One-pot synthesis of: N -methylpyrrolidine (NMPD) using Cu- and Ni-modified ZSM-5 as an efficient catalyst

Article abstract of DOI:10.1039/c8gc02640h

In this study, a green, efficient and low-cost process for the synthesis of N-methylpyrrolidine (NMPD) from 1,4-butanediol (BDO) and methylamine (MA) via a one-pot method was developed. Under the optimized reaction conditions, more than 90% yield of NMPD was achieved over a Cu and Ni modified ZSM-5 catalyst. The catalyst could be reused for several runs retaining a satisfactory catalytic performance, and the scale-up operation showed the potential of industrial application. Characterizations of BET, XPS, XRD, TEM, SEM, H₂-TPR and NH₃-TPD were conducted for the developed composite catalyst, which suggested that Cu₂O and NiO were the main species on the support. Meanwhile, it was found that the H₂ atmosphere, the high dispersion of metal oxides and the synergic effect between Cu and Ni species on ZSM-5 contributed to the excellent catalytic performance. Furthermore, a possible mechanism based on a borrowing-hydrogen process was also proposed.

Full text of DOI:10.1039/c8gc02640h

Green Chemistry
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PAPER
One-pot synthesis of N-methylpyrrolidine (NMPD)
using Cu- and Ni-modified ZSM-5 as an efficient
catalyst†
Cite this: Green Chem., 2019, 21, 141
a,b
a
a,b
a,b
a
Yan Long, Peixue Wang,
Yuqing Fei, Dawei Zhou, Shimin Liu* and
a
Youquan Deng
*
In this study, a green, efficient and low-cost process for the synthesis of N-methylpyrrolidine (NMPD)
from 1,4-butanediol (BDO) and methylamine (MA) via a one-pot method was developed. Under the opti-
mized reaction conditions, more than 90% yield of NMPD was achieved over a Cu and Ni modified ZSM-5
catalyst. The catalyst could be reused for several runs retaining a satisfactory catalytic performance, and
the scale-up operation showed the potential of industrial application. Characterizations of BET, XPS, XRD,
2 3
TEM, SEM, H -TPR and NH -TPD were conducted for the developed composite catalyst, which suggested
Received 20th August 2018,
Accepted 17th November 2018
that Cu O and NiO were the main species on the support. Meanwhile, it was found that the H2 atmo-
2
sphere, the high dispersion of metal oxides and the synergic effect between Cu and Ni species on ZSM-5
DOI: 10.1039/c8gc02640h
contributed to the excellent catalytic performance. Furthermore, a possible mechanism based on a bor-
rowing-hydrogen process was also proposed.
rsc.li/greenchem
hazardous threat to the environment and human health;
besides, such a method needs a large amount of NaOH to
1
. Introduction
N-Methylpyrrolidine (NMPD), as an important intermediate neutralize inorganic acid catalysts, which generates a great deal
compound, can be widely used in the syntheses of pharma- of waste salts. For the second and third methods, both THP and
ceutical products and agricultural chemicals due to its high NMP are relatively expensive raw materials, and sometimes
biological activities, especially for cefepime, which determines noble metal catalysts are involved in those synthetic processes.
pharmacokinetic parameters and the exchange of drugs in the
human body. Meanwhile, NMPD is also a key raw material (THF) with methylamine (MA) as the starting materials can be
for pyrrolidinium based ionic liquids. For example, 1-butyl-3- considered a green and economic route to synthesize NMPD
Fortunately, using 1,4-butanediol (BDO) or tetrahydrofuran
1
,2
methylpyrrolidinium
bis(trifluoromethyl
sulfonyl)imide (Scheme 1), because the employed raw materials are readily
(
[P14][NTf ]) has attracted a lot of attention as an electrolyte for available, cheap, and easy to handle. At the same time, only
2
electrochemical devices due to its nonvolatility, high conduc- water is the theoretical by-product in this process. However, as
3
tivity and low viscosity. At present, there are several methods far as we know, very few reports have concerned the synthesis
for preparing NMPD, mainly including (1) reductive methyl- of NMPD from BDO (or THF) and MA up to now, since it is
4
–6
ation of tetrahydropyrrole (THP) with formaldehyde;
(2) still limited by the poor performance of the catalytic system.
