Reaction of molten sebacic acid with a layered (Mg/Al) double hydroxide

Abstract

Molten decane-1,10-dicarboxylic acid, sebacic acid, reacted with the layered double hydroxide (LDH) of initial composition Mg₆Al_3.4(OH)_18.82(CO₃)_1.51(NO₃)_0.364.5H₂O, at a temperature of 150 °C which is ca. 24 °C above the melting point of the acid. The reaction was not strongly exothermic and hence may be controlled by a suitable heating programme. X-Ray powder diffraction showed that the products were polyphasic and microcrystalline materials, the compositions of which were related to the original molar ratios of acid: LDH. Typically, the dominant phases were unintercalated LDH up to an acid : LDH ratio of 0.75 : 1; intercalated dianion (ratio ca. 1 : 1); salt, e.g. magnesium sebacate (for ratios 4 : 1 and higher). Fourier-transform infrared (FTIR) and ¹³C solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies of the reaction products confirmed the presence of the dianion ^–O₂C(CH₂)₈CO₂^–. In no case was there evidence for undissociated guest acid molecules. The mechanism appears to involve the sorption of the acid, initially at the crystallite edges, followed by reaction of the molten acid with the basic solid host with evolution of CO₂. The separated LDH layers then allow the acid to diffuse in. The product from the 1 : 1 mixture was similar to the intercalation compound obtained using a modified version of Drezdzon's method, which had previously been thought to be ineffective for sebacic acid.