p-AnisEt2NH][H PO ] pro-
2
2
2
2
4
2
4
[
2
2
4
2
2
4
recalcitrance through a similar mechanism, one that is significantly Of the three biomass-derived ILs, [FurEt
2
2
4
distinct from that of [C mim][OAc].
[p-AnisEt2NH][H PO
2 4
2
net basicity, and best ability to remove lignin. Computationally,
[VanEt NH][H PO ] showed the lowest net basicity and lignin
Cost Estimates. One important consideration for the proposed
application for any IL is cost. A preliminary technoeconomic
analysis (Table S2) based on raw materials cost for the biomass-
2
2
4
removal efficiency, and the lowest sugar yields were observed
experimentally. Compared with [C mim][OAc], we found that
2
derived ILs placed these compounds at approximately $12–15/kg. [FurEt NH][H PO ] and [p-AnisEt NH][H PO ] had substantially
2
2
4
2
2
4
Replacement of diethylamine and Na(OAc)
for reductive amination (56) could significantly lower the raw removal. Sugar yields from switchgrass pretreated with these
materials cost to $4/kg. Although the methods and costs associat-
biomass-derived compounds, however, were nearly equivalent to
ed with the purification of aldehydes from depolymerized lignin yields from switchgrass pretreated with [C mim][OAc]. Taken
3
BH with NH
3
and H
2
higher β and net basicity values and were less effective toward lignin
2
and hemicellulose are still under investigation, there are industrial together, compositional analysis and molecular modeling data
precedence for such processes and our results indicate that pro- suggest that lignin removal efficiency is a better indicator of
duction of biomass-derived ILs holds significant promise. Current switchgrass pretreatment performance compared with β and net
pricing estimates for [C mim][OAc] are approximately $17–25/kg.
basicity values when comparing benzylammonium and furfur-
ylammonium ILs to those of imidazolium. These results are in
agreement with previous findings in our group whereby choline-
derived ILs with high lignin removal efficiency and high experi-
mentally obtained β values resulted in high sugar yields across
2
Conclusions
We have shown the first (to our knowledge) synthesis and evalua-
tion of a series of ILs from monomers obtained from lignin and
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Socha et al.

a broad range of pretreatment temperatures (42). It is thought that
the mechanism of action of the lignin- and hemicellulose-derived
ILs is therefore more akin to other biologically derived ILs, such as
ILs prepared from choline. Glycome-profiling experiments further
suggested that the biomass-derived ILs [FurEt NH][H PO ] and [p-
52.0 mmol, 1 equiv) in MeOH (25.9 mL) at 0 °C was slowly added H
3 4
PO (5.07 g,
5
2.0 mmol, 1 equiv). The solution was allowed to stir, warming to room
temperature for 3 h. Methanol was evaporated under vacuum and the IL 3
1
was obtained as a viscous pale yellow oil (14.9 g, 99% yield). H NMR: 1.18 (t,
6
H, J = 8 Hz), 2.85 (q, 4H, J = 8 Hz), 3.77 (s, 3H), 4.00 (s, 2H), 6.95 (d, 1H, J = 10
2
2
4
13
Hz), 7.52 (d, 2H, J = 8 Hz); C NMR: 9.1 (2C), 45.3, 48.8, 54.6, 55.2, 114.1 (2C),
24.7, 132.1 (2C), 160.0.
AnisEt
2
NH][H
2 4
PO ] act on plant cell walls in a mechanism distinct
1
2
from [C mim][OAc], and studies are underway to understand these
process implications in terms of lignin and hemicellulose de-
polymerization and IL recycling. These results indicate that
biomass-derived ILs are very effective in biomass pretreatment
and establish an important foundation for the further study of
these unique compounds in other industrial applications.
Biomass Pretreatment. Four hundred milligrams of dry switchgrass were mixed
with 3.6 g of ILs and 0.4 mL of water to give a 10 wt% biomass loading in tubular
reactors made of 0.75-inch-diameter × 6-inch-length stainless-steel (SS316) tubes
and sealed with stainless-steel caps. All pretreatment reactions were run in
triplicate. Tubular reactors were heated to 160 °C in convention oven. The heat-
up time was ∼10 min and the reactions were run for 3 h after the desired
temperature was reached. After pretreatment, the reactors were allowed to cool
to room temperature. The mixture of IL, water, and pretreated biomass was
transferred to a 50-mL Falcon tube using deionized (DI) water to a final volume
of 25 mL and then centrifuged at 3,220 × g to separate the solid and liquid
phases. An aliquant of supernatant was taken for lignin and sugar analysis. The
solid fraction was washed sequentially with 40 mL of hot water, 40 mL of 1:1
acetone:water, and three times with 40 mL of hot water to remove any residual
IL and/or sugars. Washed solids were lyophilized in a FreeZone Freeze Dry System
Experimental Section
All solvents and chemicals were reagent grade and used without purification.
