Fivefold Helical Cellulose Trapped in a Sulfuric Acid Framework

A sulfuric acid aqueous solution effectively dissolves cellulose at a concentration of 64 wt % when it is precooled at −20 °C. Although this low-temperature acid solution has a high potential as an aqueous cellulose solvent, the molecular detail of its swelling and dissolution mechanism is not known. In this study, we followed the swelling behavior of cellulose in sulfuric acid using X-ray diffraction and found a crystalline complex of cellulose and sulfuric acid at −20 °C. In this structure, the sulfuric acid molecules are organized in a similar manner to those in its tetrahydrate crystal. Cellulose molecules adopt a rare fivefold helical conformation, and their hydroxyl groups participate in the hydration of sulfuric acid. This complexation is acid-concentration dependent and occurs only above 62 wt %, indicating that the deficiency of the number of water molecules around sulfuric acid drives the sulfuric acid to recruit cellulose hydroxyl groups to form a stable tetrahydrate-like hydration state. It swells and opens the cellulose structure without completely destroying the crystalline order, as can be confirmed by the partial reversibility to regenerate cellulose I.