Thermal intercalation of layered double hydroxides: capric acid into an Mg–Al LDH

Abstract

Molten decanoic acid, capric acid, reacted with the layered double hydroxide (LDH) of initial composition Mg₆Al_3.4(OH)_18.82(CO₃)_1.51(NO₃)_0.36·4.5 H₂O, at a temperature of 150 °C using a heating rate of 1 °C min^–1. Powder X-ray diffraction (PXRD) showed that the reaction products were microcrystalline. The polyphasic nature of the reaction products was thought to derive from the presence, in all cases, of some residual unreacted LDH host plus mono- and bi-layers of caprate anion guests in the LDH. Typically, dominant phases were non-intercalated LDH (at initial capric acid:LDH ratios of up to 2:1); stacks of horizontal caprate anions (4:1); full, intercalated monolayers (6:1) and full bilayers (8:1). ²⁷Al Solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) studies indicated that the exclusively octahedral nature of the aluminium sites of the host LDH were not altered by thermal reaction with capric acid. Two-dimensional (2D)²⁷Al MAS NMR suggested that the thermal reaction products contained two octahedral aluminium environments. ¹³C MAS NMR confirmed the presence of the monoanion of capric acid, CH₃(CH₂)₈COO^–. The suggested mechanism for intercalation involves the removal of the LDH interlayer carbonate in a similar manner to that observed in the corresponding LDH-sebacic acid thermal reactions. The products of the thermal reaction were compared with material prepared via a modified coprecipitation reaction. PXRD suggested that the products of this coprecipitation reaction and the 8:1 thermal reaction product contained bilayers having similar calculated slant angles. 2D ²⁷Al MAS NMR suggested that the coprecipitated reaction differed from the thermal reaction products in that it contained a tetrahedral aluminium site in addition to the two octahedral sites also exhibited by the thermal reactions. There was no direct evidence of kinking of the acid or anion carbon chains in the thermal or coprecipitation reaction products owing to possible interaction with the alcohol during washing of the products.