A Multinuclear Solid-State NMR Study of Templated and Calcined

Jun 25, 2012 - Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom. ‡. School of Chemistry and EaStCHEM, University of ...
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A Multinuclear Solid-State NMR Study of Templated and Calcined Chabazite-Type GaPO-34 Mahrez Amri,† Sharon E. Ashbrook,*,‡ Daniel M. Dawson,‡ John M. Griffin,‡ Richard I. Walton,*,† and Stephen Wimperis§ †

Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom School of Chemistry and EaStCHEM, University of St. Andrews, St. Andrews KY16 9ST, United Kingdom § School of Chemistry and WestCHEM, University of Glasgow, Glasgow G12 8QQ, United Kingdom ‡

S Supporting Information *

ABSTRACT: The open-framework gallophosphate GaPO-34 is prepared with either 1-methylimidazole or pyridine as the structure-directing agent. 13C and 1H NMR spectra for these two variants of the as-made GaPO-34 are fully assigned, confirming the presence of the protonated amine and water within the pores of both materials. 31P MAS NMR confirms the presence of three crystallographic P sites, while 71Ga MAS and MQMAS NMR spectra reveal three crystallographic Ga sites: two tetrahedral and one six-coordinate. Simulations of 69Ga MAS NMR spectra from these results are in good agreement with spectra acquired at B0 = 20.0 T, and assignments are supported by first-principles calculations. 19F MAS NMR proves the presence of Ga-bridging fluoride within the as-made materials, leading to the six-coordinate gallium. Calcination removes the organic species and fluoride, yielding a microporous chabazite-type GaPO4, containing one tetrahedral Ga site. Exposure to moist air yields calcined, rehydrated GaPO-34 containing four-, five-, and six-coordinate gallium. Upon heating this material, loss of crystallinity is observed by powder X-ray diffraction and NMR, with the latter revealing a range of P and Ga environments. The thermal instability of calcined, rehydrated GaPO-34 contrasts with the isomorphous aluminophosphate, showing that apparently analogous materials may have important differences in reactivity.

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centers, was first prepared as a gallium phosphate by this method, later leading to the discovery of doped variants and a pure AlPO form.10 It is typical, especially when the fluoride route is employed, for five- and six-coordinate gallium environments to occur alongside four-coordinate gallium in many phosphates.7 AlPO and GaPO materials are typically prepared using hydrothermal or solvothermal synthesis in the presence of an organic structure directing agent (or ‘template’), often an amine, that must be removed by heat treatment (calcination) after isolation of the as-made solid to render the material porous. In the case of AlPOs, the removal of the organic template often gives an open-framework, truly microporous structure. For GaPOs, however, this is not always the case, and for many materials it has been observed that calcination is accompanied by collapse of the framework. A recent computational study by Girard et al. studied the relative stability of forty calcined gallophosphate structures (i.e., GaPO4 polymorphs, free of template, hydroxide or fluoride) using energy

INTRODUCTION Phosphates were the first chemical variants of silicate zeolites, prepared with the aim of extending the widely exploited properties of naturally occurring and synthetic crystalline microporous materials by the incorporation of a wider range of chemical elements.1 The aluminum phosphates (AlPOs) are perhaps now the most extensive group of such “zeotypes”, and since their discovery in the early 1980s2 new members of the family still continue to be synthesized.3 While some AlPOs have corner-sharing aluminate and phosphate tetrahedral networks that are isomorphous with zeolites, there are also a number of novel structure types not (yet) reported in silicate chemistry.4 In terms of applications, the doping of transition-metal ions into AlPOs has yielded highly selective oxidation catalysts, with potential industrial application for the processing and production of organic chemicals.5 The synthesis of gallium phosphate analogues was a natural extension to this work,6 given that the Ga−O tetrahedral bond length is only slightly longer than that of tetrahedral Al−O.7 A diverse range of GaPOs has now been synthesized, with the large number prepared by the “fluoride route” of particular note.8,9 For example, the striking tetrahedral framework material known as cloverite, which contains rings made up of 20 tetrahedral © 2012 American Chemical Society

