Oxidant free conversion of alcohols to nitriles over Ni-based catalysts

Article abstract of DOI:10.1039/c8cy01799a

Organic nitriles are significant and versatile industrial feedstocks, but their conventional synthetic protocols require hazardous starting materials and/or harsh reaction conditions posing environmental and health risks. Herein, we established a Ni-based catalytic system to convert primary alcohols to nitriles with ammonia gas as the sole nitrogen source under oxidant-free conditions at merely 190-230 °C. Based on isotope labelling experiments, in situ DRIFTS and control experiments, the reaction pathway was identified to follow a dehydrogenation-imination-dehydrogenation sequence, with α-carbon C-H bond breakage as the rate determining step. Ni is superior to all noble metal catalysts tested, due to its excellent dehydrogenation ability that is not inhibited by NH₃. The support plays an auxiliary role, promoting the reaction between aldehyde and ammonia to form imine as a critical intermediate. Ni/Al₂O₃ catalyst prepared via a deposition-precipitation method, featuring both excellent dispersion of metallic Ni and suitable acid sites, enabled alcohol transformation into nitrile under unprecedented low temperature. Various alcohols were converted into their corresponding nitriles in high conversions and yields (both up to 99%), while the catalyst kept 90% of its original activity after 48 hours in the stability test, highlighting the wide applicability and the robustness of the catalytic system.

Full text of DOI:10.1039/c8cy01799a

Volume 9 Number 1 7 January 2019 Pages 1–254
Catalysis
Science &
Technology
rsc.li/catalysis
ISSN 2044-4761
PAPER
Ning Yan et al.
Oxidant free conversion of alcohols to nitriles over Ni-based catalysts

Catalysis
Science &
Technology
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PAPER
Oxidant free conversion of alcohols to nitriles
over Ni-based catalysts†
Cite this: Catal. Sci. Technol., 2019,
9, 86
a
bc
a
Yunzhu Wang,
Shinya Furukawa,
Zhang Zhang,
d
a
and Ning Yan *a
Laura Torrente-Murciano,
Saif A. Khan
Organic nitriles are significant and versatile industrial feedstocks, but their conventional synthetic protocols
require hazardous starting materials and/or harsh reaction conditions posing environmental and health
risks. Herein, we established a Ni-based catalytic system to convert primary alcohols to nitriles with ammo-
nia gas as the sole nitrogen source under oxidant-free conditions at merely 190–230 °C. Based on isotope
labelling experiments, in situ DRIFTS and control experiments, the reaction pathway was identified to follow
a dehydrogenation–imination–dehydrogenation sequence, with α-carbon C–H bond breakage as the rate
determining step. Ni is superior to all noble metal catalysts tested, due to its excellent dehydrogenation
3
ability that is not inhibited by NH . The support plays an auxiliary role, promoting the reaction between
aldehyde and ammonia to form imine as a critical intermediate. Ni/Al
O
2 3
catalyst prepared via a deposi-
Received 28th August 2018,
Accepted 2nd November 2018
tion–precipitation method, featuring both excellent dispersion of metallic Ni and suitable acid sites, enabled
alcohol transformation into nitrile under unprecedented low temperature. Various alcohols were converted
into their corresponding nitriles in high conversions and yields (both up to 99%), while the catalyst kept
DOI: 10.1039/c8cy01799a
9
0% of its original activity after 48 hours in the stability test, highlighting the wide applicability and the ro-
rsc.li/catalysis
bustness of the catalytic system.
ten released as
environmental and health problems, requesting the develop-
ment of alternative, more sustainable synthetic routes.
a side product. These factors raise
1
. Introduction
Organic nitriles find wide applications as significant struc-
tural motifs in pharmaceuticals, fine chemicals, biological
materials and high performance polymer materials. Nitriles
are conventionally synthesized via the Sandmeyer reaction
and the Rosenmund–von Braun reaction, and more recently
Alcohol represents a better starting material for organic ni-
trile compounds that avoid the use of cyanides/halides. How-
ever, alcohols are not reactive enough and normally require in
situ conversion into ketones/aldehydes prior to further
1
,2
3
4
1
1–14
via the nucleophilic substitution of cyanides to alkyl and aryl
halides. These methods suffer from a number of drawbacks
transformation.
Currently, production of nitriles from alco-
5
hols is dominated by the ammoxidation reaction, i.e., OH
including the employment of toxic or hazardous starting ma-
terials, generation of large amounts of chemical wastes, and
the requirement of harsh reaction conditions. Both metal
group is oxidized into a carbonyl group before reacting with
15
16,17
ammonia. Strong oxidants such as iodine,
1,3-diiodo-5,5-
and (Bu N)
16,18
19
20
dimethylhydantoin,
2 2 8
K S O ,
2
MnO ,
4
2 2 8
S O
6
7
8
based cyanides, such as KCN, NaCN, K ijFeIJCN) ], and
(ref. 21) have been utilized to produce nitriles from their corre-
sponding alcohols. More recently, catalytic systems based on
4
6
9
ZnIJCN) , and non-metallic cyanides, such as trimethylsilyl cy-
anide, are highly toxic starting materials. Moreover, stoi-
chiometric waste metal salts are produced, and HCN gas is of-
2
0
1
22–25
26
27
28,29
Cu,
RuIJOH)
x
,
Mn and NiO, Fe
O
2 3
3 4
and Co O ,
and
Pd (ref. 30) catalysts employing oxygen gas as the oxidant have
been developed, but over-oxidation is difficult to avoid, and
side reactions easily occur for multi-functionalized substrates.
In addition, the current ammoxidation systems were mainly
applied on aromatic substrates, and only limited cases demon-
a
Department of Chemical and Biomolecular Engineering, National University of
Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore.
E-mail: ning.yan@nus.edu.sg
b
Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
strated the possibility to transform aliphatic alcohols, in which
c
22,23,27,28
Elementary Strategy Initiative for Catalysis and Battery, Kyoto University, Kyoto
long reaction time was necessary.
