lta and ion-exchanged lta zeolite membranes for dehydration of natural gas

Accepted Manuscript

Title: LTA and Ion-Exchanged LTA Zeolite Membranes forDehydration of Natural Gas

Author: Saeed Shirazian Seyed Nezameddin Ashrafizadeh

PII: S1226-086X(14)00344-XDOI: http://dx.doi.org/doi:10.1016/j.jiec.2014.06.034Reference: JIEC 2110

To appear in:

Received date: 2-5-2014Revised date: 25-6-2014Accepted date: 28-6-2014

Please cite this article as: S. Shirazian, S.N. Ashrafizadeh, LTA and Ion-ExchangedLTA Zeolite Membranes for Dehydration of Natural Gas, Journal of Industrial andEngineering Chemistry (2014), http://dx.doi.org/10.1016/j.jiec.2014.06.034

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http://dx.doi.org/doi:10.1016/j.jiec.2014.06.034

http://dx.doi.org/10.1016/j.jiec.2014.06.034

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LTA and Ion-Exchanged LTA Zeolite Membranes for Dehydration of Natural

Gas

Saeed Shirazian, Seyed Nezameddin Ashrafizadeh

Research Lab for Advanced Separation Processes, Department of Chemical Engineering,

Iran University of Science and Technology, Narmak 16846-13114, Tehran, Iran

Research Highlights

Synthesis and characterization of nanoporous zeolite membranes for gas separation

Tuning pore of zeolite membranes by ion-exchanging

Dehydration of natural gas using modified KA zeolite membranes

XRD, SEM, and EDX characterizations of synthesized LTA membranes

Abstract

This work presents synthesis and characterization of LTA-type zeolite membranes on α-Al2O3

substrate via secondary growth method. The membranes were prepared with the aim of applying

for dehydration of natural gas. Zeolite NaA membranes were synthesized using hydrothermal Corresponding Author: Tel: +98 21 77240496; Fax: +98 21 77240495; [email protected]

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method and the pore size of the membranes was then adjusted to 3Å by ion exchanging the

zeolite layer. Potassium chloride (KCl) aqueous solution was utilized as the ion-exchange

reagent. The molarity of KCl solution varied from 0.1 to 1 M to figure out the best conditions for

the synthesis of KA zeolite membranes. The synthesized membranes were characterized by

scanning electron microscopy (SEM), X-ray diffraction (XRD), and Energy-dispersive X-ray

spectroscopy (EDX). Separation performance of synthesized membranes was evaluated via

permeation of water vapor and methane, as a model of natural gas. The results showed that by

increasing molarity of KCl solution from 0.1 to 1 M, the permeation of CH4 was reduced. The

membranes ion-exchanged in KCl solution with molarity of 0.6 M was chosen as the optimum

condition.

Keywords: Membrane; Zeolite; Separation; Dehydration; Natural gas

1. Introduction

Natural gas mainly contains methane gas (CH4) and other impurities at low contents. Among

impurities available in the natural gas (NG), water is the most common impurity. Water exists in

the natural gas in the form of vapor and must be removed prior to the pipeline. An important unit

of the natural gas conditioning process is dehydration, in which the water vapor is separated from

NG. Separation of water vapor from the gas stream decreases the potential of corrosion, hydrate

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formation, and freezing in the pipeline [1]. Separation of water vapor from the natural gas is also

compulsory in order to meet the required specification for the sales contracts which range from

32.8 to 117 kg/106 std m3 [2]. Therefore, dehydration of natural gas is mandatory.

There are various methods for dehydration of natural gas. Among these methods, only two

dehydration processes are common at the moment, i.e. absorption by liquid desiccants and

adsorption by solid desiccants [1]. In the first process, NG is contacted with a liquid that shows

affinity towards water. In the second process, surface of a solid adsorbent is used for the capture

of water vapor. The latter processes suffer from some drawbacks including consumption of high

quantities of energy, high capital and maintenance costs, and difficult scale up.

