Multiple one-pot reaction steps using organically doped metallic hybrid catalyst

Full text of DOI:10.1002/cctc.201300261

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DOI: 10.1002/cctc.201300261
Multiple One-Pot Reaction Steps using Organically Doped
Metallic Hybrid Catalyst
Leora Shapiro and David Avnir*[a]
The recently developed family of molecularly doped metals
(molecule@metal) has opened the possibility of an interesting,
useful concept in catalysis: Entrapment of an organic catalyst
within a catalytic metal leads to a hybrid material capable of
multiple catalytic steps in one pot. In this report we show the
feasibility of this idea by effecting catalytic dehydration, cata-
lytic disproportion, and catalytic hydrogenation, all by using
Nafion@Pd. Palladium, in this case, serves not only as a metallic
heterogenization matrix—by itself a new heterogenization
methodology (see below)—but it also serves in its classical
role as a versatile catalyst. Examples of the successful use of
this methodology for metal heterogenization of homogeneous
catalysts include the acid-catalyzed pinacol–pinacolone rear-
rangement with Nafion entrapped in silver (Nafion@Ag); the
dehydration of 2-phenylethanol to styrene with the same cata-
lyst;[1] the H3[P(Mo3O10)4]@Ag- or H3[P(W3O10)4]@Ag-catalyzed
Friedel–Crafts (FC) adamantylation of toluene and anisole;[2]
the partially enantioselective hydrogenation of isophorone and
acetophenone catalyzed by (À)-cinchonidine@Pd, (+)-cinchoni-
ne@Pd, (À)-quinine@Pd, or (+)-quinidine@Pd;[3] the biocat-
alyzed hydrolysis of p-nitrophenyl phosphate with acid phos-
phatase@Au or acid phosphatase@Ag;[4] and the hydrogena-
tion of styrene or diphenylacetylene with the silver-entrapped
catalytic organometallic complex [RhCl(cod){Ph2P(C6H4SO3Na)}]
(cod=1,5-cyclooctadiene).[5] Enhanced stabilities, improved re-
action conversion rates, altered reaction product ratios, and
the obtainment of new products were all observed in these
studies. For instance, in the FC reaction, alkylation of toluene
with either 1-bromoadamantane or the less-reactive 1-chloroa-
damantane proceeded to >99% yield with H3[P(Mo3O10)4]@Ag
in comparison to only 2–3% yield with H3[P(Mo3O10)4] in
solution.[2a]
duction conditions of the metal cation in the presence of the
molecule to be entrapped (see the Experimental Section for
details). The resulting material is composed of tight aggregates
of metal crystallites, which hold the dopant in the 3D matrix,
on one hand not allowing the dopant to leach out even in its
own solvent, and on the other hand allowing free diffusion of
the substrate and product molecules in and out. We empha-
size that 3D entrapment and 2D adsorption are very different
processes, and in all of the reported applications (including
also bioapplications, altering physical properties of the metal,
and more), the unique performance was observed only for the
entrapped 3D case.
As indicated above, the ability to entrap within a metal
opens yet another very interesting possibility, namely, the ob-
tainment of heterogeneous catalysts in which both the dopant
and the entrapping heterogenizing metal are catalytic. Such
materials may thus show multiple catalytic activities, for which
two catalysts and two separation steps are usually needed.
Here we report the successful demonstration of this concept.
Specifically, the dual activity of the composite Nafion@Pd
(Nafion entrapped within palladium, Figure 1), and the fact
Figure 1. Typical HRSEM images of Nafion@Pd (scale bar: 500 nm).
The formation of catalysts@metals has been made possible
by methodology that enables the doping of metals with or-
ganic molecules,[3,6] inorganic molecules,[2a,5] polymers,[1,7] and
proteins.[4] In addition to the Ag, Au, and Pd mentioned above,
successful entrapments have also been carried out with Cu,[6h,8]
Fe,[9] and several alloys (Ag–Au, Cu–Pt, and Cu–Pd).[6d,10] Except
for the doping of Fe, which involved zero-valent Fe(CO)5 as the
precursor, the general approach utilizes carefully selected re-
that only the hybrid catalyst, but not its separate components,
could pull three dual reaction schemes to completion is report-
ed. The selection of these two catalysts was based on their
wide use: acid catalysis for the former and mainly hydrogena-
tion for the latter. It is relevant to recall that the use of Pd and
Nafion in the same context has been reported, for example, in
simple mixing, as in the case of Nafion and Pd/C;[11] as Nafion
films placed on Pd-modified electrodes by sputtering,[12] elec-
trodeposition,[13] or electroless deposition;[14] and as the use of
Pd nanoparticles immobilized in Nafion membranes for fuel
cell applications.[15] Pd doped with Nafion as developed here
has not been studied. Reactions performed in one step in one
pot with a hybrid material showing dual catalytic activity have
been demonstrated for (Schemes 1 and 2) the dehydration/dis-
proportion of 1,2,3,4-tetrahydro-1-naphthol directly to naph-
thalene and tetralin, the dehydration/hydrogenation of 1,2,3,4-
[a] L. Shapiro, Prof. D. Avnir
Institute of Chemistry
The Hebrew University of Jerusalem
Jerusalem 91904 (Israel)
Fax: (+972) 26585332
E-mail: david.avnir@mail.huji.ac.il
Homepage: http://chem.ch.huji.ac.il/avnir/
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300261.
