Reductive amination of bio-based 2-hydroxytetrahydropyran to 5-Amino-1-pentanol over nano-Ni-Al2O3catalysts

Article abstract of DOI:10.1039/d0nj04962j

The synthesis of useful amines from bio-based carbonyl compounds is highly desired owing to their mild reaction conditions and green sustainability. The reductive amination of bio-furfural-derived 2-hydroxytetrahydropyran (2-HTHP) to high-value-added 5-Amino-1-pentanol (5-AP) was carried out over efficient Ni-Al₂O₃catalysts prepared by a co-precipitation method. Among the Ni-Al₂O₃catalysts with different Ni loadings (0-100 wt%) tested, the 50Ni-Al₂O₃catalyst exhibited the highest5-APyield of 91.3% under mild conditions of 60 °C and 2 MPa H₂. This catalyst also presented good stability during a 150 h time-on-stream without appreciable deactivation. Characterization results showed that the 50Ni-Al₂O₃catalyst exhibited small Ni⁰nanoparticles (5.5 nm), a high reduction degree (up to 95%), and a large amount of strong Lewis acid sites. The cooperative catalysis of the strong Lewis acid sites and highly dispersed metallic Ni sites is suggested to play an important role in achieving high efficiency in2-HTHPreductive amination.

Full text of DOI:10.1039/d0nj04962j

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Reductive amination of bio-based
2-hydroxytetrahydropyran to 5-Amino-
1-pentanol over nano-Ni–Al2O3 catalysts†
Cite this: New J. Chem., 2021,
45, 4236
ab
Jia Zhang,
Xun Hu,
Jian Yang,ab Junying Tian,a Hailong Liu,a Xuemei Li,a Weiguo Fang,a
Chungu Xia,a Jing Chena and Zhiwei Huang
*
a
a
The synthesis of useful amines from bio-based carbonyl compounds is highly desired owing to their
mild reaction conditions and green sustainability. The reductive amination of bio-furfural-derived
2-hydroxytetrahydropyran (2-HTHP) to high-value-added 5-Amino-1-pentanol (5-AP) was carried out
over efficient Ni–Al2O3 catalysts prepared by a co-precipitation method. Among the Ni–Al2O3 catalysts
with different Ni loadings (0–100 wt%) tested, the 50Ni–Al2O3 catalyst exhibited the highest 5-AP yield
of 91.3% under mild conditions of 60 1C and 2 MPa H2. This catalyst also presented good stability during
a 150 h time-on-stream without appreciable deactivation. Characterization results showed that the
50Ni–Al2O3 catalyst exhibited small Ni0 nanoparticles (5.5 nm), a high reduction degree (up to 95%), and
a large amount of strong Lewis acid sites. The cooperative catalysis of the strong Lewis acid sites and
highly dispersed metallic Ni sites is suggested to play an important role in achieving high efficiency in
2-HTHP reductive amination.
Received 9th October 2020,
Accepted 27th January 2021
DOI: 10.1039/d0nj04962j
rsc.li/njc
The major challenge in the reductive amination of carbonyl
compounds to produce amines, particularly primary amines, is
Introduction
Amine compounds are important chemical raw materials and the control of selectivity because it usually suffers from compe-
synthetic intermediates, widely used in the synthesis of polymers, titive side reactions such as over-alkylation, carbonyl hydro-
pharmaceuticals, pesticides, food-additives, surfactants, etc.1–3 genation and cyclization.4,12,13 In view of these side reactions,
Currently, most of the industrially related aliphatic and aromatic selecting a suitable catalytic system is therefore key to achieve
amines as well as amino alcohols were produced from traditional high selectivity for the desired amines. Up to now, there are just
fossil resources.2,4 From a green and sustainable chemical per- a few reports on the synthesis of primary amines from bio-
spective, the production of bio-amines from renewable biomass is based carbonyl compounds, and these studies mainly focused
considered as an appealing approach to not only reduce the on the application of noble metal heterogeneous catalysts
reliance on increasingly scarce fossil resources, but also make thanks to their high catalytic activity and selectivity as well as
good use of the oxygen-containing functional groups in easy separation and recovery.4,6,9,14,15 For example, Chatterjee
biomass.2,5,6 To date, amines are mainly produced through the et al.16 used Rh, Ru, Pt and Pd-based catalysts for the reductive
reduction and ammonolysis processes,7 and the reductive amina- amination of furfural to furfurylamine and a high selectivity of
tion of carbonyl compounds is one of the most attractive synthesis 92% to the target furfurylamine was obtained over the Rh–Al2O3
routes.8–11 The synthesis of amines via the reductive amination of catalyst within 2 h at 80 1C. Taken the extremely high price of
carbonyl compounds only needs to selectively convert the oxygen- noble metals into consideration, it is more promising to use
containing groups into nitrogen-containing groups, and water is supported base metals, such as nickel11,12,17–19 and cobalt3,20
the main by-product. Therefore, there is a strong incentive to for the reductive amination of bio-based carbonyl compounds.
develop sustainable amines from bio-based carbonyl compounds.
