Palladium supported on an acidic resin: A unique bifunctional catalyst for the continuous catalytic hydrogenation of organic compounds in supercritical carbon dioxide

Article abstract of DOI:10.1002/adsc.200700532

1% Palladium-doped acidic resin (Amberlyst 15; styrene-divinylbenzene matrix with sulfonic acid groups) is shown to be a highly active catalyst for the continuous catalytic hydrogenation of C=C bonds in supercritical carbon dioxide (scCO₂) without affecting C=O bonds. This 1% Pd/Amberlyst-15 catalyst promotes the industrially important selective formation of 2-ethylhexanal from crotonaldehyde in a one-pot pathway involving hydrogenation and aldol condensation witha number of merits. The selectivity behavior of 1% Pd/Amberlyst-15 is strikingly different compared to that of 1% Pd/C and 1% Pd/Al₂O₃ due to its prominent bifunctional nature based on sulfonic acid groups adjacent to metallic Pd sites. Hybrid [Pd_n-H]⁺ sites are suggested to act as bothmet al and acid sites promoting the bifunctional catalysis.

Full text of DOI:10.1002/adsc.200700532

FULL PAPERS
DOI: 10.1002/adsc.200700532
Palladium Supported on an Acidic Resin: A Unique Bifunctional
Catalyst for the Continuous Catalytic Hydrogenation of Organic
Compounds in Supercritical Carbon Dioxide
a,b
a
a
a,
Tsunetake Seki, Jan-Dierk Grunwaldt, Niels van Vegten, and Alfons Baiker *
a
Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich,
Hçnggerberg, HCI, 8093 Zurich, Switzerland
Phone: (+41)-44-632-3153; fax: (+41)-44-632-1163; e-mail: baiker@chem.ethz.ch
Research Fellow of the Japan Society for the Promotion of Science
b
Received: November 9, 2007; Published online: March 12, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: 1% Palladium-doped acidic resin (Amber- ly different compared to that of 1% Pd/C and 1%
ꢀ
lyst 15; styrene-divinylbenzene matrix withsulfonic
Pd/Al O due to its prominent bifunctional nature
2 3
acid groups) is shown to be a highly active catalyst based on sulfonic acid groups adjacent to metallic Pd
+
for the continuous catalytic hydrogenation of C=C sites. Hybrid “[Pd –H] ” sites are suggested to act as
A
H
R
U
n
bonds in supercritical carbon dioxide (scCO ) with- bothmetal and acid sites promoting t he bifunctional
2
out affecting C=O bonds. This 1% Pd/Amberlyst-15 catalysis.
catalyst promotes the industrially important selective
formation of 2-ethylhexanal from crotonaldehyde in Keywords: Amberlyst 15-supported palladium cata-
ꢀ
a “one-pot” pathway involving hydrogenation and lyst; bifunctional catalysis; continuous-flow reactor;
aldol condensation witha number of merits. T he se-
hydrogenation; one-pot reaction; supercritical
lectivity behavior of 1% Pd/Amberlyst-15 is striking- carbon dioxide
Introduction
15 catalysts seem to have a great potential for envi-
ronmentally friendly chemical productions, where
Supercritical carbon dioxide (scCO ) is an attractive botha transition metal and acidic conditions are re-
2
reaction medium due to its non-toxicity, non-flamma- quired, because the use of hazardous homogeneous
bility, and low cost. Its great miscibility withgases as
acids suchas hy droc hl oric acid and sulfuric acid can
well as liquid organic compounds enhances the gas- be avoided. In addition, these catalysts may provide
liquid mass transport, and thus can accelerate those the opportunity to achieve “one-pot” synthesis
reactions whose rates are determined by gas-liquid through the transition metal-acid bifunctional cataly-
mass transfer. These features, together with the low sis pathways.
viscosity and high diffusivity, render scCO suitable
Quite recently, Hoelderichꢁs group reported an effi-
2
particularly for heterogeneous catalytic hydrogena- cient one-pot synthesis of a potential analgesic over
[
1–3]
ꢀ
tions and oxidations.
under suchconditions were conventional ones, w hi le
corresponding studies based on specifically developed quent hydrogenation. A number of acidic supports
new catalysts are still scarce. including inorganic oxides were tested, but only Am-
Most catalysts used so far Amberlyst 15-supported palladium catalyst in etha-
nol solvent, which involves dehydration and subse-
[
6]
ꢀ
ꢀ
Amberlyst 15 is a commercially available, strongly berlyst
acidic cation exchange resin composed of styrene-di- smoothly, showing that Amberlyst 15 is an excellent
vinylbenzene matrix withsulfonic acid groups and ha s support for this kind of dehydration-hydrogenation
been used as a heterogeneous acid catalyst with great one-pot reaction. However, the three-phase reaction
15 promoted the dehydration process
ꢀ
[4]
success. Laufer and Hoelderichfirst introduced tran-
system with the inherent gas-liquid-solid mass trans-
ꢀ
sition metal loaded on Amberlyst 15 as catalyst and port problem required a high reaction temperature of
reported that the support acidity assisted the catalysis 1508C, which caused unfavorable coke formation and
by surface palladium and platinum for the hydroxyl- the deterioration of the surface sulfonic acid groups.
[
5]
ꢀ
ation of benzene. The transition metal/Amberlyst
Actually, the original activity of the catalyst could not
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691

Tsunetake Seki et al.
FULL PAPERS
be restored in the second reaction cycle even after sults of these screening tests are shown in Table 1.
careful washing of the used catalyst. Commercially available 1% Pd/Al O exhibited a low
2
3
We have concurrently communicated that palladi- activity under the reaction conditions (entry 1), while
ꢀ
um-doped Amberlyst 15 directly affords 2-ethylhexa- 1% Pd/C showed excellent catalytic activity (entry 2).
nal from crotonaldehyde in scCO via hydrogenation The selectivities to 2 exceeded 99% for the commer-
2
[
7]
and aldol condensation. Owing to the great miscibil- cial catalysts. The present results with 1% Pd/C are in
ity of the substrate and gaseous hydrogen in scCO2, accordance witht he former data publis he d by Aruna-
[
2e]
the reaction could be performed at a low temperature jatesan et al.
of 608C and witha slig ht excess amount of H 2. A Palladium immobilized on an acidic resin of Am-
ꢀ
good long-term catalytic behavior was observed under berlyst 15 (Pd/Amberlyst-15) also exhibited high ac-
the optimal conditions, in which almost 100% conver- tivity (entries 3–5). The activity strongly depended on
sion was maintained for at least 12 hand t he only by-
the amount of palladium loaded, and the catalyst con-
product was butyraldehyde. The use of scCO2 taining 1% palladium showed the highest activity
medium is thus one of the promising ways to improve (entry 4). Repeating synthesis of 1% Pd/Amberlyst-15
the efficiency of the catalyst for one-pot reactions in- showed that the conversion and selectivity of the cata-
volving hydrogenation.
