Evaluation of nanostructured vanadium(v) oxide in catalytic oxidations

Article abstract of DOI:10.1039/c3cy00183k

Nanostructured V₂O₅ was prepared by electrochemical anodization in the presence of complex fluoride electrolytes. The reactivity of nanostructured V₂O₅ was compared to a commercially available V₂O₅ sample in several oxidation reactions. Catalyst nanostructuring offers improvements in yields as well as rate enhancement in oxidations.

Full text of DOI:10.1039/c3cy00183k

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Evaluation of nanostructured vanadium(V) oxide in
catalytic oxidations†
Cite this: Catal. Sci. Technol., 2013,
3, 2610
a
b
Eric T. Drew,z Yang Yang,z Julia A. Russo,a McKenzie L. Campbell,a
Samuel A. Rackley,a JoAn Hudson,c Patrik Schmuki*b and Daniel C. Whitehead*a
Nanostructured V2O5 was prepared by electrochemical anodization in the presence of complex fluoride
electrolytes. The reactivity of nanostructured V2O5 was compared to a commercially available V2O5
sample in several oxidation reactions. Catalyst nanostructuring offers improvements in yields as well as
rate enhancement in oxidations.
Received 20th March 2013,
Accepted 19th June 2013
DOI: 10.1039/c3cy00183k
www.rsc.org/catalysis
acetophenone.19 Despite these advances, V2O5 is still sparsely
employed by the synthetic community, particularly when compared
Introduction
Vanadium(V) oxide (i.e. V2O5) is a cheap and effective catalyst
for a number of synthetically useful oxidative transformations.
As early as 1937, V2O5 was explored as a catalyst for the
hydroxylation of unsaturated substrates, including olefins and
arenes, in the presence of tert-butyl hydroperoxide (tBuOOH).1
The material is an effective catalyst for the oxidation of alcohols
to ketones and aldehydes with a variety of co-oxidants including
hydrogen peroxide (H2O2),2 molecular oxygen,3 and tBuOOH.4
V2O5 is also useful for the oxidation of aldehydes5 and acetals6
to the corresponding esters in alcoholic solvents. It has been
used along with ammonium bromide as a halogen source for
several bromination protocols including the bromination of
arenes,7 b-ketoesters and 1,3-diketones,8 as well as the preparation
of organoammonium perbromides.9 Additionally, V2O5 has been
used for several miscellaneous transformations including the
deprotection of dithianes,10 the oxidation of dibenzylthiophene,11
the oxidative coupling of 2-napthols,12 the cyanation of tertiary
amines,13 the aromatization of Hantzsch 1,4-dihydropyridines,14
the deprotection of 1,3-oxathiolanes,15 the hydrolysis of thioglyco-
sides,16 the cyclization of 2-hydroxychalchones to brominated
flavones and aurones,17 and the oxidation of the opium alkaloid
thebaine, to provide 14-hydroxycodeinone.18 Very recently, V2O5
was shown to be marginally effective in the C–H oxidation of
to other more popular metal oxides of osmium, ruthenium,
rhenium, and chromium. Partly, the material has seen less
use compared to other metal oxides owing to its sparing
solubility in most organic solvents, and the limited substrate
scope of most V2O5-mediated processes. Herein we describe the
results of a proof-of-concept study aimed at enhancing V2O5
catalyst performance through catalyst nanostructuring.
When compared to the performance of bulk samples, nano-
fabricated V2O5 samples20 exhibit significant improvement in
desirable properties associated with ion intercalation, electronic,
and magnetic properties.21 They have been employed in a number
of applications including the development of chemical sensors,
field emitters, electrochromic devices, and lithium-ion batteries.22
Despite significant exploration of the effects of nanostructuring
on material performance in the aforementioned areas, relatively
few studies23 have evaluated nanofabrication as a strategy to
improve the catalytic performance of V2O5 in synthetic operations.
Further, a systematic evaluation of more than one synthetic
transformation has not been conducted.
