Alistair W. T. King

Distillable Acid–Base Conjugate Ionic Liquids for Cellulose Dissolution and Processing†

Future biorefinery concepts are seriously entertaining the use of ionic liquids (ILs) as a platform media for the processing of woody material as a second-generation biomass feedstock. The main motivation is the demonstrated efficiency of some molten salts in the dissolution of cellulose, a major structural and solvolytically resistant component of lignocellulosic materials. The first report of the use of molten salts for the modification of cellulose came in the form of a patent by Graenacher, where alkyl pyridinium chlorides were used to dissolve cellulose, thus allowing for efficient chemical modification from those media. The melting points of most alkyl pyridinium chloride salts are above 100 8C and, as such, these species do not fall under the common definition of ionic liquids. Nevertheless, the molten compounds solvated cellulose to such a state as to allow for acylation to a high degree. The next generational advance was the discovery by Rogers and co-workers that dialkyl imidazolium based ionic liquids, with melting points below 100 8C, can dissolve cellulose. The most successful of these was 1-butyl-3-methylimidazolium chloride ([bmim]Cl]). This advance was further refined by Ohno et al. into room-temperature ionic liquids capable of dissolving cellulose, such as 1-ethyl-3-methylimidazolium formate ([emim][CO2H]) [3a] or 1-ethyl-3-methylimidazolium dimethylphosphate ([emim][Me2PO4]). [3b] From the structures listed in the claims of the Rogers patent, BASF have also refined this list down to room-temperature ionic liquids, such as 1-ethyl-3-methylimidazolium acetate ([emim][OAc]). It has been reported by BASF, by oral dissemination and unofficial reports, that [emim][OAc] has higher dissolving efficiency for cellulose and has lower toxicity than structures such as [bmim]Cl. However, no detailed studies comparing chlorides with carboxylates or other such structures have been published, although certainly their undeniable high efficiency for dissolution and chemoselectivity has been demonstrated for a number of cellulose modification applications. Despite the high efficiency for the solvation of cellulose, lignin, and even wood in an increasing range of dialkyl imidazolium based ionic liquids, sustainability of prospective processes will depend on the chemical stability of solutes and ionic liquids under process and recycling conditions. There are already some indications that ionic liquids such as [emim][OAc] react chemically with lignocellulosic solutes. This reactivity may lower the recovery of the media upon recycling, although, in the case of the reaction of C2 imidazolium positions with C1 reducing end groups of cellulose, it is possible that this reaction is reversible under aqueous conditions, owing to the lability of the conjugate linkage. A bigger concern is the method of recycling to yield a pure ionic liquid. For most processes, high-purity ionic liquid will be required to maintain efficiency of dissolution and overall sustainability of the process. Decomposition of dialkyl imidazolium based ILs containing basic anions in the presence or absence of solutes proceeds according to three main pathways. From knowledge of the chemical stability of these cations, the pathways are most easily illustrated using a 1,3-diethylimidazolium cation ([eeim]) as countercation (Scheme 1).

Role of Solvent Parameters in the Regeneration of Cellulose from Ionic Liquid Solutions

The ionic liquids 1-ethyl-3-methylimidazolium acetate [emim]OAc, N,N,N,N-tetramethylguanidium propionate [TMGH]EtCO(2), and N,N,N,N-tetramethylguanidium acetate [TMGH]OAc, and the traditional cellulose solvent N-methylmorpholine N-oxide NMMO were characterized for their Kamlet-Taft (KT) values at several water contents and temperatures. For the ionic liquids and NMMO, thresholds of regeneration of cellulose solutions by water were determined using nephelometry and rheometry. Regeneration from wet IL was found to be asymmetric compared to dissolution into wet IL. KT parameters were found to remain almost constant at temperatures, between 20-100 °C, even at different water contents. Among the KT parameters, the β value was found to change most drastically, with an almost linear decrease upon addition of water. The ability of the mixtures to dissolve cellulose was best explained by the difference β-α (net basicity), rather than β alone. Regeneration of cellulose starts at thresholds values of approximately β < 0.8 (β-α < 0.35) and displayed four phases.