Here we report a systematic study of the unique biꢀ- tively, which are comparable to or even higher than
functional catalysis of palladium-doped Amberlyst those obtained with a representative hydrogenation
5 in the continuous catalytic hydrogenation in scCO2 catalyst of 1% Pd/C. In contrast to Pd/Amberlyst-15,
lyst were in the range 95–99 and 97 to >99%, respec-
1
and provide insight into the structural properties of corresponding platinum, ruthenium, and rhodium cat-
the catalyst using X-ray absorption near edge struc- alysts were far less active (entries 6, 8, and 9). Addi-
ture (XANES), X-ray photoelectron spectroscopy tion of 0.5% Pt to 0.5% Pd/Amberlyst-15 caused a
(
XPS), transmission electron microscopy (TEM), negative effect (entry 3 vs. 7). The by-products
ꢀ
powder X-ray diffraction (XRD), and CO chemisorp- formed over the metal-loaded Amberlyst 15 were
tion combined withdiffuse reflectance infrared Fouri-
er transform (DRIFT) spectroscopy.
the dimerization product 3, which was formed by the
action of Brçnsted acid sites on Amberlyst 15, and
its hydrogenated product, bicyclohexyl 4.
ꢀ [8]
Note that the 1% Pd/Amberlyst-15 catalyst showed
a long lifetime for the hydrogenation, maintaining its
high activity and selectivity for at least 12 h from the
first sampling. This indicates that this catalyst is free
from deactivation caused by unfavorable side reac-
Results and Discussion
Reaction of Cyclohexene
[9]
The hydrogenation of cyclohexene 1 to cyclohexane 2 tions such as the reverse water gas shift reaction
[
2e]
was selected as a test reaction for screening the effi- and the oligomerization of 1 . Moreover, the long
ciency of various supported transition-metal catalysts lifetime implies that the active palladium species on
for the C=C bond hydrogenations in scCO . The re- 1% Pd/Amberlyst-15 are tightly immobilized on the
2
[a]
Table 1. Reaction of cyclohexene 1 withH 2 over the supported transition-metal catalysts in scCO2.
[b]
[b]
Entry
Catalyst
Conversion [%]
Selectivity [%]
2
3
4
1
2
3
4
5
6
7
8
9
1%
1%
0.5%
1%
2%
1%
0.5%,0.5%
1%
Pd/Al O
Pd/C
26
96
86
99
56
2
59
4
1
>99
>99
95
>99
99
52
96
94
24
<1
<1
1
<1
<1
46
2
<1
<1
3
<1
<1
2
2
<1
1
2
3
Pd/Amberlyst-15
Pd/Amberlyst-15
Pd/Amberlyst-15
Pt/Amberlyst-15
Pd,Pt/Amberlyst-15
Ru/Amberlyst-15
Rh/Amberlyst-15
6
75
1%
[
a]
Reaction conditions: catalyst, 1.00 g; total pressure, 16 MPa; reactor temperature, 608C; flow rate of expanded gaseous
ꢀ
1
ꢀ1
CO , 1 Lmin ; flow rate of 1, 0.5 mLmin ; 1:H =1:1.8.
2
2
[
b]
Determined by GC.
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Palladium Supported on an Acidic Resin
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°
merization of 1 to 3 has a large negative DV value in
this pressure region because of the large isothermal
compressibility, leading to a remarkable acceleration
[10]
of the reaction with a small increase in pressure.
Although the observed yields of 3 and 4 were still
low, this phenomenon may open up a new strategy for
“one-pot” production of bicyclohexyl from cyclohex-
ene.
Reaction of Benzaldehyde
Palladium catalysts are usually effective for the hy-
drogenation of ArCHO to ArCH OH (Ar=aryl),
2
whereas they are inefficient in catalyzing the hydroge-
[
11]
nation of aliphatic aldehydes. Nevertheless, to our
knowledge, only Poliakoffꢁs group reported the heter-
ogeneous Pd-catalyzed hydrogenation of benzalde-
Figure 1. Dependence of the yield (*) and selectivity (*) on
[
2b]
the total pressure. Reaction conditions: catalyst, 0.50 g of hyde 5 in scCO2. Witha 5% Pd/aminopolysiloxane
1
% Pd/Amberlyst-15; reactor temperature, 608C; flow rate catalyst, they performed the reaction at 958C and a
ꢀ
1
of expanded gaseous CO , 1 Lmin ; flow rate of 1, total pressure of 12 MPa, employing a 5:H molar
0
2
2
ꢀ
1
.5 mLmin ; 1:H =1:1.8.
2
ratio of 1:2.0. Under the conditions applied, 5 was
completely converted into benzyl alcohol 6 and tolu-
ene 7 withselectivities of 92 and 8%, respectively.
The 1% Pd/Amberlyst-15 and the commercial Pd
catalyst surface in a stable form and that no signifi-
cant metal leaching occurred upon exposure to the re- catalysts were next applied for the hydrogenation of
action conditions. Palladium in this type of catalyst is 5, although the conditions were different from those
also stable and does not show any leaching even in employed by Poliakoff et al. The results are shown in
polar ethanol solvent and at a higher temperature of Table 2. Highly selective formation of 6 was observed
[
6]
1508C, as recently demonstrated by Wissler et al.
witht eh 1% Pd/Al O catalyst, but the conversion
2 3
Figure 1 shows the variation of the yield and selec- was unsatisfactory from a synthetic point of view
tivity as a function of the total pressure at 608C. The (entry 1). Due to the low activity observed also for
total pressure was varied by changing the CO2 the hydrogenation of cyclohexene (this study), 1%
amount in the reactor, while feed rates of CO , 1, and Pd/Al O is not suitable for multiple bond hydrogena-
2
2
3
hydrogen were constant throughout the experiments. tions under the conditions applied. Both the 1% Pd/C
At 4 MPa, the reaction hardly proceeded, giving 2 in and 1% Pd/Amberlyst-15 yielded toluene 7, dibenzyl
[
12]
a low yield of 2%. Increasing the pressure from 4 to ether 8, and an unidentified product 9 in addition
MPa led to a slight increase in the yield. Further in- to 6 (entries 2 and 3). The detailed mechanisms of the
8
crease in the pressure up to 14 MPa resulted in a re- by-product formations are still unknown. However,
markable increase in the yield due to a drastic in- even if part of 6 was consumed for the production of
crease in the miscibility of hydrogen and 1 in scCO2, by-products, it can be stated that the activity for the
which was caused by the noticeable increase in CO2 hydrogenation of 5 to 6 was muchlower for 1% Pd/
density. Above 14 MPa, the yields were almost con- Amberlyst-15 than for 1% Pd/C, because the respec-
stant. Visual observation of the phase behavior con- tive yields of 6 and the by-products were higher for
ducted in a specially designed view-cell under similar 1% Pd/C than for 1% Pd/Amberlyst-15. Note the
conditions revealed that in the total pressure region striking fact that the activity of 1% Pd/Amberlyst-15
of 10–14 MPa the mixture of 1, H , and CO trans- for the hydrogenation of the C=C bond was almost
2
2
forms from a CO -expanded liquid-gas (two-phase equal to or even higher than that of 1% Pd/C
2
system) to a single homogeneous phase; this transi- (Table 1), whereas the two catalysts differed signifi-
tion is accompanied by a drastic increase of the reac- cantly in their activity for the reduction of the C=O
tion rate (Figure 1). Thus elimination of the gas-liquid bond, indicating the high selectivity of 1% Pd/Amber-
interface is crucial to achieve the highest performance lyst-15 for C=C bond hydrogenations under the con-
of 1% Pd/Amberlyst-15 as C=C bond hydrogenation ditions applied. The low activity of 1% Pd/Amberlyst-
catalyst.
