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

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 Al-27 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.

Automated summary

The use of mineralizing agents (F- or OH-) in hydrothermal synthesis directly affects the distribution of Al sites in ZSM-5 zeolites. The Si/Al ratio impacts the proportions of Al sites for F-, whereas it remains constant for OH-. This understanding provides new and simple opportunities to control the acidity of zeolites.