Title : Deactivation/regeneration cycles of Rh/C and Ru/C for the valorization of chloromethanes by hydrodechlorination
Chloromethanes (CMs) are chlorinated volatile organic compounds, with high toxicity and carcinogenic character, which contribute to global warming, ozone layer depletion and photochemical smog. There exist several technologies available for the abatement of CMs from residual streams. Nevertheless, the valorization of these compounds, aiming their transformation into valuable hydrocarbons, is a very interesting alternative to the traditional treatments of CMs. In previous studies, the gas phase hydrodechlorination (HDC) of dichloromethane (DCM) and chloroform (TCM) with Pd/C, Pt/C, Rh/C and Ru/C catalysts was investigated, finding important differences in their activity, selectivity and stability. While Pt/C showed an exceptional stability, Pd/C exhibited the highest CM conversion and selectivity to hydrocarbons with more than one carbon atom (C1+). Rh/C and Ru/C also showed high selectivities to C1+ and olefins, which constitute very important industry feedstocks. Nevertheless, they suffered a severe deactivation with time on stream. In this work, the behavior of Rh/C and Ru/C in the HDC of DCM and TCM is studied, analyzing the relationship between their chemical and physical properties and their activity. In addition, a catalyst regeneration treatment is proposed. Rh/C and Ru/C (1 wt.%) were synthesized by incipient wetness impregnation of a commercial activated carbon, and activated by reduction with H2 at 250 °C. Their catalytic activity was determined in hydrodechlorination experiments carried out at atmospheric pressure, using an inlet CM concentration of 1000 ppm, a molar H2/CM of 100, a temperature of 250 °C and a space-time of 1,7 kg h mol-1. The catalysts were characterized before and after the hydrodechlorination reaction by adsorption-desorption of N2 at -196 °C, CO chemisorption, X-ray photoelectron spectroscopy, X-ray diffraction and thermogravimetric analysis. Catalysts deactivation was attributed to the adsorption of chlorinated compounds to the active sites, the formation of carbonaceous deposits blocking the porous structure and metal sintering. Catalysts regeneration by oxidizing treatment at 250 °C led to the complete recover of initial activity, favoring the selectivity to olefins.