15Micro-Macroporous Structured ZeoliteYa-Hong Zhang∗, Li-Hua Chen∗, Yi Tang, Xiao-Yu Yang, and Bao-Lian Su
15.1Introduction
Hierarchical micro-macroporous zeolite material is another popular bimodalporous structured material. Microporous structure generally provides an activereaction center, whereas macroporous structure provides a fast pathway for masstransportation and molecule diffusion as well as a definite shape as a catalyst,which is believed to greatly improve the catalytic performance involving reactionactivity, selectivity, and lifetime and coke resistance of conventional microporouscatalysts. The main body of this chapter comprises the review of the various bi-modal (micro–macro) porous zeolite materials, including various hollow zeolitestructures and shaped zeolite monoliths.
15.2Hollow Micro-Macroporous Structure
A hollow zeolitic structure is typical of micro/macro bimodal porous materials.Polystyrene (PS) microsphere is the first used sacrificial template for the formationof the micro/macroporous hollow structure. Wang et al. [1] used PS as a templateto prepare a hollow sphere of nanozeolite through a layer-by-layer (LbL) technique.The PS spheres were modified to form a uniform charge layer by depositing severallayers of cationic and anionic polyelectrolytes. The nanozeolite particles and oppo-sitely charged polyelectrolytes were then alternately deposited onto the charged PStemplates to form nanozeolite/poly(diallyldimethylammonium chloride) (PDDA)shells (Figure 15.1). The PS core was finally removed by calcination. By chang-ing the type of nanozeolites and the thickness of the deposited layers, a seriesof hollow nanozeolite microspheres were obtained [2, 3]. However, the hollownanozeolite spheres prepared by this technique have a limited mechanical strengthbecause there is only a weak electrostatic interaction between the building particles.Valtchev [4, 5] has proved that the secondary hydrothermal treatment in a suitablegel or clear solution could improve the mechanical stability of the LbL-constructed
Figure 15.1 (a) procedure for preparation of hollow zeolite spheres. (b) hollow nanozeoliteBEA spheres by using PS spheres with a diameter of 1.47 mm as templates via a LbL tech-nique. The inset in (b) is the SEM image of the sample by deliberately crushing. (a) and(b) reproduced from Ref. [1] and Ref. [3] by permission of The Royal Society of Chemistryand Taylor and Francis Group, respectively.
hollow nanozeolite sphere obtained from PS hard templates. Very recently, Chuand coworkers [6] synthesized nestlike hollow hierarchical MCM-22 microspheres(MCM-22-HS) (Figure 15.2). The MCM-22/C composite microspheres were ob-tained when carbon-black microspheres with a diameter of 4–8 μm were addedto a conventional MCM-22 synthetic solution under rotating crystallization. Afterstructure-directing agents and carbon-black microspheres were subsequently re-moved by calcination, the shell of MCM-22-HS was hierarchically constructed byintergrown flaky MCM-22 crystals. This method avoided presynthesis of nanozeo-lite building blocks and the LbL technique in the above PS template. Furthermore,the exceptional catalytic performance of the hollow Mo-containing MCM-22-HScatalysts in methane dehydroaromatization reaction was also demonstrated owingto their hollow and hierarchical structures.
Mesoporous silica (MS) spheres were first used as a template by Dong et al. [7] toprepare hollow nanozeolite spheres. The preparation process was fulfilled throughthe vapor-phase transport (VPT) treatment of the nanozeolite (seeds)-coated MSspheres. In this process, MS spheres acted not only as structure templates but also as
15.2 Hollow Micro-Macroporous Structure 459
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Figure 15.2 (A) Schematic representation of the growth process of the MCM-22-HS. (B)SEM images of (a, b) the MCM-22-HS synthesized in this work and (c) the MCM-22-HSwith collapse-fissure; (d) TEM images of the calcined MCM-22-HS; insets, high-resolutionSEM and TEM images of the area marked by the circle. Reproduced from Ref. [6] by per-mission of the American Chemical Society.
silica nutrients for the growth of the zeolitic shell (Figure 15.3). In addition, becauseof the adjustable morphologies of MS, hollow zeolite microcapsules with variousnonspherical shapes were also obtained through this approach [8]. As a developmentof this strategy, the seed-induced hydrothermal crystallization in alkaline aqueoussolution was applied to improve the mechanical stability and intactness of the hollowstructure [9]. A recent study showed that the match between core dissolution rateand zeolite shell intergrowth rate was crucial for the formation of perfect hollowzeolite spheres in this strategy [10]. The MS templates not only provide a diversity of
460 15 Micro-Macroporous Structured Zeolite
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Figure 15.3 Schematic illustration ofthe (A) fabrication of (a) hollow and (b)guest-encapsulated zeolite capsules. (B) SEMimages of (a) the original MS spheres, (b)MS spheres coated with monolayer seeds,and (c) hollow zeolite capsules, and (d) TEM
image of the sample shown in (c) The insetsin (B) present the corresponding blowupSEM images of one sphere surface. Repro-duced from Ref. [7] by permission of theAmerican Chemical Society.
