Yang, C; (2013) The pre-nucleation of aluminosilicate zeolites: a theoretical approach (Yang, , Trans.). Doctoral thesis , UCL (University College London).

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Abstract

Density Functional Theory (DFT) and the Conductor-like Screening Model (COSMO) have been employed to investigate the structural geometry and thermodynamical properties for the condensation reactions of aluminosilicates in the prenucleation of forming aluminosilicate zeolites. We report the relative structures and energies of clusters containing between one and six Si/Al atoms and the effect on them of the interplay of the Na+ ions and intramolecular hydrogen bonds. Our result reveals that with the exception of the dimer, “Lowensteinian” clusters (without the Al-O-Al linkage) are more energetically favourable than “non-Lowensteinian” clusters (which contain such bridges) in the gas phase. The stability of aluminosilicate clusters is strongly affected by the COSMO solvation, with the solvent influencing their conformations. In COSMO solvation, all the most stable clusters follow not only Lowenstein’s rule, but also Dempsey’s rule. The condensation reactions are involved key aluminosilicate species: the Si(OH)4 or Al(OH)4Na monomers and AlSiO(OH)6Na dimer, where we investigate reactions with the four, six, double four and double six rings to form a series of fused rings in both the gas phase and COSMO solvation. Our calculations suggests that the Al(OH)4Na monomer does not participate in these condensation reactions as such participation would generate structures that contradict Lowenstein’s rule; in contrast, on the basis of our results we propose that the condensation reactions occur via the AlSiO(OH)6Na dimer. As a result, employing the AlSiO(OH)6Na dimer, which could be responsible for forming aluminosilicate aggregates studies the nucleation mechanism of zeolite A. the rational mechanism for nucleated self-assembly for zeolite A is that the double four ring, which is probably the main initial ring in the nucleation stage could be formed by the successive condensation reactions of the AlSiO(OH)6Na dimer, tetramer, four ring, four-four ring, tri-four ring, and open double four ring. Furthermore, to consider the alkalinity in the nucleation and crystal growth of aluminosilicate zeolites, our supposition is that the AlSiO2(OH)5Na2 dimer would play the key role in the relative condensation reactions for deprotonated open clusters (the dimers, trimers and tetramers) and rings (the four rings).

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