Basic catalysis on MgO: generation, characterization and catalytic properties of active sites - Catalysis - page 22

Fig. 17 Scheme representing the formation of surface glyceroxide species from glycerol
and the FAME surface activation.

three hydroxyl groups. Species are identified as nOH(m), where n and m
represent, respectively, the amount and position of the OH groups par-
ticipating in the surface species. Thus, m is 1 or 3 for primary hydroxyls
and 2 for the secondary one. The E ads values, the intramolecular and the
Gly-MgO distances (d) were calculated at equilibrium. The Gly/MgO
distances are depicted in the scheme of Fig. 17 as d ð O Mg s Þ and d H O s

Þ , ð

considering the closest Gly hydroxyl interacting with the MgO surface.
Table 3 presents the results obtained on a perfect terrace site of MgO
(100) where Mg 2 þ and O 2 ions are five-fold coordinated (L = 5). Results
show that the d ð O Mg s Þ bond distance diminishes with the number of
hydroxyl groups (n) interacting with the surface, probably reflecting a
higher electrostatic interaction between Gly and the MgO surface that
evidences the hydrophilic properties of MgO. Shortening of d ð O Mg s Þ
suggests the presence of a more stable surface species, as indicated by
the larger E ads value.
On the other hand, the values of the intramolecular O-H distance, d (O-H) ,
in Table 3 suggest that regardless of the adsorption species structure, the
OH groups maintained their integrity which indicates that glycerol ad-
sorbs non-dissociatively on surface terrace sites. The glycerolysis reaction
requires the rupture of an O-H bond at the Gly molecule to proceed,
as depicted in Fig. 17. Then, data in Table 3 showing that Gly is non-
dissociatively adsorbed on MgO terrace sites strongly suggest that the
Mg 5c -O 5c pairs do not promote the Gly/FAME reaction.
Table 3 also presents the results obtained for the glycerol adsorption
on an edge site that models the low coordination (L = 4) base sites of
MgO. The E ads values obtained for the edge site were higher than those
determined for the terrace site, thereby indicating a stronger interaction
between Gly and the surface edge site. As a result of the stronger Gly-edge
interaction, dissociative adsorption occurred for some of the postulated
geometries such as 2OH(1,2)a (Fig. 18(b), entry 5 in Table 3), where the
d (O-H) distance was longer than in free glycerol due to O-H bond breaking
with formation of both, a new surface OH between the abstracted H and a