Sol-gel entrapped visible light photocatalysts for selective conversions

Article abstract of DOI:10.1039/c4ra01031k

A general method to heterogenize, stabilize and significantly enhance the activity of selective visible light photocatalysts for aerobic oxidation of glycerol is devised. The new method makes use of the sol-gel encapsulation of photocatalytic species in silica-based matrices to afford leach-proof catalysts capable to afford valued products at low cost with little or no waste generation. In particular, the photocatalytic selective conversion of glycerol is performed in water solvent under ambient conditions with molecular oxygen as oxidant and visible light as the driving energy source, thereby meeting all requirements for a green chemical process. Entrapment of the semiconductor-based photocatalyst inside the transparent porous silica matrix enhances substrate adsorption, thus improving its local concentration and favoring reaction with the photogenerated active species.

Full text of DOI:10.1039/c4ra01031k

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Sol-gel entrapped visible light photocatalysts for
selective conversions
Cite this: RSC Adv., 2014, 4, 18341
a
b
c
a
*
Yanhui Zhang, Rosaria Ciriminna, Giovanni Palmisano, Yi-Jun Xu
b
*
and Mario Pagliaro
A general method to heterogenize, stabilize and significantly enhance the activity of selective visible light
photocatalysts for aerobic oxidation of glycerol is devised. The new method makes use of the sol–gel
encapsulation of photocatalytic species in silica-based matrices to afford leach-proof catalysts capable
to afford valued products at low cost with little or no waste generation. In particular, the photocatalytic
selective conversion of glycerol is performed in water solvent under ambient conditions with molecular
oxygen as oxidant and visible light as the driving energy source, thereby meeting all requirements for a
green chemical process. Entrapment of the semiconductor-based photocatalyst inside the transparent
porous silica matrix enhances substrate adsorption, thus improving its local concentration and favoring
reaction with the photogenerated active species.
Received 5th February 2014
Accepted 4th April 2014
DOI: 10.1039/c4ra01031k
www.rsc.org/advances
Semiconductor-based photocatalysis, especially using the semi- the efficiency and selectivity of photocatalytic processes.7,8
conductor TiO2, is generally a non-selective process suitable for Hence, in principle, entrapment of photocatalysts in trans-
degradation of organics, rather than for organic synthesis,1 even parent SiO2 glasses might be desirable to stabilize, recover and
though some progress towards more selective photosynthetic reuse the entrapped photocatalyst. In particular, the entrap-
processes using modied titania has been achieved.2 Further ment within sol–gel silica can largely increase the surface area
progress on semiconductor-based photocatalysis highlights that of catalyst, which would increase the adsorption capacity of
the appropriate choice of semiconductors (e.g., tuning the catalyst toward reactant inside the sol–gel silica cages that
structure, morphology and interface composition) and optimi- consequently could contribute to the enhanced photoactivity.
zation of reaction parameters are able to promote the occurrence Yet, so far, only a few reports exist concerning sol–gel entrapped
of photocatalytic organic synthesis with high selectivity.3
Recently, in the context of this progress, ower-like Bi2WO6 glasses has been developed since the late 1980s.7
has been identied as a highly selective visible light photo- Two typical examples of entrapped photocatalysts are sodium
photocatalysts, even if a broad photochemistry of sol–gel doped
catalyst toward aerobic selective oxidation of glycerol to dihy- decatungstate supported on sol–gel silica employed in the oxida-
droxyacetone (DHA),4 using oxygen as oxidant in water at room tion of glycerol under UV light irradiation;8 and silica-entrapped
temperature and atmospheric pressure. Besides meeting all H4SiW12O40 UV light photocatalyst used for the aerobic oxidation
requirements for a truly green chemical process, the method of various primary and secondary benzylic alcohols into the cor-
suggests the promising potential applications of semiconductor responding carbonyl compounds.9 However, to the best of our
Bi2WO6-based photocatalysis in the eld of biomass conversion knowledge, no encapsulation of visible light photocatalyst has
to value-added ne chemicals under mild conditions.3,5
been previously reported for selective oxidation of glycerol. To
Sol–gel silica is transparent to both visible and UV light develop highly photoactive visible-light-driven semiconductor-
radiation, and the sol–gel encapsulation is known to chemically based photocatalyst is far more desirable than that of UV light
and physically stabilize the entrapped species.6 Such a hetero- photocatalyst, from a viewpoint of efficient and sustainable use of
genization strategy via sol–gel silica encapsulation provides a solar energy for practical applications. Herein, we report a simple
tunable control of microscopically conned environment and feasible method to directly transform ower-like Bi2WO6 into
surrounding the entrapped photoactive species, thereby tuning an entrapped sol–gel catalyst with enhanced photoactivity,
retaining the high selectivity. The resulting silica-entrapped
Bi2WO6 can be used as visible light photocatalyst for the aerobic
selective conversion of glycerol to related products, such as
dihydroxyacetone and glyceraldehyde, in water under ambient
conditions. The as-obtained results are not trivial because reac-
tivity, compared to the non-entrapped photocatalyst, in general is
altered and can be greatly enhanced.
