How defects and crystal morphology control the effects of desilication

The introduction of mesoporosity in zeolites by desilication has become a simple routine method to generate hierarchical materials with improved catalytic performance. The mesopore formation upon alkali leaching has been investigated employing electron microscopy, infrared spectroscopy, temperature programmed desorption of ammonia, and a catalytic test reaction. We are able to demonstrate that the mesopores are formed by two modes. They are created both as a consequence of Al-directed dissolution of siliceous areas and selective dissolution or etching along boundaries, intergrowths, and defects within each particle are important. This has allowed us to identify a preferred particle morphology for efficient desilication. Particles constructed of fused subunits appear to be very susceptible towards directed mesopore formation by desilication. The desilication may also lead to alterations of the aluminum environment, seen as a reduction in the concentration of strong Brønsted sites and the appearance of a second family of weaker sites. Introduction of mesoporosity by carbon templating rather than desilication leads to a material with a more complex distribution of surface hydroxyl groups.