Narrow micropore size distributions

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. 

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,... 

It can be concluded that microporosity in CMS prepared from the water-washed precursors is too wide, even after low activation times (low bum-ofi), as to separate N2 from O2 or CO2 from CH4. However, they can be used to separate other mixtures of gases with larger molecules, such as n-butane (0.43 nm) from i-butane (0.5 nm). Figure 6 compares the adsorption kinetics of these two gases on samples CW-1, CW-2 and CW-4. Micropore size distributions of these samples indicate that their microporosity is narrower than 0.6 nm, with a higher surface area or micropore volume accessible in CW-4. Corresponding to this, the amounts of n-butane and i-butane adsorbed are higher for CW-4. Samples CW-1 and CW-2 behaves similarly, as expected from their pore size distribution. Their porosity is hardly accessible to cyclohexane (0.46 nm) and then, the amount of i-butane (0.5 nm) adsorbed is very low. 

Chars from the initial coal, activated to low bum-off (5 % and 10%), indicate a narrow micropore size distribution in the region of widths between 0.33 and 0.41 nm (accessible for dichloromethane, inaccessible for benzene). Preoxidation of the coal shifts this distribution towards slightly wider micropores - between 0.41 and 0.54 nm (accessible for benzene, inaccessible for cyclohexane), accompanied by an increase of the volume of these pores. 

In previous sections, it was explained how different precursors can be used to produce activated carbons and the type of porosity developed depending on the type of activation method applied. Thus, thermal activation imrmally yields adsorbents with a medium to liigh adsorption capacity, a medium micropore size distribution and no mesopore formation (except in the case of high bum-off ratios, where micropores may be of a large size). Phosphoric acid activation yields a carbon with a higher adsorption capacity than thermal activation and a wider micropore size distribution (even in the low mesopore range), whereas KOH yields extremely narrow microporous carbons. 

The DA equation is corresponding to the choice of arbitrary value in the WeibulTs distribution function (eq. 4.2-1). With this additional parameter in the adsorption isotherm equation, the DA equation provides flexibility in the description of adsorption data of many microporous solids ranging from a narrow to wide micropore size distribution. The following table shows the degree of filling when the adsorption potential is equal to some fraction of the characteristic energy. 

Since the degree of sharpness of the adsorption isotherm versus adsorption potential or the reduced pressure increases as the parameter n increases, this parameter could be used as an empirical parameter to characterise the heterogeneity of the system. Since it is an empirical parameter, it does not point to the source of the heterogeneity. However, it can be used as a macroscopic measure of the sharpness of the micropore size distribution. For solids having narrow micropore size distribution such as the molecular sieving carbon, the DA equation with n = 3 is found to describe the data well. Therefore, if the parameter n of a given system is... 

Because n =3 is found to describe well data of solids having narrow pore size distribution, the DA equation with n = 3 is generally used as the local isotherm for the description of micropore size distribution as we shall discuss later in Section 4.4. 

With any form of the distribution and a particular choice of the local isotherm, eq. (4.4-2) can be in general integrated numerically to yield the overall adsorption isotherm equation. The local fractional loading 0 can take the form of either the DR or DA equation. As discussed earlier the DA equation with n = 3 describes well solids having narrow micropore size distribution, and hence this makes this equation a better candidate for a local isotherm equation rather than the DR equation. However, since the selection of a distribution function is arbitrary, this does not strictly enforce the local isotherm to reflect the intrinsic local isotherm for a specific characteristic energy. Moreover, the DR or DA itself stems from a Weibull s distribution function of filling of micropore over the differential molar work of adsorption. Thus, the choice of the local isotherm is empirical, and in this sense the procedure of eq. (4.4-2) is completely empirical. The overall result, however, provides a useful means to describe the equilibrium data in microporous solids. 

Through a simple pyrolysis of crystalline polymer (PAF-1), Qiu et al. [105] have prepared a series of nanoporous carbons having high surface areas and narrow micropore size distributions. The carbonized (at 450 °C) sample PAF-1-450 showed very excellent adsorption capacities (4.5 mmol g ) for CO2 than that of original PAF-1 at ambient conditions (Fig. 2.20). These aforementioned works exemplify a... 

