Synthesis and characterization of TiO2 nanotube supported Rh-nanoparticle catalysts for regioselective hydroformylation of vinyl acetate

Article abstract of DOI:10.1039/c4ra11156g

Three procedures: the impregnation-borohydride reduction procedure, the impregnation-alcohol reduction procedure and the impregnation-photoreducing procedure, were utilized for preparing TiO₂ nanotube supported rhodium nanoparticle catalysts, in which rhodium acetate was used as a rhodium source. Catalysts were characterized with TEM, ICP, XPS and XRD; their catalytic performances for hydroformylation of vinyl acetate were evaluated. Of these catalysts prepared by three different methods, the catalyst prepared by the impregnation-photoreducing procedure showed the highest catalytic activity under the same reaction conditions. The effects of pressure of syngas, solvents, temperature, Rh content and reaction time on the hydroformylation were examined in detail. Under the optimized reaction conditions, the conversion of vinyl acetate can reach at 100%; the chemoselectivity for aldehydes was 74.70% and the regioselectivity for the branched aldehyde was 95%.

Full text of DOI:10.1039/c4ra11156g

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Synthesis and characterization of TiO2 nanotube
supported Rh-nanoparticle catalysts for
Cite this: RSC Adv., 2014, 4, 62215
regioselective hydroformylation of vinyl acetate†
Yukun Shi, Xiaojing Hu, Baolin Zhu, Shurong Wang, Shoumin Zhang
*
and Weiping Huang
Three procedures: the impregnation-borohydride reduction procedure, the impregnation-alcohol
reduction procedure and the impregnation-photoreducing procedure, were utilized for preparing TiO2
nanotube supported rhodium nanoparticle catalysts, in which rhodium acetate was used as a rhodium
source. Catalysts were characterized with TEM, ICP, XPS and XRD; their catalytic performances for
hydroformylation of vinyl acetate were evaluated. Of these catalysts prepared by three different
methods, the catalyst prepared by the impregnation-photoreducing procedure showed the highest
catalytic activity under the same reaction conditions. The effects of pressure of syngas, solvents,
temperature, Rh content and reaction time on the hydroformylation were examined in detail. Under the
optimized reaction conditions, the conversion of vinyl acetate can reach at 100%; the chemoselectivity
for aldehydes was 74.70% and the regioselectivity for the branched aldehyde was 95%.
Received 25th September 2014
Accepted 7th November 2014
DOI: 10.1039/c4ra11156g
www.rsc.org/advances
high regioselectivity carried out under mild conditions, a lot
of effort has been made to increase the regioselectivity with
high reaction rate in the past decades.3,9 The key to achieve
high regioselectivity for target product is to create a novel
catalyst.
1. Introduction
There are several methods for preparing aldehydes, but the
dominant technology is catalytic hydroformylation, which is
an elegant method of preparing a wide range of aldehydes in
a single step by the reaction of alkenes, CO and H2 with high
selectivity.1 The reaction was rst discovered by Otto Roelen
in 1938 during investigations on the formation of oxygenated
products in cobalt-catalyzed Fisher–Tropsch reactions.
Several million tons of aldehyde, world wide, are produced
annually via the hydroformylation reaction, most of which
are reduced to alcohols or oxidized to carboxylic acids.
