纤维素吡喃糖键裂解反应的理论研究  

Theoretical study on the bond cleavages of glucopyranose in cellulose

作　　者：杨雪滢 何潇 何运华 夏桓桓 郭新良 谢小光[2] 

机构地区：[1]云南电网有限责任公司电力科学研究院,云南昆明650217 [2]云南大学化学科学与工程学院,云南昆明650091

出　　处：《云南大学学报（自然科学版）》2016年第5期797-804,共8页Journal of Yunnan University(Natural Sciences Edition)

基　　金：云南电网有限责任公司科技项目

摘　　要：以1,4-二甲氧基吡喃葡萄糖为模型分子,采用密度泛函理论方法,在B3LYP/6-311++G**水平,对纤维素分子吡喃糖单元中各键的解离能及其裂解反应的热力学性质进行了理论研究.计算结果表明,键解离能最大的是C2、C3连接羟基的2个C—O键,且比其他键的解离能大得多,而键能最小的是C4—C5键,这与该键具有最长的C—C键长是一致的.C1位糖苷C1—O4键的解离能比C4位糖苷C4—O4键的大16.92 k J/mol.从键裂解活化自由能来看,最容易断裂的4个键分别是C4位的糖苷键、C5—C6脱羟甲基、C4—C5键断裂开环和C1位糖苷键.非常有意思的是在计算C5—O5键开环断裂时发现伴随有C1—C2键的断裂.进一步研究2个键同时断裂的结果表明,该断裂方式不仅键的解离能比键能最小的C4—C5键小11.37 k J/mol,而且裂解活化自由能比其他裂解方式小得多.因此,该裂解方式是纤维素分子裂解最容易进行的方式,裂解产生五元碳链双自由基中间体和另一吡喃糖环C4位甲酸酯.在此基础上,提出了纤维素老化降解产生CO/CO_2气体,糠醛和甲醇等信号分子的可能生成机理.By using 1,4-two methoxyl glucopyranose as model molecule,the bond cleavage reactions of glu- copyranose in cellulose and their thermodynamic properties at 298.15 K and 363.15 K have been investigated u- sing density functional theory at B3LYP/6-311＋＋G＊＊ level. The calculation results show that the two C--O bonds connecting the twohydroxyl group at C2 and C3 positions have the largest dissociation energies, much larger than other bonds ,whereas the C4--C5 bond energy is the smallest in line with its longest bond-length.The C 1-- 04 glucosidic bond energy is larger than that of the C4--O4 glucosidic bond by 16.92 kJ/mol.In view of activa- ted Gibbs free energy, the four preferential cleavage bonds are the C4--O4 glucosidic bond,the C5--C6 bond with dehydroxymethylation,the C4--C5 bond with the pyran ring opening and the C1--O4 glucosidic bond.h is very interesting that the C5--O5 bond cleavage accompanied by the breaking of the C1--C2 bond was found.So, the simukaneous cleavages of both the C5--O5 and C1--C2 bonds were further investigated.The results show that this pattern of bond cleavage yielding 5-member carbochain double radical immediate and another pyran ring for- mate,is much easier to take place than others in view of whether dissociation energy or Gibbs free energy.Based on this,the possible formation mechanisms of some signal molecules,such as CO/CO2 ,furaldehyde and methanol in ageing degradation of cellulose, were proposed.

关 键 词：纤维素吡喃糖 键解离能 裂解反应热力学 密度泛函理论 

分 类 号：O29[理学—应用数学]