Page 124 - Adsorbents fundamentals and applications

CARBON MOLECULAR SIEVES 109

Monolithic composites of activated carbon fibers with phenolic resin as the
binder have been prepared for a variety of possible applications, including gas
separation (Burchell, 1999; Kimber et al., 1996). The low-density composites
3
with densities <0.25 g/cm are particularly promising for gas separation as well
as energy storage (CH 4 and H 2 ) applications. A detailed discussion on these
types of materials has been given by Burchell (1999). Methane storage will be
discussed in Chapter 10.

5.6.1. Adsorption Isotherms
A large body of experimental data on the adsorption isotherms exists in the liter-
ature (both journal publications and commercial brochures). With the exception
of methane storage, most of the data are related to purification, particularly for
environmental applications. Comparing different ACFs, the amounts adsorbed
are, in general, directly dependent on the BET surface area and the micropore
volume of the ACF. However, the heats of adsorption on ACF are higher than
those on the granulated activated carbon. This is shown in Table 5.9 for hydro-
carbons and CO 2 . The data in Table 5.9 are taken from Kuro-Oka et al. (1984),
Meredith and Plank (1967), Laukhuf and Plank (1969), and Reich et al., (1980).
Adsorption of SO 2 , NO, and various VOCs on ACF has been studied. Fig-
ure 5.19 shows the adsorption capacities of benzene, toluene, and phenol for a
number of ACFs. The corresponding capacities for commercial activated carbons
(GACs) are 0.33 g/g (benzene), 0.35 g/g (toluene), and 0.30 g/g (phenol). These
are near the lowest values of the various ACFs. The adsorption of NO by ACFs
has been studied extensively by Kaneko (1998). Comparing a number of GACs
and ACFs, the amounts adsorbed of NO were higher on the ACFs. For example,
2
for two samples with the same BET surface area (860 m /g), the amounts were
17 mg/g (at 13 kPa and 303 K) for the GAC compared with 65 mg/g for a PAN-
based ACF; and at 80 kPa and 303 K, these values were 47 mg/g for GAC and
115 mg/g for the ACF. The adsorption of SO 2 by ACF was studied extensively
by Mochida and co-workers (e.g., Mochida et al., 1997a; 1997b). However, the
focus of their studies was on the catalytic activity for oxidation of SO 2 to SO 3
in the presence of both O 2 and liquid water (to form sulfuric acid). Moderate
activities at room temperature were reported. The mechanism of the reaction is
not understood, although molecular orbital studies have indicated the importance
of surface oxides on carbon for the catalytic activity (Yang and Yang, 2002).
Enhanced adsorption capacities by the small pores of ACF have also been
observed for adsorption from aqueous solutions. Sakoda et al. (1987) reported
the adsorption isotherms of trichloroethylene and tetrachloroethylene on a number
of ACFs derived from phenolic resin, as shown in Figure 5.21. These data are
compared with that on GAC, and the enhancement is clearly seen.

5.7. CARBON MOLECULAR SIEVES
Because they are less hydrophilic than zeolites but have molecular sieving prop-
erties, carbon molecular sieves (CMS’s or MSC’s) can be used more efficiently