5 Commercial Aspects of Biomass Deconstruction with Ionic Liquids
91
Figure 5.2 shows the rapid publication growth in using ILs for cellulose, biomass,
and lignin processing and transformation.
5.2.1 Ionic Liquids and Biomass
Ionic liquids are a group of salts that are liquid at temperatures of <100 °C. ILs
are known to be designer solvents as each anion–cation pair has its own unique
properties which make them easy to be tailored to fit task-specific applications.
Ionic liquids have been used in many fields of academic research, such as chemical
synthesis [35], catalysis [36], electrochemistry [37], and biomass processing [38].
Today, ILs represent a new paradigm in the processing of biomass and biomass main
platform macromolecules: cellulose and lignin [39]. This was especially exciting
for cellulose as its highly crystalline compact structure makes it insoluble in most
conventional solvents. Traditional cellulose dissolution processes for industrial fiber
production involve the use of toxic carbon disulphide (CS 2 ) solvent to chemically
dissolve cellulose (viscose process) or the application of harsher processing conditions to achieve direct solubilization using N-methylmorpholine-N-oxide (NMMO)
as a solvent (Lyocell process) [40]. Concentrated phosphoric acid is also used as a
solvent for cellulose dissolution [41]. All these solvents possess major drawbacks,
such as toxicity (CS 2 ), instability at elevated temperatures (NMMO), or high corrosivity (phosphoric acid). The discovery (or re-discovery) that some ionic liquids
can be used as solvents for cellulose dissolution and processing opened the opportunity to develop a more stable and environmentally friendly process for biorefinery
applications.
The first patent on cellulose dissolution using an IL was issued in 1933 where
N-ethylpyridinium chloride, in the presence of nitrogen-containing bases, was shown
to be able to dissolve cellulose [42]. The innovation didn’t make any major impact on
the scientific community since very little was known about ionic liquids at the time. In
2002, Swatloski et al. revisited the innovation to re-discover the capability of ionic
liquids to solubilize cellulose [33, 42]. The study investigated cellulose solubility
using common ILs with 1-butyl-3-methylimidazolium [C 4 C 1 im]
+ cation with several
anions, such as chloride Cl
– , bromide Br
– , thiocyanate [SCN]
– , tetrafluoroborate
[BF 4 ]
– , and hexafluorophosphate [PF 6 ]
– . It was found that ILs with Cl
− and Br
−
had greater cellulose solubilities of 14 and 7%, respectively, while the large noncoordinating anions [BF 4 ]
− and [PF 6 ]
− were non-solvents. The proposed mechanism
of cellulose dissolution in [C 4 C 1 im][Cl] was attributed to the ability of the chloride
anions to break the compact intermolecular hydrogen-bond network and form new
bonds with the hydroxyl groups in cellulose. These findings paved the way for using
ILs as novel solvents for cellulose, potentially improving the efficiency and flexibility
of cellulose processing and transformation to chemicals and materials [43].
Ioncell-F is an example of an IL-based process that pioneers the use of acetatebased ionic liquid to produce cellulosic fibers with attractive properties at lower
processing temperatures compared to the Lyocell process [44].
91
Figure 5.2 shows the rapid publication growth in using ILs for cellulose, biomass,
and lignin processing and transformation.
5.2.1 Ionic Liquids and Biomass
Ionic liquids are a group of salts that are liquid at temperatures of <100 °C. ILs
are known to be designer solvents as each anion–cation pair has its own unique
properties which make them easy to be tailored to fit task-specific applications.
Ionic liquids have been used in many fields of academic research, such as chemical
synthesis [35], catalysis [36], electrochemistry [37], and biomass processing [38].
Today, ILs represent a new paradigm in the processing of biomass and biomass main
platform macromolecules: cellulose and lignin [39]. This was especially exciting
for cellulose as its highly crystalline compact structure makes it insoluble in most
conventional solvents. Traditional cellulose dissolution processes for industrial fiber
production involve the use of toxic carbon disulphide (CS 2 ) solvent to chemically
dissolve cellulose (viscose process) or the application of harsher processing conditions to achieve direct solubilization using N-methylmorpholine-N-oxide (NMMO)
as a solvent (Lyocell process) [40]. Concentrated phosphoric acid is also used as a
solvent for cellulose dissolution [41]. All these solvents possess major drawbacks,
such as toxicity (CS 2 ), instability at elevated temperatures (NMMO), or high corrosivity (phosphoric acid). The discovery (or re-discovery) that some ionic liquids
can be used as solvents for cellulose dissolution and processing opened the opportunity to develop a more stable and environmentally friendly process for biorefinery
applications.
The first patent on cellulose dissolution using an IL was issued in 1933 where
N-ethylpyridinium chloride, in the presence of nitrogen-containing bases, was shown
to be able to dissolve cellulose [42]. The innovation didn’t make any major impact on
the scientific community since very little was known about ionic liquids at the time. In
2002, Swatloski et al. revisited the innovation to re-discover the capability of ionic
liquids to solubilize cellulose [33, 42]. The study investigated cellulose solubility
using common ILs with 1-butyl-3-methylimidazolium [C 4 C 1 im]
+ cation with several
anions, such as chloride Cl
– , bromide Br
– , thiocyanate [SCN]
– , tetrafluoroborate
[BF 4 ]
– , and hexafluorophosphate [PF 6 ]
– . It was found that ILs with Cl
− and Br
−
had greater cellulose solubilities of 14 and 7%, respectively, while the large noncoordinating anions [BF 4 ]
− and [PF 6 ]
− were non-solvents. The proposed mechanism
of cellulose dissolution in [C 4 C 1 im][Cl] was attributed to the ability of the chloride
anions to break the compact intermolecular hydrogen-bond network and form new
bonds with the hydroxyl groups in cellulose. These findings paved the way for using
ILs as novel solvents for cellulose, potentially improving the efficiency and flexibility
of cellulose processing and transformation to chemicals and materials [43].
Ioncell-F is an example of an IL-based process that pioneers the use of acetatebased ionic liquid to produce cellulosic fibers with attractive properties at lower
processing temperatures compared to the Lyocell process [44].