Six single-crystal structures showing the dehydration, deamination, dealumination, and decomposition of NH4+-exchanged zeolite Y (FAU) with increasing evacuation temperature.: Identification of a Lewis acid site

Single crystals of zeolite Y, vertical bar Na-71 vertical bar[Si121Al71O384]-FAU, with diameters up to 0.20 mm were grown. They were ion exchanged to generate vertical bar(NH4)(64)Na5K2 vertical bar[Si121Al71O384]-FAU. Crystals 1 through 6, respectively, were vacuum dehydrated at 323, 373, 423, 473, 523, and 573 K, and without reexposure to the atmosphere, their structures were determined crystallographically at 294 K using synchrotron X-radiation. Their compositions (best integers) are seen to be respectively vertical bar(NH4)(59)H5Na5K2(H2O)(3)vertical bar[Si121Al71O384]-FAU (incomplete dehydration and partial deamination), vertical bar(NH4)(31)H33Na5K2 vertical bar[Si121Al71O384]-FAU (further deamination), vertical bar(NH4)(2)H62Na5K2 vertical bar[Si121Al71O384]-FAU (nearly complete deamination), vertical bar H61Na5K2(Al2O)(0.5)vertical bar[Si122Al70O384]-FAU (onset of dealumination and framework reconstruction), vertical bar H53Na5K2(Al2O)(2)vertical bar[Si124Al68O384]-FAU (further dealumination and reconstruction), and vertical bar H51Na5K2(Al2O)(2.5)vertical bar[Si124Al68O384]-FAU. The extent of deamination of NH(4)(+)increases with increasing evacuation temperature until it is complete in crystal 4. In crystal 1, 59 NH4+ ions per unit cell are found at two crystallographically distinct positions: 30 at site I' (in sodalite cavities opposite D6Rs) and 29 at site 11 (in supercages opposite S6Rs). Three water molecules coordinate to Na+ ions. In crystal 2, 17 NH4+ ions per unit cell are found at site I' and 13 at site II. In crystal 3, only 2 NH4+ ions at site II are found. The extent of dealumination of the zeolite framework, seen in crystals 4, 5, and 6, also increases with temperature: 1, 4, and 5 Al3+ ions per unit cell, respectively, are found at site I' recessed slightly into D6Rs. Each Al3+ ion coordinates trigonally (O-Al-O = 119.3 degrees) at ca. 1.85 angstrom to three framework oxygens. Pairs of these Al3+ ions are each bridged by a nonframework oxide ion at the center of a D6R to give linear Al-O-Al groups (Al-O = 1.62 angstrom). These Al3+ ions are Lewis-acid sites and should be catalytically active when accessible. Five additional crystals vacuum dehydrated at 623 k and above showed no crystallinity, due presumably to further dealumination and dehydration (loss of H+ and framework oxygen).