L. McMillan et al. / Journal of Molecular Catalysis A: Chemical 411 (2016) 239–246
245
A further point worth noting in Fig. 9 is the formation of
the secondary amine N-benzyl-2-phenethylamine (C6H5CH2CH2-
NH-CH2C6H5) via the cross-coupling of intermediate benzylimine
(C6H5CH = NH) with phenethylamine, or possibly of phenethy-
observed corresponding to the symmetrical secondary amines that
one would expect from the coupling of (i) phenethyimine with
Clearly, the co-adsorption of benzonitrile and benzyl cyanide
(Fig. 9) leads to a series of competing interactions which perturb
the reaction profiles seen in the single hydrogenation reactions
(Figs. 3 and 4). Not least, this includes competition for adsorption
sites on the carbon support by the benzyl cyanide derived amine
(phenethylamine), as well as the formation of a secondary amine
(N-benzyl-2-phenethylamine). Further, since neither benzylimine
nor phenethyimine are observed in the liquid phase, it is believed
that the coupling reactions that lead to formation of the secondary
amine must be catalyst mediated.
Fig. 9. The co-hydrogenation of benzonitrile and benzyl cyanide over 0.5 g 5% Pd/C,
333 K, 4.0 bar g, ca. 0.017 mol of benzonitrile and benzyl cyanide. A0 represents the
benzyl-2-phenethylamine.
3.3. Conclusions
In order to define the more global nature of this reaction system,
Scheme 3 needs to include a description for material spilling over
on to the carbon support. Fig. 8 schematically links the concepts
of the 3 site model with the interchange of molecules between the
liquid phase, Pd crystallites and the carbon support. Fig. 8 is defined
elementary process associated with the liquid phase hydrogenation
of aromatic nitriles over a Pd/C catalyst.
The initial mass imbalance observed in the hydrogenation of
benzonitrile (Fig. 3) is also consistent with the proposed scheme
(Fig. 8), if one acknowledges that mass transport of reactants and
products can be mediated through the carbon support. Retention
of reactants/products on the support, where no further reaction is
thought to occur, would thus render that molecule undetectable in
the liquid phase. Thus, in the case of benzonitrile hydrogenation
at least, this pathway is thought to be the origin of the low mass
balance in the initial stages of that reaction. Thereafter, reverse
spillover occurs [31].
The liquid phase hydrogenation of benzonitrile, benzyl cyanide,
3-phenylpropionitrile and cinnamonitrile over a 5 wt% Pd/C cata-
lyst was investigated in methanol at 4 bar g and 333 K. The following
observations can be made.
•
Benzonitrile hydrogenation appears as a consecutive process.
First benzonitrile is hydrogenated to form benzylamine. This
product then undergoes a hydrogenolysis reaction to form
toluene.
•
Co-hydrogenation studies on a mixture of benzonitrile and
benzylamine show the hydrogenation and hydrogenolysis reac-
tions to be occurring simultaneously and independently. This
behaviour in interpreted in terms of a 3 site model: dissocia-
tive hydrogen adsorption takes place at Site I; hydrogenation
takes place at Site II; Site III is associated with the hydrogenolysis
reaction.
•
Benzyl cyanide and 3-phenylpropionitrile hydrogenation result
in no product formation in the liquid phase. In the case of 3-
phenylpropionitrile loss of activity is attributed to amine product
poisoning Pd sites. For benzyl cyanide, converted product is
believed to partition on to the carbon support.
cyanide (C6H5CH2CN)
and hydrogenation lability, the hydrogenation of equimolar
amounts of benzonitrile and benzyl cyanide were investigated.
Fig. 9 shows the resulting reaction profile. In line with their indi-
converted, although at a slower rate than seen previously. This
is thought to reflect competition for hydrogenation sites (Site II).
Hydrogen consumption corresponds to a smooth growth curve and
displaying an intermediate profile and toluene identified as the
final product. However, the profile connected with benzyl cyanide
conversion is different to that seen in Fig. 4 because significant
quantities of phenethylamine are now seen in Fig. 9. Previously
(Section 3.1.2), the absence of this product was attributed to reten-
tion by the carbon support. Its presence in the liquid phase in Fig. 9
is observable in this case gives some credibility to the assumption
that phenethylamine is actually produced but does not partition in
to the liquid phase when only benzyl cyanide and dihydrogen are
added as reagents (Fig. 4).
•
3-phenylpropionitrile is the only product in the hydrogenation
of cinnamonitrile. The carbon–carbon double bond is selectively
reduced with respect to the carbon-nitrogen triple bond. The con-
jugation between the nitrile group and the aromatic ring does not
assist nitrile reduction in this case.
•
Hydrogenation of a mixture of benzonitrile and benzyl cyanide
indicates the competitive nature of the reaction system. The pres-
ence of phenethylamine (source = benzyl cyanide hydrogenation)
in to the liquid phase is induced. This coincides with the forma-
tion of the secondary amine N-benzyl-2-phenethylamine which,
due to the absence of imines in the liquid phase, is thought to
form at the catalyst (Pd) surface.
Acknowledgements
Syngenta and WestChem are thanked for the award of stu-
dentships (LM and LG) and research support. Hiden Analytical Ltd.
is thanked for assistance with catalyst characterisation procedures.
The EPSRC are thanked for support via awards from a Knowledge
Transfer Account [EP/H5001138/1] and an Impact Acceleration
Account [EP/K503903/1].