Chemoselective hydrogenation of unsaturated nitriles to unsaturated primary amines: Conversion of cinnamonitrile on metal-supported catalysts

Article abstract of DOI:10.1016/j.apcata.2015.01.028

The liquid-phase hydrogenation of cinnamonitrile to selectively obtain the unsaturated primary amine (cinnamylamine) was studied at 383 K and 13 bar on Ni, Co, Ru and Cu metals supported on a commercial silica. Ni/SiO₂ and Co/SiO₂ were the most active catalysts for cinnamonitrile conversion but formed only small amounts of cinnamylamine. In contrast, Cu/SiO₂ and Ru/SiO₂ presented low activity for cinnamonitrile hydrogenation but formed selectively cinnamylamine in the liquid phase; nevertheless, on both samples the carbon balance was only about 40%. In an attempt of promoting the rate and yield to cinnamylamine, additional catalytic runs were carried out at higher temperatures and H₂ pressures on a highly dispersed Cu(11%)/SiO₂ catalyst prepared by the chemisorption-hydrolysis method. Results showed that when cinnamonitrile hydrogenation was performed at 403 K and 40 bar on Cu(11%)/SiO₂, the yield to cinnamylamine was 74% giving as by-product only the unsaturated secondary amine (dicinnamylamine).

Full text of DOI:10.1016/j.apcata.2015.01.028

Applied Catalysis A: General 494 (2015) 41–47
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Chemoselective hydrogenation of unsaturated nitriles to unsaturated
primary amines: Conversion of cinnamonitrile on metal-supported
catalysts
D.J. Segobia, A.F. Trasarti, C.R. Apesteguía∗
Catalysis Science and Engineering Research Group (GICIC), INCAPE, UNL-CONICET, Santiago del Estero 2654, (3000) Santa Fe, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
The liquid-phase hydrogenation of cinnamonitrile to selectively obtain the unsaturated primary amine
(cinnamylamine) was studied at 383 K and 13 bar on Ni, Co, Ru and Cu metals supported on a commer-
cial silica. Ni/SiO2 and Co/SiO2 were the most active catalysts for cinnamonitrile conversion but formed
only small amounts of cinnamylamine. In contrast, Cu/SiO2 and Ru/SiO2 presented low activity for cinna-
monitrile hydrogenation but formed selectively cinnamylamine in the liquid phase; nevertheless, on both
samples the carbon balance was only about 40%. In an attempt of promoting the rate and yield to cinnamy-
lamine, additional catalytic runs were carried out at higher temperatures and H2 pressures on a highly
dispersed Cu(11%)/SiO2 catalyst prepared by the chemisorption–hydrolysis method. Results showed that
when cinnamonitrile hydrogenation was performed at 403 K and 40 bar on Cu(11%)/SiO2, the yield to cin-
namylamine was 74% giving as by-product only the unsaturated secondary amine (dicinnamylamine).
Received 13 November 2014
Received in revised form 10 January 2015
Accepted 16 January 2015
Available online 28 January 2015
Keywords:
Cinnamonitrile hydrogenation
Unsaturated amines synthesis
Selective hydrogenation
Metal-supported catalysts
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
have been carried out on Ni and Co-based catalysts [4,5,7,8], prob-
ably because both metals are highly active and selective to obtain
The liquid-phase selective hydrogenation of unsaturated nitriles
on solid catalysts is an important route to obtain unsaturated
primary amines that are valuable intermediates in agrochemical,
pharmaceutical, and fine chemicals industries [1,2]. Nevertheless,
the selective hydrogenation of the C≡N group in unsaturated
nitriles is still a challenging objective in heterogeneous cataly-
sis [3]. In general, the hydrogenation of unsaturated nitriles may
form three types of products: unsaturated amines (hydrogenation
of C≡N bond), saturated nitriles (hydrogenation of C C bond) and
saturated amines (hydrogenation of C C and C N bonds). Besides,
in order to obtain selectively primary amines the coupling reac-
tions leading to secondary and tertiary amines have to be avoided.
