Organic ligand and solvent free oxidative carbonylation of amine over Pd/TiO2 with unprecedented activity

Article abstract of DOI:10.1039/c9gc01577a

A highly active Pd/TiO₂ catalyst system was prepared and applied in the oxidative carbonylation of amines to ureas with ultra-low Pd content under organic ligand and solvent free conditions. The catalytic turnover frequencies (TOFs, moles of amines converted per mole of Pd per h) were 126000 and 250000 h^-1 for the production of diphenylurea and dibenzylurea, respectively. An expanded substrate scope including the electron-rich and electron-deficient anilines, primary aliphatic amines, secondary amines was also established. This work offers a straightforward, step economic, and green methodology for the efficient synthesis of valuable ureas.

Full text of DOI:10.1039/c9gc01577a

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Organic ligand and solvent free oxidative carbonylation of amine
over Pd/TiO2 with unprecedented activity
Shujuan Liu,[a,b] Xingchao Dai,[a,b] Hongli Wang,[a] and Feng Shi[a]*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
among which the highest turnover frequency (TOF) has been
achieved by the combination of Pd(OAc)2 and 3-butyl-1,2-
methylimidazolium iodide in the presence of O2 (17368 h-1).36
However, the intrinsic drawbacks of the homogeneous catalytic
systems in the separation and recycle of the catalysts severely
limited their further applications. To solve these problems, some
A highly active Pd/TiO2 catalyst system was prepared and applied
in the oxidative carbonylation of amines to ureas with ultra-low
Pd content under organic ligand and solvent free conditions. The
catalytic turnover frequencies (TOFs, moles of amines converted
per mole of Pd per h) were 126000 and 250000 h−1 for the
production of diphenylurea and dibenzylurea, respectively. An
expanded substrate scope including the electron-rich and
electron-deficient anilines, primary aliphatic amines, secondary
amines was also established. This work offers a straightforward,
step economic, and green methodology for the efficient synthesis
of valuable ureas.
supported catalysts have been developed for the oxidative
25, 37-41
carbonylation of amines to ureas and Pd,
catalysts supported on carbon materials,
Au,42 Rh,35 Co43
ZrO2,38 SiO2,37 and
39-41
zeolite44 have been investigated. Unfortunately, the corresponding
catalytic efficiency is still unsatisfactory, and large amounts of
volatile organic solvents were necessary in most cases. As an
important and classical metal oxides, titanium dioxide (TiO2)
nanomaterials possess plenty of obvious merits such as structure
stability, cheap availability, and safety to both humans and
environment.45, 46 In addition, it has been reported that amines can
preferentially adsorb on the TiO2 surface than other metal oxides,
such as Al2O3, SiO2 and Fe2O3.47 Thus, it might be a good choice
to develop an active and green heterogeneous catalyst for the
oxidative carbonylation of amines with TiO2 as a support.
In this work, the syntheses of dialkylureas by oxidative
carbonylation of a series of amines catalyzed by TiO2 supported
Pd catalysts were reported (Scheme 1). It was found that
excellent performance for the synthesis of symmetric dialkylureas
with ultra-low Pd content under organic ligand and solvent free
was achieved.
Ureas and their derivatives are very important nitrogen-containing
carbonyl compounds and are found usually in the structures of a
number of natural products and bioactive compounds, such as
2
5
plant and insect growth regulators,1, agrochemicals,3 dyes,4,
antioxidants,6 resin precursors,7 and as key intermediates in
organic synthesis. The traditional synthesis of ureas are based on
the use of toxic phosgene or triphosgene as the carbonyl source,8
which cause serious environmental pollution and equipment
corrosion. In recent years, CO and CO2 have been developed as
alternative carbonyl sources for ureas synthesis. Among them,
CO is a cheap and synthetically useful C1 building block in organic
chemistry,9 which is also the simplest C1 unit and meets the
requirements of “atom economy”,10, 11 step economy12 and “green
chemistry”.13-15 Therefore, the route using CO, amines and oxygen
as the starting materials to synthesize ureas via oxidative
carbonylation is very attractive.
