(704c) Zero Length Column (ZLC) for the Assessment of Reactant Counter-Diffusion Constants in Liquid Phase Catalytic Systems

The Zero length column (ZLC) technique is a method widely used to study gas phase diffusion constants and has many advantages over other techniques, including operating under industrial conditions and using a small sample mass. In liquid phase, pore diffusion is much more important due to the greater density of the medium, diffusion times are longer and more prone to error caused by system dead volume. In this study ZLC was applied, using a conventional HPLC system with reduced dead volume, to calculate counter-diffusion constants in porous materials and PdEnCat30 catalytic pellets. The micro and marco-porous diffusivities were calculated at the expected reaction conditions, 80 ^oC, between 0.3 - 1 mL min^-1 . Data was collected for each Heck reaction species and compared in order to determine any rate limitation. Comparisons between powder and monolithic forms of ZIF-8 yielded comparable and repeatable Iodobenzene-DMF counter-diffusion constants at ranges of 4.83 - 4.86 x 10^-10 cm²s^-1 and 5.8 - 6.3 x 10^-10 cm²s^-1 respectively. In addition, the differences between Pd(II) and Pd (0) EnCat were examined in order to determine the effects of catalytic deactivation on counter-diffusion constants. Finally, pulse field gradient NMR was utilised to validate the method and the use of the HPLC system to make ZLC measurements. This study presents novel findings on diffusivity data for liquid phase catalytic systems, much needed data in the field of reaction engineering.