Heterogeneous Pt catalysts for reductive amination of levulinic acid to pyrrolidones

Article abstract of DOI:10.1021/cs500757k

Supported platinum catalysts have been studied for the reductive amination of levulinic acid (LA) by H₂ to N-alkyl-5-methyl-2-pyrrolidones under solvent-free conditions. The activity depends on the type of metal (Pt, Re, Pd, Rh, Ru, Cu, Ni), support material, and coloaded oxides of transition metals (V, Cr, Mo, W, Re). In 24 kinds of catalyst tested, Pt and MoO_X (molybdenum oxide) coloaded TiO₂ (Pt-MoO_X/TiO₂) shows the highest activity. Pt-MoO_X/TiO₂ is effective for reductive amination of LA with wide varieties of amines under mild conditions (3 bar of H₂, 100 °C, solvent-free) to give high isolated yield of pyrrolidinones and shows higher turnover number (TON) than previously reported catalysts for reductive amination of LA with an aliphatic amine. The catalyst can be separated from the reaction mixture by filtration, and the recovered catalyst can be reused. This is the first general and reusable heterogeneous catalytic system for the reductive amination of LA. On the basis of mechanistic studies, high activity of Pt-MoO_X/TiO₂ can be attributed to acid-base interaction between the acid sites of Pt-MoO_X/TiO₂ and carboxyl groups in LA and an intermediate.

Full text of DOI:10.1021/cs500757k

Research Article
pubs.acs.org/acscatalysis
Heterogeneous Pt Catalysts for Reductive Amination of Levulinic
Acid to Pyrrolidones
Abeda Sultana Touchy,† S. M. A. Hakim Siddiki,‡ Kenichi Kon,† and Ken-ichi Shimizu*,†,‡
†Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
‡Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura, Kyoto 615-8520, Japan
S
* Supporting Information
ABSTRACT: Supported platinum catalysts have been studied for the
reductive amination of levulinic acid (LA) by H2 to N-alkyl-5-methyl-
2-pyrrolidones under solvent-free conditions. The activity depends on
the type of metal (Pt, Re, Pd, Rh, Ru, Cu, Ni), support material, and
coloaded oxides of transition metals (V, Cr, Mo, W, Re). In 24 kinds of catalyst tested, Pt and MoOX (molybdenum oxide)
coloaded TiO2 (Pt-MoOX/TiO2) shows the highest activity. Pt-MoOX/TiO2 is effective for reductive amination of LA with wide
varieties of amines under mild conditions (3 bar of H2, 100 °C, solvent-free) to give high isolated yield of pyrrolidinones and
shows higher turnover number (TON) than previously reported catalysts for reductive amination of LA with an aliphatic amine.
The catalyst can be separated from the reaction mixture by filtration, and the recovered catalyst can be reused. This is the first
general and reusable heterogeneous catalytic system for the reductive amination of LA. On the basis of mechanistic studies, high
activity of Pt-MoOX/TiO2 can be attributed to acid−base interaction between the acid sites of Pt-MoOX/TiO2 and carboxyl
groups in LA and an intermediate.
KEYWORDS: biomass, levulinic acid, reductive amination, amines, supported platinum catalysts
Pt and Pd catalysts, but the catalyst reusability was not tested.9
1. INTRODUCTION
Additionally, the catalysts suffer from disadvantages such as
Selective catalytic transformation of nonfood biomass-derived
narrow substrate scope, low turnover number (TON), and
platform compounds is a key technology in sustainable
necessities of excess amount of amine (2 equiv), high
production of chemicals. In this context, levulinic acid (LA)
temperature (150 °C), and high H2 pressure (69 bar).9 To
has been identified as one of the most important platform
our knowledge, there are no examples of a reusable and general
compounds because it can be easily and economically produced
catalytic system for reductive amination of LA by H2. We report
from lignocellulosic materials and can be converted to various
herein that Pt and MoOX coloaded TiO2 (Pt-MoOX/TiO2),
chemicals.1 In particular, N-alkyl-5-methyl-2-pyrrolidones,
which has been effective for reductive amination of CO2,10 acts
which can be produced by reductive amination of LA, are of
as heterogeneous catalyst for reductive amination of LA with a
importance as industrial solvents, surfactants, complexing
range of primary amines under solvent-free and mild (100 °C, 3
agents, and intermediates in synthesis of functional compounds
bar of H2) conditions. This is the first successful example of
such as printing ink and fiber dyes.2−4 Most of the recent
reusable and general catalytic system for reductive amination of
studies on this reaction use formic acid as a reductant,5−8
LA with high TON (2150).
