The monoclinic framework structure of zeolite H-ZSM-5. Comparison with the orthorhombic framework of as-synthesized ZSM-5

Abstract

The monoclinic framework structure of zeolite H-ZSM-5 (Si95.68Al0.32H0.32O192) has been determined by single crystal X-ray diffraction. The space group is P21/n.1.1. The unit-cell, of dimensions a = 20.107(2), b = 19.879(2), c = 13.369(1)Å and α = 90.67(1)°, has 24 independent T sites and Z = 4. The orthorhombic/monoclinic symmetry change, ascribed earlier to a shift of neighboring (010) pentasil layers alongc, is realized by a complicated displacement of the framework atoms. The SiOSi angles in the bridges connecting the T12-building units into (100) and (010) layers are hardly affected. The transition affects the large SiOSi angles within the T12 unit most prominently. The corresponding SiO distances are lengthened. The monoclinic framework seems the less strained one. The maximal pore size in the straight channel (∼ 5.8Å) is hardly affected by the symmetry change. The sinusoidal cross-sectional area changes from nearly circular (5.3 × 5.6Å) in the orthorhombic structure to elliptical in the monoclinic structure (MONO) with major axes of 5.9 and 5.8Åin the two O10-rings defining the pore window. In this way the accessibility of the sinusoidal channel in MONO, e.g., for diffusion of p-xylene, is enlarged substantially.