A nanohybrid self-assembled from exfoliated layered vanadium oxide nanosheets and Keggin Al13 for selective catalytic oxidation of alcohols

Article abstract of DOI:10.1039/c9dt04485j

A nanoscale hybrid material (V₂O₅-Al₁₃) for highly efficient alcohol oxidation is synthesized through electrostatic self-assembly between oppositely charged Keggin Al₁₃ polyoxocations and exfoliated V₂O₅ nanosheets. The analyses by X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy and structural consideration based on charge balance indicate that the Keggin Al₁₃ ions could be sparsely distributed in the nanosheet galleries. The as-prepared catalyst successfully achieved high catalytic activity toward alcohols (96.8% sel.) with the oxygen molecule as an ideal oxidant under mild conditions. Also, the nanohybrid showed an outstanding adsorption capability for benzyl alcohol (773 mg g^-1). In comparison to individual exfoliated V₂O₅ nanosheets and bulk V₂O₅, the V₂O₅-Al₁₃ nanohybrid catalyst exhibited superior catalytic activity and selectivity under the same experimental conditions. The results highlight the outstanding functionality of the V₂O₅-Al₁₃ nanohybrid as an efficient oxidation catalyst. A detailed study of its structure-activity relationship showed that the high performance of the V₂O₅-Al₁₃ nanohybrid is attributed to the adsorption-catalysis synergistic effect between V₂O₅ and Al₁₃

Full text of DOI:10.1039/c9dt04485j

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DOI: 10.1039/C9DT04485J
Nanohybrid Self‐Assembled by Exfoliated Layered Vanadium
Oxide Nanosheets with Keggin Al13 for Selective Catalytic
Oxidation of Alcohols
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
a
Shuang Wang,a Shuying Li, Rui shi, Xinyu Zou, Zhijuan Zhang, Guoyuan Fu, Lei Li, Fang
DOI: 10.1039/x0xx00000x
Luo*, a
ABSTRACT: A nanoscale hybrid material (V2O5‐Al13) for highly efficient alcohol oxidation is synthesized through electrostatic
self‐assembly between oppositely charged Keggin Al13 polyoxocations and exfoliated V2O5 nanosheets. The analysis by X‐
ray diffraction, electron microscopy, X‐ray photoelectron spectrum and structural consideration based on charge balance
indicate that Keggin Al13 ions could be sparsely distributed in the nanosheet galleries. The as‐prepared catalyst successfully
achieved high catalytic activity toward alcohols (96.8% sel.) with oxygen molecule as an ideal oxidant under mild
conditions. Also, the nanohybrid showed an outstanding adsorption capability for benzyl alcohol (773 mg g‐1). In
comparison to individual exfoliated V2O5 nanosheets and bulk V2O5, the V2O5‐Al13 nanohybrid catalyst exhibited superior
catalytic activity and selectivity under the same experiment condition. The result highlights the outstanding functionality
of the V2O5‐Al13 nanohybrid as an efficient oxidation catalyst. A detailed study of its structure‐activity relationship showed
that the high performance of V2O5‐Al13 nanohybrid is attributed to the adsorption‐catalytic synergistic effect between V2O5
and Al13.
However, it is well known that an excellent catalyst not only
has an effective catalytic ability, but also shows a good
adsorption effect on the substrate molecules.29‐31 Therefore, it
1. Introduction
The selective oxidation of alcohols to high‐valued aldehydes
and ketones represents one of the most significant organic
transformation reactions in academic and industrial fields.1‐6
Over the past several years, a great deal of catalysts has been
developed and used for catalytic oxidation of alcohols.7‐12 In
particular, the supported noble metal nanoparticles (such as
Au, Pd and Pt, et.al) are widely recognized to be one of the
most efficient catalysts for liquid‐phase oxidation of
alcohols.13‐16 Although a great number of researches has been
devoted to further improving the intrinsic catalytic activity of
the precious metal catalysts, their high cost and low thermal
stability have become a huge obstruct for their wide
applications.17‐20 Therefore, exploring catalysts that are low
cost, highly active and long‐term stability under oxidizing
conditions is very desirable.
is necessary to construct an adsorption‐catalysis bifunctional
hybrid material based on two‐dimensional vanadium oxide
nanosheets by introducing other functional guest molecules.
Polyoxocations of Keggin‐Al13 (AlO4Al12(OH)24(H2O)127+) formed
by the base hydrolysis of Al3+ aqueous solutions have been
reported to show good adsorption capacity of organic
pollutants.32 Moreover, Keggin‐Al13 polyoxocations have been
widely used as a common pillaring agent for expanding the
interlayer spacing of the host material.33,34 Therefore, it is
feasible to intercalate Keggin‐Al13 into the interlayer of
vanadium oxides nanosheets. Considering that Keggin‐Al13 has
a large amount of positive charges, it is expected to be more
easily combined with negatively charged nanosheets, as a
result, charged layered vanadium oxides nanosheets were
prepared through soft chemical exfoliation.35,36
Among many candidates as catalysts, vanadium oxides seem
to be promising because of its natural abundance, low cost,
excellent redox ability and good thermal stabilities.21‐23
Nevertheless, the real catalytic activity of vanadium oxides is
largely dependent on crystal size, morphology and dimension,
and a large number of researches have been devoted to
synthesizing low‐dimensional nanostructured vanadium
oxides.24‐26 Of particular interest is the two‐dimensional
nanosheets of vanadium oxides prepared by exfoliation.27
Compared with vanadium oxides nanospheres, two‐
dimensional vanadium oxides nanosheets with low
agglomeration possess a larger surface area, and thus more
active sites are exposed to improve catalytic performance.28
On the basis of the above considerations, we successfully
prepared the first example of a porous nanohybrid based on
electrostatic self‐assembly of layered V2O5 and Keggin Al13
(V2O5‐Al13). The introduction of Keggin Al13 could effectively
depress the aggregation of V2O5 nanosheets to expose more
catalytically active sites as well as shorten diffusion path of
organic molecules. The catalytic activity for oxidation of
alcohols with as‐prepared nanohybrid was tested and the
results showed a high selectivity for producing aldehydes or
ketones. In comparison with individual exfoliated V2O5
nanosheets and bulk V2O5, the nanohybrid had a significant
improvement in catalytic activity, which is mainly attributed to
the adsorption‐catalysis synergistic effect of the V2O5‐Al13
nanohybrid.
