Synthesis and Applications of Supramolecular-Templated

Synthesis and Applications of Supramolecular-Templated

(Parte 1 de 7)

As exemplified by Pdgrafted ultra-large pore silicate Pd-TMS11 (TEM image presented above), supramolecular-templated mesoporous materials find many uses, for example (starting at the left and going clockwise) in polymerization, for immobilization of well-defined homogeneous catalysts, in Heck catalysis, in acid catalysis, and as an encapsulation host for ferrocenyl units.

Only six years have passed since the exciting discovery of the novel family of molecular sieves called M41S was reported by the researchers at Mobil Research and Development Corporation.[1] These mesoporous (alumino)silicate materials, with well-defined pore sizes of 15–100 , break past the pore-size constraint (<15 ) of microporous zeolites. The extremely high surface areas (>1000 m2gÿ1) and the precise tuning of pore sizes are among the many desirable properties that have made such materials the focus of great interest. The M41S materials also ushered in a new approach in materials synthesis where, instead of the use of single molecules as templating agents (as in the case of zeolites), self-assembled molecular aggregates or supramolecular assemblies are employed as the structure-directing agents. Various aspects of M41S and related mesoporous materials have been reviewed by Brinker (recent advances in porous inorganic materials),[2] Vartuli et al. (synthesis of the M41S family),[3] Stucky et al. (biomimetic synthesis of mesoporous materials),[4] Raman et al. (porous silicates templated by surfactants and organo- silicate precursors),[5] Antonelli and Ying (mesoporous transition metal oxide (TMS) family),[6] Behrens (mesoporous transition metal oxides),[7] Zhao et al. (aluminosilicate MCM- 41),[8] and Sayari (catalytic applications of MCM-41).[9] Here we review the current state of mesoporous materials research from the standpoint of compositional control. The synthesis of not only (alumino)silicate M41S materials but also transition metal doped and pure transition metal oxide mesoporous materials is addressed. After discussion of the major synthesis routes and mechanisms of formation, the applications of these materials are surveyed, with particular emphasis on heterogeneous catalysis.

Inorganic solids that contain pores with diameters in the size range of 20–500 are considered mesoporous materials, according to IUPAC definition. Examples of mesoporous materials include M41S, aerogels, and pillared layered structures, as listed in Table 1.[10] In this paper, we will focus only on “mesoporous materials” that have been prepared by supramolecular 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 structures can be microporous (see Section 2.4). Mesoporous materials derived by means of surfactant templating are occasionally called “zeolites” and described as “crystalline” materials in reference to their long-range ordering of the pore packing. Such references are not correct, as the pore walls of these materials are amorphous and lack long-range order.

Synthesis and Applications of Supramolecular-Templated Mesoporous Materials**

Jackie Y. Ying,* Christian P. Mehnert, and Michael S. Wong

Research in supramolecular-templated mesoporous materials began in the early 1990s with the announcement of MCM-41 and the M41S family of molecular sieves. These materials are highly unusual in their textural characteristics: uniform pore sizes greater than 20 , surface areas in excess of 1000 m2gÿ1, and long-range ordering of the packing of pores. The mesoporous materials are derived with supramolecular assemblies of surfactants, which template the inorganic components during synthesis. Many researchers have since exploited this technique of supramolecular templating to produce materials with different compositions, new pore systems, and novel properties. This article reviews the current state of the art in mesoporous materials research in three general areas: synthesis, catalytic properties, and other applications that take advantage of bulk morphologies. Various mechanisms formulated to explain the formation of mesostructures are discussed in the context of compositional control. The catalytic applications of mesoporous materials are examined, with a significant fraction based on recent patent literature. Other directions in the utilization of mesoporous materials are also presented.

Keywords: amphiphiles · heterogeneous catalysis · mesoporosity · molecular sieves · template synthesis

Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, MA 02139 (USA) Fax: ( 1)617-258-5766 E-mail: jyying@mit.edu

[**] A list of abbreviations is provided in the Appendix.

REVIEWS J.Y. Ying et al.

