Synthesis of n-butylamine from butyronitrile on Ni/SiO2: Effect of solvent

Article abstract of DOI:10.5935/0103-5053.20140215

The effect of solvent on Ni(10.5percent)/SiO₂ activity and selectivity for the liquid-phase hydrogenation of butyronitrile to butylamines was studied at 373 K and 13 bar using ethanol, benzene, toluene and cyclohexane as solvents. In ethanol, a protic solvent, the Ni catalyst yielded n-butylamine (84percent) and dibutylamine (16percent). When non-polar solvents, such as cyclohexane, toluene or benzene, were used, the solvent-catalyst interaction strength determined the selectivity to n-butylamine: the stronger the solvent-catalyst interaction the higher the n-buylamine production. The yield to n-butylamine in non-polar solvents varied between 39percent (cyclohexane) and 63percent (benzene).

Full text of DOI:10.5935/0103-5053.20140215

http://dx.doi.org/10.5935/0103-5053.20140215
J. Braz. Chem. Soc., Vol. 25, No. 12, 2272-2279, 2014.
Printed in Brazil - ©2014 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
A
Synthesis of n-Butylamine from Butyronitrile on Ni/SiO2: Effect of Solvent
Darío J. Segobia, Andrés F. Trasarti and Carlos R. Apesteguía*
Catalysis Science and Engineering Research Group (GICIC), INCAPE, UNL-CONICET,
Santiago del Estero 2654, (3000) Santa Fe, Argentina
O efeito de solvente sobre a atividade e seletividade de Ni(10.5%)/SiO2 no processo de
hidrogenação em fase líquida da butironitrila a butilaminas foi estudado a 373 K e 13 bar usando-se
etanol, benzeno, tolueno e cicloexano como solventes. Em etanol, um solvente prótico, o catalisador
de Ni produziu n-butilamina (84%) e dibutilamina (16%). Quando solventes apolares, tais como
cicloexano, tolueno ou benzeno, foram usados, a força de interação solvente-catalisador determinou
a seletividade para formação de n-butilamina: quanto mais forte a interação solvente-catalisador,
maior era a produção de n-butilamina. O rendimento de n-butilamina em solventes apolares variou
entre 39% (cicloexano) e 63% (benzeno).
The effect of solvent on Ni(10.5%)/SiO2 activity and selectivity for the liquid-phase hydrogenation
of butyronitrile to butylamines was studied at 373 K and 13 bar using ethanol, benzene, toluene and
cyclohexane as solvents. In ethanol, a protic solvent, the Ni catalyst yielded n-butylamine (84%)
and dibutylamine (16%). When non-polar solvents, such as cyclohexane, toluene or benzene, were
used, the solvent-catalyst interaction strength determined the selectivity to n-butylamine: the stronger
the solvent-catalyst interaction the higher the n-buylamine production. The yield to n-butylamine in
non-polar solvents varied between 39% (cyclohexane) and 63% (benzene).
Keywords: butyronitrile, butylamines, selective hydrogenation, Ni catalysts
Introduction
The mechanism of nitrile hydrogenation and
condensation to higher amines accepted by many authors
is based on the pioneer work of Sabatier and Senderens,4
Von Braun et al.,5 and others.6,7 This mechanism is depicted
in Scheme 1 for butyronitrile (BN) hydrogenation to
n-butylamine (BA) and coupling reactions leading to
dibutylamine (DBA) and tributylamine (TBA).8,9 Based on
the proposal of Von Braun et al.,5 Scheme 1 includes the
initial hydrogenation of butyronitrile to butylimine that is
consecutively hydrogenated to n-butylamine. Butylimine
is a highly reactive aldimine intermediate that interacts
with n-butylamine to form 1-aminodibutylamine. This
latter intermediate gives by deamination the secondary
imine, butylidene-butylamine (BBA), that is further
hydrogenated to dibutylamine. Similarly, butylimine may
react with dibutylamine giving 1-aminotributylamine that
after consecutive deamination and hydrogenation leads to
the formation of tributylamine.
