(183ad) Li and Mg Ion-Exchange of Zeolite NaX Coatings Crystallized on Metal to Obtain Materials with Enhanced Water Sorption Capacity

Heat and mass transfer limitations present in adsorption heat pumps, which may be used in heating and cooling applications, may be eliminated to a great extent by using zeolite coatings on metal surfaces. In previous studies, zeolite NaA and NaX coatings, which had adsorption kinetics, stability, thickness and open nature easing diffusion, suitable for adsorption heat pumps, had been obtained on stainless steel surfaces [1,2]. Lately, different materials, such as metal-organic frameworks which may have surface areas higher than zeolites have been proposed for use in adsorption heat pumps. This may provide a good opportunity for improving further the power and thermodynamic efficiency of these devices. However, there may be some problems with these materials, such as kinetic restrictions as well as lack of stability in water and at high temperature and, thus, it may be difficult yet to obtain favorable effects with these materials. It may also be very useful to enhance the water adsorption capacities of the commonly used faujasite type (X and Y) zeolites. It is lately reported that some ionic forms of these zeolites have higher water adsorption capacities than the sodium form, in which they are originally prepared. However, in order to take full advantage of these materials in adsorption heat pumps and other applications, their coatings on metal supports should also be available.

In this study, Li and Mg ion-exchange was performed for NaX zeolite coatings which were directly crystallized on 5x5 cm² stainless steel plates. Solutions of 200 ml having LiCl or MgCl₂ concentrations between 0.001 M and 0.6 M were utilized for the ion-exchange process, which was performed at various temperatures for various durations. The zeolite coatings were prepared by using both conventional and substrate heating methods [3] in order to obtain materials with different properties. The stabilities of the coatings during ion-exchange were determined by weighing the samples before and after the ion-exchange process. The degree of ion-exchange was investigated by chemical analyses. The surface properties were determined by N₂ adsorption. Additionally, the water sorption capacities of the coatings were measured by thermal gravimetry (TG). These measurements were performed between ambient temperature and 350 °C under nitrogen flow. A heating rate of 20 °C/min was used.

It was observed that the ion-exchange procedure affected the stabilities of the zeolite coatings prepared on stainless steel adversely in different degrees, depending on the synthesis conditions used for obtaining the coatings. For the relatively thin coatings prepared, the amount of detachment was close to 30% of the initial coating mass while for the thicker coatings this amount was generally over 60%. In order to find a method for solving this problem, the stainless steel plates were made rougher by sanding their surfaces. In this manner, the zeolite crystals bonded to the metal surface could be protected from external impacts to a higher extent. Coatings grown on sanded stainless steel surfaces were observed to be quite more enduring and the amount of detachment decreased to about 22%.

The Na in the coatings could easily be exchanged with Li for up to 90% in this study. Only when the LiCl concentration was decreased to below 0.01 M, the exchanged amount of Li started to decrease. After Li exchange, the surface area of zeolite X increased notably by about 56% while the water sorption capacity increased by about 15%. The optimum conditions for the ion-exchange process were determined. On the other hand, it was observed that Na in the zeolite X coatings could not be exchanged with Mg so readily and only about 50% exchange could be accomplished even after a number of exchanges. Relatively high MgCl₂ concentrations in the range 0.3-0.6 M could provide this result while lower concentrations led to even lower amount of ion exchange. The water sorption capacities of MgNaX coatings were generally slightly lower than those of LiNaX coatings, possibly due to the lower amount of exchange. Table 1 shows the water sorption capacities and stabilities obtained for some of the coatings prepared.

Table 1. Stabilities and water sorption capacities of zeolite X coatings prepared under various conditions.

Coating	Synthesis method	Coating thickness (mm)	Mass loss due to ion-exchange (%)	Water sorption capacity (%)
LiNaX1	Conventional	11	27.3	26.8
LiNaX2	Conventional	17	62.0	26.9
LiNaX3	Substrate heating	112	74.6	26.9
LiNaX4	Substrate heating (on rough surface)	117	22.5	26.9
MgNaX	Conventional	12	29.8	25.9
NaX	Conventional	14	-	23.3

References

1. Schnabel, M. Tatlier, F. Schmidt, A. Erdem-Åženatalar, Adsorption kinetics of zeolite coatings directly crystallized on metal supports for heat pump applications, Appl. Therm. Eng. 30 (2010) 1409-1416.
2. Tatlier, G. Munz, G. Fueldner, S.K. Henninger, Effect of zeolite A coating thickness on adsorption kinetics for heat pump applications, Micropor. Mesopor. Mater. 193 (2014) 115-121.
3. Erdem-Åženatalar, M. Tatlier, M. Ãœrgen, M., Preparation of zeolite coatings by direct heating of the substrates. Micropor. Mesopor. Mater. 32 (1999) 331-343.