Visible-Light-Activated C?C Bond Cleavage and Aerobic Oxidation of Benzyl Alcohols Employing BiMXO5 (M=Mg, Cd, Ni, Co, Pb, Ca and X=V, P)

Article abstract of DOI:10.1002/asia.202000814

The synthesis, structure, optical and photocatalytic studies of a family of compounds with the general formula, BiMXO₅; M=Mg, Cd, Ni, Co, Pb, Ca and X=V, P is presented. The compounds were prepared by regular solid-state reaction of constituents in the temperature range of 720–810 °C for 24 h. The compounds were characterized by powder X-ray diffraction (PXRD) methods. The Rietveld refinement of the PXRD patterns have been carried out to establish the structure. The optical absorption spectra along with the colors in daylight have been explained employing the allowed d-d transition. In addition, the observed colors of some of the V⁵⁺ containing compounds were explained using metal-to-metal charge transfer (MMCT) from the partially filled transition-metal 3d orbitals to the empty 3d orbitals of V⁵⁺ ions. The near IR (NIR) reflectivity studies indicate that many compounds exhibit good NIR reflectivity, suggesting that these compounds can be employed as ‘cool pigments’. The experimentally determined band gaps of the prepared compounds were found to be suitable to exploit them for visible light activated photocatalysis. Photocatalytic C?C bond cleavage of alkenes and aerobic oxidation of alcohols were investigated employing visible light, which gave good yields and selectivity. The present study clearly demonstrated the versatility of the Paganoite family of compounds (BiMXO₅) towards new colored inorganic materials, visible-light photocatalysts and ‘cool pigments’.

Full text of DOI:10.1002/asia.202000814

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Visible Light Activated C–C Bond Cleavage and Aerobic
Oxidation of Benzyl Alcohols Employing BiMXO5 (M = Mg, Cd,
Ni, Co, Pb, Ca and X = V, P)
Authors: Anupam Bhim, Shreya Sasmal, Jagannatha Gopalakrishnan,
and Srinivasan Natarajan
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10.1002/asia.202000814
Chemistry - An Asian Journal
FULL PAPER
Visible Light Activated C–C Bond Cleavage and Aerobic
Oxidation of Benzyl Alcohols Employing BiMXO5 (M = Mg, Cd, Ni,
Co, Pb, Ca and X = V, P)
Anupam Bhim, Shreya Sasmal, Jagannatha Gopalakrishnan and Srinivasan Natarajan*
Abstract: Synthesis, structure, optical and photocatalytic studies of to the empty 3d orbitals of V5+ ions. The near IR (NIR) reflectivity
a family of compounds with the general formula, BiMXO5; M = Mg, studies indicate that many compounds exhibit good NIR reflectivity,
Cd, Ni, Co, Pb, Ca and X = V, P is presented. The compounds were suggesting that these compounds can be employed as ‘cool
prepared by regular solid-state reaction of constituents in the pigments’. The experimentally determined band gaps of the
temperature range of 720–810 °C for 24 h. The compounds were prepared compounds were found to be suitable to exploit them for
characterized by powder X-ray diffraction (PXRD) methods. The visible light activated photocatalysis. Photocatalytic C−C bond
Rietveld refinement of the PXRD patterns have been carried out to cleavage of alkenes and aerobic oxidation of alcohols were
establish the structure. The optical absorption spectra along with the investigated employing the visible light, which gave good yields and
colors in daylight have been explained employing the allowed d-d selectivity. The present study clearly demonstrated the versatility of
transition. In addition, the observed colors of some of the V5+ the Paganoite family of compounds (BiMXO5) towards new colored
containing compounds were explained using metal-to-metal charge inorganic materials, visible light photocatalysts and ‘cool pigments’.
transfer (MMCT) from the partially filled transition-metal 3d orbitals
the electron-hole recombination exhibiting good selectivity for
the products. One of the well-known and well investigated
photocatalyst has been TiO2. TiO2 has been studied as a
Introduction
Solar energy driven organic reactions, preferably under
photocatalyst for the UV light assisted oxidation of benzyl
ambient conditions, is a desirable route and has gained
alcohol to benzaldehyde in the presence of oxygen.[1,10] TiO2 has
considerable interest over the years.[1] One of the important
wide band gap in the range of 3.0-3.2 eV, depending on the
factors is to explore stable catalysts that are low cost with high
polymorph, and is active in the presence of UV light. The UV-
catalytic efficiency for the photocatalytic organic reactions. In
light activated photocatalysis, normally, over oxidizes the alcohol
general, the ruthenium and iridium based organometallic
to form acids, which may not be the favoured product.[11]
complexes have been investigated for cycloaddition, carbon-
The desirable quality in a photocatalyst is to have good
carbon and carbon-heteroatom bond formation reactions.[2] The
photocatalytic activity under visible light with good selectivity. As
use of noble metal based photocatalyst has been an impediment
mentioned above, ruthenium complexes have been anchored
for their sustained use.[3] There have been considerable efforts
over many supports for the selective oxidation of benzyl
towards developing new heterogeneous photocatalysts that
alcohol.[12] The main drawback in this reaction is the use of
would facilitate organic transformation,[4] organic pollutant
sacrificial electron donors. Many compounds that exhibit good
degradation,[5] water splitting[6] reactions employing compounds
band gap that absorbs in the visible region such as graphitic
based on oxides. Though this area offers many opportunities to
carbon nitride g-C3N4; (Eg = 2.7 eV), Bi2WO6 (Eg=2.8 eV)
explore newer photocatalysts, the re-usability, the selectivity and
converts benzyl alcohol to benzaldehyde selectively but exhibit
catalysts that are derived from earth abundant elements, which
very low conversion of 9 and 30%, respectively.[13]
can operate under visible light, are some of the challenges.