15 for the hydrogenation of the C=O bond may be
Interestingly, relatively large amounts of 3 and 4 due to the following reasons: (i) the strong interaction
were formed in the pressure range 10–12 MPa, which between sulfonic acid groups and carbonyl oxygen of
led to the decrease in selectivity. It seems that the di- benzaldehyde inhibits the adsorption of the C=O
Adv. Synth. Catal. 2008, 350, 691 – 705
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asc.wiley-vch.de
693

Tsunetake Seki et al.
FULL PAPERS
[a]
Table 2. Reaction of benzaldehyde 5 withH 2 over the supported Pd catalysts in scCO2.
[b]
[b]
Entry
Catalyst
Conversion [%]
Selectivity [%]
[
c]
6
7
8
9
1
2
3
1%
1%
1%
Pd/Al O
Pd/C
Pd/Amberlyst-15
18
54
16
100
61
33
0
2
3
0
23
55
0
14
10
2
3
[
a]
Reaction conditions: catalyst, 1.00 g; total pressure, 16MPa; reactor temperature, 608C; flow rate of expanded gaseous
ꢀ
1
ꢀ1
CO , 1 Lmin ; flow rate of 5, 0.3 mLmin ; 1:H =1:2.0.
2
2
[
[
b]
c]
Determined by GC.
Unidentified product.
[12]
bond onto palladium particles; (ii) the benzaldehyde
molecules strongly adsorbed on acid sites cause steric
hindrance for the adsorption of other free benzalde-
hyde molecules onto palladium particles; the palladi-
um particles on neutral 1% Pd/C, on the other hand,
are probably easily accessible for polar benzaldehyde
molecules; (iii) the surface Pd atoms on 1% Pd/Am-
berlyst-15 are more cationic compared to those on
1% Pd/C and presumably interact only withteh
oxygen atom in the C=O bond, leading to an unfavor-
able adsorbed conformation for the hydrogenation
(
see “Structural Properties of 1% Pd/Amberlyst-15”
Scheme 1. Industrial manufacturing routes to 2-ethylhexanal
11, X=CHO) and 2-ethylhexanol (X=CH OH). Reaction
section); and (iv) the difference in the number of
(
2
active Pd sites for the C=O bond hydrogenation types: step 1, hydroformylation; step 2, aldol condensation;
might also be related to the different activity of 1% step 3, hydrogenation; step 4, dehydrogenation; step 5, aldol
Pd/Amberlyst-15 and 1% Pd/C (see “Structural Prop- condensation; step 6, hydrogenation.
erties of 1% Pd/Amberlyst-15” section).
[13]
gas-phase hydrogenation of 2-ethylhexenal (step 3).
Highly Efficient Continuous “One-Pot” Production
of 2-Ethylhexanal from Crotonaldehyde
The major problem of the conventional multi-step
process is that high capital cost plants are required
for product purification, recovery, and waste treat-
The key catalytic properties of 1% Pd/Amberlyst-15 ment particularly in the aldol condensation step
observed in the above two test reactions are: (i) high where homogeneous strong base catalysts must be
[
14]
activity for C=C bond hydrogenations; (ii) low activity handled safely. Another drawback is the high reac-
for C=O bond hydrogenations; and (iii) palladium- tion temperature required for the gas-phase hydroge-
acid bifunctionality. This prompted us to apply the nation. Thus several research groups focused on effi-
1
“
% Pd/Amberlyst-15-scCO catalytic system for the cient bifunctional catalytic systems that convert pro-
2
one-pot” synthesis that involves selective C=C bond pylene or butyraldehyde to the C chemicals in one-
8
hydrogenations and acid-catalyzed reactions. One pot pathways. Shell (USA and UK) and Exxon (USA
such reaction that has great significance in chemical and Japan) developed the “Aldox” process that af-
industry is the one-pot production of 2-ethylhexanal fords 2-ethylhexanol directly from propylene by using
from C aldehydes, which involves aldol condensation co-catalysts suchas compounds of Zn, Sn, Ti, Al or
4
and hydrogenation(s). In Scheme 1 are shown the in- Cu or KOH in addition to the original “oxo” cata-
[
13]
dustrial synthetic routes to 2-ethylhexanal and 2-eth- lyst. However, these processes have several inher-
ylhexanol. Typically these C compounds are manu- ent disadvantages: (i) a large excess of CO is re-
8
factured by several steps including the hydroformyla- quired; (ii) the liquid space velocity in the hydrofor-
tion of propylene (step 1), base-catalyzed aldol con- mylation is relatively low; and (iii) the selectivity to 2-
[
15b]
densation of butyraldehyde (step 2), and subsequent ethylhexanol is rather low.
Recently, Kelly and co-
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Palladium Supported on an Acidic Resin
FULL PAPERS
tion of many unidentified products in non-negligible
amounts, particularly in the later retention time
region (Figure 2), as well as 11.
We next applied the 1% Pd/Amberlyst-15-scCO2
catalytic system to crotonaldehyde 10 and found that
this a,b-unsaturated aldehyde is converted to 11 with
high selectivity. Table 3 gives the reaction results with
different catalysts and under different conditions. The
1% Pd/C catalyst withinert carbon support did not
afford 11 but yielded 12, 14, and many unidentified
products suchas dimers and trimers of 10 and their
derivatives (entry 2), whereas some formation of 11
was observed over 1% Pd/Al O (entry 1). Since alu-
Figure 2. GC profile of the product sample obtained from
the reaction of butyraldehyde 12 under the following condi-
tions: catalyst, 1.00 g of 1% Pd/Amberlyst-15; total pressure,
2
3
2ꢀ
ꢀ
mina support possesses basic sites (O and OH ) as
1
6 MPa; reactor temperature, 608C; flow rate of gaseous
3+
ꢀ
1
ꢀ1
well as acidic sites (Al ) and promotes aldol conden-
CO , 1 Lmin ; flow rate of 12, 0.3 mLmin ; 12:H =1:2.0.
2
2
[
17]
sation and related reactions,
it was expected that
Column temperature conditions: 508C (10 min) ! 208C
ꢀ
1
min ! 2508C (5 min). In the inset is shown a magnified the 1% Pd/Al O catalyst functioned as palladium-
2
3
view in the retention time range 1.5–2.5 min.
base bifunctional catalyst to give 11 directly from 10
via hydrogenation and aldol condensation. However,
the yield of 11 obtained with1% Pd/Al O was too
2
3
workers reported continuous one-step synthesis of 2- low from a practical point of view. In contrast, the use
ethylhexanal from butyraldehyde using Pd/Na/SiO as of 1% Pd/Amberlyst-15 catalyst afforded 11 in much
2
[
15]
solid base hydrogenation catalyst,
et al. developed HRhCO(PPh ) supported on hydro- also 2-ethylhexenal 13 was contained in the product
talcite as a novel Aldox catalyst.
while Sharma higher yields (entries 3–5). Although not only 12 but
AHCTREUNG
3
3
[
16]
However, these samples at a lower feed amount of H (entries 3 and
2
catalytic systems require relatively high reaction tem- 4), the content of 13 was almost negligible (~0% se-
peratures (3508C in the former and 200–2508C in th e lectivity) at a 10:H molar ratio of 1:4.0 (entry 5).
