Dehydration of water/1-1-dimethylhydrazine mixtures by zeolite membranes

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MICMAT 1673 No. of Pages 8, DTD = 4.3.1

9 March 2004 Disk usedARTICLE IN PRESS

www.elsevier.com/locate/micromeso

Microporous and Mesoporous Materials xxx (2004) xxx–xxx

OFDehydration of water/1-1-dimethylhydrazine mixturesby zeolite membranes

Mansoor Kazemimoghadam, Afshin Pak, Toraj Mohammadi *

Research Lab for Separation Processes, Faculty of Chemical Engineering, Iran University of Science and Technology, Iust Narmak, Tehran, Iran

Received 25 August 2003; received in revised form 10 December 2003; accepted 10 February 2004

EDPRO

Abstract

In this research, dehydration of water/1-1-dimethylhydrazine (UDMH) mixtures by zeolite NaA and hydroxy sodalite mem-

branes has been investigated. Support of these membranes has been tubular mullites that have been made by extruding a mixture of

about 67–75% kaolin clay and 33–25% distilled water using an extruder. Zeolite NaA and hydroxy sodalite membranes have been

coated on the external surface of the porous supports by the hydrothermal synthesis.

UDMH/water mixtures have been separated at ambient temperature and pressure by pervaporation (PV) using these zeolite

membranes. These membranes showed very high selectivity of water for all UDMH mixtures. For the UDMH/water mixtures,

separation factor as high as 10 000 has been obtained for UDMH feed concentration of 2%. Total mass fluxes of 1.05–0.2 kg/(m2 h)

have been also obtained.

� 2004 Elsevier Inc. All rights reserved.

Keywords: NaA zeolite; Hydroxy sodalite zeolite; Pervaporation; Zeolite membrane; UDMH; Dehydration; Mullite support

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NCORREC1. Introduction

Polymeric membranes may not be suitable for

applications involving high water concentrations or

applications containing harsh solvents like UDMH dueto membrane stability problems and swelling effects.

However, a recent development of solvent-and-temper-

ature resistant, hydrophilic ceramic membranes has

made it possible to overcome the above limitations of

hydrophilic polymeric membranes [1].

PV is an economical separation technique compared

to conventional separation methods such as distillation

especially in processes involving azeotropes, isomers andremoval or recovery of trace substances. Due to its high

separation efficiency and flux rates, PV results in energy

cost saving and safe operation [2,3].

Removal of organic compounds from aqueous solu-

tions is important for the recovery of valuable organic

products, for the recycling of process water and for the

treatment of waste water. Generally, distillation can be

used to remove organic compounds from water. How-
U 59
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61*Corresponding author. Tel.: +98-21-7896621; fax: +98-21-

7896620.

E-mail address: [email protected] (T. Mohammadi).

1387-1811/$ - see front matter � 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2004.02.015

T

ever for low organic concentrations or thermally sensi-

tive organic compounds, distillation is neither

economical nor suitable. Furthermore, PV has several

advantages over traditional distillation: (1) reduced en-

ergy demand because only a fraction of the liquid thatneeds to be separated is vaporized, (2) simple equipment

since only a vacuum pump is used to create a driving

force and (3) lower capital cost. Thus, relatively mild

operation conditions and high effectiveness make PV an

appropriate technique for such separations. As a result

most PV studies have been focused on dehydration of

organic mixtures [2].

In PV, the feed mixture is contacted with a nonpor-ous permselective membrane. Separation is, in general,

explained by the steps of sorption into, diffusion

through and desorption from the membrane. The latter

is usually considered to be fast and taking place at

equilibrium, while diffusion is kinetically controlled and

the slowest step of the process. Permeation is dependent

on sorption and diffusion steps. The driving force for the

separation is created by maintaining a pressure lowerthan the saturation pressure on the permeate side of the

membrane. The mechanism of separation is usually ex-

plained in terms of sorption–diffusion processes [4].

mail to: [email protected]

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2 M. Kazemimoghadam et al. / Microporous and Mesoporous Materials xxx (2004) xxx–xxx



