5 Commercial Aspects of Biomass Deconstruction with Ionic Liquids
99
[C 4 C 1 im][C 1 SO 4 ] (total water 1.1 wt% including biomass moisture) produced a very
poorly enzymatically hydrolyzed pulp. On the contrary, mixing [C 4 C 1 im][C 1 SO 4 ]
with 20 wt% water produced a pulp that can be enzymatically hydrolyzed. This led to
a more elaborative and comprehensive investigation of the importance of the presence
of water during the pretreatment with this specific IL. Therefore, the first ionoSolv
study done by Brandt et al. in 2011 investigated the performance of IL/water mixtures, mainly 1-butyl-3-methylimidazolium methyl sulfate [C 4 C 1 im][C 1 SO 4 ] and
1-butylimidazolium hydrogen sulfate [C 4 im][HSO 4 ] for lignocellulosic biomass pretreatment [87]. Glucose yields obtained from M. giganteus pulp after pretreatment
were the highest when the ILs were mixed with 10–40 wt% water. This was the first
demonstration that an IL-based pretreatment can be conducted with the presence
of substantial amounts of water, eliminating the strict anhydrous conditions needed
in an IL-dissolution process. The presence of water was found to be essential for
the hydrolysis of the ether linkages in the lignin structure. Having the flexibility of
operating the process with a substantial amount of water also offers several key processing advantages, including (1) reduced overall solvent mixture cost, as water is a
cheap solvent, and (2) reduced solvent viscosity, which reduces the pumping power
and makes filtration operations easier.
5.4.2 Ionic Liquid Thermal Stability
Solvent thermal stability is one of the key parameters that impact process performance
in several ways. If a solvent has low thermal stability under process conditions, this
translates to poor solvent recovery, higher solvent make-up, higher operating cost,
accumulation of degradation products, changes in solvent properties, and, eventually,
the uncertainty of process performance. In addition, solvent degradation products
may also cause safety and process integrity issues, such as unpredictable corrosion
behavior [89, 90]. Therefore, throughout the development of a new process, such
as IL-based pretreatment, it is crucial to establish a very good understanding of the
long-term thermal stability of the IL used as well as the degradation products formed
in the process. IL thermal stability will have a direct impact on its recyclability
efficiency, which is a major economic driver in the process [91].
ILs have been long known to be thermally stable solvents. However, such a general
statement isn’t possible for every anion/cation combination, especially given the
sheer number of possible ILs [92]. There are some commonly used ILs that are
highly thermally stable, but these usually contain highly non-coordinating fluorinated
anions that are not used for biomass deconstruction [93]. IL-based pretreatment is
usually operated at temperatures of 100–190 °C with retention times of 30 min up to
24 h [94]. The operating temperature range might be high enough to cause solvent
degradation, especially in a closed-loop solvent system.
The thermal stability of ILs is influenced by both the anion and the cation, with
the anion playing a more significant role [93]. Thermal gravimetric analysis (TGA)
99
[C 4 C 1 im][C 1 SO 4 ] (total water 1.1 wt% including biomass moisture) produced a very
poorly enzymatically hydrolyzed pulp. On the contrary, mixing [C 4 C 1 im][C 1 SO 4 ]
with 20 wt% water produced a pulp that can be enzymatically hydrolyzed. This led to
a more elaborative and comprehensive investigation of the importance of the presence
of water during the pretreatment with this specific IL. Therefore, the first ionoSolv
study done by Brandt et al. in 2011 investigated the performance of IL/water mixtures, mainly 1-butyl-3-methylimidazolium methyl sulfate [C 4 C 1 im][C 1 SO 4 ] and
1-butylimidazolium hydrogen sulfate [C 4 im][HSO 4 ] for lignocellulosic biomass pretreatment [87]. Glucose yields obtained from M. giganteus pulp after pretreatment
were the highest when the ILs were mixed with 10–40 wt% water. This was the first
demonstration that an IL-based pretreatment can be conducted with the presence
of substantial amounts of water, eliminating the strict anhydrous conditions needed
in an IL-dissolution process. The presence of water was found to be essential for
the hydrolysis of the ether linkages in the lignin structure. Having the flexibility of
operating the process with a substantial amount of water also offers several key processing advantages, including (1) reduced overall solvent mixture cost, as water is a
cheap solvent, and (2) reduced solvent viscosity, which reduces the pumping power
and makes filtration operations easier.
5.4.2 Ionic Liquid Thermal Stability
Solvent thermal stability is one of the key parameters that impact process performance
in several ways. If a solvent has low thermal stability under process conditions, this
translates to poor solvent recovery, higher solvent make-up, higher operating cost,
accumulation of degradation products, changes in solvent properties, and, eventually,
the uncertainty of process performance. In addition, solvent degradation products
may also cause safety and process integrity issues, such as unpredictable corrosion
behavior [89, 90]. Therefore, throughout the development of a new process, such
as IL-based pretreatment, it is crucial to establish a very good understanding of the
long-term thermal stability of the IL used as well as the degradation products formed
in the process. IL thermal stability will have a direct impact on its recyclability
efficiency, which is a major economic driver in the process [91].
ILs have been long known to be thermally stable solvents. However, such a general
statement isn’t possible for every anion/cation combination, especially given the
sheer number of possible ILs [92]. There are some commonly used ILs that are
highly thermally stable, but these usually contain highly non-coordinating fluorinated
anions that are not used for biomass deconstruction [93]. IL-based pretreatment is
usually operated at temperatures of 100–190 °C with retention times of 30 min up to
24 h [94]. The operating temperature range might be high enough to cause solvent
degradation, especially in a closed-loop solvent system.
The thermal stability of ILs is influenced by both the anion and the cation, with
the anion playing a more significant role [93]. Thermal gravimetric analysis (TGA)