LITTLE ENERGETIC LIMITATION TO MICROPOROUS AND MESOPOROUS MATERIALS

A series of pure-silica or high-silica zeolites, several mesoporous silicas, moganite, and the conventional silica polymorphs (tridymite, cristobalite, coesite, glass) are energetically, at most, 15 kJ/mol higher than quartz, with the energy versus volume curve leveling off about 15 kJ per mol of SiO2 above quartz for large volumes. For AlPO4 polymorphs, a similar trend holds, with energy leading off at an even less metastable value, namely about 10 kJ per mol of Al0.5P0.5O2 above berlinite. These small energy differences are consistent with self assembly of these materials being geometrically and kinetically, rather than energetically, controlled. Thus there is very little energetic limitation to the possibility of synthesizing various micro- and mesoporous framework structures.