Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis

We review the current state of knowledge of phase separation and phase equilibria in porous materials. Our emphasis is on fundamental studies of simple fluids (composed of small, neutral molecules) and well-characterized materials. While theoretical and molecular simulation studies are stressed, we also survey experimental investigations that are fundamental in nature. Following a brief survey of the most useful theoretical and simulation methods, we describe the nature of gas‐liquid (capillary condensation), layering, liquid‐liquid and freezing/melting transitions. In each case studies for simple pore geometries, and also more complex ones where available, are discussed. While a reasonably good understanding is available for phase equilibria of pure adsorbates in simple pore geometries, there is a need to extend the models to more complex pore geometries that include effects of chemical and geometrical heterogeneity and connectivity. In addition, with the exception of liquid‐liquid equilibria, little work has been done so far on phase separation for mixtures in porous media.

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Phase separation in confined systems

This review provides an overview of different aspects of inclusion chemistry in ordered mesoporous host materials such as MCM-41 or MCM-48 (Mobil codes). A rich field of inclusion chemistry has been explored in this context, including sorption, ion exchange, imbibition followed by reduction, grafting of reactive metal alkoxides, halides etc., grafting of silane coupling agents (sometimes followed by subsequent reactions), grafting of reactive metal complexes, and polymerization in the channels. Finally, co-condensation of reactive species during the mesopore synthesis is a method to incorporate functionality into the walls of the channel system. Important applications of these modified and functionalized systems are heterogeneous catalysis and photocatalysis involving bulky grafted catalysts and/or the conversion of large substrates. Other potential applications include ion exchange and separations, removal of heavy metals, chromatography, stabilization of quantum wires, stabilization of dyes, and polymer...

Inclusion Chemistry in Periodic Mesoporous Hosts

The discovery of periodic mesoporous MCM-41 and related molecular sieves has attracted significant attention from a fundamental as well as applied perspective. They possess well-defined cylindrical/hexagonal mesopores with a simple geometry, tailored pore size, and reproducible surface properties. Hence, there is an ever-growing scientific interest in the challenges posed by their processing and characterization and by the refinement of various sorption models. Further, MCM-41-based materials are currently under intense investigation with respect to their utility as adsorbents, catalysts, supports, ion-exchangers, and molecular hosts. In this article, we provide a critical review of the developments in these areas with particular emphasis on adsorption characteristics, progress in controlling the pore sizes, and a comparison of pore size distributions using traditional and newer models. The model proposed by the authors for adsorption isotherms and criticalities in capillary condensation and hysteresis is found to explain unusual adsorption behavior in these materials while providing a convenient characterization tool.

Recent advances in processing and characterization of periodic mesoporous MCM-41 silicate molecular sieves

Capillary condensation of adsorbates in porous materials.

The effect of temperature on adsorption isotherms of Ar, N2, O2, C2H4, and CO2 on mesoporous MCM-41 molecular sieves with different pore sizes (pore radius rp = 1.2, 1.4, 1.8, 2.1 nm) has been measured. The occurrence of adsorption hysteresis upon capillary condensation of these molecules in mesopores depends on their capillary critical temperatures (Tcc), which are far below the corresponding bulk critical temperatures (Tc). In accord with theoretical predictions of confined geometry effects on the criticality of fluids in narrow pores, a plot of (Tc − Tcc)/Tc against d/rp for Ar, N2, O2, and C2H4 seems to form a single straight line passing through the origin, where d is the molecular diameter. The data for CO2 deviates from this relationship. These results are compared with critical point shifts of fluids in conventional mesoporous adsorbents with an interconnected network of pores available in the literature.

M. Uga

Papers

Capillary Critical Point of Argon, Nitrogen, Oxygen, Ethylene, and Carbon Dioxide in MCM-41