Full Papers
doi.org/10.1002/cctc.202002037
ChemCatChem
it thereby performed well in the next four recycling operations,
revealing the same activity level as the fresh catalyst (See
Figure 20). The excellent renewability of 1%Mo–DT is of special
significance for the industrial production of KA oil.
solution as an additive. iii) The excellent renewability of 1%Mo–
DT in the recycle experiment of photo-catalyzing cyclohexane
oxidation is of special significance for the industrial production
of KA oil. Having these interesting findings, we will be
interested to exploit more attractive HE strategy for the DTs-
based catalytic systems substituted or doped by other transition
metals.
Discussion: A generally accepted mechanism for DT-based
photocatalysis system involves light irradiation (mainly UV light)
upon the ground state of DT anion, causing an oxygen to metal
charge-transfer to yield the excited state (W10O324À *) that decays
in ca. 30 ps to a very reactive transient species (wO).[41] Actually,
the wO species having a longer lifetime[42,43] shows highly Experimental Section
strong capacity to yield its one electron reduced state and a
carbon centered radical species when oxidizing various organic
compounds.[1,42] The following re-oxidation of the reduced DT
to its starting state (catalysis cycling) and the formation of the
oxygenated products can be achieved under the participation
of O2.[44–49] The incorporating Mo, as supported by our present
studies, plays key roles in improving the redox capacity and
visible light harvesting efficiency of DT anion, especially in
enhancing the photo-excited quantum efficiency (probability
and lifetime). As a result, the visible-light-catalytic performance
of Mo–DT can be improved significantly. Notably, the contribu-
tion of water and HCl (especially under their coexistence) to the
present photocatalysis should mainly originate from their
improved effect on the photo-excited quantum efficiency of DT.
Also, water improves the photocatalysis by inhibiting the
photo-degradation of DT and enhancing its redox capacity,
respectively.[16]
Materials
Main reagents used in this work were of analytical grade, which
included sodium tungstate dihydrate (Na2WO4 2H2O), concentrated
HCl, H2SO4, and H3PO4, acetic acid (HAc), benzenesulfonic acid,
sodium molybdate dihydrate (Na2MoO4 2H2O), cyclohexane,
toluene, ethylbenzene, benzyl alcohol, ethyl acetate, acetonitrile
(MeCN). Distilled water was used throughout this experiment.
.
.
Synthesis of molybdenum substituted decatungstates (Mo–
DT)
The preparation of the special DTs replaced by molybdenum atom
is described as follows: 3.2 g (9.7 mmol) Na2WO4 ·2H2O and a range
of different amount of sodium molybdate (Na2MoO4 ·2H2O) were
dissolved with 20 mL of water in a three-necked flask (50 mL),
followed by the addition of 3 M HCl aqueous solution at 25 C,
reducing the pH to 2.05-2.10. After the mixture was heated at 85 C
for 5–8 min, an aqueous solution of tetrabutylammonium bromide
(2 M, 2 mL) was joined to the mixture slowly under continuous
stirring with stable pH value, and the tender yellow solid
precipitated from the reaction solution. Then the mixture was
reacting at 90 C for 30 min. After cooling, the precipitate was
filtered, washed with water (10 mL×3 mL) and acetone (10 mL×
°
°
Conclusions
°
To summarize, for the first time we have developed a novel
hybridizing engineering (HE) strategy for the synthesis of Mo
atoms-isomorphously substituted DTs (Mo–DTs) with the
following merits: i) This HE strategy is very simple and
productive and can finely regulate the redox capacity and
photo-physicochemical property of DT at molecular level by
adjusting the incorporated Mo atom content. ii) The newly
constructed Mo–DTs have significantly improved photo-cata-
lytic performance in visible light-triggered selective oxidation of
inert hydrocarbons with O2 in MeCN, especially with 2 M HCl
°
3 mL), and then dried at 80 C under vacuum(yield, 80–85%). After
evaluation experiments, the best Mo–DTs catalyst whose Mo and W
contents measured by ICP-AES analysis were 0.0283% and 54.35%
was obtained respectively, which were basically accordant with the
theoretical contents, written as 1%Mo–DTs ((C4H9)4N4W9.99Mo0.01O32).
Characterization of catalysts
X-ray photoelectron spectroscopy (XPS) of the samples was
measured on a VG Multi Lab 2000 system with a monochromatic
MgÀ Kα source operated at 20 kV. Powder X-ray diffraction (XRD) of
the samples was conducted on a Rigaku 2550 X-ray diffractometer
using Cu Kα radiation (λ=0.15406 nm) and a graphite monochro-
mator, Liquid UV-vis spectra of the samples in MeCN were recorded
from 200 to 800 nm on UV-2450 spectrophotometer (Shimadzu,
Japan) and their UV/Vis diffuse reflectance spectra (DRS) were
recorded on U-3310 spectrophotometer (HITACHI). Their trans-
mission FT-IR spectra recorded from 400 to 4000 cmÀ 1 on a Nicolet
Nexus 510 P FT-IR spectroscopy using a KBr disk. Photo-lumines-
cence (PL) measurements of the samples in MeCN were carried out
on fluorescence spectrophotometer (HITACHI F-7000) at room
temperature. The metal contents of the sample were measured by
inductively coupled plasma-optical emission spectroscopy (ICP-
OES) on Optima 5300DV. Transient absorption (TA) spectra were
tested on transient spectroscopy instruments (Czerny-Turner) under
anaerobic system solution.
Figure 20. Recyclability of the regenerated 1%Mo–DT in photo- catalyzing
cyclohexane oxidation.
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