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A new approach in the synthesis of LDH-type materials used in the condensation reaction
1 , 1 , 1 , 2 , * 1 , * 1
1  University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry, Biochemistry and Catalysis, 4-12 Regina Elisabeta Av., S3, 030018, Romania
2  INCDCP-ICECHIM, Splaiul Independenței 202, Bucharest, 060021, Romania
Academic Editor: Keith Hohn (registering DOI)

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. Mg0.75Al0.125Y0.125 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 pKa, 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%.

Keywords: Organic alkalis; LDH-type compounds; Memory effect; Condensation reaction