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Separation and Purification Technology 56 (2007) 378–382

Effect of post-treatment in the hydrochloric acid solution on thepervaporation performance of mordenite membranes

Gang Li ∗, Xiao-hui Su, Rui-sen LinDepartment of Chemistry, Zhejiang University, Hangzhou 310027, China

Received 8 August 2006; received in revised form 23 February 2007; accepted 3 March 2007

bstract

The influence of post-treatment of as-synthesized mordenite membranes in the hydrochloric acid solution was investigated on the membraneervaporation performance for the separation of a water/ethanol mixture. It was shown that the water/ethanol separation factor of as-synthesizedordenite membranes could be greatly improved upon post-treatment with hydrochloric acid under controlled conditions. The change in the

ervaporation performance of mordenite membranes upon post-treatment was discussed. 2007 Elsevier B.V. All rights reserved.

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eywords: Mordenite membranes; Hydrochloric acid; Pervaporation; Post-trea

. Introduction

Pervaporation is a membrane-based separation technique ofow cost and reduced energy demand for separation of azeotropicnd close-boiling liquid mixtures, and dewatering of organic sol-ents such as alcohols [1]. While polymeric membranes haveeen commercially developed for these processes, their use isimited because of their low thermal and chemical stability. Inecent years, use of zeolite membranes as an alternative for per-aporation has attracted considerable attention [2,3]. In general,ydrophilic zeolite membranes have been used for dehydrationf organic solvents [4–7]. On the other hand, hydrophobic zeo-ite membranes have been used for separation of organics fromater [8–10]. Several reports have also been published on the

eparation of organic compounds or multi-component mixtures11–13].

The separation ability of zeolite membranes by pervapora-ion relies on a molecular sieving effect and on the differencesetween the adsorption and diffusion properties of the compo-ents to be separated. For a given mixture, these properties areelated to the size and the quantities of zeolitic and nonzeolitic

intercrystalline) pores, and the surface properties (hydrophilic-ty/hydrophobicity) of the membrane. It is a prevailing way tomprove the separation performance of a zeolite membrane via

∗ Corresponding author. Tel.: +86 571 85991071; fax: +86 571 87951895.E-mail address: gli@zju.edu.cn (G. Li).

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383-5866/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2007.03.001

ptimization of synthesis conditions [14]. In most cases, how-ver, it is always difficult to optimize the parameters mentionedbove only through controlling synthesis conditions. Therefore,ost-synthetic treatment has also been employed for improv-ng the separation properties of zeolite membranes [7,15–17].or instance, Navajas et al. [17] have reported that hydrother-al post-treatment of mordenite membranes under moderately

lkaline conditions could improve the water/ethanol separationactor of the membranes from around 150 to be as high as 700.

It is generally thought that the hydrophilicity/hydrophobicityf zeolites is based on the Si/Al ratio in the framework, structuralefects and cations present in the structure [18]. This suggestshat the pervaporation performance of a zeolite membrane coulde adjusted through dealumination with acid leaching, which iscommon method to modify the properties of zeolite by vary-

ng the aluminum content in the zeolite lattice. Sawa et al. [19]ave reported that the crystallinity of mordenite could be main-ained even after dealumination in the hydrochloric acid solutionf 8 mol dm−3 for 24 h at 65–105 ◦C, indicating the higher sta-ility of mordenite structure upon acid leaching. In this work,e report the effect of post-treatment in the hydrochloric acid

olution on the pervaporation performance of mordenite mem-ranes.

. Experimental

Asymmetric porous �-Al2O3 tubes (supplied from Nanjingniversity of Technology of China, 9 mm i.d. and 13 mm o.d.)

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ith pore size of ca. 100 nm in the outer layer were useds supports. Prior to use, the supports were first treated forh at 600 ◦C and then cleansed successively in acetone andeionized water each for 30 min under ultrasonication. Theeagents employed herein include colloidal silica (40 wt% inater, Zhejiang Yuda Chemical Industry Co., Ltd.), silica gel

100–200 mesh, Sinopharm Group Chemical Reagent Co., Ltd.),odium silicate (Na2SiO3·9H2O, Rugao Jinling Reagent Fac-ory), sodium aluminate (41 wt% Al2O3, Sinopharm Grouphemical Reagent Co., Ltd.), sodium hydroxide (96.0 wt%) andeionized water.

