On the location and disorder of the tetrapropylammonium (TPA) ion in zeolite ZSM-5 with improved framework accuracy

Abstract

i11.96/~do.04024 . ½NC12H28OH + nH20 (+0.04Na+?), Mr = 822.65 + (n x 18), orthorhombic, Pnma (assumed), a = 20.022 (2), b = 19.899 (2), c = 13.383 (1) /~, V=5332(4)A 3, Z=8, Dx=2.06 (including TPAOH), Dx = 1.80 gcm -3 (for the anhy- drous framework), h(Mo Ka) =0.71069 A, /z = 6.91 cm -1, F(000) = 3343.7, T = 293 K, R = 0.042 for 4523 observed reflections with l>2.0cr(I). The framework topology agrees with earlier descriptions. Straight channels and sinusoidal channels run parallel to (010) and (100), respectively. The sym- metry of the framework atoms is too close to Pnma to permit refinement in Pn21a. In the straight channel an approximate mirror plane through N and the chan- nel axis, perpendicular to the crystallographic mirror plane, exists. The tetrapropylammonium ion lies at the intersection of the straight and sinusoidal chan- nels in two different orientations. The two orienta- tions, populated in a ratio of 3:2, are nearly related by the approximate mirror plane. Contrary to literature data the propyl-N-propyl fragments point- ing into the sinusoidal and straight channel, respec- tively, both have CNCC torsion angles around 60 °. No evidence is obtained for the tetrapropylam- monium ion breaking the rn symmetry and extra disorder around rn is assumed. The mean C-C and C-N distances and mean CCC, CCN and CNC angles, averaged over both ions, are 1.55 and 1.57 A and 109, 113 and 109 °, respectively. Contact distances smaller than 4.0/~, between terminal C atoms of adjacent template ions, are in the range 3.56 (7)- 3.75 (2) ~.