Cellulose pretreatment in subcritical water: Effect of temperature on molecular structure and enzymatic reactivity

Abstract

Microcrystalline cellulose (MCC) was pretreated with subcritical water in a continuous flow reactor for enhancing its enzymatic reactivity with cellulase enzyme. Cellulose/water suspension was mixed with subcritical (i.e., pressurized and heated) water and then fed into the reactor maintained at a constant temperature and pressure. After the reaction, product was immediately cooled in a double-pipe heat exchanger. The solid portion of the product (i.e., treated MCC) was separated and tested for molecular structure and enzymatic reactivity. Experiments were conducted at temperatures ranging from 200 to 315 °C, at 27.6 MPa, and for 3.4–6.2 s reaction times. The treated MCC was characterized for degree of polymerization ( DP v ) by viscosimetry, and crystallinity by X-ray diffraction (XRD). In addition, differential scanning calorimetry and scanning electron microscopy (SEM) analyses were carried out to study any transformation in the cellulose structure. As expected, DP v of cellulose steadily decreased with increase in the pretreatment temperature, with a rapid drop occurring above 300 °C. On the other hand, XRD analysis did not show any decrease in crystallinity upon pretreatment but, partial transformation of celluloses I–II structure was noticed in the MCC treated at ⩾300 °C. Development of surface cracks and trenches were observed in the SEM images for all the treated samples. Enzymatic reactivity was increased after the treatment at ⩾300 °C.