Use of Novel Cardanol-Porphyrin Hybrids and Their TiO2-Based Composites for the Photodegradation of 4-Nitrophenol in Water

Cardanol, a well known hazardous byproduct of the cashew industry, has been used as starting material for the synthesis of useful differently substituted “cardanol-based” porphyrins and their zinc(II), copper(II), cobalt(II) and Fe(III) complexes. Novel composites prepared by impregnation of polycrystalline TiO2 powder with an opportune amount of “cardanol-based” porphyrins, which act as sensitizers for the improvement of the photo-catalytic activity of the bare TiO2, have been used in the photodegradation in water of 4-nitrophenol (4-NP), which is a toxic and bio-refractory pollutant, dangerous for ecosystems and human health.


Introduction
Cardanol is a naturally occurring phenol obtained by vacuum distillation of cashew nut shell liquid (CNSL), a waste byproduct obtained in the cashew nut processing industry [1][2][3][4][5]. Despite the fact that cardanol could really be considered a dangerous toxic waste, mainly due to the massive amounts of CNSL produced annually, it represents a precious natural renewable resource which can be used as a starting material for the preparation of a large variety of useful chemicals [6].
In fact, the preparation of fine chemicals from natural and renewable materials is nowadays becoming an attractive topic of research especially for the purpose of recycling huge amounts of agro-industrial waste.
4-Nitrophenol (4-NP) is a harmful and bio-refractory contaminant which can cause considerable damage to the ecosystem and human health. For this reason its efficient degradation in aqueous effluents is important in order to minimize its deleterious effects as well as environmental problems [11][12][13][14].
We would also like to report studies concerning the photocatalytic activity of these compounds, once deposited onto TiO 2 , in photodegradation of 4-nitrophenol contained in the water. The advantages related to the use of cardanol-based porphyrins containing double bonds in the cardanol side chain has also been noted in this work.

Synthesis and Characterization of Cardanol Based Porphyrins
In this work, the term cardanol is used to refer mainly to 3-(pentadeca-8-enyl)-phenol, the monoolefinic component which can be obtained almost pure from the cardanol oil through distillation and chromatographic separation, the purity of which, enough for our purposes, was confirmed by GC-MS and NMR analyses. The meso-AB 3 and trans-A 2 B 2 porphyrins were obtained, using 4-[2-(3-(pentadeca-8-enyl)phenoxy)-ethoxy]-benzaldehyde (1) which was prepared from cardanol through two steps as shown in Scheme 1, following the procedure reported in the literature [6,18].  Thus, meso-AB 3 and trans-A 2 B 2 cardanol-based porphyrins 3 and 4, were synthesized respectively by acid-catalyzed condensation of compound 1 by statistical reaction with pyrrole and benzaldehyde (Method 1) or with meso-phenyldipyrrolmethane 2 (Method 2), as shown in Scheme 2 in accordance with different reaction protocols [6,7]. The resulting porphyrins 3 and 4, brown-red sticky solids, very soluble in CHCl 3 or CH 2 Cl 2 , have been characterized by FT-IR, UV-Vis, 1 H-and 13 C-NMR, and MALDI-TOF techniques. Isolated yields and UV-Vis absorption band of compounds 3 and 4 are reported in Table 1. For instance, the UV-Vis spectrum of 3 showed a Soret band at 419 nm and Q bands at 516, 552, 590 and 646 nm; the UV-Vis spectrum of 4 showed a 1 nm red shift, with a Soret band at 420 nm and Q bands at 517, 553, 591 and 647 nm. A red shift in the Q bands was also observed in the previously reported cardanol-based A 4 -porphyrin [16]. This suggested to us that the number of the substituents in the porphyrin molecule influences the value of the maximum of absorption in the UV-Vis spectra, producing a red shift when the number of substituents is increased. The MALDI-TOF analysis of the Porphyrins 3 and 4 were next used for preparation of the corresponding metallo-derivatives 3a-3d and 4a-4d (Scheme 3) in nearly quantitative yields, by reacting them with Zn(OAc) 2 , Co(OAc) 2 ·4H 2 O, CuCl 2 , and FeCl 3, respectively. FT-IR, UV-Vis, MALDI-TOF and elemental analyses of the metalloporphyrin complexes 3a-3d and 4a-4d were consistent with the proposed structures. Yields and UV-Vis absorption bands of 3a-3d and 4a-4d are also reported in Table 1.