Predicting Cellulose Solvating Capabilities of Acid-Base Conjugate Ionic Liquids

Different acid-base conjugates were made by combining a range of bases and superbases with acetic and propionic acid. Only the combinations that contained superbases were capable of dissolving cellulose. Proton affinities were calculated for the bases. A range, within which cellulose dissolution occurred, when combined with acetic or propionic acid, was defined for further use. This was above a proton affinity value of about 240 kcal mol(-1) at the MP2/6-311+G(d,p)//MP2/ 6-311+G(d,p) ab initio level. Understanding dissolution allowed us to determine that cation acidity contributed considerably to the ability of ionic liquids to dissolve cellulose and not just the basicity of the anion. By XRD analyses of suitable crystals, hydrogen bonding interactions between anion and cation were found to be the dominant interactions in the crystalline state. From determination of viscosities of these conjugates over a temperature range, certain structures were found to have as low a viscosity as 1-ethyl-3-methylimidazolium acetate, which was reflected in their high rate of cellulose dissolution but not necessarily the quantitative solubility of cellulose in those ionic liquids. 1,5-Diazabicyclo[4.3.0]non-5-enium propionate, which is one of the best structures for cellulose dissolution, was then distilled using laboratory equipment to demonstrate its recyclability.

In Situ Determination of Lignin Phenolics and Wood Solubility in Imidazolium Chlorides Using 31P NMR

Corn stover, Norway spruce, and Eucalyptus grandis were pulverized to different degrees. These samples were subjected to quantitative analyses, upon the basis of predissolution into the imidazolium chloride-based ionic liquids [amim]Cl and [bnmim]Cl followed by labeling of hydroxyl groups as phosphite esters and quantitative (31)P NMR analysis. Analysis of different pulverization degrees provided semiempirical data to chart the solubility of Norway spruce in these ionic liquids. Further method refinment afforded an optimized method of analysis of the lignin phenolic functionalities, without prior isolation of the lignin from the fiber. The lignin in these samples was further enriched using cellulase and acidolysis treatments, allowing for comparison with the fibrous samples. Analysis of all samples charts the polymerized-monomer availability for each stage of the treatment. Conditions required for adequate signal-to-noise ratios in the (31)P NMR analysis were established with a notable improvement observed upon the lignin enrichment steps.

Impact of Amphiphilic Biomass-Dissolving Ionic Liquids on Biological Cells and Liposomes

The toxicity of some promising biomass-dissolving amidinium-, imidazolium-, and phosphonium-based ionic liquids (ILs), toward two different cell lines, human corneal epithelial cells and Escherichia coli bacterial cells, was investigated. In addition, dynamic light scattering (DLS) and ζ potential measurements were used to study the effect of the ILs on the size and surface charge of some model liposomes. Capillary electrophoresis (CE) was used for determination of the electrophoretic mobilities of the liposomes and for determination of the critical micelle concentration (cmc) of the ILs. The toxicity of the phosphonium ILs was highly dependent on the longest linear chain of the IL, due to increasing hydrophobicity, with the long-chain phosphonium ILs being toxic while the shorter-chain versions were significantly less toxic or not toxic at all. Amidinium and imidazolium ILs showed no significant effect on the cells, within the concentration range used. Moreover, the more hydrophobic ILs were found to have a major effect on the surface charges and size distributions of the model liposomes, which can lead to disruption of the lipid bilayer. This indicates that the cytotoxicity is at least to some extent dependent on direct interactions between ILs and the biomembrane.

Molecular weight distributions and linkages in lignocellulosic materials derivatized from ionic liquid media.

A novel and reproducible method is described for accurately determining the molecular weight distribution by size exclusion chromatography (SEC) of whole lignocellulosic materials. This approach offers the opportunity to compare the molecular weight distributions of intact milled woods and its component fractions, lignins and holocelluloses, all from the same source, thus highlighting the potential of the technique and the contributions of the individual components to the chromatogram. The method is based on the dissolution of the ball-milled samples in the ionic liquid 1-allyl-3-methylimidazolium chloride ([amim]Cl). Under these homogeneous ionic liquid media, a derivatization reaction was performed with benzoyl chloride in the presence of pyridine. The thoroughly benzoylated wood with its associated carbohydrate and lignin components was found to be completely soluble in the THF SEC eluent with marked UV detector sensitivity. This methodology, when applied to the individually isolated holocellulose and lignin (enzymatic mild acidolysis lignin; EMAL) materials from Norway spruce ( Eucalyptus grandis ) wood and corn stover, offered a better understanding as to the possible ways the lignin and the carbohydrates may interact within these three different species. Finally, the applicability of the methodology is shown for a series of pure cellulosic samples under intense mechanical defibration conditions, offering a visualization of the molecular weight distribution changes induced during the production of nanofibrillated cellulose.

