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 Bronsted 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 Bronsted basic site. For the re-CaxAl-LDH samples, the concentration of weak Bronsted 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 Bronsted basic site serves as active center to catalyze the aldol condensation, which accelerates the product desorption and largely promotes the HPA selectivity.