Scheme 2. Synthesis of meso-AB 3 and trans-A
From the UV-Vis absorption bands it is possible to observe that in the case of metalloporphyrins 3a-3d and 4a-4d, the Soret band is only slightly shifted compared to the corresponding metal-free porphyins and the Q bands are reduced to two or at least one because the symmetry of porphyrin ring increases when the hydrogen atoms were replaced by metals.
The IR spectra of 3a-3d and 4a-4d were close to those of the corresponding metal-free porphyins 3 and 4, except for the disappearance of the NH vibration at 3317 cm −1 . MALDI-TOF mass spectrometry analysis was successfully used for the determination of the molecular weight of the metalloporphyrin complexes 3a-3cd and 4a-4d (see Experimental Section). 1 H and 13 C-NMR spectra were recorded only in the case of Zn(II) complex 3a and 4a because of the paramagnetic effect of the Cu(II), Co(II) and Fe(III) metal ions that hindered the recording of any such spectra.

Preparation of the Cardanol Based Porphyrin/TiO 2 Composites and Diffuse Reflectance (DR) Spectroscopy Characterization
TiO 2 composites used as photocatalysts were prepared by impregnating of TiO 2 with cardanol-based porphyrins according with the procedure reported in the Experimental. Figure     It is worth noting that no appreciable shift of the band gap edge of TiO 2 can be observed for any of the loaded samples. This behaviour was in accord with previously studied metal free and copper [5,10,15,20-tetra(4-tertbutylphenyl)] porphyrins [19].
Similar behavior was observed for the porphyrins H 2 Pp, 3, and MPps [M = Zn (II), Cu(II), Co(II), Fe(III)-Cl] 3a-3d) (spectra not shown in Figure 2 for clarity). Figure 3 shows the SEM pictures of bare TiO 2 and CuPp (4b)/TiO 2 . Basically, the microstructures of the bare TiO 2 and porphyrin impregnated TiO 2 composites show common features which are typical regarding this TiO 2 polymorph. In fact, both kinds of samples seem rather similar, with spherical shaped particles. (a) (b)
The efficiency of a photodegradation catalyst has been evaluated by measuring the rate of consumption of 4-NP in a slurry containing a finely dispersed semiconductor, under constant illumination. It can also be noticed that the substrate was degraded using each of the photocatalysts, following pseudo-first-order kinetics. The list of used samples is reported in Table 2, along with the initial reaction rates of 4-NP disappearance as r 0 × 10 9 (mol L −1. s −1 ), r 0 ′ × 10 9 (mol L −1. s −1. m −2 ) and % conversion of 4-NP. Figure 4 shows the diminution of 4-NP concentration vs. irradiation time using different amounts of CuPp (4b)/TiO 2 photocatalysts. These preliminary investigations were carried out in order to establish which among the differently impregnated photocatalysts exhibited the highest photoactivity.    It can be seen that the samples impregnated with 6.0-CuPp (4b)/TiO 2 exhibited the highest photoactivity. These results are in accord with those observed by using the sensitizers 3a-3d as summarized in Table 2.
As shown in the Figure 5, the Cu(II) porphyrin 4b definitely proved a more effective sensitizer in the photodegradation of 4-NP than other MPp's (M = Co, Zn) 4a, 4c, which have a slight beneficial effect. Interestingly, in contrast with previous experimental evidence [19][20][21], there is a detrimental effect observed for the free-base and Fe(III) porphyrin composites 4/TiO 2 and 4d/TiO 2 compared with bare TiO 2 which could be ascribed to the different lamp used as irradiation source.  The photocatalytic activities are also very slightly influenced by the substitutions and the spatial positions of the substitutions of porphyrins. In particular, the composites (4, 4a-4d)/TiO 2 when used as catalysts show slightly better photocatalytic activities than (3, 3a-3d)/TiO 2 , but they have a similar activity order. All the studied cases gave a conversion of 4-NP higher than 85.5%; in particular, by using the most efficient CuPps/TiO 2 photocatalysts the measured conversion was close to 98% (Table 2).
Further investigations were carried out in order to establish the photostability of the CuPp 4b impregnated onto the TiO 2 surface. Repeated recycling experiments confirmed that this porphyrin supported onto TiO 2 showed good stability under irradiation conditions and samples continued to maintain good photocatalytic activity after several cycles. Figure 6 shows how the most active photocatalyst, i.e., CuPp (4b) TiO 2 can be recycled six times, after its first use, without significant loss of activity. Typically, 3b and 4b, being effective sensitizers, were insoluble in the water and stable under UV irradiation, and the catalysts 3b/TiO 2 and 4b/TiO 2 were also reused several times without loss of the activity.
Taking into account the r 0 values reported in the Table 2 of the impregnated MPps were in the following order: CuPp > CoPp > ZnPp > bare TiO 2 > H 2 Pp > FePp. The results related to the photo-degradation of 4-NP in an aqueous heterogeneous environment suggest that the Cu(II)-Cu(I) photocatalytic redox cycle plays the main beneficial role for the occurrence of the whole process. In a previous work [20] we demonstrated that Cu(II) could be reduced to Cu(I) [see equation (1) Despite the complex mechanism of reactions the redox process reported in equation 1 seems to be the key step in the course of which is possible to increase the amounts of OH radicals and superoxide anion responsible of the degradation process of 4-NP [19,20]. Moreover, in the present case, porphyrin sensitizers containing un-saturated chains capable of being oxidized have been used for the first time. Spectroscopic analysis (UV-Vis, FT-IR, etc.) carried out in order to check the photostability of the porphyrins used as the sensitizers permitted us to prove the stability of the double bonds contained in the side cardanol chains. In fact, typical spectroscopic signals of double bond of cardanol are still present at the end of each process. This could means that the oxidizing species responsible of the photo degradation processes by oxidative demolition of the 4-NP [19,20] act in water solution far from the composite TiO 2 surface.

