General and simple approach for control cage and cylindrical mesopores, and thermal/hydrothermal stable frameworks (original) (raw)
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Advanced Materials, 2003
Mesoporous silica with large pores varying widely in size and with three-dimensional (3D) architectures are potential candidates for numerous applications. [1±5] Enlarging the pore size of mesoporous silica materials is an area that is actively researched. However, considerable attention has also been devoted to synthetic strategies for tailoring mesoporous silica dimensions in the 20±500 range through the use of various surfactant molecules, auxiliary chemicals, and synthetic conditions. [6±15] Furthermore, 3D mesoporous structures have been prepared through the phase transition of cylindrical phase assemblies by adding auxiliary molecules at ambient synthesis conditions. [7] The mesoporous silicates produced had an enlarged pore size; however, a loss of long-range order over the array was often observed, as revealed by the less well-resolved X-ray diffraction spectra and the broadening of the high-intensity peaks. In general, powdery mesoporous silica in micrometer-sized particles (1±2 lm) has been obtained; the formation of large, uniform monoliths was limited. [14±19] Furthermore, the utilization of a dilute surfactant in the synthesis of the mesoporous silica severely limited the ability to predict the mesophase topologies, because the primary phase of an amphiphile is affected by the interactions between the surfactant assemblies and the inorganic precursors. [5±7] Sol±gel synthesis of silica in a bulk lyotropic liquid-crystalline phase allows the inorganic precursors to precipitate in the synthesis solution during the solidification (i.e. rational precipitation), thereby allowing fabrication of large monoliths of the desired size and shape. [20±22] The use of a high template concentration (> 30 %) preserves the pre-existence of liquidcrystalline phases prior to solidification of the silica network, and thus allows a high degree of control over the amphiphilic phase domains and morphological organization of the mesopores. However, even in these syntheses, the pore size is limited to a maximum of 40 by the type and composition of the amphiphiles. [20±23] COMMUNICATIONS
Advanced Materials, 2005
Periodically mesoporous materials with two-and threedimensional (2D and 3D) structures are one of the most exciting new developments in materials science and technology in the last decade. Features of these mesoscale architectures include controllable mesopore size (20±300 ) which enable these materials to be promising candidates in applications such as catalysis, separations, sensing, and optical and electronic systems. Since the discovery of the prominent class of the M41S mesoporous family, surfactant-templated synthesis approaches are commonly used to direct the design of materials. [2] The nature of these amphiphiles enable successful fabrication of highly ordered nanoscale structures. [2, The resultant 3D cubic mesoporous materials will find many industrial applications.
Transparent cubic Fd3m mesoporous silica monoliths with highly controllable pore architectures
J. Mater. Chem., 2005
Ordered cubic Fd3m mesoporous silica monoliths (HOM-11) were fabricated by using lyotropic and microemulsion phases of block copolymers (EO m -PO n -EO m ) as templates. Aromatic and aliphatic hydrocarbon molecules were used in the formation of true microemulsion liquid crystal phases, in the expansion of pore sizes, and in the changes in the phase geometrical shape. Large cylinder-like pores in the range of 6-11 nm with uniform constrictions and bimodal mesopore sizes can be easily produced (within minutes) by adopting this simple and reproducible strategy. The degree of solubilization of the hydrocarbons crucially influenced the generation of the more open-pore systems with cylindrical cubic Fd3m channels. Our results also show that enlargement of pore sizes of cubic Fd3m monoliths was achieved with the use of a high concentration of copolymers in the composition phase domains, a high degree of swelling and a large size of PO-EO blocks (core-corona) of the copolymer templates. In addition, this synthetic approach is also efficient in designing cubic Fd3m silica monoliths with large-sized glass, thick-walled frameworks up to 30 nm thick and high mesopore/micropore volumes. Although XRD patterns show well-defined Bragg diffraction peaks indicative of highly ordered cubic Fd3m structures, TEM micrographs reveal that worm-like mesopore channels in large domains were observed with samples synthesized from copolymers with small EO-PO block ratios. This finding indicates that the molecular nature (i.e. the flexibility of the corona-and core-blocks) of the copolymer templates not only led to disordered pore channels but also reduced the ability to design more mesostructured phases.
