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

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. <i>T</i>₁ values for ¹³C and for ¹H, as well as <i>T</i>_CH and <i>T</i>_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-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 <i>T</i>₁ 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

Molecular-Level Consequences of Biomass Pretreatment by Dilute Sulfuric Acid at Various Temperatures

<i>Ex situ</i> room-temperature ¹³C nuclear magnetic resonance (NMR) measurements are reported on powdered poplar wood that has been pretreated with dilute sulfuric acid (concentrations up to 1 wt %) for times ranging up to 20 min and at temperatures of 120, 130, 140, and 150 °C. There are significant, albeit not dramatic, changes in the measured NMR spectra of the biomass as result of dilute sulfuric acid treatment. Values of <i>T</i>₁ for ¹³C and ¹H, as well as <i>T</i>_CH and <i>T</i>_1ρH, were measured for lignin peaks and cellulose peaks in the ¹³C NMR spectra, as potential indicators of the degree of atomic-level motion. For lignin components, one finds a trend to larger <i>T</i>_CH values as the treatment time or H₂SO₄ concentration is increased for treatment temperatures of 120 and 130 °C; however, for treatment temperatures of 140 and 150 °C, <i>T</i>_CH apparently decreases as the treatment time is increased. This higher temperature <i>T</i>_CH behavior implies that the lignin may actually become more rigid at later stages of treatment at temperatures ≥140 °C, which can be explained by cleavages of ether linkages of lignin and subsequent formation of new linkages, i.e., lignin recondensation. <i>T</i>_1C and <i>T</i>_1H measurements are consistent with this interpretation. The relationships between atomic-level mobility of lignin in biomass and treatment temperature is consistent with published relationships between the sugar yield and treatment temperature. The key role of acid treatment as a pretreatment for enzymatic digestion is evident in NMR measurements, including relaxation measurements, even after the treatment