In situ monitoring of desilication of MFI-type zeolites in alkaline medium

In situ pH and Attenuated Total Reflection ( ATR) infrared techniques have been successfully applied in order to gain insights into the dissolution process connected to mesopore formation occurring upon alkaline treatment of ZSM- 5 zeolites. Online pH measurements reveal a similar consumption of OH (-)ions in the initial stage of the reaction independent of the Si/ Al ratio of the zeolite. In view of the greatly different mesoporosity development, the extraction of polymeric silica entities is anticipated, its structure depending on the framework Si/ Al ratio. In agreement, ATR- IR experiments have confirmed dissolution of polymeric silicon- containing species that in the course of the alkaline treatment disintegrate into smaller entities. A direct relation between the type of porosity developed and the process of silicon extraction as measured in the liquid phase cannot be drawn.