because an equimolar amount of formic acid and LA is
produced by the hydrolysis of the cellulosic biomass. Ru6 and
2. EXPERIMENTAL SECTION
2.1. General. Commercially available organic and inorganic
compounds (from Tokyo Chemical Industry, WAKO Pure
Chemical Industries or Kanto Chemical) were used without
further purification. GC (Shimadzu GC-2014) and GCMS
Ir7 complexes were reported as effective catalysts for this
reaction. For a practical application, these homogeneous
methods have disadvantages such as a difficulty in catalyst/
product separation, inability of catalyst reuse, limited scope, and
6
necessities of additives (ligands, t-Bu3PHBF4 or HCOONa7),
excess amount of formic acid (>13 equiv)7 and amine (2.7
(Shimadzu GCMS-QP2010) analyses were carried out with
Ultra ALLOY capillary column UA+-1 (Frontier Laboratories
Ltd.) using N2 and He as the carrier gas.
2.2. Catalyst Preparation. TiO2 (JRC-TIO-4), MgO
(JRC-MGO-3), and CeO2 (JRC-CEO-3) were supplied from
Catalysis Society of Japan. SiO2 (Q-10, 300 m2 g−1) was
supplied from Fuji Silysia Chemical Ltd. HZSM5 zeolite with a
SiO2/Al2O3 ratio of 22.3 was supplied by Tosoh Co. Active
equiv).7 Cao et al. reported a greener method using
heterogeneous Au/ZrO2 catalyst, but the method was limited
in substrate scope and the catalyst reusability was not tested.5
Recently, catalyst-free transformation of LA to pyrrolidinones
with formic acid in DMSO with stoichiomertic amount of Et3N
was reported, but the method required a toxic solvent,
dichloromethane, for separation of the products from
DMSO.8 From economic and industrial viewpoints, use of H2
as a less expensive reductant is important. A previous patent
showed reductive amination of LA by H2 with heterogeneous
Received: April 18, 2014
© XXXX American Chemical Society
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carbon (C) was purchased from Kanto Chemical. γ-Al2O3 was
prepared by calcination of γ-AlOOH (Catapal B Alumina
purchased from Sasol) for 3 h at 900 °C. Nb2O5 was prepared
by calcination of Nb2O5·nH2O (supplied by CBMM) at 500 °C
for 3 h. ZrO2 was prepared by hydrolysis of zirconium
oxynitrate 2-hydrate by an aqueous NH4OH solution, followed
by filtration, washing with distilled water, drying at 100 °C for
12 h, and by calcination at 500 °C for 3 h.
Table 1. Reaction of LA with n-Octylamine under H2 by
Various Catalysts in Solvent-Free Conditions
a
entry
catalysts
yield (%)
Precursors of M1-MoOX/TiO2 (M1 = 5 wt % Pt, Ni, Cu, Ru,
Rh, Pd; 7 wt % Mo), Pt-M2OX/TiO2 (5 wt % Pt; M2 = 7 wt %
V, Cr, Mo, W, Re), and Pt-MoOX/M3OX (5 wt % Pt; 7 wt %
Mo; M3OX = SiO2, ZrO2, Al2O3) were prepared by sequential
impregnation method by using M1 source [aqueous HNO3
solution of Pt(NH3)2(NO3)2, Rh(NO3)3 or Pd(NO3)2 or
aqueous solution of nitrates (Ni, Cu) or RuCl3], M2 source
[(NH4)6Mo7O24·4H2O, NH4VO3, Cr(NO3)3·9H2O,
(NH4)10W12O41·5H2O or NH4ReO4] and supports (M3OX).
