Selectivity Switch in the Aerobic 1,2-Propandiol Oxidation Catalyzed by Diamine-Stabilized Palladium Nanoparticles

Article abstract of DOI:10.1002/cctc.202100309

Palladium nanoparticles stabilized by a sterically demanding secondary diamine ligand have been synthesized by hydrogen reduction of a palladium acetate complex bearing the corresponding diimine ligand. The obtained nanoparticles were used to catalyze the aerobic oxidation of 1,2-propandiol in n-hexane, and after their heterogenization onto a high surface area carbon, in water. In n-hexane (2,4-dimethyl-1,3-dioxolan-2-yl) methanol has been obtained as major product, whereas in water acetic acid with a selectivity of >85 % has been achieved. The selectivity switch observed was a clear induced by water. The robustness of diamine-stabilized palladium nanoparticles under real aerobic oxidation conditions has been proved by recycling experiments, TEM measurements of the recovered catalysts and by comparison of its performance with that of palladium nanoparticles generated by the metal vapor synthesis technique and supported onto the same carbon in the absence of the stabilizing diamine ligand.

Full text of DOI:10.1002/cctc.202100309

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Selectivity Switch in the Aerobic 1,2-Propandiol Oxidation
Catalyzed by Diamine-Stabilized Palladium Nanoparticles
[
a]
[b]
[a]
[a]
Werner Oberhauser,* Claudio Evangelisti, Laura Capozzoli, Gabriele Manca,
[c]
[a]
Maria Pia Casaletto, and Francesco Vizza
Palladium nanoparticles stabilized by a sterically demanding
secondary diamine ligand have been synthesized by hydrogen
reduction of a palladium acetate complex bearing the corre-
sponding diimine ligand. The obtained nanoparticles were used
to catalyze the aerobic oxidation of 1,2-propandiol in n-hexane,
and after their heterogenization onto a high surface area
carbon, in water. In n-hexane (2,4-dimethyl-1,3-dioxolan-2-yl)
methanol has been obtained as major product, whereas in
water acetic acid with a selectivity of >85% has been achieved.
The selectivity switch observed was a clear induced by water.
The robustness of diamine-stabilized palladium nanoparticles
under real aerobic oxidation conditions has been proved by
recycling experiments, TEM measurements of the recovered
catalysts and by comparison of its performance with that of
palladium nanoparticles generated by the metal vapor synthesis
technique and supported onto the same carbon in the absence
of the stabilizing diamine ligand.
Introduction
cetone selectivity at 33% 1,2-PD conversion at 40°C in the
absence of base).
1
,2-Propandiol (1,2-PD) is generally obtained by catalytic hydro-
Hydroxacetone is an important platform molecule for the
[1–5]
[13]
genolysis reactions of glycerol
and its further selective
synthesis of renewable diesel and the synthesis of higher
[14]
conversion under aerobic oxidation conditions to give either
selectively hydroxyacetone or acetic acid is challenging. Indeed,
aerobic metal nanoparticle (NP)-catalyzed 1,2-PD oxidation
reactions conducted in neutral water results in a mixture of
products comprising hydroxyacetone, lactic acid, pyruvic acid
and formic acid. Catalytic reactions carried out with Pd-based
catalysts in the presence of basic water solutions led to lactic
acid as the main product, due to a base mediated intra-
oxygenates. Its synthesis from glycerol occurs under harsh
reaction conditions (i.e., vapor-phase dehydration at temper-
[
15–17]
atures >250°C).
Also acetic acid, an important bulk
molecule and mainly used for the synthesis of the vinyl acetate
monomer, is produced in a more than 10 million tons scale
by the Ir-based methanol carbonylation reaction (Cativa
[18]
[19]
process).
We herein report the synthesis of Pd NPs (1) obtained upon
[6–8]
[20]
molecular Cannizzaro reaction of pyruvaldehyde.
supports (e.g. hydroxyapatite and Mg(OH) ) showed to be
Basic
hydrogenation of an organometallic Pd(II) precursor. 1 was
[
21]
efficiently stabilized by exerting the steric effect
of a
2
important for efficient dehydrogenation reactions during the
secondary diamine ligand bearing n-hexadecyl groups at both
[9,10]
1
,2-PD conversion.
The highest acetic acid selectivity in the
nitrogen donor atoms. Amines featured by long aliphatic chains
[
22]
aerobic 1,2-PD oxidation was obtained with Ag NPs in the
presence of base (i.e. 63% acetic acid selectivity at complete
have been efficiently used in Pd-based hydrogenation, Suzuki
[23,24]
[25]
Miyaura coupling
and aerobic alcohol oxidation reactions.
