Effect of lengthening alkyl spacer on hydroformylation performance of tethered-phosphine modified Rh/SiO2 catalyst

Article abstract of DOI:10.1016/S1872-2067(15)61019-1

Rh/SiO₂ catalysts with tethered-phosphines with different alkyl spacer lengths have been prepared, tested and characterized. Lengthening the alkyl spacer of the tethered-phosphine improved the flexibility of tethered-phospine, promoted the formation of active species and enhanced the activity of hydroformylation over other tethered-phosphine modified Rh/SiO₂ catalysts.

Full text of DOI:10.1016/S1872-2067(15)61019-1

Chinese Journal of Catalysis 37 (2016) 268–272
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Effect of lengthening alkyl spacer on hydroformylation performance
of tethered‐phosphine modified Rh/SiO2 catalyst
Jia Liu a,c, Li Yan a,b,#, Miao Jiang a,c, Cunyao Li a,c, Yunjie Ding a,b,
*
a Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
b State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China
c University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Rh/SiO2 catalysts with tethered‐phosphines with different alkyl spacer lengths have been prepared,
tested and characterized. Lengthening the alkyl spacer of the tethered‐phosphine improved the
flexibility of tethered‐phospine, promoted the formation of active species and enhanced the activity
of hydroformylation over other tethered‐phosphine modified Rh/SiO2 catalysts.
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Received 24 August 2015
Accepted 10 November 2015
Published 5 February 2016
Published by Elsevier B.V. All rights reserved.
Keywords:
Tethered phosphine
Chain length
Rhodium
Silica
Hydroformylation
1. Introduction
catalysts, and the heterogenization of homogeneous catalysts is
one of these hotspots.
Aldehydes, the products of olefin hydroformylation, are im‐
portant intermediates in the production of many organic com‐
pounds and have significant value in industry. Homogeneous
catalysts are the most commonly used in industrial hydro‐
formylation processes because of their high activity, high selec‐
tivity and mild reaction conditions. However, they require
complex and costly separation steps to be removed from the
reactants and products. In contrast, heterogeneous catalysts
can easily be separated from the reaction mixture but their
application in hydroformylation industrial processes has been
restricted by their low activity and low selectivity [1,2]. For
decades, researchers have explored new catalytic systems
combining the advantages of heterogeneous and homogeneous
A common strategy for heterogenizing homogeneous cata‐
lysts is immobilization or heterogenization of a transi‐
tion‐metal complex on a solid support. Immobilization of tran‐
sition‐metal complex on solid supports has achieved some
success by chemically bonding the complex to a support, but
these catalysts tend to leach the active species into the reaction
mixture and thus deactivates the catalyst [3–5].
We have reported a new tethered‐phosphine modified
Rh/SiO2 catalyst with excellent stability for use in the hydro‐
formylation of ethylene [6]. Both tethered‐phosphines and the
supported active metal nanoparticles were anchored to SiO2,
which contributed to the remarkable stability of the catalyst.
However, as the phosphines and metal nanoparticles were both
* Corresponding author. Tel/Fax: +86‐411‐84379143; E‐mail: dyj@dicp.ac.cn
# Corresponding author. Tel/Fax: +86‐411‐84379055; E‐mail: yanli@dicp.ac.cn
This work was financially supported by the National Natural Science Foundation of China (21273227, 20903090).
DOI: 10.1016/S1872‐2067(15)61019‐1 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 2, February 2016