The hydrogenation of nitriles is used to produce primary
amines that are valuable raw materials for obtaining many
chemicals, pharmaceuticals and polymers.1 At industrial
level, the reaction is performed over transition metal
catalysts at high hydrogen pressures. Nevertheless, nitrile
hydrogenation usually proceeds stepwise via consecutive
condensation/hydrogenation reactions that finally form a
mixture of ammonia and primary, secondary, and tertiary
amines. In order to obtain selectively primary amines,
the formation of secondary and tertiary amines has to be
avoided. In previous works, large amounts of ammonia
have often been employed to suppress the formation of
higher amines, since ammonia is released in condensation
reactions leading to higher amines.2,3 However, the addition
of ammonia creates waste problems that make the process
more expensive, and less easy to implement on an industrial
scale. Thus, there is a need to develop stable and selective
metal-supported catalysts for hydrogenating nitriles to
primary amines under mild operation conditions.
There is a general agreement that on metal-supported
catalysts the reaction selectivity depends mainly on the
nature of the metal component.10,11 It has been reported
that Ni, Co and Ru form predominantly primary amines
while Rh, Pt and Pd promote the formation of secondary
*e-mail: capesteg@fiq.unl.edu.ar

Vol. 25, No. 12, 2014
Segobia et al.
2273
Scheme 1. Reaction network of butyronitrile hydrogenation and coupling to amines.
and tertiary amines.12,13 Raney Co and Ni catalysts have
been extensively employed to yield primary amines
by liquid-phase nitrile hydrogenation.12,14-16 Supported
Co and Ni catalysts have been also studied in an
attempt to overcome the difficulties of handling skeletal
catalysts.8,11,17-19 Nevertheless, the reaction selectivity
depends also on other parameters such as the support acid/
base properties, the operation conditions and the nature of
the solvent. In particular, the influence of the solvent has
been little investigated, even though the choice of suitable
solvents is frequently critical to obtain high catalytic
activity and selectivity.20-22 In particular, butyronitrile
hydrogenation has been studied using different solvents
such as methanol, ethanol, ethyl acetate, hexane, heptane
and octane, and also under solvent-free conditions.7,8,10,11,13,15
In this work we studied the liquid-phase hydrogenation
of butyronitrile on Ni/SiO2 in ethanol (protic solvent),
cyclohexane (non-polar naphthenic solvent), and toluene
and benzene (non-polar aromatic solvents). The goal was
to establish the solvent effect on the catalyst activity and
selectivity for promoting the synthesis of n-butylamine.
In recent works we have discussed the influence that the
solvent-catalyst, solvent-reactant and reactant-solvent-
catalyst interactions have on the activity and selectivity of
metal-supported catalysts for liquid-phase hydrogenation
reactions.23-25 Here, the results will show that Ni/SiO2
activity and selectivity for BN hydrogenation greatly
depend on the solvent nature. Results are interpreted by
relating the catalyst performance with both the solvent-BA
interaction and the solvent-catalyst interaction strength.
powder (Sigma-Aldrich, G62, 60-200 mesh, 300 m2 g-1) by
incipient-wetness impregnation at 303 K. The impregnated
sample was dried overnight at 373 K, then heated in air at
5 K min-1 to 673 K and kept at this temperature for 2 h.
Brunauer-Emmett-Teller (BET) surface areas were
measured by N2 physisorption at its boiling point in a
Micromeritics Accusorb 2100E sorptometer. Elemental
compositions were measured by inductively coupled
plasma atomic emission spectroscopy (ICP-AES), using a
Perkin-Elmer Optima 2100 unit. Powder X-ray diffraction
(XRD) patterns were collected in the range of 2θ = 10-70°
using a Shimadzu XD-D1 diffractometer and Ni-filtered
Cu Ka radiation.
Metal dispersion (DNi, surface Ni atoms / total M atoms)
was determined by chemisorption of hydrogen.Volumetric
adsorption experiments were performed at 298 K in a
conventional vacuum unit. Catalysts were reduced in H2
at 673 K for 2 h and then outgassed 2 h at 673 K prior
to performing gas chemisorption experiments. Hydrogen
uptake was determined using the double isotherm method as
detailed in a previous work.26 A stoichiometric atomic ratio
of H/Nis = 1, where Nis implies a metal atom on surface,
was used to calculate the metal dispersion.
Temperature programmed reduction (TPR) experiments
were performed in a Micromeritics AutoChem II 2920,
using 5% H2/Ar gaseous mixture at 60 mL min-1 standard
temperature and pressure (STP) conditions. The sample size
was 150 mg. Samples were heated from 298 to 973 K at
10 K min-1. Since water is formed during sample reduction,
the gas exiting from the reactor was passed through a cold
trap before entering the thermal conductivity detector.
