Direct Amination of Biomass-based Furfuryl Alcohol and 5-(Aminomethyl)-2-furanmethanol with NH3 over Hydrotalcite-derived Nickel Catalysts via the Hydrogen-borrowing Strategy

Article abstract of DOI:10.1002/cctc.202001922

A series of hydrotalcite-derived nickel catalysts were synthesized and employed for the direct amination of biomass-based furfuryl alcohol with NH₃ via the hydrogen borrowing strategy. The effects of the Ni/Al molar ratio and calcination temperature of the NiAl hydrotalcite-like precursors on the performance of the Ni_xAl-CT catalyst were investigated. The systematic characterization showed that the synergistic catalysis of the metal and acid-base sites was of vital importance for the amination of alcohols. In particular, the Ni₂Al-600 catalyst with high amount of Ni⁰ sites (1.26 mmol g^?1) and suitable density of acid-base sites (0.71 mmol g^?1 and 1.10 mmol g^?1, respectively) exhibited the best dehydrogenation capability and therefore excellent catalytic activity. An 84.1 % yield of furfurylamine with complete conversion of furfuryl alcohol was obtained under the reaction conditions of 180 °C and 0.4 MPa NH₃ in 36 h. The presence of Ni₃N in the spent catalyst, confirmed by XRD, TEM and XPS characterizations, was demonstrated to be responsible for the deactivation of the Ni_xAl-CT catalyst. In addition, the Ni₂Al-600 catalyst exhibited satisfactory performance toward another important biomass-related transformation of 5-(aminomethyl)-2-furanmethanol to 2,5-bis(aminomethyl)furan, with a yield of 70.5 %.

Full text of DOI:10.1002/cctc.202001922

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Direct Amination of Biomass-based Furfuryl Alcohol and 5-
(Aminomethyl)-2-furanmethanol with NH3 over
Hydrotalcite-derived Nickel Catalysts via the Hydrogen-
borrowing Strategy
Kuo Zhou,[a, b] Ruihong Xie,[a] Meiting Xiao,[a] Darun Guo,[a] Zhuodi Cai,[a] Shimin Kang,[a]
Yongjun Xu,*[a] and Jinjia Wei[b]
A series of hydrotalcite-derived nickel catalysts were synthe-
sized and employed for the direct amination of biomass-based
furfuryl alcohol with NH3 via the hydrogen borrowing strategy.
The effects of the Ni/Al molar ratio and calcination temperature
of the NiAl hydrotalcite-like precursors on the performance of
the NixAl-CT catalyst were investigated. The systematic charac-
terization showed that the synergistic catalysis of the metal and
acid-base sites was of vital importance for the amination of
alcohols. In particular, the Ni2Al-600 catalyst with high amount
of Ni0 sites (1.26 mmolgÀ 1) and suitable density of acid-base
sites (0.71 mmolgÀ 1 and 1.10 mmolgÀ 1, respectively) exhibited
the best dehydrogenation capability and therefore excellent
catalytic activity. An 84.1% yield of furfurylamine with complete
conversion of furfuryl alcohol was obtained under the reaction
°
conditions of 180 C and 0.4 MPa NH3 in 36 h. The presence of
Ni3N in the spent catalyst, confirmed by XRD, TEM and XPS
characterizations, was demonstrated to be responsible for the
deactivation of the NixAl-CT catalyst. In addition, the Ni2Al-600
catalyst exhibited satisfactory performance toward another
important biomass-related transformation of 5-(aminomethyl)-
2-furanmethanol to 2,5-bis(aminomethyl)furan, with a yield of
70.5%.
Introduction
borrowing” or “hydrogen auto-transfer” mechanism (Scheme 1).
In this transformation, the alcohol is dehydrogenated at the
catalytic center to give the corresponding carbonyl intermedi-
ate; the intermediate condenses with ammonia to form an
imine, which is subsequently rehydrogenated with the catalyst-
bound hydrogen to the final primary amine.[7] As a conse-
quence, the amination of alcohols by this strategy can also be
termed “hydrogen transfer amination”. Additionally, the amina-
tion of alcohols can be catalyzed by heterogeneous metal
catalysts, mostly based on Ni,[8] Co[9] and Ru,[3b] following the
“dehydrogenation-iminization-hydrogenation” process.[10] In
contrast to the former process, this reaction process is generally
carried out under a mixture of ammonia and hydrogen.
Exogenous hydrogen is indispensable to maintain the activity
Amines are key compounds in the chemical industry, with
extensive applications in the manufacture of agrochemicals,
drugs, detergents, lubricants, food-additives, and polymers.[1]
Among amines, primary amines are the most useful intermedi-
ates for further derivatization reactions due to their nucleophilic
properties, conferring them high reactivity.[2] In principle,
primary amines can be synthesized by amination of alcohols,
which constitutes an attractive process with high atom
efficiency and with water as the main byproduct.[3] However,
most of the substrate molecules are derived from traditional
fossil resources, which are facing growing shortages and bring
many environmental problems.[4] Biomass-derived alcohols with
diverse structures can be efficiently produced from inedible
biomass resources, and therefore, they represent ideal candi-
dates for the sustainable expansion of the amine chemistry
industry.[5]
In general, the amination of alcohols with ammonia can be
catalyzed by metal complex-based homogeneous catalysts,
such as Ru- and Ir-based complexes,[6] via the “hydrogen
[a] Dr. K. Zhou, R. Xie, M. Xiao, D. Guo, Z. Cai, Dr. S. Kang, Prof. Y. Xu
College of Chemical Engineering and Energy Technology
Dongguan University of Technology
Dongguan 523808 (P.R. China)
E-mail: xuyj@dgut.edu.cn
[b] Dr. K. Zhou, Prof. J. Wei
School of Chemical Engineering and Technology,
Xi’an Jiaotong University
Xi’an 710049 (P.R. China)
Scheme 1. General mechanism for alcohol amination: “hydrogen borrowing”
and reductive amination in the presence of H2.