5-Amino-1-pentanol (5-AP) is an important pharmaceutical
intermediate that can be used in the synthesis of anti-inflammatory
and anti-cancer drugs, especially as a starting material for the
synthesis of alkaloid manzamine. With an increasing demand
for manzamine and only very expensive test doses currently
available on the market, it is of great significance to find a cheap
and large-scale preparation method for 5-AP synthesis and
promote its application.21 Recently, our group reported an
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research
Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy
of Sciences, Lanzhou 730000, China. E-mail: zwhuang@licp.cas.cn
b University of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj04962j
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Scheme 1 The synthesis of 5-AP through different routes.
efficient and clean method for the synthesis of useful 5-AP through
reductive amination of bio-furfural-derived 2-hydroxytetrahydropyran
(2-HTHP) over supported Ni catalysts (Route B, Scheme 1)22,23 in
contrast to the chlorination and ammoniation of 1,5-PD (Route A,
Scheme 1).21 The Ni–Mg3AlOx catalysts derived from hydrotalcite
precursors by co-precipitation with proper Ni loadings were found
to be more active and stable than the supported Ni–ZrO2 catalyst
prepared by impregnation.22,23 Nonetheless, obvious deactivation
of the Ni–Mg3AlOx catalyst was seen after 90 h of time-on-stream.
Nano-Ni–Al2O3 based catalysts have been reported to present high
activity and good stability in not only reductive amination of
aldehydes and ketones,11,18 but also direct amination of
alcohols.24–26 In this work, a series of nano-Ni–Al2O3 catalysts with
Ni loadings from 0 to 100 wt% were synthesized by co-precipita-
tion and were investigated in the reductive amination of 2-HTHP
to 5-AP under mild conditions with aqueous NH3. The 50Ni–Al2O3
catalyst with B50 wt% Ni was found to present the highest 5-AP
yield (91.3%) and remarkable stability during a 150 h time-on-
stream reaction. The good performance of such a catalyst was
discussed based on comparative characterization of the Ni–Al2O3
catalysts with different Ni loadings.
Fig. 1 XRD patterns of (A) calcined NiO–Al2O3 samples and (B) reduced
Ni–Al2O3 catalysts with different Ni loadings.
peaks shifted and appeared at 2y = 37.21, 43.31, 62.91, and 75.41,
corresponding to the (111), (200), (220), and (311) crystal planes of
NiO (JCPDS Card No. 71-1179).
After reduction in pure H2 at 650 1C for 3 h, the disappearance
of the NiO phase in all samples indicated that NiO has been
Results and discussion
Catalyst characterization
Fig. 1 shows the XRD patterns of the calcined NiO–Al2O3 reduced to metallic Ni (Fig. 1B), as supported by the appearance
samples and reduced Ni–Al2O3 catalyst samples with different of diffraction peaks at 2y = 44.91, 51.81, and 76.41 corresponding
Ni loadings. As can be seen from Fig. 1A, the diffraction peaks to the (111), (200), and (220) crystal planes of Ni0 (JCPDS Card
at approximately 2y = 31.91, 37.61, 39.51, 45.91, 60.91 and 67.01 No. 70-1849). In contrast, obvious diffraction peaks of NiAl2O4
observed in the pattern of Al2O3 support could be assigned to could still be observed for the 10Ni–Al2O3 and 30Ni–Al2O3
the (220), (311), (222), (400), (511) and (440) crystal planes of catalysts, implying that NiAl2O4 is more difficult to be reduced,
g–Al2O3 (JCPDS Card No. 10-0425), respectively. However, no which is in line with previous studies.29–31 The crystallite sizes of
diffraction peaks related to Al2O3 were observed after doping Ni in the reduced Ni–Al2O3 catalysts decreased gradually from
nickel, indicating that Al2O3 was in the X-ray amorphous state 9.8 nm to 5.3 nm with increasing Ni contents from 10 to 50 wt%,
in all NiO–Al2O3 samples.27 For the calcined 10NiO–Al2O3 and and then increased significantly from 5.5 nm for 70Ni–Al2O3 to
30NiO–Al2O3 samples, the diffraction peaks at 2y = 37.01, 45.01 48.1 nm for pure Ni (Table 1). The smaller Ni crystallite sizes
and 65.51 could be assigned to the (311), (400), and (440) crystal obtained over the Ni–Al2O3 catalysts with 50–70 wt% Ni loadings
planes of the NiAl2O4 spinel (JCPDS Card No. 78-0552), indicating may be related to the formation of a hydrotalcite-like precursor
that the majority of NiO in these samples were inserted into the in the dried samples (Fig. S1, ESI†), since their M2+/M3+ values
crystal lattices of Al2O3 support forming a NiAl2O4 spinel,28 as will are relatively close to the ideal 2–3.25,32 The catalysts from
be supported by the XPS characterization shown below. With hydrotalcite-like precursors have been demonstrated to possess
increasing nickel contents up to and above 50 wt%, the diffraction high metallic dispersion and high particle stability against
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Table 1 Ni loadings and textural properties of Ni–Al2O3 catalyst samples with different Ni contents
Ni contenta
(wt%)
BET surface areab
Average pore diameterc
(nm)
Ni0 crystallite sized
(nm)
Ni0 particle sizee
(nm)
Reduction degreef
(%)
Catalyst
(m2 gꢀ1
)
10Ni–Al2O3
30Ni–Al2O3
50Ni–Al2O3
70Ni–Al2O3
90Ni–Al2O3
Pure Ni
8.9
169.8
163.4
154.9
147.1
52.7
1.5
145.9
11.2
11.3
10.4
16.1
19.1
14.5
8.4
9.8
6.2
5.3
5.5
13.9
48.1
—
11.9
6.8
5.5
5.8
15.1
—
57.9
67.8
95.2
95.6
99.2
100
—
24.1
40.2
60.2
79.0
100
0
Al2O3
—
a
b
c
d
Obtained from AAS analysis. Calculated by the BET method. BJH desorption average pore diameter. Calculated from the Scherrer equation
e
f
through the Ni(200) diffraction peak. Measured from the TEM micrograph. Calculated from H2-TPR.
