Carbon-carbon double bond versus carbonyl group hydrogenation: Controlling the intramolecular selectivity with polyaniline-supported platinum catalysts

Article abstract of DOI:10.1002/adsc.200800035

The use of polyaniline (PANI) as catalyst support for heterogeneous catalysts and their application in chemical catalysis is hitherto rather poorly known. We report the successful synthesis of highly dispersed PANI-supported platinum catalysts (particle sizes between 1.7 and 3.7 nm as revealed by transmission electron microscopy, TEM) choosing two different approaches, namely (i) deposition-precipitation of H₂PtCl₆ onto polyaniline, suspended in basic medium (DP method) and, (ii) immobilization of a preformed nanoscale platinum colloid on polyaniline (sol-method). The PANI-supported platinum catalysts were applied in the selective hydrogenation of the α,β-unsaturated aldehyde citral. In order to benchmark their catalytic performance, citral hydrogenation was also carried out by using platinum supported on the classical support materials silica (SiO₂), alumina (Al₂O₃), active carbon and graphite. The relations of the structural characteristics and surface state of the catalysts with respect to their hydrogenation properties have been probed by EXAFS and XPS. It is found that the DP method yields chemically prepared PtO₂ on polyaniline and, thus, produces a highly dispersed and immobilized Adams catalyst (in the β-PtO₂ form) which is able to efficiently hydrogenate the conjugated C=C bond of citral (selectivity to citronellal=87%), whereas reduction of the C=O group occurs with polyanilinesupported platinum (selectivity to geraniol/nerol=78%) prepared via the sol-method. The complete reversal of the selectivity between the preferred hydrogenation of the conjugated C=C or C=O group is not only particularly useful for the selective hydrogenation of α,β-unsaturated aldehydes but also unveils the great potential of conducting polymer-supported precious metals in the field of hitherto barely investigated chemical catalysis.

Full text of DOI:10.1002/adsc.200800035

FULL PAPERS
DOI: 10.1002/adsc.200800035
Carbon-Carbon Double Bond versus Carbonyl Group
Hydrogenation: Controlling the Intramolecular Selectivity with
Polyaniline-Supported Platinum Catalysts
a
a
a
b
c
Martin Steffan, Florian Klasovsky, Jürgen Arras, Christina Roth, Jçrg Radnik,
d
a,
Herbert Hofmeister, and Peter Claus *
Department of Chemistry, Ernst-Berl-Institute, Darmstadt University of Technology, Petersenstr. 20, 64287 Darmstadt,
a
Germany
Fax : (+49)-6151-164-4788; e-mail: claus@ct.chemie.tu-darmstadt.de
Institute for Materials Science, Darmstadt University of Technology, Petersenstr. 23, 64287 Darmstadt, Germany
Leibniz Institute for Catalysis Berlin, Richard-Willstätter-Str. 12, 12489 Berlin, Germany
Max-Planck-Institute of Microstructure Physics Halle, Weinberg 2, 06120 Halle, Germany
b
c
d
Received: January 18, 2008; Revised: February 20, 2008; Published online: May 9, 2008
Abstract: The use of polyaniline (PANI) as catalyst genation properties have been probed by EXAFS
support for heterogeneous catalysts and their appli- and XPS. It is found that the DP method yields
cation in chemical catalysis is hitherto rather poorly chemically prepared PtO on polyaniline and, thus,
2
known. We report the successful synthesis of highly produces a highly dispersed and immobilized Adams
dispersed PANI-supported platinum catalysts (parti- catalyst (in the b-PtO form) which is able to effi-
2
cle sizes between 1.7 and 3.7 nm as revealed by ciently hydrogenate the conjugated C=C bond of
transmission electron microscopy, TEM) choosing citral (selectivity to citronellal=87%), whereas re-
two different approaches, namely (i) deposition-pre- duction of the C=O group occurs with polyaniline-
cipitation of H PtCl onto polyaniline, suspended in supported platinum (selectivity to geraniol/nerol=
2
6
basic medium (DP method) and, (ii) immobilization 78%) prepared via the sol-method. The complete re-
of a preformed nanoscale platinum colloid on poly- versal of the selectivity between the preferred hydro-
aniline (sol-method). The PANI-supported platinum genation of the conjugated C=C or C=O group is not
catalysts were applied in the selective hydrogenation only particularly useful for the selective hydrogena-
of the a,b-unsaturated aldehyde citral. In order to tion of a,b-unsaturated aldehydes but also unveils
benchmark their catalytic performance, citral hydro- the great potential of conducting polymer-supported
genation was also carried out by using platinum sup- precious metals in the field of hitherto barely investi-
ported on the classical support materials silica gated chemical catalysis.
(
SiO ), alumina (Al O ), active carbon and graphite.