For example, 26 to
Daigaku Katsura, Nishikyo-ku, Kyoto, Japan 615-8510
3
0 hours were required to achieve 65% to 82% nitrile yields
d
Department of Chemical Engineering and Biotechnology, University of
28
3 4
from aliphatic alcohols over Co O -NGr/C catalyst.
Cambridge, Philippa Fawcett Drive, Cambridge CB3 0AS, UK
In fact, the conversion of hydroxyl group to carbonyl group
can be achieved through dehydrogenation rather than
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cy01799a
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oxidation, enabling nitrile formation without any oxidant.
pore). Ruthenium on carbon (10 wt%) and rhodium on car-
bon (10 wt%) were supplied by Shanghai Kaida Chemical
Co., Ltd. (China). Deuterated compounds were supplied by
Wuhan Livika Technology Co., Ltd. (China). Commercially
available organic chemicals, metal precursors, and carbon
supported noble metal catalysts were used without further
purification. Al O -Nanorods and Al O -nanoplates were pre-
3
1,32
33
34
Metal catalysts based on Cu,
Co–Ni, FeS, Zn–Cr (ref.
3
5 and 36) etc. have been investigated to transform alcohol
substrates to corresponding nitriles, among which only Cu/m-
ZrO (ref. 31) and FeS (ref. 34) demonstrated its applicability
2
in more than one substrate while others were only applied in
converting one specific starting material. Moreover, the
reported catalysts were not efficient, requiring high tempera-
ture to function properly (280–500 °C). These conditions re-
sult in a high energy consumption, product decomposition
and severe catalyst deactivation. For instance, the Zn Cr4.5/γ-
2
3
2 3
4
6
pared via hydrothermal treatment method.
2.2 Catalysts preparation
3
0
Ni-Based catalysts were prepared by three methods, including
deposition–precipitation (DP) method, wet-impregnation (WI)
method and ion-exchange method. DP method was modified
2 3
Al O catalyst afforded 61% selectivity towards propionitrile
at 420 °C, producing considerable amounts of side products
3
6
such as 3-picoline.
4
7
from a literature. NiIJNO
O (50 mL), and the solution was divided into two portions,
0 mL and 40 mL, respectively. Into the 40 mL portion, sup-
3 2 2
) ·6H O (2.1 g) was dissolved in
Thermodynamically, dehydrogenation of alcohols does not
require such a high temperature when the reaction is
conducted in a continuous flow reactor where hydrogen is
constantly taken away from the system. For example, ethanol
H
1
2
port (1.0 g) was added (mixture A), after which the mixture
was stirred and heated at 80 °C. Urea (3.3 g) was added into
the 10 mL portion and the solution was added dropwise into
mixture A. The final mixture was stirred and heated at 90 °C
overnight. The solid was washed with deionized (DI) water
and centrifuged for separation, and freeze-dried to obtain dry
underwent dehydrogenation at 200 °C on ZnZrO supported
x
3
7
Au catalyst,
curred at the same temperature on Cu–MgO–Cr
The dehydrogenation of amine was also achieved at 200 °C
whereas cyclohexanol dehydrogenation oc-
3
8
2 3
O catalyst.
3
9
employing a Ru complex. Recently, Ni catalysts have been
widely used in many catalytic reactions, including effi-
ciently catalysing the conversion of alcohols to amines.
sample. Al
2
O
3
, SiO
2
and TiO
2
were used as supports, and the
-DP, Ni/SiO -DP
4
0
as-prepared catalysts were named as Ni/Al
O
2 3
2
4
1–45
and Ni/TiO -DP, respectively, with metal loading of ca. 20
2
However, Ni based heterogeneous catalysts for various nitrile
synthesis under mild conditions have not yet been reported.
Herein, we developed a low-temperature, oxidant-free system
based on a non-noble Ni catalyst to transform a series of ali-
phatic and aromatic alcohols to corresponding nitriles in the
presence of ammonia gas. The reaction pathway was verified
with a set of control experiments, reaction kinetics were in-
vestigated with deuterated substrates, while reactive surface
species were monitored by in situ diffuse reflectance infrared
Fourier-transform spectroscopy (DRIFTS) technique. Cleavage
of C–H bond at α-carbon in the dehydrogenation of alcohol
is the rate determining step in this reaction.
wt%.
Wet-impregnation method was also used to prepare Al
2 3
O ,
SiO , and TiO supported Ni catalysts. NiIJNO ) ·6H O (1.2 g)
2
2
3 2
2
2
was dissolved in H O (50 mL), and support (1.0 g) was added
to the solution. The mixture was stirred at room temperature
overnight. After that, the mixture was freeze-dried. The cata-
lysts based on different supports were named as Ni/Al
Ni/SiO -WI and Ni/TiO -WI, respectively, with metal loading
of ca. 20 wt%.
Ni/CaSiO
2 3
O -WI,
2
2
3
was prepared by an ion-exchange method, in
2
+
2+
48
which Ni exchanged with Ca in the CaSiO
NiIJNO ) ·6H O (1.0 g) was dissolved in DI water (50 mL), in
3
support.
3
2
2
3
which CaSiO (1.0 g) was added. The mixture was stirred at
2
. Experimental
room temperature for 12 hours. After that, the solid part was
washed with DI water for three times. The solid was dried in
an oven at 90 °C for 12 hours and calcined at 350 °C for 4
hours. The as-prepared catalyst was named as Ni/CaSiO ,
3
with metal loading of ca. 20 wt%.
