Macroscopically Oriented Porous Materials with Periodic Ordered Structures: From Zeolites and Metal-Organic Frameworks to Liquid-Crystal-Templated Mesoporous Materials

Porous materials with molecular-sized periodic structures, as exemplified by zeolites, metal-organic frameworks, or mesoporous silica, have attracted increasing attention due to their range of applications in storage, sensing, separation, and transformation of small molecules. Although the components of such porous materials have a tendency to pack in unidirectionally oriented periodic structures, such ideal types of packing cannot continue indefinitely, generally ceasing when they reach a micrometer scale. Consequently, most porous materials are composed of multiple randomly oriented domains, and overall behave as isotropic materials from a macroscopic viewpoint. However, if their channels could be unidirectionally oriented over a macroscopic scale, the resultant porous materials might serve as powerful tools for manipulating molecules. Guest molecules captured in macroscopically oriented channels would have their positions and directions well-defined, so that molecular events in the channels would proceed in a highly controlled manner. To realize such an ideal situation, numerous efforts have been made to develop various porous materials with macroscopically oriented channels. An overview of recent studies on the synthesis, properties, and applications of macroscopically oriented porous materials is presented.

Ordered mesoporous carbon coating on cordierite: synthesis and application as an efficient adsorbent

Ordered mesoporous carbon coating on the honeycomb cordierite substrate has been prepared using low-polymerized phenolic resins as carbon sources and triblock copolymer F127 as the structure directing agent via the evaporation induced self-assembly route. The high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), and nitrogen sorption techniques prove the hexagonally ordered pore arrays of carbon coating on the cordierite. The honeycomb monolith adsorbents coated by ordered mesoporous carbons are directly used without any activation, and exhibit adsorption capacities for chlorinated organic pollutants in water with 200 mg/g for p-chlorophenol and 178 mg/g for p-chloroaniline (with respect to the net carbon coating), high adsorption ratio for low-concentration pollutants, large processing volumes and reusability. More than 200 repeated times can be achieved without obvious loss in both adsorption capacity and weight.

Alignment control of self-assembled organosiloxane films derived from alkyloligosiloxane amphiphiles

Transparent and continuous organosiloxane films with macroscopically oriented mesostructures were prepared by dip-coating a substrate, on which a rubbing-treated polyimide film is formed, with hydrolyzed solutions of oligosiloxane precursors (C(n)H(2n+1)Si(OSi(OMe)(3))(3)). The structure of the films depends on the alkyl chain length of the precursors such that films with two-dimensional (2D) hexagonal and lamellar structures are obtained when n = 10 and 16, respectively. In the 2D hexagonal film, the cylindrical organic moieties are aligned perpendicular to the rubbing direction in the plane of the film over the whole film thickness. On the other hand, the lamellar film changes its orientation with increased distance from the substrate surface. While the orientation of the lamellae at the surface of the film is parallel to the film-air interface, they are perpendicularly aligned in the vicinity of the substrate with the layer normal parallel to the rubbing direction. The observed unique orientation of the mesostructures is attributed to the anisotropic hydrophobic interactions between the alkyl chains of the hydrolyzed oligosiloxane molecules and the polymer chains of the polyimide layer oriented by the rubbing treatment.

Photoactive perylenediimide-bridged silsesquioxane functionalized periodic mesoporous organosilica thin films (PMO-SBA15): synthesis, self-assembly, and photoluminescent and enhanced mechanical properties

Well-organized periodic mesoporous organosilica thin films (designated as PMO-SBA15), having covalently bonded perylene-bridged silesquioxane (PTCDBS) inside their pore channels, are successfully synthesized via sol-gel self-assembly of 1,2-bis(triethoxysilyl)ethane and perylene-bridged silsesquioxane, using micelles of pluronic surfactant (P123) as a template for the first time. The surfactant is successfully removed from the pore channels of PMO-SBA15 by an acidic solvent extraction procedure. The final PMO-SBA15 thin films are characterized by high resolution X-ray diffraction (HRXRD), transmission electron microcopy (TEM), solid-state 29Si and 13C NMR CP/MAS NMR spectroscopy, nitrogen adsorption-desorption measurements, photoluminescence (PL) spectroscopy, and nanoindentation. HRXRD data reveal the formation of well-organized hexagonal channels in the pure PMO-SBA15 films. The intensity of the diffracted X-ray, however, systematically attenuates after incorporation of the perylene functionality inside the hexagonal channels. This is attributed to the low X-ray scattering contrast between the mesostructured organosilica walls and organic moieties (perylene) inside the channels, suggesting the successful incorporation of the photoactive perylene molecules inside the nanochannels. This was further confirmed by photoluminescence spectroscopy and nitrogen adsorption-desorption measurements. Additionally, the mechanical hardness of the functionalized PMO-SBA15 thin films, measured by nanoindentation, is significantly enhanced as compared with that of the pure PMO film. Thermogravimetric analysis (TGA) and elemental analysis suggested the functionalized PMO-SBA15 materials with PTCDBS.

Self-assembly, optical, and mechanical properties of surfactant-directed biphenyl-bridged periodic mesostructured organosilica films with molecular-scale periodicity in the pore walls

Self-assembly, optical, and mechanical properties of surfactant-directed biphenyl-bridged periodic mesoporous organosilica thin films (PMOF-Bp's) with molecular-scale periodicity in the pore walls were successfully demonstrated for the first time. The biphenyl-bridged organosilica precursor, 4,4-bis(triethoxysilyl)biphenyl (Bp-TES) has been used as the sole precursor (100%) for preparing PMOF-Bp films with molecular-scale periodicity in the pore walls via the surfactant-mediated one-step mild acidic self-assembly process. High-resolution X-ray diffraction (HRXRD) patterns and transmission electron microscope (TEM) images of PMOF-Bp materials confirmed the formation of a biphenyl-bridged periodic mesophase with molecular-scale periodicity in the organosilica framework. Fourier transform infrared (FT-IR) and NMR spectroscopic data also strongly suggested that the biphenyl organic segment is covalently bonded with silicon atoms in the acidic ethanol-washed biphenyl-bridged mesoporous framework. The emission behavior is sensitive to synthesis and thermal treatment temperatures. The biphenyl-bridged PMO films show absorption and emission due to the presence of biphenyl segment in pore walls. Nanoindentation hardness of the PMOF-Bp films could be controlled by temperature, degree of pore ordering and molecular periodicity, and even thickness of films. For example, well-organized PMOF-Bp film with molecular-scale periodicity in the pore walls showed a higher hardness value (0.23 GPa) than that of less mesoordered PMOF-Bp film (0.13 GPa). For all solvent-extracted PMO samples, N(2) gas sorption experiments showed the surface area (from 714 to 688 m(2)/g), the pore volume (from 0.76 to 0.68 cm(3)/g), and pore size (2.81 to 3.1 nm). The solid-state NMR and FT-IR spectroscopic data were used to propose plausible interpretations of the formation of hydrogen-bonded molecular periodicity in the pore walls. The experimental periodicity value 1.40 nm was strongly supported by the periodicity obtained by the structural model (1.389 nm).