2 2 3
The relations of the structural characteristics and sur- Keywords: Adams catalyst; citral hydrogenation;
face state of the catalysts with respect to their hydro- platinum catalyst; polyaniline
Introduction
industry. In particular, citronellal is formed by hydro-
genation of the olefinic bond which is conjugated to
In chemical synthesis heterogeneous hydrogenation the C=O group whereas hydrogenation of the latter
catalysts are most useful for the reduction of various yields the unsaturated, allyl-type alcohols in the trans
functional groups and, thus, are widely applied in in- and cis forms (geraniol and nerol, respectively).
dustrial processes where the selective hydrogenation Under certain reaction conditions and depending on
of a,b-unsaturated aldehydes is one of the most im- the catalyst type, consecutive hydrogenation reactions
[
1]
portant reactions, especially for fine chemicals.
can occur leading to citronellol, dihydrocitronellal
Citral (3,7-dimethyl-2,6-octadienal) belongs to this and 3,7-dimethyloctanol (Scheme 1). The selectivity
class of organic compounds and has three unsaturated (C=O vs. C=C group hydrogenation) can often be
bonds (a conjugated system comprised of C=C and controlled by the nature of the individual metal, the
C=O groups as well as an isolated C=C bond). Citral presence of a second metal (bimetallic catalysts),
hydrogenation leads to industrially important com- metal particle size (dispersion), electron-donating or
pounds, for example, for the perfumery and flavoring -withdrawing ligand effects induced by the catalyst
Adv. Synth. Catal. 2008, 350, 1337 – 1348
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Martin Steffan et al.
FULL PAPERS
Scheme 1. Reaction network of citral hydrogenation, 3,7-dimethyl-2-octenal (1), dihydrocitronellal (2), 3,7-dimethyl-2-octe-
nol (3).
[
8]
support material, steric constraints in the metal envi- feedstock; the synthesis of the intrinsically conduc-
ronment and strong metal-support interactions tive macromolecule was described manifold in the lit-
[
9]
(
SMSI). The influence of the above-mentioned pa- erature. PANI is of strong interest because of its en-
rameters on activity and selectivity of catalysts for the vironmental stability, large conductivity range, good
selective hydrogenation of a,b-unsaturated aldehydes thermal stability and, especially in doped state, high
[
2,3]
has been recently reviewed
and can also be found resistance against common solvents; in this respect, it
in numerous studies of citral hydrogenation over sup- should be an appropriate support material for catalyt-
[
4]
ported metal catalysts in the past years (e.g. ).
ic processes in gas and liquid phases. The most inter-
Usually, such hydrogenation reactions are run over esting property of PANI is that, depending on the oxi-
group VIII metals which are deposited on classical dation state of the aniline polymers, several different
supports, for example, on oxides (SiO , Al O , TiO , forms may arise, which all derive from the basic struc-
2
2
3
2
zeolites) or activated carbon. Among the various stan- ture given in Scheme 2. The average oxidation state
dard preparation procedures which can be applied for can vary from fully oxidized (x=0, “pernigraniline”)
supported catalysts, deposition-precipitation, incipi- to fully reduced (x=1, “leucoemeraldine”); the term
ent-wetness or preformed metal colloids as precur- “emeraldine” is used to designate the structure with
[5]
sors are frequently used. More sophisticated meth- equal numbers of oxidized (quinonoid) and reduced
[
8c–e]
ods which allow controlled routes of catalyst synthesis (benzenoid) units (x=0.5).
The latter can, as all
are the heterogenization of organometallic com- the other possible structures, exist as a base (semi-
pounds onto the surface of zerovalent metallic parti-
cles or highly divided inorganic oxides (surface organ-
[
6]
ometallic chemistry ) and the immobilization of
metal complexes onto insoluble organic matrices such
[7]
as polymers. Among the latter, the electrically con-
ducting polymer (ECP) polyaniline (PANI) represents
a compound which is easily accessible from low cost Scheme 2. General structure of polyaniline (PANI).
1338
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Carbon-Carbon Double Bond versus Carbonyl Group Hydrogenation
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conductor) or in the protonated or doped (metallic) ized by ICP-OES, TEM, XPS and EXAFS. The ap-
form. The transition to the metallic state takes place plicability of these novel catalysts was compared with
without any change in the number of electrons; the conventional Pt catalysts supported on SiO , Al O
3
2
2
quinonoid units of the emeraldine salt are converted and activated carbon as well as graphite. To the best
to benzenoid units by a proton-induced spin-unpairing of our knowledge, no report is available where an at-
[
10]
mechanism that leads to two unpaired electrons.
tempt was made to control the selectivity of citral hy-
In the previously published works concerning the drogenation by the synthesis approach described
[
11]
synthesis of precious metal-PANI composites the using polyaniline as catalyst support.
metal was mostly introduced into the bulk of the po-
lymer matrix instead of being deposited on its sur-
face; furthermore, these materials have rarely been Results and Discussion
used as heterogeneous catalysts for chemical transfor-
mations so far. Single examples in this regard are im- Particle Size Characteristics
[
12]
mobilized molybdenum and vanadium complexes
for the oxidation of alcohols, the application of W-, Catalyst characterization by TEM revealed that both
V-, Ti- and Mo-PANI composites in epoxidation reac- synthetic routes succeed in producing Pt/PANI cata-
[13]
tions of olefins as well as the use of nanosized Pd lysts with highly dispersed particles. As depicted in
in polyaniline for Suzuki–Mijaura and Heck cou- Figure 1 for the catalyst which was prepared by depo-
[
14]
plings.
A broader field where noble metal-PANI sition-precipitation (Pt/PANI-3), the observable parti-
[15]
systems are used is electrocatalysis,
whereas they cles are uniformly distributed on polyaniline and are
are only rarely used as hydrogenation catalysts for un- marked by high dispersion [mean particle diameter of
saturated compounds. An exception is the work by d=(1.9Æ0.5) nm] in addition to a very narrow parti-
Drelinkiewicz et al. about ethylanthraquinone (with cle size distribution. The same applies for Pt/PANI
[
16]
Pd/PANI)
and Sobczak et al. about hexyne (with catalysts synthesized by the sol-method [Table 1, d=
[
17]
Pt/PANI and Pd/PANI) hydrogenation.
