Photocatalytic conversion of glucose in aqueous suspensions of heteropolyacid-TiO2 composites

Article abstract of DOI:10.1039/c5ra09894g

Commercial and home prepared TiO₂ samples were functionalized with a commercial Keggin heteropolyacid (HPA) H ₃PW₁₂O ₄₀ (PW₁₂) or with a hydrothermally home prepared K ₇PW₁₁O ₃₉ salt (PW₁₁). All the materials were characterized by specific surface area measurements (BET), XRD analyses, Raman, DRS along with SEM observations and they have been used for glucose photocatalytic conversion in an aqueous suspension. Different reaction extents and distribution of intermediate oxidation products were observed depending on the photocatalyst. Gluconic acid, arabinose, erythrose and formic acid were observed as oxidation products when bare TiO₂ or HPA/TiO₂ composite materials were used. Glucose isomerization to form fructose was also observed and in some runs traces of glucaric acid and glyceraldehyde were also found. The carbon mass balance was accomplished in the presence of the commercial Evonik P25 TiO₂ powder and the composites where TiO₂ was present, whereas the presence of the solvothermically prepared TiO₂ gave rise to a carbon unbalance, due to strong adsorption of the products on the photocatalyst surface. No reactivity was observed in the presence of PW₁₂ alone while PW₁₁ induced only isomerization of the glucose.

Full text of DOI:10.1039/c5ra09894g

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ARTICLE
Photocatalytic conversion of glucose in aqueous suspensions of
heteropolyacid-TiO composites
2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
M. Bellardita , E. I. García-López *, G. Marcì , B. Megna ,
a
a
DOI: 10.1039/x0xx00000x
F. R. Pomilla and L. Palmisano
www.rsc.org/
Commercial and home prepared TiO
2
samples were functionalized with a commercial Keggin heteropolyacid (HPA)
H
3
PW12
O
40 (PW12) or with a hydrothermally home prepared K PW11O
7
39 salt (PW11). All the materials were characterized by
specific surface area measurements (BET), XRD analyses, Raman, DRS along with SEM observations and they have been
used for glucose photocatalytic conversion in aqueous suspension. Different reaction extent and distribution of
intermediate oxidation products were observed depending on the photocatalyst. Gluconic acid, arabinose, erythrose and
formic acid were observed as oxidation products when bare TiO
isomerization to form fructose was also observed and in some runs traces of glucaric acid and glyceraldehyde were also
found. The carbon mass balance was accomplished in the presence of the commercial Evonik P25 TiO powder and the
composites where TiO was present, whereas the presence of the solvothermically prepared TiO gave rise to a carbon
2 2
or HPA/TiO composite materials were used. Glucose
2
2
2
unbalance, due to strong adsorption of the products on the photocatalyst surface. No reactivity was observed in the
presence of PW12 alone while PW11 induced only isomerization of the glucose.
biodegradable chelating agent, a water soluble cleansing agent and
an intermediate in food and pharmaceutical industries [8]. It is
currently industrially prepared by fermentation of glucose by
1
. Introduction
The search of alternative resources for synthesis of chemicals
currently produced from non-renewable sources has directed the
activities of researchers towards the use of different raw materials
such as biomass [1]. It seems particularly interesting the use of
lignocellulose (cellulose, hemi-cellulose and lignin) which can derive
from agricultural wastes. Glucose, obtained from cellulose, can be
used for the sustainable production of high value chemicals. To this
aim, catalytic processes at high pressure and temperature,
pyrolysis, gasification or conversion under supercritical conditions
have been the object of scientific research. Glucose can be used to
obtain ethanol by fermentation [2], sorbitol and mannitol by
hydrogenation [3], 5-hydroxymethyl furfural by dehydrocyclization
Aspergillus niger [9], although this process presents some
drawbacks, as the disposal of dead microbes and the slow reaction
rate. The heterogeneous catalytic oxidation of glucose has been
presented as an attempt to overcome the problems of the
biological process. The heterogeneous catalytic oxidations of sugars
are performed by using supported noble metal catalyst in aqueous
medium with batch reactors in the presence of air or oxygen under
atmospheric pressure at temperatures of 293-353 K. The reaction is
carried out at almost neutral or basic pH’s (7-9) in order to allow
the carboxylate anions to desorb from the catalyst surface avoiding
their degradation [10]. The mild reaction conditions, along with the
fact that the reactants are renewable and the products are
environmentally benign because of their biodegradability, make the
catalytic oxidation of carbohydrates a paradigm of green chemistry.
Metal catalysts as Pt, Pd, Rh, Bi or Pb supported on TiO , Al O or
2 2 3
activated carbon have been used as catalysts but Au seems to be
the most promising one [11-15]. Gold nanoparticles supported on
2
activated carbon [16] or ZrO [17] showed good catalytic activity for
oxidation of glucose to gluconic acid at 50 °C in the presence of O2
at pH’s ranging between 8 and 10.
[
4] and also to produce hydrogen [5]. Glucose is the
monosaccharide most extensively studied in oxidation reactions
particularly to obtain gluconic and glucaric acids [6]. This last
reaction and in general, the selective oxidation of alcohols to their
corresponding carbonyl compounds has attracted attention in the
field of catalysis, due to its strategic importance [7]. Gluconic acid,
4
with an annual estimated market of 6·10 ton, is used as a
In this context, heterogeneous photocatalysis can be also
considered as an alternative. The heterogeneous photocatalytic
technology by using semiconductor oxides as photomediators is
known as a process suitable to degrade organic and inorganic
pollutants both in vapour and in liquid phases under very mild
2
experimental conditions [18]. It is generally accepted that TiO is
the most reliable photocatalyst [19] and Colmenares et al.