For the preparation of Pt-MoOX/TiO2 (5 wt % Pt, 7 wt % Mo)
as an example, 5 g of TiO2, 0.88 mmol of (NH4)6Mo7O24·
4H2O, and 0.88 mmol of citric acid were added to 50 mL of
water, followed by evaporation to dryness at 50 °C, drying at 90
°C for 12 h, and calcination in air at 500 °C for 3 h. Then, the
MoO3-loaded TiO2 was added to aqueous HNO3 solution of
Pt(NH3)2(NO3)2, followed by evaporation to dryness at 50 °C,
and by drying at 90 °C for 12 h. Precursors of metal oxide-
supported Pt catalysts were prepared by the impregnation
method using aqueous HNO3 solution of Pt(NH3)2(NO3)2.
Before each catalytic experiment, catalysts were prepared by
prereduction of the precursor in a pyrex tube under a flow of
H2 (20 cm3 min−1) at 300 °C for 0.5 h.
1
2
MoOX/TiO2
Pt-MoOX/TiO2
Pt-VOX/TiO2
Pt-WOX/TiO2
Pt-ReOX/TiO2
Pt-CrOX/TiO2
Pt-MoOX/Al2O3
Pt-MoOX/ZrO2
Pt-MoOX/SiO2
Pt/TiO2
8
99
74
69
58
42
34
60
52
51
24
56
62
89
71
74
77
69
11
17
19
12
76
15
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Pt/Nb2O5
Pt/HZSM5
Pt/ZrO2
Pt/Al2O3
Pt/CeO2
Pt/MgO
Pt/SiO2
Pt/C
Ni-MoOX/TiO2
Cu-MoOX/TiO2
Ru-MoOX/TiO2
Rh-MoOX/TiO2
Pd-MoOX/TiO2
Re-MoOX/TiO2
2.3. Catalytic Tests. Pt-MoOX/TiO2 was used as a standard
catalyst. After the prereduction at 300 °C, the catalyst in the
closed glass tube with a septum inlet was cooled to room
temperature under H2. The mixture of amine (1.0 mmol), LA
(1.0 mmol), and n-dodecane (0 or 0.055 g) was injected to the
prereduced catalyst inside the glass tube through the septum
inlet, then the septum was removed under air, and a magnetic
stirrer was put in the tube, followed by inserting the tube inside
a stainless autoclave with a dead space of 28 cm3. Soon after
being sealed, the reactor was flushed with H2 from a high
pressure gas cylinder and charged with 3 bar of H2 at room
temperature. The amount of H2 present in the reactor before
heating was 3.4 mmol (3.4 equiv with respect to amine or LA).
Then, the reactor was heated at 100 °C under stirring (180
rpm) for 20 h. For the model reaction of LA and n-octylamine
in Tables 1, 2, and 3, conversion and yields of products were
determined by GC using n-dodecane as an internal standard
adopting the GC-sensitivity estimated using the isolated
product. For the scope and limitation study in Tables 4 and
5, isolated yields of products were determined as follows. After
the reaction, the catalyst was removed by filtration, and then
the reaction mixture was concentrated under vacuum
evaporator to remove the volatile compounds. Then, N-alkyl-
5-methyl-2-pyrrolidones were isolated by column chromatog-
raphy using silica gel 60 (spherical, 63-210 μm, Kanto Chemical
Co. Ltd.) with hexane/ethyl acetate (7/3) as the eluting
a
Yield was determined by GC.
