Anthony S.T. Chiang

Membranes and films of zeolite and zeolite-like materials

An ideal structure of zeolite membrane should be a slice of a perfect zeolite crystal attached on a porous metal or ceramic support. To maximize the throughput, the zeolite layer must be very thin, limited only by the cell dimension of zeolite. Separation of a mixture may then be achieved based on the molecular sieving ability of zeolite, which allows only molecules smaller than a critical size to pass through. A variety of methods have been reported for the preparation of zeolite membranes, but so far a perfect epitaxial zeolite layer is still out of reach and only a polycrystalline zeolite membrane can be obtained. The first part of this review focuses on the permeation of gases and vapors through a polycrystalline zeolite membrane as a separation means. The effect of microstructure on permeance will also be discussed, as well as the preparation methods leading to different microstructures. In addition to the usage as a shape-selective membrane, thin films of zeolite and zeolite-like molecular sieves can also serve as hosts for the encapsulation and orientation of guest atoms and molecules and their clusters. In the second part of this review, the production of layers of aligned microporous molecular sieve crystals on supports and the fabrication of supported thin zeolite-like nanoporous silica films as well as their potential applications on the preparation of advanced materials are discussed.

Mesoporous silica with short-range MFI structure

Abstract A new type of mesoporous silica has been prepared which showed 780 m 2 /g of BET surface area and 0.6 ml/g of primary mesopores narrowly distributed around 4.2 nm. More importantly however, is that it showed short-range zeolite crystallinity as demonstrated by FTIR and XRD analysis, and hydrophobicity as demonstrated by water and n -hexane adsorption. This material was synthesized via a dual-template, three-step hydrothermal–flocculation–steaming synthesis procedure developed by us recently. Briefly, MFI nanoprecursors (NPs) were first prepared by a low-temperature hydrothermal step using TPAOH as template for zeolite structure, and then flocculated using a surfactant that served as the template for the mesopores. The collected NPs are mesoporous silica exhibiting short-range MFI domains when directly calcined. However, the steaming step promoted the crystallization of the NPs and created uniform mesopores. It was found that almost every detail in these procedures affected the properties of the final product. The most important variables, however, were identified as the duration the flocculants were kept in contact with the liquid phase, and the humidity under which the steaming was conducted. By properly adjusting the procedures, the said mesoporous silica, as well as nanocrystals having high external surface area, could be produced at will.

Adsorption and diffusion of aromatics in AIPO4-5

The packing arrangements of selected hydrocarbons in AlPO4-5 molecular sieves were proposed. These molecules were of sizes comparable to the pore diameter of AlPO4-5. These packing arrangements at least qualitatively explained the results on vapor adsorption, liquid-phase counter diffusion, and binary liquid adsorption. Ortho-xylene behaved differently from its isomers in adsorption and diffusion processes. Its packing arrangement also reflected this fact.

Synthesis of silicalite nanocrystals via the steaming of surfactant protected precursors

A new scheme has been developed for the confined synthesis of silicalite nanocrystals. The scheme involves: (1) Prepare a clear solution that is known to produce colloidal TPA-silicalite upon extended hydrothermal reaction. (2) Subject the solution to hydrothermal condition but stop before the appearance of colloidal silicalite. (3) Protect the TPA-silicalite precursor nanoparticles with cationic surfactant and collect them as flocculated mass. (4) Convert the precursor/surfactant hybrid into nanocrystals via high temperature steaming. It was found that an intermediate dilution during the hydrothermal step helped to delay the appearance of colloidal silicalite, and to produce more precursor nanoparticles. A steaming temperature of 150 °C was also found enough to convert the collected precursor into nanocrystals. The obtained nanocrystals were smaller than 30 nm with good XRD and IR crystallinity, which survived after 550 °C calcination.

The fractal and percolation analysis of a polymeric Al2O3 gel

Abstract Fractal and percolation analysis have been performed on nitrogen adsorption isotherms of polymeric Al 2 O 3 xerogel, prepared from a low-water non-aqueous solution of aluminum alkoxide. These xerogels showed a distinctively low pore connectivity compared to that reported in the literature on more conventional alumina gels. The alumina xerogel exhibited a surface fractal behavior in the scale range from nitrogen size to a few nanometers. The mean pore coordination number and the surface fractal dimension of the xerogel were rather stable upon calcination, until the appearance of the α-alumina crystalline phase, despite of a large decrease in BET surface area and pore volume.