morphologies but also permit a mechanically stable zeolitically structured materialto be formed by the synchronized processes of shell growth and core consumption.Moreover, it is worth mentioning that MS templates make the guest encapsulationmuch easier because of their inherent mesoporosity, compared to the nonporousPS template. Therefore, the guest species, such as Fe2O3 [7, 8], Ag [8, 11], Pt[12], and PdO [11] nanoparticles, and micrometer-sized carbon and polymer [11],which had been preincorporated into the mesopores of the MS templates, could besuccessfully entrapped inside the generated hollow capsules along with digestionof the MS cores. In addition, the compact zeolitic shell is expected to provide aperfect protection for these encapsulated active interiors, which has already beenproved by Ren and coworkers in the reactions of Heck coupling [12] and alcoholselective oxidation [13]. Furthermore, following this procedure, Wang et al. [14]used fly ash cenosphere (FAC, an aluminosilicate-rich waste from power plants) ashollow spherical template and nutriments to prepare hollow zeolitic microsphereswith cancrinite–zeolite shell by the in situ transformation of FAC in vapor phase.Moreover, hollow zeolite composite spheres with different frameworks can beobtained if the FACs were pretreated with different seeds of nanozeolite, followedby a seed-induced in situ hydrothermal crystallization in alkaline aqueous solution[15, 16]. The hollow zeolite composite microspheres obtained from FAC exhibited animproved mechanical strength owing to their unique bilayered shells of a dense andintergrowth zeolite film on the inner mullite layers [15]. Xiong et al. [17] employedspherical mesoporous DAM-1 or SBA-15 as a silicon source and substrate to preparehollow ZSM-5 spheres by a combination of pulsed laser deposition (PLD) and VPTprocesses. Zeolite ZSM-5 fragments were predeposited on the surface of the MSsubstrates by the PLD process, which could reorganize to form a crystalline ZSM-5shell with the dissolution of the substrate. Moreover, in this method, the size ofZSM-5 crystals in the shell could be easily adjusted by changing crystallization timeor PLD coating thickness. Recently, Zheng et al. [18] used zeolite BEA as templates
15.2 Hollow Micro-Macroporous Structure 461
to prepare hollow BEA–FAU zeolite composite spheres by a two-step hydrothermalcrystallization process. Similar to that of the MS template, in this method, zeoliteBEA acted as a digestible template for the shell growth of zeolite FAU. However, thepresence of an aluminum-poor interior and an aluminum-rich outer rim in zeoliteBEA crystals results in Si extraction favorably occurring in the aluminum-poor bulkrather than in the aluminum-rich external surface. Therefore, when the preparedzeolite BEA spheres were subjected to secondary hydrothermal treatment in thepresence of sodium hydroxide, sodium aluminate, and FAU zeolite seeds, the outersurface of zeolite BEA was relatively preserved and a hollow BEA–FAU compositestructure was produced. In these digestible template methods, the template coreremoval procedure in conventional hard-template methods can be avoided becauseof their active dissoluble composition.
Besides these unique hollow spheres, other zeolitic hollow structures were alsoobtained by using other templates with various morphologies. Hollow nanozeolitefibers were obtained by depositing nanozeolite particles on micrometer-sized car-bon fiber templates [19–22]. These strategies always contain complex pretreatmentof the carbon fibers before hydrothermal synthesis. First, the carbon fibers weremodified with a cationic polymer so that the surface charge of the fibers turns frominitially negative to positive. Then, the modified fibers were immersed in a solutionof colloidal zeolite crystals so that the crystals could be electrostatically adsorbedon the carbon fibers’ surfaces via seed-induced hydrothermal growth [19] and LbL[20, 21] method. Another method using carbon fibers as the template to prepare
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Figure 15.4 SEM images of (a) zeolite-coated fibers and (b–d) hollow zeolite fibers pre-pared at pH 2.5 and step voltage 1 V, 2 × 10 min; 2 V, 2 × 10 min; The insert of (b) isSEM image of section of hollow zeolite fiber. (c) High- and (d) low-magnification micro-graphs of (b). Reproduced from Ref. [23] by permission of the Royal Society of Chemistry.
