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2.4. Possible mechanism for the hydrogenating deamination
of BN
To investigate the possible pathway of the reaction, the differ-
ent side products were identified by the GCMS analysis of the
reaction mixture (Figure S5 in the Supporting Information). In
addition to toluene (the main product of the hydrogenation
deamination of benzonitrile), the other products identified in
trace amounts are cyclohexanecarbonitrile and benzene. No
trace of benzylamine or dibenzylamine was detected during
the analysis. Direct conversion of BN to toluene was also ob-
served during the optimization studies (Figure S4). Therefore,
benzonitrile directly undergoes hydrogenating deamination to
form toluene without involving the BA intermediate. In addi-
tion, cleavage of BA was not facile in comparison to BN reduc-
tion under identical conditions (Table 3, entry 9) and the similar
Figure 2. O1s XPS spectra of Pd/ND and Pd/AC.
nanodiamond was rich in OÀC=O functionality in comparison
[
12]
to activated carbon, which is also reported in the literature.
[
26]
To see if these acidic groups might influence the reaction, we
observation has been reported previously. This also rules out
the possibility of fast cleavage of the carbon–nitrogen bond in
BA on the catalyst surface.
first prepared Pd/ND-NH (see the Experimental Section). An
3
ion-exchange reaction of acidic functional groups with aque-
ous ammonia solution can help to remove the acidic protons
from the catalyst surface. Then, we performed the reductive
cleavage of the CN bond in benzonitrile by the optimized pro-
cess and observed the yield and selectivity of toluene (Table 2,
entry 12). As expected, the yield and selectivity of toluene was
drastically decreased even when using four times of the cata-
lyst amount compare with Pd/ND. This indicated that the
acidic groups on the carbon surface might play a role during
the hydrogenating deamination reaction.
The formation of cyclohexanecarbonitrile in trace amounts
was clear evidence for the activation of the aromatic ring by
Pd, which was responsible for the ring reduction. Therefore, it
is probable that the p-electron cloud of the aromatic ring is in-
teracting with Pd during the reaction to form an intermediate
that gives toluene as the final product (Figure S6 in the Sup-
porting Information). This fact is further supported by the re-
luctance of octanonitrile and n-octylamine (which do not con-
tain aromatic rings in their structures) towards hydrogenating
deamination under similar conditions (Table 3, entries 5 and
12). Therefore, the catalyst most probably activates the aro-
matic ring in BN and cleaves the CN bond in the intermediate
(Figure S6). Cleavage of the CN bond is favored in comparison
to the carbon–carbon bond in the intermediate, which favors
the deaminated product and therefore toluene is the major
product in comparison to benzene. We have also observed
that toluene formation is minimized if the acidic groups on the
surface are exchanged with aqueous ammonia solution
(Section 2.3).
To see if the adsorption of the reactant and/or the inter-
mediates/products was responsible for the different behavior,
we studied the adsorption of benzonitrile, benzylamine, and
toluene on the nanodiamond and activated carbon surfaces.
BN, BA, and toluene (100 mg) were added to THF (25 mL) fol-
lowed by the addition of ND or AC (100 mg each). GC analysis
was performed after 24 h to determine the quantities of the re-
spective compounds remaining in the solution. No significant
difference in the adsorbed quantities of toluene and BN was
observed, but there was a significant difference in the adsorp-
tion of BA: 30 mg of BA was adsorbed on AC and only 5 mg
was adsorbed on ND. Because benzylamine formation is not
observed during the hydrogenating deamination reaction (Fig-
ure S4), its differential adsorption also did not seem to affect
the deamination reaction. However, it can certainly affect the
BA or DBA formation during the hydrogenation of BN at low
temperature and high pressure (Table 2). Because BA exhibited
more adsorption on the activated carbon surface, it may show
a greater tendency to stay on its surface along with the other
intermediates. When BA is formed at lower temperature
Based upon these facts, a mechanism is proposed for the re-
action (Figure 3). The reaction starts with the interaction of
benzonitrile with the palladium nanoparticles, where the ring
is probably activated by Pd and similar interactions could pos-
sibly be involved with the CN functional group. Well exposed
Pd nanoparticles on the surface serve as active sites for hydro-
gen absorption. Activation of the aromatic ring then may lead
to the hydrogenation of the ÀCN functional group and acidic
groups present on the surface might assist in the cleavage of
the carbon–nitrogen bond, leading to the deaminated product.
The high temperature and low pressure during this conversion
further seems to assist this cleavage (Figure 3), which seems to
operate in a synchronized way. If this arrangement is disturbed,
this ultimately disturbs the selectivity of the deamination reac-
tion. The fact is manifested by the reactions at 100 and 1208C
(
Table 2, entry 7) and high pressure on a nanodiamond surface,
most likely it leaves the surface and therefore its further deam-
ination and side reaction with possible imine intermediates is
minimized.
(
Table 2, entries 8–10): at 1008C, BA formation was observed at
a high pressure of 2 MPa, whereas at 0.5 MPa, HDA is favored
at 1208C in comparison to 1008C.
ChemCatChem 2016, 8, 922 – 928
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