investigated the glucose photocatalytic oxidation in the presence of
TiO
2
in acetonitrile-water suspensions [20-22]. They obtained
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glucaric and gluconic acids along with arabitol and reported that the An alternative method was followed to obtain TiO2 by using
presence of acetonitrile stabilized the carboxylic acids by solvation titanium isopropoxide, Ti(OPr) , (Aldrich 97%) as the precursor. The
4
suppressing their further oxidation [22]. It is rare to obtain composite materials were prepared by adding the alkoxide
acceptable selectivity values for partial photocatalytic oxidation precursor (32 mL) to the PW12 (2.3 g) aqueous solution (50 mL) and
reactions in the presence of only water as the solvent [23]. Chong et the resulting suspension was subjected to
a hydrothermal
al. studied the conversion of glucose under anaerobic conditions in treatment at 200°C for 48 h (in this case the system reached a
TiO -rutile aqueous suspensions and they found arabinose, pressure of ca. 17 atm). The resulting bluish powder was washed
2
erythrose and hydrogen as the products [24]. Heteropolyacid (HPA) three times with hot water and finally filtered and dried at 60°C.
clusters have been studied as homogeneous photocatalysts, due to This sample was denoted as PW /TiO solv. The analogous bare
1
2
2
2
their ability to absorb UV light. The absorption of light by the TiO was prepared under the same experimental conditions in the
ground electronic state of the HPA produces a charge transfer- absence of PW12 and labeled as TiO solv.
2
excited state HPA* which can behave as a better oxidant species
than the corresponding ground states [25]. Under irradiation with Another set of samples was prepared by using a monolacunary
-
light of suitable wavelengths HPA reduces to HPA , the so called PW11 Keggin salt. The heteropolyacid K
PW11O
39 has been obtained
7
“
heteropoly-blue”. The heteropoly-blue species is relatively stable, by following the Haraguchi method [31]. 20 g of commercial
absorbs visible light and is readily reoxidized to the original HPA.
40·26 H O were dissolved in 100 mL of hot water, then 1.0
This process can occur both with the plenary HPA Keggin species g of KCl was added and the pH of the solution adjusted to 5 with
40) and with the lacunary Keggin cluster (K
39) [26]. KHCO 1 M. The obtained solid was filtered and dried at room
HPA photo-reduction has been proved to be synergistically temperature. For the preparation of the K 39/TiO materials,
enhanced in HPA/TiO
composites where photo-generated 3.6 mL of titanium isopropoxide was dissolved in 24 mL of 2-
electrons can be transferred from the conduction band of TiO
to propanol under stirring at room temperature for 1 h. 0.125 or 0.250
HPA. In this way the charge-pair recombination in the TiO
is g of K 39 were dissolved in 2 mL of hot water and then added,
delayed [27,28]. Heteropolyacids, such as H
40, are also under vigorous stirring, to the alcoholic solution of the TiO
H
3
PW12
O
2
(H
3
PW12
O
7
PW11
O
3
7
PW11
O
2
2
2
7
PW11O
2
3
PW12
O
2
strong acid catalysts able to catalyze at low temperatures a wide precursor. The resulting suspension was adjusted to pH 5 with
range of catalytic processes [29]. They exhibit very strong Brönsted acetic acid 1 M, transferred to the teflonated autoclave and heated
type acidity, making them suitable for various acidic reactions, such at 433 K for 48 h. The white bluish powder obtained was washed
as esterification, transesterification, hydrolysis, Friedel-Crafts with water and eventually dried at room temperature. The obtained
alkylation and acylation and Beckmann rearrangement [30].
powders were named PW11-X/TiO
2
solv (where X = 0.125 or 0.250 g,
The present paper reports the preparation of nanometer-sized TiO
2
depending on the amount of PW11 used).
particles by a solvothermal method. The commercial saturated Bulk and surface characterizations were carried out in order to
40 (labelled as PW12) and the home prepared lacunary define some physicochemical properties of the powders. Their
monovacant K
39 Keggin salt (labelled as PW11) were coupled crystalline phase structure was determined at room temperature by
by a solvothermal treatment with TiO obtaining PW12/TiO
and powder X-ray diffraction analysis (PXRD) carried out by using a
PW11/TiO
composite materials. Moreover, also the impregnation of Panalytical Empyrean, equipped with CuKa radiation and PixCel1D
the saturated Keggin unit PW12 on commercial TiO
surface was (tm) detector. The specific surface areas (SSA) were determined in a
3
H PW12O
7
PW11O
2
2
2
2
performed for the sake of comparison. Some physico-chemical Flow Sorb 2300 apparatus (Micromeritics) by using the single-point
properties of the prepared materials were investigated along with BET method. Scanning electron microscopy (SEM) was performed
their photoactivity for glucose oxidation in aqueous medium at using a FEI Quanta 200 ESEM microscope, operating at 20 kV on
natural pH. The experiments were carried out under mild specimens upon which a thin layer of gold had been evaporated. An
conditions: room temperature, atmospheric pressure in aqueous electron microprobe used in an energy dispersive mode (EDAX) was
suspensions and by using an inexpensive material. Photocatalysis by employed to obtain information on the actual metals content
using heterogeneized heteropolyacids is a novel field, and the present in the samples. Raman spectra were obtained by means of
glucose partial oxidation by using these solids has never been a BWTek-i-micro Raman Plus System, equipped with a 785 nm
investigated before, to the best of our knowledge.
diode laser. The measurements were performed focusing the
sample by a 20x magnification lens, spot size was around 50 µm.
-1
The accuracy of Raman shift was around 3 cm . The power of the
laser used was 15% of the maximum value that was around 300
mW. Infrared spectra of the samples in KBr (Aldrich) pellets were
2
. Experimental
2.1 Photocatalysts preparation and characterization
A first set of powders was obtained by wet impregnation of obtained with a FTIR-8400 Shimadzu spectrophotometer and the
-1
commercial TiO2 (Evonik P25) with a solution of a commercial spectra were recorded with 4 cm resolution and 256 scans. The
heteropolyacid (HPA), i.e. tungstophosphoric acid
40 diffuse reflectance spectra (DRS) were recorded in air at room
Aldrich reagent grade 99.7%), labelled as PW . In particular TiO temperature in the 250-800 nm wavelength range using a Shimadzu
3
H PW12O
(
(
1
2
2
8.3 g) was added to a water solution (50 mL) containing the UV-2401 PC spectrophotometer, with BaSO4 as the reference
appropriate amount of PW12 (2.3 g). The suspension was stirred for material.
ca. 1 h and then it was divided in two parts. One of them was
hydrothermally treated in a teflonated autoclave for 48 hours at 2.2 Photocatalytic activity
2
00°C and the obtained white powder filtered and dried at 60°C.