[
11]
1
,2-PD conversion at 120°C)
and with a bimetallic AuÀ Pt
1 has been used to catalyze the aerobic oxidation of 1,2-PD in
n-hexane and supported onto a high surface area carbon in
neutral water.
catalyst supported onto carbon using neutral water solutions of
,2-PD (i.e. 66% acetic acid selectivity at 82% 1,2-PD conversion
1
[12]
at 115°C). The latter catalytic system gave also the highest
Pd NPs are known to be a sensitizing agent causing allergic
2
6]
selectivity for hydroxyacetone from 1,2-PD (i.e. 60% hydroxya-
contact dermatitis. Hence the application of a low amount of
Pd-based catalyst combined with its recyclability is mandatory
[27,28]
for a sustainable catalytic process.
[
[
[
a] Dr. W. Oberhauser, Dr. L. Capozzoli, Dr. G. Manca, Dr. F. Vizza
Istituto di Chimica dei Composti Organometallici (CNR-ICCOM)
Via Madonna del Piano 10, 50019 Sesto Fiorentino (Italy)
E-mail: werner.oberhauser@iccom.cnr.it
b] Dr. C. Evangelisti
Istituto di Chimica dei Composti Organometallici (CNR-ICCOM) U.O.S. di
Results and Discussion
Synthesis and characterization of diamine-stabilized Pd NPs
Pisa
Via G. Moruzzi 1, 56124 Pisa (Italy)
c] Dr. M. P. Casaletto
Istituto per lo Studio dei Materiali NanoStrutturati (CNR-ISMN)
Via Ugo La Malfa 153, 90146 Palermo (Italy)
The diimine ligand L (Scheme 1) reacted with a CH Cl solution
2
2
of Pd(OAc) (OAc=acetate) at 35°C in the presence of hydro-
2
gen (20 bar) for 18 h, giving a black suspension, which was
concentrated to dryness. The dark brown residue was succes-
sively suspended in n-pentane and then filtered. The n-pentane
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Scheme 1. Synthesis of 1 and 1@C .
[
29]
insoluble fraction proved to be the ammonium salt of H4L
Figure S5). The black n-pentane-soluble fraction was first
extracted with water to remove the formed acetic acid (i.e.,
HPLC analysis of the water solution confirmed the presence of
acetic acid) and then concentrated to dryness giving a black
solid (1) (Scheme 1).
48.1 and 51.9 peak area %, respectively (Table S1). The N 1s
photoelectron signal of 1 (Figure 2b) exhibited a slightly
asymmetric peak, which revealed the presence of two compo-
nents at BE=399.7 eV (75.6 peak area %) and at BE=400.7 eV
(24.4 peak area %). A comparison of the N 1s spectra for H4L
and 1 (Figure 2b) clearly showed a shift of 0.7 eV of the main
contribution (BE=399.0 eV) to higher BE in case of 1 (BE=
399.7 eV). This shift of BE was attributed to the interaction of
(
A STEM micrograph acquired for 1 (Figure 1) confirmed the
presence of spherical, well-dispersed Pd NPs characterized by
an average diameter of 3.6�0.64 nm.
[
30]
the amine functionality of H L with the Pd NPs’ surface atoms.
4
The corresponding powder X-ray diffraction (PXRD) spec-
trum (Figure S6) showed Bragg reflexes, typical for fcc Pd, which
were broad due to the small NPs’ size. In addition, a Bragg
reflex at 21.6° (2Θ) has been assigned to the crystalline fraction
The second component of the N 1s spectrum of 1 at BE=
400.7 eV was assigned to a fraction of NH-groups undergoing
hydrogen bond interactions with Pd-hydroxide species, the
presence of which was also confirmed by the O 1s peak at BE=
[31]
of the capping ligand. ICP-OES analysis of 1 exhibited a Pd
content of 23.6 wt%.
532.0 eV (Figure S7). The N to Pd atom ratio in 1 was shown
to be 3 (Table S2).
1
The XPS curve-fitting of the Pd 3d spectrum acquired for 1
The H NMR spectrum acquired for 1 in C D (Figure S8)
6
6
(Figure 2a) showed the presence of two distinct double peak
clearly revealed a notable broadening of the NMR signals
components (Pd 3d5/2 and Pd 3d3/2 spin orbit splitted). In peak 1
assigned to the nitrogen atoms-bridging methylene and the α-
Pd 3d 5/2 located at BE=335.3 eV and in peak 2 Pd3d5/2 at BE=
methylene groups of the C -alkyl chain at 2.31 and 2.68 ppm,
16
3
37.9 eV can be assigned to Pd(0) and Pd(II) surface atoms in
respectively, due to their interaction with the surface NPs’ Pd
Figure 1. STEM micrograph and histogram for as-synthesized 1 (d
m
=3.6�0.64 nm, N=514).
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Figure 2. XPS curve-fitting of Pd 3d and N 1s spectra in the investigated samples, a) and b) respectively. In the Pd 3d region the red line refers to the Pd 3d5/2
and Pd 3d3/2 spin-orbit components of peak 1 and the blue line to those of peak 2.
[
32,33]
atoms.
