硼氢化钠还原对褐腐木质素结构特征的影响

采用FTIR、UV-Visible、NMR和GPC分析手段研究了褐腐木质素被NaBH₄还原前后的化学结构变化。FTIR表明褐腐木质素还原后1 677 cm-1处与苯环共轭的羰基峰消失，1 715 cm-1处非共轭羰基峰强度减弱，1 509和1 603 cm-1处苯环骨架振动吸收峰强度变化很小;UV表明褐腐木质素还原后位于288 nm的最强吸收峰和300～400nm区域的吸收强度降低;1H NMR表明褐腐木质素还原后甲氧基和酚羟基数量减少，醇羟基数量增加，褐腐木质素芳香环和结构单元联接键上的氢质子数增加;GPC表明褐腐木质素还原后分子量分布向高分子区域扩展，数均和重均分子量增大，分子量分布明显变宽。NaBH₄在碱性环境中可以将褐腐木质素中的共轭羰基完全还原为羟基，非共轭羰基部分还原为羟基，其侧链结构部分被改变，苯环结构稳定，褐腐木质素在还原过程中发生了缩合反应。

Effect of Reduction by Sodium Borohydride on the Structural Characteristics of Brown-Rotted Lignin

The FTIR, UV-Visible, 1H NMR and GPC were used to study structural changes of brown-rotted lignin after sodium borohydride reduction. FTIR spectra showed that, after reduction, the band at 1 677 cm-1 attributed to the conjugated carbonyl groups disappeared, and the band intensity at 1 715 cm-1 attributed to the nonconjugated carbonyl groups decreased. On the other hand, the band at 1 509 and 1 603 cm-1 attributed to aromatic skeletal vibration remained almost unchanged. UV spectra showed the decreased absorptions at 288 nm and 300-400 nm after reduction. 1H NMR spectra showed that, after reduction, the number of aromatic methoxyl and aromatic hydroxyls decreased, the number of aliphatic hydroxyls and the proton number attributed to many linkage structures connecting the phenylpropane units increased. GPC results showed that the molecular weight of reduced brown-rotted lignin increased and the molecular distribution got wider, as a consequence of the large molecular weight molecules generated during the reduction reaction. Our results suggest that the conjugated carbonyl groups can be totally reduced to the hydroxyl groups, but only some nonconjugated carbonyl groups can be reduced to the hydroxyl groups. The chemical structure of the brown-rotted lignin changed, but the benzyl ring kept stable. The condensation reaction took place during the sodium borohydride reduction process.