Computational investigation of the structures and energies of microporous materials

We present a comprehensive study of calculated lattice and cohesive energies for pure silica zeolites and pure microporous alumino-phosphates (ALPOs). Molecular mechanical and quantum mechanical methodologies based on Density Functional Theory (DFT) are employed to calculate respectively lattice and cohesive energies, whose values relative to those of the dense α-Quartz (SiO2) and Berlinite (AlPO4) phases are compared to experimental values. The results confirm that the siliceous zeolites and microporous ALPOs are all metastable with respect to α-Quartz and Berlinite with the energy differences between the microporous and dense phases, calculated by the DFT methods for the siliceous systems being closer to experiment than those with the interatomic potential based methods; although calculations based on shell model potentials gave values closer to experimental values than those based on the rigid ion model and can reproduce the trends observed in both DFT and experiment at a low computational cost. For the zeolitic structures, interatomic potential based calculations tend to overestimate lattice energies which may arise from inadequacies in the modelling of charge transfer which can be modelled by the DFT studies. For the ALPO systems, DFT gives higher energies than the interatomic potential based methods which deviate appreciably from the experimental data. Possible origins of the discrepancy are discussed.