Jia Liu et al. / Chinese Journal of Catalysis 37 (2016) 268–272
269
fixed, their mobility was restricted, and some of the teth‐
ered‐phosphines could not reach, and coordinate to, the Rh
nanoparticles efficiently to form the active species, which re‐
sulted in relative low catalytic activity in the olefin hydro‐
formylation.
der the following conditions: 0.3 g of sample, the pressure of
reactant (C2H4:CO:H2 = 1:1:1) 1.0 MPa, temperature 393 K, and
the GHSV of reactant 2000 h–1. No gaseous products could be
detected in the tail gas, and the main product in the aqueous
sample was propanal. The effluent was passed through a con‐
denser filled with 70 ml of de‐ionized water giving an aqueous
solution of propanal, which remained completely dissolved for
the duration of the experiment. The activity of the catalyst was
measured by the turn‐over‐frequency (TOF) of propanal on the
basis of the Rh loaded, counting all Rh atoms as active sites.
Ligand flexibility and projection into solution are related to
the chain length of the ligand, and Zhou et al. [7,8] have re‐
ported a series of S‐Cn‐PPh2‐Rh catalysts in which Rh complex‐
es were immobilized onto SBA‐15 by anchored phosphines
with different chain lengths. The catalysts showed excellent
activities in 1‐octene hydroformylation but the P/Rh molar
ratios were variable and significant Rh leaching occurred. In
our catalytic system, lengthening the alkyl spacer of the teth‐
ered‐phosphine may enhance the coordination between the
tethered‐phosphine and the metal nanoparticles, increase the
number of active species, and thus improve the activity of the
tethered‐phosphine modified Rh/SiO2 catalyst. In this work, a
phosphine with a longer alkyl spacer was prepared, and then
used in the production of a Rh/SiO2 catalyst. The influence of
the alkyl spacer length of the phosphine on its catalytic activity
was studied and characterized by N2 adsorption‐desorption, in
situ Fourier Transform infrared (in situ FT‐IR) spectroscopy,
and solid‐state 31P nuclear magnetic resonance (31P NMR).
2.3. Catalyst characterization
N2 adsorption‐desorption isotherms of the samples were
measured using a Quantachrome Autosorb‐1 instrument to
obtain the textural properties of catalysts. The in situ FT‐IR
spectra were recorded on a Thermo Scientific iS50 instrument
equipped with a high temperature high pressure cell (Specas).
A sample of 15 mg was pressed into a self‐supporting disc. The
adsorption of CO and H2 was performed on the disc at 323 K
and atmospheric pressure. The 31P NMR spectra were acquired
using a VARIAN infinity plus spectrometer. The Rh concentra‐
tions of catalysts were analyzed by inductively coupled plasma
optical emission spectrometry (ICP‐OES).
2. Experimental
3. Results and discussion
2.1. Catalysts preparation
3.1. Catalytic studies
Rh/SiO2 was prepared by impregnating silica gel (≥99%,
amorphous silica, Ordos Chemical Co. Ltd., 20–40 mesh, BET
surface = 256.3 m2/g, total pore volume = 0.95 ml/g, average
pore radius = 7.4 nm) with RhCl3·xH2O (37.22 wt% Rh, Johnson
Matthey) in ethanol. After drying in air, the RhCl3/silica was
calcined at 573 K for 4 h and then reduced in a H2 flow at 573 K
for 4 h at atmospheric pressure. Then, it was washed to remove
Cl–, dried at 393 K, reduced again and restored in an Ar atmos‐
phere. The Rh loading was 1.2 wt%.
Fig. 1 shows the catalytic performance of the modified
Rh/SiO2 catalysts with different alkyl spacer lengths of the
tethered‐phosphine. The TOFs of the catalysts in Fig. 1 in‐
creased with the time on stream, implying a gradual formation
of the active species on the catalysts during the ethylene hy‐
droformylation reaction. The TOF reached a steady state after
400 h. The TOF of DPPETS‐Rh/SiO2 was only 20.9 h–1, while
that of DPPPTS‐Rh/SiO2 reached 40.7 h–1. Both catalysts were
higher than Rh/SiO2 (TOF = 0.8 h–1) [10]. Thus, lengthening the
alkyl spacer of the tethered‐phosphine increased the activity of
the tethered‐phosphine modified Rh/SiO2. The ICP‐OES results
2‐(Diphenylphosphino)ethyltriethoxysilane
[Ph2P(CH2)2Si(OC2H5)3, referred as DPPETS] was bought from
J&K Scientific and 3‐(Diphenylphosphino)propyl‐triethoxysi‐
lane [Ph2P(CH2)3Si(OC2H5)3, referred as DPPPTS] was prepared
according to a literature method [9]. The tethered‐phosphine
modified DPPETS‐Rh/SiO2 and DPPPTS‐Rh/SiO2 catalysts were
prepared as described in the Ref. [6]. Rh/SiO2 was added to a
solution of DPPETS (or DPPPTS) in toluene and the P/Rh molar
ratio was 2.24. The mixture was stirred for 16 h at room tem‐
perature and then for a further 6 h at reflux. After cooling to
room temperature, the toluene was removed under vacuum. All
manipulations referring to the use of phosphine were carried
out under an Ar atmosphere.
50
(1)
40
30
(2)
20
10
0
2.2. Catalytic activity
0
100
200
300
400
500
The catalytic performance of ethylene hydroformylation
over the tethered‐phosphine modified Rh/SiO2 catalysts was
tested in a stainless steel continuous flow fixed‐bed reactor
with inner diameter of 6 mm. The reaction was conducted un‐
Time (h)
Fig. 1. Catalytic performance of hydroformylation of ethylene over
DPPPTS‐Rh/SiO2 (1) and DPPETS‐Rh/SiO2 (2) catalysts at P = 1 MPa, T
= 393 K and GHSV = 2000 h–1.

