Ambient Reductive Amination of Levulinic Acid to Pyrrolidones over Pt Nanocatalysts on Porous TiO2 Nanosheets

Article abstract of DOI:10.1021/jacs.8b13024

Construction of N-substituted pyrrolidones from biomass-derived levulinic acid (LA) via reductive amination is a highly attractive route for biomass valorization. However, realizing this transformation using H₂ as the hydrogen source under mild conditions is still very challenging. Herein, we designed porous TiO₂ nanosheets-supported Pt nanoparticles (Pt/P-TiO₂) as the heterogeneous catalyst. The prepared Pt/P-TiO₂ was highly efficient for reductive amination of LA to produce various N-substituted pyrrolidones (34 examples) at ambient temperature and H₂ pressure. Meanwhile, Pt/P-TiO₂ showed good applicability for reductive amination of levulinic esters, 4-acetylbutyric acid, 2-acetylbenzoic acid, and 2-carboxybenzaldehyde. Systematic studies indicated that the strong acidity of P-TiO₂ and the lower electron density of the Pt sites as well as the porous structure resulted in the excellent activity of Pt/P-TiO₂.

Full text of DOI:10.1021/jacs.8b13024

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Article
Ambient Reductive Amination of Levulinic Acid to Pyrrolidones
over Pt Nanocatalysts on Porous TiO2 Nanosheets
Chao Xie, Jinliang Song, Haoran Wu, Yue Hu, Huizhen Liu,
Zhanrong Zhang, Pei Zhang, Bingfeng Chen, and Buxing Han
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13024 • Publication Date (Web): 10 Feb 2019
Downloaded from http://pubs.acs.org on February 10, 2019
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Ambient Reductive Amination of Levulinic Acid to Pyrrolidones
over Pt Nanocatalysts on Porous TiO2 Nanosheets
Chao Xie,†,‡ Jinliang Song,*, † Haoran Wu,†,‡ Yue Hu,†,‡ Huizhen Liu,†,‡ Zhanrong Zhang,† Pei Zhang,†
Bingfeng Chen,† and Buxing Han*,†,‡
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†Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid and Interface and
Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China.
‡School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China.
Supporting Information Placeholder
catalysts (i.e., Pt, Pd, Rh, Ru, Co, Ni),3a, 3b, 4b, 6 and homogeneous
ABSTRACT: Construction of N-substituted pyrrolidones from
7
metal (Ir, Ru) complex.5b, However, these catalysts usually
biomass-derived levulinic acid (LA) via reductive amination is
required either high H2 pressure (>3 MPa) and temperature
(>100 °C) or only provided low to moderate product yields.
Therefore, it is highly desired to develop efficient
heterogeneous catalysts for reductive amination of LA to N-
alkyl-5-methyl-2-pyrrolidones with H2 at ambient conditions.
a highly attractive route for biomass valorization. However,
realizing this transformation using H2 as the hydrogen source
under mild conditions is still very challenging. Herein, we
designed porous TiO2 nanosheets supported Pt nanoparticles
(Pt/P-TiO2) as the heterogeneous catalyst. The prepared Pt/P-
TiO2 was highly efficient for reductive amination of LA to
produce various N-substituted pyrrolidones (34 examples) at
ambient temperature and H2 pressure. Meanwhile, Pt/P-TiO2
showed good applicability for reductive amination of levulinic
esters, 4-acetylbutyric acid, 2-acetylbenzoic acid and 2-
carboxybenzaldehyde. Systematic studies indicated that the
strong acidity of P-TiO2 and the lower electron density of the
Pt sites as well as the porous structure resulted in the
excellent activity of Pt/P-TiO2.
As well-known, the catalytic activity of supported metal
catalysts can be significantly enhanced by utilizing functional
support materials.8 There is great potential to prepare
efficient heterogeneous catalysts for reductive amination of
LA by designing functional support materials. Herein, we
prepared porous TiO2 nanosheets (denoted as P-TiO2) as a
functional support for Pt nanoparticles (Pt NPs). P-TiO2
supported Pt NPs (denoted as Pt/P-TiO2) showed excellent
performance for the reductive amination of LA with a range of
primary amines even at ambient temperature and H2
pressure. To the best of our knowledge, this is the first work
to realize this kind of reactions at ambient conditions.
