Charles T. Kresge

Formation of Hollow Helicoids in Mesoporous Silica: Supramolecular Origami

In the past, helical shapes in nature have inspired inventions such as the water screw for agriculture, the retaining screw for wine presses, and architectural designs for spiral staircases. Similarly, these days helix-shaped DNA, proteins and carbon nanotubes evoke great interest in biotechnology and nanotechnology. Also biomimetic synthesis of helical morphologies of calcium carbonate, barium sulfate, and silica provides insight into morphogenesis of mineralized spiral forms in biology and ideas for new opportunities in materials science. Herein we describe the synthesis of hollow helicoids made of hexagonal mesoporous silica, a remarkable topology in the materials world. They have a hierarchical architecture comprised of 5 nm diameter channels that coil in the form of a micrometerscale tubular spiral. A population analysis of helicoid shapes defines a surprisingly narrow distribution of pitch and flute widths, pitch angles, inside and outside diameters, and significantly an equal number of leftand right-handed forms. Evidence is presented that morphogenesis involves polymerization-induced differential contraction of a patch of hexagonal silicate liquid-crystal film formed at the air± water interface, which can fold into a hollow helicoid. A supramolecular Origami theoretical model explains the creation and observed narrow distribution of mesoporous silica, hollow helicoid shapes. Mesoporous silica hollow helicoids were prepared by using cetyltrimethylammonium chloride (CTACl) as the surfactant micellar template and tetraethylorthosilicate (TEOS) as the silica precursor. An aqueous solution of CTACl, hydrochloric acid and formamide was aged for 48 h before adding TEOS, and the material was formed after 3 days in a quiescent state. The use of formamide in the synthesis is intentional because upon acid hydrolysis it yields ammonium chloride and formic acid to give an ultimate solution ca. pH 1.9 and an ionic strength that favors hollow helicoid formation. This solution pH is notably higher than the one used in the synthesis of mesoporous silica curved shapes. Control experiments demonstrate that a high concentration of ammonium and formate ions is essential for the formation of mesoporous silica at a pH close to two, which borders on the isoelectric point of aqueous silica. We believe that a low acidity and high ionic strength medium favor a slow rate of silicification, and hence polymerization-induced differential contraction of silicate micelle rods in a patch of silicate liquid-crystal film formed at the air±water interface becomes influential in hollow helicoid formation. Powder X-ray diffraction (PXRD) patterns in Figure 1 clearly define as-synthesized and calcined

THE ROLE OF DEFECTS IN THE FORMATION OF MESOPOROUS SILICA FIBERS, FILMS, AND CURVED SHAPES

The main goal of this research news article is to describe how different kinds of topological defects that exist in a silicate liquid crystal seed can initiate and direct the growth of particular forms of mesoporous silica. This avenue of investigation emerged from the results of scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and polarized optical microscopy (POM) studies of hexagonal mesoporous silica fibers, films, and curved shapes, which delineated the essential relations between synthesis conditions, morphology, bulk and surface mesostructure, and optical birefringence textures.[1–3] While SEMs of faceted mesoporous silica first appeared in 1992,[4] the recognition, understanding, and significance of morphogenesis of mesoporous silica with curved shapes emerged in a series of papers from our laboratory.

Morphokinetics: Growth of Mesoporous Silica Curved Shapes

52 O WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/0101-0052

Raman spectroscopic studies of the synthesis of faujasitic zeolites: Comparison of two silica sources

