Page 179 - Adsorbents fundamentals and applications

164 ZEOLITES AND MOLECULAR SIEVES

7.2. ZEOLITES AND MOLECULAR SIEVES: SYNTHESIS
AND MOLECULAR SIEVING PROPERTIES

At least 40 different types of naturally occurring zeolites have been found, begin-
ning with the discovery of stilbite (STI) by the Swedish mineralogist Cronstedt
in 1756, who also coined the term “zeolite.” The principal natural zeolites are
chabazite, (Ca, Na 2 )A1 2 Si 4 O 12 · 6H 2 O; gmelinite, (Na 2 ,Ca)A1 2 Si 4 O 12 · 6H 2 O;
mordenite, (Ca, K 2 ,Na 2 )A1 2 Si 10 O 24 · 6.66H 2 O; heulandite, CaAl 2 Si 6 O 16 · 5H 2 O;
clinoptilolite, (Na 2 ,K 2 ,Ca)Al 2 Si 10 O 24 · 12H 2 O; levynite, CaA1 2 Si 3 O 10 · 5H 2 O;
and faujasite, (Na 2 ,Ca, Mg,K 2 )A1 2 Si 5 O 14 · 10H 2 O. More than 150 types of zeo-
lites have been synthesized and are designated by a letter or group of letters (Type
A, Type X, Type Y, Type ZSM, etc.). Standard three-letter structure codes have
been adopted by the International Zeolite Association (IZA). (The website for IZA
provides helpful information about the codes as well as crystallographic data.)
Many more zeolite-like, crystalline molecular sieves have been successfully syn-
thesized by using amine additives as “templating” agents. Early work in zeolite
synthesis was performed largely by mineralogists using reaction conditions that
were thought likely to have arisen naturally under volcanic circumstances. The
commercial production of synthetic zeolites started with the successful develop-
◦
ment of low-temperature (25–100 C) synthesis methods by using very reactive
materials such as freshly co-precipitated gels or amorphous solids (Breck, 1974;
Milton, 1959). Two comprehensive monographs by Barrer (1978) and Breck
(1974) deal with all aspects of zeolites, including synthesis. The zeolites that
have been synthesized more recently are discussed by Szostak (1998) and others
(Chapters in Karge and Weitkamp, 1998; Jacobs and Martens, 1987; Dyer, 1988).

7.2.1. Synthesis of Zeolites A, X, and Y
Many alkali metal hydroxides and raw materials containing silica and alumina
can be used in low-temperature synthesis. The steps involving the Na 2 O-Al 2 O 3 -
SiO 2 -H 2 O system, which is used in synthesizing zeolites of types A, X, and Y,
are as follows (Breck, 1974):

NaOH(aq) + NaA1(OH) 4 (aq) + Na 2 SiO 3 (aq)
◦
T 1 = 25 C
∼
−−−−−−→
[Na a (A1O 2 ) b (SiO 2 ) c NaOH • H 2 O]gel
•
◦
∼
T 2 = 25 − 175 C
−−−−−−−−−→
Na x [(A1O 2 ) x (SiO 2 ) y ] • mH 2 O + solution(zeolite crystal)
The first step involves gel formation between sodium hydroxide, sodium
silicate, and sodium aluminate in aqueous solution at room temperature. The
gel is probably formed by the copolymerization of the silicate and aluminate