Selective hydrogenation of naphthols to tetralones over supported palladium catalysts in supercritical carbon dioxide solvent

Article abstract of DOI:10.1246/cl.2006.780

Naphthols were selectively hydrogenated to the corresponding tetralones over supported palladium metal catalysts in supercritical carbon dioxide solvent. Copyright

Full text of DOI:10.1246/cl.2006.780

780
Chemistry Letters Vol.35, No.7 (2006)
Selective Hydrogenation of Naphthols to Tetralones over Supported Palladium Catalysts
in Supercritical Carbon Dioxide Solvent
Eiichi Mine,1 Norihito Hiyoshi,1 Osamu Sato,1 Chandrashekhar V. Rode,2 and Masayuki ShiraiÃ1
1Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST),
4-2-1 Nigatake, Miyagino 983-8551
2Homogeneous Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha, Pune 411008, India
(Received April 26, 2006; CL-060498; E-mail: m.shirai@aist.go.jp)
Naphthols were selectively hydrogenated to the correspond-
ing tetralones over supported palladium metal catalysts in super-
critical carbon dioxide solvent.
higher than that of Pd/ꢀ-Al2O3. Thus, these results showed that
the Pd/C catalyst was the most active catalyst to give the highest
yield of 2. Similar to other aromatic hydrogenation in supercrit-
ical carbon dioxide,7,9 the carbon-supported metal catalysts
showed higher TON values than the ꢀ-alumina-supported ones
for the hydrogenation of 1 to 2. Hydrogen atoms spilled over
the support would take part in the hydrogenation of aromatic
rings; however, further in situ characterization is needed. Baiker
et al. reported that carbon monoxide was formed over ꢀ-alumi-
na-supported noble metal catalysts during hydrogenation in su-
percritical carbon dioxide;10 however, carbon monoxide was
not detected in the systems over both Pd/C and Pd/ꢀ-Al2O3 cat-
alysts under our reaction conditions.
For the Pd/C catalyst, the conversion of 1 was found to
increase with an increase in reaction temperature; however,
the yield of dehydroxylated compounds also increased for the
temperature > 383 K. Figure 1 shows the best results obtained
at 383 K for the hydrogenation of 1 over the Pd/C catalyst under
0.5 MPa of hydrogen and 16 MPa of carbon dioxide pressure.
The highest conversion of 1 achieved was 98% within 60 min
while from the beginning of the reaction, 1 was partially hydro-
genated to 2, 1,2,3,4-tetrahydro-1-naphthol (3) and 5,6,7,8-tetra-
hydro-1-naphthol (4) with their respective selectivities as 86, 3,
and 11% at the conversion of 40%, indicating that the hydroge-
nation of 1 was a parallel reaction. The yield of 2 as high as 71%
was obtained and almost constant after 60 min, beyond which a
part of 2 was hydrogenated to 3. Strong adsorption of carbon di-
oxide to the active site may prohibit the adsorption of 2 to form
3. Fully ring-hydrogenated products, 1-decalone (5) and decahy-
dro-1-naphthol (6), were not obtained even after 100 min. The
amounts of dehydroxylated compounds (tetralin (7) and naph-
thalene (8)) were almost negligible (<1%). The hydrogenation
Tetralones, which are important intermediates for synthesiz-
ing drugs and agrochemicals, can be obtained by the oxidation of
tetralin,1 alkylation–acylation of aromatics with ꢀ-butyrolac-
tone,2 ionic hydrogenation of naphthols,3 and intermolecular
Friedel–Craft reaction of 4-arylbutyric acids.4 1-Tetralone can
be also obtained by the hydrogenation of 1-naphthol with sup-
ported metal catalysts in organic solvents; however, large
amount of dehydroxylated products were also obtained leading
to low yield of 1-tetralone.5,6 These methods have serious envi-
ronmental problems due to use of hazardous reagents in stoichio-
metric quantities and organic solvents and tedious procedure of
separation of products from the solvents. Hydrogenation with
supported metal catalysts under supercritical carbon dioxide
have several advantages; i) higher solubility of hydrogen in su-
percritical carbon dioxide giving enhanced activity and control-
ling the product selectivity, and ii) easy separation of catalysts
and products. Recently, we have found that a hydrogenation sys-
tem with a carbon-supported rhodium catalyst and supercritical
carbon dioxide was very effective for the partial hydrogenation
of 1-naphthol to 1,2,3,4-tetrahydro-1-naphthol, 5,6,7,8-tetrahy-
dro-1-naphthol, and 1-tetralone.7 In continuation of our studies,
we now report the selective hydrogenation of naphthols to tetra-
lones with a carbon-supported palladium catalyst in supercritical
carbon dioxide solvents.
Carbon- or ꢀ-alumina-supported metal (rhodium, palladi-
um, platinum, and ruthenium) catalysts were examined. All
catalyst used in this work were commercially available from
Wako Pure Chemical Ind., Ltd., Japan and used without further
reduction for the hydrogenation. The detailed reaction procedure
are given in our previous paper.7
The conversion (in parentheses) of 1-naphthol (1) over
supported metal catalysts for 15 min at 323 K under 3 MPa of
hydrogen and 10 MPa of carbon dioxide were in the following
order: Rh/C (42.6) > Rh/ꢀ-Al2O3 (24.1) > Pd/C (11.2) >
Pt/C (5.4) > Pd/ꢀ-Al2O3 (2.7) > Ru/C (0.8) > Pt/ꢀ-Al2O3
