Chemistry Reference
In-Depth Information
Fig. 2 Illustration of hydrothermal zeolite synthesis. Adapted from Ref. 17.
from the solution.
11
The final crystal size is a function of the ratio
between rate of nucleation and rate of growth of the nuclei.
12
The zeolite
crystallization process is dependent upon a number of parameters such
as: ageing of the synthesis gel,
13
solubility of silicon,
14
crystallization
temperature,
15
and addition of seed crystals.
16
Crystal sizes play important roles in the application of zeolites as
catalysts. For example, catalyst effectiveness is larger for smaller crystals,
but filtration and recovery of very small crystals can be a practical
challenge. On the other hand, deactivation can be more severe, and
regeneration of used catalyst can be more dicult for larger crystals.
12
Zeolites have widespread applications such as catalysts in oil
refineries, adsorbents for gas separation, and in ion exchange.
18-21
However considering market values, the catalytic application of zeolites is
the most important.
22
The possibility of generating functionality within
the zeolite pores by introducing heteroatoms into the framework and/or
extra framework make them attractive for a wide range of applications.
Such functionality may have acid, base, redox or bifunctional properties,
and act as an active site to catalyze numerous reactions.
23
The name zeolite is restricted to frameworks constructed from silicon
and aluminum as central atoms (T-atoms). There are other zeotype ma-
terials with structures similar to zeolites, but different types of T-atoms.
Examples of such zeotype materials are: SAPO (in which the T-atoms are
Si, Al, and P), AlPO
4
(in which the T-atoms are Al, and P), MeAPO
(in which the T-atoms are metal cations (Me), Al, and P), and MeSAPO
(in which the T-atoms are metal cations, Si, Al, and P).
Zeolites and zeotype materials can be distinguished from denser
materials of similar type based on their framework density (FD), the
number of T-atoms per 1000 Å
3
. For zeolites and zeotype materials, values
in the range of 12.1 T-atoms up to around 20.6 T-atoms per 1000 Å
3
are
observed, while dense materials have at least 20 T-atoms per 1000 Å
3
.
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