Ordered mesoporous materials in catalysis

Porous carbon materials are of interest in many applications because of their high surface area and physicochemical properties. Conventional syntheses can only produce randomly porous materials, with little control over the pore-size distributions, let alone mesostructures. Recent breakthroughs in the preparation of other porous materials have resulted in the development of methods for the preparation of mesoporous carbon materials with extremely high surface areas and ordered mesostructures, with potential applications as catalysts, separation media, and advanced electronic materials in many scientific disciplines. Current syntheses can be categorized as either hard-template or soft-template methods. Both are examined in this Review along with procedures for surface functionalization of the carbon materials obtained.

Mesoporous Carbon Materials: Synthesis and Modification

The discovery of periodic mesoporous MCM-41 and related molecular sieves has attracted significant attention from a fundamental as well as applied perspective. They possess well-defined cylindrical/hexagonal mesopores with a simple geometry, tailored pore size, and reproducible surface properties. Hence, there is an ever-growing scientific interest in the challenges posed by their processing and characterization and by the refinement of various sorption models. Further, MCM-41-based materials are currently under intense investigation with respect to their utility as adsorbents, catalysts, supports, ion-exchangers, and molecular hosts. In this article, we provide a critical review of the developments in these areas with particular emphasis on adsorption characteristics, progress in controlling the pore sizes, and a comparison of pore size distributions using traditional and newer models. The model proposed by the authors for adsorption isotherms and criticalities in capillary condensation and hysteresis is found to explain unusual adsorption behavior in these materials while providing a convenient characterization tool.

Recent advances in processing and characterization of periodic mesoporous MCM-41 silicate molecular sieves

This review paper deals with proven and potential applications of mesoporous molecular sieves in catalysis. In addition to introduction and conclusion, the text is divided into two parts, respectively, dedicated to the design of solid catalysts and catalyst supports and to some relevant examples of catalytic processes.

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Perspectives in catalytic applications of mesostructured materials

Three different water-alcohol cosolvent systems were used to assemble mesoporous molecular sieve silicas with wormhole framework structures (previously denoted HMS silicas) from an electrically neutral amine surfactant (S{degree}) and a silicon alkoxide precursor (I{degree}). The fundamental particle size and associated textural (interparticle) porosity of the disordered structures were correlated with the solubility of the surfactant in the water-alcohol cosolvents used for the S{degree}I{degree} assembly process. Polar cosolvents containing relatively low volume fractions of C{sub n}H{sub 2n+1}OH alcohols (n = 1--3) gave heterogeneous surfactant emulsions that assembled intergrown aggregates of small primary particles with high textural pore volumes (designated HMS-HTx). Conversely, three-dimensional, monolithic particles with little or no textural porosity (designated HMS-LTx) were formed from homogeneous surfactant solutions in lower polarity cosolvents. Aluminum substituted AL-HMS-HTx analogues with high textural porosity and improved framework accessibility also were shown to be much more efficient catalysts than AL-HMS-LTx or monolithic forms of hexagonal AL-MCM-41 for the sterically demanding condensed phase alkylation of 2,4-di-tert-butylphenol with cinnamyl alcohol. Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) studies verified the textural differences between wormhole HMS and electrostatically assembled hexagonal MCM-41 and SBA-3 molecular sieves. Power law fits to the scattering data indicated a surface fractal (D{submore » s} = 2.76) for HMS-HTx, consistent with rough surfaces. A second power law at lower-q indicated the formation of a mass fractal (D{sub m} = 1.83) consistent with branching of small fundamental particles. Hexagonal MCM-41 and SBA-3 silicas, on the other hand, exhibited scattering properties consistent with moderately rough surfaces (D{sub s} = 2.35 and 2.22, respectively) and large particle diameters ({much{underscore}gt}1 {micro} m). HMS-LTx silicas showed little or no mass fractal character (D{sub m} = 2.87), and no surface fractal scattering.« less

Textural Mesoporosity and the Catalytic Activity of Mesoporous Molecular Sieves with Wormhole Framework Structures

Water:ethanol solvent mixtures of differing polarity have been used to tailor the framework and textural mesopores of HMS molecular sieve silicas through an electrically neutral (S°I°) assembly pathway (S° = dodecyl- or tetradecylamine; I° = tetraethyl orthosilicate). Mesostructure assembly from a water-rich solvent mixture, water:ethanol = 90:10 (v/v), afforded wormhole-like framework structures with a complementary textural pore volume equal in magnitude to the framework pore volume. An ethanol-rich mixture, water:ethanol = 35:65 (v/v), also formed wormhole-like frameworks, but the textural porosity was less than 20% of the framework pore volume. HMS derivatives with high textural porosity were composed of mesoscale fundamental particles that aggregate into larger particles. In contrast, HMS mesostructures with low textural porosity were assembled into much larger aggregates of macroscale spheroid to disk-shaped fundamental particles. The differences in particle textures were attributed to differences i...

and Thomas R. Pauly

Papers

Tailoring the framework and textural mesopores of HMS molecular sieves through an electrically neutral (S°I°) assembly pathway