Selective Visible Light Aerobic Photocatalytic Oxygenation of Alkanes to the Corresponding Carbonyl Compounds

Article abstract of DOI:10.1021/acscatal.9b02999

The aerobic, selective oxygenation of alkanes via C-H bond activation is an important research challenge. Photocatalysis offers the potential for the introduction of additional concepts for such reactions. Visible light photoactive semiconductors such as bismuth oxyhalides (BiOX, X = Cl and Br) used in this research typically oxidize organic compounds through photocatalyzed formation of strongly oxidizing holes. The reactive oxygen species formed react with organic compounds in one-electron processes, leading to radical intermediates and nonselective oxidation. Such oxidation reactions generally lead to total oxidation. Here, impregnation of BiOX with a polyoxometalate, H₅PV₂Mo₁₀O₄₀, as a strong electron acceptor changed the reactivity of BiOX, leading to Mars-van Krevelen-type reactivity, that is, photoactivated oxygen donation from BiOX to the organic substrate followed by reoxidation by O₂ and catalysis. This conclusion was supported by mechanistic studies involving isotope labeling studies. In this way, ethane was selectively oxidized to acetaldehyde in a flow reactor with a turnover number (24 h) of 415.

Full text of DOI:10.1021/acscatal.9b02999

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Article
Selective Visible Light Aerobic Photocatalytic Oxygenation
of Alkanes to the Corresponding Carbonyl Compounds
Miriam Somekh, Alexander M. Khenkin, Adi Herman, and Ronny Neumann
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02999 • Publication Date (Web): 13 Aug 2019
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Selective Visible Light Aerobic Photocatalytic Oxygenation of Alkanes to the Corresponding
Carbonyl Compounds
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Miriam Somekh, Alexander M. Khenkin, Adi Herman and Ronny Neumann*
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100
The aerobic, selective oxygenation of alkanes via C-H bond activation is an important research challenge. Photocatalysis
offers the potential for the introduction of additional concepts for such reactions. Visible light photoactive
semiconductors such as bismuth oxyhalides (BiOX, X= Cl, Br) used in this research, typically oxidize organic compounds
through photocatalyzed formation of strongly oxidizing holes. The reactive oxygen species formed, react with organic
compounds in one-electron processes, leading to radical intermediates and non-selective oxidation. Such oxidation
reactions generally lead to total oxidation. Here, impregnation of BiOX with a polyoxometalate, H5PV2Mo10O40, as a
strong electron acceptor changed the reactivity of BiOX, leading to Mars-van-Krevelen type reactivity, that is
photoactivated oxygen donation from BiOX to the organic substrate followed by reoxidation by O2 and catalysis. This
conclusion was supported by mechanistic studies involving isotope labelling studies. In this way ethane was selectively
oxidized to acetaldehyde in a flow reactor with a turnover number (24 h) 415.
their narrower band-gap and high charge mobility.17-19
The advantageous properties of these compounds are
attributed to hybridized band structures formed between
the Bi 6s and the O 2p orbitals that raise the energy level
of the valence band. The insertion of an oxyhalide
component instead of a metal oxide led to the discovery
of photoactive semiconductors such as BiOCl and BiOBr
that showed remarkable photocatalytic activity in dye
bleaching.20-25 It was further noticed that easily prepared
bismuth mixed oxyhalides, especially BiOClxBr1-x where
0<x<1, are even more effective for bleaching with visible
light than the corresponding mono-halides.26,27 Overall,
reports of C-H activation by bismuth based
photocatalysts are quite rare and generally yield CO2 as
Keywords. Oxygen Transfer, Photocatalysis,
Polyoxometalate, C-H bond activation, Bismuth Oxide,
Semiconductor
Introduction.
