Ionothermal Synthesis of MnAPO-SOD Molecular Sieve without theAid of Organic Structure-Directing AgentsHao Liu,†,§ Zhijian Tian,*,†,‡ Lei Wang,† Yasong Wang,†,§ Dawei Li,†,§ Huaijun Ma,† and Renshun Xu†

†Dalian National Laboratory for Clean Energy and ‡State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, ChineseAcademy of Sciences, Dalian 116023, China§University of Chinese Academy of Sciences, Beijing 100049, China

*S Supporting Information

ABSTRACT: An SOD-type metalloaluminophosphate mo-lecular sieve (denoted as SOD-Mn) was ionothermallysynthesized by introducing manganese(II) cations into thereaction mixture via MnO−acid or MnO2−reductant reactions.Composition and structure analyses results show that twokinds of manganese(II) cations exist in the SOD-Mn structure.Part of the manganese(II) cations isomorphously substitutethe framework aluminum(III) with a substitution degree of∼30%. The rest of the manganese(II) cations occupy a fractionof the sod cages in their hydrated forms. A comprehensiveinvestigation of the synthesis parameters, crystal sizes, andcrystallization kinetics indicates that the in situ releasedhydrated manganese(II) cations direct the formation of SOD-Mn. Such structure-directing effect may be inhibited by both the fluorination of manganese(II) cations and the wateraccumulation during crystallization. In the fluoride anion-containing reaction mixture with a low ionic liquid content, thecrystallization process is strongly suppressed, and large SOD-Mn single crystals of over 200 μm in size are yielded. SOD-Mn isfree from organics and shows improved thermal stability compared with metalloaluminophosphates synthesized by using organicstructure-directing agents.

■ INTRODUCTION

Zeolitic molecular sieves have crystalline microporousstructures and can provide adjustable reaction environmentfor shape-selective catalysis.1−3 These molecular sieves aretypically synthesized hydrothermally in the presence of organicstructure-directing agents, which are always poisonous,expensive, and difficult to recycle.4−6 Numerous attemptshave been made to eliminate those drawbacks, and it was foundthat organic structure-directing agents can be replaced by metalcations in some cases.6 For example, both zeolites ZSM-54,7,8

and ECR-1,9 which were originally synthesized in the presenceof tetrapropylammonium hydroxide and bis(2-hydroxyethyl)-dimethylammonium chloride, respectively, can be formed inorganic-free systems by using sodium(I) cations as thestructure-directing agents.Because of their high charge/volume ratios, metal cations in

principle can direct the formation of highly charged frame-works, such as those of aluminum-rich zeolites and highlysubstituted metalloaluminophosphates (MeAPOs). The struc-ture-directing role of alkali and some alkaline-earth (calcium-(II), strontium(II), and barium(II)) cations in the formation ofaluminum-rich zeolites has been well-established.10 However,alkali/alkaline-earth cations are seldom used as structure-directing agents in the hydrothermal synthesis of highlysubstituted MeAPOs, probably because transition metal cations

strongly compete against them for the interaction with watermolecules.11 However, although a certain amount of transitionmetal cations should exist in the synthesis mixtures forMeAPOs in free form, no definite identification has beenprovided on their structure-directing effect in hydrothermalconditions.Ionothermal method, in which either ionic liquid or deep

eutectic solvent is employed as the reaction medium, canprovide a nonaqueous condition for the synthesis andmechanism study of molecular sieves.12−15 Interestingly, itwas found that in ionothermal environment, cobalt(II)16 andmagnesium(II)17 cations added into the reaction mixtures playadditional structure-directing roles in the formation ofMeAPOs along with the ionic liquid cations. Recently, wereported the ionothermal synthesis of an open-frameworkfluorinated aluminum phosphite-phosphate (DNL-2).18

Although Mn2+ and 1-ethyl-3-methylimidazolium (EMIm)cations coexist in the cages of DNL-2, only the Mn2+ cationsdemonstrate the ability to direct the formation of DNL-2.Herein, another example of transition metal cation-directed

ionothermal synthesis of molecular sieves is presented. AnSOD-type MnAPO molecular sieve (denoted as SOD-Mn) was

