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 firstprinciples 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 topologicallydistinct 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.

Exploiting NMR spectroscopy for the study of disorder in solids

“…variation in skew reflecting the change in point symmetry at the P site. Seymour et al recently carried out a detailed experimental and computational study of the Mg and Zn substitution in STA-2and observed a similar trend in 31 P isotropic shift for both materials [81]. 1 H and27 Al NMR spectra showed that loss of bridging OHwas the charge-balancing mechanism, and Co 2+ linkage contributed around 2000 to 3000 ppm to the position of the peak maximum.…”

“…It might not be possible to generate models that correspond exactly to the "real" structure, and simplified approaches, aimed at understanding the relative variations in the NMR parameters, rather than determining their exact magnitude, have to Sn/Ti disorder in pyrochlore ceramics and F substitution into hydrous minerals. [9,11,69,[80][81][82][83][84][85][86] A better approximation for an aperiodic structure (particularly for low level disorder) can be obtained by using a repeating crystal composed of "supercells", as shown in Figure 9c, where the supercell is chosen to be sufficiently large to minimise the effect of any structural change on the neighbouring cells. It should also be noted that using a supercell increases the number of atoms to be treated explicitly, thereby increasing the computational cost.…”

Recent advances in computational 31P NMR: Part 1. Chemical shifts

“…In a continuation of their earlier studies, recently reviewed by the principal authors, Dawson, et al performed a systematic DFT investigation of the solid‐state 31 P NMR isotropic chemical shifts of the local structure of aluminophosphates, capable of providing information on the number of crystallographic phosphorus sites, their relative populations, and the positions of any dopant atoms in the framework. Based on the recently demonstrated simple relationship between the local structure around phosphorus atom (first of all, the mean P─O bond length and P─O─Al bond angle) and calculated at the DFT level 31 P NMR isotropic chemical shift, δ iso , for a series of calcined aluminophosphates, the authors extended this approach to "as‐made" aluminophosphates, illustrated in Figure .…”

Caveats and Perspectives of Computational Phosphorous Nmr: Chemical Shifts

“…In a continuation of their earlier studies, [99,100,101,102,103,104,105,106] recently reviewed by the principal authors, [107] Dawson, et al [108] performed a systematic DFT investigation of the solid-state 31 P NMR isotropic chemical shifts of the local structure of aluminophosphates, capable of providing information on the number of crystallographic phosphorus sites, their relative populations, and the positions of any dopant atoms in the framework. Based on the recently demonstrated simple relationship between the local structure around phosphorus atom (first of all, the mean P-O bond length and P-O-Al bond angle) and calculated at the DFT level 31 P NMR isotropic chemical shift, δ iso , for a series of calcined aluminophosphates, [103] the authors extended this approach to "as-made" aluminophosphates.…”