Porous texture adsorptive characteristics

Nitrogen adsorption/desorption isotherms of all the activated carbons are of Type I, i.e. characteristic of basically microporous solids. There is a lack of adsorption/desorption hysteresis. More careful analysis permits to notice significant differences in the porous texture parameters depending on precursor origin. 

The aim of this work is to test and to compare the performances of various nitrogen adsorption-desorption isotherms analysis methods. These models were applied to model samples obtained by mechanically mixing two micro- and mesoporous solids respectively in perfectly known proportions. The relevant morphological characteristics of the porous texture of the mixtures, such as the specific surface and volume, are physically additive. A criterion that allows determining the reliability of the analysis methods tested is thus to check the linearity of the relation between a given parameter and the weight percentage of the pure solids. 

Characterisation of porous texture of solids is of relevance because their properties are determined, or at least, influenced by this characteristic [1-3]. A number of techniques exist to characterise the porous texture of solids. Among them, physical adsorption of gases is the most widely used due to its simplicity [1-11], N2 adsorption at 77K [3] is, undoubtedly, the most used. One of its main advantages is that it covers reduced pressures from 10 to 1, being sensitive to the whole range of porosity. However, N2 adsorption at 77K has some limitations when used to characterise solids containing ultramicroporosity (i.e., pore sizes lower than 0.7 nm). It can be influenced by diffusional limitations in this range of porosity [4]. 

In the following, the usefulness of CO2 adsorption at 273 K to achieve a rather complete characterization of the porous texture of microporous carbons will be discussed. We will base our study on the results already published [33-35, 37] in which samples with different characteristics were used and CO2 adsorption experiments at high pressures (up to 4 MPa) were performed. In this study, the ACFs with different contents of microporosity have been very useful. The use... 

The series of 10 chapters that constitute Part 3 of the book deals mainly with the use of adsorption as a means of characterizing carbons. Thus, the first three chapters in this section complement each other in the use of gas-solid or liquid-solid adsorption to characterize the porous texture and/or the surface chemistry of carbons. Porous texture characterization based on gas adsorption is addressed in Chapter 11 in a very comprehensive manner and includes a description of a number of classical and advanced tools (e.g., density functional theory and Monte Carlo simulations) for the characterization of porosity in carbons. Chapter 12 illustrates the use of adsorption at the liquid-solid interface as a means to characterize both pore texture and surface chemistry. The authon propose these methods (calorimetry, adsorption from solution) to characterize carbons for use in such processes as liquid purification or liquid-solid heterogeneous catalysis, for example. Next, the surface chemical characterization of carbons is comprehensively treated in Chapter 13, which discusses topics such as hydrophilicity and functional groups in carbon as well as the amphoteric characteristics and electrokinetic phenomena on carbon surfaces. 

It has already been pointed out that a high surface area and an adequate pore size distribution are necessary conditions for a carbon adsorbent to perform well in a particular application. However, there are many examples of carbons with similar textural characteristics, which show a very different adsorption capacity witih the same adsorbate [12]. The reason for these different behaviours is that an adequate porous texture is a necessary but not a sufficient condition for the optimization of the adsorption capacity of activated carbons. The nature and amount of surface groups that may be present on the carbon surfaces must also be taken into account. 

The determination of the buckling constant h by calibration from mercury intrusion porosimetry, or from nitrogen adsorption-desorption, can lead in some cases to different results. It is likely that, beyond the imprecision due to the method, the differences in pore sizes observed arise from a fundamental difference in the pore size concept. lUPAC proposed to define pore size as the distance between two opposite pore walls (Rouquerol, 1994). In the case of materials from the sol-gel process, this definition is not applicable, because pores are not included between walls, but are only delimited by interconnected filaments. In practice, it is considered that the sizes obtained from analysis methods of the texture of porous materials are characteristic pore sizes. Because the different analysis methods are based on different physical phenomena, it is not astonishing that they lead to slightly different characteristic pore sizes. Discrepancies resulting from using different characterization methods appear in several publications, often when the same material is analyzed by nitrogen adsorption-desorption and mercmy intrusion porosimetry (Smith, 1990, Brown, 1974, Milburn, 1988, Minihan, 1994). Me Enaney et al. noted that the distribution profiles obtained by different characterization techniques are often similar, but that differences, sometimes important, between the absolute values of characteristic pore sizes are almost unavoidable (Me Enaney et al., 1995). 

Nitrogen adsorption/desorption isotherms on Zeolite and V-Mo-zeolite are very similar and close to a type I characteristic of microporous materials, although the V-Mo-catalysts show small hysterisis loop at higher partial pressures, which reveals some intergranular mesoporosity. Table 1 shows that BET surface area, microporous and porous volumes, decrease after the introduction of Molybdenum and vanadium in zeolite indicating a textural alteration probably because of pore blocking by vanadium or molybdenum species either dispersed in the channels or deposited at the outer surface of the zeolite. The effect is far less important for the catalysts issued from ZSM-5. 

The atomic structure of a heterogeneous catalyst determines its chemical and phase properties, but texture determines a wide range of additional features that dictate such characteristics as adsorption and capillarity, permeability, mechanical strength, heat and electrical conductivity, etc. For example, the apparent catalytic activity,. of a grain, taking into account diffusion of reagents, depends on the interrelation between the rates of reaction and diffusion, and the latter is determined by a porous structure. 

