Page 210 - Adsorbents fundamentals and applications

PREPARATION OF THREE TYPES OF SORBENTS 195

procedure of Hirai et al. (1986b), Tamon et al. (1996) have impregnated CuCl
on activated carbon by using CuCl in 1 N HCl solution. However, thorough
washing is required after impregnation to remove the HCl. The resulting sorbent
is not fully covered by CuCl, and the bare surface of carbon will participate in
adsorption of the gas mixture, thereby lowering the separation factor. Although
washing was not mentioned by Hirai et al. (1986b), the CuCl loadings in their
samples were also well below the amounts for monolayer coverage. A similar
procedure was used by Yokoe et al. (1987). In their work, both activated carbon
and activated alumina were used as the support. Before the activated alumina
was impregnated with CuCl, it was first impregnated with a hydrocarbon solu-
tion followed by coking in nitrogen such that a layer of carbon (at 1–3% wt.)
was deposited. Presumably, the carbon substrate provided a reducing environment
that helped keep the CuCl in the reduced state. Cuprous-ammonium-salt aque-
ous solutions have been widely used commercially for CO absorption (Kohl and
Riesenfeld, 1979; Safarik and Eldridge, 1998). The cuprous salts are solubilized
+
by complexing with ammonia, forming Cu(NH 3 ) 2 , which is soluble (Van Krev-
elen and Baans, 1950). Thus, the cuprous-ammonium-salt aqueous solution can
be used directly for incipient wetness impregnation. Alternatively, non-aqueous
double salts containing cuprous ion (Blytas, 1992; Safarik and Eldridge, 1998)
can be used. Of particular interest is CuAlCl 4 in toluene (Long et al., 1972;
1973; 1979). Using the CuAlCl 4 /toluene solution, Hirai et al. (1986a) success-
◦
fully prepared CuAlCl 4 supported on activated carbon. At 1 atm and 20 C, the
adsorbed amounts on the original activated carbon were (in cc/g) 9.5 (CO), 21.5
(CH 4 ), whereas and 7.2 (N 2 ), while the corresponding amounts on the CuAlCl 4
impregnated carbon were 24.1 (CO), 4.2 (CH 4 ), and 0.8 (N 2 ). This result indi-
cated that the surface of carbon was nearly fully covered by the salt. Hirai et al.
(1985) also prepared cuprous ammonium chloride supported on resin by using
the amino groups of the anion exchange resin. These sorbents showed good
selectivities for ethylene over ethane. The disadvantage of using CuAlCl 4 and
+
Cu(NH 3 ) 2 Cl is obvious. The loading of Cu is substantially reduced due to the
presence of the other salt, hence lowering the sorbent capacity.
CuCl/activated alumina and CuCl/activated carbon have been used commer-
cially for CO separation and recovery by pressure swing adsorption (Golden et al.,
1992a; 1992b; Kumar et al., 1993; Golden et al., 1998). These sorbents were pre-
pared by incipient wetness impregnation of the support with an aqueous solution of
a cupric compound with the aid of an “dispersant” (Golden et al., 1992a). Ammo-
+
nium citrate was used as the dispersant, which stabilizes Cu and should not be
decomposed. The solvent was subsequently removed by heating in an inert atmo-
◦
sphere at an elevated temperature (e.g., 200 C). For the activated carbon support,
a pre-oxidation step was found helpful for CuCl dispersion. Oxidation can be per-
formed by a number of means (see Chapter 5). Introduction of the surface oxide
groups by oxidation increased the interactions with water, or wettability, which
improved the adsorption of cupric salt and its dispersion.
Examples of the sample preparation were given by Golden et al. (1992a).
For the activated alumina support, the sample was first heat-treated in air at