Relative and inherent reactivities of imidazolium-based ionic liquids: the implications for lignocellulose processing applications

Novel methods for the fractionation of wood, as a major renewable chemical and material feedstock, are in demand. Ionic liquids, such as 1-ethyl-3-methylimidazolium acetate ([emim][OAc]), are promoted as potential media for these processes. However, the chemical stabilities of such ionic liquids are in question as they may have an effect on process sustainability or efficiency. With anion nucleophilicity and basicity being implicated more in ionic liquid reactivity, a rough scale of the relative reactivities for [emim]-based ionic liquids is demonstrated, based upon their TGA decomposition temperatures. These values are compared to the proton affinities for the anions of those ionic liquids, as a crude measure of nucleophilicity or basicity. The implications for the temperature-dependent chemical stability of imidazolium-based ionic liquids are discussed, in regard to their interactions with wood biopolymers. It is observed that for ionic liquids with less diffuse anions (more nucleophilic or basic), such as [emim][OAc], they unfortunately become more unstable. This is exhibited by a decrease in the thermal stability and an increase in the degree of interaction with the biomass, to the point of better solvation and even covalent interactions with dissolved components. The ab initio proton affinities, dipole moments, van der Waals surface area, and volumes, are presented for an extended series of anions, commonly used in ionic liquids.

Amphiphilic and Phase-Separable Ionic Liquids for Biomass Processing

One main limiting factor for the technoeconomics of future bioprocesses that use ionic liquids (ILs) is the recovery of the expensive and potentially toxic IL. We have demonstrated a new series of phase-separable ionic liquids, based on the hydrophobic tetraalkylphosphonium cation ([PRRRR](+)), that can dissolve lignin in the neat state but also hemicellulose and high-purity cellulose in the form of their electrolyte solutions with dipolar aprotic solvents. For example, the IL trioctylmethylphosphonium acetate ([P8881][OAc]) was demonstrated to dissolve up to 19 wt % of microcrystalline cellulose (MCC) at 60 °C with the addition of 40 wt % of DMSO. It was found that the MCC saturation point is dependent on the molar ratio of DMSO and IL in solution. At the optimum saturation, a ∼1:1 molar ratio of [P8881][OAc] to anhydroglucose units is observed, which demonstrates highly efficient solvation. This is attributed to the positive contribution that these more amphiphilic cation-anion pairs provide, in the context of the Lindman hypothesis. This effective dissolution is further illustrated by solution-state HSQC NMR spectroscopy on MCC. Finally, it is also demonstrated that these electrolytes are phase separable by the addition of aqueous solutions. The addition of 10 % NaOAc solution allows a near quantitative recovery of high-purity [P8881][OAc]. However, increased volumes of aqueous solution reduced the recovery. The regenerated material was found to partially convert into the cellulose II crystalline polymorph.

Enhancement of ionic liquid-aided fractionation of birchwood. Part 1: autohydrolysis pretreatment

Ionic liquid-cosolvent systems have been proposed as selective solvent media for lignocellulosic materials. We present the ionic liquid-aided fractionation of silver birch (Betula pendula) combined with an autohydrolysis pretreatment. Contrary to untreated birchwood meal, autohydrolyzed birchwood meal reveals quantitative dissolution in 1-ethyl-3-methylimidazolium acetate and distinct separation into the individual wood polymers upon regeneration in acetone/water. The process yields two main fractions, a cellulose-rich precipitate with a residual lignin content of 13–15% and another virtually pure lignin fraction. No cellulose yield loss is observed during the ionic liquid processing step. A comprehensive mass balance of the process, including insoluble material, wash waters, and soluble residues, is provided. The product fractions are characterised for their chemical compositions, molar mass distributions and structural characteristics by Klason lignin and sugar analysis, 13C NMR, GPC and WAXS. The study investigates the effects of wood particle size and autohydrolysis intensity on fractionation efficiency and selectivity.

Effect of Ionic Liquids on Zebrafish (Danio rerio) Viability, Behavior, and Histology; Correlation between Toxicity and Ionic Liquid Aggregation

The effect of 11 common amidinium, imidazolium, and phosphonium based ionic liquids (ILs) on zebrafish (Danio rerio) and Chinese hamster ovary cells (CHO) was investigated with specific emphasis on the effect of anion and cation chain length and aggregation of phosphonium based ILs. Viability and behavioral alteration in the locomotor activity and place preference, after IL treatment of 5 days postfertilization larvae, was recorded. Behavior and histological damage evaluation was performed for adult fish in order to get insight into the long-term effects of two potential biomass-dissolving ILs, [DBNH][OAc] and [P4441][OAc]. To get an understanding of how IL aggregation is linked to the toxicity of ILs, median effective concentrations (EC50) and critical micelle concentrations (CMC) were determined. The long-chain ILs were significantly more toxic than the short-chain ones, and the anion chain length was shown to be less significant than the cation chain length when assessing the impact of ILs on the viability of the organisms. Furthermore, most of the ILs were as monomers when the EC50 was reached. In addition, the ILs used in the long-term tests showed no significant effect on the zebrafish behavior, breeding, or histology, within the used concentration range.