Preparation and characterization of oriented MAPO-39 membranes

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  • Preparation and characterization of oriented MAPO-39membranes

    Lisa Washmon-Kriel, Kenneth J. Balkus Jr. *

    Department of Chemistry, University of Texas at Dallas, Richardson, TX 75083-0688, USA

    Received 22 September 1999; accepted 16 November 1999

    Abstract

    Thin films of MAPO-39, a small pore (4 A) magnesium aluminophosphate molecular sieve (ATN topology), havebeen prepared via pulsed laser deposition (PLD) for the first time. Laser ablation of MAPO-39 onto porous metal

    substrates followed by a post-hydrothermal treatment of the PLD film results in oriented crystalline MAPO-39

    membranes. The MAPO-39 crystals grow with pores oriented primarily normal to the porous metal substrate. De-

    composition of the template provides access to the membrane pores. Preliminary results for the separation of water/

    alcohol mixtures using the MAPO-39 oriented membrane are described. Selective permeation of water was observed in

    all the cases. 2000 Elsevier Science B.V. All rights reserved.

    Keywords: MAPO-39; Laser ablation; Membranes; Molecular sieve

    1. Introduction

    Microporous molecular sieve membranes oersize and shape selectivity in molecular dimensions[1]. The uniform and sometimes oriented pores ofthe molecular sieve membranes make them at-tractive for use in separations as well as emergingapplications in catalysis and chemical sensors. Theadvantages of molecular sieve membranes includethe uniformity of pore size, chemical stability andthermal stability. Molecular sieves may also bemodified to tailor the diusion and absorptioncharacteristics of the material [2].

    There are a variety of methods that can be em-ployed in the preparation of molecular sieve films,

    possessing both random and preferred orientation.Free-standing and supported films have been pre-pared. However, the fragile nature of the nanopor-ous oxide membranes generally dictates themechanically robust substrate. The most popularmethod for fabricating molecular sieve films hasbeen the seeded growth approach [313]. In thiscase, nanoseeds of the target zeolite are depositedfrom a solution onto a substrate that is typically flat.The seed crystals are then subjected to a synthesisgel in an eort to grow a thick continuous film.

    There has been some success in the preparationof oriented molecular sieve films by the secondarygrowth of the precursor layers consisting of na-nometer-sized crystals of Zeolite A [3] and silica-lite-1 [7]. In these examples, the seed layers wereapplied onto a glass or polished alumina supportby dip coating in an aqueous suspension of thezeolite nanocrystals. It has been reported that this

    Microporous and Mesoporous Materials 38 (2000) 107121

    www.elsevier.nl/locate/micromeso

    * Corresponding author. Fax: +1-972-883-2925.

    E-mail address: balkus@utdallas.edu (K.J. Balkus Jr.).

    1387-1811/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.PII: S 1 3 8 7 - 1 8 1 1 ( 9 9 ) 0 0 2 8 6 - 3

  • method circumvents the zeolite nucleation step dueto the presence of seed crystals and provides somecontrol of the film microstructure, enhances re-producibility, and scalability [8]. This approachand the related techniques have resulted in ori-ented films of zeolites having the MFI [7,1418],LTA [9,19], and AFI [20] topologies.

    We have reported a method for the preparationof molecular sieve thin films and membranes usingpulsed laser deposition (PLD) [2128]. The laserdeposition technique has, recently, yielded orient-ed films and membranes of the microporous ma-terials UTD-1 [21,29,30] and Ti UTD-1 [31], andmesoporous MCM-41 [32]. The PLD method in-volves the deposition of a densely packed layer ofmolecular sieve nanoparticles onto a variety ofsubstrates via laser ablation. The laser deposited

    film is then subjected to post-hydrothermal treat-ment (PHT) resulting in the oriented molecularsieve film. Here, we report the fabrication of anoriented MAPO-39 molecular sieve membrane forthe first time using the PLD method. MAPO-39 isa magnesium aluminophosphate molecular sievepossessing the ATN topology [33] consisting ofone-dimensional channels 4 A in diameter thatlie in parallel along the c-axis as shown in Fig. 1.Magnesium incorporation into the AlPO4 frame-work most likely replaces Al rather than P [34].Thus, MAPO-39 results in a net negative frame-work charge which is presumably balanced byprotons [35]. Therefore, the MAPO-39 frameworkshould possess ion exchange properties and thepotential for Bronsted acid sites. The small poresassociated with the MAPO-39 membrane (depos-

    Fig. 1. Structure of MAPO-39 (ATN topology) with space filling models of water, methanol, and ethanol in the eight-membered ring

    pores (4 A) calculated using Cerius2.

