j.ijhydene.2012.07.085

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Effect of synthesis conditions on performance of a hydrogen

selective nano-composite ceramic membrane

Mahdi Amanipour a, Aliakbar Safekordi a, Ensieh Ganji Babakhani b,*, Akbar Zamaniyan b,Marzieh Heidari a

a Chemical Engineering and Petroleum Faculty, Sharif University of Technology, Azadi Avenue, Tehran, IranbGas department, Research Institute of Petroleum Industry, West Blvd. Azadi Sport Complex, Tehran 14665-137, Iran

a r t i c l e i n f o

Article history:

Received 21 April 2012

Received in revised form

7 July 2012

Accepted 20 July 2012

Available online 17 August 2012

Keywords:

Nano-composite membrane

Hydrogen

CVD method

Permeance flux

a b s t r a c t

A hydrogen-selective nano-composite ceramic membrane was prepared by depositing

a dense layer composed of SiO2and Al2O3on top of a graded multilayer substrate using co-

current chemical vapor deposition (CVD) method. The multilayer substrate was made by

dip-coating a macroporousa-alumina tubular support by a series of boehmite solutions to

get a graded structure. Using DLS analysis, it was concluded that decreasing hydrolysis

time and increasing acid concentration lead to smaller particle size of boehmite sols. XRD

analysis was carried out to investigate the structure of intermediate layer and an optimized

calcination temperature of 973 K was obtained. SEM images indicated the formation of

a graded membrane with a porous intermediate layer having a thickness of about 2mm and

a dense top selective layer with a thickness of 80e100 nm. Permeation tests showed that H2permeance flux decreased from 5 105 mol m2 s1 Pa1 for a fresh substrate to

6.30 107 mol m2 s1 Pa1 after 6 h of deposition, but H2selectivity over N2increased

considerably from 5.6 to 203.

Copyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

Porous alumina-based ceramic membranes covered with

a selective layer synthesized by chemical vapor deposition

(CVD) or solegel methods have attracted great attention for

gas separation[1,2]. Because of their ability to separate smallgas molecules like hydrogen or helium, and their gas perme-

ation values larger than polymeric membranes, these kind of

membranes have great potential for applications in chemical,

petrochemical and energy industry where gas separation in

harsh conditions leads to increase in process efficiency [1].

Silica membranes prepared by different methods like CVD

or solegel, deposited on mesoporous or macroporous supports

have been shown to be effective for H2permeation with good

selectivities [3e9].OkuboandInoue [3,4] depositedsilica within

the pores of a kind of glass with 4 nm mean pore diameter,

using Tetraethylorthosilicate (TEOS) hydrolysis. Similarly, Wu

et al.[7] modified the pore size ofg-alumina membranes by

counter current CVD of TEOS, and O2as a co-reagent. Oyama

and Lee [8,9] used CVD method to deposit silica on both porousvycor glass andg-alumina supportswith pore diameter of 4 nm

and obtained silica membranes with ultra-high selectivities of

hydrogen for temperatures less than 900 K.

Initial membranes which used macroporous supports had

very high permeances, but low selectivites. Hwang et al. [10]

carried out CVD of TEOS on a porous alumina tube with pore

size of about 100 nm and obtained a selectivity of 5.2 for H2/N2at 873K after 32 h of deposition. Such low selectivities indicate

* Corresponding author. Tel.: 98 21 48252398; fax: 98 21 44739716.E-mail addresses:[email protected],[email protected](E. Ganji Babakhani).

Available online atwww.sciencedirect.com

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0360-3199/$ e see front matter Copyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.ijhydene.2012.07.085
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the presence of large pore defects. A good solution to this

problem is making a graded intermediate layer before addi-

tion of the top selective layer. Some works have been done

withg-alumina as intermediate layers[11e18]. Morooka et al.

[13,14]covered a macroporous a-alumina support which had

110e180 nm pore diameter with three layers of g-alumina

with mean pore diameter of about 8 nm.Using CVDmethod to

add silica layer to this modified support, they obtaineda membrane with selectivity for H2/N2of 100e1000 and a H2permeance of 108e109 mol m2 s1 Pa1 at 873 K.

