In situ anomalous powder diffraction study of cation distributions in bicationic zeolites

H. Palancher^1,2, J.L. Hodeau², C. Pichon¹, J.F. Bérar^2,3, J. Lynch¹, B. Rebours¹ and J. Rodriguez-Carvajal⁴

¹ Lab. de cristallographie, CNRS BP166X 38042 Grenoble, France.

² Institut Français du Pétrole, BP3 69390 Vernaison, France.

³ French CRG D2AM, ESRF, BP220 38043 Grenoble, France.

⁴ Lab. Léon Brillouin, CEA/Saclay, 91191 Gif/Yvette, France.

Adsorption properties of molecular sieves such as X, Y or A zeolites are widely used in industrial processes (in particular for separation and purification of hydrocarbon isomers). Adsorption selectivity of these materials depends highly on the type, the number and the location of cations in the structure where they compensate the negative charge of the framework. The study of the cation distributions on each site, under adsorption conditions close to their industrial use, is of great interest in the search of better molecular sieves which could be bicationic zeolites. In this work two methodological aspects will be presented: first the in situ instrumentation and second the advances in anomalous powder diffraction data analysis (required for cation distributions determination in bicationic zeolites) illustrated by the study of CaSrX and SrRbX zeolitic samples.

A cell has been especially designed for the X-ray diffraction (XRD) study of powders under gas or liquid flow, at various temperatures and pressures [1, 2]. It mainly consists of:

- a reactor whose geometry enables fix bed flow,

- a miniaturised cylindrical furnace ensuring an extremely low thermal gradient (smaller than 1°C over a 6 mm zone along the capillary at 250°C).

The limited size of this cell has enabled its installation on a synchrotron radiation beamline (D2AM at the ESRF). Thanks to this set-up, reproducible and high quality diffraction data can be measured (cf. figure I).

Figure I: Calculated and measured powder diffraction pattern on water saturated CaSrX (E=15.192keV).

In the faujasite structure, cations occupy known sites. In bicationic zeolites, two different cations are located in very close crystallographic positions. If presence of both Sr⁺⁺ and Rb⁺ cations on site II in dehydrated SrRbX can be suggested from electron density map (figure II-B), this technique appears inefficient when trying to determine Sr⁺⁺ and Ca⁺⁺ cation locations in dehydrated SrCaX: since electronic densities due to Ca⁺⁺ coincide with those of Sr⁺⁺, no site degeneracy can be observed. This kind of problem can be encountered in certain monocationic hydrated zeolites since water molecules can be adsorbed near cationic sites.

A-

B- d-SrRbX

C- d-SrCaX

4277.2 4001.3 3725.3 3449.4 3173.4 2897.5 2621.5 2345.6 2070.0 1793.7 1517.7 1241.8 965.8 689.9 413.9 138.0 -138.0

Figure II: Study of degeneracy of cation site (for example site II) on electronic density maps calculated with Fourier transforms in the P plane (represented in dark grey in the faujasite structure) (A-) in two dehydrated samples SrRbX (B-) and CaSrX (C-).

Anomalous effect could be used to distinguish these species. This phenomenon leads to strong variations with energy of the atomic scattering factor of an element under an X-ray beam of energy close to one of its absorption edges. However multi-wavelength measurements require new synchrotron sources to be performed in good conditions. Diffraction patterns have been collected over a wide angular range (sinq/l=0.57Å^-1 at least) at 15192 eV (about 900 eV under strontium K absorption edge (E_K(Sr)-900eV)) and at 16096eV (E_K(Sr)-10eV) on SrCaX. A methodology for anomalous powder diffraction data analysis has been established including simultaneous refinement of all diffraction patterns using Fullprof software package [3]. Its efficiency has been validated by the determination Sr⁺⁺ and Rb⁺ cation distributions in SrRbX sample [4, 5]. This study was a particularly difficult case for X-ray diffraction since Rb⁺ and Sr⁺⁺ have the same scattering power (Z_Rb+= Z_Sr++=35e.u.). Note that, as Rb and Sr have similar neutron scattering lengths (b_Rb≈b_Sr≈0.7.10^-12cm^-1), neutron diffraction will not help us in this case.

CaSrX sample has been characterised at two hydration levels: water saturated (measurement ex situ at 20°C) and highly dehydrated (measurement in situ under dry nitrogen flow at 250°C). Results of the refinements show the expected strong cation motions with water molecules loss [2] (figure III). If the very close distributions of Sr⁺⁺ and Ca⁺⁺ cations in the dehydrated sample could have been predicted from chemical considerations (close cationic radii and same electric charge), the very different behaviour of these cations in water saturated case underlines the complexity of bicationic zeolites. Importance of accurate measurements on these systems is demonstrated.

h -SrCaX

d- SrCaX

Figure III: Sr⁺⁺ and Ca⁺⁺ cation distributions in SrCaX sample at two hydration levels: water saturated (h-SrCaX) and highly dehydrated (d-SrCaX).

1. Palancher, H., Pichon, C., Prévot, S., Conan, G., Hodeau, J.L and Berar, J.F. (2003) Patent 03/07 641.

2. Palancher, H., Pichon, C., Hodeau, J.L., Lynch, J., Rebours, B., Berar, J.F. et al. (2004) submitted to JAC.

3. Rodriguez-Carvajal, J. (2002). Fullprof version 2.10 LLB, CEA/Saclay, France, http://www-llb.cea.fr/fullweb/poudres.htm.

4. Hodeau, J.L., Nassif, V., Palancher, H., Berar, J.F., Dooryhee, J.F., Carbonio, E. et al. (2003) ECM Durban (South Africa).

5. Palancher, H., Hodeau, J.L., Pichon, C., J. Lynch, Berar, J.F., Rebours, B. and Rodriguez-Carvajal, J. (2004) submitted.