462 15 Micro-Macroporous Structured Zeolite
zeolite hollow fibers is the electrophoretic assembly of nanozeolites on the surfaceof the carbon fibers [23] (Figure 15.4). Cotton threads [24] served as templates forthe fabrication of various hollow zeolite structures because they are flexible andeasy to handle. Recently, Liu et al. [25] used cotton threads as templates to preparesilicalite-1 hollow structures by in situ hydrothermal synthesis without surface pre-treatment. Moreover, different silicalite-1 hollow structures such as bunchy/singlehollow fiber, hollow monolith, and honeycomb structure could be synthesized bysimply controlling the configuration of the cotton thread and the synthesis cycles.Some inorganic templates with spherical and cubic morphologies, such as CaCO3
and Fe3(SO4)2(OH)5·2H2O, were also used to prepare the zeolite microcapsuleswith active interiors by hydrothermally treating the nanozeolite precoated bulkytemplates in a zeolite precursor gel [26]. In this work, the controllable release ofguest species was simulated by adjusting the thickness of zeolite shells, thanks tothe high thermal and chemical stability of zeolite shells and the acid solubility ofthe active core. Moreover, the inorganic cores in the zeolite microcapsules couldbe easily converted into their derivatives through thermal posttreatments, therebyfurther broadening the scope of their applications. Following the process using MSas a digestible template [7], Song et al. [27] reported a hexagonal hollow ZSM-5 tubeby using MS fiber as template and silica source. The difference in this work is thatthe aluminum ingredient was introduced into the zeolite framework by impregnat-ing the seeded MS fibers with Al(NO3)3 and NaCl aqueous solutions before VPTtreatment. Recently, desilication of framework Si in alkaline media was found to bean efficient method to generate hollow zeolite architectures. Mei et al. [28] utilizedthis desilication to obtain HZSM-5 zeolite microboxes with a regular hollow core bya mild alkaline treatment of ZSM-5 single crystals with Na2CO3 solution. Becauseof the mild alkalescence of Na2CO3 solution, it was relatively easier to control and itavoided excessive destruction of ZSM-5 zeolite crystals during alkaline desilication,
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Figure 15.5 (a) TEM image and (inset) electron diffraction (ED) pattern of the used par-ent single crystalline HZSM-5. (b) TEM image and (inset) ED pattern of the HZSM-5 mi-croboxes prepared by desilication in mild alkaline media (0.6 M Na2CO3 solution at 80 ◦Cfor 36 h). Reproduced from Ref. [28] by permission of the Royal Society of Chemistry.
15.2 Hollow Micro-Macroporous Structure 463
and so regular hollow microboxes formed (Figure 15.5). Moreover, the hollow struc-tures led to high catalytic activity on gas-phase cumene cracking and liquid-phaseα-pinene isomerization because of improved diffusion. Recently, Wang et al. [29,30] have reported the formation of hollow TS-1 crystals [29] and ZSM-5 nanoboxes[30] with a size <200 nm by involving a dissolution/recrystallization process. Theformation of hollow structures results from a preferential dissolution of the defectsites in TS-1 crystal or silicalite-1 cores, followed by a local recrystallization inthe presence of structure-directing agents. At present, this alkaline desilicationtechnique has already become one of the most widely used approaches for theconstruction of hierarchical (micro–meso/macro) porous materials. Valtchev et al.reported that the biomimetic zeolite Beta macrostructure that retained all morpho-logical features of the template was prepared by using a silica-containing vegetaltemplate (Equisetum arvense) [31].
In addition, the preparation of hollow zeolite structures without using hardtemplate has been reported [32, 33]. Han et al. [32] synthesized the hollow sodalitespheres and NaA crystals by introducing cross-linked polyacrylamide (C-PAM)hydrogels into zeolite synthesis gels. The three-dimensional C-PAM networks withadjustable pore sizes provided unique scaffolds for zeolite nucleation and growthand produced hollow zeolite structures by a surface-to-core zeolite crystallization
Zeolite gel entrappedin polymer hydrogel
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Figure 15.6 (A) Schematic representation of the fabrication of hollow structures includingsodalite spheres and hollow zeolite NaA crystals by crystallization in cross-linked polyacry-lamide hydrogels. (B) (a–c) SEM images of samples prepared with different amounts ofacrylamide (AM) by heating at 90 ◦C for 5 h. The inserts are their high magnification SEMimages. Reproduced from Ref. [32] by permission of the Royal Society of Chemistry.
464 15 Micro-Macroporous Structured Zeolite
process (Figure 15.6). Naik et al. [33] found that silicalite-1 hollow spheres witha diameter of 100–300 nm could be produced by self-assembly of silicalite-1nanocrystals under sonication in ammonia–ethanol solution, when the size ofzeolite nanocrystals was <30 nm.