Synthesis and characterization of cubic periodic mesoporous organosilicas with a high loading of disulfide groups

Well ordered cubic periodic mesoporous organosilicas (PMOs) with the SBA-1 motif containing a high loading of disulfide groups have been synthesized by simple co-condensation of bis[3-(triethoxysilyl) propyl]disulfide (BTEPDS) and tetraethoxysilane (TEOS) using the cationic surfactant cetyltriethylammonium bromide (CTEABr) as the template under acidic conditions. The materials thus obtained exhibited ordered and uniform mesopores up to 50 mol% of BTEPDS (based on silica) in the initial mixture. The successful incorporation of disulfide groups into the ordered mesopores was confirmed by 29Si and 13C solid-state NMR spectroscopy. These disulfide-functionalized PMO materials showed good adsorption efficiency for Hg2+ ions. The successful conversion of the disulfide moiety to sulfonic acid functionality was also demonstrated by a simple post-oxidation method using hydrogen peroxide.

An analytical solution to equilibrium PSA cycles

A general pressure swing adsorption (PSA) cycle, involving pressure equalization, forward depressurization, incomplete purge, product backfill and feed pressurization steps, has been analyzed with an isothermal local equilibrium model assuming linear isotherms. The operation conditions of such a process could be characterized by the extents of feed, backfill and purge. Analytical expressions have been derived for the recovery ratio and the cycle throughput as a function of these extents. Critical conditions that ensure the formation of a simple shock and best utilization of column have also been identified. Product backfill plus pressure equalization steps were found the best policy when both recovery and throughput are important.

Preparation of TiO2B2O3 coating by the sol-gel method

Mixed sol of TiO2 B2O3 was prepared from titanium sec-butoxide and boric acid. This sol could be aged into a clear gel, or dip coated into a transparent film. X-ray diffraction studies on the dried gel show that the transformation from anatase to rutile structure began below 500°C. This temperature is less than the case of pure TiO2. Based on Fourier transform infrared spectra, it is suggested that, after annealing, most boron is in orthoborate groups, the amount of which increases with increasing annealing temperature. The addition of B2O3 also changed the refractive index of the film produced by dip coating, and increased its transmittance in the wavelength range below 550 nm. In addition, it increased the alkaline and acid resistance of the film. Unlike the sol prepared from boron alkoxide, the sol prepared from boric acid was less affected by the atmospheric humidity during the coating process.

Grafting of cyclopentadienyl ruthenium complexes on aminosilane linker modified mesoporous SBA-15 silicates.

Cyclopentadienyl ruthenium phosphane and carbene complexes are grafted on the surface of mesoporous SBA-15 molecular sieves through an aminosilane linker. The nature of the support after the grafting is examined by powder XRD, TEM and N(2) adsorption/desorption analysis. Elemental analysis, FT-IR, DRIFTS, TG-MS and MAS-NMR studies confirm the successful grafting of the complexes on the surface. The grafted materials are applied for catalytic aldehyde olefination and cyclopropanation.

Natural zwitterionic organosulfurs as surface ligands for antifouling and responsive properties

Natural sulfur-containing zwitterionic compounds, l-cysteine (Cys), l-methionine, and glutathionine (GSH), have been employed as surface ligands to prevent protein nonspecific adsorption on planar substrates. These organosulfur compounds form self-assembled monolayers (SAMs) on gold substrates by gold-sulfur interaction. The chemical elements of SAMs were confirmed using x-ray photoelectron spectroscopy. The surface wetting tests for SAMs show that films prepared from Cys and GSH exhibited super-hydrophilicity (contact angles of θ = ~5°) due to their high coverage and strong hydration via ionic solvation and formation of hydrogen bonding. Quartz crystal microbalance with dissipation sensor was used to quantitatively and qualitatively monitor the adsorption of bovine serum albumin (BSA) from buffer onto these SAMs. It was found that the GSH film enables the resistance of BSA adsorption to the best extent at a physiological pH. Moreover, the surface charges of modified substrates were modulated by varying the pH value to control BSA adsorption. The effect of electrostatic repulsion on the antifouling behavior becomes prominent at a pH where the protein and the surface carry same charges. Consolidating the BSA adsorption measurements at different pH values, the antifouling properties of GSH-modified Au should be attributed to prevention of entropy gain and enthalpy loss, making BSA adsorption energetically unfavorable. It is believed that the surface modification with natural organosulfur ligands holds great potential in improving the biocompatibility of medical devices and in offering intelligent biointerfaces in response to environmental stimuli.