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can be attributed to the fact that the charge of the magnesi-
um ions replacing the protons is twice that of the sodium ions.
In addition, the ionic radius of Mg2+ is smaller than that of
Na+, and therefore, Mg2+ is more likely to access pores con-
taining sulfonate end groups. Further material properties of
the hybrid catalyst are given below.
The direct conversion of 1,2,3,4-tetrahydro-1-naphthol into
tetralin and naphthalene (Scheme 1, top) by Nafion@Pd (808C,
2 h) proceeds through catalyzed dehydration to afford 1,2-di-
hydronaphthalene, which then undergoes catalyzed dispropor-
tion. Figure 2 follows the kinetics of this reaction: It is seen
Scheme 1. Multiple catalyzed reactions that 1,2,3,4-tetrahydro-1-naphthol
undergoes with Nafion@Pd without (top) and with hydrogen (bottom).
Scheme 2. Catalyzed reactions that 1-phenyl-2-propanol undergoes with
Nafion@Pd and hydrogen.
tetrahydro-1-naphthol directly to tetralin, and the dehydration/
hydrogenation of 1-phenyl-2-propanol directly to 1-propyl-
benzene.
An important characteristic of the material that enables its
action as a catalyst is its porosity, which is due to its aggregat-
ed nature (Figure 1 and Figure S1, Supporting Information),
and the resulting accessibility of the entrapped sulfonic acid
groups to incoming substrate molecules. The aggregated
nature that gives rise to the interstitial porosity is revealed by
the very low density of the composite, 1.5 gcmÀ3 (compared
to the true density of Pd, 12.02 gcmÀ3), by the shape of the ni-
trogen adsorption–desorption isotherms and their small hyste-
resis opening (Figure S1, Supporting Information), by the nitro-
gen BET surface area of approximately 7 m2 gÀ1, by the Barrett–
Joyner–Halenda (BJH) average pore diameters of 19 nm for ad-
sorption and 17 nm for desorption, and by the fact that these
last values are close: all are very typical of aggregated meso-
porosity. The accessibility of the acidic protons of Nafion was
demonstrated by a cation-exchange experiment. If Nafion@Pd
is placed in 0.01m salt solutions, either sodium chloride or
magnesium sulfate, a drop in pH is observed (Table 1). The
drop in pH occurs very quickly and remains unchanged for at
least 24 h. Note that the drop in pH indicates that the final
concentration of the protons released into the solution by
MgSO4 is twice that released into the solution by NaCl. This
Figure 2. Reaction profiles of the dehydrogenation and disproportion reac-
tions of 1,2,3,4-tetrahydro-1-naphthol catalyzed by Nafion@Pd.
that when the substrate concentration drops to zero, the per-
centage of 1,2- dihydronaphthalene present reaches a maxi-
mum. The disproportion continues until naphthalene and tet-
ralin reach a similar ratio with no (NMR-)traceable 1,2-dihydro-
naphthalene.[16] The GC separation between naphthalene and
1,2-dihydronaphthalene was possible only with coupling to
a mass spectrometer.
Significantly, the dehydration did not occur at all if only
Nafion (or palladium) was used. The disproportion reaction
pulled the dehydration–rehydration equilibrium, which also
leads to 1,2,3,4-tetrahydro-2-naphthol as an intermediate
(Scheme 1), and lies heavily towards the alcohol side, to com-
pletion of the double-bond formation. As recyclability of heter-
ogeneous catalysts is one of their important properties, this
was tested four consecutive times; the result is encouraging,
as the yield did not drop below 95%.
In a second reaction scheme (Scheme 1, bottom), when1,2-
dihydronaphthalene was subjected to Nafion@Pd, this time
under hydrogen pressure (690 kPa, 808C, 24 h), full conversion
to fully hydrogenated tetralin was achieved. This shows, firstly,
that catalytic hydrogenation is by far faster than disproportion,
and secondly, that here too, the Pd-catalyzed step pulls the
more difficult Nafion-catalyzed step to completion. Palladium
alone did not show any reaction when used under the same
conditions. As catalysts are expected to effect similar reactions
of various substrates, we next tested the dehydration/hydroge-
nation of another substrate, 1-phenyl-2-propanol. Indeed, it
Table 1. The cation-exchange pH drop of salt solutions in the presence
of Nafion@Pd.
