Efficient decarbonation of carbonate-type layered double hydroxide (CO_3~(2-) LDH) by ammonium salts in alcohol medium

摘要

The deintercalation of carbonate ion (decarbonation) from CO_3 LDH (Mg/Al = 3) was conducted in alcohols at room temperature (20-25 ℃) 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 CO_3~(2-) into CO_2 during decarbonation. When using equivalent amounts of HCs with pKa = 1-8 (medium acidity), CO_3~(2-) LDH was successfully decarbonated in methanol after 2 h with the release of CO_2 to yield Cl~- LDHs containing <5% residual CO_3~(2-). For HCs with higher pKa values (8-11; weak acidity), three times the amount of HCs were needed to obtain LDHs with <5% remaining C_3~(2-). For a salt with pKa>11, no appreciable CO_2 evolution was observed and the remaining CO_3~(2-) 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 CO_3~(2-).