Preparation and structure of crystals of zeolite H-ZSM-5 loaded with p-nitroaniline

A crystal of H-ZSM-5 loaded with p-nitroaniline exhibits domains of different symmetry depending on the loading time. At very short loading time, domains with monoclinic P-2/n.1.1 symmetry, with orthorhombic P2(1)2(1)2(1) symmetry and with orthorhombic Pnma symmetry coexist. At very long loading time, crystals containing one type of domain with orthorhombic Pnma symmetry are prepared. At intermediate loading time, orthorhombic domains with P2(1)2(1)2(1) and Pnma symmetry are present. Refinement of the latter type of p-nitroaniline/H-ZSM-5 crystals leads to an apparent improvement in the structural model when the symmetry is lowered during the refinement. The structure of a single-domain crystal of H-ZSM-5, loaded with approximately four p-nitroaniline molecules per unit cell, was solved by single crystal X-ray diffraction in the orthorhombic space group Pnma with a = 19.960(1), b = 19.941(2) and c = 13.367(1) Angstrom. The structure contains eight Si(11.96)Al(0.04)O(24)0.5C(6)H(6)N(2)O(2)(+0.04H(+)) units per unit cell. The final R(R-w) = 0.050 (0.046), with w = 1/sigma(2)(F), for 5608 observed reflections with I>1.0 sigma(I) measured at 293 K. The refinement results do not justify the conclusion that the crystal structure is better described in the acentric space group Pn2(3)a. The p-nitroaniline molecules are located at the channel intersections and form loosely connected chains in the straight channel. The deformations of the channel pores are not essentially different from those caused by the adsorption of p-dichlorobenzene or naphthalene: the straight channel pores are strongly elliptically distorted while the sinusoidal channel pores are hardly affected by the adsorption.