334
J. Handzlik
sites were also found to be different each other and only in the latter case cleavage
of the Mo–O–Si linkage is predicted [107].
Similar to other supported metal oxide catalysts for alkene metathesis, active metal
alkylidene sites in the MoO x /SiO 2 system are generated in situ from the surface Mo
oxide species upon contact with alkene [82–85]. Among several initiation mechanisms considered in the literature, reduction of Mo(VI) to Mo(IV) with propene,
followed by 1,2-hydrogen shift, was recently proposed, based on experimental results
[82, 83]. It was also reported that high-temperature activation of the MoO x /SiO 2 catalyst under an alkene-containing atmosphere is highly efficient for low-temperature
alkene metathesis [85]. However, these issues were not addressed in computational
works, although the propagation mechanism for alkene metathesis was studied in
detail.
In a series of papers [109–111], DFT (B3LYP) studies of alkene metathesis over
MoO x /SiO 2 catalyst were reported. The Mo(VI) and Mo(IV) alkylidene species
(Fig. 17) were represented by various cluster models (7–49 Si atoms) derived
from the β-cristobalite structure. The largest models were developed using the twolayer ONIOM partitioning scheme [110, 111]. By exploring the pathways of ethene
metathesis over tetrahedral Mo(VI) methylidene sites, it was found that a cycloreversion step, involving trigonal bipyramidal (TBP) molybdacyclobutane intermediate,
is more favoured kinetically than a side transformation to a more stable square pyramidal (SP) molybdacyclobutane [109], in contrast to the theoretical results reported
for molybdena–alumina systems [112–115]. The TBP → SP pseudorotation was
proposed to be a reversible deactivation route. The comparison of the computed
and experimental [78] C–H stretching frequencies suggested that the experimentally observed molybdacyclobutane species were the stable SP sites, not the reactive
TBP intermediates. It was also shown that metathesis activity of the Mo(VI) alkylidene sites strongly depends on their location on the silica surface, similar to Mo(VI)
alkylidene species on γ-alumina [113–116]. The electronic and geometrical structure
analysis indicated that the main factor affecting the reactivity is the geometry of the
Mo site, determined by surface constrains [109, 111]. Especially, the more advanced
models (Fig. 18) enabled investigations of the relationship between metathesis activity of the tetrahedral Mo(VI) methylidene species (Fig. 17a) and the local structure
of partially dehydroxylated silica surface [111]. Five-coordinate Mo(VI) alkylidene
species (Fig. 17b) and Mo(IV) alkylidene species (Fig. 17c) were also examined,
and it was concluded that these sites are inactive in alkene metathesis, in contrast
to the four-coordinate Mo(VI) alkylidene sites, further additionally studied with the
paired interacting orbitals (PIO) method [117]. Thus, it was shown [109, 111] that
the metathesis activity of the Mo alkylidene sites is determined by the coordination
environment of molybdenum, its oxidation state and the geometry of the Mo species,
which is influenced by the local structure of the silica surface. The latter factor reflects
heterogeneity of the active Mo sites.
Another strategy to account for inhomogeneous distribution of active sites on
amorphous supports and investigate the structure–reactivity relationship was proposed by Goldsmith et al. [118]. They developed an algorithm which generates a
representative set of small cluster models allowing to relate the activation energy
J. Handzlik
sites were also found to be different each other and only in the latter case cleavage
of the Mo–O–Si linkage is predicted [107].
Similar to other supported metal oxide catalysts for alkene metathesis, active metal
alkylidene sites in the MoO x /SiO 2 system are generated in situ from the surface Mo
oxide species upon contact with alkene [82–85]. Among several initiation mechanisms considered in the literature, reduction of Mo(VI) to Mo(IV) with propene,
followed by 1,2-hydrogen shift, was recently proposed, based on experimental results
[82, 83]. It was also reported that high-temperature activation of the MoO x /SiO 2 catalyst under an alkene-containing atmosphere is highly efficient for low-temperature
alkene metathesis [85]. However, these issues were not addressed in computational
works, although the propagation mechanism for alkene metathesis was studied in
detail.
In a series of papers [109–111], DFT (B3LYP) studies of alkene metathesis over
MoO x /SiO 2 catalyst were reported. The Mo(VI) and Mo(IV) alkylidene species
(Fig. 17) were represented by various cluster models (7–49 Si atoms) derived
from the β-cristobalite structure. The largest models were developed using the twolayer ONIOM partitioning scheme [110, 111]. By exploring the pathways of ethene
metathesis over tetrahedral Mo(VI) methylidene sites, it was found that a cycloreversion step, involving trigonal bipyramidal (TBP) molybdacyclobutane intermediate,
is more favoured kinetically than a side transformation to a more stable square pyramidal (SP) molybdacyclobutane [109], in contrast to the theoretical results reported
for molybdena–alumina systems [112–115]. The TBP → SP pseudorotation was
proposed to be a reversible deactivation route. The comparison of the computed
and experimental [78] C–H stretching frequencies suggested that the experimentally observed molybdacyclobutane species were the stable SP sites, not the reactive
TBP intermediates. It was also shown that metathesis activity of the Mo(VI) alkylidene sites strongly depends on their location on the silica surface, similar to Mo(VI)
alkylidene species on γ-alumina [113–116]. The electronic and geometrical structure
analysis indicated that the main factor affecting the reactivity is the geometry of the
Mo site, determined by surface constrains [109, 111]. Especially, the more advanced
models (Fig. 18) enabled investigations of the relationship between metathesis activity of the tetrahedral Mo(VI) methylidene species (Fig. 17a) and the local structure
of partially dehydroxylated silica surface [111]. Five-coordinate Mo(VI) alkylidene
species (Fig. 17b) and Mo(IV) alkylidene species (Fig. 17c) were also examined,
and it was concluded that these sites are inactive in alkene metathesis, in contrast
to the four-coordinate Mo(VI) alkylidene sites, further additionally studied with the
paired interacting orbitals (PIO) method [117]. Thus, it was shown [109, 111] that
the metathesis activity of the Mo alkylidene sites is determined by the coordination
environment of molybdenum, its oxidation state and the geometry of the Mo species,
which is influenced by the local structure of the silica surface. The latter factor reflects
heterogeneity of the active Mo sites.
Another strategy to account for inhomogeneous distribution of active sites on
amorphous supports and investigate the structure–reactivity relationship was proposed by Goldsmith et al. [118]. They developed an algorithm which generates a
representative set of small cluster models allowing to relate the activation energy