(217ex) Polymeric Post-Synthesis Treatment for Improving Zeolite Coating Stability

Zeolite coatings may be grown on a wide variety of supports including metal surfaces, ceramic and polymeric plates, metallic wires, organic and inorganic fibers, which may be used in various applications, such as structured catalysts, sensors, membranes and adsorbents. One of the relatively new but strongly emerging applications of zeolites, as adsorbents, is their use in adsorption heat pumps.  These devices may provide the effects of heating and cooling whenever an appropriate source of energy, such as solar energy and waste heat is available for heating the adsorber, where the adsorbent is placed. They have the remarkable abilities of consuming no or little electricity and being environmentally friendly.

Zeolite coatings grown on metal supports should survive consecutive heating and cooling at quite rapid rates in order to be used in adsorption heat pumps. Lack of sufficient stability will result in an ineffective use of these adsorbent coatings. In this study, zeolite A and zeolite X coatings were prepared on stainless steel plates by using the substrate heating method [1]. In this synthesis method, the temperature of the metal substrate is controlled independent of from that of the reaction mixture, which is kept at a lower temperature. Relatively thick coatings exhibiting accessible sponge-like structures originating from their growth under a thermal gradient may be prepared by using this method. The zeolite coatings obtained were then covered by additional polymer films in order to improve their stabilities. Various synthesis conditions were used to obtain coatings of different thicknesses. X-ray diffraction (XRD), field emission gun scanning electron microscopy (FEGSEM), Fourier-Transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were used to characterize the materials prepared. The mechanical and thermal stabilities of the coatings were determined by the application of consecutive ultrasonic treatments and heating/cooling cycles, respectively.

Synthesis times of 18-72 h were utilized to obtain the zeolite A and X coatings while the surface temperature of the metal plate surface was kept at about 57°C and 58°C, respectively for the former and the later zeolites. At least two zeolite coatings of similar thicknesses were prepared for each synthesis condition used in this study. One of the zeolite coatings in each set was covered with a thin polymer film (polyacrylic acid ). Solutions containing 2%, 4% and 8% polymer were used to prepare the films for covering the zeolite coatings. Thermogravimetric analysis (TGA) was performed to determine the water desorption between ambient temperature and 150 °C under a nitrogen for the composite metarials obtained.  A heating rate of 20 °C/min was used to raise the temperature of the zeolite samples to 150 °C, after when the temperature was kept constant for 15 min. The mechanical and thermal stabilities of the coatings prepared were tested by using two distinct methods, namely, ultrasonic treatments and rapid heating-cooling cycles, respectively. Firstly, the coatings were treated thrice in an ultrasonic water bath for duration of 30 min each, at a temperature of 30°C. As a next step, three consecutive heating and cooling cycles were carried out for each sample investigated. The samples were heated on a hot plate from 25°C to 150°C in 1 min and then they were cooled rapidly to room temperature. Finally, another ultrasonic water bath treatment, at the same conditionsmentioned above was applied to the coatings. The samples were kept in a desiccator  under a controlled humidity atmosphere and weighed at the end of every single stage of the tests in order to determine the loss of coating mass.

The mass equivalent thickness of the zeolite A and zeolite X coatings varied between about 130 µm and 360 µm. The crystallinity and phase of the coatings were confirmed by X-ray diffraction analyses. According to TGA results, there was generally no difficulty for water desorption the samples coated with polymer films, when compared to the pure zeolite A coating. The zeolite samples coated with polymer even desorbed somewhat higher amount of water compared to the zeolite A coating. It might be speculated that the unexpected increase observed for the polymer-coated zeolite samples is related to the existence of some kind of an interphase between the polymer and zeolite [2], which has properties different than both materials. The results of the ultrasonic and heating + cooling treatments revealed quite high stabilities for all the samples coated with polymer. The post-synthesis treatment applied to zeolite coatings improved the stabilities of these materials, when compared to pure zeolite coatings. The polymer layer seemed to be well integrated with the zeolite crystals. The intercrystalline voids, which are thought to be responsible for the high effective diffusivities obtained for coatings prepared by using the substrate heating method, continued to exist after these materials were covered by the polymer.

 References

[1]        A. Erdem-Şenatalar, M. Tatlier, M. Ürgen, Microporous Mesoporous Mater. 32 (1999) 331-343.

[2]        Ş.B. Tantekin-Ersolmaz, Ç. Atalay-Oral, M. Tatlier, A. Erdem-Şenatalar, B. Schoeman, J. Sterte, J. Membr. Sci. 175 (2000) 285-288.