240
A. R. Jupp
phosphine or amine Lewis bases in combination with borane Lewis acids. Both intermolecular and intramolecular systems have been developed, and chiral systems can
be used for asymmetric hydrogenation (Fig. 7.1c) [8, 9]. In addition to hydrogenation catalysis, FLPs have been shown to activate a wide range of small molecules,
including CO 2 , CO, alkenes, alkynes, N 2 O and SO 2 . They have also found application in polymer synthesis, and providing perspective on biological systems. All of
these applications have been reviewed previously [10–24].
Across all of the varied and intensive research of FLPs to date, the vast majority
involve homogeneous systems, where the FLP components are dissolved in a solvent
and react with the dissolved substrate in the same medium. The molecular nature
of the Lewis acid and base within the FLP affords advantages for studying and
developing novel reactions in a laboratory setting. The electronic and steric nature of
the two components can be readily tuned, and the reactions can be easily monitored
in situ using a range of solution-phase techniques, of which NMR spectroscopy is
the most common and convenient.
However, the majority of catalysts used in industry are heterogeneous in nature,
typically with a solid-state catalyst and liquid or gaseous reagents [25]. Over 90%
(by volume) of the chemicals manufactured worldwide are synthesised using solid
catalysts [26]. For example, the Haber–Bosch process relies on a heterogeneous iron
catalyst for the formation of NH 3 from N 2 and H 2 , and zeolites are widely used in
the refining of crude oil [27]. The benefits of heterogeneous catalysts are manifold,
and include the ease of separation of the products, catalyst recycling and translating
scale-up by working under flow conditions instead of batch reactors. There has been
growing interest, therefore, in developing solid-state FLP systems as heterogeneous
catalysts, which may well be part of the missing puzzle piece in transforming FLPs
from academic curiosities to industrially relevant compounds. A Tutorial Review
by Qu et al. gave an excellent introduction to the topic of semi-solid and solid
FLP systems, and described the state-of-the-art at the end of 2017, with a particular
focus on two-dimensional materials and metal oxide surfaces [28]. This chapter will
therefore build on these early examples in a rapidly developing field, and give an
overview of the diverse and creative approaches that researchers are exploring to
accomplish heterogeneous reactivity and catalysis with FLPs. The account does not
give the discoveries in chronological order, but they are instead grouped according
to the type of heterogeneous system.
7.2 Semi-Immobilised Frustrated Lewis Pairs
One approach to carrying out heterogeneous catalysis with FLP systems is to immobilise either the Lewis acid or the Lewis base, and combine this insoluble species with
the complementary FLP component in solution. The solid acid or base can then be
easily separated from the product and the rest of the reaction mixture and recycled in
subsequent reactions. This approach is relatively simple to carry out, but the obvious
downside is that the soluble acid or base component still needs to be separated from
A. R. Jupp
phosphine or amine Lewis bases in combination with borane Lewis acids. Both intermolecular and intramolecular systems have been developed, and chiral systems can
be used for asymmetric hydrogenation (Fig. 7.1c) [8, 9]. In addition to hydrogenation catalysis, FLPs have been shown to activate a wide range of small molecules,
including CO 2 , CO, alkenes, alkynes, N 2 O and SO 2 . They have also found application in polymer synthesis, and providing perspective on biological systems. All of
these applications have been reviewed previously [10–24].
Across all of the varied and intensive research of FLPs to date, the vast majority
involve homogeneous systems, where the FLP components are dissolved in a solvent
and react with the dissolved substrate in the same medium. The molecular nature
of the Lewis acid and base within the FLP affords advantages for studying and
developing novel reactions in a laboratory setting. The electronic and steric nature of
the two components can be readily tuned, and the reactions can be easily monitored
in situ using a range of solution-phase techniques, of which NMR spectroscopy is
the most common and convenient.
However, the majority of catalysts used in industry are heterogeneous in nature,
typically with a solid-state catalyst and liquid or gaseous reagents [25]. Over 90%
(by volume) of the chemicals manufactured worldwide are synthesised using solid
catalysts [26]. For example, the Haber–Bosch process relies on a heterogeneous iron
catalyst for the formation of NH 3 from N 2 and H 2 , and zeolites are widely used in
the refining of crude oil [27]. The benefits of heterogeneous catalysts are manifold,
and include the ease of separation of the products, catalyst recycling and translating
scale-up by working under flow conditions instead of batch reactors. There has been
growing interest, therefore, in developing solid-state FLP systems as heterogeneous
catalysts, which may well be part of the missing puzzle piece in transforming FLPs
from academic curiosities to industrially relevant compounds. A Tutorial Review
by Qu et al. gave an excellent introduction to the topic of semi-solid and solid
FLP systems, and described the state-of-the-art at the end of 2017, with a particular
focus on two-dimensional materials and metal oxide surfaces [28]. This chapter will
therefore build on these early examples in a rapidly developing field, and give an
overview of the diverse and creative approaches that researchers are exploring to
accomplish heterogeneous reactivity and catalysis with FLPs. The account does not
give the discoveries in chronological order, but they are instead grouped according
to the type of heterogeneous system.
7.2 Semi-Immobilised Frustrated Lewis Pairs
One approach to carrying out heterogeneous catalysis with FLP systems is to immobilise either the Lewis acid or the Lewis base, and combine this insoluble species with
the complementary FLP component in solution. The solid acid or base can then be
easily separated from the product and the rest of the reaction mixture and recycled in
subsequent reactions. This approach is relatively simple to carry out, but the obvious
downside is that the soluble acid or base component still needs to be separated from