    108 L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121

  • ited on a porous stainless steel frit) could be uti-lized in the separation of a variety of small mole-cules. Preliminary results for the separation ofwater and lower alcohols such as methanol andethanol are presented.

    2. Experimental

    2.1. MAPO-39 synthesis

    MAPO-39 was synthesized hydrothermally us-ing two previously reported methods, a 96 h(Method 1) [36] and a 12 h (Method 2) synthesis[37]. For Method 1, a gel with the molar compo-sition 1.2Pr2NH : 0.80MgO : 0.8Al2O3 : 1.02P2O5 :41H2O was prepared by first adding 1.6 g boeh-mite (Vista) to 9.0 g H2O in a 50 ml beaker. Asolution of magnesium acetate was prepared sep-arately by dissolving 1.12 g of Mg(C(O)OCH3)2(Aldrich) in 5.5 g of H2O followed by the additionto the alumina/water mixture with continu-ous stirring. Subsequently, 4.0 g of 85% H3PO4(Fisher) was added to the above mixture at a rateof approximately 1 ml/min followed by stirring for15 min. Finally, 2.24 ml of dipropylamine (Ald-rich) was added dropwise and then stirred for 15min. The resulting white cloudy solution (pH 3.9)was transferred to a 23 ml Teflon-lined autoclave(Parr) and heated under static conditions at 150Cfor 96 h. The Method 2 synthesis involved dis-solving 2.5 g boehmite (Vista) in a solution of 4.6 gH3PO4 (Fisher) and 9.3 g H2O to yield a white gel.Separately, 0.72 g of magnesium acetate (Aldrich)was dissolved in 2.5 g of deionized water andadded to the above gel. The resulting gel wascombined with 2.0 g of dipropylamine (Aldrich)and stirred until it became homogeneous. The gelgradually became thinner and clearer but was stillwhite. The resulting synthesis gel of molar ratio0.87Pr2NH : 0.22MgO : 1.05Al2O3 : 1.02P2O5 : 28-H2O had a pH of 3.3. The gel was placed in aTeflon-lined autoclave (Parr) and heated at 150Cfor 424 h. Following each synthesis, the reactorwas cooled, and the white MAPO-39 crystals col-lected by vacuum filtration, washed with deionizedwater and air-dried at room temperature.

    2.2. Film deposition

    As-synthesized MAPO-39 (Method 1) waspressed into a free-standing 2.5 cm diameter targetfor laser ablation experiments. The target materialwas placed in a controlled atmosphere chamber aspreviously described [38]. Laser depositions wereperformed using a Lumonics HyperEX400 14 nslaser pulse operating at 248 nm (KrF*) with afrequency of 10 Hz. The laser flux was measuredwith a Scientech pyroelectric head (Model 380402)and was found to range from 90160 mJ/pulse.The laser radiation was rastered across theMAPO-39 target using a computer controlledmirror (Oriel) to provide a continually fresh sur-face. The laser beam was focused onto a spot ofsize 0.001 cm2 upon entering the ablation chamber.The substrates were heated in vacuo at 300C toremove any absorbed organics prior to laser ab-lation. The porous stainless steel disk substrates,6 mm 1:5 mm, having a pore size of 0.5 lm(Mott Metallurgical) were heated during laser de-position to temperatures of 175250C. Typicallaser ablation conditions were as follows: laserpower, 90160 mJ/pulse; repetition rate, 10 Hz;substrate temperature, 150225C, and back-ground oxygen pressure, 150225 mTorr whichprovided a deposition rate of 39 nm/min.

