Reconstructed La-, Y-, Ce-modified MgAl-hydrotalcite as a solid base catalyst for aldol condensation: Investigation of water tolerance

Abstract

MgAl-HT modified with La, Y, and Ce were evaluated in liquid-phase acetone self-aldolization as a model reaction. Calcined and rehydrated HT were characterized by XRD, N2 adsorption, IR spectroscopy, and calorimetry of CO2 adsorption. The results show that the introduction of rare earth elements (REE), of larger ionic radius than Al3+, can affect the degree of disorder of HT structure. Thus, La- and Y-modified HT exhibit larger surface areas and smaller particles sizes, resulting in higher specific amounts of medium/strong basic sites (Q>100kJ.mol−1). This is also correlated with the lower electronegativities of the REE ions, which may induce slight modifications of the electronic densities of the oxygen atoms, the Lewis basic centers. Calorimetry of CO2 adsorption indicates that the rehydrated HT exhibit basic sites in equivalent amounts but with more homogeneous basic strength distributions. The rehydrated HT catalysts show significant higher rate constants expressed per basic site. This confirms that acetone condensation proceeds faster over Brønsted basic sites compared with the Lewis ones. Over rehydrated HT, the rate constants per Brønsted basic sites are less dependent on the presence or nature of the REE, suggesting that the electronic effect of REE does not affect strongly the activity of the charge compensation anions OH−. The water tolerance of the catalysts was studied in the presence of 1wt% and 5wt% H2O in acetone, and the results show that rehydrated MgAl-HT modified with Y and La present a higher water tolerance for aldol condensation, which can be tentatively explained by their higher OH− content preferentially located on the defects of the HT platelets, where the solvation by interlayer water molecules could be limited.