Micropore volume porous carbon

Mcntasty el al. [35] and others [13, 36] have measured methane uptakes on zeolites. These materials, such as the 4A, 5A and 13X zeolites, have methane uptakes which are lower than would be predicted using the above relationship. This suggests that either the zeolite cavity is more attractive to 77 K nitrogen than a carbon pore, or methane at 298 K, 3.4 MPa, is attracted more to a carbon pore than a zeolite. The latter proposition is supported by the modeling of Cracknel et al. [37, 38], who show that methane densities in silica cavities will be lower than for the equivalent size parallel slit shaped pore of their model carbon. Results reported by Ventura [39] for silica xerogels lead to a similar conclusion. Thus, porous silica adsorbents with equivalent nitrogen derived micropore volumes to carbons adsorb and deliver less methane. For delivery of 150 V./V a silica based adsorbent would requne a micropore volume in excess of 0.70 ml per ml of packed vessel volume. 

Presently, the most successful adsorbents arc microporous carbons, but there is considerable interest in other possible adsorbents, mainly porous polymers, silica based xerogels or zeolite type materials. Regardless of the type of material, the above principles still apply to achieving a satisfactory storage capacity. The limiting storage uptake will be directly proportional to the accessible micropore volume per volume of storage capacity. 

Parkyns and Quinn [20] showed a linear relationship between methane uptake at 25 C, 3.4 MPa and the Dubinin-Radushkievich micropore volume from 77 K nitrogen adsorption for porous carbons,... 

From the above data, it would appear that methane densities in pores with carbon surfaces are higher than those of other materials. In the previous section it was pointed out that to maximize natural gas or methane storage, it is necessary to maximize micropore volume, not per unit mass of adsorbent, but per unit volume of storage vessel. Moreover, a porous carbon filled vessel will store and deliver more methane than a vessel filled wnth a siliea based or polymer adsorbent which has an equivalent micropore volume fraction of the storage vessel. 

Recent reports describe the use of various porous carbon materials for protein adsorption. For example, Hyeon and coworkers summarized the recent development of porous carbon materials in their review [163], where the successful use of mesoporous carbons as adsorbents for bulky pollutants, as electrodes for supercapacitors and fuel cells, and as hosts for protein immobilization are described. Gogotsi and coworkers synthesized novel mesoporous carbon materials using ternary MAX-phase carbides that can be optimized for efficient adsorption of large inflammatory proteins [164]. The synthesized carbons possess tunable pore size with a large volume of slit-shaped mesopores. They demonstrated that not only micropores (0.4—2 nm) but also mesopores (2-50 nm) can be tuned in a controlled way by extraction of metals from carbides, providing a mechanism for the optimization of adsorption systems for selective adsorption of a large variety of biomolecules. Furthermore, Vinu and coworkers have successfully developed the synthesis of... 

Detailed accounts of fibers and carbon-carbon composites can be found in several recently published books [1-5]. Here, details of novel carbon fibers and their composites are reported. The manufacture and applications of adsorbent carbon fibers are discussed in Chapter 3. Active carbon fibers are an attractive adsorbent because their small diameters (typically 6-20 pm) offer a kinetic advantage over granular activated carbons whose dimensions are typically 1-5 mm. Moreover, active carbon fibers contain a large volume of mesopores and micropores. Current and emerging applications of active carbon fibers are discussed. The manufacture, structure and properties of high performance fibers are reviewed in Chapter 4, whereas the manufacture and properties of vapor grown fibers and their composites are reported in Chapter 5. Low density (porous) carbon fiber composites have novel properties that make them uniquely suited for certain applications. The properties and applications of novel low density composites developed at Oak Ridge National Laboratory are reported in Chapter 6. 

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. 

In general, the material density of porous solids decreases with the development of porosity, as it can be seen in Figure 4 for various powdered activated carbons, where tap and packing density values are plotted versus the micropore volume. 

Porous carbon materials mostly consist of carbon and exhibit appreciable apparent surface area and micropore volume (MPV) [1-3], They are solids with a wide variety of pore size distributions (PSDs), which can be prepared in different forms, such as powders, granules, pellets, fibers, cloths,... 

A test matrix of about 20 different carbon samples, including commercial carbon fibers and fiber composites, graphite nanofibers, carbon nanowebs and single walled carbon nanotubes was assembled. The sorbents were chosen to represent a large variation in surface areas and micropore volumes. Both non-porous materials, such as graphites, and microporous sorbents, such as activated carbons, were selected. Characterization via N2 adsorption at 77 K was conducted on the majority of the samples for this a Quantachrome Autosorb-1 system was used. The results of the N2 and H2 physisorption measurements are shown in Table 2. In the table CNF is used to designate carbon nanofibers, ACF is used for activated carbon fibers and AC for activated carbon. 

Benzene was the most popular adsorptive in many early studies of the pore structure of activated carbons (Dubinin, 1958,1966 Cadenhead and Everett, 1958 Smisek and Cemy, 1970). Indeed, in order to construct the characteristic curve for a given micro-porous carbon, Dubinin and his co-workers (Dubinin, 1966) originally adopted benzene as the standard adsorptive thus, in the context of the Dubinin theory of the volume filling of micropores (TVFM), the scaling factor p (C6H6) = 1 (see Chapter 8). 

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