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Microporous and Mesoporous Materials 105 (2007) 149–155

Delaminated microporous aluminophosphate-filled polyvinyl alcoholmembrane for pervaporation of aqueous alcohol solutions

Chen Wang, Weiming Hua *, Yinghong Yue *, Zi Gao

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, PR China

Received 20 December 2006; received in revised form 24 May 2007; accepted 31 May 2007Available online 12 June 2007

Abstract

Composite poly (vinyl alcohol) membranes containing delaminated microporous aluminophosphate with [Al3P4O16]3� stoichiometry(AlP) were prepared. Their pervaporation performance in separation of isopropanol/water and tert-butanol/water systems was measuredand compared with zeolite NaA and NaX-filled membranes. Distinct improvement on the flux of permeation as well as separation factorof the polymer membranes was observed, showing that the positive contribution of the aluminophosphate filler is even greater than thatof the zeolite fillers. The effect of AlP content, temperature and feed concentration on the pervaporation performance was investigated indetail. A re-dispersion phenomenon was observed, suggesting that the dispersion of the AlP layers could even occur during swelling ofthe polymer matrix and enhance the pervaporation process effectively.� 2007 Elsevier Inc. All rights reserved.

Keywords: Layered microporous aluminophosphate; Poly (vinyl alcohol) membrane; Pervaporation; Water/alcohol mixture

1. Introduction

Pervaporation process, an effective energy-saving tech-nique for separation and purification, has gained increasingattention in many chemical industries. Separating waterfrom aqueous alcohol solutions using hydrophilic polymermembranes such as poly (vinyl alcohol) (PVA) is a well-known example of the pervaporation process. Nowadays,different casting, cross-linking and filling methods for thiskind of membranes have been extensively investigated toimprove the total permeation as well as the selectivity ofthe membrane [1–4].

Zeolites with uniform micropores and good adsorptionabilities are widely used in separation of chemical sub-stances. Introducing zeolites with low Si/Al ratio such asNaX and NaA into hydrophilic membranes can furtherimprove the pervaporation performance [3–5]. However,zeolite fillers have their own shortcomings such as low flux

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doi:10.1016/j.micromeso.2007.05.060

* Corresponding authors. Tel.: +86 21 65642409; fax: +86 21 65641740(Y. Yue).

E-mail address: yhyue@fudan.edu.cn (Y. Yue).

and separation factor due to dispersion and diffusionlimitations.

Recently, numerous layered microporous aluminophos-phates have been synthesized using solvothermal method[6]. These materials are composed of negatively chargedmicroporous inorganic sheets, and the charges are compen-sated by protonated organic ammonium cations locatedbetween the sheets. Delamination and intercalation of thiskind of layered material have been carefully investigatedbecause they are critical for the preparation of high surfacearea catalysts, thermally stable pillared materials and self-consistent films. However, the delamination and intercala-tion of these layered compounds are more difficult to bringabout than those of ordinary layered metal phosphates dueto the instability of the microporous sheets and their stronginteraction with the protonated organic amine template. Inour previous works [7–10], delamination and intercalationof layered aluminophosphates with [Al3P4O16]3� and[Al2P3O12]3� stoichiometries by C2–C12 alkyl amines usinga one-pot method were reported. It was found that control-ling the dielectric constant of the solution and the concen-tration of the alkylamine in the medium is decisive for

150 C. Wang et al. / Microporous and Mesoporous Materials 105 (2007) 149–155

gaining success. Saturated C2–C8 alkylamine intercalatesof these aluminophosphates are obtained in solutions withdielectric constant of 50–70 and an amine concentrationaround 10 mmol/g. More recently, a novel two-stepmethod for delamination and intercalation of layered alu-minophosphate has been developed [11]. In this methodthe delamination and intercalation processes proceed sepa-rately and sequentially under different conditions, and inresult the preparation of more new layered or intercalatedaluminophosphates becomes feasible and controllable. Inparticular, it was found that the exfoliated sheets of AlPare stable not only in colloidal state but also after centri-fuging and drying in air through this two-step method, sothey can be separated readily and employed to fabricatevarious other functional materials without structuraldegradation.

