Dynamic Active Site Evolution in Lanthanum-Based Catalysts Dictates Ethane Chlorination Pathways

Radical-mediated chlorination of ethane presents a low-carbon alternative for polyvinyl chloride (PVC) synthesis, yet selectivity toward 1,2-dichloroethane remains challenged by uncontrolled over-chlorination. Lanthanum oxychloride (LaOCl) has emerged as a promising catalyst, but its structural dynamics under Cl₂-rich conditions and the origin of selectivity loss remain elusive. Here, we integrate advanced spectroscopic techniques with theoretical calculations to address this knowledge gap. Our findings unveil a sequential LaOCl → LaCl₃ transformation that dictates product distribution shifting from 1,2-dichloroethane to trichloroethane. Mechanistic insights reveal that surface hydroxyl groups, generated during catalyst chlorination, promote bidentate adsorption of 1,2-dichloroethane via hydrogen-bond networks, thereby activating C─Cl over-chlorination. Additionally, by employing Al₂O₃-supported LaCl₃ model catalysts, the size-dependent chlorophilicity of the LaCl₃ species is demonstrated. The bonding of interfacial oxygen with monolayer-dispersed LaCl₃ species generates empty 4f-states above the Fermi level, creating strong Lewis acid sites that stabilize Cl radicals and selectively convert chloroethane to 1,2-dichloroethane. In contrast, aggregated nanoparticles are inactive due to their inability to stabilize chlorine radical. Our findings establish important structure sensitivity in lanthanum-catalyzed chlorination and provide guiding principles for catalyst design, highlighting the importance of stabilizing metastable LaOCl_x species and modulating surface hydroxyl chemistry to overcome selectivity limitations.