However, regioselective hydroformylation of functionalized
alkenes, e.g. vinyl acetate, is still a challenging topic and has
drawn great attention.2–6 With the regioselective hydro-
formylation of functionalized alkenes, target products having
functional groups at different positions can be synthesized.7,8
For example, the hydroformylation of vinyl acetate provides
bifunctional products (Scheme 1), which can be further
converted into synthetically useful compounds, e.g. lactic
acid and so on. To date, though there are few reports con-
cerning hydroformylation of functionalized alkenes with
Some transition metal-based catalysts involving Rh, Ir, Pd,
Co and Ru have been widely used for the hydroformylation
reaction.10–13 Due to the lower reaction temperature and pres-
sure, Rh-based catalysts are usually preferred for hydro-
formylation reaction. Nanoscale Rh particles have been holding
people's interests14 for their novel electronic, magnetic, optical
and catalytic properties.15–20 Their enhanced performances,
especially for size-dependent catalytic properties, result from
their size or shape and high surface-to-volume ratio.21 However,
pristine Rh nanoparticles are thermally unstable and tend to
agglomerate in solution when heated. In order to stabilize the
Rh nanoparticles, polymers and surfactants are oen used.22–26
However, Sayed's group and Korgel's group have reported that
surfactants or capping ligands used in the synthesis of metallic
nanoparticles can diminish the catalytic activity of the
produced metallic nanocatalysts. Somorjai and co-workers have
also reported that poly vinylpyrrolidone (PVP), the most widely
used capping agent in size-tunable and shaped nanoparticles
synthesis, interacted strongly with Rh nanoparticles resulting in
a signicant capping agent effect.27–29 Though Rh nanoparticles
can be synthesized by using capping agents and subsequently
deposited on the high-surface-area solid support, completely
removing the capping agents to expose the catalytically “clean”
metal surfaces is very difficult. Therefore, developing simple,
rapid and surfactant-free approaches for synthesizing Rh
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), The
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),
Tianjin Key Lab of Metal and Molecule-based Material Chemistry and College of
Chemistry, Nankai University, Tianjin 300071, China. E-mail: hwp914@nankai.edu.
cn; Fax: +86-22-23502669; Tel: +86-22-23502996
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra11156g
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Scheme 1 Hydroformylation of vinyl acetate.
nanoparticles deposited on the high surface area solid supports in 20 mL of Rh2(Ac)4 aqueous solution (15.0 mmol Lꢀ1, 7.50
is highly desirable for studying Rh-based catalysts.30,31 Thus, mmol Lꢀ1, 3.75 mmol Lꢀ1), and the system was vigorously
TiO2 nanotube (TNTs), a large surface area inorganic material, agitated for 24 h. Aer low-energy sonication for 2 h, the
has been widely used as support for catalytic materials.32–37
mixture was centrifuged and green TNTs were obtained. In
It has been reported that TNTs prepared via hydrothermal order to remove the Rh2(Ac)4 solution adsorbed on the outer
treatment are constructed by rolling up one (1 0 1) layer of the surface of TNTs, the green TNTs were washed with a little
anatase structure along the [1ꢀ01] direction and the (1 0 1) facet distilled water.
constructed space between walls is about 0.76 nm.38 Such
Preparation of Rh/TNTs-A. The green TNTs were transferred
multiwall structure makes TNTs have large specic surface area into 100 mL ask with 20 mL distilled water. Then 5 mL of KBH4
and nano-conne effect, which can be utilized for stabilizing Rh aqueous solution (0.72 mol Lꢀ1, 0.36 mol Lꢀ1, 0.18 mol Lꢀ1) was
nanoparticles deposited on the surfaces of TNTs. It is optimis- dropwise added, while vigorously stirred, into the mixture in
tically anticipated that multiwall TNTs supported Rh- ice-water bath. Aer addition of KBH4 aqueous solution, the
nanoparticle catalysts would be very stable, based on our mixture was continually stirred for another 1 h at 0 ꢁC and room
37
previous work,
and have a good catalytic activity for the temperature, respectively. The mixture was centrifuged and the
gray product was washed to neutral with distilled water
hydroformylation of alkenes.
In the present contribution, we report multiwall TNTs sup- repeatedly, and then washed three times with ethanol. The
ꢁ
ported Rh-nanoparticle catalysts prepared via three approaches product was dried at 40 C for 12 h in vacuum.
and the synergy effects between nano-conne effect and size-
Preparation of Rh/TNTs-B. The green TNTs were transferred
dependent catalytic activity on the regioselective hydro- into 100 mL ask with 50 mL ethanol–water solution
formylation of vinyl acetate.