Received: May 19, 2012 Revised: June 23, 2012 Published: June 25, 2012 15048

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EXPERIMENTAL AND COMPUTATIONAL METHODS Sample Preparation. Two templated samples were synthesized, GaPO-34[pyridinium fluoride] and GaPO-34[1methylimidazolium fluoride], using methods based on those published by Schott-Darie et al.15 A gallium oxide source was prepared by heating gallium nitrate hydrate (Aldrich) at 250 °C for 24 h in air. The relative molar composition of the starting mixture was Ga2O3: P2O5: HF: 70 H2O: 1.7 amine, obtained by successive addition with vigorous stirring of 0.61 g of orthophosphoric acid (85 wt % in water, Fisher), 3.18 g of deionized water, 0.5 g of Ga2O3, 0.133 g of hydrofluoric acid (40 wt % in water, Fluka), and finally 0.36 g of 1methylimidazole (Fisher, 99 wt % in water) or the equivalent amount of pyridine (0.35 g) was added with continuous stirring. The resulting gels were each stirred for 2 h at room temperature then transferred to Teflon-lined (20 mL volume) stainless steel autoclaves and placed in a fan-assisted oven for 24 h at 170 °C for crystallization. After cooling to room temperature, the solid products were recovered by suction filtration, washed with distilled water, filtered through coarse filter paper and finally dried in air at 70 °C overnight. The structure of the GaPO-34[1-methylimidazolium fluoride] is known, having been solved and refined from single crystal X-ray diffraction data,15 and also verified by computer simulation.31 The pyridine material is assumed to have a similar framework structure and comparison of the powder XRD patterns (Siemens D5000 diffractometer operating with Cu Kα1/2 radiation) of the two materials with that simulated from the crystal structure of the 1-methylimidazolium-templated material proved a successful synthesis of the two samples, as shown in Supporting Information. Thermal analysis (combined TGADSC) using a Perkin-Elmer Instrument, showed total mass loss for both materials of ∼18% to 500 °C,32 as shown in the Supporting Information. This is consistent with the chemical formula (Ga3P3O12F)·(TH)·0.5H2O (T = C4N2H5 or C5NH5, for 1-methylimidazolium and pyridinium, respectively), with total loss of the organic template, crystal water and fluoride (expected mass losses: 18.2% and 17.8%, for 1-methlyimizadolium and pyridinium templated materials, respectively). Thermodiffractometry (Bruker D8 diffractometer fitted with an Anton-Parr HTK900 heating chamber and operating with Cu Kα1/2 radiation) was used to study the formation and stability of the calcined material (denoted GaPO-34(calcined, dehydrated)). First, calcination of GaPO-34[1-methylimidazolium fluoride] was performed in situ to produce a freshly prepared calcined form of the solid: this showed the formation of a rhombohedral, template-free form of GaPO-34 (GaPO34(calcined, dehydrated)), analogous to calcined, dehydrated AlPO-3420 above ∼425 °C (Supporting Information). Upon cooling in moist air to room temperature, transformation to a calcined, rehydrated phase (GaPO-34(calcined, hydrated)) occurs rapidly after one scan while still sealed in the sample chamber. Subsequent heating of this phase, in an attempt to remove the water and revert to the calcined phase, yields instead a very poorly ordered phase, which we thus describe as GaPO-34(collapsed) (see Supporting Information). Solid-State NMR. For 71Ga, solid-state NMR spectra were acquired at magnetic field strengths of 14.1 and 20.0 T. Samples were packed into 2.5 mm rotors and rotated at MAS rates of 25−30 kHz. A spin echo was used to ensure undistorted lineshapes were acquired, with a typical echo duration, τ, of one

minimization approaches, and found that while some should in theory be stable, a number showed physically unrealistic framework distortions, such as close oxygen−oxygen distances.11 In this work we present a detailed structural study of GaPO34, a material with a CHA-type structure.4 The parent structure type is derived from the framework of the mineral chabazite,12 but structurally related forms of the material are known, including frameworks that are purely siliceous,13 as well as the aluminum phosphate (AlPO-34)14 and gallium phosphate (GaPO-34)15 analogues. AlPO-34 may be templated by a variety of organic amines14,16−19 and is known to be stable upon calcination, although it readily (and reversibly) absorbs water, giving hydrates in which water is coordinated directly to framework atoms.20 Two templated forms of GaPO-34 are known, both prepared using fluoride, which is found in the framework, balancing the charge of the template in the as-made forms.15 Although it has been reported that calcination of either of these forms is possible, it has been suggested that crystallinity is lost upon exposure to moist air.15 CHA-type structured materials have been studied for a variety of useful physical and chemical properties, including catalytic activity21−23 and, recently, negative thermal expansion.24−26 Given the interest in the family of CHA-type materials and the need for a more general experimental understanding for the possible lack of stability of calcined GaPO materials, we have undertaken a multinuclear solid-state NMR study to investigate the atomic-scale local structure of GaPO-34 in as-made, calcined, calcined hydrated and thermally collapsed forms. In addition to using the more conventional spin I = 1/2 nuclei, 1 H, 13C, 19F, and 31P, to probe the template and framework, we also utilize the two NMR-active isotopes of gallium, 71Ga and 69 Ga, both of which have spin quantum number I = 3/2. Despite apparently promising NMR properties (i.e., high γ, high sensitivity, and abundances of 60.4 and 39.6%, respectively), there has been relatively little application of Ga NMR for the study of microporous materials.27 This is largely a result of the significant second-order quadrupolar broadening observed in most NMR spectra, which can limit the feasibility of Ga MAS experiments at moderate magnetic field strengths. Recent developments in ultra fast (i.e., 60−70 kHz) MAS and the availability of higher magnetic fields have widened the applicability of 69/71Ga NMR as a probe of local structure, although high-resolution spectra (obtained using, for example, MQMAS) remain challenging to obtain unless the quadrupolar interaction is reasonably small.28 However, the presence of the quadrupolar interaction in addition to the shielding should, in principle, provide a second or alternative probe of the local environment. Indeed, the 27Al (I = 5/2) quadrupolar coupling has been demonstrated to be highly sensitive to local geometry in a range of materials.29 Furthermore, for AlPOs, solid-state NMR experiments have been successfully combined with firstprinciples density functional theory (DFT) calculations in order to validate structural models, and to aid spectral assignment and interpretation.30 Here, we demonstrate how this work can be extended in a detailed structural study of GaPO-34, using a combination of powder X-ray diffraction, multinuclear solidstate NMR, and DFT calculations. 15049