Selective photocatalytic oxidation of alkanes, preferably
with dioxygen and visible light, is a challenging research
topic. Although oxygenations of alkanes with O2 are
exergonic, room temperature photo-oxygenation
reactions
semiconductors are not selective and tend to lead to
total Mechanistically speaking,
oxidation.1,2
by
commonly
used
photoactive
photoexcitation of oxidizing semiconductors leads to the
formation of strongly oxidizing holes that then form
potent radical species such as OH• from H2O or alkyl
radicals from hydrocarbons.3 Alkyl radicals from alkanes
and alkenes and perhaps cation radicals from alkylarenes
can be formed through C-H bond activation via
hydrogen abstraction or electron transfer. In the
presence of O2 further free-radical chain autooxidation
reactions can occur leading to product formation whose
selectivity is difficult to control.4-6 Photoactive
semiconductors that have been used for alkane and
alkylarene aerobic oxidation Include TiO2,7,8 ZnO,9
main product. Nonetheless, there are reports where
alcohols and thiols were oxidized,
28
and an important
report where alkylarenes and cyclohexane were
photooxidized with O2 by combining BiOBr/TiO2 and
adding HCl.29 Evidence was presented for the formation
of chlorine radicals leading to C-H bond activation,
formation of alkyl radicals followed by oxygenation via
autooxidation. Chlorinated products were also formed.
We have had a longstanding interest in the use of a
phosphovanadomolybdate
polyoxometalate,
11
14
TiO2/MoO3,10 CuMoO4 V2O5,12 CdS,13 Fe2O3 and the
H5PV2Mo10O40, as an oxygen donor in electron transfer-
oxygen transfer or Mars-van Krevelen type reactions
where O2 re-oxidizes the reduced catalyst and completes
decatungstate polyanion,
a
soluble semiconductor
analog.15,16 Bismuth-based ternary metal oxides such as
Bi2WO6 and BiVO4 are interesting photocatalysts due to
the
catalytic
cycle.30,31
Despite
many
useful
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transformations,32,33 reactivity is limited to conditions
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preferable to both H4PVMo11O40 for a two-electron
oxygenation described here and H3PMo12O40 that is only
slowly re-oxidized by O2, Scheme 1. To the best of our
knowledge this is the first time such a photoinduced
oxygen transfer from a bismuth oxide material is being
documented. This photocatalyzed oxygen transfer
typically leads to the efficient and selective oxidation of
alkanes and to the corresponding ketones or aldehydes
through a faster reacting alcohol intermediate.
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where reaction initiation can be by electron transfer from
the substrate to the polyoxometalate, precluding the
aerobic oxidation of simple alkanes and many
alkylarenes. Since, H5PV2Mo10O40 also inhibits free-radical
autooxidation,34 the idea was not to directly oxidize the
organic substrate via a radical-O2 reaction as observed in
the past, but instead re-oxidize the catalyst after oxygen
transfer from it to the substrate. Thus, it is proposed that
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a
combination of
a
semiconductor photocatalyst,
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Results and Discussion.
BiOClxBr1-x, for C-H activation of alkanes and alkylarenes
and H5PV2Mo10O40 as oxygen donor could lead to truly
selective photooxygenation of hydrocarbons, Figure 1.
The synthesis of BiOClxBr1-x (x = ~0.875 by energy
dispersive spectroscopy, EDX) was carried out according
to a literature method.27 BiOClxBr1-x was extensively
washed with ethanol and water to remove the
quaternary
ammonium
salts
(hexadecyltrimethyl
ammonium halides) used in the synthesis and then dried
in air. The polyoxometalate was supported onto
BiOClxBr1-x
by
wet
impregnation
of
H5PV2Mo10O40•~34H2O (POM) dissolved in acetonitrile
and dried under vacuum at RT. The surface morphology
of BiOClxBr1-x, as viewed by scanning electron microscopy
(SEM), Figure S2, also showed no change after
impregnation of H5PV2Mo10O40. EDX measurements,
including element mapping, clearly shows that the
polyoxometalate is homogenously distributed on the
surface of BiOClxBr1-x, Figure 2.
Figure 1. General Scheme for the Photochemical
Oxygenation of Alkanes with a Representation of (BiOClxBr1-
x)n–H5PV2Mo10O40. X = Br, Cl (0≤x≤1); R = alkyl, aryl; X -
green, Bi – purple, O – red, Mo – black, V – blue, P – yellow.
