Lengthening alkyl spacers to increase SBA-15-anchored Rh-P complex activities in 1-octene hydroformylation

Article abstract of DOI:10.1039/b812910j

The alkyl spacer was lengthened in heterogenizing a Rh-P complex into mesoporous silicate SBA-15 to increase the immobilized catalyst activities in 1-octene hydroformylation. The Royal Society of Chemistry.

Full text of DOI:10.1039/b812910j

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Lengthening alkyl spacers to increase SBA-15-anchored Rh–P complex
activities in 1-octene hydroformylationw
Wei Zhou and Dehua He*
Received (in Cambridge, UK) 28th July 2008, Accepted 2nd September 2008
First published as an Advance Article on the web 1st October 2008
DOI: 10.1039/b812910j
The alkyl spacer was lengthened in heterogenizing a Rh–P
complex into mesoporous silicate SBA-15 to increase the
immobilized catalyst activities in 1-octene hydroformylation.
Heterogenization1 of organometallic catalysts into mesoporous
molecular sieves has been so far extensively studied. In tradi-
tional immobilized catalysts, the metal is stiffly anchored to the
support through short organic groups2,3 as spacers.4 Although
the difficulty in catalyst separation has been hence overcome, the
immobilized catalysts usually reveal decreased activities than
homogeneous counterparts.2,3 In heterogeneous cases, active sites
are localized; reactant molecules are supposed to approach to the
solid surface, to be adsorbed by the active site, and then to be
activated. Mass transfer thus greatly affects the reaction effi-
ciency. The inner diffusion of substrates in the pores will
influence the catalyst activities. Employing immobilized ionic
liquid where catalytic complex is dissolved5 may help to construct
Scheme 1 Preparation of the SARC catalysts.
a homogenous-like surroundings. The active species leaching is
TEM micrographs of the parent SBA-15 showed very ordered
however significant, since the catalyst is actually not bonded.
The present work is motivated by observing the differences
between a pinwheel and a flying kite. Imagine that the vane or the
The catalyst preparation was evidenced by IR.7 Fig. 1 shows
kite is the anchored catalyst, and the stick or the rope is the spacer.
honeycomb-like pores (Fig. E2, ESIw). XRD patterns (Fig. E3,
ESIw) reveal that all SARC catalysts have typical hexagonal
SBA-15 symmetry. The support structure was preserved.
A bird is imagined as the substrate. In case of the pinwheel, the
vanes are immobile owing to the rigid stick. The bird has to fly to
1350, 1296 cmÀ1 in P123-bearing SBA-15 (a) disappeared after
it in a specific direction. However, when a kite is flown in the sky
the IR spectra of samples with spacer C8. Bands nas(C–H) at
2933 cmÀ1, ns(C–H) at 2878 cmÀ1 and r(–CHn–) at 1457, 1375,
through a long rope, the kite is relatively free on account of the
calcination (b), indicating the template removal. These C–H
flexibility of the rope. The kite has various orientations and covers
absorptions reappeared in chlorooctylated SBA-15 (c). In addi-
tion, n(C–Si) at 725 and 687 cmÀ1, n(C–Cl) at 658 cmÀ1 were
a wider range, which offers more possibilities for the bird to
observed. The peak at 955 cmÀ1 was obviously weakened after
contact it. Enlightened by this, we took advantage of the flexibility
the grafting of silane, implying the modifiable silanols8 were
of alkyl spacers to anchor the Rh–P complex into mesoporous
SBA-15. n-Alkyl chains with different length (C1, C3, C5, C8 and
C11) were used as the spacer functions and the SBA-15-anchored
Rh–P complexes (hereafter SARC) catalysts were employed in
decreased. After the connection of diphenyl phosphine ligands
(d), n(C–Cl) disappeared, and n(C–Si) intensity was increased.
n(C–P) was observed at 741 cmÀ1. A broad band characteristic
1-octene hydroformylation6 to produce nonyl aldehydes.
As shown in Scheme 1, synthesis of SBA-15, haloalkylation,
of n(Ar CQC), which might be composed of several peaks, was
observed at 1512 cmÀ1 with p–p conjunction amplifying this
band. A tiny absorption was found at 3073 cmÀ1, which could
phosphine ligand connection, and Rh coordination were
sequentially carried out to prepare SARC catalysts using
precursor RhCl3 (see ESIw for details).
be assigned to the overtone contributed by n(Ar C–H) and
n(Ar CQC) of single-substituted benzene. After the anchoring
of RhCl3 (e), the n(Ar CQC) was weakened due to the electron-
withdrawing nature of the cation whereas n(Ar C–H) was
enhanced and observed. This indicates the interaction between
Innovative Catalysis Program, Key Lab of Organoelectronics &
Molecular Engineering of Ministry of Education, Department of
Chemistry, Tsinghua University, Beijing, 100084, China.