Previous works have reported that C C bonds are more reactive
to hydrogenation than the nitrile groups [4,5] and therefore selec-
tive C≡N hydrogenation in presence of double bonds is difficult
to achieve, in particular when both unsaturated groups are con-
jugated or in close proximity [6]. Most of the few papers dealing
with the hydrogenation of unsaturated nitriles on solid catalysts
saturated primary amines from the corresponding saturated nitrile
[9–11]. However, in the case of unsaturated nitriles, Raney Ni and
Co catalysts promote the preferential hydrogenation to the corre-
sponding saturated nitrile rather than to the unsaturated primary
amine [3]. Thus, in an attempt of tuning the Ni(Co) selectivity to the
formation of unsaturated primary amines, Cr-doped Raney Ni(Co)
catalysts and amorphous Ni(Co)–B alloys were employed, but sig-
nificant yields to the unsaturated amine where obtained only when
alkaline or ammonia solutions were co-fed to the reactor [3,6,12].
Ammonia is often employed to suppress the formation of higher
amines by shifting equilibrium to the primary amine since ammo-
nia is released in the coupling reactions leading to secondary and
tertiary amines; nevertheless, the addition of ammonia entails con-
cerns related to corrosion and disposal of spent base materials.
Cinnamonitrile has been used as a substrate molecule to study
the hydrogenation of ␣,␤-unsaturated nitriles [3,6]. In this com-
pound, the C C bond is in conjugation with the nitrile as well as
with the phenyl group. In Scheme 1 we present a possible reac-
tion network of the cinnamonitrile conversion to primary and
higher amines according to [3]. Cinnamonitrile (CN) may be ini-
tially hydrogenated in the C C bond to yield hydrocinnamonitrile
(HCN) or in the C N group producing cinnamylamine (CA), the
desired unsaturated primary amine, probably via the formation of
an imine intermediate (cinnamylimine). CA may then react with the
∗
Corresponding author. Tel.: +54 342 4555279; fax: +54 342 4531068.
E-mail addresses: capesteg@fiq.unl.edu.ar, capesteg@gmail.com
(C.R. Apesteguía).
URL: http://www.fiq.unl.edu.ar/gicic/ (C.R. Apesteguía).
http://dx.doi.org/10.1016/j.apcata.2015.01.028
0926-860X/© 2015 Elsevier B.V. All rights reserved.

42
D.J. Segobia et al. / Applied Catalysis A: General 494 (2015) 41–47
hydrocinnamyl-cinnamylamine
(HCCA)
dihydrocinnamylamine
(DiHCA)
H2
NH
NH
+ H2
+ CA
+ CA
+ H2
- NH3
- NH3
NH
H2
H2
N
H2
hydrocinnamylimine
H2
hydrocinnamonitrile
(HCN)
NH2
N
H2
H2
hydrocinnamylamine
(HCA)
cinnamonitrile
(CN)
NH2
NH
H2
cinnamylimine
cinnamylamine
(CA)
- NH3
+ H2
+ CA
+ CA
+ H2
- NH3
NH
H2
hydrocinnamyl-cinnamylamine
(HCCA)
NH
dicinnamylamine
(DiCA)
Scheme 1. Probable reaction network of cinnamonitrile conversion reactions
G62, 60–200 mesh, 300 m2/g) by incipient-wetness impregna-
tion at 303 K. Metal nitrate solutions [Co(NO3)2·6H2O Aldrich 98%,
Ni(NO3)2·6H2O Fluka 98%, Cu(NO3)2·3H2O Anedra 98%] were used
for impregnating Co, Ni, and Cu while Ru/SiO2 was prepared by
using RuCl3·H2O (Aldrich 99.98%). The impregnated samples were
dried overnight at 373 K, then heated in air at 5 K/min to 673 K
and kept at this temperature for 2 h. Cu/SiO2-I and Cu/SiO2-II cat-
alysts were obtained by supporting Cu on Grace Davison Davisil
(grade 634, 320 m2/g) and Merck (chromatographic, 366 m2/g) sil-
icas, respectively, using the chemisorption–hydrolysis method as
detailed elsewhere [13]. The support was added to a solution con-
taining [Cu(NH3)4]2+ and the slurry was slowly diluted with water.
