A new magnesium-containing aluminophosphate with a zeolite-like structure

Abstract

A novel Mg-containing aluminophosphate |C₄H₁₂N₂|[Mg₂Al₆(PO₄)₈(H₂O)₄] (JU96) has been hydrothermally synthesized using piperazine as a structure-directing agent. Its framework, constructed by the connection of AlO₄/MgO₄(H₂O)₂ polyhedra and PO₄ tetrahedra, exhibits a new 4-connected zeolite-like topology with intersecting 8-ring channels. Such a framework contains unique [4⁴·6⁶·8⁶] cages that are occupied by diprotonated piperazine cations. Study shows that the presence of Mg atoms and the pH value of the reaction gel have a vital effect on the formation of JU96.

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Introduction

Aluminophosphate molecular sieves, designated as AlPO₄-n, are a class of three-dimensional (3D) microporous crystalline materials, whose structures are constructed by the strict alternation of AlO₄ and PO₄ tetrahedra forming a neutral open framework.^1,2 Since they were first discovered in 1982,³ more than 60 different zeotype AlPO-based molecular sieves and a large variety of open-framework aluminophosphates have been synthesized under hydrothermal or solvothermal conditions by using organic amines or quaternary ammonium cations as structure-directing agents (SDAs).^4–7 Such materials have important applications in the fields of catalysis, adsorption, and ion exchange.^5,8

AlPO₄-n molecular sieves not only exhibit diverse pore structures, but also have rich framework compositions. The Al atoms in the framework can be partially replaced by other metal elements to form heteroatom-containing aluminophosphate molecular sieves (denoted as MAPO, M = a metal heteroatom other than Al).⁹ Up to now, more than 13 kinds of metal heteroatoms can be incorporated into the frameworks of AlPO₄-n molecular sieves, giving rise to various MAPO molecular sieves with about 50 different zeolite topologies.^10,11 The presence of metal heteroatoms offers Brönsted acid sites to MAPO molecular sieves, which affords such materials excellent catalytic properties. For example, some MAPOs including MAPO-18, MAPO-34 and MAPO-5 as single-site solid catalysts are widely applied in selective oxidation and MTO reactions.^12,13 On the other hand, the metal heteroatoms also play a stabilizing role on the resulting open framework, which provides a great opportunity to synthesize novel zeotype structures.^7,10,14 More than 20 kinds of MAPOs with distinct zeolite topology have been discovered, which have not been found in pure AlPO systems yet, for example APC-1 (ACO, M = Co, Fe),^15,16 MAPO-46 (AFS, M = Co, Mg, Mn, Ni, Zn),¹⁷ UCSB-6 (SBS, M = Co, Zn, Mn, Mg), UCSB-8 (SBE, M = Co, Zn, Mn, Mg), UCSB-10 (SBT, M = Co, Zn, Mn),¹⁸ MAPO-CJ40 (JRY, M = Co, Zn, Fe),¹⁹ MAPO-CJ69 (JSN, M = Co, Zn),²⁰ MAPO-CJ62 (JSW, M = Co, Zn)²¹ and so on.

Recently, more and more attention has been focused on the synthesis of MAPO molecular sieves with novel zeolite topologies, variable framework compositions, and distinctive properties.^22,23 This requires careful regulation of the synthetic system, particularly, the selection of heteroatoms and organic SDAs. Our previous study shows that the type and the amount of organic SDAs in the synthesis can influence the M/Al ratios in the frameworks of MgAPO-CJ67 (ref. 24) and FeAPO-CJ66 (ref. 15) with LEV and ACO zeotype structures, respectively. By using n-methylpiperazine as an SDA, a magnesium aluminophosphate JU92 with a novel JNT zeotype structure and luminescent properties has been synthesized.²⁵

In this work, we present a novel Mg-containing microporous aluminophosphate |C₄H₁₂N₂|[Mg₂Al₆(PO₄)₈(H₂O)₄] (denoted as JU96). This compound was synthesized using piperazine as a SDA in a similar reaction system to that of our reported magnesium aluminophosphates JU94 and JU95 which contain a basic unit, but with a slightly different gel composition and distinct structures.²⁶ JU96 possesses a new zeolite-like framework structure with intersecting 8-ring channels and unique [4⁴·6⁶·8⁶] cages. Its synthesis and structure and a structural comparison of JU94, JU95 and JU96 are discussed.

