Heterogeneous Ni catalyst for direct synthesis of primary amines from alcohols and ammonia

Article abstract of DOI:10.1021/cs3007473

This paper reports the synthesis of primary amines from alcohols and NH₃ by an Al₂O₃-supported Ni nanoparticle catalyst as the first example of heterogeneous and noble-metal-free catalytic system for this reaction without additional hydrogen sources under relatively mild conditions. Various aliphatic alcohols are tolerated, and turnover numbers were higher than those of Ru-based homogeneous catalysts. The catalyst was recoverable and was reused. The effects of the Ni oxidation states and the acid-base nature of support oxides on the catalytic activity are studied. It is clarified that the surface metallic Ni sites are the catalytically active species, and the copresence of acidic and basic sites on the support surface is also indispensable for this catalytic system.

Full text of DOI:10.1021/cs3007473

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Article
Heterogeneous Ni catalyst for Direct Synthesis
of Primary Amines from Alcohols and Ammonia
Ken-Ichi Shimizu, Kenichi Kon, Wataru Onodera, Hiroshi Yamazaki, and Junko N Kondo
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs3007473 • Publication Date (Web): 14 Dec 2012
Downloaded from http://pubs.acs.org on December 15, 2012
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Heterogeneous Ni catalyst for Direct Synthesis of Primary Amines from
Alcohols and Ammonia
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AUTHOR NAMES
Kenꢀichi Shimizu,*a,b Kenichi Kon,a Wataru Onodera,a Hiroshi Yamazaki,c and Junko N. Kondoc
AUTHOR ADDRESS
a Catalysis Research Center, Hokkaido University, Nꢀ21, Wꢀ10, Sapporo 001ꢀ0021, Japan
b
Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura, Kyoto 615ꢀ8520,
Japan
c Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriꢀku, Yokohama, 226ꢀ
8503, Japan
*Corresponding author
Kenꢀichi Shimizu
Catalysis Research Center, Hokkaido University, Nꢀ21, Wꢀ10, Sapporo 001ꢀ0021, Japan
Eꢀmail: kshimizu@cat.hokudai.ac.jp, Fax: +81ꢀ11ꢀ706ꢀ9163
ABSTRACT
This paper reports the synthesis of primary amines from alcohols and NH3 by Al2O3ꢀsupported Ni nanoparticle
catalyst as the first example of heterogeneous and nobleꢀmetalꢀfree catalytic system for this reaction without adꢀ
ditional hydrogen sources under relatively mild conditions. Various aliphatic alcohols are tolerated, and turnover
numbers (TON) were higher than those of Ruꢀbased homogeneous catalysts. The catalyst was recoverable and
was reused. Effects of the Ni oxidation states and the acidꢀbase nature of support oxides on the catalytic activity
are studied. It is clarified that the surface metallic Ni sites are the catalytically active species and the coꢀpresence
of acidic and basic sites on the support surface is also indispensable for this catalytic system.
KEYWORDS: alcohols, amines, ammonia, catalysis, nickel
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1. INTRODUCTION
Amines are important intermediates in the bulk and fine chemical industries for production of polymers, dyes,
pharmaceuticals, agrochemicals, surfactants and biologically active compounds. Among amines, primary amines
are the most useful intermediates for further derivatization reactions. Aliphatic primary amines can be syntheꢀ
sized by the reductive amination of the corresponding carbonyl compounds,1ꢀ3 but their selective synthesis is
challenging due to their high reactivity. Although heterogeneous catalysts are used for reductive aminations of
simple alcohols under H2,2ꢀ11 they suffer from drawbacks such as limited substrate scope, low selectivity for priꢀ
mary amines and needs of high temperature (>200 °C), high H2 pressure, and high pressure of NH3 (such as suꢀ
percritical NH37ꢀ9). Recently, Mizuno and coꢀworkers developed Ru12 and Cu13ꢀbased heterogeneous catalysts for
the direct synthesis of secondary and tertiary amines from alcohols and aqueous ammonia (or urea), but selective
formation of primary amines were reported. As for the homogeneous catalysts, iridiumꢀcatalyzed multialkylation
of aqueous ammonia with alcohols was reported for the synthesis of secondary and tertiary amines,14,15 but selecꢀ
tive synthesis of primary amines remains as a challenging task. Gunanathan and Milstein16 reported the first exꢀ
ample of the selective synthesis of primary amines from primary alcohols and NH3 using Ru PNP pincer complex,
thought the system was ineffective for the amination of secondary alcohols.17 Recently, Vogt et al.17 and Beller et
al.18,19 independently discovered the selective amination of secondary alcohols with NH3 to give primary amines.
