7
0
D. Laprune et al. / Applied Catalysis A: General 535 (2017) 69–76
The autoclave was then cooled to room temperature and the
solid was centrifuged, washed with water until pH = 7 and dried
◦
overnight at 90 C. Finally, the resulting solid was calcined for
◦
1
2
2 h at 525 C in air yielding silicalite-1 crystals of approximately
00 nm × 150 nm × 140 nm in size.
Ni@silicalite-1 materials were synthesized following the pre-
viously described generic method for the encapsulation of
transition metal nanoparticles in hollow silicalite-1 single crystals
[
7,10,18,19]. In brief, the 5%Ni@sil-1 was prepared by wet impreg-
nation: 2 mL of an aqueous solution of Ni(NO ) -6(H O) (99.999%,
3
2
2
Sigma-Aldrich) with a concentration of 426 mmol/L was added to
◦
1
g of silicalite-1, which had been outgassed at 300 C overnight.
◦
The mixture was stirred at 50 C until complete evaporation of
water to obtain a Ni(NO ) /sil-1. The hollow structure was then
3
2
obtained by treating 1 g of this material in a TPAOH aqueous solu-
◦
tion (7.5 mL; 0.55 M) in a Teflon-lined autoclave at 170 C under
rotating conditions for 24 h. The solution was then cooled down,
◦
washed with water until pH = 7, dried overnight at 90 C and cal-
◦
cined in air at 450 C for 6 h to obtain a NiO@silicalite-1. Finally the
solid was reduced at 750 C under H for 3 h with a heating rate of
.5 C min to yield the sample referred to as 5%Ni@sil-1.
◦
2
◦
−1
2
The citric acid post-treatment was carried out as follows. 0.5 g
of NiO@silicalite-1 was added to 50 mL of a 0.5 mol/L aqueous solu-
tion of citric acid (≥99.0%, Sigma-Aldrich). The mixture was stirred
◦
vigorously at 80 C for 2 h. The solution was then centrifuged and
◦
washed with water until pH = 7 and dried overnight at 90 C. Finally
◦
the solid was reduced at 750 C under H for 2 h to yield a sample
2
referred to as CitAc Ni@sil-1. This sample was further calcined in
◦
air at 550 C for 6 h to obtain the calc-CitAc Ni@sil-1. A reference
Fig. 1. FT-IR transmission spectra recorded in the 10 cm pathlength gas-cell dur-
ing the hydrogenation of (top) toluene and (bottom) mesitylene. The spectra in
red are those of the reactants. The blue spectra were collected after the reactor at
a conversion level of about 9%. The spectra in black are obtained from removing
the contribution of the reactant (red) from that of the reactor effluent (blue) and
corresponded to (top) methylcyclohexane and (bottom) the two stereoisomers of
catalyst named 5%Ni/sil-1 was produced by direct calcination and
reduction of Ni(NO ) /sil-1 under the same conditions as 5%Ni@sil-
3
2
1
.
2
.2. Sample characterization
−
,3,5-trimethylcyclohexane. The area of the bands between 3200 and 3000 cm
1
1
were used to quantify arene concentrations. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
Powder X-ray diffraction patterns (XRD) were recorded to assess
the crystallinity of the samples. Diffractograms were collected
◦
◦
between 4 and 90 (2ꢀ) with steps of 0.02 and 1 s per step with a
Bruker D5005 diffractometer using CuK␣ radiation at = 1.5418 Å.
Elemental analysis of the catalysts was performed using an
ICP-OES ACTIVA from HORIBA Jobin Yvon equipped with a CCD
detector for the determination of metal loadings. Nitrogen adsorp-
tion isotherms were measured at 77 K on a Belsorp-mini from
OD) containing the powdered catalyst held between quartz wool
plugs. The system was operated at atmospheric pressure and the
◦
samples were reduced in situ at 500 C in pure H for 60 min before
2
the catalytic tests. Aromatic reactants were fed individually using a
◦
saturator kept at 0 C, leading to partial pressures of 912 and 62 Pa
◦
BEL-Japan. Samples were first outgassed under vacuum at 300 C
for toluene and mesitylene, respectively. A flow of 20 mL/min of
for 4 h. The t-plot analysis was not considered here, in view of the
debate on the validity of the t-plot method to assess microporosity
in hierarchical materials [20].
pure H was used as reactant carrier gas and fed through one satu-
2
rator at a time. The catalyst powders were crushed and sieved and
the fraction 100–200 m was selected. Between 60–180 mg of cat-
alyst was used, depending on the activity of the sample, to remain
under differential conditions. The gas hourly space velocity was so
TEM pictures were obtained using a Jeol 2010 LaB6 microscope
operating at 200 kV. Nanoparticle size distributions of zeolite-
based materials were obtained by counting 500 nanoparticles using
Image J software [21]. Both number-weighted (dNW = ꢁ n d /ꢁ
−
1
varied between 6700 and 20000 h , approximating the catalyst
density as being unity.
i
i
2
3
ni) and surface-weighted (dSW = ꢁ n di /ꢁ n di ) mean diame-
The reactor effluent was then analyzed using a 10 cm path-
length FT-IR gas cell fitted in a Tensor 27 FT-IR spectrophotometer
from Bruker. Typically 32 scans were collected at a resolution of
i
i
ters were calculated from the nanoparticle size distributions. Each
distribution was then modeled using a normal law centered on
the corresponding dNW. Metal dispersions were deduced from the
corresponding dSW considering a cuboctahedral model and a cal-
culation method described by Van Hardeveld and Hartog [22]. EDX
measurements were performed using an EDX Link ISIS analyzer
from Oxford Instruments to identify the elements present in the
samples.
−
1
4 cm and averaged. The conversion of the arene reactants was
determined through integration of spectral regions correspond-
ing to the C H stretching vibration modes [6,23,24]. Note that
FT-IR spectroscopy is commonly used as an analytical technique
to investigate complex reaction mixtures as found during alkane
dehydrogenation [25] and NOx reduction [26]. The IR spectra of
toluene and mesitylene are shown in Fig. 1 (spectra in red). The
spectra in blue are those of the corresponding reactor effluents at a
conversion of ca. 9%. The spectra of the reaction products (black
spectra) could be obtained by removing the contribution of the
reactants and corresponded to that of methylcyclohexane in the
case of toluene (Fig. 1, top) and to that of the two stereoisomers
2.3. Catalytic tests
Toluene and mesitylene (i.e. 1,3,5-trimethylbenzene) hydro-
genation tests were carried out using a fixed-bed continuous-flow
reactor consisting of a quartz tube (length 400 mm, 4 mm ID, 6 mm