aState Key Laboratory of Photocatalysis on Energy and Environment, College of
Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, P.R.
China. E-mail: yjxu@fzu.edu.cn
bIstituto per lo Studio dei Materiali Nanostrutturati, CNR, via U. La Malfa 153, 90146
Palermo, Italy. E-mail: mario.pagliaro@cnr.it
c
`
Dipartimento di Fisica e Chimica, Universita degli Studi di Palermo, viale delle
Scienze, 90128 Palermo, Italy
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Data in Table 1 show that the two silica-entrapped Bi2WO6
catalysts are highly active, despite with remarkable differences
in selectivity. Results are indeed impressive because now 8 mg
of entrapped catalyst are enough to catalyze the reaction at
noticeable extent. We are dealing indeed with 8 mg of silica-
entrapped catalysts whose content of visible-light-photoactive
bismuth tungstate (Bi2WO6) varies between 5 and 10% in
weight. Hence, even using 1/20 and 1/10 the amount of 8 mg of
non-entrapped Bi2WO6 used in previous work,4 the conversion
of glycerol over silica-entrapped Bi2WO6(1) and Bi2WO6(2)
proceeds up to 41% and 58%, respectively, under visible light
irradiation for 10 h.
Fig. 1 XRD patterns of the samples of silica-entrapped Bi2WO6.
A direct comparison of glycerol conversion over 8 mg of
silica-entrapped Bi2WO6(2) (10% w/w catalyst loading, the
photoactive Bi2WO6 is 0.8 mg) and 0.8 mg of non-entrapped
The SEM pictures of the silica-entrapped Bi2WO6 samples, as
Bi2WO6 [Bi2WO6(2)] under visible light irradiation of 4 h clearly shown in Fig. 2, suggest that, due to the encapsulation of silica,
shows that the photoactivity for glycerol conversion over silica- the ower-like structure of non-entrapped Bi2WO6 cannot be
entrapped Bi2WO6(2) is indeed greatly enhanced. That is, under directly observed. In its place, matrix particles made of amor-
visible light irradiation for 4 h using one tenth the amount of phous silica are clearly observed. The ground granules obtained
non-entrapped Bi2WO6, the conversion of glycerol reaches 29%, from grinding a sol–gel monolith, furthermore, have the irreg-
which is more than 3 times higher than that over non-entrapped ular shape, angular geometry and multiple cracks, which are
Bi2WO6.4
In addition, it is worth noting that the catalyst of higher
typical of silica xerogel particles.10
Fig. 3 shows the TEM images of sol–gel entrapped Bi2WO6(2)
loading Bi2WO6(2) retains the original high selectivity to DHA of sample. It can be seen that, during the entrapping process, the
the non-entrapped Bi2WO6 photocatalyst.4 However, this is not ower-like superstructure of original Bi2WO6 is destroyed.
the case for the silica-entrapped catalyst Bi2WO6(1) with lower However, the basic unit comprising superstructure of ower-
loading of Bi2WO6. This means that the loading amount of like Bi2WO6 is still retained.4 Also, the silica encapsulating shell
bismuth wolframate entrapped within silica alters not only the for sol–gel entrapped Bi2WO6(2) sample can be clearly distin-
photoactivity but also the selectivity of the entrapped catalyst guished. The bottom image shows the outcome of electron
(see below for our tentative explanation of this nding).
