Ab initio and molecular-mechanics studies of aluminosilicate fragments, and the origin of Lowenstein's rule

Thermal evolution of metakaolin geopolymers: Part 1 – Physical evolution

“…The energy of condensation for tetrahedral linkages follows the order Si-O-Al > Si-O-Si > Al-O-Al [17]. Therefore, it may be expected that comparatively higher temperatures will be required to fully dehydroxylate geopolymeric gel containing a greater proportion of aluminol groups, than those with larger amounts of silanol groups (i.e.…”

Zeolite beta mechanisms of nucleation and growth

“…Additionally, one must consider that each anionic defect site can associate with Na + or TEA + leading to four possible defect sites each having a different free energy. It is already known that the free energy of occlusion of TEA + into zeolite beta frameworks is exergonic as has been measured by calorimetry (À32 ± 15 kJ/mol TEA + in all-silica zeolite beta) [73] and by molecular mechanics (À104 kJ/mol TEA + ) [43]. On the basis of nonbonded interactions a free energy ranking for the defects in zeolite beta (see Fig.…”

“…The surface areas of as-prepared zeolite beta products obtained from BET analysis of N 2 adsorption isotherms. We expect also that smaller or larger TAA + would direct amorphous aluminosilicate networks differently because the nonbonded interactions are optimized in geometrically different framework structures [41][42][43]. Extending this geometrical reasoning-sodium cations should direct the formation of T-atom structures other than zeolite beta, which it does to crystallize ZSM-5 [44,45], mordenite [46], and zeolite P [47] among others.…”

Cation ordering in aluminophosphates and aluminosilicates: a combinatorial and geometrical analysis of the avoidance rule

“…Avoidance rules have probably important implications for other classes of materials as well, where they are supposed to lead to cation ordering, as in the case of aluminophosphates. The rule was shown to have thermodynamic [1] or more probably kinetic [2,3] grounds and is mostly respected for materials that have been prepared at low temperature. It was proposed more recently [4] that direct Al-Al interactions are responsible for the Al distribution in zeolites.…”