7
–9
23
catalytic methylation of THP using methanol
or carbon For example, DC Hargis reported cyclodialkylation between
and (3) hydrogenation of N-methylpyrrolidone THF and MA for NMPD using titania as the catalyst, but only
However, the above-mentioned processes often 63% THF conversion with 91% NMPD selectivity was obtained.
1
0–15
dioxide;
1
6–22
(NMP).
2
4
suffer from the high cost of raw materials or they involve Subba Rao employed a CrZSM-5 catalyst for the reaction
environmental pollution. For example, using toxic formal- between BDO and MA, but the yield of NMPD was also less
dehyde as the methylation reagent in the first process is a than 65%. Therefore, the development of a catalyst with out-
standing performance is desirable for such a green process.
a
Centre for Green Chemistry and Catalysis, State Key Laboratory for Oxo Synthesis
and Selective Oxidation, State Key Laboratory of Solid Lubrication,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou,
7
30000, China. E-mail: liushm@licp.cas.cn, ydeng@licp.cas.cn
b
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc02640h
Scheme 1 Synthesis of NMPD from BDO and MA.
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2
In recent years, heterogeneous catalysts consisting of detector. TPR analysis was conducted with 10% H /Ar (30
copper and nickel oxides have been reported for the ml min− ). In a typical experiment the solid sample (100 mg
1
N-alkylation of amines with alcohols by virtue of the borrow- with particle size 160–200 μm) was pretreated at 500 °C for 1 h
−
1
ing-hydrogen method, which displayed excellent catalytic per- under air flow (30 ml min ). The profile was recorded at a
2
5,26
−1
formance.
Inspired by these results, we realized the syn- heating rate of 10 °C min from room temperature to 800 °C
thesis of NMPD from BDO and MA via borrowing-hydrogen and maintained at this temperature until the TCD signal of H
2
processes twice over Cu and Ni modified ZSM-5 catalysts in returned to the baseline.
this work. Besides the influence of the components of the cata-
The surface acid properties of the catalysts were measured
3
lyst, reaction atmosphere, temperature, time, etc., were investi- by temperature-programmed desorption (TPD) of NH and
gated in detail. It was found that a 3%Cu–3%Ni/ZSM-5 catalyst carried out on a TPD flow system equipped with a TCD detec-
displayed outstanding catalytic performance, and nearly 100% tor. In a typical experiment, the solid sample (100 mg with par-
conversion of BDO and >90% selectivity for NMPD could be ticle size 160–200 μm) was pretreated at 500 °C for 1 h under
achieved under optimum conditions. The yield of NMPD was argon gas flow (30 ml min− ) and then cooled to room tem-
1
much higher than that in previously reported results. At the perature. The sample was subsequently exposed to an NH
3
-TPR studies were stream (30 ml min− ) at room temperature for 1 h and flushed
1
same time, BET, XRD, XPS, SEM, TEM, H
2
conducted to explore the relationship between structure and again with argon gas for 1 h to remove any physico-adsorbed
performance for the developed catalyst. Moreover, a possible NH . The desorption profile was recorded at a heating rate of
3
1
mechanism was also proposed in such a catalytic system.
10 °C min− from room temperature to 800 °C and maintained
until the TCD signal of NH
3
returned to the baseline.
The BET surface areas, pore volumes and average pore radii
of the catalysts were obtained with physisorption of N
Micromeritics ASAP 2010 instrument.
2
using a
2
. Experimental
2
.1. Catalyst preparation and characterization
The Cu and Ni modified ZSM-5 catalysts were prepared by
the incipient wetness method using Cu(NO ·3H O and
Ni(NO ) ·6H O (Sinopharm Chemical Reagent Co., Ltd, China)
2.2. Synthesis of NMPD
3
)
2
2
Catalytic reactions were carried out in a 100 ml stainless steel
autoclave with a magnetic stirrer. In a typical process, BDO
3
2
2
as the precursors. The H-ZSM-5 support (Si/Al ratio of 50, 80,
00, supplied by the catalyst plant of NanKai University,
China) was calcined at 500 °C for 3 h prior to use. After
impregnation of the aqueous solutions of the precursors with
a certain concentration, the sample was dried at 110 °C for
(
0.05 mol), MA (0.1 mol, 40 wt% aqueous solution) and cata-
lyst (0.45 g) were charged in the autoclave, which was then
filled with H at 1 MPa pressure. The reactor was heated to
00 °C and magnetically stirred constantly during the reaction.
