Intercalation of linear C9–C16 carboxylates in layered Fe II–Fe III-hydroxides (green rust) via ion exchange

Abstract

Fe II–Fe III hydroxide sulfate, called green rust sulfate (GR SO4), has been intercalated with linear aliphatic monocarboxylic acids to form GR-carboxylates by direct ion exchange. The intercalation technique consisted in mixing freshly synthesized GR sulfate (GR SO4) with aqueous ethanolic solutions (30% v/v) of pure carboxylic acids, which were previously neutralized with NaOH (pH 7.3). Carboxylate anions with C, the total number of carbons, varying from 5 (pentanoic acid) to 16 (hexadecanoic acid) were used in the present study. Their starting concentrations were: a) 5 and 10-fold molar excess of C14 and C16 carboxylic acids, respectively, and b) 20-fold molar excess of C5 to C12 carboxylic acids with respect to the initial molar concentration of GR SO4. X-ray diffraction identified the presence of GR-carboxylates and the relative change in the intensity of the basal reflection of GR was used as an indicator of the extent of intercalation. GR-carboxylates were only formed for C ≥ 9. Fast and almost complete intercalation was observed for the most hydrophobic carboxylates (C14 and C16), with complete or almost complete intercalation after one day of reaction. For the carboxylic acids with shorter chains (C9 to C12) anion exchange was incomplete and GR SO4 were detected even after 6 days of reaction. FTIR spectra confirmed the intercalation of carboxylate anions into the interlayer space of GR and the loss of sulfate. Mössbauer analyses showed that the intercalation of C12 carboxylate did not alter the coordination of Fe in the GR metal hydroxide layers. The expansion in the interlayer space of the GR-carboxylates was linearly related to the number of C with an increase in 0.25 nm per methylene group. Based on XRD analyses, the intercalated carboxylate anions can be arranged as perpendicular bilayers with some degree of overlapping between the alkyl chains of opposing layers or as a double layer tilted towards the brucite-like layers with no interfingering of opposing layers. GR-carboxylates possess two important chemical/structural features: 1) hydrophobicity of the organic interlayers, and 2) presence of highly reactive Fe in their structure, that in combination makes them promising materials as reactive adsorbents for non-polar organic compounds.