INTRODUCTION
In the pursuit of a sustainable future, biomass has been
recognized as a renewable alternative for fossil resources to
produce various high-valued chemicals, and diverse strategies
have been established for biomass transformation.1 In this
context, levulinic acid, a valuable platform chemical from
lignocellulosic biomass, has attracted much interest because it
can be upgraded into some valuable chemicals,2 in which N-
alkyl-5-methyl-2-pyrrolidones, produced through reductive
amination of LA,3 are significantly attractive to be used as
industrial solvents, dispersants, and surfactants.4
RESULTS AND DISCUSSION
Preparation and Characterization of the Pt/P-TiO2. The P-
TiO2 nanosheets was prepared through a ball milling route
using KNO3 as the hard template, and Pt NPs were supported
on the prepared P-TiO2 to form Pt/P-TiO2 with NaBH4 as the
reducing agent. Detail preparation processes for P-TiO2 and
Pt/P-TiO2 was described in the Experimental Section. The
morphology of the synthesized P-TiO2 and Pt/P-TiO2 was
firstly examined by TEM and SEM (Figure 1a-d). TEM images
indicated that P-TiO2 had a porous nanosheet structure
(Figure 1a-b), which was retained after supporting Pt NPs
(Figure 1c). In Pt/P-TiO2, Pt NPs had an average size of 2.4 nm
(Figure S1). High resolution TEM (HR-TEM) image showed
that Pt NPs on the P-TiO2 had an interplanar distance of 0.19
nm, corresponding to the distinct lattice fringe of Pt (200)
surface. N2 adsorption-desorption isotherm indicated that P-
TiO2 had a BET surface area of about 259.8 m2/g (Figure 1e),
and meanwhile, the pore distribution confirmed the existence
of micropores in P-TiO2, which was consistent with the TEM
Generally, LA can be converted into N-alkyl-5-methyl-2-
pyrrolidones via reductive amination by employing formic
acid or organosilanes as the hydrogen sources.4b, 4c, 5 However,
some drawbacks hindered the large-scale application of these
two hydrogen sources, such as the corrosiveness of formic
acid, the high price and excess usage of organosilanes, and
harsh reaction conditions, etc. Compared with formic acid and
organosilanes, H2 is the more desired hydrogen source from
economic and industrial viewpoints. To dates, several
catalysts have been explored for reductive amination of LA
using H2 as hydrogen source, including supported metal
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result (Figure 1a). The XRD patterns of P-TiO2 and Pt/P-TiO2
under solvent-free conditions was not significant because the
high viscosity of the reaction system under solvent-free
conditions, which was resulted from the strong interaction
between the carboxyl group in LA and the amino group in n-
octylamine, was an important factor for the reaction
efficiency, especially at room temperature. Therefore, it is
necessary to use the solvent in the reaction using LA as the
reactant for high efficiency. Moreover, the influence of
different solvents was examined for the reductive amination
of LA with n-octylamine (Table S3). The yield of the desired
product was very low in n-hexane (Table S3, entry 1) because
the condensation of C=O in LA and NH2 in n-octylamine was
sluggish,9 significantly lowering the total reaction rate. The
aprotic polar solvents (i.e., DMF, THF, 1,4-dioxane, and
acetonitrile) were also unbeneficial for the reaction (Table S3,
entries 2-5), which resulted from the occupation of the
catalytic active sites on the Pt/P-TiO2 by the solvent molecules
due to their strong adsorption.9,10,3d In contrast, good results
could be achieved in protic polar solvents, i.e., methanol,
ethanol, and isopropanol (Table S3, entries 6-8) because the
protic polar solvents could accelerate both the condensation
of C=O and NH2 and the subsequent cyclization9 (See the
following mechanism section), resulting in higher reaction
rate. Due to the best result in methanol (Table S3, entry 6), it
was selected as the solvent. Additionally, the synthesized
Pt/P-TiO2 showed much higher activity than conventional
Pt/TiO2 when using methyl levulinate as the substrate under
solvent-free conditions (Table 1, entries 9 and 10) because the
interaction of ester group in methyl levulinate and the amino
group in n-octylamine is relatively weak and the viscosity of
the reaction system was relatively low. The results indicated
that the synthesized Pt/P-TiO2 was also more active than
Pt/TiO2 in solvent-free reaction system of low viscosity.
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suggested that these two materials had low crystallinity
(Figure 1f), and no peak for Pt species was found in the
pattern for Pt/P-TiO2, indicating no aggregation of Pt NPs.
Additionally, the content of Pt in Pt/P-TiO2 was 1.98 wt%
determined by ICP-AES (VISTA-MPX), and the Pt dispersion in
Pt/P-TiO2 was 21% determined by CO-pulse titration (Table
S1).
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Figure 1. Characterization of the obtained P-TiO2 and Pt/P-TiO2. TEM
(a) and SEM (b) images of P-TiO2, TEM (c) and HR-TEM (d) images of
Pt/P-TiO2. N2 adsorption-desorption isotherms (e) of P-TiO2 (Insert e:
the distribution of micropore), and XRD patterns (f) of P-TiO2 and
Pt/P-TiO2.
Table 1. Reductive amination of LA with n-octylamine over different
catalysts.a
O
Catalyst
N
OH
+
6
Activity of Various Catalysts. As a common used catalyst,
supported Pt showed good performance for reductive
amination of LA under high H2 pressure and temperature.3a, 3b,
NH2
6
O
O
Entry
1
Catalyst
Yield (%)
TOF (h-1)b
--
6a, 6b
Therefore, we initially evaluate the catalytic activity of
-
0
several Pt-based catalysts with similar Pt dispersion and
crystal size of Pt particles (Table S1) at ambient conditions
employing reductive amination of LA with n-octylamine as a
model reaction (Table 1). No target product was formed
without any catalysts (Table 1, entry 1). Commercial Pt/C
showed very low activity (Table 1, entries 2). To our delight,
the Pt/P-TiO2 prepared in this work could catalyze the
reaction very efficiently with the highest TOF (Table 1, entry
3). Furthermore, the Pt particles supported on commercial
TiO2 and P25 (Pt/TiO2 and Pt/P25) provided much lower
activity (Table 1, entries 4 and 5) than Pt/P-TiO2, suggesting
the intrinsic advantage of the prepared P-TiO2. In addition, the
reactants could be fully converted with a product yield of 97%
over Pt/P-TiO2 in 3 h (Table 1, entry 6, and Figure S2).