3
2
3
After reaction, the qualitative and quantitative analyses of the
resulting liquid mixture were conducted with GC-MS (Agilent
1
2 h, then calcined at 500 °C for 3 h. The catalysts with
different weight loadings of Cu and Ni were denoted x%Cu–y
6890/5973) and GC (Agilent 7890) equipped with a SE-54 capil-
%Ni/ZSM-5.
lary column and a FID detector.
The metal contents were analyzed by using graphite
furnace atomic absorption spectroscopy (GF-AAS, contrAA700,
Germany). X-ray diffraction (XRD) was measured on a Siemens
D/max-RB powder X-ray diffractometer. Diffraction patterns
were recorded with Cu Kα radiation (40 mA, 40 kV) over a 2θ
range of 5° to 90° and a position-sensitive detector using a
step size of 0.01° and a step time of 0.15 s.
3
. Results and discussion
3.1. Synthesis of NMPD from 1,4-buranediol and MA under
various conditions
Firstly, a series of catalysts were screened for the synthesis of
Surface analysis of the catalysts was performed by X-ray NMPD from BDO and MA, and the obtained results are listed
photoelectron spectroscopy (XPS) on a VG ESCALAB210 in Table 1. It was found that only 8% BDO conversion with
spectrometer using Mg Kα radiation at a pass energy of 20 eV. 89% NMPD selectivity was obtained in the absence of a catalyst
The energy scale was calibrated and corrected for charging (entry 1, Table 1), and the catalytic performance of the support
using the C 1s (285.0 eV) line as the binding energy (BE) in the absence of Cu or Ni compounds was tested (entry 2,
reference.
Table 1), and the conversion of BDO was merely 46% with 66%
Scanning electron microscopy (SEM) analysis was per- selectivity for NMPD. For 3%Cu/ZSM-5 (entry 3, Table 1), the
formed with a scanning electron microscope (JEOL-6701F) at conversion of BDO was 75%, and the selectivities for NMPD
an acceleration voltage of 5 kV. Transmission electron micro- and THF were 21% and 68%, respectively; as for 3%Ni/ZSM-5,
scope (TEM) analysis was carried out using a TF20 field emis- the conversion of BDO was 55%, and the selectivities for
sion transmission electron microscope operating at 300 kV. NMPD and THF were 34% and 47%, respectively (entry 4,
Single-particle EDX mapping analysis was performed using a Table 1). The selectivity for THF was higher than that for
TF20 field emission TEM in the STEM mode.
NMPD for all the single Cu or Ni on ZMS-5 catalysts.
Temperature-programmed reduction (TPR) of
H
2
was Meanwhile, it was found that with an increase in the loading
carried out on a TPR/TPD flow system equipped with a TCD of Cu or Ni, the conversion of BDO and the selectivity of THF
142 | Green Chem., 2019, 21, 141–148
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a
Table 1 Catalytic performance of different catalysts
which was much higher than the results of previous literature
2
4
over CrZSM-5 (yield < 65%). The obtained results implied
that the synergistic effect between Cu and Ni species on
ZSM-5 might play a key role in the synthesis of NMPD. In
addition, the impact of total loading amount of metal oxide on
the performance (holding the ratio of Cu to Ni at 1 : 1) was also
studied. As the loadings of Cu and Ni decreased to 1.5%, the
BDO conversion and NMPD selectivity were only 73% and
Sel. (%)
Con.
