Decarbonylation of water insoluble carboxaldehydes in aqueous microemulsions by some sol-gel entrapped catalysts

Article abstract of DOI:10.1016/j.molcata.2013.09.024

In the course of our attempts to replace harmful solvents in organic processes by environmentally favored media, we investigated the use of aqueous microemulsions for catalytic decarbonylation of different kinds of aldehydes. The aldehydes were solubilized in the microemulsions with the aid of the cationic surfactant, cetyltrimethylammonium bromide. The aldehydes were transferred into CO-free products by sol-gel entrapped catalysts. The best results were obtained in the presence of nanoparticles of Pd(0). The heterogenized catalyst could usually be recycled 7-8 times without loss of catalytic activity. At relatively low temperatures (140°C) the decarbonylation proceeds stepwise. Initially a mixture of saturated and unsaturated products is formed. At 180°C however, fast hydrogenation of the unsaturated compounds takes place. These investigations may be regarded as model studies for the conversion of biomass derived intermediates to fuels and chemicals.

Full text of DOI:10.1016/j.molcata.2013.09.024

Journal of Molecular Catalysis A: Chemical 380 (2013) 90–93
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Decarbonylation of water insoluble carboxaldehydes in aqueous
microemulsions by some sol–gel entrapped catalysts
Shirel Dahoaha, Zackaria Nairoukha, Monzer Fanunb, Michael Schwarzec,
Reinhard Schomäckerc, Jochanan Bluma,∗
a Institute of Chemistry, The Hebrew University, Jerusalem 91904, Israel
b Center for Colloid and Surface Research, Al-Quds University, East Jerusalem 51000, Palestinian Authority
c Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 124, D-10623 Berlin, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
In the course of our attempts to replace harmful solvents in organic processes by environmentally favored
media, we investigated the use of aqueous microemulsions for catalytic decarbonylation of different
kinds of aldehydes. The aldehydes were solubilized in the microemulsions with the aid of the cationic
surfactant, cetyltrimethylammonium bromide. The aldehydes were transferred into CO-free products by
sol–gel entrapped catalysts. The best results were obtained in the presence of nanoparticles of Pd(0). The
heterogenized catalyst could usually be recycled 7–8 times without loss of catalytic activity. At relatively
low temperatures (140 ◦C) the decarbonylation proceeds stepwise. Initially a mixture of saturated and
unsaturated products is formed. At 180 ◦C however, fast hydrogenation of the unsaturated compounds
takes place. These investigations may be regarded as model studies for the conversion of biomass derived
intermediates to fuels and chemicals.
Received 7 August 2013
Received in revised form
16 September 2013
Accepted 17 September 2013
Available online 26 September 2013
Keywords:
Aldehyde decarbonylation
Heterogeneous catalysis
Microemulsion
© 2013 Elsevier B.V. All rights reserved.
Palladium nanoparticles
Sustainable chemistry
1. Introduction
e.g., Refs. [8–10]). The decarbonylation takes place in the pres-
ence of either transition metal derived catalysts [8], by a variety
With the intention to reduce the amount of harmful organic
solvents used in many synthetic and catalytic processes through
their replacement by water we have already developed a gen-
eral system in which the substrates are solubilized in aqueous
emulsions and the catalysts can be recycled [1]. In these sys-
tems suitable surfactants promote the solubilization in the aqueous
media and sol–gel materials support the catalysts. So far, we found
that our system can be applied to a variety of hydrogenations
processes [1], transfer hydrogenation [2], regioselective hydro-
formylation [3], double bond migration in allylic compounds [4],
disproportionation of dihydroarenes [5], carbon–carbon bond cou-
pling processes [6,7] (Heck, Suzuki, Stille and multi-component
addition reactions). We now find that the combination of aqueous
emulsions of the substrates and sol–gel entrapped catalysts – the
so-called emulsion/solid-heterogenization method for transport
and catalysis (EST) – is applicable to facile aldehyde decarbony-
lation processes. Under conventional conditions (i.e., in organic
solvents) aldehyde decarbonylations are involved in a variety of
both industrial and academic studies (for some recent reviews see
of enzymes [11] or under photochemical conditions [12]. The key
step in the transformation of biomass into fuels is often assumed
to involve decarbonylation of the degradation products of plants
and animals, and some of the natural pathways have been imitated
[13]. The facile deformylation of aldehydes enables the utiliza-
tion of carboxaldehydes for gas-free carbonylation and for transfer
hydrogenation [14]. The use of an aqueous medium in aldehyde
decarbonylations by a recyclable catalyst can be regarded as a green
version of this important process and mimics the suggested route
to hydrocarbon fuels [13].
