(556d) Study of Red Oak Derived Lignin, Pyrolytic Lignin, and Hydrogenated Pyrolytic Lignin with 2D-NMR, Fticr-MS, and GPC

Pyrolytic lignin is a major component of bio-oil derived from the lignin portion of biomass during pyrolysis. Better understanding of the structure of pyrolytic lignin can aid in utilizing this material for fuels or chemicals. Analysis of lignin and lignin-derived streams cannot be done with standard analytical techniques, like gas chromatography, because it is non-volatile. In this talk, we will discuss the results from the analysis of the structure of red-oak lignin before and after pyrolysis and hydrogenation with FTICR-MS, GPC, and NMR. The lignin was isolated from red oak with a biomass fractionation method utilizing gamma-valerolactone as a solvent. Pyrolysis was performed in a fluidized bed pyrolysis reactor operated at 500 ºC and preheated nitrogen flow. An attached bio-oil collection system separated the bio-oil into five stage fractions. The pyrolytic lignin was separated with a water fractionation from the second stage fraction. Hydrogenation was performed in an ethanol solution over a 5 wt% Ru/C catalyst at 150 ºC and 500 psig of initial H₂ for 2.5 h. A five times decrease in the average molecular weight (from 4660 Da to 831 Da) was observed with GPC after pyrolysis, further decreasing to 726 Da after hydrogenation. More than 1100 distinct molecular weights were observed with FTICR-MS of the lignin stream. The FTICR-MS revealed O/C and H/C ratios decreased from 0.33 and 0.94 to 0.25 and 0.90 after pyrolysis, in line with dehydration of sidechain –OH to alkenes while the O/C ratio decreased to 0.23 and the H/C ratio increased to 1.11 after hydrogenation, supporting the hydrodeoxygenation and saturation of the aromatic units. Quantitative 13C NMR revealed a large decrease in the C-O aliphatic carbons (from 35.0% to 11.4% of carbons) during pyrolysis with a large increase in carbonyl and C-C aliphatic carbons (from 0% and 1.3% to 4.2% and 15.4% of carbons respectively). The hydrogenation decreased the carbonyl and aromatic carbons (from 4.2% and 69.0% to 2.6% and 37.0%) and increased the C-C and C-O aliphatic carbons (from 15.4% and 11.4% to 41.3% and 19.2% respectively). The methoxyl region decreased from 13.3% to 6.3% of carbons after pyrolysis and 4.2% of carbons after hydrogenation. HSQC NMR displayed the varying amounts of C-C aliphatic, C-O aliphatic, aromatic, and aldehyde functionalities. From pyrolysis, evidence supported the formation of diaryl methine linkages while the lignin β-O-4, phenylcoumaran, and resinol linkages disappeared. Hydrogenation produced cyclohexyl functionalities through saturation of the aromatics. Benzaldehyde and cinnamaldehyde were seen for the lignin and pyrolytic lignin but completely disappear after hydrogenation. These aldehyde functionalities were highly reactive during hydrogenation and potentially contributed to coke formation through aldol condensation reactions. With HMBC NMR, the lignin had small amounts of C-C aliphatic to aromatic correlations while these correlations increased after both pyrolysis and hydrogenation. C-C to C-O aliphatic and C-C to C-C aliphatic correlations increased after hydrogenation. Tertiary aromatics and C-C aliphatic to carboxylic acid correlations were present in lignin, pyrolytic lignin, and hydrogenated pyrolytic lignin. Ketone to C-C aliphatic correlations appear after pyrolysis but similarly to the aldehydes, disappear after the hydrogenation. The results from this presentation provide a more comprehensive understanding of the chemistry that occurs during lignin pyrolysis and hydrogenation.