Structure refinement of (NH4)3Al2(PO4)3 prepared by ionothermal synthesis in phospho­nium based ionic liquids – a redetermination

Nicholas, C.P.; Mowat, J.P.S.; Broach, R.W.

doi:10.1107/S2056989019015330

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Volume 75| Part 12| December 2019| Pages 1897-1901

https://doi.org/10.1107/S2056989019015330

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Structure refinement of (NH₄)₃Al₂(PO₄)₃ prepared by ionothermal synthesis in phosphonium based ionic liquids – a redetermination

Christopher P. Nicholas,^a ^* John P.S. Mowat ^b and Robert W. Broach ^b

^aExploratory Materials and Catalysis Research, Honeywell UOP, Des Plaines IL 60201, USA, and ^bAdvanced Characterization, Honeywell UOP, Des Plaines IL 60201, USA
^*Correspondence e-mail: christopher.nicholas@uop.com

Edited by M. Weil, Vienna University of Technology, Austria (Received 17 October 2019; accepted 13 November 2019; online 19 November 2019)

After crystallization during ionothermal syntheses in phosphonium-containing ionic liquids, the structure of (NH₄)₃Al₂(PO₄)₃ [triammonium dialuminum tris(phosphate)] was refined on the basis of powder X-ray diffraction data from a synchrotron source. (NH₄)₃Al₂(PO₄)₃ is a member of the structural family with formula A₃Al₂(PO₄)₃, where A is a group 1 element, and of which the NH₄, K, and Rb forms were previously known. The NH₄ form is isostructural with the K form, and was previously solved from single-crystal X-ray data when the material (SIZ-2) crystallized from a choline-containing eutectic mixture [Cooper et al. (2004 ). Nature, 430, 1012–1017]. Our independent refinement incorporates NH₄ groups and shows that these NH₄ groups are hydrogen bonded to framework O atoms present in rings containing 12 T sites in a channel along the c-axis direction. We describe structural details of (NH₄)₃Al₂(PO₄)₃ and discuss differences with respect to isostructural forms.

Keywords: powder diffraction; aluminophosphate; ionothermal synthesis; ethyltributylphosphonium diethylphosphate; Cyphos 169; redetermination.

CCDC references: 1965580; 1965580

1. Chemical context

Following the discovery of the microporous AlPO₄-n series of materials (Wilson et al., 1982 ), many efforts have been directed toward the synthesis of novel phases utilizing traditional hydrothermal (Wilson, 2007 ; Yu & Xu, 2006 ) and solvothermal syntheses (Das et al., 2012 ). Recently, ionothermal synthesis has been added to the stable of synthetic methods. Ionothermal synthesis is an extension of the solvothermal method of synthesis using an ionic liquid as the solvent (replacing, for example, water or ethylene glycol) where a portion of the organic structure-directing agent from a typical zeolite synthesis is derived from the ionic liquid (Morris, 2009 ). Many new materials have been synthesized by ionothermal synthesis, with new aluminophosphate materials among the most common (Parnham & Morris, 2007 ; Xing et al., 2008 , 2010 ).

An important issue in ionothermal synthesis is control of water (Ma et al., 2008 ). Excess water often leads to synthesis of dense AlPO₄ phases such as the one with a tridymite-type of structure, which we observed as well during syntheses utilizing 85%_wt H₃PO₄. To control the level of water in the synthesis, thereby allowing easy recycling of the ionic liquid solvent and to intentionally prepare ammonium aluminophosphates, we used (NH₄)₂HPO₄ as the phosphorous source in the synthesis. Ammonium is a good structure-directing agent for aluminophosphate frameworks; multiple ammonium aluminum phosphates are known (Byrne et al., 2009 ; Vaughan et al., 2012 ). In the current phosphonium-based ionothermal synthesis, the presence of an ammonium cation in the relative absence of water provokes the formation of a 2/3 Al/P framework with the formula (NH₄)₃Al₂(PO₄)₃. A structurally unrelated compound with the formula (NH₄)₃Al₂(PO₄)₃ has previously been synthesized via a solvothermal approach (Medina et al., 2004 ).

