5 Commercial Aspects of Biomass Deconstruction with Ionic Liquids
93
The simple acid–base neutralization reaction gives PILs a great economic advantage compared to all other ILs, which are by convention referred to as aprotic ILs
(APILs). As mentioned earlier, the key barrier towards industrial large-scale use of
commonly used ILs is their high production cost, which is usually 5–20 times more
than conventional organic solvents [20]. The high production cost of APILs mainly
stems from: (1) high cost of the starting materials, (2) low atom efficiency during their
synthesis via alkylation and salt metathesis, and (3) lengthy extensive purification
steps which lead to generation of salty wastewater that needs further treatment steps
[54]. The significant reduction in processing steps and the use of inexpensive starting
materials (e.g., mineral acids and amines) make PILs more attractive for industrial
applications. This is especially true as historically many common industrial solvents
were adapted for multi-ton industrial scale because they are generated as inexpensive
by-products in the production line of other higher value products (e.g., acetone is a
by-product of phenol production and toluene is a by-product of gasoline production).
APILs are usually hailed for their non-volatile nature as they have extremely low
vapor pressures. However, this is not always the case for PILs as many PILs have a
measurable boiling point and can be distilled [55, 56]. This feature is directly related
to a PIL’s formation mechanism. PILs exist in equilibrium with their parent acid and
amine. When heated a PIL boils rather than decomposes as the equilibrium reaction
is reversed to form the more volatile neutral acid or base species [52]. In fact, the
volatility of a PIL is highly dependent on its ionicity and degree of proton transfer.
Ideally, the proton transfer from the acid to the base must be completed such that
only ionic species exist to form a true PIL that displays ideal Walden behavior [57].
However, this is not always the case depending on the parent acid and base; the proton
transfer can be only partial [58]. Yoshizawa et al. proposed the use of pK a as a
practical qualitative indication of the proton transfer completeness of PILs (or degree
of ionicity) without conducting any direct measurement [57]. They defined pK a =
pK a (HB
+ ) − pK a (HA) where pK a (HB
+ ) and pK a (HA) represent acid dissociation
constants of HB
+ and HA in an aqueous environment. It was demonstrated that
pK a has a direct impact on the boiling point and volatility of the prospective PIL,
with greater pK a suggesting more complete proton transfer and less presence of
molecular species, and hence a higher product boiling point [59, 60].
For example, comparing two PILs that share the same α-picoline parent base and
two acids of different strengths showed that the PIL with trifluoroacetic acid had a
pK a value of 7.3 and a measurable boiling point of 175 °C. On the other hand, a
PIL synthesized with the much stronger triflic acid had a pK a value of 20 with no
observed boiling point.
One of the key properties of PILs is their Brønsted acidity which makes them
attractive media for acid-catalyzed reactions and for applications that require protonconducting media, such as fuel cells [61], batteries [61], and capacitors [62]. A PIL’s
acidity can be measured using acidity formulation methods for non-aqueous media
scales, such as potentiometric titration, calorimetric titration, or acidity functions
coupled with spectroscopy (e.g., Hammett function) [63–65].
93
The simple acid–base neutralization reaction gives PILs a great economic advantage compared to all other ILs, which are by convention referred to as aprotic ILs
(APILs). As mentioned earlier, the key barrier towards industrial large-scale use of
commonly used ILs is their high production cost, which is usually 5–20 times more
than conventional organic solvents [20]. The high production cost of APILs mainly
stems from: (1) high cost of the starting materials, (2) low atom efficiency during their
synthesis via alkylation and salt metathesis, and (3) lengthy extensive purification
steps which lead to generation of salty wastewater that needs further treatment steps
[54]. The significant reduction in processing steps and the use of inexpensive starting
materials (e.g., mineral acids and amines) make PILs more attractive for industrial
applications. This is especially true as historically many common industrial solvents
were adapted for multi-ton industrial scale because they are generated as inexpensive
by-products in the production line of other higher value products (e.g., acetone is a
by-product of phenol production and toluene is a by-product of gasoline production).
APILs are usually hailed for their non-volatile nature as they have extremely low
vapor pressures. However, this is not always the case for PILs as many PILs have a
measurable boiling point and can be distilled [55, 56]. This feature is directly related
to a PIL’s formation mechanism. PILs exist in equilibrium with their parent acid and
amine. When heated a PIL boils rather than decomposes as the equilibrium reaction
is reversed to form the more volatile neutral acid or base species [52]. In fact, the
volatility of a PIL is highly dependent on its ionicity and degree of proton transfer.
Ideally, the proton transfer from the acid to the base must be completed such that
only ionic species exist to form a true PIL that displays ideal Walden behavior [57].
However, this is not always the case depending on the parent acid and base; the proton
transfer can be only partial [58]. Yoshizawa et al. proposed the use of pK a as a
practical qualitative indication of the proton transfer completeness of PILs (or degree
of ionicity) without conducting any direct measurement [57]. They defined pK a =
pK a (HB
+ ) − pK a (HA) where pK a (HB
+ ) and pK a (HA) represent acid dissociation
constants of HB
+ and HA in an aqueous environment. It was demonstrated that
pK a has a direct impact on the boiling point and volatility of the prospective PIL,
with greater pK a suggesting more complete proton transfer and less presence of
molecular species, and hence a higher product boiling point [59, 60].
For example, comparing two PILs that share the same α-picoline parent base and
two acids of different strengths showed that the PIL with trifluoroacetic acid had a
pK a value of 7.3 and a measurable boiling point of 175 °C. On the other hand, a
PIL synthesized with the much stronger triflic acid had a pK a value of 20 with no
observed boiling point.
One of the key properties of PILs is their Brønsted acidity which makes them
attractive media for acid-catalyzed reactions and for applications that require protonconducting media, such as fuel cells [61], batteries [61], and capacitors [62]. A PIL’s
acidity can be measured using acidity formulation methods for non-aqueous media
scales, such as potentiometric titration, calorimetric titration, or acidity functions
coupled with spectroscopy (e.g., Hammett function) [63–65].