Satisfactorily, this was the observed experimental
result, however, isotope labelling studies and additional
observations indicate that BiOClxBr1-x is the actual oxygen
donor and H5PV2Mo10O40 plays a key role as an electron
acceptor in the photoactivation process. The overall
proposed reaction pathway for the first oxygenation
reaction, e.g. alkane to alcohol, is also presented in
Scheme 1.
Figure 2. Scan electron microscopy EDX analysis of
BiOClxBr1-x–5wt.% H5PV2Mo10O40 recorded at 20 kV,
magnification 25000x, WD =15.3 mm. Top to bottom, left to
right: SEM, Mo, O, Bi, Br, Cl, V, P.
The powder XRD of BiOClxBr1-x and BiOClxBr1-x–POM
Scheme 1. Proposed Reaction Pathway for the
Catalytic Oxygenation via C-H Bond Activation.
showed the retention of the structure of BiOClxBr1-x
,
Figure S1. At higher (5% wt) loadings, H5PV2Mo10O40
appears to be partially crystalline on the surface, but at
most efficient loading for catalysis (2% wt) such
crystallinity was not observed, Figure S3. The initial
analysis of the photocatalytic activity BiOClxBr1-x–5wt.%
Thus, different from previous research in using BiOX as
a
photocatalyst, we found that by combining
H5PV2Mo10O40•~34H2O with BiOClxBr1-x a change in the
reaction mechanism occurs. Instead of the common
formation of an alkyl radical through hole oxidation of
the organic substrate and then reaction with O2,
oxygenation apparently occurs by oxygen transfer from
H5PV2Mo10O40 was carried out on toluene as
a
representative alkylarene substrate. Thus, a thick slurry of
toluene and solid catalyst was continuously stirred for 24
h under 2 bar of O2 while irradiating with visible light
(white LED lamps). The results shown in Table 1 lead to
the following observations. (1) BiOClxBr1-x alone showed
very little photocatalytic activity, but in combination with
5wt% H5PV2Mo10O40 the conversion increased strikingly.
(2) H5PV2Mo10O40 alone showed no photocatalytic
the
photoactivated
BiOClxBr1-x
,
where
the
polyoxometalate acts as an electron “trap” inhibiting
charge recombination upon photoactivation of BiOClxBr1-
x. H5PV2Mo10O40 is also a limiting component in the
reaction, its re-oxidation by O2 to H2O,35 is needed to
complete the catalytic cycle. In addition, H5PV2Mo10O40 is
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activity. (3) The efficiency of the reaction decreased
decrease reactivity as may be expected due to an
increase in oxidation potential, and electron donating
substituents increase reactivity. More reactive substrates
such as 4-methoxytoluene (not shown) yielded the
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significantly in the absence of O2 indicating that for
higher turnover O2 was needed. However, the
observation that product was formed in the absence of
O2 is mechanistically important (see below). (4) Longer
reaction time led to a higher conversion of toluene but
decreased selectivity due to the further oxidation of
benzaldehyde to benzoic acid. (5) A larger amount of
H5PV2Mo10O40 supported on BiOClxBr1-x tended to inhibit
the reaction. Possibly the absorption of light by
corresponding
4-methoxybenzaldehye
with
low
selectivity (15 mol%) with co-formation of arene-arene
coupled products by an electrophilic cation radical
pathway as previously discussed for oxidation of
activated alkyarenes catalyzed by H5PV2Mo10O40.36
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Encouraged by the liquid phase oxidation of alkanes,
light alkanes were reacted in a tubular gas reactor
consisting of a sapphire tube reactor rated at 200 bar, a
GC-TCD for on line analysis and specific mass flow
controllers and a back-pressure regulator. The results are
summarized in Table 3. As shown in Table 3 high activity
was observed for propane > ethane, while methane did
not react. The reactions were highly selective with no
formation of overoxidation products, CO2, CO or
carboxylic acids. The reactions were more efficient in the
gas phase, likely because the concentration of O2 was
significantly higher, enabling faster re-oxidation of the
catalyst to complete the catalytic cycle. The catalyst was
also titurated after reaction with CD2Cl2 to verify of the
BiOClxBr1-x
is
screened,
thereby
preventing
photoactivation. The optimal loading H5PV2Mo10O40 was
2 wt%. Based on H5PV2Mo10O40 and considering each
catalytic cycle involving
a 2-electron oxidation, a
turnover number (TON, 24 h) of 156 was obtained. (6) A
weaker oxidant/electron acceptor, H3PW12O40, was
significantly less effective in promoting photocatalysis in
comparison to H5PV2Mo10O40.
