- Email: [email protected]
Outstanding response of carbon nitride photocatalysts for selective synthesis of aldehydes under UV-LED irradiation
Catalysis Today xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Outstanding response of carbon nitride photocatalysts for selective synthesis of aldehydes under UV-LED irradiation Joana C. Lopes, Maria J. Sampaio, Raquel A. Fernandes, Maria J. Lima, Joaquim L. Faria, ⁎ Cláudia G. Silva Laboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials (LSRE-LCM), Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465, Porto, Portugal
ARTICLE INFO
ABSTRACT
Keywords: Selective oxidation Photocatalysis Carbon nitride Aromatic aldehydes Glass rings
The photocatalytic conversion of alcohols into the corresponding aldehydes constitutes an important reaction in organic synthesis. Optical semiconductors based on carbon nitride were prepared by thermal condensation of dicyandiamide followed by a thermal post-treatment (gCN-T) and further sonication (gCN-TS). The efficiency of these materials was evaluated on the selective conversion of different aromatic alcohols to the corresponding aldehydes, namely anisaldehyde, benzaldehyde, tolualdehyde, piperonal and vanillin. The best performance of gCN-T (1 g L−1) was found for the conversion of anisyl alcohol to anisaldehyde (> 99% conversion and selectivity in 45 min of UV-LED irradiation). The alcohol conversion and yield for aldehyde formation was related with the number, electronic nature and position of the functional groups in the aromatic ring. The use of gCN-TS, constituted by carbon nitride nanosheets, also produced > 99% conversion and selectivity towards anisaldehyde, while reducing the reaction time to 30 min and the catalyst load to 0.2 g L−1. The immobilization of gCNTS in glass rings revealed to be a promising strategy for continuous production of aldehydes without the need of a catalyst separation step.
1. Introduction Photocatalytic technologies have been attracting wide attention of the scientific community in different fields, including degradation of organic compounds [1–4], hydrogen production [5–8], reduction of CO2 [9,10], organic synthesis [11,12], among others. In recent years, selective conversion of different alcohols into the corresponding aldehydes has emerged as a hot topic of research, due to their importance in organic synthesis reactions and fine chemical industry [13,14]. In general, conventional industrial production of aldehydes usually requires the use of hazard oxidants/reductants and harsh operation conditions, such as high temperature and pressure. Moreover, during their industrial synthesis high quantities of wastes are produced, usually presenting severe toxicity. Thus, the development of efficient alternatives operating under mild and environmentally friendly conditions (room temperature, atmospheric pressure and the use of water as solvent) is a topic of major concern. Polymeric carbon nitride (C3N4) based materials have been reported as highly efficient optical semiconductors widely used in photocatalytic processes [15–19], which is due to the low band gap (≈2.7 eV) when
⁎
compared to metal oxides (e.g., TiO2 and ZnO). Moreover, the position of valence and conduction bands of C3N4 (+1.58 V and -1.12 V, respectively) in water [14,20], makes this material a promising catalyst for selective oxidation of aromatic alcohols [21]. Other applications of C3N4 based materials in photocatalytic organic synthesis include C–H amination [22], dehydrogenation of N-heterocycles [17], cycloaddition, esterification, and transesterification reactions [23]. Carbon nitride has also been widely used for photocatalytic elimination of organic pollutants such as tetracycline [24,25], sulfamethazine [26,27], and oxytetracycline hydrochloride [28], and for photocatalytic fuel production such as hydrogen from water splitting, and hydrocarbons from CO2 reduction [29–35]. The synthesis of C3N4 may be accomplished by thermal polymerization of a variety of precursors such as urea, thiourea, melamine, dicyandiamide, cyanamide, and guanidine hydrochloride in the temperature range of 500–600 °C under air or inert atmosphere [10]. Nevertheless, bulk C3N4 materials usually exhibit low specific surface area, being a drawback due the small amount of active sites available to promote photocatalytic redox reactions. Some strategies have been employed seeking for enhanced performance of C3N4 materials namely
Corresponding author. E-mail address: [email protected] (C.G. Silva).
https://doi.org/10.1016/j.cattod.2019.03.050 Received 30 November 2018; Received in revised form 11 March 2019; Accepted 21 March 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Joana C. Lopes, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.03.050
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al.
by exposing this material to thermal, mechanical and chemical treatments [36,37]. Remarkable performance of a mesoporous C3N4, obtained by templating with silica nanoparticles, has been reported by Su et al. [38] for the photocatalytic synthesis of aldehydes under visible light. Yet, reactions were carried out using organic solvents (such as acetonitrile and toluene), and under high pressure and temperature (8 bar of O2 and 100 °C, respectively). The performance of the C3N4 material was ascribed to its high ability to activate oxygen (generating superoxide radicals, O2%―) and to the basic character of the material facilitating the deprotonation of the alcohol, followed by reaction with positively charged hole and recombination with O2%― to form the aldehyde. In addition, the authors also observed the formation of H2O2, resulting from two-electron reduction of O2. More recently, several works reported the synthesis of aromatic aldehydes in aqueous medium at mild conditions of temperature and pressure [36,39–43]. For instance, Bellardita et al. [40] used different precursors (melamine, urea and thiourea) to prepare bulk C3N4. The materials were subjected to a second thermal treatment in the presence of a phosphorus precursor, producing P-doped catalysts with increased surface area. These materials were used for the photocatalytic synthesis of piperonal, benzaldehyde and tolualdehyde, under aerated conditions, using fluorescent lamps as irradiation source, resulting in maximum selectivity of 69%, 72% and 26% for benzaldehyde, tolualdehyde and piperonal, respectively after 4 h of irradiation. In a previous work by our group [36] a C3N4 material obtained by thermal exfoliation of bulk carbon nitride at 500 °C (here denoted as gCN-T) was tested for the synthesis of benzaldehyde under deoxygenated conditions using a UV-LED system as light source. The exfoliation process leads to an increase in benzyl alcohol conversion (from 18 to 66% in 4 h of reaction), keeping the selectivity for benzaldehyde above 90%, this feature being attributed to the higher surface area of the exfoliated photocatalyst. In the present work, we have extended the use of gCN-T [36] to the photocatalytic production of benzaldehyde and other aromatic aldehydes, including anisaldehyde, tolualdehyde, piperonal and vanillin, under oxygenated conditions. Due to the several practical problems arising from the use of powder materials (e.g., separation of the catalyst), C3N4 nanosheets (gCN-TS), obtained by sonication of gCN-T, were immobilized on glass rings, which were used for the photocatalytic synthesis of anisaldehyde in continuous flow. To the best of our knowledge, this is the first study using immobilized C3N4 for the photocatalytic synthesis of aldehydes.
exposed to a second thermal step for 2 h at 500 °C and labeled as gCN-T. Additionally, an aqueous suspension containing a certain amount of gCN-T was sonicated (ultrasonic processor UP400S, 24 kHz) for 50 min. The sonicated suspension was centrifuged for 15 min at 3000 rpm to isolate the unexfoliated gCN-T material from the supernatant. The obtained colloidal suspension was denoted as gCN-TS. Glass rings (diameter = 3 mm, length = 3 mm) were cleaned as described elsewhere [44]. Before the coating with the gCN-TS suspension, the glass rings were treated with a 2% (w/V) aqueous solution of PVA by dip-coating. This procedure was directly followed by a second dipcoating of the glass rings in the gCN-TS suspension. The resulting glass rings were dried under ambient temperature for 48 h. The specific surface area (SBET) of the powder materials was determined by multipoint analysis of nitrogen adsorption isotherms at −196 °C in relative pressure range from 0.05 to 0.2 using the BrunauerEmmett-Teller method in a Quantachrome NOVA 4200e apparatus. The optical absorption was determined by diffuse reflectance UV–vis (DRUV-Vis) spectroscopy using a JASCO V-560 spectrophotometer equipped with an integrating sphere attachment (JASCO ISV-469). The results were recorded in equivalent absorption Kubelka–Munk units and used to calculate the optical bandgap energy of the photocatalysts. The morphology of the materials (as powders and immobilized) was examined by scanning electron microscopy (SEM) using a FEI Quanta 400 FEG ESEM/EDAX Genesis X4M (15 keV). Transmission electron microscopy (TEM) micrographs were conducted on a JEM 220FS microscope (Jeol, Japan), equipped with a LaB6 electron gun operating at 200 kV. 2.3. Photocatalytic experiments The photocatalytic efficiency of the different photocatalysts for the synthesis of different aldehydes, was firstly evaluated using a batch reactor. In a typical experiment, a borosilicate reactor was filled with 50 mL of a 1.5 mM aqueous solution of an aromatic alcohol. The catalyst load was fixed at 1.0 g L−1 by taking into consideration our previous studies using similar catalysts for benzaldehyde production [36]. The suspensions were continuously stirred and saturated with an air flow. Before turning on the irradiation, a 30 min period in dark was performed to establish the adsorption-desorption equilibrium between the aromatic alcohol aqueous solution and the photocatalyst. Then, the photocatalytic synthesis of the aldehydes was conducted for 4 h using a four LED system with a maximum wavelength at 370 nm as radiation source. The nominal intensity of the irradiation of each LED was c.a. 450 W m-2, which was determined using an UV-Vis spectroradiometer (USB2000+, OceanOptics, USA). Experiments using the gCN-TS suspension were performed using the system describe above. Photocatalytic experiments in continuous mode were carried out in a borosilicate cylindrical reactor (internal diameter = 2.7 cm; length =7.0 cm) with 38.5 mL of useful volume and packed with 115 glass rings coated with gCN-TS. This reactor configuration was used for the synthesis of AAD from AA. An AA solution (0.5 mM) contained in a reservoir was introduced in the reactor using a peristaltic pump at a constant flow rate (0.70 mL min−1). The irradiation source was the same one used in batch reactions. Reactions were extended for 24 h. High Performance Liquid Chromatography (HPLC) was used for identification and quantification of aromatic alcohols and aldehydes using a Shimadzu Corporation apparatus equipped with a Diode Array Detector (SPD M20A), a solvent delivery pump (LC-30AD) and a KinetexTM F5 1.7 μm 100 Å column (100 mm × 2.1 mm). The temperature of the autosampler and column oven were fixed at 4.0 °C and 35 °C, respectively. A mixture of 0.1% v/v of formic acid and methanol with volume ratio of 30:70, respectively, was isocratically eluted for 15 min at 0.150 mL min−1.
