Organization of Zeolite This Account summarizes the methods of...

Organization of ZeoliteMicrocrystals for Production ofFunctional MaterialsKYUNG BYUNG YOONCenter for Microcrystal Assembly, Department of Chemistry,and Program of Integrated Biotechnology, Sogang University,Seoul 121-742, Korea

Received May 31, 2006

ABSTRACTThis Account summarizes various methods of organizing zeolitemicrocrystals into two-dimensional functional entities such asmonolayers, multilayers, and patterned monolayers on varioussubstrates and three-dimensional functional entities such as mi-croballs and protein-zeolite composite fibrils. In the case ofmonolayer assembly, various types of linkages and substrates,factors that govern the rate, degree of coverage, degree of closepacking, degree of uniform orientation, and average bindingstrength between each crystal and the substrate are described. Thecurrent and future applications of the organized products are alsodiscussed.

IntroductionThe ability to rationally assemble complex structures frommodular components of various sizes is an essential partof life and key to success for the materials chemistry ofthe new millennium.1 Chemists have acquired enoughability to organize subnanometer building blocks such asatoms and small molecules into complex structures. Withthe dawn of a new century, chemists have begun includingnanobuilding blocks (1-10 nm) into their pools of build-ing blocks. However, micrometer-sized building blockshave not yet been considered as a class of building blocksdespite the fact that the ability to organize them will makematerials chemistry flourish.

Using zeolite microcrystals as model microbuildingblocks, we developed methods of organizing them intotwo- (2D) and three-dimensionally (3D) organized struc-tures.2-22 The use of zeolite microcrystals as microbuildingblocks has additional merit since each zeolite microcrystalhas millions of regularly spaced nanochannels and billionsof nanopores which can be filled with a variety offunctional molecules or nanoparticles. Therefore, weintended to not only develop the ‘chemistry of micro-building blocks’ but also open a gateway to applicationof zeolites as advanced materials.

This Account summarizes the methods of organizingzeolite microcrystals and the imminent and future ap-plications.

Modes of Reaction PromotionWe used three different modes of reaction promotion;reflux and stirring, sonication without stacking, and soni-cation with stacking (Figure 1).2-6,20 In reflux and stirring,the functional-group-tethering glass plates are placed ona Teflon support and a stirring bar is rotated under thesupport while reflux proceeds. In sonication withoutstacking, the functional-group-tethering glass plates areplaced individually on a comb-shaped support placed atthe bottom of a flask. Sonication is carried out using anultrasonic bath. In sonication with stacking, a functional-group-tethering glass plate is interposed between two bareglass plates and each stack of three glass plates is placedon the comb-shaped support.

Monolayer Assembly with Covalent LinkagesMallouk, Bein, and Calzaferri attempted the attach-ment of zeolite microcrystals on substrates as a means tomodify electrode surfaces23-25 and grow continuous zeolitefilms.26,27 However, the coverage, close packing, anduniform orientation were not satisfactory. We have shownthat the above factors increase dramatically when thecrystal sizes are uniform and the crystal faces are flat.Functional groups were tethered to the surfaces ofzeolite and substrates by sylilation2-4,6-13,17-22 and uretha-nation.15

Our first demonstration was the monolayer assemblyof aminopropyl-tethering zeolite-A microcrystals on 3-(2,3-epoxypropoxy)propylsilyl (EP)-coated glass plates (18 ×18 mm2) with reflux and stirring (Figure 2).2 The scanningelectron microscope (SEM) image (Figure 3a) shows thatthe microcrystals readily assemble into uniformly orientedmonolayers on the entire glass plates with high degreesof coverage (>90%), close packing, and uniform orienta-tion, indicating that amine-alcohol linkage formationreadily takes place. ZSM-5 (Figure 3b) and various othercrystals with a size larger than 2 µm also readily formedmonolayers on glass with similar degrees of coverage,close packing, and uniform orientation.

Kyung Byung Yoon received his B.S. degree in 1979 from the Department ofChemistry, Seoul National University. In 1981, he obtained his M.S. degree fromthe Department of Chemistry, Korea Advanced Institute of Science andTechnology (KAIST), Seoul. From 1981 to 1984 he was employed by ChonEngineering Co. Ltd., Seoul, Korea. In 1989, he earned his Ph.D. degree in InorganicChemistry from the Department of Chemistry, University of Houston, Houston,Texas, where his research advisor was Professor Jay K. Kochi. He has been anAssistant, Associate (1993), and Professor (1998) in Sogang University, Seoul,Korea from 1989 to the present.

