Thermochemistry of (GexSi1−x)O2 zeolites

A series of Ge-zeolites, (Ge x Si1−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 (ΔH 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 ΔH 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).