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Ceramics International 40 (2014) 5889–5896www.elsevier.com/locate/ceramint

Hydrothermal synthesis of hydroxy sodalite zeolite membrane: Separationof H2/CH4

Mohammad Sadegh Nabavi, Toraj Mohammadin, Mansoor Kazemimoghadam

Research Centre for Membrane Separation Processes, Faculty of Chemical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran

Received 28 August 2013; received in revised form 16 October 2013; accepted 7 November 2013Available online 16 November 2013

Abstract

In this study, hydroxy sodalite (HS) zeolite was synthesized via hydrothermal method. As reported in the literature, the recommended molarcomposition of H2O/Al2O3 for HS zeolite synthesis is about 1000. However in our work, HS zeolite was synthesized using new molarcompositions of H2O/Al2O3 for different synthesis times and their effects on morphology and crystallinity of the synthesized zeolites wereinvestigated. Also, HS zeolite membranes were synthesized on seeded supports using synthesized powders and the effect of seed size was alsoinvestigated. The products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Gas permeation through themembranes was also measured. First of all, it was found that synthesis time has a significant effect on crystallinity and crystal size of thesynthesized zeolite. Also, water molar ratio can control crystal size of the synthesized zeolite.It was found out that with increasing the seed size, H2 and CH4 permeations decrease, from 3224 to 279 (Barrer) and from 611 to 45 (Barrer),

respectively, while the H2/CH4 selectivity increases from 5.28 to 6.20. Finally, to obtain HS zeolite with desirable particle size, crystallinity andmorphology, optimum synthesis conditions were proposed.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: HS zeolite membrane; Crystallinity; Nanostructure; Hydrothermal synthesis; H2/CH4 separation

1. Introduction

Zeolites have crystal structures with uniform and molecularsize channels [1]. They have some applications such ascatalysts, adsorbents and ion exchangers that are useful inrefinery industries. Due to their uniform pore sizes, they havelarge internal surface area and active sites with high thermalresistance and high mechanical strength [2–9].

Although, important parts of zeolites applications are closelyrelated to their framework and shape, properties of zeolitescrystals also play a crucial role in the mode and efficiency oftheir applications. More recently, many efforts have beenfocused on some factors to control the crystallization pathwayand particulate properties of the final products. The resultsshowed that chemical composition and reaction are the mostimportant factors for controlling the zeolite particulate proper-ties [10,11].

e front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2013.11.033

g author. Tel.: þ98 21 7789 6621; fax: þ98 21 7789 6620.ss: torajmohammadi@iust.ac.ir (T. Mohammadi).

Hydroxy sodalite (HS) zeolite is one of the microporouscrystalline aluminosilicate hydrophilic zeolites with a smallpore size of about 2.8 Å. The small pore size of HS zeolitemakes separation of small molecules via difference in sizepossible. Thus, it is expected that small molecules, such ashydrogen (2.9 Å), can be separated via HS zeolite membranefrom short-chain alkanes such as methane (4 Å) via molecularsieving [12–16]. This kind of separation may have manyapplications in separation of refinery gases [17,18]. As men-tioned, controlling some synthesis parameters has significantimpacts on the zeolite performance.Nowadays, hydrogen is considered as an alternative for

fossil fuels [19]. It can play an important role to change globalsociety energy resources, specially in transport applications[20]. However, according to the literature, fossil fuelsare the main source (about 96%) of hydrogen production.However, 48% of hydrogen resources are natural gas and 30%from refinery gases, and there are some byproducts with thishydrogen [19]. For example, H2 is produced in reformationof hydrocarbons and some by products such as CO2, CO,CH4 and H2O are also produced during the reaction. Zeolite

ghts reserved.

Table 1Synthesized powders.

No. Molar composition Temperature (1C) Time (h)

1 5SiO2: Al2O3: 50NaO2: 1000H2O 100 182 5SiO2: Al2O3: 50NaO2: 700H2O 100 223 5SiO2: Al2O3: 50NaO2: 850H2O 100 224 5SiO2: Al2O3: 50NaO2: 1000H2O 100 225 5SiO2: Al2O3: 50NaO2: 1150H2O 100 226 5SiO2: Al2O3: 50NaO2: 1300H2O 100 22

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membranes have high chance to selectively separate and purifyH2 from these gas streams. This is due to the fact that zeolitemembranes exhibit both high flux and high selectivity for H2

separation in the presence of those other gas species [21].Khajavi et al. [22] reported that HS crystals can be

successfully synthesized at different synthesis temperatures(90–140 1C) for different synthesis times (3.5–24 h) via directhydrothermal synthesis. However, synthesis for short time andat low temperature leads to the co-formation of zeolites A, X,and -P in addition to HS and synthesis for long time and athigh temperature leads to the transformation of sodalite tocancrinite, and increasing either temperature or time ofsynthesis only increases the size of aggregates. Also, theyshowed that changing the Si/Al ratio of the starting gel (1–14)does not have any effect on the framework composition.