(1.6Æ0.4) nm and (3.7Æ1.4) nm, depending on the Pt
The above discussed structures of PANI should, by loading]. It must be noted that the obtained particle
subtle combination with metal precursor and synthet- size exhibits the smallest average diameter so far re-
ic route, offer the possibility of a specific design of ported on PANI. Other synthetic routes to Pt/PANI
novel catalysts which could now control the selectivity materials, where Pt was introduced either electro-
in hydrogenations of a,b-unsaturated aldehydes. This chemically into PANI films, grown also electrochemi-
fundamental consideration, based on the possibility to cally from solutions of aniline and HClO4,
manipulate a metalꢁs hydrogenation behavior by chemically by spontaneous oxidation of aniline by
[11a]
or
2À [11b]
structural influences of the support material (via elec- [PtCl ] ,
gave much larger particle sizes (200 nm–
6
tronic and/or steric effects, possesses anchor sites to 1 mm and max. 0.5–1 mm, respectively). As to the com-
[
3]
immobilize metal precursors), inspired us to use position of the particles, TEM analysis of Pt/PANI-3
polyaniline as support material for platinum catalysts shows a broad set of lattice spacings. Although size-
and to apply them in the selective hydrogenation of dependent lattice contractions up to 10% are well-
[
18]
the a,b-unsaturated aldehyde citral. The conjugated known for supported Pt particles, the measured d
polymer polyaniline has the advantage over other values could as well be smoothly attributed to hydrat-
[
19]
polymers (e.g., polystyrene) that the nitrogen atom in ed platinum dioxide species (PtO ·H O).
2
2
the backbone monomer units can anchor platinum
through coordination which leads to pre-organized
metal-polyaniline complexes and, after appropriate Catalytic Properties of PANI-Supported Platinum
pretreatment of this hybrid material, forms highly dis- Catalysts in Citral Hydrogenation
persed particles. Therefore, it can be anticipated that
catalysts prepared by contacting a Pt complex com- In order to benchmark their catalytic performance,
pound with PANI exhibit different catalytic properties the Pt/PANI catalysts as well as platinum on the clas-
compared to those which are synthesized via colloidal sical support materials SiO , Al O , activated carbon
2
2
3
metal particles immobilized onto polyaniline. For the (AC) and graphite (G) were tested in citral hydroge-
synthesis of the Pt/PANI catalysts we chose two dif- nation. The results of all catalysts are summarized in
ferent approaches, namely on the one hand the pre- Table 1. Since the hydrogenation of citral proceeds
cipitation of H PtCl as Pt precursor onto polyaniline, via a complex reaction network consisting of consecu-
2
6
suspended in basic medium, with subsequent liquid tive and parallel reaction steps (Scheme 1), selectivity
phase treatment using formalin (deposition-precipita- data (geraniol/nerol and citronellal from carbonyl and
tion, DP) and, on the other hand, the immobilization C=C group hydrogenation, respectively) have to be
of preformed nanoscale Pt colloids on polyaniline compared at the same conversion level of citral for
(
sol-method). The resulting catalysts were character- which Xcitral =30% was chosen (Table 1, data were ex-
Adv. Synth. Catal. 2008, 350, 1337 – 1348
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Martin Steffan et al.
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Figure 1. TEM images (top) and particle size distribution (bottom) of Pt/PANI-3.
Table 1. Results of citral hydrogenation with polyaniline supported Pt catalysts (Pt/PANI) and Pt on classical supports (plati-
num particle size, d , according TEM), reaction conditions: T=1408C, p(H )=70 bar, m =1.5 g, c
A
T
E
N
(citral)=0.56 mol/L,
Pt
2
Kat
0
solvent: n-hexane; all selectivities (S) at equal conversion Xcitral =30%).
[a]
[b]
[c]
[d]
Entry Catalyst
Metal
Synthesis Pt
Additive
dPt
t30
SC
=
O
SC
=
SSOP
C
content [wt%] route
precursor
[nm]
A
H
R
U
G
[%]
[%]
e
1
2
3
4
5
6
7
8
9
Pt/PANI-1
Pt/PANI-2
Pt/PANI-3[
Pt/PANI-4
Pt/G
Pt/SiO2
2.7
5.0
5.0
4.1
5.0
5.0
5.0
5.0
5.0
Sol
Sol
DP
DPH
-
IW
DPH
Sol
Na PtCl
THPC
1.6Æ0.4 120[
3.7Æ1.4 205
1.9Æ0.5 110
1.7Æ0.5 225
75
78
4
20
19
87
80
23
30
46
49
44
5
3
9
16
14
10
11
27
32
2
4
4
f]
Na PtCl
THPC
HCHO
HCHO
-
2
H PtCl
2
6
g]
2PtCl6
4
[h]
-
-
-
30
60
63
60
43
24
24
[i]
Pt
A
H
R
U
G
A
T
E
N
Pt/Al O -1
2PtCl6
HCOONa -2 .8Æ0.7 12
HCHO
2
3
Pt/Al O -2
Na PtCl
THPC
30
36
2
3
2
4
[j]
Pt/AC
DPH
2PtCl6
-
[
[
[
[
[
[
[
[
[
[
a]
Time necessary for Xcitral =30%.