On the other hand, hydrogen-selective silica materials are

hydrothermally unstable at moderate to high temperatures

[7,19e22]. Significant work has been done to improve the

stability of silica membranes. Some researchers have tried to

prepare hydrophobic silica membranes by the incorporation of

methyl groups in the silica microstructure [21]. Another

approach involves exposing coated membranes to the humid

air for a few days and calcining them in the steam[22]. Oyama

etal. [23] found thatpreparing a composite layer by silica andan

inorganic oxide like zirconia (ZrO2) gives rise to membrane

stability withsuperior permeation properties. Nomura et al. [24]reported an improvement of steam stability of a silica

membrane synthesized by counter diffusion CVD of TMOS and

O2 fromoppositesides of the support. Although this membrane

had a good selectivity for H2/N2of over 800 at 773 K, the per-

meancewaslowandintheorderof2e7108molm2s1Pa1.

Moreover, it is necessary to achieve a better understanding

of the effect of different synthesis parameters on membrane

performance. Although few attempts have been done to

systematically investigate the relation between these param-

eters and performance of the membrane [25,26], a clear

strategy should be presented in order to prepare membranes

withimprovedpermeation properties and stability. Oyama and

Gu[25]worked on the effect of some parameters such as pep-tizing agent on boehmite solutions, but they did not discuss

about some important factors like viscosity of sols, calcination

temperature and physical properties of the intermediate layer.

In this work, a nano-composite ceramic membrane has

been prepared by depositing a thin, hydrogen-selective layer

composed of silica (SiO2) and alumina (Al2O3) by co-current

CVD method on top of a graded mesoporous g-alumina

multilayer, which is supported by a macroporous a-alumina

tube. SEM and XRD analysis have been carried out to charac-

terize different layers of the membrane and effects of

synthesis conditions like changing the amount of peptizing

agent on membrane performance have been investigated

using DLS and viscometry analysis. Permeation tests wereperformed at high temperatures in the range of 873e1073 K.

2. Materials and methods

2.1. Preparation of intermediate graded layer

The nano-composite membrane in this work was prepared by

depositing a very thin, dense layer composed of silica and

alumina on a macroporous alumina support which was

modified by a g-alumina graded multilayer.Fig. 1shows the

schematic diagram of the consecutive steps that were carried

out to synthesize and characterize the membrane.

The intermediate multilayer was obtained from boehmite

(AlOOH) sols with different mean particle sizes. These sols

were prepared by carefully controlling the hydrolysis of

aluminum alkoxides and then peptization of the boehmite

precipitate with acid, as reported in literature [25]. Thegeneral

procedure is as follows: 0.1 mol of aluminum tri isopropilate

(Merck, >98%) was added to 150 ml of distilled water at 353 K

with high speed stirring and was maintained at this temper-ature for 3e20 h to hydrolyze the alkoxide. This resulted in

formation of boehmite precipitate which was then heated to

363 K and was peptized with a quantity of nitric acid (Merck,

65%) with a molar ratio ofH/alkoxide in the range of 0.08e0.2.

The resulting solution was refluxed at 363 K for 18e20 h to get

a clear sol. A series of sols with median particle size in the

range of 50e700 nm were obtained depending on their

hydrolysis time and acid concentration.

In this study, PVA (polyvinyl alcohol, Biochemical,

M.W. 7200) was used as the binder. Proper solution was

preparedby adding18 grof PVA to500 mlof water at298 K and

stirring for 30 min. The resulting solution is stable at ambient

temperature for at least one week. Viscosity of the solmeasured by a digital viscometer(Fungilab, Alpha series)with

RPM of 30. A thin and uniform g-alumina multilayer was

prepared on a macroporous a-alumina support by dip-coating

Fig. 1 e Schematic of membrane preparation and

characterization steps.