2. Experimental 2.1. Materials Dicyandiamide (DCN, 99%), benzyl alcohol (BA, > 99%), benzaldehyde (BAD, > 99%), piperonyl alcohol (PA, 98%), piperonal (PAD, 99%), p-anisaldehyde (AAD, 98%), polyvinyl alcohol (PVA), titanium (III) sulfate solution (Ti2(SO4)3) and sulfuric acid (purity ≥ 95–97%) were purchased from Sigma-Aldrich. Anisyl alcohol (AA, 98%), vanillyl alcohol (VA, > 98%) and vanillin (VAD, 99%) were supplied from Alfa Aesar. p-toluyl alcohol (TA, 98%) and p-tolualdehyde (TAD, 97%) were obtained from Acros Organics. Formic acid (98%), methanol (99.6%), 2-propanol (100%) and acetone (100%) were purchased from Fluka. Titanium dioxide Aeroxide® P25 was acquired from Evonik Industries. Ultrapure water (UP) was obtained by a Direct-Q Milipore system. 2.2. Catalyst preparation and characterization Polymeric carbon nitride was prepared by thermal decomposition of DCN, as described elsewhere [36]. Briefly, a certain amount of DCN was placed in a porcelain-capsule inside a muffle furnace under static air conditions for 4 h at 550 °C. The resulting material (here labeled as gCN), was washed and dried at 110 °C for 12 h. Then, the gCN was 2
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al.
Fig. 1. (a–b) SEM micrographs of gCN (a) and gCN-T (b) powders (insets are the corresponding TEM micrographs); (c–d) SEM micrographs of glass rings coated with gCN-TS at different magnifications (see bar scale for appropriate dimensions); (e–f) Tyndall effect for gCN-TS colloidal suspension (recipient on the right) under illumination with red laser beam (e), in comparison with artificial light (f); (g) TEM micrograph of gCN-TS.
3. Results and discussion
gCN and gCN-T (Fig. 1). While the gCN material (Fig. 1a) presents a compact stacked lamellar structure, SEM micrographs of the thermalexfoliated material (gCN-T) revealed the presence of thinner layers (Fig. 1b). TEM micrographs of the gCN and gCN-T materials (Fig. 1a and b, inset) confirm the formation of a less dense material after submitting gCN to the thermal post-treatment (gCN-T). These observations have been well discussed in the literature [16,36,45], and corroborate the results obtained from the SBET measurements, i.e., the thermal exfoliation promote the increase in the surface area of gCN-T by causing the splitting of the interlayer bonds. Representative SEM images of gCN-TS-coated glass rings show a relatively homogeneous surface at microscale (Fig. 1c and d). gCN-TS particles of smaller dimensions than gCN-T can be observed at higher magnification (Fig. 1d), which resulted from the intense ultrasonication treatment applied to gCN-T. The colloidal nature of the gCN-TS suspension was demonstrated by the scattering observed when the suspension is crossed with the red light of a laser beam – Tyndall effect (Fig. 1e). The nanometric dimensionality of the sample is in line with the TEM analysis of the colloidal suspension (Fig. 1f-g), in which is
3.1. Catalyst characterization Detailed characterization of gCN and gCN-T materials can be found in a previous work by our group [36]. The thermal treatment of gCN promotes the exfoliation of the material by separating the layers of the bulk material (gCN) which are stacked together by weak van der Waals forces. According to the results obtained from the nitrogen adsorptiondesorption isotherms, the textural properties were changed by the thermal treatment applied to the gCN material. The specific surface area of gCN and gCN-T, obtained by the Brunauer-Emmett-Teller method (SBET), were 6 and 79 m2 g−1, respectively, corresponding to an increase of more than 13 times promoted by the thermal treatment. For most photocatalytic semiconductors, an increase in the SBET is recognized as an advantage, since more active sites are available for the occurrence of redox reactions at the catalyst surface. Different morphologies were observed by microscopy analyses of 3
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al.
clearly observed the ultrathin lamellar morphology of the material.
120 min, respectively. Finally, very low yields of PAD and VAD were obtained under the used conditions (9 and 2%, respectively) in reactions with low selectivity for these compounds (Fig. 2b). In heterogeneous photocatalysis, the reactivity of benzene derivatives is widely influenced by the number, electronic mature and position of substituent groups in the aromatic ring [12,44,46]. Thus, based on the principles of photocatalytic oxidation of aromatic compounds, the relative facility of selectively synthesize the aromatic aldehydes here studied may be rationalized by the relative ability of the parent compound to be converted. The chemical structures of AA, PA, TA and VA derive from BA, the difference being the type and position of substituent groups in the aromatic ring (Fig. SD3). The photocatalytic conversion of such molecules may be accelerated or hindered by the presence of electron-donating (activating) or electron-withdrawing (deactivating) groups in the aromatic ring, respectively. In molecules with more than one functional group, this effect results from the overall contribution of each functional group. Thus, it may be expected that the relative conversion extent of each aromatic alcohol can be related with the number, position and nature of the substituent groups. Among the five alcohols tested, the AA showed the fastest conversion and the high yield for AAD formation. The presence of eCH2OH and eOCH3 functional groups (weak and strong electron-donors, respectively) in the aromatic ring of AA may explain the higher production of AAD. The combination of eCH2OH and eOCH3 functional groups made the AA more prone to be selectively oxidized to AAD. TA, with two weak (eCH2OH and eCH3) activating groups, was also easily converted to TAD. The highest selectivity (80%) to this compound was obtained at the end of 45 min of reaction. On the other hand, VA with the presence of one very strong (eOH), and (eCH2OH “weak” and eOCH3 “less strong”) activating groups was the alcohol which showed the lowest yield and selectivity for VAD production. The combination of these functional groups has resulted in the prevalence of the greatly activating nature of the eOH substituent, leading to faster VA conversion. In the case of the reactions using BA (having in common the eCH2OH group in all the studied molecules), the maximum yield for BAD production (≈50%) was obtained at 120 min. In a different case, the low yield toward PAD production may be attributed to the presence of dioxole as electron-withdrawing group, which inhibit the reactivity of PA. Additionally, the low selectivity observed for the reactions using VA and PA result from the overoxidation of the aldehydes into vanillic acid and piperonylic acid, respectively.
3.2. Photocatalytic synthesis of aromatic aldehydes using gCN-T In our previous work, gCN-T (designated in that work as T500 [36]) was tested in the photocatalytic synthesis of benzaldehyde under deoxygenated conditions and using radiation with an excitation maximum at λ =392 nm. In the present work, the main goal was the study of the photocatalytic performance of the gCN-T based materials for the synthesis of five aromatic aldehydes selected based in their importance as high value-added compounds: benzaldehyde (BAD), anisaldehyde (AAD), tolualdehyde (TAD), piperonal (PAD) and vanillin (VAD). In this case, the reactions were carried out starting from the respective aromatic alcohols, under oxygenated conditions and using radiation with λmax at 370 nm. The photochemical stability of the aromatic alcohols, in absence of catalyst, was examined. Under these conditions, the initial concentration of each alcohol remained constant after 4 h of reaction (data not shown). These results were expected since the absorption spectra of the selected molecules do not overlap the emission spectrum of the radiation source used (Fig. SD1). Reactions under dark conditions were performed for 2 h in order to establish the adsorption-desorption equilibrium between gCN-T and each aromatic alcohol. Negligible adsorption was observed disregarding the aromatic alcohol used, as exemplified for the case of AA (Fig. SD2). The concentration profiles of the five aromatic aldehydes selected in this study during the photocatalytic reactions using gCN-T as catalyst are depicted in Fig. 2a. Comparing the results obtained for the synthesis of the different compounds, the maximum yields obtained followed the order AAD > TAD > BAD > PAD > VAD (Fig. 2a). It is noteworthy that the maximum yield to AAD (99%) was obtained in merely 45 min of reaction with a selectivity of 99% (Fig. 2b). TAD and BAD yields of 60 and 50%, with selectivity of 80%, were obtained at the end of 45 and
3.3. Photocatalytic synthesis of AAD using gCN-TS Two-dimensional nanosheets constituted of few atomic layers of C3N4 have been attracting great attention due to their unique electronic and optical properties [8,47,48]. In this work, gCN-TS was prepared by sonication of gCN-T in water, producing a colloidal suspension with a catalyst load of 0.7 g L−1. To avoid a useless excess of catalyst and to ensure total absorption of efficient photons [2,49], reactions using different gCN-TS load were carried out by diluting the original suspension in order to obtain catalyst loads in the 0.05-0.70 g L−1 range. The suspensions were used for the conversion of AA into AAD. The kinetic results showed that the photocatalytic conversion of AA follows a pseudo-first order model (Fig. SD4). The values of the apparent rate constants (kapp) obtained for AA conversion with different gCN-TS loads are plotted in Fig. 3. It was observed that kapp increases with the catalyst load up to 0.10 g L−1, confirming the heterogeneous nature of the photocatalytic process. This behavior can be ascribed to an increase in the available active sites for alcohol adsorption and conversion. A plateau was observed for catalyst loads between 0.10 and 0.50 g L-1 with a negligible effect on kapp, meaning that the reaction rate becomes independent of the catalyst load. A further increase in the catalyst load to 0.70 g L-1 (i.e., using the original gCN-TS suspension) leads to a decrease in kapp, which may be assigned to light scattering resulting from a surplus
Fig. 2. (a) Concentration profiles of the aldehydes BAD, AAD, TAD, PAD and VAD after 4 h reaction using gCN-T catalyst. (b) Conversion (C) of the aromatic alcohols, selectivity (S) and yield (Y) for aldehydes formation (bars) and reaction time for the maximum yield of each aldehyde (-⬤-). 4
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al.