Received: November 23, 2015Published: January 28, 2016

Article

pubs.acs.org/IC

© 2016 American Chemical Society 1809 DOI: 10.1021/acs.inorgchem.5b02700Inorg. Chem. 2016, 55, 1809−1815

ionothermally synthesized by using hydrated manganese(II)cations as structure-directing agent. Unlike the previouslyreported cases,16−18 no EMIm cations were embedded in theSOD-Mn structure though the synthesis took place in[EMIm]Br. This work shows an efficient way to introducetransition metal cations into molecular sieves with controllablelocation and aggregation mode and, moreover, opensopportunities for the convenient synthesis and application ofMeAPOs.

■ EXPERIMENTAL SECTIONSynthesis Procedure. The reaction mixture was stirred at 90 °C

for 1 h and then transferred into a polytetrafluoroethylene (PTFE)-lined stainless autoclave (capacity: 30 mL) and heated at 200 °C for 7d. After crystallization, the product was cooled to room temperatureand washed with ethanol. Large crystals found in samples C, D, E, andG were separated from the powders by sonication and decantation. Allthe solid product fractions were filtered and then dried at 120 °Covernight. To avoid the seeding effect, the PTFE liners were cleanedby immersing in concentrated hydrochloric acid overnight after eachuse.Crystallinity Measurement. According to the methodology

provided by Alexander & Klug,19 when the internal standard isadded into the sample with a constant weight fraction (xs), the weightfraction of the component i (xi) is proportional to the ratio of thediffraction intensities Ii/Is in following manner

ρρ

=−

· = ·xK x

K xII

kII(1 )i

i

i

ii

is s

s s s s

where K is a function of both the nature of the component and thegeometry of the apparatus, and ρ is the density of the component.Therefore, the relative crystallinity of each phase can be expressed bythe ratio of Ii/Is. In our experiment, the internal standard gibbsite(Al(OH)3) was mixed into each sample (containing all the solidproduct fractions) with a weight fraction of 20.0%. The diffraction dataof these mixtures were collected by using a PANalytical X’pert Prodiffractometer with nickel-filtered Cu Kα radiation (wavelength:1.5419 Å). The step size was 0.0080° in 2θ. The powder X-raydiffraction (XRD) patterns were analyzed by using X’pert Highscoreversion 1.0e software (PANalytical). Kα2 stripping and peak searchwere performed in succession. The intensities of the Kα1 peaks with d-spacing values of 9.43 (CHA-type molecular sieve) and 3.70 Å (SOD-Mn) were measured, respectively, and then the Ii/Is values werecalculated.Structure Determination. The structure of SOD-Mn was solved

by using single-crystal XRD technique. The data were collected on anXcalibur Atlas Gemini ultra diffractometer equipped with an EnhanceX-ray source (graphite monochromatized Mo Kα radiation, wave-length: 0.71073 Å) and a CCD detector. By using SHELXT-2014/5,20

the space group of SOD-Mn was determined, and the structure wassolved. The positions of Mn(1)/Al(1), P(1), and Mn(2) wereobtained first. The structure was refined on F2 by full-matrix least-squares using SHELXL-2014/7.21 The O(1) and O(2) atoms werelocated from difference Fourier maps. The framework atoms and theextra-framework Mn(2) atom were refined with anisotropic thermalparameters. Additional crystallographic information is available in theSupporting Information.

■ RESULTS AND DISCUSSIONEffect of Reaction Mixture Composition on the

Formation of SOD-Mn. Synthesis details of samples A to Gare listed in Table 1. After crystallization at 200 °C for 7 d, amixture of fluorinated CHA-type molecular sieve and low-cristobalite type AlPO4 (sample A) was synthesized from thereaction mixture containing H3PO4, Al[OCH(CH3)2]3, HF,and [EMIm]Br. When MnCl2·4H2O was added into abovereaction mixture, the product transformed into a mixture of

AEL and fluorinated CHA-type molecular sieves (sample B).Besides being introduced into the reaction mixture in salt form,manganese(II) cations can also be in situ released by theMnO−acid (sample C) and MnO2−reductant (samples D andE) reactions. In these cases, large rhombic dodecahedral singlecrystals, which are pale yellow-green and over 200 μm in size,were formed and could be separated by sonication anddecantation (Figure 1). Powder XRD analysis shows that

these separated crystals are SOD-type molecular sieve (SOD-Mn) with high phase purity (Figure 2). SOD-Mn can also besynthesized in the fluoride anion-free (sample F) and diluted

