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Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3-loaded TiO2 catalysts
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Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3 -loaded TiO2 catalysts Agatino Di Paola a,∗ , Marianna Bellardita a , Bartolomeo Megna b , Francesco Parrino a , Leonardo Palmisano a a “Schiavello-Grillone” Photocatalysis Group, Dipartimento di Energia, Ingegneria dell’informazione, e modelli Matematici (DEIM), University of Palermo, Viale delle Scienze, 90128 Palermo, Italy b Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali (DICAM), University of Palermo, Viale delle Scienze, 90128 Palermo, Italy
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Article history: Received 7 July 2014 Received in revised form 11 September 2014 Accepted 20 September 2014 Available online xxx Keywords: Photocatalysis Trans-ferulic acid oxidation Vanillin synthesis WO3 -loaded TiO2 photocatalysts
a b s t r a c t The photocatalytic oxidation of trans-ferulic acid to vanillin in water has been studied by using various TiO2 and WO3 -loaded TiO2 samples as catalysts. Different values of selectivity were obtained depending on the physico-chemical properties of the single samples and a vanillin selectivity of 10% was reached in the presence of the commercial TiO2 Merck. Higher selectivity values were obtained by impregnation of TiO2 with H2 WO4 followed by calcination. The increased production of vanillin exhibited by the obtained WO3 -loaded TiO2 powders was attributed to a reduced further oxidation of the aldehyde caused by the presence of the practically inactive tungsten trioxide hydrate on the TiO2 surface. The best result (ca. 25% of selectivity) was obtained with the WO3 (1%)/TiO2 sample. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Vanillin (C8 H8 O3 ) is the main component of the aroma of vanilla which has become in the last decades the most important and widespread flavour in the world [1]. In 2001, 60% of world production of vanillin found utilization in food and nutraceutical industry, 33% in cosmetics and detergents, and the remaining 7% in pharmaceutical and fine chemicals industry [2]. Out of the 12,000 tonnes of vanilla yearly produced in the world, only 1% is obtained by extraction from the plant “Vanilla planifolia” which requires a very long and expensive process to obtain the “natural” product. The remaining 99% is produced by chemical syntheses which allow to obtain a product almost 100 times cheaper than the natural one [3]. However, whereas the natural aroma of vanilla is a “bouquet” of more than 200 components, the synthesized aroma of vanilla is almost exclusively constituted of vanillin and this makes the latter product of lower quality. Furthermore, the chemical processes are not environmentally safe, using toxic solvents and strong oxidizing agents, so that the final product results food-unsafe. For these reasons researchers have focused their investigations in finding “green” processes [4] to obtain a product which can be labelled as
∗ Corresponding author. Tel.: +39 091 23863729; fax: +39 091 23860841. E-mail address: [email protected] (A. Di Paola).
natural according to the European and USA laws. Literature reports a huge amount of papers on biotechnological production of vanillin which affords a natural product. However, although improvements and developments are expected, the bio-process was only once industrially applied [5–7] and there are problems related with the purification steps and with the use of virtually dangerous bacterial strains (Escherichia coli) which have to be previously accurately selected. Photocatalysis in the presence of aqueous TiO2 suspensions at room temperature and atmospheric pressure may be an attractive alternative to the biotechnological and chemical processes for the synthesis of aromatic aldehydes [8]. Recently, Augugliaro et al. [9] reported on the photocatalytic synthesis of vanillin in aqueous suspensions of TiO2 irradiated with UV light, using trans-ferulic acid, isoeugenol, vanillyl alcohol and eugenol as the starting compounds. In particular, the first two compounds are the same precursors considered for the biotechnological production of vanillin being relatively cheap and abundant. The photocatalytic process may be competitive with the biotechnological one for many reasons. Although the two processes are comparable from the economical point of view as far as the selectivity to vanillin is concerned, the photocatalytic treatment uses a non-toxic and safe catalyst which can be easily removed from the system. Furthermore, coupling photocatalysis with pervaporation affords not only the separation and purification of vanillin, but
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Please cite this article in press as: A. Di Paola, et al., Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3 -loaded TiO2 catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.09.012
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may result in process intensification in analogy to what CameraRoda and Santarelli reported for water detoxification [10]. Recently, Parrino et al. [11] investigated the possibility to use visible irradiation for the photocatalytic production of vanillin from ferulic acid when the interaction substrate-catalyst surface gives rise to formation of an absorbing complex species. Although in this case the selectivity to vanillin was quite poor, an economical balance should be considered for application, by taking into account that trans-ferulic acid is an abundant and relatively inexpensive phenolic phytochemical compound and sunlight would be the driving force of this process avoiding the use of UV-emitting lamps. This work reports a study of the influence of the catalyst on a “green” photocatalytic synthesis of vanillin. The compound was obtained by oxidation of trans-ferulic acid in aqueous suspensions at room temperature in the presence of commercial or home prepared bare and WO3 -loaded TiO2 samples. The catalysts with different WO3 -loading were tested with the aim to enhance the selectivity to vanillin.