The nitrogen adsorption / desorption isotherms (Fig. 2) are typical of well-defined porous frameworks that are characteristic of either supermicroporosity (MSU-1) or a small mesoporosity (MSU-4) without any textural porosity [14]. In these two compounds, the silica walls (deduced from x-ray diffraction and nitrogen isotherms) are quite thick (< 20 A) [5],... 

Table 3 shows the textural characteristics of three carbon monoliths, two of which were produced by dipcoating a cordierite monolith with a solution of sucrose or PFA and one of which was produced by a CVD process resulting in a CNF coating. These are referred to as Cord-SUC, Cord-PFA, and Cord-CNF, respectively. From a texture analysis, it was concluded that the sucrose-derived carbon is highly porous, with pore diameters in a favorable range (t)q5ically, 11 nm). The PFA-derived carbon was microporous and, as a consequence, not suitable for adsorption of large species, such as enzymes. 

The texture properties of the ultrathin porous glass membranes prepared in our laboratory were initially characterized by the equilibrium based methods nitrogen gas adsorption and mercury porosimetry. The nitrogen sorption isotherms of two membranes are shown in Fig. 1. The fully reversible isotherm of the membrane in Fig. 1 (A) can be classified as a type I isotherm according to the lUPAC nomenclature which is characteristic for microporous materials. The membrane in Fig. 1 (B) shows a typical type IV isotherm shape with hysteresis of type FIl (lUPAC classification). This indicates the presence of fairly uniform mesopores. The texture characteristics of selected porous glass membranes are summarized in Tab. 1. The variable texture demanded the application of various characterization techniques and methods of evaluation. 

The aim of this study is to compare pore structure characteristics of two porous catalysts determined by standard methods of textural analysis (physical adsorption of nitrogen and mercury porosimetry) and selected methods for obtaining parameters relevant to transport processes (multicomponent gas diffusion and permeation of simple gases). MTPM was used for description of these processes. 

The study of porous carbon materials using IGC is frequently carried out by adsorption of hydrocarbons at the zero surface coverage. This is doubly useful because it gives information on the textural characteristics of the carbon materials and also the adsorption capacity of these molecules is obtained. It should be pointed out that this adsorption capacity is obtained under dynamic conditions and frequently at relatively high temperatures, i.e. at the experimental conditions very close to the real situations at which the carbon materials are used (for instance to eliminate atmospheric pollutants) [6]. 

The textural characteristics of the grafted materials obtained by N2 adsorption-desorption are summarized in Table 1 and Fig. 2. The shapes of the different isotherms are similar whatever the samples indieating that no drastic modifications of the porous network occur during grafting. In particular, the absence of modification at about P/Pq = 0.4 rules out the possibility of pore blocking as a result of Al addition [10]. However, there is a marked decrease of the specific surface area with the number of graftings with a final surface of ca. 

Activated carbons are widely used as adsorbents in either the gas or the liquid phase, and also as catalysts and catalyst support. In some cases their adsorption behaviour depends basically on their textural characteristics, i.e., porous structure and pore volume. As an example carbon molecular sieves (CMS) are a kind of activated carbons that make use of a narrow pore size distribution, of a few angstroms in diameter, to selectively separate gas mixtures [1], such as N2/O2, CO2/CH4, C3H6/C3H8 and some others. But also the surface chemistry can condition in many cases [2,3] the adsorption behaviour, as well as the activity as catalyst and catalyst support [4, 5]. This means that the adsorption properties of the activated carbons cannot be easily explained only on the basis of textural characteristics (surface area and pore size distribution) the nature of the chemical surface must also be taken into account. Therefore, for a complete characterization of the activated carbon surface, textural and chemical characteristics must be assessed. 

Immersion calorimetry is a useful technique for the characterization of porous materials. The heat evolved in the immersion process is directly related to the integral enthalpy of adsorption if the experiment is carried out at constant pressure and temperature [1]. The experimental data, which are obtained by immersion measurements, are normally used to determine the textural characteristics of the adsorbent, i.e. the micropore volinne or the surface areas accessible to the wetting liquid. In relation to the former Stoeekli et al [2] consider that a thermodynamic consequence of the Dubinin theory is the equation 1. 

The measurement technique of texture of porous materials that collapse under isostatic mercury pressure is based on the mechanical behavior of the network of interconnected filaments. This method defines, as characteristic size, the length of the edge ofthe reference cubic pore that has the same resistance toward buckling that the true pore that collapses under mercury pressure. The calibration of this technique by other characterization methods is necessary, but results can differ depending on whether the adjustment has been made from nitrogen adsorption-desorption isotherms, or from mercury intrusion porosimelry. In the case of discrepancy, preference is given to this last adjustment method. 

Almazan-Almazan et al. attempted to change the textural properties of activated carbons from PET by controlling various variables and showed that these carbons can be tailored to range from molecular sieves to samples with variant pore sizes and high adsorption volume [70]. Chars were obtained after pyrolysis at 800 and 950°C with 19% yield. Subsequent activation under CO2 flow took place for 4 or 8 h. The activation resulted in further bum-offs of 81-87%. In the carbonization process at 800°C under carbon dioxide, the amorphous carbon is eliminated but the micropore system is not affected. These porous samples show molecular sieve behavior for cyclohexane/benzene as well as 2,2-DMB/benzene pairs. At 950°C, however, micropore textural characteristics are changed and the samples do not exhibit molecular sieve behavior due to constrictions at the entrance of the micropores [77]. 

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