Fast aqueous/organic hydrogenation of arenes, olefins and carbonyl compounds by poly(N-vinylpyrrolidone)-Ru as amphiphilic microreactor system

Article abstract of DOI:10.1002/adsc.200505444

Fast aqueous/organic hydrogenations of arenes, olefins and carbonyl compounds were successfully realized in a PVP-Ru amphiphilic microreactor system. The turnover frequency (TOF) of aqueous/organic hydrogenation of benzene reached 45,000 h^-1. All the hydrogenation TOFs of investigated benzene derivatives, olefins and carbonyl compounds exceeded 1000 h ^-1.

Full text of DOI:10.1002/adsc.200505444

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DOI: 10.1002/adsc.200505444
Fast Aqueous/Organic Hydrogenation of Arenes, Olefins and
Carbonyl Compounds by Poly(N-Vinylpyrrolidone)-Ru as
Amphiphilic Microreactor System
Fang Lu, Jing Liu, Jie Xu*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Graduate School of the Chinese Academy of
Sciences, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, Peopleꢀs Republic of China
Fax: (þ86)-411-84379245, e-mail: xujie@dicp.ac.cn.
Received: November 15, 2005; Accepted: March 2, 2006
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Fast aqueous/organic hydrogenations of
arenes, olefins and carbonyl compounds were suc-
cessfully realized in a PVP-Ru amphiphilic micro-
reactor system. The turnover frequency (TOF) of
aqueous/organic hydrogenation of benzene reached
45,000 hÀ1. All the hydrogenation TOFs of investi-
gated benzene derivatives, olefins and carbonyl com-
ly, at room temperature and under atmospheric hydro-
gen pressure, in an aqueous/organic system, in the pres-
ence of cyclodextrin-stabilized ruthenium nanoparti-
cles.[10]
Hydrogenation of arenes is a difficult reaction to cat-
alyze and represents an important industrial process, in
particular with the increasing demand for low-aromatic
diesel fuels for environmental constrains.[11,12] Hydroge-
nation of double bond-containing compounds such as
olefins and carbonyls are also very important processes
in the chemical industry.[13–15] Ruthenium catalysts are
often used in the hydrogenation of arenes, olefins and
carbonyl compounds.[16] However, the reported hydro-
genation TOF values usually range from several to hun-
dreds using Ru catalysts in aqueous/organic system, only
pounds exceeded 1000 hÀ1
.
Keywords: arenes; hydrogenation; multiphase cataly-
sis; nanoparticle catalysts; ruthenium; water
Water is important as a typical solvent in physiological very few reach the thousands, even the amphiphilic re-
processes and is also a favored solvent in green chemis- agents are used.[17–19]
try compared with organic solvents. The use of water-
In this communication, we report an aggregate
soluble catalysts for various reactions in the aqueous formed by water-soluble amphiphilic poly(N-vinyl-2-
phase is recognized as an effective method for catalyst pyrrolidone) (PVP K90, average Mw¼1,250,000) and
immobilization, and leads to a very efficient separation ruthenium which can serve as a microreactor in the
of the catalysts from the organic products.[1] This method aqueous/organic hydrogenation of arenes, olefins and
has been applied commercially on a large scale (600,000 carbonyl compounds. In this PVP-Ru amphiphilic mi-
tons per year) in Germany and Korea to the rhodium- croreactor system, PVP can form the framework of the
catalyzed hydroformylation of propene.[2] However, microreactor, provide a hydrophobic microenviron-
the major problem is that the reaction rate is too slow ment in water and prevent the nanoparticles from aggre-
to use as a commercial process when this method is ex- gation. The hydrophobic microenvironment will benefit
tended to poorly water-soluble substrates. One of the the enrichment of poorly water-soluble substrates. Ru
promising approaches to improve the solubility of or- nanoparticles can act as highly active catalysts for the
ganic substrates in the aqueous phase is the use of an am- hydrogenation. In addition, catalysts and substrates
phiphilic reagent, such as surfactant, ligands with alkyl are confined in the microreactor which can enhance
chains and quaternized aminoalkyl groups, cyclodex- the contact between them. With the protocol described
trin, and calix[4]arene ligands with phosphane-contain- here, fast aqueous/organic hydrogenation of arenes, ole-
ing groups, that can form micelle, vesicle or cavity struc- fins and carbonyl compounds can be achieved using this
tures.[3–9] In this way, an enhancement of activity is ob- PVP-Ru microreactor system.