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with a series of dilute boehmite solutions of different particle

sizes. Table 1 shows the synthesis parameters of three

boehmite sols with ascending median particle sizes used for

dip-coating process and denoted as A1, A2 and A3, respec-

tively. A commercial alumina membrane tube (GMITM

Corporation, i.d. 9 mm, o.d. 13 mm) with a nominal pore

sizeof 500nm was usedas the support.Firstly,the tubewascut

to a length of 5e

6 cm, soaked in an ultra-sonic bath (BANDE-LIN, Sonorex digital 10P) with 10% power for 15 min and dried

at ambient conditions for 24 h in order to clean the surface

pollution of the support. Each boehmite sol was diluted before

coating, by mixing with PVA solution in a 3 to 2 ratio to get

solutions with 1 Wt. % concentration of PVA. The resulting

solution was then stirred at 363 K for 3 h and the support was

dipped into this solution and was withdrawn after 10 s. The

coated support was then dried in ambient conditions for 24 h,

heatedto 973 K inan electric furnace ata rate of1 K min1 and

wascalcined at this temperature for2.5 h. This coating process

was carried out five times by differentsolutions in the order of

decreasing sol particle size (A1, A1, A2, A3 and A3) to obtain

a graded structure intermediate layer.

2.2. Preparation of nano-composite dense layer

A composite silicaealumina membrane was synthesized

using the graded structure substrate to deposit a very thin

layer composed of SiO2and Al2O3by chemical vapor deposi-

tion (CVD) method. This process led to coat a composite layer

on the surface of the substrate by thermal decomposition of

tetraethyl orthosilicate (TEOS, Merck, >98%) and aluminum-

tri-sec-butoxide (ATSB, Merck, >98%) at high temperature.

The experimental setup is shown in Fig. 2. The CVD process

took place in a quartz tube in where silica and alumina were

deposited into the inner side of the support. Firstly, theapparatus was heatedto 873 K ata rateof 1 K min1 in an inert

atmosphere, using two streams of argon on both sides of the

membrane module. After reaching to the desired tempera-

ture, valves were opened and two streams of argon as carrier

gas were passed through two bubblers filled with TEOS and

ATSB at 300 K and 393 K, respectively. These streams were

then mixed with argon dilution flow and were fed to the inner

side of the substrate. All streams were controlled carefully

using mass flow controllers. In general, the argon stream

passed through TEOS bubbler was fixed at 13 ml min1 and

CVD process was carried out on several substrates with

various amounts of ATSB carrier gas to produce a series of

selective composite membranes with different permeationproperties. The composition of the CVD layer was measured

using EDX (Philips, XL-30) analyzer with a 17 eV beam. The

results of the analysis are presented in Table 2.

XRD (Philips-XL10) analysis was used to recognize the

alumina phase of the intermediate layer and to determine the

proper calcination temperature of the membrane. Also, SEM

(Philips-XL30) analysis was used to investigate the

morphology of the synthesized nano-composite membrane.

Pore size of the formed layer was measured by mercury

porosimetery (Carlo Erba, 2000).

Permeation tests were carried out to investigate the effect

of preparation of dense top selective layer with a composite

structure. Gas permeation experiments were performed at

923e

1073 K, using argon as the sweep gas on the inner side ofthe membrane at atmospheric pressure and 50e200 ml min1

flow rate. Single flows of H2 and N2gases were passed indi-

vidually through the outer side of the membrane at about

160 kPa (Dp 60 kPa) and the flow rates of argon and perme-

ated gas through the membrane were measured using bubble

flow meter. The concentrations of gases were determined by

a gas chromatograph (GC, Agilent, 7890A) analyzer and

selectivity was defined as the H2/N2permeation flux.

3. Results and discussion

3.1. Effect of binder on sol viscosity

Using a sol with appropriate viscosity is important to prepare

the desired graded intermediate layer on the support. It has

been found that increasing viscosity of the sol results in

Table 1 e Synthesis parameters of boehmite sols used toprepare the intermediate layer.

Sol no. Hydrolysistime (h)

Molar ratioof H/alkoxide

Mean particlesize (nm)

A1 20 0.08 527

A2 10 0.2 148

A3 3 0.2 52

Fig. 2 e Schematic of CVD apparatus used for deposition of

the composite layer.

