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Microporous and Mesoporous Materials 217 (2015) 54e62

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate/micromeso

Ionothermal synthesis of a CHA-type aluminophosphate molecularsieve membrane and its formation mechanism

Xiaolei Li a, b, Keda Li a, Huaijun Ma a, Renshun Xu a, Shuo Tao a, b, Zhijian Tian a, c, *

a Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Chinab University of Chinese Academy of Sciences, Beijing 100049, Chinac State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

a r t i c l e i n f o

Article history:Received 26 March 2015Received in revised form29 May 2015Accepted 3 June 2015Available online 12 June 2015

Keywords:AluminophosphateIonothermal synthesisCHA membraneFormation mechanismSolid transformation

* Corresponding author. Dalian National LaboratoInstitute of Chemical Physics, Chinese Academy of ScTel./fax: þ86 0411 84379151.

E-mail address: tianz@dicp.ac.cn (Z. Tian).

http://dx.doi.org/10.1016/j.micromeso.2015.06.0051387-1811/© 2015 Elsevier Inc. All rights reserved.

a b s t r a c t

A gas permeable CHA-type aluminophosphate molecular sieve membrane was ionothermally synthe-sized on a porous d-alumina substrate through the so-called “substrate surface conversion” method. Thesynthetic conditions for this specific membrane were optimized. The temperature, time and the amountsof H3PO4 and HF in the initial synthesis solution were investigated. X-ray diffraction (XRD), scanningelectron microscopy (SEM) and gas permeation were performed to characterize the entire formationprocess for the CHA-type molecular sieve membrane. A formation mechanism of the “substrate surfaceconversion” method was proposed. The transformation of the gel layer to the CHA membrane via a solidtransformation mechanism was suggested.

© 2015 Elsevier Inc. All rights reserved.

1. Introduction

Molecular sieve membranes, which present high separationselectivities because of well-defined sub-nanometer pores andadsorption properties, possess superior thermal, mechanical andchemical properties over polymer membranes. Since the firstpreparation of molecular sieve membranes patented by Suzuki in1987 [1], extensive work has been performed on the preparationand characterization of various types of zeolite films and mem-branes [2e4]. A number of synthetic strategies have been applied tozeolite membrane synthesis, such as in-situ (without seeding)synthesis [5], secondary (seeded) growth method [6], vapor phasetransport method [7], and post-treatments of zeolite membranes[8]. The most widely used synthetic strategies are in-situ synthesisand secondary growth, both of which are derived from the strategyof zeolite hydrothermal synthesis. Thus far, over 14 zeolite struc-tures have been successfully synthesized as zeolite membranes [9].

As a novel method, ionothermal synthesis, which uses ionicliquids as the reaction media instead of water and/or other organic

ry for Clean Energy, Dalianiences, Dalian 116023, China.

solvents, is not only an efficient way to prepare aluminophosphatemolecular sieves under ambient pressure [10e17] but also apromising method for the preparation of aluminophosphate mo-lecular sieve membranes [18]. Yan and coworkers have reportedionothermal synthesis of AlPO4-11 and SAPO-11 films on aluminumalloys by microwave heating. Recently, we reported an ionothermalapproach for the synthesis of permeable aluminophosphatemolecular sieve membranes on porous alumina discs through aso-called “substrate surface conversion” method [19,20]. Duringthe synthesis, the substrate not only acted as a support for themembrane but also as an Al source. The surface of the substratewasconverted into themembrane by this novel method. Different typesof molecular sieve membranes, including CHA, AEL, AFI and LTA,were synthesized.

The CHA-type molecular sieves have structural similarity to thenatural zeolite chabazite, which has a pore diameter of 0.38 nm andinternal cage diameters of approximately 1.4 nm. These dimensionshave the potential to improve light gas separation by exploitingsize differences between the gases. After Zhang et al. [21] firstsynthesized SAPO-34 membranes on alumina disks and reportedtheir single gas permeances, efforts have been made to use thisstructure as a membrane for the separation of light gas mixtures[22e24]. A CHA membrane with an aluminophosphate structurehas also been synthesized by an in-situ hydrothermal method [25].However, the synthesis of a gas permeable membrane with a pure

Table 1Synthetic conditions for the preparation of CHA membranes.

