Cellulose dissolution in phosphoric acid: insight into the role of water from small-angle neutron scattering

Cellulose from biomass is an abundant and renewable alternative source for chemicals and fuels, yet its utilization by chemical or biological process requires pre-treatment in order to release the macromolecules from their tightly packed crystal structure. Phosphoric acid (PA) has been known for many years to be an efficient solvent for crystalline cellulose. It is also established that a certain quantity of water content in PA is required for efficient pretreatment. This study uses small-angle neutron scattering (SANS) measurements to evaluate cellulose dissolution in deuterated phosphoric acid (dPA), at different wt% dPA between 78 and 97% (different D₂O content). The results indicate that a significant part of the cellulose chains are molecularly dissolved as individual chains at water content of 81-94 wt%, in 2 wt% cellulose solutions, with insignificant structural variation at the water in this range. At 78% dPA the cellulose crystal still seems to be disrupted, yet the structure can be modeled as mass-surface fractals of small fibrils with an irregular surface. At 97% dPA evidence of undissolved fibrils is noted. The rather wide window of water content in PA in which cellulose dissolves may be rationalized by complementary thermodynamic and kinetic effects. Decreasing water content below 20 wt% enhances PA activity towards both cellulose wettability and phosphorylation reaction, yet significantly increases the solvent viscosity, decreasing the solvent mobility.