Experimental
Temperature-programmed desorption experiments
Catalyst preparation and characterization
The solvent interactions with the Ni/SiO2 catalyst
were studied by temperature-programmed desorption
(TPD) of the solvent preadsorbed at 298 K. Calcined
Ni(10.5%)/SiO2 catalyst was prepared by impregnating
Ni(NO3)2.6H2O (Fluka, 98%) on a commercial SiO2

2274
Synthesis of n-Butylamine from Butyronitrile on Ni/SiO2: Effect of Solvent
J. Braz. Chem. Soc.
samples (150 mg) were reduced at 673 K for 1 h in a
60 mL min-1 flow of H2(5%)/Ar and then cooled down
to 298 K. Afterwards, a He stream was bubbled through
the solvent in order to saturate the gaseous stream with
solvent vapor. Then, the reduced sample was exposed to
this stream for 60 min following the effluent composition
by mass spectrometry (MS) in a Baltzers Omnistar unit.
The weakly adsorbed solvent was removed by flushing
with He (60 mL min-1) at 298 K for 1 h. Temperature was
then increased at a rate of 10 K min-1 and the composition
of the reactor effluent was measured by mass spectrometry.
15-40 min for 450-650 min. The main reaction products
detected were BA, DBA, TBA, and BBA. The batch
reactor was assumed to be perfectly mixed. Interparticle
and intraparticle diffusional limitations were verified as
negligible. Conversion of butyronitrile was calculated as
XBN = CBN/(CB0N – CBN), where CB0N is the initial concentration
of butyronitrile and CBN is the concentration of butyronitrile
at reaction time t. Selectivities (Sj, mol of product j / mol
of butyronitrile reacted) were calculated as Sj = CjnBN/
(C0BN – CBN)nj where nBN and n j are the stoichiometric
coefficients of butyronitrile and product j, respectively.
Yields (hj, mol of product j / mol of butyronitrile fed) were
calculated as hj = Sj XBN.
Calorimetric determinations
Enthalpies of solvent adsorption on Ni/SiO2 and SiO2
were experimentally determined by mixing 2 g of sample
with 10 mL of each solvent in a calorimeter equipped
with a mechanical stirrer and an ERTCO-EUTECHNICS
4400 digital thermometer (0.01 K resolution). In all of
the cases, the calorimetric fluid was n-dodecane (Sigma,
99%). The catalyst samples were previously reduced and
transferred to the calorimeter in conditions similar to those
used in the catalytic activity tests. The calorimeter constant
was determined by mixing bidistilled water and absolute
ethanol (Merck, 99%). The ethanol concentration in the
final mixture was 0.05 mol L-1. The corrected temperature
increase, due to lack of adiabatic conditions and stirring
heat, was calculated by the Dickinson method.27
Results
Catalyst characterization
The BET surface area of the silica support (300 m2 g-1)
did not change significantly after metal impregnation and
the consecutive oxidation/reduction steps used for obtaining
Ni/SiO2 (290 m2 g-1). NiO (ASTM 4-835) was identified
from the XRD pattern of calcined Ni/SiO2 (not shown here).
The NiO particle size determined using the Debye-Scherrer
equation was 12 nm. The accessible Ni fraction determined
by hydrogen chemisorption was low (DNi = 1.0%), probably
because of the high Ni content.
The NiO/SiO2 TPR curve was presented in a previous
paper.11 Reduction of NiO gave rise to a single TPR
peak centered at 643 K which corresponds to the direct
reduction of Ni2+ ions to Ni0.12 No reduction peaks at
higher temperatures that would reveal the presence of less
reducible surface Ni silicates were detected.28 From these
results, it was inferred that Ni was totally in the metallic
state after the standard reduction step used prior to the
catalytic tests (reduction in pure H2 at 673 K).
Catalytic tests
The liquid-phase hydrogenation of butyronitrile
(Aldrich, > 99%) was studied at 13 bar (H2) in a Parr 4843
reactor at 373 K. The autoclave was loaded with 150 mL of
solvent, 3 mL of butyronitrile, 1.0 g of catalyst, and 1 mL of
n-dodecane (Aldrich, > 99%) as internal standard. Toluene
(Sigma-Aldrich, 99.8%), ethanol (CicarelliACS), benzene
(Merck, 99.5%) or cyclohexane (Sigma-Aldrich, 99.5%)
were used as solvents. Prior to catalytic tests, samples were
reduced ex situ in H2 (60 mL min-1) for 2 h at 673 K and
loaded immediately in the reactor at room temperature
under inert atmosphere. The reaction system was stirred at
800 rpm and heated to the reaction temperature at 2 K min-1;
the H2 pressure was then rapidly increased to 13 bar.
