V.V. Ordomsky et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 85–93
93
catalyst, while strong H-bonding of acetaldehyde to silanol groups
can be induced by interaction of CO2 with –Mg–O–Si–OH sites:
pathway involving Lewis acid–weak base pairs and Brønsted acid
sites most probably governs the conversion of acetaldehyde over
MgO/SiO2 and ZrO2/SiO2 under real conditions. This reaction path-
way accounts for the catalyst reactivity sequence observed during
catalytic runs: ZrO2/SiO2 > MgO/SiO2 ꢀ SiO2 and explains the key
role of Lewis acidic sites in aldol condensation over magnesium
and zirconium supported catalysts.
O
H
O
H
O
CO2
C
O
O
O
Mg
Si
Mg
Si
The results obtained suggest that strong basic sites existing
over MgO/SiO2 are more reactive in condensation than acidic sites
of ZrO2/SiO2. However they are quickly deactivated and further
reaction proceeds over Lewis acid–weak base –Mg–O-pairs and
for ZrO2/SiO2 catalyst. To check this, the experiment with pre-
adsorbed pyridine was carried out. As in the case of ZrO2/SiO2
catalyst, pre-adsorption of pyridine results in almost complete dis-
appearance of the bands of acetaldehyde and crotonaldehyde over
MgO/SiO2 (Fig. 11c), confirming that second reaction pathway may
play an important role over MgO/SiO2.
Thus, the IR study pointed to two mechanistic pathways of
acetaldehyde condensation which may take place over MgO/SiO2:
(i) basic reaction mechanism including intermediate carbanion
formation over strong basic sites and (ii) concerted mechanism
involving Lewis acid–weak base –Mg–O-ion pairs and silanol
groups with induced acidity.
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