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ligand-modified Rh catalysts. To our knowledge, no work has
been done with inorganic promoters on ligand-modified Rh
catalysts until now. The catalysts were tested in the eth-
ylene hydroformylation reaction and characterized by means
of N2 adsorption/desorption isotherms, transmission electron
microscope (TEM), NH3 temperature programmed desorption
(NH3-TPD), Fourier transform infrared spectroscopy (FT-IR) and
solid-state nuclear magnetic resonance (solid-state NMR).
The morphology of the catalysts was observed using an FEI Tec-
nai G2 F30 S-Twin instrument. The Rh particle size statistics was
completed by using the software (Nano Measurer 1.2).
FT-IR spectra were recorded on a Bruker Tensor 27 instrument.
The liquid samples were deposited onto a KBr window and the
solid samples were pressed into self-supporting discs of the same
amount and placed in an IR cell. The samples prepared for measur-
ing pyridine FT-IR spectra were treated under vacuum at 573 K for
0.5 h. Then, they were exposed to pyridine vapor for 0.5 h at room
temperature, followed by vacuuming. After 30 min, the spectra
were recorded. The vacuuming was a prerequisite for the subse-
quent operation and would not be mentioned specifically. Then
the temperature was raised to 423 K and held for 0.5 h. After cool-
ing to room temperature, the spectra were collected. The spectra
at 523 K were obtained after a new turn of temperature rise, reten-
tion and descent. The samples prepared for recording in situ FT-IR
spectra were treated as follows: reduction in a H2 flow for 1 h at
393 K, adsorption of a CO:H2 = 1:1 mixture gas for 0.5 h at 323 K,
then purging with N2 for 0.5 h at 323 K.
2. Experimental
2.1. Catalyst preparation
RhAln/SiO2 was prepared by co-impregnation of silica gel with
RhCl3·xH2O and Al(NO3)3·9H2O in ethanol and dried in air. Subse-
quently, it was calcined at 573 K for 4 h and then reduced in a H2
flow at 573 K for 4 h at atmospheric pressure. After washing off the
The metal loading of Rh was 1.2 wt%, and the atom ratio of Al to Rh
was n (n = 0.2, 1, 3). The Rh/SiO2 catalyst was prepared in the same
way without the addition of the Al(NO3)3·9H2O.
NH3-TPD was performed using a Micromeritics AutoChem 2910
instrument. After treating at 573 K for 0.5 h, the samples were
cooled to 373 K and adsorbed NH3 to saturation in a He flow. Then
the temperature was raised to 573 K at a rate of 10 K min−1 along
with the detection of NH3.
3-Diphenylphosphinopropyltriethoxysilane (DPPPTS) was pre-
pared according to the literature [16]. 0.9521 g lithium was cut into
pieces and added to a solution of 10.0183 g diphenylchlorophos-
phine in 40 ml tetrahydrofuran with magnetic stirring. An
exothermic reaction took place instantly, and the color of the
solution turned red-brown with a great deal of precipitation. The
mixture was stirred for 12 h and then filtered. The filtrate was added
dropwise to a solution of 15.3727 g 3-chloropropyltriethoxysilane
in 45 ml tetrahydrofuran at room temperature. The solution was
stirred for 18 h. Most of the tetrahydrofuran was removed by dis-
tillation. The produced precipitate was filtered off and the filtrate
was distilled at reduced pressure. A light yellow liquid was obtained
at 300 Pa, 459–461 K. 31P NMR (CDCl2): ı −17.2 ppm.
Solid state 31P NMR spectra and 27Al NMR spectra were acquired
using a VARIAN infinity plus spectrometer. 31P chemical shifts were
referenced to the chemical shift of H3PO4 at 0 ppm, while 27Al
chemical shifts were referenced to the chemical shift of [Al(H2O)6]
at 0 ppm.
3. Results and discussion
3.1. Hydroformylation of ethylene
The catalytic performances of ethylene hydroformylation over
the DPPPTS-RhAln/SiO2 and DPPPTS-Rh/SiO2 catalysts were eval-
uated from the TOF versus time on stream, as shown in Fig. 1.
All catalysts exhibited far higher activities than the conventional
heterogeneous Rh/SiO2 catalyst (TOF = 0.8 h−1). As there was an
induction period for the ligand-modified Rh catalyst to form in situ
the active species, the catalytic activities of all catalysts increased
with a prolonged time during the first 144 h, and then became
steady. Under identical reaction conditions, the DPPPTS-RhAln/SiO2
catalysts showed much higher activities than the DPPPTS-Rh/SiO2
Tethered diphenylphosphinopropyl/modified Rh/SiO2 and
RhAln/SiO2 catalysts were prepared as described in literature [6].
In the case of the DPPPTS-RhAln/SiO2, 0.3 g RhAln/SiO2 was added
to a solution of 0.03 g DPPPTS in 3 ml toluene, the mixture was
stirred for 16 h at room temperature and then for another 6 h
under reflux. After cooling to room temperature, the toluene was
removed by vacuum-pumping for 3 h. The DPPPTS-Rh/SiO2 had
the same P/Rh ratio with the DPPPTS-RhAln/SiO2.
All manipulations referring to the use of DPPPTS were carried
out under an argon atmosphere.
2.2. Ethylene hydroformylation
The hydroformylation of ethylene was performed in a con-
tinuous flow fixed-bed reactor with inner diameter of 4.6 mm
under the condition of P(C2H4:CO:H2 = 1:1:1) = 1.0 MPa, T = 393 K,
mcatalyst = 0.3 g, GHSV = 2000 h−1, and P/Rh = 2.2. The effluent passed
through a condenser filled with 70 ml de-ionized water. The so-
obtained aqueous solution was analyzed by a HP-6890N GC with
an FFAP column, using an FID detector and with ethanol as an inter-
nal standard. The tail gas was analyzed on-line using the same
HP-6890N GC with a Porapak-QS column and a TCD detector. The
turn-over-frequency (TOF) of the catalyst was calculated based on
the Rh content of the catalyst.
2.3. Characterization
N2 adsorption–desorption isotherms of the samples were mea-
sured using a Quantachrome Autosorb-1 instrument. Prior to the
measurements, the samples were degassed under vacuum at 573 K
for 3 h.
Fig. 1. Catalytic performance of ethylene hydroformylation over DPPPTS-Rh/SiO2
and DPPPTS-RhAln/SiO2.