A comparison between thermal expansion properties of hydrated and dehydrated orthorhombic HZSM-5 zeolite

Three different samples of orthorhombic HZSM-5 were analyzed by high-temperature X-ray diffraction and differential scanning calorimetry in order to elucidate the differences in thermal expansion behavior between its hydrated and dehydrated form. The first sample was the as-received orthorhombic HZSM-5, containing three different water species (H2O, H3O+ and OH). The second sample was previously heat treated in a tubular furnace at 550degreesC, and, contained only H2O, reabsorbed on cooling. The third one was prepared to be completely dehydrated. The dehydrated orthorhombic HZSM-5 sample showed positive thermal expansion from room temperature to 300degreesC and negative thermal expansion from 300degreesC to 800degreesC. On the other hand, the hydrated orthorhombic HZSM-5 exhibited, for the temperature range where water species are present, negative thermal expansion from room temperature to 300degreesC (for lattice parameters a and c and volume) and positive thermal expansion from 300degreesC to 500degreesC. The mechanisms that could explain the thermal expansion behavior of HZSM-5 were discussed. (C) 2004 Elsevier Inc. All rights reserved.