Table 1 Oxidation of trans-ferulic acid in water in the presence of the various TiO2 samples. Sample
SSAa (m2 g−1 )
tirr b (min)
S(max) c (%)
Xd (%)
Merck Evonik P25 Rutile Tioxide HPS
8 50 8 44
30 180 300 90
10.2 5.1 4.4 8.9
6.4 49.3 21.0 25.0
a b c d
Specific surface area. Irradiation time. Maximum selectivity to vanillin. Conversion of trans-ferulic acid.
shift range, and reported from 65 to 1200 cm−1 as all the peaks were included in this range. The signal integration time was 5 s, and every spectrum was the average of eight repetitions. The power level ranged from 10% to 50% of the maximum power according to the sensitivity of the sample to the laser exposure. 2.3. Photoreactivity experiments
2. Experimental 2.1. Catalysts 2.1.1. TiO2 samples The photoreactivity experiments were carried out by using commercial TiO2 powders (Merck, Evonik P25, Rutile Tioxide) and a home-prepared TiO2 sample prepared starting from titanium(IV) oxysulfate [12]. 20 g of TiOSO4 ·xH2 O (Riedel-de Haën) were added to 90 mL of distilled water at room temperature under continuous stirring. The obtained solution was sealed in a bottle and kept in an oven at 373 K for 48 h. The resultant precipitate was washed by removing many times the surnatant and adding water to restore the initial solution volume. The washing treatment was repeated until residual SO4 2− was not detected by a 0.5 M Ba(NO3 )2 solution. The remaining suspension was dried under vacuum at 328 K and calcined at 873 K for 10 h in air. The code used for this sample was HPS. 2.1.2. WO3 -loaded TiO2 samples WO3 -loaded TiO2 samples were prepared by an incipient wetness method [13]. TiO2 Merck was suspended in a 25% ammonia solution containing the appropriate amount of H2 WO4 (Aldrich) in order to obtain catalysts with WO3 loadings [(wt % = WO3 /(WO3 + TiO2 )] equal to 0.5, 1, 2.5, 5 and 10, respectively. The suspensions were evaporated at dryness under vacuum over a hot water bath, dried at 393 K for 12 h and finally calcined in a muffle oven at 673 K for 1 h, affording white powders of catalysts. A home prepared WO3 sample was obtained by drying an ammonia solution of tungstic acid, followed by a heat treatment analogous to that described above. 2.2. Catalysts characterization X-ray diffraction patterns of the powders were recorded at room temperature by an Ital Structures APD 2000 powder diffractometer using the Cu K␣ radiation and a 2 scan rate of 2◦ /min. The crystalline size of the samples was calculated by using the Scherrer equation. The specific surface area (SSA) of the powders was determined in a FlowSorb 2300 apparatus (Micromeritics) by using the single-point BET method. Before the measurements, the samples were degassed by a N2 /He mixture 30/70 (v/v) for 0.5 h at 523 K. Raman analyses were performed by means of a BwTek i-Raman plus system equipped with a 300 mW, 785 nm laser focalized on the sample by means of a microscope equipped with a 20× magnification lens. The spectra were collected in the 65–3200 cm−1 Raman
The experiments were carried out in a cylindrical photoreactor (CPR, internal diameter: 32 mm, height: 188 mm) containing 150 mL of an aqueous suspension of trans-ferulic acid (or vanillin) at natural pH. The initial substrate concentration was 0.5 mM. The reactor was irradiated by three external Actinic BL TL MINI 15 W/10 Philips fluorescent lamps whose main emission peak was in the near-UV region at 365 nm. The reactor was cooled by water circulating through a Pyrex thimble, so that the temperature of the suspension was about 303 K. The catalyst amounts used for the photocatalytic runs were opportunely chosen for each powder due to their different optical properties. These amounts represented the lowest limits of photocatalyst load to obtain a negligible transmission of light. So the experiments were more comparable as all the entering photons were virtually absorbed by the suspensions. The radiation intensity impinging on the suspensions was measured by a radiometer Delta Ohm DO9721 with a UVA probe. The radiation power absorbed per unit volume of suspension was about 0.76 mW/mL. Air was continuously bubbled during the experiments. During the photoreactivity runs, samples of 3 mL were withdrawn at fixed times and immediately filtered through 0.2 m membranes (HA, Millipore). The quantitative determination and identification of the starting molecules and their oxidation products were performed by means of a Beckman Coulter HPLC (System Gold 126 Solvent Module and 168 Diode Array Detector), equipped with a Phenomenex Kinetex 5u C18 (150 mm × 4.6 mm) column. The eluent consisted of a mixture of acetonitrile and 1 mM trifluoroacetic acid aqueous solution (20:80 volumetric ratio) and the flow rate was 0.8 mL min−1 . Retention times and UV spectra of the compounds were compared with those of standards. Standards of trans-ferulic acid, vanillin and some of reaction intermediates with a purity of >99% were purchased from Sigma-Aldrich. 3. Results and discussion Fig. 1 reports the X-ray diffraction patterns of the bare TiO2 samples. Merck and HPS consisted of only anatase, Rutile Tioxide consisted of only rutile, whereas Evonik P25 consisted of both anatase and rutile. As shown in Table 1, low specific surface area values (8 m2 g−1 ) were obtained for Merck and Rutile Tioxide. The home prepared sample and Evonik P25 exhibited values of surface area ranging between 44 and 50 m2 g−1 . The photocatalytic runs were carried out using air as oxidizing agent since in the presence of only oxygen, a greater mineralization of the substrate and a lower selectivity were observed [9].
Please cite this article in press as: A. Di Paola, et al., Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3 -loaded TiO2 catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.09.012
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Fig. 1. XRD patterns of the various TiO2 samples: (a) Merck, (b) Evonik P25, (c) HPS and (d) Rutile Tioxide (A = anatase, R = rutile).
