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

Abstract

Single crystals of zeolite Y, |Na 71 |[Si 121 Al 71 O 384 ]-FAU, with diameters up to 0.20 mm were grown. They were ion exchanged to generate |(NH 4 ) 64 Na 5 K 2 |[Si 121 Al 71 O 384 ]-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 |(NH 4 ) 59 H 5 Na 5 K 2 (H 2 O) 3 |[Si 121 Al 71 O 384 ]-FAU (incomplete dehydration and partial deamination), |(NH 4 ) 31 H 33 Na 5 K 2 |[Si 121 Al 71 O 384 ]-FAU (further deamination), |(NH 4 ) 2 H 62 Na 5 K 2 |[Si 121 -Al 71 O 384 ]-FAU (nearly complete deamination), |H 61 Na 5 K 2 (Al 2 O) 0.5 |[Si 122 Al 70 O 384 ]-FAU (onset of dealumination and framework reconstruction), |H 53 Na 5 K 2 (Al 2 O) 2 |[Si 124 Al 68 O 384 ]-FAU (further dealumination and reconstruction), and |H 51 Na 5 K 2 (Al 2 O) 2.5 |[Si 124 Al 68 O 384 ]-FAU. The extent of deamination of NH 4 + increases with increasing evacuation temperature until it is complete in crystal 4. In crystal 1, 59 NH 4 + 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 NH 4 + ions per unit cell are found at site I′ and 13 at site II. In crystal 3, only 2 NH 4 + 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 Al 3+ ions per unit cell, respectively, are found at site I′ recessed slightly into D6Rs. Each Al 3+ ion coordinates trigonally (O-Al-O ) 119.3°) at ca. 1.85 Å to three framework oxygens. Pairs of these Al 3+ 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 Al 3+ 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).

Synthesis of Fully Dehydrated Partially Cs+-exchanged Zeolite Y (FAU, Si/Al = 1.56), |Cs45Na30|[Si117Al75O384]-FAU and Its Single-crystal Structure

“…Further description on the synthetic procedure is available in reference. 19 The product was characterized by optical microscopy, SEM, and single-crystal X-ray technique analysis. Microscopic examination showed that the single crystals were colorless and transparent.…”

Crystal structure of Ba2+-exchanged zeolite Y (FAU), /Ba35.5/ [Si121Al71O384] (Si/Al = 1.70)

“…They have been widely studied and used as ion exchangers, adsorbents and catalysts in industrial processes, because of the properties originated from the unique crystal structure containing large pore size and void volume [1]. ion at Ba1' is located at site 1' in the sodalite cavities opposite D6Rs; the Ba 2+ ion was recessed by 1.32 Å into the sodalite cavity from their 3-oygen plane.…”

“…Large single crystals of sodium zeolite Y(FAU), stoichiometry Na 71Si121Al71O384 per unit cell, were prepared by Lim et al [1]. One of these, aclear colorless octahedron about 0.22 mm in crosssection was lodged in aP yrex capillary.…”

The dependence of Co2+-exchange into zeolite FAU on its Si/Al ratio

“…The positions of the cations would be determined in the fully dehydrated materials. [31][32][33]. Each was lodged in Pyrex capillary, one in each.…”