1
NMR spectra were obtained on an Anasazi Eft-90 instrument (90 MHz for H) in
1
DMSO-d
6
and calibrated with tetramethylsilane for H (δ = 0.00 ppm) and
13
DMSO for C (δ = 39.5 ppm), respectively (Fig. S3). Mass spectra were obtained
on an Agilent 6890 GC equipped with an Agilent 5973 mass detector. Synthetic
reactions were performed in triplicate, and average yields are reported.
(
Labconco) for composition analysis and enzymatic saccharification.
Preparation of Tertiary Amines. N-Ethyl-N-(furan-2-ylmethyl)ethanamine. To
a solution of furfural (10.0 g, 104 mmol, 1 equiv) in 1,2-dichloroethane (360
mL), cooled to 0 °C, was added diethylamine (18.3 g, 121 mmol, 1.2 equiv)
and allowed to stir for 15 min. Sodium triacetoxyborohydride (30.9 g, 1.4
equiv) was added portionwise, and the mixture was stirred and allowed to
warm to room temperature overnight under N . The solution was quenched
2
by adding aqueous 3 M HCl and the amine product was thus drawn in the
aqueous phase (pH ∼1). The organic impurities were removed with the di-
chloroethane phase, and the aqueous phase was washed with CH Cl . The pH
2 2
Enzymatic Saccharification. Enzymatic saccharification of untreated and
pretreated samples were run in triplicate following National Renewable
Energy Laboratory Laboratory Analytical Procedure 9 “Enzymatic Sacchari-
fication of Lignocellulosic Biomass” standard conditions (50 °C, 0.05 M cit-
rate buffer, pH 4.8) (57). Citrate buffer (final molarity, 50 mM), sodium azide
(
antimicrobial, final concentration of 0.02 g/L), enzymes, and DI water were
mixed with pretreated solids to achieve a final solid loading of ∼10%. En-
zyme loadings were calculated from compositional analysis data. Fifteen
milligrams of CTec2 (batch number VCN10001) per gram of untreated bio-
mass and 1.5 mg of HTec2 (batch number VHN00001) per gram of glucan
were used. The protein content of enzymes was measured by bicinchoninic
acid (BCA) assay with a Pierce BCA Protein Assay Kit (Thermo Scientific) using
BSA as protein standard. CTec2 has a protein content of 186.6 ± 2.0 mg/mL,
whereas HTec2 contains 180.1 ± 1.8 mg/mL protein. The enzyme mixtures
were gifts from Novozymes N.A. An aliquot of supernatant was taken at 2, 6,
24, and 72 h and was analyzed by HPLC for monosaccharide content as
described previously (58). Glucose yield was calculated from the maximum
potential glucose available from glucan in pretreated biomass.
of the aqueous phase was raised to ∼9.5 by addition of 3 M KOH, and the
product was extracted two times with EtOAc. The combined organic layers are
dried over Na SO and concentrated to provide the product as a light brown
2 4
liquid (13.1 g, 82% yield). All of the tertiary amine products below were
+
prepared using this method. m/z [M ] observed (Obsd.), 153.1, calculated
1
(
Calcd.), 153.12 for C
9
H
17NO; H NMR: 0.99 (t, 6H, J = 8 Hz), 2.41 (q, 4H, J = 8
13
Hz), 3.56 (s, 2H), 6.23 (m, 1H), 6.34 (m, 1H), 7.51 (m, 1H); C NMR: 11.9 (2C),
6.3 (2C), 48.3, 107.9, 109.9, 141.8, 152.7.
-((Diethylamino)methyl)-2-methoxyphenol. Following the general protocol for
the reductive amination of aldehydes to tertiary amines, 15.0 g (100 mmol,
eq) of vanillin provided the desired product as a white amorphous solid
melting point, 75–77 °C; 18.3 g; 89% yield). m/z [M ] Obsd., 209.1, Calcd.,
2
09.14 for C12H19NO ; H NMR: 0.96 (t, 6H, J = 8 Hz), 2.43 (q, 4H, J = 8 Hz),
.41 (s, 2H), 3.74 (s, 3H), 6.70 (s, 2H), 6.85 (s, 1H); C NMR: 11.6 (2C), 46.0 (2C),
5.5, 56.9, 112.6, 115.1, 120.9, 130.6, 145.3, 147.4.
