According to the literature and our results, a plausible
mechanism can be proposed (Scheme 3). It is well known that
aprotic solvents are less effective than water for stabilizing ions,
with the result that compounds tend to ionize less in
nonaqueous media. In water solution, H2O2 has a pKa of
11.8, which indicates that the formation of HO2 from H2O2 in
acetonitrile is greatly unfavorable, so that is considerably more
effective than the H2O2 is added to Mo(VI) atoms of the heptaand
octamolybdates (1) [52]. That is, hydroperoxymolybdate
group (2) can be formed by nucleophilic attack of hydrogen
peroxide on bridging oxo ligands of the hepta- and
octamolybdates species on the alumina surface (1). Subsequently,
the hydroperoxymolybdate species (2) undergoes a
reversible loss of a water molecule to produce the monoperoxo
specie (3). As a result, the peroxo group is activated
electrophilically via coordination to the high valent molybdenum
atom. By the same way, it is possible to form of the
diperoxo specie. Both mono- and diperoxo species are in
equilibrium with the starting hepta- and octamolybdate species
(Scheme 4). Eldik et al. [54] have suggested for a similar
equilibrium of rhenium peroxo species that the first step of
Scheme 4 is rate controlling. The oxidation process can be
rationalized as a mechanism that proceeds by the nucleophilic
attack of the sulfur atom in the organosulfur compounds (4) on
a peroxo group of the mono- or diperoxo specie (3) to form
sulfoxide (5) and a regenerated polymolybdate or monoperoxo
specie, respectively, Scheme 3. This attack can proceed on one
(or both) of the peroxo groups present on the molybdenum
diperoxo specie. Subsequently, the sulfoxide (5) undergoes
further oxidation by a peroxide oxygen of the molybdenum
peroxo specie (3) to form sulfone (6). On the other hand,
another possible explanation of the increase in the sulfur
elimination using catalysts containing phosphate can be the following: the present of electronegative phosphate species
adsorbed on the alumina surface would increase the electron
density withdrawn from the hepta- and octamolybdenum
species, thereby conferring a higher electrophilic character to
the Mo (VI) atoms [55]. Finally, Shaabani and Rezayan [56]
studied the selective oxidation of sulfides to sulfoxides or
sulfones with H2O2 in the presence of silica sulfuric acid as
solid acid catalyst, they proposed a oxidation mechanism where
the sulfuric group activate the H2O2 through hydrogen bonding.
Thus, taking into account that the molecular structure of
sulfuric and phosphoric groups are very similar, we can
consider an oxidation mechanism by phosphate species to our
OD system such the above-mentioned.
请高手帮帮忙啊，我英语水平有限，翻译不通，谢谢了！！！
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