The ﬁeld of porous materials is currently at an exciting stage in its technological evolution. The research on ordered −including zeolites, zeotypes, metal-organic frameworks and mesoporous silica- and dis-ordered −including ceramics, sintered metals and foamed polymers-porous solids  is among the most creative, fascinating and attractive ﬁelds of materials science. The supercritical ﬂuid technology addressed the processing of porous matter for many differed types of materials. The basis of the developments of supercritical carbon dioxide (scCO2) methodologies in porous materials is two-fold: ﬁrst, the solubility of scCO2 in polymers, with a pressure-dependent behaviour, is substantial in comparison with conventional solvents; and second, the adsorptive behaviour of scCO2 in inorganic porous systems is insigniﬁcant when compared to liquid ﬂuids, which allows the one-step design of surface grafting and impregnation processes.
scCO2 technology applied to nanopores takes proﬁt of the compressed CO2 gas-like viscosity, high diﬀusivity and null surface tension, so capillary stresses are suppressed, converting this ﬂuid in a non-damaging solvent for those structures, facilitating their synthesis and modiﬁcation. Most importantly, pore collapse can be avoided because the expansion of scCO2 directly as a gas does not give rise to a liquid-vapour interface.
When the process is carried out from a liquid solution, the possibility of competition between solvent and solute molecules for the substrate adsorption sites often leads to the incorporation of both components into the internal surface of the porous system. Competition between the solvent and the solute for the substrate adsorption sites is reduced in scCO2 with respect to liquid solvents, since supercritical ﬂuids are essentially not absorbed.
This review, explores particular cases of the use of scCO2 on polymer foaming, aerogel preparation, porous concrete densification, supercritical impregnation of zeolites and modification of mesoporous ordered silica or MOFs preparation.
Ana M. López-Periago, Concepción Domingo
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
Features of supercritical CO2 in the delicate world of the nanopores
The Journal of Supercritical Fluids, 134, 204-213 (2018)
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Classification of described porous materials along the text in regard of pore long-range order, interaction with scCO2, and tested applications.