2
.1 Materials
1
-Hexanol (99%), tert-butanol (99%), hexanal (98%),
hexylamine (99%), 1-butanol (99%), 1-heptanol (98%),
-octanol (99%), 1-nonanol (98%), cyclopentanemethanol
98%), cyclohexanemethanol (99%), benzyl alcohol (99%),
-phenylethanol (99%), 3-phenyl-1-propanol (98%), 4-iso-
propylbenzyl alcohol (97%), tetrahydrofurfuryl alcohol (99%),
furfuryl alcohol (98%), 3-pyridinemethanol (98%),
-nitrobenzyl alcohol (99%), 3-pyridinemethanol (98%),
1
(
2
2.3 General procedure for catalytic reactions
All experiments were conducted in a vertical, fixed-bed tube
reactor (SS316), with length of 0.3 m and diameter of 3/8 inch
(9.5 mm). A furnace (Yuanbang Furnace, 1 KW, 220 V, up to
1000 °C) was used to heat the reactor and a temperature
probe was placed at reactor outer surface. The furnace tem-
perature was controlled by a temperature controller. The cata-
lysts were placed at the middle part of the reactor supported
by a sieve and quartz wool. Liquid substrates were pumped
into the reactor at the top part by a glass syringe (Hamilton
81620) with a syringe pump (Harvard PHD 2000 Infusion)
4
nickelIJII) nitrate hexahydrate, silica fumed (powder, 0.2–0.3
um avg. part. size (aggregate)), aluminum oxide (activated,
neutral, Brockmann I), calcium silicate (purum, 12–22% Ca
(
as CaO) basis, ≥87% SiO basis), titaniumIJIV) oxide (nano-
2
powder, 21 nm particle size, ≥99.5% trace metals basis), urea
(
(
99%), platinum on carbon (10 wt%), palladium on carbon
10 wt%) were purchased from Sigma-Aldrich Pte. Ltd. (Singa-
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and solid substrates were dissolved in the solvent before
charged into the syringe. Flow rates of ammonia gas and ni-
trogen gas were controlled by a gas flow meter, and they were
mixed before entering the reactor and carrying the substrate
through the catalyst bed (Scheme 1).
diffraction (XRD, Bruker D8 Advance) equipped with a Cu Kα
radiation source. To maintain the metallic state of Ni-based
catalysts, passivation was conducted before XRD analysis,
using 1% O /N at room temperature for an hour after cata-
2
2
lyst reduction. Thermogravimetric (TG) analysis was
conducted on DTG-60A thermogravimetry analyser
Ni-Based catalysts or noble metal catalysts (equal-mole of
metals) were added in the reactor. All the Ni-based catalysts,
a
(Shimadzu) in air atmosphere. Electrospray ionization time-
of-flight mass (ESI-TOF-MS) spectra were obtained from a
Bruker MicroTOF-Q system. The samples were directly
−
1
except Ni/CaSiO , were pre-reduced with H (40 mL min ) at
3
2
4
60 °C for 1 hour. Ni/CaSiO
3
2
was pre-reduced with H (40 mL
−
1
−1
min ) at 600 °C for 0.5 hour. All the noble metal catalysts,
including Ru/C, Pt/C, Rh/C, and Pd/C, were treated at 190 °C
under H
injected into the chamber at 200 μL h . Typical instrument
parameters: capillary voltage, 4 kV; nebulizer, 0.4 bar; dry
−
1
−1
2
(40 mL min ) for 0.5 hour to remove moisture. Af-
gas, 2 L min at 120 °C; m/z range, 50–3000.
ter reduction, the temperature was decreased to reaction tem-
perature under hydrogen flow followed by a nitrogen purge
of the system. In this manner, there will be some hydrides
adsorbed on metal surface unless a high-temperature desorp-
tion is conducted, since the reaction temperature (130–230
IR spectroscopy of adsorbed pyridine (py-IR) was performed
on a Thermo Fisher Nicolet 5700 IR spectrometer at a resolu-
tion of 4 cm with 120 scans in the range of 1000–4000 cm
−1
−1
equipped with CaF2 windows. For the adsorption measure-
ments, 20 mg sample was pressed into a self-supporting disc
(wafer, diameter of 6.15 mm) and mounted on the sample
4
9
2
°C) is below the H desorption temperature on metallic Ni.
−6
Then, substrates together with ammonia gas and nitrogen
gas were supplied to the reactor at desired flow rates. The
products were detected with an online gas chromatography
holder. The samples were activated under vacuum (p = 10
mbar) at 450 °C for 1 h. As soon as the sample cooled to 150
°C, a spectrum of the activated sample was taken as the back-
ground. Subsequently, adsorption of pyridine was performed at
0.1 mbar for 30 min until saturation/equilibration of the sur-
face was reached and the peak area of the IR signal remained
constant. After physisorbed pyridine was removed by evacua-
(GC)-flame ionization detector (FID) system equipped with an
Agilent HP-5 capillary column with He as the carrier gas. In
case off-line GC or GC-mass spectrometry (MS) analysis is
needed, the outlet stream was passed through a bottle of eth-
anol in ice bath to trap the organic products, with 40 μL
dodecane as the internal standard.
−6
tion (p = 10 mbar) at 150 °C, another spectrum was recorded.
Finally, spectra were taken after desorption at 200 °C, 300 °C,
−6
and 450 °C for 30 min (p = 10 mbar). The IR spectra of
adsorbed pyridine were obtained by subtracting the spectrum
of the activated sample.
2
.4 Catalyst characterization
Brunauer–Emmett–Teller (BET) surface analysis was carried
out by N2 adsorption and desorption at 77 K in a Micro-
meritics ASAP 2020 surface area and pore size analyzer. H2
chemisorption were conducted over a ChemBET Pulsar TPR/
TPD automated chemisorption analyzer. For H2 chemisorp-
tion experiments, the catalyst (100 mg) was pre-reduced un-
2.5 Fourier transform infrared (FT-IR) spectroscopy
experiment
The in situ DRIFTS spectra were recorded with a Thermo Sci-
entific Nicolet iS50 FT-IR, with a liquid nitrogen cooled MCT-
A detector with ZnS window. All the experiments were carried
out under atmospheric pressure. Catalysts were loaded into
the DRIFTS cell and reduced at 460 °C under H flow for one
2
hour and then the cell temperature was decreased to the de-
sired one. Ammonia was switched on until it was fully satu-
der H
2
at 460 °C for 1 h. The temperature was reduced to 400
°
C, and the system was purged with N for 1 hour before de-
2
creasing to room temperature under N flow. After signal sta-
2
2
bilization, H pulses (159 μL per pulse) were injected every 10
min. The crystallographic information was analysed by X-ray
rated on the surface of catalyst. After that, N
to remove the gaseous ammonia as well as physically
adsorbed ammonia. Background was taken under N atmo-
2
was turned on
2
sphere, and substrate (10 μL) was injected and passed
through the catalyst cell. The spectra were recorded by accu-
−
1
mulating 32 scans with a resolution of 4 cm . Attenuated to-
tal reflectance infrared (ATR-IR) spectroscopy analysis was
carried out on a Thermo Scientific Nicolet iS50 FT-IR spectro-
meter integrated with a diamond ATR accessory. The IR spec-
−
1
tra were collected in the spectral range 4000–500 cm with a
−
1
resolution of 4 cm and scan number of 128.
