Keywords: Hydrodechlorination, Ruthenium, p-Chloroanisole, p-Chloroacetophenone, m-Chlorotoluene, p-Chloro-m-xylenol, p-Chloro-m-cresol

Introduction

Treatment of aqueous waste streams contaminated by chlorinated organic compounds is crucial. Catalytic hydrodechlorination (CHDC), which transforms such pollutants into less toxic dechlorinated products by mild hydrogenation, is a potentially attractive option (Pirkanniemi and Sillanpaa, 2002; Keane, 2005). CHDC has several advantages over other candidate destruction techniques (e.g., incineration, catalytic/chemical oxidation, biological treatment and electrochemical degradation) such as low cost, no formation of toxic byproducts and increased biodegradability of the waste streams. For this reason, many papers describing destruction of actual wastewaters and model compounds by CHDC were reported.

       In this study, CHDC of five model compounds was investigated using Ru-based catalysts in a slurry reactor: p-chloroanisole, p-chloroacetophenone, m-chlorotoluene, p-chloro-m-xylenol, p-chloro-m-cresol. Ruthenium has high activity for aqueous-phase hydrodechlorination reactions. Two Ru-based catalysts were tested in this work, viz. a commercial 5% Ru/C and lab-made 5% Ru/TiO₂. The existence of a chemical control regime was ascertained. Kinetic data over Ru/TiO₂ were experimentally obtained at T≤373 K in the 0.7-2.1 MPa H₂ pressure range. Reaction pathways were described using Langmuir-Hinshelwood-Hougen-Watson kinetics.

Experimental

The experimental setup and procedure were described in detail in our previous works (Vaidya and Mahajani, 2004; Vaidya and Dussa, 2013). Catalyst activity trials were performed in a 0.1 dm³ capacity Hastealloy C-276 autoclave (Parr Instrument Company, USA) in the 323-373 K range, using different catalyst loadings (0.1-1 kg/m³) and substrate concentrations (1-18 mM). Typically, 50 mg of Ru/TiO₂ catalyst was added to 50 mL of the feed solution inside the reactor. After purging with nitrogen, the reactor was vented and heated to the desired temperature. The stirring speed was set to 1200 rpm. When the reactor temperature stabilized, hydrogen (H₂) gas was added to initiate the reaction. Liquid samples (1-2 mL) were taken during the course of reaction. After 2 h, the reactor was cooled and the pressure was reduced. The fall in substrate concentration was recorded using high-performance liquid chromatography. In each experiment, conversion and disappearance rates were determined. The reproducibility of results was checked and the error in experimental measurements was <2%. Ru/TiO₂ was prepared by using the procedure described in our previous work (Vaidya and Mahajani, 2002). XRD, SEM and BET techniques were used to characterize the catalysts.

Results and Discussion

Heterogeneous CHDC over Ru/TiO₂ is a three-phase reaction system with several mass transport processes. The effects of external and internal diffusion processes on the rates of reaction were investigated. The gas-side mass transfer resistance was neglected, due to the high diffusivity of H₂ in the gas phase and its low solubility in the liquid phase. Knowing that the agitation speed does not influence the reaction rate, it was concluded that the gas-liquid and liquid-solid mass transfer resistances were negligible. When the catalyst particle size was varied, the reaction rate was unaffected; as a result, we concluded that the intra-particle diffusion resistance is absent. Thus, a chemical control regime existed and reaction kinetics could be investigated.

       The reactions with p-chloroanisole, p-chloroacetophenone, m-chlorotoluene, p-chloro-m-xylenol and p-chloro-m-cresol proceeded through the formation of anisole, p-chloro-phenylethanol, toluene, m-xylenol and m-cresol, respectively. These reaction intermediates were further hydrogenated to stable products. The effects of reaction variables on the efficacy of Ru/TiO₂ catalyst were investigated. The rise in temperature, H₂ partial pressure and catalyst loading resulted in increased reaction rates. Complete dechlorination was accomplished under mild reaction conditions (T≤373 K) within 1 h. To gain an insight into reaction kinetics, Langmuir-Hinshelwood-Hougen-Watson models were proposed. The kinetic data could be best described by a model proposing surface reaction between the chemisorbed species, i.e. atomic hydrogen and the reactant as the rate-determining step. Finally, it was found that the performance of Ru/C was superior to that of Ru/TiO₂.

References

1.   Pirkanniemi, K. and M. Sillanpaa, Chemosphere, 48, 1047-1060 (2002).

2.   M. A. Keane, J. Chem. Technol. Biotechnol., 80, 1211-1222 (2005).

3.   Vaidya, P. D. and V. V. Mahajani, Appl. Catal. B: Environ., 51, 21-31 (2004).

4.   Vaidya, P. D. and V. S. Dussa, Can. J. Chem. Eng., 91, 731-738 (2013).

5.   Vaidya, P. D. and V. V. Mahajani, Chem. Eng. J., 87, 403-416 (2002).