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Co-templating ionothermal synthesis and structure characterization of twonew 2D layered aluminophosphates†

Ying Wei,a Bernd Marler,a Ling Zhang,a Zhijian Tian,b Heribert Graetscha and Hermann Gies*a

Received 28th May 2012, Accepted 12th August 2012DOI: 10.1039/c2dt31150j

For the first time, the co-templating ionothermal methodology was used in the preparation of layeredaluminophosphate materials. With the addition of either 1,2-ethylenediamine or 1,6-hexanediamine to theionic liquid 1-ethyl-3-methyl imidazolium chloride, two new 2D layered aluminophosphates RUB-A1[Al3P4O16][NH3CH2CH2NH3]0.5[C6N2H11]2 and RUB-A2 [Al3P4O16][NH3(CH2)6NH3]-[NH3(CH2)6NH2]0.5[C6N2H11]0.5[H2O] have been synthesized ionothermally by co-templating. Thestructure of RUB-A1 has been determined from single-crystal X-ray diffraction data using direct methods,while the structure of RUB-A2 has been solved ab initio from powder X-ray diffraction data with limitedresolution using direct-space methods. Both of these two compounds have a 2D layered structureconsisting of macroanionic sheets of composition [Al3P4O16]

3− stacked in an AAAA sequence. Theinorganic layers are built up from alternatively vertex-sharing [AlO4]- and [PO3(vO)]-tetrahedral unitsforming a 4.6.8 and a 4.6.12 network for RUB-A1 and RUB-A2, respectively. The layer topology ofRUB-A1 is closely related to the previously known 4.6.8-layer topology but with a different sequence ofphosphoryl group orientation. Combining the results of structure analysis with the NMR, chemicalanalysis and TG-DTA experiments, we show that both the ionic liquid cation and the protonated diaminesare located in the interlayer space and together direct the formation of these two structures.

Introduction

Open-framework aluminophosphate materials constitute one ofthe most important classes of crystalline inorganic porousmaterials owing to their structure variability and compositiontunability. Broad application potential in many fields such as cat-alysis, adsorption and separation has been established.1 Topolo-gically, the frameworks of these materials are built of [AlOn]-polyhedra (n = 4–6) and [PO4]-tetrahedra connected alternativelythrough bridging oxygen atoms. Similar to silicates, alumino-phosphate materials can be classified as neutral 3-dimensional4-connected (3D, 4C) zeolite frameworks or anionic interruptedframeworks with 3D bonding network, 2D layer, 1D chain or 0Dcluster structures as the principal feature of their crystal struc-tures. The most studied materials are the microporous alumino-phosphate framework structures because of their zeolitic

properties. The family of 2D layered aluminophosphate materialshas increased significantly in the recent past and displays a richdiversity of the layer stoichiometries of AlPO4(OH)

−, AlP2O83−,

Al2P3O123−, Al3P4O16

3−, Al4P5O203− and Al13P18O72

15−, yield-ing layer networks with 4.6, 4.8, 4.6.8, 4.6.12 and 4.6.16 nets(4, 6, 8, 12 and 16 stand for n-rings) and layer stackingsequences of AAAA, ABAB, ABCABC, ABCDABCD andABCDEFABCDEF.1b The discovery of the richness of these 2Dlayered aluminophosphate compounds not only provides us withmany useful clues for mechanistic studies of aluminophosphatesynthesis but also brings about some attractive possibilities forthe preparation of specific adsorbents, novel heterogeneous cata-lysts, and advanced functional materials through delamination orintercalation treatment.2

Typically, the synthesis of open-framework aluminopho-sphates involves the crystallization of the starting gel underhydrothermal or solvothermal conditions, in which organicamines or quaternary ammonium ions are used as templates orstructure-directing agents (SDA). In recent years, an alternativemethod named ionothermal synthesis has increasingly attractedthe interest of chemists working in the field of the synthesis ofopen-framework aluminophosphates as well as other porousmaterials.3 Unlike conventional synthesis, ionothermal synthesiswhich uses ionic liquids acting both as solvents and templatesmight be performed at ambient pressure due to the negligiblevapor pressure of ionic liquids at typical synthesis temperatures,thus alleviating safety concerns associated with the high hydro/solvothermal pressure. This advantage appears to be even more