FIGURE 1. Illustration of reflux and stirring (RS), sonication withoutstacking (SW), and sonication with stacking (SS). The functional-group-tethering glass (FG) and bare glass (BG) plates are purpleand green, respectively.20

Acc. Chem. Res. 2007, 40, 29-40

10.1021/ar000119c CCC: $37.00 2007 American Chemical Society VOL. 40, NO. 1, 2007 / ACCOUNTS OF CHEMICAL RESEARCH 29Published on Web 10/04/2006

Attachment of a crystal with a size of 2 × 2 × 2 µm3

onto a glass plate through 1-nm-long molecular linkersis analogous to attachment of a rock with a size of 60 ×60 × 60 m3 onto a large wall using a large number (e2.6million) of strings with a length of 3 cm and thickness of3 mm.28 The size limit of the covalently attachable zeolitecrystals was ∼10 × 10 × 10 µm3.

Among various covalent linkages (Figure 4a-g)3,4,6,8,15,19

the ether linkage formed by reaction between surface-tethered halopropyl groups and hydroxyl groups (Figure4g)4 has been most widely used due to its simplicity.Characterization of different types of linkages in terms ofnumber density, thickness, and roughness of each linkerwas possible by X-ray reflectivity.29

Monolayer Assembly with Ionic Bonding.Sodium-butyrate-tethering silicalite-1 crystals readilyformed monolayers on trimethylpropylammonium-iodide-tethering glass plates (Figure 4h) in ethanol.9 The bindingstrengths were substantially higher than those of thezeolite crystals covalently attached onto glass, indicatingthat the amount of linkage between each zeolite crystaland glass is higher with ionic than with covalent linkagedue to the following. Ionic bonding is omnidirectional,works well regardless of the distance between the positiveand negative centers, and does not require kinetic energiesfor bond formation. Mono- or multilayers of polyelectro-lytes can also be used as the intermediate layers (Figure4i). Use of aminopropyl/negative polyelectrolyte/amino-propyl linkage even in an acidic medium leads to forma-tion of monolayers with poor coverage and close pack-ing.27

Monolayer Assembly with Hydrogen BondingThymine-tethering silicalite-1 crystals (2.5 µm) readilyformed closely packed monolayers on adenine-tetheringglass plates in an aqueous solution at room temperature(Figure 4j).12 At the annealing temperature (55 °C), atwhich bond breaking and bond reforming between thesurface-tethered complementary DNA bases become veryrapid, the assembly rate and degree of close packingincreased significantly due to faster surface migration ofthe crystals.

Monolayer Assembly with Physical AdsorptionCalzaferri,30 Tsapatsis,27,31 and Ban and Takahashi32 dem-onstrated that zeolite microcrystals can also be organizedin the form of closely packed monolayers on substratesby physical adsorption of the microparticles onto sub-strates. The adsorption methods were slow evaporation,30

dip coating,27,32 and convective assembly (vertical deposi-tion).31 Polystyrene can also be applied for strongeradhesion.26 Yan and co-workers33 elucidated that in-situmonolayer assembly of the nanocrystals in the bulk onsubstrates by physical adsorption plays an important rolefor the growth of zeolite films on substrates.

FIGURE 2. Procedure to attach zeolite-A crystals onto glass through surface-tethered aminopropyl and EP groups.2

FIGURE 3. SEM images of zeolite A (a) and silicalite-1 (b)microcrystal monolayers assembled on glass.2

Organization of Zeolite Microcrystals Yoon

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Effect of Mode of Reaction Promotion on Rate,Close Packing, and CoverageMode of reaction promotion sensitively affects the abovefactors (Figure 5).20 For instance, the rate of monolayerassembly increases in the order sonication with stackingg sonication without stacking . reflux and stirring. Bestcoverage and close packing were achieved by sonicationwith stacking for 2 min. The coverage and close packingachieved by sonication without stacking for 2 min werecomparable with those achieved by reflux and stirring for24 h.