2. Experimental
2.1. Instruments
NMR spectra were recorded on a Bruker DRX-400 instrument
in CDCl3. Infrared spectra were obtained with a Perkin-Elmer 65
FTIR spectrometer. Mass spectral measurements were performed
with a Q-TOF-II spectrometer. ICP-MS analyses were carried out
with a Perkin-Elmer model DRC II instrument. A Quantachrome
Novawin Instrument version 10.01 was used for Brunner, Emmet,
Teller (BET-N2) and Barett, Joyner, Halenda (BJH-N2) surface area
and pore diameter measurements of the sol–gel matrices. Gas
∗
Corresponding author. Tel.: +972 2 6585329; fax: +972 2 6513832.
E-mail address: jochanan.blum@mail.huji.ac.il (J. Blum).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.09.024

S. Dahoah et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 90–93
91
chromatographic determinations were carried out with a Hewlett-
Packard model Agilent 4890D by using either a 30 m long column
packed with Carbowax 20M-poly(ethylene glycol) in fused silica
(Supelco 25301-U) or a 15 m long column packed with bonded
and crosslinked (5% phenyl)methyl polysiloxane (HP-5). X-ray Pho-
toelectron Spectroscopy (XPS) measurements were performed on
a Kratos Axis Ultra X-ray photoelectron spectrometer (Karatos
Analytical Ltd., Manchester, UK). High resolution XPS spectra
were acquired with monochromatic A1K␣ X-ray radiation source
(1486.6 eV) with 90◦ takeoff angle (normal to the analyzer). The
pressure in the chamber was about 2 × 10−9 Torr. The wide (survey)
spectra were obtained for range 0–600 eV with pass energy 150 eV
and step 1 eV. The high-resolution XPS spectra were collected for C
1s, Si 2p and Pd 3d levels with pass energy 20 eV and step 0.1 eV. The
binding energies (BE) were calibrated respecting to the C1s peak
energy position as 285.0 eV. Data analyses were performed using
Casa XPS (Casa Software Ltd.) and Vision data processing program
(Kratos Analytical Ltd.). Transmission electron microscopy (TEM)
was done with a scanning transmission microscope Technai G2
F20 operated at 200 kV and equipped with an EDAX-EDS device for
identification of the elemental composition. Initial powders were
dispersed in ethanol and dropped onto a standard 400 mesh carbon
coated copper TEM grid.
2.5 wt.%) and 1-propanol (1.2 ml, 6.6 wt. %) was stirrer magnetically
at room temperature (25 ◦C) until a clear transparent mixture that
scatters laser beams was formed. In some cases the addition of a
few drops of 1-propanol was necessary.
2.5. General procedure for the catalytic decarbonylation process
The sol–gel entrapped catalyst (10–200 mg) was roughly ground
and placed together with 2,5-di-tert-butylhydroquinone (10 mg)
and a freshly prepared microemulsion of an aldehyde in a mini
autoclave equipped with a sampler through which small sam-
ples could been removed periodically. The reaction mixture was
parched with nitrogen, stirred magnetically and heated with a con-
trollable thermostat at the required temperature for the desired
length of time. The reaction mixture was cooled to 20 ◦C. For the
determining of the evolved CO the gases were transferred to a
toluene solution of excessive ClRh(PPh3)3. The transformation of
the latter complex to ClRh(CO)(PPh3)3 started immediately. After
1 h the rhodium carbonyl complex was filtered, dried and ana-
lyzed. In all experiments the amount of the isolated rhodium
carbonyl complex was above 78%. The used sol–gel entrapped cat-
alyst was filtered off from the reaction mixture, and the filtrate was
separated from the oily layer by addition of an excess of NaCl. Some-
times the microemulsions underwent spontaneous separation into
two phases without addition of the salt. The aqueous phase was
extracted with either hexane or ether and analyzed by compar-
ison with authentic samples. The used catalyst was washed and
sonicated with water and ether prior to its use in further catalytic
runs.