†Electronic supplementary information (ESI) available: Experimentaland simulated powder XRD patterns for RUB-A1, TG-DTA curves,asymmetric unit graphics, atomic coordinates with isotropic temperaturefactors and occupancies, and selected bond lengths and angles, X-raycrystallographic files (CIF), and XRD patterns for the samples obtainedfrom different water to IL ratios. CCDC 884903 anmd 884904. For ESIand crystallographic data in CIF or other electronic format see DOI:10.1039/c2dt31150j

aInstitute for Geology, Mineralogy and Geophysics, Ruhr-University-Bochum, D-44780 Bochum, Germany. E-mail: hermann.gies@rub.debDalian National Laboratory for Clean Energy, Dalian Institute ofChemical Physics Chinese Academy of Sciences, Dalian 116023,P.R. China

12408 | Dalton Trans., 2012, 41, 12408–12415 This journal is © The Royal Society of Chemistry 2012

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striking when the ionothermal approach is used for instance inmechanistic studies,4 in the large-scale production of molecularsieve films or membranes5 or in combination with the microwavetechnique.6 Meanwhile, this new synthetic methodology hasshown great potential in the preparation of hitherto unknownporous materials, in view of the rich structural diversity andspecial ionic nature of the ionic liquids as compared with themolecular solvents of the hydro/solvothermal systems. Variousnew materials with novel structures and unique compositionshave been prepared using this method in the past severalyears.3a,7

The templating and the structure directing effects are impor-tant issues in the synthesis of inorganic porous materials. Whileenormous efforts have been made in the design and utilization ofnovel organic templates with different size, geometry andpolarity to successfully synthesize new porous materials, the useof two types of organic templates cooperatively playing thestructure-directing role also has been proved to be a promisingsynthesis route.8 For example, some 2D layered alumino-phosphate materials have been hydrothermally or solvothermallysynthesized in the presence of two organic amines.9 Compu-tational studies of host–template interaction demonstrate that insome cases using mixed organic amines is more energeticallyfavorable than using solely one of them as the template.10

Recently, the strategy of adding organic amines or quaternaryammonium ions to ionothermal systems as an additional syn-thesis variant has been applied successfully in the preparation ofseveral open-framework phosphates.7e–g,11 The additionalamines appear to alter the original crystallization process andresult in the different product structures.11a In some cases, it hasbeen demonstrated that the added organic amines or the amine–IL hybrid/supermolecule forming through hydrogen bondinginteraction could facilitate the formation of a specific cage orchannel of a certain structure.4b,11b–d Remarkably, two novelaluminophosphate materials have been discovered including thefirst aluminophosphate molecular sieve with 20-ring extra-largepores, DNL-1 (with -CLO framework structure type),7e anda new 3D anionic aluminophosphate framework JIS-1 with anAl/P ratio of 6 : 7,7g in which the added organic amines act asco-templates or co-SDA together with the ionic liquid. Theseresults stimulated us to attempt this strategy in the synthesis ofnew aluminophosphate materials with 2D layered structures.

Here, we report on the co-templating ionothermal approach tocrystallize 2D layered aluminophosphate materials. With theaddition of two different amines in the 1-ethyl-3-methyl imid-azolium chloride ([emim]Cl) ionothermal system, we preparedtwo new 2D layered aluminophosphates RUB-A1 (Ruhr Univer-sity Bochum Aluminophosphate Number 1) and RUB-A2. Wehave determined their structures using the single-crystal X-raydiffraction (XRD) from the direct methods or powder XRD fromthe direct-space methods. The two materials have the same layerstoichiometry of [Al3P4O16]

3−, the same layer stacking sequenceof AAAA, but different layer topologies of 4.6.8 and 4.6.12.Interestingly, the layer topology of RUB-A1 has a novel phos-phoryl group orientation sequence never seen before in thesimilar layered compounds. In combination with nuclear mag-netic resonance spectroscopy (NMR), inductively coupledplasma (ICP) and thermogravimetric/differential thermal analysis(TG-DTA), we unambiguously demonstrate that both the amines

and ionic liquid cations are located in these two structures simul-taneously acting as co-templates.