The photoreactivity runs were carried out at room temperature and
ambient pressure in a 800 mL open reactor irradiated in the UV
region with an immersed 125 W medium pressure Hg lamp (Helios
Italquartz, Italy). The initial aqueous glucose concentration was 1
mM and the runs were carried out at natural pH. The impinging
radiation intensity was measured by a radiometer Delta Ohm
This sample has been named PW /P25 solv. The other part of the
suspension was, instead, evaporated until dryness in a vacuum-
dryer apparatus and the obtained powder labelled as PW /P25.
1
2
1
2
2
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2
DO9721, and it was 5.5 mW/cm . The amounts of photocatalyst trend, those of all of the composite materials were smaller than
needed to absorb all the photons emitted by the lamp in the those of the bare TiO
2
samples.
reacting suspension was checked by using the same radiometer and
depended on the catalyst used. In particular, in some cases 0.3 g/L XRD diffractograms of all of the prepared samples are reported in
of catalyst were sufficient (see TiO2 P25, PW /P25 solv and Figure 1. In Figure 1 (A) the diffractograms correspond to the bare
12
PW12/P25), whereas in other cases the amount was 2.0 g/L (see PW12, commercial TiO
TiO solv, PW -0.125/TiO solv and PW -0.250/TiO solv) or 2.8 g/L TiO P25) and the composites obtained with these two substances.
The PW12 presents a crystalline structure and the commercial TiO
Evonik P25 (in the following named only
2
2
11
2
11
2
2
(see PW12/TiO
2
solv).
2
Air was not bubbled during the experiments but the vessel was just P25 consists of anatase and rutile polymorphs. In the composite
opened in ambient conditions. The values of substrate materials, new peaks, attributable to the heteropolyacid, in
concentration before the addition of catalyst and before the addition to those of TiO are present and they are less intense in
2
starting of irradiation were measured in order to determine the the PW12/P25 solv sample than in the PW12/P25 one. This finding
substrate adsorption on the catalyst surface under dark conditions. can account for the best dispersion of HPA in the material prepared
During the photoreactivity runs samples were withdrawn at fixed solvothermically. Indeed, the more defined peaks can be due to the
times and immediately filtered through 0.2 µm membranes (HA, heterogeneity of the dispersion of PW12 in the P25 impregnated
Millipore) before analyses. The quantitative determination and sample, as confirmed by SEM (see in the following). Figure 1 (B)
identification of glucose and its degradation products were shows the diffractograms of PW12 along with the home prepared
performed by means of a Thermo Scientific Dionex ultimate 3000 bare TiO
2
and the composite PW12/TiO
2
solv prepared
HPLC equipped with a Diode Array and refractive index detectors. solvothermically. In the diffractogram it can be noticed the
+
The column was a REZEK ROA Organic acid H Phenomenex, the presence of anatase phase for both bare TiO
eluent an aqueous 2.5 mM H SO
solution and the flow rate 0.6 mL samples without significant differences and without the presence of
min . Retention times and UV spectra of the compounds were peaks ascribable to HPA. This result suggests a good statistic mixing
compared with those of standards purchased from Sigma-Aldrich of HPA with TiO . Figure 1 (C) reports the diffractograms of PW11
solv and PW11-X/TiO solv samples (see Experimental section
for the meaning of the code). The diffractogram of bare PW11
good crystallinity of this sample although the
intermediates (arabinose and gluconic acid) on the different characteristic peaks have not been reported in the literature. Both
photocatalysts used, some adsorption tests were carried out PW11-X/TiO solv samples do not present a strong evidence of the
following the procedure below reported. An amount of 2.8 g·L of PW11 crystalline phase segregated. The only significant difference
photocatalyst was dispersed in an aqueous solution containing 1 between PW11-0.125/TiO solv and PW11-0.250/TiO solv samples
solv is the wide peak localized at 2θ= 14°.
and PW12/TiO solv
2
2
2
4
-1
2
,
with a purity of >99%. All of the runs lasted ca. 6 h and were TiO
performed at least twice.
2
2
In order to test the adsorption extent of some reaction indicates
a
2
-1
2
2
-
1
-1
mmol·L of arabinose or 1 mmol·L of gluconic acid. The and TiO
suspension was maintained under stirring in dark condition for 6 h.
5
2
The samples were also investigated by FTIR (spectra not reported
for the sake of brevity) to check the structural integrity of the
Keggin unit after the preparation of the HPA/TiO composites. The
ml of suspension were withdrawn every hour and the
concentrations of arabinose or gluconic acid were analysed after
the separation of the photocatalyst.
2
arrangement structure of the PW12 consists of a PO
4
tetrahedron
surrounded by four W O groups formed by edge sharing octhaedra
3
9
3
. Results and Discussion
[
32]. This arrangement gives rise to four stretching bands: P-O
-1
-1
stretching mode at 1080 cm , W=O stretching observed at 990 cm
-1
-1
3.1 Characterization of the photocatalysts
and two peaks at ca. 910 cm and 810 cm attributed to two types
of W-O-W units [33]. It is difficult to characterize the Keggin
structure in the composites by IR spectroscopy because some of the
Table 1 reports some physicochemical features of the powders
investigated as photocatalysts. The specific surface area (SSA) of
bands overlap with those assigned to TiO powder; the latter, in
2
2
-
TiO
2
sample prepared under solvothermal conditions was 260 m ·g
fact, presents an intense and broad vibration band originated from
1
-1
,
a value much higher than that of the commercial TiO Evonik P25
2
Ti–O–Ti bonds located at wavenumbers lower than 900 cm .