served because substrates and catalysts are brought in
The microreactor system was synthesized by reducing
close proximity in the hydrophobic interior of the aggre- the complex of Ru ions and PVP (PVP-RuIII) in water
gates. Roucoux et al. have just reported that the hydro- with hydrogen. The micrograph (Figure 1A) clearly
genation of benzene derivatives was realized successful- shows that the microreactors, which are about 100 mm
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Fang Lu et al.
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Figure 2. FT-IR spectra of the PVP, PVP-RuIII and PVP-Ru.
is suggested that the interaction between Ru and PVP
exists inside the microreactor, which can protect the
Ru nanoparticles from aggregation.[22]
The obtained amphiphilic microreactors exist in water
on the basis of the hydrogen bonds formed between the
pyrrolidone of PVP and water. On the other hand, the
alkyl chains of PVP form the confined hydrophobic mi-
croenvironment inside the microreactors in water. The
main advantage of this hydrophobic microenvironment
is an anticipated increase in the solubility of organic sub-
strates in the aqueous phase. Here, we investigated the
Figure 1. Microscope and (HR-)TEM images of the PVP-Ru
microreactor system. A: Microscope image of the microreac-
tor; B: TEM image of microreactor; C: TEM image of micro-
reactor and Ru particles; D: TEM image of chain-like Ru
nanoparticle arrays; E: HR-TEM image of continuous Ru
nanowires; F: HR-TEM image of discrete Ru nanoparticles.
in length, are formed in the PVP-Ru aqueous solution solubility of poorly water-soluble benzene in the aque-
(after reduction by hydrogen), whereas no microreactor ous phase in the presence of the microreactors. After
was observed in the PVP-RuIII aqueous solution. It can benzene was added to the PVP-Ru microreactor system
be seen from Figures 1B and 1C that Ru particles were and stirred for 1 h, the concentration of benzene in the
entrapped inside the microreactors. Figure 1D shows aqueous phase containing microreactors reached
more details of the morphology of ruthenium in the mi- 470 g/L, which is significantly higher than the solubility
croreactors. There are chain-like Ru nanoparticle ar- of benzene in pure water (1.1 g/L, under the same condi-
rays, about 20–80 nm in length and 1–3 nm in diameter. tions). Additionally, the solubility of benzene in PVP-
HR-TEM images also show that the chain-like Ru nano- RuIII aqueous solution (before reduction by hydrogena-
particle arrays are composed of both continuous Ru tion) in the absence of microreactors was only 1.5 g/L
nanowires (Figure 1E) and discrete Ru nanoparticles under the same conditions, much close to the case in
(Figure 1F).
pure water. These results suggest that the microreactor
Compared with pure PVP, the Fourier transform in- can enrich a large amount of benzene in aqueous solu-
frared spectrum (FT-IR) (Figure 2) of a PVP-RuIII sam- tion due to its hydrophobic microenvironment.
ple exhibits an additional shoulder peak around
Aseries of hydrogenations of arenes, olefins and car-
1600 cmÀ1 along with a peak for the C O stretching vi- bonyl compounds in the PVP-Ru microreactor system
bration of the PVP amide unit at 1650 cmÀ1. The appear- was investigated in this work (Table 1). In all the inves-
ance of the shoulder peak suggests that the complex of tigated cases, the PVP-Ru microreactor system showed
PVP-RuIII forms by a part of the amide in PVP binding a very high activity and the conversion reached almost
to the ruthenium ion.[20] After the reduction process 100% within 3 h.