Experimental section

Syntheses

For the synthesis of RUB-A1, a PE beaker was charged with11.4 g ionic liquid 1-ethyl-3-methyl imidazolium chloride([emim]Cl), 0.79 g Al[OCH(CH3)2]3, 0.89 g H3PO4 (85 wt% inH2O), and 0.23 g 1,2-ethylenediamine (EDA) with a gel molarcomposition of 1Al2O3 : 2P2O5 : 40[emim]Cl : 2EDA : 3.8H2O(from H3PO4 solution). After stirring electromagnetically at100 °C in an oil bath for 30 min, the reaction mixture was trans-ferred into a PTFE-lined autoclave (volume 40 ml), and thenheated in an oven at 150 °C for 7 days. The white solid productswere filtered, washed thoroughly with distilled water and dried at95 °C overnight.

The synthesis of RUB-A2 was performed using identicalconditions and an identical gel molar composition with theexception that 1,2-ethylenediamine was replaced by 1,6-hexane-diamine and the crystallization time was decreased to 4 days.

Characterizations

Scanning electron microscopy (SEM) was performed using aLEO-1530 Gemini microscope. The Al and P contents of thetwo materials were determined by inductively coupled plasma(ICP) analysis using a Philips PU 7000 ICP-AES-spectrometer.

27Al and 31P magic-angle spinning (MAS) NMR and 1H →13C cross polarization (CP) MAS NMR experiments werecarried out at room temperature on a Bruker Advance ASX 400spectrometer using 4 mm spinner and Bruker probes.

The thermogravimetric/differential thermal analysis (TG-DTA)experiments were carried out on a Bähr STA 503 thermal analy-zer from room temperature to 1000 °C with a heating rate of10 °C min−1 under a mixture of flow gases of nitrogen and air.

The structure of RUB-A1 was solved using single-crystalXRD data. As RUB-A1 crystallized as twins, a suitable twinnedcrystal with dimensions of 0.283 mm × 0.056 mm × 0.016 mmwas carefully selected for the diffraction experiment. The inten-sity data were collected on a four-circle kappa diffractometer(Xcalibur, Oxford Diffraction) equipped with a CCD detectorand an enhanced X-ray source (graphite monochromatizedMo Kα radiation, λ = 0.71073 Å) at room-temperature. Thesample-to-detector distance was 44 mm. One phi and fouromega scans were measured at a step width of 1.0°. Theexposure times were 50, 90 and 110 s per frame. The datareduction was carried out with the program CrysAlisPro (AgilentTechnologies, 2011). The intensity data were separated for thetwo twin individuals (twin volume ratio 0.7 : 0.3) and the dataset with higher intensities was used for the subsequent structureanalysis. On the basis of systematic absences and statistics ofintensity distribution, the space group was determined to beP21/c. The structure was solved with direct methods and refinedon F2 by full-matrix least squares using SHELXTL97.12 Thepositions of the phosphorus, aluminum and oxygen atoms wereobtained first. Carbon and nitrogen atoms were located fromdifference Fourier maps. All non-hydrogen atoms were refined

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with anisotropic thermal parameters. Later, the hydrogen atomswere geometrically placed. For the refinement a common isotro-pic displacement parameter for all H-atoms was used and theC–H and N–H distances were fixed to 0.93 Å–0.97 Å and 0.89 Å,respectively. A summary of the crystallographic data, structuredetails and selected bond distances and angles is presented inTables 1, S1 and S2.† The measured X-ray powder diffractionpattern for RUB-A1 is in good agreement with that of the simu-lated one, based on the single-crystal structural data, proving thatthe as-synthesized product is a single phase (Fig. S3†).

The structure of RUB-A2 was ab initio solved using powderX-ray diffraction data. The PXRD data were collected at roomtemperature with a Siemens D5000 diffractometer operating inquasi Debye–Scherrer mode, using monochromatized Cu Kα1radiation (λ = 1.5406 Å). To prevent preferred orientation of theplatelets, sample transmission geometry was used with a capil-lary sample holder (φ = 0.3 mm). The diffraction data were col-lected with a Braun position sensitive detector over the rangefrom 7 to 95° 2θ with a step width of 0.0079° 2θ. Indexing wasperformed using the program DICVOL06.13 The space groupsymmetry was determined by analyzing the powder pattern forsystematically extinct reflections. The powder data were oflimited resolution. This is quite frequently observed for layeredmaterials because of the slight stacking disorder of layers. Sincethe data did not provide atomic resolution the structure could notbe solved by Direct Methods in reciprocal space. Instead thestructural model of RUB-A2 was determined using the realspace global optimization methods implemented in the programFOX.14 This software optimizes a structure model described bythe use of building blocks defined in terms of their internal coor-dinates, such as bond lengths and bond angles. Optimization was