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(GGA) PBE functional37 was employed, and core−valence interactions were described by ultrasoft pseudopotentials.38 Wave functions were expanded in planewaves with a kinetic energy smaller than the cutoff energy, typically ∼60 Ry, and integrals over the Brillouin zone were performed using a kpoint spacing of 0.04 Å−1. The absolute shielding tensor, σ, and electric field gradient (EFG) tensor, V, in the crystal frame are generated. From these, the isotropic chemical shift, δiso ≈ −(σiso − σref), where σiso, the isotropic shielding, is (1/3)Tr{σ}. Reference shieldings, σref, of 30.2, 172.4, 282.1, and 1704.4 ppm were used for 1H, 13C, 31P, and 71Ga, respectively, determined from previous work. For 19F, the chemical shifts were referenced according to the procedure described in ref 39, where it was shown that it is necessary to apply a scaling factor to 19F chemical shifts calculated using this approach, to ensure a good match between calculations and experiment. The magnitude (CQ) and asymmetry (ηQ) of the quadrupolar interaction can be generated from the principal components of the EFG tensor (VXX, VYY, VZZ) through CQ = eQVZZ/h and ηQ = (VXX − VYY)/VZZ. Quadrupole moments, eQ, of 171 mB and 107 mB were used for 69Ga and 71Ga, respectively.40 Although the sign of CQ is often difficult to determine experimentally, the signs of calculated CQ values are included in our tabulated data. Structural parameters (the unit cell and all atomic positions) were obtained from experimental diffraction studies. The crystal structure was reproduced from these parameters by the use of periodic boundary conditions. Where necessary, geometry optimization was also performed within the CASTEP program, with the lattice parameters and atomic coordinates allowed to vary. Calculations were carried out using CASTEP versions 4.3 and 5.5.2 on a 198-node (2376 core) Intel Westmere cluster with 2 GB memory per core and QDR Infiniband interconnect at the University of St. Andrews.

rotor period. Owing to the width of the lineshapes high-power pulses (∼200 kHz radiofrequency (rf) field strength), rather than central-transition selective pulses, were employed. Typical recycle intervals were between 0.5 and 3 s. For MQMAS experiments (20.0 T), a three-pulse z-filtered pulse sequence was employed,33 with pulse durations of 3.5 and 1.0 μs for triple-quantum excitation and conversion, and 7.5 μs for the final pulse. Spectra are shown after a shearing transformation, with the convention in ref 34 used to reference the scale in the isotropic dimension. 69Ga solid-state NMR spectra were acquired at a magnetic field strength of 20.0 T (templated materials) and 14.1 T (calcined material). Samples were packed into 1.3 mm (20.0 T) or 2.5 mm (14.1 T) rotors and rotated at an MAS rate of 62.5 kHz and 25 kHz, respectively. Spectra were acquired using recycle intervals of 0.5 s with a typical rf field strength of ∼200 kHz. In order to ensure undistorted lineshapes were acquired, a spin-echo pulse sequence was used, with rotor-synchronized τ intervals of 16 and 40 μs, respectively. 31 P solid-state NMR spectra were acquired at magnetic field strengths of 14.1 and 20 T. Samples were packed into 2.5 mm rotors and rotated at MAS rates of 10−30 kHz. Spectra were acquired using recycle intervals of 60 s (10 s calcined material) with typical rf field strengths of 90−100 kHz. 13C solid-state NMR spectra were acquired at a magnetic field strength of 14.1 T. Samples were packed into 2.5 mm rotors and rotated at MAS rates of 12.5 kHz. Spectra were acquired using crosspolarization (CP) from 1H using optimized contact pulse durations of between 1 and 2 ms (ramped 90−100% for 1H), and two-pulse phase modulation (TPPM) 1H decoupling during acquisition with an rf field strength of 110 kHz. Typical recycle intervals were 5 s. For 1H, solid-state NMR spectra were acquired at a magnetic field strength of 14.1 T, using an rf field strength of ∼110 kHz. Samples were packed into 1.3 mm rotors and rotated at MAS rates of 60 kHz. Typical recycle intervals were 3 s. Two-dimensional homonuclear 1H solid-state NMR spectra were acquired at a magnetic field strength of 14.1 T, using an rf field strength of ∼110 kHz. Samples were packed into 1.3 mm rotors and rotated at MAS rates of 55−60 kHz. Typical recycle intervals were 3 s. For 19F, solid-state NMR spectra were acquired at a magnetic field strength of 14.1 T, using an rf field strength of ∼110 kHz. Samples were packed into 2.5 mm rotors and rotated at MAS rates of 25 kHz. Chemical shift scales are shown relative to 1 M Ga(NO3)3 (aq) for 69/71Ga, 85% H3PO4 for 31P, TMS for 1H/13C and CCl3F for 19F. In order to ensure that the calcined, dehydrated GaPO-34 material did not take up any water while preparing the sample for NMR, the material was prepared by heating GaPO-34[1methylimizadolium fluoride] from room temperature to 500 °C in air, followed by cooling to ∼350 °C. The sample was then sealed in a quartz vial. The vial was transferred to a glovebox, and the sample was loaded and sealed into a 2.5 mm rotor under dry conditions. This sample was studied immediately. A second identical sample was also prepared at the same time, with a drying agent (CaO) packed at the top and bottom of the rotor. It was subsequently shown that this sample was not hydrated after ∼3 months. First-Principles Calculations. The calculation of NMR parameters was carried out using the CASTEP DFT code,35 employing the GIPAW algorithm, 36 which allows the reconstruction of the all-electron wave function in the presence of a magnetic field. The generalized gradient approximation

