Mesoporous silica scaffolds as precursor to drive the formation of hierarchical SAPO-34 with tunable acid properties

Abstract

Using a distinctive bottom-up approach, hierarchical SAPO-34 has been synthesized using CTAB encapsulated within ordered mesoporous silica (MCM-41) that serves as both the silicon source and mesoporogen. The structural and textural properties of the hierarchical SAPO-34 were contrasted against its microporous analogue, and the nature, strength and accessibility of the Brønsted acid sites were studied using a range of physicochemical characterization tools; notably probe-based FTIR and solid-state (SS) MAS NMR. Whilst CO was used to study the acid properties of hierarchical SAPO-34, bulkier molecular probes (including pyridine, 2,4,6-trimethylpyridine and 2,6-di-tert-butylpyridine) allowed particular insight into the enhanced accessibility of the acid sites. The activity of the hierarchical SAPO-34 catalyst was evaluated in the industrially-relevant, acid-catalysed Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, under vapor-phase conditions. These catalytic investigations revealed a significant enhancement in the yield of ε-caprolactam using our hierarchical SAPO-34 catalyst compared to SAPO-34, MCM-41, or a mechanical mixture of these two phases. The results highlight the merits of our design strategy for facilitating enhanced mass transfer, whilst retaining favorable acid site characteristics.

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Identifiers

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

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