Solution composition (molar ratio) T (d) T (�C) Product

M1 1H3PO4:1HF:1N-MIm:40[EMIm]Br 4 120 CHAM2 1H3PO4:1HF:1N-MIm:40[EMIm]Br 4 160 CHAM3 1H3PO4:1HF:1N-MIm:40[EMIm]Br 4 180 CHAM4 1H3PO4:1HF:1N-MIm:40[EMIm]Br 4 200 CHA þ LTA

X. Li et al. / Microporous and Mesoporous Materials 217 (2015) 54e62 55

aluminophosphate CHA structure had not yet been reported untilthe method of “substrate surface conversion” was presented. Tooptimize the synthetic conditions for a CHA-type molecular sievemembrane with high efficiency, and generalize this novel methodto other types of molecular sieve membranes, a fundamental un-derstanding of the mechanism of membrane formation is of greatimportance. In the present paper, the entire formation process ofthe CHA membrane was investigated and a formation mechanismof the CHA membrane was proposed.

2. Experimental

2.1. Membrane synthesis

The CHA-type molecular sieve membranes were prepared ac-cording to the method reported elsewhere [19]. The typical syn-thesis procedure of the “substrate surface conversion” methodcan be described as follows: a d-aluminia disc was placed into a1-ethyl-3-methyl imidazolium bromide ([EMIm]Br) solutioncontaining reagent quantities of phosphoric acid (H3PO4, 85 wt%in water, AR), hydrofluoric acid (HF, 40 wt% in water, AR) andN-methylimidazole (N-MIm). The synthesis was performed at120e200 �C for 0.5e4 d. Notably, no additional Al source wasadded to the synthesis solution. After the synthesis, the as-synthesized sample was washed several times with distilledwater and acetone by ultrasonication and dried overnight at110 �C. Before the gas permeation measurements, the templateshould be removed by calcination at 500 �C for 8 h using 0.2 �C/min as the heating and cooling rate. The CHA membrane on oneside of the disc was retained, and the membrane on the other sideof the disc was effaced prior to calcination.

2.2. Membrane characterization

The structural evolution of CHA-type molecular sieve mem-branes formed on the surface of substrate was tracked by X-raydiffraction (XRD), whichwas performed on a PANalytical X'Pert PROdiffractometer fitted with a CuKa radiation source (l ¼ 1.5418 Å)operating at 40 mA and 40 kV. The relative crystallinities representthe intensity contributions from the Bragg reflection (100) for CHA,and these data were normalized to 100%, which corresponds tocomplete crystallization. The morphological evolution of CHA-typemolecular sieve membranes was tracked by scanning electron mi-croscopy (SEM), which was performed on a Hitachi TM3000 in-strument operating at 15 kV. The integrity and continuity of thesynthesized membrane was investigated by SEM top views. Thecompactness and thickness of the membranes was demonstratedby SEM cross-sectional views.

Fig. 1. XRD patterns of the CHA-type AlPO4 membranes prepared at different tem-peratures: M1 (120 �C), M2 (160 �C), M3 (180 �C), M4 (200 �C), C indicates thediffraction peaks from the LTA phase.

2.3. Single gas permeation measurements

The single gas permeation experiments were conducted at roomtemperature in an apparatus that was fabricated in-house [19]. Thegas pressure p on the feed side of the membrane was maintained at0.1 MPa, whereas the permeation sidewas at ambient pressure. Thepermeate gas flow rate (V/t)i through the membrane was measuredwith a soap bubble flow meter. The data were obtained whensteady state conditions were attained. The permeance Pi for eachgas was calculated as:

Pi ¼�ðV=tÞi

�p��S;

where S is the efficient permeation area of the membrane. The idealseparation factor (aH2/i) is equal to the ratio of PH2 to Pi.

3. Results and discussion

3.1. Effect of the synthesis parameters

3.1.1. Effect of the synthesis temperatureThe synthetic conditions of M1e4 are listed in Table 1. The

corresponding XRD patterns are shown in Fig.1. The intensity of theCHA reflection peaks increased with the rise of temperature from120 to 180 �C. The impure phase of the LTA structure could beclearly detected when temperature rose to 200 �C. These results areconsistent with the SEM images illustrated in Fig. 2. As shown inFig. 2a, only a few crystals could be observed on the surface of thesubstrate when the synthesis was performed at 120 �C. Continuousand dense membranes were formed when the syntheses wereperformed at 160 and 180 �C (Fig. 2b, c). As shown in Fig. 2d, cubicpolyhedron of LTA crystals could be easily distinguished fromtriclinic CHA crystals.