Although much studies have been performed in synthesizingand characterizing HS zeolite powder, there are only fewstudies reporting formation of highly efficient HS zeolitemembranes. Khajavi et al. [13] were also reported synthesisand characterization of supported HS zeolite membranes. Theysynthesized HS zeolite membranes at different synthesistemperatures (90–140 1C) for different synthesis times (3.5–20 h) via in situ hydrothermal synthesis under rotation. Thebest selective membrane was synthesized for only 3.5 h at413 K with negligible flux. The other membranes exhibit poorselectivity of about 2 for helium and nitrogen separation.

In this research, HS zeolite crystals were synthesized viahydrothermal synthesis method. The effects of crystallizationtime on the final crystalline products were studied. Further-more, the influence of gel molar ratio of H2O/Al2O3 on theparticulate properties (crystal size and shape) was investigated.Finally, the effect of synthesized zeolite powder as seeds onzeolite membrane synthesis was investigated, and the HSzeolite membranes were characterized using H2 and CH4

permeation measurements.

2. Experimental

2.1. Material

The used silica source was silica foam (Evonika co. withparticle size of 12 nm) and the used alumina source wassodium aluminate (50–56% Al2O3, Riedelde Haën) and theused sodium source was sodium hydroxide (Solid white plate)that was purchased from Merck Company.

2.2. HS zeolite crystals synthesis

In order to prepare the HS powder, the synthesis solutionwas obtained by mixing aluminate and silicate solutions in apolypropylene bottle. In the first step, the aluminate solutionwas prepared by dissolving sodium aluminate in deionizedwater. Then, the silicate solution was prepared by mixing silicafoam, sodium hydroxide and deionized water. After a fewminutes, the clear solutions were obtained.

In the next step, the silica source was added to the aluminasource quickly. The resulting mixture was stirred for 30 min in

order to obtain a homogenous clear solution. The molar ratioof the resulting synthesis solution was 5SiO2:Al2O3:50Na2O:xH2O. The solution was then decanted in an autoclave andplaced in an oven. The synthesis was finally carried out attemperature of 100 1C for 18–22 h. Water molar ratio wasvaried from 700 to 1300.Water molar ratio and synthesis time for all the synthesized

seeds are presented in Table 1. After the hydrothermalcrystallization, the powders were recovered, thoroughlywashed with deionized water until a pH value of about 7was obtained, and then dried at room temperature for 24 h.

2.3. HS zeolite membrane synthesis

2.3.1. Seeding supportTo synthesize the membranes, homemade α-alumina supports

with 2 cm in diameter, 2 mm in thickness and 100–200 nm inpore diameter were employed. One side of the supports waspolished with 800 and 1500 grit-sand papers to be prepared forseeding. The supports were put in a cleaned KUDOS SK3310HPultrasonic cleaner with deionized water for about 5 min to removethe loose particles created during polishing.The zeolite powder was dispersed in distilled water. The

mixture (2 wt%) was then ultrasonicated for a period of time tohave a homogeneous mixture. The supports were dipped in thehomogeneous solution for a short time (10 sec) to cover thesupports with the zeolite particles. It must be mentioned thatone side of the supports was covered with Teflon tape to avoidseeding the both sides of the supports. After that the supportswere treated in an oven at 80 1C for a period time to be dried.Five synthesized powders as presented in Table 1 (2–6) wereused in the seeding process.

