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gave also active catalysts (60–
79% yield of benzonitrile; en-
tries 9–10).
Table 1. Synthesis of benzonitrile using iron catalysts.[a]
As previously described the
catalytic materials were system-
atically characterized by trans-
mission electron microscopy
(TEM), X-ray photoelectron spec-
troscopy (XPS), electron para-
magnetic resonance (EPR), and
Mçssbauer spectroscopy (see
the Supporting Information).[12a]
Specifically, the Fe-phenanthro-
line/C-800 catalyst is character-
ized by the formation of nano-
scale Fe2O3 particles (2–5 and
20–80 nm), which are surround-
ed by 3–5 layers of nitrogen-
doped graphene (see Fig-
ure S1A). In contrast to this
most active material, the Fe2O3
particles in Fe-bipyridyl/C-800
are much larger and consist of
several crystallites (Figure S1B).
In the latter case, the Fe2O3 par-
ticles are not surrounded by gra-
Entry
Carbon
Iron complex[b]
Pyrolysis conditions
Yield of
T [8C]
t [h]
gas
benzonitrile[c] [%]
1[a]
2[a]
3[a]
–
–
–
–
–
–
–
–
–
–
–
–
–
2
2
2
2
2
2
–
–
–
–
<1
<2
<2
2
2
5
97
20
60
79
<1
Fe(OAc)2
Fe(OAc)2-L1
Fe(OAc)2
Fe(OAc)2-L1
Fe(OAc)2
Fe(OAc)2-L1
Fe(OAc)2-L2
Fe(OAc)2-L3
Fe(OAc)2-L4
–
4[d]
5[d]
6[d]
7[d]
8[d]
9[d]
10[d]
11[d]
Vulcan XC72R
Vulcan XC72R
Vulcan XC72R
Vulcan XC72R
Vulcan XC72R
Vulcan XC72R
Vulcan XC72R
Vulcan XC72R
–
–
800
800
800
800
800
800
Ar
Ar
Ar
Ar
Ar
Ar
[a] Homogeneous catalysis reaction conditions: 0.5 mmol benzylamine, 0.02 mmol Fe(OAc)2, 0.06 mmol ligand,
200 mL aq. NH3 (28–30% NH3 basis), 3 bar O2, 4 mL t-amyl alcohol. [b] L=ligand. [c] Determined by GC. [d] Het-
erogeneous catalysis reaction conditions: 0.5 mmol benzylamine 40 mg catalyst (4 mol% Fe), 200 mL aq. NH3
(28–30% NH3 basis), 3 bar O2, 4 mL t-amyl alcohol.
Exploratory catalytic experiments were performed using
benzyl amine as model substrate, applying molecular oxygen
(Table 1). Although the heterogeneous iron catalyst proved to
be active, the corresponding secondary imine was obtained as
major product. In addition, a mixture of benzaldehyde and
benzonitrile also formed in minor amounts. It is assumed that
an initial iron oxide-catalyzed dehydrogenation of benzyl
amine to the corresponding imine occurs. This reactive inter-
mediate undergoes further dehydrogenation to give the de-
sired product or alternatively forms the more stable secondary
imine by addition of benzyl amine and elimination of ammo-
nia. Trace amounts of benzaldehyde are explained by hydroly-
sis of the imine.
phene layers. In case of simple pyrolyzed iron-acetate
(Fe(OAc)2/C-800), the Fe2O3 particles occur in well-facetted big
particles of 100–800 nm size (Figure S1C), which are also not
surrounded by graphene layers. From all these results, we pro-
pose the formation of the nitrogen-doped graphene layers to
be crucial for activity and we represent the active catalyst Fe-
phen/C-800 as Fe2O3/NGr@C. This catalyst system is highly
stable and can be conveniently recycled and re-used up to
5 times (Figure 1). For industry, catalyst re-usability is a major
issue, which is also important for the advancement of cost-ef-
fective processes.
In order to suppress the formation of the secondary imine,
aqueous ammonia was added to the reaction mixture. Indeed,
this led to a highly selective formation of benzonitrile. There-
fore, all of the following experiments were performed in the
presence of aqueous ammonia. As expected, homogeneous
iron-phenanthroline complexes were not active in the bench-
mark reaction (Table 1, entries 2–3). Similarly, pyrolyzed iron
acetate on carbon and iron-phenanthroline complex immobi-
lized on carbon under non-pyrolyzed conditions were not
active (entries 4–6). However, pyrolysis of N-ligated iron acetate
on carbon gave active catalysts with varying catalytic activities
(entries 7–10). The most active system was obtained from py-
rolysis of Fe-phenanthroline (L1) complex on carbon (Fe-phen/
C-800). With this material benzyl amine was completely con-
verted to benzonitrile in excellent yield (97%, entry 7). While,
the catalyst based on bipyridine (L2) gave poor activity
(entry 8), terpyridine (L3) and pyridinebisbenzimidazole (L4)
Figure 1. Synthesis of benzonitrile: recycling of Fe2O3/NGr@C-catalysts. Reac-
tion conditions: 1 mmol benzylamine, 80 mg catalyst (4 mol% Fe), 400 mL
aq. NH3 (28–30% NH3 basis), 3 bar O2, 4 mL t-amyl alcohol, 1108C, 15–20 h.
Yields were determined by GC using n-hexadecane standard.
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