17.50+.50/0 Adv. Mater. 1999, 11, No. 1 [3] K. C. Frisch, Rubber Chem. Technol. 1980, 126. [4] O. Bayer, E. Muller, S. Petersen, H. F. Piepenbrink, E. Windemuth, Angew. Chem. 1950, 62, 57. [5] A. J. Varma, M. D. Deshpande, V. M. Nadkarni, Angew. Makromol. Chem. 1985, 132, 203. [6] S. Demharter, J. Rosch, R. Mulhaupt, Polym. Bull. 1993, 31, 421. [7] R. Mulhaupt, J. Rosch, S. Demharter, J. de Phys. IV, Colloque C7, supplement au J. de Phys. III, 1993, 3, 1519. [8] M. A. Krenceski, H.J. Cantow, R. Mulhaupt, Polym. Mater. Sci. Eng. 1993, 70, 356. [9] G. Lagaly, in Development in Ionic Polymers, (Eds: A. D. Wilson, H. T. Posser), Applied Science, London 1986, Ch. 2, p. 77. [10] A. Akelah, in Polymers and Other Advanced Materials (Eds: N. Prasad, J. E. Mark, T. J. Fai ), Plenum, New York 1995, p. 611. [11] T. J. Pinnavaia, T. Lan, Z. Wang, H. Shi, P. D. Kaviratna, in Nanotechnology (Eds: G.-M. Chow, K. E. Gonsalves), American Chemical Society, Washington, DC 1996, ACS Symposium Ser. 622, p. 251. [12] E. P. Giannelis, Adv. Mater. 1996, 8, 29. [13] D. Smock, Mod. Plastics Int. 1998, Feb., 28. [14] R. J. Janoski, US Pat. 138562, 1995, assigned to Tremco, Inc. [15] T. M. Garrett, I. Gruzins, US Pat. 587038, 1997, assigned to MCP Industries, Inc. [16] S. Miyanaga, Y. Tsunoda, Jpn. Kokai Tokkyo Koho, JP 09255747 1997, assigned to Kao Corp. [17] A. Akelah, N. Salahuddin, A. Hiltner, E. Baer, A. Moet, Nanostructured Mater. 1994, 4, 965.

The discovery of ExxonMobil's M41S family of mesoporous molecular sieves

In this study, we have examined the influence of two common commercial silica sources, Ludox and N-Brand, on the formation of faujasitic zeolites. The preparation of zeolite X is readily accomplished from both these sources. The Raman spectra indicate that the gel preceding the formation of crystals is composed of a solid phase with disordered four-membered aluminosilicate rings and soluble monomeric and dimeric silicate species. The Ludox preparation has a more rapid crystallization rate than does N-Brand, and zeolite X crystals are observed in the Raman spectrum at earlier times than by X-ray diffraction, indicating more extensive nucleation. Synthesis of zeolite Y was only possible with Ludox, whereas preparation with N-Brand as the silica source and identical in composition as that of Ludox led to the formation of zeolite P. Raman spectroscopy in this composition indicated that the structure of the solid phase in the initial stages of aging is different for the Ludox and N-Brand preparations of zeolite Y. At later stages, the spectra are dominated in both cases by soluble silicate species. It is possible to direct the N-Brand preparation to zeolite Y either by addition of tetramethylammonium ions or by creating conditions under which zeolite X nuclei can form in the initial stage

Salted mesostructures : salt-liquid crystal templating of lithium triflate-oligo (ethylene oxide) surfactant-mesoporous silica nanocomposite films and monoliths

Publisher Summary This chapter discusses the discovery of ExxonMobils M41S family of mesoporous molecular sieves. The quest for new molecular sieves in the late 1980s led a team of mobil researchers to the discovery of a family of nanostructured mesoporous materials known as “M41S.” Mobil composition of matter (MCM)-41, the hexagonal phase, is undoubtedly the best known and most widely studied of this family of materials. Other discrete members of the M41S family are the cubic (MCM-48) and the lamellar (MCM-50) forms. Each is synthesized via a counterion initiated self assembled liquid crystal mechanism, involving oxide precursors, which form an inorganic equivalent to a liquid crystal-micelle structure. This manuscript describes the events that led to the discovery of M41S materials. It also summarizes the supporting characterization and mechanistic studies that led to a picture of how these materials are actually formed. The mechanistic and characterization studies involved many researchers from ExxonMobils Paulsboro and Princeton laboratories. ExxonMobil has very recently scaled-up the synthesis and commercialized MCM-41 for an undisclosed application.

