Development of an efficient downstream process for product separation and catalyst recycling of a homogeneous polyoxometalate catalyst by means of nanofiltration membranes and design of experiments

Extractive coupled oxidative desulfurization (ECODS) is a promising technology for efficient deep desulfurization of fuels with integrated product separation. The main objective of this contribution is the development of an efficient downstream process for the separation and recovery of the molecular HPA-5 (H8PV5Mo7O40) catalyst by means of nanofiltration membranes. We obtained optimized parameter settings for the membrane separation process using a Box, Hunter & Hunter design of experiments leading to higher rejection of catalyst components above 99%. The optimized parameters were successfully applied for recycling experiments in ECODS. In detail, model gasoline consisting of benzothiophene in iso octane could be efficiently desulfurized in six consecutive runs with product separation by the means of nanofiltration between the individual runs. Distinct changes of the catalyst structure were indicated by 51V NMR and 31P NMR spectroscopy after the second recycling step due to acidification of the aqueous reaction solution. Based on these results, we predict that a continuous process with a coupled nanofiltration separation could push the ECODS technology to industrially relevant technology readiness levels.