Accordingly, the IR spectrum of 1, acquired as KBr
diimine ligand to the corresponding diamine failed at this
molecular level.
pellet, showed the absence of the NÀ H and CÀ N stretching
À 1
bands, typically found for H L (KBr pellet) at 3238 cm and
The importance of hexadecyl units in the stabilizing ligand
of 1 for the NPs’ dispersion was underscored by carrying out an
analogous synthesis, as described in Scheme 1, using a diimine
bearing n-butyl groups. As a result, Pd precipitated on the
Teflon walls of the autoclave leading to a completely trans-
parent CH Cl solution.
4
À 1
1
130 cm , respectively (Figure S9).
A parallel synthesis of 1, following the same synthesis
protocol as described in Scheme 1, but using H L to coordinate
Pd(OAc) instead of L brought about the formation of Pd NPs
4
2
characterized by a huge size distribution (d =3.0�1.3 nm)
m
2
2
(
Figure S10). This experimental result is strongly indicating that
In order to carry out catalytic aerobic1,2-PD oxidation
the hydrogenation of L to H L does not occur at an early stage
reactions with 1 in neat water, 1 was supported onto
4
K
2
[34]
of the synthesis of 1. Indeed, DFT-based calculations conducted
Ketjenblack (C , high surface area carbon 1400 m /g) by an
1
K
with the molecular model compound M (Scheme 2, see more
impregnation method (Scheme 1) giving 1@C with a Pd
details in SI) showed that upon reaction with hydrogen in
CH Cl , acetic acid is formed through a dissociative acetate-
content of 1.28 wt% (ICP-OES analysis). A TEM micrograph of as-
synthesized 1@C (Figure 3) confirmed the presence of Pd NPs
K
2
2
based hydrogen activation process and that a final dimer Pd(I)
hydride species with a two η -N coordinating diimine ligands
identical in average size to that of 1 (i.e., d =3.3�0.6 nm
m
1
K
(1@C ) vs 3.6�0.64 nm (1)).
6
1
6
K
(
M ) was obtained. The transformation of M into M has been
Also, the Pd 3d XPS spectrum for 1@C showed a Pd(0)/
À 1
associated to a free energy gain of -15.4 kcalmol and the rate
determining step for the conversion of M to M was the de-
coordination of one acetate anion from M giving M (i.e., free
energy cost of 18.4 kcalmol ). Any attempt to find a low free
Pd(II) atom ratio of 61:39 (Figure 2a), which compared well
1
6
with that of 1. In order to validate the catalytic performance of
1
2
K
K
1@C we introduced Pd@C as catalyst reference system. The
À 1
latter catalyst was prepared by the metal vapor synthesis (MVS)
K
energy reaction path for the hydrogenation of the model
technique and supported onto C in the absence of the
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6
À 1
2 2
Scheme 2. DFT-based reaction path for the conversion of M into M in the presence of hydrogen and CH Cl . Free Gibbs energies are given in kcal mol .
stabilizing ligand H L (synthetic details are reported in the SI).
solution of 1 and 1,2-PD, are not miscible at room temper)
using air (20.0 bar) as final oxidant. The reaction products
obtained under the latter catalytic condition are shown in
Scheme 3 and the percentage of each product is compiled in
Table 1 (entries 1–6).
4
The Pd loading was of 1.5 wt% and the spherical NPs showed
an average size of 2.5�0.5 nm (Figure S11). For related NPs,
synthesized by the same technique, a Pd(II)/Pd(0) ratio of 31.6/
[35]
6
8.4 on the NPs’ surface has been found.
In order to intercept all products formed during catalysis,
methanol was added to the reaction mixture in order to
separate the n-hexane phase, which confined the catalyst, from
the reaction educts and products formed. The slightly yellow
methanol solution has been analyzed by GC (i.e., using 1-
hexanol as standard, Figure S12) and GC-MS. The carbon
1
1
-catalyzed aerobic 1,2-PD oxidation reactions in n–hexane
-catalyzed aerobic oxidation reactions of 1,2-PD were con-
ducted under biphasic reaction conditions (i.e., n-hexane
K
Table 1. Catalytic aerobic 1,2 PD oxidation with 1 in n-hexane and with 1/ Pd@C in water.
[
a]
[c]
Entry
Cat.
Pd [mmol]
T [°C]/t [h]
Conv.
Selectivity [%]
1,2 PD [%]
À 1 [b]
[
TOF(h )]
2
3
4+5+6
7
8
9+10
1
2
3
4
1
1
1
1
1
1
0.051
0.051
0.051
0.051
0.051
0.051
0.024
0.024
0.024
0.024
0.024
0.024
0.028
0.028
100/5
110/5
9.2/[n.d.]
18.2/[29]
59.7/[26]
85.7/[38]
20.0/[9]
78.0/[n.d.]
29.9[28]
45.0[42]
31.2[103]
89.1/[83]
79/[n.d.]
85.0/[118]
67.9/[43]
41.0/[n.d.]