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RESULTS AND DISCUSSION Figure 1 shows the structure of GaPO-34[1-methylimidazolium fluoride], obtained previously from X-ray diffraction,15 with the

Figure 1. Structure of GaPO-34[1-methylimidazolium fluoride], as derived from X-ray diffraction.15 Each of the two water molecules in the unit cell has an occupancy of 0.5.

protons placed in chemically reasonable orientations using idealized geometries, as they were not located in the earlier work. The framework structure contains three distinct P and Ga sites, and two methylimidazolium templates are present per unit cell. These are charged balanced by two F− anions, which bridge two Ga1 centers (resulting in one six-coordinate and two four-coordinate gallium). Water is also present within the pores 15050

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Figure 2. 1H, 13C, 19F, and 31P MAS NMR spectra of (a) GaPO-34[1-methylimidazolium fluoride] and (b) GaPO-34[pyridinium fluoride]. Spectra were acquired at B0 = 14.1 T with MAS rates of 60 kHz (1H), 12.5 kHz (13C), 55 kHz (19F) and 25 kHz (31P). For 13C, spectra were acquired using cross-polarization. Spinning sidebands are marked with asterisks.

in space to the NH proton H3 (12.3 ppm), confirming it as H2 and the peak at 7.4 ppm as H5. The remaining resonance (7.7 ppm) can therefore be assigned as H4, and displays throughspace dipolar connectivity with H3 and H5 as expected. With this assignment of the 1H spectrum, it is possible to confirm using a two-dimensional 1H−13C HETCOR experiment (Supporting Information) that the 13C resonance at 134.1 ppm results from C2, and those at 124.0 and 122.7 ppm result from C5 and C4, respectively. The 19F spectrum shows a single resonance at −98.4 ppm, in agreement with the literature15 and the presence of fluoride confirms that the organic template is indeed protonated. From a consideration of fluorinated GaPOs studied in the literature (see Supporting Information for more detail) it can be seen that this shift is consistent with the presence of bridging rather than terminal F, as consistent with the refined crystal structure from diffraction. The 31P MAS NMR spectrum shows three resonances, with isotropic chemical shifts of −0.8, −12.0, and −20.0 ppm, confirming the presence of three distinct P sites in the structure. The integrated intensity ratios are 1.17:1.0:1.09, in agreement with the 1:1:1 intensities predicted by the crystal structure (the small differences reflect small variation in T1 relaxation). Previous 31P NMR studies of AlPOs have shown a correlation between the isotropic chemical shift and the average of the four ⟨P−O−Al⟩ bond angles in the phosphate tetrahedra, with a decrease in shift as the angle increases.41 A similar observation has also been made for the P sites in GaPO MIL-50.42 Using this information, we can assign the three resonances as P3, P2, and P1 in order of decreasing shift (as shown in Table 2), with

in two sites, although refinement of the diffraction data suggests that each of the two positions has an occupancy of 0.5, i.e., there is a total of only one water molecule per unit cell, with some static disorder present. This is in contrast to a number of templated forms of AlPO-34 that are known, where water does not appear to be present in the pores.14,16−19 Figure 2a shows 1H, 13C, 19F, and 31P MAS NMR spectra of GaPO-34[1-methylimidazolium fluoride]. Four distinct resonances are observed in the 13C CPMAS spectrum, corresponding to the CH3 group and the three aromatic carbons expected for the 1-methylimizadolium template. Four resonances are observed in the fast 1H MAS spectrum, resulting from the six distinct 1H species. It is possible to assign the resonances in the 1 H and 13C spectra using a combination of two-dimensional NMR experiments, as shown in the Supporting Information. The 1H double-quantum (BABA; see Supporting Information) MAS NMR spectrum reveals the through-space proximities between 1H species. Two overlapping signals are observed between 3 and 4 ppm, corresponding to water and the CH3 group (see Scheme 1 and Table 1 for full spectral assignments). The CH3 protons (H6) display a through-space proximity with signals at 8.2 and 7.4 ppm, and the former of these is also close Scheme 1. Labeling Scheme Used for the Two Template Cations: (a) 1-Methylimidazolium and (b) Pyridinium

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Table 1. Experimental 1H, 13C and 19F Isotropic Chemical Shifts (δiso) and Assignment of the Spectral Resonances for the Spectra of GaPO-34[1-methylimidazolium fluoride] and GaPO-34[pyridinium fluoride] in Figure 2a δiso(ppm)

assignment

assignment

δiso(ppm)

assignment

δiso(ppm)

C2 C4 C5 C6

134.1 (1) 122.7 (1) 124.0 (1) 36.7 (1)

F1

−98.4 (5)