3
. Results and discussion
Transformation of 1-hexanol into hexanenitrile in the pres-
ence of NH3 gas was selected as a model reaction. The
Scheme 1 Experimental set-up for the conversion of alcohols to
nitriles.
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reaction is endothermic and its Δ G value becomes negative
To understand different catalytic activities among various
Ni-based catalysts, we first measured metal dispersion by H2
chemisorption, assuming that one surface Ni atom adsorbs
one hydrogen atom. Nickel dispersion was 9.9% and 8.3% on
Ni/SiO -DP and Ni/Al O -DP, respectively (Table S1†), higher
r
at around 200 °C and above (see page S2 in the ESI†). It is
a low pressure favoured reaction so when conducted under
low alcohol partial pressure, the reaction is thermodynami-
cally favourable at even lower temperature (i.e., 190 °C). In
addition to the desired product hexanenitrile, a small
amount of 1-hexylamine (<10%) and trace amount of
hexene and di-hexylamine (<2%) were observed as the main
side products.
2
2 3
3
than the catalysts prepared by WI methods. Ni/CaSiO also
afforded a lower Ni dispersion of 3.1%. The higher disper-
sion of Ni/SiO -DP and Ni/Al O -DP may result in their higher
2
2 3
activity. The chemisorption results are consistent with trans-
mission electron microscopy (TEM) results obtained, where
catalysts prepared by DP methods have smaller particle sizes
3
.1 Catalysts screening and optimization
2 3
than the others (Fig. S1†). Other Al O supports with different
A series of catalysts were screened under the same reaction
conditions. As presented in Table 1, commercial noble metal
catalysts (Ru, Rh, Pt, and Pd, ca. 10 wt%) supported on active
carbon did not exhibit good performance (Table 1, entry 1–4),
with the highest hexanenitrile yield of 37% on Ru/C (Table 1,
entry 2). This suggests the activities of most noble metal cata-
lysts are limited in the presence of ammonia, even though
they are widely used in dehydrogenation reactions. Ni-Based
catalysts, in sharp contrast, showed significantly higher activ-
morphology were prepared, including Al O -nanoplates and
2
3
4
6
Al O -nanorods. The same preparation methods, i.e., DP
2
3
and WI, were utilized to prepare the catalysts. The four cata-
lysts (Table 1, entry 12–15) did not outperform commercial
Al O , presumably because of the different acid sites pro-
2
3
duced on Al
ence of Na residue after their base hydrothermal synthesis.
Three peaks at 44.2°, 51.6°, and 76.2° were observed in the
XRD pattern of reduced Ni/Al -DP catalyst (Fig. S2†), ascrib-
able to (111), (200) and (220) crystal phases of Ni nano-
2 3 2 3
O -nanoplates and Al O -nanorods and the pres-
5
0
2 3
O
ity than noble metal catalysts. Nickel supported on SiO and
2
5
1
52
Al O by deposition–precipitation (DP) method (Table 1, entry
particles respectively, while no peak for NiO was observed,
2
3
5
6
and 6) gave the highest yields of hexanenitrile, reaching
9% and 66%, respectively, superior to the catalysts prepared
indicating very small amount of NiO existing, if any, under
treatment conditions.
by wet-impregnation (WI) method (entry 8 and 9). Ni catalysts
supported on TiO (entry 7 and 10) and CaSiO (entry 11)
Reaction parameters were further scrutinized using the
Ni/Al O -DP catalyst. The product yield increased linearly with
2 3
2
3
were less active, regardless of preparation methods. A control
experiment employing Al O support only was conducted and
increased catalyst loading from 15 mg to 50 mg (Fig. S3†).
Further increasing the catalyst amount did not lead to a pro-
portional increase of product yield, presumably due to the re-
action was approaching equilibrium. When the reaction tem-
perature increased from 150 °C to 230 °C (Fig. S4†), a
2
3
no activity was observed even at a higher temperature (entry
0), demonstrating the active role of Ni species in the
2
transformation.
Table 1 Conversion of 1-hexanol to hexanenitrile
4
−1
Catalyst
Catalyst amount (mg)
GHSV (× 10 h
)
Conv. (%)
Yield (%)
Selectivity (%)
a
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
Rh/C
—
—
—
—
50
50
50
50
50
50
50
50
50
50
50
—
—
—
—
92
96
72
33
94
82
41
50
31
29
61
24
26
49
41
87
97
>99
82
0
23
37
14
13
69
66
32
43
26
26
51
21
11
11
12
71
85
>99
64
0
25
39
19
39
73
80
78
86
84
90
84
88
42
22
29
82
88
>99
78
—
a
Ru/Ca
Pt/C
Pd/C
a
Ni/SiO
Ni/Al
Ni/TiO
Ni/SiO
Ni/Al
Ni/TiO
Ni/CaSiO
2
-DP
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
1.12
1.12
1.12
0.281
1.12
2
O
3
-DP
2
-DP
-WI
2
2
O
3
-WI
-WI
0
1
2
3
4
5
6
7
8
9
0
2
3
Ni/Al
Ni/Al
Ni/Al
Ni/Al
Ni/Al
Ni/Al
Ni/Al
Ni/Al
2
O
O
O
O
O
O
O
O
3
3
3
3
3
3
3
3
-Plate-DP
-Rod-DP
-Plate-WI
-Rod-WI
-DP
-DP
-DP
-DP
2
2
2
2
100
100
100
100
100
b
c
2
2
2
d
b
2 3
Al O
−
1
−1
−1
a
b
Reaction conditions: 1 μL min 1-hexanol, 10 mL min
°
N
2
, 4 mL min NH
3
, 190 °C. Same mole of metal with nickel in other entries. 210
c
d
3
C. 230 °C. Mixing catalyst (25 v/v%) with quartz sand (75 v/v%). NH : 1-hexanol = 22 : 1.
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monotonic increase of hexanenitrile yield from 26% to 82%
was observed. This is exactly as expected, because dehydroge-
nation of the substrate is favored at high temperature both
thermodynamically (see page S2 in the ESI†) and kinetically.