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Preliminary tests indicated that no significant conversion of the substrate occurred in the dark, both in the absence and in the presence of catalyst. Table 1 summarizes the results of the photocatalytic oxidation of trans-ferulic acid in the presence of the various TiO2 samples. As hypothesized by Augugliaro et al. [9], the photocatalytic oxidation of trans-ferulic acid in water proceeds through parallel reaction routes, effective from the start of the irradiation. One route is the mineralization to CO2 and H2 O that occurs through the formation of a series of intermediates that do not desorb in the bulk of the solution. Other pathways involve the partial oxidation of transferulic acid to stable intermediates that desorb in solution. The most important intermediate is vanillin that can be recovered from the aqueous suspension. However a further oxidation of these intermediates by means of consecutive adsorption–desorption steps on the catalyst surface or in the solid–liquid interface cannot be excluded [14]. The proposed reaction pathways for the photo-oxidation of trans-ferulic acid are shown in Fig. 2. The occurrence of parallel pathways for the trans-ferulic acid oxidation indicates that the substrate molecules interact with the TiO2 surface in two different ways, i.e. that the TiO2 surface possesses two types of sites which are specific for the occurrence of mineralization or partial oxidation [15]. Fig. 3 shows the variation of trans-ferulic acid concentration and conversion along with the vanillin selectivity (ratio between vanillin and the converted ferulic acid) versus irradiation time for a representative run carried out in the presence of Merck TiO2 . It can be observed that the selectivity to vanillin slightly decreased (from 10.2% to ca. 9.5%) after ca. 13% conversion of trans-ferulic acid and it remained practically constant up to conversion of ca. 38%. Table 1 reports the maximum selectivity towards vanillin and the corresponding irradiation time and substrate conversion. Commercial Merck was the most selective catalyst among the bare samples reaching a selectivity to vanillin of 10.2% after 30 min of
Fig. 2. Proposed reaction pathways for the trans-ferulic acid photooxidation [9].
Please cite this article in press as: A. Di Paola, et al., Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3 -loaded TiO2 catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.09.012
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ARTICLE IN PRESS A. Di Paola et al. / Catalysis Today xxx (2014) xxx–xxx Table 2 Oxidation of ferulic acid in water in the presence of the various WO3 /TiO2 samples. Sample
SSAa (m2 g−1 )
tirr b (min)
S (max)c (%)
Xd (%)
TiO2 Merck WO3 (0.5%)/TiO2 WO3 (1%)/TiO2 WO3 (2.5%)/TiO2 WO3 (5%)/TiO2 WO3 (10%)/TiO2
8.0 8.0 7.8 8.1 8.5 7.3
30 120 60 30 120 90
10.2 13.9 24.7 20.4 12.8 12.4
6.4 15.9 4.3 2.2 18.5 8.2
a b c d
Fig. 3. () Trans-ferulic acid concentration, () trans-ferulic acid conversion and () vanillin selectivity versus irradiation time in the presence of the TiO2 Merck sample.
irradiation corresponding to ca. 6.4% of trans-ferulic acid conversion. Evonik P25 was much more oxidant than Merck and therefore less selective, due to the rapid degradation of both trans-ferulic acid and produced vanillin. In the presence of Evonik P25 a conversion of 49.3% of trans-ferulic acid after 180 min of irradiation corresponded to a vanillin selectivity of only 5.1%. A low value of selectivity (4.4% for a 21% conversion) was obtained with Rutile Tioxide. The HPS sample prepared from TiOSO4 revealed a maximum selectivity to vanillin (8.9% for a 25% conversion) slightly slower than that of the Merck sample. When TiO2 is used in the processes of photocatalytic treatment of wastewaters, the best results from the point of view of the efficiency of contaminants removal are usually obtained by using crystalline commercial catalysts which have a high oxidizing power. However, within the selective syntheses, an anti-correlation between oxidant power and selectivity of the various samples has been often found [16]. Generally, the less crystalline is the catalyst, the lower is the oxidizing power and the higher is the selectivity towards the partial oxidation products of the starting substrate [17]. A prominent role in the selectivity is probably played by characteristic properties of the solid surface, such as the extent of surface hydroxylation [16], even if, obviously, this feature it is not the only one to come into play since other physico-chemical parameters can affect the photocatalytic reactions. Evonik P25 showed to have a high oxidizing power so that its selectivity for vanillin was relatively low. Also Rutile Tioxide exhibited a scarce selectivity that can attributed to its low photoactivity, as evidenced by the long time necessary to reach a 21% conversion of trans-ferulic acid. The best selectivity results were obtained with the Merck and HPS samples. Heterogeneous WO3 /TiO2 catalysts have shown high catalytic activity for the selective oxidation of cyclopentene to glutaraldehyde by H2 O2 [18,19]. Tsukamoto et al. [20,21] reported that mixed WO3 /TiO2 catalysts promoted the selective photocatalytic transformation of various substrates and had higher catalytic activity than pure TiO2 . In particular various aromatic alcohols were selectively oxidized to aldehydes [20] and several kinds of alkyl-substituted aromatics were converted into aldehydes and ketones [21]. Table 2 summarizes the results obtained in the presence of WO3 loaded TiO2 samples obtained modifying the TiO2 Merck powder with WO3 . The commercial TiO2 was selected because of its best photocatalytic performances and easier availability. The mixed catalysts showed higher vanillin selectivity than bare TiO2 . As shown in Fig. 4, the selectivity at 10% of trans-ferulic acid conversion increased with increasing the WO3 content and WO3 (1%)/TiO2 was the most efficient sample for the selective oxidation of trans-ferulic
Specific surface area. Irradiation time. Maximum selectivity to vanillin. Conversion of trans-ferulic acid.
