Synthesis and Applications of Supramolecular-Templated Mesoporous Materials

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<ul><li><p>As exemplified by Pd-grafted ultra-large pore</p><p>silicate Pd-TMS11 (TEMimage presented above), su-</p><p>pramolecular-templated meso-porous materials find many uses,</p><p>for example (starting at the left and go-ing clockwise) in polymerization, for</p><p>immobilization of well-defined ho-mogeneous catalysts, in Heck</p><p>catalysis, in acid catalysis,and as an encapsulation</p><p>host for ferrocenylunits.</p></li><li><p>1. Introduction</p><p>Only six years have passed since the exciting discovery ofthe novel family of molecular sieves called M41S was reportedby the researchers at Mobil Research and DevelopmentCorporation.[1] These mesoporous (alumino)silicate materials,with well-defined pore sizes of 15 100 , break past thepore-size constraint (1000 m2 g1) and the precisetuning of pore sizes are among the many desirable propertiesthat have made such materials the focus of great interest. TheM41S materials also ushered in a new approach in materialssynthesis where, instead of the use of single molecules astemplating agents (as in the case of zeolites), self-assembledmolecular aggregates or supramolecular assemblies are em-ployed as the structure-directing agents. Various aspects ofM41S and related mesoporous materials have been reviewedby Brinker (recent advances in porous inorganic materials),[2]</p><p>Vartuli et al. (synthesis of the M41S family),[3] Stucky et al.(biomimetic synthesis of mesoporous materials),[4] Ramanet al. (porous silicates templated by surfactants and organo-</p><p>silicate precursors),[5] Antonelli and Ying (mesoporous tran-sition metal oxide (TMS) family),[6] Behrens (mesoporoustransition metal oxides),[7] Zhao et al. (aluminosilicate MCM-41),[8] and Sayari (catalytic applications of MCM-41).[9] Herewe review the current state of mesoporous materials researchfrom the standpoint of compositional control. The synthesis ofnot only (alumino)silicate M41S materials but also transitionmetal doped and pure transition metal oxide mesoporousmaterials is addressed. After discussion of the major synthesisroutes and mechanisms of formation, the applications of thesematerials are surveyed, with particular emphasis on hetero-geneous catalysis.</p><p>Inorganic solids that contain pores with diameters in thesize range of 20 500 are considered mesoporous materials,according to IUPAC definition. Examples of mesoporousmaterials include M41S, aerogels, and pillared layered struc-tures, as listed in Table 1.[10] In this paper, we will focus only onmesoporous materials that have been prepared by supra-molecular templating, such as M41S. The demarcation at 20 between the micropore and mesopore regimes is convenient,in that all zeolites and related zeotypes are microporous;however, some types of the supramolecular-templated struc-tures can be microporous (see Section 2.4). Mesoporousmaterials derived by means of surfactant templating areoccasionally called zeolites and described as crystallinematerials in reference to their long-range ordering of the porepacking. Such references are not correct, as the pore walls ofthese materials are amorphous and lack long-range order.</p><p>Synthesis and Applications of Supramolecular-TemplatedMesoporous Materials**</p><p>Jackie Y. Ying,* Christian P. Mehnert, and Michael S. Wong</p><p>Research in supramolecular-templatedmesoporous materials began in theearly 1990s with the announcement ofMCM-41 and the M41S family ofmolecular sieves. These materials arehighly unusual in their textural char-acteristics: uniform pore sizes greaterthan 20 , surface areas in excess of1000 m2 g1, and long-range ordering ofthe packing of pores. The mesoporousmaterials are derived with supramo-lecular assemblies of surfactants, whichtemplate the inorganic components</p><p>during synthesis. Many researchershave since exploited this technique ofsupramolecular templating to producematerials with different compositions,new pore systems, and novel proper-ties. This article reviews the currentstate of the art in mesoporous materi-als research in three general areas:synthesis, catalytic properties, and oth-er applications that take advantage ofbulk morphologies. Various mecha-nisms formulated to explain the for-mation of mesostructures are discussed</p><p>in the context of compositional con-trol. The catalytic applications of mes-oporous materials are examined, witha significant fraction based on recentpatent literature. Other directions inthe utilization of mesoporous materialsare also presented.</p><p>Keywords: amphiphiles hetero-geneous catalysis mesoporosity molecular sieves template synthesis</p><p>[*] Prof. J. Y. Ying, Dr. C. P. Mehnert, M. S. WongDepartment of Chemical EngineeringMassachusetts Institute of TechnologyCambridge, MA 02139 (USA)Fax: (1)617-258-5766E-mail : jyying@mit.edu</p><p>[**] A list of abbreviations is provided in the Appendix.</p><p>REVIEWS</p><p>Angew. Chem. Int. Ed. 1999, 38, 56 77 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999 1433-7851/99/3801-0057 $ 17.50+.50/0 57</p></li><li><p>REVIEWS J. Y. Ying et al.