Table 2. Reaction of LA with n-Octylamine under H2 by Pt-
a
MoOX/TiO2 in Various Solvents
a
solvents
yield (%)
no solvent
o-xylene
toluene
decane
dioxane
H2O
99
40
26
15
5
4
a
Conditions are the same as in Table 1.
of Pt/carbon, 15 mg of Pt/carbon mixed with 15 mg of KBr
was used. Spectra were measured accumulating 15 scans at a
resolution of 4 cm−1. A reference spectrum of the catalyst wafer
in He taken at 40 °C was subtracted from each spectrum. Prior
to the experiment, the catalyst wafer was heated in H2 flow (20
cm3 min−1) at 300 °C for 0.5 h, followed by cooling to 40 °C
and purging with He. Then, 1 μL of acetone was injected to He
flow preheated at 150 °C, which was fed to the IR cell. Then,
the IR disk was purged with He for 500 s, and IR measurement
was carried out.
1
solvent, followed by analyses by H NMR, 13C NMR, and
3. RESULTS AND DISCUSSION
GCMS.
2.4. In Situ IR. In situ IR spectra were recorded at 40 °C
using a JASCO FT/IR-4200 with an MCT detector. The
sample was pressed into a 30 mg self-supporting wafer (ϕ = 2
cm) and mounted into the quartz IR cell (CaF2 windows)
connected to a conventional flow reaction system. For a wafer
We carried out catalyst screening using a series of supported
binary metal catalysts (M1-M2OX/M3OX) and supported Pt
catalysts containing 0.001 mmol (0.1 mol %) of M1 or Pt for
reductive amination of LA (1 mmol) with n-octylamine (1
mmol) under 3 bar of H2 at 100 °C for 20 h as a model
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Table 3. Heterogeneous Catalysts for Reductive Amination of LA with n-Octylamine to Pyrrolidones
catalyst
mol %
solvent
water
reductant
T (°C)
yield (%)
TON
reuse
ref no.
Ir complex
0.2
HCOOH
80
80
88
62
95
99
86
440
124
no
no
no
no
yes
7
Ru complex
0.5
water
HCOOH
6
a
Au/ZrO2
0.05
0.86
0.04
water
HCOOH
130
150
100
1900
115
5
9
Pt/C
water
69 bar of H2
3 bar of H2
Pt-MoOX/TiO2
no solvent
2150
this study
a
Amination of LA with n-hexylamine.
Table 4. Reaction of LA with Anilines and Benzylamines
under H2 by Pt-MoOX/TiO2
Table 5. Reaction of LA with Aliphatic Amines under H2 by
Pt-MoOX/TiO2
a
a
a
Conditions: 0.001 mmol Pt catalyst, 1.0 mmol LA, 1.0 mmol amine.
Isolated yields.
b
amination of CO2 with amines under H2,10 combined with
previous reports that oxides of transition metal with Lewis
acidity (such as MoOX, NbOX, and ReOX) acted as effective
promoters of heterogeneous catalysts for the reduction of C
O and C−O bonds,11−18 we tested TiO2-supported Pt catalysts
coloaded with metal oxides of V, Cr, Mo, W, and Re. The result
(entries 2−6, 10) shows that activity changes in the order of
Mo > V > W > Re > Cr > no promoter (Pt/TiO2). Pt-MoOX/
TiO2 (entry 2) showed the highest yield (99%). Next, the effect
of support oxides on the activity of Pt and MoOX coloaded
catalysts were studied. The result (entries 2, 7−9) shows that
TiO2 is the best support for the Pt-MoOX coloaded catalysts.
The effect of the prereduction temperature (100−500 °C) of
Pt-MoOX/TiO2 showed that the catalyst reduced at 300 °C
gave the highest activity (result not shown). We also tested
monometallic Pt-loaded catalysts (entries 10−18), but all of
a
Conditions: 0.001 mmol Pt catalyst, 1.0 mmol LA, 1.0 mmol amine.
