Application of NMR crystallography to the determination of the mechanism of charge-balancing in organocation-templated AlPO STA-2

An NMR crystallographic approach to monitoring cation substitution in the aluminophosphate STA-2

“…The Mg/P ratio was chosen to be 0.167, so full charge balance could be achieved purely by cation substitution, without any additional hydroxyls. The initial structure used was based on the dehydrated STA-2(BDAB) structure (obtained from diffraction measurements in previous work 7,9 ) with the SDA manually changed from BDAB to BQNB (i.e., replacement of the apical N with CH), to match that used in the experimental work presented here, all hydroxyls removed and two framework Al replaced with Mg (to satisfy the charge). Calculations were carried out using the CASTEP DFT code (version 5.5.2), 17 employing the GIPAW algorithm, 11 to reconstruct the all-electron wavefunction in the presence of a magnetic field.…”

“…6,7 The pure AlPO form of STA-2 was synthesized subsequently, using both BQNB and bis-diazabicyclooctanebutane (BDAB) as SDAs, and characterised using a combination of X-ray diffraction and solid-state NMR spectroscopy. 8,9 The structure of STA-2 (framework type SAT) can be described in terms of the stacking of its secondary building units (SBUs), where six-membered rings (6MRs) are stacked at different positions in the xy plane of a hexagonal unit cell, and are connected via four-membered rings (4MRs). Consequently, the framework contains single and double six-membered rings (S6Rs and D6Rs, respectively), 4MRs and both cancrinite (CAN) and elongated cages, as shown in Figure 1a.…”

“…Consequently, the framework contains single and double six-membered rings (S6Rs and D6Rs, respectively), 4MRs and both cancrinite (CAN) and elongated cages, as shown in Figure 1a. [6][7][8][9] In the calcined framework there are two topologically-distinct cation sites (occupied by Al only in the pure AlPO), and two topologically-distinct P sites, with Al2 and P1 located in D6R units, and Al1 and P2 located in S6R units. These units join to form the CAN cages, which share a 4MR face.…”

“…In the pure AlPO form, this was previously shown to be achieved by the introduction of six hydroxyl groups, occupying the CAN cages. 8,9 For the MgAPO form synthesised initially, charge balance can occur both by OH -incorporation and by Mg 2+ substitution. A fully Mg-substituted AlPO (i.e., where no hydroxyl groups are required) would have a Mg/P ratio of 1/6.…”

“…12 For phosphate frameworks, calculations have been used primarily to facilitate spectral assignment and to identify the phase(s) present and their interconversion. 8,9,13 However, the real power of such calculations lies in their ability to aid interpretation of the complicated spectral lineshapes that are observed for disordered materials, providing detailed insight into the atomic-scale structure.…”