The uniform spherical templates are easy to assemble to form an orderedmacrostructure by simple centrifugation or sedimentation. This orderedthree-dimensional (3D) monolith is expected to provide more opportunities forthe applications ranging from catalysis to electronic device. Stein et al. [34] firstpublished the works on three-dimensional ordered macroporous (3DOM) zeoliticmacrostructures by using PS spheres as templates. In this method, amorphoussilica macrostructures were first formed via close-packed PS arrays, and thenthe monolithic silica was converted in situ into silicates in the presence of thestructure-directing agent. The organic components were finally removed bycalcination to produce the micro/macroporous silicalite. This 3D interconnectivemacroporous zeolite monolith with different frameworks has also been obtained bycasting various nanozeolites into prearraying ordered PS macrostructure formedvia suction filtration [35–37] or by the self-assembly of nanozeolite particles andPS spheres with solvent evaporation [38]. Moreover, the secondary hydrothermaltreatment was used to improve the mechanical properties of the macroporouszeolite walls by the intergrowth between the nanocrystals during hydrothermaltreatment [5, 38]. Zhou and Antonietti [39] constructed a novel 3D bimodal poroussilica framework with well-defined macroporous and super-microporous lamellarnanostructure by simultaneously using three-dimensional ordered PS patternand an amphiphilic ionic liquid as templates. Moreover, because the amphiphilicionic liquid was used as both the solvent and the structure-directing templateat the same time, the typical solvent pores of the conventional sol–gel silica wereavoided. Therefore, the resulting hierarchical 3D ordered porous frameworkdisplayed a high mechanical stability. Besides, the ordered 3D nanozeolite/PSmacrostructure was obtained by assembling the core–shell PS microsphereswith nanozeolite-deposited layers into close-packed arrays by centrifugation [40]or sedimentation [5]. A monolithic zeolitic material with a close macroporousstructure was produced after calcination to remove the organic components.As we have seen, in early literature, most of the studies have focused mainlyon the preparation of hierarchical zeolites by using appropriate methods butlittle on their applications in catalysis or other fields. Recently, Xu et al. [41]studied the catalytic performance of the micro–macro dual porous zeolite ZSM-5materials prepared by Stein’s method [34] on the alkylation of phenol withtert-butanol. The authors assigned the high phenol conversion and selectivity of2,4-di-tert-butyl phenol during the reaction to the presence of the hierarchicalporosity and strong acidity of the pore walls. In addition, the above-mentioned MSspheres with uniform spherical morphology have also been applied to preparing3D-ordered zeolite monolith with closed or interconnected macropores. Donget al. [11, 42] first reported the robust 3D zeolite monolith with an ordered closedmacropore by the hydrothermal treatment of the array of nanozeolite precoatedMS spheres in a clear silica-containing solution. Wang and Caruso [43] modified
15.3 Micro-Macroporous Monoliths 465
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Figure 15.7 (a–d) SEM images of 3D interconnected macroporous zeolite monoliths atdifferent magnifications, which are prepared by using prearrayed 3D ordered MS as thetemplates and nutrient source. Reproduced from Ref. [43] by permission of John Wiley &Sons, Inc.
this process to prepare the 3D interconnected macroporous zeolite monolith(Figure 15.7). The MS spheres were prearrayed into a 3D ordered membraneby gravity sedimentation, and then the closely packed assemblies were seededwith a layer of silicalite-1 nanocrystals. The interconnected macroporous zeoliticmonolith is then produced on dissolution of the MS template and growth ofthe seeds by hydrothermal treatment. Moreover, the authors also demonstratedthat the interconnected macroporous zeolitic monolith displayed higher enzymeloadings and activities than the corresponding 3D zeolite monolith with closedmacroporous or nonporous structure.
15.3Micro-Macroporous Monoliths
Small zeolite crystals have been demonstrated to have an effect on increasing theselectivity and adsorption capacity over larger zeolite particles. However, handlingnano- or microcrystalline powders presents difficulties in comparison to larger par-ticle sizes. In particular, expensive separation steps and the corresponding healthproblems associated with their ingestion or inhalation owing to the airborne mobil-ity of fine particulate matter greatly affected their handling and disposal. Therefore,microporous materials prepared within macroporous monoliths have been an areaof research that has commanded interest in recent years. Various macroporous
466 15 Micro-Macroporous Structured Zeolite
supports were used to prepare self-supporting micro-macroporous hierarchical ma-terials. Depending on the support material used and on the composition of synthesissolution, the macroporous supports can serve as a macroporous scaffold in threeways: (i) only robust support, (ii) partial zeolitization, and (iii) complete zeolitizationduring the formation of micro-macroporous structure. In mode (i), van der Puilet al. [44] described the formation of MFI and BEA coatings on porous α-Al2O3
beads and extrudates. This zeolite monolith on the macroporous supports wasgenerally obtained using a conventional one-step hydrothermal crystallization byimmersing the support into a proper precursor solution of zeolite synthesis. Follow-ing the similar method, various micro-macroporous zeolite monoliths on differentinert supports such as stainless steel [45], cordierite honeycomb [46, 47], SiC/Al2O3,ZrO2/Y2O3 [48], and silicon oxycarbide (SiOC) ceramic foam [49, 50] have beenprepared. Wang et al. [51] obtained nanozeolite-coated diatom macrostructures bydirect LbL assembly of zeolite nanocrystals on a diatom substrate. However, itseems to provide only a low zeolite loading content in the final samples, and thezeolite nanocrystals could easily be detached from the surface of the support. The3DOM ZrO2/zeolite nanocomposites were obtained by hydrothermally treating thepolyelectrolyte-coated 3DOM activated sulfated zirconia in a synthetic solution ofzeolite NaY (Figure 15.8) [52]. Moreover, because the activated sulfated zirconiasupports can act as an isomerization catalyst, and the zeolite NaY shell is a typi-cal Fischer–Tropsch catalyst, the hierarchical micro-macroporous composites areexpected to be used in some multireaction processes as bifunctional catalysts.