Moreover, Pt/P-TiO2 showed much higher activity than the
reported catalysts (Table S2). These results indicated Pt/P-
TiO2 was a superior catalyst for reductive amination of LA,
and the reason for the excellent performance will be discussed
below. In another aspect, some control experiments were
conducted to examine the reaction efficiency under solvent-
free conditions. Without using solvents, the yield of the
desired product over the synthesized Pt/P-TiO2 and the
conventional Pt/TiO2 were 7% and 2%, respectively (Table 1,
entries 7 and 8). The difference in activity of the two catalysts
2
Pt/C
4
40 (222)
3
Pt/P-TiO2
Pt/TiO2
Pt/P25
Pt/P-TiO2
Pt/P-TiO2
Pt/TiO2
Pt/P-TiO2
Pt/TiO2
61
14
16
97
7
610 (2905)
140 (875)
160 (941)
323 (1538)
70 (333)
4
5
6c
7d
8d
9e
10e
2
20 (125)
58
10
193 (919)
33 (206)
aReaction conditions: LA, 1 mmol; n-octylamine, 1 mmol; catalyst, 0.1
mol% Pt; methanol, 2 ml; time, 1 h; temperature, r.t.; H2 pressure,
hydrogen balloon. bTOF = mol of product/mol of Pt×h-1, and values in
parentheses were the TOF calculated based on Pt dispersion. cThe
reaction time was 3 h. dThe reaction was conducted under solvent-
free conditions with a reaction time of 1 h. eThe reaction was
conducted under solvent-free conditions using methyl levulinate and
n-octylamine as reactants with a reaction time of 3 h.
Reusability of the Prepared Pt/P-TiO2. The heterogeneous
nature of Pt/P-TiO2 was evaluated by removing the catalyst
after the reaction was conducted for 1 h, and then the reaction
was continued for 4 h without solid catalyst. The product yield
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was not further increased without Pt/P-TiO2 (Figure S3a),
primary anilines with different substituted groups, including
methyl, tertiary butyl, methoxy, and halogen, could be
converted efficiently, affording the corresponding products in
high yields (Table 3, 2a-e). Meanwhile, the reactivity of the
methyl-substituted anilines followed the order: p-methyl
aniline > m-methyl aniline > o-methyl aniline (Table 3, 2e-g)
caused by the different steric effect. In addition, o-
methoxyaniline and 2,4,6-trimethylaniline needed longer
reaction time (24 h) to provide good yields due to the strong
steric effect of the substituted groups (Table 3, 2h and 2i).
Furthermore, the substrates with reduction-sensitive
substituents were examined to study the applicability of the
synthesized Pt/P-TiO2. High yields (>90%) of the desired
products could be achieved when 4-aminobenzonitrile (2j), 4-
aminoacetophenone (2k), 4-aminoacetanilide (2l) and methyl
4-aminobenzoate (2m) were used as the reactants. For
halogen (Cl, Br, I)-substituted substrates (2n-p), the
dehalogenation reaction with different extent occurred owing
to the high activity of the synthesized Pt/P-TiO2, and the
yields of the desired halogen-substituted products were 91%,
72% and 45% for 4-chloroaniline (2n), 4-bromoaniline (2o)
and 4-iodoaniline (2p), respectively. However, the desired
products (1-(4-ethynylphenyl)-5-methylpyrrolidin-2-one, and
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2
3
4
5
6
7
8
indicating no leaching of active species to the reaction
mixture. Meanwhile, the concentrations of Pt and Ti in the
reaction solution were below detection limit of ICP-AES,
further confirming the negligible leaching of Pt and Ti species
and the heterogeneous nature of Pt/P-TiO2. Furthermore, the
Pt/P-TiO2 could be recycled several times without significant
decrease in activity. The observed yield decreased slightly
after 4 recycles, which could be attributed to the catalyst loss
in the recovery process (Figure S3b). After the fourth run, 8
mg Pt/P-TiO2 (80% original amount) was recovered.
Therefore, in the fifth run, the amount of reactants was
decreased based on the recovered catalyst and the yield was
then restored. The recovered Pt/P-TiO2 was characterized by
TEM and XPS (Figure S4), and no considerable change was
found. The above results indicate the high stability of Pt/P-
TiO2 in the reaction systems.
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Scope of the Substrates. Delighted by the excellent
performance of Pt/P-TiO2 for reductive amination of LA with
n-octylamine, the reactivity of various primary amines was
subsequently investigated. Over Pt/P-TiO2, aliphatic amines
could be efficiently converted into the corresponding N-alkyl-
5-methyl-2-pyrrolidones with excellent yields (Table 2).
Different from commonly reported results,5b the chain length
of the aliphatic amines showed no obvious impact on the
reactivity (Table 2, 1a-c) in our catalytic system. Meanwhile,
Pt/P-TiO2 could also efficiently catalyze reductive amination
of LA with the aliphatic amines with branched chain (Table 2,
1d and 1e) and various functional groups (Table 2, 1f-h),
cyclic amines (Table 2, 1i-k), and benzylic amines (Table 2, 1l-
n).
1-(4-vinylphenyl)-5-methylpyrrolidin-2-one)
were
not
obtained using 4-ethynylaniline and 4-vinylaniline as the
substrates because the high activity of Pt/P-TiO2 resulted in
the complete hydrogenation of the ethynyl and vinyl groups to
generate
1-(4-ethylphenyl)-5-methylpyrrolidin-2-one.
Therefore, balancing the catalytic activity and the product
selectivity was very important for the substrates with
reduction-sensitive substituents.