¼
(PVP-Ru sample), the shoulder peak essentially disap-
For benzene, the hydrogenation was completed after
¼
pears, while the C O stretching vibration peak is wid- just 12 min, corresponding to a catalytic turnover fre-
ened, which may result from bond weakening arising quency (TOF) of 45,000 hÀ1 (entry 1), defined as the
from partial electron donation from the PVP oxygen number of moles of consumed H2 per mol of Ru per
to the vacant orbitals of ruthenium surface atoms.[21] It hour, much higher than the 888 hÀ1 obtained in a similar
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Aqueous/Organic Hydrogenation of Arenes, Olefins and Carbonyl Compounds
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system with ~1.5 nm Ru nanoparticles where no micro- reaction in microscale dimensions. This microreactor
reactors were reported.[12,23] This means that the micro- system was flexible whereby other metals can substitute
reactors make a significant contribution to the high reac- for Ru and the polymers could be adjusted to fit the new
tivity besides the small Ru particle size. It was also found reaction environment. Thus, high reactivity could also
that the conversion of benzene increased linearly along be expected in other aqueous/organic reaction systems.
with the reaction time, thus, the rate of benzene conver-
sion is independent of the benzene concentration, show-
ing zero-order kinetics to benzene in this system. How-
Experimental Section
ever, it is usually reported that the rate of conversion of
benzene is strongly dependent on the benzene concen-
tration in the hydrogenation of benzene in the presence
of water because of the low solubility of benzene in pure
water.[24] The kinetic difference can be explained by the
fact that the local benzene concentration inside the mi-
croreactors is much higher than that in pure water. The
Ru catalyst surface inside the microreactor is largely
covered with benzene molecules, regardless of the
changes of benzene concentration. Furthermore, reac-
tions defined in micrometer dimensions also improved
the reaction activity, which is usually observed in micro-
reactor systems.[25,26]
Reagents
PVP was of pharmaceutical grade (Shanghai sunpower materi-
al co.), Benzene was achromatography reagent, and all the oth-
er chemicals were analytical reagents. They were purchased
from commercial sources and used without further purifica-
tion. Hydrogen (99.99%) supplied in a high-pressure cylinder
was used through a reducing valve without further treatment.
Preparation and Characterization of the Microreactor
System
For benzene derivatives, the hydrogenation TOFs of
arenes with electron-donating substituents all exceeded
10,000 hÀ1 (entries 2–4), whereas the hydrogenation
TOFs of arenes with electron-withdrawing substituents
were about 1000 hÀ1 (entries 5 and 6), resulting from
the decrease of the electron density of the aromatic sys-
tem.
RuCl3 ·H2O (0.027 g, 0.1 mmol) and PVP (0.11 g, 1 mmol)
were dissolved in water (60 mL) in a 100-mL flask, stirred at
353 K for 2 h and a dark-brown transparent solution was ob-
tained where RuIII ions were complexed with PVP. After the
PVP-RuIII solution had been treated with 4.0 MPa H2 in a
500-mL stainless steel autoclave with a Teflon liner at 353 K
for 2 h, the catalytic microreactor system was obtained. The
PVP-RuIII solution and fresh reduced PVP-Ru solution were
It is well known that the reaction of higher olefins
(>C10) in conventional aqueous/organic system is far characterized by microscopy (Nikon Eclipse 80i microscope),
TEM on a JEM-2000EX (JEOL) electron microscope operat-
ing at 120 kVand HR-TEM on a Philips 20 TECNAI G2 field
emission gun electron microscope operating at 200 kKV. PVP,
PVP-RuIII and PVP-Ru solution samples, which were in dehy-
drated membrane form after being dried overnight at 353 K,
were analyzed using an Avatar370 E. S. P. FT-IR spectrometer
and a Nicolet continuum infrared microscope.
less effective because of their extremely low solubility
in water.[27] Here, the PVP-Ru amphiphilic microreactor
system was successfully used in the hydrogenation of ter-
minal higher olefins and cycloolefins in the aqueous/or-
ganic system. Although the TOF decreased with in-
creasing the chain length of olefins, the TOF of the ex-
tremely water-insoluble 1-hexadecene reached 3000 hÀ1
(entries 7–11). Furthermore, carbonyl compounds were
also hydrogenated efficiently using our microreactor sys-
tem (entries 12 and 13).