Table 1. Details of Synthesis and Crystal Separation Results

samplea reaction mixture compositionb (mmol) large crystals separated

A 5 H3PO4/2.5 Al[OCH(CH3)2]3/1 HF/50 [EMIm]Br

(powder)c

B 5 H3PO4/2.5 MnCl2·4H2O/2.5Al[OCH(CH3)2]3/1 HF/50 [EMIm]Br

(powder)c

C 5 H3PO4/2.5 MnO/2.5Al[OCH(CH3)2]3/1 HF/50 [EMIm]Br

SOD-Mn (>200 μm)

D 5 H3PO4/2.5 MnO2/2.5 Al(OH)3/2.5H2C2O4·2H2O/1 HF/50 [EMIm]Br

SOD-Mn (>200 μm)

E 2.5 H3PO3/2.5 H3PO4/2.5 MnO2/2.5Al(OH)3/1 HF/50 [EMIm]Br

SOD-Mn (>200 μm)

F 5 H3PO4/2.5 MnO/2.5Al[OCH(CH3)2]3/50 [EMIm]Br

(powder)c

G 5 H3PO4/2.5 MnO/2.5Al[OCH(CH3)2]3/1 HF/100 [EMIm]Br

SOD-Mn (<50 μm) +byproduct particles

aCrystallization condition: 200 °C, 7 d. bH3PO4: 85 wt % in water;HF: 40 wt % in water; [EMIm]Br: 1-ethyl-3-methylimidazoliumbromide; Al(OH)3: gibbsite.

cNo large particles could be separated outby sonication and decantation.

Figure 1. Scanning electron microscopy images of large SOD-Mnsingle crystals (from sample C).

Figure 2. Experimental and simulated powder XRD patterns of largeSOD-Mn single crystals (from sample C). Cu Kα radiation.

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(sample G) conditions. Rhombic dodecahedral single crystals,which were proved to be SOD-Mn by powder XRD analysis,could be also found in these two samples.Composition and Structure of SOD-Mn. As mentioned

above, the absence of HF in the reaction mixture does notdisturb the formation of SOD-Mn. This result indicates thatfluoride anions play no more than the role of mineralizer andare excluded from the final structure of SOD-Mn. Thisinference is supported by the X-ray fluorescence spectrum ofSOD-Mn, in which no characteristic peaks of fluorine can befound.Figure 3 shows the Fourier transform infrared (FTIR)

spectrum of SOD-Mn in selected regions. As a reference, the

spectrum of DNL-2,18 the cages of which are filled with Mn2+

and EMIm cations, is also shown. The characteristic vibrationpeaks of EMIm cations can be found in the spectrum of DNL-2but are absent in that of SOD-Mn. Considering that EMImcations are the only organic species that always exist in thereaction mixture, it can be concluded that SOD-Mn is free fromorganics.The inductively coupled plasma optical emission spectros-

copy (ICP-OES) and single-crystal XRD analysis results (Table2) show that SOD-Mn separated from different samples have

the same composition and structure within reasonable statisticalfluctuation. As the ICP-OES results show, the molar ratio (Mn+ Al)/P of SOD-Mn is above unity. In aluminophosphateframework, only aluminum(III) can be isomorphouslysubstituted by the bivalent manganese(II) cations.22 Therefore,the excess of manganese in SOD-Mn should be attributed tothe existence of extra-framework manganese-containing species.Single-crystal XRD analysis results (Table 3 and Supporting

Information) confirm that two kinds of manganese(II) cations

exist in the structure of SOD-Mn. Part of the manganese(II)cations (Mn(1)) isomorphously substitute the frameworkaluminum(III) (Al(1)) with a substitution degree of ∼30%.The disordered Mn(1)/Al(1) distribution makes the topo-logical symmetry of SOD-Mn Pm3 n (No. 223), which is lowerthan the idealized symmetry of the SOD-type framework(Im3 m, No. 229).23 The rest of the manganese(II) cations(Mn(2)) occupy the centers of the sod cages. The ratio of thetwo kinds of manganese(II) cations is ∼3.7/1. The consistencyof the Mn/Al/P ratios obtained from structure analysis andICP-OES (Table 2) suggests the reasonableness of thestructure analysis results.In SOD-Mn, the extra-framework manganese(II) cations