As such, researchers have been trying to develop another separation processes as alternatives for

the common dehydration processes of natural gas (NG). A promising alternative for NG

dehydration is the use of water selective membranes. Membrane processes offer superior

characteristics in dehydration of natural gas such as easy operation, cost effectiveness, and low

energy consumption. Membrane separation processes for dehydration of natural gas can be

organized into two categories: (i) polymeric and (ii) ceramic membranes [3]. Polymeric

membranes have some advantages such as having low costs, easy preparation, high surface area

per contact volume, and not causing significant pressure drops. However, drawbacks of

polymeric membranes such as poor thermal and mechanical strength, low chemical stability, and

swelling phenomenon reduce their usefulness for the case of NG dehydration.

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Ceramic membranes are good choices for dehydration of NG. Among ceramic membranes,

zeolite membranes have some advantages such as thermal stability and solvent resistance. These

properties of zeolite membranes make them appropriate for the dehydration of NG.

There are many reports on the preparation of zeolite membranes for various gas and liquid

separations [4-8]. Among the known zeolites, LTA-type zeolite has received extensive attentions

from researchers due to its unique performance in separation processes. LTA is not only the most

hydrophilic zeolite, but also the easiest one to synthesize. Synthesis of high quality LTA-type

zeolite membranes has been a subject of great interest in the last two decades. Due to the

complicated mechanisms behind the zeolite membrane formation, preparation of defect-free and

high quality zeolite membranes for industrial applications is still challenging [4-6]. LTA-type

zeolite membranes have been synthesized by some researchers [6-12] at various conditions and

different methods. The synthesized membranes were mostly used for the dehydration of alcohols

in pervaporation.

The original pore size of LTA zeolite containing sodium ions (Na+) in its structure is 0.41 nm,

which can be tuned via substituting the incorporated cations in the structure of LTA by a simple

ion-exchange process [3]. It has been reported that the LTA zeolite can be ion-exchanged with

different cations, including alkaline ions [12, 13]. Depending on the cation, the pore size of the Na-

LTA (NaA) zeolite could be changed. It should be pointed out that the synthesized LTA zeolite

has sodium as the counter ion because Na+ acts as a templating agent for LTA zeolite. When this

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zeolite is ion-exchanged with potassium ions, its effective pore size is reduced to 0.3 nm.

Therefore, by changing the cations, the LTA zeolite can be used for different separations [3].

Limited studies have been carried out on the synthesis of high-quality ion-exchanged zeolite

membranes. Kusakabe et al. [14] studied the permeance of different gases through the ion-

exchanged LTA-type zeolite membranes. Aoki et al. [15] synthesized NaA zeolite membranes

for gas separation. They reported that H2 and CH4 permeations at 473 K were in the range of 10-9

and 6 × 10-10 mol.m-2s-1Pa-1, respectively. In the work of Zhu et al., H2 and CH4 permeances at

temperature of 300 K were 4 × 10 -7 and 8 × 10-8 mol.m-2s-1Pa-1, respectively [16].

Jafar and Budd [17] prepared KA-type membranes by ion exchanging NaA zeolite membranes

with an aqueous KCl solution of 0.1 mol/L for a duration of 3 h at 20 °C. The separation

performance of membranes was characterized in the removal of water from an isopropanol-water

mixture by pervaporation. Both NaA and KA type zeolite membranes were found to be highly

selective at low water concentrations.

Varela-Gandia and coworkers synthesized Na-LTA/Carbon membranes for H2 separation from

H2/CO gas mixtures. They ion-exchanged zeolite Na-LTA membranes using 0.1 M potassium

nitrate aqueous solutions at 333 K during a period of 1 h [3].

There is a definite need for development of a reproducible synthesis procedure for the

preparation of high-quality ion-exchanged zeolite membranes for gas separation. To the best of

our knowledge, there is no study on dehydration of natural gas using ion-exchanged zeolite

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membranes. For gas separation, high-quality zeolite membranes should be synthesized. This

work presents the synthesis of high-quality LTA-type zeolite membranes via secondary growth

method. K-LTA zeolite membranes were also prepared by means of ion-exchange process. The

gas permeation properties of the prepared membranes were examined in separation of water

vapor from methane, as a model of natural gas.