diffraction of regions, from which it is seen that the crystalline
The XRD patterns in Fig. 1 show that crystalline, well char- structure of Bi2WO6 is clearly retained upon the sol–gel encap-
acterized Bi2WO6 crystals are encapsulated within the silica sulation, where the sol–gel silica shows its amorphous nature.
xerogels in both cases of 5% w/w and 10% w/w catalyst loading,
suggesting that the silica encapsulation does not destroy the reectance spectra (DRS) of the samples of two entrapped
crystalline phase structure of Bi2WO6 (JCPDS no. 73-1126).
catalysts in Fig. 4, clearly show that the materials can be band-
The optical properties, as reected by the UV-visible diffuse
Table 1 Photocatalytic selective oxidation of glycerol over non-entrapped Bi2WO6 and over silica-entrapped Bi2WO6 catalyst under the irra-
diation of visible light in watera
Selectivity (%)
Conversion (%)
Photocatalyst
Bi2WO6(1)
Time (h)
glycerol
Dihydroxyacetone
Glyceraldehyde
Others
2
4
6
8
10
2
4
2
4
6
11
20
28
35
41
3
63
62
58
55
53
95
93
97
95
94
92
91
95
93
28
27
25
24
24
5
7
3
5
5
9
11
17
21
23
0
0
0
0
1
Non-entrapped Bi2WO6(1)
Bi2WO6(2)
5
16
29
40
50
58
6
8
10
2
6
6
5
7
2
3
0
0
Non-entrapped Bi2WO6(2)
4
9
a
Reaction conditions: 0.1 mmol glycerol, 1.5 mL H2O, T ¼ 25 ꢀC, visible light >420 nm. For purposeful comparison, the amount of non-entrapped
Bi2WO6 used for photocatalytic oxidation of glycerol is the same as that of Bi2WO6 in silica-entrapped Bi2WO6 namely 0.8 mg for Bi2WO6(2) and
0.4 mg for Bi2WO6(1).
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Fig. 2 Typical SEM pictures of the samples of silica-entrapped Bi2WO6(1) (left) and Bi2WO6(2) (right).
Fig. 3 Top: TEM pictures of the samples of silica-entrapped Bi2WO6(2). Bottom: images showing the outcome of electron diffraction of different
sample regions.
gap-photoexcited by visible light irradiation, but at different that of Bi2WO6(1), thus suggesting the longer lifetime of pho-
extent because of their difference in optical absorption. As togenerated charge carriers. Thus, the presence of different
expected, due to the higher loading of photoactive Bi2WO6 for amounts of SiO2 affects the PL intensity for SiO2-entrapped
silica-entrapped Bi2WO6(2), the latter catalyst sample has Bi2WO6, thereby contributing to the photoactivity difference
stronger light absorption intensity than Bi2WO6(1) in both the between two different samples, i.e., silica-entrapped Bi2WO6(1)
UV and visible light regions.
and silica-entrapped Bi2WO6(2).
The photoluminescence (PL) spectra are oen employed to
Notably, the photoluminescence (PL) spectra in Fig. 5 clearly
study surface processes involving the electron–hole fate of the show that: (i) the PL intensity of silica-entrapped Bi2WO6 is
semiconductor. Following the electron–hole pairs recombina- much lower than that of non-entrapped Bi2WO6, indicating that
tion aer a photocatalyst is irradiated, photons are emitted, the sol–gel silica encapsulation contributes to the improved
which results in PL signal. As shown in Fig. 5, the PL intensity separation of photogenerated electron–hole pairs of Bi2WO6
obtained over silica-entrapped Bi2WO6(2) is clearly weaker than under visible light irradiation; (ii) the connement of Bi2WO6
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Table 2 shows the data based on the analysis of N2 adsorp-
tion–desorption isotherms of silica-entrapped catalyst Bi2WO6(1)
and Bi2WO6(2), and non-entrapped Bi2WO6. Bi2WO6(1) has huge
specic surface area (ca. 964 m2 gꢁ1) along with signicant
specic pore volume (ca. 0.56 cm3 gꢁ1). For the higher loading
sample, i.e., silica-entrapped Bi2WO6(2), the surface area is
reduced to ca. 517 m2 gꢁ1 and the pore volume is ca. 0.30 cm3 gꢁ1
.