Table 3. Reductive amination of LA with various anilines over Pt/P-
TiO2.
Table 2. Reductive amination of LA with various aliphatic amines
over Pt/P-TiO2.
R
O
O
Catalyst 0.1 mol%
OH
NH2
+
N
O
O
R
Catalyst 0.1 mol%
O
hydrogen balloon
R
r. t.
OH
N
R
NH2
1mmol
1 mmol
+
O
r. t.
hydrogen balloon
1 mmol
1 mmol
F
H3CO
O
O
O
O
O
O
N
N
O
N
N
O
N
N
( )
( )
( )
N
N
2
4
6
2c, 98%, 3h
2a, 97%, 3h
2b, 99%, 3h
2d, 97%, 3h
OCH3
1b, 99%, 3h
1a, 95%, 3h
1c,97%, 3h
1d, 94%, 3h
O
N
O
N
O
N
O
OH
O
N
N
O
N
O
N
O
N
O
2f, 98 %, 5h
N
2g, 95 %, 10h
2e, 97%, 3h
2h, 91%, 24h
O
O
H
O
N
O
N
O
1g, 91%, 3h
O
O
O
N
1h, 98%, 3h
1e, 90%, 24h
1f, 94%, 3h
N
N
O
O
O
N
O
N
N
N
2k, 92%, 24h
2j, 94%, 6h
2l, 91%, 10h
2i, 90%, 24h
Cl
Br
I
NC
O
O
O
O
1j, 99%, 5h
1l, 99%, 3h
1i, 98%, 3h
1k, 96%, 5h
N
N
N
N
O
O
N
H3CO
N
2m, 97%,10h
2n, 90%, 3h
2o, 72%, 3h
2p, 45%, 3h
Cl
1n, 99%, 3h
1m, 98%, 3h
Except for LA, Pt/P-TiO2 could be employed to efficiently
catalyze reductive amination of other substrates with the
similar structure to LA at ambient conditions (Table 4).
Various levulinic esters, a class of chemicals derived from
carbohydrates in alcohols, could also be converted with n-
Furthermore, Pt/P-TiO2 was also suitable for the
reductive amination of LA with anilines (Table 3), and most of
the examined anilines gave good yields (>90%) of
corresponding pyrrolidones at ambient conditions. The
octylamine
to
form
N-octyl-5-methyl-2-pyrrolidones.
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conditions: LA, 1 mmol; NH3 (gas), 0.5 MPa; H2, 1.5 MPa; Pt/P-TiO2, 0.1
mol% Pt; methanol, 2 mL; r.t.
Compared with LA, levulinic esters showed relative lower
1
2
3
4
5
6
7
8
reactivity, and longer reaction time (10 to 24 h) was needed to
reach high product yields (Table 4, entries 1-4). The reactivity
of levulinic esters followed the order: methyl levulinate >
ethyl levulinate > butyl levulinate owing to the different steric
hindrance effect. Meanwhile, Pt/P-TiO2 could be further
expanded to catalyze the reductive amination of 4-
acetylbutyric acid to produce N-alkyl-2-piperidinones, which
are another class of important heterocycle compounds. Taking
n-octylamine as an example, N-octyl-2-piperidinone could be
obtained from 4-acetylbutyric acid with a yield of 97% over
Pt/P-TiO2 (Table 4, entry 5). Furthermore, Pt/P-TiO2 was
applicable for reductive amination of 2-acetylbenzoic acid and
2-carboxybenzaldehyde to generate the corresponding N-
arylisoindolinones with n-octylamine, and the product yields
could reach 95% and 96%, respectively (Table 4, entries 6-7).
In another aspect, the NH-free pyrrolidone was also an
attractive compound. The synthesized Pt/P-TiO2 was used to
catalyze the reductive amination of LA using 25 wt% aqueous
ammonia as the amine source (Table 4, entry 8). To our
delight, a 85% yield of the corresponding NH-free pyrrolidone
was successfully generated with a reaction time of 72 h under
ambient conditions. Additionally, when using NH3 (gas) as the
amine source, the NH-free pyrrolidone could also be
generated with a yield of 89% over the synthesized Pt/P-TiO2
at room temperature (Table 4, entry 9). These results further
indicated the extensive applicability and high activity of the
synthesized Pt/P-TiO2.
Mechanism Discussion. To get some evidences to study the
reaction mechanism for the reductive amination of LA, some
control experiments were conducted using butyl levulinate as
the reactant (Scheme 1) because of the steric hindrance effect
of butyl in butyl levulinate (Table 4, entry 4) was beneficial for
obtaining the reaction intermediates, and the reaction process
was monitored by GC-MS method (Figure S5-6). As
reported,3b,3d,3e,6c the amine could react with ketone to form
imine. In the absence of H2, the relative imines (A and B, B was
generated from the transesterification of A with the solvent
methanol) could be easily detected. Meanwhile, the cyclization
products (C and D in Scheme 1, and D was the oxidative
product of C)1c from isomer (enamine) of imine were
determined by GC-MS (Figure S5), and C and D could also be
easily detected when using LA as the reactant no matter with
or without H2 (Figure S7-8), indicating the cyclization of the
enamine could be a possible pathway (Path Two in Scheme 2)
for the studied reductive amination to synthesize pyrrolidines.
