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

Solubility of cellulose was systematically assessed in organic onium/inium hydroxide aqueous solutions (OHAS) employing assorted cations, such as phosphonium, ammonium, piperidinium, morpholinium, pyrrolidinium, and cholinium cations. From a dissolution test of cellulose in the 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 solubility of cellulose, such as [P4444]OH and [N4444]OH, exhibited a chemical shift of water (δH2O) integrated with that of OH in a low frequency region (~4.9 ppm), while [Ch]OH with a poor cellulose solubility showed the δH2O higher than 5.2 ppm. A higher δH2O means that the protons are deshielded due to stronger hydrogen bonds around H2O and OH, which designates a high self-associating property of the OHAS that is unfavourable for the cellulose dissolution. Assuming that the high self-associating property can be reduced by improving a hydrophobicity of organic catioins, we have replaced the methyl group in [Mor14]OH by butyl chain to shield the positive charge. While [Mor14]OH dissolved only 5wt% of cellulose, the solubility in the synthesised OHAS, N,N-dibutylmorpholinium hydroxide ([Mor44]OH), was successfully improved to 20wt%. In the present paper, cellulose solubility was also analysed in relation to the Kamlet-Taft parameters.