Product concentrations were followed during the
reaction by ex situ gas chromatography using an Agilent
6850 GC chromatograph equipped with flame ionization
detector, temperature programmer and a 50 m HP-1
capillary column (0.32 mm i.d., 1.05 mm film). Samples
from the reaction system were taken by using a loop under
pressure in order to avoid flashing. Data were collected every
Solvent properties
Table 1 presents the values of several parameters
that define the nature of the solvents used in this work.
Specifically, classical polarity parameters (dipole moment m
and dielectric constant ε) and other solvatochromic scales
(hydrogen-bond donor (a) and hydrogen-bond acceptor (b)
parameters, and ET(30) scale) are included in Table 1.
Dipole moment m is a microscopic property, i.e., a property
of an individual molecule, while dielectric constant ε is a
macroscopic property that measures the ability of a bulk
material to increase the capacitance of a condenser.29
Solvatocromic parameters a and b are, respectively, related
to the electrophilic and nucleophilic properties of a given

Vol. 25, No. 12, 2014
Segobia et al.
2275
Table 1. Polarity parameters of BN, BA and the solvents used in this
compound and measure the solvent ability for hydrogen-
bond-donation (HBD) and hydrogen-bond-acceptance
(HBA), respectively.30-32 Solvatochromic scales of polarity
are based on shifts in the absorption spectrum of a reference
dye. The position of absorption bands is sensitive to
solvent polarity because the electronic distribution of the
reference molecule in the excited state is different from
that in the ground state. The ET(30) scale is based on the
charge-transfer absorption spectra of pyridinium N-phenol
betaine.29 As the data in Table 1 show, non polar solvents
(cyclohexane, toluene and benzene) exhibit low values for
ε, m and ET(30) and they have no capability to act as H-bond
donor (a = 0). In contrast, protic ethanol has high values
for ε, m and ET(30) and can act as H-bond donor (a value
higher than 0.7). The polarity properties of BN and BA are
also included in Table 1.
work.31,32
ET(30) /
Solvent
ε
m / D
a
b
(kcal mol-1)
51.9
Ethanol
Benzene
Toluene
Cyclohexane
BN
24.6
2.28
2.38
2.02
20.3
4.92
1.69
0
0.86
0.75
0.10
0.11
0
34.3
0
0
0
0
0
0.37
0
33.9
30.9
3.5
1.3
42.5
0.40
0.72
BA
37.6
rates per g of catalyst (r0BN, mmol h-1 gcat-1) that are presented
in Table 2. From the r0BN values of Table 2 we determined
the initial turnover frequencies (TOF, min-1) and the values
are included in Table 2. Data for selectivities (Si) and XBN
at the end of the runs are also presented in Table 2.
Regarding catalyst activity, Figure 1 shows that XBN
increased continuously on Ni/SiO2 in all solvents with the
progress of the reaction, reaching 100% at the end of the
runs. The initial BN conversion rate obtained in different
solvents followed the order cyclohexane > ethanol >
toluene > benzene (Table 2). The TOF value varied between
63.8 min-1 (cyclohexane) and 19.4 min-1 (benzene). Very
few papers have quantitatively determined Ni activity
Catalyst activity and selectivity
Figure 1 presents the curves of BN conversion and
yields as a function of time obtained for Ni/SiO2 in ethanol,
benzene, toluene and cyclohexane. From the XBN vs. time
curves of Figure 1 we determined by polynomial regression
and numerical differentiation the initial BN conversion
Benzene
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Ethanol
0
100
200
300
0
100
200
300
400
500
time / min
time / min
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Cyclohexane
Toluene
0
100
200
300
400
0
100
200
300
400
time / min
time / min
Figure 1. Catalytic results: £ butyronitrile conversion (XBN) and yields (hi); l butylamine (BA); r dibutylamine (DBA); ¿ butylidene-butylamine (BBA);
ê tributylamine (TBA) [T = 373 K, P = 13 bar, Wcat = 1 g].

2276
Synthesis of n-Butylamine from Butyronitrile on Ni/SiO2: Effect of Solvent
J. Braz. Chem. Soc.
Table 2. Catalytic results
Initial activity
Conversion (XBN / %) and selectivities (%) at the end of reaction
Solvent
rB0N / (mmol h-1 gcat
)
TOF / min-1
35.6
XBN
100
100
100
100
BA
84
63
52
39
DBA
16
TBA
Others
-1
Ethanol
38.2
20.8
29.6
68.7
–
2
4
9
–
1
1
2
Benzene
Toluene
19.4
34
27.6
43
Cyclohexane
63.8
50
for the liquid-phase hydrogenation of butyronitrile.
