Membrane-occluded gold-palladium nanoclusters as heterogeneous catalysts for the selective oxidation of alcohols to carbonyl compounds

Article abstract of DOI:10.1002/adsc.200700553

Pre-formed polyvinylpyrrolidone-stabilized gold-palladium clusters, consisting of 80 mol% gold and with a mean size of 1.9 nm, were immobilized quantitatively in a porous polyimide membrane via the process of phase inversion, without loss of metal nanodispersion. The obtained gold-palladium/polyimide membrane emerged as a highly active heterogeneous metal catalyst for the amide-phase and solvent-free oxidation of aliphatic, allylic and benzylic alcohols with full selectivity to the corresponding aldehydes and ketones, and could be recycled with excellently preserved catalytic activity and product selectivity. Occlusion of the optimized bimetallic clusters in the polyimide structure proved beneficial in view of their superior catalytic performance compared to the analogous colloidal gold-palladium clusters.

Full text of DOI:10.1002/adsc.200700553

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DOI: 10.1002/adsc.200700553
Membrane-Occluded Gold-Palladium Nanoclusters as
Heterogeneous Catalysts for the Selective Oxidation of
Alcohols to Carbonyl Compounds
P. G. N. Mertens,a P. Vandezande ,a X. Ye,b H. Poelman,c D. E. De Vos,a
and I. F. J. Vankelecoma,*
a
Centre for Surface Chemistry and Catalysis, Faculty of Bioscience Engineering, Katholieke Universiteit Leuven,
Kasteelpark Arenberg 23 – box 2461, 3001 Leuven, Belgium
Phone: (+32)-16-321594; fax: (+32)-16-321998
E-mail: Ivo.Vankelecom@biw.kuleuven.be
b
Department ofMetallurgy and Materials Engineering, Faculty ofEngineering, Katholieke Universiteit Leuven,
Kasteelpark Arenberg 44 – box 2450, 3001 Leuven, Belgium
Department ofSolid State Science, Faculty ofSciences, University ofGent, Krijgslaan 281 S1, 9000 Gent, Belgium
c
Received: December 22, 2008; Revised: February 19, 2008; Published online: April 15, 2008
Abstract: Pre-formed polyvinylpyrrolidone-stabi-
lized gold-palladium clusters, consisting of80 mol%
gold and with a mean size of1.9 nm, were immobi-
lized quantitatively in a porous polyimide mem-
brane via the process ofphase inversion, without
loss ofmetal nanodispersion. The obtained gold-
oxidation.[4] Successful homogeneous catalysts based
on, e.g., Pd and Ru have been introduced before.[5]
Apart from metal-based systems, efficient heterogene-
ous catalysts such as heteropolyacids, hydrotalcites,
mixed oxides and molecular sieves have been pro-
posed.[6] With respect to metal catalysts, mostly Pd-
and Pt-based catalysts have been investigated, despite
their sensitivity to deactivation through overoxidation
and chemical poisoning.[3b,7]
Recently, the suitability ofmono- and bimetallic Au
clusters deposited on carbon or oxide supports for the
liquid-phase oxidation ofalcohols has been demon-
strated.[8] The advantageous introduction ofAu result-
ed in more active, selective and durable oxidation cat-
alysts. Concurrently, Au clusters immobilized in poly-
meric structures have been introduced as promising
metal catalysts for the aerobic alcohol oxidation.[9]
These Au clusters were enclosed by polyvinylpyrroli-
done (PVP) polymers,[9a] embedded in microgels with
pyridine functional groups,[9b] incarcerated in a poly-
styrene matrix[9c] or encapsulated by vinyl ether star
polymers[9d]. Besides their often tedious synthesis,[9c,d]
the predominant disadvantage ofsuch polymer-stabi-
lized Au clusters is their recovery from the reaction
mixtures, which involves separation procedures like
centrifugation, filtration, decantation, etc.
palladium/polyimide membrane emerged as
a
highly active heterogeneous metal catalyst for the
amide-phase and solvent-free oxidation of aliphatic,
allylic and benzylic alcohols with full selectivity to
the corresponding aldehydes and ketones, and
could be recycled with excellently preserved cata-
lytic activity and product selectivity. Occlusion of
the optimized bimetallic clusters in the polyimide
structure proved beneficial in view of their superior
catalytic performance compared to the analogous
colloidal gold-palladium clusters.
