Kyung Byung Yoon

Growth of Uniformly Oriented Silica MFI and BEA Zeolite Films on Substrates

Precise control over synthetic conditions eliminates misalignment of pore channels during the growth of zeolite films. Applications of zeolite films benefit from alignment of the integrated channels, but methods for film growth have nearly always introduced orientational randomization in the direction normal to the substrate. We now report facile methods to grow silicalite-1 films and pure silica beta zeolite films on substrates with straight or sinusoidal channels positioned uniformly upright at a thickness of up to 8 micrometers. Precise gel compositions and processing temperatures are critical to promote secondary growth on pre-formed oriented crystal monolayers while suppressing self-crystallization in the bulk medium. Preliminary results highlight the potential of these uniformly oriented films in the nonlinear optical response and separation of xylene isomers.

Acidity scale for metal oxides and Sanderson's electronegativities of lanthanide elements.

Metal oxides are widely used in industry and academia. As their electron-acceptor or acidic strengths play vital roles in their applications, there needs to be a general scale that can quantitatively compare their relative acidic strengths. Conventionally, calorimetric heat measurements during adsorption of probe molecules, infrared spectroscopic analyses of adsorbed bases or acids, application of indicator dyes, and temperature-programmed desorption of the pre-adsorbed bases are standard methods for the analyses of their acidic strengths. However, these methods are not suitable for a quantitative comparison. Thus, unlike metal ions in solution, no such scales have been available for metal oxides. One of the important types of interaction between adsorbates and metal oxides is the formation of coordinate covalent bonding between adsorbates and the surface metal ions. For instance, in the case of TiO2, those compounds that have enediol, carboxylate, and nitrile groups have been shown to form coordinate covalent bonding with the surface Ti ions. In this type of interaction, the adsorbateto-metal charge-transfer interaction is often the lowestenergy electronic transition. However, in the case of alizarin (Figure 1a, inset) on TiO2, a theoretical study has suggested that the intramolecular charge-transfer (IMCT) band from the catechol moiety to the entire ring system is the lowestenergy transition. Electronegativity (EN) is one of the most important fundamental properties of an atom, which represents “the power of an atom in a compound to attract electrons to itself”. Among various EN scales that have been developed, Sanderson s scale and the associated EN equalization principle are successful in calculating the bond energies of various compounds , elucidating the acidic and basic properties of zeolites, and establishing the relationship between the reactivity and the composition of the zeolite that served as the guideline for the preparation of optimum zeolite catalysts. These methods have also been used for various other purposes. However, owing to a lack of experimental data, Sanderson s EN scale has not been extended to lanthanides (Ln) during the last five decades, despite the fact that lanthanide-containing compounds are widely used. Herein, we report that the IMCT transition of alizarin is still the lowest-energy transition when it is adsorbed on various metal oxides and sulfides, regardless of the nature of the metal ion. The charge-transfer transition serves as a highly sensitive and accurate probe for the quantitative comparison of the acidic strengths of the metal oxides and sulfides. We also report the factors that govern the surface acidity, which allows us to assign for the first time the important Sanderson s EN values of Ln ions (SLn3+) and Ce . To experimentally verify the IMCT nature of the lowestenergy electronic transition from the catechol moiety to the entire ring, we also synthesized 4-methoxyalizarin (Figure 1a, inset; Supporting Information, SI-1). The UV/Vis spectra of the two compounds (Figure 1a) show that the lowest-energy transition (2.856 eV) shifts to the lower energy region (2.617 eV) upon introducing a methoxy group at the 4 position; that is, upon increasing the donor strength of the catechol moiety, which verifies the IMCT nature of the transition. The IMCT bands of alizarin and 4-methoxyalizarin adsorbed on various metal oxides and sulfides are shown in Figure 1b, with the order of energy increasing from bottom to top. The IMCT bands appear at 2.322–2.713 eV for alizarin and 2.288–2.536 eV for 4-methoxyalizarin (Supporting Information, Table SI-2). The red-shift from alizarin to 4-methoxyalizarin also suggests that the lowest-energy transition of the adsorbed alizarin is IMCT. Furthermore, the IMCT bands of alizarin and 4-methoxyalizarin are red-shifted when they are adsorbed onto oxides and sulfides. Such coordination-induced redshifts were also observed in solution. The IMCT band of alizarin (2.398 eV) red-shifts upon coordinating to Mg in ethanol relative to its uncoordinated state (2.856 eV; Supporting Information, Figure SI-3). We attribute the red shift to a decrease in the degree of electron withdrawal of the two phenoxide groups as a result of the replacement of the two strongly electron-withdrawing protons by a less strongly electron-withdrawing metal cation (Figure 1d, inset). In this context, the gradual blue shift of the IMCT band of the adsorbed alizarin and 4-methoxyalizarin on going from MgO to Ta2O5 is attributed to the increase in the degree of electron withdrawal from the two phenoxide ligands to a surface metal ion in the following order: MgO<PbO<Y2O3<ZnO< ZnS<HfO2<Ga2O3<ZrO2<TiO2< SnO2<Ta2O5. The IMCT energies do not correlate with Sanderson s partial charges of the metal ions (dM) in metal oxides and sulfides (Figure 1c), which are expressed by Equation (1) for metal chalcogenides MxChy (Ch= chalcogen): [32] [*] N. C. Jeong, Dr. J. S. Lee, Dr. E. L. Tae, Y. J. Lee, Prof. Dr. K. B. Yoon Center for Microcrystal Assembly, Center for Nanoporous Materials, Department of Chemistry, and Program of Integrated Biotechnology, Sogang University, Seoul 121-742 (Korea) Fax: (+82)2-706-4269 E-mail: yoonkb@sogang.ac.kr

CO2 capture from humid flue gases and humid atmosphere using a microporous coppersilicate

