Processing of Citrus Nanostructured Cellulose: A Rigorous Design-of-Experiment Study of the Hydrothermal Microwave-Assisted Selective Scissoring Process.

A detailed design-of-experiment (DoE) study to investigate the cause-effect interactions of three process variables, that is, temperature (120-200 °C), holding time (0-30 min), and concentration (1.4-5.0 wt %), on the processing of citrus cellulosic matter using acid-free microwave-assisted selective scissoring (Hy-MASS) is reported. Analysis of variance (ANOVA) showed that post-microwave processing, the yield of cellulosic matter (25-72 %), decomposition temperature (345-373 °C), and crystallinity index (34-67 %) were strongly affected by temperature. SEM and TEM analyses showed that the isolated cellulosic matter was heterogeneous and consisted of a mixture of micro- and nanofibers more akin to microfibrillated cellulose (MFC) at low processing temperatures and tending towards aggregated cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs) at higher processing temperatures. The water holding capacity of the processed cellulosic matter (15-27 gH2O  g-1 ) was higher than the original feedstock or previously reported values. The average molecular weight of the cellulosic matter (113.6-1095.9 kg mol-1 ) decreased significantly by a factor of 10 at operating temperatures above 180 °C, invoking significant scissoring of the cellulosic chains. The process energy input and costs varied between 0.142-0.624 kWh and 13-373 € kg-1 , respectively, and strongly depended on the reaction time.