Characterization of cellouronic acid

以水溶性淀粉为模型化合物，用核磁共振仪研究了从0到100%氧化过程中再生纤维素生成β-1，4-连接聚葡萄糖醛酸(纤维素钠盐)的中间体结构。结果表明，有大量的C6醛作为中间结构，消耗了相应的氢氧化钠。C6醛的最大含量约为40%。在试剂充足或反应时间充足的条件下，以水溶性淀粉为原料，通过TEMPO氧化反应制得几乎纯净的α-1，4-连接聚葡萄糖酸。为了控制纤维素的相对分子质量，研究了TEMPO催化氧化过程中纤维素的解聚机理。结果表明，氧化过程中生成的羟基自由基作为副产物参与了解聚反应。一些自由基清除剂能有效地控制Ce-…的解聚。当加入到介质中时，在氧化过程中会产生更多的果糖。通过逐步加入甲醇，使纤维素酸/水溶液分步沉淀，可得到较高相对分子质量的纤维素酸，并用TEMPO以外的商用硝基自由基氧化纤维素，考察其对产物的化学结构、产率和相对分子质量的影响。一些硝基自由基可以缩短从纤维素制备纤维素酸所需的反应时间，从而使氧化产物具有更高的相对分子质量。然而，到目前为止，任何处理都不能完全控制解聚。对TEMPO介导的天然纤维素氧化进行了详细的研究。虽然天然纤维素不能高产量地转化为水溶性的纤维素酸，但一些无序的区域会发生氧化反应，形成水溶性的产物。最高产率约为20%。然而，水不溶残留物仍然具有纤维素I的晶体结构，因此很难氧化成内部的纤维素微晶。较少

Intermediate structures from regenerated cellulose to water-soluble β-1,4-linked polyglucuronic acid (cellouronic acid Na salt) by TEMPO-mediated oxidation were investigated in detail using water-soluble starch as a model compound, which was oxidized stepwise from 0 to 100% oxidation by means of NMR. The results revealed that a significant amount of C6 aldehyde groups was detected as intermediate structures, which consumed the corresponding NaOH. The maximum amount of C6 aldehyde was about 40%. Sufficient reagents or sufficient reaction time gave almost pure α-1,4-linked polyglucuronic acid from water-soluble starch by TEMPO-mediated oxidation.The mechanism of depolymerization of cellulose during TEMPO-mediated oxidation was studied to control molecular weight of cellouronic acid prepared. The results indicate that hydroxyl radicals formed during the oxidation as by-products participate in the depolymerization. Some radical scavengers are effective in controlling depolymerization of ce … More llulose during the oxidation, when added to the medium. Cellouronic acid having higher molecular weight can be obtained by fractional precipitation of cellouronic acid/water solutions by gradual addition of methanol.Commercial nitroxyl radicals other than TEMPO were applied to the cellulose oxidation to see the effects on chemical structures, yields and molecular weights of the obtained products. Some nitroxyl radicals allowed to reduce reaction time required for preparing cellouronic acid from cellulose, thus giving the oxidized products having higher molecular weight. However, complete control of depolymerization could not be achieved by any treatments examined so far.TEMPO-mediated oxidation of native cellulose was investigated in detail. Although native cellulose cannot be converted to water-soluble cellouronic acid in high yields, some disordered regions undergo the oxidation to form water-soluble products. The maximum yields were ca. 20%. However, the water-insoluble residues still had the crystal structure of cellulose I, and thus oxidation to inside cellulose crystallites is difficult. Less