One-pot synthesis of silica monoliths with hierarchically porous structure
Microporous and Mesoporous Materials, 2012
Poly(furfuryl alcohol) (PFA) and block copolymer Pluronic F127 were used as pore templates to create mechanically robust silica monoliths with a hierarchical and interconnected macro-mesoporous network in an easy, reproducible bimodal scale templating process. Control over the morphology was obtained by varying the reactant ratios. Phase separation on the submicrometer scale occurred when furfuryl alcohol was cationically polymerized and therefore became immiscible with the solvent and the silica precursor. Upon a subsequent sol-gel reaction, a silica-F127 matrix formed around the PFA spheres, leading to macropore structures with mesoporous walls. Surface areas of the final structures ranged from 500 to 989 m 2 g À1 and a maximum pore volume of 4.5 mL g À1 was achieved. Under mildly acidic conditions, micelle-templated mesopores resulted. Interconnected macropores could be obtained by increasing the pH or the block copolymer concentration. The formation mechanism and the relationship between PFA, Pluronic F127 and acidity are discussed in detail.
Journal of the American Chemical Society, 2003
FDU-1 silicas with large cage-like pores (diameter about 10 nm) were synthesized under acidic conditions from tetraethyl orthosilicate in the presence of a poly(ethylene oxide)-poly(butylene oxide)-poly-(ethylene oxide) triblock copolymer template B50-6600 (EO39BO47EO39). High-resolution transmission electron microscopy and small-angle X-ray scattering provided strong evidence that FDU-1 silica synthesized under typical conditions is a face-centered cubic Fm3m structure with 3-dimensional hexagonal intergrowth and is not a body-centered cubic Im3m structure, as originally reported. Samples synthesized in a wide range of conditions (initial temperatures from 298 to 353 K; hydrothermal treatment at 333-393 K) exhibited similar XRD patterns and their nitrogen adsorption isotherms indicated a good-quality cage-like pore structure. The examination of low-pressure nitrogen adsorption isotherms for FDU-1 samples, whose pore entrance diameters were evaluated using an independent method, allowed us to conclude that low-pressure adsorption was appreciably stronger for samples with smaller pore entrance sizes. This prompted us to examine low-pressure adsorption isotherms for a wide range of samples and led us to a conclusion that the FDU-1 pore entrance size can be systematically enlarged from about 1.3 nm (perhaps even lower) to at least 2.4 nm without an appreciable loss of uniformity by increasing the temperature of the hydrothermal treatment or the initial synthesis. Further enlargement of pore entrance size was achieved for sufficiently long hydrothermal treatment times at temperatures of 373 K or higher, as seen from the shape of nitrogen desorption isotherms. This allowed us to obtain samples with uniform pore sizes, high adsorption capacity, and with pore entrances enlarged so much that their size was similar to the size of the pore itself, resulting in a highly open porous structure. However, in the latter case, there was evidence that the pore entrance size distribution was quite broad.
Surfactant-Templated Synthesis of Ordered Silicas with Closed Cylindrical Mesopores
Chemistry of Materials, 2012
Ordered mesoporous silicas with 2-dimensional hexagonal arrays of closed cylindrical pores were synthesized via templating with block copolymer surfactant followed by calcination at appropriately high temperatures. Precursors to closed-pore silicas, including SBA-15 silicas and organosilicas, were selected based on the existence of narrow passages to the mesopores. The increase in calcination temperature to 800−950°C led to a dramatic decrease in nitrogen uptake by the materials, indicating the loss of accessible mesopores, whereas small-angle X-ray scattering (SAXS) indicated no major structural changes other than the framework shrinkage. Since SAXS patterns for ordered mesoporous materials are related to periodic arrays of mesopores, the existence of closed mesopores was evident, as additionally confirmed by TEM. The formation of closed-pore silicas was demonstrated for ultralarge-pore SBA-15 and large-pore phenylene-bridged periodic mesoporous organosilicas. The increase in the amount of tetraethyl orthosilicate in standard SBA-15 synthesis also allowed us to observe the thermally induced pore closing. It is hypothesized that the presence of porous plugs in the cylindrical mesopores and/or caps at their ends was responsible for the propensity to the pore closing at sufficiently high temperatures. The observed behavior is likely to be relevant to a variety of silicas and organosilicas with cylindrical mesopores.