The plot of amount adsorbed versus A is defined as the characteristic curve. The relationship between Eq and pore size for adsorption in micropores was studied by Chen and Yang [51]. The exponential term n, which lies in the range of 1, accounts for the structure of the adsorbent. Large fi values are found to be associated with adsorbents that have a narrow micropore size distribution, while small n values relate to adsorbents with a wide range of micropore sizes and possibly with meso- and macropores as well. 

By these means it is possible to prepare carbon sieves with effective micropore diameters ranging from about 4 to 9 A. The micropore size distribution of such sieves is much narrower than in a typical activated carbon and the porosity and therefore the adsorptive capacity are generally very much smaller, as may be seen from Figure 1.2. The ability to modify the effective pore size by adjusting the conditions of the manufacturing process makes it relatively easy to tailor a carbon sieve to achieve a particular separation. However, it is difficult to achieve absolute reproducibility between different batches, and the existence of a distribution of pore size, even if narrow, means that the molecular sieving selectivity of a carbon sieve seldom approaches the almost perfect separation achievable under fav orable circumstances with a zeolite sieve. Nevertheless, the kinetic selectivities which may be attained with a well-prepared carbon sieve are remarkably high. 

Manso et studied the formation of CMS by carbon vapor deposition (CVD) over activated carbons from four different rank coals. The deposition of carbon was carried out by pyrolyzing benzene vapors at 725°C. This produced gradual closing of the micropores, due to the formation of constrictions at their entrances. As a result the MSC with a narrow micropore-size distribution around 0.35 to 0.5 nm were obtained. Samples with diameters smaller than 0.33 mn obtained by a high degree of deposition were able to separate O2/N2 and CO2/CH4 mixtures. 

Thus, the KOH/anthracite ratio not only affects the micropore volume but also the micropore size distribution, which should be taken into account for the final use of the AC. If for a given application (e.g., for gas storage see Section V.D) both high adsorption capacity and narrow MPSD are required, the hydroxide/ carbon ratio has important limitations as a variable, and it cannot be useful for optimizing the preparation protocol of this type of AC. 

From a practical standpoint, to increase the methane adsorption capacity we need to develop not only micropore volume but also control carefully the micropore size distribution. Thus, samples with high surface areas and very narrow micropore size distributions are required for this application. For this reason, in our studies of methane storage KOH instead of NaOH was used as the activating agent for the preparation of ACs because of the narrower MPSDs that can be obtained by KOH, as shown in Sections II and III. 

Carbon molecular sieves (CMS) have played a critical role in the commercialization of the pressure swing adsorption process for the separation of nitrogen from air. They differ from activated carbon mainly in the pore size distribution and surface area. While activated carbons have a broad range of pores, with a typical average pore diameter of 20 A, carbon molecular sieves have a more narrow pore size distribution, with pore sizes in the range of 3 - 5 A. A molecular probe method is one of the best approaches to determine the effective micropore size distribution of carbon molecular sieves [3,4]. Typical surface areas for a carbon molecular sieve are in the range of 250-400 m /g, while the micropore volume is about 0.15-0.25 cm Vg [2,5]. 

Figure 5.48 includes the adsorption isotherms of N2 at 77 K for sepiolite S and the two original carbons (P and C). The shape of the isotherm for sepiolite is near to Type II, characteristic of this material, whereas the isotherms for the activated carbons are of Type I, as expected. Micropore volumes show differences in microporosity and micropore size distribution of the adsorbents. The similarity of Vq(N2 77 K) and Vo(C02 273 K) values for sepiolite indicates the existence of both narrow and homogeneous microporosity due... 

The application of the DR equation to the adsorption data isotherms of Fig. 1 leads to the micropore volume, V, of the carbons plotted in Fig. 2. There is an increase in from BO to MO and AO, the of RO being slightly lower than that of AO. The evolution of measured by CO2 is similar but the actual values are similar only for carbons BO and HO. This means that in these two carbons, the microporosity is narrow and relatively homogeneous [9] the large difference between the values deduced from N2 and CO2 for carbons AO and RO is due to the wider micropore size distribution, especially in carbon RO. 

---