Aluminum zoning in ZSM-5 as revealed by selective silica removal

Treatment of ZSM-5 crystals with an aqueous base results in partial dissolution of the sample with preferential removal of silica. The treated zeolites, which now have lower silica/alumina ratios, exhibit higher exchange capacities and enhanced catalytic activities. Scanning electron micrographs of base-treated large-crystal ZSM-5 samples revealed that highly selective dissolution of the interior of the crystals had occurred, while leaving the exterior surface relatively intact. These results provide dramatic evidence for aluminum zoning in large-crystal ZSM-5 synthesized in the presence of quaternary directing agents.