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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 , Predicting local transport coefficients at solid-gas interfacesThe regular nanoporous structure make zeolite membranes attractive candidates for separating molecules on the basis of differences in transport rates (diffusion). Since improvements in synthesis have led to membranes as thin as several hundred nanometers by now, the slow transport in the boundary layer separating bulk gas and core of the nanoporous membrane is becoming increasingly important. Therefore, we investigate the predictability of the coefficient quantifying this local process – the surface permeability α – by means of a two-scale simulation approach. Methane tracer-release from the one-dimensional nanopores of an AFI-type zeolite is employed. Besides a pitfall in determining α on the basis of tracer exchange, we, importantly, present an accurate prediction of the surface permeability using readily available information from molecular simulations. Moreover, we show that the prediction is strongly influenced by the degree of detail with which the boundary region is modeled. It turns out that not accounting for the fact that molecules aiming to escape the host structure must indeed overcome two boundary regions yields too large a permeability by a factor of 1.7 to 3.3, depending on the temperature. Finally, our results have far-reaching implications for the design of future membrane applications.Publicationtype: Journal ArticleScopus© Citations 36 261 496 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Adsorption and diffusion in zeolites : the pitfall of isotypic crystal structures(Taylor & Francis, 2011-06-27); ; ; ; The influence of isotypic crystal structures on adsorption and diffusion of methane in all-silica LTA, SAS and ITE zeolites is studied. Results obtained with the experimental structures are compared with structure predictions and approximations that are commonly employed. The results indicate that diffusion coefficients are much more affected than Henry coefficients. In fact, orders of magnitude deviations in the diffusivity can be observed and a systematic parameter study finally gives rise to the correlation between structure sensitivity and diffusion-window size.Publicationtype: Journal ArticleScopus© Citations 13 330 470 - 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 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , 188 224 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Path sampling calculation of methane diffusivity in natural gas hydrates from a water-vacancy assisted mechanism(American Chemical Society (ACS), 2008-12-03); ; ; ; Increased interest in natural gas hydrate formation and decomposition, coupled with experimental difficulties in diffusion measurements, makes estimating transport properties in hydrates an important technological challenge. This research uses an equilibrium path sampling method for free energy calculations [R. Radhakrishnan and T. Schlick, J. Chem. Phys. 121 2436 (2004)] with reactive flux and kinetic Monte Carlo simulations to estimate the methane diffusivity within a structure I gas hydrate crystal. The calculations support a water-vacancy assisted diffusion mechanism where methane hops from an occupied “donor” cage to an adjacent “acceptor” cage. For pathways between cages that are separated by five-membered water rings, the free energy landscape has a high barrier with a shallow well at the top. For pathways between cages that are separated by six-membered water rings, the free energy calculations show a lower barrier with no stable intermediate. Reactive flux simulations confirm that many reactive trajectories become trapped in the shallow intermediate at the top of the barrier leading to a small transmission coefficient for these paths. Stable intermediate configurations are identified as doubly occupied off-pathway cages and methane occupying the position of a water vacancy. Rate constants are computed and used to simulate self-diffusion with a kinetic Monte Carlo algorithm. Self-diffusion rates were much slower than the Einstein estimate because of lattice connectivity and methane’s preference for large cages over small cages. Specifically, the fastest pathways for methane hopping are arranged in parallel (non-intersecting) channels, so methane must hop via a slow pathway to escape the channel. From a computational perspective, this paper demonstrates that equilibrium path sampling can compute free energies for a broader class of coordinates than umbrella sampling with molecular dynamics. From a technological perspective, this paper provides one estimate for an important transport property that has been difficult to measure. In a hydrate I crystal at 250 K with nearly all cages occupied by methane, we estimate D ≈ 7·10-15 X m2/s where X is the fraction of unoccupied cages.Publicationtype: Journal ArticleScopus© Citations 119 512 602 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , In-depth study of the influence of host-framework flexibility on the diffusion of small gas molecules in one-dimensional zeolitic pore systemsMolecular-dynamics simulations are performed to understand the role of host-framework flexibility on the diffusion of methane molecules in the one-dimensional pores of AFI, LTL, and MTW-type zeolites. In particular, the impact of the choice of the host model is studied. Dynamically corrected Transition State Theory is used to provide insights into the diffusion mechanism on a molecular level. Free-energy barriers and dynamical correction factors can change significantly by introducing lattice flexibility. In order to understand the phenomenon of free-energy barriers reduction, we investigate the motion of the window atoms. The influence that host-framework flexibility exerts on gas diffusion in zeolites is, generally, a complex function of material, host model, and loading such that transferability of conclusions from one zeolite to the other is not guaranteed.Publicationtype: Journal ArticleScopus© Citations 61 266 387