</p><p>58 Angew. Chem. Int. Ed. 1999, 38, 56 77</p><p>2. Mechanisms of Mesostructure Formation</p><p>There have been a number of models proposed to explainthe formation of mesoporous materials and to provide arational basis for the various synthesis routes. On the mostcommon level, these models are predicated upon the presenceof surfactants in a solution to guide the formation of theinorganic mesostructure from the solubilized inorganic pre-cursors (Scheme 1). Surfactants contain a hydrophilic head</p><p>Scheme 1. Schematic representation of the general formation of MCM-41from inorganic precursors and organic surfactants.</p><p>group and a long hydrophobic tail group within the samemolecule and will self-organize in such a way as to minimizecontact between the incompatible ends. How the inorganicprecursor interacts with the surfactant is the issue whereby themodels diverge; the type of interaction between the sur-factant and the inorganic precursor will be seen as a significantdifference among the various synthesis routes, the formationmodels, and the resulting classes of mesoporous materials.</p><p>Jackie Y. Ying was born in Taiwanand raised in Singapore. She grad-uated summa cum laude from TheCooper Union in 1987, and com-pleted her Ph.D. degree as an AT&amp;TBell Laboratories Scholar at Prince-ton University in 1991. She was aNSF-NATO Postdoctoral Fellowand Alexander von Humboldt Re-search Fellow at the University ofSaarland with Prof. H. Gleiter, andshe currently holds the St. LaurentAssociate Professorship at theChemical Engineering Departmentof the Massachusetts Institute of Technology. Her research is focused on the synthesis of nanostructured inorganic materialsfor catalytic, membrane, and advanced ceramic applications. She is the author of over eighty publications, and the recipientof the American Ceramic Society Ross C. Purdy Award, David and Lucile Packard Fellowship, National ScienceFoundation and Office of Naval Research Young Investigator Awards, Camille Dreyfus Teacher-Scholar Award, RoyalAcademy of Engineering ICI Faculty Fellowship, and American Chemical Society Faculty Fellowship Award in Solid-StateChemistry. She serves on the Board of Directors of the Alexander von Humboldt Association of America as well as theeditorial boards of several journals.</p><p>Christian P. Mehnert studied chemistry at the Technische Universitt Mnchen (1985 1992). During this time he carriedout research with Dr. D. OHare at Oxford University (1990) and Prof. J. Lewis at Cambridge University (1991), before hecompleted his research thesis which focused on the synthesis of new aminocarbyne complexes of chromium with Prof. W. A.Herrmann and Prof. A. C. Filippou (1992). He then moved to England, where he became a graduate student at OxfordUniversity with Prof. M. L. H. Green to work on the synthesis of new organometallic complexes with metal vapor synthesistechniques and the investigation of the reaction chemistry of highly Lewis acidic perfluorinated borane complexes. Afterreceiving his doctorate (D.Phil.) as a student at Balliol College (1996), he moved to the United States and joined Prof. J. Y.Yings laboratory at the Massachusetts Institute of Technology as a postdoctoral research associate. The area of his currentresearch is the synthesis and application of new nanostructured materials for catalysis, with a special focus on theincorporation of organometallic catalysts into mesoporous molecular sieves.</p><p>Michael S. Wong is a Ph.D. student in Chemical Engineering at the Massachusetts Institute of Technology. He received aB.S. degree in Chemical Engineering from the California Institute of Technology (1990 1994) and a M.S. degree inChemical Engineering Practice from the Massachusetts Institute of Technology (1997). His research interests include thesynthesis and characterization of nanostructured zirconia-based material for acid catalytic applications.</p><p>J. Y. Ying C. P. Mehnert M. S. Wong</p><p>Table 1. Pore-size regimes and representative porous inorganic materials.</p><p>Pore-sizeregimes</p><p>Definition Examples Actual size range</p><p>macroporous &gt; 500 glasses &gt; 500 mesoporous 20 500 aerogels &gt; 100 </p><p>pillared layered clays 10 , 100 [a]</p><p>M41S 16 100 microporous &lt; 20 zeolites, zeotypes &lt; 14.2 </p><p>activated carbon 6 </p><p>[a] Bimodal pore-size distribution.</p></li><li><p>REVIEWSMesoporous Materials</p><p>Angew. Chem. Int. Ed. 1999, 38, 56 77 59</p><p>2.1. Pure Silicate Composition</p><p>2.1.1. Liquid Crystal Templating Mechanism</p><p>The original M41S family of mesoporous molecular sieveswas synthesized, in general, by the combination of appropri-ate amounts of a silica source (e.g. tetraethylorthosilicate(TEOS), Ludox, fumed silica, sodium silicate), an alkyltrime-thylammonium halide surfactant (e.g. cetyltrimethylammoni-um bromide (CTAB)), a base (e.g. sodium hydroxide ortetramethylammonium hydroxide (TMAOH)), and water.Aluminosilicate M41S was synthesized by the addition of analuminum source to the synthesis mixture. The mixture wasaged at elevated temperatures (100 8C) for 24 to 144 hours,which resulted in a solid precipitate. The organic inorganicmesostructured product was filtered, washed with water, andair-dried. The product was calcined at about 500 8C under aflowing gas to burn off the surfactant, to yield the mesoporousmaterial.