À1
À1
;
(
35 1C, 10 MPa, 0.714 g mL ; 40 1C, 12 MPa, 0.719 g mL
In conclusion, the present study shows that it is possible to
À1
À1 10
4
the same conversion and selectivity, but contrasts with the effect
5 1C, 14 MPa, 0.721 g mL ), the hydrogenation of I exhibits
achieved an exceptionally high reaction rate (TOF = 5051 m
)
at very low temperature of 40 1C for the hydrogenation of I in
scCO . This unusually high reaction rate might be due to the
of temperature at fixed pressure on conversion and selectivity
2
(
Fig. 3). For instance, the system at 35 1C and 10 MPa exhibits
better conversion and selectivity compared to 35 1C and 12 MPa
Fig. 3) and these remain unaltered as the pressure changes from
0 to 12 MPa at 40 1C and 12 to 14 MPa at 45 1C, which is
enhanced diffusion of the reactants and also the mass transfer
reduction as the reaction occurs under conditions of a single
(
2
phase scCO , hydrogen and the substrate. Of particular note,
1
we find that the density of the medium, which is related to
the solubility of reactant, plays an important role in increasing
the reaction rate and also the selectivity of II. Incorporation
of substituent into the substrate does not change the
selectivity but a slight decrease in the reaction rate has been
observed depending on the size of the substituent on the
substrate.
difficult to explain. So in this case it is better to suggest that the
temperature has a straightforward effect on the conversion and
selectivity at a fixed pressure. It is obvious that the solubility of
2
hydrogen has prime significance for hydrogenation in scCO .
Considering the hydrogen pressure (fixed PCO2 = 12 MPa;
T = 40 1C; t = 10 min), the conversion changes from 13.1%
to 100% as the pressure increases from 0.2 to 2.0 MPa keeping
the selectivity of II unchanged. The change in hydrogen pressure
can alter the phase behavior of the reaction mixture, which can
affect the reaction rate. An optimum hydrogen pressure of
Notes and references
1 P. G. Jessop, T. Ikariya and R. Noyori, Chem. Rev., 1999, 99, 475;
A. Baiker, Chem. Rev., 1999, 35, 453; J. Grunwaldt, R. Wandeler
and A. Baiker, Catal. Rev. Sci. Eng., 2003, 45, 1.
2
the studied reaction conditions.
MPa is necessary to maintain the high reaction rate under
2
M. Chatterjee, F. Y. Zhao and Y. Ikushima, Adv. Synth. Catal.,
2004, 346, 459; M. Chatterjee, A. Chatterjee and Y. Ikushima,
Green Chem., 2004, 6, 114; M. Chatterjee, Y. Ikushima,
T. Yokoyama and M. Sato, Adv. Synth. Catal., 2008, 350, 624;
M. G. Hitzler and M. Poliakoff, Chem. Commun., 1997, 1667;
X. Tschan, R. Wandeler, M. S. Schneider, M. M. Schbert and
A. Baiker, J. Catal., 2001, 204, 219; U. R. Pillai and E. Sahle-
Demessie, Chem. Commun., 2002, 422; V. Rode, U. D. Joshi,
O. Sato and M. Shirai, Chem. Commun., 2003, 1960.
Recycling of the catalyst is an important aspect in the
heterogeneous catalysis system. The most significant advan-
tages of the method described are that the reaction is con-
ducted at very low temperature and the reaction time is short.
Therefore, the chance of deactivation of the catalyst is low and
no leaching of Pt has been observed. To check the activity of
the used catalyst, it was recycled at least three times and the
activity and selectivity for II (Table 1; Entry 11) remained
the same.
3
E. Ronzon and G. D. Angel, J. Mol. Catal. A: Chem., 1999, 148,
1
05.
4 G. Szo
Chem., 2005, 15, 2464.
M. Rosales, A. Gonza
L. Sanchetz and H. Soscu
29, 205.
M. Chatterjee, F. Y. Zhao and Y. Ikushima, New J. Chem., 2002,
7, 510.
¨
llosi, A. Mastaliar, Z. Kiraly and I. Dekany, J. Mater.
´ ´ ´
We have extended the process to the hydrogenation of methyl
and ethoxy substituted cyclohexenone and also an acyclic
ketone, and the results are shown in Table 1 (Entries 12,
5
´
lez, M. Mora, N. Nader, J. Navarro,
n, Transition Met. Chem., 2004,
´
´
6
7
1
3, 14). In each case excellent selectivity of the corresponding
2
CQC hydrogenated product is obtained. The results show a
reasonable reaction rate but it is still lower compared to I as
observed from the TOF (reaction rate). A decrease in TOF
M. Caravati, J. D. Grunwaldt and A. Baiker, Phys. Chem. Chem.
Phys., 2005, 7, 278.
8 M. Burgener, D. Ferri, J. D. Grunwaldt, T. Mallat and A. Baiker,
À1
À1
J. Phys. Chem. B, 2005, 109, 16794.
A. Furstner, D. Koch, K. Langemann, W. Leitner and C. Six,
from 4603 min to 2816 min as the substitution changes
from methyl to ethoxy is evident. This may be attributed to the
bulkiness of the substrate, which prevents easy access of the
substrate molecule to the active sites of the mesoporous channel
and consequently the rate of the reaction decreases. However,
the acyclic ketone shows a comparable TOF to that of the
9
Angew. Chem., Int. Ed. Engl., 1997, 36, 2466.
10 The density data of supercritical carbon dioxide are from NIST
Chem Web Book (http://webbook.nist.gov/chemistry/).
1
1 R. D. Weinstein, A. R. Renslo, R. L. Danheiser and J. W. Tester,
J. Phys. Chem. B, 1999, 103, 178.
1
2 M. Chatterjee, T. Iwasaki, Y. Ondonera and T. Nagase, Catal.
Lett., 1999, 61, 199.
À1
cyclohexenone (5020 min ).
This journal is ꢀc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 701–703 | 703