The aluminophosphate database at Jilin (Li et al., 2019 ) currently lists 21 framework structures with a 2:3 ratio of Al:P. A framework with sub-stoichiometric Al content is by necessity anionically charged and must be cation-balanced, so most of the known frameworks, such as UT-3, UT-4 and UT-5 (Oliver et al., 1996 ) are charge-balanced by organoammonium cations. Low-water-content syntheses clearly favor 2:3 compounds as most of the known materials are synthesized from low-water-content preparations.

2. Structural commentary and survey of related compounds

The (NH₄)₃Al₂(PO₄)₃ phase synthesized here is related to the series of A₃Al₂(PO₄)₃ materials synthesized via high-temperature solid-state methods (Devi & Vidyasagar, 2000 ) with varying monocations on the A site. Additionally, an independent synthesis previously yielded a (NH₄)₃Al₂(PO₄)₃ material called SIZ-2 whose structure was solved and refined from single-crystal data (Cooper et al., 2004) and possesses nearly the same structure as refined from the current powder data of (NH₄)₃Al₂(PO₄)₃. A polyhedral representation of the crystal structure of (NH₄)₃Al₂(PO₄)₃ is shown in Fig. 1. SIZ-2 crystallized from a choline chloride/urea eutectic mixture where decomposition of urea was proposed to be the source of ammonium in the structure. The refinement of Cooper et al. (2004) included the ammonium N atoms, but made no attempt to find or model the corresponding H atoms.

Figure 1
Polyhedral representation of (NH₄)₃Al₂(PO₄)₃, showing the overall connectivity and ion channels in the crystal structure. Al is in the center of blue tetrahedra, P in gray tetrahedra, and N is represented by blue spheres.

Devi & Vidyasagar (2000) utilized Li, Na, K, Rb, Cs, and Tl as the A cation and succeeded in crystallizing compounds with A = Na, K, Rb, Tl. The thallium derivative yielded a completely different structure with trigonal–bipyramidal coordination of Al. The A = Na structure was not solved, but apparently crystallizes in an unrelated orthorhombic space-group type from that observed for A = K, Rb in their work, and for A = NH₄ here. Devi & Vidyasagar (2000) utilized (NH₄)₂HPO₄ as the phosphate source in their high-temperature preparations of A₃Al₂(PO₄)₃, but did not obtain (NH₄)₃Al₂(PO₄)₃, likely due to the volatility of NH₃ at high temperatures.

As in the K and Rb forms of the A₃Al₂(PO₄)₃ series, aluminum and phosphorus are both tetrahedrally coordinated and connected through corners throughout the (NH₄)₃Al₂(PO₄)₃ structure. The NH₄⁺ cations reside in a channel along the c-axis direction made from a 12 T-site ring of alternating AlO₄ and PO₄ tetrahedra (Fig. 2). The NH₄⁺ groups occupy the available space and none of the ionic liquid solvent is present within the pores of the (NH₄)₃Al₂(PO₄)₃ framework. Without the NH₄⁺ groups, the structure would have 24% void volume. The framework is triply negatively charged and charge-balanced by the ammonium cations. Three of the six phosphate groups in the ring protrude inward such that the closest contact distance between the H atom of an ammonium group and the O atom of the nearest phosphate is between 1.83 and 1.87 Å, indicating significant hydrogen-bonding interactions. The full range of H⋯O hydrogen-bond lengths is between 1.83 and 1.97 Å (Table 1).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H11⋯O9ⁱ	0.95 (4)	2.39 (4)	3.250 (10)	151 (3)
N1—H11⋯O11	0.95 (4)	2.35 (5)	3.163 (8)	143 (3)
N1—H12⋯O1ⁱⁱ	0.95 (4)	1.88 (4)	2.791 (10)	159 (3)
N1—H13⋯O5	0.95 (4)	2.14 (4)	2.934 (10)	141 (3)
N1—H14⋯O9ⁱⁱⁱ	0.95 (4)	1.83 (4)	2.776 (9)	173 (3)
N2—H21⋯O5^iv	0.95 (4)	1.96 (4)	2.896 (10)	170 (4)
N2—H22⋯O8^v	0.95 (4)	2.31 (3)	3.216 (10)	158 (4)
N2—H23⋯O9^iv	0.95 (4)	1.89 (5)	2.738 (9)	148 (4)
N2—H24⋯O11^vi	0.96 (4)	1.86 (4)	2.818 (9)	174 (5)
N3—H31⋯O5^vi	0.96 (4)	1.97 (4)	2.821 (9)	147 (3)
N3—H32⋯O11^vi	0.952 (15)	1.85 (2)	2.728 (8)	153 (4)
N3—H33⋯O1^v	0.95 (3)	1.90 (3)	2.823 (9)	164 (4)
N3—H34⋯O12	0.96 (3)	2.37 (4)	2.925 (8)	117 (4)
Symmetry codes: (i) x, y, z+1; (ii) $[x-{\script{1\over 2}}, -y+{\script{1\over 2}}, z+1]$ ; (iii) $[-x, -y+1, z+{\script{1\over 2}}]$ ; (iv) x+1, y, z; (v) $[x+{\script{1\over 2}}, -y+{\script{1\over 2}}, z]$ ; (vi) $[-x+1, -y+1, z-{\script{1\over 2}}]$ .

