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The preparation of smooth mesoporous silica layers on rough, porous stainless steel supports and the subsequent synthesis of well-intergrown b-oriented MFI films on these intermediate layers were investigated. Parameters such as surfactant−silica sol dilution, number of coating cycles, drying, and calcination were systematically varied to optimize mesoporous silica deposition results. Multiple slip- and aerosol-coating cycles with intermediate drying/calcination steps had to be employed to prepare mesoporous layers with adequate thickness and minimal cracking. b-Oriented MFI seed monolayers could be covalently attached to these mesoporous layers with high coverage using 3-chloropropyltrimethoxysilane as the coupling agent. Sonication during the final step of seed deposition was found to be necessary to achieve high seed coverage and orientation. Finally, highly b-oriented and intergrown MFI films could be produced by secondary growth of b-oriented seed layers using trimer tetrapropylammonium iodide as the structure-directing agent.

A thermodynamic equilibrium model has been developed to describe amino acid adsorption on microporous materials. The model addresses electrostatic, hydrophobic and steric interactions. A procedure for fitting the model's parameters is presented and should be applicable to the majority of the common 20 amino acids. The approach is demonstrated using experimental measurements of l-phenylalanine and l-arginine on zeolite β. Between the adsorption mechanisms of ion exchange and physisorption, the first can contribute as much as two-thirds of the phenylalanine adsorbed at saturation. For the materials tested, ion exchange is maximized when the zeolite's silicon-to-aluminum ratio is 12. When this atom ratio is raised to 100, ion exchange no longer plays a significant role, but the amount physisorbed increases by 30%.

The effect of structure-directing agents (SDAs) on zeolite particle morphology and crystal structure was studied. Dimers and trimers of tetrapropylammonium hydroxide&sbd;the SDA of siliceous ZSM-5 (silicalite-1, structure type MFI)&sbd;lead to direct growth of the same framework but with drastically different crystal morphologies. Control over relative crystal growth rates along the three main crystallographic axes is demonstrated by obtaining crystals with their longest dimension along any of these axes. Unit cell dimensions and crystal structure were determined by refinement based on powder X-ray diffraction data and electron microscopy, respectively. The importance of the new morphologies in thin film molecular sieve technology along with possible crystal growth mechanisms is discussed.

Silicalite-1 (structure type MFI) is an important zeolite that, in addition to conventional applications such as adsorption and catalysis and emerging applications such as thin films and membranes, is often used as a model system in studies of zeolite crystal growth. The surface structure of silicalite-1 crystals with two different morphologies (hexagonal prismatic and leaf-shaped platelike crystals), synthesized in the presence of two different structure-directing agents (SDAs) (tetrapropylammonium (TPA) cation and the trimer of TPA, respectively), is elucidated by high-resolution transmission electron microscopy and atomic force microscopy. The differences and similarities in crystal shape and external surface termination are interpreted based on SDA fitting in the crystal structure and adsorption kinetics of the SDAs on the external surfaces of the growing crystals.

Siliceous ZSM-5 zeolite membranes oriented with their straight 0.55-nm pores perpendicular to the support and exhibiting high separation factors for p-xylene over o-xylene were reported recently using secondary growth of an oriented seed monolayer and employing organic structure-directing agents as crystal shape modifiers (Lai et al., Science 2003, 300, 456). Here, we report additional permeation results for this type of membrane. High separation factors, as well as high permeances, were observed for benzene/cyclohexane and benzene/2,2-dimethylbutane, although the separation factors for n-/i-butane and N2/SF6 were small. The membrane stability is satisfactory. Reproducibility of the permeation properties at 220 °C is demonstrated.

The value of the silicalite-1 colloidal crystal surface zeta potential was estimated from electrophoretic mobility measurements. Potassium nitrate and tetrapropylammonium bromide have been used as background electrolytes, and the pH was varied between 2.4 and 11.6. Comparison with similar results obtained with colloidal silica particles supports the hypothesis that both potassium and tetrapropylammonium ions can adsorb in the exposed silicalite-1 pores. The results were further analyzed with the triple layer surface complexation model that captured the observed zeta potential trends. The adjustable parameters of the model have been optimized by using a combination of factorial design, solution mapping, and linear regression. Furthermore, the model was used to study the effect of composition on the apparent activation energy. It is demonstrated that when surface complexation model predictions are used as input in the silicalite-1 growth model, there is qualitative agreement between growth model results and experimental observations. This is the first time that a detailed mechanism can explain issues such as the effect of composition on apparent activation energy.