aKey Laboratory of Polyoxometalate Science of the Ministry of Education, College
of Chemistry, Northeast Normal University, Jilin 130024, P. R. China.
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
2. Experimental
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2.1 Chemicals
The nanohybrid was synthesized through mixing the aqueous
solution of Keggin Al13 and layered V2O5 nanosheets under a
constant stirring for 8 hours at room temperature. The
resulting sample (abbreviated as V2O5‐Al13) was obtained by
filtration, washed with absolute ethanol and then dried at 60
°C for 24 hours. The molar ratio of V2O5 nanosheets to Keggin
Al13 was varied to tune the chemical components on the basis
of charge balance (1:0.16).
All chemicals were of analytical grade and were used as received
without any further purification. V2O5, H2O2 (30%), NaOH, HCl, KCl
and AlCl3∙6H2O were purchased from Beijing Chemical Reagent.
Tetrabutylamine hydroxide (TBAOH) was purchased from Aladdin
Chemical Reagent. The various solvents such as 1,4‐dioxane,
acetonitrile and ethanol were obtained from Thomas Baker. The
molecular oxygen used for catalytic reaction comes from air.
2.3 Catalytic activity test.
2.2 Sample preparation
Catalytic activities of the samples were evaluated by catalyzing
oxidation of alcohols with molecular oxygen. For a typical run,
benzyl alcohol (2.89 mmol), V2O5‐Al13 catalyst (50 mg) and
isobutyraldehyde (0.329 mmol) were added into the reactor
containing acetonitrile (5 mL) under the magnetic stirring at
100 °C for 7 hours. The mixture was identified by GC‐MS, and
the product was analysed by GC with the internal standard
using biphenyl as the internal standard.
2.2.1 Exfoliation of V2O5 nanosheets.
The K0.42V2O5∙0.25H2O nanobelts were synthesized by chemical
pre‐intercalation approach that reported by Ekaterina
Pomerantseva et al.37 Typically, KCl was dissolved in 15 mL of
deionized water and added to 15 mL of H2O2 (30wt%). Next,
V2O5 powder was slowly added to the above solution under
vigorous stirring. The molar ratio of K to V was kept at 10:1 to
ensure excess of potassium in reaction mixture. After vigorous
stirring for 1 h at room temperature, the mixture was then
stirred for a further three hours at 60 °C. A powder with
brown color was produced in solution and the mixture was
aged for four days at room temperature. In the second step,
the obtained aged sol (0.25 g) was dispersed in 15 mL of 3 M
KCl aqueous solution. The mixture was placed in Teflon‐lined
steel autoclave and hydrothermally treated at 220 °C for 24
hours. The obtained K0.42V2O5∙0.25H2O powder was filtered,
washed with the deionized water and dried at 100 °C for 12
hours in air. A protonated form of H0.42V2O5∙0.3H2O was
synthesized by treating the K0.42V2O5∙0.25H2O (~10 g) with HCl
aqueous solution (1 dm3, 1 mol dm‐3) under severely stirring
for 10 days. The resulting sample (0.2 g) was then added into
200 cm3 of TBAOH solution under stirring at room temperature
for more than 2 weeks. The concentration of TBAOH ranged
from 0.00213 mol dm‐3 to 0.532 mol dm‐3. After the
unexfoliated residue was separated by centrifugation at 11000
rpm, the resulting colloidal suspension containing well‐
dispersed exfoliated V2O5 nanosheets was obtained by freeze‐
dried.
2.4. Characterization.
Powder X‐ray diffraction (PXRD) patterns were carried out
using a Rigaku D/MAX‐3 instrument with Cu Kα radiation in the
angular range 2θ=5°‐40° at 298 K. FT‐IR spectra were recorded
on an Alpha Centaurt spectrophotometer in the 400–4000 cm‐
1 range using KBr pellets. Scanning electron microscope (SEM)
images and energy dispersive X‐ray (EDX) analytical data were
obtained by using a Quanta 250 FEG equipped with an EDX
detector. Transmission electron microscope (TEM) images of
V2O5 nanosheets and V2O5‐Al13 nanohybrid were investigated
by on a JEM ‐2100F operating at 200 kV. The HR‐TEM images
of V2O5 nanosheets and V2O5‐Al13 nanohybrid were recorded
on the JEM‐2100F high‐resolution transmission electron
microscope. Surface morphology and elemental distribution of
V2O5‐Al13 nanohybrid were characterized with field emission‐
scanning electron microscopy. Organic components of the
freeze‐dried gel were recorded on an EA3000‐type CHNS/O
Elemental analyzer. Degree of proton exchange was evaluated
through induced coupled plasma, or ICP spectrometry
(LEEMAN‐Prodigy). Thermogravimetric and differential thermal
analyses (TG‐DTA) were performed on a Rigaku TAS 200
thermal analyzer in flowing nitrogen at a heating rate of 10 °C
min‐1. Raman spectra were recorded in backscattering
geometry by exciting a sample on a slide glass with an Ar+ laser
of 488 nm (HR800). X‐ray photoelectron spectroscopy (XPS)
measurements were carried out at a Thermo ESCALAB 250Xray
photoelectron spectrometer. Nitrogen adsorption‐desorption
isotherms were collected by an Autosorb volumetric gas
sorption analyzer (ASAP 2020M) at ‐196 °C. The pore size
distribution was calculated from the desorption branch of the
isotherm based on the Barrett‐Joyner‐Halenda (BJH) method.
