12-RING POCKETS ON THE EXTERNAL SURFACE OF MCM-22 CRYSTALS

MCM-22 is a synthetic high-silica zeolite which crystallizes as very t hin plates. Its internal structure contains two independent internal p ore systems accessible through 10-ring apertures. A high concentration of external zeolitic pockets, with 12-ring openings and having an app roximate depth of 7 Angstrom, covers the hexagonal faces of the thin c rystals. 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 sur face pockets unique to the MCM-22 structure have zeolitic characterist ics and may play a significant role in certain catalytic processes. (C ) 1998 Elsevier Science B.V. All rights reserved.