All of these examples are driven by Ruꢀbased organometallic catalysis16ꢀ19 via a borrowingꢀhydrogen20 (or hydroꢀ
genꢀautotransfer21) mechanism; the carbonyl compound formed by the initial dehydrogenation of the alcohol reꢀ
acts with NH3 to form the imine which is reduced to the amine. For a practical application, these homogeneous
catalysts have problems such as difficulty in the catalyst recovery and reuse of catalysts and necessity of noble
metals. The development of nonꢀnobleꢀmetalꢀbased heterogeneous catalyst for the direct amination is an imꢀ
portant research target. To our knowledge there are no nobleꢀmetalsꢀfree and ligandꢀfree catalytic systems that
are able to aminate various alcohols with NH3 to primary amines in the absence of additional hydrogen sources
(such as H2). We report herein that aluminaꢀsupported nickel metal nanoparticle catalyst, readily prepared from
inexpensive commercial materials, acts as a versatile heterogeneous catalyst for the selective synthesis of primaꢀ
ry amines directly from alcohols and ammonia under relatively mild conditions without any additives. In order to
clarify the controlling factors of the catalytic system, the effects of Ni oxidation states and acidꢀbase character of
support materials on the activity are also discussed based on the results of catalyst characterization.
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2. EXPERIMENTAL SECTION
2.1. General. Commercially available organic and inorganic compounds (from Tokyo Chemical Industry,
Wako Pure Chemical Industries, Kishida Chemical, or Mitsuwa Chemicals) were used without further purificaꢀ
tion. The GC (Shimadzu GCꢀ14B) and GCMS (Shimadzu GCMSꢀQP2010) analyses were carried out with Ultra
ALLOY capillary column UA+ꢀ5 (Frontier Laboratories Ltd.) or Rtxꢀ65 capillary column (Shimadzu) using niꢀ
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trogen as the carrier gas. HꢀNMR spectra were recorded in CDCl3 with TMS as an internal standard at ambient
temperature on JEOL JNMꢀECX 400 operating at 400 MHz.
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2.2. Catalyst preparation. γꢀAl2O3 (with surface area of 124 m2 gꢀ1), θꢀAl2O3 (112 m2 gꢀ1) and αꢀAl2O3 (17 m2
gꢀ1) were prepared by calcination of γꢀAlOOH (Catapal B Alumina purchased from Sasol) for 3 h at 900, 1000,
and 1200 °C, respectively. MgO (JRCꢀMGOꢀ3, 19 m2 gꢀ1), TiO2 (JRCꢀTIO4, 50 m2 gꢀ1), CeO2 (JRCꢀCEO3,
81.4 m2 gꢀ1) was supplied from Catalysis Society of Japan. Hydroxides of Zr, Sn, Y, and La were prepared by
hydrolysis of zirconium oxynitrate 2ꢀhydrate, SnCl4ꢁ6H2O, or metal nitrates in distilled water by gradually adding
an aqueous NH4OH solution (1.0 mol dmꢀ3), followed by filtration of precipitate, washing with distilled water
three times, drying at 100 °C for 12 h. Ca(OH)2 and Nb2O5ꢁnH2O (supplied by CBMM) were commercially supꢀ
plied. CaO, Y2O3, La2O3, ZrO2, Nb2O5 and SnO2 were prepared by calcination of these hydroxides at 500 °C for
3 h.
