130
F. Sato, S. Sato / Catalysis Communications 27 (2012) 129–133
Germany) using Cu Kα radiation (λ=0.15 nm). Tube voltage and cur-
rent were 40 kV and 40 mA, respectively. Lattice parameter a of cubic
Sc2-xYb O was calculated from the following equation:
x 3
2
2
2
2
2
2
1
d
h þ k þ l
2 þ 2 þ 2
¼
¼
ðh ¼ k ¼ l ¼ 2Þ
ð1Þ
2
2
a
a
using the lattice spacing of {222} facet, d, where h, k, and l are Miller
indices.
Temperature-programmed desorption (TPD) of CO adsorbed at 25 °C
2
was examined by neutralization titration using an electric conductivity
cell immersed in aqueous NaOH solution, and the detail procedure has
been described elsewhere [13,14]. CO
was bubbled into a dilute NaOH solution. The amount of desorbed CO
was monitored from the change in conductivity of the solution. The sam-
2 2
that desorbed with N carrier gas
2
−
1
ple was heated from 25 to 800 °C at a rate of 10 °C min in N
5
2
flow of
0 cm3 min . The density of basic sites is defined as the number of
CO molecules desorbed in the temperature range of 25–500 °C on the as-
sumption that one CO
surface. The TPD of NH
apparatus as the CO
with the desorbed NH
the amount of desorbed NH
tivity of the solution.
−1
2
2
molecule adsorbs on one basic site of the catalyst
adsorbed at 25 °C was also measured in the same
Fig. 1. XRD profiles of Sc2-xYb
e) 2.0. Diffraction data of Sc
18], respectively.
x
O
3
mixed oxides at x=(a) 0, (b) 0.5, (c) 1.0, (d) 1.5, and
and Yb were the same as those of Refs. [14] and
(
[
2
O
3
2 3
O
3
2
-TPD, as has been described previously [13]. N
was bubbled into a dilute H SO solution, and
was monitored from the change in conduc-
2
gas
3
2
4
3
0.9845 and 1.0435 nm [7]. The LP value of Sc2-xYb was linearly in-
O
x 3
creased from 0.9812 nm of Sc to 1.0445 nm of Yb O with increasing
2
O
3
2 3
x from 0 to 2.0 (Figure not shown). The tendency is consistent with the
previous report [7]. As we expected, we obtained Sc2-xYb samples
with LP values between those of Sc and Lu
Table 1 also displays specific surface area (SA) of catalysts. SA of
2
.3. Catalytic reactions
The dehydration of alkanediols, such as 1,5-pentanediol and 1,4-
x 3
O
O
2 3
2 3
O .
2
−1
butanediol, was carried out in a fixed-bed down-flow glass tube reactor
with an inner diameter of 17 mm and a length of 300 mm under the at-
mospheric pressure of nitrogen gas at the flow rate of 30 cm3 min . In
each test, 0.3 g of catalyst was loaded in the reactor. After the catalyst bed
had been heated in nitrogen flow at 500 °C for 1 h, the catalytic reaction
was performed at 350 or 400 °C. An alkanediol was fed into the reactor at
a liquid flow rate of 1.8 cm min (17 mmol h for 1,5-pentanediol).
A reaction effluent recovered every hour was analyzed by gas chroma-
tography (GC-2014, Shimadzu, Japan) with a 30-m capillary column
x
Sc2-xYb O
3
was decreased from 53.2 to 26.2 m g
with increasing
x from 0.5 to 1.5. Particle size, D, of Sc1.0Yb1.0
O
3
was calculated by
−1
the following equation, assuming that the particles are spherical:
6
D ¼
ð2Þ
3
where d′ is an estimated density of Sc1.0Yb1.0O , d′=[(density of
0
d ⋅SA
3
−1
−1
−
3
−3
Sc
2=6.5 g cm . The D of Sc1.0Yb1.0
Fig. 2 shows TEM images of Sc2-xYb
the Sc2-xYb samples were composed of nanoparticles with the
2
O
3
, 3.864 g cm
[15])+(density of Yb
was estimated to be 26 nm.