In another aspect, when the H2 was giving, the desired
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pyrrolidine,
E and F (from the hydrogenation of the
corresponding imines) became the main products (Scheme 1),
and the corresponding intermediate F (Figure S9, NMR data)
could be conveniently separated in a control experiment using
butyl levulinate and aniline as the reactants. With a prolonged
reaction time, both E and F could be further converted into the
desired pyrrolidine. These results showed that the imine or
enamine intermediate could be hydrogenated to the saturated
intermediate and further cyclized into the desired
pyrrolidone, confirming the occurrence of Path One in Scheme
2.
Table 4. Reductive amination of other substrates with n-octylamine
over Pt/P-TiO2.a
Entry Substrate
Product
Time (h)
3
Yield (%)
97
O
O
O
O
O
OH
( )
N
6
N
N
N
1
2
6
6
6
N
O
O
6
O
O
H2
O
O
46%
14%
C
N
6
27%
A
O
H 2
O
12h
Pt/P-TiO2
N
6
ut
Witho
N
6
O
93%
( )
15h
10
94
O
2
B
O
O
<1%
D
Pt/P-TiO
O
O
O
+
O
( )
( )
H
2
Pt/P-TiO
3
4
5
6
10
24
3
93
92
97
95
NH2
O
6
NH
6
6
O
O
N
6
O
O
2
H2
15h
N
6
O
O
27%
5%
E
53%
O
12h
Pt/P-TiO2
NH
94%
O
N
6
6
O
F
O
O
OH
Scheme 1. Control experiments of reductive amination of
butyl levulinate with n-octylamine.
( )
( )
N
O
O
O
O
OH
On the basis of the above experimental results and
previous reports,3c,4c,5a a possible mechanism for the reductive
amination of LA and its esters to pyrrolidinones over Pt/P-
TiO2 was proposed (Scheme 2). Initially, the corresponding
imine II was easily generated through the condensation
reaction of the carbonyl groups in LA and its esters with the
amino group in the amines.3b,3c,6c,5b,5c Subsequently, two
reaction pathways were proceeded to form the desired
pyrrolidone. In one pathway (Path One), the imine II was
hydrogenated to amino group over the Pt sites, and then the
corresponding N-substituted pyrrolidone was formed through
the intramolecular cyclization of IV catalyzed by the acidic
N
6
24
O
O
O
O
OH
( )
N
7
5
96
85
89
6
O
O
O
OH
8b
9c
72
72
HN
HN
O
OH
O
6c
sites in the Pt/P-TiO2.3b, In another pathway (Path Two),
aReaction conditions: substrate, 1 mmol; n-octylamine, 1 mmol; Pt/P-
intermediates
V and VI were firstly formed from the
b
TiO2, 0.1 mol% Pt; methanol, 2 mL; r.t.; hydrogen balloon. Reaction
cyclization of the isomer of II (III). After hydrogenated on Pt
sites, the two intermediates V and VI were converted to
generate the desired pyrrolidone.3c
conditions: LA, 1 mmol; 25 wt% aqueous ammonia, 500 ul; Pt/P-TiO2,
0.1 mol% Pt; methanol,
2
mL; r.t.; hydrogen balloon. cReaction
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TiO2 and P-TiO2 than those on Pt/TiO2 and TiO2, indicating the
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stronger interaction of the carbonyl groups with the acidic
sites of Pt/P-TiO2 and P-TiO2 than that of conventional Pt/TiO2
and TiO2.3b,6c These results indicated that the stronger acidity
of Pt/P-TiO2 and the stronger interaction of the corresponding
groups with the catalytic sites were beneficial for the
condensation of carbonyl groups and amino groups and the
corresponding cyclization (Scheme 2),13 and thus leading to
the high activity of the synthesized Pt/P-TiO2 for the reductive
amination of LA.
9
In situ CO-DRIFT measurements were performed to
determine the electronic property of the supported Pt surface.
As shown in Figure 2e, the band for CO adsorbed on metallic
Pt (CO-Pt0) in the synthesized Pt/P-TiO2 was observed at
2093 cm-1, which showed a blue-shift compared with the band
(2079 cm-1) for CO adsorbed on metallic Pt (CO-Pt0) in
conventional Pt/TiO2. The blue-shift of CO adsorption band
indicated that the electron-donating capability of Pt in the
Pt/P-TiO2 was weaker than that in the conventional Pt/TiO2
because the P-TiO2 with higher acidity (Figure 2b) had higher
ability to decrease the electron density of Pt particles than
conventional TiO2.7b,14 It has been reported that lower
electron density of the catalytic sites (Pt particles in our
systems) was beneficial for the desorption of the products and
reactants, and thus resulted in higher catalytic activity.7b,15 In
our reaction system, both the reactant (amines) and the
product (pyrrolidones) were nitrogen-containing compounds,
and they could generate strong interaction with the Pt active
sites, which could decreased the reaction rate due to the
occupation and even poison of the active sites.16 Therefore,
the lower electron density of Pt in the synthesized Pt/P-TiO2
probably resulted in a higher desorption rate of the used
amines and the generated pyrrolidones from the active sites
owing to their weaker interaction, and thus Pt/P-TiO2 showed
higher activity for the reductive amination of LA than the
conventional Pt/TiO2. Furthermore, some control experiments
using 5-methylpyrrolidin-2-one as an additive were
conducted to show the different interaction ability of
pyrrolidone with the active sites on the used Pt-based
catalysts (Table S4). It was found that by the pre-addition of
5-methylpyrrolidin-2-one, the activity of Pt/TiO2 and Pt/P25
decreased (Table S4, entries 7-8 vs. 3-4), while Pt/P-TiO2 was
not affected. (Table S4, entry 6 vs. 5). These results implied the
weaker interaction ability of pyrrolidone with the active sites
on Pt/P-TiO2 owing to the lower electron destiny of Pt sites,
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Scheme 2. A plausible mechanism for the reductive amination of LA
and its esters to pyrrolidones over the prepared Pt/P-TiO2.