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NiO/θꢀAl2O3 (Ni loading = 10 wt%) was prepared by an impregnation method; a mixture of θꢀAl2O3 (9.0 g)
and an aqueous solution of Ni(NO3)2ꢀ6H2O (0.17 M, 100 cm3) was evaporated at 50 °C, followed by drying at 90
°C for 12 h and calcination in air at 300 °C for 1 h. Before each catalytic experiment, Ni/θꢀAl2O3 was prepared
by in situ preꢀreduction of NiO/θꢀAl2O3 in a glass (pyrex or quartz) tube under a flow of H2 (20 cm3 minꢀ1) at a
reduction temperature (TH2) of 500 °C for 0.5 h. Ni/θꢀAl2O3 with different Ni loading (5 and 20 wt%), 5 wt% Ni
loaded on various supports (αꢀAl2O3, La2O3, Y2O3, CeO2, ZrO2, TiO2, Nb2O5, SnO2) were prepared by the imꢀ
pregnation method in a same manner as the preparation of Ni/θꢀAl2O3 (Ni loading = 10 wt%). CaO and MgOꢀ
supported Ni (5 wt%) were prepared by the impregnation method, followed by drying at 90 °C for 12 h, and by
in situ preꢀreduction of the precursor under H2 at 500 °C. γꢀAl2O3–supported metal catalysts, M/γꢀAl2O3 (M = Ni,
Co, Cu, Ru, Pd, Rh, Ag, Ir, Pt) with metal loading of 5 wt% were prepared by impregnation method in a similar
manner as the preparation of Ni/θꢀAl2O3 using aqueous solution of metal nitrates (for Ni, Co, Cu), RuCl3, or
aqueous HNO3 solution of Rh(NO3)3, Pd(NO3)2, or Pt(NH3)2(NO3)2. Au/γꢀAl2O3 (Au= 1 wt%) was prepared by
the depositionꢀprecipitation method with HAuCl4. Raney Ni (B113W, Ni >90%) was supplied from Evonik Inꢀ
dustries.
2.3. Typical procedures of catalytic reactions. Typically, 10 wt% Ni/θꢀAl2O3 (Ni : alcohol = 1 : 100) preꢀ
reduced at 500 °C was used as a standard catalyst. After the preꢀreduction, the catalyst in the closed glass tube
sealed with a septum inlet was cooled to room temperature under H2 atmosphere. The mixture of oꢀxylene (4.0
g), alcohol (3.0 mmol), and nꢀdodecane (0.5 mmol) was injected to the preꢀreduced catalyst inside the glass tube
through the septum inlet. Then, the septum was removed under air, and a magnetic stirrer was put in the tube,
followed by inserting the tube inside stainless autoclave with a dead space of 33 cm3. Soon after being sealed, the
reactor was flushed with NH3 from a high pressure gas cylinder and charged with 0.4 MPa NH3 at room temperaꢀ
ture. Note that the secondary pressure was reduced to 0.4 MPa using a regulator. The amount of NH3 present in
the reactor before heating was 6.7 mmol (2.2 equiv. with respect to the alcohol). Then, the reactor was heated
typically at 160 ˚C under stirring (150 rpm). Conversion and yields of products were determined by GC using nꢀ
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dodecane as an internal standard. The products were identified by GCꢀMS equipped with the same column as GC
and by comparison with commercially pure products.
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Temperature programmed reduction under H2 (H2ꢀTPR) was carried out with BELCAT (BELL Japan Inc.).
The sample, NiO/θꢀAl2O3 (Ni loading = 10 wt%, 20 mg) was mounted in a quartz tube, and the sample was heatꢀ
ed with a temperature rampꢀrate of 5 ˚C minꢀ1 in a flow of 5% H2/Ar (20 cm3 minꢀ1). The effluent gas was passed
through a trap containing MS4Å to remove water, then through the thermal conductivity detector. The amount of
H2 consumed during the experiment was detected by a thermal conductivity detector. Temperature programmed
desorption (TPD) experiments of CO2 and NH3 were carried out using BELCAT. Prior to each experiment, cataꢀ
lyst (0.1 g) was heated in a flow of He (20 cm3 minꢀ1) at 500°C for 0.5 h, followed by cooling to 40°C under He
flow. Then, the catalyst was exposed to a flow of 100% CO2 or 5% NH3/He for 10 min. After purged in He for
0.5 h, the catalyst was heated linearly at 10°C minꢀ1 to 900 °C in a flow of He, and CO2(m/e = 44) and NH3(m/e =
16 and 17) in outlet gas were analyzed by the mass spectrometer (BELMASS). The BET surface area was obꢀ
tained by measuring N2 adsorption at ꢀ196 oC by BELCAT.
