Changes in the Structure of the Cellulose Fiber Wall during Dilute Acid Pretreatment in Populus Studied by 1H and 2H NMR

Dilute acid pretreatment (DAP) is a frequently employed technique in biotite] production to increase overall sugar and subsequent ethanol yields from downstream fermentation. This is done prior to enzymatic deconstruction of cellulose to increase accessible surface area as well as to remove or redistribute hemicellulose and lignin, which have an inhibitory effect on enzymatic hydrolysis. In this study, the effect of DAP on the supramolecular and ultrastructure of lignocellulosic biomass was evaluated by both H-1 and H-2 NMR techniques. A series of DAPs were conducted on Populus using similar to 0.10 M H2SO4 at similar to 160 degrees C for varying residence times. H-2 spin lattice (T-1) times of deuterium oxide (D2O) adsorbed within the lignocellulosic biomass were measured on untreated and pretreated Populus, and an inverse Laplace transform of the T-1 decays was then used to generate pore size distributions. The resulting distributions indicate that substantial pore expansion within the cellulose fibril bundles occurs during pretreatment. H-1 Goldman-Shen (GS) spin-diffusion experiments which were also conducted, qualitatively supporting the magnitude of observed pore expansion obtained from H-2 NMR relaxation profiles. H-1 Carr-Purcell-Meiboom-Gill (CPMG) and pulse field gradient (PEG) experiments were used to investigate the altering supramolecular structure of the lignocellulose and the self-diffusive behavior of water adsorbed within the biomass as a function of DAP. Spin-spin (T-2) relaxation times indicate the nature of cellulose-water interactions change during DAP. Inverse Laplace distributions of the resulting T-2 decays demonstrate not only a shift in T-2 times to longer relaxation or a more mobile state but also indicate that the population of water with longer relaxation times increase, indicating that pretreatment begins to break down and loosen the cellulosic ultrastructure within the biomass. Lastly, the water self-diffusion experiments demonstrates that DAP increases pore tortuosity within the biomass.