Development of photocatalysts for selective and efficient organic transformations

One of the main goals of organic chemists is to find easy, environmentally friendly, and cost effective methods for the synthesis of industrially important compounds. Photocatalysts have brought revolution in this regard as they make use of unlimited source of energy (the solar light) to carry out the synthesis of organic compounds having otherwise complex synthetic procedures. However, selectivity of the products has been a major issue since the beginning of photocatalysis. The present article encompasses state of the art accomplishments in harvesting light energy for selective organic transformations using photocatalysts. Several approaches for the development of photocatalysts for selective organic conversions have been critically discussed with the objective of developing efficient, selective, environmental friendly and high yield photocatalytic methodologies.

Energy and molecules from photochemical/photocatalytic reactions. An overview

Photocatalytic reactions have been defined as those processes that require both a (not consumed) catalyst and light. A previous definition was whether such reactions brought a system towards or away from the (thermal) equilibrium. This consideration brings in the question whether a part of the photon energy is incorporated into the photochemical reaction products. Data are provided for representative organic reactions involving or not molecular catalysts and show that energy storage occurs only when a heavily strained structure is generated, and in that case only a minor part of photon energy is actually stored (ΔG up to 25 kcal·mol⁻¹). The green role of photochemistry/photocatalysis is rather that of forming highly reactive intermediates under mild conditions.

Biotechnologically obtained nanocomposites: A practical application for photodegradation of Safranin-T under UV-Vis and solar light

This research was undertaken to determine the potential of biologically obtained ZnS-TiO2 nanocomposites to be used as catalysts in the photodegradation of organic pollutants, namely, Safranin-T. The photocatalysts were prepared by modifying the surface of commercial TiO2 particles with naturally produced ZnS, using sulfide species produced by sulfate-reducing bacteria and metal contaminated wastewaters. Comparative studies using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM), prior and after photodegradation, were carried out in order to monitor possible structural and morphological changes on the particles. Adsorption properties and specific areas were determined by the Brunauer-Emmet-Teller (BET) method. The final solutions were characterized by UV-Vis and chemical oxygen demand (COD) content in order to determine Safranin-T concentration and toxicity. The influence of the catalyst amount, initial pH and dye concentration was also evaluated. Finally, the efficiency of the precipitates as catalysts in sunlight-mediated photodegradation was investigated, performing two scale experiments by using different volumes of dye-contaminated water (150 mL and 10 L). All tested composites showed potential to be used as photocatalysts for the degradation of Safranin-T, although the ZnS-TiO2_0.06 composite (0.06 g of TiO2 per 50 mL of the zinc solution) was the most effective. This substantiates the applicability of these biologically obtained materials as efficient photocatalysts for the degradation of organic pollutants, in laboratorial conditions and under direct sunlight.

Light-mediated cascade transformation of activated alkenes: BiOBr nanosheets as efficient photocatalysts for the synthesis of α-aryl-β-trifluoromethyl amides

A facile light induced, BiOBr nanosheet promoted one-pot consecutive trifluoromethylation/aryl migration/desulfonylation and N–H bond formation of activated alkenes is proposed.

Semiconductor photocatalysis type B: synthesis of unsaturated alpha-amino esters from imines and olefins photocatalyzed by silica-supported cadmium sulfide

Novel unsaturated N-phenyl-alpha-amino esters were synthesized in isolated yields of 50 to 10% by visible light irradiation of methanolic suspensions of silica-supported cadmium sulfide in the presence of methyl (2Z)-phenyl(phenylimino)acetate and various cyclic olefins. A semiconductor photocatalysis mechanism is proposed for this linear addition reaction. The light-generated electron-hole pair in the oxidative step induces a dissociative electron transfer from the olefin to CdS affording a proton and an allylic radical, whereas in the reductive step an alpha-aminobenzyl radical is formed in a proton coupled reaction. Heterocoupling of these intermediate radicals leads to the corresponding addition products. As the only by-product the hydrogenated imine is obtained in comparable amounts through the subsequent photoreduction of the alpha-aminobenzyl radical. Supporting the catalyst on silica made the reaction three times faster as compared to neat CdS. When the surface OH groups of CdS were removed through alkylation, the reaction between ArCH=NAr (Ar = p-C1C6H4) and cyclopentene was completely inhibited, but occurred again upon replacing the aldimine by its hydrochloride salt. This indicates that the protonated aldimine is involved in the reductive reaction step. Protonation makes this reduction easier by 0.1 V as indicated by the shift of the reduction potential of methyl (2Z)-phenyl(phenylimino)acetate upon addition of glacial acetic acid to the acetonitrile solution.

Synthesis of 2-methylpiperazine by photocatalytic reaction in a non-aqueous suspension of semiconductor-zeolite composite catalysts

UV irradiation onto a non-aqueous suspension of semiconductor-zeolite composite catalysts with N-(beta-hydroxypropyl)ethylenediamine (N-beta-HPEDA) produces 2-methylpiperazine and piperazine. The yield of 2-methylpiperazine and piperazine depends on the type of semiconductor and zeolite. A 5 wt% TiO2-Hbeta composite photocatalyst shows highest yield of 2-methylpiperazine.