467164 Hydrothermal Synthesis of Composite Zeolite a and X Coatings
Zeolites A and X are two of commonly investigated zeolites since they have favorable properties for different applications, ranging from separation and adsorption to ion-exchange. The use of the coatings of these zeolites in adsorption heat pumps has also received much attention. In this respect, the preparation of sufficiently thick and stable coatings has become a significant challenge. Pure zeolite A and X coatings with mass equivalent thicknesses of up to about 270 µm could be prepared on stainless steel plates by using the substrate heating method. The equivalent thickness mentioned here was calculated from (mass of zeolite deposited per coated area of substrate)/(density of zeolite). The void fraction in these coatings was quite high and actual average thicknesses up to 650 µm were observed by laser microscopy . These coatings also exhibited an accessible sponge-like structure which seemed to result in promising adsorption kinetics.
The preparation of composite materials containing zeolite has already attracted interest for a few applications. Polymer-zeolite mixed matrix membranes have been obtained by dispersion of zeolite crystals in polymeric films for use in gas separation and pervaporation . The quality of the interface between zeolite crystals and polymer are key points in related research studies. In a recent study, zeolite coatings prepared on stainless steel were covered by polyacrylic acid . An interesting observation was that composite samples desorbed somewhat higher amounts of water compared to both pure zeolite and pure polyacrylic acid coatings. In another study, three-step syntheses were performed for forming rather low-silica-zeolite coatings on aluminum surfaces. Here, a high-silica-zeolite bottom layer, a bridging layer and a low-silica-zeolite top layer were formed consecutively.
In this study, composite coatings consisting of zeolite A and zeolite X were prepared. Different synthesis conditions were used for direct crystallization of these materials on stainless steel plates in two consecutive steps. Zeolite coatings were grown on stainless steel plates (AISI 316 grade). The plates had 1 cm diameter and 0.8 mm thickness. Coatings were prepared by substrate heating  and conventional synthesis methods for obtaining zeolite A and zeolite X coatings, respectively. The syntheses were carried out by using clear solution compositions represented by 50 Na2O:Al2O3:5 SiO2:1000 H2O (zeolite NaA)  and 70 Na2O:Al2O3:15 SiO2:2100 H2O (zeolite NaX) . In order to produce zeolite A, a resistance temperature of 140 °C and a water bath temperature of 25 °C were used with the substrate heating method. The synthesis was carried out for 1 day after when the water bath temperature was increased to 60 °C and was kept there for 2.5 h. The crystallization of zeolite X was performed in an oven kept at 80 °C for 26 h of synthesis. Pure zeolite A, pure zeolite X and composite A+X and X+A coatings were prepared in this study. A+X indicates that firstly zeolite A was obtained on the substrate after which a second synthesis was performed to form an additonal layer of zeolite X on the metal plate. X+A was the opposite case where zeolite X was the first layer grown on the substrate followed by zeolite A formation. The coatings prepared were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetry (TG). TG analysis was performed to determine the water desorption between ambient temperature and 350°C under nitrogen flow. A heating rate of 20 °C/min was used.
It was possible to prepare composite crystalline zeolite coatings with different amounts of zeolites A and X in the samples, as verified by X-ray diffraction analyses. According to TG measurements, the pure zeolite X and pure zeolite A coatings exhibited the highest and lowest amounts of desorption, respectively. In general, a slightly higher amount of desorption occurred from the A+X composite zeolite coating when compared to the X+A composite. It should be noted that the ratio of zeolite X mass/zeolite A mass was higher in the former coating. It was determined that the samples exhibited almost the same amount of desorption in consecutive experiments, indicating crystallinities close to 100%.
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