2.2. Chemicals
The unsubstituted benzaldehyde, 2- and 4-chlorobenzaldehyde,
4-methyl-,
1-naphthaldehyde,
4-methoxy-,
and
4-ethylbenzaldehyde,
3-
3-phenylpropionaldehyde,
phenylbutyraldehyde, E-cinnamaldehyde, 1-decanal, as well
as all the reference compounds (CO-free hydrocarbons), and the
surfactant cetyltrimethylammonium bromide, the sol–gel precur-
sor tetramethoxysilane, the precursors of the catalysts, palladium
acetate, disodium tetrachloropalladate trihydrate, rhodium and
iridium trichloride hydrates, 3-aminopropyltrimethoxysilane,
2,5-di-tert-butylhydroquinone, 1,2-(diphenylphosphinopropane)
(dppp) and sodium borohydride were obtained from commercial
sources. 2-Phenylpropionaldehyde [8] as well as bis[1,3-
bis(diphenylphosphino)-propane]rhodium chloride [15] were
prepared according to literature processes.
3. Results and discussion
Some examples of catalytic decarbonylation of 3-
phenylpropionaldehyde by sol–gel encaged rhodium, iridium
and palladium derivatives are summarized in Table 1. Further
decarbonylations that have been studied are shown in Scheme 1.
Upon completion of the decarbonylation process metallic
nanoparticles were found within the sol–gel matrices even
when the initial heterogenized catalyst was nanoparticle-free
organometallic complexes. An example of such nanoparticles gen-
erated from sol–gel encaged ClRh(PPh3)3 during decarbonylation
of 3-phenylpropionaldehyde are shown in the TEM-micrograph in
Fig. 1. Whether the immobilized metallic nanoparticles or the resid-
ual complexes are responsible for the catalytic processes could not
be estimated at this stage of our research. At 140 ◦C the phenyl-
propanals underwent decarbonylation and dehydrogenation to
ethylbenzene and styrene by the catalysts (except by the iridium
compound, Table 1, entry 6). The ratio between the two products
changes however, by the introduction of minor alterations of the
reaction conditions. At higher temperatures (>180 ◦C) the styrene,
2.3. Preparation and entrapment of metallic nanoparticles in
sol–gel matrices
Metallic nanoparticles were prepared by the method of Bharath
et al. [16]. Typically, to a solution of Na2PdCl4·3H2O (30 mg,
0.086 mmol) and H2NCH2CH2CH2Si(OMe)3 (0.8 ml) in MeOH
(27 ml) was added under N2 at room temperature solid NaBH4
(27 mg) essentially as previously described [17]. After stirring the
mixture for 15 h the MeOH was removed by decantation and to the
residue was added THF (2 ml), triply distilled water (TDW, 2 ml) and
TMOS (3.6 ml). The stirring was continued as long as possible. The
gel was washed and sonicated with ether (2 × 20 ml) dried at 80 ◦C
and 0.5 Torr to obtain constant weight. By this procedure we usually
obtained 1.4–1.6 g of sol–gel entrapped Pd(0) nanoparticles.
Sol–gel entrapped Rh(0) and Ir(0) nanoparticles were obtained
by the same methods from the corresponding metal trichloride
hydrates.
Table 1
Decarbonylation of 3-phenylpropanal to ethylbenzene by some sol–gel entrapped
transition metal catalysts under EST conditions.a
Entry
Catalyst
Conversion of substrates,b
%
1
2
3
4
5
6
Rh(0)@sol–gel
5
13
20
70.5
14
RhCl(PPh3)3@sol–gel
RhCl(dppp)2@sol–gel
Pd(0)@sol–gel
Pd(OAc)2@sol–gel
Ir(0)@sol–gel
The surface area and pore-diameters of matrices of selected
entrapped catalysts have been measured.
44
2.4. Preparation of the microemulsions of the aldehydes
a
Reaction conditions: precatalyst (0.1 mmol) encaged within sol–gel from TMOS
(3.6 ml), water (15 ml), 1-propanol (1.1 ml), cetyltrimethylammonium bromide
(400 mg), 140 ◦C, 24 h. The percentage of water of the microemulsion was ∼90 wt.%.
