Preliminary study to use a membrane reactor in photocatalytic transfer hydrogenation of organic compounds

Hydrogenation is extensively used in industrial processes for the production of liquid fuels, e.g. hydrogenation of intermediates from biomass, hydrogenation of ketones to their corresponding alcohols intermediates in the pharmaceutical industry.Among the methods used, transfer hydrogenation avoids the direct use of H2 minimizing hazards related to its manipulation (high flammability and explosiveness) particularly on the large scale, and optimizing the costs related to its storing and transporting1.Coupling transfer hydrogenation and photocatalytic processes: i) permits to carry out the hydrogenation under mild operating conditions; ii) offers a good alternative to the energy-intensive conventional hydrogenation methods, because of the possibility to use renewable solar energy; iii) facilitates catalyst recovery.Hybrid processes coupling membrane and photocatalytic transfer hydrogenation, could represent a promising hydrogenation strategy because they allow chemical reaction and separation process to be obtained in one step, minimizing environmental and economic impacts2.In the present preliminarily work, the photocatalytic hydrogenation of acetophenone, largely used as model substrate3, to phenyl ethanol in batch has been considered. The influence of catalyst amount, type of catalyst, light intensity and distribution, type of solvent on acetophenone conversion and yield to phenyl ethanol have been determined.The experimental tests were conducted in a cylindrical reactor (V = 500 mL) with a 125 W immersed medium pressure Hg lamp at 32°C. Each test had a duration of 7 hours. Using ethanol as solvent, hole scavenger and hydrogen donor and TiO2 catalyst ranging from 0 to 3 g/L, it was obtained that acetophenone, submitted to irradiation, can react by: i) photochemical mechanism (without the catalyst) resulting in high acetophenone conversion (94%) but low yield (3.3%), because of the unselective radical reactions; ii) photocatalytic mechanism (with TiO2) characterized by higher selectivity and yield. Phenyl ethanol yield had a maximum (16.9%) at 1.5 g/L of TiO2. Increasing the catalyst amount the yield decreased, probably because of the decreased transmittance of radiation through the suspension due to scattering effect.Some tests were carried out with 1% Pd/TiO2 synthesized by the deposition-precipitation method. The results showed an high acetophenone conversion (96.2%) but yield was very low (3.1 %), because the more reactive catalytic system gave selectivity loss.The comparison of the results obtained by using two lamps with different radiation intensity (35 and 55 mW/cm2) showed that better performances were obtained at the lowest value, meaning that, upper a certain value, this parameter does not greatly influence the photocatalytic activity.The results, obtained in a water/ethanol solution (50/50 v/v), showed acetophenone conversion and yield lower than using 100% ethanol.In conclusion, the best results were obtained using 1.5 g/L of TiO2 as catalyst, the lamp with lower radiation intensity and ethanol as the solvent. Studies are in progress to design a suitable membrane reactor for this reaction.