7.8
91.3
86.1
81.8
70.0
34.0
68.0
–
–
–
–
0.9
1.4
1.8
3.6
–
3.8
–
–
–
12.5
16.4
26.4
58.0
28.2
53.2
33.7
41.0
7.0
13.1
5.0
18.9
45.0
110/18
120/18
120/18
120/18
110/18
120/18
140/5
140/18
140/18
140/24
140/18
140/18
[
[
d]
e]
5
6
7
8
9
1
1
1
1
1
1.0
6.0
1.0
K
1@CK
18.0
16.9
4.0
5.0
2.0
7.0
3.0
1.8
26.6
46.5
51.7
85.0
83.0
83.0
76.1
52.0
2.2
2.9
3.3
3.0
1.9
5.0
2.0
1.2
1@C
K
1@CK
0
1
2
3
4
1@CK
1@CK
1@C
–
[
[
e]
f]
K
Pd@CK
[
e]
Pd@C
À 1
[
[
a] Catalytic condition: 1,2-PD (13.70 mmol), solvent (50.0 mL), p(air)=20.0 bar. [b] Surface atom-related TOF (h ). [c] Selectivity (%) defined as: 100×
À 1
st
mmol(Product)]×[Σ (mmol(Products)
]
. [d] Addition of water (100.0 μL). [e] 1 recycling. [f] 1,2-PD (27.40 mmol).
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Figure 3. TEM micrographs and histograms for 1@C : as-synthesized (d
m
=3.3�0.6 nm, N=101) (upper) and after catalysis (T=140°C, t=18 h;
d
m
=4.2�1.34 nm, N=168) (lower).
balance was found to be >95%. Regardless of the reaction
temperature, (2,4-dimethyl-1,3-dioxolan-2-yl) methanol (3,
Scheme 3) was found to be the major product (Table 1).
Reaction temperatures lower than 110°C let to a low substrate
conversion (9.2%), whereas upon its increase to 120°C
1 from which emerged that the spherical Pd NPs’ size increased
only slightly in the course of the catalysis (d =4.2�1 nm
m
(recovered 1, Figure S13); d = 3.6�0.64 nm (as-synthesized 1,
m
Figure 1)). A recycling experiment with 1 showed a slight drop
of the 1,2-PD conversion to 78.0%, while the product selectivity
has found to be comparable to that obtained with as-
synthesized 1 (Table 1, entry 6 vs entry 4). ICP-OES analysis of
the separated methanol solution after catalysis showed a Pd
content of 0.1 ppm, which corresponded to a Pd leaching of
0.05%.
combined with a reaction time of 18 h, the1,2-PD conversion
increased to 85.7% (Table 1, entry 4). Under the latter exper-
imental condition, the side products (4, 5 and 6, Scheme 3)
were obtained with a selectivity of 3.6%, while 2 and 3 were
present with 26.4 and 70.0% selectivity, respectively. By
carrying out a catalytic reaction in the presence of a small
amount of water (100.0 μL), the 1,2-PD conversion dropped to
K
2
0.0% (i.e., due to a drop in the mass transfer in the
1@C -catalyzed aerobic 1,2-PD oxidation reactions in water
interphase), whereas the selectivity of 2 reached 58.0% at the
expense of the ketal (i.e., hydrolysis of the ketal). In addition,
under the latter reaction condition 8.0% of side products (7, 8,
In order to carry out aerobic 1,2-PD oxidation reactions in water
K
obtaining rational conversions, 1@C was used instead of 1 as
K
9
+10, Scheme 3) were found, with 8 being the major side
product (6%).
The stability of 1 during catalytic reactions lasting 18 h
catalyst. 1@C -mediated aerobic 1,2 PD oxidation reactions
were carried out with neutral water solutions of 1,2-PD (0.274 M
and 0.548 M) in the presence of air (20 bar) and in the
temperature range between 110 and 140°C (Table 1, entries 7–
(
110°C) was proved by carrying out STEM analysis on recovered
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Scheme 3. Reaction products obtained in the 1-and 1@C -catalyzed aerobic 1,2-PD oxidation in n-hexane and water, respectively.
1
4). All catalytic solutions have been analyzed by HPLC
selectivity for 8 increased at the expense of 2 (Table 1, entry 10
vs 9). This latter experimental fact is strongly indicating that 8
has been generated via decarboxylation of pyruvic acid,
1
13
1
(
(
Figure S14), H (Figure S15) and C{ H} NMR spectroscopy
Figure S16) and the intercepted products are shown in
[
36]
Scheme 3 and the results of the catalytic reactions in terms of
,2-PD conversion and product selectivities are compiled in
originating from oxidation of 2 and not from oxidation of 7.
