Thermal aging of 1-alkyl-3-methylimidazolium ionic liquids and its effect on dissolved cellulose

Abstract

The thermal degradation products of 1-alkyl-3-methyl-imidazolium-type ionic liquids (e.g., BMIM, EMIM), which are commonly used in cellulose chemistry, were identified. Imidazole (5), N-methylimidazole (6), and N-alkylimidazoles (7, 8) are the main products. Moreover, dimeric substituted imidazoles (9–11) were found carrying a methylene bridge formed from the N-methyl structure. The former 1-alkyl group (butyl or ethyl) was not present in any of the dimeric compounds. The N-methylene intermediate (12) acts as a precursor for the substituted imidazoles linked by a methylene group. All thermal degradation products are bases and catalyze the previously described reaction between the reducing end of celluloses and 1-alkyl-3-imidazolium cations. To minimize this side reaction, ionic liquids have to be liberated from such basic compounds prior to application as cellulose solvents. In this regard, the usage of “recycled” (and unpurified) ionic liquids has to be treated with caution.

Ionic Liquids for the Production of Man-Made Cellulosic Fibers: Opportunities and Challenges

“…For example, the dialkylimidazolium cation can show trans-alkylation, resulting in a symmetrical dialkylimidazolium structure, or undergo elimination or retro-alkylation, causing liberation of one alkyl residue and concomitant formation of an alkylimidazole [110,111]. Similar reactions were observed in a detailed study by Liebner et al, who, in addition to the aforementioned products, also identified methylene-bridged imidazole dimers [112]. The basic amine products can act as catalysts for further degradation reactions or induce secondary side reactions such as deprotonation of the imidazolium C2-H proton, resulting in carbene formation [113].…”

Synthesis and Characterization of Epoxy/MCDEA Networks Modified with Imidazolium‐Based Ionic Liquids

“…However, our system does not contain cyanamide groups. On the other hand, a nice paper reported by Liebner et al [27] on thermal degradation studies of 1-alkyl-3-methyl imidazolium ILs suggests the formation of imidazole, N-alkylimidazoles and dimeric compounds constituted by two imidazole moieties linked by a methylene bridge. Such decomposition products are relatively strong bases.…”

A cautionary note on thermal runaway reactions in mixtures of 1-alkyl-3-methylimidazolium ionic liquids and N-methylmorpholine-N-oxide

“…Highest stabilities are obtained in pure ILs from which degradation productions had previously been removed (Liebner et al 2010). Acidic degradation products, such as acetic acid, gluconic acid from cellulose or 2-furancarboxylic acid from xylan, generally destabilize NMMO, whereas basic degradation products, such as 1-methylimidazole from the IL (Liebner et al 2010) or N-methylmorpholine from NMMO, render it more stable, in complete agreement with general NMMO chemistry. There is one important exception: imidazole has a pronounced destabilizing effect in XMIm acetates (BMIM-OAc, EMIM-OAc).…”

“…In particular, problems with recycling and byproduct accumulation have cast doubts on their general usefulness in biomass fractionation or pulping, as have literature accounts showing that these solvents undergo reactions with cellulose (Köhler et al 2007;Liebert et al 2009;Ebner et al 2008;Zweckmair et al 2015) and thus lack the inertness that is often claimed as one of benefits of ILs. Also thermal stress causes slow byproduct formation in XMim ionic liquids, mainly imidazole derivatives (Wendler et al 2012;Liebner et al 2010;Dorn et al 2008).…”