(Mn(2)) occupy ∼24% of the sod cages and balance ∼54% ofthe framework charge. Around Mn(2) atom, two kinds ofresidual electron density peaks (Q peaks) can be always foundin the difference Fourier maps with Mn(2)−Q distances of2.0−2.2 Å. The sole occupancy of an sod cage with a singleMn2+ cation is energetically unfavorable. Judging by theirdistances to Mn(2) atom, these Q peaks should be assigned asoxygen atoms of the water molecules coordinating to Mn(2)atom.24 These two kinds of Q peaks occupy the Wyckoffpositions 16i and 12f and correspond to the coordinationmodes of [Mn(H2O)4]

2+ and [Mn(H2O)6]2+, respectively

(Figure 4). Although these Q peaks are too weak (0.2−0.4 e·Å−3) to be indisputably assigned in the last refinement circles,the traces of the coordinated water molecules can be found inthe FTIR spectrum of SOD-Mn (Figure 3). The sharp peak at1487 cm−1 and the broad bands in the region of 2950−2800cm−1 are attributable to the vibrations of these watermolecules.25

The difference Fourier maps show that besides the hydratedmanganese(II) cations, a group of water molecules (O(2)) alsoorderly exist in the sod cages of SOD-Mn. They may be thecarriers of protons that balance the residual negative charge inthe framework. Because the distance between Mn(2) and O(2)(∼1.5 Å) is too short for any reasonable chemical bonds, thesewater molecules should not coexist with the hydratedmanganese(II) cations in the same sod cages. The electrondistribution calculated from the difference Fourier mapindicates that each sod cage that is free from the hydratedmanganese(II) cations contains ca. four water molecules.Accordingly, the formula of SOD-Mn can be written as |H0.14[Mn(H2O)n]0.08(H2O)1.01|[Mn0.30Al0.70PO4] (n = 4 or 6).

Structure-Directing Effect in the Formation of SOD-Mn. Previously, some work has been published on theionothermal synthesis of MeAPO-SOD molecular sieves.16,26,27

Li et al. found that in the synthesis conditions they designed,tetramethylammonium cations are the necessary structure-directing agent for constructing the SOD-type framework.27

However, both the papers of Parnham & Morris16 and Han etal.26 showed that MeAPO-SOD molecular sieves can beionothermally synthesized in the absence of extra-addedamine. However, no further discussion on the formationmechanism was provided in these two papers.SOD-Mn synthesized in this work contains two kinds of

guest species, that is, hydrated manganese(II) cations and watermolecules (or their protonated form). The synthesis resultsshow that SOD-Mn can form only in the presence of in situreleased manganese(II) cations (samples C to G). Thissuggests that the hydrated manganese(II) cations are thestructure-directing agent in the formation of SOD-Mn. All thestructure-directing processes of metal cations reported herein

Figure 3. FTIR spectra of SOD-Mn (from sample C) and DNL-218 inselected regions. Characteristic vibration peaks of 1-ethyl-3-methyl-imidazolium cations in DNL-2 are indicated.

Table 2. Mn/Al/P Molar Ratiosa of SOD-Mn from DifferentSamples

Mn/Al/P molar ratio

SOD-Mn ICP-OES single-crystal XRD analysis

from sample C 0.36/0.72/1.00 0.37/0.71/1from sample D 0.36/0.74/1.00 0.38/0.70/1from sample E 0.36/0.72/1.00 0.39/0.70/1

aThe values determined by both ICP-OES and single-crystal XRDanalyses are listed.