The sonication-induced dramatic increases in rate,coverage, and close packing are attributed to a largeincrease in the surface-migration rate of the substrate-bound crystals arising from large increases in the rates ofbond breaking and bond reforming between the micro-crystals and substrates. In the case of sonication withstacking, fast influx of zeolite particles from the bottomof the glass stacks to the top is attributed to dramaticincreases because the influx inevitably ‘pushes’ the previ-ously attached crystals to the top, giving rise to very tightlateral close packing between the crystals.20

Factors that Affect Binding StrengthThe numbers of interconnecting linkages should be largeto maintain strong adhesion between microcrystals andsubstrates. Over 660 000 linkages can be formed betweenan atomically flat 1 × 1 µm2 face of a zeolite microcrystaland a substrate.8 However, the actual numbers of inter-connecting linkage are much smaller due to surfaceunevenness of zeolites and substrates. For instance, thesurface roughness of silicalite-1 was ∼15 nm, which ismuch larger than the lengths of molecular linkers (∼1nm).29

Sonication-induced detachment of the glass-boundzeolite crystals in clean, zeolite-free toluene has beenadopted as a qualitative measure for binding strengths.8

For this, the percentage of the remaining amount of zeolitecrystals after sonication for a certain period of time withrespect to the initially attached amount (%R) was obtainedand plotted with respect to sonication time. The followingthree methodologies lead to binding strength increase.

A. Use of Polymeric Linkers. Use of polyamines(dendritic polyamine or polyethyleneimine) as the inter-mediate linkers leads to a marked increase in bindingstrength (Figure 6), presumably due to their ability toposition within the nanovalleys of the solid surfaces insuch a way as to increase the amount of linkages betweenthe two surfaces.8

Although ionic linkage gives rise to stronger bindingstrength (vide supra), use of polyelectrolytes and theincrease in the number of polyelectrolyte layers furtherlead to a significant increase in binding strength.9 Bytaking advantage of this, multilayer (pentalayer) assembly

FIGURE 4. Various types of molecular linkages.

FIGURE 5. SEM images of bare silicalite crystal monolayersassembled on chloropropyl-tethering glass plates after reaction withreflux and stirring (RS) for 10 min (a) and 24 h (b), with sonicationwithout stacking (SW) for 2 min (c), and with sonication with stacking(SS) for 2 min (d).20

FIGURE 6. Plot of %R vs sonication time for the monolayers ofzeolite-A crystals assembled on glass plates through AP/EP, EP/dendritic polyamine (DPA)/EP, and EP/polyethylene imine (PEI)/EPlinkages.8


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of zeolite microcrystals on glass was successively carriedout using PSS-/PDDA+/PSS- as the repeating linkage units(Figure 7).

B. Cross-Linking between the Adjacent Microcrystals.The lateral cross-linking between the neighboring, adja-cent microcrystals leads to a marked increase in bindingstrength despite the fact that the amount of linkagesbetween the microcrystals and the underlying glass sub-strate remained unaltered.18 For instance, cross-linking ofthe neighboring aminopropyl-tethering cubic zeolite-Amicrocrystals with terephthaldicarboxaldehyde led to a7-fold increase in binding strength (Figure 8). Such aphenomenon was more pronounced with decreasingcrystal size.

C. Assembly with Strong Agitation. Under the condi-tion of reflux and stirring, the kinetic energies of the

functional groups tethered to microcrystals and substratesare very small due to very large masses of the host solids.Sonication-induced strong agitation of the microcrystalsand substrates leads to a marked increase in bindingstrength. For instance, %R of the microcrystals attachedto glass by sonication (with or without stacking) for 2 minwas 60 times higher than those of the microcrystals thatwere attached to glass by reflux and stirring for 2 min.20

Such a marked increase is attributed to the increase inamount of linkages arising from the increase in kineticenergies of the functional groups tethered to microcrystalsand substrates, which facilitates their linkage formationreactions during collision between microcrystals andsubstrates.

Sonication induces detachment as well as attachment.This contradictory behavior of sonication suggests that it

FIGURE 7. SEM images of a pentalayer of silicalite crystals assembled on a glass plate through ionic linkages with PSS-/PDDA+/PSS- asthe intermediate linkers: top view (a) and cross section (b).9



could act as a selection procedure by which only thestrongly attached crystals remain attached while theweakly attached crystals are detached. However, this doesnot explain the above large difference in binding strengthafter reaction for 2 min since detachment is not under-gone extensively during such a short period. Furthermore,no crystals are detached from the substrate for 2 min inthe case of sonication with stacking. The vibration-induced ‘strong hitting’ of the surface-bound microcrystalswith side glass plates seems to give rise to the increase inbinding strength.