UNCORREC

UDMH is an important liquid fuel, however, it also

finds many new applications every day such as an oxy-

gen scavenger for boiler-feed water, a starting material

for drug and dye intermediates, a catalyst for polymer-

ization reactions, etc. [5,6].UDMH is produced by the Rashing process which

involves reaction of ammonia with sodium hypochloride

to give chloramines which in turn are reacted with

dimethylamine to obtain UDMH. Its reaction liquor

contains low concentrations of UDMH (<2%) besides

impurities such as sodium chloride (8%) and water

(>90%). The conventional purification methods, which

involve flash vaporization followed by extractive distil-lation (at 20% UDMH, aqueous azeotrope), are highly

energy-intensive and about 210 kg of steam is required

to obtain 1 kg of UDMH. In addition, this method is

very hazardous since UDMH is highly flammable and

forms explosive mixtures with air. Moreover, UDMH is

very corrosive and its vapor is extremely toxic and car-

cinogenic. Therefore, the search for a safer and more

economical alternative technology is a challengingproblem. Membrane-based pervaporation technology

has all the requirements for completely replacing

extractive distillation for separation of the azeotrope.

This can be combined with simple distillation for

enrichment of UDMH to high purity levels [7,8].

Dehydration of hydrazine and monomethylhydrazine

(MMH) using ethyl cellulose membranes have been

previously carried out [9]. However, ethyl cellulosecannot be used to dehydrate UDMH because it is de-

graded rapidly. Also, hydrophobic polymers such as

poly dimethylsiloxane (PDMS) cannot withstand the

highly alkaline (pH¼ 13–14) medium whereas mem-

branes made of polyethylene and polypropylene give

negligible flux so that UDMH cannot be selectively ex-

tracted from its dilute reaction liquor. The problem of

chemical compatibility is also encountered in the case ofhydrophilic polymers such as poly vinyl alcohol and

poly acrylic acid [10].

Chitosan, a derivative of the naturally abundant

biopolymer chittin, is fully stable in anhydrous UDMH

and hence can be selected for its dehydration, keeping in

minds its highly hydrophilic nature and good mechani-

cal strength. The promising potential of chitosan as a

PV membrane has already been exploited for dehydra-tion of alcohols such as ethanol and isopropanol. This

polymer has recently been used to form highly selective

and permeable blend membranes with poly (vinyl alco-

hol), sodium alginate, etc. [11–13].

In recent years, however, attempts have been made to

develop zeolite membranes for separation due to their

molecular sieving properties, uniform pore size distri-

bution, high thermal resistance, high chemical inertnessand high mechanical strength. Until now, different

preparation methods, such as in situ hydrothermal

synthesis have been developed and high quality MFI, A,

TEDPROOF

Y, MOR, ETs-4 hydroxy sodalite and FER zeolite

membranes have been prepared [14–19].

The pore size of NaA zeolites is 0.41 nm. The small

pore size of these zeolites makes the separation of small

molecules possible via molecular sieving. Zeolite mem-branes have been found to be extremely effective for

dehydration of ethanol by PV, with separation factors of

104 or more being quoted. It has been observed that it is

possible to form remarkably uniform membranes of

zeolite NaA and hydroxy sodalite from a homogeneous

solution. This has many potential advantages in terms of

reproducibility and easy control [14,15].

High PV performance of zeolite NaA and hydroxysodalite membranes for separation of water/UDMH

mixtures has been observed. These dense layers have

been coated on the external surface of the porous

cylindrical mullite supports using the seeded hydro-

thermal method. These membranes have been used

successfully for dehydration of water/UDMH mixtures.

Synthesized ETS-4 membranes on a porous titania

support, by secondary growth of seed layers, have beencharacterized [20]. These ETS-4 membranes have been

evaluated for separating water from water–ethanol

mixtures by PV. The water flux of the ETS-4 membranes

was comparable to or higher that of NaA and FAU-type

zeolite membranes.

These membranes are highly water permselective and

show selectivities of water over ethanol as high as 400.

The selectivities of the ETS-4 membranes are compa-rable to those of NaX and NaY membranes and are

about one order of magnitude less than those of zeolite

NaA membranes [20].

