Porosity immersion calorimetry

Since the porosity of carbons is responsible for their adsorption properties, the analysis of the different types of pores (size and shape), as well as the PSD, is very important to foresee the behavior of these porous solids in final applications. We can state that the complete characterization of the porous carbons is complex and needs a combination of techniques, due to the heterogeneity in the chemistry and structure of these materials. There exist several techniques for the analysis of the porous texture, from which we can underline the physical adsorption of gases, mercury porosimetry, small angle scattering (SAS) (either neutrons—SANS or x-rays—SAXS), transmission and scanning electron microscopy (TEM and SEM), scanning tunnel microscopy, immersion calorimetry, etc. 

A considerable number of different techniques has been employed in the past to characterize the porosity and surface chemistry of porous carbon materials. These include gas adsorption (mostly N2 and CO2) [9-14], immersion calorimetry [9], small-angle X-ray [11,15] and neutron [14] scattering, inverse gas chromatography [12,13], differential thermal analysis [12], Fourier transform infrared [12], Raman [16] and X-ray photoelectron [17] spectroscopies and electron spin resonance [16]. It is worth mentioning that the information about the porous structure of the material provided by this array of techniques is only indirect... 

Then, it can be concluded from these results that the smaller the surface area or micropore volume accessible to benzene, the higher ability of the CMS to discriminate between CO2 and CH4. On the other hand, the N2/O2 kinetic ratio also decreases (3.6 for CS-8), as can be expected from the widening of the porosity as determined by immersion calorimetry. 

Comparison of the porosity evaluation results based on immersion calorimetry and gravimetric sorption measurements, for activated chars from a high volatile bituminous coal... 

In the following paragraphs, we discuss the use of immersion calorimetry for the assessment of the surface chemistry, wettability, surface area and porosity of carbons. 

When the adsorption of aromatic weak electrolytes is governed by nonelec-trostatic interactions, such as tt-tt dispersion or hydrophobic interactions, the area of the adsorbent occupied by the adsorbate depends on the porosity of the former and the molecular size of the latter. Thus, adsorption from diluted aqueous solution and immersion calorimetry measurements [39] showed that phenol and m-chlorophenol are adsorbed as monolayers by both porous and nonporous carbons with basic surface properties, provided that the adsorptive is undissociated at the solution pH. This did not apply where molecular sieve effects reduced the accessibility of the micropore system. 

In pores of size comparable to that of the nitrogen molecule, the low kinetic energy of the gas (77 K) may limit their accessibility. In such cases, adsorption of CO2 at 273 K can be used to reveal supermicroporosity. Narrow porosity also can be detected by using probe molecules smaller than nitrogen (often combined with immersion calorimetry) or, for example, by small angle x-ray scattering (SAXS), while several methods, very often mercury porosimetry can be chosen for materials with wider porosity (Fig. 6). 

Porosity, with the dimensions of nanometers or less, cannot be precisely imaged even in the most recent of transmission and scanning electron microscopes and recourse has to be made to the powerful experimental techniques of physical adsorption of gases, of immersion calorimetry and of small-angle scattering of X-rays (SAXS) and neutrons (SANS) to characterize porosity. Microporosity has the dimensions of molecules and such molecules, as adsorbates, become the experimental probes providing significant information about the adsorption site. Hence, the phenomena of porosity and adsorption are inseparable. 

The specific questions now required to be answered concern the dimensions of the effective porosity and how to measure them. The method of analysis of pore filling by nitrogen at 77 K, at ptp values >0.6 cannot be used, obviously. Centeno etal. (2003) indicate that immersion calorimetry provides the means to measure micropore size distributions within an activated carbon (Section 4.7). They use a rearrangement of Equation (4.3) to calculate the micropore volume (Wo(Lc)) filled by a liquid of critical molecular dimensions, L, using ... 

The use of adsorption methods to characterize porosity in carbons, and the (then considered) undisputable position of the N2 BET isotherm appeared to be permanent. However, the advent of immersion calorimetry and its application to carbon chemistry, together with the availability of commercial calorimeters, presented a significant challenge to the supremacy of the N2 (77 K) adsorption isotherm. The School of Adsorption, University of Alicante, made use of immersion calorimetry and reviewed their work over several years (Rodrfguez-Reinoso et al., 1997 Silvestre-Albero et al., 2001). Immersion calorimetry, as a method of characterization, is discussed at length in Section 4.7. 

Porosity and pore-size distributions were determined by gas adsorption and immersion calorimetry, with the measurement of helium and bulk densities. Volumes of micropores were calculated using the Dubinin-Radushkevich (DR) equation (Section 4.2.3) to interpret the adsorption isotherms of N2 (77 K), CO2 (273 K) and n-C4H o (273 K). Volumes of mesopores were evaluated by subtracting the total volume of micropores from the amount of nitrogen adsorbed at p/p° = 0.95. The two density values for each carbon were used to calculate the volume of the carbon skeleton and the total volume of pores (including the inter-particle space in monolithic disks). Immersion calorimetry of the carbon into liquids with different molecular dimensions (dichloromethane 0.33 run benzene 0.37 nm and 2,2-dimethylbutane 0.56 nm) permits the calculation of the surface area accessible to such liquids and subsequent micropore size distributions. The adsorption of methane has been carried out at 298 K in a VTI high-pressure volumetric adsorption system. Additional techniques such as mercury porosimetry and scanning electron microscopy (SEM) have also been used for the characterization of the carbons. 

The porosity of CMS is studied by adsorption of N2 (77 K) and CO2 (273 K) to determine volumes of total and narrow microporosity, respectively, and by immersion calorimetry of the carbons into liquids with different moleeular dimensions (dichloromethane, 0.33 nm benzene, 0.37 nm eyclohexane, 0.48nm 2,2-dimethylbutane, 0.56 nm and a-pinene, 0.70 nm). Adsorption kineties were studied for two-gas mixtures, nitrogen-oxygen and methane-carbon dioxide, and separation abilities were studied using columns packed with the corresponding CMS. 

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