Precursor nanoparticles that form spontaneously on hydrolysis of tetraethylorthosilicate in aqueous solutions of tetrapropylammonium (TPA) hydroxide evolve to TPA-silicalite-1, a molecular-sieve crystal that serves as a model for the self-assembly of porous inorganic materials in the presence of organic structure-directing agents. The structure and role of these nanoparticles are of practical significance for the fabrication of hierarchically ordered porous materials and molecular-sieve films, but still remain elusive. Here we show experimental findings of nanoparticle and crystal evolution during room-temperature ageing of the aqueous suspensions that suggest growth by aggregation of nanoparticles. A kinetic mechanism suggests that the precursor nanoparticle population is distributed, and that the 5-nm building units contributing most to aggregation only exist as an intermediate small fraction. The proposed oriented-aggregation mechanism should lead to strategies for isolating or enhancing the concentration of crystal-like nanoparticles.

Precursor silica nanoparticles can evolve to silicalite-1 crystals under hydrothermal conditions in the presence of tetrapropylammonium (TPA) cations. It has been proposed that in relatively dilute sols of silica, TPA, water, and ethanol, silicalite-1 growth is preceded by precursor nanoparticle evolution and then occurs by oriented aggregation. Here, we present a study of silicalite-1 crystallization in more concentrated mixtures and propose that growth follows a path similar to that taken in the dilute system. Small-angle X-ray scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM), and high-resolution transmission electron microscopy (HRTEM) were used to measure nanoparticle size and to monitor zeolite nucleation and early-stage crystal development. The precursor silica nanoparticles, present in the clear sols prior to crystal formation, were characterized using two SAXS instruments, and the influence of interparticle interactions is discussed. In addition, SAXS was used to detect the onset of secondary particle formation, and HRTEM was used to characterize their structure and morphology. Cryo-TEM allowed for in situ visual observation of the nanoparticle population. Combined results are consistent with growth by aggregation of silica nanoparticles and of the larger secondary crystallites. Finally, a unique intergrowth structure that was formed during the more advanced growth stages is reported, lending additional support for the proposal of aggregative growth.

Here, we report for the first time the fabrication of polymer/selective-flake nanocomposite membranes which can, in principle, be scaled down to submicrometer structures. A layered aluminophosphate with a porous net layer is used as a selective phase and a polyimide as a continuous phase. The microstructures of the nanocomposite membranes are investigated using various characterization techniques including X-ray diffraction, NMR, transmission electron microscopy, small-angle neutron scattering, and dynamic mechanical thermal analysis. Nanocomposite membranes with 10 wt % layered aluminophosphate show substantial enhancement in performance with oxygen selectivity over nitrogen as high as 8.9 (as compared to 3.6 for pure polymer) and carbon dioxide selectivity over methane as high as 40.9 (as compared to 13.4 for pure polymer) in room-temperature permeation measurements. This improved performance, along with permeability estimation through the aluminophosphate layers with a semiempirical model, suggests that the layered aluminophosphate plays a role as a molecular sieve favoring smaller molecules.

Nanoslabs (1): Investigations of the mechanism of formation of tetrapropylammonium (TPA)‐silicalite‐1 by TEM and atomic force microscopy (AFM) analysis (see picture) are not consistent with the existence of “nanoblocks” or “nanoslabs” previously proposed by J. A. Martens and co‐workers

Pattern formation, oscillations, and traveling fronts are widely found in nature. [1,2] Oscillatory phenomena in chemical and biochemical systems include carbon monoxide oxidation on single crystals of platinum, [3] seashell patterns, [4] aggregating slime molds, [5] banding textures in minerals, [6,7] and spiral waves in cardiac arrhythmias [8] and retinal tissue. [9] There has been an interest in the pattern formation of particles formed by self-assembly, [10][11][12] particularly those driven by physical factors, such as surface tension in evaporating colloidal solutions. [13][14][15] Herein, we present a study of oscillatory phenomena, in which inorganic nanoparticles are organized with a high degree of periodicity; these phenomena are exhibited in a similar fashion to Liesegang precipitation bands or rings, [16][17][18] or patterns of colloidal origin, which are formed in ionic or other precipitation-diffusion systems.A simple synthesis of manganese oxide colloids has been developed. [19] Transmission electron microscopy (TEM) and small-angle neutron scattering (SANS) data show that the colloidal particles are of the order of 2-8 nm in diameter, with a disklike shape, and are dispersed in solution.[19] X-ray absorption spectroscopy indicates that the particles have a layered structure consisting of edge-shared MnO 6 octahedra; the Mn centers exhibiting an average oxidation state of 3.7. [20] These colloids can be organized [21][22][23] into structures and materials with multiscale ordering. Herein, the formation of inorganic films consisting of regular micrometer-sized parallel lines is reported. The composition and phase of the manganese oxide lines can be altered by ion-exchange or thermal treatment.Micropatterns of mixed-valent manganese oxide colloids were self-assembled from dilute sols of tetramethylammonium (TMA) manganese oxide. A glass microscope slide was immersed vertically into a colloidal solution and was then placed into a preheated oven held at 85 8C until complete evaporation of the solvent had occurred (Figure 1 a). The procedure results in a film composed of parallel lines (Figure 1 b). Removal of the hydroxyl groups by silynation from the glass surface or using other types of hydrophobic

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We employ high-temperature molecular dynamics to investigate self-transport and cooperative transport of benzene in NaX (Si:Al ) 1.2). We have refined the benzene-NaX force field for use with our previously developed framework force field for aluminosilicates, which explicitly distinguishes between Si and Al atoms in the frame, and also between oxygen atoms in Si-O-Si and Si-O-Al environments. Energy minimizations and molecular dynamics simulations performed to test the new force field give excellent agreement with experimental data on benzene heats of adsorption, benzene-Na distances, and Na distributions for benzene in NaY (Si:Al ) 2.4) and NaX (Si:Al ) 1.2). Molecular dynamics simulations are performed over a range of temperatures (600-1500 K) and loadings (infinite dilution to four benzenes per supercage) to evaluate simultaneously the self-diffusivities and cooperative (alternatively Maxwell-Stefan) diffusivities. The simulated diffusivities agree well with pulsed field-gradient NMR and quasi-elastic neutron scattering data. Despite this agreement, we show in the following companion paper that membrane fluxes calculated with our diffusivities overestimate experiments by 1 order of magnitude when support resistance is accounted for in the transport model, and by about 2 orders of magnitude when support resistance is neglected. This discrepancy may arise from the polycrystalline nature of present-day NaX membranes.