Then, solids were separated by filtration, washed with water, dried
overnight at 383 K and calcined in air at 623 K for 4 h.
imine intermediate to form by deamination the unsaturated sec-
ondary amine dicinnamylamine (DiCA) and partially unsaturated
secondary amine hydrocinnamyl-cinnamylamine (HCCA). Other-
wise, CA may be hydrogenated to the saturated primary amine
(hydrocinnamylamine, HCA). Similarly, in parallel pathways, HCN
may be hydrogenated to HCA or it may react with the imine inter-
mediate (hydrocinnamylimine) to produce by deamination the
saturated secondary amine (dihydrocinnamylamine, DiHCA) and
partially unsaturated secondary amine HCCA. According to bib-
liography, the highest yields to CA (≈70%) from cinnamonitrile
hydrogenation have been obtained on Cr-doped Raney cobalt cat-
alysts at 373 K, using 14% NH3 in methanol and 80 bar H2 [3].
In this work, we investigate the selective hydrogenation of cin-
namonitrile to CA on silica-supported Co, Ni, Ru and Cu metals. In
our catalytic tests, we detected and identified all the compounds
depicted in Scheme 1, excepting the highly reactive imine interme-
diates. The goal of our work was twofold: (i) to establish the effect
of the nature of the metal on the catalyst activity and selectivity for
selectively obtaining CA from cinnamonitrile hydrogenation; (ii) to
achieve high CA yields from cinnamonitrile conversion by selecting
efficient metal-supported catalysts and optimizing reaction oper-
ating conditions, without using ammonia in the reaction media.
Results will show that Cu/SiO2 catalyst remarkably promotes the
selective formation of CA from cinnamonitrile, thereby yielding 74%
of CA at 403 K and 40 bar H2, in absence of ammonia.
2.2. Catalyst characterization
BET surface areas (Sg) 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 2ꢀ = 10–70◦ using a Shimadzu XD-
D1 diffractometer and Ni-filtered Cu K␣ radiation. Oxide crystallite
sizes were calculated using the Debye–Scherrer equation.
The temperature programmed reduction (TPR) experiments
were performed in a Micromeritics AutoChem II 2920, using 5%
H2/Ar gaseous mixture at 60 cm3/min STP. The sample size was
150 mg. Samples were heated from 298 to 973 K at 10 K/min.
The metal dispersions (DM, surface M atoms/total M atoms)
of Ni/SiO2 and Ru/SiO2 were determined by H2 chemisorption.
Volumetric adsorption experiments were performed at 298 K in
2. Experimental
2.1. Catalyst preparation
Co/SiO2, Ni/SiO2, Cu/SiO2 and Ru/SiO2 catalysts were prepared
by supporting Co, Ni, Cu, or Ru on a SiO2 powder (Sigma–Aldrich

D.J. Segobia et al. / Applied Catalysis A: General 494 (2015) 41–47
43
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 [14]. A stoichiometric atomic
ratio of H/Ms = 1, where Ms implies a metal atom on surface, were
used to calculate the metal dispersion. The copper dispersion was
measured by combining N2O oxidation with TPR, according to Sato
et al. [15]. Cu-containing samples were reduced in H2(5%)/Ar for
1 h at 623 K and then exposed to pulses of N2O at 363 K in a flow
of He. The evolution of the N2O signal (m/z = 44) was followed by
mass spectrometry in a Balzers Omnistar spectrometer. When the
N2O signal intensity was constant indicating that the Cu surface
oxidation was completed, the system was purged in Ar and then a
TPR run was carried out using a H2(5%)/Ar flow (60 cm3/min) and
by increasing the temperature from 363 K to 623 K at 10 K/min. The
H2 consumption was monitored by mass spectrometry; quantita-
tive H2 uptakes were calculated by integration of the experimental
TPR curves and the number of chemisorbed oxygen atoms was
calculated by using a Cus/N2O = 2 stoichiometry [16].
