Fabrication of zeolite coatings on stainless steel grids

Abstract

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