Predicting Cellulose Solvating Capabilities of Acid–Base Conjugate Ionic Liquids

Abstract

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.

Homogenous esterification of cellulose pulp in [DBNH][OAc]

“…The ionic liquid [DBNH][OAc] was prepared from glacial acetic acid and DBN according to Parviainen et al (2013) with slight modifications: The reaction was performed in a two necked flask with a reflux condenser, under argon atmosphere and stirred magnetically. The superbase DBN (168.5 g, 1.3 mol) was measured to the flask and one equivalent of acetic acid (81.5 g, 1.3 mol) was weighed to a dropping funnel.…”

“…The chemical properties of ILs vary according to the composition. For instance, the ability to dissolve cellulose is often dependent on the basicity of the IL (Fukaya et al 2006(Fukaya et al , 2008King et al 2012;Parviainen et al 2013), but there are exceptions, e.g., tetraalkylphosphonium salts Holding et al 2014), which are basic but require a dipolar aprotic co-solvent to dissolve cellulose. In previous literature, the use of imidazolium based ILs, with alkyl substituents (ethyl, methyl, and allyl) and chloride, bromide or acetate anions, typifies most publications on cellulose dissolving ILs (Heinze et al 2008;Swatloski et al 2002).…”

“…Above processing temperatures these conjugates dissociate to the neutral starting components, which allows for distillability of the IL. One of these that is of current interest is [DBNH][OAc], which can be recycled and purified by distillation to either pure IL or to its starting compounds (Parviainen et al 2013(Parviainen et al , 2015. [DBNH][OAc] is a robust solvent for cellulose and pulp, recycled paper and even cotton, e.g., used cotton clothes.…”

“…One of these is [DBNH][OAc], which is not only an efficient solvent for cellulose, but high-quality textile fibers can be produced upon regeneration of cellulose solutions into water (IONCELL-F process) (Michud et al 2016;Parviainen et al 2013Parviainen et al , 2015Sixta et al 2015). Therefore, it would be highly beneficial to control the modification to cellulose esters before regeneration to fibers (for textile and non-wovens) or films.…”

Efficient biomass pretreatment using ionic liquids derived from lignin and hemicellulose

“…Kamlet-Taft solvent parameters have been used to measure the ability of a solvent to donate a hydrogen bond (α), and accept a hydrogen bond (β) (51). It has been shown that basicity (β) correlates well with an IL's ability to dissolve lignocellulose (52), and that net basicity correlates with an IL's ability to dissolve cellulose (46,53). A recent experimental study on a range of cations in combination with the same anion demonstrated that cation acidity is also important for cellulose dissolution (54) PO 4 ], one would expect the differential substitution of the electron-donating groups to affect the N atom's affinity for the H 3 PO 4 proton.…”