acid to vanillin. Higher WO3 loadings, however, gave rise to a decrease of the catalytic activity. Notably, the home prepared WO3 sample obtained with the same procedure was practically inactive for the photocatalytic conversion of trans-ferulic acid. As shown in Fig. 5A, the X-ray diffraction pattern revealed that the powder did not consist of anhydrous WO3 but rather of not much crystalline tungsten trioxide hydrates. Although the exact chemical composition of the tungsten oxides is very difficult to be determined also for WO3 single crystals [22], the main peaks of the diffractogram can been attributed to WO3 ·H2 O and WO3 ·2H2 O. The experimental data were compared with the JCPDS and Glemser et al. [23] d-spacings. Anhydrous monoclinic WO3 was obtained after calcination of the sample at 600 ◦ C for 3 h. Raman spectroscopy confirmed the presence of tungsten trioxide hydrates in the home prepared tungsten oxide. Fig. 5B shows the Raman spectra of the sample before and after further calcination. The effect of the heating was the disappearance of the peak at 992 cm−1 assigned to a terminal W = O stretching band which is common for all types of tungsten trioxide hydrates [24,25]. The comparison with the spectrum of a commercial WO3 (Fluka) revealed the same peaks of the anhydrous tungsten oxide although slightly shifted to lower wavenumbers. The impregnation of TiO2 with H2 WO4 followed by calcination leads to the formation of an inactive tungsten trioxide layer that partially covers the active TiO2 surface. The selective production of vanillin occurs if its decomposition on the catalyst surface is suppressed. Fig. 6 shows the kinetics of photodegradation of pure vanillin in the presence of the various WO3 -loaded TiO2 samples. TiO2 Merck was quite efficient for the oxidation of vanillin (ca. 72% conversion after 240 min) whereas
Fig. 4. Selectivity to vanillin of the TiO2 /WO3 catalysts at 10% of trans-ferulic acid conversion.
Please cite this article in press as: A. Di Paola, et al., Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3 -loaded TiO2 catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.09.012
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Fig. 5. (A) XRD patterns and (B) Raman spectra of: (a) home prepared WO3 sample as obtained and (b) after calcination at 600 ◦ C for 3 h, (c) commercial WO3 .
practically negligible conversion of the substrate was evidenced with the home prepared WO3 sample. The conversion of vanillin decreased with increasing the WO3 content indicating that the presence of WO3 on the TiO2 surface inhibited the aldehyde decomposition.
Fig. 6. Photocatalytic degradation of vanillin in the presence of various WO3 /TiO2 catalysts: (♦) TiO2 , () WO3 (1%)/TiO2 , () WO3 (2.5%)/TiO2 , () WO3 (5%)/TiO2 , (×) WO3 (10%)/TiO2 and (*) WO3 .
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Fig. 7. Time dependence of: (a) trans-ferulic acid disappearance and (b) vanillin selectivity, in the presence of WO3 (1%)/TiO2 . Trans-ferulic acid concentration: 0.5 mM (open symbols), 1.0 mM (solid symbols).
As shown in Table 2, the best result (ca. 25% of maximum selectivity) was obtained with the WO3 (1%)/TiO2 sample. Powders with large WO3 content exhibited a low selectivity because of the presence of particles very rich in WO3 which did not show any photoactivity for the trans-ferulic acid conversion. The presence of inactive tungsten trioxide hydrate favours the release of vanillin molecules before they are attacked by oxidizing species produced under irradiation of the TiO2 surface. Some further photocatalytic experiments were carried out with the best catalyst to verify if the reaction behaviour changed by varying the initial concentration of trans-ferulic acid. As shown in Fig. 7, the same kinetics of trans-ferulic acid disappearance and practically the same values of vanillin selectivity were obtained by doubling the initial concentration of substrate. Fig. 8 shows the temporal profiles of the main intermediate products formed during the photooxidation of trans-ferulic acid in the presence of WO3 (1%)/TiO2 . The contemporaneous presence of caffeic acid and homovanillic acid and vanillyl mandelic acid, confirms that parallel pathways occur during the trans-ferulic acid oxidation [9]. The selectivity of the present photocatalytic process is low but the vanillin produced by oxidation of trans-ferulic acid can be recovered from the aqueous suspension during the runs by pervaporation through a non-porous membrane that completely retains the photocatalytic powders [26]. The membrane is very selective towards vanillin and almost impermeable with respect to most of the other organic compounds. The permeate vapours of vanillin are
Please cite this article in press as: A. Di Paola, et al., Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3 -loaded TiO2 catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.09.012
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This reduces the amount of vanillin adsorbed onto the TiO2 surface avoiding its further oxidation. Acknowledgements This work was financially supported by MIUR (Rome). The authors thank Dr. Francesco Giordano of ISMN-CNR (Palermo) for the XRD measurements. References
Fig. 8. Temporal profiles of the main products identified during the photooxidation of a 1.0 mM trans-ferulic acid aqueous solution, in the presence of WO3 (1%)/TiO2 : (♦) trans-ferulic acid, (×) homovanillic acid, () caffeic acid, () vanillyl mandelic acid, () vanillin.
separated as crystals at ambient temperature with a high degree of purity (higher than 99.8%) without the necessity to use complex extraction and re-crystallization procedures [9]. Reusability tests showed that the WO3 /TiO2 samples suffered some erosion problems under vigorous stirring in the batch reactor, revealing some decline of the selectivity (not of the photodegradation rate) during the experimental runs. Anyway, erosion problems and physical stability of the samples can be reduced if the synthesis of vanillin is carried out in a continuous reactor. 4. Conclusions The experimental results have shown that the selective photocatalytic oxidation of trans-ferulic acid to vanillin in the presence of various TiO2 samples is influenced by many factors as crystallinity, surface hydroxylation and oxidizing power of the catalysts. WO3 loaded TiO2 samples revealed a selectivity to vanillin higher than that exhibited by the most efficient bare TiO2 samples. In particular, the selectivity of WO3 (1%)/TiO2 was ca. 1.8 times higher than that obtained with TiO2 Merck. The high vanillin selectivity has been attributed to the decreased area of the TiO2 surface caused by the coating with the practically inactive tungsten trioxide hydrate.