sintering under extreme conditions even for nickel contents up oxidation state is +2 in all samples.36–39 However, the band shapes
to 75 wt%.27,33–35
and binding energies vary with Ni contents. For 10NiO–Al2O3 and
Fig. 2 displays the XPS spectra of the Ni 2p3/2 of the calcined 30NiO–Al2O3 samples, the main Ni 2p3/2 peak appeared at 856.3 eV
NiO–Al2O3 and reduced Ni–Al2O3 catalyst samples with different could be ascribed to the presence of NiAl2O4,38 indicating that the
Ni loadings. For the calcined NiO–Al2O3 sample (Fig. 2A), the majority of the Ni2+ species exist in the form of NiAl2O4. Compara-
intense and broad Ni 2p3/2 main photoelectron peaks above tively, there is an asymmetric Ni2+ overlapping peak at a lower
854 eV along with the presence of a characteristic shake-up binding energy of around 854.1–854.7 eV in Ni-rich samples with
satellite peak around 861.3–862.3 eV indicates that the nickel Ni loadings of 50–90 wt%, which was close to that of the pure NiO
(approximately 854 eV),39,40 indicating that Ni2+ species mainly
exist as NiO with different metal–support interactions.41,42 An
obvious shoulder peak at a binding energy of 855.8–856.4 eV is
attributed to the multiplet splitting interaction of NiO. In addition,
the NiO peak shifted from 854.7 to 854.1 eV with the increase of Ni
loadings from 50 to 90 wt%, indicating that the interaction
between NiO and Al2O3 was weakened to some extent due to the
agglomeration of NiO particles. The color change of the calcined
NiO–Al2O3 samples (as shown in Fig. S2, ESI†) also supported the
change in composition. After reduction at 650 1C for 3 h, as shown
in Fig. 2B, the Ni 2p3/2 peak that appeared at 852.6 eV could be
attributed to Ni0 and the intensity of this peak increased along with
Ni contents, which indicated that Ni2+ species in Ni-rich samples
were more easily reduced.41 This finding could also be further
confirmed by the H2-TPR of the calcined NiO–Al2O3 samples
(Fig. 4).
Table 1 presents the textural properties of Ni–Al2O3 catalyst
samples with different Ni loadings. The Ni contents in the Ni–
Al2O3 catalyst measured by AAS were slightly lower than the
nominal Ni loadings. The BET surface areas of the Ni–Al2O3
catalysts declined monotonically from 169.8 to 52.7 m2 gꢀ1 with
increasing Ni loadings from 10 to 90 wt%, likely due to a
decrease in the amount of Al2O3 support, which had a surface
area as high as 145.9 m2 gꢀ1. The much lower surface area of
90Ni–Al2O3 might also be ascribed to the agglomeration of a
larger amount of Ni particles, as revealed by the much lower
surface area of pure Ni, which is 1.5 m2 gꢀ1. The average pore
diameters of all samples were located within the range of 8.4–
19.1 nm (Table 1 and Fig. S3, ESI†), indicating the existence of a
mesoporous structure, which would be beneficial for the diffu-
sion of the reactants and the generated products.43
The TEM images and particle size distributions of the
reduced Ni–Al2O3 catalysts with Ni loadings of 10–90 wt% are
shown in Fig. 3. As can be seen from the TEM micrographs,
most of the nickel species showed spherical shapes with the
Fig. 2 XPS spectra for the Ni 2p3/2 core level of calcined NiO–Al2O3
samples (A) and reduced Ni–Al2O3 catalysts (B) with different Ni contents.
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Fig. 4 H2-TPR profiles of the calcined NiO–Al2O3 samples with different
Ni contents.
H2-TPR was performed on calcined NiO–Al2O3 samples with
different Ni contents to study the surface Ni species, metal–
support interaction and reducibility. As shown in Fig. 4, the
temperature of the main reduction peak of the samples gen-
erally shifted to a lower temperature with an increase of Ni
contents. It has been reported that NiO with a low interaction
with the support could be reduced at low temperatures, while
Ni2+ species with a strong metal–support interaction were
reduced at higher temperatures.44,45 According to XRD (Fig. 1)
and XPS (Fig. 2) characterization studies, the main reduction
peak of 10 NiO–Al2O3 that appeared at 765 1C could be ascribed
to the reduction of the NiAl2O4 spinel, and the two reduction
peaks of 30NiO–Al2O3 at 685 and 802 1C might be related to the
reduction of NiO and NiAl2O4, respectively.46 A similar reduction
peak observed for 50NiO–Al2O3 and 70NiO–Al2O3 samples might
be related to dispersed NiO with a strong interaction with the
Al2O3 support. A broad reduction peak that appeared at 468 1C
for the 90Ni–Al2O3 sample indicated the weak interaction
between NiO and Al2O3, which might be due to the agglomera-
tion of NiO on the support.46–49 Pure NiO showed a main
reduction peak at 360 1C and a small shoulder peak at 513 1C,
which were likely attributed to the reduction of bulk NiO with
smaller and larger domains, respectively.50 Furthermore, the
reduction degree of the calcined samples, which to some extent
reflects the availability of active metal sites, was also obtained by
Fig. 3 TEM images and particle size distributions of reduced Ni–Al2O3
catalysts with different Ni contents: (A) 10Ni–Al2O3, (B) 30Ni–Al2O3,
(C) 50Ni–Al2O3, (D) 70Ni–Al2O3 and (E) 90Ni–Al2O3.
