Lanthanide perovskite catalysts for oxidation of chloroaromatics: Secondary pollution and modifications

Abstract

Chlorobenzene (CB) was selected as a representative chloroaromatic to evaluate its oxidation process over a perovskite LaMnO3 catalyst. It was noted that in catalytic CB oxidation, severe deactivation occurred in the LaMnO3 catalyst. This was mainly ascribed to surface coke deposition (involving long-chain organics) and the loss of redox ability. The byproducts in the oxidation process involved certain polychlorinated chain organics of C2OCl4, C2HCl3, and C2Cl4, which were believed to be generated through nucleophilic substitution at the Lewis acid sites of exposed MnClx or MnOyClz. Modification of LaMnO3 by introduction of CeO2 and HF etching was found to distinctly retard catalyst deactivation. The loaded CeO2 with enriched oxygen vacancies reduced coke deposition and retained the redox ability of LaMnO3. The HF etching forced the fluoride ions (F−) to replace the surface lattice oxygen, which significantly promoted the mobility of surf-Olatt and bulk-Olatt and facilitated Cl desorption (via a Deacon reaction mechanism) from the catalyst surface, ensuring superior oxidation efficiency and stability for catalytic chloroaromatics oxidation. As calculated, the TOF of investigated catalysts followed the sequence LaMnO3-CeO2-HF (11.58 × 10−5 s−1) > LaMnO3-CeO2 (10.14 × 10−5 s−1) > LaMnO3 (5.44 × 10−5 s−1) at 120 °C in catalytic CB oxidation.