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Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3 -loaded TiO2 catalysts Agatino Di Paola a,∗ , Marianna Bellardita a , Bartolomeo Megna b , Francesco Parrino a , Leonardo Palmisano a a “Schiavello-Grillone” Photocatalysis Group, Dipartimento di Energia, Ingegneria dell’informazione, e modelli Matematici (DEIM), University of Palermo, Viale delle Scienze, 90128 Palermo, Italy b Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali (DICAM), University of Palermo, Viale delle Scienze, 90128 Palermo, Italy
a r t i c l e
i n f o
Article history: Received 7 July 2014 Received in revised form 11 September 2014 Accepted 20 September 2014 Available online xxx Keywords: Photocatalysis Trans-ferulic acid oxidation Vanillin synthesis WO3 -loaded TiO2 photocatalysts
a b s t r a c t The photocatalytic oxidation of trans-ferulic acid to vanillin in water has been studied by using various TiO2 and WO3 -loaded TiO2 samples as catalysts. Different values of selectivity were obtained depending on the physico-chemical properties of the single samples and a vanillin selectivity of 10% was reached in the presence of the commercial TiO2 Merck. Higher selectivity values were obtained by impregnation of TiO2 with H2 WO4 followed by calcination. The increased production of vanillin exhibited by the obtained WO3 -loaded TiO2 powders was attributed to a reduced further oxidation of the aldehyde caused by the presence of the practically inactive tungsten trioxide hydrate on the TiO2 surface. The best result (ca. 25% of selectivity) was obtained with the WO3 (1%)/TiO2 sample. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Vanillin (C8 H8 O3 ) is the main component of the aroma of vanilla which has become in the last decades the most important and widespread flavour in the world [1]. In 2001, 60% of world production of vanillin found utilization in food and nutraceutical industry, 33% in cosmetics and detergents, and the remaining 7% in pharmaceutical and fine chemicals industry [2]. Out of the 12,000 tonnes of vanilla yearly produced in the world, only 1% is obtained by extraction from the plant “Vanilla planifolia” which requires a very long and expensive process to obtain the “natural” product. The remaining 99% is produced by chemical syntheses which allow to obtain a product almost 100 times cheaper than the natural one [3]. However, whereas the natural aroma of vanilla is a “bouquet” of more than 200 components, the synthesized aroma of vanilla is almost exclusively constituted of vanillin and this makes the latter product of lower quality. Furthermore, the chemical processes are not environmentally safe, using toxic solvents and strong oxidizing agents, so that the final product results food-unsafe. For these reasons researchers have focused their investigations in finding “green” processes [4] to obtain a product which can be labelled as
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natural according to the European and USA laws. Literature reports a huge amount of papers on biotechnological production of vanillin which affords a natural product. However, although improvements and developments are expected, the bio-process was only once industrially applied [5–7] and there are problems related with the purification steps and with the use of virtually dangerous bacterial strains (Escherichia coli) which have to be previously accurately selected. Photocatalysis in the presence of aqueous TiO2 suspensions at room temperature and atmospheric pressure may be an attractive alternative to the biotechnological and chemical processes for the synthesis of aromatic aldehydes [8]. Recently, Augugliaro et al. [9] reported on the photocatalytic synthesis of vanillin in aqueous suspensions of TiO2 irradiated with UV light, using trans-ferulic acid, isoeugenol, vanillyl alcohol and eugenol as the starting compounds. In particular, the first two compounds are the same precursors considered for the biotechnological production of vanillin being relatively cheap and abundant. The photocatalytic process may be competitive with the biotechnological one for many reasons. Although the two processes are comparable from the economical point of view as far as the selectivity to vanillin is concerned, the photocatalytic treatment uses a non-toxic and safe catalyst which can be easily removed from the system. Furthermore, coupling photocatalysis with pervaporation affords not only the separation and purification of vanillin, but
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Please cite this article in press as: A. Di Paola, et al., Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3 -loaded TiO2 catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.09.012
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may result in process intensification in analogy to what CameraRoda and Santarelli reported for water detoxification [10]. Recently, Parrino et al. [11] investigated the possibility to use visible irradiation for the photocatalytic production of vanillin from ferulic acid when the interaction substrate-catalyst surface gives rise to formation of an absorbing complex species. Although in this case the selectivity to vanillin was quite poor, an economical balance should be considered for application, by taking into account that trans-ferulic acid is an abundant and relatively inexpensive phenolic phytochemical compound and sunlight would be the driving force of this process avoiding the use of UV-emitting lamps. This work reports a study of the influence of the catalyst on a “green” photocatalytic synthesis of vanillin. The compound was obtained by oxidation of trans-ferulic acid in aqueous suspensions at room temperature in the presence of commercial or home prepared bare and WO3 -loaded TiO2 samples. The catalysts with different WO3 -loading were tested with the aim to enhance the selectivity to vanillin.