Based on the results of the above characterization, it can beconcluded that the reaction temperature is an important factorinfluencing the synthesis of CHA membranes by the “substratesurface conversion”method. The reaction rate increases with a risein temperature under our experimental conditions. Relatively hightemperatures are favorable for the synthesis of CHA membrane. Acontinuous and dense CHA membrane can be obtained between160 and 180 �C. The unsatisfying crystallinity and integrity of M1can be explained by the low reaction rate, which was influenced bythe low temperature. The high temperature can influence thecrystallization kinetics, thus altering the product selectivity [26]. At200 �C, the impure phase of LTA appeared. In addition, the

Fig. 2. SEM top views of CHA-type AlPO4 membranes prepared at different temperatures: a) M1 (120 �C), b) M2 (160 �C), c) M3 (180 �C), d) M4 (200 �C), LTA crystals are circled.

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decomposition of ionic liquid at high temperatures has been re-ported [11,27,28]. In our system, [EMIm]Br was likely to decomposeat 200 �C, and the organic amines in the decomposed productsprobably direct the formation of LTA [29].

3.1.2. Effects of the amounts of H3PO4 and HFThe reactivity of the reactant source is a great factor influencing

the synthesis of zeolites [30]. In our previous work, the reactivity ofthe substrate has been investigated [19]. The porous d-aluminasubstrate can act as both a support and Al source because it pos-sesses appropriate reactivity. In addition to the material of thesubstrate, the influence of H3PO4 and HF was also investigated byperforming the synthesis in solution with different amounts ofH3PO4 and HF. The experimental conditions are listed in Table 2.

Fig. 3 shows the relative crystallinity of CHA membranes syn-thesized with different amounts of H3PO4 (x) in the initial solution.The corresponding SEM images are shown in Fig. 4. Compared withthe initial d-alumina substrate, no obvious changes can be detectedby either XRD or SEM when x was 0, as shown in Figs. 3 andFig. 4aeb. It can be seen from Fig. 4c that a layer, which appeared

Table 2Experimental conditions for the synthesis performed in solution with differentconcentrations of H3PO4 and HF.

Solution composition (molar ratio) T (d) T (�C) Producta

M5 0H3PO4:1HF:1N-MIm:40[EMIm]Br 3 160 amorphous (0)M6 0.25H3PO4:1HF:1N-MIm:40[EMIm]Br 3 160 amorphous (0)M7 0.5H3PO4:1HF:1N-MIm:40[EMIm]Br 3 160 CHA(14)M8 0.75H3PO4:1HF:1N-MIm:40[EMIm]Br 3 160 CHA(33.5)M9 1H3PO4:1HF:1N-MIm:40[EMIm]Br 3 160 CHA(100)M10 1.5H3PO4:1HF:1N-MIm:40[EMIm]Br 3 160 CHA(75)M11 1H3PO4:0HF:1N-MIm:40[EMIm]Br 3 160 amorphous (0)M12 1H3PO4:0.5HF:1N-MIm:40[EMIm]Br 3 160 amorphous (0)M13 1H3PO4:0.75HF:1N-MIm:40[EMIm]Br 3 160 CHA (27)

a Relative crystallinity (%).

to be an aggregation of clusters, was formed on the surface of thesubstrate when x ¼ 0.25. Seen from the cross sectional view shownin Fig. 4d, the layer was continuous and compact. The clusters wereof an amorphous structure, as indicated by XRDmeasurements. Thecrystal phase first emerged when x increased to 0.5, according tothe results of the XRD experiments shown in Fig. 3. Following thistrend, the curve increased with the growth of x until the maximumpoint was reached at x¼ 1.0. The SEM views are consistent with theXRD results, which indicate that crystals could be clearly observedon the surface of substrates, as shown in Fig. 4eej. The relativecrystallinity decreased to 75% as x grew to 1.5 because the formedcrystals were easy to exfoliate from the surface of the substrate

Fig. 3. The relative crystallinity of CHA membranes synthesized with differentamounts of H3PO4 (x). The molar composition of the synthesis solution isxH3PO4:1HF:1N-MIm:40[EMIm]Br.

