Formation and dehydration enthalpies of gallosilicate materials with different framework topologies and Ga contents

Abstract

A series of gallosilicate materials with different framework structures (i.e., Ga-MAZ, Ga-OFF, Ga-MOR, TNU-6, and TNU-7) and different Ga contents is investigated by high temperature solution calorimetry in molten lead borate at 700 degrees C. The dehydration enthalpies for the fully hydrated samples transforming to the partially dehydrated samples, which have 10-20% of the original amount of water remaining, lie in the range 2.0-14.6 kJ per mole of TO2 (T = Si or Ga) and 4.3-24.1 kJ per mole of H2O. The formation enthalpies from oxides of fully hydrated samples range from -87.9 to -4.8 kJ per mole of TO2, while the formation enthalpies of anhydrous samples are estimated to range from -87.9 to +9.8 kJ per mol TO2. The dependence of enthalpies on the framework density and Ga/(Ga + Si) ratio is examined. Similar to trends in aluminosilicate zeolites, high framework density, high T3+ content and large charge balancing cations stabilize gallosilicate materials. Enthalpies of vitrification of gallosilicate crystals are estimated by integrating differential scanning calorimetry (DSC) peaks, and formation enthalpies of gallosilicate glasses are calculated. (C) 2009 Elsevier Inc. All rights reserved.