Figure 2
Ball and stick representation of (NH₄)₃Al₂(PO₄)₃ showing the 12-membered ring with three phosphate groups protruding inward with close contact to ammonium cations.

Crystallizing in space-group type Pna2₁, (NH₄)₃Al₂(PO₄)₃ is isostructural to, but with a slightly larger unit cell than the K form synthesized by Devi & Vidyasagar (2000). Lattice expansion of ∼0.1–0.2 Å occurs along each of the three axes, leading to an overall 6.6% increase in cell volume from 1245 to 1327 Å³. A lattice expansion is no surprise as the ionic radius of NH₄⁺ is between 1.4 and 1.67 Å depending on the coordination number (Sidey, 2016 ). This is slightly larger than the reported 1.37 to 1.55 Å range for K⁺ (Shannon, 1976 ). Much of the relative lattice expansion for (NH₄)₃Al₂(PO₄)₃ occurs along the a and c axes. Tilting of tetrahedra accounts for a significantly smaller expansion of the long b axis. In addition, an isostructural K/As form is also known where two-thirds of the phosphate groups have been replaced by arsenate (Boughzala et al., 1997 ). Arsenate included on the phosphate sites increases the cell volume to 1307 Å³, just smaller than that recorded here for (NH₄)₃Al₂(PO₄)₃. The pure arsenate form K₃Al₂(AsO₄)₃ was reported by Stöger & Weil (2012 ), which has a cell volume of 1328 Å³, essentially equivalent to that here.

An overlay plot of atomic positions of (NH₄)₃Al₂(PO₄)₃ (red) versus SIZ-2 (blue) shows that although the independent refinements of the two (NH₄)₃Al₂(PO₄)₃ materials were performed via different methods at different temperatures, most atom positions are similar, with no more than about 0.004 fractional position differences along the a or c axes (for these axes, about 0.03–0.04 Å, Fig. 3). One area stands out in the A₃Al₂(PO₄)₃ series. Fig. 4 shows the key area surrounding O11 where the largest position movement is observed in the two independent refinements of (NH₄)₃Al₂(PO₄)₃.

Figure 3
Atomic position overlay plot of SIZ-2 (blue) and (NH₄)₃Al₂(PO₄)₃ (red) showing that most atom positions are within 0.03 Å of each other. The most significant difference is in the O11 position.

Figure 4
Ball and stick representation of the key area surrounding O11 where the largest position movement takes place in the two independent refinements of (NH₄)₃Al₂(PO₄)₃.

The P3—O11 bond is always among the shortest P—O bonds found in the crystal structure, here at 1.487 (5) Å. Two clusters of P—O bond lengths occur; one at about 1.49 Å and another at 1.55 Å. These distances are relatively typical for aluminophosphates (Richardson & Vogt, 1992 ; Wei et al., 2012 ). Each of the O atoms protruding into the pore possess short P—O bonds and hydrogen bonds to two ammonium ions (Table 1). In particular, N2, N3, O11, and P3 are effectively in a plane so that with the hydrogen bonding present in our refined model from N3 and N2 through the attached H atoms to O11, O11 moves closer to P3 while N2 and N3 move slightly further away versus the positions in the SIZ-2 refinement. Table 2 shows respective O—A and P—O distances for the four isostructural A₃Al₂(PO₄)₃ compounds. Other bond lengths and angles are otherwise relatively unremarkable versus other members of the structural class although we note that As/P—O distances are longer than P—O as expected.