Reagents
Cardanol oil (technical grade) was kindly provided by Oltremare S.p.A. (Bologna, Italy). TiO 2 (anatase phase, specific surface area 8 m 2 /g), kindly provided by Tioxide Huntsman was dried and crushed to obtain particles with a diameter smaller than 0.1 mm. All other starting materials were purchased from Aldrich Chemical Co and used as received. Silica gel (Merck) was used in the chromatographic separations. Solutions of 4-nitrophenol, used without further purification, were prepared by dissolving the required quantity of 4-NP in water obtained from a New Human Power I water purification system.

Analyses
FT-IR spectra were recorded on a JASCO FT-IR 430 spectrometer. UV-Vis spectra were recorded on a Cary 100scan UV-visible spectrophotometer. 1 H-and 13 C-NMR spectra were recorded on a Bruker Avance 400 instrument at room temperature and chemical shifts are reported relative to tetramethylsilane.Laser desorption/ionization time of flight mass spectrometery (LDI-TOF MS) was performed on a Reflex IV spectrometer (Bruker Daltonik, Bremen, Germany), managed by the Flex Control 2.4 software (Bruker Daltonik, Bremen, Germany), equipped with a VSL-337ND nitrogen laser (Laser Science Inc., Franklin, MA, USA) delivering 4 ns pulses with a repetition rate of 5 Hz and an average power of 200 µJ. The laser attenuation setting was typically in the range 40-50. Spectra were obtained in positive ion reflector mode, with 20-17 kV accelerating voltage and 23 kV reflection voltage. External quadratic calibration was performed with a standard mixture ranging from 757.40 to 3147.47 kDa, giving a mass error lower than 15 ppm. One µL of sample solution of CHCl 3 was spotted on a MTP 384 massive target T (Bruker Daltonik, Bremen, Germany), both in the absence and in the presence (Matrix Assisted LDI-TOF MS) of same volume of alpha-cyano-4-hydroxycinnamic acid (saturated solution in water/acetonitrile/TFA 66.9/33/0.1) as ionization adjuvant, and air-dried. Each spectrum was acquired by 100 to 200 laser shots. Diffuse reflectance (DR) spectra were obtained at room temperature in the wavelength range 200-800 nm using a Shimadzu UV-2401PC spectrophotometer with BaSO 4 as reference material. (1) was synthesized in our laboratory [6,18]; meso-phenyldipyrrolmethane (2) was synthesized with the standard procedure in the literature [7].

General procedure for synthesis of 3d and 4d
Porphyrin 3 (30.0 mg, 0.031 mmol) or 4 (30.0 mg, 0.023 mmol) were dissolved in DMF (15 mL). To this solution, an excess of FeCl 3 (30.2 mg, 0.186 mmol) was added. The reaction was heated to reflux and monitored by UV/Vis spectroscopy. The metal insertion was completed in 4 h. Then the solvent was removed under vacuum and the residue was purified by silica gel chromatography (CHCl 3 /hexane, 7/3 v/v) to give 3d and 4d respectively.
Representative data for compounds 3a-3d, 4a-4d  bubbled into the suspension when switching on the lamp. Samples (3 mL) were withdrawn from the suspension every 30 min during the irradiation. The photocatalysts were separated from the solution by centrifugation and successively filtered through 0.45-µm celluloseacetate membranes (HA, Millipore) before to perform the quantitative determination of 4-NP by measuring its absorption at 316 nm with UV-Vis spectrophotometer. Bare TiO 2 was also tested for the sake of comparison under the same experimental conditions.

Conclusions
In this  derivatives, 3a, 3b, 3c, 4a, 4b and 4c. Selected Cu(II) porphyrins used as sensitizers onto TiO 2 samples showed the best photo-catalytic activity for the photo-degradation of 4-NP in water, compared with the other MPp/TiO 2 composites.