Location of active sites for 3-pentanone formation during ethene hydroformylation on Rh/active-carbon catalysts

Article abstract of DOI:10.1006/jcat.1997.1810

In ethene hydroformylation over Rh/active carbon prepared from RhCl₃, 3-pentanone was formed in addition to propanal, which was a common product in the reaction. In the case of propene hydroformylation, however, butanal was exclusively formed and no appreciable formation of ketones was observed. On the catalyst treated with CO at 473 K, both activities for propanal and butanal formation were observed to be higher than those on the catalyst untreated. An approximate linear relation between the activity for propanal formation and that for butanal formation was observed, indicating that the active sites for aldehyde formation can catalyze the reaction for both ethene and propene. On the other hand, the activity for 3-pentanone formation was reduced by the CO treatment. These results indicate that the transformation of active species for 3-pentanone formation into the active species for aldehyde formation was induced by the CO treatment. On the catalyst treated with CO, a value for n/iso- ratio in butanal formed was observed to be higher than that on the catalyst untreated. These results suggest the formation of new active Rh particles in the narrower pores. On the basis of the results obtained, the active species for 3-pentanone formation was thought to be located in the narrow pores, where the formation of reaction intermediates of C₇-ketones from propene would be strongly suppressed because of their bulky structure compared with that for 3-pentanone.

Full text of DOI:10.1006/jcat.1997.1810

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JOURNAL OF CATALYSIS 171, 449–456 (1997)
ARTICLE NO. CA971810
Location of Active Sites for 3-Pentanone Formation during Ethene
Hydroformylation on Rh/Active-Carbon Catalysts
Hirotoshi Sakagami, Naoki Ohta, Shunsuke Endo, Teruyuki Harada, Nobuo Takahashi,1
and Takeshi Matsuda
Department of Materials Science, Kitami Institute of Technology, 165 Koencho, Kitami, Hokkaido 090, Japan
Received February 10, 1997; revised June 24, 1997; accepted June 24, 1997
decreased with an increase in CO partial pressure, while
r3-P increased; (iv) r3-P decreased when the catalyst was ex-
posed to CO at >433 K, whereas rPA was enhanced by this
treatment; and (v) the active sites for 3-pentanone forma-
tion required the presence of chlorine to be formed but
those for propanal formation did not. These results indi-
cate that the active sites for 3-pentanone formation are
different from those for propanal formation. It was al-
ready reported that the propanal formation is accelerated
by the presence of rhodium metal particles (5, 6). Chuang
et al. have proposed Rh+ sites at the surface of supported
rhodium metal particles as the active sites for the propanal
formation on Rh/SiO2 (7–9). The Rh/active-carbon cata-
lyst which showed high activity for 3-pentanone forma-
tion adsorbed a considerable amount of CO, and it showed
a CO desorption peak at almost the same position with
Rh6(CO)16-like species formed on Y-zeolite (10). In addi-
tion, EXAFS analysis of this sample suggested the presence
of Rh6(CO)16-like species (11). On the basis of the results
obtained, we suggested an important role of the Rh–CO
speciesin formingthe active speciesfor 3-pentanone forma-
