Phosgene synthesis catalysis: reaction kinetics and adsorption characteristics over Norit RX3 Extra activated carbon formulation

Abstract

The authors have recently refined a model for phosgene synthesis over industrial grade activated carbons that involves two classes of active sites: type-I and type-II. This article looks to further validate the model by examining kinetic aspects of the reaction. The work focuses on a single formulation of activated carbon, Norit RX3 Extra, and extends the applicability of the previous analysis undertaken on the Donau Supersorbon K40 formulation of activated carbon; both materials are representative industrial grade catalysts active for phosgene synthesis. The orders of this reaction, with respect to reagents and products CO, Cl₂ and COCl₂ are 1.04 ± 0.02, 0.46 ± 0.02, 0.04 ± 0.01 respectively. These findings reproduce the observations over Donau Supersorbon K40 and further validate the proposed reaction model. However, non-competitive adsorption studies over fresh catalyst reveal the following order of adsorption coefficients (K): K_COCl2 > K_Cl2 ≫ K_CO. This contrasts with the studies over Donau Supersorbon K40, suggesting a different distribution of active sites. Studies that regenerate the catalyst and re-adsorb chlorine show the concentrations of type-I and type-II Cl₂ adsorption sites are 3.2 and 0.32 mMol Cl₂ per g_(cat), respectively; the retention on type I sites being 3.5 times greater for the Norit material than is observed for the Donau sample. Mass balance profiles endorse these findings. Temperature-programmed breakthrough measurements are interpreted as indicating a degree of surface etching of the carbonaceous substrate. Whilst this work reports some distinct differences between the two catalysts, the recently amended phosgene synthesis reaction model is validated over Norit RX3 Extra, enhancing the model's credentials as being representative for phosgene synthesis over activated carbon formulations.