To overcome the electron-hole recombination during a
Aldehydes are an important intermediate as many find use
photocatalytic reaction, there have been attempts to utilize
in dyes, perfumes and in pharmaceuticals.[7] One of the simpler
composite materials containing semiconductors. Thus, the
ways to produce aldehyde would be to oxidize alcohols. Many a
following composite materials have been explored: (i) Bismuth
time the oxidation agents, (chromate, permanganate,
tungstate with rGO,[14] (ii) tungsten oxide loaded with palladium
hypochlorite etc.) employed for these reactions, are toxic and
oxide,[15] (iii) CdS@SnO2 nanorods,[16] (iv) CdS-MOFs,[17] and (v)
corrosive in nature.[8] There have been reports of the use of
CdS-graphene nanocomposites.[18] All these compounds exhibit
noble metal complexes with oxygen for this reaction as well.[9]
good selectivity and photocatalytic activity for benzyl alcohol
One of the convenient route to oxidise the alcohol is to employ
oxidation.
light source (visible light) in the presence of heterogeneous
One of the well-studied visible light photocatalyst is BiVO4
photocatalysts. The main challenges in the use of
(Eg = 2.4 eV), which has been explored for a number of
heterogeneous photocatalysts is to prepare a compound that is
reactions; (i) photoelectrochemical water oxidation,[19] (ii) C-C
an active photocatalyst under visible light along with reducing
bond cleavage of alkenes generating carbonyls,[20] and (iii)
selective photo-oxidation of benzyl amines to their
corresponding imines.[21]
[a]
Anupam Bhim, Shreya Sasmal, Jagannatha Gopalakrishnan and
Srinivasan Natarajan*
Framework Solids Laboratory,
Solid State and Structural Chemistry Unit
Indian Institute of Science, Bangalore - 560012 (India)
E-mail: snatarajan@iisc.ac.in
We have been investigating mixed oxides with a veiw to
develop new inorganic pigments. During the course of these
studies, a family of compounds related to the mineral, Paganoite,
BiNiAsO5, was found to be promising. The compounds with the
general formula, BiMXO5 (M = Mg, Ca, Cd, Co, Ni, Mn; Pb and X
= V, P and As) have been examined for their luminescence,
magnetic, microwave dielectric and optical properties.[22] Among
these compounds, bismuth cadmium vanadate BiCdVO5
Homepage: http://sscu.iisc.ac.in/people/frameworkslab/
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/asia.202000814
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(Eg=2.45 eV) appears to be promising due to its good chemical
stability and accessible band gap.[22a,23] BiCdVO5 was first
reported by Sleight and co-workers[23] and exhibit a unique
crystal structure (space group Pnc21). It occurred to us that the
Paganoite family of compounds can be explored for many
properties. It was recently shown that the empty ‘d’ orbitals can
receive electrons from partially filled ‘d’ orbitals through a metal-
metal charge transfer (MMCT).[24] The paganoite compounds
containing vanadium (V5+, d0 state) could exhibit such MMCT
transitions in addition to the usual optical transitions (d-d
transitions) associated with distorted octahedral coordination. In
this work, we have explored the paganoite family of compounds
for their optical and photocatalytic behaviour. We have also
carried out band structure calculations to learn more about the
role of different atomic orbitals near the fermi level. The results
of this study are presented in this manuscript.
BiMg0.5Co0.5VO5 is shown in Figure 3. The crystal structure of
BiMg0.5Co0.5VO5 has
a three‐dimensional network of two
edge‐sharing Mg/CoO6 octahedra that are corner linked by VO4
tetrahedra, forming large tunnels along the c‐axis. The Bi3+
occupies the tunnels and has a distorted BiO6 octahedra (Figure
S4).
Results and Discussion
As part of the present study, four distinct variants of the
Paganoite structure were examined, namely, (1) BiMXO5; M =
Mg, Ni, Co and X = V, P (space group P21/n), (2) BiMXO5; M =
Pb, Mn, Ni, Co and X = V, P, As (space group P-1), (3) BiCdVO5
(space group Pnc21) and BiCaVO5 (space group Pbca) (Table
S1). All, the compounds have centrosymmetric space group
except BiCdVO5, which has a non-centrosymmetric space group.
The Paganoite family of compounds, BiMXO5, has three
characteristic cation positions with varying coordination
geometries. Thus, Bi3+ is eight coordinated and forms a
dodecahedral geometry; the M-site has six coordination and
forms distorted octahedral geometry; the X sites have four
coordination and forms tetrahedral geometry.
Figure 2. Rietveld refinement of BiMg0.5Co0.5VO5 from the PXRD data. The
observed (O), calculated (red line), and difference (bottom blue line) profiles
are shown. The vertical bars (|) indicate Bragg reflections.
Table 1. Crystallographic data for BiMg0.5Co0.5VO5.
Atom
Site
x
y
z
Uiso(Å2)
Occupancy
Bi
4e
4e
0.180(3)
0.826(4)
0.093(1)
0.081(2)
0.111(1)
0.382(1)
0.041(2)
0.011(2)
1.0
1.0
Mg/Co
V
4e
4e
4e
4e
4e
4e
0.024(1)
0.327(2)
0.015(7)
0.825(3)
0.483(1)
0.628(2)
0.350(2)
0.929(1)
0.218(3)
0.427(2)
0.478(1)
0.172(3)
0.217(5)
0.034(2)
0.318(2)
0.086(4)
0.257(7)
0.479(1)
0.056(4)
0.001(3)
0.045(1)
0.006(2)
0.065(3)
0.032(4)
0.5/0.5
1.0
1.0
1.0
1.0
O1
O2
O3
O4
O5
BiMg1-xMxVO5 (M = Co, Ni and Cu)
The initial characterizations of all the compounds were
carried out by powder X-ray diffraction (PXRD) studies followed
by energy-dispersive X-ray (EDX) spectroscopy analysis (Figure
1, S1). The studies indicated the formation of single-phase
compounds isostructural to the parent BiMgVO5 structure. The
PXRD studies clearly indicated the formation of BiMg1-xMxVO5,
(0< x ≤ 0.5) for M = Co, Ni and BiMg1-xCuxVO5, (0< x ≤ 0.25)
(Figure 1).
1.0
Space group P21/n: a = 7.549(3) Å, b =11.621(4) Å, c = 5.308(1) Å, α= γ =
90°, β =107.48°;
Reliability Factors: Rp = 2.12%, Rwp = 3.06%, χ2 = 4.62;
Bond Lengths (Å): Mg/Co–O = 2.101(2) (average), V–O = 1.728(1)(average),
Bi–O = 2.416(3), (average)
Figure 3. Crystal structure of BiMg0.5Co0.5VO5 drawn from the Rietveld
refinement data.
Optical Absorption Studies
Figure 1. PXRD patterns of the BiMg1-xMxVO5 (M = Co, Ni, and Cu)
compounds.