Comparison to non-entrapped Bi2WO6, with low surface area of
ca. 31 m2 gꢁ1 and pore volume of ca. 0.16 cm3 gꢁ1, clearly shows
that the sol–gel entrapment signicantly increases the surface
area of the samples.
Fig. 4 The UV-visible diffuse reflectance spectra (DRS) of the samples
of silica-entrapped Bi2WO6.
Adsorption experiments in the dark shown in Fig. 6 indeed
conrm that samples of silica-entrapped Bi2WO6 have
enhanced adsorption capacity toward glycerol as compared to
the non-entrapped Bi2WO6.4 Such an enhanced adsorption
capacity reminds one that glycerol can be easily adsorbed and
concentrated at the surface of the silica sol–gel cages, with their
numerous silanol groups. The very same cages entrap the
photocatalytic crystallites and act as chemical sponges that
adsorb and concentrate reactants at their surface.11 The adsor-
bed glycerol molecules further react at the surface of entrapped
Bi2WO6 to be oxidized by the positive holes to form the corre-
sponding intermediate, which further reacts with oxygen or
activated oxygen to produce DHA and other related products.4
However, it should be noted that the surface area is not the
only factor that determines the overall photoactivity and selec-
tivity in the photocatalytic oxidation of glycerol. The sample of
silica-entrapped Bi2WO6(1) has much higher surface area and
pore volume than silica-entrapped catalyst Bi2WO6(2). But the
photoactivity of Bi2WO6(1) is lower than Bi2WO6(2). This can be
attributed to the higher amount loading of photoactive Bi2WO6
photoactive species in the silica-entrapped Bi2WO6(2), which
has a higher visible light absorption capacity and longer life-
time of photogenerated electron–hole pairs, that are the crucial
factors determining the overall efficiency of photocatalytic
process.
Fig. 5 The photoluminescence (PL) spectra of the samples of silica-
entrapped Bi2WO6 and non-entrapped Bi2WO6 with an excitation
wavelength of 340 nm.
nanocrystals resulting from sol–gel silica encapsulation leads to
the signicantly different PL emission proles, suggesting that
the synergistic interaction between silica and Bi2WO6 inu-
ences the spatial transfer kinetics (in addition to improving the
lifetime of photogenerated electron–hole pairs).
Together, these factors already mentioned above may
account for the difference in both photoactivity and selectivity
in the aerobic oxidation of glycerol over silica-entrapped
Bi2WO6 with different amounts of silica and non-entrapped
Bi2WO6. Under visible light irradiation, holes and electrons
generated inside the Bi2WO6 crystalline phase diffuse to the
surface of the inner porosity of the silica cages, where they can
react with the incoming organic molecules. In brief, the
combined effects of semiconductor connement in the silica
cages6,8–10 and synergistic interaction of Bi2WO6 with the SiO2
cage play a crucial role in affecting the selectivity and activity for
oxidation of glycerol.
In addition, as discussed above, with the variation of loading
amount of photoactive Bi2WO6 ingredients in the silica matrix,
Table
2 Textural properties of non-entrapped Bi2WO6, silica-
entrapped Bi2WO6(1) and Bi2WO6(2) based on the N2 adsorption–
desorption isotherms
SBET
Total pore volume Average pore
Sample
(m2 gꢁ1
)
(cm3 gꢁ1
)
size (nm)
Non-entrapped Bi2WO6
Bi2WO6(1)
Bi2WO6(2)
31
964
517
0.16
0.56
0.30
5 and 42
2
2
Fig. 6 Remaining fraction of glycerol after the adsorption–desorption
equilibrium in the dark over silica-entrapped Bi2WO6(1), silica-
entrapped Bi2WO6(2), and non-entrapped flower-like Bi2WO6.
