Sustainable heterogeneous platinum catalyst for direct methylation of secondary amines by carbon dioxide and hydrogen

Article abstract of DOI:10.1002/chem.201400332

Pt and MoO_x co-loaded TiO₂ is found to be highly effective for direct methylation of aliphatic and aromatic secondary amines by CO₂ and H₂ under solvent-free conditions. This is the first additive-free and reusable heterogeneous catalytic system with acceptable turnover number. Over and over: A heterogeneous Pt catalyst for direct methylation of aromatic amines by CO₂ and H₂ with high reusability and an order of magnitude higher turnover number than previous catalysts has been demonstrated (see scheme).

Full text of DOI:10.1002/chem.201400332

DOI: 10.1002/chem.201400332
Communication
&
Heterogeneous Catalysis
Sustainable Heterogeneous Platinum Catalyst for Direct
Methylation of Secondary Amines by Carbon Dioxide and
Hydrogen
[a]
[b]
[a]
[a, b]
Kenichi Kon, S. M. A. Hakim Siddiki, Wataru Onodera, and Ken-ichi Shimizu*
[9]
reaction mixture. As for heterogeneous catalysts, Baiker et al.
Abstract: Pt and MoOx co-loaded TiO2 is found to be
highly effective for direct methylation of aliphatic and aro-
matic secondary amines by CO and H under solvent-free
previously reported Cu-catalyzed gas-phase methylation of
NH by CO and H , but the system suffered from drawbacks
3
2
2
2
2
such as low yield and the necessity of high temperatures (200–
[10]
conditions. This is the first additive-free and reusable het-
erogeneous catalytic system with acceptable turnover
number.
3008C). Very recently, Shi et al. reported the first reusable
catalyst (CuAlO ) for methylation of amines with CO and H .
x
2
2
However, this system requires substoichiometric amount of Cu
38 mol% Cu with respect to amine). To our knowledge, there
(
are no examples of additive-free and reusable catalytic systems
with acceptable turnover numbers (TONs). We report herein
a new heterogeneous Pt catalyst, Pt and MoO co-loaded TiO ,
Continuous efforts have been focused on reductive fixation of
x
2
carbon dioxide (CO ), because it could be an abundant and re-
for methylation of aliphatic and aromatic secondary amines by
CO and H under solvent-free conditions that shows an order
2
newable carbon resource for the production of valuable chem-
2
2
[
1–3]
icals, such as methanol, formic acid and amides.
A recent
of higher TON than previous systems and high reusability.
Motivated by the fact that some oxide-supported transition
metal nanoparticles catalyze amide formation from CO , H ,
advance in this area is the discovery of new catalytic methods
to utilize CO as a as a methylation reagent in fine chemical
2
2
2
[
4]
[5]
[3d,f]
[11]
synthesis. Cantat et al. and Beller et al. independently re-
and amines
and hydrogenation of amides to amines, we
ported the methylation of amines by CO and silanes by homo-
screened various oxide-supported transition metal catalysts
(containing 0.02 mmol of active metals) for methylation of N-
methyl aniline (1 mmol) with CO and H at 2008C (24 h) as
2
geneous catalysts. Although the methods are not practical due
to the use of silanes as the reductant, they show a possible
2
2
use of CO as a methylation reagent. Later, Klankermayer
a model reaction. Table 1 lists the conversion and yield of N,N-
2
[
6]
et al. developed a more practical method, that is, direct
methylation of aromatic amines by CO and H using a homo-
dimethyl aniline. First, we tested a well-established CO hydro-
2
[
3c,d]
2
2
genation catalyst,
CuZnAlO , prepared by the co-precipita-
x
[3d]
geneous Ru catalyst, [Ru(triphos)(tmm)], with an acidic additive
tion method according to the literature.
The Cu catalyst
[
7]
(
HNTf ). Beller et al. also reported an improved catalyst for
(entry 1, Table 1) was nearly inactive, showing only 2% yield.
Then, we tested supported Pt catalysts (entries 2–11, Table 1)
and found that some Pt catalysts such as Pt/Nb O showed
2
this reaction with wider scope, in which aliphatic amines as
well as aromatic amines were tolerated by a homogeneous Ru
catalyst with triphos ligand and additives (methanesulfonic
acid or LiCl). This new catalytic pathway can be a practical al-
ternative to the current industrial methylation of amines with
2
5
higher yield of the desired product. Pt/C as a conventional Pt
catalyst resulted in low yield of the product partly due to aro-
matic ring reduction. Inspired by previous reports that transi-
tion metals, such as Mo and Re, act as effective promoters of
heterogeneous Ru, Rh, and Ir catalysts for hydrogenation of
[
8]
methanol or formaldehyde, since methylamines can be di-
rectly produced from inexpensive and renewable chemicals
[11–13]
and H O is the only byproduct. For practical applications, these
oxygen-containing compounds,
we tested supported Pt
2
homogeneous catalysts have serious problems, such as difficult
catalyst/product separation, the inability to reuse the catalyst,
and the necessity of additives (ligands, acids, and salts) in the
catalysts co-loaded with oxides of transition metals (V, Mo, W,
Re). Among various catalysts tested (entries 13–21, Table 1), Pt-
MoO /TiO (entry 13, Table 1) showed the highest yield (85%).