Atomic force microscopy images (AFM) were performed using
a Bruker Multimode 8 scanning probe microscope (Bruker Co.
Ltd.).
2.2.2 Synthesis of Keggin Al13 ions.
A solution containing Al13 polyoxocations was prepared by
dropwise addition of NaOH solution (100 cm3, 0.48 mol dm‐3)
to a AlCl3∙6H2O solution (100 cm3, 0.2 mol dm‐3) under stirring
at 70 °C for 2 hours. The obtained solution was aged for
overnight at room temperature.38 The obtained OH/Al ratio of
polyaluminum chloridemole solution (PAC) was 2.4.
Subsequently, 30 ml of PAC solution was poured in a 500 mL
conical flask and 300 mL of 1:5 ethanol‐dioxane mixed
solutions was slowly added under stirring for 2 hours. The
obtained solution was aged for 2‐4 hours and the white Keggin
Al13 powders were produced.
2.2.3 Synthesis of V2O5‐Al13 nanohybrid.
2 | J. Name., 2012, 00, 1‐3
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Scheme 1. Schematic diagram for the synthesis of nanohybrid of Keggin Al13 and 2D vanadate nanosheets.
respectively.41 For H0.42V2O5∙0.3H2O, the FT‐IR spectrum has
obvious adsorption bands at 1630 cm‐1 and 3400 cm‐1, which
are attributed to the bending and stretching vibrations of H3O+
ions, respectively (Figure 1B‐c and S5).42 In addition, the peaks
at 1012 cm‐1 and 748 cm‐1 occurred a significant blue shift
compared to the fresh V2O5, suggesting that there is an
interaction between the interlayer ions and the laminar V2O5
body (Figure 1B‐b). SEM image shows that the as‐prepared
3. Results and discussion
The single‐phase layered vanadate K0.42V2O5∙0.25H2O with high
purity was confirmed via PXRD analysis. As is shown in Figure
1A‐a, PXRD patterns of the sample exhibit characteristic
diffraction peaks at 2θ of 9º and 27º corresponding to the
(001) and (003) planes, respectively, which are very consistent
with the reported data.37 SEM imaging shows the layered
H
0.42V2O5∙0.3H2O is consists of micro‐sheets with a high aspect
ratio (Figure S6).
vanadate K0.42V2O5∙0.25H2O has
a nanobelt morphology
To further prepare layered vanadate nanosheets, the
swelling and exfoliating behaviors of the obtained
H0.42V2O5∙0.3H2O were further investigated by treating with
(Figure S1). To prepare protonated layered vanadate, K+ ions in
the interlayer were extracted by repeating the acid leaching.
According to ICP analysis, the removal degrees of K+ ions were
42.5%, 92.1% and 100% by reaction for 1, 3 and 10 days,
respectively (Figure S2). Mean valence of vanadium in
protonated layered vanadate was determined to be 4.79 by
redox titration (Figure S3), which is the same as a mean
valence of vanadium in K0.42V2O5∙0.25H2O. The water content
was assessed through TGA over the temperature range of 100‐
500 °C and then the weight loss of the sample by calculation is
approximately 2.85% corresponding to 0.3 water molecules in
the protonated layered vanadate (Figure S4). Therefore, the
composition of the protonated layered vanadate can be
formulated as H0.42V2O5∙0.3H2O. Figure 1A‐b shows the PXRD
patterns of the obtained H0.42V2O5∙0.3H2O. According to the
Bragg equation calculation, the interlayer spacing (d001) of as‐
prepared H0.42V2O5∙0.3H2O has enlarged to 11.2 Å compared
with the 9.6 Å interlayer spacing (d001) of K0.42V2O5∙0.25H2O,
confirming the introduction of the larger ions in the interlayer
space. Considering H+ ions are mostly present in the form of
H3O+ in water, as a consequence, the larger ions in interlayer
of H0.42V2O5∙0.3H2O are mainly H3O+.39,40 FT‐IR analysis (Figure
1B) further verified the formation of the protonated vanadate.