Alcohol selectivity of NaY and NaX zeolite mem-

branes in PV of alcohol/water mixture has also been

reported [21]. These membranes permeated water pref-

erentially. The selectivity of NaX and NaY membranes

were 170 and 130 at 75 �C and 90% EtOH, respectively.These selectivities are less than that of zeolite NaA

membranes (10 000) at the same condition. The related

flux of NaA, NaX and NaY membranes are 2.15, 1.91

and 1.59, respectively [21].

2. Experimental

2.1. Support preparation

In ceramic membranes, thin dense layers are usually

deposited on porous supports. The porous supports

provide mechanical strength for the thin selective layers.

Porous support materials include alumina, cordierite,

mullite, silica, spinel, zirconia, other refractory oxides

and various oxide mixtures, carbon, sintered metals andsilicon carbide [17,18].

In this research mullite supports have been prepared

from kaolin clay. Kaolin clay is thermally converted to

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M. Kazemimoghadam et al. / Microporous and Mesoporous Materials xxx (2004) xxx–xxx 3



RREC

mullite via high temperature calcination. The mullitiza-

tion reaction takes place when kaolin clay is utilized as

the sole source of silica and alumina. The reaction can

be represented by the following equation where the

approximate chemical formula for kaolin (without thewater of hydration) is given as Al2O3 Æ 2SiO2 and the

formula for mullite is 3Al2O3 Æ 2SiO2:

3ðAl2O3 � 2SiO2Þ ! 3Al2O3 � 2SiO2 þ 4SiO2

The term represented by 4SiO2 is the free silica gener-

ated as a result of the conversion. This free silica has

been leached and then porous mullite bodies have been

prepared. Mullite has several distinct advantages over

other bodies such as alumina. Since these bodies are

heated to high temperatures to achieve the mullite

conversion reaction, strong inter-crystalline bonds be-tween mullite crystals, are formed and this results in

excellent strength and attrition. Leaching time depends

on several factors including:

(1) the quantity of free silica to be removed,

(2) the porosity of the body prior to leaching,

(3) concentration of the leaching solution and

(4) temperature [22,23].

The kaolin material used in this study (SL-KAD

grade) has been obtained from WBB cooperation. The

analysis of the kaolin is listed in Table 1. UDMH (98%)

and sodium hydroxide (Merck) have been also used in

all experiments.Cylindrical shaped (tubular) bodies (ID: 10 mm, OD:

14 mm and L: 15 cm) have been conveniently made by

extruding a mixture of about 67–75% kaolin clay and

25–33% distilled water using an extruder.

Suitable calcination temperatures and periods are

those at which the clay converts to mullite and free sil-

ica. Good results have been achieved by calcining for

about 3 h at temperatures of about 1250 �C [24].Free silica has been removed from the calcined bodies

by leaching with strong alkali solutions. Removal of this

free silica causes mesoporous tubular supports to be

made with very high porosity. Free silica removal has

been carried out with aqueous solutions containing 20%

by weight NaOH at a temperature of 80 �C for 5 h.

UNCO

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Table 1

Analysis of kaolin clay

Component Percent (%) Phases Percent (%)

SiO2 51.9 Kaolinite 79

TiO2 0.1 Illite 8

Al2O3 34.1 Quartz 10

Fe2O3 1.4 Feldspar 3

K2O 0.8

Na2O 0.1

L.O.I 11.6

Total 100 Total 100

TEDPROOF

Supports have been washed with 2 l of water for 12 h at

a temperature of 80 �C in order to remove NaOH.

Porosity of the support before leaching is 24.3% while

after treatment it increases to 49%. Flux of the support

before and after free silica removal at 1 bar and 20 �Care 6 and 10 kg/m2 h, respectively. The porosity of

support has been measured by water absorption method

[26].

The phase identification has been performed by X-ray

diffractometry (Philips PW3710, Philips Co., The

Netherlands) with CuKa radiation.

2.2. Zeolite membrane synthesis

2.2.1. Coating of the support with seeds

Porous mullite tubes (home made) as describe above

have been used as the support. The external surface of

the supports have been polished with 600 grit-sand pa-

pers, and then the support has been washed and cleaned

with distilled water in a microwave heater for 5 min to

remove loose particles created during polishing. Then,supports have been dried at 100 �C for 3 h.