performed by comparing the PXRD patterns calculated from ran-domly generated configurations and using the “integrated Rwp”(iRwp) as the cost function. Trial structures were generated usingthe “Parallel Tempering” algorithm. In this work, the buildingblocks are represented by tetrahedral [AlO4]- and [PO4]-groupsand [emim]+ cations. It was essential to add meaningful anti-bump restraints (Al–Al/P–P = 4.0 Å, Al–P = 2.8 Å, O–O =2.4 Å, C/N–P/Al = 3.0 Å, C/N–O = 2.4 Å) and to allow fordynamical occupancy corrections in order to speed up the optim-ization process. After the optimization, the positions of four Psites and three Al sites were obtained. The positions of oxygenatoms were then relocated at halfway between Al and P atoms.The Rietveld refinement of the structural model was performedusing the Fullprof-Suite of programs.15 Extra-framework atomswere located in a difference Fourier map. Hydrogen atoms werenot included in the refinement procedure. On the basis of chemi-cally reasonable geometries, soft distance restraints for dAl–P,dAl–O, dO–O, dN–C, and dC–C were used during the refinement(see Table 2). Further details of the diffraction experiment andthe structure refinement, structure details and selected bonddistances and angles are summarized in Tables 2, S3 and S4.†The final Rietveld and difference plots are shown in Fig. 1.

Results and discussion

The SEM images (Fig. 2) show the crystal morphology ofRUB-A1 and RUB-A2, which indicate that the samples arehomogeneous and well crystallized.

The chemical composition was determined to be: forRUB-A1: Al (exp: 11.56 wt%, calcd: 11.34 wt%) and P (exp:18.50 wt%, calcd: 17.36 wt%); for RUB-A2: Al (exp: 10.88 wt%,calcd: 11.39 wt%) and P (exp: 16.81 wt%, calcd: 17.44 wt%).The calculated values are based on the results of the structureanalysis (see below).

The thermal analysis: for RUB-A1 (Fig. S1†), no distinctweight loss was observed before 300 °C, and the weight loss of34.2 wt% takes place between about 300 °C to 1000 °Caccompanied by a strong endothermic peak at 370 °C followedby an exothermic peak at 400 °C, which were attributed to thedecomposition and the combustion respectively of the organictemplates (EDA and [emim]+) in the structure (calcd: 35.0 wt%).For RUB-A2 (Fig. S2†), the first weight loss of 4.2 wt% wasobserved between 50 °C to 150 °C with an endothermic effectwhich was attributed to the removal of the physically absorbedand structural water molecules (calcd: 2.5 wt%). The remainingweight loss of 30.9 wt% between 250 °C to 1000 °C with anintense exothermic peak at 350 °C was attributed to thedecomposition and combustion of the organic templates (HDAand [emim]+) in the structure (calcd: 32.6 wt%).

RUB-A1 consists of macroanionic layers of formula[Al3P4O16]

3− with diprotonated EDA and the ionic liquid cation[emim]+ located in the interlayer region. The asymmetric unit ofRUB-A1 contains three symmetrically independent Al atomsand four symmetrically independent P atoms (Fig. S4†). All Aland P atoms are tetrahedrally coordinated by O atoms. Whereaseach [AlO4]-unit shares all the vertices with adjacent P atoms,each [PO4]-unit only shares three of four vertices with adjacentAl atoms leaving one vertex for the terminal PvO group. The

Table 1 Crystal data and structure refinement for RUB-A1

Empirical formula[Al3P4O16] [NH3CH2CH2NH3]0.5[C6N2H11]2

Formula weight(g mol−1)

714

Temperature (°C) 293(2)Wavelength (Å) 0.71073Crystal system MonoclinicSpace group P21/c (no. 14)a (Å) 10.991(2)b (Å) 16.979(3)c (Å) 14.863(2)β (°) 104.19(2)V (Å3) 2689.0(8)Z 4Densitycalc (g cm−3) 1.754R(int) 0.174M (mm−1; Mo Kα) 0.463F(000) 1468θ range (°) 2.78–25.03Index ranges −13 ≤ h ≤ 13, −20 ≤ k ≤ 20, −17 ≤ l ≤ 17Total data collected 47 477Unique data 4695Goodness of fit on F2 a 0.900R1

b[I > 4σ(I)] 0.0954wR2

c (all data) 0.2300

aGoodness-of-fit S = [∑w(Fo2 − Fc

2)2/(n − p)]1/2, where n is thenumber of reflections and p is the number of parameters. b R1 = ∑(Fo −Fc)/∑(Fo).

cwR2 = [∑[w(Fo2 − Fc

2)2]/∑[w(Fo2)2]]1/2.