2
latter) due to the low reaction rates and the yields of Then the main by-product was only 12, which is also
the C products are moderate to low. In addition, sev- industrially important and can be easily separated
8
eral by-products are formed in bothsystems, w hi ch
from 11 by distillation owing to the large difference in
renders the product purification troublesome. More- their boiling points (11, bp 1628C; 12, bp 758C). Note
over, the Pd/Na/SiO catalyst is easily deactivated by also that the undesirable C=O bond hydrogenations
2
the adsorption of the C product, 2-ethylhexanal.
were strikingly suppressed in the 1% Pd/Amberlyst-
We first attempted the one-step synthesis of 2-eth- 15 catalytic system, giving 1-butanol 14 in 1% selec-
ylhexanal 11 from butyraldehyde 12 witht he 1% Pd/ tivity and no 2-ethylhexanol regardless of the feed
Amberlyst-15-scCO catalytic system. However, GC rate of H (10:H ratio=1:2.0–4.0), while 1% Pd/
8
2
2
2
analysis of the product samples indicated the forma- Al O and 1% Pd/C afforded 14 in a higher selectivity
2
3
[a]
Table 3. Reaction of crotonaldehyde 10 withH 2 over the supported Pd catalysts in scCO2.
[b]
[b]
Entry
Catalyst
10:H2
Conversion [%]
Selectivity [%]
1
1
12
13
14
1
2
3
4
5
1%
1%
1%
1%
1%
1%
Pd/Al O
Pd/C
Pd/Amberlyst-15
Pd/Amberlyst-15
Pd/Amberlyst-15
Pd/Amberlyst-15
1:2.0
1:2.0
1:2.0
1:3.0
1:4.0
1:4.0
93
94
89
94
98
13
8
1
47
59
67
<1
70
55
46
36
32
39
<1
<1
7
4
<1
24
9
9
1
1
1
<1
2
3
[
c]
6
[
a]
Reaction conditions: catalyst, 1.00 g; total pressure, 16 MPa; reactor temperature, 608C; flow rate of expanded gaseous
ꢀ
1
ꢀ1
CO , 1 Lmin ; flow rate of 10, 0.3 mLmin .
2
[
[
b]
c]
Determined by GC.
Total pressure, 4 MPa.
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Tsunetake Seki et al.
FULL PAPERS
of 9% even at the lowest feed rate of H (10:H = gradually decreased after 7 haccompanied by an in-
2
2
1
:2.0). The high activity of the active Pd sites on 1% crease in the selectivity to 12. These results indicate
Pd/Amberlyst-15 for the C=C bond hydrogenations that the Pd remained active on the surface during the
and the low activity of them for the C=O bond hydro- long-term operation, whereas the number of active
genations are in good agreement witht he results ob-
sulfonic acid groups gradually decreased during the
served in cyclohexene and benzaldehyde hydrogena- reaction by deterioration or decomposition. Highsta-
ꢀ
tions (see the sections: “Reaction of Cyclohexene” bility of palladium on Amberlyst 15 has been ob-
and “Reaction of Benzaldehyde”). The GC profile of served also in other reactions such as the continuous
a product sample obtained under the conditions of hydrogenation of cyclohexene in scCO (this study)
2
entry 5 in Table 3 is shown in Figure 3, indicating how and the one-pot synthesis of an analgesic in ethanol
[
6]
selective the conversion of 10 to 11 and 12 was.
solvent as described in the section “Reaction of Cy-
The good long-term catalytic behavior of 1% Pd/ clohexene”. Concerning the number of sulfonic acid
Amberlyst-15 for the reaction was already shown in groups, the thermogravimetric analyses (TGA-DTG)
[
7a]
our preliminary communication.
Here we provide supported the above speculation. Figure 4 shows the
ꢀ
selected data on the variation of the yield and selec- TGA-DTG curves of the Amberlyst 15, as-made 1%
tivity withtime-on-stream (Table 4). T he catalyst kept
yielding 11 and 12 exclusively withalmost 100% con-
Pd/Amberlyst-15, and 1% Pd/Amberlyst-15 spent,
that is, after the reaction under the conditions of
version for at least 12 h. The selectivities to 11 and 12 entry 5 in Table 3 (the time-on-stream was 3 h from
did not change significantly, but the selectivity to 11 the first sampling). Each sample exhibited characteris-
tic weight losses at ca. 90, 290, and 4208C, which cor-
respond to the desorption of H O/CO , SO/H O/CO ,
2
2
2
2
and C H /NO/Cl, respectively. The much smaller loss
x
y
in mass around the temperature of 2908C for the
spent 1% Pd/Amberlyst-15 than for the as-made 1%
Pd/Amberlyst-15 implied that the deterioration or de-
composition of the sulfonic acid groups occurred
during the reaction. Although the temperature of 1%
Pd/Amberlyst-15 under the reaction conditions (95–
1
208C) was below the maximum temperature recom-
ꢀ
mended for the use of Amberlyst 15 by Fluka
1208C), the modification with palladium and the
(
Figure 3. GC profile of the product sample obtained from high pressure conditions might promote the thermal
the reaction of crotonaldehyde 10 under the conditions of
entry 5 in Table 3. Column temperature conditions: 508C
decomposition of the surface sulfonic acid groups
[
18]
even below 1208C. The greater loss in mass at ca.
208C after use of the catalyst in the reaction is at-
ꢀ
1
(
10 min) ! 208C min ! 2508C (5 min). In the inset is
4
shown a magnified view in the retention time range 1–3 min.
Note that this is a sample without an internal standard sub-
stance. The peak of toluene (internal standard substance)
appears at a retention time of ca. 3.30 min.
tributed to the thermal decomposition of adsorbed re-
actant and products as well as the hydrocarbon
ꢀ
moiety of Amberlyst 15.
The importance of the use of high-pressure CO2
medium is obvious when one compares the result of
entry 5 (total pressure: 16 MPa) witht ha t of entry 6
Table 4. Variation of the conversion and selectivity with
(
total pressure: 4 MPa) in Table 3. The retarded hy-
time-on-stream in the reaction of crotonaldehyde 10 withH
2
[a]
drogenations owing to the increased mass transport
limitation of hydrogen led to poor results under the
latter conditions, because the conversion and the se-
lectivity to 11 were far lower, while the selectivity to
in scCO over 1% Pd/Amberlyst-15.
2
[b]
[c]
[c]
Time-on-stream [h] Conversion [%] Selectivity [%]
11
12 13
14
1
3 was much higher compared to those obtained
1
5
7
1
1
>99
>99
99
>99
99
70 28 <1
71 27 <1
67 32 <1
60 39 <1
55 44 <1
1
1
1
1
1
under the conditions of entry 5. A similar tendency
was already observed in the hydrogenation of cyclo-
hexene; the hydrogenation rate decreased with de-
0
2
creasing the CO pressure (see “Reaction of Cyclo-
2
hexene” section).
[
a]
Reaction conditions: catalyst, 1.00 g; total pressure,
6 MPa; reactor temperature, 608C; flow rate of expand-
The applied method may offer the following possi-
ble advantages to an industrial large-scale production
process: (i) the separation of organic compounds
(products and recovered reactant) from scCO fluid
can be easily performed by simple expansion of the
1
ꢀ
1
ꢀ1
ed gaseous CO , 1 Lmin ; flow rate of 10, 0.3 mLmin ;
2
1
0:H =1:4.0.
2
[
[
b]
The time-on-stream started from the first sampling.