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of the catalyst, although hydrogen is not stoichiometrically
consumed in the amination of alcohols.[10–11] However, the use
of high pressure molecular hydrogen usually suffers from a
number of problems related to hydrogen availability in high
compression cost and safety.[12] Moreover, it results in some
undesirable side reaction such as hydrogenation of unsaturated
bonds, dehydroxylation, and hydrogenolysis, thereby reducing
the selectivity to the corresponding primary amines. In contrast,
direct amination of alcohols with NH3 to primary amines in the
absence of additional hydrogen exhibits higher atom efficiency,
better safety, and lower cost, and deserves increasing attention.
In recent years, a few heterogeneous metal catalysts such as Ni-
Al2O3/SiO2, Ni/Al2O3 and Ni/CaSiO3 have also been reported in
alcohol amination in the absence of hydrogen via the “hydro-
gen borrowing” strategy.[3a,12–13]
in the absence of H2. Density functional theory (DFT) calcu-
lations suggested that the adsorption energies of NH3 for noble
metals were higher than that for Ni, indicating that more metal
active sites of noble metals would be occupied by the
competitive occupation process of NH3, thereby deactivating
the catalysts. Accordingly, Raney Ni was successfully employed
in the amination of furfuryl alcohol to furfurylamine with NH3
via the “hydrogen borrowing” strategy.[7b]
Nevertheless, this process is still under development due to
the poor catalytic efficiency of Raney-type nickel. Compared
with the reductive amination of carbonyl compounds, the
amination of alcohols is much more difficult due to the
additional dehydrogenation step, which is considered to be the
rate-controlling step (Scheme 1).[7b,21] In view of this observation,
the poor efficiency of Raney Ni could be ascribed to the low
dehydrogenation activity in the presence of ammonia. Herein,
we report, for the first time to our knowledge, the direct
amination of biomass-based furfuryl alcohol to furfurylamine
over efficient Ni-based catalysts derived from hydrotalcite-like
precursors via the “hydrogen borrowing” strategy. The effects
of the Ni/Al molar ratio and calcination temperature of the NiAl
hydrotalcite-like precursors on the NixAl-CT catalyst perform-
ance were investigated. Moreover, the current Ni-based cata-
lysts were employed in the direct amination of 5-(aminometh-
Unfortunately, despite the rapid progress in the amination
of traditional fossil resource-derived alcohol compounds, the
transformation of biomass-derived alcohols into primary amines
is still in its infancy due to the lack of an efficient catalytic
amination system.[1a,5a,14] To date, only a few homogeneous
noble metal complex catalysts have shown acceptable selectiv-
ities in the amination of biomass-derived alcohols to primary
amines. For instance, Gunanathan et al.[6a] developed an
acridine-based pincer complex [RuHCl(A-iPr-PNP)(CO)] to cata-
lyze the amination of furfuryl alcohol. After refluxing for 12 h,
the yield of furfurylamine reached 94.8% under 7.5 bar NH3.
Imm et al.[15] employed [Ru3(CO)12]/CataCXiumPCy as the cata-
lyst for the same reaction, and a 71% yield of furfurylamine was
°
yl)-2-furanmethanol to 2,5-bis(aminomethyl)furan,
a novel
biomass-derived diamine that can be used in the production of
polymers, hardeners, and elastomers based on its primary
diamine and furan ring structure.[4b,22]
obtained at 170 C in 20 h. Pingen et al.[16] reported
a
[Ru3(CO)12]/A-iPr-PNP catalytic system for the amination of
isomannide, and obtained the corresponding diamine with a Results and Discussion
°
yield of 96% at 170 C in 21 h. All of these examples were
driven by Ru-based organometallic catalysis via the “hydrogen
borrowing” mechanism. For practical applications, the homoge-
neous catalysts often suffer from high cost and difficulty in
catalyst separation and recycling. Therefore, the development
of heterogeneous catalysts is urgent and highly desirable for
the amination of biomass-derived alcohols. Amination of a
natural terpene alcohol, myrtenol, with aniline over Au/ZrO2
and Au-Pd/Al2O3 catalysts has been studied by Demidova
et al.[17] Li et al.[18] and Kimura et al.[19] reported the amination of
fatty alcohols with dimethylamine over Cu-Ni/CaCO3 and Cu/Ni/
Ca/Ba colloidal catalyst, respectively. However, these catalysts
were designed for the synthesis of secondary/tertiary amines
via reaction of alcohols with primary/secondary amines.
Recently, Ruiz et al.[11b] reported the amination of dodecanol
with ammonia to dodecylamine over Ru/C catalyst, and A
dodecylamine yield of 83.8% was achieved with Ru/C catalyst
Characterization of Catalysts
The textural characteristics of the NixAl-HT precursors and NixAl-
CT catalysts were examined by N2 adsorption–desorption
analysis and the results are summarized in Table 1. The specific
BET surface areas of the Ni2Al-HT and Ni3Al-HT precursors were
97 m2 gÀ 1 and 90 m2 gÀ 1, respectively. After calcination treat-
ment, the BET surfaces, total pore volume, and average pore
size were obviously increased due to the release of H2O and
CO2.[23] However, increasing the calcination temperature in the
°
range of 400–800 C resulted in a gradual decrease in the BET
surface area from 283 to 117 m2 gÀ 1, indicating the sintering of
oxide components.[23] A similar trend was also observed from
calcined samples with different Ni/Al molar ratios. The BET
surface area of NixAl-600 decreased from 230 to 129 m2 gÀ 1 as
the Ni/Al molar ratio increased from 0.5 to 4. The much lower
surface area of Ni4Al-600 might be ascribed to a larger amount
of NiO segregated from the Al2O3 support, which would be
confirmed by XRD characterization.[24] The average pore dimeter
of the calcined samples was located within a mesopore range
of 3.0–6.8 nm.
