2
16
M. Jiang et al. / Journal of Molecular Catalysis A: Chemical 404–405 (2015) 211–217
ture of the Rh/POL-PPh3 catalyst is that one Rh ion and three
phosphorous-frame atoms are in the same plane. The rigidity of
the P-frame sites results in the distortion of the Rh P bonds due to
the bond length and orientation effect among three phosphorous-
frame atoms and one Rh ion. Thus one of the three coordination
bonds might break easily for CO inserting into HRh(CO)(P-frame)3
species to form HRh(CO) (P-frame)2 active intermediates, which
2
initiate olefin hydroformylation reaction [37].
Developing ultrastable heterogenized homogenous catalysts,
which maintain their high activity and selectivity, is a long-standing
challenge and of great interest. In this work, the Rh/POL–PPh3
catalyst showed ultrastable catalytic performance in fixed-bed
hydroformylation reactions. This superior activity and stability
performance might be originated from the following two main
features: firstly, all the Rh atoms were existed in single Rh sites
as confirmed by HAADF-STEM and EXAFS results which might
be accounted for the high TOF values of hydroformylation reac-
tion. Secondly, strong interaction between the active Rh-containing
molecular species and the highly porous POL–PPh3 support, which
has been discussed above, is beneficial to preventing the leaching
Scheme 2. Schematic diagrams of Rh/POL–PPh3 catalysts illustrating the proposed
structures of the active single sites of the Rh/POL–PPh3 catalyst: (a) Rh ion tightly
bonded to three phosphorus atoms of the Rh/POL–PPh3 support with the Rh ion at
the apex of tetragonal and (b) with the Rh ion located on the same plane as the three
phosphorus atoms.
contains an equatorial and an apical phosphine ligand). The peaks
4
. Conclusions
−1
at 2054 and 1992 cm are attributed to ee-HRh(CO) (P-frame)
2
2
−
1
species, while the peaks at 2002 and 1959 cm
are attributed
In summary, the Rh/POL–PPh3 catalyst has been demonstrated
to ea-HRh(CO) (P-frame) species. One peak of the Rh/POL–PPh3
2
2
as an ultrastable heterogenized homogenous catalyst with high
catalytic activity for fixed-bed hydroformylation of ethylene and
1-dodecene. HAADF-STEM and EXAFS results indicated that the
Rh species existed in single Rh sites. The catalyst’s stability orig-
inates from the formation of strong coordination bonds between
the Rh ion and the exposed phosphorous atoms of the highly
porous POL–PPh support. The Rh/POL–PPh catalyst provided high
−1
catalyst at 2078 cm
Another peak at 2002 cm due to HRh(CO)(P-frame) species is
overlapped with the peak at 2002 cm assigned to HRh(CO) (P-
is due to HRh(CO)(P-frame) species [34].
3
−
1
3
−
1
2
frame)2 species. In situ FT-IR spectrum showed that active species,
such as HRh(CO) (P-frame)2 and HRh(CO)(P-frame) , similar to
2
3
the conventional HRh(CO)(PPh ) , may have been formed in the
3
3
3
3
Rh/POL–PPh3 catalysts during the hydroformylation reaction.
activity and selectivity because it maintained the catalytic func-
tionalities similar to the homogeneous HRh(CO)(PPh ) complex,
3
3
as demonstrated both by 31P MAS NMR and in situ IR experiments.
The Rh/POL–PPh3 catalysts described in this report may provide a
new concept in developing ultrastable heterogenized homogenous
catalysts with high activity and selectivity.
3
.2.5. Discussion
31P MAS NMR experiments revealed that the spent Rh/POL–PPh3
catalyst after hydroformylation of ethylene showed similar spec-
trum as a HRh(CO)(PPh ) /POL–PPh sample. Strong vibration
3
3
3
bands assigned to HRh(CO) (P-frame) and HRh(CO)(P-frame)3
2
2
PPh3 catalyst with a premixing gas of C H /CO/H by in situ
Acknowledgments
2
4
31
FT-IR technique. Both the P MAS NMR and FT-IR data suggest
that the formation of Ph–P on the surface of POL–PPh3 support,
and the reaction mechanism of olefin hydroformylation over the
homogeneous HRh(CO)(PPh3)3 complex [35,36]. In the proposed
We appreciate Prof. Xiangping Hu, Mr. Xianchun Liu and Dr. Tao
Liu for technical support and helpful discussions. J.Y.L. was funded
by Arizona State University. We gratefully acknowledge the use of
facilities within the LeRoy Eyring Center for Solid State Science at
Arizona State University.
scheme, HRh(CO)(P-frame)3 and HRh(CO) (P-frame)2 might be
2
transformed into each other in dynamic.
The fitted radial distribution functions of the EXAFS spectra
(
Fig. 3 and Table 3) indicate that the Rh-P bond length is in the
range of 2.22–2.28 Å and that of the Rh-C bond is in the range of
.67–1.70 Å. According to the results of the curve-fitting analysis
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1
of the EXAFS traces for the supported Rh catalysts (Table 3), the
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active species formation. Based on all the data discussed above,
we propose two probable structures of the Rh/POL–PPh3 cata-
lyst. Both structures contain an Rh ion tightly bonded to three
phosphorous-frame atoms of the support (Scheme 2). The possi-
ble structure of the Rh/POL–PPh3 catalyst is that one Rh ion and
two phosphorous-frame atoms are in the same plane; meanwhile
another phosphorous-frame atom is nearly perpendicular to this
plane, or one Rh ion and one phosphorous-frame atom are in the
same plane; meanwhile the other two phosphorous-frame atoms
are nearly perpendicular to this plane. The other possible struc-
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