Characterization of microporous materials

The determination of an accurate pore size distribution of activated carbons is still a complex issue and several methods and adsorbates are currently used to this purpose. In this work, different methods have been applied to characterize the microporosity of activated carbons with different bum-off degrees. A deep analysis of the N2 and CO2 adsorption data of the samples has been done. The results of the different methods applied to the gas adsorption isotherms have been compared and their predictions discussed, in order to throw some light on the characterization of microporous materials. 

Progress in zeolite science and technology has gained new momentxim in recent years. Major advances have been achieved in the discovery of new molecular sieves, expanding the portfolio of known microporous crystals from zeolites and pure silica structures to new compositions. The characterization of microporous materials has greatly expanded, particularly by the application of solid-state NMR spectroscopy and by improved techniques in electron microscopy and crystallography. The large family of recently discovered new crystal structures and composi-... 

The convenient NMR observables depend on the characteristics of the system studied, but generally the protons of the liquid are readily detected. The apparent NMR linewidths are often determined by the magnetic susceptibility inhomogeneities in the sample and do not directly reflect the dynamics of the liquid. This report will focus largely on theoretical approaches to understand the spin-lattice relaxation rate constants for both classes of microporous materials. The magnetic held dependence of the spin-lattice relaxation rate constant generally provides a useful dynamical characterization of the liquid and often a structural characterization of the confining media. 

Ravikovitch PI, Vishnyakov A, Russo R, and Neimark AV. Unified approach to pore size characterization of microporous carbonaceous materials from N2, Ar, and C02 adsorption isotherms. Langmuir, 2000 16(5) 2311-2320. 

The majority of adsorbents applied in industry has porous sizes in the nanometer region. In this pore-size territory, adsorption is an important method for the characterization of porous materials. To be precise, gas adsorption provides information concerning the microporous volume, the mes-opore area, the volume and size of the pores, and the energetics of adsorption. Also, gas adsorption is an important unitary operation for the industrial and sustainable energy and pollution abatement applications of nanoporous materials. 

The characterization of porous materials exhibiting a composite pore structure encompassing micro-meso-and perhaps macro-pore sizes, is of particular significance for the development of separation and reaction processes. Among the characterization methods for materials exhibiting ultramicropore structures, DpDubinin-Radushkevich (DR)[2], Dubinin-Astakov (DA) [3], Dubinin-Stoeckli (DS) [4], as well as the Horvath-Kawazoe (HK), [5] methods are routinely used for the evaluation of the micropore capacity and the pore size distriburion (PSD). 

Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. 

However, Dubinin and co-workers do not accept the concept of monolayer formation in micropores and propose determining the microporous volume, Fq, on the basis of the thermodynamic theory of Polanyi adsorption. However, one can observe that the monolayer volume, Vm, when expressed in liquid nitrogen volume per unit mass, is very close to the Dubinin volume, Vo. The proportionality of the BET monolayer volume, Vm, and the so-called micropore volume, Va, (Vo 11 Vm) has been observed for many materials, as shown in different studies [2, 3]. This means that both variables are correlated, so determining one is equivalent to the determining the other. The discussion on the physicochemical meaning of these parameters may be interesting from a theoretical point of view but as far as practical characterization of porous materials is concerned, both methods can often be considered as equivalent. 

Transparency of gel is usually related to linear chain of polysilicates, precursors of microporous materials after calcination. This is in agreement with the below described characterization of SA. However, also the synthesis of ERS-8, performed in a basic medium, proceeds through the formation of a perfectly transparent gel. As SA, ERS-8 is microporous. 

Since the incorporation of transition metals into the frameworks of zeolites or micro-porous ahiminophosphates to form heteroatom-containing molecular sieves with important application values, the synthesis, structure, and characterization of microporous transition metal phosphates have been extensively studied in the last decade. In particular, because transition metal cations possess redox and coordination features, they are a kind of catalytic material with useful applications, and promise potential... 

L.C. de Menorval, A. Julbe, H. Jobic, J.A. Dalmon and C. Guizard, Potentialities of an innovative technique like Xe NMR and of SAXS for the characterization of micropor-ous sol-gel derived Si02. MRS Symp. Proc. Ser., Vol. 431 (Microporous and Macroporous Materials, 1996. In press. 

The atomic force microscopy technique is now widely used for the study of membrane surfaces. It has become an important tool of imaging the surface of materials to atomic-level resolution, and this technique does not need any special sample preparation, which is essential for SEM and TEM. AEM can show three-dimensional images of the surfaces. Paredes et al. has written an excellent review on the application of AEM for the characterization of microporous and mesoporous materials [16]. 

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