°
at 150 C, 4 bar ammonia and 2 bar hydrogen after 24 h
reaction. Rose et al.[20] reported the amination of biogenic
isohexides to diamines and amino alcohols with a commercially
°
available Ru/C catalyst in aqueous ammonia at 180 C with
10 bar H2, and a total amine yield of only 51% was achieved
after 24 h of reaction. However, these processes were con-
ducted under the presence of H2.Our previous work on the
amination of furfuryl alcohol indicated that noble metal
catalysts such as Pt/C, Pd/C, Ru/C, and Rh/C were totally inactive
Figure 1(a) shows the XRD patterns of the NixAl-HT
precursors. All the prepared Ni catalyst precursors showed
°
°
°
°
°
°
diffraction peaks at 2θ=13.0 , 25.8 , 34.9 , 38.4 , 46.5 , 61.8
°
and 63.7 , which were assigned to the (003), (006), (012), (015),
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Table 1. Main physiochemical properties of NiAl-HT and the NiAl-CT catalysts
[b]
[b]
[d]
[e]
Sample
Ni content[a]
[wt%]
SBET
Vp
Dp
dNiO
Amounts of
Acid amount[g]
Base amount[h]
[m2 gÀ 1
]
[cm3 gÀ 1
]
[nm]
[nm]
Ni sites[f] [mmol·gÀ 1
]
[mmolgÀ 1
]
[mmolgÀ 1
]
Ni2Al-HT
Ni3Al-HT
–
–
97
90
0.15
0.11
0.38
0.29
0.26
0.25
0.26
0.23
0.30
0.30
0.24
3.9
3.4
5.5
4.9
4.9
5.0
6.4
3.0
4.6
5.8
6.8
–
–
–
–
–
–
–
–
Ni0.5Al-600
Ni1Al-600
Ni2Al-600
Ni3Al-600
Ni4Al-600
Ni2Al-400
Ni2Al-500
Ni2Al-700
Ni2Al-800
30.9
39.7
50.5
57.6
63.8
49.0
49.8
52.5
56.4
230
192
179
162
129
283
230
172
117
14.7
9.3
8.6
8.6
9.4
8.3
8.1
8.7
9.6
0.91
1.00
1.25
1.22
1.18
2.30
1.26
1.25
0.71
0.70
0.70
0.71
0.72
0.74
0.68
0.70
0.72
0.76
1.17
1.13
1.10
1.08
1.04
1.87
1.42
0.93
0.75
[a] Obtained from ICP analysis.[b] BET surface area.[c] BJH desorption cumulative volume of pores.[d] BJH desorption average pore width.[e] Calculated by the NiO
[f]
(200) reflection (2θ=43.2 ) based on the Scherrer equation. Amounts of metal sites were determined by H2-TPD.[g] Amounts of surface acidic site were
°
determined by NH3-TPD.[h] Amounts of surface basic site were determined by CO2-TPD.
Figure 1. XRD patterns of NixAl-HT, NixAl-CT, reduced Ni2Al-600, and spent Ni2Al-600.
°
(018), (110), and (113) crystal planes of the hydrotalcite like
layered structure, respectively.[23,25] Furthermore, isolated phases
of individual metal hydroxides were undetected in all the
samples except for Ni0.5Al-HT, suggesting the formation of a
pure hydrotalcite structure.[26] For the Ni0.5Al-HT, the diffraction
peaks of Al(OH)3 (001), (100), (101), (-1-1 1) and (-1-1 2) (PDF#77-
calcination at 600 C. For all the samples, the absence of layered
hydrotalcite structures evidenced the total decomposition of
the NiAl-HT precursor. Except for Ni0.5Al-600, all the calcined
°
samples showed four obvious diffraction peaks at 2θ=37.2 ,
°
°
°
43.2 , 62.8 and 75.4 , corresponding to the NiO (111), (200),
(220) and (311) crystal planes (PDF#47-1049), respectively. The
results demonstrated the transformation of the NiAl-HT pre-
cursors into oxide compounds. However, no diffraction peaks
related to Al2O3 were observed in samples with Ni/Al molar
ratios higher than 1:1, revealing the X-ray amorphous state of
°
°
°
°
°
0114) were observed at 2θ=18.7 , 20.3 , 27.8 , 40.5 and 53.0 ,
respectively, which has also been reported by Bliek et al.[27]
Figure 1(b) shows the XRD profiles of NixAl-600 with differ-
ent Ni/Al molar ratios (1:2, 1:1, 2:1, 3:1, and 4:1) after
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Al2O3 in these samples.[23] In contrast, the diffraction peaks of
NiO (200), (220) and (311) for Ni0.5Al-600 appeared to be much
weaker than those of the other samples, and diffraction peaks
of Al2O3 (111), (400) and (440) (PDF#29-0063) were observed at
°
°
°
2θ=19.5 , 45.7 and 66.7 , respectively, suggesting the crystal-
line state of Al2O3 in the samples with low Ni content.[28] The
crystallite size of NiO did not vary obviously with the Ni/Al
molar ratio (Table 1), which gradually decreased from 14.7 to
8.6 nm with increasing Ni/Al molar ratio from 0.5 to 3 and then
slightly increased to 9.4 nm. Previous studies have demon-
strated that the catalysts prepared via hydrotalcite precursors
did not exhibit an appreciable change in metal dispersion
within a wide range of metal loadings.[24,29]
Figure 1(c) shows the XRD patterns of Ni2Al-CT samples
°
°
°
calcined at different temperatures (CT=400 C, 500 C, 600 C,
°
°
700 C, and 800 C). For all the samples, the diffraction peaks of
NiO (111), (200), (220) and (311) were observed, and the peaks
of NiO intensified slightly with increasing calcination temper-
ature, indicating a slow crystallite growth of NiO (Table 1).