Many of the Paganoite compounds have been prepared
and reported.[22] We have expanded the family of known
compounds and investigated their optical absorption behaviour.
We have attempted the substitution of transition elements such
as Co2+, Ni2+ and Cu2+ ions at the octahedral sites in place of
Mg2+ ions. The results of this investigation are described below.
We performed a Rietveld refinement of the powder XRD
data of the BiMg0.5Co0.5VO5 phase, (space group, P21/n, no. 14)
(Figure 2 and Table 1). The refinement details of the other
compounds are provided in the Supporting Information (Figures
S2, S3 and Tables S2, S3). The crystal structure of
2
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BiMg1-xMxVO5 (M = Co, Ni and Cu)
In addition, a deep valley of no absorption was observed at ~ 1.9
eV (red-orange region). The combination of two absorption and
the valley gives rise to the observed color for the compounds.
Generally, in octahedral environment, Ni2+ ions are expected to
have three absorption bands, (i) 3A2g(3F) → 3T2g(3F) (~0.8 eV) (ii)
The parent oxide, BiMgVO5, has a bright yellow color
(Figure 4). The optical absorption spectra (Figure 5) exhibits a
broad absorption band in the blue region (430–480 nm) and the
corresponding complementary color would be yellow. The broad
absorption starting from 2.5 eV extending into the UV region
could be due to the charge transfer from the filled O 2p orbital to
the empty V 3d states (ligand-to-metal charge transfer, LMCT).
On substitution of Co2+ in place of Mg2+ ions, BiMg1-xCoxVO5 (0 <
x ≤ 0.5) the color gradually change from bright yellow to a sea
green color with increasing substitution of cobalt (Figure 5). The
optical absorption spectra (Figure 5) displays two distinct
absorption maxima centered around 1.60 and 2.2 eV. The 2.2
eV band has two side bands centered around 2.0 eV and the
other 2.4 eV. Co2+ ion in octahedral geometry, generally, exhibits
3A2g(3F) → T1g(3F) (~1.4 eV) and (iii) A2g(3F) → T1g(3P) (~2.7
eV).[25] Of these, (i) falls in the IR region and (ii), (iii) transitions
fall in the visible region. The observed absorption spectra of the
BiMg1-xNixVO5 compounds are due to the allowed transitions of
Ni2+ ions in octahedral coordination. The absorption spectra also
exhibited shoulders around 1.7 eV, which could be due to the
3
3
3
3
spin-forbidden transitions from A2g(3F) to 1Eg(1D) respectively,
which are also known for Ni2+ ions in octahedral symmetry.[26]
4
4
three spin allowed transitions (i) T1g → T2g(F) (~0.83 eV) (ii)
4
4
4
4T1g → A2g(F) (~1.58 eV) and (iii) T1g → T1g(P) (~2.12 eV).[25]
Of these, (i) falls in the IR region and (ii), (iii) transitions fall in
the visible region. Thus, the broad triplet absorption can be
4
4
assigned to the T1g → T1g(P) (~2.2 eV) and the other broad
absorption to T1g
→
4A2g (~1.6 eV) transitions in the visible
4
Figure 6. Colors of BiMg1-xNixVO5 (0 < x ≤ 0.5) members in daylight.
region. In addition to the absorption peaks, we also observe two
valleys centered around 1.8 eV and 2.5 eV corresponding to the
red and the green region, respectively. Thus, the combination of
the absorption bands along with the well pronounced valleys in
the optical absorption spectrum resulted in the observed shades
of green color for the cobalt substituted compounds, which is not
observed commonly.
Figure 4. Colors of BiMg1-xCoxVO5 (0 < x ≤ 0.5) members in daylight.
Figure 7. Optical absorption (UV/Vis) spectra of BiMg1-xNixVO5 (0 < x ≤ 0.5)
members.
The Cu2+ ion substitution in BiMg1-xCuxVO5 (0 < x ≤ 0.25)
resulted in an olive green colored compound (Figure 8). The
absorption spectra of BiMg1-xCuxVO5 (0 < x ≤ 0.25) (Figure 9)
exhibits a broad maxima centered around 1.50 and 3.0 eV with a
valley of low absorption in the green-yellow region. The valley of
low absorption in the region of 2.0 to 2.25 eV, may be
responsible for the olive green color of the compounds. The
2
2
broad absorption at ~1.50 eV could be due to the E2g → T2g
transition and the high-energy absorption appears to have a
different origin.
Figure 5. Optical absorption (UV/Vis) spectra of BiMg1-xCoxVO5 (0 < x ≤ 0.5)
members
The Ni-substituted compounds, BiMg1-xNixVO5 exhibits mild
orange color under daylight (Figure 6). The optical absorption
spectra (Figure 7) of BiMg1-xNixVO5 (0 < x ≤ 0.50) also exhibits
two absorption bands: a broad absorption with maxima centered
at ~ 2.7 eV and another broad absorption centered at ~ 1.5 eV.
Figure 8. Colors of BiMg1-xCuxVO5 (0 < x ≤ 0.25) members in daylight.
3
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BiNi1-xCoxPO5 (0 < x ≤ 1.0).
BiNiPO5 also has yellow color (Figure 10). The optical
absorption spectra (Figure 11) of BiNiPO5 exhibit two absorption
bands: a broad absorption with maximum at around 2.9 eV and
another broad absorption with a maxima at around 1.6 eV with a
deep valley of no absorption around 2.2 eV (yellow region). The
observed absorption spectra of BiNiPO5 compares well with the
allowed transitions of Ni2+ ions in octahedral coordination. In
addition, the absorption spectra also exhibited shoulders around
2.45 and 1.75 eV, which could be assigned to the spin-forbidden
transitions from 3A2g(3F) to 1T2g(1D) and 1Eg(1D) respectively.[26]
On substituting Co2+ for Ni2+ ions in BiNi1-xCoxPO5, the
compounds turn from yellow to grey purple to purple color with
increasing cobalt concentration (Figure 10). The optical
absorption spectra (Figure 11) of BiNi1-xCoxPO5 (0 < x ≤ 1.0)
shows a broad absorption band that ranges from approximately
1.9 to 2.60 eV with a maximum at approximately 2.30 eV that is
Figure 9. Optical absorption (UV/Vis) spectra of BiMg1-xCuxVO5 (0 < x ≤ 0.25)
members.