Decreasing gas hourly space velocity (GHSV) by mixing cata-
lyst with quartz sands did not induce substantial change of
reactivity (Table 1, entry 19). With 100 mg catalyst, increased
temperature also induced increased activity, and at 230 °C,
quantitative formation of hexanenitrile was observed at a
GHSV of 1.12 × 10 h (Table 1, entry 16–18), mainly due to
the elimination of 1-hexylamine via further dehydrogenation
into nitrile product. Close to 100% selectivity can be achieved
under optimized conditions, highlighting the feasibility of
continuous production of organic nitrile compounds from al-
cohols without complex post-reaction treatment.
occurs, 1-butan-1-d-amine (CH IJCH ) –CDH–NH , m/z = 74)
3
2 2
2
and 1-butylamine (CH IJCH ) –CH –NH , m/z = 73) should be
3
2 2
2
2
the major products. According to GC-MS spectra (Fig. S5†),
the major molecular ion peaks were 73 and 74, correspond-
ing to none- and mono-deuterated butylamine, respectively.
This strongly supports that pathway II is dominant in the re-
action. A small peak at m/z = 75 was observed, bearing a
height of 4.1% compared with the peak at m/z = 74, in excel-
1
3
lent agreement with the predicted abundance of C isotope
4
−1
peak for CH IJCH ) –CDH–NH (4.4%, considering 4 carbons
3
2 2
2
1
3
with C abundance of 1.1% each). In another word, no
CH IJCH –CD –NH was detected in the product. Therefore,
3
)
2 2
2
2
direct substitution of hydroxyl group by NH was not occur-
3
ring at an appreciable level in the reaction system. In addi-
tion to this, tert-butanol, which is unable to be dehydro-
genated, was also applied as the substrate to identify the
validity of pathway I. Under N2 and NH , 0% conversion
was observed at 190 °C and 210 °C on Ni/Al O -DP catalyst.
2 3
3
3
.2 Reaction pathway
Two reaction pathways from alcohol to nitrile are considered,
No tert-butylamine was observed by GC-MS analysis even
with increased temperature at 290 °C, further suggesting
that direct amination followed by dehydrogenation was not
the major reaction pathway in our system. We have to point
out, that while both experiments suggest pathway II is more
likely to be the dominant pathway, neither of them are
conclusive.
To investigate the validity of pathway II, reaction rate
(mmol of substrate converted per gram of catalyst per hour)
of each step was evaluated. When ammonia was not supplied
as shown in Scheme 2. Pathway I follows an S 2 mechanism,
N
3
where a NH molecule directly attacks the α-carbon, inducing
simultaneous C–O bond cleavage and C–N bond formation.
This leads to the formation of a primary amine, which fur-
ther undergoes dehydrogenation to produce a nitrile product
together with two hydrogen molecules. In pathway II, alco-
hols undergo dehydrogenation to form aldehydes, and then
the aldehydes react with ammonia to produce the imine in-
5
3
termediates. Imines undergo either a dehydrogenation step
to form nitriles directly, or a hydrogenation step to the amine
intermediates. The transformations between imine and amine
are reversible so that the selectivity towards nitrile is not
compromised. In the latter case, the hydrogen comes from
the dehydrogenated substrate, which is reminiscent of the
pathway in “hydrogen borrowing mechanism” that is well-
2 3
(Table 2, entry 1), Ni/Al O -DP catalyst afforded aldehyde as
the main product with 4% conversion. This observation
matches pathway II, which comprises multiple reaction steps
starting from alcohol dehydrogenation into an aldehyde.
Consequently, reaction intermediates suggested in pathway II
were used as the starting materials to further probe its feasi-
bility. Hexanal was highly reactive in the system, reaching
much higher conversion (96%) under the same reaction con-
ditions (Table 2, entry 2). Interestingly, 3,5-dibutyl-2-pentyl-
pyridine (compound X, Fig. 1(a) and S6†) was detected by GC-
1
1,54,55
established in homogeneous catalysis.
To gather evidence to identify which pathway is dominant,
deuterated butanol (CH IJCH ) –CD –OH, 1,1-dideuteriobutan-
3
2 2
2
1
-ol) was converted to butylamine with ammonia and hydro-
gen at 150 °C. A low temperature was chosen and excess hy-
drogen was supplied to make sure dehydrogenation of
butylamine to butanenitrile was unfavoured. If the reaction
follows pathway I, the major product should be 1-butan-1,1-
d2-amine (CH IJCH ) –CD –NH , m/z = 75), with deuteration of
2 3
MS in the presence of Al O support only, which was formed
by one molecule of 1-hexanimine and two molecules of
hexanal, due to the instability of imine under reaction condi-
tions. This provided strong evidence for the formation of im-
ine intermediate, as predicted by pathway II.
3
2 2
2
2
both hydrogen at α-carbon. If the reaction follows pathway II,
where dehydrogenation and re-hydrogenation on α-carbon
The formation of X from hexanal and ammonia was uti-
lized as a descriptor of the ability of the support to promote
Scheme 2 Two possible reaction schemes from alcohols to nitriles.
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Table 2 Control experiments to confirm reaction pathway
Ni/Al
2
O
3
-DP
2 3
Al O
NH
(mL min
3
−1
−1
−1
Substrate
)
Product
Conversion (%)
Reaction rate (mmol g hour
)
Product
1
2
1-Hexanol
Hexanal
0
8
Hexanal
4
96
0.98
22
N.D.
X
Hexanenitrile
-Hexylamine
1
3
4
1-Hexylamine
1-Hexanol
8
8
Hexanenitrile
Hexanenitrile
19
14
4.4
3.1
N.D.
