Tuning the photocatalytic activity of bismuth wolframate: Towards selective oxidations for the biorefinery driven by solar-light

Article abstract of DOI:10.1039/c7cc04242f

The sol-gel entrapment of nanostructured Bi₂WO₆ enhances the activity and the selectivity of the short-gap semiconductor in the sunlight-driven photo-oxidation of trans-ferulic and trans-cinnamic acid dissolved in water with air as the primary oxidant. Valuable products such as vanillin, benzaldehyde, benzoic acid and vanillic acid are obtained. This provides the proof of concept that photocatalysis could be a promising technology in tomorrow's solar biorefineries.

Full text of DOI:10.1039/c7cc04242f

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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6
7
8
Tuning the Photocatalytic Activity of Bismuth Wolframate:
Towards Selective Oxidations for the Biorefinery Driven by Solar-
Light
1
2
Received 00th January 20xx,
Accepted 00th January 20xx
9
Rosaria Ciriminna,a Riccardo Delisi,a Francesco Parrino, †b Leonardo Palmisano,b Mario Pagliaro†a
3
4
DOI: 10.1039/x0xx00000x
www.rsc.org/
10 The sol-gel entrapment of nanostructured Bi2WO6 enhances the 33 chemicals and fuels under visible light, often with remarkable
5
11 acitivity and the selectivity of the short-gap semiconductor in the 34 selectivity.5 When light is excluded, the adsorbed trans-ferulic
12 sunlight-driven photo-oxidation of trans-ferulic and trans- 35 acid molecules react with the W=O active catalytic centres,
13 cinnamic acid dissolved in water with air as primary oxidant. 36 whose concentration, among all possible bismuth tungstates,
14 Valuable products such as vanillin, benzaldehyde, benzoic acid 37 reaches its maximum in Bi2WO6 phase.1 However, when the
15 and vanillic acid are obtained. This provides the proof of concept 38 visible-light conversion of trans-ferulic acid dissolved in water is
16 that photocatalysis could be
17 tomorrow’s solar biorefineries.
18
a
promising technology in 39 carried out in O2 saturated atmosphere using the same flower-
40 like Bi2WO6, the former biophenol abundant in plants where it
41 exerts numerous biological activities,6 is first converted in small
19 The aerobic oxidation of trans-ferulic acid (3-(4-hydroxy-3-42 yield (ca. 1%) into vanillin (after 1 h) and then entirely
20 methoxyphenyl)-2-propenoic acid) (1, Figure 1) dissolved in43 mineralized into CO2.
21 water at room temperature in the presence of a suspension of44 Following promising findings applied to the conversion of
22 catalytic amounts of nanostructured Bi2WO6 in the absence of45 glycerol into dihydroxyacetone,7 and aiming at finding a way to
23 light, affords vanillic acid (4-hydroxy-3-methoxybenzoic acid) (3,46 enhance the Bi2WO6 selectivity in the partial oxidation of
24 Figure 1) in high yield (60%).1 Vanillic acid is a valued fragrance47 phenolics of relevance to the bioeconomy, we entrapped
25 and flavoring agent with several beneficial effects due to its48 flower-like Bi2WO6 in two different silica-based ceramic
26 chemopreventive, hepatoprotective and cardioprotective49 matrices. The resulting photocatalysts were used in the
27 activity,2 and a global market in various application areas50 oxidation of trans-ferulic and trans-cinnamic acid (2, Figure 1)
28 exceeding $200 million3. The reaction is important because the51 under visible light and in the presence of different O2
29 acid is a precursor of vanillin, one of the most important and52 concentrations (0%, 20%, and 100% in the headspace).