Table 1 summarizes the results reported in the literature on the selective photocatalytic production of AAD from AA and includes the results obtained in this work with gCN-T and gCN-TS. The photocatalytic production of AAD from aqueous solutions of AA using the materials developed in this work appear to be very competitive when compared with those reported in the literature (Table 1), as high conversion and selectivity are achieved in a very short reaction time using UV-LEDs as high-efficient and low-cost radiation source. Moreover, using gCN-TS, we were capable to decrease the catalyst load to 1/5 of that used with gCN-T and decrease the reaction time from 45 to 30 min for obtaining > 99% conversion of AA with a > 99% selectivity to AAD. In order to clarify the mechanism and the nature of the species involved in the photocatalytic reactions, specific scavengers were added to the reaction medium and its effect in the kinetics of AA conversion to AAD using gCN-TS was assessed (Fig. SD5). In the presence of tert-butanol (t-BuOH), the AA conversion profile remained practically unchanged, revealing that hydroxyl radicals are not involved in its conversion into AAD. Yet, a decrease in the kinetics of AA conversion was observed in the presence of ethylenediaminetetraacetic acid (EDTA), indicating that holes play an important role in the mechanism of AA conversion. Another important observation is that a relatively high amount of H2O2 is formed during the photocatalytic reaction (Fig. SD5), which occurs due to two-electron reduction of oxygen at the conduction band of gCN-TS, hindering the formation of superoxide radicals (resulting from one-electron reduction of O2).
Fig. 3. Effect of catalyst load in the kapp for photocatalytic oxidation of AA using gCN-TS suspensions.
amount of catalyst particles in the suspension. Based on the previous results, we used gCN-TS suspensions with a catalyst load of 0.20 g L−1 for studying the influence of the initial concentration of AA (0.50–7.0 mM) in the efficiency of AAD production. The maximum concentration of AA was set as 7.0 mM by taking into consideration the solubility of both AA and AAD in water (≈15 mM at 20 °C [50]). As expected, higher AAD production was achieved by increasing the initial concentration of AA (Fig. 4). Above an initial concentration of AA of 3 mM, the reactions tend to be slower, due to an increase in the amount of AA molecules per active site. For the reaction with the highest initial AA concentration (7.0 mM) a maximum yield of 5.56 mM of AAD was obtained at the end of 90 min with a selectivity of 82% in 90 min, which is indicative of the high potential of gCN-TS as catalyst to produce this high added-value product.
3.4. Immobilization of gCN-TS From a technological point of view, immobilization of photocatalysts is highly desired to facilitate reuse, avoid costs related to separation and to permit continuous operation. In this view, gCN-TS was immobilized on glass rings previously treated with PVA. Before starting the photocatalytic reaction, a 0.50 mM AA solution was passed through the reactor for 2 h to establish hydrodynamic equilibrium (residence time of 55 min). Negligible AA adsorption was observed after this period. A maximum production of ≈0.27 mM AAD was observed at 90 min, this production being maintained for at least 24 h (Fig. 5). The stability of the gCN-TS-coated glass rings was assessed by repeating the reaction for two more times starting with fresh AA solutions. Before each run, a stream of ultrapure water was passed through the reactor containing the gCN-TS-coated glass rings for 4 h. The results showed that the performance of the immobilized system was maintained constant, which evidence the high stability of the gCN-TS-coated glass rings under the reaction conditions used. Globally, the results show that gCN-based materials are highly promising photocatalysts for the synthesis of aromatic aldehydes. Moreover, the possibility of immobilizing gCN-TS nanosheets opens the possibility of producing these important chemicals in continuous-flow reactors, reducing operation costs and allowing large-scale production. 4. Conclusions Carbon nitride materials, gCN-T and gCN-TS, prepared by thermal treatment of bulk gCN and further sonication, respectively, were successfully used for the photocatalytic synthesis of a series of aromatic aldehydes from the respective alcohols under UV-LED radiation. The best photocatalytic performance using gCN-T was observed for the synthesis of anisyl alcohol from anisaldehyde (C, S and Y > 99% at 45 min of reaction). The relative extent in the conversion of the alcohols was correlated with to the electronic nature (electron-donor/or withdrawing), number and position of the respective functional groups. Sonication of gCN-T in water resulted in a colloidal suspension of carbon nitride nanosheets (gCN-TS), with enhanced photocatalytic activity towards the conversion of anisyl alcohol to anisaldehyde (C, S and
Fig. 4. (a) Concentration profiles of the AAD varying the initial concentration of AA after 4 h reaction using gCN-TS (0.2 g L−1). (b) Conversion (C) of the AA, selectivity (S) and yield (Y) for AAD formation (bars); and reaction time for the maximum yield of each aldehyde (-⬤-). 5
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al.
Table 1 Results reported in the literature and obtained in the present work on the selective photocatalytic production of AAD from AA. Catalyst
Solvent
Irradiation conditions
Catalyst load (g/L)
Reaction time (h)
C (%)
S (%)
Ref.
TiO2 P25 TiO2 P25 TiO2 TiO2 TiO2 Brookite TiO2 Rutile TiO2-graphene TiO2 /DIOL/Ag WO3/TiO2 CdS g-C3N4 g-C3N4 g-C3N4 g-C3N4 gCN-T gCN-TS
Water Water Water Water Water Water Benzotrifluoride Water Water Water Water Water Water Acetonitrile Water Water
Simulated solar light UV (Hg lamp) UV (125 W Hg lamp, λ = 360 nm) Vis (λ > 420 nm) UV (Hg lamps) UV (Hg lamp, λ = 360 nm) Vis UV (fluorescent lamps, λ = 365 nm) UV (450 W Hg lamp, λ > 350 nm) Vis (λ > 420 nm) Vis (100 W halogen lamp) Vis (λ > 420 nm) Vis (λ > 420 nm) UV (300 W Xe lamp) UV-LEDs (370 nm) UV-LEDs (370 nm)
0.2 0.02 0.5 2.5 0.4 0.4 0.8 0.2 1 2.5 0.33 2.5 2.5 2 1.0 0.2
2 13.8 3.8 3 4.3 2.36 20 3 6 3 4 3 3 9 0.75 0.5
50 65 50 33 50 50 70 54 50 25 100 10 56 85.7 > 99 > 99
15 14 32 92 56 60 100 86 64 60 72 96 89 > 99 > 99 > 99
[51] [13] [52] [14] [53] [54] [55] [56] [57] [14] [40] [14] [14] [58] This study This study
