Page 313 - Adsorbents fundamentals and applications

298 SORBENTS FOR APPLICATIONS

according to whether the dioxygen is bound to one metal atom (type a), or bridges
two metal atoms (type b).
Chen and Martell synthesized and characterized a large number of O 2 -binding
cobalt Schiff base complexes (Chen and Martell, 1987; 1989). Dzugan and Busch
characterized new oxygen-binding macrocyclic cobalt complexes (Dzugan and
Busch, 1990). Ramprasad et al. (1995) reported a group of solid-state crystalline
cyanocobaltate complexes with reversible and very high oxygen-binding capac-
ity. Specifically, lithium cyanocobaltate, Li 3 Co(CN) 5 · 2DMF, was obtained by
◦
calcination at 160 CinN 2 , as opposed to the same compound with 4DMF that
was synthesized by others previously. The compound with 2DMF provided more
voids and higher surface area, hence higher O 2 diffusion rates. The O 2 capac-
◦
ity was 2.2 mmol/g at 1 atm O 2 and 25 C, and the isotherm was quite steep
◦
at pressures below 0.1 atm. O 2 /N 2 cycling tests at 25 C showed that the O 2
capacity declined steadily (linearly) to 85% of its initial value after 550 cycles
(in 410 hr). Also, it deactivated rapidly in the presence of moisture. None of
these materials, however, has shown the necessary combination of reversibility,
capacity, and stability needed for use in industrial gas separations.
Recent research efforts have focused on steric hindrance as a means of pro-
tecting the oxygen-binding complex from oxidation and dimerization. The most
promising approach has been to prepare the oxygen sorbent by entrapment or
encapsulation as a solid-state metal complex within the cages of a synthetic zeo-
lite (Lunsford, 1975; Howe and Lunsford, 1975; Imamura and Lunsford, 1985;
Herron, 1986; Drago et al., 1988; Taylor et al., 1989; Taylor et al., 1992). None
of these efforts, however, proved effective for separating oxygen from nitrogen
due to instabilities and/or inadequate O 2 -binding capacity.
Although numerous transition metal complexes with oxygen-binding abil-
ity have been reported, none of these materials has achieved commercial suc-
cess as a sorbent for air separation. All these materials have suffered from
one or more of the following drawbacks that have prevented commercializa-
tion: (1) chemical instability, (2) unacceptable adsorption characteristics, and/or
(3) unacceptable cost.
An interesting idea that has not been pursued is to immobilize the oxygen-
binding complexes on a solid support. In Wang’s classic experiment (Wang,
1970), a heme diethyl ester was embedded in a matrix of polystyrene and 1-
(2-phenylethyl)imidazole. The matrix not only prevented close approach of two
heme but also provided a hydrophobic environment. Reversible oxygen uptake
was observed (Wang, 1970).
More recently, Hutson and Yang (2000b) synthesized complexes of known
oxygen-binding ability by using a modified synthesis technique that resulted in
complexes that were attached (immobilized) to the surface of several porous
substrates. Co(salen) and Co(fluomine) were the complexes used. The O 2 -binding
capacity and the stability of these resulting sorbents were then characterized. The
modified procedure involved two steps. The first was to bond Co 2+ on anion sites
2+
of a substrate, by ion exchange, to form a stable ionically bonded Co .This
was followed by attaching ligands coordinatively to Co 2+ in order to give it the