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

Abstract

In situ 13C NMR measurements are reported on 13C-enriched powdered poplar wood that is subjected to pretreatment with 0.5 M sulfuric 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 TCH and T1ρH, 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-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 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), τc, is in the range of about 0.7–1.5 ns at various stages and temperatures of the treatment.