Keywords: alcohol oxidation; bimetallic catalyst;
catalytic membrane; gold-palladium; phase inver-
sion; solvent-free oxidation
The selective oxidation ofalcohols to aldehydes or
ketones is a key process in the production ofifne
In recent years, metal clusters have also been im-
chemicals.[1] Alcohols are traditionally oxidized by mobilized in insoluble polymer matrices to facilitate
stoichiometric amounts ofcostly metal oxidants like
catalyst recycling. Au clusters supported on, for in-
chromate and permanganate, inevitably leading to the stance, an ion exchange resin were successfully ap-
[10]
formation of environmentally noxious waste prod- plied in the oxidative carbonylation ofamines,
ucts.[2] More benign processes employ O2 or H2O2 as while an amphiphilic resin containing Pd nanoparti-
the oxidizing reagent in the presence ofa catalyst,
cles proved useful for the aqueous phase oxidation of
with H2O as the harmless by-product.[3] Both homoge- alcohols.[11]
neous and heterogeneous catalysts have already
Inspired by these recent findings, the objective of
proven their suitability for the liquid-phase alcohol this work was the immobilization ofpreo-frmed,
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PVP-protected Au0-Pd0 clusters in a porous polyimide
The Au0 nanocolloids (Ø 1.8 nm) synthesized with
(PI) matrix, thereby generating a catalytic membrane a molar PVP/Au ratio of100 emerged as the most
able to oxidize a wide scope ofalcohol substrates in
active catalysts with a turnover frequency (TOF) of
solvent-free oxidation conditions, and easily recovera- 220 hÀ1 after 1 h at a molar benzyl alcohol/Au ratio of
ble from the organic reaction medium. The research 1000. Au0 sols with similar cluster size but more PVP
approach comprised three successive experimental encapsulation were less active. This can be explained
phases. Firstly, PVP-stabilized Au0-Pd0 clusters were in terms of increased diffusion limitation and active
evaluated as quasihomogeneous metal catalysts for site blocking on the catalytic Au0 surface as a conse-
the benzyl alcohol oxidation in N,N-dimethylform- quence ofthe excess PVP. On the other hand, the less
amide (DMF), and optimized in terms ofcluster com-
protected, more reddish Au0 sols with distinct surface
position and size to maximize the oxidation rates. plasmon resonance (SPR) bands between 515 and
Subsequently, a straightforward procedure was devel- 530 nm in the UV-Vis spectrum, displayed lower cata-
oped for the quantitative immobilization of the opti- lytic activities as a result oftheir inef rior Au nanodis-
mized Au0-Pd0 colloids in a PI matrix via the well- persion.
known process ofduiffsion-induced phase inversion
Next, Pd-doped Au0 colloids were assessed in view
where a polymer solution is converted into a solidi- ofthe recent successes reported with heterogeneous
fied porous membrane after immersion in a non-sol- Au-Pd catalysts in alcohol oxidations.[17,18] Bimetallic
vent for the polymer, typically water.[13] After synthe- sols with varying Au/Pd ratios were prepared at a
sis, the obtained membrane was modified by chemical constant PVP/metal ratio of100 to investigate the po-
cross-linking to achieve stability in amidic and aro- tential synergetic effect between Au and Pd. Via XPS
matic media.[14] The potential ofthe thus developed
analysis it was found that the bimetallic sols consisted
Au0-Pd0/PI catalytic membrane as a heterogeneous ofzerovalent Au and Pd, with Au 4 f7/2 and Pd 3d5/2
catalyst was then assessed in the amide-phase oxida- levels at binding energies of84.0 eV and 335.8 eV, re-
tion ofvarious types ofalcohols and the solvent-rfee
oxidation of1-phenylethanol.
