Twelve-ring pockets on the external surface of MCM-22 crystals

Abstract

MCM-22 is a synthetic high-silica zeolite which crystallizes as very thin plates. Its internal structure contains two independent internal pore systems accessible through 10-ring apertures. A high concentration of external zeolitic pockets, with 12-ring openings and having an approximate depth of 7 Å, covers the hexagonal faces of the thin crystals. Evidence for the presence of these pockets was established by high-resolution electron microscopy. Experimental evidence to support this was obtained from ion-exchange studies using tetramethylammonium and tetraethylammonium ions, and from dynamic sorption studies using n-heptane and 2,2,4-trimethylpentane. The results suggest that the surface pockets unique to the MCM-22 structure have zeolitic characteristics and may play a significant role in certain catalytic processes.