Yongbeom Seo

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.

Tailored Mesostructured Copper/Ceria Catalysts with Enhanced Performance for Preferential Oxidation of CO at Low Temperature

Hydrogen as the most efficient and cleanest energy source for fuel cell power is produced mainly by reformation of hydrocarbons, followed by the water gas shift reaction. The CO (0.5–2%) present in the hydrogen stream must be selectively removed because CO is highly poisonous to the electrocatalyst in proton-exchange membrane fuel cells (PEMFCs). Preferential oxidation (PROX) of CO in excess H2 is therefore a key reaction for the practical use of H2 in PEMFCs. [1,2] Among the catalysts reported to be active for PROX, copper/ceria-based catalysts have been considered as promising candidates because of their low cost and high selectivity compared to catalysts based on gold or platinum. However, they usually only show noticeable activities above 100 8C, while the operating temperature of PEMFCs is around 80 8C. Furthermore, the catalytic properties depend strongly on the preparation method and the CuO/CeO2 interfacial area. Despite numerous studies about PROX catalysts, little is known concerning the influence of pore size and pore structure on the catalytic performance. Transition-metal oxides exhibiting mesoporous structures, for example, Co3O4 and CuO/Fe2O3, are active for CO oxidation at low temperature and show higher activity than the corresponding bulk materials. The high activity of mesoporous metal oxides was correlated to their ordered mesostructure and high surface area. Hard templating is a method known to enable the synthesis of materials that possess a highly defined pore architecture and a very high surface area, thus leading to unique physicochemical properties. However, studies of surface redox reactivity and the confinement of reactions near to the surface owing to the dimension of the pores have been limited to a few compositions of catalysts for CO oxidation. Herein, we report the catalytic performance in CO-PROX of Cu/CeO2 and CuM/CeO2 catalysts prepared by the nanocasting method. In this study, mesoporous catalysts with various compositions were synthesized using an improved hard templating method that we have recently developed. The pore size, specific surface area, and pore structure were tailored by changing the type of mesoporous silica used as solid template (e.g., KIT-6 aged at different temperatures, SBA-15, and MCM-48 nanospheres). The resulting metal oxide materials possess a high surface area (up to 200 mg ) and a pore size ranging from 3 nm to 12 nm. The catalytic performance of these materials is among the best reported thus far for copper/ceria-based catalysts with respect to the CO conversion and CO2 selectivity at low temperature. The effect of their mesostructure and composition on the reducibility and catalytic properties are also substantiated. Mesoporous silica templates with different pore structures (KIT-6, SBA-15, andMCM-48) were synthesized according to the literature. The pore size of the KIT-6 was varied by changing the aging temperature (40, 100, and 130 8C). The nanocast catalysts were prepared by one-step-impregnation hard templating (see Experimental Section). The samples prepared from KIT-6 were labeled as Cu(x)CeM(y)-K-T, with T representing the aging temperature of KIT-6, x (x= 10–30) and y (y= 20) are nominal molar percentages of Cu or M to Ce (M=Co or Fe), respectively. The samples using SBA-15 and MCM-48 hard templates were denoted as Cu(x)Ce-SBA and Cu(x)Ce-MCM, respectively. Representative TEM images of the nanocast materials replicated from KIT-6 and SBA-15 templates confirm the long-range periodic order of the mesopores (Figure 1A and B, and Figure S1 in the Supporting Information). The TEM image of Cu/CeO2 replicated from MCM-48 spheres clearly show the mesoporous spherical particle morphology. Mesoporosity was further confirmed by N2 adsorption–desorption measurements (Figure S2). All of the samples casted from SBA-15, as well as from KIT-6 aged at 100 and 130 8C, showed type IV isotherms with a capillary condensation step above p/po= 0.4, which are rather typical for mesoporous metal oxide nanocasts. 4d] Narrow pore size distributions were observed for all the samples except for Cu/CeO2 produced from KIT-6-40. Poresize analysis, obtained from the adsorption branch by NLDFT methods (see characterization section in the Supporting Information), indicated mesopores of approximately 5 nm [*] H. Yen, Prof. F. Kleitz Department of Chemistry and Centre de Recherche sur les Mat riaux Avanc s (CERMA), Universit Laval Quebec, G1V 0A6 (Canada) E-mail: freddy.kleitz@chm.ulaval.ca

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.

