MICROPOROUS and mesoporous inorganic solids (with pore diameters of ≤20 A and ∼20–500 A respectively)1 have found great utility as catalysts and sorption media because of their large internal surface area. Typical microporous materials are the crystalline framework solids, such as zeolites2, but the largest pore dimensions found so far are ∼10–12 A for some metallophosphates3–5 and ∼14 A for the mineral cacoxenite6. Examples of mesoporous solids include silicas7 and modified layered materials8–11, but these are invariably amorphous or paracrystalline, with pores that are irregularly spaced and broadly distributed in size8,12. Pore size can be controlled by intercalation of layered silicates with a surfactant species9,13, but the final product retains, in part, the layered nature of the precursor material. Here we report the synthesis of mesoporous solids from the calcination of aluminosilicate gels in the presence of surfactants. The material14,15 possesses regular arrays of uniform channels, the dimensions of which can be tailored (in the range 16 A to 100 A or more) through the choice of surfactant, auxiliary chemicals and reaction conditions. We propose that the formation of these materials takes place by means of a liquid-crystal 'templating' mechanism, in which the silicate material forms inorganic walls between ordered surfactant micelles.

Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism

Use of amphiphilic triblock copolymers to direct the organization of polymerizing silica species has resulted in the preparation of well-ordered hexagonal mesoporous silica structures (SBA-15) with uniform pore sizes up to approximately 300 angstroms. The SBA-15 materials are synthesized in acidic media to produce highly ordered, two-dimensional hexagonal (space group p6mm) silica-block copolymer mesophases. Calcination at 500°C gives porous structures with unusually large interlattice d spacings of 74.5 to 320 angstroms between the (100) planes, pore sizes from 46 to 300 angstroms, pore volume fractions up to 0.85, and silica wall thicknesses of 31 to 64 angstroms. SBA-15 can be readily prepared over a wide range of uniform pore sizes and pore wall thicknesses at low temperature (35° to 80°C), using a variety of poly(alkylene oxide) triblock copolymers and by the addition of cosolvent organic molecules. The block copolymer species can be recovered for reuse by solvent extraction with ethanol or removed by heating at 140°C for 3 hours, in both cases, yielding a product that is thermally stable in boiling water.

Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores

The synthesis, characterization, and proposed mechanism of formation of a new family of silicatelaluminosilicate mesoporous molecular sieves designated as M41S is described. MCM-41, one member of this family, exhibits a hexagonal arrangement of uniform mesopores whose dimensions may be engineered in the range of - 15 A to greater than 100 A. Other members of this family, including a material exhibiting cubic symmetry, have ken synthesized. The larger pore M41S materials typically have surface areas above 700 m2/g and hydrocarbon sorption capacities of 0.7 cc/g and greater. A templating mechanism (liquid crystal templating-LCT) in which surfactant liquid crystal structures serve as organic templates is proposed for the formation of these materials. In support of this templating mechanism, it was demonstrated that the structure and pore dimensions of MCM-41 materials are intimately linked to the properties of the surfactant, including surfactant chain length and solution chemistry. The presence of variable pore size MCM-41, cubic material, and other phases indicates that M41S is an extensive family of materials.

http://dns2.asia.edu.tw/~ysho/ysho-english/1000 wc/pdf/j ame che soc114, 10834.pdf

A new family of mesoporous molecular sieves prepared with liquid crystal templates

The long-standing challenge of designing and constructing new crystalline solid-state materials from molecular building blocks is just beginning to be addressed with success. A conceptual approach that requires the use of secondary building units to direct the assembly of ordered frameworks epitomizes this process: we call this approach reticular synthesis. This chemistry has yielded materials designed to have predetermined structures, compositions and properties. In particular, highly porous frameworks held together by strong metal-oxygen-carbon bonds and with exceptionally large surface area and capacity for gas storage have been prepared and their pore metrics systematically varied and functionalized.

/pdf/reticular-synthesis-and-the-design-of-new-materials-lazbx7tdux.pdf

Reticular synthesis and the design of new materials

Polymer/layered silicate nanocomposites: a review from preparation to processing

In this letter we describe a novel family of aluminophosphate-based molecular sieves denoted as VPI-5. These molecular sieves are the first to contain rings consisting of greater than twelve tetrahedral atoms. The large pores of the VPI-5 materials are long channels circumscribed by rings containing eighteen tetrahedral atoms and possess free diameters of approximately 12-13 A.

A molecular sieve with eighteen-membered rings

New nanocomposites are currently being developed that have real industrial potential—loading a polymer with a small (ca. 5 %) amount of inorganic nanoparticles (the Figure shows a polypropylene–clay hybrid composite) is expected to give rise to significant improvement to the materials properties with only a minor increase in cost. Recent developments in this area are summarized here.

Polymeric Nanocomposites for Automotive Applications

IN the rapidly developing area of advanced zeolite materials science, the development of novel optical, electronic and magnetic materials1–5 requires the synthesis of large single crystals of a well defined habit. Crystals of a specified size and shape are needed to fabricate membranes and sensor devices6,7. Large crystals of zeol-ites and molecular sieves are essential for refining structures and elucidating the intrinsic absorption and diffusion properties of these materials8,9. Here we present a method for growing uniformly large and morphologically well defined single crystals of all-silica, aluminosilicate and aluminophosphate molecular sieves with dimensions in the size range 0.4—5.0 mm. Our synthetic method relies on the use of a non-aqueous solvent, a mineralizer, an optional organic templating species and reagent amounts of water. We have prepared both large-pore and small-pore materials of various framework composition and structure type.

Juan M. Garces

Papers

A molecular sieve with eighteen-membered rings

Polymeric Nanocomposites for Automotive Applications

Non-aqueous synthesis of giant crystals of zeolites and molecular sieves

Polyethylene-layered silicate nanocomposites prepared by the polymerization-filling technique: synthesis and mechanical properties

Metamorphic materials: Restructuring siliceous mesoporous materials*