2
-1
(50 m ·g ). The SSA’s of the commercial PW12 and the home
2
-1
prepared PW11 were 15 and 80 m ·g , respectively, and as a general
Table 1. Some physicochemical data concerning the characterization of the TiO and HPA/TiO photocatalysts
2
2
Nominal
EDAX
2
Photocatalyst
TiO P25
PW12
S.S.A. [m /g]
E
gap [eV]
W atomic [%]
Ti atomic [%] W atomic [%] Ti atomic [%]
2
50
15
48
3.2
3.2
3.0
3.0
3.2
3.2
3.4
3.1
3.2
-
-
9
7
-
7
-
4
7
-
-
-
-
-
-
PW12/P25
PW /P25 solv
91
93
-
93
-
10±1*
7±1
-
10±0.1
-
90±1*
93±1
-
90±0.1
-
43
1
2
2
TiO
solv
260
176
80
206
196
PW /TiO solv
1
2
2
PW11
PW -0.125/TiO solv
96
93
5±1
7±1
95±1
93±1
1
1
2
PW11-0.250/TiO
2
solv
*Some agglomerates in the PW12/P25 sample present values of W atomic percentage and Ti atomic percentage of ca. 30% and ca. 70%, respectively.
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+
(
B)
10.2
(A)
+
1
4.5
1
7.8
34.6
23.1
46.1
5
9.9
+
+
+
+
(b)
+
2
0.6
+
+
(d)
(c)
+
*
+
+ +
+,*
*
*
+,
*
+
(b)
*
(a)
(
a)
80
1
0
20
30
40
50
60
70
10
20
30
40
50
60
70
80
2
Θ [Degrees]
2Θ [Degrees]
+
(C)
(
(
d)
c)
+
+
+
+
+
+
+
(
b)
(a)
1
0
20
30
40
50
60
70
80
2Θ [Degrees]
FIGURE 1. XRD patterns of the photocatalysts: (A) (a) PW12; (b) TiO
2 2 2
P25; (c) PW12/P25 solv; (d) PW12/P25; (B) (a) TiO solv; (b) PW12/TiO
solv and (C) (a) PW ; (b) TiO solv; (c) PW -0.125/TiO solv; (d) PW -0.250/TiO solv. ( Rutile; + Anatase).
1
1
2
11
2
11
2
*
Raman spectroscopy allowed a much better understanding of the characteristic peaks of the rutile phase, that should be located at
-
1
structural modifications induced at the surface. In Figure 2(A) both 444 and at 609 cm , are not observed in the samples, as reported
bare TiO2 samples (TiO2 solv and TiO2 P25) show Raman peaks before [27], probably because the rutile crystallites observed by
-
1
centered at 144, 197, 399, 513, and 639 cm attributable to the E
g
,
XRD are located in the bulk of the material.
E , B , A and B2g modes of anatase TiO [34]. On the contrary, the
g
1g
1g
2
144
(A)
(
B)
960
513
639
1008
3
99
960
PW12/P25 solv
PW11
PW12/TiO
2
solv
399 513
639
PW11-0.250/TiO
2
solv
solv
1
97
TiO
2
solv
PW11-0.125/TiO
2
TiO
2
P25
990
65
165 265 365 465 565 665 765 865 965 1065
Raman Shift [cm-1
]
PW12/P25
PW12
6
5
165 265
365
465
565
665
765 865
965 1065
Raman Shift [cm-1
]
FIGURE 2. Raman spectra of the samples: (A) bare TiO and PW containing materials, and (B) PW11 containing samples.
2
12
4
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FIGURE 3. SEM microphotographs of (A) two agglomerates (a, b and c, d) of bare PW11 and (e, f) bare TiO
magnifications and (B) (a-h) some selected composite materials at two different magnifications: (a, b) PW -0.125/TiO solv; (c,
2
at different
1
1
2
2 2 2
d) PW11-0.250/TiO solv; (e, f) PW12/TiO solv; (g, h) PW12/TiO P25.
The Raman spectrum of PW12 shows a sharp and intense band at SEM microphotographs of some selected materials are reported in
-
1
-1
1
008 cm and a peak at 990 cm assigned to P–O vibrations and Figures 3(A) and (B). In particular, Figure 3(A) reports some pictures
-1
bands at lower wavenumbers, attributed to W–O (925 cm ) and W– of the bare PW11 (a-d) and of the home prepared TiO
O–W (880 cm ) vibrations [33].
In the HPA/TiO
2
(e-f) samples
whereas Figure 3(B) reports some pictures of composite powders.
composites the Raman peaks attributable to The morphology of the PW11 salt appears completely different
-1
2
anatase phase are also present indicating that the crystalline form is compared with that of the other samples. This sample seems
preserved on the surface after the introduction of the consisting of very large crystals surrounded by others growing small
heteropolyacid. The four HPA characteristic bands are not present crystals. On the contrary, the TiO sample which was solvothermally
2
due to the very low amount of HPA in the samples. However, the prepared consists of agglomerates of primary particles (ca. 40-60
-1
sharper and more intense band at 1008 cm and the shoulder at nm), whose size ranges between 2.5 and 30 µm. The PW -
1
1
-
1
9
90 cm are present for the PW12/P25 composite in Figure 2(A) and 0.125/TiO
no significant shift can be observed. On the contrary, for the samples (Figure 3(B) a-b, c-d and e-f, respectively) appear very
samples treated by the solvothermal method, PW12/P25 solv and similar to the bare TiO sample (Figure 3(A) e-f), indicating that the
PW /TiO solv, the 1008 and 990 cm peaks appear as a unique small content of PW11 or PW12 did not modify the morphology of
2 2 2
solv, PW11-0.250/TiO solv and PW12/TiO solv composite
2
-1
12
2
-1
broad peak shifted to ca. 960 cm . These finding can be attributed the majority component TiO
2
. From the perusal of Figures 3(A) and
to an interaction between the oxygen atom of the Keggin anion and (B) it can be concluded that in the case of the PW -0.250/TiO solv
1
1
2
the hydroxyl groups on the TiO
Keggin is concerned, Figure 2(B) shows the high complexity of the Consequently, it seems that the presence of a higher amount of
Raman spectrum of this HPA. The spectra of the binary PW11/TiO PW11 caused a decrease of the size of the primary particles. The
samples confirm the presence of anatase along with a small morphology of PW /P25 (Figure 3(B) g-h) and PW /P25 solv (not
2
surface [35]. As far as the lacunary the size of the primary particles resulted smaller (ca. 20 nm).
