Structure/stability relations in zeolitic microporous and ordered mesoporous silica molecular sieves were investigated by high temperature solution calorimetry using lead borate (2PbO{dollar}cdot{dollar}B{dollar}sb2{dollar}O{dollar}sb3{dollar}) solvent, drop solution and transposed temperature drop calorimetry at 977 K. Thermochemistry of the new silica polymorph moganite was studied.; Six zeolitic silicas are only 7-14 kJ/mol less stable in enthalpy than quartz. This implies an entropic or kinetic rather than a large stabilizing energetic role of the template in zeolite synthesis. The small variations in energy among these structures cannot be directly related either to the degree of "openness" (framework density or molar volume), or to the mean Si-O-Si angle. Rather, the distribution of bond angles seems to dictate the energy of these structures, with Si-O-Si angles below 140{dollar}spcirc{dollar} being the major destabilizing factor.; Though the pore size in the ordered mesoporous materials varies by approximately 2.6 nm (more than a factor of 2), and different sources of silica were used in the synthesis (amorphous and crystalline), there are minimal differences in enthalpy (energy) among these structures both at 977 and 298 K. At 298 K they are destabilized by 14-15 kJ/mol relative to quartz, which implies an energy only about 1-1.5 kJ/mol higher than siliceous zeolite Y, whose pore size is 0.74 nm.; The standard molar enthalpy of formation of moganite is {dollar}-{dollar}907.3 {dollar}pm{dollar} 1.2 kJ/mol. Thus, calorimetry strongly supports results of previous studies that moganite is a distinct silica polymorph. Structurally, this thermochemical instability relative to quartz may be related to the presence of distorted 4-membered rings of SiO{dollar}sb4{dollar} tetrahedra in the moganite structure.; Enthalpies of solution of faujasite type materials become more endothermic as the Si/Al ratio decreases, implying an exothermic enthalpy of the charge coupled substitution Si{dollar}sp{lcub}4+{rcub} to{dollar} Al{dollar}sp{lcub}3+{rcub}{dollar} + Na{dollar}sp+{dollar} in these structures. Results suggest that there may be a maximum stabilization for the composition with Si/Al = 1. Using thermodynamic data for simple oxides and sodium aluminate, standard molar enthalpies of formation of dehydrated faujasite frameworks were determined. Implications for the synthesis of high silica materials and their apparent thermal stability are discussed.
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