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medicine, energy, and so on [28–35]. These valuable compounds
represent fine and bulk chemicals and serve as essential precursors
and key feedstock for advanced chemicals, pharmaceuticals, agro-
chemicals and materials [28–35]. The amine functionality consti-
tutes an integral part of many life science molecules, natural
products, and biomolecules [28–35]. It is also known that more
than 170 out of 200 drugs prescribed, in the year of 2016, contain
amine moieties, which clearly demonstrates the extreme impor-
tance of amines in the discovery of new drugs [30]. Regarding
the synthesis of higher value of amines as well as their functional-
ization, N-alkylation of amines using alcohols, the so called ‘bor-
rowing hydrogen technology’ represents more sustainable and
waste-free methodology that produces water as the only by-
product [36–63]. With respect to the potential catalysts for this
reaction, homogeneous metal-complexes have been efficiently
used [36–57]. Despite their elegant activity and selectivity, major-
ity of these homogeneous complexes are rather sensitive (to air,
moisture, or both) and/or incorporate sophisticated (synthetically
demanding) ligand systems [38–57]. In contrast, heterogeneous
catalysts are extremely stable, easily recyclable, and hence, they
are certainly vital for the advancement of cost-effective, sustain-
able industrial processes [58–66]. In this respect, heterogeneous
catalysts based on Ru [58], Ni [59], Cu-Fe [60], Ag [61] and Pd
[62–66], have been applied for N-alkylation of amines. Despite
these, still the development of stable and reusable catalysts, espe-
cially based on nanoparticles is highly desired and continues to
attract scientific interest for the base- or additive-free synthesis
of N-alkyl amines.
catalysts were synthesized using above similar procedure. The
other catalysts were denoted as follows: PdNPs supported on lab
prepared meso-SiO2 (Cat-2), PdNPs on other commercial SiO2
(Cat-3), and PdNPs on TiO2 (Cat-4). In addition, two commercial
catalysts were also used for comparison, namely Pd/C (Cat-5) and
Pd/Al2O3 (Cat-6). Meso-SiO2 was prepared by the hydrolysis of
TEOS (ꢀ50.38 mmol) in aqueous solution containing ammonia
(35.0 mL), CTAB (ꢀ10.0 mmol), and water (720 g) and stirred over-
night, followed by filtration of the precipitated white solid, wash-
ing with copious amount of water, drying at room temperature,
and finally calcination at 600 °C for three hours.
2.3. Catalyst characterization
Pd metal content in each catalyst was determined by using a
Varian Vista-MPX ICP-OES instrument. The specific surface area,
based on Brunauer-Emmett-Teller method (SBET) was measured
by N2 physisorption at À196 °C by using a Micromeritics ASAP
2010 adsorption analyzer. X-ray powder diffraction (XRPD) pat-
terns were obtained by using a D8 advance Bruker instrument,
operated at 40 mA and 40 kV by using CuKa radiation and a Ni fil-
ter, in the 2-theta range between 10⁰ and 80⁰. To investigate the
morphology and particle sizes of our catalysts, a JEM-2100F JEOL
TEM was used. The X-ray photoelectron spectroscopy (XPS) analy-
ses were carried out on a PHl 5000 VersaProbe-Scanning ESCA
microprobe (ULVAC-PHI, Japan/USA) instrument at a base pressure
of 5.5 Â 10À7 Pa, where the X-ray source of Al-K (h
m = 1486.6 eV)
a
with spot size of 200 mm was used. Temperature programmed
reduction (TPR), H2 chemisorption, CO2-TPD studies were recorded
by using a Micromeritics AutoChem II 2920. Reduction profiles
were achieved by passing a mixture of 10% H2/He at a rate of
50 mL/min (STP) over an amount of 0.1 g of catalyst. The tempera-
ture was gradually ramped from room temperature to 500 °C at a
heating rate of 10 °C/min. The total consumption of H2 was calcu-
lated by a thermal conductivity detector (TCD) as a function of
temperature.
2. Experimental section
2.1. Materials and methods
Palladium(II) acetylacetonate (Aldrich), tetraethyl orthosilicate
(TEOS; Aldrich), cetyltrimethylammonium bromide (CTAB; Alfa
Aesar), concentrated ammonia (28–30%; Aldrich), silicon dioxide
(LOBA; Cat-1), silicon dioxide (lab prepared; Cat-2), silicon dioxide
(Silica for hydrodesulfurization, in the shape of pellets provided by
Dr. Tiancun Xiao, Senior Research Fellow, Inorganic Chemistry Lab-
oratory, University of Oxford; Cat-3), titanium dioxide (Fluka, Cat-
4), and acetone (Sigma-Aldrich) were commercially accessible
(except Cat-3) and were used as received. Deionized water (18.2
2.4. Catalytic amination reactions
Dried pressure tube was charged with magnetic stir bar and
50 mg of Pd@SiO2 catalysts (1 mol% with respect to amine). Then,
1.0 mL o-xylene was added, followed by the addition of 0.5 mmol
of amine and 1 mmol of benzyl alcohol. The pressure tube was
flushed with argon was closed with screw cap. Then it was placed
in the preheated aluminum block and reaction was allowed to pro-
gress for 30 h at 150 0C. After completion of the reaction, pressure
tube was removed from aluminum block and cooled down to room
temperature. The catalyst was filtered out by ciliate and reaction
products were analyzed by GC–MS and the corresponding amines
were purified by column chromatography. The yields of selected
amines were determined by GC analysis using n-hexadecane as
standard. For this purpose, after completion of the reaction, n-
hexadecane (100 mL) as standard was added to the reaction pres-
sure tube and the reaction products were diluted with ethyl acet-
ate followed by filtration using plug of silica and then subjected GC
analysis.
MX.cm) was supplied by using Milli-Q water purification system
(Millipore). Commercial catalysts (Cat 5 and 6) were purchased
from sigma aldrich. GC and GC–MS were recorded on Agilent
6890 N instrument. GC conversion and yields were determined
by GC- 13C NMR data were recorded on a Bruker ARX 300 and Bru-
ker ARX 400 spectrometers using CDCl3, solvents.
2.2. Catalysts preparation
Palladium nanoparticles (PdNPs) supported on commercial sili-
con dioxide was obtained in two-step synthesis procedure. First,
1.0 wt% Pd/SiO2 by wet impregnation using acetone as a solvent
to mix the appropriate amounts of both of support (0.97 g) and
Pd(acac)2 (0.03 g). The resultant mixture was ultrasonicated at
room temperature for an hour by using an ultrasonic bath to dis-
perse the SiO2 support. In the second step, after solvent evapora-
tion, the obtained solid sample was oven-dried at 80 °C for two
hours, followed by H2 reduction at 150 °C for two hours to come
up with 1.0 wt/wt% PdNPs supported on SiO2 (Cat-1). For catalyst
scale up, similar procedure was followed to prepare two-kilogram
(2 Kg) catalyst using relative amount of starting materials. In order
to explore the influence of the nature of support on the catalytic
activity of PdNPs toward amination reactions, different Pd based-
3. Results and discussion
We started our investigations to develop simple Pd-based
nanocatalysts in one-gram scale. For this purpose, we used palla-
dium (II) acetylacetonate as precursor and silicon dioxide (SiO2)
as support to prepare 1 wt% Pd@SiO2 (Cat-1) (Fig. 1). First, we
immobilized Pd salt on silica support and then reduce Pd(II) to
Pd(0) under flow of molecular hydrogen at certain temperature