Thermochemistry of (Ge_xSi_1-x)O₂ zeolites

A series of Ge-zeolites, (Ge_xSi_1-x)O₂, (ITQ-7, polymorph C and zeolite Beta) with varying Ge content is investigated by high-temperature oxide melt solution calorimetry. The enthalpies of formation relative to a mixture of germania and silica in the quartz structure (Δ H_f) are in the range 14.8-21.1 kJ/mol, which implies greater metastability than for pure silica zeolites (6.8-14.4 kJ/mol relative to quartz) and aluminophosphates (5.5-8.5 kJ/mol relative to berlinite on a two-oxygen basis). All the zeolite frameworks examined become energetically less stable with increasing Ge content, but the decrease in stability is considerably different for each framework. The linear regressions of Δ H_f vs. Ge content (Ge/(Ge + >Si)) for the three structures of Ge-zeolites intersect, though the data are somewhat sparse, implying a possible crossover in framework stability with increasing germanium content. This crossover favors structures with double four member rings (D4MR) at higher Ge/(Ge + Si).