Fig. 1. X-ray diffractograms of calcined samples. ꢀ RuO2, ꢁ CuO, ꢀ NiO, ꢂ Co3O4
where ␯ and ꢁ are the stoichiometric coefficients of cinnamoni-
trile and product j, respectively. Yields (ꢂj, mol of product j/mol of
CN
j
2.3. Catalytic activity
cinnamonitrile fed) were calculated as ꢂj = SjXCN
.
Standard catalytic runs for the liquid-phase hydrogenation of
cinnamonitrile (Aldrich, >98%) were carried out at 383 K and 13 bar
(H2) in a Parr 4843 batch reactor. The autoclave was loaded with
150 mL of solvent (toluene, Cicarelli, ACS), 1.5 mL of cinnamonitrile,
0.5 g of catalyst, and 1 mL of n-hexadecane (Aldrich >99%) as inter-
nal standard. Prior to catalytic tests, samples were reduced ex-situ
in hydrogen (60 mL/min) for 2 h at 543 K (Cu) or 673 K (Co, Ni, Ru)
and loaded immediately to 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; the H2 pressure was
then rapidly increased to 13 bar. Product concentrations were fol-
lowed 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 col-
umn (50 m × 0.32 mm ID, 1.05 ␮m film). Product identification was
achieved using a Varian Saturn 2000 GC–MS unit with a VF5ht capil-
lary column. Samples from the reaction system were taken by using
a loop under pressure in order to avoid flashing. Data were collected
every 15–40 min for 300–600 min. The main reaction products
detected were saturated nitrile HCN, unsaturated primary amine
CA, saturated primary amine HCA, unsaturated secondary amine
DiCA, partially unsaturated secondary amine HCCA and saturated
secondary amine DiHCA. The presence of tertiary amines was not
detected in any case. The batch reactor was assumed to be perfectly
mixed. Interparticle and intraparticle diffusional limitations were
verified as negligible. Conversion of cinnamonitrile was calculated
3. Results and discussion
3.1. Catalyst characterization
The metal loadings and physical properties of the catalysts are
presented in Table 1. The surface area of the three different sil-
icas used as support did not change significantly after the metal
impregnation and the consecutive oxidation/reduction steps used
for obtaining metal/SiO2 catalysts. The XRD patterns of calcined
samples are shown in Fig. 1. RuO2 (ASTM 21-1172), CuO (ASTM 5-
0661), NiO (ASTM 4-835) and Co3O4 (ASTM 9-418) were identified
on Ru/SiO2, Cu/SiO2, Ni/SiO2 and Co/SiO2, respectively. In contrast,
the XRD diffractograms of Cu/SiO2-I and Cu/SiO2-II did not show
the presence of crystalline metal oxides. The oxide particle sizes
determined using the Debye–Scherrer equation were 18 nm (CuO),
12 nm (NiO) and 12 nm (Co3O4). The XRD signal obtained for RuO2
on Ru/SiO2 was too weak for determining with exactitude the RuO2
particle size.
The sample TPR profiles are shown in Fig. 2 and the corre-
sponding temperature maxima (Tmax) are included in Table 1. The
Ni/SiO2 TPR curve exhibited a broad reduction band with a max-
imum at 643 K corresponding to the direct reduction of NiO to
metallic nickel. No reduction peak at higher temperatures that
would reveal the presence of less reducible surface Ni silicates
was observed. The TPR profile of Co/SiO2 showed two reduction
peaks at 573 K and 623 K, respectively, which result from the reduc-
tion of Co3O4 following the sequence Co3+ → Co2+ → Co0 [17,18].
Reduction of CuO on Cu/SiO2 gave rise to a broad H2 consumption
as XCN = (C0 − CCN)/CC0N, where CC0N is the initial concentration
CN
of cinnamonitrile and CCN is the concentration of cinnamonitrile
at reaction time t. Selectivities (Sj, mol of product j/mol of cin-
namonitrile reacted) were calculated as Sj = CjꢁCN/(C0 − CCN)ꢁj
CN
Table 1
Catalyst characterization.
Catalyst
Metal loadinga (%)
Sg (m2/g)
Metal dispersion DM (%)
Oxide particle sized (nm)
TPR, Tmax (K)
Co/SiO2
Ni/SiO2
Ru/SiO2
Cu/SiO2
Cu/SiO2-I
Cu/SiO2-II
9.8
10.5
1.8
9.2
7.6
307
290
280
285
321
386
–
12 (Co3O4)
12 (NiO)
n.d.