    2.3. Post-hydrothermal treatment

    Laser deposited films were subjected to a post-hydrothermal treatment in which the sample wasplaced with the ablated film side down at 60angle inside the Teflon-lined reactor. A fresh syn-thesis gel (Method 2) was added to the reactor andheated statically at 150C for 424 h. To preventdepositing MAPO-39 crystals in the metal sub-strate pores, the nonablated side (back side) wasprotected by covering with Teflon tape during thepost-hydrothermal treatment. The reorganizedfilm was washed with deionized water and air-dried at room temperature. The crystallinity andmorphology of the as-synthesized MAPO-39 andthe molecular sieve membranes were characterizedby powder X-ray diraction (XRD) using a Scin-tag XDS 2000 diractometer with Cu Ka radiationand scanning electron microscopy using a Phillips

    L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121 109

  • XL60 microscope, respectively. Bulk MAPO-39and PLD films were characterized by FTIR spec-troscopy using a Nicolet Avatar 360 spectrometer.Additionally, all products were subjected to energydispersive X-ray spectroscopy using PhillipsEDAX PV 9800 for Mg : Al : P ratio determina-tion.

    2.4. Pervaporation experiments

    Reorganized MAPO-39 membranes were heat-ed at 350C in an oxygen stream for 3 h to de-compose the organic template, thus providingaccess to the one-dimensional pore system. Themembrane, supported on stainless steel, wasmounted on a coarse glass frit support in a 1 cmdiameter glass tube (Ace Glass). The membranewas secured and sealed around the edges usingepoxy cement (Devcon 2-Ton White). The epoxywas allowed to set and dry in air, overnight. Thefeed, consisting of 1.5 ml of a 1:1 (v:v) mixture ofwater and methanol or water and ethanol, wasplaced upstream at atmospheric pressure. Thedownstream-side pressure was maintained at ap-proximately 30 lm and the permeate was con-densed in a liquid nitrogen trap and, subsequently,analyzed by gas chromatography (Shimadzu GC-8A) using a Supelco 2 meter 20% DC710 packedcolumn.

    3. Results and discussion

    Highly crystalline MAPO-39 was prepared as asingle phase by the Method 1 synthesis [36] asevidenced by the powder XRD pattern shown inFig. 2. There have been only a few studies dealingwith MAPO-39 [3437,39,40]. Nevertheless, thepowder pattern is in very good agreement withthe literature data except for intensities. The as-synthesized MAPO-39 material obtained fromMethod 1 shows the (1 3 0)=(3 1 0) reflection atd 4.30 A and is the most intense, in contrast toa report that the (1 2 1)=(2 1 1) reflection has 100%peaks [36]. Methods 1 and 2, and the post-hydrothermal treatment lead to as-synthesizedMAPO-39 materials having unit-cell volumes of923, 937, and 937 A3, respectively. These volumescorrespond well to the published values of 923 and938 A3 for Methods 1 [36] and 2 [37], respectively.The crystalline MAPO-39 unit cell is also consis-tent with magnesium incorporation into the mo-lecular sieve. Methods 1 and 2 yield MAPO-39with Al : Mg ratios of 4.30 and 5.08, respectively.Single crystal X-ray diraction results for MAPO-39 having the chemical formula Al7MgHP8O32indicate an Al : Mg ratio of 6.47 [35]. Thus, onecannot rule out the presence of extra frameworkmagnesium in MAPO-39 preparations.

    The SEM image in Fig. 3(a) reveals primarilylarge aggregates (2050 lm) of platelet shaped

    Fig. 2. Powder XRD pattern of MAPO-39 prepared by the Method 1 synthesis.

    110 L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121

  • crystals, prepared by Method 1. The Method 2synthesis also yielded MAPO-39 as evidenced bypowder XRD in good agreement with the reportedpowder X-ray pattern. However, the Method 2synthesis produced MAPO-39 crystals having adierent morphology compared to Method 1 asshown in Fig. 3(b). The MAPO-39 prepared in 24 hforms dumbbell shaped aggregates of crystals

    smaller than that obtained with Method 1. Meth-od 1 MAPO-39 was used as the target material forthe pulsed laser deposition of MAPO-39 thin films,in part because of the larger crystal size. A free-standing pellet was prepared by placing 0.75 g ofMAPO-39 in a hydraulic press at 15,000 psig. ASEM image of the free-standing target is shown inFig. 4(a). It is clear that the MAPO-39 aggregates

    Fig. 3. Scanning electron micrographs of MAPO-39 prepared by the (a) Method 1 synthesis and (b) Method 2 synthesis.