N.D.
−
−1
1
−1
−1
−1
Reaction conditions: 1 μL min substrate, 20 mg catalyst, 190 °C, 8 mL min NH
3
with 20 mL min
N
2
or 28 mL min
2 3
N if no NH was
5
supplied. GHSV = 1.12 × 10 h . N.D. = not detected.
imine formation. As shown in Fig. 1(b), Al O presented
compound formation is an indirect route, following dehydroge-
2
3
much higher yield of X than SiO , CaSiO and the blank con-
nation–imination–dehydrogenation reaction sequence; 2)
2
3
trol, indicating its promotional effect in this step. This is
2 3
imination reaction (step 2) is catalysed by the Al O support,
5
6
plausibly due to the weak Lewis acid sites (LAS) on Al O ,
whereas other steps are catalysed by metallic Ni; 3) the dehy-
drogenation from alcohol to aldehyde has the smallest reaction
rate, and therefore is likely to be the rate-determining step.
2
3
suggested by Py-IR on Ni/Al O -DP (Fig. S7†). The peaks at
2
3
−
1
−1
1
447 cm and 1606 cm were ascribed to the LAS on Ni/
Al O -DP catalyst, which disappeared as temperature in-
5
7
2
3
creased to 450 °C. No obvious Brønsted acid sites (BAS) were
5
8
3.3 Reaction kinetics
observed on Ni/Al
both hexanenitrile and 1-hexylamine were produced on Ni/
Al -DP catalyst, indicating the hydrogenation and dehydro-
2 3
O -DP surface. Starting with hexanal,
Since the alcohol is vaporized under reaction temperature,
the reaction is approximated as a gas–gas reaction on a solid
catalyst surface. Therefore, the pseudo reaction rate can be
represented as:
2 3
O
genation of imine were catalysed by Ni species. Hexanenitrile
was also formed on Ni/Al O -DP when 1-hexylamine was
2
3
injected into the reactor (Table 2, entry 3), suggesting the fea-
sibility of step 5. The dehydrogenation of 1-hexylamine to
hexanenitrile was faster than the dehydrogenation of hexanol
α
β
3
r = k × [alcohol] × [NH ]
(
step 1), but much slower than step 2. Both dehydrogenation
where k is the pseudo reaction rate constant, and α and β are
the reaction orders of alcohol and ammonia, respectively.
The values of α and β were measured for 1-hexanol conver-
sion to hexanenitrile on Ni/Al O -DP catalyst. A high GHSV
(1.28 × 10 h ) is used to maintain low conversion of the
substrate. When 1-hexanol flow rate was varied while the con-
centration of ammonia was kept constant, a positive correla-
tion between the reaction rate (mmol of substrate converted
per gram of catalyst per hour) and the substrate flow rate was
steps and the overall reaction (Table 2, entry 4) were slower
than the step 2 in one order of magnitude, indicating the de-
hydrogenation limited the whole reaction rate.
Combining these control experiments, the following impor-
tant insights are obtained: 1) the major pathway for nitrile
2
3
5
−1
observed.
β
k × [NH
3
] was treated as a constant and α was determined
as 0.6 by fitting the experimental data (Fig. 2(a)). Similarly, β
was determined by maintaining 1-hexanol flow rate as a con-
stant, while altering ammonia concentration. The reaction or-
der of ammonia was close to 0 (Fig. 2(b)). The experiments
were repeated under varied conditions, while the results
obtained were essentially the same (Fig. S8†). The kinetic
measurements provided further evidence to support pathway
II, in which only the alcohol is involved in the rate-
determining step.
We further conducted the density functional theory (DFT)
calculations to investigate the adsorption energies of possible
species on Ni surface (see page S3 and Fig. S9 in the ESI†). Ad-
sorption on a Ni top site was considered for each adsorbate to
avoid any steric repulsion between molecule and surface. How-
ever, for unsaturated molecules such as aldehyde, imine, and
nitrile, side-on conformations on hollow sites were possible
(Fig. S9†), and aldehyde and nitrile were found to prefer side-
Fig.
1 (a) Scheme for formation of 3,5-dibutyl-2-pentyl-pyridine
(
compound X) and (b) yield of compound X with different supports.
−
1
Reaction conditions: 1 μL min hexanal, 200 mg catalyst, 190 °C, 8 mL
min NH
−
1
−1
4
−1
3
, 20 mL min
2
N . GHSV = 1.12 × 10 h .
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nol (CH IJCH ) –OH), 1-butanol-d (CH IJCH ) –OD, deuteration
3
2 3
3
2 3
at the hydroxyl group of 1-butanol) and CH IJCH ) –CD –OH
3
2 2
2
(
deuteration of both H at the α-carbon of 1-butanol) were
converted under the same reaction conditions at 160 °C.
Turnover frequency (TOF, calculated by molar amount of sub-
strate converted per molar amount of surface Ni atom per
hour) of CH IJCH ) –CD –OH was smaller than undeuterated
3
2 2
2
1-butanol, affording a normal KIE of 1.52 (Table 4), while no
normal KIE was observed with CH IJCH –OD (Table S2†).
3
2 3
)
The results suggest that C–H bond cleavage at α-carbon de-
termines the reaction rate. In the literature, the KIE of dehy-
3 2 3 2
drogenation of (CH ) CH–OH over (CH ) CD–OH was deter-
5
9
mined to be 2.0, which was higher than our result likely
due to the lower reaction temperature in that study (60 °C).
3.4 In situ DRIFTS investigation
In situ DRIFTS, an efficient surface species analysis tech-
6
0
nique, is employed to monitor the reaction between alco-
hols and ammonia over Al O support and supported Ni cata-
2
3
lysts. To slow down the reaction and observe possible
intermediates, the DRIFTS analysis was conducted at 130 °C.
