1
54
V. Mahdavi, S. Soleimani / Materials Research Bulletin 51 (2014) 153–160
measurement at 77 K using a Micromeritics ASAP 2010 instru-
ment. Before starting the process of nitrogen adsorption, each
sample was outgassed at 250 8C under vacuum for 8 h. Evaluation
of surface area was made according to data on nitrogen adsorption
determined by the Brunauer–Emmett–Teller (BET) method and
pore size distribution was estimated using the method of Barrett–
Joyner–Halenda (BJH). Catalyst structure was determined by X-ray
diffraction (XRD) tests. A diffractometer Philips model PW 1800
instrument with Cu Ka radiation and Ni filter was used to collect X-
ray data. The SEM images were obtained with a Philips XL30
instrument. Transmission electron microscopy (TEM) images were
collected on a JEOL 2010 electron microscope operated at an
acceleration voltage of 100 kV. Samples were made by grinding
using a pestle and mortar, followed by dispersion in ethanol,
sonicated and then dropped into a wholly carbon-coated copper
grid. The infrared spectrum was recorded on a Galaxy Ft-IR 500
spectrophotometer.
Fig. 1. Structure of K-OMS-2, which is synthetic cryptomelane KMn
8
O
16ꢀnH
2
O.
M-OMS-2 is an example of a recently developed catalyst that
improved oxidation for alcohols and side chains in organic
compounds [20].
This work is the first report on the synthesis of some vanadium
oxide containing nano porous manganese oxide octahedral
molecular sieves (V
ratios and the oxidation of alcohols investigated in the liquid phase
over V /K-OMS-2 samples using tert-butyl hydro peroxide
TBHP) or hydrogen peroxide as the oxidant. The report proposes
2 5
O /K-OMS-2) with different V/Mn molar
2
O
5
2.4. Oxidation of alcohols
(
that there is a relationship between structure and catalytic
performance and on the effect of the V/Mn molar ratio. For the
kinetic study of oxidation, benzyl alcohol was chosen as a substrate
for the model and the effects of reaction time, oxidant/alcohol
molar ratio, temperature, solvents, catalyst recycling and leaching
were investigated.
In a typical procedure, a mixture of 0.2 g catalyst with the grain
size of 200–230 mesh, 15 mL solvent (acetonitrile) and 30 mmol of
alcohol (benzyl alcohol, cyclohexanol or n-hexanol) was stirred in
a three-necked flask under nitrogen atmosphere at 50 8C for
30 min. The stirring rate of the solution was set at 750 cycle/min.
Then 30 mmol of the oxidant (TBHP) was added. The mixture was
refluxed at 90 8C for 8 h under nitrogen atmosphere (Table 2). After
2
. Experimental
2 2
filtration, the solid was washed with CH Cl and the reaction
mixture was analyzed by GC. A GC–MS model Thermo Finnigan
(60 m, RTX-1 column) was used for identification of products and a
GC (Perkin-Elmer Model 1800) was used for product analysis. The
GC was equipped with a flame ionization detector (FID) connected
to a 3% OV-17 column with length of 2.5 m and diameter of 1/8 in.
2
.1. Materials
All reagents used in the experiment were of the highest
commercial quality, and purchased from Aldrich and Merck
chemical companies and used without further purification.
3. Results and discussion
2.2. Preparation of catalysts
3
.1. Characterization of the V O /K-OMS-2 catalysts
2 5
Cryptomelane type of parent tunneled structure manganese
oxide OMS-2 was prepared by the precipitation method [7,21]. A
.4 M solution of KMnO (13.3 g in 225 mL of distilled, deionized
water, DDW) was added to a mixture of a 1.75 M solution of
MnSO O (19.8 g in 67.5 mL DDW) and 6.8 mL of concentrated
ꢀH
HNO . The resulting black precipitate was stirred vigorously and
Fig. 2(A) and (B) shows the nitrogen adsorption–desorption
isotherms of the K-OMS-2 and V /K-OMS-2(2.30) samples.
These isotherms corresponded to type II on the IUPAC classification
system [22]. The isotherm of /K-OMS-2(2.30) sample
exhibited hysteresis loops (type H3) with sloping adsorption
and desorption branches covering a large range of P/P [22] Pore
0
4
2 5
O
4
2
2 5
V O
3
refluxed at 373 K for 24 h. The precipitate was filtered and washed
0
.
with DDW until neutral pH was reached and it was then dried at
size distribution was analyzed using the BJH adsorption method
(Fig. 2(C)). The adsorption branch was located at relative pressures
in the range of 0.2–1.0. A wide pore diameter distribution with a
mean value of 13.8 nm was obtained by the BJH adsorption
method.
+
3
93 K. This gave the K form of OMS-2, which was shown to be K-
OMS-2.
Samples of vanadium containing K-OMS-2 catalysts were
prepared by impregnation described as follows; 1 g of K-OMS-2
was dispersed in water (50 mL) containing the required amount of
2 5
Since the K-OMS-2 and V O /K-OMS-2 compound contained
NH
4
VO
3
(x g) and oxalic acid (1 g). The mixture was stirred and
micropores, nitrogen adsorption isotherm data were also applied
to the D–R (Dubinin–Radushkevich) isotherm model to evaluate
evaporated at 65 8C until dry and calcined at 673 K for 4 h under
airflow. Evaluations for contents of manganese and vanadium were
determined by atomic absorption spectroscopy (AAS) using a
Perkin-Elmer Analyst instrument, after extraction of metals from
0
total micro pores volume W and D (a constant characteristic of the
pore size distribution). The linear form of the D–R isotherm
equation is:
the sample catalysts in HNO
The V/Mn molar ratio ranged from 1.15 to 3.64 (Table 2) and
/K-OMS-2(1.15) stands for the vanadium containing K-OMS-2
3
and HF acids.
ꢀ
ꢁ
ꢂ
ꢃ
2
V
2
O
5
W0
:558 ꢁ 10ꢂ3
P
ln V ¼ ln
ꢂ D ln
catalyst with V/Mn = 1.15.
1
P
0
2 5
In the next step these solids (V O /K-OMS-2) were used in
2
liquid phase to catalyze the oxidation of alcohols by TBHP.
where D is A(RT/
b
) , A is a constant and
b
is known as an affinity
2
0
coefficient. A plot of ln V vs. [ln(P/P )] gives a straight line of slope
2.3. Characterization of catalyst
D and intercept ln(W
0
/1.558 ꢁ 10ꢂ3) over the relative pressure
ꢂ5
range 1 ꢁ 10 < P/P
0
< 0.2.
Surface area and pore size distribution of K-OMS-2 and V
2
O
5
/K-
Evaluations of surface area, pore volume, W
0
and D of OMS-2
OMS-2 catalysts were determined by N
2
adsorption–desorption
and V /K-OMS-2(2.3) samples are shown in Table 1.
2 5
O