酸性分子筛催化乙烯二聚反应

应用密度泛函理论(DFT), 采用5T簇模型来模拟分子筛催化剂的酸性位, 在B3LYP/6-311+G(3df, 2p)的条件下通过理论计算研究了乙烯在酸性分子筛上的二聚反应. 对反应各驻点进行了全局优化, 经过零点能校正后, 计算得出乙烯二聚反应的活化能. 研究表明, 乙烯在分子筛上的二聚反应分三步进行: 单个乙烯分子化学吸附→第二个乙烯分子的物理吸附→两乙烯分子二聚反应. 乙烯化学吸附生成的烷氧化合物与物理吸附的乙烯分子发生二聚反应生成新的C—C键同时生成新的烷氧化合物. 计算得到的乙烯化学吸附和二聚反应的反应能垒分别为108和149 kJ·mol-1. 反应的逆过程也就是1-丁烯在酸性分子筛表面的1-丁基烷氧化合物发生β分裂反应, 计算所得相应的1-丁烯β分裂反应的能垒为217 kJ·mol-1, 远高于相应的乙烯二聚反应能垒. 此外还进一步研究了所用基组对计算结果的影响.

Dimerization of Ethene Catalyzed by Acidic Zeolites

Density functional theory (DFT) was employed to investigate the dimerization of ethene over acidic zeolites with a 5T cluster model simulating zeolite. At the B3LYP/6-311+G(3df, 2p) level, the activation energy was obtained with a zero point energy (ZPE) correction. It was found that the process of the dimerization of ethene can be divided into three steps: the chemical adsorption of first ethene molecule, the physical adsorption of second ethene molecule and the dimerization reaction. The reaction proceeds by ethene chemical adsorption to form the surface ethoxide and the C—C bond formation between the ethoxide and the second ethene molecule to form the butoxide product. The calculated reaction energy barriers of ethene chemical adsorption and dimerization reaction are 108 and 149 kJ·mol-1, respectively. The obtained activation barrier of β-scission of 1-butene is 217 kJ·mol-1 in the reverse reaction of ethene dimerization.