Chojecki et al.15 studied this reaction on Raney Ni catalysts
at 373 K and 30 bar using octane as solvent and reported a
TOF value for BN conversion of 24 min-1 that is comparable
with our data in Table 2.
(the most intense signal in the cyclohexane fragmentation
mass spectrum) and the m/z 28 and 44 signals accounting
for possible fragmentation of the cyclohexane molecule.
The m/z 2 signal (not shown in Figure 2) was also followed
during the TPD of cyclohexane on Ni. Nevertheless, no
signals of evolved compounds were detected in the TPD
of cyclohexane, thereby showing that cyclohexane was
eliminated from the catalyst by the pretreatment with He
at 298 K. This result revealed that the interaction between
cyclohexane and Ni is very weak.
Desorption of toluene, followed in Figure 2 by the
m/z 92 signal corresponding to the molecular ion, occurred
at about 350 K. Desorption of several C2 and C4 fragments
(m/z 28, 43 and 44 signals in Figure 2) formed from toluene-
derived species took place at 387 K. These results showed
that toluene adsorbs irreversibly on Ni and decomposes
at relatively low temperatures. The TPD of benzene was
followed by recording the m/z 78 signal that corresponds to
the molecular ion, and other signals representing fragments
formed from benzene-derived species. In Figure 2 no
benzene desorption (m/z 78) was observed while significant
evolutions for H2, C1 and C2 species (m/z 2, 16 and 28)
were detected at temperatures higher than 500 K. These
high-temperature evolutions reflect the decomposition of
strongly chemisorbed benzene over Ni. Finally, Figure 2
shows that the desorption temperature maximum of the
peaks corresponding to the most abundant ethanol ions
(m/z 45, 31, 29, 28 and 27) appeared at 378 K. Two
additional broad bands corresponding to high temperature
H2 and C3 hydrocarbon fragment evolutions (m/z 2 and 44,
respectively) were detected between 650 and 820 K. These
evolutions indicate the presence of surface nickel sites on
which ethanol adsorbs very strongly and decomposes at
high temperatures.
Regarding catalyst selectivity, Ni/SiO2 in ethanol formed
initially BA and BBA. The BA yield increased with the
progress of the reaction reaching 84% at the end of the run
(Figure 2). The presence of BBA among the reaction products
is predicted in the reaction mechanism of Scheme 1 as the
Schiff base leading by hydrogenation to the secondary amine.
BBA is formed at the beginning of the reaction and then goes
through a maximum because it is consecutively hydrogenated
to the secondary amine, as depicted in Scheme 1. The local
slopes of the hi curves in Figure 1 give the rate of formation
of each product at a specific BN conversion and reaction
time. The DBA formation curve in ethanol presents a zero
initial slope indicating that DBA is a secondary product as
postulated in the reaction network of Scheme 1. At the end
of the reaction, Ni/SiO2 in ethanol yielded a mixture of BA
(84%) and DBA (16%), being the carbon balance 100%
(Table 2). The Ni/SiO2 selectivity changed when ethanol
was replaced by other solvents. If we consider the sequence
ethanol → benzene → toluene → cyclohexane, the data
in Table 2 show that the selectivity to BA decreased at the
expense of DBA and TBA formation. Thus, the product
distribution in cyclohexane was DBA 50%, BA 39% and
TBA 9%, being the C balance 98%.
In order to obtain more insight on the effect of solvent
on Ni/SiO2 activity and selectivity we performed additional
studies addressing the solvent-Ni interaction strength.
Specifically, we determined the solvent adsorption
enthalpies on Ni/SiO2 by calorimetry and we also
investigated the temperature-programmed desorption of
the solvents on Ni/SiO2 by analyzing the evolved products
by mass spectrometry.
The solvent-catalyst interaction strength was also
characterized by measuring the average molar adsorption
enthalpies (DH, kcal mol-1) of the solvents on Ni/SiO2 by
calorimetry. Enthalpy values were estimated by assuming:
(i) the total coverage of the metallic surface with a monolayer
of the liquid phase species; (ii) the adsorption modes
proposed in the literature.33-35 The molar adsorption enthalpies
measured on SiO2 were much lower than those determined
Solvent-catalyst interaction
The results obtained for the TPD of solvents on Ni/SiO2
are shown in Figure 2. In the case of cyclohexane TPD,
we present in Figure 2 the evolution of the m/z 56 signal

Vol. 25, No. 12, 2014
Segobia et al.