Binding strength is a function of reaction period. Withsonication, binding strength decreases after 1 h due to agradual decrease in amount of linkage resulting fromundergoing repeated bond breaking and bond reforming.With reflux and stirring, the binding strength of themicrocrystals gradually increased during the period from0 to 18 h, indicating that the amount of linkage increasesduring the period.

D. Calcination. The binding strength between micro-crystals and glass substrates increases dramatically whenthe microcrystal-bound glass plates were calcined at 350-450 °C for 3-4 h to remove organic linkages, presumablydue to formation of direct Si-O-Si linkages between thezeolite microcrystals and the substrates during calcination.

Types of SubstratesThe substrates that have been tested so far areglass,2,3,5,8-10,12,15-22 glass fiber,8 silica,4 alumina,4 largezeolite,4 vegetable fibers (cotton, linen, and hemp) (Figure9),11 artificial fibers (nylon and polyester), and conductingsubstrates (Pt, Au, and ITO glass).19

Replacement of Attached Crystals with ThoseDispersed in Solution and Surface Migration ofAttached CrystalsGlass-bound bare zeolite microcrystals were readily re-placed by the fluorophore-incorporating zeolite micro-crystals dispersed in solution (Figure 10), indicating thatthe substrate-bound microcrystals are continuously re-placed by those dispersed in solution.20 The rate increasedwith increasing the degree of agitation. The substrate-bound zeolite microcrystals readily move around on thesurface during monolayer assembly.20 In the case of refluxand stirring, the solvent bubbles generated from the

FIGURE 8. Illustration of non-cross-linking (a) and cross-linking (b)between the neighboring microcrystals, and plot of %R vs sonicationtime (c).18 FIGURE 9. SEM images of a nano-zeolite-Y-coated cotton fiber.11

FIGURE 10. Luminescence images of the glass plates coated witha closely packed monolayer of bare silicalite crystals (SL/G) (a), aftertreatment with a fluorophore-incorporating silicalite for 30 (b) and300 s (c), respectively, with sonication with stacking (SS) and thecorresponding SL/Gs (d, e, f) with sonication without stacking (SW).λext ) 350 nm; fluoropore ) n-nonylhemicyanine.20



substrate-bound crystals help their surface migration asshown in Figure 11.3

Driving Force for Uniform OrientationSilicalite-1 crystals can be attached to substrates in twoorientations, a-orientation and b-orientation (Figure 12),that is, a and b axes pointing normal to the substrate,respectively. b-Orientation is most prevalent (>99%), anda-orientation is found only during the initial stages. Thereasons that lead to high degrees of uniform orientationare as follows. First, the attached crystals are continuouslyreplaced by those dispersed in solution. Second, thebinding strength between each microcrystal and substrateincreases with increasing contact area. Consequently, theprobability of a substrate-bound microcrystal to be re-placed by those in solution decreases when attachedthrough the largest area face. Thus, the largest area face

determines the orientation of the attached crystals. Thisphenomenon is responsible for formation of b-orientedcontinuous films on substrates during silicalite-1 filmgrowth.33

The cylindrical zeolite-L crystals formed verticallyoriented monolayers (Figure 13a), while hexagonal co-lumnar zeolite-L crystals formed horizontally orientedmonolayers (Figure 13b).21 The former occurs because thecrystals have flat faces only at the bases, and the latteroccurs because the areas of the side planes are signifi-cantly larger than those of basal planes. The methods tosynthesize flat-faceted zeolite-L in two different morphol-ogies are described in our recent report.34

Mode of reaction promotion also affects the degree ofuniform orientation. For instance, the degree of verticalorientation of cylindrical zeolite-L crystals with the aspectratio > 2 increases in the order reflux and stirring <sonication without stacking , sonication with stacking.When the aspect ratio is less than 1, sonication withoutstacking also readily produces monolayers of c-orientedcrystals.35