2. Experimental

2.1. Chemicals

The used chemicals for the preparation of LTA zeolite particles (seeds) were aluminum

isopropoxide (C9H21O3Al, Merck), silica gel (SiO2, Merck), sodium hydroxide (99% NaOH,

Merck), and deionized water. For the synthesis of zeolite membranes, sodium aluminate

(NaAlO2, purity >96 %) from BDH and sodium silicate (25.5–28.5% SiO2, 7.5-8.5 Na2O) from

Merck were used as sources of Al and Si, respectively. Potassium chloride (KCl 99%) from

Riedel-de Haen was used for ion-exchange purposes.

2.2. Synthesis of LTA zeolite particles

Na-LTA powders were required to be used as seeds on the surface of the support in the secondary

growth of zeolite membranes. LTA zeolite powders were synthesized using hydrothermal method

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with a gel formula of 1 Al2O3: 2 SiO2: 5 Na2O: 200 H2O, in a molar basis. In order to prepare the

gel, 4.5 g of NaOH was completely dissolved in 40 g of deionized water. The solution was then

divided into two equal volumes within two polypropylene beakers. An amount of 4.6 g of aluminum

isopropoxide was added to one part of the NaOH solution and was stirred for 1 h. Meanwhile,

1.3 g of silica gel was added into the rest of the NaOH solution at 323 K under stirring [18].

After cooling down the silicate solution to ambient temperature, it was slowly added to the

aluminate solution under stirring. The gel was stirred for 1 h to be thoroughly homogenized.

Afterward, the hydrogel was transferred into a teflon autoclave for hydrothermal treatment. The

hydrotheraml reactions were carried out at a temperature of 323 K and a duration of 24 h.

The syntehsized NaA zeolite particles were ion-exchanged with 1 M aqueous solution of KCl to

determine the general information about the ion-exchange condiitons, and also to investigate the

effect of ion exchange on crystallinity of synthesized LTA particles. The ion exchange was

repeated four times at temperature of 313 K.

2.3. Synthesis of zeolite membranes

Homemade porous α-alumina disks with a thickness of 1.8 mm, a diameter of 21 mm, average

pore size of ca. 500 nm, and porosity of 43% were used as supports for the synthesis of

membranes. Moreover, the used substrates were symmetric in structure, and were seeded using

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dip coating method. A suspension of 1 wt. % LTA particles was used for this purpose. Details of

the procedure of seeding on the supports are reported elsewhere [18].

Secondary growth method was applied for the synthesis of zeolite layer on the support surface.

A synthesis hydrogel with a formula of 1 Al2O3: 2 SiO2: 3.4 Na2O: 155 H2O, in a molar basis,

was prepared [18]. The membranes were synthesized at temperature of 373 K and a duration of 3 h.

Details of the procedure of membrane preparation is reported in our previous publications [18].

The prepared Na-LTA membranes were ion-exchanged in order to tune the pore size of the

membranes. The NaA zeolite membranes were thus immersed into aqueous solutions of

patassium chloride with molarities of 0.1, 0.3, 0.6 and 1 M at temperature of 313 K for 0.5 h.

The solution was mildly agitated to prevent formation of any damage to the zeolite layer during

the ion exchange process. The process was repeated to ensure the completion of ion exchange, and

the effect of ion-exchange repetition on permeation of water vapor and methane was evaluated.

2.3. Characterizations

The formation of LTA zeolite phase for the both of seeds and membranes was examined via

X-ray diffraction (XRD). XRD measurements were carried out on a JEOL diffractometer, JDX-

8030 by using Cu K radiation (wavelength of 1.54056) operating at 40 kV and 30 mA. The

step size for XRD measurements was set at 0.04. Scanning electron microscopy (SEM) was

used to examine the seeds morphology as well as to characterize the membranes. The SEM

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images were obtained using a VEGA3 TESCAN scanning electron microscope. Energy-

dispersive X-ray spectrometry (EDX) was used for elemental analysis of K-LTA membranes to

ensure the successful exchange of ions.