    L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121 111

  • are fragmented to a small degree upon targetpreparation, however, the platelets remain quitelarge and the crushing of the MAPO-39 crystalswill have little or no impact on the pulsed laserdeposition process. There has been approximatelya 15% loss in the MAPO-39 crystallinity uponpreparation of the target as evidenced by the re-

    duction in peak intensities. The MAPO-39 targetsurface changes significantly after laser irradiationas shown in Fig. 4(b). The surface now appearsless crystalline. Thus, the laser is rastered over thetarget, never striking the same surface twice.

    Upon laser ablation of MAPO-39, a yellowplume is observed and fragments of the as-syn-

    Fig. 4. Scanning electron micrographs of MAPO-39 pressed targets (a) before laser ablation and (b) following laser ablation.

    112 L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121

  • thesized molecular sieve are deposited in a verytightly packed film onto a heated substrate. In thecase of a porous stainless steel frit, shown in Fig.5(a), the substrate was heated at 150175C to helppromote adhesion. A cross-section SEM image ofthe laser deposited film reveals a continuous film

    of 140217 nm thickness. A topdown view of thelaser deposited film on a stainless steel supportreveals a fairly rough film surface, the result ofsplashing during the laser ablation [41], indicatedby the round particles ranging from 500 nm to2 lm in size. It is believed that the ablated film has

    Fig. 5. Scanning electron micrographs of the (a) blank stainless steel porous support and (b) laser deposited MAPO-39 on the stainless

    steel support.

    L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121 113

  • partially penetrated the stainless steel supportpores and the film appears to follow the topogra-phy of the support, as shown in Fig. 5(b). This filmis amorphous to X-ray diraction which is alsoconfirmed by the loss of the structure sensitivebands in the IR spectrum. The MAPO-39 productobtained from the Method 1 synthesis was morecrystalline than that obtained from Method 2 bypowder X-ray diraction peak intensities. How-ever, the one-day synthesis was preferred to thefour-day synthesis for the post-hydrothermaltreatment of the laser deposited MAPO-39 films.Post-hydrothermal treatment of laser depositedMAPO-39 results in crystalline films as evidencedby the powder XRD pattern (vide infra). A topview by SEM of the post-hydrothermally treatedlaser deposited MAPO-39 membrane indicatesthat the film is continuous, and evidence of pre-ferred orientation is observed in that the crystalsappear to be generally aligned normal to the sub-strate as shown in Fig. 6. While most of the crys-tals in the topdown SEM view appear orientednormal to the substrate, the SEM image also re-veals some MAPO-39 crystals growing at an angle.The post-hydrothermally treated MAPO-39 film

    appears to also follow the topography of the po-rous support.

    The mid-FTIR spectra of the as-synthesizedMAPO-39 in the region of lattice vibration (1500400 cm1) is compared with the laser depositedMAPO-39 in Fig. 7. The as-synthesized MAPO-39FTIR (Fig. 7(A)) is dominated by the broad, in-tense band centered around 1120 cm1 which isattributed to asymmetric AlO stretching [42]. Thebands in the region 700800 cm1 are attributed tothe symmetric stretching mode, while the bands ataround 550 and 640 cm1 are assigned to thedouble ring modes. The band at 450 cm1 is as-signed to the TO bending mode. The absence ofstructure sensitive bands in the laser depositedMAPO-39 film on a KBr disc (Fig. 7(B)) indicatesthe presence of noncrystalline MAPO-39. Bandsin the region of 1106 and 471 cm1 attributed tothe asymmetric and TO bending modes, respec-tively, of the ablated MAPO-39 correspond to theamorphous aluminum phosphates investigatedpreviously by our group [27]. However, the weakband in the region of 700750 cm1 in the ablatedMAPO-39 film is not seen in the amorphous alu-minum phosphate samples. Thus, fragments of

    Fig. 6. SEM topdown view of the laser deposited and post-hydrothermally treated MAPO-39 crystalline membrane supported on a

    stainless steel porous support.