C2 C3 C4

141.5 (1) 128.6 (1) 147.8 (1)

F1

−102.0 (5)

GaPO-34[1-methylimidazolium fluoride] H2 8.2 (1) H3 12.3 (1) H4 7.7 (2) H5 7.4 (2) H6 3.8 (1) H2O 3.4 (2) GaPO-34[pyridinium fluoride] H1 14.4 (1) H2 8.6 (1) H3 8.1 (1) H4 8.6 (1) H2O 3.3 (2) a

The numbering scheme is shown in Scheme 1.

average ⟨P−O−Ga⟩ bond angles of 127.6°, 137.2°, and 143.7°, respectively from the crystal structure.

the models, with ranges of 2−5 ppm for 31P, 2−4 ppm for 13C and 1 ppm for 1H, respectively (see Supporting Information). Given such a variation is not present in the experimental NMR spectra (e.g., linewidths in 31P MAS NMR for example are ∼1 ppm at B0 = 14.1 T and do not vary significantly with field), this may suggest the water disorder is actually dynamic, and “average” NMR parameters are observed experimentally. Although the (static) calculations cannot, therefore, predict NMR parameters with great accuracy, they are able to confirm the assignment of the 31P MAS spectrum, with P1, P2, and P3 assigned in order of increasing shift (see Table 2), in agreement with the prediction above using more simplistic geometrical arguments. The 13C, 1H, 19F, and 31P MAS NMR spectra of GaPO34[pyridinium fluoride] are shown in Figure 2b for comparison. No full structure of this material has been reported, although the similarity of the powder XRD pattern to that of GaPO34[1-methylimizadolium fluoride] suggests a close structural relationship. The 1H MAS NMR spectrum confirms the presence of pyridinium ions within the pore of the framework, with the resonance at 14.4 ppm corresponding to the NH proton. The 1H (BABA) double-quantum MAS NMR spectrum (see Supporting Information) identifies the resonance at 8.6 ppm as resulting from H2, although the relative intensity of this resonance in the MAS spectrum suggests an overlap with H4. The peak at ∼8.1 ppm is assigned to H3. The resonance at 3.3 ppm demonstrates the presence of water in this material (confirmed also by the lack of cross peaks to this resonance in the DQ spectrum). The full assignment is given in Scheme 1 and Table 1. Three resonances in the aromatic region are observed in the 13C CP MAS NMR spectrum of GaPO34[pyridinium fluoride], again in agreement with the inclusion of pyridinium ions. These are tentatively assigned (see Table 1) as C4, C2, and C3 in order of decreasing shift. The assignment is also supported by the two-dimensional 1H−13C correlation experiments shown in the Supporting Information). The 19F NMR spectrum exhibits a single resonance (at −102.0 ppm), consistent with that reported previously.15 This shift is again consistent with bridging fluoride, suggesting a similar structure to that observed for GaPO-34[1-methylimidazolium fluoride]. The difference between the 19F shifts in the two materials may possibly reflect small differences in hydrogen bonding with the template.15 The 31P MAS NMR spectrum contains three distinct resonances, although the chemical shifts (of −0.9,

Table 2. Experimental 31P and 71Ga NMR Parameters (Isotropic Chemical Shift, δiso, Quadrupolar Coupling Constant, CQ, Asymmetry, ηQ, and Quadrupolar Product, PQ), and Assignment of the Spectral Resonances for the Spectra of GaPO-34[1-methylimidazolium fluoride] and GaPO-34[pyridinium fluoride] in Figures 2 and 3 assignment P1 P2 P3 Ga1 Ga2 Ga3 P1 P2 P3 Ga1 Ga2 Ga3

δiso(ppm)

CQ/MHz

ηQ

GaPO-34[1-methylimidazolium fluoride] −20.0 (1) −12.0 (1) −0.8 (1) −29 (1) 6.3 (2) 0.30 111 (3) 7.0 (4) 0.9 121 (1) 7.9 (2) 0.35 GaPO-34[pyridinium fluoride] −20.1 (1) −13.3 (1) −0.9 (1) −27 (3) 4.4 (4) 0.3 112 (4) 5.5 (6) 0.6 126 (3) 6.1 (9) 0.6

PQ/MHz

(5) (1) (5)

6.4 (3) 7.8 (4) 8.0 (3)

(2) (3) (3)

4.4 (4) 5.8 (6) 6.5 (9)

The presence of water in the structure of GaPO-34[1methylimidazolium fluoride] is confirmed by its 1H spectrum (and two-dimensional DQ MAS spectrum), with a resonance at ∼3.4 ppm that displays no cross peaks with the other 1H in the spectrum, but is observed on the diagonal (i.e., arises from two close but identical 1H). From the crystal structure it can be seen that the water is disordered, with the two distinct sites having occupancies of 0.5. While, in principle, this should lower the symmetry of the structure, producing, e.g., six rather than three distinct P sites per unit cell, little effect is apparent on the NMR spectra. This suggests that either the differences are very slight or that the water disorder is dynamic in nature, rather than purely static as modeled by the X-ray diffraction structure refinement. Insight into this was obtained through DFT calculations, which were performed for two model structures, each with one water molecule per unit cell (placed first on O1W and subsequently on O2W), and one model with no water present. The calculated NMR parameters varied between 15052

