Preparation and characterization of oriented MAPO-39 membranes

Download Preparation and characterization of oriented MAPO-39 membranes

Post on 05-Jul-2016

218 views

Category:

Documents

6 download

Embed Size (px)

TRANSCRIPT

  • 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, an...

Recommended

View more >