ENERGETICS OF MICROPOROUS MATERIALS

The enthalpies of formation of a series of high-silica zeolites are only 8-14 kJ/mol less exothermic than that of quartz, with a very weak dependence on volume (framework density), despite a factor of two variation in those parameters. Several mesoporous silicas, with pore diameters in the range 3-5 nm, also are only 14 kJ/mol less stable energetically than quartz. Thus, in the microporous and mesoporous regime, the energy appears to level off at values only about 14 kJ/mol less stable than quartz, with the pores behaving energetically like an inert second phase. Similar trends are seen for AlPO4 zeolites, and for a dehydrated Ca-Al silicate zeolite. These data suggest that the synthesis of microporous materials is not limited by their energetics but by finding appropriate kinetic pathways for their formation.