b
Entry Catalyst
(%)
NMPD THF NMP Others
1
2
3
4
5
6
7
8
9
1
1
1
1
None
HZSM-5
3%Cu/ZSM-5
3%Ni/ZSM-5
6%Cu/ZSM-5
6%Ni/ZSM-5
1%Cu–5%Ni/ZSM-5
2%Cu–4%Ni/ZSM-5
3%Cu–3%Ni/ZSM-5
1.5%Cu–1.5%Ni/ZSM-5
6%Cu–6%Ni/ZSM-5c
3%Cu–3%Ni/ZSM-5
8
46
75
55
100
97
89
66
21
34
5
1
32
68
47
94
92
68
22
2
4
6
2
6
15
1
4
3
10
5
12
15
11
8
5
4
56%, respectively (entry 10, Table 1). If both the Cu and Ni
4
70
82
100
73
100
91
29
63
92
56
82
86
89
loadings were increased to 6%, the conversion of BDO could
be maintained at 100%, but the product selectivity reduced to
5
1
5
1
1
2
82% (entry 11, Table 1), which might be due to the agglomera-
0
1
2
3
27
2
2
tion of active sites on the surface of the ZSM-5 support.
Meanwhile, ZSM-5 supports with different ratios of Si/Al
were also tested (entries 12 and 13, Table 1). When the ratio of
d
3%Cu–3%Ni/ZSM-5
97
1
a
Reaction conditions: 0.05 mol BDO, 0.1 mol MA, 0.45 g catalyst, Si/Al was 50, the conversion of BDO was 91% with 86% selecti-
b
3
00 °C, 6 h, 1 MPa H . Other by-products include traces of THP and
2
vity for NMPD, and when the ratio of Si/Al was 300, the conver-
sion of BDO was 97% with 87% selectivity for NMPD. The
obtained yields of NMPD were lower than that of a catalyst
with an Si/Al ratio of 80. As the ZSM-5 supports with different
c
d
N-butylpyrrolidine. A ratio Si/Al of 50. A ratio Si/Al of 300.
increased, while the selectivity of NMPD declined (entries 5 Si/Al ratios possessed similar pore sizes (Table S2†), the reason
2
7
and 6, Table 1). Based on the previous literature, the de- may be that the catalyst with an 80 Si/Al ratio had suitable
hydration of BDO to THF only needs suitable acid sites. With acidity to form NMPD. According to the obtained results, it
an increase in the loading of metal species, the change in acid could be seen that adjusting the loading content of Cu and Ni
strength and amount of strong acid may be the reason for the compounds properly could allow the yield of NMPD to reach
high selectivity for THF.
an excellent level (92%).
Furthermore, it could be seen that the ratio of Cu to Ni had
In order to optimize the reaction conditions, experiments
an evident effect on the catalytic performance. As the ratio of focusing on the effects of reaction atmosphere, reaction temp-
Cu to Ni was increased, keeping the total loading amount con- erature, time, and molar ratio of substrates (MA : BDO) were
stant at 6% (entries 7–9, Table 1), both the conversion of BDO investigated using 3%Cu–3%Ni/ZSM-5 as the catalyst, and the
and selectivity for NMPD were enhanced gradually, and the by- corresponding results are summarized in Table 2. Firstly, we
product THF declined sharply. Obviously, the 3%Cu–3%Ni/ attempted to explore the influence of reaction atmosphere on
ZSM-5 catalyst expressed the highest catalytic activity for the performance. Using air as the reaction atmosphere, only 53%
synthesis of NMPD, and complete conversion of BDO with BDO conversion with 48% NMPD selectivity were obtained,
9
2% selectivity for NMPD was achieved (entry 9, Table 1), and the main by-products were THF and NMP (entry 1,
a
Table 2 Optimization of the reaction conditions for NMPD synthesis from BDO and MA
Sel. (%)
NMPD
Entry
M
MA : MBDO
Atmosphere
Air
Temperature (°C)
Time (h)
Con. (%)
THF
NMP
13
Others
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
1 : 1
3 : 1
2 : 1
2 : 1
2 : 1
300
300
300
300
300
300
230
250
270
330
300
300
300
300
300
4
4
4
2
6
8
6
6
6
6
6
6
6
6
6
53
60
81
48
64
82
83
92
91
98
97
98
35
65
86
88
89
88
37
30
3
2
2
2
6
10
8
5
6
2
3
2
2
3
10
5
8
10
1 Mpa N
2
1 Mpa H
1 Mpa H
1 Mpa H
1 Mpa H
1 Mpa H
1 Mpa H
1 Mpa H
1 Mpa H
1 Mpa H
1 Mpa H
2
2
2
2
2
2
2
2
2
2
5
7
1
1
61
100
100
21
35
88
100
87
100
89
100
100
2
0
1
2
3
4
5
63
30
4
2
3
2
5
0.5 Mpa H
2 Mpa H
2.5 Mpa H
2
2
2
b
2
a
b
Reaction conditions: 0.05 mol BDO, 0.45 3%Cu–3%Ni/ZSM-5 catalyst. The amount of substrates was magnified to 50 times, and the reaction
was carried out in a 4L reactor. The isolated yield of NMPD was 83%.