    114 L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121

  • MAPO-39 may be present in the laser depositedfilms which may provide some memory and en-courage reorganization of the film into crystallineMAPO-39.

    Energy dispersive X-ray spectroscopy of theMAPO-39 bulk material, laser deposited films and

    membranes was carried out in order to character-ize dierences in the composition of each. Theratios of each treatment are in good agreementwith one another except that of the laser depositedfilm. The MAPO-39 synthesis Methods 1 and 2,the PHT membrane and the bulk product obtainedfrom the PHT exhibited Al : P : Mg ratio averagesof 0.94 : 1.00 : 0.14. This ratio is reasonable in thatmagnesium is expected to substitute exclusively foraluminum. Thus, a slightly smaller amount ofaluminum is anticipated to be present comparedwith phosphorus. However, the laser depositedfilm Al : P : Mg ratio is 1.39 : 1.00 : 0.31 implyingthat there is relatively more aluminum thanphosphorus present in the as-deposited film thanin the crystalline MAPO-39 products. Table 1 liststhe Al : Mg ratios determined from EDS assumingP to be unity for each synthesis along with theliterature Al : Mg ratios. Also contained withinTable 1 are the observed and literature unit-cellvolumes for each synthesis. As previously stated,the unit-cell volume should increase upon incor-poration of magnesium into the aluminophos-phate structure. In support of this, AlPO4-39,having a composition of Al8P8O32, provides a unitcell of only 888 A3 [43], whereas single crystaldiraction data for MAPO-39 with an idealizedframework composition of Al7MgP8O32 [35] pro-vided a unit cell of 921 A3 and a Al : Mg ratio of6.47. The single crystal MAPO-39 synthesis wasmade using dipropylamine as the template mole-cule, but at a higher temperature (190C) and

    Table 1

    Al : Mg ratios obtained by energy dispersive X-ray spectroscopy

    Sample Al : Mg

    observed

    Al : Mg literature Unit-cell volume

    (A3) observedUnit-cell volume

    (A3) literature

    Method 1 synthesisa 6.27 4.30 923 923

    Method 2 synthesisb 6.57 5.08 937 938

    PLD film 4.48

    PHT film 7.31 937

    Bulk MAPO-39 from PHT synthesis 7.31 5.08 938 938

    Single crystal synthesisc 6.47 921

    AlPO4-39d 1 888

    a Ref. [36].b Ref. [37].c Ref. [35].d Ref. [43].

    Fig. 7. FTIR spectra of the (A) as-synthesized MAPO-39 and

    (B) laser deposited MAPO-39.

    L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121 115

  • dierent synthesis time (4860 h) compared withMethods 1 and 2. The Al : Mg ratio reported forMAPO-39 made by Methods 1 and 2 are signifi-cantly lower (4.30 and 5.08) compared to the singlecrystal sample. The Methods 1 and 2 synthesesprovided Al : Mg ratios of 6.27 and 6.57 whichmore closely approximate that reported from sin-gle crystal data. The observed Al : Mg ratios donot correlate directly to the unit-cell volume foreach synthesis method, however the observed unit-cell values compare very well with the reportedmaterials. This implies that there may be someextra framework Mg in the Method 1 and 2 syn-theses as well as in the literature syntheses. Thelaser deposited film is significantly enriched in bothaluminum and magnesium compared with phos-phorus based on the EDS results. Additionally, thePLD film showed the highest Al : Mg ratio of allthe samples. However, the PHT crystallineMAPO-39 film provided an Al : Mg ratio of 7.31,the same ratio seen in the bulk MAPO-39 obtainedfrom PHT syntheses. This result may imply thatthe magnesium enrichment of the thin PLD film(
  • tallinity when comparing the XRD intensities ofthe shorter synthesis times to the longer synthesistimes of the literature procedure [37]. However, asthe synthesis time increases, small impurity peaksat 18.4 and 34.5 two theta are observed. It ispossible that these peaks arise from the formationof an impurity of MAPO-34; however, this has notbeen verified.