The oxidative carbonylation of amines to obtain ureas has
received much interests in the past 60 years.16 Previous studies
focused on the oxidative carbonylation of amines with
homogeneous transitional metal/organic ligand complexes of Pd,
17-24
Ru,25 Co,26, 27 Mn,28 Se,29, 30 W,31 Cu,32 Ni,33 Au,34 and Rh,35
a. State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics (LICP)
Scheme 1. Oxidative carbonylation of amines to ureas.
Chinese Academy of Sciences, Lanzhou, 730000 (P.R. China)
E-mail: fshi@licp.cas.cn Address here.
b. University of Chinese Academy of Sciences
No. 19A, Yuquanlu, Beijing, 100049 (P.R. China)
Supporting information for this article is given via a link at the end of the
document.
Catalyst Preparation. The supported catalysts were prepared by
deposition-precipitation method with TiO2 as supports. First, TiO2
(1.0 g, P25, J&K Scientific, anatase/rutile = 80/20) was dispersed
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in deionized water (100 mL), and then a K2PdCl4 aqueous solution the TOF values amounted to 72000 h-1. If other supported
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DOI: 10.1039/C9GC01577A
(2.0 mL, [Pd] 0.05 mg/mL) was added dropwise into the solution catalysts including Pt/TiO2, Ru/TiO2 and Rh/TiO2 were employed,
under vigorous stirring. Subsequently, the pH value was adjusted lower TOFs of diphenylurea were obtained. Then, the catalytic
to about 10 using 0.2 M NaOH aqueous solution, and then the performance of Pd catalysts with different supports was tested
solution was stirred for another 3 h at room temperature. Next, the (Table S1). Clearly, Pd/TiO2 exhibits the best catalytic
solid sample was recovered by centrifugation, washed with performance. If other catalysts including Pd/Al2O3, Pd/SiO2, and
deionized water, dried at 80 °C for 12 h and calcined at 400 oC in Pd/Fe2O3 were employed, lower yields of diphenylurea were
o
a muffle furnace in air with a heating ramp of 5 C/min for 4 h to obtained. The superior activity should be attributed to the
obtain the final catalyst sample, which is denoted as 0.004 wt% preferential adsorption of amine on the Pd/TiO2 surface.47
Pd/TiO2. The other catalysts were prepared with the same Therefore, we applied Pd/TiO2 as the catalyst to optimize other
operations.
reaction parameters.
Characterization. XRD measurements were conducted by using
Table 1. Reaction condition optimization for oxidative carbonylation of aniline.a
a
STADIP automated transmission diffractometer (STOE)
equipped with an incident beam curved germanium
monochromator with Cu Kα1 radiation and current of 40 kV and
150 mA, respectively. The XRD patterns were scanned in the 2
Theta range of 10-90 °C. XPS were obtained using a VG ES-
CALAB 210 instrument equipped with a dual Mg/Al anode X-ray
source, a hemispherical capacitor analyzer, and a 5 keV Ar+ iron
gun. The electron binding energy was referenced to the C 1s peak
at 284.8 eV. The background pressure in the chamber was less
than 10-7 Pa. The peaks were fitted by Gaussian-Lorentzian
curves after a Shirley background subtraction. For quantitative
analysis, the peak area was divided by the element-specific
Scofield factor and the transmission function of the analyzer. The
H
H
NH2
N
N
KI
Catalyst
CO + 1/2O2
+
2
H2O
O
1a
1b
Metal content
/wt%b
Yield
/%c
TOF
/ h-1
Entry
Cat./mg
TON
1
2
Pt/TiO2 (50)
Ru/TiO2 (50)
Rh/TiO2 (50)
Pd/TiO2 (50)
Pd/TiO2 (50)
Pd/TiO2 (50)
Pd/TiO2 (50)
Pd/TiO2 (35)
Pd/TiO2 (25)
Pd(OAc)2
0.007
0.002
0.002
0.004
0.014
0.184
0.274
0.004
0.004
--
6
800
66000
100000
100000
572000
178000
14000
5
13000
13000
72000
22000
2000
3
5
BET surface area measurements were performed on
a
4
54
54
57
65
50
31
86
Quantachrome IQ2 at the temperature of 77 K. The pore size
distribution was calculated from the desorption isotherm by using
the Barrett, Joyner, and Halenda (BJH) method. Prior to
measurements, the samples were degassed at 300 °C for 3 h, at
a rate of 10 °C·min-1. TEM was carried out by using a Tecnai G2
F30 S-Twin transmission electron microscope operating at 300 kV.