Fig. 4. SEM top views of CHA-type membranes synthesized with a molar composition of xH3PO4:1HF:1N-MIm:40[EMIm]Br: a) M5 (x ¼ 0), c) M6 (x ¼ 0.25), e) M7 (x ¼ 0.5), g) M8(x ¼ 0.75), i) M9 (x ¼ 1.0); SEM cross-sectional views of M5e9: b) M5 (x ¼ 0), d) M6 (x ¼ 0.25), f) M7 (x ¼ 0.5), h) M8 (x ¼ 0.75), j) M9 (x ¼ 1.0).

X. Li et al. / Microporous and Mesoporous Materials 217 (2015) 54e62 57

Fig. 5. SEM top views of CHA-type membranes synthesized with a molar composition of 1H3PO4:yHF:1N-MIm:40[EMIm]Br: a) M11 (y ¼ 0), c) M12 (y ¼ 0.5), e) M13 (y ¼ 0.75); SEMcross-sectional views of M11e13: b) M11 (y ¼ 0), d) M12 (y ¼ 0.5), f) M13 (y ¼ 0.75).

Fig. 6. XRD patterns of CHA-type membranes synthesized at 180 �C for different du-rations with the solution composition 1H3PO4:1HF:1N-MIm:40[EMIm]Br (molar ratio).

X. Li et al. / Microporous and Mesoporous Materials 217 (2015) 54e6258

after being cleaned and dried. SEM images were unavailable whenx ¼ 1.5 because the membrane synthesized under this conditionwas unsuitable to be investigated by this method.

The synthesis of CHA membranes can be influenced by thereactivity of H3PO4. The reactivity of H3PO4 grows with the increaseof the concentration of H3PO4. A gel layer was formed at low con-centrations of H3PO4. However, the nucleation of the molecularsieve cannot occur until the reactivity grew to a higher level. Afterthat, the crystallinity and the coverage of the synthesized mem-branes increased as the concentration of H3PO4 increased. It mustbe noted that the corrosive action of the solution to the substratewould be too strong if the concentration of H3PO4 was too high.Therefore, the crystals of the membrane synthesized under thiscondition were easy to exfoliate from the surface of the substrate.This behavior can also rationalize the decrease of the relativecrystallinity.

In the synthesis of CHA-type AlPO4, fluoride ions not only play akey role as mineralizers in promoting the framework crystallizationbut also play a co-templating role [31,32]. CHA-type AlPO4 is afluorine-containing aluminophosphate material with fluorine ionsbonded to Al atoms as bridging species in 4Rs connecting D6Rs ofthe structure [33,34]. As fluoride ions are part of the framework ofCHA, the presence of fluorine in the reaction mixture is vital. Aninvestigation into the amount of HF required to optimize the re-action conditions was also performed, of which the synthetic

details and conditions are listed in Table 2. The corresponding SEMresults can be seen in Fig. 5. No distinguished changes can beobserved by XRD and SEM when HF was absent in the synthesissolution. As seen in Fig. 5c, d, a gel layer had already been formed

Fig. 7. SEM top views of CHA-type membranes synthesized at 180 �C for (a) 0.5 d, (c) 1 d, (e) 2 d, (g) 3 d and (j) 4 d with the solution composition 1H3PO4:1HF:1N-MIm:40[EMIm]Br(molar ratio); SEM cross-sectional views of the CHA-type membranes synthesized at 180 �C for (b) 0.5 d, (d) 1 d, (f) 2 d, (h) 3 d and (i) 4 d.

X. Li et al. / Microporous and Mesoporous Materials 217 (2015) 54e62 59

Fig. 8. H2/N2 permeation properties of the CHA-type molecular sieve membranesynthesized at 180 �C with different synthesis durations.

X. Li et al. / Microporous and Mesoporous Materials 217 (2015) 54e6260

on the surface of M12 with the morphology of an aggregation ofclusters. Although no apparent difference is observed by SEM be-tween M12 and M13, the characterization by XRD shows that thecrystal phase was formed on the surface of M13 whereas M12 wasamorphous. When the concentration of HF was further increased,the relative crystallinity was enhanced. In Table 2, M9 exhibits ahigher crystallinity than M13. The SEM image of M9 (Fig. 4i) showsthat most part of the substrate surface was covered with CHAcrystals.