spectively. Energy dispersive X-ray (EDX) spot anal-
ysis upon TEM characterization revealed that the bi-
PVP-stabilized monometallic Au clusters were syn- metallic sols consisted ofAu-Pd clusters as ofund
thesized by chemical hydride reduction, using molar before.[19]
vinylpyrrolidone monomer/Au ratios varying between
Using the same evaluation conditions in the benzyl
10 and 200.[9a,12] DMF was selected as the synthesis alcohol oxidation as previously applied for the mono-
medium as it facilitates the dissolution of the Au pre- metallic Au0 catalyst, a colloidal bimetallic catalyst
cursor and displays a high affinity for PVP.[15] This fa- with molar Au/Pd ratio of8:2 emerged as the most
vours the unfolding of the PVP chains and the protec- active, with a TOF of580 h À1. This sharp turnover
tion ofthe Au dispersion. X-ray photoelectron spec-
rate increase cannot be due to a metal dispersion
troscopy (XPS) analysis revealed an Au 4f7/2 core effect, as the optimal Au0-Pd0 colloids had a mean
level at a binding energy of84.0 eV, proving that all
Au species in the various Au sols are in the metallic
cluster diameter of1.9 nm (Figure 1a).
For comparison, the (8:2)Au0-Pd0 sols were pre-
state. For molar PVP/Au ratios between 10 and 100, pared with varying PVP/metal ratios and thus differ-
the gradual rise in PVP concentration at constant Au ent sizes ofthe bimetallic clusters. The ratio of100
content resulted in decreasing Au0 cluster sizes, as de- was confirmed to correspond to the most appropriate
termined by transmission electron microscopy PVP concentration to maximize the benzyl alcohol
(TEM). For the reddish brown Au0 nanosols prepared oxidation rate. Calculations on the EDX spot data, re-
with molar PVP/Au ratiosꢀ100, no further increase corded for individual metal clusters, revealed that the
in Au nanodispersion was observed. The average Au0 optimized bimetallic clusters consisted of80.5 mol%
0
cluster size ofthe Au colloids varied between 1.8 nm Au and 19.5 mol% Pd. The Au/Pd ratio ofthe clusters
(PVP/Au=200) and 6.2 nm (PVP/Au=10).
thus clearly reflects the ratio applied for the synthesis
These amidic Au0 nanosols were then tested as qua- ofthe optimal bimetallic sol.
0
sihomogeneous metal catalysts in the oxidation of
benzyl alcohol at 353 K and 2.0 MPa O2. In contrast brane was then realized by adding the optimized bi-
Immobilization ofthe Au -Pd0 clusters in a mem-
to earlier work,[16] promotion ofthe colloidal nano-
metallic nanosol to a stirred PI solution. The obtained
Au0 catalyst with an extra added base was not re- homogeneous solution was cast on a glass plate, after
quired to achieve high turnover rates. The weakly which the nascent film was solidified in water via im-
basic properties ofboth the protective polymer and
mersion-precipitation, thus forming a porous PI mem-
the amide solvent proved sufficient to promote alco- brane (Figure 1b, c) with occluded (8:2)Au0-Pd0 clus-
hol oxidation. In all experiments, benzaldehyde ters. No leaching ofdark-coloured bimetallic colloids
emerged as the sole reaction product. Typical by- from the solidifying PI film into the coagulation bath
products like, e.g., benzoic acid or benzyl ben- was detected by inductively coupled plasma atomic
zoate,[16,17] could not be detected in the product pool. emission spectrophotometry (ICP-AES), showing that
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Membrane-Occluded Gold-Palladium Nanoclusters as Heterogeneous Catalysts
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Figure 1. a) TEM picture (8:2)Au0-Pd0 nanocolloids; b) SEM picture cross-section catalytic (8:2)Au0-Pd0/PI membrane; c)
High magnification SEM picture porous matrix structure (8:2)Au0-Pd0/PI membrane; d) TEM picture (8:2)Au0-Pd0 clusters
occluded in PI membrane.
