Air oxidation of supercritical phase isobutane to tert-butyl alcohol
Li Fan,* Yuri Nakayama and Kaoru Fujimoto
Department of Applied Chemistry, School of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113, Japan
tert-2-Butyl alcohol can be synthesized efficiently on SiO
TiO or Pd–C catalyst if air is directly introduced to the
supercritical-phase isobutane.
2
–
All the catalysed reactions showed higher activity than the
non-catalytic reactions, which proved the promotional role of
the catalysts in the reaction.
More interestingly, if the total pressure of the gas phase
reaction was as low as 12 bar, the reaction did not proceed, even
2
As the initial material in methyl tert-butyl ether (MTBE)
production and methyl methacrylate (MMA) synthesis where
MTBE is high-octane-number gasoline additive and MMA is a
resin material, tert-butyl alcohol (TBA) is of increasing
importance.1 On the other hand, TBA can be used to produce
isobutene via a dehydration reaction. Commercially, production
of isobutene from isobutane via direct dehydrogenation needs
high reaction temperatures (500–600 °C) and a catalyst which is
deactivated quickly. Here we report a new synthesis of TBA
where air is used directly as oxidant to convert isobutane to
TBA. This synthesis can be conducted efficiently if isobutane is
in the supercritical phase on selected catalysts.
if SiO
2
–TiO
reaction on SiO
in Table 1.
2
was utilized. Similarly, for the liquid phase
–TiO , the reaction rate was very low, as shown
2
2
,2
Generally, the main byproduct of the reaction was acetone.
The combustion product, CO , was formed only in very small
2
amounts. CO was not detected. Lighter hydrocarbons were
formed in small amounts as well, and consisted of 98%
methane. The total amount of all C
acetone, which indicates the decomposition of C
(acetone) and C fractions. The carbon balance of the total
1
species was equal to that of
4
to C species
3
1
products was higher than 97%, indicating remarkably low
carbon deposition on the catalyst surface.
As indicated in Table 1, this reaction depended greatly on
temperature and pressure, which implied the effect of the
reaction phase. Fig. 1(a) shows a comparison of the reaction
Wide attention has recently been devoted to supercritical
fluid applications in catalytic systems, both in homogeneous
and heterogeneous systems, such as the Fischer–Tropsch
3–6
reaction, alkylation and carbon dioxide conversion.
How-
ever, organic oxidation in the supercritical phase has hardly
been studied until now, especially when air is used directly as
oxidant. It is also of theoretic interest to pursue the reaction
behaviour of oxygen species such as peroxide in supercritical
fluid. It is expected that high solubility, high diffusion
capacity—especially the cluster structure formed around solute
molecules—and the special ionization effects of the super-
critical fluid might exert a strong influence on the reaction
mechanism.
Listed in Table 1 are the reaction performance data for two
catalysts and a non-catalytic case, in the supercritical, gas and
liquid phases.† In either the non-catalytic reaction or the
catalysed reaction, changing the state of isobutane from the gas
phase (44 bar) to the supercritical phase (54 bar) gave
remarkably enhanced conversion of isobutane and oxygen.
Meanwhile, selectivities for the target products (TBA and
isobutene) increased slightly when moving from the gas phase
to the supercritical phase. Exceptionally, Pd–C catalyst ex-
hibited obvious changes in both conversion and selectivity of
the reaction if the phase changed. When the total pressure was
increased from 44 bar to 54 bar, the total yield of TBA and
isobutene was enhanced from 0.31 to 2.70%.
2 2
performance on SiO –TiO catalyst under 54 bar while the
reaction temperature was varied around the critical point (408
K). It is clear that isobutane and oxygen conversions were
enhanced remarkably when the state of the isobutane changed
from the liquid phase to the supercritical phase. Consequently,
TBA yield was enhanced in the supercritical phase. The reason
for the high isobutene selectivity on Pd–C catalyst in the
supercritical phase is not as yet clear. It is well known that the
formed TBA can be dehydrated rapidly on acidic sites of a
catalyst, but the carbon support here is generally neutral.
Concerning the selectivity of the products, as exhibited in
Fig. 1(b), TBA selectivity as well as acetone selectivity was
enhanced and isobutene selectivity was suppressed in the
2 2
supercritical phase reaction on SiO –TiO catalyst.
For the oxidation mechanism in supercritical fluid, it is
suggested that dioxygen can attack the most active hydrogen of
t
isobutane to form tert-butyl hydroperoxide (TBHP, Bu OOH).
TBHP is known as an oxygen donor in the epoxidation of
7
alkenes. It is inferred that TBHP can form in the supercritical
phase from isobutane coexisting with dioxygen. This auto-
8
oxidation step can proceed without catalyst, which happened in
the inductive period during the initial stages of the reaction.
Table 1 Reaction performance of catalytic oxidation of isobutane by air in the supercritical phase or the gas phasea
Total
pressure/
bar
Isobutane
conversion
(%)
O
2
TBA
selectivity
(%)
Isobutene
selectivity
(%)
TBA and
isobutene
yield (%)
conversion
(%)
Catalyst
none
none
44
54
44
54
12
54
44
54
0.3
1.2
2.9
4.9
0.0
0.1
0.5
3.1
2.5
9.9
24.0
40.6
0.0
1.1
4.2
25.6
55.0
58.1
59.0
61.2
0.0
55.5
61.2
64.8
7.0
8.1
5.2
6.3
0.0
7.7
2.1
20.1
0.2
0.8
1.9
3.3
0.0
0.1
0.3
2.7
SiO
SiO
SiO
SiO
2
2
2
2
–TiO
–TiO
–TiO
–TiO
2
2
2
2
b
b
Pd–C
Pd–C
a
Isobutane: air = 3:1, W/F = 10 g h mol21, catalyst weight = 0.5 g, T = 426 K. b Liquid-phase reaction where the reaction temperature was 403 K.
Chem. Commun., 1997
1179