References [1] A.L. Luna, M.A. Valenzuela, C. Colbeau-Justin, P. Vázquez, J.L. Rodriguez, J.R. Avendaño, S. Alfaro, S. Tirado, A. Garduño, J.M. De la Rosa, Appl. Catal. A Gen. 521 (2016) 140–148. [2] M.J. Sampaio, C.G. Silva, A.M.T. Silva, J.L. Faria, J. Chem. Technol. Biotechnol. 91 (2016) 346–352. [3] L. Svoboda, P. Praus, M.J. Lima, M.J. Sampaio, D. Matýsek, M. Ritz, R. Dvorský, J.L. Faria, C.G. Silva, Mater. Res. Bull. 100 (2018) 322–332. [4] Y. Cui, Z. Ding, P. Liu, M. Antonietti, X. Fu, X. Wang, Phys. Chem. Chem. Phys. 14 (2012) 1455–1462. [5] S. Cao, J. Yu, J. Phys. Chem. Lett. 5 (2014) 2101–2107. [6] C.G. Silva, M.J. Sampaio, R.R.N. Marques, L.A. Ferreira, P.B. Tavares, A.M.T. Silva, J.L. Faria, Appl. Catal. B-Environ. 178 (2015) 82–90. [7] R.D. Tentu, S. Basu, Curr. Opin. Electrochem. 5 (2017) 56–62. [8] S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P.M. Ajayan, Adv. Mater. 25 (2013) 2452–2456. [9] J. Low, J. Yu, W. Ho, J. Phys. Chem. Lett. 6 (2015) 4244–4251. [10] S. Ye, R. Wang, M.-Z. Wu, Y.-P. Yuan, Appl. Surf. Sci. 358 (2015) 15–27. [11] X. Lang, X. Chen, J. Zhao, Chem. Soc. Rev. 43 (2014) 473–486. [12] G. Palmisano, V. Augugliaro, M. Pagliaro, L. Palmisano, Chem. Commun. (2007) 3425–3437. [13] V. Augugliaro, H. Kisch, V. Loddo, M.J. López-Muñoz, C. Márquez-Álvarez, G. Palmisano, L. Palmisano, F. Parrino, S. Yurdakal, Appl. Catal. A Gen. 349 (2008) 182–188. [14] B. Long, Z. Ding, X. Wang, ChemSusChem 6 (2013) 2024-2024. [15] J. Zhu, P. Xiao, H. Li, S.A.C. Carabineiro, ACS Appl. Mater. Inter. 6 (2014) 16449–16465. [16] W.-J. Ong, L.-L. Tan, Y.H. Ng, S.-T. Yong, S.-P. Chai, Chem. Rev. 116 (2016) 7159–7329. [17] M. Zheng, J. Shi, T. Yuan, X. Wang, Angew. Chem. Int, Ed. 57 (2018) 5487–5491. [18] C. Zhou, C. Lai, C. Zhang, G. Zeng, D. Huang, M. Cheng, L. Hu, W. Xiong, M. Chen, J. Wang, Y. Yang, L. Jiang, Appl. Catal. B Environ. 238 (2018) 6–18. [19] Y. Zheng, L. Lin, B. Wang, X. Wang, Angew. Chem. Int. Ed. 54 (2015) 12868–12884. [20] X. Li, J. Yu, M. Jaroniec, Chem. Soc. Rev. 45 (2016) 2603–2636. [21] M. Zhou, P. Yang, R. Yuan, A.M. Asiri, M. Rana, X. Wang, ChemSusChem. 10 (2017) 4451–4456. [22] M. Zheng, I. Ghosh, B. König, X. Wang, ChemCatChem 11 (2019) 703–706. [23] S. Samanta, R. Srivastava, Sustain. Energy Fuels 1 (2017) 1390–1404. [24] W. Wang, P. Xu, M. Chen, G. Zeng, C. Zhang, C. Zhou, Y. Yang, D. Huang, C. Lai, M. Cheng, L. Hu, W. Xiong, H. Guo, M. Zhou, ACS Sustain. Chem. Eng. 6 (2018) 15503–15516. [25] C. Zhou, C. Lai, P. Xu, G. Zeng, D. Huang, Z. Li, C. Zhang, M. Cheng, L. Hu, J. Wan, F. Chen, W. Xiong, R. Deng, ACS Sustain. Chem. Eng. 6 (2018) 6941–6949. [26] C. Zhou, P. Xu, C. Lai, C. Zhang, G. Zeng, D. Huang, M. Cheng, L. Hu, W. Xiong, X. Wen, L. Qin, J. Yuan, W. Wang, Chem. Eng. J. 359 (2019) 186–196. [27] C. Zhou, C. Lai, D. Huang, G. Zeng, C. Zhang, M. Cheng, L. Hu, J. Wan, W. Xiong, M. Wen, X. Wen, L. Qin, Appl. Catal. B Environ. 220 (2018) 202–210. [28] Y. Yang, C. Zhang, D. Huang, G. Zeng, J. Huang, C. Lai, C. Zhou, W. Wang, H. Guo, W. Xue, R. Deng, M. Cheng, W. Xiong, Appl. Catal. B Environ. 245 (2019) 87–99. [29] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2008) 76. [30] Y. Zheng, L. Lin, X. Ye, F. Guo, X. Wang, Angew. Chem. Int, Ed. 53 (2014) 11926–11930. [31] S. Cao, J. Low, J. Yu, M. Jaroniec, Adv. Mater. 27 (2015) 2150–2176. [32] L. Lin, H. Ou, Y. Zhang, X. Wang, ACS Catal. 6 (2016) 3921–3931. [33] L. Lin, Z. Yu, X. Wang, Angew. Chem. Int, Edit. 58 (2019) 2–14. [34] S.P. Adhikari, Z.D. Hood, Vincent W. Chen, K.L. More, K. Senevirathne, A. Lachgar, Sustain. Energy Fuels 2 (2018) 2507–2515.
Fig. 5. Concentration profiles of AA and AAD using gCN-TS immobilized on glass rings during 24 h reaction; each point corresponds to the average of 3 independent runs using the same gCN-TS coated rings.
Y > 99% at 30 min of reaction). Nanosheets of gCN-TS were successfully immobilized over glass rings, which revealed to be highly stable during consecutive utilizations for the synthesis of anisaldehyde in a continuous-flow reaction system. Acknowledgments This work was financially supported by Associate Laboratory LSRELCM - UID/EQU/50020/2019 - funded by national funds through FCT/ MCTES (PIDDAC) and by projects POCI-01-0145-FEDER-031398 and POCI-01-0145-FEDER-030674, funded by European Regional Development Fund (ERDF) through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI) – and by national funds through FCT - Fundação para a Ciência e a Tecnologia. C.G.S. and M.J.L. acknowledge the FCT Investigator Programme (IF/00514/ 2014) with financing from the European Social Fund and the Human Potential Operational Programme. R.A.F. acknowledges the PhD fellowship funded by Project NORTE-08-5369-FSE-000028, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Social Fund (ESF). We are indebted to Dr. Carlos Sá and the CEMUP team (Portugal) for technical assistance with SEM measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cattod.2019.03.050. 6
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al. [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47]
J. Messinger, O. Ishitani, D. Wang, Sustain. Energy Fuels 2 (2018) 1891–1892. M.J. Lima, A.M.T. Silva, C.G. Silva, J.L. Faria, J. Catal. 353 (2017) 44–53. G. Marcì, E.I. García-López, L. Palmisano, Catal. Today 315 (2018) 126–137. F. Su, S.C. Mathew, G. Lipner, X. Fu, M. Antonietti, S. Blechert, X. Wang, J. Am. Chem. Soc. 132 (2010) 16299–16301. M.J. Lima, L.M. Pastrana-Martínez, M.J. Sampaio, G. Dražić, A.M.T. Silva, J.L. Faria, C.G. Silva, ChemistrySelect 3 (2018) 8070–8081. M. Bellardita, E.I. García-López, G. Marcì, I. Krivtsov, J.R. García, L. Palmisano, Appl. Catal. B Environ. 220 (2018) 222–233. M. Ilkaeva, I. Krivtsov, E.I. García-López, G. Marcì, O. Khainakova, J.R. García, L. Palmisano, E. Díaz, S. Ordóñez, J. Catal. 359 (2018) 212–222. A. Akhundi, E.I. García-López, G. Marcì, A. Habibi-Yangjeh, L. Palmisano, Res. Chem. Intermediat. 43 (2017) 5153–5168. M. Ilkaeva, I. Krivtsov, J.R. García, E. Díaz, S. Ordóñez, E.I. García-López, G. Marcì, L. Palmisano, M.I. Maldonado, S. Malato, Catal. Today 315 (2018) 138–148. M.J. Sampaio, C.G. Silva, A.M.T. Silva, V.J.P. Vilar, R.A.R. Boaventura, J.L. Faria, Chem. Eng. J. 224 (2013) 32–38. E.S. Da Silva, N.M.M. Moura, A. Coutinho, G. Drazic, B.M.S. Teixeira, N.A. Sobolev, C.G. Silva, M. Neves, M. Prieto, J.L. Faria, ChemSusChem 11 (2018) 2681–2694. M.H. Priya, G. Madras, J. Photochem. Photobiol. A 179 (2006) 256–262. S. Yang, X. Feng, L. Wang, K. Tang, J. Maier, K. Müllen, Angew. Chem. Int. Ed. 49 (2010) 4795–4799.
[48] J.N. Coleman, M. Lotya, A. O’Neill, S.D. Bergin, P.J. King, U. Khan, K. Young, A. Gaucher, S. De, R.J. Smith, I.V. Shvets, S.K. Arora, G. Stanton, H.-Y. Kim, K. Lee, G.T. Kim, G.S. Duesberg, T. Hallam, J.J. Boland, J.J. Wang, J.F. Donegan, J.C. Grunlan, G. Moriarty, A. Shmeliov, R.J. Nicholls, J.M. Perkins, E.M. Grieveson, K. Theuwissen, D.W. McComb, P.D. Nellist, V. Nicolosi, Science 331 (2011) 568–571. [49] N.N. Yunus, F. Hamzah, M.S. So’aib, J. Krishnan, IOP Conf. Ser. Mater. Sci. Eng 206 (2017) 012092. [50] https://pubchem.ncbi.nlm.nih.gov/compound (Accessed on October 2018). [51] S. Yurdakal, B.S. Tek, Ç. Değirmenci, G. Palmisano, Catal. Today 281 (2017) 53–59. [52] S. Yurdakal, V. Augugliaro, RSC Adv. 2 (2012) 8375–8380. [53] M. Addamo, V. Augugliaro, M. Bellardita, A. Di Paola, V. Loddo, G. Palmisano, L. Palmisano, S. Yurdakal, Catal. Lett. 126 (2008) 58–62. [54] V. Augugliaro, T. Caronna, V. Loddo, G. Marcì, G. Palmisano, L. Palmisano, S. Yurdakal, Chem. Eur. J. 14 (2008) 4640–4646. [55] M.-Q. Yang, Y.-J. Xu, Phys. Chem. Chem. Phys. 15 (2013) 19102–19118. [56] A. Abd-Elaal, F. Parrino, R. Ciriminna, V. Loddo, L. Palmisano, M. Pagliaro, ChemistryOpen 4 (2015) 779–785. [57] D. Tsukamoto, M. Ikeda, Y. Shiraishi, T. Hara, N. Ichikuni, S. Tanaka, T. Hirai, Chem. Eur. J. 17 (2011) 9816–9824. [58] J. Ding, W. Xu, H. Wan, D. Yuan, C. Chen, L. Wang, G. Guan, W.L. Dai, Appl. Catal. B Environ. 221 (2018) 626–634.
