Structure and transport characterisation of hierarchical porous solids
Chemical and Thermal Process Engineering
Final Report Abstract
Hierarchically-structured nanoscale materials are currently in the focus of active research. Among them, micro-mesoporous porous solids show promising potentials in catalytic and separation applications, which often suffer from intrinsically slow transport rates of guest species in micropore networks. It is finding increasing evidence that the introduction of mesopore spaces into microporous solids yields notable improvements of the process efficiencies, presumably due the transport improvement. In this work we explore the structure-transport correlations in micro-mesoporous materials and unlock the physical mechanisms behind these observations. We identify two different scenarios of how the catalytic or separation processes might be affected depending on structural details of the pore space organization and on the transport properties of micro- and mesopores. We show that, in the first scenario, transport remains essentially diffusive. In this case, the commonly used framework of the diffusion equations remains applicable and the particle with internal complex porosities may be described using the effective diffusivity concept. This does not hold in the second scenario, in which it is not the transport enhancement, but rather an effective reduction of the microporous regions introduces the major impact. Based on these findings, we suggest a classification of hierarchical micro-mesoporous materials regarding their transport properties. We discuss further the relationships between two different experimental approaches for transport characterization, namely of microscopic diffusion measurements, such as using the pulsed field gradient technique of NMR, and of macroscopic uptake measurements. On considering the uptake measurements, most frequently used in the laboratory studies, we identify the conditions under which their results may be analyzed in the frames of the commonly used approaches to exclude their misinterpretations. For transport characterization of micro-mesoporous solids, the interconnectivity of the mesopore space turns out to be the key parameter. Regardless of its importance, nowadays there is still lack of robust experimental approaches for quantifying the interconnectivity in complex, hierarchical materials. We suggest that the routinely used gas sorption (or thermoporometry) methods may deliver this missing information. The phase transitions in complex mesopore spaces occur via an interplay of two transition modes, namely of phase nucleation and subsequent phase growth. The latter encodes intrinsically the interconnectivity of the mesopore space. Extracting this information from the sorption data was, however, precluded due to the luck of physical models accounting for both transition modes in geometrically disordered mesoporous solids. We consider and solve exactly the problem of gas-liquid (or solid-liquid) equilibria in a disordered linear chain of pores with different pore sizes with a fixed interconnectivity being equal to two. We show that the adsorption transition is dominated by nucleation and percolation (interconnectivity) effects are negligible. In contrast, the desorption transition is a percolation-controlled phenomenon. By having the exact solution for the adsorption and desorption transitions in the linear chain model, we introduce a well-defined reference, which, by comparing to the experimentally obtained transitions, yields the interconnectivity parameter of the mesopore space.
Publications
- Transport properties of hierarchical micro-mesoporous materials. Chemical Society Reviews 45, 3439-3467
Schneider, D., Mehlhorn, D., Zeigermann, P., Karger, J. & Valiullin, R.
(See online at https://doi.org/10.1039/c5cs00715a) - Ice Nucleation in Periodic Arrays of Spherical Nanocages. The Journal of Physical Chemistry C 121, 23788-23792
Mascotto, S., Janke, W. & Valiullin, R.
(See online at https://doi.org/10.1021/acs.jpcc.7b08490) - Phase transitions in disordered mesoporous solids. Scientific Reports 7, 7216
Schneider, D., Kondrashova, D. & Valiullin, R.
(See online at https://doi.org/10.1038/s41598-017-07406-2) - Water Transport in Periodic Mesoporous Organosilica Materials. The Journal of Physical Chemistry C 122, 12673-12680
Mietner, B. J., Fröba, M. & Valiullin, R.
(See online at https://doi.org/10.1021/acs.jpcc.8b00046) - Capillary Condensation and Evaporation in Irregular Channels: Sorption Isotherm for Serially Connected Pore Model. The Journal of Physical Chemistry C 123, 16239-16249
Schneider, D. & Valiullin, R.
(See online at https://doi.org/10.1021/acs.jpcc.9b03626) - Comparative Gas Sorption and Cryoporometry Study of Mesoporous Glass Structure: Application of the Serially Connected Pore Model. Frontiers in Chemistry 7
Enninful, H. R. N. B. et al.
(See online at https://doi.org/10.3389/fchem.2019.00230)