A composite membrane containing a porous alumino-phosphate as the selective phase and a polyimide as thecontinuous phase was reported by Jeong in the literature[12], in which the enhancement in oxygen and carbon diox-ide selectivity in room-temperature permeation as com-pared to pure polymer membrane was observed. In thispaper, exfoliated microporous aluminophosphate layerswere prepared and incorporated into PVA membrane asselective-flake nanocomposite. The pervaporation of aque-ous solutions of tert-butyl and isopropyl alcohols throughthese composite membranes was investigated and com-pared with NaA and NaX zeolite-filled membranes interms of permeance and selectivity at different additive con-tents, feed temperatures and feed compositions.

2. Experimental

2.1. Preparation

2.1.1. Synthesis of AlP

A two dimensional layered aluminophosphate[Al3P4O16][CH3CH2NH3]3 (AlP) was selected as the inor-ganic molecular sieve phase. It was synthesized followingthe procedure described in the literature [13]. In a typicalsynthesis, aluminum triisopropoxide (2.0 g) was added toa mixture of ethylene glycol (EG) (15.0 ml) and 1-butanol(25.0 ml). The mixture was stirred until homogeneous.Then ethylamine (10 ml, 75 wt% in water) was added, fol-lowed by adding phosphoric acid (2.0 ml, 85 wt% in water)dropwise. The entire mixture was stirred until homoge-neous, sealed in a teflon-lined autoclave filling about 70%of the volume and heated to 180 �C for 13 days underautogenous pressure. The product was washed with dis-tilled water and then ethanol for several times and driedat ambient temperature.

2.1.2. Fabrication of composite PVA membrane

A calculated amount of AlP was added to 0.2 mol/lethylamine water–ethanol solution (the volume ratio ofethanol and water is 1:3) to form a mixture with a solid/liquid ratio of 10 mg/ml. The mixture was stirred for

several hours until a clear colloidal solution was obtained.NaCl was added to the solutions as an agglomerationagent. The obtained curd was recovered by centrifugationand washed with 0.3 M NaCl solution to remove excessethylamine.

The curd was dispersed in 30 ml 3 wt% PVA solution for30 min. Then the milk white solution was casted on a glassplate and dried under infrared radiation. The as-synthe-sized membrane was crosslinked by a solution containing5% NaOH and 10% Na2SO4 for 30 min, heated at 120 �Cfor 10 min and then soaked in water over night.

For the counterpart zeolite-filled membranes, a calcu-lated amount of NaA or NaX was added to 30 ml 3 wt%PVA solution, stirred until homogeneous and casted on aglass plate. The cross-linking process was the same as thatfor the AlP-filled membranes.

2.2. Characterization

X-ray powder diffraction patterns of the samples wererecorded on a Rigaku D/MAX-IIA diffractometer withCu Ka radiation at 30 kV and 20 mA and a Ni filter.SEM images were obtained on a Philips XL-30 scanningelectron microscope.

2.3. Pervaporation test

The membrane was mounted in the pervaporation cell,which was then connected to the collection trap and vac-uum pump. Vacuum at the downstream side was main-tained at a pressure of 13.3 Pa. Liquid nitrogen was usedas the cooling agent for the cold traps. The temperatureof the feed in the cell was kept constant and could be variedbetween 30 and 60 �C. The volume of the feed solution was120 ml, and the effective membrane area was 18.9 cm2. Thepermeate collected in the cold trap for 2 h was analyzed,and the flux and separation factor of the membranes werecalculated from the equations:

F ¼ VDtA

; a ¼ C1=C2

C01=C0

2

where V = volume (ml) of permeate, Dt = permeation time(h), A = effective membrane area (m2), C1 and C0

1 are themole fractions of water in the permeate and feed, C2 andC0

2 are the mole fractions of alcohols in the permeate andfeed.

3. Results

3.1. Preparation and characterization of AlP and AlP-filled

membranes

[Al3P4O16][CH3CH2NH3]3 is a two dimensional layeredmicroporous aluminophosphate synthesized from a non-aqueous system, and its structure has been characterizedby single-crystal X-ray diffraction [9]. The layered structure

C. Wang et al. / Microporous and Mesoporous Materials 105 (2007) 149–155 151

consists of [Al3P4O16]3� sheets with a 4 · 6 · 8 networkbuilt up from AlO4 and PO4 tetrahedral units in the ratioof 3/4, as shown in Fig. 1(A). All the AlO4 vertices areshared by adjacent PO4 units, but only three of the PO4

vertices are linked to adjacent AlO4 units and the remain-ing one forms the terminal P@O group. The terminalP@O groups are projected alternatively above and belowthe sheet and are hydrogen bonded to the ethylamine tem-plate. The XRD pattern of the as-synthesized AlP is illus-trated in Fig. 1(B (a)), displaying a distinct (001) peak at9.96�, corresponding to an interlayer spacing of 0.89 nm,which is consistent with the single-crystal X-ray diffractiondata.

