STRUCTURAL CHARACTERIZATION OF PHOTOREDUCED COMPLETE AND INCOMPLETE CUBANE-TYPE MOLYBDENUM OXIDE CLUSTERS IN CO AND THEIR CATALYTIC BEHAVIOR IN PROPENE METATHESIS REACTION
Y. Imada et al., STRUCTURAL CHARACTERIZATION OF PHOTOREDUCED COMPLETE AND INCOMPLETE CUBANE-TYPE MOLYBDENUM OXIDE CLUSTERS IN CO AND THEIR CATALYTIC BEHAVIOR IN PROPENE METATHESIS REACTION, Catalysis letters, 38(1-2), 1996, pp. 101-108
SiO2-impregnated complete and incomplete cubane-type molybdenum oxide
clusters such as [(RhCp)(4)Mo4O16] (I) and [(RhCp*)(2)Mo3O9(OMe)(4)]
(II) were photoreduced with a UHP-Hg lamp (> 365 nm) in CO, resulting
in the formation of catalytically active species for propene metathesi
s at 300 K. The CO-photoreduced Mo oxide clusters were characterized b
y EXAFS, IR, XPS and TPD technique. The results suggest that the Mo-O-
4c (four-centered bridging oxygens) in I were specifically reduced wit
h CO under the illumination to produce two sets of subcarbonyl species
, e.g., those characteristic of the IR bands at 2061 and 2021 cm(-1),
which were thermally inactive for the (CO)-C-13 exchange reaction but
very active under illumination at 300 K, possibly assignable to Mo(CO)
(x) (x = 2, 3), whereas those at 2092 and 2035 cm(-1) due to Rh(CO)(2)
, which were readily exchangeable with (CO)-C-13 at 300 K in dark. Rem
oval of both carbonyls attached on Rh and Mo in photoreduced I and II
by evacuation at 375-440 K led to the formation of oxygen-deficient Mo
4+/Mo5+ sites, which exhibited high catalytic activities in propene me
tathesis at 300 K to produce an equimolar mixture of ethene and 2-bute
nes. The CO-photoreduced incomplete cubane Mo oxide cluster (II) exhib
ited higher activities and higher trans/cis ratios of 2-butenes in the
reaction, compared with those on the photoreduced I.