Chemical Engineering July 2017 - 30

Facts At Your Fingertips
Zeolites
Department Editor: Scott Jenkins
T
he physical properties of zeolites
make them extremely important
in many industrial processes.
This one-page reference provides information
on the structure and chemistry
of zeolites that allows their use in
the chemical process industries (CPI).
Zeolite structure
Zeolites are a class of hydrated aluminum
silicates made up of silica and
alumina tetrahedra (SiO4 and AlO4
linked in a tetrahedral configuration).
The linked tetrahedra form complex
three-dimensional structures with
large, cage-like cavities and channels
(Figure 1). Zeolites have open,
non-dense structures - 20-50%
of a zeolite's volume is void space.
Zeolites' uniform framework of pores
can be exploited to control chemical
reactions by adsorbing reactant molecules
within the zeolite pores. Certain
reaction pathways will be favored
based on the shape and chemical
composition of the zeolite pores.
There are 45 types of naturally occurring
zeolites, but their structures
and properties are limited from an
industrial perspective. Most industrial
uses employ one or more of
the close to 200 synthetic zeolites
that currently exist. The International
Zeolite Association (www.iza-online.
org) Structure Commission assigns
a three-letter code for each class of
zeolites, corresponding to its threedimensional
crystal structure.
Synthetic zeolites allow control of
Zeolite platform/carrier
catalyst type
a wider range of properties, including
Si-to-Al ratio, pore size
and geometry, and the ability
to incorporate metals and
other elements into the active
sites. Most synthetic zeolites
are made using a process of
slow crystallization of a silicaalumina
gel in the presence of
alkalis and organic templates.
Many possible zeolite structures
are theorized.
Major industrial uses
Zeolites are widely used in the CPI;
the majority in the following three
general areas:
Catalysis. Because zeolites are
cation exchangers, it is possible to
introduce a variety of cations with
different catalytic properties into the
zeolite's crystal structure. Simultaneously,
their rigid pore geometry
serves as a steric influence on reactions,
where the pore size and shape
controls the access of reactants and
products. By controlling the pore
size and architecture, engineers can
suppress formation of undesired
products, for example. Zeolites serve
as catalysts for many important reactions
involving organic molecules
in the petroleum refining and petrochemical
sectors (see Table 1).
Adsorption and separation. Zeolites'
porous crystalline structure can
be exploited as a molecular sieve,
where molecules with certain properties
are preferentially adsorbed.
TABLE 1. SAMPLE ZEOLITE-CATALYZED REACTIONS IN THE PETROCHEMICAL INDUSTRY
Process
Reaction
Fluid catalytic cracking (FCC)
of crude petroleum fractions,
such as vacuum gas oil (VGO)
Hydrocracking of heavy petroleum
distillates
Catalytic dewaxing
Alkylation of benzene
Isomerization of xylenes
28
Zeolite Y (IZA code
FAU). ZSM-5 (IZA code
MFI)
Isomerization of light gasoline Acidic zeolite with
noble metal
Zeolites X and Y (IZA
code FAU)
ZSM-5 (framework
code MFI) and others
HZSM-5 (ion-exchange
sites occupied by H+)
Disproportionation of toluene ZSM-5; others may be
used also
ZSM-5
High-molecular-weight hydrocarbons are broken
into C5 to C8 hydrocarbons, as well as C3 and C4
olefins, such as propylene
Conversion of straight-chain alkanes to branchedchain
isomers to achieve higher octane numbers
Converts poor-quality feeds into gasoline and
kerosene
Selectively hydro-isomerizes or cracks straightchain
and slightly branched paraffins. Used to
improve cold-flow properties of diesel fuel and
lubricant oils
Combining ethylene with benzene to create ethylbenzene
for styrene and other purposes
Transfers methyl group to create more valuable
para-xylene and benzene from toluene
Creates para- and ortho-xylene from meta-xylene,
a less valuable product
FIGURE 1. The linked silica and alumina tetrahedra
of zeolites create cage-like cavities and channels
that can be exploited to control chemical reactions
Zeolites can be designed to separate
molecules according to differences
in size, shape and polarity,
making them useful as dessicants
and in gas separation.
Ion exchange. Hydrated cations
within the zeolite pores are bound
loosely to the zeolite framework, and
can exchange with other cations in
aqueous media. This ability makes
zeolites useful as industrial water-softeners,
where sodium and potassium
ions in the zeolite are exchanged for
calcium and magnesium ions in the
water. Also, zeolites can be used to
remove radioactive or toxic heavymetal
cations
from liquid nuclear
waste and groundwater, respectively.
Catalysis chemistry
In addition to cations, zeolites can
have protons bound into their framework.
This molecular structure gives
rise to high acidity, which can be
useful in many key organic reactions.
Protons can act as Brønsted acids
(proton donors) in reactions with hydrocarbons
that form carbocations,
for example.
n
Selected references
1. Baerlocher, C., McCuskar, L.B., and others, " Atlas of
Zeolite Framework Types, " 6th ed., Elsevier, Amsterdam,
The Netherlands, 2007.
2. Martinez, C. and Perez-Pariente, J. (editors), " Zeolites
and Ordered Porous Solids: Fundamentals and Applications, "
Valencia (Spain) Polytechnic Institute, 2011.
3. Peskov, M., Zeolites, University of Stockholm, www.
asdn.net, accessed May 2017.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2017
http://www.iza-online http://www.asdn.net http://WWW.CHEMENGONLINE.COM
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