Table 2
Key atomic distances (Å) in related A₃Al₂(PO₄)₃ structures

Compound	O11—A1	O11—A2	O11—A3	O11—P3	Reference
(NH₄)₃Al₂(PO₄)₃	3.162	2.818	2.727	1.487	This work
SIZ-2 (A = NH₄)	3.090	2.834	2.688	1.496	Cooper et al. (2004)
K₃Al₂(PO₄)₃	2.754	2.824	2.722	1.487	Devi & Vidyasagar (2000)
K₃Al₂(AsO₄)₂(PO₄)	3.025	2.743	2.621	1.673	Boughzala et al. (1997)
For each of the compounds, the atomic numbering scheme of the current (NH₄)₃Al₂(PO₄)₃ refinement has been utilized. For the first two compounds, A = NH₄, while for the second two, A = K. For the As-containing compound, the P3 site is reported to have the highest occupancy of As at 0.86.

Rb₃Al₂(PO₄)₃ is structurally related to the NH₄ and K forms, but crystallizes in a higher symmetry space-group type (Cmc2₁), accompanied with higher overall coordination numbers around Rb⁺ and a mirror plane perpendicular to a. The ionic radius of Rb⁺ is similar to that of NH₄⁺, reported as 1.52–1.63 Å (Shannon, 1976). Lithium and cesium forms of the series have not yet been synthesized, likely because of the relatively small and large, respectively, ionic radii versus those of the fitting A cations. Our initial attempts at ion-exchange of (NH₄)₃Al₂(PO₄)₃ with LiNO₃ or CsNO₃ in aqueous solution to form the Li or Cs form failed, with partial structural degradation and no ion-exchange observed.

3. Synthesis and crystallization

In a typical preparation, 1.65 g (NH₄)₂HPO₄ was added to a 125 ml polytetrafluoroethene (PTFE) lined autoclave containing 24.02 g of ethyl tri(butyl)phosphonium diethyl phosphate. The mixture was stirred at room temperature for 2 min. To this mixture were added 0.49 g of Al(OH)₃, and the contents were stirred at room temperature for 2 min. The contents of the autoclave were digested at 423 K for 24 h prior to isolating the product by filtration. Analytical results show this material has a molar ratio Al:P of 0.725. The X-ray diffraction pattern is shown in Fig. 5. Scanning electron microscopy (SEM) revealed agglomerated stacks of irregularly shaped blocky crystals of from 500 nm to 2–4 µm in length (Fig. 6). Calcination of (NH₄)₃Al₂(PO₄)₃ at temperatures of 773 K or higher causes the formation of an AlPO₄ phase with a tridymite-type structure. Ethyl tributyl phosphonium diethyl phosphate (Cyphos 169) was acquired from Cytec; aluminum hydroxide was acquired from Pfaltz and Bauer.

Figure 5
XRD pattern (λ = 0.373811 Å) of (NH₄)₃Al₂(PO₄)₃ synthesized ionothermally in ethyl tributylphosphonium diethylphosphate and Rietveld residuals following structure refinement. Part A shows the fit to the overall pattern, and inset B shows the fit to high-angle regions.

Figure 6
SEM image of polycrystalline (NH₄)₃Al₂(PO₄)₃ synthesized ionothermally in ethyl tributylphosphonium diethyl phosphate and used for structure refinement.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. Following initial survey scans on in-house Cu source powder XRD instruments, final data were acquired from samples packed in thin glass capillaries on 11-BM at the Advanced Photon Source at Argonne National Laboratory. Starting atomic positions for the refinement were adapted from the literature examples. Starting positions for the ammonium cations were located in a difference-Fourier map and subsequently refined using GSAS (Larson & Von Dreele, 2000 ) as tetrahedral rigid bodies with N—H bond lengths held at 0.9526 Å and tetrahedrality enforced, leading to H⋯H distances of 1.5556 Å. No soft constraints were applied to the framework positions. All atoms in the structure were refined with a common U_iso parameter. Two low-intensity reflections in the region 4.00–4.22°/2θ were excluded from the refinement as belonging to an impurity phase after assessment of multiple (NH₄)₃Al₂(PO₄)₃ batches. Refinement trials with a higher symmetry model (space-group type Cmc2₁) were attempted but showed poor agreement with the experimental data, with R_wp > 0.16.

