title: An NMR crystallographic approach to monitoring cation substitution in the aluminophosphate STA-2.
abstract: The substitution of the divalent cations Mg(2+) and Zn(2+) into the aluminophosphate (AlPO) framework of STA-2 has been studied using an "NMR crystallographic" approach, combining multinuclear solid-state NMR spectroscopy, X-ray diffraction and first-principles calculations. Although the AlPO framework itself is inherently neutral, the positive charge of the organocation template in an as-made material is usually balanced either by the coordination to the framework of anions from the synthesis solution, such as OH(-) or F(-), and/or by the substitution of aliovalent cations. However, the exact position and distribution of the substituted cations can be difficult to determine, but can have a significant impact upon the catalytic properties a material exhibits once calcined. For as-made Mg substituted STA-2, the positive charge of the organocation template is balanced by the substitution of Mg(2+) for Al(3+) and, where required, by hydroxide anions coordinated to the framework [27] Al MAS NMR spectra show that Al is present in both tetrahedral and five-fold coordination, with the latter dependent on the amount of substituted cations, and confirms the bridging nature of the hydroxyl groups, while high-resolution MQMAS spectra are able to show that Mg appears to preferentially substitute on the Al1 site. This conclusion is also supported by first-principles calculations. The calculations also show that (31)P chemical shifts depend not only on the topologically-distinct site in the SAT framework, but also on the number of next-nearest-neighbour Mg species, and the exact nature of the coordinated hydroxyls (whether the P atom forms part of a six-membered ring, P(OAl)2OH, where OH bridges between two Al atoms). The calculations demonstrate a strong correlation between the (31)P isotropic chemical shift and the average 〈P-O-M〉 bond angle. In contrast, for Zn substituted STA-2, both X-ray diffraction and NMR spectroscopy show less preference for substitution onto Al1 or Al2, with both appearing to be present, although that into Al1 appears slightly more favoured.
authors.affiliation: School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St Andrews, North Haugh, St Andrews KY16 9ST, UK.
authors.forename: Valerie R
authors.initials: VR
authors.lastname: Seymour
authors.affiliation: School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St Andrews, North Haugh, St Andrews KY16 9ST, UK.
authors.forename: Eike C V
authors.initials: EC
authors.lastname: Eschenroeder
authors.affiliation: School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St Andrews, North Haugh, St Andrews KY16 9ST, UK.
authors.forename: Paul A
authors.initials: PA
authors.lastname: Wright
authors.affiliation: School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St Andrews, North Haugh, St Andrews KY16 9ST, UK. Electronic address: sema@st-andrews.ac.uk.
authors.forename: Sharon E
authors.initials: SE
authors.lastname: Ashbrook
keywords: Aluminophosphates; Cation substitution and disorder; First-principles calculations; NMR crystallography; Solid-state MAS NMR