Table 2e EDX analysis results from the surface of thecomposite layer.

CVD time (h) ATSB/TEOS molar ratio Si Wt.%

6 0.04 8.24

6 0.06 8.03

6 0.1 7.72

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thicker layers, which leads to lower adhesion of the coating

and higher formation of cracks[27]. Also, use of binder has an

important effect on coating sol as it can decrease the brittle-

ness of the membrane and improves the mechanical strength;

prevent crack formation; increase adhesion of the coated layer

on the support; and change the sol viscosity[28,29].

As mentioned earlier, PVA was used as the binder in our

work. Fig. 3(a) and (b) show the light microscope (BelPhotonics) images of the graded multilayer with 1 Wt. %

concentration of PVA which dried at ambient temperature for

24 h and then calcined at 973 K for 2 h. These images present

a smooth and homogeneous surface and show no formation

of cracks. Based on the present study, addition of PVA greater

than 1 Wt. % in sols increased the sol viscosity and led to

production of thicker and non-uniform membrane layers

which tended to crack formation during drying process.

Furthermore, high viscosity resulted in formation of aggre-

gates, thus leading to separable particles on the surface of the

support. Besides, the amount of PVA in sols had great influ-

ence on the membrane characteristic, especially pore size of

the membrane.Table 3shows the effect of PVA on viscosity ofthe sol and pore size of the membrane after calcined at 973 K.

Boehmite sol A1 was used for this experiment As shown in

Table 3, by increasing the amount of PVA, pore size of the

membrane was increased. Higher amount of PVA promoted

agglomeration of the boehmite particles, which after calci-

nation resulted in wider pore size [30]. The agglomeration

causes inhomogeneous growth of particles and void forma-

tion[31]. The viscosity of the sol was increased as the amount

of PVA in solution was changed from 0.8 Wt. % to 2 Wt. %,

which is in agreement with formation of aggregates in sol.

3.2. Effect of synthesis parameters on boehmite sol

properties

It has been shown that the formation of a thin, defect-free top

selective layer depends on preparation of a substrate with

uniform structure and with pore sizes smaller than 5 nm [32].

To obtain such substrate, it is important to prepare thin and

smooth intermediate layers with small anduniform pore sizes

by the use of dilute dipping solutions containing sol particles

with appropriate size. If the sol particles are too small

compared with mean pore size of the a-alumina support, they

penetrate into the pores, which gradually cause formation of

cracks on the surface of the support [33]. The use of sols with

large particle size can overcome the problem of penetration,

but if sol particles are too large, they can give rise to defi-

ciencies. These large particles have large interstitial spaces,

and will not cover the surface uniformly leaving patches of

untreated surfaces[25]. A good solution is to use large particle

size and then successive deposition of particles of smaller size

on top. This strategy results in formation of a graded structure

multilayer with better filling of voids to obtain a smooth and

defect-free surface.The preparation of boehmite sols from alkoxide precursors

consists of several steps: firstly, the precursors are hydro-

lyzed, the alkoxides are removed and aluminum oxy

hydroxide precipitates are formed. Oyama and Gu. [25]

showed that the following reactions may occur during this

step:

Al(OR)3 H2O/ Al(OR)2(OH) R(OH), etc. (1)

2Al(OR)2(OH) H2O/ 2Al(OR)(OH) 2R(OH), etc. (2)

The second step is called peptization where precipitatesformed by hydrolysis are heated in acid in order to break up

the large precipitates and form smaller particles. The acid also

causes particles to repel eachother and allow the formation of

stable suspensions by charging the surface of particles [25].

Preparation of a stable boehmite sols depends on many

synthesis parameters like hydrolysis time, acid type and acid

concentration [25]. Aluminum alkoxides must be heated

quickly above 353 K to prevent formation of bayerite (b-Al

(OH)3) which causes instability of the sols[34]. Also, acid type

has an influence on the particle size of the sols. Acids that are

used for peptization step should have two important traits:

their anion should be noncomplexing with aluminum and

they should have sufficient strength to produce the requiredcharge effect at low concentrations[35].

