采用密度泛函理论B3LYP/6-31G(d,p)方法,研究了HZSM-5分子筛上不同酸性位以及相邻双酸性位对乙烯质子化反应的影响,考察了乙烯吸附、质子化反应过渡态和表面乙氧中间体的几何结构、原子电荷和相关能量.结果表明,存在紧邻酸性位的分子筛酸性明显减弱,乙烯分子的吸附能降低,但质子转移过渡态的活化能仅略有降低;次邻酸性位对酸性强度的影响很小,对乙烯质子化的影响也可忽略.乙烯质子化反应的活化能及表面乙氧中间体的生成能与酸强度没有必然关系,但与酸性位落位及周围的化学环境和空间环境有关.过渡态结构越接近碳正离子,活化能越高.%By the density functional theory method, the effect of different acid sites and neighboring acid sites on ethylene protonation over HZSM-5 zeolite was investigated. The calculations were based on the 20T cluster model and carried out at the B3LYP/6-31G(d,p) theoretical level. The structure, atomic charges, and the energy of the ethylene adsorption species, transition state, and the surface alkoxide products were examined. The results indicated that the next-nearest-neighboring acid sites led to obvious decrease in acid strength and adsorption energy but slight decrease in activation energy, while the next-next-nearest-neighboring acid sites had a very slender effect on acid strength and the affection on ethylene protonation could be neglected. The activation energy and reaction energy had no inevitable correlation with the acid strength but virtually depended on the location and chemical environment of the acid sites in HZSM-5 zeolite. If the transition-state structure is much resembling to the carbenium ion, the activation energy would be higher.
展开▼
