Insights on Active Sites of CaAl-Hydrotalcite as a High-Performance Solid Base Catalyst toward Aldol Condensation

Abstract

Solid base catalysts are highly demanded in many heterogeneous catalysis processes, but insights on active sites and structure–property correlation have rarely been revealed and remain a challenge. Herein, activated MxAl-layered double hydroxides (denoted as re-MxAl-LDH, M = Ca, Mg), as solid base catalysts toward the aldol condensation reaction, were prepared via a two-step procedure: calcination of the MxAl-LDH precursor to produce MxAl-mixed metal oxide (MxAl-MMO), followed by a further rehydration treatment. Structural characterizations confirm that re-CaxAl-LDH and re-MgxAl-LDH show similar morphology, particle size, specific surface area, and pore size distribution. However, a combination study including XPS, EXAFS, CDCl3–FTIR, and DFT calculations verifies that the host layer of re-CaxAl-LDH contains a distorted Ca(OH)6 octahedron with an additional Ca–OH coordination that provides weak Bronsted basic site; in contrast, re-MgxAl-LDH consists of a Mg(OH)6 octahedron structure accompanied by surface ...