Experimental section

Materials and synthesis

JU96 was synthesized under hydrothermal conditions by using piperazine (PZ) as a structure-directing agent (SDA). In a typical synthesis, magnesium acetate (Mg(CH₃COO)₂·4H₂O, Tianjin Fuchen Chemical Reagents Factory) and pseudoboehmite (Al₂O₃, 62%, Shandong Aluminium Industry) were added into a solution of orthophosphoric acid (85 wt%, Beijing Chemical Industry Group Co. Ltd) and water with vigorous stirring, followed by the addition of PZ (C₄H₁₀N₂, Aldrich). After stirring for 1 hour, a homogeneous gel with an overall molar composition of 1.0Al₂O₃ : 0.67MgO : 2.62P₂O₅ : 1.36PZ : 224H₂O was formed, which was heated at 180 °C for 3 days in a 15 ml Teflon-lined stainless steel autoclave. Pure phase JU96 was obtained by filtration, washed with distilled water and then dried in air at room temperature overnight.

Structure determination

Single crystals of JU96 with dimensions of 0.21 × 0.19 × 0.18 mm³, were selected for single-crystal X-ray diffraction analyses. The data were recorded using a Bruker AXS SMART APEX II diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at a temperature of 23 ± 2 °C. Data processing was accomplished using the SAINT processing program.²⁷ The structures were solved using direct methods and refined using a full matrix least-squares technique with the SHELXTL software package.²⁸ The heaviest atoms of Al, Mg, P and O were easily located, and the C and N atoms were subsequently located from the difference Fourier maps. The H atoms in the structures were not added. All non-hydrogen atoms were refined anisotropically. The crystal data and structure refinement for JU96 was shown in Table 1. Thermal ellipsoids of JU96 are given at 50% probability by using the SHELXTL software package. Structural details are given in Table S1.†

Characterizations

Powder X-ray diffraction (PXRD) data was collected using a Rigaku D/max-2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å, scan range from 4 to 40°, scan speed 0.5° min⁻¹, stride 0.02°). Inductively coupled plasma (ICP) analysis was performed on a Perkin-Elmer Optima 3300DV spectrometer. Elemental analysis was conducted on a Perkin-Elmer 2400 elemental analyzer. Thermogravimetric analysis (TGA) was carried out on a TA Q500 analyzer in air with a heating rate of 10 °C min⁻¹ from RT to 800 °C.

Table 1 Crystal data and structure refinement for JU96

Compounds	JU96
Empirical formula	Mg2Al6P8C4H14N2O36.5
Formula weight	1132.43
Temperature	296(2) K
Wavelength (Å)	0.71073
Crystal system, space group	Monoclinic, P21/c
Unit cell dimensions
a (Å)	12.675(2)
b (Å)	14.423(3)
c (Å)	9.5745(17)
α (deg)	90
β (deg)	100.568(2)
γ (deg)	90
Volume (Å^3)	1720.7(5)
Z, calculated density (mg m^−3)	1, 2.186
Absorption coefficient (mm^−1)	0.728
F (000)	1132
Crystal size (mm^3)	0.21 × 0.19 × 0.18
θ range (°) for data collection	1.63–28.37
Limiting indices	−10 ≤ h ≤ 16, −19 ≤ k ≤ 19, −12 ≤ l ≤ 12
Reflections collected/unique	12 203/4269
Completeness to θ (%)	28.37, 99.5%
Absorption correction	Semi-empirical from equivalents
Max and min transmission	0.8801 and 0.8621
Refinement method	Full-matrix least-squares on F^2
Data/restraints/parameters	4269/0/269
Goodness-of-fit on F^2	0.992
Final R indices [I > 2σ(I)]	R1 = 0.0487, wR2 = 0.1287
R indices (all data)	R1 = 0.0754, wR2 = 0.1461
Largest diff. peak and hole (e Å^−3)	0.772 and −0.562

