Thomas J. Pinnavaia

Polymer-layered silicate nanocomposites: an overview

An overview of polymer–clay hybrid nanocomposites is provided with emphasis placed on the use of alkylammonium exchanged smectite clays as the reinforcement phase in selected polymer matrices. A few weight percent loading of organoclay in nylon 6 boosts the heat distortion temperature by 80°C, making possible structural applications under conditions where the pristine polymer would normally fail. A similar loading of clay nanolayers in elastomeric epoxy and polyurethane matrices dramatically improves both the toughness and the tensile properties of these thermoset systems. Glassy epoxy nanocomposites exhibit substantial improvement in yield strength and modulus under compressive stress–strain conditions. The latest development in polypropylene hybrids have yielded nanocomposites with improved storage moduli. Polyimide hybrids in thin-film form display a 10-fold decrease in permeability toward water vapor at 2 wt.% clay loading. In situ and melt intercalation processing methods are effective in producing reinforced polystyrene hybrids. Nitrile rubber hybrids show improved storage moduli and reduced permeabilities even toward gases as small as hydrogen. Poly(e-caprolactone)–clay nanocomposites prepared by in situ polymerization of e-caprolactone in organoclay galleries show a substantial reduction in water adsorption. Polysiloxane nanocomposites produced from poly(dimethylsiloxane) and organoclay mixtures have improved in tensile properties, thermal stability and resistance to swelling solvents. Organoclay-poly(l-lactide) composite film was obtained by solvent casting technique. Clay nanolayers dispersed in liquid crystals act as structure directors and form hybrids composites that can be switched from being highly opaque to highly transparent by applying an electric field of short duration.

A Neutral Templating Route to Mesoporous Molecular Sieves

A neutral templating route for preparing mesoporous molecular sieves is demonstrated based on hydrogen-bonding interactions and self-assembly between neutral primary amine micelles (S�) and neutral inorganic precursors (l�). The S� l� templating pathway produces ordered mesoporous materials with thicker framework walls, smaller x-ray scattering domain sizes, and substantially improved textural mesoporosities in comparison with M41S materials templated by quaternary ammonium cations of equivalent chain length. This synthetic strategy also allows for the facile, environmentally benign recovery of the cost-intensive template by simple solvent extraction methods. The S� 1� templating route provides for the synthesis of other oxide mesostructures (such as aluminas) that may be less readily accessible by electrostatic templating pathways.

Templating of mesoporous molecular sieves by nonionic polyethylene oxide surfactants.

Mesoporous silica molecular sieves have been prepared by the hydrolysis of tetraethylorthosilicate in the presence of low-cost, nontoxic, and biodegradable polyethylene oxide (PEO) surfactants, which act as the structure-directing (templating) agents. This nonionic, surfactant-neutral, inorganic-precursor templating pathway to mesostructures uses hydrogen bonding interactions between the hydrophilic surfaces of flexible rod- or worm-like micelles and Si(OC2H5)4-x(OH)x hydrolysis products to assemble an inorganic oxide framework. Disordered channel structures with uniform diameters ranging from 2.0 to 5.8 nanometers have been obtained by varying the size and structure of the surfactant molecules. Metal-substituted silica and pure alumina mesostructures have also been prepared by the hydrolysis of the corresponding alkoxides in the presence of PEO surfactants. These results suggest that nonionic templating may provide a general pathway for the preparation of mesoporous oxides.

Intercalated Clay Catalysts

Recent advances in the intercalation of metal complex cations in smectite clay minerals are leading to the development of new classes of selective heterogeneous catalysts. The selectivity of both metal-catalyzed and proton-catalyzed chemical conversions in clay intercalates can often be regulated by controlling surface chemical equilibria, interlamellar swelling, or reactant pair proximity in the interlayer regions. Also, the intercalation of polynuclear hydroxy metal cations and metal cluster cations in smectites affords new pillared clay catalysts with pore sizes that can be made larger than those of conventional zeolite catalysts.

On the pillaring and delamination of smectite clay catalysts by polyoxo cations of aluminum

Abstract Two types of polyoxoaluminum solutions, base-hydrolyzed A1Cl 3 and aluminum chlorohydrate (ACH) with OH/A1 ratios of 2.42 and 2.50, respectively, have been used as reagents for the pillaring of the smectite clays montmorillonite and nontronite. Selective adsorption studies demonstrate that the method used to dry the flocculated clay layers is far more important than the choice of pillaring reagent or clay layer charge in determining the apparent pore size of pillared products. Air-drying leads to zeolite-like products with a pore opening ⪢6.2 A and 27 Al NMR spectroscopy indicates the polyoxo cation nuclearity to be larger in ACH than in base-hydrolyzed AlCl 3 , both reagents give pillared products with similar aluminum contents, pore sizes, thermal stabilities and catalytic properties. These similarities suggest the same type of oxo cations, probably Al 3 Keggin ions, are formed on the intracrystal surfaces. Also, the amount of Al bound per unit cell of pillared clay varies only over a small range (2.78 – 3.07), and the variation is not correlated with layer charge. This latter result suggests that a more or less uniform monolayer of hydrated polyoxo cations is formed in the interlayers, and that electrical neutrality is achieved through hydrolysis of the pillaring cations.