Determined by GC.
2
c]
696
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Palladium Supported on an Acidic Resin
FULL PAPERS
Figure 5. GC profile of the product sample obtained from
the reaction of trans-2-hexenal 15 under the conditions de-
scribed in the caption of Scheme 2. Column temperature
ꢀ
1
conditions: 708C (10 min) ! 208C min ! 2508C (5 min).
Note that this is a sample without an internal standard sub-
stance. The peak of toluene (internal standard substance)
appears at a retention time of ca. 2.46 min.
the one-pot reaction due to the great miscibility of hy-
drogen as well as crotonaldehyde in scCO ; (iv) the
2
one-pot reaction takes place satisfactorily at a low re-
actor temperature of 608C due to the rapid hydroge-
nations in scCO ; (v) the catalyst exhibits a good
2
long-term behavior and resists deactivation caused by
the adsorption of reactant, products, CO , and by-
2
products (e.g., water and oligomers); and finally (vi)
crotonaldehyde containing stabilizers such as 2,6-di-
tert-butyl-p-cresol and H O can be used owing to the
2
catalystꢁs tolerance towards them.
Our approach ha s been successfully applied also to
trans-2-hexenal 15 as shown in Scheme 2 and
Figure 5. Thus under otherwise similar conditions, 2-
butyloctanal 16 and hexanal 17 were obtained in 30
and 70% selectivity at quantitative conversion. The
selectivity to the one-pot product 16 was lower com-
pared with that to 2-ethylhexanal in the reaction of
crotonaldehyde (Table 3, entry 5), presumably be-
cause the aldol condensation of 17 proceeds slower
than that of butyraldehyde due to the increased steric
constraints.
ꢀ
Figure 4. TGA-DTG curves of the Amberlyst 15 (a), as-
made 1% Pd/Amberlyst-15 (b), and 1% Pd/Amberlyst-15
spent in the reaction under the conditions of entry 5 in
Table 3 (the time-on-stream was 3 h from the first sampling)
(
c). For analysis conditions, see “Catalyst Characterization”
section.
Structural Properties of 1% Pd/Amberlyst-15
reaction mixture; (ii) the only by-product, butyralde-
hyde, is also industrially important and easily separat- The oxidation state of the catalysts was investigated
ed from 2-ethylhexanal by distillation; (iii) a slight using a specially constructed cell for X-ray absorption
excess amount of hydrogen is sufficient to promote spectroscopy, which allows “in situ” observation of
Scheme 2. Application of 1% Pd/Amberlyst-15-scCO catalytic system to the one-step production of 2-butyloctanal 16 from
2
trans-2-hexenal 15. Reaction conditions: catalyst, 1.00 g of 1% Pd/Amberlyst-15; total pressure, 16 MPa; reactor temperature,
ꢀ
1
ꢀ1
6
08C; flow rate of expanded gaseous CO , 1 Lmin ; flow rate of 15, 0.3 mLmin ; 15:H =1:4.0.
2
2
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Tsunetake Seki et al.
FULL PAPERS
Figure 6. XANES spectrum of the pre-reduced 1% Pd/Am-
berlyst-15 (black solid line) and 1% Pd/C (black broken
line), witht eh reference spectrum of Pd foil (solid gray
line). For analysis conditions, see “Catalyst Characteriza-
tion” section.
Figure 7. XPS spectra withPd 3 d5/2 and 3d3/2 lines for the as-
made 1% Pd/Amberlyst-15 (a) and pre-reduced 1% Pd/Am-
berlyst-15 (b). For analysis conditions, see “Catalyst Charac-
terization” section.
the catalysts during the reduction in a stream of hy-
drogen. Figure 6 shows the X-ray absorption spec- transition metals suchas Pd and Pt for hy drogena-
trum of 1% Pd/Amberlyst-15 and 1% Pd/C after the tions and hydrogenolyses, the interaction between the
Since acidic supports often enhance activity of the
[19]
reduction at 1008C for 1 hunder a 5 vol% H /Ar flow metal clusters and acidic supports has been examined
2
in terms of X-ray absorption near-edge structure in detail. Koningsberger and co-workers recently pro-
(
XANES). The spectrum of Pd foil is also shown for posed that the increase in activity with increasing sup-
comparison. As emerges from these spectra, most Pd port acidity can be ascribed to an increase in the ioni-
atoms on 1% Pd/Amberlyst-15 as well as 1% Pd/C zation potential of the metal particles directly induced
existed as metallic Pd(0). It is also noteworthy that by the Coulomb potential of the support oxygen ions
the XANES spectrum of 1% Pd/Amberlyst-15 did not of which electron density remarkably changes with
[
22]
change before, during, and after the reduction with the surface acidity.
Another possibility is the ad-
H , indicating that the palladium was virtually com- sorption of a “support proton” onto the clusters,
2
pletely reduced during the preparation of the catalyst which results in the withdrawing of electron density
[
23]
presumably by the reducing action of the ammonia from the surface Pd atoms. We speculate that the
[
5]
ꢀ
ligand of the precursor complex [Pd
A
H
R
N
(NH ) ]Cl . An- oxygen atoms in sulfonic acid groups (i.e., SꢀO and
3
4
2
other interesting point is that the spectrum of 1% Pd/ S=O) and their protons are bonded to the Pd particles
Amberlyst-15 implies the presence of some cationic on 1% Pd/Amberlyst-15 through Coulomb interac-
[
20]
palladium atoms on the particles.
firmed also by X-ray photoelectron spectroscopy ficial for firm immobilization of the Pd particles and
XPS), which showed a shift of Pd 3d5/2 binding also for the control of electron density on the surface
energy (BE) from 335.0 eV of metallic Pd(0) (Table 5 Pd atoms. The electron deficiency of the surface Pd
This was con- tions as shown in Figure 8. Such interactions are bene-
(
[21]
and Figure 7).
atoms should promote the adsorption of C=C bonds
Table 5. XPS data of the as-made and pre-reduced 1% Pd/Amberlyst-15.
C
1s BE
[eV]
N
1s BE
[eV]
O
1s BE
[eV]
S
Pd
3d5/2 BE
[eV]
Atomic
%
Atomic
%
Atomic
%
Atomic
%
2p BE
[eV]
Atomic
%
As-made sample
0.9 284.6
Pre-reduced sample
4.1 284.68
6
3.1
2.0
401.9
401.9
30.0
28.0
532.1
532.0
5.8
5.7
168.8
0.15
0.20
337.6, 342.5
337.6, 342.8
[a]
6
168.8,
1
64.0
[
a]
The pre-reduction was performed at 1008C for 1 hunder a H /N flow.
2
2
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Palladium Supported on an Acidic Resin
FULL PAPERS
hydrogen treatment to form metallic palladium parti-
cles on the resin.
