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.

During the conversion of natural gas to liquified natural gas, sulfur components are separated by adsorption on zeolites. New zeolite materials may improve this adsorption process. In this paper, the adsorption of hydrogen sulfide is studied on seven faujasite (FAU) zeolites, which differ only in the number of sodium and calcium cations. From a pure NaX zeolite (13X), which contains only sodium cations, the calcium cation content was gradually increased by ion exchange. In a fixed-bed adsorber, cumulative equilibrium loadings of H 2 S on these zeolites were determined at concentrations between 50 and 2000 ppm at 25 and 85 °C and 1.3 bar (abs). Adsorption isotherms were analyzed considering the influence of cation positioning in the FAU zeolites. The experimental data indicate a superposition of a chemisorptive and a physisorptive mechanism. At a small number of chemisorptive sites, we conclude a dissociation of hydrogen sulfide and covalent bonding of the proton and the hydrogen sulfide ion to the zeolite lattice. The contribution of chemisorption exhibits a very low temperature dependence, which is typical for nearly irreversible reactions with an equilibrium strongly shifted to one side. With an increase in the proportion of Ca 2+ cations, only physisorptive adsorption by electrostatic interaction with the cations in the lattice was observed. A large number of physisorptive sites have a lower energetic value. The share of physisorption strongly depends on temperature, which is characteristic of reversible equilibrium reactions.

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