4
4
due to a spin-allowed transition T1g → T1g(P) of Co2+ ions in
distorted octahedral geometry.[25a] The substitution of Co2+ ions
at low concentrations in BiNi1-xCoxPO5 (0 < x ≤ 0.25) displays a
grey purple color. This would arise from the two valleys of low
absorption centered around 2.6 eV (green) and 1.8 eV (red-
orange) region in the absorption spectra of BiNi1-xCoxPO5
(Figure 11). The intensity of purple color of BiNi1-xCoxPO5
increases progressively from grey purple (0 < x ≤ 0.55) to
intense purple (0.5 < x ≤ 1.0) with increasing value of x; whereas
the end member, BiCoPO5 has a brilliant purple color (Figure 10).
The optial absorption spectra of BiCoPO5 exhibits two valleys
centered around 1.7 eV and 2.8 eV in addition to the absorption
which corresponds to the red and the blue region, respectively.
The combination of the absorption bands along with the well
pronounced valleys in the optical absorption spectrum resulted
in the observation of bright purple color for the compound,
BiCoPO5.
To understand the origin of the high-energy absorption
features in the Paganoite compounds, we compared the
absorption spectra of BiMgVO5, and the TM substituted
analogues (Figures 5, 7 and 9). BiMgVO5 exhibits an absorption
edge at around 2.55 eV corresponding to the band gap. This
band-gap absorption is due to charge transfer from the filled O
2p orbital to the empty V 3d states. On the other hand, TM
substituted analogues exhibit a considerable shift of the band
gap to lower energies from Mg (~2.55 eV) to Co (~2.1 eV) to Ni
(~2.25 eV) and to Cu (~2.15 eV).
Similar differences were also observed for the absorption
edges of the divalent wolframite tungstates AWO4 (A = Mg, Mn,
Co, Ni, Cu, Zn)[24] and the lyonsite molybdates Li3Al(MoO4)3 and
Li2Mg2(MoO4)3 as well as their transition-metal (TM)
analogues.[27] The smaller charge-transfer energies observed for
TM wolframite tungstates and lyonsite molybdates compared
with those of their non-transition-metal analogues were assigned
to the metal-metal charge transfer (MMCT) transitions. In MMCT
transitions, the partially occupied 3d orbitals of the TMs become
the highest occupied molecular orbital (HOMO), and the empty
4d/5d states of molybdate/tungstate become the lowest
unoccupied molecular orbital (LUMO). This MMCT gives rise to
spectral and color changes in these oxides. In the present
Paganoite compounds, the high energy absorptions of the TM
substituted compounds can be assigned to the MMCT
transitions from the partially filled 3d states to empty V 3d states.
In the case of Sapphire, the origin of the blue color can also be
considered as a case of MMCT, in which the Fe2+ transfers the
electron to the empty 3d orbitals of Ti4+ resulting in an
equilibrium: Fe2+ + Ti4+ ↔ Fe3+ + Ti3+.[28] Compounds exhibiting
MMCT transitions normally display colors that are unusual and
new.
Figure 10. Colors of BiNi1-xCoxPO5 (0 < x ≤ 1.0) members in daylight.
BiMXO5 (M = Pb, Mn, Ni, Co and X = V, P)
In these compounds also the M atoms occupy distorted
octahedral geometry. We have attempted to prepare many
variants of these phases, but with limited success (Table S1).
We have successfully prepared BiNi1-xCoxPO5 and the results
are described below. Our attempts to replace P5+ partially by V5+
was not successful. The synthesis yielded mixed phases of
compounds with P21/n space group and the phase with P-1
space group. Due to the close structural relationship with
different phases of the Paganoite structure, it was difficult to
isolate and identify single phase materials.
Figure 11. Optical absorption (UV/Vis) spectra of BiNi1-xCoxPO5 (0 < x ≤ 1.0)
members.
4
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BiCdVO5
for BiMgVO5 and BiCdVO5 as they have comparable band gap
to BiVO4 (Table S7, Figure S8). BiMgVO5 exhibits a DOS where
O-2p and Bi-6s states form the top of the valence band (VB).
The V-3d states constitute the bottom of the conduction band
(CB) with minor contributions from Bi-6p and O-2p states. This
appears to be characteristic for the Bi and V-containing
compounds.[33] The contribution of Mg states is weak. In addition,
we observed two distinct 3d blocks for V-3d states at the bottom
of CB, made of the e (dx2-y2, dz2) and t2 (dxy, dyz, dxz) manifolds,
which have been observed earlier.[22a]
Substitution of Cd, in place of Mg, would introduce 4d
states of Cd to the top of the valence band. At the same time,
the 5s states of Cd would be at the bottom of the conduction
band, (Figure S8) which is a known feature in many Cd-
containing compounds.[34] We also observed two maxima
emerging from a continuous density driven by V-3d states. This
feature of V-3d states have been observed for Scheelite bismuth
vanadate, BiVO4.[33] The specific contribution of dx2-y2, dz2
(bottom of CB) and dxy, dyz , dxz states (top of CB) according to
the tetrahedral crystal field d-splitting with some degree of
mixing due to the distortion of VO4 tetrahedra.[33] Due to this, the
band gap of BiCdVO5 (2.45 eV) appears to be slightly decreased
compared to BiMgVO5 (2.55 eV).
We have prepared BiCdVO5 as single-phase compound.
We have also made several efforts to prepare BiCd1-xMxVO5
members (M = Co, Ni and Cu), but our efforts were not
successful, and we could not isolate single-phase compounds.
As mentioned before, the Cd-containing compounds are
stabilized in an unusual space group (Pnc21) and in this
structure, Cd is located in a distorted 7-coordinated site. It is
likely that to stabilize compounds in this space group a large
cation that can take higher coordination may be required. This
could be the reason for not able to substitute 3d elements in this
structure. Our efforts to substitute 3d elements always resulted
in phase segregation forming a number of phases.
BiCdVO5 is yellow in color (Figure S5). The optical
absorption spectra (Figure S5) exhibits a broad absorption band
in the blue region (430–480 nm) and the corresponding
complementary color would be yellow. The optical absorption
behaviour of BiCdVO5 appears to be similar to BiMgVO5.