2
1
2
12
-1
shoulder at ca. 940 cm due to the presence of PW11 in the reported in Figure 3), is very similar to the bare material (SEM
composites (see the intense band observed for the bare PW at picture of bare TiO2 P25 has been already reported [28]). The
1
1
-
1
9
60 cm ).
agglomerates of these particles present the same shape and consist
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90
80
70
60
50
40
30
20
10
14
80
(
b)
(
a)
(A)
(a)
(B)
1
1
2
0
(c)
(
f)
(c)
70
(b)
(b)
8
(c)
6
0
0
6
4
2
0
(
a)
(e)
5
(d)
(e)
2
50 300 350 400 450 500
12
(
d)
40
10
8
(b)
Wavelength [nm]
30
20
10
6
(c)
(
f)
4
2
(a)
0
250
300
350
400 450
500 550
Wavelength [nm]
0
0
2
50
300
350
400
450
500
550
250
300
350
400
450
500
550
Wavelength [nm]
Wavelength [nm]
FIGURE 4. Diffuse reflectance spectra of the samples: (A) (a) TiO
PW12 and (B) (a) PW11; (b) PW11-0.125/TiO solv, and (c) PW11-0.250/TiO
applying the Kubelka-Munk function, F(R ), to the diffuse reflectance spectra.
2
P25; (b) TiO
2 2
solv; (c) PW12/P25; (d) PW12/P25 solv; (e) PW12/TiO solv; (f)
2
2
solv. The insets report the absorbance spectra obtained by
∞
of nanoparticles with similar sizes (ca. 50 nm). Table 1 reports the network to make up tungsten-oxygen octahedral lacunas.
nominal and the average EDAX values of atomic percentage of W in Consequently, the terminal nucleophilic oxygen atoms of the
PW11 or PW12 and the atomic percentage of Ti in the TiO
2
. EDAX
K
7
PW11
O39 become bridge atoms that allow to connect PW11
measurements confirmed a homogeneous content of the HPAs and TiO
2
via W-O-Ti bonds [37].
onto the catalyst with the exception of the PW12/P25 sample where 3.2 Glucose photocatalytic conversion
tungsten was present in some agglomerates in much higher content Figures 5-7 report the results of the photocatalytic glucose
with respect to the nominal one. In all of the other samples, the conversion in the presence of different catalysts. Figure 5 reports
measured amount of W and Ti was always very close to the nominal the activity of the bare TiO
one. Figure 4 (A) and (B) report the diffuse reflectance UV-Vis decrease of glucose concentration was accompanied by the
spectra (DRS) of the bare TiO and HPA/TiO composite samples. appearance of various compounds, mainly formic acid and
The insets report the absorbance spectra obtained by applying the arabinose. Other oxidation products as erythrose and gluconic acid
Kubelka-Munk function, F(R ), to the diffuse reflectance spectra. All were also formed in lower amounts. Fructose, glucose
of the spectra are characterized by a charge transfer process, from isomerization product, has been also found. It is worth to mention
2 2
powders. By using TiO P25, the
2
2
∞
a
O 2p to Ti 3d for TiO
Figure 4 (A) reports the spectra of the bare TiO and PW /TiO
2
2
or to HOMO-LUMO transition for the HPAs.
that the amount of P25 sufficient to absorb all the photons emitted
by the lamp was 0.3 g/L, under the experimental conditions used.
2
12
2
solv samples, whereas in Figure 4 (B) those concerning the TiO solv possesses a lower ability in absorbing photons (this feature
samples prepared with the lacunary HPA are shown. The can be related to the higher granulometry of the powder) and 2 g/L
spectrum of PW11 evidences a higher energy for the transition were necessary to absorb all the emitted photons. Consequently,
HOMO-LUMO compared to that of PW . The Kubelka-Munk two runs with TiO solv were carried out: the first one with 2 g/L,
1
2
2
function F(R
where the extrapolation in the linear fitting of the plot with 2 g/L of photocatalyst, the decrease of glucose concentration
∞
) has been used to obtain the Tauc plots [36] the second one with 0.3 g/L. As reported in Figure 5, for the run
1
/2
(
∞ 2
F(R )·E) vs incident light energy in eV gives the band gap was faster than in the presence of TiO P25, but the amount of
energy (see Table 1). The presence of the HPA gave rise to a formic acid was much lower. Arabinose was found as a product,
slight increase of the band gap energy, particularly where the along with low amounts of erythrose and gluconic acid. In this case,
PW11 was present. Based on the above physico-chemical the isomerization product fructose was also observed. By
2
characterization results, it can be concluded that the primary employing a lower amount of TiO powder (0.3 g/L) the glucose
Keggin structure of the saturated and lacunary HPA remained conversion decreased, the quantities of formic acid, erythrose and
virtually unchanged after the deposition of the cluster on the gluconic acid slightly decreased and arabinose was completely
oxide surface. Different kind of interactions between the absent. Only the fructose concentration increased moderately.
saturated or lacunary Keggin unit and the TiO
hypothesized. For the PW /TiO solv composite, it can be found along with the carbon mass balance calculated after 6 hours
2
surface can be Table 2 reports conversion and selectivity to the different species
1
2
2
2
suggested that the saturated Keggin unit interacts with TiO by of irradiation for the experiments reported in Figure 5. The glucose
hydrogen bonding and acid-base reaction as reported before conversion, X, and the selectivity, S, to fructose, gluconic acid,
[
35]. On the contrary, according to Ma et al., the removal of a arabinose and erythrose have been calculated as follows:
gives
3
−
tungsten–oxygen octahedral from a saturated PW O
2 40
1
7
−
rise to the lacunary anion (PW11
nucleophilic and can reacts easily with electrophilic groups
such as titanium atoms of Ti-OH groups present in TiO
O39 ) that results highly X = ([glucose]
i
– [glucose]) / [glucose]
i
×100
(1)
(2)
2
.
