Table 2 Analysis of supported catalyst used in experiments C1–5 after
various treatments
Mass of
catalyst/mg
Total Rh
Rh content (%) (1025 mol)
Chargeda
Recoveredb
Heatedc
170
205
27
21
22.4
1.04
0.74
1.7 ± 0.1
1.5 ± 0.1
Recoveredd
Expectede
a
b
To the reactor at the start of the series C experiments. From the
reactor after experiment C5. Sample of catalyst taken for heating to
200 °C. d After heating to 200 °C, MeI and MeCO2H were released; weight
c
e
loss by TGA = 13.7%. If all the mass gained during the catalytic
experiment were from adsorption of MeI and MeCO2H.
cf. 17% if all the weight gained during the catalysis were lost. In
a similar experiment carried out using thermal gravimetric
analysis (TGA), the weight loss in a transient process starting at
33 °C and complete by 70 °C was 13.7%. TGA was also carried
out on samples of the catalyst support washed with MeI and
with ethanoic acid separately. For MeI, the transient process had
an onset temperature of ca. 30 °C and was complete at 99 °C,
whilst for ethanoic acid, onset was at 22 °C and the loss was
complete at 70 °C. In all the TGA experiments, substantial
further weight loss occurred above 250 °C, probably from
decomposition of the polymer support. The observation that
MeI is held on the catalyst (even after storing for several days)
supports our suggestion4 that it interacts with the support.
The rates obtained in the supercritical experiments are of the
same order of magnitude as those obtained in the liquid phase
reactions, despite the lower concentrations of methanol and MeI
in the supercritical reactions. Turnover frequencies in the gas
phase reactions are comparable with those obtained in the
supercritical phase (up to 595 h21, at 190 °C in the gas phase
compared with up to 504 h21 at 150 °C for the supercritical
phase reactions). However, the gas phase reactions were carried
out at a higher temperature, and the concentration of methanol
achievable is very much less (1 cm3 in 9.8 dm3, as opposed to
1.5 cm3 in 50 cm3 for the supercritical reactions). Rhodium
leaching is a severe problem for the bulk liquid phase reactions,3
but is very much reduced for the reactions carried out in the
supercritical or gas4 phases.
Fig. 1 Plot of turnover frequency/h21 for repetitive catalytic reactions. The
same catalyst was reused for all runs in a series, but different samples were
used for runs B and C. For runs B, the products were condensed onto the
catalyst and removed by syringe; for runs C, the products were flushed from
the autoclave using scCO2. The asterisk means that iodomethane was not
added for this run.
Fig. 2 Possible mechanism of reaction of the catalyst with surface bound
methyl groups and remethylation of the surface.
and venting process and we surmised that the observed catalyst
leaching might occur during this period of contact between the
catalyst and the liquid products. To investigate the leaching
under supercritical conditions, we carried out the reaction in the
same way except that at the end of the reaction the stirrer was
stopped and, in a process similar to that introduced by Leitner
and co-workers5 for the separation of products from homoge-
neous catalysts after reactions in scCO2, fresh scCO2 was
passed into the bottom of the autoclave at the reaction
temperature and pressure to flush the product solution into a
second autoclave held at 40 bar and 280 °C. During this
process, the CO2 and CO were vented through an ethanol trap.
The catalyst remaining in the first autoclave was reused five
times (Table 1, Fig. 1, runs C1–5). The liquid product collected
in the second autoclave (up to 1.2 cm3) was analysed for the
organic products by GLC and for rhodium content by atomic
absorption. This series of experiments was carried out with a
slightly higher catalyst loading than that for experiments B1–5,
so the apparent yields are lower.‡ These lower apparent yields
may arise because some solution was always left on the catalyst
in the B series of experiments because the product solution was
removed by syringe. This means that the starting solution for all
the B series experiments, except B1, contained some product. In
the C series experiments, all the product apart from that
adsorbed on the support (see below) was removed by the
venting process.
None of the solutions recovered after the supercritical venting
process (series C) contained detectable rhodium ( < 0.3% of Rh
charged), apart from C2, which contained 0.08% of the rhodium
charged,§ suggesting that leaching has been minimised. To
confirm this, we analysed the catalyst before and after use
(Table 2). The mass of the catalyst recovered was higher than
that loaded, and the total rhodium content had dropped slightly
(12%, although the precision of the analyses is such that this
may not be significant) (see Table 2), confirming that rhodium
leaching is greatly reduced when the reaction is carried out and
the products are recovered in the supercritical phase.
We conclude that methanol carbonylation in the supercritical
phase has the potential to overcome the problems of liquid
phase (catalyst leaching) and gas phase (low methanol through-
put) reactions using the same supported catalyst.
We thank the EEC for a Fellowship (M. F. S.) and support
under a TMR programme and for support under the ERASMUS
programme (N. De B).
Notes and references
† The supercritical reactor has a volume of 50 cm3, whilst the liquid phase
reactions were carried out in a liquid volume of 5 cm3.
‡ Using this catalyst for an experiment under the conditions of experiment
B1, we obtained a turnover frequency of 176 ± 13 h21, within experimental
error the same as that obtained with the batch of catalyst used for the B
experiments (197 ± 20 h21).
§ In this experiment, all of the recovered solution was used for the atomic
absorption analysis, improving the detection limit for this analysis. The
amount detected is at this detection limit (0.5 ppm in the analysed
sample).
1 M. J. Howard, M. D. Jones, M. S. Roberts and S. A. Taylor, Catal. Today,
1993, 18, 325.
2 H. Sugiyama, F. Uemura, N. Yoneda, T. Minami, T. Maejima and K.
Hamato, Jpn. Pat., 1997, 235250; Chem. Abstr., 1997, 127, 279841j.
3 N. De Blasio, M. R. Wright, E. Tempesti, C. Mazzocchia and D. J. Cole-
Hamilton, J. Organomet. Chem., 1998, 551, 229.
4 N. De Blasio, E. Tempesti, A. Khaddouri, C. Mazzocchia and D. J. Cole-
Hamilton, J. Catal., 1998, 176, 253.
5 S. Kainz, A. Brinkman, W. Leitner and A. Pfaltz, J. Am. Chem. Soc.,
1999, 121, 6421 and references therein.
In order to understand the increase in weight of the catalyst,
we heated a sample of the recovered catalyst to constant weight
in a closed system and analysed the gas phase. MeI and
MeCO2H were detected by GLC and the weight loss was 22%,
Communication 9/06653E
2512
Chem. Commun., 1999, 2511–2512