Impact of Na+ and Ca2+ Cations on the Adsorption of H2S on Binder-Free LTA Zeolites

Hydrogen sulfide is removed from natural gas via adsorption on zeolites. The process operates very effectively, but there is still potential for improvement. Therefore, in this article, the adsorption of hydrogen sulfide was investigated on eight LTA zeolites with different cation compositions. Starting with the zeolite NaA (4 A), which contains only Na+ cations, the Ca2+ cation content was gradually increased by ion exchange. Equilibrium isotherms from cumulative breakthrough curve experiments in a fixed-bed adsorber at 25°C and 85°C at 1.3 bar (abs.) were determined in the trace range up to a concentration of 2000 ppmmol. From a comparison of the isotherms of the different materials, a mechanistic proposal for the adsorption is developed, taking into account the specific positions of the cations in the zeolite lattice when the degree of exchange is increased. The shape of the isotherms indicates two energetically different types of adsorption sites. It is assumed that two mechanisms are superimposed: a chemisorptive mechanism with dissociation of hydrogen sulfide and covalent bonding of the proton and the hydrogen sulfide ion to the zeolite lattice and a physisorptive mechanism by electrostatic interaction with the cations in the lattice. As the degree of exchange increases, the proportion of chemisorption sites seems to decrease. Above an exchange degree of 50%, only evidence of physisorption can be found. It is shown that this finding points to the involvement of weakly bound sodium cations at cation position III in the chemisorption of hydrogen sulfide.

Adsorption of carbon monoxide, methane and nitrogen on alkaline earth metal ion exchanged zeolite-X: structure, cation position and adsorption relationship

The present study reports adsorption of carbon monoxide, methane and nitrogen on alkaline earth metal ion exchanged zeolite-X and relationship between zeolite structure, cation position and adsorption properties.

Isotopenaustausch-Untersuchungen an Silicat-Ionenaustauschern

The isotopic exchange in Ba-and Sr-Zeolite X has been investigated over the entire range of the process. A special solution of the diffusion equation for the relevant case has been applied to evaluate self-diffusion measurements. The diffusion process was found to be non-uniform. It has been shown that 80% of the ex-changeable Ba-ions and 65% of the Sr-ions diffuse with a high diffusion coefficient and the remainder with a low one. They differ by four to five orders of magnitude at room temperature. In the temperature range 120° to 0 °C the activation enthalpy for the slow process is about twice as high as for the fast process. This behaviour is explaned in terms of cation location in the zeolite X lattice. The cations are accommodated in two types of channel systems which are assumed to be separated from each other at least with respect to alkaline earth ions. The cation position within the channel systems are discussed.