For TEM investigations, the catalysts were dispersed in ethanol
by ultrasonication and deposited on carbon-coated copper grids.
NMR spectra were measured by using a Bruker ARX 400 or ARX
100 spectrometer at 400 MHz (1H) and 100 MHz (13C). Chemical
shifts were reported in parts per million (ppm) downfield from TMS
with the solvent resonance as the internal standard. Coupling
constants (J) were reported in Hz and referred to apparent peak
multiplications. The contents of Ru, Rh, Pd and Pt in the catalysts
were measured by inductively coupled plasma-atomic emission
spectrometry (ICP-AES), using an Iris advantage Thermo Jarrel
Ash device.
5
6
7
1000
10000
8d
9d
10e
126000
110000
17000
756000
658000
4000
a Reaction conditions: 1a (20 mmol), catalyst (50 mg), KI (1 mol %), CO (35 bar),
O2 (5 bar), solvent free, 92 ± 2 °C, 8 h. TON was defined as mols aniline
b
converted per mol Pd and TOF as mols aniline converted per mol Pd per h;
c
d
e
Determined by ICP-AES; Determined by isolated yield; 122 ± 2 °C, 6h;
Pd(OAc)2 1 mg (4.45 μmol), [mmim]I 30 mg (0.13 mmol), DMF 6.0 mL, 150 °C,
15 min, 40 bar CO/O2 with molar ratio of 4, Ref. 34.
Typical procedure for oxidative carbonylation of amines. A
mixture of amines (20 mmol), 0.004 wt% Pd/TiO2 (35 mg ) and 1
mol % KI (33.2 mg) were added into a glass tube which was
placed in an 80 mL autoclave. Then the autoclave was purged
and charged with CO and O2 (CO:O2 = 35:5 bar/bar). The reaction
mixture was stirred at 90-150 oC for 6 h. After reaction, the
autoclave was cooled to room temperature and the pressure was
carefully released. Subsequently, the reaction mixture was
Following, the metal Pd loadings was optimized (Entries 4-7).
Clearly, 0.004 wt% Pd/TiO2 exhibits the best catalytic
performance (Entry 4). Subsequent research of inorganic ligands,
such as KI, NaI, I2 and (n-Bu)4NI showed that KI was the best
choice for oxidative carbonylation of aniline (Table S2-3). The
influence of the oxidant is also important for oxidative
carbonylation, and O2 is preferred as a green oxidant but some
safety issues have to be considered.48 Therefore, it is very
necessary to control the concentration of O2 in the CO/O2
mixtures to avoid potential explosive hazards (Table S4). On the
basis of the above considerations, the ratio of CO/O2 was further
adjusted and the results showed CO/O2 = 35/5 bar to be the
optimal gas phase composition. As imagined, the higher TOF was
obtained at the higher reaction temperature (Table S5). The
impact of the solvent on the reactivity of this reaction was further
investigated, and it was found that solvent-free was the optimal
choice (Table S6). Meanwhile, the reaction time was also
o
concentrated by rot-vap at 180 C for 0.5 h to remove unreacted
amines to give the desired products. Based on the NMR analysis
of the reaction mixture, no other by-products were observed
except the substrate and desired product.
To search for the best catalyst in terms of activity toward
diphenylurea, we began our exploration with oxidative
o
carbonylation of aniline with CO and O2 at 92 C (Table 1). The
influence of the active metal on the reactivity of this reaction was
first investigated (Entries 1-4). The highest diphenylurea TOF
numbers were obtained with Pd supported on TiO2 (Entry 4), and
2 | J. Name., 2012, 00, 1-3
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optimized (Table S7), and 6 h was proved to be the optimal.