Geraniol + nerol.
Citronellal.
Sum of other products (citronellol, isopulegol, dihydrocitronellal, non-identified products).
Tetrakis(hydroxymethyl)phosphonium chloride.
b]
c]
d]
e]
f]
T=1208C, p(H )=50 bar, mcat =1.0 g.
2
g]
h]
i]
Own PANI synthesis.
Pt/graphite, commercial product (Alfa).
Followed by treatments in air and H (3508C, 3 h).
Pt/activated carbon.
2
j]
tracted from the corresponding selectivity-conversion
Upon closer examination of the Pt/PANI system,
plots). Additionally, the reaction time necessary to two different selectivity patterns become apparent: (i)
obtain a citral conversion of 30% is given as a mea- the preferential hydrogenation of the carbonyl group
sure of catalyst activity.
of citral leading to the selective formation of the un-
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Carbon-Carbon Double Bond versus Carbonyl Group Hydrogenation
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saturated alcohols geraniol and nerol (SC O, max =78%) route, different active sites were generated, which will
=
in the case of Pt/PANI-sol catalysts (Table 1, entries 1 be discussed as follows.
and 2) and (ii) the complete reversal of the selectivity
towards the selective hydrogenation of the conjugated
C=C bond resulting in formation of citronellal In Depth Characterization by XPS and EXAFS/
(
(
SC
=
=87%) if Pt/PANI-DP catalysts are used XANES
C, max
Table 1, entries 3 and 4). Moreover, considering all
support materials applied, the following trend be- X-Ray Photoelectron Spectroscopy
comes apparent, namely the formation of the allylic
alcohols is favored in the sequence Pt/PANI-DP<Pt/ A key to a better understanding offer XPS investiga-
AC <Pt/Al O <Pt/SiO <Pt/G<Pt/PANI-sol. For the tions of the Pt/PANI catalysts, as they are sensitive to
2
3
2
carbon based catalysts, Pt/G and Pt/AC, the use of the surface and not only provide information about
graphite as support yields a higher selectivity towards the electronic state of platinum, but also about the ni-
geraniol and nerol compared to activated carbon trogen atoms in the PANI chains.
(
Table 1, entries 5 and 9). These results reconfirm the
For all samples C, Pt, N, O and Cl could be detect-
[
3]
findings of Gallezot for the hydrogenation of cinna- ed as components of the outermost region. XPS anal-
maldehyde with Pt-supported activated carbon and ysis of both the Pt 4d5/2 and Pt 4f7/2 binding energy
graphite catalysts, respectively. This phenomenon can (BE) in Pt/PANI-2 (prepared by the sol-method) gave
be interpreted as an electronic ligand effect exerted values of 316.2 eV and 73.6 eV, respectively
[
3]
by the “macroligand” graphite which modifies the (Figure 2). A formal ratio of 1:1.4:4.1 could be esti-
local structure of the metal in a two-fold manner: The mated for Pt:Cl:O. The binding energy and the com-
induced higher charge density on platinum gives rise position formally suggest anionic Pt-oxo-chloro-com-
to a decrease of the binding energy of the C=C bond plexes of Pt(II) existing as counterions in the PANI
via an increase of the repulsive four-electron interac- matrix or isolated reduced Pt atoms interacting with
tion and simultaneously favors the backbonding inter- the nitrogen containing group of PANI, probably
action of the antibonding orbital p*CO; thus, the car-
A
H
R
U
G
bonyl group hydrogenation is preferred over the C=C 4f7/2 binding energies of 73.0 eV are described in the
[
20]
[21]
bond hydrogenation. The electron-donating proper- literature due to the final state effect. The moder-
ties of the support decrease – analogously to the addi- ate BE shift of 0.6 eV could be explained by the inter-
[3]
tion of bases – the probability of the C=C bond hy- action between the Pt and the substrate. The distinct
drogenation which leads to a discrimination between coloration of the precursor solution after addition of
the hydrogenation of the carbonyl group and that of the reducing agent THPC indicates a reduction of the
the olefinic bond.
Pt(II) species in the precursor and hereby the exis-
However, this donation effect does not explain the tence of Pt(0). It must be noted that such isolated Pt
observed complete reversion in selectivity towards cit- species could not be resolved by TEM. Subsequent
ronellal when Pt/PANI-DP was used as catalyst and treatment with gaseous H led to a decrease of the Pt
2
suggests that, depending on the respective synthetic 4d5/2 binding energy to 315.5 eV whereas the Pt 4f
Figure 2. Normalized X-ray Pt 4f (left) and N 1s spectra (right) of Pt/PANI-2.
Adv. Synth. Catal. 2008, 350, 1337 – 1348
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Martin Steffan et al.
FULL PAPERS
Figure 3. Normalized X-ray Pt 4f (left) and N 1s spectra (right) of Pt/PANI-3.
peaks can be deconvoluted into two Pt states with a measured at a BE of 398.7 eV could not unambigu-
4
f7/2 BE of 72.4 eV and 73.4 eV, indicating a partial re- ously be attributed to an imine [=N-] (BE=
[
17,24]
duction of the platinum (Figure 2). This part is re- 398.0 eV)
or amine [-NH-] nitrogen (BE=
[
17,24]
duced to metallic Pt correlated with the BE of 399.5 eV).