18 (CuO)
n.d.
573, 623
643
470
543
497
1b
2b
1c
32c
21c
11.0
n.d.
507
n.d.: not detected.
a
Determined by ICP-AES.
Determined by H2 chemisorption.
Determined by N2O titration.
b
c
d
Determined from the XRD diffractogram using the Debye–Scherrer equation.

44
D.J. Segobia et al. / Applied Catalysis A: General 494 (2015) 41–47
(reduction in pure H2 at 543 K for Cu/SiO2 and at 673 K for the rest of
1x10-3
metal/SiO2 samples). Finally, the Ru/SiO2 TPR curve exhibited two
superimposed reduction peaks that probably reflect the existence
of a bimodal distribution of particle sizes, as reported in bibliog-
raphy for Ru-supported catalysts obtained by calcination of RuCl3
precursors [22].
Ni/SiO2
Co/SiO2
The obtained metal dispersion values are also presented in
Table 1. The metallic dispersion of Ni on Ni/SiO2 was only about
1% and this result is consistent with the large NiO crystallite sizes
determined from the X-ray diffraction pattern of Fig. 1. This low
DNi value is consequence of both the high Ni loading, and the
preparation method used to impregnate Ni nitrates onto silica. Ru
dispersion on Ru/SiO2 catalyst was also low, about 2%. The DCu value
determined on Cu/SiO2 catalyst was only 1%, which is consistent
with the large CuO crystallite size determined by XRD and proba-
bly reflects a weak interaction between the copper crystallites and
the support. In contrast, the Cu dispersion was high on Cu/SiO2-I
(32%) and Cu/SiO2-II (21%), thereby confirming that preparation of
Cu-supported catalysts by the chemisorption–hydrolysis method
leads to the formation of small Cu particles [13,23].
Cu/SiO2
Cu/SiO2-II
Cu/SiO2-I
Ru/SiO2
300
400
500
600
700
800
900 1000
Temperature (K)
Fig. 2. TPR profiles of the samples used in this work
band, which suggests a heterogeneous size distribution of the CuO
particles [19]. The temperature maximum for CuO reduction on
Cu/SiO2 (543 K) was higher than on Cu/SiO2-I (497 K) and Cu/SiO2-
II (507 K), probably reflecting that Cu/SiO2 contains bigger CuO
crystallites according to XRD characterization data in Fig. 1. The
increase of Tmax with the CuO crystallite size is consistent with the
unreacted shrinking core model predictions for non-porous metal
oxides reduction. Previous works [20,21] have effectively furnished
convincing experimental evidence for the dependence of Tmax on
particle size by modeling TPR profiles of different-sized particles
of CuO with the unreacted core model. Taking into account the
Tmax values in Table 1, it is inferred that on all the catalysts used
in this work the metal fraction is totally in the metallic state fol-
lowing the standard reduction step used prior to catalytic tests
3.2. Catalyst activity and selectivity
In Fig. 3 we have represented the CN conversion and yields as
a function of parameter W · t/n0CN, where t is the reaction time, W
is the catalyst weight, and n0CN the initial moles of cinnamonitrile.
The local slope of each curve in Fig. 3 gives the CN conversion rate
at a specific value of CN conversion and reaction time. The initial
CN conversion rate per g of metal (rC0N, mmol/h gM) determined
from the curves of Fig. 3 is shown in Table 2. The values of XCN and
selectivities Si at the end of the runs are also presented in Table 2.
Data in Table 2 show that rC0N followed the order: Ni > Co > Ru > Cu.
