Efficient decarbonation of carbonate-type layered double hydroxide (CO32−LDH) by ammonium salts in alcohol medium

The deintercalation of carbonate ion (decarbonation) from CO32−LDH (Mg/Al = 3) was conducted in alcohols at room temperature (20–25 °C) under N2 flow using ammonium salts, such as the hydrochloride (HC) salts of various amines, with pKa values between 0 and 14. These salts were expected to function as proton sources in the conversion of CO32− into CO2 during decarbonation. When using equivalent amounts of HCs with pKa = 1–8 (medium acidity), CO32−LDH was successfully decarbonated in methanol after 2 h with the release of CO2 to yield Cl−LDHs containing < 5% residual CO32−. For HCs with higher pKa values (8–11; weak acidity), three times the amount of HCs were needed to obtain LDHs with < 5% remaining CO32−. For a salt with pKa > 11, no appreciable CO2 evolution was observed and the remaining CO32− in the product was about 70% even when six equivalents of the HC were used. Thus, decarbonation behavior in alcohol depended on salt pKa values, and efficient decarbonation was observed for ammonium salts with pKa ≤ 11. Ammonium salts of organic acids were also useful for the incorporation of their conjugate bases into the LDHs. Increased temperature was very effective in reducing the residual CO32−.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► LDH was efficiently decarbonated by ammonium salts in alcohol. ► Decarbonation behavior depended on the pKa values of the ammonium salts. ► The ammonium salts acted as both proton source and anion supplier. ► Increased temperature was effective in the decarbonation reaction. ► Use of ammonium salts is more efficient than neutral salts, and safer than acids.