4
4
1
(
2
3
5
+
Glycome Profiling. To conduct glycome profiling, alcohol-insoluble residues of
cell walls derived from untreated and IL-pretreated switchgrass were sub-
jected to sequential extractions using increasingly harsh reagents (59). In the
case of native plant cell walls, mild conditions such as oxalate and carbonate
extracts, remove the most loosely bound pectic polysaccharides. Alkaline
treatment with 1 M KOH removes more tightly bound pectin and hemi-
celluloses that mainly comprise xylan and pectin, and 4 M KOH extracts
xyloglucans in addition to xylan and pectins. Treatment with acetic acid/
chlorite at high temperature (chlorite extraction) breaks down most of the
lignin, releasing lignin-associated polysaccharides into this fraction. Finally,
a 4 M KOHPC treatment removes any residual polysaccharides that remain
bound to the cell wall via association with lignin. To facilitate glycome pro-
filing, all extracts were probed with a comprehensive suite of cell wall glycan-
directed monoclonal antibodies, and the binding responses of these mono-
clonal antibodies are represented as color-coded “heat maps” (59). The total
amounts of carbohydrates recovered under each extraction condition were
also quantified gravimetrically and are represented as bar graphs atop Fig. 6.
(See SI Text, section S2 and Table S1 for full experimental details.)
1
13
N-Ethyl-N-(4-methoxybenzyl)ethanamine. Following the general protocol for the
reductive amination of aldehydes to tertiary amines, with 14.0 g (103 mmol,
1
eq) of p-anisaldehyde, provided the desired product as a pale yellow oil
+
1
(
19.0 g, 96% yield). m/z [M ] Obsd., 193.1, Calcd., 193.15, for C12
H19NO; H
NMR: 0.96 (t, 6H, J = 8 Hz), 2.43 (q, 4H, J = 8 Hz), 3.44 (s, 2H), 3.73 (s, 3H), 6.85
13
(
1
d, 2H, J = 9 Hz), 7.22 (d, 2H, J = 9 Hz); C NMR: 11.6 (2C), 45.9 (2C), 54.8, 56.3,
13.3 (2C), 129.5 (2C), 131.6, 158.2.
Preparation of ILs. N-Ethyl-N-(furan-2-ylmethyl)ethanamine, H
3
PO
], 1). To a stirred 2 M solution of N-ethyl-N-(furan-2-ylmethyl)ethanamine
10.0 g, 65.0 mmol, 1 equiv) in MeOH (32.6 mL) at 0 °C was slowly added H PO
6.4 g, 65.0 mmol, 1 equiv). The solution was allowed to stir, warming to room
4 2
salt ([FurEt NH]
[
(
(
2 4
H PO
3
4
temperature for 3 h. Methanol was evaporated under vacuum and the IL 1
was obtained as a light brown liquid (16.0 g, 98% yield). All of the dihydrogen
1
phosphate ILs described below were prepared by this method. H NMR: 1.19
(
t, 6H, J = 8 Hz), 2.79 (q, 4H, J = 8 Hz), 4.09 (s, 2H), 6.49 (m, 1H), 6.65 (m, 1H),
Computational Details and Theoretical Development of Hydrogen-Bonding
Acidity and Basicity Descriptors. The geometry optimizations of three ionic
liquids (ILs), [FurEt NH][H PO ] (1), [VanEt NH][H PO ] (2), and [p-AnisEt NH]
13
7
.69 (m, 1H); C NMR: 9.74 (2C), 46.4 (2C), 48.8, 111.1, 112.9, 144.2, 146.4.
PO salt ([VanEt NH][H PO ], 2). To
a stirred 2 M solution of 4-((diethylamino)methyl)-2-methoxyphenol (10.0 g,
4
-((Diethylamino)methyl)-2-methoxyphenol, H
3
4
2
2
4
2
2
4
2
2
4
2
[H PO ] (3), were performed using density functional theory (DFT) with the M06-
2
4
4
4
8.0 mmol, 1 equiv) in MeOH (23.9 mL) at 0 °C was slowly added H
8.0 mmol, 1 equiv). The solution was allowed to stir, warming to room
3
PO
4
(4.68 g,
2X hybrid exchange-correlation functional and the 6-311++G(d, p) basis set.
Frequency calculations were carried out to verify that the computed struc-
tures corresponded to energy minima. Interaction energies were calculated
to measure the binding strength between cation and anion of ILs and cor-
rected for basis set superposition error:
temperature for 3 h. Methanol was evaporated under vacuum and the IL 2
was obtained as a pale pink crystals (melting point, 155–157 °C; 14.4 g; 98%
yield). H NMR: 1.09 (t, 6H, J = 8 Hz), 2.70 (q, 4H, J = 8 Hz), 3.74 (s, 2H), 3.78 (s,
3
1
1
13
H), 6.78 (s, 2H), 7.07 (s, 1H); C NMR: 9.08 (2C), 45.2 (2C), 55.3, 55.9, 114.4,
15.5, 123.3 (2C), 147.2, 147.8.