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Al–O bond lengths range within 1.709(7)–1.766(7) Å, and theO–Al–O bond angles range within 107.2(4)–111.8(3)°. The P–Obond lengths range within 1.524(7)–1.556(7) Å, and the O–P–Obond angles range within 105.0(4)–113.0(4)°. The PvO bond ofthe [PO4]-units is considerably shorter and varies in lengthwithin the range of 1.464(7)–1.491(7) Å. All these values are ingood agreement with those found in other aluminophosphatecompounds.

The alternation of [AlO4]- and [PO3(vO)]-groups via brid-ging oxygen atoms forms the 2-D network with a 4.6.8 net paral-lel to the bc plane (Fig. 3a). All the 6-rings in the layers arecapped with a [PO3(vO)]-tetrahedron alternatively protruding

into the interlayer region above and below the layer. Actually,the entire layer of RUB-A1 can be solely built up from the phos-phoryl capped 6-MR, which links to four neighbors either byone bridging oxygen atom or by two bridging oxygen atomsforming a 4.6.8 network. The linking of four capped 6-rings gen-erates an approximately circular 8-ring with a window size of6.1 Å × 7.5 Å. The layer topology of RUB-A1 is similar to thatof an already known 4.6.8-net, which has been reported to bepresent in several [Al3P4O16]

3− layered compounds preparedin the presence of either one or two organic templates in hydro/solvothermal synthesis (Table 3).9a,16 Both of these two typesof 4.6.8-nets resemble the 2-D layer-like building block in

Table 2 Crystallographic data, experimental conditions for X-ray data collection and results of the Rietveld analysis of RUB-A2

Chemical formula [Al3P4O16][NH3(CH2)6NH3][NH3(CH2)6NH2]0.5[C6N2H11]0.5[H2O]

Formula weight 711Crystal system TriclinicSpace group P1̄ (no. 2)a (Å) 12.9165(2)b (Å) 13.0460(2)c (Å) 9.8873(2)α (°) 70.585(1)β (°) 99.547(2)γ (°) 119.438 (2)V (Å3) 1368.46(4)Z 2Densitycalc (g cm−3) 1.727Wavelength (Å) 1.5406Range 2θ (°) 7–95Step width 2θ (°) 0.0079Number of points 11 133Number of reflections 2674Number of structural variables 145Number of geometric restraints 121d(Al–P) 3.10(3) Åd(Al–O) 1.72(1) Åd(P–O) 1.53(1) Åd(O–O) 2.65(3) ÅPeak profile Thompson–Cox–Hastings Pseudo-Voigt*Axial divergence asymmetryBackground correction Linear interpolation of background pointsRF (%) 3.02RB (%) 2.38Rexp (%) 4.77Rwp (%) 7.26χ2 2.32

Fig. 1 The SEM images of the samples (a) RUB-A1 and (b) RUB-A2.

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aluminophosphate molecular sieve AlPO4-21 (with AWO frame-work structure type). However, it is worth mentioning that somedifferences of the phosphoryl group orientation sequence existbetween these two topologies. For the previously known

topology as observed in the compound [Al3P4O16]-(NH3(CH2)5NH3)(C5H10NH2)

9a the PvO groups orientationsequences are around the capped 6-rings UUU(D) or DDD(U)(U: up; D: down; the letter in parentheses represents the cappedPvO group orientation) and around the 8-rings UUDD, whereasfor the layer topology of RUB-A1 the PvO groups orientationsequences are around the capped 6-rings DDU(U) or UUD(D)and around the 8-rings UUUU or DDDD (Fig. 3).

