Abstract
Mechanical activation and mechanochemical reactions are the subjects of mechanochemistry, a special branch of chemistry studied intensively since the 19th century. Herein, we comparably describe two synthesis methods used to obtain the following layered double hydroxide doped with cerium, Mg3Al0.75Ce0.25(OH)8(CO3)0.5·2H2O: the mechanochemical route and the co-precipitation method, respectively. The influence of the preparation method on the physico-chemical properties as determined by multiple techniques such as XRD, SEM, EDS, XPS, DRIFT, RAMAN, DR-UV-VIS, basicity, acidity, real/bulk densities, and BET measurements was also analyzed. The obtained samples, abbreviated HTCe-PP (prepared by co-precipitation) and HTCe-MC (prepared by mechanochemical method), and their corresponding mixed oxides, Ce-PP (resulting from HTCe-PP) and Ce-MC (resulting from HTCe-MC), were used as base catalysts in the self-condensation reaction of cyclohexanone and two Claisen–Schmidt condensations, which involve the reaction between an aromatic aldehyde and a ketone, at different molar ratios to synthesize compounds with significant biologic activity from the flavonoid family, namely chalcone (1,3-diphenyl-2-propen-1-one) and flavone (2-phenyl-4H-1benzoxiran-4-one). The mechanochemical route was shown to have indisputable advantages over the co-precipitation method for both the catalytic activity of the solids and the costs.
Highlights
Licensee MDPI, Basel, Switzerland.Layered double hydroxides (LDHs), a class of anionic compounds which can be found in nature as minerals and can be synthesized, are currently generating increasing interest among scientists
Layered double hydroxides can be described as lamellar compounds similar to brucite, Mg(OH)2
The hydrotalcite-like compounds modified with cerium were prepared by the coprecipitation and mechanochemical method, respectively
Summary
Layered double hydroxides (LDHs), a class of anionic compounds which can be found in nature as minerals and can be synthesized, are currently generating increasing interest among scientists Due to their particular lamellar structure and composition flexibility, these synthetic inorganic materials exhibit unique properties that make them suitable for various applications. The following general molecular formulas describe layered double hydroxides: [M(II) 1–x M(III) x (OH)2 ][Am– x/m ·nH2 O] or [M(I) 1–x M(III) x (OH)2 ][Am– (2x+1)/m ·nH2 O] [3], where MI (Li), MII (Mg, Ni, Co, Zn), MIII (Al, Cr, Fe, Co) are metallic cations connected through the hydroxyl groups, in a cation-containing layer; Am– represents the anions which, along with n molecules of water, form the anionic layer; and x is the charge density or the anionic exchange capacity, which takes values between 0.15 and 0.5 [4,5] For their synthesis and post-synthesis modifications, various well-established solution-based methods have been developed. These conventional methods have the following serious drawbacks: the processes are longlasting, accurate pH control and subsequent heating are needed, and there is an increased consumption of energy during the ageing step, which requires significant time [6]
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