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Figure 3. 71Ga (a,b) MAS NMR spectra and (c,d) two-dimensional MQMAS NMR spectra and cross sections extracted parallel to δ2, for GaPO34[1-methylimidazolium fluoride] (a,c) and GaPO-34[pyridinium fluoride] (b,d). MAS spectra were acquired at 14.1 T (28 kHz MAS) and 20.0 T (30 kHz MAS), and MQMAS spectra at 20.0 T (30 kHz MAS). The MQMAS spectra were acquired using a triple-quantum z-filtered pulse sequence and are displayed after a shearing transformation. The scale in the indirect dimension is referenced according to the convention in ref 34. Also shown in red are spectra simulated using the parameters given in Table 2. Spinning sidebands are marked by asterisks. Ga coordination numbers are indicated by Roman numerals.

−13.3, and −20.1 ppm, as given in Table 2) are not in complete agreement with ref 15, where it is suggested there is apparently no difference in the 31P chemical shifts between the two templated materials. Given the similarity of the two spectra, however, it is reasonable to suggest that the resonances can be assigned as P3, P2, and P1 in order of decreasing shift (see Table 2), and conclude that the local bonding geometry in the two materials is very similar, with P2 exhibiting the largest change between the two (with an increase in the average ⟨P− O−Al⟩ bond angle in GaPO-34[pyridinium fluoride]). Figure 3 shows 71Ga MAS NMR spectra of both GaPO-34[1methylimidazolium fluoride] and GaPO-34[pyridinium fluoride], acquired at B0 = 14.1 and 20.0 T. In each case, four- and six-coordinate Ga sites are present, in the approximate ratio 2:1. For GaPO-34[1-methylimidazolium fluoride] the resonances exhibit features characteristic of quadrupolar broadening, although it does seem that some small additional broadening is also present. The presence of such broadening is more apparent for GaPO-34[pyridinium fluoride], as shown in Figure 3, where the spectral resonances show signs of a “tail” to low frequency, characteristic of some disorder and a (small) distribution of NMR parameters. The resonances from distinct Ga sites overlap in the MAS NMR spectra as a result of secondorder quadrupolar broadening, which, owing to its complex orientation dependence, cannot be removed completely by MAS. However, it is possible to acquire a high-resolution NMR spectrum using a two-dimensional approach, such as multiplequantum (MQ) MAS. 71Ga MQMAS spectra (20.0 T) of the two templated materials are shown in Figure 3b,d, and exhibit three clear resonances in both cases, one resulting from Ga(VI) and two from Ga(IV). This confirms the presence of F− anions

bridging between two Ga1 species in both materials, resulting in a single (crystallographically unique) six-coordinate gallium. In principle, NMR parameters may be extracted from the twodimensional spectra either using the position of the center-ofgravity of the resonance (to give δiso and the quadrupolar product, PQ (= CQ(1 + ηQ2/3)1/2)), or from cross sections extracted parallel to the δ 2 axis, giving C Q and η Q independently. As shown in Figure 3b,d, however, some distortions are present in the cross-section lineshapes, as a result of the non-uniform filtration through multiple-quantum coherences. Furthermore, the lineshapes for GaPO-34[pyridinium fluoride] also reveal additional broadening (and less characteristic lineshapes) probably as a result of disorder. NMR parameters extracted from the two-dimensional spectra, however, may be used as a starting point for an analytical fitting of the MAS spectra, where lineshapes are expected to be less distorted, but can exhibit significant overlap. The values of the NMR parameters extracted are shown in Table 2, with lineshapes simulated using those shown in Figure 3a. Note the larger estimated errors for the GaPO-34[pyridinium fluoride] as a result of the additional broadening observed. The 71Ga MAS and MQMAS spectra of the two templated GaPOs are very similar, again confirming the structural similarity between the two materials. It is possible to infer that the F− anions incorporated into the structure for charge balancing in GaPO-34[pyridinium fluoride] also bridge two Ga1 sites, resulting in one Ga(VI) and two Ga(IV) sites. The calculated NMR parameters for the three model structures of GaPO-34[1-methylimidazolium fluoride] described above (water on O1W, water only on O2W and no water) are in broad agreement with those extracted experimentally, with a 15053

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been suggested that this hydrates in air, a process that may result in the collapse of the framework structure.15 Figure 5

low chemical shift observed for the Ga(VI) species (i.e., Ga1) and a smaller shift difference (of between 3 and 12 ppm) between the two Ga(IV) species (see the Supporting Information for more detail). On the basis of these calculations, it is possible to assign the two Ga(IV) sites as Ga3 and Ga2 in order of decreasing isotropic shift. However, a large distribution of CQ values is observed for all three Ga sites between the different calculations, typically between 4 and 9 MHz (Supporting Information). As no such variation is observed experimentally, this would again suggest some dynamics of the disordered water molecule and an averaging of the NMR parameters. Recent work has demonstrated that the motion of template molecules and water within the pores of aluminophosphates can affect the 27Al NMR spectrum.43 The exact nature of the effect depends upon both the motional time scale and the magnitude of the quadrupolar interaction. Figure 4 shows 69Ga MAS NMR spectra of the two templated forms of GaPO-34, acquired at 20.0 T using 62.5