Fabrication and properties of silica monoliths with ultra large mesopores
Microporous and Mesoporous Materials, 2008
Silica monoliths with ultra large mesopores of diameters over 25 nm and pore volumes in excess of 1.5 cm 3 g À1 were obtained by the hydrothermal treatment in aqueous ammonia solution of monolithic mesophases prepared by a direct templating of lyotropic liquid phase under high surfactant concentration. The process consists in the rearrangement of silica matrix structure from a template directed mesophase to that of a typical colloidal gel and is facilitated by template and silica extraction to the solution and basic conditions of the process. Silica of the monoliths hydrothermally treated in ammonia solution is somewhat less condensed than of untreated samples. The monoliths prepared using ethyl-silicate 40 as silica precursor posses slightly higher textural parameters than those synthesized using tetraethoxysilane. The size of pores depends on the concentration of ammonia in water solution and the time of treatment.
The Journal of Physical Chemistry B, 2002
The increasing awareness of the need to create green and sustainable production processes in all fields of chemistry has stimulated materials scientists to search for innovative catalysts supports. These new catalytic supports should allow the heterogenization of most catalytic processes, increasing the efficiency and selectivity of the synthesis and reducing waste and byproducts. Following the development of several micellar templated structures, such as M41S 1 , FSM-16, 2,3 HMS, 4,5 MSU-x, 6 and SBA-x, 7,8 it is a crucial next step to create support materials, consisting of a composite matrix with combined micro-and mesoporosities and a sufficient stability to withstand most industrial treatments. We describe in this paper the very first development of a hexagonal material with large pore diameters and thick walls (4 nm), containing internal microporous silica nanocapsules. These plugged hexagonal templated silicas (PHTS) have two types of micropores (originating from the walls and the nanocapsules respectively) and a tunable amount of both open and encapsulated mesopores. The micropore volumes have a high value (up to 0.3 cm 3 /g) and the total pore volume exceeds 1 cm 3 /g. The obtained materials are much more stable than the conventional micellar templated structures known so far, and can easily withstand severe hydrothermal treatments and mechanical pressures.
Journal of Materials Chemistry, 2008
We have obtained a new class of porous silica with good structural order and additional corrugated nanopores clustered around the primary mesopores from the co-condensation of TEOS and adamantylphenol-grafted trimethoxysilane (adam-graft SQ) using a triblock Pluronic P123 (EO 20 PO 70 EO 20 , M w ¼ 5800) copolymer as a structure-directing agent. Thermally activated removal of pore-generating moieties (i.e., adamantylphenol groups) in adam-graft SQ involves the generation of secondary micro-to-small mesopores, while the block copolymer template generates 2D-hexagonal mesopores. We found that the mesostructural characteristics and the generation of secondary indented pores right next to the mesopores can be tailored by the addition order of the two silica precursors (TEOS and adam-graft SQ), varying the molar ratio between TEOS and adam-graft SQ in the starting sol mixture, and the degree of silica polymerization. The increase in the hexagonal unit cell parameters is attributed to the increment of pore size originating from the removal of adamantylphenol moieties. It is believed that the hydrophobicity of adamantylphenol groups plays a key role in its selective incorporation into the region near the PPO core blocks and the subsequent generation of corrugated pores along the silica channels resulting in the increase of pore diameter.