Besides these nonremoving inorganic supports, many sacrificial supports wereadopted for the formation of micro/macroporous monoliths. Mann et al. [53] con-structed a hierarchical zeolite fiber by filtrating nanozeolite suspension into abacterial supercellular thread. Silicalite-1 nanocrystals were aggregated specificallywithin the organized microarchitecture. After removing the bacterial template, anintact zeolite fiber with ordered macroporous channels consisting of silicalite-1nanocrystals was obtained. Starch gels have also been used by Mann et al. [54] toprepare microporous silicalite monoliths and thin films with hierarchical meso-and macroporous structures. Lee et al. [55] used polyurethane foams (PUFs) asthe template for the formation of large monolithic zeolite foams. The resultingzeolite monoliths have highly ramified networks of interconnecting macroporeswith tailorable sizes and shapes (Figure 15.9). Interestingly, this PUF templategradually decomposes with the growth of the zeolite shell in the synthetic solution,while the decomposition of PUF accelerates the crystallization of the synthetic gelinto silicalite-1 zeolite because of the influence of the decomposed fragments ofPUF on the synthetic solution. Dong et al. [56] used wood cells (cedar and bamboo)as templates for the preparation of hierarchical zeolitic tissue by a seeding-growthstrategy. The wood cells were deposited with nanozeolites by electrostatic ad-sorption and then bore a secondary hydrothermal growth. After removing thetemplate by calcination, the self-standing zeolitic tissue faithfully inherited theinitial cellular structure of wood. Wang et al. [57] used the same strategy to pre-pare a self-supporting porous zeolite membrane with a spongelike architecture byemploying cellulose acetate filter membranes as macrotemplates. Zampieri and
15.3 Micro-Macroporous Monoliths 467
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Figure 15.8 SEM images of (a) 3D ordered macroporous (3DOM) sulfated zirconia (SZ)monolith and (b) NaY-coated 3DOM SZ monolith. (c) TEM image and (d) high-resolutionTEM image of NaY/3DOM SZ. Reproduced from Ref. [52] by permission of Elsevier B.V.
coworkers [58] prepared self-supporting MFI-type zeolite frameworks with hierar-chical porosity by an in situ seeding and secondary growth route in the presenceof a Luffa cylindrica template (Figure 15.10). Moreover, the applicability of suchself-supporting biomorphous zeolitic materials was tested for the first time in amodel catalytic process, the cracking of n-hexane. Recently, a micro-/macroporousinterwoven hollow tube was synthesized by the secondary growth of preliminaryadsorbed ZSM-5 zeolite on eggshell membranes [59]. However, the porous mono-lith obtained by sacrificial natural or artificial templates generally possessed a lowmechanical stability, even though a secondary crystallization step was adopted,which badly limited their practical applications, although they could provide adiverse hierarchical structure.
In mode (ii), some supports containing active silica or alumina are employed toprepare partly zeolitized zeolite/support hierarchical structures, which provide ahigher mechanical stability because of the strong interaction between the zeolitecoating and the support. Komarneni and coworkers [60] have developed a novelhoneycomb composite ZSM-5/mullite by in situ crystallization of ZSM-5 from asintered kaolin honeycomb. The in situ crystallization of ZSM-5 results in a gradedstructure with three layers consisting of a strongly adhered ZSM-5 film at the
468 15 Micro-Macroporous Structured Zeolite
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Figure 15.9 (A) Photographic images of PUF templates in various shapes (columns 1 and3) and the corresponding monoliths of silicalite-1 foam (SLF) (columns 2 and 4). (B) SEMimages showing the morphologies and connectivity of the (a) macroporous cells of PUF,(b) the resulting SLF, and (c,d) the typical SLF strut at two different magnifications. Repro-duced from Ref. [55] by permission of John Wiley & Sons, Inc.
surface, a composite ZSM-5 and mullite layer below the pure ZSM-5 layer, andmacroporous mullite at the core. Anderson and coworkers [61] have zeolitizeddiatomite to form hierarchical macrostructures through hydrothermal treatment ofthe ultrasonic seeded or unseeded diatomite in clear synthesis solutions containingadscititious silicon or aluminum sources. Wang et al. [62] partly transformed thediatomaceous silica into zeolite by the VPT method (Figure 15.11). It is worthmentioning that the existence of over 70 000 known natural diatom species, whichare perfect silica and alumina sources for zeolite synthesis, will extend this methodto the preparation of various hierarchical zeolite structures with rationally tai-lored morphologies and porosities. Zampieri et al. [63, 64] used the bioinspiredrattan-derived macroporous SiSiC as support to prepare SiSiC/zeolite compositemonolith. Rattan stems were first pyrolyzed to form biocarbon materials and thenconverted to SiSiC replicas by a spontaneous Si-melt infiltration process. Thebimodal (micro-/macro) porous SiSiC/zeolite composite monolith was obtainedby self-transformation of the support in a reaction mixture consisting of water,
15.3 Micro-Macroporous Monoliths 469
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Figure 15.10 Silicalite-1 self-supporting (calcined) zeolite replica of Luffa sponge (highlyconcentrated precursor solution, 1-step secondary crystal growth). (a, b) SEM micrographsshowing the continuous bundles of self-supporting zeolitic microrods with high aspect ra-tio (>100). (c) Zeolitic microrods (closer view) on a fractured cross-section with texturalimprinting effects (arrows present: helical texture). (d) External film forming the zeolite scaf-fold. Reproduced from Ref. [58] by permission of Elsevier B.V.