30 expensive flavour compound in the world (about $2,400 per
31 kilogram for the natural product).4 Due to its small bandgap (2.7
32 eV), Bi2WO6 photocatalytically mediates the synthesis of
O
O
OH
OH
HO
a.Istituto per lo Studio dei Materiali Nanostrutturati, CNR, via U. La Malfa 153,
90146 Palermo, Italy;
OCH3
2
1
b.“Schiavello-Grillone” Photocatalysis Group, DEIM, Università degli Studi di
Palermo, Palermo, Italy
† Corresponding Authors:
O
O
Dr. M. Pagliaro
Y
Y
Istituto per lo Studio dei Materiali Nanostrutturati, CNR
via U. La Malfa 153
90146 Palermo (Italy)
HO
3
4
OCH3
53
E-mail: mario.pagliaro@cnr.it
54 Figure 1. Chemical structures of trans-ferulic acid (1), trans-cinnamic acid (2),
55 vanillin (3, Y=H), vanillic acid (3, Y=OH), benzaldehyde (4, Y=H), and benzoic acid
56 (4, Y= OH).
Dr. Francesco Parrino
“Schiavello-Grillone” Photocatalysis Group
DEIM, Università degli Studi di Palermo
viale delle Scienze, 90128 Palermo (Italy)
E-mail: francesco.parrino@unipa.it
57 All reactants, except the catalysts, were purchased from Sigma-
58 Aldrich and used as received. Bi2WO6 was synthesized from
59 Bi(NO3)3·5H2O and Na2WO4·2H2O via a well known hydrothermal
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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94 Table 1. BET (Brunauer-Emmet-Teller) specific surface area (SSA), particle size,
95 total pore volume (Vp), average pore diameter (Dp) of the used samples.
60 route in which careful control of reaction time (8 h) affords the
61 required flower-like Bi2WO6 microparticles. The XRD analysis
8
62 (Figure S1 in ESI) confirmed formation of crystalline Bi2WO6 with
63 the main patterns at 2θ = 28°, 33°, 47°, and 56°. SEM and TEM
64 analysis pictures revealed formation of the flower-like
65 nanostructures1. The sol-gel entrapped Bi2WO6 catalysts were
66 prepared using TEOS (tetraethylorthosilicate) and MTES
67 (methyltriethoxysilane) as silica precursors (see ESI). SEM and
68 TEM analyses of the resulting embedded methylated (SiliaSun
69 Me10%) and non methylated (SiliaSun Me0%) photocatalysts
70 (see Figure S2 and S3 in ESI) confirm that even though the
71 flower-like superstructure of original Bi2WO6 is destroyed,
72 crystalline Bi2WO6 is entrapped in amorphous glassy silica or
73 organosilica matrix, encapsulating shell clearly distinguished.
SSA
[m2·g-1]
48.2
Particle size
[nm]
Vp
Dp
[nm]
15.5
2.4
[cm3·g-1]
0.187
0.326
0.318
Bi2WO6
124.4
SiliaSun Me0%
SiliaSun Me10%
538.2
560.2
11.1
10.7
2.3
96 Indeed, the pore size distribution curve shows a broad
97 distribution up to 1000 Å centered at about 500 Å, with average
98 particle size 124.4 nm (Table 1). The isotherms of the SiliaSun
99 catalysts are very similar and may be associated to type I
100 isotherm typical for microporous solids. The pore diameter
101 indeed is slightly higher than 2 nm for both SiliaSun samples
102 with a relatively small external surface, and large inner surface
103 area typical of silica sol-gels which is 11-12 times higher than
104 that of Bi2WO6 with correspondingly smaller average particle
105 size (Table 1). Although the reported pore size of the SiliaSun
106 samples is at the upper limit of the size reported in the
107 literature for micropores, the shape of the isotherms in Figure
108 2B and Figure 2C is ascribed to microporous solids. The presence
109 of a small, almost horizontal hysteresis loop associated with H4
110 type isotherm, indicates the presence of narrow slit-like pores
111 and particles with internal voids of irregular shape. For both sol-
112 gel entrapped catalysts, the pore size distribution curves show a
113 narrow distribution up to 50 Å centered at about 25 Å.