Reasons for the High Activity of the Synthesized Pt/P-TiO2.
To investigate the reason for the excellent performance of the
synthesized Pt/P-TiO2 for the reductive amination of LA and
its esters, the influence of reaction temperature on the
catalytic performance of different catalysts (i.e., the
synthesized Pt/P-TiO2, conventional Pt/TiO2, and Pt/C) was
examined to obtain the apparent activation energy (Ea) via the
Arrhenius equation.11 The Ea of the reaction were calculated
from the slope of -ln (TOF) versus 1/T plots, and the results
are presented in Figure 2a. The Ea of the catalytic systems
promoted by different catalysts decreased in the order: the
synthesized Pt/P-TiO2 (30 kJ/mol) < conventional Pt/TiO2 (84
kJ/mol) < Pt/C (89 kJ/mol), indicating the much stronger
ability of the synthesized Pt/P-TiO2 to reduce the Ea of the
reaction than conventional Pt/TiO2 and Pt/C, and thus
achieved the highest catalytic activity. These results were
consistent with the catalytic results shown in Table 1.
Generally, both the condensation of carbonyl groups and
amino groups to form intermediate II (Scheme 2) and the
cyclization of intermediates III and IV (Scheme 2) could be
enhanced by the acidity of the catalyst. Therefore, the acidity
of the synthesized Pt/P-TiO2 and conventional Pt/TiO2 was
evaluated by different techniques in this work. NH3-TPD
examination (Figure 2b) clearly showed the stronger acidity
of P-TiO2 than the conventional TiO2. Meanwhile, the higher
binding energy values of Ti 2p in P-TiO2 indicated the higher
positive charge on Ti atoms in the P-TiO2 than in conventional
TiO2 (Figure 2c), which resulted in stronger acidity.12 To
exclude the effect of residual KNO3 on the acidity of P-TiO2, the
acidity of the mixture of commercial TiO2 (1 g) and KNO3 (20
mg) was also determined by NH3-TPD (Figure 2b). No
difference was found for the acidity of pure TiO2 and the
mixture when the NH3 desorption temperature was below
450 °C, while the peak observed at 500 °C for both P-TiO2 and
the mixture resulted from the generated gases from KNO3
decomposition rather than the acidity of the materials. In
another aspect, the addition of KNO3 showed no effect on the
catalytic activity of Pt/P25 and P/TiO2 (Table S4, entries 1-4).
These control experiments suggested that the residual KNO3
did not affect the acidity of P-TiO2. The higher acidity of the
prepared P-TiO2 may be attributed to that more acidic sites
were exposed due to its unique structure (porous, and
resulting in
a higher desorption rate of the generated
pyrrolidone from Pt/P-TiO2 to enhance the reductive
amination of LA. The discussions above indicated that the
lower electron destiny of Pt sites could improve the
desorption of the generated pyrrolidone, which was an
important parameter to affect the reaction rate.
Additionally, from the results of N2 adsorption-
desorption (Figure 2f), the prepared P-TiO2 showed a high
BET surface area (259.8 m2/g) due to its porous structure
(Figure 1a), while the BET surface area for commercial TiO2
was only 4 m2/g. Higher surface area and porous nanosheet
structure of Pt/P-TiO2 were beneficial for the exposure of
catalytic sites on the surface of Pt/P-TiO2 to interact with the
reactants, and thus improved the catalytic activity of the
synthesized Pt/P-TiO2 to some extents.
nanosheet).
Furthermore,
in
situ
acetone-DRIFT
(DRIFT=diffuse reflectance infrared Fourier transform)
spectra of P-TiO2, TiO2, Pt/P-TiO2 and Pt/TiO2 were conducted
to examine the interaction between the carbonyl groups and
the acidic sites on the catalysts (Figure 2d). The spectra in
Figure 2d showed that the C=O stretching band of the
adsorbed acetone shifted to lower wavenumber on the Pt/P-
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hot deionized water to remove KNO3 and dried at 80 °C for 12
h under vacuum.
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The preparation of Pt/P-TiO2, Pt/TiO2 and Pt/P25. In a
typical procedure for preparing Pt/P-TiO2 (Pt 2 wt%), Pt/TiO2
(Pt 2 wt%) and Pt/P25 (Pt 2 wt%), the prepared P-TiO2 (0.2 g)
or commercial TiO2 or P25 (0.2 g) was initially dispersed into
50 mL aqueous solution of H2PtCl6 (2 wt%) with stirring and
kept overnight. Then, the H2PtCl6 was reduced by NaBH4 in
ice-water bath. Subsequently, the solid was collected by
filtration and washed by water and ethanol three times.
Finally, the obtained powder was further dried at 80 °C for 12
h under vacuum. The contents of Pt in these three prepared
catalysts were determined by ICP-AES (VISTA-MPX).
