Changbum Jo

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.

Microporous aluminophosphate nanosheets and their nanomorphic zeolite analogues tailored by hierarchical structure-directing amines.

Multiamines with amphiphilic structures have been synthesized to serve as simultaneous structure-directing agents in micro- and meso-structural levels for aluminophosphate materials (AlPOs) and their analogues, such as silicoaluminophosphate, cobalt aluminophosphate, and gallium phosphate. The amine molecules are assembled into a micelle with a specific morphology to function as a meso-level structure director. Individual amine groups in the micelle are able to direct the formation of microporous crystalline AlPO structure. The resultant meso-level morphologies of the AlPOs are typically nanosheets of uniform thickness, which can be tailored in the range of 2-5 nm by the number of amine groups. Sponge-like disordered mesoporous morphologies can be generated, depending on the amine structures. Using such multiamines provides a versatile route to various phosphate materials with a structural hierarchy for enhanced porous functionalities.

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.

The effect of MFI zeolite lamellar and related mesostructures on toluene disproportionation and alkylation

The effect of MFI zeolite nanosheet morphology and related mesostructures on the accessibility of acid sites evaluated from FTIR spectra of different adsorbed probe molecules and conversions and selectivities in toluene disproportionation and alkylation with isopropyl alcohol was investigated. Two-dimensional MFI lamellar zeolite (1.5 nm thickness of layers) and related mesoporous sponge morphologies, most probably with 10-ring channel system not yet structurally fully determined, were synthesized and characterized by XRD, SEM, TEM, and adsorption measurements. FTIR spectroscopy using pyridine, deuterated acetonitrile and 2,6-di-tert-butyl pyridine evidenced high concentrations of accessible Lewis acid sites at the expense of Bronsted acid sites. Ordered mesoporous zeolites exhibited slightly higher toluene conversion in its alkylation with isopropyl alcohol than lamellar MFI or conventional MFI due to easy access to active sites. The lamellar zeolite consisting of less than a unit cell layer was slightly more active than MFI but still exhibited high selectivity to p-cymene. It evidences that alkylation of toluene with isopropyl alcohol proceeds mainly in the 10-ring channel system of MFI zeolite, although the effect of large external surface cannot be neglected. Increased para-selectivity compared with thermodynamic value is achieved even in the case of extremely thin zeolite nanosheets.

Capping with Multivalent Surfactants for Zeolite Nanocrystal Synthesis

Multiammonium surfactants exhibited a remarkable capping effect for zeolite synthesis in the forms of nanoparticles, nanorods, and nanosponges in cases where common monovalent surfactants failed. A nanorod-shaped mordenite zeolite synthesized in this manner showed significantly enhanced catalytic lifetimes in acid-catalyzed cumene synthesis reactions.

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.

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.

Mesoporous Polymeric Support Retaining High Catalytic Activity of Polyoxotungstate for Liquid‐Phase Olefin Epoxidation using H2O2

However, polymer resin basedporous supports typically have a wide distribution of pore di-ameters, ranging from micropores ( 50 nm). A large portion of the catalytic groups can be sup-ported within micropores under these circumstances, leadingto serious diffusion limitations. Another problem is that theloosely cross-linked polymeric frameworks can easily swell inmany organic solvents. This swelling can lead to the closing ofpore mouths under reaction conditions with organic solvents.In addition, water (H

Bulk crystal seeding in the generation of mesopores by organosilane surfactants in zeolite synthesis

Zeolites of MOR, CHA, and FAU-X topologies were conventionally synthesized using Na+ or an organic structure-directing agent, except that organosilane surfactants were added as a mesopore-generating agent. The organosilanes were represented by the structural formula (CH3O)3Si–C3H6–N+(CH3)2CnH2n+1, where n was varied from 12 to 16 and 18. Each zeolite synthesis was seeded with a small amount of bulk zeolites of the same structure type. The synthesis result indicated that the bulk zeolite seeds disappeared completely from the zeolite products, indicating their disintegration into tiny fragments that were undetectable by high-resolution transmission electron microscopy. Nevertheless, the zeolite seeding caused a dramatic decrease in synthesis time. More importantly, crystal seeding, in comparison to unseeded synthesis, was highly effective in extending the organosilane-directed mesopore-generating strategy to a wide range of mesoporous and zeolite structures.

Annulation of Phenols: Catalytic Behavior of Conventional and 2 D Zeolites

Catalytic behavior of MFI zeolites differing in thickness of nanosheets and ordering was studied in annulation of phenols, and compared with 3 D zeolites BEA and MFI containing large or medium pores as well as with micro/mesoporous zeolite USY. The highest conversions of phenols studied were achieved over ordered hexagonally mesostructured zeolite with 1.7 nm wall size, followed by materials possessing 2.1 and 2.7 nm of nanosheets thickness. This corresponds to decreasing surface area of materials studied. The preferences of materials with zeolitic layers and high surface areas over bulky zeolites BEA and especially MFI in annulation of phenols is more prominent for substrates with larger kinetic diameters [phenol (0.66 nm)<1‐naphthol (0.80 nm)<2‐naphthol (0.89 nm)]. USY zeolite exhibited higher conversions (32, 6, 25 % for phenol, 1‐ and 2‐naphthol, respectively, after 300 min time on stream) than BEA (23, 6, 8 %) and MFI (13, 0, 0 %) not overcoming hexagonally mesostructured MFI (45, 36, 55 %).