Enthalpy of formation and dehydration of lithium and sodium zeolite beta

Zeolite Li-BEA and Na-BEA with Si/Al = 3-4 were synthesized by alumination and ion exchange, then characterized by XRD, TG-DSC and NMR. The enthalpies of formation and dehydration of Li and Na ion exchanged zeolite beta are investigated by high temperature oxide melt solution calorimetry. For Li-BEA, the formation enthalpies of formation from oxides at 25 degrees C are 25.6 +/- 1.7 kJ/mol TO2 for the dehydrated zeolite and -8.45 +/- 0.94 kJ/mol TO2 for the fully hydrated zeolite; for Na-BEA they are -2.4 +/- 0.6 kJ/mol TO2 for the dehydrated and - 17.8 +/- 1.0 kJ/mol TO2 for the fully hydrated zeolite. The integral dehydration enthalpy at 25 degrees C is 33.2 +/- 1.8 kJ/mol H2O for Li-BEA and 16.5 +/- 1.1 kJ/moI H2O for Na-BEA. The partial molar dehydration enthalpies of both Li-BEA and Na-BEA are a linear function of water content. Molecular mechanics simulations explore the cation and water molecule positions in the framework at several water contents. (C) 2006 Elsevier Inc. All rights reserved.