Liquid phase methanol carbonylation catalysed over rhodium supported on hydrotalcite

Article abstract of DOI:10.1039/a908350b

A solid catalyst, Mg-Al hydrotalcite containing nanometer size rhodium particles, is active for liquid phase methanol carbonylation to methyl acetate at 473 K in the presence of methyl iodide.

Full text of DOI:10.1039/a908350b

Liquid phase methanol carbonylation catalysed over rhodium supported on
hydrotalcite
M. P. Kapoor and Yasuyuki Matsumura*†
Osaka National Research Institute, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
Received (in Cambridge, UK) 19th October 1999, Accepted 29th November 1999
A solid catalyst, Mg–Al hydrotalcite containing nanometer
size rhodium particles, is active for liquid phase methanol
carbonylation to methyl acetate at 473 K in the presence of
methyl iodide.
h and sealed in a glass capsule. The capsule was placed in the
reactor along with a mixture of methanol (780 mmol) and
methyl iodide (35 mmol), and CO was introduced at 0.5 MPa
(42 mmol) after displacing air inside the reactor with argon. The
capsule was broken during pressurization and the reactor was
heated to 473 K. The products were analysed with an Ohkura-
802 gas chromatograph (TCD) with PEG 1500 (2 m) and
Porapak T (2 m) columns.
Acetic acid and methyl acetate are industrially synthesised by
carbonylation of methanol with a homogeneous rhodium
catalyst promoted by methyl iodide.1 Attempts have been
made to use heterogeneous catalysts for methanol carbonyla-
tion,3 because such a system would be advantageous in
separation and recovery of the products and recycling of the
catalyst, providing greater economic advantages and minimiz-
ing the environmental impact of the process. A number of
,2
The powder XRD pattern of the parent hydrotalcite con-
firmed that the sample was well crystallised, but broadening of
the peaks was observed for the hydrotalcite containing rhodium
(Fig. 1). A change in the BET surface area after modification
with rhodium was also observed. The surface area decreased
with an increase in the rhodium content for the hydrotalcite with
,4
5
,6
polymers,7
supported rhodium systems such as zeolites,
8
9
2
21
activated carbon and inorganic oxides are reported to be active
under vapour phase conditions. In the liquid phase, polymer
an original surface area of 232 m g (Table 1). The surface
area of the sample prepared by the impregnation method was
very small in comparison with the samples prepared by
deposition–precipitation.
supported Rh(
system is unstable due to loss of rhodium.
Hydrotalcite, [Mg12xAl (OH) (CO
I
) catalysts have been reported,10 however, this
x
2
3
)
x/2·yH
2
O], consists of
Carbon monoxide was adsorbed on the samples containing
rhodium reduced at 673 K. The amount of carbon monoxide
adsorbed was saturated at ca. 1 kPa. On the basis of the amounts
saturated, the surface areas of rhodium in the samples were
tentatively calculated, assuming atomic adsorption of CO on a
positively charged brucite-like layers with charge–balancing
anions and water molecules.11 The material shows strong
basicity and is often employed as a precursor for catalysts or
catalyst supports owing to its structure.12 In the present work,
we have demonstrated that nanometer size rhodium particles
can be stabilized on hydrotalcite and the solid effectively
catalyses the liquid phase carbonylation of methanol in the
presence of methyl iodide. This may provide a realistic solution
to problems such as separation, recovery, etc. associated with
homogeneous catalytic systems.
An Mg–Al hydrotalcite (Mg/Al = 2/1) was synthesised using
the procedure reported by Reichle.13 Rhodium was loaded on
the Mg–Al hydrotalcite by a deposition–precipitation method.
In a typical procedure, Mg–Al hydrotalcite was placed in an
aqueous solution of Rh(NO
3
)
3
(Kanto Chemical) at 343 K and
3
(1 M) was added dropwise. The
an aqueous solution of Na CO
2
pH of the solution was kept at 10 for 1 h. The resulting solid was
filtered off and washed thoroughly with distilled water, dried
and then calcined in air at 673 K for 4 h. A rhodium catalyst was
also prepared by the impregnation technique. After evaporation
3 3
of a mixture of the solution of Rh(NO ) and hydrotalcite, the
wet solid was dried and calcined in air at 673 K for 4 h. The Rh
contents in the samples was analysed by ICP emission
spectrometry.
Adsorption experiments were performed with a vacuum
system equipped with Baratron vacuum gauges after the
samples were reduced with hydrogen at 673 K for 2 h. The BET
surface areas of the samples were determined from nitrogen
physisorption isotherms.
Fig. 1 XRD patterns of hydrotalcite modified with rhodium by deposition-
precipitation.
Table 1 Some surface properties of Rh/Mg–Al hydrotalcite catalysts
prepared by the deposition–precipitation method
Rhodium
Powder X-ray diffraction (XRD) patterns of the samples were
recorded with a Rigaku Rotaflex 20 diffractometer using nickel-
filtered Cu-Ka (40 kV, 150 mA) radiation.
Rhodium
content
BET
CO
uptake/
(mmol g
surface
area/
m g
surface area/
2
21
21
2
21
(wt%)
m g
3
Catalytic tests were performed in an autoclave (0.2 dm ).
Prior to the reaction, the catalyst (0.20 g) was activated under
evacuation followed by reduction with hydrogen at 673 K for 2
n.d.a
0
1.7
2
3
1.5
.8
159
116
104
86
n.d.
1.5
1.7
1.3
0.6
44
50
39
18
.1
.0
b
†
Present address: Research Institute of Innovative Technology for
80
the Earth, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan. E-mail:
yasuyuki@rite.or.jp
a
Not determined. b Prepared by impregnation.
Chem. Commun., 2000, 95–96
This journal is © The Royal Society of Chemistry 2000
95