270
Jia Liu et al. / Chinese Journal of Catalysis 37 (2016) 268–272
2005
400
300
200
100
0
1991
(1)
(2)
1948
2071
2039
1903
1900
0
5
10
r (nm)
15
(1)
(2)
(1)
(2)
0.0
0.2
0.4
0.6
0.8
1.0
2100
2050
2000
1950
1850
1800
Wavenumber (cm1)
p/p0
Fig. 2. N2 adsorption‐desorption isotherms and corresponding pore
size distributions (insert) for DPPPTS‐Rh/SiO2 (1) and DPPETS‐Rh/SiO2
(2) catalysts.
Fig. 3. In situ FT‐IR spectra of CO and H2 co‐adsorbed on DPPPTS‐Rh/
SiO2 (1) and DPPETS‐Rh/SiO2 (2) catalysts at atmospheric pressure and
323 K for 0.5 h.
the DPPPTS‐Rh/SiO2. The bands at 2071 and 1991 cm–1 have
been assigned to ee‐HRh(CO)2(DPPPTS)2/SiO2 (i.e., tethered
ee‐HRh(CO)2 (DPPPTS)2 on SiO2), while the bands at 2005 and
1948 cm–1 have been assigned to ea‐HRh(CO)2(DPPPTS)2/SiO2
[10–12].The band at 2039 cm–1 has been assigned to linear
adsorbed CO, as Rh‐CO [13–14]. This band has been signifi‐
cantly red‐shifted compared with the heterogeneous Rh/SiO2
catalyst [15–17], because of the weaker C–O bond resulting
from π* back‐donation from the Rh to the CO in the presence of
the tethered‐phosphine. There were five analogous bands in
the spectrum of DPPETS‐Rh/SiO2, which were similarly as‐
signed to ee‐HRh(CO)2(DPPETS)2/SiO2, ea‐HRh(CO)2(DPPTES)2/
SiO2 and the linear CO adsorbed on the Rh nanoparticles, re‐
spectively. Compared with the bands of DPPPTS‐Rh/SiO2, the
spectrum of DPPETS‐Rh/SiO2 was slightly blue‐shifted, a result
of the different tether length on the Rh/SiO2 catalysts.
Because HRh(CO)2(DPPPTS)2/SiO2 and HRh(CO)2 (DPPETS)2/
SiO2 are the analogous active species for the homogeneous
Wilkinson‐type hydroformylation [18], they were assumed to
be the active species that catalyzed the olefin hydroformylation
on the tethered‐phosphine modified Rh/SiO2 catalysts. Fur‐
thermore, linear CO has been shown to promote hydroformyla‐
tion in heterogeneous catalysts [19]. The FT‐IR spectrum of
DPPPTS‐Rh/SiO2 was more intense than DPPETS‐Rh/SiO2.
Rasband et al. [20] have reported the concentration of ad‐
sorbed CO is proportional to the corresponding FT‐IR band
intensity. The increased band intensity implies higher concen‐
tration of CO was adsorbed on the catalysts. Therefore, higher
concentration of CO has been adsorbed on the DPPPTS‐Rh/SiO2
catalyst than on the DPPPET‐Rh/SiO2 catalyst, which explains
the increased activity of DPPPTS‐Rh/SiO2. In addition, a band at
1903 cm–1 appeared in the DPPETS‐Rh/SiO2 catalyst, but was
not seen in the DPPPTS‐Rh/SiO2 spectrum. This band has been
assigned to bridging CO on the Rh nanoparticles [21], suggest‐
ing that some Rh atoms on DPPETS‐Rh/SiO2 were not modified
by the tethered‐phosphine and remained as Rh nanoparticles.
This would also contribute to relative low catalytic activity of
DPPETS‐Rh/SiO2.
showed that very little Rh leaching had occurred, even after
long reaction times: 11040 ppm Rh for the fresh DPPETS‐Rh/
SiO2, compared with 10060 ppm Rh for the used one, and
10450 ppm Rh for the fresh DPPPTS‐Rh/SiO2 and 10580 ppm
Rh for the used catalyst.
The preparation of pure phosphines with longer alkyl spac‐
ers was hindered by the formation of cyclic compounds that did
not contain the PPh2 group. This paper therefore focuses on the
available chain lengths, and other, longer chains will be pre‐
sented later.
3.2. N2 adsorption‐desorption results
N2 adsorption‐desorption isotherms and corresponding
pore size distributions of DPPETS‐Rh/SiO2 and DPPPTS‐Rh/
SiO2 are shown in Fig. 2, and the detailed values are listed in
Table 1. It can be seen that the catalysts had only small differ‐
ences in pore structure, pore size distribution, BET surface
(SBET), total pore volume (Vp) and average pore radius (Rp). In
other words, lengthening the alkyl spacers of the teth‐
ered‐phosphine did not change the textural properties of the
catalysts and mass transfer during the reaction. Hence, the dif‐
ference in activity of catalysts was not the result of their textur‐
al properties.
3.3. In situ FT‐IR
In situ FT‐IR is a powerful technique in identifying the active
adsorbed species on heterogeneous catalysts. Fig. 3 shows the
in situ FT‐IR spectra of H2 and CO co‐adsorbed on the surfaces
of the DPPPTS‐Rh/SiO2 and DPPETS‐Rh/SiO2 catalysts. Five
bands at 2071, 2039, 2005, 1991 and 1948 cm–1 appeared in
Table 1
Textural properties of the catalysts.
Catalyst
DPPPTS‐Rh/SiO2
DPPETS‐Rh/SiO2
S
BET (m2/g)
244.4
256.5
V
p (ml/g)
0.6
0.7
R
p (nm)
5.3
5.1