1
In fact, efficient conversions of 7 to pyruvic acid have been
reported for MoÀ VÀ Nb-based mixed oxide catalysts at reaction
Table 1. Gas-phase analysis conducted after catalysis on the
head space of the autoclave revealed the presence of CO along
with excess of air (Figure S17). A carbon balance for the catalytic
[
37]
temperatures >200°C
or for AuÀ Pt NPs supported onto
2
[38]
acidic zeolites (140°C, p(O )=3 bar. Pyruvic acid has not been
2
reactions in water in the temperature range from 110 to 140°C
intercepted as reaction intermediate in the aerobic oxidation of
1,2-PD to 8 in neat water, indicating that the decarboxylation of
pyruvic acid is a fast reaction. Compounds 9 and 10 (Scheme 3)
have been obtained in low amounts (1-5%, Table 1). Their
formation during the catalytic reactions hints to the presence of
a small amount of surface Pd-acyl species, which can recombine
to give 10 and react with surface hydride species (i.e.,
originating from alcohol dehydrogenation reaction) leading to
9.
has been found to of 95 and 90%, respectively. A blank reaction
K
with 1,2-PD in water at 140°C, 18 h without 1@C , showed no
K
appreciable 1,2-PD conversion, underscoring the neat of 1@C
for a rational substrate conversion. At low 1,2-PD-conversions
(
110°C, entry 7), 2 was the main product, whereas upon
increasing the reaction temperature to 140°C combined with a
reaction time of 18 h, 8 was the main product reaching a
selectivity of 85.0% at a 1,2-PD conversion of 89.1% (Table 1,
entry 10). The selectivity for 7 decreased with increasing
temperature and remained comparable in reactions conducted
at 140°C with different 1,2-PD conversions, whereas the
K
Pd@C showed under identical catalytic conditions (140°C,
K
18 h) to be significantly less efficient compared to 1@C in
terms of surface-related TOF and selectivity for 8 (Table 1,
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entry 13 vs entry 10). Recycling experiments conducted with
Catalytic cycle
K
K
1
@C and Pd@C lasting 18 h clearly exhibited a significantly
K
higher activity and selectivity drop for 8 in case of Pd@C
1-catalyzed aerobic 1,2-PD oxidation reactions carried out in n-
hexane comprises one dehydrogenation step to give hydrox-
yacetone (Scheme 4, reaction a), which leads upon a condensa-
tion reaction with excess 1,2-PD to the formation of (2,4-
dimethyl-1,3-dioxolan-2-yl) methanol (Scheme 4, reaction f). The
addition of water shifts the equilibrium of the condensation
reaction in favor to free hydroxyacetone (i.e., a catalytic reaction
carried out with 1 in neat n-hexane suppresses the hydrolysis of
the acetal (Table 1)), which can further undergo a 1-catalyzed
dehydrogenation reaction (Scheme 4, reaction b) giving pyr-
uvaldehyde. The latter compound is known to be a reactive
intermediate which undergoes either a base-mediated Canni-
(
Table 1, entry 14 vs entry 13 and entry 11 vs entry 10). TEM
K
micrographs acquired for recovered 1/Pd@C (i.e., reactions
lasting 18 h) confirmed a notable increase of the NPs’ size
K
during catalysis in case of Pd@C (d =5.8�1.6 nm, recovered
m
K
K
Pd@C (Figure S18) vs d =2.5�0.5 nm, as-synthesized Pd@C
m
K
(Figure S11)), whereas 1@C showed a less pronounced increase
K
of the NPs’ size (i.e., 4.2�1.34 nm, recovered 1@C vs 3.3�
K
0
.6 nm, as-synthesized 1@C (Figure 3). The same result has
been obtained by comparing the PXRD spectra of recovered 1/
Pd@C (Figure 4, trace c vs trace e). The PXRD spectrum of
recovered Pd@C showed the presence of Bragg reflexes
Pd(111), Pd(002) and Pd(022) which are typical for fcc Pd,
whereas the PXRD spectrum of recovered 1@C , which has
been acquired applying the same acquisition parameters
showed only a slight hump for Pd(111), confirming a smaller
K
K
[39]
K
zzaro reaction to lactic acid (i.e. 1@C -catalyzed aerobic 1,2-
PD (0.274 M) oxidation reaction carried out in the presence of
an equimolar amount of NaOH at 140°C for 18 h, gave lactic
K
acid as main reaction product (lactic acid, 50.4% vs acetic acid,
41.1%), or reacts with water to give the corresponding
pyruvacetal (Scheme 4, reaction c), which then dehydrogenate
to give pyruvic acid (Scheme 4, reaction d). The three alcohol
dehydrogenation reactions which converts 1,2-PD into PA
(Scheme 4, reactions a, b and d) are catalyzed by surface Pd-OH
K
NPs’ size in case of recovered 1@C .
ICP-OES analyses carried out on the water solutions,
separated from the catalysts at 80°C, after catalysis gave with
both catalysts a Pd leaching of 0.05 ppm (i.e., 0.02% of the
original Pd content), and by continuing the catalytic reaction
without catalyst gave negligible 1,2-PD conversions. The differ-
K
species of 1@C (XPS evidence), which are transformed into
K
ent catalytic activity drop found for 1/Pd@C can thus be
associated to the different NPs’ size increase observed for both
catalysts.
