Experimental energetics of large and extra-large pore zeolites: Pure silica beta polymorph C (BEC) and Ge-containing ITQ-33

Abstract

The enthalpies of formation of the large pore pure silica beta polymorph C (BEC) and the extra-large pore germanosilicate ITQ-33 zeolite are investigated by high temperature oxide melt solution calorimetry. The enthalpies of formation from quartz for two BECs synthesized with different organic structure directing agents, SDA1 and SDA9 differ by 4kJ/mol. The two SDAs produce phases with different properties as well as different energetics for the same framework and composition, due to the different amount of structural defects, while the more defective BEC is energetically less stable by 4kJ/mol. The enthalpy of formation of defect-free pure silica BEC agrees with the predicted value proposed several years ago. Moreover, the enthalpy of formation of ITQ-33 (Ge/(Ge+Si)=0.3) supports the energetic trends seen previously, namely that the enthalpy of formation becomes more endothermic as the content of double four rings (D4R) increases. The previous trend of energetics of porous materials versus molar volume is supported by the present data, with a diminishing destabilization for very open structures.