Recyclable heterogeneous Rh/SiO2 catalyst enhanced by organic PPh3 ligand

Article abstract of DOI:10.1246/cl.2004.630

Heterogeneous PPh₃-Rh/SiO₂ catalysts for hydroformylation of olefins, prepared by direct doping of phosphine onto the heterogeneous Rh/SiO₂ precursor, exhibited high activity and selectivity towards aldehydes, which originated from chemical coordination bond between the phosphine and Rh metal nanoparticles on the SiO₂ support.

Full text of DOI:10.1246/cl.2004.630

630
Chemistry Letters Vol.33, No.5 (2004)
Recyclable Heterogeneous Rh/SiO2 Catalyst Enhanced by Organic PPh3 Ligand
Hejun Zhu, Yunjie Ding,ꢀ Li Yan, Yuan Lu, Can Li, Xinhe Bao, and Liwu Lin
Dalian Institute of Chemical Physics, The Chinese Academy of Sciences, Dalian 116023, P. R. China
(Received February 12, 2004; CL-040166)
Heterogeneous PPh3–Rh/SiO2 catalysts for hydroformyla-
as nanoparticles.
Table 1 shows the results of hydroformylation of 1-hexene
tion of olefins, prepared by direct doping of phosphine onto
the heterogeneous Rh/SiO2 precursor, exhibited high activity
and selectivity towards aldehydes, which originated from chemi-
cal coordination bond between the phosphine and Rh metal
nanoparticles on the SiO2 support.
over Rh/SiO2, homogeneous HRh(CO)(PPh3)3 and our PPh3–
Rh/SiO2 catalysts in the slurry-bed reactor. The selectivity to-
ward aldehyde is very low and the turnover frequency (TOF) to-
wards aldehydes of the Rh/SiO2 catalyst is as low as 21 hꢁ1. In
contrast, the homogeneous HRh(CO)(PPh3)3 complex shows
very high activity and selectivity towards aldehydes. The TOF
of the homogeneous catalyst was detected as 1658 hꢁ1. It is
worth to note that the black PPh3–Rh/SiO2 catalyst exhibits
the TOF of 1011 hꢁ1, selectivity towards aldehyde of 84% and
n/i ratio of aldehyde of 3.1, respectively, which were almost
close to those of the homogeneous counterpart HRhCO(PPh3)3.
It is surprised that its catalytic performances are by far higher
than those of the Rh/SiO2 catalyst. This result implies that the
addition of PPh3 to Rh/SiO2 precursor exerts a significant role
on the Rh/SiO2 and thus enhanced the catalytic activity of PPh3–
Rh/SiO2 catalyst. In addition, the PPh3–Rh/SiO2 catalyst was
easily to be separated from the reaction systems and recycled
as those of the conventional heterogeneous catalysts.
Heterogeneous Rh/SiO2 modified with metal oxide or alkali
metal promoters for hydroformylation of olefins possesses the
advantage of easy separation and recycle of catalyst from reac-
tion system but has the disadvantages of low activity and selec-
tivity.1 The HRhCO(PPh3)3 (PPh3 = triphenylphosphine) as ho-
mogeneous catalysts for hydroformylation process exhibits high
activity and selectivity toward aldehydes,2 but encounters the
difficulty to separate the soluble catalyst from reaction system.3
Approaches for immobilization of homogeneous complex to
solve this problem have been explored.4 The water/oil biphasic
process using water-soluble catalyst [HRhCO(TPPTS)3]
(TPPTS = triphenylphosphanyl trisulfonate) has been developed
for the hydroformylation of propylene.5 The applicability of the
aqueous biphasic system is, however, strictly limited to sub-
strates that are slightly soluble in water, such as propylene and
1-butene. Another strategy has long been attachment of the cat-
alyst to polymer resin or silica and the separation are recyclable
by a simple filtration.6 However, low activity, selectivity, and
leaching of Rh have limited the actual application of ‘‘heterogen-
ized’’ catalysts in industry.
In this paper, we wish to report a new type of organic triphe-
nylphosphine ligand promoting heterogeneous Rh/SiO2 cata-
lysts (PPh3–Rh/SiO2) for hydroformylation of olefins. This cat-
alyst system was prepared by directly employing a phosphine
with lone-pair electron to modify heterogeneous Rh/SiO2 with
an intention to get the heterogeneous catalyst with high activity,
selectivity and easy separation.
Heterogeneous Rh/SiO2 precursor was prepared by incipi-
ent wetness impregnation of silica with a solution of RhCl3.
The impregnated sample was dried first at room temperature,
then at 393 K, and subsequently calcined at 573 K for 3 h, and fi-
nally reduced in a flow of H2 at 573 K for 3 h. The reduced black
sample was further washed with boiling deionized water to dei-
onize the Clꢁ1 ion on the Rh/SiO2. Then the obtained sample
was reduced again to remove oxygen and water. This heteroge-
neous Rh/SiO2 precursor were introduced to the solution of
phosphines in cyclohexane solvent in a Schlenk bottle (the molar
ratio of P:Rh = 15:1, unless specified), then the sample was stir-
red for 1 h at room temperature, and lastly the solvent was evac-
uated under vacuum at room temperature. The black PPh3–Rh/
SiO2 catalyst was stored under argon at room temperature. The