    A cross-sectional view of the PHT film revealsthat the MAPO-39 film is composed of tightlyintergrown individual crystals generally orientednormal to the stainless steel substrate. TheMAPO-39 film cannot be wiped o and is not re-moved by sonication. Instead, the entire film re-mains intact with no apparent formation of defectsor flaking of the film following 30 min of sonica-tion in deionized water, attesting the excellentadherence of these films to the support. Fig. 9(a)shows the MAPO-39 membrane surface at a 75tilt angle. While this angle provides a dierent viewof the membrane compared with the topdownview shown in Fig. 6, this view confirms that thecrystals are generally oriented preferentially nor-mal to the substrate. Fig. 9(b) is a cross-sectionalview of the film shown in Fig. 9(a), the film isapproximately 32 lm thick, and Fig. 9(c) is a close-up view of the interior of the MAPO-39 film. Thepreferred crystal orientation resulting from thepost-hydrothermal treatment of laser depositedfilms has been previously observed in UTD-1[21,2931] and MCM-41 [32], and this is the firsttime MAPO-39 has been laser ablated to form thinfilms, and the growth of an oriented membrane hasbeen reported for ATN topology. In the case ofMAPO-39, as well as UTD-1 and MCM-41, thekey to the preferred orientation may be in thecrystal aspect ratio. Because films prepared by la-ser ablation comprises tightly packed particles,ions, and fragments, the molecular sieve reorga-nization and growth may occur along the path ofleast resistance, that is normal to the substrate [21].The MAPO-39 products of the Methods 1 and 2syntheses have dierent morphologies. The Meth-od 1 (96 h) synthesis yields large aggregates ofplate-like crystals (Fig. 3(a)), whereas the Method2 (24 h) synthesis provides more dumbbell shapeaggregates of crystals (Fig. 3(b)). Thus, it appearsthat the MAPO-39 PHT film is composed of

    crystals similar in morphology to the Method 2synthesis; it would seem that the film is formedfrom only half of the dumbbell. This observation issimilar to that seen in the growth of oriented UTD-1 films, where the morphology of bulk UTD-1 isbundles of needle-like crystals, but the oriented filmappears to comprise of only half of the bundle[21,29,30]. Laser deposition of MAPO-39 prior tofilm growth is clearly necessary in order to ob-tain preferred crystal orientation. A control exper-iment, in which a stainless steel support was placedin a Method 2 synthesis gel for 24 h, provided a filmof randomly oriented crystals as seen in the SEMimage shown in Fig. 10. Furthermore, the XRDpattern of the MAPO-39 on this porous supportdoes not indicate preferred orientation and the filmis easily wiped o the stainless steel support.

    The as-synthesized MAPO-39 membrane wasfound to hold a vacuum, thus decomposition ofthe amine template was necessary in order to gainaccess to the MAPO-39 oriented pores. Smalltemplates such as dipropylamine are generally verytightly held in molecular sieves and often cannotbe removed by extraction. Additionally, calcina-tion may also be a problem with supported mem-branes in that the molecular sieve film may besubject to stress and cracking at elevated tem-peratures. Although it has been reported thatboth Methods 1 and 2 MAPO-39 crystals may becalcined at 500C for 24 h (Method 1) or 48 h(Method 2), we found that the MAPO-39 film wasnot stable to calcination at this temperature. Aftercalcination in air at 500C for 4 h, the membraneappearance changed from continuous white orlight gray to patchy brown and the structurecollapsed yielding an amorphous powder XRDpattern. Therefore, the template molecule was de-composed by heating the MAPO-39 membrane for3 h in a stream of oxygen at 350C resulting in amembrane that had lost no crystallinity comparedwith the as-synthesized MAPO-39 membrane asevidenced by powder X-ray diraction. The pres-ence of residual carbon in the molecular sievemembrane is expected to result in a slight decreasein the eective pore size. This may actually repre-sent a method for tailoring molecular sieve mem-brane pore size that has not been utilizedpreviously.

    L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121 117

  • Fig. 9. Scanning electron micrographs of 12 h PHT MAPO-39 membrane: (a) surface view at a 75 tilt, (b) cross-sectional view, and (c)interior of the membrane cross-section.

