Basic catalysis on MgO: generation, characterization and catalytic properties of active sites - Catalysis - page 12

that n O /n Mg-O values decreased with the calcination temperature following
a similar trend that the (U.C./B.C.) ratio determined by IR spectroscopy.
In summary, all these results show that the decomposition of Mg(OH) 2
at 673 K generates hydroxylated MgO containing predominantly high-
strength low-coordination O 2 basic sites located on defects of the crys-
talline solid surface. The increase of the calcination temperature up to
873 K removes OH groups and also surface solid defects creating more
stable structures that contain a higher concentration of medium-strength
Mg 2 þ -O 2 basic pair sites. Thus, the density, nature and strength of MgO
surface basic sites may be regulated by modifying the solid calcination
temperature.
Finally, it is significant to note here that in a previous work we have
characterized the acid properties of MgO-673 by NH 3 TPD and FTIR of
adsorbed pyridine [62]. We observed that MgO-673 contained only weak
Lewis Mg þ 2 acid sites; the density of Mg 2 þ sites as determined by NH 3
TPD was 0.14 mmol/m 2 , i.e. about 30 times lower than the density of base
sites determined by CO 2 TPD (Table 1, n b = 4.58 mmol/m 2 ).

3.2 Catalytic results on MgO-x catalysts
In order to investigate the effect of MgO calcination temperature on
catalyst activity, we carried out two base-catalyzed reactions on our MgO-x
samples: the liquid-phase aldol condensation of citral with acetone
and the gas-phase hydrogen transfer reduction of mesityl oxide with 2-
propanol. For both reactions, catalysts were treated at their calcination
temperatures prior to performing the catalytic tests.

3.2.1 Cross-aldol condensation of citral with acetone. The aldol
condensation of citral with acetone produces pseudoionone (Fig. 6), a
valuable acyclic intermediate for the synthesis of ionones which are
extensively used as pharmaceuticals and fragrances [63]. The reaction
was commercially carried out using diluted bases such as NaOH, Ba(OH) 2
or LiOH [64, 65], but it is also eciently catalyzed on solid bases [13, 66-68].
Here, the liquid-phase citral/acetone reaction was performed on the
MgO-x samples of Table 1. Figure 7 shows the evolution of pseudoionone
yields (Z PS ) as a function of reaction time. At the end of the 6-h catalytic
tests, citral conversion was 96%, 80% and 75% for MgO-673, MgO-773
and MgO-873 samples, respectively (Table 2). From the curves of Fig. 7,
we determined the initial pseudoionone formation rate (r PS , mol/h m 2 )
through the initial slopes according to:

n Cit
W cat S g

dZ PS
dt

r PS =

t ¼ 0

Fig. 6 Synthesis of pseudoionone by citral/acetone aldol condensation.