D.J. Segobia et al. / Catalysis Communications 62 (2015) 62–66
65
Table 1
Catalytic results.
Solvent
Initial rate
Reaction time
(min)
Conversion (XBN, %) and selectivities (%) at the end of reaction
r0BN
XBN
BA
DBA
TBA
Others
(mmol/h gcat
)
Methanol
Benzene
Toluene
15.1
12.4
22.1
43.7
600
630
320
340
100
100
100
100
91
79
70
60
9
20
29
39
–
–
–
–
–
1
1
1
Cyclohexane
T = 373 K, P = 13 bar (H2), 800 rpm, Wcat = 1 g, VBN = 3 mL.
clear that other solvent effects such as solvent–catalyst and solvent–re-
actant interactions have to be taken into account in order to interpret
the observed changes in Co/SiO2 activity and selectivity.
4. Discussion
Results of Fig. 2 and Table 1 show that the Co/SiO2 activity and selec-
tivity greatly depend on the solvent nature. By analyzing the results ob-
tained in non-polar solvents it is inferred that r0BN decreases while SBA
increases in the sequence cyclohexane → toluene → benzene. These
changes in catalyst activity and selectivity may be interpreted by taking
into account that according to solvent TPD experiments the solvent–
metal interaction strength on Co/SiO2 follows the order benzene N
toluene N cyclohexane. The reaction mechanism of Scheme 1 predicts
that formation of DBA requires the readsorption of BA on the Co surface
to react with butylimine and produce the secondary amine. The selec-
tive formation of BA would be favored therefore when a strong sol-
vent–Co interaction takes place and hampers the readsorption of BA
on the metal surface. This assumption explains why SBA increases
when cyclohexane is replaced by toluene or benzene following the sol-
vent–Co interaction strength trend. Moreover, the fact that Co/SiO2 ex-
hibits the highest activity in cyclohexane probably reflects also the very
weak cyclohexane–Co interaction, because this solvent will not block
any surface active sites for BN adsorption and conversion.
The BA yield obtained in methanol was higher than in non-polar sol-
vents. This result cannot be explained in terms of a stronger methanol–
metal interaction strength because our solvent TPD results showed that
the methanol adsorption on Co is weaker than that of benzene, for ex-
ample. In an attempt of interpreting the highest BA selectivity observed
in methanol, we analyzed the possible interactions occurring in the liq-
uid phase between the solvent and BA. In Table 2 we present the values
of classical polarity parameters (dipole moment μ and dielectric con-
stant ε) and hydrogen-bond-donation (α) and hydrogen-bond-
acceptance (β) properties for the solvents used in this work. The values
corresponding to BN and BA are also included in Table 2. Non-polar sol-
vents exhibit low μ and ε values and they have no capability to act as H
bond donor (α = 0). Methanol is a protic H-bond donor (β = 0.98) sol-
vent that exhibits high values for polarity parameters ε and μ. In con-
trast, BA is an H-bond acceptor molecule of β = 0.72. Thus, a strong
interaction is expected to exist between BA and protic methanol causing
the BA solvation in the liquid phase. The BA molecules would be then
surrounded by alcohol molecules that will hinder the BA adsorption
on Co and, as a consequence, also the formation of DBA that occurs by
surface condensation between BA and butylimine. The solvation of BA
in methanol, i.e. a solvent–reactant interaction, would explain then
the high selectivity to BA that Co/SiO2 exhibits in this alcohol.
3.3. Solvent–catalyst interactions
In order to obtain insight on the solvent–catalyst interaction
strength, we investigated the temperature-programmed desorption of
the solvents on Co/SiO2 by analyzing the evolved products by mass
spectrometry. Fig. 3 presents the solvent TPD profiles obtained on Co/
SiO2. The TPD of cyclohexane shows the evolution of m/z = 56 signal
(the most intense signal in the cyclohexane fragmentation mass spec-
trum) and m/z = 28 and 44 signals accounting for possible fragmenta-
tion of the cyclohexane molecule. No signals of evolved compounds
were detected thereby revealing that the interaction between cyclohex-
ane and Co is negligible.
The desorption of toluene, followed by the m/z = 92 signal corre-
sponding to the molecular ion, occurred as a small peak at 365 K. De-
sorption of several C2 and C4 fragments (m/z = 26, 28, 43 and 44
signals) associated with toluene decomposition into lighter hydrocar-
bons took place at 404 K. Evolution of C2 and C4 fragments was also ob-
served at higher temperatures, between 573 and 620 K. These results
showed that toluene adsorbs irreversibly on Co and decomposes at
low and middle 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 decomposition. Fig. 3 shows that no
benzene desorption (m/z = 78) was observed while significant evolu-
tions for H2, C1 and C2 species (m/z = 2, 16, 28) were detected at tem-
peratures higher than 500 K. These high-temperature evolutions
reflect the decomposition of strongly chemisorbed benzene over Co.
Finally, Fig. 3 shows that the desorption temperature maximum of
the peaks corresponding to two of the most abundant methanol ions
(m/z = 31, 32) appeared at 381 K. Two additional broad bands corre-
sponding to high temperature H2 were detected at about 570 K and
880 K respectively, which were accompanied by C2 and C3 hydrocarbon
fragment evolutions (m/z = 28, 44). These evolutions indicate the pres-
ence of surface cobalt sites on which methanol adsorbs very strongly
and decomposes at high temperatures.
In summary, the TPD results of Fig. 3 indicate that the solvent–cata-
lyst interaction strength in non-polar solvents follows the order
benzene N toluene N cyclohexane.
5. Conclusions
Table 2
H2 solubility and polarity parameters of BN, BA and the solvents used in this work.
The Co/SiO2 activity and selectivity for the liquid-phase synthesis of
n-butylamine from butyronitrile hydrogenation greatly depend on the
solvent nature. In protic alcohols such as methanol, the solvent–
butylamine interaction in the liquid phase controls the selectivity to n-
butylamine. Methanol is an H-bond donor solvent that strongly inter-
acts 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 the n-butylamine adsorption on Co and,
as a consequence, impede the formation of dibutylamine via surface
CHꢀ
a
Compounds
ε
μ
α
β
2
(Debye)
(mol/L)
Methanol
Benzene
Toluene
Cyclohexane
BN
32.7
1.7
0
0.37
0
3.5
1.3
0.98
0
0
0
0
0.66
0.10
0.11
0
0.40
0.72
1.21 × 10−1
5.45 × 10−2
5.37 × 10−2
5.63 × 10−2
–
2.28
2.38
2.02
20.3
4.92
BA
0
–
a
H2 solubility at 373 K and 13 bar obtained from bibliography [27,28].