In situ Study of Dilute H2SO4 Pretreatment of C-13-Enriched Poplar Wood, Using C-13 NMR

In situ ¹³C NMR measurements are reported on ¹³C-enriched powdered poplar wood that is subjected to pretreatment with 0.5Msulfuric acid as a function of time and at two temperatures. ¹³C 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. T₁ values for ¹³C and for ¹H, as well as T_CH and T _1ρH, were measured at various stages of treatment with 0.5 M H₂SO₄ for lignin peaks and for cellulose peaks in the ¹³C 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-levelmobilities 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 T₁ results by a rather simple model indicates that, for poplar wood in 0.5% H₂SO₄, 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.