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Mesoporous Silica Scaffolds as Precursor to Drive the Formation of Hierarchical SAPO‐34 with Tunable Acid Properties
Author(s) -
Miletto Ivana,
Paul Geo,
Chapman Stephanie,
Gatti Giorgio,
Marchese Leonardo,
Raja Robert,
Gianotti Enrica
Publication year - 2017
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201701978
Subject(s) - cyclohexanone oxime , beckmann rearrangement , microporous material , catalysis , mesoporous material , magic angle spinning , chemical engineering , bromide , cyclohexanone , fourier transform infrared spectroscopy , mcm 41 , molecular sieve , chemistry , materials science , caprolactam , yield (engineering) , brønsted–lowry acid–base theory , organic chemistry , nuclear magnetic resonance spectroscopy , composite material , engineering
Using a distinctive bottom‐up approach, a hierarchical silicoaluminophosphate, SAPO‐34, has been synthesized using cetyl trimethylammonium bromide (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 magic angle spinning (SS MAS) NMR spectroscopies. 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‐catalyzed 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.