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Modelling the growth of zeolitic materials

Jelfs, K.E.; (2010) Modelling the growth of zeolitic materials. Doctoral thesis, UCL (University College London).

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Abstract

Computer simulations using potential methods were used to investigate the effect of templates on zeolite crystal growth. First the adsorption sites of templates on surfaces were examined using existent computer programs and those developed here. After an examination of the features required when modelling template/zeolite surface interactions, the ZEBEDDE program was modified for this purpose. PMF calculations verified that the modified ZEBEDDE predicted the correct trends in template adsorption energies. An examination of the silicalite/TAA and sodalite/pyrocatechol systems allowed inferences to be made upon how templates modify zeolite crystal morphology. For silicalite/TAA the geometric match of a template to surface topology influenced preferred adsorption sites. Both TPA surface specificity for the {100} silicalite surface and an enhancement of the {100} growth rate by TPA are potential causes of the greater {100} growth rate relative to {010}. Furthermore, the anisotropic growth of spirals on STA-7 surfaces was explained by preferred template adsorption on one side of the step. The ZEOGON code was modified to examine the effect of an adsorbed template on the continued surface growth, to determine microscopically how a template could have a weak or strong promotion or inhibition effect. When applied to the sodalite and silicalite systems, different templates were found to promote surface growth to differing extents. Templates are vital in assisting in the closure of zeolite cages. The ReaxFF reactive forcefield was inadequately parameterised to examine the condensation barriers of silica oligomers. The barriers to (alumino)silicate cluster formation and dissolution reactions were instead calculated using DFT. In an alkaline media the barrier to incorporation of aluminium is lower than that of silicon and thus all alumina in a synthesis gel will be assimilated into the zeolite crystal. The results suggest that the RDS in surface growth is the anionic addition of a silicate oligomer.

Type:Thesis (Doctoral)
Title:Modelling the growth of zeolitic materials
Language:English
Additional information:Authorisation for digitisation not received
UCL classification:UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Chemistry

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