Brunauer classification

Brunauer classification, 1 591 and column performance, 1 604—606 gas adsorption, 1 622-623, 626-629 of nonionic surfactants, 24 142—143 predicting, 24 139-140 Adsorption kinetics... 

Brucine, 2 74 Brucite, 15 321, 323, 399 reserves of, 15 322 Brugnatelli, Luigi, 9 760 Brunauer classification of adsorption isotherms, 1 591... 

As first shown by Boehm in 1975 [41] on CDC synthesized from TaC and SiC, and later for most carbide precursors [33], the resulting carbons have type I isotherms in the Brunauer classification, which are indicative of microporous carbon having pore sizes less than 2nm and relatively high surface areas up to 2000m2/g [37,39,42], The pore size of CDC can be tailored by the selection of carbide precursors with different spatial distributions of carbon atoms in the initial carbide lattice, changing the nanotextural ordering in the CDC by varying the synthesis temperature, and posttreatment in a... 

Adsorption of Cyclohexane. The adsorption of cyclohexane was determined on synthetic lepidocrocite and its decomposition products prepared by heating it in vacuo for varying intervals of time at 190°, 300°, 400°, and 500°, and also on synthetic goethite and its decomposition products obtained in a similar manner by heating at 150°, 180°, 250°, 300°, and 500°. The adsorption was found to be physical in nature, and the isotherms are Type II of the Brunauer classification (3) in all cases except on iron oxide prepared from goethite at 300° and 500° here the isotherms are Type III. This finding is in contrast to the Type IV isotherms common to ferric oxide gels the difference may be due to the crystalline nature of the parent material. 

Adsorption isotherms are plots of the amount of gas adsorbed at equilibrium as a function of the partial pressure p/p°, at constant temperature. The quantity of gas adsorbed is mainly expressed as the mass of gas (usually g) or the volume of gas reduced to STP (standard temperature and pressure). The majority of isotherms which result from physical adsorption may conveniently be grouped into five classes — the five types I to V included in the classification originally proposed by Brunauer, Deming, Deming and Teller — sometimes referred to simply as the Brunauer classification [2]. The essential features of these types are indicated in Fig. 12.1. 

Experimental adsorption isotherms recorded in the literature, measured on a wide variety of gas-solid systems, have a wide variety of forms. Nevertheless, the majority of these isotherms which result from physical adsorption may conveniently be grouped into six classes in the IUPAC classification (cf. Figure 1.7). The first five types (I to V) of the classification were originally proposed by S. Brunauer, L.S. Deming, W.S. Denting and E. Teller as the BDDT classification (1940), sometimes referred to as the Brunauer classification (1945). 

The complete nitrogen isotherms of dealuminated Y zeolites are reported in Fig. 1. The curve of the parent H-Y zeolite corresponded to type I in the Brunauer classification, which was typical for the crystalline microporous materials [17]. As expected, the starting material showed no evidence of mesopores. Fig. 1 shows that the AHFS-treated samples with dealumination levels equal or lower than 50% were characterised by a very flat adsorption-desorption isotherm with nearly no hysteresis loop [18]. 

The adsorption isotherms from GCMC simulation are shown in figure 1 as plots of absolute adsorbate density versus fiigacity. They are all of type I in the Brunauer classification, showing a... 

Adsorption equilibria determine the thermodynamic limits of the specific amounts of adsorption (mol/g) of a pure gas or the components of a fluid mixture (gas or liquid) under a given set of conditions [pressure (P), temperature (T), and mole function (y or Xi) of component /] of the bulk fluid phase. The simplest way to describe adsorption equilibria of pure gas i is in the form of adsorption isotherms where the amount adsorbed (n ) is plotted as a function of gas pressure (P) at a constant temperature (P). The pure gas adsorption isotherms can have various shapes (Types I-V) by Brunauer classification depending on the porosity of the adsorbent (microporous, mesoporous, or nonpo-rous) and the system temperature (below or above the critical temperature of the adsorbate). However, the most common isotherm shape is Type I, which is depicted by most microporous adsorbents of practical use. These isotherms exhibit a linear section in the very low-pressure region (Henry s law region) where the amount adsorbed is proportional to the gas pressure [ n ) = KiP]. The proportionality constant is called... 

The isotherms for adsorption of pure water vapour on activated aluminas are typically Type I (microporous) or Type IV (mesoporous) in shape according to the Brunauer classification [6]. Figure 1 shows several examples. Alcoa F-1 alumina has a type I shape while Alcan AA-300 and Alcoa H-156 exhibit type IV shapes. The plots represent the specific amount of water vapour adsorbed (n, g/g) as functions of the relative vapour pressure of water (x = P/P ) at 30°C. P(atm) is the water vapour pressure over the adsorbent... 