The concentration of HF is also a key factor influencing thesynthesis of the membrane. Based on the results given above, theformation of the gel layer requires the participation of both H3PO4and HF. It is reasonable that the gel layer cannot be formed withoutthe participation of H3PO4, which is an essential component of thealuminophosphate precursor. In addition, HF may help to solubilizethe starting materials during the formation process of the gel layer.The fluoride ions may act as both mineralizers and SDAs during thenucleation and growth of the crystal, of which the concentration ofHF is a key factor influencing the synthesis of the CHA membrane.The different degrees of crystallization of the synthesized mem-branes with similar morphology indicate that the nucleation andsubsequent crystallization cannot occur with a concentration of HFthat is too low.

3.2. Formation mechanism for the substrate surface conversionmethod

In our previous report, we proposed a mechanism for the so-called “substrate surface conversion” method [19]. At the begin-ning of the process, an aluminophosphate precursor layer is formedon the surface of a d-alumina substrate. Nucleation of themolecularsieve occurs at the layer/solution interface. Subsequently, the nucleigrow to larger crystals to form a continuous membrane. In thepresent paper, the synthetic process of CHA membranes operatingat 180 �C was carefully investigated by XRD, SEM and single gaspermeation to study the formation mechanism.

Fig. 6 shows the XRD patterns of CHA-type membranes syn-thesized at 180 �C for different synthesis times with the solutioncomposition 1H3PO4:1HF:1N-MIm:40[EMIm]Br. The morpholog-ical evolution of the surface of the substrates during the sameprocess wasmonitored by SEM, as shown in Fig. 7. As seen in Fig. 7a,b, the surface of the substrate was transformed into a gel layer at0.5 d. The layer was an amorphous structure according to the XRDpattern shown in Fig. 6. As the crystallization time prolonged to 1 d,the gel layer grew much thicker, of which the outer fraction wastransformed into an incompact zone (Fig. 7d). Meanwhile, the topview of the gel layer still shows the morphology of a cluster(Fig. 7c). According to the XRD result, these clusters can beconsidered as incomplete crystalline units with low crystallinity,which appeared to be directly transformed from the gel. Subse-quently, a few molecular sieve crystals with island-like aspectsemerged on the surface of the substrate at 2 d (Fig. 7e, f). At thistime, the XRD pattern did not show significant growth, which maybe due to a lack of crystal growth. Notably, an eruption of molecularsieve crystal growth was observed at 3 d by both SEM (Fig. 7g, h)and XRD (Fig. 6). The surface of the substrate had almostcompletely been covered by CHA crystals, and grain boundariescould still be observed. Along the limited growth of crystallinity, acontinuous and dense membrane was obtained without any cracks,pinholes, or other defects after the synthesis time prolonged to 4 d,as shown in Figs. 7iej and Fig. 6.

The results of the gas separation test are shown in Fig. 8. Themembrane after a 0.5 d synthesis shows a H2 permeance of6.4� 10�7molm�2 s�1 Pa�1, and the H2/N2 permselectivity was 3.4,which is lower than the Knudsen diffusion ratio (Knudsen diffusion

ratio is the ratio of the Knudsen diffusion coefficients of twocomponents. It is proportional to the square root of the reciprocal ofthe molar mass ratio of two gases. It represents the gas selectivitywhen Knudsen diffusion dominates. For H2/N2, Knudsen diffusionratio is 3.74.). As the synthesis time increased, the permeances of H2and N2 continued to decrease and the permselectivity increased.When the duration of the synthesis prolonged to 4 d, the H2/N2permselectivity increased to 3.84, with a corresponding H2 per-meance of 4.2� 10�7 mol m�2 s�1 Pa�1. This indicates that after 4 dof synthesis the zeolite membrane was compact and free ofconsiderable defects. That is to say, the gases permeated relativelyfreely through the CHA molecular sieve channels.

Extensive efforts have been made on the formation mechanismof zeolites. Currently, there are two main mechanistic proposals: asolid hydrogel transformation mechanism and a solutionemediatetransformation mechanism [26,35]. According to the solid trans-formation mechanism, the crystalline materials are formed by abond-switching rearrangement from an amorphous precursor inthe absence of a bulk liquid phase [36e38]. For the mechanism ofsolutionemediate transformation, the amorphous precursor ma-terial is dissolved and the zeolite product crystallizes from theresulting solution [39e41].