2.4. Permeation experiments

Separation performance of the synthesized Na-LTA and K-LTA zeolite membranes were

evaluated via permeation of water vapor and methane through the membranes. Schematic

presentation of dehydration setup used in the experiments is shown in Fig. 1. Single gas

permeations were determined throughout the experiments. Permeation of methane was tested at

a pressure difference of 1 bar and a temperature of 298 K. CH4 was supplied from gas cylinder

(purity of 99.95% vol., Technical Gas Services). Constant pressure method was used to

determine the amount of permeate. The permeation measurements were performed in a PTFE

membrane module which was sealed by means of polymeric O-rings to prevent any probable

leak. An ultrasonic vapor generator was used to produce the water vapor at the ambient

temperature and pressure. The permeate side was evacuated using a vacuum pump (JB, USA),

and permeated vapor was condensed via a cold trap which was immersed in liquid nitrogen.

Fig. 1: Experimental setup used in permeation experiments.

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3. Results and discussions

3.1. Characterization of zeolite seeds

The zeolite membranes were synthesized on porous α-alumina substrates. The morphology and

pore size of substrate is of great importance for the synthesis of high-quality zeolite membranes.

There should be compatibility between support and zeolite seeds for preparation of defect-free

membranes. SEM image of the support surface is shown in Fig. 2A. As it is shown, the pore size

of the membrane can be estimated to be about 500 nm. Application of zeolite seeds with pore

sizes smaller than 500 nm would lead to growth of a zeolite film within the pores of substrate.

The latter phenomenon decreases the permeation of the gases through the membrane. SEM

image of the support surface after seeding is also illustrated in Fig. 2B. As it is observed, zeolite

particles have covered the surface of support, and no particles have penetrated into the substrate

pores. The morphology of Na-LTA particles were analyzed using SEM images. Fig. 2C

illustrates the SEM image of synthesized seeds. It is observed that the seeds have a cubic shape

which is the crystal habit of Na-LTA zeolite. It is also revealed that the particle size distribution

is not broad. This parameter is an important factor in the synthesis of high-quality zeolite

membranes.

Moreover, SEM image of ion-exchanged zeolite particles is also illustrated in Fig. 2D. The

zeolite particles are obtained with four times ion exchange with 1M KCl solution. It is revealed

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that no significant change is occurred during ion exchange. However, further analysis with XRD

is required to judge about the crystallinity of KA zeolite particles.

Fig. 2: SEM image of A: α-alumina support used for the synthesis of membranes; B: seeded

support with NaA seeds; C: NaA seeds; D: ion-exchanged LTA particles.

3.2. Characterization of membranes

XRD pattern of as-synthesized Na-LTA seeds is represented in Fig. 3. The characteristic peaks

associated with the LTA structure are detected in XRD pattern. The XRD pattern confirms that

LTA is the only present phase and no other competing phases are formed. That could be

attributed to the low synthesis temperature (323 K) which is a safe temperature for the synthesis

of LTA-type zeolite [18].

The as-synthesized zeolite membranes were also characterized using XRD. XRD patterns of Na-

LTA membrane is shown in Fig. 3. Two sets of peaks are detected in Fig. 3 including zeolite

LTA phase and substrate. The peaks observed at 2θ of 7.2, 10.1, 12.5, 16.0, 21.6, 23.9, 26.1,

27.0, and 29.9 are the characteristic peaks associated with zeolite LTA structure. The assigned

peaks which correspond to the -alumina substrate have also been detected (marked as S).

However, little amorphous phases can be detected due to the low background of XRD curve. Fig.

3 also reveals that no other zeolitic phases are formed on the surface of substrate.

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Moreover, XRD pattern of ion-exchanged zeolite LTA particles are represented in Fig. 3.

Comparison with XRD pattern of zeolite NaA seeds reveals that crystallinity of zeolite decreases

slightly. The latter could be attributed to the high duration of ion exchange (four times in this

case). Zeolite LTA is known as the most hydrophilic zeolite, and has high affinity toward water.