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Table 2). While, employing N
2
as the reaction atmosphere, the (92% yield) from MA and BDO using the composite catalyst
conversion of BDO reached 60% and the selectivity of NMPD based on Cu and Ni modified ZSM-5 at 300 °C for 6 h under a
could increase to 64% (entry 2, Table 2). To our delight, as the 1 MPa pressure of H . In this work, a green, efficient and in-
reaction atmosphere was changed to H , the BDO conversion expensive process for NMPD was thus developed.
could be further improved (81%), and the NMPD selectivity Moreover, a scale-up experiment (50 times) was carried out
was also raised to 70% (entry 3, Table 2). It was revealed that in a 4L reactor (entry 15, Table 2) under the optimum con-
the participation of H might reduce the metal oxides in situ to ditions. The results showed that 100% conversion of BDO with
2
2
2
form active species, which could make the borrowing-hydrogen 88% selectivity for NMPD was achieved. Then, the resulting
processes smoother, resulting in an improvement in the cata- solution was separated via rectification, and the isolated yield
lytic performance of the developed Cu and Ni modified ZSM-5 of NMPD (99.5% purity) was 83%, which was close to the chro-
catalyst. On the other hand, the by-produced NMP could also matographic yield. Therefore, the developed process for NMPD
be reduced to NMPD under H
2
atmosphere. Therefore, the H
2
exhibited good prospects for industrial application.
atmosphere could enhance both BDO conversion and NMPD
selectivity.
In addition, the reusability of 3%Cu–3%Ni/ZSM-5 was
studied (Fig. 1). If the catalyst was used without any treatment
Subsequently, the effects of reaction time and temperature for reusability, it showed relatively poor catalytic performance.
on the catalytic synthesis of NMPD were examined. It was For example, 87% BDO conversion with 86% NMPD selectivity
nd
found that both the conversion of BDO and the selectivity for was obtained for the 2 run, and only 42% BDO conversion
th
NMPD gradually increased with an increase in reaction time with 52% NMPD selectivity was obtained for the 5 run. The
(entries 3–6, Table 2), and full conversion of BDO with 92% reason may be the adsorption of a small amount of heavy com-
selectivity for NMPD could be achieved at 6 h (entry 5, ponents and carbon deposition on the surface of catalyst. To
Table 2). Whereas, with a further prolongation of reaction time improve the reusability, the catalyst was calcinated for 3 h at
to 8 h, no obvious improvement in the selectivity for NMPD 500 °C under an ambient atmosphere after each test, and the
was detected (entry 6, Table 2). Thus, the most suitable reac- performance could be obviously improved after the calcination
tion time was 6 h. The reaction temperature was also a very treatment. The previous results showed that the BDO could be
important parameter for this process. It could be seen that the converted completely with more than 90% NMPD selectivity
conversion of BDO could be elevated rapidly from 21% to for the first run. In the subsequent cycles, both the conversion
th
1
00% with >90% NMPD selectivity as the temperature was of BDO and NMPD selectivity declined gradually. For the 5
raised from 230 to 300 °C (entries 5, 7–10, Table 2). However, run, the conversion of BDO was 90% with 78% selectivity for
the NMPD selectivity sharply declined to 35%, as the reaction NMPD, but the yield was still higher than that in previous
temperature went up to 330 °C (entry 10, Table 2), and the reports. A possible reason for the deactivation of 3%Cu–3%Ni/
main by-product was THF, indicating that much higher temp- ZSM-5 was the loss of metal species during the reactions.
