Chemistry Reference
In-Depth Information
represent a major breakthrough in any one of the
reactions described above.
Our aim is to introduce the reader to some of the
accomplishments and challenges in the development
and use of solid acid catalysts. The literature in this
fascinating area is extensive and no attempt has been
made to provide a comprehensive review of this
topic. Our approach has been to provide brief
overviews of some of the well-known research and
development in the use of solid acid catalysts. We
then describe some selected case studies of different
reaction types or new development of materials,
reactor design and engineering. We would like to
point out also that there are a number of other
approaches based upon acid catalysts that may offer
similar advantages in terms of waste minimisation,
high yield and ease of product separation. These
would include the use of ionic liquids as catalysts and
the use of supercritical solvents as an alternative
reaction medium for enhancing the reactivity and
conversion of reagents to products. In this chapter
we will specifically describe the Mobil-Badger pro-
cess for the product of cumene, the Kvaerner process
using ion-exchange resins and novel reactor design
in esterification reactions, the use of new fixed-bed
alkylation technology to make high-octane fuels, the
development of new Nafion
®
-based nanocomposite
technology for a number of different applications
and prospects for heteropolyacids and sulfated
zirconias.
strengths of solutions of strong acids, where the
equilibrium constant for Equation 6.1 lies far to the
right, are typically measured using families of indi-
cators of varying base strengths (e.g. the neutral
bases known as Hammett indicators). In solution,
quantitative scales of Brønsted acidity are based
upon the determination of equilibrium constants for
these kinds of reactions. The well-known Hammett
acidity function for an acid is expressed as Equation
6.2:
[]
[
]
+
HK
o
=
p
+
log B
BH
(6.2)
a
where the acid (AH) protonates the indicator base
(B) to form the corresponding conjugated acid (HB
+
)
and base (A
-
). In contrast the gas-phase proton affin-
ity of a molecule M is defined as the negative of the
enthalpy for the hypothetical reaction of Equation
6.3:
M + H
+
Æ MH
+
(6.3)
in isolation from its surroundings. In practice,
experimental determination of proton affinities
involves the measurement of proton transfer of equi-
libria between two gas-phase bases. The general
concept of acidity in both solution [5] and the gas
phase [6] has been described in detail in the litera-
ture and volumes exist that describe values for a
range of compounds. A range of techniques have
been employed, such as spectrophotometric
methods, by using an appropriate series of colour
indicators, the use of nuclear magnetic resonance
methods, heats of protonation of weak bases, etc. A
useful crossover point in terms of acid strength is
based upon 100% sulfuric acid, which is known to
have a Hammett acidity constant
H
o
of -12. Thus,
acids with acidity constants greater than -12, such
as -14.1 for CF
3
SO
3
H, are often referred to as
'superacids'. One of the strongest acids known is
based upon the mixture of HSO
3
F with SbF
5
, where
the acidity can range from about -14 to -26 (
H
o
).
In the case of solid acid catalysts, similar methods
using Hammett acidity functions are possible but are
often regarded with some scepticism. This is largely
due to the fact that when in the solid state the chem-
istry is much more complicated than when in either
a solution or gas phase. Scales of solution acid
strengths are very solvent dependent. Every solid
matrix may have a solvent-like role in determining
proton transfer equilibrium constants, but the situa-
tion is even more complicated because the individ-
2 Concepts in Acidity and Solid
Acid Catalysts
In this chapter we will use the Brønsted definition
of an acid: 'An acid is a species with a tendency to
give up a proton' [4]. The species that accepts the
proton is the base. The concept of the Brønsted acid
strength of a molecule AH in solution is very well
understood. It is quantified in terms of the equilib-
rium constant for the proton transfer reaction (Equa-
tion 6.1):
AH + B Æ A
-
+ BH
+
(6.1)
where B is a reference base. When B is H
2
O and the
equilibrium is measured in water, the resulting scale
is the familiar p
K
a
scale. This is also true of the
gas-phase proton affinity of molecules. The gas-
phase measurement is essentially an unambiguous
measure of acidity because it is solvent free. Acid