We have determined the CIE 1931 chromaticity
coordinates to quantify the pigment quality of BiMg1−xMxVO5
(M=Co, Ni, Cu) and BiMPO5 (M=Co and Ni) compounds. As
expected, the results correspond to the yellow region of the
chromaticity diagram for BiMgVO5 and for the BiMg1−xMxVO5
(M=Co, Ni, Cu) compounds fall in the green, orange and yellow-
green region, respectively (Figure S6 and Table S4). The
BiCoPO5 and BiNiPO5 compounds correspond to the purple and
yellow region of the CIE 1931 chromaticity diagram (Figure S6).
The BiMgVO5 and BiCdVO5 exhibit a bright yellow color.
The observed color coordinates (L*, a*, and b* parameters) were
Phtocatalytic Studies
The present compounds, especially BiCdVO5 and
BiMgVO5, have band gaps in the visible region and appear to be
comparable to the well-known photocatalyst, BiVO4. As
mentioned before, one of the factors that reduce the
photocatalytic efficiency is the electron-hole recombination
during the photocatalytic reaction. The reduction of particle size
would help in reducing the electron-hole recombination.[35] At the
same time, the reduction of particle size would lead to an
enhancement of surface recombination. It is likely that the
overall increase in the surface area along with the available
active sites would increase the overall activity.[36] The smaller
size would also enhance the bulk diffusion of electrons and
holes towards the surface, facilitating better reaction.
PXRD pattern of bulk and nano BiMVO5 (M= Cd, Mg)
compounds are consistent with the simulated BiMVO5 (M= Cd,
Mg) (Figure S9). The structural parameters obtained from the
Rietveld refinements on the diffraction data are given in (Figure
S10, Table S8, S9). The scanning electron microscopy (SEM)
images of nano BiMVO5 (M= Cd, Mg) indicates that the particle
sizes are in the range from ca. 80 to 100 nm. The particle size
for the bulk BiMVO5 (M= Cd, Mg) was observed to the 2 to 4 µm
(Figure S11). We observed a slightly expanded unit cell for the
nano-sized BiMVO5 (M=Cd, Mg) compare to bulk-BiMVO5
(M=Cd, Mg) (Table S10). The surface area of nano-BiMVO5
(M=Cd, Mg) were found to be 7.56 m2/g and 7.21
m2/g ,respectively, as determined by N2-adsorption studies
(Figure S12). The particle sizes of the nano-BiMVO5 (M=Cd, Mg)
determined from PXRD using Scherrer's formula were found to
be marginally smaller compared to those observed by the SEM
studies.
compared with BiVO4, which is also
a yellow colored
compound.[29] The data are presented in Table S5, S6. The
BiMgVO5 and BiCdVO5 compounds compare well with the
reported BiVO4. It may be noted that the number of available
stable yellow-colored compounds are limited and the known
ones are based on toxic cadmium, lead and chromium
containing compounds.[30] The discovery of bright yellow-colored
compounds in BiMgVO5 is important as it is based on non-toxic
elements and appears to exhibit immense scope in using them
as active coloring agents for yellow pigments.
NIR Reflectance Spectra
An important aspect in the development of colored
inorganic compounds is to investigate the spectral properties in
the NIR region.[31] Compounds that exhibit NIR reflectivity is
desirable as they find use in cool pigments. Inorganic
compounds that absorbs in the visible region (origin of color in
the compound) and reflect in the near- IR region have been
investigated actively over the years.[32] We have explored the
representative members, BiMg1−xMxVO5 (M=Co, Ni, Cu),
BiMPO5 (M=Co and Ni) and BiCdVO5 for possible NIR-
reflectance behaviour (Figure S7). The NIR spectrum of
BiMgVO5 and BiCdVO5 exhibits ~ 85% NIR reflectivity, which is
comparable to BiVO4.[29e,f] The TM substituted compounds
exhibits a NIR reflectivity in the range of 40 to 50%. From the
NIR reflectivity studies, it is clear that the present Paganoite
compounds could be employed in NIR reflecting materials and
can be useful as cool pigments.
In the present study, the BiMVO5 (M = Mg/Cd) were
prepared both in bulk as well as in nano size. Our studies
indicate that the nano BiMVO5 samples have marginally lower
band gap values, BiCdVO5 (nano) = 2.42 eV; BiMgVO5 (nano) =
2.53 eV compared to the bulk samples: BiCdVO5 (bulk) = 2.45
eV; BiMgVO5 (bulk) = 2.55 eV (Figure S13). We have explored
the photocatalytic behaviour of nano BiMVO5 (M = Mg/Cd)
Electronic Band Structure Calculations
We investigated the electronic structure of the compounds
employing the first-principles density functional theory (DFT)
calculations to understand the role of electrons near the fermi
level. We computed the atom resolved density of states (DOS)
5
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10.1002/asia.202000814
Chemistry - An Asian Journal
FULL PAPER
samples. Initially, the photocatalytic activity of nano BiMVO5 was
investigated under visible light irradiation (λmax >420 nm, 60 W).
We have carried out two sets of photocatalytic organic
transformation: (i) C−C bond cleavage of alkenes to generate
carbonyl compounds and (ii) aerobic photo-oxidation of benzyl
alcohols to benzaldehydes in the presence of molecular oxygen
and water.
addition, the nano-BiCdVO5 exhibited a better performance
compared to the bulk-BiCdVO5. We have employed the nano-
BiCdVO5 for further photocatalytic studies. (Table 2).
We have also investigated the scope of the substrate on
the C–C bond cleavage to generate the carbonyls (Table 3).
Substituted styrene derivatives were employed to produce the
respective aldehyde with good selectivity. Substrates with
electron-donating substituent groups such as p-methyl, p-
methoxy groups as well as electron-withdrawing substituents
such as p-bromo, p-chloro and p-fluoro were converted into the
corresponding aldehydes (Table 3). The reaction, with chloro
and fluoro-derivatives resulted in marginally reduced yields
compared to the bromo derivatives (Table 3). The 1,1-
Disubstituted styrene derivatives (entry 7 and 8) were also
employed as the substrates, which gave the respective ketone
products in good selectivity.