Table 3
Experimental details

Crystal data
Chemical formula	(NH₄)₃Al₂(PO₄)₃
M_r	392.99
Crystal system, space group	Orthorhombic, Pna2₁
Temperature (K)	100
a, b, c (Å)	8.98884 (6), 17.01605 (10), 8.67653 (5)
V (Å³)	1327.11 (2)
Z	4
Radiation type	Synchrotron, λ = 0.373811 Å
μ (mm⁻¹)	0.12
Specimen shape, size (mm)	Cylinder, 0.70 × 0.70

Data collection
Diffractometer	11BM synchrotron
Specimen mounting	Capillary
Data collection mode	Transmission
Scan method	Continuous
2θ values (°)	2θ_min = 2.45, 2θ_max = 20, 2θ_step = 0.001

Refinement
R factors and goodness of fit	R_p = 0.082, R_wp = 0.101, R_exp = 0.060, R(F²) = 0.03552, χ² = 2.856
No. of parameters	95
No. of restraints	20
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
(Δ/σ)_max	0.17
Computer programs: local program at 11BM, GSAS (Larson & Von Dreele, 2000), coordinates from an isotypic structure, CrystalMaker (Palmer, 2005 ), publCIF (Westrip, 2010 ).

Supporting information

CCDC references: 1965580; 1965580

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2056989019015330/wm5528sup1.cif

checkCIF report

Computing details top

Data collection: local program at 11BM; data reduction: GSAS (Larson & Von Dreele, 2000); program(s) used to solve structure: coordinates from an isotypic structure; program(s) used to refine structure: GSAS (Larson & Von Dreele, 2000); molecular graphics: CrystalMaker (Palmer, 2005); software used to prepare material for publication: publCIF (Westrip, 2010).

Triammonium dialuminium tris(phosphate) top

Crystal data top

(NH₄)₃Al₂(PO₄)₃	V = 1327.11 (2) Å³
M_r = 392.99	Z = 4
Orthorhombic, Pna2₁	Synchrotron radiation, λ = 0.373811 Å
Hall symbol: P 2c -2n	µ = 0.12 mm⁻¹
a = 8.98884 (6) Å	T = 100 K
b = 17.01605 (10) Å	white
c = 8.67653 (5) Å	cylinder, 0.70 × 0.70 mm

Data collection top

11BM_synchrotron diffractometer	Scan method: continuous
Specimen mounting: capillary	2θ_min = 2.45°, 2θ_max = 20°, 2θ_step = 0.001°
Data collection mode: transmission

Refinement top

Least-squares matrix: full	Profile function: CW Profile function number 4 with 18 terms Pseudovoigt profile coefficients as parameterized in P. Thompson, D.E. Cox & J.B. Hastings (1987). J. Appl. Cryst.,20,79-83. Asymmetry correction of L.W. Finger, D.E. Cox & A. P. Jephcoat (1994). J. Appl. Cryst.,27,892-900. Microstrain broadening by P.W. Stephens, (1999). J. Appl. Cryst.,32,281-289. #1(GU) = 1.163 #2(GV) = -0.126 #3(GW) = 0.063 #4(GP) = 0.000 #5(LX) = 0.143 #6(ptec) = -0.01 #7(trns) = 0.00 #8(shft) = 0.0000 #9(sfec) = 0.00 #10(S/L) = 0.0011 #11(H/L) = 0.0011 #12(eta) = 0.7694 #13(S400 ) = 1.1E-01 #14(S040 ) = 2.8E-03 #15(S004 ) = 1.0E-01 #16(S220 ) = 1.3E-02 #17(S202 ) = -9.0E-03 #18(S022 ) = 6.9E-03 Peak tails are ignored where the intensity is below 0.0010 times the peak Aniso. broadening axis 1.0 0.0 0.0
R_p = 0.082	95 parameters
R_wp = 0.101	20 restraints
R_exp = 0.060	H atoms treated by a mixture of independent and constrained refinement
R(F²) = 0.03552	(Δ/σ)_max = 0.17
49495 data points	Background function: GSAS Background function number 1 with 11 terms. Shifted Chebyshev function of 1st kind 1: 111.086 2: -42.2923 3: 17.4011 4: -1.76183 5: -7.25556 6: 2.97020 7: 2.60010 8: -3.84672 9: 5.91765 10: -3.48127 11: 1.19076