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Characterization. The scanning electron microscopy (SEM)
measurements were performed on a Hitachi S-4800 scanning
electron microscope operated at 15 kV. The transmission
electron microscopy (TEM) images were obtained using a
TEM JEOL-1011 and TEM JEOL-2100F. Powder X-ray
diffraction (XRD) patterns were collected on a Rigaku D/max-
2500 X-ray diffractometer using Cu Kα radiation (λ = 0.154
nm). The N2 adsorption-desorption isotherm was determined
using the Micromeritics ASAP 2020M system. X-ray
photoelectron spectroscopy (XPS) measurements were
carried out on a ESCAL Lab 220i-XL spectrometer. The
content of Pt in the Pt-Ti was determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES
VISTA-MPX). NH3-TPD and CO-pulse titration were performed
on Micromeritics’ AutoChem 2950 HP Chemisorption
Analyzer. CO-DRIFT spectra were collected on Bruker Tensor
II spectroscope equipped with a MCT detector. The sample
was prereduced at 300 °C for 2h and then cooled to 25°C in a
He flow (10 mL min-1). Taking this as the background, the
sample was exposed to a 0.4% CO/He flow, and then purged
in a He flow (10 mL min-1) to remove physically adsorbed CO.
IR spectra of chemisorbed CO were recorded at 25 °C.
Acetone-DRIFT spectra were collected on Bruker Tensor 27
spectroscope with a MCT detector. The sample was pressed
into a self-supporting wafer and mounted into the quartz IR
cell connected to a conventional flow system. The sample was
prereduced at 300 °C for 2h and then cooled to 40°C in a He
flow (10 mL min-1). Taking this as the background, the acetone
was bubbled into the IR cell by He flow and then purged in a
He flow (10 mL min-1) to remove physically adsorbed acetone.
IR spectra of chemisorbed acetone were recorded at 40 °C.
Figure 2. The natural logarithm of TOF (based on the total Pt amount)
versus inversed temperature for apparent activation energy (a), NH3-
TPD of P-TiO2, commercial TiO2 and the commercial TiO2 mixed with
about 2 wt% KNO3 (b), XPS spectra of Ti 2p (c), in situ DRIFT spectra
of acetone absorbed on P-TiO2, TiO2, Pt/P-TiO2 and Pt/TiO2 (d), in situ
DRIFT spectra of CO absorbed on Pt/P-TiO2 and Pt/TiO2 (e), and N2
adsorption-desorption isotherm of P-TiO2 and TiO2 (f).
CONCLUSION
In conclusion, functional porous TiO2 nanosheets
supported Pt nanoparticles catalyst Pt/P-TiO2 was
successfully prepared, which showed excellent performance
for the reductive amination of LA and its esters to produce
various N-substituted pyrrolidones at ambient reaction
conditions. Meanwhile, N-alkyl-2-piperidinones and N-
arylisoindolinones could also be efficiently synthesized from
reductive amination of 4-acetylbutyric acid, 2-acetylbenzoic
acid and 2-carboxybenzaldehyde, respectively. The high
activity of Pt/P-TiO2 under ambient conditions resulted from
the high acidity of P-TiO2, the lower electron density of Pt sites
as well as the porous structure. This work opens the way for
efficient reductive amination of LA and its esters under
ambient conditions. We believe that the synthesized Pt/P-TiO2
has great potential of application for producing N-substituted
pyrrolidones sustainably from LA and its esters.
Typical procedures for the reductive amination of LA. In a
typical experiment, desired amounts of substrates, catalysts
and 2 mL methanol were charged into a glass bottle (10 ml)
equipped with a magnetic stirrer. After sealing, the H2 (in a
balloon) was charged into the reactor. The reactor was then
put into a water bath of 25 °C and the magnetic stirrer was
started. After a certain reaction time, the hydrogen balloon
was removed, and internal standard n-hexyl alcohol was
added into the reactor. The reaction mixture was then
analyzed quantitatively by GC (Agilent 6820) equipped with a
flame-ionized detector and identification of the products was
done by GC-MS (Shimadzu QP2010). To obtain the pure
products, the reaction mixture was concentrated and purified
by column chromatography (silica gel with petroleum
ether/EtOAc). Meanwhile, the pure products were
characterized by 1H NMR, 13C NMR, and HRMS, which were
provided in detail in the Supporting Information.
EXPERIMENTAL SECTION
The preparation of P-TiO2. In a typical procedure, tetrabutyl
titanate (6 mmol) was mixed with potassium nitrate (15 g)
completely by ball milling. The solid was further dried at 80 °C
for 12 h to vaporize the generated n-butyl alcohol. After that,
the obtained solid was calcined in air at 350 °C for 2 h. Finally,
the product (P-TiO2) was achieved after being washed with
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Sustainable routes for the synthesis of renewable heteroatom-
Reusability of the Pt/P-TiO2. To examine the reusability of
the Pt/P-TiO2, the catalyst was recovered by centrifugation
and washed with methanol and ethyl ether (5 × 5 mL). After
drying under vacuum at 80 °C for 12 h, the recovered catalyst
was reused for the next run.
containing chemicals. ACS Sustainable Chem. Eng. 2018, 6, 5694-5707.
(2) (a) Serrano-Ruiz, J. C.; Luque, R.; Sepúlveda-Escribano, A.
Transformations of biomass-derived platform molecules: from high
added-value chemicals to fuels via aqueous-phase processing. Chem.
Soc. Rev. 2011, 40, 5266-5281. (b) Zhang, Z.; Huber, G. W. Catalytic
oxidation of carbohydrates into organic acids and furan chemicals.
