Sticking Probability on Zeolites

Diffusion in nanoporous materials: fundamental principles, insights and challenges

The increasing complexity of nanoporous catalysts and adsorbents presents a challenge to both the experimental measurement and theoretical modeling of transport behavior.

Diffusion Study by IR Micro-Imaging of Molecular Uptake and Release on Mesoporous Zeolites of Structure Type CHA and LTA

The presence of mesopores in the interior of microporous particles may significantly improve their transport properties. Complementing previous macroscopic transient sorption experiments and pulsed field gradient NMR self-diffusion studies with such materials, the present study is dedicated to an in-depth study of molecular uptake and release on the individual particles of mesoporous zeolitic specimens, notably with samples of the narrow-pore structure types, CHA and LTA. The investigations are focused on determining the time constants and functional dependences of uptake and release. They include a systematic variation of the architecture of the mesopores and of the guest molecules under study as well as a comparison of transient uptake with blocked and un-blocked mesopores. In addition to accelerating intracrystalline mass transfer, transport enhancement by mesopores is found to be, possibly, also caused by a reduction of transport resistances on the particle surfaces.

In situ study on molecular diffusion phenomena in nanoporous catalytic solids

As an omnipresent phenomenon in nature, diffusion is among the rate-determining processes in many technological processes. This is in particular true for catalytic conversion in nanoporous materials. We provide a critical review of the possibilities of exploring diffusion phenomena over microscopic dimensions in such media by direct experimental observation. By monitoring the probability distribution of molecular displacements as a function of time, the pulsed field gradient technique of NMR (PFG NMR) records the rate of molecular re-distribution. By varying the observation time, PFG NMR is thus able to trace even hierarchies of transport resistances as occurring, e.g., in catalyst particles in the form of binder-compacted assemblages of zeolite crystallites. Alternatively, and complementary to this information, interference microscopy (IFM) and IR microscopy (IRM) are able to follow the evolution of intracrystalline concentration profiles during uptake and release. This allows, in particular, an accurate quantification of the transport resistances on the surface of the individual crystallites and of the probability that reactant molecules from the gas phase, upon colliding with the external surface, are able to penetrate through such "surface barriers" into the crystal bulk phase. Being able to distinguish between different molecular species, IRM is able to record the evolution of intracrystalline concentration profiles even during multi-component adsorption and catalytic reactions (169 references).

Surface adsorption isotherms and surface excess densities of n-butane in silicalite-1

We present isotherms for the adsorption of n-butane on the external and internal surfaces of silicalite, in the temperature range of 320-400 K and at pressures up to 530 kPa. The isotherms were calculated using molecular dynamics simulations. A modified thermostatting algorithm, Soft-HEX, was developed for this purpose. The results for the external as well as the internal surface were fitted to Langmuir models, and equilibrium constants are reported. Two surface structures were considered: a flat surface having only one adsorption site and a surface with a zigzag texture, characterized by two adsorption sites. The surface excess concentrations were computed. Negative surface excesses were found in some cases, indicating that the surface can serve as a barrier to transport. We show that Henry's law can be used to calculate adsorbate concentration-gas pressure relationships, and that the surface can be regarded as a thermodynamic system. These findings are important for studies of the surfaces as barriers to transport.

Exchange dynamics at the interface of nanoporous materials with their surroundings

The evolution of transient concentration profiles in nanoporous materials is shown to provide direct information about the rate of molecular exchange at the interface of these materials with the surrounding atmosphere. This includes the quantitation of a surface permeability and, related with each other, of the sticking factor, i.e., of the probability that a molecule colliding with the external surface from the outside atmosphere, will in fact enter the genuine pore system of the material under study. Owing to the recent introduction of interference microscopy to this type of systems, the relevant experimental evidence has become directly accessible and is applied to two model systems which are found to differ notably in their interface dynamics.