GEN C¸ et al./Turk J Chem
Metallic nanoparticles that are used as heterogeneous catalysts offer particular advantages in terms of
selectivity, activity, stability, and energy efficiency.12 In spite of the fact that palladium-based catalysts have a
wide range of applications in both synthetic and industrial chemistry,13 the development of nanoscale palladium
particles and widespread applications in organic synthesis were carried out only during the last few decades.14
On the one hand, homogeneous palladium catalysts are generally preferred due to their high activity, but
recovery and reuse of the catalyst are impossible in this case.15 Furthermore, when it is considered palladium
is a precious metal and thus currently used palladium catalysts are really expensive and also sensitive to
the air,16 it is clear that stable and easily reusable palladium catalysts are urgently needed for sustainable
chemistry.17 In recent years, lots of studies were carried out for this purpose.18 To be able to produce
heterogeneous palladium catalysts, a large number of solid materials (such as carbon structures,19 polymers,20
and mesoporous silica21), micelles, microemulsions, surfactants, and ionic liquids (ILs) have been utilized as
supports for palladium.22 Nevertheless, the number of reusable palladium catalysts capable of reducing by green
solvent at room temperature is still limited.
In our previous works, we carried out hydrogenation of alkenes,23 hydrogenation of vegetable oils,24 and
nitro reduction25 with an automobile catalytic convertor. Here, palladium loaded 1,3,5-triazine-pentaethylenehe-
xamine (TAPEHA) polymer nanoparticle as a novel heterogeneous catalyst was used for reduction of aromatic
nitro compounds (Figure 1).
Figure 1. Reduction of aromatic nitro compounds with TAPEHA-Pd and NaBH
4 2 4 2
or N H .H O.
2. Results and discussion
The support material used in Pd-based catalysts should undoubtedly have a high palladium-adsorption capacity,
stability, and low resolution. TAPEHA demonstrated a good affinity toward palladium by virtue of having a high
density of amine and triazine functional groups. The Pd-adsorption capacity of this polymer was found to be
4
.18 mmol/g Pd(II) (444 mg/g) by column technique and this is one of the highest values in the literature.26,27
TAPEHA-Pd is also stable and has very low solubility in water and many organic solvents.
It is known that Pd(II) is reduced to Pd(0) in the presence of a hydride source like NaBH4 or N2 H4 .2
8−30
For this reason, Pd(II)-trapped TAPEHA has been reduced to Pd(0) state during the reduction experiment.
It can be clearly observed with color change in the polymer from yellow to black31 during the first second of
adding NaBH4 or N2 H4 .H2 O. As expected, nitro reduction has been carried out with Pd(0) catalyst in our
experiments. Thus, when recycled TAPEHA-PdNPs containing Pd(0) form are used in the next reaction, the
catalyst shows the same catalytic activity. TAPEHA-PdNPs can be recovered by filtration and used again in
another reduction reaction. It was reused ten times without any loss of activity. The amount of catalyst is
taken as 0.015 g for the reduction of 1 mmol nitro compound.
2.1. Optimum reaction conditions in NaBH4 mediated reductions
To determine the optimum reaction conditions in NaBH4 -mediated reductions, a series of experiments were
performed based on nitrobenzene (Table 1).
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