In situ Study of Dilute H₂SO₄Pretreatment of¹³C-Enriched Poplar Wood, Using¹³C NMR

In situ 13 C NMR measurements are reported on 13 C-enriched powdered poplar wood that is subjected to pretreatment with 0.5 M sulfuric acid as a function of time and at two temperatures. 13 C MAS (magic-angle spinning) spectra were obtained in both the DP (direct polarization) and CP (cross-polarization) modes, the contrasts in this combination yielding valuable qualitative information on the effect of pretreatment on local molecular mobilities. T 1 values for 13 C and for 1 H, as well as T CH and T 1ρH, were measured at various stages of treatment with 0.5 M H 2 SO 4 for lignin peaks and for cellulose peaks in the 13 C NMR spectra, as quantitative indicators of the degree of molecular motion for those two structural entities. The results show that a substantial fraction of the solid/semisolid biomass is converted at elevated temperatures to (a) chemically different and more mobile structures and (b) locally similar structures with enhanced atomic-level mobilities and that some fraction of this “mobilized” biomass does not return to the original level of immobility upon cooling the biomass back to room temperature. Analysis of the T 1 results by a rather simple model indicates that, for poplar wood in 0.5% H 2 SO 4, the estimated (“global”) motional correlation time (at the multiatom level), τ c, is in the range of about 0.7–1.5 ns at various stages and temperatures of the treatment.