磷在P-ZSM-5沸石中存在的形态

Phosphorous Moieties in P-ZSM-5 Zeolites

用密度泛函理论和ONIOM(our own N-layer integrated molecular orbital molecular mechanics)方法研究磷改性的ZSM-5沸石中含磷基团的可能存在形态.计算的反应焓和自由能数据表明P-ZSM-5沸石中以磷进入骨架和在骨架外的形成磷酸根离子对是合理的稳定结构.而且,计算结果表明离子对模型F和G更适合在室温下存在,磷进入骨架的酸性结构C?在高温下更稳定,而磷进入骨架的结构C对温度变化不敏感.计算得到的27Al,31P,29Si化学位移、酸性的变化趋势和结构参数与相关实验数据吻合.

Phosphorus modified ZSM-5 （P-ZSM-5） zeolite was investigated using quantum mechanics density functional theory and our own N-layer integrated molecular orbital molecular mechanics method （ONIOM）. Extra-framework phosphate and in-framework moieties containing phosphorus were found to be plausible local structures in P-ZSM-5 zeolites based on the calculated heats of formation and the free energy data from the hypothetical reactions. Furthermore, we find that the cationic moieties are favored at room temperature. The in-framework acidic moieties are more stable at high temperatures and the stability of the in-framework phosphorus moieties is insensitive to temperature changes. The calculated 27Al, 31p, and 29Si chemical shifts, acidity changes, and structural parameters agree well with the known experimental observations.