°
Notably, a diffraction peak at 2θ=65.5 emerged over the
pattern of Ni2Al-800, which can be attributed to the NiAl2O4
(440) crystal plane (PDF#10-0339). This result demonstrated the
formation of NiAl2O4 after calcination at high temperature.
Figure 1(d) shows the XRD profiles of reduced and spent
Ni2Al-600. The freshly reduced catalyst showed diffraction peaks
Figure 2. TEM images of the reduced (a) (b) and spent (c) (d) Ni2Al-600.
depicted in Figure 3(a), Al 2p, O 1s, and Ni 2p peaks can be
observed in both survey spectra of the two samples, while the
N 1s peak can only be detected in spent Ni2Al-600, indicating
the presence of N in the spent sample. Figure 3(b) shows the
high-resolution Ni 2p spectra for the two samples. For the Ni 2p
spectra of fresh Ni2Al-600, the Ni 2p3/2 peak was present at
852.3 eV, with a weak satellite peak at 858.2 eV, and the Ni2p1/2
peak was present at 869.6 eV, with a weak satellite peak at
875.1 eV, representing a typical mixture of core level and
satellite features for metallic Ni, which indicated that the Ni
species was completely reduced to Ni0.[32] The spent Ni2Al-600
presented a different spectrum than that of the freshly reduced
sample. The Ni 2p3/2 peak at 852.6 eV and Ni 2p1/2 peak at
869.7 eV were assigned to the NiÀ N bond, suggesting that Ni3N
was formed.[31a,33] Moreover, the peaks at 855.8 eV and 873.6 eV
accompanying two Ni shake-up peaks at 861.6 eV and 880.3 eV
were attributed to the characteristic peak of NiO in Ni 2p3/2 and
Ni 2p1/2, respectively. The existence of Ni3N in the spent catalyst
was further confirmed by the N 1s peak, in which two binding
energies could be distinguished (Figure 3(c)). The peak at
399.6 eV corresponded to the CÀ N bond, which could be
ascribed to the adsorbed amino compounds. The peak at
398.1 eV was generally attributed to the NiÀ N bond, confirming
the formation of nickel nitride.[31a,33]
°
°
°
at 2θ=44.5 , 51.8 , and 76.3 , which were assigned to the (111),
(200), and (220) crystal planes of metallic Ni (PDF#04-0850),
respectively. The crystallite size of Ni0 calculated by the Ni (111)
°
reflection (2θ=51.8 ) based on the Scherrer equation was
9.0 nm, which was larger than that of NiO (8.6 nm) in the
unreduced Ni2Al-600, indicating a slight agglomeration of the
Ni species during the reduction process. The diffraction peaks
of NiO (111) and (220) were also observed at 2θ=37.2 and
62.8 , respectively, which could be ascribed to the insufficient
°
°
reduction of nickel or, most likely, partial re-oxidation of surface
Ni0 during storage before testing. However, the spent catalyst
°
exhibited completely different diffraction peaks at 2θ=38.9 ,
°
°
°
°
42.1 , 44.4 , 58.5 , and 78.3 , which corresponded to the Ni3N
(110), (002), (111), (112), and (300) crystal planes (PDF#10-0280),
respectively, indicating the formation of Ni3N.[7b]
Representative TEM images of the reduced and spent Ni2Al-
600 are shown in Figure 2 and Figure S1. After being treated in
°
H2 at 500 C, highly dispersed Ni nanoparticles were formed in
the reduced Ni2Al-600 catalyst. The average particle size of Ni
nanoparticles was approximately 8.1 nm, consistent with the
XRD results shown in Figure 1(d). The Ni metal particle shown
in Figure 2(b) exhibited an interplanar spacing of 0.20 nm,
which was assigned to the lattice fringe of the Ni (111) plane.[30]
°
After reaction at 180 C in NH3, a disordered metal distribution
Figure 4(a) shows the H2-TPR patterns of NixAl-600 with
different Ni/Al molar ratios (1:2, 1:1, 2:1, 3:1, and 4:1). All
samples displayed a broad H2 consumption peak in the temper-
was observed in the spent Ni2Al-600. The metal particles were
heterogeneously dispersed, making it difficult to determine the
average particle size. Noticeable lattice fringes for Ni3N (002)
(0.22 nm) were observed in the high-resolution TEM (HRTEM)
images (Figure 2(d)), which further confirmed the formation of
Ni3N in the spent Ni2Al-600.[31]
°
ature range of 300–800 C, which was assigned to the reduction
of the NiO phase to metallic Ni. With increasing Ni/Al molar
ratio, the total integrated area of the reduction peak (H2
consumption) gradually increased, confirming the positive
increase in surface reducible nickel species. It was interesting
that the reduction peaks slightly shifted toward a lower
In situ XPS was used to investigate the surface chemical
state of the freshly reduced and spent Ni2Al-600 catalysts. As
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Figure 3. XPS spectra of the reduced and spent Ni2Al-600: (a) full spectra, (b) Ni 2p, and (c) N 1s.
