Kanghee Cho

Directing Zeolite Structures into Hierarchically Nanoporous Architectures

Bifunctional surfactants are used to synthesize zeolites with multiple scales of porosity and enhanced catalytic activity. Crystalline mesoporous molecular sieves have long been sought as solid acid catalysts for organic reactions involving large molecules. We synthesized a series of mesoporous molecular sieves that possess crystalline microporous walls with zeolitelike frameworks, extending the application of zeolites to the mesoporous range of 2 to 50 nanometers. Hexagonally ordered or disordered mesopores are generated by surfactant aggregates, whereas multiple cationic moieties in the surfactant head groups direct the crystallization of microporous aluminosilicate frameworks. The wall thicknesses, framework topologies, and mesopore sizes can be controlled with different surfactants. The molecular sieves are highly active as catalysts for various acid-catalyzed reactions of bulky molecular substrates, compared with conventional zeolites and ordered mesoporous amorphous materials.

Zeolite nanosheet of a single-pore thickness generated by a zeolite-structure-directing surfactant

Zeolite MFI nanosheets have been hydrothermally synthesized using a surfactant with the formula [C18H37–N+(CH3)2–C6H12–N+(CH3)2–C6H12–N+(CH3)2–C18H37][Br−]3 as the zeolite structure-directing agent. These nanosheets correspond to a slice of MFI zeolite crystal of 1.5 nm thickness in the b-direction. Each nanosheet is composed of a monolayer of micropores, which is thinner than a single crystal unit-cell dimension (2.0 nm). Such a single-pore zeolite nanosheet still exhibits high thermal stability, steam stability, acid strength and shape-selective catalytic activity, comparable to the bulk crystal.

MFI zeolite nanosponges possessing uniform mesopores generated by bulk crystal seeding in the hierarchical surfactant-directed synthesis

The synthesis of a mesoporous material with uniform mesopore diameters and crystalline MFI zeolite walls has been achieved, simply by seeding the multiammonium surfactant-directed synthesis with bulk zeolite crystals. The bulk seeds disappeared in the final product. As a result of seeding, the mesoporous zeolites could be generated rapidly even at high Al content.

Intracrystalline Diffusion in Mesoporous Zeolites

Specially synthesized extra-large crystallites of zeolite LTA with intentionally added mesoporosity are used for an in-depth study of guest diffusion in hierarchical nanoporous materials by the pulsed field gradient NMR technique. Using propane as a guest molecule, intracrystalline mass transfer is demonstrated to be adequately described by a single effective diffusivity resulting from the weighted average of the diffusivities in the two (micro- and meso-) pore spaces. Gas-kinetic order-of-magnitude estimates of the diffusivities are in satisfactory agreement with the experimental data and are thus shown to provide a straightforward means for predicting and quantifying the benefit of hierarchically structured nanoporous materials in comparison with their purely microporous equivalent.

Random‐Graft Polymer‐Directed Synthesis of Inorganic Mesostructures with Ultrathin Frameworks

A widely employed route for synthesizing mesostructured materials is the use of surfactant micelles or amphiphilic block copolymers as structure-directing agents. A versatile synthesis method is described for mesostructured materials composed of ultrathin inorganic frameworks using amorphous linear-chain polymers functionalized with a random distribution of side groups that can participate in inorganic crystallization. Tight binding of the side groups with inorganic species enforces strain in the polymer backbones, limiting the crystallization to the ultrathin micellar scale. This method is demonstrated for a variety of materials, such as hierarchically nanoporous zeolites, their aluminophosphate analogue, TiO2 nanosheets of sub-nanometer thickness, and mesoporous TiO2, SnO2, and ZrO2. This polymer-directed synthesis is expected to widen our accessibility to unexplored mesostructured materials in a simple and mass-producible manner.

Exploring Mass Transfer in Mesoporous Zeolites by NMR Diffusometry

With the advent of mesoporous zeolites, the exploration of their transport properties has become a task of primary importance for the auspicious application of such materials in separation technology and heterogeneous catalysis. After reviewing the potential of the pulsed field gradient method of NMR (PFG NMR) for this purpose in general, in a case study using a specially prepared mesoporous zeolite NaCaA as a host system and propane as a guest molecule, examples of the attainable information are provided.

Large pore phenylene-bridged mesoporous organosilica with bicontinuous cubic Iad (KIT-6) mesostructure

Assembly of mesostructured silica using Pluronic P123 triblock copolymer (EO20-PO70-EO20) and n-butanol is a facile synthesis route to the MCM-48-like ordered large mesoporous silicas with the cubic Iad mesostructure, which are designated KIT-6. This synthesis route has been successfully extended to phenylene-bridged organosilicas from the limit so far for silica, using 1,4-bis(triethoxysilyl)benzene as an organosilica source and re-optimizing the synthesis conditions. In particular, optimal acid concentration and reagent ratios were determined to allow facile synthesis of the bicontinuous cubic mesophase in high yield and high phase purity. The products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy, solid state NMR spectroscopy and analysis of nitrogen sorption at −196 °C using modern non-local density functional theory methods. This synthesis procedure enabled easy fine tuning of pore volume, specific surface area and mesopore size by variation of the hydrothermal aging temperature between 80 and 130 °C. Furthermore, wide angle XRD data suggest short range molecular-scale periodicity in the framework walls originating from regular arrangement of the bridging aromatic groups. These KIT-6 organosilica materials with fully interconnected nanoporous structure would be readily available for applications in heterogeneous catalysis, selective adsorption, and nanostructure design via solid templating approaches.

Synthesis of CuCl/Boehmite adsorbents that exhibit high CO selectivity in CO/CO2 separation

We developed nanoporous adsorbent exhibiting unprecedented performance in separation of toxic carbon monoxide (CO). The adsorbent was prepared by dispersing CuCl on mesoporous boehmite via thermal monolayer dispersion route. A key point of the present synthesis is dispersing optimized amount of CuCl on the boehmite at a moderate temperature to maintain the characteristics of the boehmite. We performed a systematic study to reveal that a CuCl/boehmite composite (30wt% CuCl in total) thermally treated at 573K was the best optimized sample for CO separation. The CuCl/boehmite had a high capacity of CO adsorption (1.56mmolg-1) and an exceedingly low capacity of CO2 adsorption (0.13mmolg-1) under 100kPa of each gas at 293K. The CO/CO2 separation factor was 12.4. To the best of our knowledge, this value is the best on record. The achievement of this work is attributed to finding a new type of suitable supporting material: boehmite. The boehmite has a high affinity to CuCl, exhibits excellent dispersion of the CuCl, and achieves a superior CO adsorption capacity. However, it has a weak interaction with CO2. The CuCl/boehmite composite is a promising adsorbent for selective separation of CO from combustion exhaust and industrial off-gas streams.