Compounds containing carbonyl functionality such as
aldehydes and ketones are important as well as versatile
functional groups in organic chemistry.[7] A wide range of
synthetic protocols have been developed over the years for the
synthesis of carbonyl; in particular, oxidative cleavage of
alkenes is widely employed through ozonolysis.[6] This is a two-
step process that employs ozone as the oxidant with a special
equipment requirement, and also an additional reductant. The
other synthetic approaches involve Lemieux–Johnson or
Lemieux–von Rudloff oxidation, in which OsO4-catalyzed the
direct oxidation of alkenes to diols followed by C−C bond
cleavage using NaIO4.[37] This two-step process has inherent
disadvantages due to the toxicity of the reagents. There have
been a number of different attempts to develop environmentally
benign protocols employing oxygen in the presence of water,
preferably at room temperature for the cleavage of alkenes to
produce carbonyls.[17,37] In the present study, we attempted the
selective aerobic oxidation of alkenes to carbonyls in the
presence of BiMVO5 in water employing the photocatalytic route
(visible-light irradiation) at room temperature. To this end, we
used styrene (entry 1) as the model substrate (Table 2). The
oxidative cleavage could be achieved to produce benzaldehyde
in 81% yield with 89% selectivity (entry 1). We have attempted
the same reaction in the presence of different organic solvents,
which did not yield better results (entries 3–8). Control
experiments clearly suggested that the reaction proceeds in the
presence of the photocatalyst, visible light, and molecular
Table 3. Selective photocatalytic oxidation of various styrene derivatives over
nano BiCdVO5 under visible light irradiation.[a]
Entry
Reactant
Product
t [h]
Conversion
[%]
Selectivity
[%]
1
6.0
91.2
89
O
2
3
7.5
7.0
85.1
82
O
Me
Me
88.0
92.4
83.8
86
84
82
O
MeO
MeO
4
5
6.0
7.5
O
O
Br
Br
Cl
Cl
6
7
10.0
8.5
77.1
82.2
83
80
O
oxygen (Table 2).
Table 2. Oxidative cleavage of alkene 1 to benzaldehyde 2
F
F
Me
Me
O
O
O
photocatalyst, O2
8
12.0
72.8
76
solvent (2 mL), RT
6 h, visible LEDs (60 W)
2
1
[a]Reaction conditions: 1 (0.1 mmol) and nano BiCdVO5 (10 mg). water (2 mL), O2
bubbling for 5 min. The yields were determined by GC–MS or 1H NMR.
Entry
Solvent
H2O
Photocatalyst
nano BiCdVO5
Variation
Yield
[%]
The recyclability of the nano-BiCdVO5 catalyst was
examined under the identical reaction conditions (Table 2). The
used photocatalyst was subjected to the same reaction
conditions for 4 times (Figure 12). In all the cases, the catalyst
produced carbonyls without a noticeable loss of activity. This
suggests that the catalyst is stable for this reaction (Figure S12).
In order to establish the heterogeneous nature of the reaction,
we performed the hot filtration experiment. Thus, the reaction
was conducted under the same conditions. After 1 h of the
reaction, the catalyst was separated from the reaction mixture by
centrifugation and the filtrate was allowed to react under the
same conditions. The yield of the product was analysed with the
catalyst and without the catalyst (Figure S13). The yield of the
product appears to exhibit a marginal increase from ~ 20% at 1
h to ~ 30% at 6 h. This clearly suggests that there is no
significant progress in the reaction without the catalyst. The
1
81
2
3
4
5
6
7
8
9
10
11
12
13
14
H2O
DMSO
DMF
MeOH
EtOH
MeCN
CH2Cl2
H2O
H2O
H2O
H2O
H2O
nano BiMgVO5
nano BiCdVO5
nano BiCdVO5
nano BiCdVO5
nano BiCdVO5
nano BiCdVO5
nano BiCdVO5
nano BiCdVO5
-
nano BiCdVO5
nano BiCdVO5
bulk BiCdVO5
bulk BiMgVO5
63
22
18
0
trace
20
0
0
0
0
absence of light
no catalyst
no O2
open air
30
34
30
H2O
Reaction conditions: 1 (0.1 mmol) and nano BiCdVO5 (10 mg). The yields
were determined by GC–MS.
The studies also indicated that BiCdVO5 exhibits a much
better catalytic performance compared to BiMgVO5 (Table 2). In
6
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10.1002/asia.202000814
Chemistry - An Asian Journal
FULL PAPER
small increase in the overall yield could have resulted due to the
presence of trace residue of the catalyst.
irradiation by Fujihira et al.[1a] provided the much needed
attention towards the use of photocatalysts.[41-47]
In most of the photocatalytic oxidation of alcohols,
acetonitrile was used as the solvent, as it would provide the
“shield effect”.[48] Acetonitrile acts as a weak base to stabilize
reaction products via solvation, inhibiting proton transfer and
preventing the formation of radical species to avoid undesired
side reactions.[49] In the present study, we employed benzyl
alcohol (entry 1) as the model substrate with acetonitrile as the
solvent (Table 4). The selective oxidation could be achieved to
produce benzaldehyde in 84% yield with 95% selectivity (Table
4). We have also attempted the same reaction in the presence
of different organic solvents including water, which did not yield
better results (entries 3–8). In this study, the selective oxidation
of alcohols employing BiMVO5 (M= Mg/Cd) in acetonitrile was
extended to other substituted benzylic alcohols under identical
reaction conditions (Table 5). Control experiments indicated that
at room temperature, in the dark or in the absence of any
catalyst or in the absence of molecular oxygen no conversion
was observed (Table 4). In this reaction also, we have observed
that the photocatalytic performance of BiCdVO5 (bulk and nano)
is better than BiMgVO5 (bulk and nano) (Table 4).
Figure 12. Recyclability studies of nano-BiCdVO5 for styrene oxidation to
benzaldehyde.