2277
378 K
Ethanol
m/z
m/z
Benzene
28
27
28
18
29
44
2
31
45
16
78
2
300
400
500
600
700
800
900
300
400
500
600
700
800
Temperature / K
Temperature / K
350 K
Toluene
387 K
Cyclohexane
m/z
43
m/z
28
44
26
56
44
41
92
28
300
400
500
Temperature / K
Figure 2. TPD profiles of ethanol, benzene, toluene and cyclohexane preadsorbed at 298 K on Ni/SiO2 [He stream: 60 mL min-1, heating rate: 10 K min-1].
600
700
800
300
400
500
600
700
800
900
Temperature / K
on Ni/SiO2 and so they were considered negligible. In the
case of non-polar solvents, the average molar adsorption
enthalpies determined for benzene, toluene and cyclohexane
were 27, 16 and 3 kcal mol-1, respectively. These DH values
showed that benzene and toluene interact more strongly
than cyclohexane with the Ni/SiO2 surface, which is in
agreement with the solvent TPD results of Figure 2. As it
has been proposed, the interaction between the cyclohexane
saturated ring and metallic Ni surface should be very weak.36
Consistently with our results, previous works have reported
that the toluene adsorption on Ni metal is weaker than
benzene adsorption, probably because the repulsion forces
between the methyl group and metallic nickel surface cause
a tilted adsorption of the toluene molecule.33,34,37 Finally, we
determined that the molar adsorption enthalpy of ethanol on
Ni/SiO2 was about 10 kcal mol-1.
with the metal nature have been interpreted in terms of
the formation of different reactive intermediates and their
adsorption strength on the metal surface. In general, it is
accepted that on metal catalysts the selective formation
of primary amines from nitrile hydrogenation occurs via
nitrene intermediates, whereas the preferential formation
of secondary and tertiary amines takes place via carbene or
aldimine intermediates.13,16,38,39 In particular, the preferential
formation of BA from BN on Ni, Co, Ru has been attributed
to the fact that BN is adsorbed onto the metal via nitrene
intermediates.40,41 Huang and Sachtler40 stated that because
the Ru=N bond is very strong the condensation steps
yielding secondary and ternary amines are not favored via
“immobile” nitrene intermediates. Similarly, Chojecki et al.41
explained the selective formation of BA from BN on Raney
Co and Ni catalysts by speculating that strong binding via
the nitrogen atom may stimulate fast hydrogenation of the
carbon atom in the nitrile group, which prevents secondary
condensation reactions. Here, we observe that Ni/SiO2 in
ethanol, a protic solvent, formed initially BA and then DBA
via BBA, but the BA yield at the end of the reaction was
very high (hBA = 84%), in agreement with previous works
Discussion
Previous reports have stated that the selectivity of metal-
supported catalysts for nitrile hydrogenation depends mainly
on the metal nature.10,11,13 Differences in catalyst selectivity

2278
Synthesis of n-Butylamine from Butyronitrile on Ni/SiO2: Effect of Solvent
J. Braz. Chem. Soc.
reporting the preferential formation of BA on Ni catalysts.
However, Ni activity and selectivity significantly changed
when non-polar solvents replaced ethanol; in cyclohexane,
Ni/SiO2 formed preponderantly DBA.
interaction, would explain then the high selectivity to BA
that Ni/SiO2 exhibits in this alcohol.
In summary, if the butyronitrile hydrogenation reaction
is performed in non-polar solvents, selective BA formation
is favored when a strong solvent-Ni interaction takes place
and hinders the readsorption of BA over the metal surface.
Nevertheless, the highest BA yields are obtained in protic
alcohols, such as ethanol, that strongly interact with BA in
the liquid phase and hamper BA adsorption on the catalyst.