140
120
100
80
0.05
0.04
0.03
0.02
0.01
0.00
A
A’
60
40
20
0
0
250 500 750 1000 1250 1500
Pore Width (Å)
0.0
0.2
0.2
0.2
0.4
0.6
0.8
1.0
1.0
1.0
1.2
1.2
1.2
Relative Pressure (p/p0)
250
200
150
100
50
0.04
0.03
0.02
B
B’
0.01
0.00
114 The oxidation reactions were carried out in a tight close reaction
115 tube added with 10 mL of an aqueous solution (0.5 mM) of
116 substrate and 1.5 g·L-1 of catalyst, either as such or sol-gel
117 entrapped. The tube placed in a water bath was inserted in an
118 air cooled solar light simulator (Solar Box, CO.FO.ME.GRA.,
119 Milan, Figure S4 in ESI) equipped with a 1500 W Xenon lamp
120 irradiating the reaction mixture with simulated solar light (no
121 UV filter). The temperature of the water bath reached 60 °C and
122 did not further change until the light was kept on. Samples
123 withdrawn from the reaction mixture were filtered off through a
0
0.0
0.4
0.6
0.8
0
50
100
150
2000
Relative Pressure (p/p0)
Pore Width (Å)
250
200
150
100
50
0.04
0.03
0.02
0.01
0.00
C
C’
0
0
50
100
150
2000
0.0
0.4
0.6
0.8
Relative Pressure (p/p0)
124 nylon filter (0.2 µm) prior to HPLC analysis. Solvents of HPLC
Pore Width (Å)
125 grade were degassed prior to analysis. Total Organic Carbon
126 (TOC) analysis was conducted with a Shimadzu TOC-5000A
127 analyzer.
74 Figure 2. N2 adsorption-desorption isotherms (A, B, C), pore size distribution (A’,
75 B’, C’) of the samples: pure Bi2WO6 (A, A’), SiliaSun Me0% (B, B’), and SiliaSun
76 Me10% (C, C’).
77 The nitrogen adsorption-desorption isotherms at -196 °C and
78 the pore size distribution for all of samples are displayed in
79 Figure 2. The type IV isotherm of pure Bi2WO6, according to the
80 De Boer classification9, is associated with capillary condensation
81 taking place in mesopores (see the Bi2WO6 average pore size of
82 15.5 nm, Table 1). The initial segment of the isotherm at low
83 relative pressure values may be attributed to monolayer-
84 multilayer adsorption. Thereafter, the almost linear middle
85 section indicates that monolayer coverage is complete and
86 multilayer adsorption takes place. The shape of the hysteresis
87 loop (type H1) suggests the presence of spherical particles
88 arranged in a fairly uniform way with facile pore connectivity.
89 Although the hysteresis takes place at high relative pressure
90 (between 0.8 and 1.0) confirming the presence of mesopores,
91 the shape of the adsorption branches at p/p0 close to 1 is
92 somewhat similar to type II isotherm, indicating the presence of
93 macropores, too.10,11
128 Preliminary experiments, in the presence of the SiliaSun
129 samples, were performed in O2 atmosphere under otherwise
130 similar experimental conditions. In this case, the conversion of
131 the substrates was almost complete (above 90%) after 3 h
132 irradiation, but the selectivity towards the compounds of
133 interest was unsatisfactory (ca. 3% for vanillic acid and ca. 2%
134 for vanillin by starting from ferulic acid; and ca. 6% for
135 benzaldehyde and ca. 4% for benzoic acid by starting from
136 cinnamic acid) with CO2 being the main oxidation product with
137 both susbstrates. Notably, negligible conversion of the substrate
138 was obtained under N2 saturated atmosphere even after 6 h,
139 confirming the central role played by O2 in this kind of oxidation.
140 Blank tests in the absence of light under the same experimental
141 conditions did not show any activity after the same reaction
142 time. The results in Table 2 were obtained carrying out the
143 reaction in the presence of air.
2 | J. Name., 2012, 00, 1-3
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144 Table 2. Conversion of ferulic acid in air under visible light over free and sol-gel
145 entrapped Bi2WO6. Figures are the average of results obtained in three
146 independent runs carried out under the same experimental conditions.