Table 4. Oxidation of benzyl alcohol 3 to benzaldehyde 4
OH
O
photocatalyst, O2
solvent (2 mL), RT
3 h, visible LEDs (60 W)
4
3
Table 5. Selective photocatalytic oxidation of various benzylic alcohol
derivatives over nano BiCdVO5 under visible light irradiation.[a]
Entr
y
Solvent
Photocatalyst
Variation
Yield
[%]
Entry
Reactant
Product
t [h]
Conversi
on [%]
Selecti
vity [%]
1
2
H2O
DMSO
nano BiCdVO5
nano BiCdVO5
41
15
1
3.0
88.4
95.6
96.8
95
90
94
OH
O
3
DMF
nano BiCdVO5
18
4
5
6
7
8
EtOH
MeCN
MeCN
nano BiCdVO5
nano BiCdVO5
nano BiMgVO5
nano BiCdVO5
nano BiCdVO5
0
84
56
47
0
2
3
OH
5.0
4.0
O
Me
benzotrifluoride
MeCN
Me
absence of
light
OH
O
9
MeCN
MeCN
MeCN
MeCN
MeCN
-
no catalyst
no O2
open air
0
0
35
36
24
MeO
MeO
10
11
12
13
nano BiCdVO5
nano BiCdVO5
bulk BiCdVO5
bulk BiMgVO5
4
5
6.5
9.0
47.0
29.4
86
54
OH
O
O2
N
O2N
Reaction conditions: 3 (0.1 mmol) and nano BiCdVO5 (10 mg). The yields
were determined by GC–MS.
OH
O
HO
HO
We have carried out
a number of experiments to
6
7
8
OH
8.0
7.0
71.0
88.1
62.3
90
92
89
O
O
O
understand the possible mechanism of the conversion of the
styrenes.[38] Thus, the reaction was conducted in the presence
of different additives such as (2,2,6,6-tetramethylpiperidin-1-
F
F
OH
Br
Br
yl)oxidanyl
(TEMPO),
benzoquinone,
and
1,4-
diazabicyclo[ 2.2.2]octane (DABCO) (Table S11). The use of
TEMPO (free radical quencher) and benzoquinone (superoxide
quencher) completely prevented the transformation. In the case
of DABCO (singlet oxygen quencher), we noticed a reduction in
the overall conversion (15%). These observations helped us in
arriving at the possible mechanism (Scheme S1). The efficiency
of the present catalyst appears to be comparable to those
reported before.[39] (Table S12).
OH
10.0
Cl
Cl
9
12.0
52.6
84
OH
O
[a]Reaction conditions: 3 (0.1 mmol) and nano BiCdVO5 (10 mg). MeCN (2 mL), O2
bubbling for 5 min. The yields were determined by GC–MS or 1H NMR.
Selective oxidation of alcohols into aldehydes
The oxidation of alcohols to aldehydes and ketones
employing photocatalytic route has been well studied.[4] Many
photocatalytic aerobic alcohol oxidation suffers from low
conversion efficiency and poor selectivity.[40] The selective
oxidation of alcohols to aldehydes over TiO2 employing UV
Our studies employing nano-BiCdVO5 indicate that the
selective oxidation of all the derivatives proceeds to form the
corresponding aldehydes with good selectivity (Table 5). If the
unsubstituted benzyl alcohol was taken as a reference, then the
7
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10.1002/asia.202000814
Chemistry - An Asian Journal
FULL PAPER
studies indicated that the electron-donating groups (–OCH3 and
–CH3) increases the conversion of alcohols, whereas the
electron-withdrawing groups (–Cl, –F, and –NO2) decreases the
activity. Similar findings have been observed earlier for the
selective oxidation of alcohols.[36] The use of 4-hydroxybenzyl
alcohol in this reaction resulted a low conversion (30%) and
selectivity (52%). It is likely that there is competition between the
two hydroxyl groups in the substrate. Similar observations have
been noted before.[49] It may be noted that when R = Ph, we
observed a reduced conversion, which could be due to the
increase steric effects resulting in smaller number of alcohol
molecules binding to the catalyst surface.[49]
Halogen substitution, on the other hand, appears to show
a balance of electronic and steric effects giving yields in the
order Cl < F < Br. It may be noted that the selectivity for all the
substrates remains high (Table 5). The reusability of the nano-
BiCdVO5, was examined for up to 4 cycles of benzyl alcohol
oxidation reaction (Figure S14), which indicated catalytic activity
that is comparable to the fresh catalyst (Figure 13). We have
also performed the hot filtration test for the benzyl alcohol
oxidation reaction, which demonstrated that the catalytic
reaction does not proceed in the absence of the catalyst (Figure
S15). The catalytic efficiency of the present catalyst appears to
be comparable to those reported before.[50] (Table S13).
exhibit metal-to metal charge transfer (MMCT) effect. The
MMCT effect giving unusual colors to the 3d-transition metal
doped analogues. The NIR reflectivity studies indicate these
brightly compounds also exhibit NIR reflectivity of ~ 50%. The
present studies on the Paganoite oxides clearly suggest that
they have immense potential to be exploited as photo and
electrochemical catalysis. Studies are presently under way to
exploit these compounds further.
Experimental section:
Reagents: The chemicals, Bi2O3, MgO, CdCO3, V2O5, Bi(NO3)3·5H2O,
Mg(NO3)2·6H2O, Cd(NO3)2·4H2O, NH4VO3, citric acid (Sigma-Aldrich,
99.9%), NH4H2PO4, ethylene glycol, HNO3 (SD Fine, India), MC2O4·2H2O
(M= Co, Ni, Cu) (Alfa Aesar, 99%), were used as received. Commercially
available styrene and benzyl alcohol derivatives were purchased from
Sigma-Aldrich, Alfa Aesar and TCI companies.
Synthesis: Synthesis of all the compounds was carried out by using the
conventional ceramic method. Stoichiometric mixtures of binary oxides
were heated in air at 750 oC to 815 oC for 24 h with several intermittent
grindings, which resulted in single phase compounds.
For nano sized compounds: Bi(NO3)3·5H2O, Mg(NO3)2·6H2O,
Cd(NO3)2·4H2O and NH4VO3 were dissolved in 4M HNO3 solution
(deionized water), followed by the addition of a mixture of citric acid and
ethylene glycol. Subsequently, the obtained transparent solution was
slowly evaporated to form a gel, which was decomposed at about 500 oC
for 5 h to result in yellow powders. The precursor powders were further
heated at 770 oC for 8 h to produce BiCdVO5 as fine powders.
