Micropore size distribution

Fig. XVII-31. (a) Nitrogen adsorption isotherms expressed as /-plots for various samples of a-FeOOH dispersed on carbon fibers, (h) Micropore size distributions as obtained by the MP method. [Reprinted with permission from K. Kaneko, Langmuir, 3, 357 (1987) (Ref. 231.) Copyright 1987, American Chemical Society.]...

If a Type I isotherm exhibits a nearly constant adsorption at high relative pressure, the micropore volume is given by the amount adsorbed (converted to a liquid volume) in the plateau region, since the mesopore volume and the external surface are both relatively small. In the more usual case where the Type I isotherm has a finite slope at high relative pressures, both the external area and the micropore volume can be evaluated by the a,-method provided that a standard isotherm on a suitable non-porous reference solid is available. Alternatively, the nonane pre-adsorption method may be used in appropriate cases to separate the processes of micropore filling and surface coverage. At present, however, there is no reliable procedure for the computation of micropore size distribution from a single isotherm but if the size extends down to micropores of molecular dimensions, adsorptive molecules of selected size can be employed as molecular probes. 

Traditional adsorbents such as sihca [7631 -86-9] Si02 activated alumina [1318-23-6] AI2O2 and activated carbon [7440-44-0], C, exhibit large surface areas and micropore volumes. The surface chemical properties of these adsorbents make them potentially useful for separations by molecular class. However, the micropore size distribution is fairly broad for these materials (45). This characteristic makes them unsuitable for use in separations in which steric hindrance can potentially be exploited (see Aluminum compounds, aluminum oxide (ALUMINA) Silicon compounds, synthetic inorganic silicates). 

From these results, it can be concluded that hydrogen adsorption at 77K and at pressures up to 4 MPa requires porous solids with a very high micropore volume, such as chemically activated carbons. At these adsorption conditions, micropore size distribution does not play an important role, contrary to what happened at room temperature. 

It is especially easy to recognize that thermodesorption shows a completely different picture for both samples. This suggests a difference in micropore structure. This is confirmed by Figure 6 that shows the micropore size distribution calculated from the second peak isotherm. 

D. Lozano-Castello, D. Cazorla-Amoros, A. Linares-Solano, Can highly activated carbons be prepared with homogeneous micropore size distribution, Fuel Process. Technol. 77-78 (2002) 325-330. 

The local adsorption isotherm 0L is represented by the original D-A equation and /P(x) is the micropore size distribution ranging from xmm to xmilx (the lower and upper limits of the slit-like... 

Models to evaluate the microporous volume exist for several decades. They do not yield a micropore size distribution, but simply quantify the pore volume of all pores with a diameter < 2 nm. 

In molecular sieve adsorbents, such as zeolites and carbon molecular sieves, the micropore size distribution is extremely narrow, thus allowing the possibility of kinetic separations based on differences in molecular size. However, this feature is utilized in only a few commercial adsorption separation processes, and in the majority of such processes the separation depends on differences in the adsorption equilibrium rather than on the kinetics, even though a molecular sieve adsorbent may be used. 

Carbon molecular sieves are produced by controlled pyrolysis and subsequent oxidation of coal, anthracite, or organic polymer materials. They differ from zeolites in that the micropores are not determined by the crystal structure and there is therefore always some distribution of micropore size. However, by careful control of the manufacturing process the micropore size distribution can be kept surprisingly narrow, so that efficient size-selective adsorption separations are possible with such adsorbents. Carbon molecular sieves also have a well-defined bi-modal (macropore-micropore) size distribution, so there are many similarities between the adsorption kinetic behavior of zeolitic and carbon molecular sieve systems. 

Lozano-Castello D, Cazorla-Amoros D, Linares-Solano A, and Quinn DF. Micropore size distributions of activated carbons and carbon molecular sieves assessed by high-pressure methane and carbon dioxide adsorption isotherms. J. Phys. Chem. B, 2002 106(36) 9372-9379. 

Pantatosaki E, Psomadopoulos D, Steriotis T, Stubos AK, Papaioannou A, and Papadopoulos GK. Micropore size distributions from C02 using grand canonical Monte Carlo at ambient temperatures Cylindrical versus slit pore geometries. Colloids Surf. A Physicochem. Eng. Aspects, 2004 241(1-3) 127-135. 

To overcome these difficulties, Stoeckli (Stoeckli, 1977 Stoeckli et al., 1979) suggested that the original DR equation only holds for those carbons with a narrow micropore size distribution. According to this view, the overall isotherm on a heterogeneous microporous solid is made up of the sum of the contributions from the different groups of pores. Thus,... 