Figure 9 shows the transmission electron microsco-
py (TEM) image of the as-made and pre-reduced 1%
Pd/Amberlyst-15, respectively. As the XANES spec-
Figure 8. Surface model of 1% Pd/Amberlyst-15. The inter- trum suggested, metallic Pd particles already existed
action of palladium particles witht he sulfonic acid groups
on the as-made 1% Pd/Amberlyst-15 even though it
leads to immobilization of the particles as well as generation
of several cationic surface Pd atoms.
had not undergone the hydrogen treatment, demon-
strating again that the precursor complex [Pd-
A
H
R
U
G
during the preparation process by the action of the
[
5]
and accelerate their hydrogenations, whereas it can NH ligand. No significant change in Pd particle size
3
suppress the hydrogenation of C=O bonds because was observed before and after the reduction. Powder
the cationic Pd atoms would bond only to the oxygen X-ray diffraction (XRD) analysis also indicated the
atoms in C=O bonds, which results in unfavorable ad- presence of metallic Pd particles on the as-made 1%
sorption modes for the hydrogenations. The activity Pd/Amberlyst-15 and no drastic change in the size
of 1% Pd/Amberlyst-15 was actually high and low for after the reduction with H (Figure 10). The Pd parti-
2
the hydrogenation of cyclohexene (see “Reaction of cle size estimated by the Scherrer formula was ca.
Cyclohexene” section) and benzaldehyde (see “Reac- 15 nm for botht he as-made and pre-reduced 1% Pd/
tion of Benzaldehyde” section), respectively. As will Amberlyst-15.
be discussed in the next section, the Pd-attached
The surface properties of palladium particles were
+
proton species (denoted [Pd ꢀH] ) is considered to investigated using carbon monoxide as a probe mole-
n
be the active “hybrid” site that can promote not only cule. The CO chemisorption assuming a stoichiometry
Pd-catalyzed reactions but also acid-catalyzed reac- of CO:Pdsurface =1:1 indicated that the dispersion of
[23]
tions.
Pd metals for 1% Pd/Al O , 1% Pd/C, and 1% Pd/
2 3
The possibility of the presence of small amounts of Amberlyst-15 were 21, 21, and 12%, respectively.
immobilized Pd(II) complexes formed by the reaction Note that the average stoichiometry of the CO chemi-
of [Pd
A
C
H
T
R
E
U
N
G
(NH ) ]Cl withsulfonic acid groups can virtual-
sorption was not the same for 1% Pd/Al O and 1%
3
4
2
2 3
ly be ruled out because the XANES spectrum did not Pd/Amberlyst-15, as the diffuse reflectance infrared
change before and after the reduction with H . In Fourier transform (DRIFT) spectroscopy investiga-
2
other words, the [Pd(NH ) ]Cl complex is completely tions revealed. The spectra are shown in Figure 11.
A
H
R
U
G
3 4 2
ꢀ
decomposed during the preparation step prior to the Since Amberlyst 15 itself adsorbs a large amount of
Figure 9. TEM image of the as-made (left) and pre-reduced 1% Pd/Amberlyst-15 (right). For analysis conditions, see “Cata-
lyst Characterization” section.
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Tsunetake Seki et al.
FULL PAPERS
Figure 12. DRIFT differential spectrum between 1% Pd/
Amberlyst-15 and Amberlyst 15 after pre-reduction and
subsequent exposure to CO. For analysis conditions, see
Figure 10. XRD pattern of the as-made (a) and pre-reduced
ꢀ
1% Pd/Amberlyst-15 (b). For analysis conditions, see “Cata-
lyst Characterization” section.
“
Catalyst Characterization” section.
ꢀ
CO, the spectrum of CO-exposed Amberlyst 15 was
subtracted from that of CO-exposed 1% Pd/Amber-
lyst-15, which is shown in Figure 12. On 1% Pd/Al O ,
2
3
ꢀ
1
CO was adsorbed in bothon-top (2100–2000 cm
)
ꢀ
1
and bridge mode (2000–1700 cm ) in accordance
[
24]
withprevious reports,
whereas the on-top CO was
predominantly formed on 1% Pd/Amberlyst-15
ꢀ
1
(
Figure 12; 2200–2100 cm ). Accordingly, the actual
difference in metal dispersion between 1% Pd/Al O3
2
and 1% Pd/Amberlyst-15 will be even larger than
that obtained on the assumption of CO:Pdsurface =1:1.
This behavior may be attributed to the presence of
+
[
Pd ꢀH] hybrid sites, which seems to be unfavorable
n
for bridge CO adsorption. It is also noteworthy that
the absorption band due to the on-top CO shifted to
a higher wavenumber for 1% Pd/Amberlyst-15 com-
pared to that of 1% Pd/Al O , indicating that the CO
2
3
adsorption on 1% Pd/Amberlyst-15 was weaker than
that on 1% Pd/Al O owing to less electron transfer
2
3
from metallic Pd to CO (back donation). A possible
+
scenario is that the adjacent [Pd ꢀH] hybrid sites
n
could withdraw electrons from the metallic Pd sites
active for CO adsorption. Note that a similar IR band
shift was observed also for CO adsorbed on the acidic
[
23]
zeolite NaHY-supported Pd catalyst.
Summarizing the characterization results, it seems
ꢀ
that the properties of palladium on Amberlyst 15 are
quite similar to those of palladium clusters on acidic
zeolites, and consequently the notion established by
[
23]
Sachtler and co-workers,
namely the presence of
+
[
Pd ꢀH] hybrid sites, may be applicable also to the
Figure 11. DRIFT spectrum of CO adsorbed on the pre-re-
n
duced 1% Pd/Al O (a) and 1% Pd/Amberlyst-15 (b). For 1% Pd/Amberlyst-15.
2
3
analysis conditions, see “Catalyst Characterization” section.
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Palladium Supported on an Acidic Resin
FULL PAPERS
Mechanistic Consideration of the One-Pot Formation
of 2-Ethylhexanal
can be the active sites for both hydrogenation and
aldol condensation. The reactants and the reaction in-
termediates in the one-pot reactions can continually
The use of butyraldehyde 12 as starting compound stay near the hybrid site until they are converted to
led to the formation of many byproducts (see “Highly the final products without shuttling between Pd and
Efficient Continuous “One-Pot” Production of 2-Eth- acid sites. We cannot, however, completely rule out
ylhexanal from Crotonaldehyde” section), whereas the possibility that the “free” sulfonic acid groups par-
such by-products are negligible when crotonaldehyde ticipate in the catalysis.
1
0 was employed. During the reaction of 10, however,
a large amount of 12 was formed and the scCO2
phase became almost similar to that when 12 was Conclusions
used as starting compound. Thus it can be speculated
ꢀ
that 10 acts as poison or inhibitor for the undesirable The acidic resin (Amberlyst 15)-supported palladium
side reactions as well as substrate during the reaction (Pd/Amberlyst-15) effectively promotes the continu-
and therefore the clean conversion could be achieved. ous catalytic C=C bond hydrogenations in scCO2
A
feasible reaction mechanism is shown in without affecting C=O bonds. In addition, owing to
Scheme 3. During the reaction of 10, the Pd sites and the metal-acid bifunctionality, the industrially impor-
the “free” sulfonic acid groups that do not interact tant 2-ethylhexanal could be synthesized with this cat-
with the Pd particles should be blocked by the rela- alytic system in a one-pot pathway from crotonalde-
tively strong adsorption of 10 on these sites, inhibiting hyde via hydrogenation and aldol condensation. For
their interaction with compounds other than 10 that these reactions, the use of scCO is crucial because it
2
can be the initial steps of the undesired byproduct for- lowers the gas-liquid mass transfer resistance and en-
mations. The multi-step reaction giving 2-ethylhexanal hances the diffusion through the external fluid film of
1
1, therefore, should not involve the desorption and the catalyst. Under these conditions, the reactions
re-adsorption step of the intermediates (i.e., 12, 3-hy- proceed withsatisfactory rate even at 60 8C. The low
droxy-2-ethylhexanal, and 2-ethylhexenal) but take temperature operation is attractive from an economi-
place in adsorbed manner throughout the reaction as cal point of view. In addition, it can suppress undesir-
+
depicted in Scheme 3. Here the notion of “
A
H
R
U
[Pd ꢀH]
able side reactions and extend the lifetime of the ther-
n
adduct” proposed by Sachtler and co-workers seems mally sensitive Pd/Amberlyst-15 catalyst. Actually,
to be important. The adduct is a “hybrid” site that good long-term catalytic behaviors were observed for
is able to function as metal and as acid site and thus
[23]
Scheme 3. Plausible reaction mechanism for the one-pot production of 2-ethylhexanal 11 from crotonaldehyde 10. Step 1,
adsorption of 10; step 2, H adsorption with heterolytic bond cleavage; step 3, addition of hydride to the carbocation; step 4,
2
acid-catalyzed keto-enol tautomerization (left) and proton addition (right); step 5, aldol condensation; step 6, adsorption of
the condensate; step 7, H adsorption with heterolytic bond cleavage; step 8, addition of hydride to the carbocation; step 9,
2
desorption of 11.