(Vethanol : Vwater ¼ 9 : 1). Then, the mixture was reuxed, while
vigorously stirred, for 6 h to reduce the Rh2+ ions adsorbed on
TNTs. Aer cooling to room temperature, the mixture was
centrifuged and the gray product wasꢁwashed three times with
ethanol. The product was dried at 40 C for 12 h in vacuum.
Preparation of Rh/TNTs-C. The green TNTs were transferred
into 60 mL quartz reactor with 50 mL ethanol–water solution
(Vethanol : Vwater ¼ 9 : 1), and subsequently the mixture was
irradiated for 4 h by using a 300 W high-pressure mercury
lamp under stirring at ambient temperature. Aer irradiation,
the obtained catalyst was centrifuged, washed with distilled
2. Experimental
2.1 Chemicals
All chemicals were purchased commercially and used without
further purication. The gases, N2, CO and H2, used in the
experiments were all of 99.99% purity.
2.2 Synthesis of TNTs supported Rh-nanoparticle catalysts
ꢁ
The preparation procedures of TNTs supported Rh-nanoparticle
catalysts are described as Scheme 2. All reduction processes
were carried out under N2 atmosphere.
water and ethanol. The product was dried at 40 C for 12 h in
vacuum.
TNTs were synthesized by the hydrothermal treatment as the
previous reports.32–36
2.3 Characterization of TNTs supported Rh-nanoparticle
catalysts
The TNTs adsorbed Rh2+ ions were prepared via impregna-
tion procedure using Rh2(Ac)4 as precursor. Typically, the The morphologies and microstructure of catalysts synthesized
synthesis was performed as follows: 1.0 g TNTs were dispersed were analyzed by Transmission Electron Microscope (TEM,
Philips T20ST). The chemical states of element Rh in catalysts
were determined by X-ray Photoelectron Spectrometer (XPS,
Kratos Axis Ultra DLD multi-technique X-ray photoelectron
spectra), and all binding energies were calibrated using C 1s (Eb
¼ 284.6 eV) as reference. The phase structures of catalysts were
characterized with X-Ray Diffraction (XRD, Rigaku D/Max-2500
X-ray diffractometer with Cu Ka radiation). The bulk and
surface Rh contents of samples were measured by Inductively
Coupled Plasma (ICP-9000, USA Thermo Jarrell-Ash Corp)
Scheme 2 Schematic processes of preparing Rh/TNTs.
62216 | RSC Adv., 2014, 4, 62215–62222
and EDX.
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Fig. 1 EDX and XPS of Rh in Rh/TNTs-A (A), Rh/TNTs-B (B) and Rh/TNTs-C (C).
subsequently pressurized to the desired pressure of H2 and CO.
The reactor was heated up to reaction temperature and kept for
desired time under stirring and at the controlled reaction
temperature. Aer desired reaction time, the stirring was
stopped, and the reactor was cooled down to room temperature,
depressurized. The reaction mixtures were withdrawn for GC
analysis (Shimadzu GC-2014 gas chromatograph equipped with
a 30 m ꢂ 0.53 mm SE-30 capillary column and a FID).
2.4 Evaluation of catalytic performance of TNTs supported
Rh-nanoparticle catalysts for hydroformylation
The catalytic hydroformylation reaction was carried out in a 250
mL stainless steel autoclave reactor equipped with a magnetic
stirrer. In a typical experiment, the required amount of catalyst,
substrate and solvent were added into the reactor. The reactor
was sealed and purged three times with hydrogen, and
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Table 1 Activity and selectivity of Rh/TNTs prepared via different preparation approachesa
Rh content (wt%)
Entry
Catalyst
ICP
EDX
XPSc (Rh0)
Conversion (%)
Aldehyde (%)
b : lb
1
2
3
Rh/TNTs-A
Rh/TNTs-B
Rh-TNTs-C
0.19
0.18
0.18
0.79
0.25
0.28
85.94
71.21
87.35
22.98
72.33
99.37
68.91
74.99
72.90
>99 : 1
97 : 3
98 : 2
a
Reaction conditions: vinyl acetate ¼ 5 mL, sub/Rh ¼ 8000, catalyst ¼ 0.40 g (7.0 ꢂ 10ꢀ3 mmol Rh), temp. ¼ 100 ꢁC, syngas pressure (CO/H2 ¼ 1)
¼ 4.0 MPa, solvent (toluene) ¼ 65 mL, and reaction time ¼ 8 h. b b : l is 2-acetoxy propanal : 3-acetoxy propanal. c The ratios of Rh0 to total Rh in
samples.