Figure 5. (a) 31P, (b) 71Ga, and (c) 69Ga MAS NMR spectra of GaPO34(calcined, dehydrated), acquired at B0 = 14.1 T. The sample was packed under dry conditions and sealed into a 2.5 mm rotor to ensure no rehydration. The MAS rate was 10 kHz for 31P, and 25 kHz for 71 Ga and 69Ga.

shows 31P, 69Ga, and 71Ga MAS NMR spectra of GaPO34(calcined, dehydrated), acquired at 14.1 T (with the rotor packed under dry conditions are described above). The 31P MAS spectrum shows a single resonance demonstrating the increased symmetry present in this material, compared to the as-made materials, with a similar shift to that observed for the P1 species in the templated forms, suggesting a similar local geometrical environment, i.e., an average ⟨P−O−Al⟩ bond angle of ∼144°. The 71Ga MAS spectrum shows a quadrupolar broadened resonance that can be fitted with a single component using the quadrupolar parameters given in Table 3. The isotropic chemical shift confirms the tetrahedral nature of the calcined framework.27 The NMR parameters can be confirmed from the 69Ga spectrum, where the quadrupolar coupling is ∼1.6 times larger, reflecting the difference in the quadrupole moments.40 Using a structural model derived from the room temperature X-ray crystal structure for the calcined, dehydrated material,26 it was also possible to calculate the NMR parameters for GaPO-34(calcined, dehydrated), and these are given in Table 3. These are in excellent agreement with experiment, confirming the accuracy of the GIPAW approach for the investigation of framework gallium phosphates. It is known that excessive heating of a microporous phosphate framework during calcination may cause a collapse of the material to a dense phosphate phase such as the quartz analogue of AlPO4, berlinite.44 However, 31P and 71Ga MAS NMR spectra of a sample of quartz-type GaPO4, shown in the

Figure 4. 69Ga MAS NMR spectra of (a) GaPO-34[1-methylimidazolium fluoride] and (b) GaPO-34[pyridinium fluoride], acquired at B0 = 20.0 T. Also shown in red are spectra simulated using the parameters given in Table 2. Spectra were acquired at 62.5 kHz MAS using a spin-echo pulse sequence.

kHz MAS. A spin-echo pulse sequence was used to ensure undistorted lineshapes were observed. Despite the high rotation rate employed, broad lineshapes are observed as a result of the increased quadrupolar moment of 69Ga (∼1.6 times larger than for 71Ga).40 Also shown in Figure 4 are lineshapes simulated using the NMR parameters extracted from the 71Ga spectra in Figure 3 (neglecting any distribution of parameters). The experimental and simulated lineshapes are in very good agreement, although the additional broadening observed in the experimental spectrum for GaPO-34[pyridinium fluoride] obscures some of the characteristic features of the quadrupolar lineshapes. The close agreement between the experimental and simulated spectra provides a confirmation of the Ga NMR parameters given in Table 2. However, the presence of disorder (and the concomitant lowering of the symmetry, locally at least) does make any more detailed analysis difficult, and possible dynamics/averaging of the quadrupolar lineshapes may also contribute to any differences observed. Calcination of either of the templated frameworks produces CHA-type GaPO-34(calcined, dehydrated). However, it has 15054

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distinctively different pattern after exposure to air. The 31P MAS spectrum shows a number of resonances, between −10 and −20 ppm, while the 71Ga NMR spectrum reveals broadened lineshapes, with six-, five- and four-coordinate Ga species present. The lack of features characteristic of secondorder quadrupolar broadening suggests disorder in the material. The 1H spectrum exhibits a single, broad resonance at ∼5.5 ppm, characteristic of H2O. Although it is reported that calcination of a number of other GaPOs can ultimately result in the collapse of their framework, this does not appear to be the case when GaPO-34(calcined, dehydrated) is simply exposed to air. The complicated spectra suggest a significant lowering of the symmetry, but cross-polarization experiments (not shown) reveal that all P species are equally close to 1H, with no significant changes in the intensity of the relevant peaks. This indicates that no direct P−OH linkages have formed, i.e., that the framework structure remains intact. It would appear that hydration forms a crystalline material, denoted GaPO-34(calcined, hydrated). Water appears to be present in the pores of the material and also connected to the framework itself, with a range of Ga coordination environments. This is similar to the behavior observed for AlPO-34,16,20 where hydration of the calcined framework results in the incorporation of up to 12 water molecules per unit cell; six attached to the framework, producing three four-coordinate, one five-coordinate and two six-coordinate Al species, and six free water molecules in the pore. Interestingly, the calcined, rehydrated GaPO material does appear stable at room temperature, with no significant changes detected in the 31P NMR spectrum of a sample left on the bench at room temperature for up to 5−6 months. Despite the stability of GaPO-34(calcined, hydrated) at room temperature, powder XRD indicates that amorphization of the framework occurs if the material is subsequently heated. This is consistent with observations by NMR: the 1H, 31P and 71Ga spectra of the reheated sample, i.e., GaPO-34(collapsed), shown in Figure 7 are distinctly different from the spectra