Adsorption of 1-butanol on the Al O support was shown in
Fig. 2 Plots of lnIJreaction rate) v.s. (a) lnIJhexanol flow rate), hexanol
2
3
−
1
flow rate was in unit of μL min and flow rate of NH
3
and N
mL min and 76 mL min ; and (b) lnIJammonia concentration), flow
2
were 4
−1
−1
Fig. 3(a). The four bands between 2964 cm and 2736 cm
are attributed to C–H stretching of methyl (CH –) and methy-
−1
−1
−1
3
rate of 1-hexanol was 5 μL min and total gas flow rate was 80 mL
−
1
−
1
lene (–CH –) groups, while the bands at 1465 cm and 1383
2
min by supplementing N
2
. Reaction rate was calculated as mmol of
−
1
substrate converted per gram of catalyst per hour. 190 °C, 50 mg Ni/
cm belong to methyl C–H deformation and α-C–H deforma-
5
−1
59
−1
−1
Al
2
O
3
-DP catalyst. GHSV = 1.28 × 10
h
.
tion. The negative bands at 3742 cm , 3694 cm and 1639
−
1
cm are due to the consumption of surface O–H groups (Al–
6
1–63
OH) of Al
O
2 3
by hydroxyl group of butanol.
The spectra
on conformations. As shown in Table 3, N-containing species,
including ammonia, generally showed much more negative ad-
sorption energies than O-contacting species, reflecting the in-
trinsic azophilic character of Ni. As such, it is plausible that Ni
surface is covered by ammonia, while alcohol be adsorbed on
intensity did not change with time, suggesting that Al O
2
3
could not catalyse the reaction of alcohol. Adsorption of 1-bu-
tanol on Ni/Al -DP catalyst was performed at the same tem-
perature (Fig. 3(b)). The six bands indicating C–H stretching
2 3
O
2 3
the surface of Al O support (also suggested by DRIFTS, Fig. 3,
in section 3.4) and the interface between Ni and Al O . This
2
3
gives an explanation of the reaction orders of ammonia and al-
cohol as observed in the kinetic study.
In alcohol dehydrogenation, O–H bond cleavage in hy-
droxyl group and hydride abstraction via α-carbon C–H bond
breakage are the two key steps. To investigate the kinetic rele-
vance of each elementary step, isotope labelled substrates
were used to test the reaction activity. Undeuterated 1-buta-
Table 3 Adsorption energies of various adsorbates on Ni(111) surface
calculated by DFT
−
1
Adsorbate
-Hexanol
Conformation
Ead (kJ mol )
1
Top
Top
Hollow
Top
Top
Hollow
Top
Hollow
Top
−20.5
−25.9
−34.9
−71.7
−86.9
−72.3
−70.6
−97.3
−75.8
Hexanal
1
1
-Hexylamine
-Hexanimine
Hexanenitrile
Ammonia
Fig. 3 DRIFTS of adsorbed species on (a) Al
Al -DP surface as a function of time under N
0 min, 1-butanol was injected.
2
O
3
surface and (b) Ni/
flow at 130 °C. At t =
2
O
3
2
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Table 4 Kinetic isotopic effects for converting 1-butanol to butanenitrile
1-Butanol was subsequently added to the NH3 saturated
−1
catalysts. On NH3 saturated Al O , the adsorption bands of
2 3
Alcohol
TOF (h
)
KIE
butanol did not change with time, suggesting Al
initiate the reaction between alcohol and ammonia (Fig.
S12(a)†). As the alcohol was injected to the NH saturated Ni/
2 3
O could not
1
2
3
1-CH
1-CH
3
IJCH
IJCH
2
)
)
3
–OH
–CD
1.88
1.24
3
2
2
2
–OH
k
1-CH IJCH ) –OH/k-CH IJCH ) –CD –OH
1.52
3
3
2
3
3
2
2
2
2 3
Al O -DP (Fig. 4(a)), the two bands assigned to adsorbed am-
−
1
Reaction conditions: 160 °C, 1 μL min substrate, 200 mg Ni/Al
DP, 76 mL min
2
O
3
-
−1
−1
−1
−1
4
−1
monia at 1219 cm and 1626 cm started to decrease, indi-
cating consumption of ammonia. The new peaks at 1606
2 3
N , 4 mL min NH . GHSV = 3.21 × 10 h .
−
1
−1
cm and 1097 cm which could be ascribed as the N–H de-
formation and C–N stretching in amine species (Fig.
and deformation (2964, 2937, 2878, 2745, 1468 and 1386
cm ) were observed at the same positions with Al
suggesting that butanol was mainly adsorbed on the surface
of Al O . After 30 minutes, two new bands at 1570 cm and
419 cm , attributed to asymmetric and symmetric vibration
of surface-bound carboxylate species (–COO–),
increase with time, which was not observed on Al O surface.
The surface-bound carboxyl species were plausibly formed by
the condensation reaction between aldehyde groups and sur-
face hydroxyl groups (Fig. S10(a)†), providing indirect evi-
dence for the formation of aldehydes via dehydrogenation.
The observation suggested that Ni species provided the active
sites for dehydrogenation of alcohols.
DRIFTS were also used to study the adsorption of NH
and Ni/Al -DP (Fig. S11†). Ammonia could be
adsorbed on both LAS (1626, 1611, 1580 and 1268 cm ) and
BAS (1688 and 1454 cm ) on the Al
ing of Ni species, the peaks representing adsorbed ammonia
on LAS (1626 and 1611 cm ) maintained, and a new peak at
219 cm , which may be ascribed to ammonia adsorbed on
6
8
−
1
S10(b)†), appeared quickly after 1 min, which further in-
creased and then decreased with time. This observation con-
firmed the existence of step 4 in proposed pathway II. Within
the first 10 min, no peak relevant to aldehyde was observed,
indicating the dehydrogenation of alcohol was slow (rate-de-
termining step) and the produced aldehyde reacted immedi-
ately with ammonia. After 20 min, the peaks for vibration of
2 3
O ,
−
1
2
3
−
1
1
6
4,65
started to
2
3
−
1
−1
surface-bound carboxylate species (1562 cm and 1411 cm )
started to grow, probably because the amine species can con-
dense on the catalyst surface as well (Fig. S10(b)†).