0.6
0.5
0.4
0.3
0.2
A
Ferulic acid
Vanillic acid
Selectivity
(%)
Vanillin
Selectivity
(%)
Reaction
Time (h)
Catalyst
Bi2WO6
Conversion
(%)
42
78
80
86
21
36
37
59
21
31
35
61
93
99
1
2
3
6
1
2
3
6
1
2
3
6
1
3
5.0
2.7
0.1
0.0
13.7
15.3
15.6
10.0
30.0
32.8
26.0
12.0
38.5
33.7
31.0
-
4.1
4.5
0.0
1.0
2.0
4.0
5.0
3.0
6.0
4.9
Time(h)
1.1
45
40
35
30
25
20
15
10
SiliaSun
1.2
B
Me0%
1.8
2.0
0.8
2.1
SiliaSun
Me10%
3.6
4.1
5
0
1.2
TiO2
0.2
2.1
1.0
2.0
3.0
4.0
5.0
6.0
0.0
Time(h)
185
147 The latter results (see also Tables S1, S2, and S3 in ESI) show
148 that, in each case, switching from O2 to air significantly reduces
149 mineralization while affording formation of vanillic acid and
150 vanillin. The sol-gel entrapment of Bi2WO6 in the non photo-
151 active silica matrix reduces the overall conversion but doubles
152 the selectivity to vanillic acid with respect to bare Bi2WO6 by
153 dispersing the active Bi WO phase and lowering the overall
Figure 3. Concentration of ferulic acid during irradiation time (A) and
correspondent selectivity values towards vanillic acid (B) for pure
Bi2WO6 (□), SiliaSunMe0% (○), and SiliaSunMe10% (Δ) photocatalysts in
representative runs.
186 The commercial value of both vanillin and vanillic acid is so high
187 that the hereby reported selectivity values are of practical
188 interest. Notably, ferulic acid partial oxidation has been
189 reported in the presence of TiO2 in water under UV16 or visible
190 light17 irradiation. In the latter case, selective permeation of
191 vanillin through a membrane strongly enhanced the selectivity
192 of the process by continuously separating the product from the
193 reaction medium, thereby efficiently preventing its further
194 oxidation.18 This system allows both to completely retain the
195 photocatalytic powder into the reaction unit and to afford
196 downstream highly pure (99.98 %) crystals of vanillin.19 The
197 application of the latter integrated system to the SiliaSun, which
198 affords significantly higher yields of vanillic acid when compared
199 to TiO2, is therefore promising.
2
6
154 oxidative capability of the photocatalysts. This trend is generally
155 observed for similar systems.12 Highlighting the remarkable
156 effect of Bi WO sol-gel encapsulation, the activity per mass of
2
6
157 Bi WO is higher in the embedded samples with respect to the
2
6
158 bare photocatalyst. Indeed, the active Bi WO phase constitutes
2
6
159 only 10% in weight of the SiliaSun samples. It is worth to
160 mention that comparable selectivity values to vanillic acid were
161 obtained in the dark in the presence of bare Bi WO .1 However,
2
6
162 while in the dark almost 90 h were required, under solar light
163 irradiation the same selectivity and yield values are obtained in
164 3 h. Figure 3A and Figure 3B show the concentration of trans-
165 ferulic acid during irradiation and the corresponding selectivity
166 towards vanillic acid over pure Bi WO and over the SiliaSun
2
6
167 photocatalysts. Notably, the reduced conversion obtained for
168 the embedded photocatalysts with respect to Bi2WO6 (Figure
169 3A) translates into twice higher selectivity values (Figure 3B). It
170 is evident from Figure 3B that the selectivity reaches a
171 maximum and then decreases after a critical reaction time. This
172 general trend is due to the competition for oxidation between
173 the formed target compounds and the substrate molecules. For
174 the sake of comparison, the oxidation of trans-ferulic acid was
175 performed under the same experimental conditions in the
176 presence of TiO2 P25 (Evonik, Germany). In this case the
177 substrate is almost completely oxidized after 1 h irradiation and
178 the selectivity towards the desired products is very poor, thus
179 demonstrating the superior synthetic performance of the
