Chemistry Reference
In-Depth Information
Scheme 6.2
smaller paraffins (or hydrogen) and carbenium ions.
These react with paraffins to give additional products
(see Scheme 6.2).
The rate of
n
-butane isomerisation (e.g. sulfated
zirconia has a value of 5 ¥ 10
-9
mol s
-1
g
-1
) then can
act as a guide for relative acidity. The stronger acids
such as sulfated zirconia or heteropolyacids are
known to catalyse the isomerisation of
n
-butane,
rather than a much 'weaker' acid such as an
Amberlyst
®
ion-exchange resin (see below), which is
essentially inactive for this reaction.
a number of alternative catalysts are used in some of
these reactions in different manufacturing plants.
Full details can be found in the literature [2].
3.1 Zeolite-based solid acid catalysts
Zeolites are crystalline materials with open frame-
work structures. Approximately 200 million pounds
of zeolites are used as catalysts. The primary build-
ing blocks of zeolites are [SiO
4
]
4-
and [AlO
4
]
5-
tetra-
hedra, which are linked by their corners to form
channels and cages (or cavities) of discrete size. The
pore opening can range from 3 to 20 Å. Owing to
charge differences of the Al and Si tetrahedra, the
total charge of an aluminium-containing zeolite
(molecular sieve) is negative and hence is balanced
by cations, protons, alkali or alkaline-earth metal
ions. The beauty of these zeolite structures is the
great diversity yet fine control of the structure. Zeo-
lites offer various systems of channels and cavities,
resulting in one-, two- or three-dimensional diffu-
sion for included guest molecules. The aluminosili-
cate backbone of a zeolite can be represented in a
number of ways, including the well-known sodalite
cage structure shown in Fig. 6.1.
The sodalite cage structure (centre and left) is con-
structed of openings containing four- and six-
membered rings of [SiO
4
]
4-
and [AlO
4
]
5-
tetrahedra.
The faujasite structure (right) is constructed of
sodalite cages connected through oxygen bridges
containing six-rings and the largest pore opening is
a 12-membered ring. The variety in structure types
provides a range of pore dimensions consisting of
ring openings containing 8-membered rings, the
medium-pore zeolites with 10-membered ring open-
ings and the large-pore materials with 12-membered
ring openings. A very good discussion of zeolite
structures is described in
Inclusion Chemistry with Zeo-
lites: Nanoscale Materials by Design
[18]. Figure 6.2
shows the mordenite structure of HZSM-5. The sheer
elegance of these types of materials and their ability
to carry out shape-selective catalysis is self-evident.
It is clear how the structure of the zeolite can direct
3 Industrial Applications of Solid
Acid Catalysts
Tanabe & Hoelderich [2] recently have published a
very useful survey of the industrial applications of
both solid acid and solid base catalysis, and some of
the salient points will be described here. Two com-
plementary earlier reviews also deal with a number
of commercial applications [16,17]. Of the 127
industrial processes practiced using these catalysts,
103 are based upon solid acid catalysts. It should be
noted that this number does not reflect all of the
processes using solid acid catalysts because at least a
number of these are proprietary in nature. A number
count for the different types of reactions and types
of catalysts was described. These industrial processes
include dehydration and condensation (18), iso-
merisation (15), alkylation (13), etherification (10),
cracking (8), aromatisation (7), hydration (7),
oligomerisation (6), hydrocracking (4), esterification
(3) and disproportionation (2), with other processes
making up the total to 127. The catalysts used
include zeolites (74), oxides (54), ion-exchange
resins (16), phosphates (16), clays (4) and a number
of less-well-defined catalysts. Of all the catalysts
specified, the number of ZSM-5 plus high-silica pen-
tasil zeolites is the largest of the various zeolites.
For each of the reaction types listed, we have
selected an example of a specific industrial reaction,
a catalyst used and an example of the scale of man-
ufacturing (Table 6.3). We point out, however, that