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and previously16−18 take place in imidazolium bromide ionicliquids. This feature, along with the phenomena found inhydrothermal conditions, suggests that the structure-directingability of metal cations is strongly determined by the nature ofsolvents.One of the key steps involved in the structure-directing effect

of metal cations is the replacement of solvent molecules aroundthese cations with framework species.10 The dehydrationprocesses of beryllium(II), magnesium(II), and transitionmetal cations have relatively high energy barriers.28 As a result,in hydrothermal conditions, such replacement step for thesecations is difficult to achieve, and the overall structure-directingeffect is always inhibited.29 In imidazolium bromide ionicliquids, metal cations dissolve by weak interactions withbromide anions.30 The consequent reduction of the solvationeffect makes the interaction between metal cations andframework species much more effective and therefore improvesthe structure-directing process. Additionally, the structure-directing effect of transition metal cations may not be confinedto only ionothermal conditions. Extra-framework transitionmetal species are frequently found in the molecular sievessynthesized via conventional methods.31,32 These species canbalance the framework charge and more or less play thestructure-directing role during framework construction. How-ever, in conventional synthesis conditions, such structure-directing effect is always too weak to be clearly revealed.Crystallization Kinetics of SOD-Mn. To obtain more

details about such structure-directing effect, studies on thecrystallization kinetics of SOD-Mn were performed. Thecrystallization kinetics of samples C, F, and G were studiedby semiquantitative powder XRD analyses. The time-depend-ent relative crystallinity curves (RC∼t curves) of SOD-Mn insamples C, F, and G are shown in Figure 5. The relative heightsof these three curves indicate that the relative nutrient amountsof SOD-Mn in the reaction mixtures follow the order C < F <G. Furthermore, it can be deduced that fluoride anions (sampleC vs F) and the concentration of the reaction mixture (sampleC vs G) can influence the crystallization process of SOD-Mnsignificantly. The RCSOD‑Mn∼t curve of sample C shows a

typical sigmoidal shape with an induction period (t = 1−3 d)and a quasi-stationary region (t = 5−7 d). Similarly, a quasi-stationary region (t = 3−7 d) can also be observed in theRCSOD‑Mn∼t curve of sample G. However, this curve sharplyincreases in the beginning period (t = 1−3 d) without anobservable induction period. Unlike samples C and G, sample Fhas a linear RCSOD‑Mn∼t curve.The absence of fluoride anions (sample F) and the dilution

of the reaction mixture (sample G) cause significant decreasesin the crystallization of large SOD-Mn single crystals (Table 1).As the scanning election microscopy (SEM) images (Figure 6)

show, the sizes of single crystals collected from sample Gdecrease to less than 50 μm. During the SEM examination ofsample F, rhombic dodecahedral single crystals were occasion-ally observed but could not be separated out by sonication anddecantation due to their extremely small amount. Thesephenomena coincide with the crystallization kinetics propertiesdescribed above. In the quasi-stationary regions of theRCSOD‑Mn∼t curves of samples C and G, the nutrients forSOD-Mn are nearly exhausted, and coarsening occurs to attaina lower energy state.33 As a result, the amounts of large SOD-Mn single crystals in these two samples would increase.

Table 3. Positional and Occupancy Parametersa for SOD-Mn

atom Wyckoff position x y z occupancy

Mn(1) 6c 3/4 0 1/2 0.293(9)Al(1) 6c 3/4 0 1/2 0.707(9)P(1) 6d 1 1/4 1/2 1Mn(2) 2a 1/2 1/2 1/2 0.237(14)O(1) 48l 0.8665(5) 0.1552(5) 0.5359(6) 1/2O(2) 24k 0.615(3) 0.386(3) 1/2 0.254(5)

aFrom sample C.

Figure 4. Two possible coordination modes of the hydratedmanganese(II) cations in SOD-Mn. Large balls: Mn(2) atoms; smallballs: oxygen atoms of the coordinated water molecules.

Figure 5. Time-dependent relative crystallinity curves of SOD-Mn andthe CHA-type molecular sieve in samples C, F, and G.

Figure 6. SEM images of crystals separated from sample G. They are amixture of SOD-Mn crystals and large particles of the byproduct.