A, Rh/TNTs-B and Rh/TNTs-C are 0.79, 0.25 and 0.28 wt%,
respectively, which are all higher than ICP results (Table 1). ICP
and EDX results exhibit clearly Rh enrichment on the surfaces
3. Results and discussion
3.1 Characterization of Rh/TNTs
of samples prepared via different procedures, which is favorable
for utilizing Rh effectively and catalytic reaction.
First of all, the content and chemical state of the rhodium on
the nanotubes were studied by ICP, EDX and XPS. EDX detec-
tion (Fig. 1) shows clearly that there are peaks of Rh, which
conrms the presence of rhodium on the surface of TNTs. The
EDX shows that the contents of Rh on the surfaces of Rh/TNTs-
Also displayed in Fig. 1 are Rh 3d XPS spectrums of Rh/TNTs-
A, Rh/TNTs-B and Rh/TNTs-C. Binding energies of 307.2 and
312.0 eV are attributed to the Rh 3d5/2 and Rh 3d3/2, respectively,
which are consistent with the literature data of Rh 3d5/2 and Rh
3d3/2 for the metallic Rh.39 The peaks centered at 309.3 eV and
314.1 eV, which are also attributed to the Rh 3d5/2 and Rh 3d3/2
species, clearly show the existence of Rh in oxidation state. It is
well known that the low valent Rh is the active center for cata-
lytic hydroformylation. The ratios of Rh0 to total Rh species in
samples are calculated from the areas of the tted Gaussian
peaks and listed in Table 1.
The formation of Rh0 in the photoreducing procedure can be
illustrated as eqn (1) and (2).35,36 TNTs absorb UV light and
electron–hole pairs form Rh2+ ions capture the electrons from
the conduction band of excited TiO2, and, as a consequence,
Rh2+ ions become the metallic Rh.
TiO2 + hv / TiO2 + eꢀ + h+
Rh2+ + 2eꢀ / Rh0
(1)
(2)
Fig. 2 The TEM images of the as-synthesized Rh/TNTs-A (A), Rh/
TNTs-B (B), and Rh/TNTs-C (C).
Fig. 3 XRD patterns of Rh/TNTs-A (A), Rh/TNTs-B (B) and Rh/TNTs-C (C).
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Table 2 Effect of Rh loading in Rh/TNTs-C on hydroformylation of vinyl acetatea
Entry
Rh content (wt%)
Conversion (%)
TOFb (hꢀ1
)
Aldehyde (%)
b : lc
1
2
3
0.07 (2.7 ꢂ 10ꢀ3 mmol)
0.09 (3.5 ꢂ 10ꢀ3 mmol)
0.18 (7.0 ꢂ 10ꢀ3 mmol)
30.33
39.74
100
2080
2120
2670
83.62
74.74
74.70
>99 : 1
>99 : 1
95 : 5
a
Reaction conditions: vinyl acetate ¼ 5 mL, Rh/TNTs-C ¼ 0.40 g, temp. ¼ 100 ꢁC, syngas pressure (CO/H2 ¼ 1) ¼ 6.0 MPa, solvent (toluene) ¼ 65 mL,
and reaction time ¼ 3 h. b TOF ¼ number of moles of product formed/(number of moles of Rh ꢂ h). c b : l is 2-acetoxy propanal : 3-acetoxy propanal.