Table 1. Aerobic Visible Light Photooxygenation of
Toluene to Benzaldehyde Catalyzed by BiOClxBr1-x
POM.
–
Catalyst
Yield, mmol/g-catalyst
1
absence of non-volatile adsorbed species by H NMR
BiOClxBr1-x–2 wt %
H5PV2Mo10O40
0.64
0.38
0.12
and GC. None were found. The powder XRD of the used
catalyst was compared to that of a pristine sample,
Figure S3, and the 31P NMR of H5PV2Mo10O40 was
measured after extraction from the BiOClxBr1-x “support”
with D2O, Figure S4. Both these measurements after 48 h
reaction show no significant changes in the structure of
BiOClxBr1-x and H5PV2Mo10O40 after reaction and
indicating high catalyst stability.
BiOClxBr1-x–5 wt %
H5PV2Mo10O40
BiOClxBr1-x–10 wt %
H5PV2Mo10O40
BiOClxBr1-x
0.04
0
H5PV2Mo10O40
BiOClxBr1-x–5wt %
Table 2. Liquid Phase Photooxidation of Various
Alkylarenes and Alkanes Catalyzed by BiOX-POM.
0.16
a
H5PV2Mo10O40
BiOClxBr1-x–5 wt %
H5PV2Mo10O40
0.08
0.16
Substrate
Product (mol%)
Yield,
mmol/g-cat
TONa
b
BiOClxBr1-x–5 wt % H3PW12O40
PhCH3
PhCHO (100)
0.64
1.12
156
247
Reaction conditions: Toluene (4.75 mmol, 0.5 mL), catalyst
(250 mg), O2 (2 bar), light – 4 x 10 W white LED lamps, room
temperature, 24 h. (a) Reaction using 2 x 10 W white LED
lamps; (b) Ar (2 bar).
PhCH2CH3
PhC(O)CH3 (85)
PhCHOHCH3 (15)
4-NO2PhCH3 4-NO2PhCHO (89)
0.34
0.24
77
56
4-NO2PhCH2OH
(11)
The activity of BiOClxBr1-x–2 wt % H5PV2Mo10O40, [BiOX-
POM], was tested on a range of alkylarenes and alkanes
in the liquid phase as shown in Table 2. The results show
that (1) secondary C-H bonds are significantly more
reactive than primary ones; (2) the observation of an
alcohol co-product in most cases indicates that the
oxidation proceeds by a 2-electron oxidation to an
alcohol and then by a further faster 2-electron oxidation
to the ketone or aldehyde. In fact, cyclohexanol as
substrate was very efficiently oxidized to cyclohexanone.
