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Gas hydrates model for the mechanistic investigation of the Wittig reaction “on water”
Theoretical mechanistic details for “on water” Wittig reaction of a stabilized ylide with benzaldehyde are presented and compared with a similar reaction under neat conditions. A gas hydrate structure consisting of 20 water molecules has been applied as a water surface for the reaction. The model is chosen to capture non-bonding interactions over a larger area in order to better account for the “on water” effect. The calculated acceleration for the cis-selective Wittig reaction is more than that for the trans-selective Wittig reaction. The “on water” acceleration for the Wittig reaction is due to greater number of non-bonding interactions in the transition state, compared to the starting material. The greater acceleration for the cis-selective Wittig over the trans-selective Wittig has been rationalized on the basis of non-bonding interactions in addition to hydrogen bonding. Besides accelerating the reaction, water also affects the pathway for the reaction. Decomposition of cisOP2 to alkene is estimated as a barrierless process. Moreover OP2 is more stable than OP1 for both cis and trans-selective Wittig reactions, opposite to what is observed for the neat reaction.
Photocatalytic degradation and toxicity evaluation of diclofenac by nanotubular titanium dioxide–PES membrane in a static and continuous setup
Diclofenac is a commonly used anti-inflammatory drug, which has been found in surface waters. Advanced oxidation processes (AOPs) seem to be the most suitable technique to prevent the entry of diclofenac and other pollutants into surface waters. TiO2 is especially reliable in mineralizing many organic molecules. The combination of TiO2 nanotubes with a polymer microfiltration membrane (polyethersulfone, PES) showed high photocatalytic activity by degrading diclofenac combined with an excellent membrane performance and long-term stability. By continuously degrading pollutants from water via a cross-flow setup, the molecules to be degraded are transported right to the membrane surface so that the overall reaction rate is increased. The toxicity of diclofenac was reduced by photocatalysis and photolysis; however, photocatalysis had greater impact. Moreover, the complete degradation of pollutants is very important to avoid highly toxic intermediate products.