竹材草酸预水解过程木质素的溶出规律

Lignin removal law during bamboo prehydrolysis with oxalic acid

为了探索用草酸预处理竹材制备溶解浆,该文用不同用量的草酸对竹材进行预水解,测定预水解过程竹材木质素含量的变化,用场发射扫描电镜(field emission scanning electron microscope,FE-SEM)观察水解后假木质素在竹纤维表面的分布状况,用核磁共振技术(cross polarisation/magic angle spinning carbon-13 nuclear magnetic resonance,CP/MAS 13C-NMR)分析木质素的结构变化。研究结果表明,在竹片草酸预水解过程中,竹片预水解得率将随着预水解时间的延长而下降,并且,草酸用量越大,得率下降越多。基于水解后竹材基质的木质素含量会随着预水解时间的延长而逐渐升高,并且,草酸用量越大,木质素含量升高越多;而基于竹材原料的木质素含量则随着预水解时间的延长先下降再增加,并且,草酸用量越大,增加的速率也越大。竹材木质素由于芳基醚键(β-O-4)断裂,酚型木质素含量提高。纤维素和半纤维素降解产物在预水解过程形成的假木质素类物质会以微球的形式附着在竹片表面。该研究可为探索竹材快速高选择性去除半纤维素的预水解方法提供参考。

Dissolved pulp refers to pulp of high cellulose content which is used to manufacture various cellulose-derived products such as regenerated fibers or films （e.g., Viscose, Lyocell）, cellulose esters （acetates, propionates, butyrates, nitrates） and cellulose ethers （carboxymethyl-, ethyl-, methyl-celluloses）. With the rapid development of the global economy and a constant increase in population, the overall demand for dissolved pulp is rising. Dissolved pulp is generally made by the prehydrolysis kraft and acid sulfite processes comprising additional purification stages such as hot and cold caustic extraction. Dilute sulfuric acid is the most commonly used solvent for prehydrolysis of biomass to produce the dissolved pulp. However, cellulose is seriously degraded and the equipment is corroded while the prehydrolysis with sulfuric acid is performed. Oxalic acid, as an organic acid, has a better prehydrolysis selectivity than some mineral acids such as sulfuric acid or hydrochloric acid. The purpose of this paper is to clarify bamboo lignin removal during the bamboo prehydrolysis with oxalic acid to improve the prehydrolysis process for dissolved pulp. The acid-insoluble lignin contents were determined with 72%H2SO4 according to the TAPPI standard test method （T 222 om-06） at different prehydrolysis periods. The distribution of pseudo-lignin on the surface of bamboo fibers was observed by FE-SEM. The chemical structural changes of bamboo lignin were studied by means of CP/MAS 13C-NMR. The results show that the yield decreased with the extension of prehydrolysis time as most of the hemicellulose and a little of the lignin and cellulose were degraded under the acid prehydrolysis conditions. As an increased amount of oxalic acid was applied for prehydrolysis, an increased drop of the yield was obtained. The lignin content based on the prehydrolysed bamboo chips increased with the extension of prehydrolysis time due to a predominant degradation of hemicellulose, the increased amount of oxalic acid and the higher lignin content. However, the lignin content based on the bamboo chips increased dramatically after an initial decline with the extension of prehydrolysis time due to the condensation between the lignin degradation products and carbohydrates, and the formation of pseudo-lignin. As an increased amount of oxalic acid was used for prehydrolysis of bamboo, a greater increase in the rate of lignin content was obtained. The bamboo lignin has more phenolic hydroxyls due to the breakdown of aryl-ether bonds （β-O-4） during the oxalic acid prehydrolysis of bamboo. The pseudo-lignin originating from carbohydrates including cellulose and hemicellulose could attach to the surface of bamboo fibers in the form of microspheres.