2. Mechanisms of Mesostructure Formation

There have been a number of models proposed to explain the formation of mesoporous materials and to provide a rational basis for the various synthesis routes. On the most common level, these models are predicated upon the presence of surfactants in a solution to guide the formation of the inorganic mesostructure from the solubilized inorganic precursors (Scheme 1). Surfactants contain a hydrophilic head

Scheme 1. Schematic representation of the general formation of MCM-41 from inorganic precursors and organic surfactants.

group and a long hydrophobic tail group within the same molecule and will self-organize in such a way as to minimize contact between the incompatible ends. How the inorganic precursor interacts with the surfactant is the issue whereby the models diverge; the type of interaction between the surfactant and the inorganic precursor will be seen as a significant difference among the various synthesis routes, the formation models, and the resulting classes of mesoporous materials.

Jackie Y. Ying was born in Taiwan and raised in Singapore. She graduated summa cum laude from The Cooper Union in 1987, and completed her Ph.D. degree as an AT&T Bell Laboratories Scholar at Princeton University in 1991. She was a NSF-NATO Postdoctoral Fellow and Alexander von Humboldt Research Fellow at the University of Saarland with Prof. H. Gleiter, and she currently holds the St. Laurent Associate Professorship at the Chemical Engineering Department of the Massachusetts Institute of Technology. Her research is focused on the synthesis of nanostructured inorganic materials for catalytic, membrane, and advanced ceramic applications. She is the author of over eighty publications, and the recipient of the American Ceramic Society Ross C. Purdy Award, David and Lucile Packard Fellowship, National Science Foundation and Office of Naval Research Young Investigator Awards, Camille Dreyfus Teacher-Scholar Award, Royal Academy of Engineering ICI Faculty Fellowship, and American Chemical Society Faculty Fellowship Award in Solid-State Chemistry. She serves on the Board of Directors of the Alexander von Humboldt Association of America as well as the editorial boards of several journals.

Christian P. Mehnert studied chemistry at the Technische Universität München (1985–1992). During this time he carried out research with Dr. D. O Hare at Oxford University (1990) and Prof. J. Lewis at Cambridge University (1991), before he completed 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 Oxford University with Prof. M. L. H. Green to work on the synthesis of new organometallic complexes with metal vapor synthesis techniques and the investigation of the reaction chemistry of highly Lewis acidic perfluorinated borane complexes. After receiving his doctorate (D.Phil.) as a student at Balliol College (1996), he moved to the United States and joined Prof. J. Y. Ying s laboratory at the Massachusetts Institute of Technology as a postdoctoral research associate. The area of his current research is the synthesis and application of new nanostructured materials for catalysis, with a special focus on the incorporation of organometallic catalysts into mesoporous molecular sieves.

Michael S. Wong is a Ph.D. student in Chemical Engineering at the Massachusetts Institute of Technology. He received a B.S. degree in Chemical Engineering from the California Institute of Technology (1990–1994) and a M.S. degree in Chemical Engineering Practice from the Massachusetts Institute of Technology (1997). His research interests include the synthesis and characterization of nanostructured zirconia-based material for acid catalytic applications.

J. Y. Ying C. P. Mehnert M. S. Wong

Table 1. Pore-size regimes and representative porous inorganic materials.

Pore-size regimes Definition Examples Actual size range

[a] Bimodal pore-size distribution.

REVIEWSMesoporous Materials

2.1. Pure Silicate Composition

2.1.1. Liquid Crystal Templating Mechanism

The original M41S family of mesoporous molecular sieves was synthesized, in general, by the combination of appropriate amounts of a silica source (e.g. tetraethylorthosilicate (TEOS), Ludox, fumed silica, sodium silicate), an alkyltrimethylammonium halide surfactant (e.g. cetyltrimethylammonium bromide (CTAB)), a base (e.g. sodium hydroxide or tetramethylammonium hydroxide (TMAOH)), and water. Aluminosilicate M41S was synthesized by the addition of an aluminum source to the synthesis mixture. The mixture was aged at elevated temperatures ( 1008C) for 24 to 144 hours, which resulted in a solid precipitate. The organic–inorganic mesostructured product was filtered, washed with water, and air-dried. The product was calcined at about 5008C under a flowing gas to burn off the surfactant, to yield the mesoporous material.

