S.M. Baghbanian et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 128–136
133
Table 3 (Continued)
Entry
Substrate
Productsb (%)
Time (h)
0.5
TOFc (h−1
25324.74
)
1
2
(98.26)
a
b
c
◦
mg 2 wt% Rh/NZ-CP (0.001552 mmol Rh), 20 mmol arene, reaction temperature 60 C, reaction pressure 10 atm.
Determined by gas chromatography analysis.
Turnover frequency (TOF) defined as moles of product consumed per surface of Rh site per hour.
Under H2 pressure of 20 atm.
8
d
Table 4
Effect of Rh loading on the hydrogenation of nitrobenzene .
a
Yieldb (%)
TOF (h )c
−1
Entry
Catalyst (wt%)
Solvent
Time (h)
1
2
3
4
5
6
Rh/NZ-CP (1)
Rh/NZ-CP (2)
Rh/NZ-CP (3)
Rh/NZ-CP (2)
Rh/NZ-CP (2)
Rh/NZ-CP (2)
H2O
H2O
H2O
DMF
CH2Cl2
Toluene
0.25
0.25
0.25
2
2
2
90.45
98.67
98.67
25.38
22.43
20.18
23311.85
25430.41
16939.05
1635.31
1445.23
1300.25
a
Reaction conditions: nitrobenzene (20 mmol), H2O (10 mL), H2 (10 atm), 0.25 h.
Determined by gas chromatography analysis.
TOF was calculated as moles of product consumed per surface of Rh site per
b
c
hour.
cis/trans ratio [54]. Moreover, a noticeable decrease in the cat-
alytic activity was observed for benzene rings including either
the electron-withdrawing (-CF ), or the electron-donating (OCH )
3
3
groups (Table 3, entries 10 and 11) [55].
3.2.2. Hydrogenation of nitroarenes
The hydrogenation of nitroarenes is a reaction of remarkable
interest as the resulting anilines are versatile intermediates in the
preparation of dyes, pharmaceuticals, agrochemicals, pigments and
polymers [56]. However, the selective reduction of the nitro group
is a challenging task for chemists. Most of the reported methods
involved a reductant like hydrazine hydrate for efficient hydro-
genation in the presence of metal catalysts and required harsh
conditions like high temperature and high loading metal. In this
regard, the hydrogenation of nitrobenzene was chosen as a model
reaction to test the catalytic activity of Rh/NZ-CP in water, as a green
solvent, at room temperature. The hydrogenation of nitrobenzene
with different amounts of Rh (1–3 wt%) and 10 atm hydrogen pres-
sure was investigated (Table 4). It was observed that while the
amount of Rh loading increased from 1 to 2 wt%, the product yield
improved from 90.45 to 98.67% and TOF value increased from
Fig. 5. Survey XPS spectra for (a) 2 wt% Rh/NZ-CP before and (b) after 7 times re-
used.
genated rapidly to be 2-aminopyridine with high TOF value of
−
1
4
9,221.65 h (Table 5, entry 9).
3.2.3. Hydrogenation of alkenes
The catalytic activity of the Rh/NZ-CP (2 wt%) was also exam-
ined in the hydrogenation of alkenes under solvent-free conditions
and 10 atm H2 at room temperature. The catalyst was efficient for
the hydrogenation of various styrene derivatives in short reaction
times of 0.25–2 h (Table 6, entries 1–5). We also investigated the
hydrogenation of 1-hexene and 1-undecene. TOF values reach for
−1
−1
(Table 4, entries 1 and 2). More-
2
3,311.85 h
to 25,430.41 h
over, by further increasing the amount of Rh to 3 wt%, the product
−
1
yield was not changed, and TOF value decreased to 16,939.05 h
Table 4, entry 3). Therefore, Rh loading 2 wt% was chosen as the
optimized amount of catalyst for the further steps (Table 4, entry
). The effect of solvent in this reaction was also studied (Table 4,
(
−
1
−
1
1
-hexene 25,396.91 h and for 1-undecene 25,185.57 h (Table 6,
entries 7 and 8).
2
entries 4–6). The hydrogenation in organic solvents such as toluene,
dichloromethane and dimethylformamide actually failed (Table 4,
entries 4–6), while the reaction progressed efficiently in water.
Under these optimized conditions, the scope of hydrogenation
was studied for different nitro aromatic compounds (Table 5). In
all cases, the hydrogenation reactions proceeded rapidly to pro-
duce the corresponding aromatic amines. As shown in Table 5, nitro
arenes with both electron-donating and electron-withdrawing
substituents showed high reactivity and generated the desired
products in good to excellent yields (Table 5, entries 3–8). The
yield for a m-nitroarene is a little higher than for the corresponding
o-nitroarene and p-nitroarene. 2-nitropyridine was also hydro-
3.3. Catalyst recycling
To study the stability and the reusability of Rh/NZ-CP, it was
investigated in toluene hydrogenation under the optimized reac-
tion conditions. After completion of the reaction, the catalyst was
easily removed from the reaction mixture using centrifuge tech-
nique and recovered simply by washing with ethanol and vacuum
drying. The recovered catalyst was reused seven times with only a
reduction of less than 10% in catalytic activity (Table 7). After seven
times reused, XPS analysis were performed to study the chemical
changes in the surface of the catalyst (Fig. 5). The existence of Rd