Oxidative carbonylation of methane to methyl acetate on a rhodium-doped iron phosphate catalyst

Article abstract of DOI:10.1039/a702159c

The carbonylation of methane with carbon monoxide in the presence of nitrous oxide produces methyl acetate on a rhodium-doped iron phosphate catalyst under atmospheric pressure and at 573-673 K.

Full text of DOI:10.1039/a702159c

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Oxidative carbonylation of methane to methyl acetate on a rhodium-doped
iron phosphate catalyst
Ye Wang, Masayuki Katagiri and Kiyoshi Otsuka*
Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan
The carbonylation of methane with carbon monoxide in the
presence of nitrous oxide produces methyl acetate on a
rhodium-doped iron phosphate catalyst under atmospheric
pressure and at 573–673 K.
trile as internal standard. Other components which could not be
dissolved in the water trap were analysed by an on-line TCD-
equipped gas chromatograph.
Fig. 1 compares the reaction results over FePO
4
and various
metal-ion-doped FePO catalysts at 648 K. Using pure FePO ,
4
4
Methane is the main constituent of the abundant natural gas
resources. The direct transformation of methane to more useful
chemicals or liquid fuels has long been a desirable goal for
chemists. However, because methane is a very unreactive
molecule, the conversion of methane to useful chemicals has
proven to be a great challenge. Although there are many reports
of the functionalisation of methane, the most viable process and
a considerable amount of MeOH was formed. We have reported
that oxygen species generated from the reductive activation of
N
2
O on iron sites are responsible for the activation of CH
4
at
such a moderate temperature. However, the carbonylation
products, MeCO H and MeCO Me were not observed at all on
FePO . This result indicates that another site for the effective
8
2
2
4
activation of CO for the carbonylation is required on the catalyst
surface. Thus, many additives have been incorporated into
1–5
effective catalyst are still unsettled.
reported that the conversion of methane is enhanced and
Recently, we have
FePO
carbonylation. The results in Fig. 1 suggest that rhodium is very
specific for the formation of MeCO Me. Instead of FePO , a
large variety of materials including SiO , Al , H-ZSM-5, Ga-
H-ZSM-5, H-mordenite, active carbon, SO
2 5
V O were also used as bases for rhodium. However, none of
4
to enhance the ability for the activation of CO for the
methanol is selectively produced by co-feeding a reductant such
–8
) catalyst.6 When
as hydrogen over an iron phosphate (FePO
4
2
4
nitrous oxide is used as an oxidant in the presence of hydrogen,
methane can be selectively converted into methanol with high
2
2 3
O
22
4
2 3
/ZrO , MoO and
8
selectivity at moderate temperatures (ca. 620 K). If carbon
monoxide is used as the reductant instead of hydrogen, we can
expect that the activated carbon monoxide may react with the
methanol produced or with reaction intermediates such as
methyl species to form acetic acid or methyl acetate. The
wasteful consumption of the co-feeding reductant, which is a
common problem for many biomimetic monooxygenase cata-
lytic systems, may thus be overcome. Acetic acid is a very
important chemical and is produced mainly by the carbonyla-
tion of methanol as typically demonstrated in the Monsanto
process. If acetic acid or its derivatives such as methyl acetate
can be produced directly from methane, the impact on the
chemical industry would be significant because the production
cost of many chemical products based on acetic acid will
become remarkably cheaper. Reports concerning the direct
carbonylation of methane to acetic acid are very scarce9 and
all these studies have been performed using a high pressure
2
00
50
20
15
10
5
MeOH
MeCO Me
2
1
100
50
–11
(
> 20 bar) liquid-phase reaction system. The use of expensive
0
0
9
10
2
reagents such as FHSO
3
–SbF
5
,
K
2
S
2
O
8
and I or per-
none Co Ir
Ni Pd Pt Re Rh Ru
fluorobutyric acid11 is required in these complicated systems.
Furthermore, the separation of these expensive reagents or
M in (0.1 mol%)M doped FePO4 catalyst
catalysts such as RhCl
3
from the solvents is another drawback
Fig. 1 Oxidative carbonylation of methane over various metal-ion-doped
FePO catalysts. Reaction conditions: T = 648 K, P(CH ) = P(CO) = 25.3
kPa, P(N
for these homogeneous reaction systems. Therefore, a simple
heterogeneous catalytic system would be more desirable and
promising. This communication is the first contribution to
attempt direct carbonylation of methane using a simple
heterogeneous catalytic system.
4
4
2
1
2
O) = 12.7 kPa, W = 0.2 g, F = 80 ml min
.
Table 1 Effect of rhodium content on the formation of methyl acetate from
methanea
Formation rate/
21 21
mmol g
h
Rh content
(%)
CO conv.
(%)
MeOH
2
MeCO Me
Rh(NO , Fe(NO and NH PO and evaporating water
3
)
3
3
)
3
4
H
2
4
0
200.5
125.5
113.0
92.0
90.0
126.7
0
2.5
2.2
4.3
2.5
2.5
2.8
from the resultant solution to obtain the catalyst precursors. The
precursors were finally calcined at 823 K for 5 h in air. The
catalysts were pretreated in an oxygen flow at 723 K in the
reactor and the reaction was started by introducing the reactants
0
0
0
1
2
.1
.3
.5
.0
.0
16.0
26.5
28.5
21.2
18.3
CH
trap cooled with water/ice bath to collect the products, MeOH,
MeCO H and MeCO Me. These collected products were
analysed by a FID-equipped gas chromatograph with acetoni-
4 2
, N O and CO. The outlet gases were bubbled into a water
a
2
2
4
Reaction conditions: T = 648 °C, P(CH ) = P(CO) = 25.3 kPa,
O) = 12.7 kPa, total flow rate = 80 ml min
2
1
P(N
2
.
Chem. Commun., 1997
1187