tion.
In ethene hydroformylation over Rh/active carbon prepared
from RhCl3, 3-pentanone was formed in addition to propanal,
which was a common product in the reaction. In the case of propene
hydroformylation, however, butanal was exclusively formed and
no appreciable formation of ketones was observed. On the catalyst
treated with CO at 473 K, both activities for propanal and butanal
formation were observed to be higher than those on the catalyst
untreated. An approximate linear relation between the activity for
propanal formation and that for butanal formation was observed,
indicating that the active sites for aldehyde formation can catalyze
the reaction for both ethene and propene. On the other hand, the ac-
tivity for 3-pentanone formation was reduced by the CO treatment.
These results indicate that the transformation of active species for
3-pentanone formation into the active species for aldehyde forma-
tion was induced by the CO treatment. On the catalyst treated with
CO, a value for n/iso- ratio in butanal formed was observed to be
higher than that on the catalyst untreated. These results suggest
the formation of new active Rh particles in the narrower pores. On
the basis of the results obtained, the active species for 3-pentanone
formation was thought to be located in the narrow pores, where the
formation of reaction intermediates of C7-ketones from propene
would be strongly suppressed because of their bulky structure com-
c
pared with that for 3-pentanone.
1997 Academic Press
Formation of C7-ketones was reported by Rode et al.
in propene hydroformylation over Rh supported Y-zeolite
catalysts (12). If the Rh/active carbon could catalyze prop-
ene hydroformylation, n-butanal and isobutanal would be
formed. In addition, three kinds of ketones (4-heptanone,
2-methyl-3-hexanone, and 2,4-dimethyl-3-pentanone) are
expected to be produced. Active carbon is one of the
typical microporous materials. If the active sites for ke-
tone formation were located in the narrow pores, a ra-
tio of (ketone)/(aldehyde) would be small in propene
hydroformylation compared with that in ethene hydro-
formylation, since the intermediates of ketones are more
bulky than those for aldehydes. The ratio of (n-butanal)/
(isobutanal) may also reflect the location of active sites for
aldehyde formation, since the intermediate of isobutanal is
more bulky than that for n-butanal. In the present work,
the catalytic behavior of Rh/active carbon for propene
hydroformylation has been compared with that for ethene
INTRODUCTION
Rhodium supported on active carbon can catalyze ethene
hydroformylation as well as other supported rhodium cata-
lysts (1–4). In the reaction over the former catalyst, how-
ever, 3-pentanone is formed in addition to propanal, which
is a common hydroformylation product from ethene. The
following results were obtained in the previous work: (i)
an increase in the temperature for the pretreatment of the
catalyst with H2 reduced the activity for 3-pentanone for-
mation (denoted by r3-P), while the activity for propanal
formation (denoted by rPA) was enhanced; (ii) although r3-P
rapidly decreased with time on stream at reaction temper-
atures >413 K, rPA slowly decreased even at 453 K; (iii) rPA
1 To whom correspondence should be addressed.
449
0021-9517/97 $25.00
c
Copyright
1997 by Academic Press
All rights of reproduction in any form reserved.

