Stofftransport in mikro- und mesoporösen Materialien

Kärger, Jörg; Freude, D.

doi:10.1002/1522-2640(200112)73:12<1517::aid-cite1517>3.0.co;2-0

Cited by 3 publications

(2 citation statements)

References 6 publications

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“…In addition to accessibility, the speed at which reactants are allowed to reach active centers in the mesoporous material is also an important parameter affecting its performance. This is especially critical when reactions are strongly diffusion-controlled, for example, solid−liquid extraction 13 or electrochemical sensing, 6g,17d,, for which the effective rate is expected to be higher in organized porous structures than in nonfractal particles . Although reaction speeds were scarcely considered in the literature, 13c,d,o,r,20b detailed studies of diffusion-controlled reactions in mesoporous organosilicas are not widespread. 13g,p This was essentially applied to the accumulation of Hg II species by self-assembled mercaptan on mesoporous silica 13g and in thiol-functionalized mesoporous silica microspheres prepared according to the co-condensation route 13p.…”

Section: Introductionmentioning

confidence: 99%

Rate of Access to the Binding Sites in Organically Modified Silicates. 2. Ordered Mesoporous Silicas Grafted with Amine or Thiol Groups

2003

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Five different ordered mesoporous silica samples displaying various pore sizes and structures (two small-pore MCM-41, two large-pore MCM-41, and one small-pore MCM-48) and one amorphous silica gel have been grafted with either aminopropyl or mercaptopropyl groups. The resulting aminopropyl-grafted silicas (APS) and mercaptopropyl-grafted silicas (MPS) have been studied in solution via protonation of APS and metal ion binding on both APS and MPS. Special attention was given to characterize the accessibility to the binding sites and to the speed at which the reactants are reaching these reactive centers inside the mesoporous materials. Results have been obtained from batch experiments, by monitoring the reactant depletion in suspensions containing APS or MPS particles, and discussed with respect to the structure and porosity of the organic−inorganic hybrids. As a general trend, both accessibility and rate of access to the reactive sites were higher with ordered mesoporous solids than with amorphous materials of comparable porosity (average pore size ∼60−70 Å). The ordered mesoporous structures of smaller pore size (∼35 Å) gave rise to the same performance as that of large-pore amorphous silica, only if pore blocking can be avoided during the grafting process; if not, the advantage of uniform pore structure over the corresponding amorphous material did not exist anymore: a pore volume of at least 0.5 cm3 g-1 remaining upon grafting was necessary to keep this advantage. Increasing the amount of grafted moieties in the mesopores also led to restricted mass-transfer rates because of increasing steric hindrance. Moreover, protonation of mesoporous APS displaying uniform-sized channels was found to be dramatically slow at protonation levels higher than 50%, leading even to less-than-complete occupancy levels after 24 h of equilibration, most probably because of strong repulsive electrostatic interactions in such a confined medium. Applying a simplified diffusion model, the access rates of CuII in APS and HgII in MPS materials were quantified via calculation of apparent diffusion coefficients, D app, by appropriate fitting of kinetic curves. D app values were found to decrease slowly upon gradual completion of reaction because progressively less space was available for the ingress of reactants upon filling the mesostructures. This work would help at selecting the most appropriate conditions for target applications of grafted mesoporous solids in terms of capacity, accessibility, and especially, access rates to the active sites.

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Section: Introductionmentioning

confidence: 99%

Rate of Access to the Binding Sites in Organically Modified Silicates. 2. Ordered Mesoporous Silicas Grafted with Amine or Thiol Groups

2003

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“…Before comparing the results of different techniques, one has to be sure that the attained diffusivities are indeed comparable with each other. Table provides an overview of the different techniques developed for the measurement of intracrystalline zeolitic diffusion and their classification . The discrimination between macro-, meso-, and microscopic techniques concerns the relation between the mean diffusion path and the crystal size.…”

Section: Modes Of Measurementmentioning

confidence: 99%

The Random Walk of Understanding Diffusion

Kärger

2002

Ind. Eng. Chem. Res.

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In situ study on molecular diffusion phenomena in nanoporous catalytic solids

2010

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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).

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Stofftransport in mikro- und mesoporösen Materialien

Cited by 3 publications

References 6 publications

Rate of Access to the Binding Sites in Organically Modified Silicates. 2. Ordered Mesoporous Silicas Grafted with Amine or Thiol Groups

Rate of Access to the Binding Sites in Organically Modified Silicates. 2. Ordered Mesoporous Silicas Grafted with Amine or Thiol Groups

The Random Walk of Understanding Diffusion

In situ study on molecular diffusion phenomena in nanoporous catalytic solids

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