煤及油母中常见C—X(X=N,S)的解离能

Bond dissociation enthalpies of C—X(X=N,S) in coal and kerogen

煤及油页岩中除了碳、氢和氧原子是主要的组成元素外,氮和硫等杂原子大都以C—X(X=N,S)的键合形式存在,在其结构及转化利用中同样发挥重要的作用。认识C—X的解离能,有助于建立氮、硫热解过程中的迁移模型,丰富对煤及油母中常见化学键性质的认识,对发展高效清洁的能源利用技术至关重要。利用双杂化密度泛函方法,系统研究了煤及油母中典型C—X键的解离能(BDE)范围。研究结果表明,煤及油母中常见C—N和C—S键的BDE值范围分别是154.1~55.7 kcal·mol^(-1)和83.0~56.6 kcal·mol^(-1)。在热解过程中,苯硫类自由基以及苯胺类自由基会在初期产生,其次才是巯基、胺类自由基等侧链取代自由基脱落。C—S键的整体BDE值范围比其他类型化学键更低,各类化学键最低BDE值的高低顺序符合O—H>C—H>C—C>C—N>C—S>C—O的规律,其中,只有当有PhO·生成时,C—S>C—O,否则C—O>C—S。

The carbon, hydrogen, oxygen are the majority elements in the structure of coal and oil shale, while the nitrogen and sulfur atoms, mostly in the form of C-X （X=N, S）, still play a nonnegligible role in the structure and application of coal and oil shale. The homolytic bond dissociation enthalpies （BDE） of C-X for model compounds that are representative of the functionalities present in coal and kerogen were computed by using a double-hybrid method mPW2PLYP. The studies will be helpful for people to construct the transfer model of N and S in pyrolysis, and enrich the knowledge of chemical reaction of typical bonds in coal and kerogen. The BDE for C-N and C-S cover a range from 154.1-55.7 kcal·mol^-1 and 83.0-56.6 kcal·mol^-1, respectively. The results suggest that the thiophenyl radical or phenylamino radical are the most favorable intermediates in the early pyrolytic stage. With the increase of temperature, the loss of sulfydryl and amidogen groups become feasible, and the ring cleavage reaction of five-membered ring （pyrrole and thiophene） through homolysis of C-N and C-S bond can also occur. The phenyl radical is the most difficult to form, and the directly bond dissociation of C-N bond in pyridine with six-membered ring is extremely unfavorable in the pyrolysis of coal and kerogen. Compared to other types of bonds, C-S bond has the lowest general BDE range. The order of the lowest BDE for various bonds is O-H 〉 C-H 〉 C-C 〉 C-N 〉 C-S 〉 C-O. The bond dissociation of C-O to generate PhO· radical is easier than to generate PhS· radical through homolysis of C-S, otherwise the BDE of C-O is higher than C-S.