Diffusion in Porous Media

Kärger, Jörg; Valiullin, Rustem

doi:10.1002/9780470034590.emrstm0121.pub2

Cited by 23 publications

(17 citation statements)

References 117 publications

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“…The tortuosity is commonly used to describe the diffusion behavior of a liquid confined within the pores. 9,14,15,27,28 Figure 3a shows diffusion NMR data for lauric acid in bulk methanol solution and within two distinct silicas, SBA-15 and KIT-6 with mean mesopore diameters of 4.9 and 5.2 nm, respectively. Figure 3b is a combined DOSY spectrum of lauric acid in methanol, for bulk methanolic solution, in SBA-15, and in KIT-6.…”

Section: Resultsmentioning

confidence: 99%

Diffusion NMR Characterization of Catalytic Silica Supports: A Tortuous Path

Rottreau¹,

Parlett²,

Lee

et al. 2017

J. Phys. Chem. C

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Mesoporous silicas have found widespread application within the field of heterogeneous catalysis. Acid functionalization of such materials, through one-pot or postsynthetic grafting of sulfonic acid groups, imparts activity for fatty acid esterification, with the studious choice of pore geometry facilitating significant rate enhancements. Diffusion NMR has been utilized for the first time to characterize the structure of mesoporous silicas through the transport behavior of systematically related carboxylic acids confined within their mesopore networks. A reduced diffusion coefficient is obtained for species constrained within the 3-dimensional interconnected pores of KIT-6 relative to the 2-dimensional noninterconnected pore channels of SBA-15. The effective tortuosity of both porous silicas increases with the acid chain length, with the diffusion behavior of long-chain acids dominated by the alkyl chain and silica architecture. Carboxylic acid diffusion within these two pore networks is unlikely to be rate-limiting in catalytic esterification over sulfonic acid silica analogues. Physicochemical insights from diffusion NMR will aid the future design of optimal silica architectures for catalytic applications.

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Section: Resultsmentioning

confidence: 99%

Diffusion NMR Characterization of Catalytic Silica Supports: A Tortuous Path

Rottreau¹,

Parlett²,

Lee

et al. 2017

J. Phys. Chem. C

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“…The above equation is only valid for long diffusion times, when the molecules diffuse over distances much longer than the correlation length of the porous medium. Otherwise, a dependence of the effective diffusion coefficient on diffusion time exists …”

Section: Methodsmentioning

confidence: 99%

“…One technique is NMR relaxometry, which is generally used to probe rotational dynamics but, under certain conditions, can be also applied for translational motion . The other technique is NMR diffusometry, which allows measuring of the self‐diffusion coefficients and thus gives information on the restrictions introduced by the porous structure on the translational displacement. In the present work, the two NMR techniques will be used to probe the influence introduced on the molecule dynamics by the interaction with the surface of a mesoporous carbon xerogel.…”

Section: Introductionmentioning

confidence: 99%

Surface influence on the rotational and translational dynamics of molecules confined inside a mesoporous carbon xerogel

Cadar

Ardelean

2019

Magnetic Reson in Chemistry

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Low‐field nuclear magnetic resonance techniques are employed to extract information about the effects introduced by the interaction with the surface on the rotational and translational dynamics of molecules confined inside a mesoporous carbon xerogel. The molecules under study were water, cyclohexane, and hexane. They were chosen due to their different interaction strength with the carbonaceous matrix. Frequency dependent longitudinal relaxation measurements, using the fast field cycling technique, allowed extraction of the fractal dimension of the carbon xerogel surface. It was observed that the measured value is influenced by the molecule affinity to the surface. Diffusion measurements, using the pulse field gradient technique, have revealed that the stronger interaction with the surface of cyclohexane and hexane molecules leads to an increased diffusive tortuosity, as compared with water.

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“…Thus, PFG NMR has been successfully applied to characterize the diffusion properties of pure substances like alkanes, alcohols or water. To date there are quite a few fundamental studies on the transport properties in zeolithes and mesoporous silica as hosts using PFG NMR, and the interested readers are referred to one of the recent, excellent review articles by Kärger et al [9][10][11] The investigation of compound mixtures or solutions is much more demanding, because now the NMR activity of the surrounding matrix becomes a disadvantage. 12 Magic angle spinning (MAS) PFG NMR is needed for observing sufficiently resolved spectra for different components, and so far only few molecules and their diffusion in solvents could be studied in this manner, [13][14][15] in particular dissolved species inside a solid, porous host.…”

Section: Introductionmentioning

confidence: 99%

Multiple scale investigation of molecular diffusion inside functionalized porous hosts using a combination of magnetic resonance methods

Wessig

Spitzbarth

Drescher

et al. 2015

Phys. Chem. Chem. Phys.

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Mass transport of molecular compounds through porous solids is a decisive step in numerous, important applications like chromatography or heterogeneous catalysis. It is a multi-scale, hierarchical phenomenon: macrodiffusion (>μm) is influenced, in addition to parameters like grain boundaries and particle packing, by meso-scale (>10 nm, <μm) factors like particle size and the connectivity of pores. More importantly, meso-scale diffusion and macro-scale diffusion are first and foremost determined directly by processes on the molecular scale (<10 nm), which depend on numerous factors like pore-size, interactions of the host with the solid surfaces and with the solvent. Due to the high complexity of the latter and the fact that current analytical techniques enable only limited insights into solvent-filled pores with sufficient spatial and temporal resolution, the knowledge about the molecular origins of diffusive processes in porous materials is still restricted. The main focus of the current paper is on the development of continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy into a tool shedding some new light on molecular diffusion inside mesoporous silica materials differing systematically in pore size and surface functionalities. The advantages of CW-EPR are that its spatial resolution fits ideally to the size of mesopores (2-10 nm), it is fast enough for spotting molecular processes, and any conventional solvent and the porous matrix are EPR silent. Diffusion coefficients have been calculated considering spin exchange occurring from the diffusive collision of radicals, and are compared to complementary analytical techniques like MAS PFG NMR (sensitive for meso-scale) and EPR-imaging (sensitive to macroscale diffusion). Our results show that the choice of surface bound functional groups influences diffusion much stronger than pore-size. There are indications that this is not only due to different guest-surface interactions but also due to an altered mobility within the solvent under confinement.

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Diffusion in Porous Media

Cited by 23 publications

References 117 publications

Diffusion NMR Characterization of Catalytic Silica Supports: A Tortuous Path

Diffusion NMR Characterization of Catalytic Silica Supports: A Tortuous Path

Surface influence on the rotational and translational dynamics of molecules confined inside a mesoporous carbon xerogel

Multiple scale investigation of molecular diffusion inside functionalized porous hosts using a combination of magnetic resonance methods

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