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Results and discussion

Synthesis and characterization of JU96

JU96 was synthesized using piperazine as an SDA under hydrothermal conditions. It is noted that two other magnesium aluminophosphates, JU94 and JU95, can also be produced in such a reaction system but by using different gel compositions. In the synthesis, the pH value determined by the amounts of SDA and H₃PO₄ in the reaction gel influences the final product. For instance, 1.0Al₂O₃ : 0.67MgO : 2.62P₂O₅ : 1.36PZ : 224H₂O is the best gel proportion for synthesizing pure JU96. Under such conditions, a slight change in the amount of piperazine may produce magnesium aluminophosphate JU95. When the molar ratio of piperazine/Al₂O₃ is 1.36, this results in the formation of JU96, whereas if the ratio is 2.63 this leads to JU95 formation. When the molar ratio of piperazine/Al₂O₃ is between 1.36 and 2.63, a mixture of JU95 and JU96 could be produced. As for JU94 and JU96, a pH value of 1 favours the formation of JU96 and a higher pH value of 1.5–2 facilitates the formation of JU94. Furthermore, the content of Mg also plays an important role in the synthesis of JU96. Pure JU96 could be synthesized when the molar ratio of Mg/Al is 1 : 3, increasing the Mg/Al ratio results in the formation of a mixture of JU94/JU96 or JU95/JU96. However, JU96 could not be produced without Mg atoms or in the presence of other divalent metals including Co and Zn. In summary, compared with the synthetic conditions for JU94 and JU95, a low pH value and low piperazine/Al₂O₃ ratio are better for the formation of pure JU96.

The powder X-ray diffraction pattern of JU96 is consistent with the simulated one based on the single-crystal structure of JU96 (Fig. 1), which indicates the pure phases of the as-synthesized samples. The structure of JU96 can be stable at up to 400 °C which is suggested by the XRD analysis. ICP and elemental analyses show that the amounts of Mg, Al, P, C, H and N of JU96 are 4.18, 14.11, 21.60, 4.18, 1.92 and 2.43 wt% (calcd: Mg, 4.20; Al, 14.05; P, 21.83; C, 4.18; H, 1.92; N, 2.44 wt%), giving Mg/Al ratios of 1/3 and (Mg + Al)/P ratios of 1/1. The results are in good agreement with the empirical formula |C₄H₁₂N₂|[Mg₂Al₆(PO₄)₈(H₂O)₄] given by single-crystal analysis.

Fig. 1 XRD patterns of (a) simulated JU96 based on the single-crystal structure, (b) synthesized JU96 and (c) calcined JU96 at 400 °C.

The TG curve in Fig. 2 shows three continuous weight losses for JU96. The first weight loss of 1.5 wt% at around 130 °C is attributed to the removal of adsorbed water in channels, the second weight loss of 6.4 wt% from 130 to 300 °C is due to the removal of coordinated water (calcd 6.3 wt%), and the third weight loss of 4.6 wt% from 450 to 800 °C is due to the decomposition of organic amines (cal. 7.6 wt%). This result indicates that the organic SDAs could not be completely removed, and some organic species may still be occluded in the calcined sample after calcination. This phenomenon has also been observed in JU94 and JU95.²⁵

Fig. 2 TG analysis of JU96.

Crystal structure of JU96

Single-crystal structure analysis shows that JU96 crystallizes in the monoclinic space group P2₁/c (no. 14). The structure of JU96 consists of a [Mg₂Al₆(PO₄)₈]²⁻ anionic framework and diprotonated piperazine cations to achieve charge balance. Thermal ellipsoids of JU96 are shown in Fig. 3. Its asymmetric unit contains eight crystallographically distinct framework positions: one Mg site, three Al sites and four P sites. All of the Al and P atoms are tetrahedrally coordinated. The Al–O and P–O bond distances are in the range of 1.715(2)–1.749(2) Å and 1.484(2)–1.545(2) Å, respectively. The Mg–O_f (O_f: framework O atom) bond lengths vary from 1.999(2) to 2.107(2) Å, and the two terminal Mg–O_w bond distances are 2.100(2) and 2.237(3) Å.

