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 CO 2 - and NH 3 -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. • Oxalic acid treatment on LDHs is suitable for removing M(II) or M(OH) x specimens. • Stable layered structures with considerable number of vacancies can be produced. • The treated samples showed improved basicity/acidity; and porous feature. • The treated MgAl-LDHs promote aldol condensations efficiently as reusable catalysts. • Maximum FMIBK yield of 80 % achieved over the catalysts with vacancies.