Chemistry Reference
In-Depth Information
or on products of fairly broad importance (e.g.
certain pharmaceuticals). Promising non-commer-
cial pilot- or laboratory-scale research is included
where appropriate. The reference list will direct the
reader to additional examples of reactions that are
applicable to industrial use but are still in the devel-
opmental stage or are being applied in specialised or
low-volume applications.
Table 14.2
Comparison of homogeneous and heterogeneous
catalysts
Homogeneous
Heterogeneous
catalyst
catalyst
Activity (relative to
High
Variable
metal content)
Selectivity
High
Variable
Service life
Variable
Long
Reaction conditions
Mild
Harsh
Sensitivity toward
Low
High
4 Catalysis
poisons
Catalyst recycling
Only recently has catalysis, either heterogeneous or
homogeneous, become green. In the absence of
attention now focused on the environmental impact
of a chemical process, catalysis was seen simply as a
highly efficient and economical way to make chem-
ical products. Catalysis has been practiced widely for
many years in the oil refining and bulk chemical
industry, even before green technology was recog-
nised. Catalysts offer a number of advantages,
including [19]:
Expensive
Not necessary
Diffusion problems
None
May be important
Variability of steric
Possible
Not possible
and electronic
properties
Mechanistic
Relatively good
Relatively poor
understanding
4.1 Examples of heterogeneous catalysis
in practice
• The elimination of stoichiometric reagents and the
resulting large amounts of waste associated with
these reagents.
• The ability to carry out transformations that are
difficult or impossible using conventional stoichio-
metric technology (from transformations as simple
as catalytic hydrogenation to more complex C-C
bond-forming reactions such as Suzuki, Stille or
Heck couplings).
• The ability to combine several transformations
into a single step by acting as a 'template' for the
assembly of different molecular fragments.
• The replacement of toxic or problematic reagents.
Heterogeneous systems are the cornerstones of
industrial catalysis. Many large-scale examples exist,
such as catalytic cracking or isomerisation, and
Fischer-Tropsch synthesis in the Sasol hydrocarbon
process [20]. Heterogeneous catalysts are key in the
selective Ag-catalysed oxidation of ethylene with
oxygen to give ethylene oxide and the direct ethyl-
ation of benzene and subsequent dehydrogenation
of ethylbenzene for the production of styrene (3.3 ¥
10
6
and 5.2 ¥ 10
6
t, respectively, in 1995 in the USA)
[21]. A wide range of materials have been used as
heterogeneous catalysts, including clays, both acidic
and basic zeolites, ion-exchange resins and metals on
various support materials [22]. These species have
led to new catalytic oxidations and reductions, or
replacement of stoichiometric reagents such as AlCl
3
or mineral acids, notorious for the production of
large amounts of waste salts.
A significant breakthrough in industrial heteroge-
neous catalysis has been achieved with the develop-
ment of the TS-1 family of titanium silicalite zeolites;
TS-1 is the first example of a 'redox molecular sieve'
[23]. In general, use of a zeolite to support an active
metal centre has the advantage of inhibiting both
leaching of the metal into the bulk solution and
preventing deactivation of the metal centres via
oligomerisation. The pore size can be defined by
Heterogeneous catalysis is by far the major player
in the chemical industry. However, more industrial
uses of homogeneously catalysed reactions are
appearing. A comparison of the strengths and weak-
nesses of both are shown in Table 14.2 [9].
The choice of one over the other obviously will be
determined by the needs of the process. It is inter-
esting that homo- and heterogeneous catalysis fre-
quently are viewed as competing, even when both
offer a unique set of properties for chemical pro-
cessing. The following sections will highlight recent
examples where significant benefits have been or
might be realised by the use of catalysis as part of
green technology.
