*7AI Solid-state NMR Spectra of Ultrastable Zeolite Y with Fast Magic-angle Spinning and 1Hd7AI Cross-polarization

Fast (>I2 kHz) magic-angle-spinning NMR with IH' 7AI cross-polarization quantitatively monitors several aluminous species in dealuminated zeolite Y; the non-framework matter contains 4-, 5-and 6-coordinated A1 and the *7AI signal at 30 ppm is an independent resonance probably due to 5-coordinated non-framework Al and not part of a second-order quadrupole lineshape as previously reported. Since Bronsted acid groups in zeolites are associated with 4-coordinated framework aluminium, their catalytic activity strongly depends on the concentration and location of A1 in the structure. Upon hydrothermal treatment of zeolite NH4-Y, the process known as 'ultrastabilization,'lJ part of the aluminium is ejected from the framework into the intracrystal-line space, and the framework vacancies are reoccupied by silicon from other parts of the crystal. As a result, thermal stability of the zeolite is greatly increased, so that the product retains crystallinity at temperatures in excess of 1000 "C. Ultrastable faujasitic catalysts, better suited for hydrocracking reactions than the 'as-prepared' zeolites, are a cornerstone of the petroleum industry, and much effort has been devoted to the study of their properties and methods of preparation. Ultrastable zeolite Y has been extensively examined by 29Si and 27Al solid-state NMR.3-9 2% magic-angle-spinning (MAS) NMR can measure directly the framework Si/A1 ratio of thermally treated samples, while 27Al MAS NMR shows how 6-coordinated non-framework A1 species build up at the expense of the 4-coordinated framework A1 as the calcination temperature is increased. However, the precise nature of non-framework aluminium entities, which are amorphous and subject to large second-order quadrupolar interactions, is unknown. It has been shown that special care needs to be taken to obtain quantitatively reliable spectra of non-framework aluminium in zeolites.5.6 More recently, quadrupole nutation 27Al NMR has provided information about the coordination number of non-framework aluminium, but the actual compounds present in the intracrystalline space have not been identified.4-7 We demonstrate that very fast MAS combined with 1H27Al cross-polarization (CP) makes it possible to gain further insights into the chemical status of non-framework A1 species. Our starting material was 80% NH4-exchanged zeolite Y with Si/Al= 2.41 (sample 1) prepared by threefold contact of Na-Y with a saturated aqueous solution of NH4N03 at 80 "C followed by washing with water. The zeolite was heated in a tubular quartz furnace at 550 "C for 2 hours under deep-bed conditions with water being slowly injected into the tube by a peristaltic pump so that the partial pressure of H 2 …