Mordenite membranes were prepared by seeded hydrother-al synthesis on the outer surface of tubular �-alumina

upports. The colloidal suspension of 0.2 or 0.02 g L−1 nano-ized mordenite crystals for seeding was prepared fromordenite powder, which was hydrothermally synthesized for

8 h at 180 ◦C using a gel with the molar composition ofNa2O:Al2O3:30SiO2:780H2O (using sodium silicate and sil-ca gel as Si sources). An organic-free synthesis mixture withhe molar composition of 10Na2O:0.15Al2O3:36SiO2:440H2Ousing colloidal silica as Si source) was used for the secondaryrowth of mordenite seeds to form a membrane. Hydrother-al crystallization processes were all conducted at 180 ◦C for

8–24 h without agitation. The experimental details have beenescribed in a previous work [6].

The post-synthetic hydrothermal treatment in the hydrochlo-ic acid solution was performed in the Teflon-lined autoclaves.or each run, 50 mL of hydrochloric acid solution of 1 mol dm−3

as used. The concentrations of Si and Al in the hydrochloriccid solution that recovered from the autoclave after treatmentf the membrane were analyzed by use of inductively coupledlasma mass spectroscopy (ICP-MS).

The phase of the materials formed on the support sur-ace was identified using X-ray diffraction (XRD, Rigaku/max-rA) analysis. The morphology of mordenite mem-

ranes was characterized using field emission scanning electronicroscopy (FE-SEM, Hitachi S-4700) analysis. Differential

canning calorimetric (DSC) and thermogravimetric (TG) anal-ses were carried out on a Netzsch STA 409 PG/PC instrument

Atcs

able 1omparison of pervaporation performance of mordenite membranes before and after

embrane Synthesis conditions, time/seedconcentration (h/g L−1)

Treatm

OR01 24/0.02 As-syn100 ◦C,100 ◦C,

OR02 18/0.2 As-syn120 ◦C,120 ◦C,

OR03 18/0.02 As-syn140 ◦C,

OR04 24/0.02 As-syn160 ◦C,

a A fresh hydrochloric acid solution of 1 mol dm−3 was used for each treatment.b Pervaporation tests were carried out at 75 ◦C for the separation of a mixture conta

ion Technology 56 (2007) 378–382 379

or powdery mordenite samples before and after post-treatmentt 100 ◦C in the hydrochloric acid solution of 1 mol dm−3. Theowdery mordenite samples were collected from the bottomf the autoclave after membrane preparation. Prior to TG/DSCxperiments, the powdery samples were washed thoroughly witheionized water and dried at 50 ◦C for 12 h.

The pervaporation performance of mordenite membranesas evaluated by the separation of a mixture containing 15 wt%ater in ethanol at 75 ◦C. One end of the tubular membraneas sealed with a nonporous glass plate and the other end was

onnected to a nonporous glass tube with epoxy. The mem-rane was immersed in the liquid mixture while the inner sidef the membrane was evacuated to maintain a pressure of ca.66 Pa. The effective membrane area was ca. 4 cm2. The per-eate was condensed in a cold trap cooled by liquid nitrogen,eighed and analyzed after the permeation of 1–2 h. The feed

nd permeate compositions were determined using a gas chro-atograph (REX, GC-8810) equipped with a 2 m long stainless

teel column packed with Porapak Q and a thermal conductiv-ty detector (TCD). The separation factor, αw/e, was defined asw/e = (Yw/Ye)/(Xw/Xe), where Yw/Ye and Xw/Xe are the weightatio of water to ethanol in the permeate and in the feed, respec-ively.

. Results and discussion

Table 1 summarizes the pervaporation results of severalordenite membranes before and after post-treatment in the

ydrochloric acid solution. It can be seen that the water/ethanoleparation factor for MOR01 membrane increased from 107o 363 after hydrothermal treatment for 2 h at 100 ◦C in theydrochloric acid solution of 1 mol dm−3. Upon post-treatmentor another 2 h at 100 ◦C, however, the water/ethanol sep-ration factor decreased. On the other hand, the total fluxncreased gradually with post-treatment for this membrane.

similar variation trend in the water/ethanol separation fac-or was observed for MOR02 membranes while the total fluxhanged little. The results for MOR03 and MOR04 membraneshow that the water/ethanol separation factor decreased to be

hydrothermal treatment in the hydrochloric acid solutiona

ent conditions Total fluxb (kg m−2 h−1) �w/eb

thesized 0.07 1072 h 0.10 3632 h 0.18 30

thesized 0.12 2782 h 0.10 3682 h 0.13 73

thesized 0.10 1452 h 2.79 1

thesized 0.06 1032 h 12.96 1

ining 15 wt% water in ethanol.