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
P1	0.1689 (3)	0.21458 (16)	−0.0138 (5)	0.0086 (3)*
P2	0.3100 (3)	0.29952 (16)	0.4927 (5)	0.0086 (3)*
P3	0.2594 (3)	0.5033 (2)	0.0861	0.0086 (3)*
Al1	0.3631 (4)	0.33375 (18)	0.1361 (5)	0.0086 (3)*
Al2	0.1295 (4)	0.17048 (19)	0.6462 (5)	0.0086 (3)*
O1	0.2886 (7)	0.1552 (3)	0.0117 (8)	0.0086 (3)*
O2	0.0384 (8)	0.2046 (3)	0.1014 (8)	0.0086 (3)*
O3	0.2289 (7)	0.3006 (3)	0.0100 (8)	0.0086 (3)*
O4	0.1017 (7)	0.2138 (4)	0.8252 (8)	0.0086 (3)*
O5	0.1953 (7)	0.3602 (3)	0.5464 (8)	0.0086 (3)*
O6	0.4625 (7)	0.3234 (3)	0.5425 (8)	0.0086 (3)*
O7	0.2776 (8)	0.2165 (3)	0.5518 (7)	0.0086 (3)*
O8	0.3087 (7)	0.2962 (4)	0.3126 (8)	0.0086 (3)*
O9	0.1072 (5)	0.4838 (3)	0.1404 (8)	0.0086 (3)*
O10	0.3228 (7)	0.5700 (4)	0.1747 (7)	0.0086 (3)*
O11	0.2604 (6)	0.5218 (3)	0.9186 (6)	0.0086 (3)*
O12	0.3716 (7)	0.4348 (3)	0.1188 (9)	0.0086 (3)*
N1	0.0164 (8)	0.3978 (4)	0.8194 (9)	0.0086 (3)*
H11	0.076 (5)	0.425 (3)	0.893 (4)	0.0086 (3)*
H12	−0.058 (4)	0.368 (3)	0.871 (5)	0.0086 (3)*
H13	0.077 (5)	0.364 (2)	0.760 (5)	0.0086 (3)*
H14	−0.030 (5)	0.435 (2)	0.753 (5)	0.0086 (3)*
N2	0.9630 (8)	0.3743 (4)	0.3168 (9)	0.0086 (3)*
H21	1.036 (4)	0.363 (3)	0.393 (5)	0.0086 (3)*
H22	0.915 (6)	0.3268 (16)	0.287 (6)	0.0086 (3)*
H23	1.009 (5)	0.397 (3)	0.229 (4)	0.0086 (3)*
H24	0.891 (5)	0.410 (3)	0.358 (6)	0.0086 (3)*
N3	0.6786 (6)	0.4917 (4)	0.1118 (7)	0.0086 (3)*
H31	0.755 (4)	0.530 (2)	0.095 (6)	0.0086 (3)*
H32	0.668 (5)	0.483 (3)	0.2196 (14)	0.0086 (3)*
H33	0.704 (5)	0.4438 (18)	0.062 (5)	0.0086 (3)*
H34	0.587 (3)	0.511 (3)	0.071 (5)	0.0086 (3)*

