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

• Published in: ACS omega 2018-06, Vol.3 (6), p.7086-7095
• Main Authors: Devi, Rasna, Begum, Pakiza, Bharali, Pankaj, Deka, Ramesh C
• Format: Article
• Language: English
• ISSN: 2470-1343; 2470-1343
• DOI: 10.1021/acsomega.8b00767

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.