Reconstructed La-, Y-, Ce-modified MgAl-hydrotalcite as a solid base catalyst for aldol condensation: Investigation of water tolerance
Résumé
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, N-2 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 > 100
kJ.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 Bronsted basic sites compared with the
Lewis ones. Over rehydrated HT, the rate constants per Bronsted 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 1 wt% and 5 wt% 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.