Ana Sanjuán

2,4,6-Triphenylpyrylium ion encapsulated within Y zeolite as photocatalyst for the degradation of methyl parathion

Abstract 2,4,6-Triphenylpyrylium ion (TP + ) encapsulated inside the supercages of Y zeolite (TPY) can efficiently effect the photocatalytic degradation of methyl parathion in aqueous solutions. Comparison of the performance of TPY with that of TP + deposited on amorphous silica has shown that the zeolite host contributes to the efficiency of the system by stabilizing TP + avoiding its nucleophilic ring opening and by absorbing the organic pollutant from the aqueous phase and concentrating it in the interior of the solid. Although TPY does not undergo irreversible bleaching after three consecutive irradiations, its activity is largely reduced probably due to blockage of the zeolite micropores by adventitious reagents and products. Up to 75% of the original activity can be regained by simple solid–liquid extraction. Concerning the mechanism of the photodegradation, it has been ruled out the intermediacy of singlet oxygen as well as a direct photoinduced electron transfer between excited TP + and methyl parathion. Instead, evidence has been presented for the need of O 2 as co-reagent as well as the involvement of radicals.

2, 4, 6-Triphenylpyrylium ion encapsulated in Y zeolite as photocatalyst. A co-operative contribution of the zeolite host to the photodegradation of 4-chlorophenoxyacetic acid using solar light

Abstract 2, 4, 6-Triphenylpyrylium ion (TP+), a well-known single electron transfer photosensitizer, encapsulated inside the voids of a microporous Y zeolite (TPY) (spherical cages of 1.3 nm diameter, tetrahedrally interconnected through four openings of 0.74 nm diameter) has been tested as heterogeneous photocatalyst for the degradation of 4-chlorophenoxyacetic acid as model probe molecule. The results were compared under simultaneous irradiation with the performance of TiO2, TP+BF−4 supported on amorphous silica and malachite green (another dye) also imprisoned inside Y zeolite. It was found that the activity of TPY is higher than that of the other photocatalytic systems, especially using visible light excitation or direct solar irradiation. These results have been interpreted as being due to a combination of factors, including appropriate absorption spectrum of TP+, favorable thermodynamics of the electron transfer pathway and a co-operative contribution of the zeolite host. Thus, we have observed that an initial rapid step consisting in the adsorption of the pollutant inside the zeolite micropores takes place before the actual slower photoreaction enables detection of photoproducts. This adsorption produces a favorable high concentration of the target substrate in the proximity of the reactive photocatalytic center, thus increasing the efficiency of the overall degradation process.

Degradation of propoxur in water using 2,4,6-triphenylpyrylium–Zeolite Y as photocatalyst: Product study and laser flash photolysis

Abstract Direct UV photolysis of aerated aqueous solutions of propoxur leads to an almost complete disappearance of the starting material and the formation of the photo-Fries rearrangement products. The degree of mineralization estimated by the difference between the initial and the combined final moles is, however, small. Laser flash photolysis techniques established that the key intermediate in the direct aqueous photolysis is 2-isopropoxyphenoxyl radical. Photocatalyzed mineralizations using 2,4,6-triphenylpyrylium ion encapsulated inside the supercages of Zeolite Y under irradiation conditions in which propoxur is not directly altered, produces a degree of mineralization comparable to that achieved using TiO2. Formation of minor amounts of 2-isopropoxyphenol, 1,2-dihydroxybenzene and isopropoxy-dihydroxybenzene was observed in photocatalyzed degradations. Laser flash photolysis established that a mechanism involving an initial electron transfer between excited TP+ and propoxur is possible.

Hydroxyalkylation of benzene derivatives by benzaldehyde in the presence of acid zeolites

Condensation of benzene, anisole and N,N-dimethylaniline with benzaldehyde has been carried out in the presence of a USY zeolite (Si/Al 17.5). A combined study of the product distribution present in the liquid phase (di- and triarylmethanes together with N-benzylated byproducts in the case of dimethylaniline) as well as the organic material entrapped inside the zeolite micropores (triphenylmethane or trityl ions) has established that there is a direct relationship between the reactivity of the arene and the conversion of benzaldehyde, percentage of diarylmethanes in the liquid phase and population of trityl cations. A hydride transfer from triarylmethanes to diarylmethyl cation intermediates has been proposed as a key step for the simultaneous presence of trityl cations adsorbed in the solid and diarylmethanes in the liquid phase.