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them showed activity for the carbonylation of CH
Rh/FePO is a unique catalyst. Further detailed studies were
thus carried out using the Rh/FePO catalyst.
In order to increase the rate of the carbonylation reaction,
catalysts with various mol% Rh were investigated. As shown in
Table 1, an increase in Rh content up to 0.5% increased the
4
. Therefore,
Rh to FePO
CO into CO
Fig. 2 shows the effect of CO pressure on the carbonylation
reaction over the Rh (0.5%)/FePO catalyst. No MeCO Me was
produced in the absence of CO. The introduction of CO caused
the formation of MeCO Me. It should be noted that MeOH was
4
. This result suggests that the direct oxidation of
4
2 4
occurs mainly on the host FePO .
4
4
2
2
formation rate of MeCO
Rh (0.5%)/FePO catalyst, the formation rate of MeCO
reached ca. one third of that of MeOH. Further increase of Rh
content decreased the formation rate of MeCO Me as well as
2
Me at the expense of MeOH. For the
also remarkably increased at low partial pressures of CO
[P(CO) < 5 kPa]. This result suggests that CO functions not
4
2
Me
only as a reactant in the carbonylation reaction but also as an
III
2
4
activator for the conversion of CH . Probably, CO reduces Fe
to Fe on the FePO
II
II
the ratio of MeCO
2
Me to MeOH. This fact probably indicates
4
surface and the produced Fe site acts as
that highly dispersed Rh3 on the surface of FePO
+
is
the active site for the generation of the active oxygen species
4
6–8
responsible for the carbonylation reaction.
from N
2
O. On the other hand, higher pressures of CO are not
Me.
with the active
Naturally, the conversion of CO shown in Table 1 was mainly
ascribed to the formation of CO . Only 1–5% of CO converted
favourable for the formation of both MeOH and MeCO
2
2
Competitive oxidation between CO and CH
oxygen species must be the main reason.
4
took part in the carbonylation reaction. Table 1 shows that the
conversion of CO is not changed appreciably by the addition of
Whether MeCO
2
Me is formed from the direct carbonylation
of CH or from the carbonylation of the produced MeOH is a
4
key point in the reaction mechanism. Thus, the influence of
1
20
00
600
500
400
300
200
100
0
WF21 (weight of catalyst/flow rate) on the yields of MeOH and
MeCO
2
Me has been investigated. As shown in Fig. 3, the yields
Me increased almost linearly with
increase in WF . This result indicates that MeOH and
MeCO Me are formed in parallel from CH
The results described above allow us to speculate on a
reaction mechanism as follows. CH must be activated by the
of both MeOH and MeCO
2
1
2
1
(
a)
2
4
.
80
60
40
20
0
4
active oxygen species generated on the iron site, producing an
Me species as a transient intermediate. This Me species may
easily be oxidized to an MeO species on the surface of FePO .
4
It must also migrate and adsorb on a Rh site where CO is
activated. The insertion of CO into Rh–Me may occur on this
(
b)
Rh site, producing Rh–COMe. MeCO
reaction of Rh–COMe with the MeO species produced on
FePO near this Rh site, while the addition of a proton (probably
2
Me is then formed by the
4
from OH groups) to the adsorbed MeO produces MeOH.
According to this mechanism, the high dispersion of Rh at sites
surrounded by iron sties is a key factor for the carbonylation
reaction. Further confirmation of this mechanism and the design
of catalysts with high concentration of Rh sites with surround-
ing iron sites are underway.
0
10
20
P(CO) / kPa
30
40
Fig. 2 Effect of CO pressure on the formation of MeOH and MeCO
Rh (0.5%)/FePO catalyst. (a) MeCO Me, (b) MeOH. Reaction conditions:
T = 648 K, P(CH ) = P(N O) = 25.3 kPa, W = 0.2 g, F = 160 ml
2
Me on
4
2
4
2
2
1
min
.
Footnote
*
E-mail: kotsuka@o.cc.titech.ac.jp
0
0
0
0
0
.8
.6
.4
.2
.0
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Fig. 3 Effect of WF21 on the formation of MeOH and MeCO
0.5%)/FePO catalyst. (a) MeCO Me, (b) MeOH. Reaction conditions:
T = 648 K, P(CH ) = P(N
O) = 25.3 kPa, F = 300 ml min2
2
Me on Rh
(
4
2
1
4
2
.
Received in Cambridge, UK, 1st April 1997; Com. 7/02159C
1188
Chem. Commun., 1997