In this study the effect of acid type on particle size of

boehmite sols was studied. Acetic acid (CH3COOH), nitric acid

(HNO3) and hydrochloric acid (HCl) were used with an H/

alkoxide molar ratio of 0.08 and resulted boehmite sols were

analyzed using DLS (Dynamic Light Scattering, MALVERN,

nano-ZS) analysis. The analyzer was calibrated by a standard

latex solution with mean particle size of 65 nm and a value of

1.65 was used as the refractive index for boehmite sols. Fig. 4

shows the particle size distribution of these sols. It was found

that inorganic acids like nitric acid and hydrochloric acid give

smaller mean particle size (527 nm and 310 nm, respectively)

Fig. 3e

Light microscope images of the graded multilayer (3800): (a) dried at ambient temperature; (b) calcined at 973 K.


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compared to acetic acid (712 nm). Since the mean pore size of

the a-alumina support was 500 nm, HNO3 seemed to be proper

and was used as the peptizing agent.

Nitric acid with various H/alkoxide ratios in the range of

0.08e0.2 was used to investigate the effect of acid concentra-

tion on particle size of boehmite sols, and as shown inFig. 5,

the amount of acid had considerable effect on the mean

particle size of the sols. As H/alkoxide molar ratio increased

from 0.08 to 0.2, mean particle size of the resulting boehmite

sol decreased from about 1000 nm to less than 300 nm. This

result could be related to the fact that nitric acid causes

fracture of large agglomerates and results in formation of

smaller particles in sol.

3.3. Effect of calcination temperature on the membrane

phase structure

Calcination temperature of the intermediate layer has

a dominant effect on the microstructure of the membrane. If

the calcination temperature is low, boehmite (AlOOH) would

still exist in the membrane crystal structure, which is lessstable than gamma alumina (g-Al2O3). On the other hand,

calcination temperature should not be very high because the

tubular support used as the substrate cannot endure

temperatures higher than 1123 K and deformation occurs in

macroporous structure of the support. So, it is important to

find an optimum temperature for calcination process.Fig. 6

shows three XRD patterns of the membrane calcined at

various temperatures. First pattern is obtained from analysis

of the support which shows only a-alumina peaks. The second

pattern presents a support which is coated with sols and

calcined at 923 K. This pattern indicates the existence of

boehmite in membrane structure and also a-alumina peaks

related to the support. Calcination process was carried out atvarious ascending temperatures and as shown in third

pattern, at 973 K there is no boehmite remained in membrane

structure and all boehmite is changed into g-alumina phase.

Therefore, this temperature is chosen as an appropriate

temperature for calcination of intermediate layer.

3.4. Morphology of the membrane

Cross-sectional and surface images obtained from interme-

diate and top selective composite layers by SEM are shown in

Fig. 7.Fig. 7(a) and (b) shows a porous g-alumina multilayer

with graded structure which is coated on the surface of

a macroporous a-alumina support. The support has a meanpore size of around 500 nm which is in agreement with the

nominal value reported by the supplier. The thickness of the

intermediate multilayer is observed to be around 2 mm and

meanpore sizeto beless than10 nm. Asshownin Fig. 7 (c) and

(d), top selective composite layer deposited on top of inter-

mediate multilayer has a uniform, dense structure with

a thickness around 80e100 nm. These images indicate that gas

permeation follows different mechanisms through various

layers of the membrane because of a fundamental difference

Fig. 4 e Particle size distribution of boehmite sols peptized

with three different acids at a constant HD/alkoxide molar

ratio of 0.08.

Fig. 5 e Particle size distribution of boehmite sols peptized

with various molar ratios of nitric acid(R).

Fig. 6 e XRD patterns of a multilayer membrane calcined at

two different temperatures.

Table 3e Effect of PVA on viscosity of boehmite sol A1and pore size of the coated layer.

Amount of PVA insol (Wt. %)

Viscosity (cP) Pore size (nm)

0.8 1.97 14.10

0.9 2.19 15.47

1.0 2.35 16.211.4 3.14 27.15

1.7 4.05 36.39

2.0 4.95 44.11


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in structure between the composite dense layer and the

porous substrate.