2.4 eV. This binding energy could be explained by Taking into account both the Pt 4f and N 1s spec-
small metallic Pt particles. The second peak at tra, it can be clearly concluded from the XPS analysis
3.4 eV could be assigned to the reduced Pt-PANI of the catalyst Pt/PANI-3 that its surface exclusively
7
7
species described formerly, which seems to be stable consists of highly oxidized platinum which is deposit-
under the applied hydrogen pre-treatment conditions. ed on the nearly fully oxidized form of PANI, i.e.,
The observed N 1s binding energy (BE=399.3 eV, pernigraniline (Scheme 3). In conjunction to the
[
10,22]
Figure 2), which could be assigned to [-NH-],
def- aforementioned TEM results, the visible nanoparticles
initely reveals the existence of the benzenoid, fully re- in this catalyst type consist of hydrated platinum
duced form of PANI, and this is consistent with the oxide whose water content might be diminished at the
application of the reducing agent THPC during the outermost shell. Note that during treatment with gas-
syntheses. Thus, one could assume that this catalyst, eous H a considerable reduction of Pt took place;
2
prepared by the sol-method, mostly comprises very two states at Pt 4f7/2 BE=71.8 eV and BE=73.2 eV in
small metallic Pt species or even atoms on the leuco- a ratio of 3:1 could be observed (Figure 3). The
emeraldine form (Scheme 2; x=1) of PANI.
former could be correlated with the metallic form,
A completely different behavior, especially con- whereas a part of Pt seems to remain as oxide during
cerning the Pt oxidation state and also with respect to H treatment. At the same time, the N 1s binding
2
the Pt-N interaction, is found for the XPS analysis of energy is shifted to a peak which can be deconvoluted
Pt/PANI-3, which was synthesized via the deposition- to 399.4 eV and 398.1 eV indicating that the nitrogen
precipitation (DP) method. Here, obviously highly in PANI presumably exists mainly in the amine form
oxidized platinum must dominate as the main compo- (82%), the imine remains only to a much lesser con-
nent. In fact, the existence of only one valence state tent (18%). As expected, the amount of chlorine and
of Pt could be concluded from the Pt spectra especially of oxygen decreased during the H treat-
2
(
4
Figure 3) with BE of 75.0 eV and 317.2 eV for the Pt ment.
f7/2 and 4d5/2 electrons, respectively, pointing to the
presence of platinum oxides (PtO : P t 4f BE=74.1–
2
7/2
7
7
5.6 eV, Pt 4d5/2 BE=318.1 eV; PtO: P t 4f BE= EXAFS/XANES Studies
7/2
[23]
2.4–74.6 eV, Pt 4d5/2 BE=317.1 eV). The near-sur-
face composition was determined as 1:1:14.7 To leave no doubt that Pt/PANI-3 catalyst is indeed
Pt:Cl:O). composed of highly oxidized platinum particles, addi-
Furthermore, analysis of the N 1s core-level spec- tional analysis by X-ray absorption fine structure
trum of Pt/PANI-3 revealed that positively charged (EXAFS) spectroscopy also gives evidence for this
(
+
3
nitrogen [-N -] is present in this catalyst by = (BE= structure.
4
[
16,23]
1
400.4 eV, Figure 3).
The remaining = of N states
4
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Carbon-Carbon Double Bond versus Carbonyl Group Hydrogenation
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Scheme 3. Different interactions between Pt and PANI showing the favored hydrogenation product, Pt-PANI-2 (right),
Pt/PANI-3 (left).
XANES Analysis
A comparison with literature data further substanti-
ates the above assumption: XANES spectra reported
2+
[25]
In Figure 4, the XANES region of Pt/PANI-3 has for [Pt(H O) ] (aq) appear very similar to that of
A
H
R
U
G
2 4
been plotted and compared to a commercial Pt/C cat- Pt/PANI-3, whereas only little resemblance seems to
2À
alyst in its as-received state and after exposure to 5% be there with [PtCl ] (aq). In particular, the white
4
hydrogen in a nitrogen atmosphere at room tempera- lines for the aqua complexes are very pronounced
ture. The most obvious changes occur both in the with only little structure beyond the absorption edge.
white-line and the edge position. In contrast to the
hydrogen-reduced, but also to the as-received Pt/C,
the white-line intensity of Pt/PANI-3 is significantly EXAFS Analysis
enhanced and the Pt L absorption edge position is
3
shifted to higher energies indicative of a higher oxida- In a first step, the raw spectra were background-sub-
tion state. Furthermore, the oscillatory structure in tracted, normalized and Fourier transformed. A quali-
the extended edge region of the spectrum is far less tative comparison of hydrogen-reduced Pt/C and
pronounced for Pt/PANI-3 than for Pt/C. Both obser- Pt/PANI-3 on the one hand is displayed in Figure 5,
vations lead us to propose a platinum oxide structure while a comparison of as-received Pt/C and Pt/PANI-3
with very small particle sizes for Pt/PANI-3.
is shown in Figure 6.