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Transmission electron microscopy (TEM) measurements were carried out by using a JEOL JEMꢀ2100F TEM
operated at 200 kV. Ni Kꢀedge Xꢀray absorption fine structure (EXAFS) was measured in transmission mode at
the BL01B1 in the SPringꢀ8 (Proposal No. 2011B1137). The storage ring was operated at 8 GeV. A Si(111) sinꢀ
gle crystal was used to obtain a monochromatic Xꢀray beam. The catalyst preꢀreduced in a flow of 100% H2 (20
cm3 minꢀ1) for 0.5 h at 500 °C was cooled to room temperature in the flow of H2 and was sealed in cells made of
polyethylene under N2, and then the EXAFS spectrum was taken at room temperature. The EXAFS analysis was
performed using the REX version 2.5 program (RIGAKU). The Fourier transformation of the k3ꢀweighted
EXAFS oscillation from k space to R space was performed over the range 3.0ꢀ16.0 Åꢀ1 to obtain a radial distribuꢀ
tion function. The inversely Fourier filtered data were analyzed with a usual curve fitting method in the k range
of 3.0ꢀ16.0 Åꢀ1. The parameters for the Ni–O and Ni–Ni shells were provided by the FEFF6. The number of free
parameters for curve fitting can be estimated as Pfree = 2ꢂkꢂR/π ≈ 13, indicating that we can model the EXAFS
data with three shells.
IR (infrared) spectra were obtained by use of a Jasco Fourier transform infrared (FTꢀIR) 4100 spectrometer
equipped with an MCT detector at a 4 cmꢀ1 resolution. 64 scans were collected to obtain each spectrum. The real
temperature of the sample was estimated from a calibration curve of temperatures measured inside and outside
the IR cell. A catalyst disc was preꢀreduced under H2 (20 kPa) at 500 oC for 0.5 h, followed by evacuation at the
same temperature for 0.5 h. IR spectra of clean samples before CO adsorption experiments were measured under
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evacuation at ꢀ180 C and were used as background spectra. Subtracted IR spectra of adsorbed CO were measꢀ
ured after the sample was exposed to CO (500 Pa) at ꢀ180 oC.
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3. RESULTS AND DISCUSSION
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As summarized in Table 1, we first screened the effect of various catalyst parameters on the catalytic activity
for the amination of 2ꢀoctanol 1a with NH3 (P = 0.4 MPa, 6.7 mmol) to the corresponding primary amine 2a and
byꢀproducts (secondary amine 3a and 2ꢀoctanone 4a) under the same conditions (Ni = 1 mol% with respect to
alcohol, T = 160 °C, t = 4 h). Note that the metalꢀloaded catalysts (M/M’Ox) were prepared by H2ꢀreduction (500
°C) of metal oxideꢀloaded precursors (MOy/M’Ox), and then the catalytic tests were carried out without exposing
the catalyst to air. First, various transition metal (5 wt%)ꢀloaded γꢀAl2O3 catalysts were tested (entries 4ꢀ14).
Among the firstꢀrow transition metals (entries 4ꢀ6), only Ni/γꢀAl2O3 showed high yield of 2a (81%), while Co
and Cu catalysts showed low activity. Noble metal (Pt, Ir, Re, Au, Ru, Rh, Pd, Ag)ꢀloaded γꢀAl2O3 catalysts (enꢀ
tries 7ꢀ14) showed lower yield of 2a (0ꢀ46%) than Ni/γꢀAl2O3. The Cuꢀ and Agꢀloaded catalysts preꢀreduced at
lower temperature (300 °C) were also tested, but the yield of 2a was lower than _%. Use of a large amount (10
mol%) of conventional Ni catalysts (Ni powder and Raney Ni, entries 16,17) resulted in lower yield than Ni/γꢀ
Al2O3 (1 mol%).
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Next, we tested the reaction with 1mol% of various Ni (5 wt%)ꢀloaded catalysts (Ni : alcohol = 1 : 100). Aluꢀ
mina with different crystal phase was tested as a support of Ni (entries 2ꢀ4). The alumina with spinel structure (θꢀ
Al2O3 and γꢀAl2O3) gave similar yields (80ꢀ81 %), while that with corundum structure (αꢀAl2O3) was ineffective.
To study the support effects on the activity of Ni catalysts, purely basic (MgO, CaO), amphoteric (γꢀAl2O3, Y2O3,
La2O3, CeO2, ZrO2), and relatively acidic (TiO2, Nb2O5, SnO2) supports were selected according to well known
classifications of acidꢀbase character of metal oxides.22ꢀ24 Amphoteric supports (entries 4, 20ꢀ23) gave good to
moderate yields of 2a (42ꢀ81%), while purely basic supports (entries 18,19) and relatively acidic supports (enꢀ
tries 24ꢀ26) showed no activity. For these catalysts, the initial rate of 2a formation was measured under the conꢀ
ditions where conversion was below 30%, and the rate was converted to the turnover frequency (TOF) based on
the total number of Ni atom. Figure 1 plots TOF as a function of the electronegativity of the support metal oxide,
which is generally used as a parameter of acidity of metal oxides.25ꢀ28 The electronegativity of metal oxide is calꢀ
culated according to the concept of Sanderson29 based on the electronegativity of the element.30 Among the supꢀ
ports with basic to amphoteric nature (MgO∼γꢀAl2O3), the activity increases with the electronegativity of the
support oxide. These results suggest that both acidic and basic sites in the support are necessary for this reaction.