Early results showed that the NaA zeolite membrane

synthesized without seeding has a very low separation

factor (about 10) even after five times of coating.

Therefore, seeded supports have been used for mem-

brane synthesis. As Kondo explained the suitable seed

crystal size must be less than 200 lm and more prefer-

ably in the range of 1–5 lm [30]. In other words, in orderto form a thin and uniform zeolite membrane on the

mullite support, the nucleation seeds should be small

and uniform in size. In order to inhibit the formation of

zeolites into the support pores, the seeds should not

penetrate into the pores. The high purity nucleation

seeds have been synthesized by hydrothermal method.

Size of the seeds is about 2 lm [25]. The seeds should be

dispersed homogeneously on the support surface and theamount of seeds on the support surface should not be

too much. Otherwise, the synthesized zeolite membrane

is heterogeneous or too thick.

The seeded supports have been prepared by dipping

the mullite supports in an 8% NaA zeolite suspension in

a single step. The 8% NaA zeolite suspension has been

prepared by mixing 8 g NaA zeolite in 92 ml distilled

water. After dipping procedure, the supports have beendried at 100 �C for 3 h.

2.2.2. NaA synthesis

NaA membrane has been grown hydrothermally on

the surface of the porous support. The hydrothermal

synthesis of NaA zeolite membrane has been performed

as follows:

The synthesis solution has been prepared by mixingaluminate and silicate solutions. 4.87 g of sodium

hydroxide has been dissolved in 76 ml of distilled water.

The solution has been divided into two equal volumes

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UNCORREC

and kept in polypropylene bottles. Aluminate solution

has been prepared by adding 6.23 g sodium aluminate

(Aldrich, 50–56% Al2O3) to one part of the NaOH

solution. It has been well mixed until cleared. Silicate

solution has been prepared by adding 16.57 g sodiumsilicate solution (Merck, 25–28% SiO2) to another part

of the NaOH solution. Silicate solution has then been

poured into aluminate solution and well mixed until a

thick homogenized gel has been formed. Composition of

the homogeneous solution for zeolite NaA membrane is

represented by the following molar ratio 1.926SiO2:

Al2O3:3.165Na2O:128H2O [27].

Two ends of the support have been closed with rub-ber caps to avoid any precipitation of zeolite crystals on

the internal surface of the support during membrane

synthesis. The seeded support has been placed vertically

in a Teflon autoclave. The solution has been carefully

poured in the autoclave and then the autoclave has been

sealed. The crystallization has been carried out in an

oven at a temperature of 100 �C for 3 h. Then, the

sample has been taken and the synthesized membraneshave been washed several times with distilled water. The

pH of the washings solution must be 7. Then, the sam-

ples have been dried in air at room temperature for 12 h.

2.2.3. Hydroxy sodalite synthesis

Hydroxy sodalite membrane has been grown hydro-

thermally on the surface of the porous support. The

synthesis solution has been prepared by mixing alumi-nate and silicate solutions. 16.33 g of sodium hydroxide

has been dissolved in 67.63 ml of distilled water. The

solution has been divided into two equal volumes and

kept in polypropylene bottles. Aluminate solution has

been prepared by adding 0.623 g sodium aluminate

(Aldrich, 50–56% Al2O3) to one part of the NaOH

solution. It has been well mixed until cleared. Silicate

solution has been prepared by adding 0.86 g sodiumsilicate solution (Merck, 25–28% SiO2) to another part

of the NaOH solution. Silicate solution has then been

poured into aluminate solution and well mixed until a

homogenized gel formed. Composition of the homoge-

neous solution for zeolite NaA membrane is represented

by the following molar ratio SiO2:Al2O3:55Na2O:

1000H2O [28].

Two ends of the support have been closed with rub-ber caps to avoid any precipitation of zeolite crystals on

the internal surface of the support during membrane

synthesis. The seeded support has been placed vertically

in a Teflon autoclave. The solution has been carefully

poured in the autoclave and then the autoclave has been

sealed. The crystallization has been carried out in an

oven at a temperature of 100 �C for 12 h. Then, the

sample has been taken and the synthesized membraneshave been washed several times with distilled water. The

pH of the washings solution must be 7. Then, the sam-

ples have been dried in air at room temperature for 12 h.

