The structure and dispersion of the molybdenum sulfides supported
on a NaY zeolite were studied
using XAFS techniques. It was found that molybdenum sulfide
species prepared by sulfiding
vapor deposited Mo(CO)6 or by sulfiding molybdenum
oxide dimer species encaged in the zeolite
are highly dispersed and thermally stabilized against sintering or
restructuring. These
molybdenum species are formed via molybdenum sulfide dimer species as
an intermediate. On
the other hand, with the molybdenum sulfide catalysts prepared by an
impregnation method,
the sulfidation of molybdenum oxides was incomplete. The
molybdenum oxide species are
suggested to be mainly located in the zeolite cavities after
calcination, forming isolated
molybdenum oxides in tetrahedral configurations. The molybdenum
sulfide species prepared
from Mo(CO)6 showed much higher catalytic activities
for thiophene hydrodesulfurization and
butadiene hydrogenation than the molybdenum sulfides prepared by the
impregnation, in
conformity with a higher dispersion and higher fraction of the
molybdenum sulfide species. It
is demonstrated that in combination with metal carbonyl techniques,
zeolite supports are very
suitable for the preparation of highly dispersed molybdenum sulfides at
a high Mo loading.
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