The absence of benzoic acid and derivatives in the
oxidation of corresponding toluene compounds is also
notable. Electron withdrawing substituents on toluene
2-NO2PhCH3 2-NO2PhCHO (78)
2-NO2PhCH2OH
(22)
4-BrPhCH3
4-FPhCH3
4-BrPhCHO (84)
4-BrPhCH2OH (16)
4-FPhCHO (68)
0.40
0.12
88
24
4-FPhCH2OH (32)
1,3-(CH3)2Ph 3-CH3PhCHO (100)
1,4-(CH3)2Ph 4-CH3PhCHO (100)
0.72
0.80
172
191
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potentials as surrogates of the redox potential,31
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cyclohexane cyclohexanone (68)
cyclohexanol (32)
0.26
0.063
0.05
65
15
12
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correlate very well for an electron transfer initiated PCET
oxygenation where PhCH2OH (8.27 eV) > PhCH3 (8.83 eV)
>> PhCHO (9.50 eV).37 Reactivity did not correlate with
the homolytic C-H bond disassociation energies (BDE).38
Notably, the aldehydic C-H is bond is the weakest, but
the aldehyde formed is quite inert under reaction
conditions.39 A PCET mechanism also explains the
observation that alcohols are intermediate products that
are typically more reactive than the initial alkane. The
apparent PCET reaction is also associated with a fairly
large kinetic isotope effect (KIE). Thus, a competitive
oxidation of 1:1 ethylbenzene/ethylbenzene-d10 under
conditions described in Table 2 yielded a KIE = 5.4 ± 0.2.
This relatively high KIE also explains the preference of
reaction at a secondary carbon atom versus a primary
carbon atom. In addition, the relative reactivities, e.g. in
the toluene series, combined with the relatively high KIE
suggest a sequential PCET mechanism where the proton
transfer may be rate limiting.40
n-hexane
2-hexanone (51)
3-hexanone (49)
2-decanone (29)
3-decanone (35)
n-decane
4-&5-decanone
(36)
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Reaction conditions: Substrate (2.0 mmol), BiOX-POM
(250 mg), O2 (2 bar), light – 4 x10 W white LED lamps, room
temperature, 24 h. (a) TON (24 h) was calculated relative to
the amount of H5PV2Mo10O40 considering that each catalytic
cycle involves a 2-electron oxidation.
As indicated above we were additionally interested
how the oxygenation reaction occurs and support of the
mechanistic scheme as presented in Figure 1 and
Scheme 1. Accordingly, the reaction is initiated by
photoactivation of BiOClxBr1-x where hole/electron
separation can be ameliorated by the presence of
H5PV2Mo10O40 as electron acceptor/oxidant. Indeed,
upon illumination of [BiOX-POM] with white light, the
polyoxometalate turns to its classic blue color of the
reduced compound. The isotropic eight-line room
temperature EPR spectrum, Figure S5, is indeed
commensurate with the formation of a 1-electron
The key further issue is the source of the oxygen atom
in the product where, in principle there are three
possibilities. (1) From O2, as is common, and as was
observed in the past in BiOX photocatalyzed
oxygenation in the presence of chloride;29 (2) from
H5PV2Mo10O40 as an oxygen donor which has been
documented for many reactions in the past,30-33 or (3)
from BiOClxBr1-x, which as far as we know, has not been
documented for any BiOX photocatalysts. Oxygen
labelling can also come indirectly from residual H218O
reduced V(IV) species in
octahedral coordination. In the absence of light, no such
species is formed.
a tetragonally distorted
18
Table 3. Gas Phase Photooxidation of Light Alkanes
Catalyzed by BiOX-POM.
used in the preparation of H5PV2Mo10 O40, especially
through exchange reactions once a carbonyl product has
been formed. This was verified by reacting products with
H218O in the presence of [BiOX-POM]. We have tested
the source of oxygen using 18-O labelled O2 and
H5PV2Mo10O40. The large amounts of water needed to
synthesize BiOClxBr1-x made its 18-O labelling cost
prohibitive. Both liquid phase reactions using neat
ethylbenzene as substrate, where residual water is always
present and gas phase reactions using n-butane as
substrate were carried out. In the case of the n-butane to
2-butanone transformation only CH3CH2C(16O)CH3 was
Substrate
Product
Rate
Yield
TONc
µmol/mi
mmol/g
b
na
CH3CH2CH3 CH3COCH3 4.8
13.83
1.73
3300
415
CH3CH3
CH3CHO
0.6
CH4
none
Reaction conditions: Reactor volume – 1.7 cm3; 0.5 g
[BiOX-POM]; alkane/O2 ratio – 0.5; flow rate 3.0 -
standard cm3/min; Ptotal = 5 bar; room temperature; light
– 8 x 10 W white LED lamps; room temperature; 24 h. (a)
steady state rate of formation of product. (b)Yield
assuming constant product formation for 24 h. (c) TON
(24 h) was calculated relative to the amount of
H5PV2Mo10O40 considering that each catalytic cycle
involves a 2-electron oxidation.
