Xingdong Wang

Zeolitization of diatomite to prepare hierarchical porous zeolite materials through a vapor-phase transport process

In this study, we report a new, simple approach to the preparation of hierarchical structured zeolites through transforming the diatomaceous silica into zeolite by a vapor-phase transport (VPT) method. The morphology and macro-porosity of the diatomite are well preserved even in the samples with zeolite content higher than 50%. The products possess high mechanical strength and hydrothermal stability, and are thus promising for application in catalysis, adsorption and separation. The influence of the zeolite structures, the amount of adsorbed seeds, and the VPT treatment time and temperature on the crystallinity of the resulting materials are discussed. Powder XRD, SEM, TEM, IR and N2 adsorption–desorption measurements are employed to monitor the VPT treatment process.

LAYER-BY-LAYER ASSEMBLY OF NANOZEOLITE BASED ON POLYMERIC MICROSPHERE: ZEOLITE COATED SPHERE AND HOLLOW ZEOLITE SPHERE

ABSTRACT Zeolite β, silicalite-1, ZSM-5, and TS-1 coated spheres have been prepared successfully through layer-by-layer assembly of nanozeolite/polymer multilayers on polystyrene (PS) microspheres, and hollow zeolite spheres have been obtained by removal of the core by calcination. In the adsorption process of nanozeolites onto the polyelectrolyte-modified template spheres, it has been found that zeta potential of the zeolite colloidal solution that determines the electrostatic interaction has important effects on the zeolite adsorption procedure. The charge on the original sphere template is not necessary, because proper polyelectrolyte modification could change the surface property of the template and make it suitable for layer-by-layer adsorption. The influences of zeolite type and/or size (40–120 nm), the number of zeolite/poly(diallyl-dimethyl-ammonium) layer pairs, and the template sphere size (0.53–10.3 µm) on the preparation of the hollow zeolite spheres were also discussed.

Fabrication of hollow zeolite spheres

Hollow spheres of zeolite have been fabricated through a layer-by-layer technique using polystyrene spheres as templates and nanozeolites as ‘building blocks’, followed by calcination.