Isolated yield. GC yield.
b
c
reaction. Table 1 lists the yield of 5-methyl-1-octyl-pyrrolidin-2-
one as a desired product. On the basis of our recent finding that
Pt-MoOX/TiO2 showed high catalytic activity for reductive
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these conventional catalysts resulted in lower yield than Pt-
MoOX/TiO2. Pt/Al2O3 (entry 14) showed 89% yield of the
product, although the other supports (TiO2, Nb2O5, HZSM5
zeolite, ZrO2, CeO2, MgO, SiO2, and C) resulted in moderate
yields. Finally, we compared a series of transition metal and
MoOX coloaded TiO2 (entries 2, 19−24). Among various
metals tested (Pt, Re, Pd, Rh, Ru, Cu, Ni), Pt-MoOX/TiO2
showed the highest yield. In summary, Pt-MoOX/TiO2 was
found to show the highest catalytic activity for this reaction in
24 types of catalysts in Table 1. MoOX-loaded TiO2 (entry 1)
was nearly inactive and Pt/TiO2 (entry 10) showed lower yield
(51%) than Pt-MoOX/TiO2 (99%). These results indicate that
copresence of Pt metal nanoparticles and Mo species on TiO2
is indispensable.
The reductive amination of LA with n-octylamine with small
amount of Pt-MoOX/TiO2 (0.04 mol %) resulted in 86% yield
of the product, corresponding to TON of 2150 (Table 3). This
TON is higher those of Au/ZrO2 (TON = 1900),5 Pt/C
(115),9 and complexes of Ir (440)7 and Ru (124)6 for reductive
amination of LA with n-octylamine (or n-hexylamine) with
formic acid or H2. These facts clearly demonstrate the high
catalytic efficiency of the present system.
Under the optimized conditions with Pt-MoOX/TiO2, we
studied general applicability of the present catalytic system.
Table 4 shows the isolated yields of pyrrolidinones from LA
with anilines or benzylamines under 3 bar of H2 in the presence
of 0.1 mol % of the catalyst. Aniline (entry 1) and its derivatives
with both electron-poor (entries 2, 3) and electron-rich
substituents (entries 4−6) were tolerated to give high yields
(83−91%) with 100% conversions of anilines and LA. Note
that the reaction of a sterically hindered 2,4,6-methyl-
substituted phenylamine (entry 6) was successful. Benzylamine
(entry 7) and its derivatives with electron-donating (entries 8−
12), electron-withdrawing groups (entries 13,14), and a
sterically hindered 1-phenyl-ethylamine (entry 15) were also
converted to the desired products in high yields (84−95%). For
4-aminobenzylamine (entry 12), the NH2 group at benzyl
position was exclusively reacted. It is important to note that
heteroaromatic amines with pyridinyl (entry 16) and benzo-
[1,3]dioxolyl (entry 17) groups were also tolerated with good
yields (78% and 82%). Considering the fact that previous
methods5−8 do not tolerate heteroaromatic amines, our system
is the first method of pyrrolidinones synthesis from LA and
heteroaromatic amines.
In our recent report,10 we discussed the structure of Pt-
MoOX/TiO2 (prereduced at 300 °C). On the basis of
characterization results of temperature-programmed reduction
in H2 (Figure S1 in the Supporting Information), X-ray
absorption spectroscopy (Figure S2), and CO adsorption, the
dominant Pt species in Pt-MoOX/TiO2 were shown to be Pt
metal nanoparticles with average size of 4.1 nm.10
With the most effective catalyst, Pt-MoOX/TiO2 reduced at
300 °C, next we optimized the reaction conditions. Table 2 lists
the result of the model reaction in different solvents. The
reaction in o-xylene, toluene, and decane gave poor to moderate
yield (15−40%), and dioxane and water gave low yield of the
pyrrolidinone. The reaction under neat condition gave the
highest yield (99%) of the desired product. In the absence of
solvent, the reactions at different temperatures (90 and 150 °C)
gave lower yields than the standard condition (100 °C).
Then, we confirmed the heterogeneous nature and
reusability of this catalytic system by the following results.