particles in catalysts containing 30–70 wt% Ni distributed more H2-TPR. As shown in Table 1, the reduction degree of the
uniformly. The mean particle sizes of Ni nanoparticles calculated samples increased sharply from 57.9% for 10NiO–Al2O3 to
from TEM decreased from 11.9 to 5.5 nm upon increasing the Ni 95.2% for 50NiO–Al2O3, and then increased slightly to 99.2%
loading from 10 to 50 wt%, and then clearly increased to 14.8 nm for 90NiO–Al2O3.
for the 90Ni–Al2O3 catalyst. Obviously, the TEM findings agreed
It has been reported that the acid sites of the catalyst could
well with the results of XRD analysis (Table 1). The representative activate the CQO group of the aldehydes/ketones, and thus
HAADF-STEM and EDX mapping of Ni–Al2O3 catalysts (Fig. S4, promote the imine formation by the condensation of carbonyl
ESI†) also revealed a highly homogeneous Ni elemental distribu- groups with NH3.6,51,52 NH3-TPD was carried out on Ni–Al2O3
tion in 50Ni–Al2O3, which may be attributed to a strong metal– catalysts to study their surface acidity. As shown in Fig. 5, Al2O3
support interaction between Ni and Al2O3 support. However, a support showed an NH3 desorption peak at around 128 1C,
further increase in Ni loading leads to the agglomeration of Ni while the NH3 desorption peak shifted to a high temperature
nanoparticles in 90Ni–Al2O3.
after the incorporation of Ni. One major NH3 desorption peak
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Table 2 The catalytic performance of Ni–Al2O3 catalysts with different Ni
contents toward the reductive amination of 2-HTHPa
Selectivity (%)
Conversion
Entry Catalyst
(%)
5-AP 1,5-PD 5-IP THPOPI 5-HPIP DPA
1
2
3
4
5
6
7
8
No catalyst 93.1
0
0
0
0
50.5 31.3
69.1 22.5
20.9 0
0
0
0
0
b
Al2O3
94.3
c
10Ni–Al2O3c 98.6
30Ni–Al2O3c 100
50Ni–Al2O3c 100
70Ni–Al2O3c 100
90Ni–Al2O3 100
32.4 1.5
35.9 1.5
84.6 2.3
81.0 2.7
63.2 2.8
33.7
33.9
2.4
4.9
19.2
5.4
1.9
2.1
2
5.1
2.4
0
14.7 0
0.6
1.2
5.5
0
0
0
Pure Nic
95.4
0.5
0
65.5 0
Fig. 5 NH3-TPD patterns of the reduced Ni–Al2O3 catalysts with different
Ni loadings.
a
Reaction conditions: 15 g of 21.3 wt% 2-HTHP aqueous solution, 15 g of
b
c
25 wt% aqueous ammonia, 60 1C, 2 MPa H2, 1 h. 0.1 g. 0.05 g active Ni.
at 186 1C was observed for the 10Ni–Al2O3 catalyst, and two or
three NH3 desorption peaks could be seen in the range of 190–
440 1C upon increasing the Ni loading up to and above 30 wt%
for the Ni–Al2O3 catalysts. In comparison with 50Ni–Al2O3 and
70Ni–Al2O3 catalysts, the NH3 desorption peaks for 90Ni–Al2O3
substantially weakened, revealing that the former two catalysts
presented larger amounts of acid sites than the latter. In
contrast, almost no NH3 desorption peaks could be detected
for pure Ni. It has been reported that the NH3 desorption peaks at
around 200 1C, 300–350 1C and above 400 1C are associated with
weak, medium-strong and strong acid sites, respectively.6,53
Accordingly, mainly the weak or medium-strong acid sites were
present on the surface of the Ni–Al2O3 catalysts with Ni loadings
below 50 wt%, and the Ni-rich catalysts (50–90 wt% loadings)
additionally showed strong acid sites, while pure Ni showed
almost no acid sites. The reduction degrees compiled in Table 1
imply that a small amount of Nid+ (Ni2+ 4 Nid+ 4 Ni0) may still
exist even in 90Ni–Al2O3 reduced at 650 1C, which may be
formed by the partial reduction of Ni2+ species with a strong
metal–support interaction.54 Although the insufficiently
reduced low valent metal sites were often reported to present
Lewis acid sites,6,55 the reduction of the dispersed metal oxide
phase to the metallic state was also found to contribute to the
profoundly increased Lewis acidity of the catalyst.56 Pyridine-
FTIR shown in Fig. S5 (ESI†) confirmed the presence of Lewis
acid sites on the 50Ni–Al2O3 catalyst. Therefore, it is likely that
the strong Lewis acid sites of the reduced catalysts with Ni
loadings of 50–90 wt% were enhanced by the presence of a large
amount of dispersed Ni0.