The TEM images of the as-synthesized catalysts are shown in bulk Rh contents are almost identical, the Rh/TNTs-A has the
Fig. 2. As showed in Fig. 2, all of the catalysts take on perfect lowest activity, which is a typical size-dependent catalytic
nanotubular morphology; their diameters are about 6 nm and property. As discussed above, the size of Rh0 particles in Rh/
lengths up to hundreds of nanometers. The Rh nanoparticles TNTs-A is the largest, which results in that the amount of
are well-dispersed on the surfaces of TNTs. By comparison with catalytically Rh active species is small in spite of the fact that the
each other, the size of Rh particles in Rh/TNTs-A (Fig. 3-A) is the percentage of Rh0 in Rh/TNTs-A is not the lowest. However, for
largest, the average size is about 3 nm. In the reducing process, both Rh/TNTs-B and Rh/TNTs-C, though the difference in Rh
the Rh atoms will be preferentially agglomerated on the just- nanoparticle size is not obvious, Rh/TNTs-C shows higher
formed Rh nuclei to generate multiple Rh islands on TNTs. activity than Rh/TNTs-B. The reasonable interpretation is that
This island growth mode can be attributed to the strong Rh–Rh the Rh/TNTs-C has a higher percentage of Rh0, which would be
interaction due to the high interatomic binding energy and more favorable for coordination of reactants to the metallic Rh,
surface free energy of Rh.40 When the strength of reductant is resulting in good activity. The leaching of transition-metal
weak, the reduced Rh atoms have enough time to diffuse to nanoparticle catalyst is frequent, especially for the catalytically
other regions of TNT rather than agglomerating on the just- active group VIII metals. Therefore, the Rh leaching of Rh/TNTs-
formed Rh nuclei. In the case of photoreducing process, C in hydroformylation of vinyl acetate was studied. The leaching
though photogenerated electrons, the photoreductant, are of Rh in solution was detected by ICP-AES and the results
strong reductant, their amount is not large enough to reduce indicated that the leaching of Rh was 1.013 ppm (0.071 mg),
Rh2+ ions rapidly on a large scale, which results in that Rh which is the 10 percent of Rh species in Rh/TNTs-C. According
atoms is homogeneously and instantaneously produced during to the data of XPS, the ratio of Rh2+ to total Rh species in Rh/
the irradiation thus providing favorable conditions for the TNTs-C is 12.65%, indicating that the leaching of Rh species
formation of nearly monodispersed particles.41 However, is merely the TNTs adsorbed Rh2+ and Rh nanoparticles on
compared with the photoreducing process, the concentration of TNTs are stable in the hydroformylation. This result is in
strong reductant is far higher in the KBH4 reducing procedure, accordance with our previous work of Co–B/TNTs for hydro-
which reduces Rh2+ ions quickly and causes multiple Rh islands formylation of cyclohexene.37
formation on the surfaces of TiO2 nanotubes. Compared with
Shown in Table 2 are the catalytic performances of Rh/TNTs-
KBH4, ethanol is a weak reducing agent, and it is reasonable C catalysts with various Rh loadings. It can be seen that
that Rh0 particles in Rh/TNTs-B are smaller than that in Rh/ increasing Rh loading can remarkably enhance the conversion
TNTs-A. It is now obvious that synthesizing size-tunable Rh0 of vinyl acetate and TOF, but the selectivity for aldehyde and
particles can be achieved via the three methods, which is regioselectivity decrease slightly with the increase of Rh loading
signicant for preparing catalysts of size-dependent catalytic from 0.07 wt% to 0.18 wt%. The optimum Rh loading should be
activity.
Fig. 3 shows the XRD patterns of Rh/TNTs-A, Rh/TNTs-B and
about 0.18 wt%.
Rh/TNTs-C. It can be seen from Fig. 3 that all of catalysts display
the diffractions of anatase TiO2 (JCPDS21-1272); the Rh/TNTs-C
has more perfect anatase phase, which is the result of UV irra-
diation.36 However, there is not any diffraction related to Rh0
particles due to the relatively low content and very small size of
Rh particles.