Finally, the reusability of this catalyst was explVoierwedArtwicliethOntlhinee
DOI: 10.1039/C9GC01577A
Noteworthy, even if the amount of Pd/TiO2 was reduced to 35 mg oxidative carbonylation of benzylamine as the model reaction with
and 25 mg (Entries 8-9), good catalytic performance was still toluene as the solvent. Gratifyingly, the Pd/TiO2 catalyst can be
maintained. Unfortunately, the yield of diphenylurea would not recycled and reused for at least ten runs without loss in catalytic
increase significantly if further increasing the reaction temperature activity, providing the desired product in 89% yield at the tenth run.
or prolonging the reaction time, which might be due to that the (Fig. 2). The palladium loadings of 0.004, 0.002, 0.002, and 0.002
generated diphenylurea with high melting point covered the wt% (±10% error) were detected in fresh Pd/TiO2 and after 1st, 2nd,
catalyst and prevented the contacting of it with starting materials. and 3rd use (Table S9). This shows that, apart from slight Pd
Finally, a 126000 h-1 TOF was achieved when the reaction was leaching during the first run, the catalyst is stable.
o
performed with 35 mg Pd/TiO2 at 120 C for 6 h(Entry 8), which
indicated our catalytic system to be highly efficient. A direct
evidence is that the TOF value reported in this work is ∼7 times
higher than the best result with homogeneous Pd(OAc)2/[mmim]I
catalyst system (Entries 8, 10).36
To explore the difference of catalysts in terms of activity
toward oxidative carbonylation of aniline, the supported catalysts
were characterized by XRD, TEM, XPS and N2 adsorption-
desorption analysis to reveal their structure differences. XRD
measurements showed neither metal oxide nor metal diffraction
pattern was detectable in all the catalysts (Fig. 1 (a), Fig. S1),
which might be attributed to the low metal loadings or that the
metal species were amorphous and highly dispersed. The
morphologies of the catalysts were further characterized by TEM
and STEM (Fig. S2). The TEM and STEM images of 0.274 wt%
Pd/TiO2 exhibit that the Pd nanoparticles well dispersed on TiO2
Fig.
2 Recycling experiments for the Pd/TiO2 catalysts under optimized
(Fig. S2 (b), (c)) and the HR-TEM image of Pd particles on TiO2
shows a lattice spacing of 0.23 nm, which can be indexed to the
Pd(111) plane.49 However, no palladium nanoparticles was
detectable in the 0.004 wt% Pd/TiO2, which might be attributed to
the lower palladium loadings.
conditions: benzylamine (5 mmol), 0.004 wt% Pd/TiO2 (35 mg), CO (35 bar), O2
(5 bar), KI (1 mol %), toluene (2 mL), 122 ± 2 oC, 2 h.
With the optimized conditions in hand, the substrate scope
and limitation of this catalyst system was further investigated. As
shown in Table 2, the anilines with both electron-withdrawing and
electron-donating groups on the phenyl ring were well tolerated
under the current conditions. Oxidative carbonylation of electron-
rich methyl-, and tert-butyl-substituted anilines proceeded
smoothly to provide the corresponding products in 50−82% yields
and TOFs in 126000–178000 h-1 (Entries 1–4). The anilines
bearing electron-deficient groups, such as halogen groups, also
proceeded well, affording the corresponding product in good yield
and the corresponding TOF in 162000 h-1 (Entry 5). Other primary
aliphatic amines, including n-hexylamine, n-heptanamine, n-
octylamine, hexadecylamine and benzylamine were also tested
with Pd/TiO2 catalyst (Entries 6–10), and the corresponding
products were obtained in 58−88% yields. Noteworthy, the directly
isolated yield of the desired product would be increased
remarkably with the increasing of the C number or the chain
length of the attached alkyl. For example, 88% of directly isolated
yield could be achieved when using hexadecylamine as the
substrate. This may be partially due to the lower solubility of those
amines containing the higher C number or the longer chain length
of the attached alkyl.38 In addition, cyclohexylamine was also
converted smoothly into the desired product in 149000 h-1 TOF
and 60% yield (Entry 11). In order to examine the scope and
limitation of this catalyst system, secondary amines as substrates
were also tested. In comparison with the primary amine, the
secondary amine were proved to be less active and 45-62%
yields with 28519-153000 h-1 TOFs were obtained when N-ethyl
aniline, morpholine, piperidine, N-methyl piperazine, N-ethyl
piperazine and dibutylamine were used as the strating materials
(Entry 12-17), which suggested that the catalytic performance
Moreover, XPS studies of the catalysts were performed to
determine the chemical state of Pd nanoparticles. As shown in Fig.