A mixture of freshly distilled aldehyde (1 mmol which amounts
to ca. 0.8 wt.% of the expected microemulsion), TDW (15–20 ml,
90.1 wt.%), cetyltrimethylammonium bromide (CTAB, 0.4–0.8 g, ca.
b
These figures represent the total yield of ethylbenzene (decarbonylation prod-
uct) and of styrene (dehydrodecarbonylation) except for entry 6.

92
S. Dahoah et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 90–93
Scheme 1. Decarbonylation of carboxaldehydes.
Fig. 1. TEM images of sol–gel entrapped ClRh(PPh3)3 (a) before and (b) after decarbonylation of 3-phenylpropionaldehyde.
in the presence of 2,5-di-tert-butylhydroquinone, undergoes fast
transfer hydrogenation to ethylbenzenes.
in quantitative yields by heating them either for 22 h at 200 ◦C or
for 45 h at 180 ◦C in aqueous microemulsions. The aforementioned
aldehydes gave lower yields either because of steric effects (cf.
entries 5 and 6) or because of electronic reasons (compare entries
4 and 6). We do not fully understand the diminished activity of the
fluorinated compound 7. We assume that the formations of some
stable complexes with the fluorine moiety are formed. The yields
of decarbonylation of compounds 4, 5, 7, 11 and 13 after 45 h at
180 ◦C were 47, 71, 44, 50 and 22%, respectively. Some comparative
aldehyde decarbonylations for 5 h at 180 ◦C are summarized in
Table 2. The further progress of the reaction at extended reaction
time shows that the catalyst is not deactivated by the substrates 4,
5, 7, 11 and 13. The yields of the decarbonylation products were
determined, in addition to conventional GC and NMR analyses,
also by transferring the evolved gases into a toluene solution of
ClRh(PPh3)3 that gives in the presence of the CO, the sparingly
soluble E-ClRh(CO)(PPh3)3. Kinetic measurements revealed that
under the EST conditions all the decarbonylations studied follow
the first order rate law in the substrate but only some of them
can be expressed in terms of a Hammett plot (see Table 3 and
Fig. 2). Thus, the decarbonylations in aqueous microemulsions
resemble the features of the reactions in diglymes [18,19]. In both
systems the Hammett plots have positive slopes, which suggest
that electron-attracting groups enhance the reaction and electron
withdrawing ones cause the decarbonylation to slow down. The
effect in the aqueous medium (ꢀ = 1.65) surpasses that in the
The metallic pre-catalysts were prepared simply by
H2N(CH2)3Si(OMe)3–NaBH4 reduction of the corresponding
metal chlorides in analogy to the method of Bharath et al. [16].
Their entrapment within the silica sol–gel matrices was per-
formed as previously reported [17]. Their BET-N2 and BJH-N2
analyses revealed high surface areas before the decarbonylation
processes, which changed, however, during the catalytic reactions
(vide infra). The immobilized catalyst proved to be an efficient
decarbonylation catalyst for a variety of aldehydes. Except for
compounds 4, 5, 7, 10 and 13, the aldehydes listed in Table 2 were
transferred completely into the respective carbonyl-free products
Table 2
Decarbonylation of some aldehydes by sol–gel encaged Pd(0) particles under com-
parable reaction conditions.a
Entry
Substrates
Main productb (conversion, %)
1
2
Benzaldehyde (1)
4-Toluealdehyde (2)
Benzene (60)
Toluene (42)
3
4
5
6
7
8
9
10
11
12
13
4-Ethylbenzaldehyde (3)
4-Methoxybenzaldehyde (4)
2-Chlorobenzaldehyde (5)
4-Chlorobenzaldehyde (6)
4-Fluorobenzaldehyde (7)
1-Naphthaldehyde (8)
2-Phenylpropionaldehyde (9)
3-Phenylpropionaldehyde (10)
3-Phenylbutyraldehyde (11)
E-Cinnamaldehyde (12)
1-Decanal (13)
Ethylbenzene (67)
Anisole (15)
Chlorobenzene (3)
Chlorobenzene (85)
Fluorobenzene (11)
Naphthalene (66)
Ethylbenzene (99)
Ethylbenzene (10)c
Cumene (10)d
Table 3
Substituent effect on the catalytic decarbonylation of some benzaldehydes by het-
Styrene (9)
Nonane (4)
erogenized Pd(0) nanoparticles under the EST conditions.a
a
Entry
Substrate
k [10−5 ms−1
]
ks/ka
log (ks/ka)b
Typical reaction conditions: sol–gel entrapped Pd(0) nanoparticles (0.086 mmol,
roughly ground); microemulsion of aldehyde (1 mmol, ∼0.8–1.0 wt.%), CTAB
(2.5 wt.%), 1-PrOH (6.6 wt.%), TDW (90.1 wt. %); 180 ◦C, 5 h within a glass lined
autoclave. The products were analyzed by comparison with standard solutions of
authentic samples of the corresponding saturated and unsaturated products.
