Basic catalysis on MgO: generation, characterization and catalytic properties of active sites - Catalysis - page 23

Fig. 18 Optimized geometrical structures of species 2OH(1,2) of Gly adsorbed on a MgO
(100) surface: (a) non-dissociative adsorption on terrace sites; (b) dissociative adsorption
on defective edge sites; (c) non-dissociative adsorption on a defective O-apical corner site.

cluster oxygen, O s , and a surface glyceroxide on a cluster cation, Mg s
(Fig. 17). Thus, the d ð O Mg s Þ and d H O ð Þ distances for species 2OH(1,2)
considerably shortened compared to 1OH(2) and 3OH surface structures
of Table 3 for which non-dissociative adsorption took place.
Glycerol adsorption was studied also on an O-apical corner (Table 3).
Comparison between the Gly adsorption through the primary OH,
structure 1OH(1), on the terrace (entry 1) and on the O-corner (entry 7)
sites, indicated that a more stable species was obtained on the O-corner
with shortening of the d ð O Mg s Þ and d H O ð Þ distances. However, it seems
that for the other two structures of Table 3, the Gly molecule tended to
rotate and interacted to a greater extent with the Mg 5c -O 5c sites of the
cluster than with O 3c at the cluster corner, probably due to a steric effect
that hampered the molecule arrangement on the oxygen corner. In this
regard, Fig. 18 illustrates the adsorption of structure 2OH(1,2) of Gly on
the terrace, edge and O-corner sites of MgO (100). The increase of the
surface oxygen unsaturation from the terrace to the edge site would cause
the Gly molecule O-H bond dissociation forming much more stable
species. However, a further oxygen unsaturation increase from the edge
to the O-corner site, would give rise to a non-dissociated 2OH(1,2) species
with an E ads value in between those of the terrace and edge sites.
Consistently, the d (O-H) distances of adsorbed Gly structures on an
O-apical corner were similar to those of free Gly, suggesting that no O-H
bond breaking took place.

3.3.4 FAME adsorption on MgO. DFT calculations were also
carried out for FAME adsorption on representative terrace, edge, and
Mg- and O-corner sites of MgO. The FAME molecule used in the catalytic
experiments was methyl oleate that contains eighteen carbon atoms and
one unsaturation (C18:1). For modeling purposes, a shorter molecule
containing just five carbon atoms in the acyl chain was used (C5:0). The
optimized geometrical structures of free methyl oleate as well as of
the FAME used in the calculations are shown in Fig. 19. In the free short
C5:0 FAME molecule

the calculated intramolecular interatomic

distances (d) were: C ¼ O(d (C ¼ O) = 1.212 Å), C-O (d C OCH 3

Þ ¼ 1:355 Å and ð

d O CH 3

Þ ¼ 1:436 Å), C-H (d (C-H) E

1.09 Å) and C-C (d (C-H) E 1.53 Å).

ð