Therefore, long contact of zeolite LTA with water results in reduction of zeolite crystallinity.

Therefore, the membranes are ion exchanged with special care, and two times of ion exchange is

carried out for synthesis of KA membranes.

Fig. 3: XRD pattern of A: NaA zeolite seeds; IE: ion-exchanged LTA particles; M:

zeolite NaA membrane.

SEM images of the as-synthesized NaA membrane are shown in Fig. 4. A dense and integrated

zeolite layer can be seen on the surface of the substrate. Fig. 4A shows that some amorphous

phases are present in the membrane layer. These amorphous phases reduce the membrane

performance and are formed due to the use of a concentrated hydrogel for the membrane

synthesis. Application of a clear gel might avoid the formation of these amorphous phases.

Furthermore, from Fig. 4B the membrane thickness can be estimated to be about 5 microns.

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Fig. 4: SEM images of the as-synthesized Na-LTA zeolite membrane. A: top surface; B:

cross surface of the membrane.

3.3. Permeation results

The as-synthesized Na-LTA zeolite membranes were subjected to ion-exchange with potassium

chloride aqueous solutions for preparation of K-LTA zeolite membranes with smaller pore sizes.

The Na-LTA membranes were contacted with aqueous solutions of KCl at various concentrations.

The concentrations of solution were 0.1, 0.3, 0.6, and 1 M. The ion-exchange process was also

repeated to evaluate the effect of repetition on the permeation and selectivity of the membranes.

Ion-exchanged membranes were tested against gas permeation to determine the best conditions

for preparation of K-LTA membranes. The results of gas permeation tests are listed in Table 1.

Table 1: Permeance of water vapor and methane through NaA and KA zeolite

membranes.

As it can be seen from data of Table 1, Na-LTA membrane provides an ideal separation factor

of 11.4 for water vapor over methane. By ion-exchanging Na-LTA membrane with 0.1 M KCl

solution, although the permeation of both vapor and methane decreases, the ideal selectivity

increases. It seems that the molarity of ion-exchange solution is not sufficient for a successful

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preparation of K-LTA membrane. By enhancement of molarity of KCl solution from 0.1 to 1 M,

permeance of CH4 and water vapor reduces considerably. According to Table 1, there is not too

much difference between permeations of KA-6 and KA-7. The KA-6 membrane which was ion-

exchanged in a solution with the molarity of 0.6 M has the best performance while increment of

molarity is ineffective. Separation factor of KA-6 membrane at ambient temperature was 19.7.

According to data of Table 1, increasing the time of repetition results in enhancement of ideal

selectivity. The reason for this phenomenon is that ion exchanging the membrane with a fresh

KCl solution increases the chemical potential, which is the driving force for the ion exchange.

It should be noted that increasing the time of repetition is not suggested because the crystallinity

of LTA zeolite decreases when it is subjected to water molecules. By considering the lower

crystallinity of the ion-exchanged zeolite, the lifetime of the membranes will decrease with ion-

exchange. Therefore, it is not suggested to use the ion-exchange more than two times in order to

improve the performance of the membrane and prevent the reduction of the zeolite layer stability.

To prevent the destruction of zeolite structure during the ion exchange process, the duration of

ion exchange was reduced to 0.5 h in this work, which is smaller than other similar works [3, 17].

3.4. Characterization of KA membrane

The cations and other elements present in the membrane structure were determined using

Energy-dispersive X-ray spectroscopy (EDX). EDX analysis was performed for KA-8 membrane

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(see Table 1) to calculate the degree of ion exchange for this membrane. Fig. 5A shows the EDX

results for KA-8 membrane. It is confirmed that the ion-exchange has been successfully carried

out, and no significant sodium cations are detected in the membrane structure. The prepared Na-

LTA membranes were ion-exchanged for 0.5 h during which the extent of ion-exchange could be

semi-quantitatively measured by means of EDX analysis of the zeolite layer. According to Breck

et al. [13], ion-exchange of zeolitic materials is related to the size of the various cations and their

respective location in the zeolite cages. Other authors have studied the ion-exchange with Rb

ions [19, 20]. Yan and Bein [19] have studied the ion-exchange with different cations in a thin

film of LTA crystals and silica over a gold surface modified with siloxane.

