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Y. Nakamura et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 137–144
nucleophilic substitution (SN2) of hydroxyl group of alcohols by
a hydride ion [7].
followed by an exposure in air. Thus obtained MoO2 was provided
for catalysis.
are several research reports on equimolar formation of toluene and
benzyl aldehyde. Jayamani et al. and Ganesan et al. reported that
toluene and benzaldehyde were produced in an equal amount from
benzyl alcohol over alumina catalyst [8–10] and Ganesan et al. pro-
posed that the disproportionation of di-benzyl ether is a pathway
to form the toluene and benzaldehyde in a 1:1 ratio in addition
Mathew et al. reported that the conversion of benzyl alcohol to
toluene and benzaldehyde occurred over molybdenum supported
Al(OH)3 [11,12]. ABBꢀO3 (A = Ba, B = Pb, Ce, Ti and Bꢀ = Bi, Cu, Sb)-
type perovskite oxides [13] and Au–Pd nanoparticle [14,15] are also
found active for the reaction. In the case of the former, hydrogena-
tion of benzyl alcohol to form toluene was proposed, while in the
Au–Pd led to the equimolar formation of toluene and benzaldehyde
in benzyl alcohol reaction under He.
2.2. Catalytic test
Catalytic reactions were carried out in a continuous flow fixed
bed reactor (Pyrex). A similar volume mixture of the catalyst
(0.03–1.5 g) and SiO2 sands (1.3–2.6 g) as a diluent, which showed
no catalytic activity in the alcohol reaction, was placed in the reac-
tor and heated to a desired reaction temperature (533–633 K) under
a N2 flow (21.4 ml/min). Then, the catalytic reaction was started
by the introduction of ethanol (99.5%, Wako Pure Chemical Indus-
tries) with N2 carrier into the reactor. The total flow rate of the
reactant gas was kept constant (21.4 ml/min) for all the reactions.
Ethanol concentration was changed from 1.8–7.5 mol%. The con-
centrations of methanol (99.8%, Wako Pure Chemical Industries),
1-propanol (99.5%, Wako Pure Chemical Industries) and 2-propanol
(99.7%, Wako Pure Chemical Industries) were 2.7, 1.7, and 4.3 mol%,
respectively. For study on kinetic isotopic effect, two types of
isotope-labeled ethanol, CH3CH2OD (99%, Wako Pure Chemical
Industries) and CD3CD2OD (99%, Wako Pure Chemical Industries),
were used as reactant.
Among the catalytic systems mentioned above, 12-
molybdophosphate [3], Pt-supported catalysts [4] are the unique
cases that simultaneously produced equivalent amounts of alkanes
and aldehydes from aliphatic alcohols, although some other prod-
ucts were formed along with them and the proposed reaction
mechanisms are still controversial. In addition, it seems have been
considered that higher oxidation states of metal elements in the
case of oxide catalysts favor the alkane formation. Meanwhile, our
results reported recently using vanadium oxides and molybdenum
oxides as catalysts for ethanol conversion evidently showed that
equivalent amounts of alkanes and aldehydes were formed from
corresponding alcohols as main products without H2 formation
and clarified that lower oxidation states of metal elements in
these oxide catalysts are active for the equimolar formation
[1]. Apparently vanadium oxides and molybdenum oxides in
low valence states are now one of the representative catalysts for
equimolar formation of alkanes and aldehydes from corresponding
alcohols. In order to further confirm the simultaneous formation
of equivalent amounts of alkanes and aldehydes and to elucidate a
plausible reaction mechanism for the reaction, we conducted the
reaction under different conditions and kinetic analysis. Here in
this report, an intermolecular hydrogen-transfer dehydration of
aliphatic alcohols will be described based on observed results.
The reaction products were analyzed by gas chromatography.
Two gas chromatographs, Shimadzu GC-8A equipped with a ther-
mal conductivity detector and a packed column Porapack-QS and
bon and molecular sieve 5A, were used. N2 gas was used as internal
standard for quantitative GC analysis. Alcohol conversion, the prod-
uct selectivity, and carbon balance were defined as the following
Eqs. (1)–(3), respectively.
X
Conversion(%) =
Selectivity(%) =
× 100
(1)
X0
A
× 100
(2)
(3)
X
Carbon balance = Selectivitytotal
where X0, X, and A refer to the amount of alcohol feed, the amount
of reacted, and amounts of products, respectively.
2.3. Catalyst characterization
Powder X-ray diffraction (XRD) measurements were performed
with a RINT Ultima+ diffractometer (Rigaku) with Cu-K␣ radiation
(ꢀ = 0.1540 nm) and X-ray power of 40 kV/20 mA. Specific surface
areas were measured by the BET method from N2 adsorption at
77 K using a BELSORP MAX (BEL Japan Inc.). XPS measurements
were performed using a JPS-9010 MC (JEOL). An Mg-K␣ radiation
source (1253.3 eV) operated at a power of 100 W (10 kV, 10 mA)
was employed. Vacuum in the analysis chamber was <5 × 10−6 dur-
ing all measurements. Pass energy of 30 eV was used to acquire
all survey scans. The binding energy (BE) was corrected for sur-
face charging by taking the C1s peak of carbon as a reference at
248.7 eV. Data were analyzed using the SpecSurf including Shirley
background subtraction and fitting procedure. Quantification of
the components for the surface oxidation state of vanadium and
molybdenum was made using the SpecSurf. The binding energies
2. Experimental
2.1. Catalyst preparation
V2O5 was prepared by the calcination of NH4VO3 (99% Wako
Pure Chemical Industries) at 773 K for 2 h in air. V2O3 was then
prepared by the reduction of the prepared V2O5 (0.3 g) in a tubu-
lar furnace under a H2 stream (30 ml/min) at 773 K for 2 h. The
reduced samples were then exposed to air when they cooled at
room temperature before use for catalysis. MoO3 was prepared by
the calcination of (NH4)6Mo7O24 (99% Wako Pure Chemical Indus-
tries) at 773 K for 2 h in air. MoO2 was obtained by the reduction of
the obtained MoO3 (0.3 g) in a tubular furnace under a H2 stream
(30 ml/min) at 773 K for 2 h. Subsequently, the reduced molybde-
num oxide samples were exposed to air once after they cooled
at room temperature. Then the samples were treated again in a
tubular furnace under a 5%H2/Ar (30 ml/min) stream at 773 K for
2 h. Finally the reduced sample was cooled to room temperature,
of 517.2, 516.0 and 515.2 eV were attributed to V5+, V4+ and V3+
,
respectively in the V2O3 [16,17], and Mo6+, Mo5+ and Mo4+ oxida-
tion states (Mo3d3/2 and Mo3d5/2) were identified at 235.8, 234.7
and 232.7 eV, and 232.2, 231.9 and 229.1 eV, respectively [18,19].