Figure 22.5(a) shows the variety of isotherms (Type I by Brunauer classification [19]) available for adsorption of pure CO2 on the activated carbons of Table 22.3 and on 5A zeolite [18]. Table 22.4 gives the corresponding Henry s law constants and selectivities for adsorption of CO2 and H2 at 303 K as well as the isosteric heats of adsorption of pure CO2 in the Henry s law regions [18]. These data show that (a) the strength of CO2 adsorption and the coadsorption of Hj from CO2 + H2 mixtures can vary significantly, and (b) the adsorption of COj on the 5A zeolite is too strong to be useful in a PSA process. 

Even limiting the goal to a purely numerical exercise, the study of the behaviours of eq. (51) is manifestly a difficult task, because of the numerous parameters it contains. Such an analysis, carried out in a preliminary form in ref. [74], shows however that eq. (51) is able to account for the Brunauer classification of adsorption isotherms. 

Figure 5. How the C M equation accounts for Brunauer classification of adsorption isotherms.

Figure 4 shows a type of isotherm shape that is seen with crystalhne ion exchangers such as molecular sieves and clay minerals, but is nevertheless relatively uncommon. The shape resembles the type II vapour adsorption isotherm of the Brunauer classification, having a clear plateau region and inflexion point. An example is the Na/K exchange in zeohte P [48] that was found to be reversible over the whole range of equivalent fraction of potassium in the crystal Zeohte P has the gismondine-type structvue (CIS [49]). More commonly, isotherms of this type are found to be partiaUy irreversible in the plateau... 

Fig. I.l The five types of adsorption isotherm, I to V, in the classification of Brunauer, Deming, Deming and Teller (BDDT), together with Type VI, the stepped isotherm.

Classification of Isotherms by Shape Representative isotherms are shown in Fig. 16-5, as classified by Brunauer and coworkers. 

The advantage of equation 17.14 is that it may be fitted to all known shapes of adsorption isotherm. In 1938, a classification of isotherms was proposed which consisted of the five shapes shown in Figure 17.5 which is taken from the work of Brunauer et alSu Only gas-solid systems provide examples of all the shapes, and not all occur frequently. It is not possible to predict the shape of an isotherm for a given system, although it has been observed that some shapes are often associated with a particular adsorbent or adsorbate properties. Charcoal, with pores just a few molecules in diameter, almost always gives a Type I isotherm. A non-porous solid is likely to give a Type II isotherm. If the cohesive forces between adsorbate molecules are greater than the adhesive forces between adsorbate and adsorbent, a Type V isotherm is likely to be obtained for a porous adsorbent and a Type III isotherm for a non-porous one. 

Thus, in terms of a, the sorption isotherm for osmotically ideal solutions is of Type III in Brunauer s classification (29) and reduces to Henry s law for (Mi/M2)a< 1. 

Figure 5.5 Brunauer s classification of adsorption isotherms (pn saturated vapour pressure)...

Many adsorption isotherms are borderline cases between two or more of the above types. In addition, there are some isotherms which do not fit into Brunauer s classification at all, the most notable being the stepwise isotherms, an example of which is given in Figure 5.6. Stepwise isotherms are usually associated with adsorption on to uniform solid surfaces, each step corresponding to the formation of a complete monomolecular adsorbed layer (see page 133). 

The sorption isotherms can be grouped into five types, according to the classification of Brunauer, Emmet and Teller. l,2 3A However, we prefer a classification, based on the pore size of the adsorbent.5 The IUPAC classification6 of pores is given in table 2.1. 

The equilibrium isotherms for microporous adsorbents are generally of type I form in Brunauer s classification (Fig. 1). Such isotherms are commonly represented by the Langmuir model,... 

FIGURE 1 Brunauer s classification of equilibrium isotherms. P, sorbate pressure Ps, saturation vapor pressure. 

Intracrystalline sorption is normally of Type 1 in Brunauer s classification ( V7) and isotherm contours therefore resemble those according to Langmuir s isotherm equation. This can describe actual isotherms well enough (18) to be of value in predicting, through Equations 5 or 6, some features of zeolite chemistry. 

Equilibrium sorption of water (solubility) is described by the different isotherms of the Brunauer-Emmett-Teller classification. 

Two types of isotherms are most common. The first is type I of Brunauer s classification (6) with Langmuirs equation being the simplest example. The curves corresponding to this type are convex everywhere and have a v = 1 asymptote. This type was studied in the framework of the moving bed reactor by Viswanathan and Aris (1). According to the result of the previous section we cannot have more than one internal discontinuity. 

The forms of the isotherms and hysteresis loops have been subject to a classification initially proposed by Brunauer and taken up by the lUPAC. This classification shows the relationship between the form of the isotherms, the average radius of the pores and the intensity of the adsorbate-adsorbant interactions. Four types of isotherms out of the six proposed by the lUPAC are commonly encountered (Fig. 1.1). Similarly, the hysteresis loops correspond-... 

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