Investigation of the Adsorption of Hydrogen Sulfide on Faujasite Zeolites Focusing on the Influence of Cations

Abstract

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 H₂S 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 Ca²⁺ 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.

Figure 1

Process scheme of the experimental plant.

Figure 2

Schematic view of (a) a sodalite cage and (b) a super cage with the distribution of cation positions in the FAU zeolite (according to refs (9) and (20). Position I: This position is centered inside the double six-membered ring that connects the sodalite cages. Per super cage, this position is present 16 times. Position I’: Position I′ is located on the double six-membered ring surfaces of the sodalite cages. In total, this position is present 32 times per supercage. Position II: Cation site II is located on the six-membered ring surfaces of the sodalite cages, which are oriented toward the supercage. These are present 32 times per supercage and are accessible to adsorptive molecules. Position II’: This cation position corresponds to position II and is also present 32 times per supercage, but cation position II′ has shifted into the sodalite cage. Thus, this position is not accessible from the supercage and therefore is inaccessible to many adsorptive molecules. Moreover, positions II and II′ cannot be occupied simultaneously. Position III: This position is located inside the supercage near the four-membered rings. From this, a total of 48 sites are available per supercage. Adsorptive molecules can easily reach cations at this position. Position III’: While position III is centrally located on the surface of the four-membered ring, position III′ is located offset from the four-membered ring. Similarly, this position is described as lying in the 12-member ring, which forms the entrance to the supercage. Cation position III′ occurs up to 96 times per supercage.

Figure 3

Position of the cation sites I and I′ in the double six-membered ring.

Figure 4

Changes in the unit cell during the exchange of Na⁺ cations for Ca²⁺ cations (according to ref (18)).

Figure 5

H₂S isotherms on calcium-exchanged FAU X zeolites at 25 °C.

Figure 6

H₂S isotherms on FAU X zeolites at 25 °C: (a) exchange rates 0–50% and (b) exchange rates 56–90%.

Figure 7

H₂S isotherms on calcium-exchanged FAU zeolites at 85 °C.

Figure 8

H₂S isotherms on FAU zeolites at 85 °C: (a) exchange rates of 0–50% and (b) exchange rates of 56–90%.

Figure 9

H₂S isotherms at 25 and 85 °C on (a) NaX and (b) CaNaX 56%.

Figure 10

H₂S isotherms normalized to the input concentration at 25 and 85 °C on (a) NaX and (b) CaNaX 56%.

Figure 11

Mechanistic proposal of the interaction of an adsorbed H₂S molecule on Na⁺ cations on position III.

Figure 12

Isotherms of H₂S on zeolites NaX and NaA at (a) 25 °C and (b) 85 °C.