糠醇树脂裂解过程中呋喃环的开环机理研究

糠醇树脂在高温炭化过程中的结构演变，对其在高温下的结构/性能稳定性具有极为重要的影响。本文基于密度泛函理论方法，利用Gaussian09程序包，过QST2(Quadratic Synchronous Transit)和IRC(Intrinsic Reaction Coordinate）等方法，研究了糠醇树脂在单呋喃环，双呋喃环，以及三呋喃环等不同层次结构单元的开环裂解反应路径。此外，在B3LYP/6-31G**水平下对开环过程中的各种结构进行优化，并在MP2/6-31G**水平下计算各结构的单点能，进而比较了各种反应路径的活化能。结果表明：位于呋喃环间的亚（次）甲基桥上的氢原子较为活泼，易形成氢自由基。伴随着氢自由基向呋喃环上的碳原子的迁移，呋喃环开环。而氢自由基的迁移主要表现出两种方式：(1) 如产生的氢自由基与呋喃环上的氧原子较近，则先与氧原子相结合形成羟基，并导致呋喃环的开环；然后再从氧原子上迁移到碳原子上，从而形成链式的酮结构，此方式要经过两个过渡态和一个中间体才能完成开环。以这种方式开环需要的活化能相对较小。(2) 当氢自由基与呋喃环上的氧原子较远，则直接迁移到碳原子形成C—H键。该过程只经过一个过渡态即完成呋喃环的开环。此外，通过能量比较发现：有水参与的开环反应活化能，要低于无水参与的开环反应活化能；而随着呋喃环的增多，开环反应的活化能逐渐增大。

Theoretical study of the ring-opening mechanisms of constituent units with different numbers of furan rings during the degradation of furfuryl-alcohol resin

The structural evolution of furfuryl alcohol resin at elevated temperatures has a great influence on its structure and performance stability. Density functional theory calculations have been employed to investigate the thermal degradation reactions of furfuryl alcohol resin by studying different constituent units composed of one, two, and three furan rings. The results showed that the C—H bond was cleaved initially, producing hydrogen radicals at the methane or methyne bridges between the furan rings. The migration of the hydrogen radical to the carbon atom of the furan ring and the ring-opening reaction occurred simultaneously. Depending on the distance between the hydrogen atom on the methane or methyne bridge and the oxygen atom on the furan ring, two possible pathways were found for the migration of the hydrogen radical and the ring-opening reaction: (i) In the case of the hydrogen atom being adjacent to the oxygen atom, the hydrogen radical initially becomes bonded to the oxygen atom, resulting in the formation of a hydroxyl group. Finally, a chain ketone structure is obtained after the re-migration of the hydrogen radical to the carbon atom on the furan ring. Two transition states and one intermediate occur during such a ring-opening process. The activation energy of this pathway is lower than that of the other pathway. (ii) If the hydrogen radical is far from the oxygen atom, the hydrogen radical migrates to the adjacent carbon directly. Only one transition state is observed. In addition, optimization of the structures of every transition state and intermediate product was carried out using the B3LYP method, and the corresponding single point energies were subsequently calculated at the MP2/6-31G**level. The results showed that the participation of H2O decreases the activation energy of the ring-opening reaction. In addition, the activation energies of the ring-opening reactions increase as the number of furan rings in the constituent units increases.