Expanding beyond the micropore: harnessing pendant silanols within hierarchically porous aluminophosphates for targeted catalysis

Abstract

Discrete solid-acid centres within hierarchically porous (HP) architectures, which contain micropores with interconnected mesopores, offer the potential to overcome the diffusional limitations in the conventional microporous zeotypes. This thesis presents a design strategy to synthesise hierarchically porous isomorphously metal (Me) substituted aluminophosphates (AlPOs) and silicoaluminophosphates (SAPOs), with AFI and CHA frameworks, as a means to create a solid acid catalyst for both the liquid and vapour phase Beckmann rearrangement with enhanced catalyst lifetimes, catalyst activity and substrate versatility. By employing a one-step soft templating approach coupled with detailed physicochemical and spectroscopic characterisation, isolated solid acid sites can be suitably tailored and discretely modulated within the micropores and mesopores. The resulting HP SAPOs and AlPOs were designed to possess an auxiliary mesoporous network (as confirmed via pXRD, BET and N2 isotherms), to mimic the microporous analogues active sites and to have additional active silanol sites lining the mesopores (as confirmed via FTIR spectroscopy, NMR spectroscopy and TPD). Owing to the interplay between mesoporosity, Brønsted acidity and silanol sites the HP catalysts were shown to have superior catalytic properties to the microporous analogues. The HP AlPO’s potential as scaffolds for the heterogenisation of bulky functional groups was also assessed. The pendant silanols were harnessed to covalent anchor an imidazolium organocatalyst to the HP mesopores. The resulting catalyst was active in the formation of cyclic carbenes from epoxide and CO2. Thus, this further highlighted the efficacy of our design rationale in the synthesis of HP AlPOs and its amenability for creating a range of active sites for catalysis.

More information

Identifiers

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

Catalogue record

Export record