3.5. Permeation properties of nano-composite membrane

In this part, performance of the prepared membrane with and

without the top selective composite layer was studied and the

results are shown inFig. 8. The composite membrane used forpermeation tests was synthesized by 6 h deposition of silica

and alumina with constant ATSB/TEOS molar ratio of 0.1. The

high-temperature gas permeation results show that perme-

ation values of 5.0 105 mol m2 s1 Pa1 for H2 and

9.0 106 mol m2 s1 Pa1 for N2were obtained at 1073 K,

using a support coated with a gamma alumina graded multi-

layer (Fig. 8(a)). These values indicate an H2/N2selectivity of

about 5.6, which is not high enough and should be improved.

The overall transport properties of this substrate were

controlled mainly by the transport properties of intermediate

layer [32]. This is due to the larger diffusion resistance

imposedby small pores of the g-alumina multilayer compared

to those of larger pores of the a-alumina support. Knudsendiffusion is the dominant transport mechanism in this

multilayer substrate, because this mechanism mostly takes

place through porous materials when the mean pore size is

much smaller than the mean free path of gas molecules[36].

The results confirm that permeation is proportional to the

inverse square root of the temperature which is well in

agreement with Knudsen diffusion relations.

After deposition of silica and alumina composite layer by

CVD method, H2selectivity increased considerably, although

there was a large decrease in permeation fluxes. Fig. 8(b)

shows gas permeance of H2and N2at a temperature range of

923e1073 K. Permeation values of 9.0 107 mol m2 s1 Pa1

and 1.3 108 mol m2 s1 Pa1 were obtained at 1073 K after

6 h of deposition for H2 and N2, respectively. These values

resulted in a H2/N2 selectivity of 203,which was about 36 times

more than that obtained before CVD and is quite acceptable

compared to the works reported in the literature [10]. The

Fig. 7 e SEM images of the nano-composite membrane: (a) cross-sectional image of the graded intermediate multilayer,

(b) surface image of the porous multilayer, (c) cross-sectional image of the final composite membrane, (d) surface image of

the top selective silicaealumina dense layer.

Fig. 8 e Gas permeation of membrane at different

temperatures: (a) permeation in graded multilayer without

dense layer; (b) permeation in nano-composite membrane.


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increase of permeance by temperature indicates an activated

diffusion transport mechanism through dense layer. This can

be explained by a theory based on a mechanism based on

jumps between solubility sites [25]. Briefly, the permeating gas

molecules move in solubility sites and jump randomly from

site to site. These sites in composite membrane have a size of

around 0.3 nm[25], which is larger than kinetic diameter of

most gases and can explain high H2 selectivity over N2 indense layer.

4. Conclusions

A nano-composite ceramic membrane with a graded structure

was successfully synthesized to purify hydrogen at high

operating temperatures. This membrane was prepared by

depositing a dual element thin layer composed of SiO2 and

Al2O3on top of a graded substrate by co-current CVD method.

The graded substrate was synthesized by dip-coating the

macroporous a

-alumina support with three different sizecontrolled boehmite sols using solegel method. Boehmite sols

of different particle size in the range of 50e700 nm were ob-

tained by carefully hydrolysis of aluminum tri isopropilate

followed by peptization with nitric acid. It was found that the

hydrolysis time and acid concentration would affect sol

particle size, so that longer hydrolysis time and lower acid

concentration gave larger particle size which was supposed to

be related to the formation of larger agglomerates in sols. SEM

images obtained from cross section of the membrane indi-

cated formation of a multilayer structure with an interme-

diate thickness of about 2.5 mm and a top selective layer of

80e100 nm. Permeation tests showed a reduction in gas per-

meance values after6 h of CVD, but H2

/N2

selectivity increasedfrom 5.6 to a high value of 203. This happens because gas

permeation mechanism changes from Knudsen diffusion in

intermediate layer to an activated mechanism of hopping

between solubility sites in dense top layer.

Acknowledgment

The authors would like to thank SUNA (Renewable Energy

Organization of Iran) as the financial supporter of this study.

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