Catalytic dechlorination

Recently, both catalytic hydrodechlorination and catalytic oxidation have been examined as alternate methods for treating or recycling chlorinated waste streams. The hydrodechlorination reactions of 1,3-dichloropropene, a component of the waste stream from epichlorohydrin manufacturing, were examined over a variety of catalysts in a packed bed microreactor. Pt/γ-Al2O3 and Ni/SiO2-Al2O3 catalysts were found to exhibit significant activity and no signs of deactivation over the course of the experiments, while other catalysts such as NiMo/γ-Al2O3 exhibited substantial deactivation. Many dechlorination studies use only a single model compound. However, in this work a pair of waste streams from the manufacture of vinyl chloride monomer was analyzed by GC/MS to reveal their complexity. Significant variations between the two waste samples were found, particularly with regard to aromatic content. xi To compliment the microreactor studies, additional experiments were performed in an ultra high vacuum molecular beam apparatus to examine some of the details of oxidative chemistry on the catalytic surface. The production of phosgene was observed when a pure carbon tetrachloride molecular beam was impinged on the oxygen modified Ir (111) and Ir(110) surfaces. Phosgene formation is thought to proceed through the partial decomposition of the CCl4 molecule to a CCl2 surface intermediate, which subsequently reacts with adsorbed oxygen to form phosgene. Although the mechanism of phosgene formation is believed to be identical on the two surfaces, important differences in reactivity were observed. The phosgene production on oxygen modified Ir(111) was greater than the oxygen modified Ir(110) surface. Formation of the surface oxide on Ir(110) (which begins to occur if the surface is heated above 550 K) reduces adsorption of carbon tetrachloride, which necessarily decreases phosgene formation. Additionally, the Ir(110) surface may cause a more rapid decomposition of the adsorbed CCl2 intermediate. Phosgene production on oxygen modified Ir(110) also demonstrated a maximum at 500 K, whereas phosgene production on oxygen modified Ir(111) decreased with increasing temperature. This maximum in production may arise from the diffusion of oxygen from the chemisorbed state to the surface oxide state. Furthermore as the surface temperature increased, competing reactions became important, especially the rapid and more complete decomposition of CCl4 on the surface prior to reaction with oxygen (ultimately resulting in the formation of CO and CO2).