x
2
Next, we tested a series of transition metal and MoO co-
x
loaded TiO (entries 13, 22–26, Table 1). Among various metals
2
[
a] Dr. K. Kon, W. Onodera, Dr. K.-i. Shimizu
Catalysis Research Center, Hokkaido University
N-21, W-10, Sapporo 001-0021 (Japan)
Fax: (+81)11-706-9163
(
Pt, Pd, Rh, Ru, Ni, Cu), Pt-MoO /TiO showed the highest yield.
x 2
MoO -loaded TiO (entry 12, Table 1) was nearly inactive and
x
2
Pt/TiO (entry 4, Table 1) and Pt/MoO (entry 5, Table 1) showed
2
3
E-mail: kshimizu@cat.hokudai.ac.jp
markedly lower yield (6–10%) than Pt-MoO /TiO . These results
x
2
[b] Dr. S. M. A. H. Siddiki, Dr. K.-i. Shimizu
indicate that co-presence of Pt and Mo species on TiO is criti-
2
Elements Strategy Initiative for Catalysts and Batteries
Kyoto University, Katsura, Kyoto 615-8520 (Japan)
cal. The effect of the prereduction temperature (200–6008C) of
Pt-MoO /TiO showed that the catalyst reduced at 3008C gave
the highest activity (result not shown).
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400332.
x
2
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&
&
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Communication
neat conditions (without solvent) gave the highest yield of
2 2
Table 1. Methylation of N-methyl aniline with CO and H by various cat-
alysts.
85%. The time-conversion profile (Figure S3) under the neat
conditions shows that a reaction time of 24 h is appropriate.
The heterogeneous nature and reusability of this catalytic
system is confirmed by the following results. For the standard
reaction (entry 1, Table 2), the catalyst was removed from the
[
a]
Entry
Catalysts
Conv. [%]
Yield [%]
1
2
3
4
5
6
7
8
9
Cu-ZnO/Al
2
O
3
6
41
27
7
10
16
16
11
3
2
39
10
6
10
4
3
3
2
Pt/Nb
Pt/HMFI
Pt/TiO
Pt/MoO
Pt/SiO
Pt/ZrO
Pt/Al
Pt/CeO
2 5
O
Table 2. Methylation of amines with CO
2
2 x 2
and H by Pt-MoO /TiO .
2
3
2
2
2
O
3
Entry Substrate
Product
T
Pt
Conv. Yield
[
a]
2
[8C] [mol%] [%]
[%]
1
0
Pt/MgO
Pt/C
5
100
1
2
0
1
1
1
[
b]
[c]
1
200
200
2
1
85
98
85(82)
90
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
MoO
Pt-MoO
Pt-VO /TiO
Pt-WO /TiO
Pt-ReO /TiO
x
/TiO
2
x
/TiO
2
85
49
27
16
34
32
25
34
43
25
54
20
29
9
85
48
22
12
32
20
11
29
39
10
49
0
x
2
x
2
2
x
2
Pt-MoO
Pt-MoO
Pt-MoO
x
x
x
/Al
/ZrO
/SiO
2
O
3
2
3
4
230
200
1
1
87
85
85
77
2
Pt-WO
Pt-MoO
x
/ZrO
/Nb
2
x
2 5
O
Pd-MoO
Rh-MoO
Ru-MoO
x
x
x
/TiO
/TiO
/TiO
/TiO
/TiO
2
2
2
Ni-MoO
Cu-MoO
x
2
1
8
5
6
7
250
170
180
1
2
3
96
84
92
96
67
64
x
2
[
a] GC yield based on amine (using n-dodecane as an internal standard).
Next, we show the characterization results of the representa-
tive catalyst, Pt-MoO /TiO prereduced at 3008C. Figure S1 (in
the Supporting Information) show X-ray absorption near edge
x
2
8
9
200 0.1
100
100
99
63
spectroscopy (XANES) spectra of Pt-MoO /TiO2 (reduced at
x
200
220
2
2
3008C) and a reference compound (Pt foil). The XANES spec-
trum of Pt-MoO /TiO is nearly identical to that of Pt foil, which
x
2
10
100
96
indicates that the electronic state of the Pt species in Pt-MoOx/
TiO is metallic. Temperature-programmed reduction of the un-
[a] GC yield based on amine (using n-dodecane as an internal standard).
[b] 40 bar H . [c] Yield of isolated product.
2
reduced catalyst precursor under H2 (Figure S2) shows a H2
consumption peak due to the reduction of oxidic Pt to metallic
Pt below 2008C. Thus, Pt species in Pt-MoO /TiO are com-
2
x
2
pletely metallic after the standard prereduction at 3008C,
which is consistent with the XANES result. From the CO ad-
sorption experiment on Pt-MoO /TiO , the mean diameter of Pt
reaction mixture at 26% yield (t=3 h). Then, further heating of
the filtrate under 5 bar CO and 40 bar H for 21 h at 2008C
2
2
did not increase the yield. inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) analysis of the filtrate con-
firmed that the content of Pt and Mo in the solution was
below the detection limits. These results confirm that the reac-
tion is attributed to the heterogeneous catalysis of Pt-MoOx/
x
2
metal particles is estimated to be 4.1 nm, based on the as-
sumption that CO is adsorbed on the surface of spherical Pt
particles at a stoichiometry of CO/(surface Pt atom) of 1:1.