As listed in Table S1 (in the supporting information), densities of surface acidic and basic site on each catalyst
was estimated based on the surface area of the catalyst and the number of desorbed NH3 and CO2 during NH3ꢀ
and CO2ꢀTPD experiments. In Figure 1, the densities of acidic and basic site as experimental indexes of the acidꢀ
basicity of the catalyst are plotted versus the support electronegativity. For the supports with basic to amphoteric
nature (CaO∼γꢀAl2O3), the increase in the electronegativity generally results in a decrease in the density of the
basic sites and an increases in the acid site density. The supports with the electronegativity higher than 2.5
(TiO2∼SnO2) show negligible value of basic site density. Comparing the catalytic and characterization results in
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Figure 1, we can conclude that the coꢀpresence of acidic and basic sites on the surface of support oxides is indisꢀ
pensable for this reaction.
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Ni/θꢀAl2O3 with various Ni loadings (1ꢀ20 wt%) were also tested (Figure S1 in the supporting information),
and the catalyst with 5 or 10 wt% Ni were found to be optimal. From the above results, θꢀ or γꢀAl2O3ꢀsupported
Ni (5 or 10 wt%) catalysts were found to be the optimal catalysts.
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To discuss the relationship between the structure of Ni species and catalytic activity, we carried out various
spectroscopic characterization of the standard catalyst, Ni/θꢀAl2O3 (10 wt% Ni). Transmission electron microsꢀ
copy (TEM) analysis of Ni/θꢀAl2O3 (10 wt% Ni) showed spherical or semispherical Ni particles with the average
particle sizes of 6.5 ± 2.2 nm (Figure 2). Next, we studied the effect of oxidation states of Ni on the activity. The
catalytic results showed that NiO/θꢀAl2O3 as well as the support (θꢀAl2O3) showed no activity, indicating that the
support and NiO were completely inert. NiO/θꢀAl2O3 was reduced in a flow of H2 at various reduction temperaꢀ
tures (TH2 = 300∼800 °C), and their TOF for the amination of 1a was measured. The result showed that TOF inꢀ
creased with TH2 upto 500 °C and did not markedly change with further increase in TH2 (Figure S2). In Figure 3,
TOF is plotted as a function of the fraction of metallic Ni0 in each catalysts estimated by the temperature proꢀ
grammed reduction under H2 (H2ꢀTPR, Figure S3). It is clear that the activity increases with the fraction of meꢀ
tallic Ni0, which indicates that the metallic Ni nanoparticles are the active species. The structure of Ni species in
the standard catalyst, Ni/θꢀAl2O3 (TH2 = 500 °C), was confirmed by EXAFS (Figure S4). The curveꢀfitting of the
EXAFS of Ni/θꢀAl2O3 gave a NiꢀO shells and two NiꢀNi shells listed in Table S2. By comparison with the crysꢀ
tallographic data of NiO and Ni metal, these are assigned as follows. The NiꢀNi shell at a distance (R) of 2.48 Å
with coordination number (N) of 5.2 indicates the presence of metallic Ni. A relatively low coordination numbers
of the NiꢀO shell at 2.00 Å (N = 1.8) and the NiꢀNi shell at 2.94 Å (N = 1.3) indicates the coꢀpresence of NiO as
a minor Ni species. This result is consistent with the Ni0/NiO ratio of 0.75/0.25 (Figure 3) for the same catalyst
estimated by H2ꢀTPR.