180 Bi2WO6 photocatalysts. Indeed, differently from TiO2 mediated
181 photooxidations, Bi2WO6 induced photocatalytic oxidation
182 processes do not proceed through ˙OH radicals but via direct
183 holes and via more selective peroxidic species including
184 activated oxygen (e.g., O2˙−).13-15
200 The same selectivity trend was observed also in the oxidation of
201 trans-cinnamic acid (Table 3 and Figure S5 in ESI). Hence, while
202 reaction over a suspension of Bi2WO6 under O2 resulted in
203 complete mineralization after 3 h, the replacement of O2 with
204 air afforded complete photocatalytic conversion into a mixture
205 of benzoic acid and benzaldehyde in 6 h, with predominance of
206 benzoic acid (see Tables S1, S2, and S3 in ESI). Again, replacing
207 Bi2WO6 with the SiliaSun catalysts alters both the activity and
208 the selectivity of bismuth tungstate. Now, after 6 h a large
209 amount of benzaldehyde is obtained along with unreacted
210 substrate both in the original trans configuration, and
211 isomerized into the cis form. Indeed, upon irradiation with
212 visible light trans-cinnamic acid dissolved in water
213 photochemically isomerizes into cis-cinnamic acid.20
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214 Table 3. Conversion of trans-cinnamic acid under visible light over free and sol-gel
215 entrapped Bi2WO6. Figures are the average of results obtained in three
216 independent runs carried out under the same experimental conditions.
255 may justify the observed high activity of the embedded samples
256 even if they contain only 10% in weight of optically active
257 Bi2WO6 phase (See Figure S7 in ESI).
Cinnamic
Benzoic
Acid
Selectivity
(%)
Benzaldehyde
Selectivity
(%)
Reaction
Time (h)
Acid
Conversion
(%)
77
258 In conclusion, we have discovered that trans-ferulic acid and
259 trans-cinnamic acid dissolved in water can be selectively
260 oxidized with significant yield into a mixture of valued acid and
261 aldehyde derivatives (vanillic acid and vanillin, and benzoic acid
262 and benzaldehyde, respectively) under simulated solar light
263 irradiation over flower-like nanostructured Bi2WO6 and, even
264 better, over the SiliaSun sol-gel entrapped forms. The sol-gel
265 encapsulation of the photocatalyst significantly enhanced the
266 selectivity towards the aldehyde or the acid, depending on the
267 nature of the substrate. These findings could be relevant from
268 both scientific and technology viewpoints. From the application
269 viewpoint, in light of the forthcoming broad utilization of solar
270 photocatalysis to chemical synthesis,17 the robustness, low cost
271 and highly porous nature of the sol-gel glassy catalyst make it
272 ideally suited for continuous practical applications. Further
273 experiments are ongoing on different substrates and different
274 SiliaSun catalysts.
Catalyst
Bi2WO6
1
2
3
6
1
2
3
6
1
2
3
6
1
3
5.5
7.4
8.5
92
8.5
98
13.6
20.1
1.2
10.3
13.4
3.7
100
28
SiliaSun
37.7
59.5
73
1.5
8.0
Me0%
2.9
9.7
6.5
16.7
6.0
23
2.7
SiliaSun
55
2.8
8.0
Me10%
79
2.9
11.0
17.1
1.50
2.4
82
6.4
87
4.4
TiO2
99
3.8
275 References
276
1
2
3
4
5
R. Delisi, R. Ciriminna, F. Parrino, L. Palmisano, Y.-J. Xu, M. Pagliaro,
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217 Again, similarly to what observed with trans-ferulic acid, using
277
278
218 the 10%-methylated SiliaSun catalyst slightly enhances the
279
219 selectivity towards the aldehyde (by considering the figures at
280
220 the various reaction times), which now becomes the
281
221 predominant reaction product (Table 3 and Figure S4 in ESI).
282
222 Indeed, the silica entrapment addressed the reaction selectivity
283
223 to the aldehyde or the acid depending on the affinity with the
284
224 used substrate. For all of the runs TOC analysis confirmed a
285
5276-5287.