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the selectivity is also affected, suggesting that a synergy inter-
Two sol–gel entrapped catalysts were prepared using TEOS
action between the silica matrix and photoactive Bi2WO6 exists (tetraethylorthosilicate, Si(OCH3)4) as silica precursor. In one
in the silica-entrapped Bi2WO6 photocatalysts. Silanols at the typical preparation, 150 mg of ower-like Bi2WO6 was sus-
surface of the sol–gel silica cages favour adsorption of glycerol, pended in 1 mL H2O and the suspension was kept stirring for
enhancing its local concentration and thus easing its reaction 30 minutes. The required amount of TEOS (0.025 mol for the
with photogenerated active charge carriers including activated 10 wt% catalyst and 0.05 mol for the 5 wt% catalyst) was added
molecular oxygen at the cage surface. However, when an to an aqueous solution of HCl 0.05 N and stirred for 30 minutes.
insufficient amount of photoactive Bi2WO6 is encapsulated, the Aer this pre-hydrolysis step, the suspension of Bi2WO6 was
glycerol molecules are preferentially adsorbed through their added and the resulting mixture was kept under mechanical
more reactive and less hindered primary hydroxyl groups, stirring for 30 minutes, aer which an aliquot of NaOH 1 N
thereby affording considerable formation of glyceraldehyde (0.1 mL for 10 wt% and 0.18 mL for 5 wt%) was added. Gel-
(Table 1). In contrast, when a sufficient amount of encapsulated ication rapidly occurred.
Bi2WO6 is available for reactivity at the sol–gel cages, glycerol is
The resulting alcogel was le in a closed vessel for 24 h aer
again preferentially oxidised at the secondary OH group, which the vessel was opened and le for 3 days at room
showing the remarkable effect of both the encapsulating silica temperature and pressure (ambient conditions). The doped
matrix and loading of photoactive Bi2WO6 species on selectivity silica glass monolith thereby obtained was mildly powdered in a
toward selective photocatalytic oxidation of glycerol. This effect, mortar and used as such. The sol–gel encapsulated catalyst
we argue, is due to the templating action of the dopant species sample Bi2WO6(1) has a 5% w/w catalyst loading, and Bi2WO6(2)
during the sol–gel hydrolytic polycondensation, in which the has a 10% w/w catalyst loading. To check the materials synthesis
dopant species dictate the size and the shape of the encapsu- reproducibility, similar catalysts (not shown) were prepared also
lating silica cage in the nal xerogel.12
in China using the same methodology. Almost identical results
In conclusion, we have shown that the sol–gel encapsulation were obtained both for the materials and their synthetic appli-
of ower-like Bi2WO6 microparticles in SiO2 xerogels leads to cation, conrming full reproducibility of both sol–gel materials
the formation of excellent visible-light-driven photocatalytic synthesis and hydrothermal synthesis of ower-like Bi2WO6.
materials with great potential to act as solar energy transfer
The morphology information was determined by a eld-
materials. The transparent, stabilizing and high surface area of emission scanning electron microscope (FESEM, FEI Nova
the encapsulated silica matrix allows light penetration, NANOSEM 230). Transmission electron microscopy (TEM)
promoting the efficiency of the photogenerated electron–hole images were collected using a JEOL model JEM 2010 EX micro-
separation and enhancing adsorption capacity toward glycerol. scope at an accelerating voltage of 200 kV. The crystalline struc-
As a result, the photoactivity in the aerobic selective oxidation of ture of the catalysts was determined by powder X-ray diffraction
glycerol under visible light irradiation over silica-entrapped (XRD), using Ni-ltered Cu Ka radiation in the 2q range from 5ꢀ
Bi2WO6 is greatly enhanced. The synergy between the silica to 80ꢀ with a scan rate of 0.08ꢀ per second. The optical properties
matrix and photoactive entrapped Bi2WO6 allows the tunable of the catalysts were analyzed by UV-vis diffuse reectance spec-
control of both activity and selectivity for photocatalytic oxida- troscopy (DRS) using a Cary-500 spectrophotometer over a
tion of glycerol under visible light irradiation.
wavelength range of 230–800 nm, during which BaSO4 was
Glycerol demonstrated herein is just one example of a employed as the internal reectance standard. Nitrogen adsorp-
biomass-derived chemical suitable for chemical upgrade.13,14 tion–desorption isotherms and the Brunauer–Emmett–Teller
Many other biomass molecules could react, including organic (BET) surface area were collected at 77 K using Micromeritics
molecules that are not water soluble, and be readily converted to ASAP 2010 equipment. The photoluminescence (PL) spectra were
value-added ne chemicals using this SiO2 xerogels trapped measured on an Edinburgh FL/FS900 spectrophotometer. For the
semiconductor composite photocatalyst. Further work is in PL analysis of solid samples of Bi2WO6, the excitation wavelength
progress to expand the scope of this simple and feasible method. is 340 nm. The PL spectra are oen employed to study surface
processes involving the electron–hole fate of semiconductor.