Electric field-driven alignment of long ZSM-5 crystalswas demonstrated.36 This physical method is limited tothose crystals having net sizable intrinsic dipole moments.Growth of vertically oriented aluminophosphate molecularsieves using anodized alumina discs was also demon-strated.37 However, the degrees of coverage, close packing,

FIGURE 11. SEM images of various spots on a glass plate showingdifferent degrees of coverage during monolayer assembly withzeolite-A crystals.3

FIGURE 12. Orientations of a silicalite crystal and its channels.16

FIGURE 13. Monolayers of closely packed and vertically alignedcylindrical zeolite-L crystals (a) and horizontally aligned hexagonalcolumnar zeolite-L crystals (b) assembled on a 3-chloropropyl-tethering glass plate. Insets show cross sections; scale bar ) 2µm.21



and uniform orientation cannot compete with thoseshown in Figure 13a.

Driving Force for Close PackingDuring monolayer assembly with hydrogen bonding, theweak hydrogen-bonding and hydrophobic interactionbetween the surface-tethered thymine groups seems tobe the driving force for close packing. During monolayerassembly of bare or functional-group-tethering micro-crystals in toluene or in ethanol, the hydrophilic interac-tion between the crystals in the relatively hydrophobicsolvent seems to be an important contribution.

Factors that Affect Close PackingSurface migration of attached crystals is an importantfactor for close packing. Figure 14 illustrates the proposedmechanism: initial random attachment of the crystals,surface migration leading to close packing, and subse-quent random attachment.3 For close packing to occur,the crystals should repeatedly undergo the cycle of bondbreaking between the existing linkages and bond formingbetween the new, undamaged functional groups. There-fore, close packing becomes easier with decreasing bind-ing strength (covalent bonding < amine-metal bonding< H-bonding; Figure 15) and increasing degree of agita-tion.

Patterned Monolayer AssemblyMicropatterned monolayers of zeolite microcrystals havethe potential to be applied as combinatorial catalysts andlow-dielectric packing materials for integrated circuits.38,39

The first step is preparation of micropatterned monolayersof functional groups on substrates using microcontactprinting5 or photopatterning.10,22 Figure 16a illustrates amicrocontact printing-based procedure. A typical SEMimage of a micropatterned monolayer of ZSM-5 crystalsis shown in Figure 17a. The related micropatternedmicroporous and mesoporous silica films were preparedby Stucky, Yan, Zhao, Ozin, and co-workers.40-42

Photochemical degradation of organic linker groupstethered to glass surfaces is a highly versatile and effectiveway for preparing glass plates patterned with organiclinker groups.10 Thus, when a 3-halopropyl-tethering glassplate was exposed to UV light under a photomask, theUV-exposed halopropyl groups were selectively degraded(Figure 16b). While 3-halopropyl-tethering crystals formedmonolayers only on the UV-exposed areas (Figure 17b),the bare crystals formed monolayers only on the unex-posed areas (Figure 17c).

Patterned continuous silicalite-1 films are obtained byimmersing photopatterned glass plates into the corre-sponding synthesis gel.10 The weaker attractive forcebetween the colloidal seed crystals and gold was alsoutilized to produce patterned continuous silicalite-1 filmson silicon wafers.38 Photoinduced decarboxylation of glass-bound silver butyrate (C3H7-CO2

-Ag+) is an efficient wayfor micropatterned monolayer assembly of zeolite micro-crystals on substrates through ionic linkages.22

Synthesis of Zeolite as Ordered Multi-CrystalArraysMonolayer assembly of zeolite crystals does not allow usto freely control the orientation of attached zeolite crystalssince it is determined by the largest area face. Stemmingfrom the facile transformation of polyurethane spongesinto self-supporting silicalite-1 foams,43,44 we discoveredthat uniformly aligned polyurethane films serve as tem-plates for orientation-controlled synthesis of 2D and 3Darrays of silicalite-1 crystals, where the crystal orientationis controlled by varying the nature of polyurethane.16

For this, we prepared glass plates coated with 500 layersof uniformly aligned poly-(PDI/BDO) [(PDI/BDO)500/G]and poly-[PDI/TBE] [(PDI/TBE)500/G] (Figure 18). Whenthe hydrothermal reaction of silicalite-1 was carried outin the presence of (PDI/BDO)500/G, closely packed 2Darrays of c-oriented silicalite-1 crystals covered the glasssubstrates (Figure 19a). There were some areas coveredwith second layers of c-oriented crystals (Figure 19b),indicating that production of even 3D arrays of uniformly

