A new approach in the synthesis of LDH-type materials used in the condensation reaction

Traditionally, the synthesis of LDH-type materials is performed by the co-precipitation method and inorganic alkalis presence. However, Tetra Methyl Ammonium Hydroxide seems to be a viable alternative for the synthesis of these materials. Mg_0.75Al_0.125Y_0.125 LDH-type materials were prepared by two methods, co-precipitation and mechano-chemical, in the presence of TMAH. The corresponding mixed oxides were obtained by calcination at 460 °C in air. The memory effect was used in order to reconstruct the LDH layered structure. The samples were characterized by XRD, DRIFT, basicity by irreversible adsorption organic molecules with different pK_a, DTA-TG, N₂ adsorption - desorption isotherms. XRD patterns confirm the obtaining of LDH structure with some Y(OH)₃ impurities while mixed oxides illustrates the forming of an MgO-periclase like solid solution with Y₂O₃ which remains stable in the reconstructed structure. The catalytic activity of samples was determined in the aldol condensation reaction between benzaldehyde and cyclohexanone. The sample c-LDH-MgAlY-TMAH-MC presents 56% cyclohexanone conversion after 2h at 120°C. The activities keep the trend: mixed oxides > reconstructed > dried samples, regardless of the preparation method used. This variation linearly depends on the total basicity of the samples. Also, the selectivity towards to the di-condensate product, namely 2,6-dibenzylidene-cyclohexanone, is more than 99%.