Grabbing CO2 from wet gas streams It is a challenge to extract CO2 from typical gas streams, such as the flue gas from a power plant. This is because any water in the stream tends to prevent CO2 absorption and may also degrade the absorbing material. Datta et al. developed a microporous copper silicate that avoids these problems. Most other materials have sites that absorb both water and CO2 at the same sites, and in that fight, the water tends to win. Although their material still absorbs water, it has separate sites for the CO2 absorption. It also shows good stability despite the absorbed water and can be reused. Science, this issue p. 302 A material with different sites for CO2 and water absorption can extract CO2 from moist gas streams. Capturing CO2 from humid flue gases and atmosphere with porous materials remains costly because prior dehydration of the gases is required. A large number of microporous materials with physical adsorption capacity have been developed as CO2-capturing materials. However, most of them suffer from CO2 sorption capacity reduction or structure decomposition that is caused by co-adsorbed H2O when exposed to humid flue gases and atmosphere. We report a highly stable microporous coppersilicate. It has H2O-specific and CO2-specific adsorption sites but does not have H2O/CO2-sharing sites. Therefore, it readily adsorbs both H2O and CO2 from the humid flue gases and atmosphere, but the adsorbing H2O does not interfere with the adsorption of CO2. It is also highly stable after adsorption of H2O and CO2 because it was synthesized hydrothermally.

Oriented MFI membranes by gel-less secondary growth of sub-100 nm MFI-nanosheet seed layers

A zeolite membrane fabrication process combining 2D-zeolite nanosheet seeding and gel-free secondary growth is described. This process produces selective molecular sieve films that are as thin as 100 nm and exhibit record high permeances for xylene- and butane-isomers.

Synthesis of diamond-shape titanate molecular sheets with different sizes and realization of quantum confinement effect during dimensionality reduction from two to zero

Synthesis of semiconductor nanoparticles with uniform shapes, sizes, and compositions in series with a gradual size reduction has not been achieved for two-dimensional molecular sheets. We report a large-scale (>2.6 g) synthesis of 0.75-nm-thick diamond-shape lepidocrocite-type titanate molecular sheets with the sizes decreasing from (27.3, 19.1) to (7.7, 5.5), where the numbers in parentheses represent the long and short diagonal lengths, respectively, in nm. This is the first example of synthesizing semiconductor nanoparticles in series with the dimensionality reduction from two to zero, without coating the surfaces with surface-passivating ligands. The titanate molecular sheets showed three exciton-absorption bands in the 4.0-6.5 eV region, the absorption energies of which increased with decreasing the area. Contrary to the common belief, the per-unit cell oscillator strengths gradually increased with increasing area and the per-particle oscillator strengths increased in proportion to the area. The average reduced exciton masses along the two diagonal axes were 0.10 and 0.11 m e, respectively, which were much smaller than those of bulk titanates (by 60-130 times). The estimated average Bohr radii along the two-diagonal axes were 4.8 and 4.3 nm, respectively.

Gel‐Free Secondary Growth of Uniformly Oriented Silica MFI Zeolite Films and Application for Xylene Separation

Gel-Free Secondary Growth of Uniformly Oriented Silica MFI Zeolite Films and Application for Xylene Separation Zeolite membranes : A promising method is reported for the fabrication of oriented silica MFI zeolite films (see picture; TPAOH = tetrapropylammonium hydroxide). The films synthesized using this method exhibit an outstanding performance for the separation of pand oxylene. Angewandte Chemie

Diisocyanates as novel molecular binders for monolayer assembly of zeolite crystals on glassElectronic supplementary information (ESI) available: experimental details. See http://www.rsc.org/suppdata/cc/b2/b205046c/

Isocyanate groups readily form urethane linkages with surface hydroxy groups on glass and zeolites and this phenomenon was utilized in the assembly of monolayers of zeolite microcrystals on glass by employing diisocyanates as novel molecular binders.

Core–shell strain structure of zeolite microcrystals

Zeolites are crystalline aluminosilicate minerals featuring a network of 0.3-1.5-nm-wide pores, used in industry as catalysts for hydrocarbon interconversion, ion exchangers, molecular sieves and adsorbents. For improved applications, it is highly useful to study the distribution of internal local strains because they sensitively affect the rates of adsorption and diffusion of guest molecules within zeolites. Here, we report the observation of an unusual triangular deformation field distribution in ZSM-5 zeolites by coherent X-ray diffraction imaging, showing the presence of a strain within the crystal arising from the heterogeneous core-shell structure, which is supported by finite element model calculation and confirmed by fluorescence measurement. The shell is composed of H-ZSM-5 with intrinsic negative thermal expansion whereas the core exhibits a different thermal expansion behaviour due to the presence of organic template residues, which usually remain when the starting materials are insufficiently calcined. Engineering such strain effects could have a major impact on the design of future catalysts.

Aligned inclusion of dipolar dyes into zeolite channels by inclusion in the excited state.

Unlike octadecyl-tethering hemicyanine (HC-18), nonyl-tethering hemicyanine (HC-9) entered the hydrophobic channels of silicalite-1 with a very small preference to the inclusion with the alkyl part...

Synthesis, characterization and catalytic properties of TS-1 monoliths

Titanium silicalite-1 monolith (TS-1M) was prepared using polyurethane foam as the template. The characterization of the TS-1M was carried out using XRD, SEM, UV–vis and FT-IR spectroscopies, and water vapor adsorption. Catalytic activity was measured for 1-hexene and 2,5-dihydrofuran epoxidation using H2O2 as an oxidant. These studies revealed that TS-1 monoliths have essentially identical chemical properties to the TS-1 powders, but the crystal morphology was different and diffusion plays an important role in the catalysis of the liquid-phase epoxidation reactions.