Figure 4. H2-TPR results of the NixAl-600 and Ni2Al-CT catalysts.
temperature, indicating that a lower Ni/Al molar ratio led to a
stronger interaction between Ni and its surroundings.[24,34]
Figure 4(b) shows the H2-TPR patterns of the Ni2Al-CT samples
calcined at different temperatures. It was observed that the
maximum peaks of the TPR profile significantly shifted toward
higher temperatures from Ni2Al-400 to Ni2Al-800, indicating an
enhanced interaction between NiO and Al2O3 species.[23,35]
Another reason for the excessively high reduction temperature
of Ni2Al-800 could be ascribed to the formation of NiAl2O4
in Ni0 site abundance could be attributed to the depressed
dispersion of Ni species, as evidenced by the XRD results. This
finding was in line with a previous report by Zhang et al.[36a]
Figure 5(b) shows the H2-TPD profiles of reduced Ni2Al-CT
°
°
catalysts calcined at different temperatures (CT=400 C, 500 C,
°
°
°
600 C, 700 C, and 800 C). Obviously, calcination at higher
temperatures rapidly decreased the number of surface Ni0 sites,
which could be ascribed to the hindered reducibility and
decreased dispersion degree of Ni species.[23,35]
(confirmed by the XRD results in Figure 1(c)), which was difficult
NH3-TPD and CO2-TPD were employed to probe the surface
acidity and basicity of all the reduced samples, respectively. As
shown in Figure 6(a), (b), the NixAl-600 catalysts with different
Ni/Al molar ratios (1:2, 1:1, 2:1, 3:1, and 4:1) exhibited similar
acid-base strengths and densities. Variation in the Ni/Al molar
ratio appeared to exhibit a negligible effect on the surface
acidity and basicity (Table 1). However, the calcination temper-
ature significantly affected the surface acidity and basicity of
the Ni2Al-CT catalysts. Figure 6(c), (d) shows the NH3-TPD and
CO2-TPD profiles of Ni2Al-CT calcined at different temperatures.
[36]
°
to reduce at temperature lower than 700 C.
To quantitatively determine the amount of surface metallic
Ni sites, H2-TPD measurements were carried out. The H2-TPD
profiles for the reduced NixAl-600 catalysts with different Ni/Al
molar ratios (1:2, 1:1, 2:1, 3:1, and 4:1) are shown in
Figure 5(a). The H2 desorption peak intensified with increasing
Ni/Al molar ratio from 0.5 to 2 and then slightly weakened as
the Ni/Al molar ratio further increased from 2 to 4. This result
indicated that Ni2Al-600 possessed the highest number of
surface Ni0 sites (Table 1). It should be noted that the nickel
content of both Ni3Al-600 and Ni4Al-600 was higher than that of
Ni2Al-600, while the reduction temperatures of Ni3Al-600 and
Ni4Al-600 were lower than that of Ni2Al-600; thus, the decrease
°
As the calcination temperature increased from 400–800 C, the
density of acid sites increased slightly, while the density of basic
sites diminished significantly (Table 1). It has been reported that
the increased amount of surface acid sites was mainly due to
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Figure 5. H2-TPD results of the NixAl-600 and Ni2Al-CT catalysts.
Figure 6. NH3-TPD and CO2-TPD profiles of the NixAl-600 and Ni2Al-CT catalysts.
the increased amount of NiO species caused by the hindered
reducibility and defective Alδ+ sites derived from dehydration
of surface hydroxyl groups.[23,37] The diminishment of basic sites
could be attributed to the loss of surface low-coordinated
oxygen anions due to calcination.
Catalytic performance
The direct amination of furfuryl alcohol was conducted in the
absence of hydrogen over hydrotalcite-derived nickel catalysts
with different Ni/Al molar ratios (1:2, 1:1, 2:1, 3:1, and 4:1)
°
°
°
and different calcination temperatures (400 C, 500 C, 600 C,
°
°
700 C, and 800 C); the results are summarized in Table 2.
Initially, commercially available Raney Ni was investigated as
the catalyst for comparison, and a furfuryl alcohol conversion of
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Table 2. Direct amination of furfuryl alcohol over different catalysts
Entry
Catalyst
Conversion
[%]
Yield [%]
Specific rate
(mmolFAM/(gCat h))
Others[a]
1
2
3
4
5
6
7
8
9
Raney Ni
Ni0.5Al-600
Ni1Al-600
Ni2Al-600
Ni3Al-600
Ni4Al-600
Ni2Al-400
Ni2Al-500
Ni2Al-700
Ni2Al-800
22.0
2.0
13.8
0.2
5.0
1.7
–
–
0.6
3.2
0.1
4.2
4.9
5.0
3.9
3.5
5.1
6.2
–
0.23
0
24.1
48.0
35.5
24.3
22.8
30.8
48.1
10.6
19.9
43.1
29.9
20.3
17.9
24.6
41.2
8.1
0.34
0.73
0.51
0.35
0.30
0.42
0.70
0.14
–
1.4
1.1
0.7
2.6
10
°
Reaction conditions: furfuryl alcohol, 0.5 g; THF, 20 mL; catalyst, 0.25 g; NH3 pressure, 0.4 MPa; reaction temperature, 180 C; reaction time, 12 h; stirring
speed, 1000 rpm. [a] Undetectable products.
22.0% with a furfurylamine yield of 13.8% was obtained
(Table 2, entry 1). For the hydrotalcite-derived nickel catalysts,
the conversion of furfuryl alcohol and yield of furfurylamine
varied with the Ni/Al molar ratio and calcination temperature.
The yield of furfurylamine was directly proportional to the
conversion of furfuryl alcohol, which increased with increasing
Ni/Al molar ratio from 0.5 to 2 and then gradually decreased as
the Ni/Al molar ratio further increased to 3 and 4 (Table 2,
entries 2–6). Thus, among the NixAl-600 series catalysts, Ni2Al-
600 was the most active for the direct amination of furfuryl
ation/hydrogenation capability of the catalyst. In addition, it has
been widely recognized that the dehydrogenation capability
may be significantly affected by the surface acidity and basicity
of the catalyst.[35,38]
The H2-TPD results of the Ni2Al-CT series catalysts suggested
that the amount of Ni0 sites decreased sharply with increasing
calcination temperature. However, the NH3-TPD and CO2-TPD
results showed that the density of acid sites increased with
increasing calcination temperature, while the change trend of
the density of basic sites showed the opposite pattern (Table 1).