Characterization of mesoporous molecular sieves: differences between M41s and pillared layered zeolites

A lyotropic lithium triflate-silicate liquid crystal is utilized as a supramolecular template for a ‘one-pot’ synthesis of a novel ionically conducting nanocomposite material, denoted meso-SiO 2 -C 12 (EO) 10 OH-LiCF 3 SO 3 , in the form of a film or monolith. In this structure Li + ions interact, in a crown ether-like fashion, with oligo(ethylene oxide) head groups of a non-ionic surfactant assembly that is imbibed within the channels of hexagonal mesoporous silica. Details of the acid catalyzed polymerization of the silicate oligo(ethylene oxide) surfactant co-assembly in the presence of lithium triflate have been investigated using polarized optical microscopy (POM), powder X-ray diffraction (PXRD), multinuclear nuclear magnetic resonance (NMR) spectroscopy and impedance spectroscopy. Insight into the incorporation of Li + and CF 3 SO 3 – ions into meso-SiO 2 -C 12 (EO) 10 OH-LiCF 3 SO 3 was obtained using NMR and Fourier transform (FT) Raman spectroscopy. Collectively the results indicate that lithium ions coordinate to oxygens of the oligo(ethylene oxide) head group, maintain the structural integrity of both the silicate liquid crystal and templated mesoporous silica, and are essentially completely dissociated with respect to the triflate counteranion. ac Impedance spectroscopy, which bodes well for their use in the fields of polymer electrolytes and battery technology for meso-SiO 2 -C 12 (EO) 10 OH-LiCF 3 SO 3 have demonstrated high ionic conductivities at room temperature measurements. Salt-liquid crystal templating may offer a general approach for synthesizing diverse kinds of salted mesostructures including redox active transition metal complexes, which may be reductively-agglomerated to form metal cluster-based meso-SiO 2 -C 12 (EO) 10 OH-M n or sulfided with H 2 S to produce semiconductor cluster-based meso-SiO 2 -C 12 (EO) 10 OH-(MS) n nanocomposites.

Method for preparing a pillared layered oxide material

Abstract Two major classes of mesoporous molecular sieves are currently of great interest: M41S materials prepared by direct synthesis and pillared materials synthesized by swelling and/or pillaring of layered solids. The pillared sieves derived from layered zeolite precursors and represented by MCM-36 complement the extensively studied M41S class and are of interest due to their much stronger acid activity. Both types are prepared under similar conditions (high pH, presence of surfactants) and share some common characteristics. This presents a problem of differentiating between them, especially because of possible contamination of the pillared product with M41S and the parent zeolite. The pillared zeolite case is particularly critical and requires proof that mesoporosity is not the result of M41S impurity. The typical methods employed to characterize both classes of mesoporous materials are X-ray diffraction, microscopy, static and dynamic adsorption/desorption techniques and catalytic testing. No single determination appears to provide an unambiguous answer concerning purity of the pillared phase. The absence of significant contaminants and identity of the product can be determined by combining results from all the mentioned techniques. The evidence of mesoporosity and pillaring are found by static sorption measurements and X-ray diffraction, respectively. Sorption isotherms and dynamic sorption in conjunction with microscopic images provide further indications of successful exfoliation of the layered zeolite precursor.

The Synthesis and Properties of M41S and Related Mesoporous Materials

There is provided a method for preparing a pillared layered material, designated MCM-36, with a characteristic X-ray diffraction pattern. Upon calcination of the swollen, non-pillared form of this material, the layers collapse and condense upon one another in a somewhat disordered fashion to form a non-swellable material. However, when the swollen layered material is intercalated with polymeric oxide pillars, the layer separation is maintained, even after calcination. A quaternary ammonium silicate, such as tetramethylammonium silicate, is used as a pillaring agent for treating the swollen material.

Use of silica gels and mesoporous molecular sieves as supports for the solid-phase Claisen rearrangement.

Microporous and mesoporous inorganic materials form the backbone of many heterogeneous catalysts and separations media. Because of the extensive commercial applications of these classes of materials, substantial efforts on the part of both academic and industrial researchers have been made to unlock the hidden secrets of the mechanisms of their formation and, through the exploitation of this understanding, to synthesize novel materials with important properties. Much of the mechanistic work has focused on understanding the role of organic directing or templating agents which play a complex, cooperative role of spatial ordering through the filling of void space, balancing charge, and stabilizing structural units.