The delamination of AlP was carried out in a solutionwith 1:3 water/ethanol ratio, and a sufficient amount ofethylamine was added into the solution to guarantee thefast exfoliation and to prevent the exfoliated sheets fromreassembly [7]. The SEM images of the AlP sample beforeand after delamination are shown in Fig. 2. The original

5 10 1

0

1000

2000

3000

4000

5000

6000

7000

Inte

nsity

/ a.

u.

2 θ

A

B

Fig. 1. (A) Structure of [Al3P4O16]3� sheets with a 4 · 6 · 8 network

AlP is composed of large regular thin platelike crystals withthe size of about 7 · 15 · 0.1 lm. These platelike crystalsbreak into small thin flakes with the size of about 1 lmafter delamination, which are stable at rest.

The small exfoliated sheets of AlP in the colloidal solu-tion were agglomerated after adding an electrolyte solutionsuch as the NaCl solution. They were washed with diluteNaCl solution to get rid of the residual ethylamine beforeredispersing in PVA solution. SEM images in Fig. 3 showthat the exfoliated sheets distributed quite homogeneouslyin the PVA matrix after drying and cross-linking. Fromboth the top view and the cross-sectional view, no segrega-tion of the exfoliated AlP sheets in the PVA polymermatrix is observed. On the contrary, the zeolites are sedim-entated in the membrane obviously from the cross-sectionview due to their larger particulates and heavier density.The smaller dimension and higher homogeneity of the thinAlP layers in the membrane should be advantageous to thepervaporation process.

5 20 25 30

b

a

/ degree

. (B) XRD patterns of the (a) original and (b) delaminated AlP.

Fig. 2. SEM images of the (a) original and (b) delaminated AlP.

Fig. 3. SEM images of AlP-filled (5 wt%) PVA membrane (a) top view (b)cross-sectional view, NaX-filled (5 wt%) PVA membrane (c) top view and(d) cross-sectional and NaA-filled (5 wt%) membrane (e) top view and (f)cross-sectional.

152 C. Wang et al. / Microporous and Mesoporous Materials 105 (2007) 149–155

3.2. Pervaporation performance

3.2.1. The effect of AlP content

A series of AlP-filled PVA membranes with differentAlP contents were prepared, and their pervaporation per-formances in the 80 vol% aqueous solutions of tert-butylalcohol and isopropyl alcohol at 40 �C were investigatedand compared with NaA and NaX-filled membranes.The results are shown in Fig. 4. It was found that boththe flux and separation factor of the system wereincreased with the initial increasing of AlP content, butfell down abruptly while too much AlP was incorporated.This phenomenon was quite different from the zeolite-filled membranes, whose flux increases with the increaseof zeolite content while the separation factor decreased,indicating that the AlP filler in the polymer matrix mightnot behave the same as zeolites. The flux and the separa-tion factor of the AlP-filled membrane with 5 wt% AlP aremuch greater than those of the NaA and NaX counter-parts, showing that this kind of membrane could be supe-rior to the latter in the industrial separation of water/alcohol mixtures.

3.2.2. The effect of temperature

Temperature is an important influential factor for per-vaporation process. The pervaporation of 80 vol% aqueousisopropanol and tert-butanol solutions was carried out at30–60 �C for PVA and AlP-filled PVA membranes. Thefluxes and separation factors as a function of temperature

Fig. 3 (continued)

C. Wang et al. / Microporous and Mesoporous Materials 105 (2007) 149–155 153

are shown in Fig. 5. As the temperature is raised, the flux ofall the membranes increases but the increasing rate is nothomologous for each kind of membrane. On the otherhand, the separation factors of the filled and unfilled mem-branes change differently with the rise of temperature. Forall the filled membranes there is an individual optimal tem-perature, whereas the separation factor of the unfilled

membrane decreases almost linearly [3,5]. The optimal tem-perature shifts with the AlP content in the membrane, i.e.,40 �C for the 5 wt% and 10 wt% AlP-filled membranes and50 �C for the 15 wt% AlP-filled one.