Fig. 3 Thermal ellipsoids given at 50% probability, showing the atomic labelling scheme of JU96.

JU96 exhibits a new 4-connected zeolite-like topological structure. Its open-framework is constructed by the connection of AlO₄ tetrahedra, MgO₆ octahedra and PO₄ tetrahedra, and possesses two-dimensional intersecting 8-ring channels. One 8-ring channel runs along the [100] direction (Fig. 4a) with a free diameter of pore opening of 5.9 × 2.3 Å (O⋯O distances). Two 8-ring channels run along the [001] direction (Fig. 4b) with different free diameters of 5.4 × 2.5 Å and 4.8 × 2.0 Å (O⋯O distances). Notably, a new cage [4⁴·6⁶·8⁶] is found in JU96, which has not been observed in the previously reported zeolite structures. Diprotonated PZ cations are occluded in these [4⁴·6⁶·8⁶] cages, and interact with the inorganic framework through hydrogen bonds.

Fig. 4 Topological structure of JU96 with tiles showing intersecting 8-ring channels along the (a) [100] and (b) [001] directions.

The 4-connected framework structure of JU96 can be described as a (4, 2)-connected three-periodic net. This three-periodic net is carried by a unique natural tiling with a transitivity of (8 16 19 10). Four different tiles are observed in this tiling, which include [4·6²], [4·8²], [6·8²], and [4⁴·6⁶·8⁶]. The signature of this tiling is 8[4·6²] + 4[4·8²] + 6[6·8²] + [4⁴·6⁶·8⁶]. The intersecting channel system of JU96 can also be viewed as a construction by different tiles. The 8-ring channel along the [100] direction (Fig. 4a) is defined by the linear arrangement of [4⁴·6⁶·8⁶] tiles (channel A). Two 8-ring channels along the [001] direction (Fig. 4b) are composed by the [4⁴·6⁶·8⁶], [6·8²] and [4·8²] tiles (channel B) and the [4⁴·6⁶·8⁶] and [6·8²] tiles (channel C), respectively.

Interestingly, the structures of JU96 and the reported JU94 and JU95 (ref. 26) are all composed by the strict alternation of AlO₄/MgO₆ polyhedra and PO₄ tetrahedra, but their structures are quite different. Meanwhile, all of them possess 2D interconnected 8-ring channels. However, their zeolitic frameworks are quite different. In detail, the structures of JU94 and JU95 are constructed of a characteristic building unit, Mg₂Al₃P₅O₃₀, composed of four 4-rings and two 6-rings, but their connection modes are different. As shown in Fig. 5, the structure of JU96 is featured by a 2D (4, 8)-net; such layers are connected by bridging O atoms to form the 3D framework. In the (4, 8)-net, two edge-sharing 4-rings composed by MgO₆, 3 PO₄ and 2 AlO₄ polyhedra can be found as a building unit (BU). The BUs are linked with each other through two PO₄ tetrahedra, giving rise to the 2D layers.

Fig. 5 Open-framework structure of JU96 constructed by the 2D (4, 8)-net.

Conclusions

A new magnesium aluminophosphate |C₄H₁₂N₂|[Mg₂Al₆(PO₄)₈(H₂O)₄] (JU96) has been hydrothermally synthesized in the presence of piperazine as a structure-directing agent. The connection of AlO₄ tetrahedra, MgO₄(H₂O)₂ octahedra and PO₄ tetrahedra forms the 3D open framework of JU96 with intersecting 8-ring channels along the [100] and [001] directions. Unique [4⁴·6⁶·8⁶] cages are observed in such a 4-connected topology, which are occupied by diprotonated piperazine cations. Mg atoms are in an octahedral geometry due to two coordinated water molecules, which occupy a distinct site in the framework. The synthesis and structure comparison of JU94 with other known magnesium aluminophosphates JU95 and JU96 obtained in the same reaction system will provide more insight into the relationship between synthesis and structure, which makes it possible to synthesize more novel MAPO zeolitic materials.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21271081).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The crystallographic information files (CIF) of JU96; bond lengths and angles for JU96. CCDC 1432162. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra22194c

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