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The catalyst named Ni/θꢀAl2O3–air (Table 1, entry 32) was prepared by exposing the asꢀreduced Ni/θꢀAl2O3
catalyst to the ambient conditions for 0.5 h. This catalyst showed no activity. This suggests that the metallic Ni0
species on the surface of Ni nanoparticles are the active species and reꢀoxidation of them by O2 under ambient
conditions results in the catalyst deactivation. This hypothesis is confirmed by the following results. The EXAFS
result (Table S2) showed that airꢀexposure of the Ni/θꢀAl2O3 catalyst resulted in an increase in the NiꢀO coordiꢀ
nation number from 1.8 to 2.3, which indicates that the metallic Ni was partially reꢀoxidized in air. The oxidation
states of surface Ni species can be studies by CO adsorption IR study at ꢀ180 ˚C. IR spectra of adsorbed CO speꢀ
cies are shown in Figure 4. The bands in the 2150ꢀ2210 cmꢀ1 region are usually attributed to COꢀAl3+ and/or COꢀ
Ni2+ species,31ꢀ33 as is confirmed by the experiments on pure θꢀAl2O3 and unreduced precursor (NiO/θꢀAl2O3).
The spectrum for the NiO/θꢀAl2O3 showed a weak shoulder at 2106 cmꢀ1 due to CO adsorbed on Ni+ species
(COꢀNi+).31ꢀ33 The spectrum of the reduced Ni/θꢀAl2O3 catalyst exhibits bands around 1940 cmꢀ1 due to the
bridged CO on Ni0 site and a band at 2042 cmꢀ1 due to the linear CO on Ni0 site,31ꢀ33 indicating the presence of
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metallic Ni0 species on the surface of Ni/θꢀAl2O3. The strong band due to COꢀNi+ species (2106 cmꢀ1) is also obꢀ
served. Upon exposure to O2 (20 kPa) at room temperature for 0.5 h, the bands due to CO adsorbed on the Ni0
sites completely disappeared, while the shoulder band due to COꢀNi+ (2106 cmꢀ1) remained. Combined with the
result of catalytic test, the result indicates that the surface metallic Ni0 sites are the catalytically active species.
Under ambient conditions, the surface metallic Ni0 species are completely converted to the inactive Ni2+ species.
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Next, we examined the catalytic properties of Ni/θꢀAl2O3 (10 wt% Ni) under the conditions shown in Table 2.
The amination of 2ꢀoctanol by Ni/Al2O3 (1 mol%) for 20 h resulted in 88% yield of the corresponding primary
amine with a low yield (5%) of the secondary amine. The reaction was completely terminated by a removal of
the catalyst from the reaction mixture after 32% conversion of 1 (t = 1 h); further heating of the filtrate for 19 h
under NH3 (0.4 MPa) at 160 °C did not increase the yield. ICP analysis of the filtrate confirmed that the content
of Ni in the solution was below the detection limit. These results confirm that the reaction is attributed to the hetꢀ
erogeneous catalysis of Ni/θꢀAl2O3. After the reaction, the catalyst was easily separated from the reaction mixꢀ
ture by a centrifugation. The filtered catalyst was washed with acetone, followed by drying in air at 90 ˚C for 12
h, and by reducing in H2 at 600 ˚C for 0.5 h. As shown in Table 2 (entries 2,3), the recovered catalyst was reused
at least two times.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The scope of the Ni/θꢀAl2O3ꢀcatalyzed amination of various alcohols with NH3 at 140ꢀ160 °C is shown in Taꢀ
ble 2. In the presence of Ni/θꢀAl2O3 (Ni : alcohol = 1 : 100), various cyclic aliphatic secondary alcohols were
selectively converted to the corresponding primary amines with good to high yield (77ꢀ96%), while the yields of
the secondary amines were below 5%. It should be noted that primary amines were selectively produced even at
high conversion level. The amination of 2ꢀAdamantanol resulted in 99% conversion of the alcohol. After the reꢀ
action, the catalyst was separated from the mixture by a centrifugation and the solvent was removed under vacuꢀ
um. Then, the product (2ꢀadamantylamine) was isolated by column chromatography on silica gel, followed by
removal of solvent under vacuum. The isolated yield was 90%, corresponding to turnover number (TON) of 90.
This value is larger than those of the stateꢀofꢀtheꢀart homogeneous Ru catalysts for the amination of secondary
alcohols: 46 for Ru3(CO)12]/CataCXiumPCy,18 16 for [Ru(CO)ClH(PPh3)3]/Xantphos.19 Note that our heterogeꢀ
neous system requires an order of magnitude lower amount of ammonia (2.23 equiv.) with respect to alcohols
than the Ru catalysts (35ꢀ59 equiv.).5 To the best of our knowledge, our result represents the first example of hetꢀ
erogeneously catalyzed selective synthesis of primary amines from various alcohols and NH3 without any addiꢀ
tives such as H2. For the amination of primary alcohols (benzylalcohol and a linear aliphatic alcohol) with 5
mol% of the catalyst, moderate yields of the corresponding primary amines (70, 78%) were obtained with relaꢀ
tively high yields of secondary amine (11, 7%).