In order to understand how the permeation properties of synthesized membranes are affected by

ion-exchange with K+ cations, size and location of the exchanged cations in zeolite structure are

of great importance. In the case of K-LTA zeolite membranes, the potassium cations are situated

in the same position as the corresponding Na+ ions. Since the size of K+ ions are larger than Na+

ions, therefore, the pore size zeolite membrane is reduced to 0.28 nm. This statement is in

agreement with the results obtained in the permeation tests (see Table 1). In this case, the

reduction in the pore size of the membrane causes the water vapor and CH4 permeance to be

reduced in comparison with that in the Na-form membrane.

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EDX analysis for NaA zeolite membrane is also provided to calculate the yield of ion exchange.

The yield of ion exchange is calculated to be more than 99 %. EDX analysis of NaA membrane

is also represented in Fig. 5B.

Fig. 5: EDX analysis of synthesized zeolite membranes. A: KA membrane; B: NaA

membrane.

The effect of ion exchange on morphology of zeolite membrane is also investigated. Fig. 6

illustrates SEM images of ion-exchanged membranes (KA-8). It is clearly observed that the

morphology of ion-exchanged zeolite membrane does not change. A dense and cubic integrated

zeolite layer can be observed on the surface of support.

Fig. 6: SEM images of KA zeolite membranes. A: top surface; B: cross section.

4. Conclusions

High-quality LTA-type zeolite membranes were synthesized and ion-exchanged in potassium

chloride solutions at various molarities. The effects of the molarity of KCl solutions and the

repetition of ion-exchange on the membrane performance were investigated to produce high-

quality KA membranes suitable for dehydration of natural gas. The zeolite powder and

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membranes were characterized by XRD, SEM and EDX. The membranes performance was

evaluated via gas permeation experiments. The results showed that the membrane ion-exchanged

in 1 M KCl solution for two times had the best separation performance.

Acknowledgments

The research council at Iran University of Science and Technology (IUST) is highly

acknowledged for its financial support during the course of this research.

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[7] S. Aguado, J. Gascon, D. Farrusseng, J.C. Jansen, F. Kapteijn, Microporous and Mesoporous

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List of Tables:

Table 1: Permeance of water vapor and methane through NaA and KA zeolite membranes.

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List of Figures:

Fig. 1: Experimental setup used in permeation experiments.

Fig. 2: SEM image of A: α-alumina support used for the synthesis of membranes; B: seeded

support with NaA seeds; C: NaA seeds; D: ion-exchanged LTA particles.

Fig. 3: XRD pattern of A: NaA zeolite seeds; IE: ion-exchanged LTA particles; M: zeolite NaA

membrane.

Fig. 4: SEM images of the as-synthesized Na-LTA zeolite membrane. A: top surface; B: cross

surface of the membrane.

Fig. 5: EDX analysis of synthesized zeolite membranes. A: KA membrane; B: NaA membrane.

Fig. 6: SEM images of KA zeolite membranes. A: top surface; B: cross section.

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Table 1

Sample Molarity of

KCl solution

(M)

Ion-exchange

repetition

Vapor Permeance

(mol/(m2.s.pa)×10-6)

CH4 Permeance

(mol/(m2.s.pa)×10-7)

Vapor/CH4

ideal

selectivity

NaA - 0 5.7 5.0 11.4

KA-1 0.1 1 4.3 3.1 13.8

KA-2 0.1 2 4.1 2.9 14.1

KA-3 0.3 1 1.5 1.0 15.1

KA-4 0.3 2 1.4 0.9 15.9

KA-5 0.6 1 1.1 0.7 16.9

KA-6 0.6 2 0.7 0.3 19.7

KA-7 1 1 0.6 0.3 20.6

KA-8 1 2 0.6 0.3 20.7

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Graphical Abstract (for review)

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Figure 1

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lta and ion-exchanged lta zeolite membranes for dehydration of natural gas

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