Kinetics of Catalyst Poisoning during Capillary Condensation of Reactants

The influence of the capillary condensation of reactants on the poisoning of Pt/SiO₂ catalysts by thiophene is studied experimentally for p-xylene hydrogenation at T = 60 and 80°C. The poisoning kinetics is independent of a catalyst and its rate decreases with a decrease in temperature. Poisoning during capillary condensation is 1.5–6 times slower than that in the gas phase, depending on the fraction of surface platinum in the pores filled with a liquid. The poisoning of the catalyst active sites in the pores filled with a liquid requires less sulfur at the same deactivation degree. The number of sulfur atoms per one platinum atom necessary for the complete poisoning of platinum in the gas phase is higher than that in the case of capillary condensation by a factor of 1.4–1.5.     

Catalytic reactions accompanied by capillary condensation. 1. Formulation of the problems

Capillary condensation was found to accompany gasphase catalytic reactions, in particular motor fuel hydrotreatment. Reaction conditions appear to differ significantly in large pores (filled with gas) and small ones (filled with liquid). Regarding capillary condensation we explain the strange dependences observed in this study or described in the literature. We have simulated jet fuel dearomatization taking into account capillary condensation as well.     

Catalytic reactions accompanied by capillary condensation 2. Mass transfer inside a pellet

The influence of mass transfer inside a catalyst pellet is analyzed in the case when a catalytic reaction accompanied by capillary condensation takes place. It is shown that gas-liquid interfacial transfer as well as diffusion in the liquid does not limit the process. The overall rate is determined by diffusion in macropores filled with gas, or by the reaction rate in the liquid phase.     

Catalytic reaction accompanied by capillary condensation, 3. Influence on reaction kinetics and dynamics

The influence of capillary condensation of reagents on the catalytic reaction kinetics and dynamics was studied. The hydrogenation ofp-xylene over Pt/SiO₂ was used as a model reaction. Two types of SiO₂ were used (KCK-1 with large pores and KCM-5 with small pores). It was shown that capillary condensation could modify the kinetics and the transition regimes. The proposed mathematical model demonstrates good agreement with experimental results for both steady-state and dynamic regimes, including reaction rate—temperature hysteresis.