The anionic layers are held together in an AAAA stackingsequence by a network of H-bonds involving the diprotonatedEDA located between the aluminophosphate layers. As shown inFig. 4, the protonated ammonium groups of the EDA cations arenot coplanar with the alkyl chain, and form two H-bonds withtwo phosphoryl groups in one adjacent layer with distances ofd(N1–O2) = 2.759 Å and d(N1–O4) = 2.834 Å and one H-bondwith a phosphoryl group in the opposite layer with the bond dis-tance of d(N1–O10) = 2.769 Å, thus forming a fairly stable 3D-bonded structure. Additionally, there are two symmetrically inde-pendent [emim]+ cations, which are inside the 8-ring channelsgenerated by the alignment of the 8-ring-units as seen in the[100] direction. The [emim]+ cations supply no hydrogenbonding interaction with the layer, but the positive charge bal-ances the residual negative charges of the aluminophosphatesheets.

Similarly to RUB-A1, RUB-A2 also consists of anionic layersof formula [Al3P4O16]

3− built from alternating vertex-sharing[AlO4]- and [PO3(vO)]-tetrahedral units as shown in Fig. 5.The asymmetric unit of RUB-A2 also contains three symmetri-cally independent Al atoms and four symmetrically independentP atoms (Fig. S5†). The Al–O bond lengths are within 1.65(3)Å–1.73(4) Å, and the O–Al–O bond angles range from 104(3)°to 116(2)°. The P–O bond lengths are within 1.52(2) Å–1.60(2)Å, and the O–P–O bond angles range from 102(2)° to 118(2)°.The fourth P–O bond length of the [PO4]-units has typical PvOdistances of 1.47(2) Å, 1.51(3) Å and 1.51(2) Å, except forP2–O15 having an elongated length of 1.54(2) Å, which can beattributed to the charge–dipole interaction (see below). All thesebond lengths and angles are within a reasonable range for alumi-nophosphate compounds.

The layers of RUB-A2 can also be built up solely from thephosphoryl capped 6-rings, the capped PvO groups of whichpoint to the interlayer region alternately above and below.However, the connection mode of the capped 6-ring is differentfrom that in RUB-A1. In RUB-A2, each of the capped 6-ringslinks to three neighbors by two bridging oxygen atoms, which

Fig. 2 Observed (red dots) and calculated (black lines) XRD patternsof RUB-A2 as well as the difference profile (blue lines). The short tickmarks below the patterns give the positions of Bragg reflections.

Fig. 3 Polyhedral view of the 4.6.8 network in (a) RUB-A1 and (b)[Al3P4O16](NH3(CH2)5NH3)(C5H10NH2)

9a along the a axis (above) orthe c/b axis (below). (PO4 tetrahedra filled with black or white color rep-resent PvO groups pointing up or down the layers, respectively.)

Table 3 Layered aluminophosphates with [Al3P4O16]3− stoichiometry and 4.6.8-net

L-n

Organic templates PvO orientation sequence

Ref.Template-1 Template-2 Around capped 6-ring Around 8-ring

L-1 1,2-Ethylenediamine Ethylene glycol UUU(D)/DDD(U) UUDD 16aL-2 1,5-Diaminopentane Piperidinium UUU(D)/DDD(U) UUDD 9aL-3 Co(1,3-diaminopropane)3

3+ — UUU(D)/DDD(U) UUDD 16bL-4 Cyclobutylamine Piperidinium UUU(D)/DDD(U) UUDD 16cL-5 Ethylamine — UUU(D)/DDD(U) UUDD 16dL-6 Propylamine — UUU(D)/DDD(U) UUDD 16eL-7 Tetramethylethylenediamine — UUU(D)/DDD(U) UUDD 16fL-8 1,2-Ethylenediamine 1-Ethyl-3-methyl-imidazolium UUD(D)/DDU(U) UUUU/DDDD This work

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generates a different layer-type having 4.6.12 topology (Fig. 5).The linkage of six capped 6-rings forms a circular 12-ring with awindow size of 9.3 Å × 9.5 Å. The layer topology of RUB-A2 isthe same as the 4.6.12 network previously reported in the seriesof [Al3P4O16]

3− layered compounds,17 which resembles the 2-Dbuilding block of the framework of aluminophosphate molecularsieve AlPO4-5 (with AFI framework structure type).

