USE OF THE TERT-PLOT-DE BOER METHOD IN PORE VOLUME DETERMINATIONS OF ZSM-5 TYPE ZEOLITES

Nitrogen adsorption isotherms at 77 K have been measured of H-ZSM-5 samples of different crystal size and of moderate as well as of very low aluminum content. Dealumination of the latter crystal samples caused the isotherms to change from type I to a new type related to type IV, in which the hysteresis loop was located at very low relative pressure (p/p0 = 0.1). From t-plots, derived from the isotherms, pore volumes as well as external crystal surface areas have been calculated. Comparison of the results with densities determined by both helium pycnometry and mercury displacement revealed that the sudden change in the adsorbed volume was caused by a liquid-like to solid-like phase transition of adsorbed nitrogen inside the channels of the near-silicalite crystals. External areas derived from t-plots were much larger than calculated from crystal morphology and size. Because of the fact that the 'filled pore' method, described in the literature, does indeed give consistent results, from suggestions are made regarding nitrogen sorption on the external surface of a ZSM-5 crystal.