the heterogenization procedure proceeded quantita- p-xylylenediamine solution for cross-linking of the po-
tively. TEM characterization confirmed that the bi- lymer (Figure 2).[14,20] This is required for polymeric
metallic nanodispersion was unaffected throughout membranes to ensure chemical stability in demanding
the immobilization (Figure 1d). A typical membrane media, such as aromatics or amides that would cause
had a metal loading of40 mmol per g, or 0.16 mmol excessive swelling or even full dissolution of the mem-
per cm2 ofPI membrane.
branes.[13a] After 6 h, a sufficient number of imide
After synthesis, the PI membrane with occluded bi- groups ofthe PI chains had been opened and cross-
metallic clusters was immersed in a methanolic linked to allow complete solvent stability, as proven
Figure 2. Scheme ofthe synthesis ofthe catalytic membrane.
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P. G. N. Mertens et al.
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by diffuse reflectance infrared Fourier transform spec- from the catalytic membrane occurred during the oxi-
troscopy (DRIFTS). In comparison with a non-cross- dation experiments, as determined with ICP-AES.
linked PI membrane, a gradual increase ofthe IR
The superior performance of the heterogenized
signal around 3250 cmÀ1 was observed, characteristic (8:2)Au0-Pd0 clusters is probably ascribable to the par-
À
for the N H stretch vibrations ofamidic bonds. The
tial withdrawal ofPVP rfom the bimetallic clustersꢁ
cross-linking procedure did not affect the physical surface as a result of the high affinity between PVP
pore structure ofthe catalytic membrane as con-
firmed via scanning electron microscopy (SEM).
The (8:2)Au0-Pd0/PI membrane was then assessed
and PI, and moreover to the very open structure of
the catalytic membrane.
Different types of substrates were then studied,
as a heterogeneous metal catalyst in alcohol oxida- that is, primary and secondary alcohols ofthe benzyl-
tions. The effects of O2 pressure (0.5–4.0 MPa O2) and ic, allylic and aliphatic type. In each case, a total che-
reaction temperature (353–433 K) were studied for moselectivity to the derived aldehydes and ketones
the catalytic oxidation ofbenzyl alcohol in DMF. A
slight decrease in oxidation rate ofat most 3% was
was observed, and no isomerisation products were
discerned for the allylic alcohol substrates, as repre-
observed for pressures below the optimum pressure sented in Table 1. The oxidation rate over the catalyt-
of2.0 MPa O , while higher O2 pressures did not lead ic membrane was found to be highly dependent on
2
to higher TOF values. While the O2 pressure had a the alcohol structure, with a higher reactivity for sec-
limited effect, a temperature increase strongly raised ondary alcohols in comparison with primary alcohols
the oxidation rate, even though the selectivity some- and moreover a decrease in alcohol reactivity in the
what declined at the highest temperatures due to for- order benzylic>allylic>aliphatic.
mation ofbenzene. At 2.0 MPa O
pressure and
The catalytic (8:2)Au0-Pd0/PI membrane was also
2
413 K, the TOF for the benzyl alcohol oxidation applied in solvent-free conditions for the oxidation of
amounted to 16520 hÀ1 at 99% conversion (Table 1), 1-phenylethanol (Table 2). Full selectivity to aceto-
meaning a significant improvement as compared to phenone was maintained over the temÀp1erature range
the
non-heterogenized
(8:2)Au0-Pd0
catalyst studied. A maximum TOF of11720À01h
was reached
was obtained at
(12870 hÀ1), with a maintained full selectivity to ben- at 433 K, while a value of49170 h
zaldehyde. Under the same conditions, the deter- 413 K (Table 2, entry 4), constituting a 10% increase
mined TOF values were 22190 hÀ1, 20650 hÀ1 and compared to the 1-phenylethanol oxidation in DMF
18830 hÀ1 at, respectively, 25%, 50% and 75% benzyl (Table 1, entry 2). At 353 K an oxidation rate of
alcohol conversion. No leaching ofAu or Pd species
2130 hÀ1 was found, corresponding to a 40% increase
with respect to the 1-phenylethanol oxidation in DMF
over the optimized (8À:21 )Au0-Pd0 nanocolloids which
Table 1. Alcohol oxidation[a] over Au0-Pd0/PI.
