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 degrees C) under N-2 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 MC 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-. (C) 2012 Elsevier B.V. All rights reserved.