H. Jiang et al.
Molecular Catalysis 504 (2021) 111490
without visible light irradiation (Table 1, entry 7). For comparison, urea
doped TiO2/Ti3C2 was also tested as the carrier, poor activity and
moderate chemoselectivity to acetanilide was attained (Table 1, entry
8). In comparison with 3%Pt/N-TiO2/Ti3C, 3%Pt/N-TiO2@C reached
67.6 % conversion and 86.7 % chemoselectivity to A1. This result
indicated that the combination of Ti3C, TiO2 and Pt NPs together with
the formation of heterojunction, with Ti3C acting as a transporter for
electrons to inhibit the backward diffusion of electrons, significantly
improved one-pot nitrobenzene hydrogenation as well as amidation
with acetic acid. We further monitored the reaction between nitroben-
zene and acetic acid with 1 atm hydrogen over visible light promoted
photocatalysis (Fig. 7). Nitrobenzene was firstly reduced to aniline in-
termediate followed closely with amidation. No other byproduct was
detected. The 3%Pt/N-TiO2/Ti3C could be reused by straightforward
centrifugation followed by vacuum drying. Catalyst could be recycled 5
times with no obvious activity and chemoselectivity decrease (Fig. 8).
No obvious Pt loss was detected by ICP-OES in the reaction solution.
In the visible light induced 3%Pt/N-TiO2/Ti3C catalytic system,
nitrobenzene one-pot hydrogenation and amidation with carboxylic
acids could also be extended to various carboxylic acids (Table 2). 3%
Pt/N-TiO2/Ti3C showed excellent activity and chemoselectivity to cor-
responding amides (>90 %) with acetic acid, propanoic acid, butyric
acid, valeric acid as well as isobutyric acid (Table 2, entries 1, 3–6).
However, formic acid, with lower pKa value of 3.77, gave lower che-
moselectivity to the corresponding amide (Table 2, entry 2); meanwhile,
octanoic acid gave a chemoselectivity of 27.9 % and aniline was the
byproduct (Table 2, entry 7). Either the chemoselectivity decrease for
formic acid or the photocatalytic decline for octanoic acid should be
ascribed to the steric hindrance.
Fig. 8. Cycling visible light promoted one-pot hydrogenation and amidation of
nitrobenzene with acetic acid over 3%Pt/N-TiO2/Ti3C. The reaction conditions
are the same as in Table 1 entry 3.
The as-prepared 3%Pt/N-TiO2/Ti3C catalyst was used for one-pot
hydrogenation and amidation of nitro compounds with carboxylic
acids under visible light irradiation. Nitrobenzene and acetic acid was
chosen to test the catalytic performance (Table 1). No product was
detected without catalyst (Table 1, entry 1). Without alkaline additive
K3PO4, one-pot hydrogenation and amidation of nitrobenzene was
absolutely converted but just with 64.0 % chemoselectivity to acetani-
lide (A1) (Table 1, entry 2), which suggested that the base was required
for the deprotonation of amine [56]. The decreased conversion in the
absence of base should be owing to aniline (a primary amine) itself could
assist the progress of reaction by acting as a base. In the presence of
K3PO4 additive, hydrogenation and amidation reaction could reach 100
% conversion with 100 % chemoselectivity to A1 (Table 1, entry 3). The
amount of melamine used during TiO2/Ti3C doping was further opti-
mized (Table 1, entries 3–5). When the melamine:TiO2/Ti3C2 increased
from 0.25:1 to 0.5:1 during the doping, slightly increase in activity and
chemoselectivity was observed. Further increase the melamine:TiO2/-
Ti3C2 ratio had no obvious influence on the catalytic performance.
Above results indicated that N doping (N 2p generate new energy levels
by hybridizing with O 2p, thus reducing the apparent band gap) with
appropriate amount of melamine could effectively form hybrid energy
levels in the TiO2 band gap [57]. To gain deep insight into the role of
light on the reaction, we investigated the hydrogenation and amidation
reaction with 0.15 W cmꢀ 2 blue LED, just 31.0 % conversion with 29.0 %
chemoselectivity to A1 was achieved (Table 1, entry 6). The one-pot
hydrogenation and amidation reaction could not be carried out
To further extend the scope of the use of aromatic nitro compounds
in the one-pot hydrogenation and amidation with atmosphere H2 upon
visible light irradiated 3%Pt/N-TiO2/Ti3C, other aromatic nitro com-
pounds having different substituents were chosen as substrates
(Table 3), good to excellent activity and chemoselectivity were ach-
ieved. When the aromatic nitro compounds all had methyl substituent
(Table 3, entries 2–4), the chemoselectivity order, o-nitrotoluene < m-
nitrotoluene < p-nitrotoluene, should be ascribe to the steric hindrance
effect. As for stronger electron-donating methoxy substituent substrates,
o-nitroanisole showed 99.8 % chemoselectivity to corresponding amide
(Table 3, entry 5). However, only 85.3 % chemoselectivity was obtained
with p-nitroanisole (Table 3, entry 6), this should be explained that both
electron donating as well as steric hindrance effect were favorable for
the p-nitroanisole conversion, and the monoamidation product was
further converted into bisamidation product (Table 3, entry 6). For the
electron withdrawing group substituted nitrobenzenes, the slightly
chemoselectivity decrease to corresponding amides should be owing to
the electronic effect (Table 3, entries 7–9). Additionally, m-
Table 2
Visible light promoted one-pot hydrogenation and amidation of nitrobenzene with carboxylic acids over 3%Pt/N-TiO2/Ti3C.a
Entry
Substrate
Conv. / %
A sel. / %
B sel. / %
C sel. / %
1
2
3
4
5
6
7
acetic acid
100.0
100.0
99.8
100.0
85.8
91.9
96.1
93.6
90.8
27.9
–
–
formic acid
14.2
–
propionic acid
butyric acid
valeric acid
isobutyric acid
octanoic acid
–
–
–
–
–
8.1
3.9
6.4
9.2
72.1
100.0
95.2
100.0
95.4
a
The reaction conditions were the same as in Table 1 (entry 3) except the carboxylic acids.
6
Jiang, Heyan