led to a TOF of1480 h
.
Finally, recycling experiments were performed to
assess the durability ofthe catalytic membrane at two
reaction temperatures (Table 3). In both cases the cat-
alytic membrane retained its activity in the solvent-
free oxidation of 1-phenylethanol very well. In the
fourth reuse, >97% ofthe oxidation rate ofthe ifrst
run was still preserved, while total selectivity to aceto-
phenone was maintained.
Substrate (S)
S/Me[b]
[mol/
mol]
Time[c]
[min]
C[d]
[%]
TOF[e]
[hÀ1]
G
benzyl alcohol 50000
20000
1-phenyletha- 50000
32
72
12
66
15
60
19
78
25
60
53
60
30
66
24
99
24
99
26
93
25
94
24
98
25
96
25
95
22470
16520
58980
45030
2050
1850
1610
1420
1140
980
nol
50000
crotyl alcohol 2000
2000
1-octen-3-ol
3-octanol
1-octanol
1-butanol
2000
2000
2000
1000
2000
500
Table 2. Solvent-free 1-phenylethanol oxidation[a] over Au0-
Pd0/PI.
570
480
990
860
T (K)
S/Me [mol/mol]
C [%]
TOF[b] [hÀ1]
2000
1000
353
373
393
413
433
20000
20000
50000
200000
200000
11
33
39
26
59
2130
6540
19360
49170
117200
[a]
Conditions: (8:2)Au0-Pd0/PI (PVP/Me=100); DMF;
413 K; 2.0 MPa O2.
Me: metal.
Time: reaction time.
C: conversion.
[b]
[c]
[d]
[e]
[a]
Conditions: (8:2)Au0-Pd0/PI (PVP/Me=100); 1-phenyl-
ethanol; 2.0 MPa O2; 1 h.
TOF: turnover frequency at reported reaction time and
conversion level.
[b]
TOF: turnover frequency at reported reaction time and
conversion level.
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Table 3. Recycling experiments in the solvent-free 1-phenyl-
nificantly superior to the results achieved with mono-
metallic catalysts, and in the same activity range as
obtained with the PdAu/TiO2 catalyst (269000 hÀ1)[8b]
under identical oxidation conditions.
Summarizing, the evaluation ofPVP-stabilized
metal clusters as quasihomogeneous catalysts in alco-
hol oxidations, with experimental control over the
cluster composition and size, enabled to discern Au0-
Pd0 clusters with a mean size of1.9 nm and consisting
of80 mol% Au as the most active and selective metal
ethanol oxidation[a] over Au0-Pd0/PI.
T [K]
353
Run
S/Me [mol/mol]
C [%]
TOF[b] [hÀ1]
1
5
1
5
20000
20000
200000
200000
11
10
59
57
2110
2070
116400
113300
433
[a]
Conditions: (8:2)Au0-Pd0/PI (PVP/Me=100); 1-phenyl-
ethanol; 2.0 MPa O2; 1 h.
