Photocatalytic applications of titanium dioxide coated with phthalocyanine

Recently, environmental contaminants spread over a wide area has become one of the most serious problems for the environment and requires treatment of a large amount of pollutants in water, air and soil. The goal of this work has been to realize degradation of organic pollutants in aqueous suspension with Visible light. Degradation of various pollutants assisted with Visible light have been broadly divided into two main categories. In the first category, TiO2 semiconductors can be used to facilitate the photooxidative degradation of organic pollutants. A major aim of these treatments is to maximize the range of wavelength range in the Visible light region for wastewater treatment. In the second category, various ways of degrading organic pollutants without the use of TiO2 semiconductor. In recent years polycrystalline TiO2 samples have been used as effective photocatalytic systems for degradation of organic pollutants in water because of its economic benefits and environmental friendly nature. Phthalocyanines (Pcs) have been used as photocatalysts for a large variety of oxidation reactions of phenol derivatives. In 1907, Braun and Tcherniac obtained phthalocyanine compound as a dark-coloured insoluble by-product in the preparation of 2-cyanobenzamide. In 1934, Linstead and his group clarified the structure of phthalocyanines after intensive research studies including X-ray diffraction. Afterwards, many research groups have prepared a big number of derivatives carrying different substituents in peripheral and non-peripheral positions together with different metals in the cavity. Due to their high photostability and ability to absorb light with high molar absorptivity, metal-phthalocyanines (MPcs) are productive photosensitizers and catalysts for many reactions. Carboxylated phthalocyanines are especially prone to be adsorbed onto TiO2 type semiconductors due to interaction with the hydroxylated oxide surface. Metallophthalocyanines (MPc) are very stable dyes with high absorption coefficients in the Visible region. By changing the central metal ion, it is possible to vary their redox ability. The photophysical and photochemical properties of carboxylated MPcs in homogeneous solutions and in micellar systems have been studied previously in our laboratory. A number of different precursors including phthalonitriles, phthalic acids, phthalic anhydrides, phthalimides and o-cyanobenzamides have been used in synthesizing phthalocyanine derivatives. Relatively cheaper phthalic acid derivatives are preferred in industrial processes. However, phthalonitrile derivatives might be chosen when high quality and high purity products of lower amount are required in high technological applications without taking the costs of the materials into account. Additionally, the possibility of obtaining them in a few steps and relative ease of the condensation when compared with phthalic acid derivatives will add to prefer phthalonitriles as precursors. In the first part of this work, four phthalonitrile derivatives were prepared starting from 4-nitrophthalonitrile: substitution of the nitro group with thioglycolic acid (TGA), 3-mercaptopropionic acid (3-MKPA), 4-mercaptobenzoic acid (4-MKBA) or 4-hydroxybenzoic acid (4-HBA) gave four precursors namely, 4-(karboksimetilsülfanül)-ftalonitril (1), 3-[(3,4dicyanophenyl)sulfanyl]propanoic acid (2), 4-[(3,4- dicyanophenyl)sulfanyl]-benzoic acid (3) and 4-(3,4- dicyanophenoxy)benzoic acid (4). All of compounds were prepared as reported in the literature. In the next part of this work, cyclotetramerisation reaction of 1 in the presence of Zn(CH3COO)2 or CoCl2 led to metallophthalocyanines 5, 6. Similarly, compounds 7, 8 were prepared starting from 2, compounds 9, 10 from 3 and compounds 11, 12 from 4 was prepared according to the literature. In the second part of this work, towards the first category, TiO2-MPC composites (compounds 13, 14, 15, 16, 17, 18, 19, 20) from treatment of TiO2 with metallophthalocyanines (MPC) 5, 6, 7, 8, 9, 10, 11 or 12 were prepared. In the final stage of this work, Photocatalytic properties of TiO2-MPC composites were investigated by using a photoreactor on degradation of a persistent organic pollutant, namely 4-Chlorophenol (4-CP), in the water. Spectral data obtained from FT-IR, UV-Vis, EI-MS, 1H-NMR, X-ray diffraction and DR (diffuse reflectance) have been used to confirm the structures of the products. As expected, for the metallo phthalocyanines B bands are observed between 300-400 nm and Q bands are observed between 650-700 nm in the UV-Vis spectraA typical change in going from phthalonitrile derivatives to metallo phthalocyanines is the diappearence of the intense CN peak around 2230-2240 cm-1 in the case of the latter. Due to the dimerization of –COOH groups, the peaks of OH groups are not observed in both of IR and 1H-NMR spectra of some compounds. The chemical shifts concerning the aromatic hydrogens of phthalonitrile derivatives and phthalocyanines are observed between 7,5-8,8 ppm in 1H-NMR spectra. While chemical shifts of phthalonitrile derivatives are observed as narrow sharp peaks, the chemical shifts of phthalocyanines are seen as bands due to aggregation behaviour. XRD patterns indicate that the structure of TiO2 remains unchanged after sensitization process, indicating that the anchoring process did not change the characteristic crystal structure of TiO2 as reported in similar works. All of the photocatalytic process is performed within 6 hours. Photocatalytic activity was studied in the presence of visible light and darkness. At the end of the 6 hours, all examples of the 4-chlorophenol 99 % fragmented was monitored by GC-MS. For more specific study, all MPC/ TiO 2/ 4-CP mixtures were prepared again to identical process, when similar photocatalytic processes carrying out at different periods of time (0, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours) photocatalytic process was sampled from the actual ambient and degradation of 4-chlorophenol (4-CP) of the amount was determined by GC-MS. Consequently, the 4-chlorophenol in first 30 minutes, it was observed that 99 % of fragmentation and fixed after 30 minutes. Unmodified pure TiO2 powder did not show significant degradation yield under visible irradiation, however a significant increase in photocatalytic activity was observed when we used sensitized/TiO2 powders. The photo-degradation tests did not show substantial degradation (near 3 %) when 4-CP was exposed to condition of darkness during 360 min, this result is relevant because it confirms that TiO2 is not photocatalytically active under Visible radiation. These findings can be rationalized by the presence of MPc as sensitizer on the TiO2 surface: The electrons in the ground state of MPc are excited by the absorption of photons and then injected into the conduction band of TiO2. In this way, highly active species such as OH· and O2·- are generated at the TiO2 surface, where the photoelectrons and holes become effectively separated. Moreover, it is known that the aggregation of the MPc species shortens the excited triplet state lifetimes by enhancement of internal conversion and therefore reducing their photosensitizing efficiency. Consequently, a monomolecular distribution of the dye on the TiO2 surfaces favours improved photocatalytic activity. As a result, when 4-CP was exposed to Visible radiation in the presence of ZnPc–TiO2 and CoPc–TiO2 heterogeneous catalysts, the photo-degradation of 4-CP was increased and nearly complete degration of 4-CP have been achieved in about 30 minutes.