Geometric parameters (Å, º) top

P1—Al1	2.975 (5)	O3—P1	1.573 (6)
P1—Al2ⁱ	3.064 (5)	O3—Al1	1.724 (7)
P1—O1	1.493 (6)	O4—P1^iv	1.522 (7)
P1—O2	1.550 (7)	O4—Al2	1.737 (7)
P1—O3	1.573 (6)	O5—P2	1.532 (6)
P1—O4ⁱ	1.522 (7)	O5—H21^viii	1.953 (13)
P2—Al2	3.038 (4)	O5—H31^ix	1.97 (3)
P2—O5	1.532 (6)	O6—P2	1.493 (7)
P2—O6	1.493 (7)	O6—Al2ⁱⁱⁱ	1.753 (7)
P2—O7	1.532 (6)	O7—P2	1.532 (6)
P2—O8	1.563 (6)	O7—Al2	1.748 (7)
P3—Al1	3.062 (5)	O8—P2	1.563 (6)
P3—Al2ⁱⁱ	3.060 (5)	O8—Al1	1.730 (7)
P3—O9	1.484 (5)	O9—P3	1.484 (5)
P3—O10	1.485 (6)	O9—H14^x	1.828 (10)
P3—O11ⁱ	1.487 (5)	O9—H23^viii	1.88 (3)
P3—O12	1.567 (6)	O10—P3	1.485 (6)
Al1—P1	2.975 (5)	O10—Al2ⁱⁱ	1.780 (7)
Al1—P3	3.062 (5)	O11—P3^iv	1.487 (5)
Al1—O2ⁱⁱⁱ	1.732 (7)	O11—H24^ix	1.868 (10)
Al1—O3	1.724 (7)	O11—H32^ix	1.84 (2)
Al1—O8	1.730 (7)	O12—P3	1.567 (6)
Al1—O12	1.727 (6)	O12—Al1	1.727 (6)
Al2—P1^iv	3.064 (5)	N1—H11	0.9526 (1)
Al2—P2	3.038 (4)	N1—H12	0.9526 (1)
Al2—P3^v	3.060 (5)	N1—H13	0.9526 (1)
Al2—O4	1.737 (7)	N1—H14	0.9526
Al2—O6^vi	1.753 (7)	N2—H21	0.9526 (1)
Al2—O7	1.748 (7)	N2—H22	0.9526 (1)
Al2—O10^v	1.780 (7)	N2—H23	0.9526 (1)
O1—P1	1.493 (6)	N2—H24	0.9526 (1)
O1—H12^vii	1.88 (2)	N3—H31	0.9526 (1)
O1—H33^vi	1.897 (15)	N3—H32	0.9526 (1)
O2—P1	1.550 (7)	N3—H33	0.9526 (1)
O2—Al1^vi	1.732 (7)	N3—H34	0.9526 (1)

O1—P1—O2	112.1 (4)	O6^vi—Al2—O10^v	109.6 (3)
O1—P1—O3	111.3 (4)	O7—Al2—O10^v	108.1 (3)
O1—P1—O4ⁱ	114.6 (4)	P1—O2—Al1^vi	147.3 (5)
O2—P1—O3	106.1 (4)	P1—O3—Al1	128.9 (4)
O2—P1—O4ⁱ	106.9 (4)	P1^iv—O4—Al2	140.1 (4)
O3—P1—O4ⁱ	105.4 (4)	P2—O6—Al2ⁱⁱⁱ	161.9 (5)
O5—P2—O6	110.3 (4)	P2—O7—Al2	135.6 (5)
O5—P2—O7	113.1 (4)	P2—O8—Al1	150.4 (4)
O5—P2—O8	108.9 (4)	P3—O10—Al2ⁱⁱ	139.0 (5)
O6—P2—O7	109.2 (4)	P3—O12—Al1	136.7 (5)
O6—P2—O8	107.8 (4)	H11—N1—H12	109.4713 (9)
O7—P2—O8	107.4 (4)	H11—N1—H13	109.4719 (6)
O9—P3—O10	111.1 (4)	H11—N1—H14	109.4706
O9—P3—O11ⁱ	111.3 (4)	H12—N1—H13	109.4715 (1)
O9—P3—O12	111.7 (4)	H12—N1—H14	109.4704
O10—P3—O11ⁱ	110.0 (4)	H13—N1—H14	109.4715 (5)
O10—P3—O12	103.2 (3)	H21—N2—H22	109.4716 (7)
O11ⁱ—P3—O12	109.3 (4)	H21—N2—H23	109.4710 (4)
O2ⁱⁱⁱ—Al1—O3	113.8 (4)	H21—N2—H24	109.4716 (4)
O2ⁱⁱⁱ—Al1—O8	105.8 (4)	H22—N2—H23	109.4707
O2ⁱⁱⁱ—Al1—O12	108.6 (4)	H22—N2—H24	109.4717 (8)
O3—Al1—O8	104.1 (3)	H23—N2—H24	109.4706 (7)
O3—Al1—O12	107.6 (4)	H31—N3—H32	109.4709
O8—Al1—O12	117.2 (4)	H31—N3—H33	109.4710 (5)
O4—Al2—O6^vi	108.1 (3)	H31—N3—H34	109.4709 (1)
O4—Al2—O7	109.8 (3)	H32—N3—H33	109.4717
O4—Al2—O10^v	108.5 (4)	H32—N3—H34	109.472
O6^vi—Al2—O7	112.6 (4)	H33—N3—H34	109.4709

