Facile preparation of cellulose IVII using ionic liquids

Abstract

A characteristic feature of cellulose is crystal polymorphism. Among the cellulose allomorphs, cellulose IV_II has been the most time-consuming and energy-intensive one to prepare from native cellulose with a cellulose I structure. In this work, we report a facile preparation method of cellulose IV_II by regeneration from an ionic liquid (IL) solution. Although regeneration from arbitrary IL solutions is known to produce only cellulose II and amorphous cellulose, we found that appropriate selections of regeneration temperature (373 K), IL structure (dimethylphosphate anion paired with imidazolium cation), and cellulose concentration (5 mol%) induced the formation of cellulose IV_II. With the optimal conditions, cellulose IV with crystallinity of 31% was obtained without contamination of other cellulose allomorphs. Wide angle X-ray scattering and ¹³C solid-state NMR measurements on cellulose/IL mixtures demonstrated that the cations and anions were rather ordered and the gt conformer of cellulose was preferred at a high temperature, which would be relevant for the formation of cellulose IV_II.