FIGURE 14. Illustration of close packing of zeolite microcrystalsthrough surface migration on the substrate.3

FIGURE 15. Monolayer of thymine-tethering zeolite-A microcrystalsassembled on an adenine-tethering glass plate (a), and monolayerof aminopropyl-tethering zeolite-A monolayers assembled on plati-num (b).12,15



aligned silicalite-1 crystals is also possible by optimizationof the condition. In strong contrast, only b-orientedsilicalite-1 crystals grew on bare glass plates in the samegel. Randomly oriented poly(PDI-BDO) fibers gave onlyrandomly aggregated crystals.

In the case of [(PDI/TBE)500/G], closely packed 2Darrays of a-oriented silicalite-1 crystals covered the glasssubstrates (Figure 19c). Supramolecular organization ofthe hydrolyzed organic products and the reactive siliconspecies seems to be responsible for the above phenom-enon. We originally proposed that nanoslabs45 were thereactive silicon species. However, the disproval of theexistence of nanoslabs46,47 suggests that systematic inten-sive research is necessary to elucidate the mechanism ofthe above highly intriguing phenomenon.

The organic species in synthesis gels (such as TPA+)have been known as ‘structure directors’ for creation ofnanopores in certain shapes, sizes, and networks inzeolites. Our result indicates that the organic species canalso serve as ‘orientation directors’ for the produced

FIGURE 16. Schematic procedures for the microcontact-basedmicropatterned monolayer assembly of zeolite microcrystals on glass(a) and for photochemical methods for micropatterned monolayerassembly of microcrystals or direct growth of continuous zeolite filmson glass (b).5,10

FIGURE 17. SEM images of micropatterned monolayers of zeolitemicrocrystals.5,10

FIGURE 18. Structures of phenylene diisocyanate (PDI), 2-butyne-1,4-diol (BDO), and terephthalic acid bis-(2-hydroxy ethyl) ester (TBE).



crystals. Bein and co-workers reported the growth ofrandomly scattered zincophosphate and AlPO4-5 crystalsin certain orientations on alkyl phosphate-coated gold.48

Preparation of Surface-Aligned ZeoliteMicroballsZeolite nanocrystals (150 nm) can be organized into self-supporting zeolite microballs (1-20 µm) by adding a smallamount of water into a toluene solution predispersed withzeolite nanocrystals followed by vigorous sonication.14 Thewater droplets dispersed in toluene acted as templates toattract hydrophilic zeolite crystals (Figure 20a). The de-veloped strong bonding between the bare zeolite nano-

crystals suggests that direct siloxyl linkages form betweenthe nanocrystals during sonication by undergoing dehy-dration reaction between the surface hydroxyl groups.

When produced from pure water, the microballs con-sisted of randomly oriented nanocrystals (Figure 20a).Sodium dodecylsulfate induces the zeolite nanocrystalsin the two outermost surface layers of microballs uni-formly aligned (Figures 20b and 21a,b). The following mayexplain the above phenomenon. Dodecylsulfate ions self-assemble at the water-toluene interface with the nega-tively charged polar heads pointing to water droplets(Figure 20b). To minimize electrostatic repulsion with thenegative heads, zeolite nanocrystals uniformly align insuch a way as to minimize surface area. At higherconcentrations of dodecylsulfate, the zeolite nanocrystalsassembled into anisotropic structures (Figures 20c and21c) due to the decrease of the surface energy.

Perforated zeolite microballs were produced during theinitial periods of sonication (Figure 21d). The shaded spotsin Figure 21a represent the internal voids covered withthin layers (usually monolayers) of zeolite. The 3D net-

FIGURE 19. Monolayer (a) and double layer (b) of closely packed,c-oriented silicalite-1 crystals grown on (PDI/BDO)500/G, and amonolayer of closely packed a-oriented silicalite-1 crystals grownon (PDI/TBE)500/G (c).16

FIGURE 20. Illustrations of a water droplet in toluene acting as atemplate to attract hydrophilic zeolite crystals (a) and the alignedanionic surfactant molecules (sodium dodecylsulfate, SDS) actingas nanotools for the alignment of zeolite-A crystals at the water-toluene interface at an intermediate concentration of SDS (b), anda deformed structure of a water pool caused by high concentrationsof SDS (c).14

FIGURE 21. SEM images of a microball composed of octahedralzeolite-X crystals (size ) ∼150 nm) and its uniformly aligned surface(a), an egg-shaped ball composed of cubic zeolite-A crystals (size) ∼150 nm) and its uniformly aligned surface (b), a highly deformedball with aligned surface (c), and a perforated ball showing the 3Dnetwork of voids (d).14



works of the perforated spherulites can be seen fromFigure 21d (right).

