Locations of deuterobenzene molecules in the synthetic zeolite ZSM-5 at 77 K by powder neutron diffraction

Abstract

Locations of adsorbed deuterobenzene molecules in the synthetic zeolite ZSM-5 have been determined at 77 K by powder neutron diffraction (λ = 1.893 Å) with difference Fourier syntheses and constrained least-squares refinement. The number of C 6D 6 molecules found per unit cell is 8.05 (Ref. 8). ZSM-5 — C 6D 6 is orthorhombic, Pnma, with a = 19.961 (2), b = 19.824 (2), c = 13.398 (4) A ̊ . The C 6D 6 molecules lie in three distinct locations A, B and C. A and B lie at the intersection of the straight and wavy channels approximately in the planes x = −0.08 and x = 0.08 respectively, and C is at the symmetry centre (0, 1 2 , 1 2 ) in the straight channel. B and C are modelled with disorder positions. The profile intensity is very sensitive at low 20 angles to the contributions from A, B and C and R (profile) increases from 0.22 to 0.32, 0.38 and 0.39 if A, B and C are left out of the calculations, respectively. The molecules fit neatly into the zeolite cavities. The total number of atomic sites in the structure of ZSM-5C 6D 6 is 124, giving 357 structural parameters. The Fourier syntheses show some evidence for a small cation and water content in the channels; these 14 additional sites refined and were included in the profile calculation. The structure is possibly the largest studied with powder data, and the first ZSM-5 sorbent to undergo structural analysis.