Despite the higher number of Ni0 sites and basicity over Ni2Al-
400 and Ni2Al-500, their lower activity could be attributed to
the lower proportion of acidity and basicity. Similarly, the very
close specific rates of Ni2Al-600 and Ni2Al-700 could be
attributed to the similar amount of Ni0 sites and surface acid-
base properties. The catalytic results of the Ni2Al-CT series
catalysts demonstrated that the catalytic activity depended not
only on the metal Ni sites but also the surface acid-base
properties. Given that the dehydrogenation of the alcohol was
the rate-limiting step, we can conclude that the surface acid-
base properties significantly influenced the activity of the
catalyst by affecting the dehydrogenation step.
To confirm the effect of the surface acid-base properties on
the dehydrogenation step, the dehydrogenation of furfuryl
alcohol with styrene as the H2 acceptor was conducted over
Raney Ni and Ni2Al-CT series catalysts. As shown in Figure S1,
Raney Ni gave a furfural (the dehydrogenation product) yield of
only 14.8%. For the Ni2Al-CT series catalysts, a volcano-type
relationship between the yield of furfural and the calcination
temperature of Ni2Al-CT was obtained. The highest yield of
furfural (36.3%) was obtained over Ni2Al-600. Moreover, the
°
alcohol, giving a furfurylamine yield of 43.1% at 180 C in 12 h
(Table 2 entry 4). On the other hand, the calcination temper-
ature also showed a remarkable impact on the catalytic activity.
As shown in Table 2 (Entries 4, 7–10), the yield of furfurylamine
increased rapidly with increasing calcination temperature,
reached a maximum of 43.1% with Ni2Al-600, and then
decreased as the calcination temperature further increased. The
effect of calcination temperature on the yield of furfurylamine
had a trend similar to that of the Ni/Al ratio. The only difference
that should be noted was that the conversion and furfurylamine
yield of Ni2Al-600 and Ni2Al-700 were very close. Additionally,
the specific rates of the catalysts for producing furfurylamine (in
mmol based on 1 g of catalyst per hour) were calculated to
evaluate the catalytic performance. As seen, Ni2Al-600 had a
specific rate as high as 0.73 mmolFAM/(gCat·h), which was superior
to that of conventional Raney Ni and other catalysts with
different Ni/Al molar ratios and calcination temperatures.
From the physiochemical properties of the NixAl-600 series
catalysts (x=0.5–4) in Table 1, the specific rates could be well
correlated with the amount of available Ni0 sites on the
catalysts. The superior performance of Ni2Al-600 could be
attributed to this catalyst possessing the most abundant Ni0
sites. In contrast, Ni0.5Al-600 was almost inactive in the direct
amination of furfuryl alcohol because it possessed the fewest
Ni0 sites. Based on our previous research, during the direct
amination in the absence of H2, furfuryl alcohol and ammonia
compete for adsorption on the same metal active sites.[7b] Thus,
the low activity of Ni0.5Al-600 was ascribed to the occupation of
ammonia at the Ni0 sites, which suppressed the dehydrogen-
°
Ni2Al-CT catalysts calcined at the lowest temperatures (400 C
°
°
and 500 C) and highest temperature (800 C) exhibited a lower
°
dehydrogenation capability than those calcined at 600 C and
°
700 C, which was in line with the results of furfuryl alcohol
amination over these catalysts. From the above experiments,
the synergistic catalysis of metal and acid-base sites in direct
amination was well verified. Compared with the Raney Ni
(almost pure nickel) catalyst, the NiAl hydrotalcite-derived nickel
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Scheme 2. A possible mechanism of synergistic catalysis of Ni0and acid-base sites.
catalysts with rich Ni0 sites and proper acid-base properties
showed better performance in hydrogen transfer amination.
Based on previous studies on the dehydrogenation of
alcohols,[35,39] a possible mechanism of synergistic catalysis of
Ni0 and acid-base sites was proposed (Scheme 2): (1) Reaction
of an alcohol with a Lewis acid (Alδ+)-base (Al-OδÀ ) pair site of
alumina yielded an alkoxide on the Alδ+ site and a proton on
the Al-OδÀ site, (2) CÀ H dissociation of the alkoxide by the low
coordinated Ni0 site to form NiÀ H and aldehyde, (3) protolysis
of NiÀ H by a neighboring proton to hydrogenate the imine
(formed from the condensation of aldehyde with ammonia),
accompanied by regeneration of the Ni0 site, acidic Alδ+ and
basic Al-OδÀ site.
Considering the above observations, Ni2Al-600 was chosen
for the following experiments to obtain further insight into the
direct amination of furfuryl alcohol. First, the reaction temper-
Figure 7. Effect of temperature on the direct amination of furfuryl alcohol.
Reaction conditions: furfuryl alcohol, 0.5 g; THF, 20 mL; catalyst, 0.25 g; NH3
pressure, 0.4 MPa; reaction time, 12 h; stirring speed, 1000 rpm. Abbrevia-
tion: FAM, furfurylamine; THFA, tetrahydrofurfuryl alcohol; Secondary Amine,
bis(furan-2-ylmethyl)amine; Other, other by-products and undetectable
products.