Heterogeneous Gif oxidation of cyclohexane using Fe3+-picolinate complex encapsulated within zeolites

Abstract A series of catalysts containing the Fe3+-picolinate (Fe-PA) complex inside the micropores of zeolite Y and mordenite has been synthesized by treating pre-exchanged Fe3+ zeolites with a solution with picolinic acid in dichloromethane. The Fe3+-content of the solids was purposely low to avoid the presence of uncomplexed, free Fe3+ ions that could decrease the selectivity of the catalysts. Formation of the Fe-PA complex was ascertained by diffuse-reflectance UV-vis (characteristic absorption at 340 nm), FT-IR (vibrations at 1705 and 1480 cm−1) and luminescence (emission at 540 nm) spectroscopies. These solids have been found to act as heterogeneous catalysts for the oxidation of cyclohexane by H2O2 with a similar selectivity to that reported in the literature as much higher conversion for the same complex in solution. The beneficial influence of a high PA Fe 3+ ratio in the solid reflects the need to use a large excess of PA to ensure the absence of uncomplexed Fe3+ ions. In addition, mordenite (channels, 0.74 nm) was found more efficient (93% selectivity at 49.1% of the maximum conversion) as a support than the more open Y zeolite (spherical cavities, 1.3 nm diameter).

Product studies and laser flash photolysis of direct and 2,4,6-triphenylpyrylium-zeolite Y photocatalyzed degradation of fenvalerate.

Direct irradiation of fenvalerate (FV) in aqueous solution (4 x 10(-3) M) gives rise to 2,3-diarylisohexanitrile through the homolytic CO-CH(i-Pr) bond cleavage, followed by a rapid decarboxylation of the primary carbonyloxy radical, and radical recombination. The unusual exclusive formation of the cross combination product suggests that the persistent free radical effect is operative in this system and controls product formation. Spectroscopic evidence of the intermediacy of benzylic radicals decaying in microsecond time scale has been obtained by laser flash photolysis. These studies also reveal a remarkable attenuation of radical reactivity toward oxygen resulting from cyano substitution at the radical center. TPY photocatalyzed degradation of FV also leads to decarboxylation products accompanied by oxidative mineralization (38%). The TPY-photocatalyzed degradation can occur through two operating mechanisms involving the generation of OH(*) radicals and/or FV(*+) radical cations. Evidence for the latter intermediate has been obtained by laser flash photolysis in acetonitrile solution (detection of TP(*) radical) and by the quenching by FV of the emission from the TPY solid.

Topological quenching of 2,4,6-triphenylpyrylium tetrafluoborate in anionic micelles

Abstract Topological quenching of the excited states of the photosensitizer 2,4,6-triphenylpyrylium tetrafluoborate bound to an anionic SDS micelle has been observed by fluorescence spectroscopy. Spectroscopic data suggests that the triphenylpyrylium cation is micellized with the cationic pyrylium moiety slightly projecting out, experiencing the aqueous polar environment. Accordingly, the quenching mechanism varied from static for neutral quenchers and dynamic for the cationic quenchers whereas no quenching was observed for anionic quenchers. Upon irradiation, micellized TP+ undergoes a photoinduced hydrolysis to 1,3,5-triphenyl-1-pentene-1,5-dione, possibly through an electron transfer between triplet excited pyrylium cation and water molecules. This electron transfer leads to the generation of hydroxyl radicals producing the build-up of 10−2 M hydrogen peroxide.

An organic sensitizer within Ti-zeolites as photocatalyst for the selective oxidation of olefins using oxygen and water as reagents

A new positive photocatalytic system comprising an organic dye within the micropores of a titanium zeolite is able to effect the catalytic dihydroxylation of alkenes using water and molecular oxygen as reagents; the dye acts as an antenna to absorb the energy of visible light, generating hydroxyl radicals from water; subsequently, the OH· radicals react with olefins in the presence of oxygen to form allylic hydroperoxides that in the presence of the Ti atoms of the zeolite promote the epoxidation of the alkene.

Evolution of computing in spanish-speaking countries

Two decades ago, people used computers as an information resource for many fields. These fields include library information, climatic information, medicine, transportation schedules, banking, and other areas. The use of international networks at that time enabled people to communicate globally in a rapid and accurate fashion not only to experts, but to the public also. In regions such as South and Central America, however, the uses of information resources were not as widespread as they were in developed countries. Indeed, estimates showed that Latin America contained two percent of the world’s informatics equipment. At that time, computers appeared in commercial and governmental agencies as well as universities that used global networks such as the internet, BITNET, FidoNet, and other similar networks.