Coking and Ex Situ Catalyst Reactivation Using Supercritical CO₂:  A Preliminary Study

Abstract

High initial conversions for the cyclododecatriene (CDT) hydrogenation in a slurry reactor at 433 K and 1.2 MPa were obtained from a supercritical (sc) CO 2 -cleaned 0.5% Pd/Al 2 O 3 catalyst. A recovered intermediate selectivity was achieved by the sc CO 2 -cleaned catalyst at the higher CDT conversion values, suggesting an effective cleaning of Pd sites and a possible reconstruction of the residual coke layers onto the surface caused by the solvent nature of sc CO 2 . SEM pictures show that the cleanup of the used catalyst with sc CO 2 removes most of the coke and oily materials from the surface. Pd SEM X-ray microanalysis evidenced a decrease of the Pd dispersion on the surface of the used-activated catalyst. Coke was also characterized using elemental analysis and TGA in air, indicating that about 80 wt % of the deactivating materials were efficiently removed from the surface by sc CO 2 . The formation of the oligomerized and stabilized residual coke (remaining 20%) should be presumably avoided if the catalytic hydrogenation had taken place at sc (in situ) conditions where oligomerization times are minimized. The SCF conditions are then highly challenging at the catalytic hydrogenation of polyunsaturated hydrocarbons.

Cleaning Using CO2‐Based Solvents

“…The efficiency of the removal of contaminants is; however, strongly dependent on the physiochemical properties of the soils to be cleaned, such as its total organic content, porosity, pore size distribution, water content, etc. Regeneration of catalysts was another area of Sc CO2 cleaning [24]. Recently, CO2-based solvent have been used to clean patterned circuit wafers.…”

Reactivation of spent Pd/AC catalyst by supercritical CO2 fluid extraction

“…As to the application of supercritical fluid to the regeneration of deactivated catalysts, while most works have been focused on the removal of oligomers on solid acid catalysts,12–16 less attention has been received by the reactivation of metal catalysts for hydrogenation reactions by this technique. Available examples include scCO 2 reactivation of a Pd/Al 2 O 3 catalyst for cyclododecatriene hydrogenation,17 and scCO 2 regeneration of a spent Pd/AC catalyst used in the hydrogenation of a variety of organic compounds 18, 19. Provided that the carbonaceous species on the spent Pd/AC catalyst for the hydrogenation of benzoic acid can be eliminated by scCO 2 , the environmental impact and cost for the regeneration of the spent Pd/AC catalyst can be substantially reduced.…”