Table
3 Effect of reaction medium on Rh/TNTs-C catalyzed
hydroformylationa
Entry Solvent
Conversion (%) Aldehyde (%) Acetal (%) b : lb
1
2
3
Toluene
Isooctane
Tetrahydrofuran 33.94
Ethanol
100
96.53
74.70
79.60
77.90
26.51
74.81
0
0
0
95 : 5
95 : 5
95 : 5
95 : 5
99 : 1
3.2 Activity and selectivity of Rh/TNTs
4
78.96
19.50
69.69
0
5c
—
To investigate the catalytic performances of Rh/TNTs-A, Rh/
TNTs-B and Rh/TNTs-C, hydroformylation of vinyl acetate was
chosen as a model reaction so that both catalytic activity and
regioselectivity of catalysts could be evaluated in detail; the
results are listed in Table 1. Of the three catalysts, though their
a
Reaction conditions: vinyl acetate ¼ 5 mL, sub/Rh ¼ 8000, Rh/TNTs-C
¼ 0.40 g (7.0 ꢂ 10ꢀ3 mmol Rh), temp. ¼ 100 ꢁC, syngas pressure (CO/H2
b
¼ 1) ¼ 6.0 MPa, solvent ¼ 65 mL, and reaction time ¼ 3 h. b : l is 2-
acetoxy propanal : 3-acetoxy propanal. c No solvent, vinyl acetate ¼ 70 mL.
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Table 4 Effects of temperature and syngas pressure on Rh/TNTs-C catalyzed hydroformylationa
Entry
Pressure (MPa)
Temperature (ꢁC)
Conversion (%)
TOFc (hꢀ1
)
Aldehyde (%)
b : ld
1
2
4.0
5.0
6.0
6.0
6.0
100
100
100
60
75.08
88.89
100
66.87
99.92
2000
2370
2670
890
77.84
75.21
74.70
91.49
86.23
99 : 1
98 : 2
95 : 5
>99 : 1
97 : 3
3
4b
5b
80
1330
a
Reaction conditions: vinyl acetate ¼ 5 mL, sub/Rh ¼ 8000, Rh/TNTs-C ¼ 0.40 g (containing 7.0 ꢂ 10ꢀ3 mmol Rh), CO/H2 ¼ 1, solvent (toluene) ¼
65 mL, and reaction time ¼ 3 h. b Reaction conditions: vinyl acetate ¼ 5 mL, sub/Rh ¼ 8000, Rh/TNTs-C ¼ 0.40 g (containing 7.0 ꢂ 10ꢀ3 mmol Rh),
syngas pressure (1 : 1), solvent (toluene) ¼ 65 mL, and reaction time ¼ 6 h. c TOF ¼ number of moles of product formed/(number of moles of Rh ꢂ
d
h). b : l is 2-acetoxy propanal : 3-acetoxy propanal.
For the heterogeneous catalysis catalyzed liquid-phase reaction time. It is can be seen from the data in Fig. 4 that
hydroformylation reaction, the inuence of reaction medium prolonging reaction time led to an increase in conversion.
should be a crucial factor to consider. Listed in Table 3 are Before 1 h, the conversion of vinyl acetate increases quickly with
catalytic performances of Rh/TNTs-C in toluene, isooctane, prolonging reaction time; almost all of vinyl acetate is
tetrahydrofuran, ethanol and neat vinyl acetate without solvent. consumed aer 2 h. The chemoselectivity for aldehyde and
As the results showed, Rh/TNTs-C displays higher catalytic regioselectivity for 2-acetoxy propanal keep nearly unchange-
activity in solvents of weaker Lewis basicity (Table 3, entries 1 able in the catalytic process for 4 h, which means that the Rh/
and 2) than in coordinative solvents (Table 3, entries 3 and 5); TNTs-C has high regioselectivity for 2-acetoxy propanal.
