From CO oxidation to CO2 activation: An unexpected catalytic activity of polymer-supported nanogold

Article abstract of DOI:10.1021/ja042207o

A simple, clean, safe, and reproducible catalyst system, polymer-supported nanogold, was successfully developed for the fixation of CO₂ to cyclic carbonate and for the carbonylation of amines to disubstituted ureas with unprecedented catalytic activity (TOF > 50000 mol/mol/h and TOFP ≈ 3000 mol/mol/h, respectively). To the best of our knowledge, it was the first to report that nanogold catalysts have exclusive catalytic activity for activation of carbon dioxide, and that the catalytic activity of the polymer-immobilized nanogold catalysts could be controlled by the particle size of the nanogold. Copyright

Full text of DOI:10.1021/ja042207o

Published on Web 03/04/2005
From CO Oxidation to CO2 Activation: An Unexpected Catalytic Activity of
Polymer-Supported Nanogold
Feng Shi, Qinghua Zhang, Yubo Ma, Yude He, and Youquan Deng*
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, China
Received December 27, 2004; E-mail: ydeng@ns.lzb.ac.cn
Scheme 1. Formation of Carbamate from Amines and Carbon
Dioxide
Since the discovery that highly dispersed nanogold particles
supported on metal oxides could be highly active catalysts for CO
oxidation,1 there has been an explosion in the interest shown in
catalysis by gold;2 for example, several researchers have made great
efforts to expand the application of nanogold catalysts for some
reactions, such as hydrogenation of unsaturated substrates,3 epoxi-
dation of propene using H2 and O2,4 selective oxidation of alcohol,5
6
removal of NOX and methane combustion,7 etc. Until today, less
Scheme 2. Cyclization of Epoxides with CO2 over Nanogold
Catalysts
effort, however, was made to investigate the possibility of nanogold
being used as a catalyst for CO2 activation, which would exploit a
safe and green carbonyl source, reduce the release of green house
gas, and benefit the recycling of carbon when its use is linked with
other processes that emit CO2.
With our continuous effort to expand the application of gold
catalysis8 and carbon dioxide activation,9 we report our new findings
that the nanogold could be a highly effective catalyst for cyclization
of epoxides (Scheme 1) or carbonylation of aliphatic amines
(Scheme 2) with carbon dioxide to yield cyclic carbonate10 and
disubstituted ureas,9a,11 respectively. Polymer was used as the
support to immobilize the nanogold catalysts because the polymer-
immobilized catalysts were closer to homogeneous catalysts in
chemical character and were attractive for technological applica-
tions.12
The basic properties of polymer-supported nanogold catalysts
are given in Table S1 (Supporting Information). The cyclization
reaction of epoxide and carbon dioxide can be conducted with an
unprecedented high turn over frequencies (TOF), ca. >50 000 mol/
mol/h (Table 1, entry 1), which is significantly superior to those
obtained by a classical acid-basic catalyst system (∼20-5000 mol/
mol/h).10 All of the selectivities for the desired product nearly
reached ∼100%, so the yield could be used to indicate the catalytic
performances. It is worth noting that catalyst with 0.05 wt % gold
loading exhibited the highest yield (71.2%), and lower yields were
obtained over catalysts with higher gold loadings due to the
formation of larger gold particles (entries 2-5). For 0.05 wt %
Au/Poly2, poor results were obtained (entry 6), indicating that the
properties of the polymer support had great impact on the catalytic
activity of nanogold, and the reason for such tremendous difference
in the activities should be attributed to the different functional
groups of polymers. The functional group in Poly1 was quaternary
ammonium, which may stabilize the nanogold particles, while the
functional group in Poly2 was sodium sulfonate, which could not.
Poor results were obtained over catalysts Pd/Poly1 and Rh/Poly1
with 3.1 and 1.1% yields, respectively, although palladium and
rhodium complexes were usually regarded as more effective
catalysts in comparison with gold complexes for carbonylation
reactions. Therefore, these gold catalysts had exclusive catalytic
activity for carbonylation of epoxides with carbon dioxide, that is,
activation of carbon dioxide. To achieve higher yield, longer
reaction time (10 h) was adopted over catalyst 0.05 wt % Au/Poly1,
and 89.5% yield was obtained (entry 7). The result of catalyst 0.05
Table 1. Cyclization Reactions of Epoxypropane with CO2 over
Au/Polya
entry
catalyst
GC yields (%)
TOFb
1
2
3
4
Poly1
trace
53
0.01 wt % Au/Poly1
0.05 wt % Au/Poly1
0.1 wt % Au/Poly1
0.5 wt % Au/Poly1
0.05 wt % Au/Poly2
0.05 wt % Au/Poly1
0.05 wt % Au/Poly1
57900
15400
4800
820
2800
9700
7100
71.2
44.2
38.3
12.9
89.5
66.4
5
6
7c
8d
a Reaction conditions: 90 mL autoclave, 0.2 g of catalyst, 4 mL of
epoxides, 30 atm CO2, 150 °C, 5 h; the yield was obtained with an out-
standard method. b The mol of epoxides converted per mol gold per hour.
