Mechanistic investigation of catalytic COS formation on ion-exchanged LTA zeolites during fixed-bed adsorption

Abstract

In this study, the catalytic formation of COS from CO₂ and H₂S during the adsorptive treatment of natural gas is investigated. Therefore, experiments with a model gas (CO₂, H₂S and H₂O in N₂) in a fixed bed were carried out. Breakthrough experiments were performed on five zeolites of the NaA and CaNaA type at 25 °C, 85 °C and 145 °C. From pre-loading experiments with the feed components, a mechanistic idea of multi-component adsorption in the fixed bed is derived in the form of generalized loading and concentration profiles. Furthermore, it is shown that at high excess of CO₂ in the feed the chemisorptive dissociation of H₂S is rate-determining and a transition state for this step is proposed. The analysis of the COS breakthrough curves proves that the kinetics of the catalytic reaction is governed by temperature and the availability of physisorbed H₂S at catalytically active sites. When sodium is exchanged for calcium, the number of catalytically active cations decreases. On the other hand, cations with double the charge number are introduced. In breakthrough experiments with materials of different exchange ratios, at 25 °C the influence of the type of cation dominates the catalytic activity of the zeolite, while at 85 °C and 145 °C the decreasing number of cations dominates the kinetics. Finally, the advancing front of the slowly migrating feed water stops the reaction. Migration velocity increases with temperature. At 85 °C, the fastest kinetics with the highest amount of COS is observed due to the already high reaction speed and the still slow water front. This article is a follow-up of previous article [1].