Diffusion in Porous Materials: Insights, Surprises and Challenges

After, with the first measurement of guest diffusion in nanoporous materials over microscopic dimensions, pulsed field gradient NMR gave rise to a paradigm shift in our understanding of zeolitic diffusion, with the potentials of microimaging to observe the evolution of molecular ensembles, finally also microscopic diffusion measurement under non-equilibrium conditions became possible1. Examples of the novel insight thus accessible include

  1. the measurement of transport resistances of the external surface of the adsorbent particles (“surface barriers”) as a crucial, but so far inaccessible parameter deciding about the technological feasibility of these materials where, depending on the host-guest system under study, the surface barriers are found to be caused by essentially impermeable layers with dispersed “holes” or by quasi-continuous layers of dramatically reduced permeability2,
  2. the potentials for immediately deciding about the applicability of Fick’s diffusion laws for describing mass transfer in a given nanoporous host-guest system 3,
  3. simultaneous recording of adsorption and conversion within catalytically active nanoporous crystals/particles, with direct (“one-shot”!) measurement of effectiveness factors as the key number describing the efficiency of the industrial application of such materials4,
  4. measurement of two-component adsorption and diffusion, with the option of the direct recording of “uphill” fluxes and “overshooting”, i.e. of profiles with concentrations temporarily exceeding the equilibrium ones5, and
  5. recording of pore filling and emptying upon gas pressure variation as a function of space and time as a novel route towards exploring the fundamentals of sorption hysteresis 6.
  1. J. Kärger, T. Binder, C. Chmelik, F. Hibbe, H. Krautscheid, R. Krishna, and J. Weitkamp, Nat. Mater. 13 (2014) 333–343.
  2. J.C.S. Remi, A. Lauerer, C. Chmelik, I. Vandendael, H. Terryn, G.V. Baron, Denayer, Joeri F. M., and J. Kärger, Nat Mater 15 (2015) 401–406.
  3. T. Titze, A. Lauerer, L. Heinke, C. Chmelik, N.E.R. Zimmermann, F.J. Keil, D.M. Ruthven, and J. Kärger, Angew. Chem. Int. Ed. 54 (2015) 14580–14583.
  4. T. Titze, C. Chmelik, J. Kullmann, L. Prager, E. Miersemann, R. Gläser, D. Enke, J. Weitkamp, and J. Kärger, Angew. Chem. Int. Ed. 54 (2015) 5060–5064.
  5. A. Lauerer, T. Binder, C. Chmelik, E. Miersemann, J. Haase, D.M. Ruthven, and J. Kärger, Nat. Comms. 6 (2015) 7697.
  6. A. Lauerer, P. Zeigermann, J. Lenzner, C. Chmelik, M. Thommes, R. Valiullin, and J. Kärger, Microporous and Mesoporous Materials 214 (2015) 143–148.