1 (b), no signal peaks of Pd 3d5/2 and Pd 3d3/2 were found in 0.004
wt% fresh and used Pd/TiO2. In order to check the chemical state
of Pd nanoparticles clearly in Pd/TiO2, a 0.274 wt% Pd/TiO2
catalyst was prepared by the same method. The two signal peaks
appeared at 336.4 and 341.7 eV, respectively, which suggested
that Pd2+ was mainly formed on the surface of catalysts.50 The N2
adsorption-desorption tests (Fig. S3) showed that the specific
surface areas of Pd/TiO2 is 103.0 m2/g (Table S8).
Fig. 1 (a) XRD diffraction patterns of the catalysts; (b) XPS spectra of the
catalysts.
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could be remarkably inhibited by the steric hindrance of the
substrate molecules. In order to show the practical value of the
present procedure, 6-fold scale transformation of aniline was
carried out and 5.554 g diphenylurea could be obtained in 44%
isolated yield (Entry 18), the same yield was obtained compare
with the 1.86 g aniline (Entry 1). The synthesis of heteroaromatic
ureas and asymmetrical ureas were tested. However, the desired
heteroaromatic ureas were not observed, and normally a mixture
of symmetric and asymmetric disubstituted ureas was obtained.
Therefore, the current methodology can not be applied in the
O
View Article Online
NH
N
DOI: 10.1039/C9GC01577A
N
N
16
62
153000
920000
N
N
H
3
3
N
N
N
17
30
44
72000
27762
432000
666286
3
3
3
3
O
O
H
N
H
N
NH2
18d
synthesis of heteroaromatic ureas and asymmetrical ureas.
a
Table 2. Results of the oxidative carbonylation of aminesa.
Reaction conditions: amine (20 mmol), catalyst (35 mg), KI (1 mol %), CO (35
bar), O2 (5 bar), solvent free, 122 ± 2 oC, 6 h, TON as mols amine converted per
b
mol Pd, TOF as mols amine converted per mol Pd per h; Determined by
Yield
/%b
TOF
/ h-1
Entry
1
Substrate
Product
TON
d
c
isolated yield; hexadecylamine (10 mmol); 120 mmol aniline, catalyst (210
mg), KI (1 mol %), CO (60 bar), O2 (15 bar), solvent free, 122 ± 2 oC, 24 h.
H
N
H
N
NH2
50
54
126000
129000
758000
O
O
H
N
H
N
NH2
Conclusions
2
3
772000
In summary, we have developed extraordinarily highly active
oxidative carbonylation reaction starting from amines with CO and
O2 using Pd/TiO2 as catalyst with ultra-low Pd content under
organic ligand and solvent free. A series of functional symmetrical
ureas with diverse structures were synthesized by applying this
simple catalyst system. Up to 250000 h-1 was achieved in this
work, to the best of our knowledge, this is the reported highest
TOF numbers. The superior activity should be attributed to the
highly dispersed PdO nanoparticles on the surface of TiO2 support.
The recycling experiments showed the active Pd/TiO2 catalyst
was stable during the reaction and could be reused for ten times
without obvious activity loss.
NH2
H
H
N
N
But
82
74
78
178000
175000
162000
1070000
O
But
tBu
H
H
N
NH2
N
4
5
1050000
970000
O
O
NH2
H
N
H
N
Cl
Cl
Cl
NH2
H
N
H
N
6
7
80
62
58
66
88
60
182000
147000
140000
156000
94000
1094000
880000
838000
938000
562000
894000
O
O
O
O
H
H
N
NH2
NH2
NH2
N
4
5
4
4
Acknowledgements
H
N
H
N
8
5
5
This work was supported by NSFC (21633013, 91745106), the
National Key Research and Development Program of China
(2017YFA0403103), the Youth Innovation Promotion Association
(2019409), “Light of West China” Program and Fujian Institute of
Innovation of CAS.
H
N
H
N
6
9
6
6
H
N
H
N
NH2
NH2
10c
11
14
14
14
O
O
H
N
H
N
Data availability
149000
The data supporting the findings of this study are available within
the Article and its Supplementary Information files. All other
relevant source data are available from the corresponding author
upon request.
H
N
N
N
12
48
110000
658000
O
O
NH
13
14
15
O
38
42
56
90000
108000
147000
542000
646000
884000
N
N
N
N
N
N
O
O
Notes and references
O
O
NH
NH
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N
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