Toward a Better Understanding of Cellulose Swelling, Dissolution, and Regeneration on the Molecular Level

Abstract

The combination of 13C-perlabeled compounds and NMR spectroscopy allowed insights into the detailed changes of the hydrogen bond in celluloses and cellulose model compounds during swelling, dissolution, and regeneration (precipitation). 13C-perlabeled cellulose allomorphs I and II as well as the model compound methyl 4′-O-methyl-β-d-cellobioside-13C12 were studied as solutes, in combination with the perdeuterated solvents N,N-dimethylacetamide/lithium chloride (DMAc/LiCl), N-methylmorpholine N-oxide monohydrate (NMMO), 1-ethyl-3-methylimidazolium acetate (EMIm-OAc), 1-butyl-3-methylimidazolium acetate (BMIm-OAc) and D2O/urea. Swelling is a reversible stepwise process that mainly involves hydrogen bonds of C2O and C6O while C3O hydrogen bonds remain largely unchanged. For dissolution, the latter bonds are broken so that dissolution mainly depends on C3O. This behavior was the same for all solvents and for both cellulose allomorphs. With regard to H-bond changes, deswelling/precipitation occurs roughly in opposite order of the swelling/dissolution. Upon swelling, solvent molecules increasingly approach C2 and C6, but not C3. Only in dissolved state, the solvent molecules interact also closely with C3. Interestingly, nonclassical-hydrogen bonds are established regioselectively between CH-1 and CH-3, and the solvent in dissolved state so that these CH-hydrogen bonds can be assumed to be a prerequisite to cellulose dissolution, at least for the solvents studied. While more work has to be done to go from idealized model cases to “real-world” pulps, the studies offer for the first time a detailed picture on dynamic changes of H-bonds during swelling, dissolution, and regeneration of cellulose, and help us to better understand the nature of cellulose solvents.