Table 1 Oxidation of trans-ferulic acid in water in the presence of the various TiO2 samples. Sample
SSAa (m2 g−1 )
tirr b (min)
S(max) c (%)
Xd (%)
Merck Evonik P25 Rutile Tioxide HPS
8 50 8 44
30 180 300 90
10.2 5.1 4.4 8.9
6.4 49.3 21.0 25.0
a b c d
Specific surface area. Irradiation time. Maximum selectivity to vanillin. Conversion of trans-ferulic acid.
shift range, and reported from 65 to 1200 cm−1 as all the peaks were included in this range. The signal integration time was 5 s, and every spectrum was the average of eight repetitions. The power level ranged from 10% to 50% of the maximum power according to the sensitivity of the sample to the laser exposure. 2.3. Photoreactivity experiments
2. Experimental 2.1. Catalysts 2.1.1. TiO2 samples The photoreactivity experiments were carried out by using commercial TiO2 powders (Merck, Evonik P25, Rutile Tioxide) and a home-prepared TiO2 sample prepared starting from titanium(IV) oxysulfate [12]. 20 g of TiOSO4 ·xH2 O (Riedel-de Haën) were added to 90 mL of distilled water at room temperature under continuous stirring. The obtained solution was sealed in a bottle and kept in an oven at 373 K for 48 h. The resultant precipitate was washed by removing many times the surnatant and adding water to restore the initial solution volume. The washing treatment was repeated until residual SO4 2− was not detected by a 0.5 M Ba(NO3 )2 solution. The remaining suspension was dried under vacuum at 328 K and calcined at 873 K for 10 h in air. The code used for this sample was HPS. 2.1.2. WO3 -loaded TiO2 samples WO3 -loaded TiO2 samples were prepared by an incipient wetness method [13]. TiO2 Merck was suspended in a 25% ammonia solution containing the appropriate amount of H2 WO4 (Aldrich) in order to obtain catalysts with WO3 loadings [(wt % = WO3 /(WO3 + TiO2 )] equal to 0.5, 1, 2.5, 5 and 10, respectively. The suspensions were evaporated at dryness under vacuum over a hot water bath, dried at 393 K for 12 h and finally calcined in a muffle oven at 673 K for 1 h, affording white powders of catalysts. A home prepared WO3 sample was obtained by drying an ammonia solution of tungstic acid, followed by a heat treatment analogous to that described above. 2.2. Catalysts characterization X-ray diffraction patterns of the powders were recorded at room temperature by an Ital Structures APD 2000 powder diffractometer using the Cu K␣ radiation and a 2 scan rate of 2◦ /min. The crystalline size of the samples was calculated by using the Scherrer equation. The specific surface area (SSA) of the powders was determined in a FlowSorb 2300 apparatus (Micromeritics) by using the single-point BET method. Before the measurements, the samples were degassed by a N2 /He mixture 30/70 (v/v) for 0.5 h at 523 K. Raman analyses were performed by means of a BwTek i-Raman plus system equipped with a 300 mW, 785 nm laser focalized on the sample by means of a microscope equipped with a 20× magnification lens. The spectra were collected in the 65–3200 cm−1 Raman
The experiments were carried out in a cylindrical photoreactor (CPR, internal diameter: 32 mm, height: 188 mm) containing 150 mL of an aqueous suspension of trans-ferulic acid (or vanillin) at natural pH. The initial substrate concentration was 0.5 mM. The reactor was irradiated by three external Actinic BL TL MINI 15 W/10 Philips fluorescent lamps whose main emission peak was in the near-UV region at 365 nm. The reactor was cooled by water circulating through a Pyrex thimble, so that the temperature of the suspension was about 303 K. The catalyst amounts used for the photocatalytic runs were opportunely chosen for each powder due to their different optical properties. These amounts represented the lowest limits of photocatalyst load to obtain a negligible transmission of light. So the experiments were more comparable as all the entering photons were virtually absorbed by the suspensions. The radiation intensity impinging on the suspensions was measured by a radiometer Delta Ohm DO9721 with a UVA probe. The radiation power absorbed per unit volume of suspension was about 0.76 mW/mL. Air was continuously bubbled during the experiments. During the photoreactivity runs, samples of 3 mL were withdrawn at fixed times and immediately filtered through 0.2 m membranes (HA, Millipore). The quantitative determination and identification of the starting molecules and their oxidation products were performed by means of a Beckman Coulter HPLC (System Gold 126 Solvent Module and 168 Diode Array Detector), equipped with a Phenomenex Kinetex 5u C18 (150 mm × 4.6 mm) column. The eluent consisted of a mixture of acetonitrile and 1 mM trifluoroacetic acid aqueous solution (20:80 volumetric ratio) and the flow rate was 0.8 mL min−1 . Retention times and UV spectra of the compounds were compared with those of standards. Standards of trans-ferulic acid, vanillin and some of reaction intermediates with a purity of >99% were purchased from Sigma-Aldrich. 3. Results and discussion Fig. 1 reports the X-ray diffraction patterns of the bare TiO2 samples. Merck and HPS consisted of only anatase, Rutile Tioxide consisted of only rutile, whereas Evonik P25 consisted of both anatase and rutile. As shown in Table 1, low specific surface area values (8 m2 g−1 ) were obtained for Merck and Rutile Tioxide. The home prepared sample and Evonik P25 exhibited values of surface area ranging between 44 and 50 m2 g−1 . The photocatalytic runs were carried out using air as oxidizing agent since in the presence of only oxygen, a greater mineralization of the substrate and a lower selectivity were observed [9].
Please cite this article in press as: A. Di Paola, et al., Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3 -loaded TiO2 catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.09.012
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Fig. 1. XRD patterns of the various TiO2 samples: (a) Merck, (b) Evonik P25, (c) HPS and (d) Rutile Tioxide (A = anatase, R = rutile).
