Page 119 - Adsorbents fundamentals and applications

104 ACTIVATED CARBON

5.6. ACTIVATED CARBON FIBERS
Carbon fibers have been produced commercially since ca. 1960. The precur-
sors for carbon fibers were polymeric fibers (polyacrylonitrile, or PAN; phenolic
formaldehyde, or Novalak; polyvinylidene chloride, or Saran), celluose (and
rayon derived from cellulose), and pitches (coal tar pitch and petroleum pitch).
These carbon fibers have high tensile strength and high elasticity and are con-
siderably more graphitic than activated carbon because a mesophase is usually
formed during the carbonization process of the fibers. Gas activation (usually
with carbon dioxide) of these carbon fibers results in ACFs. Consequently there
are as many types of ACFs as there are precursors. Comprehensive reviews on
ACFs have been made by Suzuki (1994) and Mays (1999), including a history of
its development and the present commercial production. The pioneering develop-
ment work on ACF (Abbott, 1962; Rodgers et al., 1965; Doying, 1966; Peters,
1966; Boucher et al., 1970; Economy and Lin, 1973; Bailey and Maggs, 1972)
has also been reviewed.
Activated carbon fibers have high surface areas, with BET surface areas rang-
2
2
ing from around 1000 m /g to well over 2000 m /g. Beside their fibrous form,
they have the following unique properties (compared with GAC and PAC):
(1) Narrow and uniform pore size distribution (hence stronger interactions
with sorbates).
(2) Small and uniform fiber diameter (hence faster uptake and desorption).
(3) Graphitic (hence more conductive and more heat resistant).
(4) High strength and elasticity (allowing greater flexibility in the shapes and
forms of the sorbent such as ACF clothes and papers)
As a result of these unique properties, there are many advantages for using
them as sorbents. Despite these properties, their present application is limited by
its high cost. The total annual worldwide production of carbon fibers in 1993
was 7300 tons, of which less than 2% was ACF (Mays, 1999). Five compa-
nies in Japan (Suzuki, 1994) provide the main supply of ACFs. However, as
production processes improve further and new demands arise for environmental
applications, as well as consumer products applications, the use of ACF will
undoubtedly increase.
As a consequence of the four unique properties above, the adsorption proper-
ties of ACF are discussed accordingly.
(1) Narrow and uniform pore size distribution: The pore sizes in ACF are
generally <20 ˚ A and mostly in the 8–10 ˚ A. range. Table 5.8 shows the pore size
ranges for three commercial ACFs (Kaneko et al. 1994; Suzuki, 1994).
From their N 2 adsorption data, Jaroniec et al. (1991) determined the pore sizes
of PAN-based and cellulose-based ACFs, and found that 85% of the pore volumes
were composed of uniform micropores with a diameter ∼10 ˚ A. In fact, the pore
sizes of ACF in all reported literature are ∼10 ˚ A.
There are two likely reasons for the fine pore sizes and uniformity (or nar-
rowness) of their size distribution. First, the ACFs have essentially zero “ash”