Electrostatic properties of the molecular sieve AlPO4-15

 

E. Aubert, F. Porcher, M. Souhassou and C. Lecomte

 

Laboratoire de Cristallographie et Modlisation des Matriaux Minraux et Biologiques, CNRS UMR 7036, Universit H. Poincar Nancy I, BP 239, 54506 Vanduvre-ls-Nancy, France (aubert@lcm3b.uhp-nancy.fr)

 

 

Charge density and electrostatic potential analyses are of special interest in characterizing host / guest systems like zeolites or AlPO systems[1].

Single crystal of AlPO4-15 (NH4Al2(OH)(H2O)(PO4)2.H2O) were obtained by hydrothermal synthesis (180C); the X-ray diffraction data were collected on a Nonius K CCD diffractometer, Mo(Ka) radiation, P21/n, a=9,556(1) , b=9,563(1) , c=9,615(1) , b=103,58(1). The charge density was modeled according to the multipolar formalism of Hansen and Coppens using program MoPro[2]. The final residual indices are R=1,02%, Rw=0,66%, Nobs=4725, Npar=510, (sinq/l)max=0,90Ǻ-1. The analysis of the topology of the electron density was performed on the multipolar and IAM models in order to characterize interactions between framework atoms (P,Al-O) and between framework and guest molecules (hydrogen bonds).

The electrostatic potential (EP) was derived from two representations. The first one consists of point charges placed at atomic positions. The potential was obtained trough a summation in direct space V(r)=SiQi/rr,i using different set of charges (issued from a Pv/K refinement or topological analysis). The second one used the charge density issued from the multipolar model and calculations were performed with summation in direct and reciprocal spaces[3,4]. From these two representations, electrostatic interaction energies were derived. For the second one, it was evaluated for a molecule A as EA=Vt-A(r)rA(r)dr, where Vt-A(r) is the EP that is felt by molecule A (generated by all the crystal except molecule A) and rA(r) is the charge density of molecule A. Thus, electrostatic interaction energies for water molecules, ammonium cation and hydroxyl group occluded in the molecular sieve are compared with results obtained from a differential scanning calorimetry experiment.

 

References

1       Lecomte, C. (1999).  In Implications of Molecular and Materials Structure for New Technologies, pp. 23-44. Kluwer Academic Publishers.

2       Guillot, B.; Viry, L.; Guillot, R.; Lecomte, C.; Jelsch, C. (2001) J. Appl. Cryst., 34, 214-223.

3       Stewart, R. F. (1979) Chem. Phys. Lett. 65, 335-338.

4       Ghermani, N. E., Bouhmaida, N. & Lecomte, C. (1992) ELECTROS, STATDENS. Internal report URA CNRS 809. Universit Henri Poincar, Nancy 1, France.