Microstructure and thickness of the membranes have

been observed by scanning electron microscopy (SEM,

ISI ABT, SRT-50, Japan).

TEDPROOF

3. Pervaporation tests

The zeolite membranes have been used for long term

dehydration of UDMH. Since UDMH is very hazard-

ous, most experiments have been carried out using

ethanol mixtures with a concentration of 50–95%.

However, to make sure the mixture are suitable for

dehydration of UDMH mixtures some experiments havebeen carried out using a UDMH mixture with a con-

centration of 2%.

The experiments have been carried out at a temper-

ature of 30 �C and a pressure of 1.5 mbar at the per-

meate side, within a period of 30–60 min.

The pervaporation setup is presented in Fig. 1(a) and

(b). Any change of feed concentration due to perme-

ation is negligible because the amount of permeate issmall (max 2 ml) compared to total feed volume in the

system (0.5 l). A three stage diaphragm vacuum pump

(vacuubrand, GMBH, Germany) has been employed to

evacuated the permeate side of the membrane to a

pressure of approximately 1.5 mbar while the feed side

has been kept at room pressure. The permeate side has

been connected to a liquid nitrogen trap via a hose to

condense the permeate (vapor). Permeate concentra-tions have been measured by a GC (TCD detector,

Varian 3400). It must be mentioned that the TCD

detector can measure 10�4 wt.% of UDMH (i.e. a sep-

aration factor of 10 000 for a feed (water) concentration

of 99%).

Performance of PV is usually evaluated by total flux

(kg/m2 h) and separation factor (dimensionless). Sepa-

ration factor of any organic aqueous solution can becalculated from the following equation:

Separation factorðaÞ ¼XH2O=XOrganic

� �permeate

XH2O=XOrganic� �

feed

where XH2O and Xorganic are weight fractions of water andorganic compound, respectively.

4. Results and discussions

Fig. 2(a) and (b) shows the XRD patterns of the

membrane support before and after sodium hydroxidetreatment, respectively. Fig. 2(c) is XRD pattern of the

seeded support after treatment.

The XRD pattern shows that kaolin after calcination

at 1250 �C for 3 h is converted into mullite and free

silica. Free silica can be observed in crystalline and

amorphous form as shown in Fig. 2(a). In primary

C

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Fig. 1. (a) Pervaporation cell: 1––feed tank, 2––membrane module, 3––

membrane, 4––O-ring, 5––teflon fitting, 6––stainless steel vacuum fit-

ting, 7––vacuum hose, 8––cap, 9––feed tank cap. (b) Pervaporation

setup: 1––feed container and pervaporation cell, 2––nitrogen cold trap,

3––filtrate container, 4––tree stage vacuum pump.




UNCORRE

support (Fig. 2(a)), the XRD pattern is very noisy be-

cause of the amorphous phase. After treatment by so-

dium hydroxide, the noisy base is completely omitted

(Fig. 2(b)) and intensity of mullite peaks increases. This

shows that silica has been removed from the support.

Removal of this free silica causes the support porosity to

increase from 24.3% to 49%. By increasing the leachingtime, all free silica has been removed as shown in the

Fig. 2(c). As seen, only mullite and zeolite (seed) can be

observed.

The XRD patterns of NaA and hydroxy sodalite

zeolite membranes confirm that crystal of zeolite NaA

and hydroxy sodalite have been formed. Figs. 3 and 4

shows XRD of the membranes synthesized using the

above mentioned methods. In these two figures the onlyphases which can be observed are zeolite NaA and

mullite and hydroxy sodalite and mullite, respectively.

The results show that repeating zeolite NaA forma-

tion procedure on the same support by the hydrother-

TEDPROOF

mal method improves membrane quality. The XRD

intensities of zeolite peaks are greater after additional

coating (as shown in Fig. 5). Also significant changes in

separation factor can be observed as shown in Table 2.

A promising result (defect-free membrane) has beenobtained after five times of coating as shown in Table 2.

