No abstract

In recent times, it has been shown that the microporous ETS-10 titanosilicate can be used as a shape-selective photocatalyst for the decomposition of aromatic molecules (Chem. Commun. 2001, 2131). Its actual use on practical grounds is however discouraged by its too low activity, when compared with that of TiO(2) photocatalysts. In the present work, we show how an ad hoc mild treatment with HF enhances the activity of ETS-10 toward the photodegradation of large aromatic molecules that are unable to penetrate inside the zeolitic pores, such as 2,5-dichlorophenol, 2,4,5-trichlorophenol, 1,3,5-trihydroxybenzene, and 2,3-dihydroxynaphthalene (DHN). The photoactivity of the acid-treated materials is comparable or even greater than that of the nonselective TiO(2) catalyst. Moreover, the enhancement of the photoactivity is accompanied by a remarkable parallel increase of the shape selectivity, particularly toward DHN (k(DHN)/k(P) = 127, where P = phenol). A complete characterization (by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, ultraviolet-visible spectroscopy, and X-ray aborption spectroscopy techniques) of a set of ETS-10 samples which have undergone a progressively severe HF treatment allows us to propose an explanation of the photocatalytic activity and selectivity of the modified materials.

In the first part of this work we report an exhaustive characterization of the bulk and surface properties of ETS-4 and ETS-10 microporous titanosilicates by means of combined N 2 volumetric measurements, SEM, IR, Raman, and UV-Vis techniques. The structure of the surface titanols, derived from literature XRD studies, is shown using a molecular graphic approach. UV-Vis titration experiments using H 2 O 2 , and catechol molecules, allowed us to directly measure the total number of available titanols (both in the channels and on the crystal external surface) and the surface titanols, respectively. In the second part of the paper, the ability of ETS-4 and ETS-10 (and of the standard P25) in the photodegradation of phenol (P), 4-chlorophenol (CP), 2,5-dichlorophenol (DCP), 2,4,5-trichlorophenol (TCP), 1,3,5-trihydroxybenzene (THB), and 2,3-dihydroxynaphthalene (DHN) is investigated, using both UV and visible lights, exciting above and below the materials energy gap, respectively. While microporous ETS-4 and ETS-10 exhibit a significant selectivity in the photodegradation of the above-mentioned molecules using both lights, P25 selectivity is observed with visible light only. This means that besides the inverse shape selectivity effect already observed for the microporous materials [Xamena et al. J. Am. Chem. Soc. 2003, 125, 2264, selectivity may be achieved also by selecting the excitation light in accordance with the electronic transition of the adsorbed molecule (determined by a previous systematic UV-Vis study). In such a case the photodegradation may occur if the conduction band of the Ti-based material is opportunely matched with the LUMO level of the adsorbed molecule so that it can receive the electron of the excited adsorbate. The concept of band alignment, well-established in the field of solid-state physics applied to semiconductor heterostructures [Margaritondo, G.

Phenol chlorination was studied in the presence of dissolved Fe(III) and chloride under irradiation and of hydrogen peroxide and chloride in dark acidic solutions. In the former case phenol photochlorination is most likely due to the formation of Cl2*- as a consequence of Fe(III) irradiation in the presence of chloride. The most efficient pathway is the photolysis of FeOH2+ producing hydroxyl, which oxidizes chloride to Cl*. The latter finally yields Cl2*- upon further reaction with chloride. The importance of the pathway involving FeOH2+ is higher at higher pH and moderately low chloride concentration. At pH 2.0 and [Cl-] > 0.03 M chlorophenol generation rate decreases with increasing [Cl-], due to the formation of the much less photoactive species FeCl2+/FeCl2+. The photolysis of FeCl2+/ FeCl2+ yielding Cl* is likely to play an important role at pH 0.5 and high chloride, but under such conditions chlorophenol formation rates are about an order of magnitude lower than at pH 2.0. Due to pH and kinetic constraints, under most environmental conditions the photochemistry of FeCl2+/FeCl2+ can be expected to play a minor role toward chlorination when compared with the one of FeOH2+, which leads to hydroxyl-mediated chloride oxidation. Hydrogen peroxide and chloride react in dark acidic solutions to yield HClO, involved in electrophilic chlorination processes. Chlorophenol formation rates under such conditions are directly proportional to [H+]. The described chlorination and photochlorination processes can take place in acidic aerosols of marine origin, naturally rich in chloride and Fe(III). Antarctic aerosol is also rich of hydrogen peroxide and often strongly acidic due to the presence of sulfuric acid of biogenic origin.

A zeolitic material containing Ti chains activated by light allows a shape-selective transformation of organic compounds; the molecules that can enter into the zeolitic cavities are protected from the light-induced processes, while the other are degraded.