Synthesis of the large pore aluminophosphate STA-1 and its application as a catalyst for the Beckmann rearrangement of cyclohexanone oxime

The preparation of stable large pore aluminophosphate (AlPO) zeotypes offers materials for applications in adsorption and catalysis. Here we report the synthesis of the pure AlPO with the SAO topology type (AlPO STA-1) using N , N ′-diethylbicyclo[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidine (DEBOP) as the...

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Published in:Journal of materials chemistry. A, Materials for energy and sustainability Materials for energy and sustainability, 2024-06, Vol.12 (25), p.15398-15411
Main Authors: González-Camuñas, Nuria, Cantín, Ángel, Dawson, Daniel M, Lozinska, Magdalena M, Martínez-Triguero, Joaquín, Mattock, James, Cox, Paul A, Ashbrook, Sharon E, Wright, Paul A, Rey, Fernando
Format: Article
Language:English
Subjects:
Acidity
Aluminum phosphate
Caprolactam
Catalysis
Catalysts
Chemical synthesis
Chemistry
Crystallography
Cyclohexanone
Dehydration
Fluorides
Infrared spectroscopy
Ions
NMR
NMR spectroscopy
Nuclear magnetic resonance
Roasting
Topology
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Summary:The preparation of stable large pore aluminophosphate (AlPO) zeotypes offers materials for applications in adsorption and catalysis. Here we report the synthesis of the pure AlPO with the SAO topology type (AlPO STA-1) using N , N ′-diethylbicyclo[2.2.2]oct-7-ene-2,3:5,6-dipyrrolidine (DEBOP) as the organic structure directing agent in the presence of fluoride. The AlPO STA-1 can be rendered microporous (pore volume 0.36 cm 3 g −1 ) via calcination and the calcined form remains stable in the presence of moisture. The structure of the dehydrated form has been established by Rietveld refinement (tetragonal P 4&cmb.macr; n 2, a = 13.74317(10) Å, c = 21.8131(5) Å, V = 4119.94(16) Å 3 ). Multinuclear 27 Al and 31 P MAS NMR, together with 2D COSY and CASTEP NMR calculations, enables resolution and assignment of the signals from all crystallographically distinct Al and P framework sites. Structural elucidation of the as-prepared aluminophosphate-fluoride is more challenging, because of the presence of partially protonated OSDA molecules in the 3D-connected channel system and in particular because the fluoride ions coordinate with positional disorder to some of the Al atoms to give 5-fold as well as tetrahedrally-coordinated framework Al species. These are postulated to occupy Al-F-Al bridging sites, where they are responsible for distortion of the framework [ P 4&cmb.macr; n 2, a = 13.3148(9) Å, c = 22.0655(20) Å, V = 3911.9(7) Å 3 ]. Calcination and removal of fluoride ions and OSDAs allows the framework to expand to its relaxed configuration. The SAO topology type aluminophosphate can also be synthesised with small amounts of Si and Ge in the framework, and these SAPO and GeAPO STA-1 materials are also stable to template removal. IR spectroscopy with CO as a probe at 123 K indicates all have weak-to-mild acidity, increasing in the order AlPO < GeAPO < SAPO. These STA-1 materials have been investigated for their activity in the Beckmann rearrangement of cyclohexanone oxime to -caprolactam at 598 K: while all are active, the AlPO form is favoured due to its high selectivity and slow deactivation, both of which are a consequence of its very weak acid strength, which is nevertheless sufficient to catalyse the reaction. Microporous AlPO 4 STA-1 has been synthesised and studied by NMR crystallography; it is an effective catalyst for the Beckmann rearrangement of cyclohexanone oxime.
ISSN:2050-7488
2050-7496
DOI:10.1039/d4ta01132e