Lanthanum-catalysed synthesis of microporous 3D graphene-like carbons in a zeolite template

Three-dimensional graphene architectures with periodic nanopores—reminiscent of zeolite frameworks—are of topical interest because of the possibility of combining the characteristics of graphene with a three-dimensional porous structure. Lately, the synthesis of such carbons has been approached by using zeolites as templates and small hydrocarbon molecules that can enter the narrow pore apertures. However, pyrolytic carbonization of the hydrocarbons (a necessary step in generating pure carbon) requires high temperatures and results in non-selective carbon deposition outside the pores. Here, we demonstrate that lanthanum ions embedded in zeolite pores can lower the temperature required for the carbonization of ethylene or acetylene. In this way, a graphene-like carbon structure can be selectively formed inside the zeolite template, without carbon being deposited at the external surfaces. X-ray diffraction data from zeolite single crystals after carbonization indicate that electron densities corresponding to carbon atoms are generated along the walls of the zeolite pores. After the zeolite template is removed, the carbon framework exhibits an electrical conductivity that is two orders of magnitude higher than that of amorphous mesoporous carbon. Lanthanum catalysis allows a carbon framework to form in zeolite pores with diameters of less than 1 nanometre; as such, microporous carbon nanostructures can be reproduced with various topologies corresponding to different zeolite pore sizes and shapes. We demonstrate carbon synthesis for large-pore zeolites (FAU, EMT and beta), a one-dimensional medium-pore zeolite (LTL), and even small-pore zeolites (MFI and LTA). The catalytic effect is a common feature of lanthanum, yttrium and calcium, which are all carbide-forming metal elements. We also show that the synthesis can be readily scaled up, which will be important for practical applications such as the production of lithium-ion batteries and zeolite-like catalyst supports.

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.

Design of multicomponent photocatalysts for hydrogen production under visible light using water-soluble titanate nanodisks†

We report the design of efficient multicomponent photocatalysts (MPs) for H2 production under visible light by using water-soluble ultrathin titanate nanodisks (TNDs) stabilized by tetraethylammonium cations (TEA(+)) as building blocks. The photocatalysts are designed in such a way to significantly enhance simultaneously the efficiency of the three main steps in the photocatalytic process i.e., light absorption, charge separation and catalytic reaction. We show, as an example, the construction of water-soluble CdS-TND-Ni MPs. The designed CdS-TND-Ni MPs, in which CdS nanoparticles and TNDs are intimately assembled to enhance the charge transfer and surface area, are controlled in composition to optimize visible light absorption. The conception of the MPs allows them to be highly dispersed in water which markedly improves the photocatalytic H2 production process. Most importantly, a Ni co-catalyst is selectively located on the surface of TNDs, enabling vectorial electron transfer from CdS to TND and to Ni, which drastically improves the charge separation. Consequently, under visible light illumination (λ ≥ 420 nm), the optimally designed CdS-TND-Ni MPs could generate H2 from ethanol-water solution with rate as high as 15.326 mmol g(-1) h(-1) during a reaction course of 15 h and with an apparent quantum yield of 24% at 420 nm. Moreover, we also demonstrate that TNDs can be combined with other single or mixed metal sulfide to form water-soluble metal sulfide-TNDs composites which could also be of great interest for photocatalytic H2 production.

Non‐Topotactic Transformation of Silicate Nanolayers into Mesostructured MFI Zeolite Frameworks During Crystallization

Mesostructured MFI zeolite nanosheets are established to crystallize non-topotactically through a nanolayered silicate intermediate during hydrothermal synthesis. Solid-state 2D NMR analyses, with sensitivity enhanced by dynamic nuclear polarization (DNP), provide direct evidence of shared covalent 29 Si-O-29 Si bonds between intermediate nanolayered silicate moieties and the crystallizing MFI zeolite nanosheet framework.

