Chemical modification of cellulose by TEMPO-mediated oxidation

本项目研究了2,2,6,6-四甲基胡椒啶-1-氧自由基（TEMPO）介导氧化对天然纤维素的化学修饰。通过tempo介导氧化引入天然纤维素的羧酸和醛基团根据氧化条件（如反应时间和次氯酸钠作为共氧化剂添加到纤维素/水悬浮液中的量）进行测定。本研究采用毛毡纤维素作为起始天然纤维素。显微观察表明，经tempo氧化前后，棉纤维素的原始纤维形态基本不变，经过滤后，经tempo氧化后的棉纤维素的回收率几乎为100%。经tempo氧化后的绒毛纤维素中羧酸盐和醛的含量分别达到原始绒毛纤维素的35倍和1000倍。通过对氧化产物的x射线衍射和固态^<13>C-NMR分析，证实了在tempo介导氧化后，更多的er纤维素的结晶度和晶体尺寸仍未改变。因此，tempo介导的氧化可以非常有效地将羧酸盐和醛基引入纤维素微原纤维表面，保持天然纤维素的微原纤维结构。通过tempo氧化法制备的表面改性纤维素具有对金属离子的特异性吸附行为。特别是，在tempo氧化纤维素中，羧酸离子与钙、银或铅离子以1:1的摩尔比形成金属盐结构。当tempo介导氧化作用于木材化学浆、细菌纤维素、被膜纤维素、苎麻纤维素和木材全纤维素时，即使在相同的tempo介导氧化条件下，每种纤维素的羧酸盐和醛含量也不同，这取决于所使用的原始纤维素。特别是，我们已经成功地在相当温和的条件下，通过tempo介导的氧化，然后进行机械处理，制备了透明的纤维素微纤维分散凝胶。这些透明凝胶由独立分散的纳米级宽度的纤维素微原纤维组成，不同于传统的分散在水中的纤维素微晶或分子溶解在水中的纤维素衍生物，因此即使在低浓度下也表现出相当高的粘度。这些分散在水中的新型纤维素纳米材料有望显示出一些独特的功能。少

Details about chemical modifications of native cellulose by 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO)-mediated oxidation were studied in this project. Carboxylate and aldehyde groups introduced into native celluloses by the TEMPO-mediated oxidation were determined in terms of oxidation conditions such as reaction time and the amount of sodium hypochloride added as a co-oxidant to the cellulose/water suspensions. Linter cellulose was used to begin with as the starting native cellulose. Microscopic observation showed that the original fibrous morphology of linter cellulose was unchanged before and after the TEMPO-mediated oxidation, and recovery ratios of the TEMPO-oxidized linter cellulose were almost 100% by filtration. Carboxylate and aldehyde contents in the TEMPO-oxidized linter cellulose reached to 35 times and 1000 times as much as those of the original linter cellulose, respectively. Crystallinity and crystal size of the native cellulose I structure of the original lint … More er cellulose were unchanged even after the TEMPO-mediated oxidation, which was proved by X-ray diffraction and solid-state ^<13>C-NMR analyses of the oxidized products. Thus, TEMPO-mediated oxidation allows to introduce carboxylate and aldehyde groups only on the surface of cellulose microfibrils quite efficiently, maintaining the microfibril structure of native cellulose. Such surface-modified celluloses prepared by the TEMPO-mediated oxidation showed specific adsorption behavior of metal ions. Especially, carboxylate ions in the TEMPO-oxidized cellulose form metal salt structures at molar ratio of 1:1 with either calcium, silver or lead ion. When the TEMPO-mediated oxidation was applied to wood chemical pulp, bacterial cellulose, tunicate cellulose, ramie cellulose and wood holocellulose, each cellulose had characteristic carboxylate and aldehyde contents even under the same TEMPO-mediated oxidation conditions, depending on the original cellulose used. Particularly, we have succeeded in preparation of transparent cellulose microfibril dispersion gels by TEMPO-mediated oxidation under quite mild conditions followed by mechanical treatments. These transparent gels consisted of independently-dispersed cellulose microfibrils with nano-size width in water, differing from the conventional cellulose microcrystals dispersed in water or cellulose derivatives molecularly dissolved in water, and thus behaved quite high viscosities even at low consistencies. These new cellulose nano-materials dispersed in water are expected to show some unique functionalities. Less