The above methodology can be applied to organizenonspherical nanoparticles in high ordering. The highlyperforated microballs of zeolite nanocrystals can beutilized as highly effective catalysts and adsorbents. Themicroballs also show the anatomies of emulsions.

Self-Assembly of Substrate-Tethering ZeoliteCrystals with ProteinsExamples of self-assembly of substances into structuredaggregates, in particular by enzyme-substrate complex-ation, are very rare. We discovered a novel phenomenonthat â-glucosidase and D-glucose-tethering zeolite micro-crystals (Figure 22a) and avidin and D-biotin-tetheringzeolite microcrystals (Figure 22b) readily self-assembleinto thin (2-20 µm) and long (>1 cm) fibrils (Figure 23a,b)upon stirring them in aqueous buffer solutions at whichthe protein activities are highest.7,13 The morphology ofthe fibrils is sensitively governed by the protein/zeoliteratio. In the case of avidin and D-biotin-tethering zeolitemicrocrystals, discrete clusters of zeolite crystals with sizesof 5-10 µm (Figure 23c) are produced when the protein/zeolite ) 0.1.

The above results represent an unprecedented phe-nomenon that a protein and its substrate-tethering inor-ganic microcrystals self-assemble into structured aggre-gates. Although the mechanism is unclear, in particularas to why the two components grow into axial symmetry,this method is a way to organize zeolite microcrystals withprotein.

Current and Future ApplicationsThe uniformly aligned zeolite monolayers can be im-mediately used as excellent precursors for molecular sievemembranes.49 They can also be used to characterizeparamagnetic species in zeolites.17 The natural and syn-thetic fabrics and papers coated with Ag+-exchangedzeolite crystals can be used as novel antibacterial func-tional fabrics and papers.11 The optical fibers coated with

zeolite microcrystals can be used as high-efficiency pho-tocatalysts.50 The monolayers of vertically oriented fluo-rophore-incorporating cylindrical zeolite-L crystals gaveanisotropic photoluminescence (dichroic ratio ) 8.9),21

which is higher than the previously reported value (4.5)from the fluorescent polymer-incorporating mesoporoussilica.51 Thus, the uniformly aligned mono- and multilayersof zeolite crystals can also be used as media for generationof anisotropic photoluminescence,21 supramolecularlyorganized light-harvesting systems,35,52 and nonlinearoptical films.53 The micropatterned monolayers can beused as high-throughput combinatorial catalysts and low-dielectric packing materials for integrated circuits.38,39 Thehighly perforated microballs of zeolite nanocrystals canbe utilized as effective catalysts and adsorbents. Althoughthe assembly of zeolite microcrystals into 3D supercrystalsis still a challenge, we believe that this is the direction towhich the zeolite microcrystal organization should movesince the resulting supercrystals could find many impor-tant optical and other applications. The zeolite micro-crystal organization has been regarded as one of the futuredirections of zeolite research.54,55

Conclusion and Future DirectionsThe microbuilding blocks can be readily organized into2D and 3D functional entities. The chemistry of micro-crystals is doable and a highly promising area. We believeour findings set a new direction to which zeolite researchshould move, which will help zeolites be applied asinnovative materials and devices.

FIGURE 22. Zeolite-A microcrystals tethering â-D-glucose (a) andbiotin (b) groups.7,13

FIGURE 23. SEM images showing the fibrous zeolite-A-â-D-glucosidase composite material (a) and a discrete cluster of biotin-tethering zeolite-A crystals (b).7,13



I thank the graduate students and postdoctoral fellows whosenames appear in the references for their hard work which madethis Account possible and the Ministry of Science and Technologyof Korea and Sogang University for financial support.

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