Clayson, Ivan George; (2023) Insights into the Ion-Exchange of Metals in Zeolites. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

Metal zeolite catalysts are crucial for a wide-range of commercially relevant processes with Cu-SSZ-13 notably being employed for the NOx abatement of automotive exhausts. However, the speciation of these metal sites is dependent on the ion-exchange procedure during the catalyst's synthesis with the atomistic ion-exchange mechanism of transition metals into zeolites remaining elusive. Therefore, this work presents a novel scheme for modelling the migration of metal complexes with explicit hydration, evaluated at the quantum mechanical level in a computationally tractable manner, referred to as partial relaxation-geometry optimisation (PRGO). From this, exhaustive pro les of mass transfer in the bulk crystal and (001) surface of CHA for a range of divalent metals along with their accessible spin states have been modelled. From these migrations, it was found that FeII unexpectedly undergoes a reduction in spin from M = 5 to M = 3 as it migrates between the cages as well as the transition state for CoII being predicted to have a degenerate spin preference in the bulk crystal. Moreover, a supervised machine-learning procedure has been developed, referred to as optimal feature nder with optional functional variance (OFF-OFV), that has been employed to determine the key features and thermodynamic drives of these migrations. Additionally, experimental work coupled with these PRGO and OFF-OFV methods demonstrates that the rate-limiting step for ion-exchange is surface mediated with a tentative mechanism being presented for the uptake of metals from the external solution to the CHA zeolite. Finally, a high-throughput work ow for generating expanded valence framework Al species has been developed for a range of topologies where examination of these structures has demonstrated that MOR exhibits a T-site preference for forming 5-fold structures, which causes framework cleavage to become exothermic. Therefore, these Al framework sites with additional coordination will form selectively within the MOR crystal and introduce weakness into the framework, resulting in additional defects being present that will inevitably a ect catalysis. This work presents a range of novel methods to examine a vast array of industrially relevant zeolite systems in a tractable manner to provide unique insight into the ion-exchange mechanism and the zeolite structure which would be unattainable without the approaches developed here.

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