Application of NMR Crystallography to Highly Disordered Templated Materials: Extensive Local Structural Disorder in the Gallophosphate GaPO-34A

Abstract

We present an NMR crystallographic investigation of two as-made forms of the recently characterised gallophosphate GaPO-34A, which has an unusual framework composition with a Ga : P ratio of 7 : 6 and contains both hydroxide and fluoride anions and either 1-methylimidazolium or pyridinium as the structure-directing agent. We combine previously reported X-ray crystallographic data with solid-state NMR spectroscopy and periodic density functional theory (DFT) calculations to show that the structure contains at least three distinct types of disorder (occupational, compositional and dynamic). The occupational disorder arises from the presence of six anion sites per unit cell, but a total occupancy of these of five, leading to full occupancy of four sites and partial occupancy of the fifth and sixth (which are related by symmetry). The mixture of OH and F present leads to compositional disorder on the occupied anion sites, although the occupancy of some sites by F is calculated to be energetically unfavorable and signals relating to F on these sites are not observed by NMR spectroscopy, confirming that the compositional disorder is not random. Finally, a combination of highfield 71 Ga NMR spectroscopy and variable-temperature 13 C and 31 P NMR experiments shows that the structure directing agents are dynamic on the microsecond timescale, which can be supported by averaging the 31 P chemical shifts calculated with the SDA in different orientations. This demonstrates the value of an NMR crystallographic approach, particularly in the case of highly disordered crystalline materials, where the growth of large single crystals for conventional structure determination may not be possible owing to the extent of disorder present.

17O NMR spectroscopy of crystalline microporous materials

“… 97–99 Fig. 4 shows a summary of calculated 17 O NMR parameters for T–O–T′ linkages in a range of silicate/aluminosilicate zeolites and substituted phosphate frameworks from our previous work 86,91,97–99 (with charge-balancing H added where required). Fig.…”

Thermal Dehydrofluorination of GaPO-34 Revealed by NMR Crystallography

“…45 While as-made zeolitic frameworks tend not to exhibit the same level of NTE owing to the pores being occupied by the SDA, 51,52 it has been demonstrated in as-made phosphate frameworks including GaPO-34A and CHA-type AlPO-34 that the SDA can be dynamic. 24,42,53 Therefore, in the partially-calcined GaPO-34[mim], it seems very likely that dynamic effects will influence the average structure at experimentally-relevant temperatures and that these will not be present in the calculated structural model. The 13 C, 31 P and 71 Ga NMR spectra calculated from the optimized structural model of are shown overlaid with the experimental spectra in Figure 6.…”

“…[25][26][27] In particular, we recently reported the application of this approach to GaPO-34A, a material that can be prepared from the same starting gel as GaPO-34, but which contains multiple levels of compositional, orientational and dynamic disorder that could not have been revealed or understood using a single characterisation technique. 24 In this work, we combine solid-state NMR spectroscopy, thermogravimetry, in-situ variable-temperature pXRD and density functional theory (DFT) calculations to investigate the calcination of GaPO-34 containing mimHF and pyHF within the pores. We show that, for GaPO-34[pyHF] (where the square brackets denote the extraframework species present), calcination occurs as a single step, whereas, for GaPO-34[mimHF], the calcination is a two-stage process, involving first dehydrofluorination to GaPO-34[mim], followed by loss of neutral 1-methylimidazole (mim).…”

“…In recent years, interest has grown in the use of strategies such as NMR crystallography and “SMARTER” crystallography, which combine information from NMR spectroscopy (and often first-principles calculations) with X-ray (or neutron) crystallography to provide an in-depth picture of both the local and long-range structure of a material, where these techniques could not individually have provided such information. − These approaches have been especially successful for microporous materials, ,− particularly with recent advances in both computational power and the accuracy and efficiency of calculations available. − In particular, we recently reported the application of this approach to GaPO-34A, a material that can be prepared from the same starting gel as GaPO-34, but which contains multiple levels of compositional, orientational, and dynamic disorder that could not have been revealed or understood by using a single characterization technique …”

Field‐stepped ultra‐wideline NMR at up to 36 T: On the inequivalence between field and frequency stepping

“…The National High Magnetic Field Laboratory of the United States has built a seriesconnected hybrid (SCH) magnet that can reach fields up to 36 T ( 1 H resonance frequency higher than 1.5 GHz) [4]. Openly accessible to researchers worldwide, this magnet has demonstrated its resolution and sensitivity benefits for quadrupolar nuclei in a variety of samples since being commissioned [5,[8][9][10][11][12][13][14][15][16][17][18].…”