PdÀ H (i.e., Pd-OH reacts with the alcohol to give the
corresponding Pd-alcoholate and water and the former under-
goes then a Pd-β-hydride elimination reaction giving PdÀ H
along with the release of the corresponding carbonyl or
carboxyl species). PdÀ H species on Pd NPs of an average size of
2.6 nm have been intercepted by a combination of X-ray
K
The significantly higher robustness of 1@C compared to
K
Pd@C under aerobic 1,2-PD oxidation conditions let us to carry
out an 1,2-PD oxidation with the former catalyst using a 1,2-PD
water solution of 0.548 M (i.e., molar ratio of 1,2-PD and surface
Pd atoms of 3358). As a result, a surface atom related TOF value
of 118 h along with an acetic acid selectivity of 83.0% was
obtained (Table 1, entry 12).
[
40,41]
absorption spectroscopy and X-ray diffraction.
The PdÀ H
species is retransformed into the Pd-OH species upon insertion
À 1
of an oxygen molecule into the PdÀ H species giving a Pd-
[42]
hydroperoxide intermediate, which reacts with water to give
K
K
K
K
Figure 4. PXRD spectra acquired at room temperature for: C (a); 1@C as-synthesized (b); 1@C after catalysis (140°C, 18 h) (c); Pd@C as-synthesized (d) and
K
Pd@C after catalysis (140°C, 18 h) (e).
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Scheme 4. Proposed catalytic cycle for the 1-mediated aerobic 1,2-PD oxidation.
free hydrogen peroxide and Pd-OH. Hydrogen peroxide syn-
thesis with Pd NPs in the presence of a H /O mixture has been
observed by operand spectroscopy.
Hydrogen peroxide then nucleophilic attacks pyruvic acid
leading to an oxidative decarboxylation of the latter and
ketal formed between hydroxyacetone and 1,2-propandiol),
whereas in the presence of water the same Pd-NPs converted
1,2-propandiol to acetic acid with a selectivity of 85% at 89%
1,2-propandiol conversion (i.e. 140°C, 18 h, p(air)= 20 bar),
2
2
[43]
outperforming the best known catalytic system for the con-
version of 1,2-propandiol to acetic acid, which uses a AuPt@C-
based catalyst, 10 bar of oxygen at 115°C for 24 h, leading to
bringing about the formation of acetic acid, CO and water.
2
Indeed, the decarboxylation of pyruvic acid in water by hydro-
[44]
K
gen peroxide occurs already at 25°C. The need of 1@C for a
an acetic acid selectivity of 66% at a 1,2-propandiol conversion
[12]
selective pyruvic acid decarboxylation reaction to occur has
been proved by conducting parallel reactions with water
solutions of pyruvic acid at 140°C in the presence of 20 bar of
of 82%. Water has an important role in the conversion of 1,2-
propandiol to acetic acid under aerobic oxidation conditions,
since it avoids the ketalization of hydroxyacetone by 1,2-
propandiol. The pyruvic acid decarboxylation reaction to acetic
acid is only selective in the presence of the palladium catalyst.
The notable stabilizing effect of the diamine ligand on the Pd
NPs is mainly steric in nature. On the other hand, the amine
functionalities of the ligand might be involved in hydrogen
bond interactions with water molecules close to the NPs’
surface, fostering hence all reactions steps of the aerobic
oxidation of 1,2-propandiol to acetic acid which need water to
occur.
K
air, with and without 1@C . As a result, in the presence of the
Pd-based catalyst pyruvic acid was quantitatively converted
giving acetic acid with a selectivity of 99%, whereas in its
absence 70% of pyruvic acid was converted to a 1:1 mixture of
lactic acid and acetic acid. Hence in the absence of the catalyst
pyruvic acid functions as hydride acceptor.
Conclusions
Pd NPs capped by secondary diamine ligand bearing C -alkyl
1
6
chains at the nitrogen atoms (1) have been synthesized and Experimental Section
successfully applied to catalyze the aerobic oxidation of 1,2-
propandiol under biphasic catalytic conditions (i.e., using n-
hexane solutions of 1) and in water, after having supported 1
onto a carbon (1@C ) featured by a high surface area. In the
Materials
Glyoxal (40 wt% water solution), hexadecylamine, iso-propyl alco-
hol, Pd(OAc) , dichloromethane, methanol, n-hexane, n-pentane,
K
2
absence of water (2,4-dimethyl-1,3-dioxolan-2-yl) methanol (i.e.
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[46]
C D , and CDCl were purchased from Aldrich. Dichloromethane
Wagner’s energy dependence of attenuation length
and a
6
6
3
was distilled over CaH , methanol over Mg, while n-hexane and n-
pentane were used as received. Ketjenblack (C ) was purchased
from Cabot Corp. USA.
standard set of VG Escalab sensitivity factors. The uncertainty on
the atomic quantitative analysis is about �10%. Assignment of
photoelectron signals and peak components was determined
2
K
[47]
according to literature reference database.
1
13
1
H and C{ H} NMR spectra were acquired in C D at room
6
6
Analytical methods
1
temperature by using a Bruker Avance 400 MHz spectrometer. H
NMR spectra were acquired at 400.12 MHz and C{ H} NMR spectra
at 100.0 MHz.