eratures promoted competitive dehydration and had an
adverse effect on the formation of NMPD. Hence, to obtain 3.2. Results of the catalyst characterization
more target product, the optimum reaction temperature was
The contents of Cu and Ni species in the catalysts were deter-
3
00 °C.
mined by atomic absorption spectroscopy (AAS, Table S1†). It
could be seen that the contents of Cu and Ni in all samples
were very close to their theoretical values. For example,
In addition, the impact of amount of MA with the molar
ratio (MA : BDO) ranging from 1 : 1 to 3 : 1 was also studied
under the same condition (entries 5, 11 and 12, Table 2).
Based on the obtained results, it was necessary to have an
excess of MA for this process to achieve a high yield of NMPD.
But, when the molar ratio of MA : BDO increased to 3 : 1, the
yield of NMPD was slightly lower than that for a molar ratio of
2
.92 wt% Cu with 2.89 wt% Ni were detected for the 3%Cu–3%
Ni/ZSM-5 catalyst. In addition, both the Cu and Ni contents in
%Cu–3%Ni/ZSM-5 decreased obviously after 5 cycles, which
was a possible reason for the decline in catalytic activity. The
3
2
: 1 (entry 5, Table 2). The reason for this phenomenon might
be that a large excess of basic MA destroyed the skeleton struc-
ture of the support, leading to a decrease in catalytic
performance.
The pressure of H was also optimized in the synthesis of
2
NMPD over the 3%Cu–3%Ni/ZSM-5 catalyst. When the
pressure of H
2
was adjusted from 0.5 MPa to 1 MPa (entry 5,
3 and 14, Table 2), the conversion of BDO was promoted from
9% to 100%, and the selectivity for NMPD was also improved.
went up to 2Mpa, although
1
8
Nonetheless, as the pressure of H
2
the conversion of BDO was still maintained at 100%, the
selectivity for NMPD decreased to 89%, which might be owing
to excessive reduction of the metal oxides by the superfluous
H . Therefore, NMPD could be produced with high efficiency Fig. 1 The reusability of 3% Cu–3%Ni/ZSM-5.
2
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loss of Cu was more than that of Ni, which might be due to
the coordination between MA and Cu.
The physical properties of HZSM-5, 3%Cu–3%Ni/ZSM-5 and
6
%Cu–6%Ni/ZSM-5 samples were analysed by the BET tech-
nique, including surface area (SA), pore volume, and pore size,
and the data are filed in Table S2.† It can be seen that the SA,
pore volume and pore radius of the HZSM-5 support were
2
−1
3
−1
3
88 m
g
, 0.355 cm
g
and 2.549 nm, respectively. As
expected, the BET surface area, pore volume and pore size of
the loaded catalysts decreased with an increase in total load-
ings of Cu and Ni compounds. For example, 3%Cu–3%Ni/
2
−1
ZSM-5 displayed 285 m g SA with a 2.348 nm radius. As the
loadings of both Cu and Ni were further raised to 6%, the Fig. 3 XRD patterns of different catalysts.
2
−1
decline in SA to 260 m g might be due to the agglomeration
phenomenon of the oxides on the surface of the ZSM-5
support, resulting in a decline in its catalytic performance.
metal oxides could also be detected at 37° and 43°, which cor-
The chemical states of the metal oxides for 3%Cu–3%Ni/ responded to the (111) facets of Cu O and the (200) facets of
2
ZSM-5 were examined by XPS analyses, which revealed that the NiO (JCPDS: 34-1354 and JCPDS: 47-1049), respectively. The
binding energies of Cu 2p3/2 and Ni 2p3/2 were 932.9 and 855.8 XRD results were accordance with the XPS analyses. It was
eV, respectively (Fig. 2), suggesting that Cu O and NiO were found that the peak intensity became stronger with an
2
2
5
the main species on the catalyst surface. In addition, the XPS increase in loading content. Furthermore, the relatively wide
results showed that the atomic ratio of Cu to Ni was 1.13 peaks of the metal oxides indicated that the Cu O and NiO
Table S3†), which was slightly higher than the theoretical were poorly crystallized or consisted of smaller nanoparticles.