Characterization: All the prepared compounds were characterized by
PXRD, Scanning Electron Microscopy (SEM) and Energy-Dispersive X-
ray Spectroscopy (EDX). PXRD patterns were recorded with a Philips
X’pert Diffractometer (Ni-filtered Cu Kα radiation, λ = 1.5418 Å). PXRD
data for Rietveld refinement of the structures were collected at room
temperature employing the same diffractometer (λ = 1.5418 Å) in the 2θ
range 5−120° with a step size of 0.02° and step duration 50 s. The PXRD
patterns were refined using the program GSAS-EXPGUI.[53] Lattice
parameters, scale factors, background (Fourier polynomial background
function), pseudo-Voigt (U, V, W, and X), and isothermal temperature
factors (Uiso) were refined. Thermal parameters were constrained to be
the same for different atoms occupying the same sites. PXRD patterns
were simulated with the help of the program POWDERCELL.[54] The
diffuse reflectance spectra for all powdered samples were recorded by
using a Perkin–Elmer Lambda 950 UV/Vis double beam spectrometer
over the spectral region of 250–1200 nm. The reflectance data were
converted to the Kubelka–Munk[55] function by using the equation
F(R) = (1-R)2/2R = α/S
Figure 13. Recyclability studies of nano-BiCdVO5 for benzyl alcohol oxidation
to benzaldehyde.
Based on the present studies a possible mechanism for
the conversion of alcohols has been proposed (Scheme S2).
Thus, on photon absorption, the electron-hole separation occurs
with the carriers becoming trapped in surface defect states.[51]
Adsorption of the substrates occurs at those sites, which are
also responsible for possible emissive recombination. The
trapped electrons are transferred to surface adsorbed molecular
oxygen, while the holes transfer to the surface adsorbed benzyl
alcohol molecules. This includes the reduction and the oxidation,
respectively. Benzaldehyde is formed by liberating the protons,
in which R is the reflectance, α is absorptivity, and S is the scattering
factor. NIR reflectance at RT in the range, λ =500–2500 nm were
collected by using the same instrument. For characterization of pigment
quality of the samples, the CIE 1931 chromaticity coordinates at RT in
the λ =380–750 nm range and NIR reflectance in the λ =500–2500 nm
range were collected by using the same instrument. CIE-1931
chromaticity coordinates were determined by using the gocie.exe
program[56] from http://www.geocities.com/krjustin/gocie.html. SEM
images and EDX data were recorded by using an Ultra 55 field-emission
1
scanning electron microscope (Karl Zeiss). H NMR data were collected
using a Bruker 400 MHz spectrometer. Chemical shifts are reported in
which gives rise to water from the reduction of molecular oxygen. ppm using tetramethylsilane as the internal reference. Gas sorption
measurements were carried out using the AUTOSORB IQ2 instrument
on the dehydrated samples.
The formed benzaldehyde can either desorb from the surface or
can be further oxidised forming the benzoic acid. The formation
of benzoic acid would inhibit the binding of fresh alcohol towards
the surface and retard the overall reaction.[52]
Electronic Structure Calculations: The electronic band structure
calculations for the prepared samples were carried out by using the
density functional theory (DFT) implemented in the planewave
pseudopotential method CASTEP.[57,58] Under the norm-conserving
pseudopotential (NCP),[59] the following orbital electrons were treated as
valence electrons: Bi-4f14 5d10 6s2 6p3, Cd-4d10 5s2, Mg-3s2, V-3d3 4s2,
O-2s22p4. The exchange correlation interaction was treated by the
Conclusions
The visible light activated photocatalytic C−C bond
cleavage of alkenes and benzyl alcohols under aerobic
conditions have been accomplished using (BiMXO5; M = Mg, Cd,
Ni, Co, Pb, Ca and X = V, P) Paganoite oxides. The optical
absorption studies indicate that the V5+ containing compounds
Generalized
Gradient
Approximation
(GGA)[60,61]
with
the
Perdew−Burke−Ernzerhof (PBE) functional.[62,63] To achieve energy
convergence, the plane-wave basis set energy cutoff values were set at
8
This article is protected by copyright. All rights reserved.

10.1002/asia.202000814
Chemistry - An Asian Journal
FULL PAPER
958–964; d) M. Y. Zheng, Y. S. Wei, G. Fan, Y. Huang, Asian J. Chem.
2012, 24, 161–164.
610 eV. The Monkhorst−Pack scheme k-point grid sampling was given
by 3 × 3 × 6 in the Brillouin zone. The other calculation parameters and
convergent criteria were the default values of the CASTEP code.[64,65]
Photocatalytic reactions: The photocatalytic reactions were carried out
employing a slightly modified procedure reported before.[66] The catalyst
(10 mg) in H2O (2 mL) was taken in a round bottom flask with a magnetic
stir bar and the reaction vessel was sealed with a silicone septum screw
cap. Molecular oxygen was bubbled through the reaction mixture for 5
min; then, styrene derivatives (0.1 mmol) were added (styrenes were
added after oxygen bubbling due to the volatility of some styrenes). An
oxygen-filled balloon was fixed to the tube, which was then placed under
60 W visible LED (>420 nm) at room temperature, with stirring for 6-20 h.
For oxidation of alcohols, benzyl alcohols were taken in place of styrenes
and MeCN (2 mL) was taken as solvent. The progress of the reaction
was monitored by gas chromatography or 1H NMR spectroscopy. After
completion of the reaction, dichloromethane or ethyl ether was added to
the mixture, and the organic phase was extracted and concentrated on a
rotary evaporator.
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The authors declare no conflict of interest.
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Keywords: Photocatalysis • Paganoite • Inorganic pigments •
Structure elucidation • Structure elucidation • Optical properties
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Entry for the Table of Contents
FULL PAPER
Family of Paganoite oxides
were prepared and investigated
for color, optical and visible light
photocatalytic behaviour. The
compounds were active as
Synthesis, structure, optical and
photocatalytic
studies
of
Paganoite oxides
Anupam Bhim, Shreya Sasmal,
Jagannatha Gopalakrishnan and
Srinivasan Natarajan*
visible
light
assisted
photocatalyst for the C−C bond
cleavage of alkenes and aerobic
oxidation of benzyl alcohols.
Presence of transition metal
elements in these oxides, gives
Page No. – Page No.
Visible Light Activated C–C
Bond Cleavage and Aerobic
Oxidation of Benzyl Alcohols
Employing BiMXO5 (M = Mg, Cd,
Ni, Co, Pb, Ca and X = V, P)
rise
materials
to
brilliantly
rendering
colored
them
attractive candidates for new
inorganic pigments.
Inorganic pigments • Visible Light Photocatalysis • Paganoite • Transition metals
11
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