100
A. R. Abouelela et al.
is a common analytical technique used to characterize solvent thermal stability by
correlating the solvent mass loss with temperature. Using the step-tangent method,
the TGA scan of different solvents can be compared based on their onset decomposition temperature, T onset . The study of George et al. showed that all [HSO 4 ]-based
ILs with different ammonium cations have higher thermal stabilities (T onset of 290–
320 °C) compared to the famously used [C 2 C 1 im][Ace] (T onset = 215 °C) [29].
Another study evaluating the thermal stability of 66 ILs has also categorized acetatebased ILs among the least stable ILs (T onset < 250 °C) while hydrogen sulfate-based
ILs were classified to be more stable (250 °C ≤ T onset < 300 °C) [93]. It should be
noted that T onset represents the decomposition temperature; however, the significant
mass loss still occurs at much lower temperatures [92]. For example, Rogers and
co-workers have observed up to 15% weight loss within 10 min at 185 °C when
treating bagasse and pine with [C 2 C 1 im][Ace] [16]. Brandt et al. have also reported
that [C 4 C 1 im][Ace] lost 10% of its weight after 60 h at 120 °C, which is much lower
than T onset [71]. As a rough guide, process engineers typically expect a solvent lifetime of around 4 months or 2500 h of operation. As such, under process operating
conditions, the combination of IL losses and degradation should not exceed 0.04%
per hour of operation, which provides an upper bound for an acceptable solvent
degradation rate.
In addition, since T onset values are strongly subject to set-up (e.g., heating rate, gas
flow, and pan material), some authors have proposed a more practical measurement
parameter, T x/z, where x represents a given decomposition extent (e.g., 1% weight
loss) in z length of time (e.g., 10 h) [95]. Applying this to [C 4 C 1 im][Ace] IL, T 0.01/10h
was 102 °C and [C 2 C 1 im][Ace] is expected to have a very similar value. Although it
was suggested that a maximum stable operating temperature for an ionic liquid-based
process should be 10 °C lower than T 0.01/10h , a more realistic value would be at least
30 °C lower to maintain high solvent recovery and integrity. In both temperature
limits, acetate ILs cannot be thermally stable, especially at the elevated temperatures
required for biomass pretreatment.
The IL cation also has an important effect on its thermal stability. Achinivu
et al. have recently reported the use of three acetate-based PILs with pyridinium,
1-methylimidazolium, and pyrrolidinium cations to selectively extract lignin from
corn stover [96]. All three PILs suffered from low thermal stability with significant mass loss observed at temperatures <100 °C, making them easily susceptible
to degradation at processing conditions. The poor thermal stability of acetate-based
ILs is also largely attributed to the presence of dissociated molecular species due to
incomplete protonation of the cation by the weak acetic acid [96].
The ionoSolv process typically uses ILs with ammonium-based cations, such
as triethylammonium [22]. The most dominant degradation mechanism of an
ammonium-based IL is the loss of the alkyl chain from the cation (dealkylation) before the breaking of C–H and C–C bonds at higher temperatures [97].
For [N 0 2 2 2 ][HSO 4 ], diethylammonium or monoethylammonium are the expected
dealkylation products (Scheme 5.2). A recent ionoSolv study recorded the
1 H-NMR
spectra for recycled triethylammonium hydrogen sulfate [N 0 2 2 2 ][HSO 4 ] (4th cycle
at 120 °C and 9 h) to investigate the extent of IL degradation during pretreatment.
A. R. Abouelela et al.
is a common analytical technique used to characterize solvent thermal stability by
correlating the solvent mass loss with temperature. Using the step-tangent method,
the TGA scan of different solvents can be compared based on their onset decomposition temperature, T onset . The study of George et al. showed that all [HSO 4 ]-based
ILs with different ammonium cations have higher thermal stabilities (T onset of 290–
320 °C) compared to the famously used [C 2 C 1 im][Ace] (T onset = 215 °C) [29].
Another study evaluating the thermal stability of 66 ILs has also categorized acetatebased ILs among the least stable ILs (T onset < 250 °C) while hydrogen sulfate-based
ILs were classified to be more stable (250 °C ≤ T onset < 300 °C) [93]. It should be
noted that T onset represents the decomposition temperature; however, the significant
mass loss still occurs at much lower temperatures [92]. For example, Rogers and
co-workers have observed up to 15% weight loss within 10 min at 185 °C when
treating bagasse and pine with [C 2 C 1 im][Ace] [16]. Brandt et al. have also reported
that [C 4 C 1 im][Ace] lost 10% of its weight after 60 h at 120 °C, which is much lower
than T onset [71]. As a rough guide, process engineers typically expect a solvent lifetime of around 4 months or 2500 h of operation. As such, under process operating
conditions, the combination of IL losses and degradation should not exceed 0.04%
per hour of operation, which provides an upper bound for an acceptable solvent
degradation rate.
In addition, since T onset values are strongly subject to set-up (e.g., heating rate, gas
flow, and pan material), some authors have proposed a more practical measurement
parameter, T x/z, where x represents a given decomposition extent (e.g., 1% weight
loss) in z length of time (e.g., 10 h) [95]. Applying this to [C 4 C 1 im][Ace] IL, T 0.01/10h
was 102 °C and [C 2 C 1 im][Ace] is expected to have a very similar value. Although it
was suggested that a maximum stable operating temperature for an ionic liquid-based
process should be 10 °C lower than T 0.01/10h , a more realistic value would be at least
30 °C lower to maintain high solvent recovery and integrity. In both temperature
limits, acetate ILs cannot be thermally stable, especially at the elevated temperatures
required for biomass pretreatment.
The IL cation also has an important effect on its thermal stability. Achinivu
et al. have recently reported the use of three acetate-based PILs with pyridinium,
1-methylimidazolium, and pyrrolidinium cations to selectively extract lignin from
corn stover [96]. All three PILs suffered from low thermal stability with significant mass loss observed at temperatures <100 °C, making them easily susceptible
to degradation at processing conditions. The poor thermal stability of acetate-based
ILs is also largely attributed to the presence of dissociated molecular species due to
incomplete protonation of the cation by the weak acetic acid [96].
The ionoSolv process typically uses ILs with ammonium-based cations, such
as triethylammonium [22]. The most dominant degradation mechanism of an
ammonium-based IL is the loss of the alkyl chain from the cation (dealkylation) before the breaking of C–H and C–C bonds at higher temperatures [97].
For [N 0 2 2 2 ][HSO 4 ], diethylammonium or monoethylammonium are the expected
dealkylation products (Scheme 5.2). A recent ionoSolv study recorded the
1 H-NMR
spectra for recycled triethylammonium hydrogen sulfate [N 0 2 2 2 ][HSO 4 ] (4th cycle
at 120 °C and 9 h) to investigate the extent of IL degradation during pretreatment.