2
(
value of 1.08, indicating that Cu and Ni particles were relatively
evenly distributed on the support surface.
The SEM images of HZSM-5 and 3%Cu–3%Ni/ZSM-5 are
shown in Fig. S1,† and it can be seen that the morphological
The XRD patterns for the prepared catalysts with different features of HZSM-5 and 3%Cu–3%Ni/ZSM-5 were similar,
loadings are described in Fig. 3. As shown in Fig. 3, the corres- which means that the introduction of Cu and Ni species did
ponding peaks of ZSM-5 (appearing at 8, 8.9, 14.8 and 23°, not destroy the structure of the support. Furthermore, the
JCPDS: 44-0002) could be observed in all samples. Except for high-resolution morphology of 3%Cu–3%Ni/ZSM-5 was carried
the diffraction peaks of the support, the characteristic peaks of out by TEM. Fig. 4a shows that no coagulation could be
observed on the surface of the catalyst, meaning that both
Cu O and NiO particles were mostly well dispersed on the
2
ZSM-5 support. Moreover, from the high resolution TEM
image (HRTEM) in Fig. 4b, lattice fringes can be clearly dis-
cerned with d-spacings of 0.24 and 0.21 nm. Since the diffrac-
2
tion peaks of the Cu O (111) and NiO (200) facets could be
observed in the XRD patterns, and the values of the d-spacing
from the TEM results were also close to the values of the PDF
card (JCPDS: 34-1354, JCPDS: 47-1049), so the lattice fringes
2
could be assigned to the Cu O (111) and NiO (200) facets. And
such speculation was also obtained from previously reported
2
8,29
literature.
EDX mapping was also conducted in order to
identify the distribution of the elements (Fig. 4c). The results
revealed that the Cu and Ni species were uniformly distributed
in the catalyst. Additionally, the EDX pattern (Fig. S2†) of the
catalyst also demonstrated the existence of Cu and Ni oxides.
So, the obtained HR-TEM results agreed well with the obser-
vations from the XRD characterizations.
In order to explore the interaction between Cu and Ni
species, H -TPR examinations were conducted for 3%Cu–3%
2
Ni/ZSM-5, and compared with individual loadings of 3% Cu
and Ni on ZSM-5. As shown in Fig. 5, for the sample of 3%Cu/
ZSM-5, the reduction temperature occurred at 323 °C; for 3%
Ni/ZSM-5, there are three reduction peaks at 313, 442, 578 °C,
respectively, which was in accordance with the previous
3
0
Fig. 2 XPS spectra of (a) Cu 2p3/2, (b) Ni 2p3/2 for 3%Cu–3%Ni/ZSM-5.
report. The two former peaks could be assigned to the
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Fig. 5
H
2
-TPR curves of 3%Cu/ZSM-5, 3%Ni/ZSM-5 and 3%Cu–3%Ni/
ZSM-5.
addition, the peak at 348 °C was possibly contributed by some
Cu O particles having a relatively weaker interaction with NiO.
2
Therefore, the presence of the synergistic effect between metal
species might be one of the reasons for the high catalytic per-
formance of 3%Cu–3%Ni/ZSM-5 in the synthesis of NMPD
from MA and BDO.
3
NH -TPD was used to characterize the strength of accessible
acid sites in the catalysts, and the results are shown in
Fig. S3.† It can be seen that two characteristic peaks could be
observed for HZSM-5, and the peak at around 100 °C indicated
the weak acid sites, and the ones locating at 500 °C meant the
strong acid sites. After the loading of metal, the peak at high
temperature changed obviously. For 6%Cu/ZSM-5, although
the peak for strong acid sites shifted to a lower temperature,
the amount of strong acidity increased; while the peak of 6%
Ni/ZSM-5 for the strong acid sites shifted mainly to higher
31
temperature with a small portion to lower temperature. The
increase in acid strength or the amount of strong acid sites
may be the reason for the high selectivity for THF. As for 3%
Cu–3%Ni/ZSM-5, both the amount of strong acid and the
strength decreased compared with HZSM-5. Hence, the suit-
able acidity for the catalysts may also be an important reason
for the formation of NMPD.
