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Chapter 12. Properties of Some Novel Adsorbents

Chapter 12. Properties of Some Novel Adsorbents

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clear, but it seems likely that the chromatographic results are in part due to a very

high degree of interconnectivity within the pore structure of AX21.

In this book we have generally adopted the convention of expressing the a d s o p

don as the amount adsorbed by unit muss of the outgassed adsorbent. From a technological standpoint, however, it is sometimes more important to consider the amount

adsorbed per unit volume of the adsorbent. It is evident that when this change is made

for a powder of low bulk density such as AX21 (bulk density = 0.3 g ~ r n - the

~ ) available adsorbing capacity is much less impressive (Sing, 1989). For this reason, for a

particular purpose it may be necessary to employ an adsorbent of moderate activity

rather than one of very high specific activity.

12.2.2. Activated carbon fibres and carbon cloth

The first high-strength carbon fibres were produced in the 1950s (see Donnet and

Bansal, 1984). The early carbonized products were rayon-based, but it was soon

found that the mechanical properties and the carbon yield could be improved by the

use of polyacrylonitrile (PAN) as the precursor. Also, less expensive fibres of

somewhat lower strength and modulus could be made from various other precursors including petroleum pitch and lignin. However, cotton and other forms of

natural cellulose fibres possess discontinuous filaments and the resulting mechanical properties were consequently found to be inferior to those of the rayon-based


It was not long before the first activated carbon fibres (ACFs) were developed. In

the work of Economy and Lin (1971, 1976) highly porous carbon fibres were prepared from .Kynol, a fibrous phenolic precursor. Carbonization was carried out in

nitrogen at 800°C and activation occurred in steam at 750-1000°C. The products

appeared to be predominantly microporous and were found to be effective for the

removal of low levels of certain pollutants (e.g. phenol and pesticides) from air or

aqueous solutions.

Another important development was the disclosure by Bailey and Maggs (1972;

Bailey et al., (1973) of a novel procedure for the manufacture of 'charcoal cloth'. The

continuous process developed in the laboratories of the Chemical Defence

Establishment, Porton Down, England, involved three main stages: (1) immersion of

the roll of viscose rayon cloth in an aqueous solution of inorganic chlorides (e.g.

ZnC1,. AlCl, and NH,Cl); (2) oven drying in nitrogen; (3) carbonization and activation in carbon dioxide.

The Type I character of the nitrogen and toluene isotherms displayed in Figure

12.1 indicates that a typical sample of activated charcoal cloth produced by the original Porton process had novel adsorbent properties. The activated material was strong

and flexible and had a BET area of over 1200 m2 g-', a wide distribution of micropores (no detectable mesoporosity) and a small external surface area (Atkinson et al.,

1982; Hall and Williams, 1986). In view of its early promise, it was logical to attempt

to control the pore structure of charcoal cloth. A systematic study of the development

of porosity was therefore undertaken by Atkinson et al. (1982, 1984) and Freeman

et al. (1987-1991).

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Chapter 12. Properties of Some Novel Adsorbents

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