ENTHALPY CHANGE AND TEMPERATURE OF THE REVERSIBLE MONOCLINIC-ORTHORHOMBIC PHASE-TRANSITION IN MFI TYPE ZEOLITIC MATERIALS

Calorimetric investigations show that the enthalpy change correspondin g to the reversible and displacive monoclinic (P2(1)/n.1.1)<->orthorho mbic (Pnma) solid state phase transition in MFI type zeolitic material s presenting several Si/Al ratios is very sensitive versus the framewo rk aluminum content. For approximate to 0.01 < Al/uc < 4.57 the enthal py change Delta H corresponds to 18.3 > E > 1.8 kJ/mol. Delta H rapidl y decreases with increasing Al and the transition temperature T-t for strictly anhydrous materials is always above 50 degrees C. In the case of air-equilibrated phases, the extra-framework water content increas es with Al/uc and beyond approximate to 1.3-1.4 Al/uc T-t is below 20 degrees C. T-t is also sensitive versus the nature and the amount of s orbed or encaged guest molecules (e.g., water, aliphatic and aromatic hydrocarbons).