On Ni/SiO2 and Co/SiO2, XCN rapidly increased with the progress
of the reaction reaching 100% at the end of the runs. In contrast,
Ni/SiO2
Co/SiO2
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
0
100
200
300
400
500
600
0
100
200
300
400
W t/noCN (g h/mol)
W t/n0CN
(g h/mol)
0.4
0.3
0.2
0.1
0.0
0.4
0.4
0.3
0.2
0.1
0.0
0.4
Cu/SiO2
Ru/SiO2
0.3
0.2
0.1
0.0
0.3
0.2
0.1
0.0
0
100
200
300
400
0
100
200
300
W t/n0CN (g h/mol)
W t/n0CN (g h/mol)
Fig. 3. Catalytic results: cinnamonitrile conversion (XBN) and yields (ꢂi). ᭹ HCN, HCA,
HCCA, CA, DiHCA, DiCA [T = 383 K, P = 13 bar, Wcat = 0.5 g].

D.J. Segobia et al. / Applied Catalysis A: General 494 (2015) 41–47
45
Table 2
Catalytic results.
Catalyst
Reaction length (min)
Initial activity
Conversion (XCN,%) and selectivities (%) at the end of reaction
rC0N (mmol/h gM
)
XCN
CA
HCA
DiCA
DiHCA
HCAA
Others
Co/SiO2
Ni/SiO2
Ru/SiO2
Cu/SiO2
500
800
500
430
90
102
33
100
100
8
–
–
40
46
52
33
–
10
–
–
–
63
–
30
–
–
8
4
60
54
23
10
–
–
–
–
T = 383 K, rC0N = 13 bar, Wcat = 0.5 g, VCN = 1.5 mL, solvent: toluene (150 mL).
the CN conversion on Ru/SiO2 and Cu/SiO2 slowly increased with
reaction time reaching only about 10% after 360 min of reaction.
Regarding catalyst selectivity, Fig. 3 shows that CN was rapidly
converted on Co/SiO2 but formation of primary products HCN and
CA was detected only after a long induction period of about 100 min,
probably reflecting a strong adsorption of the reactant and inter-
mediates. HCN and CA reached a maximum yield as they converted
to saturated primary amine HCA and secondary amines, HCCA and
DiCA, with increasing reaction time (Scheme 2A). At the end of the
run, Co/SiO2 yielded 52% of HCA and the carbon balance was 92%
(Table 2). Initially, Ni/SiO2 selectively hydrogenated CN to HCN and
we did not detect unsaturated primary amine CA among the reac-
tion products. Then, the HCN yield curve in Fig. 3 went through a
maximum because HCN was converted to saturated primary amine
HCA and to secondary amines HCCA and DiHCA. Unsaturated sec-
ondary amine HCCA was completely hydrogenated to DiHCA with
the progress of the reaction so that at the end of the catalytic test
the product mixture contained only DiHCA (63%) and HCA (33%).
Scheme 2B summarizes the consecutive reaction pathways leading
from CN to final products on Ni/SiO2 catalyst. Finally, on Ru/SiO2
and Cu/SiO2 we observed the formation of only one product: unsat-
urated primary amine CA. However, CN conversion did not exceed
10% on both catalysts revealing the sample deactivation with the
progress of the reaction, probably because of a strong adsorption of
reactant, intermediate and products on the metals since the carbon
balance was very low on these catalysts (40–46%, Table 2).
The different catalytic performances of the samples in Table 2
may be interpreted by considering the ratio of repulsive and attrac-
tive forces (FR/FA) existing between the emergent d-orbitals of the
metal and the adsorbate, as frequently observed in bibliography
for hydrogenation reactions involving ␣,␤-unsaturated compounds
[24–26]. In a simple way, the relative intensity of FR and FA forces is
usually related to the combination of extent and filling of the metal
d bands. The order of d-orbital filling for the non-noble metals used
in this work is Co < Ni < Cu, while the d-band extent follows the
opposite order Cu < Ni < Co. In our case, the repulsive forces exist-
ing between metal d-orbitals and the phenyl group of CN molecule
would increase with the filling of d-orbitals. It can be expected
therefore that a low interaction between the CN phenyl group and
the metallic Cu surface takes place because in Cu0 the d-orbitals are
completely filled (Fig. 4). The CN adsorption on Cu would occur then
mainly via its C N group, thereby favoring the formation of nitrene
intermediates (i.e. formation of N Cu bonds) which would lead to
the preferential hydrogenation of CN to unsaturated amine CA. Pre-
vious works have reported, in fact, that the nitrene intermediates
promotes the selective formation of primary amines at the expense
of the parallel condensation to secondary amines [27,28]. In the
case of Ni, the FR/FA ratio for CN adsorption is lower as compared to
Cu because the d-band of Ni is only partially filled. Consequently,
the interaction between the substrate and Ni probably occurs via
the C C group of the CN molecule which would promote the selec-
tive CN hydrogenation to HCN, as we observed in Fig. 3. Finally,
it is expected a weak interaction between the CN phenyl group
and cobalt because this metal presents the largest d-orbital extent
A
N
HYCN
NH
HCCA
N
HCA
CN
NH
DiCA
CA
B
N
(Fig. 4). This would favor the CN adsorption on Co through its C
N
HYCN
group which turns more accessible the C C group of CN to the metal
surface and allows its rapid hydrogenation to saturated amine HC.
