Page 184 - Adsorbents fundamentals and applications

ZEOLITES AND MOLECULAR SIEVES: SYNTHESIS AND MOLECULAR SIEVING PROPERTIES 169

(AlPO 4 s) and silicoaluminophosphates (SAPO 4 s). Although the main interest in
their synthesis was in catalysis, there are many potential applications for their
use as sorbents, one of which has already been recently commercialized (i.e.,
ETS-4, a titanium silicate, for N 2 /CH 4 separation, by Kuznicki et al., 2001).
The successful syntheses of transition metal-containing silicates have been
accomplished by incorporating transition metal atoms in the pentasil (MFI) frame-
work (shown in Figure 7.6). The large amount of literature on their syntheses has
been discussed by Perego et al. (1998). The “mixed alkoxide” method appeared to
be the most successful. For example, titanium-silicalite-1 (TS-1) was synthesized
by controlled hydrolysis of an aqueous solution containing tetraethylorthosilicate
(TEOS), tetraethylorthotitanate (TEOT), and tetrapropylammonium hydroxide
(TPAOH) as the template (Taramasso et al., 1983). Different templates were used
for the syntheses of TS-2, TS-3, etc., as reviewed by Perego et al. (1998). The
Ti atom is coordinated tetrahedrally in these structures. Many forms of vana-
dosilicates and ferrisilicates have also been synthesized (Perego et al., 1998).
Transition-metal containing silicates can also be prepared without the use of
templates. Young claimed hydrothermal synthesis of titano- and zirconosilicate
frameworks in 1967 (Young, 1967). Kuznicki (1990) was able to synthesize a
small pore titanosilicate molecular sieve, named ETS-4, by reacting a solution
◦
of sodium silicate/TiCl 3 /NaOH//KF at 150 C. KF was used to increase the crys-
tallization rate, and the pH of the solution was 10.5. ETS-4 has a Si/Ti ratio
of 2.6 and pore dimensions of 3–5 ˚ A, depending on the calcination temperature
◦
(Kuznicki et al., 2001). Its structure collapses near 350 C. Unlike TS-1, the Ti
atom in ETS-4 is octahedrally coordinated. A form of titanosilicate, named TAM-
5, has very high selectivities for Cs and Sr 2+ over Na (Anthony et al., 1994;
+
+
Hritzko et al., 2000). Using this sorbent, Wang and co-workers have designed
a highly efficient carousel process for removal of radioactive 137 Cs + from a
simulated nuclear waste (Hritzko et al., 2000).
The syntheses of aluminophosphate (AlPO 4 ) molecular sieves were first report-
ed in 1982 (Wilson et al., 1982a; 1982b and 1984). These molecular sieves have
a very narrow range of chemical composition (i.e., rather invariant ratio of P/Al
compared with the wide range of Si/Al ratio in zeolites), but exhibit a rich
diversity of framework structures. The chemical composition of AlPO 4 is

xR · Al 2 O 3 · 1.0 ± 0.2P 2 O 5 · yH 2 O

where R is an amine or quaternary ammonium ion. The average of the ionic radii
of Al 3+ (0.39 ˚ A) and P 5+ (0.17 ˚ A) is 0.28 ˚ A, which is similar to the ionic radius
of Si 4+ (0.26 ˚ A). This similarity apparently is responsible for the narrow range
of the chemical composition (i.e., P/Al ≈ 1). A large number of amines and
quaternary ammonium ions have been used as the templates for their syntheses.
AlPO 4 -5, AlPO 4 -11, AlPO 4 -17, and AlPO 4 -20 were among the first synthesized.
They are formed in both cage-type and channel-type framework structures, and
the sizes of their pore apertures vary from the size of 6-ring (2.8 ˚ A, in AlPO 4 -20)
to 7.3 ˚ A of 12-ring (in AlPO 4 -5). Tetrapropyl ammonium ion was a typical tem-
plate used for AlPO 4 -5. The structural diversity apparently reflects a dominant