S = [product]/ ([glucose] – [glucose]) ×100
i
Therefore, in the PW /TiO composite, K PW O presents
1
1
2
7
11 39
vacant sites, which allow connecting two TiO
4
units of the TiO
2
6
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1
1
1
TiO solv (0.3 g/L)
TiO solv (2 g/L)
2
TiO2 P25 (0.3 g/L)
2
0
0
0
0
.8
.6
.4
.2
0.8
0.8
0.6
0.4
0.2
0
0.6
0.4
0.2
0
0
0
60 120 180 240 300 360
Irradiation time [min]
0
60 120 180 240 300 360
Irradiation time [min]
0
60 120 180 240 300 360
Irradiation time [min]
FIGURE 5. Evolution of glucose and photocatalytic reaction products versus irradiation time in the presence of bare TiO samples. (ꢀ)
2
glucose, (
▲) fructose, (▼) erythrose, (ꢁ) arabinose, (ꢀ) gluconic acid, and (ꢂ) formic acid. The average oscillation percentage of the
experimental data was ca. ± 2%.
where [glucose] is the initial glucose concentration, [glucose] the TiO2 P25 with PW , i.e. PW /P25 and PW /P25 solv, convert
i
12
12
12
concentration after 6 hours of irradiation and [product] denotes the glucose faster than
product concentration analysed at the same time. It is important to
the correspondent bare TiO P25. In both cases fructose was the
2
highlight that the carbon atom mass balance has been satisfied main species observed during the run, but also a significant amount
when bare TiO P25 has been used but not in the presence of the of gluconic acid was formed, particularly in the presence of
2
bare home prepared TiO
sample turned pale orange after all of the runs, indicating that formed by using these two composite materials was always higher
some species remained strongly adsorbed on its surface. The than that observed in the presence of bare TiO P25. Small amounts
greater ability of TiO2 solv sample to adsorb the reaction of formic acid and erythrose were also found along with traces of
intermediates compared to that of TiO P25 sample has been glucaric acid. By using PW12/TiO solv as the photocatalyst (2.8 g/L
2
solv sample. In fact, the color of the latter PW12/P25 solv. It is worth noting that the amount of gluconic acid
2
2
2
confirmed by adsorption tests carried out in the presence of were needed to absorb all the photons emitted by the lamp), the
arabinose and in the presence of gluconic acid chosen as conversion of glucose was lower with respect to PW12/P25 and
representative intermediates. These tests indicate that the TiO solv PW /P25 solv. Fructose resulted to be the main species, along with
2
12
sample was able to adsorb the compounds above mentioned ca. gluconic acid and formic acid. Table 3 collets the X, S and B data
seven times more than TiO P25 sample. obtained for the runs reported in Figure 6. The presence of PW12
Figure 6 reports the evolution of glucose and reaction products in along with TiO P25, in both samples obtained by impregnation or
photocatalytic experiments carried out in the presence of by solvothermal treatment, caused an increase in the glucose
2
2
PW12/TiO
2
composites. The samples obtained by impregnation of conversion, the absence of arabinose and a strong decrease of
formic acid and erythrose in comparison to TiO P25 (see Table 2).
2
TABLE 2. Photoreactivity assessment of bare TiO powders: glucose conversion, X, selectivity to the identified reaction products, S, and
2
carbon mass balance, B, after 360 minutes of irradiation.
TiO
2
P25
TiO
2
solv
2
TiO solv
(0.3 g/L)
(0.3 g/L)
(2 g/L)
X [%]
S [%]
Glucose
Fructose
Gluconic acid
Arabinose
Erythrose
Carbon
41±1
40±1
20±1
3±0.2
-
13±1
75±2
72±1
6±0.5
1±0.1
26±1
5±0.5
55±1
14±1
0.6±0.1
42±2
28±1.5
99±1
B [%]
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1
1
PW /P25 (0.3 g/L)
PW /P25 solv (0.3 g/L)
12
PW /TiO solv (2.8 g/L)
12
1
2
2
0
0
0
0
.8
.6
.4
.2
0.8
0.6
0.4
0.2
0
0
.8
.6
.4
.2
0
0
0
0
0
0
60 120 180 240 300 360
Irradiation time [min]
0
60 120 180 240 300 360
Irradiation time [min]
0
60 120 180 240 300 360
Irradiation time [min]
FIGURE 6. Evolution of glucose and their photocatalytic reaction products versus irradiation time in the presence of the binary materials
composed of PW12 and TiO . (ꢀ) glucose, (▲) fructose, (▼) erythrose, ( ) arabinose, (ꢀ) gluconic acid, ( ) glucaric acid, and (ꢂ) formic
acid. The average oscillation percentage of the experimental data was ca. ± 2%.
2
ꢁ
ꢃ
On the contrary, conversion to fructose and gluconic acid increased. solvothermically, was able to modify the reactivity and selectivity of
In particular, in the presence of PW /P25 solv the amount of the bare TiO towards the formation of less degraded products in
1
2
2
valuable gluconic acid was the highest obtained in this work. the glucose oxidation process. Also in the PW12/TiO
Notably this last sample did not give rise to leaching of PW12 in the the presence of PW12 in the bulk of TiO influenced the reactivity of
liquid phase, contrary to what observed for PW12/P25 prepared by TiO giving rise to a decrease in the formation of formic acid and an
impregnation. increase of fructose.
