CL-160875
Received: September 24, 2016 | Accepted: October 17, 2016 | Web Released: October 22, 2016
Heterogeneous Platinum Catalysts for Direct Synthesis of Trimethylamine
by N-Methylation of Ammonia and Its Surrogates with CO /H
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Takashi Toyao,*1,2 S. M. A. Hakim Siddiki, Keisuke Ishihara, Kenichi Kon, Wataru Onodera, and Ken-ichi Shimizu*
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1
Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo, Hokkaido 001-0021
Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura, Kyoto 615-8520
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(
E-mail: toyao@cat.hokudai.ac.jp, kshimizu@cat.hokudai.ac.jp)
Direct synthesis of trimethylamine through N-methylation of
NH3 or its surrogate (NH4HCO3) with both CO2 and H2 has been
achieved by employing Pt and MoO coloaded TiO (Pt-MoO /
TiO2). Pt-MoOx/TiO2 was found to be superior to other
supported Pt and transition-metal-loaded MoOx/TiO2 catalysts
for the trimethylamine synthesis process.
under a flow of H2 (20 mL min¹1) at 300 °C for 0.5 h in a glass
tube. The prepared Pt-MoOx/TiO2 was characterized by X-ray
diffraction (XRD) and transmission electron microscopy (TEM)
measurements (see the Supporting Information). XRD patterns
of TiO2, MoOx/TiO2, and Pt-MoOx/TiO2 were found to be
essentially identical, with peaks arising from TiO2 having both
anatase and rutile phases (Figure S1). This fact suggests that the
x
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x
Keywords: Carbon dioxide (CO2)
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N-Methylation
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introduced MoO and Pt species exist in amorphous and/or as
very small particles that cannot be identified by XRD. TEM
x
Trimethylamine synthesis
images shown in Figure S2 revealed that Pt-MoO /TiO con-
x
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Much effort has been devoted to the utilization of carbon
dioxide (CO2) as a renewable carbon resource for the production
of chemicals such as methanol, formic acid, and value-added
tains Pt nanoparticles with average size of 4.7 « 1.1 nm, as
previously reported. Other supported catalysts were prepared
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in a similar manner. After the H -reduction, the catalyst
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chemicals. One of the important advances in this research area
is catalytic methods for the utilization of CO2/H2 mixtures as a
methylation reagent in fine chemical synthesis.2 Recent studies
have shown the direct N-methylation of amines by CO /H
(5 mol % on the basis of the Pt loading amount), and a mixture
of ammonium hydrogen carbonate (NH4HCO3) and octane
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(0.29 mmol) were added to a stainless steel autoclave (10 cm ).
Soon after being sealed, the reactor was flushed with H and
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mixtures using homogeneous Ru catalysts or heterogeneous
Cu, Pd, Pt, or Au catalysts. More recently, Klankermayer
charged with 1 MPa CO2 and 5 MPa H2 at room temperature.
The autoclave was heated at 250 °C under stirring (500 rpm) for
24 h. The yields of trimethylamine were determined on the basis
of N content in the substrate by GC using octane as the internal
standard.
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et al. have succeeded in the direct synthesis of an industrially
important chemical, trimethylamine, by N-methylation of am-
monia (NH3) and its surrogate (NH4Cl) by CO2/H2 using a
homogeneous Ru catalyst, [Ru(triphos)(tmm)] (triphos: tridentate
phosphine; tmm: trimethylene methane) under batch conditions.
Since methylamines are industrially produced by zeolite-
In the initial phase of this effort, we carried out catalyst
screening for the methylation of an NH3 surrogate (NH4HCO3)
using various oxide-supported transition-metal catalysts, con-
taining 5 wt % of active metals. Table 1 shows the yield of
trimethylamine from 1 mmol of NH4HCO3 under 1 MPa of CO2
and 5 MPa of H2 in the presence of the catalyst containing
0.05 mmol (5 mol %) of active metals. Pt-MoOx/TiO2 showed
the highest yield (65%) among the supported Pt catalysts
explored in this study (Entries 112). We also tested a series of
transition metal and MoOx coloaded TiO2 catalysts (Entries 1
and 1320). Among various metals (Pt, Pd, Re, Ir, Rh, Ru, Ni,
Co, and Cu) tested, Pt-MoOx/TiO2 exhibited the highest yield
for the trimethylamine synthesis. It was confirmed that MoO3/
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catalyzed N-methylation of NH by methanol, this catalytic
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system can be a sustainable alternative route for the synthesis of
methylamines using CO2 as a renewable carbon resource. For
a practical application, however, it is strongly desired that a
heterogeneous catalytic system is developed. Previously, Baiker
et al. reported heterogeneous Cu catalysts for the direct synthesis
of methylamines from NH3/CO2/H2 in gas phase at 200300 °C
using a high-pressure flow reactor.12 Although these reports
represent important contributions to the synthesis of methyl-
amines, the system suffered from drawbacks such as low yield
(
<1%) and low selectivity to trimethylamine. Herein, we report
TiO (Entry 21) showed 0% yield, indicating that the presence
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a heterogeneous Pt catalyst, Pt and MoOx coloaded TiO2 (Pt-
MoOx/TiO2), for the direct synthesis of trimethylamine through
N-methylation of NH or its surrogate (NH HCO ) with CO /H
of Pt is essential for progression of the reaction.
Next, we further optimized the reaction conditions by
employing the best catalyst, Pt-MoO /TiO . Table 2 shows
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under batch conditions.
Pt-MoOx/TiO2 (5 wt % Pt, 7 wt % Mo) was prepared by
a sequential impregnation method. First, MoO3-loaded TiO2
the catalytic results employing different solvents. The reactions
in 1,4-dioxane, glyme (1,2-dimethoxyethane), THF (tetrahydro-
furan), heptane, and H2O resulted in low to moderate yields
(2053%). The reaction under solvent-free conditions gave the
highest yield of 65%. Under the solvent-free conditions, a
reaction at a lower temperature (200 °C) gave 54% yield.
Furthermore, a reaction using a lower amount of the catalyst
(3 mol % Pt) was found to result in a lower product yield (54%).
Reactions using various NH3 surrogates such as (NH4)2CO3,
urea, and NH Cl instead of using NH HCO as a N source were
(
MoO /TiO ) was prepared by impregnation of TiO (JRC-TIO-
x
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supplied from the Catalysis Society of Japan) with an aqueous
solution of (NH4)6Mo7O24¢4H2O, followed by evaporation to
dryness, and by calcination in air at 500 °C for 3 h. Subse-
quently, for the deposition of Pt nanoparticles, MoOx/TiO2 was
added to an aqueous HNO3 solution containing Pt(NH3)2(NO3)2.
After stirring at room temperature for 15 min, the solution was
subjected to evaporation to dryness and subsequent reduction
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also performed, as shown in Table 3. The reactions were carried
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