(567e) Transformation of Corn Stover Lignin During Wet Aerobic Oxidation: Lessons for Making High-Value Added Aromatic Aldehydes

Lignin is the second most abundant biopolymer available in nature after cellulose. Unfortunately, it is often seen as a hindrance to cellulose isolation for cellulosic ethanol production and paper processing instead of a readily available resource for producing high-value chemicals with an aromatic ring in their structure.  Many specialty chemicals can be made from oxidation of lignin including the valuable aromatic aldehydes vanillin (3-methoxy-4-hydroxybenzaldehyde) and syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde).  Lignin is a heterogeneous polymer which varies in its monomer ratios from one plant species to another and therefore will produce different amounts of aldehydes depending on the nature of the feed that is used in the reaction.

Sugar cane bagasse has been used to produce vanillin and syringaldehyde, but the production of the most economically valuable compound, syringaldehyde, was lower than desired.  Corn stover lignin has more syringyl units than sugar cane bagasse and should produce more syringaldehyde. The catalytic system, Pd/γ-Al₂O₃, previously used to produce syringaldehyde, showed promising increases in aldehyde yields. Here, we use lignin from corn stover and report different reactivity trends than those for lignin from sugar cane bagasse. To better understand these findings, we examined the composition of corn stover lignin and the influence of wet aerobic oxidation conditions, both in the presence and absence of a heterogeneous catalyst, in order to understand the impact of these parameters on the reactivity of lignin transformation to aromatic aldehydes.

In this study, thermogravimetric analysis (TGA) in air was performed on washed lignin feedstock to identify the composition of the lignin.  The EDS study (energy dispersive x-ray spectroscopy) of residual powder after TGA confirmed the presence of Fe, known to be catalytically active in catalytic wet aerobic oxidation (CWAO), as well as significant amounts of Si, Al and K. To confirm the catalytic roles of Fe, various amounts of Fe(III) nitrate were impregnated onto the lignin feed stock that was used in CWAO to determine the impact of iron concentration on the reaction rate and the selectivity to aromatic aldehydes. The resulting aldehydes were analyzed by GCMS and residual lignin was quantified by TGA in air. The role of pH was also examined to distinguish the roles of dioxygen and hydroxyl ions in CWAO of lignin.