In principle, C-H bond activation by BiOX•+ could
proceed either by a proton coupled electron transfer
(PCET) or hydrogen abstraction (HAT) pathway. The
relative reactivity observed for toluene derivatives and
ethane was RCH2OH > RCH3 >> RCHO. For example, in
the toluene series, that is R = Ph, the ionization
formed in the presence of 18O2 (>95% purity) or/and
18
H5PV2Mo10
O
40
(~85% purity). A largely similar result
was observed in the liquid phase oxygenation of
ethylbenzene, where oxidation yielded 1-phenyethanol
and acetophenone with only ~5% 18-O labelled
products. The results show that that the source of the O
atom in the product in the gas phase reaction is neither
from O2 nor from H5PV2Mo10O40 leaving BiOClxBr1-x as the
only possible source of the oxygen atom in the product.
This result additionally suggests that after formation of
the caged as opposed to free alkyl/alkylarene radical an
oxygen transfer reaction from BiOClxBr1-x to the substrate
occurs. Further oxidation from the alcohol to the major
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min, the solvent was removed by rotovapor and the BiOClxBr1-x
2 % wt H5PV2Mo10O40 was dried at RT overnight under high
-
carbonyl
product
occurs
via
an
oxidative
1
2
3
4
5
6
7
8
dehydrogenation reaction without O transfer. The small
amounts of 18-O labelled products in the case of the
liquid phase oxidation reaction are likely due to more
minor secondary pathways: (1) reaction of the benzylic
vacuum.
18O-labeled H5PV2Mo10O40. H5PV2Mo10
O was prepared by
40
18
drying H5PV2Mo10O40•34H2O at 120 °C for 24 h and then adding
50 equiv of H218O (98%) and mixing for 5 h. The drying-
exchange cycle was repeated 3 times. The 18O enrichment was
estimated by deconvolution of the IR spectra assuming a
theoretical 40 cm-1 isotope shift and expected equal peak
intensities for labeled and unlabeled M-O bonds. Total
enrichment excluding the four internal oxygen atoms was ~
95%, ~85% overall.
Liquid phase photocatalytic oxidation reactions. Substrate and
catalysts in the amounts noted in the Tables were placed in 15
mL Pyrex pressure tube under 2 bar O2. The slurry obtained was
stirred for 24 h under irradiation with four cold-white LED lamps
(GU10 LED, 10W, 5500-6500K, 730 lumens, 120⁰ beam spread,
produced by “Spectrum LED”; see Figure S6 for spectrum)
placed on four sides of the pressure tube at a distance of 4 cm.
After reaction the slurry was filtered and the liquid analyzed by
GC-MS for qualitative analysis of products and, GC-FID for
quantitative measurements. A 30 m, 0.32 mm ID 5% cross-
linked phenylmethyl siloxane capillary column with a 0.25 m
film thickness and a split injection mode was used with He as
carrier gas. Liquid phase isotope labelling experiments were
carried out in the same way and analysed for relative m/z by
GC-MS.
radical intermediate with 18O2; (2) oxygen transfer from
18
H5PV2Mo10
O
40
to the benzylic cation [PhCHCH3]+ after
ET oxidation of the intermediate radical as shown in the
past.31
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Conclusions.
