Introducing copper ions into zeolite Y by the thallous ion exchange method: single crystal structure of |Cu21.6Tl39.2|[Si121Al71O384]–FAU

Journal of Porous Materials, 2014, Volume 21, Issue 3, pp 321-330.

Dipak Sen, Cheol Woong Kim, Nam Ho Heo, Karl Seff.

 

1. Laboratory of Structural Chemistry, Department of Applied Chemistry, College of Engineering, Kyungpook National University, Daegu, 702-701, Korea and
2. Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, HI, 96822-2275, USA.

 

Abstract

The thallous ion exchange (TIE) method was used for the first time in an attempt to introduce copper ions into zeolite Y (FAU, Si/Al = 1.69). |Cu _10.9 ⁺Cu _10.7 ²⁺Tl _39.2 ⁺|[Si₁₂₁Al₇₁O₃₈₄]–FAU was prepared by reacting fully dehydrated and fully Tl⁺-exchanged zeolite Y (Tl₇₁–Y) with CuCl₂(g) and its decomposition products CuCl(g) and Cl₂(g) at 673 K under anhydrous conditions. Its structure was determined using single-crystal crystallography with synchrotron X-radiation and was refined in the space groupFd3¯m (a = 24.769(1) Å) with all 903 unique data; the final error index, R ₁ = 0.075, was calculated using only the 858 reflections with F _o > 4σ(F _o). About 45 % of the Tl⁺ ions were replaced by 21.6 copper ions per unit cell at the following sites (distances to nearest framework oxygen atoms are given): 10.7 Cu²⁺ at site I′ in the sodalite cavity opposite double 6-rings (Cu²⁺–O = 2.093(9) Å), 3.5 Cu⁺ at site II opposite single 6-rings in the supercage (Cu⁺–O = 2.24(3) Å), and 7.4 Cu⁺ at site III near 12-rings in the supercage (Cu⁺–O = 2.45(7) Å). All Cu⁺ ions are in supercages where they are easily accessible to guest molecules. The remaining ca. 39 Tl⁺ ions per unit cell occupy three distinct positions: 12 are at a second site I′ (Tl⁺–O = 2.571(9) Å), 23 are at a second site II (Tl⁺–O = 2.732(10) Å), and 4 are at site III′ (Tl⁺–O = 2.871(16) Å) near triple 4-rings in the supercages.

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Additional Information.

Ion exchange from aqueous solution into zeolites and other porous refractory solids sounds simple, and is easy to do experimentally, but the products are almost always complex.  A few of the metal ions in the periodic table, however, can be simply and quantitatively ion exchanged into many of these materials.  One of them is Tl⁺, the thallous ion.  If a Tl⁺ exchanged material is fully dehydrated and exposed to the vapor of a volatile compound (e.g., SnCl₂(g)) of the incoming cation (here Sn²⁺), the products can be the ion-exchanged material and a volatile compound of Tl⁺ (here TlCl(g)) which is readily removed, leaving behind a pure ion exchanged product.  This is TIE, the thallous ion exchange method.

     Here are six problems, largely avoidable by TIE, that are often encountered when ion exchange is attempted from aqueous solution. 1. the incoming cation hydrolyzes and the resulting H⁺ ions can exchange into the solid; OH^– or O^2- ions coordinating to the incoming metal ions may also enter the solid. 2. Even moderate levels of H⁺ exchange into the material can cause it to decompose, either directly or later upon heating. 3. Ion exchange may be sparse to incomplete due to disadvantageous selectivity coefficients. 4. The anion of the salt of the incoming cation may accompany it non-stoichiometrically into the porous material. 5. Impurity cations in the ion-exchange salt may concentrate in the porous solid, and 6. More profoundly, some cations cannot be exchanged into solids from aqueous solution at all because a water soluble salt of the incoming monatomic cation (e.g., V, Mo, P, and S) does not exist.

    TIE will have its own set of complications, but it is clear that many new compositions of matter, each with their own unique properties, can be synthesized by this method.  The first fully exchanged material to be prepared by TIE was indium-Y (see Figure): zeolite Y, as synthesized in its Na⁺ form, was easily and quantitatively ion exchanged with Tl⁺ from aqueous solution at 294 K, fully dehydrated under vacuum at 673 K, and treated with 510 Pa of InCl(g) at 623 K.

 

Figure Legend

  Structures of the dehydrated zeolites Tl-Y and In-Y as determined by single crystal synchrotron X-ray crystallography.