Chemistry Letters Vol.34, No.3 (2005)
425
2.5
2.0
1.5
1.0
0.5
100
100
80
80
60
40
20
0
60
40
20
0
0
2
4
6
8
10
0
H2 pressure/MPa
0
50
100
150
200
Reacton time/min
Figure 2. Effect of hydrogen pressure on the hydrogenation of
naphthalene over 5 wt % Rh/C (0.10 g) under carbon dioxide
pressure 10 MPa and initial naphthalene 2.3 mmol at 333 K
for 30 min. ( ) Conversion of naphthalene; ( ) selectivity to
decalin.
Figure 1. The hydrogenation of naphthalene under hydrogen
pressure 6 MPa carbon dioxide pressure 10 MPa, and initial
naphthalene 2.3 mmol at 333 K for 0.10 g of 5 wt % Rh/C. (
Naphthalene; ( ) tetralin; ( ) decalin.
)
was almost constant in each case. The conversion of naphthalene
and decalin selectivity increased linearly with increase in hydro-
gen pressure (2 to 6 MPa) and both remained constant beyond
6 MPa hydrogen pressure. The increase in activity and decalin
selectivity would be caused by increase in concentration of sur-
face hydrogen with increasing hydrogen pressure. Over 6 MPa of
hydrogen pressure, catalyst surface would be saturated with hy-
drogen, then the conversion and decalin selectivity were con-
stant regardless of the increase in hydrogen pressure.
Naphthalene was hydrogenated over 5 wt % Rh/C catalyst
under supercritical carbon dioxide solvent at very low tempera-
ture (333 K) to give high selectivity to decalin in a shorter reac-
tion time. The formation of tetralin and decalin were parallel re-
actions unlike the consecutive reactions in the case of hydroge-
nation at high temperature. In this study, we used commercial
supported metal catalysts and did not add any promoter, or did
not perform any special treatment on the catalysts. Optimiza-
tions of catalyst preparation method and of catalyst components
are currently under investigation.
der to compare the catalyst activity for naphthalene hydrogena-
tion under non supercritical conditions, several experiments
were separately carried out in n-heptane solvent at 333 K and
the results are given in Table 1. Under non supercritical condi-
tions also, the 5 wt % Rh/C catalyst was the most active catalyst,
however, the activity was about one half times lower than that
under the supercritical conditions. Also, it is important to note
that the selectivity to decalin was only 11.3% over the 5 wt %
Rh/C catalyst under non-supercritical conditions. The maximum
selectivity to decalin achieved was only 30% (for 1.7% conver-
sion of naphthalene) over the 5 wt % Ru/C catalyst in liquid
phase conditions (Table 1). When the amount of n-heptane sol-
vent in the reactor was changed from 20 to 10 mL under 6 MPa
of hydrogen, the rate of hydrogenation and selectivity to decalin
were not changed over the 5 wt % Rh/C and 5 wt % Ru/C cata-
lysts. The higher naphthalene conversion and higher decalin se-
lectivity in supercritical carbon dioxide than that in n-heptane is
mainly due to the enhanced solubility and diffusivity of hydro-
gen in supercritical carbon dioxide.
The hydrogenation profile under supercritical carbon diox-
ide at 333 K over 5 wt % Rh/C was also examined (Figure 1).
Decalin formation was observed along with tetralin without
any induction period and its formation increased with increase
in reaction time, and the ratio of decalin (60%) to tetralin
(40%) was almost constant for about 90 min initially, indicating
that both tetralin and decalin are the primary products in naph-
thalene hydrogenation. After 80% of naphthalene was hydrogen-
ated, the hydrogenation of tetralin to decalin proceeded. Finally
almost all naphthalene was hydrogenated to decalin. This pattern
is dramatically different from that reported in gas/liquid phase
hydrogenation of naphthalene at high temperature, in which first
tetralin is formed followed by its hydrogenation to decalin indi-
cating these to be consecutive reactions.5–7 Also, it is reported
that residual naphthalene inhibits the hydrogenation of tetralin
to decalin.11 Thus, in the hydrogenation at high temperature,
decalin is not formed at low naphthalene conversion. It is impor-
tant to note that the formation of decalin becomes a parallel re-
action in hydrogenation of naphthalene under supercritical car-
bon dioxide over supported rhodium catalyst at very low temper-
ature (333 K).
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Figure 2 shows the dependence of conversion and selectiv-
ity on hydrogen pressure over 5 wt % Rh/C under 10 MPa of su-
percritical carbon dioxide solvent. The ratio of decalin to tetralin
Published on the web (Advance View) February 22, 2005; DOI 10.1246/cl.2005.424