Enthalpies of formation of Ge-zeolites: ITQ-21 and ITQ-22

A series of Ge-zeolites (ITQ-21 and ITQ-22 with varying Ge content) has been investigated by high-temperature drop solution calorimetry using lead borate solvent at 973 K. The enthalpies of formation relative to a mixture of germania and silica in the quartz forms (ΔH f) at 298 K are in the range 18.9–21.9 kJ/mol. For ITQ-22, the ΔH f values are constant, ∼14.8±0.4 kJ/mol, independent of Ge-content for 0.11<Ge/(Ge+Si) < 0.24. For ITQ-21, ΔH f becomes progressively more endothermic with increasing Ge content (which corresponds to decreasing framework density or increasing molar volume), similar to the trends seen previously for ITQ-7 and ITQ-17. The relationships between the ΔH f and Ge content for various structures are discussed in terms of concentration of double four membered rings (D4MR) and Ge location in the framework.