Figure 1B‐a represents the characteristic peaks of the fresh
V2O5. Among them, the peaks at 1012 cm‐1, 748 cm‐1 and 522
cm‐1 are ascribed to the stretching vibration of V=O and V‐O‐V,
various
concentrations
of
aqueous
solution
of
tetrabutylammonium hydroxide (TBAOH). The molar ratio of
TBA+ ions to H+ ions (abbreviated as TBA+/H+) in ions exchange
process of H0.42V2O5∙0.3H2O was ranged from 0.1 to 25. Among
them, lots of visible solids were observed in the sample with a
low TBA+/H+ (~0.1) by separation. In contrast, samples with
TBA+/H+
≧ 1 produced stable colloidal suspensions without
noticeable sedimentation, which signifies the appearance of
colloidal suspensions depending on the content of TBAOH. In
order to investigate the degree of exfoliation of the
H0.42V2O5∙0.3H2O under different content of TBAOH in detail,
the obtained colloidal suspension was centrifuged at high
speed and then the upper liquid was freeze‐dried to obtain
exfoliated solid samples. It's worth noting that the volume and
color of samples after treated with TBAOH (abbreviated as
TBA‐V2O5) have obvious changes compared to the protonated
vanadate (Figure S8). The conspicuous increase of volume
from the pristine H0.42V2O5∙0.3H2O to TBA‐V2O5 has been
observed as the proportion of TBA+/H+ increases from 0.1 to 2,
which clearly confirmed that the increment of TBAOH is
beneficial to improve the interior space of the product. But
what disappoints us is that the volume of TBA‐V2O5 sample
slightly decreased as the ratio of TBA+/H+ exceeds 2. This
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phenomenon makes us suspect that the higher concentration 0.1, the PXRD patterns of the sample showed that there are
of TBAOH does not support better exfoliation of V2O5. To test two immiscible phases, that is expanded (12.6 Å) and non‐
the conjecture, typical XRD patterns of these freeze‐drying expanded (11.2 Å), simultaneously appeared. When the value
samples obtained at different ratio of TBA+/H+ were depicted of TBA+/H+ was further increased to 1, there is a sole phase in
in detail (Figure S9). A distinct sharp basal diffraction peak at the freeze‐drying sample. It is worth noting that when TBA+/H+
about 7º was observed as the value of TBA+/H+ exceeds 5, and is equal to 2, a noticeable amorphous‐like drum kit in a 2θ
when the value of TBA+/H+ is equal to 25, the intensity of basal range of 5‐10º was present (Figure 1A‐c), which is ascribed to
diffraction peak reaches the maximum. The Bragg peak scattering from the agglomeration of exfoliated V2O5 sheets.
corresponding to gallery height of the sample is 11.85 Å, which On the basis of the above discussion, an important conclusion
is only slightly larger than the origin value of 11.2 Å for the was educed, that is, a 2‐fold excess to an equivalent amount of
H
0.42V2O5∙0.3H2O. Based on the previous study, we inferred TBA ions (abbreviated as TBA‐V2O5‐2) to the exchangeable
that this expansion of the gallery height maybe due to the protons in H0.42V2O5∙0.3H2O is the most optimized value for
uptake of water into interlayer space and the pattern obtained achieving complete delamination. The preparation process of
above is ascribed to osmotic swelling.43 In addition, a series of the above material is illustrated in Scheme 1.
samples with TBA+/H+ <5 were also analyzed. For TBA+/H+ =
Figure 1. (A) Powder X‐ray diffraction patterns of K0.42V2O5∙0.25H2O, H0.42V2O5∙0.3H2O and TBA‐V2O5‐2 nanosheets, respectively. (B) FT‐IR measurements of
V2O5, K0.42V2O5∙0.25H2O, H0.42V2O5∙0.3H2O and TBA‐V2O5‐2 nanosheets, respectively. (C) Raman spectra of H0.42V2O5∙0.3H2O and TBA‐V2O5‐2 nanosheets,
respectively. (D) SEM images of V2O5 nanosheets. (E) Edge curls view of the TBA‐V2O5‐2 nanosheets. (F) TEM images of TBA‐V2O5‐2 nanosheets. (G) Tyndall
effect of TBA‐V2O5‐2 nanosheets. (H) HR‐TEM images of TBA‐V2O5‐2 nanosheets. (I) Zeta potential curve of the colloidal suspension of exfoliated V2O5
nanosheets. (J) AFM image and corresponding height profiles of TBA‐V2O5‐2 nanosheets along the red line.
4 | J. Name., 2012, 00, 1‐3
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Figure 2. (A) Raman spectra measurements of Al13 salt, TBA‐V2O5‐2 nanosheets and V2O5‐Al13 nanohybrid, respectively. (B) Powder X‐ray diffraction patterns of Al13 salt,
TBA‐V2O5‐2 nanosheets and V2O5‐Al13 nanohybrid, respectively. (C) XPS full survey of V2O5‐Al13 nanohybrid. (D) SEM images of V2O5‐Al13 nanohybrid. (E) N2 adsorption–
desorption isotherms V2O5‐Al13 nanohybrid. (F) HR‐TEM images of a flaky aggregate. (G) Cross‐sectional HR‐TEM images for the as‐prepared V2O5‐Al13 nanohybrid (H)
elemental mapping analysis of V2O5‐Al13 nanohybrid .
