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Item type:Publication, Access status: with files , Transport in nanoporous materials including MOFs : the applicability of Fick’s laws(Wiley-VCH, 2015); ; ; ; Diffusion in nanoporous host-guest systems is often considered to be too complicated to comply with such “simple” relations as Fick’s first and second laws of diffusion. However, we show here that the microscopic techniques of diffusion measurement, notably the pulsed field gradient (PFG) technique of NMR and micro-imaging by interference microscopy (IFM) and IR microscopy (IRM), provide direct experimental evidence of the applicability of Fick’s laws to such systems. This remains true in many situations, even when the detailed mechanism is complex. The limitations of the diffusion model are also discussed with reference to the extensive literature of this subject.Publicationtype: Journal ArticleScopus© Citations 84 450 1449 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Transport at gas–zeolite interfaces probed by molecular simulations(2013-08); In this work, the so-called surface-barriers phenomenon is investigated for hydrocarbons entering siliceous zeolites. Using molecular simulation methods (transient and equilibrium molecular dynamics, Monte Carlo, reactive flux simulations) and comparison to experimental data from literature, conditions (systemic, thermodynamic, and chemical) were identified under which such barriers have significant impact on diffusion measurements.Publicationtype: Doctoral Thesis337 976 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Transport into nanosheets : diffusion equations put to testUltrathin porous materials, such as zeolite nanosheets, are prominent candidates for performing catalysis, drug supply, and separation processes in a highly efficient manner due to exceptionally short transport paths. Predictive design of such processes requires the application of diffusion equations that were derived for macroscopic, homogeneous surroundings to nanoscale, nano-structured host systems. Therefore, we tested different analytical solutions of Fick’s diffusion equations for their applicability to methane transport into two different zeolite nanosheets (AFI, LTA) under instationary conditions. Transient molecular dynamics simulations provided hereby concentration profiles and uptake curves to which the different solutions were fitted. Two central conclusions were deduced by comparing the fitted transport coefficients. First, the transport can be described correctly only if concentration profiles are used and the transport through the solid-gas interface is explicitly accounted for by the surface permeability. Second and most importantly, we have unraveled a size limitation to applying the diffusion equations to nanoscale objects. This is because transport-diffusion coefficients, DT, and surface permeabilities, α, of methane in AFI become dependent on nanosheet thickness. Deviations can amount to factors of 2.9 and 1.4 for DT and α, respectively, when, in the worst case, results from the thinnest AFI nanosheet are compared with data from the thickest sheet. We present a molecular explanation of the size limitation that is based on memory effects of entering molecules and therefore only observable for smooth pores such as AFI and carbon nanotubes. Hence, our work provides important tools to accurately predict and intuitively understand transport of guest molecules into porous host structures, a fact that will become the more valuable the more tiny nanotechnological objects get.Publicationtype: Journal ArticleScopus© Citations 40 288 784 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Surface barriers of hydrocarbon transport triggered by ideal zeolite structuresShedding light on the nature of surface barriers of nanoporous materials, molecular simulations (Monte Carlo, Reactive Flux) have been employed to investigate the tracer-exchange characteristics of hydrocarbons in defect-free single-crystal zeolite membranes. The concept of a critical membrane thickness as quantitative measure of surface barriers is shown to be appropriate and advantageous. Nanopore smoothness, framework density, and thermodynamic state of the fluid phase have been identified as the most important influencing variables of surface barriers. Despite the ideal character of the adsorbent, our simulation results clearly support current experimental findings on MOF Zn(tbip) where a larger number of crystal defects caused exceptionally strong surface barriers. Most significantly, our study predicts that the ideal crystal structure without any such defects will already be a critical aspect of experimental analysis and process design in many cases of the upcoming class of extremely thin and highly oriented nanoporous membranes.Publicationtype: Journal ArticleScopus© Citations 42 263 819 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the effects of the external surface on the equilibrium transport in zeolite crystalsWith the aid of molecular simulation techniques (molecular dynamics [MD], Grand-Canonical Monte Carlo [GCMC], and reactive flux correlation function [RFCF]), the influence of the external surface on the equilibrium permeation of methane and ethane into and out of an AFI-type zeolite crystal has been studied. In particular, “extended dynamically corrected transition state theory”, that has been proven to describe the transport of tracers in periodic crystals correctly, has been applied to surface problems. The results suggest that the molecules follow paths that are close to the pore wall in the interior, and also at the crystal surface. Moreover, the recrossing rate at the surface turns out to be non-negligible, yet, in contrast to the intracrystalline recrossing rate, remains almost constant over loading which gives indication to diffusive barrier crossing at the crystal surface. As a consequence of very different adsorption and desorption barriers, the corresponding permeabilities are shown to be not equal for one and the same condition (T , p). The critical crystal length, beyond which surface effects can be certainly neglected, is computed on basis of flux densities. Entrance/exit effects, in the present cases, are practically important solely for ethane at low pressures. The influence of the type of external surface on the surface flux is, hereby, rather small, because the transport at the surface is controlled by the slow supply from the gas phase. This has been evidenced by a simplified thermodynamic model that has been derived within this work and which is based on rapidly assessable simulation data. Finally, we propose a procedure for estimating the importance of different factors that have an impact on surface effects.Publicationtype: Journal ArticleScopus© Citations 41 288 551