main products in the absence of a catalyst with selectivities of
50.5% and 31.3%, respectively. The 5-IP imine product was
formed by the dehydration condensation of 5-HP with NH3,
while THPOPI was likely generated by the condensation of 5-IP
with 2-HTHP.22 It seems that NH3 promoted the ring-opening
tautomerization of 2-HTHP to form the active 5-HP and then
the in situ formed 5-HP intermediate condensed with ammonia
to form imine intermediates. As expected, no hydrogenation
products including the target 5-AP product were detected in the
absence of a catalyst. Similar results were obtained when
the Al2O3 support was solely used as a catalyst (entry 2). In
the presence of Ni–Al2O3 catalysts, the conversion of 2-HTHP
increased to 100% on 30Ni–Al2O3 and remained constant until
pure Ni was used as a catalyst and the target product 5-AP was
obtained (entries 3–8). The product selectivity of the Ni–Al2O3
catalysts largely depended on Ni loadings. The selectivity to the
target 5-AP product increased rapidly to a maximum of 84.6%
on 50Ni–Al2O3 and then dropped to 0.5% on pure Ni. The
selectivity to 5-IP showed an opposite trend, which declined to a
minimum of 0.6% and then increased to 65.5% over pure Ni,
revealing that 5-IP is a primary imine intermediate for the
target 5-AP. The selectivity to THPOPI dropped to 0 in the
presence of an Ni–Al2O3 catalyst, inferring its rather high
reactivity. The increase of 5-AP selectivity at the expense of
the selectivity of 5-[(5-hydroxypentyl)imino]-1-pentanol (5-HPIP)
for the Ni–Al2O3 catalysts with Ni loadings from 10 to 90 wt%
(entries 3–7) suggested that it is a secondary imine that could
be further transformed into the target 5-AP product. In the
presence of NH3, all the Ni–Al2O3 catalysts showed very low
selectivity (o2.8%) to 1,5-PD, which is a direct hydrogena-
tion product of 2-HTHP through the 5-HP intermediate.23,57
Catalytic performance
Table 2 shows the catalytic performance of Ni–Al2O3 catalysts In addition, a small amount of di-1-pentanolamine (DPA) pro-
with Ni loadings from 0 to 100 wt% on the reductive amination duced by 5-HPIP hydrogenation was also detected (o5.1%). The
of 2-HTHP at 60 1C and 2 MPa H2. The reactant 2-HTHP was low selectivity to the direct hydrogenation by-products 1,5-PD
very reactive in the presence of NH3 and the conversion reached and DPA for Ni–Al2O3 catalysts, especially those with Ni loadings
93% even without any catalyst (entry 1). 5-Imino-1-pentanol around 50–70 wt%, accounts for their high selectivity to the
(5-IP) and THP-oxypentanimine (THPOPI) were obtained as the target 5-AP product.
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Obviously, the above findings indicated that the Ni–Al2O3 on the reductive amination of 2-HTHP over the 50Ni–Al2O3
catalysts with 50–70 wt% Ni loadings presented much better catalyst. These reaction parameters had a weak effect on
performance in the synthesis of 5-AP from the clean reductive 2-HTHP conversion, all of which were above 93% owing to its
amination of bio-2-HTHP. It has been suggested that the Lewis high reactivity,22,23,57 while significantly influencing the pro-
acid sites promotes the formation of imine by activating the duct selectivity. The conversion of 2-HTHP reached 100% even
CQO group of the aldehydes/ketones via the electron lone pair at a low temperature of 40 1C (Fig. 6(a)). At such a low
in oxygen, while the metal sites are responsible for the hydro- temperature, the selectivity of the target 5-AP was only 28.6%
genation of imine to amine.6,45 Although 2-HTHP could readily and large amounts of imine products of 5-IP and 5-HPIP were
react with NH3 through 5-HP to form imine intermediates of detected with selectivities of 17.8% and 46.0%, respectively.
5-IP and THPOPI (Table 2, entry 1),22 the promotion effect of The 5-AP selectivity increased sharply with increasing temperature
the strong Lewis acid sites on the surface of the 50Ni–Al2O3 and and reached a maximum of 88.0% at 70 1C, which then slightly
70Ni–Al2O3 catalysts could not be excluded at present. In decreased to 84.6% upon further increasing the temperature to
addition, because the hydrogenation of the imine intermediates 80 1C. The selectivities to 5-IP and 5-HPIP decreased rapidly with
over metallic sites is likely a rate-determining step for the increasing temperature and almost disappeared at temperatures
reductive amination of aldehydes/ketones,17,22 the presence of up to 70 1C and above. The selectivity of 1,5-PD decreased
a large amount of small Ni0 particles (Table 1, Fig. 1 and 3) would gradually from 3.2% to 1.1% with an increase of temperature,
contribute to the high activity of these catalysts. Due to its implying that high temperature is more favorable for the reductive
outstanding performance, the 50Ni–Al2O3 catalyst was chosen amination of 2-HTHP rather than its direct hydrogenation. The
for further studies.
formation of large amounts of DPA and other undetected side-
Fig. 6 shows the effect of reaction parameters such as products at a higher temperature of 80 1C may account for a slight
temperature, H2 pressure, reaction time, and catalyst amount decrease of 5-AP selectivity at such a temperature. Therefore, an
Fig. 6 Effects of the reaction parameters on the reductive amination of 2-HTHP over the 50Ni–Al2O3 catalyst: (a) temperature, (b) H2 pressure,
(c) reaction time, and (d) catalyst amount. Reaction conditions: 15 g of 21.3% 2-HTHP aqueous solution; 15 g of 25 wt% aqueous ammonia; (a) 0.1 g of 50Ni–
Al2O3 catalyst, 2 MPa H2, 1 h; (b) 0.1 g of 50Ni–Al2O3 catalyst, 60 1C, 1 h; (c) 0.1g of 50Ni–Al2O3 catalyst, 60 1C, 2 MPa H2; and (d) 60 1C, 2 MPa H2, 1 h.