Summarizing the structural results, the dominant Pt species in
Pt-MoO /TiO are Pt metal nanoparticles with average size of
TiO . Figure 1 shows the result of catalyst recycling. After the
x
2
2
4
.1 nm.
standard reaction for 24 h, the catalyst was separated from re-
action mixture by centrifugation and was dried at 908C for 3 h
and then reduced in H2 at 3008C for 0.5 h. The catalyst
showed high yield (80–90%) for at least 10 cycles. A total TON
during the successive 10 cycles reached 433, which is higher
With the most effective catalyst, Pt-MoO /TiO2 reduced at
x
3
008C, we studied the optimization of reaction conditions.
First, we tested the reaction in different solvents (Table S1 in
the Supporting Information). The reaction in dodecane, water,
and toluene gave moderate to good yield (45–72%), and
methanol gave the lowest yield (34%). The reaction under
[6]
[7]
than TONs (40 and 80 ) of the state-of-the-art homogeneous
Ru catalysts for the same reaction.
&
&
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Communication
(
55% yield) as the major product. These results suggest that
formic acid and N-methylformanilide can act as key intermedi-
ates. A plausible mechanism is given in Scheme 1. Formic acid,
first formed by Pt-catalyzed hydrogenation of CO , reacts with
2
amine to give N-methylformanilide, which undergoes Pt-cata-
lyzed hydrogenation to give tertiary amine. Mechanistic studies
are ongoing to clarify the role of Mo in the catalytic cycle.
Figure 1. Catalyst reuse for methylation of N-methyl aniline with CO
2 2
and H
by Pt-MoO /TiO . Conditions are the same as those in entry 1 in Table 2.
x
2
Scheme 1. Reaction pathway of Pt-catalyzed methylation of N-methyl aniline
Next, we studied general applicability of the present catalyt-
ic system. Table 2 shows the scope of methylation of various
2 2
with CO and H .
secondary amines under 5 bar CO2 and 50 bar H using Pt-
2
MoO /TiO (2 mol% Pt with respect to amine). When methyl
In conclusion, we have found that Pt and MoO co-loaded
x
2
x
anilines with electron-donating groups (entries 2–4, Table 2)
and tetrahydroquinoline (entry 5, Table 2) were used, the de-
sired products were obtained in good to high yields (77–96%).
Benzylic and aliphatic secondary amines (entries 6–10, Table 2)
also underwent catalytic methylation with CO and H , giving
TiO is effective for catalytic methylation of aliphatic and aro-
2
matic secondary amines by CO and H under solvent-free con-
2
2
ditions. This method is the first additive-free and reusable het-
erogeneous catalytic system with an acceptable TON. If hydro-
gen is provided from renewable sources, this system can pro-
vide a sustainable and practical method for production of
tertiary amines.
2
2
good to high yield of the tertiary amine products (63–99%).
Methylation of pyrrolidine with small amount (0.1 mol%) of
the catalyst resulted in 99% yield of 1-methylpyrrolidine, corre-
sponding to a TON of 990. This value is 470 times higher than
[
10]
that of the CuAlO catalyst (2.1).
Acknowledgements
x
To discuss the key intermediates and pathway of this reac-
tion, we studied the following control experiments. First, we
tested Pt-MoO /TiO -catalyzed reductive methylation of N-
This work was supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas “Nano Informatics” (25106010)
from JSPS and a MEXT program “Elements Strategy Initiative to
Form Core Research Center”.
x
2
methyl aniline with formic acid or methanol, because formic
acid or methanol are general products of heterogeneous CO
2
[
3]
hydrogenation. The reaction with formic acid for 4 h resulted
in 35% yield of N,N-dimethyl aniline [Eq. (1)], which is close to
the yield (38%) of the standard reaction of N-methyl aniline,
Keywords: carbon dioxide
·
heterogeneous catalysis
·
hydrogenation · methylation · platinum
CO and H2 for 4 h. In contrast, the reaction with methanol
2
gave only 2% yield of the product [Eq. (2)].
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Published online on && &&, 0000
&
&
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4
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ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
Over and over: A heterogeneous Pt cat-
alyst for direct methylation of aromatic
amines by CO and H with high reus-
&
Heterogeneous Catalysis
K. Kon, S. M. A. H. Siddiki, W. Onodera,
K.-i. Shimizu*
2
2
ability and an order of magnitude
higher turnover number than previous
catalysts has been demonstrated (see
scheme).
&
& – &&
Sustainable Heterogeneous Platinum
Catalyst for Direct Methylation of
Secondary Amines by Carbon Dioxide
and Hydrogen
Chem. Eur. J. 2014, 20, 1 – 5
www.chemeurj.org
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