Effect of the cation structure on cellulose dissolution in aqueous solutions of organic onium hydroxides

Abstract

The solubility of cellulose was systematically assessed in organic onium/inium hydroxide aqueous solutions (OHAS) having assorted cations, such as phosphonium, ammonium, piperidinium, morpholinium, pyrrolidinium, and cholinium. From a dissolution test of cellulose in OHAS, it was confirmed that the single most important factor in the dissolution is the high concentration of OHAS. In addition, having a weaker hydrogen bond network around OH and H₂O was found to be important to facilitate the cellulose dissolution. In NMR analysis, the OHAS with an excellent cellulose solubility, such as tetrabutylphosphonium hydroxide ([P₄₄₄₄]OH), exhibited a chemical shift of water (δ_H2O) integrated with that of OH in the low frequency region (∼4.9 ppm), while choline hydroxide ([Ch]OH) with poor cellulose solubility showed δ_H2O higher than 5.2 ppm. A higher δ_H2O means that the protons are deshielded due to a stronger hydrogen bond network around H₂O and OH, which indicates a strong self-associating property of OHAS that is unfavourable for the cellulose dissolution. Assuming that the strong self-associating property can be reduced by improving the hydrophobicity of organic cations, the methyl group in N-butyl-N-methylmorpholinium hydroxide ([Mor₁₄]OH) was replaced by a butyl chain to shield the positive charge. While [Mor₁₄]OH dissolved only 5 wt% of cellulose, the solubility in the synthesised OHAS, N,N-dibutylmorpholinium hydroxide ([Mor₄₄]OH), was successfully improved to 20 wt%. In the present paper, cellulose solubility was also analysed in relation to the Kamlet–Taft parameters.