Chemistry Reference
In-Depth Information
Table 4 Adsorption energies (E
ads
), carbonyl oxygen charge difference
Dq
ð
O
C
¼
O
Þ
and
bond distances (d) for FAME adsorption on terrace, edge and Mg- and O-apical corner
sites of MgO (100).
a
Entry
Cluster site
E
ads
(eV)
Dq
ð
O
C
¼
O
Þ
(a.u.)
d
ð
O
C
¼
O
Mg
s
Þ
(Å)
d
(C
¼
O)
(Å)
0.05
0.02
1
Terrace (L
=
5)
2.474
1.219
0.02
0.04
2
Edge (L
=
4)
2.298
1.224
0.56
0.07
3
O-corner (L
=
3)
2.189
1.227
4
Mg-corner (L
=
3)
0.67
0.07
2.148
1.223
a
Cluster sites as in Fig. 16.
ions in the cluster decreased from L
=
5toL
=
3, suggesting a stronger
FAME-surface interaction. Consistently, the O-Mg
s
bond distance,
d
ð
O
C
¼
O
Mg
s
Þ
, decreased when FAME was adsorbed on lower coordination
surface ions. Nevertheless, in all the cases the d
(C
¼
O)
distances remained
close to that of the free molecule (1.212 Å) which shows that the integrity
of the adsorbed FAME molecule is preserved. In line with these results,
the q
O
C
¼
ð Þ
values in Table 4 indicated that the oxygen of the C
¼
O bond
gained some negative charge as a consequence of the adsorption process.
However, in all the cases low q
O
C
¼
ð Þ
values were obtained thereby sug-
gesting that polarization does not proceed to a significant extent on any
cluster geometry.
In summary, DFT calculations predict that the proton abstraction
from the glycerol hydroxyl groups required in the glycerolysis reaction
(Fig. 16) would preferentially occur on low coordination O
2
(strong base
O
4c
sites located on edges), in agreement with the catalytic results
presented in Fig. 15 and Table 2. FAME adsorption on MgO is weak, even
on low coordination Mg
3c
and O
3c
sites. Therefore, the Gly/FAME reaction
would proceed through a mechanism in which the most relevant
adsorption step is that of glycerol.
4 Conclusions
The density, nature and strength of surface basic sites on MgO obtained
from Mg(OH)
2
decomposition may be regulated by modifying the solid
calcination temperature. Decomposition of Mg(OH)
2
at 673 K generates
hydroxylated MgO containing mainly strong O
2
basic sites located in
surface defects such as corners and edges of the crystalline solid surface.
The increase of the calcination temperature 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.
The effect of calcination temperature on the MgO activity and
selectivity for a given base-catalyzed reaction depends on the basicity
requirements for the rate-limiting step of the reaction mechanism. For
example, the activity for the liquid-phase synthesis of monoglycerides by
glycerolysis of methyl oleate as well as that of pseudoionones by
condensation of citral with acetone diminish with MgO calcination
temperature because both reactions occur predominantly on strong basic
O
2
sites. In contrast, the gas-phase hydrogen transfer reduction of
