Chemistry Reference
In-Depth Information
Chapter 6: Industrial Processes using
Solid Acid Catalysts
MARK A. HARMER
1 Introduction
one of the primary tools for achieving all twelve of
the principles of green chemistry. One good example
of using substances that possess little or no toxicity
to human health or the environment is in the man-
ufacture of cumene. Traditionally, solid phosphoric
acid or aluminium trichloride catalyse the benzene
alkylation with propene. These catalysts are corro-
sive and are categorised as hazardous wastes. This
process has been improved using the Mobil-Badger
cumene process, where the catalyst used (a zeolite)
is environmentally inert and gives high yields of
product.
This chapter outlines the use of solid acid catalysts
as an alternative to or as a replacement for homo-
geneous (liquid) acid catalysts such as hydrofluoric
acid. There has been a strong interest in the use of
solid acid catalysts as a replacement to homogeneous
catalysts such as HF, AlCl
3
and H
2
SO
4
. Although
these latter catalysts are very effective, they produce
highly corrosive media with chemically reactive
waste streams. Purification can be both difficult and
hazardous. The major challenge in this area is in the
development of cost-effective, highly active, selec-
tive and stable solid acid replacements. By contrast,
the solid acid counterparts are easier to handle,
purification is simpler and cheaper and the general
operation of a large chemical process is safer. Some
of the well-known industrial processes today use
homogeneous acid catalysts and the preference
would be to use a solid rather than a liquid. Two
examples are in the synthesis of ibuprofen (more
commonly used in the form of Advil
TM
) and in
the synthesis of high-'octane' fuels, which are based
almost exclusively on the use of HF (and sulfuric
acid for high-octane fuels). In the pharmaceuticals
industry, aluminium chloride often is used as a stoi-
chiometric reagent in a number of chemical trans-
formations, such as acylation chemistry. For every
pound of the desired product there can be several
pounds of waste (from spent, stoichiometric
reagent). Clearly, the ability to use a solid that is
highly active, catalytic and yet safe to handle would
Catalysis lies at the very heart of waste minimisation.
Indeed, one of the main goals of scientists working
in the area is to produce products with 100% yield
and 100% selectivity. By careful choice and design
of catalysts, this eventually will allow us to manu-
facture chemicals with zero waste. As pointed out by
Anastas
et al
. [1], catalysis has manifested its role as
a fundamental tool in pollution prevention. The
general topic of catalysis is intimately linked with the
design, development and implementation of green
chemistry. The term 'green chemistry' refers to the
design of chemical products and processes that
reduce or eliminate the use and generation of haz-
ardous substances. In this chapter we draw attention
to the growing role that solid acid catalysis is playing
within this arena. At the outset we point out an often
overlooked simple fact that reducing waste due to
increased yield represents a huge economic driver.
Improving the economics of a process while reduc-
ing waste often go hand in hand.
Throughout this chapter we will guide the reader
to recent key publications that describe in detail
some of the concepts that will be discussed. In the
area of solid acid catalysis, for example, we are for-
tunate to have an excellent collection of review arti-
cles covering the many different kinds of solid acid
catalysts. In total there are approximately 103
known industrial processes that use solid acid cata-
lysts [2]. The kind of reactions practiced include
alkylation, etherification, cracking, dehydration,
condensation, hydration, oligomerisations, esterifi-
cation, isomerisation and disproportionation. The
types of catalysts used include zeolites, oxides, ion-
exchange resins, phosphates, clays, complex oxides,
heteropolyacids and sulfated zirconia. Anastas [3]
has helped to design a set of principles (twelve in
total) that help to define how we can reduce or
eliminate the use or generation of hazardous sub-
stances in the design, manufacture and application
of chemical products. It was shown that catalysis is
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