For the standard reaction (entry 2 in Table 1), the catalyst was
removed from the reaction mixture by filtration after 3 h (32%
yield). Then, further heating of the filtrate under 3 bar of H2 for
17 h at 100 °C did not increase the yield. ICP-AES analysis of
the filtrate confirmed that the Pt content in the solution was
below the detection limit. These results confirm that the
reaction is attributed to the heterogeneous catalysis of Pt-
MoOX/TiO2. However, Mo content in the filtrate was 29 ppm,
corresponding to 0.3% of the Mo in the Pt-MoOX/TiO2
catalyst used. Figure 1 shows the result of the catalyst recycle
test. After the reaction of LA with aniline for 20 h, the catalyst
was separated from the reaction mixture by filtration and was
dried at 90 °C for 3 h and then reduced in H2 at 300 °C for 0.5
h. The recovered catalyst showed high yield (90−97%) at least
four cycles.
Table 5 shows yields of pyrrolidinones from aliphatic amines
and LA under H2. Various aliphatic amines including linear
(entries 1−3) and branched (entries 4, 5) and cyclic amines
(entries 6−10) were successfully reacted with LA to form
desired products in good to excellent isolated yields (77−95%).
Patents by Manzer3,4 showed that the 5-methyl-1-octyl-
pyrrolidin-2-one (entry 1) and 1-cyclohexyl-5-methyl-pyrroli-
din-2-one (entry 8) can be industrial solvents, surfactants,
complexing agents, and additives in functional materials, such as
pharmaceutical, agrochemical, cleaning compositions, and
printing ink. For the first time, an aminol was successfully
transformed to the corresponding pyrrolidinone (entry 11).
Amines with hydrogen-accepting groups, allylamine and
propargyl amine, were not tolerated; 85% and 81% yields of
the pyrrolidinones were obtained with complete reduction of
the CC and CC bonds (result not shown).
To discuss a possible mechanism (Scheme 1), we carried out
the reaction of n-octylamine (2.0 mmol) and LA (1.0 mmol) at
Scheme 1. Possible Reaction Pathway of Pt-MoOX/TiO2-
Catalyzed Reductive Amination of LA with Amines under H2
room temperature under N2 in the absence of the catalyst. After
10 min, the mixture was characterized by GCMS and NMR
(Figure S3). Two characteristic peaks (169.35 and 50.87 ppm)
Figure 1. Catalyst reuse for reductive amination of LA with aniline
under H2 by Pt-MoOX/TiO2 catalyst. Conditions are the same as in
Table 4.
1
in the 13C NMR chart and one peak (3.19 ppm) in the H
NMR chart assignable to the corresponding imine 1 (4-
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octylimino-pentanoic acid) were observed together with the
peaks due to LA and n-octylamine. A GC-MS result supported
the presence of the imine 1 (m/e = 227.195) in the mixture. 1H
NMR analysis showed around 14% yield of the imine based on
LA. Thus, catalyst-free condensation of amine and LA to the
imine 1 can be the initial step of the reductive amination of LA.
Then, the CN bond of the imine 1 can undergo Pt-catalyzed
hydrogenation to give the amine 2, which is dehydrated to give
the cyclized product, pyrrolidones.
hypothesize that the catalytic activity in Table 6 depends on the
acid−base interaction between the catalyst and carbonyl
groups. To evaluate the acid−base interaction between the
catalyst and a carbonyl group, we carried out IR experiment of
acetone adsorbed on the catalysts in Table 6. The spectrum in
Figure 2 shows that the CO stretching band of the acetone
On the basis of the mechanism proposed, lastly we discuss
mechanistic reasons why Pt-MoOX/TiO2 showed higher
activity than the other Pt catalysts. Table 6 lists the yields of
Table 6. Summary of IR Results and Product Yields for the
Reactions I, II, and III
Figure 2. IR spectra of acetone adsorbed on the catalysts at 40 °C.
adsorbed on Pt-MoOX/TiO2 is centered at lower wavenumber
(1691 cm−1) than those on nonacidic catalyst, Pt/C (1710 and
1740 cm−1) and other Pt catalysts (1696−1710 cm−1). This
indicates that acid sites of Pt-MoOX/TiO2 interact more
strongly with the carboxyl oxygen than those of the other
catalysts. On the basis of these results, we can conclude that
effective polarization of the carbonyl groups via Lewis acid−
base interaction between Pt-MoOX/TiO2 and carboxyl oxygen
leads to its high activity for reductive amination of a carbonyl
group in LA to the intermediate 2 and for cyclic amidation of 2
to pyrrolidones, resulting in high catalytic activity for the
reductive amination of LA.