The chemical composition of the TBA‐V2O5‐2 sample was exhibited micron‐thick layered structure; when the value of
further determined to be (C4H9)4N)0.67H0.42V2O5∙0.3H2O TBA+/H+ reached 25, a large number of stacked layers were
through element analysis. Vibrational spectra in FT‐IR (Figure obtained, demonstrating that high dose of TBA+ ions would
1B‐d) also confirmed that the existence of TBA+ ions and H2O induce the production of osmotically swollen phase (Figure
species simultaneously present in TBA‐V2O5‐2 sample. FT‐IR S10). TEM images of the TBA‐V2O5‐2 clearly showed exfoliated
peaks corresponding to 3336 and 1627 cm‐1 are attributed to ultrathin nanosheets with curls or wrinkles (Figure 1F). The
the stretching and bending vibration for H2O, respectively. dispersion of TBA‐V2O5‐2 nanosheets was further confirmed by
Sharp adsorptions in the range of 1000‐1500 cm‐1 and at 3000 the light scattering experiment and a clear Tyndall effect was
cm‐1 belong to the vibration of TBA+ ions.43 Raman spectrum also observed clearly in upper bright yellow solution obtained
(Figure 1C) shows the superposition of bands from TBA+ and by centrifugation (Figure 1G). Furthermore, close examination
the V2O5 framework.44 SEM picture shows that the TBA‐V2O5‐2 of the HR‐TEM revealed a lattice spacing of 0.28 nm
sample possesses thin enough and flexible nanosheets (Figure corresponding to the (301) plane of orthorhombic V2O5, which
1D and 1E). However, when the value of TBA+/H+ is less than 2, proved that the ultrathin TBA‐V2O5‐2 nanosheets still
samples consist of a large number of agglomerated particles; maintained high crystallinity (Figure 1H).45 In addition, to deep
and when the value of TBA+/H+ exceeds 5, these samples
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understand the oxidation state of the vanadium in the TBA‐ seriously restacking phenomenon between exfoliated
V2O5‐2 nanosheets,
nanosheets.
Table 1. Catalytic Performance of Different Catalysts for the Oxidation of Benzyl Alcohol
to Benzaldehydea
Based on the results shown above, TBA‐V2O5‐2 nanosheets
were selected as the most suitable matrix to combine with
Keggin Al13 polyoxocations. Among them, the V2O5‐Al13
nanohybrid was synthesized based on the charge balance ratio
of V2O5 nanosheets/ Keggin Al13 polyoxocations (V2O5
nanosheets: Al13 = 1:0.16). FT‐IR spectra (Figure S14) of V2O5‐
Al13 nanohybrid display the characteristic peaks at 1625, 967,
818, 731, 546, and 457 cm‐1. Among them, the characteristic
peaks of Keggin Al13 polyoxocations in nanohybrid changed
significantly from 987 cm‐1, 1643 cm‐1 to 967 cm‐1 and 1625
cm‐1, respectively. Meanwhile, the characteristic asymmetric
vibration of V‐O‐V shifts from 843 cm‐1 in V2O5 nanosheets to
818 cm‐1 for V2O5‐Al13, suggesting that a strong electrostatic
interaction was presented between the exfoliated V2O5
nanosheets layers and the guest Al13 polyoxocations. As is
shown in Figure 2A, Raman detection of the high purity Al13
salts shows three bands, that is a sharp one at 300 cm‐1, a
broader one at 635 cm‐1, and a smaller one at 987 cm‐1,
respectively. The above bands can be accordingly assigned to
the vibrational modes of Al‐OXs or the hydroxo group.47‐49 The
Raman spectrum of V2O5‐Al13 nanohybrid simultaneously
shows characteristic signal peaks of Al13 polyoxocations and
V2O5 nanosheets, which confirmed the successful combination
of the two materials. It is a remarkable fact that these Raman
peaks are not too strong, which may be due to their high
signal‐to‐noise ratios. Figure 2B presents the PXRD patterns of
the as‐prepared V2O5‐Al13 nanohybrid. In comparison with that
of the TBA‐V2O5‐2 nanosheets, a broad diffraction envelope at
20° disappeared in V2O5‐Al13 nanohybrid, which can be
Time Conversion Selectivity
Catalysts
Yield (%)
(h)
(%)
81.9
5.4
(%)
96.8
0
V2O5‐Al13
Al13
7
79.3
0
7
Bulk V2O5
7
20.6
63
12.9
V2O5
nanosheets
7
32.9
67.5
22.2
VO(acac)2
VOCl3
No
15
16
12
18.5
24.9
‐
54.1
69.2
‐
10
17.2
‐
aReaction conditions: 2.89 mmol of benzyl alcohol. 50 mg of catalysts, 5 mL of attributed to the combination of V2O5 nanosheets with Al13
acetonitrile, reaction temperature 100 °C, 1 atm O2, 0.329 mmol of isobutyraldehyde.
Conversion and yield were calculated on the basis of the formed product and
consumed reactant as measured by GC‐MS. Biphenyl was used as the internal standard
polyoxocations. Furthermore, two diffraction peaks at 6.58°
and 9.12° were observed in V2O5‐Al13 nanohybrid. In light of
the Bragg equation calculation, the former possesses an
expanded basal spacing of 1.34~1.51 nm. Considering that the
XPS measurements were carried out and the spectra of V2p
crystallographic thickness of V2O5 nanosheet is 0.42 nm50 and
are shown in Figure S11. The peaks of both V2p3/2 and V2p1/2
the diameter of Keggin Al13 is 0.86 nm,51 thus the repeating
distance of V2O5‐Al13 should be 1.28 nm, which is lower than
the experimental value. This stupendous difference
demonstrated that the layer‐by‐layer restacking structure was
were identified at 517.3, 516.1 eV and 524.5, 522.7 eV,
corresponding to V4+ (516.1 and 522.7 eV) and V5+ (517.3 and
524.5 eV), respectively, confirming the presence of mixed‐
valence vanadium.46 Meanwhile it is notable that the content
present in V2O5‐Al13 nanohybrid. Since the V2O5 nanosheet has
ratio of V4+ peak and V5+ peak is about 1:2.5 by calculating the
a negative charge of 1.09 to each chemical formula and one
peak areas of V4+ and V5+. It was because of the existence of
Al13 polyoxocation has a high positive charge of 7+, in other
V4+ that V2O5 nanosheets were negatively charged and the zeta
words, 7 unit cells of V2O5 nanosheets are required to balance
potential measurement (Figure 1I) also demonstrates the
the positive charge of one Al13 ion, Al13 ions were distributed
electronegativity of the exfoliated V2O5 nanosheets. In order to
sparsely in V2O5 nanosheets through electrostatic interactions.
