HIGH-TEMPERATURE (350-K) ORTHORHOMBIC FRAMEWORK STRUCTURE OF ZEOLITE H-ZSM-5

Si11.96Al0.04O24 (+ 0.04H+), M(r) = 720.96, orthorhombic, Pnma, a = 20.078 (6), b = 19.894 (7), c = 13.372 (3) angstrom, V = 5431 (9) angstrom-3, Z = 8, D(x) = 1.80 g cm-3, lambda-(Mo K-alpha) = 0.71069-angstrom, mu = 6.64 cm-1, F(000) = 2879.7, T = 350 K, R = 0.040 for 3957 observed reflections with I > 2.0-sigma-(I). The high-temperature 'empty' orthorhombic framework of (calcined) H-ZSM-5 is not essentially different from the room-temperature orthorhombic framework of as-synthesized ZSM-5, containing the tetrapropylammonium cation as a template. The organic template hardly influences the orthorhombic framework geometry: the maximum observed differences between corresponding Si-O distances, and OSiO and SiOSi angles in both orthorhombic frameworks are 0.017 (4) angstrom, 1.4 (3) and 3.9 (4)-degrees, respectively; in both frameworks the maximum pore size in the straight channels is approximately 5.7-angstrom and the diameter of the nearly circular effective cross-sectional area in the sinusoidal channels is approximately 5.3-angstrom. Therefore, two recent reports on H-ZSM-5, describing the atomic scale mechanism of the orthorhombic/monoclinic phase transition [van Koningsveld, Jansen & van Bekkum (1990). Zeolites, 10, 235-242] and a possible diffusion pathway of p-xylene [van Koningsveld, Tuinstra, van Bekkum & Jansen (1989). Acta Cryst. B45, 423-431] by comparing the orthorhombic framework of as-synthesized ZSM-5 with the monoclinic framework of H-ZSM-5, are essentially correct.