Besides, it has been reported that formation of carbenes from the
NH
N
DiHCA
HCA
CN
C
N group adsorption leads also to the production of secondary
amines [27,29].
In summary, Ni/SiO2 and Co/SiO2 were the most active catalysts
for CN conversion but formed only small amounts of unsaturated
primary amine CA. Ni/SiO2 yielded a mixture of saturated primary
(HCA) and secondary (DiHCA) amines while Co/SiO2 produced a
mixture of saturated primary amine HCA, unsaturated secondary
amine DiCA, and partially unsaturated secondary amine HCCA. In
contrast, Cu/SiO2 and Ru/SiO2 presented low activity for CN hydro-
genation at 383 K but formed selectively unsaturated amine CA in
the liquid phase; nevertheless, on both samples the carbon bal-
ance was only about 40%. Taking into account that our goal was to
efficiently produce unsaturated primary amine CA via CN hydro-
genation, we decided to perform additional catalytic runs using
more dispersed Cu/SiO2 catalysts at higher reaction temperatures
in order to substantially increase the catalyst activity. We selected
Cu instead of Ru to perform this study because previous work
C
NH
N
CA
CN
D
NH
N
DiCA
24
CA
CN
Scheme 2. Main reaction pathways for cinnamonitrile conversion reactions on: (A)
Co/SiO2, (B) Ni/SiO2, (C) Cu/SiO2 or Ru/SiO2, (D) Cu/SiO2-II. The line width indicates
the relative relevance of each reaction.

46
D.J. Segobia et al. / Applied Catalysis A: General 494 (2015) 41–47
Fig. 4. Schematic representation of the interaction between the cinnamonitrile molecule and the transition metal surface
Table 3
Catalytic results on Cu catalysts.
Catalyst
Temperature (K)
Pressure (bar)
Reaction length (min)
Initial activity rC0N
Conversion (XCN, %) and selectivities (%)a
(mmol/h gCu
)
XCN
CA
DiCA
Others
Cu/SiO2-I
Cu/SiO2-II
Cu/SiO2-II
Cu/SiO2-II
Cu/SiO2-II
383
383
403
403
403
13
13
13
25
40
540
560
1170
650
39
75
201
359
580
19
40
96
98
100
52
59
60
67
74
–
11
28
23
20
48
30
12
10
6
600
Wcat = 0.5 g, VCN = 1.5 mL, solvent: toluene (150 mL).
a
Determined at the end of the reaction.
[30,31] showed that Cu exhibits poor activity for hydrogenating
isolated C C bonds of ␣,␤-unsaturated compounds, which is a
desirable metal property in our case for selectively obtaining unsat-
urated primary amines from unsaturated nitriles hydrogenation.
at 403 K, reaching SDiCA a value of 28% at the end of the run. These
later results show that at 403 K CN is converted on Cu/SiO2-II via a
single reaction pathway involving the initial CN hydrogenation to
unsaturated primary amine CA that forms consecutively the unsat-
urated secondary amine DiCA (Scheme 2D). We confirm then that
Cu does not form saturated amines from CN because is inactive for
hydrogenating isolated C C bonds. The global carbon balance also
improved with temperature, from 70% (383 K) to 88% (430 K).
