Chlorine recovered from industrial tail gas streams could be directly upcycled into useful chemicals using a commercially available ion exchange resin. The reversible adsorption of chlorine gas onto the polystyrene-based resin could enable better use of large quantities of chlorine that are currently lost as waste.
Many industrial processes create chlorine-containing tail gas streams. One large-scale example is chloralkali electrolysis, which converts sodium chloride and water into sodium hydroxide, hydrogen and chlorine and is used to manufacture around 8 million tonnes of chlorine in Europe each year. While most of this chlorine is captured through processes like low-temperature condensation, an estimated 2–3% is lost.
Scrubber systems that recover chlorine from these facilities’ waste streams tend to generate low-value by-products, while attempts to use membranes for chlorine recapture have been limited by their selectivity and durability.
Now, researchers in Germany have tackled the problem with a commercially available polystyrene-based ion exchange resin. By soaking the resin in hydrochloric acid, they convert it to a chloride form that can selectively bind chlorine from mixtures of several gases that are commonly present in chloralkali plants’ waste streams. The researchers suggest this is enabled by halogen bonding involving the resin’s chloride ions leading to the formation of a trichloride species.
When the fully loaded resin is heated to 80°C for one hour, 71% of chlorine was released. ‘A one-hour cycle is quite reasonable for an industrial process [for example] using multiple columns in a swing-bed system,’ says Hongyuan Chuai, a catalysis researcher from Hong Kong Polytechnic University, China, who was not involved in the research.
The team also demonstrated direct upcycling of adsorbed chlorine into the more valuable commodity chemicals 1,2-dichloroethane and phosgene. ‘Due to weakening of the chlorine–chlorine bond by the population of anti-bonding orbitals, the trichloride is considered to show a higher reactivity than neat chlorine gas,’ explains Freie Universität Berlin’s Gesa Dreyhsig, who helped develop the system.
Chuai calls the work a convincing proof-of-concept. ‘[The researchers] demonstrated the process in a flow setup designed with 3D-printed and commercial parts, showing that the process works dynamically, not just in batch,’ she explains. And because the process uses an off-the-shelf material, Chuai says that it could be ‘implemented immediately using existing supply chains’. However, she warns that managing the heat that the process releases could prove challenging. ‘Scaling to industrial dimensions requires careful design to ensure heat is dissipated efficiently to prevent runaway reactions or degradation,’ she notes.
