Porous materials are used in a variety of industrial processes, from the heterogeneous separation of chemical products to waste-water remediation, and catalysis. Often macro-scale mass transfer events are characterised by the microscale diffusion phenomena both in and out of the pores of a material. In this work, the Zero-Length Column (ZLC) method is used to determine liquid phase diffusion constants, mass transfer coefficients, and adsorption equilibrium isotherms.
Limitations for applying the ZLC method to the liquid phase phenomena on a HPLC instrument were reviewed, with a particular focus on materials/adsorbents within which the adsorbate produces large interactions, with both macropore and micropore diffusion constants calculated at 2.7 x 10-7 and 5.8 x 10-8 cm2 s-1 respectively. Where appropriate, experimental procedures were proposed to decouple the surface and kinetic diffusion.
The current liquid phase ZLC model has been adapted for cylindrical pellets, such as extrudates, which are commonly used in industrial applications such as isomer separations. This novel model incorporates a complete consideration of the fluid phase hold-up effects, which is necessary for liquid phase measurements. Effective diffusion constants calculated at ~1 x 10-6 cm2 s-1 compared acceptably to those molecular pore diffusion constants obtained from Pulsed-Field Gradient (PFG) NMR ~5 x 10-6 cm2 s-1.
The biomass conversion of levulinic acid to γ-valerolactone by transfer hydrogenation was used as a case study for the reaction and ZLC characterisation. The post-synthetic modification of MIL-100 with decyl-phosphonic acid increased the material stability by 60% and increased the LA conversion by 20%, owing to an increase in MOF hydrophobicity. Treatment with octadecyl-phosphonic acid caused significant pore blockages, reflected by the ZLC macropore diffusion at 1.4 x 10 -7 cm2 s-1, resulting in a decrease in conversion by approximately 20%.