The deposition of films within a porous substrate is modeled in a countercurrent reactor configuration. Two transient models are solved:  a diffusion−one-step reaction chemistry model and a diffusion−four-step reaction model that incorporates precursor formation followed by nucleation and growth events. It is shown that the quasi-steady-state/infinitely fast reaction assumptions employed in diffusion-reaction models are not valid in either the slow or fast kinetics regime due to the transient evolution of a deposit. The diffusion−one-step reaction model results in continuous deposits, which are relatively thick. The interplay of nucleation and growth with diffusion and reaction can result in quite thinner deposits. The demonstrated strong dependence of deposition features on initial conditions provides an additional means of controlling the deposition on one hand but indicates possible reduced experimental reproducibility on the other. Application to Pd membranes in alumina under supercritical CO2 conditions is discussed.

We report the hydrothermal synthesis of 10 μm × 100 μm × 10 μm single crystals of the microporous titanosilicate sodium-ETS-4. The crystallization of large needle-shaped Na-ETS-4 crystals is preceded by the formation and subsequent dissolution of the titanosilicate GTS-1. Large Na-ETS-4 crystals are obtained with purity higher than 97%. The structures of Na-ETS-4 and its Sr ion-exchanged form are solved from single-crystal X-ray diffraction data. It is suggested from the structure solution and further supported by the results of the empirical bond valence model that the bridging unit of ETS-4 contains five-coordinate titanium(IV). Infrared spectroscopy is also used to support the crystallographic results, with regard to the presence of charge-balancing protons and five-coordinate titanium. ETS-4 is, thus, one of only two synthetic titanosilicate materials containing five-coordinate titanium. We also discuss some structural differences between ETS-4 and its natural analogue, zorite.

Continuous palladium films were synthesized at controlled depths within porous alumina disks by H2 reduction of organopalladium compounds dissolved in supercritical CO2 at 60 °C using an opposing reactants deposition geometry. Film position was controlled by adjusting the relative concentrations of H2 and the palladium precursor (π-2-methylallyl(cyclopentadienyl)palladium (II) or palladium(II) hexafluoroacetylacetonate) on opposite sides of the alumina substrate. Because of a disparity in the diffusivity of the metal precursor and H2 in the support, a temporary barrier of poly-4-methyl-1-pentene on the H2 side of the alumina substrate was used to reduce H2 flux in a controlled manner. Guided by a simple mass transport model, Pd films between 2 and 80 μm thick were deposited at prescribed depths between 80 and 600 μm as measured from the precursor side. Electron probe microanalysis indicated complete pore filling of the porous alumina at the reaction zone and X-ray diffraction revealed that the structure of the deposit is nanocrystalline. The flux of N2 through the alumina disk was reduced by over 4 orders of magnitude after deposition and annealing at 500 °C.

Oriented MFI membranes made by secondary growth on the surface of α-Al2O3 support disks have been evaluated for the separation of xylene isomers vapors in the temperature range 22−275 °C and feed partial pressures up to ~0.7−0.9 kPa. It is found that the separation performance of these membranes is directly related to the synthesis conditions and the resulting membrane microstructure. Thick (12−18 μm) and c-oriented membranes (type A membranes) made by 24-h growth at 175 °C exhibit single-component p-xylene/o-xylene permselectivity as high as 150 at 100 °C but very modest separation factors (usually <5) are obtained with binary feed mixtures, a result of a drastic increase in the flux of o-xylene in the presence of p-xylene. Thin (2−3 μm) and (h0h)-oriented membranes (type B membranes) made by 120-h growth at 90 °C exhibit comparable single-component and binary permeation behavior but suffer from lower selectivities/separation factors (up to 12) as a result of cracks formed after calcination for template removal. It is found that the addition of n-hexane in a binary p-xylene/o-xylene feed in the case of type A membranes or the dip-coating of type B membranes with a surfactant-templated silica sol results in a considerable improvement of mixture separation factors up to 60 or 30−300, respectively. This improvement is attributed to preferrential adsorption of n-hexane in nonzeolitic micropores/grain boundaries in the case of type A membranes or to selective sealing of cracks by mesostructured silica in the case of type B membranes. Both type A and type B membranes exhibit, at 100−125 °C, p-xylene permeance [(2−5) × 10^-8 mol m^-2 s^-1 Pa^-1] and p-xylene/o-xylene separation factors (60−300) that are comparable to or higher than the values reported for other MFI membranes in the literature.