template, and NaOH via a continuous dissolution–crystallization process. In thismethod, because of the high Si content of the biomorphic SiSiC ceramic support(∼40%), it was used as the unique source of Si for the zeolite crystallization.No additional Si source was used in the synthesis mixture. Johnson and Worrall[65] employed inexpensive, abundant, macroporous pumice granules to preparemicro-macroporous hierarchical materials by the zeolitization of pumice super-structures. Most of the pumice granules were transformed into zeolite, whiletheir macroporous structure was retained. The resulting hierarchical monolith wasintroduced as a granular material without the use of binders.
Except the advantage of micro-macroporous zeolite materials as catalysts in masstransportation and molecule diffusion, the motivation of preparing a binder-freezeolite catalyst also triggers the development of mode (iii). The widely used poroussupports in mode (iii) have porous glass granules/beads, bimodal pore silica oraluminosilicate monolith, and so on. Dong et al. [66, 67] prepared boron-containingMFI-type and TON-type zeolite granules by in situ transformation of porous glassgranules in vapor phase. Shimizu and Hamada [68] synthesized MFI-type zeolitetubes from porous glass tubes by a bulk-material dissolution technique, in whicha porous quartz tube was converted to MFI in an aqueous solution of tetrapropy-lammonium hydroxide and hydrogen fluoride. Bimodal (micro–macro) porousZSM-5 zeolite beads [69] and granules [70] were also prepared by hydrothermally
470 15 Micro-Macroporous Structured Zeolite
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Figure 15.11 (a) SEM image of the initial diatomite. (b–e) SEM and TEM images of thecalcined diatomite seeded through one layer of nanozeolite after VPT treatment for 10 days.Reproduced from Ref. [62] by permission of the Royal Society of Chemistry.
treating porous glass beads or granules in the presence of dipropylamine or mono-propylamine. The bimodal pore silica monolith is the most widely used supportfor the complete zeolitization to prepare various hierarchical all-zeolite monolithsbecause of its interconnected macroporous and active silica composition. Lei et al.[71–73] fabricated hierarchical MFI- and BEA-type zeolite monoliths by trans-forming the skeletons of a preseeded bimodal pore silica gel into a zeolite byusing a steam-assisted conversion method. Furthermore, the hierarchically struc-tured ZSM-5 monoliths also were synthesized from this silica monolith throughthe nanocasting method, and their high catalytic activity in catalytic crackingof large molecules was displayed [74]. Very recently, this bimodal silica mono-lith was used to prepare complete crack-free sodalite zeolite monoliths with ahierarchical pore system by a pseudomorphic transformation process [75]; itscrack-free body and homogeneous macroporous network allowed it to be used as aflow-through microreactor (Figure 15.12). Some alumina-containing aluminosili-cate extrudates/monoliths were used to prepare different shaped binderless zeoliteextrudates with an interconnected macroporous and microporous skeleton by theseed-induced liquid- or vapor-phase hydrothermal transformation [76–78]. Yanget al. [79] used amorphous silicoaluminophosphate (SAPO) monoliths to preparehierarchical SAPO-34 monolith by the dry-gel conversion technique. Moreover,the hierarchical SAPO-34 shows higher catalytic activity and stability than theconventional SAPO-34 in the catalytic conversion of methanol to light olefins.It is worth mentioning that the above-prepared hierarchical materials generallypossess trimodal (micro–meso–macro) pores because of the bimodal porosity of
15.4 Conclusion and Remarks 471
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Figure 15.12 Photograph of (a) the crack-free sodalite zeolite monolith with a heat shrink-able DERAY-PTFE clad. SEM images of (b) the original bimodal silica monolith and (c) itspseudomorphic transformed sodalite all-zeolite monolith. Reproduced from Ref. [75] by per-mission of the American Chemical Society.
the porous supports used, which further provides pathways for mass transportationand molecule diffusion.
Besides these transferable supports, hierarchical all-zeolite monoliths were alsodirectly prepared from nanozeolite colloid or zeolite powders by a physical methodor self-assembly strategy. Wang et al. [80] prepared hierarchical zeolite structureswith designed shapes by gel casting the colloidal nanocrystal suspensions. Fol-lowing this gel-casting method, macroporous NaP zeolite monoliths (M-ZPMs)with designed shapes such as cylinder, rectangular prism, and donut were syn-thesized by gel casting the aged zeolite gel with colloidal silica as a binderand by subsequent VPT treatment [81]. Sebastian et al. [82, 83] directly preparedmicro-/macroporous hierarchical titanosilicate microspheres with umbite struc-ture by a direct liquid-phase hydrothermal synthesis under a range of synthesisconditions. The precondition of forming microspheres with hierarchical structureis to require agitation by rotation and use TiO2 anatase as a Ti source during thesynthesis. However, the whole process needs neither organic agents for structuringthe microporous titanosilicate nor macrotemplates for forming the macroporosity.Hua and Han [84] proposed an one-step method to prepare MFI-zeolite micro-spheres without the need for presynthesizing zeolite nanocrystals in the presence ofthe Pluronic triblock copolymer F127. The formation of micrometer-sized spheresis attributed to the use of F127 in the synthesis. Moreover, the macroporosity ofthe zeolite microspheres can be increased and adjusted by simply adding styrenein the initial mixture. Vasiliev et al. [85] used a pulsed current processing (PCP)technique to directly prepare binderless hierarchically porous monoliths from zeo-lite powders. The PCP-treated binderless ZSM-5 monoliths displayed mechanicalstability and high selectivity in xylene isomer separation.