. Judging from the TEM images,
2
O
3
, 9.2 g cm
[15])]/
−
3
(Rtx-Wax, RESTEK, USA). A gas chromatography mass spectrometer
O
3
O
x 3
(GCMS-QP5050A, Shimadzu, Japan) equipped with a 30-m capillary col-
umn (DB-WAX, Agilent Technologies, USA) was used for identification
of compounds in the effluent.
x 3
O
particle size ranging from 20 nm to 40 nm. Table 1 also summarizes
particle size estimated from the diffraction peak of {222} facet in Fig. 1.
The particle size, on the other hand, is smaller than the size observed
in TEM photos of Fig. 2. The size is similar to the D value calculated
from the SA value (Table 1). Thus, it is reasonable that the nanoparticles
Since the catalytic activity is stable, as a previous paper has reported
[11], both the conversion of diol and the selectivity to each product are
averaged in the initial 5 h to evaluate the catalytic activity. The conver-
sion of diol is defined as the amount of diol consumed in the reaction,
and the selectivity to each product is defined as the molar selectivity.
observed in Fig. 2 are composed of primary particles of Sc1.0Yb1.0
Fig. 3 shows TPD profiles of NH and CO adsorbed on Sc2-xYb
catalysts. In the NH -TPD (Fig. 3a–c), NH was not adsorbed on the
samples even at 25 °C. This indicates that Sc2-xYb samples have
no acidic sites which can be estimated by NH adsorption. This is con-
sistent with the previous TPD results of Yb [13]. In contrast, CO
3
O .
3
2
x 3
O
3
. Results and discussion
3
3
x 3
O
3
.1. Characterization of catalyst samples
3
2
O
3
2
Fig. 1 shows XRD profiles of Sc2-xYb
x
O
3
(0≤x≤2) calcined at 800 °C.
was adsorbed on the samples, and desorption profiles were obtained
(Fig. 3d–h). Table 1 lists density of basic sites and desorption temper-
All the samples had cubic bixbyite phase: the largest diffraction appeared
at 2θ=ca. 30°, which is assigned {222} facet (Fig. 1A). The diffractions
assigned {211}, {400}, {440}, and {622} facets also appeared. Fig. 1B
shows enlarged view of Fig. 1A around the main diffraction peak of
222} facet. Diffraction angle of {222} facet, 2θ, was decreased from
1.56 to 29.60° with increasing x from 0 to 2.0. In the sample at x=0.5
and 1.0 (Fig. 1B, b and c), however, a small peak at 2θ=ca. 29.6° was ob-
served, and this could be attributed to the segregation of Yb phase in
2
ature observed in the CO -TPD profiles. The density of basic sites of
−
2
Sc2-xYb
The basic sites, estimated by desorption temperature around 130 °C, is
regarded as weak basic sites. The TPD results indicate that Sc2-xYb
x 3 2 3
O is ca. 5 μmol m , which is comparable to that of Yb O .
{
3
x 3
O
surface is weak basic rather than acidic.
2
O
3
3.2. Catalytic reactions
the samples. Diffraction angle of other facets also depended on Yb con-
tent. We calculated a LP value from the diffraction angle of {222} facet.
Table 2 summarizes catalytic activities of various catalysts in the de-
hydration of 1,5-pentanediol at 400 °C. Sc2-xYb catalysts showed
high selectivity to 4-penten-1-ol higher than 80% at the conversion rang-
ing from 57% to 68% regardless of Yb content. Over Sc2-xYb , the selec-
tivity to 4-penten1-ol was higher than those of simple Sc and Yb
Table 1 summarizes the calculated LP values of Sc2-xYb
of Sc0.5Yb1.5 and Sc1.0Yb1.0 are close to the values (Sc1.494Yb0.506
.0009 nm; Sc1.015Yb0.985
values of Sc and Yb
x 3
O . LP values
x 3
O
O
3
O
3
3
O ,
1
O
3
, 1.0149 nm) reported previously [7]. LP
correspond with the reported values of
x 3
O
O
2 3
2
O
3
O
2 3
2 3
O ,