Comparative Study of Potassium Salt-Loaded MgAl Hydrotalcites for the Knoevenagel Condensation Reaction

Abstract

A series of potassium salt-loaded MgAl hydrotalcites were synthesized by wet impregnation of KNO₃, KF, KOH, K₂CO₃, and KHCO₃ salts over calcined MgAl hydrotalcite (Mg-Al = 3:1). The samples were characterized by X-ray diffraction, Fourier transform infrared, thermogravimetry-differential thermal analysis, scanning electron microscopy, and N₂ absorption-desorption techniques to investigate their structural properties. The results showed formation of well-developed hydrotalcite phase and reconstruction of layered structure after impregnation. The prepared hydrotalcites possess mesopores and micropores having pore diameters in the range of 3.3-4.0 nm and Brunauer-Emmett-Teller surface area 90-207 m² g^-1. Base strengths calculated from Hammett indicator method were found increasing after loading salts, where KOH-loaded hydrotalcite showed base strength in the range of 12.7 < H_- < 15, which was found to be the preferred catalyst. Subsequently, KOH loading was increased from 10 to 40% (w/w) and catalytic activity was evaluated for the Knoevenagel condensation reaction at room temperature. Density functional theory calculations show that among all of the oxygen atoms present in the hydrotalcite, the O atom attached to the K atom has the highest basic character. In this study, 10% KOH-loaded hydrotalcite showing 99% conversion and 100% selectivity was selected as the preferred catalyst in terms of base strength, stability, and catalytic efficiency.