Acid-etched layered double hydroxides with defect sites as mesoporous acid/base bifunctional catalysts for aldol condensation reactions

Layered double hydroxides (LDHs) treated with organic carboxylic acids, including the most effective oxalic acid, were prepared and tested as catalysts in industrially important aldol condensation reactions of furfural and methyl isobuthyl ketone (MIBK) and isobutyraldehyde with formamide. The acid treatment can significantly reduce the actual M-to-Al (M = Ca or Mg) molar ratio of the as-prepared LDHs in such a way that the long-range ordered structures are preserved and a phase-pure products with cationic vacancies/defect sites are obtained, which was underpinned by ICP-MS, XRD, Raman, SS-NMR and XPS measurements. Furthermore, this etching process can be readily controlled by systematically changing the contact time. As shown by the comprehensive structural analysis, including CO2- and NH3-TPD, BET, SEM and TEM techniques, this method is capable of modifying the acid/base properties as well as the porosity and morphology of the as-prepared clays. LDHs with modified acid/base and porosity properties proved to be highly active and particularly selective towards the condensation products in the observed test reactions. They clearly outperformed the generally used LDHs activated conventionally, especially in terms of selectivity (86 mol% vs. 65 mol%). In addition, the treated MgAl-LDHs appeared to be recyclable and regenerable, in contrast to the conventionally activated LDHs, where significant leaching of the cationic components was observed.