Jia Liu et al. / Chinese Journal of Catalysis 37 (2016) 268–272
271
confirms the previous results where the TOF increased with
time on stream. The r value of the used DPPPTS‐Rh/SiO2 (1.31)
was significantly higher than that of the used DPPETS‐Rh/SiO2
(1.05). This observation suggests that the proportion of the
tethered‐phosphine coordinated to the Rh nanoparticles in
DPPPTS‐Rh/SiO2 was higher than in DPPETS‐Rh/SiO2, corrob‐
orating the assumption that tethered‐phosphines with longer
alkyl spacers can more easily coordinate to Rh nanoparticles to
form a more active Rh‐phosphine complex than those with
shorter alkyl spacers.
-16.0
-16.9
+38.1
(1)
(2)
r = 0.77
r = 1.31
+35.1
-9.6
+40.5
+36.6
(3)
(4)
r = 0.26
r = 1.05
-9.4
4. Conclusions
70 60 50 40 30 20 10
0
-10 -20 -30 -40
This study shows that the catalytic activity of teth‐
ered‐phosphine modified Rh/SiO2 catalyst for olefin hydro‐
formylation could be improved by lengthening the alkyl spacer
of the tethered‐phosphine. In situ FT‐IR results confirmed that

Fig. 4. 31P NMR spectra of the fresh DPPPTS‐Rh/SiO2 (1), used
DPPPTS‐Rh/SiO2 (2), fresh DPPETS‐Rh/SiO2 (3), and used
DPPETS‐Rh/SiO2 (4) catalysts.
the active species were
a HRh(CO)2(DPPPTS)2/SiO2 or
HRh(CO)2(DPPETS)2/SiO2 complex. The intensity of the IR
bands showed higher concentration of active species were
formed on the catalyst modified by the tethered‐phosphine
with the longer alkyl spacer. 31P NMR results suggest that it is
easier for the tethered‐phosphine with longer alkyl spacer to
coordinate to Rh nanoparticles and produce the active
Rh‐phosphine complex. In conclusion, lengthening the alkyl
spacer of the tethered‐phosphine was beneficial in forming
more active species and enhancing the performance of teth‐
ered‐phosphine modified Rh/SiO2 catalysts for olefin hydro‐
formylation.
3.4. 31P NMR results
Fig. 4 displays the 31P NMR spectra of fresh and used
DPPPTS‐Rh/SiO2 and DPPETS‐Rh/SiO2 catalysts. The spectrum
of the fresh DPPPTS‐Rh/SiO2 shows bands at −16.0 and +38.1
ppm, assigned to free DPPPTS tethered to the SiO2 carrier, and
tethered‐DPPPTS coordinated to Rh nanoparticles, respectively
[6,10,22]. Two bands at −16.9 and +35.1 ppm can be observed
in the spectrum of used DPPPTS‐Rh/SiO2, which have shifted
up‐field compared with the fresh one. This shift is because of a
change in the chemical environment of the phosphorous in the
used sample. Similarly, the fresh and used DPPETS‐Rh/SiO2
catalysts also show two bands assigned to free DPPETS‐SiO2
and coordinated tethered‐DPPETS, respectively.
The down‐field band (α) and up‐field band (β) were inte‐
grated and the ratios of the areas of band α to band β (r) are
shown in Fig. 4. For both catalysts, the r value of the used cata‐
lyst was higher than that of the fresh one, which means more
phosphine was coordinated to the Rh nanoparticles after the
hydroformylation reaction. The gradual formation of the active
tethered Rh‐phosphine complex as the reaction proceeded
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Graphical Abstract
Chin. J. Catal., 2016, 37: 268–272 doi: 10.1016/S1872‐2067(15)61019‐1
50
Effect of lengthening alkyl spacer on hydroformylation performance
Ph2
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of tethered‐phosphine modified Rh/SiO2 catalyst
Si
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Jia Liu, Li Yan *, Miao Jiang, Cunyao Li, Yunjie Ding *
Dalian Institute of Chemical Physics, Chinese Academy of Sciences;
University of Chinese Academy of Sciences
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