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appropriate reaction temperature of B70 1C is selected for the
efficient synthesis of 5-AP.
The changes in conversion and product selectivity with H2
pressure from 0 MPa to 4 MPa are shown in Fig. 6(b). In the
absence of H2, the conversion of 2-HTHP was 93.1%, and the
main products were 5-IP and THPOPI with selectivities of
69.2% and 22.5%, respectively. The presence of 1 MPa and
above H2 resulted in the full conversion of 2-HTHP and the
generation of hydrogenation products of 5-AP, 1,5-PD and DPA.
The selectivity of 5-AP increased sharply from 58.4% at 1 MPa
H2 to 87.5% at 3 MPa H2 and almost leveled off at 4 MPa H2.
The selectivities of 1,5-PD and DPA increased linearly from
1.1% and 1.4% at 1 MPa H2 to 3.4% and 7.5% at 4 MPa H2,
respectively. With an increase of H2 pressure, 5-IP and 5-HPIP
were quickly converted to 5-AP or DPA and completely disap-
peared at 3 MPa H2. These results suggested that increasing the
H2 pressure promoted the increase of both the reductive amina-
tion and hydrogenation products, which in line with previous
studies.22,23 Clearly, an appropriate H2 pressure (2 MPa) is
reasonable for the efficient production of the target 5-AP.
Fig. 6(c) shows the product distribution as a function of the
Fig. 7 Reductive amination of 2-HTHP with NH3 over the 50Ni–Al2O3
catalyst in a continuous flow reactor. Reaction conditions: a mixed
solution of 21.3 wt% 2-HTHP aqueous solution and 25 wt% concentrated
ammonia at a weight ratio of 1 : 1, 60 1C, 2 MPa H2, weight hourly space
velocity (WHSV) of 2-HTHP = 0.5 hꢀ1, and H2/2-HTHP molar ratio = 30.
reaction time from 0 min to 4 h. Due to its high reactivity, full aldehydes and ketones to form the corresponding primary
conversion of 2-HTHP was achieved even upon increasing the amines with high yields of 88–99% (Table S1, ESI†).
temperature up to 60 1C. In addition, a small amount of 5-AP
The stability of the 50Ni–Al2O3 catalyst on the reductive
(17.1%) was detected during the increasing temperature process amination of 2-HTHP was investigated via a fixed-bed flow
and large amounts of imine intermediates of 5-IP and 5-HPIP reactor and the result is shown in Fig. 7. To our delight, this
with selectivities of 30.2% and 42.3% were also detected, respec- catalyst presented very good stability during a 150 h time-on-
tively. As the reaction went on, the selectivity of 5-AP significantly stream. The 2-HTHP conversion maintained 100% during the
increased to 84.6% within 1 h, and then slowly rose up to 90.6% whole process, which is in line with our previous study using
with the reaction time further extending to 4 h. The imine the 30Ni–Mg3AlOx catalyst22 due to the high reactivity of
intermediates of 5-IP and 5-HPIP were considerably high at the 2-HTHP. The yield of 5-AP maintained at B86% and no
beginning but then disappeared completely within 2 h following appreciable decline in 5-AP yield could be seen during the
the increase of 5-AP. On the other hand, the DPA selectivity 150 h time-on-stream. In contrast, obvious deactivation was
slightly increased from 0 to 4.0% within 4 h. These findings observed for the Ni–ZrO2 catalyst after 22 h of reaction23 and for
suggest that the 5-IP and 5-HPIP intermediates were more the 30Ni–Mg3AlOx catalyst after 90 h of reaction22 under similar
favorably converted to 5-AP in the presence of sufficient ammonia conditions, revealing the remarkable stability of the 50Ni–Al2O3
and H2 than to DPA. 1,5-PD was detected with a low selectivity of catalyst in the reductive amination of 2-HTHP.
B2.0% and remained almost constant during the process, pro-
bably as a result of the very fast competitive reaction of the used 50Ni–Al2O3 catalyst after the 150 h time-on-stream was
condensation of 2-HTHP and NH3. characterized by XRD, XPS and TEM. XRD results (Fig. S6, ESI†)
For a better understanding of the stability of the catalyst, the
Fig. 6(d) shows the influence of catalyst amounts from 0.05 g showed that there were almost no perceptible changes for the
to 0.4 g. 2-HTHP could be fully converted with the presence of diffraction peak of the Ni crystals as compared with the freshly
even 0.05 g of catalyst. The selectivity to 5-AP increased sharply reduced one, and the Ni crystallite size of the used catalyst just
from 48.3% to a maximum of 91.3% with increasing catalyst slightly enlarged from 5.5 nm to 7.0 nm, showing the good
amounts from 0.05 g to 0.2 g, and then decreased slowly to stability of the catalyst against sintering during the reaction.