further explore the synthesis of ultrathin nanosheets and
understand their thickness, TBA‐V2O5‐2 nanosheets were
examined by AFM and the results revealed that the thickness
of most of the TBA‐V2O5‐2 nanosheets were approximately 3‐5
nm (Figure 1J and Figure S12). The surface areas and pore
structure of TBA‐V2O5‐2 nanosheets are examined with N2
adsorption‐desorption isotherm measurements. As is shown in
Figure S13, the as‐prepared V2O5 nanosheets possess smaller
Of more importance, sparsely distributed Al13 polyoxocations
could act as the inorganic linker to stabilize the multilayer
assembly according to previous report,33 therefore we inferred
that the stacked V2O5 nanosheets can be opened through Al13
polyoxocations, which effectively increases the stacking space
of the sheets and produces corresponding holes. From the
second diffraction peak of 9.12°, a layer spacing can be
calculated about 9.68 Å, which is lower than the sum of the
BET surface area of 36 m2 g‐1, which may be due to the
thickness of two components, suggesting that Al13
polyoxocations and V2O5 nanosheets are assembled in other
6 | J. Name., 2012, 00, 1‐3
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Table 2．
Selective Oxidation of Various Alcohols with V2O5‐Al13 nanohybrida
Conversion
Selectivity
(%)
Entry
1
Substrate
Product
(%)
Yield (%)
72.5
72.5
81.9
71.5
72.4
100
96.8
90.0
90.5
2
3
4
79.3
64.5
65.5
5
6
7
62.6
63.5
61.2
100
100
100
62.6
63.5
61.2
8
62.4
96
59.9
aReaction conditions: 2.89 mmol of alcohols. 50 mg of catalysts, 5 mL of acetonitrile, reaction temperature 100 °C, reaction time 7 h, 1 atm O2, 0.329 mmol of isobutyraldehyde.
Conversion and yield were calculated on the basis of the formed product and consumed reactant as measured by gas chromatography. Biphenyl was used as the internal standard
forms other than the layer‐by‐layer mode. The reason for this an average diameter of 56 Å (Figure S15). TEM (Figure 2F)
behavior may be due to the relative lower charge of V2O5 confirms that the as‐prepared V2O5‐Al13 nanohybrid still
nanosheets cannot accommodate Keggin Al13 polyoxocations maintain the sheet morphology. Moreover, the formation of
in high packing density so that Keggin Al13 polyoxocations V2O5‐Al13 nanohybrid structure is approved through the cross‐
would not be completely inserted into the layer of V2O5 sectional HR‐TEM analysis. As illustrated in Figure 2G, a cross‐
nanosheets.52 XPS analysis was performed to further validate sectional view of the as‐prepared V2O5‐Al13 nanohybrid shows
the elemental composition in V2O5‐Al13 nanohybrid. As is an assembly of well‐developed parallel dark stripes, which is
shown in Figure 2C, the O 1s, V 2p and Al 2p signals are attributed to layered V2O5 lattice. To further verify the
observed from the full survey. SEM images of the V2O5‐Al13 hybridization between V2O5 nanosheets and Keggin Al13, the
nanohybrid show that there are porous structures by slice spatial distributions of oxygen, vanadium and aluminum
stacking in the as‐prepared nanohybrid (Figure 2D). N2 elements are evidenced with EDX (Figure S16) and elemental
sorption isotherms (Figure 2E) of synthesized V2O5‐Al13 mapping analysis. As is plotted in Figure 2H, all elements of
nanohybrid exhibit weak adsorption of N2 molecules in a low V2O5‐Al13 nanohybrid are well distributed in nanohybrid,
relative pressure region and a distinct hysteresis in the region implying that two components are homogeneous mixed.
of p/p0 > 0.5 reveals that most of porosity in the present
The catalytic performance of as‐prepared V2O5‐Al13
nanohybrid originates from mesoporous. Meanwhile, pore size nanohybrid has been examined with respect to the selective
analysis showed that the as‐prepared V2O5‐Al13 nanohybrid has oxidation of alcohols using molecule oxygen as a clean oxidant
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Figure 3．(A) Time profiles of catalytic oxidation of benzyl alcohol using different catalysts. (B) FT‐IR spectra of single Al13 and benzyl alcohol‐adsorbed on Al13. (C) The amount of
benzyl alcohol adsorbed by V2O5‐Al13, single solid Al13 and pure V2O5 nanosheets.
in presence of isobutyraldehyde. Among them, benzyl alcohol confirming the notable positive effect of hybridization on the
as a model substrate was catalyzed in a series of solvents. catalytic activity (Figure 3A). When the reaction time was
According to a large number of literature reports, acetonitrile, further extended to 12 h, the change of catalytic reaction rate
toluene, THF and ethanol are the most commonly organic based on V2O5 analogs is not obvious. The benzyl alcohol
solvents used to selective catalytic oxidation of alcohols. The conversion profile between 7 and 12 hours of V2O5‐based
experimental results showed that benzyl alcohol exhibited the analogs are shown in Figure S18. It can be seen that there is
best catalytic oxidation activity in acetonitrile (Table S1). This is very little benzyl alcohol conversion between 7 and 12 hours.
