Synthesis of thick zeolite 4A coatings on stainless steel Lucio Bonaccorsi, Edoardo Proverbio * Department of Industrial Chemistry and Materials Engineering, Universita ` di Messina Salita Sperone, 31––98166 S. Agata di Messina (Messina), Italy Received 27 March 2003; received in revised form 16 June 2004; accepted 16 June 2004 Abstract Zeolite coatings on metal supports having thickness ranging from few to several tens of microns could have important technical applications in adsorption heat pumps. Direct growth of zeolites on metal surfaces could improve the physical contact between the substrate and adsorbent leading to an improvement in the heat transfer performance on which pump operation is based. One of the major problems in zeolite coating technique is the final film thickness obtained by in situ synthesis, generally limited to few microns. In this work, thick zeolite A coatings have been prepared on stainless steel supports by direct synthesis. The metal supports were pre- treated before coating. Two surface pre-treatments have been compared: a surface oxidation at high temperature in air and an alum- inizing process obtained by coating the metal surface with an aluminum–silicon eutectic alloy (Al–10%Si). After this, a layer of zeolite A seeds was deposited by dip-coating on the pre-treated supports. A first zeolite coating was obtained by secondary growth of the zeolite seeds. Coating thickness of up to 25 lm were obtained on aluminized stainless steel. Several procedures for further growth and accretion of the initial deposit were attempted. Addition of triethanolamine to the synthesis mixture allowed the zeolite coatings obtained on aluminized stainless steel to reach a thickness of 80 lm. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Coating; Zeolite; Adsorption heat pumps; Deposition; Crystal growth 1. Introduction In recent years the preparation of zeolite films, mem- branes and coatings has gained increasing interest for new applications in catalysis, separation and adsorption processes [1–6]. Synthesis of a zeolite membrane for gaseous or liquid separation requires the formation of a thin, in the range of microns, pinhole free, continuous layer, ideally a monocrystalline zeolite film. Since it is very difficult to prepare a membrane made of a single large zeolite crys- tal, much effort has been devoted growing polycrystal- line structures with aligned, well packed, crystals [4–6]. For some applications, such as adsorption heat pumps, zeolites are used for their adsorption rather than separation properties. In this case, continuity is less important than coating thickness. A zeolite layer di- rectly grown on the heat exchanger tubes is the ideal arrangement to remove some critical limitations of adsorption heat pumps [7]. The optimum layer thickness is a function of the operating conditions and parameters including the wall thickness of the exchanger tubes and the adsorbent/adsorbate pair used [7–9]. Tatlıer and Erdem-Senatalar [9] using in a mathematical model the zeolite 4A–water pair, calculated that the optimum layer thickness should range between 75 and 150 lm. In a recent debate [10,11] it was discussed if zeolite layers thinner than 500 lm and large void volume in the adsorber could negatively influence the heat pump performances. In spite of this, it should be pointed out that the growth of zeolite coatings with a thickness of a few tens 1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.06.024 * Corresponding author. Tel.: +39 090 6765608; fax: +39 090 391518. E-mail address: proverbi@ingegneria.unime.it (E. Proverbio). www.elsevier.com/locate/micromeso Microporous and Mesoporous Materials 74 (2004) 221–229