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Tsunetake Seki et al.
FULL PAPERS
the hydrogenation of cyclohexene and the one-pot
synthesis of 2-ethylhexanal.
A
H
R
U
(NH
3
)
4
]Cl
2
·H
2
O (ABCR, 99%, 99.99% Pt), [Ru
A
H
R
U
G
(
ABCR, 99%), and RhCl ·3H O (Merck, 38% Rh) were
3
2
+
used as metal precursors.
Metal-acid hybrid sites, [Pd ꢀH] species, are
n
The following commercially available catalysts were em-
ployed in order to compare the catalytic performance of 1%
Pd/Amberlyst-15 witht ho se of conventional catalysts. 1%
Pd/Al O (Engelhard, 5376) and 1% Pd/C (Engelhard, 5109)
speculated to be the active site for both hydrogena-
tion and aldol condensation, promoting the one-pot
synthesis of 2-ethylhexanal from crotonaldehyde.
However, further work will be necessary to elucidate
the role of these sites.
2
3
were crushed and used as sieved fractions (ca. 0.5–1.4 mm).
Commercially available high-grade cyclohexene (Fluka,
ꢁ
99.5%, stabilizer: ~0.01% 2,6-di-tert-butyl-p-cresol), ben-
zaldehyde (Aldrich, ꢁ99.5%, redistilled), butyraldehyde
Experimental Section
(Aldrich, ꢁ99.5%, redistilled), crotonaldehyde (Fluka,
ꢁ
99.5%, cis:trans=~1:20, stabilizer: ~0.1% 2,6-di-tert-
butyl-p-cresol and ~1% H O), and trans-2-hexenal (Acros,
Catalysts and Reagents
2
9
9%) were used without further purification.
As a typical example, preparation method of 1% Pd/Amber-
lyst-15 is described in detail. The palladium precursor of
Continuous-Flow Reactor System and Reaction
Procedures
[
Pd(NH ) ]Cl (ABCR, 99%; 0.0466 g, 0.190 mmol) and de-
A
C
H
T
R
E
U
N
G
3 4 2
ionized water (20.0 g) were mixed in a 50-mL flask contain-
ing a stirring bar. After stirring the mixture for homogeniza-
Safety note: The present experiments involve the use of rela-
tively high pressures and require equipment with the appro-
priate pressure rating.
ꢀ
tion, Amberlyst 15 (Fluka, 20–50 mesh, dry (moisture
~
5%), product number: 06423, lot number: 1166065; 2.00 g)
was added, and the resultant heterogeneous mixture was
stirred at 808C for 24 h. The solid material was separated by
filtration, washed with deionized water (15 mL), and dried
at 100–1108C overnight. The as-made material thus obtained
already had metallic palladium on the surface (see “Struc-
tural Properties of 1% Pd/Amberlyst-15” section) presuma-
bly through the auto-reduction by the ammonia resulting
The continuous-flow reactor system (NWA, Lçrrach, Ger-
many) employed in the present research is shown in
Figure 13. Commercially available CO2 (Pangas, 99.90%)
and H (Pangas, 99.995%) were used as received. The reac-
2
tions were performed witha tubular reactor (designed by
the ETH workshop) of which inner diameter and volume
were 13 mm and 35 mL, respectively. Glass beads were
placed above the catalyst to ensure an optimum flow distri-
bution and mixing of the substrate at the entrance of the
catalyst bed. Hydrogen was introduced through a 6-port
switching valve (Rheodyne, model 7000) dosing 0.05 mL
pulse at high pressure and constant frequency. Freshly
opened liquid substrate was fed to the reactor using an
HPLC pump (Gilson, model 305). The total pressure was
[5]
from the decomposition of the tetramine complex. Never-
theless, it was pre-reduced under an H /N flow at 1008C for
2
2
1
h just before the use for a catalytic reaction. The induc-
tively coupled plasma-optical emission spectroscopy (ICP-
OES) showed that the exact content of palladium in the
pre-reduced 1% Pd/Amberlyst-15 was 0.80 wt%. For the
synthesis of other supported transition metal catalysts, [Pt-
Figure 13. Schematic view of the continuous-flow high-pressure fixed-bed reactor system used in the present research.
702
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Palladium Supported on an Acidic Resin
FULL PAPERS
adjusted to a constant value during the reactions with the Catalyst Characterization
pressure regulator connected to the CO compressing unit
2
TGA-DTG was performed on a STA 449 C thermoanalyzer
Netzsch) with an OmniStar mass spectrometer (Pfeiffer
and the total fluid flow was controlled at the expansion unit.
The catalysts were first reduced in a stream of H /N at
(
2
2
Vacuum) attached to the exhaust via heated Teflon and
stainless steel lines (ca. 1508C). Typically, 50 mg of the
sample were placed in a crucible and heated in a stream of
1
008C for 1 hin t he reactor and stored under an N atmos-
2
phere overnight. Then the stream of CO /H /substrate mix-
2
2
ture was fed into the reactor to start the reaction.
ꢀ
1
He (50 mLmin ) from 30 to 8508C witha he ating ramp of
The product samples, which were taken every 20 min,
were collected in a glass vial containing a known amount of
n-decane or toluene (internal standard substance). The mix-
ture was subsequently diluted withet hy l acetate and subject-
ed to GC analysis to determine the composition. A GC ap-
paratus equipped withFID (Hewlett Packard, model HP
ꢀ
1
1
58C min .
X-ray absorption spectroscopy was performed witha spe-
cially constructed in situ cell at a synchrotron laboratory
beamline X1, HASYLAB at DESY, Hamburg, Germany)
(
[19]
using Si(311) double-crystal monochromator. The pre-re-
A
H
R
U
G
duction of the samples was performed at 1008C for 1 hin a
6
0
890) that had an HP-5 column (J&W Scientific, 30 m”
.32 mm”0.25 mm) was used employing the following
stream of 5 vol% H /Ar. The spectra of the samples were re-
2
corded before, during, and after the reduction at room tem-
perature, 1008C, and room temperature, respectively.
column temperature conditions: 508C (10 min) ! 208C
ꢀ
1
min ! 2508C (5 min) for the reaction of butyraldehyde
ꢀ
1
X-ray photoelectron spectroscopy (XPS) was performed
withan LHS11 MCD instrument (Leybold-Heraeus) using
Mg Ka radiation. The pre-reduction of 1% Pd/Amberlyst-15
was then performed at 1008C for 1 hunder an H /N flow.
and crotonaldehyde; 708C (10 min) ! 208C min ! 2508C
(
5 min) for the reaction of cyclohexene, benzaldehyde, and
trans-2-hexenal. After steady state conditions had been
reached after ca. 3 h, conversion and selectivity were deter-
mined using the following equations. Average conversion
and selectivity obtained from four samples were reported.