As previously postulated, the present reaction could proceed through the hydrogenꢀborrowing pathway.16ꢀ21
Thus, the initial stage of the reaction would be the oxidation of alcohol to a carbonyl intermediate accompanied
by a transitory generation of Ni hydride species. A time course of the amination of cyclododecanol with NH3 by
Ni/θꢀAl2O3 is shown in Figure 5. A typical timeꢀconversion profile of the consecutive reaction was observed; the
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carbonylꢀintermediate 4b, formed at an initial period, was gradually consumed, and the yield of the primary
amine 2b increased with time. Based on the above results, a plausible catalytic cycle is given in Scheme 1. The
reaction begins with the dehydrogenation of alcohol by Ni sites to a carbonyl compound, which reacts with amꢀ
monia to give an imine. Finally, hydrogen transfer from NiꢀH species to the imine gives the primary amine. We
tentatively assume that basic sites of the alumina34,35 promote the dehydrogenation step possibly by abstraction of
proton from the alcohol, while acidic nature of alumina might be relevant to the hydrogen transfer step.
1
2
3
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5
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8
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60
4. CONCLUSIONS
In conclusion, we have reported the selective synthesis of primary amines from alcohols and NH3 by Al2O3ꢀ
supported Ni nanoparticle catalyst as the first example of heterogeneous and nobleꢀmetalꢀfree catalytic system
for this reaction without additional hydrogen sources. This method provides an atomꢀefficient, versatile, and ecoꢀ
nomic method for the synthesis of primary amines. Studies on structureꢀactivity relationship show that coꢀ
presence of acidꢀbase sites of alumina and surface metallic Ni0 species is responsible for the high activity. The
fundamental information will lead to a design of ligandꢀ and nobleꢀ metalꢀfree recyclable catalysts for environꢀ
mentally benign and practical production of amines.
ASSOCIATED CONTENT
Supporting Information
Characterization data for catalyst and products. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
kshimizu@cat.hokudai.ac.jp
ACKNOWLEDGMENT
This work was supported by a "Grant for Advanced Industrial Technology Development" in 2011 from the New
Energy and Industrial Technology Development Organization (NEDO) and a MEXT program "Elements Strategy
Initiative to Form Core Research Center" (since 2012), Japan. The authors thank Mr. Naomichi Imaiida (Nagoya
University) for his contribution in preliminary experiments.
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1
2
3
4
5
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7
8
γ-Al2O3
40
30
20
10
0
ZrO2
CeO2
Y2O3
La2O3
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10
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60
Nb2O5 SnO2
TiO2
CaO
MgO
2.2
Electronegativity of support metal oxide
2
2.4
2.6
2.8
CaO
4
3
2
1
0
20
Y2O3
La2O3
MgO
10
CeO2
ZrO2
Nb2O5
SnO2
γ-Al2O3
TiO2
0
2
2.2
2.4
2.6
2.8
Electronegativity of support metal oxide
Figure 1. TOF(ꢁ) and the surface densities of basic (ꢂ) and acidic (ꢃ) sites vs. electronegativity of support
metal oxide.
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1
2
30
5 nm
3
4
5
6
20
10
0
7
0
5
10
15
Particle size/ nm
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9
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Figure 2. A representative TEM image and particle size distribution of Ni/θꢀAl2O3 (Ni loading = 10 wt%, TH2 =
500 oC).
50
500 oC
800 oC
40
30
20
10
600 oC
400 oC
NiO/Al2O3
300 oC
0
0.2 0.4 0.6 0.8
Ni0/(Ni0+NiO)
1
Figure 3. TOF vs the fraction of metallic Ni (from H2ꢀTPR) for 10wt% Ni/θꢀAl2O3 catalysts reduced in H2 at varꢀ
ious temperatures.
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1
2
3
4
2042 cm-1
linear CO
on Ni0
1940 cm-1
bridged CO
on Ni0
0.1
5
6
7
8
d
c
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b
a
2200 2100 2000 1900
Wavenumber /cm-1
Figure 4. IR spectra of CO adsorbed on 10 wt% Niꢀloaded catalysts at ꢀ180 oC: (a) NiO/θꢀAl2O3, (b) Ni/θꢀAl2O3
(TH2 = 500 oC), (c) Ni/θꢀAl2O3 after reꢀoxidation in 20 kPa O2 at room temperature, (d) θꢀAl2O3.