With the electron–hole pairs recombination aer a semi-
Experimental section
conductor photocatalyst is irradiated, photons are emitted, thus
The catalyst hydrothermal sol–gel synthesis starts from ower- resulting in PL. Thus, the PL analysis reects the fate of electron–
like Bi2WO6 prepared following the published procedure.4 In hole pairs photogenerated from semiconductor Bi2WO6 encap-
detail, the simple hydrothermal synthesis starts with sonication sulated into the sol–gel silica matrix.
of Bi(NO3)$5H2O (0.98 g) in 40 mL of 0.3 M HNO3 aqueous
The aerobic oxidation of glycerol in water was carried out in a
solution. Then, 20 mL of 0.05 M Na2WO4$2H2O solution was 10 mL Pyrex glass bottle under the irradiation of visible light
added with vigorous stirring and a white precipitate was using a 300 W Xe arc lamp with a UV cutoff lter (l > 420 nm). In
formed. Subsequently, a NaOH solution (20 mL, 0.2 M) was a typical process, a mixture of 8 mg solid catalyst and 0.1 mmol
added under stirring, mixing the resulting solution for 24 h. of glycerol were added to 1.5 mL water. The mixture, saturated
This mixture was then transferred to a 100 mL Parr autoclave with pure molecular oxygen from a gas cylinder, was transferred
and maintained at 160 ꢀC for 8 h. The resulting sample was into a 10 mL Pyrex glass bottle and stirred for 10 min. Aer the
recovered by ltration, washed by water, and fully dried at 60 ꢀC reaction, the mixture was centrifuged at 12 000 rpm for 20
in oven to get the nal ower-like Bi2WO6 samples.
minutes to separate and remove the catalyst particles. The
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remaining solution was analyzed with a Shimadzu Liquid
Chromatograph (DGU-20A3, equipped with a 512 Diode Array
Detector and a C18 analysis column).
Chem., Int. Ed., 2012, 51, 4062; (d) Y. Zhang, N. Zhang,
Z.-R. Tang and Y.-J. Xu, ACS Nano, 2012, 6, 9777; (e)
B. Weng, S. Liu, Z.-R. Tang and Y.-J. Xu, RSC Adv., 2014, 4,
12865.
4 Y. Zhang, N. Zhang, Z.-R. Tang and Y.-J. Xu, Chem. Sci., 2013,
4, 1820.
Acknowledgements
This article is dedicated to University of Palermo's Professor
Giuseppe Aiello for his excellent teaching of physics at
chemistry students spanning his lifetime career. We thank
5 C.-H. Zhou, X. Xia, C.-X. Lin, D.-S. Tong and J. Beltramini,
Chem. Soc. Rev., 2011, 40, 5588.
6 M. Pagliaro, R. Ciriminna and G. Palmisano, Chem. Soc. Rev.,
2007, 36, 932.
`
PhD student Piera Demma Cara for preparing preliminary
catalytic samples. The support by the NSFC (21173045,
20903023), the Award Program for Minjiang Scholar Profes-
sorship, the NSF of Fujian Province for Distinguished Young
Investigator Grant (2012J06003), and Program for Returned
High-Level Overseas Chinese Scholars of Fujian Province is
gratefully acknowledged.
7 (a) A. Slama-Schwok, D. Avnir and M. Ottolenghi, Photochem.
Photobiol., 1991, 54, 525; (b) B. Dunn and J. Zink, J. Mater.
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8 A. Molinari, A. Maldotti, A. Bratovcic and G. Magnacca,
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10 R. Ciriminna, A. Fidalgo, F. Beland, V. Pandarus,
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