The yield of cellulose precipitate from sub- and supercritical water treatment of various microcrystalline celluloses

Subcritical and supercritical water are known to dissolve crystalline cellulose, offering a simple way to produce low molar mass cellulose which precipitates at ambient temperatures. However, the yield of precipitate is limited by concomitant degradation reactions. In this study, the formation of cellulose precipitate from six different microcrystalline celluloses (MCC) was investigated in 0.2 s treatments at 250–380 °C. The results were elucidated with a simple two-step kinetic model. The maximum yield of cellulose precipitate depended on the choice of starting material, which had a more important role than the optimized treatment time or temperature. Wood-derived MCCs dissolved faster and resulted in a higher yield of precipitate than those prepared from cotton linter. The highest yield of precipitate, 68 %, was recovered from a MCC prepared from mercerized prehydrolysis hardwood Kraft pulp. The high yield of precipitate could not be attributed to any single factor but in general the wood-derived MCCs had a smaller particle size, smaller crystallite dimensions, and lower molar mass than their cotton-derived equivalents. The analysis of molar mass distributions indicated heterogeneous dissolution mechanism at 250 and 320 °C whereas at 380 °C the cellulose crystallites were subjected to random chain cleavage.