E-mail: hedeh@mail.tsinghua.edu.cn; Fax: +8610 6277 3346;
Tel: +8610 6273 3346
w Electronic supplementary information (ESI) available: SARC cata-
phosphine ligands and rhodium; n(CQO) was found at
1969 cmÀ1 (see inset). The stretching of Si–O–Si in the range
1290–996 cmÀ1 and d(Si–O–Si) at 467 cmÀ1 were unchanged
through the SARC catalyst preparation, indicating the SBA-15
framework was not affected. SARC catalysts prepared by other
lyst preparation, characterization, 1-octene hydroformylation, Rh
leaching, and other Rh precursor results. See DOI: 10.1039/b812910j
alkyl spacers revealed similar IR spectra.
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Fig. 2 1-Octene hydroformylation over the SARC catalysts (precursor
RhCl3): (&) 1-octene TON, (J) selectivity of nonyl aldehydes. TON
(turnover number) = mol converted 1-octene/immobilized Rh.
concentration. The ratio of linear and branched nonyl aldehyde
is 0.72 (C1), 0.68 (C3), 0.57 (C5), 0.56 (C8), and 0.53 (C11). Smaller
pores seem to favor formation of the linear product. This might
because carbonyl is more easily added to the b position of 1-octene
owing to the steric hindrance of the adjacent spacers.
SARC catalyst (precursor RhCl3) with spacer C11 showed
lower activity than RhCl(CO)(PPh3)2 and RhCl(PPh3)3
complexes in 1-octene hydroformylation (Table E1, ESIw).
However, if these more active Rh precursors are anchored to
S–C11–PPh2, the immobilized catalyst also reveals comparable
activity to the corresponding homogeneous counterpart.
Moreover, the selectivity is higher. If less active precursor
[Rh(Ac)2]2 is employed, the immobilized catalyst showed
lower activity as well (Table E1, ESIw).
Fig. 1 IR spectra monitoring of SARC catalyst preparation (C8).
N2 sorption analysis (Fig. E4, ESIw) of the SARC catalysts
revealed typical type IV isotherms with H1 hysteresis loop.9
The pore size distribution was narrow. This indicates all the
SARC catalysts maintained ordered mesoporous structure.
Textual properties and immobilized Rh contents of SARC
catalysts are listed in Table 1.
1-Octene hydroformylation was carried out in an autoclave
(reaction temperature 393 K, initial syngas (CO/H2 = 1) pressure
5 MPa, reaction time 2.5 h, see ESIw). Reaction catalyzed by
RhCl3 was also performed. As is shown in Fig. 2, the TON
revealed by the SARC catalysts increased with the spacer length.
SARC catalyst with spacer C11 showed comparable activity to the
homogeneous process. SARC catalyst with shortest spacer C1
showed lowest TON level. All SARC catalysts revealed high
selectivity to nonyl aldehydes (497%). On the contrary, RhCl3
showed a selectivity of only 91%. The high selectivity revealed by
SARC catalysts might be attributed to their confining by the
mesoporous molecular sieves. By-products (discussed later) were
restricted in the pores, and in the liquid they were of low
Fig. E5 (ESIw) showed Rh leaching in the reaction liquid
after every SARC catalyst recycle. Sum Rh leaching on
Table 1 Textual properties and Rh contents of SARC catalysts
a
a0
nm
/
SBET
/
DBJH
/
VP/
TW
/
Rhb
m2 gÀ1 nm
cm3 gÀ1 nm (wt%)
SBA-15
10.96 673
10.20 208
11.21 283
10.41 242
10.20 199
6.2
5.2
3.7
3.6
3.5
3.2
0.75
0.33
0.34
0.29
0.30
0.32
4.8
—
S–C1–PPh2–Rh
S–C3–PPh2–Rh
S–C5–PPh2–Rh
S–C8–PPh2–Rh
5.0 0.7
7.5 1.5
6.8 1.2
6.7 1.0
7.1 0.9
S–C11–PPh2–Rh 10.30 255
a
Calculated by a0
=
(2/O3)d100
.
SBET = BET specific surface
area; DBJH = BJH primary pore diameter; VP = single-point total pore
volume; TW = hexagonal phase wall thickness (a0 À DBJH). b Immobilized
Rh contents were determined by ICP-AES analysis.
Fig. 3 IR spectra of fresh and recycled SARC catalysts. Spacer n = 1
(a, a*), 3 (b, b*), 5 (c, c*), 8 (d, d*), 11 (e, e*). Fresh catalysts shown as
dash-dot lines and recycled ones labeled with asterisks (*).
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n(CQO) in the –(CQO)–CQC– was observed (Fig. 3e*).
However, when a shorter spacer was involved, the active sites
were confined within in a larger circle and the immobilized Rh–P
complexes maintained a relatively longer distance with each
other, which favored the by-product coordination and such
complexes were inert towards hydroformylation.