7
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Outstanding response of carbon nitride photocatalysts for selective synthesis of aldehydes under UV-LED irradiation Joana C. Lopes, Maria J. Sampaio, Raquel A. Fernandes, Maria J. Lima, Joaquim L. Faria, ⁎ Cláudia G. Silva Laboratory of Separation and Reaction Engineering - Laboratory of Catalysis and Materials (LSRE-LCM), Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465, Porto, Portugal
ARTICLE INFO
ABSTRACT
Keywords: Selective oxidation Photocatalysis Carbon nitride Aromatic aldehydes Glass rings
The photocatalytic conversion of alcohols into the corresponding aldehydes constitutes an important reaction in organic synthesis. Optical semiconductors based on carbon nitride were prepared by thermal condensation of dicyandiamide followed by a thermal post-treatment (gCN-T) and further sonication (gCN-TS). The efficiency of these materials was evaluated on the selective conversion of different aromatic alcohols to the corresponding aldehydes, namely anisaldehyde, benzaldehyde, tolualdehyde, piperonal and vanillin. The best performance of gCN-T (1 g L−1) was found for the conversion of anisyl alcohol to anisaldehyde (> 99% conversion and selectivity in 45 min of UV-LED irradiation). The alcohol conversion and yield for aldehyde formation was related with the number, electronic nature and position of the functional groups in the aromatic ring. The use of gCN-TS, constituted by carbon nitride nanosheets, also produced > 99% conversion and selectivity towards anisaldehyde, while reducing the reaction time to 30 min and the catalyst load to 0.2 g L−1. The immobilization of gCNTS in glass rings revealed to be a promising strategy for continuous production of aldehydes without the need of a catalyst separation step.
1. Introduction Photocatalytic technologies have been attracting wide attention of the scientific community in different fields, including degradation of organic compounds [1–4], hydrogen production [5–8], reduction of CO2 [9,10], organic synthesis [11,12], among others. In recent years, selective conversion of different alcohols into the corresponding aldehydes has emerged as a hot topic of research, due to their importance in organic synthesis reactions and fine chemical industry [13,14]. In general, conventional industrial production of aldehydes usually requires the use of hazard oxidants/reductants and harsh operation conditions, such as high temperature and pressure. Moreover, during their industrial synthesis high quantities of wastes are produced, usually presenting severe toxicity. Thus, the development of efficient alternatives operating under mild and environmentally friendly conditions (room temperature, atmospheric pressure and the use of water as solvent) is a topic of major concern. Polymeric carbon nitride (C3N4) based materials have been reported as highly efficient optical semiconductors widely used in photocatalytic processes [15–19], which is due to the low band gap (≈2.7 eV) when
⁎
compared to metal oxides (e.g., TiO2 and ZnO). Moreover, the position of valence and conduction bands of C3N4 (+1.58 V and -1.12 V, respectively) in water [14,20], makes this material a promising catalyst for selective oxidation of aromatic alcohols [21]. Other applications of C3N4 based materials in photocatalytic organic synthesis include C–H amination [22], dehydrogenation of N-heterocycles [17], cycloaddition, esterification, and transesterification reactions [23]. Carbon nitride has also been widely used for photocatalytic elimination of organic pollutants such as tetracycline [24,25], sulfamethazine [26,27], and oxytetracycline hydrochloride [28], and for photocatalytic fuel production such as hydrogen from water splitting, and hydrocarbons from CO2 reduction [29–35]. The synthesis of C3N4 may be accomplished by thermal polymerization of a variety of precursors such as urea, thiourea, melamine, dicyandiamide, cyanamide, and guanidine hydrochloride in the temperature range of 500–600 °C under air or inert atmosphere [10]. Nevertheless, bulk C3N4 materials usually exhibit low specific surface area, being a drawback due the small amount of active sites available to promote photocatalytic redox reactions. Some strategies have been employed seeking for enhanced performance of C3N4 materials namely
Corresponding author. E-mail address: [email protected] (C.G. Silva).
https://doi.org/10.1016/j.cattod.2019.03.050 Received 30 November 2018; Received in revised form 11 March 2019; Accepted 21 March 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Joana C. Lopes, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.03.050
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al.
by exposing this material to thermal, mechanical and chemical treatments [36,37]. Remarkable performance of a mesoporous C3N4, obtained by templating with silica nanoparticles, has been reported by Su et al. [38] for the photocatalytic synthesis of aldehydes under visible light. Yet, reactions were carried out using organic solvents (such as acetonitrile and toluene), and under high pressure and temperature (8 bar of O2 and 100 °C, respectively). The performance of the C3N4 material was ascribed to its high ability to activate oxygen (generating superoxide radicals, O2%―) and to the basic character of the material facilitating the deprotonation of the alcohol, followed by reaction with positively charged hole and recombination with O2%― to form the aldehyde. In addition, the authors also observed the formation of H2O2, resulting from two-electron reduction of O2. More recently, several works reported the synthesis of aromatic aldehydes in aqueous medium at mild conditions of temperature and pressure [36,39–43]. For instance, Bellardita et al. [40] used different precursors (melamine, urea and thiourea) to prepare bulk C3N4. The materials were subjected to a second thermal treatment in the presence of a phosphorus precursor, producing P-doped catalysts with increased surface area. These materials were used for the photocatalytic synthesis of piperonal, benzaldehyde and tolualdehyde, under aerated conditions, using fluorescent lamps as irradiation source, resulting in maximum selectivity of 69%, 72% and 26% for benzaldehyde, tolualdehyde and piperonal, respectively after 4 h of irradiation. In a previous work by our group [36] a C3N4 material obtained by thermal exfoliation of bulk carbon nitride at 500 °C (here denoted as gCN-T) was tested for the synthesis of benzaldehyde under deoxygenated conditions using a UV-LED system as light source. The exfoliation process leads to an increase in benzyl alcohol conversion (from 18 to 66% in 4 h of reaction), keeping the selectivity for benzaldehyde above 90%, this feature being attributed to the higher surface area of the exfoliated photocatalyst. In the present work, we have extended the use of gCN-T [36] to the photocatalytic production of benzaldehyde and other aromatic aldehydes, including anisaldehyde, tolualdehyde, piperonal and vanillin, under oxygenated conditions. Due to the several practical problems arising from the use of powder materials (e.g., separation of the catalyst), C3N4 nanosheets (gCN-TS), obtained by sonication of gCN-T, were immobilized on glass rings, which were used for the photocatalytic synthesis of anisaldehyde in continuous flow. To the best of our knowledge, this is the first study using immobilized C3N4 for the photocatalytic synthesis of aldehydes.
exposed to a second thermal step for 2 h at 500 °C and labeled as gCN-T. Additionally, an aqueous suspension containing a certain amount of gCN-T was sonicated (ultrasonic processor UP400S, 24 kHz) for 50 min. The sonicated suspension was centrifuged for 15 min at 3000 rpm to isolate the unexfoliated gCN-T material from the supernatant. The obtained colloidal suspension was denoted as gCN-TS. Glass rings (diameter = 3 mm, length = 3 mm) were cleaned as described elsewhere [44]. Before the coating with the gCN-TS suspension, the glass rings were treated with a 2% (w/V) aqueous solution of PVA by dip-coating. This procedure was directly followed by a second dipcoating of the glass rings in the gCN-TS suspension. The resulting glass rings were dried under ambient temperature for 48 h. The specific surface area (SBET) of the powder materials was determined by multipoint analysis of nitrogen adsorption isotherms at −196 °C in relative pressure range from 0.05 to 0.2 using the BrunauerEmmett-Teller method in a Quantachrome NOVA 4200e apparatus. The optical absorption was determined by diffuse reflectance UV–vis (DRUV-Vis) spectroscopy using a JASCO V-560 spectrophotometer equipped with an integrating sphere attachment (JASCO ISV-469). The results were recorded in equivalent absorption Kubelka–Munk units and used to calculate the optical bandgap energy of the photocatalysts. The morphology of the materials (as powders and immobilized) was examined by scanning electron microscopy (SEM) using a FEI Quanta 400 FEG ESEM/EDAX Genesis X4M (15 keV). Transmission electron microscopy (TEM) micrographs were conducted on a JEM 220FS microscope (Jeol, Japan), equipped with a LaB6 electron gun operating at 200 kV. 2.3. Photocatalytic experiments The photocatalytic efficiency of the different photocatalysts for the synthesis of different aldehydes, was firstly evaluated using a batch reactor. In a typical experiment, a borosilicate reactor was filled with 50 mL of a 1.5 mM aqueous solution of an aromatic alcohol. The catalyst load was fixed at 1.0 g L−1 by taking into consideration our previous studies using similar catalysts for benzaldehyde production [36]. The suspensions were continuously stirred and saturated with an air flow. Before turning on the irradiation, a 30 min period in dark was performed to establish the adsorption-desorption equilibrium between the aromatic alcohol aqueous solution and the photocatalyst. Then, the photocatalytic synthesis of the aldehydes was conducted for 4 h using a four LED system with a maximum wavelength at 370 nm as radiation source. The nominal intensity of the irradiation of each LED was c.a. 450 W m-2, which was determined using an UV-Vis spectroradiometer (USB2000+, OceanOptics, USA). Experiments using the gCN-TS suspension were performed using the system describe above. Photocatalytic experiments in continuous mode were carried out in a borosilicate cylindrical reactor (internal diameter = 2.7 cm; length =7.0 cm) with 38.5 mL of useful volume and packed with 115 glass rings coated with gCN-TS. This reactor configuration was used for the synthesis of AAD from AA. An AA solution (0.5 mM) contained in a reservoir was introduced in the reactor using a peristaltic pump at a constant flow rate (0.70 mL min−1). The irradiation source was the same one used in batch reactions. Reactions were extended for 24 h. High Performance Liquid Chromatography (HPLC) was used for identification and quantification of aromatic alcohols and aldehydes using a Shimadzu Corporation apparatus equipped with a Diode Array Detector (SPD M20A), a solvent delivery pump (LC-30AD) and a KinetexTM F5 1.7 μm 100 Å column (100 mm × 2.1 mm). The temperature of the autosampler and column oven were fixed at 4.0 °C and 35 °C, respectively. A mixture of 0.1% v/v of formic acid and methanol with volume ratio of 30:70, respectively, was isocratically eluted for 15 min at 0.150 mL min−1.
