While catalysis of lignin conversion in ILs has not received the same attention as cellulose and monosaccharides, there is growing interest in the application of ILs to the catalysis of lignin conversion. The main goal of much of the research relating to lignin in ILs have been for the purpose of pretreatment [76, 80, 85, 133]. Sun et al. demonstrated the ability to fractionate wood into cellulose rich and lignin rich samples using EMIMAc and acetone/water as an antisolvent for cellulose followed by evaporation of acetone to precipitate the lignin [74]. Other groups then used this discovery as a stepping stone to pretreatment of biomass. While some use ILs as a path towards delignification for pretreatment [76], others have shown that a loss of cellulose crystallinity is also a source of IL pretreatment efficacy [14, 77, 79]. The other work in catalysis of lignin conversion has covered thermal and chemical depolymerization and hydrodeoxygenation. As lignin is a complex, amorphous polymer, most studies on the catalysis of lignin conversion work with lignin model compounds as a way to test a process while keeping analytical complications to a minimum.

One of the simplest treatments of lignin in ILs is to simply dissolve lignin and heat the IL/lignin mixture for a period of time. Kubo et al. performed a series of experiments with the lignin model compound guaiacylglycerol-p-guaiacyl ether (GG) mixed with BMIMCl, EMIMAc, or AMIMCl at 120 °C. This model compound simulates the p-O-4 ether linkage, which is the most common struc­ture in lignin. The main product of this reaction was 3-(4-hydroxy- 3methoxyphenyl)-2-(2-methoxyphenoxy)2-propenol, which is an enol ether (EE). EE is the dehydration product of GG, and an analogous process has been implicated as an intermediate in the depolymerization of lignin under both acidic and alkaline conditions [134]. This intermediate has been detected in other studies involving lignin model compounds.

The cleavage of the P-O-4 ether linkage is a possible pathway both to general delignification of biomass and to the utilization of lignin as a feedstock for fuel and chemical production [135, 136]. A number of methods have been explored to sever this bond. In one study, N-bases were used in 1-butyl-2,3-dimethylimidazolium chloride (BDMIMCl). It was demonstrated that the base 1,3,5-triazabicyclo[4.4.0] dec-5-ene (TBD) was effective at cleaving the p-O-4 ether linkage in GG with a yield of up to 23 %. As in the study by Kubo et al., EE was observed as an intermediate. Other N-bases, including 7-methyl-1,3,5-triazabicyclo[4.4.0]dec-5- ene (MTBD), did not show the same activity as TBD, indicating a unique func­tionality for TBD. It was suggested that TBD could act as a combination base and nucleophile to break down lignin using the same mechanism as kraft pulping, although the process was not shown to be catalytic [137]. The trialkylimidazolium IL was utilized in this study as opposed to the more common dialkylimidazolium ILs because the hydrogen at the 2 position on the dialkylimidazolium ring can be extracted under basic conditions to form a reactive carbene [49]. Another method, using metal chlorides as catalysts, was demonstrated to be effective at the hydro­lysis of both phenolic and non-phenolic lignin model compounds. In this study, GG and veratrylglycerol-p-guaiacyl ether (VG) were used to model the P-O-4 ether linkage in lignin. FeCl3, CuCl2, and AlCl3 were shown to be effective at catalyzing the hydrolysis of the ether linkage with AlCl3 showing the highest yield of 80 % for GG and 75 % for VG. The metal chlorides most likely acted as acid catalysts to break the bond through the same mechanism of other acid promoted systems [138].

The task-specific acidic IL HMIMCl has been shown to catalytically hydrolyze the ether bonds in both GG and VG up to a 71.5 % yield. The mechanism for this hydrolysis starts with dehydration into an enol ether structure, which is then susceptible to acidic attack of the P-O-4 ether linkages. This process, as shown in Fig. 8.8, occurs both in the individual model compounds and dimers of the model compound that form under reaction conditions [139]. This method was extended to a number of other acidic ILs. ILs composed of 1-H-methylimidazolium cations and chloride, bromide, hydrogensulfate, and tetrafluoroborate anions, along with BMIMHSO4 were used to hydrolyze the P-O-4 ether linkage in GG and VG. HMIMCl was found to be the most effective of these ILs. The Hammett acidity of each of these ILs was measured using UV-vis measurements to determine protonation of 3-nitroanaline added to the ILs, but the acidity of the IL did not correlate with the yield of hydrolysis products. The efficacy of acid catalyzed hydrolysis in these ILs was determined by the ability of the anion to hydrogen bond with the hydroxyl groups on the lignin model compounds [88]. Further study demonstrated the ability of HMIMCl to depolymerize lignin through acid catalyzed hydrolysis of the P-O-4 ether linkage. The lignin used was extracted from oak wood using EMIMAc. Treated lignin was demonstrated to be reduced in size from the untreated lignin and the disappearance of the ether structures was observed through NMR and IR spectroscopy [136].

Other transformations of lignin have been demonstrated in ILs as well. Binder et al. performed work with many catalysts in EMIMCl and 1-ethyl-3-methylimi — dazolium triflate (EMIMOTf). While a number of catalysts were able to dealkylate the lignin model compound eugenol, these catalysts failed to produce monomeric products from organosolv lignin [140]. Further use of metals for catalysis of lignin in ILs was demonstrated by Jiang and Ragauskas. This study

Hibberts Ketones +Unknowns

Hibbert’s Ketones

Fig. 8.8 Proposed acid-catalyzed mechanism for hydrolysis of P-O-4 bonds lignin model com­pounds (Adapted with permission from [138]. Copyright 2010 John Wiley and Sons) dealt with the use of vanadyl acetylacetonate in BMIMPF6 along with Cu(II) or Cu(I) co-catalysts to selectively oxidize aromatic alcohols into carbonyl or carboxylic acid groups. While most of this work focused on a wide variety of alcohols, 3,4-dimethoxybenzyl alcohol and 1-(3,4-dimethoxyphenyl)ethanol were specifically noted as being model compounds for lignin [141]. Other work has been performed with metal catalysts in ILs for the purpose of deoxygenation of lignin model compounds. In a study by Yan and coworkers, cyclohexanol was dehydrated into cyclohexene with Br0nsted acidic ILs. Then, by combining the acidic ILs with Ru, Rh, or Pt nanoparticles, phenolic lignin model compounds were hydrogenated and deoxygenated to non-aromatic hexane species [142]. Other work on hydrodeoxygenation of lignin in ILs have been limited because the temperature at which traditional hydrodeoxygenation catalysts function exceeds the stability limit of IL, especially of those that have the ability to solubilize biomass.