1
3
1
STEM (scanning transmission electron microscopy) images were
acquired with a TESCAN GAIA 3 FIB/SEM apparatus using STEM
detector operating at an accelerating voltage of 30 kV. TEM
analyses were carried out on a TEM PHILIPS CM 12 instrument,
equipped with an OLYPMPUS Megaview G2 camera, and using an
accelerating voltage of 100 keV. Samples suitable for the analyses
were prepared by depositing a drop of a n-pentane solution of 1 or
IR spectra were acquired as KBr pellets at room temperature in the
wave number interval 4000–400 cm using a Perkin Elmer instru-
ment.
À 1
ICP-OES analysis were carried out with an ICP-Optical emission dual
view Perkin Elmer OPTIMA 8000 apparatus.
K
water suspension of 1/Pd@C on a holey film of a 300 mesh Cu
grid, followed by evaporation of the solvent at room temperature.
HPLC analyses were carried out with a Shimadzu apparatus
equipped with a RID 10 A detector and an Aminex HPX-87H column
The mean particle diameter (d ) was calculated by using Equa-
m
tion (1)
(
300 mm×7.8 mm, BIO RAD) using H SO (0.005 M) as eluent with a
2 4
flow rate of 0.4 mL/min and an oven temperature of 35°C.
dm ¼ Sd n =Sn
(1)
i
i
i
GC analyses were carried out with a GC-2010 apparatus from
Shimadzu, equipped with an FID and a capillary column of the type
VF WAXms (30.0 m×0.25 mm×0.25 μm) from Agilent and He as
carrier gas.
with n corresponding to the number of particles with diameter d.
i
i
The total number of particles (N) used for the histogram is reported
in the caption of the corresponding figure.
Powder X-ray diffraction (PXRD) spectra were acquired at room
temperature with a PANalytical X’PERT PRO powder diffractometer,
using CuKα radiation (λ=1.5418 Å) and a parabolic MPD-mirror
and a PIXcel RTMS detector. The spectra were recorded in the 2Θ
range from 5.0 to 80.0° with a step size of 0.105° and a counting
GC-MS analyses were carried out with a QP2010SE apparatus from
Shimadzu, using the same capillary column as for GC analysis.
K
Syntheses of 1and 1@C
time of 428.9 s, using a Si zero background as sample holder.
K
Detailed protocols for the syntheses of L, H L and Pd@C are
4
reported in the Supporting Information (SI).
X-ray photoelectron spectroscopy (XPS)-analyses of the samples
was performed by using a XPS VG MicrotechESCA3000 Multilab
spectrometer, equipped with a twin anode X-Ray source (Mg and
Al) and a five channeltrons detection system. Photoemission
spectra were acquired by using a standard non-monochromatized
Al Kα (hν=1486.6 eV) excitation source, an ultrahigh vacuum
L
0
(343 mg, 0.680 mmol) (Scheme 1) and Pd(OAc)2 (152.6 mg,
.680 mmol) were added to a Teflon-lined autoclave, which was
sealed and evacuated. Afterwards dichloromethane (50.0 mL) was
deaerated and introduced into the autoclave by suction. The
autoclave was then successively stirred at room temperature for
one hour, followed by its pressurization with hydrogen (20 bar) and
mechanical stirring at 35°C for 18 hours. The autoclave was then
successively cooled to 10°C by means of a water-ice bath, the gas
À 6
chamber (base pressure lower than 1×10 Pa) and a hemispherical
analyser operating in CAE mode. The binding energy (BE) scale was
calibrated with Au 4f7/2 core level at 83.9 eV. C 1s peak from the
adventitious carbon (BE=285.1 eV) was used for charge correction
of all spectra. The accuracy of the energy measure is �0.1 eV.
Photoemission data were collected and processed by VGX900 and
XPSPEAK4.1 software, respectively. XPS data analysis was performed
by a nonlinear least square curve-fitting procedure in order to
extract chemical information from the spectral data (i.e., to separate
the photoemission signal originating from distinct elemental or
chemical states). A curve fitting model defined in terms of
component peaks, as a properly weighted sum of Lorentzian and
Gaussian component for the shape of Voight curves, and a Shirley
pressure released, and the dark brown suspension transferred into
a round bottom flask. The solvent was removed by vacuum and the
brown residue suspended in n-pentane (50.0 mL), followed by
filtration over celite. The obtained dark brown solution was
extracted with water (3×10 mL) (i.e., to remove acetic acid, which
was formed during the reaction), dried over Na SO , filtered and
2
4
concentrated to dryness. In order to obtain the product as powder,
the solid was dissolved in dichloromethane (10.0 ml) and then
concentrate again to dryness, obtaining a dark brown solid. Yield:
1
1
10 mg. ICP-OES analysis of 1 showed a Pd content of 23.6 wt%. H
[
45]
background subtraction according to Sherwood has been used.