The anionic layers are held together in an AAAA stackingsequence by a network of H-bonds involving the diprotonatedHDA and water molecules located between the layers (Fig. 6).There are two symmetrically independent diprotonated HDAcations in the structure. One is located between the layers, par-tially occupying the 12-ring pore. Each protonated ammoniumgroup of this HDA forms two H-bonds with two phosphorylgroups in one adjacent layer with the distances of d(N2–O15) =2.699 Å, d(N2–O16) = 2.930 Å and d(N3–O13) = 2.726 Å, d(N3–

O15) = 2.820 Å, respectively, and one H-bond with a phosphorylgroup in the opposite layer with the distances of d(N2–O16) =2.515 Å and d(N3–O14) = 2.601 Å respectively. The other HDAwhich is subject to a twofold symmetry occupies the center ofthe 12-ring voids as viewed from the [100] direction. A weakhydrogen bond exists between the amino group of this HDA andthe surrounding layer with a distance of d(N1–O4) = 2.917 Å.There are also two water molecules in the 12-ring channel,which have two hydrogen bonds with the phosphoryl groups ofthe layers above and below with the distances of d(Ow–O13) =2.714 Å and d(Ow–O14) = 2.849 Å. Moreover, there is one[emim]+ cation adopting two different orientations and locatingbetween the 4-rings with the ethyl groups protruding into the12-ring pore. The [emim]+ cation has a charge–dipole interactionwith two O15 atoms in the layer above and below as obviousfrom the distances of d(N4–O15) = 2.954 Å and d(N5–O15) =2.990 Å, thus resulting in the elongated P2–O15 group.

To support the results obtained from structure analysis, wefurther carried out multi-nuclear NMR experiments with the as-synthesized RUB-A2 sample. From the 27Al and 31P MAS NMRspectra (Fig. 7), we see the isotropic resonance signals at δ =42.7 ppm and δ = −25.3 ppm respectively, which are assignedto the typical 4-coordinated Al and P tetrahedra in the alumino-phosphate frameworks.18 From the 1H → 13C CP MAS NMRspectrum (Fig. 8), we can clearly identify the resonance signalsof the [emim]+ cation of the ionic liquid, and we can also unam-biguously distinguish the partially resolved resonance signals ofthe protonated HDA centered at δ = 25.6 ppm.7e All these NMR

Fig. 5 Polyhedral view of the 4.6.12 network of RUB-A2 along the (a)c and (b) b axis (PO4 tetrahedra filled with black or white color representthe PvO groups pointing up or down the layers, respectively).

Fig. 4 View of templates in the structure of RUB-A1 along the (a) aand (b) c axis (dashed lines indicate H-bonds; some symmetricallyequivalent templates are omitted for clarity).

Fig. 6 View of templates in the structure of RUB-A2 along the (a) cand (b) tilted a axis (dashed lines indicate H-bonds or charge–dipoleinteraction; some symmetrically equivalent templates are omitted forclarity).

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results correspond well with the structure analysis based onX-ray powder data of RUB-A2. It further confirms that both theIL-cations and the added amines reside in the structure, coopera-tively acting as co-templates in the formation of RUB-A2.

In studying the time and temperature dependence of the crys-tallization of RUB-A2, we found that the synthesis productchanged to another phase for extended crystallization times orelevated synthesis temperature. This phase has been identified asSIZ-6 according to the powder X-ray diffraction pattern. SIZ-6was first reported by Morris et al. as an ionothermal productusing the same type of ionic liquid ([emim]Br).7a It is also alayered aluminophosphate in which the [emim]+ cation is locatedin the interlayer region as a template. The 1H → 13C CP MASNMR result (Fig. 8) demonstrates that the template occluded inthe structure of SIZ-6 from our synthesis is exclusively the[emim]+ cation. It indicates that in the ionothermal system withamines added, there is a competition of the structure directingeffect between the amine and the ionic liquid, and only specificsynthesis conditions lead to material containing both SDAtaking advantage of the co-templating effect. Microporous com-pounds and related materials are kinetically rather than thermo-dynamically stable crystalline products. In our example and forthe synthesis conditions described, RUB-A2 is a metastableintermediate phase in the formation of SIZ-6.