13C CP-MAS NMR spectra (Fig. E6, ESIw, C3 shown for
example) of recycled SARC catalysts revealed a carbon signal
from CQO in the a,b-unsaturated aldehyde at 185.8 ppm,
while the coordinated carbonyl of fresh catalyst gave the signal
at 179.9 ppm. The phenyl gave a resonance at 126.8 ppm
(fresh) and 129.1 ppm (recycled). This difference may arise
from CQC in the unsaturated aldehyde. In addition, the
recycled catalyst gave more resonance peaks characteristic of
alkyls, indicating the long chain of the dehydrated product.
In conclusion, Rh–P complex (using precursor RhCl3) was
anchored to SBA-15 through linear alkyls with different length
(C1, C3, C5, C8 and C11, respectively) as spacers. The prepared
SARC catalysts reveal remarkable selectivity of nonyl alde-
hydes in 1-octene hydroformylation, satisfying low metal loss,
and several catalyst runs. The catalyst activity in 1-octene
hydroformylation increases with the increasing length of alkyl
spacer. SARC catalyst prepared by longest alkyl C11 as
connecting spacer shows highest TON comparable to the
homogeneous process. SARC catalyst activity can be further
promoted by choosing a more active rhodium precursor (e.g.
RhCl(CO)(PPh3)2 or RhCl(PPh3)3) anchoring through spacer
C11. Furthermore, SARC catalyst with Rh immobilized
through spacer C11 reveals stable activity during the catalyst
recycling. A 18-carbon a,b-unsaturated aldehyde formed by
the dehydration of the aldol condensation adduct may stably
chelate with Rh owing to the high degree of conjugation,
which is inert towards 1-octene hydroformylation. Longer
spacers helped the SARC catalyst maintain the activity since
the coordination of the linear long aldehyde was disfavored by
steric hindrance.
Scheme 2 The dehydrated product of the aldol condensation adduct
coordinates with Rh so as to poison it.
recycling was less than 3% for all catalysts and the leached Rh
might originate from the external surface2 of SBA-15. Fig. 2
reveals that SARC catalyst with shorter alkyl spacers showed
more significant activity decrease upon recycling and
S–C1–PPh2–Rh was only recycled three times as the third
TON sharply decreased to 16% of its initial value. However,
S–C11–PPh2–Rh revealed a relatively stable activity. The tenth
TON was 93% of its initial value. Clearly, Rh leaching does
not account for the SARC catalyst activity decrease.
To investigate the reasons for catalyst activity decrease, IR
was performed on recycled catalysts (S–C1–PPh2–Rh 3 runs;
others 10 runs). In Fig. 3, n(alkyl C–H), n(Ar CQC), n(C–Si),
and n(C–P) were obviously enhanced after SARC catalyst
recycles. n(Ar C–H) was found as a shoulder peak on n(alkyl
C–H). In the range 1530–1320 cmÀ1, a series of absorptions was
observed as the fingerprint of oligomeric alkane rocking.
No coordinated carbonyl was found. A peak located at ca.
1705 cmÀ1 appearing as shoulder peak on d(O–H), was ob-
served. This peak could be assigned to the n(CQO) in the
–(CQO)–CQC– structure.7 This means an a,b-unsaturated
species existed in the recycled catalyst. The aldol condensation
is regarded as the side reaction of olefin hydroformylation.6 The
dehydrated product of the aldol condensation adduct is an
acyclic a,b-unsaturated compound. On the basis of the literature
and the IR spectra, we assume an 18-carbon a,b-unsaturated
aldehyde, the dehydration product of the aldol condensation
adduct of nonyl aldehydes, was produced and coordinated with
Rh (Scheme 2). It was also detected by GC-MS in the liquid.
The phosphorus p electrons, phenyl p electrons, p electrons from
CQC and CQO in the a,b-unsaturated aldehyde, and rhodium
d electrons together led to a strong conjugation. The conjugation
should enhance all the stretching vibrations as observed. The
stably coordinated a,b-unsaturated aldehyde was difficult to
release during the catalytic process, and thus SARC catalysts
were poisoned. All SARC catalysts have mesopores (Table 1),
and mass transfer is therefore eliminated. It is shown, however,
the dimer of nonyl aldehydes was not efficiently diffused out of
the pores. Molecular sieve-based catalysts are superior in poly-
merization,10 which is probably attributable to the confining of
porous materials and their high adsorption capacity. The
by-product coordinated with Rh in SARC catalysts and showed
only a low concentration in liquids. When a longer spacer was
employed, the pore size was smaller. The terminal active sites
(Scheme 2) were confined into a smaller central circle, which
disfavored the long linear aldehyde coordination owing to the
steric hindrance. SARC catalyst with spacer C11 showed no
obvious activity decrease during the catalyst recycles, and no
This work is financially supported by the National Natural
Science Foundation (Grant No. 20673064).
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