Table 3. Experimental and Calculated 31P, 71Ga and 69Ga NMR Parameters (Isotropic Chemical Shift, δiso, Quadrupolar Coupling Constant, CQ, Asymmetry, ηQ, and Quadrupolar Product, PQ) for GaPO-34 (calcined, dehydrated) assignment

δiso(ppm)

Experimental (Figure 5) P1 −19.5 (2) 71 Ga1 107.5 (5) 69 Ga1 107.5 (5) Calculated P1 −20.3 71 Ga1 114.3 69 Ga1a 114.3

CQ/MHz

ηQ

PQ/MHz

3.20 (5) 5.03 (3)

0.80 (5) 0.9 (1)

3.60 (5) 5.6 (2)

3.01 4.81

0.89 0.89

3.38 5.41

a

The calculated 69Ga quadrupolar parameters are determined from those calculated for 71Ga with the substitution of the correct quadrupole moment.

Supporting Information, confirm that this is not the case in the present study: although the spectra show a single resonance for both 31P and 71Ga, and confirm the structure as a tetrahedrally coordinated network, there is a significant (∼9 ppm) difference in the 31P shift compared to GaPO-34(calcined, dehydrated), and the 71Ga line shape exhibits a much greater CQ value. DFT calculations of the NMR parameters for this material are in excellent agreement with experimental data and were used to determine the reference 31P and 71Ga shielding for this work (see Supporting Information). If GaPO-34(calcined, dehydrated) is exposed to the atmosphere it appears to hydrate readily, with significant changes in the 71Ga, 31P and 1H NMR spectra, as shown in Figure 6. This is consistent with powder XRD that shows a

Figure 6. (a) 1H, (b) 31P and (c) 71Ga MAS NMR spectra of GaPO34(calcined, hydrated), acquired at B0 = 14.1 T (1H) and 20.0 T (31P and 71Ga). Ga coordination numbers are indicated by Roman numerals. The MAS rate was 60 kHz (1H), 20 kHz (31P), and 30 kHz (71Ga).

Figure 7. (a) 1H, (b) 31P, and (c) 71Ga MAS NMR spectra of GaPO34(collapsed), acquired at B0 = 14.1 T. The MAS rate was 60 kHz (1H), 20 kHz (31P), and 30 kHz (71Ga). 15055

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studentship to D.M.D. We also thank the Leverhulme Trust for funding (F/00/268/BJ). The UK 850 MHz solid-state NMR Facility used in this research was funded by EPSRC and BBSRC, as well as the University of Warwick including via part funding through Birmingham Science City Advanced Materials Projects 1 and 2 supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF). We thank EaStCHEM for computational support through the EaStCHEM Research Computing Facility. We are grateful to Dr. Ross Hatton for the use of his drybox, and to Drs. Luminita Duma and Teresa Kurkiewicz for help with the recording and analysis of the 71Ga MQMAS NMR spectra.

presented earlier. A very broad Gaussian-like resonance is observed in the 31P MAS spectrum, covering the region between 0 and −20 ppm, indicating the amorphous nature of the structure. The 71Ga spectrum also shows very broad resonances, although the majority of the Ga appears to be fourcoordinate, with lower intensity signal covering the five- and six-coordinate regions. It would appear that the collapse of the material is brought about by exposure to water at increased temperature; the calcined material left exposed to air at room temperature simply incorporates water into the structure, while heating in the absence of water of calcined GaPO-34 to 460 K in previous work26 also left the framework intact.

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CONCLUSIONS Solid-state NMR spectroscopy has provided a detailed structural understanding of microporous gallium phosphate materials, making use of the less-studied 71Ga and 69Ga nuclei, enabled by the use of high field which, in particular, provides access to 71Ga MQMAS spectra for unambiguous assignment of crystallographic sites. Thus, we have been able to confirm the structural similarity of GaPO-34 prepared in the presence of 1methylimizadole and pyridine, in the absence of a crystal structure for the latter; this includes verification of the presence of fluoride in a bridging position, resulting in a single sixcoordination gallium site and the inclusion of (disordered) water within the microporous framework. The microporous CHA-type structure, formed by calcination, shows water uptake to give a rehydrated phase analogous to that found for the corresponding aluminium phosphate phase. However, unlike the isomorphous AlPO phase, the gallium phosphate is inherently unstable toward water upon heating, resulting in a poorly ordered phase that NMR shows to contain a range of both P and Ga environments. There is potential for future work here studying the rehydration of the calcined material and the possibilities for low temperature dehydration. Furthermore, we hope to investigate the exact structure of rehydrated GaPO-34 and the number and position of water molecules it contains using DFT calculations and a comparison to the related AlPO analogue. This will add to the new insight obtained in the present work on the instability of calcined microporous gallium phosphate materials.

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ASSOCIATED CONTENT

S Supporting Information *

Further information on XRD and TGA of as-made GaPO-34, in situ XRD of calcination of GaPO-34, further information on 1 H/13C/19F MAS NMR of as-made GaPO-34, DFT calculations for GaPO-34[1-methylimidazolium fluoride] and 31 71 P/ Ga NMR and DFT calculation for quartz-type GaPO4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.I.W.); sema@st-andrews. ac.uk (S.E.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the EPSRC for funding some of this work (EP/C156591 and EP/E041825/1), and for the award of a 15056

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