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Preliminary tests indicated that no significant conversion of the substrate occurred in the dark, both in the absence and in the presence of catalyst. Table 1 summarizes the results of the photocatalytic oxidation of trans-ferulic acid in the presence of the various TiO2 samples. As hypothesized by Augugliaro et al. [9], the photocatalytic oxidation of trans-ferulic acid in water proceeds through parallel reaction routes, effective from the start of the irradiation. One route is the mineralization to CO2 and H2 O that occurs through the formation of a series of intermediates that do not desorb in the bulk of the solution. Other pathways involve the partial oxidation of transferulic acid to stable intermediates that desorb in solution. The most important intermediate is vanillin that can be recovered from the aqueous suspension. However a further oxidation of these intermediates by means of consecutive adsorption–desorption steps on the catalyst surface or in the solid–liquid interface cannot be excluded [14]. The proposed reaction pathways for the photo-oxidation of trans-ferulic acid are shown in Fig. 2. The occurrence of parallel pathways for the trans-ferulic acid oxidation indicates that the substrate molecules interact with the TiO2 surface in two different ways, i.e. that the TiO2 surface possesses two types of sites which are specific for the occurrence of mineralization or partial oxidation [15]. Fig. 3 shows the variation of trans-ferulic acid concentration and conversion along with the vanillin selectivity (ratio between vanillin and the converted ferulic acid) versus irradiation time for a representative run carried out in the presence of Merck TiO2 . It can be observed that the selectivity to vanillin slightly decreased (from 10.2% to ca. 9.5%) after ca. 13% conversion of trans-ferulic acid and it remained practically constant up to conversion of ca. 38%. Table 1 reports the maximum selectivity towards vanillin and the corresponding irradiation time and substrate conversion. Commercial Merck was the most selective catalyst among the bare samples reaching a selectivity to vanillin of 10.2% after 30 min of
Fig. 2. Proposed reaction pathways for the trans-ferulic acid photooxidation [9].
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ARTICLE IN PRESS A. Di Paola et al. / Catalysis Today xxx (2014) xxx–xxx Table 2 Oxidation of ferulic acid in water in the presence of the various WO3 /TiO2 samples. Sample
SSAa (m2 g−1 )
tirr b (min)
S (max)c (%)
Xd (%)
TiO2 Merck WO3 (0.5%)/TiO2 WO3 (1%)/TiO2 WO3 (2.5%)/TiO2 WO3 (5%)/TiO2 WO3 (10%)/TiO2
8.0 8.0 7.8 8.1 8.5 7.3
30 120 60 30 120 90
10.2 13.9 24.7 20.4 12.8 12.4
6.4 15.9 4.3 2.2 18.5 8.2
a b c d
Fig. 3. () Trans-ferulic acid concentration, () trans-ferulic acid conversion and () vanillin selectivity versus irradiation time in the presence of the TiO2 Merck sample.
irradiation corresponding to ca. 6.4% of trans-ferulic acid conversion. Evonik P25 was much more oxidant than Merck and therefore less selective, due to the rapid degradation of both trans-ferulic acid and produced vanillin. In the presence of Evonik P25 a conversion of 49.3% of trans-ferulic acid after 180 min of irradiation corresponded to a vanillin selectivity of only 5.1%. A low value of selectivity (4.4% for a 21% conversion) was obtained with Rutile Tioxide. The HPS sample prepared from TiOSO4 revealed a maximum selectivity to vanillin (8.9% for a 25% conversion) slightly slower than that of the Merck sample. When TiO2 is used in the processes of photocatalytic treatment of wastewaters, the best results from the point of view of the efficiency of contaminants removal are usually obtained by using crystalline commercial catalysts which have a high oxidizing power. However, within the selective syntheses, an anti-correlation between oxidant power and selectivity of the various samples has been often found [16]. Generally, the less crystalline is the catalyst, the lower is the oxidizing power and the higher is the selectivity towards the partial oxidation products of the starting substrate [17]. A prominent role in the selectivity is probably played by characteristic properties of the solid surface, such as the extent of surface hydroxylation [16], even if, obviously, this feature it is not the only one to come into play since other physico-chemical parameters can affect the photocatalytic reactions. Evonik P25 showed to have a high oxidizing power so that its selectivity for vanillin was relatively low. Also Rutile Tioxide exhibited a scarce selectivity that can attributed to its low photoactivity, as evidenced by the long time necessary to reach a 21% conversion of trans-ferulic acid. The best selectivity results were obtained with the Merck and HPS samples. Heterogeneous WO3 /TiO2 catalysts have shown high catalytic activity for the selective oxidation of cyclopentene to glutaraldehyde by H2 O2 [18,19]. Tsukamoto et al. [20,21] reported that mixed WO3 /TiO2 catalysts promoted the selective photocatalytic transformation of various substrates and had higher catalytic activity than pure TiO2 . In particular various aromatic alcohols were selectively oxidized to aldehydes [20] and several kinds of alkyl-substituted aromatics were converted into aldehydes and ketones [21]. Table 2 summarizes the results obtained in the presence of WO3 loaded TiO2 samples obtained modifying the TiO2 Merck powder with WO3 . The commercial TiO2 was selected because of its best photocatalytic performances and easier availability. The mixed catalysts showed higher vanillin selectivity than bare TiO2 . As shown in Fig. 4, the selectivity at 10% of trans-ferulic acid conversion increased with increasing the WO3 content and WO3 (1%)/TiO2 was the most efficient sample for the selective oxidation of trans-ferulic
Specific surface area. Irradiation time. Maximum selectivity to vanillin. Conversion of trans-ferulic acid.