2. Experimental 2.1. Materials Dicyandiamide (DCN, 99%), benzyl alcohol (BA, > 99%), benzaldehyde (BAD, > 99%), piperonyl alcohol (PA, 98%), piperonal (PAD, 99%), p-anisaldehyde (AAD, 98%), polyvinyl alcohol (PVA), titanium (III) sulfate solution (Ti2(SO4)3) and sulfuric acid (purity ≥ 95–97%) were purchased from Sigma-Aldrich. Anisyl alcohol (AA, 98%), vanillyl alcohol (VA, > 98%) and vanillin (VAD, 99%) were supplied from Alfa Aesar. p-toluyl alcohol (TA, 98%) and p-tolualdehyde (TAD, 97%) were obtained from Acros Organics. Formic acid (98%), methanol (99.6%), 2-propanol (100%) and acetone (100%) were purchased from Fluka. Titanium dioxide Aeroxide® P25 was acquired from Evonik Industries. Ultrapure water (UP) was obtained by a Direct-Q Milipore system. 2.2. Catalyst preparation and characterization Polymeric carbon nitride was prepared by thermal decomposition of DCN, as described elsewhere [36]. Briefly, a certain amount of DCN was placed in a porcelain-capsule inside a muffle furnace under static air conditions for 4 h at 550 °C. The resulting material (here labeled as gCN), was washed and dried at 110 °C for 12 h. Then, the gCN was 2
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al.
Fig. 1. (a–b) SEM micrographs of gCN (a) and gCN-T (b) powders (insets are the corresponding TEM micrographs); (c–d) SEM micrographs of glass rings coated with gCN-TS at different magnifications (see bar scale for appropriate dimensions); (e–f) Tyndall effect for gCN-TS colloidal suspension (recipient on the right) under illumination with red laser beam (e), in comparison with artificial light (f); (g) TEM micrograph of gCN-TS.
3. Results and discussion
gCN and gCN-T (Fig. 1). While the gCN material (Fig. 1a) presents a compact stacked lamellar structure, SEM micrographs of the thermalexfoliated material (gCN-T) revealed the presence of thinner layers (Fig. 1b). TEM micrographs of the gCN and gCN-T materials (Fig. 1a and b, inset) confirm the formation of a less dense material after submitting gCN to the thermal post-treatment (gCN-T). These observations have been well discussed in the literature [16,36,45], and corroborate the results obtained from the SBET measurements, i.e., the thermal exfoliation promote the increase in the surface area of gCN-T by causing the splitting of the interlayer bonds. Representative SEM images of gCN-TS-coated glass rings show a relatively homogeneous surface at microscale (Fig. 1c and d). gCN-TS particles of smaller dimensions than gCN-T can be observed at higher magnification (Fig. 1d), which resulted from the intense ultrasonication treatment applied to gCN-T. The colloidal nature of the gCN-TS suspension was demonstrated by the scattering observed when the suspension is crossed with the red light of a laser beam – Tyndall effect (Fig. 1e). The nanometric dimensionality of the sample is in line with the TEM analysis of the colloidal suspension (Fig. 1f-g), in which is
3.1. Catalyst characterization Detailed characterization of gCN and gCN-T materials can be found in a previous work by our group [36]. The thermal treatment of gCN promotes the exfoliation of the material by separating the layers of the bulk material (gCN) which are stacked together by weak van der Waals forces. According to the results obtained from the nitrogen adsorptiondesorption isotherms, the textural properties were changed by the thermal treatment applied to the gCN material. The specific surface area of gCN and gCN-T, obtained by the Brunauer-Emmett-Teller method (SBET), were 6 and 79 m2 g−1, respectively, corresponding to an increase of more than 13 times promoted by the thermal treatment. For most photocatalytic semiconductors, an increase in the SBET is recognized as an advantage, since more active sites are available for the occurrence of redox reactions at the catalyst surface. Different morphologies were observed by microscopy analyses of 3
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al.
clearly observed the ultrathin lamellar morphology of the material.
120 min, respectively. Finally, very low yields of PAD and VAD were obtained under the used conditions (9 and 2%, respectively) in reactions with low selectivity for these compounds (Fig. 2b). In heterogeneous photocatalysis, the reactivity of benzene derivatives is widely influenced by the number, electronic mature and position of substituent groups in the aromatic ring [12,44,46]. Thus, based on the principles of photocatalytic oxidation of aromatic compounds, the relative facility of selectively synthesize the aromatic aldehydes here studied may be rationalized by the relative ability of the parent compound to be converted. The chemical structures of AA, PA, TA and VA derive from BA, the difference being the type and position of substituent groups in the aromatic ring (Fig. SD3). The photocatalytic conversion of such molecules may be accelerated or hindered by the presence of electron-donating (activating) or electron-withdrawing (deactivating) groups in the aromatic ring, respectively. In molecules with more than one functional group, this effect results from the overall contribution of each functional group. Thus, it may be expected that the relative conversion extent of each aromatic alcohol can be related with the number, position and nature of the substituent groups. Among the five alcohols tested, the AA showed the fastest conversion and the high yield for AAD formation. The presence of eCH2OH and eOCH3 functional groups (weak and strong electron-donors, respectively) in the aromatic ring of AA may explain the higher production of AAD. The combination of eCH2OH and eOCH3 functional groups made the AA more prone to be selectively oxidized to AAD. TA, with two weak (eCH2OH and eCH3) activating groups, was also easily converted to TAD. The highest selectivity (80%) to this compound was obtained at the end of 45 min of reaction. On the other hand, VA with the presence of one very strong (eOH), and (eCH2OH “weak” and eOCH3 “less strong”) activating groups was the alcohol which showed the lowest yield and selectivity for VAD production. The combination of these functional groups has resulted in the prevalence of the greatly activating nature of the eOH substituent, leading to faster VA conversion. In the case of the reactions using BA (having in common the eCH2OH group in all the studied molecules), the maximum yield for BAD production (≈50%) was obtained at 120 min. In a different case, the low yield toward PAD production may be attributed to the presence of dioxole as electron-withdrawing group, which inhibit the reactivity of PA. Additionally, the low selectivity observed for the reactions using VA and PA result from the overoxidation of the aldehydes into vanillic acid and piperonylic acid, respectively.
3.2. Photocatalytic synthesis of aromatic aldehydes using gCN-T In our previous work, gCN-T (designated in that work as T500 [36]) was tested in the photocatalytic synthesis of benzaldehyde under deoxygenated conditions and using radiation with an excitation maximum at λ =392 nm. In the present work, the main goal was the study of the photocatalytic performance of the gCN-T based materials for the synthesis of five aromatic aldehydes selected based in their importance as high value-added compounds: benzaldehyde (BAD), anisaldehyde (AAD), tolualdehyde (TAD), piperonal (PAD) and vanillin (VAD). In this case, the reactions were carried out starting from the respective aromatic alcohols, under oxygenated conditions and using radiation with λmax at 370 nm. The photochemical stability of the aromatic alcohols, in absence of catalyst, was examined. Under these conditions, the initial concentration of each alcohol remained constant after 4 h of reaction (data not shown). These results were expected since the absorption spectra of the selected molecules do not overlap the emission spectrum of the radiation source used (Fig. SD1). Reactions under dark conditions were performed for 2 h in order to establish the adsorption-desorption equilibrium between gCN-T and each aromatic alcohol. Negligible adsorption was observed disregarding the aromatic alcohol used, as exemplified for the case of AA (Fig. SD2). The concentration profiles of the five aromatic aldehydes selected in this study during the photocatalytic reactions using gCN-T as catalyst are depicted in Fig. 2a. Comparing the results obtained for the synthesis of the different compounds, the maximum yields obtained followed the order AAD > TAD > BAD > PAD > VAD (Fig. 2a). It is noteworthy that the maximum yield to AAD (99%) was obtained in merely 45 min of reaction with a selectivity of 99% (Fig. 2b). TAD and BAD yields of 60 and 50%, with selectivity of 80%, were obtained at the end of 45 and
3.3. Photocatalytic synthesis of AAD using gCN-TS Two-dimensional nanosheets constituted of few atomic layers of C3N4 have been attracting great attention due to their unique electronic and optical properties [8,47,48]. In this work, gCN-TS was prepared by sonication of gCN-T in water, producing a colloidal suspension with a catalyst load of 0.7 g L−1. To avoid a useless excess of catalyst and to ensure total absorption of efficient photons [2,49], reactions using different gCN-TS load were carried out by diluting the original suspension in order to obtain catalyst loads in the 0.05-0.70 g L−1 range. The suspensions were used for the conversion of AA into AAD. The kinetic results showed that the photocatalytic conversion of AA follows a pseudo-first order model (Fig. SD4). The values of the apparent rate constants (kapp) obtained for AA conversion with different gCN-TS loads are plotted in Fig. 3. It was observed that kapp increases with the catalyst load up to 0.10 g L−1, confirming the heterogeneous nature of the photocatalytic process. This behavior can be ascribed to an increase in the available active sites for alcohol adsorption and conversion. A plateau was observed for catalyst loads between 0.10 and 0.50 g L-1 with a negligible effect on kapp, meaning that the reaction rate becomes independent of the catalyst load. A further increase in the catalyst load to 0.70 g L-1 (i.e., using the original gCN-TS suspension) leads to a decrease in kapp, which may be assigned to light scattering resulting from a surplus
Fig. 2. (a) Concentration profiles of the aldehydes BAD, AAD, TAD, PAD and VAD after 4 h reaction using gCN-T catalyst. (b) Conversion (C) of the aromatic alcohols, selectivity (S) and yield (Y) for aldehydes formation (bars) and reaction time for the maximum yield of each aldehyde (-⬤-). 4
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al.
