An Investigation of the Roles of Surface Aluminum and Acid Sites in the Zeolite MCM-22

Ammonia adsorption studies reveal that the observed Lewis acidity in the zeolite MCM‐22 is derived from at least two types of framework aluminum sites (AlF), that is, octahedral AlF and three‐coordinate AlF. Comparative ammonia or trimethylphosphine (TMP) adsorption experiments with MCM‐22 confirm that octahedral Al species gives rise to the signal at δiso≈0 in the 27Al NMR spectrum; this is a superposition of two NMR signals from the different Al species on the water‐reconstructed zeolite surface. A sharp resonance assigned to framework Al reversibly transforms on ammonia adsorption to δiso27Al≈55 from tetrahedral AlF, while the broad peak is assigned to nonframework aluminum which results from hydrothermal treatment. This study also demonstrates the effectiveness of 27Al magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectroscopy as a technique for the study of zeolite reactions. Modern high‐field and multiple‐quantum MAS NMR spectroscopy have been used to investigate the dealumination process of zeolite MCM‐22 (see diagram). The nature of surface aluminum and acidity of MCM‐22 is studied by the adsorption of basic molecules to the zeolite. We conclude that octahedral AlF and three‐coordinate Al contribute to the Lewis acid sites.