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 KNO 3 , KF, KOH, K 2 CO 3 , and KHCO 3 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 N 2 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 m 2 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.

Layered double alkoxides a novel group of layered double hydroxides without water content

“…MgAl-LDHs prepared by the conventional aqueous co-precipitation method only displayed appreciable catalytic activities in the Knoevenagel condensation between benzaldehyde and malononitrile (SScheme 1) after heat-treatment [38] or doping with base [39] or in the presence of water [40]. Even after doping, the selectivity towards the condensation product was not 100%.…”

“…no Michael adduct could be observed. Hydrotalcites and hydrocalumites needed activation [37][38][39][40][41], while our water-free LDHs worked in their as-synthesized forms and under mild conditions (small amounts of catalysts, low reaction temperature, free of solvent and short reaction time).…”

Synthesis of Mg–Al Mixed Oxides with Markedly High Surface Areas from Layered Double Hydroxides with Organic Sulfonates

“…Layered double hydroxides (LDHs) consist of brucite-like positively charged layers and interlayer anions and possess the general formula of [M 1– x 2+ M x 3+ (OH) 2 ] x + [A x / n n – ]· m H 2 O, where M 2+ , M 3+ , and A n – represent divalent and trivalent metal cations and interlayer anions, respectively . These materials serve as precursors to materials used for a wide range of applications such as catalysts and adsorbents. − Anion exchanges of interlayer carbonates in Mg–Al LDHs with isethionate (HO­(CH 2 ) 2 SO 3 – , Ise) and chlorides in Zn–Al LDHs with dodecyl sulfate (CH 3 (CH 2 ) 11 SO 4 – for Zn–Al LDHs) are known to allow facile delamination in water and butanol, respectively. However, calcination of the resulting solids in a flow of air to obtain mixed metal oxides converts interlayer organic sulfur species to inorganic sulfate species .…”

Spectroscopy and Microreactor Technology for Single Catalyst Particle Diagnostics

“…The basicity can be further increased via impregnation of the calcined hydrotalcite with salts like KOH or KNO . [78][79][80] The use of different solvents also affects the catalytic performance of hydrotalcites in terms of their activity and selectivity. Aprotic polar solvents are reported with the best activity and selectivity.…”

“…For non-polar solvents, the conversion is lower but a good selectivity is preserved. [79] Hydrotalcites are known to be active heterogeneous base catalysts for C-C bond formations or the transesterification of oils. Furthermore, they are used as supports for various metals that act in dehydrogenation and deoxygenation reactions.…”