The fitting parameters that were allowed to vary for a component
peak included the energy position, the full width at half maximum
NMR (C D ) and IR (KBr) spectra are shown in SI, Figure S8 and S9,
6
6
respectively.
K
K
(
FWHM), the percentage of Lorentzian/Gaussian lineshape and area.
1@C was synthesized by suspending Ketjenblack (C ) (1.0 g) in n-
pentane (180.0 mL) followed by sonication for 5 min. at room
temperature, addition of 1 (54.2 mg) dissolved in n-pentane
(30.0 mL), stirring for an hour under a nitrogen atmosphere and
then evaporating the solvent by vacuum. The obtained black solid
was continued drying at 50°C under vacuum. The Pd content of
The software used fitted the experimental data by adjusting all the
parameters of the component peaks until a χ minimum was
2
obtained. The BE position of a component peak resulting from the
curve fitting provided evidence for assignment to an elemental or
chemical environment. Quantitative information concerning the
concentration of the chemical states was inferred by measuring the
relative area of each component peak. C 1s, N 1s and O 1s spectra
showed a single XPS peak, whereas Pd 3d showed a double peak
with well separated Pd 3d5/2 and Pd 3d3/2 spin-orbit components
the isolated solid was analyzed by ICP-OES, giving a Pd content of
1.28 wt%.
(
energy split of ~5.4 eV). Surface relative atomic concentrations
were calculated by a standard quantification routine, including
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K
Catalytic aerobic 1,2-PD oxidation reactions
the same amount of 1@C as used for 1,2-PD oxidation reactions
(Table 1) has been used).
Catalytic reactions in n-hexane
1
(23.22 mg, 0.051 mmol) was dissolved in n-hexane (50.0 mL) and
Acknowledgements
transferred into a Teflon-lined steel autoclave (320.0 mL). To the
clear dark brown solution was added 1,2-PD (1.0 mL, 13.7 mmol) at
room temperature. The autoclave was then successively sealed,
heated to the desired reaction temperature, pressurized with air
The authors thank the Italian National Research Council (CNR)
microscopy facility “Ce.M.E.-Centro Microscopie Elettroniche Laura
Bonzi” for providing the facilities for the Gaia 3 (Tescan s.r.o, Brno,
Czech Republic) instrument acquired thanks to “Ente Cassa di
Risparmio di Firenze” Grant Number n. 2013.0878, Regione
Toscana POR FESR 2014–2020 and the project FELIX (Fotonica ed
Elettronica Integrata per l’ Industria), Grant Number 6455.
(
20 bar) and magnetically stirred (1000 rpm). After the catalytic
reaction, the autoclave was cooled to 10°C by means of a water-ice
bath, slowly vented and methanol (10.0 mL) added. The resulting
suspension was stirred, the methanol phase separated from n-
hexane solution and n-hexanol (standard, 200 μL) was added to the
former solution. The methanol solution was analyzed by GC and
GC-MS and each product was quantified by means of GC analysis
using calibration curves. For the recycling experiment, the recov-
ered n-hexane solution of 1 was reused.
Conflict of Interest
Catalytic reactions in water
The authors declare no conflict of interest.
A Teflon-lined autoclave (320.0 mL) was successively charged with
K
K
1
@C (200.0 mg), Pd (1.28 wt%, 0.024 mmol) or Pd@C (200.0 mg)
Pd (1.5 wt%, 0.028 mmol) and a water solution (50.0 mL) of 1,2-PD
0.274 M or 0.548 M) sealed, heated to the desired reaction temper-
ature, pressurized with air (20 bar) and mechanically stirred
1000 rpm) for the desired reaction time. Afterwards, the autoclave
Keywords: heterogeneous catalysis · air · oxidation · 1,2-
propandiol · palladium
(
(
was cooled to 10°C and the gas slowly vented. A fraction of this
venting gas was collected for gas analysis, which was carried out
with a GC apparatus from Shimadzu, equipped with a TCD.
Thereafter the autoclave was opened, and the suspension filtered.
[
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[
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FULL PAPERS
Dr. W. Oberhauser*, Dr. C. Evangelis-
ti, Dr. L. Capozzoli, Dr. G. Manca,
Dr. M. P. Casaletto, Dr. F. Vizza
1
– 12
Selectivity Switch in the Aerobic
,2-Propandiol Oxidation
1
Catalyzed by Diamine-Stabilized
Palladium Nanoparticles
Switch it up! Hexane solutions of the
diamine-stabilized palladium nanopar-
ticles efficiently catalyzed the aerobic
oxidation of 1,2-propandiol, giving
oxidized in water 1,2-propandiol to
acetic acid. The selectivity switch was
triggered by water. Selectivity switch
in the aerobic 1,2-propandiol
(
2,4-dimethyl-1,3-dioxolan-2-yl)
oxidation catalyzed by diamine-stabi-
lized palladium nanoparticles (Werner
Oberhauser CNR-ICCOM CNR-ISMN)
methanol as major compound,
whereas the same nanoparticles