Dissolution mechanism of crystalline cellulose in H3PO4 as assessed by high-field NMR spectroscopy and Fast Field Cycling NMR relaxometry

Many processes have been proposed to produce glucose as a substrate for bacterial fermentation to obtain bioethanol. Among others, cellulose degradation appears as the most convenient way to achieve reliable amounts of glucose units. In fact, cellulose is the most widespread biopolymer, and it is considered also as a renewable resource. Due to extended intra- and interchain hydrogen bonds that provide a very efficient packing structure, however, cellulose is also a very stable polymer, the degradation of which is not easily achievable. In the past decade, researchers enhanced cellulose reactivity by increasing its solubility in many solvents, among which concentrated phosphoric acid (H3PO4) played the major role because of its low volatility and nontoxicity. In the present study, the solubilization mechanism of crystalline cellulose in H3PO4 has been elucidated by using high- and lowfield NMR spectroscopy. In particular, high-field NMR spectra showed formation of direct bonding between phosphoric acid and dissolved cellulose. On the other hand, molecular dynamics studies by low-field NMR with a fast field cycling (FFC) setup revealed two different H3PO4 relaxing components. The first component, described by the fastest longitudinal relaxation rate (R1), was assigned to the H3PO4 molecules bound to the biopolymer. Conversely, the second component, characterized by the slowest R1, was attributed to the bulk solvent. The understanding of cellulose dissolution in H3PO4 represents a very important issue because comprehension of chemical mechanisms is fundamental for process ameliorations to produce bioenergy from biomasses.