Salt solution Initial pH
(0.1m)
pH after 2 h
pH after 24 h
(H+ concentration) (H+ concentration) (H+ concentration)
MgSO4
NaCl
6.5 (3.2ꢁ10À7
6.5 (3.2ꢁ10À7
)
)
4.9 (1.3ꢁ10À5
5.1 (7.9ꢁ10À6
)
)
4.8 (1.6ꢁ10À5
5.1 (7.9ꢁ10À6
)
)
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was fully converted under similar conditions into 1-phenylpro-
pane (via 1-phenyl-2-propene, Scheme 2). As with the previous
observation, treatment of 1-phenyl-2-propanol with Nafion
only did not yield detectable levels of (1-propenyl)benzene. If
the reaction is performed under H2 pressure, the minute quan-
tity of (1-propenyl)benzene that is in equilibrium with the
starting material is consumed by the much faster hydrogena-
tion reaction, which pulls the reaction to completion. It is im-
portant to note that full conversion of this reaction required
the addition of molecular sieves to hinder the hydration back
reaction.
We conclude with some additional comments on Na-
fion@Pd: In search for the proper conditions for the successful
entrapment of the catalytic polymer, it was found that slowing
down the reduction rate of PdII was needed; otherwise, sepa-
rate phases formed. Successful slowing down was achieved by
replacing water-soluble Pd complexes (such as K2PdCl4, which
was a first, natural choice), with sparingly soluble PdCl2. That
Nafion does exist in the composite was proven by the detec-
tion of fluorine (ꢀ2.4%) by energy-dispersive X-ray spectros-
copy (EDAX) measurements (Figure S2, Supporting Informa-
tion) and by SEM elemental mapping (Figure 3). Residual Zn is
Figure 4. XRD spectra of pure Pd powder (top) and Nafion@Pd (bottom).
average palladium crystallite size (calculated from the Scherrer
equation) was found to be somewhat larger in Nafion@Pd,
that is, 9.6 nm compared to pure Pd at 7.8 nm. This may be at-
tributed to growth inhibition of the metal nanocrystals by the
coating polymer chains.
In conclusion, we displayed here a heterogeneous hybrid
catalyst capable of performing two catalytic steps in one pot.
The two steps can be carried out only with the dual catalyst,
as the first reaction must be pulled to completion by the
second step. The new concept of a catalyst entrapped within
a metal framework that is itself a catalyst was demonstrated,
which opens the possibility to form many new catalysts that
are of this composite nature.
Figure 3. Elemental mapping of Nafion@Pd performed on the micrograph
depicted (scale bar: 600 nm).
seen in Figure S2, probably too buried to be washed away, in
the form of zinc oxide (in passing, we note that alloying of
metals with Zn is known to improve their catalytic activity[17]).
Oxygen (ꢀ6.5%) originates from Nafion, and probably from
some PdO and ZnO, although backscattered electrons and
XRD measurements did not show these oxides. Mapping the
elements (Figure 3) revealed the homogeneous nature of the
composite, at least on the scale of the diameter of the ray of
electrons, which was approximately 1 mm area-wise and depth-
wise. XRD measurements (Figure 4) were performed on the
composite to compare its crystallinity to that of pure elemental
palladium powder produced by using the same reduction pro-
cess. Comparison of the two samples revealed that the matrix
crystalline structures are essentially identical, except that the
Experimental Section
Preparation of the dual Nafion@Pd catalyst
Nafion-H (0.6 g), EtOH (100 mL), and H2O (100 mL) were placed in
a flat-bottomed flask. PdCl2 (1.6 g, 0.96 g free palladium) was
added to the stirred solution (Nafion to palladium ratio was 3% w/
w) and then zinc (0.7 g) was added. The slurry turned black almost
immediately, which indicated that the reduction had started to
progress. The reaction mixture was left for 24 h to enable the com-
pletion of the reaction. The product suspension was then filtered
under vacuum by using 0.8 mm filter paper. The solid was washed
with 0.1m HCl (30 mL) to dissolve the remaining zinc and with
triply distilled water (3ꢁ) and dried overnight under vacuum. Mi-
croanalysis indicated 1.75 wt% of Nafion in the doped Pd. Before
use, each batch was homogenized with a mortar and pestle.
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Dehydration–disproportion of 1,2,3,4-tetrahydro-1-naphthol
to naphthalene and tetralin as a typical one-step dual
catalytic reaction
1,2,3,4-Tetrahydro-1-naphthol (0.2 mL) was placed in cyclohexane
(8.0 mL) along with Nafion@Pd (0.24 g) in a round-bottomed flask.
The mixture was brought to boiling and heated at reflux for 2 h.
The reaction was followed every 10 min by GC until full conversion
was achieved, which occurred after 2 h. All of the components of
the reaction were determined by NMR spectroscopy.
Please see the Supporting Information for the other catalytic reac-
tions.
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Keywords: heterogeneous catalysis · organic–inorganic hybrid
composites · multiple catalysis · mesoporous materials ·
supported catalysts
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Received: April 7, 2013
Published online on May 24, 2013
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