As membranes prepared with less number of coating

behaved low separation factor, they have not been

evaluated by UDMH because of its very high toxic

properties. In other words, only membranes with high

separation factor have been evaluated by UDMH. It

must be mentioned that the aim of this study was

making a high performance zeolite membrane forUDMH purification. It must also be noted that hydroxy

sodalite zeolite membranes after one time of coating

behaved very high separation factor for the UDMH

mixture. Zeolite NaA membrane thickness increases

with increasing the number of coating processes. It has

been confirmed by a SEM micrograph as shown in Fig.

6. However, zeolite hydroxy sodalite membranes

showed promising result even after one time of coating.It must be mentioned that flux of this membrane is lower

than that of zeolite NaA membrane. The separation

mechanism of zeolite membranes is molecular sieving

and water adsorption is the key factor since UDMH

cannot enter into the pores of the membranes. Also

interaction of water/organic affects the adsorption and

causes lower fluxes [31]. It must be mentioned that pore

size and as a result adsorption of hydroxy sodalitezeolite is less than those of NaA zeolite (18 wt.% water

adsorption of hydroxy sodalite against 25% water

adsorption of NaA) [29]. This causes the flux of hydroxy

sodalite zeolite membrane to be less. As adsorption is

the rate controlling process and the permeate side

pressure is very low (1.1 mm Hg) and less than water

vapor pressure (20 mm Hg at room temperature), it may

be concluded that the thickness of the membrane has nosignificant effect on flux (due to a very high driving

force).

It must be mentioned that membrane flux decreases

with increasing membrane thickness. Thickness of NaA

membrane after two times and four times of coating and

that of zeolite hydroxy sodalite membrane after one

time of coating are 50, 555 and 71 lm, respectively. Thezeolite hydroxy sodalite membrane is uniform and de-fect free and as a result its separation factor is high.

Preparation time of hydroxy sodalite membrane has

been found to be 12 h for making a uniform membrane.

However increasing preparation time of zeolite NaA

membrane for more than 3 h causes other forms of

zeolite such as sodalite and NaX to be formed. Lower

flux of the hydroxy sodalite membrane can be also

attributed to its smaller pore size.

UNCORRECTEDPROOF

Fig. 2. XRD pattern of the membrane support (a) before treatment, (b) after treatment, (c) seeded support after treatment.

Fig. 3. XRD pattern of the zeolite NaA membrane with two times synthesis.

Fig. 4. XRD pattern of the zeolite hydroxy sodalite membrane.




CORRECTEDPROOF

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Fig. 5. XRD pattern of the zeolite NaA membrane with four times synthesis.

Table 2

Flux and separation factor of the membranes

Example Feed Type of zeolite Number of

coating processes

Concentration

of water (%)

Flux

(kg/m2 h)

Separation

factor

1 Ethanol NaA 2 50 1.05 >10 000

2 3 10 0.71 786

3 3 50 0.75 >10 000

4 4 10 0.68 >10 000

5 4 5 0.56 884

6 5 5 0.45 >10 000

7 Hydroxy sodalite 1 10 0.32 >10 000


9 UDMH NaA 5 98 0.3 >10 000


Fig. 6. SEM micrograph of zeolite NaA membrane thickness (a) after two times of coating, (b) after four times of coating.




UN5. Conclusion

Tubular porous mullite supports have been prepared

from kaolin clay by extruding and calcining at a tem-

perature of 1250 �C for 3 h. All free silica in the forms of

crystalline phase and amorphous phase has been re-

moved using 20% sodium hydroxide solution at a tem-

perature of 80 �C. Free silica removal from the primary

support increases its porosity from 24.3% to 49%. Zeo-

lite NaA and hydroxy sodalite have been coated on the

above support. Separation factor of hydroxy sodalite

membrane is higher than that of NaA membrane,

however, its flux is less. Therefore, for low concentration

of UDMH, zeolite NaA membrane is better than zeolite

hydroxy sodalite membrane but for high concentration

452

453

454

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501502




of UDMH, zeolite hydroxy sodalite membrane can be

recommended.
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Dehydration of water/1-1-dimethylhydrazine mixtures by zeolite membranes

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