In the formation of microporous compounds and relatedmaterials, organic amines are the most common guest species

residing inside pores of the host framework, functioning as tem-plates and space-fillers, or more generally as structure directors.19

From the above results of structure characterization, we can seethat the amines as additional SDA along with the ionic liquidcations reside either between the layers or within the channelsystems and cooperatively direct the formation of 2D layeredstructures. They interact through weak bonds, such as H-bonds,van der Waals forces and Coulomb interaction with the alumino-phosphate layers. It is noteworthy that the layer topologies ofthese two compounds are different from those of the previouslyreported 2D layered aluminophosphate structures, (co-)templatedby a single amine EDA/HDA or together with another organicamine in hydro/solvothermal synthesis.10,16a Especially, thephosphoryl group orientation sequences on the 4.6.8-layer ofRUB-A1 have never been seen before in similar hydro/solvother-mal layered aluminophosphate.9a,16 Generally, the templating orstructure-directing effect of organic amines in open-frameworkmaterials synthesis is not specific to a certain structure. Thesame amines can direct the formation of different structuresdepending on the synthesis conditions such as solvent, startinggel composition, crystallization temperature, time, etc. The suc-cessful synthesis of these two new materials by the co-templat-ing ionothermal method indicates that this strategy is promisingin the preparation of new 2D layered structures. Furthermore, inthe mixed-templates system, the synthesis chemistry is morecomplicated and the product is most likely a mixed-phase. Com-pared with classical synthesis, ionothermal synthesis to a certaindegree removes the competition between solvent–framework andtemplate–framework interaction. The ionic liquids act as bothsolvents and templates, which simplifies the synthesis chemistryand benefits the high phase selectivity of the co-templatessystem. Further experiments demonstrate that a large quantity ofwater (molar ratio water to IL ≥ 2) added to the system results inlower phase selectivity on RUB-A1/A2 for the appearance ofunidentified impurities in the products (Fig. S6 and S7†). Thehigher water content (molar ratio water to IL ≥ 3) even leads tothe complete disappearance of the RUB-A1 phase, which furtherproves the influence of solvent on the templating effect in thehydrothermal and ionothermal synthesis.3a

From RUB-A1 to RUB-A2, the layer topologies vary withincreasing length of the alkyl chain of the amines added to theionothermal system. From our results it is indicated that theamine with a longer alkyl chain length leads to the less densenetwork which was also observed in the hydro/solvothermal syn-thesis previously.16d Various organic amines such as mono-amines, diamines and cycloamines have been shown to besuccessful in the preparation of 2D layered structures in thehydro/solvothermal synthesis. We believe that an extensive studyof other amines or ionic liquids will lead to more novel 2Dlayered structures. This conclusion is also supported by numeri-cal experiments which postulated many hypothetical layer struc-tures as energetically feasible.20

Conclusions

In conclusion, for the first time the co-templating ionothermalstrategy has been successfully used in the preparation of new 2Dlayered aluminophosphate materials. With the addition of EDA

Fig. 8 1H → 13C CP MAS NMR spectra of (a) RUB-A2 and (b)SIZ-6.

Fig. 7 (a) 27Al and (b) 31P MAS NMR spectra of RUB-A2.

12414 | Dalton Trans., 2012, 41, 12408–12415 This journal is © The Royal Society of Chemistry 2012

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or HDA in the [emim]Cl ionothermal system, two new 2Dlayered aluminophosphates RUB-A1 and RUB-A2 have beenprepared. Their structures have been determined from single- orpowder-crystal X-ray diffraction data. Complementary experi-ments such as NMR, ICP and TG-DTA confirmed and substan-tiated the structure analyses. The structures of RUB-A1/A2contain the 4.6.8/4.6.12-net of anionic layers built from alterna-tively vertex-sharing [AlO4]- and [PO3(vO)]-tetrahedral unitsand stacked in an AAAA sequence. The layer topology ofRUB-A1 has a different phosphoryl group orientation sequenceas compared with those of structures which have already beenreported to belong to the 4.6.8-topology. Both the amines andionic liquid cations reside within these two structures coopera-tively acting as co-templates. The increased chain length of theamine as second templates leads to the two different layer struc-tures, nicely highlighting the structure directing effect of the co-template. These results demonstrate that co-templating ionother-mal synthesis is an effective method for the preparation of new2D layered aluminophosphate materials. We believe that othernovel 2D layered structures will be obtained applying this strat-egy in future studies.

Acknowledgements

We gratefully acknowledge BASF SE for providing us with ionicliquids.

Notes and references

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