15.4Conclusion and Remarks
The micro-macroporous zeolite materials already obtained by various methods aresummarized in Table 15.1. As can be seen, the wide diversity of templates/supports
472 15 Micro-Macroporous Structured Zeolite
Tabl
e15
.1Su
mm
ary
ofth
eva
riou
sm
icro
-mac
ropo
rous
zeol
item
acro
stru
ctur
es.
Stru
ctur
eTe
mpl
ate/
supp
ort
Met
hod
Fram
ewor
k/ch
emic
alco
mpo
sitio
nof
poro
usm
ater
ials
Ref
eren
ces
Hol
low
sph
ere
PS
LbL
Sili
calit
e-1,
BE
A,L
TA
,FA
U[1
–3]
LbL
+se
con
dary
grow
thLT
A,s
ilica
lite-
1[4
,5]
MS
Seed
-indu
ced
VP
TS
ilica
lite-
1[7
]Se
ed-in
duce
dh
ydro
ther
mal
crys
talli
zati
onS
ilica
lite-
1[9
]
FA
CV
PT
CA
N[1
4]Se
ed-in
duce
dh
ydro
ther
mal
crys
talli
zati
onLT
A,F
AU
[15,
16]
DA
M-1
orS
BA
-15
PLD
+V
PT
ZS
M-5
[17]
Car
bon
blac
kR
otat
ing
hyd
roth
erm
alcr
ysta
lliza
tion
MC
M-2
2[6
]
BE
Aze
olit
eH
ydro
ther
mal
crys
talli
zati
onF
AU
-BE
A[1
8]C
aCO
3Se
ed-in
duce
dh
ydro
ther
mal
crys
talli
zati
onS
ilica
lite-
1[2
6]
3DC
-PA
Mh
ydro
gels
Surf
ace-
to-c
ore
zeol
ite
crys
talli
zati
onS
OD
[32]
Self
-ass
embl
yu
nde
rso
nic
atio
nin
amm
onia
–et
han
olso
luti
onS
ilica
lite-
1[3
3]
Hol
low
fibe
rC
arbo
nfi
ber
Seed
-indu
ced
hyd
roth
erm
alcr
ysta
lliza
tion
Sili
calit
e-1
[19,
22]
LbL
Sili
calit
e-1,
BE
A[2
0,21
]E
lect
roph
oret
icte
chn
iqu
eS
ilica
lite-
1[2
3]M
esop
orou
ssi
lica
fibe
rSe
ed-in
duce
dh
ydro
ther
mal
crys
talli
zati
onZ
SM
-5[2
7]
15.4 Conclusion and Remarks 473
Cot
ton
thre
ads
Hyd
roth
erm
alcr
ysta
lliza
tion
Silic
alit
e-1
[24,
25]
Hol
low
box
Fe 3
(SO
4) 2
(OH
) 5·2H
2O
See
d-in
duce
dh
ydro
ther
mal
crys
talli
zati
onSi
lical
ite-
1[2
6]Z
eolit
eD
esili
cati
onof
fram
ewor
kZ
SM-5
,TS-
1[2
8–
30]
3Dor
dere
dh
ollo
wst
ruct
ure
Arr
ayed
PS
mac
rost
ruct
ure
Hyd
roth
erm
alcr
ysta
lliza
tion
Silic
alit
e-1
[34]
Nan
ocas
tin
gLT
A,F
AU
,BE
A,Z
SM-5
,si
lical
ite-
1,LT
L[3
5–
37]
Am
phip
hili
cio
nic
liqu
idas
solv
enta
nd
stru
ctu
re-d
irec
tin
gag
ent
Lam
ella
rsi
lica
[39]
PS
sph
ere
Self
-ass
embl
yof
nan
ozeo
lite
and
PS
sph
eres
wit
hso
lven
teva
pora
tion
Sili
calit
e-1
[38]
Cor
e–sh
ell
nan
ozeo
lite/
PS
sph
ere
Cen
trif
uga
tion
Silic
alit
e-1
[5,4
0]
Arr
ayed
nan
ozeo
lite/
MS
sph
eres
Seed
-indu
ced
hyd
roth
erm
alcr
ysta
lliza
tion
Silic
alit
e-1
[11,
42]
Arr
ayed
MS
sph
ere
Seed
-indu
ced
hyd
roth
erm
alcr
ysta
lliza
tion
Silic
alit
e-1
[43]
Mic
ro-
mac
ropo
rou
sm
onol
ith
α-A
l 2O
3H
ydro
ther
mal
crys
talli
zati
onin
the
pres
ence
ofsu
ppor
tZ
SM
-5B
EA
[44,
48]
Stai
nle
ssst
eel
ZSM
-5[4
5]C
ordi
erit
eT
S-1,
FE
R[4
6,47
]Si
C/A
l 2O
3Z
SM
-5[4
8]Z
rO2/Y
2O
3Z
SM
-5[4
8]S
iOC
Sili
calit
e-1,
ZS
M-5
[49,
50]
ZrO
2N
aY[5
2]P
UF
Sili
calit
e-1
[55]
Cel
lulo
seac
etat
eS
eedi
ng-
grow
thst
rate
gySi
lical
ite-
1[5
7]W
ood
Sili
calit
e-1
[56]
Luffa
cylin
dric
aS
ilica
lite-
1,Z
SM
-5[5
8]
(con
tinu
edov
erle
af)
474 15 Micro-Macroporous Structured Zeolite
Tabl
e15
.1(c
ontin
ued)
Stru
ctur
eTe
mpl
ate/
supp
ort
Met
hod
Fram
ewor
k/ch
emic
alco
mpo
sitio
nof
poro
usm
ater
ials
Ref
eren
ces
Egg
shel
lmem
bran
eSi
lical
ite-
1,Z
