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

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 Brønsted basic site; in contrast, re-MgxAl-LDH consists of a Mg(OH)6 octahedron structure accompanied by surface adsorbed hydroxyl group via noncovalent interaction, resulting in a medium Brønsted basic site. For the re-CaxAl-LDH samples, the concentration of weak Brønsted basic sites can be enhanced by tuning the Ca/Al molar ratio in the LDH precursor. The optimized re-CaxAl-LDH catalyst (re-Ca4Al-LDH) exhibits prominent catalytic performance toward the condensation of isobutyraldehyde (IBD) and formaldehyde (FA) to produce hydroxypivaldehyde (HPA), with a HPA yield of 61.5%. This is much higher than that of re-MgxAl-LDH (12.2%) and conventional solid base catalysts (4–33%), and even comparable to the level of liquid alkali (55–72%). Studies on the structure–property correlation reveal that the weak Brønsted basic site serves as active center to catalyze the aldol condensation, which accelerates the product desorption and largely promotes the HPA selectivity.