[b]
TOF: turnover frequency at reported reaction time and clusters. Via a straightforward synthesis procedure in-
conversion level.
volving simple immersion-precipitation, a robust sol-
vent-stable PI membrane was obtained, containing oc-
cluded Au0-Pd0 clusters with fully preserved metal
The benefits of the developed Au0-Pd0/PI mem- nanodispersion. The bimetallic nanoclusters immobi-
brane in comparison with polymer-stabilized Au lized in the PI membrane displayed enhanced activity
nanoclusters described in literature,[9] are obvious in compared to the analogous colloidal Au0-Pd0 catalysts.
view ofits easy synthesis and simple recovery rfom
Moreover, the catalytic membrane allowed working
the reaction mixture and thus its straightforward recy- in solvent-free conditions leading to an additional in-
cling with preserved catalytic performance in consecu- crease in alcohol oxidation rates, and could be recy-
tive oxidation runs. Moreover, the significantly higher cled with excellently preserved performance. These
0
activity ofthe membrane-occluded (8:2)Au -Pd0 clus- findings clearly demonstrate the potential of porous
ters as compared to the corresponding colloidal bi- polymeric membranes as supports for metal clusters.
metallic clusters reveals that the immobilization re-
sulted in a reduced diffusion limitation around the bi-
metallic clusters. Previous studies with polymer-stabi- Experimental Section
lized Au clusters did also not consider doping with Pd
and thus the synergetic effect observed here, which Preparation of Quasihomogeneous Metal Catalysts
clearly induces a significant increase in oxidation rate.
Furthermore, the catalytic membrane is applicable
in amidic media, with the inherent advantage ofufll
Mono- and bimetallic sols were prepared by addition of
varying amounts ofAuCl and PdCl2 (total metal (Me) con-
3
tent of0.1 mmol) to a stirred DMF (8 mL) solution contain-
selectivity to aldehyde products in the oxidation of
primary alcohols. This contrasts with the pronounced
formation of acid products reported elsewhere as a
ing PVP (MW=10000; 0.111–2.220 g with quantity incre-
ments of0.111 g). These variations in PVP content resulted
in molar ratios ofmonomeric vinylpyrrolidone to Me rfom
10 to 200. The applied variations in the composition ofthe
bimetallic sols led to molar Au/Pd ratios ranging from 10:0
to 0:10 with 10 equal intervals. After 12 h, a DMF solution
(2 mL) containing NaBH4 (0.4 mmol; molar ratio NaBH4/
MeClx =4) was added to generate the Me sol.
[9a,b]
consequence ofthe aqueous reaction medium.
Moreover, basic promotion ofthe membrane-occlud-
ed Au0-Pd0 clusters proved no pre-requisite to achieve
high oxidation rates, contrary to PVP-stabilized Au
clusters,[9a] and amphiphilic resin-anchored Pd clus-
ters.[11] The Au0-Pd0/PI membrane even affords the
opportunity to run the alcohol oxidation in solvent-
free conditions, which facilitates product purification
and further improves the specific oxidation activity of
the bimetallic clusters.
Preparation of Catalytic Membrane
A PI (Matrimidꢂ 9725 US; Huntsman) based catalytic mem-
brane was prepared as schematically shown in Figure 2.
First, Me sol (6 mL) was added to a stirred PI solution
(10 g; 15 wt%) in N-methylpyrrolidone (NMP). After ho-
In addition to these benefits, results obtained with
the developed (8:2)Au0-Pd0/PI membrane in the aero-
bic alcohol oxidation compare favourably with state- mogenization, this solution was cast (1.2 mminÀ1) on a glass
plate using an automatic film applicator, leading to a wet
film thickness of 250 mm. The solvent was allowed to evapo-
rate for 30 s after which the nascent film was immersed in
deionized H2O for 15 min. After synthesis, the obtained cat-
alytic membrane was cross-linked through immersion in a
methanolic p-xylylenediamine solution (12.5 wt%) for typi-
cally 6 h at 393 K. Finally, the modified catalytic membrane
was washed with methanol to remove excess cross-linker,
after which it was kept in DMF or 1-phenylethanol, depend-
of-the-art metal catalysts in terms of oxidation rates.