Table 2 Methanol carbonylation over Rh/Mg–Al hydrotalcite catalysts at
formation of a small amount of DME. Hence, methanol cannot
be directly carbonylated over the catalyst, and the presence of
methyl iodide is essential in the carbonylation. This suggests
that carbonylation of methyl iodide to acetyl iodide is a key step
of the carbonylation as also found for homogeneous carbonyla-
tion with rhodium complexes.14 Formation of MeOAc was low
after reaction for 1 h with the catalyst containing 1.7 wt% of Rh
while DME formation was considerable (Table 2). This result
indicates an induction period for the formation of MeOAc.
Formation of homogeneous rhodium species during the induc-
tion period might be possible. However, no acetic acid, which is
473 K for 4 h
Rhodium MeOH
Products/mmol
MeCHO
content
wt %)
conversion
(%)
(
MeOAc
DME
0.8
1.7
1.7
2.1
3.0
1.5
12.7
16.3
5.8
16.2
15.0
7.7
0.1
0.2
0.1
0.5
0.3
0.2
21
30
2
32
23
9
29
33
20
31
35
21
a
b
2
one of the major products in homogeneous carbonylation, was
a
Reaction time = 1 h. b Prepared by impregnation.
detected in the reaction with the heterogeneous catalyst. In the
mechanism for homogeneous carbonylation, methyl acetate is
formed as a product of a subsequent esterification of acetic acid
2
2
rhodium site and a site density of 0.058 nm (Table 1). The
mean particle sizes of rhodium were calculated from the data in
Table 1 assuming that all the rhodium particles are spherical and
on the surface, and gave values of 5, 6 and 11 nm, respectively,
for deposition–precipitation samples containing 1.7, 2.1 and 3.0
wt% of rhodium. The Rh particle size for the sample prepared
by the impregnation method (1.5 wt% of Rh) was calculated as
and methanol. The product distribution with the heterogeneous
catalys is similar to that found for the vapour phase carbonyla-
tion of methanol in the presence of methyl iodide over rhodium
8
supported on activated carbon while methyl acetate appears to
be directly produced in the reactions. The similarity suggests
that the carbonylation with rhodium containing hydrotalcite
takes place on the surface of rhodium particles. Carbonylation
of DME to MeOAc has been proposed in the vapour phase
reaction, and can account the selective formation of MeOAc and
the induction period.15
In summary, nanometer sized rhodium particles can be
stabilised on hydrotalcite, and methyl acetate is directly formed
in the liquid-phase methanol carbonylation with supported
rhodium catalysts. A detailed kinetic study is in progress, which
should provide stimulating results for practical modifications of
the methanol carbonylation process.
1
2 nm. Particles of > 10 nm are expected to be large enough to
enable detection by XRD. However, no peaks attributed to
rhodium metal were evident in the patterns of any of the samples
after reduction, with the XRD patterns being similar to those of
Fig. 1. Since the stoichiometry of the adsorption varies with the
chemisorption mode of CO (for bridge-type adsorption a CO
molecule occupies two rhodium sites), the rhodium surface
areas in Table 1 may be underestimates and would result in
larger particle sizes. Hence, the actual particle sizes of rhodium
may well be smaller than the values estimated by adsorption of
carbon monoxide. The reduction of the BET surface area after
modification of rhodium suggests that some of the rhodium
particles are incorporated in the bulk phase of the hydrotalcite
and so will be another reason for the apparent contradiction of
the adsorption and XRD results.
Notes and references
1
F. E. Paulik, A. Hershman, W. R. Knox and J. F. Roth, U.S. Pat.,
769 329, 1973.
3
Mg–Al hydrotalcites containing rhodium were found to be
active for methanol carbonylation at 473 K. The main products
were methyl acetate (MeOAc) and dimethyl ether (DME) with
a small amount of acetaldehyde (Table 2). Neither acetic acid,
nor other decomposition products such as hydrogen were
detected. Loss of methyl iodide was not observed and no trace
of rhodium was detected in the liquid phase after the reaction.
The XRD patterns of the catalyst recovered after the reaction
were very similar to those prior to the reactions, showing that
the catalysts retain the hydrotalcite structure during reaction. No
reaction products were obtained with rhodium-free Mg–Al
hydrotalcite. An increase in rhodium content to 2.1 wt%
enhanced formation of MeOAc while a further increase of
rhodium to 3.0 wt% resulted in a lower yield of MeOAc. The
catalyst prepared by the impregnation method with a rhodium
content of 1.5 wt% was the least active. The activity towards
MeOAc mainly depends on the estimated surface area of
rhodium (cf. Tables 1 and 2).
2 F. E. Paulik and J. F. Roth, Chem. Commun., 1968, 1578.
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B. K. Nefedov, N. S. Segeeva and Ya. T. Eidus, Izv. Akad. Nauk, SSSR,
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1
1
1
1
1
1 R. Allmann, Acta Crystallogr., Sect B, 1968, 24, 972.
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983, 7, 361.
No methanol carbonylation occurred in the absence of methyl
iodide for the catalyst containing 1.7 wt% of rhodium except for
Communication a908350b
96
Chem. Commun., 2000, 95–96