the regioselectivities are almost identical in two types of
As discussed above, the high regioselectivity (2-acetoxy
solvents. The results are very reasonable. It is well accepted that propanal : 3-acetoxy propanal > 95 : 5) is an unchanged result in
coordination of reactants to the catalytically active species is the Rh/TNTs catalyzed vinyl acetate hydroformylation.
rst and a necessary step for catalytic hydroformylation, the
catalytic activity, rather than selectivity, of catalyst is certainly
affected when catalytically active centers is occupied by coor-
On the basis of experimental results and reported litera-
dinative solvent. It is noteworthy that the conversion of vinyl
3.3 Mechanism
ture1,2,42–44 on vinyl acetate hydroformylation, the proposed
acetate is 78.96%, but the main production is acetal and the
reaction procedures of vinyl acetate hydroformylation over Rh/
selectivity for acetal is 69.69% when ethanol is used as solvent
TNTs catalysts is shown in Scheme 3. Firstly, the Rh particles
(Table 3, entry 4). For neat vinyl acetate without solvent in the
turn into RhHy(CO)x/TNTs in the atmosphere of H2 and CO. The
reaction system (Table 3, entry 5), the conversion of vinyl acetate
catalytic cycle involves successively the coordination of the vinyl
decreases to 19.50%, which is due to the increase of sub/Rh
from 8 ꢂ 103 to 112 ꢂ 103, and the selectivity for aldehyde is
acetate, the formation of intermediate (I) or (II), and so on;
furthermore, the formation of (I) or (II) dominantly decides the
still maintained with good regioselectivity for 2-acetoxy propa-
regioselectivity of the reaction for 2-acetoxy propanal and 3-
nal (2-acetoxy propanal : 3-acetoxy propanal is 99 : 1).
acetoxy propanal. It is well understandable that the formation
Besides reaction medium and Rh loading, the reaction
temperature and pressure may also inuence reaction rate and
selectivity. The effects of reaction temperature and CO/H2
pressure on the hydroformylation are examined in detail and
the results are summarized in Table 4. In the pressure range of
4.0–6.0 MPa (Table 4, entries 1–3), the conversion of vinyl
acetate increases with the increase of the syngas pressure, while
the regioselectivities for 2-acetoxy propanal decreases slightly at
the same reaction temperature. As showed in Table 4, the
reaction temperature also affects conversion and regiose-
lectivity of reaction. With the increase of temperature from 60
ꢁC up to 100 ꢁC, the conversion of vinyl acetate increases along
with decrease in regioselectivity for 2-acetoxy propanal. At lower
reaction temperature (60 ꢁC), high selectivity for aldehyde is
observed (91.49%), however the TOF of vinyl acetate is rather
low (890 hꢀ1). At 100 ꢁC, the conversion of vinyl acetate
increases up to 100% with TOF increased to 2670 hꢀ1 and
selectivity for aldehyde decreased to 74.70%.
Fig. 4 shows the variations of conversion, selectivity for
aldehyde and 2-acetoxy propanal/3-acetoxy propanal ratio with
Fig. 4 Variations of conversion, chemoselectivity and regioselectivity
with reaction time.
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Scheme 3 Proposed mechanism for Rh/TNTs catalyzed hydroformylation of vinyl acetate.
of (I) and (II) is related to the inductive effect and chelating
Acknowledgements
effect of the ester carbonyl group, as well as the relative stability
of (I) and (II). It has been reported that (I) with a ve-membered
ring is a more stable intermediate45 and the next intermediate is
a stable six-membered ring, which should be the reason that the
dominant 2-acetoxy propanal : 3-acetoxy propanal ratio (>9 : 1)
comes into being in the catalytic hydroformylation of vinyl
acetate over Rh/TNTs.
This work is supported by the National Natural Science Foun-
dation of China (21373120, 21301098, 21071086 and 21271110),
the Applied Basic Research Programs of Science and Tech-
nology Commission Foundation of Tianjin (13JCQNJC02000
and 12JCYBJC13100) and project IRT13022, B12015.
4. Conclusion
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