Worm-Like Superparamagnetic Nanoparticle Clusters for Enhanced Adhesion and Magnetic Resonance Relaxivity

Nanosized bioprobes that can highlight diseased tissue can be powerful diagnostic tools. However, a major unmet need is a tool with adequate adhesive properties and contrast-to-dose ratio. To this end, this study demonstrates that targeted superparamagnetic nanoprobes engineered to present a worm-like shape and hydrophilic packaging enhance both adhesion efficiency to target substrates and magnetic resonance (MR) sensitivity. These nanoprobes were prepared by the controlled self-assembly of superparamagnetic iron oxide nanoparticles (SPIONs) into worm-like superstructures using glycogen-like amphiphilic hyperbranched polyglycerols functionalized with peptides capable of binding to defective vasculature. The resulting worm-like SPION clusters presented binding affinity to the target substrate 10-fold higher than that of spherical ones and T2 molar MR relaxivity 3.5-fold higher than that of conventional, single SPIONs. The design principles discovered for these nanoprobes should be applicable to a range of other diseases where improved diagnostics are needed.

Engineering the Surface of Therapeutic “Living” Cells

Biological cells are complex living machines that have garnered significant attention for their potential to serve as a new generation of therapeutic and delivery agents. Because of their secretion, differentiation, and homing activities, therapeutic cells have tremendous potential to treat or even cure various diseases and injuries that have defied conventional therapeutic strategies. Therapeutic cells can be systemically or locally transplanted. In addition, with their ability to express receptors that bind specific tissue markers, cells are being studied as nano- or microsized drug carriers capable of targeted transport. Depending on the therapeutic targets, these cells may be clustered to promote intercellular adhesion. Despite some impressive results with preclinical studies, there remain several obstacles to their broader development, such as a limited ability to control their transport, engraftment, secretion and to track them in vivo. Additionally, creating a particular spatial organization of therapeutic cells remains difficult. Efforts have recently emerged to resolve these challenges by engineering cell surfaces with a myriad of bioactive molecules, nanoparticles, and microparticles that, in turn, improve the therapeutic efficacy of cells. This review article assesses the various technologies developed to engineer the cell surfaces. The review ends with future considerations that should be taken into account to further advance the quality of cell surface engineering.

Stretchable, anti-bacterial hydrogel activated by large mechanical deformation

&NA; Hydrogels have been used extensively to deliver functional molecular cargos in response to external mechanical force. However, the intrinsic brittleness of gels restricts the applicable range of strain to 0.1, thus limiting the range of molecular release rate that may be controlled. Also, uncontrollable molecular diffusion, which is especially prominent in small molecules, reduces the role of mechanical stimulus on the release rate. As such, we hypothesized that these challenges would be resolved by combining cyclodextrin, which may form guest‐host complexes with small molecular cargos, with a stretchable hydrogel system. We examined this hypothesis by synthesizing cyclodextrin acrylate and incorporating it into a polyacrylamide gel that can be stretched by 100% of its original length. In the absence of external stretching, hydrogels containing cyclodextrin acrylate with a degree of acryloyl group substitution (DSA) of 2.3 presented a lower molecular release rate than hydrogels without cyclodextrin acrylate. More interestingly, the polyacrylamide‐cyclodextrin hydrogel system displayed an increased molecular release rate corresponding to the degree of stretching, particularly in the gels containing cyclodextrin acrylate with a DSA of 2.3. As such, this stretchable gel loaded with quinine was used to inhibit the growth of E. coli in lysogeny broth only when the gel was stretched. We believe the results of this study would be valuable for improving the quality of controlled molecular delivery and subsequent efficacy of molecular cargos. Graphical abstract Figure. No Caption available.