SM-5
[59]
Bac
teri
alN
anoc
asti
ng
Sili
calit
e-1
[53]
Star
chge
lsG
elat
ion
wit
hn
anoz
eolit
esor
nan
ocas
tin
gSi
lical
ite-
1[5
4]K
aolin
Hyd
roth
erm
alcr
ysta
lliza
tion
ZSM
-5/m
ulli
te[6
0]S
iSiC
Sili
calit
e-1
[63,
64]
Pu
mic
egr
anu
leN
aX/Y
,SO
D[6
5]D
iato
mit
eS
eed-
indu
ced
hyd
roth
erm
alcr
ysta
lliza
tion
Sili
calit
e-1/
diat
omit
e[6
1]V
PT
Sili
calit
e-1/
diat
omit
e[6
2]P
orou
sgl
ass
gran
ule
s/be
ads
VP
TZ
SM-5
,TO
N[6
6,67
]
Alk
alin
eh
ydro
ther
mal
crys
talli
zati
onZ
SM-5
[68
–70
]B
imod
alpo
resi
lica
mon
olit
hSt
eam
-ass
iste
dco
nve
rsio
nSi
lical
ite-
1,Z
SM-5
,BE
A[7
1–
74]
Nan
ocas
tin
gZ
SM-5
[74]
Pse
udo
mor
phic
tran
sfor
mat
ion
SOD
[75]
Bim
odal
pore
alu
min
osili
cate
Hyd
roth
erm
altr
eatm
ent
LTA
,ZSM
-5[7
6,78
]
Seed
-indu
ced
VP
TZ
SM-5
[77]
Silic
oalu
min
oph
osph
ate
Dry
-gel
con
vers
ion
SAP
O-3
4[7
9]G
elca
stin
gof
nan
ozeo
lites
Silic
alit
e-1,
NaP
[80,
81]
Rot
atin
gh
ydro
ther
mal
crys
talli
zati
onT
itan
osili
cate
um
bite
[82,
83]
Hyd
roth
erm
alcr
ysta
lliza
tion
inth
epr
esen
ceof
F12
7S
ilica
lite-
1[8
4]
PC
Pte
chn
iqu
eZ
SM-5
[85]
References 475
employed in the construction of bimodal porous zeolites resulted in materialswith various morphological features, such as hollow spheres, fibers, boxes, and3D-ordered hollow array, as well as diverse hierarchical macrostructured mono-liths. Moreover, the variety of compositions of the resulting hierarchical materialsoffers enormous application possibilities in catalysis, adsorption, and other non-conventional fields. However, so far, only a few examples of the application ofhierarchical micro-macroporous structures have been reported. One of the mainobstacles to their application is their mechanical stability. Most of the hollowmicro-macroporous materials and monoliths obtained from sacrificial artificial ornatural supports possess poor mechanical strength so that they are difficult tohandle even in the laboratory. In addition, although these materials provide a novelmorphology and multilevel porous structures, these approaches are still an artrather than a science. They are difficult to standardize for the industry and fac-tory applications. Many factors influence the morphology and porous structure ofthe materials formed by such bottom-to-up template/assembly techniques. Amongthem, the substrate-supported zeolite and binderless all-zeolite materials withmicro-macroporous structures are the most promising for application in catalysisbecause of their high mechanical strength and preshaped characteristic. Still, theinterest in preparing various multilevel porous zeolites by more novel templates ishigh because of the strong desire for the hierarchical zeolite structures with specialfunctionality. Therefore, an effort to develop exciting zeolite materials with a novelhierarchical porous structure and their applications will continue, and the potentialrelationship between the structure and performance of these hierarchical zeolitematerials will continue to be studied.
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