P. Schärringer et al. / Journal of Catalysis 253 (2008) 167–179
175
4.2. Role of the strength of adsorption
proposed. In fact, the rate of nitrile conversion in the presence of
amine was approximately equal to the sum of the rates of nitrile
consumption in the absence of amine and the rate of formation
of the unsymmetric dialkylimines. Hence, faster reaction of the
nitrile is mostly due to enhanced by-product formation. This
observation again suggests that the by-product formation takes
place on other sites than the hydrogenation. Note that a hydro-
genation step is involved in the formation of the precursor of
the condensation product. Either, a partially hydrogenated in-
termediate migrates to the condensation sites as proposed by
Verhaak et al. [20] or, which is considered less likely, hydro-
genation also takes place on the condensation sites resulting in
a surface intermediate more susceptible to condensation than to
further hydrogenation.
In both, hydrogenation of the single nitriles and co-hydro-
genation of C1–C≡N and C3–C≡N, the rate for C1–C≡N con-
sumption was higher than for C3–C≡N. The difference may be
caused by a stronger adsorption of C1–C≡N and/or by a faster
intrinsic reaction rate [10]. In the co-hydrogenation of the two
compounds, the initial rate of C3–C≡N consumption was re-
duced much more than that of C1–C≡N, when compared to
the reactions with only one nitrile as reactant. With decreas-
ing amount of C1–C≡N, the reaction rate of C3–C≡N hydro-
genation increased. Both observations indicate that C1–C≡N
adsorbs more strongly on the surface of Raney-Co. In conse-
quence, the surface concentration of C1–C≡N is higher than
of C3–C≡N during the initial phase of the reaction, when both
nitriles are present in equal concentration in the liquid phase.
When the reaction mixture becomes depleted of C1–C≡N, the
surface concentration of C3–C≡N increases and C3–C≡N hy-
drogenation becomes faster. Note that according to our experi-
mental results, C1–C≡N adsorbs more strongly on the surface
than C3–C≡N, whereas it is the other way around with the
amines. Analysis of the proton affinity showed that butyroni-
trile is more basic than acetonitrile and butylamine more basic
than ethylamine [24]. However, the higher steric demand of the
propyl group might explain the weaker adsorption of butyroni-
trile on cobalt compared to acetonitrile.
4.3. Reaction steps during co-hydrogenation of two nitriles
In the co-hydrogenation of C1–C≡N and C3–C≡N, the de-
velopment of by-products can roughly be divided into two peri-
ods. Formation of the imine intermediates occurred during the
first period, while the further reaction of these intermediates
giving rise to secondary amines constitutes the second period.
The primary nature of all imine intermediates suggests that—at
least in the initial phase of the reaction—they were formed in
the same way. Formally, the formation of dialkylimine interme-
diates can be explained by the overall reactions,
In parallel to the hydrogenation of the nitrile, dialkylimines
are formed, which were shown to be primary reaction products.
This strongly suggests that the condensation reactions occur on
the catalyst surface, in agreement with other studies [8,19,20].
For both nitriles investigated, the selectivity to primary amines
was lowered upon an increase of the starting concentration of
the reactants. Thus, it is concluded that the surface concen-
tration of precursors necessary for the formation of secondary
amines increases with the concentration of nitrile. For exam-
ple, in the hydrogenation of C1–C≡N the rate of hydrogenation
increased by a factor of 1.13, whereas the rate of formation
of C1–HC=N–C2 was a factor of 4.7 higher, when raising the
starting concentration from 4.08 to 9.52 mol dm−3. While the
hydrogenation was almost independent of the nitrile concentra-
tion, indicating an order close to zero due to full coverage of the
sites participating in the hydrogenation, a positive order was ob-
served for the formation of dialkylimines. Note that the factor
of 4.7 ≈ 22 is indicative of a bimolecular reaction.
C1–HC=NH + C4–NH2 → C1–HC=N–C4 + NH3,
C1–HC=NH + C2–NH2 → C1–HC=N–C2 + NH3,
C3–HC=NH + C4–NH2 → C3–HC=N–C4 + NH3,
C3–HC=NH + C2–NH2 → C3–HC=N–C2 + NH3,
or by disproportionation of the respective amines as, e.g.,
2C2–NH2 → C1–HC=N–C2 + NH3 + H2.
As a dehydrogenation step is involved, the later reaction
(Eq. (14)) appears unlikely at high hydrogen pressures. The pri-
mary nature of the dialkylimines suggests that surface species
were involved, which did not desorb from the surface before
the condensation reaction occurred. In the second period of the
reaction the dialkylimines were transformed to dialkylamines,
which is possible by hydrogenation of the dialkylimines as, e.g.,
(10)
(11)
(12)
(13)
(14)
C1–HC=N–C4 + H2 → C2–NH–C4.
(15)
The different influence on the rate of hydrogenation and by-
product formation indicates that the two processes take place on
different sites. While the sites for hydrogenation were almost
saturated at lower nitrile concentrations, those for condensation
had remaining sorption capacity. It has been reported that ad-
dition of amines to the reaction mixture has no effect [19,21]
indicating that amines are adsorbed either weakly or on other
sites than those used for hydrogenation. Also a retarding effect
on the rate of hydrogenation has been described [22] suggest-
ing that amines are more strongly adsorbed on the metal than
nitriles. In this study, in the presence of an equimolar amount of
amine, the rate of hydrogenation was slightly increased for both
nitriles, which is another indication for the dual site mechanism
An alternative reaction is the reaction of an amine with a
dialkylimine to 1-alkylamino-dialkylamine followed by either
formation of an imine and subsequent hydrogenation or by
direct hydrogenolysis resulting in another amine and dialkyl-
amine as, e.g.,
C1–HC=N–C4 + C2–NH2 + H2 → C2–NH–C2 + C4–NH2,
(16)
C3–HC=N–C4 + C2–NH2 + H2 → C4–NH–C2 + C4–NH2,
(17)
C1–HC=N–C2 + C4–NH2 + H2 → C2–NH–C4 + C2–NH2,
(18)