because that acetonitrile is a polar solvent with a very high Therefore, the catalytic performance study of V2O5‐based
dielectric constant and it may readily dissolve analogs for the first 7 hours was more meaningful.
isobutyraldehyde along with the benzyl alcohol and increase
To understand the role of each component in V2O5‐Al13
the efficiency of the catalytic system. Also, highly polar nanohybrid, the catalytic performance of single Al13 was in
solvents like acetonitrile may facilitate formation of active detail explored. Table 1 showed that Keggin Al13 has a
oxygen species and thereby enhance the catalytic activity. The negligible catalytic activity, which suggested that the true
blank test was performed in the absence of catalysts, during catalytic active centers are V2O5 nanosheets. But it is now
which no product was observed. Meanwhile, the catalytic apparent that the amount of benzyl alcohol has a certain
experience in the absence of isobutyraldehyde was also decrease. We suspected that the phenomenon may be due to
explored and the result showed that not any product was the fact that benzyl alcohol was adsorbed on Keggin Al13 rather
detected, which manifested that isobutyraldehyde plays an than being catalyzed. To confirm the above speculation, the
essential role in the catalytic oxidation reaction. To clearly solid Al13 after reaction was recovered and characterized by
clarify the oxidant role of molecule oxygen, the N2 and Ar‐ FT‐IR spectrum. As is shown in Figure 3B, a significant
purged the oxidation of benzyl alcohol was used as control absorption peak around 1010 cm‐1 on the surface of recovered
experiments and the results displayed that benzyl alcohol did Keggin Al13 was observed, which is attributed to C‐O vibrations.
not undergo any conversion (Figure S17). Therefore, we Meanwhile, the Ar‐H and O‐H stretching vibrations were
speculated that O2 is actually a key substance for initiating the appeared in 700, 750 and 3300 cm‐1, respectively. The above
occurrence of oxidation. Table 1 lists the catalytic oxidation of analysis stated that there is an interaction between Keggin Al13
benzyl alcohol by V2O5‐Al13 and a series of related vanadium‐ and the benzyl alcohol molecule. Moreover, compared to that
based analogues. V2O5‐Al13 exhibits the best catalytic oxidation of Keggin Al13 before the reaction, the characteristic peaks of
performance with 81.9% conversion and 96.8% selectivity. the recovered Keggin Al13 significantly occurred to red‐shift.
Under identical experimental conditions, the catalytic activities This phenomenon also indicated the presence of
of bulk V2O5 and the exfoliated V2O5 nanosheets are estimated chemisorption between Keggin Al13 and benzyl alcohol. In
and the result showed that the exfoliated V2O5 nanosheets order to more intuitively clarify the adsorption of benzyl
have a higher catalytic performance than that of bulk V2O5, alcohol by Keggin Al13, the adsorption amount of benzyl
indirectly signifying that the exfoliated V2O5 nanosheets can alcohol by different catalysts was detected (Figure 3C). Among
expose more catalytically active sites. In addition, the catalytic them, the adsorption capacity of benzyl alcohol by Keggin Al13
performance of the exfoliated V2O5 nanosheets is still lower reached 353 mg/g. In contrast, that by V2O5 nanosheets is
than that of V2O5‐Al13 nanohybrid, which explained that the about 121 mg/g, which is only one third of that by single Al13.
hybridization of V2O5 nanosheets and Keggin Al13 is beneficial What's even more surprising is that the adsorption value for
for improving the catalytic performance. Furthermore, the benzyl alcohol by V2O5‐Al13 nanohybrid is up to 773 mg/g. In
kinetic behavior analyses of different catalysts were also comparison to the single component of nanohybrid, the high
investigated in detail. The results illustrated that the catalytic adsorption ability of V2O5‐Al13 nanohybrid is not only related to
reaction rate of the V2O5‐Al13 is obviously quicker than that of the adsorption performance of Keggin Al13, but also attributed
bulk V2O5 and exfoliated V2O5 nanosheets in the first 7 hours, to the porous structure of the nanohybrid. On the basis of
8 | J. Name., 2012, 00, 1‐3
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Figure 4． (A) Benzyl alcohol conversion over V2O5‐Al13 nanohybrid in the presence of the different trapping species. (B) Recyclability of V2O5‐Al13 nanohybrid in aerobic
oxidation of alcohols. (C) PXRD patterns of V2O5‐Al13 nanohybrid after recycling 5 times.
above analysis, we inferred that the high catalytic performance species in the present catalytic system, at the same time the
is attributed to synergistic effect of adsorption‐catalysis GC test results showed the presence of isobutyricacid.
between the exfoliated V2O5 nanosheets and Keggin Al13.