°
°
ature was tested in the range of 140 C to 220 C, and the results
are shown in Figure 7. When the reaction proceeded under
°
milder conditions (140 C), only trace amounts of furfurylamine
were detected, with a furfuryl alcohol conversion of 2.4%. With
increasing reaction temperature, the furfuryl alcohol conversion
and furfurylamine yield continuously increased until a max-
of 0.1 MPa resulted in a product mixture mainly composed of
furfurylamine (12.5%), secondary amine (bis(furan-2-ylmethyl)
amine, 6.2%), and Schiff base (N-(2-furanylmethylene)- 2-
furanmethanamine, 4.3%), accompanied by a furfuryl alcohol
conversion of only 28.3%. Additionally, increasing the ammonia
pressure had a positive effect on the furfuryl alcohol conversion
and furfurylamine yield. When ammonia was pressurized to
0.2 MPa, the conversion of furfuryl alcohol dramatically in-
creased to 67.3%, and a 54.8% yield of furfurylamine was
obtained. Interestingly, the furfuryl alcohol conversion and
furfurylamine yield did not continuously increase with increas-
ing ammonia pressure under higher pressure. As mentioned
above, due to the competitive adsorption of substrate mole-
cules and ammonia, a variation in the ammonia pressure greatly
impacted the activation of the alcohol. Therefore, the decrease
in furfuryl alcohol conversion and furfurylamine yield with
excessively high ammonia pressure was ascribed to the
occupation of the metal sites by ammonia, which suppressed
°
imum furfurylamine yield of 43.1% was obtained at 180 C. This
result indicated that direct amination of furfuryl alcohol could
be better activated at higher temperatures. However, further
°
°
increasing the reaction temperature to 200 C and 220 C
resulted in a decreased furfurylamine yield due to the formation
of secondary amine (bis(furan-2-ylmethyl)amine) and much
more undetectable products, decreasing the carbon balance.
°
Notably, the yield of secondary amine at 220 C was lower than
°
that at 200 C, which was caused by the decomposition of
secondary amine into N-methyl-2-furanmethanamine. Accord-
°
ingly, the reaction temperature was maintained at 180 C for
the following reaction.
The effect of ammonia pressure on the direct amination of
°
furfuryl alcohol over Ni2Al-600 was investigated at 180 C, and
the results are shown in Figure 8. As seen in Figure 8(a), the
ammonia pressure significantly influenced the furfuryl alcohol
conversion and product distribution. A low ammonia pressure
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Figure 8. Direct amination of furfuryl alcohol under different ammonia pressure: (a) Effect of ammonia pressure on the direct amination of furfuryl alcohol;
°
Reaction conditions: furfuryl alcohol, 0.5 g; THF, 20 mL; catalyst, 0.25 g; reaction temperature, 180 C; reaction time, 12 h; stirring speed, 1000 rpm. (b) Reaction
kinetics of the direct amination of furfuryl alcohol under 0.2 MPa NH3, (c) Reaction kinetics of the direct amination of furfuryl alcohol under 0.3 MPa NH3, and
(d) Reaction kinetics of the direct amination of furfuryl alcohol under 0.4 MPa NH3. Reaction conditions: furfuryl alcohol, 0.5 g; THF, 20 mL; catalyst, 0.25 g;
°
reaction temperature, 180 C; stirring speed, 1000 rpm. Abbreviation: FAM, furfurylamine; THFA, tetrahydrofurfuryl alcohol; Secondary Amine, bis(furan-2-
ylmethyl)amine; Schiff base, N-(2-furanylmethylene)- 2-furanmethanamine; Other, other by-products and undetectable products.
the activity of the metal catalyst. However, it should be noted
that the formation of secondary amine was remarkably sup-
pressed with increasing ammonia pressure. This finding was
more clearly clarified by the reaction kinetics under different
ammonia pressures. As seen in Figure8(b), (c), and (d), although
the reaction rates with 0.2 MPa NH3 (b) and 0.3 MPa NH3 (c)
were faster than that with 0.4 MPa NH3 (d), the yield of
secondary amine was also increased, with the reaction proceed-
ing under relatively lower pressure, consequently decreasing
the selectivity to furfurylamine. As a result, the highest yield of
furfurylamine (84.1%) was obtained with complete conversion
of furfuryl alcohol after reaction for 36 h under 0.4 MPa NH3.
Given the effectiveness of hydrotalcite-derived nickel cata-
lysts in the direct amination of furfuryl alcohol, another
important biomass-derived platform chemical, 5-(aminomethyl)-
2-furanmethanol, was also subjected to this catalytic system for
the production of 2,5-bis(aminomethyl)furan. As shown in
Table 3, the temperature was also found to play a key role in
the transformation of 5-(aminomethyl)-2-furanmethanol into
2,5-bis(aminomethyl)furan over Ni2Al-600 (Entries 1, 2, 3, 6, and
7 in Table 3). Compared with that of furfuryl alcohol, the direct
amination of 5-(aminomethyl)-2-furanmethanol appeared eas-
ier, as it needed a lower temperature to activate the hydroxyl
group. For instance, furfuryl alcohol was almost unreactive at
°
the reaction temperature of 140 C, while a conversion of 37.0%
was observed in the direct amination of 5-(aminomethyl)-2-
furanmethanol.
A
satisfactory 70.5% yield of 2,5-bis
(aminomethyl)furan was obtained in the presence of 0.4 MPa
°
NH3 at 160 C in 18 h. However, 5-(aminomethyl)-2-furanmetha-
nol was more likely to undergo hydrogenolysis and polymer-
ization during the amination process, which resulted in a lower
yield of the corresponding amination product.[21]
Finally, the reusability of Ni2Al-600 for the direct amination
of furfuryl alcohol was investigated. After the reaction, the
catalyst was centrifuged, washed with THF, and returned to the
reactor for reuse under the previous reaction conditions. As
shown in Figure 9, the conversion of furfuryl alcohol and yield
of furfurylamine sharply dropped with increasing catalyst
recycling times. The XRD, TEM and XPS characterizations
showed that a new Ni3N crystalline phase was formed in the
spent Ni2Al-600, which resulted in a serious decline in the
abundance of Ni0 sites and ultimately deactivated the catalyst.
