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Storing and processing anhydrous hydrogen chloride as an ionic liquid

| By Gerald Ondrey

Hydrogen chloride is an important byproduct of the chemical industry, with over 9-million ton/yr generated by industrial chlorination processes, such as the production of chloromethanes and polymers (polyurethanes and polycarbonates). Although HCl can be easily recovered by gas scrubbing with water to form hydrochloric acid, the HCl solution (typically 20%) is not suitable for many processes that require anhydrous HCl gas.

Now, an alternative approach is being developed by researchers at the Freie Universität Berlin (FU Berlin; Germany;, led by inorganic-chemistry professor Sebastian Hasenstab-Riedel, in collaboration with partners from the Technische Universität Berlin ( In the method, described in a recent issue of Science Advances, HCl is converted into an ionic liquid, which makes it easier and safer to store, handle and process anhydrous HCl.

The researchers discovered that HCl gas can be safely bound to the triethylmethylammonium chloride salt, [N(C2H5)3 CH3]Cl, to create an ionic liquid, called bichloride, [N(C2H5)3 CH3][Cl(HCl)n], under ambient conditions. HCl can then be safely released from this bichloride following transportation or storage.

Although HCl release is possible from this ionic liquid by heat or vacuum, the bichloride can be used directly to produce base chemicals, such as vinyl chloride, the chemists report. The study also showed that [N(C2H5)3 CH3][Cl(HCl)n] can be electrolyzed under anhydrous conditions, using a membrane-free cell, to generate H2 and the corresponding chlorination agent [N(C2H5)3 CH3][Cl(Cl2)n], enabling the combination of these ionic liquids for the production of base chemicals.