2
solv sample
2
2
In the light of the results above presented, some general Moreover, the presence of PW12 favoured the products desorption
considerations can be done. The presence of PW12 in PW12/P25 and from the photocatalyst surface making possible the achievements
PW12/P25 solv samples, resulted beneficial. On the contrary, of the carbon mass balance contrarily to what observed with the
PW12/TiO
obtained in the presence of PW12/P25 samples giving rise mainly to discussed for TiO
fructose. This finding can be explained by taking into account the presence of PW12 drastically reduced the adsorption ability of TiO
different preparation methods used to obtain this composite in solv. This finding was also observed in the presence of PW11, as it
2
solv composite showed a lower reactivity than that bare TiO
2
solv sample. Adsorption tests carried out as those
2
P25 and TiO solv sample indicated that the
2
2
comparison to those containing P25.
will be discussed later. It is worth to mention that both HPA species,
In the latter cases, PW12 was added to TiO
2
already crystallized, PW11 and PW12, are soluble in water. PW12 used as photocatalyst in
whereas PW12/TiO
organometallic precursor to the PW12 solution; consequently, PW12 PW11 gave rise to a slight conversion of glucose to fructose. The
was dispersed not only on the TiO surface but mainly in the bulk. products formed in the presence of samples consisting of the PW11
This fact has been evidenced by XRD investigations where the lacunary cluster and TiO (PW11-0.125/TiO solv and PW11
pattern of the PW12/TiO solv does not indicate the presence of the 0.250/TiO solv) are reported in Figure 7. Also for these two
heteropolyacid. Consequently, it is evident that the presence of samples different amounts were used (0.3 g/L and 2 g/L), due to
PW12 on the TiO P25 surface, both prepared by impregnation or their scarce ability to absorb all the photons emitted by the lamp.
2
solv sample was prepared by adding the Ti homogeneous regime did not show to be photoactive; conversely,
2
2
2
-
2
2
2
2
Table 3. Photoreactivity assessment of composites containing PW12 and TiO : glucose conversion, X, selectivity to the identified reaction
products, S, and carbon mass balance, B, after 360 minutes of irradiation.
PW12/P25
PW12/P25 solv
(0.3 g/L)
PW12/TiO
(2.8 g/L)
2
solv
(0.3 g/L)
X [%]
S [%]
Glucose
64±1
68±2
22±1
-
85±1
55±2
34±1
1.5±0.2
11±1
99±1
39±1
56±2
20±1
-
Fructose
Gluconic acid
Glucaric acid
Erythrose
Carbon
-
-
B [%]
97±1
92±1
8
| J. Name., 2012, 00, 1-3
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ARTICLE
1
1
PW -0.125/TiO solv (0.3 g/L)
1
PW -0.250/TiO solv (0.3 g/L)
11 2
1
2
0
0
0
0
.8
.6
.4
.2
0
0.8
0.6
0.4
0.2
0
0
60
120 180 240 300 360
PW -0.125/TiO solv (2 g/L)
0
60
120 180
240 300 360
1
1
PW -0.250/TiO solv (2 g/L)
11
1
1
2
2
0
.8
0.8
0.6
0.4
0
0
0
.6
.4
.2
0.2
0
0
0
60
120
180 240 300 360
0
60
120 180 240 300
Irradiation time [min]
360
Irradiation time [min]
FIGURE 7. Evolution of glucose and their photocatalytic reaction products versus irradiation time in the presence of the binary materials
composed of PW11 and TiO . (ꢀ) glucose, (▲) fructose, (▼) erythrose, ( ) arabinose, (ꢀ) gluconic acid, ( ) glucaric acid, and (ꢂ) formic
acid. The average oscillation percentage of the experimental data was ca. ± 2%.
ꢁ
ꢃ
2
No leaching of PW11 occurred during the experiments, indicating It is worth noting that the presence of PW11 in the composite
that PW11 played a role exclusively in heterogeneous phase. In the PW /TiO solv samples favoured the desorption of the products
11
2
runs carried out with 0.3 g/L, the materials were scarcely active (see from the photocatalyst surface, however it caused a strong increase
Figure 7) and some decrease in the concentration of glucose was of the overoxidation product, i.e. gluconic acid and formic acid.
observed along with the formation of small amounts of fructose,
On the basis of the previous considerations we can conclude that
gluconic acid, erythrose and formic acid. The use of 2 g/L of
the presence of HPA (PW12 or PW ) changed the glucose reaction
mechanism; indeed, in the runs carried out by using bare TiO the
2
11
photocatalyst gave rise to a high increase of reactivity. PW11
-
0
.250/TiO solv converted glucose faster than PW -0.125/TiO solv
2
11
2
main products were arabinose and erythrose, whereas fructose and
gluconic acid were preferentially formed when the PW12
composites were used as photocatalysts. When the PW11
composites were employed, the formation of formic acid was
favoured. Probably, HPA modified the surface properties of TiO2
and in particular the type and the strength of acid site along with
their distribution. This finding can justify the higher amounts of
fructose obtained when composite catalysts containing PW12 were
used. Indeed, in the literature it is reported that the isomerization
of glucose to fructose is catalysed by Lewis acids [38]. In particular,
but in both cases the most important product was formic acid that
was surprisingly found in higher amount in the presence of the
latter sample. In both systems, the presence of gluconic acid,
erythrose and fructose was also detected. The reactivity of this two
photocatalysts differed only because arabinose was found in the
presence of PW -0.125/TiO solv while small amounts of glucaric
1
1
2
2
acid were detected in the PW11-0.250/TiO solv. For the composite
PW /TiO solv samples the conversion of glucose was lower than
1
1
2
for TiO
2
solv, whereas the selectivity to gluconic acid higher. The
largest amount of photocatalyst (2 g/L vs. 0.3 g/L) caused a higher
conversion of glucose but also a higher formation of formic acid.
2
this effect was evident in the case of P25/TiO samples where HPA
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2
Table 4. Photoreactivity assessment of composites containing PW11 and TiO : glucose conversion, X, selectivity to the identified products, S,
and carbon mass balance, B, after 360 minutes of irradiation.
