ORIENTATION OF NAPHTHALENE IN H-ZSM-5 AS DETERMINED FROM POWDER AND SINGLE CRYSTAL XRAY DATA. H. van Koningsveld and J. C. Jansen, Labs. of Applied Physics and Organic Chemistry and Catalysis, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands

The adsorption properties of frameworks with the MFI topology (H-ZSM-5 and its Al-free analogue silicalite-1) have received much attention. There is a general agreement that in low-loaded MFI/adsorbent systems the preferred adsorption site is at the intersection of channels.

Recently published papers on the localization of naphthalene (nph) in H-ZSM-5 determined by Xray Powder Diffraction (hereafter referred to as XPD1 [Mentzen et al.; Zeolites, 13 (1993) 485] and XPD2 [Klein et al.; Microporous Materials, 3 (1994) 291]) show inconsistent results.

In both papers the structure is described in the orthorhombic space group Pnma. In XPD1 as well as in XPD2 the nph molecules (3.8 and 3.0 mols/u.c., respectively) are at the intersection of channels. However, the inversion of the unit cell axes a and b (a/b < 1; the empty orthorhombic HZSM-5 framework has a/b > 1), as observed in XPD1, is not reported in XPD2. In addition, the orientation of the nph molecules at the intersection of channels in XPD1 and XPD2 is quite different.

We succeeded in preparing a single crystal of the H-ZSM-5 zeolite loaded with 3.68(2) molecules nph per u.c., large enough to allow a single crystal X-ray diffraction study of the material.

The paper describes the structure of the H-ZSM-5/nph complex, gives the ensuing deformation of the channel pores and compares the orientation of nph as determined from powder and single crystal X-ray diffraction. The inversion of the a and b axes is confirmed and the orientation of nph is yet different from the orientations reported in the XPD-papers.