Synthesis and catalytic performance of poly(amidoamine) dendrimer encapsulated lanthanum nanoparticle in claisen-schmidt condensation

Abstract

A series of poly(amidoamine) (PAMAM) dendrimers with ethylenediamine (EDA) core, having generation numbers 0.5 to 5.0 were successfully synthesized via divergent method. Dendrimers were prepared starting from EDA followed by consecutive Michael addition and ester amidation reaction in methanol. Lanthanum was then encapsulated in the PAMAM dendrimers by reduction of lanthanum(III) nitrate hexahydrate with sodium borohydride to yield the corresponding catalysts, having general formula LaPGx (x= 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 3.5, 4.0, 4.5 and 5.0). The lanthanum encapsulated PAMAM dendrimers were characterized by using FTIR, 1H NMR, fluorescence spectroscopy, FESEM, TEM microscopic techniques and cyclic voltammetry. The 1H NMR spectra indicated that the PAMAM dendrons consist of defect site in their structure and the number of defective molecules increases exponentially with each generation. The excess of EDA reduced the occurrence of these defects, however they cannot be eliminated. In the fluorescence spectra, the appearance of the strong charge transfer band, attributed to n ! "* transition of lanthanum within PAMAM dendrimer encapsulated lanthanum nanoparticle in the range 200 – 250 nm was observed. The morphology of PAMAM dendrimer encapsulated lanthanum nanoparticles was investigated using FESEM and the size of particle was observed to be in the range of 30 – 100 nm. The cyclic voltammetry was used in this study to explore the effect of generation number towards the electrochemical properties of synthesized catalysts. The catalytic properties of PAMAM dendrimer encapsulated lanthanum nanoparticles were tested in the Claisen-Schmidt condensation of acetophenone with benzaldehyde at different temperatures (140ºC, 100ºC and 60ºC) and various mole ratio of acetophenone to benzaldehyde (1:1, 2:1 and 1:2) for 24 hour to produce chalcone. The optimum reaction temperature for the entire catalysts was 140ºC with the acetophenone to benzaldehyde mole ratio of 1:1 where the highest percentage conversion (89.1%) and selectivity (98.2%) were achieved using La(5)P3.0 catalyst. Reaction temperature was found to affect the catalytic activity probably due to the viscosity of reaction mixture that is lower at high temperature such that it facilitates heat and mass transfer of the catalytic system. With regards to La(5)P3.0, the selectivity and conversion for chalcone was substantially increased to 100% and 94.7% respectively, when the acetophenone to benzaldehyde ratio was 2:1. For reusability test, the catalytic performance of the used samples was maintained ! 80% after three cycles without significant loss in activity.