a
b
c
d
catalysts
νCO of acetonead (cm−1
)
I (%) II (%) III (%)
Pt-MoOX/TiO2
Pt-MoOX/SiO2
Pt/TiO2
1691
1696
1710
1703
20
11
10
9
35
15
15
7
34
32
24
5
Pt/Al2O3
Pt/C
1740, 1710
14
2
9
a
b
From Figure 2 (acetone-IR). Conditions: 1 mmol LA, 1 mmol
aniline, 0.1 mol % catalyst (3.9 mg), 100 °C, 3 bar of H2, 3 h.
Conditions: 1 mmol 2-octanone, 1 mmol aniline, 1 mL of hexane, 0.1
mol % catalyst, 80 °C, 1 bar of H2, 3 h. Conditions: 1 mmol 4-
aminobutyric acid in 1 mL of o-xylene, 0.1 mol % catalyst, reflux, 1 bar
c
d
4. CONCLUSION
of N2, 3 h.
We have found that Pt and MoOX coloaded TiO2 is effective for
the reductive amination of LA by H2 under solvent-free
conditions. This method is the first general and reusable
heterogeneous catalytic system for the reductive amination of
LA by H2. Considering the fact that the previous homogeneous
catalytic systems have difficulties in catalyst/product separation
and need expensive organic ligands, toxic solvent, or excess
amount of reductant or aminating agent, as well as the fact that
the previous heterogeneous systems show no catalyst reuse and
quite limited scope, the present method can be the most atom-
efficient method for the reductive amination of LA. High
catalytic activity of Pt-MoOX/TiO2 can be attributed to acid−
base interaction between the acid sites of Pt-MoOX/TiO2 and
carboxyl groups in LA and the intermediate 2.
the pyrrolidinone for reaction of LA with aniline in H2
(reaction I) under low conversion conditions using five
representative Pt catalysts: Pt-MoOX/TiO2, Pt-MoOX/SiO2,
Pt/Al2O3, Pt/TiO2, and Pt/C. Pt-MoOX/TiO2 showed higher
yield (20%) than the other Pt catalysts (9−14%). The
mechanism in Scheme 1 consists of two catalytic reactions:
reductive amination of a carbonyl group in LA to the
intermediate 2 and its dehydrative cyclization (cyclic
amidation) to pyrrolidones. To tests catalytic activity of each
catalytic step, we carried out a model reaction of 2-octanone
with aniline in H2 (reaction II) and cyclic amidation of 4-
aminobutyric acid to pyrrolidones (reaction III), and the results
are included in Table 6. For these model reactions, Pt-MoOX/
TiO2 showed higher activity than the other Pt catalysts, and this
trend was similar to that in the reductive amination of LA
(reaction I). This suggests that high activity of Pt-MoOX/TiO2
for the reductive amination of LA can be due to its high activity
for reductive amination of a carbonyl group in LA to the
intermediate 2 and its cyclic amidation to pyrrolidones.
ASSOCIATED CONTENT
* Supporting Information
Characterization data for catalyst, an intermediate, and
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: kshimizu@cat.hokudai.ac.jp. Fax: +81-11-706-9163.
■
Molybdenum oxides are known as acidic prompters in
heterogeneous catalysis.19,20 Considering that both of the
elementary steps (II) and (III) can be promoted by increasing
electrophilicity of the carbonyl groups via Lewis acid−base
interaction between the catalyst and carboxyl oxygen, one can
Notes
The authors declare no competing financial interest.
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ACS Catalysis
Research Article
ACKNOWLEDGMENTS
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Funding sources include a project entitled “Informatics”
(25106010) from the Japan Society for the Promotion of
Science and a Ministry of Education, Culture, Sports, Science,
and Technology program entitled “Elements Strategy Initiative
to Form Core Research Center”.
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