CRYSTAL CHEMISTRY AND ENERGETICS: A CASE STUDY IN ZEOLITES

Crystallography and thermodynamics are intimately related; details of short and long range atomic environments control both. Using zeolites as a case study, the following issues are addressed: (1) how the very small energetic difference between different framework types at a given composition (e.g. SiO₂) relates to local geometry and the energy of internal and external surfaces, (2) the similarity of vibrational heat capacities and entropies for different structure types, (3) structural and energetic basis for new frameworks in Si-Ge zeolites. A comparison of major issues in thermodynamic stability of mineralogical and technological zeolites is presented.

Specifically, the following has been found. The enthalpies of all silica zeolites studied so far lie in a narrow range spanning 7 kJ/mol and lying 8-15 kJ/mol about quartz. They can be related to the increased internal surface area of the pores and define an internal surface energy near 0.09 J/m², similar to the external surface energy of amorphous silica. This small surface energy explains why there is no dependence of measured enthalpy of solution on particle size, in contrast to behavior for other oxides like TiO₂. For SiGe zeolites, the concentration of Ge in double-four-rings is a major stabilizing factor for new structures. For zeolite minerals, hydration/dehydration reactions probably dominate phase stability.