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450
SAKAGAMI ET AL.
hydroformylation in order to clarify the location of active monolayer coverage of a Rh metal surface, we used a single
sites for 3-pentanone formation.
H2 or CO isotherm. Namely, the value for the monolayer
coverage was estimated by extrapolating the flat part of an
adsorption isotherm to the zero of the equilibrium pressure
EXPERIMENTAL
for adsorption. The amounts of H2 and CO adsorbed on the
6
The active-carbon support (obtained from Wako Pure active carbon support itself (<1 10 mol g-cat 1) were
Chemical Ind., Ltd., specific surface area 930 m2/g, average found to be much less than those on the Rh/active-carbon
pore diameter (dav) 4 nm) was washed with 0.1 mol/dm3 catalysts. Assuming that each surface rhodium atom can
of hydrochloric acid solution and thoroughly washed with adsorb one H atom or one CO molecule, the dispersion of
deionized water. Rh/active carbon with 2.0 wt% Rh load- rhodium was evaluated from the value for the monolayer
ing was prepared by a conventional impregnation method, coverage.
where rhodium trichloride trihydrate (RhCl3 3H2O) or
Transmission electron micrographs of the samples were
rhodium trinitrate (Rh(NO3)3) was deposited on the sup- obtained by using a JEOL JEM-2010F electron microscope
port from the aqueous solution of them. The solid was with an energy dispersive X-ray spectrometer (EDS) in co-
dried at 383 K for 12 h in an oven. In the same manner, operation with JEOL Ltd. (at 200 kV and direct magnifica-
˚
Rh/molecular sieve carbon 3A (dav < 0.5nm, obtained from tion of 400,000). Samples were exposed to air and crushed
Takeda Chemical Industries, Ltd.) was prepared. Rh/SiO2 into fine particles for the measurements by TEM.
catalysts were prepared in the same manner by using SiO2
with different pore sizes (A, obtained from Nishio Industry
Co., Ltd.; B–D, obtained from Fuji Sylicia Chemical Ind.
as CARIACT Nos. 10, 15, 30, and 50, respectively). Values
RESULTS AND DISCUSSION
In ethene hydroformylation over Rh/active carbon (de-
for average pore diameter of SiO2 used were measured by noted by Rh/AC), propanal, 3-pentanone, and ethane were
Carlo Erba Sorptmatic 1900.
formed (1–4). For the evaluation of the catalytic activity, we
Pretreatment of the catalyst with H2 at 573 K for 3 h was tried to determine the number of surface rhodium atoms on
followed by ethene or propene hydroformylation at 393 K the catalysts by using H2 or CO chemisorption technique.
–
with a reactant gas mixture of He C2H4(or C3H6)(20% )– The amounts of H2 and CO adsorbed on the rhodium cata-
CO(20% )–H2(20% ) (total flow rate 100 cm3(STP)/min) in lysts prepared are shown in Table 1. In the case of SiO2 sup-
a fixed-bed type reactor with a continuous flow operated at port, the values for the ratio of the amount of CO to that
atmospheric pressure. If necessary, the pretreatment with of H2 were observed to be around 2.0, suggesting that the
H2 was followed by a treatment with CO at 473 K for 1 h normal adsorption of H2 and CO to surface rhodium atoms
prior to the reaction performance.
(each surface rhodium atom can adsorb one H atom or one
The amounts of H2 or CO adsorbed on the catalyst were CO molecule) is predominant on the Rh/SiO2 catalysts. On
measured at 296 K by a conventional static method using the other hand, the values of CO/H2 were observed to be
a glass-made apparatus connected to a vacuum line. For much higher than 2.0 on the Rh/AC catalysts, particularly
the estimation of the amount of H2 or CO required for a on the Rh(C)/AC prepared from RhCl3. The excess amount
TABLE 1
Adsorption of Hydrogen and Carbon Monoxide on the Catalysts Used
H2
CO
adsorbed
(10 6 mol g-cat
Rh loading
Rh dispersion (% )
adsorbed
(10 6 mol g-cat
1
1
1
Catalyst
(wt% )
(10 6 mol g-cat
)
)
)
CO/H2
H2
CO
Rh(C)/AC
Rh(C)/AC
Rh(C)/AC
Rh(C)/AC
Rh(N)/AC
Rh(N)/AC
Rh/SiO2(A)
Rh/SiO2(B)
Rh/SiO2(C)
Rh/SiO2(D)
Rh/SiO2(E)
Rh(C)/MSC
1.0
2.0
3.0
4.0
2.0
4.0
4.0
4.0
4.0
4.0
4.0
2.0
97
194
292
389
194
389
389
389
389
389
389
194
15.0
31.6
46.4
54.7
15.5
45.3
66.9
69.0
53.0
36.2
28.8
6.9
139.4
267.5
384.0
488.8
48.8
189.3
141.8
137.0
90.9
9.3
8.5
8.3
8.9
3.1
4.2
2.1
2.0
1.7
1.8
1.9
2.8
31
33
32
28
16
23
34
35
27
19
15
7
144
138
132
126
25
49
36
35
23
63.8
54.4
19.5
16
14
10

