醋酸溶液中Pd(OAc)_2催化甲烷选择氧化制甲醇机理研究

Mechanism study of Pd(OAc)_2-catalyzed selective oxidation of methane to methanol in acetic acid solution

利用自制实验系统,进行了醋酸体系中甲烷选择氧化制甲醇的实验研究。结果表明,氧气和对苯醌本身不具备氧化甲烷的能力,但在醋酸钯和对苯醌同时存在的条件下,可发生甲烷催化氧化制甲醇的反应,且在反应体系中添加CO以后,目标产物的生成量大幅增加。在实验的基础上,结合量子化学计算结果,分析了Pd(OAc)_2催化甲烷选择氧化制甲醇的反应机理。分析认为,甲烷活化机理为氧化加成反应,即甲烷和对苯醌借助于Pd^(2+)的d、f轨道作为过渡轨道实现了有机加成反应,生成的产物经质子转移生成更为稳定的对羟基苯甲醚,对羟基苯甲醚可被体系中的氧气氧化为对苯醌,同时,生成甲醇。CO的作用主要是调节反应平衡。

A self-made experimental system was used for experimental study of selective oxidation of methane to methanol in the presence of acetic acid. Results showed that both of oxygen and p-benzoquinone themself had no methane-oxidizing activity, but catalytic oxidation of methane to methanol could occur in the presence of both palladium acetate and p-benzoquinone and there was a sharp increase in the yield of the interested product after adding carbon monoxide in the reaction system. The mechanism of Pd（OAc）2-catalyzed selective oxidation of methane was analyzed on the basis of experiments and results of quantum chemistry calcul-ations. It was found that the activation mechanism of methane is oxidative addition reaction, namely, methane and p-benzoquinone realized the organic addition reaction by means of d- and f-orbitals of Pd-（2＋）as transition orbitals, in which the obtained product further changes into stable p-hydroxyanisole by proton transfer, which then could be oxidized to p-benzoquinone and methanol by the oxygen in the system. Carbon monoxide mainly functions as a regulator of reaction equilibrium.