380 G. Li et al. / Separation and Purification Technology 56 (2007) 378–382

Fig. 1. Effect of post-treatment time on pervaporation performance for MOR05membrane. The post-treatment was performed at 100 ◦C in the hydrochloric acidst

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olution of 1 mol dm−3. The pervaporation tests were carried out at 75 ◦C forhe separation of a mixture containing 15 wt% water in ethanol.

nity when post-treatment was carried out for 2 h at 140 ◦C origher.

Fig. 1 shows the dependence of pervaporation performancen the duration of post-treatment for MOR05 membrane withhe initial water/ethanol separation factor of 53. It can beeen that the effects of post-treatment with hydrochloric acidn the separation factor and on the total flux were differ-nt. The water/ethanol separation factor increased initially andent through a maximum, whereas the total flux decreased

nitially and went through a minimum with the duration of post-reatment. The water/ethanol separation factor of more than 800as obtained after post-treatment for ca. 1.5 h at 100 ◦C.Fig. 2 shows the dependence of pervaporation performance

n the duration of post-treatment for MOR06 membrane with thenitial water/ethanol separation factor lower than that of MOR05

embrane. It can be also seen that the water/ethanol separation

actor increased whereas the total flux decreased with the dura-ion of post-treatment. This trend is similar to that for MOR05

embrane. Unlike the case of MOR05 membrane, however, nei-her a maximum for the separation factor nor a minimum for the

ig. 2. Effect of post-treatment time on pervaporation performance for MOR06embrane. The post-treatment was performed at 100 ◦C in the hydrochloric acid

olution of 1 mol dm−3. The pervaporation tests were carried out at 75 ◦C forhe separation of a mixture containing 15 wt% water in ethanol.

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ig. 3. DSC patterns for powdery mordenite samples before and after post-reatment at 100 ◦C in the hydrochloric acid solution of 1.0 mol dm−3.

otal flux was observed for MOR06 membrane upon treatmentor as long as 5 h. This could be attributed to the difference inhe initial properties of these two membranes.

The dependence of the water/ethanol separation factor withhe duration of post-treatment could be attributed to the changen the adsorption intensity of water molecules in mordeniterystals upon post-treatment. Fig. 3 shows the DSC patternsor powdery mordenite samples before and after post-treatmentith hydrochloric acid. The endothermic peak around 300 ◦C

orresponds to water desorption from the crystals. The des-rption temperature is similar for all samples, but the amountf the heat was different for these samples. The endothermicuantity for the sample upon post-treatment for 1 h was largerhan the untreated one, indicating that the interaction between

ordenite crystals and water molecules became stronger uponost-treatment. This can account for the corresponding increasen the water/ethanol separation factor upon post-treatment. Withncreasing the duration of post-treatment, however, the endother-

ic quantity reduced. Namely, the endothermic quantity for theample after 2 h of post-treatment was smaller than that for theample after 1 h of post-treatment. This is also in agreement withhe variation trend in the water/ethanol separation factor with theuration of post-treatment. For the sample that was subjected forost-treatment for 3 h at 100 ◦C, both the endothermic quantitynd the water/ethanol separation factor were the smallest amongll samples. These results imply that the higher the endother-ic quantity, the stronger the interaction between the crystals

nd the water molecules, and the higher the water/ethanol sep-ration factor. Thus, it could be concluded that the increase inhe water/ethanol separation factor upon post-treatment was dueo the increased interaction between the crystals and the water

olecules.The variation of the total flux upon post-treatment with

ydrochloric acid solution could be related to the adsorptionapacity of water molecules in mordenite crystals. Fig. 4 showshe TG patterns for powdery mordenite samples before and after

ost-treatment with hydrochloric acid. It can be clearly seen thathe weight loss (due to water desorption) decreased monotoni-ally with the duration of post-treatment. This is in accordanceith the change in the total flux of mordenite membranes with

G. Li et al. / Separation and Purification Technology 56 (2007) 378–382 381

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ig. 4. TG patterns for powdery mordenite samples before and after post-reatment at 100 ◦C in the hydrochloric acid solution of 1.0 mol dm−3.

ost-treatment, except for the membrane that was subjected forh of post-treatment. The much larger total flux of the latterembrane could be attributed to the formation of defects due to

erforation of the mordenite layer by the action of hydrochloriccid [20].