Biomimetic templating of porous lamellar silicas by vesicular surfactant assemblies

A biomimetic templating approach to the synthesis of lamellar silicas is demonstrated. The procedure is based on the hydrolysis and cross-linking of a neutral silicon alkoxide precursor in the interlayered regions of multilamellar vesicles formed from a neutral diamine bola-amphiphile. Unlike earlier surfactant-templating approaches, this method produces porous lamellar silicas (designated MSU-V) with vesicular particle morphology, exceptional thermal stability, a high degree of framework cross-linking, unusually high specific surface area and pore volume, and sorption properties that are typical of pillared lamellar materials. This approach circumvents the need for a separate pillaring step in building porosity into a lamellar host structure and offers new opportunities for the direct fabrication of adsorbents, catalysts, and nanoscale devices.

Porous clay heterostructures prepared by gallery templated synthesis

THE mesoporous molecular sieves developed recently1,2 have stimulated a great deal of interest in large-pore materials for selective catalysis3–5 and other applications6. The synthesis of these materials involves the use of ionic surfactants which interact with the inorganic ions in a cooperative process to form a range of ordered mesostructures7,8. Many of these same surfactants can be intercalated into layered inorganic materials9,10, and we explore here the possibility of using intercalated surfactants for the templated synthesis of structures within the interlayer spaces (galleries) of clays. The surfactants act in concert with aqueous silicate species to form a silica framework between the layers. Removal of the surfactant by calcination then leaves a mesoporous solid with thermally stable pores of widths in the range 14–22 Å. These materials, which we call porous clay heterostructures, might provide new opportunities for the rational design of heterogeneous catalysts.

Epoxy self-polymerization in smectite clays

Abstract The diglycidyl ether of bisphenol A, epoxy resin Epon-828, undergoes self-polymerization when heated with acidic onium ion exchanged forms of montmorillonite to form polyether-clay nanocomposites. Nanocomposite formation is accomplished by a dramatic transition from a gel state to a dry powder with a very low bulk density. XRD and TEM studies indicated that the final product contains a uniform dispersion of exfoliated 10 A thick clay plates separated by a few nanometers of polyether polymer, thus verifying the nanocomposite structure. DSC results showed that two exothermic catalytic processes occur during the reaction. The lower temperature process is attributed to polymerization of pre-intercalated epoxide on the internal gallery surfaces where the proton concentration is a maximum. Polymerization of the extragallery monomer on the external and internal surfaces of the clay particles occurs at the higher temperature. The acidity of the clay exchange cation plays a major role in catalyzing epoxy self-polymerization, especially the intragallery polymerization process. The activation energies for polymerization of intra- and extragallery monomer, as determined by DSC measurements, are 84 and 130 KJmol −1 , respectively.

Aluminosilicate mesostructures with improved acidity and hydrothermal stability

Significant improvements in both the hydrothermal stability and the acidity of mesostructured aluminosilcates have been reported recently. New assembly pathways, along with post-synthesis treatment methods, have made it possible to form structures with thick and more highly crosslinked framework walls. The resulting structures are slow to degrade under hydrothermal conditions in comparison to conventional analogs. Also, improved methodologies for grafting Al centers into the walls of pre-assembled frameworks have afforded aluminosilicate mesostructures with enhanced acidity. The most promising strategy for improving the hydrothermal stability and acidity of aluminosilicate mesostructures, however, is based on the use of protozeolitic nanoclusters. These so-called “zeolite seeds” can be directly assembled into hexagonal, cubic, wormhole, and foamlike framework structures under a variety of assembly conditions. They also can be grafted into the walls of pre-assembled frameworks to form more stable acidic derivatives.

Iron oxide pillared clay with large gallery height: Synthesis and properties as a Fischer-Tropsch catalyst

New iron oxide pillared montmorillonites have been prepared by the reaction of Na+ montmorillonite with base-hydrolyzed solutions of Fe3+ salts and subsequent thermal conversion of the intercalated polycations. Depending on the hydrolysis conditions used to generate the pillaring solutions, pillared products with basal spacings in the range 18 to 25 A were obtained. Under optimum hydrolysis conditions (base/metal = 2.0 meq/mol, aging time = 23–147 hr) the pillared products contained 6.8–9.8 Fe3+ ions per O20(OH)4 unit cell and exhibited basal spacings of 25–29 A. These latter spacings corresponded to exceptionally large gallery heights of 15–19 A. Upon calcination at 300°C, the spacings decreased to 23–27 A. N2 BET surface areas after outgassing at 350°C were in the range 270 to 350 m2/g. The pillared products are active catalysts that have undergone Fischer-Tropsch synthesis of hydrocarbons at 275°C and 120-psi (CO: H2 = 1 : 2). The hydrocarbon distribution in the C1–C6 range (1.3% conversion) followed Anderson-Schulz-Flory statistics with a chain propagation probability of α = 0.49. X-ray energy dispersive analysis indicated that substantial amounts of the intercalated iron migrated to the edge sites of the clay particles under reaction conditions. The redistribution of iron resulted in a distribution of gallery heights sufficiently heterogeneous to preclude Bragg X-ray scattering along the 001 direction. Iron migration also occurred upon exposure of the pillared products to the ambient atmosphere for prolonged periods (≥3 months).