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, |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).