Understanding the catalytic benefits of multi=metallic active sites in microporous aluminophosphates

Abstract

Structure-property correlations, which direct the formation of specific active-acid sites, in nanoporous materials have been explored with the aim to further the notion of rational catalyst design. It is demonstrated that the influence of the specific zeotype framework extends beyond modulating Brønsted acidity, with the framework topology playing a fundamental role in reaction kinetics. The structural integrity and nature of the active site has been probed using a combined catalysis and multi-technique characterisation study, aimed at optimising the sustainable production of ?-Caprolactam, the precursor of the recyclable Nylon 6 polymer. The effect of bimetallic substitution, with an aim to generate novel acid sites, has been explored using Mg2+, Zn2+ and Si4+ dopants. Through a combined CO and lutidine probed FT-IR study, the nature of the acid site has been examined and correlated to the nature of the dopants present. The presence of multiple dopant atoms has had a significant effect on catalytic activity, modulating the presence of acid sites to allow new active species to be discovered, and prompting synergistic interactions in multi-step pathways. A combined X-ray absorption spectroscopy and ab initio DFT theoretical study was used to probe the nature of the active sites responsible for catalytic synergy in Co-Ti bimetallic nanoporous frameworks. Contrasting the bimetallic and analogous monometallic species has led to the discovery of a unique bimetallic Co-O-Ti bond. The properties of this bond have been explored and contrasted with the analogous monometallic species, with specific reference to their catalytic applicability for sustainable oxidation reactions. The catalytic synergy prompted by the bimetallic substitution in the CoTiAlPO-5 material has been investigated and contrasted to the monometallic system. Specific focus was placed on the in-situ production of hydroxylamine as part of the sustainable aerobic ammoximation of cyclohexanone. Periodic DFT calculations have revealed subtle mechanistic differences between the mono- and bimetallic systems which account for energetic differences in the rate determining step of the reaction pathway.

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ORCID for Robert Raja: orcid.org/0000-0002-4161-7053

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