A “liquid crystal templating” (LCT) mechanism was proposed by the Mobil researchers, based on the similarity between liquid crystalline surfactant assemblies (i.e., lyotropic phases) and M41S.[1] The common traits were the mesostructure dependence on the hydrocarbon chain length of the surfactant tail group,[1] the effect of variation of the surfactant concentrations, and the influence of organic swelling agents. With MCM-41 (which has hexagonally packed cylindrical mesopores) as the representative M41S material, two mechanistic pathways were postulated by the Mobil researchers (Scheme 2): 1) The aluminosilicate precursor species occupied the space between a preexisting hexagonal lyotropic liquid crystal (LC) phase and deposited on the micellar rods of the LC phase. 2) The inorganics mediated, in some manner, the ordering of the surfactants into the hexagonal arrangement. In either case, the inorganic components (which were negatively charged at the high pH values used) preferentially interacted with the positively charged ammonium head groups of the surfactants and condensed into a solid, continuous framework. The resulting organic–inorganic mesostructure could be alternatively viewed as a hexagonal array of surfactant micellar rods embedded in a silica matrix; removal of the surfactants produced the open, mesoporous MCM-41 framework. It is now known that pathway 1 did not take place because the surfactant concentrations used were far below the critical micelle concentration (CMC) required for hexagonal LC formation.[12] This mechanistic pathway was shown possible recently under different synthesis conditions (see Section 2.1.5).

The second mechanistic pathway of LCT was vaguely postulated as a cooperative self-assembly of the ammonium surfactant and the silicate precursor species below the CMC. It has been known that no preformed LC phase was necessary for MCM-41 formation but, to date, the actual details of MCM-41 formation have not yet been fully agreed upon. Several mechanistic models have been advanced which share the basic idea that the silicate species promoted LC phase formation below the CMC.

Davis and co-workers[13] found that the hexagonal LC phase did not develop during MCM-41 synthesis, based on in situ 14N NMR spectroscopy. They proposed that, under the synthesis conditions reported by Mobil, the formation of MCM-41 began with the deposition of two to three monolayers of silicate precursor onto isolated surfactant micellar rods (Scheme 3). The silicate-encapsulated rods were randomly

Scheme 3. Assembly of silicate-encapsulated rods (adapted from ref. [13]).

ordered, eventually packing into a hexagonal mesostructure. Heating and aging then completed the condensation of the silicates into the as-synthesized MCM-41 mesostructure.

2.1.1.2. Silicate Layer Puckering

Instead of the formation of silicate-covered micellar rods,

Steel et al.[14] postulated that surfactant molecules assembled directly into the hexagonal LC phase upon addition of the silicate species, based on 14N NMR spectroscopy. The silicates were organized into layers, with rows of the cylindrical rods intercalated between the layers (Scheme 4). Aging the

Scheme 2. Two possible pathways for the LCT mechanism.

REVIEWS J.Y. Ying et al.

Scheme 4. Puckering of silicate layers in the direction shown (adapted from ref. [14]).

mixture caused the layers to pucker and collapse around the rods, which then transformed into the surfactant-containing MCM-41 hexagonal-phase mesostructure.

2.1.1.3. “Charge Density Matching”

A “charge density matching” mechanistic model was proposed by Monnier et al.[15] and Stucky et al.[16] and suggested that MCM-41 could be derived from a lamellar phase. The initial phase of the synthesis mixture was layered (as detected by X-ray diffractometry (XRD)), and was formed from the electrostatic attraction between the anionic silicates and the cationic surfactant head groups (Scheme 5).

Scheme 5. Curvature induced by charge density matching. The arrow indicates the reaction coordinate.

As the silicate species began to condense, the charge density was reduced. Accompanying this process, curvature was introduced into the layers to maintain the charge density balance with the surfactant head groups, which transformed the lamellar mesostructure into the hexagonal mesostructure.

2.1.1.4. “Folding Sheets”

The lamellar-to-hexagonal phase motif also appeared in materials called FSM prepared from the intercalation of the ammonium surfactant in kanemite, a type of hydrated sodium silicate composed of single-layered silica sheets.[17] After the surfactants were ion-exchanged into the layered structure, the silicate sheets were thought to fold around the surfactants and condense into a hexagonal mesostructure (Scheme 6). The

Scheme 6. Folding of silicate sheets around intercalated surfactant molecules. a) Ion exchange, b) calcination.

final product was claimed to be very similar to MCM-41, with no resemblance to the original kanemite structure. However, Vartuli et al.[12] found that the layered structures were still retained in the kanemite-derived mesoporous materials.

2.1.1.5. “Silicatropic Liquid Crystals”

(Parte 1 de 7)

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