Both FTs in Figure 5 look distinctly different. In
the case of reduced Pt/C, the most prominent contri-
bution appears at a distance of approximately 2.8 Š,
which is consistent with a metallic PtÀPt distance. For
Pt/PANI-3, however, the main feature is centered at
about 1.9 Š attributed to a PtÀO contribution. No
evidence of a PtÀCl contribution is observed, which
should occur at approximately 2.4 Š according to the
[
26]
literature. The same is true for a comparison of the
chi data of Pt/C and Pt/PANI-3 (Figure 6). At values
À1
below k=5 Š in k space, the data appear almost
identical, whereas at higher k values significant differ-
ences are observed. In the higher k range, the metallic
contribution becomes predominant, and consequently,
Pt/PANI-3 seems to be much more oxidized even
than Pt/C in its as-received state, which was used as a
reference.
Figure 4. Near-edge region of Pt/PANI-3 (black) compared
to Pt/C in its as-received state (dark gray) and after reduc-
The above information was used to generate two
tion at room temperature in 5% hydrogen/N2 flow (light different start models for the fitting of the EXAFS
gray).
data: a) model I including PtÀPt, PtÀO and an addi-
Adv. Synth. Catal. 2008, 350, 1337 – 1348
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1343

Martin Steffan et al.
FULL PAPERS
Figure 5. FT of Pt/PANI-3 (black) compared to Pt/C after reduction at room temperature in 5% hydrogen/N flow (dark
2
gray).
model I (Figure 7a) as well as for the sample fitted
with model II (Figure 7b).
For model I, PtÀO was found to be the predomi-
nant contribution with a coordination number N
À
O
Pt
of 4.0 at a standard PtÀO distance of 2.00 Š. Both
PtÀPt and PtÀCl contributions were less pronounced
with coordination numbers of N
Pt Pt Pt Cl
À À
=0.7 and N =
0
.3, respectively. Using model II for the fit, the PtÀO
shell at a distance of 2.01 Š remains the dominant
feature with a coordination number NPt O of 4.0,
while the second PtÀO shell at 2.18 Š and the PtÀPt
contribution at 3.02 Š are less pronounced (N
À
À
Pt Pt
=
1
.8 and NPt O
larger PtÀPt distance observed is in good agreement
with PtO data reported in the recent literature.
À
=1.4, respectively). The significantly
[28]
2
Figure 6. Data of Pt/PANI-3 in k space (black) compared to The obtained values are reasonable for both models
Pt/C in its as-received state (dark gray).
used. However, the quality of the fit was significantly
smaller, when model II was used.
From the X-ray absorption data and in good agree-
ment with the XPS data, it is concluded that the
tional PtÀCl coordination and b) model II with PtÀPt sample consists largely of b-PtO with very small par-
2
and two PtÀO shells according to b-PtO as reported ticle sizes. PtO in bulk form, usually prepared by
2
2
[27]
in the literature.
Both models were applied to fit fusion of H PtCl with NaNO3 at 450–5008C, is
2 6
the EXAFS data, and the fit parameters are listed in known in organic synthesis and heterogeneous cataly-
Table 2. In Figure 7, the data in k space and the corre- sis as Adams catalyst which exhibits excellent hydro-
[
29]
sponding fit are shown for the sample fitted with genation properties.
Detailed XRD analysis of
À1
Table 2. Summary of the EXAFS results using either model I or model II for the fit; 3 <k <14 Š , 1.5 <R <3.2 Š.
2
2
Model
N (Æ10%)
R [Š] (Æ0.02)
s [Š ] (Æ5%)
E [eV] (Æ10%)
0
model I
NPtÀPt =0.7
NPtÀO =4.0
NPtÀCl =0.3
NPtÀPt =1.8
NPtÀO =4.0
NPtÀO =1.4
RPtÀPt =2.77
RPtÀO =2.00
RPtÀCl =2.50
RPtÀPt =3.02
RPtÀO =2.01
RPtÀO =2.18
0.006
0.006
0.006
0.009
0.003
0.004
À15.0
3.0
À15.0
6.6
model II
À0.4
4.7
1344
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1337 – 1348

Carbon-Carbon Double Bond versus Carbonyl Group Hydrogenation
FULL PAPERS
Figure 7. Chi data and fit for Pt/PANI-3; fit obtained when using model I including a PtÀCl contribution (left), data fitted
with model II for b-PtO (right).
2
Adams catalysts revealed that they are a mixture of particles, that is, selectivity has been found to be
a-PtO as main component besides Pt and platinum structure-sensitive in that case. According to the re-
2
bronze (Na Pt O ). Proof of the latter is a clear hint sults of the present study, polyaniline seems to take
x
3
4
for introducing sodium into the platinum oxide which the role of the charge density donor, in line with the
[
31]
may affect the hydrogenation properties of the works of Kaner et al., and, thus, is a suitable sup-
Adams catalyst. The preparation method applied in port for selective citral hydrogenation towards the un-
the present study constitutes a simple and new way to saturated alcohols provided that metallic platinum
fabricate an immobilized Adams catalyst (in the b- particles are present which have been prepared from
PtO form) which is an efficient C=C group hydroge- preformed colloids in the present study (Pt/PANI-2).
2
nation catalyst (Table 1). It is assumed that the reduc- Then, electron transfer from PANI to Pt allows an in-
tion of b-PtO easily occurs in the presence of polyan- crease of the hydridic character of the chemisorbed
2
iline (see Figure 3 and the associated discussion in the hydrogen and, thus, the rate of the nucleophilic addi-
À
XPS part) fabricating highly dispersed, PANI-support- tion of H on the positively charged carbon of the
ed platinum black under the reaction conditions. carbonyl group is enhanced.