100
80
60
40
20
0
20
40
60
t / h
Figure 5. Timeꢀconversion profile for the amination of 1b (3 mmol) by NH3 with 1 mol% of Ni/θꢀAl2O3 (Ni = 10
wt%) at 150 C in oꢀxylene (4 g): yields of unreacted 1b (ꢄ), primary amine 2b (), secondary amine 3b (ꢀ)
o
and cyclododecanone 4b (ꢁ).
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Scheme 1. A plausible pathway of Niꢀcatalyzed amination of alcohols.
OH
NH2
R2
Ni0
R1
R2
R1
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9
10
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NH
R2
O
Ni-H
R1
R1
R2
NH3
H2O
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1
2
Table 1. Amination of 2-octanol with NH3.a
3
NH2
O
4
5
6
7
cat. (1 mol%)
2
2
OH
+ NH3
2a
4a
2
o-xylene(4 g)
160 oC
H
0.4 MPa
3 mmol
1a
N
4 h
2
2
3a
8
9
conv.
(%)
yield (%)
2a 3a 4a
entry
catalyst
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1
2
3
NiO/θꢀAl2O3
Ni/θꢀAl2O3
Ni/αꢀAl2O3
Ni/γꢀAl2O3
Cu/γꢀAl2O3
Co/γꢀAl2O3
Pt/γꢀAl2O3
Ir/γꢀAl2O3
Re/γꢀAl2O3
Au/γꢀAl2O3
Ru/γꢀAl2O3
Rh/γꢀAl2O3
Ag/γꢀAl2O3
Pd/γꢀAl2O3
γꢀAl2O3
Ni powder
Raney Ni
Ni/CaO
Ni/MgO
Ni/La2O3
Ni/Y2O3
Ni/CeO2
Ni/ZrO2
Ni/TiO2
0
83
0
83
18
10
95
80
6
0
80
0
81
14
5
41
46
1
0
28
2
3
1
0
0
22
0
0
42
56
68
54
0
0
3
0
0
0
0
9
11
0
0
1
0
0
0
0
0
0
0
0
1
1
1
1
0
0
0
0
0
0
0
2
2
2
16
4
1
0
3
0
0
1
0
0
0
1
0
8
8
3
6
0
0
0
0
4
5 b
6
7
8
9
10
11
12
13 b
14
15c
16d
17d
18
19
20
21
22
23
24
25
26
27e
0
37
12
3
2
0
0
22
25
0
52
66
74
71
1
Ni/Nb2O5
Ni/SnO2
Ni/θꢀAl2O3ꢀair
0
0
0
0
0
0
a Conversion of 1a and yields of 2a, 3a and 4a were determined by GC based on 1a. b TH2 = 300 °C c Al2O3 = 0.1
g. d Ni = 10 mol% e Preꢀreduced Ni/θꢀAl2O3 was exposed to air at room temperature for 0.5 h.
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Table 2. Amination of various alcohols with NH3.a
7
8
9
Ni/Al2O3 (1 mol%)
R1
OH
R1
O
R1
NH2
+
NH3
o-xylene (4 g), 160 oC
R2
R2
R2
R1
4
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
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55
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58
59
60
0.4 MPa
3.0 mmol
H
1
N
R1
R2 R2
3
t
conv.
(%)
yield (%)
entry
alcohols
/ h
20
2
3
4
OH
1b
93
88
5
0
2
2b
3b
4c,d
1st reuse
20
20
92
92
88
89
4
4
0
0
2nd reuse
OH
28
29
96
94
96
77
5
4
0
OH
5c
12
OH
6c
22
95
82
96
0
3
12
OH
7
63
20
24
99
0
4
0
(90)g
OH
8e
9
100
81
85
0
6
OH
68
OH
10c
11e,f
12e
72
72
13
94
99
90
85
78
70
3
11
7
6
0
0
OH
OH
3
a Conversion and yields were determined by GC based on alcohol. TH2 = 600 °C. c T = 140 °C. d in nꢀdecane
b
(4 g). catalyst = 5 mol%. f T= 150 °C. g isolated yield.
e
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For TOC Only
3
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5
6
1mol%
Ni/Al2O3
10 examples
68-96% yields
Rusable catalyst
R1 OH
R1 NH2
+
NH3
R2
140-160 oC
R2
0.4 MPa
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