Different views regarding the formation mechanism of zeolitemembranes were also proposed. A homogeneous nucleation modelwas reported for the synthesis of LTA [42] and MFI zeolite mem-branes [43]. According to this model, nucleation occurs homoge-neously in the bulk solution and the formed zeolite nuclei orcrystals move and deposit onto the substrate surface via electro-static and van der Waals interactions. A heterogeneous nucleationmodel was also reported for the synthesis of zeolite membranes[44,45]. With this model, a gel layer is formed on the substratesurface at an early stage of preparation by the agglomeration ofprimary gel particles present in solution. Heterogeneous nucleationand crystal growth occur at the interface between the gel and so-lution, where both the nutrition and SDAs are present in abun-dance. In the present study, based on the characterization of theentire formation process of CHA-type molecular sieve membranes,we propose that the “substrate surface conversion” method occursvia a solid transformation mechanism, as illustrated in Fig. 9.

(i) Formation of the amorphous aluminophosphate precursorlayer. At the beginning of the synthetic process, the surface of

Fig. 9. Schematic illustration of the formation mechanism of CHA-type molecular sieve membranes synthesized ionothermally.

X. Li et al. / Microporous and Mesoporous Materials 217 (2015) 54e62 61

the substrate is converted into a gel layer by the solutionduring the induction time. According to the results of XRDand EDX [19], this gel layer is composed of a heterogeneousamorphous aluminophosphate precursor. The P/Al ratio ofthe gel layer decreased from 1 to 0 along the permeation axisof solution. The formation of the layer is a gradual and dy-namic process, which requires the participation of both HFand H3PO4.

(ii) Nucleation by solidesolid transformation. Nucleation of theCHA membrane occurs by the reorganization of the amor-phous network in the outer region of the gel layer, as shownin Fig. 9. The nutrients required for the crystallization,including the SDA, the P source and F�, are all present inabundance in this area. The distribution of cavities in theinner region of this nucleation zone observed by SEM can beexplained by the difference in densities of the precursor andmolecular sieve. This result indicates that nucleation pri-marily occurs via the solidesolid transformation [46]because the solution phase can barely support the forma-tion of nuclei in the inner region of the gel layer [47].

(iii) Crystal growth. The molecular sieve crystals with an island-like aspect, which emerged on the surface of the substrate,are transformed by the incorporation and aggregation of thenucleated clusters, followed by a densification process. Acontinuous membrane is finally obtained by the growth ofthese island crystals, of which the process must consume thealuminophosphate precursor contained in the gel layer. Thenutrient demand of the crystal growth is supplied by thesustained formation of the gel, which occurs until theinteraction between the substrate and the solution isobstructed by the formation of a continuous membrane.Crystallization occurs only on the surface area, and no crystalappears in the solution. The d-alumina support does notdissolve; thus, the possibility that the Al source dissolves andcrystallizes back to a support surface is excluded. Therefore,the strength and shape of the substrate can be maintainedduring the synthesis.

The probable synthesis of the membrane by a solid trans-formation mechanism enlightens a promising route for the prep-aration of other types of molecular sieve membranes by the“substrate surface conversion” method. Moreover, a study on thesynthesis of the membrane can offer some useful findings con-cerning the mechanism of molecular sieve crystallization underionothermal conditions.

4. Conclusion

A gas permeable CHA-typemembrane is obtained at 180 �C after4 d of synthesis with the solution molar composition1H3PO4:1HF:1N-MIm:40[EMIm]Br. The formation mechanism ofthe “substrate surface conversion” method can be rationalized asfollows: the surface of the substrate, which acts as both an Al sourceand support, is converted into a heterogeneous gel layer by thesolution at the beginning of the synthesis. Nucleation and growth ofmolecular sieve crystals then occur by a solidesolid transformationinvolving the aluminophosphate precursor contained in the gellayer. The formation of the layer is a gradual and dynamic process,such that the nutrient demand for the membrane synthesis issatisfied until the interaction between the substrate and the solu-tion is obstructed by a continuous membrane.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Grant No. 21373214).

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