J.A. Cortes-Concepcion et al. / Applied Catalysis A: General 386 (2010) 1–8
7
affected by the introduction of lithium. Thus, the promotional effect
exhibited by lithium is probably limited to the increased ability
for abstracting a proton from 2ꢀ-hydroxyacetophenone during the
initial adsorption.
4. Conclusions
Results obtained with MgO samples modified with different
amounts of Li indicate a good correlation between Li content, sur-
face basicity and the initial reaction rates for the Claisen–Schmidt
condensation reaction, which represents the first step in the
synthesis of flavanone. MgO undergoes structural changes upon
modification with Li. At low lithium loadings, a higher concentra-
tion of surface defects becomes evident, with the 0.1 wt.% Li/MgO
catalyst exhibiting a maximum in both defect concentrations and
catalytic activity. The increase in activity is likely the result of the
increased ability of Li–O− pairs to abstract a proton from the 2ꢀ-
hydroxyacetophenone molecule, which represents the first step
of the adsorption and surface reaction processes that constitute
the Claisen–Schmidt condensation mechanism. While this promo-
tional effect is still present at higher Li loadings (i.e., above 0.1 wt.%),
a substantial decrease in surface area becomes dominant in such
samples, leading to lower overall weight-normalized reaction rates.
Fig. 10. Intensity of the 1368 cm−1 band of adsorbed 2ꢀ-hydroxyacetophenone dur-
ing exposure of a saturated surface to benzaldehyde at 433 K: (ꢀ) MgO and (♦)
0.1 wt.% Li/MgO.
are associated with four-fold- and three-fold-coordinated O2− sites
(i.e., Lewis sites). In the presence of lithium the number of both
“medium” and “strong” basic sites increases substantially.
Fig. 9 shows the relationship between the initial reaction rates
observed and the amount of “medium” and “strong” basic sites.
Indeed, a good correlation exists between the initial reaction rates
from “medium” and “strong” basic sites. As discussed previously,
this effect can be attributed to the ease of the deprotonation of
2ꢀ-hydroxyacetophenone, which is generally believed to be the
first step of the Claisen–Schmidt condensation reaction mechanism
[22,33].
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Fig. 10.
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