    118 L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121

  • The small pore MAPO-39 molecular sievemembrane is potentially attractive for size andshape selective separation of a variety of smallmolecules such as water and lower alcohols. Asshown in Fig. 1, MAPO-39 has two dierent eight-membered ring pores. In the center of Fig. 1, thewider pore is composed of an eight-membered ringand surrounded by four four-membered rings, andfour narrower saddle-shaped (not flat) pores madeof an eight-membered ring [35]. The narrowereight-membered ring pores have a diameter of only2.9 A [35]. Thus, they may enhance selectivity forwater over the lower alcohols, methanol and eth-anol which should fit in the 4 A pores as shown inFig. 1. In the present work, the MAPO-39 mem-brane was evaluated for the separation of 1:1 (v:v)mixtures of water/methanol and water/ethanol.The pervaporation process has been used eec-tively in the separation of azeotropic mixtures [44].As a test for the presence of cracks or large defectspresent in the membrane, a separation of 1:1 (v/v)methanol and cyclohexanol was also performed.Based on the kinetic diameter, the permeability ofthe above liquids to the MAPO-39 membrane wasexpected to follow the order: water 2:65A >methanol 3:8A > ethanol 4:3A cyclohexanol(6 A). Contrary to the kinetic diameter of

    ethanol, it was found by modeling (Cerius2) thatan ethanol molecule can fit through unoccludedpores of MAPO-39 with minimal close contacts(see Fig. 1); thus, the presence of carbon in thepores may enhance the molecular sieving eectthereby increasing the water/ethanol selectivity.The separation of a methanol and cyclohexanolmixture yielded only methanol in the permeateafter a total run time of 101

    2h, as expected based on

    the diameter of MAPO-39 pores. These results areconsistent with a continuous membrane free oflarge defects. Table 2 compares the preliminaryresults of flux and selectivity for each separationexperiment. The separation of water/alcohol mix-tures aorded greater selectivity for water overethanol than for methanol, as expected based onthe kinetic diameter of these molecules. Theseseparation results compare with pervaporationresults using a silicalite membrane in which the

    Fig. 10. Scanning electron micrograph of MAPO-39 crystals grown on a blank stainless steel porous support.

    Table 2

    Comparison of MAPO-39 membrane separations

    Separation

    experiment

    Total flux

    (kg m2 h1)

    Water/alcohol sep-

    aration selectivity

    Water/methanol 0.5 13

    Water/ethanol 0.6 96

    Methanol/cyclohexanol 0.2 1

    L. Washmon-Kriel, K.J. Balkus Jr. / Microporous and Mesoporous Materials 38 (2000) 107121 119

  • best performance was obtained in the separa-tion of a 5 vol% ethanol solution providinga EtOH=H2O 120 [45]. In a similar experiment,the separation of methanol and water yielded aseparation factor a of value 23. A pervaporationexperiment providing separation factors of 5000for water over ethanol at temperatures rangingfrom 50120C have been reported from a 5 wt.%water feed using a NaA zeolite membrane grownon aluminosilicate supports [9]. The NaA mem-brane was found to be a composite of three layers,the outer layer being the zeolite film, the innerlayer the support, and the layer between was foundto have an intermediate composition. NaA andMAPO-39 each have 4 A diameter pores; thus, thedierence in selectivity for water/alcohol separa-tion between the two membranes may reflect thedierence in support and conditions for the sepa-rations. Also, one cannot rule out small defectsthat were not made evident by the cyclohexanolpervaporation experiment. Nevertheless, theMAPO-39 films hold promise for the separation ofsmall molecules.

    4. Conclusions

    MAPO-39 molecular sieves have been laserablated for the first time. Post-hydrothermaltreatment of the PLD MAPO-39 film produces amostly oriented membrane. The mechanism oforiented MAPO-39 film growth is the subject ofcontinuing investigation, but there is some simi-larity with UTD-1 in that both materials formaggregates of small plank- or needle-shaped crys-tals. This appears to be the first report on an ori-ented film having ATN topology and the firstreport on a MgAlPO4 molecular sieve film. It isanticipated that other structures and compositionswill yield similar films and membranes using thePLD technique.

    Acknowledgements

    The authors thank the National Science Foun-dation, the Robert A. Welch Foundation and theACS-PRF for financial support of this work.

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