The chlorine industry is currently characterized by linear material flows arising, among others, from difficulties in handling, storage, and transport of chlorine (Cl2) and hydrogen chloride (HCl). This limits the needed rethinking of processes towards a more sustainable approach focusing on circularity and resource efficiency. Herein, the potentials of bichlorides and polychlorides as alternative chemical platforms for HCl and Cl2 were investigated to valorize industrial side streams and support a more sustainable chlorine industry. Ammonium-based poly(hydrogen chloride) chlorates, so-called bichlorides, were introduced as stable, easy to handle HCl storage systems with significantly reduced vapor pressures. Among the investigated compounds, triethylmethylammonium bichloride ([NEt3Me][Cl(HCl)n]) was identified as promising candidate, reversibly binding up to n = 2.5 equivalents of HCl under ambient conditions. Beyond storage, the bichloride can be directly used as reagent, increasing process safety by shifting, e.g., the hydrochlorination of acetylene to vinyl chloride from the gas to the liquid phase. In combination with palladium(II)chloride, a highly active and stable catalytic system was obtained as viable alternative to currently used toxic and environmentally harmful mercury(II)chloride. Furthermore, technologies of Cl2 recovery have been investigated. The bichloride enables the efficient electrochemical regeneration of Cl2 from HCl in form of the corresponding polychloride. Using a simple undivided cell setup, no parasitic side reactions were observed overcoming difficulties associated with the electrolysis of neat HCl or hydrochloric acid. Complementarily, ion exchange resins providing ammonium chloride functions have been introduced as selective adsorber materials reversibly binding Cl2 under the formation of polymer-based polychlorides. Cl2 can be efficiently recovered from tail and waste gas streams by also enabling its upcycling to base chemicals like 1,2-dichloroethane or phosgene. In this way, the further use of otherwise downgraded Cl2 can be realized as an energy- and resource-efficient alternative to current performed neutralization processes. Overall, the presented concepts demonstrate how rethinking Cl2 chemistry through circular process design could enhance safety, resource efficiency, and sustainability in the chemical industry.
