J. Zhao et al. / Catalysis Communications 16 (2011) 30–34
31
2
.2. Characterization of catalysts
(
a)
The chemical compositions of catalysts were analyzed by an ARL-
9
800 X-ray fluorescence spectrometer (XRF). The surface areas and
Ni/Al O -B
pore sizes were measured by Micromeritics Gemini V 2380 autosorp-
tion analyzer. Experiments were performed at 77.3 K using N as an
adsorbate. The samples were degassed at 573 K for 1 h before the
measurements. H and O adsorptions were performed on the
home-made volumetric adsorption system at room temperature and
2
3
2
Ni/Al O
2
3
Ni/MgO-B
2
2
7
2
23 K, respectively. The catalysts were reduced in H
h and evacuated at 723 K for 1 h before the measurements. After
were admitted
was measured until the equilibrium
pressure reached about 50 kPa. The uptake of H for the saturation cov-
2
at 723 K for
cooling the catalysts to room temperature, doses of H
sequentially and coverage of H
2
2
2
0.0
0.2
0.4
0.6
0.8
1.0
erage of hydrogen on a nickel surface was determined by extrapolating
the coverage of isotherm to PH2=0. The surface area of metallic nickel
Relative pressure (P/P0)
was calculated by the uptake of H
H/Nisurf =1 and a surface area of 6.5 Å for a Ni atom. Microcalorimetric
2
2
assuming the molar ratio of
(
b) 4
3 2
adsorption of NH and CO was performed on a Tian–Calvet heat-flux
apparatus. A C-80 calorimeter (Setaram, France) was connected to a
volumetric system equipped with a Baratron capacitance manometer
3
2
1
0
(
USA) for pressure measurements and gas handling. Prior to the micro-
calorimetric adsorption, the samples were typically reduced in H at
23 K for 2 h followed by the evacuation at the same temperature for
h.
X-ray diffraction (XRD) patterns were collected in ambient atmo-
2
Ni/Al2O3-B
Ni/Al O
7
1
2
3
Ni/MgO-B
sphere by an X-ray diffractometer (Shimadzu XRD-6000) with Cu Kα
radiation (λ=1.5408 Å) generated at 40 kV and 30 mA. Diffraction
intensities were recorded from 10˚ to 80˚ at a rate of 7˚/min. Transmis-
sion electron microscopic measurements (TEM) were carried out
using a JEOL electron microscope (JEM-2010), with an accelerating
voltage of 200 keV.
0
10
20
30
40
50
Pore diameter (nm)
Fig. 1. N
Ni/Al
2
adsorption–desorption isotherms (a) and pore size distributions (b) for the
-B, Ni/Al and Ni/MgO-B catalysts reduced in H at 723 K. The reduced sam-
at room temperature for 48 h before
2
O
3
2
O
3
2
2
.3. Catalytic tests
ples were passivated in N
the measurements.
2 2
flow containing 0.1% O
The hydrogenation reactions were carried out in a stainless steel
fixed-bed reactor. The catalysts were pelletized, crushed and sieved
to a fraction of size 0.18–0.25 mm. A catalyst diluted with inert silicon
oxide was placed in the middle of the vertical trickle-bed reactor (the
inner diameter of 10 mm). Particles of silicon oxide were placed at
the both ends of catalyst bed to fill the remaining reactor volume.
The catalyst was reduced in the flowing hydrogen at 723 K for 2 h.
A n-hexane solution containing toluene (toluene:n-hexane=2:1,
w/w) or pure DOP was delivered into the reactor using a 2ZB-1 L10
dual-plunger infinitesimal quality metering pump, and flowed down-
ward with hydrogen through the packed catalyst. The reaction of hy-
drogenation of toluene (0.1 g catalyst) was performed at atmospheric
pressure. The products were analyzed by a gas chromatograph with
an FID equipped with an HP-FFAP capillary column. The hydrogena-
tion of DOP (0.65 g catalyst) was performed at the pressure range of
method, almost no micropores were found in the catalysts. At the rel-
atively higher pressure range of 0.9–0.99, the isotherms for Ni/Al
B went up, indicating the presence of some larger pores. Thus, the av-
erage pore size of Ni/Al -B is larger than that of Ni/Al . The nickel
loading and surface area of the Ni/Al -B were 62.5% and
respectively, much higher than the corresponding
2 3
O -
2
O
3
2 3
O
2 3
O
3
−1
301 cm g
values for Ni/Al
Table 2 summarizes the information obtained by H
2
−1
2 3
O (48.8% and 191 m g ).
2
2
and O chemi-
sorption, according to which the dispersion, reducibility and active
nickel surface area of the supported catalysts could be calculated. Re-
sults showed that the dispersion of reduced nickel was about 30% and
14% for the Ni/Al
Ni/Al -B possessed higher dispersion of metallic nickel although the
loading of nickel was higher in Ni/Al -B than in Ni/Al . Active nick-
el surface areas of the Ni/Al -B and Ni/Al were 70 and 32 m g
respectively. Thus, the density of surface active sites was much higher
on Ni/Al -B than on Ni/Al . The reduction degrees of the
Ni/Al -B and Ni/Al were about 65% and 82%, respectively. As
2 3 2 3
O -B and Ni/Al O , respectively. It is clear that the
2 3
O
2
–5 MPa. The products were analyzed by an HPLC with a differential
detector equipped with a XDB-C18 column. Turnover frequency
TOF) was calculated by dividing the number of molecules converted
O
2 3
2 3
O
2
−1
O
2 3
O
2 3
,
(
per second by the number of active nickel atoms measured by H
adsorption.
2
O
2 3
2 3
O
O
2 3
2 3
O
3
. Results and discussion
Table 1
Composition, surface area and pore structure of the Ni/Al
catalysts.
2 3 2 3
O -B, Ni/Al O and Ni/MgO-B
3
.1. Textural and structural properties of catalysts
Catalyst
Catalyst composition
wt.%)
S
BET
V
total pore
V
micropore
Pore
size
Fig. 1 shows the N
size distributions of the catalysts Ni/Al
B. The compositions, BET surface areas, pore structure parameters
are listed in Table 1. The catalysts exhibited type IV adsorption iso-
therms with obvious hysteresis loops at relative pressures between
2
adsorption–desorption isotherms and pore
-B, Ni/Al and Ni/MgO-
(
2
O
3
2 3
O
Ni
Al
2
O
3
MgO SiO
2
(m2 g−1
)
(cm3 g−1
)
(cm3 g−1
)
(nm)
Ni/Al
Ni/Al
2
O
O
3
-B 62.5 37.5
48.8 50.3
0
0
0
301
0.27 191
152
0.98
0.34
0.40
0.009
0
0
9.1
5.7
9.1
2
3
Ni/MgO-B 62.7
0
37.3
0
0
.6 and 0.95, characteristics of mesopores. According to t-plot