2.3.2. Membrane synthesisThe gel solution for synthesizing the HS zeolite membranes

was prepared. It should be mentioned that the preparationprocedure of the prepared gel solution for the zeolite powdersand the zeolite membranes were completely the same and therewas no difference between these two gels. The same molarratio (5SiO2: Al2O3:50Na2O:1000H2O) was used for all of theexperiments.The seeded supports were put in PTFE holders and placed

vertically in an autoclave. The synthesis solution was againpoured into the autoclave and the autoclave was put in anoven. The membrane synthesis was carried out at temperature

M.S. Nabavi et al. / Ceramics International 40 (2014) 5889–5896 5891

of 100 1C for 15 h and the autoclave was put at roomtemperature for 2 h to avoid any defect formation. Finally,the solution was decanted off and the membranes were washedwith deionized water until a pH value of about 7 was obtained,and then dried in an oven at 120 1C for 3 h.

2.4. Characterization

XRD experiments were performed to identify the HSzeolites. The X-ray data were collected using an INEL modelEQuinox 3000 with Cu-kα radiation in the range of 90–50with 40 kV and 30 mA.

The exact framework of the HS zeolites structures wasobserved using SEM. The SEM images were obtained usingVEGA ll device, TESCAN Company. The membranes werecoated with gold layers before observation.

3. Results and discussion

3.1. Effect of synthesis time

The samples 1 and 2 (in Table 1) were synthesized using thesame molar composition and at the same temperature but fordifferent synthesis times. The samples XRD patterns arepresented in Fig. 1. The obtained results show that the bothsamples are HS zeolite. However, in compare with thestandard XRD pattern, the peaks intensity of sample 2 is more

Inte

nsity

a.u

. In

tens

ity a

.u.

2θ

2θ

Fig. 1. (a) The XRD of the 1st sample in Table 1. (b) The XRD of the 2ndsample in Table 1.

Fig. 2. (a) The SEM of sample 1 (in Table 1). (b) The SEM of sample 2(in Table 1).

Inte

nsity

a.u

.

2θ

Fig. 3. The XRD of 3rd to 6th samples (in Table 1).

M.S. Nabavi et al. / Ceramics International 40 (2014) 5889–58965892

desirable in compare with those of sample 1. Also, the SEMmicrographs of the two samples are presented in Fig. 2indicating that synthesis time is effective on morphology ofthe synthesized zeolite and the zeolite crystal size. Whensynthesis time increases, the particulate size also increases, and

Fig. 4. (a) The SEM of sample 3 (in Table 1) with (5SiO2: Al2O3: 50NaO2: 700H50NaO2: 850H2O) molar ratios. (c) The SEM of sample 2 (in Table 1) with (5SiO2:with (5SiO2: Al2O3: 50NaO2: 1150H2O) molar ratios. (e) The SEM of sample 6 (

this is because the longer synthesis time the more crystalgrowth as reported in the literature [2,22].Crystal size and crystallinity of the HS zeolite seeds are two

important factors in various applications, especially in synth-esis of zeolite membranes [2]. So, the optimum conditions

2O) molar ratios. (b) The SEM of sample 4 (in Table 1) with (5SiO2: Al2O3:Al2O3: 50NaO2: 1000H2O) molar ratios. (d) The SEM of sample 5 (in Table 1)in Table 1) with (5SiO2: Al2O3: 50NaO2: 1300H2O) molar ratios.

Fig. 5. The particle size distribution of 2nd to 6th samples (in Table 1).

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should be selected to synthesize HS zeolite seeds. In order tosynthesis uniform defect free HS zeolite membranes on thesupports, the nucleation zeolite seeds should be uniformlysmall. Also the zeolite seeds should have high crystallinity andthis plays an important role to avoid non-zeolitic pores. As aresult, the maximum synthesis time of 22 h was selected forfurther investigations.

3.2. Effect of water molar content

The XRD patterns of the remaining samples (in Table 1)for the same synthesis time and temperature at different watermolar ratios are shown in Fig. 3.

HS zeolite with 5SiO2: Al2O3:50Na2O:1000H2O gel for-mula was purely synthesized in our previous work [9]. Tostudy the effect of water molar content on quality of thesynthesized powder, it was varied for the first time. Bycomparing XRD patterns of the samples with the standardXRD pattern, it can be observed that most the synthesizedsamples are HS zeolite. The most important thing to say is thefeasibility of HS zeolite synthesis in a wide range of watermolar content, and this has not been reviewed in the literatureyet. Also, crystallinity of the samples which means that theextent of the crystalline solid phase (which is not amorphous),was checked and it was found out that variation of water molarcontent does not have any significant effect on crystallinityin the studied range (700–1300). In the other words, there isno difference between them with respect to the presence ofthe amorphous phase and the amount of non-zeolites andamorphous phases are minor and similar in all of the 2nd to 6thsamples.

