Microporosity models

A New Method for Microporosity Detection Based on the Use of the Corrugated Pore Structnre Model (CPSM). 

The objective of the present work was to study and compare by scanning tunneling microscopy (STM) the microporosity and mesoporosity of several different carbon materials with various types and amounts of pores highly oriented pyrolytic graphite with artificially-generated model pores, activated carbon fibers, nonporous thermally treated carbon black and nonactivated carbon fibers with an ultramicroporous texture. 

In this study, we have prepared and characterized montmorillonite pillared with Al and La in different proportions. The structural and textural parameters of the materials were compared with those of montmorillonite pillared only with Al. We have applied classical and new models to low pressure nitrogen adsorption data to obtain a quantitative evaluation of the microporosity of the synthesized materials and their evolution under thermal treatments. 

Whole biomass samples are characterized by rather low microporosity and surface areas when freshly prepared. This is in contrast to pure cellulose, which exhibits significant microporosity initially. The biomass samples do. however, develop a significant amount of microporosity (activate) with modest amounts of burnoff in oxygen. In this regard, cellulose is a good model for whole biomass behavior. 

Correlation between specific surface areas measured using a simple and robust technique such as gas permeametry and SSA determined by more sophisticated techniques such as PSD, MIP and BET have been carried out. A good agreement between Sbf (surface measured by gas permeametry, Blaine Fisher, Sbf) and measured Sbet was obtained. This is due to the fact that the DS studied does not exhibit intraparticular microporosity (both Krypton and air can access all the surface developed by the powder). Sbf compared with estimated Sng (estimated from MIP results) and Spsd (estimated from PSD results) show a good linearity, but Shb and Spsd values are overestimated. This arises due to the simplifying approximations for particles shape included in the theoretical models for PSD and PIM... 

Everett, D.H. and Fowl, J.C. (1976). Adsorption in sht-like cylindrical micropores in the Henry s law region. A model for the microporosity of carbons. J. Chem. Soc., Faraday Trans. 1 Phys. Chem. Condens. Phases, 72(3), 619—36. 

Due to the small size of the micropores (i.e., size below 2nm), adsorbate filling at low relative pressures may occur. The presence of micropores in ACFs, and in most of the ACs, causes that most of the adsorption takes place within them and, at least, 90 % of the total surface area corresponds to micropores. The adsorption in microporosity is not so well understood and simple to interpret as for adsorption in mesopores or nonporous solids, which has led to an important research effort since more than 50 years ago, trying to establish experimental methods and refined models useful to explain adsorption in micropores (i.e., assessment of micropore volume and micropore size distribution [28, 35, 36, 47-49]. 

To quantitatively model reaction kinetics of geochemical systems, reliable estimates of the physical and reactive surface areas of the system are needed. The physical surface areas have been measured on the basis of either the macroscopic nature of the surface, i.e. estimates of its bulk geometry, or the microscopic nature, i.e. the areal extent of coverage by atoms or molecules, as in the BET method. In the latter case, comparisons with water sorption isotherms indicate that BET-determincd surface areas produce reliable estimates of the mineral/water interface, except for materials with high microporosity such as expandable clays. 

The N2 adsorption-desorption isotherms were collected on an ASAP 2010 analyser (Micromeritics). Prior to analysis, the samples were degassed (palumina-based materials) during 5 h. The contributions of microporosity to the overall surface area were estimated fi om a t-plot (Harkin-Jura) analysis of the adsorption curve (0.3 nm < t < 0.5 nm, with t being the statistical thickness). The pore size distribution was calculated from the desorption branch of the isotherm using the B JH model. 

The experiments showed strong effect of reservoir permeability on oil recovery. For example, the model of the petroliferous bed of microporosity tjrpe but with permeability of 294 Darcies gave an oil recovery factor of 60%, whereas the same porosity-type reservoir but with permeability of 5.2 Darcies yielded only 30% of oil in place. 

A combination of x-ray diffraction data with nitrogen adsorption and electron microscopy has been employed by Clerc et al. [71] to describe the structure of SBA-15 mesoporous sifica. The model they propose is similar to the one presented earlier by Edler et al. [25]. The presence of a fining, according to Edler et al., or a corona, according to Clerc et al., of lower density than the walls and the existence of occluded molecules in this region is enough to explain the origin of microporosity when those molecules are removed. SBA-15 structure will be further discussed in the next section. 

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