1
2
3
4
4-H3COC6H4CHO
4-H3CC6H4CHO
C6H5CHO
1.30
3.00
5.17
9.67
0.26
0.58
1.00
1.87
−0.588
−0.236
0
4-ClC6H4CHO
0.272
b
The conversions are the average of at least two experiments that did not differ
a
by more than 3%.
Reaction conditions as in Table 2.
c
b
Accompanied by 7% styrene.
Accompanied by 5% isopropenylbenzene.
ka and ks represent the reaction constants for the composition of the unsubsti-
tuted and substituted benzaldehyde, respectively.
d

S. Dahoah et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 90–93
93
4. Conclusions
The EST system is applicable to the decarbonylation of a vari-
ety of aldehydes in aqueous media. During the decarbonylation
the immobilized catalysts form metallic nanoparticles that can be
recycled at least seven times without loss of catalytic activity. The
most efficient catalyst studied is the one derived from palladium
nanoparticles. At 140 ◦C the decarbonylations lead in part to the for-
mation of alkenes. At 180–200 ◦C the saturated alkane derivatives
are the predominate products.
Acknowledgement
We gratefully acknowledge the financial support of this trilat-
eral study by the Deutsche Forschungsgemainschaft (DFG) through
grant SCHO687/8-2. We also thank Dr. Inna Popov for her help in
the TEM studies, Dr. Vitally Gutkin for his help in the XPS work and
Gilad Zafriri for the BET and NJH measurements.
Fig. 2. Hammett plot for the decarbonylation of some benzaldehyde derivatives
using sol–gel encaged Pd(0) nanoparticles at 180 ◦C as catalyst. The ꢁ values were
taken from Ref. [19]; Y = 1.65; R2 = 0.9631.
organic solvent. The fact that metallic palladium particles pro-
mote the decarbonylation could suggest that during the process
the metallic nanoparticles are transformed into a catalytically
active complex (as e.g., in the arene hydrogenation process) by
the nanoparticle formed from Rh2Co2(CO)12 [20,21]. However,
we were unable to trace by IR any metal-carbonyl bands that
may confirm this suggestion. Furthermore, XPS studies on the
oxidation state of the palladium after the decarbonylation process
indicate only the presence of Pd(0) species. In the high resolution
spectrum in the 300–346 eV range appeared a sharp peak of Pd
3d5/2 on 335.5 eV and of Pd 3d3/2 on 340.7 eV which corresponds to
Pd(0) [22].
As indicated above the BET-N2 and BJH-N2 analyses of
the sol–gel matrices in which the metallic nanoparticles were
entrapped, reveal structural changes during the decarbonylation
processes. While the average surface area of the Pd(0) particle
loaded sol–gel materials was 559 m2/g before the decarbonyla-
tion and the pore diameter 33 nm, the surface area decreased to
215–255 m2/g after the reactions. But no change in the pore vol-
umes was detected. We recall that in other cases where sol–gel
entrapped palladium catalyst was applied (e.g., in the catalytic
Heck coupling [6]) the surface area decreased upon recycling of the
immobilized catalyst owing to the Si O bond cleavage by hydrol-
ysis during the catalysis in the aqueous media. The recyclability of
the Pd(0) particles was found to be unusually high, in most cases
no decrease in the rate could be observed. In a typical series of
experiments of decarbonylation of 4-chlorobenzaldehyde for 18 h
at 180 ◦C the yields were 96–100% for the first 7 runs. Only at the
eighth run the yield dropped sharply to 76%.
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