Symmetry codes: (i) x, y, z−1; (ii) −x+1/2, y+1/2, z−1/2; (iii) x+1/2, −y+1/2, z; (iv) x, y, z+1; (v) −x+1/2, y−1/2, z+1/2; (vi) x−1/2, −y+1/2, z; (vii) x+1/2, −y+1/2, z−1; (viii) x−1, y, z; (ix) −x+1, −y+1, z+1/2; (x) −x, −y+1, z−1/2.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H11···O9^iv	0.95 (4)	2.39 (4)	3.250 (10)	151 (3)
N1—H11···O11	0.95 (4)	2.35 (5)	3.163 (8)	143 (3)
N1—H12···O1^xi	0.95 (4)	1.88 (4)	2.791 (10)	159 (3)
N1—H13···O5	0.95 (4)	2.14 (4)	2.934 (10)	141 (3)
N1—H14···O9^xii	0.95 (4)	1.83 (4)	2.776 (9)	173 (3)
N2—H21···O5^xiii	0.95 (4)	1.96 (4)	2.896 (10)	170 (4)
N2—H22···O8ⁱⁱⁱ	0.95 (4)	2.31 (3)	3.216 (10)	158 (4)
N2—H23···O9^xiii	0.95 (4)	1.89 (5)	2.738 (9)	148 (4)
N2—H24···O11^xiv	0.96 (4)	1.86 (4)	2.818 (9)	174 (5)
N3—H31···O5^xiv	0.96 (4)	1.97 (4)	2.821 (9)	147 (3)
N3—H32···O11^xiv	0.952 (15)	1.85 (2)	2.728 (8)	153 (4)
N3—H33···O1ⁱⁱⁱ	0.95 (3)	1.90 (3)	2.823 (9)	164 (4)
N3—H34···O12	0.96 (3)	2.37 (4)	2.925 (8)	117 (4)

Symmetry codes: (iii) x+1/2, −y+1/2, z; (iv) x, y, z+1; (xi) x−1/2, −y+1/2, z+1; (xii) −x, −y+1, z+1/2; (xiii) x+1, y, z; (xiv) −x+1, −y+1, z−1/2.

Key atomic distances (Å) in related A₃Al₂(PO₄)₃ structures top

Compound	O11—A1	O11—A2	O11—A3	O11—P3	Reference
(NH₄)₃Al₂(PO₄)₃	3.162	2.818	2.727	1.487	This work
SIZ-2 (A = NH₄)	3.090	2.834	2.688	1.496	Cooper et al. (2004)
K₃Al₂(PO₄)₃	2.754	2.824	2.722	1.487	Devi & Vidyasagar (2000)
K₃Al₂(AsO₄)₂(PO₄)	3.025	2.743	2.621	1.673	Boughzala et al. (1997)

For each of the compounds, the atomic numbering scheme of the current (NH₄)₃Al₂(PO₄)₃ refinement has been utilized. For the first two compounds, A = NH₄, while for the second two, A = K. For the As-containing compound, the P3 site is reported to have the highest occupancy of As at 0.86.

Acknowledgements

We thank UOP for funding and allowing this publication. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02–06CH11357. We acknowledge E. Fulmer, C. L. Nicholas and S. T. Wilson for helpful discussions and J. F. Kotek and A. Stolarski for the ICP and SEM results.

Funding information

Funding for this research was provided by: UOP LLC.

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