6
2
The deuterated substrates were also studied via in situ
DRIFTS. Fig. 4(b) presented the reaction of CH IJCH ) –CD –
3
2 2
2
OH with ammonia on the surface of Ni/Al
2
O
3
-DP. The two
3
on
−
1
−1
peaks at 2192 cm and 2074 cm were ascribed to α-C–D
Al O
2 3
2 3
O
−
1
−
1
stretching, the peak at 1241 cm represented α-C–D defor-
5
9
−
1
66
mation. The cleavage of α-C–D bond was relatively slow, in-
2 3
O surface. With load-
dicating this step determined the overall reaction rate. As the
−
1
−1
−
1
peak at 2192 cm decreased, a new peak at 2153 cm in-
creased accordingly, which might be ascribed to vibration of
C–D bond in deuterated imine/amine group. In addition, the
−
1
1
6
7
Ni surface,
2 3
appeared. The BAS of Al O were largely
−
1
peak assigned to ammonia adsorbed on Ni at 1219 cm
suppressed after loading of Ni species, therefore inhibiting
undesired dehydration reaction catalysed by BAS. This obser-
vation is consistent with the Py-IR results (Fig. S7†).
disappeared rapidly at 1 min. This is probably due to the pro-
ton exchange between ammonia and deuterium from the
substrate, and thus the peak for deuterated ammonia moved
to a lower wavenumber. This was also observed with
3 2 3 2 3
CH IJCH ) –OD adsorbed on ammonia saturated Ni/Al O -DP
surface (Fig. S12(b)†). Similarly, the bands for amine (1606
−
1
−1
−1
cm and 1083 cm ) and carboxylate species (1558 cm and
1
−
1
417 cm ) were observed. In Fig. S12(b),† the broad peak at
Fig. 4 DRIFTS of adsorbed species on NH
surface as a function of time under N flow at 130 °C. At t = 0 min, (a)
-butanol and (b) 1,1-dideuteriobutan-1-ol was injected.
3
saturated Ni/Al
2
O
3
-DP
2 3
Fig. 5 DRIFTS of adsorbed butanenitrile on Ni/Al O -DP surface as a
2
function of time under N
was injected.
2
flow at 100 °C. At t = 0 min, butanenitrile
1
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Table 5 Conversion of various primary alcohols to corresponding nitriles
Entry
1
Substrate
Product
Temperature (°C)
Yield/selectivity (%)
230
210
86/89
86/86
a
2
a
3
210
210
70/82
79/79
a
4
a
5
210
210
86/93
95/99
a
6
a
7
230
250
84/94
83/88
a
8
9
230
68/90
−1
−1
−1
a
−1
4
−1
Reaction conditions: 1 μL min substrate, 200 mg Ni/Al
2 3
O -DP, 20 mL min
N
2
, 8 mL min NH
3
.
0.5 μL min substrate. GHSV = 1.12 × 10 h .
−
1
2
589 cm represented the vibration of deuterated O–D bond,
yield (68%). 2-Phenylethanol and other primary alcohols with
heterocyclic rings, such as 3-pyridinemethanol, tetra-
hydrofurfural alcohol, and furfuryl alcohol (Table S3, entry 1–
4†) were tested but lower yields were obtained. 4-Nitrobenzyl
alcohol was transformed to 4-aminobenzonitrile, instead of
4-nitrobenzonitrile (Table S3, entry 5†), since the nitro group
was hydrogenated on Ni catalyst.
and its rapid decrease suggested fast cleavage of O–D bond
of hydroxyl group. With deuteration at the hydroxyl group,
the reaction rate was fast that only carboxyl species (1566
cm ) were observed.
Finally, the adsorption of the product (using
butanenitrile as an example) has been conducted on Ni/
−
1
Al
250 cm decreased with time, suggesting either desorption
or conversion of butanenitrile under mild condition (Fig. 5
2
O
3
-DP. At merely 100 °C, the intensity of CN peak at
The catalytic stability test over a period of 48 hours
was conducted (Fig. 6(a)), with initial yield of
hexanenitrile at around 70%. The Ni/Al O -DP catalyst kept
2 3
90% of its original activity after 48 hours, and satisfactory
carbon balance was maintained throughout the stability
test. Indeed, TG analysis (Fig. 6(b)) over spent catalyst
suggested 13 wt% carbon deposition, which corresponds
to only about 1 mol% of carbon in the starting material
accumulatively injected in 48 hours. ATR-IR (Fig. S13†)
−
1
2
6
9
).
This explains why the characteristic peak for CN was
not observed in in situ DRIFTS analysis at 130 °C. The two
−
1
−1
peaks at 1665 cm and 1608 cm could be assigned to
7
0
CN stretching and N–H bending. The hydrogenation of
nitrile to imine was observed probably because the hydride
adsorbed during catalyst reduction step or produced in de-
hydrogenation step remained on the catalyst was added to
the nitrile group.
was further employed to analyse fresh and spent Ni/Al
2 3
O -
DP catalyst. The major bands for the spent catalyst were
−
1
ascribed to the surface-bound carboxyl species (1554 cm
−
1
and 1458 cm ), which, at least partially, accounted for
the 1 mol% of carbon loss.
3
.5 Substrate scope and catalyst stability
The Ni/Al O -DP catalyst is able to effectively convert a series
2
3
of alcohols to nitriles. Straight-chain aliphatic alcohols gave
nitriles as the only detectable products, regardless of carbon
chain length (Table 5, entry 1–4). Cyclo-aliphatic alcohols,
such as cyclohexanemethanol and cyclopentanemethanol (en-
try 5 and 6), were also easily converted into corresponding ni-
triles with high yield (86% and 95%) and excellent selectivity
Conclusions
We report in this study the development of a highly effi-
cient catalytic system to promote nitrile production from al-
cohols and ammonia gas at low temperature in the absence
of any oxidant. A simple, conventional Ni on alumina cata-
lyst prepared by deposition–precipitation method is identi-
fied to be most active. The reaction progresses mainly via a
(93% and 99%). Benzyl alcohol and substituted benzyl alco-
hol were effectively converted to aromatic nitriles (entries 7–
8
), while 3-phenyl-1-propanol (entry 9) afforded moderate
tandem
dehydrogenation–imination–dehydrogenation
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NH , while alumina is the most effective support promoting
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