Combining previously reported findings, we speculated the
Owing to V2O5‐Al13 nanohybrid exhibiting the best catalytic carbon‐centered radical was the acyl free radical. Since oxygen
activity among these catalysts, therefore we used it as the molecule alone has not ability to oxidize alcohols, therefore we
model catalyst to further evaluate the catalytic oxidation inferred that there may be other reactive oxygen species to
performance through extending the substrate scope. As is oxidize alcohols in the system. To confirm this point of view,
shown in Table 2, V2O5‐Al13 nanohybrid can successfully induce Ph2NH as an oxygen‐radical scavenger was added to catalytic
a wide range of examples of selective alcohols oxidation. system and an obvious inhibitory effect was observed,
Among them, the yield of aliphatic alcohols exceeds 60%, suggesting that oxygen active radicals are generated in this
significantly higher than those reported in the literature, system.62 In order to more clearly validate which oxygen active
where yields of < 44% were only able to be achieved for the radicals actually exist in the system, we add 1,4‐benzoquinone
selective oxidation of 1‐heptanol using a V2O5 catalyst in the (PBQ) and 2,6‐di‐tert‐butyl‐4‐methylphenol (BHT) to the
presence of base at 100 °C.53 Using the as‐prepared catalytic system to specifically inhibit the generation of
nanohybrid described here, the conversion rates of aromatic superoxide free radical and peroxyl free radical. As is shown in
alcohols are excellent and the selectivity is up to 100% for the Figure 4A, only BHT could obviously suppress the oxidation of
desired products. Such selectivity and efficacy are far superior alcohols, which revealed that peroxyl free radical is the main
to those reported previously in the literature using other V2O5‐ oxygen active species. Summing up the previous reports, the
based catalysts (Table S2). 54‐58
generation of peroxyl free radical is inferred from the
To in‐depth explore the reason for high selectivity of the combination of acyl free radical with molecular oxygen. The
V2O5‐Al13 nanohybrid for oxidation of alcohols, the catalytic generated peroxyl free radical typically combine with
mechanism was investigated in detail. According to the hydrogen ions to form isobutyric acid for oxidizing lots of
previous reports, the oxidation pathway of alcohols generally organic substrates, including cycloolefins, alcohols and sulfides.
63, 64
was divided into two types, one is free radical path and
another is non‐radical path.59 In order to test possible reaction
In view of the above exploration results, we have proposed
pathway of this system, hydroquinone as an efficient radical a possible reaction pathway for oxidation of alcohols over the
scavenger was added to the catalytic system. The results V2O5‐Al13 nanohybrid catalyst. Here, it can undergo redox
showed that not any product peaks were detected by GC, thus reaction with VV centers in the nanohybrid catalyst to generate
we deduced that the catalytic reaction followed the free a large amount of acyl radicals, at the same time VV centers
radical reaction path. Subsequently, to further define possible would be reduced to VIV centers. Subsequently, the produced
free‐radicals produced in this system, we tried to add a series acyl radicals react with molecular oxygen to afford the peroxy
of reasonable free‐radical scavengers to the reaction solution free radical. The unstable peroxy free radical could quickly
under the standard conditions. Among them, considering that abstract hydrogen from the aldehyde to give peroyacid. The
isobutyraldehyde as a common sacrificial reagent usually could obtained peroxyacid could oxidize adsorbed alcohol molecules
react with high valent metal centers to generate acyl radicals, on the one hand, and on the other hand they would react with
which can act as a radical initiator to promote rapid process of VIV centers to give high‐valent vanadium centers so as to keep
reaction.60 Hence
tetramethylpiperidine‐1‐oxyl (TEMPO) as an efficient carbon
we
choose
to
add
2,2,6,6‐ the catalyst structure unchanged.
In the final set of experiments, the reusability of the V2O5‐
radical scavenger to the catalytic system (Figure 4A).61 The Al13 catalyst was investigated because it is important to signify
experimental results showed that the reaction basically has no that the highly active catalyst is stable under oxidizing
conversions after 11
h
of reaction, which strongly conditions. After five catalytic cycles, the nanohybrid was
demonstrated the involvement of carbon‐centered radical recycled from the reaction mixture by centrifugation. Catalytic
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7
F. Ferretti, A. P. C. Ribeiro, E. Alegria, A. M. Ferraria, M. N.
Kopylovich, M. F. C. Guedes da Silva, F. Marchetti and A. J. L.
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results revealed that no clear loss of activity was observed in
five runs (Figure 4B). PXRD characterization showed that the
structure of catalyst was mostly retained (Figure 4C). ICP
measurements indicated that only traces of metal (< 1% of the
total vanadium or aluminum) were detected. SEM image of
V2O5‐Al13 nanohybrid after five catalytic runs was shown in
Figure S19, which revealed a certain degree of aggregation
through drying. The HR‐TEM image also indicated that the
hybrid material after catalysis still showed the shape of the
sheet stack (Figure S20). The above results commonly
confirmed that the V2O5‐Al13 nanohybrid was a stable and
highly efficient catalyst for aerobic oxidation of alcohols.
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Conclusion
In summary, a porous V2O5‐Al13 nanohybrid was for the first
time synthesized through electrostatic self‐assembly between
positively charged Keggin Al13 and negatively charged
exfoliated V2O5 nanosheets for selective catalytic oxidation of
alcohols under oxygen atmosphere. The experiment results
exhibited that the V2O5‐Al13 nanohybrid has a marked
superiority (86.2% con.) over catalytic activity than bulk V2O5
and exfoliated V2O5 nanosheets, which is attributed to a
synergistic coupling of adsorption‐catalysis between the V2O5
nanosheets and Keggin Al13. Moreover, the V2O5‐Al13
naohybrid maintained a high conversion and selectivity during
five recycles and the recycled catalyst still has a high stability.
The combination of highly catalytic performance and high
stability enables the V2O5‐Al13 nanohybrid as an excellent
catalyst for replacing previously expensive metals‐based
catalyst.
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Conflicts of interest
There are no conflicts to declare.
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Table of Contents
A porous V2O5‐Al13 nanohybrid based on self‐assembly of Keggin Al13 and exfoliated V2O5 nanosheets for selective oxidation of alcohols.
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