Porosity, molecular probes

The first moment of the molecular probe has to depend on all the different porosities. Therefore, the molecular probe has to folfill the following conditions (i) its size should allow to enter the micropores (no steric exclusion) and (ii) it must be able to easily replace the molecules of the solvent on the adsorption sites. For this latter condition, the molecular probe must have, at least, the same adsorption properties of the solvent. We may choose for the feed and the solvent two molecules of the same chemical family, that is to say two paraiSnes or two aromatics. A better choice would be a molecular probe which adsorbs more strongly on the adsorption sites than the molecules of the solvent. It would be the case with an aromatic as the feed and a paraffine as the solvent the aromatic adsorbs preferentially because of the stronger interaction between the electronic cloud of the aromatic and the adsorption sites (the cations and the -OH groups) of the zeolite. The Gurvich rule [12] says that densities in the adsorbed phase and the liquid phase are close to that of the bulk liquid adsorptive. If we make this hypoAesis (C, = Cm = Co), the equation (4) becomes ... 

Figure 18-4. Fractional porosity vs. molecular probe size for silica sol gel films annealed at various temperatures as indicated (after Yeatman, 1994).

From these studies with SynChropak SEC packings and controlled porosity glass, it is concluded that the silica packing contains a population of micropores which are differentially accessible to low molecular weight probes of total permeation volume. It is not known, however, if the microporosity in the 100 and 300A SynChropak SEC packings is the result of the rather wide pore-size distribution and whether all silicas contain micropores. 

A variety of spin probe methods have also been used to study the morphological features of the nano-channels present within MCM 41, as well as dynamical aspects connected to molecular diffusion in the inner pores,186-188 EPR has been used to investigate the adsorption and interactions of nitroxide-labelled de-ndrimers within porous silica.181 This method allows one to investigate the effective porosity of a solid surface (as a host) which is determined by the accessibility of the host surface to an adsorbed guest molecule. Information on the adsorption and interaction of dendrimers with the porous surface arises from computer-aided analysis of the EPR spectra based on of the well-established procedure proposed by Schneider and Freed.189... 

The purpose of the present work is to incorporate aluminum into the framework of SBA-15 during the synthesis in order to create acid sites on the surface of the material directly and to enhance its activity in acid-catalyzed reactions and to study the stability of SBA and AlSBA molecular sieves under various treatments. The influence of these treatments on the pore size, wall thickness and the environment of Al in these materials are investigated in detail. X-ray diffraction (XRD), Electron Microscopy (TEM) and N2 adsorption were used to characterize the structure, the porosity and the stability of these materials. 27Al MAS NMR was used to ascertain the nature and environment of Al, cumene cracking to test the catalytic activity of parent materials and ammonia chemisorption to probe their surface acidity. 

Since carbon molecular sieves are amorphous materials, the dimensions of their pore structures must be measured phenomenologically by the adsorption of small probe molecules with different critical dimensions. There is insufficient long range order to utilize standard x-Ray diffraction methods for characterization. The earliest reports of molecular sieving carbons dealt primarily with coals and charcoals. Sorption of helium, water, methanol, n-hexane, and benzene was measured and related to the porosity of the carbon. Pore-sizes were estimated to be two to six angstroms (3-6). In a classic paper P.H. Emmett described methods for tailoring the adsorptive properties and pore size distributions of carbon Whetlerites. 

Other researchers [68, 69] have reached similar conclusions. In particular, studies on freeze-dried organic gels led to A 2.55 when the FHH isotherm equation was applied to N2 adsorption data and A > 2.6 when SAXS was used [68]. For three shale samples, Ma et al. [69] found that the A values obtained from N2 adsorption data were significantly lower than those obtained from SANS experiments. The authors [69] suggested that the discrepancies were due to the different properties of the dense liquid phase in the small and large pores and to the different volumes that were occupied by the adsorbed film with respect to that probed by SANS. In conclusion, while SANS (and SAXS) sees the total porosity of a system (including inaccessible pores), adsorbate molecules can only penetrate pores that are both larger than their molecular diameter and accessible. 

There is no doubt that methods of adsorption of gases and vapors, overall, are used for the characterization of porosity in carbons. This is because die molecular-sized porosity responds to the presence of the similar-sized molecules of an adsorptive. Another effective, but much less used (because of the need for specialty equipment), approach is that of SAXS and SANS. These two approaches provide informations which, generally, are in agreement with adsorption methods described above, as well as additional informations. They are non-destructive approaches where the probes are photons of X-ray radiation and neutrons. SAXS involves scattering of photons at interfaces with pores and tends to be more diffuse (because electron density is not zero within pores) than the scattering of neutrons. The technique is quite different to wide-angle X-ray diffraction (XRD), a technique most suitable for crystalline materials. An in-depth review of this subject area of scattering phenomena is provided by Hoinkis (1997). 

Liquid chromatography breakthrough curves have been used to characterize the porosity of a dealuminated Y zeolite, and more precisely the mesopores in cavities and the cylindrical mesopores. The methodology presented in this paper shows that the use of several probe molecules with different molecular size and adsorption strength can give an estimation of the zeolite porosity. 

Pores contained in solid samples can be classified into closed and open pores. The meaning of closed and open depends on the size of pores into which a probe fluid (gas or liquid) can diffuse. In any case, when the probe fluid cannot penetrate into certain spaces, they are denoted as closed pores. Materials such as foamed metals or polymers have a considerable amount of isolated voids, individually surrounded by a dense matrix. Most of their porosities are denoted as closed pores. Gels prepared from a relatively soft network tend to retain molecular scale pores. They are open pores in nature, but virtually... 

This made it possible to identify surface hydroxyl groups in zeolites [4,5], characterize Brpnstead acidity [6], porosity, and adsorption sites with adsorbed probe molecules [7-13]. Later on, the adsorbed reaction products were characterized with MAS NMR [14], which provided a stimulus of application of MAS NMR to characterize the chemical reaction occurrence on zeolites [15] with the aim of unraveling the mechanisms of the reactions. Note, however, that researchers sometimes do not have to narrow the NMR signals with MAS to characterize the solids. For example, analysis of the line shape evolution with temperature for wide-line NMR spectra of adsorbates in zeolites affords a valuable information on the adsorbate molecular dynamics [16,17],... 

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