c Reacted for 10 h. d This catalyst was reused at the fifth time.
Table 2. Cyclization Reactions of Epoxides with CO2 over 0.05 wt
% Au/Poly1a
a Same reaction conditions as Table 1. b Reacted for 15 h.
wt % Au/Poly1, reused at the fifth time, indicated that the reusability
of the catalyst system was possible, although relatively lower yield
(66.4%) was given (entry 8), which should be attributed to the loss
of gold species during reaction (only 0.03 wt % gold loading
maintained after reused five times). 2,3-Epoxypropyl phenyl ether,
2,3-epoxypropyl isopropyl ether, and epichlorohydrin were further
tested, and satisfactory results could also be achieved with relatively
longer reaction time for epichlorohydrin, which may be attributed
to the presence of electro attractive chloride methyl (Table 2, 1-3).
The polymer-immobilized nanogold could also be an effective
catalyst for carbonylation of amines with carbon dioxide to yield
9
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10.1021/ja042207o CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 3. Carbonylation of Amines with CO2 over 0.05 wt %
Au/Poly1a
Figure 1. Picture and HRTEM image of 0.05 wt % Au/Poly1.
a Reaction conditions: 90 mL autoclave, 0.2 g of catalyst (0.05 wt %
Au/Poly1), 4 mL of amines, 1 mL of water, 50 atm CO2, 180 °C, 20 h.
b Turn over frequencies for products, mol product per mol gold per hour.
Scheme 3. Carbonylation of Amines with CO2 over Nanogold
Catalysts
corresponding disubstituted ureas (Table 3). It can be seen that the
isolated yields of dicyclohexyl urea and dibenzyl urea were 85 and
83%. Here, we use TOFP (turn over frequency for product) in
instead of the TOF concept because the alkylamines could react
with carbon dioxide to yield the corresponding carbamate under
any conditions (Scheme 3), and TOF, therefore, could not denote
the difference of catalytic activity of different catalysts. The TOFP
numbers of 0.05 wt % Au/Poly1 catalyst in the carbonylation of
amines, therefore, were 2900 and 3000 mol/mol/h, respectively,
which were much better than that of previously reported homoge-
neous catalysts (10-20 mol/mol/h).9a These results suggested that
the activation of carbon dioxide over polymer-supported nanogold
catalysts could be universal.
Figure 2. Possible mechanisms for (I) disubstituted urea synthesis and
(II) cyclization reactions of epoxides.
Acknowledgment. This work was financially supported by The
National Natural Science Foundation of China (No. 20173068).
Supporting Information Available: Details for the preparation of
polymer-supported nanogold catalysts, XPS, TEM, and XRD charac-
terizations of polymer-supported nanogold catalysts, and complete ref
2b. This material is available free of charge via the Internet at http://
pubs.acs.org.
References
For 0.05 wt % Au/Poly1, XPS analysis showed that the gold
content on the surface of the polymer was 8.7 wt % and the
chemical state of gold species was partially cationic (BE ) 84.3
eV) (Figures S1,2). These results indicated that the nanogold
particles were assembled on the surface of the polymer support,
which ensured the high catalytic activity with lower gold content,
and some interactions occurred between the nanogold particles and
the cationic terminal group of the polymer. Further TEM charac-
terization (Figure 1) indicated the formation of nanogold particles,
and the average particle sizes of the nanogold were ∼3, ∼6, ∼10,
and ∼12 nm (Figures S3-5) for 0.01 wt % Au/Poly1, 0.05 wt %
Au/Poly1, 0.1 wt % Au/Poly1, and 0.5 wt % Au/Poly1. Therefore,
the catalytic activity of the polymer-supported nanogold catalysts
was regulated by the particle size of nanogold particle, that is, higher
catalytic activity exhibited over a smaller nanogold particle. XRD
characterization confirmed the formation of crystal gold species
when the gold loadings were high enough (Figure S6).
As to the reaction mechanism, it is still not clear at this stage,
but the activation of carbon dioxide at the nanogold particle should
be the key step, and the synergism between nanogold species and
the peculiar microenvironment of the polymer surface should be
indispensable (Figure 2), which may be completely different from
previous reported processes for carbon dioxide activation with the
acid-base catalysts.9a,10
In conclusion, we have shown that polymer-immobilized nano-
gold catalysts had unprecedented catalytic activity for activation
of carbon dioxide, with TOF > 50 000 mol/mol/h for the synthesis
of cyclic carbonate and TOFP ≈ 3000 mol/mol/h for the synthesis
of disubstituted ureas. The synergism between the nanogold species
and the peculiar microenvironment of the polymer surface resulted
in the exclusive catalytic activity for these two kinds of reactions.
The particle size of the nanogold, other than gold loading, had much
stronger impact on the catalytic performance. Further investigation
is now underway.
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