Diffusion in hierarchical silica monoliths: impact of pore size and probe molecule
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ORIGINAL
Diffusion in hierarchical silica monoliths: impact of pore size and probe molecule Alexey Zhokh 1
&
Peter Strizhak 1 & Katarzyna Maresz 2 & Agnieszka Ciemięga 2 & Julita Mrowiec-Białoń 2
Received: 31 December 2019 / Accepted: 22 July 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The silica monoliths were prepared by a combination of sol-gel synthesis, pore templating, and a process of phase separation. The as-prepared monoliths are characterized by meso- and macroporosity. The pore size significantly differs for each sample depending on the preparation conditions. The transport properties of the monoliths were investigated using benzene, isopropanol, hexane, and methane as probing molecules. The effective diffusion coefficients in the monoliths were estimated based on the second Fick’s law of diffusion. The obtained diffusivities are quite similar for different samples, demonstrating that the diffusion occurs mainly in the macropores of the monolith. The values of the diffusion coefficients were found to fall outside the Knudsen prediction. In addition, it was investigated that the benzene and hexane transport in the monolith sample with a bimodal mesopores distribution does not follow the Fick’s diffusion law. To describe the mass transfer of the benzene and hexane in this sample the time-fractional diffusion equation was utilized on a phenomenological basis. Several scenarios concerning the role of the relatively small mesopores in the hydrocarbons diffusion are discussed.
1 Introduction Silica monolithic materials with hierarchical pore size distribution are the objective of research in recent years due to their exceptional properties, i.e. high porosity, large specific surface area and extended continuous network of meso and macropores [1–5]. The monoliths were prepared with the use of the sol-gel process of silicon alkoxide precursor under acidic conditions and phase separation induced by a watersoluble polymer. The composition of the initial reaction mixture and process conditions have impact on the porous structure both in meso and macro scales. A final mesoporous structure of these materials also strongly depended on the post-gelation treatment in an aqueous ammonia solution due Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00231-020-02929-3) contains supplementary material, which is available to authorized users. * Alexey Zhokh [email protected] 1
L.V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, prospect Nauki 31, Kiev 03028, Ukraine
2
Institute of Chemical Engineering, Polish Academy of Sciences, Bałtycka 5, 44-100 Gliwice, Poland
to Ostwald ripening mechanism [6, 7]. The monoliths appeared to be good candidates for application in continuous flow processes e. g. for a chromatographic analysis [8–12], as supports of active species for catalytic and sorption processes and in the fabrication of microreactors [13–19]. In order to meet the expectations for these applicat
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