6.5 Interaction of Mo Species with Brønsted …
145
M
(O
Si
O
O
O
O
O
Al
O
O
Species (IV)
H
Si
O
O
O
O
O
Al
O
O
+
oO 2 H) 2
Mo O
O
OH
+ H 2 O
(6.14)
Species (IV) further reacts with acidic protons to produce Species (II) and Species
(III).
A similar condensation reaction of the surface OH groups in zeolites leads to the
extraction of Al
3+ ions from the framework, accompanied by the formation of H 2 O
and the disappearance of two Brønsted acid sites; this process is typically referred
to as dealumination. The tendency for Al atoms to be extracted from the framework
became more significant with increasing Mo loading [67]. The extra framework Al
atoms form small domains of not only Al 2 O 3 , but also Al 2 (Mo 4 ) 3 via the reaction of
MoO 3 with Al 2 O 3 .
Although Mo supported on Al 2 O 3 showed low catalytic activity in the MDA
reaction (Table 6.2), the structure of Mo species on various Al 2 O 3 structures, such
as γ-Al 2 O 3 , has been studied. For example, Giordano et al. proposed the formation
of aluminum-molybdic complexes based on IR spectra and ultraviolet reflectance
spectra [76]. Thus, the condensation of molybdate ions with the surface OH groups
of Al 2 O 3 proceeded to produce Mo species, namely, the monomeric and dimeric
surface structures of Species (V) and Species (VI).
Al
O
O O
Al
O
O
O
Mo
O
O
Species (V)
O
O
Al
O
Mo
O
O
Al
O
Al
O
Species (VI)
O
(6.15)