Al原子分布对Y型分子筛酸强度的影响

Effects of Al Atom Distribution on Acid Strength of Y Zeolite

构建了120T Y型分子筛簇模型,采用量子化学方法研究了Al原子落在酸中心NNN(Next nearest neighbor,3N)位、NNNN(Next next nearest neighbor,4N)位以及更远位置时对Y型分子筛酸强度的影响。结果表明:Al原子落位在NNN和NNNN位时,Y型分子筛酸强度降低;当落在更远位置时,对Y型分子筛酸强度的影响很小,可以忽略不计。这主要是因为Al与Si电负性不同,Al取代近距离Si后会发生电子转移,使得酸中心近邻原子O所带负电荷增加,对酸中心H的吸引力增强,不利于H-O键异裂释放出H质子,因此酸强度降低;随着取代距离越来越远,电子诱导效应越来越弱,Al原子对酸强度的影响也越来越小。

A 120T Y zeolite cluster model was constructed,and the effect of Al atom distribution on acid strength of Y zeolite was studied by quantum chemistry method when it located at NNN(next nearest neighbor,3N)position,NNNN(next next nearest neighbor,4N)and further positions.The results show that acid strength of Y zeolite decreases when Al atoms locate at NNN and NNNN positions,and the effect on acid strength of Y zeolite is negligible when Al atoms locate at further positions.This is mainly because the electronegativity of Al is different from that of Si.The electron transfer occurs when Al replaces Si which are close to the acid center.As a result,the negative charge of the O atom near the acid center increases,showing stronger attraction force to the acid center H,which is disadvantageous for the heterolytic cleavage of H-O bond and releasing of H proton.Hence,the acid strength is reduced.When the substitution distance getting further,the electron induction effect becomes weaker,and the effect on acid strength gets smaller.