NMR研究超稳Y分子筛水热老化过程中结构与酸性的变化

Structure and Acidity Changes in Ultra-Stable Y Zeolites During Hydrothermal Aging: A Solid-State NMR Spectroscopy Study

利用固体核磁共振(NMR)结合探针分子技术,探讨了超稳Y分子筛在水热老化过程中酸中心结构和酸性的变化规律.结果表明,水热老化初期,大量的骨架铝脱出最终形成了非骨架五配位铝,使得丙酮探测到的Br?nsted酸(B酸)酸量随着老化时间的增加而减少.而非骨架五配位铝会向表面迁移并形成多聚铝,使得分子筛中Lewis酸(L酸)酸量也随老化时间的增加而减少.在老化过程的前3 h内,酸量的变化幅度较大.整体硅/铝原子比相等的分子筛,其B酸酸量的变化幅度与新鲜剂的晶胞大小呈正比;随着老化时间的继续增加,不同配位的铝物种的含量趋于稳定,B酸和L酸的酸量变化幅度很小.在整个水热老化过程中,超稳Y分子筛中B酸和L酸的协同效应一直存在,由协同效应产生的一种酸性增强的B酸位的稳定性最好.

Structure and acidity changes of the acid sites in ultra-stable Y zeolites of different cell sizes during the hydrothermal aging process were studied by solid-state nuclear magnetic resonance(NMR)combined with molecular probes.The experimental results demonstrated that,during the initial stage of hydrothermal aging,parts of the framework aluminum species were removed from the framework and formed five-coordinate extra-framework aluminum species.In the meanwhile,the five-coordinate extra-framework aluminum species migrated to the surface and formed poly-aluminum species,resulting in decreases of both Br?nsted acid sites and Lewis acid sites.The acidity of ultra-stable Y zeolites changed considerably during the first 3 h of the aging process,and the change of Br?nsted acid content was proportional to the unit cell dimension of the fresh zeolites agent.With the increase of aging time,the aluminum distribution tended to become stable,with little further changes in the amounts of Br?nsted acid and Lewis acid sites.The synergy between Br?nsted/Lewis acids in the ultra-stable Y zeolites existed during the entire aging process,resulting in a stably enhanced acidity for the Br?nsted acid sites.