Framework Stability of Nanoporous Inorganic Structures upon Template Extraction and Calcination:  A Theoretical Study of Gallophosphate Polymorphs

Abstract

A systematic computational study of gallophosphates was undertaken. First, lattice energy minimization calculations using a formal-charge shell model potential have been carried out on a series of hypothetical gallium phosphates derived from their metallogallophosphate, aluminophosphate, or aluminosilicate analogues through atomic substitution. The minimized structures show the typical features in terms of bond angles and distances as expected in zeolitic gallophosphates. Second, the crystal structures of several gallophosphates in their calcined forms have been predicted, using for each compound lattice energy minimization and an initial model derived from its as-synthesized templated form. All the modified structures thus have the same GaPO(4) composition. The lattice energies of all the simulated gallophosphate structures were compared to that of GaPO(4)-quartz as a reference structure. Interestingly, among all predicted calcined structures, various zeolitic topologies were found. The study of the energetics of these zeotypic structures showed a linear dependence of lattice energy upon density. Strikingly, a few simulated structures showed unrealistic structural features, such as important framework distortions, often associated with the occurrence of a hexameric unit in the original as-synthesized structures. Also, those gallophosphates with structural faults were found in the upper part of the energy/density plot. To address the validity of our force field calculations in these special cases, first principles calculations were undertaken on ULM-4, chosen as a typical representative structure. Indeed, the qualitative agreement found between our results and those obtained with the nonlocal density functional theory demonstrates the robustness of our force field. Further minimization also showed that the inclusion of polarizability is crucial for yielding results comparable with those obtained using first principles methods.

Crystalline oxyfluorinated open-framework compounds: Silicates, metal phosphates, metal fluorides and metal-organic frameworks (MOF)

“…The framework stability of such solids was estimated by means of computational method using lattice energy minimization calculations. These theoretical studies [77] indicated that the structures are not energetically viable and agree with the experimental data showing the decomposition of the solid after removal of the template.…”

Computing the Properties of Materials from First Principles with SIESTA

“…The more extensive use of SIESTA in this field is due to Gale and coworkers [144][145][146]. An illustrative example is the study of the binding of palladium atoms and dimers in the zeolite gmelinite.…”

“…For instance, Girard et al [146] have used SIESTA type calculations in order to validate a force field put forward to study the structure of zeolitic gallophosphates. Borowski et al [148] used a combination of experimental and theoretical techniques in order to solve the structure of the layered silicic acid H-RUB-18.…”

“…This is probably an indication that more sophisticated approaches than Tersoff-Hamman theory are needed to reproduce the STM images in this system. 146 D. Sµnchez-Portal et al…”

Crystallization, stability and possible application of the molecular sieve cloverite

“…In a recent paper Girard et al [21] have theoretically investigated the stability of a great number of existing and hypothetical gallophosphate structures. However, the authors could not apply their model to cloverite due to the fact that the as-synthesized cloverite framework is interrupted by terminal oxygens (P-OH and Ga-OH groups).…”