An NMR crystallographic approach to monitoring cation substitution in the aluminophosphate STA-2
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The substitution of the divalent cations Mg2+ and Zn2+ 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 Mg2+ for Al3+ and, where required, by hydroxide anions coordinated to the framework.  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 31P 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 31P isotropic chemical shift and the average 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.
Seymour , V R , Eschenroeder , E C V , Wright , P A & Ashbrook , S E M 2014 , ' An NMR crystallographic approach to monitoring cation substitution in the aluminophosphate STA-2 ' Solid State Nuclear Magnetic Resonance . DOI: 10.1016/j.ssnmr.2014.10.007
Solid State Nuclear Magnetic Resonance
Copyright © 2014 Published by Elsevier Inc. This work is made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at http://dx.doi.org/10.1016/j.ssnmr.2014.10.007
The authors thank the Leverhulme Trust (F/00 268/BJ), EPSRC (EP/E041825/1, EP/J501542/1, and EP/F018096/1) and the European Commission FP6 Marie Curie Research Training Network “INDENS” (MRTN-CT-2004–005503), and the Leverhulme Trust (F/00 268/BJ) for support. Accepted 23rd October 2014.
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