Fig. 4 TEM images of 3%Cu–3%Ni/ZSM-5. Scale bars: (a) 50 nm, (b)
2
nm. (c) EDX mapping of 3%Cu–3%Ni/ZSM-5 for O, Cu and Ni
elements.
reduction of NiO of different crystal sizes located on the outer
zeolite surface. For 3%Cu–3%Ni/ZSM-5, there were three peaks
3.3. Possible reaction mechanism
at 228, 348 and 508 °C, respectively. It can be seen that the A possible reaction pathway through the borrowing-hydrogen
reduction peak of Cu O shifted from 323 °C to 228 °C, and the process twice for the synthesis of NMPD was proposed in this
main reduction peak of NiO also shifted from 578 °C to study (Scheme 2). Firstly, with the help of Cu species being
08 °C. So, the corresponding reduction peaks for both Cu
reduced in situ under the H atmosphere, the dehydrogenation
2
5
2
O
2
and NiO declined dramatically due to the interaction between of BDO formed the intermediate carbonyl compound (I),
Cu and Ni. Moreover, compared to 3%Ni/ZSM-5, more NiO in which underwent addition–elimination with MA to generate
3
%Cu–3%Ni/ZSM-5 was reduced in the presence of Cu species the imine (II). Then, the generated molecular hydrogen in the
under the same conditions, which might also reflect the syner- dehydrogenation step overflowed on the Ni species and
3
2
gistic effect between Cu and Ni species. It should be noted reduced imine to amino alcohol (III). Next, the amino
that the Cu O on the composite catalyst could be partially alcohol transformation could advance the ‘hydrogen shuttling’
2
reduced to Cu (0) at the reaction temperature (300 °C), which process, in which the hydroxyl group dehydrogenated to the
was favourable for the borrowing-hydrogen process. In carbonyl compound (IV) followed by condensation and cycliza-
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4. Conclusions
A one-pot method to synthesise NMPD from BDO and MA over
Cu-Ni/ZSM-5 was realized by virtue of the borrowing-hydrogen
methodology. NMPD could be obtained with high efficiency
(
92% yield) over a 3%Cu–3%Ni/ZSM-5 catalyst at 300 °C for 6 h
under 1 MPa pressure of H . Meanwhile, the AAS BET, XPS,
XRD, SEM and TEM characterization results indicated that the
main species on the catalyst surface were Cu O and NiO,
which exhibited a high level of dispersion. H -TPR analyses
2
2
2
revealed that the reductive atmosphere and the synergic effect
of Cu and Ni on the ZSM-5 support enhanced the catalytic
activity. Moreover, a possible mechanism was also proposed,
and the developed Cu–Ni/ZSM-5 could act as an excellent cata-
lyst for the ‘borrowing-hydrogen’ reaction. This process offers
a green and economic route for the synthesis of NMPD, which
also has a promising prospect for industrial application.
Conflicts of interest
Scheme 2 Possible reaction mechanism for NMPD and THF.
There are no conflicts to declare.
Acknowledgements
tion to form an imine (V). Finally, the imine (V) converted to
NMPD via a hydrogenation reaction. For the intermediary com-
pounds in the proposed possible mechanism, we did not find
them in the present stage, and more powerful evidence
requires our continued efforts. The intermediates were specu-
lated on via the mechanism of the borrowing-hydrogen
process according to the previous literature.
Meanwhile, a spot of NMP was detected during the syn-
2
thesis of NMPD in the absence of H , meaning that there was
This work was financially supported by National Key Research
and Development Program of China (No. 2017YFA0403101)
and the National Natural Science Foundation of China
(
No. 21761132014).
2
5,33
Notes and references
another route to produce NMPD. Some of the BDO could
readily generate γ-butyrolactone (VI) under test conditions.
The formed γ-butyrolactone easily converted to NMP (VII) in
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through hydrogenation over the Ni species. So, the hydrogen
atmosphere could not only promote the borrowing-hydrogen
progress, but also facilitate the by-product NMP turning into
the target product. Consequently, the route for NMPD from
BDO and MA was likely to undergo primary Route A with a sec-
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