By analyzing the results obtained in non-polar solvents
in the sequence benzene → toluene → cyclohexane, it is
inferred that the BN conversion rate increases while the
selectivity to BA decreases at the expense of DBA and
TBA formation. These changes in catalyst activity and
selectivity may be explained by considering that the TPD
experiment results of Figure 2 and solvent adsorption
entalphies measurements showed that the solvent-metal
interaction strength on Ni/SiO2 follows the order benzene >
toluene > cyclohexane. According to the reaction network
of Scheme 1, formation of DBA requires the readsorption
of BA over the Ni surface to react with butylimine and
produce the secondary amine. Thus, selective formation of
BA would be favored when a strong solvent-Ni interaction
takes place (benzene-Ni interaction, for example) and
hinders the readsorption of BA over the metal surface. This
assumption explains why the selectivity to DBA increases
when benzene is replaced by toluene or cyclohexane,
because the reaction between butylimine and adsorbed
BA is easier to proceed when the solvent does not compete
with both reactants for metal active sites. Similarly, it can
be expected that formation of TBA, which is formed by
condensation of DBA with butylimine, would be favored in
solvents having a weaker interaction with Ni. This is exactly
what we observed here because the highest selectivity to
TBA was obtained in cyclohexane. This interpretation
also explains why the highest BN conversion rate on Ni/
SiO2 was observed in cyclohexane; in fact, because the
interaction between cyclohexane and Ni is negligible,
the solvent will not block any surface active sites for BN
adsorption and conversion.
Conclusions
Solvent nature plays a crucial role in controlling
the Ni/SiO2 activity and selectivity for the synthesis of
n-butylamine from butyronitrile hydrogenation. When non-
polar solvents such as cyclohexane, toluene or benzene are
used, the solvent-catalyst interaction strength determines
the selectivity to n-butylamine: the stronger the solvent-
catalyst interaction the higher the n-buylamine yield. This
is because a strong solvent-Ni interaction hinders the
readsorption of n-buylamine over the metal surface and,
as a consequence, impedes the formation of dibutylamine
that occurs by surface condensation between n-butylamine
and butylimine.
If the reaction is carried out in protic alcohols, such as
ethanol, the solvent-butylamine interaction strength in the
liquid phase controls the selectivity to n-butylamine on
Ni/SiO2. Ethanol is an H-bond donor solvent that strongly
interacts with H-bond acceptor n-butylamine and causes its
solvation in the liquid phase. The n-butylamine molecules
are then surrounded by alcohol molecules that hinder
n-butylamine adsorption on Ni and, as a consequence,
also decrease the consecutive formation of dibutylamine.
Acknowledgements
The highest BA yield on Ni/SiO2 was obtained in
ethanol. This result cannot be explained in terms of
stronger ethanol-Ni interaction strength because our
results showed that the adsorption enthalpy of ethanol
on Ni is lower than those of benzene or toluene. Data
in Table 1 shows that ethanol is a protic H-bond donor
(a = 0.86) solvent that exhibits high values for polarity
parameters ε, m and ET(30). In contrast, BA is an H-bond
acceptor molecule of b = 0.72. Thus, it can be expected
that a strong interaction will exist between BA and protic
ethanol causing BA solvation in the liquid phase. The
BA molecules would be then surrounded by alcohol
molecules that would hinder BA adsorption on Ni and,
as a consequence, also the formation of DBA that occurs
by surface condensation between BA and butyilimine.
The solvation of BA in ethanol, i.e., a solvent-reactant
We thank the Universidad Nacional del Litoral
(UNL), the Agencia Nacional de Promoción Científica
y Tecnológica (ANPCyT) and the Consejo Nacional
de Investigaciones Científicas y Técnicas(CONICET),
Argentina, for the financial support of this work.
References
1. Turcotte, M. G.; Johnson, T. A. In Kirk-Othmer Encyclopedia
of Chemical Technology; Kroschwitz, J. I., ed.; Wiley: New
York, 1992, pp. 369.
2. Kukula, P.; Studer, M.; Blaser, H. U.; Adv. Synth. Catal. 2004,
346, 1487.
3. Das, S.; Zhou, S.;Addis, D.; Enthaler, S.; Junge, K.; Beller, M.;
Top. Catal. 2010, 53, 979.

Vol. 25, No. 12, 2014
Segobia et al.
2279
4. Sabatier, P.; Senderens, J. B.; C. R. Hebd. Seances Acad. Sci.
1905, 140, 482.
25. Bertero, N. M.; Trasarti,A. F.;Apesteguía, C. R.; Marchi,A. J.;
Appl. Catal., A 2014, 475, 282.
5. Von Braun, J.; Blessing, G.; Zobel, F.; Ber. Dtsch. Chem. Ges.
1923, 36, 1988.
26. Borgna, A.; Garetto, T. F.; Apesteguía, C. R.; Le Normand, F.;
Moraweck, B.; J. Catal. 1999, 186, 433.