Chem. Soc. Rev. 2018, 47, 1351-1390. (c) Mika, L. T.; Cséfalvay, E.;
Németh, Á. Catalytic conversion of carbohydrates to initial platform
chemicals: Chemistry and sustainability. Chem. Rev. 2018, 118, 505-
613. (d) Luo, W.; Sankar, M.; Beale, A. M.; He, Q.; Kiely, C. J.; Bruijnincx,
P. C. A.; Weckhuysen, B. M. High performing and stable supported
nano-alloys for the catalytic hydrogenation of levulinic acid to γ-
valerolactone. Nat. Commun. 2015, 6, 6540.
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ASSOCIATED CONTENT
Supporting Information
9
The Supporting Information is available free of charge on the
ACS Publications website.
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(3) (a) Vidal, J. D.; Climent, M. J.; Concepcion, P.; Corma, A.; Iborra, S.;
Sabater, M. J. Chemicals from biomass: Chemoselective reductive
amination of ethyl levulinate with amines. ACS Catal. 2015, 5, 5812-
5821. (b) Touchy, A. S.; Siddiki, S. M. A. H.; Kon, K.; Shimizu, K. ACS
Catal. 2014, 4, 3045-3050. (c) Wu, C.; Zhang, H.; Yu, B.; Chen, Y.; Ke, Z.;
Guo, S.; Liu, Z. Lactate-based ionic liquid catalyzed reductive
amination/cyclization of keto acids under mild conditions: A metal-
free route to synthesize lactams. ACS Catal. 2017, 7, 7772-7776. (d)
Siddiki, S. M. A. H.; Touchy, A. S.; Bhosale, A.; Toyao, T.; Mahara, Y.;
Ohyama, J.; Satsuma, A.; Shimizu, K. Direct synthesis of lactams from
keto acids, nitriles, and H2 by heterogeneous Pt catalysts
ChemCatChem 2018, 10, 789-795. (e) Vidal, J. D.; Climent, M. J.; Corma,
A.; Concepcion, D. P.; Iborra, S. One-pot selective catalytic synthesis of
pyrrolidone derivatives from ethyl levulinate and nitro compounds.
ChemSusChem 2017, 10, 119-128.
(4) (a) Das, S.; Addis, D.; Knöpke, L. R.; Bentrup, U.; Junge, K.;
Brückner, A.; Beller, M. Selective catalytic monoreduction of
phthalimides and imidazolidine-2,4-diones. Angew. Chem. Int. Ed.
2011, 50, 9180-9184. (b) Du, X.-L.; He, L.; Zhao, S.; Liu, Y.-M.; Cao, Y.;
He, H.-Y.; Fan, K.-N. Hydrogen-independent reductive transformation
of carbohydrate biomass into γ-valerolactone and pyrrolidone
derivatives with supported gold catalysts. Angew. Chem. Int. Ed. 2011,
50, 7815-7819. (c) Ogiwara, Y.; Uchiyama, T.; Sakai, N. Reductive
amination/cyclization of keto acids using a hydrosilane for selective
production of lactams versus cyclic amines by switching of the indium
catalyst. Angew. Chem. Int. Ed. 2016, 55, 1864-1867.
The used materials, size distribution of Pt in the prepared
Pt/P-TiO2, CO-pulse titration results of the used Pt catalysts,
time-yield plots for the reductive amination of LA with n-
octylamine, activity of different catalysts, solvent effect,
reusability of the Pt/P-TiO2, TEM image and XPS spectra of the
recovered catalyst, GC-MS spectra of the generated
intermediate from butyl levulinate and n-octylamine, GC-MS
spectra of the intermediate generated from LA and n-
1
octylamine, H NMR and 13C NMR of the intermediate butyl 4-
(phenylamino)pentanoate, effect of different additive on the
catalytic activity of different catalysts, and the 1H NMR, 13C
NMR, and HRMS data for all synthesized products.
AUTHOR INFORMATION
Corresponding Author
*songjl@iccas.ac.cn
*hanbx@iccas.ac.cn
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
(5) (a) Wu, C.; Luo, X.; Zhang, H.; Liu, X.; Ji, G.; Liu, Z.; Liu, Z.
Reductive amination/cyclization of levulinic acid to pyrrolidones
versus pyrrolidines by switching the catalyst from AlCl3 to RuCl3
under mild conditions. Green Chem. 2017, 19, 3525-3529. (b) Wei, Y.;
Wang, C.; Jiang, X.; Xue, D.; Li, J.; Xiao, J. Highly efficient transformation
of levulinic acid into pyrrolidinones by iridium catalysed transfer
hydrogenation. Chem. Commun. 2013, 49, 5408-5410. (c) Wei, Y.;
Wang, C.; Jiang, X.; Xue, D.; Liu, Z.-T.; Xiao, J. Catalyst-free
transformation of levulinic acid into pyrrolidinones with formic acid.
Green Chem. 2014, 16, 1093-1096.
This work was supported by National Natural Science
Foundation of China (21673249, 21733011), the National Key
Research
and
Development
Program
of
China
(2017YFA0403003), Beijing Municipal Science & Technology
Commission (Z181100004218004), Key Research Program of
Frontier Sciences of CAS (QYZDY-SSW-SLH013), and Youth
Innovation Promotion Association of CAS (2017043).
(6) (a) Manzer, L. E.; Herkes, F. E. Production of 5-methyl-1-
hydrocarbyl-2-pyrrolidone by reductive amination of levulinic acid.
U.S. Patent 2004/0192933A1, 2004. (b) Manzer, L. E. Production of 5-
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