Why did the adsorption intensity of water molecules increasehereas the adsorption capacity decreased in mordenite crystalspon post-treatment with hydrochloric acid solution? Table 2resents the concentrations of Si and Al in the hydrochloriccid solution after post-treatment. The results show that the Al-

ich species were readily dissolved upon post-treatment withydrochloric acid solution. Since the Si/Al ratio of the gel forreparing mordenite membranes was as high as 120, the Si/Alatio in the hydrochloric acid solution would be around this

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ig. 6. Top view and cross-section FE-SEM images for: (a and b) an as-synthesized mh at 100 ◦C in the hydrochloric acid solution of 1.0 mol dm−3.

ig. 5. XRD patterns for: (a) mordenite powder, (b) an as-synthesized mordeniteembrane and (c) MOR05 membrane after post-treatment for 3 h at 100 ◦C in

he hydrochloric acid solution of 1.0 mol dm−3.

alue if the dissolved species were the amorphous gel. Buthis is not the case, because the Si/Al ratio in the hydrochloriccid solution was only 0.02–0.1, as shown in Table 2. There-ore, it could be concluded that the dissolved species in theydrochloric acid solution were mainly resulted from the extrac-ion of tetrahedrally coordinated aluminum in the frameworkpon acid leaching (dealumination). Upon dealumination, theeolite framework became less charged and the exchangeableations in the extra-framework were reduced, which leads to theecrease in the adsorption capacity of water molecules in theeolite crystals [20]. The reason for the increase in the adsorp-

ion intensity of water molecules in the crystals is not clear so far.his might be due to the formation of hydrogen bond betweenater molecules and the terminal silanol groups resulted fromealumination.

ordenite membrane and (c and d) MOR05 membrane after post-treatment for

382 G. Li et al. / Separation and Purification Technology 56 (2007) 378–382

Table 2Al and Si concentrations in the hydrochloric acid solution after post-treatment of mordenite membranesa

Membrane Treatment conditions Al concentration (�g mL−1) Si concentration (�g mL−1) Al/Si

MOR01 100 ◦C, 2 h 20.36 1.03 20.5MOR02 120 ◦C, 2 h 30.96 0.97 33.1MOR03 140 ◦C, 2 h 39.14 0.84 48.3MOR04 160 ◦C, 2 h 66.60 1.49 46.4M

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[19] M. Sawa, M. Niwa, Y. Murakami, Change of pore-opening structure of

OR05 100 ◦C, 1 h 3.74

a A fresh hydrochloric acid solution of 1 mol dm−3 was used for each treatme

Fig. 5 shows the XRD patterns of two mordenite membranesith and without post-treatment, together with that of morden-

te powder. It can be seen that pure mordenite was the onlyeolitic phases present in the membrane, and the crystallinity ofordenite crystals was hardly changed upon post-treatment in

he hydrochloric acid solution, in agreement with the literatureeport [19]. This suggests that the structure of mordenite wasundamentally unaltered upon dealumination. The considerablehange in the water/ethanol separation factor and in the totalux for MOR03 and MOR04 membranes upon post-treatmentith hydrochloric acid could therefore be attributed to the for-ation of lattice defects via dealumination instead of lattice

ollapse.Fig. 6 shows the top view and cross-section FE-SEM images

or an as-synthesized mordenite membrane and a post-treatedne. It can be seen that the crystal surface in the membrane post-reated with hydrochloric acid is much rougher than that in thes-synthesized mordenite membrane, which could be considereds a result of dealumination.

. Conclusions

Mordenite membranes with good pervaporation performanceould be synthesized by seeded hydrothermal synthesis on theuter surface of tubular �-alumina supports. It was demonstratedhat the water/ethanol separation factor could be effectivelymproved upon post-treatment with hydrochloric acid underontrolled conditions. Compared with the as-synthesized mem-ranes, mordenite membranes prepared via this technique areore water-selective and of high stability in an acidic environ-ent, which is desirable in membrane reactors such as in situ

emoval of water from the esterification reaction system to shifthe equilibrium towards the ester products.

cknowledgements

This work was sponsored by Zhejiang Provincial Naturalcience Foundation of China (No. Y406098) and by the Sci-ntific Research Foundation for the Returned Overseas Chinesecholars, State Education Ministry of China.

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