This finding was in consistent with our previous observation on
a Raney Ni catalyst.[7b] Moreover, the TG results (Figure S2)
indicated the formation of carbon deposits, which also resulted
in the catalyst deactivation.[3a]
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Table 3. Direct amination of 5-(aminomethyl)-2-furanmethanol into 2,5-bis(aminomethyl)furan over Ni2Al-600
Entry
Temperature
Time
[h]
Conversion
[%]
Yield [%]
°
[ C]
Others[a]
1
2
3
4
5
6
7
120
140
160
160
160
180
200
12
12
12
18
24
12
12
22.8
37.0
65.4
83.9
94.6
70.1
81.4
–
–
–
3.4
5.6
8.9
3.0
15.2
–
–
–
–
–
1.5
4.6
–
–
–
–
–
1.9
1.9
18.7
28.9
54.3
70.5
65.7
42.5
25.4
4.1
7.0
6.2
6.2
15.3
19.2
23.3
1.1
1.5
1.6
4.7
2.0
11.0
Reaction conditions: 5-(aminomethyl)-2-furanmethanol, 0.5 g; THF, 20 mL; catalyst, 0.25 g; stirring speed, 1000 rpm. [a] unidentified compounds and
undetectable products.
and surface acid-base properties were suggested to play
important roles in the direct amination of alcohol, and an
84.1% yield of furfurylamine was obtained over Ni2Al-600 under
°
the reaction conditions of 180 C and 0.4 MPa NH3 in 36 h. In
addition, a satisfactory 70.5% yield of 2,5-bis(aminomethyl)
furan was obtained from 5-(aminomethyl)-2-furanmethanol
°
over Ni2Al-600 under the reaction conditions of 160 C and
0.4 MPa NH3 in 18 h. Unfortunately, the NiAl hydrotalcite-
derived nickel catalysts still suffered from serious deactivation
caused by ammonia corrosion. Further studies to explore a
more active and stable catalyst for the hydrogen transfer
amination process are under-way.
Figure 9. Reusability test of Ni2Al-600 for the direct amination of furfuryl
alcohol. Reaction conditions: furfuryl alcohol, 0.5 g; THF, 20 mL; catalyst,
Experimental Section
°
0.25 g; NH3 pressure, 0.4 MPa; reaction temperature, 180 C; reaction time,
12 h; stirring speed, 1000 rpm. Abbreviation: FAM, furfurylamine; THFA,
tetrahydrofurfuryl alcohol; Other, undetectable products.
Materials
Furfuryl alcohol (FA, 98%) and furfurylamine (FAM, 99%) were
purchased from Shanghai Macklin Biochemical Co., Ltd., and used
It has been reported that Ni catalyst can be regenerated in a
hydrogen containing atmosphere. Accordingly, the spent cata-
as received. 2,5-Bis(aminomethyl)furan (BAMF, 95%) was procured
from Shanghai Shaoyuan Co., Ltd. Ni(NO3)2 ·6H2O, Al(NO3)3 ·9H2O,
NaOH and Na2CO3 were purchased from Shanghai Aladdin Industri-
al Co. Ltd. 5-(Aminomethyl)-2-furanmethanol was synthesized in
our laboratory by reductive amination of 5-hydroxymethylfurfural
with ammonia and hydrogen according to our previous work,[41]
with a purity of more than 98%. Ammonia and nitrogen gases were
provided by Dongguan Dongke Special Gas Co., Ltd., with purity
higher than 99.99%. Other chemicals were analytical reagents and
purchased from Sinopharm Reagent Co., Ltd.
[10–11,40]
°
lyst was regenerated by reduction in H2 at 500 C.
However, the initial catalytic activity was not completely
recovered (Table S2), indicating that there are other factors that
deactivated the catalyst. This probably caused by the Ni
sintering as displayed in the TEM results (Figure 2(c) and
Figure S1(c), (d)).
Conclusion
Catalyst preparation
Five samples of nickel-based hydrotalcite like precursors with Ni/Al
molar ratios of 1:2, 1:1, 2:1, 3:1, and 4:1 were synthesized by a
constant-pH coprecipitation method employing NaOH/Na2CO3 as
the precipitating agent. An aqueous solution containing Ni-
(NO3)2 ·6H2O and Al(NO3)3 ·9H2O and a solution containing Na2CO3
and NaOH were simultaneously added to a flask under vigorous
stirring and constant temperature, the pH value was kept within
10�0.1 by adjusting the dropping rate of the two solutions. The
We have demonstrated an efficient catalytic system for the
direct amination of biomass-based furfuryl alcohol and 5-
(aminomethyl)-2-furanmethanol with ammonia via the hydro-
gen borrowing mechanism over a NiAl hydrotalcite-derived
nickel catalysts. The reduced NixAl-CT catalysts were found to
be more active than the conventional Raney Ni catalyst in the
direct amination of furfuryl alcohol, and a maximum activity
°
precipitate was kept in suspension at 70 C for 12 h. Then the
0
°
was obtained over Ni2Al-600 reduced at 500 C. The Ni sites
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precipitate was filtered, washed with distilled water and dried at
Keywords: Direct amination
(aminomethyl)-2-furanmethanol
Catalysts
·
·
Furfuryl Alcohol
·
5-
°
100 C for 12 h. A green material was obtained and denoted as
NixAl-HT, where x was the Ni/Al molar ratio. The precursor was
Primary Amine
· Nickel
°
°
°
°
calcined at different temperatures (400 C, 500 C, 600 C, 700 C,
°
and 800 C) in air. The calcined products were denoted as NixAl-CT,
where CT represents the calcination temperature. Prior to the tests,
the calcined catalysts were reduced in 10% H2/Ar flow at 500 C for
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Ai Ms
As M
_
Yield ðiÞ ðmol %Þ ¼ fi s
(1)
where Ai and As are the peak areas of the component and n-
dodecane, respectively; Ms and M are the molar concentrations of
n-dodecane and furfuryl alcohol added in the reactor, respectively;
and fi refers to the correction factor of the component, which was
determined using a self-prepared solution of known concentration.
Acknowledgements
This work was supported by the funding of Key Projects of Social
Science and Technology Development of Dongguan
(20185071401607 and 2019507140211), the Scientific Research
Youth Team of Dongguan University of Technology
(TDQN2019006), and the Research start-up funds of DGUT
(196100040004).
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