6. Juday, R.; Adkins, H.; J. Am. Chem. Soc. 1955, 77, 4559.
7. Greenfield, H.; I&EC Prod. Res. Dev. 1967, 6, 142.
8. Witte, P. T.; Collect. Czech. Chem. Commun. 2007, 72, 468.
9. Ortiz-Hernandez, I.; Williams, C. T.; Langmuir 2007, 23, 3172.
10. Huang, Y.; Sachtler, W. M. H.; Appl. Catal., A 1999, 182, 365.
11. Segobia, D. J.; Trasarti,A. F.;Apesteguía, C. R.; Appl. Catal., A
2012, 445-446, 69.
27. Coops, J.; Jessup, R. S.; van Nes, K. In Experimental
Thermochemistry; Rossini, F. D., ed.; Interscience: NewYork,
1956, pp. 6.
28. Trasarti, A. F.; Marchi, A. J.; Apesteguía, C. R.; Catal. Comm.
2013, 32, 62.
29. Reidchardt, C.; Solvents and Solvent Effects in Organic
Chemistry; Wiley VCH: New York, 2004.
12. Volf, J.; Pasek, J.; Stud. Surf. Sci. Catal. 1986, 27, 105.
13. Krupka, J.; Pasek, J.; Curr. Org. Chem. 2012, 16, 988.
14. Hochard, F.; Jobic, H.; Massardier, J.; Renouprez,A. J.; J. Mol.
Catal. A: Chem. 1995, 95, 163.
30. Kamlet, M. J.; Abboud, J. L. M.; Abraham, M. H.; Grellier,
P. L.; Doherty, R. M.; Taft, R.; Can. J. Chem. 1988, 66, 2673.
31. Marcus, Y.; Chem. Soc. Rev. 1993, 22, 409.
32. Reichardt, C.; Chem. Rev. 1994, 94, 2319.
33. Friend, C. M.; Muetterties, E. L.; J. Am. Chem. Soc. 1981, 103,
773.
15. Chojecki,A.;Veprek-Heijman, M.; Müller, T. E.; Schärringer, P.;
Veprek, S.; Lercher, J. A.; J. Catal. 2007, 245, 237.
16. Scharringer, P.; Muller, T. E.; Jentys,A.; Lercher, J.A.; J. Catal.
2009, 263, 34.
34. Myers, A. K.; Schoofs, G. R.; Benziger, J. B.; J. Phys. Chem.
1987, 91, 2230.
17. Huang,Y.;Adeeva,V.; Sachtler, W. M. H.; Appl. Catal., A 2000,
196, 73.
35. Bigot, B.; Delbecq, F.; Milet,A.; Peuch,V.; J. Catal. 1996, 159,
383.
18. Li, H.; Wu,Y.; Luo, H.; Wang, M.; Xu,Y.; J. Catal. 2003, 214,
15.
36. Rautanen, P. A.; Aittamaa, J. R.; Krause, A. O. I.; Ind. Eng.
Chem. Res. 2000, 39, 4032.
19. Yadav, G. D.; Kharkara, M. R.; Appl. Catal., A 1995, 126, 115.
20. Rylander, P. N.; Catalytic Hydrogenation over Platinum Metals;
Academic Press: New York, 1967.
37. Myers, A. K.; Benziger, J. B.; Langmuir 1987, 3, 414.
38. Coq, B.; Tichit, D.; Ribet, S.; J. Catal. 2000, 189, 117.
39. Poupin, C.; Maache, R.; Pirault-Roy, L.; Brahmi, R.; Williams,
C. T.; Appl. Catal., A 2014, 475, 363.
21. Hegedus, L.; Máthé, T.; Kárpáti, T.; Appl. Catal., A 2008, 349,
40.
40. Huang, J. Y.; Sachtler, W. M. H.; J. Catal. 1999, 184, 247.
41. Chojecki,A.; Jobic, H.; Jentys,A.; Müller, T. E.; Lercher, J.A.;
Catal. Lett. 2004, 7, 155.
22. Besson, M.; Bonnier, J. M.; Joucla, M.; Bull. Soc. Chim. Fr.
1990, 127, 5.
23. Bertero, N. M.;Apesteguía, C. R.; Marchi,A. J.; Appl. Catal., A
2008, 349, 100.
Submitted: June 26, 2014
24. Bertero, N. M.; Trasarti,A. F.;Apesteguía, C. R.; Marchi,A. J.;
Appl. Catal., A 2011, 394, 228.
Published online: September 16, 2014