2
2
The sample was put on a sample holder, evacuated in a load
ꢀ
6
lock to 10 mbar, and transferred to the analysis chamber
ꢀ
9
of which the pressure was typically <10 mbar. The peaks
were energy-shifted to the binding energy of Al 2s
(
118.5 eV) to correct for the charging of the material. The
surface composition of the sample was determined from the
peak areas that were computed after subtraction of the Shir-
ley-type background by empirically derived cross-section
[26]
factors.
Powder X-ray diffraction (XRD) analysis was performed
witha D5000 diffractometer (Siemens) using Cu K a radia-
tion in step mode between 15 and 658 2q witha step size of
ꢀ
1
The products were identified by comparing the GC reten-
tion times witht ho se of commercially available aut he ntic
samples, GC-MS (Hewlett Packard, model HP 5973) with
an analyzing program (Enhanced ChemStation, G1701BA,
Version B.01.00, Hewlett Packard), and NMR spectroscopy.
All the products described in this work are known com-
pounds.
0
.018 and 0.3 s step . The 1% Pd/Amberlyst-15 previously
used in the CO chemisorption experiment (vide infra) was
employed as the pre-reduced sample.
TEM investigations were performed on a Tecnai F30 mi-
croscope (FEI (Eindhoven); field emission cathode, operat-
ed at 300 kV). Then the pre-reduction of 1% Pd/Amberlyst-
1
5 was performed at 1008C for 1 hunder a H /N flow.
2 2
The CO chemisorption was carried out with an ASAP
Phase Behavior Measurements
2010 instrument (Micromeritics). The sample was flushed
withHe at 100 8C for 1 h, evacuated at the same tempera-
ture, and cooled down to 408C after which a leak test was
performed. Then the sample was pre-reduced at 1008C for
Visual observation of phase behavior under the reaction
conditions was performed with a high-pressure view-cell
which was designed similarly to a commercial screw-type
1
hin a stream of pure H 2 and then evacuated at the same
[25]
manual pump (SITEC-Sieber Engineering, Switzerland).
temperature. Two isotherms, each composed of 11 points,
were then measured at 408C. The first isotherm correspond-
ed to the physisorbed and chemisorbed CO. After evacua-
tion at 408C for 1 h, the second isotherm was measured,
which corresponded to the physisorbed CO. From the differ-
ence between the two isotherms, the fraction of chemisor-
bed CO was determined. For the calculation of metal disper-
sion, a stoichiometric CO:Pdsurface ratio of 1:1 was assumed
and the exact Pd content of 0.80 wt% determined by ICP-
OES was applied for 1% Pd/Amberlyst-15.
Into the cell adjusted to 63 mL volume, cyclohexene 1 was
introduced at room temperature, followed by the addition
of gaseous H and compressed liquid CO in that order. The
2
2
stirred mixture was heated up from room temperature to
08C and then the molar ratio of 1:H :CO in the cell was
6
2
2
approximately 1:1.8:9.1 corresponding to the ratio of the
flow rates (see caption of Figure 1). Under the conditions,
the total pressure reached 11 MPa and the system was com-
posed of gaseous and liquid phase. The total pressure was
increased by introducing additional amounts of compressed
DRIFT spectra were recorded at room temperature with
liquid CO witha constant cell volume of 63 mL to make
ꢀ1
2
a resolution of 4 cm using an EQUINOX 55 spectrometer
the conditions similar to those of Figure 1.
(
Bruker Optics) equipped witha liquid N -cooled MCT de-
2
tector and an HVC-DRPZ reaction chamber (Harrick). Typ-
ically, 10 mg of sample diluted with90 mg of KBr was pre-
reduced at 1008C for 1 hin a stream of 10 vol% H /Ar and
2
cooled down to room temperature under pure Ar flow.
Adv. Synth. Catal. 2008, 350, 691 – 705
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
703

Tsunetake Seki et al.
FULL PAPERS
Then the flow was switched to 10 vol% CO/Ar for 45 min
and subsequently to pure Ar for 45 min, followed by the IR
measurement. Spectra were collected by adding 200 scans
[4] G. Gelbard, Ind. Eng. Chem. Res. 2005, 44, 8468.
[5] a) W. Laufer, W. F. Hoelderich, Chem. Commun. 2002,
1684; b) W. Laufer, J. P. M. Niederer, W. F. Hoelderich,
Adv. Synth. Catal. 2002, 344, 1084.
ꢀ
for 1% Pd/Al O and 1000 scans for Amberlyst 15 and 1%
2
3
Pd/Amberlyst-15.
[6] M. C. Wissler, U.-P. Jagusch, B. Sundermann, W. F.
Hoelderich, Catal. Today 2007, 121, 6.
[
7] a) T. Seki, J.-D. Grunwaldt, A. Baiker, Chem.
Commun. 2007, 3562; b) Chem. Technol. 2007, 4, T75.
Acknowledgements
[8] M. Di Girolamo, M. Lami, M. Marchionna, E. Pescar-
ollo, L. Tagliabue, F. Ancillotti, Ind. Eng. Chem. Res.
1997, 36, 4452.
T.S. thanks Prof. Dr. K. Domen and Ms. M. Tomita (The
University of Tokyo) for supporting his activity as a Research
Fellow of the Japan Society for the Promotion of Science.
This work is partially supported by the grant-in-aid from the
Japan Society for the Promotion of Science (project code:
[
9] M. Burgener, D. Ferri, J.-D. Grunwaldt, T. Mallat, A.
Baiker, J. Phys. Chem. B 2005, 109, 16794.
[
[
10] J. F. Brennecke, J. E. Chateauneuf, Chem. Rev. 1999,
9, 433.
9
070300000755). Beamtime granted by HASYLAB and finan-
11] A. O. King, R. D. Larsen, E.-i. Negishi, in: Handbook
of Organopalladium Chemistry for Organic Synthesis,
cial support by the European Community (Contracted RII3-
CT-2004–506008) is gratefully acknowledged. The authors
also thank Dr. Frank Krumeich for the electron microscopic
measurements performed at the electron microscopy center of
ETH Zurich (EMEZ). The ICP-OES analysis was per-
formed at ENVILAB (Zofingen, Switzerland).
(
2
Ed.: E.-i. Negishi), John Wiley and Sons, New York,
002, p. 2719.
[
12] We could not determine the precise structure of 9 due
to the low yield. However, the GC-MS analysis implied
that 9 is an acetal formed by the reaction of 5 with 6.
The GC-MS data was as follows: m/z=51, 65, 77, 91,
1
07, 197.
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ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 691 – 705

Palladium Supported on an Acidic Resin
FULL PAPERS
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asc.wiley-vch.de
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