88.3% with further increasing the catalyst amounts to 0.4 g. Such a finding was also confirmed by TEM characterization
Simultaneously, the selectivities to other hydrogenation products (Fig. 8), as Ni particles were still well dispersed on the support
of 1,5-PD and DPA also gradually increased from 2.5% and 2.7% with a mean particle size of 6.7 nm. The good resistance to the
to 4.1% and 7.7% upon increasing the catalyst amounts from sintering of the catalyst might be attributed to the strong
0.05 g to 0.4 g, respectively. The selectivities of 5-IP and 5-HPIP interaction between the Ni2+ species and the Al2O3 support,58
decreased sharply with increasing catalyst amounts and both as revealed by the H2-TPR characterization shown in Fig. 4. Our
disappeared at catalyst amounts up to 0.2 g. These results previous studies22,23 showed that surface oxidation of active Ni0
indicate that appropriate active Ni sites played a crucial role in might be another important reason along with sintering that
the reductive amination of 2-HTHP to generate the target 5-AP accounted for the deactivation of Ni catalysts in 2-HTHP reduc-
with high selectivity. In addition, the Ni–Al2O3 catalyst was also tive amination. XPS characterization of the used 50Ni–Al2O3
active for the reductive amination of other biomass-derived catalyst (Fig. S6, ESI†) showed the presence of an obvious Ni0
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the samples were dissolved using a mixture of HCl and HNO3
before the measurements. The BET surface area and average pore
diameter of the calcined samples were measured by using a
TriStar II 3020 (Micromeritics) surface area and porosity analyzer.
Prior to the measurements, the samples were degassed in N2 flow
at 300 1C for 4 h to remove adsorbed water and impurities.
The X-ray powder diffraction (XRD) patterns of the samples
were obtained on a Rigaku D/MAX-2400 diffractometer operating
at 40 kV and 30 mA, using Ni-filtered Cu Ka radiation with a
Fig. 8 TEM image of the used 50Ni–Al2O3 catalyst after the 150 h time-on-
stream.
scanning angle (2y) from 101 to 801 at a scan rate of 51 minꢀ1
.
The mean crystallite size of Ni is calculated using the Scherer
equation. X-ray photoelectron spectra (XPS) were performed on
an ESCALAB250xi spectrometer, using a monochromatized Al Ka
X-ray radiation source with a pass energy of 20 eV. All binding
energies were referenced to the C 1s peak at 284.8 eV of the
surface adventitious carbon with an uncertainty of 0.2 eV.
The transmission electron microscopy (TEM) images of the
catalysts were obtained on a JEM2010 electron microscope
operating at 200 kV and the pre-reduced catalyst was suspended
in ethanol by sonication and deposited on the copper mesh for
TEM inspection. The particle size distribution of Ni particles
was statistically obtained from the TEM micrographs by counting
at least 200 particles. High angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) and energy-
dispersive X-ray spectroscopy (EDX) mapping measurements were
obtained using an FEI Talos 200ꢃ transmission electron micro-
scope (USA) operated at an acceleration voltage of 200 kV.
H2 temperature-programmed reduction (H2-TPR) measure-
ments were carried out on a Quantachrome automated chemi-
sorption analyzer (ChemBET pulsar TPR/TPD). The calcined
sample containing around 10 mg of Ni was charged into a
quartz reactor and pretreated at 200 1C for 1 h under He flow. After
cooling to room temperature, it was heated to 900 1C at a heating
rate of 10 1C minꢀ1 under 10% H2–N2 flow (30 mL minꢀ1).
H2 consumption was collected using a thermal conductivity
detector (TCD). For determining the reduction degree of the
catalyst, the same amount of calcined sample was pre-reduced
at 650 1C for 2 h under 10% H2–N2 flow (30 mL minꢀ1). After
cooling to below 50 1C, the obtained reduced sample was sub-
jected to another H2-TPR to obtain the calcined sample upon
increasing the temperature to 900 1C, and the reduction degree
was calculated as follows:60
peak similar to the freshly reduced and passivated catalyst. Such
a finding infers that the 50Ni–Al2O3 catalyst also presents good
resistance to surface oxidation, contributing to the remarkably
enhanced reaction stability of the catalyst.
Experimental section
Materials
Ni(NO3)2ꢁ6H2O, Al(NO3)3ꢁ9H2O, Na2CO3 and NaOH were pur-
chased from West Long Science Co., Ltd. 1,2-Pentanediol (98%)
was purchased from Maclean Biochemical Technology Co., Ltd.
5-Amino-1-pentanol (95%) was purchased from Aladdin
Chemical Reagent Co. Ltd. All of the reagents were of analytical
grade and were used without further purification.
An aqueous solution of 21.3 wt% 2-HTHP was obtained by the
autocatalytic hydration of dihydropyran.59 The reaction was per-
formed in a 2 L autoclave equipped with mechanical stirring. After
adding 100 g of dihydropyran and 400 g of deionized water, the
autoclave was sealed and flushed 3 times with N2, and then
pressurized to 2 MPa. The reaction was carried out at 100 1C for 1 h.
Catalyst preparation
The Ni–Al2O3 catalysts with Ni loadings from 0 to 100 wt% were
prepared by the co-precipitation method. The specific prepara-
tion process was described below. A mixed salt solution of
Ni(NO3)2ꢁ6H2O (0.1 M) and Al(NO3)3ꢁ9H2O, and a mixed alkali
solution of NaOH (5 M) and Na2CO3 (1 M) were added dropwise
concurrently into a three-necked flask under vigorous stirring.
