Ion exchange of zeolite coatings for adsorption heat pump applications

Coatings of NaA and NaX zeolites with various thicknesses were grown by direct crystallization on stainless steel plates. As a next step, ion exchange was applied to obtain the Li forms of the coatings. The materials obtained were characterized by X-ray diffraction (XRD), field emission gun scanning electron microscopy (FEGSEM), inductively coupled plasma (ICP), thermal gravimetry (TG), and N₂ adsorption. The stabilities of the coatings in the ion exchange process were also determined. Li⁺ ions were observed to be readily exchanged with Na⁺ ions in zeolite coatings of various thicknesses but the percentage of exchange did not attain 100%. Favorable ion exchange conditions for obtaining both zeolite LiNaA and LiNaX coatings were determined. After Li exchange, the surface area of zeolite X increased by about 56% while the water capacities of zeolites X and A were enhanced by about 15 and 14%, respectively, for the conditions investigated. The ion exchange process resulted in some detachment from the coatings, which increased with enhanced coating thickness. Significant improvement was obtained in coating stability by using plates with roughened surfaces. Zeolite coatings in Li form may improve the performances of adsorption heat pumps, provided that relatively thick coatings remain stable to a desired extent during ion exchange. [Figure not available: see fulltext.]