The support of H5PV2Mo10O40 polyoxometalate as an
electron acceptor onto
a
BiOClxBr1-x visible light
photoactive semiconductor has enabled apparent
improved charge separation upon photoactivation. The
h+ species formed, initiated a PCET C-H bond activation
of alkylarenes and alkanes as deduced from relative
reactivity combined with the KIE = 5.4 observed in the
oxidation of 1:1 ethylbenzene/ethylbenzene-d10. Instead
of a radical coupling reaction between the alkyl radical
formed and O2, which is typically considered a diffusion-
controlled reaction,41 reactions with 18O2 and
H5PV2Mo10
18
O strongly suggest that oxygenation takes
40
place through oxygen transfer from BiOClxBr1-x. This
novel mechanistic change in reactivity leads to the
conclusion that in the presence of H5PV2Mo10O40,
BiOClxBr1-x acts as a photoactivated oxygen donor, in
what can be generalized as a Mars-van Krevelen type
oxygenation, where BiOClxBr1-x is re-oxidized with O2. The
direct reaction of O2 with C-H bonds was not observed in
the gas phase and was a minor pathway in the liquid
phase. On the more practical side, gas phase reactions
were particularly efficient with high TONs for selective
oxygenation of ethane and propane to acetaldehyde and
acetone, respectively. Methane, however did not react,
possibly because ET or the sequential proton transfer
from methane to the photoactivated BiOClxBr1-x was
energetically unfavorable.
Gas phase photocatalytic reactions. Reactions were carried
out in a sapphire tube reactor (5 mm ID, 85 mm long) rated at
200 bar and loaded with 250 mg BiOClxBr1-x-2 % wt
H5PV2Mo10O40 specific mass flow controllers and a back-
pressure regulator. 1:1 Light alkanes (99.99%) and O2 were
reacted under a total flow of 3.0 - standard cm3/min at Ptotal = 5
bar. The tube was irradiated by eight cold-white LED lamps as
described above, Figure S7. Products were quantified by on-line
sampling using a GC-TCD. The columns used for separation
were two 1/8” ID, 6 foot columns in series packed with
molecular sieves 13X and Hayesep D using He as eluant.
Photocatalytic oxidation of n-butane with 18O2. The sapphire
tube used in the flow system described above was loaded with
250 mg BiOClxBr1-x-2 % wt H5PV2Mo10O40 with 1 bar n-butane
and 3 bar O2, capped and then irradiated for 24 h as described
above. The reaction mixture was sampled by injection in GC-
MS.
Other measurements. Powder XRD Diffraction measurements
were carried out in reflection geometry using an Ultima III
diffractometer equipped with a sealed Cu anode X-ray tube
operating at 40 kV and 40 mA. A bent graphite monochromator
and a scintillation detector were aligned in the diffracted beam.
θ/2θ scans were performed under specular conditions in the
Bragg-Brentano mode with variable slits. The samples were
scanned from 2 to 90 degrees with step size of 0.025 degrees
and scan speed of 1 degree per minute. EM analysis was
performed using a Gemini SEM 500 operating at 15 kV. EDX
measurements were performed using the same instrument,
equipped with an EDX detector from Bruker (60mm), at 20 kV. A
sample of BiOClxBr1-x-2 % wt H5PV2Mo10O40 was irradiated
under Ar for 30 min in an EPR capillary tube and analyzed at RT
by a X-band CW EPR using a Bruker ELEXSYS 500 spectrometer
equipped with a Bruker ER4102ST resonator at 20 K with
microwave power of 20 mW, 0.1 mT modulation amplitude and
Experimental Section.
Preparation of BiOClxBr1-x: Synthesis of BiOClxBr1-x (x=~0.875)
was carried out according to a literature method.26 Deionized
water (85 mL), glacial acetic acid (45 mL), and bismuth nitrate
(14.69 g, 30 mmol) were placed into a 250 mL flask and stirred
at room temperature for 15 min until a clear, transparent
solution is formed. Then hexadecyltrimethyl ammonium
bromide (1.378 g dissolved in 10 mL of water, 3.5 mmol) and
hexadecyltrimethyl ammonium chloride (2.12 g dissolved in
6.36 mL of water, 6.6 mmol) were added to the above solution
in one batch, and the mixture is stirred for an additional 30 min
at room temperature. The precipitate formed was filtered and
washed five times with hot ethanol (300 mL) and five times with
boiling water (1L) to remove the unreacted quaternary
ammonium salts and then dried in air.