Recently, some of us disclosed a novel method for the
preparation of nanoporous and nanotubular V2O5 samples
(e.g. Fig. 1A) by the electrochemical anodization of vanadium
metal in the presence of complex fluoride electrolytes.24 We
then set out to compare the reactivity of V2O5 samples prepared
in this manner to bulk, commercial V2O5 samples. The details
of this study are described herein.
a Department of Chemistry, Clemson University, Clemson, SC 29634, USA.
E-mail: dwhiteh@clemson.edu; Fax: +1-864-656-6613; Tel: +1-864-656-5765
b Department of Materials Science and Engineering, WW4-LKO, University of
Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany.
E-mail: schmuki@ww.uni-erlangen.de
Results and discussion
c Clemson University Microscopy Laboratory, 91 Technology Dr., Anderson, SC,
Nanoporous V2O5 samples (B20 nm pores) were prepared as
described previously24 (Fig. 1, panel A) and then cleaned from the
vanadium foil substrate by gentle stripping with a knife (see ESI†
for details). We initially sought to evaluate the morphological
29625, USA
† Electronic supplementary information (ESI) available: General experimental
details and procedures. See DOI: 10.1039/c3cy00183k
‡ These authors contributed equally.
c
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Fig. 1 Panel A: SEM of nanoporous V2O5 (20 nm pores); panel B: SEM of
commercial V2O5 sample; panel C: SEM of nanostructured V2O5 sample; panel
D: SEM close-up of nanostructured V2O5 sample.
differences between the nanostructured V2O5 samples and bulk
powder samples for a commercial source. Scanning Electron
Microscopy (SEM) images of the two samples revealed marked
differences in surface morphology. The commercial, bulk V2O5
samples were large particle aggregates ranging from B1 to
5 mm in diameter (Fig. 1, panel B). The surface of the commercial
particle aggregates is strikingly smooth. The large particle aggre-
gate size coupled with the smooth surface present a small surface
area available for catalysis relative to the volume of the particles.
In stark contrast, the nanostructured material was more uniform
in size (particles ranging from 1 to 1.5 mm). Further, the surface of
the particles was decorated with intricate crypts and valleys
(Fig. 1, panel C, panel D for close-up). The nanostructuring
achieved by anodic growth served to significantly increase the
available surface area relative to the volume of the particle. We
hypothesized that the increase in surface area of the fabricated
particles might translate into an observable rate enhancement in
several V2O5-mediated oxidation reactions.
We chose to conduct a comparative study of a series of six
test reactions comprised of known V2O5-mediated oxidation
methodology in order to evaluate the potential for rate accelera-
tion due catalyst nanofabrication. The reactions were chosen to
represent a variety of different transformations employing a
panel of different co-oxidants with V2O5 (Scheme 1). Reactions
included the oxidative deprotection of acetals to give methyl
esters (eqn (1)),6 the TBHP mediated oxidation of alcohols to
ketones (eqn (2)),4 the C–H oxidation of ethyl benzene to
acetophenone (eqn (3)),19 the oxidation of aldehydes to esters
mediated by aq. H2O2 (eqn (4)),5a the air-mediated oxidation of
alcohols to ketones (eqn (5)),3 and the oxidative deprotection of
dithianes (eqn (6)).10
Scheme 1 Test reactions for comparative evaluation of nanostructured V2O5
samples.
case, the GC yields of products were compared when nano-
fabricated V2O5 was employed to control reactions utilizing an
equimolar loading of conventional, bulk V2O5. Reactions were
carried out in triplicate. Average yields and standard deviations
for the six test reactions are collected in Table 1.
