184
Chemistry Letters Vol.37, No.2 (2008)
Selective Oxidation of Alcohols by Orthorhombic Mo–V–O Phase
with Molecular Oxygen
Feng Wangꢀ1 and Wataru Uedaꢀ2
1CREST, Japan Science and Technology Corporation (CREST-JST), Kawaguchi 332-0012
2Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021
(Received November 12, 2007; CL-071251; E-mail: wangfeng@cat.hokudai.ac.jp; ueda@cat.hokudai.ac.jp)
We have investigated heterogeneous oxidation of alcohols
MoO6
Octahedron
MoO7
Pentahedron
using crystalline molybdenum vanadium oxide (Mo–V–O) as
catalyst with molecular oxygen as oxidant. The major product
from primary alcohol was aldehyde, the secondary alcohol was
mainly dehydrated to olefin, and the oxidation of cyclic alcohols
chiefly afforded ketones. A Hammett plot suggested that hydride
abstraction might be involved in the reaction step. We have dis-
cussed possible reaction mechanism based on substrate adsorp-
tion and activation on catalytically active sites.
7MR
6MR
VO6 or MoO6 Octahedron
6MR
7MR
a
c
b
4 Å
Heterogeneous oxidation of alcohols has been extensively
reported on precious metal-based catalysts such as Pd, Ru, Au,
etc.1 However, catalyst of transition-metal oxide is far less
studied except octahedral manganese oxide and nickel hydrotal-
cite.2
Figure 1. The crystal structure and SEM image of the ortho-
rhombic molybdenum vanadium oxide. (left upper) In view of
crystal from the c axis direction. six- and seven-membered rings
(6MR and 7MR) were labeled; (left lower) in view of crystal
from the a axis direction.
We ever reported the catalysis by Mo–V–O oxide in the
selective oxidation in gas phase.3 Inspired by the reports of
Ishii et al. and Neumann and Levin on Mo- and V-containing
polyoxometalate (POM) catalysts in alcohol oxidation,4 we
showed interests in using the crystalline Mo–V–O oxide in the
reaction. As a result, excellent selectivity for benzaldehyde in
the oxidation of benzyl alcohol was achieved in the presence
of crystalline Mo–V–O oxide. The reaction was conducted
without cocatalysts, cooxidants, and additives. To the best of
our knowledge, this is the first report regarding crystalline
Mo–V–O oxide catalyst in the oxidation of alcohol although
elements of Mo and V were used to prepare POM catalysts.5
The detailed preparation procedure was reported else-
where.6 In brief, a mixture of (NH4)6Mo7O24 and VOSO4
was prepared at ambient condition and then transferred to a
Teflon-lined autoclave for crystallizing at 448 K for 48 h. The
obtained black solid was filtered off, washed with oxalic acid
solution and then degassed at 673 K in a N2 atmosphere for
2 h. Our analysis indicates that the crystal is an orthorhombic
phase with lattice constants: a ¼ 21:19, b ¼ 26:57, and
POM catalysts;4 however, the same oxidation using the crystal-
line Mo–V–O as catalyst afforded the conversion of 83% (Entry
13).5 We conducted competitive oxidation of p-substituted ben-
zyl alcohols at 383 K for 4 h. As depicted in Figure 2, a plot of
þ
Hammett ꢀp against logðkX=kHÞ gave a moderate ꢁ value of
ꢁ0:249 (r2 ¼ 0:98), suggesting that one of reaction steps might
involve hydride abstraction from benzylic carbon and thus
forming benzylic carbocation.7
The Mo–V–O catalyst was reused for two times in the oxi-
dation of benzyl alcohol. As expected, 21 and 20% conversion
were acquired with 98 and 97% selectivity of benzaldehyde, re-
spectively, which were comparable to those of fresh catalyst.
The XRD characterization confirmed that the crystal structure
did not change after third recycling (Figures S1–S3).8 Further-
more, the oxidation was completely stopped if the catalyst was
removed after 9 h run (Figure S4),9 indicating that the reaction
occurs on solid catalyst. The inductively coupled plasma analy-
sis showed that the concentrations of Mo and V in the filtered re-
action mixture were below detection limit [Mo < 0.5 ppm; V <
1.0 ppm], suggesting no Mo and V leached into reaction media.
Distinctive difference of product distribution was observed
in the oxidation of various alkanols in terms of the chain length
and the position of hydroxy group (Table 1, Entries 15–22). The
major products from primary alcohols were aldehydes with the
selectivities more than 90% and the remaining 10% for olefins.
The conversion of primary alcohol greatly depended on the
chain length. For instance, from alcohols of C4 to C8, the conver-
sion decreased from 33 to 1%. The plot of carbon number against
catalytic reaction rate resulted in a decreasing line (Figure S5),10
suggesting that a steric hindrance by the tail CH3(CH2)n
(2 < n < 6) chain played a key role. On the other hand, secon-
dary alcohols were mainly dehydrated on acid sites with higher
˚
c ¼ 4:00 A. The anisotropic growth along the c axis direction
forms rod shape crystal (Figure 1). The rod crystal contains
˚
4-A height slabs, which are close to the dimension of MoO7
pentagonal bipyramid. The slab consists of six- or seven-mem-
˚
bered rings with a pore size of ca. 4 A (Figure 1).
The oxidation of substituted benzyl alcohols by dioxygen
was first examined. Table 1 shows the results of the oxidation.
Among all benzyl alcohols investigated (Entries 1–14), the
selectivities for benzaldehydes were more than 95% and did
not change greatly with substituents. However, the conversions
of benzyl alcohols were remarkably dependent on substrates.
As the substituent varied from electron-withdrawing to elec-
tron-donating, the conversion was increased from 10 to 99%.
It was noted that 4-hydroxybenzyl alcohol was not oxidized over
Copyright ꢀ 2008 The Chemical Society of Japan