Studies of catalytic asymmetric transfer hydrogenation in batch and continuous reactors

• Main Author: Sun, Xiuyan
• Published: University College London (University of London) 2008
• Subjects: 660.2832
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.497811

The aim of this work was to design novel reactors to increase the reaction conversion and enantioselectivity for the asymmetric transfer hydrogenation (ATH) of acetophenone. The reactor designs should also be amenable to scale-out for increase of productivity. Asymmetric transfer hydrogenation of acetophenone with isopropanol is a reversible reaction and backwards reaction limits the reaction conversion and enantioslectivity. Novel reactor design aims to efficient acetone removal, which is a byproduct in the reaction system. By gas stripping, reaction equilibrium can be shifted and hence conversion and enantioselectivity improved. Reaction conditions optimization was initially conducted in a laboratory batch reactor and a simple kinetic model was built. The catalyst deactivation, the effect of the temperature, substrate concentration, substrate/catalyst concentration ratio and acetone concentration on the ATH were investigated. The metal-ligand bifunctional mechanism was considered for the kinetic model. The gPROMS/gEST commercial software was used for kinetic parameters estimation. Three continuous reactors (tubular reactor, rotating disc reactor and micromesh reactor) were designed and fabricated, each one encompassing a different gas/liquid contacting method. In the tubular reactor, gas/liquid contact is achieved through slugs. In the rotating disc reactor, gas/liquid contact is achieved through a thin film formed on the disc which rotates. In the micromesh reactor, a micromesh forms and stabilizes the gas/liquid interface. Acetone removal and asymmetric transfer hydrogenation studies were carried out under different conditions in these reactors and the performance of the different reactors were compared. The tubular reactor showed similar performance as the batch reactor. The rotating disc reactor enhanced acetone removal, thus improved the conversion and enantioselectivity. Acetone was removed most efficiently in the micromesh reactor. Therefore, the highest conversion and enantioslectivity were also obtained in this reactor. By simplified calculations, it was established that in order to increase the acetone removal efficiency, the ratio of gas to liquid flowrate and the gas-liquid interfacial area has to be increased. The scale out/up concept was also demonstrated with the micromesh reactor. Scale out/up was achieved by increasing the number of meshes in parallel and enlarging the single mesh reactor. The conversion and enantioselectivity was slightly lower in the scaled out version reactor which is probably due to the fact that the flow distribution inside the reactor was not uniform.