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

Ex situ 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 T₁ for ¹³C and ¹H, as well as T_CH and T_1H, 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 T_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, T_CH apparently decreases as the treatment time is increased. This higher temperature T_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. T_1C and T_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.