IE = −½EIL − ðEanion + EcationÞꢀ
[1]
N-Ethyl-N-(4-methoxybenzyl)ethanamine, H
a stirred 2 M solution of N-ethyl-N-(4-methoxybenzyl)ethanamine (10.0 g,
3
PO
4
salt ([p-AnisEt
2
NH][H
2
PO
4
], 3). To
where EIL refer to the energies of cation and anion pair (for IL), and Eanion
and Ecation refer to the energies of the anion and cation monomer molecule,
Socha et al.
PNAS
|
Published online August 18, 2014
|
E3593

respectively. In the present study, DFT-based global reactivity descriptors,
such as chemical hardness, chemical potential, and electrophilicity were
calculated (60). Quantum chemical reactivity descriptors predicted acidity,
basicity, and net basicity, and showed good correlation with experimentally
observed values for [C mim][OAc] (4). These results provide a basis to de-
2
scribe the observed experimental delignification trend for a set of ILs (42).
These descriptors were used to derive the molecular basicity, acidity, and net
Upon formation of a hydrogen bond, electron density is transferred from
the hydrogen bond acceptor toward the hydrogen bond donor. In the light of
the “hard and soft acids and bases” concept (62), the propensity of molecule
to donate the electron density depends inversely on the electronegativity
and hydrogen atom-attracting sites are formed through the interaction of
intramolecular sites of different electronegativity. Hence, we define the
IL
hydrogen-bonding basicity B of an IL using a simple working equation
basicity values for the [C
2
mim][OAc] and biomass derived ILs. According to
based on DFT-based descriptors as follows:
the DFT (61), the chemical potential (μ), and chemical hardness (η) are de-
fined as follows:
ꢂ
BIL = η χ:
2
[8]
ꢀ
ꢁ
∂
E
An increased electrostatic component and the high electronegativity of
a molecule influences the hydrogen-bonding ability. In the case of basicity,
electronegativity is very important for hydrogen bonding. Therefore, molecular
acidity dependence on the reduced orbital electronegativity of the molecule is
considered. The formula for the acidity is written as follows:
χ = −μ = −
[2]
[3]
∂
N
vð ~r Þ
and
!
ꢀ
ꢁ
2
1
2
∂ E
1
2
∂μ
∂N
η =
=
,
ꢂ
2
AIL = χ2 2η:
∂N
[9]
vð ~r Þ
vð ~r Þ
where E is the total energy of the system, N is the number of electrons in the
system, and vð ~r Þ is the external potential. μ is identified as the negative of
IL
IL
IL net basicity values were reported as a difference in B − A . Proton affinity
values were calculated at M06-2X/6-311++G(d, p) level from the cationic
conjugated acids and neutral unconjugated bases for ILs 1–4. All quantum
chemical calculations were performed using the Gaussian 09 suite of pro-
grams (63).
the electronegativity ðχÞ. By applying finite difference approximation to Eqs.
2
and 3, we get the operational definition for η and μ as follows:
μ = −
ðIP + EAÞ,
[4]
[5]
2
ACKNOWLEDGMENTS. We thank Novozymes for their generous donation
of Ctec2 and Htec2 enzymes, and Christian Rodriguez [Bronx Community
College (BCC)] for his kind assistance with NMR measurements. This work
conducted by the Joint BioEnergy Institute was supported by the
Office of Science, Office of Biological and Environmental Research, US
Department of Energy, under Contract DE-AC02-05CH11231. Additional
funding was provided by Research Foundation, City University of New
York (65102-00 43). This research used resources of the National Energy
Research Scientific Computing Center and BCC. The glycome profiling was
supported by the BioEnergy Science Center administered by Oak Ridge
National Laboratory and funded by Grant DE-AC05-00OR22725 from the
Office of Biological and Environmental Research, Office of Science, US
Department of Energy. The generation of the CCRC series of plant cell
wall glycan-directed monoclonal antibodies used in this work was
supported by the National Science Foundation Plant Genome Program
(DBI-0421683 and IOS-0923992).
IP − EA
η =
:
2
Chemical potential and chemical hardness can be rewritten using Koopmans’
theorem in terms of the vertical ionization potential and electron affinity as
follows:
ELUMO − EHOMO
η =
,
[6]
2
ELUMO + EHOMO
μ =
,
[7]
2
where ELUMO is the lowest unoccupied molecular orbital’s energy and EHOMO is
the highest occupied molecular orbital’s energy.
1
2
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