EPR investigations of the effects of inorganic additives on the charring and char/air interactions of cellulose

EPR (electron paramagnetic resonance) behaviors of cellulose chars during exposure to air at room temperature strongly depend on the identities of any inorganic additives and on the charring temperature. The air exposure of low-temperature (300-400 degreesC) cellulose/alkali carbonate chars or alkali bicarbonate chars appears to involve two different chemical processes, the production and annihilation of free radicals. The new radicals formed during air exposure have larger g-values than those generated by pyrolysis. In contrast, the corresponding low-temperature chars of pure cellulose and cellulose/NaCl show only an apparently small, slow decrease in radical concentration upon air exposure. For the high-temperature (>400 degreesC) chars of cellulose and cellulose/additive mixtures used in this study, air exposure is accompanied by a large reduction in apparent radical concentration. However, this decrease is found to depend dramatically on the presence of water in the exposure environment. Adding Na2Co3 into the cellulose not only causes a higher aliphatic-to-aromatic ratio of carbons in the char (assessed by C-13 NMR), but also changes the distribution of functional groups. Carboxyl groups appear in the cellulose/Na2CO3 char prepared at 350 degreesC for 1 h, while in the corresponding pure-cellulose char and cellulose/ NaCl char they were not detected.