Wun-gwi Kim

Swelling, Functionalization, and Structural Changes of the Nanoporous Layered Silicates AMH-3 and MCM-22

Nanoporous layered silicate materials contain 2D-planar sheets of nanoscopic thickness and ordered porous structure. In comparison to porous 3D-framework materials such as zeolites, they have advantages such as significantly increased surface area and decreased diffusion limitations because the layers can potentially be exfoliated or intercalated into polymers to form nanocomposite materials. These properties are particularly interesting for applications as materials for enhancing molecular selectivity and throughput in composite membranes. In this report, the swelling and surface modification chemistry of two attractive nanoporous layered silicate materials, AMH-3 and MCM-22, were studied. We first describe a method, using long-chain diamines instead of monoamines, for swelling of AMH-3 while preserving its pore structure to a greater extent during the swelling process. Then, we describe a stepwise functionalization method for functionalizing the layer surfaces of AMH-3 and MCM-22 via silane condensation reactions. The covalently attached hydrocarbon chain molecules increased the hydrophobicity of AMH-3 and MCM-22 layer surfaces and therefore allow the possibility of effectively dispersing these materials in polymer matrices for thin film/membrane applications.

Silylated Mesoporous Silica Membranes on Polymeric Hollow Fiber Supports: Synthesis and Permeation Properties

We report the synthesis and organic/water separation properties of mesoporous silica membranes, supported on low-cost and scalable polymeric (polyamide-imide) hollow fibers, and modified by trimethylsilylation with hexamethyldisilazane. Thin (∼1 μm) defect-free membranes are prepared, with high room-temperature gas permeances (e.g., 20,000 GPU for N2). The membrane morphology is characterized by multiple techniques, including SEM, TEM, XRD, and FT-ATR spectroscopy. Silylation leads to capping of the surface silanol groups in the mesopores with trimethylsilyl groups, and does not affect the integrity of the mesoporous silica structure and the underlying hollow fiber. The silylated membranes are evaluated for pervaporative separation of ethanol (EtOH), methylethyl ketone (MEK), ethyl acetate (EA), iso-butanol (i-BuOH), and n-butanol (n-BuOH) from their dilute (5 wt %) aqueous solutions. The membranes show separation factors in the range of 4-90 and high organic fluxes in the range of 0.18-2.15 kg m(-2) h(-1) at 303 K. The intrinsic selectivities (organic/water permeability ratios) of the silylated membranes at 303 K are 0.33 (EtOH/water), 0.5 (MEK/water), 0.25 (EA/water), 1.25 (i-BuOH/water), and 1.67 (n-BuOH/water) respectively, in comparison to 0.05, 0.015, 0.005, 0.08, and 0.14 for the unmodified membranes. The silylated membranes allow upgradation of water/organics feeds to permeate streams with considerably higher organics content. The selective and high-flux separation is attributed to both the organophilic nature of the modified mesopores and the large effective pore size. Comparison with other organics/water separation membranes reveals that the present membranes show promise due to high flux, use of scalable and low-cost supports, and good separation factors that can be further enhanced by tailoring the mesopore silylation chemistry.

A Mesoporous Cobalt Aluminate Spinel Catalyst for Nonoxidative Propane Dehydrogenation

A mesoporous CoAl2O4 spinel (Co‐Al) is synthesized by a one‐step evaporation‐induced self‐assembly (EISA) method. N2 physisorption and TEM are used to demonstrate the presence of mesopores within the Co‐Al material. The spinel crystal structure of Co‐Al, in which Co occupies tetrahedral (Td) sites, is confirmed by using XRD and UV/Vis spectroscopy. In nonoxidative propane dehydrogenation at 550 °C, a propane conversion of approximately 8 % is observed for Co‐Al with a >80 % propylene selectivity, which corresponds to a turnover frequency of 5.1 h−1 based on an estimation of the number of active Co sites by using NH3 temperature‐programmed desorption. A much higher propane conversion rate and a circa 80 % propylene selectivity is observed upon reaction at 600 °C. Continuous deactivation of the catalyst is observed for Co‐Al at this elevated temperature. In situ X‐ray absorption spectroscopy results suggest that Co remains as a Td Co2+ species under the reaction conditions. The Td Co2+ sites within the Co‐Al material are thus proposed to act as Lewis acidic active sites; this acidity is verified using IR spectroscopy with pyridine as a probe molecule.