225 satisfactory carbon mass balance, indicating that no other
286
6
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226 intermediates were produced during reaction with the
287
227 exception of CO and the species detected by HPLC. Finally, to
288
4
2
228 check for possible leaching of active Bi2WO6 reaction centres in289
8
9
4, 1820-1824.
290
J. H. De Boer, The Structure and Properties of Porous Materials,
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229 solution, the solid catalyst was removed by filtration using a
291
230 0.20 μm nylon filter after 90 min (50% of total conversion). The
231 filtrate monitored for activity within subsequent 2 h showed no
232 further reaction, indicating that no catalytically active species
292 10 S.W. Liu, J.G. Yu, J. Solid State Chem, 2008, 181, 1048-1055.
293 11 J. G. Yu, Q. J. Xiang, J. R. Ran, S. Mann, Cryst. Eng. Commun, 2010, 12
,
294
872-879.
233 were present in the filtrate. Moreover, the filtrate analyzed by
234 ICP-MS showed no presence of bismuth or tungsten, thereby
235 excluding leaching of the insoluble Bi2WO6 or its derivatives.
295 12 M.A. Lazar, W.A. Daoud, RSC Adv., 2013, 3, 4130-4140.
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Eur. J. 2016, 22, 7063–7067.
236 Further pointing to no changes in the catalyst structure upon
298 14 S. Zhu, T. Xu, H. Fu, J. Zhao, Y. Zhu, Environ. Sci. Technol., 2007, 41
,
237 reaction, the XRD analysis of the used photocatalysts revealed
299
6234-6239.
238 that in each case the crystal structure was not affected by the300 15 Y. Zhang, Y.-J. Xu, RSC Adv., 2014,
4, 2904–2910.
301 16 V. Augugliaro, G. Camera-Roda, V. Loddo, G. Palmisano, L. Palmisano,
240 selectivity values remained virtually unchanged after three
239 photocatalytic reaction. Furthermore, the conversion and
302
F. Parrino, M.A. Puma, Appl. Catal. B: Environ, 2012, 111–112, 555-
561.
303
241 reaction runs performed with the same catalyst filtered, washed
242 with distilled water and reused as such in a subsequent reaction
304 17 F. Parrino, V. Augugliaro, G. Camera-Roda, V. Loddo, M.J. López-
305
Muñoz, C. Márquez-Álvarez, G. Palmisano, L. Palmisano, M.A. Puma, J.
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243 run. All these evidences indicate the catalytic nature of the
306
244 reaction and the recyclability of the catalysts. In analogy to
307 18 G. Camera Roda, V. Augugliaro, V. Loddo, G. Palmisano, L. Palmisano,
245 silica-supported TiO2, in which formation of Ti-O-Si linkages
308
EP2580182 (2016).
246 induces remarkable optical modification ascribed to electronic
309 19 G. Camera-Roda, A. Cardillo, V. Loddo, L. Palmisano, F. Parrino,
247 semiconductor support interaction,21,22 we make the hypothesis
310
Membranes, 2014, 4, 96-112.
248 that the sol-gel encapsulation of Bi2WO6 likely induces formation311 20 F. Parrino, A. Di Paola, V. Loddo, I. Pibiri, M. Bellardita, L. Palmisano,
312
Appl. Catal. B: Environ, 2016, 182, 347-355.
249 of oxygen bridges between Si and W with different intermediate
250 energy states. Indeed, the photoluminescence spectra of the
313 21 H Weiss, A Fernandez, H. Kisch, Angew. Chem. Int. Ed., 40, 2001, 3825-
251 SiliaSun Me0% catalyst7,23 displays much lower intensity with
314
3827.
315 22 M. Gärtner, V. Dremov, P. Müller, H. Kisch, ChemPhysChem, 6, 2005,
252 respect to bare Bi2WO6, (see Figure S6 in ESI) suggesting an
316
714-718.
253 interaction between silica and Bi2WO6 that markedly influences
254 the spatial electron and hole transfer kinetics. This hypothesis
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