The SEM images of these five samples are presented inFig. 4. As can be observed, by increasing water molar content,the crystal size reduces. Low concentration of materials (withincreasing water molar content) creates more space for theprimary nucleus and more distance between them and it causesthat the nucleus grows less in compare with the concentratedgel. So, it is possible to synthesis zeolite powder with highcrystallinity and small crystal size, together.

To confide the powders particle size, microstructure mea-surement software was obtained from Nahamin Pardazan AsiaCo. The zeolite powders SEM images were investigated withthis software. The results are shown in Fig. 5. As seen, whenthe gel water molar content increases, mean size of the zeoliteparticles significantly decreases.

It should be mentioned that the main goal of zeolite powdersynthesis is using it to synthesize zeolite membrane. From theliterature [3], it can be said that the zeolites begin to transforminto other types of zeolites before a special kind of acontinuous zeolite layer is formed on an unseeded supportunder this synthesis condition. So seeding can help andimprove the zeolite membrane synthesis. It should be alsomentioned that there are some peaks in the HS zolite XRDpattern which can be observed similarly in XRD patterns ofsome other zeolites, likes 4A. In other words, the presence ofneighbor zeolites cannot be confirmed without further inves-tigations. Synthesizing the zeolite membranes on seeded

supports is the best way to identify whether the correct zeoliteis synthesized or not. As mentioned, the effects of crystal sizeand also crystallinity of the synthesized zeolite seeds on thesupport surface were further investigated. The first synthesizedparticles after ultrasonication were used for seeding. It shouldbe mentioned that it was tried to ultrasonicate as short aspossible to have a homogenous solution without making theparticles significantly smaller. The particle size of the zeolitewas almost unchanged after ultrasonication.

3.3. Zeolite membrane characterization

3.3.1. SEM and XRDThe synthesis conditions of all the membranes were the

same at 100 1C synthesis temperature and for 15 h synthesistime. The best XRD patterns and the SEM images of thezeolite membrane surfaces are shown in Figs. 6 and 7,respectively. The XRD results show that HS zeolite mem-branes can be purely synthesized except for the membraneseeded with the 6th powder with molar content of 1300. It wasfound out that this membrane is partially 4A zeolite membrane.As reported in the literature, zeolites 4A and HS are in thesame zeolite family [22]. The water molar content of 1300 istoo high. This causes zeolite 4A is partially synthesized alongwith the main zeolite (HS), and because of some similar peaksin the XRD patterns, it is not found in the XRD analysis of thepowder. In the membrane synthesis, due to the presence ofzeolite 4A seeds along with HS zeolite seeds (as a minorphase), zeolite 4A membrane is formed.The SEM images show that continuous HS zeolite layers

can be obtained after using the seeds (2–5 in Table 1) and acontinuous 4A zeolite layer using the seed (6 in Table 1). Fromthe SEM images, it can be concluded that with decreasing theseed size, a more continuous zeolite layer can be formed. Also,from the SEM images (Fig. 6), it is clear that the zeolite layeris more continuous with smaller grains.However, the XRD and the SEM analysis can only indicate

the presence of zeolite layers. They cannot guaranty formationof a continuous (defect free) zeolite layers. Thus, selectivity

Fig. 6. The XRD of membrane of sample 3 in Table 2.

M.S. Nabavi et al. / Ceramics International 40 (2014) 5889–58965894

and permeability are two important properties to characterizethe synthesized zeolite membranes.

3.3.2. H2 and CH4 permeationConsidering the pore size and the hydrophilic nature of the

HS zeolite membrane, this can be nominated for H2/CH4

separation. The ideal selectivity can be calculated using thepermeabilities. Because the kinetic diameter of CH4 (0.35 nm)is larger than the pore size of the HS zeolite channels(0.28 nm), only H2 molecules can permeate through a defect-free HS zeolite membrane. Permeability through the membrane(P1) can be calculated as follows:

p1 ¼Q1; l

Δp;Að1Þ

where Q1, Δp, l and A are volumetric flow rate of gas ‘1’ atstandard temperature and pressure, pressure drop, effectivethickness of the membrane and surface area of the membrane,respectively. Permeabilities are reported in Barrer, where 1Barrer equals to 10�10 (cm3 STP. cm cm�2 cmHg�1 s�1).

