Effects of fractality on the accessible surface area values of zeolite adsorbents

The effects of the molecular sizes of adsorbates on the accessible surface area values of the zeolites 13X, 5A, silicalite and NaY as well as the modified forms of NaY are investigated by taking into account the concept of fractality. For this aim, a relationship developed by combining the Pfeifer–Avnir and the surface area equations, which relates the surface areas of adsorbents to the molar volumes and the cross-sectional areas of adsorbates is utilized. The expected relationship, signifying that the accessible surface areas of adsorbents having fractal dimensions above and below 2, increase and decrease, respectively, with decreasing size of the adsorbate, is quantified in this study for the above zeolite adsorbents. Modifying the properties of adsorbents by using various treatment methods is seen to be potentially useful for enhancing the performances of processes involving adsorption, e.g. adsorption heat pump applications. Since the treatments employed may change the fractal dimension of the original sample, adsorbates having proper sizes should be used with the modified forms in order to achieve a good result. The modified hydrogen form of NaY provides the opportunity to increase the efficiency of the adsorption heat pumps by almost 40% with respect to the utilization of the original sample when methanol is used as the adsorbate.