The pH of the solution was maintained as 10 ꢂ 0.1. After that,
the precipitate was aged at 80 1C for 24 h, and then filtered and
washed with deionized water until the pH was B7. The
obtained solid was dried at 110 1C overnight and calcined in
a muffle furnace at 700 1C for 4 h. The calcined sample was
reduced in flowing H2 at 650 1C for 3 h to obtain active xNi–
Al2O3 catalysts with different Ni contents, where x represents
the theoretical mass fraction of active Ni in the catalyst. For
comparison, the calcined samples were denoted as xNiO–Al2O3.
NH3 temperature programmed desorption (NH3-TPD) measure-
ments were performed on a Huasi DAS-7200 automatic chemical
adsorption instrument. The samples (200 mg) were firstly
reduced at 650 1C for 3 h under 5% H2–Ar flow (30 mL minꢀ1).
Then, the sample was cooled to 100 1C followed by adsorbing
NH3 for 1 h under 10% NH3–N2 flow (30 mL minꢀ1). After
purging the system with He (30 mL minꢀ1) for 1 h, the system
was heated to 600 1C at a ramp rate of 10 1C minꢀ1 under He
flow, and the desorption of NH3 was detected by TCD. Pyridine
Catalyst characterization
The Ni contents in the Ni–Al2O3 catalysts were detected using a Fourier transform infrared (py-IR) spectra were recorded on a
fluorescence spectrometer (AAS, Analytik Jena ContrAA 700), Tensor 27 Fourier Infrared Spectrometer produced by Bruker.
TPR peak area of calcined sample ꢀ TPR peak area of reduced sample
Reduction Degree ð%Þ ¼
TPR peak area of calcined sample
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After pretreating at 400 1C under 10ꢀ3 Pa for 1 h, the spectrum of with different Ni contents from 0 to 100 wt% prepared by
the sample disk was collected as a blank experiment. Pyridine co-precipitation. The selectivity to the desired product 5-AP
adsorption was carried out at room temperature, and the follow- strongly depended on Ni loadings and the highest 5-AP selectivity
ing desorption steps were performed at 30 1C, 100 1C and 150 1C of 91.3% at full conversion was achieved over the 50Ni–Al2O3
for 30 min, respectively. The spectrum of each sample was catalyst under mild conditions of 60 1C and 2 MPa H2. This
acquired after cooling the sample.
catalyst also exhibited good stability during a 150 h time-on-
stream without appreciable deactivation. The outstanding perfor-
mance of the 50Ni–Al2O3 catalyst could be ascribed to the highly
Catalytic tests
The catalyst activity test was performed in a 100 mL stainless dispersed Ni0 particles with strong interaction with the support
steel autoclave reactor at a stirring speed of 800 rpm. Prior to and good resistance to surface oxidation during the reaction. The
the reaction, the calcined NiO–Al2O3 samples were reduced at important findings of this work would be helpful for developing
650 1C in pure H2 for 3 h to obtain the active catalysts. In a
mol of 2-HTHP charged ꢀ mol of 2-HTHP left
Conversion ð%Þ ¼
Selectivity ð%Þ ¼
ꢃ 100
mol of 2-HTHP charged
mol of product ꢃ C atoms in product
ꢃ 100
ðmol of 2-HTHP charged ꢀ mol of 2-HTHP leftÞ ꢃ C atoms in 2-HTHP
typical experiment, 15 g of 21.3 wt% 2-HTHP aqueous solution novel base-metal catalysts for the synthesis of useful amines by
was firstly added into the reactor, and then the reduced active reduction amination of carbonyl compounds.
catalyst containing 0.05 g of Ni was loaded into the reactor in a
glove box filled with Ar, followed by the addition of 15 g of 25%
aqueous ammonia. After flushing the reactor 3 times with H2, it
Conflicts of interest
was pressurized to 2 MPa H2, then heated to 60 1C, and
remained stable during the reaction.
There are no conflicts to declare.
The catalyst stability test was performed in a tubular fixed bed
flow reactor (length 36 cm, inner diameter 0.9 cm) at 60 1C and Acknowledgements
2 MPa H2. 1.0 g of the prepared sample (40–60 mesh) was charged
This work was financially supported by the National Natural
into the flow reactor, and the catalyst was pre-reduced under the
Science Foundation of China (21872155, 21902164), the Strategic
Pilot Science and Technology Project of the Chinese Academy of
Sciences (XDA21010700), the Cooperation Foundation of Dalian
flow of pure H2 (40 mL minꢀ1) at 650 1C for 3 h. After cooling to
the reaction temperature, the reactor was pressurized to 2 MPa
with H2, and then a solution mixed with 21.3 wt % 2-HTHP
National Laboratory for Clean Energy (DNL 180303) and the
aqueous solution and 25 wt % aqueous ammonia at a mass ratio
Lanzhou Chengguan District Science and Technology Plan Project
of 1 : 1 was pumped into the flow reactor at a flowing rate of
(2020JSCX0051).
around 5 g hꢀ1. Next, the reaction effluents was collected from a
stainless steel gas–liquid separator every 5 to 10 h. The reaction
products were identified using a GC-MS system (Agilent 7890A/
5975C) and the detected products included 5-amino-1-pentanol
References
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