3.2.3. The effect of feed concentration

The feed composition is another changeable parameterin pervaporation. Its influence on the separation ofwater/isopropanol and water/tert-butanol solutions in thealcohol concentration range from 50% to 90% (vol%) wasstudied at 40 �C. The results are displayed in Fig. 6. Theflux of the unfilled PVA membrane increases as the waterconcentration is increased, but the separation factordecreases due to the swell characteristic of the polymermatrix. For the AlP-filled membranes, the fluxes alsoincrease with the increase of water concentration, but theseparation factors first increase with the increase of waterconcentration and then decrease when excessive water areadded. The optimal water concentration for the membranefilled with 5 wt% AlP is 20 vol% and higher for 10 wt% and15 wt% filled membranes. The best separation factor isobtained over the 5 wt% filled membrane in the aqueoussolution containing 80 vol% alcohol.

4. Discussion

In general, the improvement of pervaporation perfor-mance of AlP-filled membrane can be attributed to the highhydrophilicity caused by both the P negative electricitycenters and the P@O� � �HOH hydrogen bonding and themolecular sieving effect of AlP related to 8 member ringson the layers. The exfoliated layers of AlP are much thinnerand smaller than the zeolite particulates and they can behomogeneously colloidized in water solution, which areundoubtedly advantageous to the fabrication of highquality pervaporation membranes. Furthermore, the thintwo-dimensional AlP sheets facilitate the adsorption anddiffusion of the water and alcohol molecules and enhancethe pervaporation process. Hence, the flux and separationfactor of the AlP-filled membranes are better than thoseof the zeolite-filled ones.

Considering the way of the AlP layers working in themembrane, it can be noted that the well dispersion ofAlP layers in the membrane plays a key role for the suc-cess of the pervaporation of aqueous alcohol solutions.An excess amount of AlP in the membrane maycause the agglomerization of the exfoliated AlP layersand retard the pervaporation process, which explains thedrop down of the flux and separation factor when toomuch AlP is added into the membrane. Since the dispersionof AlP layers is difficult to be observed through commonmethods, the details of this phenomenon deserve furtherstudies.

It is interesting to note that the dispersion of AlP inPVA is altered under different prevaporation conditions,such as different temperature and feed composition. When

Fig. 4. Pervaporation result of PVA membrane filled with different amount of AlP, NaA and NaX in aqueous (a) isopropanol and (b) tert-butanolsolutions.

Fig. 5. The effect of temperature on the separation of aqueous (a) isopropanol and (b) tert-butanol solutions with unfilled membrane and membrane filledwith different amount of AlP.

154 C. Wang et al. / Microporous and Mesoporous Materials 105 (2007) 149–155

the pervaporation temperature or the water content in themixture is increased, the free space in the membraneincreases due to the swelling of the polymer, and morewater is infiltrated into the membrane, which gives rise tothe redispersion of the AlP in the fabricated membraneand leads to an enhancement of flux and separation factor.The redispersion should be easier to occur at low AlP con-tent, so the increase in flux and separation factor is moreobvious for the membrane containing 5 wt% AlP.

The performance of the AlP-filled membranes dependson the characteristics of both PVA and the AlP filler.Since the permeation rate of the PVA membrane itselfincreases as the water content of the feed or the pervapora-tion temperature increases but the selectivity decreases dueto swelling of the membrane, the appearance of the max-ima in Figs. 4–6 could be explained as the result of thecombined effect of various conditions on PVA and theAlP filler.

Fig. 6. The effect of feed concentration on the separation of aqueous (a) isopropanol and (b) tert-butanol solutions with unfilled membrane and membranefilled with different amount of AlP.

C. Wang et al. / Microporous and Mesoporous Materials 105 (2007) 149–155 155

5. Conclusion

A new kind of high quality composite PVA membranecontaining layered microporous aluminophosphate was pre-pared. The pervaporation dehydration of isopropanol andtert-butanol aqueous solutions performance of the mem-brane was measured. It was found that the efficiency of per-vaporation of the PVA membrane was greatly improvedafter incorporated with AlP due to the hydrophilicity andshape selectivity of the dispersed AlP sheets. The AlP-filledmembrane with low AlP content performs much better thanordinary zeolite-filled membranes. The dispersion of AlP inthe PVA matrix is the key factor for pervaporation perfor-mance of AlP-filled membranes. Increasing the temperatureor water content may facilitate the redispersion of AlP layersin the fabricated polymer membrane and improve the per-vaporation performance of the membrane.

Acknowledgments

This work was supported by the State Basic ResearchProject of China (2006CB806103) and the National Natu-ral Science Foundation of China (20633030).

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