Saadoune, Iman; (2005) Computational study of doped aluminophosphate catalysts. Doctoral thesis , UCL (University College London).

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Abstract

In this thesis, a combination of computational techniques, namely periodic ab-initio QM methods, and interatomic potentials (IP) have been applied to study metal substituted aluminophosphate (MeAlPOs) catalysts. This computational study has focused on the following topics: (1) The structural, electronic, acid and redox properties of substitutional dopants in AlPO frameworks: The heteroatoms investigated include Mg2+, B3+, Cr2+/3+,Fe2+/3+, Co2+/3+ Ni2+, Ca2+, Ga2 and Sr2. Structural data for the defect centres are in agreement with the available experimental data obtained by in situ EXAFS studies. The calculated energetic results, which include the substitutional and redox energies of the dopant ions, are able to rationalise the experimental evidence concerning the framework instability at high metal loading, and the redox activity during catalytic cycles. Results suggest that among the transition metal ions investigated, Fe is the most stable in the 3+ oxidation state, while Mn is the most stable as the 2+ ion. The acid strength is due to a complex combination of the structural and electronic features of the dopant ion, and does not show appreciable correlation with the local environment or electronic properties of the metal dopant in the framework. (2) Site ordering of trivalent substitutional ions in AIPOs: Results show that the size of 3+ metal dopants has a major influence upon their site ordering in AlPO frameworks. Bigger metal dopants prefer to substitute 'free' A1 sites, located in unconstrained regions of the framework, whereas small metal dopants are energetically stable when replacing A1 sites situated in smaller cages. This site preference increases on increasing the size difference between host ion (Al3+) and dopant, and is explained via the topological features of the host AlPO framework. (3) Hydration in MeAlPO-34 catalysts Upon hydration, the presence of a metal dopant in the AlPO framework activates the framework towards hydration. Substitutional Me3+ ion interacts preferentially with water molecules; the adsorbed water is energetically stable when coordinated to the heteroatom rather than the framework A1 species. This indicates that the Me3+ dopant is a better Lewis acid than framework aluminium. In divalent metal doped AlPO-34 framework, the dopant is screened by the acid OH group from direct interaction with the adsorbed water. The water-dopant Lewis interaction is from the region behind the Bronsted acid site. The stable adsorption site in the Me2+ doped AlPO-34 is the acid proton, which forms a strong hydrogen bond with the water oxygen. The Me -OH substitutional defect activates the framework A1 ions towards hydration, in particular the A1 ion located opposite to the Me2+ dopant and in the same cage containing the defect. The latter result is important to explain the low resistance of AIPOs and MeAlPOs to steaming and chemical treatment at high temperature, which may limit their applicability in heterogeneous catalysis compared to zeolites.

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