These distinct platinum clusters could now interact
with the conjugated C=C bond, resulting in the for- increased due to strong interaction with the PANI
mation of citronellal. support. Further catalyst characterization by electron
The reducibility of the highly oxidized Pt species is
Although information about selective hydrogena- spin resonance (ESR) should also confirm the pres-
tion with polyaniline-supported precious metal cata- ence of easily reducible, highly oxidized Pt species,
lysts and especially regarding supported nanodis- which are able to withdraw electron density and de-
persed Adams catalysts is very limited, there are a plete the number of polarons in the PANI support
few hints which support the findings of the present due to strong interaction.
study. The strong catalytic activity of the novel immo-
bilized platinum oxide towards C=C group hydroge-
nation is in line with the increased surface reactivity Conclusions
2À
of polyaniline-supported [PdCl4] complexes towards
the hydrogenation of olefins and alkynes compared to In the present article the applicability of chemically
[17]
Pd(0). On the other hand, a commercial (bulk) Pt- synthesized platinum/polyaniline catalysts for selec-
Adams catalyst exhibiting either large agglomerates tive hydrogenation of the a,b-unsaturated aldehyde
of 2–5 nm crystallites when reduced at 333 K or large citral was presented and the role of different platinum
facetted particles in the size range 20–200 nm due to species affecting selectivity elucidated. It is found that
extensive sintering and recrystallization at a reduction supporting chemically prepared PtO on polyaniline
2
temperature of 373 K was used for cinnamaldehyde produces a highly dispersed, supported Adams cata-
[30]
hydrogenation. The large particles were more selec- lyst which is able to efficiently hydrogenate the conju-
tive towards the carbonyl group hydrogenation yield- gated C=C bond of citral whereas reduction of the
ing cinnamyl alcohol whereas the C=C bond was hy- carbonyl group occurs with polyaniline-supported Pt.
drogenated much more rapidly in the case of smaller This approach, hitherto not described in the literature,
Adv. Synth. Catal. 2008, 350, 1337 – 1348
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1345

Martin Steffan et al.
FULL PAPERS
is not only particularly useful for the selective hydro- water. After drying (room temperature overnight, then at
8
08C for 2 h), the catalyst was calcined in air and reduced in
genation of a,b-unsaturated aldehydes by controlling
the selectivity of this also industrially significant reac-
tion, but also unveils the great potential of conducting
polymer supported precious metals in the field of
hitherto barely investigated chemocatalysis. Precisely
in this regard, we expect that polyaniline as support
material offers novel alternatives for the rational
design of catalysts with desired features – not only for
flowing hydrogen at 3508C for 3 h.
The catalysts were characterized by several methods. The
metal content was estimated by ICP-OES (Perkin–Elmer
Optima 3000XL) after dissolving the materials in a mixture
of HF/HNO3 by means of a MDS-2000 microwave unit
(CEM).
The size of the polyaniline-supported particles and their
morphology was determined by transmission electron micro-
hydrogenations, but also for partial oxidations – the scopy, TEM (JEOL JEM-3010, 300 kV, LaB -cathode).
6
latter remained unused to a large extent so far.
X-ray photoelectron spectroscopy (XPS) has been applied
to obtain information on the chemical environment of sur-
face atoms, in particular the oxidation state of platinum
loaded to the PANI support and the chemical state of the
PANI nitrogen. Surface analytical measurements were per-
formed by means of a VG ESCALAB 220 iXL spectrometer
Experimental Section
Catalysts Synthesis and Characterization
with an Mg K radiation source. The C 1s-peak at 284.8 eV
a
was used as a reference for the binding energy. After satel-
lite and background subtraction the peaks were fitted with
Gaussian–Lorentzian curves. The peak positions could be
determined with a precision of Æ0.1 eV. The quantitative
composition was elucidated from the peak areas, which were
divided by the transmission function of the spectrometer
and element specific Scofield factors.
To elucidate the chemical nature of platinum and to
obtain quantitative structural information, we carried out X-
ray absorption fine structure (EXAFS) spectroscopy. The X-
The graphite-supported catalyst 5Pt/G was a commercial
product (Alfa Aesar); other materials used for preparation
were obtained from Chempur (Na PtCl ), Merck {tetrakis-
2
4
A
C
H
T
R
E
U
N
G
(hydroxymethyl)phosphonium chloride [THPC], formalde-
hyde [37 wt%], NaOH}, Aldrich {polyaniline [PANI, M =
w
6
5.000], Pt
GL25]), Fluka (sodium formate [HCOONa]), Heraeus
H PtCl6 solution), Alfa Aesar (large pore silica) and
A
C
H
T
R
E
U
N
G
(NH ) (NO ) }, LaRoche (alumina [Versal
A
C
H
T
R
E
U
N
G
3 4 3 2
(
2
Arkema (activated carbon [AC, Acticarbone]).
The synthesis of the catalysts Pt/PANI-1, Pt/PANI-2 and
Pt/Al O -2 was carried out using preformed Pt colloids
ray absorption spectrum of the Pt L -edge was obtained at
3
2
3
[5]
the beamline X1 at Hasylab, Hamburg, with the synchrotron
source operated at an energy of 4.45 GeV and an initial
positron beam current of 120 mA. The measurements were
carried out in transmission mode in an energy range from
which were prepared according to a modified sol-method.