</p><p>A liquid crystal templating (LCT) mechanism wasproposed by the Mobil researchers, based on the similaritybetween liquid crystalline surfactant assemblies (i.e. , lyotrop-ic phases) and M41S.[1] The common traits were the meso-structure dependence on the hydrocarbon chain length of thesurfactant tail group,[11] the effect of variation of the surfactantconcentrations, and the influence of organic swelling agents.With MCM-41 (which has hexagonally packed cylindricalmesopores) as the representative M41S material, two mech-anistic pathways were postulated by the Mobil researchers(Scheme 2):1) The aluminosilicate precursor species occupied the space</p><p>between a preexisting hexagonal lyotropic liquid crystal(LC) phase and deposited on the micellar rods of the LCphase.</p><p>2) The inorganics mediated, in some manner, the ordering ofthe surfactants into the hexagonal arrangement.</p><p>In either case, the inorganic components (which werenegatively charged at the high pH values used) preferentiallyinteracted with the positively charged ammonium headgroups of the surfactants and condensed into a solid,continuous framework. The resulting organic inorganicmesostructure could be alternatively viewed as a hexagonalarray of surfactant micellar rods embedded in a silica matrix;removal of the surfactants produced the open, mesoporousMCM-41 framework. It is now known that pathway 1 did nottake place because the surfactant concentrations used were</p><p>far below the critical micelle concentration (CMC) requiredfor hexagonal LC formation.[12] This mechanistic pathway wasshown possible recently under different synthesis conditions(see Section 2.1.5).</p><p>The second mechanistic pathway of LCT was vaguelypostulated as a cooperative self-assembly of the ammoniumsurfactant and the silicate precursor species below the CMC.It has been known that no preformed LC phase was necessaryfor MCM-41 formation but, to date, the actual details ofMCM-41 formation have not yet been fully agreed upon.Several mechanistic models have been advanced which sharethe basic idea that the silicate species promoted LC phaseformation below the CMC.</p><p>2.1.1.1. Silicate Rod Assembly</p><p>Davis and co-workers[13] found that the hexagonal LC phasedid not develop during MCM-41 synthesis, based on in situ 14NNMR spectroscopy. They proposed that, under the synthesisconditions reported by Mobil, the formation of MCM-41began with the deposition of two to three monolayers ofsilicate precursor onto isolated surfactant micellar rods(Scheme 3). The silicate-encapsulated rods were randomly</p><p>Scheme 3. Assembly of silicate-encapsulated rods (adapted fromref. [13]).</p><p>ordered, eventually packing into a hexagonal mesostructure.Heating and aging then completed the condensation of thesilicates into the as-synthesized MCM-41 mesostructure.</p><p>2.1.1.2. Silicate Layer Puckering</p><p>Instead of the formation of silicate-covered micellar rods,Steel et al.[14] postulated that surfactant molecules assembleddirectly into the hexagonal LC phase upon addition of thesilicate species, based on 14N NMR spectroscopy. The silicateswere organized into layers, with rows of the cylindrical rodsintercalated between the layers (Scheme 4). Aging the</p><p>Scheme 2. Two possiblepathways for the LCTmechanism.[1b]</p></li><li><p>REVIEWS J. Y. Ying et al.</p><p>60 Angew. Chem. Int. Ed. 1999, 38, 56 77</p><p>Scheme 4. Puckering of silicate layers in the direction shown (adaptedfrom ref. [14]).</p><p>mixture caused the layers to pucker and collapse around therods, which then transformed into the surfactant-containingMCM-41 hexagonal-phase mesostructure.</p><p>2.1.1.3. Charge Density Matching</p><p>A charge density matching mechanistic model wasproposed by Monnier et al.[15] and Stucky et al.[16] andsuggested that MCM-41 could be derived from a lamellarphase. The initial phase of the synthesis mixture was layered(as detected by X-ray diffractometry (XRD)), and wasformed from the electrostatic attraction between the anionicsilicates and the cationic surfactant head groups (Scheme 5).</p><p>Scheme 5. Curvature induced by charge density matching.[15] The arrowindicates the reaction coordinate.</p><p>As the silicate species began to condense, the charge densitywas reduced. Accompanying this process, curvature wasintroduced into the layers to maintain the charge densitybalance with the surfactant head groups, which transformedthe lamellar mesostructure into the hexagonal mesostructure.</p><p>2.1.1.4. Folding Sheets</p><p>The lamellar-to-hexagonal phase motif also appeared inmaterials called FSM prepared from the intercalation of theammonium surfactant in kanemite, a type of hydrated sodiumsilicate composed of single-layered silica sheets.[17] After thesurfactants were ion-exchanged into the layered structure, thesilicate sheets were thought to fold around the surfactants andcondense into a hexagonal mesostructure (Scheme 6). The</p><p>Scheme 6. Folding of silicate sheets around intercalated surfactant mole-cule...</p></li></ul>