N-methylmorpholine-N-oxide (NMMO) categorizes as a family of cyclic, aliphatic, tertiary amine oxides [287, 288]. Tertiary amine oxide systems were first patented by Graenacher and Sallmann in 1939 to dissolve cellulose for enhanced chemical processing [289]. However, Johnson, for the first time in 1969, introduced a cyclic mono(N-methylamine-N-oxide) compound to dissolve strong intermolecular hydrogen bonding networks (e.g., cellulose, wool, silk, hair, and feather), which are insoluble in commonly used solvents [290]. Since the late 1970s, the research on the dissolution of cellulose in NMMO was initiated when McCorsley and Varga [291] produced a highly concentrated yet economical cellulose solution by dissolving cellulose in a NMMO-water …show more content…
9) [288, 292, 312]. Cellulose dissolution in NMMO leads to a tertiary phase of cellulose-NMMO-water system [292, 313]. Hydration with 1-1.2 water molecule per NMMO (water content 13.3-17 wt %) significantly improves its solvation strength, while increasing water content to 19–24% and 25–30% results heterogeneous swelling by forming balloons and ballooning, respectively [314]. Higher water contents (above 35%) make fibers swell homogeneously and precipitate, because in the tertiary system water is further preferred to from hydrogen bond with NMMO than cellulose [288, 292, 314]. Ballooning and swelling modes of cellulose dissolution is more efficient for biogas production, while for ethanol production pretreatment with 85% NMMO leads to a better lignocellulose bioprocessing …show more content…
The crystalline structure of regenerated lignocellulose as well as the untreated one are usually expressed by Total Crystallinity Index (TCI) and Lateral Order Index (LOI) using FTIR [318]. Purwandari et al. [310] reported that TCI (the absorbance ratio A1427/A898 calculated by FTIR spectra) of oil palm empty fruit bunch (OPEFB) reduced by up to 78%, during the pretreatment in 85% NMMO at 120°C for 3 h. In addition, ballooning and swelling modes of NMMO result in lower total crystallinity indexes at 120°C rather than 90°C. This finding is in contrast with ballooning and swelling modes of NMMO pretreated of cotton that result in lower crystallinity indexes at 90°C than 120°C [297]. However, crystallinity indexes decrease slightly for different modes of dissolution, ballooning, and swelling. Besides, the intramolecular hydrogen-bonding OH stretching at about 3,350 cm-l in pretreated cotton is broadened and shifted to a higher wave number, which is an indication of transforming cellulose I to cellulose II [319, 320]. Moreover, it is in accordance with other reports [303] and also confirms that the pretreatment reduced the structural lignin content. NMMO pretreatment of bagasse at 130°C for 1 h transformed crystalline structure into amorphous form, since the TCI and LOI decrease from 1.39 and 1.44 to 1.18 and 1.10, respectively [298]. LOI, a