Enthalpy change and temperature of the reversible monoclinic-orthorhombic phase transition in MFI type zeolitic materials

Calorimetric investigations show that the enthalpy change corresponding to the reversible and displacive monoclinic (P2(1)/n.1.1)<->orthorhombic (Pnma) solid state phase transition in MFI type zeolitic materials presenting several Si/Al ratios is very sensitive versus the framework aluminum content. For approximate to 0.01 < Al/uc < 4.57 the enthalpy change Delta H corresponds to 18.3 > E > 1.8 kJ/mol. Delta H rapidly 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 increases 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 sorbed or encaged guest molecules (e.g., water, aliphatic and aromatic hydrocarbons).