Table 1 summarizes the results reported in the literature on the selective photocatalytic production of AAD from AA and includes the results obtained in this work with gCN-T and gCN-TS. The photocatalytic production of AAD from aqueous solutions of AA using the materials developed in this work appear to be very competitive when compared with those reported in the literature (Table 1), as high conversion and selectivity are achieved in a very short reaction time using UV-LEDs as high-efficient and low-cost radiation source. Moreover, using gCN-TS, we were capable to decrease the catalyst load to 1/5 of that used with gCN-T and decrease the reaction time from 45 to 30 min for obtaining > 99% conversion of AA with a > 99% selectivity to AAD. In order to clarify the mechanism and the nature of the species involved in the photocatalytic reactions, specific scavengers were added to the reaction medium and its effect in the kinetics of AA conversion to AAD using gCN-TS was assessed (Fig. SD5). In the presence of tert-butanol (t-BuOH), the AA conversion profile remained practically unchanged, revealing that hydroxyl radicals are not involved in its conversion into AAD. Yet, a decrease in the kinetics of AA conversion was observed in the presence of ethylenediaminetetraacetic acid (EDTA), indicating that holes play an important role in the mechanism of AA conversion. Another important observation is that a relatively high amount of H2O2 is formed during the photocatalytic reaction (Fig. SD5), which occurs due to two-electron reduction of oxygen at the conduction band of gCN-TS, hindering the formation of superoxide radicals (resulting from one-electron reduction of O2).
Fig. 3. Effect of catalyst load in the kapp for photocatalytic oxidation of AA using gCN-TS suspensions.
amount of catalyst particles in the suspension. Based on the previous results, we used gCN-TS suspensions with a catalyst load of 0.20 g L−1 for studying the influence of the initial concentration of AA (0.50–7.0 mM) in the efficiency of AAD production. The maximum concentration of AA was set as 7.0 mM by taking into consideration the solubility of both AA and AAD in water (≈15 mM at 20 °C [50]). As expected, higher AAD production was achieved by increasing the initial concentration of AA (Fig. 4). Above an initial concentration of AA of 3 mM, the reactions tend to be slower, due to an increase in the amount of AA molecules per active site. For the reaction with the highest initial AA concentration (7.0 mM) a maximum yield of 5.56 mM of AAD was obtained at the end of 90 min with a selectivity of 82% in 90 min, which is indicative of the high potential of gCN-TS as catalyst to produce this high added-value product.
3.4. Immobilization of gCN-TS From a technological point of view, immobilization of photocatalysts is highly desired to facilitate reuse, avoid costs related to separation and to permit continuous operation. In this view, gCN-TS was immobilized on glass rings previously treated with PVA. Before starting the photocatalytic reaction, a 0.50 mM AA solution was passed through the reactor for 2 h to establish hydrodynamic equilibrium (residence time of 55 min). Negligible AA adsorption was observed after this period. A maximum production of ≈0.27 mM AAD was observed at 90 min, this production being maintained for at least 24 h (Fig. 5). The stability of the gCN-TS-coated glass rings was assessed by repeating the reaction for two more times starting with fresh AA solutions. Before each run, a stream of ultrapure water was passed through the reactor containing the gCN-TS-coated glass rings for 4 h. The results showed that the performance of the immobilized system was maintained constant, which evidence the high stability of the gCN-TS-coated glass rings under the reaction conditions used. Globally, the results show that gCN-based materials are highly promising photocatalysts for the synthesis of aromatic aldehydes. Moreover, the possibility of immobilizing gCN-TS nanosheets opens the possibility of producing these important chemicals in continuous-flow reactors, reducing operation costs and allowing large-scale production. 4. Conclusions Carbon nitride materials, gCN-T and gCN-TS, prepared by thermal treatment of bulk gCN and further sonication, respectively, were successfully used for the photocatalytic synthesis of a series of aromatic aldehydes from the respective alcohols under UV-LED radiation. The best photocatalytic performance using gCN-T was observed for the synthesis of anisyl alcohol from anisaldehyde (C, S and Y > 99% at 45 min of reaction). The relative extent in the conversion of the alcohols was correlated with to the electronic nature (electron-donor/or withdrawing), number and position of the respective functional groups. Sonication of gCN-T in water resulted in a colloidal suspension of carbon nitride nanosheets (gCN-TS), with enhanced photocatalytic activity towards the conversion of anisyl alcohol to anisaldehyde (C, S and
Fig. 4. (a) Concentration profiles of the AAD varying the initial concentration of AA after 4 h reaction using gCN-TS (0.2 g L−1). (b) Conversion (C) of the AA, selectivity (S) and yield (Y) for AAD formation (bars); and reaction time for the maximum yield of each aldehyde (-⬤-). 5
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al.
Table 1 Results reported in the literature and obtained in the present work on the selective photocatalytic production of AAD from AA. Catalyst
Solvent
Irradiation conditions
Catalyst load (g/L)
Reaction time (h)
C (%)
S (%)
Ref.
TiO2 P25 TiO2 P25 TiO2 TiO2 TiO2 Brookite TiO2 Rutile TiO2-graphene TiO2 /DIOL/Ag WO3/TiO2 CdS g-C3N4 g-C3N4 g-C3N4 g-C3N4 gCN-T gCN-TS
Water Water Water Water Water Water Benzotrifluoride Water Water Water Water Water Water Acetonitrile Water Water
Simulated solar light UV (Hg lamp) UV (125 W Hg lamp, λ = 360 nm) Vis (λ > 420 nm) UV (Hg lamps) UV (Hg lamp, λ = 360 nm) Vis UV (fluorescent lamps, λ = 365 nm) UV (450 W Hg lamp, λ > 350 nm) Vis (λ > 420 nm) Vis (100 W halogen lamp) Vis (λ > 420 nm) Vis (λ > 420 nm) UV (300 W Xe lamp) UV-LEDs (370 nm) UV-LEDs (370 nm)
0.2 0.02 0.5 2.5 0.4 0.4 0.8 0.2 1 2.5 0.33 2.5 2.5 2 1.0 0.2
2 13.8 3.8 3 4.3 2.36 20 3 6 3 4 3 3 9 0.75 0.5
50 65 50 33 50 50 70 54 50 25 100 10 56 85.7 > 99 > 99
15 14 32 92 56 60 100 86 64 60 72 96 89 > 99 > 99 > 99
[51] [13] [52] [14] [53] [54] [55] [56] [57] [14] [40] [14] [14] [58] This study This study