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3-PENTANONE FORMATION OVER Rh/ACTIVE CARBON
451
TABLE 2
Comparison of C3H6 Hydroformylation with C2H4 Hydroformylation over Rh/AC
Reaction of C2H4
Reaction of C3H6
Rh
loading
(wt% )
1
1
(TOF, 10 3 min
)
(TOF, 10 3 min
)
Treatment
with COa
b
Run
Catalyst
rPA
r3-P
rE
rn-BA
ri-BA
rBA
rP
n-BA/i-BA
1
2
3
4
5
6
7
8
9
Rh(C)/AC
Rh(C)/AC
Rh(C)/AC
Rh(C)/AC
Rh(C)/AC
Rh(C)/AC
Rh(C)/AC
Rh(C)/AC
Rh(N)/AC
Rh(N)/AC
Rh(N)/AC
Rh(N)/AC
1.0
1.0
2.0
2.0
3.0
3.0
4.0
4.0
2.0
2.0
4.0
4.0
No
Yes
No
Yes
No
Yes
No
Yes
No
32
252
66
357
87
383
100
407
85
19
6
9
5
5
3
3
2
0
0
1
0
67
265
114
376
147
405
142
311
114
107
139
139
11
80
16
102
19
115
26
116
15
15
25
36
10
48
13
59
14
65
18
63
10
10
16
21
21
128
28
160
33
180
44
178
25
66
186
55
172
52
175
52
127
60
59
50
63
1.1
1.7
1.2
1.7
1.3
1.8
1.4
1.8
1.5
1.5
1.6
1.7
10
11
12
Yes
No
Yes
80
113
132
25
41
57
Note. Catalytic activities, rates of formation at time-on-stream of 8 h.
a Pretreatment of the catalyst with H2 was followed by the treatment with CO at 473 K for 1 h.
b rBA = rn-BA + ri-BA
.
of CO adsorbed over Rh atoms on the Rh/AC catalysts was the three products are denoted by rn-BA, ri-BA, and rP, respec-
already observed by means of a temperature-programmed tively. However, no appreciable formation of C7-ketones
desorption technique (TPD) (3, 4, 11). The higher values was observed. In the present work, the catalytic activities
of CO/H2 indicate the presence of highly dispersed phase are evaluated by the rates of formation at time on stream
of Rh. In the present work, the number of surface rhodium of 8 h.
atoms was evaluated by using the amount of H2 required
for the molayer coverage.
In Table 2, the catalytic behaviors of Rh/AC for propene
hydroformylation are compared with those for ethene hy-
In the ethene hydroformylation at 393 K, the rates of droformylation. On both Rh/AC catalysts, Rh(C)/AC and
propanal and ethane formation slowly decreased with time Rh(N)/AC (prepared from Rh(NO3)3), the activity for
on stream. The rate of 3-pentanone formation, on the other butanal formation (rBA = rn-BA + ri-BA) in propene hydro-
hand, increased with time on stream at the early stage of formylation increased with an increase in the amount of
the reaction for about 8 h, and then it decreased slowly. As Rh loaded. An approximate linear relation was observed
shown in Fig. 1, n-butanal, isobutanal, and propane were between rBA and rPA as shown by open circles for Rh(C)/AC
observed to be the products in propene hydroformylation and open squares for Rh(N)/AC in Fig. 2. Namely, the ac-
over Rh(C)/AC (2.0 wt% ). The rates for the formation of tive sites for aldehyde formation can catalyze the reaction
FIG. 1. Propene hydroformylation over Rh(C)/AC (2 wt% ) with H2
pretreatment at 573 K ( , ᮀ, 4) and with CO treatment at 473 K after
the H2 treatment (᭹, ᭿, ᭡). ( , ᭹, n-butanal; ᮀ, ᭿, isobutanal; 4, ᭡,
propane).
FIG. 2. Plots for rPA vs rBA on Rh(C)/AC ( , ᭹) and on Rh(N)/AC
(ᮀ, ᭿). ( and ᮀ, with H2 pretreatment at 573 K; ᭹ and ᭿, with CO
treatment at 473 K after the H2 treatment).

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452
SAKAGAMI ET AL.
FIG. 3. (a) A TEM photograph on Rh(C)/AC (2 wt% ) with H2 pretreatment at 573 K; (b) an energy-dispersive X-ray spectrum on the spot shown
in (a).