Fabrication of zeolite coatings on stainless steel grids

During the last decade considerable research enthusiasm has been aroused for the preparation of zeolite coatings on various substrates, which might be used as catalysts, adsorbents, chemical sensors, selective electrodes and corrosion-resistant films [1–7]. Stainless steel is one of the ideal supports for preparing such kinds of materials due to its low cost, high thermal stability, good thermal conductivity, mechanical strength, facility of forming arbitrary shape and resistance to chemical corrosion [4–7]. The zeolite coatings on stainless steel have generally been prepared using in situ crystallization [4–6] and seed film [7] method. However, these methods can hardly control over the zeolite particle size, film thickness and are difficult to avoid the concurrently crystallizing of zeolites in the synthesis solution [5]. Recently, many endeavors have been paid to developing the layer-by-layer (LbL) zeolite assembly technique, which can easily control the zeolite film composition, structure and thickness by alternately electrostatic adsorbing zeolite nanocrystals and polyelectrolytes [8–10]. By this convenient and versatile approach the zeolite coatings on latex spheres [8, 9] and carbon fibers [10] have been successfully prepared. In this paper, the LbL zeolite assembly technique was first used to fabricate zeolite coatings on stainless steel grids. Because of the unique features of steel grids and the controllability of thickness and composition of the nanozeolite films, the novel materials are expected to have even more favorable conditions for mass transfer, thermal conductivity and lessening pressure drop. Furthermore, these kinds of materials can also be applied as the monolithic catalysts in de-NOx , dehydrogenation, catalytic distillation and other important catalytic processes if the corresponding active nanozeolites were used as the building blocks. Nanocrystals of silicalite-1 (80 ± 10 nm and 300 ± 40 nm), TS-1 (80 ± 10 nm), ZSM-5 (80 ± 10 nm) and beta (40 ± 5 nm) were prepared as described in the literature [11–15], and characterized by means of XRD, IR and SEM. The products were purified by repeated centrifugation and washing, then dispersed in distilled water to form a stable zeolite suspension with a concentration of approximately 1.0 wt% at pH 9.5 (adjusted with NH4OH) In order to provide a smooth and positively charged surface to aid subsequent adsorption of nanozeolites, the stainless steel grids (wire diameter of ∼22 μm and mesh size of ∼5 μm as shown in Fig. 1a) were precoated with three layers of polyelectrolytes of cationic poly(diallyldimethylammonium chloride) (PDDA) and anionic poly(styrene-sulfonate, sodium salt) (PSS) in the order of PDDA/PSS/PDDA. Then, the nanozeolites and PDDA were alternately deposited on the surface of the modified steel grids to form homogeneous nanozeolite/PDDA multi-layer films (20 min each adsorption step). After each adsorption step was completed, the grids were rinsed with 0.1 mol/L NH4OH solution for four times to remove the excess nanozeolites or PDDA. After certain deposition cycles had attained, the nanozeolite-coated grids were calcined at 823 K (heating rate 5 K/min) for 5 h in air to remove the organic species. The proper electrostatic interaction between substrate and zeolite particles is crucial to forming the perfect zeolite coatings on stainless steel grids by LbL method. Thus the charge character of nanozeolite, which may be scaled by zeta potential, is the fundamental parameter that affects the LbL process. The curves of the zeta potential of the 80 nm silicalite-1 colloids vs. pH value and salt concentration were shown in Fig. 2. We could find that, if positively charged steel grids were used as substrates, it was prerequisite to make the nanozeolite particles oppositely charged (i.e., negatively) by keeping the deposition solution in basic condition. At pH 9.5 and 0.1 mol/L salt concentration, the coatings on the grid surface were very uniform and dense after one layer of nanozeolites was deposited. However, uncovered regions could be observed on the grids at the same pH value but without adding salts (Fig. 1b), which may be explained as the fact that too high negative zeta potential on the zeolite particles would lead to the mutual repulsion among them. On the other hand, if the zeta potential of the nanosilicalite-1 particles was not high enough, e.g., at pH near 7, zeolite particles preferred to coalesce and form aggregates in the solution. These phenomena indicate that appropriate amount of surface charge of the nanozeolites determines the density of zeolite coating. To further study the effect of electrostatic attraction, the pH value of the dipping solution was also adjusted below the isoelectric point, e.g. pH = 3.0, where the particle surface is positively charged. Only few particles were deposited on the positively charged grids after one nanosilicalite-1 deposition step. However, when

Morphology modification by high energy ion beam bombardment concurrent with TiN film growth

Ion bombardment during film growth has been shown to be useful in controlling the growth kinetics and thus the physical properties of the film. One of the more spectacular aspects of how ion beams modify the growing film concerns topographical changes. There have been a number of reports in the literature concerning the development of surface morphology of films growing under concurrent low energy (<1 keV) ion bombardment [1, 2], but few works dealing with morphology modification by high energy ion beam bombardment concurrent with film growth. There are remarkable differences between low energy ion/surface interaction and high energy ion/surface interaction in terms of range and energy deposition etc. In assessing the role of low energy ion irradiation on the surface morphology, ion energy and ion flux were often considered, but the effect of ion species was usually neglected, because the mean ion ranges in the film for different ions at low energies are very similar. However, at higher energies, ions with different masses penetrate to quite different depths, and thus their influence on topography modification should be detectable. On the other hand, there are also differences between morphology modification by high energy ion bombardment concurrent with film growth and after film deposition [3-5]. The present authors have reported the fabrication of TiN films by vapour deposition of Ti in a N2 atmosphere concurrent with high energy inert Xe + ion beam bombardment [6] and studied the effect of ionic mass on the intrinsic stress of the formed TiN films [7]. In this letter, the morphologies of the synthesized TiN films were examined by atomic force microscopy (AFM), which can provide quantitative three-dimensional surface morphological information with high resolution. In order to investigate the mechanism of surface morphology modification by high energy ion beam irradiation during film growth, both Xe + ions and lighter Ne + ions were employed to prepare TiN films. A detail description of the film synthesis facility has been given elsewhere [6]. The TiN films were deposited onto optically polished silicon substrates by electron beam evaporation of Ti at a rate of 1 nms -a in a N2 atmosphere. The angle of incidence of the vapour stream to the substrate was 45 °. During deposition the substrates were water-cooled and did not reach atemperature greater than 200 °C.