Chemistry Letters Vol.37, No.7 (2008)
735
tetralin hydrogenation over all the supported catalysts used were
higher in scCO2 than in n-heptane, the TOF values decreased
steeply from a maximum over the catalysts having 26 and
28% of dispersion in scCO2 compared with n-heptane.
50
(A)
(B)
40
30
20
10
0
It has been reported that carbon monoxide is formed on
supported noble metal catalysts during hydrogenations in sc-
CO2 by reverse water–gas shift reaction,2d,9 and that dissociation
of carbon dioxide proceeds over stepped surfaces of rhodium
metal.10 Selective poisoning of stepped or kinked sites on a sup-
ported platinum catalyst by carbon monoxide was also observed
for the hydrogenation of chloronitrobenzene in scCO2.2d Hence,
one probable explanation for the decrease of the TOF value over
the rhodium catalysts having higher dispersion values in scCO2
is the catalyst poisoning by carbon monoxide formed during the
hydrogenation in scCO2 on low coordination rhodium sites,
whose proportion is larger in higher rhodium dispersion. Howev-
er, we cannot show any direct evidence for the formation of car-
bon monoxide during the hydrogenation in scCO2 at this stage.
Another possible explanation is that the effect of particle size
on the electronic states of rhodium particles could be different
in the two solvents and further research is needed in this regard.
Higher reaction rates were observed in scCO2 than those
in organic solvents for several hydrogenation reactions over
supported metal catalysts;2,3,4a,4b however, exceptions were also
reported.4c One of the reasons behind this contradiction would be
the particle-size effect in scCO2 demonstrated in this study. The
present results also suggest that the tuning of metal particle size
and metal dispersion is very important for heterogeneous catalyt-
ic reactions in scCO2.
0
10
20
30
0
10
20
30
Dispersion/%
Figure 2. TOF as a function of the metal dispersion of activated
carbon-supported rhodium catalysts for the hydrogenation of
naphthalene (A) and tetralin (B) in 15 MPa of carbon dioxide
(
gen: 3 MPa; reaction temperature: 313 K; reactor capacity:
) and in 20 cm3 of n-heptane ( ). Partial pressure of hydro-
50 cm3; initial substrate: 2.34 mmol; catalyst weight; 0.003 g.
larger than those from TEM and XRD results. This difference
could be explained by i) the location of rhodium particles within
the pores of activated carbon support, and the surface of rhodium
particles being covered with carbon, and/or ii) residual chloride
ions derived from the precursor, which inhibit the adsorption
of hydrogen atoms.7 The number of adsorbed hydrogen atoms
is considered to be the number of surface metal atoms which
could act as active sites.
The influence of metal dispersion and particle size on naph-
thalene hydrogenation was examined by the initial consumption
rate of naphthalene in both scCO2 and n-heptane.11 The amount
of naphthalene decreased linearly from the beginning of the
reaction in both the solvents, and mainly tetralin (91–95% of
selectivity depend on catalysts and solvents), small amounts of
cis- and trans-decalin (3–7% and 0.3–0.8%, respectively) and
octahydronaphthalene (0.8–1.5%) were formed. Figure 2A
shows TOF as a function of the metal dispersion for the hydro-
genation of naphthalene in both scCO2 and n-heptane at 313 K.
The TOF values for naphthalene hydrogenation in scCO2
decreased from 43 to 26 minꢁ1 with an increase in the metal
dispersion from 10 to 28%, while TOF values remained almost
constant in the range between 31 and 35 minꢁ1 in the same
dispersion range for naphthalene hydrogenation in n-heptane,
indicating a strong influence of metal dispersion on the catalyst
activity of naphthalene hydrogenation in scCO2.
We also studied the hydrogenation of tetralin in scCO2 and
in n-heptane. The amount of tetralin decreased linearly from the
beginning of the reaction, and mainly cis-decalin (77–81% of
selectivity), small amounts of trans-decalin (11–13%) and
octahydronaphthalene (8–11%) were the products formed in
both the solvents. Figure 2B shows the TOF values as a function
of metal dispersion for the hydrogenation of tetralin in both
scCO2 and n-heptane at 313 K. The 5 wt % Rh/AC-0.3u catalyst
having a dispersion value of 20%, which corresponded to 2.3 nm
particle size determined by TEM, showed maximum TOF
among the supported catalysts used in both the solvents. The
different tendencies were observed between the hydrogenation
of naphthalene and tetralin, though both reactants were com-
pletely dissolved in scCO2 under the reaction conditions, which
was confirmed by direct observation. It is reported that silica-
supported rhodium metal particles between 1.4 and 2.0 nm
showed a maximum TOF value in the hydrogenation of benzene
because the most appropriate ensembles of rhodium atoms were
formed.8 It should be noted that, although the TOF values of
This study was supported by the Industrial Technology Re-
search Grant Program in 2006 from New Energy and Industrial
Technology Development (NEDO) of Japan.
References and Notes
1
2
M. Boudart, Adv. Catal. 1969, 20, 153.
a) Chemical Synthesis Using Supercritical Fluids, ed. by P. G.
Jessop, W. Leitner, Wiley, New York, 1999. b) A. Baiker,
3
4
a) K. Matsui, H. Kawanami, Y. Ikushima, H. Hayashi, Chem.
a) N. Hiyoshi, R. Miura, C. V. Rode, O. Sato, M. Shirai, Chem.
a) S. Hodoshima, S. Takaiwa, A. Shono, K. Satoh, Y. Saito,
5
6
7
8
9
M. Burgener, D. Ferri, J.-D. Grunwaldt, T. Maiiet, A. Baiker,
11 Supporting Information is available ellectoronically on the
Hiyoshi, Norihito