(0.8) > Ru/ꢀ-Al2O3 (0.1) as we reported in our earlier commu-
nication.7 Supported rhodium catalysts were highly active; how-
ever, supported palladium catalysts showed higher selectivity to
1-tetralone (2) than rhodium catalysts. The order of catalysts for
the selectivity of 2 was as follows: Pd/ꢀ-Al2O3 (38.0) > Pd/C
(36.1) > Rh/C (7.8) > Rh/ꢀ-Al2O3 (6.4). Considering the
metal dispersion of Pd/ꢀ-Al2O3 and Pd/C as 18 and 3%, respec-
tively,8 the turnover number ((1-naphthol molecules reacted)/
(surface palladium atoms of a catalyst)) of Pd/C was four times
100
80
60
40
20
0
100
80
60
40
20
0
OH
OH
O
(a)
(b)
1
7
1
O
2
OH
2
3
OH
OH
OH
4
4
3
7
0
20 40 60 80 100
0
20 40 60 80 100
Reaction time/min
Figure 1. The hydrogenation of 1-naphthol under hydrogen
pressure 0.5 MPa over 5 wt % Pd/C (0.075 g). Initial 1-naphthol
1.4 mmol; reaction temperature 383 K; carbon dioxide pressure
16 MPa (a); n-heptane 10 mL (b).
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.7 (2006)
781
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
OH
(b)
(a)
OH
O
O
OH
OH
OH
OH
0
0.5 1.0 1.5 2.0
0
50
100
150
0
50
100
150
Hydrogen pressure/MPa
100
80
60
40
20
0
100
80
60
40
20
0
OH
OH
(c)
(d)
Figure 2. Effect of hydrogen pressure on the hydrogenation
of 1-naphthol over 5 wt % Pd/C (0.075 g) at 383 K for 10 min.
Initial 1-naphthol 1.4 mmol; carbon dioxide pressure 16 MPa
OH
O
OH
OH
(
,
); 10 mL of n-heptane ( , ).
OH
of 1 was also examined over the Pd/C catalyst at 383 K under
0.5 MPa of hydrogen in n-heptane (10 mL) and methanol
(10 mL) solvents. The hydrogenation did not proceed in metha-
nol as indicated by <1% conversion of 1 even after 60 min of the
reaction time. As against this, almost complete conversion of 1
was obtained in about 40 min in n-heptane, indicating that the
rate of hydrogenation in n-heptane was faster than that in carbon
dioxide solvent (Figure 1). From the beginning of the reaction, 2,
3, and 4 were also formed simultaneously in n-heptane. The
product selectivities to these partial ring-hydrogenated com-
pounds viz. 2, 3, and 4 were 75, 5, and 20% in n-heptane at
the conversion of 40%. The maximum yield of 2 was 61% and
the selectivity to 2 decreased after 30 min because of further hy-
drogenation of 2 to 3 and dehydroxylation to 7. The yield of 7
was as high as 18%, indicating higher extent of dehydroxylation
taking place in n-heptane solvent. Fully ring-hydrogenated prod-
ucts, 5 and 6 were also obtained in n-heptane. The above results
indicate that dehydroxylation was considerably suppressed un-
der supercritical carbon dioxide solvent unlike the hydrogena-
tion in organic solvents even at the same reaction temperature.
We have also examined the influence of carbon dioxide
pressures on the catalyst activity and selectivity for Pd/C-cata-
lysed hydrogenation of 1. The activity and selectivity to partial
hydrogenated products for 2, 3, and 4 were almost constant
regardless of carbon dioxide pressure from 16 to 22 MPa at
1 MPa of hydrogen. The density of supercritical carbon dioxide
and solubility of 1 in supercritical carbon dioxide solvent in-
crease with increasing pressure; however, the hydrogenation
rates were not affected by carbon dioxide pressure, indicating
that the diffusion of the reactant and products was not a rate-de-
termining step. In addition, we also examined the hydrogen pres-
sure effect (Figure 2) at a constant carbon dioxide pressure of
16 MPa and in n-hentane solvent. The conversion of 1 increased
with an increase in the hydrogen pressure from 0.5 to 1.5 MPa of
hydrogen in both carbon dioxide and n-heptane solvents, indicat-
ing that surface hydrogen concentration increased with increase
in hydrogen pressure. On the other hand, the initial selectivities
to 2 decreased with an increase in the hydrogen pressure from
0.5 to 1.5 MPa in both carbon dioxide and n-heptane solvent.
It is noteworthy that the selectivity to 2 in carbon dioxide was
higher than that in n-heptane under the similar conversion of 1.
The hydrogenations of 2-naphthol and 2-methyl-1-naphthol
O
0
50
100
150
0
50
100
150
Reaction time/min
Figure 3. The hydrogenation of 2-naphthol and 2-methyl-1-
naphthol under hydrogen pressure 0.5 MPa over a Pd/C catalyst.
Initial 2-naphthol and 2-methyl-1-naphthol 1.4 mmols; carbon
dioxide pressure 16 MPa ((a) and (c)); n-heptane 10 mL ((b)
and (d)).
in supercritical carbon dioxide and n-heptane solvents were also
investigated over the Pd/C catalyst (Figure 3). From the begin-
ning of the hydrogenation, tetralones, 1,2,3,4-tetrahydronaph-
thols, and 5,6,7,8-tetrahydronaphthols were parallely formed in
the four systems. Among them, tetralones were highly obtained
and their yields were almost constant even though most of naph-
thols were hydrogenated in carbon dioxide. The yields of tetra-
lones were also high in n-heptane; however, decreased after
the maximum at 50 and 60 min, respectively because of further
hydrogenation of tetralones to the corresponding 1,2,3,4-tetrahy-
dronaphthols.
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Published on the web (Advance View) June 10, 2006; doi:10.1246/cl.2006.780