5 Commercial Aspects of Biomass Deconstruction with Ionic Liquids
97
ionic liquid and a cellulose-rich pulp remains highly intact during the process. The
selective dissolution of lignin and hemicellulose in the IL medium gives the process
the “fractionation” feature, with lignin removal (or delignification) being the main
deconstruction mechanism. Lignin was subsequently recovered by adding an antisolvent (e.g., water), changing the solution pH or temperature. Another patent was filed
in 2008 by Varanasi et al. describing the same biomass fractionation effect using ionic
liquids and the subsequent successful enzymatic hydrolysis of cellulose to glucose
[83]. Interestingly, the ionic liquids used for the fractionation were [C 2 C 1 im][Ace]
and [C 4 C 1 im][Cl], which are the same ionic liquids reported earlier for their ability
to dissolve all the biomass components, including cellulose. The major difference
was the use of a much higher biomass loading (i.e., the weight percent of biomass in
the reactor relative to IL; 33 wt% versus <5 wt% for dissolution [16]) that weakens
the ability of the ionic liquid to dissolve the entire biomass components. The crystallinity of the cellulose-rich pulp recovered from the fractionation process is usually
intact or is slightly increased due to the lignin and hemicellulose removal [29]. The
extraction of lignin (i.e., delignification) increases the exposed surface of cellulose
making it easier for the enzymes to access the cellulose fibers, thereby significantly
increasing its hydrolysis to glucose. In general, ILs used in fractionation processes
do not need to have high hydrogen-bond basicity to solubilize lignin [71]. Thus, a
larger number of ILs are capable of dissolving lignin than cellulose. This includes
ILs with moderate to strong hydrogen-bonding anions, such as halides (chlorides and
bromides) [84, 85], acetate [Ace]
− , triflate [OTf]
− , methyl sulfate [C 1 SO 4 ]
− [85],
xylenesulfonate [XSO 3 ]
− [86], and hydrogen sulfate [HSO 4 ]
− [87].
The ionoSolv process is a biomass fractionation process that uses ionic liquid and
water mixtures to selectively dissolve lignin, producing a cellulose-rich pulp [22, 88].
The extracted lignin is recovered via precipitation with the addition of an anti-solvent,
usually water, and the IL is recycled back for another pretreatment. The process has
mainly focused on using protic ammonium-based ILs with a [HSO 4 ]
− anion because
they provide key technical and economic benefits, which will be discussed in detail
in Sect. 5.4. Figure 5.4 shows the main steps involved in the lab-scale version of the
ionoSolv process [22].
5.4 Early Technical Challenges
Historically, the biomass dissolution process has been the most prevalent ionic liquid
pretreatment approach studied since the process was developed as a direct consequence of discovering cellulose-dissolving ILs [16]. As understanding the process
continued to grow, technical challenges were associated with the practical implementation of biomass dissolution using ILs. These challenges were mainly related to the
hyper-sensitivity of water in the process as well as to the thermal stability of the ILs
used [71]. While thermal stability might be overcome by a more careful design of the
IL, the water sensitivity problem cannot be solved the same way because cellulose
97
ionic liquid and a cellulose-rich pulp remains highly intact during the process. The
selective dissolution of lignin and hemicellulose in the IL medium gives the process
the “fractionation” feature, with lignin removal (or delignification) being the main
deconstruction mechanism. Lignin was subsequently recovered by adding an antisolvent (e.g., water), changing the solution pH or temperature. Another patent was filed
in 2008 by Varanasi et al. describing the same biomass fractionation effect using ionic
liquids and the subsequent successful enzymatic hydrolysis of cellulose to glucose
[83]. Interestingly, the ionic liquids used for the fractionation were [C 2 C 1 im][Ace]
and [C 4 C 1 im][Cl], which are the same ionic liquids reported earlier for their ability
to dissolve all the biomass components, including cellulose. The major difference
was the use of a much higher biomass loading (i.e., the weight percent of biomass in
the reactor relative to IL; 33 wt% versus <5 wt% for dissolution [16]) that weakens
the ability of the ionic liquid to dissolve the entire biomass components. The crystallinity of the cellulose-rich pulp recovered from the fractionation process is usually
intact or is slightly increased due to the lignin and hemicellulose removal [29]. The
extraction of lignin (i.e., delignification) increases the exposed surface of cellulose
making it easier for the enzymes to access the cellulose fibers, thereby significantly
increasing its hydrolysis to glucose. In general, ILs used in fractionation processes
do not need to have high hydrogen-bond basicity to solubilize lignin [71]. Thus, a
larger number of ILs are capable of dissolving lignin than cellulose. This includes
ILs with moderate to strong hydrogen-bonding anions, such as halides (chlorides and
bromides) [84, 85], acetate [Ace]
− , triflate [OTf]
− , methyl sulfate [C 1 SO 4 ]
− [85],
xylenesulfonate [XSO 3 ]
− [86], and hydrogen sulfate [HSO 4 ]
− [87].
The ionoSolv process is a biomass fractionation process that uses ionic liquid and
water mixtures to selectively dissolve lignin, producing a cellulose-rich pulp [22, 88].
The extracted lignin is recovered via precipitation with the addition of an anti-solvent,
usually water, and the IL is recycled back for another pretreatment. The process has
mainly focused on using protic ammonium-based ILs with a [HSO 4 ]
− anion because
they provide key technical and economic benefits, which will be discussed in detail
in Sect. 5.4. Figure 5.4 shows the main steps involved in the lab-scale version of the
ionoSolv process [22].
5.4 Early Technical Challenges
Historically, the biomass dissolution process has been the most prevalent ionic liquid
pretreatment approach studied since the process was developed as a direct consequence of discovering cellulose-dissolving ILs [16]. As understanding the process
continued to grow, technical challenges were associated with the practical implementation of biomass dissolution using ILs. These challenges were mainly related to the
hyper-sensitivity of water in the process as well as to the thermal stability of the ILs
used [71]. While thermal stability might be overcome by a more careful design of the
IL, the water sensitivity problem cannot be solved the same way because cellulose