Hierarchical HZSM-5 was synthesized using a steam assisted crystallization method with a SiO2/Al2O3 ratio of 100.  The roles of steam during the synthesis were investigated by varying the steam time from 2 to 216 hours.  The materials were characterized by N2 physisorption, X-ray diffraction (XRD), combination focused ion beam with scanning electron microscopy (FIB/SEM), energy dispersive X-ray analysis (EDAX), X-ray photoelectron spectroscopy (XPS) depth profiling, ammonia temperature programmed desorption (NH3-TPD), pyridine-TPD, and trimethylpyridine-TPD. Crystallization time was found to affect both physical and chemical properties of the materials.  Although the BJH mesopore diameters increased with increased steam time, the BET surface area and BJH mesopore volume decreased with longer steam time. The distribution of aluminum across individual particles was analyzed with EDAX after a portion of each particle was milled using FIB/SEM to reveal a cross section of the material.  Although aluminum was well distributed across the center of the particles steamed for 18 hours and edge enrichment was observed after 24 hours, XPS depth showed aluminum enrichment at the edge even after 18 hours. A local maxima in total acidity as measured by NH3 and pyridine-TPD was observed after 18 hours of steaming, while accessibility of acid sites to larger molecules and total acidity decreased with increased time.  These observations suggest the need to consider the desired material properties in synthesis condition selection, because although longer steam time increased mesopore size, mesopore volume and acid site accessibility decreased. Thus, crystallization time is an important synthesis parameter that needs to be considered when using a steam assisted crystallization method with soft templates.

Effects of steam-assisted crystallization synthesis conditions on the mesopore properties of hierarchical ZSM-5

246th National Meeting (2013)

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