Finally, we investigated the effect of increasing the H2 pressure
from 13 bar to 25 bar and 40 bar on the catalytic performance of
Cu/SiO2-II catalyst at 403 K. Fig. 5 illustrates the evolution of CN
conversion and yields as a function of time obtained on Cu/SiO2-
II at 40 bar. Results in Table 3 show that rC0N increased almost
3.3. Selective synthesis of CA on silica-supported Cu catalysts
Additional catalytic runs for CN hydrogenation were carried out
on Cu/SiO2-I and Cu/SiO2-II catalysts. Results obtained at 383 K and
13 bar are given in Table 3 (two first rows). The initial CN con-
version rates determined on Cu/SiO2-I (r0 =39 mmol/h gCu) and
CN
Cu/SiO2-II (r0 = 75 mmol/h gCu) were higher than that obtained on
CN
Cu/SiO2 (rC0N = 23 mmol/h gCu) at similar reaction conditions. Con-
sistently, the XCN conversion at the end of the runs increased from
10% (Cu/SiO2) to 19% (Cu/SiO2-I) and 40% (Cu/SiO2-II). The final
selectivity to CA also increased from 46% (Cu/SiO2) to 52% (Cu/SiO2-
I) and 59% (Cu/SiO2-II). Formation of unsaturated secondary amine
DiCA was observed on Cu/SiO2-II (SDiCA = 11%) but not on Cu/SiO2-
I. On the other hand, the carbon balance at the end of the runs
improved from 46% (Cu/SiO2) to 52% (Cu/SiO2-I) and 70% (Cu/SiO2-
II). All these results showed that the catalyst activity, selectivity,
and stability for CA formation were clearly improved when the Cu
dispersion was increased.
proportionally with PH indicating that the reaction order with
2
respect to hydrogen was about one. Complete CN conversion was
obtained at the end of the 600-min run performed at 40 bar (Fig. 5).
The final selectivity to CA increased from 60% (13 bar) to 74%
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
In an attempt of promoting further the rate and yield to CA,
we performed catalytic tests on Cu/SiO2-II (the most active and
selective catalyst) at higher temperatures and H2 pressures. Results
obtained at 403 K and 13 bar are presented in Table 3 (third row).
As expected, the initial CN conversion rate increased with temper-
ature, from 75 mmol/h gCu (383 K) to 201 mmol/h gCu (430 K) and
XCN reached 96% at the end of the run. As a consequence, the yield
to CA on Cu/SiO2-II increased from 23.6% (383 K) to 57.6% (430 K), in
spite that the CA selectivity did not change with the increase of tem-
perature. Significant amounts of DiCA were obtained on Cu/SiO2-II
0
100 200 300
600 700
Time
(
m
400in)500
Fig. 5. Cinnamonitrile conversion (XBN) and yields (ꢂi) on Cu/SiO2-II catalyst CA,
DiCA [T = 403 K, P = 40 bar, Wcat = 0.5 g].

D.J. Segobia et al. / Applied Catalysis A: General 494 (2015) 41–47
47
(40 bar) while SDiCA decreased from 28% to 20%; i.e., the increase of
H2 pressure enhanced the CN hydrogenation to CA at the expense of
the competitive condensation reaction leading to secondary amine
(CONICET), and Agencia Nacional de Promocion Cientifica y Tec-
nologica (ANPCyT), Argentina, for the financial support of this
work.
DiCA (Scheme 1). The carbon balance also improved with PH
,
2
from 88% (13 bar) to 94% (40 bar). In summary, Cu/SiO2-II prepared
by chemisorption–hydrolysis is an efficient catalyst for promoting
the selective hydrogenation of cinnamonitrile to the unsaturated
primary amine because combines a significant activity for BN con-
version (it contains a high amount of well dispersed copper) with
the ability of Cu for selectively hydrogenating the CN group of cin-
namonitrile. Thus, our results show that Cu/SiO2-II yields 74% of CA
at 403 K and 40 bar H2, without the presence of ammonia.
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Authors thank the Universidad Nacional del Litoral (UNL),
Consejo Nacional de Investigaciones Cientificas
y Tecnicas