In most cases, the nanostructured V2O5 catalysts produced a
small, but reproducible acceleration in rate when compared to
the conventional, commercial V2O5 source. The oxidative
deprotection of acetals (eqn (1)) produced methyl benzoate 1
in yields of 32% with conventional V2O5 and 46% with nano-
structured V2O5 (2 h quench time). When followed to the t1/2 of the
reaction, this difference in yield corresponded to B1.5 times rate
acceleration. The TBHP mediated oxidation of 1-phenylethanol
(eqn (2)) returned acetophenone 2 in yields of 27% and 38% for
conventional and nanostructured V2O5 samples, respectively
Table 1 Reactivity comparison of conv. V2O5 and nano V2O5
Reactiona
Product
Quench time
Conv. V2O5
Nano V2O5
b
b
Eqn (1)
Eqn (2)
Eqn (3)
Eqn (4)
Eqn (5)
Eqn (6)
1
2
2
2 h
17 h
5 d
40 min
1 h
30 min
32 ꢀ 0.2
27 ꢀ 1
46 ꢀ 0.5
38 ꢀ 3
5 ꢀ 0.4
48 ꢀ 1
23 ꢀ 0.1
45 ꢀ 0.3
41 ꢀ 0.6
11 ꢀ 0.8
3
5, 6c
30 ꢀ 0.4
11 ꢀ 2.0
For each test reaction, GC standard curves were generated
for the target product in concentrations ranging from 0.1 to
0.01 M. Tetraglyme (0.05 M) was used as an internal standard.
Reactions were quenched prior to completion by the addition of
4
a
b
See Scheme 1. Average yields of triplicate runs employing tetraglyme
c
as an ISTD. Products 5 and 6 were isolated as a 1 : 1 mixture. Yields
were calculated based on disappearance of starting material. See
a saturated aqueous solution of sodium metabisulfite. In each Scheme 2 and text for more details.
c
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differences when compared to commercially available V2O5
samples. The reactivity of the nanofabricated samples was
compared to that of commercial, bulk V2O5 in a series of six
known transformations. Significant rate acceleration was
observed when nanofabricated V2O5 was used in four of the
six test reactions. Yields at the prescribed quench times were
increased by at least 10 percentage points in these four exam-
ples. This study serves as a valuable proof-of-concept indicating
that catalyst nanostructuring is a relatively untapped parameter
that is available for reaction optimization and tuning. Current
efforts in the laboratory include conducting a thorough evalua-
tion of nanofabricated V2O5 materials of a variety different
nanopore sizes. The fruits of these studies will be reported in
due course.
Scheme 2 V2O5 mediated formation of ethers 5 and 6.
(17 h quench time). After a 5 day quench time, the C–H oxidation
of ethyl benzene resulted in 23% yield of acetophenone with
nanostructured V2O5 (cf. just 5% yield with conventional catalyst
samples). The rate of the oxidative esterification of p-anisalde-
hyde (eqn (4)) did not benefit from catalyst nanostructuring.
In fact, the conventional V2O5 sample slightly out performed
(cf. 48% vs. 45% yield) the fabricated materials at the prescribed
quench time. This result seems to suggest that the rate-determining
step of the reaction may not involve the V2O5 catalyst. Alternatively,
the strongly acidic reaction medium (70% perchloric acid) may
promote the decomposition of the V2O5 bulk materials into soluble
vanadates, thus destroying any nanostructuring present in larger
particles.
The air-mediated benzylic oxidation of 1-phenylethanol
(eqn (5))3 surprisingly did not return any detectable amount
of the desired acetophenone 2. In our hands, we isolated an
inseparable 1 : 1 mixture of the diastereomeric ethers 5 and 6 as
the only detectable GC products (Scheme 2). The isolated ether
products were confirmed by comparison of their 1H NMR
spectrum to data reported previously.25 Despite the formation
of unexpected products, the nanostructured V2O5 samples
conferred a rate acceleration in the formation of 5 and 6 when
compared to conventional samples (cf. 40% yield with nano-
structured material vs. 31% with conventional samples at 1 h
quench time). The meso ether 5 and racemate 6 might arise
from the trapping of a stabilized 21 benzylic carbocation with
the starting 1-phenylethanol 2. In this context, the V2O5 serves
as a Lewis acid catalyst to facilitate the ionization of 2 to form
the benzylic cation intermediate. Prolonged heating of 5 and 6
under the reaction conditions did not result in conversion to
the reported product 2.
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catalyst choice. This result is not surprising given that the
oxidative hydrolysis of the dithiane is likely the rate determin-
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