Reinvestigating the concurring reactions in early-stage cellulose pyrolysis by solution state NMR spectroscopy

Low temperature pre-treatments increase the char yield during cellulose carbonization. Although this effect is mostly understood on a kinetic basis, the formed chemical structures leave room for scrutiny. In ongoing ambitions to enhance the char yield of bioderived precursors, the thereby occurring chemistry was reinvestigated. A set of isothermal heating protocols ranging from 150° to 250°C was applied to man-made Ioncell® cellulose fibers and to Avicel® PH-101 microcrystalline cellulose. The prepared cellulosic samples were examined by solution state NMR using a tetra-n-butyl phosphonium acetate ([P₄₄₄₄][OAc]): DMSO-d₆ (1:4 wt%) electrolyte as dissolving medium. Complementary, IR spectroscopy, size-exclusion chromatography (SEC) and thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) measurements were performed. The NMR spectra evidenced the formation of levoglucosan end capped moieties as being the first and major occurring reaction during low temperature pre-treatments. In contrast to other mechanistic proposals, no signs for carbonyl or alkene functionalities were discernible in the cellulosic material soluble in the NMR electrolyte, even after treatment at 250 °C for several hours. Thermal cross-linking was observed in SEC already at temperatures not known to significantly influence the overall char yield. In the solution state analytics only partial solubility was observed, owing to the formation of a reluctant fraction previously described as “thermostable condensed phase”. This resulted in analytical blind spots and led to discrepancies between NMR results and FTIR spectra in which carbonyl and alkene vibrations were clearly discernible. Those discrepancies might also imply the co-existence of different fractions in the early stages of cellulose pyrolysis.