Abstract
A comparative study of degradation of wheat straw lignin by alkaline nitrobenzene and cupric compound (CuO, Cu(OH) 2, CuSO 4 5H 2O) oxidation was carried out at 170 °C for 2.5 h in 2 N NaOH (7 ml) and 6.8% nitrobenzene. The major products of such a degradation were the phenolic aldehydes of vanillin and syringaldehyde in both of the reactions. A relative high guaiacyl syringyl ratio value was obtained by nitrobenzene oxidation, whereas a slightly high syringyl guaiacyl ratio appeared in the cupric(II) oxidation. A suite of up to 13 phenols was detected in the oxidation mixtures except that no acetosyringone appeared in the nitrobenzene oxidation. Gallic acid, protocatechuic acid and acetovanillone were firstly identified in both of the oxidation products from wheat straw lignin. The main difference between the two oxidant reagents was twice yield of nitrobenzene oxidation than that of cupric(II) oxidation. The effects reaction time, temperature, concentration of sodium hydroxide and nitrobenzene, and the amounts of sample used on the reaction yield and components of oxidation products from wheat straw were performed at 38 various conditions and the nature of the aldehyde and acid products determined by high-performance liquid chromatography (HPLC). The results obtained showed that the percentages of various products were dependent on the conditions of alkaline nitrobenzene oxidation. The reaction condition (0.10 g straw in 2 N NaOH (7 ml) with 6.8% nitrobenzene at 170 °C for 4 h in a steel autoclaves) optimised for maximum recovery of products from wheat straw. Meanwhile, ferulic and p-coumaric acid released in amounts during the nitrobenzene oxidation, especially, at 170 °C for 2.5 h in 2 N NaOH without nitrobenzene, which suggested that more than 50% of ferulic acid was in the etherified form while p-coumaric acid was predominant in the esterified bond with lignin in wheat straw.
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