Also, selectivity (α12) of the membranes can be calculatedwith dividing permeability of the first permeant (p1) by thesecond one (p2):

α12 ¼p1p2

ð2Þ

Permeability and selectivity of the membranes are presentedin Table 2. As observed in Table 2, the membrane seededusing the 5th powder with water molar content of 1150exhibits the higher selectivity of H2/CH4 and lower CH4 andH2 permeabilities, while the membrane seeded using the 6thpowder with water molar content of exhibits minimumselectivity and maximum CH4 and H2 permeabilities. It is

confirmed that the synthesized membrane is 4A zeolite and haslarger pores than HS zeolite (about 0.41 nm) [3].As observed in Fig. 8, permeabilies of the both gases

decrease with increasing water molar content. H2 permeabiltychanges more significantly than CH4. With paving themembrane surface, due to increasing water molar content inthe seed synthesis, H2 permeability comes down more sig-nificantly than CH4. It is because of molecular size ofhydrogen and due to the fact that it cannot permeate throughevery zone on the membrane surface. But this is not significantfor the bigger molecule of CH4.So, the surface crystals of the zeolite membrane are formed

smaller and more continuous, gas permeability stronglydecreases and membrane selectivity slightly increases. This isthe reason for variation in selectivity and permeability of themembranes as shown in Fig. 8. As observed in Fig. 8, themaximum deviation of the results is less than 5%. Finally, itcan be said that seed size reduction, an optimum seed sizeshould be selected to have an optimum selectivity andpermeability.

4. Conclusion

HS zeolite crystals and HS zeolite membranes were synthe-sized successfully via hydrothermal synthesis method. Theeffects of different parameters such as water molar content,synthesis time and seeding on membrane synthesis wereinvestigated.It was shown that by increasing synthesis time, crystallinity

and average particle size of the synthesized HS zeolite particlesincrease dramatically. Therefore, to obtain HS zeolite seedswith suitable particle size and desirable crystallinity, themaximum synthesis time (22 h) is proposed.

Fig. 7. (a) The SEM of the 1st membrane seeded with (5SiO2: Al2O3: 50NaO2: 700H2O) molar ratios powder. (b) The SEM of the 2nd membrane seeded with(5SiO2: Al2O3: 50NaO2: 850H2O) molar ratios. (c) The SEM of the 3rd membrane seeded with (5SiO2: Al2O3: 50NaO2: 1000H2O) molar ratios. (d) The SEM ofthe 4th membrane seeded with (5SiO2: Al2O3: 50NaO2: 1150H2O) molar ratios. (e) The SEM of the 5th membrane seeded with (5SiO2: Al2O3: 50NaO2: 1300H2O)molar ratios.

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Also, HS zeolite crystals were synthesized using new watermolar contents based on the original molar composition forsynthesis of HS zeolite with emphasis on the SiO2/Al2O3 ratio

of 5. The obtained results showed that water molar content insynthesis of HS zeolite controls the morphology and the phasebehavior. However, both XRD patterns and SEM images results

Table 2Synthesized membranes.

No. Molar compositionsof synthesized seed

Samplecode

H2

permeability(barrer)

CH4

permeability(barrer)

H2/CH4

idealselectivity

1 5SiO2: Al2O3:50NaO2: 700H2O

700 3224 611 5.28

2 5SiO2: Al2O3:50NaO2: 850H2O

850 2378 415 5.72

3 5SiO2: Al2O3:50NaO2: 1000H2O

1000 1189 202 5.89

4 5SiO2: Al2O3:50NaO2: 1150H2O

1150 279 45 6.2

5 5SiO2: Al2O3:50NaO2: 1300H2O

1300 77,156 45,008 1.71

Fig. 8. (a) The gas permeability of the membranes. (b) H2/CH4 selectivity ofthe membranes.

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showed that the purest HS zeolite could be synthesized correctlyusing Al2O3:50Na2O: 5SiO2: (700, 850, 1000 and 1150) H2Omolar composition.

Also, it was shown that the seeds size has a significant effect onmorphology and performance of the synthesized zeolite mem-branes. With smaller seeds and higher crystallinity, continuous anddefect free zeolite layers on the membranes can be formed and themembranes perform better selectivity. However, permeabilities ofH2 and CH4 significantly decrease and an optimum point shouldbe selected. But it should be mentioned that the obtained selec-tivities are higher than the selectivities reported in the literature.

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