Thermochemistry of pure-silica zeolites

A series of pure-silica molecular sieves (structural codes AST, BEA, CFI, CHA, IFR, ISV, ITE, MEL, MFI, MWW, and STT) is investigated by high-temperature drop solution calorimetry using lead borate solvent at 974 K. The enthalpies of transition from quartz at 298 K (in kJ/mol) are AST, 10.9 ± 1.2; BEA, 9.3 ± 0.8; CFI, 8.8 ± 0.8; CHA, 11.4 ± 1.5; IFR, 10.0 ± 1.2; ISV, 14.4 ± 1.1; ITE, 10.1 ± 1.2; MEL, 8.2 ± 1.3; MFI, 6.8 ± 0.8; MWW, 10.4 ± 1.5; and STT, 9.2 ± 1.2. The range of energies observed is quite narrow at only 6.8-14.4 kJ/mol above that of quartz, and these data are consistent with and extend the earlier findings of Petrovic et al.¹ The enthalpy variations are correlated with the following structural parameters: framework density, nonbonded distance between Si atoms, and framework loop configurations. A strong linear correlation between enthalpy and framework density is observed, implying that it is the overall packing quality that determines the relative enthalpies of zeolite frameworks. The presence of internal silanol groups is shown to result in a slight (≤2.4 kJ/mol) destabilization of the calcined molecular sieves by comparing calorimetric data for MFI and BEA samples synthesized in hydroxide (containing internal silanol groups) and fluoride (low internal silanol group density) media.