The solvent-free oxidation of 1-phenylethanol to ace-
tophenone at 433 K was considered as the reference
reaction here. With respect to supported monometal-
lic clusters, a Pd/hydroxyapatite catalyst led to a TOF
À1 [20]
value of9800 h
,
a Au/CeO2 catalyst resulted in an
À1 [21]
oxidation rate of12500 h
,
and Au/Cu5MgAl2Ox
catalyzed the 1-phenylethanol oxidation with a rate of
11750 hÀ1.[8d] The catalytic membrane introduced here
ing on its further use. Prior to application in the oxidation
À1
2
displays a TOF of117200 h
(Table 2), which is sig- experiments, the membrane was cut into pieces of0.25 cm .
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P. G. N. Mertens et al.
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ples by GC-MS. The TOF (hÀ1) represents the molar ratio
ofconverted alcohol substrate and Me, divided by the reac-
tion time.
Characterization of Catalysts and Catalytic
Membrane
Non-diluted monometallic Au sols were characterized by
UV-Vis spectroscopy to determine the wavelength position
ofthe SPR band maxima. Colloidal Me clusters were stud-
ied with TEM at 200 kV, after deposition of a droplet of a
diluted Me sol on a TEM grid. The catalytic membrane was
fixated in a resin after which microtome slices of the mem-
brane were characterized with TEM. The mean Me cluster
size was estimated as the mean diameter for an ensemble of
200 Me particles. For each Me sol, the composition of20 in-
dividual Me clusters was determined by EDX spot analysis
upon TEM analysis. Additionally, the Me sols were immobi-
lized on a glass plate to allow XPS analysis using a mono-
chromated 450 W Al Ka source.
Acknowledgements
PGNM thanks K.U.Leuven for a postdoctoral fellowship
grant. This research was done in the framework of an I.A.P.-
P.A.I. grant (IAP 6/27) on Supramolecular Catalysis spon-
sored by the Belgian Federal Government, and of a G.O.A.
grant from the Flemish Government.
References
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metal detection limits below 1 ppm. DRIFTS allowed us to
investigate the post-synthesis modification of the catalytic
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À
membrane. The presence ofN H stretching vibrations
around 3250 cmÀ1 was monitored since the cross-linking pro-
À
cedure results in the substitution ofimide groups (no N
H
À
bond) by amidic bonds (N H bond). For SEM, membrane
cross-sections were obtained by breaking the catalytic mem-
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Alcohol Oxidation Experiments
For the oxidations with quasihomogeneous Me0 catalysts,
DMF (0.6 mL), Me0 sol (0.4 mL; equivalent with 4 mmol of
Me) and alcohol substrate (S; 4 or 8 mmol) were mixed,
leading to molar S/Me ratios of1000 and 2000. Reaction
mixture for heterogeneously catalyzed amide-phase oxida-
tions consisted ofDMF (5 mL), the appropriate amount of
0
S and pieces ofthe cross-linked Au -Pd0/PI membrane, con-
stituting a total surface of 1.25 cm2 (0.2 mmol Me; S/Me=
50000, 20000) or 5.0 cm2 (0.8 mmol Me; S/Me=2000, 1000,
500). For the solvent-free oxidation of 1-phenylethanol, the
solvent was replaced by S (4.89 g; 40 mmol), with an S/Me
ratio of 20000 for the oxidations performed at 353 KꢁTꢁ
383 K, 50000 for the oxidations at 393 KꢁTꢁ403 K and
200000 for the oxidations at Tꢀ413 K. For the recycling ex-
periments, the reaction was analyzed after 1 h, and the
membrane pieces were washed three times in pure 1-phenyl-
ethanol. Fresh 1-phenylethanol was then added and the next
oxidation run was started. The oxidation experiments were
performed at reaction temperatures between 353 and 433 K
and O2 pressures from 0.5 to 4.0 MPa, with continuous O2
supply and magnetic stirring at 1000 rpm. All oxidation and
recycling experiments were performed in triplicate. A blank
experiment in the absence ofthe metal catalysts was always
performed to evaluate potential product formation as a
result ofautoxidation.
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