Measurement of the Solubility of Benzene
Benzene (60 mL) and the prepared aqueous solution of PVP-
Ru microreactors (200 mL) were introduced into a 500-mL
stainless steel autoclave with a Teflon liner. The mixture was
stirred at 1000 rpm under 0.1 MPa N2 at 298 K for 1 hour.
About 6 mL of the mixture were taken from the autoclave ev-
ery 10 min and left to stand for about 1 h until the organic and
aqueous phases had separated. The concentration of benzene
in the dark-brown layer was analyzed using an Agilent 4890
GC equipped with an FID and an OV-1 capillary column. Tol-
uene was used as the internal standard. For comparison, the
solubilities of benzene in the PVP-RuIII aqueous solution and
in water were measured under the same conditions.
In addition to the high hydrogenation reactivity, this
microreactor system also had several merits. The PVP-
Ru microreactor system can be recycled three times
without losing activity. No ruthenium was detected in
the product solution after reaction as checked by induc-
tively coupled plasma emission spectroscopy analysis,
which could also be noticed by the dark-brown aqueous
and colorless organic phases obtained. This meant that
no leaching of the Ru occurred. Moreover, the prepara-
tion procedure of the microreactor system was quite
simple.
In summary, fast aqueous/organic hydrogenation of
arenes, olefins and carbonyl compounds was successful-
ly realized in the PVP-Ru amphiphilic microreactor sys-
tem. The very high TOF values were attributed to the
enrichment of poorly water-soluble organic substrates,
Hydrogenation Reactions
After the PVP-Ru microreactor system had been prepared
(0.1 mmol Ru) in the autoclave, the substrate (Ru/substrate
highly active Ru nanoparticle catalysts and the localized mole ratio¼1/3000) and 60 mL dilutent (cyclohexane or dec-
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Fang Lu et al.
Table 1. Hydrogenation of arenes, olefins and carbonyl compounds in aqueous PVP-Ru microreactor system.[a]
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Entry
Substrate
Product and Selectivity[d] [%]
Initial TOF[e] [hÀ1
]
1
45,000
100
2
3
30,000
29,000
100
100
70,
16,
14
4
5
16,000
1100
100
83,
17
6
7
2600
23,000
100
100
100
8
9
11,800
5500
100
100
10
11
4000
3800
12
7400
7300
100
100
13
[a]
Conditions: microreactor system (0.1 mmol Ru), Ru/substrate¼1/3000. Solvent: cyclohexane (60 mL), T (353 K), H2
(4.0 MPa), agitation rate (1000 rpm).
Solvent: decane (60 mL).
[b]
[c]
[d]
[e]
Microreactor system (0.02 mmol Ru), Ru/substrate¼1/1000, solvent: cyclohexane (100 mL).
Determined by GC-MS.
Tested within 5 minutes.
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Aqueous/Organic Hydrogenation of Arenes, Olefins and Carbonyl Compounds
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ane) were added directly. Considering the possibility of the pol-
ymerization of long-chain alkenes during the hydrogenation
process, the amount of Ru catalyst was reduced to
0.02 mmol, cyclohexane as dilutent was added to 100 mL and
the Ru/substrate mole ratio was 1/1000. After air had been
flushed out of the reactor, the autoclave was heated to 353 K
and then was pressurized with H2 to 4.0 MPa. Then the mixture
was stirred at 1000 rpm and the reaction was considered to
start. Samples were periodically taken from the organic phase
after the stirring was interrupted to separate the organic phase
and the aqueous phase. The samples were qualitatively ana-
lyzed by GC-MS on an Agilent 6890 GC equipped with an Agi-
lent 5973 mass selective detector. Quantitative analysis was
performed on an Agilent 4890 GC equipped with an FID and
an HP-5 capillary column or an OV-1 capillary column by the
area normalization method. The amount of Ru in the organic
phase after hydrogenation was analyzed by using PLASAM-
SPEC-II inductively coupled plasma emission spectroscopy.
The detection limit of the equipment to Ru is 0.03 ppm.
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