PW11
PW -0.125/TiO solv
PW -0.250/TiO solv
PW -0.125/TiO solv
PW -0.250/TiO solv
11 2
11
2
11
2
11
2
(
0.3 g/L)
(2 g/L)
(2 g/L)
(0.3 g/L)
(0.3 g/L)
X [%]
S [%]
B [%]
Glucose
Fructose
20±1
44±2
24±1
4±0.2
-
38±2
37±1
99±1
53±2
31±1
19±1
1±0.1
-
20±1
34±1
9±0.5
-
28±1
24±1
35±2
-
99±1
-
-
-
-
Gluconic acid
Glucaric acid
Arabinose
Erythrose
Carbon
-
-
19±1
90±1
23±1
95±1
16±1
95±1
99±1
2
is preferentially localized on the TiO surface. The presence of PW12 catalytic oxidation of carbohydrate molecules is the regio- and
was able to change the reaction mechanism, probably because it chemoselectivity, but these aspects have been considered out of
modified the acid properties of the catalyst surface. This finding the scope of this work. In Scheme 1 it has been summarized a
indicate that PW12 was able to explicate its acid function (see the possible reaction sequence by considering the Hoffman structures
higher formation of fructose) reducing the oxidizing ability of TiO
2
of the analysed molecules in the D-form.
(
see the disappearance of arabinose and erythrose and the
The oxidation of glucose can occur by the oxidation of the anomeric
center (C1) giving rise to gluconic acid. Successively, an oxidative
attack to the C2 carbon gives rise to a formation of formic acid and
arabinose. A further oxidative attack to the C2 carbon in arabinose
molecule releases another formic acid molecule and erythrose, and
in turn, the erythrose molecule can be transformed into
glyceraldehyde (analysed in traces) by an oxidative attack. Finally
formic acid, as the over-oxidation species, was obtained. The
presence of glucaric acid has been also observed indicating the
oxidation of both C1 and C6 atoms of glucose. The
contemporaneous or successive oxidation of C1 and C6 can be
explained by considering different glucose adsorption modes on the
surface of the photocatalysts. The primary oxidation at the C6 of
glucose to give glucuronic acid, reported in literature as one of the
intermediates in catalytic oxidation of glucose [1], was not observed
under the experimental condition used in this work, however in
scheme 1 it has been also hypothesized, and represented in
brackets.
2
formation of gluconic acid). Indeed, when TiO was used alone,
according to Chong et al [24], the most favoured reactions were
due to the α scission (C -C cleavage) giving rise to the formation of
arabinose, erythrose and glyceraldehyde (this last observed only in
traces). On the other hand, the oxidation of glucose to gluconic acid
1
2
(
as first step) and to glucaric acid (as second step) were observed
only during the runs carried out by using PW12 containing
composite photocatalysts. On the contrary, the presence of PW11
that does not show any acid function, induces an increase of the
oxidizing ability of TiO , probably acting through the formation of
the heteropoly-blue species [26] and consequently reducing the
electron-hole recombination on TiO . However, it is worth noting
that the sample PW11-0.125/TiO solv is more oxidizing than the
sample PW11-0.250/TiO solv. This fact can be explained by
considering that an increase of the amount of PW11 over a certain
value could cover the TiO surface or favour the electron-hole
,
2
2
2
2
2
recombination (two effects that in any case are detrimental for the
reactivity). It is worth to remark that an important feature in the
1
0 | J. Name., 2012, 00, 1-3
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ARTICLE
D-Glucuronic acid
D-Glucaric acid
H
O
C
C
C
C
C
C
H
OH
H
D-Glucose
HO
H
HO
O
OH
H
O
C
C
C
C
C
C
C
C
C
C
C
H
OH
H
OH
H
H
OH
H
Photocat.
Photocat.
hυ
, O2
O
HO
H
Other intermediates species
HCOOH
OH
hυ
, O2
HO
OH
OH
H
H
OH
OH
H
HO
O
C
O
CH OH
2
OH
H
C
C
C
C
OH
H
D-Glyceraldehyde
HO
H
H
O
H
O
OH
C
C
C
C
C
Photocat.
, O2
H
OH
H
H
OH
OH
OH
hυ
H
CH OH
2
H
O
H
CH OH
2
-
HCOOH
D-Gluconic acid
C
C
C
C
CH OH
2
OH
D-Eritrose
H
H
OH
OH
CH OH
2
HCOOH
COOH
C
C
C
C
O
C
C
C
C
O
Photocat.
CH OH
2
HO
H
H
OH
H
H
hυ
OH
OH
D-Arabinose
OH
OH
H
H
CH OH
2
CH OH
2
D-Fructose
2
-keto-D-
Gluconic
acid
Scheme 1. Hypothesis of reaction sequence for the photocatalytic glucose oxidation.
4
. J.N. Chheda, Y. Roman-Leshkov, J.A. Dumesic, Green Chem.,
007, , 342.
5. R.R. Davda, J.W. Shabaker, G.W. Huber, R.D. Cortright, J.A.
2
9
Conclusions
Dumesic, Appl. Catal. B, 2005, 56, 171.
. M. Comotti, C.D. Pina, M. Rossi, J. Mol. Catal. A, 2006, 251
9.
We can conclude that, although fructose, gluconic acid,
arabinose, erythrose and formic acid were observed as
6
,
8
2 2
oxidation products when bare TiO or HPA/TiO
composite 7. T. Mallat, A. Baiker, Chem. Rev., 2004, 104, 3037.
8
9
1
. S. Ramchandran, P. Fontanille, A. Pandey, C. Larroche, Food
Technol. Biotechnol., 2006, 44, 185.
. J.Z. Liu, L.P. Weng, Q.L. Zhang, H. Xu, L.N. Ji, Biochem. Eng. J.,
materials were used, depending on the photocatalyst used a
different reaction extent and distribution of intermediate
oxidation products were observed. Moreover, traces of
glucaric acid and glyceraldehyde were also found in some runs.
2
0. M. Besson, P. Gallezot, Catal. Today, 2000, 57, 127.
003, 14, 137.
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The C mass balance was virtually accomplished in the presence
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