acid to vanillin. Higher WO3 loadings, however, gave rise to a decrease of the catalytic activity. Notably, the home prepared WO3 sample obtained with the same procedure was practically inactive for the photocatalytic conversion of trans-ferulic acid. As shown in Fig. 5A, the X-ray diffraction pattern revealed that the powder did not consist of anhydrous WO3 but rather of not much crystalline tungsten trioxide hydrates. Although the exact chemical composition of the tungsten oxides is very difficult to be determined also for WO3 single crystals [22], the main peaks of the diffractogram can been attributed to WO3 ·H2 O and WO3 ·2H2 O. The experimental data were compared with the JCPDS and Glemser et al. [23] d-spacings. Anhydrous monoclinic WO3 was obtained after calcination of the sample at 600 ◦ C for 3 h. Raman spectroscopy confirmed the presence of tungsten trioxide hydrates in the home prepared tungsten oxide. Fig. 5B shows the Raman spectra of the sample before and after further calcination. The effect of the heating was the disappearance of the peak at 992 cm−1 assigned to a terminal W = O stretching band which is common for all types of tungsten trioxide hydrates [24,25]. The comparison with the spectrum of a commercial WO3 (Fluka) revealed the same peaks of the anhydrous tungsten oxide although slightly shifted to lower wavenumbers. The impregnation of TiO2 with H2 WO4 followed by calcination leads to the formation of an inactive tungsten trioxide layer that partially covers the active TiO2 surface. The selective production of vanillin occurs if its decomposition on the catalyst surface is suppressed. Fig. 6 shows the kinetics of photodegradation of pure vanillin in the presence of the various WO3 -loaded TiO2 samples. TiO2 Merck was quite efficient for the oxidation of vanillin (ca. 72% conversion after 240 min) whereas
Fig. 4. Selectivity to vanillin of the TiO2 /WO3 catalysts at 10% of trans-ferulic acid conversion.
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Fig. 5. (A) XRD patterns and (B) Raman spectra of: (a) home prepared WO3 sample as obtained and (b) after calcination at 600 ◦ C for 3 h, (c) commercial WO3 .
practically negligible conversion of the substrate was evidenced with the home prepared WO3 sample. The conversion of vanillin decreased with increasing the WO3 content indicating that the presence of WO3 on the TiO2 surface inhibited the aldehyde decomposition.
Fig. 6. Photocatalytic degradation of vanillin in the presence of various WO3 /TiO2 catalysts: (♦) TiO2 , () WO3 (1%)/TiO2 , () WO3 (2.5%)/TiO2 , () WO3 (5%)/TiO2 , (×) WO3 (10%)/TiO2 and (*) WO3 .
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Fig. 7. Time dependence of: (a) trans-ferulic acid disappearance and (b) vanillin selectivity, in the presence of WO3 (1%)/TiO2 . Trans-ferulic acid concentration: 0.5 mM (open symbols), 1.0 mM (solid symbols).
As shown in Table 2, the best result (ca. 25% of maximum selectivity) was obtained with the WO3 (1%)/TiO2 sample. Powders with large WO3 content exhibited a low selectivity because of the presence of particles very rich in WO3 which did not show any photoactivity for the trans-ferulic acid conversion. The presence of inactive tungsten trioxide hydrate favours the release of vanillin molecules before they are attacked by oxidizing species produced under irradiation of the TiO2 surface. Some further photocatalytic experiments were carried out with the best catalyst to verify if the reaction behaviour changed by varying the initial concentration of trans-ferulic acid. As shown in Fig. 7, the same kinetics of trans-ferulic acid disappearance and practically the same values of vanillin selectivity were obtained by doubling the initial concentration of substrate. Fig. 8 shows the temporal profiles of the main intermediate products formed during the photooxidation of trans-ferulic acid in the presence of WO3 (1%)/TiO2 . The contemporaneous presence of caffeic acid and homovanillic acid and vanillyl mandelic acid, confirms that parallel pathways occur during the trans-ferulic acid oxidation [9]. The selectivity of the present photocatalytic process is low but the vanillin produced by oxidation of trans-ferulic acid can be recovered from the aqueous suspension during the runs by pervaporation through a non-porous membrane that completely retains the photocatalytic powders [26]. The membrane is very selective towards vanillin and almost impermeable with respect to most of the other organic compounds. The permeate vapours of vanillin are
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This reduces the amount of vanillin adsorbed onto the TiO2 surface avoiding its further oxidation. Acknowledgements This work was financially supported by MIUR (Rome). The authors thank Dr. Francesco Giordano of ISMN-CNR (Palermo) for the XRD measurements. References
Fig. 8. Temporal profiles of the main products identified during the photooxidation of a 1.0 mM trans-ferulic acid aqueous solution, in the presence of WO3 (1%)/TiO2 : (♦) trans-ferulic acid, (×) homovanillic acid, () caffeic acid, () vanillyl mandelic acid, () vanillin.
separated as crystals at ambient temperature with a high degree of purity (higher than 99.8%) without the necessity to use complex extraction and re-crystallization procedures [9]. Reusability tests showed that the WO3 /TiO2 samples suffered some erosion problems under vigorous stirring in the batch reactor, revealing some decline of the selectivity (not of the photodegradation rate) during the experimental runs. Anyway, erosion problems and physical stability of the samples can be reduced if the synthesis of vanillin is carried out in a continuous reactor. 4. Conclusions The experimental results have shown that the selective photocatalytic oxidation of trans-ferulic acid to vanillin in the presence of various TiO2 samples is influenced by many factors as crystallinity, surface hydroxylation and oxidizing power of the catalysts. WO3 loaded TiO2 samples revealed a selectivity to vanillin higher than that exhibited by the most efficient bare TiO2 samples. In particular, the selectivity of WO3 (1%)/TiO2 was ca. 1.8 times higher than that obtained with TiO2 Merck. The high vanillin selectivity has been attributed to the decreased area of the TiO2 surface caused by the coating with the practically inactive tungsten trioxide hydrate.
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Please cite this article in press as: A. Di Paola, et al., Photocatalytic oxidation of trans-ferulic acid to vanillin on TiO2 and WO3 -loaded TiO2 catalysts, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.09.012