A "camel through the eye of a needle": direct introduction of the TPP+ ion inside Y-zeolites by formal ion exchange in aqueous medium

Properties and applications of zeolites

Zeolites are aluminosilicate solids bearing a negatively charged honeycomb framework of micropores into which molecules may be adsorbed for environmental decontamination, and to catalyse chemical reactions. They are central to green-chemistry since the necessity for organic solvents is minimised. Proton-exchanged (H) zeolites are extensively employed in the petrochemical industry for cracking crude oil fractions into fuels and chemical feedstocks for other industrial processes. Due to their ability to perform cation-exchange, in which the cations that are originally present to counterbalance the framework negative charge may be exchanged out of the zeolite by cations present in aqueous solution, zeolites are useful as industrial water-softeners, in the removal of radioactive Cs+ and Sr2+ cations from liquid nuclear waste and in the removal of toxic heavy metal cations from groundwaters and run-off waters. Surfactant-modified zeolites (SMZ) find particular application in the co-removal of both toxic anions and organic pollutants. Toxic anions such as arsenite, arsenate, chromate, cyanide and radioactive iodide can also be removed by adsorption into zeolites that have been previously loaded with co-precipitating metal cations such as Ag+ and Pb2+ which form practically insoluble complexes that are contained within the zeolite matrix.

Direct time-resolved detection of singlet oxygen in zeolite-based photocatalysts

Singlet oxygen has been characterized spectroscopically as a product of the exposure of suspensions of zeolites containing oxidation catalysts. Spectroscopic and lifetime studies show that a part of the singlet oxygen formed reacts within the zeolite porous structure, while a significant fraction escapes and becomes available for reaction in the bulk media. The liquid phase plays a key role in determining intra- and extracavity dynamics.

Zeolite Mediated Reactions: Mechanistic Aspects and Environmental Applications

An overview is presented of basic research and applications for the use of zeolites in environmentally-related topics. The use of these materials to promote oxidation reactions, and other catalytic processes is surveyed along with ship-in-a-bottle synthesis of particular zeolite-encapsulated catalysts. Methods for probing the surfaces of zeolites using spin-probes and ESR detection are mentioned. The ability of some thermally activated zeolites to produce spontaneously radical cations by single-electron oxidation of organic molecules is reviewed. The issue of the degree of acidity of H-zeolites is discussed, which demonstrates them not to be super-acids. Some ESR determinations of the adsorption of NOx species onto selected zeolites are reviewed and the use of zeolites to catalyse desulfurisation of hydrocarbons is considered. Finally, there is mention made of the use of zeolites to catalyse hydrodechlorination of CFC's and other organic chlorine-containing compounds. Surfactant-modified zeolites (SMZ) for use as polyfunctional decontamination agents are further discussed. Theoretical methods, e.g. Density Functional Theory (DFT), are finding their own application to studying the mechanisms of reactions that are mediated by zeolites as is shown throughout.

Involvement of triplet excited states in the electron transfer photodegradation of cinnamic acids using pyrylium and thiapyrylium salts as photocatalysts

The mechanistic pathway for degradation of cinnamic acids using 2,4,6-triphenylpyrylium as well as 2,4,6-triphenyl(thia)pyrylium salts (,) as solar photocatalysts has been unambiguously established. Results obtained in steady-state experiments have been correlated with time-resolved photophysical studies. High percentages of photodegradation (60-70%) were achieved when aqueous solutions of caffeic and ferulic acids (,) as model pollutants were submitted to irradiation in the presence of ,. Electron-transfer quenching of both the singlet and triplet excited states of , by , has been proved, and the quenching rate constants (close to diffusion control) have been determined. However, the percentages of singlet quenching by ,, even at relatively high concentrations of the model pollutants, is lower than 5%. In addition to this, growth of the signal corresponding to the pyranyl radical occurs in the microsecond timescale, incompatible with the singlet state as precursor. Thus, photodegradation of , mainly involves the triplet state of the photocatalysts.

Oxidative degradation of 2,4-xylidine by photosensitization with 2,4,6-triphenylpyrylium: homogeneous and heterogeneous catalysis

The possibility of using zeolites containing the 2,4,6-triphenylpyrylium cation as photocatalysts for the degradation of pollutants has been tested on aqueous xylidine (2,4-dimethylaniline) solutions as models for contaminated wastewaters. The influence of the photocatalyst and substrate concentrations on xylidine oxidation has been investigated in homogeneous solution, by performing a series of experiments chosen according to the experimental design methodology (Doehlert uniform array). The empirical models and the corresponding response surfaces obtained from data analysis have been used for simulating and predicting degradation efficiency. The results have shown that conversion increases with increasing amounts of photocatalyst and decreasing concentration of the model pollutant. The fluorescence of 2,4,6-triphenylpyrylium was quenched by xylidine with a rate constant k(q) of 3.1x10(9)M(-1)s(-1). This result suggests a direct electron transfer between the excited pyrylium salt and xylidine. Because of the limited stability of the photocatalyst in homogeneous media, a pyrylium containing Y-zeolite has been tested for the photocatalytic oxidation of xylidine under heterogeneous conditions. The results suggest that the supported catalyst has a much improved stability and that xylidine oxidation rates remain nearly constant during the whole reaction time. An additional advantage of the pyrylium containing zeolite photocatalyst is that it can be recycled and used for further experiments.