After immobilization onto the supports, the materials were
used as catalysts without any further treatment. More pre-
cisely, for the preparation of Pt/PANI-1 an aqueous solution
of Na PtCl (0.5 g in 100 mL H O) was combined with a so-
E=11300 eV up to 12800 eV (Pt L -edge at 11564 eV),
3
2
4
2
using a thin Pt metal foil as reference. An Si(111) double-
A
H
R
U
G
lution of THPC (2.5 mL, 1.3 wt%) and NaOH (3.75 mL, 0.2
mol/L) in water (117.5 mL). After 1 h aging time, the as-pre-
pared sol was added to a suspension of 10 g PANI in 1 L
H O, stirred overnight, filtered, washed (50 mL H O) and
crystal monochromator was used and detuned to 50% inten-
sity to avoid higher harmonics present in the X-ray beam.
The intensities of the focused beam and the transmitted
beam were detected by three gas-filled ion chambers in
series. Extraction of the EXAFS data from the measured
raw data was either performed with the program package
2
2
dried (1108C, 2.5 h). The intermediate Pt/PANI composite
thus obtained was loaded with a second sol portion as de-
scribed above. Pt/PANI-2 was prepared in a one-step immo-
bilization using a sol derived from an Na PtCl (0.4 g in
[32]
WinXAS (for qualitative comparison only) or the XDAP
2
4
[33]
code developed by Vaarkamp et al. The pre-edge was ap-
proximated by a modified Victoreen curve, and normaliza-
tion was carried out by dividing the absorption spectrum by
the height of the absorption edge at 50 eV above the
4
0 mL H O) and a THPC solution (3 mL 0.2 mol/L NaOH,
2
4
mL 1.3 wt% THPC, 94 mL H O), whereas in the case of
2
Pt/Al O -2, 4.75 g alumina, suspended in 500 mL water,
2
3
served as support for a sol prepared according to Pt/PANI-
[34,35]
edge.
The background was approximated by a spline
1.
function defined by:
During the preparation of Pt/PANI-3, Pt/PANI-4, Pt/AC
and Pt/Al O -1 via deposition-precipitation, 9.5 g of support
2
3
were suspended in 50 mL diluted aqueous Na CO (10
2
3
wt%). The suspensions were combined with 20 mL H PtCl
2
6
solution (containing 0.5 g Pt) and stirred for 15 min at 908C
with a pH maintained neutral by further addition of aqueous
Na CO . After addition of (i) a mixture containing formal-
2
3
dehyde solution and NaOH solution (10 wt%) in a volume
ratio of 4.8/1 (for PANI and AC supports) or (ii) a dilute with SM=smoothing parameter. The data as finally shown
HCOONa solution (for alumina support), the suspensions
were stirred for further 20 min, filtered, washed and dried in
air overnight.
in this paper were obtained with a multiple shell R-space fit
with k weighting, Dk=3.0–14 Š and DR=1.4–3.2 Š. To
1
À1
obtain the final fit, all parameters were fully optimized in
0
1
2
3
Pt/SiO was obtained via incipient-wetness impregnation of
k , k , k and k weighting. Theoretical phase shifts and
2
the support (4.75 g, pre-dried at 2508C for 2.5 h) with a 0.5 g
backscattering amplitudes for the PtÀPt, PtÀCl and PtÀO
portion of Pt
A
C
H
T
R
E
U
N
G
(NH )
A
T
E
N
(NO ) (Aldrich) dissolved in 9.5 mL
absorber-scattering pairs were used in EXAFS data analy-
3
4
3 2
1346
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1337 – 1348

Carbon-Carbon Double Bond versus Carbonyl Group Hydrogenation
FULL PAPERS
[
36]
[34]
sis,
which were generated utilizing the FEFF7 code.
peano, C. Milone, S. Galvagno, B. Corain, J. Catal.
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Sample preparation was done by diluting 180 mg of the
powder sample in 200 mg of boronitride and pressing it into
a dense pellet (calculated to have an absorbance of 1.5).
The pellet was then fixed onto the multiple sample holder,
which was cooled down to liquid nitrogen temperature. Two
spectra were recorded with recording times of approximate-
ly 45 min each and added to improve the spectra quality.
Data analysis was carried out in two steps: first of all, the
near-edge region is discussed qualitatively and compared to
literature data. In a second step, an EXAFS analysis is per-
formed fitting the Fourier-transformed spectrum to a rea-
sonable model using the software package XDAP.
1
82, 186; k) A. M. Silva, O. A. A. Santos, M. J. Mendes,
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The near-edge region, FT and data of polyaniline-support-
ed platinum in k space are referred to a commercial carbon-
supported platinum catalyst (Pt/C) which was purchased
from E-TEK.
[
7] Y. Wang, H. Liu , Y. Huang, Polym. Adv. Technol.
1
998, 7, 634.
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The catalytic runs were carried out in a stainless steel batch
reactor (Parr Instr., 300 mL). The catalyst (mcat =1–1.5 g)
was filled into the reactor before addition of the reaction
mixture containing the educt (10 mL citral, Merck), the sol-
vent (90 mL n-hexane) and an internal GC standard (5 mL
n-tetradecane). The mixture was flushed three times with
2
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DB-Wax column (l=30 m, d =0.25 mm, t =0.25 mm).
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