References [1] A.L. Luna, M.A. Valenzuela, C. Colbeau-Justin, P. Vázquez, J.L. Rodriguez, J.R. Avendaño, S. Alfaro, S. Tirado, A. Garduño, J.M. De la Rosa, Appl. Catal. A Gen. 521 (2016) 140–148. [2] M.J. Sampaio, C.G. Silva, A.M.T. Silva, J.L. Faria, J. Chem. Technol. Biotechnol. 91 (2016) 346–352. [3] L. Svoboda, P. Praus, M.J. Lima, M.J. Sampaio, D. Matýsek, M. Ritz, R. Dvorský, J.L. Faria, C.G. Silva, Mater. Res. Bull. 100 (2018) 322–332. [4] Y. Cui, Z. Ding, P. Liu, M. Antonietti, X. Fu, X. Wang, Phys. Chem. Chem. Phys. 14 (2012) 1455–1462. [5] S. Cao, J. Yu, J. Phys. Chem. Lett. 5 (2014) 2101–2107. [6] C.G. Silva, M.J. Sampaio, R.R.N. Marques, L.A. Ferreira, P.B. Tavares, A.M.T. Silva, J.L. Faria, Appl. Catal. B-Environ. 178 (2015) 82–90. [7] R.D. Tentu, S. Basu, Curr. Opin. Electrochem. 5 (2017) 56–62. [8] S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai, X. Wang, P.M. Ajayan, Adv. Mater. 25 (2013) 2452–2456. [9] J. Low, J. Yu, W. Ho, J. Phys. Chem. Lett. 6 (2015) 4244–4251. [10] S. Ye, R. Wang, M.-Z. Wu, Y.-P. Yuan, Appl. Surf. Sci. 358 (2015) 15–27. [11] X. Lang, X. Chen, J. Zhao, Chem. Soc. Rev. 43 (2014) 473–486. [12] G. Palmisano, V. Augugliaro, M. Pagliaro, L. Palmisano, Chem. Commun. (2007) 3425–3437. [13] V. Augugliaro, H. Kisch, V. Loddo, M.J. López-Muñoz, C. Márquez-Álvarez, G. Palmisano, L. Palmisano, F. Parrino, S. Yurdakal, Appl. Catal. A Gen. 349 (2008) 182–188. [14] B. Long, Z. Ding, X. Wang, ChemSusChem 6 (2013) 2024-2024. [15] J. Zhu, P. Xiao, H. Li, S.A.C. Carabineiro, ACS Appl. Mater. Inter. 6 (2014) 16449–16465. [16] W.-J. Ong, L.-L. Tan, Y.H. Ng, S.-T. Yong, S.-P. Chai, Chem. Rev. 116 (2016) 7159–7329. [17] M. Zheng, J. Shi, T. Yuan, X. Wang, Angew. Chem. Int, Ed. 57 (2018) 5487–5491. [18] C. Zhou, C. Lai, C. Zhang, G. Zeng, D. Huang, M. Cheng, L. Hu, W. Xiong, M. Chen, J. Wang, Y. Yang, L. Jiang, Appl. Catal. B Environ. 238 (2018) 6–18. [19] Y. Zheng, L. Lin, B. Wang, X. Wang, Angew. Chem. Int. Ed. 54 (2015) 12868–12884. [20] X. Li, J. Yu, M. Jaroniec, Chem. Soc. Rev. 45 (2016) 2603–2636. [21] M. Zhou, P. Yang, R. Yuan, A.M. Asiri, M. Rana, X. Wang, ChemSusChem. 10 (2017) 4451–4456. [22] M. Zheng, I. Ghosh, B. König, X. Wang, ChemCatChem 11 (2019) 703–706. [23] S. Samanta, R. Srivastava, Sustain. Energy Fuels 1 (2017) 1390–1404. [24] W. Wang, P. Xu, M. Chen, G. Zeng, C. Zhang, C. Zhou, Y. Yang, D. Huang, C. Lai, M. Cheng, L. Hu, W. Xiong, H. Guo, M. Zhou, ACS Sustain. Chem. Eng. 6 (2018) 15503–15516. [25] C. Zhou, C. Lai, P. Xu, G. Zeng, D. Huang, Z. Li, C. Zhang, M. Cheng, L. Hu, J. Wan, F. Chen, W. Xiong, R. Deng, ACS Sustain. Chem. Eng. 6 (2018) 6941–6949. [26] C. Zhou, P. Xu, C. Lai, C. Zhang, G. Zeng, D. Huang, M. Cheng, L. Hu, W. Xiong, X. Wen, L. Qin, J. Yuan, W. Wang, Chem. Eng. J. 359 (2019) 186–196. [27] C. Zhou, C. Lai, D. Huang, G. Zeng, C. Zhang, M. Cheng, L. Hu, J. Wan, W. Xiong, M. Wen, X. Wen, L. Qin, Appl. Catal. B Environ. 220 (2018) 202–210. [28] Y. Yang, C. Zhang, D. Huang, G. Zeng, J. Huang, C. Lai, C. Zhou, W. Wang, H. Guo, W. Xue, R. Deng, M. Cheng, W. Xiong, Appl. Catal. B Environ. 245 (2019) 87–99. [29] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2008) 76. [30] Y. Zheng, L. Lin, X. Ye, F. Guo, X. Wang, Angew. Chem. Int, Ed. 53 (2014) 11926–11930. [31] S. Cao, J. Low, J. Yu, M. Jaroniec, Adv. Mater. 27 (2015) 2150–2176. [32] L. Lin, H. Ou, Y. Zhang, X. Wang, ACS Catal. 6 (2016) 3921–3931. [33] L. Lin, Z. Yu, X. Wang, Angew. Chem. Int, Edit. 58 (2019) 2–14. [34] S.P. Adhikari, Z.D. Hood, Vincent W. Chen, K.L. More, K. Senevirathne, A. Lachgar, Sustain. Energy Fuels 2 (2018) 2507–2515.
Fig. 5. Concentration profiles of AA and AAD using gCN-TS immobilized on glass rings during 24 h reaction; each point corresponds to the average of 3 independent runs using the same gCN-TS coated rings.
Y > 99% at 30 min of reaction). Nanosheets of gCN-TS were successfully immobilized over glass rings, which revealed to be highly stable during consecutive utilizations for the synthesis of anisaldehyde in a continuous-flow reaction system. Acknowledgments This work was financially supported by Associate Laboratory LSRELCM - UID/EQU/50020/2019 - funded by national funds through FCT/ MCTES (PIDDAC) and by projects POCI-01-0145-FEDER-031398 and POCI-01-0145-FEDER-030674, funded by European Regional Development Fund (ERDF) through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI) – and by national funds through FCT - Fundação para a Ciência e a Tecnologia. C.G.S. and M.J.L. acknowledge the FCT Investigator Programme (IF/00514/ 2014) with financing from the European Social Fund and the Human Potential Operational Programme. R.A.F. acknowledges the PhD fellowship funded by Project NORTE-08-5369-FSE-000028, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Social Fund (ESF). We are indebted to Dr. Carlos Sá and the CEMUP team (Portugal) for technical assistance with SEM measurements. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cattod.2019.03.050. 6
Catalysis Today xxx (xxxx) xxx–xxx
J.C. Lopes, et al. [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47]
J. Messinger, O. Ishitani, D. Wang, Sustain. Energy Fuels 2 (2018) 1891–1892. M.J. Lima, A.M.T. Silva, C.G. Silva, J.L. Faria, J. Catal. 353 (2017) 44–53. G. Marcì, E.I. García-López, L. Palmisano, Catal. Today 315 (2018) 126–137. F. Su, S.C. Mathew, G. Lipner, X. Fu, M. Antonietti, S. Blechert, X. Wang, J. Am. Chem. Soc. 132 (2010) 16299–16301. M.J. Lima, L.M. Pastrana-Martínez, M.J. Sampaio, G. Dražić, A.M.T. Silva, J.L. Faria, C.G. Silva, ChemistrySelect 3 (2018) 8070–8081. M. Bellardita, E.I. García-López, G. Marcì, I. Krivtsov, J.R. García, L. Palmisano, Appl. Catal. B Environ. 220 (2018) 222–233. M. Ilkaeva, I. Krivtsov, E.I. García-López, G. Marcì, O. Khainakova, J.R. García, L. Palmisano, E. Díaz, S. Ordóñez, J. Catal. 359 (2018) 212–222. A. Akhundi, E.I. García-López, G. Marcì, A. Habibi-Yangjeh, L. Palmisano, Res. Chem. Intermediat. 43 (2017) 5153–5168. M. Ilkaeva, I. Krivtsov, J.R. García, E. Díaz, S. Ordóñez, E.I. García-López, G. Marcì, L. Palmisano, M.I. Maldonado, S. Malato, Catal. Today 315 (2018) 138–148. M.J. Sampaio, C.G. Silva, A.M.T. Silva, V.J.P. Vilar, R.A.R. Boaventura, J.L. Faria, Chem. Eng. J. 224 (2013) 32–38. E.S. Da Silva, N.M.M. Moura, A. Coutinho, G. Drazic, B.M.S. Teixeira, N.A. Sobolev, C.G. Silva, M. Neves, M. Prieto, J.L. Faria, ChemSusChem 11 (2018) 2681–2694. M.H. Priya, G. Madras, J. Photochem. Photobiol. A 179 (2006) 256–262. S. Yang, X. Feng, L. Wang, K. Tang, J. Maier, K. Müllen, Angew. Chem. Int. Ed. 49 (2010) 4795–4799.
[48] J.N. Coleman, M. Lotya, A. O’Neill, S.D. Bergin, P.J. King, U. Khan, K. Young, A. Gaucher, S. De, R.J. Smith, I.V. Shvets, S.K. Arora, G. Stanton, H.-Y. Kim, K. Lee, G.T. Kim, G.S. Duesberg, T. Hallam, J.J. Boland, J.J. Wang, J.F. Donegan, J.C. Grunlan, G. Moriarty, A. Shmeliov, R.J. Nicholls, J.M. Perkins, E.M. Grieveson, K. Theuwissen, D.W. McComb, P.D. Nellist, V. Nicolosi, Science 331 (2011) 568–571. [49] N.N. Yunus, F. Hamzah, M.S. So’aib, J. Krishnan, IOP Conf. Ser. Mater. Sci. Eng 206 (2017) 012092. [50] https://pubchem.ncbi.nlm.nih.gov/compound (Accessed on October 2018). [51] S. Yurdakal, B.S. Tek, Ç. Değirmenci, G. Palmisano, Catal. Today 281 (2017) 53–59. [52] S. Yurdakal, V. Augugliaro, RSC Adv. 2 (2012) 8375–8380. [53] M. Addamo, V. Augugliaro, M. Bellardita, A. Di Paola, V. Loddo, G. Palmisano, L. Palmisano, S. Yurdakal, Catal. Lett. 126 (2008) 58–62. [54] V. Augugliaro, T. Caronna, V. Loddo, G. Marcì, G. Palmisano, L. Palmisano, S. Yurdakal, Chem. Eur. J. 14 (2008) 4640–4646. [55] M.-Q. Yang, Y.-J. Xu, Phys. Chem. Chem. Phys. 15 (2013) 19102–19118. [56] A. Abd-Elaal, F. Parrino, R. Ciriminna, V. Loddo, L. Palmisano, M. Pagliaro, ChemistryOpen 4 (2015) 779–785. [57] D. Tsukamoto, M. Ikeda, Y. Shiraishi, T. Hara, N. Ichikuni, S. Tanaka, T. Hirai, Chem. Eur. J. 17 (2011) 9816–9824. [58] J. Ding, W. Xu, H. Wan, D. Yuan, C. Chen, L. Wang, G. Guan, W.L. Dai, Appl. Catal. B Environ. 221 (2018) 626–634.
7