Equation (4.47) and obtain an isotherm equation in which the distribution function, (B) was expressed in an analytical form (Huber et al., 1978 Bansal et al., 1988). In principle, f(fl) provides an elegant basis for relating the micropore size distribution to the adsorption data. However, it must be kept in mind that the validity of the approach rests on the assumption that the DR equation is applicable to each pore group and that there are no other complicating factors such as differences in surface heterogeneity. 

It is apparent that any limited range of linearity of a simple DR plot cannot be used to give a reliable evaluation of the pore size distribution. In order to describe a bixnodal micropore size distribution, Dubinin (1975) applied a two-term equation, which we may write in the form... 

Immersion of dry samples in liquids of different molecular size This method is designed to take advantage of molecular sieving. The basic data are simply in the form of a curve of the specific energy of immersion versus the molecular size of the immersion liquid. This provides immediate information on the micropore size distribution. For room-temperature experiments one can use the liquids listed in Table 8.1, which are well suited for the study of carbons. Because of the various ways of expressing the critical dimension of a molecular probe or its molecular size , one must be careful to use a consistent set of data (hence the two separate lists in Table 8.1). Again, one can process the microcalori-metric data to compare either the micropore volumes accessible to the various molecules (see Stoeckli et a ., 1996), or the micropore surface areas, as illustrated in Figure 8.5. 

By computing a series of model single-pore isotherms for nitrogen adsorption at 77 K, Seaton, Gubbins, Olivier, Quirke and their co-workers (see Gubbins, 1997 Olivier, 1995) have been able to make use of Equation (8.6) in order to determine the micropore size distribution. It is assumed that all the pores are of the same shape (i.e. slits or cylinders of semi-infinite length) and that the distribution,... 

As explained in Chapter 7, since the multilayer isotherm path is rather insensitive to differences in surface chemistry, for routine mesopore analysis it is possible to make use of a universal form of nitrogen isotherm. However, most activated carbons are highly microporous and the determination of the micropore size distribution remains a more difficult problem. Indeed, as discussed in Chapter 8, even the assessment of the total micropore volume presents conceptual difficulties. We should therefore regard the measurement of a nitrogen adsorption isotherm as only the first stage in the characterization of a microporous carbon. 

For the pore size analysis of microporous carbons, a series of liquids of different molecular size should be employed (see Section 8.3.1). The energy of immersion can be converted into an effective area, which is accessible to each liquid, and the micropore size distribution obtained from the plot of surface area versus molecular size (see Figure 8.5). 

Another procedure proposed for micropore evaluation is by nonane pre-adsorption, which is dependent on the strong retention of n-nonane molecules in the ultramicropores. However, it is now apparent that the extent of the pore blocking is determined by the network connectivity as well as by the micropore size distribution. 

Micropore Size Distribution Analysis. Low pressure nitrogen adsorption and desorption of fresh and coked catalysts were carried out using an Omicron Technology Omnisorp lOOCX. The data for the adsorption isotherms were collected at very low partial pressures of nitrogen (P/Pq < 10 ) to determine the BET surface area and the micropore volume. The micropore volume was estimated from the t-plots. The desorption isotherm was obtained to measure meso and macro pore volume which correspond to the pore volume larger than pore radius of Inm. 

Three commercial activated carbons were used BPL, CAL and GAe, manufactured by Chemviron, Calgon and CECA respectively. In addition, sample GAe-oxl was prepared by oxidation of GAe in aqueous solution of (NH4)2S20g and further pyrolysis in N2 flow at 773 K [5]. The specific surface areas were obtained applying the BET and Dubinin-Asthakov equations to the adsorption of N2 at 77 K and CO2 at 273 K respectively. Moreover, the C02 adsorption data permitted the evaluation of the micropore size distributions and the mean value of pore width using the Dubinin-Stoeckli equation [6] which supposes a gaussian distribution of pore sizes. 

The micropore size distribution curves obtained by applying the Dubinin-Stoeckli equation to the CO2 adsorption data are shown in Fig. 1. CAL has the narrowest micropore size distribution, BPL and GAe show very similar curves and GAe-ox 1 has the most open distribution that extends beyond the microporosity limit (2 nm). The mean value of the micropore size distribution ranges between 1.47 and 1.77 nm for CAL and GAe-oxl respectively. 

Figure 1 Micropore size distributions obtained from Dubinin-Stoeckli equation...

From these results it can be said that the shape of water isotherm up to relative pressures around 0.6 is mainly related to the micropore size distribution. On the other hand, the adsorption at higher P/Po is due to the presence of mesoporosity. Depending on the pore size distribution and the contribution of the different pore volumes, the shape of water isotherms can be more similar to the sample 1A or 3B. 

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