Impact of Mineralizing Agents on Aluminum Distribution and Acidity of ZSM-5 Zeolites

Abstract

Intensive research on improving the catalytic properties of zeolites is focused on modulating their acidity and the distribution of associated Al sites. Herein, by studying a series of ZSM-5 zeolites over a broad range of Al content, we demonstrate how the nature of the mineralizing agent (F− or OH−) used in hydrothermal syntheses directly impacts Al sites distribution. The proportions of Al sites, probed by 27Al NMR, depend on the Si/Al ratio for F−, but remain identical for OH− (from Si/Al=30 to 760). This leads to contrasting variations in weak and strong acidities. Such opposite effect of mineralizers is explained by the spatial location of negative charges and the resulting balance between short- and long-range electrostatic interactions. This understanding paves the way for additional and simple opportunities to control zeolites’ acidity.