Preparation of BiOClxBr1-x-2 % wt H5PV2Mo10O40:
H5PV2Mo10O40•34H2O (20 mg) prepared by a literature
procedure,42 was dissolved in 5 mL of CH3CN. BiOClxBr1-x (980
mg) was added as a fine powder; the slurry was stirred for 30
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Page 6 of 8
100 kHz modulation frequency. 31P NMR were measured on a
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AUTHOR INFORMATION
Corresponding Author
* Email: ronny.neumann@weizmann.ac.il
ORCID
Ronny Neumann: 0000-0002-5530-1287
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Author Contributions
12. Wada, K.; Yamada, H.; Watanabe,Y.; Mitsudo, T. Selective
Photo-Assisted Catalytic Oxidation of Methane and Ethane to
Oxygenates using Supported Vanadium Oxide Catalysts. J.
Chem. Soc. Faraday Trans. 1998, 94, 1771-1778.
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into an Efficient Visible Light Photocatalyst for Selective
Oxidation of Saturated Primary C–H Bonds under Ambient
Conditions. Chem. Sci. 2012, 3, 2812-2822.
14. Ide, Y.; Iwata, M.; Yagenji, Y.; Tsunoji, N.; Sohmiya, M.;
Komaguchi, K.; Sano T.; Sugahara, Y. Fe Oxide Nanoparticles/Ti-
Modified Mesoporous Silica as a Photo-Catalyst for Efficient
and Selective Cyclohexane Conversion with O2 and Solar Light.
J. Mater. Chemistry A, 2016, 41, 15829-15835.
15. Tzirakis, D. M.; Lykakis, I .N.; Orfanopoulos, M.
Decatungstate as an Efficient Photocatalyst in Organic
Chemistry. Chem. Soc. Rev. 2009, 38, 2609–2621.
16. Ravelli, D.; Fagnoni, M.; Fukuyama, T.; Nishikawa, T.; Ryu, I.
Site-Selective C-H Functionalization by Decatungstate Anion
Photocatalysis: Synergistic Control by Polar and Steric Effects
Expands the Reaction Scope. ACS Catal. 2018, 8, 701-713.
17. Liu, Y.; Chen, L.; Yuan, Q.; He, J.; Au, C.-T.; Yin, S.-F. A
Green and Eficient Photocatalytic Route for the Highly-Selective
Oxidation of Saturated Alpha-Carbon C–H Bonds in Aromatic
Alkanes over Flower-Like Bi2WO6, Chem. Commun. 2016, 52,
1274-1277.
18. Abdi, F. F.; Han, L.; Smets, H. M.; Zeman, M.; Dam, B.; van
de Krol, R. Efficient Solar Water Splitting by Enhanced Charge
Separation in a Bismuth Vanadate-Silicon Tandem
Photoelectrode. Nature Commun. 2013, 4, 2195-2202.
19. Cao, X.; Chen, Z.; Lin, R.; Weng-Chon Cheong, W.-C.; Liu,
S.; Zhang, J.; Peng, Q.; Chen, C.; Han, T.; Tong, X.; Wang, Y.; Shen,
R.; Zhu, W.; Wan, D.; Li, Y. A Photochromic Composite with
Enhanced Carrier Separation for the Photocatalytic Activation of
Benzylic C–H bonds in Toluene. Nature Catal. 2018, 1, 704-710.
20. Di, J.; Xia, J.; Li, H.; Guo, S.; Dai, S. Bismuth Oxyhalide
Layered Materials for Energy and Environmental Applications.
Nano Energy 2017, 41, 172-192.
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
Notes
The authors note no competing financial interest
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on
the ACS Publications website.
Electron microscope images, powder XRD measurements,
and EPR and 31P NMR spectra, and other data (PDF).
ACKNOWLEDGMENT
This research was supported by the Israel Science
Foundation grants 2046/14 and 1237/18. Srinivasa Rao
Amanchi is thanked for his input on this topic. Tong Bian is
thanked for his help in EM measurements. R.N. is the
Rebecca and Israel Sieff Professor of Organic Chemistry.
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