Crystal diversity of nanoporous materials -a fundamental parameter of mass transfer

Nanoporous materials, like zeolites and metal-organic frameworks, find widespread applications in chemical processes involving catalytic reactions and molecular separations. The efficient design of these processes crucially depends on the detailed understanding of the mechanisms of molecular exchange between the pores of the materials and their surroundings. For instance, mass transfer resistances are found at the surface of the materials and due to the intracrystalline porenetwork.
The governing resistances are typically determined from the analysis of so-called uptake or release curves, expressing the amount adsorbed in the pores of the materials in function of time.
These uptake curves are conventionally measured with batches of porous crystals, assuming that all crystals are identical. However, in this presentation, we will show that this assumption may lead to inaccurate conclusions on the material properties. The crystal diversity, which is omitted in conventional analysis, may cause the uptake to mimic mass transfer mechanisms and to
obscure the real governing uptake mechanism[1]. This will be demonstrated by presenting several experimental and theoretical cases. Experimental results obtained from single-crystal and multiple-crystals uptake measurements[1], carried out with the micro-imaging techniques[2], will be shown. In addition, during this presentation the results from an extensive characterization of the material, namely the zeolite SAPO-34, including scanning-electron-microscopy (SEM), energy-dispersive X-rays (EDX) and X-rays photo-electron spectroscopy (XPS), as well as atomic force microscopy (AFM), will be shown, correlating the material properties with the mass transfer mechanisms.