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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

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journal contribution
posted on 13.12.2007, 00:00 by Woo Taik Lim, Sung Man Seo, Ghyung Hwa Kim, Heung Soo Lee, Karl Seff
Single crystals of zeolite Y, |Na71|[Si121Al71O384]-FAU, with diameters up to 0.20 mm were grown. They were ion exchanged to generate |(NH4)64Na5K2|[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 |(NH4)59H5Na5K2(H2O)3|[Si121Al71O384]-FAU (incomplete dehydration and partial deamination), |(NH4)31H33Na5K2|[Si121Al71O384]-FAU (further deamination), |(NH4)2H62Na5K2|[Si121Al71O384]-FAU (nearly complete deamination), |H61Na5K2(Al2O)0.5|[Si122Al70O384]-FAU (onset of dealumination and framework reconstruction), |H53Na5K2(Al2O)2|[Si124Al68O384]-FAU (further dealumination and reconstruction), and |H51Na5K2(Al2O)2.5|[Si124Al68O384]-FAU. The extent of deamination of NH4+ 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 II (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°) at ca. 1.85 Å 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 Å). 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).

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