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However, because the amount of SOD-Mn crystals in sample Gis much larger than that in sample C, the coarsening effect ofsample G is less pronounced than that of sample C. Therefore,the sizes of crystals separated from sample G are smaller thanthose from sample C. The linear RCSOD‑Mn∼t curve of sample Findicates that at the end of the crystallization time (t = 7 d), thenucleation and growth of SOD-Mn is still in process and thatthe exhaustion of nutrient is not yet reached. Therefore, forsample F, the formation of small SOD-Mn crystals is dominant.CHA-type molecular sieve, which is the main byproduct in

samples C to G, is selected as reference for the crystallizationkinetics studies. Similar to SOD-Mn, the CHA-type molecularsieve can be synthesized in the absence of fluoride anions, andtherefore, it is a fluoride anion-free molecular sieve. Expansionof its unit cell suggests the incorporation of manganese(II)cations into its framework. Crystallization process of the CHA-type molecular sieve is not markedly influenced by fluorideanions. During the observation time (t = 1−7 d), the RCCHA∼tcurves of samples C and F nearly coincide with each other andvary only slightly (Figure 5). Because the CHA-type molecularsieve and SOD-Mn possess similar framework compositions toeach other, the difference between their crystallization kineticsshould be attributed to the different sensitivities of theirstructure-directing agents to fluoride anions. EMIm cations areof suitable size to occupy cha cages and can direct theformation of CHA structure.34 Because of their abundance inthe reaction mixture, their existence mode and relative amountare little influenced by the addition of fluoride anions. But influoride anion-containing conditions, the fluorination ofmanganese(II) cations is inevitable. These fluorinated ionshave lower positive charge compared with the hydratedmanganese(II) cations and, as a result, are less suitable tobalance the framework negative charge. Furthermore, unlikewater molecules, fluoride anions cannot form hydrogen bondswith the framework oxygen atoms. Therefore, the metalcation−framework species interaction and the consequentstructure-directing effect are depressed in such situation.Comparably, when manganese(II) cations are introduced intothe reaction mixture in the form of MnCl2·4H2O (sample B),the strong Mn−Cl interaction fully inhibits the formation ofhydrated manganese(II) cations in a way similar to thefluorination effect. Thus, no SOD-type molecular sieve canform in such condition. Similar result was also obtained in thestudy on the ionothermal synthesis of DNL-2.18

An unusual phenomenon of the crystallization kinetics ofSOD-Mn is that the formation of SOD-Mn is stronglyimproved by diluting the reaction mixture (sample G).According to the general rules of molecular sieve crystallization,the decrease of supersaturation of the reaction mixture wouldslow the rates of both crystal nucleation and growth, while thenucleation rate decreases more sharply than the growth rate.35

Therefore, the dilution of the reaction mixture should inprinciple result in the formation of larger crystals but with alower yield. However, as the RCSOD‑Mn∼t curve of sample Gshows, when the reaction mixture is diluted, the nutrientamount of SOD-Mn indeed increases. This phenomenonshould be also attributed to the influences on hydratedmanganese(II) cations. When reaction mixture is diluted byincreasing the [EMIm]Br amount, the fluorination degree ofmanganese(II) cations is reduced because of the isolation offluoride anions. Therefore, the amount of hydrated manganese-(II) cations increases, and consequently the crystallization ofSOD-Mn is improved. But if the crystallization kinetics of

SOD-Mn were influenced by only the fluorination effect, thefluoride anion-free reaction mixture (sample F) would containmore nutrient of SOD-Mn than the fluoride anion-containingones (samples C and G). Such assumption is inconsistent withthe observed order of relative nutrient amounts (C < F < G).This suggests that dilution brings some other improvement onthe structure-directing ability of hydrated manganese(II)cations. In ionothermal conditions, the existence of water isunavoidable because water may be introduced into the reactionmixture accompanying raw materials, and will be in situgenerated during molecular sieve crystallization.36 A largeamount of water accumulation may disturb the ionothermalcharacters of the reaction environment.37 The addition of alarger amount of [EMIm]Br (sample G) may delay thisdisturbing effect and, as a result, improve the SOD-Mncrystallization.Xu et al.’s extensive studies on the synthesis of molecular

sieves have shown that structure-directing agents influence thestructure of the final product through the roles such as charge-balancing, space-filling, and structure-stabilizing,38 and theseeffects are strongly determined by the local reaction environ-ment.39−41 The hydrated manganese(II) cations have suitablecharge and size to direct the formation of sod cages and,therefore, can compete with EMIm cations to play thestructure-directing role. The present work, along with Li etal.’s study,27 suggests that EMIm cations are not the properstructure-directing agent for the formation of sod cages, andthose ions are always excluded from the MeAPO-SODstructures. This phenomenon should be attributed to thesize-mismatch between the EMIm cations and the sod cages.Accordingly, it can be deduced that the structure-directingeffect of transition metal cations in ionothermal synthesis canbe strengthened by optimizing the local environment for themetal cation-directed crystallization process.

