IMPROVED PROTOCOL FOR ALKALINE NITROBENZENE OXIDATION OF WOODY AND NON-WOODY BIOMASS

The protocol of alkaline nitrobenzene oxidation was investigated to improve its ability to identify the different lignin structures for both woody and non-woody biomass. The survival factors of all six oxidation products-syringaldehde (Sr), vanillin (V), p-hydroxybenzaldehyde (B) and their corresponding acids, syringic acid (Sa), vanillic acid (Va), and p-hydroxybenzoic acid (Ba)-were studied at 170, 180, and 190 degrees C for several residence times. Under similar conditions, various lignin model compounds-a softwood (loblolly pine), a hardwood (red maple), and a non-wood raw material (corn stover)-were oxidized. Molar yields of oxidation products were determined and the ratios of (Sr+Sa)/(V+Va), (Sr/V), and B/(V+Va) (B/V) were calculated. All oxidation products were relatively stable at 170 and 180 degrees C but showed some degradation at 190 degrees C, especially at long residence time. In all cases, p-hydroxybenzoic acid was barely detectable. While yields of oxidation products reach a maximum at 170 degrees C for pine and maple, maximal yields of corn stover require 190 degrees C. Consequently, we recommend that nitrobenzene oxidation be carried out at 170 degrees C for 2.5 h for softwood and hardwood, but at 190 degrees C and 4 h with correction for the survival factors for corn stover and other non-woody biomass. Alternatively, a protocol of oxidation at two temperatures is recommended for non-woody biomass.