Rh loading of Rh/SiO2 was 1 wt%. The uptake data of H2 pulse
adsorption of the Rh/SiO2 precursor has showed that the disper-
sion of metals on the surface of SiO2 is 67% and Rh metal exists
Table 1. The results of 1-hexene hydroformylation over different
catalysts
Selectivity
Catalysts
Conv.
/%
TOF
Hexene-
2 and -3
93.1
12.9
16.5
0
Alde
Hexane
n/i
Rh/SiO2
HRhCO(PPh3)3
PPh3–Rh/SiO2
PPh3–Rh/SiO2
PPh3–Rh/SiO2
97.5
18.4
23.1
29.4
30.6
6.9
87.1
83.5
100
0
0
0
0
0
1.1
3.2
3.1
–
21
1658
1011
–
a
b
100
0
–
–
aRection conditions: T = 373 K; P = 1.0 MPa; the stirring rate was 900 RPM;
Rh/Si2 catalyst was tested for 4 h, the other catalysts were tested 0.5 h; In the
hydroformylation of 1-hexene, the gas pressure ratio of CO and H2 is
0.5 MPa:0.5 MPa. In the hydroformylation of C2H4, the gas pressure ratio
of CO, H2 and C2H4 is 0.33 MPa:0.33 MPa:0.33 MPa; TOF is defined as mo-
les of aldehydes formed per mole of rhodium per hour; PPh3–Rh/SiO2
(3:25 ꢂ 10ꢁ2 mmol of Rh); HRh(CO)(PPh3)3 (2:17 ꢂ 10ꢁ2 mmol of Rh);
Rh/SiO2 (10:83 ꢂ 10ꢁ2 mmol of Rh); 1-hexene (15 mL); aThe catalytic per-
formance for hydroformylation of ethylene over PPh3–Rh/SiO2 in the
fixed-bed reactor at 75th hour; bthe catalytic performance for hydroformyla-
tion of ethylene over PPh3–Rh/SiO2 in the fixed-bed reactor at 950th hour.
Hydroformylation of ethylene over PPh3–Rh/SiO2 catalyst
was carried out in the fixed-bed reactor at 373 K and 1.0 MPa
for more than 1000 h. Some representative data are shown in
Table 1. The conversion of ethylene at 75th h was almost equiv-
alent to those at 950th h, which demonstrated good stable activ-
ity for PPh3–Rh/SiO2 catalyst over a long period of time. It was
very important that no Rh leaching of the black PPh3–Rh/SiO2
catalyst was observed in the course of the operation. Another ad-
vantage was that this reaction system could be conducted in a
continuous manner and the product could be very easily separat-
ed from the reactor. From above observations, we can conclude
that the hydroformylation of olefin mostly takes place on the sur-
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.5 (2004)
631
face of the heterogeneous PPh3–Rh/SiO2 catalyst.
The interaction between phosphine and Rh/SiO2 was inves-
tigated by means of 31P MAS NMR and FTIR techniques. Figure
1 shows the 31P MAS NMR spectra of the PPh3/SiO2, PPh3–Rh/
SiO2 and HRhCO(PPh3)3/SiO2 catalysts. In the spectrum A,
there was a sharp signal at ꢁ6.2 ppm assigned to free PPh3 on
the PPh3/SiO2. While in the spectrum B, a broad peak at ca.
34.1 ppm appeared and the signal at ca. ꢁ6.0 ppm became weak
when the PPh3 ligands was added onto Rh/SiO2. There was a
broad signal at 37.2 ppm in the 31P MAS NMR spectrum of
HRhCO(PPh3)3/SiO2 sample (Figure 1C), which could be at-
tributed to the arose coordination bond between Rhþ ion and
PPh3 of rhodium complex.8 By comparison, the chemical shift
as well as the appearance of the broad peak at 37.2 ppm in the
spectrum C was close to both of that at ca. 34.1 ppm in the spec-
trum B. This result suggested that chemical coordination bond
was formed between PPh3 and surfacial Rh metal nanoparticle
on the PPh3–Rh/SiO2 sample.
Figure 2. The FTIR spectra of the CO absorbed Rh/SiO2 and PPh3–Rh/
SiO2 (the molar ratio of PPh3 to Rh = 3:1) and the rhodium–phosphine car-
bonyl complex, and the in situ hydroformylation of CO/H2/C2H4 on Rh/
SiO2 and PPh3–Rh/SiO2 in an IR cell (A) rhodium–phosphine carbonyl
complex; (B) Rh/SiO2; (C) PPh3–Rh/SiO2; (D) CO/H2/C2H4 hydrofor-
mylation on RhSiO2 in an IR cell after 90 min; (E) CO/H2/C2H4 hydrofor-
mylation on PPh3–Rh/SiO2 after 5 min.
ylene occurred on the heterogeneous PPh3–Rh/SiO2 catalyst,
not the derived homogeneous rhodium–phosphine species.
In conclusion, the heterogeneous PPh3–Rh/SiO2 catalysts
for hydroformylation of olefins, prepared by modifying the
Rh/SiO2 precursor with organic phosphines, demonstrated the
advantages of high activity, selectivity towards aldehydes and
easy separation. The chemical bond of the catalysts was not only
obviously distinguished from those of hererogeneous Rh/SiO2
catalyst, but also different from those of homogeneous counter-
parts.
Figure 1. 31P MAS NMR spectra of samples containing PPh3 (the molar
ratio of PPh3 to Rh = 3:1) (A) PPh3/SiO2; (B) PPh3–Rh/SiO2; (C)
HRhCO(PPh3)3/SiO2.
Figure 2 shows the FTIR spectra of the CO adsorbed on the
Rh/SiO2 and PPh3–Rh/SiO2, and the rhodium–phosphine car-
bonyl complex. The signals at 2050 cmꢁ1 and 1892 cmꢁ1 were
assigned to linear and bridged CO adsorbed on the Rh/SiO2
This work was financially supported by the Science and
Technology Ministry of China (Grant no. 2003CB615803)
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Published on the web (Advance View) April 24, 2004; DOI 10.1246/cl.2004.630