Thermal Stability of SOD-Mn. MeAPOs are of specialresearch interest because they can be used, beyond the scope ofacid catalysis, to the selective redox reactions for fine chemicalsynthesis.42−44 However, in practice the wide application ofMeAPOs is seriously hindered by their limited thermal stability,which significantly decreases with the increase in theirsubstitution degrees. Calcination of the as-synthesized molec-ular sieves is always required to remove the embedded organicsthoroughly and make the active centers accessible. To achievethis aim, the calcination temperature must reach at least 450°C.45 Such high temperature often causes hydrolytic breakdownof the frameworks. Moreover, the damage should be furtherexacerbated by the heat and steam generated during thecombustion of the organics.5,46 For example, Li et al.’s studyshows that frameworks of the SOD-type molecular sieves(C4NH12)0.88[Me0.88Al2.12P3O12] (Me = Co and Zn) stronglycollapse when calcined at 550 °C.27 We also examined thethermal stability of SOD-Mn, which has a similar frameworksubstitution degree compared with those synthesized by Li etal. As the powder XRD pattern (Figure 7) shows, theframework of SOD-Mn partially remains even after calcinationat 550 °C for 12 h. Though the thermal decompositionresulting from the inherent instability of the substitutedframework is unavoidable, the absence of organics in SOD-Mn makes the combustion effect on the stability negligible. Bycomparing the powder XRD patterns of the calcined samples, itcan be deduced that SOD-Mn has improved thermal stabilitycompared with the samples synthesized by using organicstructure-directing agents.

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The calcination temperature of MeAPOs required for theremoval of organics is always much higher than the operatingtemperature of MeAPO-based catalysts. Therefore, therestriction on thermal stability of MeAPOs is indeed originatedfrom the use of organic structure-directing agents. In somesenses, the organic-free molecular sieves can be applied directlywithout calcination. For example, one of the most significantchallenges in the preparation of zeolitic membranes is theminimization of defects and cracks formed during postsynthesisthermal treatment.47,48 However, the organic-free zeolitemembranes such as NaA,49 NaY,50 and hydroxy sodalite51

can be directly used for separation without calcination and thusshow great convenience for application. Similar convenienceseems can be brought by the organic-free molecular sieve SOD-Mn reported herein.

■ CONCLUSIONSSOD-Mn, the organic-free MnAPO-SOD molecular sieve withframework composition [Mn0.30Al0.70PO4]

0.30− was synthesizedin [EMIm]Br by using the in situ released hydratedmanganese(II) cations as structure-directing agent. Thisstructure-directing effect relies on the relatively weak solvationability of [EMIm]Br and could hardly take place in hydro-thermal conditions. As an organic-free molecular sieve, SOD-Mn shows improved thermal stability compared with themolecular sieves synthesized by using organic structure-directing agents. The synthesis strategy of SOD-Mn could beextended to the preparation of other transition metal-containing zeolitic catalysts and zeolite-based devices andenhance convenience in the application of them.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.inorg-chem.5b02700. CCDC 1446533−1446535 contain the supple-mentary crystallographic data for this paper. The data can beobtained free of charge from The Cambridge CrystallographicData Centre via www.ccdc.cam.ac.uk/structures.

Crystallographic information files (CIFs) for SOD-Mnfrom samples C, D, and E. (CIF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: tianz@dicp.ac.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors acknowledge National Natural Science Foundationof China for its funding support (Grant No. 21373214).

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Figure 7. Powder XRD pattern of SOD-Mn (from sample C) aftercalcination at 550 °C for 12 h. Cu Kα radiation.

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